1 Introduction
Molecular sieve catalysis is essential for the development of the national economy, not only for the traditional oil refining and conversion process, but also for methanol conversion and NO
x elimination in diesel vehicles in recent years
[1⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~30][11⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~27]. In these molecular sieve catalytic processes, different catalytic reactions require different molecular sieve structure catalysts. For example, in the fluid catalytic cracking (FCC) reaction, the crude oil molecules are relatively large, and the Y-type molecular sieve with a 12-membered ring structure with relatively large pore channels and pore volumes is required
[11⇓⇓⇓~15]; In the reaction of methanol to light olefins (MTO), because the molecules of light olefins are small, in order to avoid the formation of relatively large molecules, the molecular sieve catalyst used is an 8-membered ring pore structure
[19]; The hydroisomerization dewaxing reaction of straight-chain alkanes mainly occurs at the orifice of the molecular sieve, so the molecular sieve structure with one-dimensional straight pores is needed
[28⇓~30].
In the conventional molecular sieve catalytic process, the reaction temperature is relatively high. Under these conditions, the reactants and reaction products are usually gas molecules, and the condensed chemical processes involved mainly include: (I) the adsorption of gas reactants in the pores of molecular sieves; (ii) adsorption and conversion of reactants on molecular sieve catalytic centers; (iii) Desorption of reaction products. In the above process, if the gas reactant can be quickly adsorbed, the conversion rate can be increased, or the reaction product can be quickly desorbed, these characteristics will greatly accelerate the reaction rate in the molecular sieve catalytic process, because most of the molecular sieve catalytic reactions are reversible processes. This paper introduces the progress in recent years and looks forward to the future work in the above three aspects.
2 Adsorption of Gas Reactants in Molecular Sieve Pore
The gas reactant is rapidly adsorbed and condensed in the molecular sieve pore channels, so that the concentration of the reactant around the catalytic center of the molecular sieve can be greatly increased, and the conversion rate of the catalytic reaction can be effectively increased. Generally speaking, zeolite molecular sieves have uniform microporous pore structure, which has a certain restrictive effect on the diffusion of reactant molecules and the adsorption in the pore of zeolite molecular sieves, thus affecting the regulation of catalytic performance. For this reason, some solutions have been proposed, such as nanocrystallization of zeolite molecular sieve crystals to shorten the diffusion and condensation path of gas reactant molecules in the molecular sieve channels. On the other hand, additional mesoporous structures have been introduced into molecular sieve crystals to improve the diffusion and adsorption properties of gas reactant molecules. In addition, the thickness of zeolite crystals can be controlled to prepare zeolite nanosheets, which can not only control the diffusion rate of reactants, but also effectively change the diffusion selectivity of reactants. These methods are very conducive to the adsorption and conversion of specific reactant molecules in molecular sieve pores.
2.1 Nanocrystallization of zeolite molecular sieve crystal
FCC is one of the most important processes in molecular sieve catalysis, which processes billions of tons of crude oil worldwide every year. In this process, the FCC catalyst is mainly composed of a silica-alumina carrier and Y-type molecular sieve crystals as the active components (fig. 1), where the smaller the size of the molecular sieve crystals, the larger the external surface area, and the higher the performance of the reactant molecules to adsorb faster in the molecular sieve pores and further crack
[31]. For this reason, people try to prepare Y-type molecular sieves with the smallest possible grain size to accelerate the diffusion of reactant molecules and the adsorption in the pore channels of molecular sieves.
Vuong et al. studied the relationship between the crystal size of Y-type zeolite and FCC catalytic performance. In this work, the crystal size of Y-type zeolite was 25, 40, and 100 nm, respectively, and the prepared catalysts were represented by FCC-25, FCC-40, and FCC-100, respectively. They found that the smaller the grain size, the higher the conversion rate, which is obviously related to the rapid adsorption and condensation of gas reactants and catalytic conversion in the zeolite pores with smaller grains.
In the process of FCC catalyst preparation, the adhesive is usually needed to prepare microspheres of about 70 μm, and the adhesive inevitably covers these nano-Y zeolite grains. Therefore, a new method has been proposed to prepare microspheres first and then grow nano-Y zeolite grains in situ, which can expose more nano-zeolite particles to gas reactants and make the gas reactants more quickly adsorbed in the pore structure of Y zeolite for catalytic cracking
[33].
Nanocrystallization of zeolite molecular sieve crystals can significantly improve the activity of molecular sieve catalysts. Catizzone et al. Reported that the conversion of methanol to dimethyl ether could be effectively improved by reducing the crystal size of ferrierite (FER), in which the crystal sizes of NC-FER, NP-FER and M-FER were 5 ~ 10, 0.3 ~ 0.5 and 0.1 μm, respectively
[34]. Considering that the conversion of methanol to dimethyl ether can be used as an alternative fuel for diesel engines because of its high cetane number and smokeless emissions, the catalytic application of this nano-FER is of great significance
[35].
If the reaction conditions are carefully controlled, even molecular sieve crystals as small as 10 nm can be prepared. For example, Mintova et al. Prepared high-quality FAU zeolite crystals in the range of 10 ~ 400 nm by controlling the composition of the starting gel, and the crystal sizes were about 10 nm and 50 ~ 70 nm, respectively
[36]. These Y zeolites with different sizes showed different catalytic activities in 1,3,5-triisopropylbenzene cracking reaction. The crystal size of LZY-62 and Y-400 zeolites was about 400 nm, and the catalytic conversion was much lower than that of Y-10 and Y-70 under the same reaction conditions. The difference of their catalytic properties is obviously directly related to the difference of diffusion and adsorption of gas reactants in the pore channels.
Up to now, many different methods have been developed to synthesize nano-zeolite crystals, which show better catalytic properties than conventional zeolite molecular sieves in a series of reactions
[37⇓~39].
