Acidity and porosity properties of zeolites affect their catalytic performance in thymol synthesis

Thymol is synthesized from m-cresol alkylation reaction with 2-propanol. A few commercial zeolites have been tested in this reaction, but the influence of acidity and porosity of a zeolite on the m-cresol conversion and thymol yield is scarcely studied. Isomorphously-substituted Al-, Ga-, Fe-, B-, and In-UTL with different heteroelements and thus different ratio between strong and weak sites give insight into the effect of acidity. Isoreticular zeolites Al-UTL, Al-IPC-7, and Al-IPC-2 with different pore sizes reveal the effect of porosity. In this study, m-cresol conversion was far from comparable over Al-UTL or Ga-UTL (X = 18–19 %) and Fe-UTL, B-UTL, or In-UTL (X = 2–3 %) having different concentration of strong Brønsted acid sites. Thymol yield was also higher over Al- and Ga-UTL (Y = 11–13 %) than over Fe-, B-, and In-UTL (Y = 1–2 %). The easier accessibility to the strong Brønsted acid sites in Al-UTL, provided by bigger pore sizes, resulted in higher m-cresol conversion and thymol yield compared to Al-IPC-7 (X = 15 % and Y = 8 %) and Al-IPC-2 (X = 6 % and Y = 2 %). In addition, the stronger Brønsted acidity and large and extra-large porosity in Al-UTL and Ga-UTL facilitated the rearrangement of isopropyl-3-methylphenyl ether - the product of the O-alkylation pathway - to thymol. Thus, Al-UTL and Ga-UTL (along with Al-FAU reference material) confirmed the optimum structural properties for a high selectivity to thymol.