The research progress of the preparation of multi - hole zsm-5 zeolite molecular sieve

The reprocessing method is used to synthesize multistage zsm-5 zeolite molecular sieve
Post-processing method refers to the traditional method of ZSM - 5 zeolite by water vapor or processing, acid and alkali solution makes the parts of the ZSM - 5 zeolite framework Si/Al atoms are removed, produce certain methods of mesoporous channel.
1.1
Take off the aluminium method
Dealuminated method is to point to by high temperature heat treatment, water vapor treatment, acid leaching, or chemical reagent treatment methods such as the parts of the micro Kong Feishi molecular sieve frame aluminum leaching, the zeolite in the lattice defects, the formation of mesoporous channel method, this method is currently the most widely used Y type zeolite, however the ZSM - 5 zeolite aluminum has reported.
Water heat treatment can effectively increase the movement of silicon aluminum atoms in zeolite crystals, so it is the most commonly used deal-removal method.
Water heat treatment temperature is not lower than 500 ℃, molecular sieve using hydrogen or ammonia.
Pure water will produce a lot of non-framework aluminum, heat treatment and is not conducive to molecular sieve catalytic properties and the transmit performance, so water after heat treatment is often acid pickling to remove these non-framework species.
L. a ntonio et al. [2] treated zsm-5 zeolite with water heat treatment and water heat treatment, and examined the acid change of the molecular sieve after treatment.
The study found that compared with pure water heat treatment method, the water, with the method of heat treatment and HCl leaching photo development make the ZSM - 5 zeolite after processing acid strength and acid density changes, not only can reduce the secondary pore structure at the same time the diffusion resistance, improve the stability of catalyst aromatization reaction.
Research suggests that inorganic acid (hydrochloric acid, for example) processing makes further depth of aluminum, molecular sieve so X.Y.L in [3] the hydrothermal - such as citric acid ZSM - 5 molecular sieve to citric acid acid properties and pore structure of the molecular sieve ZSM - 5, results show that after heat treatment by the citric acid water can make some non-framework aluminum reinsert the skeleton, modulated the type of acid strength and acid molecular sieve, improves the catalyst in catalytic cracking reaction selectivity and hydrogenation of aromatics isomerization performance.
Using chemical reagent treatment on zeolite molecular sieve can be removed the zeolite framework section aluminum atoms, chemical reagent generally choose some strong chelate, such as ethylenediamine tetraacetic acid (EDTA) or a compound containing silicon fluosilicic acid ammonium, four fluoride materials such as silicon.
S.Kumar et al. [4] treated zsm-5 molecular sieves with hydrochloric acid, acetylacetone and ammonium fluorosilicate, and investigated the effect of dealumination on the molecular sieve and its acidity and activity.
The total surface area of zsm-5 zeolite increased slightly after treatment, but the surface area of the middle hole increased obviously and the micropore was slightly less than the surface.
Acetylacetone is the most effective dealitant, while ammonium fluorosilicate treatment is the least obvious.
The activity of ZSM-5 zeolite applied to dimethylbenzene was significantly increased.
1.3
The coagulation mechanism
The method of alkali solution was used to remove some of the silicon in the zsm-5 zeolite and also introduce the mesoporous, and the multi-level hole zsm-5 zeolite molecular sieve was obtained.
Unlike dealumination, the separation method can selectively remove the silicon in the molecular sieve skeleton, and the mesoporous is first formed at the edge of the crystal or the defect of the surface.
J.C.G roen [5] in treated with NaOH solution ZSM - 5 zeolite, desilication of mesoporous the ZSM - 5 zeolite framework silica alumina ratio influence, only when the zeolite n (Si)/n (Al) = 25 ~ 50 can get 5 ~ 20 nm mesoporous aperture, the mesoporous surface area of 250 m2 / g.
Research results show that the zeolite of Al atoms in the lattice in adjacent Si atoms have a protective effect of skeleton, when n (Si)/n (Al) 15 or less, the protection under high density of Al Si atoms removal skeleton, and when n (Si)/n (Al) 50 or more, the Si no selective removal, led to the generation of macroporous skeleton even collapse.

By SEM, EDX (figure 1) can be further proved the skeletons of Al distribution of non-uniform (Al are mainly distributed in the outer surface of the ZSM - 5 zeolite crystals), so the desilication occurs mainly in the interior of the crystal, which can lead to desilication after got the contains a lot of mesoporous hollow ZSM - 5 zeolite crystals.

