Zeolites in Industrial Separation and Catalysis: Novel Separators, Innovative Catalysts and Future P

Abstract:Zeolites with their unique properties find applications in various fields, including medicine, agronomy, ecology, production of detergents and drying agents, and in a number of industrial processes. Among zeolites, mordenite is particularly widespread because of its high silica/alumina ratio, which allows it to resist exposure to high temperatures and to acidic gases and liquids. Mordenite is commercially available as a natural mineral and as a synthesized material. This zeolite is mostly used in its synthetic form as an acid catalyst in the petrochemical industry for the isomerization of alkanes and aromatics. In this review, we consider the scientific literature on the structure, synthesis, and two main types of modifications that solve the diffusion difficulties during catalytic processes. The first type of modifications is related to a reduction of the size of the mordenite crystals obtained to submicron or nanometric range, whereas the second ones aim to obtain hierarchical mordenite samples by appropriate post-synthetic treatments. Both types of modifications find many other applications besides solving diffusion constraints in catalytic processes. Attempts to fine-tune and control the particle size in the first type of modifications or the pore size in the second ones by adjusting various parameters during the synthesis are described.Keywords: zeolite; synthesis; nanosized mordenite; hierarchical material

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Zeolites with a Si/Al ratios higher about 3 are classified as high-silica zeolites, which are rarely found in nature and are synthesized industrially. They possess high physical and chemical stability due to the large covalent bonding contribution. They have excellent hydrophobicity and are suited for adsorption of builky, hydrophobic molecules such as hydrocarbons. In addition to that, high-silica zeolites are H+ exchangeable, unlike natural zeolites, and are used as solid acid catalysts. The acidity is strong enough to protonate hydrocarbons and high-silica zeolites are used in acid catalysis processes such as fluid catalytic cracking in petrochemical industry.

Although more than 200 types of zeolites are known, only about 100 types of aluminosilicate are available. In addition, there are only a few types that can be synthesized in industrially feasible way and have sufficient thermal stability to meet the requirements for industrial use. In particular, the FAU (faujasite, USY), *BEA (beta), MOR (high-silica mordenite), MFI (ZSM-5), and FER (high-silica ferrierite) types are called the big five of high silica zeolites,[17] and industrial production methods have been established.

In synthetic chemistry, homogeneous catalysts are preferred because of availability, low cost, and excellent catalytic activity as all the catalytic sites are readily available. But these homogeneous catalysts have several disadvantageous, such as being non-reusable, and require more than the stoichiometric amount. Also, some other drawbacks in its use include the potential dangers in handling, toxicity, corrosive nature, difficulty in separation and recovery, and disposal problems due to the acidic effluent. In addition to that, hydrolysis and purification of the resultant complex results in corrosive by-products. So, the basic idea is to find alternative heterogeneous solid catalysts which are stable, reusable, and nature-friendly, and there has been much attention to finding new ones which will also allow a better work up of reaction products. Among these different solid catalysts, zeolites were found to be superior due to their shape selectivity, thermal stability, and reusability .

Abstract: Catalytic conversion of methanol to propylene (MTP) by HZSM-5 zeolite is of great importance in industrial applications. In this paper, a series of HZSM-5 zeolites with different crystal sizes were synthesized by adjusting the initial gel composition, crystallization temperature, and crystallization time. The crystal structure, size, morphology, pore structure, and acidity of HZSM-5 were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, and temperature-programmed desorption of ammonia (NH3-TPD). The catalytic activity and stability of HZSM-5 with different crystal sizes for MTP were evaluated on a continuous flowing fixed-bed reactor. Coke deposited on HZSM-5 was analyzed by thermogravimetric (TG) analyzer. Results indicated that with smaller crystal size, HZSM-5 zeolite had larger surface area and pore volume, higher density of pore openings, and shorter path length of micropore channels that prevent side reactions. For MTP reaction, smaller crystal sizes of HZSM-5 showed a higher resistance and better tolerance to coke, and longer catalytic lifetime. The lowering of both the total and strong acidity on HZSM-5 with smaller crystal size favored a higher selectivity of target product, propylene.

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