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* Downloaded via UNIV OF SOUTHERN QUEENSLAND on July 18, 2020 at 08:57:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 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: 22133 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 22134 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 22135 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 Review 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− 22137 (9) DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 22138 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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. 22139 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 materials compared with non-nitrogen doped porous car22140 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22141 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 → CO2 (ads) (18) CO (ads) → 2CO* (19) CO* + 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. 22142 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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, CO, 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 2NHSHNH 2R (23) 22143 H 2S + 2R 2NH 2 ↔ R 2HNHSHNHR 2 (24) H 2S + 2R3H ↔ R3NHSHNR3 (25) DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 2NHSHOH 2 (26) H 2S + R 2NH + H 2O ↔ R 2HNHSHOH 2 (27) H 2S + R3N + H 2O ↔ R3NHSHOH 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 22144 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 22145 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22146 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review 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 22147 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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. 22148 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22149 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22150 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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. 22151 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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− 22152 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22153 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 (VO···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 22154 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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). 22155 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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]. 22156 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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 22158 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research 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. 22159 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research ■ (19) Westmoreland, P. R.; Harrison, D. P. Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases. Environ. Sci. Technol. 1976, 10 (7), 659−661. (20) Yumura, M.; Furimsky, E. Comparison of Calcium Oxide, Zinc Oxide, and iron (III) Oxide Hydrogen Sulfide Adsorbents at High Temperatures. Ind. Eng. Chem. Process Des. Dev. 1985, 24 (4), 1165− 1168. (21) Abbasian, J.; Slimane, R. B. A Regenerable Copper-Based Sorbent for H2S Removal from Coal Gases. Ind. Eng. Chem. Res. 1998, 37 (7), 2775−2782. (22) Wang, J.; Liang, B.; Parnas, R. Manganese-Based Regenerable Sorbents for High Temperature H2S Removal. Fuel 2013, 107, 539− 546. (23) Yasyerli, S. Cerium−manganese Mixed Oxides for High Temperature H2S Removal and Activity Comparisons with V−Mn, Zn−Mn, Fe−Mn Sorbents. Chem. Eng. Process. 2008, 47 (4), 577− 584. (24) Liu, X.; Meng, X.; Zhao, J. Synthesis of Nanocrystalline Iron Oxides with Mesostructure as Desulfurizer. Mater. Lett. 2013, 92, 255−258. (25) Su, Y.-M.; Huang, C.-Y.; Chyou, Y.-P.; Svoboda, K. Sulfidation/ regeneration Multi-Cyclic Testing of Fe2O3/Al2O3 Sorbents for the High-Temperature Removal of Hydrogen Sulfide. J. Taiwan Inst. Chem. Eng. 2017, 74, 89−95. (26) Cimino, S.; Lisi, L.; de Falco, G.; Montagnaro, F.; Balsamo, M.; Erto, A. Highlighting the Effect of the Support during H2S Adsorption at Low Temperature over Composite Zn-Cu Sorbents. Fuel 2018, 221 (December 2017), 374−379. (27) Daneshyar, A.; Ghaedi, M.; Sabzehmeidani, M. M.; Daneshyar, A. H2S Adsorption onto Cu-Zn−Ni Nanoparticles Loaded Activated Carbon and Ni-Co Nanoparticles Loaded γ-Al2O3: Optimization and Adsorption Isotherms. J. Colloid Interface Sci. 2017, 490, 553−561. (28) Tran, D. T. Synthesis of Porous ZnO Based Materials Using an Agarose Gel Template for H2S Desulfurization. RSC Adv. 2016, 6 (2), 1339−1345. (29) Pahalagedara, L. R.; Poyraz, A. S.; Song, W.; Kuo, C. H.; Pahalagedara, M. N.; Meng, Y. T.; Suib, S. L. Low Temperature Desulfurization of H2S: High Sorption Capacities by Mesoporous Cobalt Oxide via Increased H2S Diffusion. Chem. Mater. 