A quantum‐chemical study of the formation mechanism and nature of tert‐butyl carbenium ions in 100% sulfuric acid

Ab initio quantum‐chemical calculations reveal that the interaction of isobutene with H₃SO ₄ ⁺ ions produced by self‐dissociation of sulfuric acid occurs virtually without an activation barrier, whereas the reactions involving neutral species of sulfuric acid are characterized by considerable activation barriers: 14.9 kcal/mol at the MP2(full)/6‐31+G*//6-31+G* level and 16.9 kcal/mol^-1 at the MP2(full)/6‐31+G*//3‐21+G* level. It is also concluded that the species resulting from interaction of isobutane with H₃SO ₄ ⁺ ions of protonated tert‐butyl sulfuric acid are ion‐molecular complexes which should be considered as tert‐butyl carbenium ions weakly solvated by H₂SO₄ molecules. Although the concentration of these species in concentrated sulfuric acid is very low, presumably, they play a role of active intermediates in isobutene conversions catalyzed by sulfuric acid. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highly efficient catalyst system for the synthesis of 2‐arylpropionic acids by carbonylation

Carbonylation of 1‐(4‐isobutylphenyl)ethanol has been studied using a homogeneous catalyst system consisting of PdCl₂(PPh₃)₂/ TsOH/ LiCl. Higher reaction rates with TOF up to 1200 h⁻¹ and ibuprofen selectivity >95% have been achieved at 388 K under a CO partial pressure of 5.4 MPa. The reaction proceeds through the formation of 4‐isobutylstyrene and 1‐(4‐isobutylphenyl)ethyl chloride as intermediates. The same catalyst system is shown to be effective for carbonylation of various α-arylethanols, vinyl aromatics and corresponding chloro derivatives. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Methane transformation into aromatic hydrocarbons by activation with LPG over Zn‐ZSM‐11 zeolite

Methane (C₁) can be activated by interaction with liquefied petroleum gas (LPG) even at very high C₁ molar fractions in the feed (C₁/(C₁ + LPG)=0.85) at temperatures of 450–550°C, GHSV(LPG)=2240 and 810 ml/g h, over Zn‐ZSM‐11 (molar fraction Zn²⁺/(Zn²⁺H⁺=0.86) and total pressure of 1 atm. The isobutane (i‐C₄) of LPG could be the main responsible of this interaction. Aromatic hydrocarbons were the main products in the whole range of C₁ molar fractions (0.4–0.85) studied, reaching excellent levels of 10–45%. This revised version was published online in July 2006 with corrections to the Cover Date.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ring‐expansion of methylcyclopentane to cyclohexane and ring‐contraction of cyclohexane to methylcyclopentane on silica‐supported 12‐tungstophosphoric acid and stoichiometric and nonstoichiometric microporous silver 12‐tungstophosphate

Methylcyclopentane is ring‐expanded to cyclohexane and cyclohexane is ring‐contracted to methylcyclopentane on 12‐tungstophosphoric acid (HPW) supported on silica and on stoichiometric microporous silver 12‐tungstophosphate. The conversions of the two reactants increase with the loading of HPW on SiO₂ while achieving significantly higher values on silver 12‐tungstophosphate which pass through a maximum as the Ag/H⁺ preparative ratio increases. The ring‐expansion and ring‐contraction processes on the supported HPW and the microporous AgPW are catalyzed by the acidic protons in the solid acid and the residual protons in the silver salt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of water on the reduction of NOx with propane on Fe‐ZSM‐5. An FTIR mechanistic study

Adsorption of NO on Fe‐ZSM‐5 leads to formation of Feⁿ⁺–NO (n = 2 or 3) species (1880 cm^-1), Fe²⁺(NO)₂ complexes (1920 and 1835 cm^-1) and NO⁺ (2133 cm^-1). Water strongly suppresses the formation of NO⁺ and Feⁿ⁺(NO)₂ and more slightly the formation of Feⁿ⁺ –NO. Introduction of oxygen to NO converts the nitrosyls into surface nitrates (1620 and 1575 cm^-1) and this process is almost unaffected by water. The nitrates are thermally stable up to ca. 300^°C, but readily interact with propane at 200^°C, thus forming surface C–H–N–O deposit (bands in the 1700–1300 cm^-1 region). Here again, water does not hinder the process. The C–H–N–O deposit is relatively inert (it does not interact with NO or NO + O₂ at ambient temperature) but, at temperatures higher than 250 °C, it is decomposed to NCO^- species (bands at 2215 (Fe–NCO) and 2256 cm^-1 (Al–NCO)). In the presence of water, however, the Fe–NCO species only are formed. At ambient temperature the NCO^- species are inert towards NO and O₂, but easily react with a NO + O₂ mixture. The mechanism of the selective catalytic reduction of nitrogen oxides on Fe‐ZSM‐5 and the effect of water on the process are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the acidity of liquid and solid acid catalysts. Part 3. Esterification of benzoic and mesitoic acids

