Recent Advances in Processing and Characterization of Periodic

Ind. Eng. Chem. Res. 2001, 40, 3237-3261

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REVIEWS Recent Advances in Processing and Characterization of Periodic Mesoporous MCM-41 Silicate Molecular Sieves Parasuraman Selvam,† Suresh K. Bhatia,* and Chandrashekar G. Sonwane Department of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane QLD 4072, Australia

The discovery of periodic mesoporous MCM-41 and related molecular sieves has attracted significant attention from a fundamental as well as applied perspective. They possess welldefined cylindrical/hexagonal mesopores with a simple geometry, tailored pore size, and reproducible surface properties. Hence, there is an ever-growing scientific interest in the challenges posed by their processing and characterization and by the refinement of various sorption models. Further, MCM-41-based materials are currently under intense investigation with respect to their utility as adsorbents, catalysts, supports, ion-exchangers, and molecular hosts. In this article, we provide a critical review of the developments in these areas with particular emphasis on adsorption characteristics, progress in controlling the pore sizes, and a comparison of pore size distributions using traditional and newer models. The model proposed by the authors for adsorption isotherms and criticalities in capillary condensation and hysteresis is found to explain unusual adsorption behavior in these materials while providing a convenient characterization tool. 1. Introduction In the past decade, there has been a significant breakthrough in the synthesis of open-framework inorganic materials of well-defined geometry. The development of such composite materials, e.g., silica and carbon, with precise and easily controlled pore shapes and sizes is of great importance in many areas of modern science and technology.1-4 These materials are best appreciated in systems where molecular recognition is needed, e.g., shape-selective catalysis, selective adsorption and separation processes, chemical sensors, and nanotechnology. According to IUPAC nomenclature,5 porous solids are broadly classified, on the basis of pore size (diameter), into three categories, viz., microporous (50 nm). There has been continued research activity on the synthesis and characterization of both periodic and nonperiodic mesoporous solids. However, a new area in periodic porous materials began after the discovery6-9 of ordered mesophases. The advantages and limitations of various microporous (zeolite) molecular sieves,10-14 as well as of the recently discovered ordered/disordered macroporous solids,15-20 have been described in the literature. A discussion of these materials, however, is beyond the scope of this study. In this review, we focus mainly on the recent progress as well as the various aspects of these novel mesoporous silica materials. (Note that the terms “silica(s)” and “silicate(s)” are interchangeably * To whom correspondence should be addressed. E-Mail: [email protected]. Tel.: (+61) 7 3365 4263. Fax: (+61) 7 3365 4199. † Permanent Address: Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India.

used in this review.) We discuss here the various aspects dealing with MCM-41 and outline only the major trends in this field. This paper also presents a comprehensive overview of the current developments and the various key issues related to the determination of pore size, pore wall thickness, and pore size distribution of MCM-41, with a particular emphasis on the evaluation of various models. 2. Categorization of Mesophases 2.1. M41S-Type Silica Mesophases. The invention of a new family of mesoporous silicate/aluminosilicate molecular sieves6,8,21 has attracted worldwide interest in many areas of physical, chemical, and engineering sciences. In the early 1990s, Mobil scientists6,8,21 as well as Japanese researchers7,9 reported the first successful synthesis of novel periodic mesostructured materials. The former employed a new concept in the synthesis of these porous composite materials, viz., the use of a selfassembled molecular aggregate or supramolecular assembly of surfactant molecules such as cetyltrimethylammonium (CTA) cation as the structure-directing agent, rather than the conventional single- (amine-) molecule-templated microporous (zeolites) structures. This led to the discovery of the so-called M41S family of mesoporous materials of greatest interest.6,8,21 They have been grouped into four main categories, as depicted in Figure 1. Two have been identified as the thermally stable MCM-41 having a hexagonal structure with unidimensional pore structure and MCM-48 displaying a cubic structure with a three-dimensional pore system. The two other phases, viz., lamellar MCM-50 and molecular organic octomer [a surfactant-silica compos-

10.1021/ie0010666 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001

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Figure 1. Phase formation from C16TMA/SiO2/H2O.23

Figure 2. Powder X-ray diffraction patterns of (a) disordered MCM-41, (b) ordered MCM-41, (c) MCM-48, (d) MCM-50, and (e) octamer.

ite species, (surf‚SiO2.5)8, are unstable.22 All of these phases exhibit well-defined X-ray diffraction (XRD) patterns. Figure 2 illustrates the typical diffraction patterns, along with the pattern of disordered MCM41.23 On the other hand, Inagaki and co-workers7,24 used a quite different approach wherein the mesoporous materials were prepared from a layered form of sodium silicate and CTA surfactant cation via an ion-exchange process followed by hydrothermal treatment. Subse-

quently, it was shown that the mesoporous materials can also be obtained using amphiphilic block copolymer templates.25-30 More recently, novel organic-inorganic hybrid mesophases have been reported using 1,2-bis(trimethoxysilyl)ethane and octadecyltrimethylammonium chloride.31 Among the different methodologies reported for the synthesis of mesoporous materials, that based on Mobil’s strategy, i.e., M41S-type, is the most popular.6,8 The unique pore topology and high surface area of these mesophases identify them as the exceptional porous solids. Of the two crystalline forms of the M41S family, viz., hexagonal MCM-41 and cubic MCM48, the former has attracted considerable attention because of its possible industrial applicability as well as scientific interest in the challenges posed by the synthetic strategies, processing conditions, and characterization methods. Therefore, a great deal of effort has been directed toward the development of the various aspects of MCM-41 for its commercial use. Recently, several groups have reported25,32-35 alternate methods of preparation of mesoporous materials that crystallize in the MCM-41 or MCM-41-type structure. Table 1 summarizes the various silica mesophases based on the above methodologies. 2.2. Other Silica-Based Mesophases. Inagaki and co-workers7,24 independently prepared, virtually at the same time as M41S materials were synthesized, mesoporous silica designated as FSM-16 from the sodium form of layered polysilicate, kanemite (NaHSi2O5‚3H2O), and CTA cation. Although the FSM-16 is structurally similar to MCM-41, the adsorption properties36 and surface chemistry37 of these two materials are slightly different. Although a majority of the studies have focused on MCM-41, there has been continued interest in developing new mesophases. It is interesting to note that different choices of the inorganic precursor, organic template, reaction conditions, and synthetic pathway have resulted in a variety of novel mesostructured materials.1,11,38-55 Many of them have structures similar to MCM-41 or can be derived from MCM-41. Indeed, Mobil’s self-organized surfactant procedure6,8,21 has been extended using anionic-, neutral-, and acidtreated surfactant assemblies. The use of different types of surfactant templates results in numerous silicate mesophases.27,29,32,56-59 All of these approaches are mainly based on intermolecular forces between the various silica sources and surfactant molecules via electrostatic, hydrogen-bonding, covalent, and van der Waals interactions. Pinnavaia and co-workers27,29,58 used neutral surfactants such as primary amines and poly(ethylene oxide)s to prepare a series of (disordered) mesoporous silicate structures through hydrogen-bonded interactions. They include HMS,27,46 MSU-n (n ) 1-4),29 MSU-V60 and MSU-G.61 Stucky and coworkers19,32,33,44,62-64 reported a series of acid-prepared mesostructured silicas, termed SBA-n (n ) 1-3, 8, 11, 12, 14-16) structures, using cationic surfactant and halogen acids through balanced Coulombic, hydrogenbonding, and van der Waals interactions. Yet another approach by Ryoo et al.65 involving the addition of ethylenediaminetetraacetic acid to a high-pH synthesis gel resulted in hydrothermally stable mesoporous silicates designated KIT-1, with disordered interconnected pore channels. Several types of micelle-forming amphiphilic block copolymers as well as glycosilicate surfactants have also been used as templates for the preparation of mesoporous silicates.25,66-68

Ind. Eng. Chem. Res., Vol. 40, No. 15, 2001 3239 Table 1. Physical Properties of Various Mesoporous Silica Materials structural data

e

mean pore size (nm)

sample code

reference

dimensionality, crystal system, and space group

unit cell dimension (nm)

reported

SB modela

MCM-41 MCM-48 FSM-16 SBA-1 SBA-2 SBA-3 SBA-8 SBA-11 SBA-12 SBA-14 SBA-15 SBA-16 HMMb HMMb MSU-1 MSU-2 MSU-3 MSU-4 MSU-V MSU-G HMS KIT-1 CMK-1e APOf

Beck et al.8 Beck et al.8 Inagaki et al.24 Huo et al.32 Huo et al.64 Huo et al.32 Zhao et al.19 Zhao et al.26 Zhao et al.26 Zhao et al.26 Lukens et al.63 Zhao et al.26 Inagaki et al.31 Inagaki et al.31 Bagshaw et al.29 Bagshaw et al.29 Bagshaw et al.29 Prouzet et al.291 Tanev et al.60 Kim et al.61 Zhang et al.292 Ryoo et al.65 Ryoo et al.45 Kimura et al.82,83

2D hexagonal (P6mm) cubic (Ia3 h d) 2D hexagonal (P6mm) cubic (Pm3 h n) 3D hexagonal (P63/mmc) 2D hexagonal (P6mm) 2D Rectangular (cmm) cubic (Pm3 h m) 3D hexagonal (P63/mmc) cubic (Pm3 h n) 2D hexagonal (p6mm) cubic (Im3 h m) 2D hexagonal (P6mm) 3D hexagonal (P63/mmc) hexagonal (disordered) hexagonal (disordered) hexagonal (disordered) hexagonal (disordered) lamellard lamellard hexagonal (disordered) hexagonal (disordered) cubic (I4132) 2D hexagonal (P6mm)

a ) 4.04 a ) 8.08 a ) 4.38 a ) 7.92 a ) 5.40, c ) 8.70 a ) 4.75 a ) 7.57, b ) 4.92 a ) 10.64 a ) 5.40, c ) 8.70 a ) 4.47 a ) 11.6 a ) 17.6 a ) 5.70 a ) 8.86, c ) 5.54 a ) 4.73 a ) 7.16 a ) 7.04 a ) 6.01 a ) 3.87 a ) 6.54 a ) 4.55 a ) 4.80 a ) 8.33 a ) 4.27

3.70 3.49 2.80 2.00 2.22 2.77 1.87 2.50 3.10 2.40 7.80 5.40 3.10 2.70 3.10 3.50 5.80 -c -c 3.20 2.80 3.52 3.00 2.80

3.25 3.50 3.52 3.05 3.41 3.48 2.98 3.60 4.09 3.46 8.73 8.35 3.85 3.58 3.60 3.63 3.61 3.85 3.89 3.84 3.35

a Sonwane-Bhatia model. For details, see section 5. b Hybrid mesoporous material. c Data not available. Mesoporous carbon molecular sieve. f Mesoporous aluminophosphate.

