Concentration Of Lewis Acid Sites Research Articles

Acylation of Cyclohexene and 1-Methylcyclohexene Over Zeolites and Mesoporous Molecular Sieves

Liquid phase Friedel-Crafts acylation of cyclohexene and 1-methylcyclohexene was investigated over zeolites and mesoporous molecular sieves. The effect of the structure and concentration and type of active sites on the conversion and selectivity of cyclohexene and 1-methylcyclohexene was investigated. In addition, the effect of the chain length of acylating agent was studied. Catalytic experiments were carried out at temperature 80 °C and at atmospheric pressure. The highest conversion was reached over zeolite Beta with Si/Al ratio 37.5 (cyclohexene conversion 78.7%, 1-methylcyclohexene conversion 54.1%). It was found that with increasing chain length of the acylating agent cyclohexene as well as 1-methylcyclohexene conversions decrease. No direct relationship between the conversion of cyclohexene or 1-methylcyclohexene and the concentration of Lewis acid sites was found.

Comparison of Activity and Selectivity of SSZ-33 Based Catalyst with other Zeolites in Toluene Disproportionation

SSZ-33 based-catalyst, after modification with Mo and alumina binder, was evaluated in long-run tests in the toluene disproportionation in the presence of hydrogen as a carrier gas. The activity and selectivity of this catalyst were compared with those of ZSM-5 and mesoporous ZSM-5 prepared with the same concentration of Mo and alumina. Formation of the final catalysts decreased the void volume of micropores in order SSZ-33 > ZSM-5 > ZSM-5/Meso. Simultaneously, the concentration of Lewis acid sites increased due to the addition of alumina to the catalyst. The highest toluene conversion was achieved with SSZ-33 catalyst comprising 12-12-10-ring channels, which is the result of high acidity of this zeolite together with increased mass transport through large pores. ZSM-5 zeolite exhibited lower toluene conversion while only a low activity was found for mesoporous ZSM-5 probably due to the limitations of the access to the zeolite channels after modification with alumina.

Catalytic activity and magnetic properties of Co–ZSM-5 zeolites prepared by different methods

Abstract The selective catalytic oxidation of styrene with hydrogen peroxide was studied over Co–ZSM-5 catalysts which differ in the preparation methods. The catalysts were obtained from NH 4 + –ZSM-5 by wetness impregnation and ion-exchange techniques followed by different post-thermal treatments: oxidation and reduction. The characterization of acidic properties by FTIR of adsorbed pyridine revealed that new and strong Lewis sites (1452 cm −1 ), different from those characteristics of aluminum in the matrix (1456 cm −1 ) were generated by cobalt introduction. Styrene conversion shows an important increment over the exchanged samples as compared to the impregnated ones. This behavior may be explained by the high Lewis acid sites concentration. Benzaldehyde was the main product in all the samples under study. The samples’ magnetic properties were improved with the cobalt incorporation and the applied thermal treatments. The catalytic and magnetic properties of Co–ZSM-5 samples are discussed.

Structure and acid-base properties of hexaaluminates

Hexaaluminates SrAl12O19, BaAl12O19 and LaAl11O18 were prepared by co-precipitation of soluble nitrates of Sr, Ba, or La and Al using NH4HCO3 as a precipitating agent with subsequent calcination at 700–1400°C. The samples were characterized by adsorption methods, thermal analysis (TA and DTG), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules (CO and CDCl3). It was shown that calcination at 1200°C resulted in the formation of single-phase hexaaluminates in all the samples except for LaAl11O18, where an additional phase was found. The specific surface area of the samples was obtained in the range 16–22 m2/g. After the treatment at comparable conditions, the total concentrations of both surface Lewis acid sites and basic sites were found to increase in the series: BaAl12O19 < SrAl12O19 < LaAl11O18. However, the strongest basic sites were detected on the surface of BaAl12O19.

Oligomerization of isobutene over dealuminated Y zeolite catalysts

Abstract Oligomerization of isobutene to produce oligomers such as trimers and tetramers has been investigated over dealuminated Y zeolite catalysts that were obtained by calcination at temperatures in the range of 500–700 °C in the presence of steam. Relatively stable isobutene conversion and high selectivity for trimers and tetramers are attained over a dealuminated Y zeolite by steam-treatment at 600 °C. The observed stable performance with the increased selectivity is attributed to the high concentration of Lewis acid sites. Moreover, a deactivated catalyst can be regenerated easily by simple calcination in air. The present study demonstrates that a steamed zeolite having high concentration of Lewis acid sites, even with decreased crystallinity and porosity, is one of the potential catalysts to exhibit stable and high conversion, high selectivity (for trimers and tetramers) and facile regeneration for the oligomerization of isobutene. However, the contribution of a strong or new acid site for the stable catalysis cannot be ruled out.

