Early stationary phases of gas-solid chromatography—Molecular sieve adsorbent

In 1925, people found that people have discovered that natural zeolite (such as zeolite) has a strong adsorption effect on vapors such as water, methanol, and ethanol，but it does not adsorb vapors such as acetone, ether, and benzene, and this type of zeolite is a natural molecular sieve. Then people imitated the generation conditions of natural zeolite foam and continuously improved the synthesis process, finally, various types of artificial molecular sieves were synthesized. The reason why it is called molecular sieve is that zeolite has a cage like crystalline structure, the pore size of the cage is consistent and exactly matches the size of the molecule. If the molecular size is smaller than the pore size of zeolite, it is easier to adsorb, but if it is the opposite, it is not adsorbed.

Molecular sieves have geometric selectivity: The crystalline structure of molecular sieves has a certain size, and different types of molecular sieves have different sizes. Therefore, the selectivity of molecular sieves is related to the critical size of the separated compound using the type of molecular sieve used. The critical dimension refers to the diameter of the maximum cross-section perpendicular to its length. Molecular sieves have strong interactions with polar molecules and molecules with high polarizability, have a high affinity for polar molecules and unsaturated hydrocarbon molecules, if the ability to adsorb the following gases on 4A molecular sieve increases sequentially:O₂< n₂< ch₄< co< c₂h₆< c₂h₄< co₂< c₂h₂

Molecular sieves have strong forces on compounds with hydrogen bonds that can be formed, For example, molecular sieves have irreversible adsorption effects on water, CO₂, and NO₂.

Molecular sieves have some characteristics that other adsorbents don’t have, for example, even at low concentrations, it has a relatively high adsorption capacity for the adsorbed substance; At high temperatures, it also has a high adsorption capacity for the adsorbed substance; At high flow rates, it still has a high adsorption capacity for the adsorbed substance.

Issues need pay attention to when using molecular sieves: Before using molecular sieves, it is necessary to activate them, usually by drying them under vacuum at 300-400 ℃ for 3 hours or at 550 ℃ for 2 hours. The separation performance of different types of molecular sieves also varies greatly. Molecular sieves have the characteristic of irreversible adsorption of some active gases, such as H₂O, CO₂, NO₂, H₂S, SO₂, Cl₂, HCl, etc., which are reversible adsorbed on molecular sieves.

The application of molecular sieves in gas-solid chromatography: mainly used for the separation of permanent gases such as O₂, N₂, CO, CH₄, etc. Due to the appearance of carbon porous microspheres, the role of molecular sieves has been reduced to a certain extent.

However, in recent years, due to the emergence of mesoporous molecular sieves, the pore size of molecular sieves has been increased to 30nm, expanding the scope of their applications. In 1992, Kresge et al. synthesized mesoporous molecular sieves such as MCM-41 by alkyl quaternary ammonium cations as surfactants for the first time. This type of mesoporous molecular sieve has the characteristics of large specific surface area, uniform pore size, and adjustable pore size, which breaks through the pore size limitation of microporous materials (such as zeolite) and has broad application prospects in catalytic separation and other fields. However, due to the small pore size, thin pore wall, poor hydrothermal and chemical stability of MCM-41, its application is greatly limited. In 1998, Zhao Dongyuan and others (now professors and academicians at Fudan University) who conducted postdoctoral research at the University of California, Santa Barbara in the United States, prepared ordered two-dimensional hexagonal mesoporous molecular sieve SBA-15 (SBA stands for Santa Barbara Amorphous) by using a hydrophilic triblock copolymer of polyethylene oxide, propylene oxide, and ethylene oxide (i.e. P123), its wall thickness can reach 6.4nm, pore size can reach 30nm, and it has high hydrothermal performance (100 ℃, 50h).SBA-15 not only compensates for the shortcomings of MCM-41 in terms of hydrothermal performance, but also has the characteristics of biodegradability, non toxicity, and low cost of triblock copolymers, It meets the needs of environmental protection and economic development, and has become one of the research hotspots in recent years. It has been widely applied in catalysis, adsorption, separation, nanoassembly, biomedicine, and sensing, among others.