Hybrid Mesoporous Materials withFunctionalized Monolayers**

By Jun Liu,* Xiangdong Feng, Glen E. Fryxell, Li-Qiong Wang,Anthony Y. Kim, and Meiling Gong

Mesoporous materials have great potential for environmental and industrial processes, but many applications require the ma-terials to exhibit specific surface chemistry and binding sites. A new approach has been developed so that organized func-tional monolayers are covalently bound to mesoporous supports. The functionalized hybrid materials show exceptional selec-tivity and capacity for removing heavy metals from waste streams. Tailored hybrid materials have also shown potential toselectively bind anions and radionuclides. Rational design of the surface properties of mesoporous materials will lead to moresophisticated functional composites.

1. Introduction

In 1992, scientists at Mobil Oil Research successfullysynthesized ordered mesoporous materials using surfactantmicellar structures as templates.[1] These materials have veryhigh surface area (> 1000 m2/g), ordered pore structure(mostly hexagonal packed cylindrical pore channels), andextremely narrow pore size distribution. The pore diametercan be adjusted from 2 to 15 nm. The preparation methodsinvolve mixing ceramic precursors (such as sodium alumi-nate, tetramethyl ammonium silicate, and silica) in a surfac-tant (cetyltrimethylammonium chloride, CTAC, or cetyltri-methylammonium bromide, CTAB) solution and reactingthe agents at temperatures below 150 �C. In principle, surfac-tants form ordered micellar phases. The most common phaseconsists of rod-like micelles packed in hexagonal arrays. Theceramics precursors bind to the head groups of the surfac-tant molecules, and finally condense together, forming acontinuous ceramic phase. Subsequently, the surfactant mol-ecules can be removed by thermal or chemical treatment.

Since 1992, mesoporous materials research has become avery active area because of the great potential for applica-tions in environmental and industrial processes. Numerouspapers have been published on the preparation of mesopor-ous materials of novel chemical compositions and on the fun-damental understanding of the reaction processes.[2] A widerange of mesoporous materials have been prepared, includingalumina, zirconia, titania, niobia, tantalum oxide, and manga-

nese oxide. With a few exceptions (for example, manganeseoxide[3]), the pore structure of non-silica-based mesoporousmaterials is not as well-defined as silica-based materials, andis not stable at elevated temperatures. Mesoporous silica hasalso been doped with elements possessing catalytic proper-ties[4] and with conducting polymers.[5] Recently, progress hasbeen made in the fabrication of oriented mesoporous films onvarious substrates,[6] in making free-standing films,[7]

spheres,[8] and single-crystalline mesoporous materials inwhich all the pore channels are aligned.[9]

Although the potential of mesoporous materials has beenwidely recognized, progress on the practical use of thesenovel materials has been slow. Many applications, such asadsorption, ion exchange, catalysis, and sensing, require thematerials to have specific binding sites, stereochemical con-figuration or charge density, and acidity.[10] Most mesopor-ous materials do not themselves have the appropriate sur-face properties. For example, mercury and heavy-metalcontamination is a serious problem at waste-contaminatedsites of the Department of Energy.[11] Industrial and civiliansources also deposit a large amount of mercury into the envi-ronment every year.[12] It is necessary to develop a methodto systematically modify the surface chemistry and tailor themolecular recognition process of mesoporous materials to-ward these targets.

2. Approach

Recently, a class of hybrid mesoporous materials havebeen developed, based on organized monolayers of func-tional molecules covalently bound to the mesoporous sup-port.[13] The functional molecules are attached to the meso-porous support similarly to the preparation of self-assembled monolayers (SAMs) on flat substrates (Fig. 1).This approach provides a unique opportunity to rationallyengineer the surface properties. The hybrid mesoporous ma-

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[*] Dr. J. Liu, Dr. X. Feng, Dr. G. E. Fryxell, Dr. L.-Q. Wang,A. Y. Kim, Dr. M. GongPacific Northwest National LaboratoryPO Box 999, Richland, WA 99352 (USA)

[**] Pacific Northwest National Laboratory is operated by Battelle Memori-al Institute for the US Department of Energy under Contract DE-AC06-76RL01830. The EXAFS study was conducted by Dr. K. Kemnerfrom Argonne National Laboratory.

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terials demonstrate exceptional selectivity and capacity foradsorbing heavy metal ions from contaminated wastestreams. Materials capable of recognizing other species andmolecules are also under development.

