ZSM-5分子筛的合成与应用

总结分析了ZSM-5分子筛合成方面的研究进展,主要包括ZSM-5分子筛的合成,ZSM-5复合分子筛以及含有杂原子的ZSM-5分子筛的合成.在介绍各具体合成方法原理的同时,还讨论了各合成方法的优势与缺陷,这为今后ZSM-5分子筛在合成方面的研究提供了一定的借鉴.其次,还介绍了ZSM-5分子筛在炼油工业、精细化工及环境保护...     

Acidity and catalytic activity of mesoporous ZSM-5 in comparison with zeolite ZSM-5, Al-MCM-41 and silica–alumina

Acidity of mesoporous HZSM-5 prepared using amphiphilic organosilane template molecules was measured. Brønsted acid sites were observed in the prepared sample, and the number and the strength of Brønsted acid sites were determined quantitatively by a method of infrared-mass spectroscopy/temperature-programmed desorption (IRMS-TPD) of ammonia. ΔH for ammonia adsorption as an index of the strength was ca. 150 kJ mol⁻¹ that was almost the same as on usual HZSM-5, but the number was smaller than that of HZSM-5. From the measured acidity, it was concluded that the mesoporous materials contained a smaller concentration of Brønsted acid site notable on the structure of HZSM-5. Measurements of turnover frequency (TOF) in the catalytic cracking of octane supported the conclusion. Density functional calculations showed that the defect sites Al–OH and Si–OH attached to the Brønsted acid site changed the strength of the acid sites to show some possible structures of the weak and strong Brønsted acid sites included in the mesoporous HZSM-5. Acidities of Al-MCM-41 and silica–alumina were also measured, and the difference in the solid acidities of these materials was discussed.     

Thermal-catalytic degradation kinetics of polypropylene over BEA, ZSM-5 and MOR zeolites

In this study, thermal degradation of additive-free polypropylene powder over different type of zeolite catalysts was investigated. BEA, ZSM-5 and MOR with different surface areas, pore structures, acidities and Si/Al molar ratios were used as solid catalysts for degradation of polypropylene (PP). Degradation rate of the PP over zeolites was studied by thermogravimetric analysis (TGA) employing four different heating rates and apparent activation energies of the processes were determined by the Kissinger equation. The catalytic activity of zeolites decreases as BEA > ZSM-5a (Si/Al = 12.5) > ZSM-5b (Si/Al = 25) > MOR depending on pore size and acidity of the catalysts. On the other hand, initial degradation is relatively faster over MOR and BEA than that over both ZSM-5 catalysts depending on the apparent activation energy. It can be concluded that acidity of the catalyst is the most important parameter in determining the activity for polymer degradation process as well as other structural parameters, such as pore structure and size.     

Modulating acid site spatial location of Zn/ZSM-5 catalyst to boost the stable aromatization of hexane

Engineering acid site spatial location of zeolite catalyst harbors tremendous potential to boost catalytic performance but still remains a grand challenge for aromatization reaction. Herein, we successfully manipulate acid site spatial location inside the ZSM-5 channels to promote the hexane aromatization via the simple SiO₂ coating treatment. Multi-techniques demonstrated that the medium strong L acid in the channel originated from the Si(Al) O Zn structure is mainly retained in the Zn/ZSM-5-Si catalyst by covering the acid sites outside the channel. Surprisingly, there is a good linear relationship between BTEX yield and this medium strong L acid content. Based on the reaction kinetics, in situ FT-IR and theoretical calculations, it is found that the L acid with confinement effect served as the main active site could prominently enhance the cyclization of propene intermediate and the dehydrogenation of cycloalkane due to the strong affinity between the C/H and ZnOx, and control the desorption of BTEX (mainly benzene, toluene, ethylbenzene, and p-xylene) by weakening the binding of hydrogen proton to π electrons. Compared with the ordinary Zn/ZSM-5 catalyst, the yield of BTEX is increased by nearly 10%, and the Zn/ZSM-5-Si catalyst exhibits excellent anti coking deactivation ability. This strategy together with mechanistic results may pave the rational design of efficient Zn/ZSM-5 catalysts for aromatization reaction.     

