Review
Cite This: Ind. Eng. Chem. Res. 2019, 58, 22133−22164
pubs.acs.org/IECR
Removal of Hydrogen Sulfide from Gas Streams Using Porous
Materials: A Review
Maryam Khabazipour and Mansoor Anbia*
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Department of Chemistry, Research Laboratory of Nanoporous Materials, Iran University of Science and Technology, Farjam Street,
Narmak, P.O. Box 16846-13114, Tehran, Iran
ABSTRACT: Removal of hydrogen sulfide (H2S) released from various
source processes is crucial because this compound can cause corrosion and
environmental damage even at low concentration levels. The porous materials
offer a wide variety of chemical architectures with tunable pore size and high
surface area that are very promising for the adsorption of H2S molecules. This
review attempts to comprehensively compile the current studies in the
literature on hydrogen sulfide removal in gas purification processes using
highly porous materials such as zeolites, carbon materials, activated carbon,
porous metal oxides, mesoporous silica, and metal−organic frameworks as
highly effective adsorbents. Possible interactions between the H2S and active
adsorption sites of these materials are also discussed. Surface functionality and
porosity play a crucial role in the H2S removal performance by virgin or
modified porous materials. However, tailoring these materials to obtain high
adsorption capacity, good selectivity, and suitable stability and regenerability
and to retain structural integrity under high temperatures or in the presence of
moisture are still the major challenges in the practical applications. According
to the extensive background knowledge for H2S removal by different porous
materials in this review, it can be expected that readers will gain insight into the further developments in this area and the design
of new cost-effective sorbents.
1. INTRODUCTION
Hydrogen sulfide is a highly toxic, colorless, and odorous
(rotten egg) pollutant that occurs in a variety of hydrocarbon
sources such as natural gas, biogas, and crude oil.1 This
compound is lethal to humans with a threshold limit value of 1
ppm for 8 h exposure2 whereas the acceptable ambient limits
for H2S are between 20 and 100 ppb.3 Hydrogen sulfide can
lead to blood poisoning even at short-term exposures to low
levels as it is rapidly absorbed by the lungs. It inhibits the
cytochrome oxidase enzyme, causing a lack of oxygen for use in
cells.4 Hydrogen sulfide mainly originates from several
processes including coke ovens, sewage treatment, food
processing industries, coal or natural gas manufacturing, and
oil refining.5−7 The occurrence of H2S in various energy
resources is one of the major problems in industry since it is
corrosive to piping and facilities in the form of gas or in the
solution and can poison catalysts (reduce the activity of the
catalysts) used in fuel cells or the oil reformers even at low
concentrations.8−10 The H2S levels below 1 ppm are preferred
for reformers and fuel cells.11 In addition, the combustion of
H2S containing fuels will cause the release of toxic sulfur oxides
to the atmosphere, and thus, the production of acid rains by
reaction with water molecules, which have adverse impacts on
human health and the environment.12 Therefore, it is necessary
to remove unacceptable amounts of H2S from hydrocarbon
resources such as natural gas, syngas, or biogas for pollution
control and operational safety.
© 2019 American Chemical Society
In order to remove H2S effectively, several technological
approaches have been developed such as adsorption,
scrubbing, hydrodesulfurization, biological treatment, and
catalytic oxidation.13 Among these developed methods applied
to remove H2S, adsorption processes are the most applied
techniques for the removal of H2S as they are cost-effective
approaches and allow deep H2S removal (down to H2S
concentration less than 1 ppmv).14,15 Currently, many studies
have been devoted to developing effective and high performing
solid sorbent materials with high sulfur capacity and selectivity,
thermal durability, and good reproducibility for use in H2S
adsorptive removal processes, particularly at low temperature.
The rapid development of nanotechnology and material
science in recent years resulted in the design of novel materials
with outstanding properties for gas desulfurization. Porous
materials with the features of high surface area and large pore
volume are the most suitable materials used as adsorbent or
support for H2S removal from different gas streams. Porous
materials are a wide class of materials (micro-, meso-, and
macro-porous) which are structurally composed of numerous
well-ordered or disordered pores. Zeolites, mesoporous silicas,
porous carbon materials, activated carbons, and porous metal
Received:
Revised:
Accepted:
Published:
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July 25, 2019
October 30, 2019
November 6, 2019
November 6, 2019
DOI: 10.1021/acs.iecr.9b03800
Ind. Eng. Chem. Res. 2019, 58, 22133−22164
Review
Industrial & Engineering Chemistry Research
Table 1. Textural Parameters and Breakthrough Capacity of Porous Metal Oxides for H2S Removala
porous metal
oxide
SBET
(m2 g−1)
Vt
(cm3 g−1)
Da (nm)
Fe2O3/Al2O3
137.89
0.56
6.3
ZnO
Ni-ZnO
CoO
Cr2O3
CuO
Mn2O3
Fe2O3
3DOM-Fe2O3
3DOM-Fe2O3/
SiO2
Co3O4 (3DSCN57)
Co3O4 (3DSCE57)
3DOM- γFe2O3/SiO2
3DOM ZnO/
SiO2
3DOM-CuO
10.2
6.8
23−143
0.029
0.025
0.05−0.3
∼3, >50 nm
macropore
3DOMZnFe2O4
16−44
80−220
crystal size
(nm)
23
T (°C)
feed gas composition
1% H2S, 10% H2, 30% CO, 5%
CO2, N2 balance
51% H2, 30% He, 10%
H2O, 8% CO2, 8% H2S
1% H2S, He balance
4.2−17.2
9.3−19.7
20−33
6−17
25
5
300 ppm H2S, 5% H2, N2 balance
3% H2O, 500 mg/m3 H2S, N2
balance
44.7
25
400
457.3
730.0
14−692
196
246
431
248
37.18
38.92
28
75
31
175−250
200
200
200
200
350
30
0.319
1.999
170
275.4
0.490
3.523
140
7.50−9.00
3.0−9.9
500 mg/m3 H2S, N2 balance
19−30
3% H2O, 500 mg/m3 H2S, N2
room
balance
temperature
room
500 mg/m3 H2S, vapor-saturated
N2
temperature
0.1% H2S, 5.0% H2, 3.0% H2O, N2 500
balance
142−357
1.402− 5.572
275−325
0.25−
0.70
0.99
112.42−
245.30
0.30−
0.44
7.18−12.10
1.44−1.57
ref
700
159.5
72−125
breakthrough
capacity (mg g−1)
29
30
189
80
406.1
32
47−170
33
92−147
34
∼10−92
35
a
SBET: specific surface area. Vt: total pore volumes. Da: average pore diameter.
surface oxygen species to form sulfates or elemental sulfur via a
redox reaction.
Liu et al.24 investigated the potential ability of nanocrystalline mesoporous iron oxide (Fe2O3) for removing H2S and
found that the sulfur capacity was strongly dependent on the
pore volume and hydroxyl groups of the sorbent. The higher
surface area, greater number of mesopores, and smaller crystal
size of mesoporous Fe2O3 compared to the iron oxide resulted
in higher sulfur capacities. It was found that the pore size
ranged from 2 to 4.5 nm was favorable for better sulfur
capacity.
The utilization of unsupported metal oxides may lead to the
reduction of desulfurization performance because they would
rapidly sinter and aggregate during the cycles of sulfidation/
regeneration processes at high temperatures. An ideal way to
overcome the above limitation is to disperse the metal oxides
onto supports with high surface area and porous structure.
Materials used as support (template) should have inert nature,
high surface area, large pore size and volume, and good
mechanical and thermal stability during the sulfidation−
regeneration process. Table 1 summarized some results of
the H2S removal by porous metal oxides. Various porous
materials have been used as support to load different metal
oxides. Su et al.25 prepared efficient sorbents by loading 20 wt
% Fe2O3 on porous alumina (Al2O3) substrate and investigated
the multicycles of sulfidation/regeneration performance of
sorbents. The sorbent was able to remove H2S down to the
parts per million level at 700 °C and maintain 64% of its initial
sulfur capacity (4.47 g S/100 g sorbent) after the seventh cycle.
Dynamic H2S adsorption with mixed Zn and Cu active
phase supported onto mesoporous γ-alumina was carried out
by Cimino et al.26 at room temperature in a lab-scale fixed-bed
adsorption column to explore the possible synergic effect of the
support and metal phases on the adsorption performance. The
characterization studies revealed that the active phase
oxides are well-known for their superior desulfurization
performance under different working conditions.16,17 Besides
these conventional porous adsorbents, porous metal−organic
frameworks (MOFs) have recently attracted great attention for
the effective adsorptive removal of H2S because of their high
adsorption capacities and easy tenability.13,18
This review offers the aspects and issues attributed to the
application of various porous materials for the removal of H2S
acidic gas. It has been shown that the in situ or postsynthetic
modification is an effective way to improve the H2S adsorption
capacity of porous materials. Therefore, the effectiveness of
their structural modification with different functionalities such
as amine materials, caustic, chemical compounds, etc., is
discussed here, and it is described how such modification can
enhance H2S uptake capacity.
2. POROUS METAL OXIDES
Since Westmoreland and Harrison first reported the potential
ability of various metal oxides for high temperature
desulfurization in the early 1970s,19 a great deal of effort has
been paid to gas desulfurization based on a variety of metal
oxides sorbents such as oxides of Zn, Cu, Fe, Mn, Co, Mo, Ca,
etc.20−23 In this review, we focus only on porous metal oxide
structures that have been used for H2S removal. Bulk metal
oxides show limited sulfidation rates because of their large
particle sizes, low surface areas, poor dispersion, and
insufficient porosity. Thus, developing new materials with
open macro- and/or mesopores could improve the desulfurization performance. Porous metal oxides with large surface areas
and high porosity offer additional H2S adsorption sites and
provide better diffusion of it through the internal open pores.
The hydrogen sulfide removal mechanism by metal oxides may
involve its physisorption onto the surface of a solid sorbent or
the liquid water film deposited thereon and then reaction with
the active phase to generate sulfides or a combination with
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Industrial & Engineering Chemistry Research
resulting in the decrease of the H2S adsorption reaction with
metal oxides and higher diffusion resistance. Therefore,
researchers have tried to develop suitable strategies for
lowering the operating temperatures of the desulfurization
process without losing the sulfur capacity. Pahalagedara et al.29
investigated the desulfurization performance of mesoporous
cobalt oxides (M-Co-X, X = 150, 250, 350, and 450 °C) in the
temperature range of 25−250 °C. Different calcination
temperature resulted in the formation of various sorbents
with different surface areas and pore sizes. A significantly high
H2S adsorption capacity was obtained even at room temperature (134 mg g−1), and the maximum sulfur capacities of
650−689 mg g−1 were achieved in the temperature range of
175−250 °C. The interconnected mesopores were found to be
the main reasons for the high sorption capacity of the
mesoporous cobalt oxides at low temperatures as pores
increase the H2S diffusion through the sorbent. The authors
also demonstrated that sulfur capacities of mesoporous Cr2O3,
Mn2O3, CuO, and Fe2O3 were 2−200 times higher than their
nonporous analogs which were due to their mesostructure. In
addition, the sorbent was approximately 100% recovered upon
heating at 800 °C and H2O/N2 stream without losing its
porosity.
In recent years, H2S adsorption in three-dimensionally
ordered macroporous (3DOM) materials with a lot of uniform
large pore size has attracted great attention. These materials
showed much higher breakthrough H2S capacity than that of
the corresponding bulk materials. The 3DOM materials
contain high ordered open and interconnected macropores
which significantly enhance the diffusion of gaseous adsorbate
from the surface to the interior matrix of the sorbent. In
addition, it can improve the metal oxides dispersion resulted in
a better adsorption−reaction kinetics of H2S metal oxides. Fan
et al.30 prepared a series of Fe2O3 and Fe2O3/SiO2 based
3DOM sorbents (named 3DOM-F and 3DOM-FS respectively), ranging in size from 60 to 550 nm by the colloidal
crystal template method and employed the sorbents for H2S
removal of at moderate temperatures (300−350 °C). The
3DOM-FS samples revealed smaller particle size (5 nm) than
that of 3DOM-F sorbents (25 nm) since SiO2 prevented the
iron grains from aggregation and further growing of them
during calcination. The results demonstrated that BET surface
areas of 3DOM-FS samples (80−220 m2 g−1) were higher than
that of the iron 3DOM-F samples (16−44 m2 g−1) while the
average pore size was increased from 20−33 for 3DOM-FS to
6−17 nm for 3DOM-F samples. The difference in the textural
parameters of the two sorbents caused large differences
between the desulfurization performance of 3DOM-F and
3DOM-FS.
The sorbent with large macropores and a high surface area
exhibited better performance because of faster gas diffusion
and availability of more active sites for adsorption. In addition,
iron oxide-3DOM structures showed much higher desulfurization performance than the conventional iron oxide sorbent.
The regenerability of 3DOM-FS sorbent using 2% O2/N2 at
200 °C showed that in spite of some breakage at the wall, the
whole macroporosity of the structure was still retained after
regeneration.
In another study, two types of cobalt oxide−silica
composites using n-butyl alcohol (SCN57-500) and ethylene
glycol (SCE57-500) and the corresponding 3DOM adsorbents
were prepared by Wang et al. and used for H2S removal at
room temperature.31 It was found that SCN57-500 had an
contained a mixture of metal (hydro)oxides and hydroxynitrates because of a strong stabilizing effect of the alumina
support against the complete decomposition of metal nitrate
precursors during the calcination step. In spite of a minor
reduction of the pore size distribution and surface area after
depositing metal active phases on mesoporous γ-Al2O3 spheres,
a greatly improved H2S adsorption capacity was observed in
comparison with the virgin support. The authors observed the
highest H2S adsorption capacity of 0.82 mmol g−1 over
Cu0.5Zn0.5/Als sorbent under both dry and humid conditions. Moreover, the Cu0.5Zn0.5/Als sorbent showed a
significantly lower breakpoint time and adsorption capacity
than the sorbent by similar active phase composition but
supported on activated carbon (Cu0.5Zn0.5/AC). This was
due to the greater dispersion of metal active phases on the
large surface area of AC and lower reactivity of metal phases on
the surface of the γ-Al2O3 due to the high stabilization of metal
(hydroxyl)nitrates. The results exhibited that the copper and
zinc sulphates formation occurred on the surface of Cu−Zn/
alumina by oxidation of H2S during the adsorption process
through various oxygen sources from the Cu/Zn (hydro)oxides, metal (hydroxy)nitrates, and −OH groups of the Al2O3
substrate.
In another study, Daneshyar et al.27 fabricated different
nanocomposites based on copper, zinc, and nickel nanoparticles loaded on activated carbon (Cu-Zn-Ni/AC) and
cobalt and nickel supported on γ-alumina (Ni-Co/γ-Al2O3)
and compared their performances for the removal of H2S from
natural gas under various operational parameters. They
observed better desulfurization performance for Cu-Zn-Ni/
AC adsorbent in comparison with Co-Ni/γ-Al2O3 which can
be attributed to the stronger sulfide binding capability of
copper and zinc active sites. The authors suggested that the
hydrogen bonding between H2S and functional group of AC
and −OH groups and oxygen atoms of γ-Al2O3 as well as soft−
soft interaction of H2S with metallic centers are involved in the
H2S adsorption mechanism. In the case of Cu-Zn-Ni/AC, the
number and strength of such interactions seems to be more
favorable than the Ni-Co/γ-Al2O3.
Tran used agarose gel as a template to prepare a highly
porous network of ZnO and Ni-doped ZnO materials with
interconnected mesopore/macropore and well dispersed active
phases for H2S desulfurization.28 The comparison of H2S
desulfurization performances of two synthesized materials
showed that the size and morphology of particles were two
important parameters affecting sulfur adsorption capacity.
Based on the results, porous ZnO revealed much higher sulfur
capacity (457 mg g−1) than that of the commercial ZnO (245
mg g−1) and over doping with 4 wt % Ni, the sorbent exhibited
a higher adsorption capacity of 730 mg g−1 due to the creation
of additional active sites for reacting with H2S. The Ni-doped
ZnO sorbent was then easily regenerated by heat treatment in
the air, recovering 100% of its initial capacity. Other porous
materials such as mesoporous silica (SBA-15 and MCM-41),
activated carbon, zeolites, etc., used as metal oxide supports are
discussed in the next sections.
Recently, studies have been focused on low working
temperatures for the removal of H2S by metal oxides.
Compared to high-temperature desulfurization, low temperatures (<400 °C) are favorable owing to the lower cost of
operation. In addition, high temperatures may lead to metal
oxides sintering, reducing the activity. However, sulfidation
kinetics is greatly reduced at low operating temperatures
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Industrial & Engineering Chemistry Research
Figure 1. SEM (a−f) and TEM (g−l) images, at different magnifications, of the prepared 3DOM composites: 3D-SZ20-500 (a, d, g, j); 3D-SZ50500 (b, e, h, k); 3D-SZ73-500 (c, f, i, l). (a−c) Large area view of the materials [Reproduced with permission from ref 33. Copyright 2014
American Chemical Society].
ordered mesoporous structure with a very large surface area
(314.5 m2 g−1) and a diameter of 2−5 nm. Whereas the
SCE57-500 sorbent showed smaller surface area but welldistributed particles which resulted in a better H2S breakthrough capacity of 28 mg g−1 compared to the SCN57-500
(12 mg g−1). By introducing 3DOM structure into the bulk
counterpart, an increase in the surface area of the 3D-SCE57500 structure (275.4 m2 g−1) was observed due to the excellent
dispersion of the metal oxide nanoparticles. However, the 3DSCN57-500 sorbent revealed a decreased surface area (159.5
m2 g−1) compared with SCN57-500 which was due to the
reduction of pores as a result of the sintering of the metal oxide
during the hard template burning. The calcination temperature
and Co3O4 content had also a great effect on the 3DOM
structure and desulfurization performance. The 3DOM
sorbents presented much higher breakthrough H2S capacity
compared to their bulk analogs. The 3D-SCE57-500 structure
with smaller particles and larger surface area than the 3DSCN57-500 sorbent demonstrated a better breakthrough H2S
capacity of 189 mg g−1 (with cobalt oxide utilization of 63%).
