Catalysts for oxidative sulfur removal and methods of making and using thereof

Abstract

Catalysts for oxidative sulfur removal and methods of making and using thereof are described herein. The catalysts contain one or more reactive metal salts dispersed on one or more substrates. Suitable reactive metal salts include those salts containing multivariable metals having variable valence or oxidation states and having catalytic activity with sulfur compounds present in gaseous fuel streams. In some embodiments, the catalyst contains one or more compounds that function as an oxygen sponge under the reaction conditions for oxidative sulfur removal. The catalysts can be used to oxidatively remove sulfur-containing compounds from fuel streams, particularly gaseous fuel streams having high sulfur content. Due to the reduced catalyst cost, anticipated long catalyst life and reduced adsorbent consumption, the catalysts described herein are expected to provide a 20-60% reduction in annual desulfurization cost for biogas with sulfur contents ranges from 1000-5000 ppmv compared with the best adsorbent approach.

Claims

1. A method for removing sulfur-containing compounds from a fluid fuel stream, the method comprising (a) contacting the fluid fuel stream with a catalyst in the presence of an oxygen-containing gas, wherein the catalyst comprises one or more substrates and one or more reactive metal salts, and wherein the metal in the one or more reactive metal salts is in its lowest possible oxidation state, wherein step (a) occurs under conditions in which greater than 70% of the sulfur-containing compounds convert to elemental sulfur in the gaseous state, wherein the one or more reactive metal salts are selected from the group consisting of chlorides of transition metals having multiple oxidation states, sulfates of transition metals having multiple oxidation states, and combinations thereof, and wherein the method does not involve a Claus reaction.

2. The method of claim 1, wherein the oxygen-containing gas is selected from the group consisting of oxygen, air, ozone, hydrogen peroxide, and combinations thereof.

3. The method of claim 1, wherein the fluid fuel stream has a sulfur content of 4% vol. or less, prior to contacting the catalyst.

4. The method of claim 1, wherein the fluid fuel stream is a gaseous fuel stream selected from the group consisting of land fill gases, natural gas from natural gas wells, flammable gases from oil wells, flammable gases from tar sands, syngas, and flare gas.

5. The method of claim 1, wherein the sulfur-containing compounds are selected from the group consisting of H.sub.2S, CS.sub.2, mercaptans, thiols, COS, and combinations thereof.

6. The method of claim 1, wherein oxygen to sulfur atomic ratio inside the fluid fuel stream is from about 1 to about 100, prior to contacting the catalyst.

7. The method of claim 1, wherein the fluid fuel stream contacts the catalyst at a temperature from about 160 C. to about 300 C., wherein an oxidized state of the catalyst converts sulfur-containing compounds to elemental sulfur.

8. The method of claim 7, wherein the temperature is from about 180 C. to about 250 C.

9. The method of claim 1, wherein less than 5% of sulfur species are converted to SO.sub.2.

10. The method of claim 1, wherein the method has a single pass conversion of from about 80% to about 99%.

11. The method of claim 1, wherein the method has a conversion rate of at least 80% at a gas hourly space velocity (GHSV) of 500-15,000 hr.sup.1 or a resident time of 0.24-7.2 seconds.

12. The method of claim 1, further comprising passing the fluid fuel stream through a sorbent material, a scrubber, or other sulfur removal materials or systems to remove remaining sulfur-containing compounds.

13. The method of claim 12, wherein following the passing step, the amount of remaining sulfur-containing compounds is from about 1 ppm to about 0.1 ppm.

14. The method of claim 1, wherein the one or more substrates comprise a metal oxide or mixed metal oxides.

15. The method of claim 14, wherein the metal oxide or mixed metal oxides are selected from the group consisting of aluminum oxide, titanium dioxide (TiO.sub.2), silicon dioxide (SiO.sub.2), and combinations thereof.

16. The method of claim 1, wherein the one or more metal salts are selected from the group consisting of CuCl, MnCl.sub.2, MnSO.sub.4, FeCl.sub.2, FeSO.sub.4, NiCl.sub.2, NiSO.sub.4, and combinations thereof.

