Gas-Phase Reactions of O- and OH-Containing Cr Cluster Ions with Ethylene Molecules:  Isolation of Reaction Sites of Cr-Containing Catalysts for Ethylene Polymerization

Reactions of ethylene molecules on Cr n + ( n = 1−4), Cr n O + ( n = 1−4), and Cr n OH + ( n = 1, 2) were studied in a tandem mass spectrometer equipped with a reaction cell. The mass spectra of the product ions showed that (1) under a single collision condition, oxidation of ethylene and chromium pickup (Cr-pickup) by ethylene take place on Cr n + ( n = 2−4), Cr n O + ( n = 2−4), and Cr n OH + ( n = 2), and (2) under a multiple collision condition, two ethylene molecules are chemisorbed exclusively on CrO + and CrOH +; the ethylene molecules seem to be dimerized in comparison with our preliminary density functional theory calculation. Size and collision-energy dependences of the reaction cross sections lead us to conclude that the Cr-pickup reaction proceeds via an intermediate in which C 2 H 4 is chemisorbed on a parent ion. The preferential ethylene dimerization on CrO + and CrOH + implies that, on a silica-supported chromium catalyst (Philips catalyst), ethylene polymerization proceeds preferentially on active centers consisting of atomic chromium bonded to O or OH of the substrate.