CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] The present application is related to and claims the priority benefit of U.S.

Provisional Patent Application No. 62/831,617, filed April 9, 2019, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to systems and methods for alkene generation. More particularly, the present disclosure relates to systems and methods for alkene generation using reducible metal sulfide particles.

INTRODUCTION

[0003] Alkanes exhibit a tendency to dehydrogenate to alkenes at high temperatures through an endothermic reaction. Industrially, this is accomplished by the steam cracking process and is commonly used for non-catalytic conversion of ethane to ethylene. Thermal cracking or steam cracking relies on thermally activating the hydrocarbon feedstock to produce cracked or smaller hydrocarbons or unsaturated hydrocarbons. The cracking process takes place by gas phase radical mechanism, where the hydrocarbon radicals undergo initiation, propagation and termination steps. Typically, longer hydrocarbon chain cracking reactions to smaller

hydrocarbons are preferred over unsaturated hydrocarbons. Thus, propane or higher alkanes tend to produce ethylene instead of their respective alkenes, and therefore require a catalyst to ensure that the desired alkene product is the kinetically favored product.

[0004] Taking the example of propylene production, the most common and commercially available method is propane dehydrogenation (PDH). The basic principle involves

dehydrogenation of propane over a catalyst to form propylene and hydrogen, as shown in equation 1 below. [0005] This reaction is performed at a lower temperature than in steam cracking reactions, catalyzing the C-H bond activation in propane with no or minimal C-C bond activation. These PDH processes typically are run in either fixed bed reactors or fluidized bed reactors at temperatures ranging from 500-700°C and pressures from 0.5-3 bar. Out of the several commercially available systems, two processes have been highlighted in this section. The Catofin process, by Lummus, which uses a CrO _x on A1 ₂ 0 ₃ catalyst with Na/K as promoters and the Oleflex process, by UOP, which uses a Pt-Sn alloy on A1 ₂ 0 ₃ catalyst with Na/K promoters. Both of these processes suffer from carbon deposition on the catalyst, and subsequent gradual catalyst deactivation.

[0006] Reactivation of a deactivated catalyst either requires reducing the catalyst with hydrogen, or using chlorine gas to disperse the sintered active sites, where the carbon is typically burnt off with air oxidation. Additionally, as seen from Figure 1, the PDH process for all alkanes is limited by the thermodynamic equilibrium of the reaction in equation 1. Thus, in order to achieve higher propane conversions, the reaction would need to be run at a higher temperature. However, higher temperatures tend to favor C-C bond activation, reducing the selectivity and limiting the operational matrix of the process.

[0007] In order to address this trade-off, several catalytic technologies have been developed which introduce an oxidizing gas into the system, thus creating a sink for hydrogen. This allows for higher conversion of the alkane in order to restore the dehydrogenation equilibrium. This process is known as oxidative dehydrogenation (ODH) and is widely used for ethane and propane dehydrogenation in the presence of molecular oxygen. This molecular oxygen assisted ODH process relies on utilizing oxygen to extract H from an alkane, such as propane, to convert it to propylene and have water and heat as by-products. Due to the electronegativity difference, this reaction, shown in equation (2), theoretically occurs at a lower temperature than PDH technology.

C ₃ H ₈ +0.50 ₂

[0008] However, using a strong oxidant, such as 0 ₂ , negatively affects the selectivity due to the formation of undesired products, such as CO and C0 ₂ . As a result, a majority of the 0 ₂ -ODH catalysts fail to meet the performance of PDH catalysts, where selectivity drops sharply with an increase in propane conversion. As an alternative, sulfur or sulfur derivatives, such as H ₂ S, are used which resemble a softer oxidant. Transition state metal sulfide catalysts have been shown to be active towards conversion of butane to iso-butene. These sulfide catalysts have a lower activation energy barrier for C-H activation than C-C bond activation, making them much more effective than the PDH catalysts. However, as these catalysts react with the alkane, some sulfur is lost as H ₂ S, thus reducing the catalyst activity. Some sulfide catalysts have been reported for propane to propylene conversion which operate by co-feeding H ₂ S and H ₂ with propane. However, these catalysts also require a regeneration step with air followed by H ₂ S and H ₂ mixture to regain the active metal sulfide catalyst.

[0009] A major drawback of the catalytic ODH system is that the oxidant stream and the alkane stream must be co-fed in the reactor. This results in the formation of undesired side products, which decrease the selectivity of the desired alkene. Also, in the case of sulfur, the metal sulfide catalyst may lose its activity as the catalyst reduction reactions dominate the catalyst oxidation reactions. This imbalance results in the use of extreme catalyst regeneration steps, limiting the efficiency and turnover of the process.

SUMMARY

[0010] Generally, the instant disclosure relates to alkene generation using metal sulfide particles. In one aspect, a method can include providing a gaseous alkane input stream to a first reactor and providing a metal sulfide (MSx) particle to the first reactor, whereupon the metal sulfide (MSx) particle reacts with an alkane in the gaseous alkane input stream to generate an alkene, a reduced metal sulfide (MSx-i) particle, and at least one of: hydrogen sulfide (H2S) and a sulfur containing compound. The method can also include collecting a product stream from the first reactor including the alkene, hydrogen sulfide (H2S) and/or the sulfur containing compound, providing the reduced metal sulfide (MSx-i) particle to a second reactor, providing a sulfur stream to the second reactor, whereupon the reduced metal sulfide (MSx-i) particle reacts with sulfur in the sulfur stream to generate the metal sulfide (MSx) particle and hydrogen (H2). Then a second reactor output stream including hydrogen (H2) can be collected.

[0011] In another aspect, a method can include providing a gaseous alkane input stream to a reactor, the reactor including a metal sulfide (MSx) particle, whereupon the metal sulfide (MSx) particle reacts with an alkane in the gaseous alkane input stream to generate an alkene, a reduced metal sulfide (MSx-i) particle, and at least one of: hydrogen sulfide (H2S) and one or more sulfur containing compounds selected from: S2, CS, and CS2. The method can also include collecting a product stream from the reactor including the alkene, hydrogen sulfide (H2S) and/or the one or more sulfur containing compounds, after collecting the product stream, providing an inert gas stream to the reactor, after providing the inert gas stream to the reactor, providing a sulfur stream to the reactor, whereupon the reduced metal sulfide (MSx-i) particle reacts with sulfur in the sulfur stream to generate the metal sulfide (MSx) particle and hydrogen (H2), and collecting a reactor output stream including hydrogen (H2).

[0012] There is no specific requirement that a material, technique or method relating to alkene generation include all of the details characterized herein, in order to obtain some benefit according to the present disclosure. Thus, the specific examples characterized herein are meant to be exemplary applications of the techniques described, and alternatives are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. l is a graph showing thermodynamic equilibrium for alkane conversion to alkene via dehydrogenation.

