Brpnsted acid sites interaction

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. 

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. 

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... 

That is, the Brpnsted acid site can be thought of as a hydrated Lewis acid site. Since the Brpnsted site can donate a proton to another species, we can represent its reaction with ammonia simply as the interaction of a proton with the unshared electron pair on the nitrogen atom of the ammonia molecule ... 

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. 

Fig. 16. Schematic representation of the interaction between H3PO4 and surface OH groups of alumina, (a) Formation of two Brpnsted acid sites (B sites) from the adsorption of one H3PO4 on one OH site of alumina (b) formation of two B sites from two H3PO4 and two OH sites (interaction between neighboring P-OH groups) (c) formation of one B site from one H3PO4 and two OH sites (d) formation of one site with no B sites from one H3PO4 and three OH sites [adapted from Stanislaus et al. (46) and Petrakis et al. (47) reprinted with permission, copyright 1998 Elsevier Science and 1995 the Royal Society of Chemistry].

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

Zirconia and titania both contain Lewis-acid and Lewis-base sites, with the latter having stronger adsorption properties. The titania phase also has strong Brpnsted acidic sites. Basic compounds are less retained on zirconia and titania phases, due to their basic nature. Neutral compounds such as poly aromatic hydrocarbons (PAH), due to their rr-electron system, behave as Lewis bases and the interactions with Lewis acid sites on the zirconia and titania packing materials become dominant for retention. 

Scheme 5. Interactions of water molecules with Brpnsted acid sites on zeolite surface.

As described above, favorable effects of higher alcohols were widely observed with large-pore zeolites [40], although the use of 1-hexanol was not so beneficial to 10- or 8-MR zeolites as to 12-MR zeolites [41]. An IR study revealed that the adsorption/interaction of methanol on the H-BEA (Si/Al = 12.8) catalyst was stronger than that of 1-hexanol. The relatively strong Brpnsted acid sites, probably... 

This is caused by dispersion interaction between the carbon atoms of the reactant and the oxygen atoms of the zeolite framework. Interaction of the reactants with the Brpnsted acid site further reduces the potential. The intrinsic energy barrier for the forward reaction is the difference between the bottom of the well for the coadsorption of A and B at the active site and the top of the transition state. First, the... 

Probe molecules such as, pyridine, ammonia, carbon monoxide, piperidine, and -butylamine are used to investigate acid-base surface properties due to their capability to distinguish between surface Lewis and Brpnsted acid sites of oxide materials. These molecules possess the ability to interact as distinct adsorbed surface species according to the nature of the acid site. 

The VPO catalyst acidity is also very important as H-abstraction occurs on Lewis acid sites (i.e., and the n-butane C-H bond cleavage results from the interaction between Lewis and Brpnsted acid sites (P-OH). Brpnsted acidity is reported to favor the stabilization of reaction intermediates, the stabilization of adsorbed oxygen species or in the generation of organic surface species, involved in oxygen activation and transport [27],... 

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