Fig. 1. Catalyst models and general reaction steps simulated in this work. View of the H-ZSM-5 zeolite model from the straight (a) and sinusoidal (b) channels. (c) Schematic illustration of the transition states of the typical reaction steps involved in the chain growth of olefin intermediates, their aromatization, and methylation of the benzene ring. (d) 5-Ring (R5) and 6-ring (R6) routes for the cyclization reaction of the C6 species in the aromatization step with the calculated reaction barriers shown in eV.

Fig. 2. Reaction pathway for aromatics formation from propene and methanol catalyzed by the acidic site of H-ZSM-5. The reaction processes considered include the growth of the carbon chain in propene to form long-chain C6 species, its further aromatization to yield benzene, and the formation of heavier aromatics by methylation of the benzene ring. ZOH, ZO-, and ZOCH3 after @denote protonated, deprotonated acid sites of zeolite, and the zeolite with surface methoxy species (SMS) formed by methanol, respectively.

Fig. 3. Formation of C6 species via stepwise methylation of propene by methanol. The elementary steps consist of methylation with methanol, intramolecular hydride transfer (IHT), and deprotonation of the carbenium ion, resulting in a C6 carbenium ion. The rate-determining steps are chain methylation with methanol, and their transition state (TS) structures are shown in the inserts. Numbers in the paratheses are the calculated energy barriers. The asterisk (*) refers to the acid site of the H-ZSM-5 zeolite. Color code: Si (yellow), Al (purple), O (red), H (white), C (gray).

Fig. 4. Aromatization of C6 species for benzene formation via the R5 route. The elementary steps include the deprotonation of the carbenium ions, hydride transfer (HT), cyclization, intramolecular hydride transfer (IHT), and expansion of the 5-member ring. Transition state (TS) structures of the rate-determining steps (i.e., hydride transfer) are shown in the inserts. Energy barriers of related reaction steps are shown after the TS. Color code: Si (yellow), Al (purple), O (red), H (white), C (gray).

Fig. 5. Methylation of the benzene ring with methanol to form polymethylbenzene. The elementary steps in this process consist of successive ring methylation with methanol and deprotonation of the benzenium ions. Transition state (TS) structures for the rate-determining steps (i.e., ring methylation with methanol) in this process are shown in the inserts. Energy barriers of the related reaction steps are shown after the TS. Color code: Si (yellow), Al (purple), O (red), H (white), C (gray).

Fig. 6. Rate-determining steps in aromatics formation from propene in H-ZSM-5. (a) Comparison of the energy barriers (Ea, eV) for the rate-determining steps in chain growth, ring formation, and ring methylation, which are the chain methylation, hydride transfer, and ring methylation steps, respectively. Analysis of the distances (?) between the carbon atom undergoing methylation in olefins and aromatics (denoted as COl and CAr, respectively) and the carbon atom of methanol (CMe), as well as the distance (?) between the carbon and oxygen (OMe) atoms of methanol in the transition state structures for the methylations of carbon chain (b) and benzene ring (c) with methanol.

Fig. 7. Mechanism of methanol-mediated aromatics formation from propene in H-ZSM-5. The relationship of the aromatics formation with the induction period and aromatic cycle are also indicated in the figure.