Selectivity as a Function of Nanoparticle Size in the Catalytic Hydrogenation of Unsaturated Alcohols

Abstract

Layer-by-layer adsorption of poly(acrylic acid)-Pd(II) complexes and poly(ethylenimine) on alumina powder followed by reduction of Pd(II) with NaBH(4) yields Pd-nanoparticle catalysts embedded in multilayer polyelectrolyte films. The use of different ratios of poly(acrylic acid) to Pd(II) in deposition solutions gives a series of films with Pd nanoparticles whose average diameters range from 2.2 to 3.4 nm, and the catalytic selectivities of these nanoparticles vary dramatically with their size. Turnover frequencies (TOFs) for the hydrogenation of monosubstituted unsaturated alcohols increase with decreasing average nanoparticle size, whereas multisubstituted unsaturated alcohols show the opposite trend. Hence, the ratio of TOFs for the hydrogenation of allyl alcohol and crotyl alcohol is 39 with average particle diameters of 2.2 nm and only 1.3 with average particle diameters of 3.4 nm. Ratios of TOFs for hydrogenation of allyl alcohol and beta-methallyl alcohol are as high as 240 with the smallest nanoparticles, but substantial isomerization of beta-methallyl alcohol complicates this comparison. Increasing selectivity with decreasing average particle size occurs with both films deposited on alumina powder and nanoparticles stabilized by polyelectrolytes in solution. Presumably, high selectivities occur on the smallest nanoparticles because the active sites on the smallest Pd nanoparticles are less available for binding and hydrogenation of multisubstituted double bonds than are active sites on larger particles.

Directing reaction pathways by catalyst active-site selection using self-assembled monolayers

“…Using a dense SAM, such as one created by octadecanethiol, access to the threefold sites can be severely limited, drastically reducing the rate of DC leading to selectivity enhancement for aldehyde hydrogenation and HDO. Thus, active-site selection using SAM modifiers in catalysis can potentially serve as an alternative (or complement) to recent efforts [33][34][35][36] aimed at modifying shape, and thus surface-site distribution, of supported metal nanoparticles to control selectivity.…”

Hydrogen (H2) sensing and catalysis with organic-stabilized Pd and Pd alloy nanoparticles.

“…With a long chain length, but weak coordinating ligand like the amine, the allyl alcohol readily reacts with the Pd NPs, but with a short chain length, but strong coordinating thiol, the reactivity is greatly diminished by a factor of 20 down to a TOF value of 47 mol substrate/mol Pd/h. NP size is also an important factor, 35,116,117,124,134 but was not controlled in this study. Figure 6.6 UV-vis spectra of a solution of ½x C6S Pd, 12x C16NH 2 Pd, and 12x C16NH 2 Pd 91 Ag 9 exposed to 100% H 2 (A, C, E) and to 100% H 2 plus allyl alcohol (B, D, E) for 60 min and then the solutions were sitting in air for 20 min and 3 h. reactivity with allyl alcohol.…”

“…Catalytic selective hydrogenation of allyl alcohol to produce 1-propanol, which is a precursor in organic synthesis and the pharmaceutical industry, has been studied as a function of particle size and shape of Pd NPs coated with polymers, 134,135 biopolymers (collagen fiber), 136 surfactants, 35 or dendrimers 29,30,32,123,124,137,138 either in solution or immobilized on solid supports such as alumina, 35,134 composites, 32 silica, 139 and magnetic NP cores. [140][141][142] However, the transformation of allyl alcohol to carbonyl compounds (isomerization reaction) has not been widely explored using Pd NPs as catalysts.…”

“…Because this allyl alcohol is the simplest α,β-unsaturated alcohol and has been used as a substrate in prior studies, 29,30,32,35,123,124,[134][135][136][137][138][139][140][141][142] it was a good starting point for this work.…”

“…[143][144][145][146][147] Although heterogeneous Pd catalysts, such as Pd/TiO 2 , 151,179 polymer-Pd(0) and Pd(II) complexes, 152 and Pd NPs stabilized by polymers and immobilized on composites or embedded in polyelectrolyte films, 134,[180][181][182] have been used to study the activity and selectivity for hydrogenation and isomerization of allyl alcohols, the favored hydrogenation process prohibited formation of the isomer. An important challenge is to controllably obtain the alcohol, or the saturated carbonyl that the substrate also plays a role in NP stability.…”