Pentane isomerization and disproportionation catalyzed by sulfated zirconia promoted with iron and manganese

Fe- and Mn-promoted sulfated zirconia was used to catalyze the conversion ofn-pentane in a flow reactor at temperatures in the range of –25 to 40C andn-pentane partial pressures in the range of 0.005 to 0.01 atm. The rates of reaction increased with time on stream during an induction period and then decreased rapidly. The predominant reaction at –25C and short times on stream was isomerization to give isopentane; no dibranched products were observed. The selectivity for isomerization decreased and that for disproportionation increased with increasing temperature, with disproportionation becoming predominant at 40C; the principal product was then isobutane. The product distribution data are consistent with acid-base catalysis and carbocation intermediates. However, there appears to be more to the reaction mechanism than acid-base chemistry, and the roles of the iron and manganese promoters are not yet explained.     

Promotive effect of isopentane on cyclohexane isomerization catalyzed by sulfated zirconia

Cyclohexane isomerization to methylcyclopentane over sulfated zirconia is markedly enhanced in the presence of isopentane which acts as a hydride transfer agent to facilitate the slow step of hydride transfer from cyclohexane to isopropyl cation. This was revealed by deuterium tracer studies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of hydrogen on n-butane isomerization over sulfated zirconia catalysts

This work investigates the influence of hydrogen on the catalytic activity of Fe- and Mn-promoted sulfated zirconia catalysts. It was found that the effect of hydrogen on the activity of Fe-promoted SZ in n-butane isomerization significantly depended on the Fe content of the catalyst. It was also discovered that the negative effect of hydrogen is more significant at lower temperatures. The reason for the decreased activity in hydrogen is thought to be due to the interaction of hydrogen with reaction intermediates.     

Competitive mechanisms ofn-butane isomerization on sulfated zirconia catalysts

Isotopic analysis ofn-butane isomerization over sulfated zirconium oxide, using double¹³C-labelledn-butane, shows that at low temperature this isomerization is an inter-molecular process. Probably, a C₈ surface intermediate is formed which isomerizes and undergoes -fission; the iso-C₄ fragments are desorbed asi-butane. Previously, the same mechanism was indicated for Fe, Mn promoted catalysts. The isomerization rate at 130°C is drastically lowered by gaseous H₂, because the concentration of the unsaturated species, required for the formation of the C₈ intermediate, is low under such conditions. Whereas₁₃C-scrambledi-butane is a true primary product, isotopic scrambling ofn-butane continues after chemical equilibrium betweenn-butane andi-butane has almost been reached; i.e.¹³C scrambledn-butane is a secondary product. Intra-molecular rearrangement of carbon atoms inn-butane precedes intermolecular scrambling. The similarity of the isomerization mechanism over unpromoted sulfated zirconia and Fe, Mn promoted sulfated zirconia is paralleled by an equal strength of the acid sites in both catalysts. The shift in the FTIR band of CO adsorbed on the Lewis sites indicates that these sites, presumably surface Zr⁴⁺ ions, are weaker acids than A1³⁺ in dehydrated alumina.     

Alkane isomerization over sulfated zirconia and other solid acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C₈ intermediate. Easily accessible Brønsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Brønsted acid sites is demonstrated by results on deuterated catalysts.     

Neopentane cracking catalyzed by iron- and manganese-promoted sulfated zirconia

Cracking of neopentane was catalyzed by a sulfated oxide of zirconium promoted with iron and manganese. Reaction at 300–450°C, atmospheric pressure, and neopentane partial pressures of 0.00025–0.005 bar gave methane as the principal product, along with C₂ and C₃ hydrocarbons, butenes, and coke. The order of reaction in neopentane was determined to be 1, consistent with a monomolecular reaction mechanism and with the formation of methane andt-butyl cations; the latter was presumably converted into several products, including only little isobutylene. At 450°C and a neopentane partial pressure of 0.005 bar, the rate of cracking at 5 min onstream was 5×10^–8 mol/(g of catalyst s). Under the same conditions, the rates observed for unpromoted sulfated zirconia and USY zeolite were 3×10^–8 and 6×10^–9 mol/ (g of catalyst s), respectively. The observation that the promoted sulfated zirconia is not much more active than the other catalysts is contrasted to published results showing that the former catalyst is more than two orders of magnitude more active than the others forn-butane isomerization at temperatures <100°C. The results raise a question about whether the superacidity attributed to sulfated zirconia as a low-temperature butane isomerization catalyst pertains at the high temperatures of cracking.     

