Nanofibrous and nanotubular supports for the immobilization of metalloporphyrins as oxidation catalysts

Journal of Colloid and Interface Science 315 (2007) 142–157 www.elsevier.com/locate/jcis

Feature article

Nanofibrous and nanotubular supports for the immobilization of metalloporphyrins as oxidation catalysts Shirley Nakagaki, Fernando Wypych ∗ Universidade Federal do Paraná, Departamento de Química, CP 19081, CEP 81531990, Curitiba, Paraná, Brazil Received 4 April 2007; accepted 19 June 2007 Available online 22 June 2007

Abstract Nanofibrous and nanotubular materials, natural and synthetic, are important alternative matrices for the immobilization of metallocomplexes, especially metalloporphyrins, as oxidation catalysts. The process permits a regular and controllable distribution of the active phase at the outer and/or inner surfaces of the tubes, promoting a special environment for the approximation of a substrate to the catalytic active species. The immobilization also prevents the molecular aggregation and bimolecular self-destruction reactions, facilitates the recovery and reuse of the catalyst, reduce de cost of material preparation and environmental concerns. A variety of nanofibrous and nanotubular structures are presented and specific examples of immobilization of iron porphyrins in different supports and their oxidation catalytic activities are presented and discussed. © 2007 Elsevier Inc. All rights reserved. Keywords: Porphyrins; Immobilization; Oxidation catalysis; Fibrous and tubular matrixes

1. Introduction The strategy of mimic enzymatic systems has become an extensive research area of synthetic porphyrins, and other coordination complexes, as models of enzyme active sites. The main efforts are concentrated especially on monoxygenase enzymes of the cytochrome P-450 family since the catalytic activity behavior of this enzyme family has inspired the design and synthesis of various mineral (inorganic) complexes with potential application in the chemical industry for oxidation reactions and many other uses such as in electrochemical applications and chemical sensors [1,2]. Studies of selectivity and stability presented by the synthetic models compared to natural systems (cytochrome P-450), in the biomimetic oxidation of hydrocarbons, suggested that selectivity not only arises from the steric effects imposed by the environment of the enzyme active site upon substrate approach, but also from specific binding at the active site, amongst other factors [3,4]. The monooxygenase cytochrome P-450 has a heme-enzime catalytic site that catalyses a variety of aerobic * Corresponding author.

E-mail address: [email protected] (F. Wypych). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.032

oxidations mediated by a reducing agent. The catalytic cycle of the enzyme has been proposed for many different research groups based on biological studies, synthesis of model compound studies and spectroscopic analyses. An example of the mechanism of the action of the enzyme is illustrated in Fig. 1. The active specie assumed in the catalytic oxidation of different substrate by the enzyme is a high valence iron-oxo, (6) resulting by the previously iron–dioxygen interaction following by the O–O bond cleavage (2–5). Besides the dioxygen molecule as oxygen donor to the effective cytochrome P-450 oxidation mechanism, the enzyme (1) and its metalloporphyrin model complexes can promote reactions by oxidants such as iodosylbenzene, alkyl and hydrogen peroxide or peracid (Fig. 1, shunt path). In the last 30 years different porphyrins structures were synthesizes in order to reproduce the shunt path in the catalytic active cycle. Many research groups modified the porphyrin structure in order to design and synthesize resistant structures that mimic the efficiency and selectivity observed in the biological systems. Many of these structural modifications were done in order to mimic the protein cavity of natural enzymes [5]. The immobilization of metalloporphyrins carrying electronwithdrawing substitutes on mineral supports has been found efficient when used as a selective catalyst for oxidation of hy-

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Fig. 1. Schematic representation of the reaction mechanism for cytochrome P-450.

drocarbons because the matrix support can impose shape selectivity besides promoting a special environment for the approximation of the substrate to the catalytic active species [6–8]. In addition, the immobilization may prevent molecular aggregation or bimolecular self-destruction reactions, which lead to deactivation of catalytically active metalloporphyrin species, besides exposing the active catalytic sites. Furthermore, heterogeneous catalysts with immobilized metalloporphyrins allow continuous operation in flow reactors, and in batch reactors facilitate their recovery from the reaction media, for reuse and recycling, reducing wastes and costs. A constant theme among these challenging issues is the creation of structured assemblies containing porphyrins located in well-defined chemical environments. The creation of supported metal centres that are accessible to large organic substrates is an essential characteristic for the biomimetic catalytic applications of supported metallomacrocycles such as porphyrins and phthalocyanines [9–12]. The immobilization of metalloporphyrins associated with obtaining an easily recyclable solid such as an inorganic support, for example, silica, clay or layered double hydroxide (LDH), alternative tubular [13,14] and fibrous supports and many others in natura (clays or others mineral solids) or modified inorganic [15–31] or organic supports [32–37] would greatly enhance the usefulness of such complexes for a variety of traditional and new uses. An immediate application of the special materials could be in control of environmental pollution. The functionalization of layered mineral supports and nanofibers (grafting) is a useful strategy frequently employed for im-

mobilization of catalyst molecules such as metallocomplexes. Grafting reactions can occur by establishing covalent bonds between the reactive groups of the layer and an adequate reactant molecule, which ensures higher chemical, structural and thermal stability to the compound. As the reactions are restricted to the surfaces and the nanolayers, and the nanofibers are inert, have very high surface area and are resistant to chemical attack, the obtained materials are very useful for heterogeneous catalytic processes. Considering the lack of information compiled in one single text about the structural characteristics of nanofibrous materials to be used as supports for the immobilization of metalloporphyrins, the objective of this article is to show the structural aspects of the nanofibers, followed by a discussion of some specific results obtained with the use of nanofibers-immobilized metalloporphyrins, and finally some alternative nanofibrous materials are proposed. 2. Tubular and fibrous matrices 2.1. Halloysite Halloysite is a clay mineral similar to kaolinite, having a 1:1 structure in which a silica tetrahedral sheet is joined to a gibbsite (Al(OH)3 ) octahedral sheet [38]. However, unlike kaolinite, the structure is disordered in both “a” and “b” axis directions in successive layers. The mineral has different colours ranging from white, yellowish, pink, greenish or brownish, depending on the substitution metals and on min-

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Fig. 2. Schematic representation of a halloysite nanotube wall [40,41].

