Organic Base-functionalized Hybrid Materials: Synthesis And Catalytic Properties In Transesterification Reaction

Diphenyl carbonate (DPC) is an important intermediate for the production of many organic compounds, especially as the precursor for the synthesis of engineering thermoplastics, such as polycarbonates (PC). DPC can also be used to increase the viscosity of polycondensates (e.g. polyamides and polyesters). Because PC possesses excellent mechanical, optical, electrical, and heat resistance properties, which have been used in many fields, there have been increasing demands for DPC. Therefore, DPC synthesis has received much attention as an alternative precursor to PC production. The traditional process for DPC synthesis was based on the reaction of phosgene (or chloroformic acid trichloromethyl ester (diphosgene), bis(trichloromethyl) carbonate) and phenol. Again, the obvious disadvantage of this method is the use of highly toxic phosgene as a raw material. With increasing demands for safer and cleaner processes, various non-phosgene approaches for DPC synthesis have been explored and investigated, including oxidative carbonylation of phenol and transesterification reaction. Among them, transesterification of dimethyl oxalate (DMO) with phenol to diphenyl oxalate (DPO), following a decarbonylation reaction, is an attractive and environmentally friendly route for the synthesis of DPC. The crucial problem of this route is the study on transesterification of DMO with phenol. For this transesterification reaction could not carry out without any catalyst, it is necessary to develop the highly efficient catalysts. Recently, it was reported that a variety of metal oxides bearing weak acidity and/or weak basicity were effective for the transesterification of DMO with phenol. However, the utility of these metal oxides is often limited by metal leaching or catalyst deterioration in liquid phase reaction.Recently, more attention has been drawn on organic-inorganic hybrid materials containing catalytic activities, which can be synthesized by grafting organic molecules bearing acid and/or basicity onto inorganic supports. Compared with other kinds of heterogeneous catalysts, organic-inorganic hybrid materials have been applied in catalytic fields and have excellent application prospects, because of several advantages, such as easy separation of products, high stability and easy reuse.In this thesis, the transesterification of DMO with phenol has been investigated over some different kinds of organic base-functionalized organic-inorganic hybrid materials. The effects of the type of the support and organic base on the catalytic properties of transesterification of phenol with DMO have been investigated. Both the structure and acid-base properties of catalysts were studied by different characterization means of XRD, N2-sorption, CO2-TPD, FT-IR and TGA, etc. Some other problems have also been discussed, including the nature of active sites and the reaction mechanism of the catalysts, and so on. The main experimental results and conclusions are as follows:1. Cyclopentadienyl-functionalized mesoporous Si-MCM-41 catalysts for the transesterification of dimethyl oxalate with phenol Three kinds of hybrid inorganic-organic catalysts (denoted as Cp'-Si-MCM-41, Cp''-Si-MCM-41 and Cp*-Si-MCM-41) are prepared by grafting methylcyclopentadienyl (Cp'), pentamethylcyclopentadienyl (Cp'') or 1,2-phenyl-4-methylcyclopentadienyl (Cp*) group onto the surface of mesoporous Si-MCM-41 by a post-grafting method, which can be characterized by means of X-ray powder diffraction (XRD), N2 sorption, thermogravimetric analysis (TGA), Fourier transform-infrared spectroscopy (FT-IR) and 29Si MAS NMR. It's found that such cyclopentadienyl-functionalized materials are active for the transesteri?cation of DMO with phenol. Under test conditions, Cp''-Si-MCM-41 bearing the relatively strong Lewis basicity and small steric hindrance gives the relatively high conversion of DMO and selectivity of DPO for the transesterification. Furthermore, the hybrid materials of Cp'-Si-MCM-41 and Cp''-Si-MCM-41 are more active than the corresponding homogeneous catalysts (Cp'H and Cp''H), in our opinion, there are at least two possible explanations: First, the strength of