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

• Published in: Applied catalysis. B, Environmental Environmental, 2018-07, Vol.227, p.266-275
• Main Authors: Ji, Tuo, Tu, Rui, Li, Licheng, Mu, Liwen, Liu, Chang, Lu, Xiaohua, Zhu, Jiahua
• Format: Article
• Language: English
• Subjects: Acidity; Acids; Alkalies; Alkalis; Biomass; Catalysis; Catalysts; Chemical reactions; Chemical synthesis; Crystal structure; Energy conversion efficiency; Energy efficiency; Fructose; Heat generation; Heating; HMF; Hydrothermal reactions; Interlayers; Intermediates; Irradiation; Microwave; Microwave absorption; Nanostructure; Nanotubes; Organic chemistry; Polarity; Polarization; Porosity; Power efficiency; Saccharide; Studies; Surface polarization; Titanates; Titanium dioxide
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2017.12.073

[Display omitted] •Surface polarization enables localized microwave heating at catalyst surface.•Open crystal structure allows interlayer polarization and better microwave response.•Microwave heat can be directly used for on-site catalytic reactions.•Engineering catalyst surface polarity greatly improves reaction energy 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, H2TinO2n+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.