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  • ZSM-5 - Zeolite Socony Mobil–5
    Jul 24, 2019 | ACS MATERIAL LLC

    ZSM-5

    The ZSM-5 molecular sieve boasts unique and exceptional properties, which are widely employed in important processes and applications across a broad range of industries. Commonly used in converting methanol to gasoline and diesel as well as oil refining, ZSM-5 has proved superior to amorphous solid acid catalysts in reactions such as xylene isomerization, toluene disproportionation and toluene alkylation etc. Upon ion exchange or modification, H-ZSM-5 zeolites can be derived to also possess enhanced para-selectivity. All in all, the high versatility of this zeolite makes it a truly indispensable material across many industries.

    Introduction

    The ZSM-5 molecular sieve is a highly siliceous aluminosilicate zeolite with an intersecting and three-dimensional channel system. Its chemical formula, NanAlnSi96-nO192 · 16H2O, shows the unit cell composition of the zeolite. In this formula the variable, n, can range from 0 to 27. This means the ratio of the amount of silicon molecules and aluminum molecules can be changed within a pretty large range with the total number of silicon and aluminum molecules at 96.1,2

    ZSM-5 is a shape selective catalyst with a unique structure, and it is one of the most important microporous materials characterized by a 10-membered ring (10-MR) pore structure. Kokotailo et al. reported the structure of ZSM-5 in 19783 and the structural description of ZSM-5 is shown in the table in Figure 1.

    The ZSM-5 framework contains a novel configuration of linked tetrahedra, which are the primary building units of three-dimensional channel systems within zeolite structures. The secondary building unit that forms from these tetrahedra display a 5-1 arrangement, a principal characteristic of Pentasil Zeolites, by which 5-membered oxygen rings are formed. Fourteen tetrahedra combine to form a sub-unit (referred to as the Pentasil Unit) with eight faces, all pentagonal in shape. The pentagonal sub-units exhibit the 41 m2 (D2d) symmetry and the linear linkage of these units results in the formation of chains extended along the z-axis. The framework structure parallel to orientations <010> and <100> is shown in Figure 2.4

    Fig.1

    Figure 1. Structural description of ZSM-5 Zeolite

    The ZSM-5 framework contains two sets of 10-MR channels, running perpendicular to one another, through the lattice. One set of channels is straight with a slightly elliptical cross section (5.2 x 5.7 Å) and the other channels run in zig-zag formation (sinusoidal) with a circular cross section (5.3 x 5.6 Å).5

    Fig.2

    Figure 2. Framework Structure

    X-ray diffraction (XRD) is a powerful and readily available technique for determining the atomic arrangements within a zeolite. Like any other crystalline material, ZSM-5 zeolite exhibits unique diffraction patterns. Such XRD analyses are helpful for structural identification. The XRD analysis for ZSM-5 shows five characteristic peaks, as shown in Figure 3, clearly indicating the MFI structure.

    Fig.3

    Figure 3. XRD analysis of ZSM-5 Zeolite

    SEM images of ZSM-5 change according to the synthesis method and precursors. Several different morphologies and forms of ZSM-5 are available for purchase on the ACS Material online store.

    Synthesis

    From the moment the ZSM-5 molecular sieve was successfully synthesized and its excellent performance in applications was discovered, researchers have worked tirelessly on developing various methods for producing ZSM-5.6-8 Currently, the most popular synthesis methods of ZSM-5 can be roughly divided into the following three categories: synthesis in organic amine and inorganic amine systems; synthesis in load systems; and synthesis in hydro-thermal systems/ non-hydro-thermal systems.9-12 Although the aforementioned methods involve different templates, the raw materials (including different silicon sources and aluminum sources) and principles remain the same. The structure of raw materials is rearranged to form unique pore channel structures we know as molecular sieves.13-14

    TPA+ and SiO2/Al2O3 ratios are two important factors that influence the synthesis of the ZSM-5 molecular sieve. Alkalinity of the gel mixture is an additional parameter that plays a dominant role. The high silica content of this structure makes the material particularly sensitive to solubilizing in highly alkaline media. At high hydroxide concentrations, crystal growth and dissolution phenomena compete, resulting in the formation of smaller crystals.

