Synergy between Bronsted acid sites and Lewis acid sites

Article abstract of DOI:10.1039/b807953f

Synergy between Bronsted acid sites and Lewis acid sites in mesoporous Al-Zr-TUD-1 was demonstrated to exist in Bronsted acid catalysed reactions, but not in Lewis acid catalysed reactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807953f

View Article Online / Journal Homepage / Table of Contents for this issue
Chemical Communications
www.rsc.org/chemcomm
Number 38 | 14 October 2008 | Pages 4509–4648
ISSN 1359-7345
FEATURE ARTICLE
Phillip A. Gale
Synthetic indole, carbazole, biindole
and indolocarbazole-based receptors:
COMMUNICATION
Ulf Hanefeld et al.
Synergy between Brønsted acid sites applications in anion complexation and
and Lewis acid sites sensing
1359-7345(2008)38;1-W

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Synergy between Brønsted acid sites and Lewis acid sites
Selvedin Telalovic,a Jeck Fei Ng,b Rajamanickam Maheswari,c
Anand Ramanathan,d Gaik Khuan Chuahb and Ulf Hanefeld*a
Received (in Cambridge, UK) 9th May 2008, Accepted 29th May 2008
First published as an Advance Article on the web 24th June 2008
DOI: 10.1039/b807953f
Synergy between Brønsted acid sites and Lewis acid sites in
mesoporous Al-Zr-TUD-1 was demonstrated to exist in
Brønsted acid catalysed reactions, but not in Lewis acid cata-
lysed reactions.
Heterogeneous acidic catalysts are of great importance for the
environmentally benign synthesis of bulk and fine chemicals.1
Scheme 1 Meerwein–Ponndorf–Verley reduction of 4-tert-butyl
cyclohexanone.
Zeolites and mesoporous silicates (MPS) in particular have
been widely used for this purpose.2,3 While purely siliceous
zeolites and MPSs show low acidity, Brønsted and Lewis
acidity can be imparted by the incorporation of metals such
as Al into the framework of these materials. The influence of
the Brønsted acid sites on the Lewis acid sites (and vice versa)
in the thus obtained materials has long been a topic of debate.
In particular, possible synergy was and is a focus of interest. A
recent study on dealuminated HY zeolite supports the notion
that synergy does indeed exist, demonstrating interactions
between Al induced Brønsted and Lewis acid sites.4 While
Al induces both Brønsted and Lewis acid sites, tetrahedrally
coordinated Zr tends to impart only Lewis acidity to the
material.5,6 It should therefore be possible to vary the ratio
of Brønsted and Lewis acid sites in a material by incorporating
both metals in different ratios. When Al-Zr-beta was prepared,
only marginal synergy was observed in the isomerisation of
m-xylene,7 while adding Zr to Al-MCM-41 only allowed for
an increase of its Lewis acidity in line with the amount of Zr
added, no experiments to probe for a possible synergy effect
were performed.8
overall Si/M ratio of 25. By comparing the activity of these
catalysts with physical mixtures of Al-TUD-1 and Zr-TUD-1
with Si/M of 25, synergy should become detectable.
Al-Zr-TUDs with Si/M ratios of 25 and Al : Zr ratios of
3 : 1, 2 : 2 and 1 : 3 (denoted Al-Zr-3 : 1-TUD-1, Al-Zr-
2 : 2-TUD-1 and Al-Zr-1 : 3-TUD-1) were prepared in an
analogous manner to Al- and Zr-TUD-1, via a one-pot
procedure based on the sol–gel technique.10,12 Triethanola-
mine as a complexing reagent for both Al and Zr ensured their
incorporation as isolated species, a process also known as the
‘‘atrane route’’.19 The three catalysts obtained were mesopor-
ous with surface areas, pore volumes and pore diameters
comparable to the earlier prepared Al-TUD-1 and Zr-TUD-1
(Table 1). While the composition of the M-TUD-1s is identical
to the synthesis mixture and thus highly predictable when only
one metal is introduced, some deviation from the composition
of the synthesis mixture was observed for the Al-Zr-TUDs.
