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1.
The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. A new picture of the formation and evolution of mesopores during hydrothermal treatment has emerged from recent electron microscopy studies on hydrothermally dealuminated USY materials. Laboratory steam treatments give rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are observed among different USY grains as well as within single grains. In regions with high defect concentration, mesopores “coalesce” to form channels and cracks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boundaries. High-silica Y materials, having little or no nonframework Al, exhibit poor catalytic activity. The results of recent studies provide compelling evidence that the critical nonframework Al species are (1) highly dispersed, and (2) quite possibly exist as cationic species in the small cages of dealuminated H-Y. Investigations of Lewis acidity in mildly dealuminated zeolites indicate that the origin of the high catalytic activity is a synergistic interaction between Brønsted (framework) and highly dispersed Lewis (nonframework) acid sites. The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity, as, for example, by calorimetry, or by NMR spectroscopy. The enhanced activity of mildly steamed protonated zeolites is not due to an increase in acidity of the bridging hydroxyl or Brønsted sites. 相似文献
2.
Zuzana Musilová Naděžda Žilková Sang-Eon Park Jiří Čejka 《Topics in Catalysis》2010,53(19-20):1457-1469
Catalytic behavior of mesoporous ZSM-5 was investigated in toluene disproportionation, toluene alkylation with isopropyl alcohol, and p-xylene alkylation with isopropyl alcohol to understand the effect of the presence of mesopores. Three ZSM-5 zeolites (conventional one and two mesoporous differing in the mesopore volume) having similar Si/Al ratio were synthesized and characterized as for their acidity (internal and external) as well as their micropore/mesopore volume. No substantial differences among three samples were observed as for the type and concentration of Brønsted and Lewis acid sites as well as their location in zeolite channels or on external surface of zeolite crystals. Conversions of toluene and p-xylene increased with increasing volume of mesopores in ZSM-5 zeolite while the selectivity to individual products depended on the type of reaction. In general, selectivity to sum of xylenes in toluene disproportionation, sum of isopropyltoluenes in toluene alkylation and to 1-isopropyl-2,5-dimethylbenzene in p-xylene alkylation increased due to a shorter contact time molecules spent in mesoporous ZSM-5 catalysts. In contrast, para-selectivity decreased as diffusion pathways were shorten due to the presence of mesopores. 相似文献
3.
An original quantitative method based on H/D exchange between H2O/D2O molecules and the OH groups of different zeolites has been developed for the titration of the Brönsted acid sites present on the solid surface. The measured Brönsted acid sites density appears to be in good agreement with the theoretical amount estimated by the Si/Al ratio. In contrary to classical methods, this non-destructive anhydride method titrates the whole quantity of Brönsted acid sites of zeolites. 相似文献
4.
Yuechao Zhang Yingjie Li Junjie Gu Senlin Tian Ping Ning 《Korean Journal of Chemical Engineering》2018,35(9):1932-1940
Supercritical water (SCW) impregnation is an efficient and feasible method that has been used to prepare highly dispersed supported catalysts, but few studies have investigated the stability of support materials in supercritical water. Thus, our aim was to investigate the hydrothermal stability of zeolite supports (ZSM-5, TS-1, ZSM-35, HY, 13X, Beta, SAPO-11 and SAPO-34) as model compounds in supercritical water. Results showed that almost all of zeolites suffered from crystallinity change, structural properties degradation, obvious desilication and dealumination. The decrease of surface areas and the collapse of crystalline structures in HY, 13X, Beta, SAPO-11 and SAPO-34 were more serious compared to ZSM-5, ZSM-35 and TS-1. The micropore areas and acidity of all SCW-treated zeolites were reduced. 13X with lower Si/Al ratio had higher hydrothermal stability than HY due to the formation of extra-framework Al (EFAL). EFAl also generated strong Lewis acid sites determined by ammonia temperature-programmed desorption and 27Al magic angle spinning nuclear magnetic resonance. Desilication and dealumination were simultaneous, and led to the increase of framework Si/Al ratio. ZSM zeolites (ZSM-5, ZSM-35 and TS-1) had higher hydrothermal stability than HY, 13X, Beta, SAPO-11 and SAPO-34 in SCW. 相似文献
5.
