Beta zeolite has BEA topology, formed by the random intergrowth of three polymorphs (A, B and C). It features a 3D interconnected 12-membered ring pore system. Two sets of straight channels run along the [100] and [010] directions with pore apertures of 0.66 nm × 0.67 nm, and a third set of sinusoidal channels extends along the [001] direction. The unit cell formula can be simplified as \(\ce{|Na_{n}(H_{2}O)_{m}|[Al_{n}Si_{64-n}O_{128}]-BEA}\). Unlike FAU-type Y zeolite with supercages, beta zeolite has no oversized cavity inside the framework. Its 12-MR large pores are less restrictive for bulky organic molecules compared with medium-pore ZSM series zeolites. The stacking faults and crystal intergrowth in beta are inherent structural features, which affect molecular diffusion and the accessibility of internal acid sites. High-crystallinity beta with fewer stacking defects shows better mass transfer performance.
Framework aluminum atoms generate negative framework charges. After ammonium exchange and calcination, H-beta zeolite forms Brønsted acid sites, while extra-framework aluminum species produce Lewis acid sites. The Si/Al ratio of beta zeolite can be tuned in a broad range during hydrothermal synthesis. Lower Si/Al increases Brønsted acid density, but excessive framework Al reduces hydrothermal stability and tends to cause coke accumulation. High-silica beta has lower acid density and weaker acid strength, with better hydrophobicity. The acid strength distribution of beta is mostly medium‑strong. Compared with HY zeolite, H-beta has fewer super-strong acid sites, which suppresses non-selective cracking side reactions in many hydrocarbon conversion processes.
The three-dimensional 12-MR pore network allows bulky reactant and product molecules to diffuse in and out of the crystal. This property makes beta suitable for reactions involving large aromatic molecules, such as aromatic alkylation. Stacking faults in the beta crystal may act as diffusion barriers; constructing hierarchical pores via desilication or dealumination can introduce mesopores, shorten diffusion paths and improve the utilization of internal active sites.
The density, strength and distribution of acid sites directly determine catalytic activity, selectivity and coke resistance. Matching acid property with reaction requirement is critical:
Small crystal beta zeolite has shorter pore channels, faster molecular diffusion and better anti-coking performance than large crystal samples. Nano-sized beta exposes more external surface acid sites, which is beneficial for reactions limited by intracrystalline diffusion.