[0014] FIG. 2 is a schematic diagram showing an exemplary two reactor system for generating alkenes.

[0015] FIG. 3 is a schematic diagram showing an exemplary single reactor system for generating alkenes.

[0016] FIG. 4 is a schematic diagram of an exemplary method for operating the two reactor system shown in FIG. 2.

[0017] FIG. 5 is a schematic diagram showing an exemplary method for operating the single reactor system shown in FIG. 3.

[0018] FIG. 6 is a graph showing experimental data for propane conversion and propylene selectivity values over time for an oxidative dehydrogenation reaction including propane, H2S, and Feo.89S at 650°C.

[0019] FIG. 7 is a graph showing experimental data for H2S conversion over time for the sulfidation reaction of FeS to form Feo.89S at 850°C. [0020] FIG. 8 is a graph showing experimental data for temperature dependence of iron vacancy in Fe-S system for a sulfidation reaction.

[0021] FIG. 9 is a graph showing experimental data for temperature dependence of iron vacancy in Fe-S system for an oxidative dehydrogenation reaction, assuming Fh to be the reactive species from alkane dehydrogenation.

[0022] FIG. 10 is a graph showing experimental data for propane conversion and propylene selectivity values over time for an oxidative dehydrogenation reaction including propane, FhS, and Feo.89S+Si02 at 600°C.

[0023] FIG. 11 is a graph showing experimental data for FhS conversion over time for the sulfidation reaction of FeS to form Feo.89S at 850°C.

[0024] FIG. 12 is a graph showing experimental data for moles of pyrite (FeS2) and Fe ^~ Sx at sulfur loadings greater than 1.

[0025] FIG. 13 is a graph showing experimental data for moles of pyrrhotite phase across temperatures 200-900 °C for sulfur loading less than 1.

[0026] FIG. 14 is a graph showing experimental data for moles of pyrrhotite and trendlines for sulfur loadings 1-2 in the T350-650 zone.

[0027] FIG. 15 is a graph showing experimental data for moles of pyrite (FeS2) and trendlines for sulfur loadings 1-2 in the T350-650 zone.

[0028] FIG. 16 is a graph showing experimental data for pyrrhotite sulfidation extent for sulfur loading 0-1 and 1-2 across temperatures 200 °C -900 °C.

[0029] FIG. 17 is a graph showing experimental data for FhS generation capacity for different metal sulfides.

[0030] FIG. 18 is a graph showing experimental data for regenerability with FhS for different metal sulfides.

[0031] FIG. 19 is a graph showing experimental data for FhS generation capacity of a Co-S system for different temperatures.

[0032] FIG. 20 is a graph showing experimental data for FhS generation capacity of a Pb-S system for different temperatures.

DETAILED DESCRIPTION [0033] Systems and methods disclosed and contemplated herein relate to alkene generation. Disclosed systems and methods employ reducible metal sulfides during conversion of alkanes to alkenes, typically in a chemical looping reactor system. Some implementations can utilize two reactor systems. Some implementations can utilize single, fixed bed reactor systems.

[0034] In certain aspects, systems and methods disclosed herein address one or more drawbacks of catalytic ODH reactions by splitting an oxidant stream and an alkane stream. In some instances, those streams are provided to two reactors operating independent of each other. In some instances, those streams are sequentially provided to a single reactor. Generally, an alkane or a mixture of alkanes reacts with a metal sulfide (MS ) to form the alkene, H ₂ S and/or a sulfur containing compound in a reactor. Here M is the metal component of the metal sulfide and S represents the sulfur in the solid lattice. Thus, the metal sulfide acts as the sulfur source that carries out the oxidation of H ₂ to H ₂ S, thus improving the alkane conversion.

[0035] During exemplary operation of a two reactor system, the MS _x can reduce to MSx-i, which is sent to the sulfidation reactor where a sulfur source regenerates the metal sulfide into its original form, i.e. MS ₂ . This regeneration is different from the regeneration steps in a catalytic system, because this operation is a part of the chemical looping structure. The regeneration step in a catalytic system is carried out to address the loss of reactivity of the catalyst under non-ideal and unstable conditions. However, an ideal catalyst would portray a stable performance, without requiring a regeneration step. The chemical looping mode, however, intentionally carries out the reduction and oxidation reactions and the oxidation or regeneration reaction is performed to complete the loop. In other words, the metal sulfide can be considered as a sulfur carrier between the two reactors, where the two reactors follow very different reaction mechanisms. The regeneration reactor is also capable of producing a value-added product such as H ₂ , which is not the case in the catalytic system. The chemical looping mode thus allows for the two reactors to be governed by different thermodynamic and kinetic factors based on their operating parameters.

I. Chemical Aspects

[0036] Systems and methods of the present disclosure may include input streams provided to reactor systems and output streams generated by reactor systems. The sections below discuss various chemical aspects of exemplary systems and methods. A. Input Streams

[0037] Exemplary reactor systems may receive a gaseous alkane input stream and a sulfur stream. In two reactor configurations, exemplary reactors may also receive metal sulfide particles.

[0038] Gaseous alkane input streams may include one alkane species or may be a mixture of alkane species. As implied, alkanes in gaseous alkane input streams are in a gaseous phase.

[0039] Alkanes usable in gaseous alkane input streams may be linear, branched, or cyclic. In some implementations, gaseous alkane input streams may include at least one C2-C ₆ alkane. In some instances, gaseous alkane input streams may include only C2 alkanes, only C3 alkanes, only C4 alkanes, only C5 alkanes, or only G alkanes. In some instances, gaseous alkane input streams may include a mixture of C2-C5 alkanes; a mixture of C3-C ₆ alkanes; a mixture of C2-C4 alkanes; a mixture of C ₃ -C5 alkanes; a mixture of C4-C6 alkanes; a mixture of C2 and C ₃ alkanes; a mixture of C ₃ and C4 alkanes; a mixture of C4 and G alkanes; or a mixture of G and G, alkanes. Example alkanes may include, but are not limited to, ethane, propane, n-butane, n-pentane, and n-hexane.

[0040] In some instances, gaseous alkane stream input may also contain CEE as an alkane component. CEE may or may not react with the metal sulfide depending on the operating conditions.

[0041] In some instances, gaseous alkane input streams may also include one or more non alkane components, such as inert components. Example non-alkane components that may be present in gaseous alkane input streams include, but are not limited to, hydrogen (EE), nitrogen (N2) and argon (Ar).

