100% selective cyclotrimerization of acetylene to benzene on Ag(111)

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.


Explanation of Ethylene Peak
In order to probe the origin of the ethylene peak in Fig. S2 and support the claim of 100% selectivity of acetylene trimerization we used mass spectrometry 1 to study isotope scrambling of the reactant and product molecules.Specifically, if ethylene is to be reactively formed on the Ag(111) surface, C-H bonds in acetylene must be broken to supply H.We therefore deposited C6D6 and regular acetylene on Ag(111) and monitored H/D exchange between molecules that would indicate C-H activation.Fig. S3A UPPER shows a mass spectrum recorded when 5×10 -9 mbar of C6D6 was introduced to the UHV chamber showing that the ratio of C6D6 (m/z = 84) to C6D5H (m/z = 83) in the incoming gas was 31.9.Similarly, after a 2 L C6D6 dose on Ag(111), the desorbing molecules in TPD have a C6D6 (m/z = 84) to C6D5H (m/z = 83) ratio 33.5 as shown in Fig. S3B UPPER.Then, TPD of co-adsorbed C6D6 and C2H2 on Ag(111) shown in Fig. S3C UPPER illustrates that the same C6D6 (m/z = 84) to C6D5H (m/z = 83) ratio (32.5) is observed, indicating no isotope scrambling occurs between C2H2 and fully deuterated benzene, and hence C-H bonds are not broken.
Furthermore, the expected ratio of m/z 26 to 28 according to the NIST 1 database is 999.9.However, as seen in Fig. S3 LOWER, during the acetylene dose the ratio of m/z 26 to 28 was 8.74.This indicates that acetylene (m/z 26) must be hydrogenating on the chamber walls to produce ethylene (m/z 28).Furthermore, after pump down to UHV the ratio of m/z 26 to 28 is 0.21 indicating that a significant amount of ethylene remains in the chamber background after the introduction of acetylene.This result indicates that the observed desorption of ethylene at low temperatures (around 95 and 120 K) in the cyclotrimerization experiments originates from background ethylene adsorption.

Figure S3 :
Figure S3: UPPER (A) Mass spectra for C6D6 and the ratio of C6D6 (m/z = 84) to C6D5H (m/z = 83) in the 5 × 10 -9 mbar C6D6 background dose.(B) TPD data for C6D6 desorption from Ag(111) shows the ratio of C6D6 to C6D5H adsorbed on Ag(111) after a 2 L C6D6 exposure (C) TPD data for co-adsorbed C6D6 and C2H2 on Ag(111) illustrates the same C6D6 (m/z = 84) to C6D5H (m/z = 83) ratio, indicating no isotope scrambling between C2H2 and fully deuterated benzene occurred.LOWER Mass spectra for (A) UHV chamber background before 100 L acetylene dose showing prominent peaks at m/z 18 and 28 for water and CO, (B) 1×10 -6 mbar of acetylene showing a prominent peak at m/z 26 and (C) UHV chamber background after 100 L acetylene dose showing a diminished m/z 26 signal but enhanced m/z 28 consistent with acetylene hydrogenation to ethylene on the UHV chamber walls.Background spectrum collected in panel A before was subtracted from the spectrum in panel B to distinctly show the changes induced by the introduction of acetylene.

Fig. S6
Fig. S6 The C4 formation step with different packing densities.Barriers are calculated from the lowest energy state to the transition state (TS1).(a) Free energies for a single C2H2 layer with various structures on a Ag(111) surface, all with packing density ≤ 1/3.(b) Free energies for different amounts of C2H2 in a 2x3 unit cell, forming multi-layers with packing density ≥ 1/3.Note that energies cannot be compared across different numbers of acetylene molecules per unit cell.

Fig. S7
Fig. S7Average adsorption energy of physisorbed C2H2 molecules (i.e., not including any chemisorbed or reacting molecules).As the packing density increases, the physisorbed molecules adsorb more strongly in the two-chemisorbed state and the transition state as compared to the allphysisorbed state.This counteracts the increase in energy for the chemisorbed or reacting molecules and facilitates reaction.

Fig. S8
Fig. S8 Acetylene C-H bond-breaking pathway on the Ag(111) surface.C-H bond breaking is both kinetically and thermodynamically unfavorable.