Synthesis of glycerol carbonate from glycerol and urea using gold-supported catalysts
Au/MgO-type catalysts for the carbonylation of glycerol with urea. Effect of the morphology and macro/meso-porous structure of the support
M. Van Oudenhove1, W.Y. Hernández2*, A. Verberckmoes1 and P. Van Der Voort2
1 Industrial Catalysis and Adsorption Technology (INCAT), Department of Industrial Technology and Construction, Faculty of Engineering & Architecture, Ghent University, Valentin Vaerwyckweg 1, 9000 Ghent, Belgium.
2 Center for Ordered Materials, Organometallics & Catalysis (COMOC), Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium.
Keywords: Gold catalysts, magnesium oxide, macro/meso-porous structure, glycerol conversion
1 Introduction
The reaction of glycerol with urea to form glycerol carbonate has become a relevant research topic during the last few years. This process utilizes two inexpensive and readily available compounds and additionally provides a route to upgrade the surplus production of glycerol formed in large quantities as byproduct during the production of biodiesel [1]. Although the reaction of glycerol with urea can proceed by increasing the temperature of the system, several heterogeneous catalysts (mostly Zn-based ones) have been found to improve its rate and selectivity to glycerol carbonate [2]. It is accepted that the existence of well-balanced acid-base properties of the catalyst are responsible of the activity and selectivity during the reaction. Hutchings et al. [3] presented a gold supported on magnesia catalyst as a high active and selective system for the glycerolysis of urea. These authors suggest that the combination of the basic properties of the support with the Lewis acidity of the gold-supported nanoparticles results in the improvement of the yield and selectivity. Moreover, the support can also play an additional role concerning to the stabilization of the gold nanoparticles and/or the occurrence of different metal-support interaction effects (e.g. charge withdrawing effects). Thus, the rational design of MgO-type supports (considering parameters such as morphology and porous structure) and the study of the gold-support interactions originated in these types of materials represent a feasible way to improve the catalytic efficiency and stability of Au/MgO-type catalysts.
This work describes the synthesis of MgO-type supports by a hydrothermal synthesis route, employing Pluronic P123 block copolymer surfactant or cetyltrimethylammonium bromide (CTAB) as soft-templates. Gold nanoparticles were deposited on the most relevant supports (in terms of surface and morphology) and used as catalysts for the synthesis of glycerol carbonate.
2 Experimental/methodology
In a typical synthesis, P123 or CTAB were dissolved in water at 60 ºC and under vigorous stirring to form a transparent solution. After that, the Mg(NO3)2.6H2O was added the to the clear solution (surfactant/Mg molar ratio equal to 0.03). An aqueous ammonia solution (25 wt%) was added dropwise at room temperature to the resulting liquid mixture under stirring until having a final pH close to 10. After precipitation, the slurry was transferred to a 50-mL Teflon-lined stainless steel autoclave for hydrothermal treatment at the selected temperature (120 ºC) for 12 or 24 h. The obtained solid was filtered out and washed three of four times with distilled water and ethanol (for the removal of the majority of the surfactant) and then dried overnight at 100 ºC. MgO was formed by calcination at 500 ºC, 3h, using a very slow calcination program.
3 Results and discussion
Figure 1 shows the XRD patterns of the samples calcined at 500 ºC. All the materials present the reflections characteristic of a pure MgO periclase-phase. Nevertheless, depending on the surfactant used, the MgO particles exhibit a different morphology. The SEM micrographs of the samples prepared with P123, CTAB and without addition of surfactant are shown in figure 2-a, 2-b and 2-c, respectively. In the absence of surfactant or using CTAB, a similar morphology of hexagonal nanoplates is observed. Higher agglomeration is seen without using surfactant. On the other hand, the presence of P123 provokes the formation of randomly piled aggregates of sheets. Those types of structures allow the formation of flower-like agglomerates with an open macroporous structure.
In all the prepared samples, the textural analysis reveals the formation of mesoporous structures (isotherm adsorption IV-type). However, the material synthesized in presence of P123 is the one with the highest surface area and pore volume.
Fig. 2. SEM images of a). P-120-12-500, b). C-120-12-500 and c). W-120-12-500 materials
Table 1. Textural properties of the synthesized materials
Code
SBET (m2/g)
Pore Vol. (cm3/g)
Av. Pore size (Å)
W-120-12-500
136
0.22
64
C-120-12-500
97
0.23
96
P-120-12-500
163
0.31
76
4 Conclusions
The combination of an open macro and mesoporous structure and a relevant surface area make the P-120-12-500 solid an interesting material to support gold-nanoparticles. Such textural and morphological properties are expected to influence the stabilization and dispersion of the deposited metallic phase and the diffusional process involved during the catalytic reaction.
References
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