Effect of coating with silica on acidic and catalytic properties of SAPO-18 in MTO conversion

Abstract

Abstract Silicoaluminophosphate with AEI structure has been modified by silica coating using polydimethylsiloxane (PDMS) and tetraethoxysilane (TEOS) as silica sources. The single step procedure including deposition of 0.5, 1.0, 1.5 and 2.0 wt% of silicon in one step and the multistep procedure including stepwise deposition of silica source by small portions corresponding to 0.5 wt% of silicon with intermediate calcination steps have been applied. The modified materials have been characterized by X-ray fluorescence spectroscopy, X-ray diffraction analysis, low temperature nitrogen adsorption, 29Si MAS NMR, thermoprogrammed desorption of ammonia and FTIR spectroscopy of adsorbed 2,6-ditertbutylpyridine (2,6-DTBPy). The results show that multistep procedure leads to the gradual increase of silicon content and results in the deposition of 2 wt% of Si in 4 steps, whereas single step procedure does not lead to the desired result. Coating with silicon leads to poisoning of Bronsted acid sites located on the external surface of SAPO-18 crystal. This effect is more pronounced in the case of PDMS due to more uniform silica deposition in this case. Coated materials show higher stability of catalytic activity with time on stream in methanol to olefins conversion (MTO) and higher selectivity to light olefins. The mechanism of silica coating with PDMS and TEOS is discussed.