Development and reactivity tests of Ce-Zr-based Claus catalysts for coal gas cleanup

Claus reaction (2H₂S + SO₂ ↔ 3/nS_n + 2H₂O) was used to clean the gasified coal gas and the reactivity of several metal oxide-based catalysts on Claus reaction was investigated at various operating conditions. In order to convert H₂S contained in the gasified coal gas to elemental sulfur during Claus reaction, the catalysts having the high activity under the highly reducing condition with the moisture should be developed. CeO₂, ZrO₂, and Ce_1−xZr_xO₂ catalysts were prepared for Claus reaction and their reactivity changes due to the existence of the reducing gases and H₂O in the fuel gas was investigated in this study. The Ce-based catalysts shows that their activity was deteriorated by the reduction of the catalyst due to the reducing gases at higher than 220 °C. Meanwhile, the effect of the reducing gases on the catalytic activity was not considerable at low temperature. The activities of all three catalysts were degraded on the condition that the moisture existed in the test gas. Specifically, the Ce-based catalysts were remarkably deactivated by their sulfation. The Ce-Zr-based catalyst had a high catalytic activity when the reducing gases and the moisture co-existed in the simulated fuel gas. The deactivation of the Ce-Zr-based catalyst was not observed in this study. The lattice oxygen of the Ce-based catalyst was used for the oxidation of H₂S and the lattice oxygen vacancy on the catalyst was contributed to the reduction of SO₂. ZrO₂ added to the Ce-Zr-based catalyst improved the redox properties of the catalyst in Claus reaction by increasing the mobility of the lattice oxygen of CeO₂.     

Improvement of the method of obtaining gas sulfur

An improved method of obtaining gas sulfur using the Claus and Sulfren processes, which provides an increase in its yield to 99.6–99.8%, is suggested. To realize this method, new catalysts are developed, namely, the alumina catalyst for the Claus process, the catalyst for the reduction of SO₂, the catalyst for the Sulfren process, and the low-temperature catalyst for the direct oxidation of H₂S, which provided for the utilization of previously not used components of the gas medium H₂ and CO forming at the thermal stage. This method is recommended for introduction at the enterprises of OAO GAZPROM, Orenburg and Astrakhan, gas processing plants. No substantial changes in the hardware implementation of technological lines will be necessary; it will be sufficient to reconstruct the reactors of the Sulfren process.     

A study on the reactivity of Ce-based Claus catalysts and the mechanism of its catalysis for removal of H2S contained in coal gas

In this study, Claus reaction was applied for the selective removal of H₂S contained in the gasified coal gas, and the characteristics of Claus reaction over the Ce-based catalysts were investigated to propose the reaction mechanism. The Ce-based catalysts showed a high activity on Claus reaction. Specially, Ce_0.8Zr_0.2O₂ catalyst had a higher activity than CeO. On the basis of our experimental results, it was proposed that the selective oxidation of H₂S was carried out by the lattice oxygen in the Ce-based catalysts and that the reduction of SO₂ was performed by the lattice oxygen vacancy in the reduced catalyst. Since the mobility of the lattice oxygen in Ce_0.8Zr_0.2O₂ composite catalyst was better than the one in CeO₂, Ce_0.8Zr_0.2O₂ provided more lattice oxygen for the selective oxidation of H₂S. It was presumed that the reaction mechanism to convert H₂S and SO₂ into elemental sulphur over our prepared catalysts was different from the mechanism over the solid-acid catalysts. It is believed that Claus reaction over the Ce-based catalysts was carried out by the redox mechanism. Since the moisture was contained in the major components, CO and H, of the gasified fuel gas, the effects of CO and H₂O on the catalytic reaction were investigated over a Ce-based catalyst. The conversion of H₂S and SO₂ was decreased in Claus reaction over the Ce-based catalysts as the concentration of either H₂O or CO in the gasified coal gas was increased. Under the circumstances of the coexistence of both moisture and CO, however, the conversion was increased as the concentration of CO was increased. The reactivity of Claus reaction was varied in terms of the concentration ratio of CO to H₂O. The maximum conversion of H₂S and SO₂ was achieved in the condition of that the concentration of CO contained in the reacting gas was higher than the one of H₂O. The conversions of H₂S and SO₂ did not match to the stoichiometric ratios of Claus reaction. The higher conversion of H₂S was obtained in the higher concentration of H₂O, while the higher conversion of SO₂ was achieved in the higher concentration of CO. It was another evidence to indicate that the Claus reaction over the Ce-based catalysts was carried out by the redox mechanism.     

