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Zeolite Composition and Structure
Published in Subhash Bhatia, Zeolite Catalysis: Principles and Applications, 2020
Zeolite ZSM-5 is a member of a new class of shape-selective catalysts with unique channel structures which differ from the familiar large pore faujasite and small pore zeolites such as Linde type A and erionite. They also possess unusual catalytic properties and have high thermal stability. The framework of ZSM-5 contains a novel configuration of linked tetrahedra shown in Figure 2.7 and consisting of eight five-membered rings. These ZSM-5 units join through edges to form chains as shown in Figure 2.7. The chains can be connected to form sheets and the linking of the sheets lead to a three-dimensional framework structure. The chains extend along the Z-axis. The crystal structures of zeolites ZSM-5 and ZSM-11 which were determined by Kokotailio and Meier22 are closely related and it appears that silicon can be substituted for aluminum without affecting the basic structure until the lattice consists of pure SiO2 (Olson et al.23). The generic name “pentasil” has been given to designate these solids, irrespective of minor differences in crystal structure. A schematic structure of ZSM-11 is shown in Figure 2.8. The straight channels have diameters of about 5.4 x 5.6 Å free diameter and the sinusoidal channels are about 5.1 x 5.5 Å free diameter. ZSM-5 crystallizes in the idealized orthorhombic system with space group Pnma and lattice constant a = 20.1, b = 19.9, and c = 13. Monoclonic symmetry has also been observed. The unit cell contents of the Na form are
From Nano- to Macro-engineering of Nanocomposites and Applications in Heterogeneous Catalysis
Published in Mahmood Aliofkhazraei, Advances in Nanostructured Composites, 2019
In recent years, the methanol-to-olefins process has been attracting particular attentions as an alternative route for the light olefins production from non-petroleum sources such as coal, natural gas, and biomass (Mokrani and Scurrell 2009). To date, large-scale implementation of this process has been successful in ethylene operation mode on a SAPO-34 zeolite catalyst in a fluidized bed reactor. However, the global demand for propylene is growing faster than for ethylene (Traa 2010). Hence, it is particularly desirable to develop catalysts that can selectively convert methanol to propylene (MTP). ZSM-5 zeolite-based catalysts for methanol-to-olefins process have been extensively studied to orient product selectivity toward light olefins and especially propylene and to further improve the catalyst stability, although Lurgi’s MTP process based on a packed bed with a ZSM-5 particulate catalyst has been industrially demonstrated (Mokrani and Scurrell 2009). Most of the effort has been focused on ZSM-5 modification such as tuning the acidity, size- and/or morphology-controllable synthesis, and hierarchical design of the pore structure. In some cases, high selectivity toward propylene is obtainable with the increased propylene to ethylene ratio on ZSM-5 zeolite catalysts (Bleken et al. 2012, Liu et al. 2009). Despite these promising results, their practical use as catalysts in a fixed bed reactor is still particularly challenging, as microgranules or extruded pellets a few millimeters in size are required in real-world, macroscopically shaped forms rather than as-made powders. As a result, some frustrating problems are inevitable and unavoidable in these cases including mass/heat transfer limitations, high pressure drop, non-regular flow pattern, and adverse effects of the binders used, which will always reduce the intrinsic catalyst selectivity and activity.
Response surface methodology approach for the optimization of tartrazine removal by heterogeneous photo-Fenton process using mesostructured Fe2O3-suppoted ZSM-5 prepared by chitin-templating
Published in Chemical Engineering Communications, 2018
Fernanda Caroline Drumm, Jivago Schumacher de Oliveira, Edson Luiz Foletto, Guilherme Luiz Dotto, Erico Marlon Moraes Flores, Michele Stéfani Peters Enders, Edson Irineu Müller, Sérgio Luiz Janh
To increase the interaction between the pollutant molecules and the HO• radicals formed on the surface of the catalyst, providing a higher reaction rate, several iron-based catalysts have been impregnated on different solid supports such as zeolite (MacDonald et al., 2014; Oliveira et al., 2016), activated carbon (Sheydaei et al., 2014), clay (Gao et al., 2015), and silica (Kumar et al., 2016). In the present work, ZSM-5 was prepared and used as a support of iron oxide particles, because several studies have shown that ZSM-5-supported iron-based catalysts are highly efficient for the degradation of organic pollutants (Kasiri et al., 2008; Chen et al., 2008; Sashkina et al., 2014). Due to microporous characteristic of the ZSM-5 zeolite, which limits the accessibility of large molecules on its active surface, different templates have been used to obtain a ZSM-5 with mesoporous structure (Chou et al., 2006; Jin et al., 2012). However, the preparation of mesostructured Fe2O3-supported ZSM-5 using chitin as a mesopore-directing agent for use as a photo-Fenton catalyst has not been reported yet.
