SnTUD-1: A solid acid catalyst for three component coupling reactions at room temperature

Article abstract of DOI:10.1039/c3gc40792f

A novel wormhole structured mesoporous material containing tin, SnTUD-1, was prepared by a direct hydrothermal synthesis method using triethanolamine (TEA) as an organic inexpensive mesoporous structure directing agent. XRD and N₂ sorption studies of SnTUD-1 confirmed the amorphous mesoporous nature of SnTUD-1, which possessed a large surface area of 555 m² g^-1 and a pore diameter of 7.4 nm. HR-TEM further ascertained the disordered pores in their morphology and the presence of nano-domains of SnO₂ species. The nature of the Sn⁴⁺ ion co-ordination with the silica matrix was evaluated by using different techniques including diffuse reflectance UV-Vis, FTIR, ²⁹Si MAS NMR and XPS. SnTUD-1 had an interesting Lewis acidity as measured by FTIR of pyridine adsorption which provided excellent activities in one-pot three-component Mannich-type reactions of ketones with aldehydes and amines at room temperature.

Full text of DOI:10.1039/c3gc40792f

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SnTUD-1: a solid acid catalyst for three component
coupling reactions at room temperature†
Cite this: DOI: 10.1039/c3gc40792f
Muthusamy Poomalai Pachamuthu,a Kannan Shanthi,a Rafael Luque*b and
Anand Ramanathan*c
A novel wormhole structured mesoporous material containing tin, SnTUD-1, was prepared by a direct
hydrothermal synthesis method using triethanolamine (TEA) as an organic inexpensive mesoporous
structure directing agent. XRD and N2 sorption studies of SnTUD-1 confirmed the amorphous meso-
porous nature of SnTUD-1, which possessed a large surface area of 555 m2 g−1 and a pore diameter of
7.4 nm. HR-TEM further ascertained the disordered pores in their morphology and the presence of nano-
domains of SnO2 species. The nature of the Sn4+ ion co-ordination with the silica matrix was evaluated
by using different techniques including diffuse reflectance UV-Vis, FTIR, 29Si MAS NMR and XPS.
SnTUD-1 had an interesting Lewis acidity as measured by FTIR of pyridine adsorption which provided
excellent activities in one-pot three-component Mannich-type reactions of ketones with aldehydes and
amines at room temperature.
Received 26th April 2013,
Accepted 18th June 2013
DOI: 10.1039/c3gc40792f
www.rsc.org/greenchem
redox and solid acid catalysts for a number of industrially rele-
vant chemistries.
1. Introduction
Heterogeneous solid acid catalysts have been increasingly
Among the mesoporous silicate family, a variety of catalyti-
investigated in recent years due to their important applications cally active TUD-1 type materials have been reported in recent
in greener organic synthesis and more environmentally sound years following the original report of the purely siliceous
catalytic processes in replacement of mineral homogeneous TUD-1.23 TUD-1 preparation featured a simple, low cost tem-
acids.1,2 Ordered porous silicates constitute a relevant family plating (TEA) synthesis route which was able to produce
of materials which have found extensive applications as cata- materials with three dimensional interconnected pores,
lyst supports, separation, adsorption etc. due to their large tunable pore sizes and the possibility of higher incorporation
surface area and uniform pore sizes.3 Pure siliceous silicates of active metal ions (M+).24 In view of the recent interests of
have been conveniently modified in various ways (e.g. incorpor- the groups in terms of the design of novel highly active nano-
ation of Al3+, Fe3+, Ga3+, Zr4+, Sn4+ either during the synthetic porous materials for catalytic applications, we have been inter-
procedure or post-synthesis) to develop acidity rendering cata- ested in exploring Sn incorporation into TUD-1 to develop
lytically active materials in acid-catalysed processes. Tin based SnTUD-1 materials with catalytic applications in multicompo-
silicates have been utilized as Lewis acid catalysts in a range of nent reactions. These chemistries have been carried out using
important organic transformations,4–10 with tin containing homogeneous as well as heterogeneous catalyst systems.25,26
mesoporous molecular sieves employed in the conversion of One type of such important multicomponent chemistries are
bulkier substrates. Particularly, tin based mesoporous Mannich type reactions, as β-amino carbonyl compounds are
materials Sn-MCM-41,11–14 Sn-MCM-48,15 Sn-SBA-15,16–19 Sn- some of the most essential intermediates for the construction
HMS,20,21 and Sn-PMO22 have been utilised in recent years as of various nitrogen containing natural products and
pharmaceuticals.27–29 For this type of C–C bond forming reac-
tion, different types of acid catalysts have been reported,
aDepartment of Chemistry, Anna University, Chennai, 600025, India
including bismuth triflate,30 WOx–ZrO2,31 NbCl5,32 HClO4–
bDepartamento de Quimica Organica, Universidad de Cordoba, Campus de
SiO2,33 adenine (aminocatalyst),34 quaternary ammonium salt
Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, 14014 Cordoba,
(QAS) Gemini fluorosurfactants,35 Nafion-H®,36 SiO2–AlCl3,37
Spain. E-mail: q62alsor@uco.es; Fax: +34 212066; Tel: +34 957211050
cCenter for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence,
heteropoly acids,38 indium trichloride,39 Cu-nanoparticles40
KS 66047, USA. E-mail: anand@ku.edu; Fax: +1-785-864-6051;
and inorganic–organic hybrid silica based tin(II) catalyst.41
Tel: +1-785-864-1631
Herein, we report for the first time the direct incorporation
of Sn4+ into TUD-1 material that produced a catalytically active
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc40792f
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SnTUD-1 in Mannich-type three component coupling reactions atmospheric pressure to remove the physisorbed pyridine
at room temperature.
