(689e) Facile Formation of Methyl Acrylates By Cross Coupling of Unsaturated Alcohols-Aldehydes with Methanol on Au(110)
In the present study we use temperature programmed reaction spectroscopy (TPRS) and scanning tunneling microscopy (STM), to study the esterification of methanol due to cross-coupling reactions with small unsaturated oxygenates on a model single crystal surface - Au(110). We identify reaction intermediates and their surface stability on the Au single crystal and compare the ester optimum selectivity with studies under flow conditions on npAu.
In the first part of this research we demonstrate the catalytic production of methyl acrylate and methyl methacrylate over O/Au(110) interface, via the coupling of unsaturated alcohols (allyl alcohol and methallyl alcohol) with methanol under ultra high vacuum (UHV) conditions. We show that the alcohols can be used to generate aldehydes in-situ with high selectivity, without oxidation of the unsaturated C=C bond, from which coupling with methanol to form acrylates follows. The product selectivity can be tuned by changing the relative concentrations of the unsaturated alcohol and methanol; the high relative concentrations of methanol which were found to be necessary to maximize acrylate selectivity, indicate that the C=C bond and methyl group stabilize the allyloxy intermediate on the surface (compared to saturated alcohols).
In the second part of our research we study the oxygen-assisted esterification of acrolein and methacrolein with methanol on Au to form their respective acrylates. Following the same experimental procedure with the alcohol cross-coupling part, TPRS was used to establish the mechanism and conditions for the facile esterification of acrolein and methacrolein. Selectivities of ~95% were achieved on oxygen pre-covered Au (110) with methanol conversions of nearly 100 % under appropriate conditions. Under flow conditions on npAu, methyl acrylate and methyl methacrylate were formed with optimum selectivities ~20%. The origin of this low activity is shown to be due to poisoning of active sites by adsorbed acrylates.
Pursuant to this poisoning effect, STM studies were performed on the acrylate and methacrylate on Au(110) at room temperature, formed by the reaction of acrolein and methacrolein with oxygen. In both cases ordered structures of the adsorbed intermediates were observed, indicating strong intermolecular interactions. The reactive intermediates are present on the surface at room temperature, and lead to the destruction of the 1x2 reconstruction of Au(110), confirming that they are strongly bound with Au atoms. The formation of these dense structures almost certainly accounts for the higher stability of these intermediates on gold, therefore, contributing to its blocking of the surface sites.
It is shown that model studies of the methanol cross-coupling reactions on single-crystal gold surfaces, provided information about the oxidative cross-coupling of allyl and methallyl alcohols with methanol which can be achieved under mild conditions without competing attack of the unsaturated C=C bond. Optimum cross-coupling selectivities are achieved when the surface concentrations of methoxy and allyloxy/methallyloxy intermediates are relatively equal, requiring methanol mole fractions above 0.99 due to the stabilizing effects of the C=C bond (on allyloxy) and an additional methyl group (on methallyloxy).
The esterification of unsaturated aldehydes with methanol can also be achieved selectively at low temperature by oxygen-assisted coupling on Au(110). However. Lower conversion and selectivity was observed in steady flow under catalytic conditions with nanoporous Au catalysts. The lower activity observed for npAu (vs Au(110)), is due to the buildup of adsorbed acrylates. The model studies of the reaction mechanism indicate that facile esterification of acrolein and methacrolein with methanol can be achieved with ~95% selectivity provided the oxidation of the aldehyde can be suppressed.
The critical aspect of designing efficient heterogeneous catalytic reactions by determining the factors responsible for the relative stability of intermediates on the surface of the catalyst is addressed. It was found that the presence of highly stable species, which do not lead to productive reactions, inhibits desired reactions and leads to the suppression of catalytic activity. The catalytic processes proposed in this research can provide an alternative, green route for the production of acrylates and methacrylates and can further extend the principles governing coupling activity/selectivity on gold surfaces to unsaturated organic molecules.