Controlling Reaction-Induced Loss of Active Sites in ZnO_x/Silicalite-1 for Durable Nonoxidative Propane Dehydrogenation

Abstract

ZnO-based catalysts are promising for nonoxidative propane dehydrogenation (PDH) to propene owing to their low cost and environmental friendliness but experience serious loss of the active component because of the reduction of ZnO to metallic Zn that evaporates. Here, we demonstrate that MgO-modified ZnO x /silicalite-1 materials prepared through one-pot hydrothermal method are active, selective, and durable in the PDH reaction. The undesired loss of Zn could also be successfully suppressed without negative effect on the PDH performance owing to a strong interaction between Mg2+ and ZnO x , as concluded from the results of X-ray photoelectron and Fourier-transform infrared spectroscopic measurements as well as temperature-programmed reduction with CO. X-ray absorption spectroscopy revealed that atomically dispersed Zn2+ sites are responsible for PDH. Using an industrially relevant feed with 40 vol % propane, propene selectivity between 88 and 95% at propane conversion between 15 and 32% was achieved over six PDH/oxidative regeneration cycles lasting for about 20 h on stream at 550 °C without loss in the initial activity, while some deactivation occurred after longer (up to about 60 h) time on stream. The deactivation (caused by Zn loss) constant of Mg-modified ZnO x /silicalite-1 considering the 2nd and 20th cycles is more than 3 times lower than that of its Mg-free counterpart.

Effect of Diffusion Constraints and ZnOx Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts

“…Regardless of the calculation method, the TOF value of the sub-nanometer 3D ZnO x cluster is higher than that of binuclear ZnO x species. Considering the lower reactivity of single Zn 2+ sites on the surface of SiO 2 (0.77 h –1 ) and in silicalite-1 (39.6 h –1 ), we can conclude that the degree of oligomerization of the ZnO x species is an activity-governing descriptor if the ZnO x clusters are in sub-nanometer range.…”

“…In the iBDH reaction, however, the deactivation rate constants of the most active three catalysts, i.e., ZnO−deAl beta, ZnO− S-1(2), and ZnO−S-1(1), are comparable (0.117 vs 0.120 vs 0.123 h −1 ). According to our previous studies, 31,32,35 the deactivation of ZnO-based catalysts is mainly caused by (i) coke formation during the DH reaction and (ii) irreversible Zn loss to a minor extent within 3 h on alkane stream. The prepared catalysts contain excessive ZnO (8 wt % ZnO), which could be transferred into catalytically active sites under reaction conditions and compensate their thermally induced loss.…”

“…These catalysts, however, suffer from high costs and necessity of using Cl-containing compounds to redisperse large Pt particles or the toxic nature of Cr­(VI) compounds. Encouraged by the requirements of environmental sustainability, many efforts have been made both in academia and industry to develop alternative supported or bulk DH catalysts based on oxides of V, , Ga, , Co, Fe, , or Zr. , ZnO-containing catalysts based on, e.g., SiO 2 , Al 2 O 3 , ZrO 2 -based materials, , and zeolites, e.g., HZSM-5, , dealuminated (deAl) beta, and silicalite-1 (S-1) − were widely investigated. However, based on the abovementioned five general steps in the gas–solid reactions, the diffusion effect on the catalyst activity and product selectivity could not be ignored when zeolites are used as catalysts or supports.…”

Recent Progress in Metal-Molecular Sieve Catalysts for Propane Dehydrogenation

“…26 The optimized 0.2Ni0.06Zn-S1 catalyst exhibits an initial propane conversion of 32.5% with a propylene selectivity of 90%. Recently, Zhao et al 92 developed a MgO modified ZnOx/S-1 system to suppress the volatilization of ZnOx species during PDH reaction. The formation of stable Zn-O-Mg bonds was confirmed by FTIR, XPS, and XAFS measurements.…”

“…154 Besides, the introduction of alkali metal ions to form multimetal/oxide active sites is another viable pathway to improve the regeneration capacity, as validated in MgO-ZnOx/S-1 and PtZn 4 @S-1-Cs systems. 23,92 Apart from acting as supports, the MFI and CHA topologic zeolites were reported to show H 2 -permeable capacity at the PDH reaction window. The selective removal of H 2 products potentially improves the propylene yield from the reaction engineering aspect.…”