Interplay of Lewis and Brønsted Acid Sites in Zr-Based Metal-Organic Frameworks for Efficient Esterification of Biomass-Derived Levulinic Acid

We report the performance of UiO-66 and its Brønsted acid functionalized derivative, UiO-66-(COOH)₂, as heterogeneous catalysts for levulinic acid esterification with ethanol. Importantly, compared with UiO-66, UiO-66-(COOH)₂ displayed superior catalytic performance (up to 97.0 ± 1.1% yield of ethyl levulinate) attributed to the synergistic effect between Lewis acidic Zr clusters and Brønsted acidic -COOH groups. Furthermore, UiO-66-(COOH)₂ was stable and reusable without an appreciable loss in catalytic activity for at least five consecutive cycles. This study demonstrates that the interplay of Brønsted and Lewis acid sites in zirconium metal-organic frameworks leads to more efficient catalytic conversion of a biomass feedstock to biofuel, and with further hypothesis driven research, additional materials that show promise as candidates for catalytic conversion of biomass feedstocks to biofuels and valuable chemicals can be developed.

Designing UiO-66-Based Superprotonic Conductor with the Highest Metal-Organic Framework Based Proton Conductivity

Metal-organic framework (MOF) based proton conductors have received immense importance recently. The present study endeavors to design two post synthetically modified UiO-66-based MOFs and examines the effects of their structural differences on their proton conductivity. UiO-66-NH₂ is modified by reaction with sultones to prepare two homologous compounds, that is, PSM 1 and PSM 2, with SO₃H functionalization in comparable extent (Zr:S = 2:1) in both. However, the pendant alkyl chain holding the -SO₃H group is of different length. PSM 2 has longer alkyl chain attachment than PSM 1. This difference in the length of side arms results in a huge difference in proton conductivity of the two compounds. PSM 1 is observed to have the highest MOF-based proton conductivity (1.64 × 10^-1 S cm^-1) at 80 °C, which is comparable to commercially available Nafion, while PSM 2 shows significantly lower conductivity (4.6 × 10^-3 S cm^-1). Again, the activation energy for proton conduction is one of the lowest among all MOF-based proton conductors in the case of PSM 1, while PSM 2 requires larger activation energy (almost 3 times). This profound effect of variation of the chain length of the side arm by one carbon atom in the case of PSM 1 and PSM 2 was rather surprising and never documented before. This effect of the length of the side arm can be very useful to understand the proton conduction mechanism of MOF-based compounds and also to design better proton conductors. Besides, PSM 1 showed proton conductivity as high as 1.64 × 10^-1 S cm^-1 at 80 °C, which is the highest reported value to date among all MOF-based systems. The lability of the -SO₃H proton of the post synthetically modified UiO-66 MOFs has theoretically been determined by molecular electrostatic potential analysis and theoretical p K_a calculation of models of functional sites along with relevant NBO analyses.

Organocatalytic asymmetric synthesis of both cis- and trans-configured pyrano[2,3-b]chromenes via different dehydration pathways

The enamine-catalyzed [3 + 3]-cycloaddition between chroman-2-ols and β,γ-unsaturated α-ketoesters is developed to access both enantiomers of cis- and trans-fused pyrano[2,3-b]chromene derivatives.

A Perspective on Reversibility in Controlled Polymerization Systems: Recent Progress and New Opportunities

The field of controlled polymerization is growing and evolving at unprecedented rates, facilitating polymer scientists to engineer the structure and property of polymer materials for a variety of applications. However, the lack of degradability, particularly in vinyl polymers, is a general concern not only for environmental sustainability, but also for biomedical applications. In recent years, there has been a significant effort to develop reversible polymerization approaches in those well-established controlled polymerization systems. Reversible polymerization typically involves two steps, including (i) forward polymerization, which converts small monomers into macromolecule; and (ii) depolymerization, which is capable of regenerating original monomers. Furthermore, recycled monomers can be repolymerized into new polymers. In this perspective, we highlight recent developments of reversible polymerization in those controlled polymerization systems and offer insight into the promise and utility of reversible polymerization systems. More importantly, the current challenges and future directions to solve those problems are discussed. We hope this perspective can serve as an "initiator" to promote continuing innovations in this fairly new area.