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Study on zeolite molecular sieve catalyst

The introduction

Zeolite is a material containing precise and single small holes that can be used to absorb gases or liquids.Small enough molecules can be absorbed through the channel, and larger molecules cannot.Unlike a normal sieve, it operates at the molecular level.Zeolite is a new type of catalyst with specific spatial structure.Because the pore size of different zeolite is different, it is often used as the carrier of alternative catalyst.

1. Alternative catalytic theory [1-4]

Selective catalytic chemistry is the science of combining chemical reaction with the adsorption and diffusion properties of zeolite, which can change the choice of known reaction pathways and products.The traditional selective catalytic theory is mainly reflected in zeolite effect, mass transfer selectivity and transition state selectivity.

(1) zeolite effect.In selective catalysis, this effect is reflected by the selectivity of reactants or product selectivity.In mixed material, only can enter the carrier channel and come in contact with the active center of the pore, molecules involved in response to as reactants, and greater than zeolite pore size of molecules will be excluded from the channel, not participate in the reaction, this is shown by the selectivity of the reactants.However, the larger molecules formed in the channel, or the equilibrium transformation into smaller molecules escape, or in situ clog the channel, resulting in the inactivation of the catalyst, which shows the selectivity of the product.

(2) mass transfer selectivity.In the selective catalysis, not only because the molecule penetrates the orifice of the zeolite, but also has a selective effect, and it is also restricted by the mass transfer after the molecule enters the inner hole.Especially when the reactant or product zeolite orifice diameter and the diameter of molecular approaches, due to the effect of inner hole wall of the field and all kinds of energy barrier, molecules in the intracrystalline diffusion will be various restrictions.The diffusion in this condition is different from that of Kundsen, which is common in non-crystalline porous materials.Small changes in the diameter of zeolite or diffusible molecules can lead to significant changes in the diffusion coefficient.This change is due to the change in the configuration of the molecules through the orifice of zeolite.At this point, diffusion is not only related to the length of the molecules, but also to the internal motions of the molecules.This diffusion, which Weisz calls the configuration diffusion, occurs mostly at 0. 4 ~ 1.0 nm, and the reaction rate will be affected by the grain size and activity of the catalyst.

(3) selectivity of transition state.As reactant and product molecules can spread in the channel, but generated transition state needed for the final product (intermediate), due to the size of the reaction intermediate or directional need larger space, and the effective space of the zeolite pore is small, can't provide the required space, cannot form transition state in the zeolite pore, reaction can't right now, so as to show the transition state reaction selectivity.This selectivity is different from mass transfer selectivity, which is independent of the size or activity of zeolite crystals, but only depends on the pore diameter and structure of zeolite.With high relative molecular mass, the corresponding normal alkanes was due to its within the zeolite pore diffusion is slow, and cyclopropyl are intermediates in the channel is not easy to form carbon ions, the traditional theory of shape-selective catalysis is difficult to explain very well.Martens et al. proposed the concept of pore catalysis on the basis of detailed analysis of the hydrogenation products of i-c17h36 on Pt/ zsm-22.They think the reactant molecules in one chain reaction and not through the channel, but part of the inserted into the zeolite pore, skeleton isomerization reaction occurred in the adsorption on the surface orifice and zeolite molecular.When one side chain of molecular adsorption in a zeolite crystal at the end of the tunnel, the other end of the chain of reactant molecules can also be drilled into the adjacent pore zeolite crystals and isomerization reaction, this mechanism is called a key lock catalysis.Although this mechanism is only a speculation, the concept of pore catalysis and key chain catalysis is a good explanation for the distribution of hydrocracking/isomerization of long-chain paraffin molecules.

2. Several common zeolite catalysts.

2.1 sapo-11 [5-6] zeolite catalyst.

SAPO 11 Kong Feishi zeolite catalysts, ten yuan of two-dimensional cross part of elliptical hole, aperture 0.39 nm * 0. 64 nm, physical and chemical properties similar to silicon aluminum zeolite, but also has some features of phosphorus aluminate zeolite.

Sapo-11 zeolite exhibits unique catalytic performance because of its different acid strength due to its different synthesis conditions.It has been applied in the process of cracking, alkylation, isomerization and other petroleum refining and petrochemical processes.The acidity, REDOX characteristics, cleanliness and pore structure of zeolite can be changed by means of load and doping to achieve the modification of zeolite.

2.2 zsm-5 [8] zeolite catalyst.