Alkali because NaOH is strong, in the process need to strictly control the concentration of NaOH solution, so many researchers use Na2CO3 solution for desilication process preparation of multistage aperture ZSM - 5 zeolite molecular sieve, the study found that after treated with Na2CO3 solution, specific surface area and ZSM - 5 zeolite mesoporous volume are improved obviously, after the processing of the ZSM - 5 zeolite used in methanol to propylene reaction, improve the selectivity of propylene [7].
In recent years, S.A bello, etc. [8] of the ZSM - 5 zeolite by organic alkali treatment can also get a multistage aperture ZSM - 5 zeolite, compared to inorganic base, the organic alkali desilication strength is weak, therefore in the process often need higher temperature.
The most important advantage of organic base treatment is that it is hydrogen structure without ion-exchange roasting. However, the higher cost of organic alkali will also cause a large amount of aluminum dissolution in the process of desilicon removal.
Recently, Z.H asan etc. [9] the microwave radiation method is adopted to desilication of ZSM - 5 zeolite, the study found that: compared with the traditional electric heating method, microwave radiation method is used to deal with not only the mesoporous size is relatively small;
At the same time, the pore diameter distribution is narrower and the processing time is shortened effectively.
Different from the process of dealumination, the effect of desilicon on the acid bit is not great. Meanwhile, the interface of mesoporous and zeolite is better.
However, the method of desulphation is limited by the silica alumina ratio of zeolite skeleton, while the loss of zeolite in the process of desilicon is more.
The reprocessing method is a relatively simple and relatively low cost preparation method for the preparation of a multistage pore zeolite, which is also the most widely used in industry.
Demanding post-processing method of acid and alkali concentration, however, in the process will cause the loss of zeolite crystallinity and crystal skeleton collapse, and post-processing method of uneven distribution of mesoporous general, mesoporous and microporous connectivity is poor.
2. Add mesoporous template agent to synthesize multi-level hole zsm-5 zeolite molecular sieve
2.1
Hard template
Hard template method is refers to the use of the space limit the thinking of the gel of the synthesis of molecular sieves is injected into the ordered mesoporous materials, not with gel templates, after the crystallization by roasting and other means to remove the template agent can get contains multistage aperture of mesoporous molecular sieves, research and application of carbon materials is the most hard template.
A.H.J. anssen et al. [10-11] successfully synthesized zsm-5 molecular sieves with mesoporous carbon black, multi-walled carbon nanotubes (MWNTs) and carbon nanofibers.
The mesoporous zsm-5 molecular sieve of the carbon materials is disordered and the mesoporous distribution is not uniform. In the catalytic reaction of large molecules, there is often no outstanding characteristic.
Since the technology of mesoporous molecular sieve preparation of mesoporous carbon materials (cmk-n) has been developed, multi-hole molecular sieves have been gradually attracted by the introduction of mesoporous carbon materials.
To avoid in the process of crystallization from inside the mesoporous zeolite precursors emigrated from the surface to lead to a large zeolite crystal formation and carbon separation phenomenon such as template agent, H.S.C ho, etc. [12] under the condition of steam assisted by a series of CMK as the template agent synthesis of ordered mesoporous molecular sieve ZSM - 5, and examines the various influence factors in the process of synthesis in detail, the research indicates that the aperture size of mesoporous template agent, topology structure and humidity is the key to the synthesis of mesoporous molecular sieves.
Using carbon materials synthesis multilevel Kong Feishi molecular sieve as hard template agent, due to the hydrophobic and hydrophilic silica alumina carbon materials precursors prone to two-phase separation, this is liable to occur in the synthetic process of uneven distribution of template agent, synthetic complex step, synthetic cycle is long, in recent years, many researchers have proposed using ethanol and other organic solvent instead of water as solvent, gas phase transfer method is used to synthesis (DGC) or microwave synthesis;
At the same time, it is used to synthesize materials such as preoxidized carbon black or cheaper sucrose to synthesize the template.
To sum up, the hard template method with high crystallinity of multistage aperture can be prepared ZSM - 5 zeolite molecular sieve, and mesoporous aperture can be modulated, however, synthetic zeolite mesoporous channel with poor connectivity, synthesis process is often more complex as template agent and the high cost limits its application in practical production.
2.2
Soft templating
Soft template method is the use of surfactants, polymer and organic material such as silane as a template, with silicon or aluminum sources templates were assembled to form ordered mesoporous structure.
The addition of surfactant to silicon aluminum gel with microporous guide is the conventional method to synthesize multistage pore molecular sieve.
