| Ethanol is an important solvent and industrial chemical. As a high-octane rating and pollution free fuel, it can be mixed with gasoline to fuel vehicles. The United States and Brazil have been promoting the development of ethanol fuel for decades, and China is also quickening her steps in this field and has gained remarkable fruits. Traditionally, ethanol was produced by means of ethylene hydration and fermentation. Both routes are unattractive for China. The feed of the former one is ethylene, which depends on petrochemical; and the latter one consumes large quantity of food. On one hand, China is a country with limited farm land and a large population. On the other hand, the distribution of China’s energy resources is "abundant in coal and lack of oil". Thus, it’s a topic of significance to promote the research on the production of ethanol from acetic acid hydrogenation via coal based syngas and methanol to relieve the tense of oil shortage and save food, and provide support for the development of national economy.Presently, the studies on acetic acid hydrogenation are mainly focused on the adsorption behavior of acetic acid and the reaction mechanism. Among the catalysts used for acetic acid hydrogenation, the noble metals, especially Pt, have attracted much attention due to its high activity, low loading and relative price advantage. But, other improvements are still needed because that the reaction network of acetic acid on Pt is complicate and will lead to the production of by products. What’s more, the addition of promoters and the choice of operation conditions are also critical to the performance of Pt catalysts in acetic acid hydrogenation.In this paper, the effect of Sn addition was studied in the gas phase hydrogenation of acetic acid on Pt/Al2O3. The catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), transmission electron microscope (TEM) to investigate the influence of Sn addition on the physical property, the reducibility, the dispersion of Pt and the property of particles on the surface of the catalysts. The impacts of preparation procedure, shaping method on the acetic acid hydrogenation performance of the catalysts were studied. The reaction dynamics experiment was performed in a fixed-bed reactor and the stability of the selected catalyst was evaluated. The role of K promoter was explored.(1) The effect of Sn addition on the performance of Pt/Al2O3in acetic acid hydrogenationMonometallic Pt/Al2O3and Sn/Al2O3and bimetallic PtSn/Al2O3with different Pt/Sn ratio were prepared by impregnation and their performance in acetic acid hydrogenation was tested in a fixed-bed reactor. Furthermore, the effects of operation conditions on the performance of the1Pt1Sn/Al2O3were evaluated. Sn addition is beneficial to the performance of the catalysts, the conversion of acetic acid and the selectivity of ethanol are improved while the occurrence of side reactions is inhibited. The performance of the catalysts is related to the Pt/Sn ratio,1Pt1Sn/Al2O3shows the highest activity. The conversion of acetic acid is enhanced with the rise of temperature and pressure, the rise of temperature causes the decrease of ethanol selectivity and the increase of ethyl acetate selectivity and the effect of pressure on the selectivity of the products is opposite that of temperature. When the LHSV of acetic acid increased, the conversion of acetic acid and the selectivity of ethyl acetate decreased while the selectivity of ethanol increased.The catalysts were characterized by XRD, TPR and DRIFTS and the results are as follow: there are strong interactions between Pt and Sn and the introduction of Sn will lead to the coverage of Pt; their co-reduction can form PtSn alloy and Pt-SnOx species. This phenomenon lowered the dispersion of Pt and inhibited the decarbonylation of acetic acid, which can be attributed to the remarkable improvement of PtSn/Al2O3in acetic acid hydrogenation.(2) The effect of preparation procedure and shaping method on the performance of PtSn/Al2O3in acetic acid hydrogenationCatalysts with same loading were prepared by co-impregnation and sequential impregnation and characterized by N2adsorption, H2-TPR, CO-DRIFTS and so on. The procedure of catalyst preparation can affect the pore distribution, the reducibility and Pt dispersion of the catalysts, as well as their catalytic performance. Best acetic acid hydrogenation performance was achieved on catalyst prepared by co-impregnation. Two groups of co-impregnated catalysts with industrial size were shaped by different methods and tested in the hydrogenation of acetic acid. Results showed that catalysts shaped by tablet machine show better performance, and the smaller the size the higher the activity of the catalysts.(3) The reaction dynamics and the stability of PtSn/Al2O3The reaction kinetics experiments were carried out at239.9～284.2℃,0.5～4.5MPa, LHSV0.73～2.18h-1, n(H2)/n(CH3COOH)1.6～13.53. The conversion of acetic acid is in the range of5%～65%, the main products are ethanol and ethyl acetate and the total selectivity of aldehyde and decomposition products is less than2%. The kinetic equation derived from a Langmuir-Hinshelwood type mechanism was proposed. The estimation of the values of the kinetics parameters was determined with a multivariable nonlinear regression method, the Levenberg-Marquardt algorithm, and the kinetic model has the following rate expression:The results of statistical tests and relative error analysis show that the finial kinetic model is acceptable. The stability test performed at255℃,2.5MPa, LHSV1.8h"’and n(H2)/n(CH3COOH)=10during30days showed that the catalyst is stable.(4) The effect of K promoter on the performance of PtSn/Al2O3PtSnK/Al2O3catalysts with different K content were prepared by the combination of co-impregnation and sequential-impregnation and tested in the hydrogenation of acetic acid. Characterizations of the catalysts show that the addition of K can lower the acidity of the catalysts, cover the surface of Pt. Consequently, the activity and product selectivity on PtSnK/Al2O3catalysts is different from that of PtSn/Al2O3, and the disparity is dependent on the content of K. |
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