Synthesis and catalytic properties of Ti-substituted SAPO molecular sieves

Abstract

The effect of wall polarity on liquid-phase oxidation reactions catalyzed by Ti-substituted SAPO molecular sieves was examined. Titanium and silicon were incorporated into two different aluminophosphate molecular sieves of relatively large pores, namely AlPO 4-5 and VPI-5, through hydrothermal synthesis. The synthesized compounds were characterized with XRD, XANES, SEM, Raman, UV–Vis and FT-IR spectroscopies, as well as surface area measurements. The hydrophilicity of aluminophosphate was retained by keeping the Si/Al atomic ratio in the synthesis gel at 0.1. Using hydrogen peroxide as the oxidant, the Ti-substituted SAPO molecular sieves demonstrated marked catalytic activity in phenol hydroxylation reaction. However, little catalytic activity was observed in oxidation of organic substrates of low polarity, such as alkenes. The parameters which might affect the catalytic activities in phenol hydroxylation were investigated. These included the titanium content, the crystalline structure of the catalyst, the relative concentration of the reactants, as well as the polarity of the solvent. The catalytic reactions were considered to proceed mainly on the external surfaces of the aluminophosphate molecular sieves.