Catalytic processes and catalysts for production of elemental sulfur from sulfur-containing gases

The modern technologies for production of elemental sulfur are considered. It is demonstrated that along with the further wide application of the conventional Claus process with conventional alumina catalyst in the observable future some new trends which may significantly influence the technological picture of recovered sulfur manufacturing may be formulated: active development of Claus tail gas cleanup processes with the stress on replacement of subdewpoint Sulfreen-type processes by processes of hydrogen sulfide selective oxidation by oxygen; development of novel highly-efficient technologies for hydrogen sulfide decomposition to sulfur and hydrogen; application of new catalysts forms, first of all — at microfiber supports for Claus and H₂S oxidation processes; wider application of titania and vanadia catalysts at the newly constructed Claus units; development of technologies and catalysts for direct purification of H₂S-containing gases and for catalytic reduction of SO₂ for sulfur recovery from smelter gases. All these prospective routes are actively developed by Russian science and some of them are completely based on domestic developments in this area.     

The Effect of Sulfur on ZrO<Subscript>2</Subscript>-Based Biomass Gasification Gas Clean-Up Catalysts

The effect of sulfur on biomass gasification gas clean-up over ZrO₂, Y₂O₃–ZrO₂ and SiO₂–ZrO₂ catalysts was examined. Experiments were carried out at the temperature range of 600–900 °C with sulfur free and 100 ppm H₂S containing simulated gasification gas feeds. A mixture of toluene and naphthalene was used as a tar model compound. Results revealed that the sulfur addition affected positively on the catalyst properties mainly at 600 and 700 °C: over Y₂O₃–ZrO₂ and ZrO₂ sulfur addition improved naphthalene and ammonia conversion. However, over SiO₂–ZrO₂ no clear effect with H₂S addition was observed. The effect of sulfur addition on the catalyst properties was connected to the formation of SO₂ from H₂S when oxygen was available. The intensity of the sulfur effect increased with the Lewis basicity strength of the catalysts. This indicates that the sulfur adsorption has a role in generating new type of active sites and/or plays role in changing the redox properties of the zirconia. Since the biomass gasification gas contains usually significant amounts of H₂S the sulfur tolerance of the zirconia based catalysts is a remarkable benefit.     

Analysis of the deactivation of Claus alumina catalyst during its industrial exploitation

Change in the activity of AO-NKZ-2 (AO-MK-2) alumina catalyst in the Claus reaction and transformations of carbonyl sulfide during operation over four years in the Claus reactor at the Magnitogorsk Metallurgical Combine’s coke-oven gas purification shop were studied at an average temperature of 245–260°C and a volume velocity of ∼2000 h⁻¹. The rate constants of the Claus reactions and COS transformation were determined, and the changes in the active surface area of the catalyst were investigated. Fundamental discrepancies in the rate and deactivation mechanism of the Claus catalysts were revealed with respect to the reactions of the conversion of hydrogen sulfide and carbonyl sulfide.     

Catalytic activity of faujasite-type zeolites towards the claus reaction

Commercial NaY and NaX zeolites showed significant catalytic activity towards the Claus reaction. Bronsted acidity retarded the catalytic activity to a much greater extent than Lewis acidity. Electron Spin Resonance (ESR) spectroscopic studies indicated the formation of SO₂^? anion radicals on the catalyst surface upon SO₂ adsorption. The reactivity of SO₂^? towards H₂S depended upon many factors. When the electron donating property of NaX was increased by impregnation with a small amount of NaOH, the Claus activity was enhanced and the activity of NaX containing 2.0 wt% NaOH was similar to that of a commercial Claus alumina catalyst. Infrared (IR) spectroscopic studies indicated physical and dissociative adsorption of H₂S; SO₂ was found to chemisorb on the zeolites.     

