Synthesis Of ZSM-5 Zeolites With Different Morphology In Polyacrylamide Hydrogels

Zeolites with different morphology have different pore sizes, surface area and acidity sites distribution, which affects the catalytic activity and adsorption performance. Many approaches have been reported for preparing zeolites with different morphology. Most of them can be classified into two categories: changing the synthetic condition and adding organic or inorganic additives. Changing synthetic condition includes changing the type or concentration of the template, changing the molar ratio of the initial precursors or changing the alkalinity of the synthesis solution. In principle, the orientation and morphology of the zeolite is determined by the different growth rate of different crystal faces, which have different surface energies. Additives could interact with the crystal face and change the relative surface energy, which leads to the final zeolite with different morphology. In this paper, the polyacrylamide(PAM) hydrogels were introduced into ZSM-5 synthesis gel, the growing orientation of ZSM-5 zeolite was successfully changed and the catalytic performance in MTO reaction was obviously improved. The effect of PAM on the morphology of zeolite was studied by varying the type of PAM, the adding way and the synthesis conditions.B-orientated ZSM-5 zeolite was successfully prepared in the presence of PAM. The effect of various paramaters was investigated. The results showed that the appropriate condition was crystallizating at 180 ℃for 48 h and the molar ratio of organic monomers, crosslinking agent and initiator was determined 100:1:0.5. Conventional ZSM-5 sample(Z-0) was obtained without PAM, hexagonal prism structure was got in 1% PAM(Z-1) and plate-shaped was prepared in 2% PAM(Z-2). The formation mechanism of plate-shaped crystals may be explained as follows. The abundant Si-OH groups on the surface of the layered ZSM-5 precursor species could interact with PAM to form hydrogen bonding. Therefore, the condensation reaction of the Si-OH groups was inhibited. The plate-shaped ZSM-5 zeolite was developed by growing faster in c-axis than that of a-axis and b-axis. The amount of weak acid and total acid of the samples with different morphology have the same tendency of Z-0>Z-1>Z-2. And only the sample synthesized in 2% PAM has strong acid sites. Therefore, Z-2 sample with plate-shaped ZSM-5 had the highest catalytic activity and the methanol conversion approached 100%, the selectivity of C2-C4 olefins achieved 81.19%. Moreover, the conversion and selectivity could also reached 100% and 50.74% even after reaction for 23 h, respectively. Compared to Z-0 and Z-1, the catalytic performance of Z-2 significantly improved and the lifetime of Z-2 prolonged. This may be attributed to the straight channels of plate-shaped ZSM-5 are favorable for mass transfer and inhibiting coke formation.Then, the influence of the structure and adding way of PAM as well as the synthesis conditions on the synthsis of ZSM-5 was investigated. The results indicated that the synthesized ZSM-5 samples had lower relative crystallinity and showed irregular morphology compared to Z-0 by varying the type of PAM and adding it directly into the ZSM-5 synthesis gel.The morphology of ZSM-5 was also changed by changing the synthesis condition and adding PAM directly into the ZSM-5 mixture. High crystallization temperature or appropriate PAM concentration was beneficial for the formation of ZSM-5 sample with regular morphology. Plate-shaped ZSM-5 zeolite was formed at high temperature without PAM. In 0.1 g PAM, conventional ZSM-5 zeolite was obtained at lower temperature, but hexagonal and conventional morphology both appeared at higher temperature. In this condition, ZSM-5 synthesis gel would be entrapped in the three-dimensional network of individual PAM and then limited the growing by c-axis.