Lin Liang

Novel urea derivative-based ionic liquids with dual-functions: CO2 capture and conversion under metal- and solvent-free conditions

Several urea derivative-based ionic liquids (UDILs) with superior thermal stability were facilely synthesized, structurally analyzed, and applied to CO2 capture and conversion under mild conditions. These UDILs reversibly capture CO2 with double molar CO2 absorption and exhibit outstanding catalytic activity for the conversion of CO2 and various epoxides to cyclic carbonates under metal- and solvent-free conditions. The effects of reaction parameters on the catalytic activity for propylene carbonate (PC) synthesis from propylene oxide (PO) and CO2 were thoroughly investigated. Because water is inevitably contained in real gases requiring treatment, the influence of water on CO2 capture and conversion was also studied. Furthermore, reaction kinetic studies were also undertaken and a cation–anion synergistic catalytic mechanism was proposed. The single-component, metal-free, dual functional, stable, and easily recyclable ionic liquids reported herein are interesting materials, displaying a good performance for both CO2 capture and conversion.

Experimental and theoretical insights into binary Zn-SBA-15/KI catalysts for the selective coupling of CO2 and epoxides into cyclic carbonates under mild conditions

The combination of a metal-modified SBA-15 catalyst with potassium iodide was developed as heterogeneous dual catalysts for chemical fixation of CO2 to cyclic carbonates. It was observed that the binary Zn-SBA-15/KI catalysts were the most efficient among various metal-modified SBA-15/KI catalysts and showed an excellent synergetic effect in promoting the reaction under mild conditions. Moreover, the effects of reaction parameters on cycloaddition of CO2 with propylene oxide (PO) to propylene carbonate (PC) were optimized. Under the optimal conditions determined, the Zn-SBA-15/KI catalytic system was also versatile in CO2 cycloaddition with other epoxides. Additionally, the mechanistic details for the fixation of CO2 into a cyclic carbonate catalyzed by SBA-15/KI and Zn-SBA-15/KI were also contrastively elucidated using the density functional theory (DFT) method. The DFT results suggested that the zinc-modified and unmodified catalysts showed different coupling modes of CO2, and the ring-opening reaction was the rate-determining step in the SBA-15/KI catalyzed cycloaddition reaction, but the zinc-modified SBA-15/KI catalysts could enhance the CO2 cycloaddition, as the formation of a stable complex was beneficial to CO2 trapping. As a result, the ring-closing reaction became the rate-determining step in the Zn-SBA-15/KI catalyzed cycloaddition reaction, which was a promising result to guide the catalyst design for CO2 conversion.

Heterostructured g-C3N4/Ag–TiO2 composites with efficient photocatalytic performance under visible-light irradiation

g-C3N4/Ag–TiO2 composites with close interfacial contact among Ag, TiO2 and g-C3N4 were facilely fabricated and thoroughly characterized. As the deposited Ag nanoparticles played important roles as an electron-conduction bridge and in the surface plasmon resonance effect, Ag modification feasibly improved the separation efficiency for photoinduced electron–hole pairs and enhanced visible-light response. Furthermore, due to the further enhanced separation for photogenerated charges resulting from the existing build-in electric field of the heterojunction and the superposed light response from the hybridization of TiO2 and g-C3N4, g-C3N4/Ag–TiO2 composites exhibited remarkably improved photocatalytic activities for degrading rhodamine B dye compared with pristine TiO2 and g-C3N4 and single-component modified photocatalysts. Additionally, the g-C3N4/Ag–TiO2 catalyst retained excellent stability even after five recycles.

Zn-based ionic liquids as highly efficient catalysts for chemical fixation of carbon dioxide to epoxides

The novel Zn-based task-specific ionic liquids (Zn-TSILs) catalysts were developed for the coupling of carbon dioxide and epoxides to form cyclic carbonates under mild reaction conditions without using additional organic solvents and cocatalysts. Due to the synergistic effects of the cation and anion in this catalytic system, excellent yields and selectivities to cyclic carbonates were achieved with high TOF values up to 794 h−1. Among the catalysts investigated, OH-containing Zn-TSILs showed better activity than COOH-containing Zn-TSILs, and [(CH2CH2OH)Bim]ZnBr3 was found to be the best. Additionally, the influences of CO2 pressure and catalyst concentration were also investigated over [(CH2CH2OH)Bim]ZnBr3. In addition, the rate constants as well as the activation energies for the cycloaddition reaction catalyzed by Zn-TSIL and TSIL were comparatively determined. The activation energy was calculated to be 34.1 kJ mol−1 for bare TSIL catalyst, whereas the Zn-TSIL reduced the activation energy value by 14.7 kJ mol−1. Moreover, the Zn-TSIL was easily recyclable without significant loss of activity, representing the exceptionally promising candidate for the effective fixation of CO2 to epoxides.

