Gang Liu,Zhi-Jian Zhao,Tengfang Wu,Liang Zeng,Jinlong Gong
Supported VOx catalysts are promising for use in propane dehydrogenation (PDH) because of the relatively superior activity and stable performance upon regeneration. However, the nature of the active sites and reaction mechanism during PDH over VOx-based catalysts remains elusive. We examined active species by attaining various fractions of V5+, V4+, and V3+ ions by adjusting the surface vanadium density on an alumina support. The results reveal a close relationship between TOF and the fraction of V3+ ion, indicating that V3+ was more active for PDH. In situ diffuse reflectance infrared Fourier transform spectroscopy showed the same strong adsorbed species during both propane dehydrogenation and propylene hydrogenation. The results indicated that such an intermediate may correspond to V species containing a C═C bond, i.e., V–C3H5, and a reaction mechanism was proposed accordingly.
The relatively low cost and mild toxicity of vanadium makes it an alternative to commercially applied Pt and Cr based catalysts in propane dehydrogenation (PDH). Conventional VO x /Al 2 O 3 catalyst shows high initial activity, but deactivates fast due to severe coke formation, which requires frequent regeneration by coke combustion and causes massive CO 2 emissions. In this context, we report on the phosphine (PH 3 ) surface modification of VO x /Al 2 O 3 catalyst leading to substantial improvement of its stability. The phosphorus considerably modifies the properties of the VO x /Al 2 O 3 catalyst: (i) tunes surface acidity, (ii) separates polymerized vanadium oxide species and (iii) weakens interaction between vanadium species with alumina support. Synergy among all these factors facilitates desorption of produced propylene and inhibit the oligomerization of coke precursors, which contributed to the enhancement of the catalyst stability during propane dehydrogenation.
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