and FTIR analysis was carried out in the DRIFT mode. X-ray
photoelectron spectroscopy (XPS) measurements were per-
formed in an ultra-high vacuum (UHV) multipurpose surface
analysis system (Specs™ model, Germany) using a conven-
tional X-ray source (XR-50, Specs, Mg-Kα, 1253.6 eV) in a
“stop-and-go” mode. The survey and detailed O, Si and Sn
were recorded at room temperature with a Phoibos 150-MCD
energy analyzer. The sample was deposited on a sample
holder using double-sided adhesive tape and afterwards evacu-
ated under vacuum (<10−6 Torr) overnight. The binding
energies were referenced to the C1s line at 284.6 eV from
adventitious carbon. The 29Si MAS NMR spectra (pulse: 6 μs;
2. Experimental
2.1. Synthesis of SnTUD-1
Tin containing mesoporous TUD-1 with a Si/Sn molar ratio
of 50 was synthesized by a sol–gel method followed by
hydrothermal treatment based on our previously reported
procedure.42 SnTUD-1(50) was synthesized using tetraethyl-
orthosilicate (TEOS, Sigma Aldrich, 99%) and tin tetrachlo-
ride (SnCl4·5H2O, Sigma Aldrich, 98%) as silicon and tin
precursors, respectively. Triethanolamine (TEA, SRL) was
used as a simple mesoporous structure directing agent. Tetra-
ethyl ammonium hydroxide (TEAOH, 35%, Sigma Aldrich)
was used as a base. In a typical catalyst preparation 20 g of
TEOS and 0.67 g of tin chloride in 2 g of water were stirred
for 10 minutes. A mixture of TEA (14.3 g) and water (3 g)
was then added to the above mixture and stirred vigorously
for 1 h. Finally, TEAOH (13 g) was added dropwise to the
previous solution under continuous stirring for another 2 h.
The final gel composition was 1SiO2 : 0.02SnO2 : 0.3TEAOH :
1TEA : 10H2O. The obtained gel was aged at 30 °C for 16 h
and then dried at 100 °C for 20 h. The final solid was trans-
ferred into an autoclave for hydrothermal treatment at
180 °C for 6 h and subsequently calcined at 600 °C for 10 h
to remove the organic moieties.
recycle delay: 600 s) were recorded on
a
Bruker
ACP-400 multinuclear spectrometer at 79.45 MHz. The
chemical shifts have been referenced to tetramethylsilane
(TMS) as an external standard.
ICP-OES leaching studies were conducted on a Perkin-
Elmer Optima 5300 DV instrument. Samples (the catalyst,
the filtrate after first use and subsequent filtrates and
recovered catalysts, 10 mg each) were digested in 0.5 mL HF
and then completed with 25 mL distilled water prior to ana-
lysis. A wavelength of 189.927 nm for Sn was used in the
analysis.