2.2 Introducing mesoporous structure into zeolite molecular sieve crystal
Because the separation of nano-zeolite molecular sieve crystals from the synthesis system is relatively complex, the preparation cost is high, and sometimes the high-speed centrifugal method is needed to prepare the nano-zeolite molecular sieve crystals, which is time-consuming and energy-consuming, people propose to introduce the mesoporous structure into the zeolite molecular sieve crystal to facilitate the mass transfer and adsorption of gas reactants in the catalytic reaction.
Initially, the introduction of mesoporous structures into zeolite molecular sieve crystals was achieved by post-treatment methods, with typical strategies including steam treatment, acid or alkali solution etching
[40⇓~42]. However, these post-treatment methods sometimes lead to the decrease of crystallinity of molecular sieves, so the template method is proposed to introduce mesoporous structure.
Carbon nanoparticles were the first template used in the synthesis of mesoporous zeolite molecular sieve crystals. Jacobsen et al. Used nano-carbon particles to add to the initial synthesis system, and after calcination to eliminate the nano-carbon template, a series of mesoporous zeolite molecular sieve crystals containing holes were prepared
[43⇓~45]. These mesoporous zeolite crystals can greatly promote the diffusion and condensation of gaseous reactants, so they show more excellent performance in butane isomerization and benzene alkylation reactions than conventional zeolite crystals without mesoporous structure
[46]. Because these cavity-like mesoporous structures do not communicate with each other, the mass transfer and condensation of relatively large gas molecules are still difficult. For this reason, carbon or polymer aerogel was used as a template to synthesize mesoporous zeolite crystals, and partial continuous mesoporous structures were obtained, which showed excellent performance in a series of catalytic reactions
[47⇓⇓~50]. For example, Sch Schüth et al. Used polymer aerogel as a template to synthesize all-silica MFI zeolite with mesoporous structure, which showed a higher reaction rate in the Beckmann rearrangement reaction of cyclohexanone oxime than zeolite without mesoporous structure, which was attributed to the rapid diffusion and adsorption of reactant molecules in the pores
[50].
Compared with carbon nanoparticles and aerogel templates, soft templates are more conducive to the preparation of zeolite crystals with completely continuous mesoporous structure. In 2006, three independent research groups in the world (Xiao, Ryoo and Pinnavaia) used three different soft templates, namely, polyquaternary ammonium salt solution, amphiphilic surfactant containing silane species and polymer containing oxysilane, to synthesize mesoporous zeolite crystals
[51⇓~53]. Subsequently, different soft templates were employed and a large number of different mesoporous zeolite crystals were successfully prepared
[54⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~65].
Fig. 2 is a transmission electron microscope image of mesoporous Beta zeolite synthesized using poly-diallyldimethylammonium chloride (PDADMAC) as a mesoporous template, which clearly shows that there are not only disordered continuous mesoporous structures of 5 ~ 20 nm, but also regular microporous structures of 0.7 nm in size. The continuity of the mesoporous structure is further confirmed by electron microscope tomography
[51][54].
Ryoo et al. Synthesized mesoporous LTA zeolite using an amphiphilic surfactant [(CH
3O)
3SiC
3H
6N
+(CH
3)
2C
n ] containing silicon species as a mesoporous template, and high-resolution scanning electron microscopy images confirmed that the zeolite crystals contained abundant mesoporous structures
[52]. By using this kind of mesoporous template, different kinds of zeolite crystals can be synthesized, which show more excellent catalytic performance in different reactions than those without mesoporous structure.
Fig. 3 shows the difference in the performance of benzene alkylation with propanol between mesoporous Beta zeolite synthesized by polyquaternary ammonium salt solution as mesoporous template and conventional Beta zeolite. After the introduction of mesoporous structure, the conversion and selectivity of both were significantly improved
[51]. Garcia-Martinez et al. Reported the use of surfactants to precisely control the mesoporous structure of Y zeolite and used it in the FCC process, and found that Y zeolite with mesoporous structure as the active component of FCC can obtain higher gasoline yield and lower carbon deposit, as shown in Figure 4
[55]. When the mesoporous ZSM-5 is added to the FCC catalyst instead of the traditional ZSM-5 additive, a higher propylene yield can be obtained while obtaining a similar gasoline yield, from 9. 0% for the conventional E-USY/ZSM-5 catalyst to 12. 2% for the E-USY/Meso-Z catalyst. Obtaining more propylene from the FCC process can better balance the current propylene shortage problem
[56]. When the mesoporous ZSM-5 catalyst was used for vacuum gas oil (VGO) cracking, the conversion of VGO was increased from 33% to 48% and the gasoline yield was increased from 12% to 19% at 550 ℃ compared with the conventional ZSM-5 catalyst, which showed the great effect of mesoporous structure
[57].
In addition to the use of mesoporous templates for the preparation of mesoporous zeolite crystals, mesoporous zeolites can also be prepared by the oriented growth of different crystal planes of zeolites. A typical example is that MFI-SDS zeolite with mesoporous structure is prepared by adding MFI seed crystal to make zeolite grow directionally without any organic template. Its mesoporous pore volume can reach 0.29 cm
3/g, and the existence of mesoporous structure can be clearly observed, as shown in Figure 5
[58]. In the cumene catalytic cracking reaction, the mesoporous MFI-SDS zeolite showed a higher conversion than the conventional ZSM-5 zeolite (tetrapropylammonium hydroxide to prepare ZSM-5 is denoted as ZSM-5-TPA,), as shown in Fig. 6
[58]. The introduction of mesoporous structure can effectively adjust the diffusion rate and adsorption state. With the increase of mesopore size, the diffusion rate will also be significantly improved, which is more conducive to the adsorption of reactant molecules in the zeolite micropores, enriched around the catalytic center, and more conducive to catalytic conversion, thus greatly improving the catalytic performance.
