Continuous Catalytic Production of Methyl Acrylates from Unsaturated Alcohols by Gold: The Strong Effect of C═C Unsaturation on Reaction Selectivity

Abstract

Here we demonstrate the gas-phase catalytic production of methyl acrylates by oxygen-assisted coupling of methanol with the unsaturated alcohols allyl alcohol and methylallyl alcohol over nanoporous gold (npAu) at atmospheric pressure. Analogous investigations on O-activated Au(110) exhibit the same pattern of reactivity and are used to establish that the competition between methoxy and allyloxy (or methallyloxy) reaction intermediates for adsorption sites, mediated by the reactants themselves, determines the selectivity of reaction. Our results clearly show that the CC bond substantially increases the binding efficacy of the allyloxy (or methallyloxy), thus requiring extremely high methanol mole fractions (>0.99) in order to achieve comparable surface concentrations of methoxy and produce optimum yields of either methacrylate or methyl methacrylate. Allyloxy and methallyloxy were favored by factors of ∼100 and ∼450, respectively, vs methoxy. These values are more than 1 order of magnitude greater than those measured for competitive binding of ethoxy and 1butoxy vs methoxy, demonstrating the strong effect of the carbon−carbon bond unsaturation. The 4.5-fold increase due to the addition of the methyl group in methylallyl alcohol vs allyl alcohol indicates the significant effect of the additional van der Waals interactions between the methyl group and the surface. Gas-phase acidity is also shown to be a good qualitative indicator for the relative binding strength of the alkoxides. This work provides insight into the control of reaction selectivity for coupling reactions and demonstrates the value of fundamental studies on single crystals for establishing key principles governing reaction selectivity. Notably, these oxygen-assisted coupling reactions occur without oxidation of the CC bond.

Noncovalent Bonding Controls Selectivity in Heterogeneous Catalysis: Coupling Reactions on Gold

“…The stabilization effect of CC bond unsaturation is shown by the displacement of 2-propoxy by allyl alcohol on Au(110) (Figure S2B). Further methallyloxy displaces allyloxy, showing that even the addition of a single methyl group to the unsaturated alkoxy results in increased stabilization of the surface intermediate. Further, benzyl alkoxy displaces methallyloxy (Figure S2C); the aromatic ring results in stronger attractive interactions with the substrate than the CC unsaturated bond (see Figures S2 and S3).…”

“…The work described herein is motivated by previous studies under catalytic conditions that illustrate the effect of competitive binding in the selectivity for the oxygen-assisted cross-coupling of methanol with a second dissimilar, short-chain alcohol − to form methyl esters on metallic gold-based catalysts (Figure ). , To a good approximation, the optimum cross coupling occurs when the surface concentration of the two different alkoxy intermediates is equal, as the cross-coupling originates from the surface reaction between the dissimilar alkoxy species. If the O–Au bond strength dictates the binding energy, methoxy and ethoxy, for example, would have nearly identical surface stabilities.…”

Selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen on highly dilute NiCu alloys

“…It has also reported that unsupported gold, prepared in nanoporous form from AuAg alloys after leaching out most (~97%) of the silver, is an effective catalyst for the selective oxidation of methanol, and other alcohols. [30,31] Since unsupported gold is inert, it was apparent that the active sites must be at the interface of silver domains and gold. These sites also catalyze the oxidative self-coupling of methanol to methyl formate [32] and the cross coupling of methanol with higher alcohols or aldehydes to form esters.…”

Hydrodebromination of Aromatic Bromides Catalyzed by Unsupported Nanoporous Gold: Heterolytic Cleavage of Hydrogen Molecule

“…The unsupported nanoporous gold (AuNPore) catalyst is an efficient, versatile, and reusable catalyst for oxidation, [14] reduction, [15] addition, [16] annulation, [17] and CÀ H bond oxidative coupling. [18] In the course of our continuous research on AuNPore catalysis, we found that the AuNPore can also be used as a catalyst for selective hydrodebromination of aromatic bromides, revealing a new application for AuNPore catalyst.…”