The crystallization mechanism of pure-silica ZSM-22 was investigated in the presence of seeds. Seed crystals played a significant role in the nucleation and crystal growth. Experimental findings revealed that the seed crystals acted as a substrate for crystal crystallization, and nucleation only occurred via the interaction between the organic structure-directing agent (OSDA), 1,6-hexanediamine, and seed crystal surface. Oligomeric silicate species in the liquid phase as nutrients directly crystallized on the crystals. The crystal growth occurred via layer-by-layer construction on the external surface of the seed crystals. The major chemical process occurring in the crystallization process was (i) dissolution of the large seed crystals into many thin needle-like crystals, (ii) dissolution of silica particles to form oligomeric silicate species, (iii) interaction between template molecules and the crystal framework which promoted nucleation, and (iv) formation of fully crystalline pure-silica ZSM-22 via layer growth. The morphology of pure-silica ZSM-22 can be controlled by the interaction between OSDA and the crystal framework which affected crystal growth in the a- and b-orientation.
Pure EMT-type zeolite (EMC-2) was synthesized using 18-crown-6 ether as structure direction agent and first applied for deeply purification the trace polar-oxygenated compounds from light olefins as well as studying the removal mechanism. This EMC-2 zeolite crystals exhibited high crystallinity , regular hexagonal platelet morphology mainly with diameter of 3–4 μm, thickness of 350 nm. The N 2 adsorption/desorption analyzer indicated the EMC-2 zeolite had both micropores and inter-particle mesopores . The BET surface area , total pore volume and average pore size were 599.5 m 2 /g, 0.347 cm 3 /g and 2.32 nm, respectively. By dynamic adsorption experiment at room temperature , the results demonstrated that the EMC-2 zeolite could deeply purify the trace polar-oxygenated compounds to below 1 × 10 −6 mol/mol in N 2 or C 2 H 4 flow, and exhibited more excellent purification performance than that of 13X zeolite. The breakthrough time for methanol to 1.18 g EMC-2 zeolite was 73.0 h in N 2 system, and 6.0 h to 0.31 g EMC-2 zeolite in C 2 H 4 system. The cumulative adsorption amounts of EMC-2 zeolite for methanol and propanal in N 2 and C 2 H 4 system were 55.99 mg/g, 113.93 mg/g and 98.17 mg/g, 171.61 mg/g, respectively. Moreover, the regeneration experiment also demonstrated the EMC-2 zeolite had more stable structure and longer lifetime. The in-situ FTIR spectra indicated both the purification processes of EMC-2 zeolite for methanol and propanal were physi-sorption mainly via the interaction between the oxygen lone-pair electrons of impurities and the Na + of EMC-2 zeolite. Therefore, the EMC-2 zeolite had great potentials for future industrial application of trace polar-oxygenated compounds removal from light olefins .
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