Selective Conversion of Syngas into Tetramethylbenzene via an Aldol-Aromatic Mechanism

Selectivity control in the single-step conversion of syngas to a single aromatic product is a big challenge. Here, we report an aldol-aromatic mechanism composed of aldol, phenolic, and aromatic cycles, that gave high selectivity >70% of a single product, tetramethylbenzene (TeMB) in hydrocarbons, at a reaction temperature as low as 275 °C. We evidently found the existence of oxygenated-aromatic compounds in the carbon pool, which remained active throughout the reaction and acted as key intermediates for the formation of the aromatics. The physical contact of ZnCr2O4 with H-ZSM-5 exhibited a strong coupling effect that promoted surface diffusion of C1 oxygenates (i.e., formaldehyde and methanol) from ZnCr2O4 into H-ZSM-5 and transformed into aromatics via an aldol-aromatic reaction pathway, thus overcoming the most difficult step for first carbon–carbon bond formation. In addition, ZnCr2O4 promoted the aromatics desorption by lowering the desorption activation energy and prevented the oversaturation of carbon pool species. Furthermore, it was found that a combination of thermodynamic equilibrium, surface methylation, and static repulsion are the key factors for giving high selectivity of TeMB in both carbon pool and final aromatics. This aldol-aromatic mechanism will open an efficient reaction pathway to upscale the process for selective aromatic synthesis in high yield from syngas.