Huang et al. studied the performance of the γ-Fe2O3-based
3DOM materials for H2S capturing at low temperature (20−80
°C) and compared their desulfurization performance with that
of α-Fe2O3 and the commercial amorphous hydrated iron
oxide (HXT-1).32 The decreasing activity of materials for H2S
removal (at 60 °C) was γ-Fe2O3 > HXT-1 > α-Fe2O3. They
suggested that the vacancy site in the framework of γ-Fe2O3
facilitated the diffusion of H2S molecules. In addition, 3DOM
γ-Fe2O3/SiO2 sorbents (Fe:Si mole ratios of 2−8) showed
better sulfur adsorption capacity and utilization than the parent
γ-Fe2O3 and HXT-1. The authors observed that the humidity
∼10% favored H2S removal since water increases hydrogen
sulfide uptake by promoting the dissociation of H2S. The
experiments showed that the high-temperature heating of the
sorbent under air was not a suitable approach to regenerate the
adsorbent since γ-Fe2O3 could be converted to α- Fe2O3
resulted in a decrease in the surface area of the regenerated
sorbent. However, simultaneous desulfurization and regeneration and by using a mixture of 5% O2 and H2S in nitrogen
resulted in a breakthrough sulfur capacity two times that of
observed when O2 was absent the feed stream. Based on the
results, the authors suggested a mechanism for the
desulfurization/regeneration process as follows (eqs 1 and 2):
Fe2O3 + 3H 2S → Fe2S3 + 3H 2O
(1)
2Fe2S3 + 3O2 → 2Fe2O3 + 3/4S8
(2)
In addition, because of its thermodynamic instability, Fe2S3
might decompose to FeS2 and Fe3S4 (eq 3) where FeS2 could
also be converted into Fe2(SO4)3 in the presence of oxygen
and moisture (eq 4)
22136
2Fe2S3 → FeS2 + Fe3S4
(3)
FeS2 + 15O2 + 2H 2O → 2Fe2(SO4 )3 + 2H 2SO4
(4)
DOI: 10.1021/acs.iecr.9b03800
Ind. Eng. Chem. Res. 2019, 58, 22133−22164
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Industrial & Engineering Chemistry Research
Figure 2. Schematic of H2S sulfurization/desulfurization on 3DOM zinc ferrite composited silica sorbent [Reproduced with permission from ref
35. Copyright 2019 Elsevier].
Zinc oxide-based sorbents have shown good desulfurization
performance at high temperatures. Zinc oxide can react with
H2S to produce the zinc sulfide via an exothermic process.
Therefore, lower temperatures will result in higher desulfurization efficiency. Wang et al.33 prepared a series of zinc oxide
silica-based 3DOM structures with various weight ratios of
ZnO (3D-SZx-500, x (ZnO weight ratio) = 20%, 50%, and
73%) at calcination temperature of 500 °C, and different
calcination temperatures ranged from 400 to 700 °C for 50%
of ZnO content (3D-SZ50-y, y = the calcination temperature)
and used these materials for H2S removal at room temperature.
The prepared materials had a well-ordered 3D-interconnected
network composed of macro-, meso-, and micropores (Figure
1). The results exhibited that both the Zn/Si ratio and
calcination temperature had a crucial role in the properties of
structures. With increasing the Zn/Si ratio, the surface areas
and total pore volumes decreased due to the aggregation of
ZnO particles in the pores. Moreover, the surface areas and
pore volumes of sorbents were decreased gradually at high
calcination temperature (>500 °C) because the channels
shrunk. Lower calcination temperature (400 °C) produced
carbon impurities which could cover the ZnO surface and
block the pore channels resulting in the sulfur capacity
reduction. In addition, with increasing the ZnO content, the
breakthrough time and the breakthrough capacity were
increased. The highest utilization of 69% and sulfur capacity
of 135 mg g−1 was achieved with 50 wt % ZnO loaded 3DOM
sorbents. Moisture in the feed gas favored the removal process
of H2S.
The authors suggested the desulfurization mechanism over
the 3DOM sorbent in the presence of moisture at room
temperature as the following reactions:
The 3DOM ZnO/SiO2 sorbent exhibited good stability even
after four times regeneration and reached 67.4% of the fresh
sorbent capacity which would improve by optimizing the
regeneration conditions.33
Studies have shown that moisture is a crucial parameter to
keep metal oxides activity for H2S removal at ambient
temperature. The positive effect of water vapor on H2S
capture by metal oxides was studied by different researchers.30,32 It has been suggested that the interaction between
metal oxides and H2S in the presence of moisture involved the
H2S adsorption on the metal oxide surface and dissociation to
HS−/S2−, and subsequent migration of these ions to the
sorbent matrix. However, high moisture can lead to the
decrease of H2S removal capacity since the thick water film
deposited on the surface of solid may block the pores,
preventing H2S to diffuse through the sorbent and access the
active sites. Wang et al.34 used a series of highly porous Cubased 3DOM sorbents for H2S removal from a vapor-saturated
N2 gas (containing 500 mg m−3 H2S) at ambient temperature.
The large surface area, good dispersion of CuO nanoparticles,
and the interconnected macropores led to a higher breakthrough H2S capacity (147 mg g−1) for the CuO-3DOM
sorbent compared to the CuO sorbent (25.4 mg g−1).
Moisture was found to have a crucial role in desulfurization
performance. By increasing the relative humidity from 16.3%
to 47.6%, the breakthrough time, sulfur capacity, and CuO
utilization increased from 5.3 to 8 h, 94 to 147 mg g−1, and
47% to 73.5%, respectively. However, the further increase of
the relative humidity to 100%, led to the reduction of the sulfur
capacity and CuO utilization which was due to the blocking of
pores with water. The authors suggested a simple reaction
scheme for H2S removal under the moister condition as
follows eq 9−14:
ZnO + H 2O → Zn 2 + + 2OH−
(5)
H 2S + OH− → HS− + HS− + OH−
(6)
HS− + OH− → S2 − + H 2O
(7)
H 2S + H 2O → HS− + H3O+
(10)
Zn 2 + + S2 − → ZnS
(8)
H 2S + OH− → HS− + H 2O
(11)
CuO + H 2O → Cu 2 + + 2OH−
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(9)
DOI: 10.1021/acs.iecr.9b03800
Ind. Eng. Chem. Res. 2019, 58, 22133−22164
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Industrial & Engineering Chemistry Research
Table 2. Textural Parameters and Breakthrough Capacities of Porous Carbon Materials for H2S Removala
structure
SBET
(m2 g−1)
Vt
(cm3 g−1)
CA-1 (14.7%
Na2CO3)
C100-I
C100-CO2-3h-I
MCS-MgO-15
MCS-Na2CO3
MCS-NaOH
MCS-K2CO3
MCS-KOH
PCSs-3
651
832
660
674
669
689
603
1057
2.33
2.60
1.74
1.61
1.62
1.72
1.68
1.180
NPC-1.0-700
NMC-2.0-600
M/P-2
962
332
770
0.543
0.57
2.4
Da
(nm)
15.64
15.31
12
12
12
8
13
8−12
10.9
T
(°C)
feed gas composition
H2S breakthrough capacity
(mg g−1)
H2S saturation
capacity (mg g−1)
ref
1% O2, 0.1% (1000 ppm) H2S, 80%
H2O, 99% N2
1% O2, 0.1% (1000 ppm) of H2S, 80%
H2O, 99% N2
25
3140
3450
37
30
30
1640
2260
-
38
0.1% (1000 ppm) of H2S, 1% of O2,
80% H2O, N2 balance
100 ppm H2S, 500 ppm benzene, N2
balance
1000 ppm H2S, N2 balance
1000 ppm H2S, N2 balance
1000 ppmv, H2S, 80% H2O, air
25
1040
1610
2460
1290
1160
860
930
40.1 (61.5: mixture gas)
25
25
30
27.62
11.59
2770
39
40
33.08
16.37
41
42
43
a
SBET: specific surface area. Vt: total pore volumes. Da: average pore diameter.
HS− + OH− → S2 − + H 2O
(12)
HS− + H 2O → S2 − + H3O+
(13)
Cu 2 + + S2 − → CuS
(14)
the repeated phase change in sulfidation/regeneration cycles
could decrease the desulfurization performance.
In another study, good desulfurization performances
obtained by different Cu-ZnO-Al2O3 adsorbents with ordered
mesoporous structures highlighted the synergetic effect of
multicomponent sorbents and highly porous structure.36 The
as-synthesized materials with highly ordered mesopores, large
surface areas ranging from 226 to 386 m2 g−1, and large pore
volume (0.46−0.60 cm3 g−1) provided a high sulfur saturation
capacity up to 49.4 mg g−1 which was greater than that of the
bulk Cu-ZnO-Al2O3 sorbent (13.5 mg g−1). The mesoporous
structure of the sorbent enabled the high dispersion of ZnO
and Cu, facilitating the sulfidation of ZnO by H2S. In addition,
the ordered mesoporous structure could prevent the sintering
of Cu nanoparticles enhancing the desulfurization performance
and the sorbent stability.
After modification of the sorbent by ammonia, high H2S
removal capacity (137 mg g−1) was achieved by the modified
3DOM sorbent and a low existing H2S concentration (below
0.07 mg m−3) was maintained for 2.5 h in the outlet before
breakthrough without the help of moisture. They stated that
the presence of ammonia enhanced the H2S adsorption on the
sorbent surface and accelerated the H2S dissociations. The
results of experiments for five cycles desulfurization/regeneration at 650 °C showed that the regenerated CuO-3DOM
sorbents could not perform well compared to the corresponding fresh structures; however, they had still better desulfurization performance than the bulk structures.
Multicomponent sorbents can offer synergetic effects for
H2S elimination. Li et al.35 fabricated a series of 3DOM zinc
ferrites and silica composites which were superior to the zinc
oxide or iron oxide for removal of H2S. The as-prepared
materials presented a well-ordered and interconnected macroporous structure with a pore size of ∼230 nm which were
contained the textual mesopores within the wall of macropores,
contributing to the high surface area of the 3DOM structure.
Sulfates (Fe2(SO4)3 and ZnSO4) could be produced during
both sulfidation and regeneration processes (Figure 2).
The 3DOM structure with 75 wt % ZnFe2O4 showed the
highest breakthrough time and capacity whereas the 50 wt %
ZnFe2O4-3DOM structure revealed the much higher utilization
ratio than those of the nanosorbent and other 3DOM
composites. At low content of ZnFe2O4, there were insufficient
active sites to interact with H2S while the excessive ZnFe2O4
loading led to the particles aggregation and pore blocking
which limited the mass diffusion of H2S through the sorbent.
The multiple regenerations of the sorbent with 5% O2/N2 at
550 °C indicated that, after the seventh regeneration, the
3DOM structure preserved its breakthrough capacity over
72.40% of the fresh one which was still much higher than those
of the fresh bulk nanosorbents. The authors suggested that the
spalling or cracking of the interconnected pores in the sorbent
skeleton and the occurrence of particle agglomeration during
3. CARBON-BASED SORBENTS
3.1. Porous Carbon Materials. Several researchers have
investigated the performance of porous carbon materials
(PCMs) for H2S removal due to interesting characteristics
such as large surface area and high micropores. Table 2
summarized the important results of some studies for H2S
adsorption and elimination through these materials. Despite
the large volume of pores on PCMs structure, the access of the
analyte to adsorption sites can be limited by the microporous
structure. The introduction of larger pores such as mesopores
and macropores into the PCM structure can enhance the
adsorption performance by providing plentiful channels that
improve the intrapore mass transfer.
Besides porosity, the surface chemistry of PMCs is another
crucial factor when applied for polar pollutants such as H2S.
Considering their hydrophobic nature, PCMs exhibit relatively
poor adsorption behavior toward acidic gases like H2S.
Therefore, extensive research efforts have been devoted to
overcome this problem and increase their H2S removal
efficiency through structural modification such as impregnation
with alkaline compounds and doping with heteroatoms.
3.1.1. Chemical Impregnation. Long et al.37 revealed that
with increasing the Na2CO3 loading content of threedimensional mesoporous carbon aerogel from 0 to 14.7 wt.
%, the H2S breakthrough capacity was enhanced from 0.056 to
3.14 g g−1. Chen et al.38 observed an increase in the saturation
sulfur capacity of Na2CO3 impregnated porous carbon aerogel
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Figure 3. Optical (a), SEM (b), and TEM images (c) of MCSs; SEM images at low (d) and high (e) magnification and TEM image (f) of MCSMgO-15; (g) SEM elemental mapping images of MCS-MgO-15. The lattice fringe space indicated by the red arrows in (c) is 0.21 nm,
corresponding to the (200) facet of periclase MgO [Reproduced with permission from ref 39. Copyright 2016 Elsevier].
up to 2.26 g g−1 (related to the material with a high pore
volume and relatively small pore size) over increasing the
Na2CO3 mole ratio in the presence of O2 at a low temperature
of 30 °C. The authors stated that the presence of Na2CO3
could increase the pH value of water films deposited on the
carbon surface and then assist the formation of HS− ions.
These ions could be then oxidized to elemental sulfur or sulfur
oxides (SOx) depending on the carbon aerogel pore sizes. It
was shown that small micropores allowed strong oxidation of
HS− to sulfur oxides, whereas mesopores were favored for
producing elemental sulfur. However, a possible drawback of
using Na2CO3 impregnated porous carbon aerogels for
catalytic oxidation removal of H2S is that they cannot be
regenerated due to the irreversible consumption of Na2CO3.
Zhang et al.39 fabricated millimeter-sized mesoporous
carbon spheres (MCSs) loaded with MgO and other basic
impregnants (Na2CO3, NaOH, K2CO3, and KOH) for catalytic
oxidation of H2S at room temperature. The scanning electron
microscopy (SEM) and transmittance electron microscopy
(TEM) images (Figure 3a−c) demonstrated that the MCSs
had a porous structure with interconnected but disordered
mesopores and macropores. After impregnation, MCSs showed
a smooth external surface with a uniform distribution of MgO
particles (Figure 3d−g).
A high removal capacity of 2.46 g g−1 was achieved for the
MgO-impregnated MCSs, which was much greater than that of
commercially available activated carbons (0.4−0.6 g g−1). This
should be attributed to the easy diffusion of H2S molecules and
products through the interconnected mesoporous channels of
the MCSs without sulfur aggregation or pore blockage. As
shown in Figure 4a−c, the exhausted MCSs still had a smooth
surface without obvious sulfur agglomerations and the sulfur
Figure 4. SEM images at low (a) and high (b) magnification, SEM
elemental mapping images (c) of MCS-MgO-15 after H2S oxidation
at 30 °C and relative humidity of 80%, and (d) schematic illustration
of the H2S oxidation process over MCSs with MgO and soluble salts
[Reproduced with permission from ref 39. Copyright 2016 Elsevier].
and Mg atoms (formed by dissolution of MgO) were
uniformly enriched on the surface of the exhausted MCSs.
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Figure 5. Schematic fabrication of NMC for the direct oxidation of H2S and the possible reaction process [Reproduced with permission from ref
43. Copyright 2013 American Chemical Society].
bon.41,42 This was due to the presence of nitrogen heteroatoms
in the carbon matrix that could provide high surface polarity
and Lewis-base sites for binding the acidic gases resulted in
increased adsorption performance for acidic H2S gas. The
authors observed that besides the nitrogen content, nitrogen
configuration and structural porosity had also a significant
influence on the sulfur capacity.
Sun et al.43 observed high H2S oxidation/removal activity
and selectivity at low temperature using the nitrogen-rich
mesoporous carbons (NMCs). A high breakthrough H2S
capacity of 2.77 g g−1 was obtained for the nitrogen-doped
material which was considerably higher than that of the
undoped material (0.05 g g−1). The authors believed that not
only the content of nitrogen but also the bonding
configurations had a significant role in the H2S oxidation.
Humidity was also a crucial factor in the H2S oxidation
mechanism. When H2S molecules diffused into water film
deposited on the surfaces of pores, the presence of nitrogen
functionalities as Lewis basic sites, especially pyridinic N, could
assist the dissociation of H2S. Subsequently, the reaction
between HS− ions and oxygen radicals adsorbed by the active
sites resulted in the formation of the elemental sulfur which
condensed within the mesopores (Figure 5).