2.3. Catalytic application of SnTUD-1 for Mannich-type
reactions
The Mannich-type reaction was carried out at room tempera-
ture in a 10 mL closed reaction tube. SnTUD-1 catalyst
(100 mg) was added to a solution containing a mixture of alde-
hyde (1 mmol), ketone (1 mmol) and amine (1 mmol) in
ethanol (5 mL) solvent. The progress of the reaction is moni-
tored using TLC. The reaction mixture was stirred at 750 rpm
for an appropriate time and then filtered. The filtrate was
washed with a sodium bicarbonate (NaHCO3) solution fol-
lowed by distilled water and dried over sodium sulphate
(Na2SO4). The final organic layer was evaporated and the crude
product was purified by silica gel column chromatography
with petroleum ether–ethyl acetate (5 : 1). The structures of
2.2. Characterization of SnTUD-1
Powder X-ray diffraction (XRD) measurements were carried out
on a Brucker D8 diffractometer with a nickel filtered Cu Kα
(λ = 1.5418 Å) radiation source, operating at 40 kV and 40 mA.
Transmission electron microscopy (TEM) was performed on
FEI Tecnai G2 fitted with a CCD camera. The sample was sus-
pended in ethanol and deposited on a copper grid prior to
analysis. Scanning electron microscopy (SEM) was performed
using an ESEM Quanta 200, FEI microscope with a resolution
of 20 kV. The surface area, pore volume and pore size distri-
bution were evaluated from N2 sorption carried out at −196 °C
1
products were confirmed by H NMR and FTIR in comparison
with the literature. Examples of product characterisation are
shown below:
using
a Quantochrome porosimeter (QuadrasorbSI). The
sample was degassed at 200 °C for 3 h before the measure-
ment. Temperature programmed desorption (NH3-TPD) experi-
ments were performed under NH3 desorption monitoring
from 100 °C to 600 °C in a Micromeritics TPR/TPD 2900 instru-
ment with a Thermal Conductivity Detector (TCD). Diffuse
reflectance UV-Vis spectra were recorded in the range of
200–800 nm with Thermoscientific Evolution 600 equipment
with diffuse reflectance attachments, using BaSO4 as a refer-
1
ence. FTIR spectra of KBr-diluted pellets of the sample were (Table 1, entry 1) A white solid, H NMR (500 MHz, CDCl3): δ
recorded on a Bruker instrument at room temperature with a 3.56–3.43 (m, 2H), 5.03 (m, 1H), 6.62–6.57 (d, 2H), 6.81–6.66
resolution of 4 cm−1 averaged over 100 scans. For pyridine (m, 1H), 7.08–7.15 (m, 2H), 7.22–7.34 (m, 4H), 7.55–7.58 (m,
FTIR analysis, the catalyst was dried at 200 °C for 2 h to 2H), 7.89–7.93 (m, 2H); 13C NMR (500 MHz, CDCl3): 46.4, 54.9,
remove the adsorbed species. Further, 100 mg of the oven 59.6, 118.0, 127.5, 128.3, 128.9, 133.5, 136.8, 147.0, 198.4; FTIR
dried sample was wetted with pyridine and equilibrated for (KBr, cm−1): 3380, 3059, 3021, 2921, 1671, 1596, 1500, 1218,
5 h. The sample was then heated at 120 °C for 1 h at 1180, 748.
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Table 1 Mannich-type three component couplings catalysed by SnTUD-1a
7.29 (m, 2H), 7.46–7.36 (d, 2H), 7.56–7.46 (m, 1H); FTIR (KBr,
cm−1): 3380, 3030, 2932, 1669, 1597, 1507, 1281, 1107, 842,
740, 687, 551.
Aldehyde Ketone
Amine
R3
Entry
R1
R2
t (h) Yieldb
c
Blank
1
2
3
4
5
6
7
8
9
10
11
12
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4-Cl
12
12
10
10
12
12
24
10
24
24
12
6
—
89
80
82
75
65
—
80
60
—
70
40
73
4-Cl
4-CH3
4-C2H5
4-OCH3
2-NO2
4-Br
H
H
H
H
H
Cyclohexanone
4′-Methyl acetophenone
4-Cl
Acetone
H
(Table 1, entry 5) A pale yellow solid, 1H NMR (500 MHz,
CDCl3): δ 3.47–3.41 (b, 2H), 3.79 (s, 3H), 5.01 (t, 1H), 6.49 (d,
J = 8.1 Hz, 2H), 6.63 (d, 1H), 6.90–6.86 (m, 2H), 7.26 (d, J = 8.0
Hz, 2H), 7.43 (m, 2H), 7.56–7.52 (m, 1H), 7.89 (d, 2H); FTIR
(KBr, cm−1): 3396, 3042, 2924, 1662, 1596, 1501, 1282, 752.