SAMs have been widely explored for engineering the sur-face and interfacial properties of materials, such as wetting,adhesion, and friction.[14] These monolayers are also used tomediate the molecular recognition processes and to directoriented crystal growth.[15] On crystalline surfaces that willnot chemically react with the SAMs (alkyl thiols on gold),the overlayer structure is influenced by the substrate sym-metry. These kinds of SAMs have been used as model sys-tems to study surface properties and conformations of mol-ecules in other SAMs.[14] On oxide surfaces, such as silica,the packing and ordering are defined by the intermolecularinteractions.[15,16] In this approach, bifunctional moleculescontaining a hydrophilic head group and a hydrophobic tailgroup adsorb onto a substrate as closely packed monolayers.The tail group and the head group can be chemically modi-fied to contain specific functional groups. The hydrocarbontails provide the driving force (van der Waals interaction)for the self-assembly of the molecules into close packed ar-rays on the substrate. For example, the chlorosilane end ofthe molecule can be covalently bonded to the oxide surfaceand cross-linked to adjacent silanes through hydrolysis andcondensation reactions.

In the hybrid materials, short alkyl chains are used. For ex-ample, the van der Waals forces between the alkyl chainsplay a less important role, and the main driving force for themolecules to anchor to the substrate is chemical bondingthrough condensation reactions. Compared with SAMs onflat substrates, it is more difficult to attach an organic mono-layer to mesoporous supports. The functional moleculesmust be able to access the interior surface of the pore chan-nels (a few nanometers wide), and must not prematurely hy-drolyze and condense with themselves. The quality of thefunctional monolayers on the mesoporous materials isgreatly affected by the population of silanol groups and ad-

sorbed water molecules on the mesoporous silica sur-face.[17,18] The silanols are needed to anchor the organic mol-ecules to the silica surface, and physically adsorbed water isrequired for the hydrolysis of siloxanes. However, excessfree water from capillary action is also detrimental to the ef-ficient formation of a clean monolayer, due to polymeriza-tion in the solution.

The mesoporous silica that we used had been calcined at550 �C, which removed most of the hydroxyl groups on thesurface. The initial strategy was to rehydrate the mesoporoussilica surface. This process involved boiling a weighed sam-ple of mesoporous silica in pure water for several hours, col-lecting the silica by filtration, weighing it again, and remov-ing the surplus water content via azeotropic distillation withtoluene. This method, although successful in the depositionof high-quality monolayers up to 75 % surface coverage, wastime-consuming and laborious. Recently, we have developeda more efficient approach by wetting the silica surface with 2to 2.5 monolayers of water (based on available surface area).Experimentally, this approach is accomplished by adding therequisite amount of water to a suspension of mesoporous sil-ica in toluene and stirring the mixture for an hour to allowcomplete dispersal of the aqueous phase across the ceramicinterface. When the mesoporous ceramic interface is prop-erly hydrated, construction of the monolayer is accom-plished by adding one equivalent (or a slight excess) of thedesired alkoxysilane (based on available surface area), stir-ring the mixture, and heating it in toluene reflux for severalhours. Currently, we can systematically vary the populationdensities of functional groups on the mesoporous materialsfrom 10 % up to 100 % of the full surface coverage.

3. Characterization and Application of HybridMesoporous Materials

The initial applications involve alkylthiols [tris(meth-oxy)mercaptopropylsilane, TMMPS] as the functional mol-ecules. TMMPS was selected because it has been previouslyused to make functional monolayers,[14] and the thiol groupshave a high affinity for binding heavy metals. The thiol±silicahybrid mesoporous materials thus produced can efficientlyremove mercury and other heavy metals (such as lead andsilver) from contaminated aqueous and organic solutions.The distribution coefficient, Kd, has been measured to be ashigh as 108. (Kd is defined as the amount of adsorbed metal[mg] on 1 g of adsorbing material, divided by metal concen-tration [mg/mL] remaining in the treated waste stream.)

The pore structure and chemical composition of the hy-brid mesoporous materials can be studied by transmissionelectron microscopy (TEM), low angle X-ray diffraction(XRD), and the Brunauer±Emmett±Teller (BET) tech-niques. TEM studies suggest that the hexagonal structuresremain the same after the functional molecules have beenattached and the metal ions absorbed. BET shows that themonolayers inside the pore channels reduced the pore diam-

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Fig. 1. Preparation and schematic illustration of hybrid mesoporous materials.The functional molecules are attached to the mesoporous support similarly tothe preparation of self-assembled monolayers (SAMs) on flat substrates. Oneend group of the functional monolayers is covalently bonded to the silica sur-face, and the other end group can be used to bind heavy metals or other func-tional molecules.

