Mesoporous silica with site-isolated amine and phosphotungstic acid groups: A solid catalyst with tunable antagonistic functions for one-pot tandem reactions

Article abstract of DOI:10.1002/anie.201101449

Coniuga et impera: A bifunctional solid catalyst is prepared by combining acid and base functions on mesoporous silica supports (see picture). The co-existence of these functions is shown by a two-step reaction sequence in one pot. Excellent product yields, which cannot be obtained by separated acid and base functions in one pot, show the validity of our concept. Copyright

Full text of DOI:10.1002/anie.201101449

DOI: 10.1002/anie.201101449
Bifunctional Catalysis
Mesoporous Silica with Site-Isolated Amine and Phosphotungstic Acid
Groups: A Solid Catalyst with Tunable Antagonistic Functions for
One-Pot Tandem Reactions**
N. Raveendran Shiju,* Albert H. Alberts, Syed Khalid, David R. Brown, and Gadi Rothenberg*
Scientists engaged in heterogeneous catalysis often cite
enzymes as their model catalysts. Enzymes can efficiently
catalyze multistep processes that give various types of
biomolecules.[1] Remarkably, many enzymes combine two
antagonistic catalytic functions, such as acid and base
functions.[2] Attracted by this challenge, several groups
synthesized homogeneous catalysts that can successfully
combine chemically hostile functions.[3] However, the diffi-
culty lies in controlling the separation between these groups
and simultaneously working at practical catalyst concentra-
tions. In principle, both of these problems can be solved using
heterogeneous catalysis. But synthesizing solid catalysts that
combine hostile functions is no easy task. Not surprisingly,
examples of solids with both acidic and basic functions are
limited. The reported examples combine acids such as
sulfonic acids, silanols, ureas, and thiols with amines.[4]
However, problems such as low catalytic activity because of
weak acidity, complicated preparation, and lack of a contin-
uous range of acidic and basic catalytic sites often significantly
limit their application in organic reactions.
We demonstrate the catalytic efficiency and robustness of this
new system in two cascade reactions: a tandem deprotection–
Henry reaction, and a tandem deprotection–aldol reaction.
Excellent yields and selectivities are obtained in both cases.
Figure 1 shows the synthesis of the catalyst. We started by
making a mesoporous silica support S (Figure 1, top; pore
diameter of 4.9 nm and surface area of 870 m2 gÀ1, see the
Experimental Section). The base functions were then added
Here, we present a simple and straightforward route to
such bifunctional solids. We combine amine base functions
and heteropolyacid functions on periodic mesoporous silica,
obtaining an efficient and robust bifunctional acid–base solid
catalyst. This new material enables one-pot cooperative
catalysis. Moreover, the synthesis permits easy tailoring of
the acid/base properties by controlling the number and
surface concentration of the acid and base sites, respectively.
[*] Dr. N. R. Shiju, Dr. A. H. Alberts, Prof. Dr. G. Rothenberg
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904
1098 XH Amsterdam (The Netherlands)
E-mail: n.r.shiju@uva.nl
Figure 1. Catalyst synthesis.
by grafting 3-aminopropyl groups, giving the base catalyst SB
(Figure 1, middle). This material was then immersed in a
methanolic solution of phosphotungstic acid (H3PW12O40).
The 3-ammoniumpropyl groups immobilized the acid poly-
anions, creating the acid/base catalyst SAB (Figure 1,
bottom). By controlling the ratio of immobilized heteropo-
lyacids and aminopropyl tethers, we succeeded in reacting
only part of the amino groups, creating bifunctional catalytic
sites inside the silica mesopores.
g.rothenberg@uva.nl
Homepage: http://home.medewerker.uva.nl/n.r.shiju/
Prof. Dr. D. R. Brown
Materials and Catalysis Research Centre
Department of Chemical and Biological Sciences
University of Huddersfield, Queensgate HD1 3DH (UK)
Dr. S. Khalid
National Synchrotron Light Source
Brookhaven National Laboratory, Upton, NY 11973 (USA)
The catalysts were characterized before and after each
functionalization step. Figure 2 shows the N2 adsorption
isotherms of S and SAB. Both materials are mesoporous,
with average pore diameters of 4.9 and 3.4 nm, respectively.
