Zeolites template synthesis

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Three methods can be followed for the synthesis of a SIB catalyst (i) zeolite synthesis around the metal complex (ii) template synthesis and (iii) the flexible ligand method. 

The template synthesis method involves the diffusion of ligand precursors into the pores of a zeolite where they can assemble around an intrazeolite metal ion that acts as a template. This approach was first used in 1977 for the intrazeoHte synthesis of Cu, Co and Ni phthalocyanines by the condensation of four molecules of dicyanobenzene around the metal cation within the cages of NaY [132, 174]. 

Fe(CO)s], [Fe2(CO)g], [Co2(CO)8] and [Os3(CO)i2]) have been reacted with dicyanobenzene to form intrazeolite [M(Pc)] complexes [140]. Another class of materials prepared by the intrazeolite template synthesis method has been mixed ligand metal carbonyls and metal carbonyl clusters, frequently by reductive car-bonylation of metal ions in zeolite cages [175]. However, because these are frequently decomposed in situ to form, for example, nanoparticles, they are outside the scope of this chapter, and will be considered here only when they are used as precursors for metal complexes. 

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

The immobilization of Ru-phthalocyanines follows routes similar to those employed for the analogous Fe complexes. Particularly, the perfluorinated Ru phthalocyanines were immobilized in zeolites by ship-in-a-bottle synthesis or by template synthesis, or in MCMs after surface modification. The materials display extremely high activities for the oxygenation of paraffins with r-BuOOH as the oxidant (128,288). 

Kyotani, T., Ma, Z.X., and Tomita, A. Template synthesis of novel porous carbons using various types of zeolites. Carbon 41, 2003 1451-1459. 

L. E. Scriven Yes, I had zeolites in mind, not as catalysts themselves but because of the role of templates in the synthesis of zeolites, templates that catalyze the desired supramolecular structure. That would serve as a point of discussion on this notion. 

In addition to the opportunities for new materials synthesis and characterization along these lines, transport properties, rheology, and processing techniques for liquid crystal polymers are essentially unexplored. Experiences with synthesis of polymer structure based on these liquid crystal templates may open up other creative avenues for template synthesis, for example, inside other crystalline structures, chlathrates, or zeolites, or on surfaces [4], Composites, alloys, or mixtures of liquid crystalline and flexible polymers may produce new materials. 

For this reason the synthesis, originally used for the preparation of the fi ee Co-salophen complexes was modified and the template synthesis method was tried to use for the preparation of the Co-salophen / zeolite catalyst. 

The Co-salophen / zeolite catalyst, prepared by template synthesis method was active in the oxidation of hydroquinone to benzoquinone (Fig. 2) and produced similar oxygen uptake curves as the free complex. It was also possible to reuse the catalyst in a subsequent run with a similar activity as in the first run. 

We have also prepared the Co-salophen/zeolite catalyst, using the template synthesis and the flexible ligand method. The Co-salophen/zeolite catalyst prepared by the template synthesis method proved to be active in the oxidation of hydroquinone and in the aerobic oxidation of 1-octene and the acetoxylation of cyclohexene. The zeolite-encapsulated catalyst was active and produced the same selectivity and yield as the free complex. It was also possible to remove the catalyst and to reuse it in subsequent experiments. 

Host-guest chemistry has also given some direction to the template synthesis of zeolite materials. Notable examples are the synthesis of extra-large zeolites SSZ-53 (SFH) and SSZ-59 (SFN) by means of bulky rigid quaternary ammonium SDAs.11111 Burton et al. 

Key-words Raman spectroscopy, TEOS, synthesis, zeolite, templates, framework vibrations... 

A Co(salophen)/zeolite catalyst was prepared by the template synthesis method. This catalyst proved to be active in the ruthenium catalyzed oxidation of benzyl alcohol. The heteroge-nized Co(salophen), having the same amount of complex produced a higher rate in the oxidation reactions than the free complex. It can be explained by the sites isolation theory. In the case of the heterogenized catalyst it was not necessary to use an extra axial ligand such as triphenylphosphine. It was also found that in the case of Co(salophen)/zeolite catalyst the choice of the solvent was not so critical, as in the case of the free complex. 

Cobalt(saiophen) encapsulated in zeolite. The template synthesis method was used for the preparation of the Co(salophen) /zeolite catalyst (salophen = N,N - Bis(salicylidene) -1,2-phenylenediamine). The Co - exchanged zeolite was prepared by stirring 6g of NaY zeolite and 0.9 g of Co(OAc)2 4 H2O, dissolved in 150 ml deionized water for 48 h at room temperature. The slurry was then filtered and the pink solid obtained was washed with deionized water and dried overnight at 523 K. 0.62 g salicylaldehyde was added to 6g of Co-exchanged zeolite. 0.28 g 1,2-phenylenediamine was dissolved in 20 ml of methanol and the solution was slowly added to the mixture of the zeolite and salicylaldehyde. Having added the solution, the reaction mixture was refluxed for 1 hour and then allowed to stand at room temperature overnight. The product was filtered, washed with methanol and dried. 

The Co(salophen) complex encapsulated in zeolite was prepared by two different methods. The flexible ligand method involves the diffusion of the Schiff base ligand into the zeolite, where upon complexation with the Co ion becomes too large to exit. However, this method was not useful for the preparation of Co(salophen)/zeolite, because the catalyst was not active in the oxidation reaction. For this reason the synthesis, originally used for the preparation of the free Co(salophen) complexes was modified and the principles of template synthesis method was used to prepare the Co(salophen)/zeolite catalyst. 

The cost of the support element and the lengthy steps involved in zeolite film synthesis makes zeolite membranes very expensive compared to the well established polymeric membranes. MFI membranes are relatively expensive and their price (about 5.000 /m2) is typically due by 50% to the support and by 50 % to chemicals. Consequently the development of template-free synthesis on cheaper supports (multi-channels, ceramic foils/plates, capillaries, hollow fibers) is seriously considered, in parallel with the development of continuous, automatic, reliable, and reproducible production methods. 

In comparison to the zeolite synthesis approach there are many disadvantages associated with the preparation of intrazeolite complexes by the flexible ligand and template synthesis methods. The complexes are difficult to characterize, especially if the ligand has multiple coordination modes available and some of the target metal ions may remain uncomplexed which will complicate any reactivity studies. Additionally, there are limitations to the types of metal complexes that might be encapsulated in a zeolite. The only criteria for incorporating metal complexes... 

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