More recently, Yu et al.44 produced two-dimensional Ndoped mesoporous carbon nanosheets (NMCS) by pyrolyzing
of microporous Zn-based zeolitic imidazolate framework (ZIF8) in a molten salt medium for H2S catalytic oxidation
removal. The thin nanosheet structure provided an easier and
short length of diffusion for the reactants wherein small
mesopores (2.73 nm) prevented the blocking of active sites
due to the growth of produced sulfur clusters. Interestingly, the
authors noticed that Na2CO3 impregnated NMCS exhibited a
good H2S removal performance with a breakthrough sulfur
capacity of 1.37 g g−1, which was much greater than that of
Na2CO3 impregnated activated carbon (0.31 g g−1), Na2CO3
impregnated activated carbon fibers (0.05−0.71 g g−1),45 and
Na2CO3 impregnated multiwalled carbon nanotubes (0.38−
0.98 g g−1).46
3.2. Activated Carbons. Due to the large specific surface
area, high pore volume, and possibility of modification,
activated carbon (AC) is extensively used as a superior
adsorbent in separation processes. Moreover, this material is
relatively cheap as compared with other adsorbents such as
zeolites, alumina, and silica. Activated carbon can be produced
The breakthrough time of the sorbents were different and
followed an order of MgO > Na2CO3 > NaOH > KOH >
K2CO3 which might be due to their differences in basicity and
solubilities in the water film. The highly soluble basic salts such
as Na2CO3 in the adsorbed water films immediately generate
OH− ions which facilitate the dissociation of H2S to HS− but
also are readily consumed through an acid−base reaction. In
contrast, basic MgO salts are partially soluble in the water film,
which continuously releases the OH− for a certain long time,
improving the removal capacity (Figure 4d).
3.1.2. Nitrogen Doping. Although good desulfurization
performance can be acquired by the impregnation, the
reduction of available porosity is unavoidable due to the
blockage of pores with impregnating materials. Doping PCMs
with nitrogen heteroatoms seems to be a better choice to
enhance the H2S removal efficiency via changing the polarity
without reducing or blocking the structural porosity. Qi et al.40
designed porous carbon spheres (PCSs) doped with nitrogen
(graphitic carbon nitride as nitrogen precursor) and used the
prepared materials for simultaneous removal of benzene and
H2S from polluted air. The obtained PCSs exhibited a high
surface area (1017−1083 m2 g−1), uniform millimeter-scale
spherical shape with micro/meso/macro hierarchical pores and
plentiful basic functionalities (mainly pyrrolic and quaternary
nitrogen). They indicated that varying the doped nitrogen
heteroatoms had little effect on the adsorption performance of
nonpolar benzene and its adsorbance by PCSs mainly through
physisorption on micropores, while a positive effect was
observed for H2S adsorption capacity (1.6−40.1 mg g−1)
among the increase of doped nitrogen content (from 0 to
5.1%). Unlike benzene, the adsorption of polar H2S involved
both physisorption and chemisorption, wherein the latter was
controlled by chemical polarity. In addition, it was shown that
the electron distribution change on nitrogen atoms caused by
the adsorbed benzene on micropores could enhance H2S
adsorption from the mixture gas. The introduced large volume
of meso- and macropores could assist the adsorbate diffusion
and mass transfer through the adsorbent efficiently with less
resistance.
Yu’s group prepared different nitrogen-doped porous carbon
materials with micro/meso/macropores through different
methods such as the molten salt and soft-template approach
and detected superior H2S adsorption performance over these
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Table 3. Textural Parameters and Breakthrough Capacities of Activated Carbons for H2S Removala
material
SBET (m2 g−1)
Vt (cm3 g−1)
Vmic (cm3 g−1)
Vmeso
(cm3 g−1)
feed gas composition
T (°C)
60% CH4 and 40% CO2, 0.1%
H2S, 70% H2O
H2S breakthrough capacity
(mg g−1 sorbent)
ref
BAX
CBAX-BMb
CBAX-BUOc
ACS-1
2176
1435
963
1854
1.519
0.954
0.574
0.97
0.818
0.560
0.411
0.90
0.701
0.390
0.163
0.07
W (H3PO4
activation)
M (KOH
activation)
C (steam
activation)
COFAC-1d
1700
1.18
0.41
0.77
2240
0.98
0.609
0.371
12
1040
0.49
0.361
0.129
9
905
0.492
0.399
0.093
S
S-BMb
S-BUc
NaOH-AC
CuO-AC
KI-AC
K2CO3-AC
AC-15
AC-Cu-Cr
AC-KOH-KI
Desorex K43K2CO3
Desorex K43Fe2O3
Desorex K43NaOH
AC
AC- Na2CO3
Desorex K43
Desorex K43Fe2O3
Desorex K43NaOH
Virgin GAC
MgO-GAC
AC Darco
Cu0.5Zn0.5/
AC Darco
AC
Zn/AC
Cu/AC
Cu0.5Zn0.5/
AC
AC
ZnOSiO2(7%)/
AC
ZnO0.5-N20/
AC
898
732
808
930
1050
1250
1000
1200
1599
1042
1005
0.483
0.386
0.402
0.454
0.372
0.026
0.029
0.014
0.432
0.50
0.66
0.42
0.36
952
0.43
0.36
815
0.38
0.30
732
715
1003
952
0.416
0.406
0.48
0.43
0.226
0.218
0.37
0.36
815
0.38
0.30
0.748 ± 0.03
0.595 ± 0.01
0.81
0.76
0.629 ± 0.01
0.515 ± 0.01
0.23
0.2
80% H2O, 1% H2S, air
25
3000 ppmv H2S, N2 balance
30
641
558
559
570
0.81
0.8
0.67
0.76
0.23
0.19
0.22
0.2
3000 ppmv H2S, N2 balance
30
6.82
33.08
46.4
49.79
1
148.2
174.6
0.436
0.539
800 ppm H2S, 3% H2O, N2
balance
30
104.2
160.95
67
495.1
0.28
600 ppm H2S, 3% H2O, N2
balance
30
62.5
63
1678 ± 41
1358 ± 39
641
570
0.15
20000 ppmv H2S, 10000 ppmv
SO2, N2 balance
80% H2O, 1% H2S, Air
47
30
5.7
64.1
51.6
64.27
20
47
52
moist air (1000 ppm H2S, 80%
H2O
60% CH4 and 40% CO2, 0.1%
H2S, 70% H2O
127
18.75% H2, 6.25% CO, 75.0%
N2, 50 ppm H2S
400
H2S, N2, CO2, CH4, H2O, air
balance
45
1000 ppm H2S, 0−70% H2O,
N2 balance
30
27.2
71.9
54.1
11
12
8
13
4
26.50
34.87 (65.60, O2 2%)
87.98
48
53
54
55
56
57
27.96
156.18 (520.36: wet)
mixed gases containing 100−
1000 ppm H2S
100 ppm H2S, He balance
30−60
75
3.0
9.3
0.34
4.01
59
61
15.64
53
275
4.1
29
62
65
a
SBET: specific surface area. Vt: total pore volumes. Vmeso: mesoporous volumes. Vmic: microporous volumes. bMelamine doped. cUrea doped.
ZnCl2 impregnated.
d
from many types of carbon sources such as wood,47 coconut
shell,48 etc., with different pores, fixed carbon content, and ash
depending on the properties of the initial source and
carbonizing process. Several studies have been reported over
the H2S adsorption and removal by porous activated carbons.
Some results about H2S removal by various types of porous
activated carbons are summarized in Table 3. The specific
surface area, pore size and volume, and the surface
functionality can influence on the adsorption capacity of
ACs. The H2S removal by activated carbon can occur via
several mechanisms. It can be physically or chemically
adsorbed by activated carbon. Nonspecific physisorption
involved weak van der Waals’ forces while the chemisorption
of H2S by functional groups present on the activated carbon
can convert it to elemental sulfur or sulfur oxide in the
presence of oxygen. The adsorption performance of activated
carbons is highly controlled by their surface chemistry which is
originally attributed to the content of ash and some elements
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following steps: (1) formation of thin water films in the
micropores and even mesopores of impregnated ACs in the
presence of moisture; (2) dissolution of H2S in the water films
and then dissociation into HS− ions (eq 15). Dissociation can
also occur on alkaline active sites (eqs 16 and 17); (3)
dissociation of captured O2 into dissociatively adsorbed oxygen
(O*) (eqs 18 and 19); (4) reaction of HS− with O* to form
elemental sulfur or sulfur oxides (eqs 20−22).
presented such as oxygen, nitrogen, and sulfur.49 In a density
functional theory (DFT) study, the role of various oxygen
groups on the H2S adsorption on the AC surface was
investigated.50 The results indicated that except for hydroxyl
groups, other oxygen functional groups such as pyrone,
carbonyl, ester, and carboxyl are active sites for H2S
adsorption. H2S can react directly with carbonyl oxygen
atoms in these oxygen groups leading to the formation of C−S,
C−OH, and C−SH species. The reactivity of oxygen groups
for H2S adsorption was related to their acid−base properties. A
pyrone group could enhance the activity of surface C sites for
H2S adsorption while the presence of carboxyl and hydroxyl
groups had negative effect on the H2S adsorption on AC
surface. Carbonyl and ester were weak active sites to react with
H2S and had no obvious effects on its adsorption. This
calculation proved the complexity of H2S interactions with
oxygen groups on the AC surface. In another study, Shen et al.
employed DFT calculation and cluster models to study the
adsorption mechanism of H2S on activated carbon at the
molecular level.51 The results exhibited that the adsorption of
H2S on the AC surface is highly thermally favorable. The
adsorption energies of H2S on zigzag (−664.9 kJ mol−1) and
armchair (−349.6 kJ mol−1) edges indicated that the active
sites on the zigzag edge are more active for H2S adsorption
than those on the armchair edge. The adsorption of H2S on the
carbon surface was based on a dissociative process and
chemisorption. The direct adsorption of the H2S molecule led
to the formation of C−S, C−S−C, and C−SH. During the H2S
adsorption process, electrons of S and H atoms in the
molecular H2S transferred to the surface C atoms. The results
indicated that AC acts as a catalyst by facilitating the
dissociation of H2S molecule besides providing active sites
for H2S adsorption. Further surface modification of ACs can be
performed via various methods such as heteroatom doping,
chemical impregnation, and metal oxide deposition.
3.2.1. Heteroatom Doping. Bandosz’s group has extensively
studied the effect of surface chemistry, acidity, and pore
structure of various porous activated carbons on H2S
removal.47,52−54 The acidic environment was shown to be
favor the conversion of H2S to sulfur oxides and sulfuric acid,
while a basic media promoted the formation of elemental
sulfur. In addition, more water or O2 captured on the activated
carbon surface significantly increased the adsorption/oxidation
of H2S. Nitrogen, as one of the most common heteroatoms
incorporated into a carbon matrix, led to an increase in the
surface basicity. Nitrogen functionalities can be mainly
introduced to activated carbons via their treatment with
nitrogen-rich precursors such as melamine and urea47,54 or
carbonization of nitrogen-containing carbon precursor. The
experiments proved that the presence of nitrogen functionalities in activated carbons, especially quaternary and pyridinic
nitrogen, has a catalytic role in H2S oxidation resulting in an
improvement in the desulfurization performance. It was shown
that both elemental sulfur and its oxide could be formed
depending on the pore size and availability of oxygen in the
system. In the presence of enough oxygen and small pores
(micropores), more sulfur oxide was formed.54
3.2.2. Alkaline Impregnation. Several studies have reported
better performance for H2S removal after impregnating
activated carbon with different alkaline materials such as
NaOH, KOH, K2CO3, Na2CO3, etc.55−57 The mechanism of
H2S removal over AC impregnated with K2CO3 or Na2CO3
and in the presence of oxygen can be described as the
H 2S + H 2O → HS− + H3O+
(15)
H 2S + CO32 − → HS− + HCO3−
(16)
H 2S + HCO3− → HS− + H 2CO3
(17)
C + O2 → CO2 (ads)
(18)
CO (ads) → 2CO*
(19)
CO* + HS− → C + S* + OH−
(20)
xS* → Sx
(21)
S* + O2 → SO2
(22)
58
Hui et al. investigated the effect of various types of alkaline
(NaOH, KOH, and K2CO3) and their impregnation ratio
toward H2S adsorption capacity by porous coconut shell
activated carbon (CSAC). All alkaline impregnated activated
carbons showed better performance than unimpregnated
activated carbon wherein, impregnation by K2CO3 with ratio
2.0 showed the highest adsorption capacity which was 25 times
higher than that of unimpregnated activated carbon (1.5 mg
g−1). After impregnating with alkaline chemicals, the surface
was basic which enhanced chemisorption of H2S as well as
dissociation of hydrogen sulfide to HS− and H+ and
subsequent formation of sulfide product.
Similar to that mentioned earlier, Xiao et al.59 observed an
increase of more than three times in the adsorption capacity of
H2S on microporous activated carbon after impregnation with
Na2CO3, though the surface area and micropore volume was
reduced slightly. They believed that although impregnants
might occupy some pores of AC and thus limited physical
adsorption, chemisorption was the main mechanism for H2S
adsorption on impregnated AC and the reaction would be
continued until most of the Na2CO3 was consumed. The
authors also found that H2S adsorption capacities increased
considerably on both AC and impregnated AC upon increasing
the relative humidity. Li et al.60 applied a series of
predominantly microporous walnut shell-based activated
carbon (WSBAC) materials impregnated with different
activator including KOH, K2CO3, ZnCl2, and H3PO4 for
simultaneous removal of H2S, COS, and CS2. The activity
order of these materials was KOH > K2CO3> H3PO4 > ZnCl2.
The highest sulfur capacity of 45.25 mg g−1 was obtained with
KOH impregnation whereas the sulfur capacities of WSBAC
impregnated with K2CO3, ZnCl2, and H3PO4 were 29.27, 3.64,
and 2.5 mg g−1, respectively. The authors explained that the
KOH could provide a lot of OH− functional groups in the
activation process which increased the alkaline adsorption sites.
In addition, KOH and K2CO3 produced molten metal K at
high-temperature conditions, which could be reacted with the
carbon material, decreasing the surface tension of the carbon
and making a large number of micropores inside the carbon
matrix.
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Menezes61 revealed a significant increase in H2S uptake
capacity after chemical impregnation of AC with NaOH (from
0.34 to 15.64 mg g−1) or Fe2O3 (from 0.34 to 4.01 mg g−1), in
spite of the reduction in the textural properties (surface area
and pore volume). Both physisorption and chemisorption were
observed for the nonimpregnated AC (physisorption at lower
temperatures and chemisorption at higher temperatures) while
the dominant mechanism for H2S removal by impregnated
ACs was related to the chemisorption. The authors found that
the nonimpregnated and impregnated adsorbents could
recover only 50% and 20% of their initial capacity respectively
under heat treatment.
3.2.3. Metal Oxide Deposition. The synergetic adsorption
effect of metals incorporated activated carbons has been
observed by several studies. In particular, Siriwardane et al.,62
prepared nano magnesium oxide impregnated in a porous
activated carbon, which provided both chemical and physical
adsorption of H2S. The H2S adsorption capacity of MgO
incorporated AC was increased from 20 to 275 mg g−1 by
enhancing the MgO content from 0.2 to 1.2 wt % whereas for
virgin activated carbon, it was found to be 53 mg g−1. It was
explained that though the MgO impregnation had a negative
effect on the surface area and total pore volume by blocking
the micropores (the macro- and mesopore had not been
clogged), the chemical adsorption was improved by increasing
the MgO content caused enhanced H2S adsorption capacity.
The effect of both nitrogen doping and metal oxide
incorporating on H2S uptake was investigated through a
study by Yang et al.63 They selected a coal-based AC as the
support for incorporating ZnO nanograins and tuned the
ZnO−AC interactions by introducing N species on the AC
surface to alter the acidity/basicity of the surface as well as its
electronic structure and obtained high H2S adsorption capacity
over the fabricated materials. They used a green soft nitriding
strategy for introducing N species (pyrrolic N, pyridine N, and
graphitic N) on the AC surface which was based on a mild
annealing of AC with urea at 300 °C. H2S breakthrough
experiments showed a low H2S uptake over pristine AC due to
the weak physical adsorption while nitrogen-modified AC with
ZnO could significantly increase the H2S removal capacity up
to 62.5 mg g−1 which was two times larger than the sorbent
without N-modification (30.5 mg g−1). They observed that
elemental sulfur and ZnS were the main products of the
desulfurization process and thereby suggested chemical
adsorption (ZnO + H2S → ZnS + H2O) and catalytic
oxidation (H2S + 1/2O2 → S + H2O) for the desulfurization
mechanism. In the proposed mechanism, the water vapor in
the feedstock first creates a thin water film on the sorbent
surface. Then the hydroxylation process occurs on the surface
of ZnO nanograins and provides a weak alkali environment.
Besides the hydroxylated ZnO nanograins, the incorporated N
species attached to the AC surface can also increase the local
basicity of the water film, accelerating the dissociation rate of
the diffused H2S molecules into HS− or S2− on the water film
surface. Finally, the dissociated HS− and S2− are easily oxidized
into elemental S by the adsorbed oxygen species on the AC
surface. In addition, the dissociated HS− and S2− can also react
with ZnO to form ZnS. In addition, it was found that the high
BSC of the sorbent was also related to the doped N
concentrations, ZnO dispersion, and the material porosity.
Activated carbons were also widely used as highly porous
supports to disperse nanosized active metal oxides. Lau et al.64
selected a microporous palm shell activated carbon (PSAC) as
a support for loading active species of cerium oxide (CeO2)
and NaOH and used the fabricated adsorbent for H2S removal
from biogas. The CeO2/NaOH/PSAC adsorbent combined
the physical adsorptive property of AC and catalytic oxidative
effect of CeO2. NaOH not only helps to convert cerium
precursor to CeO2 but also increases the basicity of the
adsorbent, improving the adsorption of H2S. The conversion
redox reaction of CeO2 to Ce2O3 provides oxygen species for
H2S oxidation reaction and creates some new oxygen
functional groups such as C−O, CO, and COOH to the
adsorbent surface, increasing the H2S removal by catalytic
oxidation. Superior H2S removal performance was also
reported for the AC impregnated with a mixed metal oxides
of ZnO−CuO,1,65 coal, and walnut shell-based AC incorporated with manganese oxides66 and mesoporous ZnO/SiO2
functionalized commercial activated carbon.67 The overall
results indicated a better dispersion of active component and
formation of the smaller particle size of the active component
which resulted in the higher desulfurization performance of
metal oxides supported on AC.
