Microwave-assisted synthesis of MOFs: Rational design via numerical simulation

Microwave (MW) irradiation is known to enable facile synthesis of MOFs. However, the complex distribution of MW field causes the generation of hot spots in the MW reactor; this phenomenon causes local overheating that is unfavorable for the nucleation and growth of MOFs crystals. The present study uses numerical simulation to achieve rational design of MW-assisted synthesis by eliminating hot spots. First, we evaluate the influence of hot spots by analyzing the morphology of MIL-88B (Fe) synthesized in the presence and absence of hot spots, respectively. SEM images show that nearly 50% of the hexagonal bipyramidal-shaped particles deteriorate into irregular shapes in the presence of hot spots, revealing the detrimental effect of overheating phenomenon on the synthesis of MOFs. Then, the regulation of hot spots occurring is investigated by simulating the temperature distribution in the reactant solution under various operating parameters, including the input power, the shape and dimension of cavities, the dielectric property of reactants, as well as the stirring rate. Based on the simulating results, several strategies are proposed to weaken hot spots, including the adjustment of MW power, the optimization of cavity structure and the enhancement of liquid turbulence. The simulating results show that these strategies eliminate hot spots and equalize liquid temperature in the solution. Moreover, the effect of these strategies is validated experimentally, where the regular and uniform particles are obtained when these strategies are employed. The present study can provide guidance for the further design of MW-assisted reactor for materials synthesis.