1. Introduction
Silicoaluminophosphate (SAPO) molecular sieves are a series of microporous crystalline materials first developed in the 1980s, which break through the traditional aluminosilicate zeolite composition system. Among various SAPO materials, SAPO-34 with CHA structure has attracted the most extensive attention in industrial catalysis and academic research. Unlike aluminosilicate zeolites, the SAPO-34 framework is composed of alternating AlO₄ and PO₄ tetrahedra, forming a neutral backbone. The introduction of silicon atoms replaces partial phosphorus sites and produces adjustable weak-to-moderate Brønsted acid sites, which endow SAPO-34 with excellent catalytic activity for methanol conversion reactions.
In the MTO reaction, the unique structural advantage of SAPO-34 is reflected in its typical “large cage-small window” feature. The supercage structure can accommodate hydrocarbon pool intermediates to promote the continuous conversion of methanol, while the narrow eight-membered ring pore windows effectively limit the diffusion of large molecular by-products, thereby achieving high selectivity of ethylene and propylene. At present, SAPO-34 is the core catalyst of domestic and international coal-based methanol-to-olefin industrial units. However, rapid coke deposition, short service life and limited hydrothermal stability still restrict its long-cycle stable operation. Therefore, in-depth understanding of the structure, acidity and catalytic mechanism of SAPO-34 is of great significance for guiding catalyst modification and industrial upgrading.
2. Crystal Structure and Physical Properties of SAPO-34
SAPO-34 belongs to the CHA topological structure with a rhombohedral crystal system. Its basic structural units include double six-rings (D6R) and single six-rings, which are stacked in a regular arrangement to form a complete three-dimensional microporous framework. The entire crystal structure is composed of interconnected supercages, and each supercage is connected with six adjacent cages through eight-membered ring pore openings.
The key structural parameters of SAPO-34 are as follows: the pore window size of the eight-membered ring is 0.38 nm × 0.38 nm, and the inner diameter of the ellipsoidal supercage is about 0.94 nm. This pore-cage matching structure is the fundamental reason for its excellent shape-selective performance. Small-molecule reactants such as methanol and water can freely enter and exit the channels, while long-chain hydrocarbons and aromatic macromolecules generated during the reaction are trapped in the cages, which inhibits the formation of heavy by-products and significantly improves light olefin selectivity.
In terms of physical properties, SAPO-34 has a high specific surface area ranging from 450 to 600 m²/g and a regular micropore structure. The material exhibits good thermal stability and can maintain complete crystal structure below 700 °C. Nevertheless, long-term high-temperature hydrothermal environment is easy to cause the hydrolysis of Al-P bonds, resulting in framework damage and irreversible deactivation of the catalyst.
3. Acid Formation Mechanism of SAPO-34
The acidity of SAPO-34 is completely different from that of traditional ZSM-series aluminosilicate zeolites. Pure aluminum phosphate AlPO-34 has a neutral framework without any acidity. When silicon atoms are introduced into the framework to replace pentavalent phosphorus atoms, the original charge balance is broken, and negative charges appear on the framework. After protonation, bridging hydroxyl groups (Si-OH-Al) are formed, which are the source of Brønsted acid sites of SAPO-34.
There are two typical silicon substitution mechanisms in SAPO-34 crystallization process. The SM2 mechanism refers to the substitution of a single Si atom for one P atom, which effectively generates independent Brønsted acid sites with moderate strength. The SM3 mechanism means that two Si atoms replace one pair of Al-P atoms to form Si-O-Si bonds, which does not produce effective acid sites. Excess silicon content easily leads to the formation of silicon island structures, which reduces the number of effective acid sites and deteriorates the catalytic performance. Therefore, uniform dispersion of silicon atoms is the key to preparing high-quality SAPO-34 catalysts.
Compared with strong-acid zeolites such as ZSM-5, SAPO-34 has moderate acid strength and suitable acid density, which can effectively avoid excessive cracking and hydrogen transfer side reactions, and greatly reduce the selectivity of alkanes and aromatics in MTO products.
