Synthesis, Characterization Of NaY Zeolite Encapsulated Cosalen Complex And Its Catalytic Performance In Cyclohexane Oxidation

Cyclohexone is the most important petrochemical intermediate. Up to now, cyclohexane oxidation is the main method to prepare cyclohexone in industry. However, the current process requires elevated temperature and often shows lower conversion per pass (about 4%) and selectivity (about 80%) as for the inertia of molecule oxygen. Considerable research effort has been spent on how to realize the cyclohexane oxidation under the milder conditions. Mimicry for biological oxygenation may be an inspirational approach for selective oxidation of cyclohexane with molecule oxygen. N,N'-Bis (salicylidene)ethylenediiminocobalt (Cosalen) is analogue of metalloporphyrin with ability to activate oxygen effeciency, what's more, it was synthesized with higher yield by simple reaction routes from facile raw material. Whereas the Cobalt Schiff base complex is easily deactivated due to the formation ofμ-oxo-dimers and simple dimmers resulted in the decreased stability and deactivity. Immobilizing the complex on microprous solids can solve these problems. NaY zeolite has the special bottle-liking framework with the large supercage of 1.2 nm and relatively small open window of 0.7 nm, resembled the protein moiety of nature enzyme properly. It provided a new route for aerobic oxidation of cyclohexane catalyzed by the mimic model compound using the Cosalen complex imitates the role of the active site, the zeolite presumably replaces the surrounding protein moiety. In this work, the Cosalen complex encapsulated in the supercage of NaY zeolite by the technique of"Ship-in-a-bottle", the synthesis, characterization of zeolite encapsulated Cosalen complex (Cosalen/NaY) and its catalytic performance for cyclohexane oxidation were studied.As the ligand Bis (salicylidene) ethylenediimine (salen) diameter is smaller than the apertures of the NaY zeolite, the flexible ligand method was used to synthesis the Cosalen/NaY. The effects of synthesis conditions were studied and the conventional method was ameliorated. First, the Co exchanged NaY zeolite (CoNaY) was obtained by the NaY zeolite exchanged with the solution of cobalt salt, and then proceeded with the complex reaction between the CoNaY and salen. The final coordination complex became too large and rigid to escape out of the zeolite cages once assembled inside. When confined in the zeolite voids, the Cosalen may lose some of its degrees of freedom. By the conventional method the ligand is first melt at high temperature 150℃and then allowed to diffuse for 24 h into the zeolite to complex with ion-exchanged NaY zeolite. It was found that adding t-butyl alcohol as solvent can also realize the encapsulation even at relatively low temperature 80℃for shorten periods of time 16 h.The synthesized samples were characterized by the technique of FT-IR, UV-Vis, X-ray diffraction (XRD), TG/DTA and scanning electron microscope (SEM). At the same time, the impregnated Cosalen immobilized on the external surface of the zeolite was also prepared to compare with the encapsulated complex. The characterization results supported the fact that the Cosalen was encapsulated into the zeolite supercage and not on its surface.The catalytic oxidation performances of Cosalen/NaY in the cyclohexane oxidation by oxygen were investigated systemically. Cosalen/NaY showed the relatively higher catalytic activity during the oxidation reaction. In the absence of any solvent and initiator, the conversion of cyclohexane was up to 13.1% at the temperature 150℃and 0.85 MPa oxygen for 3 h, the ratio of cyclohexone to cyclohexanol was 3.6, which has the comparatively activity compared with the neat Cosalen complex. The solvent effect was existed in the oxidation reaction catalyzed by the Cosalen/NaY. CH3CN was the proper reaction solvent. Adding CH3CN as solvent, the ability of active oxygen was improved and the blocking of the zeolite pore was decreased. The conversion of cyclohexane was improved to 28.3% at 130℃for 3 h, the ratio of cyclohexone to cyclohexanol was 2.73, which was obviously higher than the result catalyzed by the neat complex.The effects of the reaction temperature, reaction time, usage of catalyst, initial oxygen preesure were reviewed. Among the products, the amount of cyclohexylhydroperoxide (CHHP) was small, it showed that the Cosalen/NaY had the ability to add the decomposition of the CHHP. Initial oxygen pressure was the important factor effected the reaction. With increasing oxygen pressure, the conversion of cyclohexane was improverd and in favor of the production of cyclohexone, the ratio of cyclohexone to cyclohexol was also improved. The oxidation reaction can proceeded effectively when the usage of catalyst was 0.5% of the adding amount of cyclohexane. In the system without solvent, the reaction temperature must over 130℃to initial the reation. With both the catalyst systems with or without solvent, the amount of acid in the products was found to observably increase with increasing reaction time, while the total amount of cyclohexone and cyclohexol had no obvious change. Cosalen/NaY showed the ability to oxidize cyclohexane into acid by one step. The reasonable explain was that the oxidation speed of cyclohexone and cyclohexol was higher than the one of cyclohexane; on the other hand, NaY zeolite with small ratio of Si/Al has the hydrophilic performance easier to absorb the polar material such as the cyclohexone and cyclohexol, which is in favor of the production of acid.The analysis of LC-MS indicated that adipic acid was the main diacid in the products of the cyclohexane oxidation catalyzed by Cosalen/NaY, there was a small amount of glutaric acid produced, while no succinic acid was found. When the conversion of cyclohexane was up to 37.2% at 130℃for 4.5 h under the initial oxygen pressure 0.85 MPa using CH3CN as solvents, the concentration of adipic acid in the total acid was over 50% and the yield was 7.24%. The total selectivity of cyclohexone, cyclohexol, CHHP and adipic acid was reached 68.5%. It provided a new method to prepare the adipic acid directly from cyclohexane.The reuse performance of the catalyst Cosalen/NaY was also studied. No obvious leaching of the Cosalen complex in encapsulated systems were observed in the oxidation reaction and the catalyst can be used repeatedly. The formation of simple dimmers between the Cosalen and theμ-oxo-dimers can be prevented effectively by the site isolation effect of the carrier's matrices when encapsulated into the supercage of the NaY zeolite.The effect of catalyst structure to catalytic performance was investigated. NaY zeolite was the proper carrier to encapsulate the complex. The catalytic activity sequence for the electron withdrawing group substituted salens of cobalt complex encapsulated into the zeolite was 5FCosalen/NaY>5ClCosalen/NaY >5BrCosalen/NaY. Cosalen complex was more active than the Agsalen/NaY for the ability to activate dioxygen.