4. MESOPOROUS SILICA
Mesoporous silica structures are a particular class of silica
materials which possess interesting features like uniform
channels with large pore size, high surface area (>700 m2
g−1), high porosity, narrow pore size distributions, good
mechanical and thermal stability, and tunable structure.68,69
These remarkable properties make these materials very
attractive for many applications. A large number of studies
have been devoted to mesoporous silica after its discovery by
the Mobil Corporation in 1992.70 There are various types of
mesoporous silica structures that vary in the pore organization
though little differences in pore diameter size or wall thickness.
Depending on synthesis conditions, it is possible to obtain
various mesostructures: hexagonal parallel packed nanochannels such as MCM-41 and SBA-15, cubic 3D structures
with interconnected short channels such as MCM-48, and
lamellar structures such as MCM-50.69 These structures with
highly opened pores enable easy access of reactants and
facilitate their diffusion throughout the channels without pore
clogging. However, the frameworks of mesoporous silica
materials are neutral, and thus, their affinity toward H2S is
limited. Therefore, the functionalization of mesoporous silica
materials with active components or immobilization of active
particles can be interesting ways to obtain superior materials
with better desulfurization performance since they combine
physicochemical features of incorporated active phase with the
outstanding properties of well-ordered mesoporous silica
structures.
4.1. Amine Modification. Amine modification is one of
the most investigated routes for this application, and several
researchers, in recent years, have been tried to prepare aminefunctionalized mesoporous silica structures for adsorptive
removal of H2S. In general, H2S adsorption by amine-modified
mesoporous silicas strongly depends on the type and amount
of amine functionalities. The main reaction mechanism
between H2S and amine groups under the dry conditions or
as follows:
H 2S + 2RNH 2 ↔ TH 2NHSHNH 2R
(23)
22143
H 2S + 2R 2NH 2 ↔ R 2HNHSHNHR 2
(24)
H 2S + 2R3H ↔ R3NHSHNR3
(25)
DOI: 10.1021/acs.iecr.9b03800
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Industrial & Engineering Chemistry Research
Table 4. Textural Parameters and Adsorption Capacities of Amine Modified Mesoporous Silicas for H2S Removala
material
MDEA(0.6)-SBA15
PEI(50)/SBA-15
PEI(50)/MCM-48
PEI(50)/MCM-41
PEI(50)/SBA-15
PEI(50)/SBA-15
MAPS(2)/SBA-15
TEA(23)/SBA15
Hexamine(50)/
SBA-15
APTMS/MCM-41
AAPTS/MCM-41
SBET
(m2 g−1)
Vt
(cm3 g−1)
Vmic
(cm3 g−1)
Vmeso
(cm3 g−1)
Da
(nm)
217
0.65
4.9
80
13
11
80
0.20
0.02
0
0.20
6.06
0
0.03
6.06
80
0.20
6.1
392
326.7
115
0.67
0.35
0.34
873.7
823.9
0.64
0.60
0.67
5.5
5.75
2.89
2.80
T
(°C)
H2S breakthrough capacity
(mg g−1)
ref
100−800 ppmv H2S, CH4
balance
4000 ppmv H2S, 20% H2, N2
balance
25
3.7
71
22
72
6000 ppmv H2S, 20% H2, N2
balance
14.9% CO2, 4.3% O2, N2
balance
1% H2S, N2 balance
1% H2S, N2 balance
0.9% H2S, CH4 balance
22
26.9
27.6
15.7
50.5
75
70
75
30
25
22
5.5
0.1441
30.7
76
78
80
20 ppm H2S, O2, N2 balance
25
feed gas composition
134.4
46.6
73
81
a
SBET: specific surface area. Vt: total pore volumes. Vmeso: mesoporous volumes. Vmic: microporous volumes. Da: average pore diameter.
was shown in another study by this group.75 The authors
suggested an innovative two-stage sorption process consisted
of the CO2 removal at the first column operated at 75 °C and
H2S removal at the second column worked at 22 °C. The
lower kinetic barrier for diffusion of the adsorbed H2S from the
surface to the bulk of PEI resulted in higher H2S breakthrough
capacities at a lower temperature compared to the CO2,
although low temperature thermodynamically preferred the
CO2 adsorption.
Okonkwo76 modified the SBA-15 with different aminosilanes including the following: primary amine 3-aminopropyltrimethoxysilane, secondary amine (N-methylaminopropyl)
trimethoxysilane, and tertiary amine (N,N-dimethylaminopropyl) trimethoxysilane. They observed the best
efficiency for the H2S adsorption over the secondary aminemodified SBA-15 in dilute H2S concentrations while the
tertiary amine was more efficient for H2S uptake at higher
concentrations. Belmabkhout et al.77 revealed the high
adsorption capacity and good selectivity of H2S and CO2
acidic gases versus methane over a triamine-modified poreexpanded mesoporous silica (TRI-PE-MCM-41) and proposed
that depending on the feed gas composition, CO2 and H2S can
be removed simultaneously or sequentially. However, the
adsorptive performance of amine-functionalized CMC-41
toward H2S was barely affected by the presence of the
moisture in the feed gas.
An improvement in H2S sorption capacity from 0.07426 to
1441 mg g−1 was observed after modification of an ordered
SBA-15 with triethanolamine at a loading ratio of 23%.78
Abdouss et al.79 compared the H2S adsorption performance of
three different porous silica structures, i.e. MCM-41, SBA-15,
and UVM-7 which were functionalized with three different
aminosilanes. They found that the breakthrough curves of the
amine grafted UVM-7 were higher than those of the amine/
MCM-41 and amine/SBA-15, which was due to the pore size
distribution of the adsorbents. The smaller pore size in MCM41 (2.6 nm) might cause pores blocking by grafting different
types of aminosilanes. Experimental results exhibited that even
though SBA-15 (4.6 nm) had larger mesopores than the
MCM-41, its pore size was not large enough for grafting a very
large size of aminosilan groups resulting in a decrease in the
adsorption. However, the UVM-7 contained very large pores
(52.2 nm) which resulted in the free diffusion of aminosilanes
In the presence of moisture, the reaction mechanisms are
based on the following equations:
H 2S + RNH 2 + H 2O ↔ RH 2NHSHOH 2
(26)
H 2S + R 2NH + H 2O ↔ R 2HNHSHOH 2
(27)
H 2S + R3N + H 2O ↔ R3NHSHOH 2
(28)
The interaction between H2S and amine limits the sorption
capacity to one mole of H2S for every two moles of amine
groups while in the presence of moisture, one mole of amine
group can sorb one mole of H2S. A summary of amine loaded
mesoporous silica sorbents available for the sorptive removal of
H2S and their adsorption capacities is provided in Table 4. Xue
and Liu71 modified mesoporous molecular silica of SBA-15
with methyl-diethyl-amine (MDEA) and tested its potential
performance for removal of the minor concentration of H2S in
a dynamic setup. The amine loading to SBA-15 showed an
enhanced effect in H2S removal performance while the SBA-15
structure remained unchanged after loading of MDEA. The
highest breakthrough capacity of 0.109 mmol g−1 was obtained
with an amine loading ratio of 0.6 which was about ten times
greater than that of the unmodified SBA-15 (0.010 mmol g−1).
The reason for this was attributed to the possible reaction
mechanisms between H2S and amines (eq 25 and 28). In
addition, they observed good regenerability and stability of the
MDEA-modified SBA-15 over 15 cycles of adsorption/
regeneration.
Song’s group extensively studied the H2S removal performance of different molecular sieves including SBA-15, MCM-41,
and MCM-48 loaded with a polymeric amine named
polyethyleneimine (PEI).72−74 Higher H2S breakthrough
capacity was detected using the PEI modified mesoporous
molecular sieves with larger pore size and three-dimensional
channel structure (MCM-41). The authors believed that the
low sorption capacity of unloaded mesoporous structures
might be due to the limited physisorption of H2S while PEI
modified sorbents contained numerous basic amine groups
which had a higher affinity toward acidic H2S via the strong
acid−base interactions (eq 23−25). In addition, it was
reported that moisture had an enhanced effect on the sorption
removal of H2S through the PEI modified sorbents (eqs
26−28). An exceptional dependence of PEI/SBA-15 sorbent
on temperature for the competitive adsorption of CO2 and H2S
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Industrial & Engineering Chemistry Research
Figure 6. Possible mechanism of APTMS/MCM-41 (a), PEI/MCM-41 (b), and AAPTS/MCM-41 (c) adsorbents [Reproduced from ref 81. 2018
Published by the Royal Society of Chemistry].
load metal oxides in their matrix. The large mesopores and
high surface area allow high dispersion of the metal oxides into
the mesoporous silica frameworks, preventing the agglomeration or sintering of particles under exposure to high
temperatures. The metal oxide can be loaded into mesoporous
silica matrix either during its synthesis (direct synthesis)82 or
by postsynthesis (two steps approach)83 in which the
mesoporous silica matrix is first synthesized and then the
desired metal oxide is loaded to the structure through different
methods. The latter is often preferred since it is difficult to
maintain the ordered mesoporous structure by simultaneous
condensation of two phases. Furthermore, the postsynthesis
method allows control over the size and the shape of the
particles and dispersion of the metal oxide active phases on the
support in a wide range of loading. In recent years, much effort
has been made to functionalize mesoporous silicate structures
with various metal oxides of zinc, iron, copper, manganese, or
mixed metal oxides for the adsorption of H2S. Table 5
summarizes some of the metal oxide modified mesoporous
silica sorbents that are used to remove H2S.
4.2.1. Zinc Oxide Loading. Because of its outstanding
desulfurization ability, zinc oxide is one of the most widely
used metal oxides for loading to the mesoporous silica
supports. Improved H2S desulfurization activity of ZnO/
mesoporous silica nanocomposites prepared through various
strategies has been demonstrated in different studies. Wu et
al.82 synthesized ZnO supported mesoporous silica MCM41
with bimodal pore structure via a one-step hydrothermal
approach and observed high sulfur capacity up to 110 mg g−1
at 35% ZnO loading. In the preparation method, the zinc
source was inserted already during the synthesis of MCM-41.
Both ZnO and ZnSiO4 were observed during the adsorbent
regeneration that the latter could be decomposed into ZnO
under the desulfurization process. The regeneration of the
sorbent with 6% O2/N2 at 650 °C showed that the sorbent
preserved its mechanical stability and high sulfur capacity (98
mg g−1) even after five cycles of sulfidation/regeneration.
Wang et al.84 investigated low-temperature H2S removal of
mesoporous SBA-15 supported zinc oxide. The prepared
into the pores independent of their size. Moreover, UVM-7
had a bimodal pore system (meso/macropores) constructed
from connected mesoporous silicate nanoparticles which
favored the H2S mass transfers in its meso/macropores during
the adsorption. According to the results, the author suggested
that pore size had a more significant effect on the H2S
adsorption of aminosilane functionalized sorbents rather than
their surface area and the number of grafted amine groups.
In another study, hexamethylenetetramine was grafted on
SBA-15 by a wet impregnation method for H2S removal from a
gas mixture containing 9000 ppm H2S in CH4.80 The sorbent
showed a very low H2S adsorption capacity (0.015 mmol g−1)
before amine loading while a high breakthrough capacity of 0.9
mmol g−1 was observed after amine loading at 50 wt %. The
sorbent exhibited high stability during ten cycles when it was
regenerated by N2 at 100 °C, though the breakthrough sulfur
capacity reduced about 83% of the first value.
In a recent work by Zhang et al.81 modified MCM-41
materials with different amines including (3-aminopropyl)trimethoxysilane (APTMS), PEI, and N-ethyl-g-amino propyl
trimethoxysilane (AAPTS) which exhibited higher adsorptive
desulfurization performance upon all amine modified MCM-41
compared to the unmodified MCM-41, even though the
specific surface area of MCM-41 was decreased after amine
modification. The APTMS/MCM-41 showed the highest
desulfurization performance among all amine-modified adsorbents which was attributed to the smaller molecular size of
APTMS compared to the PEI and AAPTS, resulting in an
easier diffusion of APTMS to the inner pores of MCM-41 and
thus loading more active sites. In addition, APTMS/MCM-41
had higher alkalinity, which could lead to more H2S
adsorption. They described that the diffusion of H2S molecules
from the bulk gas to the external surface of the adsorbent, and
subsequent diffusion to the inside of the pores leads to the
chemisorption of H2S molecules and formation of sulfurcontaining organic compounds through the sulfidation of the
active phase (Figure 6).
4.2. Metal Oxide Loading. Another interesting approach
to prepare highly efficient mesoporous silica materials is to
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Table 5. Textural Parameters and H2S Adsorption Capacities of Metal Oxides Loaded Mesoporous Silicasa
material
SBET
(m2 g−1)
Vt
Vmic
(cm3 g−1) (cm3 g−1)
Vmeso
(cm3 g−1)
Da
(nm)
ZnO/MCM-41
565
0.561
2.1ZnO1Al/SBA-15
3.04ZnO/SBA-15
15.5ZnO/SBA-15
15ZnO/SBA-15-F
15 ZnO/MCM-41
15 ZnO/KIT-6
20ZnO/MCM-41
20ZnO/SBA-15
30ZnO/MCM-48
30ZnO/MCM-48
30ZnO/KIT-6
20ZnO/SBA-16
MnxOy/MCM-48
434
270.6
79.6
377
183
333
686
213
323
419.57
308.89
306.09
443
0.69
0.96
0.24
0.558
0.199
0.412
0.7
0.4
0.3
0.36
0.49
0.26
0.3140
0.064
0.014
0.014
0.1443
0.296
0.476
0.246
0.1697
5.9
5.4
13.4
5.3
3.4
4.8
2.7
5.6
3.7
3.14
7.27
3.97
2.8
Mn2O3/MCM-41
Mn2O3/HMS
Mn2O3/KIT-1
50%4Mn1Fe-3%
Mo/FSM-16
50%1Zn2Fe2Mn/
MCM-48
1Cu9Mn/SBA-15
236
316
335
340
0.146
0.330
0.295
0.280
0.070
0.094
0.116
0.090
0.076
0.236
0.179
0.180
2.5
4.1
3.5
3.23
365
0.2448
0.1099
0.1349
2.7
243
0.260
0.070
0.190
2.1
4Mn1Ce/HMS
275
0.260
0.090
0.170
1.86
50%Mn90Mo10/
KIT-1
90Mn10Ca/MSU-H
372
0.311
0.104
0.207
3.5
369
0.40
0.14
0.26
4.33
Mn/MCF
250
0.5929
0.0896
0.5033
9.5
La3Mn97/KIT-6
214
0.381
0.072
0.309
3.56
CuO/SBA-15
CuO/MCM-41
30Cu/MSU-1
10Zn/MSU-1
31.3%Fe2O3/SBA15
Fe2O3/SBA-15
ZnO/SBA-15
Fe2O3/MCM-41
Fe2O3/MCM-48
Fe2O3/MCM-41micro
LaFeO3/MCM-41
651
411
179
263
420.2
0.794
0.276
0.14
0.31
0.58
454
482
744
911
768
0.87
0.90
0.60
0.65
0.53
389
0.310
0.140
0.170
3.2
362
0.37
0.14
0.23
4.1
LaFeO3/SBA-15
3.83
0.01
0.007
0.01
0.04
0.011
0.015
0.68
0.96
0.23
0.54
0.589
0.635
feed gas composition
0.25% H2S, 39% H2, 27% CO, 12%
CO2, N2 balance
0.1% H2S, air balance
0.1% H2S, air balance
0.1% H2S, air balance
Biogas (N2/H2S mixture)
T
(°C)
600
25
25
25
25
500 ppmv H2S, N2 balance
30
800 ppmv H2S, N2 balance
25
0.33% H2S, 10.5% H2, 18% CO, N2
balance
0.33% H2S, 10.6% H2, 28% CO, N2
balance
550
0.33% H2S, 10.5% H2, 18% CO, N2
balance
0.33% H2S, 10.5% H2, 18% CO, N2
balance
0.33% H2S, 10.5% H2, 17.1% CO, N2
balance
0.33% H2S, 10.6% H2, 28% CO, N2
balance
0.33% H2S, 10.6% H2, 28% CO, N2
balance
0.33% H2S, 10.5% H2, 18% CO, N2
balance
0.33% H2S, 10.5% H2, 18% CO, N2
balance
1000 ppm H2S in N2, 0- 70%
humidity
0.2% H2S, 20% H2, N2 balance
0.2% H2S, 20% H2, N2 balance
H2S/CH4 (5% H2S)
800
H2S breakthrough
capacity (mg g−1)
ref
110
82
103.2
436.0
177.3
21.8
9.6
5.8
54.9
41.0
53.2
14.8
37.6
8.0
124
83
84
85
86
87
88
89
90
600
152.1
158
160
181.5
550
132
92
800
138
93
600
121.7
94
700
168
95
750
186.9
96
700
16.9
97
91
800
116
98
515
515
25
15.7
29.0
19.4
42.3
701
99
100
101
102
5.5
0.1% H2S, air balance
25
103
3.6
6.2
2.4
2.4
2.2
1.5% H2S, He balance
300
15200 ppm H2S, Helium balance
300
15200 ppm H2S, Helium balance
300
80
10
17
17
38
0.33% H2S, 10.5% H2, 17.1% CO (or
CO), 72.1% N2
0.33% H2S, 10.5% H2, 17.1% CO (or
CO), 72.1% N2
500
32.4
106
500
48.5
107
104
105
a
SBET: specific surface area. Vt: total pore volumes. Vmeso: mesoporous volumes. Vmic: microporous volumes. Da: average pore diameter.
material exhibited superior H2S removal ability down to less
than 0.1 ppm with the highest H2S breakthrough capacity of
436 mg g−1 at 3.04 wt % zinc loading which was attributed to
the combination of the high surface area of SBA-15 and the
good desulfurization properties of ZnO nanoparticles (ZnO +
H2S → ZnS + H2O). However, it was shown that the reaction
products condensed in the channels and on the surface of the
adsorbent can limit the gas diffusion, resulting in a decrease of
H2S uptake. In another study, the SBA-15 loaded with the zinc
contents higher than 15.5 wt % showed the breakthrough
sulfur capacity up to 177.3 mg g−1.85
Hussain et al.,86 compared the potential ability of ZnO
loaded MCM-41, KIT-6, and SBA-15 (spherical and fiber-like)
with different structural frameworks for H2S removal at room
temperature. According to the results, 15 wt % ZnO loaded on
both spherical and fiber-like SBA-15 structures showed the
better desulfurization performance compared to the MCM-41,
KIT-6, and some other adsorbent reported in the literature
which was due to the retained superior physical characteristics
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Industrial & Engineering Chemistry Research
its lower surface area and irregular pores (Figure 7). All Mn2O3
supported mesoporous silicas demonstrated high breakthrough
and the better dispersion of ZnO nanoparticles on the surface
of these materials. Although this study highlighted the
importance of the support structure on metal oxide loading
and thus H2S removal, the main factors that might influence
the desulfurization performance of ZnO supported mesoporous silicas were not investigated obviously.
