Zeolites: structure, framework types and uses

A zeolite is a crystal whose framework is made only of TO₄ tetrahedra, where T is silicon or aluminium, each corner oxygen shared between two tetrahedra. The tetrahedra join into rings, the rings into cages and channels, and the result is a rigid solid full of regular pores, typically narrower than 2 nm and so about the size of small molecules (Li J. et al. 2022).

Silicon is 4+ and aluminium 3+. A framework of pure SiO₂ is neutral, but every Al that takes the place of a Si leaves one negative charge on the oxygens around it, and a cation sitting in the pores balances it (Li J. et al. 2022). Na⁺ or K⁺ from the synthesis can be exchanged for other cations, which is why zeolites are ion exchangers. When the balancing cation is a proton, the bridging Si–OH–Al group is a strong Brønsted acid site, the active site of zeolite cracking and methanol conversion (Li J. et al. 2022; Vogt and Weckhuysen 2015).

Zeolite pores are named by the number of tetrahedra (T atoms) in the ring that forms the narrowest opening. Small-pore zeolites have 8-ring windows about 4.0 Å across, medium-pore zeolites 10-rings about 5.5 Å, large-pore zeolites 12-rings about 7.5 Å, and extra-large-pore zeolites larger rings still (Shan et al. 2021). Because these openings are about the size of molecules, a zeolite lets some molecules in and keeps others out: molecular sieving and shape-selective catalysis (Shan et al. 2021). Newer zeolites include odd 11- and 15-ring pores and extra-large pores of 16 to 30 rings (Li et al. 2015).

Each distinct framework topology gets a three-letter code from the Structure Commission of the International Zeolite Association; more than 250 have been recognised, and about 20 are used in industry, including MFI, MOR, FAU, *BEA, MWW, LTA, FER and CHA (Li J. et al. 2022). The code names the net of tetrahedra, not the composition: ZSM-5 and its all-silica form silicalite share the MFI framework (Vogt and Weckhuysen 2015), and the aluminosilicate SSZ-13 and the silicoaluminophosphate SAPO-34 are both CHA (Beale et al. 2015).

The Si/Al ratio sets how many acid sites and exchangeable cations a zeolite has, and through them its acidity, its affinity for water and its stability (Li J. et al. 2022). Lower Si/Al means more Al, more cations and more water uptake; higher Si/Al means fewer but stronger acid sites and a more hydrophobic, steam-stable framework (Li J. et al. 2022). The ratio cannot fall below 1, the limit set by the Löwenstein rule (Li J. et al. 2022).

• Zeolite A (LTA) made without organic templates has Si/Al = 1 (Li J. et al. 2022).
• Zeolite Y (FAU) made without templates usually has Si/Al below 3; steaming removes framework Al to make the more stable ultra-stable Y (USY) used in FCC (Li J. et al. 2022).
• ZSM-5 (MFI) is called high-silica above Si/Al = 20, and with tetrapropylammonium it can be made from Si/Al 20 to 220 (Li J. et al. 2022).
• SSZ-13 (CHA) typically has Si/Al from 10 to 54 (Li J. et al. 2022).

• Catalysis and oil refining. Fluid catalytic cracking converts heavy oil fractions to gasoline, diesel and light gases (Li J. et al. 2022) and is the largest commercial process using zeolites; zeolite Y has been its main cracking component since 1964, and additives containing ZSM-5 raise the propylene yield (Vogt and Weckhuysen 2015).
• Ion exchange and detergents. Zeolites are used as detergents and ion exchangers; a low Si/Al gives a high exchange capacity, suited to water softening and removing heavy-metal cations from water (Li J. et al. 2022).
• Adsorption and separation. Zeolites dry natural gas, separate air, purify hydrogen, separate xylene and paraffin isomers and capture CO₂; Al-rich zeolites such as low-silica X (Si/Al = 1) are used for air separation (Li J. et al. 2022).
• Emissions control. Copper-exchanged small-pore SSZ-13 reduces NOₓ with ammonia (NH₃-SCR) in diesel exhaust (Shan et al. 2021; Beale et al. 2015).
• Methanol to hydrocarbons. Zeolite and zeotype catalysts turn methanol into gasoline or olefins, the choice of pore and cavity size steering the products (Olsbye et al. 2012).

Most zeolites are crystallised hydrothermally from a gel of a silicon source, an aluminium source, an alkali hydroxide and water; hydroxide acts as the mineraliser that forms Si–O–Si and Si–O–Al bonds, and the aluminosilicate species assemble around the cations into the crystal (Li J. et al. 2022). Alkali cations have a high charge density, so zeolites made with them alone are Al-rich (Li J. et al. 2022).

Organic structure-directing agents (OSDAs), usually quaternary ammonium cations, change that. An OSDA carries less charge per volume than Na⁺, so fewer Al are needed to balance it and the zeolite comes out with a higher Si/Al; fluoride media push further, to pure-silica frameworks (Li J. et al. 2022). Tetrapropylammonium directs ZSM-5 and N,N,N-trimethyl-1-adamantylammonium directs SSZ-13 (Li J. et al. 2022; Shan et al. 2021), and designing new OSDAs is one of the main routes to new framework types (Li et al. 2015). Seeds, interzeolite transformation and dry-gel conversion are other levers on the product and its Si/Al (Li J. et al. 2022). The organic template is burned out of the pores by calcination, for example at 550 °C for 6 h in one ZSM-5 synthesis (Feng P. et al. 2024).

• ZSM-5 (MFI): a high-silica, medium-pore zeolite with intersecting 10-ring channels about 5.1–5.6 Å across, the shape-selective workhorse of FCC additives and methanol conversion (Vogt and Weckhuysen 2015).
• Zeolite A (LTA): the most Al-rich framework, Si/Al = 1 when made without templates (Li J. et al. 2022), and so the highest ion-exchange capacity a zeolite can have (Li J. et al. 2022).
• Faujasite (FAU, zeolites X and Y): a large-pore zeolite whose ~7.3 Å windows open into 13 Å supercages; zeolite Y is the main cracking component of FCC catalysts (Vogt and Weckhuysen 2015).
• SSZ-13 (CHA): a small-pore chabazite with 8-ring windows of about 3.8 × 3.8 Å; as Cu-SSZ-13 it cleans NOₓ from diesel exhaust (Shan et al. 2021).

Metal–organic frameworks borrowed the zeolite idea. In zeolitic imidazolate frameworks (ZIFs) the tetrahedral Si(Al) of a zeolite is replaced by a transition-metal ion and the bridging oxygen by an imidazolate linker, so ZIFs take the nets of aluminosilicate zeolites; ZIF-8 is the best-known example (Park et al. 2006).

For a paper, slide or poster, draw a zeolite at two scales: the TO₄ tetrahedron and its shared corners, then the rings and pores they form. The LabCanvas figure editor Library has a zeolite object that draws a framework of rings (4- and 8-rings like LTA, or 6-rings) with Si/Al and oxygen atoms and optional cations in the pores; set the ring size and colours and export a high-resolution image. For real frameworks, the Library's crystal files for MFI (ZSM-5), LTA (zeolite A) and FAU (faujasite) draw the crystal structure, where you can switch between atoms and polyhedra and look down the channels, and the LabCanvas 3D Studio has zeolite models you can turn and render.