100% Selective Cyclotrimerization of Acetylene to Benzene on Ag(111)

Result of the Month

STM images of (A) pure Ag(111) inset atomic resolution Ag(111), (B) 3 L C2H2 on Ag(111) inset shows the square packing of C2H2 at lower acetylene coverage, (C) 10 L C2H2 on Ag(111). STM images in panels (D) and (E) shows the hexagonal packing of the C2H2 at higher coverage (10 L C2H2 ) and the appearance of dumbbell shaped intermediates. C2H2 was exposed to the surface at ∼25 K and the images were taken at ∼13 K. Imaging conditions: (A) 0.2 nA and 600 mV, (B) 0.1 nA and −700 mV, inset 0.1 nA and −500 mV, (C) and (D) 0.1 nA and 800 mV, and (E) 0.08 nA and 800 mV. All STM images were captured by Elizabeth Happel.

Author: Volkan Çınar, Shengjie Zhang, Elizabeth E. Happel, Nipun T. S. K. Dewage, Matthew M. Montemore and E. Charles H. Sykes Institute: ''Tufts University, MA, USA'' Chemical Science
URL: https://pubs.rsc.org/en/content/articlelanding/2024/sc/d4sc01053a
Date: 7/2024
Instruments: INFINITY SPM Lab

Benzene, a high-volume chemical, is produced from larger molecules by inefficient and environmentally harmful processes. Recent changes in hydrocarbon feedstocks from oil to gas motivate research into small molecule upgrading. For example, the cyclotrimerization of acetylene reaction has been demonstrated on Pd, Pd alloy, and Cu surfaces and catalysts, but they are not 100% selective to benzene. We discovered that acetylene can be converted to benzene with 100% selectivity on the Ag(111) surface. Our temperature programmed desorption experiments reveal a threshold acetylene surface coverage of ∼one monolayer, above which benzene is formed. Furthermore, additional layers of acetylene increase the amount of benzene produced while retaining 100% selectivity. Our scanning tunneling microscopy images show that acetylene prefers square packing on the Ag(111) surface at low coverages, which converts to hexagonal packing when acetylene multilayers are present. Within this denser layer, features consistent with the proposed C4 intermediates of the cyclotrimerization process are observed. Density functional theory calculations demonstrate that the barrier for forming the crucial C4 intermediate generally decreases as acetylene multilayers are formed because the multilayer interacts more strongly with the surface in the transition state than in the initial state. Given that acetylene desorbs from Ag(111) at ∼90 K, the C4 intermediate on the pathway to benzene must be formed below this temperature, implying that if Ag-based heterogeneous catalysts can be run at sufficiently high pressure and low enough temperature, efficient and selective trimerization of acetylene to benzene may be possible. STM experiments were performed on a low-temperature scanning tunneling microscope (Infnity by Scienta Omicron) in UHV.