This might be due to differences in ionic radii of the tetra-
hedral Al(III), Zr(IV) and Si(IV) in the framework.
HR-TEM and powder X-ray diffraction studies unambigu-
ously demonstrated the material to be structured and meso-
porous, containing no crystalline phase (Fig. 1). This strongly
indicates that both Al and Zr are framework incorporated in
Recently we prepared Al-TUD-1, a sponge like MPS with a
three dimensional pore structure.3,9 This Brønsted and Lewis
acidic material proved to be a good catalyst in the Friedel–
Crafts alkylation.10 In contrast, the novel Zr-TUD-1 was
demonstrated to have exclusively Lewis acidity.11,12 It cata-
lysed the Meerwein–Ponndorf–Verley (MPV) reduction of 1, a
reaction that is catalysed by Lewis acids only (Scheme 1).13–15
It also showed activity in the Prins cyclisation of citronellal
3,
a
Brønsted and Lewis acid catalysed reaction
(Scheme 2).12,16–18 Based on these results we designed a series
of mixed Al-Zr-TUD-1s with different Al/Zr ratios and an
a Gebouw voor Scheikunde, Technische Universiteit Delft, Julianalaan
136, 2628 BL Delft, The Netherlands. E-mail: u.hanefeld@tudelft.nl;
Fax: þ31 15 278 1415; Tel: þ31 15 278 9304
b Department of Chemistry, National University of Singapore,
3 Science Drive 3, Singapore 11754, Republic of Singapore.
E-mail: chmcgk@nus.edu.sg; Fax: þ65 6779 1691;
Tel: þ65 6516 2839
c Department of Chemistry, Anna University, Chennai – 600025,
India. E-mail: maheswarianand@annauniv.edu
d Department of Chemistry, National Institute of Technology,
Tiruchirappalli – 620015, India. E-mail: ranand@nitt.edu
Scheme 2 Prins cyclisation of citronellal yields isopulegol and its
stereoisomers.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4631–4633 | 4631

Table 1 Physical properties of Al-TUD-1, Zr-TUD-1 and the different Al-Zr-TUD-1s
Synthesis mixture
After calcinationa
SBETb/m2 gꢁ1 dP,BJHc/nm
VP,BJHd/cm3 gꢁ1 Uptake NH3/mmol gꢁ1e
nSi/n(Al1Zr) nSi/nAl
nSi/nZr nSi/n(Al1Zr)
M-TUD-1
Zr-TUD-1
25
25
25
25
25
—
92
47
31
25
45
69
135
—
25
30
28
25
764
877
686
705
956
8.8
3.3
4.6
4.2
3.7
1.23
0.70
0.85
0.70
0.95
d
0.69
0.38
0.32
0.33
0.40
Al-Zr-1 : 3-TUD-1
Al-Zr-2 : 2-TUD-1
Al-Zr-3 : 1-TUD-1
Al-TUD-1
26.6
26.6
a
b
c
e
From elemental analysis. Specific surface area. Mesopore diameter from adsorption branch. Specific mesopore volume. Data from
NH3-TPD.
the Al-Zr-TUD-1s, similar to Al-TUD-1 and Zr-TUD-1.
Temperature-programmed desorption (TPD) of ammonia re-
vealed that Al-TUD-1 and all the Al-Zr-TUD-1 samples have
a very similar total acidity, while Zr-TUD-1 differs signifi-
cantly (Table 1). This is surprising and might indicate that Al
strongly influences the overall acidity of the materials.
Al-Zr-2 : 2-TUD-1 to 0.7 for Al-Zr-1 : 3-TUD-1. Thus a range
of MPS materials with either Al or Zr or mixtures thereof was
available to study the influence of Brønsted acid sites on Lewis
acid sites and vice versa.