The isotherms show that grafting the base and immobilizing
the acid do not change the structure. SAB adsorbs less
[**] We thank the EPSRC Solid-state NMR service, University of Durham
for assistance with the solid-state NMR spectra.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101449.
Re-use of this article is permitted in accordance with the Terms and
Conditions set out at http://angewandte.org/open.
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Figure 2. N2 adsorption isotherms at standard temperature and pres-
sure (left) and pore-size distribution (right) of S and SAB.
nitrogen than S, because a fraction of the pores is already
occupied by the aminopropyl and phosphotungstic groups.
However, the pores are still wide enough to allow easy
diffusion of reactant and product molecules. The structure
retention after immobilization was also confirmed by X-ray
diffraction (see the Supporting Information).
13C cross-polarization (CP) magic angle spinning (MAS)
NMR spectra showed peaks at 11, 22, and 43 ppm (Figure 3).
These peaks are typical for the SiCH2CH2CH2NH2 chain,
Figure 4. 29Si CP-MAS NMR spectrum of SAB. Peaks at À110, À101,
À91, À68, and À58 ppm were assigned to the Q4 (Si(OSi)4), Q3
(Si(OH)(OSi)3), Q2 (Si(OH)2(OSi)2), T3 (SiR(OSi)3), and T2 (Si(OH)R-
(OSi)2) sites, respectively.
Figure 3. 13C CP-MAS NMR spectrum of SAB, indicating successful
grafting of the aminopropyl groups on S.
Figure 5. The tungsten L3 edge XANES spectra of SAB (straight line)
and bulk phosphotungstic acid (dotted line), indicating retention of
the Keggin structure after immobilization.
indicating the successful grafting of the aminopropyl groups
on silica support S. The presence of T3 and T2 functionalities
in the 29Si CP-MAS NMR spectra (Figure 4) confirmed a
strong covalent linkage between the organic groups and the
silica surface (see also Figure S3 in the Supporting Informa-
tion). A comparison of the W-L3 edge region in the X-ray
absorption near-edge structure (XANES) spectrum of SAB
with that of bulk phosphotungstic acid showed no major shifts
in position and amplitude. This confirms the absence of
geometrical or electronic changes in the immobilized groups
(Figure 5). Moreover, 31P NMR spectroscopy also showed
that the Keggin structure of the phosphotungstic acid was
maintained after immobilization (singlet at À15.5 ppm, see
the Supporting Information).
We then investigated the bifunctionality of catalyst SAB
in the one-pot tandem conversion of dimethoxymethylben-
zene (benzaldehydedimethylacetal) 1 to trans-1-nitro-2-phe-
nylethylene 3 (Scheme 1; the functionality that catalyzes each
step is shown in blue). The first step of this reaction is the
acid-catalyzed deacetalization of 1 to benzaldehyde 2. In the
second step, which is base-catalyzed, the benzaldehyde reacts
with nitromethane, giving the nitro product 3. Table 1 shows
the results. High conversions and yields were observed in the
presence of the SAB catalyst (entry 1 in Table 1). When SB
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for the tandem deacetalization–nitroaldol reaction is the ratio
between amino groups and polyanions. If all the amino groups
are used for polyanion immobilization, the reaction stops at
the first step and benzaldehyde is the sole product (entry 3,
Table 1). In our bifunctional catalyst, only part of the amine
groups are used for immobilizing polyacids; the remaining
half are free. We also confirmed that immobilization does not
affect the intrinsic activity of polyacid groups by comparing
the catalytic activities of SAB and bulk phosphotungstic acid
in ethyl acetate hydrolysis reaction. The catalytic activities,
calculated based on the amount of acidic protons (measured
by pyridine adsorption), were similar (102 and 95 mmol
molÀ1 minÀ1).
Control reactions confirmed that S alone did not catalyze
the reaction of 1 under the same conditions (entry 5, Table 1).
Moreover, the SAB catalyst was easily recovered by filtration
and reused several times without the loss of the catalytic
activity and selectivity (entry 6, Table 1; the reactions stopped
as soon as the catalyst was filtered off the hot reacting
mixture). This rules out the possibility of leaching of active
species into the reaction solution.
