Removal of Hydrogen sulfide (H 2 S) using MOFs: A review of the latest developments

: Removal of hydrogen sulfide (H 2 S) from gas streams with varying overall pressure and H 2 S concentration is a long-standing challenge faced by the oil and gas industries. The present work focuses on H 2 S capture using metal-organic frameworks (MOFs), in an effort to shed light on their potential as adsorbents in the field of gas storage and separation. MOFs hold great promise as they make possible the design of structures from organic and inorganic units but also, they have provided an answer to a long-time challenging objective, i.e., how to design extended structures of materials. Moreover, the functionalization of the MOF’s surface can result in increased H 2 S uptake. For example, the insertion of 1% of a fluorinated linker in MIL-101(Cr)-4F(1%) allows for enhanced H 2 S capture. Although noticeable efforts have been made in studying the adsorption capacity of H 2 S using MOFs, there is a clear need for gaining a deeper understanding in terms of their thermal conductivities and specific heats in order to design more stable adsorption beds, experiencing high exothermicity. Simply put, the exothermic nature of adsorption means that sharp rises in temperature can negatively affect the bed stability in the absence of sufficient heat transfer. The work presented herein provides a detailed discussion, by thoroughly combining the existing literature, on new developments in MOFs for H 2 S removal, and tries to provide insight into new areas for further research.


Introduction
The removal of hydrogen sulfide (H2S), released from different industrial sources is a matter of great importance as it can cause corrosion and environmental damage even at low concentrations.This work thoroughly focuses on H2S capture using a relatively new type of material, namely, Metal Organic Frameworks (MOFs), with a view to shed light on their desulfurization performance (i.e., principally via adsorption) [1].Crystalline MOFs are formed by reticular synthesis, resulting in strong bonds between organic and inorganic units.Proper selection of MOF constituents can lead to crystals of ultrahigh porosity and high chemical, thermal, and mechanical stability.These properties allow the interior of MOFs to be chemically modified for the use in the field of gas storage and separation, among other applications.The ability to expand their metrics without changing the underlying topology and the precision generally exercised in their chemical modulation has not been met in other materials.The work presented herein discusses the existing literature on new developments in MOFs for H2S removal opening new avenues for further research in terms of desulfurization processes [2].
H2S capture using MIL-53(Al), in both powder and pellet form, was also examined by Heymans et al. [5] who carried out a joint experimental/theoretical approach.Their focus was on synchronous removal of H2S and CO2 from biogas streams.Results showed that MOF-53(Al) was fully regenerable at moderate temperatures (200 °C), indicating that no chemisorption took place.It was also reported that the powdered form of MOF-53(Al) exhibited higher desulfurization performance in comparison to that of the pelleted form, probably due to its increased specific surface area (SSA) and pore volume.
MIL-68(Al) was probed at high H2S pressures up to 12 bar at room temperature by Yang et al. [6] using both experimental and theoretical (Grand Canonical Monte Carlo-GCMC) approaches.Given the results obtained, one can conclude tat the triangular pores of this MOF were locked by some remaining organic or solvent molecules because of the incomplete activation of the material.This partially activated sorbent was proven to be fully regenerable for at least five consecutive sulfidation cycles.Notwithstanding, it needs further elucidation whether the MIL-68(Al) can resist corrosiveness following H2S exposure if fully activated.
Vaesen et al. [7] tested the adsorption performance of the amino-functionalized titanium terephthalate MIL−125(Ti)-NH2 towards its parent MIL−125(Ti) analogue, for the simultaneous removal of CO2 and H2S from biogas and natural gas, applying a joint experimental/modeling approach.The pure-component adsorption runs at low temperature (30 °C) and low pressures showed decreased H2S capture for both MOFs.A key finding was that compared to other sorbents, such as 13X zeolites, these materials exhibited lower H2S adsorption enthalpies, thus lower energy footprint for recycling the sorbent.

