Ab Initio and Density Functional Study of the Activation Barrier for Ethane Cracking in Cluster Models of Zeolite H-ZSM-5

Abstract

Protolytic cracking of ethane in zeolites has been investigated using quantum-chemical techniques and a cluster model of the zeolite acid site. An aluminosilicate cluster model containing five tetrahedral (Si, Al) atoms (5T) was used to locate all of the stationary points along a reaction path for ethane cracking at the HF/6-31G(d), B3LYP/6-31G(d), and MP2(FC)/6-31G(d) levels of theory. The cracking reaction occurs via a protonated structure that is a carbonium-like ion and is a transition state on the potential energy surface. The activation barrier for cracking calculated at each level of theory was refined by including (i) vibrational energies at the experimental reaction temperature of 773 K, (ii) electron correlation and/or an extended basis set at the B3LYP/6-311+G(3df,2p) or MP2(FC)/6-311+G(3df,2p) levels, and (iii) the influence of the surrounding zeolite lattice from a 58T cluster model of the zeolite H-ZSM-5. The barrier is especially sensitive to the long-range electrostatic effect of the lattice, which reduces it by 14.5 kcal/mol from the value obtained with the 5T cluster. The final calculated barrier of 54.1 kcal/mol at the MP2(FC)/6-311+G(3df,2p)//MP2(FC)/6-31G(d) level, including corrections, is significantly smaller than values obtained by previous theoretical studies and is in reasonable agreement with typical experimental values for short alkanes. The other levels of theory give similar values for the barrier.