17. The method of claim 1, wherein the one or more metal salts are present in an amount from about 1% to about 20% by weight of the catalyst.

18. The method of claim 1, wherein the one or more substrates comprise one or more metal oxides in an amount from about 80% to about 99% by weight of the catalyst.

19. The method of claim 1, wherein the catalyst further comprises a compound which is an oxygen sponge.

20. The method of claim 19, wherein the compound is a lanthanide oxide.

21. The method of claim 20, wherein the compound is cerium oxide.

22. The method of claim 19, wherein the compound is magnesium oxide and/or nickel oxide.

23. The method of claim 19, wherein the compound is present in an amount of about less than 10% by weight of the catalyst.

24. The method of claim 19, wherein the compound is present in an amount from about 2% to about 7% by weight of the catalyst.

25. The method of claim 1, wherein the metal ions are oxidized to the next higher oxidation state by oxygen in the oxygen-containing gas at a temperature in the range of 100-300 C.; and wherein the metal ions at the higher oxidation state oxidize molecules containing one or more SH groups to form elemental sulfur.

26. The method of claim 25, wherein the molecules containing one or more SH groups are selected from the group consisting of H.sub.2S and mercaptans.

27. The method of claim 1, wherein in step (a) less than 5% of the sulfur-containing compounds are converted to sulfur dioxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the percent conversion of H.sub.2S as a function of temperature ( C.) using gamma-alumina as catalyst in the presence of oxygen and sulfur dioxide. The percent conversion was performed with an H.sub.2SH.sub.2 mixture at GHSV of 5000 hr.sup.1. The O/S was 1.5.

(2) FIG. 2 is a graph showing the percent conversion of H.sub.2S as a function of time (minutes) for various catalysts. The rate of conversation was measured at 200 C. and O/S of 1.2 in the presence of 0.4% H.sub.2S-20% H.sub.2-79.6% CO.sub.2 at a GHSV of 1000 h.sup.1.

(3) FIG. 3 is a graph showing the percent conversion of sulfur compounds COS and butyl thiol as a function of time (minutes) for KI/MgOAl.sub.2O.sub.3 at 200 C. and GHSV of 1000 hr.sup.1 and O/S of 1.2.

(4) FIG. 4 is a graph showing the rate of conversion of sulfur compounds as a function of time (minutes) for KI/MgOAl.sub.2O.sub.3 at different O/S at 200 C. with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2 mixture at GHSV of 1000 h.sup.1.

(5) FIG. 5 is a graph showing the rate of conversion of sulfur compounds as a function of time (minutes) for KI/MgOAl.sub.2O.sub.3 at different temperatures. The catalyst was tested initially at 200 C. with 4000 ppmv H.sub.2S in H.sub.2N.sub.2 mixture at a GHSV of 1000 hr.sup.1.

(6) FIG. 6 is a graph showing the rate of conversion of sulfur compounds as a function of time (minutes) for KI/MgOAl.sub.2O.sub.3 at different GHSVs. The test was conducted at 200 C. with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2 mixture.

(7) FIG. 7 is a graph showing H.sub.2S conversion for FeSO.sub.4/CeO.sub.2TiO.sub.2 at high GHSV of 10186 hr.sup.1. The catalyst was tested at 200 C. with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2N.sub.2 mixture.

(8) FIG. 8 is a graph showing H.sub.2S conversion as a function of temperature for FeCl.sub.2/CeO.sub.2Al.sub.2O.sub.3 at GHSV of 10186 hr.sup.1. The test was conducted initially at 200 C. with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2 mixture.

(9) FIG. 9 is a graph showing H.sub.2S conversion for MnCl.sub.2/CeO.sub.2Al.sub.2O.sub.3 at 200 C. and GHSV of 1000 hr.sup.1. The test was conducted with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2 mixture.

(10) FIG. 10 is a schematic describing the sulfur content of various gaseous fuel streams and the sulfur requirements for various applications.

DETAILED DESCRIPTION OF THE INVENTION

I. Definitions

(11) Oxidative sulfur removal, as used herein, generally means the oxidation of sulfur-containing compounds to elemental sulfur. In particular embodiment, sulfur-containing compounds are oxidized to elemental sulfur with little or no generation of sulfur dioxide.

