Supramolecular-Templated Synthesis of Nanoporous zirconia-silica

Supramolecular-Templated Synthesis of Nanoporous zirconia-silica

(Parte 1 de 5)

Supramolecular-Templated Synthesis of Nanoporous Zirconia-Silica Catalysts

Michael S. Wong,† Howard C. Huang, and Jackie Y. Ying*

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307

Received January 29, 2001. Revised Manuscript Received October 2, 2001

Thermally stable, mesoporous, and microporous zirconium-doped silica were prepared via supramolecular templating under low pH conditions. Zirconium loadings of up to 20 wt % were successfullyincorporatedinto the porous silica-basedframework,giving materialswith very high surface areas, uniform pore sizes, and pore ordering. The zirconium cations were found dispersed throughout the material, and additionally, were preferentially located at the pore wall surface. The extent of Zr incorporation was found to be highly dependent on the synthesispH and the nature of the Zr salt precursorand could be controlledby modifying the S+X-I+ synthesis route. These Zr-doped materials showed catalytic activity for 1-butene isomerization and cis-cyclooctene oxidation reactions.


There has been a great deal of research in developing supramolecular-templated mesoporous silicas for heterogeneous catalysis.1-3 While pure mesoporous silicas, e.g., MCM-41, contain unique structural features, they are catalyticallyinactive, like amorphous silica gel. The general method of introducing catalytically active sites into such materials has been the chemical modification of the silica pore walls. Successful examples include the incorporation of a second metal cation into the silicabased framework;4,5 the surfaceadditionof metal films,6 metal cations,7 metal oxo species,8 or metal oxide clusters;9 and the surface anchoring of organometallic complexes.10

From a composition standpoint, mixed metal oxides have been investigated for a variety of catalytic reactions because of their enhanced acidity relative to pure metal oxides.1 Of these materials, those composed of zirconia and silica have been found among the most strongly acidic.12 The surface acidity comes from the difference in valence or coordination states of adjacent metal cations11 and is affected by the relative metal amounts,13 the molecular homogeneity of the mixed oxide,14 and the presence of surface anions.15 These zirconia-silicates tend to be amorphous and have low surface areas. Silicas in the form of zeolitesthat contain smallamountsof frameworkZr have generatedinterest as oxidation catalysts because of the importance of Tidoped zeolites (e.g., TS-1) for commercial oxidation reactions and the similar chemistry shared by Zr and Ti.16,17 Zirconia-silicamaterialswith a well-definedpore structure,controllableporesizes,and high surfaceareas would be advantageous for acid and oxidation catalysis.

The preparation of MCM-41 doped with a variety of metal cations has been reported by numerous research-

* To whom correspondence should be addressed.Present address: Department of Chemical Engineering, Rice

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10.1021/cm01076+ C: $2.0 © 202 American Chemical Society Published on Web 04/03/2002

Downloaded by UNIV EST PAULISTA UNESP on October 30, 2009 | Publication Date (Web): April 3, 2002 | doi: 10.1021/cm010076+ ers.4,5 The dopingprocedureis straightforward,in which the precursor of the desired metal is added to the silicate/surfactant/water/base synthesis mixture. The metal cations are generally thought to be distributed homogeneouslythroughoutthe silicateframework,with the totalamountof incorporatedmetalcationsbeinglow ( 2-4w t% ).3 Higher loadings are desirable for increased catalytic activity but are difficult to achieve in doped mesoporous silicates. The highly basic solution (pH g 10) required for MCM-41 synthesis appears to limittheincorporationof secondarymetalcationdopants. Under such conditions, the dopant metal can separate into a second phase because of incompatible condensation and precipitation rates. Also, high levels of doping can weaken mesostructure, resulting in structural collapse after surfactant removal.5 Careful manipulation of the templating chemistry is thus required.

The preparation of Zr-doped mesoporous silicas at high18 and neutral19 pH’s has been studied by other researchers,althoughthesematerialswerefoundto lose long-range pore ordering with increasing Zr loading. Still, such materials were shown to possess acidic18,20 and oxidative properties.19 A low-pH route to mesoporous silicas was developed by Stucky and co-workers,21 and early work done on doped silicas via this approach had found doping levels of 1 wt %.2

In this article, we investigated the preparation of mesoporous Zr-doped silicas under highly acidic conditions (pH < 0). Understanding of the synthesis chemistry allowed the derivation of mesoporous silicas with as much as 20 wt % Zr, while maintaining pore ordering and high surface areas. The effect of Zr doping on the mesostructure was examined, and the nature of Zr in the framework was elucidated. The catalytic activity of these nanoporous materials was demonstrated for acid-catalyzed and oxidation reactions.

