Catalytic Activity of Li_1+xHf_2–xIn_x(PO₄)₃-Based NASICON-Type Materials for Ethanol Conversion Reactions

Li_{1+ x}Hf_2–xIn_x(PO₄)₃ (x = 0, 0.05, 0.1) materials have been prepared by solid-state reactions and characterized by X-ray diffraction, low-temperature nitrogen adsorption measurements, and scanning electron microscopy. The materials consist of NASICON-type lithium hafnium double phosphates with a hexagonal structure. Milling in a planetary mill has been found to increase the specific surface area of the Li_{1+ x}Hf_2–xIn_x(PO₄)₃ materials by almost one order of magnitude (from 1.5 to 13 m²/g in the case of LiHf₂(PO₄)₃). The materials with a larger surface area exhibit catalytic activity for ethanol dehydration reactions and are less active for ethanol dehydrogenation. Ethanol conversion predominantly yields diethyl ether at low temperatures and ethylene at higher temperatures. The diethyl ether selectivity of the catalytic processes reaches 85% at 350°C, with 60% conversion, and the ethylene selectivity reaches 96% at 510°C, with 100% conversion. Indium doping raises the high-temperature acetaldehyde selectivity from 4 to 8% and leads to the formation of C₄ hydrocarbons as reaction products. C₄ selectivity reaches 15 and 17% in the case of the Li_1.05Hf_1.95In_0.05(PO₄)₃ and Li_1.1Hf_1.9In_0.1(PO₄)₃ materials, respectively (420°C, 97 and 92% conversion, respectively).