(12) Catalyst, as used herein, refers to one or more substrates in combination with one or more compounds or materials that have catalytic activity.

(13) Substrate, as used herein, refers to one or more support materials that may be non-reactive when contacted by sulfur containing fuel streams and oxygen-containing gases. Some reaction of the substrate material can be tolerated provided it does not adversely affect the oxidative sulfur removal reaction.

(14) Reactive metal salts, as used herein, generally refers to metal salts responsible for the catalytic activity with sulfur-containing species found in the fuel stream.

(15) Physically absorbed, as used herein, generally means that the one or more reactive metal salts are physically associated with (e.g., physically adsorbed to), not chemically bound to, the one or more substrates.

(16) Oxygen sponge, as used herein, refers to a compound or compounds that facilitate oxygen adsorption, transport, and/or reaction.

(17) Impregnating, as used herein, generally means the process of placing the reactive metal salts, oxygen sponges, and/or their precursors on and/or in the supports. In some embodiments, this is done by allowing the substrate to interact with these components, typically in a solution. The impregnation step can be followed by thermal treatments to generate the catalyst in its final form.

(18) Fluid, as used herein, generally means a substance that has no fixed shape and yields easily to external pressure, such as a liquid or gas. The fluid fuel stream can be in the form of a liquid or gas.

(19) Gaseous fuel stream, as used herein, generally refers to a fuel stream that is in the form of a gas.

(20) Oxygen-containing gas(es), as used herein, generally refers to oxygen, oxygen-enriched gases, mixtures of gases containing elemental oxygen, such as air, or gases containing oxygen-containing molecules, such as ozone (O.sub.3), hydrogen peroxides (H.sub.2O.sub.2), SO.sub.2.

(21) High sulfur content, as used herein, generally means fuel streams, such as gaseous fuel streams, which contain sulfur-containing compounds in an amount from at least about 300 ppm to about 40,000 ppm.

(22) Stable, as used herein, generally means the catalysts are thermally stable and chemically stable. Thermally stable, as used herein, means the catalysts can oxidize sulfur-containing compounds at 200-300 C. with little or no decomposition of the catalyst. Chemically stable, as used herein, generally means that the catalysts will not react with H.sub.2, CO.sub.2, and halogenated compounds, and will not react with ammonia at its typical concentration found in gaseous fuel sources (e.g., <1%) at 200-300 C. or will react to such a small degree that it does not effective catalytic efficiency.

II. Catalysts

(23) A. Substrate

(24) The catalysts described herein contain one or more substrates. Suitable substrates include activated carbon, metal or metalloid oxides, and combinations thereof. In some embodiments, the substrate is two or more metal or metalloid oxides, herein referred to as mixed metal oxides. Suitable metals include transition metals such as titanium, and metalloids, such as silicon and aluminum. Exemplary metal and metalloid oxides include, but are not limited to, aluminum oxide (Al.sub.2O.sub.3), titanium dioxide (TiO.sub.2), silicon dioxide (SiO.sub.2), ceria, and combinations thereof.

(25) The one or more substrate materials are generally present in an amount from about 80% to about 99% by weight of the catalyst, preferably from about 80% to about 95% by weight of the catalyst, more preferably from about 90% to about 95% by weight of the composition.

(26) In order to load sufficient amounts of the metal salts on the substrates, substrates with large pore volume are preferred. Typical pore volume is around 0.2-1.2 cc/g of support. Typical medium pore diameter is in the range of 10-200 . High surface area helps to facilitate the oxidation reaction. Typical surface area is in the range of 40-600 m.sup.2/g of support.

(27) Particle size is typically less than 3 mm due to slow mass transfer of sulfur vapor inside the catalyst particles. Minimal particle size is selected such that significant pressures drops are avoided. Accordingly, the particle size is typically in the range of 14-20 mesh (0.8-1.4 mm).

