Characterization Of Lewis Acids On Metal-organic Frameworks By Computational Chemistry

Metal-Organic Frameworks (MOFs) is a new type of nanoporous material.Due to their ultra-high specific surface areas and porosities, adjustable poresizes and controllable properties, acceptable thermal stability, MOFs arepromising materials for a wide range of application, such as gas adsorptionand separation, catalyst, and pharmacy, etc. As one of the most popular MOFs,Cu-BTC which has unsaturated metal ions is a very good candidate for Lewisacids in catalytic process. Because of its high specific surface area, it ispossible for transition metal loaded on the surface of this MOF dispersedly,and promise this material be a good industrial catalyst. Solvent environmentcan influence the structure of the MOFs and their catalytic properties, so it isvery important to study the influence of the solvent on acids of the MOFs.In this paper, quantum chemical calculations were used to calculate theLewis acids of Cu-BTC and MOP-15. The main contents and findings are summarized as follows.(1) The structure of small metal clusters Pdn(n=2,3,4) were optimized, andthe most stable structures with different number of metal atoms werediscussed. Then we studied the strength of Lewis acids of these stableclusters. The computational results show that all the most stable clustersare the most symmetrical configurations, and the fewer metal atoms inthe cluster, the higher strength of the Lewis acids is. So when loading themetal in the MOF, improve the dispersion of metal nanoparticles in theMOF can improve the material’s catalytic properties.(2) DFT and COSMO model were used to investigate the solvent effects onthe structure and charge distribution of the cluster model and theinfluence on the Lewis acids of the material. The calculation resultsshow that the geometrical construction was not seriously changed, whilethe electron transferred from the metal atoms of Cu-BTC to its organiclinker which caused the unsaturated metal had a much higher Lewisacids strength. The Lewis acids of the material which has a largerelectronegative group on its organic linker could be influenced muchmore remarkably by the solvent environment.(3) DFT were performed to optimize the location of different metal atomson the Cu-BTC and to calculate their Lewis acids. The results show thatloading transition metal on the MOF can increase the number of Lewisacid sites, and lower the strength of original sites on the Cu-BTC. The Cu-BTC cluster that loads a gold atom showed the highest strength ofLewis acids.