Abstract
Metal oxides such as Nb2O5, Cr2O3, and especially a Zn-II-Cr-III mixed oxide are demonstrated to be highly active and recyclable heterogeneous catalysts for Prins condensation, which provides a clean, high-yielding route for the synthesis of nopol through the condensation of biorenewable beta-pinene with paraformaldehyde. Zn-Cr mixed oxide with an optimum Zn/Cr atomic ratio of 1: 6 gave 100% nopol selectivity at 97% beta-pinene conversion, with the catalyst easily recovered and recycled. The acid properties of Nb2O5 and Zn-Cr mixed oxide were characterized by the diffuse reflectance IR Fourier transform spectroscopy of adsorbed pyridine and ammonia adsorption microcalorimetry. An appropriate combination of acid-base properties of Zn-Cr mixed oxide is believed to be responsible for its efficiency.