2014, 26 (22), 6613−6621. (30) Fan, H. L.; Sun, T.; Zhao, Y. P.; Shangguan, J.; Lin, J. Y. ThreeDimensionally Ordered Macroporous Iron Oxide for Removal of H2S at Medium Temperatures. Environ. Sci. Technol. 2013, 47 (9), 4859− 4865. (31) Wang, J.; Yang, C.; Zhao, Y.-R.; Fan, H.-L.; Wang, Z.-D.; Shangguan, J.; Mi, J. Synthesis of Porous Cobalt Oxide and Its Performance for H2S Removal at Room Temperature. Ind. Eng. Chem. Res. 2017, 56 (44), 12621−12629. (32) Huang, G.; He, E.; Wang, Z.; Fan, H.; Shangguan, J.; Croiset, E.; Chen, Z. Synthesis and Characterization of γ-Fe2O3 for H2S Removal at Low Temperature. Ind. Eng. Chem. Res. 2015, 54 (34), 8469−8478. (33) Wang, L.-J.; Fan, H.-L.; Shangguan, J.; Croiset, E.; Chen, Z.; Wang, H.; Mi, J. Design of a Sorbent to Enhance Reactive Adsorption of Hydrogen Sulfide. ACS Appl. Mater. Interfaces 2014, 6 (23), 21167−21177. (34) Wang, J.; Wang, L.; Fan, H.; Wang, H.; Hu, Y.; Wang, Z. Highly Porous Copper Oxide Sorbent for H2S Capture at Ambient Temperature. Fuel 2017, 209 (April), 329−338. (35) Li, L.; Zhang, H.; Zhou, P.; Meng, X.; Liu, L.; Jia, J.; Sun, T. Three Dimensional Ordered Macroporous Zinc Ferrite Composited Silica Sorbents with Promotional Desulfurization and Regeneration Activity at Mid-High Temperature. Appl. Surf. Sci. 2019, 470 (October 2018), 177−186. (36) Liu, Y.; Pan, Y.; Wang, H.; Liu, Y.; Liu, C. Ordered Mesoporous Cu-ZnO-Al2O3 Adsorbents for Reactive Adsorption Desulfurization with Enhanced Sulfur Saturation Capacity. Chinese J. Catal. 2018, 39 (9), 1543−1551. ACKNOWLEDGMENTS The authors are grateful to Research Council of Iran University of Science and Technology for this support. ■ REFERENCES (1) De Falco, G.; Montagnaro, F.; Balsamo, M.; Erto, A.; Deorsola, F. A.; Lisi, L.; Cimino, S. Synergic Effect of Zn and Cu Oxides Dispersed on Activated Carbon during Reactive Adsorption of H2S at Room Temperature. Microporous Mesoporous Mater. 2018, 257, 135− 146. (2) Chen, Y. J.; Gao, X. M.; Di, X. P.; Ouyang, Q. Y.; Gao, P.; Qi, L. H.; Li, C. Y.; Zhu, C. L. Porous Iron Molybdate Nanorods: In Situ Diffusion Synthesis and Low-Temperature H2S Gas Sensing. ACS Appl. Mater. Interfaces 2013, 5 (8), 3267−3274. (3) Meng, F. N.; Di, X. P.; Dong, H. W.; Zhang, Y.; Zhu, C. L.; Li, C.; Chen, Y. J. Ppb H2S Gas Sensing Characteristics of Cu2O/CuO Sub-Microspheres at Low-Temperature. Sens. Actuators, B 2013, 182, 197−204. (4) Carpenter, T. S.; Rosolina, S. M.; Xue, Z. L. Quantitative, Colorimetric Paper Probe for Hydrogen Sulfide Gas. Sens. Actuators, B 2017, 253, 846−851. (5) More, P. S.; Raut, R. W.; Ghuge, C. S. Room Temperature H2S Gas Sensing Characteristics of Platinum (Pt) Coated Porous Alumina (PoAl) Thick Films. Mater. Chem. Phys. 2014, 143 (3), 1278−1281. (6) Shi, J.; Cheng, Z.; Gao, L.; Zhang, Y.; Xu, J.; Zhao, H. Facile Synthesis of Reduced Graphene Oxide/hexagonal WO3 nanosheets Composites with Enhanced H2S Sensing Properties. Sens. Actuators, B 2016, 230, 736−745. (7) Zhang, Z.; Jiang, W.; Long, D.; Wang, J.; Qiao, W.; Ling, L. A General Silica-Templating Synthesis of Alkaline Mesoporous Carbon Catalysts for Highly Efficient H2S Oxidation at Room Temperature. ACS Appl. Mater. Interfaces 2017, 9 (3), 2477−2484. (8) Bao, J.; Krishnan, G. N.; Jayaweera, P.; Lau, K.; Sanjurjo, A. Effect of Various Coal Contaminants on the Performance of Solid Oxide Fuel Cells: Part II. J. Power Sources 2009, 193, 617−624. (9) Lee, S.; Lee, T.; Kim, D. Adsorption of Hydrogen Sulfide from Gas Streams Using the Amorphous Composite of α-FeOOH and Activated Carbon Powder. Ind. Eng. Chem. Res. 2017, 56 (11), 3116− 3122. (10) Nam, S.; Hur, K.; Lee, N. Effects of Hydrogen Sulfide and Siloxane on Landfill Gas Utility Facilities. 2011, 16 (3), 159−164. (11) Crespo, D.; Qi, G.; Wang, Y.; Yang, F. H.; Yang, R. T. Superior Sorbent for Natural Gas Desulfurization. Ind. Eng. Chem. Res. 2008, 47 (4), 1238−1244. (12) Zhang, D.; Jing, X.; Sholl, D. S.; Sinnott, S. B. Molecular Simulation of Capture of Sulfur-Containing Gases by Porous Aromatic Frameworks. J. Phys. Chem. C 2018, 122 (32), 18456− 18467. (13) Vellingiri, K.; Deep, A.; Kim, K. H. Metal-Organic Frameworks as a Potential Platform for Selective Treatment of Gaseous Sulfur Compounds. ACS Appl. Mater. Interfaces 2016, 8 (44), 29835−29857. (14) Shah, M. S.; Tsapatsis, M.; Siepmann, J. I. Hydrogen Sulfide Capture: From Absorption in Polar Liquids to Oxide, Zeolite, and Metal-Organic Framework Adsorbents and Membranes. Chem. Rev. 2017, 117 (14), 9755−9803. (15) Habeeb, O. A.; Kanthasamy, R.; Ali, G. A. M.; Sethupathi, S.; Yunus, R. B. M. Hydrogen Sulfide Emission Sources, Regulations, and Removal Techniques: A Review. Rev. Chem. Eng. 2018, 34 (6), 837− 854. (16) Ozekmekci, M.; Salkic, G.; Fellah, M. F. Use of Zeolites for the Removal of H2S: A Mini-Review. Fuel Process. Technol. 