H4PMo11VO40-Catalyzed β-Citronellal Condensation Reactions

In this work, vanadium replaced molybdenum atoms generating catalysts with general formulae H3+nPMo12-nVnO40 (n = 0,1, 2 and 3), which were used in the acetalization of alkyl alcohols with terpene aldehyde (beta-citronellal and methyl alcohol were model substrates). The initial focus was evaluating how the vanadium load impact the catalytic activity of phosphomolybdic acids and trying to link this effect with their structural properties. A comparison of performance achieved by the catalysts revealed that among phosphomolybdic acids (i.e., with V-1, V-2, and V-3 atoms/per anion), the vanadium monosubstituted phosphomolybdic acid was the most active and selective toward the formation of beta-citronellyl acetal. The effects of main reaction variables such as time, temperature, catalyst load, type of alcohol, and vanadium load on conversion and selectivity of the reactions were investigated. Remarkably, while in methyl alcohol, only acetal was formed, in the presence of other alkyl alcohols terpene ethers (geranyl and beta-citronellyl) were also obtained. Their highest activity of H4PMo11VO40 was attributed to the greatest Bronsted acidity strength, as demonstrated by the acidity measurements and infrared spectroscopy analysis. This catalyst has advantages over traditional liquid mineral acid catalysts and provides an alternative route to synthesize acetal and terpene ethers. [GRAPHICS]