共查询到20条相似文献,搜索用时 460 毫秒
1.
2.
3.
4.
弱酸性介质中Fe-ZSM-5型分子筛的合成与鉴定 总被引:3,自引:0,他引:3
在弱酸性介质中,NH_4F存在下,以四丙基溴化铵(TPABr)为模板剂,采用水热合成法成功地合成了Fe-ZSM-5型分子筛。合成中pH值为5—7。该分子筛原料为铵型,经高温灼烧后可直接转化为氢型。X射线衍射、扫描电镜及电子探针、红外光谱、顺磁共振谱、Si高分辨固体核磁共振谱、差热分析等对Fe-ZSM-5型分子筛的的研究确证了Fe对Fe-ZSM-5型沸石骨架的同晶取代。X射线光电子能谱和穆斯堡尔谱对Fe-ZSM-5分子筛原粉的研究表明,Fe-ZSM-5分子筛中铁的价态是以三价和二价共存。 相似文献
5.
分子筛材料具有酸性可调节和择形选择性的优势, 因而在多相催化反应中表现出优异的性能. 本文综述了利用固体核磁共振(ssNMR)光谱对分子筛的酸性和择形选择性进行研究的最新进展. 通过采用或不采用探针分子的ssNMR技术, 探测了分子筛中酸性位的数量、 酸强、 酸类型及酸位之间的协同作用. 此外, 通过直接观察多相催化反应中关键中间体的存在, ssNMR光谱提供了分子筛择形选择性的证据. 酸性和择形选择性的协同作用有助于更好地设计分子筛材料, 以实现更好的多相催化. 相似文献
6.
7.
8.
《高等学校化学学报》2021,(1)
分子筛材料具有酸性可调节和择形选择性的优势,因而在多相催化反应中表现出优异的性能.本文综述了利用固体核磁共振(ssNMR)光谱对分子筛的酸性和择形选择性进行研究的最新进展.通过采用或不采用探针分子的ss NMR技术,探测了分子筛中酸性位的数量、酸强、酸类型及酸位之间的协同作用.此外,通过直接观察多相催化反应中关键中间体的存在, ssNMR光谱提供了分子筛择形选择性的证据.酸性和择形选择性的协同作用有助于更好地设计分子筛材料,以实现更好的多相催化. 相似文献
9.
10.
采用X射线粉末衍射(XRD)、固体核磁共振(MAS NMR)、扫描电镜(SEM)等分析手段研究了金属离子(Y、Ga、Cr、Zn、Cu)对Y型分子筛水热稳定性的影响。结果表明,金属离子Y的引入能抑制Y型分子筛晶胞收缩,避免骨架铝的脱除,显著提高了分子筛的水热稳定性;金属离子Zn、Cu的引入在一定程度上能较好地稳定分子筛骨架结构,但是其耐高温水热性能较差,经过800℃水热处理后,分子筛的骨架结构基本倒塌;金属离子Ga、Cr不同于Y、Zn、Cu,不能很好地稳定分子筛骨架结构,其耐高温水热性能介于两者之间。 相似文献
11.
Zhu Q Kondo JN Yokoi T Setoyama T Yamaguchi M Takewaki T Domen K Tatsumi T 《Physical chemistry chemical physics : PCCP》2011,13(32):14598-14605
A series of boron- and aluminium-containing MFI zeolites were synthesized and various characterization techniques, such as NMR ((27)Al, (29)Si and (11)B), were employed to study the acidities of zeolites. Moreover, in situ IR was applied to investigate the interaction of methanol and ethene with the acid sites, and those catalytic materials were used for co-reaction of methanol and ethene to produce propene. The production of propene was related to the Al content of the zeolites with Si/Al ratios of higher than 90. It is implied that the presence of boron during the synthesis directed the aluminium to occupy certain tetrahedral sites in the zeolite framework, thus preventing the formation of ethene oligomers, and resulting in increased propene selectivity. 相似文献
12.
