Influence of final calcination temperature and calcium addition on the properties of titania-based sulfur recovery catalysts

Compared to traditional alumina Claus catalysts, titania based sulfur recovery catalysts demonstrate improved initial activity for the recovery of elemental sulfur from both hydrogen sulfide and sulfur dioxide and are less prone to aging by sulfation. The influence of the preparation mode on the properties of titania catalysts is studied in detail: With increasing calcination temperature, surface area drastically decreases, whereas mechanical strength goes through a minimum, with only minor modifications of total pore volume and catalytic activity. Addition of calcium during catalyst preparation hinders the loss of mechanical properties while allowing a higher calcination temperature. Hydrothermal aging of such catalysts is therefore limited during operation in the plant.     

Elucidation of hydrodesulfurization mechanism on molybdenum‐based catalysts using 35S radioisotope pulse tracer methods

Elucidation of the hydrodesulfurization (HDS) mechanism on molybdenum‐based catalysts using radioisotope tracer methods and reaction kinetics is reviewed. Firstly, to investigate the sulfidation state in Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts, presulfiding of these catalysts has been performed using a ³⁵S pulse tracer method. Secondly, HDS of radioactive ³⁵S‐labeled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts and cobalt‐promoted molybdena–alumina catalysts in a pressurized flow reactor to estimate the behavior of sulfur on the working catalysts. Finally, sulfur exchange of a ³⁵S‐labeled catalyst with hydrogen sulfide was performed to estimate the relationship between the amount of labile sulfur and catalytically active sites.     

Analysis of the activity of an aluminum oxide Claus catalyst in commercial operation

Activity dynamics of an AO-MK-2 aluminum oxide catalyst in conversion of hydrogen sulfide and carbonyl sulfide during four years of service in an industrial Claus reactor was analyzed. The rate constants of the Claus reaction were determined and changes in the active surface area of the catalyst were examined.     

The molecular mechanism of low-temperature decomposition of hydrogen sulfide under conjugated chemisorption-catalysis conditions

The molecular mechanism of interaction of two hydrogen sulfide molecules with the (Co^III-H_o)₂S₂(SH₂)₄ model active center containing occluded hydrogen was studied by the density functional theory method with the B3P86 hybrid exchange-correlation functional. The reaction was found to occur in the following elementary steps: molecular adsorption of hydrogen sulfide ? dissociative chemisorption ? S-S bond formation in the surface intermediate {2Co^III ? (μ-S₂) + 2H(ads)} with the release of the first hydrogen molecule into the gas phase H₂(g) ? the release of the second hydrogen molecule into the gas phase H₂(g) ? the formation of cyclooctasulfur in the reaction 4S₂(ads) → S₈(ads). The first three steps occur spontaneously at room temperature, the thermodynamic driving force of the process being the stoichiometric reaction of S-S bond formation at the stage of conjugated chemisorption of two hydrogen sulfide molecules on two adjacent metal ions with the release of the first hydrogen molecule into the gas phase. The catalytic cycle is terminated by the recombination of molecular sulfur S₂ into cyclooctasulfur S₈ in the adsorption layer and the release of the second hydrogen molecule into the gas phase.     

纳米分子筛的合成及硫化氢吸附性能

利用无模板水热法合成纳米尺寸的小孔SOD型和大孔X型分子筛, 并考察了其硫化氢脱除性能. 通过对比两种分子筛的硫化氢吸附性能, 证实分子筛对硫化氢的吸附主要集中于孔道内和可及的活性位. 考察了晶粒尺寸、 体积空速和吸附温度等因素对硫化氢吸附的影响, 发现纳米尺寸X型分子筛具有更大的硫容, 并且在低温和低空速下分子筛中的硫化氢分子易于脱除. 随后对纳米X型分子筛进行了金属离子交换改性, 发现Cu改性的分子筛硫容优于其它离子改性的吸附剂, 能达到20.6 mg/g. NaX-N和Cu-NaX-N再生后的硫容分别为新鲜分子筛的62.4%和78.5%.     