(28) B. Reactive Metal Salts

(29) The catalysts described herein contain one or more reactive metal salts which are primarily responsible for the catalytic activity of the catalyst. In some embodiments, the metal salts are multivariable metals having variable valence or oxidation states and having catalytic activity with sulfur compounds in the fuel stream. Examples of these classes of salts include, but are not limited to, chlorides of transition metals having multiple oxidation states, sulfates of transition metals having multiple oxidation states, and combinations thereof. Examples of species of these salts include, but are not limited to, CuCl, MnCl.sub.2, MnSO.sub.4, and FeSO.sub.4/Fe.sub.2(SO.sub.4).sub.3.

(30) For the reactive metal salts described herein, it is preferred that (1) the metal ions are in the lowest or a lower oxidation state; (2) the metal ions can be oxidized to the next higher oxidation state by oxygen in the temperature range of 100-300 C.; (3) the metal ions at higher oxidation states can oxidize H.sub.2S to elemental sulfur; (4) the cations will not be oxidized by the metal ions at the next higher oxidization state; and (5) the salts must be thermally and chemically stable under various operation conditions.

(31) For example, CuCl.sub.2 is not a suitable reactive metal salt. Cu is in its highest oxidization state, therefore, it can only be reduced to CuCl with generation of hydrochloric acid (HCl). HCl, however, adversely affects the H.sub.2S adsorption on the surface of the catalyst. Moreover, HCl is also poisonous and corrosive to most catalysts and reactor equipment, such as pipings. For the same reason, other metal chloride salts, such as FeCl.sub.3 are not suitable catalysts.

(32) In other embodiments, the reactive metal salts are salts of alkaline and alkali earth metals, particularly those metals after the third row in the periodic table, such as bromide and iodide salts. Examples include, but are not limited to, KI, CaI.sub.2, and combinations thereof. For salts such as MnI.sub.2 and MnBr.sub.2, Mn cannot be oxidized to its highest oxidization state without oxidizing I.sup. and perhaps Br.sup.. As a result, elemental iodine and bromine are generated as intermediates. These intermediates can oxidize H.sub.2S and generate elemental sulfur. Therefore, salts which, upon oxidation, generate elemental halogen, e.g., iodine and/or bromine, which in turns oxidizes H.sub.2S to elemental sulfur may also be used.

(33) The reactive metals salts can be used alone or in combinations of one or more of the salts described above. The one or more reactive metal salts are present in an amount from about 1% to about 20% by weight of the catalyst, preferably from about 1% to about 10% by weight of the catalyst, more preferably from about 5% to about 10% by weight of the catalyst. In some embodiments, the amount of the one or more reactive salts is about 7% by weight of the catalyst.

(34) C. Oxygen Sponge

(35) In some embodiments, the catalyst contains one or more compounds that function as an oxygen sponge under the reaction conditions for oxidative sulfur removal. In some embodiments, the oxygen sponge is one or more metal oxides. Examples of suitable metal oxides include, but are not limited to, lanthanide oxides, such as cerium oxide and alkaline earth oxides, such as magnesium oxide.

(36) D. Form of the Catalyst

(37) The catalysts described herein can be prepared in any form. Exemplary forms include, but are not limited to, powder, granules, pellets, slabs, rings, trilobes, saddles, extrudates, or monoliths.

III. Methods of Making the Catalysts

(38) The catalysts are prepared by dispersing the one or more reactive metal salts on the one or more substrates. The metal salts have weak interactions with the substrates compared to similar catalysts. For example, the metal salts in the catalysts described herein can be easily removed by dissolving the catalyst in aqueous solution. The one or more reactive metal salts are physically adsorbed to the substrate, not chemically bound to the substrate.

(39) The catalysts are prepared by impregnating the one or more reactive metal salts into the substrate. A typical impregnation can be carried out by uniformly contacting an impregnation salt solution onto the support particles while stirring. Multiple reactive salts can added in one impregnation step or in multiple steps.

(40) Once the impregnation step is complete, the catalyst is dried. Methods of drying include passing air through the catalyst at an elevated temperature, e.g., 200 C., for quick drying or passing dry gas through the catalyst at room temperature for an extended period of time to remove crystallized water resulting from impregnation. In contrast, catalysts having a similar composition, such as catalysts for the Deacon process, require a high-temperature calcination (500-800 C.) step, which enables the formation of strong chemical bonds between active catalysts and the metal oxide substrate.

