Silica-alumina catalyst active protons

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The carbonaceous material which was retained by the catalysts after evacuation was held in 2 forms 1 could be recovered as butene molecules (12) by exchange with isotopically labelled 1-butene the other could be removed only by combustion to CO2. With silica-alumina catalysts, the latter (residues) is thought to form the active sites for car-bonium ion activity (3,8,9). The present results showed that the activity correlated with the degree of decationation or the Ca content of the catalyst. Moreover, the amount of residue retained by the catalyst (nonexchangeable ) was about 2 orders of magnitude smaller than the number of decationated sites of the sample (12). It seems probable, therefore, that residues do not play an important role in the development of catalytic activity of these materials in the presence of H2O however, residues may supply the necessary protons for catalytic activity in its absence. 

The assumed presence of an active proton in silica-alumina catalysts... 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

Further evidence for the need of protons for carboniogenic activity is given by Matsumoto et al. (58), who have shown that NaY, which is inactive for cumene cracking, can be made into an active catalyst by the addition of HCl. This effect of HCl is reversible. In contrast, silica gains no activity on HCl addition, and y-alumina is activated irreversibly by HCl. Similar observations were made by Kolesnikov et al. (51), who found that in the propylation of benzene, a treatment with propyl chloride promotes the activity of NaY and of type X zeolites. 

Further evidence that the active centers on silica alumina-are Lewis rather than protonic acids was provided by the spectral response to catalyst pretreatment shown in Fig. 28. Here curves A, B, and C represent the spectra of p-phenylenediamine chemisorbed on silica-alumina samples which were heated at increasing temperatures. The regular increase in the intensity of the 4680 A band, due to the cation radical, was taken as spectroscopic evidence for an increasing number of Lewis-acid sites. Although these spectra are in qualitative agreement with the known effect of thermal treatment on the relative abundance of Lewis and Bronsted acidity, quantitative conclusions cannot be drawn since the concurrent increase in intensity at 3240 A indicates that these measurements were not made under conditions of constant surface coverage. 

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