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Mesoporous Materials in Heterogeneous Catalysis
Published in Varun Rawat, Anirban Das, Chandra Mohan Srivastava, Heterogeneous Catalysis in Organic Transformations, 2022
Meenal Batra, Ashutosh Sharan Singh
Stability is a prominent factor in ensuring better performance of a heterogeneous catalyst. Thermal stability of MOF can be increased either by using mixed ligand or by using “paddle-wheel” as SBU. Thermal stability of MOF also increases by the interpenetration of the network. Apart from network stability, the catalytic site also must be stable for obtaining better output. Jacobsen’s catalyst, used for the epoxidation process is very sensitive and its performance depends upon several factors [94]. Loss of catalytic activity (with time) of Jacobsen’s catalyst typically, is associated with oxidation of the salen ligand. If salen oxidation is facilitated by reactive encounters with other catalyst molecules, immobilisation of the catalytic site should prevent encounters. It will also extend the catalyst lifetime. An example for improvement of epoxidation process was reported by Hupp, Nguyen and co-workers [95] with Mn-salen based MOF and biphenylene dicarboxylate as secondary ligand. Hupp, Nguyen and co-workers have attempted to sort out this drawback by immobilisation of salen unit as part of the framework.
Review: Recent advances of one-dimensional coordination polymers as catalysts
Published in Journal of Coordination Chemistry, 2018
Edward Loukopoulos, George E. Kostakis
Ever since the introduction of a Mn(II) salen-based coordination compound (also known as the Jacobsen catalyst [60, 61]) with catalytic activity toward alkene epoxidation, more transition metal complexes have been explored to support this reaction [62–67]. In 2011, Huang and co-workers [68] incorporated 1-D Ni(II) salen-based homochiral coordination polymers as self-supported heterogeneous catalysts in alkene epoxidation. These CPs were constructed in a two-step synthesis: First, the chirality of the compounds was functionalized through the use of salen-based R,R- or S,S-H2cybps (H2cybps = 1,2-cyclohexanediamino-N,N′-bis(3-tert-butyl-5-(4-pyridyl)salicylidene) as the main organic ligand. The resulting zero-dimensional Ni(cybps) coordination compounds were then used as nodes, as the use of 4,4′-biphenyldicarboxylic acid (H2bpdc) as a secondary linker ensured the targeted synthesis of the 1-D CPs [Ni3(bpdc)(RR-cybps)2]·DMF and [Ni3(bpdc)(SS-cybps)2]·DMF (5 and 6, respectively). Both CPs are isostructural: Each chiral Ni(cybps) node coordinates to a terminal pyridyl group to form a helical chain which contains two Ni(cybps) units almost perpendicular to each other and a linking Ni-bpdc component (Figure 5). The metal center of the Ni(cybps) units are unsaturated in both CPs and thus the compounds were identified as potential catalysts in alkene epoxidation. The reactions were carried out at room temperature under the non-coordinating solvent dichloromethane with 2 mol% of 5 or 6. The catalytic activity of the compounds for styrene oxidation delivers moderate yields of the epoxide (21% for 5, 19% for 6), however the catalyst may be reused for at least two cycles before any loss in activity is observed. For comparison, homogeneous counterparts were also synthesized, showing similar activity in the alkene epoxidation reaction.