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The microscopic world hidden in ZSM-5: Why are its pores so special?
ZSM-5, also known as Zeolite Socony Mobil–5, is a pentacyclic aluminosilicate molecular sieve. Since it was patented by Mobil Oil Company in 1975, it has been increasingly used in the petroleum industry, primarily as a heterogeneous catalyst for isomerization reactions. However, why does the secret hidden in this microscopic structure still attract the attention of countless scientists? A careful observation of the unique pore structure of ZSM-5 may reveal its mystery.
Structure
The structure of ZSM-5 is composed of multiple five-ring units connected by oxygen bridges to form a so-called five-ring chain, and each five-ring unit contains eight pentagonal rings. In these rings, the vertices are aluminum (Al) or silicon (Si), and it is assumed that there are oxygen atoms connecting each vertex. These five-ring chains are further connected to form a corrugated surface containing 10-ring holes.
The vertices of each 10-ring hole are also made of aluminum or silicon, and it is assumed that there are oxygen connections between the vertices.
According to the study, the pore size of the channels parallel to the corrugations of ZSM-5 was estimated to be between 5.4–5.6 Å. Its crystal unit cell has 96 T sites (Si or Al), 192 oxygen sites, and different numbers of compensating cations depending on the Si/Al ratio. This unique structure exhibits a highly ordered property and remains a hot topic in scientific research today.
Synthesis process
As a synthetic molecular sieve, the synthesis process of ZSM-5 is quite important. Its common synthesis method involves mixing hydrated silicon, sodium aluminate, sodium hydroxide and tetrapropylamino bromide to form supersaturated tetrapropylamino ZSM-5, which can be heated and recrystallized to obtain a solid.
ZSM-5 can be synthesized under high temperature and high pressure by mixing various compounds in appropriate ratios.
This method was first proposed by Robert Argauer and George Landolt in 1969. Subsequent studies have shown that ZSM-5 can still be synthesized even without expensive organic amine templates, and the possibility of using substitutes from it has been explored.
Purpose
ZSM-5 is known for its high silicon-to-aluminum ratio, which makes it an important player in many catalytic reactions. Once aluminum trivalent cations (Al3+) replace silicon tetravalent cations (Si4+), the material takes on an additional positive charge. If protons (H+) are used as cations, the material becomes very acidic, so the acidity is proportional to the aluminum content.
The highly ordered three-dimensional structure of ZSM-5 and its acidic properties can be used in acid-catalyzed reactions such as isomerization and alkylation of hydrocarbons.
For example, ZSM-5 can effectively catalyze the isomerization reaction of p-xylene. The p-xylene in its pores has a higher diffusion coefficient, so that during the catalytic reaction, p-xylene can quickly pass through the molecular sieve, thereby improving the reaction efficiency and yield.
Catalytic properties and the future
In addition to its application in conversion reactions, ZSM-5 is also used as a catalytic support material. In one example, copper is deposited on a molecular sieve and steam is passed through it to effect an oxidation reaction, ultimately producing acetaldehyde. The specific pore size allows this reaction to proceed smoothly, and it is also effective for other alcohols and oxidation reactions.
For example, the process of converting alcohol directly into gasoline is called the methanol-to-gasoline (MTG) process, which is a technology patented by Mobil Corporation.
With the continuous improvement of molecular sieve technology, the application scope of ZSM-5 continues to expand, showing unlimited potential and value both in energy conversion and chemical synthesis. So, how can ZSM-5 be used innovatively in future catalytic research and applications?