2.3 Zeolite nanosheet
Zeolite nanosheets mainly refer to the crystal morphology in which the thickness of zeolite crystal is less than 100 nm and the length and width of the crystal are much greater than 100 nm. In 2009, Ryoo et al. First reported that MFI zeolite nanosheets were synthesized by using a template (C
22H
45-N
+(CH
3)
2-C
6H
12-N
+(CH
3)
2-C
6H
13) containing a bis-quaternary ammonium salt and a long-chain alkyl group, and the thickness of MFI zeolite nanosheets was even only 2 nm
[66]. Subsequently, TS-1 zeolite nanosheets were synthesized using the same surfactant as a template
[67]. A particularly noteworthy phenomenon is that these nanosheets are partially rebonded together while the organic template is eliminated by calcination. In order to solve this problem, Ryoo et al. Reported a silica pillared method
[68]. Later, in order to keep the structure of zeolite nanosheets unchanged, Tsapatsis et al. Used quaternary phosphonium salt template to synthesize "house of cards" nanosheets
[69]. Even if the organic template is eliminated by calcination, they can still maintain their zeolite nanosheet morphology. Considering the relatively high price of the organic template for the synthesis of zeolite nanosheets and the high cost of industrial application, Xiao et al. Proposed to add urea additive to inhibit the b-axis growth of MFI crystals, and successfully prepared MFI nanosheets with short b-axis
[70]. Up to now, zeolite nanosheets with different structures have been synthesized by many different strategies
[71⇓⇓⇓⇓⇓⇓⇓⇓~80].
Wu et Al. Prepared MWW zeolite nanosheets (Al-ECNU-7) using a zeolite layered precursor and a surfactant, and in the cumene cracking reaction, the conversion rate of TIPB was about three times that of the conventional MWW zeolite structure (72% vs 24%), as shown in Figure 7
[71]. Zhang et al. Reported that the conversion of ZSM-5 zeolite nanosheets in the catalytic cracking reaction of TIPB was as high as 90.5%, which was much higher than that of conventional ZSM-5 (15.7%), which showed the advantages of zeolite nanosheets in the mass transfer and catalytic conversion of macromolecules
[72]. Xu et al. Reported that FER zeolite nanosheets as 1-butene isomerization catalysts exhibited higher reactivity and selectivity than conventional FER zeolite (Fig. 8)
[73]. It can be observed that the activity and selectivity of the nanosheets are higher than those of the corresponding conventional zeolite molecular sieves at each stage of the reaction. Zhang et al. Prepared SAPO-11 nanosheets, which showed higher conversion and higher isomerization products than the conventional SAPO-11 supported catalyst in the hydroisomerization reaction of n-dodecane after supporting Pt nanoparticles (Fig. 9)
[74].
In addition, by introducing heteroatoms into the zeolite framework, the prepared heteroatom zeolite nanosheets also showed better catalytic performance than the corresponding conventional heteroatom zeolite catalysts. For example, Hensen et al. Reported that in the oxidation of benzene with N
2O to produce phenol, Fe-ZSM-5 zeolite nanosheets gave an initial reaction rate much higher than that of the corresponding conventional Fe-ZSM-5
[75]. Shan et al. Reported that TS-1 zeolite nanosheets with different thicknesses could be synthesized by adding different amounts of urea into the synthesis gel system. The thinner the thickness, the higher the conversion and yield of cyclohexanone oxime, confirming the great influence of pore diffusion and adsorption on the catalytic reaction performance, as shown in Figure 10
[70].
3 Adsorption and Conversion of Gaseous Reactants on Zeolite Catalytic Centers
When the gas reactant is adsorbed in the pore of zeolite molecular sieve, it can interact with the catalytic center through migration and further catalytic conversion. In this process, the size of the molecular sieve pore determines the size of the adsorbed gas molecules. If the diameter of the gas molecule is larger than the micropore of the molecular sieve, it is difficult for the gas reactant to be adsorbed in the molecular sieve pore, which is called "molecular sieve pore shape selection". Due to space limitations, these concepts are not discussed in this section. In the catalytic reaction of molecular sieves, the main active sites include the acid sites and redox sites in the framework of molecular sieves, as well as the metal oxides and metal nanoparticles added on the basis of the acid sites and redox sites.
3.1 Acid site of zeolite molecular sieve
The acid center of zeolite molecular sieve comes from the four-coordinated Al species in the zeolite framework to produce a proton matched with the negative charge of the framework, so its concentration is directly related to the Si/Al ratio in the zeolite framework. If the Si/Al ratio is low, there is a higher concentration of acid sites. However, a higher concentration of acid sites results in a decrease in the intensity of the acid sites. If the acid zeolite is dehydrated, the protonic acid sites can be converted to Lewis acid sites. Therefore, the acid concentration and acid strength of zeolite can be controlled by adjusting the Si/Al ratio of the zeolite framework, and the Lewis acid properties of zeolite can be controlled by post-treatment.
Protonic acid sites can protonate with alkene molecules to form carbonium ions, while Lewis acid sites can react with alkane molecules to form carbonium ions, which can be further transformed as reaction intermediates, as shown in Figure 11
[33]. Obviously, there are many possibilities for the carbonium ion intermediate, such as cracking, isomerization, etc. Therefore, the higher acid site concentration of zeolite is beneficial to the cracking of reactants, while the lower acid site concentration is beneficial to the isolated catalytic site reaction.
