Desilication

Likewise, the desilication procedure has been extended to other zeolites besides ZSM-5, such as MOR [153] and ZSM-12 [154]. The alkali treatment can also be applied using organic bases (TPAOH and tetrabutylammonium hydroxide (TBAOH)) instead of NaOH. In this way, the mesopore volume can be optimized by incorporating both NaOH and quaternary ammonium cations [155], such as TPA+ or TBA, in the alkaline treatment. These cations act as pore growth moderators, and a fact that is essential for the success of this method is that these do not enter the zeolite micropores (hence TMA does not work). The addition of these cations together with the alkaline solution 

---

By demetallation, one constituent is preferentially extracted from a preformed zeolite material to form mesoporous zeolite crystals. Existing and emerging demetallation strategies basically comprise dealumination, detitanation, and desilication. 

Figure 2.3 3D-TEM reconstruction of (a) severely steamed and subsequently acid-leached Y zeolite [22] and (b) desilicated ZSM-5 zeolite crystal [24]. The mesopores in the crystal are shown as lighter gray tones.

Not only the concentration but also the nature of the aluminum and its location (next section) plays a role in desilication by alkaline treatment. The presence of a... 

Most of the papers available on desilication are devoted to the alkaline treatment of ZSM-5 zeolites. Since a large number of zeolites consist of aluminosilicate frameworks and framework Al plays a crucial role in the mesoporosity development during desilication, this methodology should be suitable for extrapolation to other... 

Tunable Intracrystalline Mesoporosity by Partial Detemplation-Desilication... 

Figure 2.7 Schematic representation of the partial detem-plation and desilication treatment to tailor mesoporosity development in zeolite crystals.

Table 2.1 Benefits of mesoporous zeolites prepared by desilication in catalytic applications with respect to the purely microporous counterparts.

Kwauk, M., Fluidized Calcination of Aluminum Ore for Pre-Desilication, (unpubl.), Inst. Chem. Metall., 1964-11-6 (1964c)... 

Desilicated samples were obtained by alkaline treatment of calcined H-MCM-22 (Si/Al = 15). For this purpose the zeolite was suspended in 0.05-0.50 M NaOH solutions for 45 min at 323 K, followed by filtering and washing and an additional ion-exchange step to regain the H-form of the modified zeolite. 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

Both ketals66 and enol ethers67 can be used in place of aldehydes with the selection of appropriate catalysts. Trimethylsilyl iodide causes addition to occur.68 The trimethylsilyl iodide can be used in catalytic quantity because it is regenerated by recombination of iodide ion with silicon in the desilation step. 

Fluoride-induced desilation has also been used to effect ring closures.75 76... 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

Figure 1.4 SEM-EDX images of polished nontreated (a) and alkaline-treated (b) ZSM-5 crystals. Desilication predominantly occurs in the core, where the A1 concentration is lowest. Reproduced from Groen et alJ /4i by permission of American Chemical Society...

Groen, J. C., Peffer, L. A. A., Moulijn, J. A. and Perez-Ramirez, J. Mechanism of hierarchical porosity development in MFI zeolites by desilication the role of aluminium as a pore-directing agent, Chem. Eur. J., 2005, 11, 4983 1994. 

Direct reductive cleavage by catalytic hydrogenolysis of a cyclopropane C-C-bond is only possible in exceptional cases, where the three membered ring is further activated by a phenyl or a vinyl group. With unpoisoned catalyst a subsequent reductive desil-oxylation occurs to afford esters like 124 (Eq. 48), whereas addition of small amounts of triethylamine allows isolation of the desired siloxy compounds (e.g. 125, Eq. 49). Interestingly, both reactions demonstrate that the cleavage of the cyclopropane bond proceeds non-stereoselectively with inversion and retention at C-l and C-2, respectively 79). 

---