Fig. 3. TEM micrographs of a sample of halloysite. Left picture scale bar = 0.5 µm.

eral origin. Halloysite occurs mainly in two different polymorphs, the hydrated form (basal distance of 10 Å) with the formula Al2 Si2 O5 (OH)4 ·2H2 O and the anhydrous (basal distance of 7 Å), with identical composition to kaolinite, with the formula Al2 Si2 O5 (OH)4 . The hydrated form converts spontaneously and irreversibly into the anhydrous form when dried at temperatures around 100 ◦ C. Because the 1:1 layers in halloysite generally are separated from each other by a water layer, halloysite has a larger cation exchange capacity and surface area, than kaolinite. Due to structural mismatch between the silicate and gibbsite sheets, the most common morphology of halloysite is a more or less ordered curled empty tubes (shape of a papyrus), exposing the oxygen planes to the outer surface. Other less common morphologies have also been observed such as platy and spheroidal crystals, among others. The empty spaces inside the halloysite tubes can be occupied by a hydrophylic solution carrying complexes with catalytic activity, and the restriction of the movement of the substrate to be oxidized can bring interesting selectivity to the catalyzed reactions [39]. The process of forcing the entrance of the active molecules into the tubes can be done by vacuum pumping the solid to remove water molecules and mixing the solution with the halloysite in pressurized reactors. For the insertion of a hydrophobic specie, firstly a chemical modification of the inner surface of the tubes should be performed with hydrophobic molecules. This is possible because the inner surface of

the tubes is populated with highly reactive aluminol (Al–OH) groups. Fig. 2a shows a schematic representation of a halloysite nanoscroll wall [40,41]. Fig. 3 shows the transmission electron micrographs of a halloysite, in two different magnifications [42]. The measurements were performed with a JEOL 1200 EX-II microscope, operating at 100 kV. It can be observed that halloysites have a high aspect ratio, a length of a few of thousand angstroms and an outside diameter of at least 400 Å. The internal diameter ranges from 50 to 150 Å, large enough to allocate molecules of metalloporphyrins having the highest diameter around 15 Å [43]. 2.2. Chrysotile Chrysotile belongs to the serpentine group and represents several polymorphic members [38]. These minerals have essentially the same chemical composition but different structures and morphologies. The planar morphologies are represented by lizardite: Mg3 Si2 O5 (OH)4 ; amesite: (Mg, Fe2+ )2 AlSiAlO5 (OH)4 , alternative waves represented by antigorite (Mg, Fe)3 Si2 O5 (OH)4 and cylindrical rolls represented by clino/ortho/parachrysotile: Mg3 Si2 O5 (OH)4 . Clinochrysotile is the most important of this class, being composed by silicate tetrahedral sheets bound to octahedral brucite-like (Mg(OH)2 ) sheets. Since the bonding distances of the two sheets are not exactly the same this mismatch has the effect of curling the lay-

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Fig. 4. Schematic representation of a chrysotile nanotube wall [40,41].

Fig. 5. TEM micrographs of a sample of chrysotile.

Fig. 6. Chrysotile functionalization of the outside tube wall with isopenthanol [40,41].

ers, producing concentric nanotubes. The other consequence of the mismatch of the two sheets is that the curling process occurs in opposite way of the halloysite, with the hydroxyl groups in the outer surface of the tubes, which can be easily modified by different chemical species (Fig. 4) [44]. Fig. 5 shows the micrograph of chrysotile nanotubes, obtained with transmission electron microscopy, with the same equipment and conditions as those of Fig. 3. It can be observed that chrysotile also has a high aspect ratio and similar sizes to those observed for halloysite. The differences are related to the highest length of the single nanotubes. The internal diameter is relatively smaller ranging from 20 to 50 Å, but still sufficient to allocate single molecules of metalloporphyrins, having a diameter of around 15 Å [43].

The process of grafting can attribute to the surface, the desired function as described in Fig. 6 [44]. In this case, the grafting can be performed with isopenthanol but many different reactions can be used, especially when pendant reactive groups are desired to attach other positively or negatively charged molecules. The pendant groups can be from any desired functionality such as amine, carboxylic acid, etc. 2.3. Tubular kaolinite Tubular kaolinite is also an alternative to be used as support for the immobilization of metallocomplexes, especially because these tubes can be obtained “in situ” in the presence of soluble molecules, entrapping the active phase inside the tubes. Al-

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though tubular kaolinite is very similar to halloysite, the process of preparation is relatively laborious, with the intercalation and grafting process prior to subjecting to high energy ultrasound treatment. In the example of Fig. 7, the process of grafting was performed with 1-hexanol, followed by the intercalation of dodecylamine [45]. The advantage of the process is rolling a small number of layers or a single layer of kaolinite, forming nanotubes with the external diameter of 250 Å and an internal diameter higher than 100 Å and populated with organic grafted molecules inside the tubes, creating a hydrophobic environment (see Fig. 8). The process of just intercalating kaolinite and trying to roll the layers also generates the tubes, but the process is not so efficient. The advantage of not using the grafting process is the possibility of obtaining empty tubes and the immobilization of molecules during the rolling process. Fig. 8 shows the transmission electron micrographs obtained with the grafting of 1,3-butanediol (a) and di(ethylene glycol) 2-hexyl ether (b) followed by the intercalation of octadecylamine [45].

groups on the outside. This can be an interesting alternative because both outer and inner surfaces of the tube can be grafted, being halloysite like in the inside and chrysotile like in the outside. The tubes have also interesting external diameters ranging from several angstroms to several micrometers and an internal average diameter being smaller than halloysite and chrysotile, ranging from 5 to 15 Å [46] and consequently, difficult to be populated by the metalloporphyrin molecules, whose diameters exceed the internal diameter. Fig. 9 shows the perspective view of the unit cell of an imogolite nanotube with 24 aluminum atoms in the circumference (b) and the hexagonal arrangement of aluminum atoms, bridging oxygen atoms and pendant silanol groups (a). Fig. 10 shows the TEM micrograph of an imogolite synthesized from tetraethoxysilane, aluminum-sec-butoxide and perchloric acid, in water, according to the procedure described in [47].

