Research News

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 havevery high surface area (> 1000 m2/g), ordered pore struc-ture (mostly hexagonal packed cylindrical pore channels),and extremely narrow pore size distribution. The pore di-ameter can be adjusted from 2 to 15 nm. The preparationmethods involve mixing ceramic precursors (such as so-dium aluminate, tetramethyl ammonium silicate, and silica)in a surfactant (cetyltrimethylammonium chloride, CTAC,or cetyltrimethylammonium bromide, CTAB) solution andreacting the agents at temperatures below 150 �C. In princi-ple, surfactants form ordered micellar phases. The mostcommon phase consists of rod-like micelles packed in hex-agonal arrays. The ceramics precursors bind to the headgroups of the surfactant molecules, and finally condense to-gether, forming a continuous ceramic phase. Subsequently,the surfactant molecules can be removed by thermal orchemical 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

fundamental understanding of the reaction processes.[2] Awide range of mesoporous materials have been prepared, in-cluding alumina, zirconia, titania, niobia, tantalum oxide,and manganese oxide. With a few exceptions (for example,manganese oxide[3]), the pore structure of non-silica-basedmesoporous materials is not as well-defined as silica-basedmaterials, and is not stable at elevated temperatures. Meso-porous silica has also been doped with elements possessingcatalytic properties[4] and with conducting polymers.[5] Re-cently, progress has been made in the fabrication of orientedmesoporous films on various substrates,[6] in making free-standing films,[7] spheres,[8] and single-crystalline mesopor-ous materials in which all the pore channels are aligned.[9]

Although the potential of mesoporous materials hasbeen widely recognized, progress on the practical use ofthese novel materials has been slow. Many applications,such as adsorption, ion exchange, catalysis, and sensing, re-quire the materials to have specific binding sites, stereo-chemical configuration or charge density, and acidity.[10]

Most mesoporous materials do not themselves have theappropriate surface properties. For example, mercury andheavy-metal contamination is a serious problem at waste-contaminated sites of the Department of Energy.[11]

Industrial and civilian sources also deposit a large amountof mercury into the environment every year.[12] It is neces-sary to develop a method to systematically modify thesurface chemistry and tailor the molecular recognitionprocess of mesoporous materials toward 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-

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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 Memo-rial Institute for the US Department of Energy under Contract DE-AC06-76RL01830. The EXAFS study was conducted by Dr. K. Kem-ner from Argonne National Laboratory.

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port.[13] The functional molecules are attached to the meso-porous support similarly to the preparation of self-as-sembled monolayers (SAMs) on flat substrates (Fig. 1).

This approach provides a unique opportunity to rationallyengineer the surface properties. The hybrid mesoporousmaterials demonstrate exceptional selectivity and capacityfor adsorbing heavy metal ions from contaminated wastestreams. Materials capable of recognizing other species andmolecules are also under development.

SAMs have been widely explored for engineering thesurface and interfacial properties of materials, such as wet-ting, adhesion, and friction.[14] These monolayers are alsoused to mediate the molecular recognition processes and todirect oriented crystal growth.[15] On crystalline surfacesthat will not chemically react with the SAMs (alkyl thiolson gold), the overlayer structure is influenced by the sub-strate symmetry. These kinds of SAMs have been used asmodel systems to study surface properties and conforma-tions of molecules in other SAMs.[14] On oxide surfaces,such as silica, the packing and ordering are defined by theintermolecular interactions.[15,16] In this approach, bifunc-tional molecules containing a hydrophilic head group and ahydrophobic tail group adsorb onto a substrate as closelypacked monolayers. The tail group and the head group canbe chemically modified to contain specific functionalgroups. The hydrocarbon tails provide the driving force(van der Waals interaction) for the self-assembly of themolecules into close packed arrays on the substrate. For ex-ample, the chlorosilane end of the molecule can be cova-lently bonded to the oxide surface and cross-linked to adja-cent silanes through hydrolysis and condensation reactions.

