Microwave synthesis method, zeolite

Microwave processing of zeolites and their application in the catalysis of synthetic organic reactions has recently been excellently reviewed by Cundy [23] and other authors [24], The microwave synthesis of zeolites and mesoporous materials was surveyed, with emphasis on those aspects which differ from conventional thermal methods. The observed rate enhancement of microwave-mediated organic synthesis... 

In the microwave synthesis of zeolites, a mixture of a precursor and a zeolite support is heated in a microwave oven. The sample is then tested for its catalytic activity and the results compared with those from the sample obtained by the conventional method. Microwave irradiation at the calcination stage led to samples with more uniform partide-size distribution and microstructure and to bimetallic catalysts with different morphology. 

We also developed a novel synthesis method ( in situ aging - microwave synthesis method) for microwave synthesis of zeolite membranes without seeding, which was adapted to the scale-up of the membrane synthesis to integrated supports (e.g. multi-channel tube modules and hollow fibers) [14],... 

Microwave technology as a way to induce rapid heating of solutions has been applied to the crystallization of zeolite phases. Zeolites A, Y, and ZSM-5 can all be prepared under microwave radiation [53,54]. Aliuninophosphate gels which, under normal thermal heating procedures, produce AIPO4-5 were also foimd to produce the same phase under microwave heating [55,56]. Crystal formation was observed as early as 60 seconds after irradiation. The introduction of an additional framework component such as cobalt to produce CoAPO-5 has also been demonstrated utiHzing microwave synthesis methods [57]. 

Importantly, ionic liquids show excellent properties on the adsorption of microwave. The combination of microwave-assisted synthesis and ionothermal synthesis opens up an efficient and safe route to prepare zeolite materials [60]. Tian and coworkers reported the microwave-assisted ionothermal synthesis of AlPO-ll using [EmimJBr [61]. Silicon element was also successfully introduced to the framework forming SAPO-11 with potential catalytic applications. In addition, Yan and coworkers developed the ambient pressure synthesis method for silicate zeolites by combining the advantages of ionothermal synthesis, dry-gel conversion, and microwave irradiation [62]. With the assistance of microwave-assisted synthesis, the safe and fast process of ionothermal synthesis has shown to be a promising synthetic route for a variety of zeolite structures. 

The synthesis of MOFs is a moderately young field, and its evolution from well-developed solid-state/zeolite chemistiy is noteworthy. Different types of synthesis methods were reported in the literature such as classical hydro (solvo) thermal synthesis, microwave heating [13], diffusion [14], ultrasonic [15], mechanochemical [16], electrochemical synthesis [17], and more recently... 

In order to reduce the total time of synthesis ( 30 min), it has been reported to employ two stage synthesis processes, an initial microwave heating for around 30 min. followed by the conventional synthesis method [8, 40, 41]. Although, the purity of zeolite has been of much concern, the final yield used to contain unreacted (read residual) fly ash, which can affect the characteristics of synthesized zeolite. In fact, the presence of considerable quantity of unreacted fly ash residue in the final yield can be attributed to the factors, which are being discussed in the next chapter The mechanism of zeolite formation . 

The state-of-the-art for synthesis of fly ash zeolites from fly ash employs various chemicals like NaOH, KOH, NaaCOa, KNO3, NaNOa, NH4F and NH4NOa as weU as different techniques like hydrothermal, fusion, combination of both fusion and hydrothermal, molten salt method, microwave irradiation method. However, these methods yield final end products blended with zeolites, which comprise of some... 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

The Williamson reaction, discovered in 1850, is still the best general method for the preparation of unsymmetrical or symmetrical ethers.The reaction can also be carried out with aromatic R, although C-alkylation is sometimes a side reaction (see p. 515). The normal method involves treatment of the halide with alkoxide or aroxide ion prepared from an alcohol or phenol, although methylation using dimethyl carbonate has been reported. It is also possible to mix the halide and alcohol or phenol directly with CS2CO3 in acetonitrile, or with solid KOH in Me2SO. The reaction can also be carried out in a dry medium,on zeolite-or neat or in solvents using microwave irradiation. Williamson ether synthesis in ionic liquids has also been reported. The reaction is not successful for tertiary R (because of elimination), and low yields are often obtained with secondary R. Mono-ethers can be formed from diols and alkyl halides. Many other... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

In a word, ion-exchange with the assistance of microwaves is feasible, convenient, and fast. It can reach a higher exchange degree than can traditional methods and make the inaccessible ions in traditional methods exchangeable. This method is especially appropriate for the laboratory preparation of ion-exchanged zeolite molecular sieves. The microwave technique is very successful in the synthesis of microporous crystals, modification of the properties of zeolites, secondary synthesis of microporous materials, and the preparation of ultra-fine particles and films, and has attracted the wide interest of chemists in the field of molecular sieves. 

The following methods have been used for the synthesis of Ti-SBA-15 (1) Direct synthesis, (2) treatment of SBA-15 with a Ti-containing compound, (3) synthesis of Ti-SBA-15 with microwave heating, (4) Ti-containing zeolite seeds as precursor, e.g., MTS-9.[204]... 

In order to accelerate the synthesis of both zeolite seeds and membranes, microwave-assisted hydrothermal synthesis (MW-HT) revealed an attractive method. Colloidal zeolite seeds can be successfully prepared by MW-HT synthesis [100,112,113]. Starting from a seeded support,... 

The use of ionic hquids (ILs) or eutectic mixtures as solvent in zeolite synthesis has been reported [178]. The IL, viz. l-methyl-3-ethylimidazolium bromide, was used both as solvent and as SDA. It solubilizes all components involved in the synthesis, while the electrostatic interaction between cations, IL and framework species form the basis of a strong templating effect [165, 178]. It was shown that the IL also works as a microwave absorber [172] and is suited for microwave-assisted zeolite synthesis. Because of the low vapor pressure of IL, the synthesis occurs at low pressure. The addition of amines to such synthesis mixtures yields pure API and ATV structures. The amine as well as the IL seem to exert a structure-directing effect [179]. This method is particularly useful for preparation of transition-metal functionalized frameworks, viz. Co-AlP04-n [180]. New zeotypes were obtained in the Al, P system, viz. SIZ-7, as well as in the Si system, viz. SIZ-12. 

Figure 3.4 Schematic diagram for synthesis of NaA zeolite membrane together with microwave heating and conventional heating method. Reproduced from [26]. With permission from Elsevier.

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