2.3. Non-Silica-Based Mesophases. Shortly after the discovery of mesoporous M41S-type materials, the use of surfactant species to organize various non-silicabased mesostructured oxides was explored over a wide range of conditions. A large number of mesoporous oxides, sulfides and phosphates have been reported.32,40,41,44,45,69-74 Stucky and co-workers32,44,64,75 developed a generalized synthesis procedure for a large range of main group and transition element oxide mesostructures with cationic and anionic surfactants, using both acidic and basic conditions with low surfactant concentrations and low reaction temperatures. They also reported28 numerous mesoporous binary metal oxides, as well as some (disordered) ternary metal oxides, using amphiphilic polyalkylene oxide block copolymers as the structure-directing agents. On the other hand, Antonelli and Ying76,77 prepared mesoporous oxides of niobium and tantalum by employing an alternate synthesis methodology that relied on the covalent interaction between the surfactant headgroup and the respective metal alkoxide precursor in the absence of water. Nearly at the same time, Ozin and co-workers78 reported a novel procedure for the synthesis of mesoporous titania, zirconia, and niobia based on the use of glycometalate surfactants. The method is also extended to produce silcon mixed oxides of titanium, zirconium and niobium. Other studies have demonstrated that stabilized-emulsion57,79 or latex-sphere30,80,81 templating can also be used to create mesostructured oxides of titanium and zirconium with pore sizes ranging over 100 nm. However, it is noteworthy that the mesoporous oxides prepared using block copolymers28 as well as glycometalate surfactants78 are found to be thermally more stable than those obtained using amphiphilic surfactants or other approaches. Aluminophosphates. The supramolecular templating method originally developed for the mesoporous silicates/ aluminosilcates has lately been used for the synthesis of novel mesoporous aluminophosphates.70,82-91 There

d

Thermally stable mesophase.

have been several reports on the preparation of mesostructured lamellar86,92,93 and hexagonal70,82,83,87-89 AlPO materials. The former was obtained using a neutral surfactant, e.g., decylamine or dodecylamine, as the structure-directing agent and Catapal B alumina and phosphoric acid as the aluminum and phosphorus sources, respectively. In contrast, for the latter, cationic (CTA) surfactant was employed. Yet another approach85 uses a hydrogen phosphate/dihydrogen phosphate buffer with a layered mesoscopic salt of aluminum in the presence of an anionic surfactant, viz., sodium dodecyl sulfate. More recently, a novel mesostructured aluminophosphate/surfactant (dodecyl phosphate) composite material has been reported.91 Carbons. One of the most interesting periodic mesoporous materials reported in recent times is based on a carbon framework structure.94,95 Ryoo et al. reported the first successful synthesis of ordered mesoporous carbon, using MCM-48 as the template. It was achieved by converting sucrose (carbon source) inside the mesopores of MCM-48 through a mild carbonization process in the presence of sulfuric acid. Mesostructured carbon molecular sieves were then obtained by the removal of the framework silica (MCM-48) using aqueous sodium hydroxide. A similar synthesis of carbon mesophases was also reported later by Lee et al.95 employing AlMCM-48 as the template. 2.4. Significance of MCM-41. Perhaps, MCM-41 is one of the most widely studied among the numerous mesoporous materials reported so far. This is simply because of its structural simplicity and ease in preparation with negligible pore-networking and pore-blocking effects. MCM-41 has also been identified as the most suitable model mesopore adsorbent presently available for studying some of the fundamental features of adsorption such as the effects of pore size, hysteresis, etc., owing to its relatively uniform cylindrical/hexagonal pore channels. This distinction among all of the known periodic mesophases also results from other

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outstanding and unusual properties. The prominent features of MCM-41, and in general of most periodic mesoporous materials, are as follows: well-defined pore shapes (hexagonal/cylindrical); narrow distribution of pore sizes; negligible pore networking or pore blocking effects; very high degree of pore ordering over micrometer length scales; tailoring and fine-tuning of the pore dimensions (1.5-20 nm); large pore volumes (>0.6 cm3 g-1); exceptional sorption capacity (64 wt % of benzene at 50 Torr and 298 K) as a result of the large pore volume; very high surface area (∼700-1500 m2 g-1); large amount of internal hydroxyl (silanol) groups (∼40-60%); high surface reactivity; ease of modification of the surface properties; enhanced catalytic selectivity in certain rections; and excellent thermal, hydrothermal, chemical, and mechanical stability. A number of reviews1,11,38-44,47,48,50-55,96-102 have appeared on various aspects of different mesoporous materials. However, it is quite remarkable that only one report43 has appeared, as early as 1996, exclusively on MCM-41, considering its synthesis, characterization, formation mechanism, physicochemical properties, and applications. Since then, considerable progress has been made in pore size engineering, hydrothermal stability, and accurate determination of pore sizes of MCM-41. Numerous gas adsorption studies have been conducted, and several adsorption models have been put forward. At this juncture, it is therefore appropriate to review the recent progress on these important materials. 3. Applications of MCM-41 The high surface area, well-defined regular pore shape, narrow pore size distribution, large pore volume, and tunable pore size, in conjunction with the high thermal, hydrothermal, chemical, and mechanical stability of MCM-41 are highly conducive for a number of important applications such as adsorption and separation, ion exchange, catalysis, and molecular hosts. In the past few years, the scientific community has witnessed a great deal of work and rapid expansion of the activities pertaining to this versatile material. We briefly describe below some of the important applications of MCM-41. Other developing applications include the separation of bulky molecules,103 conversion of fly ash into M41S-type materials,104 membranes,62,105,106 chromatography,107-109 and electron-transfer materials,110 as well as the sorption of methane111 and of hydrogen.112 3.1. Heterogeneous Catalyst and Catalyst Support. Stringent environmental regulations, increasing public concern, and legislation governing the disposal of hazardous wastes have become a central theme of the modern chemical industry. There are increasing restrictions on the use of traditional stoichiometric and conventional homogeneous catalytic processes because of their inherent problems such as cost, separation, handling, waste disposal, etc. This has accelerated the tendency to shift toward more viable alternatives such as heterogeneous catalysis, which offers the advantages of simple separation and easy recovery, reuse, waste reduction, and elimination of hazardous chemicals, in addition to their use in both liquid- and gas-phase operations. The development of eco-friendly, environmentally compatible, and recyclable solid (heterogeneous) catalysts for the production of fine chemicals and the synthesis of building blocks for pharmaceuticals and agrochemicals is becoming an area of growing interest.

In this context, MCM-41 is considered promising for a variety of catalytic applications including photocatalysis and enzyme catalysis.11,96,100,101,113-120 Silicious MCM41’s possess a neutral framework, which limits their applications in catalysis, but they have great utility as adsorbents, molecular sieves, and supports. On the other hand, the siliceous materials can easily be modified by incorporation of heteroions into the silicate framework, thereby creating catalytically active sites.117,121-124 Brønsted (solid) acid sites are generated by isomorphous substitution of trivalent cations of boron, aluminum, gallium, and iron for tetravalent silicon in the mesoporous matrix.117,122-124 They can function as monofunctional acid catalysts. Once the acid sites are created, they can be exchanged with alkali/ alkaline-earth metal ions, thereby introducing mild basicity that will serve as catalysts for base-catalyzed reactions.125-127 Likewise, the simultaneous incorporation of trivalent aluminum and tetravalent titanium in the silicate framework opens new possibilities for producing bifunctional acid/oxide catalysts.11,96 On the other hand, the incorporation of tetravalent metal ions of titanium, vanadium, zirconium, and tin into the mesoporous matrix produces redox catalysts that are useful for selective oxidation as well as air pollution abatement.11,100,128-130 Extensions of this activity, viz., MCM-41 supported mono- and bimetallic catalysts, as well as certain catalytically active phases such as heteropoly acids, amines, transition metal complexes, metals, and metal oxides deposited novel catalyst systems, have also been realized.114,131-134 Furthermore, the use of MCM-41 for a variety of applications that include hydrogenation,114 hydrocracking,135 hydrodesulfurization135,136 hydrodenitrogenation,135 hydroxylation,130 nitric oxide reduction,133 carbon monoxide oxidation,134 and polymerization reactions131,135,136 has been established. 3.2. Heterogenization of Homogeneous Catalysts. The presence of a high density of surface hydroxyl groups37,137-141 in MCM-41 provides convenient anchoring sites on which certain homogeneous catalysts such as organic moieties, inorganic complexes, and organometallic species can be grafted or immobilized.115,121,126,127,142-144 This approach isolates the catalytically active sites at the molecular level, leading to the so-called heterogenization of homogeneous catalysts. In such a situation, the mesopores allow for the fixation of active species that also restrict the reactants and enable reactions to take place within the cavities, as depicted in Figure 3. The spatial freedom within the mesopores can be further hindered by the use of tethers (spacers)120,143 that deliberately restrict the freedom (of the reactants) in the vicinity of the active center present at the extremity of the tether. This facilitates the freedom of the catalytically active sites to flutter in the molecular breeze during the process of catalytic conversion, leading to high stereoselectivity of the products.115,121 Two approaches have generally been followed for the grafting of the MCM-41 pore walls with organic functional compounds, viz., postsynthesis and cocondensation. 3.3. Separation and Adsorption Processes. The large pore volume, pore size flexibility, and structural diversity of MCM-41 can be widely exploited for the selective adsorption of a variety of gases, liquids, and solids. Early work has shown an extremely high sorption capacity for benzene.6,8 Extensive work has been un-