In order to probe for synergy between Brønsted acid sites
and Lewis acid sites, the Lewis acid catalysed MPV reduction
of 1 was investigated (Scheme 1). Reaction conditions were
chosen to match earlier optimisation studies.12 Reduction of 1
in all cases yielded cis-2 and trans-2 in ratio cis : trans of
B13 : 87, in line with a non-selective reduction.12 As expected
for a Lewis acid catalysed reaction, Zr-TUD-1 catalysed the
reaction significantly better than Al-TUD-1, which has a less
Lewis acidic character (Fig. 3A). With increasing Zr content
Al-Zr-3 : 1-TUD-1, Al-Zr-2 : 2-TUD-1 and Al-Zr-1 : 3-TUD-1
display increasing activity in the MPV reduction (Fig. 3B). In
order to evaluate whether the Al-induced and Zr-induced
acidity influence each other in the Al-Zr-TUD-1s, physical
mixtures of Al-TUD-1 and Zr-TUD-1 with the same metal-
concentration as in Al-Zr-TUD-1 were also tested (Fig. 3A).
This revealed that the activities of all samples were dependent
only on the concentration of Zr in the sample and virtually
independent of the Al-concentration. This clearly demons-
trates that there is no synergy between Brønsted acid sites
and Lewis acid sites in the MPV reduction of 1.
FT-IR spectra of Al-Zr-TUD-1 samples after pyridine
desorption at 200 1C clearly show the presence of Brønsted
(1547 cmꢁ1
1448 cm
)
and Lewis acidity (Al: 1456 cmꢁ1 10
,
Zr:
;
ꢁ1 12 Fig. 2). With an increase of Zr the signal for the
Lewis acidic site due to Al decreases, as does that for
Brønsted acidity. Overall the ratio Brønsted/Lewis acidity
decreases from approximately 1.1 for Al-Zr-3 : 1-TUD-1 and
In dealuminated zeolite HY, synergy between Brønsted acid
sites and Lewis acids sites was shown to be present.4 However,
no reaction demonstrating this was performed. Since no
synergistic influence of Brønsted acid sites on Lewis acid sites
could be detected in the MPV reduction, we then turned to the
Prins cyclisation of citronellal (3), yielding isopulegol (4) and
its stereoisomers 5–7. Employing earlier described reaction
Fig. 1 HR-TEM (top, the bar represents 9 nm) and XRD (bottom) of
Fig. 2 FT-IR spectra after pyridine desorption at 200 1C: top Al-Zr-
3 : 1-TUD-1, middle Al-Zr-2 : 2-TUD-1 and bottom Al-Zr-1 : 3-TUD-1.
Al-Zr-1 : 3-TUD-1.
ꢀc
This journal is The Royal Society of Chemistry 2008
4632 | Chem. Commun., 2008, 4631–4633

Fig. 4 Cyclisation of 3 over Al- and Zr-TUD-1 catalysts. Reaction
conditions: 4 mmol 3, 5 g toluene, 80 1C, 50 mg catalyst. (J) Zr-
TUD-1, (ꢂ) Al-TUD-1, (’) Al-Zr-1: 3-TUD-1, (E) Al-Zr-2 : 2-TUD-1
and (m) Al-Zr-3 : 1-TUD-1.
Stimulating discussions with Prof. Bert Weckhuysen (Univer-
siteit Utrecht) are gratefully acknowledged.
Notes and references
1 R. A. Sheldon, I. W. C. E. Arends and U. Hanefeld, Green
Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007.
2 Fine Chemicals through Heterogeneous Catalysis, ed. R. A. Sheldon
and H. van Bekkum, Wiley-VCH, Weinheim, 2001.
3 Y. Wan and D. Zhao, Chem. Rev., 2007, 107, 2821–2860.
4 S. Li, A. Zheng, Y. Su, H. Zhang, L. Chen, J. Yang, C. Ye and F.
Deng, J. Am. Chem. Soc., 2007, 129, 11161–11171.
5 M. Boronat, A. Corma and M. Renz, J. Phys. Chem. B, 2006, 110,
21168–21174.
Fig. 3 Reduction of 1 over Al- and Zr-TUD-1 catalysts. Reaction
conditions: 2 mmol 1, 4 ml isopropanol, 80 1C, 50 or 100 mg catalyst.
