QUANTUM-CHEMICAL STUDIES OF ACID SITES IN ZEOLITE ZSM-5

Semiempirical quantum chemical calculations have been performed using cluster models for the acid sides of zeolite ZSM-5. The effects of lat tice relaxation and oxygen protonation at T sites where Al has been su bstituted for Si have been treated using 12 clusters containing about 100 atoms to represent the 12 distinct T sites. The calculations show a wide range (1.2 eV) in the (Al,H)/Si substitution energies, but near ly every site gives rise to at least one acid site whose energy is les s than 0.2 eV from that of the lowest-energy T9 site. The calculations also show that there exists a direct correlation between the substitu tion energy and the proton affinity of a given site-those sites that e xhibit the largest proton affinity are the ones for which the aluminum is most stable. From comparing the most favorable location for a prot on at each of the 12 T sites in ZSM-5, the proton affinities are found to vary by 0.3 eV (29/kJ/mol). In addition, both the (Al,H)/Si substi tution energy and the proton affinity depend on the geometric paramete rs, such as bond lengths and angles, of the optimum configurations for the acid sites. The most stable sites for protons correspond to ''por e'' sites where the proton is oriented into the major pores of the zeo lite lattice. The most important conclusion from this study is that th ere is a direct relationship between the experimental T-O-T angle and the proton affinity. This suggests that the T-O-T angle can be used as a measure of the acidity of the site. This conclusion, although obtai ned for the 48 sites of zeolite ZSM-5, would be expected to be transfe rable to other zeolites.