TiO2 solid acid catalysts with different niobium (Nb) doping ratios (8, 16, and 24 mol%) were synthesized for microwave-assisted catalytic conversion of fructose to hydroxymethylfurfural (HMF). Nb doping has been demonstrated effective to enhance the quantity of acid sites and increase the acid strength. At optimal Nb loading of 16 mol%, it was found that the reaction rate constant and energy efficiency were significantly enhanced by 32 and 49%, respectively. The enhancement mechanism of Nb-doping was investigated by both experiment and density functional theory (DFT) calculation. It was revealed that Nb doping promoted the electron transfer from the bulk catalyst to surface sulfonic groups, resulting in enhanced acidity and polarity of sulfonic groups. The electron transfer could strengthen the sulfonic group-water interaction at catalyst surface and created a unique polarized interface region where heat generation and the catalytic reaction can be coupled for enhanced energy efficiency of such reactions.