Synthesis Of G-C3_N4 Based Materials And Mesoporous Ceria And Their Application Towards Catalytic Transesterification Of Ethylene Carbonate

Dimethyl carbonate(DMC) is one of the most important intermediate extensively used in organic chemistry. DMC has various physicochemical properties, including high oxygen content, low toxicity, and biodegradation, and therefore it has been widely used as a promising additive for gasoline, and candidate to substitute the highly toxic phosgene and dimethyl sulphate in carbonylation and methylation processes. Traditionally, DMC is manufactured by phosgenation and oxidative carbonylation of methanol, which involve high-risk compounds. By contrast, transesterification of ethylene carbonate(EC) with simple alcohols has been regarded as a mild, green and highly selective route for the synthesis of DMC. Meanwhile, the byproduct of the transesterification, i.e. ethylene glycol, is also an important chemical reactant. In this regard, the transesterification process has emerged as the most promising route to produce DMC. Wherein, the key issue associated with the catalytic activity and selectivity lies in the development of efficient heterogeneous catalysts.Recently, graphitic carbon nitride(g-CN) materials have attracted tremendous attention in the fields of catalytic synthesis and functional materials. g-CN possesses abundant basic N-containing species at the edges of graphitic-like layers, which enable it to be a typical solid base catalyst. Compared with the traditional bulk g-CN materials prepared by direct thermal condensation, mesoporous g-CN materials have large surface areas(> 200 m2 g-1) and rich mesopores, which promise their upgradation in terms of heterogeneous catalytic activity.In the present thesis, we have prepared 3D mesoporous g-CN(CN-MCF) using 3D mesocellular silica foam(MCF) as a hard template, and carbon tetrachloride(CTC) and ethylenediamine(EDA) as precursors through a nanocasting approach. Several techniques including XRD, N2 adsorption–desorption, SAXS, TEM, FT-IR, and XPS have been employed to characterize the crystalline, surface area and porous structure, micro-morphology, and chemical composition of CN-MCF. The corresponding results indicate that CN-MCF have successfully negatively replicated the 3D mesostructures of MCF. In the transesterification of EC with methanol, CN-MCF demonstrates a remarkable catalytic performance, affording a maximum DMC yield of 78% at 6 h under 160 °C. Furthermore, the catalytically active sites have been proposed as uncondensed N-containing species at the graphitic-like layers of CN-MCF.Despite the successful finding in the above section, it should be noted that CN-MCF was prepared using expensive MCF as a template, and the preparation route is complicated and time-consuming. In the second section, a series of Zn-doped g-C3N4 materials have been fabricated using inexpensive and low toxic dicyandiamide as a precursor and zinc halides as dopants, by a simple doping and calcination procedure. The Zn-g-C3N4 catalyst shows outstanding DMC yield of 83.3% at 4 h under 160 °C, and reliable catalytic recyclability. Furthermore, other transition metal halides have been also used as dopants towards g-C3N4, and the synthesized metal-doped g-C3N4 materials reveal high catalytic activity. The results of XPS and FT-IR characterization verify that the transition metal cations react with the terminal amine of g-C3N4 by coordination, thereby increasing the concentration of bridging amine species, finally leading to enhancement of the basic intensity of g-C3N4.Given the relatively harsh reaction conditions required in the above transesterification catalyzed by g-CN materials, in the third section, we have also investigated the catalytic performance of mesoporous ceria materials(CeO2-meso), which were prepared using Ce(NO3)3·6H2O as a precursor, and cetyltrimethylammonium bromide(CTAB) as a soft template through a soft-templating method. The resultant CeO2-meso materials have tunable surface areas(109–182 m2 g-1) and concentrated pore sizes(5.1–5.4 nm). The profiles of N2 adsorption–desorption and CO2-TPD reveal that the surface area and basic intensity are responsible for the catalytic activity of CeO2-meso. Among the various CeO2-meso samples with different surface areas, CeO2-meso-400 offers the highest catalytic activity, affording a DMC yield of 73.3 at 2 h under 140 °C. The possible active sites of CeO-meso in the transesterification of EC to DMC are suggested as the basic Ce–OH groups.