Direct incorporation of B, Al, and Ga into medium-pore ITH zeolite: Synthesis, acidic, and catalytic properties

Abstract

Direct crystallization of B-, Al- and Ga-substituted medium-pore ITH zeolites was carried out using hexamethonium hydroxide (HMH) and N,N,N′,N′,-tetramethyl-1,6-hexanediamine (TMHDA) as structure-directing agents. The type of organic SDA was found determining the crystal size and the nature of Brønsted acid sites in isomorphously substituted ITH zeolites. The use of HMH resulted in formation of tiny (length of 0.5–3μm) needle-like ITH crystals having bridging Si(OH+)E(−) (E=B, Al, Ga) groups with acidic strength increasing in the following sequence B<Ga<Al. In contrast, ITH zeolites with bulky (length of 40–50μm) crystals prepared using TMHDA possess loosely bond framework and [(SiO)3AlOH]−H+ and (SiO)2BOH groups (absorption band at 3670cm−1) perturbing upon interaction with pyridine. B-, Al- and Ga-ITH possessing tiny crystals showed improved catalytic performance in tetrahydropyranylation of 1-hexanol in comparison with non-acidic heteroelement-free germanosilicate zeolites and isomorphously substituted ITH zeolites with bulky crystals. The yield of target ether is enhanced with increasing strength of acid sites (e.g. B-<Ga-<Al-ITH) or with growth in their concentration.