In the hybrid materials, short alkyl chains are used. Forexample, the van der Waals forces between the alkyl chainsplay a less important role, and the main driving force forthe molecules to anchor to the substrate is chemical bond-ing through condensation reactions. Compared with SAMson flat substrates, it is more difficult to attach an organic

monolayer to mesoporous supports. The functional mol-ecules must be able to access the interior surface of thepore channels (a few nanometers wide), and must not pre-maturely hydrolyze and condense with themselves. Thequality of the functional monolayers on the mesoporousmaterials is greatly affected by the population of silanolgroups and adsorbed water molecules on the mesoporoussilica surface.[17,18] The silanols are needed to anchor the or-ganic molecules to the silica surface, and physically ad-sorbed water is required for the hydrolysis of siloxanes.However, excess free water from capillary action is alsodetrimental to the efficient formation of a clean monolayer,due to polymerization 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 mesopor-ous silica surface. This process involved boiling a weighedsample of mesoporous silica in pure water for severalhours, collecting the silica by filtration, weighing it again,and removing the surplus water content via azeotropic dis-tillation with toluene. This method, although successful inthe deposition of high-quality monolayers up to 75 % sur-face coverage, was time-consuming and laborious. Re-cently, we have developed a more efficient approach bywetting the silica surface with 2 to 2.5 monolayers of water(based on available surface area). Experimentally, this ap-proach is accomplished by adding the requisite amount ofwater to a suspension of mesoporous silica in toluene andstirring the mixture for an hour to allow complete dispersalof the aqueous phase across the ceramic interface. Whenthe mesoporous ceramic interface is properly hydrated,construction of the monolayer is accomplished by addingone equivalent (or a slight excess) of the desired alkoxysi-lane (based on available surface area), stirring the mixture,and heating it in toluene reflux for several hours. Currently,we can systematically vary the population densities of func-tional groups on the mesoporous materials from 10 % up to100 % 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 pre-viously used to make functional monolayers,[14] and thethiol groups have a high affinity for binding heavy metals.The thiol±silica hybrid mesoporous materials thus pro-duced can efficiently remove mercury and other heavy me-tals (such as lead and silver) from contaminated aqueousand organic solutions. The distribution coefficient, Kd, hasbeen measured to be as high as 108. (Kd is defined as theamount of adsorbed metal [mg] on 1 g of adsorbing materi-al, divided by metal concentration [mg/mL] remaining inthe treated waste stream.)

Fig. 1. Preparation and schematic illustration of hybrid mesoporous materi-als. The functional molecules are attached to the mesoporous support simi-larly to the preparation of self-assembled monolayers (SAMs) on flat sub-strates. One end group of the functional monolayers is covalently bonded tothe silica surface, and the other end group can be used to bind heavy metalsor other functional molecules.

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 di-ameter by about 8 �. In addition, the structure of the func-tional monolayers and the chemical bonding can be studiedby solid-state nuclear magnetic resonance (NMR) and ex-tended X-ray absorption fine structure (EXAFS) tech-niques. Multinuclear solid-state NMR measurements pro-vide direct information on the local environment ofnuclear-active elements (such as 13C and 29Si) throughchemical shifts, coupling between different nuclear spins,and electric field gradient.[19] EXAFS probes the distribu-tion of neighboring atoms at a particular 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 canadapt a wide range of conformations, as indicated by a sin-gle broad 13C NMR resonance formed from the two carbonatoms next to the thiol group. Under this condition, the si-loxane groups can adopt three different conformations: i)isolated groups that are not bound to any neighboring si-loxanes, ii) terminal groups that are only bound to oneneighboring siloxane, and iii) crosslinked groups that arebound to two neighboring siloxanes. Among the threegroups, the terminal conformation (ii) is dominant. Athigher population densities, all of the carbon chains arenear one another, closely packed, and have a vertical orien-tation with respect to the silica surface. The NMR reso-nance peaks from all three carbon atoms in the backboneare well-resolved. NMR spectra for 29Si show the predomi-nance of only crosslinked bonding conformation for the si-loxanes, rather than a distribution of isolated, terminal, andcrosslinked groups. When heavy metal (mercury) binds tothe thiol group, the peak position and shape of the terminal

head group in 13C spectra is also affected. From the EX-AFS data, mercury±sulfur and mercury±oxygen bondlengths are calculated as 2.4±0.01 � and 2.14±0.01 �, re-spectively. The mercury atoms on the two adjacent thiolgroups are linked by the same oxygen atom with a mer-cury±mercury separation of 3.99±0.05 �, and the bond an-gle of mercury±oxygen±mercury is calculated as 137�. Thefunctional molecules are estimated to be about 4 � apart,with each molecule occupying 16 �2 on the surface. Thisnumber is consistent with the lateral dimension of theTMMP molecules. The various molecular conformations inthe hybrid mesoporous silica are illustrated in Figure 2.

