Version of Record: https://www.sciencedirect.com/science/article/pii/S2949908923001322
Manuscript_1074cf5818abfe57d1ada7af812a8a50
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Review of the Adsorbents/Catalysts for the Removal of Sulfur Compounds from Natural
Gas
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Percival Soni Castro1,2, Gerson Martinez Zuniga1,2, William Holmes1,2, Prashanth R
Buchireddy1,2, Daniel D. Gang2,3, Emmanuel Revellame2,4, Mark Zappi1,2, Rafael Hernandez1,2*
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Abstract
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This paper aims to provide a compilation of studies on adsorptive removal of H2S and organic
sulfur compounds from natural gas at relatively low temperatures and pressures. The manuscript
compares sulfur uptakes among different materials, including zeolites, activated carbons,
metallic organic frameworks (MOFs), and metal oxides, and the impact of modifications to these
materials. Best sulfur uptakes were achieved by adding metallic or nitrogen species to the
adsorbent surfaces. This contribution to the peer-reviewed literature could provide a better
understanding of the improvements necessary to achieve greater sulfur capacities and the
mechanisms of sulfur interactions with adsorbents' surfaces.
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Keywords: activated carbons, catalysts, desulfurization, metal-organic frameworks, metal oxides, zeolites
1Department of Chemical Engineering, University of Louisiana at Lafayette, USA
2Energy Institute of Louisiana, University of Louisiana at Lafayette, USA
3Department of Civil Engineering, University of Louisiana at Lafayette, USA
4Department of Engineering Technology, University of Louisiana at Lafayette, USA
*Corresponding author’s email address: rafael.hernandez@louisiana.edu
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© 2023 published by Elsevier. This manuscript is made available under the Elsevier user license
https://www.elsevier.com/open-access/userlicense/1.0/
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Nomenclature
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AC - Activated Carbon
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AMP (Aminomethyl propanol)
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BET - Brunauer-Emmett-Teller
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CA: cellulose acetate
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CoRE MOF - Computation Ready Experimental MOF
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DDEC - Density Derived Electrostatic and Chemical
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DFT - Density Functional Theory
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DGA: Di-glycol amine
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DMDS: dimethyl disulfide
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ED: ethylenediamine
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EDS (Energy Dispersive Spectroscopy)
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EDS: ethyl disulfide
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EM: ethyl mercaptan
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ERI: Erionite
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FAU: faujasite
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FTIR (Fourier transform infrared spectroscopy)
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GC (Gas Chromatography)
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GHSV: gas hourly space velocity
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LNG (Liquified natural gas)
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MDEA (Methyl diethanolamine)
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MIL - Material Institute Lavoisier
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MM: methyl mercaptan
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MOFs - Metal Organic Frameworks
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MOR: Mordenite
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MS :(Mass Spectrometry)
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PIXE (Particle-induced X-ray emission)
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PSA (Pressure swing adsorption)
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PTSA (Pressure-temperature swing adsorption)
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PVSA (Pressure-vacuum swing adsorption)
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RH: relative humidity
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SEM (Scanning Electron Microscopy)
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SEM-EDS (Scanning electron microscopy-energy dispersive X-ray spectroscopy)
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SNG: synthetic natural gas
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TBM; Tert-butyl mercaptan
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THT: tetra hydrothiophene
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TSA (Temperature swing adsorption)
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UHV: ultrahigh-vacuum
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VSA (Vacuum swing adsorption)
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XPS (X-ray photoelectron spectroscopy)
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XRD: X-ray diffraction
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Table of Contents
1. Introduction ....................................................................................................................................................................................................................... 5
2. Process description ........................................................................................................................................................................................................... 6
2.1 Claus process ............................................................................................................................................................................................................. 6
2.2 Absorption ................................................................................................................................................................................................................. 7
2.3 Adsorption ................................................................................................................................................................................................................. 9
3. Type of adsorbents/catalysts ......................................................................................................................................................................................... 16
3.1 Zeolites ..................................................................................................................................................................................................................... 16
3.1.1 Ion exchange zeolites ..................................................................................................................................................................................... 19
3.1.2 X and Y zeolites ............................................................................................................................................................................................... 23
3.1.3 Zeolite composite materials .......................................................................................................................................................................... 27
3.2 Activated Carbon ..................................................................................................................................................................................................... 29
3.2.1 Modified activated carbons with acids, caustics, and/or salts ................................................................................................................... 31
3.2.2 Modified activated carbons with metals/metal oxides ............................................................................................................................... 33
3.2.3 N2-rich modified activated carbons............................................................................................................................................................... 34
3.3 Metal-Organic Frameworks .................................................................................................................................................................................... 36
3.3.1 Unmodified and modified MOFs ................................................................................................................................................................... 37
3.3.2 Composite of MOFs ........................................................................................................................................................................................ 39
3.3.3 Theoretical approaches for desulfurization using MOFs ............................................................................................................................. 42
3.4 Metal Oxides ............................................................................................................................................................................................................ 44
3.4.1 Pure metal oxides ........................................................................................................................................................................................... 45
3.4.2 Mixed metal oxides ........................................................................................................................................................................................ 48
4. Reaction mechanisms ..................................................................................................................................................................................................... 57
4.1 zeolite ....................................................................................................................................................................................................................... 57
4.1.1 Rection mechanism ........................................................................................................................................................................................ 57
4.2 Activated carbons.................................................................................................................................................................................................... 59
4.2.1 Reaction mechanism ...................................................................................................................................................................................... 59
4.3 MOFs ........................................................................................................................................................................................................................ 60
4.3.1 Reaction mechanism ...................................................................................................................................................................................... 60
4.4 Metal oxides ............................................................................................................................................................................................................ 62
4.4.1Reaction mechanism ....................................................................................................................................................................................... 62
5. Sulfur capacities performances ...................................................................................................................................................................................... 63
5.1 Comparison of sulfur capacities of different zeolites ........................................................................................................................................... 63
5.2 Comparison of Sulfur capacities and surface areas of different activated carbons ........................................................................................... 65
5.3 Comparison of sulfur capacities of different MOFs .............................................................................................................................................. 69
5.4 Comparison of sulfur capacities of different metal oxides .................................................................................................................................. 71
5.5 Comparison of the highest sulfur capacities from best performing materials ................................................................................................... 72
6. Conclusions ...................................................................................................................................................................................................................... 74
Acknowledgments ............................................................................................................................................................................................................... 75
References ........................................................................................................................................................................................................................... 75
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1. Introduction
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Supply and demand for natural gas have increased significantly over the last decade due
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to the implementation of innovative fracking technologies (see Fig. 1)(Dismukes 2019), and the
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recognition that natural gas is more environmentally friendly when compared to other fossil
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fuels, such as coal and crude oil. Natural gas can be used for power generation, but also to
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produce chemicals, which may require more effective technologies to remove potential catalyst
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poisons, such as organic and inorganic sulfur compounds. Natural gas is categorized as dry or
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wet (containing more than 10 vol% of C2+ hydrocarbons), and as sweet or sour, depending on the
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amount of acid gases in the mixture (sour if the amount of hydrogen sulfide is greater than 1
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vol% and/or the amount of CO2 is greater than 2 vol %) . The type of distribution as well as the
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final usage of natural gas depends on its chemical composition (Table 1) (Tagliabue et al. 2009).
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Fig. 1. Oil and natural gas production trends in the US (Dismukes 2019)
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Table 1. Typical specifications on the composition of natural gas. Total sulfur is the total amount of H2S, Carbonyl
sulfide (COS), and Organic Sulfur (Tagliabue et al. 2009).
Impurity
H2O
H2S
CO2
Total Sulfur
N2
Feed to LNG Plant
<0.1 ppmv
<4 ppmv
<50 ppmv
<20 ppmv
<1 vol.%
Pipeline Gas
150 ppmv
5.7–22.9 mg Sm−3
3–4 vol.%
115–419 mg·Sm−3
3 vol.%
Hg
C4
C5+
Aromatics
<0.01 mg/Nm3
<2 vol.%
<0.1 vol.%
<2 ppmv
–
–
–
–
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The composition of natural gas can vary considerably depending on the country and
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region (see Table 2) (Ratnasamy et al. 2012). Removal of H2S and mercaptans has become
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important because relatively high concentrations of these compounds can result in severe
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corrosion of pipes and equipment in addition to being harmful to health and the environment.
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Mercaptans (or Thiols) are toxic, colorless, and corrosive compounds. These groups are
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identified by having SH functional groups attached to a hydrocarbon chain. Mercaptans must be
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removed from natural gas to comply with environmental regulations. However, small quantities
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of mercaptans are added to natural gas at the point of sale as a way of detecting leaks (Taheri,
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Babakhani, and Towfighi 2017). Previous research has focused on the removal of mercaptans
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using materials, such as γ-Fe2O3, activated carbon, and zinc oxide at elevated temperatures,
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generally between 300–400 °C (de Angelis 2012; Huang et al. 2015).
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2. Process description
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2.1 Claus process
The Claus process, which converts between 94-98% of H S to elemental sulfur (Zhang et
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al. 2015), is one of the most applied methods used by companies such as Shell and BP. The
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following equations describe the reactions in the Claus process (de Angelis 2012; Zhang et al.
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2015):
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H2S + 3/2O2 ―› SO2 + H2O
ΔH°= -519 kJ/mol (1)
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SO2 + 2H2S ―› 3/8S8 + 2H2O
(2)
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From the equations above, it is shown that the Claus (see Fig. 2 ) process involves 2 stages,
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being combustion the first one, where the temperature range is 1000 – 1200 °C, followed by
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catalytic reaction using mainly alumina or titania based-catalysts with a temperature range
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between 230 to 300 °C, (de Angelis 2012; Ghahraloud et al. 2017).
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Fig. 2. A schematic illustration of the Clauss process(Basu et al. 2012.)
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The super-Claus process was developed to remove the remaining H2S from the Claus
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process, using direct oxidation of the H S above the dew point (Ghahraloud et al. 2017; Lee et al.
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2005). The super-Claus process can achieve yields up to 99.5% of sulfur recovery (de Angelis
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2012). The process is carried out by the reaction of H2S with O2 to form directly elemental sulfur
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and water (de Angelis 2012). In addition, process temperatures can be as low as 200 °C by using
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Fe2O3/SiO2 as the catalyst, and the H2S concentrations that can be treated by the super-Claus
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process are below 2 %vol (Zhang et al. 2015).
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2.2 Absorption
Operation of sulfur removal technologies at lower temperatures and pressures could make
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the process more economically effective, there are several other processes to remove hydrogen
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sulfur and organic sulfur compounds from natural gas, such as reactive and non-reactive
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absorption, membranes, cryogenic distillation, and adsorption (Shah, Tsapatsis, and Siepmann
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2017). These systems could benefit from more research on the development of new absorbents,
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and catalysts for more cost-effective operation.
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Table 2. Composition (%vol) of raw natural gas depending on the location (Taheri et al. 2017).
Compound
North
America #1
North
America #2
Europe #1
Europe #2
Asia
Methane
Ethane
Propane
Butane
Carbon dioxide
95.0
3.0
0.4
0.1
1.0
95.0
3.0
0.4
0.1
1.0
89.0
5.0
2.0
0.5
1.0
67.0
0.5
0.5
13.0
0.5
89.0
5.0
3.5
2.5
0.0
Nitrogen
Oxygen
DMS (ppm)
TBM
THT
COS
H2 S
Other
Mercaptans
0.5
0.0
0.0
3.0
0.0
0.0
0.0
1.0
0.5
0.0
2.0
4.0
0.0
1.0
1.0
0.0
2.5
0.0
0.0
0.0
8.0
2.0
0.5
0.1
14.5
4.0
0.0
0.0
8.0
2.0
0.5
0.1
0.0
0.0
6.0
5.0
0.0
0.0
0.0
0.0
Amines and sodium hydroxide have been utilized as absorbents for the removal of sulfur
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compounds, especially mercaptans, from natural gas (Anyanwu et al. 2021; Huang et al. 2003;
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Kazemi et al. 2014; Shoukat et al. 2019). One of these amine processes is the amine scrubbing
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system for removing of acidic gases (Fig. 3). Sour Gas is introduced into a contactor vessel
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(Absorption step), where it is brought into contact with a circulating amine solution. The gas and
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liquid flow in opposite directions, maximizing the contact between the two. The amine solution
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selectively absorbs the acidic gases (H2S, CO2), which are then removed from the gas stream.
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The clean gas exits the contactor vessel and is discharged to the atmosphere or further
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processing. The spent amine solution, now containing absorbed acidic gases, is sent to a
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regeneration vessel (Desorption step). Here, it is heated at 100°C- 120°C to release the acidic
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gases, which are then sent to a gas treatment unit for further processing. The regenerated amine
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solution is cooled and returned to the contactor vessel to continue the scrubbing process
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(Rochelle 2009).
A mass transfer rate-based model of the absorption process has been developed to
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describe the removal of methyl, ethyl, propyl, and n-butyl mercaptans from gas streams using
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different concentrations of methyl diethanolamine (MDEA) or caustic soda. The results
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demonstrated a relationship between the molecular weight of a mercaptan and its absorption with
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amines. Caustic soda solutions are more effective in removing lighter mercaptans than heavier
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mercaptans due to their solubility in water . On the other hand, MDEA solutions are more
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effective at removing heavier mercaptans than lighter mercaptans due to the attractive
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interactions between the hydrocarbon parts of the mercaptans and MDEA molecules (Jones et al.
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2014).
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Fig. 3. Amine Scrubbing process (Rochelle 2009).
A study by Ghanbarabadi et al. (2015) simulated the removal of acid gases (H2S, CO2)
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and organic sulfur compounds (methyl and ethyl mercaptans, dimethyl sulfide, COS) from
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natural gas using Sulfinol (Sulfolane + MDEA + H2O), di-glycol amine (DGA), and MDEA
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mixed with aminomethyl propanol (AMP) as solvents. Removal effectiveness was compared
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with MDEA. The simulation showed that more than 30-40% of mercaptans along with sour gas
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are absorbed by Sulfinol-M solvent at lower flow rates. In addition, regeneration of Sulfinol-M
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was achieved by using 10-25% less energy when using different composition of Sulfolane,
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MDEA, and water. This can be translated into economic savings due to the use of less amount of
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solvent as well as less energy for regeneration.
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2.3 Adsorption
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Fig. 4. Pressure swing adsorption (PSA) process flow diagram (Costa et al. 2018)
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Adsorption processes are practical and economically feasible to remove sulfur
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compounds in low concentrations, such as mercaptans, from natural gas (Yousefi et al. 2017).
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Pressure swing adsorption (PSA), temperature swing adsorption (TSA), and vacuum swing
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adsorption (VSA) are examples of adsorption processes. In addition, there are some variations of
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these processes such as pressure-vacuum swing adsorption (PVSA), and pressure-temperature
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swing adsorption (PTSA). The PSA process works the adsorption step at pressures above
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atmospheric, and the desorption at pressures near atmospheric (see Fig. 4), while the VSA
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process uses near-ambient pressure for the adsorption step, and the desorption is carried out at
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vacuum (Sircar 2002). TSA process, which is preferred to PSA when species are strongly
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adsorbed, carries out the adsorption process at low temperatures, while the regeneration process
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is done at high temperatures (Bonjour et al. 2002). PTSA is a process that combines both
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techniques, TSA, and PSA. Similarly, PVSA combines PSA and VSA techniques. PSA process
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has been widely applied for the removal of sulfur compounds (T. Liu et al. 2016; Rallapalli et al.
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2020; Riboldi, Procedia, 2017; Tagliabue et al. 2009). PTSA and PVSA have been investigated
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for deep desulfurization of natural gas (Qazvini, Technology et al 2015; Yousefi et al. 2017).
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These processes can also be combined for even higher sulfur removal (Qazvini et al. 2015;
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Tohidi et al. 2015). In general, these processes are carried out by placing adsorbents in columns
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to form packed bed reactors to remove the sulfur compounds from gas streams. Plenty of
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materials have been investigated for adsorptive desulfurization. These materials can be
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categorized as zeolites, activated carbons, metal-organic frameworks (MOFs), and metal oxides.
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In addition, composites from those materials have been tested in the quest to find the best
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performing material with the highest sulfur uptakes as well as their ease for regeneration.
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This literature review describes the chemical and physical characteristics of different
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adsorbents used for the removal of sulfur from natural gas. These characteristics could be
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correlated with performance (See Table 3). Additionally, common properties could be used as a
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guide for synthesizing and selecting materials for removing sulfur from gas matrices in industrial
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or environmental applications. The removal of H2S at elevated temperatures and pressures has
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been widely investigated and reviewed by other researchers (Georgiadis, et al. 2020; Shah et al.
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2017; Tagliabue et al. 2012). This paper focuses on presenting information about investigations
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related to the removal of H2S as well as organic sulfur compounds like mercaptans, COS, and
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CS2 from natural gas at low temperatures and pressures.
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Table 3. Comparison of adsorption capacities of different adsorbent from literature
Order
Adsorbent
Composition
T
°C
P
atm
Sulfur capacity
mgS-mgcat-1
25
25
25
25
25
25
25
RT
25
25
25
25
35
50
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
1
0-.01
1.28
1.28
1
1.28
1.28
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
182.8
242.3
186.3
168.3
140.4
55.91
24
102.9
168
19
193.9
164.5
122.8
86.1
67.5
24.7
18.6
59
29.7
51.3
64.8
60.8
60.8
55.5
54.9
44.8
43
41.9
34.6
34
16.7
30.3
24.5
18.4
Reference
Zeolites
1
1
1
1
1
1
1
2
2
2
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
13X (NaX)
13X (NaX)
5A
5A
10X (CaX)
Silica-Alumina Gel
Silica-Alumina Gel
PEI on SBA-15
SAPO-43
BASF ZnO
IMS
IMS
IMS
IMS
IMS
13X
5A
Cu/13X 450 C
Cu/13X 450 C
Cu/13X 450
Ag/13X
Ni/13X
Ce/13X
CuCl2/13X
Zn/13X
Cu/13X 400 C
Cu (.05 mol/L)/13X
Cu (.01 mol/L)/13X
Cu/13X 350 C
CuBr2/13X
Hß
Cu (1 mol/L)/13X
Cu/13X 150 C
Cu/13X 550 C
0-2000 ppm CH3SH in CH4
5% CH3SH in CH4 .
0-2000 ppm CH3CH2SH in CH4
0-2000 ppm CH3SH in CH4
5% CH3SH in CH4 .
0-2000 ppm CH3CH2SH in CH4
0-2000 ppm CH3SH in CH4
6000 ppm H2S
H2S pure
1% H2S in H2
10000 ppm H2S
3000 ppm H2S
3000 ppm H2S
3000 ppm H2S
3000 ppm H2S CO2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g CH3CH2SH in N2
200 μg-S/g CH₃SSCH₃ in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
200 μg-S/g (CH₃)₂S in N2
(Taheri et al.,2017)
(Shah et al., 2017)
Georgiadis et al., 2021)
(Zhu et al., 2019)
4
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
8
9
9
10
10
10
10
10
10
10
HY
Cu(I)Y
Ag-Y
Cu(II)Y
NaX
NaX
NaX
CaX
CaX
MgNaX
MgNaX
ZnNaX
ZnNaX
BaX
BaX
NiNaX
NiNaX
NaY
CsNaX
CsNaX
CsY
NiY
ZnO/SiO2-Ag2O
ZnO/SiO2-Ce
ZnO/SiO2-CuO-La2O3
ZnO/SiO2-La2O3
ZnO/SiO2-MnOx
ZnO/SiO2-NiO
Cu-ZnO/SiO2
Clinoptilolite
Activated carbons
IAC
Granular AC (GAC)
AirDep CKC
SulfaTrap R8G
AirDep CKI
Red soil
Biochar
GAC
ZnAc2-CAC
200 μg-S/g (CH₃)₂S in N2
.5ppm H2S
.5ppm H2S
.5ppm H2S
.05% CH3CH2SH in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05% CH3SH in CH4
.05%COS in CH4
.05% CH3SH in CH4
.05% CH3SH in CH4
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
2 vol % H2S in H2
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
20
20
20
20
20
20
20
20
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7.4
243.6
216.4
173.1
244
183
30
140
55
132
4.5
115
4.5
111
55
88
7.5
78
66
11
69
62
189
177
161
140
132
113
213
87
200 ppm H2S, RH:80% in O2/H2S = 2:1
200 ppm H2S, RH:80% in O2/H2S = 2:1
20 ppm-H2S, .1% O2, CH4/CO2=1.66
20 ppm-H2S, .1% O2, CH4/CO2=1.66
20 ppm-H2S, .1% O2, CH4/CO2=1.66
2000 ppm H2S in N2
2000 ppm H2S in N2
2000 ppm H2S in N2
1000 ppm H2S in N2/CO2
30
30
30
30
30
RT
RT
RT
RT
1.97
1.97
1.97
0.98
407
140
84.8
49.9
27.0
18.8
10.4
5.9
1.6
(Crespo et al., 2008)
(Ryzhikov et al., 2011)
(Yang et al., 2010)
(Yasyerli et al., 2002)
(Bashkova et al., 2005)
(Georgiadis et al., 2020)
10
10
10
10
10
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
12
13
13
13
13
13
13
13
13
13
13
14
14
14
14
14
Na2CO3-CAC
KOH–CAC
>KI–CAC
CAC
>CuSO4–CAC
FeCl3-AC
HNO3-AC
Virgin Activated Carbon
FeCl3-AC
HNO3-AC
Virgin Activated Carbon
FeCl3-AC
HNO3-AC
Virgin Activated Carbon
FeCl3-AC
HNO3-AC
Virgin Activated Carbon
FeCl3-AC
HNO3-AC
Virgin Activated Carbon
FeCl3-AC
HNO3-AC
Virgin-AC
NMC-M/P-2
Cu0.5Zn0.5/AC
Cu0.25Zn0.75/AC
Cu0.1Zn0.9/AC
Cu0.05Zn0.95/AC
Cu0Zn1.0/AC
ZnOAC Darco Norit
ZnO (+KI)AC ROZ3 Norit
Cu-CrAC RGM1 Norit
KOH-AC RBAA1 Norit
Darco AC
MOFs
MIL-101
MIL-100(Cr)
MIL-47 (V)
MIL-53(Cr)
MIL-53(Al)
1000 ppm H2S in N2/CO2
1000 ppm H2S in N2/CO2
1000 ppm H2S in N2/CO2
1000 ppm H2S in N2/CO2
1000 ppm H2S in N2/CO2
SNG H2S
SNG H2S
SNG H2S
SNG CH3SH
SNG CH3SH
SNG CH3SH
SNG CH3CH2SH
SNG CH3CH2SH
SNG CH3CH2SH
SNG (CH3)2S DMS
SNG (CH3)2S DMS
SNG (CH3)2S DMS
SNG CH₃SSCH₃ DMDS
SNG CH₃SSCH₃ DMDS
SNG CH₃SSCH₃ DMDS
SNG (CH₂) ₄S THT
SNG (CH₂) ₄S THT
SNG (CH₂) ₄S THT
1000 ppm H2S -RH=80%
H2S = 3000 ppm in N2
3000 ppm H2S in N2
3000 ppm H2S in N2
3000 ppm H2S in N2
3000 ppm H2S in N2
3000 ppm H2S in N2
200 ppm in N2
200 ppm in N2
200 ppm in N2
3000 ppm H2S in N2
RT
RT
RT
RT
RT
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
RT
30
30
30
30
30
30
28
30
30
30
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
1
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.98
0.9
0.8
0.6
0.5
0.5
0.02
0.03
0.02
0.15
0.09
0.02
0.45
0.23
0.07
0.03
0
0
0.29
0.29
0.03
2.4
3.4
0.9
2770
49.7
46.3
44.6
43.6
33
33
29
27
20
6.8
H2S
H2S
H2S
H2S
H2S
30
30
30
30
30
20
20
20
20
20
1322
568
498
446
402.1
(Cui et al., 2009)
(Sun et al., 2013)
(Balsamo et al., 2016)
(Shah et., 2017)
14
14
15
15
15
15
15
15
15
16
16
16
16
16
16
16
16
17
17
17
17
17
17
17
17
18
18
18
18
18
18
18
18
18
18
19
19
19
19
19
MIL-53(Fe)
rho-Zmof
Cu-BTC
MIL-53(Al)
UiO-66(Zr)
NaY
Beta
ZsM-5
MIL-53 (Al)
UVM-7-ZIF-8
UVM-7-ZIF-8
SBA-15-ZIF-8
MCM-41-ZIF-8
ZIF-8
MCM-41-ZIF-8
SBA-15-ZIF-8
ZIF-8
UiO-67(bipy)-CuCl2
UiO-67(bipy)-Cu(acac)2
UiO-67(bipy)-CuSO4
UiO-67(bipy)-Co(NO3)2
UiO-67(bipy)-CuSO4
UiO-67(bipy)-Cu(NO3)2
Ni(bpb)
Zn(bpb)
MAC-3
MAC-1
MAC-2
MOF-199
MAC-3
MAC-2
MAC-1
MOF-199
ED-ZIF-8
WS-ZIF-8
IRMOF-3
Y-fum-fcu-MOF
Y-FTZB-fcu-MOF
IRMOF-3
Y-1,4-NDC-fcuMOF
H2S
0.2% H2S, 29.8% CO2,70% CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
60 ppm-TBM in CH4
1300 ppm CH3CH2SH in butane
4500 ppm H2S in N2
4500 ppm H2S in N2
4500 ppm H2S in N2
4500 ppm H2S in N2
1300 ppm CH3CH2SH in butane
1300 ppm CH3CH2SH in butane
1300 ppm CH3CH2SH in butane
1000 ppm H2S
1000 ppm H2S
1000 ppm H2S
1000 ppm H2S
1000 ppm H2S
1000 ppm H2S
30 ppm -C4H4S in He:CO2:CH4
30 ppm -C4H4S in He:CO2:CH4
600 mgm-3-CH3SCH3
500 mgm-3-H2S
500 mgm-3-H2S
500 mgm-3-H2S
500 mgm-3-H2S
600 mgm-3-CH3SCH3
600 mgm-3-CH3SCH3
600 mgm-3-CH3SCH3
88.8%CH4,7.3%CO2,3%H2S,1%He
88.8%CH4,7.3%CO2,3%H2S,1%He
.071 ppm DMS
.071 ppm H2S
.071 ppm H2S
.071 ppm H2S
.071 ppm H2S
30
30
35
35
35
35
35
35
35
30
30
30
30
30
30
30
30
25
25
50
50
50
50
50
50
50
50
25
25
30
25
25
30
25
20
49.3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.97
1.97
0.98
0.98
0
0.98
290
17.