The major drawback of mesoporous silica supported metal
oxides is the small portion of active components compared to
the overall mass of the material which caused the low amount
of H2S adsorbed per mass of the adsorbent. Therefore, more
research to increase the loading content of metal oxide in the
mesoporous structure without losing utilization is required.
More recently Geng et al.87 used a melt infiltration method to
prepare three ZnO supported molecular sieves (MCM-41,
SBA-15, and MCM-48) for H2S removal at ambient temperature and compared these materials with those prepared by an
impregnation method. The results showed that the melt
infiltration produced more uniformly dispersed ZnO in the
pores of mesoporous materials compared with the impregnation. The highest sulfur capacity for the ZnO (20 wt %)/SBA15, ZnO (20 wt %)/MCM-41, and ZnO (30 wt %)/MCM-48
were found to be 41.0, 54.9, and 53.2 mg g−1, respectively
which were higher compared to the most ZnO-based
adsorbents reported in the literature. In addition, the
corresponding ZnO utilization ratio under the best ZnO
loading contents obtained values of 69.8%, 52.2%, and 45.1%
for MCM-41, SBA- 15, and MCM-48, respectively, while the
utilization ratio of adsorbents prepared by impregnation was
only 26.8%, 38.2%, and 28.3%.
It has been shown that because of faster mass transfer and
more resistance to pore blockage, 3-dimensional (3-D)
structural mesoporous silica materials are superior for metal
oxide loading compared to the corresponding one- or twodimensional structures. Li et al.88 prepared a series of 3Dmolecular sieves, SBA-16, MCM- 48, and KIT-6, supported
with ZnO at different loading content (10, 20, 30, and 40 wt
%) for H2S removal at room temperature. All synthesized ZnO
supported materials showed good H2S removal capacities from
800 to 0.1 ppmv (0.34−3.76 g g−1). The best ZnO loading was
20 wt % on SBA-16 and 30 wt % on MCM-48 and KIT-6.
Further increasing ZnO loading might lead to the reduction of
adsorption capacities due to the pore collapse or blockage as
well as the formation of large ZnO clusters. The H2S
adsorption capacities changed in the order of KIT-6 >
MCM-48 > SBA-16, which was attributed to their pore
volume and average pore size. KIT-6 with the largest pore size
among these materials allowed better dispersion of ZnO and
facilitated faster gas diffusion.
4.2.2. Manganese Oxide Loading. Huang et al.89 prepared
a series of 50% MnxOy/MCM-48 sorbents under different
synthesis conditions to improve the desulfurization efficiency.
The results showed that the synthesis method and precursor
type controlled the dispersion of active particles while the pore
structure and specific surface area of MCM-48 depended on
the heating rate and calcination temperature.
Zhang et al. fabricated a series of 50 wt % Mn2O3 supported
various silica materials, diatomite, MCM-41, HMS, and KIT-1
and investigated their desulfurization performances at the
temperature range of 600−850 °C using a simulated hot coal
gas containing 0.33 vol % H2S.90 The results demonstrated that
Mn2O3 was dispersed uniformly in the pore channels of the
mesoporous materials due to their high surface area while the
agglomeration of Mn2O3 observed over the diatomite owing to
Figure 7. Schematic drawing of Mn2O3 loaded on different silica
supports [Reproduced with permission from ref 90. Copyright 2014
Elsevier].
sulfur capacity compared with the Mn2O3/diatomite which was
due to the high surface area and large pore volume of the
mesoporous materials. The Mn2O3/KIT-1 showed the best
desulfurization performance because of its 3D wormhole-like
channels which could allow the fast diffusion of gas molecules.
The high sulfur capacity of the Mn2O3/KIT-1 (16.0 g S/100 g
adsorbent) was retained even after eight desulfurization−
regeneration cycles.
To further improve the desulfurization performance of
mesoporous manganese-based materials, transition metals
(e.g., Zn, Fe, Mo, Cu, and Ce) and rare-earth metals (e.g.,
Ce, La, and Sm) have been incorporated into Mn2O3. Xia et
al.91 evaluated the desulfurization performance of hot coal gas
by Mn-Fe-Mo supported FSM-16 mesoporous silica sorbent
(50% 4Mn1Fe−3% Mo/FSM-16) at 600 °C over a fixed-bed
reactor. The adsorbent exhibited better desulfurization
performance with a high breakthrough sulfur capacity of
181.5 g g−1 compared to the corresponding sorbent without
Mo loading which was due to the incorporation of
molybdenum species that effectively enhanced the dispersion
of particles and prevented the reduction of Mn and Fe oxides.
The main reaction of H2S adsorption was Mn−Fe−Mo−O +
H2S → MnS + FeS + MoS2 + H2O. After the sulfidation
process, the sulfided sorbent was regenerated at 700 °C in 5
vol % O2/N2. The results showed that the sorbent could retain
its high sulfur capacity (90% of the initial sulfur capacity) after
eight sulfidation−regeneration cycles, indicating its durability.
The final product of the regeneration process was elemental
sulfur which avoided the process for further treatment of SO2
(eqs 29 and 30).
2x MeS(s) + (2x + y)O2(g) → 2MexOy(s) + 2xSO2(g)
(29)
2MexOy(s) + 2xSO2(g) → 2x MeO(s) + 3/2xSO2(g )
(30)
92
Huang et al. synthesized 50% Zn-Fe-Mn/MCM-48
sorbents for hot coal gas desulfurization. The high breakthrough sulfur capacity of 132 mg g−1 and utilization of 66.1%
were achieved over 50% 1Zn2Fe2Mn/MCM-48 sorbent
(1Z2F2M denotes a Zn:Fe:Mn molar ratio = 1:2:2) at 550
°C. The results showed that ZnMn2O4, MnFe2O4, and
ZnFe2O4 were mainly active particles which were highly
dispersed on the support. After eight desulfurization/
regeneration cycles, the mesoporous channel structure of
MCM-48 remained intact; however the breakthrough sulfur
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tion performance. In addition, the enhanced chemisorption of
H2O by Ca species resulted in the outstanding H2O-resistance
ability of the adsorbent.
Huang et al.97 examined the effect of different supports
(SBA-15, MCM-41, MCM-48, and MCF) and rare-earth
oxides (Ce, La, Sm) doping on the desulfurization performance of a series of Mn supported mesoporous silica sorbents.
Among four Mn/mesoporous silica structures, Mn/MCF
exhibited the highest desulfurization capacity of 16.9 g S/100
g sorbent due to the better dispersion of active particles in 3-D
ultralarge mesopores and high thermal stability of MCF at 700
°C. Doping rare earth oxides into Mn/MCF caused the high
dispersion of active particles. However, the breakthrough sulfur
capacities over rare earth oxides loaded Mn/MCF sorbents
were significantly lower than undoped Mn/MCF sorbents. The
effect of doping rare earth oxides was in the order of CeO2 >
La2O3 > Sm2O3.
Xia et al.98 used KIT-6 synthesized La-modified mesoporous
Mn2O3/KIT-6 sorbents with different La/Mn atomic ratios to
investigate their H2S removal performance in hot coal gas at
700−850 °C. 3La97Mn/KIT-6 showed the best performance
at 800 °C with a breakthrough sulfur capacity of 115.6 mg g−1.
The author suggested that the 3D double gyroidal channel of
KIT-6 prevented the sintering of active La/Mn particles and
pore blockage which improved remarkably the gas diffusion as
well as the stability of the sorbent at high temperature (Figure
8).
capacity was reduced to 70% of its initial value. It might be
attributed to the migration and accumulation of Zn onto the
sorbent surface and then and vaporization to the exterior from
the surface during the sulfidation/regeneration cycles resulting
in a dramatic increase in the gas diffusion resistance and
therefore lower gas desulfurization performance.
Zhang et al.93 investigated the hot coal gas desulfurization
performance of a series of mesoporous xCuyMn/SBA-15
sorbents in a fixed-bed quartz reactor in the range of 700−850
°C. The high breakthrough sulfur capacity of 13.8 g S/100 g
sorbent with the utilization of 71.54% was achieved over
1Cu9Mn/SBA-15 at 800 °C which suggested the high thermal
stability of the Cu/Mn supported SBA-15. The successive nine
desulfurization−regeneration cycles at 800 °C revealed the
high desulfurization performance and durable regeneration
ability of the adsorbent due to the high dispersion of Mn2O3
particles incorporated with copper oxides. The authors stated
that the incorporation of a small amount of copper to Mn/
SBA-15 could inhibit the aggregation of Mn2O3 particles.
In another study, Zhang et al.94 examined the performance
of a series of Ce-doped Mn2O3/hexagonal mesoporous silica
(xMnyCe/HMS) sorbents with a wormhole-like structure for
hot coal gas desulfurization at 600 °C. All Ce-doped sorbents
exhibited higher breakthrough sulfur capacity compared with
unloaded Mn/HMS which was due to the synergetic effects of
loaded manganese oxide and ceria oxide, and the wormholelike porous structure in sorbents provided better dispersion of
active phase and improved the diffusion of H2S molecules. The
best breakthrough sulfur capacity of 121.7 mg S/g sorbent was
obtained over 4Mn1Ce/HMS with the high utilization of
82.4%. No obvious deactivation and alteration in the
mesoporous channel structure of the sorbent observed after
eight consecutive desulfurization−regeneration cycles. The
desulfurization process in the 4Mn1Ce/HMS was explained by
the following reactions:
Mn3O4 + H 2 → 3MnO + H 2O
(31)
MnO + H 2S → MnS + H 2O
(32)
CeO2 + 0.5H 2 → 0.5Ce2O3 + 0.5H 2O
(33)
Ce2O3 + H 2S → Ce2O2 S + H 2O
(34)
Figure 8. Scheme of desulfurization process over xLayMn/KIT-6 with
double gyroidal structure [Reproduced with permission from ref 98.
Copyright 2015 Royal Society of Chemistry].
Zhang et al. 95 fabricated Mo-doped Mn 2 O 3 /KIT-1
(MnxMoy/KIT-1) sorbents for high-temperature H2S removal
for hot coal gas. This indicated that the introduction of a small
amount of Mo in Mn/KIT-1 sorbents could promote
considerable sulfur capacity of sorbents up to 168.4 mg S/g
sorbent (for 50% Mn90Mo10/KIT-1) due to the synergetic
effects of Mn−Mo oxides. The presence of steam in hot coal
gas had a negative effect on the desulfurization performance of
the adsorbent because of the competition between MnO
(MoO2)/H2S and MnS (MoS2)/H2O reactions as well as the
destruction of the KIT-1 3D structure.
Xia and Liu96 designed Mn-Ca supported MSU-H sorbents
with large framework pore for H2S removal from hot coal gas
at 600−800 °C. The 90Mn10Ca/MSU-H sorbent exhibited
the best desulfurization performance at 750 °C with a
breakthrough sulfur capacity of 186.9 mg g−1 that was higher
than that of the supported Mn-based sorbent (132 mg g−1) in
similar conditions. The authors believed that the introduction
of Ca to Mn-based sorbents promoted effectively the
dispersion of active components and increased the surface
basicity, which resulted in an improvement in the desulfuriza-
4.2.3. Copper Oxide Loading. Karvan’s group investigated
the hot gas desulfurization performance of CuO/SBA-1599 and
CuO/MCM-41100 through sulfidation−regeneration cycles in
a laboratory scale fixed-bed reactor operating at atmospheric
pressure and the temperature range of 788−838 K. After three
cycles, the average breakthrough point sulfur uptake capacities
of CuO/MCM-41 and CuO/SBA-15 sorbents were obtained
in the range of 23.8−29.1 and 6.3−15.7 mg g−1 adsorbent,
respectively. Montes et al.101 observed a significant improvement in the H2S removal performance over MSU-1 supported
ZnO or CuO at room temperature while the unloaded MSU-1
demonstrated no activity. They found that the adsorption
capacity strongly affected by the structure porosity (both
micro- and mesopores), the active phase dispersion inside the
pores and the crystal size of metal oxide nanoparticles. The
best performance was achieved by 10 wt % of Zn (sulfur
capacity of 42.3 mg g−1) and 20 wt % of Cu (sulfur capacity of
19.2 mg g−1) impregnation.
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Table 6. Hydrogen Sulfide Adsorption Capacities of Zeolites
material
Clinoptilolite
4A
natural zeolite (Tuff)
13X
13X
AgX
0.15-CoX-600
ZnX-0.5 M (14% Zn)
13X Ex-Cu
Fe-NaA
7Cu2Mo1Mn-SP115
20Zn-NaA
5-TiO2/zeolite
13X-Ex-Cu (4.66%
cu)
Cu-ETS-2
Cu-ETS-2
Cu-ETS-2
CuCr2O4/Cu-ETS-2
Cu-ETS-2
surface area
(m2 g−1)
34.2
49.5
567
700
333
382
427
239
305
7.99
93.42
370
187
250
feed gas composition
59.95 CH4, 39.95 CO2, 0.1 H2S
1000 ppmv H2S, N2 balance
898 ppmv H2S, N2 balance
20000 ppmv H2S, 10000 ppmv SO2, 15% H2O, N2 balance
landfil biogas (∼4000 ppmv H2S)
2% H2S, 200 ppmv COS, 35% CO2, 63% N2
100 ppm H2S, N2 balance
100 ppm H2S, N2 balance
1000 ppmv H2S, N2 balance
1% H2S (or 3% NH3), N2 balance
42.5% N2, 11% CO2, 12.5% CO, 13.8% H2, 1% CH4, 19% H2O,
0.2% H2S
200 ppmv H2S, N2 balance
35% N2, 65% CH4, dry, 0.1% H2S
8 ppmv H2S, He balance
10 ppm H2S, N2 balance
500 ppmv H2S, He balance
10 ppm H2S, He balance
500 ppmv H2S, He balance
500 ppmv H2S, 69% He, 14% H2, 17% H2O
4.2.4. Iron Oxide Loading. Different composites of iron
oxide and mesoporous silica materials such as Fe2O3-SBA15,102,103 Fe2O3-MCM-41,104,105 and Fe2O3-MCM-48104 have
been developed for H2S removal. High surface area and porous
structure of all supports allowed the homogeneous dispersion
of the active phase resulting in an enhanced H2S removal
activity. However, a reduced sulfur capacity was observed by
varying the structure from hexagonal (MCM-41) to cubic
(MCM-48).104 This behavior was probably due to the fragility
of the cubic porous structure built up of very thin silica walls
that caused the collapse of the structure after repeated
sulfidation−regeneration cycles, limiting the access of H2S to
the active phase.
Liu et al. prepared LaMeOx/MCM-41106 and LaMeOx/
SBA-15 (Me = Co, Zn, Fe)107 sorbents and investigated their
ability for H2S removal. They achieved high breakthrough
capacities over LaFeO3 supported mesoporous silica materials
which were significantly higher than those of obtained over
unsupported materials. It was greatly attributed to the high
surface area and mesoporous channel structure of MCM-41
and SBA-15, improving the diffusion of H2S molecules and the
sulfidation rate. The results of successive sulfidation/
regeneration cycles indicated the good stability and durability
of the synthesized mesoporous sorbents.
T
(°C)
H2S breakthrough capacity
(mg g−1)
ref
25
50
25
150
25
25
25
25
120
30
871
1.4
8.36
0.04
179.7
142
52.17
4.42
23.5
40.12
15
112.8
113
114
119
120
123
127
128
129
130
133
134
28
25
40
15.75
4.43
39.9
135
136
137
650
25
950
350
47
60
29.7
61
41
138
139
140
141
142
the structure. The adsorption applicability of zeolite materials
depends on the Si/Al ratio. Zeolites containing lower Si/Al
ratios are more hydrophilic and have good affinity to adsorb
polar substances, whereas high-silica zeolites often possess
fewer structural defects and become more hydrophobic and
hydrothermal stable.110 These latter structures are favored in
the adsorption of nonpolar compounds. Several forms of
synthetic and natural zeolites have been used for adsorptive
desulfurization.16,111 Some of the most important results of
H2S removal over zeolites are summarized in Table 6. It has
been shown that the adsorptive desulfurization by zeolites is
based on physisorption or chemisorption process. Two distinct
mechanisms have been reported for H2S chemisorption:
sulfide-sorbent π-complexation and sulfur−metal (S−M)
bond formation based on the direct coordinate bonding.