In situ bubble template promoted facile preparation of porous g-C3N4 with excellent visible-light photocatalytic performance

Porous graphitic carbon nitride (pg-C3N4) was facilely and economically prepared through in situ bubble templates such as (NH4)2S2O8, and the surface area of the resultant pg-C3N4 was controlled from 6.4 to 55.0 m2 g−1 by adjusting the mass ratio of (NH4)2S2O8/melamine. Moreover, pg-C3N4 was also obtained by other gas-generating porogens of ammonium salts, displaying the generality of the preparation method. The as-prepared g-C3N4 presented a porous structure with a higher surface area and displayed an improved separation efficiency for photogenerated electron–hole pairs. These integrative positive factors contributed to pg-C3N4 possessing more excellent photocatalytic activity for degrading Rhodamine B and phenol pollutants, and splitting water to H2 than bulk g-C3N4 under visible-light. Moreover, the as-prepared pg-C3N4 exhibited unexceptionable stability and reusability even after four photocatalytic runs. The simple, economical and general fabrication strategy with bubble-generated porogen agents for porous graphitic carbon nitride with superior visible-light photocatalytic performance is attractive for environmental and energy application fields.

Melamine–ZnI2 as heterogeneous catalysts for efficient chemical fixation of carbon dioxide to cyclic carbonates

In this contribution, the combination of melamine with Lewis acid ZnI2 was developed as heterogeneous dual catalysts for the cycloaddition of carbon dioxide with epoxides yielding the corresponding cyclic carbonates. With molar ratio of 1 : 3.3 of ZnI2 to melamine, high yield (96%) and selectivity (99%) of propylene carbonate was obtained at 150 °C and 3.0 MPa for 4.0 h. The binary catalysts were also effectively versatile for CO2 cycloaddition to other epoxides, especially the less active epoxides such as styrene oxide and cyclohexene oxide. Additionally, the catalysts could be separated easily from the products after reaction and then reused efficiently. Furthermore, a possible synergistic catalytic mechanism was proposed, wherein, melamine played the dual roles to activate CO2 and epoxide simultaneously, ZnI2 activated epoxide and subsequently attacked the activated epoxide, the synergetic effects from melamine and ZnI2 promoted the reaction smoothly. The binary catalysts showed the advantages of simple preparation, low cost, abundant availability and high catalytic activity for CO2 chemical fixation into valuable chemicals.

Facile synthesis of a g-C3N4 isotype composite with enhanced visible-light photocatalytic activity

A highly active g-C3N4 isotype composite was prepared with a precursor mixture of melamine and thiourea by a one-step thermal treatment. The differences in the electronic band structure of the as-prepared isotype g-C3N4 made the band levels match with each other. This novel metal-free isotype composite was demonstrated to promote the separation of photoinduced charges, leading to a significant enhancement in the photocatalytic activity for degrading rhodamine B under visible-light irradiation. Moreover, the resultant g-C3N4 isotype composite possessed excellent stability after three recycles. Finally, the relevant mechanism for the photodegradation process was also proposed.

The crystal phase transformation of Ag2WO4 through loading onto g-C3N4 sheets with enhanced visible-light photocatalytic activity

A g-C3N4/Ag2WO4 composite photocatalyst was synthesized via a simple co-precipitation method at room temperature and was thoroughly characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, UV-visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. The characterization results indicated that upon depositing Ag2WO4 onto the surface of g-C3N4 its morphology changed from primary rod-like particles to globular nanoparticles and its phase changed from α-type to β-type Ag2WO4. When the composite was used as the catalyst in the photodegradation of Rhodamine B, 40%-g-C3N4/Ag2WO4 exhibited the highest photocatalytic activity with its rate constant was about 10.8 times larger than that of pure g-C3N4 and 3.12 times larger than that of bare Ag2WO4. The enhanced photocatalytic activity can be attributed to the complementary potentials of the conduction bands and valence bands of g-C3N4 and Ag2WO4, which could not only realize the effective separation of photoinduced electron and hole pairs, but also retained the catalysts high stability and photocatalytic performance even after five recycles. Furthermore, a possible photocatalytic mechanism for the degradation of RhB over the g-C3N4/Ag2WO4 composite was proposed according to the results of active species quenching experiments.