Effects of the preparation method on the performance of the Cu/ZnO/Al2O3 catalyst for the manufacture of l-phenylalaninol with high ee selectivity from l-phenylalanine methyl ester

Abstract

The effects of the preparation method on the properties of Cu/ZnO/Al₂O₃ catalysts for L-phenylalanine methyl ester hydrogenation to L-phenylalaninol were investigated in detail, including the precipitation method and conditions (the aging time, calcination temperature and so on), with the help of ICP-OES, N₂ and N₂O adsorption, XRD, H₂-TPR and TEM techniques. The results show that physicochemical properties of the catalysts are greatly affected by the preparation method and conditions. The uniform size distribution of CuO species can be obtained by fractional co-precipitation. The appropriate aging time is 2 h, and the catalyst aged for 2 h has the largest metallic copper surface area (S_Cu) and surface copper amount and the smallest CuO crystallites. The lower calcination temperature is favorable for increasing the surface area and metallic copper surface area of the catalyst. The spinel structure CuAl₂O₄ phase can form after calcination at 550 °C. The turnover frequency (TOF) values of L-phenylalaninol formed using different catalysts indicate the structurally sensitive character of the title reaction, and S_Cu is not the sole cause affecting the catalytic activities of the catalysts. B-TOF on the basis of the active sites (Cu⁰) in the boundary between CuO and ZnO or Al₂O₃ was proposed; the relationships of B-TOF with d_CuO (particle size of CuO) and S_Cu were established. Using the Cu/ZnO/Al₂O₃ catalyst prepared by fractional co-precipitation with aging at 70 °C for 2 h and calcination at 450 °C for 4 h, 83.6% selectivity to L-phenylalaninol without racemization was achieved.