Zeolites Brdnsted acid sites

In the case of solid catalysts, any atomic (ionic) group at the surface that can donate a proton is a Brdnsted acid while any place where one empty electron orbital exists is Lewis acid. For example, in the case of zeolites, Brdnsted acid site is a part of microporous aluminosilicate framework—a bridging [= Si (OH) A1 =] configuration which is able to donate a proton to an acceptor while Lewis acid site is either tri-coordinated A1 atom or charge-balancing cation Me " " which are able to accept the electron pair. Accordingly to the same theories, any place at the solid surface which can accept proton is a Brdnsted base while any place which can donate electron(s) is a Lewis basic site. For example, in the case of MeOjt (metallic oxides), the oxygen ions (0 ) behave as Brpnsted bases (because they are proton acceptors) while cations at the surface possess Lewis acidity (they are electron acceptors) [27, 28],... 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/Al ratios calculated using equation (23) and their comparison with bulk values [14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Brdnsted acid sites on the external surface of the zeolite crystals. The measurement of... 

The FR results are not correlated with the OH bands observed in the FTIR spectra of the numerous zeolites studied (second column in Fig. 1). For exanq>le X- and Y-faujasites have the same structure and OH-bands, but their FR spectra are quite strikingly different. The FR method seems to be more sensitive to distinguidiing Brdnsted acid sites with different strengths especially when the dependence on temperatme of the FR spectra is taken into consideration. 

IR spectra of H-boralites show four hydroxyl bands at 3450, 3680, 3720 and 3740 cm. These bands were assigned [7,11] to Si—OH—O, B—OH, Si—OH -B, and terminal Si—OH respectively. In the case of H-ZSM-5, two bands were observed 3610 (Si—OH— Al) and 3740 cm (terminal Si—OH). The sorption of pyridine resulted in a reaction of Si—OH - B and Si—OH—A1 groups and the formation of PyH ions. The maximal intensity of PyH band was used to calculate the content of Brdnsted acid sites in the boralites and zeolites studied. These concentrations are presented in Table 2. After the neutralization of all Bronsted acid sites pyridine was desorbed. This was accompanied by the diminishing of PyH and reappearance of acidic hydroxyl bands (3720 cm in the case of boralites or 3610 cm in the case of H-ZSM-5). The intensities of both PylT and OH bands are presented in Figure 1. 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

Acceding to this mechanism, thiophene is adsorbed on Brdnsted acid site of Pt/HZSM-5 and hydrogen is activated on R to form spillover hydrogen. The spillover hydrogen formed on R attacks the reaction intermediate, e.g., species such as S=C=CH-CH=CH2, formed by the decomposition of thiophene adsorbed on the strong Brdnsted acid site of the zeolites [16]. 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

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. 

Brdnsted acid sites) ratioed against the 1870 cm zeolite lattice overtone band. These results are shown in Table 1. 

The unique catalytic properties of zeolites are mainly attributed to their acidic properties. An important characteristic of zeolites, and other acidic molecular sieves, is that each material contains a well-defined, discrete number of acid sites. However, the acidity of zeolites is difficult to characterize because these materials can contain both Lewis and Brdnsted acid sites and may exhibit a heterogeneous distribution of acid-site strengths. 

Fraenkel et al. postulated that H-ZSM-5 crystals contain two types of Brdnsted acid sites [43]. The internal sites are accessible for molecules which can cross the 10-MR barriers and have kinetic diameters lower than 0.58 run. A second type of sites is accessible for molecules with kinetic diameters up to 0.62 nm, and was assigned to half chatmel intersections located on (001) crystal planes. These special pore mouths were thought to be responsible e.g. for the discrimination between ortho- and meta-ethyltoluene isomers and cymene. The key observation at the basis of this hypothesis was that in the alkylation of naphthalene with methanol the "slim" isomers 2-methyl- and 2,6- and 2,7-dimethylnaphthalene were dominating [44,45] over HZSM-5 and HZSM-11 as catalysts, in contrast to what was observed on zeolites with larger pores as H-Mordenite. lire authors suggest that this shape selectivity occurs at the special sites at the external surface of the ZSM-5 or 2 M-11 crystals and advanced the concept of shape selectivity at the external surface. Derouane et al. [46,47] generalized this concept and coined to it the term "nest effect" [46]. 

Two decades ago, when zeolites were new, one frequently heard statements such as "finally we will understand acid catalysis because now we know the exact structure of the acid sites of the catalyst..." No one makes such claims today. Our concepts have moved from a static view to a dynamic one. As a consequence, we have more questions today about the nature of Brdnsted acid sites than twenty years ago. We may even know less about Lewis sites. 

Concerning acidity, RECH O) cations undergo hydrolysis upon calcination, generating Brdnsted acidic sites. Such a hydrolysis reaction allows one to explain the higher acid site concentration in CREY (Calcined RE-Y) zeolites and their higher activity. The Brdnsted sites seem to be a function of the type of RE cations introduced in the zeolite. 

Influence of structure on the number and strength of available Brdnsted acid sites was investigated on a series of dealuminated zeolites (HY, mordenite, ZSM-5 and... 

These results strongly pointed toward the involvement of the acidic hydroxyl groups in the catalytic reaction as suggested by Benesi (157), since the maximum activity was obtained when the zeolite was completely deammoniated. In addition, catalysts which had been dehydroxylated by high-temperature calcination demonstrated low activity. Thus, Benesi proposed that the Brtfnsted acid sites rather than the Lewis acids were the seat of activity for toluene disproportionation. This conclusion was supported by the enhancement in toluene disproportionation activity observed when the dehydroxylated (Lewis acid) Y zeolite was exposed to small quantities of water. As discussed previously, Ward s IR studies (156) indicated a substantial increase in Brdnsted acidity upon rehydration of dehydroxylated Y sieve. 

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because... 

Microcalorimetric studies of several zeolites (H-mordenite, USY, H-ZSM-5), treated in such a way as to contain a noticeable amount of extra-framework aluminum, have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the I wis sites (Freundlich isotherm) and linear for the Brdnsted sites (Temkin isotherm) [82,83]. In most cases, the catalytic activity is related to the number of Brdnsted sites rather than Lewis acid sites. However, the influence of acidic Lewis sites in catalytic reactions over zeolites is still subject to controversy and cannot be neglected. 

Both Lewis and Brdnsted acidity are involved in the dehydration reactions over acid catalysts, and selectivity control to limit the dehydration of DME to olefins and aromatics requires that the surface acidity not be too hi and the reaction temperature be below 300°C [65]. The olefins are generally thou t to be produced by a consecutive reaction in which methanol is first converted to DME, which in turn is converted to olefins and aromatics. Reaction mechanisms for DME formation have been proposed by various investi tors. According to Kubelkova et aL [78], the mechanism over Si-Al zeolites involves protonation of the hydroxyl group of methanol on a Bronsted acid site to form a skeletal methoxyL This methoxyl group reacts with a -phase methanol molecule to form DME at 180300°C and C2C5 aliphatics and aromatics above 300°C. According to these authors, Lewis acid sites (Al -OH), associated with nonskeletal alumina, can also form methoxyls according to the reaction... 

Zeolites become solid Brdnsted acids if protons play the role of charge-compensating cations. Proton forms of zeolites are exceedingly important as industrial catalysts, e.g., for hydrocarbon conversion in the petroleum industry. The most active catalysts include relatively few H/Al sites per Si, typically the Si/Al ratio, x — y)/y, is 10 or larger. The proton forms of zeolites are obtained from the ammonium form by heating ... 

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