In recent years, mordenite molecular sieves with various properties have been successfully synthesized by template method. Although the template method has received a lot of attention, the addition of the templating agents especially organic templating agents, would increase the cost of raw materials, and could be harmful to environment. Therefore, the method without organic template is more economical and environmentally friendly. Recently, the synthesis and applications of mordenite modified by hetero-atom have getting more and more attentions, and the performance of modified mordenite molecular sieve shows better. The common methods for modifying mordenite molecular sieve include ion exchange method and impregnation method. Some researchers have synthesized monolithic iron-containing mordenite (Fe-MOR) by template-free method, which has excellent CO₂/N₂ separation performance and high carbon dioxide capture performance. In addition, Fe-MOR zeolite can catalytically decompose N₂O into N₂ and O₂, which can effectively reduce the N₂O content in the atmosphere. In this work, Fe-MOR zeolite was synthesized by hydrothermal method without organic template. A novel rod-cluster-shaped Fe-MOR zeolite was controllably synthesized by fine adjustment of pH and crystallization time, and its CO₂ adsorption properties were investigated.

Deionized water, tetraethyl orthosilicate, concentrated hydrochloric acid, ferric nitrate, aluminum sulfate and sodium hydroxide were used. A mixture with n(Fe) : n(SiO₂) : n(Na₂O) : n(H₂O) : n(Al₂O₃) : n(HCl)=0.002 5∶1.000 0∶x∶30.000 0∶0.066 5∶y (n is molar ratio, x=0.300 0~0.580 0, y=0.100 0~0.150 0) was obtained, then the mixture was transferred into a polytetrafluoroethylene lined stainless steel autoclave and crystallized at 180°C for 7 to 14 days. When the reaction finished, a white powder sample was obtained after post-treatment.

The crystalline phase and crystallinity of the samples were analyzed by PANalytical X'Pert Powder (XRD). S-3400N scanning electron microscope (SEM) was used to observe the morphology of the samples. The elemental composition of the samples was analyzed by S8-tiger X-ray fluorescence spectrometer (XRF). The static CO₂ adsorption performance of the samples was measured by ASAP-2460 physical adsorption analyzer. A fixed-bed apparatus and a Hiden Analytical HPR-20 on-line mass spectrometer were used to determine the dynamic CO₂ adsorption performance of the samples.

The crystalline phase of the product is effected by the amount of sodium hydroxide or the concentration of hydrochloric acid used in the initial gel mixture. The pH value of the initial gel mixture is crucial for the synthesis of pure phase Fe-MOR. The XRD results show that the peak intensity of MOR decreases gradually with the decrease of Na₂O molar ratio from 0.4 to 0.36, indicating the crystallinity of samples related tod the basicity in a certain range. When the molar ratio of Na₂O/SiO₂ is 0.4 or 0.375, the pure phase MOR with rod-shaped cluster was obtained, in which the single rod is approximately hexagonal and the morphology is uniform. When the molar ratio of Na₂O/SiO₂ is in the range of 0.3-0.58, the analcite will arise if the alkalinity of the system is too high, and the amorphous phase will appear if the alkalinity is too low. This means that moderate alkalinity is conducive to the interaction of silicon, aluminum and other components, promoting the nucleation and growth of the crystallization, contributing to the synthesis of pure mordenite. Moreover, when the molar ratio of Na₂O/SiO₂ is 0.4, the pure phase of Fe-MOR can be controllably synthesized by adjusting the molar ratio of concentrated hydrochloric acid.

The pure phase Fe-MOR molecular sieves can also be synthesized by shortening the crystallization time under the constant pH. The axial size of the synthesized samples also decreases with the decrease of the crystallization time. The results show that the longer the crystallization time is, the better the axial growth of the crystals, but when the crystallization time is less than 7 days, the hetero-crystalline phase of analcite appears.

The static adsorption capacity of CO₂ on Fe-MOR samples was similar because the adsorption mainly depends on the atmosphere pressure. The capacity of dynamic adsorption of CO₂ is due to the difference of the suitable site of electric field in the 8-membered ring channel of MOR, which determines the adsorption capacity in the dynamic CO₂ penetration under low partial pressure of CO₂. The microstructure of Fe-MOR molecular sieves synthesized under different conditions are different, which results in the difference of CO₂ dynamic adsorption capacity of several samples.

The results show that the pH of the gel mixture and crystallization time affected the synthetic products. When the amount of hydrochloric acid is constant, the high alkalinity results in analcite crystals while the low alkalinity results in amorphous phase. When the amount of sodium hydroxide is high, increasing the amount of hydrochloric acid will result in the pure Fe-MOR molecular sieves. The axial size of the crystal will decrease with the shortening of the synthesis time under appropriate pH value. When the crystallization time was less than 7 days, the hetero-crystalline phase of analcite appeared, which was attributed to the short synthesis time leading to incomplete crystallization. Rod cluster Fe-MOR molecular sieves were synthesized by hydrothermal method without organic template and showed good static and dynamic adsorption capacity for CO₂, and the static adsorption capacity was up to 3.57 mmol/g.