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Formation of a Porous Zirconium Oxo Phosphate with a High Surface Area by a Surfactant-Assisted Synthesis.

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Formation of a Porous Zirconium 0 x 0
Phosphate with a High Surface Area by
a Surfactant-Assisted Synthesis**
Ulrike Ciesla, Stefan Schacht, Galen D. Stucky,
Klaus K. Unger, and Ferdi Schiith*
110
4.16
2.39
200
210
2.06
1.57
100
The synthesis of MCM-41, a silica with a hexagonal arrangement of cylindrical pores, the sizes of which are adjustable from
2 to 10 nm, and related materials"] has stimulated a considerable amount of interest in this new class of mesoporous materials. Shortly after its synthesis, different mechanisms were developed to explain the formation of this porous material.[21The
mechanism suggested by Monnier et al.['"] implies the possibility of substituting the silicate with other metal oxides to prepare
a wide range of mesostructured oxidic materials. Subsequently,
mesostructured surfactant composites of tungsten oxide, anti],
mony oxide, and other metal oxides have been synthesi~ed.~~.
However, a major problem of these non-siliceous materials is
the removal of the template: It was neither possible to remove
the surfactant by conventional methods like calcination or extraction nor by oxygen plasma calcination without destroying
the pore structure. An exception is the recently described mesoporous TiO,, in which the hexagonal pore structure remains
stable even after calcination, but the BET surface area is only
about 200 m2g-'.[51 The thermal instability of the mesostructured metal oxide composites is probably due to the different
0x0 chemistry of the metals in comparison with silicon. For
instance, the walls in the tungsten oxide surfactant composite
are formed by Keggin ions, and a complete condensation seems
improbable.@'Another reason is probably the existence of several relatively stable oxidation states of the metal centers. Thus,
during the calcination, reduction by the surfactant and/or oxidation by air oxygen could occur leading to structural collapse.
We have chosen zirconium as the metal species because it has
only one stable oxidation state and exhibits a pronounced poly0x0 ion chemistry in aqueous solutions.['] Here we present two
different syntheses leading to zirconia compounds with high
surface areas and regular pore systems, in which either zirconium sulfate or zirconium propoxide is used as the Zr source.
The syntheses were carried out in aqueous solution at 373 K
with a surfactant-unless otherwise stated hexadecyltrimethylammonium bromide-as template and a zirconium compound.
We initially used Zr(SO4);4H,O as Zr source. Zirconium sulfate forms polymeric [Zr(OH),SO,OH,] units in water that are
capable of crosslinking through sulfate bridges.[81 Charge
interactions between the sulfate groups and the positively
charged surfactant headgroups lead to the formation of the
ZrO,(SO,),
surfactant composite.
The X-ray diffraction (XRD) pattern of the colorless solid
product shows three reflections at very low angles (Fig. 1,
trace A). These reflections correspond to d spacings of 4.06 nm,
[*I
[**I
Prof. Dr. F. Schuth, Dipl.-Chem. U. Ciesla. DipLChem. S. Schacht
Institut fur Anorganische Chemie der Universitat
Marie-Curie-Strasse 11, D-60439 Frankfurt (Germany)
Fax: Int. code +(69)798-292.60
Prof. Dr. G. D. Stucky
Chemistry Department, University of California at Santa Barbara (USA)
Prof. Dr. K. K. Unger
Institut fur Anorganische und Analytische Chemie der Universitat Mainz
(Germany)
This work was supported by the Deutsche Forschungsgemeinschaft (SCHU
74418-l), the Fonds der Chemischen Industrie, the U. S. National Science
Foundation (DMR 95-20971), and the NATO Research Grant Programme
(CRG.950169). We thank Dr. B. Matthiasch and U. Junges for the chemical
analysis and W. Jacobs for recording the IR spectra.
Angew. Chem. Inl. Ed. Engl. 1996. 35. No. 5
0 VCH
f
110
200
.2
4
100 dlnm
4.05
6
2Qrl
-
0
,
2.36
2.04
12
10
Fig. 1. X-ray diffraction pattern of the zirconium oxide compound synthesized with
zirconium sulfate as Zr source. As-synthesized containing surfactant (A), after
treatment with phosphoric acid (B), and sample after calcination at 773 K (C).
