Edge-Activating CO2-Mediated Ethylbenzene Dehydrogenation by a Hierarchical Porous BN Catalyst

The metal-free BN catalyst is a competitive candidate in oxidative dehydrogenation (ODH). The popular oxidant used is molecular O-2, and the alternative soft oxidant, CO2 with better selectivity, is difficult to activate and has been less addressed. It is expected that porous BN with plenty of active edges result in an improved activity, and a clarified reaction mechanism can also guide the rational design of efficient CO2-mediated dehydrogenation (O-2 DH) catalysts. Herein, the hierarchical porous BN (hpBN, 657-785 m(2)/g) with nearly uniform mesopores (similar to 22 nm) and rich exposed edges is derived from an "evaporation-assembly"-like pyrolytic approach. Its high synthetic temperature or reductive atmosphere leads to a suppressed edge -BOx degree as well as a decreased styrene production activity. Combined with the decreased N-H signal below 300 degrees C, enhanced B-O signals, BOC, BOCO, and the carboxylate-like bonds suggested by the diffuse reflectance infrared Fourier transform spectra, the CO2 DH activities and -BOx are found to be correlated. The depressed activity, enhanced C-C bond cleavage, and activity loss of the supported B2O3 catalyst can be attributed to its strong acidity and weak binding. According to the density functional theory calculations and experimental results, the reaction routes are estimated. The O atom of CO2 can be bonded to the exposed B edge of BN; then, the >BO-CO bond cleavage happens along with the alpha/beta-H of EB followed by styrene/CO/>BOH production or with the help of a nearby oxygen radical/OH group in a stepwise manner. It is suggested that the concerted route is more favored, and the coexistence of DDH and ODH might originate from -BOH dehydration or dehydrogenation.