Transition metals-incorporated zeolites as environmental catalysts for indoor air ozone decomposition
Published in Environmental Technology, 2018
E. F. Mohamed, G. Awad, H. Zaitan, C. Andriantsiferana, M-H. Manero
On the other hand, the transition metals and their oxides have been found to be the most active substances for ozone decomposition [2]. Zeolite is characterized by its ability to participate in ion-exchange reactions [10]. The incorporation of various transition metal ions, such as iron, copper, and cobalt ions, into these materials can significantly change the chemical properties of zeolites making them active and efficient catalysts for several redox processes [18]. The crystal structure of ZSM-5 is composed of two types of channel systems with similar size and 10-membered oxygen ring. Straight channels have an oval cross-section of 5.3 × 5.6 Å and sinusoidal channels with cross-section of 5.1 × 5.5 Å. These two types of channels are perpendicular to each other and generate microporous volume network of diameters of 8.9 Å. Na ZSM-5 zeolite is basic and a typical catalyst, but its catalytic ability is poor. Through ion exchange, that is, Na+ from the framework is substituted by H+ and forms HY zeolite which has unselective acid sites located on the external surface. Furthermore, the hydrothermal stability of HY zeolite is very poor. The presence of protons gives the ZSM-5 zeolite the ability to exchange with transition metal ions. Modification of ZSM-5 including incorporation of transition metal ions leads to good catalytic activity and good hydrothermal stability. The catalytic properties of transition metal zeolites are strongly influenced by the composition, location, and structures of the reactive metal species introduced into the microporous space [19].
Catalytic performance of modified beta zeolite on the synthesis of styrene and xylene: a kinetic study
Published in Indian Chemical Engineer, 2020
Yashika Raparia, Gopinath Halder, RajKumar Arya, Sanghamitra Barman
The commercial production of styrene and xylene are restricted by thermodynamical and environmental limitations, by market limitations/factory requirements. This has invoked extensive research towards novel routes for the production of styrene. Side-chain alkylation takes place at intermediate electronegativity [9]. The direction to side-chain or ring alkylation lay on the state of the adsorbed methanol in the zeolite [10, 11]. The basic catalysts were modified by the addition of Cu or Ag [12] B, P, Cu, Zn or other metals [13–15] KOH or CsOH [16] or cesium acetate [17]. Other authors [18] studied alkali X zeolites modified with Cs and K. It has been shown that the selectivity towards side-chain alkylation is in the order CsX > RbX > KX > NaX > LiX [19]. ZSM-5 zeolites exchanged with strongly basic metals like Cs, catalyse only the side-chain methylation of toluene [20]. Toluene methylation was performed over alkaline earth (Mg2+, Ca2+) exchanged zeolite X and Y which followed path B in Scheme 1 [17]. This high activity and xylene selectivity of alkaline earth exchanged X and Y zeolites are related to the presence of acidic surface hydroxyl groups (Brönsted acid sites), although these catalysts were prepared from Na-X and Na-Y zeolites. These Brönsted acid sites are formed by hydrolysis of water during the preparation, resulting from the strong electrostatic field imposed by the alkaline earth cation exchanged into the zeolite pores. This produces basic alkaline earth metal hydroxyls and Brönsted acid sites [17, 20, 21]. ZSM-5 type zeolites are used for selective formation of p-Xylene by alkylation of toluene [22–24] with methanol. ZSM-5 is used because of its activity and shape selectivity.