2.37 nm, and 2.04 nm. Three reflections are usually not sufficient to index a pattern; however, in analogy to MCM-41 these
reflections can be indexed as (loo), (110), and (200), assuming
a hexagonal unit cell. There are no more reflections in the X-ray
diffraction pattern at higher angles. Only around 30" (28) a
broad halo can be distinguished which is typical for amorphous
material and which can also be detected in the siliceous MCM41 material. Whether this halo is due to large amorphous regions or to disorder in the pore walls, as discussed for MCM-41,
is at present uncertain. All available structural data suggest that
the zirconia compound prepared as described above has a structure that is similar to MCM-41 .['I We believe that the structure
possesses a hexagonal arrangement of cylindrical pores that are
filled with surfactant. The pore walls consist of ZrO,(SO,),-,.
The rodlike pores build up a two-dimensional system with no
long-range order in the c direction-similar to the pure surfactant liquid crystal.
Thermal treatment of such samples in air at 773 K for five
hours leads to a collapse of the ordered pore structure which is
noticeable in the loss of intensity or disappearance of the low
angle reflections. This may be due to incomplete condknsation
of the walls because of the very low pH value during the synthesis.
To stabilize the structure the as-synthesized surfactant composite was treated with phosphoric acid solution. During this
treatment the uncondensed Zr-OH groups probably react with
the phosphate ions, leading to complete crosslinking. The XRD
pattern of this sample shows four sharp reflections, which, in
comparison to the reflections of the untreated sample, are shifted to lower angles (Fig. 1, trace B and Table 1). This pattern can
also be indexed assuming a hexagonal unit cell as (100) , (1lo),
(200), and (210) reflections. After calcination (6 h at 773 K) a
Table 1. Summary of the properties of the zirconium oxide synthesized with zirconium sulfate as Zr source.
Surfactant
dspacing (100) [a] dspacing (100) [b] BET
[nml
[nml
surface area
[m'g-']
Pore
volume
[cm3g-']
C,,
4.16
4.62
4.90
0.122
0.153
0.218
c,,
c20
2.81
3.38
3.86
230
320
390
[a] After H,PO, treatment. [b] After calcination at 773 K
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541
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porous material is obtained that shows only one reflection,
which has a d spacing of 2.81 nm (Fig. 1, trace C). The higher
order reflections are no longer observed which is probably due
to a decrease in the long-range order of the hexagonal pore
arrangement. Nevertheless, nitrogen adsorption measurements
(see below) show that the pores still exist in the calcined
materiaI.
Variation of the chain length of the surfactant changes the
dimensions of the system: If trimethyl(octadecy1) or icosyltrimethyl ammonium bromide is used, the corresponding reflections are shifted to lower angles, both for the as-synthesized
product and the calcined form (Table 1). Similar results have
also been obtained for MCM-41. Moreover, nitrogen adsorption measurements at 77 K show that the surface areas increase
with longer surfactant chain length. From nitrogen adsorption
analysis, Brunauer Emmett -Teller (BET) surface areas of
230 m2g- ' for the sample synthesized with C,,-surfactant,
320 m2g-' for that with the C,,-surfactant, and 390 m2g- ' for
the sample prepared with the C,,-surfactant can be calculated.
To the best of our knowledge these are the highest surface areas
obtained for zirconia. In a recently described surfactant-assisted
synthesis for zirconia by M. J. Hudson et al., BET surface areas
up to 329 m2g- were obtained.["] However, these materials do
not possess a regular pore system, neither in the as-synthesized
nor in the calcined form.
Figure 2 shows the nitrogen isotherms for the samples synthesized with C,,-surfactant (Fig. 2A), C,,-surfactant (Fig. 2B),
and C,,-surfactant (Fig. 2C). The difference in the position of
~
determined by X-ray diffraction which would be 3.5 nm for
MCM-41. This is probably due to the existence of thicker walls
in the zirconia material than in the silicate MCM-41. The structural parameters obtained by analysis of the nitrogen adsorption are in good agreement with the data from X-ray diffraction:
An increase of the lattice parameter corresponds to an increase
of the pore diameter, the pore volume, and the BET surface
area.