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loading and encapsulation of various metals, metal oxides, semiconductor clusters, and nanowires.99,132,168-175 The encapsulation of graphite-type169 or polyaniline170,174 wires in the hexagonal channels of mesoporous MCM41 has the potential for creating a revolution in materials chemistry and advanced materials.54,99 Other guest molecules encapsulated within the mesopore channels of MCM-4 include numerous organic and inorganic polymers, photocatalysts, chemically active colloids, semiconducting nanoparticles, luminescent silicon clusters, conducting carbon wires, fullerenes, long-chain alkylthiolates, immobilized enzymes, and a variety of organic, inorganic, and organometallic compounds.69,99,113,116,172,176-178 4. Synthesis and Formation of MCM-41 Figure 3. Schematic of the immobilization of chromium binaphthyl Schiff base complex inside a mesopore of MCM-41 (Adapted from Zhou et al.143).

dertaken on sorption properties of several adsorbates, e.g., nitrogen, argon, oxygen, water, benzene, cyclopentane, toluene, and carbon tetrachloride, as well as certain lower hydrocarbons and alcohols on MCM41.6,8,137,145-161 It was also demonstrated that the adsorptive capacity of the mesoporous materials is more than an order of magnitude higher than that of conventional sorbents. MCM-41 is therefore promising as a selective adsorbent in separation techniques, e.g., high-performance liquid chromatography and supercritical fluid chromatography.107-109 Furthermore, the replacement of the surface hydroxyl groups in the pore wall with trimethylsilyl groups creates a more hydrophobic environment that substantially reduces the sorption capacity of polar molecules. Interestingly, MCM-41 offers a method for the recovery of some heavy metal ions such as mercury, lead, and silver from liquid pollutants.162-166 For example, grafting of thiol-functionalized tris-(methoxy/propoxy)-mercaptopropylsilane on MCM-41 has shown a remarkable ability to sop up heavy metal ions from wastewater, thus opening a viable route for related environmental and industrial pollution control processes. 3.4. Molecular Host. The area of quantum structures in confined environments provides several potential applications.99,167 In this regard, MCM-41 is promising as a host material because of its mesopore structure. Thus, it is considered as an ideal candidate for the

4.1. Mechanism. The originally proposed mechanistic pathways of the formation of the MCM-41 structure6,8 are illustrated in Figure 4. The existence of M41S materials having structures, viz., hexagonal (MCM-41), cubic (MCM-48), and lamellar (MCM-50), that mimic well-known liquid-crystal phases179,180 strongly suggest the proposed mechanisms. In the first, the presence of the liquid-crystal mesophase prior to the addition of the reagents, i.e., preexistence of surfactant aggregates (rodlike micelles), followed by the migration and polymerization of silicate anions, results in the formation of the MCM-41 structure. The second postulates selfassembly of the liquid-crystal-like structures as a result of the mutual interactions between the silicate anion and the surfactant cations in the solution, i.e., the silicate species generated in the reaction mixture influence the ordering of the surfactant micelles to the desired liquid-crystal phase. This was suggested by the identification of silica aggregates in the synthesis mixture using 14N NMR spectroscopy.181 Further, the formation of hexagonal, cubic, and lamellar structures through variations in the silica concentration at constant surfactant concentration is strong support for the latter mechanism.182 Huo et al.32,44 and Inagaki et al.183 carried out a more detailed analysis on the formation of these mesophases. They observed the formation of a layered silica-surfactant phase upon mixing the reactants that, with time, produced an MCM-41 structure (pathway 3 in Figure 5). Davis and Burkett23 proposed a somewhat similar mechanism (pathway 2 in Figure 5) where, depending on the reaction temperature, the

Figure 4. Possible mechanistic pathways for the formation of MCM-41: (1) liquid-crystal-phase-initiated and (2) silicate-anion-initiated (Vartuli et al.182).

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Figure 5. Proposed formation mechanistic pathways by (1) stacking of silicate surfactant rods, via the formation of an initial (2) lamellar intermediate and (3) silicate bilayer (Vartuli et al.182).

formation of MCM-41 occurs via a disordered or lamellar structure.184 4.2. General Synthesis Procedure. In a typical MCM-41 synthesis, an aqueous solution containing silica (e.g., fumed silica, sodium silicate, or tetraethyl orthosilicate as the source) is added to a clear aqueous (alkaline) solution of a micelle-forming surfactant (e.g., long-chain quaternary ammonium halides) under constant stirring. For example, low-molecular-weight amphiphiles, e.g., CnH2n+1(CH3)3N+ (n ) 8-22) or CnH2n+1C5H5N+ (n ) 12 or 16),6,185,186 which have a hydrophilic (water-soluble) headgroup and a hydrophobic (waterinsoluble) tail group, were employed as structuredirecting agents. The silica source is hydrolyzed and condensed to form multidentate, multicharged anions that can coordinate with the surfactant headgroups. The silica species and the surfactants assemble into a silicasurfactant phase, and a gel mixture is formed. The pH is then adjusted (in the range of 8-11) using a mineral acid or a base such as sodium hydroxide/tetramethyl-

ammonium hydroxide. Over the extended period of time, the silica condenses to form a siloxane framework. Alternatively, the gel is transferred into a Teflon-lined autoclave or polypropylene bottle and then heated (under autogenous pressure) in an air oven or in a steam box at 343 K for 1-3 days. After crystallization, the solid product is filtered from the mother liquor, washed with deionized water, and dried in air at room temperature or higher. This sample is designated as-synthesized MCM-41. 4.3. Alternative Synthesis Procedures. The nature and relative amounts of various ingredients used for the synthesis of MCM-41 can be varied greatly, thus offering a high degree of flexibility for the design of new materials. Depending on the actual synthesis strategies, viz., hydrothermal, microwave or ambient synthesis, the final product can typically be obtained in the temperature range 293-423 K in a few minutes to several days.6,32,44,187-189 The use of simple and elegant microwave heating methods in the synthesis is currently


gaining much importance. The novelty of the microwaveassisted preparation of MCM-41 (heating the precursor gel at 373-423 K) over the conventional hydrothermal method concerns acceleration of the process, i.e., a remarkable decrease in reaction times, as well as homogeneous heating. The addition of a small amount of ethylene glycol to the synthesis mixture during microwave-assisted synthesis improves the crystallinity, forming uniform and fine particles, ca. 0.1-0.2 µm. On the other hand, the use of different silica sources, e.g., layered kanemite9,24,34 or a fluoride-containing silica such as (NH4)2SiF6189 or H2SiF6,190 and novel surfactants, such as poly(ethylene oxide) oligomers,25,26,59 triblock copolymers,25,32,33 or glycosilicate,68 produces MCM-41-type mesophases. That is, the resultant materials exhibit typical XRD and TEM patterns characteristic of MCM-41 structure. Some examples of the latter are illustrated in Figure 6. Table 2 lists some of these structures with their synthesis conditions. In Table 3, the various physical properties are summarized. It is interesting to note that, although the newer materials and many other structures are classified under different structure types by various research groups, their diffraction patterns resemble that of the MCM-41 structure, and the only notable variation is the unit cell dimension resulting from the use of different source materials. Davis and co-workers34 reported that the formation of MCM-41 takes place from the kanemite-CTA cationexchange process at higher pH. It was proposed that the silicate species are dissolved from kanemite at higher pH in the presence of CTA cations and simply act as a silica source. This dissolution mode was further confirmed by the synthesis of ZSM-5 starting with kanemite and quaternary alkylammonium cations. On the other hand, Patarin and co-workers189 reported fluoride-ion-mediated synthesis of mesoporous silica at room temperature using CTA+ and (NH4)2SiF6 or mixtures of TMOS and NH4F. Jeong et al.190 synthesized silica-based mesoporous molecular sieves from fluorosilicon compounds in an ammonia-surfactant mixed solution. More recently, Ozin and co-workers35 reported a novel multistep nonaqueous synthesis procedure for the formation of (hexagonal) mesoporous silica using Cab-O-sil silica and glycosilicate surfactants. In the first step, a cetyltrimethylammonium glycosilicate building block, CTA2(Si2(OCH2CH2)5), is synthesized under nonaqueous conditions by solubilizing silica with sodium hydroxide in ethylene glycol in the presence of CTACl at 353 K for 5 days in a closed polypropylene container. Alternatively, CTA2(Si2(OCH2CH2)5) can also be formed by reacting Na2(Si2(OCH2CH2)5) with CTACl in ethylene glycol. In the nonaqueous medium, the glycosilicate surfactant self-assembles into a lamellar mesophase containing bilayers of CTA+ and (Si2(OCH2CH2)5)2-. The second step is simply a controlled hydrolysis of the lamellar glycosilicate phase with water at room temperature, leading to a well-ordered mesophase. 4.4. Template Removal Methods. The conventional method of surfactant removal by calcination (g773 K under a flow of N2, O2, or air) of the as-synthesized MCM-41 affects the surface area, pore size, and pore volume of the material. To avoid such shortcomings, several methods have been employed to extract the occluded surfactant molecules. The alternative template extraction methods include acid treatment,114 oxygen plasma treatment,114 liquid extraction,27,181,191,192 and