Ramamurthy V Shailaja J Kaanumalle LS Sunoj RB Chandrasekhar J 《Chemical communications (Cambridge, England)》2003,(16):1987-1999
The alkali ions present in the supercages of zeolites X and Y interact with included guest molecules through quadrupolar (cation-pi), and dipolar (cation-carbonyl) interactions. The presence of such interactions can be inferred through solid-state NMR spectra of the guest molecules. Alkali ions, as illustrated in this article, can be exploited to control the photochemical and photophysical behaviors of the guest molecules. For example, molecules that rarely phosphoresce can be induced to do so within heavy cation-exchanged zeolites. The nature (electronic configuration) of the lowest triplet state of carbonyl compounds can be altered with the help of light alkali metal ions. This state switch (n pi*-pi pi*) helps to bring out reactivity that normally remains dormant. Selectivity obtained during the singlet oxygen oxidation of olefins within zeolites illustrates the remarkable control that can be exerted on photoreactions with the help of a confined medium that also has active sites. The reaction cavities of zeolites, like enzymes, are not only well-defined and confined, but also have active sites that closely guide the reactant molecule from start to finish. The examples provided here illustrate that zeolites are far more useful than simple shape-selective catalysts. 相似文献
13.
Synergic Effect of Active Sites in Zinc‐Modified ZSM‐5 Zeolites as Revealed by High‐Field Solid‐State NMR Spectroscopy 下载免费PDF全文
Dr. Guodong Qi Dr. Qiang Wang Prof. Jun Xu Dr. Julien Trébosc Prof. Olivier Lafon Dr. Chao Wang Prof. Jean‐Paul Amoureux Prof. Feng Deng 《Angewandte Chemie (International ed. in English)》2016,55(51):15826-15830
Understanding the nature of active sites in metal‐supported catalysts is of great importance towards establishing their structure–property relationships. The outstanding catalytic performance of metal‐supported catalysts is frequently ascribed to the synergic effect of different active sites, which is however not well spectroscopically characterized. Herein, we report the direct detection of surface Zn species and 1H–67Zn internuclear interaction between Zn2+ ions and Brønsted acid sites on Zn‐modified ZSM‐5 zeolites by high‐field solid‐state NMR spectroscopy. The observed promotion of C?H bond activation of methane is rationalized by the enhanced Brønsted acidity generated by synergic effects arising from the spatial proximity/interaction between Zn2+ ions and Brønsted acidic protons. The concentration of synergic active sites is determined by 1H–67Zn double‐resonance solid‐state NMR spectroscopy. 相似文献
14.
Yang XY Tian G Chen LH Li Y Rooke JC Wei YX Liu ZM Deng Z Van Tendeloo G Su BL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(52):14987-14995
Preparation and characterization of well-organized zeolitic nanocrystal aggregates with an interconnected hierarchically micro-meso-macro porous system are described. Amorphous nanoparticles in bimodal aluminosilicates were directly transformed into highly crystalline nanosized zeolites, as well as acting as scaffold template. All pores on three length scales incorporated in one solid body are interconnected with each other. These zeolitic nanocrystal aggregates with hierarchically micro-meso-macroporous structure were thoroughly characterized. TEM images and (29)Si NMR spectra showed that the amorphous phase of the initial material had been completely replaced by nanocrystals to give a micro-meso-macroporous crystalline zeolitic structure. Catalytic testing demonstrated their superiority due to the highly active sites and the presence of interconnected micro-meso-macroporosity in the cracking of bulky 1,3,5-triisopropylbenzene (TIPB) compared to traditional zeolite catalysts. This synthesis strategy was extended to prepare various zeolitic nanocrystal aggregates (ZSM-5, Beta, TS-1, etc.) with well-organized hierarchical micro-meso-macroporous structures. 相似文献
15.
High-resolution 17O/1H double resonance NMR spectra were obtained for two zeolites, one with a low Si/Al ratio (zeolite HY) and one with a high Si/Al ratio (HZSM-5), to investigate their local structure and Br?nsted acidity. Two different oxygen signals, corresponding to Br?nsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D) 1H-17O heteronuclear correlation (HETCOR) NMR spectra allowing the 17O isotropic chemical shift (deltaCS) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, eta) for the two oxygen atoms to be extracted. Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY. 17O-1H rotational echo double resonance (REDOR) NMR was applied to probe the O-H distances in zeolites HY and HZSM-5. Weaker 17O-1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY, consistent with longer O-H bonds and/or increased proton mobility. 相似文献
16.
Mengting Jin Dr. Manoj Ravi Chen Lei Dr. Christopher J. Heard Dr. Federico Brivio Dr. Zdeněk Tošner Dr. Lukáš Grajciar Prof. Jeroen A. van Bokhoven Prof. Petr Nachtigall 《Angewandte Chemie (International ed. in English)》2023,62(31):e202306183
While the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework-associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental 27Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework-associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion-exchange to non-acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework-associated octahedral aluminium in zeolites. 相似文献
17.
