Modified melamine-based porous organic polymers with imidazolium ionic liquids as efficient heterogeneous catalysts for CO2 cycloaddition.

The chemical conversion of carbon dioxide (CO2) into highly value-added products not only alleviates the environmental issues caused by global warming but also makes an impact on economic benefits in the world. The synthesis of cyclic carbonates by the cycloaddition of CO2 with epoxides is one of the most attractive methods for CO2 conversion. However, the development of green and highly efficient heterogeneous catalysts is considered to be a great challenge in catalysis. In this work, alkenyl-modified melamine-based porous organic polymer (MPOP-4A) was firstly synthesized by a one-pot polycondensation method, and it was again modified with imidazolium-based ionic liquids to obtain final modified catalyst (MPOP-4A-IL). Various analytical techniques were used to confirm structure and chemical composition of the prepared materials. The MPOP-4A-IL catalyst synthesized by the post-modification strategy with imidazolium-based ionic liquids exhibited enhanced catalytic activity for CO2 cycloaddition reaction. The enhanced catalytic performance could be attributed to the presence of abundant active sites in their structure such as hydrogen bond donors (HBD), nitrogen (N) sites, and nucleophilic groups for an effective chemical reaction. The MPOP-4A-IL catalyst was found to be metal-free, easy to recycle and reuse, and has good versatility for a series of different epoxides. The interaction of MPOP-4A-IL catalyst with epoxide and CO2 was further verified by density functional theory (DFT) calculations, and the possible mechanism of the CO2 cycloaddition reaction was proposed.

A facile method for in situ fabrication of silica/cellulose aerogels and their application in CO2 capture.

Old corrugated containers-based cellulose and fly ash-based fresh wet silica gel were used as raw materials for in situ synthesis of a series of silica/cellulose aerogels in NaOH/urea solution. At a silicon to cellulose ratio of less than 2.5:1, the skeleton structure of the synthesized composite material was dominated by fibrils decorated with spherical silica nanoparticles. At a silicon to cellulose ratio of higher than 2.5:1, the skeleton structure of the composite material was dominated by spherical silica particles interspersed with cellulose. The synthesized composite material was applied to capture CO2 at ambient temperature and pressure. We observed that with increasing silicon content, the CO2 adsorption capacity of the composite material decreased (regardless of its dominant structure), while its selectivity for CO2/N2 increased. This work presents a facile method for the synthesis of adsorption material that has high capacity and selectivity for CO2.

CO2 capture performance and characterization of cellulose aerogels synthesized from old corrugated containers.

Old corrugated containers with low recyclability were used as raw materials to synthesize a series of aerogels with varying cellulose concentrations in NaOH/urea solution via a freeze-drying process. The resulting aerogels had a rich porous structure with specific surface areas in the range of 132.72-245.19 m2.g-1 and mesopore volumes in the range of 0.73-1.53 cm3.g-1, and were tested for CO2 sorption at ambient temperature and pressure, displaying excellent CO2 adsorption capacities in the range of 1.96-11.78 mmol.g-1. Furthermore, the CO2/N2 selectivity of aerogels decreased with decreasing specific surface area, which was mainly caused by the decrease in CO2 capture. In addition, the CO2 sorption capacity of the sample with 2% cellulose content, CA-2, exceeded the values reported so far for many other sorbents with higher specific surface areas, and showed reasonable cyclic stability for CO2 capture. Therefore, this adsorbent represents an attractive prospect for CO2 uptake at room temperature.

Magnetically separable and reusable rGO/Fe3O4 nanocomposites for the selective liquid phase oxidation of cyclohexene to 1,2-cyclohexane diol.

A series of magnetically separable rGO/Fe3O4 nanocomposites with various amounts of graphene oxide were successfully prepared by a simple ultrasonication assisted precipitation combined with a solvothermal method and their catalytic activity was evaluated for the selective liquid phase oxidation of cyclohexene using hydrogen peroxide as a green oxidant. The prepared materials were characterized using XRD, FTIR, FESEM, TEM, HRTEM, BET/BJH, XPS and VSM analysis. The presence of well crystallized Fe3O4 as the active iron species was seen in the crystal studies of the nanocomposites. The electron microscopy analysis indicated the fine surface dispersion of spherical Fe3O4 nanoparticles on the thin surface layers of partially-reduced graphene oxide (rGO) nanosheets. The decoration of Fe3O4 nanospheres on thin rGO layers was clearly observable in all of the nanocomposites. The XPS analysis was performed to evaluate the chemical states of the elements present in the samples. The surface area of the nanocomposites was increased significantly by increasing the amount of GO and the pore structures were effectively tuned by the amount of rGO in the nanocomposites. The magnetic saturation values of the nanocomposites were found to be sufficient for their efficient magnetic separation. The catalytic activity results show that the cyclohexene conversion reached 75.3% with a highest 1,2-cyclohexane diol selectivity of 81% over 5% rGO incorporated nanocomposite using H2O2 as the oxidant and acetonitrile as the solvent at 70 °C for 6 h. The reaction conditions were further optimized by changing the variables and a possible reaction mechanism was proposed. The enhanced catalytic activity of the nanocomposites for cyclohexene oxidation could be attributed to the fast accomplishment of the Fe2+/Fe3+ redox cycle in the composites due the sacrificial role of rGO and its synergistic effect with Fe3O4, originating from the conjugated network of π-electrons in its surface structure. The rapid and easy separation of the magnetic nanocomposites from the reaction mixture using an external magnet makes the present catalysts highly efficient for the reaction. Moreover, the catalyst retained its activity for five repeated runs without any drastic drop in the reactant conversion and product selectivity.