Low-temperature catalytic decomposition of hydrogen sulfide into hydrogen and diatomic gaseous sulfur

The thermodynamics of three pathways of the hydrogen sulfide decomposition reaction is considered. In the thermal process, the gas-phase dissociation of hydrogen sulfide yields hydrogen and diatomic singlet sulfur. Over sulfide catalysts, the reaction proceeds via the formation of disulfane (H₂S₂) as the key surface intermediate. This intermediate then decomposes to release hydrogen into the gas phase, and adsorbed singlet sulfur recombines into cyclooctasulfur. Over metal catalysts, H₂S decomposes via dissociation into surface atoms followed by the formation of gaseous hydrogen and gaseous triplet disulfur. The last two pathways are thermodynamically forbidden in the gas phase and can take place at room temperature only on the surface of a catalyst. An alternative mechanism is suggested for hydrogen sulfide assimilation in the chemosynthesis process involving sulfur bacteria. To shift the hydrogen sulfide decomposition equilibrium toward the target product (hydrogen), it is suggested that the reaction should be conducted at room temperature as a three-phase process over a solid catalyst under a layer of a solvent that can dissolve hydrogen sulfide and sulfur. In this case, it is possible to attain an H₂S conversion close to 100%. Therefore, hydrogen sulfide can be considered as an inexhaustible source of hydrogen, a valuable chemical and an environmentally friendly energetic product.     

On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures

Activated carbons of various origins (bituminous coal, wood, coconut shells, and peat) were studied as adsorbents of hydrogen sulfide. Before the experiments the surface of the adsorbents was characterized by using the sorption of nitrogen, Boehm and potentiometric titrations, thermal analysis, and FTIR. The adsorbents were chosen to differ in their surface areas, pore volumes, and surface acidities. To broaden the spectrum of surface acidity, carbons were oxidized by using nitric acid and ammonium persulfate. After hydrogen sulfide adsorption the species present on the surface were analyzed using thermal analysis, ion chromatography, and elemental analysis. The H(2)S breakthrough capacity tests showed that the performances of different carbons differ significantly. For a good performance of carbons as hydrogen sulfide adsorbents a proper combination of surface chemistry of carbon and porosity is needed. It was demonstrated that a more acidic environment promotes the formation of sulfur oxides and sulfuric acid despite yielding small H(2)S removal capacities. On the other hand, a basic environment favors the formation of elemental sulfur (sulfur radicals) and yields high capacities. The presence of a sufficient amount of water preadsorbed on the carbon surface to facilitate dissociation also plays an important role in the process of H(2)S adsorption/oxidation. The results showed that there is a critical value in carbon surface acidity, which when exceeded results in a negligible hydrogen sulfide breakthrough capacity. This is consistent with the mechanism of H(2)S adsorption on unmodified carbons, where the rate-limiting step is the reaction of adsorbed hydrogen sulfide ion with dissociatively adsorbed oxygen. When the acidity is expressed as pH, its value should be higher than 5 to ensure the effective removal of hydrogen sulfide from the gas phase. Study of carbon regeneration using water washing and heat treatment showed that the adsorbents can be regenerated to about 40% of their initial capacity.     

Oxidative addition of dihydrogen as the key step of the active center formation in the HDS sulfide bimetallic catalysts: ab initio MO/MP2 study

The electronic structure of Ni in the sulfide bimetallic species (SBMS), which is the active component of the sulfide HDS catalysts, is studied with the ab initio molecular orbital calculations. In the previous paper [I.I. Zakharov, A.N. Startsev, G.M. Zhidomirov, J. Mol. Catal. 119 (1997) 437], we have shown that the d⁸ Ni(II) electronic state in the SBMS composition cannot be active in HDS reaction because of the lack of possibility to coordinate S-containing molecule. Therefore, this paper deals with the study of the possibility to stabilize d⁶ electron configuration with the formal Ni(IV) oxidation state. With this in mind, the reaction of oxidative addition of dihydrogen to square–planar complex Ni(II)Cl₂(PH₃)₂ has been studied, which allowed to predict a stabilization of the octahedral complex Ni(IV)H₂Cl₂(PH₃)₂ with d⁶ configuration. This allows us to assume a possibility of an oxidative adsorption of dihydrogen to the Ni atom entering the SBMS composition. Ab initio calculations have shown that such type of oxidative addition is thermodynamically favorable resulting in stabilization of the Ni(IV) d⁶ electronic state. Consequently, the dihydrogen molecule is assumed to dissociate on the Ni atom resulting in the formation of `surface' H<sub>s</sub> and `occluded' H_o hydrogen, which is located under the Ni atom in the center of the trigonal sulfur prism. The structure of the active centers is optimized and the stretching modes of the hydrogen atoms are calculated, which appear to be close to the literature data. The H₂S adsorption on the active center was also investigated and it was shown that the hydrogen disulfide molecule benefits to stabilization of the active Ni(IV) d⁶ state. The conclusion is drawn that the deciding factor in the formation of the active centers of sulfide HDS catalysts is the `occluded' hydrogen.     

