Yong-Xin Li

A new and environmentally benign precursor for the synthesis of mesoporous g-C3N4 with tunable surface area

An environmentally benign and widely available precursor, guanidinium chloride, has been employed, for the first time, for the synthesis of mesoporous g-C3N4 materials via a nanocasting method using 12 nm colloid silica as a hard template. A series of mesoporous g-C3N4 (C3N4-G-r) samples with bimodal pore systems (1.8 and 13.4 nm), and high surface areas (146-215 m(2) g(-1)) along with large pore volumes (0.57-0.84 m(3) g(-1)), depending on the amount of templates, have been successfully prepared. Several techniques including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and CHN elemental analysis have confirmed the formation of graphitic structures and extremely high nitrogen content of the synthesized C3N4-G-r materials. In Friedel-Crafts acylation reaction between hexanoyl chloride and benzene, C3N4-G-r samples, especially C3N4-G-1.0 with a large surface area of 215 m(2) g(-1), have demonstrated high catalytic conversions, affording a maximum yield of 86% of n-hexanophenone in 4 h at 80 °C.

Mesoporous carbon nitride grafted with n-bromobutane: a high-performance heterogeneous catalyst for the solvent-free cycloaddition of CO2 to propylene carbonate

A new type of mesoporous-C4N4-based catalyst (n-butBr/mp-C3N4) was prepared by simple grafting of n-bromobutane. The N2 adsorption–desorption and X-ray diffraction characterizations indicate that, in comparison with the parent mp-C3N4, the textual and structural properties have been well retained by n-butBr/mp-C3N4. In the cycloaddition of CO2 with propylene epoxide, the n-butBr/mp-C3N4 exhibits high catalytic conversion as well as a high selectivity to propylene carbonate. The maximum TOF value obtained over n-butBr/mp-C3N4 is 10.7 h−1 at 6 h under 140 °C, which compares favorably to other reported C3N4-based catalysts. In addition to n-bromobutane, mp-C3N4 materials grafted with other alkyl halides also provide high catalytic activities. According to the Fourier transform infrared and X-ray photoelectron spectroscopy measurements, it is speculated that the catalytic active sites of n-butBr/mp-C3N4 are uncondensed amines and Br anions which originate from the reaction between n-bromobutane and the N-containing heterocycles of mp-C3N4.

Vanadia supported on mesoporous carbon nitride as a highly efficient catalyst for hydroxylation of benzene to phenol

Mesostructured V/mp-C3N4 catalysts were prepared using NH4VO3 as a precursor and mesoporous carbon nitride as a support through a wet impregnation method. A series of techniques, including N2 adsorption–desorption, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared and UV-vis diffuse reflectance spectra, temperature-programmed desorption, etc., were employed to characterize physicochemical properties of the synthesized catalysts. In the hydroxylation of benzene to phenol in the presence of H2O2, the V/mp-C3N4 materials showed remarkable catalytic performance, affording a maximum phenol yield of 18% at 60 °C, superior to the previously reported C3N4-based catalytic systems in terms of their TOF values. The catalytic activity of V/mp-C3N4 is attributed to high benzene-activation capability of mp-C3N4 and dispersion of vanadia species.

Utilization of Environmentally Benign Dicyandiamide as a Precursor for the Synthesis of Ordered Mesoporous Carbon Nitride and its Application in Base-Catalyzed Reactions

Assisted by a new dissolution procedure, dicyandiamide (DCDA), an environmentally benign and cheap precursor, has been employed for the synthesis of mesoporous carbon nitride (CN) materials through a nanocasting approach. The synthesized mesoporous materials possessed high specific surface areas (269-715 m(2) g(-1)) with narrow pore-size distributions (about 5 nm) and faithfully replicated the mesostructures of the SBA-15 and FDU-12 templates. Several characterization techniques, including XRD, SAXS, TEM, Raman and FTIR spectroscopy, XPS, and CO2-TPD, were used to analyze the physicochemical properties of these materials and the results showed that the mesoporous CND materials had graphitic-like structures and consisted of CN heterocycles, as well as amino groups. In a series of Knoevenagel condensation reactions, as exemplified by the reaction of various aldehydes and nitriles, these mesoporous CND materials demonstrated high and stable catalytic activities, owing to an abundance of basic sites.

Mesostructured graphitic carbon nitride as a new base catalyst for the efficient synthesis of dimethyl carbonate by transesterification

Mesostructured graphitic carbon nitride (CN-MCF) material has been prepared using carbon tetrachloride and ethylenediamine as precursors and mesocellular silica foam as a hard template, and characterized by XRD, N2 adsorption–desorption, TEM, FT-IR, and XPS techniques. The material was employed as a catalyst for the production of dimethyl carbonate (DMC) via transesterification of ethylene carbonate (EC) with methanol (MeOH). The influences of reaction conditions, including time, temperature, and the molar ratios of MeOH to EC, on the catalytic performance have been investigated in detail. Catalytic results revealed that CN-MCF could catalyze the transesterification reaction with high efficiency, affording a high DMC yield of 78% and stable catalytic activity for several running cycles. Furthermore, a possible reaction mechanism for the g-CN-catalyzing transesterification of EC with MeOH has been proposed.