A: K Al-TUD-1 (50 mg), ’ Zr-TUD-1 (50 mg), J Al-Zr-2 : 2-TUD-1
(50 mg), . Al-TUD-1 (25 mg) and Zr-TUD-1 (25 mg), n Al-TUD-1
(50 mg) and Zr-TUD-1 (50 mg); B: . Al-Zr-3 : 1-TUD-1 (50 mg), ’
Al-Zr-2 : 2-TUD-1 (50 mg), K Al-Zr-1 : 3-TUD-1 (50 mg).
6 Y. Zhu, G. Chuah and S. Jaenicke, J. Catal., 2004, 227, 1–10.
7 B. Rakshe, V. Ramaswamy and A. V. Ramaswamy, J. Catal.,
1999, 188, 252–260.
8 I. Eswaramoorthi, V. Sundaramurthy and N. Lingappan, Micro-
porous Mesoporous Mater., 2004, 71, 109–115.
9 C. Simons, U. Hanefeld, I. W. C. E. Arends, R. A. Sheldon and T.
Maschmeyer, Chem.–Eur. J., 2004, 10, 5829–5835.
10 R. Anand, R. Maheswari and U. Hanefeld, J. Catal., 2006, 242,
82–91.
11 A. Ramanathan, D. Klomp, J. A. Peters and U. Hanefeld, J. Mol.
Catal. A: Chem., 2006, 260, 62–69.
12 A. Ramanathan, M. C. Castro Villalobos, C. Kwakernaak, S.
Telalovic and U. Hanefeld, Chem.–Eur. J., 2008, 14, 961–972.
13 G. K. Chuah, S. Jaenicke, Y. Z. Zhu and S. H. Liu, Curr. Org.
Chem., 2006, 10, 1639–1654.
conditions, this Brønsted acid catalysed reaction was per-
formed with Zr-TUD-1, Al-TUD-1, Al-Zr-3 : 1-TUD-1,
Al-Zr-2 : 2-TUD-1 and Al-Zr-1 : 3-TUD-1 (Fig. 4). Zr-TUD-
1 had the lowest activity, while Al-TUD-1 displayed signifi-
cantly higher activity. Remarkably, the activities of all Al-Zr-
TUD-1s were higher than that of either Zr-TUD-1 or
Al-TUD-1. As the metal concentration in the mixed Al-Zr-
TUD-1s is the same as in either Zr-TUD-1 or Al-TUD-1 and
given the fact that the overall acidity is virtually identical to
Al-TUD-1 and lower than Zr-TUD-1 this is unambiguous
proof of synergy between the Al induced Brønsted acid sites
and the Zr induced Lewis acid sites.
14 D. Klomp, T. Maschmeyer, U. Hanefeld and J. A. Peters,
Chem.–Eur. J., 2004, 10, 2088–2093.
15 D. Klomp, J. A. Peters and U. Hanefeld, Transfer hydrogenation
including the Meerwein–Ponndorf–Verley reduction, in Handbook
of Homogeneous Hydrogenation, ed. J. G. de Vries and C. J.
Elsevier, Wiley, Weinheim, 2007, ch. 20, vol. 3, pp. 585–630.
16 H. J. Prins, Chem. Weekbl., 1917, 14, 932–939.
17 H. J. Prins, Chem. Weekbl., 1917, 14, 627–630.
18 Y. Z. Zhu, Y. T. Nie, S. Jaenicke and G.-K. Chuah, J. Catal., 2005,
229, 404–413.
In conclusion, mixed Al-Zr-TUD-1 samples clearly demon-
strate synergy for the Brønsted acid catalysed Prins cyclisation
of 3. In contrast, no synergy was observed in the Lewis acid
catalysed MPV reduction of 1.
The authors thank Dr P. Kooyman of DCT/NCHREM,
TU Delft, for the electron microscopy investigations. ST
thanks NWO (Mozaıek fellowship) for financial support.
19 J. El Haskouri, S. Cabrera, C. Guillem, J. Latorre, A. Beltran, D.
Beltran, M. D. Marcos and P. Amoros, Chem. Mater., 2002, 14,
5015–5022.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4631–4633 | 4633