Xiantai Zhou

Highly efficient synthesis of cyclic carbonates from epoxides catalyzed by salen aluminum complexes with built-in “CO2 capture” capability under mild conditions

A series of monometallic salen aluminum complexes were prepared by covalent linkage of the imidazolium-based ionic liquid moieties containing various polyether chains with the salen ligand at the two sides of the 5,5′-position. The salen aluminum complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst. The catalysts presented excellent “CO2 capture” capability due to the molecules containing polyether chains and the metal aluminum center, in which >90% yield of cyclic carbonate could be obtained under mild conditions. The catalysts can be easily recovered and six times reused without significant loss of activity and selectivity. Moreover, based on experimental and previous work, the “CO2 capture and activation” cycloaddition reaction mechanisms by monometallic or bimetallic salen aluminum complexes were both proposed.

Metallosalen-Based Ionic Porous Polymers as Bifunctional Catalysts for the Conversion of CO2 into Valuable Chemicals.

A series of new metallosalen-based ionic porous organic polymers (POPs) were synthesized for the first time using a simple unique strategy based on the free-radical copolymerization reaction. Various techniques were used to characterize the physicochemical properties of these catalysts. These well-designed materials endowed high surface area, hierarchical porous structures, and enhanced CO2 /N2 adsorptive selectivity. Moreover, these POPs having both metal centers (Lewis acid) and ionic units (nucleophile) could serve as bifunctional catalysts in the catalytic conversion of CO2 into high value-added chemicals without any additional co-catalyst under mild and solvent-free conditions, for example, CO2 /epoxides cycloaddition and Nformylation of amines from CO2 and hydrosilanes. The results demonstrated that the irregular porous structure was very favorable for the diffusion of substrates and products, and the microporous structural property resulted in the enrichment of CO2 near the catalytic centers in the CO2 -involved transformations. Additionally, the superhydrophobic property could not only enhance the chemoselectivity of products but also promote the stability and recyclability of catalysts.

Immobilization of β-Cyclodextrin as Insoluble β-Cyclodextrin Polymer and Its Catalytic Performance

Abstract Insoluble β-cyclodextrin polymers were prepared from β-cyclodextrin (β-CD) using epichlorohydrin (EPI) as crosslinking agent under basic conditions. The polymers were characterized by Fourier Transform Infrared (FTIR), Thermogravimetry (TG), X-ray diffraction (XRD) and TG-FTIR. The results demonstrated that the polymerization between EPI and β-CD indeed occurred, and a number of CD rings were interconnected to form a three-dimensional network. Moreover, different factors influencing the polymerization, e.g. molar ratio of EPI to β-CD, the concentration of NaOH and reaction temperature, have been investigated. The polymer prepared under the optimal conditions (the molar ratio EPI: β-CD of 44, the NaOH concentration 50% in mass, and the temperature at 65 °C) showed excellent thermal stability and insolubility in organic solvents or strong acid/base. In addition, the β-cyclodextrin polymers also presented high catalytic activity for aqueous oxidation of benzyl alcohol with hypochlorite as oxidant.

State‐of‐the‐Art Aluminum Porphyrin‐based Heterogeneous Catalysts for the Chemical Fixation of CO2 into Cyclic Carbonates at Ambient Conditions

A series of metalloporphyrin‐based hyper‐crosslinked polymers (M‐HCPs: M=Al, Co, Fe, Mn) has been directly synthesized through Friedel–Crafts alkylation reactions. The M‐HCPs afforded abundant permanent nanopores, high Brunauer–Emmett–Teller (BET) surface area, and exceptional CO2/N2 adsorptive selectivity. The experimental results suggested that the hollow tubular Al‐HCP exhibited extraordinary catalytic performance in the solvent‐free synthesis of cyclic carbonates from epoxides and CO2 by using tetrabutylammonium bromide as a cocatalyst under mild conditions, which was clearly superior to the corresponding homogeneous analogue. Surprisingly, a high turnover frequency (TOF) value of 14 880 h−1 was achieved with propylene oxide at 100 °C and 3.0 MPa, which was a promising result for industrial production compared with previously reported heterogeneous catalysts. More interestingly, Al‐HCP could smoothly catalyze the cycloaddition reaction, producing the corresponding cyclic carbonates by using simulated flue gas (15 % CO2 and 85 % N2 in volume) as the raw material under ambient conditions. Moreover, Al‐HCP could be readily recycled and efficiently reused more than ten times, exhibiting excellent stability.

Enhanced catalytic activity and recyclability for oxidation of cinnamaldehyde catalysed by β-cyclodextrin cross-linked with chitosan

Selective oxidation of cinnamaldehyde to benzaldehyde under mild conditions has been carried out for the first time in aqueous phase with β-cyclodextrin–chitosan polymer (β-CD–CTS) as a heterogeneous catalyst. Compared with β-cyclodextrin polymer (β-CDP) catalyst, β-CD–CTS showed significant promotion for the oxidation of cinnamaldehyde to benzaldehyde, in which 78% yield of benzaldehyde could be obtained. In addition, the mechanism of the oxidation was proposed according to the experimental results and theoretical calculation. The results indicated that chitosan had played a key role, and the significant promotion of conversion and selectivity for the oxidation could be attributed to inclusion complex formed via intermolecular weak interactions, e.g. hydrogen bonding between cinnamaldehyde and β-CD–CTS. The heterogeneous catalyst could be reused, and its catalytic efficiency remained unchanged, which suggests that the catalyst is efficient for the oxidation of cinnamaldehyde. The results reported herein may be a promising method for the production of benzaldehyde from natural cinnamaldehyde.