Figure 6. Transmission electron micrographs of MCM-41 prepared by various routes: (a) Alfredson et al.,293 (b) Feng et al.,33 (c) O’Brien et al.,294 (d) Feng et al.,33 (e) Jeong et al.,190 (f) Luan et al.,295 (g) Chen et al.,34 (h) Zhao et al.,25 (i) Zhao et al.,19 and (j) Huo et al.32

supercritical fluid extraction.193 In the case of solvent extraction, dry as-synthesized MCM-41 (∼1 g) was added to the extraction media, e.g., acid, ethanol, ammonium acetate, a mixture of ethanol and ammonium acetate, or a mixture of ethanol (95%, 25 mL) and hydrochloric acid (37%, 0.5 mL). It was then stirred at elevated temperatures for a few hours followed by calcination at 823 K for a few hours. For example, for a 1-g sample, a mixture of 0.5 mL of 37% hydrochloric acid and 20 mL of 95% ethanol is used to extract the surfactant. It was then calcined in air at 823 K for a few hours. On the other hand, the use of supercritical fluid extraction method results in significant savings

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Table 2. Various Mesoporous Silicas Based on 2D Hexagonal (P6mm) MCM-41 Structure research group

mesophase terminology

silica source

Beck et al.8

MCM-41

sodium silicate

Inagaki et al.9,24

FSM-16

sodium kanemite

Chen et al.34

FSM-16

sodium kanemite

Huo et al.44

SBA-3

tetraalkyl orthosilicate

Huo et al.32 Zhao et al.25

SBA-15


Feng et al.33

MMSb


Attard et al.59

HMPc

tetramethyl orthosilicate

Voegtlin et al.189

MSMd

(NH4)2SiF6

Jeong et al.190

MMSb

H2SiF6

Khushalani et al.68

HMPc

Cab-O-Sil

surfactant/template alkyltrimethylammonium halides cetyltrimethylammonium chloride cetyltrimethylammonium chloride cetyltrimethylammonium bromide + HBr poly(ethylene oxide) oligomers, C16EO10a triblock copolymer EO20PO70EO20a triblock copolymers, EO106PO70EO106 C12EO8 (octaethylene glycol monododecyl ether) cetyltrimethylammonium bromide cetyltrimethylammonium bromide glycosilicate surfactante

reaction conditions hydrothermal synthesis at 373 K for 144 h silica/surfactant/water mixture heated at 343 K with stirring; after 3 h, pH adjusted to 8.5, and mixture again heated at 343 K for 3 h silica/surfactant ratio ) 1:20 (w/w); final pH ) 11.3 (without adjustment); stirred at 353 K for 3 h silica/surfactant/acid/water mixture stirred at room temperature (RT) for g 30 min silica/surfactant/water mixture heated at 373 K for 24 h silica/surfactant/water mixture heated at 313 K for 24 h silica/surfactant/butanol/water mixture; hydrothermal synthesis at 373 K for 24 h silica/surfactant/water mixture aged for 18 h at room temperature; surfactant/water ) 50 wt % silica/surfactant/water mixture (pH ) 8) stirred at RT for 1 h silica/surfactant/water mixture (pH ) 7-8) aged at 343 K for 5 h nonaqueous synthesis: mixture aged at 353 K for 3 days

a Commercial surfactants: EO ) poly(ethylene oxide); PO ) poly(propylene oxide). b Monolithic mesoporous silica. c Hexagonal mesophase. d Mesoporous silica material. e Solubilization of silica with sodium hydroxide in ethylene glycol in the presence of CTACl.

Table 3. Physical Properties of Various Mesoporous Silicas Based on 2D Hexagonal (P6mm) MCM-41 Structure reference

mesophase terminology

d100 (nm)

a (nm)

Beck et al.8 Inagaki et al.24 Inagaki et al.31 Huo et al.44 Lukens et al.63 Feng et al.33 Attard et al.59 Voegtlin et al. 189 Jeong et al.190 Khushalani et al.68

MCM-41 FSM-16 HMMa SBA-3 SBA-15 MMSb HMPc MSMd MMSe HMPc

3.50 3.79 4.94 4.11 10.05 11.30 3.72 3.94 4.07 3.81

4.04 4.38 5.70 4.75 11.6 13.05 4.30 4.55 4.70 44.0

a

b

c

mean pore size (nm) reported SB model 3.70 2.80 3.10 7.80 7.70 2.80 2.60 3.00 -f

Hybrid mesoporous material. Monolithic mesoporous silica. Hexagonal mesophase. molecular sieves. f Data not available.

as a result of recovery of the surfactant, and the materials show more uniform pore size distribution than the calcined MCM-41.193 In fact, the supercritical-fluidextracted MCM-41 has a larger pore size (D ) 2.94 nm) than the calcined sample (D ) 2.64 nm). Recently, Antochshuk and Jaroniec194 employed an alternative procedure to remove the surfactants from the assynthesized MCM-41 using trialkylchlorosilanes. This procedure not only extracts the template molecules but also results in the formation of a highly uniform pore structure. 4.5. Product Crystallinity. Mesoporous materials usually have long-range order; however, the pore walls are amorphous. In the absence of such short-range order, they can simply be interpreted as semicrystalline solids. In general, the surfactants with 12, 14, and 16 carbon atoms, i.e., C12, C14 and C16, give good MCM-41 samples. The high-molecular-weight surfactants (gC18) are difficult to dissolve and hence rarely used. On the other hand, the lower-molecular-weight surfactants (eC10) seem to be difficult to self-organize and, hence, give less ordered materials with broader pore size distributions. However, the use of a mixture of Gemini [C20H41N(CH3)2(CH2)3N(CH3)2C20H41] and n-alkyltrimethylammonium (n ) 20) surfactants results in the formation of good-quality MCM-41 with five or more well-indexed XRD reflections.56 Highly ordered MCM41 can also be obtained with various pore diameters if micelle packing is suitably controlled with a mixture of

3.25 3.52 3.85 3.48 8.73 8.51 3.19 3.35 -f d

wall thickness (nm) reported SB model 0.34 1.58 2.60

0.79 0.86 1.85 1.27 2.87 4.54 1.11 1.20

3.80 5.35 1.50 1.95 1.60 -f

-f e

Mesoporous silica material. Mesoporous

n-alkyltrimethylammonium and n-alkyltriethylammonium surfactant cations (n ) 12-22).45 Another parameter that influences the crystallinity and quality of MCM-41 is the pH of the synthesis gel, as well as the nature of the pH adjustment. For example, the pH of the gel mixture is adjusted by adding the required amount of acid or base at once. The immediate adjustment of pH results in poorly crystallized/less ordered MCM-41.186 Nevertheless, a gradual or delayed adjustment of pH leaves more time for the micelles and silica polyionic species to assemble into a more ordered structure.186 On the other hand, Ryoo and Kim195 reported the synthesis of a highly crystallized MCM-41 by repeated adjustment of pH using the following procedure. The synthesis gel was initially heated to 370 K for 1 day. The resulting mixture was cooled to room temperature, and the pH was adjusted to 10 by dropwise addition of 30 wt % acetic acid with vigorous stirring followed by heating as before. The latter procedure was repeated twice before the solid product was removed from the reactor. Edler and White196 reported a further development of the above method by optimizing the various acids for maintaining the pH. The best result was achieved with sulfuric acid and a pH of 9-10, with sharp diffraction peaks up to seven orders. The longrange structural ordering, as well as the pore size uniformity, can also be improved by careful choice and control of synthesis conditions such as the pH of the


Figure 7. Change in XRD pattern of MCM-48 by compression: (a) under air and (b) under dry nitrogen. (i) 0, (ii) 441, (iii) 882, (iv) 1323, and (v) 1764 kg/cm2.