H2S Capture via HKUST−1 (Hong King University of Science and Technology)
Petit et al. [9] prepared HKUST-1 composites with GO (5 to 46 wt% graphite oxide) and reported a synergistic effect on H2S capture for the hybrid materials.The authors reported that the composite material GO/MOF (5 wt% of GO) outperformed both GO and HKUST-1 sorbents, with H2S adsorption capacity of 199 mg g -1 .The enhanced desulfurization performance of GO/MOF solid was ascribed to the formation of newly formed pores in its structure.H2S molecules were captured through physisorption and reactive adsorption.Nevertheless, it was claimed that HKUST-1 suffers from structural collapse due to H2S molecules that strongly bind to the unsaturated copper centers of the MOF resulting in the formation of CuS.
Pokhrel et al. [10] also studied the H2S adsorption on HKUST-1 and HKUST-1/GO.The authors claimed that GO did not enhance the adsorption of H2S molecules, but it was the presence of well dispersed crystals of the MOF that promoted the H2S uptake.It was also showed that both physical and reactive adsorption took place, due to the unsaturated Cu sites in the MOF structure, which interact with H2S.Howbeit, since physical adsorption predominates, an increasing temperature resulted in favoring kinetics, but reducing H2S uptake.In the presence of water, the stability of both materials presented gradual degradation, indicating that chemisorption occurred.
An interesting theoretical study was carried out by Watanabe et al. [11] who tried to calculate the binding energies of different molecules, namely, H2S, H2O, CO, NO, pyridine, C2H2, and NH3) using HKUST−1.Results showed that H2S exhibited a binding strength of 0.49 eV on Cu dimers, quite close to that of H2O, which demonstrated a large affinity for the metal center of HKUST-1.
In general, the results obtained from the theoretical studies differ from the ones adopted from the experimental results.Modeling suggests that H2S physisorption prevails on HKUST-1, thus failing to explain the conversion of H2S to CuS in the presence of moisture.Contrary to experimental approaches, theoretical studies do not consider the host-guest interactions which are required to explain the CuS formation.
All things considered, experimental studies denote that H2S molecules bind stronger with Cu atoms in the center of HKUST-1, thereby displacing the existing H2O molecules.

H2S Capture via Isoreticular Metal-Organic Frameworks (IRMOF-n)
Isoreticular MOFs (IRMOF-n, where n = 1-16) based on a skeleton of Zn-based MOF were first prepared by Eddaoudi et al. [1] To exemplify the structure of IRMOF−1 (also known as MOF-5) Figure 1 is presented; this solid has a stable cube-like structure with a regular, three dimensional cubic lattice with BDC as edges and Zn4O cluster as vertexes 5.In a theoretical study by Gutiérrez-Sevillano et al. [12], the adsorption of H2S on MOF-5 was examined.Results showed lower heat of adsorption for the material under consideration (~ −15 kJ mol −1 ) compared to that of HKUST-1 (~ −30 kJ mol −1 ) probably because of the wider pores of the MOF-5.Moreover, the energy of adsorption of H2O (~ −22.5 kJ mol −1 ) on MOF-5 was higher than that of H2S (~ −16.7 kJ mol −1 ), suggesting that the presence of moisture negatively affects the H2S uptake.
Another study was carried out by Huang et al. [13], who prepared composites of Zn-based MOF (MOF-5) and GO in the presence of glucose for H2S removal (Figure 2).Results showed that the glucose-promoted Zn-based sample exhibited increased H2S uptake at 5.25% of GO loading, reaching a maximum capacity of 130.1 mg g −1 .Nevertheless, even though the loading of GO enhanced the dispersive force in the porous structure, when GO loading surpassed the optimum value of 5.25%, it led to the crystal distortion of the MOF-5.It was also mentioned that the insertion of glucose can help maintain structural stability and prevent distortion.
Chavan et al. [15] also studied H2S removal (relative pressures, 10 mbar) on Ni-MOF-74 and reported the formation of H2S adducts on almost 80% of the Ni sites.It was also claimed that Ni-MOF-74 had a reversible behavior upon thermal activation at 200 °C for 12 h and that after desorption, there was an increase in H2S adsorption capacity probably because of additional active sites produced by means of heat treatment.
Li et al. [17] performed a theoretical study to probe the adsorption performance of the pristine UiO-66(Zr) and its functionalized derivatives in removing sulfur from binary gas mixtures.UiO-66-(COOH)2 and UiO-66-COOH displayed the highest H2S uptake compared to that of other tested solids probably owing to their higher adsorption isosteric heats.The isosteric heat of adsorption at infinite dilution and radial distribution functions suggest that the hydrophilic groups and polar H2S molecules strongly interact with one another, favoring H2S removal.
Huang et al. [18] studied the antagonistic adsorption between CO2 and H2S by synthesizing core-shell-structure H2S imprinted polymers (PMo12@UiO-66@H2S-MIPs) based on the surface of UiO-66 modified by phosphomolybdic acid hydrate.At the outset, it was mentioned that the use of H2O as substitution template for H2S can surmount limitations associated with H2S molecules, such their toxic and instable nature.It was reported that PMo12@UiO-66@H2S-MIPs presented increased H2S adsorption capacity (24.05 mg g −1 ) compared to that of carrier PMo12@UiO-66, suggesting that the capacity of the latter in capturing H2S was further improved by the H2S imprinted polymers.In addition, PMo12@UiO-66@H2S-MIPs exhibited decent H2S adsorption capacity at room temperature and in the presence of water, while it successfully separated H2S/CO2 mixtures.