2.4. Imogolite

Fibrous silica can be obtained by acid leaching of chrysotile. The process consists of removing the brucite sheet from the chrysotile structure (Fig. 11) and maintaining the silicate sheets, in an amorphous structure. The morphological modification consists of opening the nanotubes and forming nanoribbons, which are larger then the original tubes. The nanoribbons are

Imogolite with the ideal formula Al2 SiO3 (OH)4 is a natural fibrous aluminosilicate, which occurs in volcanic ash solids. The structure consists of curved gibbsite-like sheets with siloxane (Si–OH) groups on the inside and aluminol (Al–OH)

2.5. Fibrous silica

Fig. 7. Schematic representation of a tubular kaolinite nanotube [40,41].

Fig. 8. TEM micrographs of a tubular kaolinite [45]. Reprinted with kind permission from the author.

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Fig. 9. Schematic representation of the imogolite nanotube structure [46]. Reprinted with permission from S. Konduri, S. Mukherjee, and S. Nair (2006). © 2006 American Physical Society

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two-dimensional layers, compensated by intercalated potassium cations, positioned in two different crystallographic positions named I and II. After exchanging potassium cations by protons, the layered structure can be swollen in different media separating individual layers (exfoliation). After 50% exchange of the potassium cations by protons (H2 K2 Nb6 O17 ) and exfoliation, the interaction between the layers is reduced, the pH and ionic strength are adjusted, the suspended nanolayers acquire an energetically favourable configuration, curling the layers. The results are nanotubes with variable diameters and with nanometric internal diameters. Recently [51], the empty nanotubes were used as matrix for the immobilization of nonmetallated tetramethypiridylporphyrin (TMPyP), where the immobilization was confirmed to be at the outer surface of the nanotubes and also between the walls of the tubes (Fig. 12). The developed procedure can also be used to immobilize metalloporphyrins with catalytic activity, especially at the outer and inner surfaces of the tubes. The same procedure can also be applied to other exfoliable niobates, titanates, among others. 2.7. Fibrous hydroxides

Fig. 10. TEM micrograph of a synthetic imogolite [47]. Reprinted with permission from G.H. Koenderink, S.G.J.M. Kluijtmans, and A.P. Philipse (1999). © 1999 Elsevier

classified as disordered layered silica [48], specially based on the fact that they can be grafted with different substances, producing packed layered derivatives [49,50]. Fig. 11 shows the process of destroying the nanotubes of chrysotile, generating the nanoribbons of amorphous silica [48], and the corresponding TEM micrograph of chrysotile (a) and the derived nanoribbons (b). As the scale of both micrographs is almost the same, it can be clearly seen that the process of acid leaching increase the width of the silica ribbons, by flattening of the tubes. It can also be seen in picture (b) that the ribbons are twisted, a feature not observed in the parent nanotubes, confirming the different morphologies. 2.6. Tubular niobates [51,52] Orthorhombic potassium hexaniobate (K4 Nb6 O17 ) structure consists of octahedral NbO6 units, which form negative charged

Most metal hydroxides have layered structures consisting of metal atoms coordinated to six hydroxyl groups, in an octahedral geometry. These octahedral units are connected through the edges, producing charge neutral two-dimensional layers, which are stacked along the basal direction and separated by van der Waals gaps. Many metal hydroxides when produced in controlled conditions [53,54] can produce “unusual” morphologies, most of them being composed of empty nanotubes. Fig. 13 shows the scanning electron micrographs of orthorhombic copper hydroxide, obtained electrochemically by anodization of a copper foil in an aqueous solution of potassium hydroxide (a) and the field emission scanning electron micrograph of hexagonal yttrium hydroxide obtained by treating yttrium nitrate hydrothermally in the presence of poly(ethylene glycol) (b). It can be observed that in two preparation conditions and considering two different compounds, hollow nanotubes can be obtained with micrometric lengths and nanometric diameters. These hollow nanotubes can be used as nanoreactors to immobilize metalloporphyrins as described for other fibrous structures. Changing the synthetic conditions, tubes with different lengths, internal and external diameters can be easily obtained, which opens new alternatives to immobilize complexes of different species, especially catalytic active metalloporphyrins. One important aspect is the easy way to produce the hydroxide nanotubes and nanofibers, whose synthesis can be conducted in the presence of the metalloporphyrins, which can be trapped inside and between the tubes, occupying the empty macropores. Another alternative for the case of hydroxides, is to use the hydroxylated surface to perform grafting reactions similar to those reported for chrysotile and other matrices with hydroxylated surfaces. Fig. 13c presents, the structure of the layered copper hydroxide, showing the layers and coordination of copper atoms to hydroxyl groups [55]. Similar fibrous hydroxides have also been reported for aluminium hydroxide [55] and magnesium hydroxide [56].

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Fig. 11. Schematic representation of the acid leaching of chrysotile [48] and the corresponding TEM micrographs of chrysotile (a) and obtained amorphous silica nanoribbons (b). Reprinted with permission from F. Wypych, L.B. Adad, N. Mattoso, A.A.S. Marangon, and W.H. Schreiner (2005). © 2005 Elsevier

Fig. 12. Morphological structure of H2 K2 Nb6 O17 after interaction with the porphyrin [51]. Reprinted with permission from M.A. Bizeto and V.R.L. Constantino (2005). © 2005 Elsevier

2.8. Fibrous hydroxide salts Another class of compounds derived from single hydroxides are the layered hydroxide salts (LHS), whose general formula is M2+ (OH)2−x (Am− )x/m ·nH2 O, where M2+ is a metallic cation (e.g., Mg2+ , Ni2+ , Zn2+ , Ca2+ , Cd2+ , Co2+ , or Cu2+ ), A is an anion of charge “m” and “x” ranging from 0.3 to 1.0 [57]. The LHS could also contain two types of cations in the octahedral sites and thus forming a compound is called “double hydroxide salt” (DHS) and its composition is described by the formula:

Ma1−y Mby (OH)2−x (Am− )x/m ·nH2 O, where Ma and Mb are divalent cations. It is rare but also possible to find LHSs formed by three different divalent cations. Frequently, LHSs can be obtained in the form of whiskers and fibers that can exchange interlayer anions and also have the surfaces grafted, as reported for other hydroxylated surfaces. Fig. 14 shows the copper hydroxide benzoate fibers obtained by reacting copper hydroxide acetate with benzoic acid (a) [58] and magnesium hydroxide sulfate whiskers obtained hydrothermaly (b) [59]. The fibers are several micrometers long and have diameters of dozens of mi-

