Zeolites Brpnsted acid sites

Krossner, M., Sauer, J., 1996, Interaction of Water With Brpnsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1 1 and 2 1 Surface Complexes , J. Phys. Chem., 100, 6199. 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

It has been proposed that hydride transfer in zeolites requires the presence of two adjacent Brpnsted acid sites (69). In light of the above-mentioned theoretical examinations and also adsorption isotherms of 1-butene and n-butane on USY zeolites with various aluminum content (70), this proposition seems unlikely. 

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

The theoretical modeling of the activation and reaction of methanol by Brpnsted acid sites within zeolites has attracted a wide interest. This is in part a consequence of the industrial importance of the interaction—as the first step of the conversion of methanol into gasoline in the MTG process (213). However, a great deal of the theoretical interest has arisen because of the possibility of proton transfer from the zeolite lattice to methanol. An early investigation was that of Vetrivel et al. (214), who employed ab... 

In conclusion, extensive research has revealed that the Lewis and Brpnsted acid sites on the promoted sulfated zirconia catalysts are not necessarily stronger acids than the corresponding sites in zeolites, but sulfated zirconia circumvents the energetically unfavorable monomolecular reaction path by following a bimolecular mechanism. The question of superacidity of sulfated zirconia, however, is still debated.312... 

The relationship of Lewis and Brpnsted acid site concentrations on H—Y zeolite was explored further in a study by Ward (156) of the effect of added water. At low calcination temperatures (<500°C) only a small increase in the Brpnsted acid site concentration occurred upon addition of water to the sample. Rehydration of samples dehydroxylated by calcination above 600°C resulted in a threefold increase in the amount of Brpnsted-bound pyridine. However, no discreet hydroxyl bands were present in the infrared spectrum after rehydration. Thus, the hydroxyl groups reformed upon hydration must be in locations different from those present in the original H—Y zeolite, which gave rise to discreet OH bands at 3650 and 3550 cm-1. 

Whereas the number of Brpnsted acid sites can be easily determined,146 149 their acidity may vary depending on their position in the framework and also by interaction with Lewis acid sites. For these reasons and despite the various techniques tested, there are no general and reliable methods to measure the acidity of solid acids. The synthesis of active acidic zeolites used in industry still relies on a very empirical base, the most important character being their catalytic activity. 

Recently, the same authors reported the nitration of toluene and 2-NT using the nitric acid-acetic anhydride system, and using different batches of zeolite Beta.1391 The results showed that the number of Brpnsted acid sites and diffusion rates of the products out of the pores play a major role in determining the performance. Lewis... 

The role of Brpnsted acid sites in the oligomerization of ethylene over HZSM-5 has been studied.[30,31] Amin and Anggoro1311 concluded that dealumination of HZSM-5 led to higher ethylene conversion, but the gasoline selectivity was reduced compared with a nondealuminated HZSM-5 (Si/Al = 15) zeolite sample. 

Zeolites are not typically used in Lewis acid type catalysis due to the absence of Lewis acid centers in zeolites. This is due to the coordination of the Al-site to four lattice-oxygens in a perfect zeolite framework. It has, however, been shown for zeolite Beta that the aluminum atom can reversibly move between a framework Brpnsted acid site and a framework-grafted Lewis-acid site.70 Accordingly, Creyghton et al. showed that zeolite Beta is active in the Meerwein-Ponndorf-Verley reduction (MPV) of ketones (scheme 4).71 In this reaction a hydrogen hydride transfer reaction between an alcohol and a ketone takes place. 

Ti-Beta was also applied in the selective transformation of a-pinene oxide to camphenolenic aldehyde.78 A selectivity of over 98% was observed in the gas-phase reaction that was explained as a combination of a Lewis catalysed reaction in the absence of Brpnsted acid sites. Furthermore the concentration of a-pinene oxide in the zeolite pores was found to be an important factor not ruling out additional transition state selectivity as well. 

---