(264e) Template-Free Preparation of EMT Zeolite and Its Coatings

EMT type zeolites have favorable properties, especially regarding adsorption and catalysis applications. The three-dimensional large pore of EMT is significant for catalysis, such as in alkylation, isomerization and aromatization. Remarkable adsorption capacities, especially for water have also been mentioned for these zeolites. EMT type of zeolite is a hexagonal polymorph of the cubic faujasite with somewhat different framework structure. When EMT is observed as an intergrowth with FAU-type zeolites, it is called ZSM-3 or ZSM-20. On the other hand, pure EMT zeolite may be represented by the EMC-2 material. Due to the relatively large pore sizes, surface areas and high water capacities of both FAU and EMT zeolites, their intergrowth structures ZSM-3 and ZSM-20 are also advantageous to be used in catalysis and adsorption.

Zeolites are known to be metastable and may transform into other phases as the synthesis time is extended due to the changes in the composition of the reaction mixture. Some zeolites are more metastable than others and this may present a lot of difficulties in obtaining them in pure form. Preparing relatively thick coatings of such zeolites may also be very difficult. Quite thick coatings of the metastable zeolite A could be prepared in one synthesis step only after using the substrate heating method, in which the substrate was heated to a higher temperature than the reaction mixture which was cooled in a water bath [1]. EMT zeolite may be regarded as one of the most metastable zeolites and has been recognized to be difficult to obtain in its pure phase. Crown ethers, which fit well in the supercage of faujasite, may act as organic structure directing agents (templates) for the hydrothermal synthesis of EMT. Unfortunately, these materials are generally quite expensive and not environmental friendly. Such drawbacks have made the widespread practical use of EMT zeolite difficult. The preparation of EMT coatings of notable amount has not been possible, either.

Recently, studies performed to meet the challenge of reducing and even completely removing templates in EMT synthesis have become popular. Reducing the amount of the template [2], preparing nano-sized (6-15 nm) EMT without any template [3], utilizing EMC-2 seeds for the template-free synthesis of EMT zeolites [4] provided some improvements. However, there are still some problems regarding the generally low yield and crystallinity, as well as the difficulty for recovery, in the case of nano-sized EMT crystals. The developments achieved up to date do not seem sufficient for the template-free preparation of coatings of EMT zeolite, at least, with notable amount of mass/thickness. In this study, it was aimed to prepare coatings of EMT type zeolites by using the substrate heating method without using any organic template. For this purpose, various synthesis conditions (reaction mixture composition, synthesis time and temperature) were investigated. The phase of the zeolite obtained, as well as the thickness, stability and homogeneity of the coating were taken into consideration. The coatings obtained were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEGSEM), energy dispersive X-ray spectroscopy (EDX) and thermogravimetry (TG).

As a result, the preferential heating of the substrate was shown to be useful for preparing coatings of EMT type zeolites, without using any organic template. Coatings of ZSM-3 of about 45 mm could be prepared by using the substrate heating method. Under similar synthesis conditions, conventional synthesis in the oven did not allow obtaining coatings with much notable thickness (<a few microns), due to the highly metastable nature of EMT zeolites. The coatings were observed to be quite homogeneous and stable as the reaction mixture used for synthesis became clearer to the eye. Thermogravimetric analysis indicated the presence of relatively high water capacities as well as relatively low regeneration temperatures for some EMT coatings that may be useful for various applications, such as adsorption heat pumps.

References

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