(58e) Toluene Alkylation with Methanol in Brønsted Acid Zeolites

The selective production of p-xylene is commercially interesting, because p-xylene is a valuable intermediate with high demand in the chemical industry. It is primarily used in the large-scale synthesis of various polymers, including polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Currently, xylenes are produced from cracking of naphtha and the isomers are subsequently isolated from the resulting mixture. Since the physical properties of xylenes are nearly identical, their separation by distillation is unfeasible, and fractional crystallization or selective adsorption are challenging and costly.¹ Thus, it is desirable to selectively synthesize p-xylene in an effective and inexpensive process.

Toluene, a major component produced in petroleum refineries, can be upgraded to xylenes through alkylation by methanol over acid zeolites.^2-3 Shape selectivity of zeolite pores and channels can provide useful means for selectively synthesizing different xylene isomers.⁴

In this contribution we employ density functional theory and statistical mechanics to revisit the important toluene alkylation reaction with methanol in Brønsted acid zeolites. Using surface and bulk models of zeolites with MFI and related frameworks with medium-sized pores, we investigate the catalytic activity of different T-sites located on the surface, channels and cages towards the synthesis of xylenes by toluene methylation. We pay particular attention to catalytic factors and mechanistic differences that determine the selectivity to the most valuable isomer of the xylene products, i.e. p-xylene. To that end, we investigated three different mechanisms in detail: (i) methanol binds to the active site first, (ii) toluene binds to the active site first, and (iii) both reactants co-adsorb to the zeolite active site. In order to accurately calculate the free energy profiles we employ a range of available entropy models. Microkinetic models are then used to estimate temperature and pressure effects on the reaction rate.

This study shows that the zeolite framework can play a significant role in the selectivity of this reaction toward different xylene isomers. Moreover, pressure and temperature effects on the reaction rate are discussed. Finally, we propose the relevant features of zeolite frameworks that seems to correlate with high activity and selectivity toward p-xylene.

References

1. Yang, Yuxi, Peng Bai, and Xianghai Guo. "Separation of xylene isomers: A review of recent advances in materials." Industrial & Engineering Chemistry Research56, no. 50 (2017): 14725-14753.
2. Zhu, Zhirong, Qingling Chen, Zaiku Xie, Weimin Yang, and Can Li. "The roles of acidity and structure of zeolite for catalyzing toluene alkylation with methanol to xylene." Microporous and mesoporous materials88, no. 1-3 (2006): 16-21.
3. Vos, Ann M., Xavier Rozanska, Robert A. Schoonheydt, Rutger A. van Santen, Francois Hutschka, and Juergen Hafner. "A theoretical study of the alkylation reaction of toluene with methanol catalyzed by acidic mordenite." Journal of the American Chemical Society123, no. 12 (2001): 2799-2809.
4. Young, L. B., S. A. Butter, and W. W. Kaeding. "Shape selective reactions with zeolite catalysts: III. Selectivity in xylene isomerization, toluene-methanol alkylation, and toluene disproportionation over ZSM-5 zeolite catalysts." Journal of Catalysis76, no. 2 (1982): 418-432.