Investigation of the Crystallization Process of SAPO-35 and Si Distribution in the Crystals

The hydrothermal crystallization of SAPO-35 molecular sieve is examined in the present study. Many characterization methods including XRD, SEM, XRF, XPS, EDS, and various solid-state NMR techniques are applied to monitor the formation process of SAPO-35 and the development of the local environments of Si, P, and Al atoms as a function of crystallization time. The results show that an intermediate phase with ordered structure and AlPO compositions first forms. Subsequently, SAPO-35 crystallizes at the expense of the intermediate phase. Si takes part in the formation of SAPO-35 since the initial crystallization. The Si concentration in SAPO-35 shows an increasing trend with time, which leads to the change of both Si and Al coordination environments. More information is obtained from two-dimensional ²⁷Al MQ MAS and ²⁷Al → ³¹P MQ-MAS HETCOR NMR. Moreover, XPS analysis reveals the enrichment of Si on the crystal surface. On the basis of the characterization results, it is supposed that the distribution of Si in the SAPO-35 crystals is not uniform, showing a gradual increase from the core to the external surface.