The sulfur content of the calcined sample determined by
X-ray fluorescence analysis is less than 1 wt %, the phosphorus
content about 10 wt%. From this a nominal wall composition
of 45 wt YOzirconium oxide (based on ZrO,), 54 wt YOzirconium
phosphate (based on Zr3(P04)4),and only 0.5-1 wt% zirconium sulfate (based on Zr(SO,),) can be calculated.
Starting from zirconium propoxide, the synthesis is carried
out analogous to the acidic synthesis of MCM-41 developed by
Huo et al, 14) in which strong HC1 acidic surfactant solutions are
used. In contrast to the MCM-41 synthesis, the addition of
(NH,),SO, is necessary for the formation of the corresponding
zirconia surfactant composite. After isolation and drying of the
product at 363 K, three reflections were observed in the XRD
pattern (4.51 nm, 2.60 nm, and 2.24 nm); however, the intensities of the latter two corresponding to the (110) and (200) reflections are very weak (Fig. 3 trace A). The reason for the broad
signal at 6-8" (20) is not yet clear.
1
A3.00 nm
I
A
I
I
0
0.2
0.4
PIP0
0.6
0.8
1.0
2
Fig. 2. Nitrogen adsorption isotherms at 77.4 K for calcined and post-treated samples synthesized with hexadecyltrimethylammonium bromide (A), trimethyl(octadecy1)ammonium bromide (B), and icosyltrimethylammonium bromide ( C ) .
the steep increase in the volume adsorbed is apparent from the
isotherms. The increase for the sample synthesized with C,,-surfactant is at low pressure and thus is an indication for the presence of micropores, whereas the step of the isotherm for the
C,,-surfactant sample is around a relative pressure of 0.2. N,
isotherms of this type are typical for pores with sizes between
micro- and mesopores. Similar adsorption isotherms have been
described for MCM-41 with pore sizes of 2.5 nm;["] however,
in this case the step in the isotherm was obtained at a slightly
higher relative pressure ofp/p, = 0.3. Therefore a pore diameter
of 2 nm can be estimated for the zirconium compound. However, the pore size determination by nitrogen adsorption measurements is not very precise in this region because BJH analysis
(BJH = Barrett-Joyner-Halenda) does not yield reliable data
and other algorithms do differ in their results. The relatively
small pore diameter seems to be inconsistent with the d spacings
542
0 VCH
Verlagsgesel/schaft mhH. 0-69451 Weinherm. 1996
4
-
6
28["]
8
I
10
12
Fig. 3. X-ray diffraction pattern of the zirconium oxide compound synthesized
with zirconium propoxide as Zr source; as-synthesized (A) and after calcination at
773 K (B).
In contrast to the material from zirconium sulfate, it is possible to calcine the material from zirconium propoxide at 773 K
without the structure collapsing. However, also in this case the
(100) reflection is shifted to higher angles (Fig. 3, trace B) and
the intensity decreases. The (110) and (200) reflections disappear
completely. The nitrogen adsorption isotherm is typical for a
microporous material ; the BET surface area can be calculated
to 280 m2g- '. The sulfur content of this zirconia after calcination at 500 "C is about 2 wt % ; thus, this material essentially
consists of zirconium oxide.
The fact that the samples contain sulfur even after calcination
is interesting for potential applications. The sulfur remains in
the calcined products in form of sulfate groups which could be
detected by IR spectroscopy. Zirconias are important as catalysts and catalyst supports. Sulfated zirconias are used as super
0570-OS3319613505-0542 8 15 O O i 2510
Angew Chem Int Ed Engl 1996,35, N o 5
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acidic catalysts in isomerization reactions. High surface areas
are often the main requirement of such materials. The new zirconias prepared by surfactant-controlled syntheses show interesting structures and could become important for technical applications.
