Selective Grafting of Ga(i-Bu)3 on the Silanols of Mesoporous H-ZSM-5 by Surface Organometallic Chemistry

Abstract

The reaction of triisobutylgallium toward meso-H-ZSM-5250, prepared by desilication of H-ZSM-5, has been studied by IR, high field solid-state NMR, and EXAFS spectroscopies as well as elemental analysis. Grafting occurs selectively on silanol groups, resulting in the monopodal gallium species [(≡SiO)Ga(i-Bu)2]. Since the Brønsted acidic protons remain largely unreacted, control of the surface organometallic chemistry affords a zeolitic material with both gallium sites and significant Brønsted acidity. The resulting bifunctional Ga(i-Bu)3/meso-H-ZSM-5250 shows higher activity and selectivity in propane aromatization than monofunctional meso-H-ZSM-5250. The aromatic products arise from alkane dehydrogenation on the gallium sites, followed by oligomerization, cyclization, and aromatization of the resulting olefins on the Brønsted acid sites.