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Fig. 13. SEM micrographs obtained for copper hydroxide (a) [54] and yttrium hydroxide (b) [24]. Scale bar in b = 1 µm. Fig. 12c shows the layered structure of Cu(OH)2 [40,41]. Reprinted with permission from Q. Tang, Z.P. Liu, S. Li, S.Y. Zhang, X.M. Liu, and Y.T. Qian (2003). Copyright 2003 Elsevier. Reprinted with permission from X.F. Wu, H. Bai, J.X. Zhang, F.E. Chen, and G.O. Shi (2005). © 2005 American Chemical Society

crometers depending on the compound and on the method of preparation. 2.9. Fibrous and tubular layered double hydroxides Another important class of compounds with many different potential applications is the layered double hydroxides (LDH) also known as hydrotalcite-like compounds. In these materials of variable compositions and mainly of synthetic origin, the layered structure is intimately related to that of brucite, where hydroxyl ions are hexagonally close packed and magnesium (or aluminum) ions occupy octahedral sites. LDHs have 3+ x+ m− ) a generic formulation [M2+ x/m ·nH2 O, 1−x Mx (OH)2 ] (A 2+ 3+ where M and M represent metal ions in octahedral sites, Am− represents the interlayer anion and “x” ranging from 0.08 to 0.5. In these compounds, the trivalent cation substitutes isomorphically the divalent cation in the hydroxide structure, generating charges that are compensated by the intercalation of hydrated anions. LDHs can also be prepared with a single metal in two different oxidation states (e.g., 3+ x+ m− ) [Fe2+ x/m ·nH2 O) producing very well1−x Fex (OH)2 ] (A known “green rusts.” Exfoliating these materials in different solvents and by rolling the layers, nanotubes and/or nanoscrolls can be produced. Recently, the nanoscrolls of a Mg/Al LDH have been reported [60]. The process consists of synthesizing the carbonate LDH in a water/ethanol solution, followed by hydrothermal treatment at 160 ◦ C for 24 h. Fig. 15 shows the transmission electron micrographs of the so obtained materials.

Nanoscrolls with external diameters in the range 50–100 nm and lengths of several nanometers were obtained. This material is certainly a good alternative for the immobilization of negatively charged metalloporphyrins through exchange reactions and reconstruction after calcination of the nanoscrolls. The outer surface is probably also very active for grafting and exchange reactions. 3. Surface grafting reactions Normally the catalytic active metallocomplexes are not retained at the raw surface of a supports but through interfacial layer obtained by a preliminary grafting reaction. Grafting reactions occur by establishing covalent bonds between the reactive groups of the layer outer surface and an adequate reactant molecule. Choosing the correct functional groups at the pendant grafted molecule, the interfacial layer can be successfully used to immobilize catalytic active species. In the case of the layered compounds, these reactions can be restricted to the outer crystal surface (the basal spacing remains unchanged) or layer surface (in this case an interlayer expansion occurs) and in the case of exfoliated layered compounds and single fibres, the outer surface is totally populated with grafted molecules. Fig. 16 shows a schematic representation of a functionalization of layered hydroxide salts and layered double hydroxides single layers with 4-aminobutanoic acid and tioglycolic acid, respectively. Both examples can be used to immobilize negative and positive charged metalloporphyrins, after a proper protonation

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and deprotonation reaction, respectively. It is important to say that many different chemical functions can be used to graft the surface of the above mentioned materials and also given different functionalities to the pendant molecules. More details of the reactions involved can be seen in a recently published book [61] and other systems studies in the author’s laboratory [44,49,50, 62–65]. Covalent grafted metalloporphyrins into hydroxylated surfaces can also be easily performed, when substituted porphyrins bearing carboxylic groups in the 5, 10, 15, and 20 position are reacted with hydroxyl groups from the surface (see R indicated in Fig. 16). Through this reaction, ester linkage can be established and the metalloporphyrin will be positioned perpendicularly to the fibrous or layered surface. Specific details of the catalytic performance of some of these grafted and metalloporphyrins immobilized surfaces will be discussed in the topic below. 4. Metalloporphyrins immobilizations

Fig. 14. SEM micrographs obtained from copper hydroxide benzoate (a) [58] and TEM micrographs of magnesium hydroxide sulfate (b). Insert in (b), SAED spectra [59]. Reprinted with permission from Y. Ding, H.Z. Zhao, Y.G. Sun, G.T. Zhang, H. Wu, and Y.T. Qian (2001). © 2001 Elsevier

The oxidation of cyclohexane with iodosylbenzene in the presence of metalloporphyrins as a biomimetic cytochrome P450 model (Fig. 1) in general is found to yield cyclohexanol and cyclohexanone as major products. High selectivity for the alcohol product is also observed. Metalloporphyrins, other complexes and transition metal ions either in solution or immobilized have been used for oxidation of linear alkanes such as n-heptane [62,66–70]. The regioselectivity in the oxidation process using metalloporphyrins and in the absence of steric restraints is essentially under thermodynamic control [71]. The product distribution observed is consistent with a radical intermediate species produced by hydrogen atom abstraction [72]. Consequently, the rate of the products observed is related to the C–H bond strength, with more prod-

Fig. 15. TEM micrographs of Mg/Al–CO3 nanoscrolls [60]. Insert in (a), higher magnification of the image. Reprinted with permission from L. Ren, J. Hu, L. Wan, and C. Bai (2007). © 2007 Elsevier

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Fig. 16. Schematic representation of grafting and immobilization of cationic and anionic porphyrins in zinc hydroxide nitrate (a) (layered hydroxide salt) and layered double hydroxides (b), respectively.

ucts being expected from oxidation at the tertiary and secondary carbon atoms instead of the primary carbon atom [68]. On the other hand, in the presence of steric constraints, caused by the support when the metalloporphyrin is immobilized [62] or by the porphyrin structure itself [73], efficiency and selectivity in the oxidation of linear alkanes can be changed. This effect was observed in the author’s laboratory using second generation iron porphyrins in solution or immobilized in tubular and fibrous supports [74,75]. Some examples of the successful application of these catalysts are described in the following sections. The immobilization of porphyrins and metallated porphyrins in carbon nanotubes have been also described by several research groups [13,76–86]. The processes of immobilization are also similar to the above described systems, normally being conducted by adsorption or surface covalent functionalization. As most of them have been designed for noncatalytic purposes, these systems will not be discussed in this article. 4.1. Tubular kaolinite In order to obtain the catalytic iron porphyrin immobilized on solid kaolinite, the kaolinite mechanochemically intercalated with urea was stirred in n-hexylamine, obtaining the n-hexylamine intercalated kaolinite. After that, the solid intercalated with the catalytic species is obtained by subjecting the intercalated kaolinite to an ultrasonic bath in the presence of the neutral ([Fe(TPFPP)]Cl) and charged ({Na4 [Fe(TDFSPP)]}Cl)) iron porphyrins (Fig. 17). After the processing, kaolinite nanotubes are produced (or scroll-like structures) as confirmed by transmission electron

