Design of palladium-based heterogeneous catalysts for selective oxidation of alcohols
Date of Issue2016
School of Chemical and Biomedical Engineering
Selective oxidation of alcohols into their corresponding aldehydes and acetones have always been an important issue in chemical engineering as the obtained products are essential raw materials for the synthesis of other fine chemicals. Particularly, the selective oxidation of benzyl alcohol to benzaldehyde has attracted much interest in the last several decades due to the importance of benzaldehyde as a precursor for the synthesis of other fine chemicals. Similarly, ethanol selective oxidation produces a variety of oxide products including acetaldehyde, acetone, acetic acid, ethylene oxide, diethyl ether and ethyl acetate, many of which are of paramount importance in chemical industry. Much efforts have been devoted to producing highly selective carbonyl compounds with high yields in the past several decades. One significant improvement in alcohol transformation reaction is the evolution of oxidants. Primarily, stoichiometric quantities of expensive inorganic reagents including ruthenium oxides, chromium reagents, permanganates and some others are employed as oxidants. Now inexpensive, environmentally benign oxidants like pure oxygen, hydrogen peroxide are used. Another big improvement, also the most important step in catalytic engineering is to choose the appropriate catalysts. In this dissertation, a series of palladium catalysts supported on commercial available titanium dioxide nanoparticles and titanium dioxide nanowires synthesized by a molten salt flux method are prepared by two common methods, urea-assisted deposition-precipitation and incipient wetness impregnation methods. Selective oxidation of benzyl alcohol was employed as a model reaction to investigate the performance of as-prepared catalysts. It is found that catalysts prepared by the impregnation method have larger metal loadings with a broader size distribution than that prepared by the deposition-precipitation method, resulting in much higher activity with lower selectivity towards benzaldehyde. The lower loadings of palladium nanoparticles using deposition-precipitation method was due to the greater repulsion between support materials and palladium complex at elevated pH. The influences of reaction parameters including reaction temperature, reaction duration, catalysts/substrate molar ratio and oxygen flow rate are studied. For all the catalysts, benzyl alcohol can be transformed by two seemly independent routes, with benzaldehyde the main product. No further oxidative compounds are obtained, suggesting the excellent controllability of as-prepared catalysts in inhibiting deeper oxidation reaction. It is believed initially only the disproportionation route is active and as the reaction proceeds, the oxidation reaction is triggered, gradually match and finally exceeds the disproportionation route. Due to the existence of mass transfer problems, all of them have their own optimum reaction parameters for catalysts/substrate weight ratio and oxygen flow rates. Also, catalytic results showed that water is the main reason for catalysts deactivation and removal of the toluene produced at early stages in time might be able to fully inhibit the disproportion route, resulting in almost 100% selectivity towards aldehyde. Although catalysts based on titanium dioxide nanoparticles have better activity compared to their nanowire counterparts, much lower aldehyde selectivity is its main disadvantage. With 100% selective towards benzaldehyde and excellent stability, further modification of palladium catalysts supported on titanium dioxide nanowire have the potential to be used in large-scale synthesis of benzaldehyde through benzyl alcohol oxidation. The introduction of basic sites can greatly improve the adsorption and dispersion of noble metal catalysts to the supporting materials, thus modify the catalytic activity. In this thesis, a series of nitrogen-doped carbon nanotubes were successfully prepared using commercial available carbon nanotubes under a mild hydrothermal reaction system. Different kinds of nitrogen sources, including hydrazine hydrate coupled with ammonia, urea, hexamethylenetetramine, dicyandiamide, hydroxylamine chloride, are employed to modify the interesting tubular structure. It was demonstrated that different nitrogen sources can change the chemical and electronic properties of pristine carbon nanotubes with different extents. Palladium ions have a greater tendency to absorb on the modified CNTs due to the existence of various types of nitrogen atoms, resulting much higher activity and higher selectivity towards aldehyde compared to its unmodified counterparts. To better understand the evolution of nitrogen doping, nitrogen-doped carbon nanotubes prepared with increased weight ratios of hydroxylamine chloride towards carbon nanotubes are prepared. All the palladium nanoparticles supported on modified carbon nanotubes have a smaller mean size and narrower distribution compared to unmodified ones. Even quite a small amounts of HACl can decrease both of them by half. With the increase of HACl, gradually bigger particles are obtained while the distribution can be maintained. When appropriate amounts of nitrogen sources are used, dramatic changes resulting abnormally larger particles are achieved. Such changes can also be reflected by the activity towards benzyl alcohol oxidation. Changes in benzyl alcohol conversion have exactly the same pattern as that of the mean particle size, with almost 2.5 times enhancement for 2Pd@CNT-HACl-3. Different from the trend in conversion, monotonically increase in benzaldehyde selectivity is observed until 100% is realized over 2Pd@CNT-HACl-5 catalysts. These results suggest gradually changes in carbon nanotubes can be realized by choosing appropriate type of nitrogen source and tuning reaction conditions through a simple and easy hydrothermal reaction, and the resulting carbon nanotubes are promising supporting materials for other oxidation reactions. A series of Pd/HY bifunctional catalysts have been successfully prepared with deposition-precipitation method for selective oxidation of ethyl alcohol using molecular oxygen as an efficient oxidant. The properties of these catalysts were examined through TEM, XRD, FTIR, TGA and stability test. Characterization results proved that Pd nanoparticles are effective in suppressing the coke deposition in ethanol selective oxidation and a small amount of coke deposited on the catalyst may enhance the stability due to the suppressed Pd nanoparticle agglomeration. In conclusion, Pd/HY bi-functional catalyst was demonstrated as a unique catalyst for ethanol oxidation to afford high yield one-step synthesis of acetaldehyde and ethyl acetate, especially the ethyl acetate formation, 91.8% of ethyl acetate selectivity at 150 oC and 84.2 % of ethyl acetate yield were achieved over 2Pd/HY catalyst at 210 oC. The product selectivity can be readily tuned by changing the reaction condition parameters. The reverse hydrogen spillover effect was identified to take place between Pd nanoparticles and zeolite support, which can well balance the oxidative activity and acidity at an appropriate reaction temperature. Pd/HY catalyst was remarkably stable at low reaction temperature during the reaction and only a small amount of coke was deposited.