[0042] Example sulfur streams may include one or more allotropes of sulfur. For instance, exemplary sulfur streams may include, but are not limited to, S2, S ₃ , S4, and Ss. In some instances, example sulfur streams may include hydrogen sulfide (EES) and/or mercaptans like CEE SEE In some instances, example sulfur streams may include one or more inert carrier gases including, but not limited to, nitrogen (N2) and argon (Ar).

[0043] In single reactor configurations, example reactors may also receive inert gas streams. Example inert gas streams may include, but are not limited to, nitrogen (N2) and/or argon (Ar). B. Output Streams

[0044] Exemplary reactor systems may generate various output streams. In two reactor configurations, one reactor may provide an output stream including one or more desired products and the other reactor may provide a second output stream.

[0045] Exemplary output streams may include one or more desired products. For instance, a metal sulfide (MSx) particle reacting with an alkane in the gaseous alkane input stream may generate an alkene, a reduced metal sulfide (MSx-i), and one or more of: hydrogen sulfide (EbS) and other sulfur containing products like S2, among other products. In single reactor and two reactor configurations, a product stream may include the generated alkene and one or more of hydrogen sulfide (EbS) and other sulfur containing products, among other products. In two reactor configurations, the reduced metal sulfide (MSx-i) may be provided to the other reactor.

[0046] As another example, a reduced metal sulfide (MSx-i) particle reacting with sulfur in a sulfur stream may generate a metal sulfide (MSx) particle. In some instances, a reactor output stream can include the generated hydrogen (Eb) when the input sulfur stream to the reactor contains a hydrogen feed such as EbS. In some instances, a reactor output stream may include hydrogen sulfide (EbS). In two reactor configurations, the metal sulfide (MSx) particle may be provided to the first reactor.

C. Reactions

[0047] Various reactions may occur in exemplary reactor systems. For example, alkane(s) and metal sulfide (MSx) may be provided to a reactor. The metal sulfide MSx may be capable of donating its sulfur to Eb to form EbS and alkene(s). In this process, MSx converts to MSx-i, which may be sent to a second reactor (or which may remain in the reactor in single reactor configurations). An input stream that includes sulfur may be used to regenerate the MSx-i to MSx, where the MSx-i reacts with sulfur in the input stream to form MSx.

[0048] Exemplary reactions are provided below without limitation. In implementations where propane is provided as an alkane, reaction (3) may occur in a reactor that includes a metal sulfide (MSx) particle.

CiHs + MSx CiHe + EbS + MSx-i (3)

[0049] In implementations where a reactor includes a reduced metal sulfide (MSx-i) particle and receives a sulfur stream that includes EbS, reaction (4) may occur: MSx-i + EbS H ₂ + MSx (4)

[0050] In implementations where butane is provided as an alkane, reaction (5) may occur in a reactor that includes a metal sulfide (MSx) particle.

C4H10 + MSx C4H8 + EbS + MSx-i (5)

[0051] In implementations where a reactor includes a reduced metal sulfide (MSx-i) particle and receives a sulfur stream that includes Ss, reaction (6) may occur:

MS _X -I + (1/8)S ₈ MS _X (6)

D. Metal Sulfide Particles

[0052] Various types of metal sulfide particles may be utilized in exemplary systems and methods. Generally, metal sulfide particles used in exemplary systems and methods are either in a reduced form or in an oxidized form. The reduced or oxidized terms refer to the change in oxidation state of the metal, lattice sulfur species, or both. Oxidized metal sulfide particles can react with an alkane, dehydrogenate the alkane, and form EbS, which reduces the oxidized metal sulfide particle into a reduced metal sulfide or a metal/metal alloy. The reduced metal sulfide particle or metal/metal alloy can accept sulfur in the solid lattice from a sulfur source. Upon sulfur addition/oxidation, reduced metal sulfide particles can form oxidized metal sulfide particles.

[0053] Exemplary metal sulfide particles have an active metal capable of forming sulfides where active metal, sulfur, or both display one or more than one oxidation states. Generally, example metals (M) may be transition state, metalloid, or rare earth metals. In some instances, example metal sulfide particles may be bimetallic or trimetallic. Example metals (M) include, but are not limited to, Fe, Co, Ni, Cu, W, La, Ce, Ti, Zn, Cd, Ru, Rh, and Pb. The metals may include sulfide (S ² ), persulfide (S2 ² ), or another sulfur species.

[0054] There may be more than one active metal in a metal sulfide either in the form of a mixed metal sulfide or as a promotor or dopant. Dopants and promoters may be alkali metals, alkaline earth metals, transition state metals, metalloid metals, or rare earth metals. Supports may be inert oxides of alkali metals, sulfides of alkali metals, alkaline earth metals, transition state metals, metalloid metals, or rare earth metals. The amount of support, promotor, or dopant material may vary from 0.01 wt%, 10 wt%, 20 wt%, 30 wt% 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90wt% or any value in between.

[0055] The metal sulfide may contain metal sulfides from group I or group II in the form of promotor, dopant, or to form mixed metal sulfides. Inert sulfides such as, but not limited to M0S2, Ce2S3, MgS, Na2S may be used as supports and dopants and promotors as well. Inert oxides that do not react with the metal sulfide may be used as promotor, dopant, or as a support. Examples of promotors, dopants, or supports may include, but not limited to, K2O, MgO, S1O2, and AI2O3, as well as mixed metal oxides such as Mg AI2O4.

[0056] Oxides that do react with the sulfide to form metastable structures can also be considered as a metal sulfide. Dopants, promotors, and supports, in addition to other

components, may provide high surface area, highly active sulfur vacancies.

[0057] Exemplary metal sulfide particles may be synthesized by any suitable method including, but not limited to, wet milling, extrusion, pelletizing, freeze granulation, co precipitation, wet-impregnation, sol-gel, and mechanical compression. Certain techniques may be used to increase the strength and/or reactivity of exemplary metal sulfide particles, such as sintering synthesized particles or adding a binder or sacrificial agent with synthesis methods such as sol-gel combustion.

[0058] Exemplary metal sulfide particles may be provided as powders or pellets. Example powders may include metal sulfide particles having a size of about 100 pm. Example pellets may include metal sulfide particles having a size of about 2 mm.

[0059] Example metal sulfide particles may be bulk structures or mesoporous supported nanoparticles. Example bulk structures may have random orientations of large grains, cage-like structures for added physical strength, layered structure, or similar configurations. Example mesoporous supported metal sulfide particles may have a mesoporous support such as Santa Barbara Amorphous- 15 silica (SB A- 15), Santa Barbara Amorphous- 16 silica (SBA-16), and other SB A variants, Mesoporous-AhCb, Mesoporous CeCk, etc., which have micro and meso pores, in which metal sulfide nanoparticles may be embedded.