Isobutene Trimerization over FeCl3-modified HY Zeolites: Effect of Lewis Acid Sites

Abstract Oligomerization of isobutene has been investigated using FeCl3-modified HY zeolite catalysts in order to produce triisobutenes or remove/separate isobutene from C4 streams. Stable isobutene conversion and high selectivity for trimers are attained over the modified zeolite with high concentration of Lewis acid sites, suggesting that a potential trimerization catalyst can be obtained easily by loading a Lewis acid into an acidic zeolite catalyst.

N-Hexane skeletal isomerization over sulfated zirconia catalysts with different Lewis acidity

Abstract The n -hexane skeletal isomerization over Pt/SO 4 2− /Al 2 O 3 /ZrO 2 catalysts with different density of Lewis acid sites has been studied in a fixed-bed reactor. The acidity of the catalysts was characterized by FTIR spectroscopy of low temperature CO adsorption. The calculation of the rate constants, characterizing the catalyst activity and selectivity was performed on the base of mathematical model of plug flow reactor. It was shown that the rate of hexane isomerization is mainly determined by the density of Lewis acid sites on the surface of sulfated zirconia catalysts: rate constant of n -hexane isomerization for catalyst with Lewis acidity 250 μmol g −1 is four-fold as that for catalyst with concentration of Lewis acid sites 150 μmol g −1 and the rate constant of desired product (2,2-dimethylbutane) formation is also higher by two times.

Dehydration of Methanol to Dimethyl ether, Ethylene and Propylene over Silica-Doped Sulfated Zirconia

Two types of catalyst samples were prepared, one sulfated zirconia and the other silica doped sulfated zirconia. The acidity tests indicate that sulfated zirconia doped with silica has higher concentration and strength of acidic catalyst sites than undoped sulfated zirconia. The acidic surface sites have been characterized using FTIR, NMR, pyridine adsorption, TPD, XRD and nitrogen adsorption. Doping with silica increased the concentration of surface Lewis and Brfnsted acid sites and resulted in generation of proximate acid sites.The activity test indicates that doping sulfated zirconia with silica increases both the acidity and catalytic activity for liquid phase dehydration of methanol at 413-453 K. Methanol is sequentially dehydrated to dimethyl ether and ethylene over both catalysts. Significant amounts of propylene are also formed over the silica-doped catalyst, but not over the undoped catalyst.

Tailoring Raney-catalysts for the selective hydrogenation of butyronitrile to n-butylamine

The influence of LiOH promotion on Co-based Raney-catalysts for the selective hydrogenation of butyronitrile to n-butylamine was explored. Doping with LiOH led to an increase in the fraction of metallic surface area and reduced concentration of Lewis acid sites resulting from alumina particles decorating the metal surface. Two factors were found to be crucial to achieving high selectivity to primary amines. These factors include a low adsorption constant of n-butylamine relative to butyronitrile (because adsorbed butylamine is necessary for byproduct formation) and a low concentration of Lewis acid sites catalyzing condensation reactions.

Trimerization of isobutene over a zeolite beta catalyst

Oligomerization of isobutene has been investigated using several zeolite catalysts to produce triisobutenes that are useful as chemical feedstocks. Stable isobutene conversion and high selectivity for trimers are attained over a beta zeolite with high concentration of Lewis acid sites. Isobutene is quantitatively oligomerized over a beta zeolite with selectivity for trimers > 50 % up to 100 h at a high isobutene WHSV of 10 h −1. Moreover, a deactivated catalyst can be easily regenerated by calcining in flowing air. Zeolite beta's high stability, quantitative conversion, high selectivity, and facile regeneration make it an attractive potential catalyst for isobutene trimerization.

Hydrophobic, solid acid catalysts for production of biofuels and lubricants

A novel application of Fe–Zn double-metal cyanide (DMC) complexes as solid catalysts in the preparation of fatty acid alkyl esters (biodiesel/biolubricants) from vegetable oils is reported. The catalysts are hydrophobic (no H2O adsorption at reaction temperatures) and contain only Lewis acidic sites (NH3 and pyridine adsorption). Brönsted acid sites are absent (absence of 1546 and 1639 cm−1 bands on adsorption of pyridine). Basic sites are also absent (no CO2 adsorption). Unlike the homogeneous or other solid catalysts (like ZnO–Al2O3, for example), the Fe–Zn, DMC catalysts are highly active even for the simultaneous transesterification of triglycerides and esterification of the free fatty acids (FFA) present in unrefined and waste cooking oils as well as non-edible oils. They are also tolerant of water, probably, due to their surface hydrophobicity. A relationship between the transesterification activity and the concentration of strong Lewis acid sites has been observed. Coordinatively unsaturated Zn2+ ions in the structure of the Fe–Zn complex are the probable active sites.