H2S Capture via Zeolitic Imidazolate Frameworks (ZIFs)
It is widely known that MOFs exhibit hydrothermal stability.For example, MOF-5's structure irreversibly collapses after only 10 min of H2O exposure, even at mild conditions (low pressure and temperature) [19].Given that MOFs have hydrophilic properties, the strongly interact with H2O.That said, even small amounts of moisture, can disintegrate the coordination bonds, resulting in the framework degradation.Another downside that is associated with the hydrophilic properties of MOFs is that the access of hydrophobic organic substrates is hampered, compromising the catalytic activity of some reactions.In this regard, many scientists trying to take advantage of the stability of zeolites, combined with the diverse structures and the MOFs tailorability in terms of chemical functionality, synthesized zeolitic imidazolate frameworks (ZIFs), which are classified as a MOF subclass.
ZIFs, are zeolite-like structures which are built of transition metal ions that replace aluminum or silica atoms and maintain the topology of a zeolitic material.Furthermore, the organic ligands displace the oxygen atoms in the lattice of the zeolite.For instance, ZIF-8, was thoroughly examined, owing to its high thermal stability (up to 550 °C), high surface area (1630 m 2 g −1 ) and notable chemical resistance to boiling organic solvents and alkaline H2O [20].

Conclusions
H2S removal using MOFs can be limited by the formation of strong and often irreversible bonds.To avoid this issue, one can regulate the host-guest binding interaction between MOFs and H2S.
Reversibility after H2S sulfidation can be achieved through noncovalent bonding between functionalized ligands H2S molecules.However, the need to elucidate further the preferred H2S adsorption sites arises to optimize this kind of H2S separations.
Moreover, a deeper understanding for the structural characteristics of MOFs is key.For example, the structure of MOFs with open metal sites, (i.e., HKUST−1, IRMOF-3 and MIL-53(Fe)) degrades when challenged with the toxic H2S, generating metal sulfides.A solution to this shortcoming may be the use of MOF-composites, such as MOF/GO, where graphene oxide is used as a support.However, these composite materials suffer from poor H2S uptake.
Conversely, mild interactions between H2S and the open metal sites, can promote the adsorption of H2S molecules without breaking the material's structure offering the opportunity of reversible adsorption processes.
In addition, the functionalization of the MOF's surface can lead to increased H2S adsorption capacities, as in the case of MIL−101(Cr)-4F(1%), which exhibited noticeably H2S uptake at low temperature and pressure.
At this point, it worth mentioning the lack of studies in the literature in terms of MOFs thermal conductivities.The designing of adsorption beds with high stability when experiencing high exothermicity is critical.
Further research should focus on the benefits provided by reticular chemistry for the developments of stable porous MOFs, suitable for sweetening applications.
Moreover, several theoretical studies studied the mechanism of the H2S capture in MOFs in the presence of moisture.However, the results obtained are somehow misleading, as they contradict with the ones obtained from the experimental H2S adsorption tests.
Finally, even though significant improvements have been made in terms of MOFs structural characteristics in the past two decades, the development of MOF structures with high H2S selectivity, higher H2S adsorption capacities, regenerability, long-term stability, and lower cost remains a challenge to be addressed to reach industrialization standards.

Figure 1 .
Figure 1.The cubic structure of MOF-5.The lattice constant at 27 °C is 25.85 Å .The diameter is 7.16 Å .Reproduced with permission from Ref. [1].Copyright 2007 International Journal of Heat and Mass Transfer.