Structure of zeolites

The framework of zeolites includes channels with dimensions less than 1 nm; therefore, zeolites are typical microporous materials. The elementary building block of zeolites is TO₄ tetrahedra, in which the T atom is Si or Al. This aluminosilicate framework is formed when the adjacent tetrahedra share the common oxygen atom. The building unit is illustrated in Figure 2: a tetrahedron is comprised of either a silicon cation (Si⁴⁺) or an aluminum cation (Al³⁺) that is surrounded by four oxygen anions (O^2-), resulting in the formal electron valency [SiO₄]^4- and [AlO₄]^5-. 

When aluminum cations substitute silicon cations, the net charge in the framework becomes negative. The resulting negative-charged sites in the framework are compensated by extra-framework cationic species such as alkaline or alkaline earth metals (Na+, K+, or Ca2+ are the most common cases)

. These cations connect with the aluminosilicate structure by weaker electrostatic bonds and are found on the external surface of the zeolite

The chemical composition of a zeolite can be represented by the following formula

Al^{m+}_{y/m}[(SiO_2)_{x}.(AlO^-_2)_{y}].zH_2O

where A (e.g., Na) is a cation with the charge m, (x+y) is the number of tetrahedra per crystallographic unit cell, and x/y is the so-called framework silicon/aluminum cation ratio. According to Löwenstein’s rule (shown in Figure 3), two continuous tetrahedra containing aluminum on tetrahedral positions (Al–O–Al linkages) are prohibited.

Figure 3. An illustration of Löwenstein’s rule (Figure by Linh Tong)

Case study: Structure of ZSM-5

ZSM-5 is short for Zeolite Socony Mobile Five, one of the most important catalysts  with unique channel structures. The unit cell contents of the Na form are Na_nAl_nSi_96-n.16H₂O, where n < 27, and ZSM-5 crystallizes in an orthorhombic system with a space group Pnma

. The orthorhombic system would shift to the monoclinic framework structure when Brønsted acid sites are balanced instead of cationic species, and such zeolite is called HZSM-5. HZSM-5 has higher acidity than ZSM-5, and the space group is P2₁/n.1.1. 

ZSM-5 processes an MFI structure, and the MFI framework can be described based on the pentasil unit (a), a pentasil chain (b), and a pentasil sheet (c) illustrated in Figure 4. A pentasil unit comprises eight five-membered rings with a symmetry element of 4₁m2, and the units join through edges to form a pentasil chain. These chains are connected via oxygen bridges to form a pentasil sheet with 10-membered pores. 

Figure 4. Schematic models of the pentasil structure. a) Pentasil unit. b) Pentasil chain (run parallel to [001]). c) A single pentasil sheet

(Figure by Linh Tong, inspired from

The sheets are further connected to form a three-dimensional framework structure and are parallel to [010] and [100], shown in Figure 5. The projection along [010] reveals the 10-membered pore which are the entrance to the straight channels, and the projection along [100] shows the enter to the sinusoidal channels (Figure 6).

Catalytic properties of zeolites

(include general properties)

Acid sites in zeolites

Both Brønsted acid sites and Lewis acid sites exist in zeolites due to the charge difference when aluminum cations substitute silicon cations (as discussed in section Structure of zeolites). However, solid-state IR and 1H-NMR  have repeatedly detected the  Brønsted OH group; therefore, the catalytic activity in zeolites is claimed mostly attributed to the Brønsted rather than the Lewis acid site

(write more) 

To understand how the acid sites enable catalytic conversion, a mechanistic study on methanol conversion to toluene by ZSM-5 is illustrated in Figure 7. First, methanol migrates on the ZSM-5 surface, reacts with the acid sites of the zeolite to form a methoxy group,  and water is released as a by-product. The methoxy group then enters the reaction cycle and reacts with benzene, leading to the formation of toluene through the alkylation of benzene with the methoxy group. This reaction is the final step of the 2-step conversion of syngas (CO and H2) and benzene to toluene and xylene with methanol as an intermediate. [7] 

Figure 7. Proposed mechanism of alkylation of benzene with syngas catalyzed by ZSM-5 

Shape selective catalysis

Zeolites are considered molecular sieves, meaning only selective reactants and products with proper size and shape are allowed to diffuse the pore channel system of the zeolites due to their uniform pore sizes. The term “shape-selective catalysts” are used to describe catalysts in which the selectivity of the reactions is based on the shape or size of the reactants, products, or intermediates corresponding to the pore with or the pore architecture of the zeolites. This property of zeolite has been utilized in many large-scale processes, and ZSM-5 has been the model for shape-selective catalysts.

Shape-selective zeolite-catalyzed reactions can be categorized into three distinct types:

(i) Reactant shape selectivity occurs when there are at least two reactants with different molecular sizes. One reactant can be too bulky to diffuse through the channels, therefore, left unconverted. The less bulky molecule would proceed to the catalytic conversion instead. For example, in a mixture of straight-chain paraffin, cycloparaffin, and branched and aromatic hydrocarbons, only straight-chain paraffin is cracked into smaller molecules by ZSM-5. At the same time, the other reactants are not affected.

(ii) Product shape selectivity occurs when the catalytic reaction results in at least two products with different molecular dimensions. If the diffusion of the bulkier product inside the pores is hindered, the less bulky product would be formed. When toluene reacts with methanol, the possible products are ortho-xylene, meta-xylene, and para-xylene. The preferred product of this alkylation reaction is para-xylene, as para-xylene is a valuable starting material for the synthesis of polymer precursors. Therefore, ZSM-5 was used as a heterogeneous catalyst to maximize the conversion rate of para-xylene due to its shape-selective potential.

(iii) Restricted transition state shape selectivity is different from the two shape selectivity mentioned above. If the chemical reactions involve transition states which are too bulky to fit inside the pore of the zeolites, products of the smaller intermediates would be formed preferentially.  In both cases (i) and (ii), the diffusion inside the pores depends on the nature (e.g. molecular size) of the reactants or products; therefore, the diffusion can be improved by e.g. choosing larger (or smaller) crystals of the same zeolites.  However, case (iii) is merely a chemical effect, meaning the selectivity of the reaction does not depend on the crystal size of the zeolites.

Challenges and recommendation

Zeolites are crystalline materials with well-defines pores, leading to its catalytic properties in highly selective chemical transformations in numerous industrial processes. Besides the initial Si and Al atoms, the framework-building atoms includes Ti, Sn, Fe, etc leading ot the diversity of zeolites framework structure

. To improve the catalytic perfomances, the synthetic zeolites are further modified, typically to increase the active acid sites by loading metals such as (Fe, Ni, Zn) in zeolites

. For example, Zn-modified ZSM-5 enhanced the BTX (benzene, toluene, and xylene) yield from biomass by 19.2% compared to ZSM–5

Recent studies in zeolite catalysis are based on the same mechanistic studies and fundamentals of zeolites material science in early 2000s. The key difference is the focus on more sustainable starting materials such as biomass derivatives and synthesis gas instead of petroleum-based materials

[14,15]. Zeolites as catalysts also possess challenges as other catalysts; therefore, studies on improving stability and active sites and reducing coking effects are also required

Bibliography