Investigation on kinetic processes of zeolitic imidazolate framework-8 film growth and adsorption of chlorohydro-carbons using a quartz crystal microbalance

Abstract

In this work, the kinetic processes of zeolitic imidazolate framework-8 (ZIF-8) film growth and the adsorption of dichloromethane (DCM), trichloromethane (TCM) and carbon tetrachloride (CTC) on ZIF-8 film are monitored in real time using a quartz crystal microbalance sensor. It is shown that the growth kinetics of ZIF-8 film from a mixture of zinc nitrate and 2-methylimidazole consists of a rapid initial growth (0.22–1.1 μg cm⁻² min⁻¹) and later slow rearrangement stages. The time to obtain 50% of total equilibrium grown mass is in the range of 5.1–6.8 min. The mass deposited in the second, third and fourth cycles is 2.37, 3.36 and 4.08 times that in the first one (3.11 μg cm⁻²), respectively, and approaches a stable level after the fourth cycle. The as-prepared ZIF-8 film (0.70 μm) exhibits adsorption capacities of 568.3, 496.7 and 212.8 mg g⁻¹ to DCM, TCM and CTC (20 °C), respectively. The initial adsorption rate of DCM is much higher than that of TCM and CTC. The difference in the adsorption behavior of the three chlorohydrocarbons is ascribed to the sieve effect of the pore apertures in ZIF-8 to the adsorbates. The adsorption kinetic data are in agreement with the pseudo-second-order model. For the adsorption in ZIF-8 film, the adsorption activation energies are 34.9 ± 5.6, 66.9 ± 5.5 and 49.1 ± 3.8 kJ mol⁻¹; standard enthalpy changes of −63.2, −89.1 and −119 J mol⁻¹, and standard entropy changes of −41.9, −45.3, and −50.1 kJ mol⁻¹ K⁻¹ have been evaluated for DCM, TCM and CTC respectively. The adsorption isotherms of DCM and TCM at 20 °C are fitted well by the Langmuir model while those of CTC and TCM at 60 °C follow the Freundlich model.