In the catalytic cracking reaction of crude oil, the reactant molecules are large, and the catalytic reaction mechanism requires a high concentration of acid sites, so in industrial applications, the main catalytic active component is Y-type zeolite molecular sieve with macroporous structure and rich framework aluminum. However, it should also be noted that more framework aluminum species cause a decrease in the hydrothermal stability of the catalyst. Therefore, Y-type zeolite catalysts generally need to be dealuminized by a variety of post-treatment methods to improve their hydrothermal stability
[33]. In addition, these post-treatment methods can also create more mesoporous structures and Lewis acid sites, which are more conducive to the mass transfer of reactants and the adsorption of reactants on catalytic sites, forming reaction intermediates such as carbonium ions.
In the reaction of methanol to light olefins (MTO), the mechanism basically follows the carbon pool route of side chain methylation
[81]. The main role of the acid site is the formation of the intermediate. In order to avoid side reactions as little as possible, people use zeolite structure with highly isolated acid sites to form carbon pool reaction intermediates. Therefore, the zeolite catalyst used is a high silica-alumina ratio molecular sieve with CHA and MFI structures, and the acid concentration is relatively low. For example, the silica-alumina ratio of SSZ-13 and ZSM-5 is generally in the range of 60 ~ 150, which can show better catalytic performance.
In addition to the change of acid concentration in zeolite, the position of acid center in zeolite framework also plays an important role. For example, in the decarbonylation of methyl acetate to ethanol, the acid sites show a very large difference between the 8-membered ring and the 12-membered ring channels in the MOR zeolite structure. If the acid sites are distributed in both channels of the 8-membered ring and the 12-membered ring, the catalytic performance is poor. When the acid sites in the 12-membered ring channels of MOR zeolite were poisoned by pyridine base, only the acid sites in the remaining 8-membered ring channels showed excellent catalytic performance, as shown in fig. 12
[82]. These results show that the adsorption of reactant molecules on the pore acid sites at different positions has a significant effect on the catalytic performance.
Yokoi et al. Reported the precise regulation of the acid sites in the pore of ZSM-5. When only TPA
+ is used as the cation of the synthesis system, all the Al species are located at the intersection of the two-dimensional channels of ZSM-5. When the synthesis system contains not only TPA
+ but also Na
+, the Al species is located not only at the cross site, but also in the middle of the pore (Fig. 13). The toluene disproportionation reaction showed that the activation energy of ZSM-5-TPA was 60. 0 kJ/mol, which was much lower than that of ZSM-5-Na, TPA (78. 9 kJ/mol).These results confirmed that the position of the acid center in the zeolite structure had a good control on the condensation state of the reactant molecules, which had a great impact on the catalytic reaction performance
[83].
3.2 Heteroatom catalytic center in zeolite framework
Heteroatomic zeolite molecular sieves are formed when atoms other than silica and alumina enter the zeolite framework. If these heteroatoms have a catalytic function, a heteroatom molecular sieve catalytic material is formed. Fig. 14 gives several possibilities for the incorporation of heteroatoms into the silica zeolite framework
[84]. When a tetravalent element such as a Ti
4+ substitutes for the tetracoordinated silicon in the framework, the heteroatom molecular sieve framework remains neutral; When a trivalent element such as a Fe
3+ replaces the tetracoordinated silicon into the framework, the heteroatom molecular sieve framework charge is -1, which requires a cation to balance; When a divalent element such as a Zn
2+ replaces the four-coordinated silicon into the framework, the heteroatom molecular sieve framework charge is -2, requiring two cationic charges to balance. Considering the energy factor, it is easier for trivalent and tetravalent elements to enter the silica zeolite framework. Thus, to date, there have been a large number of successful examples of incorporation of trivalent and tetravalent elements into the silica zeolite framework, and less often of incorporation of divalent elements into the silica framework. It should be very difficult from the energy point of view to introduce pentavalent and hexavalent elements into the zeolite framework, because the zeolite framework will produce a positive charge, and the silica-based zeolite with a positive charge framework has not been successfully verified experimentally so far. If the heteroatom is not present in a four-coordinate state, it is not an isomorphous substitution, but a chemical ligation mode.
For the catalysis of heteroatom molecular sieves, most of the studies are liquid-solid phase catalytic oxidation reactions, mainly involving heteroatom molecular sieves doped with tetravalent elements and keeping the framework neutral, such as TS-1 and Sn-Beta
[85⇓⇓~88]. In recent years, with the development of natural gas and shale gas, more and more heteroatom molecular sieves have been used for the dehydrogenation of light alkanes. For example, Yang et al. Synthesized Fe-doped iron-silicon MFI zeolite molecular sieve (FeS-1-EDTA) in a gel system containing EDTA complexing agent. Various characterizations showed that the catalyst had uniform and stable isolated Fe sites. The modification of the siliceous zeolite framework stabilized the isolated Fe sites and avoided the formation of carbon deposits and the deactivation of the catalyst caused by the reduction of iron species
[89]. This unique condensed state active site promotes the rapid desorption of hydrogen and olefin products in zeolite micropores, thus improving the dehydrogenation performance of ethane, which shows higher yield and lifetime than the current industrial dehydrogenation catalyst PtSn/Al
2O
3 in ethane catalytic dehydrogenation to ethylene, as shown in Fig. 15. Zhou et al. Reported the preparation of borosilicate MFI zeolite molecular sieve (BS-1) by solvent-free technology, which showed the highest reaction rate so far in the catalytic oxidative dehydrogenation of propane to propylene, and still maintained a high conversion rate (43.7%) and a high selectivity (82.8%) after the reaction for more than 210 H, as shown in Figure 16
[90]. The key of this work is to use the porous silicate zeolite (MFI) framework to separate boron, which hinders the complete hydrolysis and leaching of boron, thus greatly improving the durability of the catalyst. The boron center in the MFI framework has a boron dihydroxy structure and forms a -B[OH…O(H)-Si]
2 coordination with the adjacent disilanol group. In the propane dehydrogenation reaction, the boron dihydroxy group and one of the silanol groups cooperate to activate propane and oxygen, thereby forming a stable intermediate which is subsequently converted into propylene, and the reaction energy barrier is superior to that of a single boron hydroxy group structure, so the catalytic performance of the catalyst is significantly improved. In addition, the Si — O — B bond can undergo reversible hydrolysis-condensation during the reaction, which can effectively inhibit the deboronation of the molecular sieve to form boric acid and improve the stability of the catalyst. However, when B-MWW zeolite is used, there is only a single boron hydroxyl (B-OH) center in the framework, and the catalytic activity is very low. This study shows that different condensed active sites have a significant effect on the reaction performance.
