Protonic acid sites

Does Ae generation of protonic acid sites originating from molecular hydrogen occur for the catal ts other than Pt/S042--ZrO2 ... 

For determination of the number of protonic sites and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-i (due to pyridine chemisorbed on Lewis acid sites, L-Py) and 1490 cm i (due to both the L-Py and pyridine chemisorbed on protonic acid sites, B-Py) were used with the tangent background for all samples. The values obtained were normalized to the weight of the sample wafer. To obtain the apparent absorption coefficients of the bands, a known amount of pyridine was adsorbed on the sample, and the absorbance of each band was measured. Then, a small quantity of water which is sufficient to convert all Lewis acid sites into protonic acid sites was introduced into the IR-... 

The recovery of the Lewis add sites and the decrease in the protonic acid sites to the original values by outgassing gas phase hydrogen are rather slow and require a high temperature. The addition of Pt/Si02 did not affect much the restoration of Ae Lewis add sites of H-ZSM-5 by outgassing gas phase hydrogen. 

It has been revealed that the formation of protonic acid sites from molecular hydrogen is observable for the catalysts other than Pt/S042--Zr02, and the protonic acid sites thus formed act as catalytically active sites for acid-catalyzed reaction. We propose the concept "molecular hydrogen-originated protonic acid site" as a widely applicable active sites for solid acid catalysts. 

Zeolites exhibit a considerably lower proton (acid site) concentration than liquid acids. For example, 1 g of H2SO4 contains 20 X 10-3 moles of protons, whereas 1 g of zeolite HY, with a Si/Al atomic ratio of five, contain no more than 3 X 10-3 moles of protons. (Note that this is a cmde approximation of the acidic sites available for catalysis, because it assumes that with both materials all protons are available and catalytically active.) Moreover, 1 g of H2SO4 occupies far less volume (i.e., 0.5 cm3) than the equivalent mass of zeolite (4-6 cm3). 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

In recent years, modification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Peng (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (AljOs, SiOj) and zeolite catalysts... 

Infrared spectra of pyridine adsorbed on kaolinite indicated that the dry clay (110°C) contained both Brtfnsted and Lewis acid sites (235). At 1% water content only protonic acid sites were observed. It was not possible to assign the polymerization activity to either type of acid site, since both were present on samples which were catalytically active. 

The rich variety of their active sites [2,3] protonic acid sites of course, which play a major role in acid catalysed reactions, but also Lewis acid sites acting often in... 

Activation of solid catalysts under well-specified conditions is a key step for obtaining the desired catalytic performance. It is particularly the case with zeolites, which are hygroscopic solids and for which the efficiency can be significantly reduced by the presence of water (e.g. change in the characteristics of the protonic acid sites, loss of reactant by hydrolysis). Polar organic molecules (even present in low amounts in the atmosphere of the chemical laboratories) can also be rapidly and strongly adsorbed over zeolites causing a decrease of their catalytic efficiency. Pretreatment of the zeolite in the reactor is preferable. This in situ pretreatment is easier to carry out in fixed bed than in batch reactors. 

Reaction mechanism It is generally admitted that, over zeolites, acetylation of arenes with AA is catalysed by protonic acid sites. Comparison of the activity of a series of dealuminated HBEA samples allows one to exclude any direct participation of Lewis acid sites in 2-MN acetylation with AA. Indeed, two HBEA samples with similar protonic acidities but with very different concentrations of Lewis acid sites (170 and 16 pmol g ) have practically the same acylating activity.1271 The role of Brpnsted sites is also clearly expressed in Spagnol et a/.131... 

The initial rate of 2-MN acetylation depends on the framework Si/Al ratio of the zeolite catalyst.[27] For a series of dealuminated BEA samples (by treatment with hydrochloric acid or with ammonium hexafluorosilicate), the acetylation rate passes through a maximum for a number of framework A1 atoms per unit cell (/VA() between 1.5 and 2.0 (Si/Al ratio between 30 and 40). The activity of the protonic sites (i.e. the TOF) increases significantly with Si/Al from 420 h 1 for Si/Al = 15 to 2650 h 1 for Si/Al = 90. It should be noted that similar TOF values could be expected from the next nearest neighbour (NNN) model. Indeed all the framework A1 atoms of the zeolite (hence all the corresponding protonic acid sites) are isolated for Si/Al ratio 10.5. Therefore the acid strength of the protonic sites is then maximal as well as their activity.[57,58] This was furthermore found for m-xylcnc isomerization over the same series of BEA zeolites.1271 This increase in TOF for... 

Figure 3. Change in fraction of P-Py (proton acid site-pyridine) and L-Py (Lewis acid site-pyridine) caused by exposure to hydrogen at different temperatures.

Bifunctional catalytic reactions involve a series of catalytic steps over acidic and hydrogenating-dehydrogenating sites with formation of intermediate compounds. Thus n-hexane (hydro)isomerization involves successively n-hexane dehydrogenation in n-hexenes (metal catalyzed), skeletal isomerization of n-hexenes into isohexenes over protonic acid sites followed by the (metal catalyzed) hydrogenation of isohexenes into isohexanes (Figure 1.4). 

The participation of protonic acid sites in xylene isomerization is clearly demonstrated by correlations between the isomerization rate and the concentration of protonic sites of silica alumina with various alumina contents (13), alkaline-earth and rare earth FAU zeolites (14, 15), MFI zeolites (16), etc. Evidence is also provided by the fact that protonic sites participate in alkylbenzene disproportionation. On the other hand, it seems most unlikely that Lewis acid sites play a direct role in xylene isomerization and disproportionation (8). 

The surface acidity of zeolites appears to be an important ingredient for polymerization of acetylene and derivatives. Thus, when diazomethane was used to remove the protonic acidic sites on HZSM-5, no evidence for acetylene polymerization remained, compared to the original acidic form. ... 

Moljord et al [12] have shown, for protonic Y zeolites, that density of acid sites is the most important fector in determining the rate of coke oxidation and that the larger the number of Al atoms or protonic acid sites per unit cell, the easier the coke combustion. This was not observed in the present work, since coke formed on USY (high acid sites density) was more difficult to bum than that formed on CREY-2 (low acid sites density). Although the role of rare earth cations in coke combustion has yet to be further explored, this observation suggests that these cations present a catalytic effect on promoting coke oxidation. 

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