Pillared layered microporous materials

Cheng and coworkers at Taiwan University1611 have discussed the silica- and alumina-pillared derivatives of four layered silicates with different charge density and thickness, as well as related synthetic problems. Here, this system will be presented as a typical example. In general, its synthesis includes three steps. 

The hexylamine-intercalation method was usually used four layered compounds were treated by the exchange of acid (IM-HNO3 with stirring for several hours at 70 °C) to form H+-type product, which was dried in air, and then suspended in the excess of hexylamine solution at room temperature and the mixture was stirred for 28 d. Hexylamine was removed by filtration and the hexylamine-expanded silicates were used immediately for the pillaring reaction in order to prevent hexylamine evaporation. 

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The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Synthesis of Pillared, Layered Crystalline Microporous Materials... 

Both natural clays and their alnminium oxide pillared analogues have also been tested for the catalytic cracking of polyethylene [49-51]. The clays investigated include mont-morillonite and saponite. They possess a layered structure which can be converted into a two-dimensional network of interconnected micropores by intercalation of molecular moieties. In the case of alnmininm pillared clays, these materials show a mild acidity... 

Pillared Layered Structures (PLS) From Microporous to Nano-phase Materials... 

The starting material K2Ti O, is nonporous, and has a fairly low BET surface area as 3.3 m /g. In contrast, the resulted silica-pillared layered titanate has a relatively high surface area of 45.9 m /g, indicating the existence of an appreciable intracrystal surface area. The M2 adsorption-desorption isotherm of the silica-pillared layered titanate is shown in Figure 3. The adsorption isotherm of the pillared sample is between type I and ZZ at low pressures[14]. This type of isotherm is characteristic of materials in which both micropores and mesopores are present. The hysteresis loop of the curve demonstrates that this material is porous. Moreover, the loop is somewhat like type H3 and H4 loops as classified by K.S.H. Sing, et al.[14]. This type of loop... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

More recently, various attempts have been made to develop cracking catalysts from pillared smectite clays, in which the layers are separated and held apart by the intercalation of large cations. Pillared clays (PILCs) have large surface areas within fairly well-ordered micropore structures (pore widths in die approximate range 0.6-1.2 nm). It is not surprising that these materials have attracted considerable interest with the prospect of an alternative type of catalytic shape selectivity (Thomas, 1994 Thomas etal., 1997 Fripiat, 1997). 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Advancements in the preparation of new PLS s nearly parallels that of the zeolite and zeolite-like phases. Initially the pillared smectite clays were investigated but the quest for new materials with new properties led to e qiloring the pillaring of other layered phases. These include, most notably, the layered zirconium phosphates, double hydroxides (hydrotalcites), sihcas and metal oxides. The parallel paths of discovery in new material compositions for the layered phases and the microporous (zeoUte) phases are summarized in Table 1. A conq>arison between the pore architectures of the zeohtes and the two dimensional PLS is shown in Table 2. 

Cations based on substituted silsesquioxanes have also been intercalated [7]. On heating the ion-exchanged clay minerals water is evolved, but oxycation or oxide pillars keep the siliceous layers apart. These materials have enhanced thermal stability compared with clay minerals expanded with organic cations. Expanded clay minerals cover at least as wide a range of accessibilities to the interlamellar micropore spaces as the zeolites, but the pore characteristics of clay minerals with inorganic pillars need more detailed investigation. 

The so-called template-based technique has been found to be particularly suitable for the synthesis of carbons whose porosity is not only uniform in size and shape, but also periodically ordered in some cases. In this approach, the porous carbon is prepared through infiltration of an organic precursor into the nanochannels of an appropriate inorganic material (the template), followed by carbonization and then liberation of the resultant carbon from the template. Different nanospaces in templates have been used to confine the carbon precursors. The first templates used included, e.g., silica gel or porous glass [84,85], layered clays such as montmorillonite ortaeniolite [86,87], or pillared clays [88-90]. Several detailed reviews on this topic have been published [75,91-95] that cover the areas of microporous and, especially, mesoporous solids. Here, some illustrative examples will he presented in some detail rather than reviewing systematically the literature. 

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