Oxidative co-dehydrogenation of ethane and propane over h-BN as an effective means for C-H bond activation and mechanistic investigations

doi:10.1016/S1872-2067(21)64042-1

Abstract

Abstract:

Hexagonal boron nitride (h-BN) is a highly selective catalyst for oxidative dehydrogenation of light alkanes to produce the corresponding alkenes. Despite intense recent research effort, many aspects of the reaction mechanism, such as the observed supra-linear reaction order of alkanes, remain unresolved. In this work, we show that the introduction of a low concentration of propane in the feed of ethane oxidative dehydrogenation is able to enhance the C₂H₆ conversion by 47%, indicating a shared reaction intermediate in the activation of ethane and propane. The higher activity of propane makes it the dominant radical generator in the oxidative co-dehydrogenation of ethane and propane (ODEP). This unique feature of the ODEP renders propane an effective probe molecule to deconvolute the two roles of alkanes in the dehydrogenation chemistry, i.e., radical generator and substrate. Kinetic studies indicate that both the radical generation and the dehydrogenation pathways exhibit a first order kinetics toward the alkane partial pressure, leading to the observed second order kinetics of the overall oxidative dehydrogenation rate. With the steady-state approximation, a radical chain reaction mechanism capable of rationalizing observed reaction behaviors is proposed based on these insights. This work demonstrates the potential of ODEP as a strategy of both activating light alkanes in oxidative dehydrogenation on BN and mechanistic investigations.

Key words: Hexagonal boron nitride, Oxidative dehydrogenation, Radical chain reaction, Reaction order, C-H activation

Hao Tian, Bingjun Xu. Oxidative co-dehydrogenation of ethane and propane over h-BN as an effective means for C-H bond activation and mechanistic investigations[J]. Chinese Journal of Catalysis, 2022, 43(8): 2173-2182.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(21)64042-1

https://www.cjcatal.com/EN/Y2022/V43/I8/2173

Figures/Tables 9

Fig. 1. ODE promoted by the addition of C3H8, 580 °C, pC2H6 = 0.25 atm, pO2 = 0.125 atm with N2 as balance, total gas flow = 40 mL·min-1. Red columns represent C2H6 conversion and blue columns represent the formation rate of C2H4.

Fig. 2. Catalytic performance of ODEP at 540 °C, (pC2H6 + pC3H8) = 0.25 atm, pO2 = 0.125 atm with N2 as balance, total gas flow = 40 mL·min-1. (a) product distribution and alkane conversion, carbon balances in the experiments are >98%. (b) Alkane conversion rates at different ratio of pC3H8/pC2H6.

Fig. 3. The influence of pC3H8 on the catalytic performance of ODEP and ODP with steam (pH2O = 1.8 kPa) at fixed pC2H6 = 0.25 atm at 520 °C. pO2 = 0.125 atm with N2 as balance, total gas flow = 40 mL·min-1. (a) Alkane conversion rates. (b) The reaction order of -rC3H8 with respect to pC3H8.

Fig. 4. Alkane conversion rates of ODEP with steam at 520 °C, (pC2H6 + pC3H8) = 0.25 atm, pO2 = 0.125 atm, pH2O = 1.8 kPa with N2 as balance, total gas flow = 40 mL·min-1.

Scheme 1. Radical chain reaction mechanism of ODH over BN. Green area: chain initiation; blue area: chain propagation; yellow area: chain termination.

Fig. 5. The MS signals in temperature programmed reaction. (a) On-line MS signals (m/z = 50-70 amu) of ODP over h-BN at different temperature. (b) The MS signals of temperature programmed reaction between C3H8 and O2 over h-BN. (c) The MS signals of temperature programmed reaction between C3H6 and O2 over h-BN. (d) The MS signal of propylene oxide (m/z = 58 amu) in temperature programmed reaction under C3H8 + O2 and C3H6 + O2. Temperature programmed reaction condition: pC3H8 or pC3H6 = 0.25 atm, pO2 = 0.125 atm with N2 as balance, total gas flow = 40 mL·min-1.

Fig. 6. Curve fitting to determine kinetic parameters of the “wet” ODEP at 520 °C. Fitted curves were obtained by global fitting of Eqs. (13)-(16) with alkane conversion rates measured with steam at 520 °C via minimizing the overall root of mean square error. The fitting results are shown in Table 1.

Table 1 Global fitting results of the “wet” ODEP at 520 °C.

k₂^’/mL·atm^-1	k₃^’/mL·atm^-1	k₄/mL·min^-1·atm^-1	k₅/mL·min^-1·atm^-1	Root of mean square error (RMSE)	R²
1.042	5.125	0.633	1.562	0.0204	0.9972

Fig. 7. The results of simulation with kinetic parameters obtained from global fitting and measured alkane conversion rate. Kinetic parameters (k3’, k4’, k6 and k7) are obtained by global fitting Eqs. (13)-(16) with data measured at 520 °C with steam. The solid red and blue points are measured at 520 °C and palkane = 0.25 atm with steam (pH2O = 1.8 kPa) and different ratio of pC3H8/pC2H6.

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