Zeolites loaded with different metal ions such as K+, Ag+, Cu+,
Zn2+, and Ni2+ have the potential to form π-complexation
between metal ions and sulfur.
Although the good features of natural zeolites may not offset
the effect of impurities and structural defects on their
adsorption performance compared to the synthetic ones,112 a
study by Alonso-Vicario’ s group demonstrated the importance
of zeolite activation to overcome the above-mentioned
drawback.113 They compared the behavior of two synthetic
molecular sieves (5A and 13X) and natural zeolite
(Clinoptilolite) for H2S removal and upgrading of biogas.
Prior to use, Clinoptilolites were activated by washing and
calcination to remove the soluble impurities and volatile
compounds presented in their cavities or channels. The
experiments indicated that washing at 40 °C and calcination
at 220 °C were crucial for the removal of impurities and
enhanced the adsorption capacity of Clinoptilolite almost 14
times after the activation process due to the improved access of
the adsorbate molecules to the adsorption sites maybe by a
chemical adsorption mechanism. Activated Clinoptilolite
showed the breakthrough adsorption capacity of 1.39 mg g−1
at 1 cm s−1 biogas (CH4/CO2/H2S) feeding velocity, the
pressure of 7 bar and 25 °C. Moreover, although all of the
5. ZEOLITE
Zeolites are porous crystalline aluminosilicates composed of a
three-dimensional framework of SiO44− and AlO45− tetrahedra
being linked to each other through bridging all the oxygen
atoms.108,109 The aluminosilicate structure is negatively
charged and a variety of exchangeable cations (univalent or
bivalent) are present in the zeolite matrices to keep the overall
framework neutral. The typical micropores size of zeolites is
similar to that of many small molecules making zeolites as
versatile materials for numerous applications. Zeolites have
large vacant spaces or “cages” in their structures that are
interconnected in some structures to form large pore channels
allowing easy diffusion of the gust ions or molecules through
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indicated the efficient recovery of the sorbents without
significant changes in their adsorption performance.
A 13X zeolite was applied to simultaneously remove H2S
and SO2 in the simulated Claus tail gas with high water
vapor.120 The breakthrough sulfur capacity of 13X was
achieved 179.7 mg g−1 at 150 °C, which was three times
more than that of activated carbon (64.3 mg g−1). H2S and
SO2 could adsorb by 13X through an adsorption−redox
mechanism and oxidized to elemental sulfur and sulfuric acid,
respectively. After five adsorption−regeneration cycles, a small
number of sulfate residues accumulated in the 13X pores
caused the incomplete recovery of the sorbent. The authors
suggested adding a reducing gas in the regeneration process or
using a two-step low-temperature dewatering and hightemperature nitrogen purging process to overcome the
above-mentioned problem and improve the regeneration
efficiency.
Shah et al. investigated the adsorption behavior of seven allsilica zeolite frameworks (CHA, DDR, FER, IFR, MFI, MOR,
and MWW) for H2S and CH4, both pure-component and
mixture, over a wide range of temperatures, pressures, and
compositions through Gibbs ensemble Monte Carlo simulations.121 The simulations demonstrated that selectivities
increased with increasing H2S concentration due to favorable
sorbate−sorbent interactions. When the H2S concentration
was low, MOR showed the highest selectivity and the most
favorable enthalpy of adsorption for H2S; which was attributed
to the favorable interactions between H2S and the adsorbent in
its smaller pores. However, MFI demonstrated the highest
degree of selectivity for H2S at a high H2S concentration. They
also carried out a computational study of the adsorption of
binary H2S/CH4 and H2S/C2H6 mixtures in the all-silica forms
of 386 zeolitic frameworks using Monte Carlo simulations122
and found that depending on the fractions of CH4, C2H6, and
CO2, different sorbents allow for optimal H2S removal.
The effect of moisture on the H2S removal performance of
zeolite was investigated by Sigot et al.123 for the first time when
they tried to compare the efficiency of a 13X zeolite and an
impregnated activated carbon for H2S removal from biogas.
The presence of moisture dramatically decreased the
adsorption capacity of 13X zeolite down to zero but
significantly enhanced that of IAC. This could be due to
their different affinity for water adsorption. The H2S
adsorption capacity of 13X was 142 mg g−1. The Sigato
groups later proposed a mechanism for H2S adsorptive removal
by the zeolite 13X.124 This mechanism involved H2S
adsorption at the surface of zeolite (H2S(g) → H2S(ads)),
dissolution (H2S(ads) → H2S(aq)) and dissociation of H2S in
water existing in zeolite pores or a water film deposited on the
surface (H2S(aq) + H2O(l) → HS−(aq) + H3O+(aq)), and
oxidation of HS− with adsorbed oxygen to form elemental
sulfur (HS−(aq) + O(ads) → S(ads) + OH−(aq)) that can further
arrange in more stable linear or cyclic sulfur polymers such as
S8(xS(ads) → Sx(ads)). The formation of elemental sulfur was the
major limitation for the thermal regeneration of the studied
zeolite and authors suggested other regeneration methods such
as high-temperature desorption in an inert atmosphere, lowtemperature (<400 °C) desorption−oxidation in air, desorption under reduced pressure, and chemical dissolution with
organic solvents.
5.1. Ion-Exchanged Aluminosilicate Zeolites. Kumar et
al.125 studied the H2S adsorption from mixtures containing He,
N2, CO2, CO, and H2O over zeolites X and Y in sodium form
investigated zeolites were totally recovered, a surprising
behavior was observed for Clinoptilolite as its H2S adsorption
capacity was improved significantly with increasing the number
of cycles and after 4 cycles it was much higher than the fresh
Clinoptilolite.
Liu and Wang114 synthesized 4A molecular sieve zeolite in
different synthesis conditions to investigate the effect of each
synthesis parameter on the H2S removal performance. The
H2S adsorption results indicated that the silicon to aluminum
ratio had a significant influence on the H2S removal
performance while the ratio of sodium to silicon displayed a
weak effect. The optimum value of both ratios was 1.5. In
addition, high crystallization temperatures up to 90 °C and a
crystallization time of 4 h favored the H2S removal of 4A
zeolites. A low water to sodium ratio (30) was also beneficial
to increase the H2S adsorption capacity. This was possibly
caused by increasing the impurities present in the zeolite
structure (such as oxides of iron, calcium, and magnesium)
which had a positive influence on the reaction with H2S. The
affecting degree of different synthesis factors on H2S removal
performance was shown as follows: crystallization temperature
> SiO2/Al2O3 > crystallization time > H2O/Na2O > Na2O/
SiO2. The highest breakthrough and saturation sulfur capacities
under optimum conditions were found to be 8.36 and 12.4 mg
g−1 at 50 °C.
Melo et al.115 compared the applicability of Zeolite 13X and
Zinox 380 (88% ZnO) as H2S adsorbents at 25 °C. Zeolite
13X demonstrated better H2S adsorption (53 mg g−1) than
Zinox at 25 °C which was probably due to the difference in the
porosity of Zinox 380 (micropores) and zeolite (mesopores)
that caused a longer saturation time for Zeolite 13X (36.5 h at
25 ◦C). The adsorptive removal of the small amounts of H2S
from propane with a storage capacity more than 62% (at 25 bar
and 298 K) through nanoporous molecular sieve 13X was also
confirmed the satisfactory ability of zeolites for H2 S
removal.116
Besides the H2S impurity, biogas also contain other
impurities such as CO2, CH4, and H2O, and these molecules
may have a tendency to compete with H2S adsorption.
Suthanyawatchai and Onthong investigated the adsorption of
H2S, CO2, and CH4 on H-FER zeolites (5T tetrahedral
quantum cluster, 12T, and 34T extended framework) under
computational chemistry using the embedded ONIOM
method and showed that zeolite can be considered as a good
adsorbent for purifying biogas before being used as fuel.117 The
result of calculations indicated that the zeolite could adsorb
H2S better than CO2 and CH4. The adsorption energies were
decreased in the sequence H2S (−10.78 kcal/mol) > CO2
(−9.67 kcal/mol) > CH4 (−7.79 kcal/mol). The H2S molecule
could adsorb on the adsorbent by having a sulfur atom end of
the molecule pointing to the Brønsted acid site of the ferrierite
zeolite.
The separation of binary mixtures of H2S and CO2 toward
high concentration streams of both gases on zeolite materials
was not yet investigated when Tomadakis et al. reported a high
selectivity of H2S adsorption over CO2 for 13X, 5A, and 4A
zeolites up to 11.9, 5.4, and 2.0, respectively.118 In another
study by Paolini et al.119 simultaneously selective adsorption of
CO2 and H2S toward CH4 on thermally activated natural
zeolites (from Tuffs) was confirmed. The loading capacities
were found to be 40 and 45 mg kg−1 for H2S and CO2,
respectively. Thermal and vacuum regeneration processes
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Figure 9. SEM images of NaX (a), CoX (b), ZnX (c), and AgX (d) [Reproduced with permission from ref 127. Copyright 2018 Elsevier].
Consequently, these ions occupied less space in the zeolite
cages as compared to Na+. According to the breakthrough
curves, AgX displayed the best adsorption performance among
all tested sorbents, with the highest H2S breakthrough
adsorption capacity of 1.53 mmol g−1. The H2S breakthrough
capacities were in the following order: AgX > ZnX > CoX >
NaX. The regeneration experiments revealed that with thermal
treatment at 350 °C under air atmosphere, the adsorption
performance of AgX zeolite could be recovered well. The DFT
study confirmed that a chemical reaction (sulfur−metal bond
formation and π-complexation) rather than physical interactions plays the crucial role in the adsorption mechanism of
H2S on X-zeolites during the sulfidation process and AgX had
the strongest interaction.
Tran et al.128 realized that the Co(II) exchange and heat
treatment processes (up to 600 °C) considerably enhanced the
H2S uptake capacity of the NaX zeolite with the highest value
of 4.42 mg g−1 for 0.15-CoX-600 due to the increase in the
mesopore volume. Besides the physisorption, H2S adsorption
on the thermally treated cobalt(II)-exchanged NaX zeolite also
occurred by chemisorption (an acid−base reaction) according
to the following equation:
and ion-exchanged with silver and copper cations. Among the
investigated sorbent, just Ag exchanged both X and Y zeolites
were able to remove H2S in presence of all other gases
mentioned earlier while CuX and CuY were failed to remove
H2S in presence of 2% CO. H2S adsorption capacities reduced
in the order of AgX > AgY > CuX > CuY > NaX at room
temperature. The result of this study was in good agreement
with the DFT calculations carried out by Sung et al.126 The
authors investigated the capabilities of Cu-Y and Ag-Y zeolites
for selective H2S removal from Claus process tail gas and
compared their adsorption energies to those of alkali metalexchanged Y zeolites (metal ion = Li, Na, and K). The
adsorption energies indicated that Ag-Y had the best-predicted
selectivity for the H2S adsorption while Cu-Y and alkali metalexchanged Y zeolites were favorable for strong adsorption of
CO and H2O, respectively. For Cu- and Ag-exchanged Y
zeolites, the charge transfers (CT) with the adsorbed CO and
H2S mainly involved the valence s and d orbitals of Cu and Ag
cations. On the other hand, for alkali metal-containing systems,
where d electrons are missing, adsorption was dominated by
non-CT terms due to electrostatics, polarization, and structural
distortions.
Different Zn, Co, and Ag modified NaX zeolites were
prepared by the ion-exchange method and the dynamic
adsorption performance of the modified sorbents for H2S
and COS from Claus tail gas were evaluated at 25 °C.127 The
SEM images of the NaX zeolite and modified samples (Figure
9) revealed that the crystals of the ion-exchanged samples have
clear shape and similar size (2−3 μm).
However, NaX demonstrated much a smoother surface
compared to the other ion-exchanged zeolites. In addition,
some particles of X-zeolites were broken after ion-exchange
processes. On the other hand, the total pore volume of ZnX
and CoX were slightly increased compared to the NaX zeolite
when the porosity of AgX was reduced about 33%. This was
attributed to the production of more vacant spaces in the
zeolite framework by replacement of two monovalent sodium
ions with one divalent zinc or cobalt ions. In addition, the ionic
radius of Zn2+ and Co2+ was smaller than that of Na+.
H 2S + [Co(Ox)6 ]2 + X2 − → CoS ↓ +H 2X + Ox
(35)
Long et al.129 investigated the adsorptive removal of H2S at
room temperature over ion-exchanged zeolite X sorbents
including NaX, ZnX, CuX, MnX, CaX, NiX, and CoX and
found that Zn2+, Cu2+, and Co2+ ion-exchanged zeolites could
improve the H2S adsorption capacity. In particular, ZnX
demonstrated the best desulfurization performance among all
tested sorbents with the adsorption capacity of 23.5 mg g−1
which was 24 times more than that of the NaX zeolite.
According to the experimental data, they proposed the
mechanism of the H2S removal by the ZnX zeolite as: (1)
Zn2+ + H2S → (Zn−SH2)2+ads and (2) (Zn−SH2)2+ads → ZnS
+ 2H+. They suggested reaction 1 as a coordination bond
(donor−acceptor bond) that can be formed between H2S
(donor) and Zn2+ (acceptor) and the reaction 2 because the
ZnX could not be easily regenerated by heating.
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highest capacity (15.75 mg g−1) was achieved when 20 wt %
ZnO was loaded on Na-A zeolite.
Liu et al.136 succeeded to prepare a hybrid TiO2/zeolite
composite for selectively H2S removal and simultaneously SO2
emission reduction in the biogas purification process. Figure 10
In another study, the performance of a 13X zeolite modified
with Cu ions (13X Ex-Cu) for H2S removal from biogas was
completely investigated under various operating conditions
including gas space velocity (850−16941 h−1), reactor
temperature (30−120 °C), gas matrix composition (N2,
CH4, CO2 and 60% CH4/40% CO2), inlet H2S concentration
(200 and 1000 ppmv), and particle size (pellet or powder).130
13X Ex-Cu zeolite presented the highest H2S adsorption
capacity of 40.12 mg g−1 under 850 h−1 space velocity, 120 °C,
and an H2S concentration of 200 ppmv and in the presence of
CH4 as the feed gas. The improved desulfurization performance was ascribed to the presence of a large amount of Cu2+
ions that allowed a physical and chemical (π-complexation)
adsorption mechanism. In addition, a significant adsorption
capacity improvement was observed using the 13X zeolite in
pellets (1.6 mm). Furthermore, a comparison study with
several conventional sorbents for dynamic H2S adsorption
exhibited the decreasing H2S adsorption capacity in the order
of AC Cu-Cr > AC KOH > Zeolite 13X Ex-Cu > AC KOH-KI
> alumina KMnO4> virgin AC > iron oxide SCX > natural
zeolite sepiolite.
The activity of various metal ion sites in ZSM-12 zeolite (M
= Fe, Co, Ni, Cu, and Zn) for the H2S adsorption was
investigated using DFT calculations with B3LYP formalism.131
The results exhibited that the Cu-ZSM-12 cluster has the
lowest chemical potential, minimum adsorption energy value,
highest electronegativity and lowest energy gap between
HOMO and LUMO compared with other M-ZSM-12 clusters
indicating that Cu-ZSM-12 is a promising candidate adsorbent
for the H2S removal via activation of the S−H bond.
5.2. Metal Incorporated Aluminosilicate Zeolites. It
has been shown that the adsorption efficiency of zeolites may
be improved by the use of another host molecule. Higher
sulfur capacity is observed over Zeolites modified with metals
or metal oxides.132 According to the literature, several reaction
mechanisms might be occurred during the H2S adsorption by
zeolites impregnated with metal oxides. In fact, H2S can be
adsorbed by the metal cation site and oxide site presented in
the structure.
Lee et al.133 prepared iron incorporated Na-A zeolites and
found that the adsorption capacities depended on the
concentration of Fe3+ solution and calcination temperature.
The adsorbent composed of 78 mM Fe3+ solution and
calcination temperature of 200 °C showed the highest H2S
adsorption capacity of 1.5% which was found to be higher than
commercial zeolites such as 4A (0.15%) and 13X (0.91%).
Gasper-Galvin et al.134 described the synergetic effect of
mixed-metal oxides (Cu, Mo, and Mn) supported high silicazeolite (SP-115, SiO2 > 99 wt %) on the enhancing of the hotgas desulfurization performance at 871 °C and 205 kPa. The
copper oxide was the main active component that produces
metal sulfides, while molybdenum (to enhance the initial
desulfurization activity) and manganese oxides (to enhance the
crush strength) acted as catalysts/promoters. The ternary
metal oxide supported SP-115 maintained its reactivity and
stability during multicycle sulfidation/regeneration.
