Solid acid catalysts acidity characterization

The dehydration of glycerol to acrolein has been carried out in the gas phase using various solid catalysts with a wide range of acid-base properties. The solid base catalysts were characterized by their... 

In summary, the Avada process is an excellent example of process intensification to achieve higher energy efficiency and reduction of waste streams due to the use of a solid acid catalyst. The successful application of supported HP As for the production of ethyl acetate paves the way for future applications of supported HP As in new green processes for the production of other chemicals, fuels and lubricants. Our results also show that application of characterization techniques enables a better understanding of the effects of process parameters on reactivity and the eventual rational design of more active catalysts. 

The apparent acidities of zeolite catalysts are characterized by Av0h values induced by adsorption of hexane (Av0h.C6) under the same conditions than those applied during separate catalytic experiments. The Avoh,c6 values for the different zeolite samples shown in fig. 2 were determined as Figure 1. DRIFT spectra measured in the above for the nitrogen probe molecule. vOH region before (solid lines) and after (dashed lines) contacting the samples with N2 at 298 K and 9 bar equilibrium pressure. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

The use of zeolites as selective catalysts in organic syntheses is a field of growing importance. Zeolites are salts of solid silicoaluminic acids characterized by a strictly regular structure of their crystalline lattice (ref. 1) and by their high acidity and shape selectivity (ref. 2). Attention to the broad potential of zeolites in organic reactions was first drawn in the sixties by Venuto (ref. 3) and various applications of their catalytic properties have been recently reviewed (ref. 4). 

Attempts have recently been made to prepare solid acids by loading triflic acid into various inert oxides including silica,184 titania,185,186 and zirconia.187,188 Silica functionalized with anchored aminopropyl groups was also used to immobilize triflic acid.189 These new catalysts have been tested in a variety of organic transformations, such as alkane-alkene alkylation, Friedel-Crafts acylation, alkene dimerization, and acetalization. Silica nanoboxes prepared by dealumination of Na-X- and Ca-A-type zeolites were also loaded with triflic acid up to 32 wt%.190 The materials were thoroughly characterized but have not been tested as catalysts. 

Silico-alumina zeolites are an important class of catalyst, serving both as solid acids and as supports in bifunctional catalysts. The acidity of the zeolite can be modified by substituting a heteroatom for the Si or A1 atoms in the zeolitic framework. Whenever a framework substitution is attempted, the first question is always whether the heteroatom is indeed in the framework, or instead exists as an extra-framework species. Then, if it can be demonstrated that the heteroatom is in the framework, the question arises as to the exact crystallographic site in the lattice where the substitution has occurred. Detailed knowledge of the site (the so-called T-site in a zeolite) is needed for a complete characterization of the catalyst. 

Zeolites are widely used as solid acid catalysts for a number of organic transformations, such as the cracking of n-paraffins which are catalysed by Bronsted acid sites. " In the case of zeolites, the so-called bridging hydroxyl groups in the i-OH-Als configuration as shown in Eq. (1) are known to act as Brdnsted acid sites and they are responsible for the ability of zeolites to catalyse the reactions. Therefore, the characterization of acidic properties of solid acids is of great importance in discussing the catalytic properties of solid acids. 

The concept of acid site is based on the idea that protons are fixed at definite position. Thus, the measures of the acid strength, which are described so far, are basically based on the static properties of OH groups. However, the solid acid catalysed reactions are often carried out at higher temperatures than room temperature. In general, the catalysts undergo structural and chemical change under reaction conditions. Therefore, the characterization of properties of zeolites at high temperatures is more desirable. 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

Membrane-like heteropoly acid-blended polymer film catalysts were prepared using a common solvent (or mixed solvents) and they were tested as fixed-bed catalysts for the ethanol conversion reaction in a continuous flow reactor. It was found that heteropoly acid catalyst was finely and uniformly distributed through the polymer matrix. All the film catalysts showed the higher selectivity to acetaldehyde than the bulk solid catalyst. Conversion and selectivity over the film catalysts were also affected by the nature of solvent and polymer. Microporosity of the film catalyst was controlled by the phase separation method. The microporous film catalyst could be regarded as a highly dispersed heteropoly acid catalyst supported on polymer matrix. The film catalysts were characterized by IR, TPD, SEM, EDX, DSC, and ESCA. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

An efficient single-step catalytic process was recently developed for the conversion of glucan-type polysaccharides, especially starch, to sorbitol [15]. This process is characterized by the simultaneous hydrolysis of the polysaccharide and hydrogenation of the liberated monosaccharide. The catalyst used is Ru-loaded H-USY zeolite (3 % wlw Ru) in which the zeolitic material fulfils the role of metal carrier (Ru) and solid-acid catalyst. The zeolite provides the Brpnsted acidity required for the hydrolysis reaction either because of its outer surface or by introducing some homogeneous acidity, and the Ru catalyzes the hydrogenation of D-glucose to sorbitol (Scheme 2). 

A general introduction to the subject is followed by a discussion of basic principles regarding types of catalyst and their preparation and characterization and types of catalytic reactors. Chapter 3 deals with the different types of solid acids. In the following chapters (4-6) various (solid) acid-catalyzed transforma-... 

Isobutane Alkylation. The deactivation of solid acid catalysts due to coke deposition is the cause of not having as yet, a commercially available process for isobutane alkylation with C4 olefins, using solid acid catalysts. The coke on these catalysts have been characterized with TPO analyses . The TPO profiles on zeolites used in this reaction, displayed two well defined burning zones. One peak below 300°C, and the other at high temperatures. The relative size of these peaks depends on the zeolite and the reaction temperature. In the case of the mordenite, the first peak was the most important, and in the case of the Y-zeolite, at 50°C or... 

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

J. V. Sinisterra, J. M. Marinas, and A. Llobura [Can. J. Chem., 61, 230 (1983)] employed an extended form of the Hammett equation to correlate data for esterification of ethanol with substituted benzoic acids in the gas phase over a solid AIPO4 catalyst. The focus of their efforts was correlation of values of the adsorption equilibrium constants and apparent rate constants. Use the information tabulated below to prepare a Hammett plot of experimental absorption equilibrium constants versus the corresponding values of the a parameters for the substituted benzoic acids indicated. Employ the a values in Table 7.2 of the text. In principle the intercept of the best straight line through these data should correspond to the adsorption equilibrium constant for benzoic acid. By how much does this value differ from the experimental value below What value of the parameter p characterizes the chemisorption of substituted benzoic acids ... 

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