Surface acidity zeolites

The next step is the ahstraction of a hydride ion hy a Lewis acid site from the zeolite surface to form the more stable allylic carhocation. This is again followed hy a proton elimination to form a cyclohexadiene intermediate. The same sequence is followed until the ring is completely aromatized. 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Busca, G. (2006) The surface acidity and basicity of solid oxides and zeolites, Chemical Industries (Boca Raton, FL, United States), 108 (Metal Oxides), 247. 

The type of treatment applied significantly affected the benzene/naphthalene (B/N) product ratio (Fig. 6). Thus, treatment with oxalic and citric acids, steaming plus HC1 washing, and partial Cs-exchanged increased the B/N ratio with respect to the untreated sample. In the first three cases, this effect may be explained by a preferential removal of the acid sites at the external zeolite surface, where naphthalene is predominantly formed [6], The reason of the increased B/N ratio in the 3Mo/CsHZ5 sample still needs to be elucidated, but a decrease in surface acidity in combination with an enhanced shape selectivity effect due to presence of voluminous Cs+ cations inside the micropores (a decrease in Vmicrop was noticed in Table 1) may be hypothesized. 

As for 10MR ID zeolites, the isomerization selectivity improvement is correlated with the microporosity plugging, it is proposed that EB isomerization on these coked catalyst mainly occurs on the outer surface acid sites. 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

Our third hypothesis, i.e., that the activity enhancement involves the proximity of the zeolite s acid sites, appears to be consistent with the hydrocarbon adsorption experiments, but may also be due to differences in the nickel dispersion arising from surface area differences between the two types of particles. Clearly, the adsorption of hexane at lower temperature on the nickel contaminated zeolitic particles suggests a significantly altered environment from both the uncontaminated and the non-zeolitic materials. 

FIGURE 9.1 Correlation between Bronsted acidity and Zeolite surface area for all the trials of CPS and ADV-CPS in the presence of metals. 

Humphries et al. [104] have given an excellent description of the surface acidity of zeolites and its influence on FCC. This topic has been comprehensively reviewed also by Shen and Auroux [105] and Auroux [103]. 

Zeolites consist of linked tetrahedra of Si04 and AIO4. The substitution of the aluminum by other trivalent atoms such as B, Ga, and In in order to modify the surface acidity of such solids has aroused considerable interest [34,213-215],... 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

MTO supported on acidic metal oxides was quickly discovered to form metathesis catalysts that are active without the need for additives, even for functionalised olefins [70]. Standard supports are zeolites and niobia (Nb205), and the activity was reported to be related to the surface acidity [79]. 

Ethylene and dimethylamine would result successively from the formation of ylide by deprotonation, an intramolecular carbanionic attack, and finally reprotonation. These mechanisms in which the methyl derivatives have been used are different from those proposed for montmorillonite where the ethylammonium cations were mainly implied. The origin of these differences may be partially the reactivity of the hydrogen as well as the nature of the surface acid sites. These considerations prompted us to repeat our previous experiments (1) for ethylammonium-exchanged Y zeolites. 

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