Synthesis, characterization of COK-5 with different Si/Al ratios and their catalytic properties for the tert-butylation of phenol

The synthesis, characterization and catalytic properties of COK-5 with different Si/Al ratios are described. Various key parameters (including the ratios of Si/Al and/or Na/Si, the type/concentration of alkali cations, and the structure of dialkylammonium) are investigated in detail for the successful synthesis of COK-5. Under our synthesis conditions, it is found that COK-5 zeolites could be synthesized only in a narrow range of Si/Al ratios and Na/Si ratios. The concentration of Na+ in the initial gel should be carefully controlled for the successful synthesis of COK-5 with lower Si/Al ratios. Rising the crystallization temperature (<453 K) or adding seed (including heterogeneous seeds ZSM-57 and ZSM-22) can shorten the crystallization time of COK-5. N2 adsorption–desorption results suggest that the COK-5 samples synthesized from gels with different Si/Al ratios show different texture properties. NH3-TPD characterization demonstrates the presence of some relatively strong acid sites on the surface of H-form COK-5 zeolites. The resultant COK-5 zeolites with different Si/Al ratios are catalytic active in the liquid-phase tert-butylation of phenol. The acid sites located on the external surface and near the entrance of pore channels of COK-5 zeolites are the main active centers for the reaction. Compared with other 10-ring and/or 12-ring zeolites, different product selectivities are observed on COK-5. We speculate that the nature of less stronger acids and the unusual pore systems of COK-5 zeolites, especially with distorted 12-ring pore structures, should be highly correlated with their catalytic properties.