Synthesis of ZSM-5 zeolite and silicalite from rice husk ash

Local rice husk was precleaned and properly heat treated to produce high purity amorphous SiO₂ for use in the synthesis of ZSM-5 zeolite and silicalite by hydrothermal treatment (150 °C) of the precursor gels (pH 11) under autogenous pressure in a short reaction time (4–24 h). A wide range of SiO₂/Al₂O₃ molar ratios (30–2075) and a small template content were employed to fully exploit the potential of rice husk ash (RHA). The mineralogical phases, morphology, specific surface area and pore volume of the synthesized products were investigated by XRD, FT-IR, SEM and BET analyses, respectively. Under the employed conditions, it was found that the gels with a low range of SiO₂/Al₂O₃ molar ratios (<80) produced an amorphous phase to poorly crystalline ZSM-5 zeolite; those with a medium range (80–200) favored well crystalline ZSM-5 zeolite production with a large surface area; whilst those with a high range of SiO₂/Al₂O₃ molar ratios (>200) yielded silicalite. The increase in Na₂O content, which was derived from the addition of NaAlO₂ to attain the desired SiO₂/Al₂O₃ molar ratio of the gel, did not significantly enhance the crystallization rate, crystallinity, or yield of products. On the contrary, these properties were greatly affected by the increase in the SiO₂/Al₂O₃ molar ratio.     

Dealumination kinetics of composite ZSM-5/mordenite zeolite during steam treatment: An in-situ DRIFTS study

To get a better understanding of structural deactivation of ZSM-5/MOR during the catalytic cracking of n-heptane in the steam atmosphere, a comprehensive mechanism of hydrothermal dealumination was proposed through in-situ diffuse reflectance Fourier transform infrared spectroscopy(DRIFTS) in this work. The mechanism can be divided into two steps: firstly, the hydrolysis of four Al\\O bonds, and secondly, the self-healing of Si\\OH bonds accompanied with partial condensation of the extra-framework Al species. Accordingly, the kinetics of dealumination process has also been fully discussed. In the IR spectra, the range of 3450–3850 cm~(-1) could be deconvolved to distinguish the hydroxyl groups on the different position and calculate the consumption of each hydroxyl group during the reaction. Based on results from the in-situ DRIFTS, the kinetics of dealumination was hence developed and also in well agreement with the kinetics of deactivation of ZSM/MOR catalysts during the reaction in the presence of little coke deposits.     

ZSM-5沸石分子筛脱硫脱氮机理及改性方法

综述了近年分子筛的脱硫脱氮机理及动力学,从表面酸性和孔结构两方面评述了ZSM-5分子筛的改性。重点综述了基于ZSM-5的杂原子分子筛、分子筛膜、纳米分子筛的研究动态和发展趋势,指出了研究应用中存在的问题以及今后的研究方向。     

Theoretical studies of IM-12 zeolite for acidic catalysts

We theoretically investigate the properties of the IM-12 to address a catalyst for acidic conversion reaction of larger organic molecules. The acidic characteristics of the IM-12 are investigated by density functional theory (DFT) considering both the local density and generalized gradient approximations, LDA and GGA, respectively. Based on quantum mechanical (QM) calculation results, we find that the zeolite with Al element prefers the tetrahedral (T) sites, T4 and T6, when replacing Si in IM-12 framework. Isomorphously substituted IM-12 on the T4 and T6 sites by B, Al, and Ga is studied, respectively. Both of the sites give the Brönsted acidity order: B–IM-12 < Ga–IM-12 < Al–IM-12, which is the same as other zeolites. The calculated NH₃ adsorption energies are compared with the calculated and experimental results of H–[Al]MOR [M. Elanany, D.P. Vercauteren, M. Koyama, M. Kubo, P. Selvam, E. Broclawik, A. Miyamoto, J. Mol. Catal. A 243 (2006) 1; C. Lee, D.J. Parrillo, R.J. Gorte, W.E. Farneth, J. Am. Chem. Soc. 118 (1996) 3262]. Molecular dynamics (MD) results show that IM-12 zeolite allows the large molecules such as diisopropylbenzene (DIPB) and triisopropylbenzene (TIPB) to diffuse faster than those in MOR zeolite and IM-12 may have significant selectivity for TIPB over DIPB. We conclude that the IM-12 with Al impurity would be a good candidate for large organic molecule acidic conversion reaction.     

