Construction Of Hydrophobic Alumina Material And Its Application On Phenol Hydrogenation

Alumina(Al2O3)is widely used in the petrochemical industry as an adsorption material,catalyst carrier,and catalyst due to its high specific surface area,abundant pore size distribution,and chemical stability.However,hydration has become a crucial problem when Al2O3 is used in high temperature and high-pressure water environments.The hydration results in a sharp decrease in specific surface area and collapse of pore structure,which significantly affects the performance of Al2O3.Especially as a catalyst carrier,the structural collapse of the Al2O3 material caused by hydration will lead to the coating and agglomeration of active sites,and affect the mass transfer process of reactants and products,thereby affecting the catalyst’s activity,selectivity,and stability.Consequently,developing structurally stable alumina materials is of great significance for improving the efficiency of the petrochemical process.This study explored the hydrothermal stability of Al2O3 materials.Hydrophobic Al2O3 materials were prepared by the self-assembly method in this paper.The hydrothermal stability of the materials was tested under the condition of an aqueous reaction.This study explored the relationship between the hydrothermal stability of Al2O3 and the surface hydrophobicity of materials.Based on hydrophobic Al2O3 structure,a hydrophobic Pd/Al2O3 catalyst was constructed and applied to phenol hydrogenation reaction to prepare high valueadded cyclohexanol.In this paper,we discussed the reactivity of hydrophobic catalysts in the hydrogenation of phenol,which revealed the mechanism of hydrophobic modification on the surface of the catalyst to improve its catalytic performance.(1)Dodecyl phosphoric acid(PA)is applied to modify the surface of Al2O3 to obtain a series of hydrophobic Al2O3 materials(Al2O3-PAs).The characterization results of nuclear magnetic,X-ray photoelectron spectroscopy,and inductively coupled plasma emission spectroscopy show that PA is mainly modified by P-O-Al bonds on the Al2O3 surface to form a monolayer film structure.The maximum modification amount is 1.45 molecules/nm2,and the contact angle test results show that the hydrophobicity of the modified Al2O3 material can achieve 140°.The construction of the hydrophobic monolayer film on the surface effectively isolates the contact between water and the material during the reaction,where Al2O3-3PA and Al2O3-5PA can be maintained at a hydrothermal temperature of 200℃ for 10 hours without the hydration reaction of aluminum oxide.As for Al2O3-1PA,only a small part of the Al2O3 undergoes a crystalline transformation under the same conditions.The results obtained by BET to characterize the materials before and after modification show that as the amount of modification increases,the specific surface area and pore volume of the material decrease rapidly.Specifically,the specific surface area and pore volume of Al2O3-5PA decrease to 24%and 21%of the original Al2O3,while the specific surface area and pore volume of Al2O3-1PA retained 60%and 43%of the original alumina.The decrease in pore volume will greatly affect its performance.Therefore,we studied the effect of temperature treatment on the hydrophobic properties of Al2O3 based on Al2O3-1PA.It was found that with the increase of the treatment temperature,the alkyl chain gradually broke,and the hydrophobicity of the material decreased.However,the hydrothermal stability of the material first increases and then decreases.This problem mainly lies in that when the alkyl chain is partially broken,the alkyl chain changes from an orderly distribution to a disorderly distribution on the surface of the material,effectively isolating the contact between water and the material a tendency to increase the hydrothermal stability.While the alkyl chain is further broken,the surface of the material will gradually come into contact with water,which will cause the Al2O3 to undergo a hydration reaction.The experiment results of different temperature treatments also revealed that the reaction temperature of PA in the modification of Al2O3 materials is 200℃.(2)To alleviate the catalyst deactivation problem in the phenol hydrogenation reaction,we prepared the hydrophobicity surface on Pd/Al2O3PA catalyst.The phenol hydrogenation was carried out under the conditions of 90℃ and 0.4 MPa water solvent.Pd/Al2O3-PA catalysts exhibited good catalytic activity and stability in the process of four utilizations.At the fourth utilization,the phenol conversion rate was 87%,and the selectivity of cyclohexanol was 87%.In comparison,the phenol conversion rate of the Pd/Al2O3 catalyst was only 26%,and the selectivity of cyclohexanol was only 41%.The phenol conversion rate of the Pd/Al2O3 catalyst is only 26%,and the cyclohexanol selectivity is only 41%.On the one hand,the improvement of catalytic performance can be attributed to the modification of the surface alkyl chains to form a "barrier" between the active components,which effectively alleviates the agglomeration of the active components.The particle size distribution shows that the size increase of Pd/Al2O3-PA Pd NPs by 7%is much smaller than 15%of Pd/Al2O3 after four times uses.On the other hand,the modification of the surface alkyl chain makes it difficult for the Pd/Al2O3-PA catalyst to adsorb the cyclohexanol product,which effectively alleviates the coverage of the active site by cyclohexanol.The strategy of obtaining hydrophobic catalysts by modification with PA is also suitable for constructing monolithic hydrophobic catalysts.It provides a feasible scheme for the preparation of monolithic hydrophobic catalysts.In this paper,the hydrophobicity of the Al2O3 material is controlled,and a hydrophobic monolayer film is formed on the surface of the Al2O3 through chemical bonding,which effectively improves the hydrothermal stability of the material.The surface hydrophobicity modification changes the adsorption properties of the catalyst surface and alleviates the agglomeration of active components.The surface hydrophobicity modification effectively improves the reusability of the catalyst under the condition of an aqueous solution.The research results in this paper provide a new theoretical basis and research ideas for the preparation and application of hydrophobic catalysts.