Localizing microwave heat by surface polarization of titanate nanostructures for enhanced catalytic reaction efficiency

Microwave has been traditionally used as fast and uniform heating source in chemical reactions, where polar solvent is often required to generate heat under microwave irradiation. In this work, solid acid catalysts were synthesized with engineered surface polarity and microwave absorption. Therefore, heat generation and reaction can be coupled at catalyst surface to improve the overall energy efficiency of reactions. Specifically, titanate nanostructures (nanocube, nanotube and nanobelt) were synthesized by using different alkalis in hydrothermal reactions. The titanate intermediates (protonated titanates, H₂Ti_nO_2n+1, n = 3, 5) have been demonstrated critical in controlling the catalyst pore structure, surface area, crystal composition and the quantity of acid sites. Especially, the open crystal structure of H₂Ti₃O₇ allowed interlayer polarization of titanates, which was critical to enable a large number of Ti-O-SO₄²⁻ acid sites. The Ti-O-SO₄²⁻ not only serves as catalytic active site, but also offers heat generation capability under microwave irradiation. Among the titanate nanostructures, titanate nanotube shows the best heat generation capability and gives the largest rate constant of 0.31 min⁻¹. The reaction equilibrium of fructose to HMF conversion can be reached within a few minutes at 140 °C. Benefited from the surface acidity and microwave heating ability, the energy efficiency of the reaction by titanate nanotube (5.6 mmol (KJ L)⁻¹) is 9 times higher than commercial TiO₂ solid acid (0.6 mmol (KJ L)⁻¹). The interlayer polarization is revealed as the major reason for the enhanced microwave response of titanate catalyst and energy efficiency of the reactions.