Evidence for non-site-isolation in ZSM-5 from ion-exchange and catalytic studies

Ion exchange of HZSM-5 samples with alkali metal cations, using metal chloride solutions, results in partially exchanged zeolites, MHZSM-5, M = Li, Na, K or Cs. The degree of exchange is found to increase with increasing ionic radius of the cations. The catalytic properties of the alkalized zeolites were evaluated using the reaction conditions under which the catalytic activity of the HZSM-5 samples in terms of n-hexane cracking is proportional to the aluminium content. From the residual catalytic activity exhibited by the Na-, K- and CsHZSM-5 samples it is concluded that each of the larger Na⁺, K⁺ and Cs⁺ ions is influencing more than one AlO ₄ ^– tetrahedron, implying that the aluminium sites in ZSM-5 are not isolated. The ion-exchange results are then interpreted in terms of non-isolated aluminium sites. The ion-exchange and catalytic properties of the zeolites as a function of aluminium content are also discussed.     

Thermogravimetric and microcalorimetric studies of ZSM-5 acidity

The combination of thermogravimetry, microcalorimetry and infrared spectroscopy studies of pyridine adsorption has been used to characterize the acidity of a ZSM-5 catalyst. The majority of the acid sites are Brønsted acid centers associated with framework Al species, with heats of pyridine adsorption equal to 140 kJ/mol. Non-framework Al species in the zeolite sample of this study eliminate an approximately equal number of Brønsted acid sites. These nonframework Al species also produce strong Lewis acid sites with pyridine adsorption heats greater than 140 kJ/mol, as well as weak adsorption sites (e.g., weak Bransted acid sites or hydrogen bonding sites) with heats equal to 90–140 kJ/mol.     

Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM5

Copper ion-exchanged zeolites ZSM5 with SiO₂:Al₂O₃ molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h⁻¹). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO₂ formation in the gas phase.

A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO₂:Al₂O₃ ratio, enhances the conversion of NO. A vibrational band at 1631 cm⁻¹ is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu²⁺–O–Cu²⁺ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N₂O₃, also binds in a bridge configuration to the same Cu²⁺–O–Cu²⁺ dimers.

A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm⁻¹ probes isolated copper ions.

A third infrared band at 2130 cm⁻¹ confirms previous observations of -ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for adsorption are blocked by Cu²⁺ ion-exchange. The 2130 cm⁻¹ species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice.

Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers.

A high SiO₂:Al₂O₃ ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.     

Conventional and microwave hydrothermal synthesis of zeolite ZSM-5 from the cupola slag

ZSM-5 type zeolites have been prepared from cupola slag waste using both conventional hydrothermal and microwave syntheses at 130–200 °C. The ZSM-5 was synthesized by conventional heating by taking advantage of the high silica content of cupola slags. Microwave heating increased the rate of ZSM-5 formation by 4 times at 150 °C compared with conventional heating. The Si/Al ratio of the ZSM-5 produced by the conventional heating and the microwave crystallization were similar 28 and 29, respectively. The conventional-heating produced ZSM-5 particles 3 μm in diameter, while, microwave-heating produced smaller ZSM-5 particles only 0.3 μm in size.     

Fundamental studies of aluminum corrosion in acidic and basic environments: Theoretical predictions and experimental observations

Using quantum electrochemical approaches based on density functional theory and cluster/polarized continuum model, we investigated the corrosion behavior of aluminum in HCl and NaOH media containing phenol inhibitor. In this regard, we determined the geometry and electronic structure of the species at metal/solution interface. The investigations revealed that the interaction energies of hydroxide corrosive agents with aluminum surface should be more negative than those of chloride ones. The inhibitor adsorption in acid is more likely to have a physical nature while it appears as though to be chemical in basic media. To verify these predictions, using Tafel plots, we studied the phenomena from experimental viewpoint. The studies confirmed that the rate of corrosion in alkaline solution is substantially greater than in HCl media. Moreover, phenol is a potential-molecule having mixed-type inhibition mechanism. The relationship between inhibitory action and molecular parameters was discussed and the activity in alkaline media was also theoretically anticipated. This prediction was in accord with experiment.     