300
220
90
160
100
30
220
1470
6120
5210
4920
3720
1320
1300
920
760
23
10
10
9
7.8
300
340
58.1
61.5
84.6
55.9
54.6
85.3
66.3
60.1
3305.7
1499.5
5.8
37.4
30.6
10.6
51
(G. Chen et al., 2015)
(Saeedirad et al., 2018)
(Vellingiri et al., 2016)
(Jameh et al., 2017)
(Deng et al., 2018)
20
20
20
20
20
20
20
HK-02
HKG-02
HK-02/PHGO
HK-01/PLGO
HK-01
PHHK-02
PLHK-01
Metal oxides
21
Fe/Mgo
22
10Mn−45Zn−45Ti−O
22
TRGZ-3
22
TRGZ-2
22
Ce-Mn
22
5Fe−15Mn−40Zn−
40Ti−O feed 1% H20
22
ZnO
22
10Mn−45Zn−45Ti−O u
22
TRGZ-1
22
20Fe−40Zn−40Ti−O
23
(UCI-4169
24
ZnO SudChemie
265
266
267
268
99.6 ppm H2S in N2
99.6 ppm H2S in N2
99.6 ppm H2S in N2
99.6 ppm H2S in N2
99.6 ppm H2S in N2
99.6 ppm H2S in N2
99.6 ppm H2S in N2
H2S wet
600 ppm H2S,25% H2, 7.5% CO2 in He
with 1%H20
99.4%CH4, 0.41%CO2, O.15%H2S
99.4%CH4 ,0.41%CO2, O.15%H2S
H2S
0.06% H2S,7.5%CO2,25%H2 in He
99.4%CH4, 0.41%CO2, O.15%H2S
600 ppm H2S,25% H2, 7.5% CO2 in He
with 1%H20
600 ppm H2S,25% H2, 7.5% CO2 in He
with 1%H20
600 ppm H2S,25% H2, 7.5% CO2 in He
with 1%H20
H2S
10000 ppm in H2
25
25
25
25
25
25
25
25
50
RT
RT
25
RT
50
RT
25
350
20
0.98
0.98
0.98
0.98
0.98
0.98
0.98
56.2
55.8
55.5
30.6
17
10.2
8.8
(Bhoria et al., 2020)
-
2600
(Lee et al., 2005)
160
114
88
85
(Shah et al., 2017)
-
82
53
41
35
17
10.5
32
(Polychronopoulou et al.,
2005)
(Balsamo et al., 2016)
269
270
271
3. Type of adsorbents/catalysts
3.1 Zeolites
Zeolites (also called molecular sieves) are microporous aluminosilicate minerals that
272
select adsorbate molecules based on their size due to a regular pore structure (Georgiadis et al.
273
2021). Alex Frederick Cronstedt named zeolites in 1756 combining the Greek words ‘zein’ (to
274
boil) and ‘lithos’ (rock) after he observed how a natural zeolite seemed to bubble when heating it
275
up (Masters and Maschmeyer 2011). Zeolites were used commercially as adsorbents in 1905 by
276
Robert Gans to soften water (Masters and Maschmeyer 2011). The type of molecules that can
277
flow through the pores of the zeolites depends on the dimensions of the pore channels (Keil et
278
al., 2013)-(Weber et al. 2008). Depending on the Si/Al ratio, zeolites can be categorized as low
279
silica zeolites (Si/Al molar ratio between 1 to 2), medium silica zeolites (Si/Al molar ratio
280
between 3 to 10), and high silica zeolites (Si/Al ratio of 10 and above) (Wang et al. 2019).
281
Zeolites have been applied as catalysts in the petrochemical industry for cracking and alkylation
282
for several decades (Shah et al. 2017). In general, the lower the Si/Al ratio in the zeolite, the
283
more suitable it is for adsorption of polar compounds, whereas the greater the Si/Al ratio is, the
284
more hydrothermally stable and hydrophobic the zeolite becomes, favoring the adsorption of
285
nonpolar molecules (Georgiadis et al. 2021). It has been stated elsewhere that the adsorption of
286
H2O in zeolites is related to the aluminum adsorption sites (Halasz et al. 2005). With aluminum-
287
rich zeolites, polar molecules are allowed to access the zeolite channels due to the presence of
288
extra lattice cations (Al+3) which produces a heterogeneous negative electrostatic field of oxygen
289
ions (Halasz et al. 2005). Moreover, the catalytic activity of zeolites has been attributed to the
290
presence of Brønsted and Lewis acid sites (Bailleul et al. 2019; Masters and Maschmeyer 2011;
291
Wang et al. 2014). While Brønsted acid sites are attributed to surface Si-OH and Al-OH groups,
292
three coordinate aluminum atoms act as Lewis acids to the hydroxyl groups on adjacent silicon
293
atoms (Masters and Maschmeyer 2011). In other words, Brønsted acid sites are proton donors
294
produced by hydroxyl groups, and Lewis acid sites are formed from unsaturated cationic sites,
295
leading to M+ (M = metal) signatures to interact with adsorbate molecules as an acceptor of the
296
electron pair (Weingarten et al. 2011).
297
One way to determine Brønsted and Lewis acid sites is through applying FTIR
298
spectroscopy using pyridine as a probe molecule, where the zeolite is saturated with pyridine
299
vapor (Bailleul et al. 2019) . Using the band at 1545 cm-1 for Brønsted acid sites and 1456 cm-1
300
for Lewis acid sites, and assuming that one molecule of pyridine is adsorbed on one acid site, the
301
concentration of Brønsted and Lewis acid sites can be calculated using the following equations
302
(Bailleul et al. 2019; Selli and Forni 1999):
303
CBAS= 1.88*IAB*R2/W
(3)
304
CLAS= 1.88*IAL*R2/W
(4)
305
Whereas CBAS is the concentration of Brønsted acid sites (µmol/g), CLAS is the concentration of
306
Lewis acid sites (µmol/g), IAB is the integrated absorbance of Brønsted acid sites (cm-1), IAL is
307
the integrated absorbance of Lewis acid sites (cm-1), R is the radius of the catalyst disk (cm), and
308
W is the mass of the catalyst sample (mg). The coefficients give the conversion to obtain the units
309
for the concentrations (µmol/g).
310
Currently, there are over 190 zeolites frameworks and more than 40 naturally formed
311
zeolites (Dehghan et al.,2017; Ozekmekci et al. 2015). Clinoptilolite (HEU), Erionite (ERI), and
312
Mordenite (MOR) are some examples of the most commercially used natural zeolites, while the
313
most common types of synthetic zeolites are based on faujasite (FAU) (Dehghan et al. 2017;
314
Georgiadis et al. 2021; Ozekmekci et al. 2015; Shah et al. 2017). Zeolites can be synthetized by
315
different methods such as ion exchange and impregnation (Ozekmekci et al. 2015). In addition,
316
hydrothermal synthesis and post synthesis methods can be used to synthesize zeolites with
317
different SiO2/Al2O3 ratios (Wang et al. 2019). Hydrothermal synthesis can be used to prepare
318
zeolites with a range of Si/Al ratio ≥ 1, but it cannot be used to modify the Si/Al ratio of natural
319
zeolites, which can be done using post synthesis method (Wang et al. 2019). Additionally, post-
320
synthesis methods such as dealumination, ion exchange, and functionalization, allow for a greater
321
degree of control over the properties of the zeolite, and can be used to modify the material for
322
specific applications. Dealumination is a process by which aluminum ions are removed from the
323
zeolite framework. This can be done by using chemical or thermal methods. It can change the
324
pore size of the zeolite, making it more selective for certain molecules, and increase its catalytic
325
activity. Similarly, ion exchange is a method by which the cations (positively charged ions) in a
326
zeolite framework are replaced with other cations. Functionalization is a process by which
327
chemical groups are attached to the surface of a zeolite. In conclusion, post-synthesis methods are
328
widely used in zeolite research and development to tailor the properties of these materials for
329
specific applications in industry. These methods can improve the zeolite performance in catalytic
330
cracking, removal of impurities from natural gas, and water treatment(Kuwahara et al. 2012; Wang
331
et al. 2019).
332
333
334
Fig. 5. Representation of the structural characteristics of Zeolite X and Y(Dantas de Oliveira et al. 2019).
335
X and Y zeolites are the two main types among FAU type zeolites. Zeolite X means that the
336
Si/Al ratio ranges between 1 and 1.5. Zeolite Y has Si/Al ratios higher than 1.5, both zeolites
337
have essentially the same crystalline structure (Fig. 5) (Dehghan et al. 2015.). As mentioned
338
above, zeolites, which have good sieving properties at a molecular level, can be modified by
339
introducing metal ions to the structure as a way of improving their selectivity towards polar gas
340
molecules (Shah et al. 2017; Zhu et al. 2019). Extra lattice cations (M+), which produces a more
341
heterogenous negative electrostatic field of oxygen ions, allow polar molecules to access
342
zeolites.
343
3.1.1 Ion exchange zeolites
344
Ion exchange zeolites (see Fig. 6) have compensating ions outside of their framework.
345
These ions are generally protons, alkaline earth metals, or alkali metals (Zhu et al. 2019). A
346
phenomenon called chemical complexation or π-complexation is produced between metal ions
347
such as Zn+2, Ni+2, Cu+, Fe+, or Na+ and sulfur compounds in the ion exchange zeolites (Dehghan
348
et al. 2017).
349
350
Fig. 6. Typical structure of ion exchange zeolites(Sobuś et al. 2020).
351
Copper based adsorbents, such as Cu(I)Y zeolite, Cu (II)Y zeolite, CuCl/MCM-41
352
(MCM-41 stands for Mobil Composition of Matter No. 41), and CuCl/SBA-15 (SBA-15 stands
353
for Santa Barbara Amorphous No. 15) have been compared with the best commercial adsorbents
354
AgY and activated carbon for desulfurization of natural gas at ambient temperature (Crespo et al.
355
2008). Hydrogen sulfide and dimethyl sulfide (DMS) were the main sulfur compounds. The
356
results showed that hydrogen sulfide is adsorbed more strongly than DMS, showing adsorption
357
energies Cu(I)Y>AgY>Cu (II)Y. In addition, the researchers found that Cu(I) performed better
358
than CuCl/SBA-15, and CuCl/MCM-41 in the following order: Cu(I)> CuCl/SBA-15>
359
CuCl/MCM-41. The regeneration process was carried out by heating up the spent sorbent using
360
He gas at 200°C, 300°C, and 450°C. In contrast to AgY, Cu(I)Y, CuCl/MCM-41, and
361
CuCl/SBA-15 were fully regenerable. The adsorption capacity of the regenerated Cu(I)Y,
362
CuCl/MCM-41, and CuCl/SBA-15 was similar to fresh sorbents, while the adsorption capacity
363
of AgY was reduced by almost half of its initial capacity after the regeneration step.
364
A previous investigation studied the adsorption of methyl mercaptan (CH3SH) and
365
carbonyl sulfide (COS) by metal exchanged zeolites (MeX) and layered double hydroxides
366
(LDH) from natural gas (Ryzhikov et al. 2011). NO3-M(II)/Al LDHs (M(II) = Mg, Ni, or Zn)
367
were the layered double hydroxides used by the investigators. The LDHs were synthetized using
368
co-precipitation method with nitrate salts M(II)NO3 under ambient conditions. The experiment
369
was carried out in a fixed-bed reactor, at atmospheric pressure and composition of 0.05%
370
CH3SH. A breakthrough was defined as 5 ppm of CH3SH in the effluent. The effluent was
371
monitored online by a gas chromatograph. According to the investigators, the affinity of CH3SH
372
was directly linked to the basicity for metal exchanged zeolites. In contrast, the affinity of COS
373
for Me-zeolite (Me = Metal) was lower compared to CH3SH. In addition, basicity controlled the
374
hydrolysis of COS on metal exchanged zeolites and LDHs. It has been reported elsewhere that
375
cationic zeolites with low Si/Al ratio (≈1) such as NaX contain orthosilicate species
376
characterized by relevant basicity, possessing a large number of acid-base sites, but access to
377
these sites is highly obstructed (Busca et al.,2017). Moreover, the exchange of Na+ with heavier
378
alkali ions such as Cs can increase the basic sites, but it increases also the obstruction of
379
molecular diffusion (Busca et al.,2017). The adsorption capacities of exchange zeolites for methyl
380
mercaptan were given by: NaX> CaX> MgNaX> ZnNaX> BaX> NiNaX> NaY> CsY>
381
CsNaX> NiY. The adsorption capacity of NaX for CH3SH was 183 mg/g.
382
Another study investigated the selective adsorption of ethyl mercaptan on NaX zeolite
383
(Weber et al. 2008). Binary mixtures of ethyl mercaptan with toluene or n-heptane were analyzed
384
at 298 K. The researchers observed that the adsorption-desorption isotherms for all the gases on
385
NaX zeolite were type I (characteristic for microporous solids), meaning they were reversible
386
and without hysteresis upon desorption. It was concluded that NaX zeolite is selective for
387
adsorption of ethyl mercaptan even if it is mixed with a non-polar gas.
388
Several other studies have been done related to the use of zeolites for the removal of
389
sulfur compounds from natural gas. AgNa-Y zeolite has been used as an adsorbent to test the
390
adsorption of dimethyl sulfide (DMS) and t-butylmercaptan (TBM) under ambient conditions
391
(Satokawa et al. 2005). The test was carried out in a fixed bed flow tubular reactor of 8 mm
392
internal diameter with 1 cubic centimeter of the zeolite (about 0.52–0.62g) at ambient
393
temperature and pressure. The influent concentrations of DMS, TBM, and water were 1.8 ppm,
394
1.2 ppm, and 10 ppm, respectively. Adsorbent capacities were determined by the breakthrough at
395
0.1 ppm of DMS and TBM in the outlet gases. The authors found that the breakthrough times for
396
adsorption on Ag(15)Na-Y (15 wt.% silver content) of DMS and TBM were 72.9 h and 82.2 h,
397
respectively. In addition, the sulfur capacity on Ag(15)Na-Y was 64 mg/g for DMS.
398
Regeneration of the adsorbents was carried out by a heat treatment at 500 °C for 2 h in air. The
399
degree of regeneration was evaluated by measuring the change of sulfur adsorption capacities
400
between the fresh and the regenerated samples. It was concluded that DMS and TBM were
401
adsorbed efficiently on AgNa-Y at ambient conditions independently of the presence of water in
402
the natural gas (high breakthrough times for DMS and TBM of 72.9 h and 82.2 h, respectively).
403
The adsorption capacity increased with an increase in the silver content of AgNa-Y under dry
404
and wet conditions. However, the sulfur capacities of the regenerated adsorbents decreased with
405
Ag content, which could be attributed to the formation of silver sulfate (observed by the
406
investigators using XRD), being difficult to remove during the regeneration process.
407
Another study measured the selective adsorption of methyl mercaptan from natural gas
408
using ion-exchanged zeolites Y (Chen et al. 2017). The investigators used a series of transition-
409
metal (Cu2+, Co2+, Zn2+, Ni2+) to modify NaY zeolite. The adsorbent materials were characterized
410
by N2 adsorption, X-ray photoelectron spectroscopy (XPS), energy disperse spectroscopy (EDS),
411
X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The study
412
investigated the competition between CO2 and methyl mercaptan as well as the multiple
413
adsorption-regeneration cyclic progress of a fixed-bed containing Cu-Y. The investigators used a
414
0.2g adsorbent sample in a quartz fixed-bed adsorption column (3 mm of internal diameter and
415
100 mm length). Nitrogen gas was first fed to the column followed by methane with a methyl
416
mercaptan concentration of 500 mg/m3. Five different flow rates (50, 100, 150, 200, and 250
417
mL/min) were applied to evaluate the impact of residence time with a constant temperature of 25
418
°C. Then, the temperature was varied (25, 40, 60, and 80 °C) with a constant flow rate of 100
419
mL/min to study the effect of temperature variation on methyl mercaptan adsorption on ion-
420
exchanged zeolites. For the regeneration step, the fixed-bed reactor containing the samples
421
saturated with methyl mercaptan was treated by flowing air into the reactor at 80 mL/min (Chen
422
et al. 2017). The results showed that the modified zeolites (except Ni-Y) had longer adsorption
423
breakthrough times than NaY. Cu-Y resulted in the best performance for methyl mercaptan
424
adsorption with a breakthrough time of 280 min. The highest sulfur capacity achieved by Cu-Y
425
was 70 mg S/g adsorbent. The adsorption capacity decreased with increasing the gas hourly
426
space velocity (GHSV). Furthermore, when the Cu-Y adsorbent was tested for adsorption using a
427
mixture of methyl mercaptan/4% CO2, its sulfur capacity barely decreased, suggesting that
428
methyl mercaptan was selectively adsorbed over CO2. Using density functional theory (DFT)
429
calculations, the researchers found that the adsorption energies for methyl mercaptan on NaY,
430
Cu(I)-Y, and Cu(II)-Y were -81.2 kJ/mol, -119.9 kJ/mol, and -136.3 kJ/mol, respectively.
431
Further explaining the better selectivity Cu-Y zeolite for methyl mercaptan in a real natural gas
432
influent compared to NaY zeolite.
433
3.1.2 X and Y zeolites
434
Other studies have been conducted to model the adsorption mechanism of sulfur
435
compounds from natural gas. For example, the 13X zeolite was tested as an adsorbent due to its
436
high selectivity to water and mercaptans (mostly ethyl mercaptan) (Shirani, Kaghazchi, and
437
Beheshti 2010). The TSA technique was applied, taking into consideration the pressure effect,
438
bed height, and operating time effects on the breakthrough time. Three parallel beds loaded with
439
13X zeolite were used, 2 beds for the adsorption process (12 hours each cycle) and 1 bed for the
440
regeneration process (5.5 hours). Table 4. shows the adsorbent characteristics. The following
441
conditions were used for the feed gas: gas flow rate of 6.646 kmol/s, gas pressure of 6400 kPa,
442
gas temperature of 295 K, gas molecular weight of 18.91, gas viscosity of 0.0137 cp, water mole
443
fraction of 0.0018, mercaptan mole fraction of 0.00025 and other component mole fractions of
444
0.9879. The researchers determined that the outlet concentration was proportional to the pressure
445
and the breakthrough time was proportional to bed height.
446
Table 4. Characteristics of adsorbent and process parameters (Shirani et al. 2010).
Parameter
Bed height, L
Bed diameter, d
Bed porosity, ε
Particle diameter, dp
Particle density, ρp
Bed temperature, T
Operating time, tcycle
Bed pressure, P
Adsorbent internal porosity
Adsorbent bulk dry density
Adsorbent surface area
Adsorbent sorptive capacity
Unit
M
M
M
kg/m3
K
Hr
kPa
%
g/mL
m2/g
g/g (dry)
Values
5.5
2.5
0.034
0.0026
630
295
12
6400
38
0.58-0.64
0.6
0.25-0.36
447
448
13X zeolite and other zeolites, such as 5A, Hβ, HY, and ZSM-5 were tested and
449
compared with activated carbon (AC) and silica gel for the adsorption of organic sulfur
450
compounds from natural gas (Zhu et al. 2019). The difference between the zeolites is the Si/Al
451
ratio in the following order ZSM-5> Hβ>HY>5A (Poly et al. 2018; Shao et al. 2009; Xu et al.
