Competitive Hydrogenation between Linear Alkenes and Aromatics on Close-Packed Late Transition Metal Surfaces

Selective hydrogenation of linear alkenes in the presence of aromatics is desired to prevent gum formation in pyrolysis gasoline (PYGAS) upgrading. To examine the competitive hydrogenation between linear alkenes and aromatics, we investigate ethylene and benzene competitive hydrogenation on different catalysts. Through density functional theory (DFT) calculations, we show the adsorption energies of benzene and ethylene correlate on monometallic close-packed surfaces, with benzene binding stronger for the same C to surface metal atom ratio. DFT calculations demonstrate Brønsted-Evans-Polanyi and scaling relationships hold, and these are fed into microkinetic modeling to predict the rate of ethylene and benzene hydrogenation with only ethylene and hydrogen binding energies as the surface descriptors. Due to stronger binding, benzene adsorption will dominate the surface. Higher barriers for benzene hydrogenation versus ethylene hydrogenation lead to benzene poisoning at temperatures at which ethylene hydrogenation would otherwise have been fast. Experimental studies using a Pd foil catalyst agree with microkinetic model predictions that benzene will poison the surface during ethylene hydrogenation while not being hydrogenated itself. Computational results predict bimetallic surfaces can avoid benzene poisoning during ethylene hydrogenation with the addition of an inert metal to disrupt the binding of benzene on 3-fold sites.