Abstract
The gas phase hydrogenation of 3-butyne-2-one, an alkynic ketone, over two alumina-supported palladium catalysts is investigated using infrared spectroscopy in a batch reactor at 373K. The mean particle size of the palladium crystallites of the two catalysts are comparable (2.4±0.1nm). One catalyst (Pd(NO3)2/Al2O3) is prepared from a palladium(II) nitrate precursor, whereas the other catalyst (PdCl2/Al2O3) is prepared using palladium(II) chloride as the Pd precursor compound. A three-stage sequential process is observed with the Pd(NO3)2/Al2O3 catalyst facilitating complete reduction all the way through to 2-butanol. However, hydrogenation stops at 2-butanone with the PdCl2/Al2O3 catalyst. The inability of the PdCl2/Al2O3 catalyst to reduce 2-butanone is attributed to the inaccessibility of edge sites on this catalyst, which are blocked by chlorine retention originating from the catalyst’s preparative process. The reaction profiles observed for the hydrogenation of this alkynic ketone are consistent with the site-selective chemistry recently reported for the hydrogenation of crotonaldehyde, an alkenic aldehyde, over the same two catalysts. Thus, it is suggested that a previously postulated structure/activity relationship may be generic for the hydrogenation of α,β-unsaturated carbonyl compounds over supported Pd catalysts.
Highlights
A major goal in contemporary heterogeneous catalysis is to define structure/activity relationships for specified reactions [1,2]
The present study considers how the morphology of Pd crystallites can influence selectivity branching in gas phase hydrogenation reactions of ␣,-unsaturated carbonyl compounds
The fact that the trends observed for crotonaldehyde hydrogenation over Pd/Al2O3 are reproduced for 3-butyne-2-one suggest that the proposed structure/activity model [7] may be generically applicable for ␣,-unsaturated carbonyl compounds over supported Pd catalysts
Summary
A major goal in contemporary heterogeneous catalysis is to define structure/activity relationships for specified reactions [1,2]. The present work seeks to discover if the proposed structure/activity relationship can be extended to ␣,-unsaturated ketones This will provide insight on the possible generic nature of the postulated site-selective model for hydrogenation reactions over supported Pd catalysts. Whereas it is conceded that the high chloride residue evident in Table 1 relates to the catalyst pre-activation, infrared measurements of CO chemisorption after a catalyst reduction treatment establish chloride to remain present at the metal surface after a reduction stage [8] It is the blocking of specific Pd crystallite sites by residual chloride originating from the catalyst preparative stage that is thought to be influencing the crotonaldehyde reaction profiles over these two catalysts [7]. A heightened awareness of metal crystallite morphological contributions to product distributions should lead to more efficient catalysis
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