eter by about 8 �. In addition, the structure of the functionalmonolayers and the chemical bonding can be studied by sol-id-state nuclear magnetic resonance (NMR) and extendedX-ray absorption fine structure (EXAFS) techniques. Multi-nuclear solid-state NMR measurements provide direct infor-mation on the local environment of nuclear-active elements(such as 13C and 29Si) through chemical shifts, coupling be-tween different nuclear spins, and electric field gradient.[19]

EXAFS probes the distribution of neighboring atoms at aparticular atomic position (Hg).[20]

Based on the TEM, NMR, and EXAFS experiments, themolecular conformation of the monolayers has been estab-lished. At low surface coverage, the carbon chains can adapta wide range of conformations, as indicated by a single broad13C NMR resonance formed from the two carbon atoms nextto the thiol group. Under this condition, the siloxane groupscan adopt three different conformations: i) isolated groupsthat are not bound to any neighboring siloxanes, ii) terminalgroups that are only bound to one neighboring siloxane, andiii) crosslinked groups that are bound to two neighboring si-loxanes. Among the three groups, the terminal conformation(ii) is dominant. At higher population densities, all of thecarbon chains are near one another, closely packed, andhave a vertical orientation with respect to the silica surface.The NMR resonance peaks from all three carbon atoms inthe backbone are well-resolved. NMR spectra for 29Si showthe predominance of only crosslinked bonding conformationfor the siloxanes, rather than a distribution of isolated, term-inal, and crosslinked groups. When heavy metal (mercury)binds to the thiol group, the peak position and shape of theterminal head group in 13C spectra is also affected. From theEXAFS data, mercury±sulfur and mercury±oxygen bondlengths are calculated as 2.4±0.01 � and 2.14±0.01 �, respec-tively. The mercury atoms on the two adjacent thiol groupsare linked by the same oxygen atom with a mercury±mer-cury separation of 3.99±0.05 �, and the bond angle of mer-cury±oxygen±mercury is calculated as 137�. The functionalmolecules are estimated to be about 4 � apart, with eachmolecule occupying 16 �2 on the surface. This number isconsistent with the lateral dimension of the TMMP mol-

ecules. The various molecular conformations in the hybridmesoporous silica are illustrated in Figure 2.

The exceptional selectivity and capability of hybrid mate-rials to remove mercury and other heavy metals from con-taminated solutions have been demonstrated under a widerange of conditions (water, oil, acidic, neutral, and basic sol-vents). Figure 3 shows the mercury concentration remainingin the waste solution as a function of treatment time. Distri-bution coefficients as high as 108 and a loading capacity of600 mg(Hg)/g (absorbing materials) have been obtained. Asingle treatment of highly contaminated water usually re-duced the mercury concentration to well below US Environ-mental Protection Agency elemental limits for hazardouswastes and even drinking water standards. Similar resultshave also been obtained for lead and silver, which are majorconcerns in drinking water. The performance of the materi-als is not affected by the presence of background electrolytes(ions of barium, zinc, sodium, or nitrate).

The hybrid materials have other attractive attributes, suchas stability and recyclability. In situ NMR experiments indi-cated that the bonding between the mercury and thiol groupand the structure of the organic monolayers is stable up to125 �C in air. The mercury-loaded materials heated in waterat 70 �C released little mercury. The mercury-loaded materi-als can be regenerated by washing in a concentrated HCl(12.1 M) solution. This procedure results in 100 % removalof the loaded mercury. The regenerated materials retainnearly half of the original loading capacity. These materialsremain effective even after several regeneration and reusecycles. The prospect of the mercury-loaded material beingdisposed of as a permanent waste form is also under studydue to its potential long-term durability. The small pore size(< 20 nm) should prevent bacteria (at least 2000 nm in size)from putting the bound mercury into solution.

The hybrid materials have also shown high efficiency intreating different species, such as methylmercury. Similarloading capacities have been obtained for mercury ions(Hg2+ in mercury nitrate) and methylmercury, one of themost toxic forms of mercury. It exists in the environmentthrough methylation of mercury by methanogenic bacteria

Chem. Eng. Technol., 21 (1998) 1, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0930-7516/98/0101-0099 $ 17.50+.50/0 99

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Fig. 2. Schematic conformationsof functional monolayers on thesurface under different condi-tions: a) disordered molecules atlow surface coverage, b) close-packed at high coverage, c) con-taining mercury at high surfacecoverage.

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that are widely distributed in the sediments of ponds and inthe sludge of sewage beds.[12a] A very small amount ofmethylmercury can be fatal to humans.[12b]

4. Potential and Future Research

The novel hybrid porous materials allow us to tailor thepore size and surface chemistry on a molecular scale. Keymaterials parameters can be adjusted and independentlyevaluated, including:± Pore channel size. The actual pore channel spacing is de-

termined by the pore size of the support and the chainlength of functional molecules on the surface. Therefore,the pore channel sizes will be varied from angstrom levelto nanometer scale by adjusting the pore size of the sup-port and the molecular size of the functional groups.