[0060] Example metal sulfide particles may have various densities. For instance, example metal sulfide particles may have a density of from 1.5 g/cm ³ to 6 g/cm ³ . In various

implementations, example metal sulfide particles may have a density of from 1.5 g/cm ³ to 3 g/cm ³ ; 3 g/cm ³ to 6 g/cm ³ ; 2 g/cm ³ to 4 g/cm ³ ; 4 g/cm ³ to 6 g/cm ³ ; 1.5 g/cm ³ to 2 g/cm ³ ; 2 g/cm ³ to 3 g/cm ³ ; 3 g/cm ³ to 4 g/cm ³ ; 4 g/cm ³ to 5 g/cm ³ ; or 5 g/cm ³ to 6 g/cm ³ .

II. Reactor Configurations and Operating Conditions

[0061] Exemplary reactor systems may be single reactor system configurations or two reactor system configurations. In single reactor system configurations, example reactors may be configured to be fixed bed reactors. In two reactor system configurations, example reactors may be configured to be moving beds, ebullated beds, fluidized beds, or combinations thereof.

[0062] Exemplary reactor systems disclosed and characterized herein can operate under temperatures and pressures sufficient for alkene generation and metal sulfide regeneration.

[0063] Temperatures within exemplary reactors during oxidative dehydrogenation

(performed in the first reactor in two reactor systems) are typically between 200°C and 1200°C. In various implementations, a temperature of an exemplary reactor during oxidative

dehydrogenation can be between 300°C to 1100°C; 400°C to 1000°C; 200°C to 500°C; 500°C to 800°C; 800°C to 1100°C; 400°C to 800°C; 800°C to 1200°C; 500°C to 700°C; 700°C to 900°C; 900°C to 1100°C; 600°C to 800°C; 400°C to 500°C; 500°C to 600°C; 600°C to 700°C; 700°C to 800°C; 800°C to 900°C; or 900°C to 1000°C.

[0064] Temperatures within exemplary reactors during sulfidation (performed in the second reactor in two reactor systems) are typically between 200°C and 1000°C. In various

implementations, a temperature of an exemplary reactor during sulfidation can be between 300°C to 900°C; 400°C to 800°C; 200°C to 600°C; 600°C to 1000°C; 300°C to 500°C; 500°C to 700°C; 700°C to 925°C; 300°C to 400°C; 400°C to 500°C; 500°C to 600°C; 600°C to 700°C; 700°C to 800°C; or 800°C to 1000°C.

[0065] Pressures within exemplary reactors during oxidative dehydrogenation (performed in the first reactor in two reactor systems) are typically between 1 atm and 30 atm. In various implementations, a pressure of an exemplary reactor during oxidative dehydrogenation can be between 1 atm and 15 atm; 15 atm and 30 atm; 2 atm and 25 atm; 5 atm and 20 atm; 1 atm and 5 atm; 5 atm and 10 atm; 10 atm and 15 atm; 15 atm and 20 atm; 20 atm and 25 atm; 25 atm and 30 atm; 1 atm and 3 atm; 3 atm and 6 atm; 6 atm and 9 atm; 9 atm and 12 atm; 1 atm and 2 atm;

2 atm and 3 atm; 3 atm and 4 atm; 4 atm and 5 atm; 5 atm and 6 atm; 6 atm and 7 atm; 7 atm and 8 atm; 8 atm and 9 atm; or 9 atm and 10 atm. [0066] Pressures within exemplary reactors during sulfidation (performed in the second reactor in two reactor systems) are typically between 1 atm and 30 atm. In various

implementations, a pressure of an exemplary reactor during sulfidation can be between 1 atm and 15 atm; 15 atm and 30 atm; 2 atm and 25 atm; 5 atm and 20 atm; 1 atm and 5 atm; 5 atm and 10 atm; 10 atm and 15 atm; 15 atm and 20 atm; 20 atm and 25 atm; 25 atm and 30 atm; 1 atm and 3 atm; 3 atm and 6 atm; 6 atm and 9 atm; 9 atm and 12 atm; 1 atm and 2 atm; 2 atm and 3 atm; 3 atm and 4 atm; 4 atm and 5 atm; 5 atm and 6 atm; 6 atm and 7 atm; 7 atm and 8 atm; 8 atm and 9 atm; or 9 atm and 10 atm.

[0067] Various flow rates may be used within exemplary reactors during oxidative dehydrogenation (performed in the first reactor in two reactor systems) and sulfidation

(performed in the second reactor in two reactor systems). Specific flow rates can vary, particularly depending upon the scale of the operation, based on the stoichiometry and reaction kinetics of particular alkane and MSx pairs or sulfur-containing MS pairs. For illustration, example gas hourly space velocities can vary from 1 ml/g hr to 5000 ml/g hr.

[0068] For the single reactor configuration, the temperature, pressure and gas hourly space velocities mentioned for the two-reactor system are applicable.

[0069] For the single reactor configuration, the outlet gas composition may be measured or estimated to determine the segment times of the alkane dehydrogenation step or the sulfidation step.

[0070] The threshold value for the alkane dehydrogenation step may be determined by the conversion of the alkane, selectivity of the desired alkene, FhS/Sulfur containing compounds produced or a combination of these parameters.

[0071] The threshold value for the inert purging step may be determined by the volume of the reactor. The time for this segment can be determined by sending the inert gas into the reactor where the volume of the gas inside the reactor is replaced by anywhere between 2 to 10 times to ensure the gas has been purged.

[0072] The threshold value for the sulfidation step is determined by the amount of sulfur that reacted with the reduced metal sulfide. This may be estimated by measuring the difference between the sulfur in the inlet and outlet streams through gas analyzers.

III. System Arrangements [0073] FIG. 2 shows a schematic diagram of an exemplary reactor system 100. As shown, reactor system 100 includes reactor 102, reactor 104, alkane source 106, and sulfur source 114. Reactor system 100 is an example embodiment of a two reactor system that may be used for alkene generation using metal sulfides. Reactor system 100 may be configured for continuous operation. Other embodiments may include more or fewer components.

[0074] Reactor 102 receives gaseous alkane input stream 108 and metal sulfide (MSx) particles via input 118. The metal sulfide (MSx) particles react with alkane from gaseous alkane input stream 108 to generate an alkene, a reduced metal sulfide (MSx-i) particle, hydrogen sulfide (FhS) and other sulfur containing streams formed during the reaction. The reduced metal sulfide (MSx-i) particles are provided to reactor 104 via input 110.

[0075] Alkane source 106 provides one or more alkanes to reactor 102 in gaseous alkane input stream 108. Alkane source 106 may be configured to adjust a flow rate of gaseous alkane input stream 108. In some instances, the flow rate of gaseous alkane input stream 108 may be adjusted based on conversion data for an output stream 112 of reactor 102.