Thiophene hydrodesulfurization over modified alumina-supported molybdenum sulfide catalysts

Methanethiol has been synthesized by one‐step catalytic reaction from H₂S‐content syngas on K₂MoS₄/SiO₂ catalyst with selectivity over 95% under the optimum reaction conditions of 563 K, 2.0–3.0 MPa and 5–6% H₂S content in the feed syngas. The results of XRD and XPS showed that Mo–S–K phase on the surface of the catalyst K₂MoS₄/SiO₂ was responsible for the high activity and selectivity to methanethiol, and which may be restrained by the existence of (S–S)^2- species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalyst’s Sulfur Displacement by Thiophene, as Monitored by Exchange of 35S Radiotracer

The isotopic exchange has been studied between catalyst radiosulfur and H₂S, formed in thiophene hydrodesulfurization (HDS) (named S-displace) on alumina supported molybdena, on CoMoO_x, PdMoO_x, PtMoO_x and on silica–alumina supported NiWO_x. S-displace was compared with radiosulfur exchange data between catalyst radiosulfur and gas phase H₂S (S^exc) determined previously. The extent of S^exc was higher than that of the S-displace for Mo, CoMo in and NiW, whereas the extent of S-displace from PdMoO and PtMoO was significantly higher, than that of S^exc. Thiophene HDS product distribution data are discussed in terms of increased C=C hydrogenation and C–C hydrogenolysis activity, explained by increasing H₂S production with longer circulation time of the thiophene/H₂ mixture, The C₁/C₃<1 ratios among C₄-hydrogenolysis products indicate some coke formation. The decrease of thiophene HDS activity is presumably a consequence of increasing site-blocking with the formation of more H₂S and coke with longer duration of thiophene treatment.     

Synthesis of New Ferrocenylenesilylene Polymers by Direct Hydrosilylation/Polymerization of [FC–SiRH], FC?=?(η5-C5H4)Fe(η5-C5H4), Using Organometallic Alkenes and Alkynes

Synthesis of new ferrocenylenesilylene polymers was effected by direct hydrosilylation/polymerization of 1,1′-methylsilyl-ferrocenophane [FC–SiMeH] (FC = (η⁵-C₅H₄)Fe(η⁵-C₅H₄) with acetylenes and organometallic olefins using a Pt⁰ catalyst. The reaction of [FC–SiMeH] with HC₂Ph, HC₂SiMe₃, CH₂=CHSiMe₂Fp, Fp = (η⁵-C₅H₅)Fe(CO)₂ and CH₂=CHCH₂SiMe₂Fp in the presence of a platinum catalyst resulted in high yields of the corresponding hydrosilylated polymers. In the case of the acetylenes only β products were obtained, both E and Z isomers, whereas for the olefins both α and β-isomers were noted. Only in the case of the more bulky allylsilicon material was any unreacted SiH functionality retained in the polymer, an effect also noted when using [FCSiPhH] as the starting ferrocenophane. Cyclic voltammetric studies on these polymers revealed metal–metal interaction with two redox processes associated with the ferrocenylene Fe center along with an irreversible oxidation at higher potential for the Fp Fe atom. Ian: A pleasure to contribute; keep up the good work-aptp, cheers, Keith.     

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H₂SO₄/HZSM−5 catalyst prepared from a H₂SO₄‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH₄)₂WO₄ showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO₆)^n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H₂‐TPR results showed that the reduction of precursor of the 3% W–H₂SO₄/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W⁵⁺ and W⁴⁺, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W⁵⁺. The results seem to imply that the observed high DHAM activity on the W–H₂SO₄/HZSM‐5 catalyst was closely correlated with (WO₆)^n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H₂SO₄/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H₂SO₄/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH₄) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Regeneration behaviors of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for C2H6 dehydrogenation with CO2 to C2H4

The catalytic performance of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for conversion of C₂H₆ with CO₂ to C₂H₄ was examined in a continuous‐flow and fixed‐bed reactor. The results show that the Fe–Mn/Si‐2 catalyst exhibits much better reaction activity and selectivity to C₂H₄ than those of the Fe/Si‐2 catalyst. Furthermore, the coking–decoking behaviors of these catalysts were studied through TG. The catalytic performances of the catalysts after regeneration for conversion of C₂H₆ or dilute C₂H₆ in FCC off‐gas with CO₂ to C₂H₄ were also examined. The results show that both activity and selectivity of the Fe–Mn/Si‐2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si‐2 catalyst to refresh its reaction behavior after regeneration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reduced kinetic mechanism for combustion of synthesis gas at elevated temperatures and pressures