Acid-base properties of alumina prepared from a hydrated product of centrifugal thermal activation of hydrargillite (cta-product)

Acid-base properties of aluminas prepared by thermal treatment of a hydrated CTA-product at 600°C were studied. The CTA-oxides, representing γ-Al2O3, were shown to contain terminal and bridged OH-groups. The concentration of the terminal OH-groups in the CTA-oxides was found to exceed their concentration in γ-Al2O3 prepared by dehydration of the “precipitated” pseudoboehmite, whereas the concentration of the bridged OH-groups in the CTA-oxides was lower than that in γ-Al2O3 prepared from pseudoboehmite. The total concentration of the surface Lewis acid sites in CTA-oxides varies within the limits of 2.80-4.14 mmol/m2 and is essentially above that in g-Al2O3 (2.25 mmol/m2). The distinctive feature of the CTA-oxides is that their surface contains strong Lewis acid sites with nCO = 2220 and 2238 cm-1. The total concentration of basic sites in the CTA-oxides is lower than that in g-Al2O3, however, in contrast to g-Al2O3,they contain strong basic sites with nCDCl3 = 2200 cm-1.

Acid–base reactions on alumina-supported niobia

Catalysts containing from 8 to 28% wt. of niobia supported on γ-alumina by impregnation with niobium pentaethoxide solutions were characterized by infrared spectroscopy of adsorbed pyridine as a probe for acidic sites and CO 2 adsorption as a probe for basic sites. The catalysts had their activity measured in isopropanol dehydration at 453 K, 1-butene isomerization at 348 K and in cumene dealkylation at 728 K. The density and strength of the Lewis acid sites and of the basic sites decreased with niobium content. On the other hand, the density of Brønsted acid sites increased in the same direction. Different reactions responded differently to niobium addition. In isopropanol dehydration, the main factor responsible for the observed decrease in catalytic activity was the decrease in concentration of basic sites. Activity for 1-butene isomerization also decreased with niobium addition, but the main factor seemed to be the decrease in concentration of alumina-associated Lewis acidic sites. Evidence from cis-/ trans-2-butene ratio indicated that niobium addition modifies the properties of neighboring aluminum sites. Brønsted acidic sites created by niobium addition are responsible for the development of activity in the cumene dealkylation reaction, but the sites associated with tridimensional niobia species seem to be more effective in this reaction.

α-Pinene isomerization to campheneEffect of thermal treatment on sulfated zirconia

Abstract The isomerization reaction of α-pinene produces bicyclic and monocyclic compounds and other products. The purpose of this study is to investigate the influence of the pre-treatment temperature of the catalyst on the concentration of Lewis and Bronsted acid sites and the activity and selectivity of the catalyst. Catalyzed reactions in heterogeneous and homogeneous phases were performed. In heterogeneous reaction, sulfated zirconia (SZ) and pre-treated SZ catalyst at several temperatures were used. The thermal treatment affects the ratio between Bronsted and Lewis sites. Results were contrasted with those obtained with SZ catalyst without thermal treatment. In homogeneous reaction the catalyst is sulfuric acid, and in the reaction medium Bronsted sites are only present coming from the H+ of the acid. The ratio between bi/monocyclic compounds for H2SO4, SZ catalyst and pre-treated catalyst at several temperatures, SZ250, SZ350 and SZ500, varies between 0.4 and 4.3. It can be observed that there exists an optimal thermal treatment where the ratio is higher. FT-IR spectra of adsorbed pyridine on catalysts pre-treated thermally show the presence of Lewis and Bronsted acid sites. The amount of acid sites was determined by TPD of adsorbed pyridine.

Involvement of active sites of promoted vanadyl pyrophosphate in selective oxidation of propane

Involvement of active sites in the selective oxidation of propane was investigated using (VO) 2P 2O 7 catalysts promoted by Sb, Ti, Zr, Hf, Cu, and Ce. Based on X-ray diffraction patterns, infrared spectra, and Raman spectra, the formation of a (VO) 2P 2O 7 phase was confirmed for all of the promoted catalysts; no other phases were detected. After the addition of Sb, Ti, and Zr, the selectivity to acrylic acid increased, whereas Cu-, Ce-, and Hf-promoted catalysts showed lower selectivity than pure (VO) 2P 2O 7. The effect of promoters on the activity and selectivity for the propane oxidation was investigated on the basis of the nature of surface active sites. The surface V O species, evaluated by the nitric oxide–ammonia rectangular pulse technique, decreased with the addition of the promoters. A proportional correlation between the number of surface V O species and reaction rate of propane clarified that the surface V O species is the controlling factor for the catalytic activity of propane oxidation. In the case of Sb-, Ti-, and Zr-promoted catalysts, the selectivity to acrylic acid increased significantly. The selectivity was compared with the number and strength of Brønsted and Lewis acid sites estimated from dimethylpyridine temperature-programmed desorption. A good correlation was observed between the selectivity to acrylic acid and the surface concentration of Lewis acid sites. It was experimentally demonstrated that Lewis acid sites represent the key factor in selective oxidation.