Hammond KD Dogan F Tompsett GA Agarwal V Conner WC Grey CP Auerbach SM 《Journal of the American Chemical Society》2008,130(45):14912-14913
Nanoporous acid catalysts such as zeolites form the backbone of catalytic technologies for refining petroleum. With the promise of a biomass economy, new catalyst systems will have to be discovered, making shape-selective base catalysts especially important because of the high oxygen content in biomass-derived feedstocks. Strongly basic zeolites are attractive candidates, but such materials are notoriously difficult to make due to the strong inherent acidity of aluminosilicates. Several research groups have endeavored to produce strongly basic zeolites by treating zeolites with amines, but to date there is no compelling evidence that nitrogen is incorporated into zeolite frameworks. In this communication, we detail synthesis, NMR spectroscopy, and quantum mechanical calculations showing that nitrogen adds onto both surface and interior sites while preserving the framework structure of zeolites. This finding is crucial for the rational design of new biomass-refinement catalysts, allowing 50 years of zeolite science to be brought to bear on the catalytic synthesis of biofuels. 相似文献
18.
高硅ZSM-5分子筛和二次合成Ti-ZSM-5分子筛的结构 总被引:1,自引:0,他引:1
运用度角旋转(MAS)和交叉极化(CP)等核磁共振波谱和红外光谱技术,分别对采用不同模板剂制备的高硅ZSM-5分子筛和以此为母体,经气固同晶取代法得到Ti-ZSM-5分子筛的结构特性进行了研究.结果表明:在水热法合成高硅ZSM-5分子筛过程中,模板剂明显制约着所制得的分子筛的结构特性.通过选用合适的模板剂,可直接获得具有正交晶系结构的高硅ZSM-5分子筛.在随后的气固相同晶取代过程中,钛原子通过占据分子筛骨架中的缺陷位进入骨架,形成具有一定Ti/Si比例的钛硅分子筛(Ti-ZSM-5)。骨架结构中缺陷位的含量直接影响分子筛的载钛量。 相似文献
19.
A. A. Kubasov L. E. Kitayev S. V. Malyshev Yu. V. Novakovskaya 《Moscow University Chemistry Bulletin》2010,65(5):279-284
The effect of a phosphorus atom introduced in a zeolite cluster involving ten silicon and aluminum atoms on the state of active
catalytic structure sites is estimated. Zeolite clusters modified with boron are also considered. The effect of the electron
density redistribution upon zeolite modification with boron and phosphorus, as well as the effect upon the coordination of
probe water and ammonia molecules to the clusters, is analyzed. The bond energies of water or ammonia molecules coordinated
to acceptor sites of phosphorus-containing zeolites are estimated. Experimentally discovered changes in the acidic properties
of zeolites resulting from their modification with boron and phosphorus are interpreted. 相似文献
20.
Various boron only ([B]-BEA) as well as aluminum- and boron-containing beta zeolites ([Al,B]-BEA) have been prepared and modified by ion exchange of ammonium, sodium, and nickel ions. The zeolite samples have been characterized by 11B, 27Al, and 29Si MAS as well as three of them by 11B and 27Al 3Q-MAS NMR spectroscopy. The quantitative contributions of defect-free Si(nX) (n = 2, 1, 0; X = Al, B) and Si(OH)x (x = 2, 1) sites to the NMR signal intensities were calculated from the various Si/(Al + B) ratios and relative 11B, 27Al, and 29Si NMR signal intensities using the special distribution of aluminum and boron in different periodical building units of the zeolite framework. The boron atoms are sitting exclusively in diagonal positions in the four-membered rings of [B]-BEA zeolites, while the aluminum atoms are situated both in diagonal and lone positions in the four-membered rings of [Al,B]-BEA zeolites. A higher part of boron atoms are positioned in framework-related deformed tetrahedral boron species than in lattice positions in the [B]-BEA than in the [Al,B]-BEA zeolites. All extraframework octahedral aluminum species are transformed back to lattice positions due to ion exchange from the protonated form to ammonium-, sodium-, or nickel-ions containing zeolites. Oppositely, trigonal boron leaves the zeolite structure completely during ion exchange. 相似文献