Zhenhao Wei Tengfei Xia Minghui Liu Qingsheng Cao Yarong Xu Kake Zhu Xuedong Zhu 《Frontiers of Chemical Science and Engineering》2015,9(4):450-460
The effects of alkaline treatment on the physical properties of ZSM-5 catalysts and on their activities for methanol to aromatics conversion have been investigated. A mild alkaline treatment (0.2 and 0.3 mol/L NaOH) created mesopores in the parent zeolite with no obvious effect on acidity. The presence of mesopores gives the catalyst a longer lifetime and higher selectivity for aromatics. Treatment with 0.4 mol/L NaOH decreased the number of Brønsted acid sites due to dealumination and desilication, which resulted in a lower deactivation rate. In addition, more mesopores were produced than with the mild alkaline treatment. As a result, the lifetime of the sample treated with 0.4 mol/L NaOH was almost five times that of the parent ZSM-5. Treatment with a higher alkaline concentration (0.5 mol/L) greatly reduced the number of Brønsted acid sites and the number of micropores resulting in incomplete methanol conversion. When the alkaline-treated catalysts were washed with acid, some of the porosity was restored and a slight increase in selectivity for aromatics was obtained. 相似文献
6.
Yasunori Oumi Jou Takahashi Kazuyoshi Takeshima Hery Jon Tsuneji Sano 《Journal of Porous Materials》2007,14(1):19-26
Dealuminated zeolites Y were treated with aqueous solutions of various acids and ammonium salts to investigate the realumination behavior under acidic conditions. From the results of 27Al MAS NMR, 29Si MAS NMR and FT-IR measurements, it was found that a part of non-framework aluminum species in the dealuminated zeolite Y is effectively reinserted into the zeolite framework in CH3COONH4 and C6H5COONH4 aqueous solutions. Pyridine adsorption experiments also revealed that most of incorporated aluminum species generate tetrahedrally coordinated framework aluminum species, namely Brönsted acid sites. Although the realumination also proceeded in H2SO4 and CH3COOH aqueous solutions, large amounts of incorporated aluminum species were not necessarily responsible for generation of Brönsted acid sites. Framework connected aluminum species, presumably as 3-fold-coordinated Lewis acidic framework aluminum species, were mainly generated. In the TEM image of the realuminated zeolite Y, needle-like crystals with ca. 25–80 nm in length were observed, which are probably due to AlOOH generated from non-framework aluminum species. 相似文献
7.
A series of Al–Fe-SBA-15 catalysts were synthesized by microwave methods with various iron and aluminum contents. The samples were characterized by XRD, SEM, TEM, BET, NH3-TPD, FT-IR. Results indicated that all the samples that exhibited well-ordered hexagonal arrays of mesopores, disk-like and petal-like morphologies were observed for different catalysts. NH3-TPD indicated that the introduction of Al results in the generation of Brönsted acid originated from Al–OH. Brönsted acid played important roles in the selective catalytic reduction of NO by NH3, NOx conversion over Al–Fe-SBA-15 samples increased rapidly as temperature increased and achieved 95% at about 360 °C. 相似文献
8.
《Catalysis communications》2007,8(3):539-542
A thermal dealumination method was applied to modify HZSM-5 zeolites, and the Mo/HZSM-5 catalyst pre-dealuminated in N2 stream exhibited rather high catalytic activity and stability in the methane dehydroaromatization reaction (MDA). 29Si NMR, FT-IR and TPO measurements show that the thermal treatment of the HZSM-5 in inert atmosphere induced partial removal of tetrahedral coordinated Al from the zeolite lattices leading to elimination of the original excess strong Brönsted acid sites (known as responsible for the coke formation), and thus significantly promoted the coke-resistance of the Mo/HZSM-5 catalyst. 相似文献
9.
10.