Graphene oxide immobilized with ionic liquids: facile preparation and efficient catalysis for solvent-free cycloaddition of CO2 to propylene carbonate

Functionalized imidazolium-based ionic liquids (ILs) with different halides (Cl, Br, and I) were successfully immobilized on the surface of graphene oxide materials by one step through covalent condensation between alcoholic hydroxyl groups of GO and alkoxyl groups of functionalized ILs. Several characterization including TG, Raman, AFM, FT-IR, and XPS techniques have been applied to characterize the physicochemical properties of the synthesized GO-[SmIm]X materials. In the solvent-free cycloaddition reactions of CO2 to propylene oxide, GO-[SmIm]I showed remarkably catalytic activity, affording a maximum yield of propylene carbonate as ca. 96%. The heterogeneous catalyst could be reused for at least four runs without any significant loss in activity, and demonstrated versatile catalysis for a wide range of substrates. A possible catalytic mechanism has been proposed, wherein epoxides were activated by the oxygen-containing groups of GO and the halide anions of the grafted ILs acted as key active species for the catalytic cycloaddition reactions.

Synthesis of mesoporous carbon nitride via a novel detemplation method and its superior performance in base-catalyzed reactions

A series of mesoporous graphitic carbon nitride materials have been synthesized using dicyandiamide as a precursor by a nanocasting method through a new detemplation process. Alkaline solutions could effectively eliminate hard siliceous templates, and ordered mesostructures have been well retained over the final synthesized CND materials. Fourier transform infrared and X-ray photoelectron spectroscopy characterization revealed that the employment of alkaline solution during the synthesis of the CND samples facilitated the condensation of tri-s-triazine fragments and thus enhanced the amount of bridging N species. According to the profiles of CO2 temperature programmed desorption and elemental analysis, the detemplating agent could not only preserve the original basic sites but also improve the overall basic intensity of the mesoporous CND samples. In base-catalyzed reactions, represented by Knoevenagel condensation and transesterification reactions, the mesoporous CND materials demonstrated excellent and stable catalytic performance, which is especially much higher than that of the mesoporous sample detemplated by traditional HF solution.

Facile alkali-assisted synthesis of g-C3N4 materials and their high-performance catalytic application in solvent-free cycloaddition of CO2 to epoxides

A series of graphitic carbon nitride materials were synthesized using guanidine hydrochloride (GndCl) as a precursor with the aid of alkali treatment. The introduction of alkali successfully enabled GndCl to be transformed into g-C3N4 at much lower calcination temperatures (450–475 °C). The g-C3N4 samples synthesized under various conditions have been characterized by several techniques including XRD, FT-IR, UV-vis, 13C NMR, and XPS spectroscopy. The results confirmed that the alkali could effectively accelerate further condensation of melem-like fragments to g-C3N4. Meanwhile, a possible mechanism of alkali-assisted synthesis of g-C3N4 from GndCl has been proposed. In solvent-free catalytic cycloaddition of CO2 to propylene oxide to propylene carbonate (PC), g-C3N4-NaOH and g-C3N4-KOH materials demonstrated high and stable catalytic performances, affording PC yields of ca. 90% under optimized reaction conditions. Moreover, the activities were superior to those obtained over g-C3N4 prepared without alkali treatment. In addition, the catalytic activity along with preparation method for the present g-C3N4 has also been compared with other reported g-C3N4-based catalysts.

A hybrid sol―gel synthesis of mesostructured SiC with tunable porosity and its application as a support for propane oxidative dehydrogenation

Porous silicon carbide (SiC) is of great potential as catalyst support in several industrially important reactions because of its unique thermophysical characteristics. Previously porous SiC was mostly obtained by a simple sol–gel or reactive replica technique which can only produce a material with low or medium surface area (< 50 m2 g(−1)). Here we report a new hybrid sol–gel approach to synthesize mesostructured SiC with high surface area (151–345 m2 g(−1)) and tunable porosity. The synthesis route involves a facile co-condensation of TEOS and alkyloxysilane with different alkyl-chain lengths followed by carbothermal reduction of the as-prepared alkyloxysilane precursors at 1350 °C. The resulting materials were investigated by X-ray diffraction, N2 adsorption-desorption, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. A mechanism for the tailored synthesis of mesostructured SiC was tentatively proposed. To demonstrate the catalytic application of these materials, vanadia were loaded on the mesostructured SiC supports, and their catalytic performance in oxidative dehydrogenation of propane was evaluated. Vanadia supported on the mesostructured silicon carbide exhibits higher selectivity to propylene than those on conventional supports such as Al2O3 and SiO2 at the same propane conversion levels, mainly owing to its outstanding thermal conductivity which makes contributions to dissipate the heat generated from reaction thus alleviating the hot spots effect and over-oxidation of propylene.

A Schiff-base-type vanadyl complex grafted on mesoporous carbon nitride: a new efficient catalyst for hydroxylation of benzene to phenol

Mesoporous graphitic carbon nitride (g-CN) was utilized as a new support to immobilize vanadyl(IV) acetylacetonate ([VO(acac)2]). The immobilized vanadyl complex materials (VOac-CND) have been thoroughly characterized by various techniques including N2 adsorption–desorption, XRD, SAXS, TEM, Raman, FT-IR, XPS and benzene-TPD. The characterization results showed that [VO(acac)2] had been successfully grafted on the surface of g-CN via the reaction between the carbonyl group of the acetylacetonate ligand and the amino groups of g-CN and thus transformed into a Schiff-base-type complex. Moreover, after the immobilization of [VO(acac)2], the ordered natures of the mesoporous structures and graphitic structures of the g-CN support have been well retained. The immobilization temperature has been found to be sensitive to the immobilization effect. As heterogeneous catalysts, the immobilized catalysts exhibited high performances in the direct hydroxylation reaction of benzene to phenol, affording a maximum phenol yield of ca. 20.0%. The catalytically active site for hydroxylation was proposed as the grafted vanadyl complex while the mesoporous g-CN played a crucial role in activating benzene.