In particular, it is worth pointing out that the catalytic rate of heteroatom molecular sieve catalysts is much higher than that of the corresponding supported catalysts, because each heteroatom of heteroatom molecular sieve as a catalytic center can be exposed to reactants and adsorbed on the catalytic center, which is also a hot topic in the research of single-atom catalysts in recent years
[91⇓~93]. In addition, an additional advantage of heteroatom molecular sieve catalysts over supported single atom catalysts is their thermal stability, as the thermal stability of the silica zeolite framework can reach 1000 ° C,As a result, the heteroatom active center is firmly fixed in the high-stability zeolite framework, which naturally endows it with excellent thermal stability, which is very important for industrial catalytic applications.
3.3 Multiple catalytic centers of zeolite molecular sieves
Zeolite molecular sieves can incorporate additional catalytic centers in addition to their acid and redox centers. The simplest method is ion exchange. For example, when partial protons of H-type SSZ-13 are exchanged by Cu
2+, Cu-SSZ-13 zeolite molecular sieve is obtained, which is the best national IV catalyst for selective reduction of diesel engine exhaust NO
x and NH
3 to N
2(SCR-NH
3), and is widely used in different diesel engines
[94⇓~96].
In the SCR-NH
3 reaction, at least two kinds of catalytic centers are needed: one is acidic center, whose main function is to adsorb alkaline ammonia; The other type of center is the Cu
2+ center, which mainly adsorbs NO
x, so that the activated ammonia reacts with the activated NO
x to form N
2. In this reaction, of course, the more acid sites and Cu
2+ sites there are, the more favorable the reaction is. However, more catalytic centers require the presence of more tetra-coordinated aluminum in the framework, which in turn reduces the hydrothermal stability of zeolite molecular sieves. Considering these factors, the organic template used in the preparation of Cu-SSZ-13 zeolite molecular sieve is ammonium adamantane (N,N,N-trimethyl-1-adamantanammonium,TMAda
+), and the Si/Al ratio of the prepared product is about 12.After calcination and ion exchange, H-type SSZ-13 can be obtained, and then the final product can be obtained after ion exchange with copper salt. The whole process not only requires expensive organic templates, but also is complex and environmentally unfriendly. In order to reduce the cost of preparing Cu-SSZ-13, Ren et Al. Proposed to use Cu-tetraethylenepentamine (Cu-TEPA) as an organic template to synthesize Cu-SSZ-13 (ZJM-1) in one step. Its Si/Al ratio can be lower, so it can accommodate more acid sites and copper species. It shows excellent catalytic performance in the SCR-NH
3 reaction. After treatment at 750 ℃ for 16 H, under the condition of space velocity 400 000 h
-1, it still
[97][98].
In addition to the introduction of catalytic centers by ion exchange, catalytic centers or metal nanoparticles can also be introduced by means such as loading. Tian et al. Supported nanoparticles on mesoporous and conventional SAPO-11 molecular sieves, respectively, and the obtained catalysts showed excellent performance in long-chain alkane hydroisomerization reactions
[99]. Among them, the introduction of a certain amount of mesoporous structure into SAPO-11 molecular sieve can effectively improve the conversion of the reaction and the selectivity of the isomeric products. In these reactions, not only the adsorption and activation of reactants on acid sites to form reaction intermediates, but also the interaction of reactant intermediates with metal nanoparticles and the regulation of their adsorption state are required to form isomeric products.
Chen et al. Reported a study on the complete oxidation of toluene to CO
2 by zeolite supported metal nanoparticles
[100]. In the catalytic combustion reaction of toluene, the Pt nanoparticle catalyst supported on ZSM-5 zeolite molecular sieve can completely oxidize toluene to CO
2 at a lower temperature, especially when the ZSM-5 zeolite is introduced into the mesoporous structure and the catalyst is reduced, the reaction temperature of the T
98 can be reduced to 185 ° C (fig. 18), which is much lower than that of the catalyst supported on alumina with the same amount of Pt. These results indicate that the performance of zeolite-supported Pt catalysts is better than that of conventional supported catalysts, which may be attributed to the unique condensation effect of zeolite structure on toluene and other gases, which is of great significance for the preparation of catalysts with high efficiency to eliminate environmental pollution.
In that preparation of zeolite molecular sieve support metal nanoparticle catalysts, the metal particle are generally located on the outer surface of the zeolite. Because the size of metal nanoparticles commonly used in industry is between 2 and 5 nm, it is difficult to enter the microporous channels of zeolite (microporous channels are generally less than 0.8 nm). When the metal nanoparticles on the external surface of the zeolite are applied to a reaction performed at a higher temperature, the metal nanoparticles are easily sintered, thereby causing deactivation of the catalyst. In order to solve this problem, Zhang et al. Proposed that metal nanoparticles were added to the initial system of synthesis, and the metal nanoparticles were embedded in the zeolite crystal by seed-directed synthesis or solvent-free method.The prepared molecular sieve catalytic material not only has the high activity of metal nanoparticles, but also has the high stability and pore channel shape selectivity of a zeolite structure,It combines the advantages of both zeolite molecular sieves and metal nanoparticles perfectly, showing excellent activity and selectivity and long life, which provides a new strategy for the design and preparation of efficient catalytic materials
[101]. Figs. 19 and 20 show the performance of methane reforming with oxygen using S-1 zeolite supported or embedded Pd metal nanoparticles as the catalytic component. It can be seen that the embedded Pd nanoparticles show more excellent anti-sintering properties than the supported nanoparticles, and also give longer catalyst life.