[0076] Reactor 102 provides a product stream 112 that includes alkene and hydrogen sulfide (FhS). Product stream 112 can also include one or more sulfur-containing compounds. In some instances, product stream 112 includes one or more monitoring units to monitor conversion rates in reactor 102. Based on measured conversion rates, flow rates of the gaseous alkane input stream 108 and/or metal sulfide particles (MSx) may be adjusted to achieve desired conversion rates.

[0077] Reactor 104 may receive a sulfur stream 116 from sulfur source 114 and reduced metal sulfide (MSx-i) particles 110 from reactor 110. In reactor 104, the reduced metal sulfide (MSx-i) particles may react with sulfur in the sulfur stream to generate the metal sulfide (MSx) particle and hydrogen (Fh). One or more additional components may be generated depending upon constituents in sulfur stream 116.

[0078] Sulfur source 114 provides a sulfur stream 116 to reactor 104. Sulfur source 114 may be configured to adjust a flow rate of sulfur stream 116. In some instances, the flow rate of sulfur stream 116 may be adjusted based on conversion data for an output stream 120 of reactor 104. In various implementations, and as discussed in greater detail above, sulfur stream 116 may include one or more allotrope of sulfur and/or hydrogen sulfide (FhS). [0079] Reactor 104 provides metal sulfide (MSx) particles to reactor 102. Reactor 104 also provides an output stream 120 that includes one or more gaseous components. For instance, output stream 120 can include hydrogen (Fh).

[0080] Output stream 120 may include hydrogen sulfide (FhS). In some instances, reactor system 100 may also include one or more separation units (not shown in FIG. 2) that can separate hydrogen sulfide (FhS) from output stream 120. Then, the separated hydrogen sulfide (FhS) may be recycled to reactor 104.

[0081] In some instances, reactor system 100 may include one or more separation units (not shown in FIG. 2) that can separate hydrogen sulfide (FhS) from product stream 112 generated by reactor 102. Then, the separated hydrogen sulfide (FhS) may be recycled to reactor 104.

[0082] FIG. 3 shows a schematic diagram of example reactor system 200. As shown, reactor system 200 includes reactor 208, alkane source 202, inert gas source 212, and sulfur source 218. Reactor system 200 is an exemplary embodiment of a single reactor system that may be used for alkene generation using metal sulfides. Reactor system 200 may be configured for batch operation. Other embodiments may include more or fewer components.

[0083] Alkane source 202 may provide a gaseous alkane input stream 204 to reactor 208.

One or more flow regulation units 206 may be used to selectively provide gaseous alkane input stream 204 to reactor 208 and/or control a flow rate of gaseous alkane input stream 204.

Exemplary components that may be included in gaseous alkane input stream 204 are discussed in greater detail above.

[0084] Reactor 208 may be configured as a fixed bed reactor including metal sulfide (MSx) particles. In reactor 208, the metal sulfide (MSx) particles may react with alkane in gaseous alkane input stream 204 to generate an alkene, a reduced metal sulfide (MSx-i) particle, and hydrogen sulfide (FhS).

[0085] Reactor 208 may generate a product stream 224 that includes alkene and hydrogen sulfide (FhS), and, in some instances, sulfur-containing compounds. Gas analyzer unit 211 may monitor alkane conversion and/or alkene selectivity. One or both of those values can be compared to a threshold value and, upon reaching the value, flow regulation unit 206 may stop the flow of gaseous alkane input stream 204 to reactor 208.

[0086] Inert gas source 212 may provide an inert gas stream 214 to reactor 208. One or more flow regulation units 216 may be used to selectively provide inert gas stream 214 to reactor 208 and/or control a flow rate of inert gas stream 214. Exemplary components of inert gas stream 214 are discussed in greater detail above. Generally, inert gas stream 214 can purge alkane(s), FhS, and alkene gas from reactor 208.

[0087] Gas analyzer unit 211 may monitor alkane(s), FhS, and alkene gas content in output stream 224. Upon detecting that most or all of those components are not present in output stream 224, flow regulation unit 216 may be configured to stop a flow of inert gas stream 214.

[0088] Sulfur source 218 may provide a sulfur stream 220 to reactor 208. One or more flow regulation units 222 may be used to selectively provide sulfur stream 220 to reactor 208 and/or control a flow rate of sulfur stream 220. Example components of sulfur stream 220 are discussed in greater detail above.

[0089] Gas analyzer unit 211 may be used to monitor hydrogen (Fh) content in output stream 224. Upon detecting a desired conversion of metal sulfide, flow regulation unit 222 may be configured to stop a flow of sulfur stream 220. Another purge of reactor 208 can be subsequently run by providing the inert gas stream 214 to reactor 208.

[0090] Usually, reactor 208 receives only one of gaseous alkane input stream 204, inert gas stream 214, and sulfur stream 220 at a time. That is, those streams are usually not mixed together and provided to reactor 208.

IV. Methods of Operation

[0091] FIG. 4 shows example method 300 for operating a reactor system. In some instances, method 300 may be used to operate example two reactor system 100 discussed above with reference to FIG. 2. Other embodiments of method 300 may include more or fewer operations.

[0092] Method 300 may begin by providing a gaseous alkane input stream (operation 302) to a first reactor. The alkane in the gaseous alkane input stream may include at least one C2-C6 alkane. Other aspects of the gaseous alkane input stream are discussed in greater detail above.

[0093] Metal sulfide (MSx) particles also may be provided (operation 304) to the first reactor. Various aspects of exemplary metal sulfide (MSx) particles are discussed in greater detail above. In the first reactor, the metal sulfide (MSx) particles may react with alkane in the gaseous alkane input stream to generate an alkene, reduced metal sulfide (MSx-i) particles, and hydrogen sulfide (FhS) and/or one or more other sulfur containing compounds. [0094] In some instances, during operation, a temperature of the first reactor may be maintained to be between 200°C and 1200°C. In some instances, during operation, a pressure of the first reactor may be maintained to be between 1 atm and 30 atm.

[0095] During operation, a product stream may be collected (operation 306) from the first reactor. Typically, the product stream includes the alkene generated in the first reactor and hydrogen sulfide (FhS). In some instances, exemplary method 300 may also include separating the hydrogen sulfide (FhS) from the product stream and recycling the separated hydrogen sulfide (FhS) to the second reactor.

[0096] The reduced metal sulfide (MSx-i) particles may be provided (operation 308) to the second reactor. A sulfur stream also may be provided to the second reactor (operation 310). Various aspects of example sulfur streams, including example components, are discussed above in greater detail. In the second reactor, the reduced metal sulfide (MSx-i) particle reacts with sulfur in the sulfur stream to generate the metal sulfide (MSx) particle and hydrogen (Fh).