452
2007). This experiment was carried out in a fixed bed reactor at 25 °C and normal pressure. The
453
first part of this research was to determine the best performing adsorbent for methyl sulfide
454
adsorption. The highest adsorption capacity (24.7 mg S/g) was achieved by 13X zeolite. The
455
performance was given by: 13X> Hβ>5A>AC>>HY>Silica gel. The pore size seems to play an
456
important role in the performance of these adsorbents. The 13X zeolite had the largest pore size
457
(10 Å) compared to the other adsorbents (ranging from 2 to 8 Å). Thus, there is less resistance to
458
flow for methyl sulfide on 13X than the other adsorbents. Impregnating 13X zeolite with
459
Cu(NO3)2 improved the adsorption capacity for dimethyl sulfide. Adsorption performance
460
increased with Cu ion concentration. However, Cu positive impact on adsorption capacity
461
decreases when the metal ion concentration is above 0.1 mol/L. A large concentration of Cu
462
appeared to block the pores and reduced the distribution of Cu inside the zeolite, and thus the
463
concentration of active sites inside the pore structure. Impregnation of other metal ions on 13X
464
Zeolite resulted in the following adsorption performance:
465
Ag/13X>Ce/13X>Cu/13X>Zn/13X>Ni/13X>13X. The adsorption capacity for Ag/13X was
466
64.8 mg S/g. Experiments also evaluated the selectivity of ethyl mercaptan (EM), methyl sulfide
467
(MS), and dimethyl disulfide (DMDS) on Cu/13X, resulting in EM>MS>DMDS. The modified
468
and unmodified 13X zeolites can be fully regenerated by calcination at 350 °C.
469
Another study was performed on the modeling and optimization of an industrial
470
desulfurization process using the 13X zeolite (Khademi et al. 2015; Yousefi et al. 2017). The
471
TPSA process was applied using the 13X zeolite (Yousefi et al. 2017). Table 5. shows the 13X
472
zeolite specifications.
Table 5. Adsorbent specifications (Yousefi et al. 2017).
473
Specification
Bulk density (g/cm3)
Particle radius (mm)
Particle void fraction
Heat capacity (kJ/kg*K)
Tortuosity
Pore size (Å)
474
Zeolite 13X
0.65
2.2
0.24
1.07
1.38
9
475
Three parallel columns of 13X zeolite were configured: one for the adsorption process
476
and two for the regeneration step. Each cycle operated for 30 hours (18 hours adsorption and 12
477
hours regeneration). In this simulation, the operating pressure varied from 21 to 64.4 bar, and the
478
operating temperature ranged from 110 °C to 310 °C (Yousefi et al. 2017). Table 6. shows the
479
feed composition of the natural gas for this research.
480
Table 6. Feed composition of natural gas mixture with impurities (Yousefi et al. 2017).
Component
Nitrogen
CO2
H2S
Methane
Ethane
Propane
i-Butane
n-Butane
i-Pentane
n-Pentane
C6
C7
(ppm)
35511
8993
3
869198
55421
20370
3277
4614
1058
813
434
125
Component
C8
C9
C10
C11
C12
C13
COS
Methyl mercaptan
Ethyl mercaptan
Propyl mercaptan
Butyl mercaptan
H2O
(ppm)
31
4
0
0
0
0
3
20
105
8
2
11
481
482
Investigators concluded that mercaptans were the last to be adsorbed due to their low
483
concentration (between 2-105 ppm) (Yousefi et al. 2017). The relatively low adsorption
484
efficiency of methyl mercaptan could be caused by the small size of CH3SH molecule compared
485
to the zeolite pores, which reduces retention and interaction with the surface of adsorbents (Ma et
486
al. 2018). Heavy hydrocarbons did not prevent mercaptans adsorption. The adsorption capacities
487
increased with temperature. Moreover, at the regeneration stage, heavy hydrocarbons with weak
488
adsorption were removed first, followed by carbon dioxide and finally mercaptans. The
489
adsorption capacity was fully recovered after regeneration.
490
The performance of 13X zeolite was compared with the mixed metal oxide Zinox 380
491
(87.1 wt.% ZnO, 0.2 wt.% Na2O, 22.7 wt.% CaO) for adsorptive removal of 100 ppm of H2S
492
mixed with CH4 (Melo et al. 2006). The operating conditions were as follows: 25 °C and 75 °C,
493
and a pressure of approximately 5 bar. The maximum adsorption capacity for Zinox and 13X
494
Zeolite were 9.5 mg S/g and 53 mg S/g, respectively. Even though Zinox had a higher specific
495
surface area (34.54 m2/g) than 13X zeolite (16.15 m2/g), 13X zeolite performed better than
496
Zinox. Investigators concluded that performance differences were likely due to the different
497
pore size structures (13X has mesopores, and Zinox 380 has micropores).
498
3.1.3 Zeolite composite materials
499
Polymers combined with zeolites have also been investigated for sulfur removal. Zeolite
500
NaY and metal organic frameworks MIL-53(Al) and UiO-66(Zr) were combined with cellulose
501
acetate (CA) polymer to form hybrid porous composite fibers for the selective adsorption of
502
mercaptans from natural gas streams (Chen et al. 2016). The capacities of the sorbents were
503
measured using different desorption temperatures to evaluate the potential for lower temperature,
504
energy, and cost-efficient system operation. Both fibers yielded high adsorbent loadings, having
505
a high selectivity for t-butyl mercaptan (TBM) while being stable for multiple regeneration
506
cycles. The materials were evaluated for equilibrium adsorption capacity and cyclic stability at
507
35 °C and atmospheric pressure with natural gas containing 60 ppm TBM. In addition, the
508
materials were pretreated at different temperatures (120, 200 and/or 300°C) for 1 hour in helium.
509
The desorption of the sulfur compounds from the materials was carried out using the same
510
conditions as the pretreatment. The NaY/CA fibers had higher overall adsorption than UiO-
511
66(Zr)/CA fibers, but UiO-66(Zr)/CA fibers had higher stability. Table 7. shows the adsorbents
512
properties used in this research (Chen et al. 2016).
513
Table 7. Properties of adsorbent sieve materials and hybrid fiber (Chen et al. 2016).
514
Sample
NaY sieve
75 wt% NaY/CA fiber
50 wt% NaY/CA fiber
UiO-66(Zr) SB sieve
UiO-66 (Zr) LB sieve
75 wt% UiO, 66(Zr)/CA fiber
50 wt% UiO, 66(Zr)/CA fiber
MIL-53(Al) sieve
50 wt% MIL, 53(Al)/CA fiber
Sorbent-free CA fiber
BET surface
area (m2/g)
868
598
339
1002
1063
652
481
1329
334
60
Micropore volume
(cm3/g)
0.35
0.26
0.15
0.40
0.38
0.26
0.18
0.54
0.14
0.02
Particle size (µm)
1-3
1-3
1-3
1-4
1-4
1-4
1-4
1-4
1-4
515
516
The removal of mercaptans from natural gas for use in fuel cells has also been
517
investigated (Roh et al. 2004). The adsorption of TBM and THT from natural gas was evaluated
518
using BEA (Beta) zeolite, Fe/BEA, activated carbon (AC), and Fe/AC. BEA zeolite has a higher
519
Si/Al ratio than zeolites X and Y in the following order: BEA>zeolite Y>zeolite X. The
520
operating conditions were the following: 200 mg of adsorbent pretreated at 300 °C for 1 hour in
521
He before the experiment, a temperature of 150 °C on lines to avoid sulfur condensation, a quartz
522
tube as reactor at 50 °C, and natural gas with 50 ppm of TBM. The breakthrough time was
523
considered at 10 ppb of TBM and THT. The researchers found that 3% Fe/BEA (prepared by
524
ion-exchanged method) had the highest sulfur adsorption among all the tested sorbents.
525
Chemisorption due to the Lewis acid sites on the Fe/BEA zeolite was considered as the possible
526
reason for the high sulfur removal. It is important to highlight that Lewis acid sites are generated
527
by the introduction of a cation (Fe2+ in this research) in the zeolite structure.
528
SBA-15 zeolite used as support for Cu-Zn oxide nanoparticles for the simultaneous adsorption
529
of H2S and Hg0 from natural gas was studied by Zhang, et. al. (Zhang et al. 2021). The adsorbent
530
xCuyZn/SBA-15 was prepared by sol gel method loading 30 wt% of metal oxides, where x and y
531
were the molar ratios x:y of 1:4, 1:2, 1:1, and 2:1. From characterization of the sorbent, it was
532
confirmed that the hexagonal arrangement of SBA-15 structure did not change after the loading
533
with Cu-Zn oxides. Moreover, the sorbent with the different molar ratios followed a type IV
534
isotherm, which is reversible on the IUPAC classification. The researchers observed that the best
535
performing adsorbent was 1Cu1Zn/SBA-15 (ratio 1:1) for the simultaneous removal of H2S and
536
Hg0 with adsorption capacities of 296.78 mg S/g and 12.75 mg Hg0/g, respectively. The high
537
adsorption of this adsorbent could be explained by the affinity of H2S to Cu and Zn oxides on the
538
surface of the SBA-15. It seems that the addition of these oxides significantly increases the number
539
and availability of adsorptive sites. Investigators indicated that H2S first reacts with the metal
540
oxides, then the resulting sulfur and sulfide species are selective for Hg0 to form stable HgS. It can
541
be observed that Cu and Zn oxides have a synergy in balanced doping amounts that improves the
542
adsorptive removal of H2S and Hg0. The regeneration of the 1Cu1Zn/SBA-15 was carried out at
543
600 °C for 1 h, using 5% O2 and 95 % N2 to prevent sulfate formations. After 5 regeneration cycles,
544
removal capacities of H2S and Hg0 decreased to 254.8 mg S/g, and 12.08 mg Hg0/g, respectively.
545
While the adsorption capacity of Hg0 decreased just 6%, the adsorption capacity of H2S decreased
546
14%, probably due to partial sintering because of the relatively high regeneration temperature (600
547
°C).
548
3.2 Activated Carbon
549
Activated carbons are amorphous materials well known due to their high surface area
550
(approximately 1000 m2/g), surface chemistry, and great porosity (Fig. 7), (Fang et al. 2013;
551
Georgiadis et al. 2021). Coconut shell, wood, coal, peat, and rice husks are some materials
552
utilized to produce activated carbons at high temperatures in an inert environment (Georgiadis et
553
al. 2021; Xiao et al. 2008). There are several factors that influence the adsorption capacity of
554
activated carbons such as their physical, chemical, and porous structures (Cui et al. 2009;
555
Shafeeyan et al. 2010). Surface pH, surface chemistry, pore size, and pore volume are important
556
features that determine the adsorption of sulfur compounds on activated carbons (Aguiar et
557
al.,2017; Bashkova et al. 2005).
558
559
Fig. 7. Basic chemical structure of activated carbons(Yahya et al. 2018).
560
Even though unmodified activated carbon is a suitable cost-effective material for the
561
adsorption of sulfur compounds, its adsorption performance can be improved by modifying its
562
surface (Cui et al. 2009; Georgiadis et al. 2021.; Xiao et al. 2008). Surface modifications of
563
activated carbon can be done by impregnation with acids (H2SO4, HNO3, etc.), caustics (NaOH,
564
KOH, NH3, etc.), salts (sodium and potassium carbonates, FeCl3, etc.), metals, and/or metal
565
oxides (Georgiadis et al. 2021; Meshkat et al. 2018; Xiao et al. 2008). Several studies have been
566
conducted for the removal of H2S and other sulfur compounds using activated carbon (Fang et al.
567
2013; Georgiadis et al. 2021; Xiao et al. 2008). The formation of elemental sulfur is favored by
568
surface basicity on activated carbons, while acidity promotes sulfur oxides and sulfuric acid
569
formations (Georgiadis, et al. 2021). Basicity can be promoted by the impregnation of activated
570
carbons with basic compounds such as caustics, urea, and amines (Georgiadis et al. 2021).
571
572
3.2.1 Modified activated carbons with acids, caustics, and/or salts
573
Sulfur removal from synthetic natural gas (SNG) was studied using modified activated
574
carbons (Cui et al. 2009). In this research, different sulfur compounds such as H2S, methyl
575
mercaptan (MM), ethyl mercaptan (EM), dimethyl sulfide (DMS), dimethyl disulfide (DMDS),
576
tetra hydrothiophene (THT) and ethyl disulfide (EDS) were studied for their simultaneous
577
adsorptive removal from SNG at ambient conditions. Coconut shell activated carbon (AC) was
578
modified with different substances such as HNO3, H2O2, CuSO4, Cu(NO3)2, FeCl3, NaOH,
579
NaCO3, KOH, K2CO3, and KCl. The results showed that modified activated carbons performed
580
better than unmodified activated carbon. In addition, the best performing activated carbons were
581
AC-CuSO4 for H2S (0.17 mg S/g), AC-CuSO4 and AC-FeCl3 for MM (0.15 mg S/g), AC-FeCl3
582
for EM (0.45 mg S/g), AC-FeCl3 for DMS (0.03 mg S/g), AC-HNO3 and AC-FeCl3 for DMDS
583
(0.29 mg S/g), AC-HNO3 for THT (3.43 mg S/g), and AC-FeCl3 for EDS (0.75 mg S/g). Almost
584
all the sulfur compounds had more affinity to modified AC with Fe, Cu, and Zn ions, except for
585
THT, which obtained the highest sulfur capacity with AC-HNO3. Moreover, DMS appeared to
586
be the most difficult sulfur compound to be removed with both the unmodified and modified
587
activated carbons.
588
Activated carbons prepared from different materials were investigated for the adsorption
589
of methyl mercaptan (Bashkova et al. 2005). Among the studied activated carbons were BAX-
590
1500 (wood-based carbons-Westvaco), S208 (coconut shell-based carbon-Waterlink Barnabey
591
and Sutcliffe), Centaur (bituminous coal based catalytic carbon-Calgon) and PCB (coconut shell-
592
Calgon). The activated carbons were impregnated with FeCl3. Each experiment was carried out
593
at room temperature under wet conditions (80 % relative humidity). Adsorbent samples were
594
ground and packed into a glass column and pre-humidified with moist air. The breakthrough of
595
CH3SH was monitored using a Micromax monitoring system (Lumidor) with an electrochemical
596
sensor calibrated with CH3SH. The test was stopped at the breakthrough concentration of 50
597
ppm. It was found that the introduction of Fe enhances the removal capacity as a result of the
598
electron transfer reaction in which thiolate ion is oxidized to dimethyl disulfide (DMDS).
599
Researchers have also studied activated carbon adsorbents that offer the flexibility of
600
regeneration (Aguiar et al. 2017). The use of modified activated carbon as an adsorbent for the
601
removal of sulfur compounds and the use of supercritical carbon dioxide for the regeneration
602
step have been investigated (Aguiar et al. 2017). Unmodified activated carbon as well as
603
impregnated with ferric chloride and nitric acid were used as adsorbents. The adsorbent
604
characteristics for this study are shown in Table 8.
Table 8. Adsorbent characteristics from BET analysis (Aguiar et al. 2017).
605
Adsorbent
AC-Virgin
AC-FeCl3
AC-HNO3
Area BET
(m2/g)
149.88
116.62
100.01
Micropore area
(m2/g)
108.76
89.42
75.85
Micropore
volume (cm3/g)
0.050
0.041
0.035
Medium pore
diameter (Å)
23.89
24.25
24.20
606
607
Experiments were conducted using a 10 cm length (about 6.7 mL of volume) stainless
608
steel tube as adsorption column, and 0.3 g of adsorbent. Glass spheres were used to pack the
609
column and maintain the adsorbent in place. The flow rate of natural gas (containing sulfur
610
compounds, see Table 9) was fed 5 mL/min. Breakthrough was observed after 5 hours (when the
611
sulfur compounds reached a value next to the equilibrium). CO2 was pressurized and used for the
612
regeneration step, using a volumetric flow of 350 mL/min. All the modified activated carbons
613
had higher sulfur removal than the virgin activated carbon. The use of supercritical CO2
614
improved the regeneration of the spent unmodified activated carbon by 60%. Ethyl mercaptan
615
and dimethyl sulfide were completely desorbed at 333.15 K and 100 bar of carbon dioxide
616
pressure. Table 9. describes the adsorption capacities of the different adsorbents. It is noted that
617
the adsorption capacities of the activated carbon modified with FeCl3 were the highest among the
618
3 activated carbons for all the tested sulfur compounds.
Table 9. Adsorption capacities for all the sulfur compounds (Aguiar et al. 2017).
619
Sulfur compound
EM1
DMS2
IPM3
TBM4
MES5
SBM6
THT7
620
621
622
623
Virgin activated
carbon
q(mg/g)
>0.34 ± 0.0130
>1.39 ± 0.1300
0.65 ± 0.0510
0.79 ± 0.0590
>1.01 ± 0.0450
0.98 ± 0.0410
2.02 ± 0.1300
FeCl3 activated
carbon
q(mg/g)
>0.46 ± 0.0230
>2.06 ± 0.1010
1.19 ± 0.0820
1.84 ± 0.1400
>1.99 ± 0.1500
1.35 ± 0.0950
3.62 ± 0.2600
HNO3 activated
carbon
q(mg/g)
>0.43 ± 0.0081
>1.76 ± 0.0530
0.91 ± 0.0580
0.98 ± 0.0510
>1.71 ± 0.0830
1.03 ± 0.0460
3.01 ± 0.0440
1
Ethyl mercaptan, 2Dimethyl sulfide, 3Isopropyl mercaptan, 4Tertbutyl mercaptan, 5Methyl ethyl sulfide, 6Sec-butyl
mercaptan, and 7Tetrahydrothiophene.
3.2.2 Modified activated carbons with metals/metal oxides
624
Activated carbons modified with transition metals were utilized for the removal of methyl
625
mercaptan from gas streams (Zhao et al. 2015). A commercial coconut shell AC (surface area of
626
606.1 m2/g) was impregnated by sol-gel method with different solutions containing transition
627
metals such as Zn, Ni, Fe, Cu, and Al. Then, the samples were calcinated using air at 450 °C for
628
3h to obtain the modified activated carbons Fe-AC, Al-AC, Zn-AC, Ni-AC, and Cu-AC. The
629
experiments were carried out at 50 °C and atmospheric pressure. The investigators observed that
630
the modified activated carbons performed better than the unmodified AC, meaning that the active
631
sites increased on the modified activated carbons after deposition and calcination of the
632
transition metals on their surface. In addition, Cu-AC achieved the highest sulfur removal,
633
followed by Ni-AC, Al-AC, Fe-AC, and Zn-AC. According to the researchers, methyl mercaptan
634
was oxidized to form dimethyl disulfide. However, products such as sulfate species
635
(CH3S· + CuO + Oads →CH3SO3Cu) and methyl thiolate (CH3S· + Cu → CH3SCu) were formed
636
on the AC surface. Excessive accumulation of sulfates and methyl thiolate is expected to inhibit
637
the adsorption and conversion of methyl mercaptan.
638
3.2.3 N2-rich modified activated carbons
639
The effect of nitrogen on the surface of activated carbon has been studied for the removal
640
of methyl mercaptan ( Liu et al. 2017). A coconut shell activated carbon was modified with
641
HNO3 to oxidize the surface, then it was impregnated with two substances, urea, and melamine,
642
and heated under a nitrogen atmosphere at 950 °C. The adsorption of methyl mercaptan was
643
tested on the modified adsorbent with a relative humidity of 80% at 25 °C and atmospheric
644
pressure. The unmodified and modified AC surface areas were calculated by BET. The surface
645
areas for each activated carbon were given by 1508 m2/g for unmodified AC, and 1358 m2/g for
646
modified AC with melamine, and 1433 m2/g for modified AC with urea. The modified AC
647
showed a smaller surface due to erosion caused by the acid treatment. However, the acid
648
treatment impregnated the AC with nitrogen. This addition achieves greater sulfur capacities
649
compared to the unmodified AC, achieving a maximum sulfur capacity of 602.1 mg S/g
650
adsorbent for modified AC with HNO3 and melamine, 465.7 mg S/g adsorbent for modified AC
651
with HNO3 and urea, and 161.8 mg S/g adsorbent for unmodified AC. Investigators concluded
652
that pyridinic nitrogen and quaternary nitrogen promote sulfur removal due to better electron
653
transfer. Similar to previous studies (Bashkova et al., 2002), water content enhanced sulfur
654
removal via oxidation mechanisms. In this case, the best performing activated carbon was
655
modified for surface nitrogen enrichment and the relative humidity of the feed was 80%. Methyl
656
mercaptan was oxidized to dimethyl disulfide in the presence of water and nitrogen.
657
A commercial coconut shell activated carbon modified with active sites of iron and
658
nitrogen (Fe-N activated carbon) was investigated for the removal of ethyl mercaptan (Lyu et al.
659
2020). The activated carbon was impregnated with Fe(NO3)3·9H2O and calcinated at different
660
temperatures under N2 or ammonia flow. Ethyl mercaptan (500 ppm) was mixed with N2 and
661
water. The mixture was passed through a fixed-bed reactor loaded with the modified activated
662
carbon at room temperature under atmospheric pressure. The best performing activated carbon
663
was calcinated at a temperature of 500 °C under ammonia flow. Researchers demonstrated that
664
the best performing Fe-N activated carbon, had Fe3O4, pyridinic-N, pyrrolic-N, and quaternary
665
type-N on its surface, which enhanced the sulfur removal achieving a capacity of 283.3 mg S/g.
666
Pyridinic-N and pyrrolic-N are considered basic sites, and thus, can interact with the acid ethyl
667
mercaptan. Researchers suggested that metal oxides also contributed as active sites for the
668
removal of ethyl mercaptan. Oxides can act as catalyst active sites, increasing the electron
669
transfer from sulfur to oxygen. The regeneration of the spent modified activated carbon was
670
carried out via calcination at 550 °C under N2 atmosphere. The regenerated activated carbon
671
achieved 80 % of the sulfur capacity compared to the fresh activated carbon, suggesting the
672
formation of strongly adsorbed oxidation products with high stability at that temperature. The
673
sulfur capacity of the regenerated activated carbon at 600 °C decreased, possibly due to the
674
sintering of the iron species at high temperatures. Diethyl disulfides and metal sulfonates were
675
observed as the main products of adsorption and catalytic processes on the surface of the
676
activated carbon.
677
Theoretical approaches to study the adsorption mechanism of sulfur compounds on activated
678
carbons have been applied and published elsewhere (Golebiowska et al. 2012; Li et al. 2014). Li
679
et al. (2014) studied the dynamics of hydrogen bonding on ethyl mercaptan adsorption onto AC
680
using density functional theory (DFT) and time-dependent density functional theory (TDDFT)
681
calculations. Results showed that bonds of the atoms S-H-O (oxygen from the AC) are formed,
682
observing that that the distance between atoms was shortened from 2.3409 Å in the ground state
683
to 2.2480 Å in the excited state. This result indicated that the intramolecular hydrogen bond S-H-
684
O was strengthened upon excitation to the singlet state.
