A molecular beam study of olefin adsorption on ultrathin ionic liquid films on Pt(111)

We investigate fundamental aspects concerning selectivity in hydrogenation reactions for Solid Catalyst with Ionic Liquid Layer (SCILL)-type systems. Some of us recently reported that the adsorption behavior of 1,3-butadiene and 1-butene on Pt(111) can be tuned with ultrathin layers of the ionic liquid (IL) 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C1Im][Tf2N]): Increasing the IL coverage leads to increased blocking of olefin adsorption sites. Notably, a smaller IL amount is needed to prevent 1-butene adsorption as compared to 1,3-butadiene adsorption, leading to a selectivity window, in which 1,3-butadiene still can be adsorbed while 1-butene cannot. With the aim to evaluate whether this is a general behavior, we study an IL with a longer alkyl chain at the cation, 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C8C1Im][Tf2N]), and an IL with a different anion, 1-methyl-3-octylimidazolium hexafluorophosphate ([C8C1Im][PF6]). Indeed, we observe a selectivity window for all ILs and thus demonstrate that this concept also applies for other ILs with different chain lengths and anions. Nevertheless, there are pronounced differences for the three ILs in terms of the IL coverages required for full blocking and the width of the selectivity window. Different explanations are discussed, e.g., the structure of the IL layer and the interaction strengths of olefins and ILs with the substrate.