The key combustion reactions of synthesis gas at elevated initial temperatures (T ₀ = 500–700 K) and pressures (p = 10–30 atm) are identified by analyzing the kinetic mechanism. A reduced mechanism of the oxidation reactions of synthesis gas consisting of 14 elementary reactions involving 13 species is proposed which adequately describes the results of experimental data on the burning velocities of mixtures of synthesis gas with oxygen and inert diluents at T ₀ = 300–700 K, p = 10–30 atm, and ratios CO/H₂ = 0.05–0.95, and satisfactorily predicts the flame structure and the dependence of the flammability limits on the initial temperature at atmospheric pressure.     

Intrinsic kinetics of one-step dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst

The reaction kinetics of the dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst was investigated using an isothermal integral reactor at 220–260°C temperature, 3–7 MPa pressure, and 1,000–2,500 mL/g·h space velocity. The H₂/CO ratio of the synthetic gas was chosen between 3 : 1 and 6 : 1. The bi-functional catalyst was prepared by physically mixing commercial CuO/ZnO/Al₂O₃ and γ-alumina, which act as methanol synthesis catalyst and dehydration catalyst, respectively. The three reactions, including methanol synthesis from CO and CO₂ as well as methanol dehydration, were chosen as independent reactions. The Langmuir-Hinshelwood kinetic models for dimethyl ether synthesis were adopted. Kinetics parameters were obtained using the Levenberg-Marquardt mathematical method. The model was reliable according to statistical and residual error analyses. The effects of different process conditions on the reactor performance were also investigated.     

On the acidity of liquid and solid acid catalysts: Part 1. A thermodynamic point of view

The protonation equilibria of weak bases (B) in solid acids (HClO₄/SiO₂, CF₃SO₃H/SiO₂, H₂SO₄/SiO₂) were studied by UV spectroscopy and the results were compared to those obtained for analogous compounds in concentrated aqueous solutions of strong acids (HClO₄, CF₃SO₃H, H₂SO₄). The behaviour of B in liquid (L) and solid (S) phase was analysed by titration curves, log[BH⁺]/[B] ratios and thermodynamic pK _BH+ values. It has been shown that the proton transfer process acid → base (i.e., from (H+A^-)_(L,S) to (BH+A^-)_(L,S)} can be described by the relationship observed between the activity coefficient terms that are to be taken into account for acid–base equilibria occurring in nonideal systems ( – log(f _B f _B+/f _BH+)_(L,S)= -n _BA log(f _A–f _H+/f _HA)_(L,S)) and can be estimated by the n _BA values. Two “activity coefficient functions” (i.e., Mc(B) = – log(f _B f _B+/f _BH+)and Mc(s) = – log(f _A–f _H+/f _HA)) were used to describe, respectively, the equilibria of B and the equilibria of the acids in concentrated aqueous solutions and the meaning of terms “activity coefficient function” and “protonating ability of an acid” were discussed. The difference between “acidity functions”, determined for solutes (Ac(i)) and solvents (Ac(s)) in aqueous acids, and the H_x acidity functions, the latter developed for solutes in analogous media by the Hammett procedure, was also shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidative reforming of methane on structured Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite catalysts

The catalytic properties of Ni/Al₂O₃ composites supported on ceramic cordierite honeycomb monoliths in oxidative methane reforming are reported. The prereduced catalyst has been tested in a flow reactor using reaction mixtures of the following compositions: in methane oxidation, 2–6% CH₄, 2–9% O₂, Ar; in carbon dioxide and oxidative carbon dioxide reforming of methane, 2–6% CH₄, 6–12% CO₂, and 0–4% O₂, and Ar. Physicochemical studies include the monitoring of the formation and oxidation of carbon, the strength of the Ni-O bond, and the phase composition of the catalyst. The structured Ni-Al₂O₃ catalysts are much more productive in the carbon dioxide reforming of methane than conventional granular catalysts. The catalysts performance is made more stable by regulating the acid-base properties of their surface via the introduction of alkali metal (Na, K) oxides to retard the coking of the surface. Rare-earth metal oxides with a low redox potential (La₂O₃, CeO₂) enhance the activity and stability of Ni-Al₂O₃/cordierite catalysts in the deep and partial oxidation and carbon dioxide reforming of methane. The carbon dioxide reforming of methane on the (NiO + La₂O₃ + Al₂O₃)/cordierite catalyst can be intensified by adding oxygen to the gas feed. This reduces the temperature necessary to reach a high methane conversion and does not exert any significant effect on the selectivity with respect to H₂.     