Iron and manganese promoted sulfated zirconia: acidic properties and n-butane isomerization activity

Sulfated zirconia catalysts promoted with Fe and Mn have been synthesized and the acidity characterized by IR spectroscopy of adsorbed pyridine. The catalytic isomerization of n-butane was investigated in a fixed bed reactor operated at low conversion at atmospheric pressure and 250 °C. The main effect of the promoters was to change the ratio Brönsted:Lewis acidity of the samples. The catalytic activity was found to be correlated with the number of strong Brønsted acid sites.     

对溴苯酚歧化异构化反应动力学

测定了在 15 0～ 180℃范围内对溴苯酚歧化异构化反应的动力学数据 ,提出了对溴苯酚的歧化异构化反应为一级不可逆平行反应 ,并选用 Powell法和定步长龙格库塔法相结合的数学方法拟合实验数据 ,估算了动力学模型参数 ,模型计算值与实验值拟合良好     

Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n-butane isomerization reaction in the presence of hydrogen

Palladium supported on sulfated zirconia (PdSZ) has been characterized by the n-butane isomerization reaction in the presence of hydrogen, X-ray absorption spectroscopy (XAS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) of adsorbed carbon monoxide. Catalyst calcination at 873 K followed by hydrogen reduction at 513 K results in the formation of 30–40 Å Pd metal clusters, but the surface can only weakly adsorb CO, though stronger than Pd-free, sulfated zirconia catalysts. In the presence of hydrogen, PdSZ has a lower n-butane isomerization activity than SZ, and the Pd function cannot stabilize the reaction at low H₂/n-butane ratios.     

A comparison of cesium-containing heteropolyacid and sulfated zirconia catalysts for isomerization of light alkanes

The heteropolyacid H₃PW₁₂O₄₀ and its cesium salts Cs_xH_3-x PW₁₂O₄₀ (x = 1, 2, 2.5, 3) were synthesized, characterized and tested as catalysts for hydrocarbon reactions. All samples were characterized by a variety of techniques including elemental analysis, X-ray diffraction, dinitrogen adsorption, thermal gravimetric analysis and ammonia sorption. Results from these methods confirmed that pure cesium salts were prepared without significant contamination by amorphous oxide phases. Incorporation of cesium into the heteropolyacid decreased the acidic protons available for catalysis, increased the specific surface area, and increased the thermal stability. The heteropolyacids were tested as catalysts for butane skeletal isomerization, pentane skeletal isomerization and 1-butene double bond isomerization. For comparison, the activity of sulfated zirconia, a well-studied strong acid catalyst, was also evaluated for the three probe reactions. On a per gram basis, the Cs₂HPW₁₂O₄₀ sample was the most active heteropolyacid, presumably due to its high surface area. This sample was more active than sulfated zirconia for pentane skeletal isomerization and 1-butene double bond isomerization. However, sulfated zirconia was more effective for butane skeletal isomerization. Since the pentane and 1-butene reactions were monomolecular in nature, whereas butane isomerization was bimolecular, restrictions inside the micropores of the heteropolyacid may inhibit the formation of long chain intermediates. Interestingly, trace butenes were required to initiate butane isomerization reactions on sulfated zirconia, whereas heteropolyacids catalyzed the reaction in the absence of butenes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Optimization of synthesis variables in the preparation of active sulfated zirconia catalysts

The synthesis of a series of sulfated zirconia catalysts was optimized using the isomerization of n-butane as a reaction probe. The normality of the H₂SO₄ solution used in the sulfation step was found to be the most important variable. A systematic change in the concentration of the H₂SO₄ solution showed that the optimum acid concentration was 0.25 N. When a catalyst prepared with this acid concentration was used, the conversion of n-butane at 200 °C was 35% at 5 min t-o-s. This was close to the thermodynamic equilibrium value of 56% conversion. This maximum was coincident with a catalyst with the highest specific surface area. An increase in the concentration of the H₂SO₄ solution above 0.25 N resulted in a decrease in both surface area and zirconia crystallinity. XPS studies showed a linear relationship between the H₂SO₄ solution concentration and the surface sulfur concentration. Bulk concentrations were determined by elemental analysis. The surface area increased to a maximum for a H₂SO₄ concentration of 0.25 N, while the concentration of bulk sulfur continued to increase when the acid concentration was progressively increased to 2.00 N. The use of a mordenite trap in the reactant stream resulted in an increase in n-butane conversion and a decrease in the rate of catalyst deactivation. XPS studies showed that the sulfur was present as sulfate species and that the oxidation state was not affected by the reaction.     