Similarly, we evaluated the SAB catalyst in the tandem
reaction of 1 through 2 to benzylidene malononitrile 4
(Scheme 2). Following the first deacetalization step, 2 under-
goes a base-catalyzed aldol condensation with malononitrile.
SAB efficiently catalyzed this tandem deacetalization–aldol
reaction as well, which yielded 91% of 4 and shows the
versatility of our catalyst (Table 2). Control reactions in the
presence of homogeneous p-toluenesulfonic acid gave no
product because of the neutralization of the amino groups of
the catalyst.
Scheme 1. One-pot tandem conversion of benzaldehydedimethylacetal
1 to trans-1-nitro-2-phenylethylene 3. The acid sites of SAB catalyze the
deacetalization first and the base sites then catalyze the Henry
condensation.
Table 1: Tandem deprotection–Henry reaction.[a]
Entry Catalyst Substrate Conversion [%] Yield GC/isolated [%]
2
tr
–
95/–
–
tr
3
97/92
–/–
5/–
>99/91
tr
1
2
3
4
5
6
SAB[b]
SB
1
1
1
2
1
1
ꢀ98
–
ꢀ99
>99
tr
SAB[c]
SB[e]
S
SAB[d]
95
tr
94/90
[a] Reaction conditions: benzaldehyde dimethyl acetal (1 mmol),
CH3NO2 (10 mL), 508C, 12 h. [b] Phosphotungstic acid (HPW)/amino-
propyl (AP) ratio around 0.5:1 (molar). [c] HPW/AP ratio around 1:1
(molar). [d] Fourth recycle of the catalyst of entry 1. [e] For Henry reaction
alone starting from entry 2. tr: trace.
was used instead of SAB, no reaction occurred with 1 (entry 2
in Table 1). However, control reactions starting from 2
confirmed that SB is an excellent base catalyst for the
second step, converting benzaldehyde with nitromethane to 3
in near-quantitative yield (entry 4 in Table 1). These results
show that both the amino groups and the acid polyanions in
SAB retain their reactivity. The immobilized polyanions,
which are still acidic, catalyze the deacetalization of 1 to 2
(Figure 6). This is followed by the Henry reaction of 2 with
nitromethane at the free amino groups. The key requirement
In conclusion, we showed that phosphotungstic acid
immobilized on amine-grafted mesoporous silica is an effi-
cient bifunctional catalyst. The catalyst combines two antag-
onistic active functions on one solid catalyst, allowing acid–
Scheme 2. Tandem deacetalization–aldol reaction of dimethoxymethyl-
benzene 1 through 2 to benzylidene malononitrile 4.
Table 2: Tandem deprotection–aldol reaction.[a]
Entry Catalyst Substrate Conversion [%] Yield GC/isolated [%]
2
tr
–
4
99/91
–/–
5/–
>99/92
tr
1
2
3
4
5
6
SAB[b]
SB
1
1
1
2
1
1
>98
–
ꢀ99
>99
tr
SAB[c]
SB[e]
S
ꢀ95/–
–
tr
tr
SAB[d]
95
ꢀ95/91
[a] Reaction conditions: benzaldehyde dimethyl acetal (1 mmol),
CH2(CN)2 (10 mL), 508C, 12 h. [b] HPW/AP ratio around 0.5:1 (molar).
[c] HPW/AP ratio around 1:1 (molar). [d] Fourth recycle of the catalyst in
entry 1. [e] For the aldol reaction alone starting from 2. tr: trace.
Figure 6. The conceptual diagram showing the bifunctional catalysis
by free amino groups and phosphotungstic acid immobilized inside
the SBA-15 nanoreactors.