In situ time-resolved characterization of novel Cu–MoO2 catalysts during the water–gas shift reaction

A novel and active Cu–MoO₂ catalyst was synthesized by partial reduction of a precursor CuMoO₄ mixed-metal oxide with CO or H₂ at 200–250 °C. The phase transformations of Cu–MoO₂ during H₂ reduction and the water–gas shift reaction could be followed by in situ time resolved XRD techniques. During the reduction process the diffraction pattern of the CuMoO₄ collapsed and the copper metal lines were observed on an amorphous material background that was assigned to molybdenum oxides. During the first pass of water–gas shift (WGS) reaction, diffraction lines for Cu₆Mo₅O₁₈ and MoO₂ appeared around 350 °C and Cu₆Mo₅O₁₈ was further transformed to Cu/MoO₂ at higher temperature. During subsequent passes, significant WGS catalytic activity was observed with relatively stable plateaus in product formation around 350, 400 and 500 °C. The interfacial interactions between Cu clusters and MoO₂ increased the water–gas shift catalytic activities at 350 and 400 °C.     

Performance of an oxygen-permeable membrane reactor for partial oxidation of methane in coke oven gas to syngas

Perovskite-type oxygen-permeable membrane reactors of BaCo_0.7Fe_0.2Nb_0.1O_3−δ packed with Ni-based catalyst had high oxygen permeability and could be used for syngas production by partial oxidation of methane in coke oven gas (COG). The BCFNO membrane itself had a poor catalytic activity to partial oxidation of CH₄ in COG. After the catalyst was packed on the membrane surface, 92% of methane conversion, 90% of H₂ selectivity, 104% of CO selectivity and as high as 15 ml/cm²/min of oxygen permeation flux were obtained at 1148 K. During continuously operating for 550 h at 1148 K, no degradation of performance of the BCFNO membrane reactor was observed under the condition of hydrogen-rich COG. The possible reaction pathways were proposed to be an oxidation-reforming process. The oxidation of H₂ in COG with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and H₂O reacts with CH₄ by reforming reactions to form H₂ and CO.     

Combinatorial approach to the preparation and characterization of catalysts for biomass steam reforming into syngas

A number of catalysts based on Al₂O₃ loaded with doped Ce-Zr mixed oxides and different active components (Cu, Cu-Ni, Ru, Pt, etc.) were synthesized via standard wet impregnation method using the robotic workstation. Ethanol (EtOH) was taken as a model compound of bio-oil and steam reforming of ethanol (ESR)—as a model reaction. Activity screening experiments performed at 600–700 °C in 0.5 vol.% C₂H₅OH + 2.5 vol.% H₂O + 97 vol.% He mixture revealed that the most effective catalyst composition is Ru/Ce_0.4Zr_0.4Sm_0.2/Al₂O₃. Catalytic activity investigations at high reagent concentrations (10 vol.% C₂H₅OH + 40 vol.% H₂O + 50 vol.% N₂) at 650–800 °C confirmed this fact, revealing also that at high temperatures the activity of Cu-Ni catalysts is comparable with that of Ru-containing catalyst.     

Alumina supported tungsten hydrides, new efficient catalysts for alkane metathesis

Silica and alumina supported tantalum and tungsten hydrides were tested in alkane metathesis, for comparison of their catalytic properties. In propane metathesis [W]–H/Al₂O₃ proves to be twice more efficient than the usual [Ta]–H/SiO₂ catalyst which is still better than [Ta]–H/Al₂O₃ and [W]–H/SiO₂. Tungsten based catalysts lead to a narrower distribution in the products selectivity and to a higher amount of linear products than [Ta]–H/SiO₂. [W]–H/Al₂O₃ is also a better catalyst than [Ta]–H/SiO₂ for butane metathesis but is less efficient with ethane. Whereas the results in the case of propane or butane can involve the higher initial activity and a lower deactivation of the tungsten catalyst, in the case of ethane, mechanistic or kinetic aspects can be envisaged in particular concerning the difficulty to dehydrogenate ethane into ethylene.