Charged Metalloporphyrin Polymers for Cooperative Synthesis of Cyclic Carbonates from CO2 under Ambient Conditions

A facile and one-pot synthesis of metalloporphyrin-based ionic porous organic polymers (M-iPOPs) was performed through a typical Yamamoto-Ullmann coupling reaction for the first time. We used various characterization techniques to demonstrate that these strongly polar Al-based materials (Al-iPOP) possessed a relatively uniform microporosity, good swellable features, and a good CO2 capture capacity. If we consider the particular physicochemical properties, heterogeneous Al-iPOP, which bears both a metal active center and halogen anion, acted as a bifunctional catalyst for the solvent- and additive-free synthesis of cyclic carbonates from various epoxides and CO2 with an excellent activity and good recyclability under mild conditions. Interestingly, these CO2 -philic materials could catalyze the cycloaddition reaction smoothly by using simulated flue gas (15 % CO2 in N2 , v/v) as a raw material, which indicates that a stable local microenvironment and polymer swellability might promote the transformation. Thus, the introduction of polar ionic liquid units into metalloporphyrin-based porous materials is regarded as a promising new strategy for the chemical conversion of CO2 .

Direct aerobic liquid phase epoxidation of propylene catalyzed by Mn(III) porphyrin under mild conditions: evidence for the existence of both peroxide and Mn(IV)-oxo species from in situ characterizations

Generally, gaseous propylene is hard to oxidize directly in liquid phase by dioxygen under mild conditions. Here, the liquid phase epoxidation of propylene to propylene oxide (PO) using molecular oxygen catalyzed by manganese porphyrins (MnTPPCl) in the presence of benzaldehyde was developed. Manganese(III) porphyrin exhibited excellent activity for the selective oxidation of propylene under mild conditions. The conversion of propylene and selectivity towards PO can reach 38% and 80%, respectively. The turnover frequency (TOF) of MnTPPCl catalyst reached 1840 h−1. Experiment evidences that the generation of peroxide and Mn(IV) oxo species during propylene epoxidation occurred, which was confirmed by in situ IR, in situ UV and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). On the basis of the experimental results, the mechanism for the epoxidation of propylene in the presence of metalloporphyrins and benzaldehyde with dioxygen was proposed. The mechanism is also supported by density functional theory (DFT) calculations.

β-Cyclodextrin polymer promoted green synthesis of cinnamaldehyde to natural benzaldehyde in aqueous solution

An environmentally benign synthesis of natural benzaldehyde from cinnamaldehyde under mild conditions has been investigated with sodium hypochlorite as oxidant and β-cyclodextrin polymer as phase-transfer catalyst. The polymer showed excellent catalytic activity exhibiting 92% conversion and 62% selectivity to benzaldehyde at ambient pressure and at 70°C. This catalyst could be recovered and reused six times, and the catalyst efficiency remained unchanged, which suggests that the catalyst is an efficient and green catalyst for oxidation of cinnamaldehyde. The results reported herein may be a promising method in industry for the synthesis of natural benzaldehyde.

Recyclable bifunctional aluminum salen catalyst for CO 2 fixation: the efficient formation of five-membered heterocyclic compounds

A variety of unique Al(salen) complexes functionalized by imidazolium-based ionic liquid (IL) moieties with the salen ligand at the two sides of 3,3′-position have been successfully prepared, rather than familiar 5,5′-position reported previously. The catalytic activity obtained by these bifunctional catalysts could be superior to those of the binary type catalysts in the formation of five-membered heterocyclic compounds from the cycloaddition reaction of CO2 and three-membered heterocyclic compounds (including terminal epoxides and N-substituted aziridines), presumably due to the distinguished intramolecularly synergistic catalysis, which might lead to perform the cycloaddition reaction at ambient conditions and retain excellent yield and unprecedented chemo- or regioselectivity. Moreover, the polyether-based trifunctional Al(salen) catalysts with the best catalytic performance could be regenerated and reused at least eight times without any obvious decreases in catalytic activity. Finally, the kinetic investigation suggested the structure of catalysts had important influences on the catalytic activity, thereby proposing the possible reaction mechanism.

Oxidative cleavage of C = C bond of cinnamaldehyde to benzaldehyde in the presence of β-cyclodextrin under mild conditions

Efficient oxidative cleavage of cinnamaldehyde to benzaldehyde by hydrogen peroxide in the presence of β-cyclodextrin (β-CD) and sodium bicarbonate in water has been developed. β-CD showed excellent activity for the oxidation under mild conditions. The effects of different reaction conditions, e.g. reaction temperature, the amount of β-CD, sodium bicarbonate and hydrogen peroxide concentrations, etc. have been investigated. The yield of benzaldehyde could reach 78% under the optimum conditions (333 K, 2 h, molar ratio for cinnamaldehyde:β-CD:sodium bicarbonate = 1:1:12). Moreover, a plausible mechanism including the formation of inclusion complex and the corresponding epoxide was proposed.