2015, 139, 49−60. (17) Peluso, A.; Gargiulo, N.; Aprea, P.; Pepe, F.; Caputo, D. Nanoporous Materials as H2S Adsorbents for Biogas Purification: A Review. Sep. Purif. Rev. 2019, 48 (1), 78−89. (18) DeCoste, J. B.; Peterson, G. W. Metal−Organic Frameworks for Air Purification of Toxic Chemicals. Chem. Rev. 2014, 114 (11), 5695−5727. 22160 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research KI Activated Carbon Performance toward H2S Removal. Int. J. Hydrogen Energy 2017, 42 (15), 10341−10353. (57) Castrillon, M. C.; Moura, K. O.; Alves, C. A.; Bastos-Neto, M.; Azevedo, D. C. S.; Hofmann, J.; Möllmer, J.; Einicke, W.-D.; Gläser, R. CO2 and H2S Removal from CH4-Rich Streams by Adsorption on Activated Carbons Modified with K2CO3, NaOH, or Fe2 O3. Energy Fuels 2016, 30 (11), 9596−9604. (58) Hui, S. C.; Lee, C. L.; Rahman Mohamed, A.; Teong Lee, K. Hydrogen Sulfide Adsorption by Alkaline Impregnated Coconut Shell Activated Carbon. J. Eng. Sci. Technol. 2013, 8 (86), 741−753. (59) Xiao, Y.; Wang, S.; Wu, D.; Yuan, Q. Experimental and Simulation Study of Hydrogen Sulfide Adsorption on Impregnated Activated Carbon under Anaerobic Conditions. J. Hazard. Mater. 2008, 153 (3), 1193−1200. (60) Li, K.; Ruan, H.; Ning, P.; Wang, C.; Sun, X.; Song, X.; Han, S. Preparation of Walnut Shell-Based Activated Carbon and Its Properties for Simultaneous Removal of H2S, COS and CS2 from Yellow Phosphorus Tail Gas at Low Temperature. Res. Chem. Intermed. 2018, 44 (2), 1209−1233. (61) Menezes, R. L. C. B.; Moura, K. O.; De Lucena, S. M. P.; Azevedo, D. C. S.; Bastos-Neto, M. Insights on the Mechanisms of H2S Retention at Low Concentration on Impregnated Carbons. Ind. Eng. Chem. Res. 2018, 57 (6), 2248−2257. (62) Siriwardane, I. W.; Udangawa, R.; de Silva, R. M.; Kumarasinghe, A. R.; Acres, R. G.; Hettiarachchi, A.; Amaratunga, G. A. J.; de Silva, K. M. N. Synthesis and Characterization of Nano Magnesium Oxide Impregnated Granular Activated Carbon Composite for H2S Removal Applications. Mater. Des. 2017, 136, 127−136. (63) Yang, C.; Yang, S.; Fan, H.; Wang, Y.; Shangguan, J. Tuning the ZnO-Activated Carbon Interaction through Nitrogen Modification for Enhancing the H2S Removal Capacity. J. Colloid Interface Sci. 2019, 555, 548−557. (64) Lau, L. C.; Nor, N. M.; Lee, K. T.; Mohamed, A. R. Hydrogen Sulfide Removal Using CeO2/NaOH/PSAC: Effect of Preparation Parameters. J. Environ. Chem. Eng. 2018, 6 (1), 386−394. (65) Balsamo, M.; Cimino, S.; de Falco, G.; Erto, A.; Lisi, L. ZnOCuO Supported on Activated Carbon for H2S Removal at Room Temperature. Chem. Eng. J. 2016, 304, 399−407. (66) Wang, J.; Ju, F.; Han, L.; Qin, H.; Hu, Y.; Chang, L.; Bao, W. Effect of Activated Carbon Supports on Removing H2S from CoalBased Gases Using Mn-Based Sorbents. Energy Fuels 2015, 29 (2), 488−495. (67) Yang, C.; Wang, J.; Fan, H.; Hu, Y.; Shen, J.; Shangguan, J.; Wang, B. Activated Carbon-Assisted Fabrication of Cost-Efficient ZnO/SiO2 Desulfurizer with Characteristic of High Loadings and High Dispersion. Energy Fuels 2018, 32 (5), 6064−6072. (68) Wu, S.-H.; Mou, C.-Y.; Lin, H.-P. Synthesis of Mesoporous Silica Nanoparticles. Chem. Soc. Rev. 2013, 42 (9), 3862. (69) Alothman, Z. A. A Review: Fundamental Aspects of Silicate Mesoporous Materials. Materials 2012, 5 (12), 2874−2902. (70) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature 1992, 359 (6397), 710− 712. (71) Xue, Q.; Liu, Y. Removal of Minor Concentration of H2S on MDEA-Modified SBA-15 for Gas Purification. J. Ind. Eng. Chem. 2012, 18 (1), 169−173. (72) Wang, X.; Ma, X.; Xu, X.; Sun, L.; Song, C. MesoporousMolecular-Sieve-Supported Polymer Sorbents for Removing H2S from Hydrogen Gas Streams. Top. Catal. 2008, 49 (1−2), 108−117. (73) Wang, X.; Ma, X.; Sun, L.; Song, C. A Nanoporous Polymeric Sorbent for Deep Removal of H2S from Gas Mixtures for Hydrogen Purification. Green Chem. 2007, 9 (6), 695−702. (74) Xu, X.; Novochinskii, I.; Song, C. Low-Temperature Removal of H2S by Nanoporous Composite of Polymer-Mesoporous Molecular Sieve MCM-41 as Adsorbent for Fuel Cell Applications. Energy Fuels 2005, 19 (5), 2214−2215. (37) Long, D.; Chen, Q.; Qiao, W.; Zhan, L.; Liang, X.; Ling, L. Three-Dimensional Mesoporous Carbon Aerogels: Ideal Catalyst Supports for Enhanced H2S Oxidation. Chem. Commun. 2009, No. 26, 3898−3900. (38) Chen, Q.; Wang, J.; Liu, X.; Li, Z.; Qiao, W.; Long, D.; Ling, L. Structure-Dependent Catalytic Oxidation of H2S over Na2CO3 impregnated Carbon Aerogels. Microporous Mesoporous Mater. 