6678
J. Phys. Chem. 1996, 100, 6678-6690
Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules C. Otero Area´ n* and G. Turnes Palomino Departamento de Quı´mica, UniVersidad de las Islas Baleares, 07071 Palma de Mallorca, Spain
F. Geobaldo† and A. Zecchina* Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniVersita` di Torino, Via P. Giuria, 7, 10125 Torino, Italy ReceiVed: October 5, 1995; In Final Form: January 31, 1996X
Three MFI-type, H-GaZSM-5 gallosilicates were synthesized with Si/Ga ratios of 25, 50, and 75, respectively. Incorporation of gallium into the zeolite framework was checked by powder X-ray diffraction, which showed an approximately linear variation of d spacings with Si/Ga ratio. Fourier transform IR spectroscopy of CO and N2, adsorbed at liquid nitrogen temperature, was used to characterize these gallosilicates after activation in vacuo at temperatures from 673 to 973 K. It was found that both probe molecules formed stable adducts with polarizing centers having either Bro¨nsted or Lewis acid character. On samples activated at a temperature smaller than 700 K Bro¨nsted sites were found to be predominant; apart from regular Si(OH)Ga sites, adsorbed carbon monoxide also revealed the presence of a small fraction of Bro¨nsted acid groups associated with nonregular sites. These could be located either at the mouth of small cavities (presumably associated with defective regions of the zeolite framework where gallium was segregated) or on amorphous regions. On the sample with Si/Ga ) 25 activated at temperatures higher than 700 K, gallium showed a marked tendency to go into either partial or total extraframework positions, thus generating coordinatively unsaturated Ga3+ centers with strong Lewis acid character. These centers were revealed by formation of CO and N2 adducts where stretching frequencies of the adsorbed molecules showed a strong hypsochromic shift, ∆ν. Values of ∆ν as high as 82 and 25 cm-1 were found for CO and N2, respectively. In the case of dinitrogen, interaction with Lewis acid centers also brings about a pronounced increase of IR band intensity, and this effect can advantageously be used to monitor extraframework species.
1. Introduction Isomorphous substitution of Si or Al in zeolite frameworks by other tetrahedrally coordinated elements (e.g., B, Fe, Ga, Ge, or Ti) provides a means to enhance useful properties, among which catalytic behavior is perhaps the best exponent. Specific interest in gallium-containing zeolites stems from their high selectivity to aromatics in the catalytic conversion of olefins and paraffins.1-13 Thus the so-called Cyclar process,14,15 where C3-C5 alkanes are dehydrocyclized to yield aromatic hydrocarbons, proceeds over GaZSM-5 zeolites whereas conventional ZSM-5 aluminosilicates yield mainly paraffins. There is evidence8,10,11,16-19 that enhanced aromatization involves a bifunctional catalytic process where gallium species are responsible for dehydrogenation (possibly via H- abstraction from alkanes) while oligomerization could take place on both gallium species and Bro¨nsted acid sites of the zeolite framework. The nature of the gallium species involved in this process is not well-known, and enhanced aromatization may be associated with either framework or nonframework gallium (or both).7,12,20,21 Moreover, partial extraframework gallium species (vide infra) can also be generated during thermal treatment of GaZSM-5, and this complicates the interpretation of catalytic mechanisms. Detailed characterization of the zeolite, including extraframework species, is a prime requirement to understanding catalytic behavior. * Authors to whom correspondence should be addressed. † And Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6678$12.00/0
On low-temperature (