synthesis gel such that the polymerization equilibrium is shifted toward the mesophase region of MCM-41.195,196 4.6. Thermal and Hydrothermal Stability. For practical applications, the materials under consideration should display good thermal and hydrothermal stabilities. MCM-41 exhibits high thermal stability (∼1173 K) in dry air or in air having a low water vapor pressure.181,195,197-200 In addition, it is relatively stable under acidic conditions, whereas it degrades readily in basic environments.199 The latter is, however, expected as it is known that (amorphous) silica dissolves partially in water, especially at high pH.201 On the other hand MCM-41 has a low hydrothermal stability in water, in aqueous solutions, and even in air saturated with water vapor.25,199,202-206 Furthermore, the structure collapses by mechanical compression through the hydrolysis of siloxane bonds in the presence of adsorbed water, as is evident from the XRD patterns depicted in Figure 7207 for MCM-48. However, the hydrothermal stability can be improved by increasing the hydrophobicity, i.e., by decreasing the number of silanol groups in the framework structure. For example, synthesis in the presence of various salts204,205 or ion exchange after synthesis200 have been shown to stabilize MCM-41. In addition, postsynthesis treatment with acid,192,208,209 modification by silylation,43,206 or removal of silanol groups by alumina210 have also been shown to improve both the chemical and mechanical stabilities. Recently, Igarashi et al.211 reported increased stability of organically modified MCM-41 synthesized by a single-step procedure, in comparison to the unfunctionalized species. On the other hand, the so-called hydrothermal restructuring method is a convenient method for preparing large mesopore samples of MCM-41. It has been

Figure 8. X-ray powder diffraction patterns of calcined MCM-41 prepared using surfactants of different carbon chain length. (a) C8 TMAB, (b) C10 TMAB, (c) C12 TMAB, (d) C14 TMAB, (e) C16 TMAB, (f) C18 TMAB, (g) C20 TMAB, and (h) C22 TMAB (Lin and Mou186).

shown that pore size enlargement is accompanied by an improvement in pore size uniformity, a decrease in structural ordering, and a gradual decrease in specific surface area, as well as pore wall thickening. The restructuring of the as-synthesized material in the mother liquor212-215 or water216,217 over longer crystallization times or at higher reaction temperatures215,218,219 results in expansion in pore size from 3 to 7 nm. However, the pore size distribution of the samples becomes broader. On the other hand, with an increase of the hydrothermal reaction temperature from 373 to g438 K or with a prolonged reaction time at 443 K, MCM-41 transforms into the MCM-50 and/or ZSM-5 structures.220 The addition of certain cations, such as tetraalkylammonium or sodium ion, to the synthesis gel results in considerable improvement of the hydrothermal stability of MCM-41.221 4.7. Pore Size Modification. The generation of different pore sizes (2-5 nm) in MCM-41 can be achieved by varying of the alkyl chain length of the surfactant molecules,6,185,186,222,223 as illustrated by the XRD patterns in Figure 8. The pore size can also be controlled by employing certain auxiliary organic (expander) molecules such as trialkylbenzenes6,21,223,224 during synthesis (Figure 9). Somewhat larger diameter pores (3-5 nm) can be obtained by using large-headgroup cationic surfactants, such as alkyltriethylammonium cations, with acidic or basic synthesis conditions32,56 or by adjusting the initial pH of the synthesis mixture from 11.5 to 10.0.225 Careful control of the reaction temperature,6,218 reaction time,11 and calcination conditions6,21,213 can also produce modest pore sizes (2-3 nm). The increase of the d spacing with reaction

3246


Figure 9. Formation and swelling of surfactant micelle during synthesis of MCM-41 (Vartuli et al.182).

Figure 10. X-ray diffraction pattern of MCM-41 samples with different pore diameters after 24-, 104-, and 166-h reaction times (Corma et al.214).

time is illustrated in Figure 10. Newer methods are being developed to tune much larger mesopores with relatively narrow pore size distributions. For example, pore size enlargement (4-10 nm) can be achieved by swelling surfactant aggregates with certain other organic molecules, viz., aromatic hydrocarbons, alkanes, trialkylamines, and alkyldimethylamines,6,21,56,226-228 or

by adjusting the concentrations of surfactant and cocations, e.g., tetraalkylammonium or sodium, as well as by increasing the crystallization times.214 Other methods use high temperature (423-438 K) during direct synthesis (2-7 nm)212,213,218,229,230 or postsynthesis treatment (4-11 nm), i.e., restructuring at elevated temperatures.6,56,212,214,215,219,230 In many of the procedures, the use of expanders plays an important role and is responsible for the enlargement of the pore sizes. However, in the case of high-temperature synthesis, the swelling action is due to hexadecyldimethlyamine that is generated in situ by the decomposition of part of the surfactant cations.218 Indeed, it has been shown that hexadecyldimethlyamine forms during hydrothermal restructuring in the mother liquor and, therefore, during the expansion.212,223,227,230 Another method uses a mixture of surfactants and a (mild) hydrothermal treatment (373 K) of the product in water to produce highly crystalline large-mesopore MCM-41.56,185,222 Recently, a novel method, viz., condensation of the silicate network using commercially available alkyl poly(ethylene oxide) and polyalkylene oxide block copolymers in acid media,25,26 which is a variation of the surfactant template route, has been followed to prepare much larger-mesopore (2-30 nm) materials. The type and concentration of cosolvents, e.g., alcohols, ethers, carboxylic acids, glycols, ketones and amides, can affect the pore diameter of MCM-41,231-233 as they change the solution thermodynamics, which either alters the packing or the number of surfactant molecules in the micelle. Further, the addition of cosolvent readily dissolves the silica, leading to the formation of a homogeneous gel; nearly quantitative yields with respect to silica and surfactant can be achieved. However, it is noteworthy that a wide variety


Figure 11. Scanning electron micrographs of MCM-41 samples: (a) Lin et al.,296 (b) Ogawa et al.,106 (c)-(e) Lin et al.,252 (f) Lin et al.,253 (g) Shio et al.,237 and (h) Wang et al.187

of functionalizing groups, including silanes, tinanocene dichloride, aluminum and vanadium alkoxides, etc., grafted on the MCM-41 structure have led to a pore size reduction.6,21,115,194,234-236

4.8. Morphological Control. Knowledge of microstructural characteristics allows for an understanding of the evolution of the catalyst and catalyst support during preparation procedures and provides useful

3248


feedback for modifying the method to obtain the desired results. Different synthetic processes usually yield materials with different particle sizes and shapes, as illustrated in Figure 11. MCM-41 can be prepared in the form of spherical and rodlike powders,6,8,237,238 discoids and gyroids,239 millimeter- to micrometer-sized particles and hollow spheres,108,187,240,241 monolithic gels,242 and thin films.243,244 Ordered mesoporous films of 0.2-1.0 µm were grown with and without the use of (solid or liquid) substrates.244-247 For certain special applications such as biomolecular separations and sensors for large molecules, it would be particularly useful to have pores oriented vertically, rather than horizontally, with respect to the substrate surface. This can, however, be achieved by using a continuous-flow method248 or by the application of a strong magnetic field (11.7 T)249 during the co-assembly of the films. Dipcoating the substrates with dilute precursor sols and slow evaporation of the solvent250,251 also grow oriented films. Hollow tubules of (0.3-3 µm) MCM-41 can be prepared by careful control of the synthetic conditions.252-254 Such hierarchical order of this tubuleswithin-a-tubule organization is similar in nature to that of the frustules of marine diatoms. Huo et al.240 demonstrated the preparation of ∼1-mm spherical MCM41 based on an emulsion synthesis technique similar to that reported by Schacht et al.57 Preparation of large crystals (0.4-0.8 µm) has been achieved by using calcined small crystallite MCM-41 as the seed in a multistage synthesis method.255,256 5. Characterization of MCM-41 Reliable experimental methods are essential for characterizing tailored pore sizes in a reproducible manner. Mesoporous materials, in general, are characterized by a variety of techniques including X-ray diffraction (XRD), electron diffraction (ED), small-angle X-ray/ neutron scattering (SAXS/SANS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), gas adsorption measurements, and nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies, to name a few. The study of the melting and freezing of liquids inside the mesopores of MCM-41 has given new insight into the structure of these fluid molecules in confined media and the pore size dependency of melting- and freezing-point depression.257-260 Various techniques such as XRD, SAXS, and TEM combined with ED have proven useful for the elucidation of structure and wall thickness. On the other hand, gas adsorption measurements261 have normally been used to determine surface area, pore size, and pore size distribution (PSD). A combination of these methods has demonstrated262 that MCM-41 has a multilevel structure comprising mesopores, crystallites, and grains within the parent particles, as depicted in Figure 12a. Typically, the crystallites have sizes of 30-50 nm, while the grain diameter is around 0.3-0.8 µm. The fractal interpretation of adsorption data for various compounds has also shown the mesopore surface to be smooth on molecular scales. This is evident from the correlation of monolayer capacity with molecular area263 depicted in Figure 12b, yielding a slope of -1 on the logarithmic coordinates. Several techniques are available for estimating the pore sizes of MCM-41. These include gas adsorption, SAXS/SANS, TEM and porosimetry. Although each method has a limited length-scale of applicability for probing the structure, N2 adsorption at

Figure 12. (a) Meso- and macro-structure of MCM-41. (b) Fractal characterization of MCM-41 by the molecular tiling method.

its normal boiling point is the most popular because of its utility for both micropores and mesopores and because of its convenience and low cost. 5.1. Key Results from Gas Adsorption Methods. Quantitative evaluation of the pore structure in porous solids is a crucial aspect of the processes involved in the design and application of materials for adsorption and catalysis. The transport and reaction properties of porous solids, e.g., adsorbents and catalyst supports, are determined by their pore morphology, i.e., PSD and connectivity of the pore network. Conventionally, nitrogen sorption isotherms represent the most widely used method, often providing the input data for the analysis models for determining pore structure information.261 For MCM-41, typical sorption measurements follow the type IV isotherm, as illustrated in Figure 13, with a high porosity (0.7-1.2 cm3 g-1) and a large surface area exceeding 900 m2 g-1. At low relative pressures (P/P0 < 0.25), the formation of a monolayer of adsorbed molecules is the prevailing process. At higher pressures (P/P0 > 0.25), the adsorption in mesopores leads to multilayer formation until condensation takes place, giving a sharp increase in adsorption volume. As the mesopores are filled, the adsorption continues on the external surface. The isotherms are


