Abstract
Alkenols are commercially important chemicals employed in the pharmaceutical and agro-food industries. The conventional production route via liquid phase (batch) alkynol hydrogenation suffers from the requirement for separation/purification unit operations to extract the target product. We have examined, for the first time, the continuous gas phase hydrogenation (P = 1 atm; T = 373 K) of primary (3-butyn-1-ol), secondary (3-butyn-2-ol) and tertiary (2-methyl-3-butyn-2-ol) C4 alkynols using a 1.2% wt. Pd/Al2O3 catalyst. Post-TPR, the catalyst exhibited a narrow distribution of Pdδ- (based on XPS) nanoparticles in the size range 1-6 nm (mean size = 3 nm from STEM). Hydrogenation of the primary and secondary alkynols was observed to occur in a stepwise fashion (-C≡C- → -C=C- → -C-C-) while alkanol formation via direct -C≡C- → -C-C- bond transformation was in evidence in the conversion of 2-methyl-3-butyn-2-ol. Ketone formation via double bond migration was promoted to a greater extent in the transformation of secondary (vs. primary) alkynol. Hydrogenation rate increased in the order primary < secondary < tertiary. The selectivity and reactivity trends are accounted for in terms of electronic effects.
Highlights
The bulk of research on -C≡C- bond hydrogenation has been focused on the transformation of acetylene over Pd catalysts where the main challenge is to selectively promote semi-hydrogenation with -C=C- formation [1]
We have examined the effect of -OH group position on catalytic gas phase hydrogenation of
C4 alkynols over Pd/Al2 O3 (Pdδ- nanoparticles with mean size = 3 nm)
Summary
The bulk of research on -C≡C- bond hydrogenation has been focused on the transformation of acetylene (to ethylene) over Pd catalysts where the main challenge is to selectively promote semi-hydrogenation with -C=C- formation [1]. Associative adsorption (through a π/σ double bond) on Pd planes [2] follows the Horiuti-Polanyi model, consistent with a stepwise alkyne → alkene → alkane transformation [3,4]. Π-allyl specie [5] on electron deficient edges/corners of palladium nanoparticles [6] can lead to direct alkyne → alkane hydrogenation [7] or double bond migration [8]. The electronic properties of the palladium phase and the electron density of the -C≡C- bond functionality can influence the alkyne adsorption/activation which, in turn, impact on olefin selectivity. Taking an overview of the published literature, unwanted over-hydrogenation and double migration are prevalent over electron deficient (Pdδ+ ) nanoparticles that promote strong complexation with the (electron-rich) -C≡C- bond [9,10]
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