It should also be pointed out that the different condensed state active sites of supported and mosaic metal nanoparticles lead to completely different adsorption States of reactant molecules on different catalyst structures. On the supported metal nanoparticle catalyst, the adsorption of reactant molecules on the metal nanoparticle is only related to the metal nanoparticle, while on the mosaic catalyst,The reactant molecules are not only affected by the metal nanoparticles, but also by the confinement of the zeolite pore structure. Usually, the reactant molecules have a higher concentration of adsorption state, so the catalytic activity is higher.
4 Desorption of adsorbed reaction product
In gas-solid zeolite catalytic reactions, carbon deposition often occurs, which leads to reaction deactivation. This is mainly due to the fact that the reaction products adsorbed on the zeolite catalyst do not desorb in time. If these reaction products can be quickly desorbed and the aggregation of reaction products on the catalytic center can be reduced, the carbon deposition rate of the catalyst can be greatly reduced and the reaction life of the catalyst can be improved. The main methods to solve this problem are to improve the mass transfer rate of reaction products, including nanocrystallization of zeolite crystals, introduction of mesoporous structure into zeolite crystals, or preparation of zeolite nanosheets. In addition, it has been found that changing the wettability of the catalyst can also increase the desorption rate of the reaction products
[102].
4.1 Nanocrystallization of zeolite crystal, preparation of mesoporous zeolite and zeolite nanosheet
Nanocrystallization of zeolite crystals and preparation of mesoporous zeolite and zeolite nanosheets can not only improve the mass transfer rate of gas reactants, but also accelerate the desorption rate of reaction products and reduce the aggregation and coverage of reaction products on catalytic centers. In Section 2, we have briefly described their synthesis and their role in different catalytic reactions. In the discussion of this section, to save space, the effects of zeolite crystal nanocrystallization, preparation of mesoporous zeolites, and zeolite nanosheets on the methanol to olefin (MTO) reaction are mainly discussed.
Yu and Liu et al. Systematically studied the effect of the grain size of SAPO-34 molecular sieve on the lifetime of MTO catalytic reaction
[103]. Using the same starting gel and TEAOH as a structure-directing agent, they synthesized four SAPO-34 with the same composition but significantly different crystallites, named SP-S, SP-F, SP-M, and SP-C, with crystallites of approximately 20 × 250 × 250 nm flakes, 80 nm spheres, 1 μm, and 8 μm cubes, respectively, as shown in Figure 21. The MTO reaction shows that the lifetime of the catalytic reaction is related to the grain size, especially the nanosheet of SAPO-34 shows the longest reaction lifetime (Fig. 22), which is directly related to the fastest desorption of the reaction product of the catalyst, which can effectively avoid the adsorption of the reaction product on the catalytic center and the formation of carbon deposits.
Yu et al. Prepared hierarchically porous SAPO-34 molecular sieves using silane surfactants, which showed very different reaction lifetimes in the MTO reaction
[104]. Those SAPO-34 with rich hierarchical pore structure have far longer reaction lifetime in MTO than ordinary SAPO-34, although they all have the same high crystallinity and micropore volume. Schwieger et al. Also introduced the macroporous structure into the mesoporous ZSM-5 zeolite during the preparation of the catalyst, and the reaction life of the obtained MTO catalyst was further improved
[105].
In addition to SAPO-34 and high-silica ZSM-5 used in industry as MTO catalytic materials, the catalytic properties of a large number of different types of molecular sieves have been explored in recent years. For example, it has been found that the pore size of ZSM-34 is larger than that of SAPO-34, but smaller than that of benzene ring, which can improve the diffusion rate of propylene and inhibit the formation of benzene compounds, as shown in Table 1
[106]. These results indicate that the pore structure of zeolite also has a direct effect on the MTO reaction performance. Recently, Lei et Al. Reported that aluminosilicate ITH zeolite (Al-ITH) was directly synthesized in a germanium-free system. Compared with ZSM-5 zeolite, Al-ITH zeolite can not only obtain more propylene yield, but also has a longer reaction life in MTO reaction, as shown in Fig. 23. Al-ITH zeolite has 9 × 10 × 10 three-dimensional pore structure, large specific surface area, fully four-coordinated aluminum species, excellent hydrothermal stability and suitable acidity, and the nanosheet morphology is conducive to the condensation and diffusion of reactant and product molecules in the pore, thus effectively increasing the yield of propylene.
4.2 Regulation of wettability of zeolite molecular sieves
The control of catalyst wettability can accelerate the desorption of reaction products and reduce the aggregation of reaction products on the catalytic center, thus speeding up the reaction and effectively improving the selectivity and yield of products. Although little research has been done in this area, the results obtained show great potential for improving catalytic performance. For example, Zhang et al. Prepared three different Beta zeolites loaded with Pt nanoparticles to study the catalytic oxidation of formaldehyde to carbon dioxide at low concentrations, which is also the best way to eliminate indoor formaldehyde pollution. In these studies, it was found that the catalyst prepared from pure silica Beta zeolite (Pt/Beta-Si, Pt 0.2 wt%) was the most active and could catalyze the total oxidation of 80 ppm formaldehyde to CO
2 at -20 ℃. In contrast, the temperature for complete decomposition of formaldehyde reached at least 40.d egree. C., as shown in fig. 24, whether the high-silica Beta zeolite (Beta-TEA) synthesized by organic template TEAOH or the aluminum-rich Beta zeolite (Beta-SDS) synthesized by zeolite seeding method was loaded with the same Pt loading
[108]. Generally speaking, the presence of acidic sites is beneficial to the conversion of formaldehyde, but compared with the three catalysts, the catalyst without acidity at all has the best catalytic performance. To understand this issue, the authors conducted a catalytic kinetic study (Figure 25) and found that Pt/Beta-Si showed a high ability to convert formic acid to carbon dioxide and water products. That is to say, on the catalyst, the desorption ability of water is the strongest, which can quickly desorb the reaction product water species condensed on the metal catalytic center.The condensation state of the reaction product water on the metal catalytic center is effectively changed, so that the complete oxidation ability of formaldehyde is greatly improved, and the hydrophobicity of the catalyst is essential for improving the complete oxidation performance of formaldehyde.
