Designation And Investigation Of The Catalystic System For The Selective Hydrogenation Of Phenol

Selective hydrogenation of phenol is an important reaction pathway in industry for production of cyclohexanone/cyclohexanol. Cyclohexanone is of great commercial significance as a key raw material in the production of both caprolactam for nylon 6 and adipic acid for nylon 66. The manufacture of cyclohexanone has the alternative routes of the oxidation of cyclohexane or hydrogenation of phenol. The former route requires high temperatures, high pressures and generates undesirable byproducts that lower the product yield and complicate the recovery/separation steps. Cyclohexanone made by the selective hydrogenation of phenol has better quality. Under mild conditions, this route has much broader industrial prospects. And the cyclohexanone could also be obtained through the dehydrogenation of another productive cyclohexanol. However, designing highly efficient, environmental benign and low-cost catalytic system for the selective hydrogenation of phenol and investigating its related mechanism are still challenging in this field. In this work, the catalysts Pd@mpg-C3N4 with palladium nanoparticles deposit on mpg-C3N4 and the CoOx@CN with cobalt oxide loaded on the carbon nitride material have been developed. They have exhibited excellent catalytic activity and selectivity in the selective hydrogenation of phenol. Besides, the selectivity of this reaction can be tuned by the acid-base properties of the reaction medium.C3N4 is a kind of carbon nitride material with graphite flake structure. Its high content of nitrogen can change the charge distribution of the loaded metal, making it a good supporter. We selected the disordered mesoporous silica as template and obtain the carbon nitride material (mpg-C3N4) with mesoporous structure. An efficient and convenient ultrasonic-dispersion method was used to deposit the palladium nanoparticles uniformly on the mpg-C3N4. Upon treatment of phenol under mild condition (0.1 MPa H2 at 65℃), a 100% conversion of phenol with a 99% selectivity of cyclohexanone was obtained by the catalyst which possessed proper surface area and lower calcination temperature. Its catalytic activity is obviously higher than the common used Pd@C, Pd@Al2O3 and Pd@MgO. The activity of this catalyst is closely related to the temperature and its involved reaction kinetics has been represented by a standard pseudo-first-order approximation with the apparent activation energy of 35.9 kJ/mol. Experiments show that the mpg-C3N4 absorbs the phenol much stronger than the adsorption of cyclohexanone, which is favorable for the selective hydrogenation of phenol to product cyclohexanone. In the large scale reaction, the catalyst can also promote the formation of cyclohexanone effectively with better reusability. Besides, the resonance rearrangement of cyclohexenol and cyclohexanone has been confirmed by the isotopic tracing experiment. Moreover, the water, could accelerate the reaction via proton exchange according to the DFT calculation.Due to the resonance rearrangement of cyclohexenol and cyclohexanone in the selective hydrogenation of phenol is a sensitive balance to the acid or base, we pretend to tune the activity and selectivity of this hydrogenation by using variety of acid or base system. Results show that, organic carboxylic acid could effectively bond with the intermediates cyclohexenol in the reaction. The cyclohexenol could immediately convert into the stable cyclohexanone via the double hydrogen transformation, thereby giving the high selectivity. Besides, some carbon atoms in the organic carboxylic are lack of the electron, which present similar nature as the Lewis acids. This property could polarize the benzene ring of phenol, resulting in the higher conversion. The more deficient electron the carbon atoms have, the higher conversion of this reaction will be. In the basic system, the phenol could easily interact with the hydroxyl anion, which decrease the concentration of substrate and lead to the lower conversion of phenol. Besides, the a-H atom of cyclohexanone could be abstracted by the bases. This is favorable for the cyclohexanone to convert into cyclohexenol, resulting to higher yield of cyclohexanol.As the noble metal-based catalysts are expensive, we try to develop the low-cost non-noble metal Co-based catalyst. Economic and green biological sugar glucosamine hydrochlorides are selected as the precursor. With hydrates of CoCl2 as the metal precursor and carbon nitride as template, the carbon nitride catalysts supported on Co (CoOx@CN) can be obtained by one step with high temperature calcination. The catalyst has the structure of tube and sheet with 27% content of Co, which exists mainly in the form of Co3O4. Its surface area reaches 340 m2/g. With 3 MPa H2 at 150℃, this catalyst reaches perfect activity and the selectivity of cyclohexenol (98%) in the selective hydrogenation of phenol. This catalytic activity is obviously higher than the Co supported on carbon nanotubes or that on the activated carbon. The high catalytic efficiency of CoOx@CN is attributed to the active species Co3O4. Through the comparison of the catalytic activity of Co, CoO and Co3O4, and the relationship between the reaction rate of Co3O4, monitoring at different reaction time, and the metal valence state, we found that the Co based catalyst adsorbs phenol by Co3O4. Then the Co3O4 was situ reduced into Co and the hydrogen was activated with the completion of the hydrogenation of phenol. In addition, the effect of reaction pressure, reaction temperature and solvent on the catalytic activity of hydrogenation was discussed. Results show that higher conversion of phenol could be achieved with high pressure, high temperature and the protic solvent.In this work, the catalysts Pd@C3N4 and CoOx@CN were obtained by loading noble/non-noble metals on the carbon nitride material, respectively. High selectivity of cyclohexanone in the hydrogenation of phenol catalyzed by Pd@C3N4 has been achieved, while high selectivity of cyclohexenol for that catalyzed by CoOx@CN. The reactivity of Pd@C in the different acid/base system for the hydrogenation of phenol was also studied to explore the reaction mechanism. This research results provide valuable information for designing high efficient catalytic system for the hydrogenation of phenol in the future.