New insights into the crystallization mechanism of microporous AlPO4-21

Abstract

Abstract This paper deals with the investigation of the crystallization mechanism of a microporous aluminophosphate AlPO 4 -21, with dimethyl amine as a template agent in organic solvents. The dependence of the crystallization rate of AlPO 4 -21 on water/organic solvents ratio is examined by calculating crystallization rates (referred to as k *) in different organic solvents involving ethylene glycol, ethylene glycol monoether, diethylene glycol and 1,4-butanediol. The solvation effect as a quantitative indication of the relation between k * and the polarities of the organic solvents under Koppel's solvation effect model shows that the templating effect in the crystallization of AlPO 4 -21 may be considered as a bimolecular process in which the formation of the crystallization transition state is related to the initial aluminophosphate gel and the steric–electronic effect of the amine. The templating effect appears to be similar to an organic reaction following nucleophilic mechanism, and is confirmed by linear free energy relationship analysis for both substituent constants of amines and crystallization rate. In order to design the electronic transfer model in the crystallization transition state of AlPO 4 -21, various coordination states for Al and P atoms are characterized by 27 Al and 31 P MAS NMR spectra. A steric–electronic model is proposed for the templating effect on the crystallization process of AlPO 4 -21.