The exceptional selectivity and capability of hybrid ma-terials to remove mercury and other heavy metals fromcontaminated solutions have been demonstrated under awide range of conditions (water, oil, acidic, neutral, and ba-sic solvents). Figure 3 shows the mercury concentration re-maining in the waste solution as a function of treatmenttime. Distribution coefficients as high as 108 and a loadingcapacity of 600 mg(Hg)/g (absorbing materials) have beenobtained. A single treatment of highly contaminated waterusually reduced the mercury concentration to well belowUS Environmental Protection Agency elemental limits forhazardous wastes and even drinking water standards. Simi-lar results have also been obtained for lead and silver,which are major concerns in drinking water. The perfor-mance of the materials is not affected by the presence ofbackground electrolytes (ions of barium, zinc, sodium, ornitrate).

The hybrid materials have other attractive attributes,such as stability and recyclability. In situ NMR experimentsindicated that the bonding between the mercury and thiolgroup and the structure of the organic monolayers is stableup to 125 �C in air. The mercury-loaded materials heated inwater at 70 �C released little mercury. The mercury-loadedmaterials can be regenerated by washing in a concentratedHCl (12.1 M) solution. This procedure results in 100 % re-moval of the loaded mercury. The regenerated materials re-tain nearly half of the original loading capacity. These ma-terials remain effective even after several regeneration and

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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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reuse cycles. The prospect of the mercury-loaded materialbeing disposed of as a permanent waste form is also understudy due to its potential long-term durability. The smallpore size (< 20 nm) should prevent bacteria (at least2000 nm in size) from putting the bound mercury into sol-ution.

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 bacteriathat 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

determined by the pore size of the support and thechain length of functional molecules on the surface.Therefore, the pore channel sizes will be varied fromangstrom level to nanometer scale by adjusting the poresize of the support and the molecular size of the func-tional 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 thehybrid materials can be tailored for these applications. Theanion 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,materials 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 providesa unique opportunity to introduce molecular binding sitesand to rationally design the surface properties (wettabilityand charge density distribution) of the mesoporous materi-als. Functional monolayers have already been widely inves-tigated in materials synthesis.[15] Specific groups in thefunctional monolayers can be used to attach new functionalgroups or stimulate mineral deposition. The novel ap-proach discussed in this article will benefit many applica-tions involving materials synthesis, catalysis, and sensing.

±[1] a) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,

K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B.McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc. 1992, 114,10 834. b) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S.Beck, Nature 1992, 359, 710.

[2] For reviews, see: a) J. S. Beck, J. C. Vartuli. Curr. Opin. Solid State Ma-ter. Sci. 1996, 1, 76. b) J. Liu, A. Y. Kim, L. Q. Wang, B. J. Palmer,Y. L. Chen, P. Bruinsma, B. C. Bunker, G. J. Exarhos, G. L. Graff,P. C. Rieke, G. E. Fryxell, J. W. Virden, B. J. Tarasevich, L. A. Chick.Adv. Colloid Interface Sci. 1996, 69, 131. c) N. K. Raman, M. T. An-derson, C. J. Brinker, Chem. Mater. 1996, 8, 1682.

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

[4] P. T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 1994, 368, 6469.[5] C. H. Wu, T. Bein, Science 1994, 264, 5166.[6] a) H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afrara, G. A. Ozin,

Nature 1996, 379, 703. b) I. A. Aksay, M. Trau, S. Manne, I. Honma, N.Yao, L. Zhou, P. Fenter, P. M. Eisenberger, S. M. Gruner, Science1996, 273, 892.

[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. Schier-baum, 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 forMixed Waste Tritium Pump Oils (U), Westinghouse Savannah RiverCompany Inter-Office Memorandum, SRT-HTS-94-0235, July 11,1994.

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

[12] a) S. Mitra, Mercury in the Ecosystem, Trans Tech Publications, Lan-caster, 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. Kem-ner, Science 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,

2663. b) A. Badia et al., Langmuir 1996, 12, 1262.[20] a) E. A. Stern, S. M. Heald, Rev. Sci. Instrum. 1979, 50, 1579. b)

D. E. Sayers, B. A. Bunker, X-Ray Absorption, Principles, Applica-tions, Techniques of EXAFS, SEXAFS, and XANES (Eds: D. C.Koningsberger, R. Prins), Wiley, Chichester, UK 1988, pp. 211±256and Ch. 6.

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

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