Sealing and reopening of micropores of mordenite and ZSM-5 by disilylbenzene compounds

Regularly-structured solid matrices are potent materials for regulating molecular permeation and diffusion. Zeolite materials bearing ordered micropores are very advantageous to control the diffusion of molecules. When some disilylbenzene compounds were grafted on mordenite and ZSM-5 zeolites, the penetrations of small molecules such as α-methylstyrene (AMS) and molecular nitrogen were prevented by the direct formation of the crosslinking with two grafting sites on the exterior surfaces of those zeolites. The dimerization reaction of α-methylstyrene effectively promoted by mordenite was not catalyzed by these grafted mordenite samples. No adsorptions of molecular nitrogen were observed in the grafted mordenite and ZSM-5 samples. When these grafted samples were calcined, both the catalytic activity for the dimerization and the permeation of molecular nitrogen were recovered. The reopening of the micropores of zeolites was also achieved by the treatment with HCl acidic solutions. Thus, the diffusion and the permeation of various small molecules can be perfectly and reversely controlled by the sealing and the reopening of the micropores of zeolites by the disilylbenzene compounds.     

Exploring suitable ZSM-5/MCM-41 zeolites for catalytic cracking of n-dodecane: Effect of initial particle size and Si/Al ratio

In this study, various ZSM-5/MCM-41 micro/mesoporous zeolite composites have been prepared by alkalidesilication and surfactant-directed recrystallization of ZSM-5. The effects of particle size and Si/Al ratio of initial ZSM-5 zeolites on the structure and catalytic performance of ZSM-5/MCM-41 composites are studied. The results of XRD, TEM N₂-adsorption-desorption, NH₃-TPD and in situ FT-IR revealed that ordered hexagonal MCM-41 mesopores with 3-4 nm pore size were formed around ZSM-5 crystals, and the specific surface area and mesopore volume of composites increased with increasing the Si/Al ratio of initial ZSM-5. Catalytic cracking of n-dodecane (550 ℃, 4 MPa) showed that the ZSM-5/MCM-41 composites obtained from the high Si/Al ratio and nano-sized initial ZSM-5 zeolites exhibited superior catalytic performance, with the improvement higher than 87% in the catalytic activities and 21% in the deactivation rate compared with untreated zeolites. This could be ascribed to their suitable pore structure, which enhanced the diffusion of reactant molecules in pores of catalysts.     

Theoretical and experimental studies of the pitting of type 316L stainless steel in borate buffer solution containing nitrate ion

Passivity breakdown on Type 316L stainless steel (UNS S31603) in the presence of aggressive (Cl⁻) and inhibitive (NO₃⁻) species has been studied and the experimental data are interpreted in terms of the Point Defect Model (PDM). Combining the PDM with competitive adsorption of Cl⁻ and NO₃⁻ into surface oxygen vacancies at the passive film/solution interface predicts that the critical breakdown potential (V_c) will vary linearly with log[Cl⁻] or log([Cl⁻]/[NO₃⁻]) for systems containing only chloride ion or chloride plus nitrate ion, respectively. The experimental measurements confirm these linear relationships and fundamental parameters contained within the PDM have been determined. The transfer coefficient, α₀, of the reaction in which a cation is ejected from the barrier layer to form the surface vacancy pair, , is about 1.00, indicating that the transition state resembles the products. Furthermore, the dependence of V_c on potential scan rate has been measured, and the critical, areal concentration of condensed cation vacancies (ζ) at the metal/barrier layer interface been calculated. That value is in good agreement with the theoretical value calculated from the structure of Cr₂O₃, of which the barrier layer is postulated to comprise, demonstrating that the PDM successfully accounts for passivity breakdown on UNS S31603 in aqueous solutions containing both aggressive and inhibitive species.     

