Study On The Catalytic Performance Of Transition Metal Catalysts For The Reaction Of Direct Transformation Of Ethanol To Ethyl Acetate

Ethyl acetate is an important chemical materials and a useful solvent that is widely used in adhesives, medicines, and extraction solvent of organic acids. Three main commercially processes are currently available to achieve the production of ethyl acetate, including esterification of ethanol with acetic acid, addition of ethylene to acetic acid, the Tishchenko reaction of acetaldehyde. However, there are still several problems associated to these processes, such as the toxicity of acetaldehyde or the corrosion of acetic acid. The process of direct transformation of ethanol to ethyl acetate can solve these problems, because of using only ethanol as feedstock.In this work, various transition metal catalysts are used in the reaction of direct transformation of ethanol to ethyl acetate, including molybdenum-based and copper-based catalysts, the catalytic performances of above catalysts have been investigated. The relationship between the structure, surface texture and the catalytic performance of catalysts was studied by combining different characterization means. The nature of active sites on the catalysts and the reaction mechanism were also discussed in this article. The main results and conclusions are as follows.1. Study on the catalytic performance of MoSx/C catalysts for ethyl acetate synthesisIn this part of work, we studied the activity of molybdenum sulfide catalysts in the reaction of direct transformation of ethanol to ethyl acetate. The results showed that the conversion of ethanol can reach 91.5% and the selectivity to ethyl acetate can reach 49.4% over MoSx/C (Mo=8.7 wt%) catalyst under following reaction condition: T=320℃, P=1 atm, LHSV=1 h-1. According to the literatures and our experimental facts, we believe that CUS can provide dehydrogenation active sites and SH- can provide acid sites for this reaction. The study has also indicated that formation of highly dispersed molybdenum species, which were observed on the activated carbon support with HNO3 pretreatment, was favorable for high catalytic activity. The treatment of activated carbon with HNO3 produced an increase of the surface oxygen groups. Interactions between metal and oxygen functional groups were developed and then favored the dispersion of active phase.The effect of preparation parameter on the catalytic performance of MoSx/C catalysts has been systemically investigated. The results show that MoSx/C catalysts which were prepared by sulfidation of (NH4 ) 6M o 7 O2 4 at 400℃for 2 h have a good catalytic performance. At lower sulfurization temperature (300℃), there are two molybdenum species (Mo5+ and Mo4+) on the surface of the catalyst, Mo5+ species is not conducive to the formation of ethyl acetate. The particle sizes of the active species become larger at higher sulfurization temperature (500℃or 600℃), so the catalytic performance decrease. MoSx/C catalyst which was sulfided at 400℃obtained the best catalytic performance. At shorter sulfidation time (1 h), the concentration of SH- on the catalyst surface is relatively small, lack of Br?nsted acid sites resulting in lower catalytic performance. MoSx/C catalyst which was sulfided for 2 h obtained the best catalytic performance. The drying atmosphere did not have much effect on the catalytic performance of the catalyst.2. Study on the catalytic performance of Cu-Zr-O catalysts for ethyl acetate synthesisIn this part of work, we studied the activity of copper-zirconium catalysts in the reaction of direct transformation of ethanol to ethyl acetate. The results showed that the conversion of ethanol can reach 70.9% and the selectivity to ethyl acetate can reach 78.8% over Cu-Zr-O (Cu︰Zr = 1) catalyst under following reaction condition: T=270℃, P=1 atm, LHSV=1 h-1. According to the literatures and our experimental facts, we believe that metal Cu can provide dehydrogenation active sites and ZrO2 can provide acid sites for this reaction. The prereduction temperature has a great influence on the particle size of Cu0 species. With an increase of reduction temperature, the particle size of Cu0 species increases. Changes in Cu0 particle size not only affect the rate of ethanol conversion, but also the selectivity of ethyl acetate. The result shows that Cu0 species not only involved in the dehydrogenation of ethanol to acetaldehyde, but also the process of ethyl acetate synthesis from acetaldehyde. So we believe that the process of ethyl acetate synthesis from acetaldehyde occurred on the interface of Cu0 and ZrO2 .The catalytic performances of Cu-Zr-O mixed oxides were considerably influenced by changing the molar ratio of Cu to Zr. The highest selectivity to ethyl acetate was found over catalyst which has the highest ZrO2 content. However, all the Cu-Zr-O catalysts show similar conversion of ethanol, which should be due to the fixed Cu content for these catalysts during the catalytic test. The characterization results show that the concentration of acid sites and also Cu+ species increase gradually with increasing ZrO2 content. These results suggest the positive correlation between the selectivity to ethyl acetate on Cu-Zr-O catalysts and the concentration of Cu+ species. According to the literatures, we believe that the presence of Cu+ species may be regarded as Lewis acidic sites, which are involved in acetaldehyde chemisorption, thus resulting in the improvement of ethyl acetate selectivity. Therefore, we suppose that the co-existence of Cu0 and Cu+ species in the Cu-Zr-O catalyst might play a synergic interaction for converting ethanol to ethyl acetate. The effect of preparation parameter on the catalytic performance of Cu-Zr-O catalysts has been systemically investigated. The results show that Cu-Zr-O catalysts which were prepared by coprecipitation of Cu(NO3)2 and Zr(NO3)4 with the addition of Na2CO3 at 60℃have a good catalytic performance. The characterization results show that the good catalytic performance can be attributed to smaller particle size of copper species and higher surface area of the catalyst. The calcination atmosphere did not have much effect on the catalytic performance of the catalyst. 3. Study on the other catalysts in ethyl acetate synthesis reactionIn this part of work, Cu-Zr-Co-O, Cu-Al-O and Mo2C/C catalysts were applied to the reaction of direct transformation of ethanol to ethyl acetate. We found that addition a small amount of CoO to Cu-Zr-O catalyst can improve the catalytic performance. The characterization results showed that the good catalytic performance can be attributed to the smaller particle size of copper species and also the appropriate Lewis acidic sites on the surface of the catalyst. Cu-Al-O catalysts have good stability. According to our experimental results, the conversion of ethanol can reach 55.1% and the selectivity to ethyl acetate can reach 63.5% over Cu-Al-O (Cu︰Al = 1) catalyst under following reaction condition: T=270℃, P=1 atm, LHSV=1 h-1. Mo2C/C catalysts were prepared by carbonization of Mo/D293 sample at 900℃under nitrogen. The results showed that the conversion of ethanol can reach 82.5% and the selectivity to ethyl acetate can reach 40.1% over this catalyst under following reaction condition: T=300℃, P=1 atm, LHSV=1 h-1. Although the catalytic performance of Cu-Al-O catalyst and Mo2 C/C catalyst is not good enough, it is still a useful attempt to explore the good catalytic materials for the reaction of direct transformation of ethanol to ethyl acetate.