Methane is abundant in nature, and it is the main component of natural gas, shale gas and combustible ice. If methane can be directly converted into methanol at low temperature, a large number of natural gas resources can be added value. Jin et al. coated the long-chain hydrophobic silane on the outside of the zeolite crystal embedded with AuPd nanoparticles, and formed a layer of hydrophobic molecular fence on the outside surface of the zeolite crystal, which allowed hydrogen and oxygen to pass through. Once hydrogen peroxide was formed, it condensed inside the zeolite crystal and was difficult to diffuse through the fence to the outside of the catalyst
[109]. The hydrogen peroxide in the crystal continues to react with methane to form methanol. Since methanol has a certain hydrophobicity, it can penetrate the molecular fence of the catalyst to obtain a high yield of methanol product, as shown in fig. 26. Thanks to these reaction processes, 17.2% methane conversion was obtained and methanol selectivity was maintained at 92% at 70.d egree. C. (fig. 27). In this process, the adsorption state of various reaction intermediates and reaction products can be effectively improved by controlling the structure of catalytic materials, which shows the importance of condensed state research in catalytic reaction process.
4.3 Selective Adsorption of Reaction Products by Zeolite Molecular Sieve Additive
In addition to improving the mass transfer of the reaction product and changing the wettability of the catalyst as described above, additional promoters can be used to selectively desorb the product adsorbed on the catalyst surface,These products are quickly moved away from the catalytic center, thereby reducing the aggregation of reaction products on the surface of the catalyst, promoting the chemical equilibrium on the surface of the catalyst to move to the right, and accelerating the catalytic reaction. One example is the catalytic reaction for the conversion of syngas to olefins, in which the olefin product on the catalyst surface can be selectively adsorbed by adding a zeolite molecular sieve that can selectively adsorb olefins,Therefore, the aggregation of olefin molecules on the catalyst surface is reduced, the desorption of olefin products on the catalyst surface is accelerated, and the local chemical equilibrium on the surface of the catalyst is effectively changed, so that the chemical equilibrium on the surface of the catalyst is moved to the right, and the reaction activity is improved, as shown in fig. 28
[110].
It should be pointed out that the system studied by people is a macroscopic whole, and few people consider the chemical equilibrium of the microscopic system, especially for the chemical equilibrium of the catalyst surface. In this example, only physically mixing the molecular sieve additive reduces the aggregation and coverage of olefin molecules on the catalyst surface, realizes the selective desorption of olefin molecules, and effectively improves the yield of olefins.This indicates that the yield of the desired product can be greatly improved by regulating the local chemical equilibrium and the adsorption state of the reaction product molecules in the catalytic process in the future, which also needs more experimental results to verify.
5 Conclusion and prospect
In this work, we briefly review the condensed state chemistry of gas molecules in zeolite catalytic reactions,Comprises the adsorption and condensation of gas molecular reactant in a zeolite molecular sieve pore channel, the migration of the condensed reactant molecule to a catalytic reaction center for reaction, and the further migration and desorption of the reactant molecule from the catalytic center to a gas phase system. In the whole catalytic reaction process, the zeolite molecular sieve is used as a catalyst to greatly improve the diffusion of gas-phase reactants no matter the size of zeolite crystals is reduced, mesoporous structures are introduced, or zeolite nanosheets are prepared,Accelerate their condensation in zeolite pore channels and desorption of reaction products, thereby greatly improving the catalytic reaction activity, selectivity and reaction life, and preparing the high-efficiency zeolite molecular sieve catalytic material.
The main catalytic species in the framework of zeolite molecular sieve include acid sites and redox sites, and the acid sites include protonic acid and Lewis acid sites.Not only the type and concentration of acid, but also the position of acid sites in the zeolite structure can significantly affect the condensation state of reaction molecules, thus affecting the catalytic reaction performance. In the zeolite framework, the doping of heteroatoms such as redox centers has certain rules, which generally follow L'Löwenstein's rule (Al — O — Al bonds and M — O — M bonds cannot be formed,M represents heteroatom), trivalent and tetravalent elements are easy to enter the zeolite framework structure, divalent elements are relatively difficult, and pentavalent and hexavalent elements are almost impossible to enter the zeolite molecular sieve framework, all of which are related to the energy of the zeolite structure
[111].
Because the zeolite framework structure is very stable, even up to 1000 ℃, the heteroatoms doped in the zeolite molecular sieve structure are endowed with high thermal stability and hydrothermal stability, which provides a new method for the preparation of anti-sintering catalysts. Considering that the catalytic species doped in the molecular sieve structure are in a monoatomic dispersion state, this method is also an effective route to prepare monoatomic catalysts with high stability. These single-atom catalytic centers with different structures can effectively regulate the adsorption state of molecules, thus effectively regulating the catalytic performance of heteroatom molecular sieves.
By calculating the energy of different zeolite framework structures, the structure of molecular sieves can be effectively designed, and the directional synthesis can be carried out according to these designed structures, especially with the rapid development of computer technology in recent years.It provides a new opportunity for the directional synthesis of zeolite catalytic materials through theoretical calculation, and then these zeolite molecular sieves with new structures can control the adsorption state of reaction molecules and affect their catalytic performance
[112].