[0097] A second reactor output stream may be collected (operation 312). The second reactor output stream may include, at least, hydrogen (Fh). In some instances, the second reactor output stream may include hydrogen sulfide (FkS). Optionally, example method 300 may include separating the hydrogen sulfide (FkS) from the second reactor output stream and recycling the separated hydrogen sulfide (FkS) to the second reactor.

[0098] FIG. 5 shows exemplary method 400 for operating a reactor system. In some instances, method 400 can be used to operate a single reactor system 200 discussed above with reference to FIG. 3. Typically, exemplary method 400 is performed with a fixed bed reactor that includes metal sulfide (MSx) particles. Other embodiments of method 400 may include more or fewer operations.

[0099] Method 400 may begin by providing a gaseous alkane input stream (operation 402) to the reactor. The alkane in the gaseous alkane input stream may include at least one C2-C6 alkane. Other aspects of the gaseous alkane input stream are discussed in greater detail above. The metal sulfide (MSx) particles react with alkane in the gaseous alkane input stream to generate an alkene, a reduced metal sulfide (MSx-i) particle, and hydrogen sulfide (FkS). A product stream is collected (operation 404) that includes, at least, the alkene and hydrogen sulfide (FkS) and/or one or more other sulfur-containing compounds. [00100] While providing the gaseous alkane input stream, the product stream can be monitored for whether alkane conversion is below a predetermined threshold (operation 406). If alkane conversion is above the predetermined threshold, the gaseous alkane input stream may be continually provided (operation 402) to the reactor. In some instances, while providing the gaseous alkane input stream (operation 402), a temperature of the reactor may be maintained to be between 200°C and 1200°C and a pressure of the reactor can be maintained to be between 1 atm and 30 atm.

[00101] If alkane conversion is below the predetermined threshold, then the gaseous alkane input stream may be stopped, and an inert gas stream is provided (operation 408) to the reactor. Providing the inert gas stream can purge alkane, alkene, and ThS from the reactor. In some instances, a reactor output stream may be monitored, and inert gas may be provided until alkane, alkene, and/or ThS content drops below a predetermined threshold. In some instances, the hydrogen sulfide (ThS) may be separated from the reactor output stream and recycled back to the reactor.

[00102] After providing the inert gas stream, a sulfur stream may be provided (operation 410) to the reactor. The sulfur stream may include one or more sulfur-containing components, such as an allotrope of sulfur or hydrogen sulfide (ThS). Additional details about the sulfur stream are provided above. In some instances, while providing the sulfur stream (operation 410), a temperature of the reactor may be maintained to be between 200°C and 1000°C and a pressure of the reactor may be maintained to be between 1 atm and 30 atm.

[00103] After providing the sulfur stream (operation 410), the reduced metal sulfide (MSx-i) particle may react with sulfur in the sulfur stream to generate the metal sulfide (MSx) particle and hydrogen (Th). The reactor output stream may be collected (operation 412), which includes, at least, hydrogen (Th).

[00104] The reactor output stream may be monitored (operation 414) for whether metal sulfide conversion in the reactor is above a predetermined threshold. If the metal sulfide conversion is below the predetermined threshold, the sulfur stream may be continued to be provided (operation 410) to the reactor.

[00105] If the metal sulfide conversion is above the predetermined threshold, then sulfur stream may be stopped. Then, the inert gas stream may be provided (operation 416) to the reactor. Providing the inert gas stream can purge the reactor of gaseous species generated while sulfur was provided to the reactor. Then, method 400 may return to operation 402 and gaseous alkane input stream can be provided to the reactor.

V. Experimental Examples

[00106] Experimental examples were conducted, and various aspects are discussed below. In particular, two experiments were conducted where metal sulfide particles were iron (Fe)-based, propane was the alkane, and EbS was the sulfur source.

A. Exemplary Fe-S System

[00107] An example first reactor was operated at 650°C with a propane space velocity of 300 ml/g hr and Feo.89S metal sulfide particles. An example second reactor was operated at 800°C with an FhS space velocity of 15 ml/ g hr. The reaction was carried out in a u-tube reactor and a mass spectrometer was used to measure the gas composition for the alkane dehydrogenation step. For the sulfidation step, a FhS gas analyzer was used. The mass spectrometer and the FhS analyzer were calibrated with known concentrations of gas mixtures. These continuous gas analyzers analyzed a slip stream of the product gas. Results for this example system are shown in FIG. 6 and FIG. 7. FIG. 6 shows data for the oxidative dehydrogenation reaction in the first reactor, and FIG. 7 shows an FhS sulfidation reaction of FeS to Feo.89S.

[00108] A characteristic trend seen in FIG. 6 for propylene selectivity and propane conversion can be attributed to the change in the surface species with the reaction time. A maximum yield of propylene was calculated to be 17.2%. A loss in sulfur from the iron sulfide in the first reactor is seen in the form of FhS production in gas phase along with the propylene produced. The sulfidation reaction converts FhS into Fh, re-sulfating the iron sulfide into the original state of Feo.89S.

[00109] To measure the performance of metal sulfides, thermodynamic studies were conducted using Fh as reactant gas. There are two ways in which alkanes can interact with metal sulfides in S-ODH reactor, alkanes react directly with metal sulfides to form alkenes and FhS or alkanes can thermochemically crack over metal sulfide surface forming alkenes and Fh. This Fh then reacts with metal sulfide to form FhS. In both these ways, formation of FhS drives the reaction and pushes dehydrogenation equilibrium forward. Hence, to asses metal sulfides for the proposed process, its ability to convert H2 to H2S should be measured as it is the equilibrium determining reaction. In view of this, all thermodynamic calculations on metal sulfides are performed with H2 as reactant rather than any alkanes.

[00110] Generally, FIG. 8 and FIG. 9 show thermodynamic data for the Fe-S system. FIG. 8 shows temperature dependence of iron (Fe) vacancy in the Fe-S system for the sulfidation reaction. FIG. 9 shows temperature dependence of iron (Fe) vacancy in the Fe-S system for the oxidative dehydrogenation reaction, assuming Fh to be the reactive species from alkane dehydrogenation. Under the current reaction conditions and temperatures, the system favorably forms Fe _(i -x ₎ S or pyrrhotite phase, where x varies between 0 and 0.2. The vacancy‘x’ directly correlates to the amount of sulfidation of a particular phase.

B. Exemplary Fe-S-SiCh System

[00111] An example first reactor was operated at 600°C with a propane space velocity of 60 ml/g hr and Feo.89S+Si02 metal sulfide particles (SiCk present in the particles at 20 wt%). An example second reactor was operated at 800°C with an FkS space velocity of 15 ml/ g hr. The instruments and methodology used are similar to Example A. Results for this example system are shown in FIG. 10 and FIG. 11. FIG. 10 shows data for the first reactor, and FIG. 11 shows an FkS sulfidation reaction of FeS to Feo.89S.

