Acidity of Metallosilicate

The band position of OH groups changes in conformity with TPD spectra. Thus, the OH band appears at 3610, 3620, 3630, and 3725 cm for [Al] -, [Ga] —, [Fe] -, and [B]-ZSM-5, respectively. The fact that the acid strength of [B]-ZSM-5 is much weaker than that of [Al] - ZSM-5 has been reported by several authors.  

Weaker acid strength of [B] —ZSM-5 is confirmed also by catalytic reactions. Table 3.29 shows the product distributions of 1-butene reaction over [B] — ZSM-5 and [Al] — ZSM-5 at 773 It is clear that there is a great difference in the product dis-tirubution. Thus, over [Al] —ZSM-5, the main products are lower alkanes and aromatic hydrocarbons, while over [Bj —ZSM-5, lower alkenes are the main products. This indicates that the hydride transfer reactions from alkene to carbenium ion does not proceed over [Bj-ZSM-5. 

For the same reasons, alkenes are the main products in the conversion of methanol over [Bj-ZSM-5, while [Alj-ZSM-5 is a unique catalyst for gasoline production.  

The yield of aromatic hydrocarbons gready increases by introducing zinc cations into [Bj —ZSM-5 (Table 3.29). In this case, however, the yield of alkanes remains low. This is because the tiromatics are formed by the direct dehydrogenation of olefins by the action of zinc species. As exemplified by this case, it is possible to achieve catalysis by metal cations at the same time suppressing catalysis by acid. 

The acid strength of [Fe] — ZSM-5 can be inferred to be weak from the very low yield of alkanes and aromatics in the conversion of methanol or olefins. Holderich reported that ketone can be isomerized to aldehyde in a high yield over [Bj —ZSM-5. ZSM-5 gave only low selectivity.  

---

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

---