Synthesis of nanostructured SiC by magnesiothermal reduction of silica from zeolite ZSM-5 and carbon: The effect of carbons from different sources

Nanostructured SiC was synthesized by magnesiothermal reduction of silica derived from zeolite ZSM-5 with a Si/Al ratio of about 80 and four carbons from different sources. The carbons were carbon black and three carbons synthesized by impregnating furfuryl alcohol or sucrose into three porous templates (ZSM-5, clinoptilolite and MCM-48). Magnesiothermal reduction was carried out by reacting mixtures of silica, carbon and magnesium powder at 600–800 °C in a flowing argon atmosphere. The starting materials and final products were characterized by X-Ray diffraction, thermogravimetric analysis, scanning electron microscopy and BET analysis. The results indicated that the synthesis of nanostructured SiC is influenced by the chemical nature of the carbon and its surface area.     

Direct partial oxidation of methane over ZSM-5 catalyst: Zn-ZSM-5 catalyst studies

Extended studies on Zn-ZSM-5 catalyst for the production of liquid hydrocarbons in the direct partial oxidation (DPO) of CH₄ with O₂ are reported. Previously, it was reported that metal-containing ZSM-5 catalysts could produce C₅₊ hydrocarbons from pure CH₄/O₂ feeds without feed additives. Zn-ZSM-5 produced the highest C₅₊ yields of the catalysts tested. This work shows that the method of introducing Zn onto the catalyst, ion-exchange versus impregnation, does not significantly alter C₅₊ yields if low Zn content is maintained ( 0.4–0.5 wt%). Liquid hydrocarbon yields in this system doubled after 8 h on stream while overall C₂₊ yields increased by over 300%. Mechanistic implications of these findings are discussed. Finally, processing a natural gas feed over Zn-ZSM-5 gave higher C₅₊ yields over CH₄ feed but these yields were not improved over previously published results using HZSM-5.     

Storage of molecular hydrogen into ZSM-5 zeolite in the ambient atmosphere by the sealing of the micropore outlet

The storage of molecular hydrogen into ZSM-5 zeolite in the ambient atmosphere was examined by hydrogen filling into the micropore and the following sealing of the micropore outlet to prevent the release of hydrogen to the outside. The surface grafting of 1,4-bis(hydroxydimethylsilyl)benzene onto ZSM-5 zeolite was applied to the sealing of the micropore outlet. Pressurized hydrogen (10 MPa) was filled into the micropore of ZSM-5 at liquid nitrogen temperature (−196 °C), and then the sample was heated at 150 °C for forming strong binding between the zeolite surface and the disilane compound under the hydrogen pressure. The hydrogen sorption isotherm at −196 °C showed that the adsorption of hydrogen onto the disilane-grafted ZSM-5 thus obtained was reduced to less than 20% from the original ZSM-5. The remarkable hysteresis between the adsorption and desorption branches of the isotherm indicated that the kinetic trap of hydrogen occurred by the narrowed outlets of the micropores of ZSM-5 with the disilane compound. Even after exposing the disilane-grafted ZSM-5 to the atmosphere over a few months, hydrogen could be discharged by heating over 150 °C. This result demonstrated that molecular hydrogen was successfully stored into ZSM-5 zeolite in the ambient atmosphere for a long time.     

Flexibility in zeolites:29Si NMR studies of ZSM-5 frame transitions

Zeolites are most often perceived as rigid solids. Recent evidence has demonstrated that the temperature or the adsorption of molecules whose dimensions approach the pore dimensions induce changes in the solid structure, i.e., a flexing of the solid lattice. For pentacil zeolites, as ZSM-5, a transition between monoclinic and orthorhombic forms of the crystalline structure is found with a change in temperature or the adsorption of ring containing molecules. We find that the temperature at which this transition occurs depends on the Si/Al ratio within ZSM-5 and the discrete, reversible transition can be measured by²⁹Si NMR, confirming prior X-ray studies. A similar dependence is found for ZSM-11. Phosphorous modification of the ZSM-5 does not change the transition temperature; however, steam treating of the zeolite does. The implications of these measurements to the flexibility and to the potential for transport is discussed. An analogy between transport and flexibility in one-dimensional polymers and that in the three-dimensional, open, solid network of zeolites is suggested.We wish to thank Union Carbide, Mobil, Haldor Topsøe, Johannes Lercher, peter Jacobs and Mark Davis for generously providing samples for this study.