Control of Redox Events by Dye Encapsulation Applied to Light‐Driven Splitting of Hydrogen Sulfide

Solar production of hydrogen by consuming low‐value waste products is an attractive pathway that has both economic and environmental benefits. Inspired by the reactive pocket of enzymes, a synthetic platform to combine photocatalytic hydrogen evolution with sulfide oxidation in a one‐pot process via control over the location of the electron‐transfer steps is developed. The redox‐active coordination vessel Ni‐ TFT , which has an octahedral pocket, encapsulates an organic dye to pre‐organize for photocatalytic proton reduction via an oxidative quenching pathway using the nickel corners as catalysts, generating molecular hydrogen and the oxidized dye. The oxidized dye is displaced by a neutral dye and oxidizes sulfide once outside the pocket to give element sulfur. The overall reaction constitutes hydrogen sulfide splitting, forming molecular hydrogen and elemental sulfur, which is analogous to the water‐splitting reaction.     

A SERS spectroelectrochemical investigation of the interaction of O-isopropyl-N-ethylthionocarbamate with copper surfaces

The interaction of the sulfide mineral flotation collector, O-isopropyl-N-ethylthionocarbamate (IPETC), with copper surfaces has been investigated by surface enhanced Raman scattering (SERS) spectroscopy. Adsorption of IPETC has been shown to involve a charge transfer process in which the sulfur atom in the organic species becomes bonded to a copper atom in the metal surface and the hydrogen is displaced from the nitrogen atom to form a hydrogen ion in solution. IPETC and copper IPETC compounds were characterised by ¹³C NMR and Raman spectroscopy to provide a basis for identifying surface species. The formal potential for the Cu ∣ IPETC system in acid and neutral solutions was found to be 0.131 V and the dependence of the reversible potential on IPETC concentration and on pH to be in agreement with the proposed mechanism. The SERS investigations showed that adsorption of IPETC commenced at a potential more than 0.31 V below the reversible value for the formation of the bulk copper compound; this behaviour is analogous to that previously found for the adsorption of other thiol collectors on metal and sulfide mineral surfaces. An estimate is made of the potential dependence of the interaction of IPETC with chalcocite.     

ADVANCES IN CLAUS REACTION AND RELATED REACTIONS

Abstract

During the sweetening of sour natural gas H2S and other contaminants are separated from natural gas. The conversion of H,S and other sulfur conipounds to sulfur is accomplished by the well known Claus process in which H2S and SOz are allowed to react catalytically over an alumina-based catalyst at around 250°C. The Claus reaction is thermodynamically limited so that 2–4 catalytic stages, with intervening sulfur removal, are required to achieve total conversions of 95–98%. There is considerable research activity into all phases of sulfur recovery operations with the major emphasis on maximizing the overall sulfur recovery. This report summarizes the developments in Claus reaction and attempts to focus attention on potential areas for future research.     

活性炭孔隙结构对催化氧化脱除单质汞的影响

研究活性炭在硫化氢存在条件下催化氧化脱除煤气中单质汞的吸附机理和探讨提高其吸附能力的方法,在模拟煤气气氛下对3种活性炭和一种活性焦进行汞的吸附性能实验,并进一步分析活性炭(焦)的孔隙结构。用BET方程处理N₂等温吸附数据,计算比表面积;用HK法进行微孔分析;用BJH法计算中孔孔径分布。结果表明,硫化氢被催化氧化后,生成吸附在活性炭孔壁上的活性硫促进了对汞的吸附;随着活性炭微孔和中孔体积的增大,活性炭对汞的吸附能力得到提高。     