Fig. 17. Schematic representation of iron porphyrins. (1) {Na4 [Fe(TDFSPP)]}Cl = {tetrasodium [5,10,15,20–Tetrakis (2,6-difluorphenyl-3-sulfonatophenyl) porphyrinate Fe(III)]} chloride, (2) {Na4 [Fe(TCFSPP)]}Cl = {tetrasodium [5, 10,15,20–Tetrakis (2-chloro-6-fluor-3-sulfonatophenyl) porphyrinate Fe(III)]} chloride, and (3) [Fe(TPFPP)]Cl = [5,10,15,20–Tetrakis (pentafluorphenyl) porphyrinate Fe(III)] chloride. In order to simplify the representation of the iron porphyrin, in the text will be used for (1) Fe(TDFSPP), (2) Fe(TCFSPP), and (3) Fe(TPFPP).

microscopy, but with the predominance of layered or nonexfoliated kaolinite. This can be attributed to the low power of the ultrasonic bath used in the process, since when repeating the procedure, the nanotubes are more frequently observed. The tubes have different external diameters, with some of them still having a ragged structure attributed to the incomplete curling process. Considering that the raw kaolinite has a crystal size

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around 0.2–3 µm, it can be supposed that the curling process occurs by detaching the outer layers of the crystals (small amounts or even single layers) and curling them until the crystal layers are totally transformed into rolled-up structures. Actually, in kaolinite the curling process is thermodynamically favorable, when the hydrogen bonds of the kaolinite structure are weakened by the intercalation of large molecules like n-hexylamine or other long chain amines. The inner diameter of the tubes is variable, with the smallest being around 250 Å, in which is perfectly possible to trap a single molecule of iron porphyrin with diameters of less than one tenth of that size. We cannot infer much about the position of the iron porphyrin molecules in the solid, but we can exclude intercalation between the layers and suppose the presence inside the rolledup tubes as the inner diameter is large enough to lodge single iron porphyrin molecules. As the surface area of the compound is probably high, the presence of the catalytic active site between the crystals and/or tubes is also possible, especially for the charged iron porphyrin, where the interaction with aluminol surface groups is the most probable. Fig. 18 shows a schematic representation of the intercalation and preparation of the catalyst. Step 1 refers to the mechanochemical intercalation of urea, step 2 to the intercalation of n-hexylamine, and step 3 after addition of iron porphyrins in the presence of an ultrasonic bath. Step 4 refers to the process of washing the urea-intercalated kaolinite and step 5 to the washing of the solid after contact with the iron porphyrins. The phases obtained from steps 4 and 5 are also present in step 3, curled or not. The catalytic activity of both second generation iron porphyrins (homogeneous catalysis) and the corresponding supported catalysts of both iron porphyrins (heterogeneous catalysis) was investigated in the oxidation of two different substrates, cyclohexane (cyclic) and linear n-heptane [75]. In homogeneous catalysis, as expected, both iron porphyrins (Fe(TPFPP), (Fe(TDFSPP)) presented catalytic activity for oxidation of cyclohexane since both are robust and efficient structures due to the presence of electronegative substituents on the phenyl ring that reduce the electronic density on the porphyrin ring stabilizing it against oxidative degradation [87]. In general, when charged groups are added to the structure of second generation porphyrins, as observed for Fe(TDFSPP), the catalytic efficiency can be modified because the macrocycle can suffer of insolubility problems. In fact, high yield (69%) and high selectivity for cyclohexanol have been observed for the Fe(TPFPP), a neutral second generation iron porphyrin used as homogeneous catalyst in the oxidation of cyclohexane by iodosylbenzene [75]. The opposite behavior has been observed for the charged porphyrin Fe(TDFSPP) (19% yield), and that effect can be attributed to the poor solubility of this complex in the solvent mixture used for the catalytic reactions. In general, the insolubility problems observed with metalloporphyrins in homogeneous catalysis are minimized when they are immobilized on a mineral matrix [5,6,8]. It has been observed in this situation, that the efficiency could be improved and only alcohol is produced (28% yield) (Fe(TPFPP) and Fe(TDFSPP)) in heterogeneous catalysis. This is one of the ad-

vantages of the heterogeneization of porphyrin catalysts using supports such as tubular kaolinite. The strong interaction that probably occurs between the charged iron porphyrin (Fe(TDFSPP)) and the kaolinite support (layered crystals or tubes) keeps the catalyst on the solid during the catalytic reaction, consequently no iron porphyrin traces have been detected in the solution after the catalytic reaction using the immobilized iron porphyrin. This behavior creates the opportunity to reuse and recycle the catalysts. On the other hand, immobilization of the neutral Fe(TPFPP) on the kaolinite (K/Fe(TPFPP)) mostly produced a deactivated catalyst and only a yield of 8% of alcohol was observed. Since no iron porphyrin trace was observed after any of the catalytic reactions using both immobilized iron porphyrins (anionic or neutral), it is reasonable to suppose that there is also a strong interaction between the neutral complex and the support. The very low yields observed suggest that the catalytic site is inaccessible to the substrate and oxidant. This could arise because the iron porphyrin is firmly attached inside the tubes or in layered kaolinite crystal agglomerates. The site could even be located in pores obtained by the single layer agglomeration with a “castle of cards” morphology, resulting from the exfoliation process. Another possibility could be that the fifth and sixth coordinative positions of the iron are bound to the support, which avoids the interaction with the oxidant iodosylbenzene and consequently deactivate the catalytic site [66,88]. Yet another possibility is the coordination of the iron porphyrin to hexylamine/urea, which can also deactivate the catalyst. The catalytic performance of the iron porphyrins immobilized on exfoliated kaolinite can be seen in Table 1. The yields of heptane oxidation products reflect the same result observed for cyclohexane oxidation during homogeneous catalysis, Fe(TPFPP) is more efficient than Fe(TDFSPP) for converting the substrate to alcohol at positions C-2 and C-3 (quantitative yields below 2% were not observed for products at positions C-1 or C-4 as expected for the oxidation of the linear alkane). A tendency for selectivity toward alcohol instead of ketone was also observed. For the neutral iron porphyrin, the rate of the products observed is as expected, with more alcohol (61% of the total) and more ketone (2.6% of the total) produced at position C-2 than C-3. In spite of the lower efficiency, the charged Fe(TDFSPP) presented high selectivity for the C-2 position of the alcohol and the C-3 position of the ketone. Normally, in metalloporphyrins, there is no discrimination between one secondary site and another (C-2 and C-3), with the ratio of yield products at these sites (2-heptanol/3-heptanol) close to 1, as observed for the unhindered metallotetraphenylporphyrin [3,5,6]. The selectivity observed for the homogeneous catalysis for the most accessible secondary site (C-2) is higher for Fe(TDFSPP) (catalyst 4, Table 1, 31% of alcohol yield with 100% of selectivity to 2-heptanol) in comparison with Fe(TPFPP) (catalyst 2, Table 1, 46% of alcohol yield with 61% of selectivity to 2-heptanol). The presence of the sulfonate groups at the meta position of the meso phenyl groups of the porphyrin rings in Fe(TDFSPP) and the negative charges probably would make it difficult for the substrate to access the metal center of this iron