2011, 142 (2−3), 641−648. (39) Zhang, Z.; Wang, J.; Li, W.; Wang, M.; Qiao, W.; Long, D.; Ling, L. Millimeter-Sized Mesoporous Carbon Spheres for Highly Efficient Catalytic Oxidation of Hydrogen Sulfide at Room Temperature. Carbon 2016, 96, 608−615. (40) Qi, J.; Wei, G.; Li, Y.; Li, J.; Sun, X.; Shen, J.; Han, W.; Wang, L. Porous Carbon Spheres for Simultaneous Removal of Benzene and H2S. Chem. Eng. J. 2018, 339, 499−508. (41) Yu, Z.; Wang, X.; Song, X.; Liu, Y.; Qiu, J. Molten Salt Synthesis of Nitrogen-Doped Porous Carbons for Hydrogen Sulfide Adsorptive Removal. Carbon 2015, 95, 852−860. (42) Yu, Z.; Wang, X.; Zhou, S.; Yang, L.; Zhao, Z.; Qiu, J. A Facile Soft-Template Synthesis of Nitrogen Doped Mesoporous Carbons for Hydrogen Sulfide Removal. Adsorption 2016, 22 (8), 1075−1082. (43) Sun, F.; Liu, J.; Chen, H.; Zhang, Z.; Qiao, W.; Long, D.; Ling, L. Nitrogen-Rich Mesoporous Carbons: Highly Efficient, Regenerable Metal-Free Catalysts for Low-Temperature Oxidation of H2S. ACS Catal. 2013, 3 (5), 862−870. (44) Yu, Z.; Wang, X.; Hou, Y. N.; Pan, X.; Zhao, Z.; Qiu, J. Nitrogen-Doped Mesoporous Carbon Nanosheets Derived from Metal-Organic Frameworks in a Molten Salt Medium for Efficient Desulfurization. Carbon 2017, 117, 376−382. (45) Chen, Q.; Wang, Z.; Long, D.; Liu, X.; Zhan, L.; Liang, X.; Qiao, W.; Ling, L. Role of Pore Structure of Activated Carbon Fibers in the Catalytic Oxidation of H2S. Ind. Eng. Chem. Res. 2010, 49 (7), 3152−3159. (46) Chen, Q.; Wang, J.; Liu, X.; Zhao, X.; Qiao, W.; Long, D.; Ling, L. Alkaline Carbon Nanotubes as Effective Catalysts for H2S Oxidation. Carbon 2011, 49 (12), 3773−3780. (47) Seredych, M.; Bandosz, T. J. Desulfurization of Digester Gas on Wood-Based Activated Carbons Modified with Nitrogen: Importance of Surface Chemistry. Energy Fuels 2008, 22 (2), 850−859. (48) Shi, L.; Yang, K.; Zhao, Q.; Wang, H.; Cui, Q. Characterization and Mechanisms of H2S and SO2 Adsorption by Activated Carbon. Energy Fuels 2015, 29 (10), 6678−6685. (49) Azargohar, R.; Dalai, A. K. The Direct Oxidation of Hydrogen Sulphide over Activated Carbons Prepared from Lignite Coal and Biochar. Can. J. Chem. Eng. 2011, 89 (4), 844−853. (50) Shen, F.; Liu, J.; Gu, C.; Wu, D. Roles of Oxygen Functional Groups in Hydrogen Sulfide Adsorption on Activated Carbon Surface: A Density Functional Study. Ind. Eng. Chem. Res. 2019, 58 (14), 5526−5532. (51) Shen, F.; Liu, J.; Zhang, Z.; Dong, Y.; Gu, C. Density Functional Study of Hydrogen Sulfide Adsorption Mechanism on Activated Carbon. Fuel Process. Technol. 2018, 171 (November 2017), 258−264. (52) Bandosz, T. J. Effect of Pore Structure and Surface Chemistry of Virgin Activated Carbons on Removal of Hydrogen Sulfide. Carbon 1999, 37 (3), 483−491. (53) Kante, K.; Nieto-Delgado, C.; Rangel-Mendez, J. R.; Bandosz, T. J. Spent Coffee-Based Activated Carbon: Specific Surface Features and Their Importance for H2S Separation Process. J. Hazard. Mater. 2012, 201−202, 141−147. (54) Seredych, M.; Bandosz, T. J. Role of Microporosity and Nitrogen Functionality on the Surface of Activated Carbon in the Process of Desulfurization of Digester Gas. J. Phys. Chem. C 2008, 112 (12), 4704−4711. (55) Köchermann, J.; Schneider, J.; Matthischke, S.; Rönsch, S. Sorptive H2S Removal by Impregnated Activated Carbons for the Production of SNG. Fuel Process. Technol. 2015, 138, 37−41. (56) Barelli, L.; Bidini, G.; de Arespacochaga, N.; Pérez, L.; Sisani, E. Biogas Use in High Temperature Fuel Cells: Enhancement of KOH22161 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research (75) Ma, X.; Wang, X.; Song, C. “Molecular Basket” Sorbents for Separation of CO2 and H2S from Various Gas Streams. J. Am. Chem. Soc. 2009, 131 (16), 5777−5783. (76) Okonkwo, C. N.; Okolie, C.; Sujan, A.; Zhu, G.; Jones, C. W. Role of Amine Structure on Hydrogen Sulfide Capture from Dilute Gas Streams Using Solid Adsorbents. Energy Fuels 2018, 32 (6), 6926−6933. (77) Belmabkhout, Y.; De Weireld, G.; Sayari, A. Amine-Bearing Mesoporous Silica for CO2 and H2S Removal from Natural Gas and Biogas. Langmuir 2009, 25 (23), 13275−13278. (78) Chu, X.; Cheng, Z.; Zhao, Y.; Xu, J.; Zhong, H.; Zhang, W.; Lü, J.; Zhou, S.; Zhu, F.; Zhou, Y.; et al. Study on Sorption Behaviors of H2S by Triethanolamine-Modified Mesoporous Molecular Sieve SBA15. Ind. Eng. Chem. Res. 2012, 51 (11), 4407−4413. (79) Abdouss, M.; Hazrati, N.; Miran Beigi, A. A.; Vahid, A.; Mohammadalizadeh, A. Effect of the Structure of the Support and the Aminosilane Type on the Adsorption of H2S from Model Gas. RSC Adv. 2014, 4 (12), 6337−6345. (80) Anbia, M.; Babaei, M. Novel Amine Modified Nanoporous SBA-15 Sorbent for the Removal of H2S from Gas Streams in the Presence of CH4. Int. J. Eng. 2014, 27 (11), 1697−1704. (81) Zhang, J.; Song, H.; Chen, Y.; Hao, T.; Li, F.; Yuan, D.; Wang, X.; Zhao, L.; Gao, J. Amino-Modified Molecular