XPS studies of the effects of modification pH on the interaction of methylacetoacetate with (S)-aspartic acid-modified Ni surfaces

The adsorption, from aqueous solution, of (S)-aspartic acid onto polycrystalline Ni was investigated as a function of pH via X-ray photoelectron spectroscopy in order to investigate why the hydrogenation of β-ketoesters over aspartic acid-modified Ni catalysts exhibits a very sharp maximum in enantioselectivity at modification pH 5. Following similar modification and washing procedures to those used in the real catalyst preparation, it was found that, under the optimum preparation conditions, the coverage of aspartic acid on the Ni surface drops below detectable levels. The implications for understanding the mechanism of the enantioselective hydrogenation reaction are discussed.

Ni catalysts modified by (S)-aspartic acid exhibit a sharp maximum in enantioselectivity following modification at pH 5.0 and 300 K. XPS shows that under these conditions the coverage of aspartate is vanishingly small implying that enantioselectivity, in this case, may be derived from chiral metal sites created by the corrosive leaching of Ni by aspartic acid.Figure optionsDownload high-quality image (38 K)Download as PowerPoint slideResearch highlights
► Washing modified catalysts removes nickel aspartate leaving chiral nickel ensembles.
► The best enantioselective Ni/aspartate catalyst has negligible aspartate coverage.
► The tautomeric form of the pro-chiral reagent depends on modification pH.