Hydroisomerization of n-hexane over Pt-ETS-10

The hydroisomerization of n-hexane has been investigated at atmospheric pressure in the temperature range 523-623 K over Pt-H-ETS-10 containing different amounts of Pt. The influence of Pt content and reaction parameters on the isomerization efficiency is reported. The optimum Pt content for the reaction was found to be around 0.3% Pt. The moderate acidity of the molecular sieve (H-form) results in high selectivities for isomerization. The studies suggest that n-hexane transformation over Pt-H-ETS-10 proceeds through a bifunctional route.     

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.     

Electrophilic chlorination of methane over superacidic sulfated zirconia

Catalytic chlorination of methane was studied over SO ₄ ^2– /ZrO₂, Pt/SO ₄ ^2– /ZrO₂, and Fe/Mn/SO ₄ ^2– /ZrO₂ solid superacid catalysts. The reactions were carried out in a continuous flow reactor under atmospheric pressure, at temperatures below 240°C, with a gaseous hourly space velocity of 1000 ml/g h and a methane to chlorine ratio of 4 to 1. At 200°C with 30% chlorine converted the selectivity in methyl chloride exceeds 90%. At more elevated temperatures, the selectivity decreases but stays above 80% in methyl chloride at 225°C using the sulfated zirconia catalysts. The selectivity can be enhanced by adding platinum to sulfated zirconia catalysts. An iron and manganese-doped catalyst exhibited excellent selectivities at somewhat lower conversions. Methyl chloride is obtained at 235°C in selectivities greater than 85%. No chloroform or carbon tetrachloride is formed. The electrophilic insertion involves electron-deficient metal-coordinated chlorine into the methane C-H bond.Catalysis by solid superacids, 29. For part 28 see ref. [14].     

Transformation of C6 hydrocarbons over sulfated zirconia catalysts

The conversion of the C6 saturated hydrocarbons hexane, methylcyclopentane and cyclohexane was studied in a flow microreactor in the temperature range 120–240°C. Sulfated zirconia with different sulfate contents were used as catalysts. Catalysts were characterized before and after reactant exposure using Raman spectroscopy and XPS. The obtained results confirm data on the reaction mechanism already reported in the literature. The first step is the hydrogen abstraction which could occur to a more or less advanced extent as a function of the sulfate density species. This process is the main cause of the catalyst reactivity but also of the catalyst deactivation. More advanced dehydrogenated species lead to polymerized species which remain strongly chemisorbed on the catalyst surface. XPS data also indicated that during these redox processes a part of the sulfur remained on the catalysts surface. Higher sulfate densities lead to the cyclization of hexane.     

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.     

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 acylation of aromatics with carboxylic anhydrides over sulfated zirconia

Two sulfated zirconia samples have been synthesized using different preparation procedures. Textural and acid properties of these catalysts have been characterized and compared with commercial zeolite Hβ by means of nitrogen low-temperature adsorption, temperature-programmed desorption of ammonia and microcalorimetry. The two sulfated zirconias proved to be appropriate catalysts for acylations of aromatic compounds containing electron donating substituents, e.g., for syntheses of several aromatic ketones. This revised version was published online in June 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.     

Vapour phase tertiary butylation of phenol over sulfated zirconia catalyst

Vapour phase butylation of phenol was carried out over a sulfated zirconia solid superacid catalyst in the temperature range 448–473 K using t-butyl alcohol as the alkylating agent. A good substrate (phenol) conversion and excellent product (para-tertiary BP) selectivity was obtained. The catalytic activity remains nearly the same on repeated use of the catalyst. Further, the catalyst was not deactivated when the reaction was carried out for a longer duration, i.e., even after several hours of reaction.     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

Transformation of propane,n-butane and n-hexane over H3PW12O40 and cesium salts. Comparison to sulfated zirconia and mordenite catalysts

Over H₃PW₁₂O₄₀ and its acidic cesium salts at 250°C, alkane transformations occur through the mechanisms previously proposed for sulfated zirconia and mordenite catalysts: propane is mainly transformed into butanes through a trimerization–isomerization–cracking process, n-butane into isobutane, propane and pentanes through a dimerization–isomerization–cracking process, n-hexane into methylpentanes and 2,3-dimethylbutane through a monomolecular mechanism. With all the samples, n-butane transformation is initially much faster than propane transformation, the difference in rate increasing significantly with the Cs content: from 25 times with H₃PW₁₂O₄₀ to 350 times with Cs_2.4H_0.6PW₁₂O₄₀. On the other hand, n-hexane transformation is 2.3 to 7 times faster than n-butane transformation. A decrease in acid strength and in acid site density with Cs introduction is proposed to explain the increase in the rate ratios. For all the reactions, sulfated zirconia pretreated at 600°C is 2–3 times more active than the heteropolycompounds. HMOR10 which is the most active catalyst for n-hexane transformation is the least active for n-butane and especially propane transformation. This very low activity of mordenite for these bimolecular processes can be related to particularities of its pore system: bimolecular reactions are strongly unfavoured in the narrow non-interconnected channels of this zeolite. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Oxidative dehydrogenation of ethane over alkali‐metal doped sulfated zirconia catalysts