Sieves for Adsorptive Removal of H2S from Natural Gas. RSC Adv. 2018, 8 (66), 38124− 38130. (82) Wu, M.; Jia, L.; Fan, H.; Mi, J. Hot Coal Gas Desulfurization Using Regenerable ZnO/MCM41 Prepared via One-Step Hydrothermal Synthesis. Energy Fuels 2017, 31 (9), 9814−9823. (83) Wang, X.; Jia, J.; Zhao, L.; Sun, T. Chemisorption of Hydrogen Sulphide on Zinc Oxide Modified Aluminum-Substituted SBA-15. Appl. Surf. Sci. 2008, 254 (17), 5445−5451. (84) Wang, X.; Sun, T.; Yang, J.; Zhao, L.; Jia, J. Low-Temperature H2S Removal from Gas Streams with SBA-15 Supported ZnO Nanoparticles. Chem. Eng. J. 2008, 142 (1), 48−55. (85) Wang, X. H. Chemical Characterization of Mesoporous Material Supported ZnO Nanoparticles for Hydrogen Sulfide Capture from Gas Streams. Adv. Mater. Res. 2010, 129−131, 143−148. (86) Hussain, M.; Abbas, N.; Fino, D.; Russo, N. Novel Mesoporous Silica Supported ZnO Adsorbents for the Desulphurization of Biogas at Low Temperatures. Chem. Eng. J. 2012, 188, 222−232. (87) Geng, Q.; Wang, L. J.; Yang, C.; Zhang, H. Y.; Zhao, Y. R.; Fan, H. L.; Huo, C. Room-Temperature Hydrogen Sulfide Removal with Zinc Oxide Nanoparticle/molecular Sieve Prepared by Melt Infiltration. Fuel Process. Technol. 2019, 185 (September2018), 26− 37. (88) Li, L.; Sun, T. H.; Shu, C. H.; Zhang, H. B. Low Temperature H2S Removal with 3-D Structural Mesoporous Molecular Sieves Supported ZnO from Gas Stream. J. Hazard. Mater. 2016, 311, 142− 150. (89) Huang, Z. B.; Liu, B. S.; Wang, X. H.; Tang, X. Y.; Amin, R. Different Preparation Process versus Performance of MnxOy/MCM48 Sorbents for Hot Coal Gas Desulfurization. Ind. Eng. Chem. Res. 2015, 54 (45), 11268−11276. (90) Zhang, Z. F.; Liu, B. S.; Wang, F.; Wang, W. S.; Xia, C.; Zheng, S.; Amin, R. Hydrogen Sulfide Removal from Hot Coal Gas by Various Mesoporous Silica Supported Mn2O3 Sorbents. Appl. Surf. Sci. 2014, 313, 961−969. (91) Xia, H.; Liu, B.; Li, Q.; Huang, Z.; Cheung, A. S. High Capacity Mn-Fe-Mo/FSM-16 Sorbents in Hot Coal Gas Desulfurization and Mechanism of Elemental Sulfur Formation. Appl. Catal., B 2017, 200, 552−565. (92) Huang, Z. B.; Liu, B. S.; Wang, F.; Amin, R. Performance of Zn−Fe−Mn/MCM-48 Sorbents for High Temperature H2S Removal and Analysis of Regeneration Process. Appl. Surf. Sci. 2015, 353, 1− 10. (93) Zhang, F. M.; Liu, B. S.; Zhang, Y.; Guo, Y. H.; Wan, Z. Y.; Subhan, F. Highly Stable and Regenerable Mn-based/SBA-15 Sorbents for Desulfurization of Hot Coal Gas. J. Hazard. Mater. 2012, 233−234, 219−227. (94) Zhang, Z. F.; Liu, B. S.; Wang, F.; Li, J. F. Fabrication and Performance of xMnyCe/hexagonal Mesoporous Silica Sorbents with Wormhole-like Framework for Hot Coal Gas Desulfurization. Energy Fuels 2013, 27 (12), 7754−7761. (95) Zhang, Z. F.; Liu, B. S.; Wang, F.; Zheng, S. High-Temperature Desulfurization of Hot Coal Gas on Mo Modified Mn/KIT-1 Sorbents. Chem. Eng. J. 2015, 272, 69−78. (96) Xia, H.; Liu, B. High H2O-Resistance CaO-MnOx/MSU-H Sorbents for Hot Coal Gas Desulfurization. J. Hazard. Mater. 2017, 324, 281−290. (97) Huang, Z. B.; Liu, B. S.; Tang, X. Y.; Wang, X. H.; Amin, R. Performance of Rare Earth Oxide Doped Mn-Based Sorbent on Various Silica Supports for Hot Coal Gas Desulfurization. Fuel 2016, 177 (March), 217−225. (98) Xia, H.; Zhang, F.; Zhang, Z.; Liu, B. Synthesis of Functional xLayMn/KIT-6 and Features in Hot Coal Gas Desulphurization. Phys. Chem. Chem. Phys. 2015, 17 (32), 20667−20676. (99) Karvan, O.; Atakül, H. Investigation of CuO/mesoporous SBA15 Sorbents for Hot Gas Desulfurization. Fuel Process. Technol. 2008, 89 (9), 908−915. (100) Karvan, O.; Sirkecioglu, A.; Atakul, H. Investigation of NanoCuO/mesoporous SiO2 materials as Hot Gas Desulphurization Sorbents. Fuel Process. Technol. 2009, 90 (12), 1452−1458. (101) Montes, D.; Tocuyo, E.; González, E.; Rodríguez, D.; Solano, R.; Atencio, R.; Ramos, M. A.; Moronta, A. Reactive H2S Chemisorption on Mesoporous Silica Molecular Sieve-Supported CuO or ZnO. Microporous Mesoporous Mater. 2013, 168, 111−120. (102) Mureddu, M.; Ferino, I.; Musinu, A.; Ardu, A.; Rombi, E.; Cutrufello, M. G.; Deiana, P.; Fantauzzi, M.; Cannas, C. MeOx/SBA15 (Me = Zn, Fe): Highly Efficient Nanosorbents for MidTemperature H2S Removal. J. Mater. Chem. A 2014, 2 (45), 19396−19406. (103) Wang, X.; Jia, J.; Zhao, L.; Sun, T. Mesoporous SBA-15 Supported Iron Oxide: A Potent Catalyst for Hydrogen Sulfide Removal. Water, Air, Soil Pollut. 