The Synthesis Of Nano-gold Catalyst In The Reaction Of Diol Oxidation Of Lactone Applied Research

The lactones and their derivatives are widely distributed in nature. The lactone ring exists in the molecules of many bioactive substances and metabolic intermediates. Due to their high boiling point, solubility, conductivity and stability, the lactones are widely used as solvent, extraction agent and also can be used for the synthesis of a variety of polymers. In this dissertation, the nano gold catalysts were used to catalyze the aerobic oxidative dehydrogenation of diols to lactones. The reaction was carried out at low temperature with air as oxidant, which is a clean route in accordance with the demand of green chemistry and sustainable development.The nano gold is a catalyst with high performance in the low temperature CO oxidation, oxidation of alcohols, water-gas shift reaction and selective hydrogenation ofα,β-unsaturated aldehydes or ketones. Due to their low temperature activity and high selectivity, the gold catalysts are becoming more and more important. The gold particle size, gold electronic state and support effect was studied in this dissertation to investigate the active sites of gold catalyst and the reaction mechanism.Gold catalysts prepared in this dissertation showed high activity in the oxidative dehydrogenation of a series of diols such as 1,4-butanediol,1,5-pentanediol and 1,2-benzenedimethanol to y-butyrolactone,δ-valerolactone and phthalanone. The preparation conditions of the catalysts, support compositions, support morphology and support crystal phase were optimized, and finally multi-component oxide support were prepared to give catalysts with high activity, selectivity and stability. At the same time, the active sites of the gold catalysts, as well as the support effect were discussed and the following results were obtained.1. Influence of the preparation conditions and catalyst compositions on the catalytic performance of Au/TiO2 catalysts in the oxidative dehydrogenation of diols to lactonesThe commercial Degussa P25 was utilized as support, and the gold catalysts were prepared by the homogeneous deposition-precipitation method (HDP) using urea as precipitation agent. Nano sized gold particles were obtained with average particle size of about 5 nm. There was interaction between gold and the support. The catalysts showed high activity in the oxidation of 1,4-butanediol and 1,5-pentanediol to the corresponding lactones. Calcination temperature had significant effect on the catalytic activity of the catalysts, and it was found that the catalyst with 3-8% gold loading calcined at 573-673 K gave high activity. From the characterization results it can be found that when calcined at high temperature, gold particles tended to aggregate and form large particles, but when the calcination temperature was below 573 K, gold species were not completely reduced, and there were still some oxidized gold (Au3+). For catalysts with gold loading higher than 8%, gold particles tended to aggregate and catalytic activity dropped. From the above results it can be concluded that the surface metallic gold species with small particle size were the active species.2. Properties and catalytic performance of gold catalysts on different oxide support (Au/MOx)The transition metal oxides and main group metal oxides were used as support for gold catalysts, and most of them showed high activity in the oxidative dehydrogenation of 1,4-butanediol to y-butyrolactone.Among the transition metal oxide supported gold catalysts, Au/TiO2, Au/MnOx, Au/Fe2O3, Au/Co3O4, Au/ZnO, Au/CeO2 and Au/ZrO2 were highly active with both diol conversion and lactone selectivity above 90% after 8 hours'reaction, especially Au/TiO2 and Au/MnOx, whose conversion were above 80% after 4 hours'reaction. Due to its large gold particle size and low surface gold content, Au/NiO showed much lower activity.Activity of Au/SnO2 was even superior to some of the transition metal oxide supported gold catalysts, with conversion of 82% after 4 hours'reaction and 99% after 8 hours'reaction. When catalyzed by Au/Mg(OH)2, the formed y-butyrolactone would be hydrolyzed due to the basicity of Mg(OH)2. Although A12O3 was always considered as inert in gold catalysis, Au/Al2O3 showed high activity with 4 hour and 8 hour conversion of 58% and 99%, respectively.For the above gold catalysts on active support, the average gold particle size was in the range of 4-7 nm, and the support particle diameter was about 10-30 nm. The surface gold content was higher than that in bulk, and gold particles were highly dispersed on the surface of the support. Most of the gold can be deposited onto the support to form gold-support interaction. From the BET results it can be inferred that although surface area of some support oxides were low, gold catalysts with high activity can still be obtained on them.From the above results, it can be concluded that gold particle size must be small enough to give high activity. However, the gold particle size was not the decisive factor and the type of the support was also important. Pore properties were not as important as support effect, and there was interaction between gold species and the support.3. Support effect of Au/FeOx catalysts and their catalytic performance in the oxidative dehydrogenation of diolsThe commercial nano iron oxidesγ-Fe2O3,α-Fe2O3 and Fe3O4 were utilized as support for gold catalysts, and it was found that the smallest gold particles were obtained on Fe3O4 with average gold diameter of 4.7 nm, and then the Au/α-Fe2O3 (5.5 nm), and the largest gold particles were got on Au/γ-Fe2O3 (6.7 nm). TOF values of the catalysts were in the order:Au/Fe3O4> Au/α-Fe2O3> Au/γ-Fe2O3. Although Au/Fe3O4 had the highest initial activity, Fe3O4 would be oxidized