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Fig. 18. Schematic representation of the steps followed during the preparation of the K/Fe(por) catalyst (iron porphyrin immobilized on kaolinite) [74,75]. Reprinted with permission from S. Nakagaki, K.A.D.F. Castro, G.S. Machado, M. Halma, S.M. Drechsel, and F. Wypych (2006). © 2006 Brazilian Chemical Society. Reprinted from S. Nakagaki, G.S. Machado, M. Halma, A.A.S. Marangon, K.A.D.F. Castro, N. Mattoso, and F. Wypych (2006). © 2006 Elsevier Table 1 Oxidation of cyclohexane and heptane catalyzed by iron porphyrins and immobilized iron porphyrins [75] Catalyst

1 2 3 4 5

Cyclohexane oxidationa

Heptane oxidationa

Alcohol yield (%)b

Ketone yield (%)b

Alcohol yield (%)b,c

Ketone yield (%)b,c

Total yield (%)b

Alcohol ketone ratioa

8 69 28 19 Trace

– 5 – 7 Trace

0 46 71 31 –

0 11 4 6 –

0 69 79 43 –

– 4 18 5 –

Regioselectivity (%)d C-2

C-3

ol

one

ol

one

– 61 61 100 –

– 83 – – –

– 39 39 – –

– 17 100 100 –

a Conditions: catalyst:oxidant:cyclohexane molar ratio = 1:20:2000; solvent mixture dichloromethane/acetonitrile (1:1, v/v) at room temperature under argon. Homogeneous catalyses reactions were performed under identical conditions as heterogeneous catalyses. b Yield based on starting PhIO (it was assumed that 2 mol of iodosylbenzene were used for the ketone formation). c C-2 and C-3 heptane products of oxidation (alcohol and ketone). Products corresponding to C-1 and C-4 heptane positions were not observed. d Relative proportions of products (%) at positions C-2 and C-3 of heptane. ol = heptanol and one = heptanone. Catalysts: 1 = K/[Fe(TPFPP)]; 2 = [Fe(TPFPP)]; 3 = K/[Fe(TDFSPP)]; 4 = [Fe(TDFSPP)]; 5 = kaolinite (K).

porphyrin, more hindered for the secondary C-3 heptane position. In case of 3-heptanol production, this was probably further converted to 3-heptanone. After immobilization (Table 1, catalysts 1 and 3), the catalytic behavior changed for the two catalyst complexes. For the supported neutral iron porphyrin (catalyst 1) no activity was observed in the oxidation of heptane. The catalytic efficiency for the hydroxylation seems to arise by controlling the access of the substrate to the active oxidant site or the access of iodosylbenzene to generate the oxidant site. The access of the alkane to the metal site would be restricted by the kaolinite support in the case of the immobilized neutral iron porphyrin since both substrates showed low or zero activity for catalytic oxidation,

even for the more thermodynamically favorable C-2 and C-3 position in linear alkanes. The immobilized anionic iron porphyrin on the other hand (Table 1, catalyst 3), presented high yields of alcohol and ketone (total yield 79%) but differently from the iron porphyrin in solution, alcohol at positions C-2 and C-3 was observed with rates similar to those observed for iron porphyrins in general. Since no Fe(TDFSPP) leaching was observed during the heterogeneous catalytic reactions with the immobilized anionic iron porphyrin which might justify the drastic catalytic behavior changes observed, it is reasonable to assume that the immobilization of Fe(TDFSPP) happens through the strong charge interaction between the solid support and the porphyrin ring

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sulfonate groups. This interaction would expose the catalytic active site, facilitating mostly the access of (C-2) and to a lesser extent the secondary site (C-3).

Table 2 Hydroxylation of cyclohexane by PhIO catalyzed by iron(III) porphyrins, Fe(TDFSPP) and Fe(TCFSPP), in homogeneous media and by the supported iron(III) porphyrins in heterogeneous media [74]