2008, 193 (1−4), 247−257. (104) Cara, C.; Rombi, E.; Mameli, V.; Ardu, A.; Sanna Angotzi, M.; Niznansky, D.; Musinu, A.; Cannas, C. γ-Fe2O3 -M41S Sorbents for H2S Removal: Effect of Different Porous Structures and Silica Wall Thickness. J. Phys. Chem. C 2018, 122 (23), 12231−12242. (105) Cara, C.; Rombi, E.; Musinu, A.; Mameli, V.; Ardu, A.; Sanna Angotzi, M.; Atzori, L.; Niznansky, D.; Xin, H. L.; Cannas, C. MCM41 Support for Ultrasmall γ-Fe2O3 Nanoparticles for H2S Removal. J. Mater. Chem. A 2017, 5 (41), 21688−21698. (106) Liu, B. S.; Wan, Z. Y.; Zhan, Y. P.; Au, C. T. Desulfurization of Hot Coal Gas over High-Surface-Area LaMeOx/MCM-41 Sorbents. Fuel 2012, 98 (3), 95−102. (107) Liu, B. S.; Wei, X. N.; Zhan, Y. P.; Chang, R. Z.; Subhan, F.; Au, C. T. Preparation and Desulfurization Performance of LaMeOx/ SBA-15 for Hot Coal Gas. Appl. Catal., B 2011, 102 (1−2), 27−36. (108) Koohsaryan, E.; Anbia, M. Nanosized and Hierarchical Zeolites: A Short Review. Chinese J. Catal. 2016, 37 (4), 447−467. (109) Auerbach, S. M.; Carrado, K. A.; Dutta, P. K. Handbook of Zeolite Science and Technology; CRC press, 2003. (110) Kristóf, T. Selective Removal of Hydrogen Sulphide from Industrial Gas Mixtures Using Zeolite NaA. Hung. J. Ind. Chem. 2017, 45 (1), 9−15. (111) Dehghan, R.; Anbia, M. Zeolites for Adsorptive Desulfurization from Fuels: A Review. Fuel Process. Technol. 2017, 167, 99−116. (112) Ackley, M. W.; Rege, S. U.; Saxena, H. Application of Natural Zeolites in the Purification and Separation of Gases. Microporous Mesoporous Mater. 2003, 61 (1−3), 25−42. (113) Alonso-Vicario, A.; Ochoa-Gómez, J. R.; Gil-Río, S.; GómezJiménez-Aberasturi, O.; Ramírez-López, C. A.; Torrecilla-Soria, J.; Domínguez, A. Purification and Upgrading of Biogas by Pressure Swing Adsorption on Synthetic and Natural Zeolites. Microporous Mesoporous Mater. 2010, 134 (1−3), 100−107. (114) Liu, X.; Wang, R. Effective Removal of Hydrogen Sulfide Using 4A Molecular Sieve Zeolite Synthesized from Attapulgite. J. Hazard. Mater. 2017, 326, 157−164. 22162 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research Zeolites Synthesized from Melting Slag. Mater. Trans. 2009, 50 (10), 2476−2483. (134) Gasper-Galvin, L. D.; Atimtay, A. T.; Gupta, R. P. ZeoliteSupported Metal Oxide Sorbents for Hot-Gas Desulfurization. Ind. Eng. Chem. Res. 1998, 37 (10), 4157−4166. (135) Abdullah, A. H.; Mat, R.; Somderam, S.; Abd Aziz, A. S.; Mohamed, A. Hydrogen Sulfide Adsorption by Zinc OxideImpregnated Zeolite (Synthesized from Malaysian Kaolin) for Biogas Desulfurization. J. Ind. Eng. Chem. 2018, 65, 334−342. (136) Liu, C.; Zhang, R.; Wei, S.; Wang, J.; Liu, Y.; Li, M.; Liu, R. Selective Removal of H2S from Biogas Using a Regenerable Hybrid TiO2/zeolite Composite. Fuel 2015, 157, 183−190. (137) Micoli, L.; Bagnasco, G.; Turco, M. H2S Removal from Biogas for Fuelling MCFCs: New Adsorbing Materials. Int. J. Hydrogen Energy 2014, 39 (4), 1783−1787. (138) Rezaei, S.; Tavana, A.; Sawada, J. A.; Wu, L.; Junaid, A. S. M.; Kuznicki, S. M. Novel Copper-Exchanged Titanosilicate Adsorbent for Low Temperature H2S Removal. Ind. Eng. Chem. Res. 2012, 51 (38), 12430−12434. (139) Yazdanbakhsh, F.; Bläsing, M.; Sawada, J. A.; Rezaei, S.; Müller, M.; Baumann, S.; Kuznicki, S. M. Copper Exchanged Nanotitanate for High Temperature H2S Adsorption. Ind. Eng. Chem. Res. 2014, 53 (29), 11734−11739. (140) Rezaei, S.; Jarligo, M. O. D.; Wu, L.; Kuznicki, S. M. Breakthrough Performances of Metal-Exchanged Nanotitanate ETS-2 Adsorbents for Room Temperature Desulfurization. Chem. Eng. Sci. 2015, 123, 444−449. (141) Yazdanbakhsh, F.; Alizadehgiashi, M.; Bläsing, M.; Müller, M.; Sawada, J. A.; Kuznicki, S. M. Cu−Cr−O Functionalized ETS-2 Nanoparticles for Hot Gas Desulfurization. J. Nanosci. Nanotechnol. 2016, 16 (1), 878−884. (142) Roller, D.; Bläsing, M.; Dreger, I.; Yazdanbakhsh, F.; Sawada, J. A.; Kuznicki, S. M.; Müller, M. Removal of Hydrogen Sulfide by Metal-Doped Nanotitanate under Gasification-Like Conditions. Ind. Eng. Chem. Res. 2016, 55 (14), 3871−3878. (143) Rowsell, J. L. C.; Yaghi, O. M. Metal-Organic Frameworks: A New Class of Porous Materials. Microporous Mesoporous Mater. 2004, 73 (1−2), 3−14. (144) Anbia, M.; Hoseini, V.; Sheykhi, S. Sorption of Methane, Hydrogen and Carbon Dioxide on Metal-Organic Framework, Iron Terephthalate (MOF-235). J. Ind. Eng. Chem. 2012, 18 (3), 1149− 1152. (145) Anbia, M.; Sheykhi, S. Preparation of Multi-Walled Carbon Nanotube Incorporated MIL-53-Cu Composite Metal-Organic Framework with Enhanced Methane Sorption. J. Ind. Eng. Chem. 2013, 19 (5), 1583−1586. (146) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C. E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. O.; Hupp, J. T. Metal−Organic Framework Materials with Ultrahigh Surface Areas: Is the Sky the Limit? J. Am. Chem. Soc. 2012, 134 (36), 15016−15021. (147) Babaei, M.; Salehi, S.; Anbia, M.; Kazemipour, M. Improving CO2 Adsorption Capacity and CO2/CH4 Selectivity with Amine Functionalization of MIL-100 and MIL-101. J. Chem. Eng. Data 2018, 63 (5), 1657−1662. (148) Hamon, L.; Serre, C.; Devic, T.; Loiseau, T.; Millange, F.; Ferey, G.; Weireld, G. D. Comparative Study of Hydrogen Sulfide Adsorption in the MIL-53(Al, Cr, Fe), MIL-47(V), MIL-100(Cr), and MIL-101(Cr) Metal-Organic Frameworks at Room Temperature. J. Am. Chem. Soc. 2009, 131 (25), 8775−8777. (149) Hamon, L.; Leclerc, H.; Ghoufi, A.; Oliviero, L.; Travert, A.; Lavalley, J.