Quantum chemistry and chemical thermodynamics methods have been used to calculate the energy characteristics of the direct alkylation reaction of benzene with ethane catalyzed by H-ZSM-5, an unmodified zeolite in the decationized form. It has been shown that the reaction of ethylbenzene synthesis from benzene and ethane with a small yield is thermodynamically feasible at temperatures below 400A degrees C. The catalyst H-ZSM-5 significantly reduces the energy of the formation reaction of the benzyl and ethyl radicals.