Thermochemical sulfate reduction a review

The high concentrations of hydrogen sulfide found in many oil and gas fields is thought to arise from the oxidation of petroleum hydrocarbons by sulfate—a reaction that reduces the value of the resource. This review, undertaken in order to better understand the geochemistry of TSR reaction in oil field sediments, covers the relevant information on thermochemical sulfate reduction (TSR) to 1991. The theoretical and experimental aspects of TSR reactions (including sulfur and carbon isotope studies) are reviewed and their significance to the geochemical system discussed. The present review agrees with previous suggestions that biochemical reduction of sulfate dominates in sedimentary environments below 120°C, and supports the possibility that reactive sulfur species will oxidize certain organic molecules at meaningful rates in geochemically reasonable reaction periods at temperatures above 175°C. We conclude that under typical petroleum reservoir reaction conditions, both elemental sulfur and polysulfides are capable of oxidizing some organic molecules under basic conditions. But that sulfate alone will not react unless lower oxidation state sulfur is present. The possible interaction of low-valence-state sulfur with sulfate to form TSR active oxidants is examined. both H₂S and SO ₄ ²⁻ are required for the formation of active polysufide reductants (e.g. thiosulfate or polythionates) in TSR systems. Such intermediates can serve to lower the overall activation energy of the oxidation of hydrocarbons by sulfate via thermal generation of sulfur radicals that can function as TSR active oxidants in many oil field sediments. We suggest that some proposed chemical mechanisms for TSR need to be experimentally verified and the results re-interpreted with respect to TSR relations in geologic systems. Dedicated to Professor Lisa Heller-Kallai on the occasion of her 65^th birthday     

噻吩在猝冷骨架Ni上吸附脱硫的XPS研究

采用X射线光电子能谱(XPS)研究了室温下噻吩在猝冷骨架Ni吸附剂上的吸附及受热分解行为. 研究结果表明, 298 K时噻吩首先在吸附剂表面发生C—S键断裂, 生成原子硫及含金属的有机环状化合物. 当吸附剂表面完全被解离物种覆盖后, 发生噻吩的多层物理吸附. 加热至373 K, 大部分物理吸附的噻吩直接脱附, 其余部分在碳物种脱附后暴露的Ni表面上发生解离. 473 K时表面的碳物种消失, 而残留在样品上的硫均转化为硫化镍.      

Pathways to Better Stability of Catalysts for Reforming of Gasoline Fractions

Adsorbents AS-31 and AKD-981 based on zinc oxide and alumina are described. They are used in the purification of hydrogen-containing gas of reforming from hydrogen sulfide at 20–120°C and pressures up to 4 MPa. The second of these adsorbents is capable of forming weak adsorption complexes with hydrogen sulfide and can be recovered 2–3 more times than the first one. To remove sulfur-containing compounds from the gaseous feed of reforming at 350–450°C, the KAS-50 catalyst/adsorbent is proposed, which is prepared by mixing manganese dioxide and aluminum hydroxide. The sulfur capacity of this catalyst is 20%. New platinum–rhenium reforming catalysts KR-200 and KR-201 are proposed, which have higher stability when they work with purified feedstock. The concentration of the active catalyst is the same or lower, and these catalysts show better activity than their predecessors. All catalysts and adsorbents are tested and work in industry.     

Thiophene Hydrogenation to Tetrahydrothiophene over Tungsten Sulfide Catalysts

Independent reactions of thiophene reduction to tetrahydrothiophene and thiophene hydrogenolysis to form hydrogen sulfide and C₄ hydrocarbons are shown to occur over supported tungsten sulfide catalysts and unsupported tungsten sulfide at an elevated temperature and a high pressure. The highest rate of tetrahydrothiophene formation over the supported catalysts is observed when alumina was used as a support, and the lowest reaction rate is found when silica gel was used as a support. Both catalysts are less active than unsupported tungsten disulfide. The rate of thiophene hydrogenation over tungsten disulfide increases with increasing thiophene concentration and hydrogen pressure and is inhibited by tetrahydrothiophene. The selectivity to tetrahydrothiophene is constant (70–90%) in the whole range up to high thiophene conversions. The high selectivity over tungsten sulfide catalysts is suggested to be due to the reaction pathway through thiophene protonation mediated with the surface SH groups and to the inhibition of hydrogenolysis.     