Acid-base properties of single-phase aluminum oxides

It was found that the surface of single-phase aluminum oxides contains terminal, bridging, and hydrogen-bonded hydroxyl groups, which differ in both concentration and position of related absorption bands. A transition from η-Al2O3 to θ-Al2O3 does not change the number of absorption bands, but the intensities of these bands decrease. The total concentration of Lewis acid sites in single-phase oxides increases from 2.5 to 5.34 µmol/m2 for γ-Al2O3 and θ-Al2O3, respectively. As distinct from other species, η-Al2O3 contains strong Lewis acid sites (ν(CO) = 2238 cm−1). The total concentration of basic sites in aluminum oxides prepared by boehmite dehydration at 600, 800, and 1000°C decreases from 4.86 to 3.72 µmol/m2.

N2 and CO as probe molecules for determining the properties of acid sites on the surface of zeolites

The applicability of molecular nitrogen as a probe for the Bronsted and Lewis acid sites of HNaY and HZSM-5 zeolites was studied by Fourier transform IR spectroscopy. The integrated absorption coefficients of bands due to N-N vibrations in complexes with Bronsted and Lewis acid sites were determined. The correlation between the integrated absorption coefficients and the positions of bands due to N-N vibrations in nitrogen interacting with the acid sites of test samples is discussed. We propose using the low-temperature adsorption of nitrogen for express determination of the concentrations of strong Lewis and Bronsted acid sites in zeolites.

Toluene disproportionation and coking on zeolites Y modified with Lewis-connected InO+ acid sites

Abstract The effect of introduction of Lewis acid sites on the reactions of toluene transformation and catalysts coking has been studied over parent (Si/Al = 2.5) and dealuminated (Si/Al = 3.7) Y zeolites modified with Lewis connected InO+ cationic acid sites. The catalysts with prevalent amount of Bronsted acid sites (less than 40% of protons exchanged by InO+) possess the typical for the proton directed reaction of methyl transfer upon toluene disproportionation long period of activation, the longer the higher the concentration of bridging protons. In contrast, at predominant Lewis sites concentration, the activation period disappear, an enhanced initial activity is observed, followed by deactivation on the expense of a rapid process of accumulation of strongly held reaction products and intermediates. Their further condensation leads to catalysts aging, the faster the higher the concentration of electron acceptor Lewis acid sites. A relation has been found between the initially accelerated processes of alkyl transfer and the reinforced formation of carbonaceous deposits over the In-modified catalysts.

Simultaneous removal of SO 2 and NO x from flue gas using a CuO/Al 2O 3 catalyst sorbent : II. Promotion of SCR activity by SO 2 at high temperatures

Promoting effect of SO 2 on selective catalytic reduction (SCR) of NO by NH 3 over a CuO/Al 2O 3 catalyst sorbent has been investigated. Transient experiment, X-ray photoelectron spectroscopy (XPS), in situ DRIFT, NH 3-TPD, and temperature-programmed reduction (TPR) were used to evaluate the promoting mechanism. The results show that the presence of SO 2 in the reaction stream can significantly promote catalytic activity of NO reduction at temperatures above 300 °C due to the formation of sulfate species on the catalyst surface. Sulfate species affect SCR activity by changing the acidity and redox property of the catalyst sorbent. In situ DRIFT shows that only ammonia coordinated on Lewis acid sites is found on the CuO/Al 2O 3 catalyst sorbent. Sulfation of CuO/Al 2O 3 catalyst sorbent greatly increases the concentration and strength of Lewis acid sites and the concentration of Brønsted acid sites. However, the ammonia bound to Brønsted acid sites does not play an important role in the SCR reaction in the catalyst system. Moreover, there is a close relationship between the SCR activity and ammonia oxidation activity. Ammonia oxidation experiments and TPR in H 2 suggest that sulfation of the catalyst sorbent weakens the catalyst's oxidation ability and inhibits NH 3 oxidation to NO in the process of SCR, which may be another main promoting effect of SO 2 on SCR.

Common Mechanistic Aspects of Liquid and Solid Acid Catalyzed Alkylation of Isobutane with N‐butene

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