Porous clay heterostructure

Mesostructured clay catalysts a new porous clay heterostructure (PCH) derived from synthetic saponite... 

A novel mesoporous intercalate belonging to the class of mesostructured solid acids known as porous clay heterostructures (PCH) has been synthesized through the surfactant - directed assembly of silica in the two - dimensional galleries of saponite. The new saponite PCH, denoted SAP-PCH, exhibits a basal spacing of 32.9 A, a BET surface area of 850 m2/g and pore volume of 0.46 cm3/g. SAP-PCH is an effective catalyst for the condensed phase Friedel-Crafts alkylation of bulky 2,4-di-tert-butylphenol (DBP) with cinnamyl alcohol to produce a large flavan, namely, 6,8-di-tert-butyl-2,3-dihydro[4H]benzopyran. 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

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. 

The synthesis method used to prepare the saponite porous clay heterostructure (SAP-PCH) was similar to the one earlier reported from our laboratory [7]. The clay was transformed to a quaternary-ammonium exchange form (organoclay) by ion exchange with a CTAB solution, followed by washing and drying. A mixture of organoclay, decylamine and TEOS at molar ratio 1 20 100 was stirred for 4 hours at room temperature. 

Al-modified porous clay heterostructures with combined micro- and mesoporosity. 

Al-Modified Porous Clay Heterostructures with Combined Micro- and 409... 

L. Mercier and T. J. Pinnavaia, A functionalized porous clay heterostructure for heavy metal ion (Hg2+) trapping, Micropor. Mesopor. Mater. 20(1-3), 101-106 (1998). 

Reversible Type lb isotherms have been reported by Galameau et al. (1995) for the adsorption of nitrogen on some novel porous clay heterostructures (PCHs). A... 

M. Polverejan, Y. Liu, and T. J. Pinnavaia. Mesostructured clay catalysts a new porous clay heterostructure(PCH) derived from synthetic saponite. Stud Surf. Sci. Catal., 129(2000), pp.401-408. 

Galameau et al. [81] used a similar approach to design porous clay heterostructures. They intercalated layered fluorohectorite by CjgTMA cations followed by treatment in a solution of neutral amine and TEOS, then drying and calcination. In brief, the authors proposed that the interlayer galleries of the intercalated clay are further swollen by the amine followed by insertion of TEOS, formation of rod-like micelles and silica polymerization. 

Qu, F., Zhu, L. Yang, K. (2009). Adsorption behaviors of volatile organic compounds (VOCs) on porous clays heterostructures (PCH). Journal of Hazardous Materials, vol.170, pp. 7-12. ISSN 0304-3894. 

Six of the eleven chapters are devoted primarily to the intercalation chemistry of smectite clays, the most versatile among all lamellar compounds. Two of these chapters are devoted to the experimental and theoretical aspects of the clay structures and surface chemistry, including chemical catalysis. Organo clays and polymer-clay nanocomposites, the adsorption of nitroaromatic compounds of environmental significance onto clay surfaces, photochemical processes, and pillared clays and porous clay heterostructures are the subjects of the remaining four chapters. These six chapters provide detailed discussions of the factors that influence access to the intragallery surfaces of the clay host and the materials properties of the resulting intercalates. 

The content of this Handbook reflects the fact that the bulk of knowledge in this field concerns clays. In Part 1, which concentrates on clays, there is considerable focus on clay-organic interactions because of their relevance to catalysis. A chapter on nitroaromatic compound sorption is included because it provides an excellent example of clay-organic interactions. Other pertinent topics include molecular modeling of surface chemistry and photochemical processes, including photocatalysis. Pillared clays and porous clay heterostructures are the subject of an entire chapter. Chapters in Part 2 cover synthesis, characterization, host-guest pillaring, sorption, and catalysis for each class of layered material. 

Pillared Clays and Porous Clay Heterostructures Pegie Cool and Etienne F. Vansant... 

True mesoporous clay derivatives can be obtained via two routes, resulting in different structures, depending on the clay dimensions. The first one, proposed by Pinnavaia s group, describes the synthesis of porous clay heterostructures, or PCHs (24). These materials will be discussed in detail in Section III. 

Since the porosity of pillared clays was limited primarily to the microporous region (max. 2 nm), in 1995 researchers started to intercalate the same surfactants into layered clay hosts to perform a templated reaction in the clay interlayer region, in order to obtain true mesoporous clay materials (pore diameter > 2 nm), the so-called porous clay heterostructures, or PCHs. Because this research on PCHs is very young, many challenges still remain. In what follows, an overview of what is known about these materials in the literature will be given. 

Porous Clay Heterostructures Derived from High-Charge-Density Clays... 

FIGURE 18 Synthesis mechanism of the formation of porous clay heterostructures. 

Both synthesized PCH materials exhibit supermicropores or mesopores between 1.5 and 3.0 nm and surface areas over 500 m /g. Figure 19 illustrates a typical N2 adsorption-desorption isotherm at 77 K of a porous clay heterostructure, with the corresponding mesopore size distribution in the inset. The adsorption curve clearly exhibits a nearly linear portion in the partial pressure region 0.03-0.4, indicative of supamicropores (1.5-2.0 nm) and small mesopores (2.0-3.0 nm). The increase in adsorbed volume at very small pressures (<0.03) is indicative of the filling of micropores. The mesopore size distribution shows a maximum at a pore diameter of 2.2 nm. 

FIGURE 19 Typical N2 adsorption-desorption isotherm at 77 K of a porous clay heterostructure, with its corresponding mesopore size distrihution in the inset. 

Table 8 Physical Properties and Acidity of Porous Clay Heterostructures Derived from Montmorillonite (PCH) and Acid-Activated Montmorillonite (PAACH) Clays... 

For environmental adsorption applications, porous clay heterostructures can be functionalized with suitable surface groups in order to immobilize heavy metal... 

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