685
686
3.3 Metal-Organic Frameworks
Metal-organic frameworks (MOFs) are advanced crystalline materials with high porosity
687
(typical pore diameters range 3 to 20 Å), large surface area (range from 1000 to 10000 m2/g),
688
and wide range of physical and chemical properties (Ahmed et al, 2016.; Gangu et al. 2016;
689
Georgiadis et al. 2020). MOFs have a hybrid inorganic-organic structure, where the inorganic
690
part consists of metal ions or clusters of metal ions, while the organic part is a linker molecule
691
with covalent bonds, such as benzenedicarboxylic acid (BCD), benzenetricarboxylic acid (BTC),
692
and imidazole (Ahmed et al. 2016; Chen et al. 2015; Wang et al. 2014).
693
694
Fig. 8. Metal-organic framework (MOF) basic structure (Heo et al. 2020).
695
MOFs can be redesigned by changing the inorganic moiety or the organic linkers to
696
obtain a unique material with a specific pore shape, pore size, and chemical functionality
697
(Saeedirad et al. 2018). In order to understand and predict the MOFs structure, the concept of
698
secondary building units (SBUs) was adopted (Gangu et al. 2016). SBUs are geometrical figures
699
comprised of inorganic clusters, which form frameworks when joined by organic components
700
(Fig. 8) (Gangu et al. 2016). MOFs can be formed using most of the metals from the periodic
701
table as the inorganic component, making the development of these materials practical and
702
potentially beneficial for numerous applications (Ahmed et al. 2006). Many MOFs have been
703
developed and tested, such as Materials of Institute Lavoisier (MILs) frameworks, Hong Kong
704
University of Science and Technology (HKUST-1) framework, Isoreticular MOF-n (IRMOF-n,
705
where n = 1 – 16 is used to classify this series of MOFs with similar network topology but
706
different organic linkers), Universitetet I Oslo (UiO-66) framework, and Zeolitic Imidazolate
707
Frameworks (ZIFs) (Georgiadis et al. 2020; Saeedirad et al. 2018; Vellingiri, Deep, and Kim
708
2016). The use of MOFs has been studied for different applications such as gas storage, catalysis,
709
electrochemical energy storage, and gas separation (Georgiadis et al. 2020; Fan et al. 2017). In
710
addition, adsorptive desulfurization using MOFs has been investigated due to their high capacity
711
and selectivity for sulfur compounds (Chen et al. 2015).
712
3.3.1 Unmodified and modified MOFs
713
Zinc based metal organic framework has been studied for the removal of dimethyl
714
sulfide, ethyl mercaptan, and hydrogen sulfide from a gas mixture of pure nitrogen (Wang et al.
715
2014). Application IRMOF-3, at a range of temperatures (80 °C – 200°C), and ambient pressure
716
resulted in a limited adsorption capacity with removal of 1.06 % for H2S, 0.59% for ethyl
717
mercaptan, and 0.58% for dimethyl sulfide.
718
Other investigators also evaluated ZIF-8 zeolite for the removal of H2S from natural gas
719
(Jameh et al. 2018). Unmodified and modified ZIF-8 zeolite nanoparticles were analyzed for the
720
adsorptive removal of H2S and CO2 from a mixture of CH4/CO2/H2S/He (88.8/7.3/3.0/1.0 %mol)
721
at 25 °C and 2 bar. ZIF-8 was modified via wet impregnation (active precursor is dissolved in an
722
organic or aqueous solution) with ethylenediamine (ED). Excess ED on the sureface was
723
washed with methanol. The H2S and CO2 adsorption capacities (55.7 g CO2/g sorbent, 3.3 g
724
H2S/g sorbent) were higher on the modified ZIF-8, which could be related to the improved
725
surface area (412 m2/g for unmodified ZIF-8 compared to 1389 m2/g modified ZIF-8), and the
726
functional group from the amine that facilitates polar interaction on the surface with H2S and
727
CO2. The modified ZIF-8 adsorbs both, H2S and CO2, demonstrating a higher selectivity toward
728
H2S.
729
A study on the selective removal of t-butyl mercaptan (TBM) from natural gas by several
730
adsorbents, including MOFs has been published elsewhere (Chen et al. 2015). Among other
731
adsorbents, the investigators evaluated NaY, Cu-BTC, MIL-53 (Al) and UiO-66 (Zr) for
732
adsorption capacity of t-butyl mercaptan. These materials were tested at 35 °C and atmospheric
733
pressure, using a methane feed containing 60 ppm of TBM and a flow rate of 90 mL/min. After
734
adsorption, the materials were regenerated at 200 °C for 1 h in flowing helium. Table 10. shows
735
BET results for all the tested adsorbents.
736
737
Table 10. Surface areas, pore volumes, and particle size ranges of adsorbent materials (Chen et al. 2015).
Adsorbent
Cu-BTC
MIL-53 (Al)
UiO-66 (Zr)
Zeolite NaY
Zeolite Beta
Zeolite ZSM-5
ZIF-8
BET surface area
(m2/g)
1504
1329
1002
868
530
648
1528
Micropore volume
(cm3/g)
0.55
0.54
0.40
0.35
0.15
0.15
0.68
Particle size (µm)
3-7
1-4
1-4
1-3
1-3
1-3
1-5
738
739
The authors found that Cu-BTC and MIL-53 (Al) were better adsorbents, yielding TBM
740
uptakes of 0.30 and 0.22 g/g. These values are higher than NaY, which had a TBM adsorption
741
capacity of 0.16 g/g. MIL-53 (Al), and UiO-66 (Zr) lost their TBM uptake capability by
742
approximately 20% after 5 cycles of regeneration. Cu-BTC lost 100% of the adsorptive capacity
743
after the first cycle. MIL-53 (Al) had the lowest reduction of adsorption capacity after each cycle
744
(about 4-7%) without stabilization. In contrast, UiO-66(Zr) showed a stabilized adsorption
745
capacity after the third cycle, losing about 23% of its initial uptake, but then stabilizing at 0.14
746
g/g.
747
Joshi et al. also conducted a study for the removal of H2S from natural gas using different
748
MOFs (Joshi et al. 2018). UiO-66 (Zr), MIL-125 (Ti), and MIL-101 (Cr) were modified using
749
amine functionalization (with 2-aminoterephthalic acid) and tested for the removal of H2S from
750
mixtures of 1% H2S/99% CH4, and 1% H2S/10% CO2/89% CH4 at ambient conditions. All the
751
adsorbents were activated by N2 flow at 150 °C for 3 h before the adsorption experiments. The
752
results showed higher sulfur removals for all the modified MOFs. The maximum sulfur capacity
753
(0.559 mmol S/g adsorbent) was achieved by the modified MIL-125 (Ti), or MIL-125-NH2 (Ti),
754
from the mixture of 1% H2S/99% CH4. However, all the MOFs obtained lower sulfur capacities
755
for H2S from the mixture of 1% H2S/10% CO2/89% CH4, which suggested competitive
756
adsorption between H2S and CO2. This competitive behavior was probably due to the hydroxyl
757
groups observed on the MOFs structures, which could have promoted physisorption of CO2
758
(Joshi et al. 2018). According to the investigators, the amine functionalization in smaller pores
759
of the MOFs improved H2S sulfur capacity, so it was suggested that the formation of linker-
760
based amines in MOFs mesopores could enhance the adsorption of H2S and CO2 from sour
761
mixtures of natural gas.
762
3.3.2 Composite of MOFs
763
A study tested ZIF-8 supported on zeolites for adsorptive desulfurization (Saeedirad et al.
764
2018). MCM-41 (MCM stands for Mobil Composition of Matter), SBA-15, and UVM-7
765
mesoporous zeolites were investigated as supports for ZIF-8. Researchers applied hydrothermal
766
methods for the removal of ethyl mercaptan (from mixture of butane and ethyl mercaptan) and
767
H2S (from mixture of N2 and H2S) under atmospheric pressure and a temperature range of 30 °C
768
to 50 °C. The composites of zeolites/ZIF-8 were activated at 100 °C under a N2 atmosphere for 1
769
h to remove possible small molecules of solvents and water that are sometimes left behind inside
770
the pores of MOFs during synthesis. All the experiments showed that the composites of
771
zeolites/ZIF-8 achieved higher sulfur capacity than unmodified ZIF-8 as follows: UVM-7/ZIF-
772
8>SBA-15/ZIF-8>MCM-41/ZIF-8>ZIF-8. The sulfur capacities for UVM-7/ZIF-8 were given by
773
6.12 g S/g and 1.47 g S/g at 30 °C for H2S and ethyl mercaptan, respectively. The highest sulfur
774
capacity for all the modified ZIF-8 adsorbents was achieved at 30 °C, sulfur capacity decreased
775
as a function of temperature. The adsorption of the sulfur compounds was accompanied by
776
chemical reactions characterized by acid-base interactions, coordination bond formation, and
777
hydrogen-bonding. Regeneration of the zeolites/ZIF-8 adsorbents was conducted under a N2
778
atmosphere at 150 °C. After 4 cycles of regeneration, the sulfur capacities for H2S and ethyl
779
mercaptan were 98% and 88% for UVM-7/ZIF-8, 93% and 91% for SBA-15/ZIF-8, 91% and
780
88% for MCM-41/ZIF-8 compared to their initial sulfur capacities.
781
Copper based MOFs have been widely studied for the removal of sulfur compounds: H2S,
782
methyl mercaptan, ethyl mercaptan, dimethyl sulfide, and dimethyl disulfide (Deng et al. 2018;
783
Li et al. 2015; Fan et al. 2017). MOF-199 is a Cu-based MOF with a surface area approximately
784
1350 m2/g (Fan et al. 2017). MOF-199 was modified with AC at different loading weights for the
785
removal of H2S and dimethyl sulfide (Fan et al. 2017). The feed had 9% humidity. AC loading
786
enhanced the sulfur capacity of MOF-199. The modified adsorbent showed larger surface area
787
and ordered crystallinity structure compared to the unmodified MOF-199, with loadings of AC
788
less than 2 wt.%. Higher loadings of AC resulted in saturation of MOF-199 pores. The
789
performance of modified MOF-199 has also been studied by several other investigators (Deng et
790
al. 2018; Li et al. 2015). MOF-199 was activated in all the studies with heat-treatment under a N2
791
atmosphere. This activation procedure is common in most MOFs syntheses. It has been
792
suggested that the copper in the MOF-199 structure promotes adsorption and chemical reaction
793
with sulfur compounds, forming CuS and the decomposition of the MOF-199 structure (Deng et
794
al. 2018.;Li et al. 2015; Fan et al. 2017). In addition, MOF-199 can be regenerated by N2 at 180
795
°C. Sulfur adsorption capacities of 94% of the initial sulfur capacity were achieved after 3
796
regeneration cycles (Fan et al. 2017).
797
HKUST-1 is a Cu-based framework that has been proven effective for adsorptive
798
desulfurization. The study compared a pure HKUST-1 framework to a modified HKUST-1 with
799
graphene oxide (GO) and polyethyleneimine (PEI) for hydrogen sulfide removal (Bhoria et al.
800
2020). A concentration of 99.6 ppm of H2S in N2 was used for desulfurization experiments over
801
the unmodified and modified MOFs at room temperature, 150 °C, and 1 bar. Adsorbent samples
802
were activated at 150 °C overnight under an argon flow of 43 ml/min to remove moisture or any
803
other adsorbed molecules. Investigators observed the characteristic octahedral shape of the
804
HKUST-1 with a crystal size distribution from 2 to 15 µm. For modified HKUST-1 with GO and
805
PEI, HKUST-1/GO-PEI, they observed a uniform distribution of Cu, which is typical for MOF
806
crystals. They also observed an improved distribution of nitrogen groups originated from the PEI
807
functional groups. Modified HKUST-1 with GO, HKUST-1/GO showed improvement in carbon
808
content, which prevented agglomeration of the HKUST-1 crystallites. For the experiment
809
conducted at room temperature, the HKUST-1, HKUST-1/GO, and HKUST-1/GO-PEI achieve
810
very similar sulfur capacities of approximately 1.65 mmol S/g adsorbent (56.26 mg S/g
811
adsorbent), suggesting that GO and PEI just provided a uniform dispersion of the HKUST-1
812
molecules (including the Cu atoms that are part of the HKUST-1 framework). On the other hand,
813
when using 150 °C, the sulfur capacities were 1.65 mmol S/g adsorbent (56.26 mg S/g
814
adsorbent) for HKUST-1, 2.1 mmol S/ g adsorbent (71.61 mg S/g adsorbent) for HKUST-1/GO,
815
and 0.9 mmol S/g adsorbent (30.69 mg S/g adsorbent) for HKUST-1/GO-PEI. The researchers
816
observed that the sulfur capacity of HKUST-1/GO adsorbent was improved when increasing
817
temperature, which suggested the adsorption was kinetically limited. The reduction in sulfur
818
capacity HKUST-1/GO-PEI compared to unmodified HKUST could be attributed to the
819
obstruction of MOF pores and MOF active centers due to amine molecule’s mobility at high
820
temperatures.
821
3.3.3 Theoretical approaches for desulfurization using MOFs
822
Other studies have focused on predicting the best framework to effectively remove sulfur
823
compounds from natural gas using theoretical approaches (Z. Li et al. 2015; Liang et al. 2019;
824
Nazarian, Camp, and Sholl 2016). The application of a composite formed using ionic liquids
825
(ILs) with MOFs has been simulated for the removal of H2S from natural gas (Z. Li et al. 2015).
826
In this computational research, different ionic liquids supported on Cu-TDPAT were studied,
827
where TDPAT (2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) is the organic linker
828
bounded to Cu2(COO)4. For the ILs, four different anions were analyzed,
829
bis[(trifluoromethyl)sulfonyl]imide [Tf2N]−, tetrafluoroborate [BF4]−, hexafluorophosphate
830
[PF6]−, and chloride [Cl]− with the same cation, and 1-n-butyl-3-methylimidazolium [BMIM]+.
831
Using Monte Carlo simulations, the results showed that the anions from the ILs are favorably
832
incorporated near the Cu atoms of the MOF structure, while the cations are located with the
833
organic linkers. The incorporation of ILs on Cu-TDPAT pores increased the selectivity of H2S
834
from a mixture of H2S/CH4 over the MOFs. The ionic liquid with chloride [Cl]− anion resulted in
835
the highest selectivity using a loading of 25 molecules of [Cl]− per unit cell of MOF. From the
836
simulations, it was observed that the selectivity decreased when 30 molecules per unit cell are
837
incorporated in the MOF. This could be associated with a reduction in porosity.
838
Another study analyzed theoretically the selective removal of tertbutyl mercaptan (TBM)
839
in CH4 using approximately 3000 experimentally synthesized MOF frameworks with density
840
functional theory (DFT) calculations and the density derived electrostatic and chemical (DDEC)
841
charge partitioning method (Nazarian et al. 2016). According to the researchers, most of the
842
MOFs structures used in their research were taken from the Computation Ready Experimental
843
MOF (CoRE MOF) database of Chung et al. (2014) (Chung et al. 2014). Simulations were
844
conducted using a binary mixture of 10 ppm TBM in CH4 at 25°C and 18.1 atm, using 3x104
845
initialization and 4x105 production Monte Carlo cycles. A total of 354 MOFs were found to
846
achieve sulfur capacities greater than 200 mg S/g of TBM. Some of the highest capacities were
847
obtained by BIBXUH (a nickel-based MOF) with 691 mg S/g adsorbent, MFU-4 (a zinc and
848
chlorine-based MOF) with approximately 300 mg S/g adsorbent, ZIF-8 with 270 mg S/g
849
adsorbent, and Cu-BTC with approximately 580 mg S/g adsorbent.
850
A theoretical approach has been applied to analyze the removal of methyl mercaptan
851
(MM) and ethyl mercaptan (EM) using machine learning (back propagation neural network and
852
partial least square methods) to predict the best MOFs based on the highest adsorption capacities
853
for both mercaptans (Liang et al. 2019). The investigators evaluated 137953 hypothetical MOFs
854
(hMOFs) which were computationally generated from another researcher (Wilmer et al. 2012)
855
using a wide crystallographic data set from MOFs with 6 topologies. MOFs (4764) from the
856
CoRE MOF database were also analyzed. These are MOFs experimentally synthesized using 350
857
unique topologies. The adsorption experiments for MM and EM over the MOFs were simulated
858
using the grand canonical Monte Carlo (GCMC) method at 298 K and 1 kPa. Results showed
859
approximately 40 hMOFs with sulfur capacities greater than 600 mg S/g adsorbent for MM (up
860
to 700.4 mg S/g adsorbent), and 850 mg S/g adsorbent for EM (up to 980.5 mg S/g adsorbent).
861
These results were achieved when the optimal value of ϕ (void fraction or porosity) was in the
862
range of 0.76-0.88 for MM, and 0.82-0.88 for EM as well as isosteric heat (Q0st) of adsorption in
863
the range of 32-42 kJ/mol for MM, and 32-48 kJ/mol for EM. According to the investigators, at
864
low pressures (1 kPa), the adsorption of MM is smaller than EM because weaker affinity with
865
MOFs. Conversely at high pressures (>10kPa), MM has larger adsorption capacity than EM
866
because of its smaller molecular size, thus more MM can be accommodated in MOFs pores. The
867
researchers also concluded that the main interactions affecting the adsorption of MM and EM on
868
the MOFs was the framework-adsorbate (F-A) interactions instead of the adsorbate-adsorbate
869
interactions (A-A). A key conclusion is that the molecules of MM or EM are going to interact
870
mainly with the MOF molecules, instead of interacting with each other.
871
3.4 Metal Oxides
872
Metal oxides (MOs) have also played an important role in desulfurization processes both
873
as active phases and supports (Gawande et al. 2012). The application of metal oxides depends on
874
their acid-base properties. MOs have been applied for the desulfurization of coal gas, syngas,
875
hydrogen for fuel cells, Claus tail gas, and natural gas (Georgiadis et al. 2020; Shah et al. 2017).
876
Transition metals are commonly used as metal oxide catalysts due to their selectivity, easy
877
regeneration, and low cost of production (Gawande et al. 2012). Pure metal oxides are
878
characterized by low porosity, low surface area, low resistance to sintering at high temperatures,
879
and attrition (which is a mechanical strength problem) (Georgiadis et al. 2020; Jangam et al.
880
2021; Vaziri et al., 2019). In order to improve the properties of single metal oxides, several
881
studies have synthesized and evaluated the performance of mixed metal oxides (Georgiadis et al.
882
2020;Vaziri et al. 2019). Mixed metal oxides can be prepared by different methods, such as sol-
883
gel, wet impregnation, mechanochemical synthesis, hydrothermal method, co-precipitation, and
884
microwave irradiation (Gawande et al. 2012). Several studies have been performed using metal
885
oxides and mixed metal oxides for gas desulfurization in temperatures ranging from 25-1200 °C
886
(Santos et al. 2016; Shah et al. 2017). There are limited studies for adsorptive desulfurization of
887
natural gas at low temperatures. Among sulfur compounds, H2S has been widely studied
888
(Georgiadis, et al. 2020;Vaziri et al. 2019; Shah et al. 2017). Among metal oxides, ZnO and CuO
889
based adsorbents have been intensively investigated for the removal of H2S due to their high
890
equilibrium sulfidation constant, which results in low equilibrium H2S concentrations
891
(Georgiadis, et al. 2020). The following equation describes the general reaction between H2S and
892
metal oxides (Georgiadis,et al. 2020;Vaziri et al. 2019):
893
894
895
MxOy(s) + yH2S(g) ―› MxSy(s) + yH2O(g)
(5)
3.4.1 Pure metal oxides
896
A study was conducted to understand the interaction between metal oxides and sulfur
897
compounds (H2S and S2) using a synchrotron-based high-resolution photoemission (Rodriguez et
898
al. 1998). Investigators were mainly focused on analyzing the band-gap size, which is an important
899
property correlated with more effective desulfurization (Thimsen et al. 2009), and chemical
900
reactivity. The study compared the reactivity of H2S and S2 on different metal oxides, such as
901
Al2O3, Cr2O3, Cr3O4, ZnO, and Cu2O. H2S and S2 were passed through the different metal oxides
902
at 300 K in an ultrahigh-vacuum (UHV) chamber. The S2 gas was obtained in situ by decomposing
903
Ag2S in a Pt/Ag/AgI/Ag2S/Pt solid-state electrochemical cell. The results showed the following
904
order of reactivity: Al2O3 < ZnO < Cu2O. According to investigators, the results could not be
905
explained by the number of defect sites on the surface, since the alumina possessed the surface
906
with the largest roughness, but it achieved the lowest reactivity toward H2S. The low reactivity of
907
Al2O3 compared to Cu2O and ZnO did not correlate with electrostatic interactions. If that was the
908
case, the strength of the bond metal-sulfur had to follow Cu2O (Cu1+) < ZnO (Zn2+) < Al2O3 (Al3+).
909
The investigators indicated that reactivity was dependent on the band gap. The smaller the band
910
gap the higher the reactivity toward H2S as demonstrated by the following oxide: Cr3O4 (BG∼0
911
eV), Cu2O (BG ∼ 2.2 eV), ZnO (BG∼3.4 eV), Cr2O3 (BG~4.8 eV), and Al2O3 (BG ∼ 9 eV).
912
Perturbation theory was used to predict the dependence of adsorption energy on the top of the
913
valence band energy of the metal oxide (ETVB) and the energy of the orbitals partially occupied in
914
HS and S (Eacceptor). The proportionality is given by the expression:
915
Q ∝ (ETVB – Eacceptor)
(6)
916
HS and S are well known for exhibiting high electron affinities, a key property for electron-
917
acceptor interactions. Eq. 6. suggests that the bonding interactions of the oxide with HS and S
918
would be higher with less stable oxide valence band, since smaller band gaps yield less stable
919
valence band. The researchers suggested that mercaptans would follow the same principle,
920
increasing reactivity toward metal oxides with small band gaps such as SnO2, Cu2O, and ZnO and
921
less reactivity toward metal oxides with larger band gaps such as Al2O3, MgO, and CaO. It was
922
also observed that the rate of adsorption of H2S and S2 on mixed metal oxides surfaces (e.g.,
923
Cu/Al2O3 and Cu/ZnO) is faster than on the pure oxides because the supported metal could provide
924
more electron transfer pathways when combined with mixed metal oxides. If the metal oxide is
925
characterized by a big band gap, such as Al2O3, the sulfur molecules react mostly with the
926
supported metal (Cu, Zn, Fe, etc.), because electron conduction through the band gap could be
927
energetically inefficient. The metal used to support metal oxides (e.g., copper in Cu/ZnO)
928
contributes to electronic states that are utilized for bonding interactions with the sulfur molecules.
