Acid]Base Catalysis in Zeolites from First Principles

Zeolite materials are microporous aluminosilicates with various uses, including acting as important catalysts in many processes. One such process is the methanol to gasoline reaction, used widely in industry. This reaction is known to be associated with Brønsted acid sites in the zeolite, formed when Si is substituted by Al in the framework, with an associated Hq being bound nearby to maintain charge neutrality. However, it is not clear exactly what role the proton plays in this reaction. Because of the large unit cell Ž . generally 50]300 atoms, depending on the particular zeolite of such structures, most ab initio calculations of these materials have focused on studying small clusters representing just a portion of the framework. However, by choosing the chabazite zeolite structure, which has only 36 atoms in the primitive unit cell, we have been able to perform a full periodic ab initio calculation. This has used density functional theory with a generalized gradient approximation for the exchange-correlation energy, a plane-wave basis set, and norm-conserving optimized pseudopotentials. Using these methods we have examined the geometry and electronic structure of a zeolite acid site and considered one way in which a methanol molecule may bind to such a site. Q 1997 John Wiley & Sons, Inc. eolites are microporous aluminosilicate strucZ tures, important for a variety of applications w x Ž . 1 , for example, the methanol to gasoline MTG conversion process, which accounts for one-third w x of New Zealand’s gasoline supply 2 . Although U To whom correspondence should be addressed. many experimental studies have been made of this w x process 3 , the exact microscopic mechanism is still unclear. Methanol is known to be initially adsorbed at Brønsted acid sites associated with Al atoms in the structure, but the geometry and nature of adsorption have been matters for debate. ( ) International Journal of Quantum Chemistry, Vol. 61, 393]398 1997 Q 1997 John Wiley & Sons, Inc. CCC 0020-7608 / 97 / 030393-06 SHAH, PAYNE, AND GALE Experimental data have been interpreted as suggesting proton transfer from the zeolite framework to the methanol molecule, creating a ‘‘chemiw x sorbed’’ complex 4 . Calculations on model clusters used to represent zeolite fragments, using w x density functional theory 5 as well as w x Hartree]Fock and MP2 calculations 6 , have shown that the proton remains on the zeolite, and the molecule is simply adsorbed as a ‘‘physisorbed’’ species, forming hydrogen bonds with the zeolite framework. However, results have been found to converge very slowly with respect to w x cluster size 7 . This is probably due to the longrange interactions that are present in the zeolite, in particular the electrostatic potential. Although attempts have been made to mimic this potential by embedding a cluster in an array of point charges w x 8 , there are problems of boundary effects, and the ability of point charges to accurately reproduce the potential. In this work we have adopted a different approach, using periodic boundary conditions to represent the full zeolite crystal structure. In general zeolite structures have very large unit cells, making this approach impracticable; for example, ZSM-5, the commercially used catalyst for the MTG process, has a unit cell of 288 atoms. However, we have instead studied the chabazite w x structure 9 , also known to be an active catalyst w x for methanol conversion 10 , but with a unit cell of only 36 atoms. In the next section we outline the methods used for our calculations, and in the third section the results obtained for calculations of Brønsted acid sites in zeolites are presented. The fourth section presents initial results for methanol adsorbed onto the zeolite, and a brief conclusion ends the study.