Zusammenfassung
Colloidal zeolites, with particle sizes in the range of about 10 to 100 nm, which have very high external surface areas, have been extensively investigated in the last decade. It has been suggested that even smaller entities of several nanometers in size exist in synthesis solutions under certain conditions.
Small particle sized catalysts are known to be advantageous for some reactions due to their high external surface areas. Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam can be given as an example for these reactions. If catalyst samples prepared by crosslinking the smallest entities are used, it might be possible to have an extremely high surface area catalyst, which might be useful for the Beckmann rearrangement reaction.
In this study, it is aimed to synthesize colloidal sized zeolite catalysts, crosslinked with a silicone linker, and to characterize and test these catalysts in the Beckmann rearrangement reaction. Investigation of the synthesis parameters such as aging time, aging temperature, linker amount and their effects on the catalyst characteristics are also within the scope of this study.
Thus, in this study, colloidal Silicalite-1, TS-1, and ZSM-5 samples were synthesized and crosslinked with 1,7-dichloro-octamethyl-tetrasiloxane. The effects of preparation parameters such as aging time, aging temperature and linker amount on the characteristics of the samples were investigated by N2 adsorption, X-ray Diffraction, TEM, 13C MAS NMR, and 29Si MAS NMR techniques. Crosslinked catalysts were compared with a commercial ZSM-5 sample, and with crosslinked Aerosil, for their performances in the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam.
All solid products recovered from the colloidal solutions without crosslinking were X-ray amorphous with the exception of silicalite-1 aged at 363 K. Samples were also analysed by using 13C MAS NMR and TEM techniques, however, none of the ZSM-5 samples showed any crystallinity. On the other hand, it was documented by XRD and TEM analyses that the Silicalite-1 samples aged at 363 K for 24 hours were crystalline. Since the ZSM-5 samples did not exhibit any crystallinity, they are called ZSM-5 (prep.) indicating that they were prepared from a solution, which would result in the formation of ZSM-5 crystals under suitable conditions.
It was possible to observe that crosslinking was indeed achieved by using N2 adsorption analysis. Crosslinking invariably produced a higher porosity, with the additional pores forming predominantly in the mesopore range as was shown by N2 adsorption. Spacer molecules with different lengths were also tried but any pronounced effect on pore size distribution could not be observed.
29Si MAS NMR analyses also provided significant evidence on the interaction of the crosslinker with the silanols on the zeolite surfaces. After crosslinking, the concentration of Q2 and Q3 groups decreased, while the intensity of the Q4 group increased. This was ascribed to the reaction between the silanol groups of the zeolite and the silicone crosslinker. 29Si MAS NMR results made it possible to follow the extent of consumption during the crosslinking reaction, of the silanols on the zeolite surfaces, which are desirable for the Beckmann rearrangement reaction.
In all cases, these crosslinked materials were found to have substantially better catalytic performance than the non-crosslinked samples and than a commercial ZSM-5 catalyst (ALSI-PENTA® SH-27, SiO2/Al2O3 = 23-27). About tenfold increase of the caprolactam yield was typical for many of the crosslinked colloidal zeolites, and typically, the crosslinked materials performed better than the commercial ZSM-5.
It was observed that, aging temperature had a positive effect on the catalytic performances of all materials. Although increasing the linker amount above a certain level did not increase the reaction rates of ZSM-5 (prep.) samples further, the stabilities of the samples crosslinked with large amounts of linker were higher resulting in slower deactivation rates.
The best performance was obtained by the catalysts prepared by crosslinking of Silicalite-1 aged at 363 K. A strong relationship was observed to exist between the reaction rates and mesopore areas of the crosslinked Silicalite-1 samples aged at this temperature. Reaction rate increased parallel to the increase in the mesopore area. Reaction rates were related to the mesopore areas for the samples aged at room temperature too. The only difference between the samples aged at 363 K and room temperature was in their behaviour when linked with high amounts of linker. For the samples aged at 363 K, mesopore areas and the reaction rates of the samples continued to increase at high linker amounts, while for those aged at room temperature, mesopore area and reaction rate started to decrease when a certain level of linker amount was exceeded.
There was no clear relation between the reaction rates and mesopore areas for ZSM-5 (prep.) samples, if they were calcined and acid treated (CAT) or calcined and ion exchanged (CIE) before the catalytic tests. On the other hand, for the samples, which were acid treated without calcination (UAT), the reaction rates and the mesopore areas increased after crosslinking up to a certain amount of linker usage. Further increase in linker amount did not improve the reaction rate while it decreased the mesoporosity.
Aerosil crosslinked using the same linker did not exhibit any activity in the Beckmann reaction, indicating that the crosslinker had no catalytic activity on its own and that all catalytic activity originated from the zeolite surfaces.
Effect of crosslinking was also studied for TS-1 samples. Very high mesopore area was detected for the TS-1 samples crosslinked with large amounts of linker. Crosslinked TS-1 samples were tested both in the Beckmann rearrangement and the epoxidation of propene reactions. Although these samples, in general, did not exhibit a significant catalytic activity in the epoxidation reaction, the TS-1 sample crosslinked with a large amount of linker had a high catalytic activity in the Beckmann rearrangement reaction. Another advantage of crosslinked TS-1 was its long lifetime in the Beckmann rearrangement reaction. The catalyst life was longer in case of TS-1 samples crosslinked with large amounts of linker. After 4 h of TOS, almost no decrease in the reaction rate was observed for these catalysts, whereas the rates obtained with ZSM-5 and Silicalite-1 samples decreased by 49 % and 48 %, respectively, under similar conditions.
The selectivity toward caprolactam varied for the different materials, but was mostly above 65 %. The higher reactivity of the crosslinked materials was attributed to improved mass transfer and a high concentration of readily accessible catalytic sites on or close to the external surfaces of the colloidal zeolite particles.