929
A previous study reported the application of nanoparticles of zinc and iron for the
930
removal of sulfur compounds from natural gas (Sekhavatjou, Moradi, and HOSSEINI 2014).
931
Two different particle sizes of zinc and iron oxides were tested for the adsorption of hydrogen
932
sulfide, carbonyl sulfide, methyl mercaptan, ethyl mercaptan, dimethyl sulfide and carbon
933
disulfide from natural gas. The particle sizes for ZnO were 80 nm and 119 nm, while Fe2O3 were
934
20 nm and 166 nm. The adsorption experiments were carried out at 26°C and 15 psi. For ZnO
935
with particle size of 119 nm, the removal percentages obtained were 83.8%, 26.2%, 68.4 %,
936
64.4%, 40% and 20% for H2S ,COS, CH3SH, CH3CH2SH, DMS and CS2, respectively. For the
937
nanoparticles of ZnO (80 nm), the removal percentages were 20%, 40%, 84.2 %, 78.9%, 21.5%
938
and 93.2 % for H2S ,COS , CH3SH, CH3CH2SH, DMS and CS2, respectively. For the iron oxides
939
with particle size of 166 nm the removal efficiencies were 20%, 20%, 31.6 %, 31.6%, 7.6% and
940
58% for H2S ,COS , CH3SH, CH3CH2SH, DMS and CS2, respectively. In the case of 20 nm iron
941
oxide nanoparticles, the removal efficiencies were 80%, 80 %, 94.7%, 89.5% 56.9% and 95.9 %
942
for H2S ,COS , CH3SH, CH3CH2SH, DMS and CS2, respectively. When the zinc oxide particle
943
size was reduced by 33%, the removal of COS, CS2, methyl and ethyl mercaptans was increased
944
by 16%. However, the removal efficiencies of H2S and DMS decreased, which may be due to
945
increase in selectivity of ZnO for mercaptans and carbon-type sulfides, leaving less available
946
active sites for H2S and DMS. In contrast to zinc oxide, the reduction of particle size in iron
947
oxide improved adsorption capacity for all the sulfur compounds.
948
Another study using nanoparticles was performed with MnO2 for the removal of dimethyl
949
sulfide (DMS) and ethyl mercaptan (EM) (He et al. 2012). The research focused on developing
950
an adsorption mechanism for DMS and EM combining experimental results and theoretical
951
calculations. The nanoparticles of MnO2 were prepared by the sol-gel method using manganese
952
acetate as precursor. Experiments were conducted at room temperature and 1.43 kPa.
953
Investigators suggested that DMS could be partially oxidized to formate when adsorbed on the
954
surface of MnO2. EM was oxidized to sulfonate, sulfate, and sulfonic acid (−SO3H), an
955
indication of a greater interaction between MnO2 and adsorbed EM compared to MnO2 and
956
DMS. According to the investigators, theoretically the C–S bond (bond energy = 272 kJ/mol)
957
should be easier to break and oxidize further than the S–H bond (bond energy = 347 kJ/mol).
958
However, density functional theory indicates that S in DMS has a positive density, while S in
959
EM has a more negative charge density. Therefore, due to the positive charge density of the S
960
atom in DMS it is more difficult to oxidize through electron loss than EM, which has a negative
961
charge density on S. Electrons can be transferred easier from S in EM to Mn. Based on these
962
theoretical calculations, investigators proposed an adsorption mechanism. In the case of DMS,
963
interaction of Mn+ and -CH3- leads to oxidation of the C atom, but not oxidation products of S
964
on the MnO2 surface, because S in DMS has a positive charge density, making difficult to
965
oxidize it through electron loss. In the case of EM, S has a negative charge density, allowing the
966
transfer of electrons from S to Mn+, resulting in oxidation of S-H (probable oxidation products of
967
S on MnO2 surface such as sulfonate, sulfate, and sulfonic acid). Sulfur adsorption capacities
968
were out of the scope for this research.
969
3.4.2 Mixed metal oxides
970
Some MOs adsorbents have been evaluated at low pressure and temperature (Kim et al.
971
2006). For example, co-precipitated Cu/ZnO/Al2O3 was used as adsorbent for the removal of
972
TBM and Tetrahydrothiophene (THT) from natural gas. The researchers also evaluated the effect
973
of sodium content on the TBM and THT adsorption. The experiment was conducted in a fixed
974
bed tubular reactor loaded with 1 mL of sorbent at ambient pressure. The sorbent was activated
975
by a reduction process at 200 °C for 3 hours in a flow of 5% H2 balance nitrogen. The test gas
976
flow rate was 100 mL/min with a constant GHSV of 6000 h-1. The breakthrough time was
977
established when the concentration of TBM and THT reached 10 ppb in the outlet gas. The
978
concentration of TBM and THT in the natural gas was 24 ppm and 56 ppm, respectively. Results
979
indicated that Cu/ZnO/Al2O3 is a competitive sorbent compared to others in the literature
980
displaying an excellent adsorption capacity at low and high temperatures. The maximum sulfur
981
adsorption capacity was 0.5 mmol/g at 250 °C. According to the investigators, sodium content
982
controlled the dispersion of Cu metal particles as well as interactions with ZnO and Al2O3. The
983
maximum sulfur capacity (0.5 mmol S/g adsorbent) was achieved with 0.006 wt.% of sodium
984
content, with a maximum surface area of 104.26 m2/g and pore diameter of 16.54 nm. Sodium
985
content was also important to achieve the maximum surface area and pore diameter and thus
986
maximum sulfur capacity.
987
988
989
990
Fig. 9. Graphical representation of metal oxide support (γ-Al2O3) impregnated with different metals (Yi
et al. 2021).
The adsorption of methyl mercaptan using γ-Al2O3 as support (Fig. 9) catalyst loaded
991
with Cu, Mn, Fe, and Co was studied by (Yi et al. 2021). The catalyst was prepared by the
992
impregnation method, using γ-Al2O3 and M(NO3)2 salt solution. “M” was the representative of
993
the metals: Cu, Fe, Mn, or Co. The mixture was treated by an ultrasonic oscillator for 30 minutes
994
at 40 °C, and dried at 110 °C. The calcination of the catalyst was carried out in air at 500 °C in a
995
muffle furnace for 3 h. The adsorption experiments were conducted using a fixed-bed reactor at
996
room temperature, a mixture of 400 ppm of CH3SH in nitrogen and oxygen, and a flow of 200
997
ml/min. Unmodified γ-Al2O3 catalyst resulted in the lowest CH3SH removal. The best removal
998
was achieved by Cu/γ-Al2O3 followed by Co/γ-Al2O3. γ-Al2O3 impregnated with Fe and Mn
999
performed as poorly as unmodified γ-Al2O3. Even though Cu impregnation enhanced the sulfur
1000
removal of γ-Al2O3, the removal effect of CH3SH decreased when the Cu load increased above
1001
20%. This effect was most likely due to agglomeration/pore obstructions because of the
1002
excessive amount of Cu. The calcination temperature (from catalyst preparation) also impacted
1003
sulfur removal. The highest sulfur removal was observed at a calcination temperature of 500 °C.
1004
Temperatures of 650 °C or higher adversely impacted CH3SH removal, probably due to catalyst
1005
sintering, and thus reduction of catalyst surface area and active sites.
1006
Introducing Mn on the surface of Cu/γ-Al2O3 improved the CH3SH removal efficiency
1007
while adding Co had little effect. According to the investigators, MnOx on γ-Al2O3 magnified
1008
the dispersion of CuO on the support surface. An optimum amount of Mn led to enhance activity
1009
of Cu/γ-Al2O3. The investigators reported that Cu–Mn/γ-Al2O3 enhanced activity could be
1010
associated with the presence of numerous strong alkali sites, which adsorbed CH3SH molecules.
1011
An excessive loading of Mn was found to hinder CuO active sites. For the regeneration step, the
1012
catalyst was placed under a nitrogen atmosphere at 300 °C and purged for 2 h. Cu–Mn/γ-
1013
Al2O3 demonstrated excellent regeneration performance, maintaining almost 100% removal of
1014
CH3SH after regeneration multiple times.
1015
1016
An adsorbent based on γ-Fe2O3 with three-dimensionally ordered macropores (3DOM)
was analyzed for the removal of H2S at low temperature (Huang et al. 2015). 3DOM γ-
1017
Fe2O3/SiO2 adsorbents were produced by the colloidal crystal templating method with a
1018
calcination temperature of 300 °C for 1 h and then 500 °C for 2 hours. The synthesized 3DOM
1019
sorbents were named FS-1, FS-2, FS-4, FS-8, and FS-16. Number identifiers are the molar ratios
1020
of Fe/Si. Sulfur removal tests were conducted at atmospheric pressure and 20–80 °C in a fixed
1021
bed reactor. The test gas mixture was 500 mg/m3 of H2S balanced with high-purity nitrogen
1022
(nitrogen content higher than 99.999% in gas). Sulfur removal increased at 80 °C compared to
1023
lower temperatures. Thus, more experiments were conducted at 80 °C for the desulfurization
1024
performance of the synthesized 3DOM sorbents FS-1, FS-2, FS-4, FS-8, and FS-16.
1025
Investigators observed an improvement in the performance of all sorbents with a 3DOM
1026
structure compared to the unmodified γ-Fe2O3. FS-8 resulted in the highest sulfur removal
1027
(40.61%) among the evaluated sorbents. The poor performance of FS-1 was likely caused by the
1028
low iron oxide content compared to the other 3DOM sorbents. However, FS-16, which had the
1029
highest iron content and lowest SiO2 content, achieved a low adsorption capacity compared to
1030
the other 3DOM sorbents. This was explained by the presence of α-Fe2O3 as revealed by XRD.
1031
According to the researchers, SiO2 helps to impede the formation of α-Fe2O3, promoting the
1032
formation of γ-Fe2O3. It has been reported elsewhere that α-Fe2O, which has a surface area of 36
1033
m2/g and grain size of 54.2 nm, has a low H2S adsorption capacity (Huang et al. 2015). γ-
1034
Fe2O3 possesses lower density (4.87 g/m3), larger surface area (82 m2/g), and smaller grain size
1035
(11.6 nm), which improves the performance in H2S capture. Ion migration in the lattice of oxide
1036
(lattice diffusion) is necessary for the reaction of a metal oxide with H2S. Investigators have
1037
determined that this diffusion is enhance by the presence of defects and vacancies (Huang et al.
1038
2015). The vacancy facilitates lattice diffusion in the capture of H2S by γ-Fe2O3. The 3DOM
1039
sorbents, which contain macropores and mesopores, gave easy accessibility to H2S molecules to
1040
the inner part of the sorbent. Moreover, these adsorbents have large surface area as well as active
1041
particles in the nanoscale range, providing more active sites for H2S interactions.
1042
Another important result was the influence of water content on H2S adsorption.
1043
Unmodified γ-Fe2O3 was tested at different relative humidity. The best performance was
1044
achieved at a relative humidity of 10% with a sulfur removal of 14%. Investigators observed an
1045
adverse impact of water content on H2S removal. Sulfur removal was only 5% using a relative
1046
humidity of 41%. Results indicated that excessive water content (more than 19% relative
1047
humidity in this research) can form a thin film on the adsorbent surface as well as pore blockage,
1048
avoiding the diffusion of H2S onto the adsorbent.
1049
Regeneration of the spent adsorbents was carried out at 300 °C using 5% O2 in nitrogen
1050
with a flow of 200 mL/min. The regenerated sorbent reached a sulfur removal of 11.28%, which
1051
was less than one-third of that of the fresh material. This poor performance was attributed to the
1052
formation of α-Fe2O3 (which is less reactive than γ-Fe2O3) during regeneration, combined with
1053
the decreased surface area (α-Fe2O3 has greater particle size and lower surface area than γ-
1054
Fe2O3). Furthermore, a revivifying in situ method, which is a simultaneous sulfur removal and
1055
regeneration process, was done on FS-8 for H2S removal using a mixture of oxygen (5%) and
1056
500 mg/m3 H2S in nitrogen at 80 °C. The results showed that the addition of O2 in the feed gas
1057
increased the sulfur adsorptive capacity of the adsorbent to 79.1%, which was almost twice
1058
compared to the test in the absence of O2.
1059
Mixed metal oxides-based Zn have also been investigated for the removal of H2S at low
1060
temperatures (Jiang et al. 2010; Polychronopoulou et al.,2005). For example, Mn-Zn-Ti oxides
1061
have been tested for the removal of H2S (Polychronopoulou et al. 2005). The adsorbents were
1062
prepared by sol–gel method, using air for calcination at 200 °C for 2 h, then at 500 °C for 4 h.
1063
The results showed that 10Mn–45Zn–45Ti–O adsorbent, where the coefficients are the molar
1064
percentage, achieved the highest sulfur capacity of H2S at 25 °C (1.21 mmol H2S/g). The effect
1065
of the operating temperature was studied on 10Mn–45Zn–45Ti–O in the range of 25–100 °C.
1066
The increase of temperature resulted in an increase on the sulfur capacity of the adsorbent,
1067
achieving the highest sulfur capacity of 4.6 mmol S/g adsorbent (156.8 mg S/g adsorbent) at 100
1068
°C, suggesting that the sulfidation process was faster at this temperature. Investigators also
1069
observed that the temperature of calcination in the preparation of metal oxides impacted the
1070
sulfur capacity of the adsorbents. The effect of water content on the mixed metal oxide was also
1071
studied. When the 10Mn–45Zn–45Ti–O adsorbent was tested in the absence of water, the sulfur
1072
capacity increased with the increase of temperature from 25 °C to 100 °C. However, the presence
1073
of 1 vol.% H2O in the feed increased the sulfur capacity of the adsorbent by a factor of 3
1074
(approximately 109.12 mg S/g adsorbent) compared to the adsorbent with no water content at 25
1075
°C. The sulfur capacity decreased when experiments were conducted at 100 °C in the presence of
1076
water. Investigators suggested that a metal hydroxide shell is formed to a greater extent at 100 °C
1077
compared to the case at 25 °C. The hydroxide limited the diffusion of HS− and S2− into the bulk
1078
of the metal oxide, decreasing the sulfur capacity of the adsorbent.
1079
Jiang et al. (2010) studied a mixed metal oxide composed of Cu and Zn oxides for the
1080
removal of H2S. Cu-Zn-O and Cu-Zn-Al-O adsorbents with different Cu/Zn molar ratios were
1081
prepared using co-precipitation and multi-precipitation methods, followed by calcination at 350
1082
°C for 3 h. The adsorption experiments were carried out using 3000 ppm H2S in N2 and a flow
1083
rate of 34.4 ml/min at 40 °C, and 1 atm. The sulfur capacity increased with Cu/Zn molar ratio for
1084
each series of adsorbents. This result indicated that the addition of copper to the adsorbents
1085
greatly enhanced sulfidation. Thermodynamically this could be explained by the Gibbs free
1086
energies for the reaction of CuO with H2S (ΔG298, −126 kJ/mol) and the reaction between ZnO
1087
and H2S (ΔG298, −76 kJ/mol).
1088
The mp-CuxZnyAl10 (mp stands for multi-precipitation) adsorbents, where x and y are
1089
molar percentage, revealed higher sulfur capacities than the cp-CuxZnyAl10 (cp stands for co-
1090
precipitation) adsorbent. In the co-precipitation method, all the precursors (CuO/ZnO/Al2O3)
1091
were obtained in one precipitate with the required Cu/Zn/Al molar ratio. In the multi-
1092
precipitation method, the precursor of CuO/ZnO was obtained in one precipitate, and the
1093
precursor of Al2O3 was obtained in another precipitate, then both precipitates are mixed together
1094
to finally produce CuO/ZnO/Al2O3 with the required molar ratio.
1095
Addition of aluminum improved H2S adsorption on Cu–Zn mixed oxides. The adsorbents
1096
mp-Cu80Zn10Al10 and cp-Cu90Zn0Al10 exhibited superior capacities for removing H2S resulting in
1097
sulfur capacities of 234 and 257 mg S/g adsorbent, respectively.
1098
An interesting finding was that the addition method of aluminum (mp or cp) impacted the
1099
sulfur uptake. The mp-CuxZnyAl10 adsorbents achieved higher sulfur capacities than the cp-
1100
CuxZnyAl10 adsorbents. According to the investigators, the mp-CuxZnyAl10 adsorbents might
1101
contain less Al3+ ions in the lattice of CuO and/or ZnO. Thereby, the high sulfur uptakes of the
1102
mp-CuxZnyAl10 adsorbents might be explained by the increase of lattice diffusion resulting from
1103
less obstruction of Al3+ ions, allowing sulfur anions to move more freely. It is generally believed
1104
that pore or lattice diffusion controls the overall rate of reaction between oxides and H2S. For
1105
pore diffusion, the diffusion of H2S into the pores of the oxides controls the overall rate of
1106
reaction. For lattice diffusion , the HS− and S2− diffusion into the oxide lattice and migration of
1107
O2− and water to the surface control the overall rate. The cp-CuxZny adsorbents possessed large
1108
average pore diameters in comparison with the cp-CuxZnyAl10 or mp-CuxZnyAl10 adsorbents, but
1109
lower sulfur capacities. This is an indication that the lattice diffusion determined sulfur uptakes.
1110
The sulfur capacities for the cp-CuxZnyAl10 and mp-CuxZnyAl10 adsorbents were higher
1111
compared to the cp-CuxZny adsorbents due to smaller crystalline sizes of the CuO and/or ZnO,
1112
higher specific surface area, and greater interface area for the exchange of HS−/S2− and O2−.
1113
Results also showed that the Cu-rich adsorbent were more sensitive to the change of
1114
temperature than the Zn-rich adsorbent. The mp-Cu80Zn10Al10 adsorbent exhibited an increase of
1115
sulfur capacity from 234 to 347 mg S/g with an increase in temperature from 40 to 80 °C.
1116
Operating temperature for the adsorption process also determined the sulfur uptake, increasing
1117
the rate of lattice diffusion of the anions when the temperature increased. It was seen that the
1118
lattice diffusion was more sensitive to the increase of temperature in CuO than with ZnO.
1119
Other investigators conducted a study using HNb3O8 nanosheets for the removal of ethyl
1120
mercaptan (EM) (Zhang et al. 2021). The adsorption experiments were carried in a fixed bed
1121
reactor using 500 ppm EM/methane gas at 25 °C and a flow rate of 10 ml/min. The adsorbent
1122
was pretreated with N2 at a flow rate of 60 mL/min at 120 °C for 1 h before the adsorption
1123
experiments. The sulfur adsorption capacity of HNS (HNb3O8) reached 21.63 mg/g adsorbent.
1124
This is approximately 50 times higher compared to HN (H3ONb3O8). The better performance is
1125
attributed to the surface area (108.2 m2/g)and pore structure (0.3011 cm3/g). The spent HNS was
1126
regenerated under an N2 atmosphere at 100 °C. After 9 regeneration cycles, the sulfur adsorption
1127
capacity of the HNS only diminished slightly to 94% of the initial adsorption. The
1128
characterization results indicated good stability of the HNS active sites (the adsorbent could be
1129
regenerated, keeping same active sites after regeneration). This research also intended to explain
1130
the adsorption mechanism of EM from natural gas onto H3ONb3O8 by investigating the hydrogen
1131
bond interaction. HNb3O8 nanosheet (HNS) was chosen due to its basic structural unit
1132
NbO6 octahedron charge imbalance resulting in unsaturated oxygen sites and hydroxyl groups on
1133
its surface. Hydrogen bond interactions can be formed between the adsorbate and the adsorbent.
1134
DFT calculations corroborated that the adsorption mechanism of EM on the HNS surface is
1135
through hydrogen bonding interaction.
1136
Mixed metal oxides can be synthesized from layered double hydroxides (LDHs) as
1137
precursors to develop efficient adsorbents for the removal of sulfur compounds (Othman et al.
1138
2013.; Wen et al. 2022.). The structure of LDHs consists of positively charged layers owing to
1139
the presence of two metal hydroxides in different oxidation states (Othman et al. 2013.; Wen et
1140
al. 2022.). In general, LDHs can be formulated as [M1−x2+Mx3+(OH)2]Ax/nn−·mH2O, where A is an
1141
anion, such as CO32− > HPO42− > SO42− (Othman et al. 2013.). The selectivity of several layered
1142
double hydroxides and their derivative mixed metal oxides was investigated for the adsorption of
1143
H2S (Othman et al. 2013.). Three divalent metals (Cu2+, Ni2+, and Zn2+) and three trivalent
1144
metals (Fe3+, Al3+, and Cr3+) were selected to prepare the LDHs. The anion was obtained from
1145
Na2CO3. The tests were performed at 20 °C and atmospheric pressure in a tubular acrylic glass
1146
fixed bed reactor column using 0.5 g of material. The sorbent was held in place by plugs of glass
1147
wool. The feed contained 10,760 ppm of H2S in N2 and air with a relative humidity of 40–45%.
1148
The results revealed that mixed metal oxides have better adsorption for H2S than their precursors
1149
LDHs. The best performing adsorbent was the mixed metal oxide Ni0.64Fe0.36 with an adsorptive
1150
capacity of 136 mg S/g adsorbent. For regeneration, spent adsorbents were mixed with 500 mL
1151
of an aqueous solution of acetate-buffer (sodium acetate and acetic acid) (0.05 M, with acetic
1152
acid ratio of 0.1), NaCl (2 M), and NaOH (0.1 M). The adsorbent (0.5 g) was stirred in the
1153
solution for 24 h at room temperature. The adsorbent lost 22% of its original sulfur capacity
1154
after 2 cycles. The investigators suggested that the adsorption of H2S onto Ni0.64Fe0.36 mixed
1155
metal oxides followed an oxidation process forming a sulfate, which eventually migrated to the
1156
interlayer region. This research confirmed the catalytic characteristics of Ni/Fe mixed metal
1157
oxides to oxidize H2S into sulfate.
1158
4. Reaction mechanisms
1159
The desulfurization process involves several mechanisms for the removal of sulfur compounds,
1160
including physisorption, and chemisorption. Physisorption is characterized by surface
1161
interactions between sulfur compounds and the adsorbent material. In chemisorption, the
1162
formation of end products such as NiS, CuS, and H2O can lead to structural changes in the
1163
adsorbent material due to the active interaction between the metal ions and ligands. The specific
1164
mechanism employed in the desulfurization process is determined by the composition of the
1165
target sulfur species, such as reduced or oxidized sulfur compounds, and the type of framework
1166
structure of the adsorbent material.
