Acidity of silicon modified CoAPO-5 molecular sieves

Abstract

Cobalt substituted AlPO 4-5 molecular sieve was synthesized hydrothermally. Silicon modified CoAPO-5 samples were obtained by treating the CoAPO-5 sample with different quantities of ammonium hexa-fluorosilicate solution. The modified and unmodified CoAPO-5 samples were characterized for acidic properties using pyridine as a probe molecule. The XRD data of modified samples showed that the unit cell parameter a increases and c decreases with overall increase in the unit cell volume as compared to the unmodified CoAPO-5 sample. The IR spectra in the mid-infrared region indicated that silicon atoms were present in the framework position. Temperature-programmed desorption data showed an increase in the retention capacity for pyridine molecule with increased silicon content. The IR and N 1s, X-ray photoelectron spectra of chemisorbed pyridine were used to determine the relative concentration of Brønsted and Lewis acid sites present in these molecular sieves. Comparison of IR data with XPS, allowed us to conclude that on the surface of the molecular sieve crystallites the concentration of the Lewis sites is higher than Brønsted acid sites and for the bulk it is vise versa. In view of the above evidence it is concluded that the ammonium hexafluorosilicate treatment leads to the isomorphous substitution of phosphorus by silicon in the CoAPO-5 framework accompanied by a small but significant amount of simultaneous substitution for aluminum of cobalt plus phosphorus.