RESUMEN
An Al3+-based metal-organic framework (MOF), CAU-11-COOH, with a V-shaped ligand, DPSDA (3,3'-4,4'-diphenylsulfonetetracarboxylic dianhydride), was prepared using the solvothermal method, and was characterized using powder X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, elemental analysis, thermogravimetric analysis, Brunauer-Emmett-Teller analysis, and CO2 adsorption. The catalytic efficiency of CAU-11-COOH was investigated in the solvent-free cycloaddition of carbon dioxide with epoxides, which yielded five-membered cyclic carbonates under mild reaction conditions. CAU-11-COOH with a co-catalyst, tetrabutylammonium bromide (TBAB), gave higher than 98% yield of epichlorohydrin carbonate at 80 °C without a solvent. A plausible reaction mechanism in which the Lewis acidic metal center, an uncoordinated carboxyl group, and a nucleophilic bromide anion operate synergistically is proposed. The CAU-11-COOH catalysts were found to exhibit high thermal stability and could be reused more than four times without any significant reduction in activity.
RESUMEN
A series of highly thermally and hydrolytically stable porous solids with intriguing properties of zirconium- and hafnium-based metal-organic frameworks (MOFs) [Dresden University of Technology (DUT) series] was synthesized. The DUT MOFs were found to be effective catalysts for both epoxide-CO2 cycloaddition reactions and the catalytic transfer hydrogenation (CTH) of ethyl levulinate (EL). In particular, 12-connected DUT-52(Zr) showed higher catalytic activity than eight- and six-connected catalysts in the synthesis of cyclic carbonates as well as in the production of γ-valerolactone (GVL). The secondary building unit connectivity, coexistence of a moderate number of acidic and basic sites, Brunauer-Emmett-Teller surface area, and combined effects of the pores of the MOFs seem to influence the catalytic activity. The reaction mechanism for the DUT-52(Zr)-mediated cycloaddition reaction of CO2 and the CTH reactions were investigated in detail by using periodic density functional theory calculations. To the best of our knowledge, this is the first detailed computational study for the formation of GVL from EL by using MOF as the catalyst. In addition, grand canonical Monte Carlo simulations predicted the strong interaction of CO2 molecules with the DUT-52(Zr) framework. Remarkably, the DUT-series catalysts possess extraordinary tolerance toward water. Further, DUT-52(Zr) is recyclable and is an efficient catalyst for cycloaddition and CTH reactions for at least five uses without obvious reductions in the activity or structural integrity.
RESUMEN
We synthesized two new adenine-based Zn(II)/Cd(II) metal-organic frameworks (MOFs), namely, [Zn2(H2O)(stdb)2(5H-Ade)(9H-Ade)2]n (PNU-21) and [Cd2(Hstdb)(stdb)(8H-Ade)(Ade)]n (PNU-22), containing auxiliary dicarboxylate ligand (stdb = 4,4'-stilbenedicarboxylate). Both MOFs were characterized by multiple analytical techniques such as single-crystal X-ray diffraction (SXRD), powder X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscopy, as well as temperature program desorption and Brunauer-Emmett-Teller measurements. Both MOFs were structurally robust and possessed unsaturated Lewis acidic metal centers [Zn(II) and Cd(II)] and free basic N atoms of adenine molecules. They were used as heterogeneous catalysts for the fixation of CO2 into five-membered cyclic carbonates. Significant conversion of epichlorohydrin (ECH) was attained at a low CO2 pressure (0.4 MPa) and moderate catalyst (0.6 mol %)/cocatalyst (0.3 mol %) amounts, with over 99% selectivity toward the ECH carbonate. They showed comparable or even higher catalytic activity than other previously reported MOFs. Because of high thermal stability and robust architecture of PNU-21/PNU-22, both catalysts could be reused with simple separation up to five successive cycles without any considerable loss of their catalytic activity. Densely populated acidic and basic sites in both Zn(II)/Cd(II) MOFs facilitated the conversion of ECH to ECH carbonate in high yields. The reaction mechanism of the cycloaddition reaction between ECH and CO2 is described by possible intermediates, transition states, and pathways, from the density functional theory calculation in correlation with the SXRD structure of PNU-21.
RESUMEN
A facile approach for modifying the UiO-66-NH2 metal-organic framework by incorporating imidazolium-based ionic liquids (ILs) to form bifunctional heterogeneous catalysts for the cycloaddition of epoxides to CO2 is reported. Methylimidazolium- and methylbenzimidazolium-based IL units (ILA and ILB, respectively) were introduced into the pore walls of the UiO-66-NH2 framework through a condensation reaction to generate ILA@U6N and ILB@U6N catalysts, respectively. The resultant heterogeneous catalysts, especially ILA@U6N, exhibited excellent CO2 adsorption capability, which makes them effective for cycloaddition reactions producing cyclic carbonates under mild reaction conditions in the absence of any cocatalyst or solvent. The significantly enhanced activity of ILA@U6N is attributed to the synergism between the coordinately unsaturated Lewis acidic Zr4+ centers and Br- ions in the bifunctional heterogeneous catalysts. The size effect of the ILs on coupling between the epoxide and CO2 was also studied for ILA@U6N and ILB@U6N. A periodic DFT study was performed to provide evidence of possible intermediates, transition states, and pathways, as well as to gain deeper insight into the mechanism of the ILA@U6N-catalyzed cycloaddition reaction between epichlorohydrin and CO2 .