    Studies have indicated the readily crystallizable nature of ZSM-5 zeolite from gel systems can be formed using different organic templating agents. Van der Gaag shows that, 6-hexanediol, 1,6-hexanediamine, 1-propanol, 1-propylamine, and pentaerythritol all encourage the formation of this structure. The TPA+ cation is a preferred additive to the synthesis mixture in this case, as it strongly encourages ZSM-5 structure formation over the widest range of SiO2/Al2O3 ratios. The downfalls of this method, however, include the high cost of the additives, corrosivity and the need for the removal of the additives before employing the zeolite for its catalytic activity. Attempts have been made to synthesize ZSM-5 zeolite from a template-free gel medium and some results have been reported to yield highly crystalline ZSM-5 material. Continuous attempts are being made to efficiently produce good crystalline ZSM-5 zeolites at low costs.

    Applications

    Theproperties of a particular ZSM-5 zeolite depend on its crystalline framework arrangement, the uniformity of its channel size and acidity. ZSM-5 zeolites have uniformly sized pores which comes to an advantage when molecules larger than the size of the channel cannot form within the zeolite with exceptions, at times, at the intersections. The pore dimensions of the ZSM-5 zeolite are also uniquely suitable for the formation of C7 and C8 olefins, and their cyclization to corresponding aromatic compounds. This unique property of ZSM-5 restricts the formation of di- and tri- cyclic aromatic compounds, which are coke precursors.

    The particular property which makes ZSM-5 zeolites especially useful for commercial applications is shape selectivity. The term “shape selectivity” was coined in 1960 by Weisz and Frilette to describe the unique catalytic properties of small pore molecular sieves.15 It wasn’t until later that the availability of synthetic medium 6Å pore zeolites expanded the realm of shape selectivity. Ultimately, it was the uniformity and medium sized pore opening of the ZSM-5, along with the probability of forming product molecules, that made pentasil family zeolites suitable for shape selective catalysis. ZSM-5 zeolite differs greatly from most other molecular sieves in that its shape selectivity has a very wide dynamic range.16

    Generally, shape selectivity can be classified into the following categories: (1) Reactant Selectivity, (2) Restricted Transition State Selectivity and (3) Product Selectivity.

    (1) Reactant Selectivity

    Reactant selectivity is when only a certain type of reactant molecule, smaller in size compared to the others, is diffused and passes through the catalyst pores. The distillate dewaxing process of Mobil, for instance, is a reactant shape selective process in which only the straight chain or slightly branched paraffins present in a distillate are able to enter the ZSM-5 pores, where they get cracked to lighter products. This yields a less “waxy” product with a lower pour point.

    (2) Restricted Transition State Selectivity

    This occurs when both the reactant molecules and product molecules are small enough to diffuse through the channel, but the reaction intermediates are larger than either reactants or products and are specially constrained. Monomolecular rather than bimolecular transition states are favored at these conditions. The most important example of restricted transition state selectivity is the absence of early cooking in ZSM-5 type molecular sieves. This type of shape selectivity plays a major role in the selective cracking of paraffins in the ZSM-5 family of zeolites. For example, the steric strain of the larger transition states complex required to crack 3-methyl pentane in ZSM-5 is the proposed cause of its lower activity than that of n-hexane. The methanol to gasoline (MTG) conversion is another important example of transition state shape selectivity, where the available space in the cavities of the ZSM-5 determines the largest bimolecular reaction complex that can be formed.

    (3) Product Selectivity

    This occurs when some of the products formed within the pores are too bulky to diffuse out and appear as observed products. They are either converted to less bulky molecules (e.g. by equilibration) or eventually deactivate the catalyst by blocking the pores. Disproportionation of m-xylene is the best example of this. Among the alkylated products, 1, 3, 5-trimethy benzene will be preferentially formed to the bulky products molecule 1,3,5-trimethy benzene. Similarly, in xylene isomerization, p-isomer is preferentially formed compared to o-isomer.

    One of the unique shape selective characteristics of H-ZSM-5 is its para-selectivity in electrophillic substitution reactions such as alkylation and disproportionation of alkyl aromatics. By adjusting the acid site activity of the zeolite and controlling the diffusion parameters, high para-selectivity can be achieved.

    Conclusion

    The aforementioned features of the ZSM-5 zeolite makes it a highly suitable catalyst for an incredibly wide variety of industrial processes including shape selective cracking such as M-forming, distillate dewaxing, and lube de-waxing processes. Aromatization processes like M-2 forming, cyclar, aroforming, and methanol to gasoline (MTG) conversion also benefit highly from the ZSM-5 zeolite as well as shape selective conversion processes like xylene isomerization, toluene disproportionation, ethylbenzene synthesis, and para ethyl toluene synthesis. It goes without saying that the the ZSM-5 zeolite is a highly valuable material in many industries across the board.

    ACS Material Products:

    Molecular Sieves

    References

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