It appears that the added SiCk, which played the role of a support, improved the surface area and the dispersion of active sites. The lower temperature and lower space velocity in the first reactor (compared to the experimental example above), appears to improve the overall selectivity and conversion of the system. The highest yield for propylene with this system is 39%, which the sulfidation reaction showing a similar trend as compared to the example above. The volcano trend of the yield depicts a strong dependence of the performance parameters with sulfur vacancies in the solid lattice. This provides insight into a mechanism of the first reactor (where the oxidative dehydrogenation occurs), which can be leveraged to synthesize sulfides that yield higher propylene selectivity.

C. Exemplary Fe-S System at various conditions

[00112] Following the example depicted in FIG. 8 and FIG. 9, several configurations of the

MSx-MSx-i pair could be envisioned. In this example, thermodynamic studies using FactSage 7.3 were done on Fe-S system to validate regenerability across a temperature range of 200°C - 1000°C for both the reactors. 1 mol of Fe was sulfidized using 10 mols of H2S at a given temperature and the subsequent formed metal sulfide was reacted with 1 mol of H2 to simulate system performance. Further, MSx has been dubbed as FeSx and MSx-i has been dubbed as FeS _y. In the following reactions (7)-(9), x is the sulfur present in the most sulfidized metal phase, y is sulfur present in metal sulfide post reaction with Fh, and m is the FhS required to regenerate FeSy.

[00113] Reaction (7) shows a sulfidation step to set up the calculation.

Fe + 10 H ₂ S ® FeS _x + x H ₂ + (10 - x) H ₂ S (7)

[00114] Reaction (8) shows a reaction of metal sulfide with Fh (S-ODH).

FeS _x + H ₂ ® FeS _y + (x - y) H ₂ S + (l - (x - y)) H ₂ (8)

[00115] The reduced metal sulfide was reacted with FhS in incremental steps till it was completely regenerated, as shown in reaction (9).

FeS _y + m Fl ₂ S ® FeS _x + (x— y) H ₂ + (m— (x— y)) H ₂ S (9)

[00116] The results for this example are given below in table 1.

Table 1. Regenerability of Fe-S System

[00117] In this experiment, it was observed that in the temperature range of 200 °C - 1000°C, iron metal sulfides swing between three phases: FeS2, Fe7Ss and FeSz (pyrrhotite z = 1-1.25). Fe7S8 is not formed at 400°C and above and FeS2 is not formed at 600°C and beyond [x] and [y] values are calculated based on these phases. At lower temperatures of 200°C and 300°C, metal sulfide swings only between FeS2 and Fe ^~ Sx. As the temperature reaches 400°C metal sulfide swings between FeS2 and FeSz. At 500°C, Fe is no longer sulfidized completely to FeS2, and swing occurs between mixture of FeS2 and FeSz and pure FeSz. Beyond 500°C, swing occurs only in pyrrhotite phase with change in [z] value, for instance at 600°C [z] value changes from 1.167 to 1.1050 as indicated from [x] and [y] values in Table 1.

[00118] It appears from Table 1 that regenerability is achieved for entire temperature spectrum under different x and y values. Regeneration requires high partial pressure of FhS and hence, higher amount of FhS is needed even if all of it does not get converted.

D. Exemplary Fe-S System with S as the sulfur in the sulfur stream

[00119] In all the following experimental examples, the iron loading was kept constant at 1 mole and sulfur was used as the sulfur stream. The temperatures studied were divided into three zones based on the formation of iron sulfide phases. The temperature zones are 200-300 °C, 300- 650 °C and 650-900 °C. In all the temperature zones, sulfur loading was varied to understand the product distribution and sulfidation extent.

1. Zone 200 °C -300 °C (T200-300)

[00120] In this temperature zone, when the sulfur loading is less than 1, it was observed that the product consists predominantly of the pyrrhotite phase. As the temperature was increased, the pyrrhotite phase decreased (FIG. 12) at a fixed sulfur loading. At these low loadings, there was no unreacted sulfur left in the solution (S-MATT) phase since it was the limiting reactant.

Limiting reactant is defined with respect to a mole of stoichiometric pyrrhotite (FeS).

[00121] Upon increasing the sulfur loading beyond 1 until 2, it was observed that the pyrrhotite phase decomposed completely into two phases of constant molar quantities of pyrite (FeS2) and Fe ^~ Sx without any unreacted sulfur across the entire temperature range. However, it is worth noting that with an increased sulfur loading (from 1 towards 2), the molar quantities of pyrite increased and pyrrhotite decreased across the entire temperature range as illustrated in FIG. 12. This suggests a higher sulfide product (pyrite) is favored over FevSs on increasing temperature when the sulfur is in excess with respect to iron.

[00122] Upon further increasing the sulfur loading beyond 2, it was observed the pyrrhotite fully decomposed into pyrite and the excess unreacted sulfur was left in the solution (MATT) phase. There was also no formation of pyrrhotite or FevSs phase at these sulfur loadings. The trend is consistent across the entire temperature range.

2. Zone 350 °C - 650 °C (T350-650)

[00123] In this temperature zone, when the sulfur loading is less than 1, the trends are similar to the T200-300 zone’s sulfur loading<l. The product consisted of only pyrrhotite which decreased as the temperature is increased from 350 °C to 650 °C illustrated in FIG. 13.

[00124] When the sulfur loading is increased beyond 1 till 2, the product consisted of two phases here i.e. pyrrhotite and pyrite (FeS2). There is no formation of FevSs in this zone unlike previous case. The molar quantities of pyrrhotite increased while the pyrite decreased as illustrated in FIG. 14 which is attributed to the pyrite decomposition into pyrrhotite phase upon increasing the temperature. It is worth noting that in this temperature zone, for the sulfur loadings of 1 to 1.5 the dominant phase is pyrrhotite while from 1.5 to 2 it is pyrite as shown in figure 14. In addition, the pyrite decreases and pyrrhotite increases for sulfur loadings (1-2) with increase in temperature. This is illustrated through the trendlines in the FIG. 15.

[00125] For sulfur loadings beyond 2, no pyrrhotite is observed furthermore across the entire zone. The products obtained at excessive sulfur is a mole of pyrite and the excess unreacted sulfur in found in the solution(S-MATT) phase till 450 °C. While beyond 450 °C the unreacted sulfur is present in the gas phase in the form of S2 since the temperature is well beyond the boiling point of sulfur.

3. Zone 700 °C - 900 °C (T700-900)

[00126] In this temperature zone, the only phase is pyrrhottite across all ranges of sulfur loading. Herein too the pyrrhottite phase decreased with increasing the temperature up until sulfur loading equals 1. For sulfur loadings beyond 1, the product consists of 1 mole of pyrrhotite and the excess unreacted sulfur is found in the gaseous phase in form of S2.