-C.; Devic, T.; Serre, C.; Ferey, G.; De Weireld, G.; Vimont, A.; Maurin, G.; et al. Molecular Insight into the Adsorption of H2S in the Flexible MIL-53(Cr) and Rigid MIL-47(V) MOFs: Infrared Spectroscopy Combined to Molecular Simulations. J. Phys. Chem. C 2011, 115 (5), 2047−2056. (150) Wang, X. L.; Fan, H. L.; Tian, Z.; He, E. Y.; Li, Y.; Shangguan, J. Adsorptive Removal of Sulfur Compounds Using IRMOF-3 at Ambient Temperature. Appl. Surf. Sci. 2014, 289, 107−113. (115) Melo, D. M. A.; De Souza, J. R.; Melo, M. A. F.; Martinelli, A. E.; Cachima, G. H. B.; Cunha, J. D. Evaluation of the Zinox and Zeolite Materials as Adsorbents to Remove H2S from Natural Gas. Colloids Surf., A 2006, 272 (1−2), 32−36. (116) Bandarchian, F.; Anbia, M. Conventional Hydrothermal Synthesis of Nanoporous Molecular Sieve 13X for Selective Adsorption of Trace Amount of Hydrogen Sulfide from Mixture with Propane. J. Nat. Gas Sci. Eng. 2015, 26, 1380−1387. (117) Suthanyawatchai, N.; Onthong, U. Adsorption of Hydrogen Sulfide, Carbondioxide and Methane by Zeolite (Ferrierite; H-FER): Computational Chemistry Method. Adv. Mater. Res. 2011, 356−360, 707−711. (118) Tomadakis, M. M.; Heck, H. H.; Jubran, M. E.; Al-Harthi, K. Pressure-Swing Adsorption Separation of H2S from CO2 with Molecular Sieves 4A, 5A, and 13X. Sep. Sci. Technol. 2011, 46 (3), 428−433. (119) Paolini, V.; Petracchini, F.; Guerriero, E.; Bencini, A.; Drigo, S. Biogas Cleaning and Upgrading with Natural Zeolites from Tuffs. Environ. Technol. 2016, 37 (11), 1418−1427. (120) Yang, K.; Su, B.; Shi, L.; Wang, H.; Cui, Q. Adsorption Mechanism and Regeneration Performance of 13X for H2S and SO2. Energy Fuels 2018, 32 (12), 12742−12749. (121) Shah, M. S.; Tsapatsis, M.; Siepmann, J. I. Monte Carlo Simulations Probing the Adsorptive Separation of Hydrogen Sulfide/ methane Mixtures Using All-Silica Zeolites. Langmuir 2015, 31 (44), 12268−12278. (122) Shah, M. S.; Tsapatsis, M.; Siepmann, J. I. Identifying Optimal Zeolitic Sorbents for Sweetening of Highly Sour Natural Gas. Angew. Chem., Int. Ed. 2016, 55 (20), 5938−5942. (123) Sigot, L.; Fontseré Obis, M.; Benbelkacem, H.; Germain, P.; Ducom, G. Comparing the Performance of a 13X Zeolite and an Impregnated Activated Carbon for H2S Removal from Biogas to Fuel an SOFC: Influence of Water. Int. J. Hydrogen Energy 2016, 41 (41), 18533−18541. (124) Sigot, L.; Ducom, G.; Germain, P. Adsorption of Hydrogen Sulfide (H2S) on Zeolite (Z): Retention Mechanism. Chem. Eng. J. 2016, 287, 47−53. (125) Kumar, P.; Sung, C.-Y.; Muraza, O.; Cococcioni, M.; Al Hashimi, S.; McCormick, A.; Tsapatsis, M. H2S Adsorption by Ag and Cu Ion Exchanged Faujasites. Microporous Mesoporous Mater. 2011, 146 (1−3), 127−133. (126) Sung, C. Y.; Al Hashimi, S.; McCormick, A.; Tsapatsis, M.; Cococcioni, M. Density Functional Theory Study on the Adsorption of H2S and Other Claus Process Tail Gas Components on Copperand Silver-Exchanged Y Zeolites. J. Phys. Chem. C 2012, 116 (5), 3561−3575. (127) Chen, X.; Shen, B.; Sun, H.; Zhan, G. Ion-Exchange Modified Zeolites X for Selective Adsorption Desulfurization from Claus Tail Gas: Experimental and Computational Investigations. Microporous Mesoporous Mater. 2018, 261, 227−236. (128) Tran, H.-L.; Kuo, M.-S.; Yang, W.-D.; Huang, Y.-C. Hydrogen Sulfide Adsorption by Thermally Treated Cobalt (II)-Exchanged NaX Zeolite. Adsorpt. Sci. Technol. 2016, 34 (4−5), 275−286. (129) Long, N. Q.; Vuong, H. T.; Ha, P.; et al. Preparation, Characterization and H2S Adsorptive Removal of Ion-Exchanged Zeolite X. ASEAN Eng. J. Part B (AEJ: part B). 2015, 5 (1), 4−14. (130) Barelli, L.; Bidini, G.; Micoli, L.; Sisani, E.; Turco, M. 13X ExCu Zeolite Performance Characterization towards H2S Removal for Biogas Use in Molten Carbonate Fuel Cells. Energy 2018, 160, 44− 53. (131) Fellah, M. F. Adsorption of Hydrogen Sulfide as Initial Step of H2S Removal: A DFT Study on Metal Exchanged ZSM-12 Clusters. Fuel Process. Technol. 2016, 144, 191−196. (132) Ma, Y.; Mao, J.; Xiao, C.; Zhao, T.; Zang, L. Zeolite Supported Ionic Liquid Improved by Cyclodextrins for Efficient Removal Capacity of H2S. Chem. Lett. 2019, 48, 234. (133) Lee, S.-K.; Jang, Y.-N.; Bae, I.-K.; Chae, S.-C.; Ryu, K.-W.; Kim, J.