A highly effective and stable CuZn0.3MgxAlOy catalyst for the manufacture of chiral l-phenylalaninol: the role of Mg and its hydrotalcite-like precursor

Abstract

Highly effective CuZn_0.3Mg_xAlO_y (x = 0–0.2) catalysts for the synthesis of chiral L-phenylalaninol derived from Cu-rich hydrotalcite-like precursors were prepared by a co-precipitation method with Na₂CO₃ as the precipitant, and their physicochemical and catalytic properties were characterized. The results show that the presence of Mg²⁺ ions can promote the formation of hydrotalcite-like (htl) precursors, and the Mg²⁺ content would affect the phase purity of the prepared htl precursors. The BET surface area, exposed copper surface area and amount of acid sites of the samples decreased with the increase in the molar ratio of Mg²⁺/Al³⁺. Also, the dense layered htl precursors are beneficial to the atomically uniform distribution of the corresponding metal oxides in the prepared catalysts, promoting the stronger interaction between Cu⁰ and Al₂O₃ after the catalysts were reduced (SMSI effect). The activity of the CuZn_0.3Mg_xAlO_y catalysts is greatly dependent on not only the metallic copper surface area, but also the SMSI effect and the acidity of the catalysts. When Mg²⁺/Al³⁺ = 0.1 (mol), a phase-pure htl precursor could be obtained, and after calcination, the prepared CZA-0.1 catalyst exhibited very excellent catalytic performance for the hydrogenation of L-phenylalanine methyl ester to chiral L-phenylalaninol. After 5 h of reaction at 110 °C and 4 MPa H₂, 100% conversion of L-phenylalanine methyl ester and 91.1% yield of L-phenylalaninol with an ee value of ~100% were achieved. After recycling 13 times, the L-phenylalaninol selectivity of the CZA-0.1 catalyst only decreased by 7.2%.