Zeolites in organic reactions

Alkylation of benzene with formaldehyde to give diphenylmethane was carried out at 30–80°C in the presence of a series of HY zeolites at three levels of Na+ exchange (HY-21, HY-50 and HY-100), several framework Si-to-Al ratios from 4 to 90 and two zeolite crystal sizes (0.30 and 0.80 μm). Only strong acid sites, i.e., those with p K a < 0, which correspond to centres with 0 and 1 aluminium in the next nearest neighbours, are active in catalysing this reaction. The turnover number of the acid zeolites increases steadily with increasing framework Si-to-Al ratio. Under the conditions studied there is no diffusion control of the reaction in the pores of the zeolite. There is a decay in the catalyst activity during the course of the formaldehyde condensation, which is greater for samples with higher Al content. This catalyst decay is due not only to poisoning by oligomers, but also to the strong adsorption of polar products.