F.J.L iu etc. [15] with polystyrene - 4 - vinyl pyridine (PSt - co - P4VP) as the mesoporous template agent for the synthesis of mesoporous ZSM - 5 zeolite, although the synthesis of mesoporous arranged disorderly, yet the study found that the molecular sieve in a series of condensation reaction of benzaldehyde showed superior catalytic properties.
K.N, moreover, such as a [16] through the unique design of the structure of the hydrophilic surfactant was prepared with nanometer layered structure of the mesoporous ZSM - 5, surfactant with microporous and mesoporous dual template, micro distributed wall thickness by surfactant adjusted microporous part of the chain length, the material surface has more acidic, the macromolecular showed higher activity in the cracking reaction.
Recently, G.T.M.K adja TPABr etc. [17] using NaOH instead of part, in the synthesis of ratio of n (SiO2) : n (Al2O3) : n (TPABr) : n (NaOH) : n (H2O) = 1: (0.004 ~ 0.04) (0.012 ~ 0.19) (0.12 ~ 0.149) (5.3 ~ 23.8) system, the low temperature (< 100 ℃) step in the synthesis of the ZSM - 5 zeolite molecular sieves with multi-stage pore structure, greatly reduces the amount of organic template agent and synthetic process more concise, studies have shown that TPAOH ACTS as mesoporous support stands the role of matrix, the ratio of raw material is the key to synthesis, ratio of raw material only within a narrow range can get a multistage aperture ZSM - 5.
Compared with hard template method, soft template method general ordered degree to the synthesis of mesoporous good ZSM - 5 zeolite, however soft template agent there is also a method of synthesis of experimental conditions are demanding, complex synthetic steps, mesoporous template agent problems also higher costs, these are the key to limit its industrial application.
3. The precursors self-assembly composite multistage zsm-5 zeolite molecular sieve

Precursor self-assembly method, a new method for synthesis of multistage Kong Feishi most, the principle of the primary and secondary structure of zeolite unit or the precursors of nano zeolite and surfactant micelle self-assembly, finally with the structure of the mesoporous molecular sieve, was first put forward by Y.L iu etc. [18].
Y.Liu et Al. [18] developed a series of mesoporous materials in acidic and alkaline conditions with zeolites in different structures such as zsm-5.
At the same time Z.Zhang et al. [19] have also obtained a series of mesoporous molecular sieves with a series of six - square phases for the precursor of zeolites such as zsm-5.
Precursor self-assembly synthesis of mesoporous molecular sieves synthesis commonly low temperature (373 ~ 423 k), and this makes the synthesis of mesoporous distributed wall Al content is low, the difference of crystallinity of hole wall, recently, H.X.V u [20], such as using the two-step crystallization method through modulation synthesis system of pH and under the condition of high temperature on the ZSM - 5 kinds of self-assembled nanocrystals increased the content of Al in mesoporous materials have better stability and acidity of SAZ mesoporous materials.
Precursor self-assembly synthesis of mesoporous materials are generally has a larger specific surface area and interface distributed capacitance, however because it is assembled by the secondary structural unit, so the degree of crystallinity is generally low and water thermal stability is relatively poor.
4 conclusion
Multistage aperture ZSM - 5 zeolite molecular sieve due to both the advantages of mesoporous molecular sieves and microporous molecular sieve has important application value in the areas of oil refining chemical industry, so more and more get the attention of researchers in recent years, the people study reported the various synthesis methods of multistage aperture ZSM - 5 zeolite molecular sieves, these methods are widespread complex step, on synthesis and processing conditions were hard problem.
Different preparation methods of the multistage aperture ZSM - 5 zeolite molecular sieve crystal morphology, catalytic performance and water thermal stability and so on have very big difference, and each method exists some deficiencies: 1) post-processing method can get a second hole at the same time can also be modulated silica alumina ratio and acidic zeolite molecular sieve, this method is the most method in industrial application;
However, the method can easily cause the collapse of zeolite skeleton and the acid and alkali pollution in the process.
2) the template agent method is the most frequently reported method, but the hard template agent often has high cost, and the interface of the mesoporous channel is relatively poor.
However, the cost of soft templating agent is generally higher and the synthesis conditions are more complex.
3) precursor self-assembly method while it is possible to create specific surface and interfaces only electives are larger zeolite molecular sieve, molecular sieve, however, had lower crystallinity and water thermal stability is relatively poor.
Therefore, it is the main research direction for researchers to find a simpler and cheaper synthetic method or recycle the template agent effectively.