Imaging of liquid temperature distribution during microwave heating via thermochromic metal organic frameworks

Abstract

Microwave has been widely used as an efficient and clean method for energy supply in chemical engineering, due to its unique heating characteristics. The industrialization of microwave is obstructed by the problem of uneven heating; the inhomogeneous temperature distribution results in inaccurate prediction of reaction kinetics, uncertain product yields, and unstable energy utilization efficiencies. With the aim to overcome this problem, we use a non-contact nano-thermometer to achieve in situ imaging of liquid temperature distribution during microwave heating. A zinc-based metal organic framework material (ZMOF) is employed as the temperature indicator in microwave field, due to its microwave transparency, thermal stability and good dispersibility in solvents. A case study was conducted by dispersing the ZMOF particles in a 95% ethanol solution, where the color of ZMOF changes from yellow to orange with the increase of temperature in the range of 30–120 °C with the sensitivity of 1.512%/°C. By linearly correlating the hue value of ZMOF suspension with the temperature, the color images of liquid heated in microwave cavity were transformed into temperature distribution maps. ZMOF also has shown the advantages of good dispersion, cyclic temperature measurement, continuous linear discoloration, and visible light excitation. This facile route of temperature field imaging can provide experimental validation for numerical simulation and guide the equipment optimization in microwave-assisted chemical engineering.