Methyl esters of benzoic and mesitoic acid have been prepared with high yields (>98 wt%) from the corresponding carboxylic acids + methanol in aprotic solvents over samples of H₂SO₄/SiO₂ at 60°C. The results show a high catalytic efficiency of the solids but also suggest an acid strength comparable to that observed in concentrated aqueous H₂SO₄ (range >90 wt%) when the acid requirements for the esterification of analogous compounds in aqueous acid solutions are taken into account. Indeed, different reacting species, i.e., ArC(OH) ₂ ⁺ from benzoic acid and 2,4,6‐triMe‐ArC=O⁺ from mesitoic acid are involved in the esterification, but the mesitoyl cation can be formed and esterified in the acidity ranges between 92 and 98 wt% H₂SO₄. This revised version was published online in July 2006 with corrections to the Cover Date.     

n‐butane isomerization on sulfated zirconia: isotopic transient kinetic analysis of reaction at the site level

By using SSITKA (steady‐state isotopic transient kinetic analysis), n‐butane (n‐C₄) isomerization on sulfated zirconia (SZ) has been studied for the first time at the site level. Accurate measures of the average residence time and the concentration of the most active surface intermediates leading to isobutane (iso‐C₄) were able to be determined. As has previously been observed, a fast initial deactivation of the catalyst followed by a slow steady‐state deactivation was observed over 400 min time‐on‐stream (TOS). It was shown that even though a large amount (∼100 μmol/g) of n‐C₄ was adsorbed on the catalyst, the concentration of active surface intermediates leading to iso‐C₄ was only ∼10 μmol/g at 30 min TOS. The continuous decrease in indicated that the decrease in catalytic activity was due to loss of active sites. An increase in the average residence time of active surface intermediates leading to iso‐C₄ was observed only between 30 and 100 min TOS. This suggests a possible presence of two types of active sites for n‐C₄ isomerization on SZ under the reaction condition studied. It is suggested that the more‐active sites contributed to the high initial activity, while the majority of active sites after 100 min TOS were the less active sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photodecomposition of amino acids and photocurrent generation on TiO2/OTE electrodes prepared by pulse laser deposition

The photoelectrical degradation of the amino acids L‐aspartic acid (L‐Asp), L‐glutamic acid (L‐Glu), L‐leucine (L‐Leu), L‐α‐alanine (L‐α‐Ala) and β‐alanine (β‐Ala) was examined on TiO₂/OTE electrodes prepared by a pulse laser deposition method. The disappearance of amino acids and their mineralization into CO₂ were determined with and without an applied anionic bias of 0.3 V. The generation of photocurrent was also measured during the photoelectrical degradation of substrates on the TiO₂/OTE electrode assembly considering it as a possible type of solar cell. The relationship between the photoelectrodegradation rate, the photocurrent and the structure of the amino acids was established. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrothermal deactivation of Ce‐ZSM‐5, Ce‐beta, Ce‐mordenite and Ce‐Y zeolite deNOx catalysts

The hydrothermal stability of Ce³⁺ zeolite catalysts used for selective catalytic reduction of NO _x was investigated. Aging of Ce‐ZSM‐5, Ce‐beta, Ce‐mordenite and Ce‐Y catalysts consisted of steaming in 10 or 12 vol% water at 600°C for 3–99 h. Ce‐ZSM‐5 (Si/Al ratios: Si/Al = 17.1, 22.6 and 146.6) and Ce‐mordenite (Si/Al = 6.4, IE = 77.2%) showed fast deactivation. Ce‐beta (Si/Al = 12, IE = 68.4%) and Ce‐Y (Si/Al = 2.8, IE = 122%) are significantly more stable zeolite catalysts, Ce‐beta being the most active of these two. Ce‐beta and Ce‐ZSM‐5 catalysts – both having high initial activities – were characterized with ²⁹Si‐NMR and ²⁷Al‐NMR. Especially Ce‐ZSM‐5 showed an increase of non‐framework Al, meaning that the zeolite suffered from dealumination. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective oxidation of propane on MgO/γ‐Al2O3‐supported molybdenum catalyst: influence of promoters