铝促进的SO_4~(2-)/ZrO_2催化剂上正戊烷低温异构化

轻质烷烃 (C4 -C6 )骨架异构化是生产优质高辛烷值清洁汽油组分的重要反应。目前工业上应用的烷烃异构化催化剂主要有两类:一类是低温型Pt-Cl/Al2O3催化剂,异构化温度一般在 110 ~150℃,活性高,由于此类催化剂含氯化物助剂,对反应环境要求苛刻,要求原料中硫和水分含量必须在1×10-6以下,并且需要在原料中补充有机氯化物,异构化产品中含有相当量的氯化氢,对设备的腐蚀较严重。另一类是中温型Pt(或Pd) /分子筛系列催化剂,需在较高温度 (250~280℃)下反应。烷烃骨架异构化反应属于轻度放热反应,受平衡限制,低温对异构产物有利。SO2-4 /…     

Catalytic behavior of nanostructured sulfated zirconia promoted by alumina: Butane isomerization

Promotion of sulfated zirconia with alumina (ASZ) improves its catalytic activities in n-butane isomerization. The activity and stability of the sulfated zirconia catalysts are investigated in three different nanostructures: ASZ supported on MCM-41, ASZ nanoparticles, and Al-promoted mesoporous sulfated zirconia. The increase of activity was determined primarily by the amount of aluminum addition and the temperature of calcination. The remarkable activity and stability of the Al-promoted catalysts are due to an improved distribution of acid sites strength. The Al loadings in all three catalysts can be adjusted so that optimum catalytic activities for butane isomerization could be found. The increase of butane conversion can be as high as 6 times of that in un-promoted SZ catalysts. This is due to an enhanced amount of weak Brønsted acid sites with intermediate strength on the optimal catalysts. For nanoparticle form of sulfated zirconia, the activity is most steady which is related to the optimum distribution of weak Brønsted acid. On the other hand, too much strong Brønsted acid leads to rapid decay of activity because of coking and cracking. The overall reaction mechanism of the isomerization of n-butane over sulfated zirconia was discussed to understand the details in product distribution.     

Hydrocarbon deposits formed during n-butane isomerization on sulfated zirconia

The hydrocarbon deposits formed during n-butane isomerization on sulfated zirconia are spectroscopically characterized by infrared, in situ Raman and in situ UV-VIS spectroscopy. Evidence is summarized that suggests the formation of alkenyl and cycloalkenyl ions which presumably are precursors for aromatics that lead to catalyst deactivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solid superacid catalysis: kinetics of butane isomerization catalyzed by a sulfated oxide containing iron,manganese, and zirconium

Kinetics of the isomerization ofn-butane and of isobutane catalyzed by sulfated zirconium oxide containing 1.5 wt% Fe, 0.5 wt% Mn, and 4.0 wt% sulfate at 60°C are well represented by a Langmuir-Hinshelwood equation accounting for the reaction equilibrium and for adsorption of both butanes. The adsorption equilibrium constants estimated from the kinetics data are nearly the same for the two butanes. The form of the rate equation and the observation that disproportionation accompanies isomerization suggest that the reaction proceeds via a C_g intermediate.     

State of platinum in zirconia and sulfated zirconia catalysts

Platinum is present in a metallic state following activation in air at 725C of both 5 wt% Pt/ZrO₂ and 5 wt% Pt/SO ₄ ^2– /ZrO₂. Reduction of either catalyst at 725C produces a Pt-Zr alloy, and these reduced catalysts, upon recalcination in air at 725C, form metallic Pt crystallites. Likewise, reduction of these uncalcined catalysts at 725C in H₂ leads to a Pt-Zr alloy formation. However, treatment of these uncalcined catalysts in H₂ at 450C does not produce Pt crystallites large enough to detect by XRD.     

