Oxidation of a wood extractive betulin to biologically active oxo-derivatives using supported gold catalysts
Liquid-phase oxidation of betulin aiming to obtain its biologically active oxo-derivatives (betulone, betulonic and betulinic aldehydes) was demonstrated for the first time over gold-based catalysts. Gold was deposited on pristine TiO2 and the same support modified with ceria and lanthana, followed by pretreatment with H2 or O2 atmosphere. The catalysts were characterized by XRD, BET, ICP, TEM, XPS, DRIFT CO, TPD of NH3 and CO2 methods. The nature of the support, type of modification and the pretreatment atmosphere significantly influenced the average particle size of gold, its distribution and the electronic state of gold, as well as the acid-base properties and, thereby, the catalytic performance (activity and selectivity) in betulin oxidation. Au/La2O3/TiO2 pretreated in H2 displayed the highest catalytic activity in betulin oxidation among the studied catalysts with selectivities to betulone, betulonic and betulinic aldehydes of 42, 32 and 27%, respectively, at 69% conversion. Side reactions resulting in oligomerization/polymerization products were occurring on the catalyst surface with the participation of strong acid sites, diminishing the yield of the desired compounds. The latter was improved by adding hydrotalcite with the basic properties to the reaction mixture containing the catalyst. Kinetic modelling was performed to quantify the impact of such side reactions.