4.2. Chrysotile and fibrous silica

Catalysta

Recently, we have reported the catalytic activity of anionic iron(III) porphyrins Fe(TDFSPP) and Fe(TCFSPP) (Fig. 17) immobilized in raw and silanized chrysotile (3-APTS grafted chrysotile) [74], and in organo functionalized fibrous silica derived from chrysotile (3-APTS grafted disordered silica, obtained from acid leached chrysotile, where 3-APTS is 3aminopropyltrietoxysilane) [89]. The solids were characterized by FTIR and powder X-ray diffraction. The presence of the iron porphyrin in the solids was confirmed by UV–vis spectra, in Nujol mineral oil. The measurements suggested that no demetallation (characterized by a blue-shift of the Soret band that is associated with a significant amount of free base porphyrin) [23,90,91] nor significant exchange of the Fe(III) ion with the supports occurred during the preparation process. The Soret peaks of all immobilized iron porphyrins were red-shifted when compared to those of iron porphyrins in solution. A similar behavior was observed previously [92], when metalloporphyrins were immobilized in different inorganic supports [8,90–93]. This behavior was attributed to steric constraints caused by the support, which substantially modified the iron porphyrin structure in these supported catalysts [94]. The TEM measurements of Fe(TCFSPP) immobilized in 3APTS grafted chrysotile and raw chrysotile have shown that the fibers did not experiment major changes in the morphology of the single fibers. In high resolution images, a corrugation of the surface was observed, attributed to the grafting and immobilization of the iron porphyrins at the surface of the fibers, with probable single layer coverage. The catalytic activities of both iron porphyrin supported catalysts (heterogeneous catalysis) and in solution (homogeneous catalysis) were investigated on the oxidation of weakly reactive alkanes such as cyclohexane. The catalytic results are presented in Table 2, and compared with the same porphyrins immobilized on 3-APTS grafted disordered silica, obtained from acid leached chrysotile [89]. When using 1.1 to 1.4 mmol of substrate in the solvent mixture CH3 CN:CH2 Cl2 (1:1 v/v ratio), the supported catalysts did not release their iron porphyrin, and different proportions of PhIO (iodosylbenzene) relative to the iron porphyrin supported catalyst led to selective formation of alcohol (based on PhIO) within 1 and 24 h. The consumption of PhIO in all reactions was monitored by the presence of iodobenzene (PhI). The low solubility of the iron porphyrins in a CH2 Cl2 :CH3 CN (1:1 v/v ratio) solvent mixture during homogeneous catalysis certainly is an important factor responsible for the low yields (and low turnover number, TON = mol products/mol catalyst) with the iron porphyrin itself. The possibility of the molecular interaction of the Fe(TDFSPP) species in solution could be another reason for the low catalytic activity observed in the homogeneous catalysis because it can be accompanied

Runb

Time (h)

Fe(TDFSPP)

1

1

13

9

Fe(TDFSPP)/Si-APTS

2 3

1 24

36 61

19 34

Fe(TDFSPP)/Chrys

4 5

1 24

12 35

7 19

Fe(TDFSPP)/Chrys-APTS

6 7

1 24

16 25

9 15

Fe(TCFSPP)

Alcohol yield (%)c

TON

8

1

48

3

Fe(TCFSPP)/Si-APTS

9 10

1 24

15 33

8 18

Fe(TCFSPP)/Chrys

11 12

1 24

49 44

29 23

Fe(TCFSPP)/Chrys-APTS

13 14

1 24

60 42

30 22

a Chrys (raw chrysotile), Chrys-APTS (3-APTS grafted chrysotile and Si-APTS (3-APTS grafted disordered silica, obtained from acid leached chrysotile). b Conditions: catalyst:oxidant:cyclohexane molar ratio = 1:50:5000; solvent mixture dichloromethane/acetonitrile (1:1, v/v) at room temperature under argon. Homogeneous catalyses reactions were performed under identical conditions as heterogeneous catalyses. c Yield based on starting PhIO (it was assumed that 2 mol of iodosylbenzene were used for the ketone formation). The standard error is smaller than 5%.

by inactive μ-oxo complex formation [95–97]. The oxidative degradation of the catalyst in solution is frequently responsible for the low yield in catalytic reactions using metalloporphyrins [87], but not for second generation iron porphyrins and in cases with a low Fe(Por)/oxidant molar ratio such as that used in run 1 (iron porphyrin:iodosylbenzene:cyclohexane molar ratio = 1:50:5000) (Por = porphyrin). The hydroxylation reaction selectivity (% alcohol:% ketone) was higher than 10 for all reactions and conditions. Control reactions carried out using Si-APTS (silica derivates from raw chrysotile), Chrys (raw chrysotile) or Chrys-APTS (pure 3-APTS grafted chrysotile) as catalyst show alcohol yields below 2%. For the iron porphyrin Fe(TDFSPP) immobilized in the three different supports (run 2 to 7, Table 2) Chrys (raw chrysotile), Chrys-APTS (3-APTS grafted chrysotile) and Si-APTS (3APTS grafted disordered silica, obtained from acid leached chrysotile) at the optimum condition (24 h), 61% of alcohol yield was observed when the inorganic support used was SiAPTS (run 3). When the three different supports were compared using the same porphyrin (Fe(TDFSPP)), it is clearly seen that the best performance is obtained for Fe(Por)/SiAPTS, compared with the same porphyrin immobilized in raw chrysotile (run 5) and grafted chrysotile (run 7) (for 24 h of reaction). This behavior can be explained by the higher surface area of the silica obtained from the acid leached chrysotile, leading to a better contact of the grafted active site with the oxygen donor (PhIO) and the substrate. Although the surface areas of the supports are not available, the increase of the sur-

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face area after the acid treatment is explained by the opening and flattening of the tubes into the form of nanobelts [49]. These nanobelts of disordered silica are produced from concentric tubes of chrysotile, exposing surfaces initially within the tubes and therefore not available for the grafting reactions of raw chrysotile. Another important consequence of the opening of the tubes is that both sides of the silica nanobelts become available to the grafting reaction, which is otherwise limited to the outer surface area of the raw chrysotile nanotubes. Considering the same amount of matrix and a similar fixed amount of iron porphyrin distributed on the surface of the matrix (2.0–2.6 × 10−2 mmol of iron porphyrin/g of solid), in the case of acid-leached chrysotile the active sites will be better dispersed, and consequently more available to the access of the substrates. The grafted 3-APTS molecule seems to contribute negatively to the active site (run 7) in comparison with the immobilization on raw chrysotile (run 5), possibly due to the bonding of the NH2 surface groups to the iron porphyrin reducing the activity of the catalytic site. In the case of Fe(TCFSPP), the effect is not so pronounced as the catalytic activities after 24 h are always similar and independent of the surface area. This behavior can perhaps be related to the polarity of the solvent used during the catalytic experiments. For the Fe(TCFSPP) iron porphyrin, the best yield for the immobilized system was obtained in run 13 (60% alcohol). In general, the Fe(TDFSPP) catalyst and its Fe(TCFSPP) counterpart display opposite behavior regarding the following: Fe(TCFSPP) presents a similar or better yield in homogeneous catalysis (run 8) when compared with the cases where this iron porphyrin is immobilized. In homogeneous catalysis, one ortho-chlorine substituent in each meso-phenyl porphyrin groups in the Fe(TCFSPP) can avoid molecular interactions, which can deactivate (by destruction of the iron porphyrin or dimerization) and/or avoid the catalytic active species formation [98,99]. Therefore, higher yields in oxidation reactions are expected for this iron porphyrin in comparison with Fe(TDFSPP). In contrast, after immobilization, the better catalytic results observed for Fe(TDFSPP) could be due to an easy access of PhIO and substrate to the iron site, based on the small size of the two ortho-fluorine substituents in comparison with the ortho-chlorine substituents from Fe(TCFSPP) [99]. Besides, the immobilization process avoids any molecular interaction possible during homogeneous catalysis, mainly when a low percentage of iron porphyrin is immobilized in the Si-3-APTS solid. 4.3. Halloysite Raw halloysite characterized by powder X-ray diffraction (basal distance = 7.4 Å), IR spectroscopy (two bands characteristics in the 3700–3600 cm−1 region) and TEM (tubular form of the clay mineral) was used to immobilize the anionic [Fe(TDFSPP)], the neutral [(Fe(TDFPP)]Cl ([5,10,15,20– Tetrakis (2,6-difluorphenyl) porphyrinate Fe(III)] chloride) and the anionic Zn(TDFSPP) (the same porphyrin (1) represented in Fig. 17 but metalated with Zn(II) ion instead Fe(III)). Two