1167
1168
4.1 zeolite
4.1.1 Rection mechanism
1169
Previous studies have sought to investigate the mechanism of adsorption of sulfur compounds on
1170
zeolites. Utilizing infrared spectroscopy examined the adsorption of ethanethiol and n-
1171
butanethiol on Na-ZSM-5 and ZSM-5 zeolites found that the presence of metal cation such as
1172
Na+ had a significant impact on the adsorption of these compounds. Furthermore, the study
1173
observed that the thiols selectively interacted with SiOHAl groups on H-ZSM5, and that
1174
ethanethiol and butanethiol can be adsorbed in various forms. The study determined that the
1175
adsorption structure of thiols on Na-ZSM-5 was akin to that on SiOH groups, with the thiol
1176
molecules coordinating with the cations via the sulfur atom. The described adsorption can be
1177
seen in Fig. 10 (Garcia and Lercher 1991).
1178
1179
Fig. 10. Proposed adsorption mechanism for NA-ZSM-5 zeolite. (Garcia and Lercher 1991)
1180
A previous study conducted by(Karge and Raskó 1978) investigating the adsorption of H2S on
1181
faujasite-type zeolites with systematically varied Si/Al ratios (NaX 1≤Si/Al<2.50 and NaY SiAl
1182
greater 2.5) were conducted to gain insight into the mechanism of adsorption and the impact of
1183
the Si/Al ratio on the process. It was determined that OH groups and S2− are generated because
1184
of the initial dissociative adsorption of H2S molecules when NaX-type zeolites (1≤Si/Al<1.50 )
1185
are used.(Fig.11). The study also found that an increase in coverage results in not only the
1186
dissociation of H2S into HS− and H+, but also physical adsorption. However, on NaY-type
1187
zeolites (Si/Al > 2.5) H2S molecules are adsorbed without dissociation. The study concluded that
1188
the H2S adsorption capacity of X-type zeolites is four times higher in comparison to Y-type
1189
zeolites.
1190
1191
1192
Fig.11. Dissociative H2S adsorption on FAU NaX zeolite to form -OH and -SH groups (Karge and Raskó 1978)
4.2 Activated carbons
1193
4.2.1 Reaction mechanism
1194
A study to compare the effect of impregnation of activated carbon on sulfur removal
1195
capacity was carried out by Xiao, et. al. (Xiao et al. 2008.). A coal-based granular AC was
1196
modified by impregnation with Na2CO3, and it was compared to the unmodified AC for the
1197
removal of H2S at 30 °C. This experiment was carried out in different ways. First, it was tested
1198
with fixed 80 % relative humidity (RH) in the absence of oxygen, and with an O2/H2S ratio of
1199
2:1. Secondly, it was tested with different RH (0 and 80%) with a fixed O2/H2S ratio of 2:1. For
1200
the first part, the results showed that the sulfur capacities of both modified and unmodified
1201
activated carbons was higher in the oxidation environment than in the absence of oxygen,
1202
achieving 407 mg S/g and 140 mg S/g (with O2/H2S ratio of 2:1), respectively. According to the
1203
researchers, these results were due to adsorption and catalytic oxidation, the latter having the
1204
strongest impact. For the second part, the sulfur capacities for modified and unmodified
1205
activated carbons were 47.6 mg S/g and 8.8 mg/g, respectively. These results indicated that the
1206
catalytic activity of both activated carbons was undermined in the absence of water. The sulfur
1207
capacities of the activated carbons were higher in a dry oxidation environment than in the
1208
absence of oxygen. However, the sulfur capacities were improved when RH increased on both
1209
unmodified and modified activated carbons, confirming that water content can enhance oxidative
1210
removal of H2S on activated carbons. Modified activated carbon performed better than
1211
unmodified activated carbon, confirming the modification benefits on adsorptive capacity. The
1212
investigators concluded that NaCO3 modified the surface chemistry of the activated carbon,
1213
forming a basic solution film with water, and promoting the dissociation of H2S to HS- ions (
1214
Fig.12) which allowed the oxidation process to form elemental sulfur, and partial formation of
1215
sulfuric acid.
1216
1217
1218
1219
Fig.12.The dissolution of adsorbed hydrogen sulfide within water film (Xiao et al. 2008.).
4.3 MOFs
4.3.1 Reaction mechanism
1220
The adsorption of dimethyl sulfide, ethyl mercaptan, and hydrogen sulfide by IRMOF-3 at
1221
ambient temperature as well as a possible mechanism for sulfur capture was explored by several
1222
investigators ( Georgiadis, et al., 2021). The study applyed infrared spectroscopy (IR) and X-ray
1223
photoelectron spectroscopy (XPS) to understand the adsorption mechanism. Results from IR
1224
showed that the -NH2 groups in IRMOF-3 had a weak interaction with dimethyl sulfide and ethyl
1225
mercaptan, but a stronger interaction with hydrogen sulfide. XPS results showed that the spent
1226
catalyst with hydrogen sulfide (H2S) revealed the formation of ZnS, S-H, and elemental sulfur. It
1227
was concluded that the adsorption of dimethyl sulfide and ethyl mercaptan is due to the weak
1228
interaction between the amino group in the metal-organic frameworks and the sulfur atom of the
1229
adsorbate, while the interaction between hydrogen sulfide and the sulfur atom in MOFs is a
1230
result of the interaction between the amino group and the zinc site in the MOFs.
1231
HKUST-1 was tested to remove hydrogen sulfide and nitrogen dioxide. The results revealed that
1232
the adsorption capacity of HKUST-1 was affected by the presence of water, and that the porosity
1233
of HKUST-1 and the composites decreased significantly. This decrease in porosity is believed to
1234
be the result of chemical reactions between the MOF and the adsorbates. Further analysis using
1235
Fourier transform infrared spectroscopy (FT-IR) showed changes in the chemistry of the
1236
materials, indicating a reactive adsorption process. Thermal analysis showed the presence of a
1237
peak at around 200-250 °C, which was attributed to the formation of copper sulfide because of
1238
the interaction between HKUST-1 and H2S. This conclusion was supported by the observation
1239
that the materials turned black after exposure to H2S, which is consistent with the color of
1240
copper sulfide. Other studies have also reported the formation of metal sulfides upon reaction
1241
with H2S and MOFs. A reaction mechanism of the adsorption of H2S on HKUST-1 is proposed
1242
as a two steps process (see Fig. 13). First, H2S molecules bind to the copper sites on the
1243
HKUST-1, and then, the sulfide ions react with the MOF framework to form copper sulfide
1244
(CuS) (Petit et al. 2012)
1245
1246
1247
Fig. 13. A proposed mechanism for the adsorption of H2S on HKUST-1 MOF (Petit et al. 2012).
4.4 Metal oxides
1248
4.4.1Reaction mechanism
1249
A proposed a reaction mechanism, where CH3SH is converted to dimethyl disulfide is
1250
given by (Yi et al. 2021):
2RSH + 1/2O2 → RSSR + H2O
1251
1252
(7)
In this reaction mechanism, the active sites are provided by the metal oxide, which
1253
supplies energy to break up a hydrogen bond, promoting the generation of O-. Then, the
1254
asymmetric oxygen vacancies allow the activation and dissociation of H2O and production of
1255
active oxygen (Fig. 14).
1256
1257
1258
Fig. 14. Proposed reaction mechanism for the conversion of CH3SH to dimethyl disulfide on a metal oxide support
impregnated with copper and manganese (Yi et al. 2021).
1259
5. Sulfur capacities performances
1260
5.1 Comparison of sulfur capacities of different zeolites
1261
Fig. 15 shows a comparison of sulfur capacities of the zeolites studied for adsorptive removal
1262
of H2S. It can be observed that the best performing adsorbents for H2S removal are metal exchange
1263
type Y zeolites (Cu(I)Y followed by AgY) with Si/Al ratio of 2.4. It seems that Cu and Zn are the
1264
most used metals for zeolite modifications.
250
200
150
100
50
0
1265
1266
1267
Cu(I)Y
Ag-Y
Cu-ZnO/SiO2
IMS 10000 ppm H2S
ZnO/SiO2-Ag2O
ZnO/SiO2-Ce
Cu(II)Y
SAPO-43
IMS 25C
ZnO/SiO2-CuO-La2O3
ZnO/SiO2-La2O3
ZnO/SiO2-MnOx
IMS 35C
ZnO/SiO2-NiO
PEI on SBA-15
Clinoptilolite
IMS 50C
11.2%Cu-6%Zn-SBA-15
.2Cu-.21ZnO/SiO2
.05Cu-.21ZnO/SiO2
.05Fe-.21ZnO/SiO2
IMS 3000 ppm H2S
0.05Co-.21Zn-SiO2
hierarchical porous silica…
13X
Undoped Zn-SiO2
.05Ni-.21Zn-SiO2
0.05Mn-.21Zn-SiO2
.05Cu-.15Zn-SiO2
0.25%Fe-0.25%Mn-…
0.59%Cu-11.4%Zn-SiO2
0.25%Fe-11.75%Zn-SiO2
0.25%Mn-11.75%Zn-SiO2
12.1%Zn-SiO2
Sud-Chemie ZnO
fSi-TETA-20
fSi-PEI800-20
BASF ZnO
fSi-TEPA-20
fSi-AEEA-20
Zinox 380
fSi-TRI-20
mg S/g adsorbent
300
Fig. 15. Sulfur capacities of different zeolites evaluated in numerous studies for H2S removal (Crespo et al. 2008;
Elyassi et al. 2014; Georgiadis et al. 2021; Shah et al. 2017; Yang and Tatarchuk 2010; Yaşyerli et al. 2002).
1268
Fig.16 shows the comparison of sulfur capacities of zeolites for removing methyl and ethyl
1269
mercaptans. There is a more limited number of studies compared to H2S, which is a more abundant
1270
sulfur compound in raw natural gas. Again, metal exchange zeolites have been widely studied.
1271
For methyl and ethyl mercaptans, the adsorbent with the highest sulfur capacity was NaX zeolite
1272
with Si/Al ratio of 1.2 followed by 13X zeolite. Zeolites X are characterized by having low Si/Al
1273
ratio, which favors the selectivity towards polar molecules. In addition to the relatively high
1274
surface areas of the adsorbents, their pore structure, and the surface chemistry low Si/Al ratio could
1275
be one factor that contributed to the high sulfur capacities achieved by NaX and 13X for the
1276
removal of methyl and ethyl mercaptans.
300
mg S/g adsorbent
250
200
150
100
50
0
Ethyl mercaptan
Methyl mercaptan
1277
1278
1279
Fig.16. Sulfur capacities of zeolites for methyl and ethyl mercaptans (Ryzhikov et al. 2011.; Taheri et al. 2017;
Weber et al. 2008.; Zhu et al. 2019.).
1280
Fig. 17 summarizes adsorption capacity performance for other organic sulfur compounds.
1281
Again, the number of studies with other organic compounds is limited. The best performing
1282
adsorbents were metal exchange zeolites.
CaX
BaX
NaX
CsNaX
NiNaX
MgNaX
ZnNaX
Cu/13X 450 C
Ag/13X
Ni/13X
Ce/13X
CuCl2/13X
Zn/13X
Cu/13X 450 C
Cu/13X
Cu (.1 mol/L)/13X
Cu/13X 400 C
Cu (.05 mol/L)/13X
Cu (.01 mol/L)/13X
Cu/13X 350 C
CuBr2/13X
Hß
Cu (1 mol/L)/13X
13 X
Cu/13X 150 C
5A
Cu/13X 550 C
Hß
HY
Na-Y
Na-Y
mg S/g adsorbent
70
60
50
40
30
20
10
0
COS
Dimethyl disulfide
Dimethyl sulfide
methyl sulfide
tert-butyl mercaptan
1283
1284
1285
Fig. 17. Sulfur capacities of zeolites for COS, dimethyl disulfide, dimethyl sulfide, methyl sulfide, and tert-butyl
mercaptan (Chen et al. 2016; Crespo et al. 2008; Ryzhikov et al. 2011.; Zhu et al. 2019.).
1286
1287
5.2 Comparison of Sulfur capacities and surface areas of different activated carbons
1288
Fig. 18 summarizes the sulfur capacities of activated carbons for H2S published by
1289
numerous investigators. It can be observed that the best performing activated carbon for H2S
1290
removal was NMC-M/P-2, which is a mesoporous N2-rich activated carbon. The high sulfur
1291
capacity of the NMC-M/P-2 activated carbon demonstrated that N2-rich activated carbons can
1292
improve sulfur capacities. Similar to studies with zeolites, Cu and Zn are the main metallic
1293
species used to modify the surface of activated carbon with the objective of enhancing adsorption
1294
capacity.
mg S/g adsorbent
3000
2500
2000
1500
1000
500
0
1295
CAC
>CuSO4–CAC
>KI–CAC
KOH–CAC
Na2CO3-CAC
GAC
ZnAc2-CAC
Biochar
Darco AC
Red soil
KOH-AC RBAA1 Norit
AirDep CKI
ZnO (+KI)AC ROZ3…
Cu-CrAC RGM1 Norit
1303
IAC
NMC-M/P-2
1800
1600
1400
1200
1000
800
600
400
200
0
ZnOAC Darco Norit
activated carbon in combination with surface pH, pore size, and surface chemistry.
Cu0Zn1.0/AC
1302
Cu0.1Zn0.9/AC
sulfur capacity. However, surface area is an important factor to consider when selecting an
Cu0.05Zn0.95/AC
1301
Cu0.25Zn0.75/AC
these studies, there is no direct correlation between surface area of an activated carbon and its
SulfaTrap R8G
1300
Cu0.5Zn0.5/AC
Fig. 19 shows the comparison of surface areas from the activated carbons in Fig. 18. From
AirDep CKC
1299
coalbased…
Fig. 18. Sulfur capacity of activated carbons for H2S from previous studies (Balsamo et al. 2016.; Georgiadis et al.
2020.; Hernández et al. 2011.; Katz et al. 2013; Park et al. 2011; Rout, Bhunia, and Dash 2015; Shah et al. 2017;
Sun et al. 2013; Xiao et al. 2008.; Zulkefli et al. 2019).
Surface area m2/g
1296
1297
1298
1304
1305
1306
Fig. 19. Surface areas of activated carbons for H2S from literature (Balsamo et al. 2016.; Georgiadis et al.2020.;
Hernández et al. 2011.; Katz et al. 2013; Park et al. 2011; Rout et al. 2015; Shah et al. 2017; Sun et al. 2013; Xiao et
al. 2008.; Zulkefli et al. 2019).
1307
Fig. 20 shows the sulfur capacities of the different activated carbons for methyl and ethyl
1308
mercaptans presented by previous investigators. The limited number of studies for the removal of
1309
methyl and ethyl mercaptan is clear. As Fig. 20 shows there are promising activated carbons that
1310
have demonstrated high sulfur uptakes for these compounds. The surface of the best performing
1311
activated carbons was modified using nitrogen.
mg S/g adsorbent
700
600
500
400
300
200
100
0
FeCl3-AC
HNO3-AC
Virgin Activated
Carbon
Ethyl mercaptan
Fe N2 AC
AC N2 rich
Methyl mercaptan
1312
1313
1314
Fig. 20. Sulfur capacities of activated carbons for methyl and ethyl mercaptans published elsewhere (Aguiar et al.
2017.; Liu et al. 2017.; Lyu et al. 2020.)
1315
Fig. 21 shows the surface areas of the activated carbons for the removal of ethyl and methyl
1316
mercaptans. In these cases, the surface area of the adsorbents clearly correlates with better
1317
performance, in addition to modification with N2 and/or surface chemistry.
1600
Surface area m2/g
1400
1200
1000
800
600
400
200
0
FeCl3-AC
HNO3-AC
Ethyl mercaptan
Virgin Activated
Carbon
Fe N2 AC
AC N2 rich
Methyl mercaptan
1318
1319
1320
Fig. 21. Surface areas of activated carbons for methyl and ethyl mercaptans found in previous studies (Aguiar et al.
2017.;Liu et al. 2017.; Lyu et al. 2020.)
1321
The activated carbons with the highest sulfur capacities for methyl and ethyl mercaptans
1322
were N2-rich. A limited number of studies have been conducted for other organic sulfur
1323
compounds (see Fig. 22).
4
3.5
3
2.5
2
1.5
1
0.5
0
DMS
Ethyl mercaptan
Isopropyl mercaptan
Methylethyl sulfide
TBM
Tetrahydrothiophene
1324
1325
Fig. 22. Sulfur capacities of activated carbons for organic sulfur compounds (Aguiar et al. 2017.).
1326
1327
Fig. 23 shows the surface area of the activated carbons for organic sulfur compounds. It is
1328
observed that surface area is not the only factor to determine the highest sulfur capacity of the
1329
activated carbons (as observed in Fig. 19 and Fig. 21).
Surface area m2/g
160
140
120
100
80
60
40
20
0
DMS
Ethyl mercaptan
Isopropyl mercaptan
Methylethyl sulfide
TBM
Tetrahydrothiophene
1330
1331
1332
1333
1334
Fig. 23. Sulfur capacities of activated carbons for organic sulfur compounds (Aguiar et al. 2017).
5.3 Comparison of sulfur capacities of different MOFs
Fig. 24 shows the sulfur capacities of the MOFs for H2S removal from different
investigations in the peer reviewed literature.
mg S/g adsorbent
7000
6000
5000
4000
3000
2000
1000
1335
UVM-7@ZIF-8
SBA-15@ZIF-8
MCM-41@ZIF-8
ZIF-8
ED-ZIF-8
WS-ZIF-8
MIL-101
UiO-67(bipy)-CuCl2
MIL-100(Cr)
MIL-47 (V)
MIL-53(Cr)
MIL-53(Al)
MIL-53(Fe)
MAC-2
MAC-1
HK-02
MOF-199
HKG-02
HK-02/PHGO
MAC-3
Y-1,4-NDC-fcuMOF
Y-fum-fcu-MOF
HK-01/PLGO
Y-FTZB-fcu-MOF
UiO-67(bipy)-Cu(acac)2
rho-Zmof
HK-01
IRMOF-3
PHHK-02
UiO-67(bipy)-CuSO4
UiO-67(bipy)-Co(NO3)2
UiO-67(bipy)-CuSO4
PLHK-01
UiO-67(bipy)-Cu(NO3)2
0
1336
1337
1338
Fig. 24. Sulfur capacities of different MOFs for H2S from literature (Bashkova et al. 2002; Bhoria et al. 2020; Deng
et al. 2018; Jameh et al. 2019; Saeedirad et al. 2018; Shah et al. 2017; Fan et al. 2017; Vellingiri et al. 2016; Wang
et al. 2014).
1339
Fig. 25 shows the surface areas of these MOFs (Fig. 24) for H2S removal. Surface areas of
1340
the MOFs are not the only factor determining the high sulfur capacities. The high sulfur capacities
1341
achieved by the adsorbents is probably due to the combination of the surface chemistry, pore
1342
structure, and surface area of the MOFs.
Surface area m2/g
3000
2500
2000
1500
1000
500
UVM-7@ZIF-8
SBA-15@ZIF-8
MCM-41@ZIF-8
ZIF-8
ED-ZIF-8
WS-ZIF-8
MIL-101
UiO-67(bipy)-CuCl2
MIL-100(Cr)
MIL-47 (V)
MIL-53(Cr)
MIL-53(Al)
MIL-53(Fe)
MAC-2
MAC-1
HK-02
MOF-199
HKG-02
HK-02/PHGO
MAC-3
Y-1,4-NDC-fcuMOF
Y-fum-fcu-MOF
HK-01/PLGO
Y-FTZB-fcu-MOF
UiO-67(bipy)-Cu(acac)2
rho-Zmof
HK-01
IRMOF-3
PHHK-02
UiO-67(bipy)-CuSO4
UiO-67(bipy)-Co(NO3)2
UiO-67(bipy)-CuSO4
PLHK-01
UiO-67(bipy)-Cu(NO3)2
0
1343
1344
1345
1346
Fig. 25. Sulfur capacities of different MOFs for H2S from literature (Bashkova et al. 2002; Bhoria et al. 2020; Deng
et al. 2018; Jameh et al. 2019; Saeedirad et al. 2018; Shah et al. 2017; Fan et al. 2017.; Vellingiri et al. 2016; Wang
et al. 2014).
1347
Fig.26 shows MOFs sulfur capacities for methyl and ethyl mercaptans presented by
previous investigators. Several MOFs have demonstrated experimental sulfur uptakes of over
1349
1000 mg S/g adsorbent. This is considered exceptional.
mg S/g adsorbent
1348
1600
1400
1200
1000
800
600
400
200
0
Ethyl mercaptan
methyl mercaptan
1350
1351
1352
1353
1354
Fig.26. Sulfur capacities of MOFs for methyl and ethyl mercaptans (Ghanbarabadi, et al.,2015; Saeedirad et al.
2018; Fan et al. 2017; Taheri et al. 2017).
Fig. 27 presents sulfur capacities of MOFs for several organic sulfur compounds contained
in natural gas. Most of these studies focused on the removal of tert-butyl mercaptan (TBM).
400
mg S/g adsorbent
350
300
250
200
150
100
50
0
IRMOF-3 MOF-199 IRMOF-3
dimethyl sulfide.
Cu-BTC MIL-53(Al) UiO-66(Zr) Zn(bpb)
DMS
TBM
Ni(bpb)
Thiophene (C4H4S)
1355
1356
1357
Fig. 27. Sulfur capacities of MOF for different organic sulfur compounds (Chen et al. 2015; Ghanbarabadi et al.
2015; Fan et al. 2017; Vellingiri et al. 2016).
1358
1359
5.4 Comparison of sulfur capacities of different metal oxides
Fig. 28 shows the sulfur capacities of the different metal oxides and mixed metal oxides
1361
reported from previous studies (Jiang et al. 2010; Lee et al. 2005; Shah et al. 2017; Zhang et al.
1362
2015). Limited information was obtained from desulfurization of organic sulfur compounds from
1363
natural gas using metal oxides.
mg S/g adsorbent
1360
3000
2500
2000
1500
1000
500
0
1364
1365
1366
1367
1368
Fig. 28. Sulfur capacities of metal oxides for H2S (Jiang et al. 2010; Lee et al. 2005; Shah et al. 2017; Zhang et al.
2015).
1369
1370
5.5 Comparison of the highest sulfur capacities from best performing materials
A comparison of sulfur capacities of different material categories was prepared taking
1371
into consideration the best performing materials for the adsorption of sulfur from natural gas (see
1372
Fig. 29-Fig. 31.
1373
Fig. 29 shows the comparison of the best performing materials (zeolites, activated
carbons, MOFs, and metal oxides) for removing H2S. MOFs demonstrated the highest sulfur
1375
uptakes compared to the other 3 categories. The ZIF-8/UVM-7, which is a modified MOFs (ZIF-
1376
8) with a zeolite (UVM-7) achieved the best sulfur removal.
mg S/g adsorbent
1374
7000
6000
5000
4000
3000
2000
1000
0
Zeolites
AC
MOF
Metal oxides
1377
1378
1379
1380
Fig. 29. Comparison of the 5 highest sulfur capacities using zeolites, activated carbon, MOFs, and metal oxides for
removing H2S (Crespo et al. 2008; Georgiadis et al. 2021; Jameh et al. 2019; Jiang et al. 2010.; Lee et al. 2005;
Saeedirad et al. 2018; Shah et al. 2017; Xiao et al. 2008).
