Adsorption of dihydroxybenzenes inside the UiO-66-SO3H, UiO-66 and defect-UiO-66 Metal Organic Framework: An ONIOM study

Pattraporn Srirattanasakunsuk, Pemikar Srifa, Varangkana Jitchum, Thana Maihom, Bundet Boekfa

Abstract

The confinement effect on the adsorption of benzene-1,2-diol, benzene-1,3-diol and benzene-1,4-diol on the UiO-66 Metal Organic Framework (MOF), has been investigated by a multi-layered chemistry calculation approach called our own n-layered integrated molecular orbital and molecular mechanics (ONIOM). The effect of two Brønsted acid sites of the UiO-66-SO₃H and UiO-66 MOF has been studied together with the effect of the Lewis acid site at the defected UiO-66 MOF. The high level calculation area covered a reactive site which included a functionalized benzene sulfonic group with two metal-oxide clusters. This area was calculated through M06-L method. The low level area was the outer six metal-oxide clusters which were treated with PM6 semi-empirical method of calculation. To improve the adsorption energies, the overall optimized structures from the ONIOM were recalculated using single point energy calculations using ONIOM(M06-L/6-311+G(2df,2p)+Lanl2DZ:M06-L/Lanl2DZ). Dihydroxybenzene molecule adsorbed inside the pore via a hydrogen bond between hydroxyl group and acid sites of MOF. The strong interactions were found more on the Lewis acid site than on the Brønsted acid sites.  Natural bond orbital analysis indicated that electrons from ion pair of hydroxyl group transfer to the antibonding orbital of the Brønsted acid while electrons from Lewis acid site Zr-O bond transfer to the hydroxyl group of dihydroxybenzene. This data is able to reflect tendency in diffusion coefficients which corresponded to chemical reactions for dihydroxybenzene and its derivatives.