Porous aluminum-based DUT metal-organic frameworks for the transformation of CO2 into cyclic carbonates: A computationally supported study

Two aluminum-based DUT (Dresden University of Technology) metal organic frameworks were synthesized, and their structural and chemical properties were exploited for the cycloaddition reaction of CO2 and epoxide under solvent-free condition. Both the catalysts have the same supramolecular architecture but different ligands. DUT-5 (Al(OH)(bpdc), bpdc = 4,4'-biphenyldicarboxylate) has a higher number of acidic and basic sites and larger BET surface area and specific pore volume compared with DUT-4 (Al(OH)(ndc), ndc = 2,6-naphthalenedicarboxylate). The highly porous DUT-5 exhibited better catalytic conversion of epichlorohydrin (ECH) than DUT-4, with > 99 % selectivity in the presence of tetrabutylammonium bromide (TBAB). Short linkered MIL-53 (MIL = Materials Institute Lavoisier) (Al(OH)(bdc), bdc = 1,4-benzenedicarboxylate) exhibited lower ECH conversion compared with DUT-4 and DUT-5, even though all the catalysts possessed the same crystal structure. DUT catalyst could be recycled at least five times without any noticeable loss in the ECH conversion. In addition, the mechanism for the DUT-5/TBAB mediated cycloaddition of CO2 to ECH, affording the five-membered ECH carbonate, has been investigated in detail using the density functional theory. The energy barrier for the ECH ring opening in the presence of DUT-5/TBAB (15.14 kcal/mol) is significantly lower than those of un-catalyzed (61.96 kcal/mol) and TBAB-catalyzed (39.60 kcal/mol) reactions, clearly showing the vital role of the Al3+/Br(-)bifunctional catalyst system.