-K. Adsorption of Toxic Gases on Iron-Incorporated Na-A 22163 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164 Review Industrial & Engineering Chemistry Research (168) Fan, H. L.; Shi, R. H.; Zhang, Z. R.; Zhen, T.; Shangguan, J.; Mi, J. Cu-Based Metal−organic Framework/activated Carbon Composites for Sulfur Compounds Removal. Appl. Surf. Sci. 2017, 394, 394−402. (169) Nickerl, G.; Leistner, M.; Helten, S.; Bon, V.; Senkovska, I.; Kaskel, S. Integration of Accessible Secondary Metal Sites into MOFs for H2S Removal. Inorg. Chem. Front. 2014, 1 (4), 325−330. (170) Zhang, H.-Y.; Yang, C.; Geng, Q.; Fan, H.-L.; Wang, B.-J.; Wu, M.-M.; Tian, Z. Adsorption of Hydrogen Sulfide by AmineFunctionalized Metal Organic Framework (MOF-199): An Experimental and Simulation Study. Appl. Surf. Sci. 2019, 497, 143815. (171) Xu, J.; Xing, W.; Wang, H.; Xu, W.; Ding, Q.; Zhao, L.; Guo, W.; Yan, Z. Monte Carlo Simulation Study of the Halogenated MIL47(V) Frameworks: Influence of Functionalization on H 2 S Adsorption and Separation Properties. J. Mater. Sci. 2016, 51 (5), 2307−2319. (151) Chen, Z.; Ling, L.; Wang, B.; Fan, H.; Shangguan, J.; Mi, J. Adsorptive Desulfurization with Metal-Organic Frameworks: A Density Functional Theory Investigation. Appl. Surf. Sci. 2016, 387, 483−490. (152) Liu, J.; Wei, Y.; Li, P.; Zhao, Y.; Zou, R. Selective H2S/CO2 Separation by Metal-Organic Frameworks Based on ChemicalPhysical Adsorption. J. Phys. Chem. C 2017, 121 (24), 13249−13255. (153) Li, Y.; Wang, L. J.; Fan, H. L.; Shangguan, J.; Wang, H.; Mi, J. Removal of Sulfur Compounds by a Copper-Based Metal Organic Framework under Ambient Conditions. Energy Fuels 2015, 29 (1), 298−304. (154) Peterson, G. W.; Britt, D. K.; Sun, D. T.; Mahle, J. J.; Browe, M.; Demasky, T.; Smith, S.; Jenkins, A.; Rossin, J. A. Multifunctional Purification and Sensing of Toxic Hydride Gases by CUBTC MetalOrganic Framework. Ind. Eng. Chem. Res. 2015, 54 (14), 3626−3633. (155) Yang, Q.; Vaesen, S.; Vishnuvarthan, M.; Ragon, F.; Serre, C.; Vimont, A.; Daturi, M.; De Weireld, G.; Maurin, G. Probing the Adsorption Performance of the Hybrid Porous MIL-68(Al): A Synergic Combination of Experimental and Modelling Tools. J. Mater. Chem. 2012, 22 (20), 10210. (156) Al-Jadir, T. M.; Siperstein, F. R. The Influence of the Pore Size in Metal-Organic Frameworks in Adsorption and Separation of Hydrogen Sulphide: A Molecular Simulation Study. Microporous Mesoporous Mater. 2018, 271, 160−168. (157) Chavan, S.; Bonino, F.; Valenzano, L.; Civalleri, B.; Lamberti, C.; Acerbi, N.; Cavka, J. H.; Leistner, M.; Bordiga, S. Fundamental Aspects of H2S Adsorption on CPO-27-Ni. J. Phys. Chem. C 2013, 117 (30), 15615−15622. (158) Belmabkhout, Y.; Bhatt, P. M.; Adil, K.; Pillai, R. S.; Cadiau, A.; Shkurenko, A.; Maurin, G.; Liu, G.; Koros, W. J.; Eddaoudi, M. Natural Gas Upgrading Using a Fluorinated MOF with Tuned H2S and CO2 Adsorption Selectivity. Nat. Energy 2018, 3 (12), 1059− 1066. (159) Belmabkhout, Y.; Pillai, R. S.; Alezi, D.; Shekhah, O.; Bhatt, P. M.; Chen, Z.; Adil, K.; Vaesen, S.; De Weireld, G.; Pang, M.; et al. Metal−organic Frameworks to Satisfy Gas Upgrading Demands: FineTuning the Soc -MOF Platform for the Operative Removal of H2S. J. Mater. Chem. A 2017, 5 (7), 3293−3303. (160) Heymans, N.; Vaesen, S.; De Weireld, G. A Complete Procedure for Acidic Gas Separation by Adsorption on MIL-53(Al). Microporous Mesoporous Mater. 2012, 154, 93−99. (161) Petit, C.; Mendoza, B.; Bandosz, T. J. Hydrogen Sulfide Adsorption on MOFs and MOF/Graphite Oxide Composites. ChemPhysChem 2010, 11 (17), 3678−3684. (162) Petit, C.; Levasseur, B.; Mendoza, B.; Bandosz, T. J. Reactive Adsorption of Acidic Gases on MOF/graphite Oxide Composites. Microporous Mesoporous Mater. 2012, 154, 107−112. (163) Pokhrel, J.; Bhoria, N.; Wu, C.; Reddy, K. S. K.; Margetis, H.; Anastasiou, S.; George, G.; Mittal, V.; Romanos, G.; Karonis, D.; et al. Cu- and Zr-Based Metal Organic Frameworks and Their Composites with Graphene Oxide for Capture of Acid Gases at Ambient Temperature. J. Solid State Chem. 2018, 266, 233−243. (164) Ebrahim, A. M.; Jagiello, J.; Bandosz, T. J. Enhanced Reactive Adsorption of H2S on Cu-BTC/S- and N-Doped GO Composites. J. Mater. Chem. A 2015, 3 (15), 8194−8204. (165) Huang, Z.; Liu, G.; Kang, F. Glucose-Promoted Zn-Based Metal−Organic Framework/Graphene Oxide Composites for Hydrogen Sulfide Removal. ACS Appl. Mater. Interfaces 2012, 4 (9), 4942− 4947. (166) Joshi, J. N.; Zhu, G.; Lee, J. J.; Carter, E. A.; Jones, C. W.; Lively, R. P.; Walton, K. S. Probing Metal−Organic Framework Design for Adsorptive Natural Gas Purification. Langmuir 2018, 34 (29), 8443−8450. (167) Sheikh Alivand, M.; Hossein Tehrani, N. H. M.; ShafieiAlavijeh, M.; Rashidi, A.; Kooti, M.; Pourreza, A.; Fakhraie, S. Synthesis of a Modified HF-Free MIL-101(Cr) Nanoadsorbent with Enhanced H2S/CH4, CO2/CH4, and CO2/N2 Selectivity. J. Environ. Chem. Eng. 2019, 7 (2), 102946. 22164 DOI: 10.1021/acs.iecr.9b03800 Ind. Eng. Chem. Res. 2019, 58, 22133−22164