1381
1382
1383
Fig. 30 shows the comparison among materials with the highest ethyl mercaptan uptakes.
ZIF-8/UVM-7 also demonstrated the best performance for ethyl mercaptan removal.
mg S/g adsorbent
1600
1400
1200
1000
800
600
400
200
0
zeolites
AC
MOF
Metal oxides
1384
1385
1386
1387
Fig. 30. Comparison of the highest sulfur capacities using zeolites, activated carbon, MOFs, and metal oxides for
removing ethyl mercaptan (Aguiar et al. 2017; Ghanbarabadi et al. 2015; Saeedirad et al. 2018; Fan et al. 2017;
Taheri et al. 2017; Weber et al. 2008; Zhu et al. 2019).
1388
Fig. 31 shows the best performing materials for the removal of methyl mercaptan. Only
1390
one MOF (MIL-53 (Al)) performed better than zeolites, achieving the highest sulfur capacity. No
1391
previous studies were found in the literature using activated carbons and metal oxides for the
1392
removal of methyl mercaptan from natural gas.
mg S/g adsorbent
1389
500
450
400
350
300
250
200
150
100
50
0
zeolites
MOF
1393
1394
1395
1396
Fig. 31. Comparison of the highest sulfur capacities using zeolites and MOFs for removing methyl mercaptan
(Ryzhikov et al. 2011; Fan et al. 2017; Taheri et al. 2017).
1397
It is important to highlight that several factors should be considered when selecting an
1398
adsorbent for desulfurization of natural gas. These factors are high sulfur capacity, regeneration
1399
of the adsorbent, good stability, and low costs of manufacturing. For commercially available
1400
adsorbents, it is noted that MOF materials have the highest production costs ($8000/kg),
1401
followed by zeolites ($590/kg), and activated carbons ($86/kg) (Vellingiri et al. 2016). For this
1402
reason, great performance of MOFs for desulfurization of natural gas could be diminished by
1403
their high manufacturing costs, preventing their use for industrial applications
1404
1405
6. Conclusions
1406
A comprehensive literature review was conducted on the current state of the materials
1407
used for the adsorptive desulfurization of natural gas. Materials characteristics, such as surface
1408
area, pore size, pH, molecular structure, and particle size are important considerations when
1409
selecting a material for adsorption of sulfur compounds from natural gas. Modifications of the
1410
materials (zeolites, activated carbons, MOFs, and metal oxides) have demonstrated to improve
1411
sulfur uptakes. Reaction mechanisms were presented to facilitate an understanding of how the
1412
sulfur compounds react on the surface of adsorbents. Materials from every category have been
1413
tested in the presence of water, which favors reactions of sulfur compounds on adsorbents.
1414
However, excessive amounts of water can form water shells on an adsorbent surface as well
1415
within their pores, impeding the sulfur compounds to interact with the active sites.
1416
Transition metals loadings are widely used for surface modifications, enhancing sulfur
1417
capacities. However, excessive loads can lead to particle agglomerations and pore
1418
blockage. Calcination temperatures for adsorbents during synthesis can also have a significant
1419
impact on sulfur removal. Most of the previous studies show an effective calcination
1420
temperature of 500 °C. Temperatures higher than this value can result in sintering of the
1421
adsorbents and reduced performance. Development of low-cost adsorbents with high sulfur
1422
capacities should be targeted in future research. Advancements in this regard would enable the
1423
use of these technologies on larger-scales. In addition, more research focused on the removal of
1424
organic sulfur compounds such as methyl mercaptan and ethyl mercaptan is needed to have a
1425
larger database on the sulfur capacities from different adsorbents. This review paper could serve
1426
as a guide for other researchers to improve the performance of current adsorbents for sulfur
1427
removal from natural gas.
1428
Acknowledgments
1429
The authors acknowledge the financial support of the U.S. – Israel Fossil Energy Center (FEC-
1430
19) administered by the BIRD Foundation and funded by the Israeli Energy Ministry and the
1431
U.S. Department of Energy for this work.
1432
References
1433
1434
1435
Aguiar, M. F. D., Coelho, G. L. V., 2017. Adsorption of sulfur compounds from natural gas by different
adsorbents and desorption using supercritical CO2. J. Environ. Chem. Eng., 5(5), 4353–4364.
https://doi.org/10.1016/J.JECE.2017.07.079
1436
1437
Ahmed, I., & Jhung, S. H., 2016. Adsorptive desulfurization and denitrogenation using metal-organic
frameworks. J. Hazard. Mater., 301, 259–276. https://doi.org/10.1016/J.JHAZMAT.2015.08.045
1438
1439
Angelis, A., 2012. Natural gas removal of hydrogen sulphide and mercaptans. Appl. Catal. B Environ.
113e (114), 37e42. https://doi.org/10.1016/J.APCATB.2011.11.026
1440
1441
1442
Anyanwu, J. T., Wang, Y., Yang, R. T., 2021. SBA-15 functionalized with amines in the presence of water:
Applications to CO2 capture and natural gas desulfurization. Ind. Eng. Chem. Res., 60(17), 6277–6286.
https://doi.org/10.1021/ACS.IECR.1C00415/ASSET/IMAGES/LARGE/IE1C00415_0010.JPEG
1443
1444
1445
Bailleul, S., Yarulina, I., Hoffman, A. E., Dokania, A., Abou-Hamad, E., Chowdhury, A. D., Van Speybroeck,
V., 2019. A supramolecular view on the cooperative role of Brønsted and Lewis acid sites in zeolites for
methanol conversion. J. Am. Chem. Soc., 141(37), 14823-14842. https://doi.org/10.1021/jacs.9b07484.
1446
1447
1448
Balsamo, M., Cimino, S., de Falco, G., Erto, A., Lisi, L., 2016. ZnO-CuO supported on activated carbon for
H2S removal at room temperature. J. Chem. Eng., 304, 399–407.
https://doi.org/10.1016/J.CEJ.2016.06.085
1449
1450
1451
Bashkova, S., Bagreev, A., Bandosz, T. J., 2002. Effect of surface characteristics on adsorption of methyl
mercaptan on activated carbons. Ind. Eng. Chem. Res., 41(17), 4346–4352.
https://doi.org/10.1021/IE020137T/ASSET/IMAGES/LARGE/IE020137TF00007.JPEG
1452
1453
1454
Bashkova, S., Bagreev, A., Bandosz, T. J., 2005. Catalytic properties of activated carbon surface in the
process of adsorption/oxidation of methyl mercaptan. Catal. Today, 99(3–4), 323–328.
https://doi.org/10.1016/J.CATTOD.2004.10.007
1455
1456
1457
1458
Basu, Arun & Meyer, Howard & Leppin, Dennis & Zhou, Jim & Makkuni, A.,2012. UCSRP: A Flexible Sulfur
Removal Process for Sweetening Natural Gas.
https://www.researchgate.net/publication/265068701_UCSRP_A_Flexible_Sulfur_Removal_Process_for
_Sweetening_Natural_Gas
1459
1460
1461
1462
Bhoria, N., Basina, G., Pokhrel, J., Kumar Reddy, K. S., Anastasiou, S., Balasubramanian, V. v., AlWahedi,
Y. F., Karanikolos, G. N., 2020. Functionalization effects on HKUST-1 and HKUST-1/graphene oxide hybrid
adsorbents for hydrogen sulfide removal. J. Hazard. Mater., 394, 122565.
https://doi.org/10.1016/J.JHAZMAT.2020.122565
1463
1464
Bonjour, J., Chalfen, J. B., Meunier, F., 2002. Temperature swing adsorption process with indirect cooling
and heating. Ind. Eng. Chem. Res., 41(23), 5802-5811. https://doi.org/10.1021/ie011011j.
1465
1466
Busca, G., 2017. Acidity and basicity of zeolites: A fundamental approach. Microporous Mesoporous
Mater., 254, 3-16.https://doi.org/10.1016/j.micromeso.2017.04.007.
1467
1468
1469
Chen, G., Koros, W. J., Jones, C. W., 2016. Hybrid polymer/UiO-66 (Zr) and polymer/NaY fiber sorbents
for mercaptan removal from natural gas. ACS Appl. Mater. Interfaces, 8(15), 9700-9709.
https://doi.org/10.1021/acsami.6b00897.
1470
1471
1472
Chen, G., Tan, S., Koros, W. J., & Jones, C. W., 2015. Metal organic frameworks for selective adsorption
of t -butyl mercaptan from natural gas. Energy Fuels, 29(5), 3312–3321.
https://doi.org/10.1021/ACS.ENERGYFUELS.5B00305/SUPPL_FILE/EF5B00305_SI_001.PD
1473
1474
1475
1476
Chen, X., Shen, B. X., Sun, H., Zhan, G. X., 2017. Ion-exchanged zeolites Y for selective adsorption of
methyl mercaptan from natural gas: Experimental performance evaluation and computational
mechanism explorations. Ind. Eng. Chem. Res., 56(36), 10164-10173.
https://doi.org/10.1021/acs.iecr.7b01982.
1477
1478
1479
1480
Chung, Y. G., Camp, J., Haranczyk, M., Sikora, B. J., Bury, W., Krungleviciute, V., Yildirim, T., Farha, O. K.,
Sholl, D. S., Snurr, R. Q., 2014. Computation-ready, experimental metal-organic frameworks: A tool to
enable high-throughput screening of nanoporous crystals. Chem. Mater., 26(21), 6185–6192.
https://doi.org/10.1021/CM502594J/SUPPL_FILE/CM502594J_SI_002.XLSX
1481
1482
1483
Costa, N. E. G., P. C. P. Patrício, R. v. Ferreira, M. R. Vianna Neto, M. Cardoso. 2018. “SIMULATION AND
ANALYSIS OF A HYDROGEN GENERATION UNIT.” Braz. j. pet. gas 12(3):159–68. doi: 10.5419/bjpg20180015.
1484
1485
Crespo, D., Qi, G., Wang, Y., Yang, F. H., & Yang, R. T., 2008. Superior sorbent for natural gas
desulfurization. Ind. Eng. Chem. Res., 47(4), 1238-1244. https://doi.org/10.1021/ie071145i.
1486
1487
1488
Cui, H., Turn, S. Q., & Reese, M. A., 2009. Removal of sulfur compounds from utility pipelined synthetic
natural gas using modified activated carbons. Catal. Today, 139(4), 274–279.
https://doi.org/10.1016/J.CATTOD.2008.03.024
1489
1490
1491
De Oliveira, A.D.; Beatrice, C.A.G. Polymer Nanocomposites with Different Types of Nanofiller. In
Nanocomposites—Recent Evolutions; Sivasankaran, S., Ed.; IntechOpen: London, UK, 2019; pp. 103–
128.10.5772/intechopen.81329.
1492
1493
Dehghan, R., & Anbia, M., 2017. Zeolites for adsorptive desulfurization from fuels: A review. Fuel
Process. Technol., 167, 99-116. https://doi.org/10.1016/j.fuproc.2017.06.015.
1494
1495
1496
Deng, Y., Vellingiri, K., Kim, K. H., Boukhvalov, D. W., & Philip, L., 2018. Activation strategies of metalorganic frameworks for the sorption of reduced sulfur compounds. J. Chem. Eng., 350, 747–756.
https://doi.org/10.1016/J.CEJ.2018.06.006
1497
1498
1499
1500
dos Santos, J. P. L., de Carvalho Lima Lobato, A. K., Moraes, C., de Lima Cunha, A., da Silva, G. F., dos
Santos, L. C. L., 2016. Comparison of different processes for preventing deposition of elemental sulfur in
natural gas pipelines: A review. J. Nat. Gas Sci. Eng., 32, 364–372.
https://doi.org/10.1016/J.JNGSE.2016.04.045
1501
1502
1503
1504
Elyassi, B., Wahedi, Y. al, Rajabbeigi, N., Kumar, P., Jeong, J. S., Zhang, X., Kumar, P., Balasubramanian, V.
v., Katsiotis, M. S., Andre Mkhoyan, K., Boukos, N., Hashimi, S. al, Tsapatsis, M., 2014. A highperformance adsorbent for hydrogen sulfide removal. Microporous Mesoporous Mater., 190, 152–155.
https://doi.org/10.1016/J.MICROMESO.2014.02.007
1505
1506
1507
Fan, H. L., Shi, R. H., Zhang, Z. R., Zhen, T., Shangguan, J., Mi, J., 2017. Cu-based metal–organic
framework/activated carbon composites for sulfur compounds removal. Appl. Surf. Sci., 394, 394–402.
https://doi.org/10.1016/J.APSUSC.2016.10.071
1508
1509
1510
Fang, H. bin, Zhao, J. T., Fang, Y. T., Huang, J. J., Wang, Y., 2013. Selective oxidation of hydrogen sulfide
to sulfur over activated carbon-supported metal oxides. Fuel, 108, 143–148.
https://doi.org/10.1016/J.FUEL.2011.05.030
1511
1512
1513
Gangu, K. K., Maddila, S., Mukkamala, S. B., Jonnalagadda, S. B., 2016. A review on contemporary Metal–
Organic Framework materials. Inorganica Chim. Acta, 446, 61–74.
https://doi.org/10.1016/J.ICA.2016.02.062
1514
1515
1516
Garcia, C. L., Lercher, J. A., 1991. Adsorption and surface chemistry of light thiols on Na-ZSM5 and HZSM5. J. Phys. Chem., 95(26), 10729–10736.
https://doi.org/10.1021/J100179A040/ASSET/J100179A040.FP.PNG_V03
1517
1518
1519
Gawande, M. B., Pandey, R. K., Jayaram, R. v., 2012. Role of mixed metal oxides in catalysis science—
versatile applications in organic synthesis. Catal. Sci. Technol, 2(6), 1113–1125.
https://doi.org/10.1039/C2CY00490A
1520
1521
1522
Georgiadis, A. G., Charisiou, N. D., Gaber, S., Polychronopoulou, K., Yentekakis, I. V., Goula, M. A., 2021.
Adsorption of hydrogen sulfide at low temperatures using an industrial molecular sieve: an experimental
and theoretical study. Acs Omega, 6(23), 14774-14787. https://doi.org/10.1021/acsomega.0c06157.
1523
1524
1525
Georgiadis, A. G., Charisiou, N. D., Goula, M. A., 2020. Removal of hydrogen sulfide from various
industrial gases: A review of the most promising adsorbing materials. Catalysts, 10(5), 521.
https://doi.org/10.3390/catal10050521.
1526
1527
Georgiadis, A. G., Charisiou, N., Yentekakis, I. v., Goula, M. A., 2020. Hydrogen Sulfide (H2S) Removal via
MOFs. Materials 2020, Vol. 13, Page 3640, 13(16), 3640. https://doi.org/10.3390/MA13163640
1528
1529
1530
Ghahraloud, H., Farsi, M., Rahimpour, M. R. 2017. Modeling and optimization of an industrial Claus
process: Thermal and catalytic section. J. Taiwan Inst. Chem. Eng., 76, 1–9.
https://doi.org/10.1016/J.JTICE.2017.03.005
1531
1532
1533
Ghanbarabadi, H., & Khoshandam, B., 2015. Simulation and comparison of Sulfinol solvent performance
with Amine solvents in removing sulfur compounds and acid gases from natural sour gas. J. Nat. Gas Sci.
Eng., 22, 415-420. https://doi.org/10.1016/j.jngse.2014.12.024.
1534
1535
1536
Golebiowska, M., Roth, M., Firlej, L., Kuchta, B., Wexler, C., 2012. The reversibility of the adsorption of
methane–methyl mercaptan mixtures in nanoporous carbon. Carbon, 50(1), 225–234.
https://doi.org/10.1016/J.CARBON.2011.08.039
1537
1538
1539
Halasz, I., Agarwal, M., Marcus, B., Cormier, W. E., 2005. Molecular spectra and polarity sieving of
aluminum deficient hydrophobic HY zeolites. Microporous Mesoporous Mater., 84(1-3), 318-331.
https://doi.org/10.1016/j.micromeso.2005.05.040.
1540
1541
1542
He, J., Liu, X., Li, L., Wang, B., Hu, J., 2012. Experimental investigation of the interaction of dimethyl
sulfide/ethyl mercaptan with nano-manganese dioxide. Ind. Eng. Chem. Res., 51(49), 15912–15917.
https://doi.org/10.1021/IE302097X/ASSET/IMAGES/LARGE/IE-2012-02097X_0010.JPEG
1543
1544
Heo, Do Yeon, Ha Huu Do, Sang Hyun Ahn, Soo Young Kim., 2020. “Metal-Organic Framework Materials
for Perovskite Solar Cells.” Polymers 12(9). doi: 10.3390/POLYM12092061
1545
1546
1547
Hernández, S. P., Chiappero, M., Russo, N., Fino, D., 2011. A novel ZnO-based adsorbent for biogas
purification in H2 production systems. J. Chem. Eng., 176–177, 272–279.
https://doi.org/10.1016/J.CEJ.2011.06.085
1548
1549
1550
1551
Hernández-Maldonado, A. J., Yang, R. T., Chinn, D., Munson, C. L., 2003. Partially calcined gismondine
type silicoaluminophosphate SAPO-43: Isopropylamine elimination and separation of carbon dioxide,
hydrogen sulfide, and water. Langmuir, 19(6), 2193–2200.
https://doi.org/10.1021/LA026424J/ASSET/IMAGES/LARGE/LA026424JF00015.JPEG
1552
1553
1554
Huang, G., He, E., Wang, Z., Fan, H., Shangguan, J., Croiset, E., & Chen, Z. 2015. Synthesis and
Characterization of γ-Fe2O3 for H2S Removal at Low Temperature. Ind. Eng. Chem. Res., 54(34), 8469–
8478. https://doi.org/10.1021/ACS.IECR.5B01398/ASSET/IMAGES/LARGE/IE-2015-01398X_0009.JPEG
1555
1556
1557
Huang, H. Y., Yang, R. T., Chinn, D., & Munson, C. L., 2003. Amine-grafted MCM-48 and silica xerogel as
superior sorbents for acidic gas removal from natural gas. Ind. Eng. Chem. Res., 42(12), 2427
2433.https://doi.org/10.1021/IE020440U/ASSET/IMAGES/LARGE/IE020440UF00012.JPEG
1558
1559
1560
Jameh, A. A., Mohammadi, T., Bakhtiari, O., Mahdyarfar, M., 2019. Synthesis and modification of Zeolitic
Imidazolate Framework (ZIF-8) nanoparticles as highly efficient adsorbent for H2S and CO2 removal
from natural gas. J. Environ. Chem. Eng., 7(3), 103058. https://doi.org/10.1016/J.JECE.2019.103058
1561
1562
Jangam, K., Chen, Y. Y., Qin, L., Fan, L. S., 2021. Perspectives on reactive separation and removal of
hydrogen sulfide. Chem. Eng. Sci.: X, 11, 100105. https://doi.org/10.1016/J.CESX.2021.100105
1563
1564
1565
Jiang, D., Su, L., Ma, L., Yao, N., Xu, X., Tang, H., Li, X., 2010. Cu–Zn–Al mixed metal oxides derived from
hydroxycarbonate precursors for H2S removal at low temperature. Appl. Surf. Sci., 256(10), 3216–3223.
https://doi.org/10.1016/J.APSUSC.2009.12.008
1566
1567
1568
1569
Jones, C.E., Hatcher, N.A., Weiland, R.H., 2014. Mercaptans Removal from Gases by Absorption into
Amines and Caustic. Optimized Gas Treating, Inc., Buda.
https://www.ogtrt.com/files/publications/65/Revised_2013_Brimstone_Vail_Mercaptans_Removal_fro
m_Gases_Final.pdf (Accessed: June 23, 2022).
1570
1571
1572
Joshi, J. N., Zhu, G., Lee, J. J., Carter, E. A., Jones, C. W., Lively, R. P., Walton, K. S., 2018. Probing MetalOrganic Framework Design for Adsorptive Natural Gas Purification. Langmuir, 34(29), 8443–8450.
https://doi.org/10.1021/ACS.LANGMUIR.8B00889/ASSET/IMAGES/LARGE/LA-2018-00889W_0005.JPEG
1573
1574
1575
Karge, H. G., Raskó, J., 1978. Hydrogen sulfide adsorption on faujasite-type zeolites with systematically
varied Si-Al ratios. J. Colloid Interface Sci., 64(3), 522–532. https://doi.org/10.1016/00219797(78)90394-6
1576
1577
1578
Katz, L. E., Speitel, G. E., Richard, C.-S., Corsi, L., Liljestrand, H. M.,Stevenson, K. J. 2013. Removal of
Formaldehyde from Indoor Air: Enhancing Surface-Mediated Reactions on Activated Carbon
.http://hdl.handle.net/2152/26091
1579
1580
1581
Kazemi, A., Malayeri, M., Gharibi kharaji, A., Shariati, A., 2014. Feasibility study, simulation and
economical evaluation of natural gas sweetening processes – Part 1: A case study on a low-capacity
plant in iran. J. Nat. Gas Sci. Eng. 20, 16–22.https://doi.org/10.1016/J.JNGSE.2014.06.001
1582
1583
Keil, F. J., 2013. Complexities in modeling of heterogeneous catalytic reactions. Comput. Math. Appl.,
65(10), 1674-1697. https://doi.org/10.1016/j.camwa.2012.11.023.