IV. Methods of Oxidative Sulfur Removal

(41) Catalysts containing one or more reactive metal salts impregnated in one or more substrates are described herein. The catalysts can be used to selectively oxidize sulfur-containing compounds, such as H.sub.2S, to elemental sulfur according to Equation 1.
H.sub.2S(g)+0.5O.sub.2(g).fwdarw.H.sub.2O(g)+S.sub.8(g)

(42) The catalyst described herein can be used to treat a variety of fuel streams, particularly gaseous fuel streams, such as biogas, frac gas, gasified biomass, and gasified coal/bitumen. Many of the gaseous fuel streams have a high sulfur content. In order to be suitable for use as transportation fuels and/or power generation, the sulfur content must be significantly decreased. FIG. 10 describes the sulfur content of various gaseous fuel streams and the sulfur requirements for various applications.

(43) The catalysts described herein can be used to reduce the amount of inlet sulfur-content species by about 5% to about 10%, preferably from about 5% to about 20%, preferably from about 5% to about 50%, more preferably from about 5% to about 75%, most preferably from about 5% to about 95% after condensation of liquid elemental sulfur. In some embodiments, the amount of inlet sulfur-containing species is decreased at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% after condensation of liquid elemental sulfur. If additional desulfurization is required, the outlet gas can be passed through a sorbent bed to remove remaining sulfur-containing species.

(44) In a typical oxidative sulfur removal system, the system contains a catalysts bed containing one or more of the catalysts described herein; a condensation unit which condenses the elemental sulfur in the gaseous fuel stream into liquid elemental sulfur, and optionally a sorbent unit or scrubber which can remove more of the sulfur-containing compounds if necessary. Commercially available sorbents/systems, such as ZnO, activated carbon, water scrubbing, etc. are suitable for the removal of the sulfur-containing compounds.

(45) In some cases, liquid sulfur can accumulate inside the pores of the catalysts. This accumulation can cause severe deactivation of the catalyst. In these cases, periodically increasing the temperature to a higher level will remove the liquid sulfur from the pores while maintaining the reaction at ideal conditions. This approach can extend the life of the catalyst.

(46) The catalysts can be used in combination with oxygen or an oxygen-containing gas. Suitable oxygen-containing gases include, but are not limited to, oxygen, sulfur dioxide, air, ozone, hydrogen peroxide, or combinations thereof. The concentration of oxygen or oxygen-containing gas can vary. In some embodiments, the atomic ratio of oxygen to sulfur is from about 0.5 to about 100, preferably from about 1.0 to about 100, more preferably from about 1.0 to about 25, most preferably from about 1.0 to about 5. In particular embodiments, the atomic ratio of oxygen to sulfur is from about 1.0 to about 4.0.

(47) The catalysts described herein generally maintain a very high single-pass conversion rate. For example, the catalysts exhibited a single pass conversion rate of 85-99% at 200 C. and an O/S of 1.2 in the presence of 0.4% H.sub.2S-20% H.sub.2-79.6% CO.sub.2. Similar results were achieved using CH.sub.4 instead of CO.sub.2. Moreover, there was no SO.sub.2 detected with any of the metal salt-based catalysts.

(48) The catalysts described herein are active over a variety of temperatures. For example, the catalysts exhibit a percent conversion of 85-90% at 200 C., 80-85% at 180 C. and 90-95% at 220 at an O/S of from about 1 to about 2, preferably about 1.2. In the 180-220 C. temperature range, no SO.sub.2 was detected by GC-PFPD and elemental sulfur was the only product. The data shows that the catalyst results in efficient sulfur conversion even with significant temperature variations with little or no production of SO.sub.2. The ideal operative temperature range is from about 160 C. to about 300 C., preferably from about 180 C. to about 250 C.

(49) Since the oxidative sulfur removal (OSR) catalysts uses oxygen as the oxidizer, the catalysts have extremely high equilibrium for each reaction. This is a significant advantage over Claus reaction catalysts that rely on SO.sub.2 as the oxidizer and suffer from equilibrium limitations.