Development of novel and sensitive methods for the determination of sulfide in aqueous samples by hydrogen sulfide generation-inductively coupled plasma-atomic emission spectroscopy

Two new, simple and accurate methods for the determination of sulfide (S²⁻) at low levels (μg L⁻¹) in aqueous samples were developed. The generation of hydrogen sulfide (H₂S) took place in a coil where sulfide reacted with hydrochloric acid. The resulting H₂S was then introduced as a vapor into an inductively coupled plasma-atomic emission spectrometer (ICP-AES) and sulfur emission intensity was measured at 180.669 nm. In comparison to when aqueous sulfide was introduced, the introduction of sulfur as H₂S enhanced the sulfur signal emission. By setting a gas separator at the end of the reaction coil, reduced sulfur species in the form of H₂S were removed from the water matrix, thus, interferences could be avoided. Alternatively, the gas separator was replaced by a nebulizer/spray chamber combination to introduce the sample matrix and reagents into the plasma. This methodology allowed the determination of both sulfide and sulfate in aqueous samples. For both methods the linear response was found to range from 5 μg L⁻¹ to 25 mg L⁻¹ of sulfide. Detection limits of 5 μg L⁻¹ and 6 μg L⁻¹ were obtained with and without the gas separator, respectively. These new methods were evaluated by comparison to the standard potentiometric method and were successfully applied to the analysis of reduced sulfur species in environmental waters.     

Support effects on the chemical property and catalytic activity of Co-Mo HDS catalyst in sulfur recovery

Effects of carbon nanotubes (CNT) and alumina (γ-Al2O3) supports on the catalytic activities of hydrodesulfurization (HDS) process over CoMo catalyst have been studied. XRD results indicated that the main active phases in CNT and γ-Al2O3 supported Co-Mo catalysts are MoO2 and MoO3, respectively. The TPR results reveal that the reduction peak temperatures of the active species on CNT supported Co-Mo catalyst is lower than those on alumina supported Co-Mo catalyst, indicating that the CNT supports favor the r...     

Influence of Gas Components on the Formation of Carbonyl Sulfide over Water-Gas Shift Catalyst B303Q

Water-gas shift reaction catalyst at lower temperature (200—400℃) may improve the conversion of carbon monoxide. But carbonyl sulfide was found to be present over the sulfided cobalt-molybdenum/alumina catalyst for water-gas shift reaction. The influences of temperature, space velocity, and gas components on the formation of carbonyl sulfide over sulfided cobalt-molybdenum/alumina catalyst B303Q at 200—400℃were studied in a tubular fixed-bed quartz-glass reactor under simulated water-gas shift conditions. The experimental results showed that the yield of carbonyl sulfide over B303Q catalyst reached a maximum at 220℃with the increase in temperature, sharply decreased with the increase in space velocity and the content of water vapor, increased with the increase in the content of carbon monoxide and carbon dioxide, and its yield increased and then reached a stable value with the increase in the content of hydrogen and hydrogen sulfide. The formation mechanism of carbonyl sulfide over B303Q catalyst at 200—400℃was discussed on the basis of how these factors influence the formation of COS. The yield of carbonyl sulfide over B303Q catalyst at 200-400℃was the combined result of two reactions, that is, COS was first produced by the reaction of carbon monoxide with hydrogen sulfide, and then the as-produced COS was converted to hydrogen sulfide and carbon dioxide by hydrolysis. The mechanism of COS formation is assumed as follows: sulfur atoms in the Co9S8-MOS2/Al2O3 crystal lattice were easily removed and formed carbonyl sulfide with CO, and then hydrogen sulfide in the water-gas shift gas reacted with the crystal lattice oxygen atoms in CoO-MoO3/Al2O3 to form Co9S8-MoS2/Al2O3. This mechanism for the formation of COS over water-gas shift catalyst B303Q is in accordance with the Mars-Van Krevelen's redox mechanism over metal sulfide.     

Elucidation of hydrodesulfurization mechanism on molybdenum‐based catalysts using 35S radioisotope pulse tracer methods

Elucidation of the hydrodesulfurization (HDS) mechanism on molybdenumbased catalysts using radioisotope tracer methods and reaction kinetics is reviewed. Firstly, to investigate the sulfidation state in Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts, presulfiding of these catalysts has been performed using a ³⁵S pulse tracer method. Secondly, HDS of radioactive ³⁵Slabeled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts and cobaltpromoted molybdena–alumina catalysts in a pressurized flow reactor to estimate the behavior of sulfur on the working catalysts. Finally, sulfur exchange of a ³⁵Slabeled catalyst with hydrogen sulfide was performed to estimate the relationship between the amount of labile sulfur and catalytically active sites.