Relationships Between the Structure and the Surface Activity of Nano-Sized Alumina in Acid Catalysis
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*Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201 ABSTRACT
As shown in another communication to Symposium V, nano-sized aluminas enriched in pentacoordinated aluminum (Alv) are obtained through the limited hydrolysis of aluminum alkoxides. Here the surface properties of these aluminas are investigated from the catalytic point of view. The goal is to obtain geometrical information on the Lewis acid sites. 'H-,. 27A1 (crosspolarization) high resolution MAS NMR, using chemisorbed NH 3 which acts as a 'H spin reservoir, allows the direct observation of surface Al involved in the catalytic site. The polarization transfer from these protons to surface Al is remarkably efficient because of the relatively short Al:N-H distance. It is observed that the main Lewis acid sites are located on distorted tetrahedral aluminum and pentacoordinated Al. These results are compared to low temperature infrared data of adsorbed CO. INTRODUCTION
Aluminas play an important role in catalysis as catalysts as well as metal catalyst supports. On thoroughly dehydrated transition aluminas the acid sites are (electron acceptors) Lewis sites. Commercial aluminas have surface areas between 200 and 400 m2g- . Upon dehydroxylation coordinately unsaturated aluminum (CUS) sites appear on the surface.' Aluminas obtained by dehydroxylating boehmite or bayerite precursors are generally considered as pseudo-spinel containing in their bulk twice as many octahedrally coordinated Al (Alv') than tetrahedrally 2 coordinated (Ally).
In fact, the aluminas used as catalysts have very disordered structures, and
particles in the nano-range, which make them X-ray quasi-amorphous. The topology of the3 surface is largely unknown. The main recent progress in this area is due to Ellis and coworkers who have shown that the surfaces of aluminas can be studied by 2 7A1 nuclear magnetic resonance spectroscopy using a cross-polarization procedure. This procedure may be briefly illustrated as follows. Suppose that the surface is hydrated because the alumina has been exposed to atmospheric moisture. Upon i) polarizing the OH proton, ii) transferring the polarization to aluminum, and iii) observing the aluminum resonance, only those aluminum close (:2 Mhz), an intense magnetic field is necessary to decrease the second-order quadrupolar effect. Our goal here is to study by 27 A1 CP MAS NMR from chemisorbed ammonia the surfaces of thoroughly dehydrated aluminas with different bulk compositions and to compare these results with the information obtained from the Fourier transform infrared spectroscopy (FTIR) of chemisorbed CO. The absolute number of Lewis sites has been semi-quantitively estimated by measuring the integrated absorbance of CO adsorbed at -150 K and using the known specific absorbances in the range of wave numbers (2050 to 2250 cm-1) where vibrational bands attributable to CO adsorbed on Lewis sites are observed. 4 961 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
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