(50) The OSR catalysts described herein use the same catalyst supports used in Claus reaction catalysts. However, Claus reaction catalysts do not contain reactive metal salts. Therefore, the Clause reaction requires much higher temperatures (300-400 C.) to activate the reaction (see FIG. 1). Under these conditions, the reaction can reach a conversion as high as 90% at 350 C. at a high O/S ratio of 1.5; however, a significant amount of H.sub.2S is converted to SO.sub.2. SO.sub.2 is highly corrosive and a health and environmental hazard in its own right. Production of significant amounts of SO.sub.2 can also damage sensitive components in fuel cells and other reactors.

(51) Other oxidative desulfurization processes also utilize very high oxygen to atomic sulfur (O/S) ratios in order to achieve high sulfur conversions. The excess oxygen reacts with sulfur species to form SO.sub.2, a troublesome contaminant on the anode side due to the formation of stable metal sulfates. In some embodiments, the amount of SO.sub.2 generated using the catalysts described herein is less than about 5%, 4%, 3%, 2%, 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1% at an oxygen to sulfur ratio of 1-100, 1-50, 1-25, 1-20, or 1-10.

(52) The catalysts described herein exhibited a sulfur conversion rate of 0.83-0.95 over O/S ratio of 1.2-2.4. At a ratio of 1.2, 20% above the stoichiometric amount, there was no SO.sub.2 detected. After the O/S was increased to 2.4 (or 140% excess oxygen above the stoichiometric amount), SO.sub.2 still was not detected. This suggests the catalyst has a very high selectivity to elemental sulfur formation and that SO.sub.2 formation is negligible making these catalysts excellent candidates for fuel cell applications.

(53) Besides H.sub.2S, the OSR catalysts described herein can also oxidize other sulfur species such as mercaptans and COS. Several metal salts catalysts were tested for the oxidation of butyl thiol and COS. The oxidation of butyl thiol and COS is shown in reactions 2 and 3. Both reactions have high equilibrium constants: 7.3910.sup.20 for n-butyl thiol and 2.0110.sup.23 for COS at 200 C.
RSH(g)+0.5O.sub.2(g)=ROH(g)+S.sub.8(g)(2)
COS(g)+0.5O.sub.2(g)=CO.sub.2(g)+S.sub.8(g)(3)
The test results show that the catalysts described herein convert a high percentage of mercaptans, such as butyl thiol. The catalyst reduced butanethiol from 500 ppmv to less than 100 ppmv.

(54) The catalysts were less active for COS; the conversion remained at 60%. This suggests that the catalysts are most active against molecules containing one or more SH groups. In cases of high concentrations of COS a COS hydrolysis reactor can be used prior to the gas entering the catalyst bed. Since most gas mixtures for fuel cells, such as natural gas and reformate, contain extremely low levels of COS, the catalysts described herein can be used to remove high concentrations sulfur compounds from gas streams for fuel cell applications. Moreover, the sulfur conversion at various O/S ratios maintained stable. The sulfur conversion was in the range of 0.83-0.95. The O/S ratio appeared to have no significant effects on the sulfur conversion rate.

(55) The efficiency of the catalyst was also evaluated as a function of flow velocity. The same amount of catalyst was tested in a varying flow velocity or gas hourly space velocity (GHSV) at a constant temperature of 200 C. The catalyst was initially tested at a GHSV of 1000 h.sup.1. The rate of sulfur conversion was maintained at 0.87-0.93 with the GHSV varying from 500 to 2000 h.sup.1. The data shows that the catalyst bed can tolerate significant variation in the flow rate.

(56) The OSR catalysts described herein typically convert 85-95% of sulfur species in gaseous fuel streams, such as biogas, to elemental sulfur. The catalysts are formulated to improve catalyst life for desulfurization of high sulfur containing gaseous fuel streams. The catalysts are thermodynamically and chemically stable in the presence of contaminants such as CO.sub.2, NH.sub.3, halogenated compounds and the process temperatures, according to analyses performed with HSC Chemistry v7.0.