Alkali‐metal doped sulfated zirconia catalysts were tested for the oxidative dehydrogenation of ethane into ethene. The effects of metal precursor compounds and acidic anion promoters on the catalytic activity in this reaction were studied. It was found that sulfation of zirconia increases the selectivity of ethane towards ethene. Lithium‐, sodium‐, and potassium‐doped sulfated zirconia catalysts showed quite different activities in this reaction. Sulfated zirconia doped with lithium catalysts were found to be effective for the oxidative dehydrogenation of ethane, giving over 90% selectivity to ethene and 25% ethene yield at 650 °C. © 1999 Society of Chemical Industry     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

Selective conversion of cellulose to hexitols over bi-functional Ru-supported sulfated zirconia and silica-zirconia catalysts

We report a process of selective conversion of microcrystalline cellulose to hexitols over bi-functional Ru-supported sulfated zirconia and silica-zirconia catalysts. A 58.1% yield of hexitols and a 71.0% conversion of cellulose were achieved over Ru/SZSi(100:15)-773 catalyst at 443 K. The as-synthesized catalysts were characterized by X-ray diffraction (XRD), BET, thermogravimetric analysis and pyridine adsorption Fourier transform infrared spectroscopy (FTIR). XRD results indicated that the sulfated catalysts were pure tetragonal phase of ZrO₂ when calcined at 773 K. Monoclinic zirconia appeared at the calcination temperature of 873 K, and the content of monoclinic phase increased with the elevating temperature. Compared with sulfated zirconia catalyst, sulfated silica-zirconia catalysts possessed a higher ratio of Brønsted to Lewis on the surface of catalysts, as shown from pyridine adsorption FTIR results. The reaction results indicated that the tetragonal zirconia, which is necessary for the formation of superacidity, was the active phase to cellulose conversion. The higher amounts of Brønsted acid sites can remarkably accelerate the cellulose depolymerization and promote side reactions that convert C5–C6 alcohols into the unknown soluble degradation products.     

Oxidative desulfurization by chromium promoted sulfated zirconia

Chromium promoted sulfated zirconia (CSZ), prepared by wetness impregnation technique, was characterized by various techniques like Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Electron probe microscopic analysis (EPMA). Bulk density and Brauner-Emmett-Teller (BET) surface area of CSZ were found to be 0.996 kg/m³ and 116.2 m²/g, respectively. CSZ was further used as catalyst for the oxidative removal of sulfur from model oil (dibenzothiophene, DBT, dissolved in iso-octane). Optimum CSZ dose was found to be 5 g/l. The catalytic oxidation of DBT by CSZ was found to be gradual process with optimum reaction time of 6 h. The activation energy for DBT conversion by CSZ was found to be 3.8 kJ/mol.     

Active sites on Pt containing sulfated zirconia

The influence of the Pt and sulfate concentration on the activity of Pt containing sulfated zirconia for n-heptane conversion was investigated. Pt was deposited on the support by impregnation and by photocatalytic deposition. The amount deposited was 2.5 and 0.4 wt% respectively. For comparison a hybrid catalyst consisting of sulfated zirconia and Pt on SiO₂ was prepared. As supports a commercial sulfated zirconia with a fixed sulfate concentration, a commercial and self synthesized Zr(OH)₄ were used. The sulfate content varied between 20 and 60% of a monolayer. The shifts to higher frequency in the IR spectra of CO adsorbed on Pt correlate with the increasing amounts of sulfates on zirconia and are attributable to the changes in the electron density of the supported metal, i.e. the electron deficiency of Pt increases with increasing concentration of acid sites. After activation in air and reduction in hydrogen two SO₂ peaks were detected by a temperature programmed heating procedure (TPE—temperature programmed evolution). The lower the desorption temperature of the first SO₂ peak, the higher the activity. The shift to lower temperature is connected with a higher Pt and sulfate concentration, furthermore with the proximity of the metal to acid sites. The catalysts with a low sulfate concentration possess only Lewis acid sites and are inactive for n-heptane conversion. At higher sulfate concentration, Br?nsted acid sites are present and the catalysts are active. The concentration of these acid sites is related to the concentration of sulfates, which desorb at lower temperature. Dedicated to Professor Konrad Hayek.     

Influence of hydrogen and of reaction temperature on the mechanism of n-butane isomerization over sulfated zirconia

Over HMOR zeolites, hydrogen inhibits n-butane isomerization which occurs through a bimolecular pathway and has practically no effect on n-hexane isomerization, of which the reaction mechanism is intramolecular. The large inhibiting effect found with butane isomerization is certainly related to the demanding character of the bimolecular process: two sec-butyl carbenium ions are necessary for the alkylation step. Hydrogen could react with carbenium ions limiting their concentration. Over sulfated zirconia hydrogen has also an inhibiting effect on n-butane isomerization, this effect being particularly pronounced at low temperature, and has no effect on n-hexane isomerization. It is suggested that the differences in the butane isomerization mechanisms proposed in the literature in the case of sulfated zirconia are mainly due to the ``diluent' gas used (nitrogen and hydrogen) and for a small part to the reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

ESR study of paramagnetic sites in sulfated zirconia

ESR spectroscopy was used to investigate paramagnetic sites in sulfated zirconia. Catalysts derived from zirconium oxide and zirconium hydroxide were studied. It was demonstrated that paramagnetic sites assigned to near-surface F-centers were formed during activation at temperatures above 573 K. The catalyst derived from zirconium hydroxide shows after activation at 873 K two types of paramagnetic sites: F-centers and Zr³⁺ sites. Both F-centers and Zr³⁺ sites in this catalyst form complexes with reagents upon n-butane or hydrogen adsorption at range of 423–523 K in contrast to paramagnetic sites of the oxide-derived catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.