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 degrees 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 C-1 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.