(57) Because the majority (85-95%) of sulfur is removed by OSR in the form of elemental sulfur, the adsorbent consumption and solid waste generation have decreased by a factor of 7-20 compared with traditional adsorbent approaches. Moreover, due to the relatively high operating temperatures of the catalysts, the OSR reaction takes place rapidly, with GHSVs typically in the range of 500-2000 h.sup.1. As a result, the OSR reactor is typically small, ca. 168-670 liters for a 500 kWe system. Therefore, the catalytic approach can significantly reduce the overall size, weight, and cost of the desulfurization process. The annual costs of the OSR and adsorbent combination are less than the cost of the best available commercial sulfur adsorbent with a high sulfur capacity of 0.35 g/g adsorbent.

(58) The catalysts described herein exhibit one or more of the properties described above. The catalysts can exhibit any combination of the properties listed above.

(59) Due to the reduced catalyst cost, anticipated long catalyst life and reduced adsorbent consumption, the catalysts described herein are expected to provide a 20-60% reduction in annual desulfurization cost for biogas with sulfur contents ranges from 1000-5000 ppmv compared with the best adsorbent approach.

EXAMPLES

Example 1. Preparation of Oxidative Sulfur Removal (OSR) Catalysts

(60) 10 grams of Al.sub.2O.sub.3 with pore volume of 0.6 cc/g was impregnated with 6 ml of cerium nitrate solution (0.5 mol/L). The impregnated particles were dried in a flowing air stream (100 ml/min) for 8 hours. The particles were then calcined in a furnace at 500 C. in air. After the supported particles were cooled to room temperature, they are impregnated again with 6 ml of a FeSO.sub.4 solution (1 mol/L). The impregnated particles were dried in a flowing air stream at room temperature. After the particle weight dropped to 12.5 grams, the catalyst particles were dried at 200 C. for 30 minute in a flowing air stream. The drying temperature can be the same temperature used for the OSR reaction.

Example 2. Oxidative Sulfur Removal Performance of Metal Salt-Based OSR Catalysts

(61) The following catalysts were evaluated for oxidative sulfur removal: (1) CuClAl.sub.2O.sub.3; (2) MnCl.sub.2/CeO.sub.2Al.sub.2O.sub.3; (3) FeSO.sub.4/CeO.sub.2Al.sub.2O.sub.3; (4) KI/MgOAl.sub.2O.sub.3; and (5) CeO.sub.2Al.sub.2O.sub.3. The catalysts were tested at 200 C. at an O/S ratio of 0.6 in the presence of 0.4% H.sub.2S-20% H.sub.2-79.6% CO.sub.2 at a GHSV of 1000 h.sup.1. The results are shown in FIG. 2.

(62) All of the catalysts, with the exception of catalyst (5), showed greater than 85% conversion of H.sub.2S over a period of almost 5 hours. Catalyst (5), CeO.sub.2Al.sub.2O.sub.3, which lacks a reactive metal salt, showed signs of deactivation at starting at less than 100 minutes. This parallels reports in the literature that mixed metal oxide catalysts are prone to deactivation after a short period of time.

Example 3. Desulfurization Activity for Sulfur-Containing Species Other than H2S

(63) The catalyst KI/MgOAl.sub.2O.sub.3 was evaluated for sulfur-containing compounds other than H.sub.2S. The catalyst was tested at 200 C. at an O/S ratio of 1.2 in the presence of (1) 500 ppmv butanethiol-methane and (2) 1000 ppmv COSN.sub.2 at a GHSV of 1000 h.sup.1. The results are shown in FIG. 3.

(64) FIG. 3 shows that the catalysts convert a high percentage of mercaptans, such as butyl thiol. The catalyst reduced butanethiol from 500 ppmv to less than 100 ppmv. The catalyst was less active for COS; the conversion remained at 60%. This suggests that the catalysts are most active against containing SH group. Moreover, the sulfur conversion at various O/S ratios maintained stable. The sulfur conversion was in the range of 0.83-0.95. The O/S ratio appeared to have no significant effects on the sulfur conversion rate.

