Why Can the Molecular Sieve Be Used As a Solid Acid Catalyst?
Ⅰ. Introduction to the molecular sieve catalyst
A natural or synthetic chemical substance with a network structure. For example, when cross-linked dextran and zeolite are used as chromatographic medium, the mixture can be fractionated according to molecular size. Molecular sieves are crystalline silicates or aluminosilicates, formed by silico-oxygen tetrahedra or alumino-oxygen tetrahedra connected by oxygen bridges to form a pore and cavity system of molecular sieve molecular size (usually 0.3~2.0 nm). So it has the characteristics of sieving molecules. Also known as zeolite catalyst, it refers to a catalyst with molecular sieve as the catalytically active component or one of the main active components. The largest industrial use is the molecular sieve cracking catalyst, which is a solid acid catalyst.
Ⅱ. Molecular sieve solid acid catalyst
PSA molecular sieve has ion exchange performance, uniform molecular size pores, acid catalytic activity, and has good thermal and hydrothermal stability. It can be made into a catalyst with high activity and high selectivity for many reactions.
Generally speaking, solid acid can be understood as any solid that can change the color of an alkaline indicator, or any solid that can chemically adsorb alkaline substances. According to the definition of Bronsted and Lewis, solid acid is A solid capable of producing protons or accepting electron pairs, while the PSA molecular sieve catalyst has acid catalytic activity. Its acidity comes from the decomposition of exchanged ammonium ions, hydrogen ion exchange, or the hydrolysis of the contained multivalent cations during dehydration.
For example:
NH4M─→NH3+HM
H++NaM─→HM+NaCe
3+M+HOMHTCeOIHM+HM
In the formula, M represents molecular sieve. The number of protic acid centers and acid strength produced are of great significance to the acid catalytic activity of molecular sieves. The dehydration of the two hydroxyl groups of the molecular sieve will form a Lewis acid center, the structure of which is a three-coordinated aluminum atom and a positively charged silicon atom generated at the same time. There is a view that Lewis acid is produced from the six-coordinated aluminum atom formed at the cation site. The composition of PSA molecular sieve expressed by the ratio of silicon to aluminum has a great influence on its acidity and acid strength, so molecular sieve can be used as a solid acid catalyst.