Abdullah et al.135 investigated the adsorption of H2S from
the biogas using zinc oxide (10−30 wt %) impregnated on NaA zeolite prepared from local kaolin. Although they observed a
reduction in the micropore surface area and micropore volume
of Na-A zeolite after impregnation with ZnO, the H2S
adsorption capacities were increased during this process. The
Figure 10. Schematic diagram of the experimental setup for H2S
removal [Reproduced with permission from ref 136. Copyright 2015
Elsevier].
presented a schematic diagram of the biogas purification
system designed by the authors. Experiments were performed
on three parallel columns filled with zeolite, TiO2, and TiO2/
zeolite. A UV-light was placed in the center of the columns to
supply irradiation (3 mW cm−2, 365 nm) for photocatalytic
reaction, and a light concentrator was used to supply
irradiation in the back of the columns. Four different
compositions of simulated biogas were passed through the
columns and the concentrations of H2S and SO2 in the outlet
gas were measured with an electrochemical sensor. The
synthesized TiO2/zeolite composite showed the highest H2S
removal capacity (0.13 mmol g−1) than those of the virgin
zeolite (0.05 mmol g−1) and TiO2 (0.07 mmol g−1) and
released the lowest SO2. Little change was observed in the
capacity of H2S removal and SO2 capturing in the presence of
CO2 in biogas while the moister favored these processes.
Additionally, the TiO2/zeolite sorbent could be easily
regenerated by washing with NaOH solution and calcination
at 400 °C.
13X zeolites modified by Cu and Zn through ion exchange
or impregnation methods were studied as H2S sorbents for
biogas purification.137 The adsorption properties of 13X zeolite
were strongly enhanced with the addition of Cu or Zn.
Although Zn had a better degree of ion exchange with zeolite
13X, 13X-Ex-Cu zeolite showed a much longer breakthrough
time of 580 min, which was 12 times longer than the bare
zeolite (47 min). Moreover, the H2S adsorption values of both
impregnated and exchanged 13X-Cu were higher than those of
the 13X-Zn and bare zeolite.
5.3. Metal Incorporated Titanosilicate Zeolites. The
Engelhard Titanosilicate (ETS) is a molecular sieve zeolite
with pore size about 8 Å and a titania/silica mole ratio of 2.5−
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Table 7. MOFs Used for the Adsorptive Removal of Gaseous H2S
material
MIL-100(Cr)
MIL-101(Cr)
MIL-47(V)
MIL-53(Cr)
MIL-53(Al)
MIL-53(Fe)
MIL-47(V)
MIL-53(Cr)
IRMOF-3
Mg-MOF-74
MIL-101(Cr)
UiO-66
UiO-66-NH2
ZIF-8
Ce-BTC
Zn-MOF-74
MOF-5
Cu-BTC
Cu-BDC(ted)0.5
MIL-100(Fe) gel
MOF-199
AlFFIVE-1-Ni
MIL-53 (Al)pellet
surface area
(m2 g−1)
feed gas composition
H2S in CH4
T
(°C)
H2S adsorption capacity
(mg g−1)
30
0.1 ppm H2S (or CH3SCH3 or C2H5SH), N2 balance
1% H2S, 10% CO2, 89% He
30
25
400 mg/m3 H2S (or 600 mg/m3 CH3CH2SH or 600 mg/m3 CH3SCH3),
N2 balance
5% CO2, 5% H2S, 90% CH4
CO2, CH4, H2S, N2
80
534.4
908.8
467.2
419.8
376.6
273.0
467.2
419.4
10.61
7.7
12.8
7.5
29.1
1.6
4.0
52.5
35.5
35.5
32.0
28.8
57.2
25
30
73.6
528
30
1244
3203
1322
1097
1602
930
920
2250
1590
1045
816
601.4
ref
148
149
150
152
153
158
160
inactive sites, and lower thermodynamically favorable
sulfidation reactions.
ETS-2 was also functionalized with copper and chromium in
a study by Yazdanbakhsh et al.141 It was found that using ETS2 as a support, the H2S adsorption capacity increased from 35
mg g−1 for Cu−Cr−O to 61 mg g−1 for Cu-Cr-ETS-2. The
H2S capacity of Cu-Cr-ETS-2 remained stable up to temperatures as high as 950 which was due to the presence of
chromium that effectively stabilized the copper against
reduction to metallic copper under high temperatures.
Despite the above-mentioned studies that used an inert
atmosphere, Roller et al. executed a comparative study under
simulated gasification conditions by ETS-2 doped with
different metals (Cu, Cu−Cr, and Ce) for H2S removal in
the temperature range of 75−950 °C.142 The gas mixture was
composed of He (69%), H2 (14% or 28%), H2O (17% or 3%),
and 500 ppm H2S. Cu-ETS-2 exhibited the highest H2S
capacity (41 mg g−1 at 350 °C) while Cu−Cr-ETS-2 and CeETS-2 were able to reach 50% (20 mg g−1) and 5% (2−3 mg
g−1) of this capacity, respectively, at their best. In addition, the
H2S removal performance was strongly dependent on the gas
composition. The water-rich gas resulted in a higher H2S
capacity compared to the hydrogen-rich gas.
25 that has both tetrahedrally and octahedrally coordinated
framework sites.16 Metals can be well-dispersed on the ETS
supports to provide highly reactive sites for H2S adsorption.
Rezaei et al.138 demonstrated the good H2S removal capacity
of copper supported on ETS-2 (Cu-ETS-2) at room
temperature because of its high cation exchange capacity and
copper dispersion. In addition, the adsorption capacities
remained almost constant over a wide temperature range. A
similar result was observed by Yazdanbakhsh et al.139 while
they tried to investigate the H2S removal by Cu-ETS-2 at the
temperature between 250 and 950 °C. The H2S capacity for
Cu-ETS-2 remained almost stable up to 650 °C with
breakthrough capacity of 0.7 mol of H2S per copper. In this
region, CuO was the main active species and the sulfidation
reaction was governed as: CuO + H2S → CuS + H2O. At
higher temperatures, the reduction of Cu2+ into its metallic
form in the presence of H2, formed from the thermal
dissociation of H2S, caused a reduction in the H2S adsorption
capacity of Cu-ETS-2 to nearly half of its capacity (0.35 mol
H2S per Cu) according to the overall reaction of: 2CuO + H2S
+ H2 → Cu2S + 2H2O.
To further improve the H2S adsorption capacity and
utilization of loaded metal-oxide, a series of Na-ETS-2 sorbents
exchanged with Ag, Ca, Cu, and Zn were prepared by mixing
the ETS-2 with Ag/Cu nitrate and Zn/Ca chloride in the
solution.140 The high external surface area and cation exchange
capacity of ETS-2 allowed the high dispersion of active ions
which were easily available to H2S molecules. The H2S
breakthrough capacities were in the following trend: Cu-ETS >
Ag-ETS > Zn-ETS > Ca-ETS > Na-ETS-2 wherein the H2S
capacity of Cu-ETS-2 achieved 29.7 mg g−1 at room
temperature. Lower H2S removal capacities for other metal
ions could be attributed to the lower surface area, formation of
6. METAL−ORGANIC FRAMEWORKS
MOFs are porous materials in which metal ions or small
metallic clusters are chemically coordinated to multifunctional
organic ligands within a porous framework.143 Numerous
MOFs display unique properties including highly porosity,
tunable pore size, large surface area (reported up to 7140 m2
g−1) and diversity of metal centers and ligands, making these
materials ideal candidates for gas uptake.144−147 MOFs have
been examined extensively in the literature for H2S removal
and outstanding results are observed which arises from the
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In another study by Wang et al., a zinc-based MOF
containing a 2-amino-1,4-benzenedicarboxylate ligand
(IRMOF-3) was exposed to a tertiary mixture of S gases
including dimethyl sulfide (DMS), ethyl mercaptan, and
hydrogen sulfide in a fixed bed reactor at ambient temperature
and exhibited the adsorption capacity in order of H2S (16 mg
g−1) > ethyl mercaptan (5.92 mg g−1) > DMS (5.80 mg
g−1).150 This behavior relied on the interaction strength
between IRMOF-3 and sulfur compounds. In the case of
hydrogen sulfide, strong chemical adsorption between the
sulfur atom and the amino group (an acid−base interaction) or
zinc site in the MOFs was the main mechanism wherein the
latter produced ZnS and H2O which can cause serious
destruction of the MOF structure. In the case of dimethyl
sulfide and ethyl mercaptan, the interactions were based on the
weak hydrogen bond (H-donor and H-acceptor mechanism)
between the amino groups of the MOF and the sulfur atoms of
adsorbates that resulted in lower adsorption capacities. In
order to obtain better insights on the desulfurization
performance of MOFs, Wang’s group investigated the
contribution of each part of MOFs (organic ligand, metal
center structures, and metal ions) to the adsorption of H2S,
DMS, and ethyl mercaptan using DFT.151 The DFT results
revealed that the binding strength of MOFs with sulfur
compounds followed the order of MOFs with coordinatively
unsaturated sites > MOFs with NH2-BDC ligands > MOFs
with saturated metal centers > MOFs with organic ligands
without substituent groups (Figure 11). Moreover, it was
presence of metallic sites and organic ligands providing each
unit as a potential coordination−adsorption site for H2S
capture. The results of some of these studies summarized in
this review are shown in Table 7.
In a pioneering work by Hamon et al., the adsorption of H2S
at room temperature was studied by various MIL-series MOFs,
including MIL-47(V), MIL-53(Al, Cr, Fe), MIL-100(Cr), and
MIL-101(Cr).148 They observed that larger pore MOFs such
as MIL-100 (16.7 mmol g−1) and MIL-101 (38.4 mmol g−1)
generally had higher adsorptive capabilities compared with
smaller pore MOFs including MIL-47 (14.6 mmol g−1) and
MIL-53 (Al, Cr, and Fe, 13.1, 11.8, and 8.5 mmol g−1,
respectively). However, large-pore MIL-100 and MIL-101
MOFs showed irreversible adsorption, which was either due to
the strong interaction of H2S with the framework or structural
destruction upon adsorption of H2S. In contrast, MIL-47 and
all of the MIL-53 MOFs except MIL-53(Fe) were perfectly
regenerable under the same conditions. This indicated that the
type of metal in the MOFs had an important role in the
stability and regenerability of the structure and thus the
adsorption performance. The author proposed that the
hydroxyl groups located at the pore opening of the MIL-53
structure resulted in a strong interaction by the polar H2S
molecules leading to block the pores. The high selectivity
(∼30) of MIL-47(V) for H2S versus CH4 (at 1.0 MPa and 303
K) also confirmed that this adsorbent is a good candidate for
industrial gas purification processes.
The sulfur removal by MOFs can be performed through
various mechanisms such as physisorption, chemisorption, and
hydrogen binding (H-donor and H-acceptor) interactions.13
Physisorption is fundamentally based on surface adsorption. In
chemisorption, the sulfur adsorption yields metallic sulfides
and H2O which can cause the collapse of the initial MOF
framework. In the latter mechanism, the weak interactions
generally occur between the electron-rich framework and the
electron-poor sulfur atoms. Hence, the composition of target
sulfur species and the MOF features (e.g., framework topology
and metallic cluster and the organic ligands types) structure is
crucial in determining the removal mechanism.
Hamon et al., in a study based on in situ IR spectroscopy
measurements and molecular simulations, tried to verify that
the type of metal centers presented in the MOF framework was
crucial as it determined the interaction pathways of
adsorbate.149 It was shown that when the MIL-47(V) structure
remained rigid over H2S adsorption even at high pressure up to
1.8 MPa, the MIL-53(Cr) presented a structural switching
between the large pore form (LP) to its narrow pore version
(NP) at very low pressure and then from the NP to the LP
form at higher pressure. The microscopic arrangements of H2S
molecules within the MILs pores revealed that at the initial
stage of adsorption, the H2S behaved either as hydrogen donor
(VO···H−S−H) or hydrogen-acceptor (Cr−OH···SH2)
over MIL-47(V) and MIL-53(Cr), respectively with the μ2-O
and μ2-OH groups located at the MOF surfaces. Furthermore,
the MIL-53 showed lower hydrogen sulfide adsorption at low
pressures as compared to MIL-47; however, both frameworks
showed almost similar H2S adsorption capacity of 446 mg g−1
for MIL-53(Cr) and 498 mg g−1 for MIL-47(V) at 303 K and
2.03 MPa. At high loading, the H2S molecules could be
condensed inside of the pores. The H2S interaction over the
MOFs was decreased based on the following sequence: MIL53(Cr) NP > MIL-47(V) > MIL-53(Cr) LP.
Figure 11. Effect of different organic ligands (NH2-BDC, BDC, and
NDC), metal center structures (M, M−M, and M3O), and metal ions
(Zn, Cu, and Fe) on sulfur species adsorption [Reproduced with
permission from ref 151. Copyright 2016 Elsevier].
found that MOFs containing unsaturated Fe exhibited stronger
adsorption for sulfur compounds than that with Cu and Zn.
Strong adsorption strength was observed for metal centers with
structure M and M−M. The binding energies indicated that
sulfur compounds adsorption on MOF-74 and MOF-199 was
based on the chemical interaction.
To investigate the stability and adsorption mechanism of
MOFs during the H2S adsorption process, Liu et al. examined
11 MOF-based sorbents with various metal sites, ligands,
surface areas, and porous structures for H2S capture.152
Reversible physical sorption was observed on Mg-MOF-74,
UiO-66, MIL-101(Cr), Ce-BTC, and ZIF-8 wherein high H2S
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adsorption obtained on the MIL-101(Cr) with open metal
sites and high surface area. Irreversible H2S chemisorption
obtained on HKUST-1 (Cu-BTC), Zn-MOF-74, and MOF-5,
resulted in high H2S adsorption and high H2S/CO2 selectivity.
However, the formation of metal sulfide by reaction with H2S
can damage the structure and limit their future applications.
The same situations were found with UiO-66(NH2) and CuBDC(ted)0.5, although their structures were stable. In the case
of MIL-100(Fe) gel, the oxidation−reduction reaction of
Fe(III) to Fe(II) by H2S produces S8. According to the
breakthrough experiments, UiO-66, Mg-MOF-74, and MIL101(Cr) displayed superior performance on the H2S removal.
Li et al.153 evaluated the desulfurization performance of
MOF-199, also known as Cu-BTC (BTC: 1,3,5-benzenetricarboxylic acid) or HKUST-1, by breakthrough experiments of
the activated sorbent material. It was found that preheating
treatment was efficient to activate the MOF as the breakthrough capacity of MOF-199 for hydrogen sulfide removal
increased by 38% upon activation at 180 °C compared to the
unactivated sorbent. The authors suggested a chemisorption
mechanism for H2S removal based on the strong acid−base
interaction between sulfur atoms in H2S and copper atom of
MOF-199 that could lead to the formation of CuS and collapse
of the MOF structure by releasing carboxylic groups. This
caused the unrecoverability of the MOF after adsorption of
H2S.
The effect of moisture on the adsorption performance of
MOF materials is another important concept however the role
of moisture in desulfurization performance of MOFs remains
unresolved until now. Peterson et al.154 investigated the
performance of Cu-BTC under dry and high humid (80%
relative humidity) conditions and observed no significant
difference between H2S adsorption capacity under dry (3.4
mol kg−1) and wet (3.5 mol kg−1) conditions while Li
groups153 observed a competitive adsorption between H2O
and H2S wherein the lower moisture could enhance the H2S
capacity.
Yang et al. reported H2S adsorption in porous aluminumbased terephthalate MIL-68, containing two kinds of triangular
(6.0−6.4 Å) and hexagonal (16−17 Å) pore channels (Figure
12).155 They found that the MIL-68(Al) sample was not fully
activated under the experimental conditions and only some of
the hydroxyl groups of the MOF were available to interact with
H2S due to the possible blocking of triangular pores by organic
or solvent molecules. The authors further indicated that
functionalizing MIL-68(Al) with amino groups might also
enhance its adsorption performance significantly. In addition,
they observed no destruction of the MIL-68(Al) framework
upon H2S adsorption. The effect of the pore size in a range of
Zr-MOFs (UiO-66, UiO-67, and UiO-68) with different ligand
lengths on the adsorption of H2S was also studied using
molecular simulations.156 Detailed analysis of the distribution
of molecules in the cages and the radial distribution functions
(RDFs) showed that the effect of the linker length is negligible
at low pressures. However, the effect of the linker size was
obvious at high pressures. At low pressure, H2S molecules were
favorably adsorbed in the tetrahedral cages, with increasing
pressure some molecules started to be populated the
octahedral cages. Extending linkers led to a decrease in the
strength of the interaction between H2S and UiO-MOFs.
Based on the adsorption selection parameter, UiO-67 exhibited
better performance for H2S adsorption from natural gas in a
wider range of conditions than the other materials.
Figure 12. View of the crystalline structure of the MIL-68(M) (M =
V, Ga, Fe, or Al) along the c axis: green and red circles denote the
triangular and hexagonal channels respectively (metal polyhedra pink;
C, gray; O, red, H, white) [Reproduced with permission from ref 155.
Copyright 2012 Royal Society of Chemistry].
In another study, Chavan et al. demonstrated the
reversibility of CPO-Ni-27 (also known as MOF-74-Ni)
upon thermal activation at 473 K for 12 h. Surprisingly, the
adsorption capacity of H2S was increased after desorption
which was probably due to the newly produced active sites by
the thermal activation.157 Medium-strong physisorption
interactions (between Ni atom and sulfur) were also confirmed
by a high adsorption enthalpy of 56−58 kJ mol−1.
Belmabkhout et al.158 explored several fluorinated MOFs for
the removal/adsorption of CO2 and H2S. The developed
materials showed a different range of H2S/CO2 selectivity that
among them, AlFFIVE-1-Ni offered simultaneous and equally
selective removal of H2S and CO2 from a methane-rich stream
in a single adsorption step while SIFSIX-2-Ni-I acted as an
H2S-selective MOF (Figure 13). The authors also found a
100% regenerating efficiency for AlFFIVE-1-Ni under very
mild conditions (reactivation at 105 °C).