1584
1585
1586
1587
Khademi, S. S., Anisi, H., Sadighi, S., Khademi, S., Mirian, S., Yu, X., 2015. Synthesis of SBA-15 View
project Adsorption Processes View project A MATHEMATICAL MODEL TO DESIGN MERCAPTANS
REMOVAL UNIT (MRU) USING REAL COMPONENT LUMPING APPROACH. Retrieved January 19, 2023,
from www.vurup.sk/petroleum-coal
1588
1589
1590
Kim, H.-T., Jun, K.-W., Kim, S.-M., Shankar Potdar, H., Yoon, Y.-S., 2006. Co-Precipitated Cu/ZnO/Al 2 O 3
Sorbent for Removal of Odorants t-Butylmercaptan (TBM) and Tetrahydrothiophene (THT) from Natural
Gas. Energy Fuels 2006, 20, 5, 2170–2173. https://doi.org/10.1021/ef060236
1591
1592
1593
1594
Kuwahara, Y., Aoyama, J., Miyakubo, K., Eguchi, T., Kamegawa, T., Mori, K., Yamashita, H., 2012. TiO2
photocatalyst for degradation of organic compounds in water and air supported on highly hydrophobic
FAU zeolite: Structural, sorptive, and photocatalytic studies. J. Catal., 285(1), 223–234.
https://doi.org/10.1016/J.JCAT.2011.09.031
1595
1596
1597
Lee, E. K., Jung, K. D., Joo, O. S., & Shul, Y. G. 2005. Support effects in catalytic wet oxidation of H2S to
sulfur on supported iron oxide catalysts. Appl. Catal. A-Gen, 284(1–2), 1–4.
https://doi.org/10.1016/J.APCATA.2004.12.034
1598
1599
1600
Li, H., Liu, Y., Yang, Y., Yang, D., Sun, J., 2014. Influences of hydrogen bonding dynamics on adsorption of
ethyl mercaptan onto functionalized activated carbons: A DFT/TDDFT study. J. Photochem. Photobiol. A.,
291, 9–15. https://doi.org/10.1016/J.JPHOTOCHEM.2014.06.017
1601
1602
1603
Li, Y., Wang, L. J., Fan, H. L., Shangguan, J., Wang, H., & Mi, J., 2015. Removal of sulfur compounds by a
copper-based metal organic framework under ambient conditions. Energy Fuels, 29(1), 298–304.
https://doi.org/10.1021/EF501918F/ASSET/IMAGES/LARGE/EF-2014-01918F_0012.JPEG
1604
1605
1606
Li, Z., Xiao, Y., Xue, W., Yang, Q., Zhong, C., 2015. Ionic liquid/metal-organic framework composites for
H2S removal from natural gas: A computational exploration. J. Phys. Chem. C, 119(7), 3674–3683.
https://doi.org/10.1021/ACS.JPCC.5B00019/SUPPL_FILE/JP5B00019_SI_001.PDF
1607
1608
1609
Liang, H., Yang, W., Peng, F., Liu, Z., Liu, J., Qiao, Z., 2019. Combining large-scale screening and machine
learning to predict the metal-organic frameworks for organosulfurs removal from high-sour natural gas.
APL Mater., 7(9), 091101. https://doi.org/10.1063/1.5100765
1610
1611
1612
Liu, Q., Ke, M., Liu, F., Yu, P., Hu, H., & Li, C., 2017. High-performance removal of methyl mercaptan by
nitrogen-rich coconut shell activated carbon. RSC Advances, 7(37), 22892–22899.
https://doi.org/10.1039/C7RA03227G
1613
1614
1615
Liu, T., First, E. L., Hasan, M. F., & Floudas, C. A., 2016. A multi-scale approach for the discovery of
zeolites for hydrogen sulfide removal. Comput. Chem. Eng., 91, 206-218.
https://doi.org/10.1016/j.compchemeng.2016.03.015.
1616
1617
1618
LSU Center of energy studies, 2019. Natural gas outlook: supply, demand and prices.
https://www.lsu.edu/ces/presentations/2019/2019_dismukes_nasuca-gas-committee-julymtg_final.pdf
1619
1620
Lyu, Y., Liu, X., Liu, W., Tian, Y., Qin, Z., 2020. Adsorption/oxidation of ethyl mercaptan on Fe-N-modified
active carbon catalyst. Chem. Eng. J., 393, 124680. https://doi.org/10.1016/J.CEJ.2020.124680
1621
1622
1623
Ma, X., Peng, S., Li, W., Liu, H., & Chen, Y., 2018. Efficient removal of low concentration methyl
mercaptan by HKUST-1 membrane constructed on porous alumina granules. CrystEngComm, 20(4), 407–
411. https://doi.org/10.1039/C7CE01922J
1624
1625
Masters, A. F., & Maschmeyer, T., 2011. Zeolites–From curiosity to cornerstone. Microporous
Mesoporous Mater.142(2-3), 423-438. https://doi.org/10.1016/j.micromeso.2010.12.026.
1626
1627
1628
Melo, D. M. A., de Souza, J. R., Melo, M. A. F., Martinelli, A. E., Cachima, G. H. B., Cunha, J. D., 2006.
Evaluation of the zinox and zeolite materials as adsorbents to remove H2S from natural gas. Colloids
Surf. A Physicochem. Eng. Asp., 272(1–2), 32–36. https://doi.org/10.1016/J.COLSURFA.2005.07.005
1629
1630
1631
Meshkat, S. S., Rashidi, A., Tavakoli, O., 2018. Removal of mercaptan from natural gas condensate using
N-doped carbon nanotube adsorbents: Kinetic and DFT study. J. Nat. Gas Sci. Eng., 55, 288–297.
https://doi.org/10.1016/J.JNGSE.2018.04.036
1632
1633
1634
1635
Nazarian, D., Camp, J. S., Sholl, D. S., 2016. A Comprehensive Set of High-Quality Point Charges for
Simulations of Metal-Organic Frameworks. Chem. Mater., 28(3), 785–793.
https://doi.org/10.1021/ACS.CHEMMATER.5B03836/ASSET/IMAGES/LARGE/CM-201503836S_0005.JPEG
1636
1637
1638
Othman, M. A., Zahid, W. M., Abasaeed, A. E., 2013. Selectivity of layered double hydroxides and their
derivative mixed metal oxides as sorbents of hydrogen sulfide. J. Hazard. Mater., 254–255(1), 221–227.
https://doi.org/10.1016/J.JHAZMAT.2013.03.030
1639
1640
Ozekmekci, M., Salkic, G., Fellah, M. F., 2015. Use of zeolites for the removal of H2S: a mini-review. Fuel
Process. Technol., 139, 49-60. https://doi.org/10.1016/j.fuproc.2015.08.015.
1641
1642
1643
Park, J. H., Choppala, G. K., Bolan, N. S., Chung, J. W., Chuasavathi, T., 2011. Biochar reduces the
bioavailability and phytotoxicity of heavy metals. Plant Soil., 348(1–2), 439–451.
https://doi.org/10.1007/S11104-011-0948-Y/METRICS
1644
1645
1646
Petit, C., Levasseur, B., Mendoza, B., & Bandosz, T. J., 2012. Reactive adsorption of acidic gases on
MOF/graphite oxide composites. Microporous Mesoporous Mater., 154, 107–112.
https://doi.org/10.1016/J.MICROMESO.2011.09.012
1647
1648
1649
Poly, S. S., Siddiki, S. H., Touchy, A. S., Yasumura, S., Toyao, T., Maeno, Z., Shimizu, K. I., 2018. High-silica
Hβ zeolites for catalytic hydration of hydrophobic epoxides and alkynes in water. J. Catal., 368, 145-154.
https://doi.org/10.1016/j.jcat.2018.10.004.
1650
1651
1652
Polychronopoulou, K., Fierro, J. L. G., Efstathiou, A. M., 2005. Novel Zn–Ti-based mixed metal oxides for
low-temperature adsorption of H2S from industrial gas streams. Appl. Catal. B, 57(2), 125–137.
https://doi.org/10.1016/J.APCATB.2004.10.017
1653
1654
1655
Qazvini, O. T., Fatemi, S., 2015. Modeling and simulation pressure–temperature swing adsorption
process to remove mercaptan from humid natural gas; a commercial case study. Sep. Purif. Technol.,
139, 88-103. https://doi.org/10.1016/j.seppur.2014.09.031.
1656
1657
1658
Rallapalli, P. B. S., Cho, K., Kim, S. H., Kim, J. N., Yoon, H. C., 2020. Upgrading pipeline-quality natural gas
to liquefied-quality via pressure swing adsorption using MIL-101 (Cr) as adsorbent to remove CO2 and
H2S from the gas. Fuel, 281, 118985. https://doi.org/10.1016/j.fuel.2020.118985.
1659
1660
1661
Ratnasamy, C., Wagner, J.P., Spivey, S., Weston, E., 2012. Removal of sulfur compounds from natural gas
for fuel cell applications using a sequential bed system. Catal. Today 198, 233e238..
http://dx.doi.org/10.1016/j.cattod.2012.04.069.
1662
1663
1664
Riboldi, L., Bolland, O., 2017. Overview on pressure swing adsorption (PSA) as CO2 capture technology:
state-of-the-art, limits and potentials. Energy Proc., 114, 2390-2400.
https://doi.org/10.1016/j.egypro.2017.03.1385.
1665
1666
Rochelle, G. T., 2009. Amine Scrubbing for CO2 Capture. Science, 325(5948), 1652–1654.
https://doi.org/10.1126/science.1176731
1667
1668
1669
Rodriguez, J. A., Chaturvedi, S., Kuhn, M., Hrbek, J., 1998. Reaction of H2S and S2 with metal/oxide
surfaces: Band-gap size and chemical reactivity. J. Phys. Chem. B., 102(28), 5511–5519.
https://doi.org/10.1021/JP9815208/ASSET/IMAGES/LARGE/JP9815208F00010.JPEG
1670
1671
1672
Roh, H. S., Jun, K. W., Kim, J. Y., Kim, J. W., Park, D. R., Kim, J. D., Yang, S. S., 2004. Adsorptive
desulfurization of natural gas for fuel cells. J. Ind. Eng. Chem., 10(4), 511-515.
https://www.cheric.org/PDF/JIEC/IE10/IE10-4-0511.pdf
1673
1674
1675
Rout, P. R., Bhunia, P., Dash, R. R., 2014. A mechanistic approach to evaluate the effectiveness of red soil
as a natural adsorbent for phosphate removal from wastewater. Desalin. Water Treat., 54(2), 358–373.
https://doi.org/10.1080/19443994.2014.881752
1676
1677
1678
Ryzhikov, A., Hulea, V., Tichit, D., Leroi, C., Anglerot, D., Coq, B., Trens, P., 2011. Methyl mercaptan and
carbonyl sulfide traces removal through adsorption and catalysis on zeolites and layered double
hydroxides. Appl. Catal. A., 397(1-2), 218-224. https://doi.org/10.1016/j.apcata.2011.03.002.
1679
1680
1681
Saeedirad, R., Taghvaei Ganjali, S., Bazmi, M., Rashidi, A., 2018. Effective mesoporous silica-ZIF-8 nanoadsorbents for adsorptive desulfurization of gas stream. J. Taiwan. Inst. Chem. Eng., 82, 10–22.
https://doi.org/10.1016/J.JTICE.2017.10.037
1682
1683
1684
Satokawa, S., Kobayashi, Y., Fujiki, H., 2005. Adsorptive removal of dimethylsulfide and t-butylmercaptan
from pipeline natural gas fuel on Ag zeolites under ambient conditions. Appl. Catal. B., 56(1-2), 51-56.
https://doi.org/10.1016/j.apcatb.2004.06.022.
1685
1686
1687
Sekhavatjou, M. S., Moradi, R., HOSSEINI, A. A., TAGHINIA, H. A., 2014. A new method for sulfur
components removal from sour gas through application of zinc and iron oxides nanoparticles. Int. J.
Environ. Res., 8(2):273-278. https://doi.org/10.22059/ijer.2014.716
1688
1689
1690
Selli, E., & Forni, L., 1999. Comparison between the surface acidity of solid catalysts determined by TPD
and FTIR analysis of pre-adsorbed pyridine. Microporous Mesoporous Mater., 31(1-2), 129-140.
https://doi.org/10.1016/S1387-1811(99)00063-3.
1691
1692
1693
Shafeeyan, M. S., Daud, W. M. A. W., Houshmand, A., Shamiri, A., 2010. A review on surface
modification of activated carbon for carbon dioxide adsorption. J. Anal. Appl. Pyrolysis., 89(2), 143–151.
https://doi.org/10.1016/J.JAAP.2010.07.006
1694
1695
1696
1697
Shah, M. S., Tsapatsis, M., & Siepmann, J. I. 2017. Hydrogen Sulfide Capture: From Absorption in Polar
Liquids to Oxide, Zeolite, and Metal-Organic Framework Adsorbents and Membranes. Chem. Rev.,
117(14), 9755–9803.https://doi.org/10.1021/ACS.CHEMREV.7B00095/ASSET/IMAGES/LARGE/CR-201700095Q_0020.JPEG
1698
1699
1700
Shao, D. D., Fan, Q. H., Li, J. X., Niu, Z. W., Wu, W. S., Chen, Y. X., Wang, X. K., 2009. Removal of Eu (III)
from aqueous solution using ZSM-5 zeolite. Microporous Mesoporous Mater.,123(1-3), 1-9.
https://doi.org/10.1016/j.micromeso.2009.03.043.
1701
1702
1703
Shirani, B., Kaghazchi, T., Beheshti, M., 2010. Water and mercaptan adsorption on 13X zeolite in natural
gas purification process. Korean J. Chem. Eng., 27(1), 253-260. https://doi.org/10.1007/s11814-0090327-z.
1704
1705
1706
Shoukat, U., Pinto, D. D. D., & Knuutila, H. K. 2019. Study of Various Aqueous and Non-Aqueous Amine
Blends for Hydrogen Sulfide Removal from Natural Gas. Processes, Vol. 7, Page 160, 7(3), 160.
https://doi.org/10.3390/PR7030160
1707
1708
Sircar, S., 2002. Pressure swing adsorption. Ind. Eng. Chem. Res., 41(6), 1389-1392.
https://doi.org/10.1021/ie0109758.
1709
1710
1711
Sobuś, Natalia, Izabela Czekaj, Volodymyr Diichuk, and Igor M. Kobasa., 2020. “Characteristics of the
Structure of Natural Zeolites and Their Potential Application in Catalysis and Adsorption Processes.”
Technical Transactions 1–20. doi: 10.37705/techtrans/e2020043.
1712
1713
1714
Sun, F., Liu, J., Chen, H., Zhang, Z., Qiao, W., Long, D., Ling, L., 2013. Nitrogen-rich mesoporous carbons:
Highly efficient, regenerable metal-free catalysts for low-temperature oxidation of H2S. ACS Catal., 3(5),
862–870. https://doi.org/10.1021/CS300791J/SUPPL_FILE/CS300791J_SI_001.PDF
1715
1716
1717
Tagliabue, M., Bellussi, G., Broccia, P., Carati, A., Millini, R., Pollesel, P., Rizzo, C., 2012. High pressure
hydrogen sulphide adsorption on silica–aluminas. Chem. Eng. J., 210, 398-403.
https://doi.org/10.1016/j.cej.2012.08.076.
1718
1719
1720
Tagliabue, M., Farrusseng, D., Valencia, S., Aguado, S., Ravon, U., Rizzo, C., Corma, A., Mirodatos, C.,
2009. Natural gas treating by selective adsorption: material science and chemical engineering interplay.
Chem. Eng. J. 155, 533e566. https://doi.org/10.1016/j.cej.2009.09.010
1721
1722
Taheri, A., Babakhani, E. G., & Towfighi, J. 2017. Methyl mercaptan removal from natural gas using MIL53(Al). J. Nat. Gas Sci. Eng. 38, 272–282.https://doi.org/10.1016/J.JNGSE.2016.12.029
1723
1724
1725
Thimsen, E., Biswas, S., Lo, C. S., Biswas, P., 2009. Predicting the band structure of mixed transition
metal oxides: Theory and experiment. J. Phys. Chem. C., 113(5), 2014–2021.
https://doi.org/10.1021/JP807579H/ASSET/IMAGES/LARGE/JP-2008-07579H_0003.JPEG
1726
1727
1728
Tohidi, Z., Fatemi, S., Qazvini, O. T., 2015. Mercaptan removal from natural gas by the efficient cyclic
adsorption process; a simulation study. J. Nat. Gas Sci. Eng., 26, 758-769.
https://doi.org/10.1016/j.jngse.2015.07.010.
1729
1730
1731
Vaziri, R. S., Babler, M. U., 2019. Removal of Hydrogen Sulfide with Metal Oxides in Packed Bed
Reactors—A Review from a Modeling Perspective with Practical Implications. Appl. Sci. 2019, Vol. 9,
Page 5316, 9(24), 5316. https://doi.org/10.3390/APP9245316
1732
1733
1734
Vellingiri, K., Deep, A., Kim, K. H., 2016. Metal-Organic Frameworks as a Potential Platform for Selective
Treatment of Gaseous Sulfur Compounds. ACS Appl. Mater. Interfaces, 8(44), 29835–29857.
https://doi.org/10.1021/ACSAMI.6B10482/ASSET/IMAGES/LARGE/AM-2016-10482C_0010.JPEG
1735
1736
1737
Wang, C., Leng, S., Guo, H., Cao, L., Huang, J., 2019. Acid and alkali treatments for regulation of
hydrophilicity/hydrophobicity of natural zeolite. Appl. Surf. Sci., 478, 319–326.
https://doi.org/10.1016/J.APSUSC.2019.01.263
1738
1739
1740
Wang, C., Leng, S., Guo, H., Yu, J., Li, W., Cao, L., Huang, J., 2019. Quantitative arrangement of Si/Al ratio
of natural zeolite using acid treatment. Appl. Surf. Sci., 498, 143874.
https://doi.org/10.1016/j.apsusc.2019.143874.
1741
1742
1743
Wang, X. L., Fan, H. L., Tian, Z., He, E. Y., Li, Y., & Shangguan, J., 2014. Adsorptive removal of sulfur
compounds using IRMOF-3 at ambient temperature. Appl. Surf. Sci.289, 107–113.
https://doi.org/10.1016/J.APSUSC.2013.10.115
1744
1745
Wang, Z., Wang, L., Jiang, Y., Hunger, M., Huang, J., 2014. Cooperativity of Brønsted and Lewis acid sites
on zeolite for glycerol dehydration. Acs Catal., 4(4), 1144-1147. https://doi.org/10.1021/cs401225k.
1746
1747
1748
Weber, G., Benoit, F., Bellat, J. P., Paulin, C., Mougin, P., Thomas, M., 2008. Selective adsorption of ethyl
mercaptan on NaX zeolite. Microporous Mesoporous Mater., 109(1-3), 184-192.
https://doi.org/10.1016/j.micromeso.2007.04.044.
1749
1750
1751
Weingarten, R., Tompsett, G. A., Conner Jr, W. C., Huber, G. W., 2011. Design of solid acid catalysts for
aqueous-phase dehydration of carbohydrates: The role of Lewis and Brønsted acid sites. J. Catal. 279(1),
174-182. https://doi.org/10.1016/j.jcat.2011.01.013.
1752
1753
1754
Wen, Y., Zhao, S., Yi, H., Gao, F., Yu, Q., Liu, J., Tang, T., Tang, X., 2022. Efficient catalytic oxidation of
methyl mercaptan to sulfur dioxide with NiCuFe mixed metal oxides. Environ. Technol. Innov., 26,
102252. https://doi.org/10.1016/J.ETI.2021.102252
1755
1756
1757
Wilmer, C. E., Leaf, M., Lee, C. Y., Farha, O. K., Hauser, B. G., Hupp, J. T., & Snurr, R. Q., 2011. Large-scale
screening of hypothetical metal–organic frameworks. Nat. Chem. 2011.4:2, 4(2), 83–89.
https://doi.org/10.1038/nchem.1192
1758
1759
1760
Xiao, Y., Wang, S., Wu, D., Yuan, Q., 2008. Catalytic oxidation of hydrogen sulfide over unmodified and
impregnated activated carbon. Sep. Purif. Technol.59(3), 326–332.
https://doi.org/10.1016/J.SEPPUR.2007.07.042
1761
1762
1763
Xu, B., Bordiga, S., Prins, R., Van Bokhoven, J. A., 2007. Effect of framework Si/Al ratio and extraframework aluminum on the catalytic activity of Y zeolite. Appl. Catal. A., 333(2), 245-253.
https://doi.org/10.1016/j.apcata.2007.09.018.
1764
1765
1766
Yahya, M. A., Mansor, M. H., Zolkarnaini, W. A. A. W., Rusli, N. S., Aminuddin, A., Mohamad, K., Ozair, L.
N.,2018. “A Brief Review on Activated Carbon Derived from Agriculture By-Product.” AIP Conf Proc. Vol.
1972. American Institute of Physics Inc. https://doi.org/10.1063/1.5041244
1767
1768
Yang, H., & Tatarchuk, B., 2010. Novel-doped zinc oxide sorbents for low temperature regenerable
desulfurization applications. AIChE J., 56(11), 2898–2904. https://doi.org/10.1002/AIC.12201
1769
1770
1771
Yaşyerli, S., Ar, İ., Doabreveu, G., & Doğu, T., 2002. Removal of hydrogen sulfide by clinoptilolite in a
fixed bed adsorber. Chem. Eng. Process. 41(9), 785–792. https://doi.org/10.1016/S02552701(02)000090
1772
1773
1774
Yi, H., Tao, T., Zhao, S., Yu, Q., Gao, F., Zhou, Y., Tang, X., 2021. Promoted adsorption of methyl
mercaptan by γ-Al2O3 catalyst loaded with Cu/Mn. Environ. Technol. Innov., 21, 101349.
https://doi.org/10.1016/J.ETI.2020.101349
1775
1776
1777
Yousefi, H., Mehrpooya, M., & Naeiji, E., 2017. Modeling and optimization of currently in operation
natural gas desulfurization process using adsorption separation method. Chem. Eng. Process. Process
Intensif., 120, 220-233. https://doi.org/10.1016/j.cep.2017.06.015.
1778
1779
1780
Zhang, H., Wang, J., Liu, T., Zhang, M., Hao, L., Phoutthavong, T., Liang, P., 2021. Cu-Zn oxides
nanoparticles supported on SBA-15 zeolite as a novel adsorbent for simultaneous removal of H2S and
Hg0 in natural gas. Chem. Eng. J., 426, 131286. https://doi.org/10.1016/J.CEJ.2021.131286
1781
1782
Zhang, X., Tang, Y., Qu, S., Da, J., & Hao, Z. 2015. H2S-selective catalytic oxidation: Catalysts and
processes. ACS Catal., 5(2), 1053–1067. https://doi.org/10.1021/cs501476p
1783
1784
1785
Zhang, X., Wang, L., Hu, L., He, J., 2021. Adsorption and Separation of Ethyl Mercaptan from Methane
Gas on HNb3O8Nanosheets. Ind. Eng. Chem. Res., 60(23), 8504–8515.
https://doi.org/10.1021/ACS.IECR.1C00460/ASSET/IMAGES/LARGE/IE1C00460_0015.JPEG
1786
1787
1788
Zhao, S., Yi, H., Tang, X., Gao, F., Zhang, B., Wang, Z., Zuo, Y., 2015. Methyl mercaptan removal from gas
streams using metal-modified activated carbon. J. Clean. Prod., 87(1), 856–861.
https://doi.org/10.1016/J.JCLEPRO.2014.10.001
1789
1790
1791
Zhu, L., Lv, X., Tong, S., Zhang, T., Song, Y., Wang, Y., Hao, Z., Huang, C., Xia, D., 2019. Modification of
zeolite by metal and adsorption desulfurization of organic sulfide in natural gas. J. Nat. Gas Sci. Eng., 69,
102941. https://doi.org/10.1016/J.JNGSE.2019.102941
1792
1793
1794
Zulkefli, N. N., Masdar, M. S., Roslam Wan Isahak, W. N., Jahim, J. M., Md Rejab, S. A., & Lye, C. C., 2019.
Removal of hydrogen sulfide from a biogas mimic by using impregnated activated carbon adsorbent.
PLOS ONE, 14(2), e0211713. https://doi.org/10.1371/JOURNAL.PONE.0211713