The influence of promoters, potassium and samarium, on molybdenum supported over MgO–γ‐Al₂O₃ catalyst has been investigated in the oxidative dehydrogenation of propane. The acidities of catalysts were determined by temperature‐programmed desorption of NH₃ and by decomposition of 2‐propanol. The K‐promoted catalyst showed the lower acidity followed by the Sm, whereas the unpromoted sample showed the highest acidity. The higher the acid character of the catalyst, the lower the selectivity to propene. Redox properties determined from EPR spectra change with the addition of the promoter. A parallelism between Mo⁶⁺ reducibility and catalytic activity was found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of anisaldehyde via hydroxymethylation of anisole over SnO2–CeO2 catalysts

Hydroxymethylation of anisole has been carried out over SnO₂–CeO₂ catalysts in the temperature range 623–723 K. Methoxybenzaldehyde (anisaldehyde) and condensation products were formed along with minor quantities of methoxybenzyl alcohol, o‐cresol, phenol and 2,6‐xylenol. A maximum anisaldehyde selectivity of 64% was obtained at 623 K at an anisole conversion of 46% under optimized conditions. Catalytic activity of these systems in the formation of aldehyde is ascribed to the presence of weak acid sites and redox metal sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vibrational spectroscopy of carbon monoxide and dinitrogen adsorbed on magnesium‐exchanged ETS‐10 molecular sieve

Carbon monoxide and dinitrogen adsorbed, at nominally liquid‐nitrogen temperature, on Mg²⁺‐exchanged ETS‐10 were found to form Mg²⁺...CO and Mg²⁺...N₂ adducts involving cations located in the main channels (12‐membered rings) of the titanosilicate molecular sieve. These adducts gave main IR absorption bands at 2190 and 2203 cm^-1 for CO, and at 2336 and 2339 cm^-1 for N₂, which were assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by Mg²⁺ ions. Corresponding adducts with Na⁺ and K⁺ ions, not exchanged with Mg²⁺, were also present. The results, which are relevant to the potential use of ETS‐10 as a catalyst, lend support to previous structural studies suggesting the existence of two different cation sites along the main channel of ETS‐10. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox and photo‐redox properties of isolated Mo5+ ions in MoH‐ZSM‐5 and MoH‐beta zeolites: in situ ESR study

Redox and photo‐redox properties of isolated Mo⁵⁺ ions stabilized in H‐ZSM‐5 and H‐beta zeolites are studied by in situ ESR in flowing O₂, NO, H₂, and C₃H₆. Upon oxidation of pre‐reduced samples at 20 °C, NO demonstrates a higher oxidative ability, as compared with O₂. Interaction of Mo⁵⁺ ions with propene at 20 °C results in formation of a chemisorption complex with enhanced reactivity of Mo(V) toward NO. Illumination of the Mo⁵⁺/HZSM‐5 sample with UV‐visible light causes measurable acceleration of Mo(V) oxidation by NO at 20 °C. Therefore, photochemical activation of the oxidation step could be realized, in principle, for Mo/zeolite catalysts. At 500 °C in the reaction mixture NO + H₂, the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(VI) ↔ Mo(V) for MoH‐ZSM‐5 and MoH‐beta seems to be strongly shifted to Mo⁵⁺. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conversion of methanol to hydrocarbons on zeolite HZSM‐5 investigated by in situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography

In situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography have been applied to study the onset of the conversion of methanol on zeolite HZSM‐5 at temperatures between 373 and 573 K. In the steady states of methanol conversion at T ⩾ 523 K, by on‐line gas chromatography mainly the formation of ethene and propene was observed. Simultaneously recorded in situ ¹³C MAS NMR spectra show signals at 12–25 ppm and at ca. 125–131 ppm indicating the presence of adsorbed C₄–C₆ olefins. The observation of these adsorbates on a working catalyst supports the “hydrocarbon pool” mechanism previously proposed for the methanol‐to‐hydrocarbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K and subsequent purging of the catalyst with dry nitrogen at 293 K led to a ¹³C MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presence of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium ions were found by in situ ¹³C MAS NMR spectroscopy under flow conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partial oxidation of ethane to syngas over nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide

The catalytic activity, thermal stability and carbon deposition of various modified NiO/γ‐Al₂O₃ and unmodified NiO/γ‐Al₂O₃ catalysts were investigated with a flow reactor, XRD, TG and UVRRS analysis. The activity and selectivity of the NiO/γ‐Al₂O₃ catalyst showed little difference from those of the modified nickel‐based catalysts. However, modification with alkali metal oxide (Li, Na, K) and rare earth metal oxide (La, Ce, Y, Sm) can improve the thermal stability of the NiO/γ‐Al₂O₃ and enhance its ability to suppress carbon deposition during the partial oxidation of ethane (POE). The carbon deposition contains graphite‐like species that were detected by UVRRS. The nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide have excellent catalytic activities (C₂H₆ conversion of ~100%, CO selectivity of ~94%, 7 × 10⁴ l/(kg h), 1123 K), good thermal stability and carbon‐deposition resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gas‐phase pinacol conversion on AlPO4, γ‐Al2O3 and SiO2 catalysts

Pinacol (2,3‐dimethyl‐2,3‐butanediol) conversion over AlPO₄ (Al/P = 1) and γ‐Al₂O₃ catalysts proceeded in two parallel reaction pathways with formation of 2,3‐dimethyl‐1,3‐butadiene (by 1,2‐elimination) and 3,3‐dimethyl‐2‐butanone (by rearrangement), the latter being the main reaction product. The activity was in accordance with the surface acidity data as measured versus cyclohexene skeletal isomerization reaction. Thus, AlPO₄ showed the highest activity (almost total conversion at 523 K). The 1,2‐elimination/rearrangement ratio depended on the type of catalyst used and diene formation increased with reaction temperature. Moreover, pinacolone was a reaction intermediate for diene production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Ba and rare earths cations on the properties and dehydrocyclization activity of Pt/K‐LTL catalysts

Several Pt/L‐zeolite catalysts were prepared by the impregnation method from L‐zeolite samples previously exchanged with Ba²⁺, La³⁺, Pr³⁺, Nd³⁺ and Sm³⁺. Lanthanides (Ln³⁺) increase the overall dispersion of platinum, measured by H₂–O₂ titration. TPR and CO/FTIR measurements indicate that these ions modify the distribution of Pt in the zeolite lattice, increasing the fraction of metal on the external surface. Additionally, both CO/FTIR and competitive hydrogenation of toluene–benzene mixtures indicate that, in the presence of Ln³⁺, the electron density of the platinum decreases in comparison with Pt/KL‐zeolite. On the other hand, Ba²⁺ does not substantially modify neither the distribution nor the electronic state of Pt. The reactivity measurements in the hydroconversion of n‐heptane show that Pt/BaKL and Pt/KL exhibit similar catalytic behaviour with a high dehydrocyclization activity. However, the exchange of K⁺ by Ln³⁺ increases the production of heptane isomers, while selectivities to aromatic and terminal hydrogenolysis products significantly decrease. This revised version was published online in July 2006 with corrections to the Cover Date.     

FTIR study of CO adsorption on coked Pt–Sn/Al2O3 catalysts

Infrared spectra of adsorbed CO have been used as a probe to monitor changes in Pt site character induced by the coking of Pt/Al₂O₃ and Pt–Sn/Al₂O₃ catalysts by heat treatment in heptane/hydrogen. Four distinguishable types of Pt site for the linear adsorption of CO on Pt/Al₂O₃ were poisoned to different extents showing the heterogeneity of the exposed Pt atoms. The lowest coordination Pt atoms (ν(CO) < 2030 cm⁻¹) were unpoisoned whereas the highest coordination sites in large ensembles of Pt atoms (2080 cm⁻¹) were highly poisoned, as were sites of intermediate coordination (2030–2060 cm⁻¹). Sites in smaller two‐dimensional ensembles of Pt atoms (2060–2065 cm⁻¹) were partially poisoned, as were sites for the adsorption of CO in a bridging configuration. The addition of Sn blocked the lowest coordination sites and destroyed large ensembles of Pt by a geometric dilution effect. The poisoning of other sites by coke was impeded by Sn, this effect being magnified for Cl‐containing catalyst. Oxidation or oxychlorination of coked catalyst at 823 K followed by reduction completely removed coke from the catalyst surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.