Solid State NMR Studies of the Adsorbed States of Formic Acid on Y Zeolites
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SOLID STATE NMR STUDIES OF THE ADSORBED STATES OF FORMIC ACID ON Y ZEOLITES
T. MICHAEL DUNCAN* and ROBERT W. VAUGHAN** California Institute of Technology Division of Chemistry and Chemical Engineering Pasadena, CA 91125
ABSTRACT Several multiple-pulse double-resonance NMR techniques have been applied to isolate and characterize the spectra of the adsorbed states of formic acid on two Y zeolites. The two surface states, bidentatT3 and ynidentate, possess different motional properties and C - H cross-polarization 13 techniques may be used to separate the spectra. The C chemical shift anisotropy is founa to iorrelate with the symmetry of the formate species. The H spectrum of the carbonyl hydrogen, selectively observed with the dipolardifference method, indicates that this hydrogen becomes more acidic upon adsorption.
INTRODUCTION An understanding of the reaction mechanisms on heterogeneous catalysts is essential to optimizing existing processes and designing new catalysts. This understanding requires a knowledge of the adsorbed state and the adsorption site of both the reactant and product species. This paper describes the application of several multiple-pulse NMR techniques to isolate and characterize the spectra of a catalytic system. The system studied is the decomposition of formic acid on Y zeolites (Si/Al = 2.8). Formic acid adsorbs irreversibly on the zeolites through che loss of the acidic proton. The adsorbed species decomposes almost exclusively to carbon monoxide and water. The nature of the adsorbed formate can be described by the type of bonding to the oxide surface. The unidentate structure is bonded through one oxygen atom with the other oxygen atom doublebonded to the carbon. The bidentate formate is chelated to the adsorption site through both oxygen atoms, yielding symmetric carbon-oxygen bonds. The two zeolites investigated are an ammonium-Y (NH4 -Y) and an ultrastable hydrogen-Y (u-H-Y) zeolite. A discussion of these molecular sieves can be found elsewhere [1]. Briefly, the NH4 -Y and u-H-Y (derived from the NH4 -Y by heating to 775 K) have the same crystal structure, except the u-H-Y has a fraction of the Al ions that have been displaced from the crystal framework. Correspondingly, the u-H-Y zeolite has 12-15 new Bronsted sites per unit cell (unit cell contains 48 Al atoms) and increased catalytic activity with respect to hydrogenation and cracking.
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260 The multiple-pulse NMR experiments are used to quantify the dipolar interactions and the chemical shift anisotropy of the 3C nuclei of the The strength of the dipolar couplings is used to adsorbed formate species. determine the extent of motion of the adsorbed molecules and the nature of the adsorption site. The principal comp
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