± Stereochemical interactions. This stereochemical rela-tionship can be adjusted by manipulating the arrange-ment of the functional groups on the surface (for exam-ple, the population density or chain length).

± Functionality of the surface groups. The functionalgroups can be substituted and tailored for a particularapplication.

Besides heavy metals, more efficient materials are alsoneeded for remediation involving anions such as chromateand arsenate, and radionuclides. We have shown that the hy-brid materials can be tailored for these applications. The an-ion selectivity can be greatly enhanced by systematicallychanging the head group structures of the functional groupson the monolayers. In collaboration with Professor K. N.

Raymond from the University of California at Berkeley, ma-terials have been developed to remove actinides (AmIII,ThIV, NpV, and UVI) from nitrate solutions with Kd valuesthat are two orders of magnitude higher than those of exist-ing commercial materials.[21]

In addition to the immediate applications in environmen-tal cleanup, synthesis of the new hybrid materials provides aunique opportunity to introduce molecular binding sites andto rationally design the surface properties (wettability andcharge density distribution) of the mesoporous materials.Functional monolayers have already been widely investi-gated in materials synthesis.[15] Specific groups in the func-tional monolayers can be used to attach new functionalgroups or stimulate mineral deposition. The novel approachdiscussed in this article will benefit many applications in-volving materials synthesis, catalysis, and sensing.

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[3] Z.-R. Tian, W. Tong, J.-Y. Wang, N.-G. Duan, V. V. Krishnan, S. L. Suib,Science 1997, 276, 926.

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[7] H. Yang, N. Coombs, I. Sokolov, G. A. Ozin, Nature 1996, 381, 589.[8] Q. Hua, J. Feng, F. Scheth, G. D. Stucky, Chem. Mater. 1997, 9, 14.[9] D. M. Antonelli, A. Nakahira, J. Y. Ying, Inorg. Chem. 1996, 35, 3126.

[10] a) A. Sayari, Chem. Mater. 1996, 8, 1840. b) R. G. Anthony, C. V. Phil-ips, R. G. Dosch, Waste Management 1993, 13, 503. c) K. D. Schierbaum,Science 1994, 265, 1413.

[11] US Department of Energy (DOE), Mixed Waste Focus Area, TechnicalBaseline Results, World Wide Web: http://wastenot.inel.gov/mwfa/results.html 1996. US Department of Energy (DOE), FY91 Waste andHazard Minimization Accomplishments, DOE Report MHSMP-91-37,Pantex Plant, Amarillo, TX 79177, 1991. J. E. Klein. R&D Needs for Mi-xed Waste Tritium Pump Oils (U), Westinghouse Savannah River Com-pany Inter-Office Memorandum, SRT-HTS-94-0235, July 11, 1994.

[12] a) S. Mitra, Mercury in the Ecosystem, Trans Tech Publications, Lancas-ter, PA 1986. b) Chem. Eng. News 1997, June 16, 11.

[13] a) X. Feng, G. E. Fryxell, L.-Q. Wang, A. Y. Kim, J. Liu, K. M. Kemner, Sci-ence 1997, 276, 923. b) L. Mercier, T. J. Pinnavaia, Adv. Mater. 1997, 9, 500.

[14] a) G. M. Whitesides, Sci. Am. 1995, 273, 146. b) A. Ulman, Chem. Rev.1996, 96, 1533.

[15] B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. A. Campbell, G. E. Fry-xell, G. L. Graff, L. Song, J. Liu, J. W. Virden, G. L. McVay, Science1994, 264, 48.

[16] a) G. E. Fryxell et al., Langmuir 1996, 12, 5064. b) A. R. Bishop, R. G.Nuzzo, Curr. Opin. Colloid Interface Sci. 1996, 1, 127.

[17] W. Gao, L. Reven, Langmuir 1995, 11, 1860.[18] C. P. Tripp, M. L. Hair, Langmuir 1992, 8, 1120.[19] a) L.-Q. Wang, J. Liu, G. J. Exarhos, B. C. Bunker, Langmuir 1996, 12,

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[21] S. F. Marsh, Z. V. Svitra, S. M. Bowen, Los Alamos National Laborato-ry Report LA-12654, Los Alamos, NM 1993.

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Fig. 3. Mercury composition in the waste stream as a function of mixing time.As compared with one of the best commercial materials, the mercury concen-tration in the solution decreases much faster with the new hybrid materials.Starting mercury concentration: a) 500 ppb, b) 10 ppm.