|Other Abstract||Pyruvic acid and its derivatives are organic intermediates, widely applied in chemical, pharmaceutical and food industries. With the development of social economy, the commercial demand for pyruvate has been increasing. Currently，pyruvic acid is produced mainly via dehydrative decarboxylation of tartaric acid. A large amount of SOx is produced in the process, which results in serious environmental pollution. Moreover, high energy consumption, high cost, low atomic efficiency and low yield are the main defective problems in this technology. So this process will be eliminated. However, a process for production of pyruvic acid or its esters by the oxidative dehydrogenation (ODH) of lactic acid (LA) or esters is developed. The reaction pathway shows excellent atomic efficiency and provides an eco-friendly route and has characteristic of low cost, which is recognized as the most ideal production technology and the most promising process.
In this thesis, the MoVNbOx based composite oxides were synthesized by a slurry method and used as catalysts for the catalytic oxidative dehydrogenation of ethyl lactate (EL) to ethyl pyruvate (EP) in a continuous fixed-bed reactor under atmospheric pressure. Firstly, the catalytic activities of MoVNbOx/TiO2 composite oxides and the selectivity of productions had been studied. In addition, we studied the reaction pathways in the catalytic system and the reaction kinetics. Secondly, through to the modification of the MoVNbOx catalysts by changing preparation conditions, we studied the relationship between the catalytic characterization and performance. The main conclusion of this thesis is summarized as follows:
(1) MoVNbOx/TiO2 catalyst presented much higher conversion rate of ethyl lactate and relatively lower apparent activation energy in comparison with that over MoVNbOx. With the increase of the ethyl lactate conversion, the selectivity for ethyl pyruvate as the target product maintained stable in the range of 90–93%, and ethanol was the most abundant byproduct with the selectivity around 4.0%. The selectivities of other products were all below 1.5%. According to the distribution of reaction products, the reaction path network was established, in which the oxidative dehydrogenation of ethyl lactate into ethyl pyruvate was the dominant reaction. Besides, the hydrolysis and further oxidation of ethyl lactate and ethyl pyruvate were side effects, which resulted in the production of byproducts，such as ethanol, lactic acid, pyruvate and acetic acid. The reaction kinetics of the experimental results showed that the conversion rate of ethyl lactate was first order reaction towards the O2 concentration but none of any relation with the concentration of ethyl lactate in the kinetic condition. The analysis of elementary reaction steps of ethyl lactate into ethyl pyruvate and the derivation of reaction rate equation were performed based on Mars-van Krevelen (MvK) redox mechanism. The results showed that the conversion rate of ethyl lactate was first order reaction, which was only related to the partial pressure of O2 in the precondition of considerably abundant surface oxygen vacancies. The results were in good accordance with the experimental kinetics results.
(2) The MoVNbOx composite metal oxides were prepared at calcination atmosphere of 600 ºC in nitrogen and air atmosphere, respectively. The studied showed that N2-calcined catalyst exhibited remarkable better activity than air-calcined catalyst. The conversion of EL and the selectivity of EP were 90% and 91%, respectively. The reaction was carried out continuously for 50h, the conversion of EL and selectivity of EP were maintained, which showed that the catalyst had a remarkable catalytic stability. Through a series of characterization of catalytic materials found that N2-calcined MoVNbOx catalysts were significantly different to the recorded of air-calcinated MoVNbOx catalysts. N2-calcinated MoVNbOx catalysts existed MoO2 phases and rutile structure of Mo-V phase. Air-calcinated MoVNbOx catalysts existed Mo5O14 phase, α-MoO3 phase and V2MoO8 phase. Compared to air-calcinated MoVNbOx catalysts, N2-calcinated MoVNbOx catalysts presented relatively higher low-temperature reducibility and specific surface area and had more available V4+ species on the surface than air-calcinated MoVNbOx catalysts. These were mainly responsible for the remarkable enhancement of the catalytic activity.
(3) A series of MoVNbOx catalysts were prepared by changing the calcination temperatures at N2 atmosphere. The results showed that the conversion of ethyl lactate first increased and then decreased as the calcination temperatures from 400 ºC to 700 ºC. Among all samples, the catalyst at 600 ºC of calcination temperatures exhibited the best catalytic performance. The results of catalytic material characterization showed that the structure of catalysts, which were calcinated at 500 ºC, 600 ºC and 700 ºC, was no significant difference. However, the catalysts were amorphous at calcination temperature of 400 ºC. MoVNbOx catalysts at calcination temperatures of 600 ºC presented relatively higher lowe-temperature reducibility and specific surface area than other catalysts. These reasons may be lead to high catalytic activity of the N2-calcinatied catalyst.|