Example 4. Sulfur Conversion as a Function of O/S Ratio

(65) The catalyst KI/MgOAl.sub.2O.sub.3 was evaluated for percent sulfur conversion as a function of O/S ratio. The catalyst was tested at 200 C. with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2 at a GHSV of 1000 h.sup.1. The O/S ratio was varied from 1.2 to 2.4 and from 2.4 to 1.2. The results are shown in FIG. 4.

(66) The catalysts exhibited a sulfur conversion rate of 0.83-0.95 over an O/S ratio 1.2-2.4. At a ratio of 1.2, 20% above the stoichiometric amount, there was no SO.sub.2 detected. After the O/S was increased to 2.4 (or 140% excess oxygen above the stoichiometric amount), SO.sub.2 still was not detected. This suggests the catalyst has a very high selectivity to elemental sulfur formation and that SO.sub.2 formation is negligible.

Example 5. Sulfur Conversion as a Function of Temperature

(67) The KI/MgOAl.sub.2O.sub.3 catalyst was evaluated for percent sulfur conversion as a function of temperature. The catalyst was tested with 4000 ppmv H.sub.2S in H.sub.2N.sub.2 at a GHSV of 1000 hr.sup.1. The initial temperature was 200 C. The temperature was lowered to 180 C. and then raised to 220 C. The results are shown in FIG. 5.

(68) At 200 C., the catalyst had a sulfur conversion rate of 0.88-0.9. At a lower temperature, 180 C., the sulfur conversion dropped slightly to 0.83; at a higher temperature, 220 C., the conversion increased to 0.93. In the 180-220 C. temperature range, no SO.sub.2 was detected by GC-PFPD and elemental sulfur was the only product. The data shows that the catalyst results in efficient sulfur conversion even with significant temperature variations with little or no production of SO.sub.2.

Example 6. Sulfur Conversion as a Function of Flow Rate

(69) The KI/MgOAl.sub.2O.sub.3 catalyst was evaluated for percent sulfur conversion as a function of flow rate (GHSV). The catalyst was tested at 200 C. with 4000 ppmv H.sub.2S in H.sub.2N.sub.2. The GHSV was 1000 hr.sup.1. The GHSV was increased to 2000 hr.sup.1 and then reduced to 500 hr.sup.1. The results are shown in FIG. 6.

(70) The rate of sulfur conversion was maintained at 0.87-0.93 with the GHSV varying from 500 to 2000 h.sup.1. The data shows that the catalyst bed can tolerate significant variation in the flow rate.

Example 7. Iron (II) Sulfate (FeSO4) Based Catalyst

(71) An iron sulfate-based catalyst was supported on CeO.sub.2TiO.sub.2. The catalyst was evaluated for percent sulfur conversion at 200 C. and a GHSV of 10186 h.sup.1. The challenge gas was 4000 ppmv H.sub.2S in H.sub.2CO.sub.2N.sub.2. The results are shown in FIG. 7.

(72) The sulfur conversion was maintained at 0.84-0.93 and the desulfurization performance was very stable at this GHSV, which is 10 times higher than the GHSV used in other studies. This result suggests that the OSR catalyst can be further reduced.

Example 8. Iron (II) Chloride-Based Catalyst

(73) The catalyst FeCl.sub.2/CeO.sub.2Al.sub.2O.sub.3 was evaluated for percent sulfur conversion as a function of temperature. The catalyst was tested with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2N.sub.2. The GHSV was 10186 hr.sup.1. The initial temperature was at 200 C. The temperature was decreased to 180 C. and then increased to 200 C. and 220 C. The results are shown in FIG. 8.

(74) At this high GHSV, the catalyst maintained a high conversion of 0.8-0.94 with the significant temperature variation. The data suggests that this catalyst formulation has good temperature tolerance.

Example 9. Manganese (II) Chloride Based Catalyst

(75) The catalyst MnCl.sub.2/CeO.sub.2Al.sub.2O.sub.3 was evaluated for H.sub.2S oxidation. The catalyst was tested with 4000 ppmv H.sub.2S in H.sub.2CO.sub.2N.sub.2. The GHSV was 1000 hr.sup.1. The temperature was at 200 C. The results are shown in FIG. 9. MnCl.sub.2 demonstrated a very stable and high conversion of 0.88-0.97 during the test.