Tuning the isomer composition is a key to overcome the performance limits of commercial benzyltoluene as liquid organic hydrogen carrier

Abstract

This work is the outcome of a three-year journey to develop a technically feasible way to overcome the limits of the benzyltoluene (H0-BT) and perhydro benzyltoluene (H12-BT) pair, recognized as an attractive liquid organic hydrogen carrier (LOHC) system. In the first attempt to synthesize and evaluate three H0-BT regioisomers, the para isomer showed faster H2 storage and release rates than the ortho and meta counterparts. This was attributed to its favorable adsorption capacity and strength on the catalyst surface. The subsequent evaluation of binary and ternary H0-/H12-BT isomeric mixtures, prepared by physical mixing the synthesized isomers, suggested that abundant amounts of para isomer in the mixture were needs to maximize the LOHC performance. Thus, a H0-BT mixture with 72 % para isomer was successfully synthesized using the zeolite H-Beta under optimized conditions, resulting in a faster kinetics of both H2 storage and release than commercial Marlotherm LH. Cycling tests on this mixture indicated that the formation of methylfluorene as a dehydrogenation byproduct was detrimental to the hydrogenation reaction. By reducing the formation of methylfluorene to a level of 3 % using a sulfur-decorated Pt/Al2O3 catalyst, a gravimetric H2 storage density of 6 wt% could be maintained for 10 cycles. Finally, a cost analysis confirmed that the isomeric composition of H0-BT in the mixture should be tuned to increase the para isomer content, while the raw materials for H0-BT synthesis are cheap. The step-by-step approach reported herein will pave the way for the development of novel aromatic LOHC systems in the near future.