The effect of cavitating ultrasound on the aqueous phase hydrogenation of cis-2-buten-1-ol and cis-2-penten-1-ol on Pd-black

We have studied the effect of cavitating ultrasound on the heterogeneous aqueous hydrogenation of cis-2-buten-1-ol (C4 olefin) and cis-2-penten-1-ol (C5 olefin) on Pd-black to form the trans-olefins (trans-2-buten-1-ol and trans-2-penten-1-ol) and saturated alcohols (1-butanol and 1-pentanol, respectively). Silent (and magnetically stirred) experiments served as control experiments. As described in an earlier publication by our group, we have added an inert dopant, 1-propanol, in the reaction mixture to ensure the rapid onset of cavitation in the ultrasound-assisted reactions that can lead to altered selectivity compared to silent reaction systems [R.S. Disselkamp, Ya-Huei Chin, C.H.F. Peden, J. Catal. 227 (2004) 552]. The motivation for this study is to examine whether cavitating ultrasound can reduce the [trans-olefin/saturated alcohol] molar ratio during the course of the reaction. This could have practical application in that it may offer an alternative processing methodology of synthesizing healthier edible seed oils by reducing trans-fat content. We have observed that cavitating ultrasound results in a [(trans-olefin/saturated alcohol)ultrasound/(trans-olefin/saturated alcohol)silent] ratio quantity less than 0.5 at the reaction mid-point for both the C4 and C5 olefin systems. This indicates that ultrasound reduces trans-olefin production compared to the silent control experiment. Furthermore, there is an added 30% reduction for the C5 versus C4 olefin compounds again at reaction mid-point. We attribute differences in the ratio quantity as a moment of inertia effect. In principle, the C4 versus C5 olefins has a ∼52% increase in moment of inertia about C2C3 double bond slowing isomerization. Since seed oils are C18 multiple cis-olefins and have a moment of inertia even greater than our C5 olefin here, our study suggests that even a greater reduction in trans-olefin content may occur for partial hydrogenation of C18 seed oils.