Generation of a Heterocyclic 1,3-Cyclopentanediyl Radical Cation by Chemically Induced Electron Transfer Oxidation and
Pulse Radiolysis**
Experimental Procedure
Waldemar Adam,* Thomas Kammel, and
Steen Steenken*
Synthesis with Zr(SO4);4H,O: Hexadecyltrimethylammonium bromide (2.5 g,
6.87 mmol) was dissolved in H,O (85 g). and Zr(SO4),.4H,0 (4.55 g. 0.0128 mol)
dissolved in H,O (1 5 gj was added. This led to a colorless solid precipitate. The
mixture was stirred for 2 h at room temperature and then heated to 100°C for two
days in a closed polypropylene beaker. The precipitate was filtered and dried a t
100°C. The dried precipitate was stirred in a phosphoric acid solution ( 0 . 8 7 ~
0.87 Mj for 2 h, then filtered and dried again a t 100 "C The colorless product was
then calcined at 500°C for 5 h. S-content after calcination: 1-2 wt%, P-content
after H,PO, treatment and calcination: 10 wt%.
Synthesis with Zr(OC,H,),: Hexadecyltrimethylammonium bromide (2.5 g,
6.87 mmol) was dissolved in a solution of water (1 15 g) and HCI (24 4 g) (37 wt%)
After the hydrolysis products had dissolved, (NH,)SO, (2.04 g, 0.0155 mol) in water
(23 gj was added and the mixture was stirred at room temperature for 1 h. The
initially clear solution was then heated at 100°C for two days The colorless precipitate was filtered. dried, and calcined at 500°C for 5 h. S-content after calcination:
1-2 wt %. The as-synthesized samples contain 40-50 wt% surfactant, as detected
by thermogravimetric analysis between 200 and 600°C.
X-ray diffraction Scintag PADX diffractometer with nitrogen-cooled Ge-detector.
= 1.54056 nmj. step 0.02'. scan 2.4 s.
Cu,, radiation (i
Nitrogen adsorption measurements: Micromeritics ASAP 2000, 77.4 K, equilibration time 60s.
Received September 6, 1995 [Z8370IE]
German version Angew. Chem. 1996, 108, 597-600
Keywords: mesoporous materials . surfactants . zeolites . zirconium compounds
[ l ] C. T. Kresge. M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Narure
1992. 359, 710; 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, 10834.
[2] a) A. Monnier. F. Schuth. Q. Huo, D . Kumar, D Margolese, R. S. Maxwell,
G. D. Stucky. M . Krishnamurty, P. Petroff, A. Firouzi. M. Janicke, B. F.
Chmelka. ScirnLe 1993,261, 1299; b) Q. Huo, D. L. Margolese, U. Ciesla, D.
Demuth, P Feng. T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth,
G . D Stucky. Chem. M a w . 1994, 6 , 1176; c) S . Inagaki, Y Fukushima, K.
.
Chrm. Commun. 1993, 680; d ) C. Y. Cheng, S. L. BurKuroda. J. C h ~ mSoc
kett, H.-X. Li, M. E. Davis, Micropor. Muter. 1993, 2, 27.
[3] U. Ciesla. D . Demuth, R. Leon, P. Petroff, G. D. Stucky. K. Unger. F. Schiith,
J. Chem. Sol Chem. Commun. 1994. 1387.
[4] Q. Huo. D. L. Margolese, U. Ciesla, P. Feng, T. E. Gier, P. Sieger, R. Leon,
P. M. Petroff, F. Schiith, G. D. Stucky, Nature 1994, 368, 317.
[5] D. M. Antonelli. J. Y. Ying, Angew. Chem. 1995,107.2202; Angew. Chem. Inr.
Ed. Engl. 1995. 34. 2014.
[6] A. Stein. M. Fendorf, T. P. Jarvie, K. T. Mueller, A. J. Benesi, T. E. Mallouk,
Chmm. Murcr. 1995, 7, 304.
[7] C. J. Brinker. G. W. Scherer. Sol-GelScience, Academic Press, New York, 1990.
[XI J. Livage. M. Henry, C. Sanchez, Prog. Solid Stare Chem. 1988, 18, 259.
[9] The mesoporous silicates could also only be characterized by X-ray diffraction,
chemical analysis, and nitrogen adsorption measurements, sometimes assisted
by transmission electron microscopy (TEM) and *'Si and 27AI MAS NMR.