Experimental Section

Synthesis.A solutionof zirconiumsulfatetetrahydratewas prepared by dissolving a desired amount of the salt (Zr(SO4)2â

4H2O, 9.9+%, Strem) in 15 mL of deionizedwater. This was combinedwith a solutioncontainingcetyltrimethylammonium bromide(CTAB,9+%,AlfaAesar)andhydrochloricacid(37%, Mallinckrodt). Tetraethyl orthosilicate (TEOS, 98%, Aldrich) was immediatelyadded to the mixture. The molar ratio of this

1690 H2O. Samples prepared with y ) 54 (at a calculated pH of -0.25) were labeled ZrSi1-x, and samples prepared with y

) 5.4 (at a higher pH of 0.75) were labeled ZrSi2-x (Table 1).

Zirconylchlorideoctahydrate(ZrOCl2â8H2O, 98%,Aldrich)was used as an alternative Zr precursor, resulting in a sample labeledZrSi1-3C.These sampleswere collectivelynamed ZrSi. The mixture was stirred for 2 h and then aged at room temperature for 2 days. The resulting white powder was filtered and washed 3 times with water, and left to air-dry overnight. For surfactant removal, the mesostructured ZrSi sampleswere calcinedat 540 °C for 3 h under flowingair, with a ramp rate of 3 °C/min. A pure mesoporous silica labeled ZrSi1-0 was prepared without addition of any Zr precursors. Pure silica ZrSi2-0 was also prepared without addition of any Zr precursors, but an aging period of two weeks was required for precipitation of a product.

ZrSi with larger mesopores was prepared by using a much larger surfactant than CTAB. An acidic solution of the sur- factant P123 [(ethylene oxide)20(propylene oxide)70(ethylene oxide)20,F W ) 5880, BASF]23 was combined with a 30 wt % solution of zirconyl chloride octahydrate. After TEOS was added, the synthesismixturewas stirredat room temperature for 1.5 h and at 35 °C for 1 day. Then, the mixture was left at 80 °C for 2 days before product recovery and calcination at 540°C.Themolarratioof theprecursormixturewas10 TEOS/

10 ZrOCl2/0.17 P123/57 HCl/1600 H2O. This sample was termed ZrSBA-15.

Characterization. Powder X-ray diffraction (XRD) data were recorded on a Siemens D5000 ı-ı diffractometer (45 kV, 40 mA) using nickel-filteredCu KR radiation with wavelength ì ) 1.5406 Å. Diffraction patterns were collected under ambient conditions in the 2ı range of 1.5° to 40.0° with a resolution of 0.04°. Analysis of ZrSBA-15 was carried out through small-angle X-ray scattering (SAXS) on a Siemens small-angle diffractometer with a Siemens HI-STAR area detector, operating at 40 kV and 30 mA (ì ) 1.5406 Å).

Nitrogen adsorption isotherms were obtained at 7 K on a

Micromeritics ASAP 2010 Gas Sorption and Porosimetry system. Samples were normally prepared for measurementby degassing at 150 °C under vacuum until a final pressure of 1 10-3 Torr was reached. BET surface areas were determined over a relative pressure range of 0.05 to 0.20.24 Pore size distributionswerecalculatedusingtheHK (Horvath-Kawazoe) method.25

Transmission electron micrographs (TEM) were taken on a JEOL2000FXtransmissionelectronmicroscopeequippedwith a lanthanumhexaboride(LaB6) gun operatingat an accelerating voltage of 200 kV and with an objective aperture of 50 ím.

Samples for TEM studies were ground and sprinkled onto a carbon-coated copper grid.

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Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (b) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (2) Zhang, W.; Wang, J.; Tanev, P. T.; Pinnavaia, T. J. Chem. Commun. 1996, 979.

(23) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.;

Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (b) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (24) Gregg, S. K.; Sing, K. S. W. Adsorption, Surface Area and

Porosity, 2nd ed.; Academic Press: London, 1982. (b) Webb, P. A.; Orr, C. Analytical Methods in Fine Particle Technology; Micromeritics Instrument Co.: Norcross, 1997. (25) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470.

Table 1. Synthesis Conditions and Elemental Analysis Results of ZrSi Materials sample precursor

Zr/Si molar ratio precursor HCl/Si molarratio calculated pHd measured

Zr content (wt %)e expected

Zr content (wt %)e,f a Nonmesostructured after calcination. b Zr precursor:

(Parte 1 de 5)