155

processes of immobilization were investigated: (1) under pressure and high temperature and (2) stirring and reflux at room pressure. The immobilization results of different metalloporphyrins obtained under pressure in the halloysite showed that: Fe(TDFSPP) was 100% immobilized (denominated by Fe-Hallo), Fe(TDFPP) was not immobilized at all, and Zn(TDFSPP) was 47% immobilized. The results under pressure suggested that the immobilization process occurred mainly between the interaction of the sulfonate groups presents in the anionic porphyrins and the surface of the halloysite, but some interaction between the metal and the solid surface is also possible. The results obtained under reflux at room pressure and stirring showed that: Fe(TDFSPP) was 80% immobilized, Zn(TDFSPP) was 35% immobilized and none of the neutral Fe(TDFPP) was immobilized. An immobilization of only 80% of the charged iron porphyrin can be attributed to the presence of interfacial water in the solid which makes difficult the immobilization process. The solid Fe(TDFSPP)/clay immobilized under pressure (Fe-Hallo), was used to investigate the catalytic activity in an oxidation reaction in similar conditions used for the investigation of the activity of the iron porphyrins immobilized on chrysotile. Preliminary results showed a high activity of the catalyst in the epoxidation of cyclooctene (99% yield), in the oxidation of cyclohexane (39% yield of cyclohexanol), and in the oxidation of heptane with high selectivity to the alcohol at positions C-2 and C-3 if compared to the respective ketones (50% of alcohol and 8% of ketones). The preliminary results were superior to those previously reported for the porphyrin under homogeneous catalysis [100], mainly the efficiency and selectivity for heptane oxidation (alcohol and ketones at positions C-2 and C-3 are 31 and 12%, respectively), suggesting that the low efficiency and selectivity results for the oxidation of the most inert linear alkanes can be changed if the adequate combination of support and macrocyclic structure is found. Finally, no iron porphyrin traces were detected in the reaction mixture after the catalytic reactions, showing an efficient immobilization. 4.4. Other fibrous and tubular structures The immobilization of metalloporphyrins in imogolite, fibrous niobates, fibrous hydroxides, fibrous hydroxide salts and fibrous layered double hydroxides, to our knowledge has not yet been reported in the literature which opens a great and interesting research area of inorganic supports, for the immobilization of this versatile and efficient oxidation catalyst. However, the use of this class of inorganic synthetic materials as supports to obtain oxidation heterogeneous compounds is perfectly possible based on the results of the immobilization of a nonporphyrinic copper complex in scrolled particles of niobates [101]. The immobilized Cu-complex was used as a heterogeneous catalyst for catechol oxidation and showed promising results as a mimetic system, for catechol oxidase enzymes. The authors commented that the benefits of using the immobilized complex on scrolled particles instead of intercalated on the pristine layered niobate are mainly related to the enhancement of the surface area of the catalyst solid obtained and also due to the

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channels that the scrolled niobate solid presents. The presence of the channels depends on the substrate, can promote interesting selectivity in heterogeneous oxidation reactions. Recently [78], a review was also published reporting the immobilization of porphyrins in different inorganic supports, emphasizing especially their structures and photochemical reactions. 5. Conclusions, challenges, and future perspectives The immobilization of active metallocomplexes is the key factor for the large scale application of these compounds. Several supports have been extensively investigated but in most of them the versatility of the fibrous and tubular structures cannot be reached, being of natural or synthetic origin. This class of compounds is very versatile, of very high surface areas, low cost, broadly available or of easy synthesis, easy control of compositions and reactive surface. The immobilization can be performed at the raw surface or after chemical grafting of the surface, leaving pending groups that can be used to retain the active molecule. This gives the chance to control the concentration of the active sites at the surface of the matrix and minimize steric constrains during the approaching process of the reactive molecules to the catalyst surface. Another important factor is to minimize the interactions between the metallocomplexes, which can deactivate the catalytic sites and maybe one of the most important aspects is related with the reuse of the catalyst. In the case of tubular structures, the active molecule can be allocated inside the tubes, having a pre-defined diameter which can then selectively allow the entrance of molecules with specific sizes, giving shape selectivity to the catalyst. Regioselectivity can also be achieved when only part of the molecule can penetrate the tube and come in contact with the active site, allowing reactions which are not selective in homogeneous media. Crucial aspects to consider when environmentally benign chemical processes for industrial applications are designed and high cost catalysts are used can be summarize in some key words: improve the catalytic activity, reduce the cost, facilitate the catalysts separation of the reaction media, allow an easy recycling and disposal of the catalysts after use. As the described matrices are from natural origin and those from synthetic origin are based on very common elements available in soils, consequently known for their nontoxicity, most of them will satisfied the pre-requisites above and certainly will find new and broad application. Nowadays, where functional and nanodimensional materials are the key words in top publications, researchers need to look at the potential of old and very common substances. Especially now in the nanoera, the fibrous and nanotubular natural silicates and cellulose, broadly available in several countries will soon stand out and show their tremendous potential, bringing new surprises. References [1] G.A. Schick, I.C. Schreiman, R.W. Wagner, J.S. Lindsay, D.F. Bocian, J. Am. Chem. Soc. 111 (1989) 1344.

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