MCM-41 adsorbs much greater amounts of organic species than water. For example, the amount adsorbed exceeds 0.4 g/g for cyclohexane but is less than 0.05 g g-1 for water at P/P0 ) 0.4,181 which reveals that the internal surface is quite hydrophobic albeit with the presence of silanol groups. On the other hand, trimethylsilation renders the materials repellent even to liquid water.6,21 Surface Area. The commonly used BET surface area (SBET) represents the total surface area rather than the mesopore area. Recent studies 264 have indicated that the BET method overestimates the surface area of MCM-41, possibly as a result of the considerable overlap of monolayer and multilayer adsorption in the recommended range of relative pressures (P/P0 ) 0.05-0.35). Estimates of the primary (mesopore) surface area (SP) were obtained here using the following equation,264 with the surface area and mesopore radius (r) calculated from geometrical considerations and with MCM-41 considered to be made of infinitely long cylindrical pores arranged on a two-dimensional hexagonal lattice

SP ) Figure 13. Nitrogen adsorption isotherms at 77.4 K on various MCM-41 samples.

usually reversible for pores smaller than a critical size and exhibit a sharp inflection at P/P0 ) 0.25-0.5, depending on the pore size of the material. Qualitatively, the step in the isotherms reflects a narrow and uniform distribution of the pore size, while its height indicates the pore volume. The desorption occurs via evaporation of the adsorbate from mesopores and usually takes place at a pressure lower than that of capillary condensation, resulting in hysteresis. For disordered samples, the step of the isotherm becomes less sharp for the samples with the largest pores, suggesting a widening of the pore size distribution. The hysteresis is, in general, attributed to the different sizes of the pore mouths and pore bodies or to the different adsorption and desorption behaviors in near-cylindrical pores. Materials with uniform pore sizes and shapes exhibit type H1 hysteresis, whereas those with nonuniform pore sizes and shapes give type H2 hysteresis. In the latter case, condensation takes place in each section at the relative pressure provided by the Kelvin equation, but evaporation from the pore body cannot occur while the pore mouth remains filled. On the other hand, in the former case, the meniscus is cylindrical during condensation and hemispherical during evaporation. Gas adsorption isotherms for a wide variety of gases, including nitrogen, oxygen, argon, water, etc., have been measured for MCM-41. It is interesting to note that

2VP ) r F

4πr

SiO2[x3(2r

(1)

+ W)2 - 2πr2]

where VP is the primary mesopore volume, FSiO2 is the skeletal density, and W is the wall thickness. The value of W was taken as 1 nm for the constant wall thickness (CWT) approach, whereas for the variable wall thickness (VWT) approach,152 it was estimated using

W)

2 dXRD - 1.2125dXRD x3

x

FSiO2VP

1 + FSiO2VP

(2)

where dXRD is the lattice constant obtained from the XRD measurements. This geometrical approach is preferable to some of the prevailing alternatives based on conventional adsorption models (Dubinin-Astakov, Saito-Foley, and BJH), in view of their observed inaccuracies in describing adsorption in MCM-41.265 Table 4 presents the results of the surface area estimations, indicating that the values obtained by the CWT approach are lower than those estimated by the BET technique, whereas those from the VWT approach with the wall thickness estimated from the primary mesopore volume are closer. The surface area obtained by Hg porosimetry is much lower than that found by the BET method (Table 4). Because the contact angle of Hg with most solids is large, it penetrates the pores only when forced under pressure. However, working under pressure requires some care because it can cause breakage of pore walls, as is evident from the nitrogen isotherms on MCM-41 compressed to different pressures depicted in Figure 14a.266 For Hg porosimetry, it has been shown

Table 4. Surface Area (m2/g) Estimated by Various Techniques sample code

N2

O2

SBET (at 77 K) Ar

Ara

CO2b

SHg

X-ray

C8 C10 C12 C14 C16 C18

937 1318 1280 1162 1240 1123

1063 1150 1163 888 909 1043

1100 973 1027 1015 895 920

873 965 963 954 1125 942

946 868 1123 1018 1013 1338

19 143 194 299 365 295

939 1665 1835 2579 2599 2745

a

At 87 K. b At 195 K.

SSAS neutron 1333 2616 2588 3941

SWT constant

variable

645 648 656 606 615 607

661 1024 1016 933 986 902

3250

Ind. Eng. Chem. Res., Vol. 40, No. 15, 2001 Table 5. Structural and Adsorption Data of Various MCM-41 Samples Prepared Using Surfactants with Different Carbon Chain Lengths surface area (m2 g-1) sample structural data (nm) VP 3 -1 code d100 a (cm g ) SBET STOT SEx SP C8 C10 C12 C14 C16 C18

Figure 14. (a) Nitrogen adsorption isotherms on MCM-41 compressed at different mechanical pressures (i) calcined, (ii) 86, (iii) 224, and (iv) 1200 MPa. (b) Mercury intrusion porosimetry along with the percentage crystallinity left after compression.

that the crystallinity of MCM-41 starts to decrease considerably at about 18 Mpa, as is evident in Figure 14b.262 Hence, the data obtained using Hg porosimetry is highly unreliable for MCM-41. Surface areas have also been estimated265 by SAXS and SANS using the well-known Debye equation

area )

4 × 104φsφp Rdap

(3)

where φs and φp represent the solid and pore volume fractions, respectively, while R and dap represent the correlation length and apparent density, respectively. As seen in Table 4, the values are much higher than those from gas adsorption methods. However, estimates of the surface area of C8 from the Debye equation are close to those from gas adsorption, indicating surface randomness, which is consistent with HRTEM.265 This is not the case for the C10-C18 samples, for which the earlier suggestions267 of a geometrically heterogeneous bilayered surface have been modified268 to indicate a smooth surface. This is consistent with our own fractal characterization263 using molecular tiling. Consequently,

2.89 2.92 3.12 3.47 3.79 4.33

3.34 3.37 3.60 4.01 4.37 5.00

0.36 0.70 0.75 0.79 0.93 0.98

937 1318 1280 1162 1240 1123

691 51 640 1087 61 1026 1100 51 1049 1029 81 948 1117 104 1013 1064 125 939

further studies overcoming the structural limitations of the Debye equation are necessary to obtain an unequivocal model of the microstructure for the C10-C18 samples using SAS data. A further feature is that SAXS- and SANS-based characterizations also include the closed pore space inaccessible to adsorptive molecules. Consequently, a comparison of the results with those from adsorption can also yield information on the closed porosity. Pore Volume. The BET model still remains one of the most widely used methods for estimating the pore volume from adsorption measurements. The density of adsorbed N2 has been assumed to be equal to the density of liquid N2 at the same temperature.261 However, the t-plot method is often used to obtain accurate and reliable pore volume data. The method is based on the observation that a plot of the adsorbed volume per unit surface area, i.e., the statistical thickness (t) of the adsorbed layer, versus pressure follows a single curve independent of the solid. For mesoporous materials such as MCM-41, an upward deviation from linearity is observed on the Vads vs t plot, corresponding to capillary condensation. The slope of the straight line is directly proportional to the total surface area (ST). After complete filling of the mesopores, a new straight line with a slope corresponding to the external surface area (SEx) is observed, with the mesopore volume (VP) corresponding to the intercept of the line with ordinate. The success of the method, however, depends on the choice of the reference isotherm, i.e., the isotherm used to determine the dependence of the t on P/P0. The best choice of reference isotherm would be to obtain data for a solid having structural properties similar to those of the substance under study. Alternatively, the RS-plot method, which is a variant of the t-plot method, can be employed to obtain both the surface area and the pore volume of MCM-41. This uses the quantity RS, which is the ratio between the adsorbed volume and the adsorbed volume at P/P0 ) 0.4. The surface area can then be calculated directly from the surface area of the reference material. The mesopore volumes (VP) and surface areas (SP) of six different samples of MCM41 whose nitrogen isotherms are given in Figure 13 were determined using different methods, and the results are summarized in Table 5. Pore Size. Several geometrical and classical methods have been applied to the determination of the mesopore diameter (D) of MCM-41. In the case of mesoporous and macroporous solids, the Kelvin equation provides a useful model for the transformation of adsorption data into a PSD.269,270 This model is widely applied but is limited to pore diameters greater than 2 nm. Below this pore size, the liquid cannot be considered a fluid with bulk properties because of the forces exerted by the pore wall. Theoretical calculations suggest that the properties of fluids in microporous structures are highly

Ind. Eng. Chem. Res., Vol. 40, No. 15, 2001 3251 Table 6. Pore Diameters Determined by Geometrical and Gurvitsch Methods sample code

