Hydroxyl groups in phosphorus-modified HZSM-5

The effects of post-synthesis modification on the hydroxyl groups of HZSM-5 were investigated by IR spectroscopy and temperature-programmed desorption. Three model compounds [phosphoric acid, trimethylphosphine and trimethyl phosphite] were used for modification. Six different hydroxyl groups were observed with the samples investigated. They were assigned to silanol groups terminating the crystals (3745 ^cm-1), SiOH in the interior (3725 cm^-1), AlOH and POH groups (3683 and 3668 cm^-1), bridged hydroxyls (3610 cm^-1) and hydrogen-bonded internal SiOH groups (3500 cm^-1). Phosphorus treatment was found to decrease the concentration of the strong Brønsted acids sites (bridging hydroxyl groups) by dealumination (H₃PO₄ and trimethyl phosphite treatment) or by chemisorption of the phosphorus compound (trimethylphosphine treatment). After H₃PO₄ and trimethyl phosphite treatment, the formal replacement of strong Brønsted acidic hydroxyls by H₂PO₄ ^- groups occurs to some extent. The concentration of weak acid sites was found to increase after H3PO4 and trimethyl phosphite treatment and did not vary after trimethylphosphine treatment. Strong Brønsted acid sites are bridging hydroxyl groups, weak Brønsted acid sites, hydrogen-bonded SiOH groups at defect sites and, to some extent, POH groups.