Sites proton-donating

Figure 11.1 Relief map of the electron density calculated for the charge-assisted dihydrogen complex H20H+- HBeBeH shown in the plane of the HBeBeH molecule. The electron density of the HBeBeH molecule is located on the left side of the figure, and the electron density of the H-0 bond, which is the proton-donating site, is shown on the right side. (Reproduced with permission from ref. 2.)...

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The object of this work was to study the influence of pretreated, decationized NH4-zeolites on adsorbed A,iV-dimethylaniline molecules. Such influence is caused by, proton-donating and electron-deficient active sites in decationized zeolites. Interaction of an aromatic amine molecule (M) with the proton-donating site leads to the formation of the MH+ molecule ion interaction with the electron-deficient site results in the M+ cation radical. Stabilization of these states by adsorption leads to the... 

Scheme I illustrates the appearance of proton-donating sites in the temperature region for stable OH groups when the NH4+ ions are still partially in the zeolites at this point electron-deficient sites are not formed (low temperature region). Schemes II and III describe the successive breakdown of proton-donating sites when the temperature is raised, according to ...

Schemes I—III do not differ significantly from those reported in the literature (8,12). First, the electron-deficient centers in the zeolites must arise at the expense of proton-donating sites. Secondly, the nonproton centers formed in decationized zeolites are essentially different from each other. Both facts are confirmed by the results of our investigations on the electronic spectra of decationized zeolites.

Interaction of an aromatic amine molecule with the proton-donating site... 

Figure 1 shows that photoirradiation of the samples pretreated at 350°, 450°, 550° C causes an increase in intensity of the 540-600-nm H+M-M+ band. This indicates the formation of additional M+ cation radicals under these conditions. The slight increase in intensity of the 540-600-nm band for the sample treated at 550° C, compared with the samples treated at 350° and 450° C, is apparently limited by the number of proton-donating sites and MH+ ions associated with it. The absence of H+M-M+ after M+ cation radicals are formed (sample treated at 750° C) can be caused by the complete absence of proton-donating sites and consequently by the impossibility of forming MH+ ions. Special attention should be paid to the effects caused by photoirradiation of the samples heat treated at 200° and 650°C. The appearance of H+M-M+ in the first case can be explained by assuming that the photoirradiation itself produces some M+ cation radicals from excess of MH+ ions. In the second case excess M+ cation radicals are observed on photoirradiation. The 540-600-nm band was observed after treatment at 650° C in type Y zeolites only (see Table I). 

In these four sections the terms Bronsted and Lewis acid sites have been widely used. While Bronsted sites can be reasonably well defined as proton-donating sites, the properties of Lewis acids are not so clear. A status report on Lewis acid-base definitions is highly relevant for clarifying these problems. 

Low-temperature adsorption of weak CH proton-donating molecules such as CHFj, acetylene and its derivatives or HCN, enables one to chai acterize the basicity of surface electron-donating sites. 

Homocitrate is bound to the molybdenum atom by its 2-carboxy and 2-hydroxy groups and projects down from the molybdenum atom of the cofactor toward the P clusters. This end of FeMoco is surrounded by several water molecules (5, 7), which has led to the suggestion that homocitrate might be involved in proton donation to the active site for substrate reduction. In contrast, the cysteine-ligated end of FeMoco is virtually anhydrous. 

This will increase the basicity of the quinoline system from about pATa 5, but almost certainly not as far as represented by pATa 10.8. However, it solves our problem, since it means the 4-amino substituent is donating its lone pair into the aromatic ring system and is not, therefore, available for bonding to a proton. This site is going to be less basic than a tertiary amine. pATa 10.8 must represent the terminal -NEt2. 

The important conclusion drawn from the above studies on PS(OH)/PMMA in solution and bulk is that complexes formed in dilute solutions can be preserved during the process of film casting. In particular, when we use an inert solvent whose Ejp is close to zero, the critical hydroxyl contents in proton-donating polymers for complexation estimated by viscosity or LLS are comparable to that for the miscibility-to-complex transition in bulk, which can be easily detected by DSC or TEM. Therefore, by combining the results from both solution and bulk, it should be possible to construct a map for a given blend system visualizing how the immiscibihty, miscibihty and complexation of the blend depend on the content of interacting sites. 

Concentrations of proton and non-proton sites in zeolites were changed by thermal treatment of Na, NH zeolites at different temperatures (.100°, 250°, 350°, 450°, 550°, 650°, and 750°C). Molecules of N,N-dimethylaniline interact at 20°C with both the proton-donating and electron-deficient zeolite sites. Effects of these interactions are evident in the spectra of the adsorbed species. 

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