DCWT

C8 C10 C12 C14 C16 C18

2.34 2.37 2.60 3.01 3.37 4.00

mesopore diameter (nm) D[VP + XRD] D[4VP/SBET] 2.31 2.73 2.96 3.38 3.78 4.36

1.54 2.12 2.34 2.72 3.00 3.49

D[4VP/SP] 2.25 2.73 2.86 3.33 3.67 4.18

d(∆G) dA ) µ a - µg + γ )0 dN dN

sensitive to the size of the pores. The established methods for the determination of the pore size are based on the Kelvin model or its variants, such as the Barrett-Joyner-Halenda (BJH)271 and Broekhoff and de Boer (BdB)272 methods, or other approaches such as the Horva´th-Kawazoe (HK)273 and Saito-Foley (SF)274 methods and the density function theory (DFT) of Tarazona et al.275 that consider the fluid-solid interaction. The pore diameter of MCM-41 can be most conveniently determined from the lattice dimensions and the mesopore volume following eq 2. This method considers the pore shape to be cylindrical. In the case of a hexagonal pore geometry, the pore diameter is multiplied by a factor 0.95 to take into account the hexagonal geometry of the mesopore. The results of pore diameters calculated by this method are presented in the second column of Table 6 for all six samples. The pore diameter of MCM-41 can also be estimated from the mesopore volume and the measured surface area following the Gurvitsch approach, which is based on the relation: D4V/S ) 4VP/S, where VP is the mesopore volume and S is the specific surface area. Table 6 shows the results of the pore diameter values of the various samples when the BET surface area, SBET, and mesopore area, SP, are used for S. Clearly, the latter agrees with the results of the geometric method, attesting to the fact that the BET method overpredicts the mesopore area. 5.2. Pore Size Distribution. The pore size distribution of MCM-41 is another important characteristic that has received significant attention. In numerous applications, such as the use of MCM-41 as a host for encapsulation of nanowires and molecular clusters, it is important that the pore sizes be relatively uniform. Accordingly, several studies have been conducted to determine pore size distributions of MCM-41 materials using the adsorption of nitrogen or other adsorptives such as argon.35,63,157,264,276-278 These methods utilize the generalized adsorption integral (GAI)

Ca(P) )

∫0∞ F(r,P) f(r) dr

In addition to density functional theory, other methods utilized for the local isotherm at any pore size appear to be modifications of the classical BdB model. In this method, the multilayer isotherm is obtained using the equilibrium condition

(4)

with a suitable choice of the local isotherm F(r,P) to fit experimental isotherm data for measured values of Ca(P) in order to determine the unknown pore size distribution f(P). Although regularization techniques are often used for the inversion of the integral,276,277,279 because of the ill-posed nature of the problem, the classical BJH method of solution has also been adopted by other workers.63,264 The most rigorous form of the local isotherm is obtained from density functional theory,157,276 although, as will be discussed subsequently, there is a weakness in the method because of its inability to correctly predict the critical size below which hysteresis disappears.

(5)

in which ∆G is the free energy change and N is the adsorbed amount, while µa and µg are the adsorbate and gas-phase chemical potentials, respectively. Further, the last term represents the surface free energy contribution from the adsorbate-gas interface of area A. In the original BdB model, the adsorbate chemical potential in the mesopores is taken to be independent of the curvature and based on the isotherm on a flat surface, leading to

µa - µg ) RT ln

( )

P0 - f(t) P

(6)

which combines with eq 5 to provide the multilayer isotherm for a cylindrical pore

( )

γvl P0 d(∆G) ) RT ln - f(t) )0 dN P (r - t)

(7)

Here, t is the thickness, and it is assumed that this multilayer has a liquidlike density (i.e., molar volume vl) and surface tension (γ). On the adsorption branch, the onset of capillary condensation is obtained by considering the instability of the multilayer following

d2(∆G) dN2

)0

(8)

which provides

df dt

|

)-

t)tc

γvl (r - tc)2

(9)

where tc is the critical thickness at which capillary condensation occurs. The overall adsorption isotherm is then obtained by considering the different portions of the pore size distribution in the multilayer and capillary condensation regions, assuming liquid-like densities for the adsorbate. Most of the various modifications introduced to this model have been aimed at improving eq 7 and/or eq 8, which suffer from the deficiency that the effect of curvature on the adsorbate chemical potential is neglected. To this end, Bhatia and Sonwane280 and Sonwane and Bhatia281 related f(t) to the incremental fluidsolid interaction potential φ˜ (t,r) at the multilayer surface and also considered the effect of curvature on the surface tension to obtain

RT ln

( )

vlγ∞(r - t) P0 + φ˜ (t,r) ) P (r - t - λ/2)2

(10)

for the multilayer isotherm, where γ∞ is the surface tension at a flat interface and

|

∂φ˜ ∂t

t)tc

)

vlγ∞(r - tc + λ/2) (r - tc - λ/2)3

(11)

3252


Figure 15. Pore size distributions of MCM-41 samples by regularization using nitrogen adsorption data at 77.4 K. Model, s; XRD peak position, - - -.

for its stability boundary. Here, λ is the interlayer spacing, and the fluid-solid interaction potential is based on Lennard-Jones centers in the fluid and solid phases. For low pressures where a multilayer does not exist, Sonwane and Bhatia used a heterogeneous Unilan model, following their approach282 for constructing hybrid models applicable over a wide pressure range. Figure 15 depicts the pore size distributions obtained by Sonwane and Bhatia277 for their various MCM-41 samples, based on the nitrogen adsorption isotherms given in Figure 13. As is evident from the figure, the distributions are quite narrow, with a spread of only about 4 Å for the C18 sample. With decreasing surfactant chain length this spread increases, reaching about 11 Å for the C10 sample, which is consistent with the observation of increased disorder with decreasing chain length.262,263 Such a consistent pattern in the pore size distribution is, however, not obtained from an analysis of argon isotherms.35,276,277 Other modifications of the BdB model have also been proposed for the characterization of materials. Among these, Zhu et al.278 used the empirical formulation

µa - µg ) RT ln

( )

C2 C3 P0 + C1 + n - 2 P r t

Figure 16. Pore size distributions of sample AM-1 of Neimark et al.157 determined by various models.

used the classical Cohan relation for a cylindrical meniscus261

r ) tc +

RT ln(P0/Pc)

(13)

which, however, is not consistent with the stability criterion obtained from the application of eq 8 to this model. Indeed, when the stability criterion of the multilayer is applied using the constants specified by Zhu et al., values of (P/P0) for condensation exceed unity at large pore sizes,277 indicating the thermodynamic inconsistency of this model. Another model is that of Lukens et al.,63 who utilized eqs 7 and 9 with the simplified Hill’s approximation261

f(t) )

R t3

(14)

for the adsorption on a nonporous substrate. Here, R is a constant. This method suffers from the same drawback as the original BdB model in that the effect of curvature on the adsorbate chemical potential is neglected. Finally, Kruk et al.35,264 applied an empirically adjusted form of the Kelvin equation

(12)

in which C1, C2, C3, and n are constants obtained by fitting the multilayer region of the isotherm. For the onset of capillary condensation (at pressure PC), they

γvl

r)

2γ∞vl RT ln(P0/P)

+t+C

(15)

where C is a constant, taking the values of 3 Å for nitrogen and 4.38 Å for argon. The values of C are

Table 7. Pore Diameters (nm) Derived Using Various Models Based on the Modified Kelvin Method BJH sample code

SFa

C8 C10 C12 C14 C16 C18

1.90 2.51 2.81 3.50 4.09 5.07

a

HKb

original BJH

Naona et al. (1997)

1.65 2.19 2.45 3.06 3.57 4.36

1.21 1.47 1.70 2.13 2.42 2.97

1.61 1.90 2.11 2.61 2.98 3.42

BdB KJS

original BdB

Zhu et al. (1998)

Lukens et al. (1999)

SBc

2.30 2.76 2.94 3.37 3.78 4.26

2.15 2.22 2.40 2.80 3.20 3.75

1.68 2.12 2.32 2.68 3.02 3.40

1.69 2.12 2.29 2.68 3.05 3.47

2.55 2.80 3.00 3.36 3.75 4.25

Saito-Foley method. b Horvath-Kawazoe method. c Sonwane-Bhatia model. For details, see section 5.


Figure 17. Pore size distributions of various mesoporous materials. Curves with solid lines represent results of SB model, whereas those with dashed lines represent the reported PSD based on a BJH or HK analysis. The solid vertical line (when reported) represents the XRD diameter.

obtained from the observation that the usual Kelvin equation, when applied to the condensation pressure, underpredicts the pore radius compared to the corresponding XRD values by these amounts. However, at this time, it is not clear whether the Kelvin equation, representing equilibrium on a hemispherical meniscus, is applicable to the adsorption branch. Indeed, Ravikovitch et al.276 and Neimark et al.157 used their DFTbased equilibrium branch on the desorption branch for the several samples that they analyzed. Sonwane and Bhatia277 compared the various approaches discussed above using the nitrogen isotherms of Neimark et al.157 for their sample AM1. Figure 16 depicts the pore size distributions obtained using the different methods, along with the XRD pore diameter.