Zeolite molecular sieve crystals can further incorporate other catalytic centers, typically metal nanoparticles and metal oxide nanoparticles. Zeolite molecular sieves have high hydrothermal stability and shape-selective reaction, while metal nanoparticles have excellent catalytic performance, the combination of which can develop a variety of efficient catalysts.The adsorption and desorption States of these catalysts to reactants and reaction products can be effectively controlled by their structure and composition, and a new generation of catalysts with excellent catalytic performance can be prepared, in which zeolite supported noble metal catalysts have been widely used in industrial processes of hydrocracking and hydroisomerization.
It is worth pointing out that when metal nanoparticles are added to the initial synthesis system of zeolite molecular sieve, these metal nanoparticles will be embedded in the interior of zeolite crystals. Because the zeolite crystal structure is very stable, these metal nanoparticles embedded in the zeolite crystal structure are also endowed with excellent thermal and hydrothermal stability, showing good anti-sintering performance of metal nanoparticles.This is of great significance for the development of efficient and long-life metal nanocatalytic materials, and also provides a new means for the regulation of the adsorption and desorption state of reactants and reaction products in the reaction process
[113].
In addition to the incorporation of metal nanoparticles into zeolite crystals, other catalytically active components such as oxide nanoparticles can also be incorporated into zeolite crystals
[114]. Combined with the fact that the zeolite framework can also be doped with single-atom metal species, zeolite molecular sieve catalytic materials can form a new generation of catalytic system with a variety of catalytic centers, which provides a new way to regulate the adsorption and desorption of reactants and reaction products in the reaction process and to design efficient catalysis.
Considering the wide application of zeolite molecular sieve catalytic materials, people especially expect to find low-cost green synthesis technology, in which solvent-free and template-free synthesis can greatly reduce the emission of waste water and gas, which provides a new method for the green production of zeolite molecular sieve catalytic materials
[115⇓⇓~118].
Controlling the morphology of zeolite crystals can effectively control the catalytic performance, because the change of crystal morphology can effectively change the mass transfer and condensation state of reactants, reaction products and reaction intermediates in the catalytic reaction process. Taking the pyrolysis of waste polyethylene plastics as an example, their traditional catalytic pyrolysis is above 500 ℃, the quality of reaction products is low, and accompanied by a large number of carbon deposits, so it is difficult for traditional means to reduce the cost and increase the value of waste plastics such as polyethylene. Recently, zeolite nanosheets have been successfully used to rapidly condense and transfer the intermediates in the reaction process, and polyethylene can be converted into C
3~C
6 olefins with high added value at 280 ℃, and the formation of carbon deposits can hardly be detected, as shown in Figure 29
[119]. This low-temperature coke-resistant polyethylene cracking process not only overturns the general view that coke is readily formed in zeolite catalytic cracking, but also demonstrates that polyethylene-based plastics can be recycled into high-value chemicals. In addition to polyethylene, the reaction system can effectively depolymerize a wide range of practical polyethylene-rich plastics, thus providing an industrially and economically viable route for the disposal of plastic waste.
Recently, the adsorption and condensation of guest molecules in zeolite channels have been studied by high-resolution electron microscopy, and unexpected results have been obtained. When benzene was adsorbed on the straight channel of ZSM-5 zeolite as a probe molecule, the stretching change of the channel size along the longest direction of benzene molecule was up to 15%, and the deformation of the adjacent channel was caused
[120]. Because of the compensation effect of pore deformation, the stability and stiffness of the whole unit cell are kept within the deformation range of 0.5%, which is also the reason why the local deformation of zeolite pore structure is not observed in the structural characterization usually carried out by conventional X-ray. It is very interesting that the local pore deformation phenomenon only occurs in the Si — O — Si or Si — O — T chain, while the SiO
4 unit is basically rigid because of its tetrahedral structure, which is the rigidity of the whole zeolite framework and the flexibility of the local pore. These results indicate that the adsorption and desorption mechanisms of guest molecules in zeolite channels still need to be further studied.
In a word, the catalytic reaction performance of gas molecules can be effectively improved by regulating the composition, size, multi-level structure and morphology, and wettability modification of the molecular sieve catalyst, which is mainly based on the rapid diffusion and adsorption of reactant molecules, the efficient conversion of adsorbed reactant molecules into reaction products, and the rapid desorption of reaction products. In the future, there are still some key scientific problems to be solved in condensed matter chemistry of molecular sieve catalytic reactions, such as whether the physical and chemical properties of gas molecules themselves (kinetic size, mass fraction, spatial configuration, wettability, etc.) And the physical and chemical structure of molecular sieve pores (pore diameter, topology, pore wall properties, etc.) Can be correlated.A quantitative or semi-quantitative relation is used to describe the adsorption and condensation behavior of gas molecules in the pore channels of the molecular sieve; What is the intrinsic relationship between the adsorption and condensation behavior of gas molecules and the catalytic conversion on the active sites of condensed matter? Whether the condensed state behavior of gas-solid phase reaction in molecular sieve catalytic reaction is applicable to liquid-solid phase reaction; How to dynamically monitor the condensed state behavior of guest molecules and reaction intermediates in the pores of molecular sieves in real time by means of modern advanced characterization technology, and establish appropriate theoretical models to accurately simulate these catalytic processes. These are important issues and challenges in condensed matter chemistry research, and a lot of research and in-depth development are needed to create more efficient molecular sieve catalytic materials matching the reaction process through the regulation and optimization of these factors, so as to make a contribution to energy saving and carbon reduction.
The National Natural Science Foundation of China(U21B20101)
The National Natural Science Foundation of China(22288101)
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