In another study, Belmabkhout et al.159 examined In3+-,
Fe3+-, Ga3+-, and Al3+-based soc-MOFs for the production of
high-quality hydrocarbons from H2S-containing gas streams
(Figure 14). The authors noticed that only Ga3+- and Al3+based soc-MOFs could preserve their structure under
interaction with H2S. The breakthrough measurements over
the Ga-soc-MOF sorbent and a ternary mixture of 5% CO2/5%
H2S/90% CH4 showed an H2S breakthrough time of 40 min
versus CH4 and CO2 breakthrough times of nearly 0 and 5
min, respectively, reflecting very high H2S selectivity of the
adsorbent over CH4 and CO2.
Heymans et al. investigated the removal performance of pure
H2S, CO2, and CH4 over MIL-53(Al) and compared the
adsorption and selectivity among these gases by both
experimental and computational studies.160 They obtained a
high H2S uptake compared with CO2 and CH4, highlighting
the good preference of MIL-53(Al) for H2S removal.
Furthermore, they exhibited that the adsorbent was completely
recoverable under relatively mild conditions (473 K).
6.1. Modification with Graphene Oxide. In order to
improve the sorption performance of MOF materials toward
H2S, they have been modified with various materials such as
graphene oxide (GO), metal oxides, etc. (see Table 8).
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adsorption as the main adsorption mechanisms. H2S molecules
could strongly bind to the unsaturated copper centers of the
MOF resulting in the formation of CuS and the collapse of the
MOF structure. In addition, water actually enhanced the H2S
adsorption in the composites as it assisted the dissolution of
H2S.
Enhanced capacities and kinetics of H2S sorption over a
composite of HKUST-1 and GO (50 wt%) was also confirmed
by Pokhrel et al.163 The authors indicated that the GO was not
contributing to the H2S adsorption and the observed capacity
of the GO/HKUST-1 composite was due to the presence of
the well-dispersed MOF crystals. They also proposed a
physisorption mechanism based on the interaction between
unsaturated copper sites in the HKUST-1 and H2S molecules
since the temperature favored the kinetics of H2S sorption but
reduced the saturation capacity. The stability of HKUST-1 and
GO/HKUST-1 composite under a moist condition showed
that the materials slowly degraded upon humidity since the
unsaturated Cu atoms in MOF frameworks with coordination
numbers of four could chemisorb water molecules.
Ebrahim et al.164 observed better H2S adsorption performance over Cu-BTC composites with S- and N-doped graphite
oxides compared to the parent MOFs due to the introduction
of polar reactive groups and formation of new microporosity
by linkages between the sulfur or nitrogen groups of modified
GO and copper centers of Cu-BTC. The synergetic effect of
the modified graphene phase resulted in high H2S breakthrough capacities of 241 ± 6 and 125 ± 2.17 mg g−1 for Sdoped-GO/Cu-BTC and N-doped-GO/Cu-BTC composites
respectively under moist conditions. The authors suggested
that, under moist conditions, water prevented the direct attack
of H2S on the copper centers of the framework by dissolving
the H2S molecules, resulted in higher sulfur adsorption
capacity.
In another study, Huang et al. prepared composites of zincbased MOF (MOF-5) and GO in the presence of glucose and
achieved higher H2S adsorption by GO loading up to 5.25%
Figure 13. Correlation between pore volume and H2S/CO2 selectivity
of fluorinated MOFs. (a) Structure of SIFSIX-2-Ni-i (brick red
polyhedra represent SiF62-pillar). (b) Structure of SIFSIX-3-Ni (brick
red polyhedra represent SiF62-pillar). (c) Structure of NbOFFIVE-1Ni (blue polyhedra represent NbOF52-pillar). (d) Structure of
AlFFIVE-1-Ni (yellow polyhedra represent AlF52-pillar). The optimal
structural features allow it to have H2S/CO2 selectivity close to 1, as
indicated by the dashed line. Unless otherwise mentioned, all atoms
follow the CPK coloring scheme [Reproduced with permission from
ref 158. Copyright 2018 Springer Nature].
Graphene oxide is one of the most used materials to prepare
composite with MOFs that can increase their H2S sorptive
performance. Petit et al. synthesized HKUST-1 composites
with GO (5 to 46 wt % graphite oxide) and found a synergistic
effect on H2S adsorption capacity for GO/MOF composites.161,162 The highest H2S breakthrough capacity of 199 mg
g−1 was obtained by composite with 5 wt % of GO while the
capacities of GO and HKUST-1 were 9 and 92 mg g−1,
respectively. The enhanced adsorption capacity of composites
was attributed to the formation of new small pores in the
structures. They suggested the physisorption and reactive
Figure 14. Structure of the soc-MOF: (left-top) polyhedral representation of the μ3-oxygen-centered trinuclear metal carboxylate clusters
([M3O(O2C−)6], where M = In3+, Fe3+, Ga3+, and Al3+, which can be viewed as a 6-connected node having a trigonal-prismatic geometry); (leftbottom) representation of the organic ligand (ABTC, which is shown as a 4-connected rectangular-planar geometry); crystal structure of the
cuboidal cage-type soc-MOF (M, plum; C, gray; N, light blue; and O, red; the cavity space is indicated by yellow vdW spheres; hydrogen atoms,
Cl− and NO3− ions are omitted for clarity) [Reproduced with permission from ref 159. Copyright 2017 Royal Society of Chemistry].
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Table 8. Textural Parameters and H2S Adsorption Capacities of Modified MOFsa
material
GO(5%)/ HKUST
GO(5%)/ HKUST
S doped GO/ CuBTC
N doped GO/ CuBTC
glucose-GO
(5.25%)/
MIL-125-NH2(Ti)
MIL-101(Cr)HNO3-1
AC (2%)/MOF199
Cu/UiO-167
SBET
(m2 g−1)
Vt
(cm3 g−1)
Vmic
(cm3 g−1)
Vmeso
(cm3 g−1)
989
989
1419
0.515
0.515
0.682
0.478
0.478
0.581
0.037
0.037
0.101
feed gas composition
1000 ppm H2S, moist air
1000 ppm H2S, air
1000 ppm H2S, air
T
(°C)
breakthrough capacity
(mg g−1)
ref
25
25
25
199
199 (wet), 100 (dry)
241 (wet), 133 (dry)
161
162
164
1722
125 (wet), 109 (dry)
0.35
100 ppmv H2S, N2 balance
25
130.1
165
20
19
166
3841
1.72
1.42
0.3
1% H2S, 99% CH4 and 1% H2S, 10% CO2,
89% CH4
H2S, He balance
25
27.16
167
1415
0.66
0.54
0.12
500 ppm H2S, 9% H2O
25
84.6
168
1000 ppm H2S, 70% H2O, air
20
78
169
1062
1612
a
SBET: specific surface area. Vt: total pore volumes. Vmeso: mesoporous volumes. Vmic: microporous volumes.
Figure 15. XRD patterns for MOF-199 and the composite before and after exposure to H2S (a) and FT-IR patterns for MOF-199 and the
composite before and after exposure to H2S (b) [Reproduced with permission from ref 168. Copyright 2017 Elsevier].
with the maximum capacity of 130.1 mg g−1.165 Although GO
loading enhanced the dispersive force in the porous structure
and increased available Zn sites, beyond the optimal loading of
5.25% it caused the collapse of the MOF-5 structure. The
authors suggested that glucose loading can maintain structural
stability and prevent distortion. The removal mechanism was
based on the bonding of H2S to the Zn sites of the MOF-5,
which led to the formation of ZnS.
6.2. Modification with Other Materials. Joshi et al.166
compared the H2S removal efficiency of parent and aminefunctionalized of different MOFs, UiO-66(Zr), MIL-125(Ti),
and MIL-101(Cr), using H2S/CH4 and H2S/CO2/CH4 gas
mixtures. The MIL-101(Cr) exhibited a partial degradation
after H2S exposure, while other frameworks retained their
structures. It was found that amine functionalities increased
H2S adsorption capacities. In the gas mixture containing CO2,
mesoporous MIL-101(Cr), and MIL-101-NH2(Cr) showed
higher selectivity toward H2S as compared to microporous
UiO-66(Zr) and MIL-125 (Ti) frameworks. The experiments
suggested that hydroxyl and amine functionalities of MOFs
acted as H2S physisorption sites. However further investigation
in the presence of other common natural gas species such as
H2O was required.
Sheikh Alivand et al.167 prepared a series of MIL-101 (Cr)
MOFs in the presence of nitric acid as a less hazardous
alternative for hydrofluoric acid and then tested the resulted
MIL-101-HNO3 nanoadsorbents for H2S removal over a wide
range of pressure (1−35 bar) and different temperatures (273,
283, and 293 K). They obtained high H2S uptake of 8.80 mmol
g−1 at 1 bar and 27.16 mmol g−1 at 35 bar (298 K) for MIL101-HNO3 which were much higher than the 13X and 4A
zeolites. In addition, MIL-101-HNO3 exhibited higher H2S/
CH4 selectivity (12.56 at 1 bar and 61.06 at 35 bar) compared
to the conventional MIL-101-HF. The adsorbent showed good
recyclability (93.6% of initial capacity after the fourth cycle) as
well as thermal and water stability.
Fan et al. tried to improve the ability of a Cu-based metal−
organic framework, MOF-199, for the H2S removal by
incorporating MOF with AC.168 They fabricated different
MOF/AC compositions with the AC content of 1, 2, and 3.5%
of the weight of the Cu (NO3)2·3H2O (as the precursor of
MOF-199) and observed the highest sulfur capacity of 8.46%
upon the composite with 2% AC incorporation (MAC-2)
which was increased by 51% compared to that of parent MOF199. They found that this enhancement was attributed to the
formation of more micropores, the enhanced surface area and
the increased number of unsaturated copper metal sites. To
further understand the adsorption mechanisms, they analyzed
the MOF-199 and composite by XRD and FT-IR before and
after exposure to the H2S. According to the obtained XRD
patterns (Figure 15), no diffraction peak could be found after
H2S adsorption for both of them, confirming the collapse of
the MOF structure due to the chemical reaction between H2S
and Cu ions (Cu-S formation) during the adsorption. The FT22157
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Figure 16. Postsynthetic insertion of metal salts into UiO-67(bipy) by adsorption from the liquid phase. Green octahedra represent the
[Zr6O4(OH)4]12+ cluster; gray, red, and blue spheres represent carbon, oxygen, and nitrogen atoms, respectively. The orange spheres represent the
respective metal complex [Reproduced with permission from ref 169. Copyright 2014 Royal Society of Chemistry].
Figure 17. Properties of different porous materials compiled in this review for H2S removal.
comparative study over postsynthetic functionalized of the
UiO-67(bipy) (bipy: 2,2′-bipyridine-5,5′-dicarboxylic acid)
with different metal salts (Cu2+, Ni2+, Co2+, counter onions
Cl−, NO3−, SO42−, acac2−) (Figure 16). Due to the highly
dispersed metal ions, the developed MOFs exhibited good H2S
adsorption performance wherein the structure loaded with
Cu2+ had the highest adsorption capacity of 7.8 wt % in the
presence of moisture. They showed that the H2S adsorption
capacity also depended on the counteranions. For instance, the
IR results also confirmed obvious changes of the material after
being subjected to the H2S (Figure 14). The appearance of two
new bands at 1700 and 1278 cm−1 associated with the released
BTC ligands (not coordinated to copper) and another new
band at 690 cm−1 attributed to the Cu−S vibration confirmed
the serious destruction of the MOF structure.
The introduction of additional metal ions into the MOF
structures can be an effective way of enhancing the H2S
adsorption capacity. Nickerl et al.169 explored this behavior in a
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surface chemistry (oxygen-containing functionalities), or from
experimental conditions such as the gas composition, moisture
content, and operating temperature and pressure. It has been
shown that functionalization of the surface of these porous
materials with organic or inorganic functional groups permits
the tuning of the physical and chemical surface properties and
alteration of the surface reactivity. Amine grafting, heteroatom
(nitrogen) doping, alkaline impregnation, and metal oxide
incorporation are most applied approaches for the surface
functionalization of the porous structures reviewed in this
paper. Amine functionalities, nitrogen moieties, and alkaline
active phases provided require basicity for better dissociation
of H2S to HS− and S2−, resulting in higher adsorption
capacities; however, controlling the loading content of these
functionalities and also obtaining a good H2S-selectivity over
CO2 are major challenges in this area. Loading metal oxides or
mixed metal oxides into porous structures enhanced the H2S
adsorption capacity even at low (room) temperatures due to
the strong metal−sulfur bond formation. However, the limited
loading amount of metal active phases is usually a major
problem as the excessive loading of metal oxide would result in
pore blocking or particles aggregation and a lower desulfurization performance. So, the development of more efficiently
porous support materials containing large voids or channels to
load active sites as high as possible without losing the
adsorbent porosity is crucial to reach a better gas−solid
reaction and avoid the resistance against the H2S diffusion.
Compared with other porous structures, the number of
experimental studies which are devoted to H2S removal by
MOFs is still low in the literature. So, we expect that a large
number of applications based on MOFs or MOF-modified
materials will be described for H2S removal in the near future,
due to the almost endless possibilities to build MOFs and their
further postsynthetic modification.
According to the experimental results, during the desulfurization process, breakage of the sorbent skeleton and pore
collapse of some porous structures such as MOFs are often
unavoidable which would lead to the poor regenerability of
these structures and loss of the adsorption capacity. Therefore,
another significant challenge is developing more stable and
recoverable porous structures for providing long-term stability
and lowering the operational costs in industrial applications.
So, we envision that more studies in the future would be
devoted to the commercialization of the porous sorbents. It
should be noted that, in our opinion, there is a major problem
in finding a porous sorbent that offers all desired features
including a high H2S adsorption capacity, significant selectivity,
and full regeneration capabilities and considering the fact that,
in most cases, promising results can be obtained with materials
which are not commercially available.
adsorption capacity of copper (II) sulfate@UiO-67(bipy) was
found to be 9 mg g−1, whereas the uptake value for nitrate was
38 mg g−1. On the other hand, a framework collapse was
observed for metal ions loaded with chloride anions after
adsorbing H2S.
Many studies have reported that the amino functionalization
is beneficial for H2S adsorption and the improvement of the
adsorption performance of sorbent materials. The effect of
amine functionalization on the adsorption performance of H2S
by MOF-199 was investigated in an experimental and
simulation study by Zhang et al.170 They modified MOF-199
with amine groups by direct grafting of them on metal centers.
The experimental results exhibited that tertiary amine
triethanolamine (TEA) functionalized-MOF-199 had a higher
H2S adsorption capacity (2.74 mmol g−1) than that of parent
MOF-199 (1.67 mmol g−1). Whereas, primary amine and
secondary amine modification of MOF-199 led to the decrease
of the H2S adsorption capacity due to the strong interaction
between them and consequently the destruction of the MOF
structure. The DFT calculation revealed that the binding
energies of H2S adsorption on the TEA/MOF-199 were larger
than that of the bare MOF-199, which was responsible for the
improved adsorption. The improved binding energy of H2S on
the TEA/MOF-199 was attributed to the hydroxyl in TEA.
H2S can interact with the hydroxyl of TEA to form a strong
OH···S bond. In addition, H-atom of H2S could also interact
with O-atom of parent MOF-199 or the nitrogen of TEA. In
fact, H2S molecule was subjected to multiple adsorption
interactions, such as simultaneous interactions by the MOF199 copper center and oxygen, hydrogen, carbon, and nitrogen
atoms of TEA molecule, contributing to the binding energies.
In another study by Xu et al.,171 the grand canonical Monte
Carlo (GCMC) simulation calculations showed that halogen
(F, Cl, Br) functionalization of MIL-47(V) MOFs could
enhance the H2S adsorption especially in low pressures with
the order of MIL-47(V)-Br > MIL-47(V)-Cl > MIL-47(V)-F >
MIL- 47(V) due to the generation of a new stronger halogen
binding sites for H2S adsorption. All of four halogenated MIL47(V) MOFs exhibited high H2S selectivity for both H2S /CH4
and H2S /N4 gas mixtures under conditions of low temperature, high pressure, and high H2S mole fraction.
7. SUMMARY AND PERSPECTIVES
Considering the toxic and corrosive nature of hydrogen sulfide,
a need for efficient removal processes is evident. Development
of novel and highly porous adsorbent materials with unique
textural properties (high specific surface area, large pore
volume) and surface chemistry makes adsorption processes
promising methods for removal of H2S. This review briefly
summarized already reported experimental studies in the
literature for H2S adsorptive removal by different porous
materials such as porous metal oxides, PCs, ACs, mesoporous
silicas, zeolites, and MOFs. The overall results of this review
indicate that a proper combination of surface chemistry and
porosity offers a promising performance for H2S removal by
porous materials based on physisorption or chemisorption
mechanisms. It seems that some structures such as porous
metal oxides, MOFs, and PCs can provide higher H2S
adsorption capacities compared with other porous structures
(Figure 17). However, it is difficult to determine a common
trend for H2S removal by these porous adsorbents since H2S
adsorption is strongly affected by many factors that arise from
adsorbent properties such as surface area, pore size, pH, and
■
AUTHOR INFORMATION
Corresponding Author
*Email: anbia@iust.ac.ir. Tel.: 0098 21 77240516. Fax: 0098
21 77491204.
ORCID
Mansoor Anbia: 0000-0002-0180-5244
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors are grateful to Research Council of Iran University
of Science and Technology for this support.
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