Effects of commercial HFAU structural parameters over m-cresol transformation

Abstract

The transformation of m-cresol was studied on a series of commercial HFAU zeolites (Si/Al=3.9–100) at 653 K, and reactant partial pressure of 0.1 bar. Whatever the catalyst samples the m-cresol was transformed into isomerisation products ( p- and o-cresol) and disproportionation products (phenol and xylenols). The catalyst pore volume distribution was investigated by nitrogen adsorption at 77 K, it was found that the micropores decreases, while meso and ultramicropores increase with decreasing N FAL, as a result of the partial collapse of the micropore structure during dealumination procedure. The acidity was investigated by IR in the OH stretching vibrations (3800–3400 cm −1) and by adsorption/desorption of pyridine followed by IR. For this series of catalysts it was found, that both the Bronsted and the Lewis acid site concentrations grow with the number of framework aluminium atoms per unit cell. The nature and amount of extraframework aluminium species vary from one sample to another and are strongly determined by the sample treatment history. A portion of the extraframework aluminium species is of cationic nature capable of synergetic interaction with nearby Bronsted acid sites, producing stronger sites, which are responsible for the 3600 and 3525 cm −1 bands in the IR spectra. This is the case of the less dealuminated samples ( N FAL=34.9–39.2), consequently, the more active. The highly dealuminated samples ( N FAL=1.9–4), contrary to what is commonly observed, has a weak Bronsted acidity, and therefore, lower activity, due to inhomogeneous dealumination that leaves adjacent OH groups. The initial rates of isomerisation reaction and p-cresol formation are higher than of disproportionation and o-cresol formation. The selectivity ratios I/ D and p/ o increased with increasing framework aluminium atoms. The stronger acid sites, of the less dealuminated samples, are capable of retaining the diphenylmethane intermediate of disproportionation, transforming it to coke precursors, instead of to phenol and xylenol, as is the case for the weak acid sites of the more dealuminated samples. A close relationship between o-cresol formation and the disproportionation was observed, this could be associated to a possible bimolecular path to o-cresol formation via transalkylation of m-cresol with xylenol.