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Communications
powder form. To monitor the reaction, samples of the reaction
mixture were taken periodically and analyzed by gas chromatography
(GC, Perkin–Elmer Clarus 500) using a 50 m BP5 capillary column
and an free induction decay (FID) detector. After the completion of
the reaction, the catalyst was separated by filtration. The filtrate was
analyzed by GC and the products were confirmed by GC-MS and
1H NMR spectroscopy. The recovered catalyst was washed with
nitromethane and acetone, and was then reused for the above one-pot
reaction. Deacetalization–aldol reaction of benzaldehyde dimethyla-
cetal and malononitrile was also conducted in a similar way. Ethyl
acetate hydrolysis was carried out in a round bottom flask, fitted with
a condenser and a magnetic stirrer. A dilute aqueous solution of ethyl
acetate (10 wt%) was stirred with catalyst powder (2 wt%) at 343 K
and the catalytic activity was measured by GC.
base tandem conversions in a single pot. We can prepare this
catalyst as well as tune it easily. The catalyst can be made
predominantly basic, or predominantly acidic, or equally
acidic and basic by changing the ratio of polyacid and amine
groups. Similar types of catalysts can be synthesized using
other heteropolyacids, which further widens the scope of
these materials.
Experimental Section
Catalyst synthesis: Mesoporous silica SBA-15 (S) was synthesized by
route using block copolymer templates following a literature report.[5]
Pluronic P123 (average molecular weight of 5800, EO20PO70EO20,
BASF; 4.0 g) was dissolved in a solution of distilled water and 2m HCl
(125 and 25 g, respectively) and stirred at 258C. After stirring for 3 h,
tetraethyl orthosilicate (TEOS, Aldrich; 8.6 g) was added slowly
under stirring. The solution was vigorously stirred at 408C for 24 h.
The mixture was aged at 808C for 48 h. The resulting white solid was
filtered off, washed, and air-dried at room temperature for 24 h. The
sample was calcined in air at 5508C for 5 h.
Received: February 27, 2011
Revised: July 13, 2011
Published online: September 15, 2011
Keywords: heterogeneous catalysis · mesoporous materials ·
.
NMR spectroscopy
SBA-15 was heated at reflux with 3-aminopropyltriethoxysilane
in dry toluene for 24 h to obtain aminopropyl-grafted mesoporous
silica (SB). The amount of grafted amino groups were determined by
elemental analysis.
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a
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Characterization of the catalyst: Powder X-ray diffraction
patterns were collected on a Bruker powder X-ray diffractometer
(D5000) operated at 40 kV and 20 mA using nickel-filtered Cu Ka
radiation (1.5406 ꢀ). N2 adsorption–desorption isotherms were
measured at 77 K on a micromeritics ASAP-2000 after evacuation
at 473 K for 5 h. The surface areas and the average pore sizes were
calculated by the Brunauer–Emmett–Teller (BET) and Barrett–
Joyner–Halender (BJH) methods, respectively. The solid-state 13C
and 29Si NMR spectra were obtained using a Varian VNMRS
spectrometer operating at 100.56 mhz for 13C and 79.44 mhz for 29Si
equipped with a 6 mm (outside diameter of the rotor) MAS probe.
The spectral referencing is with respect to neat, external tetrame-
thylsilane.31P NMR spectra were obtained with a Varian VNMRS
instrument operating at 161.88 mhz. A direct polarization pulse
sequence was used to record the spectra, with a long recycle (300 s).
The spectra were referenced with respect to 85% H3PO4.
The powder X-ray diffraction pattern of the calcined material (S)
shows the typical pattern of SBA-15 (Figure S1 in the Supporting
Information). The retention of the SBA-15 structure after immobi-
lization was also confirmed by X-ray diffraction (see Figure S2a in the
Supporting Information). A major signal at À15.5 ppm was observed
in the 31P NMR spectrum of SAB (see Figure S2b in the Supporting
Information). Phosphotungstic acid (HPW) has a symmetrical cubic
structure with a central P atom.[6] The signal at À15.5 ppm confirms
that the Keggin structure of the HPW was maintained after
immobilization.[6] There is a minor peak at À13.7 ppm, which may
be due to distortion of the HPW cubic structure because of the
functionalization. However, this is only a negligible fraction of the
immobilized HPW.
X-ray absorption spectra around the W-L3 edge were recorded at
the X18 A beam line at the National Synchrotron Light Source,
Brookhaven National Laboratory. We used a Si(111) double-crystal
monochromator. The energy was calibrated using a W foil.
Catalytic testing: The reactions were carried out in liquid phase in
a 50 mL glass reactor equipped with a condenser and a magnetic
stirrer. Benzaldehyde dimethylacetal (1 mmol) and nitromethane
(10 mL) were stirred at 508C under N2 with 30 mg of the catalyst in
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