Alkali-treatment of ZSM-5 zeolites with different SiO2/Al2O3 ratios and light olefin production by heavy oil cracking 总被引:1,自引:0,他引:1
Four kinds of ZSM-5 zeolites with different SiO2/Al2O3 ratios are alkali-treated in 0.2 M NaOH solution for 300 min at 363 K. Changes to the compositions, morphologies, pore sizes, and distributions of the zeolites are compared before and after alkali-treatment. The changes observed are largely influenced by the SiO2/Al2O3 ratios with which the zeolites are synthesized. A possible mechanism of desilication during alkali-treatment is proposed. The SiO2/Al2O3 ratio of zeolites is found to influence the yield of light olefins that use heavy oil as feedstock. Alkali-treated ZSM-5 zeolites produce higher yields of light olefins compared to either untreated zeolites or the industry catalyst CEP-1. It is believed that alkali-treatment introduces mesopores to the zeolites and improves their catalytic cracking ability. ZSM-5 zeolites with SiO2/Al2O3 ratios of 50 also present superior selectivity toward light olefins because of their optimized hierarchical pores. 相似文献
11.
The effect of synthesis conditions on the preparation and properties of three novel zeolites TUN, IMF, and -SVR was investigated and compared with MFI. X-ray powder diffraction, infrared spectroscopy, chemical analysis, sorption measurements, and scanning electron microscopy were employed to characterize the textural and chemical properties of these zeolites. FTIR spectroscopy in combination with pyridine as probe molecule was used to describe the type, concentration and acid strength of these zeolites. Zeolite -SVR was synthesized with Si/Al ratios higher than 100 in contrast with TUN and IMF having Si/Al ratios about 20–30, respectively. TUN, MFI and IMF zeolites exhibit similar concentrations of Brønsted and Lewis acid sites and also similar acid strength of Brønsted sites. In contrast, the presence of one vacancy/unit cell in the case of -SVR zeolite substantially changes its acidic properties. Generally, zeolites TUN and IMF exhibit similar acidic properties in terms of acid strength and Brønsted to Lewis acid site ratio like MFI and due to their slightly larger pores higher activities in catalytic reactions are expected. 相似文献
12.
Won Cheol YooXueyi Zhang Michael Tsapatsis Andreas Stein 《Microporous and mesoporous materials》2012,149(1):147-157
ZSM-5 zeolite crystals with secondary mesopores were synthesized by alkaline desilication and surfactant-induced re-assembly of dissolved species (i.e., silicates, aluminosilicates and zeolite crystal fragments) originating from the parent ZSM-5 crystals. The meso-zeolite products exhibit a dual-mesopore structure in which the smaller mesopores (ca. 3 nm) are attributed to surfactant-induced micelle formation involving dissolved species, and larger mesopores (ca. 10-30 nm) result from desilication processes occurring under the alkaline reaction conditions. The external surface area (i.e., the surface area due to mesopores, macropores and the external particle surface) of the meso-zeolite materials depends on the Si/Al ratio, the hydroxide concentration and the presence of surfactant, and it reaches values as high as 327 m2 g−1 when a surfactant is used. The crystallinity of the highest surface-area meso-zeolite is well preserved, maintaining values of ca. 83% (on the basis of micropore volume) or ca. 77% (on the basis of X-ray diffraction intensities) of the parent zeolite structures. Further physicochemical characterization by 27Al and 29Si magic-angle-spinning solid-state NMR spectroscopy, scanning and transmission electron microscopy, temperature-programmed ammonia desorption measurements, and inductively coupled plasma elementary analysis support the hypothesis that re-assembly of dissolved species of zeolite crystals occurred by surfactant-induced micellization, resulting in the high external surface area of the meso-zeolite materials. 相似文献
13.
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. 相似文献
14.
《Catalysis communications》2003,4(6):295-302
Metal analogs of Ferrierite (FER) have been prepared by incorporating Fe by single route and Al by three different routes (a) using pyrrolidine (b) in presence of promoting media (perchloric acid) and (c) in presence of anionic surfactant sodium bis-(2-ethylhexyl) sulpho succinate, (AOT) and characterized by FTIR using CD3CN as probe molecule to study acid sites of these samples. D3-acetonitrile adsorption on these samples showed a band at 2297 cm−1 corresponding to the interaction of nitrile group with Brönsted acid sites and a band at 2322 cm−1 due to interaction with Lewis acid sites. The concentrations of Brönsted and Lewis acid sites were calculated by using the reported extinction coefficients. The concentration of Brönsted acid sites was found to decrease in the order Al-FER (PER) > Al-FER (AOT) > Al-FER (H2SO4) > Fe-FER. The TPD of ammonia of FER samples was carried out to find the acid strength of the samples which decreased in the order Al-FER (PER) > Fe-FER > Al-FER (AOT) > Al-FER (H2SO4). The catalytic activity was tested by m-xylene isomerization on Al-FER prepared by three different routes and Fe-FER catalyst and was found to decrease in the order Al-FER (PER) > Al-FER (H2SO4) > Fe-FER > Al-FER (AOT). 相似文献
15.
