The Study Of Copper-Based Catalysts In Dehydrogenation Of Cyclohexanol To Cyclohexanone

Cyclohexanone is a major intermediate for producing caprolactam and Nylon. It is also an excellent organic solvent, and also a good industrial raw material. So the catalytic dehydrogenation of cyclohexanol has gained much important in recent year. Copper-based catalyst, which is an excellent catalyst system for dehydrogenation, has been used for this specific reaction. It would be important in academic and actual meaning for studying this catalyst system.In the paper, the SiO₂ was used as support. An impregnation method, a Sol-Gel method and a deposition-precipitation method were used in the preparation of the Cu/SiO₂ catalysts. The alkali metal oxides and alkali earth metal oxides, etc. were used as dope to modify the Cu/SiO₂ catalyst prepared by impregnation method. These copper-based catalysts were characterized by XRD, BET, EPR, XPS, H₂-TPR, H₂-N²O titration and NH3-TPD techniques. We studied the catalysts properties changing with the different catalysts preparation, the catalytic behaviors in the reaction of cyclohexanol dehydrogenation, and investigated the active sites of dehydro- genation, dehydration and aromatic reaction in detail. We emphasized to study the rule of copper-based catalyst made by impregnation method, and contrast study with catalysts made by other method.The major results are in following:1. It was showed that the nanjing-SiO₂ was a good support, high conversion of cyclohexanol and selectivity of cyclohexanone were made on the Cu/SiO₂ catalyst prepared with it. Copper metal is the active component for the dehydrogenation of cyclohexanol. The catalytic activity was affected by the amount of dispersed copper metal and its state. The best amount of supported copper was 10% 15%. The promoter is an important factor for effecting on catalytic activity, selectivity and stability property. The addition of alkali metal oxides and alkali earth metal oxides effected decreased the amount of acidic sits on catalysts surface, so the dehydration of cyclohexanol was restrained, and the selectivity of cyclohexanone was increased, the Cu-K₂O/SiO₂ catalyst showed good catalytic property.2. The preparation condition can affect the state of active site. The short impregnation time is favorable for producing big copper particle. The impregnation for 5 h is favor for getting high dispersion Cu, so the catalysts showed good catalytic property. The preparation of the high dispersed CuO precursors is very important for getting catalysts with high catalytic activity, the low calcined temperature can preparation high dispersion CuO, and the favorable temperature is the range between 300 500℃. The reduction temperature is another important factor for affecting the particle size and dispersion of Cu⁰. The high reduction temperature can make particle size of Cu⁰ increase, and show low conversion of cyclohexanol. At the low reduction temperature, the CuO can not be reduced in full, so the catalytic property was poor. The CuO can not be reduced completely during the period of short reduction, but the catalytic property would be decreased when the reduction time beyond 3 h, for the size of Cu increasing. The results of the reaction showed that the catalyst which was reduced at 300℃for 3 h is the best. The regular study results of effects of the preparation on the structure and the property of catalysts were little in the literature.3. We used the Sol-Gel method in the first to prepare Cu/SiO₂ catalyst, and the catalyst was studied in the dehydrogenation of cyclohexanol. High dehydrogenation activity, selectivity and stability were obtained.4. The results of the reaction showed that the appropriate reaction temperature is in the region of 260 280℃. Although the conversion of cyclohexanol was high on high reaction temperature, but the size of copper will quick sinter, which made catalysts inactively. The appropriate GHSV was 3000 h^-1. The inert gas carrier also can increase the catalytic activity of catalysts.5. The active sites for dehydrogenation of cyclohexanol, and the reason for cyclohexanol dehydration and cyclohexanol aromatization. The results showed that Cu⁰ is the active sites for dehydrogenation of cyclohexanol, and the catalytic dehydrogenation activity is relation with the copper metal surface. The acidic sites over catalysts lead to dehydration of cyclohexanol, and the more acid-sites the higher selectivity of cyclohexene. The addition of appropriate basic oxide can decrease the acid-sites on catalysts surface, it restrain the dehydration of cyclohexanol, and improve the selectivity of cyclohexanone. For the aromatization of cyclohexanol to phenol, the results on Cu-ZnO/SiO₂ catalyst showed that the Cu+ was the active sites for phenol, it was the interaction between copper species and ZnO that made some Cu+ existed in the surface of catalyst after reduction. It was first report that the Cu+ was responsible for the aromatization of cyclohexanol. In order to get a performance catalyst, it was important that make the Cu⁰ in high dispersion, and adjust the catalyst's acid-base property.6. We prepared and investigated the copper-based catalysts using sol-gel and deposition-precipitation methods. We also compared these results with those obtained from the impregnation method. The structure and the state of species over catalysts were characterized by BET, XRD, FTIR, EPR, H₂-TPR, H₂-N²O titration and XPS techniques. The catalytic properties of these catalysts were studied on cyclohexanol dehydrogenation. The results show that the Cu⁰ clusters and a lot of isolated Cu²⁺ are both existed on the surface of catalysts prepared by Sol-Gel method, the Cu⁰ clusters are the active site for cyclohexanol dehydrogenation. But for the isolated Cu²⁺, they are no activity, although they exist as high dispersion in the SiO₂, because they was enveloped within the SiO₂ matrix and can not be reduced during reaction. The catalysts prepared by impregnation and deposition-precipitation method showed high conversion of cyclohexanol, because the Cu²⁺ clusters are dominant on the catalyst and it can be easily reduced to Cu⁰ after reduction, leading to higher conversion of cyclohexanol, the catalyst prepared by impregnation showed the highest catalytic activity. For the Na was introduced in the process of deposition-precipitation, which decreased the acidity of catalyst surface, this catalyst showed the highest selectivity of cyclohexanone.7. It is the key that copper high disperse on the support for the good catalytic activity. We attempted that MCM-41 and MCM-48 which have big surface area used as support, and the catalysts were used in dehydrogenation of cyclohexanol. The catalysts showed good catalytic properties.8. We also explored Cu nanoparticle, ZnO nanoparticle and mixed Cu, ZnO nanoparticles as catalysts using in dehydrogenation of cyclohexanol. The Cu nanoparticle showed low catalytic activity and stability, it is because that the particle size of Cu increased during reaction. The mixed Cu and ZnO catalyst showed high catalytic dehydrogenation activity and stability, but some cyclohexene was also obtained, it may be related with Cu²⁺. The mixed catalyst was characterized by EPR, XPS, XRD, BET techniques. The results showed that there is some interaction between Cu and ZnO nanoparticles. This is a new exploring study, this study in the dehydrogenation of cyclohexanol is not reported up to now.