The reason for this is nature of the material itself: It has no order in the walls
and therefore diffraction methods are limited, except for the information about
the periodiciry of the pore system. TEM analysis gives only selective information about the particle but not about the whole sample. By using TEM we
detected hexagonal pore arrangements in the zirconium compounds. However,
X-ray diffraction and nitrogen adsorption appear to 11sto be more important
than TEM lor the characterization of the structure.
[lo] J A. Knowlcs. M J. Hudson, J. Chem. Soc. Chem. Commun. 1995, 2083.
[ I l l P. Llewellyn. Y. Grillet. F. Schiith, H. Reichert, K. K. Unger, Micropor. Mazer.
1994. 3, 345
Angeic. Chem. in[ E d Engl. 1996, 35, No. 5
8 VCH
Although the chemistry of 1,3-diradicals derived from cyclic
azoalkanes and their corresponding strained hydrocarbons
(housane = bicyclo[2.1 .O]pentane) has been extensively studied,"] their electron transfer oxidation". 31 was investigated
only recently. The reaction of 1,3-disubstituted bicyclo[2.1.O]pentanes with catalytic amounts of the oxidant tris(4-bromopheny1)aminium hexachloroantimonate (TBA'+SbCI;) led
to the corresponding cyclopentenes after 1,Zhydrogen o r -alkyl
migration in the intermediary 1,3-radical cations [Eq. (a)].[4J
Since no examples of heterocyclic analogues of these distonic
1,3-radical cations were known, it was of interest, particularly
with regard to the reaction mechanism, to investigate the chemical behavior of these systems. Arnold et
had synthesized
the urazole-substituted azoalkanes (urazole = 1,2,4-triazolidine-3,5-dione) by Diels-Alder reaction of 4-pheny1-3,2,4-triazoline-3,5-dione (PTAD) with substituted isopyrazoles. Direct
photolysis of these azoalkanes led to the corresponding housanes. The latter are suitable precursors for the desired heterocyclic 1,3-radical cations. By replacing PTAD with 4-methyl1,2,4-triazoline-3,5-dione
(MTAD), the solubility of the radical
cation was significantly improved, which allowed a thorough
examination of these urazole-annelated azoalkanes and their
corresponding housanes.[6J Moreover, it is known from the
work of Nelsen et aI.['] that substituted hydrazine derivatives
yield stable radical cations upon oxidation at the nitrogen
atoms. Consequently, it was of interest to assess the effect of
heteroatoms on the mechanism of the chemically induced electron transfer (CET) reaction of these housanes. Herein, we report on the generation and spectroscopic detection of the first
urazole-bridged 1,3-radical cation and its unusual chemical
transformations.
Direct photolysis of azoalkane 1 (Scheme 1, path a) led exclusively to the corresponding housane 2.[*'The latter is heat and
acid labile and rearranges subsequently to the olefin 3
(Scheme 1, pathways c and d). In contrast to carbocyclic bicyclo[2.1 .O]pentane derivatives (heteroatom-free housanes)
[Eq. (a)], stoichiometric amounts of tris(4-bromopheny1)aminium hexachloroantimonate are necessary for the CET reaction of 2 (Scheme 1, path e). The CET product 4'') could only
[*I Prof. Dr. W Adam, DipLChem. T. Kammel
Institut fur Organische Chemie der Universitdt Wiirzburg
Am Hubland. D-97074 Wiirzburg (Germany)
Fax: Int code +(931) 888-4756
e-mail: adam(a'chemie.uni-wuerzburg.de
Prof. Dr. S. Steenken
Max-Planck-Institut fur Strahlenchemie
Stiftstrasse 34-36, D-45470 Miilheim a n der Ruhr (Germany)
[**I This work was supported by the Volkswagenstiftung [Photoelektronentransferchemie (PET) von Azoalkanen und gespannten Kohlenwasserstoffen]. We
thank Dip1:Chem. H. M. Harrer for performing the PM3 calculations and
M. Toubartz for experimental assistance with the pulse radiolyses at the MaxPlanck-Institut, Mulheim an der Ruhr
Verlugsgesellschufi mbH, 0-69451 Weinhem. 1996
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