Isomerization of n-butane by sulfated zirconia: the effect of calcination temperature and characterization of its surface acidity

The distributions of Brønsted and Lewis acid sites of different acid strengths on sulfated zirconia calcined at 450–650°C were measured by IR of adsorbed pyridine to elucidate the active sites for butane isomerization. The total numbers of Brønsted acid sites were largest when the catalyst was calcined at 500°C. The total numbers of Lewis acid sites increased with increasing calcination temperature to a maximum at 650°C. The catalytic activity in skeletal isomerization of butane correlated well with the number of Brønsted acid sites but not with the number of Lewis acid sites. The active sites were completely blocked by pyridine irreversibly absorbed at 350°C. We suggest that the strong Brønsted acid sites, which are able to retain pyridine against evacuation at 350°C, act as active sites for butane isomerization on sulfated zirconia.     

Methane conversion over sulfated zirconia

In a continuous-flow differential microreactor, sulfated zirconia (SZ), deliberately activated in situ by water, has converted methane at 673 K to a C₂–C₆ hydrocarbon mixture of which 65–70% was ethene and isobutane. Maximum conversion activity of ∼4.6%, corresponding to 4 × 10⁴ mole methane reacted per mole sulfate per second, was attainable at S/(added H₂O) molar ratio of 3.0 and methane flow rate of 5.6 × 10⁶ mol (g-SZ)⁻¹ s⁻¹. This methane conversion could be catalytic and may involve superacidic sites. This revised version was published online in November 2006 with corrections to the Cover Date.     

n-Butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal and sulfate content

n-Butane isomerization has been investigated over transition metal-promoted sulfated zirconia catalysts. Fe alone was a more efficient promoter than Mn and Cr, and mixtures of Fe–Mn, Fe–Cr, and Fe–V. Promotion of a commercial sulfated zirconia (4.8% sulfate) with iron amounts in excess of 2% depressed the conversion and increased the deactivation rate. Increasing reaction temperatures improved the conversion and decreased the induction period. At 70°C and above, the induction period was suppressed. The influence of activation temperature was studied over a Fe-sulfated zirconia catalyst (with 2% Fe and 4.8% SO₄²⁻ pre-calcined at 600°C). The best conversion was achieved when the catalyst was activated at 350°C in air. This temperature was apparently sufficient to generate the redox sites active at low reaction temperature. Activation in flowing helium depressed the catalytic activity and increased the induction period. The sulfate content had a significant effect on the catalyst performance. For 2% Fe-promoted zirconia, a maximum conversion was found for 7.5% SO₄²⁻, probably related with a better balance (or synergism) of the redox and acid sites involved in a bimolecular mechanism. The time required to reach a maximum of conversion (induction period) decreased with increasing total acidity, i.e. with sulfate content. The series of catalysts with different amounts of sulfate has been characterized by X-ray diffraction, nitrogen adsorption–desorption isotherms, TGA, ammonia-TPD, DRIFT, Raman, and X-ray photoelectron spectroscopy.     

Surface characterization and catalytic activity of sulfated-hafnia promoted zirconia catalysts for n-butane isomerization

Sulfated-hafnia promoted zirconia containing various compositions of hafnia (1-10 wt.%) were prepared by precipitation method in an attempt to ultimately carry n-butane isomerization reaction. The catalyst samples were calcined under dry air flow at 600 °C. The structural and crystal changes were monitored by FTIR and XRD whereas the textural changes were estimated by low temperature N₂ adsorption. FTIR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted and Lewis acid sites from pyridine adsorption. The catalytic activity, stability and selectivity of the catalyst samples were tested for n-butane isomerization at 250 °C. The results reveal that the existence of a small content of hafnia increases the surface sulfate density, stabilizes the tetragonal phase of zirconia, increases the amount and strength of Brönsted acid sites and enhances the initial catalytic activity of the samples. Increasing hafnia content to 5 and 10 wt.% is accompanied with the formation of hafnia monoclinic phase which causes a drastic decrease in the amount of Brönsted acid sites and the initial activity of the catalyst. The isomerization of n-butane occurred through bimolecular pathway with the formation of iso-butane, propane and pentanes as primary products. In spite of the high activity of the samples, the catalyst deactivates rapidly during the first hour of the reaction due to coke formation, retreating the catalyst at 450 °C in the presence of dry air before the catalytic reaction lead to regeneration of the initial catalytic activity of the catalyst which implies that the coke formation is the main source for catalyst deactivation.