From these results, it is evident that their method and the NLDFT of Neimark et al. match the XRD result the closest. Considerable deviation of the modal pore size is seen for the models of Zhu et al.278 and Lukens et al.63 and for the original BdB method. The result from the method of Kruk et al. (KJS)35,264 is also close, with an error of about 2 Å. However, for their own samples, Kruk et al. reported even smaller deviations, indicating some sample-to-sample variations for MCM-41 necessitating modification of the value of the parameter C. Such refinements are, of course, also suggested for solidphase LJ parameters in the DFT and Sonwane-Bhatia approaches for more accurate results for different samples. In addition, surface hydroxyls and contamination due to remaining template can also influence the

3254


Figure 18. Adsorption isotherms of nitrogen in MCM-41 samples of different pore sizes. Closed symbols, adsorption; open symbols, desorption.

adsorption, with sample-to-sample variations that are not considered in these approaches. Another comparison of the various approaches based on the Kelvin equation and its modifications is provided in Table 7, where pore sizes have been estimated from the points of inflection of the nitrogen isotherms (assumed as the points of capillary condensation) in Figure 13. It is evident that only the KJS and Sonwane-Bhatia models yield satisfactory results for the pore diameter that compare with the XRD-based values listed in Table 4. Included in the table are also results based on the method of Naona et al.,156 a modification of the BJH technique, indicating large errors. In the above results in Figures 15 and 16 and Table 7, the adsorption isotherms were used for all of the methods, except for the NLDFT for sample AM1 of Neimark et al.157 in Figure 16, where the equilibrium branch was used with the desorption isotherm. As indicated earlier, hysteresis is often found for adsorption in MCM-41 for pore sizes larger than a critical value. For nitrogen at 77.4 K, this corresponds to a pore diameter of about 38 Å. Below this critical pore size, the isotherms are reversible, as will be discussed in the next section. In their recent work, Ravikovitch and Neimark283 also analyzed the adsorption branch, based on the spinodal condensation branch of their NLDFT isotherm. It can be noted that this spinodal condensation is esentially the microscopic instability of the multilayer, represented by eq 11 of the SonwaneBhatia model. However, Ravikovitch and Neimark find that, for pores below the critical size for hysteresis, the reversible isotherm is better modeled by the equilibrium curve of the NLDFT rather than the spinodal condensation curve. From a theoretical point of view, there is therefore a controversy about which is the experimental equilibrium branch, adsorption or desorption, for pores larger than 38 Å. Nevertheless, for pores smaller than this value, consistent pore size distributions are obtained for all three methods, i.e., KJS, SonwaneBhatia, and NLDFT. In addition to MCM-41, Sonwane and Bhatia277 have also applied their approach to analyses of published isotherms for a variety of other mesoporous materials among those referred to in Tables 1 and 3. Figure 17

presents their results for the pore size distributions of various mesoporous materials, along with the result based on the BJH method or the XRD peak where reported. The results clearly indicate that the conventional method is in error and that the actual pore sizes are significantly larger. This is also reflected in Tables 1 and 3, which provide the mean pore diameters evaluated from the point of inflection of the isotherm using the Sonwane-Bhatia model, as well as the reported diameter based on conventional methods. As indicated in Table 3, the error of the classical methods also leads to a larger estimated wall thickness. 5.3. Adsorption Hysteresis. One of the unexpected observations from adsorption studies in MCM-41 is that of reversibility of the isotherms below a critical pore size. Figure 18 depicts the normalized isotherms of nitrogen in MCM-41 samples of various pore diameters281 illustrating this feature, with hysteresis being absent for the mesopores of C16 MCM-41 and smaller-pore-size materials and present only for the C18 material of about 42 Å diameter. On the other hand, for oxygen, hysteresis is found265 for C14 and larger-pore MCM-41’s, indicating a critical pore diameter for appearance of hysteresis of about 30 Å. Thus, the critical pore diameter (DCH) below which reversibility is observed depends on the adsorbate. In addition to this critical size, there also exists a critical pore size (DCP) below which capillary condensation is absent, as is evident from Figure 13, in which the capillary condensation rise becomes less significant with a reduction in pore size. For nitrogen, this pore size is slightly less than 25 Å, the diameter of C8 MCM41. Morishige and Shikimi284 have analyzed a great deal of data for various adsorbates and presented correlations of the form

(TC - T) d )a TC D

(16)

relating the temperature to the critical pore diameter (D, representing DCP or DCH) and molecular diameter (d). Here, a is an empirical constant having the value of 4 for criticality of hysteresis, whereas a ) 2 for criticality of condensation. Figure 19 presents a compilation of data285 for critical sizes at various temperatures for different adsorbates, along with these correlations. It can be noted that the form of the correlation for the condensation critical size is suggested by the density functional theory study of Evans et al.286 The disappearance of hysteresis for pore sizes smaller than the critical value DCH is rather intriguing as it is not anticipated in classical capillary condensation theory.261 According to this theory, capillary condensation occurs on the cylindrical meniscus of the adsorbed multilayer, whereas desorption occurs from a hemispherical meniscus of the condensed pore fluid, so that hysteresis is always expected. Density functional theory157,276,287 is unable to predict this disappearance and yields a single critical value of about 19 Å for the pore diameter for nitrogen at 77.4 K, corresponding to the critical size below which abrupt condensation is absent. On the other hand, Maddox et al.,288 using a simulation approach, found that the introduction of surface heterogeneity into the fluid-solid potential permits reversible condensation of nitrogen in MCM41 below a critical pore diameter of about 28-30 Å, somewhat lower than the experimental value of about 38 Å. A somewhat different approach was taken by


Figure 19. Variation of hysteresis (TCH) or pore critical (TCP) temperature with pore diameter (D). TC and d are the bulk critical temperature and molecular diameter of the adsorbate, respectively. Predictions: 1, Evans et al.;286 2, empirical correlation of Morishige and Shikimi;284 3-6, Sonwane and Bhatia.285

Sonwane and Bhatia,285 who found the classical tensile stress hypothesis,289,290 when combined with their multilayer isotherm for adsorption and condensation or the model of evaporation from a hemispherical meniscus, to yield values of DCP ) 24 Å and DCH ) 34 Å, in good agreement with experiment. Thus, for condensation, eqs 10 and 11 are combined with the Young-Laplace equation at the cylindrical gas-multilayer interface

P + τ0 g

γ∞ (R - t - λ/2)

(17)

where the minimum fluid pressure (-τ0) is given by the tensile stress obtained from the metastable region. A similar approach can be adopted for the hemispherical meniscus during desorption. Figure 19 also depicts the results of their calculations based on this approach, demonstrating the correct trends with respect to the effect of temperature. Nevertheless, unequivocal resolution of this issue can only be obtained from microscopic theories such as DFT, and definitive developments along these lines are awaited. 6. Conclusion It is clear that a vast field of new and exciting periodic mesoporous MCM-41 materials is rapidly emerging from the world of the analogous microporous counterparts. Diverse applications of these ordered open-framework materials continue to stimulate interest in various disciplines of science, engineering, and technology, but the emphasis will continue to be on their use in adsorption, catalysis, separation, and ion-exchange processes. However, various other potential applications in microelectronics, sensors, and molecular wires are currently being exploited and will continue to grow, especially with the development of materials with new compositions, structures, and surface modifications. We can look forward to a future in which the materials and their uses become far more diverse. Despite the large amount of work done on various aspects spanning synthesis, characterization, and applications, relatively

little attention has been paid to the identification of high-quality materials by means of various physicochemical techniques. This is likely to be one of the most important aspects in choosing materials for various applications. The combined use of various experimental techniques, such as XRD, TEM, and SAS, with gas adsorption measurements along with the proper choice of adsorption model will provide a better understanding of these unique materials. From a fundamental standpoint, the unconnected parallel mesopores of MCM-41 provide ideal “test tubes” for the development and validation of models of adsorption in mesopores. Several models have been developed that are extensions of the classical Kelvin equation and the Broekhof-de Boer analysis of surface layering and capillary condensation. However, many of these approaches suffer from the same weakness that the adsorbate chemical potential is assumed to be curvatureindependent. The most successful models appear to be one based on a semiempirical modification of the Kelvin equation,35,264 a multilayer approach incorporating the fluid-solid interaction potential,277,280,281 and the density functional theory approach.157,276,283,287 All of these models produce pore size distributions that agree with each other and match the XRD-determined pore size. However, some methods consider the adsorption branch as an equilibrium branch, others as a branch of metastability, while still others consider the desorption branch as the equilibrium branch. Although each method is successful, primarily because of appropriate choice of model parameters, further work is needed to unequivocally determine the mechanisms operating on the adsorption and desorption branches. Another concern is that of the critical pore size below which the adsorption is reversible. Presently, only the Sonwane-Bhatia approach, combining the tensile stress hypothesis with their multilayer model, appears to be successful in correctly predicting this size. Further development of microscopic models such as DFT is needed to unequivocally resolve this issue. Literature Cited (1) Barton, T. J.; Bull, L. M.; Klemperer, W. G.; Loy, D. A.; McEnaney, B.; Misono, M.; Monson, P. A.; Pez, G.; Scherer, G. W.; Vartuli, J. C.; Yaghi, O. M. Tailored Porous Materials. Chem. Mater. 1999, 11, 2633. (2) Ma, Y.; Tong, W.; Zhou, H.; Suib, L. A Review of Zeolitelike Porous Materials. Microporous Mesoporous Mater. 2000, 37, 243. (3) Cheetham, A. K.; Fe´rey, G.; Loiseau, T. Open-Framework Inorganic Materials. Angew. Chem., Int. Ed. Engl. 1999, 38, 3269. (4) Behrens, P. Mesoporous Inorganic Solids. Adv. Mater. 1993, 5, 127. (5) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Evertt, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D.; Sing, K. S. W.; Unger, K. K. Recommendations for the Characterization of Porous Solids. Pure Appl. Chem. 1994, 66, 1739. (6) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesised by a Liquid-Crystal Template Mechanism. Nature 1992, 359, 710. (7) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. The Preparation of Alkyltrimethylammonium-Kanemite Complexes and Their Conversion to Mesoporous Materials. Bull. Chem. Soc. Jpn. 1990, 63, 988. (8) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10835.

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Received for review December 11, 2000 Revised manuscript received March 29, 2001 Accepted April 11, 2001 IE0010666

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