Enhanced hydration efficiency of cyclohexene to cyclohexanol over acid-modified HZSM-5 catalysts under solvent-free conditions

In this work, several HZSM-5 catalysts with different Si/Al ratios treated with acids are selected as catalysts and used for hydration of cyclohexene to cyclohexanol. The results indicated that HZSM-5 (Si/Al = 38) modified with 4 mol·L⁻¹ nitric acid was selected as an efficient catalyst for improving the hydration efficiency of cyclohexene. Furthermore, the microstructures and properties of fresh, used and regenerated acid-modified catalysts have been characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption/desorption isotherm, Fourier transform infrared, thermal gravimetric analyzer, ammonia temperature programmed desorption and pyridine adsorbs fourier transform infrared. The characterization results indicated that the total surface areas and pore volume of HZSM-5 zeolite increased after nitric acid treatment due to the formation of mesoporous structure. This benefits the diffusion rate of reactants and products, which improves the hydration efficiency and stability of the catalyst. Under the catalysis of HZSM-5, the conversion of cyclohexene was found to be 9.0%. However, treatment of HZSM-5 with nitric acid enhanced the conversion of cyclohexene to 12.2%, achieving a selectivity of 99.7% for cyclohexanol under optimal reaction conditions. This work affords a mild and efficient approach for improving the hydration efficiency and has potential industrial application value.