Brpnsted acid sites

Fig. XVIII-20. Spectra of pyridine adsorbed on a water-containing molybdenum oxide (IV)-Al203 catalyst L and B indicate features attributed to pyridine adsorbed on Lewis and Brpnsted acid sites, respectively. (Reprinted with permission from Ref. 191. Copyright 1976 American Chemical Society.)...

In addition, if we accept the conventional mechanism of Brpnsted acid site generation by hydrogen spillover from Pt sites, as proposed for Pt-S04=/Zr02, we must recognize that this would be in fact a reduction process (zerovalent hydrogen yielding -OH groups). 

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. 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

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. 

Conclusions, some of them contrary to the above, were reached more recently by Zhuang et al. (145) from a combination of 31P and 1H MAS NMR spectroscopy of adsorbed trimethylphosphine. These authors found not only Lewis acid sites (vide infra), but also Brpnsted acid sites in TS-1 (145). They claimed that the 1H, 29Si MAS NMR spectra and the resonance related to Brpnsted acid sites in the 31P MAS NMR demonstrated clearly that the presence of Ti in the framework results in the formation of a new OH group, titanols, which is more acidic than the silanols of silicalite-1 (145) . The peak at 4.3 ppm in the 31P MAS NMR spectra was assigned to a ((CH3)3P-H)+ complex arising from the interaction of (CH3)3P with Brpnsted acid sites present on TS-1. The origin of this proton is not clear at present, especially because the MAS NMR spectra of the same TS-1 samples did not differ significantly from those of silicalite-1 (145) the latter, when free from impurities, is not known to be a Brpnsted acid. 

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. 

The catalyst is faujasite derived, with a high concentration of sufficiently strong Brpnsted acid sites and a minimized concentration of Lewis acid sites. It also contains a hydrogenation function. The process operates at temperatures of about 323-373 K with a molar isobutane/alkene ratio between 6 and 12 and a higher alkene space velocity than in the liquid acid-catalyzed processes. Preliminary details of the process concept have been described (240). 

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. 

Bond Thompson mechanistic proposal, 46 bridges, 6 bromide, 185 brookite, 56, 68 Bmst method, 93 Bronsted acidic site, 77 Brpnsted acid sites, 144 buta-1,3-diene hydrogenation, 283... 

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 ... 

Brpnsted acid sites can be further dehydroxylated to form Lewis acid sites as shown also in Figure 13.2. The elimination of water by dehydroxylation should lead to the creation of one Lewis acid site from every two Brpnsted sites. 

Sato et al. [195] have studied the surface borate structures and the acidic properties of alumina-boria (3-20 wt.%) catalysts prepared by impregnation method using B(MAS)-NMR measurements and TPD of pyridine, as well as their catalytic properties for 1-butene isomerization. The number of Brpnsted acid sites was found to increase with increasing boria content, and the catalytic activity was explained by the strong Brpnsted acid sites generated by BO4 species on the surface of alumina. 

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. 

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