Vapor phase acylation of guaiacol with acetic acid was explored over a series of micro, nanocrystalline, and hierarchical MFI and BEA zeolites with different acidic and textural properties using a continuous flow reactor. The main products formed over these zeolites were i) guaiacol acetate, which is produced via O-acylation; ii) Hydroxymethoxyacetophenones (HMAPs), which are of pharmaceutical interest and mainly formed by C-acylation of guaiacol and to lesser extent by Fries rearrangement of guaiacol acetate and iii) catechol, which derives from the demethylation/demethoxylation of guaiacol. Both microcrystalline zeolites activity was poor because of the steric/diffusion limitation. Nanocrystalline and hierarchical MFI zeolites with improved accessibility and optimised acid strength and Lewis/Bronsted acid sites ratio were the most suitable catalysts for the continuous production of HMAPs. Although the guaiacol conversion slightly declines along the time on stream, the HMAPs selectivity and yield are enhanced due to deactivation affecting in higher extent to secondary reactions..
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