Lewis basicity of cyclopentadienyl groups may be changed after introducing it onto the surface of mesoporous Si-MCM-41. Second, the structure and the surface properties of the support are in favor of the adsorption and activation of reactant.Moreover, the recyclabilities of these two hybrid catalysts are also investigated. It is found that the catalytic activity of Cp''-Si-MCM-41 decreases gradually with the increase of recycle time. And Cp'-Si-MCM-41 shows relatively high stability since the catalytic activity of the recycled sample does not change considerably, which might be mainly due to that Cp'- group bears the relatively weak Lewis basicity and the adsorption of organic molecules on the active sites does not easily occurs on Cp'-Si-MCM-41 under our experimental conditions.2. Amino-functionalized mesoporous Si-MCM-41 catalysts for the transesterification of dimethyl oxalate with phenolA series of hybrid mesoporous Si-MCM-41 materials containing different amino groups (e.g. 3-aminopropyl (AP-), (2-aminoethylamino)propyl (AAP-), [2-(2-aminoethylamino)ethylamino]propyl (AEP-) group) were prepared by a post-grafting method, and characterized by means of XRD, N2 adsorption, FT-IR, TGA and temperature-programmed desorption of CO2 (CO2-TPD). The catalytic properties of these hybrid materials were investigated for the transesterification of DMO with phenol. All the samples were active for the transesterification. Among them, the sample AEP-Si-MCM-41 exhibited the highest activity under test conditions, which might be assigned to the relatively strong Lewis basicity of AEP-Si-MCM-41. On the basis of some related literatures, a proposed mechanism for the transesterification of phenol with DMO catalyzed by the amino-functionalized materials was proposed. In short, the amino group could act as Lewis base to assist hydrogen transfer from phenol to the carbonyl oxygen of DMO to form a hexahedral intermediate and then the intermediate decomposes to form the main product along with the release of methanol. Moreover, both the density and the basic strength of the amino groups have notable effects on the catalytic properties. And only a slight decrease in activity could be observed after a few runs, which might be mainly because the adsorption of organic molecules on the active sites occurs on this material. Notably, the activity of the used catalyst could be regenerated by simple treatment with methanol solution of tetramethylamonium hydroxide under a mild condition. By designing several additional reactions, we suppose that the formation of the by-product anisole should be mainly due to the reaction of methyl phenyl oxalate (MPO) with phenol.3. The effect of the support on the catalytic performance of Transesterification over organoamine-functionalized catalystsThe nature of the supports (aluminosilicate and aluminophosphate sieves) have an influence on catalytic performance over organoamine-functionalized hybrid catalysts. Under the same loadings of organic amine, the activity of different amino-functionalized materials decreases in the order: 0.5AEP-Si-MCM-41 > 0.5AEP-AlP1.0Os > 0.5AEP-Al-MCM-41, which might be ascribed to the mutual effects between acidic sites with basic sites under test conditions. In order to discern the above point, the surface M-OH of 0.5AEP-Al-MCM-41 becomes M-OSiMe3 via silazane silylation. The experimental results suggest that the activity of the silylated mesoporous material can clearly be improved.Two kinds of aminopropyl-functionalized magnetic materials containing different supports (e.g. Hydroxyapatite-encapsulatedγ-Fe2O3 nanoparticles, Iron oxide nanoparticles coated with mesoporous silica) were synthesized by a post-grafting method, and used for catalyzing the transesterification of DMO with phenol. The results suggest that 0.75AP/HAP-γ-Fe2O3 shows much higher activity than either 0.75AP/γ-Fe2O3@SiO2 or 0.75AP-Si-MCM-41. A reasonable explanation is that the cooperative activation between base carrier (HAP-γ-Fe2O3) and basic group (AP-). Considering magnetic hydroxyapatite-encapsulatedγ-Fe2O3 nanoparticles with cheap raw material, easy preparation and recovery, therefore, this kind of organo-functionalized hydroxyapatite-encapsulatedγ-Fe2O3 materials will have excellent application prospects.