4. Zone 950 °C - 1000 °C (T950-1000)

[00127] In this temperature range, to avoid the MATT phase, the sulfur loading was kept at excess with respect to iron (>1). The pyrrhotite phase was formed and any unreacted sulfur was found in the gas phase in form of S2.

5. Extent of sulfidation

[00128] The sulfidation extent is measured in the pyrrhotite phase across the entire temperature range (200 °C -900 °C). It was observed that the sulfidation increased with an increase of the temperature for sulfur loadings up till 1 while the sulfidation extent is peaked at 700 °C for sulfur loadings beyond 1. This is shown in FIG. 16.

E. Experimental Example with Mixed Metal Sulfides

[00129] In this example, thermodynamic analysis using FactSage 7.3 was done on Fe-Ni-S and Fe-Cu-S system to determine improvement over Fe-S system. 1 mol of Fe along with 1 mol of Ni/Cu was sulfidized using 10 mols of H2S at 600°C. The formed bimetallic sulfide was reacted with 1 mol of H2 at 600°C. The reactions were similar to those given in Fe-S section. Comparison based on H2S generation for different metal sulfides is depicted in FIG. 17. H2S generation is normalized with respect to sulfur present in metal sulfide.

[00130] Fe-Cu-S system shows a 53% improvement in H2S formation over an Fe-S system. This means that Fe-Cu-S can push the equilibrium of alkane dehydrogenation using less amount of material. To confirm the regenerability of these bimetallic sulfides, sulfides post H2 reaction were reacted with incremental amounts of H2S. As an amount of Fe remains constant, H2S addition and sulfur content was normalized based on Fe to keep consistent with Fe-S single sulfide system. Both the Fe-Cu-S and Fe-Ni-S sulfides show complete regenerability at 600°C as indicated by FIG. 18.

[00131] Like the pyrrhotite phase of Fe, bimetallic sulfides form phase of FeMSz where M is either Cu or Ni. The swing occurs between different [z] values. The change of [z] value for each sulfide can be calculated from FIG. 18 by subtracting sulfur content at zero addition of FLS with constant sulfur content achieved after addition of enough FLS.

F. Experimental Example with Co-S System

[00132] Metals other than Fe can exhibit multiple sulfidation states which can be exploited for alkane dehydrogenation. In this example, thermodynamic study on another transition metal Co is performed to estimate its overall performance. Co cannot be sulfidized with FLS, but it reacts with pure sulfur to form sulfides. 1 mol of Co was reacted with 10 mols of S at various temperatures. At every temperature, C0S2 was obtained as the most sulfidized phase which was reacted with 1 mol of Fh. Based on the temperature, mixture of CoS and C0S2 is obtained with generation of FLS and S.

[00133] Reaction (10) below shows sulfidation.

Co + 105 ® CoS ₂ + 85 (10)

[00134] Reaction (11) below shows reaction with Fh.

CoS ₂ + H ₂ ® a CoS + (1— a)CoS ₂ + b H ₂ S + (1— a— b) S (11)

[00135] Results for FbS and S generation are shown in FIG. 19. At low temperatures, entire conversion of C0S2 is not obtained which results in poor FLS generation. Sulfur is also emitted in very low quantities at 700 °C and 800 °C.

[00136] At 600°C, Co-S system is better than Fe-S system by a factor of 3.76. This huge enhancement is possible because C0S2 is very easily reduced to CoS by Fb. CoS can be regenerated back to C0S2 using sulfur for temperature range of 200 °C -800 °C as shown in Table 2 Table 2. Regenerability of Co-S System

[00137] As C0S2 is always achieved as the most sulfidized phase, [x] value is always 2. [y] value is calculated based on amount of CoS and C0S2 present, shown below:

[y] = mols of CoS + 2 * mols of CoS ₂

[00138] As stated above, the table clearly shows low sulfide conversion at lower temperatures. In contrast to Fe-S system, excess amount of sulfidizing agent (S) is not required to fully regenerate the sulfide.

G. Experimental Example with Pb-S System

[00139] Similar to transition metals, even metalloids such as Pb can display multiple oxidation states. Using FhS, Pb can be sulfidized only until PbS. PbS is a stable phase and does not react with Fh in temperature range of 200 °C -700 °C and shows little reactivity at

temperatures above 700°C. Hence, to achieve greater sulfidation, S is used to sulfidize and regenerate Pb metal sulfide. 1 mol of Pb was reacted with 10 mols of S at various temperatures. A mixture of PbS and PbSz (z > 1) is obtained which is then further reacted with 1 mol of H2. The reaction scheme is similar to Co. The result for H2S formation is depicted in FIG. 20.

[00140] The sulfided form of Pb tends to lose a lot of sulfur. However, as the analysis was restricted to 1 mol of H2, entire potential of this metal sulfide is not captured in the above figure. Above 800°C, some Pb evaporates in form of PbS and hence temperatures only up till 700°C are considered. PbS phase is formed only till temperatures below 400°C while PbSz is formed in entire temperature range. The reduced metal sufide can be regenerated using stoichiometric amount of S as seen in Table 3.

Table 3. Regenerability of Pb-S System

[00141] The tendency of Pb to retain S decreases as temperature increases and as almost complete conversion of Th to ThS is obtained, the ThS produced/S input parameter increases with temperature as seen in FIG. 20. However, as mentioned earlier, metal sulfides at lower temperatures are capable of processing more ¾, which is not studied to keep the study consistent with other metal systems. This can be seen by the difference in sulfur content between initial and reduced sulfided form (amount to be regenerated) which is being emitted as pure sulfur in this experimental example.

[00142] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Example methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the present disclosure. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting. [00143] The terms“comprise(s),”“include(s),”“having,”“has, ”“can,”“contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional acts or structures. The singular forms“a,” “an” and“the” include plural references unless the context clearly dictates otherwise. The present disclosure also contemplates other embodiments“comprising,”“consisting of’ and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not.

[00144] The modifier“about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier“about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression“from about 2 to about 4” also discloses the range“from 2 to 4.” The term“about” may refer to plus or minus 10% of the indicated number. For example,“about 10%” may indicate a range of 9% to 11%, and“about 1” may mean from 0.9-1.1. Other meanings of“about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.

[00145] Definitions of specific functional groups and chemical terms are described in more detail below. For purposes of this disclosure, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 75 ^th Ed., inside cover, and specific functional groups are generally defined as described therein.

[00146] For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated. For example, when a pressure range is described as being between ambient pressure and another pressure, a pressure that is ambient pressure is expressly contemplated.