Abstract
A series of ZnSnPt supported defective MFI zeolites with different SiO2/Al2O3 ratios (30, 110, 700, and ∞) and hydroxyl nests concentration were prepared and characterized by multiple techniques including scanning electron microscopy (SEM), nitrogen physisorption, NH3-TPD, transmission electron microscopy (TEM), hydrogen temperature programmed reduction (H2-TPR), and Fourier transform infrared spectrometer (FT-IR). It was found that Brønsted acid sites (Si(OH)Al) with strong acid strength and the hydroxyl nests with weak acid strength coexisted over the defective ZSM-5 zeolites and ZnSnPt Lewis acid sites preferentially located on the hydroxyl nests. The increase in the concentration of hydroxyl nests and SiO2/Al2O3 ratios apparently improved the distribution of ZnSnPt Lewis acid sites. The hydroxyl nest incorporated ZnSnPt Lewis acid sites showed extraordinary dehydrogenation ability. Specially, operando dual beam Fourier transform infrared spectrometer (DB-FTIR) was applied to characterize the propane transformation under reaction conditions. At low SiO2/Al2O3 ratios, the propane efficiently transforms into propene and aromatics (total selectivity of 93.37%) by the cooperation of Brønsted acid sites and ZnSnPt Lewis acid sites. While at high SiO2/Al2O3 ratios, the propane mainly transforms into propene (selectivity of above 95%) and hydrogen. This study provides guidance for the preparation of highly efficient propane dehydrogenative transformation catalyst.
Highlights
IntroductionFriendly and non-corrosive, solid acid catalysts (zeolites and zeolite-based catalysts) have been widely applied in numerous petrochemical processes [1]
Friendly and non-corrosive, solid acid catalysts have been widely applied in numerous petrochemical processes [1]
−1 )(3740 spectra suggested that the defective-1ZSM-5 contained Brønsted acid groups (3610 cm associated with internal silanol groups (3720 cm ), and hydroxyl nests that consist of a framework aluminum (Si(OH)Al), isolated external silanol groups (3740 cm ), free internal silanol number of silanol groups interacting through extended hydrogen bonding [3,34,39]
Summary
Friendly and non-corrosive, solid acid catalysts (zeolites and zeolite-based catalysts) have been widely applied in numerous petrochemical processes [1]. According to literature and our previous work, Zn modified HZSM-5 zeolites are effective catalysts for the dehydrogenative aromatization of light alkanes and exhibit comparable catalytic performance to Ga–Pt loaded ZSM-5 zeolite [20,21,22,23,24,25,26,27,28,29,30,31]. It is found that ZnSnPt Lewis acid sites are preferentially located on the hydroxyl nests with weak acidity rather than on the Si(OH)Al groups with strong acidity, and the size of metal particles is smaller and much more uniform at higher concentration of the hydroxyl nests These ZnSnPt Lewis acid sites located on the hydroxyl-nest defects show superior propane dehydrogenation performance. Fourier transform infrared spectrometer (FTIR) study unravels that the acidity of neighboring active sites over ZnSnPt Lewis acid sites located hydroxyl nests determines the transformation paths
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