《Catalysis communications》2007,8(11):1567-1572
Enhanced performance of methane dehydro-aromatization reaction (MDA) were achieved on a Mo-based HZSM-5 zeolite catalyst in which HZSM-5 were pretreated by a proper amount of NH4F (Mo/HZ(F)). The results of NH3-TPD and 27Al MAS NMR demonstrated that the number of Brönsted acid sites decreased on the HZSM-5 zeolite and Mo/HZSM-5 catalyst after NH4F treatment. TGA and TPO measurements showed that the Mo/HZ(F) catalysts were highly resistant to coke deposition, which resulted mainly from the elimination of the Brönsted acid sites after the pretreatment of the HZSM-5 zeolite with NH4F. 相似文献
16.
Limited information is available on the isomerization of α-pinene proceeding on zeolites and related desilicated materials. We wish therefore to report on the title reaction proceeding over ZSM-5, ZSM-12 and MCM-22 type zeolites, parent and modified by the sodium hydroxide treatment. The NaOH solutions of various concentrations (0.05–1 M) were used as a desilicating agent. Such treatments with basic solutions were applied: (i) under atmospheric pressure, and (ii) under hydrothermal conditions. It was shown that ZSM-12 was more resistant towards the basic solutions treatment, and its structure was retained over a whole range of NaOH concentrations studied. Nitrogen sorption revealed strong influence of the desilication process on the pore structure of modified materials—the mesopores system was formed in the zeolite crystals. Finally, catalytic studies were carried out using ZSM-5, ZSM-12, MCM-22, their desilicated derivatives and a ZSM-12/MCM-41 composite material. Catalytic properties of the samples studied were affected to a large extent by the NaOH treatment. 相似文献
17.
Zhipeng Chen Jiajin Huang Qiaowen Mu Huiyong Chen Feng Xu Yanxiong Fang Baoyu Liu 《中国化学工程学报》2019,27(8):1981-1987
The modification of pillared MFI zeolites was performed by nitridation of silica pillared MFI zeolite nanosheets under NH3 atmosphere with different time. The resultant zeolites were characterized by a complementary combination of X-ray power diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), pyridine-IR spectroscopy and N2 adsorption–desorption isotherms. The analyses showed that the nitridation didn't destroy the crystallinity and specific surface area of zeolites, and the acidity of zeolites can be tailored by tuning the time of nitridation, resulting in the different concentration ratios of Brønsted-to-Lewis (B/L) acid sites. Moreover, the nitrided zeolites exhibited high selectivity to 2-benzyl-1,3,5-trimethylbenzene than parent silica pillared MFI zeolite nanosheets in benzylation of mesitylene with benzyl alcohol. A balance between Brønsted acid sites and Lewis acid sites can inhibit the self-etherification of benzyl alcohol and enhance the selectivity of alkylated product. These experimental data implied that nitridation was an effective method to modulate the acidity of zeolites and the synergy between Brønsted acid sites and Lewis acid sites was a decisive factor to determine the selectivity. 相似文献
18.
Lara Heeribout Robert Vincent Patrice Batamack Claudine Dorémieux-Morin Jacques Fraissard 《Catalysis Letters》1998,53(1-2):23-31
1H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H3O+ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength. 相似文献
19.
NH3‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of 1H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo5+ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization. 相似文献
20.
A. van Miltenburg J. Pawlesa A. M. Bouzga N. Žilková J. Čejka M. Stöcker 《Topics in Catalysis》2009,52(9):1190-1202
Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This ‘desilication’ process led for mild alkaline concentrations (~0.10–0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3-acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Brønsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Brønsted acid site concentration. Decomposition and deconvolution of 29Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q3 SiOH groups were formed at the expense of the Q4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite. 相似文献