toγ-Fe2O3 during the reaction with high temperature and oxidative atmosphere, leading to the loss in activity and final lactone yield. For Au/Fe3O4 catalysts treated under different atmospheres, Au/Fe3O4-H2 had the smallest gold particle size (2.7 nm), but its catalytic behavior was similar to that of Au/Fe3O4-Ar. Therefore it was inferred that the different activity of the Au/FeOx with different crystal phase was due to the variation in support properties other than the gold particle size.FeOx-NF was synthesized by a hydrothermal method, and showed different crystal phase and morphology when calcined at different temperatures. The room temperature treated sample was in the form of Fe3O4, and it was oxidized toγ-Fe2O3 when calcined at 573 K in air. Further increase in calcination temperature led to the crystal transformation fromγ-Fe2O3 toα-Fe2O3, and the simultaneous transformation of iron oxide particles to nano flakes. The FeOx-NF supported gold catalysts were more active than those supported on commercial iron oxides. When FeOx-NF in the. form ofγ-Fe2O3 was used as support for gold catalysts, it showed high TOF value (624 h-1 for Au/FeOx-573). According to the characterization results, the gold particles were evenly distributed on the support with average particle size about 5 nm, and the support had significant influence on the gold particle size distributions and electronic properties. There was strong gold-support interaction, and on Au/γ-Fe2O3, electrons were transferred from the support to gold, leading to the formation of negatively charged gold (Auδ-). On contrary,α-Fe2O3 can stabilize the oxidized gold species. Support was responsible for the variation in gold particle size and oxidation state, and thus the differences in catalytic activity. In contrast to commercial iron oxide supported gold catalysts, the Au/γ-Fe2O3 showed higher activity than Au/α-Fe2O3, and this may be due to the morphology change and support synthetic method utilized in this dissertation.4. Catalytic properties of AuAlT catalysts in the oxidative dehydrogenation of diols to lactonesγ-AlOOH andγ-Al2O3 (AIT) were utilized as support for gold catalysts to investigate the influence of support basicity. The support was prepared by a simple deposition method, and by elevating pretreating temperature from room temperature to 973 K, AIT dehydroxylated fromγ-AlOOH toγ-Al2O3.γ-AlOOH was stable below 573 K, and then transformed toγ-Al2O3 at 773 K. According to the CO2-TPD results, there were mainly weak basic sites onγ-Al2O3 (A1773, A1873 and A1973), while forγ-AlOOH, there were medium basic sites on A1573 and strong basic sites on A1373. Small gold particle size can be obtained onγ-Al2O3 with average diameter about 4nm. Larger gold particles were got on y-AlOOH (A1373, A1573), and the average gold diameter was 5-8nm. Catalytic activity of AuAlT catalysts was higher than that of other oxide supported gold catalysts (Au/TiO2 and Au/FeOx). Conversion of AuA1773 reached 90% after 1 hour's reaction. Although with larger gold particle size, AuA1373 still showed high activity with 74% conversion after 1 hour's reaction. Conversion of all the catalysts can achieve 100% after 8 hours' reaction. Due to the basicity of the support, the y-butyrolactone product would be hydrolyzed, which led to the drop in lactone selectivity as compared to Au/TiO2 and Au/FeOx. Selectivity of AuAlT increased firstly and then dropped with the prolonging of reaction time. Therefore, it was concluded that basicity was beneficial to the oxidation of alcohols; however, it was detrimental to the lactone selectivity. AuAlT can also be used for the oxidation of other diols, for example,1,5-pentanediol and 1,2-benzedimethanol.5. Synthesis of multi-component Fe-Al-O oxides and their application as support for gold catalysts in the oxidative dehydrogenation of 1,4-butanediol to y-butyrolactoneThe multi-component Fe-Al-O oxides were prepared by encapsulation (FeAlOx) and impregnation methods (Fe2O3/Al2O3 and Al2O3/Fe2O3). According to the characterization results, it was found that when prepared by the impregnation method, there was no interaction between Fe2O3 and Al2O3, and the support was the simple mixture of Fe2O3 and Al2O3. For FeAlOx, Fe2O3 was encapsulated by Al2O3, and there were no isolated Fe2O3 particles observed, indicating that interaction was formed between Fe2O3 and Al2O3. It was also confirmed by the XPS results that FeAlOx surface was enriched with Al, due to its low surface Fe/Al ratio (1:18), which was far lower than that of the bulk (1:3.4) and the other two support prepared by the impregnation method. Crystal phase of FeAlOx wasγ-Fe2O3 andγ-Al2O3. Gold can be highly dispersed on the support surface with average diameter of 3.5 nm, and gold-support interaction was observed. Au/FeAlOx showed high activity in the oxidative dehydrogenation of 1,4-butanediol, and its catalytic activity was better than that of the mixture of Au/Fe2O3 and Au/Al2O3, very close to that of the most active Au/Al2O3 catalyst. In addition, its selectivity was higher than that of Au/Al2O3 and Au/Fe2O3. For Fe-Al-O supported gold catalysts, the activity was in the order: Au/FeAlOx> Au/Fe2O3/Al2O3-Au/Fe2O3+Au/Al2O3> Au/Al2O3/Fe2O3.By adjusting the Fe/Al molar ratio of Au/FeAlOx catalysts from 1:1 to 1:16, the surface Fe/Al ratio changed from 1:8 to 1:41. When Fe/Al was 1:1, the amount of Al2O3 was not enough to encapsulate all the Fe2O3 particles and it can be seen from the TEM images that there were some isolated Fe2O3 particles. Increasing Al content to Fe/Al ratio of 1:4, Al2O3 can cover all the Fe2O3, however, further increase in Al content would lead to the weakening of the Fe-Al interaction and the stronger support basicity, which was not beneficial to the reaction. The optimized Fe/Al was found to be 1:4.