Zsm-5 zeolite catalysts have unique pore structure and pore size, stable skeleton and large range of adjustable silica alumina ratio.Excellent catalytic performance.There are two dimensional 10 - yuan ring channel, the pore diameter is about 0.55nm, and the thermal stability and catalytic activity are higher.Due to its unique pore structure and surface acid base, its catalytic reaction is mainly carried out in the center of acid and alkali, which can be used to convert methanol into hydrocarbon process and low carbon alkanes dehydrogenation process.At the same time, high silica zsm-5 zeolite is hydrophobic, and the methanol conversion is good for the activity and thermal stability of hydrocarbons.Zsm-5 can be modified by water vapor method, ion exchange method and chemical vapor deposition method, and the modified zeolite can improve its catalytic performance.

2.3 zeolite catalyst for aluminium phosphate [9].

The skeleton of such zeolite catalyst is strictly alternating between AlO4 and PO4 tetrahedron, and the skeleton is electrically neutral.Aluminum phosphate zeolite catalyst tetrahedron in the center of Al3 + and P5 + can be many different valence state of metal or nonmetal elements, the formation of impurity atoms with different structure and performance MeAlPO -n zeolite catalyst.Because the alpo4-5 zeolite catalyst has three-dimensional micropore crystal structure, it is composed of tetrahedron and aluminum oxygen tetrahedron, which is neutral.Therefore, as a carrier, there are unique advantages that other substances do not have.Adding iron ion to the catalyst can effectively limit the generation of inactive graphite carbon and improve the stability of the catalyst.

2.4 TS zeolite [10] catalyst.

There are Ti4+ centers in the zeolite skeleton structure of TS, and the zeolite titanite is a mesoporous zeolite with crystal structure.Due to the mesoporous structure of the TS zeolite catalyst not only has the REDOX properties and can have weak Lewis acid, combined with the zeolite mesoporous structure, can be used in the selective oxidation of macromolecular compounds, photocatalytic reaction and acid catalytic reaction, is a kind of environmental friendly solid catalyst.TS zeolite catalyst can be successfully applied to catalytic oxidation of cyclic olefin, cycloalkanes and unsaturated alcohols.With the improvement of the zeolite, the size limit of the microporous inorganic skeleton was broken, and the possibility was provided for the catalysis of organic macromolecules.It has good application prospect in fine chemical industry and pharmaceutical industry.

2.5 MCM zeolite [7] catalyst.

MCM series catalyst is a mesoporous catalyst, and its mesoporous is disordered and amorphous.Their channels are arranged in order and the size distribution is narrow.After the optimized synthesis conditions or post-processing, have certain water thermal stability, good thermal stability, larger specific surface area (> 400 m2 / g), high porosity, particle shape of rules, adjustable, etc.And it can maintain a high order in the micron scale.It can be modified by changing the template agent, adding augmentation agent, adjusting the carbon chain length of surfactant, adding adjuvant and so on.Modification of MCM can also be used as adsorbent, catalyst and catalyst carrier, also can be applied to environmental protection, organic macromolecular synthesis, REDOX reaction and oil refining industries.

2.6 SBA zeolite catalyst.

SBA zeolite catalyst has mesoporous structure, with uniform pore diameter distribution, adjustable pore diameter, high wall thickness and high hydrothermal stability.Has a larger specific surface area (up to 2500 m2 / g) and pore volume (up to 2.25 cm3 / g) by increasing the wall thickness and the hole wall, doping metal atoms and ions in the surface coating film such as water imbibition method of its modification, can make its stability is improved, so as to improve its application in the field of catalysis.

Third, the conclusion

Zeolite catalysts have been applied in petrochemical, environmental protection, bioengineering, food industry, pharmaceutical and chemical industries.Some zeolite catalysts are not very active in catalytic oxidation, so some researchers modified them.Micro Kong Feishi due to its smaller aperture, large molecules into the hole is difficult, diffusion resistance is larger at the same time, in the channel of the products, can not escape quickly, greatly limits the application of micro Kong Feishi in macromolecular catalysis.The mesoporous zeolite can compensate for the deficiency of microporous zeolite and provide a favorable space configuration for the large molecule reaction.However, because the pore wall of mesoporous zeolite is in an amorphous state, its hydrothermal stability is poor and its acidity is weak, which also limits its application range.Therefore, the synthesis of mesoporous composite materials with strong acid and zeolite pore wall structures and microporous mesoporous zeolite composites is being studied.