Glycerol carbonation with urea over Zn-containing mixed oxide catalysts

Abstract

In this millennium, climate change is one of the biggest challenges of the human’s future. The major aspect of the current climate change is global warming which is the result of human activities. The most influence activity of humans is the emission of greenhouse gas including the carbon dioxide (CO2) by the burning of fossil fuel. If humans do not have effective actions to control the problem, the impact of climate change can be extended not just in this century but also in the next 10 millennia more. Replacing conventional fossil fuel by other renewable energy (including biofuel, biodiesel) is recognized as a solution to mitigate the impact of climate change. However, one problem of biodiesel is its byproduct- glycerol which is abundant and needs to be converted to a value-added product. In this thesis, I focus on preparing the catalysts for the reaction of glycerol and urea to glycerol carbonate; and investigating the catalysis mechanism of the reaction. Catalytic conversion to glycerol carbonate (GC) from glycerol and urea was investigated with Zn/Al catalysts supported by activated red mud (ARM), a waste material. Compared to an unsupported catalyst, ARM-supported Zn/Al catalysts exhibited higher GC yields. ARM-supported Zn/Al catalysts showed a volcano curve for the GC yield as a function of the Zn/Al loading. FTIR analysis revealed the ARM-supported Zn/Al catalysts to be more selective, resulting in higher GC yield. The balance of active sites from ARM and Zn/Al was related to rates of each reaction step in GC synthesis, which eventually influenced the selectivity and yield of GC. In the second research, we prepared ZnO, ZnAl2O4, and ZnAl mixed oxides with different metal molar ratios and applied them for synthesizing glycerol carbonate (GC) from glycerol and urea. The reaction routes related to the Zn species over the ZnAl mixed oxides were investigated. The ZnAl mixed oxides were found to consist of two Zn crystalline phases: ZnO and ZnAl2O4. From the reaction results, the ZnAl mixed oxides showed much higher glycerol conversion and GC yield than the ZnO and ZnAl2O4. During the reaction, the dissolution of the Zn species from the ZnO phase over the ZnAl mixed oxides was observed while the ZnAl2O4 phase remained insoluble. The ZnO phase provided a homogeneous reaction route via the dissolved Zn species, resulting in the formation of a Zn complex containing the isocyanate (NCO) and zinc glycerolate. In contrast, the insoluble ZnAl2O4 phase was responsible for not only a heterogeneous reaction route, but also adsorption of the Zn NCO complex on the catalyst. We proposed that the adsorbed Zn NCO complex could play a role as an active site for an additional heterogeneous reaction route. Therefore, the ZnAl mixed oxides exhibited high GC yields through the dual catalysis routes: the homogeneous reaction route over the ZnO phase and the heterogeneous reaction route over the ZnAl2O4 phase. To understand the reaction mechanism, two different Zn-based catalysts - ZnO, and ZnAl mixed oxide (ZnAlO or Zn7Al3) - were employed to investigate Zn-phase-dependent catalysis in the reaction of glycerol with urea as a function of reaction times. Zn7Al3 catalyst exhibited higher selectivity and yield of glycerol carbonate (GC) over a wide range of glycerol conversion than the ZnO catalyst. The time-dependent Zn species and reaction intermediates were observed in the solid and liquid phases at various reaction times through FTIR and XRD measurements in order to understand Zn-containing intermediates and corresponding reaction routes over each catalyst. The low GC selectivity in the reaction over the ZnO catalyst was closely connected to the formation of zinc glycerolate (ZnGly) in the solid phase. For the ZnO catalyst, ZnGly was formed in the solid phase even at an initial reaction time by the reaction between Zn NCO complex and glycerol, resulting in the loss of GC selectivity. Alternatively, over a Zn7Al3 catalyst, the formation of the Zn isocyanate (NCO) complex was dominant up to 2 hr of reaction time in both the liquid and solid phases. After 2 hr of reaction time, ZnGly was observed in the spent Zn7Al3 catalyst along with decreasing GC selectivity. The relative formation rates of Zn-containing reaction intermediates (ZnGly and Zn NCO complex) over the Zn7Al3 catalyst were affected by the Zn phases over the solid catalysts and the ratio of urea to glycerol in the liquid phase during the reaction time. The effect of a disordered ZnAl2O4 spinel structure on the reaction of glycerol with urea was investigated with pure ZnAl2O4 (c-ZnAl2O4) and ZnAl mixed oxide (c-ZnAlO) prepared by a citrate complex method, and ZnAl physically mixed oxide (p-ZnAlO). During catalysts preparation, a disordered bulk ZnAl2O4 phase generated disordered sites on the surface: the Al3+ cations substituting for Zn2+ cations at the tetrahedral sites, and the surface oxygen vacancy corresponding to the Zn2+ cations substituting for Al3+ cations at the octahedral sites. The disordered surface sites increased in order of p-ZnAlO < c-ZnAlO < c- ZnAl2O4, which was proportional to the surface acidity. c-ZnAlO exhibited the best reaction performance due to the existence of a solid zinc isocyanate (Zn NCO) complex on the disordered sites. Here, we proposed that the solid Zn NCO complex preferentially generated glycerol carbonate (GC), while the liquid Zn NCO complex produced both GC and zinc glycerolate. Finally, we investigated the glycerolysis of urea over various ZnMeO (Me = Co, Cr, and Fe) mixed oxide catalysts. ZnMeO mixed oxide catalysts were prepared by a co-precipitation method for two Zn/Me ratios, resulting in Zn-rich mixed oxide (Zn2MeO) and Zn-poor mixed oxide (ZnMe2O). In the glycerolysis of urea, the Zn2MeO catalysts exhibited higher glycerol conversion and glycerol carbonate yields than the ZnMe2O catalysts due to the predominance of homogeneous catalysis through Zn isocyanate (NCO) complexes from the Zn2MeO catalysts. Specifically, Zn2CrO was the best catalyst, with the highest yield of glycerol carbonate. Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) results of the spent catalysts clearly demonstrated the dominant formation of a solid Zn NCO complex over the spent Zn2CrO catalyst, a unique feature indicating that the better catalytic performance of Zn2CrO was due to the additional heterogeneous reaction route through the solid Zn NCO complex.