Selectivity in Catalytic Alkyne Cyclotrimerization over Chromium(VI): Kinetic Evaluation Using the Characteristics of Radioactive Carbon-11 Decay for Nondisruptive Ultrasensitive Delection of Adsorbed Species

Article abstract of DOI:10.1021/j150655a016

The application of carbon-11 to kinetic measurements of molecular sorption is reported using positron annihilation surface detection (PASD).The technique is nondisruptive to dynamic processes and has the sensitivity to detect 1E-8 of a monolayer.In studies of alkyne cyclotrimerization on silica-alumina-supported Cr(VI), a high selectivity toward p-xylene formation was observed when acetylene-propyne mixtures were cotrimerized at monolayer total alkyne coverages.This selectivity was enhanced to 84percent p-xylene, as the partial acetylene coverage was reduced to 1.0 percent of a monolayer.Competitive sorption studies utilized PASD to measure the surface concentration of <11C>-acetylene coupled with macroscopic sorption measurements of propyne.Surface displacement of sorbed acetylene by propyne was observed with subsequent readsorption.The kinetics of this displacement were evaluated by using PASD in pulse-flow studies with various acetylene and propyne coverages and were modeled to a calculation of the isomeric xylene distribution.A near-idenitcal fit was obtained between experimental and modeled results.This strongly suggested that the observed selectivity for p-xylene formation was due to sorbate interactions resulting in a specific molecular ordering of the alkyne mixture on the catalyst surface.