12
a Reaction conditions: benzaldehyde (1 mmol), acetophenone
(1 mmol), aniline (1 mmol), ethanol (3 mL), catalyst (100 mg), room
temperature. b Isolated yield, products are confirmed by 1H NMR and
FTIR. c Reaction carried out with purely siliceous TUD-1 as a catalyst.
(Table 1, entry 7) A yellowish solid, 1H NMR (500 MHz, CDCl3):
1H NMR (500 MHz, CDCl3): δ 3.58 (br, 2H), 4.99 (t, 1H), 6.50
(d, J = 8.4 Hz, 2H), 6.65 (m, 1H), 6.89–6.96 (m, 2H), 7.15 (t,
2H), 7.32–7.22 (m, 1H), 7.40–7.35 (m, 2H), 7.50–7.46 (m, 1H),
7.56–7.52 (m, 2H); FTIR (KBr, cm−1): 3390, 3047, 1665, 1600,
1280, 1092, 756, 684.
1
(Table 1, entry 2) A white solid, H NMR (500 MHz, CDCl3): δ
3.58 (br, 2H), 5.01 (t, 1H), 6.66 (d, J = 8.1 Hz, 2H), 6.78 (m, 1H),
6.87–6.97 (b, 2H), 7.15 (t, J = 7.5 Hz, 2H), 7.32–7.22 (m, 1H),
7.40–7.35 (m, 2H), 7.56–7.52 (m, 1H), 7.88 (d, J = 7.2 Hz, 2H);
FTIR (KBr, cm−1): 3392, 3047, 2921, 1664, 1602, 1501, 1287,
1093, 754.
1
(Table 1, entry 8) A white solid, H NMR (500 MHz, CDCl3): δ
1.80–1.62 (m, 3H), 1.98 (m, 3H), 2.44–2.30 (m, 2H), 2.80–2.74
(m, 1H), 4.62 (d, J = 7.4 Hz, 0.99H), 4.81–4.78 (d, 0.16H),
6.56–6.54 (d, 2H), 6.65–6.62 (t, 1H), 7.05–7.01 (t, J = 8.0 Hz,
2H), 7.24–7.20 (t, 1H), 7.34–7.30 (m, 2H), 7.58 (d, 2H), 8.16 (d,
2H); FTIR (KBr, cm−1): 3391, 1689, 1088, 762.
1
(Table 1, entry 3) A white solid, H NMR (500 MHz, CDCl3): δ
2.4 (s, 3H), 3.50–3.40 (m, 2H), 4.98 (t, 1H), 6.25 (d, J = 7.9 Hz,
2H), 6.90 (br, 1H), 7.1–7.0 (m, 2H), 7.18 (d, 2H), 7.26 (d, 2H),
7.43–7.33 (m, 2H), 7.55–7.46 (m, 1H), 7.92 (d, 2H); FTIR (KBr,
cm−1): 3388, 3037, 2923, 1668, 1603, 1505, 1287, 993, 752.
1
(Table 1, entry 10) A white solid, H NMR (500 MHz, CDCl3): δ
3.42 (m, 2H), 5.1 (s, 1H), 6.25 (d, J = 7.9 Hz, 2H), 6.78 (m, 1H),
1
(Table 1, entry 4) A white solid, H NMR (500 MHz, CDCl3): δ 7.2–7.1 (m, 2H), 7.29 (m, 2H), 7.46–7.36 (d, 2H), 7.56–7.46 (m,
1.3–1.2 (t, J = 7 Hz, 3H), 2.72 (m, 2H), 3.49–3.40 (m, 2H), 4.99 1H), 7.82 (d, 2H); FTIR (KBr, cm−1) 3386, 3045, 2921, 1668,
(t, 1H), 6.25 (d, J = 7.9 Hz, 2H), 6.78 (m, 1H), 7.2–7.1 (m, 2H), 1604, 1501, 1289, 759, 619.
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(Table 1, entry 12) A pale yellow solid, 1H NMR (500 MHz,
CDCl3): δ 3.42 (m, 2H), 5.01 (t, 1H), 6.42 (m, 2H), 6.86 (d, J =
8.1 Hz, 2H), 7.14–7.10 (m, 2H), 7.25 (t, 2H), 7.34–7.29 (m, 2H),
7.42 (m, 2H), 7.86 (d, 2H); FTIR (KBr, cm−1): 3380, 2921, 1670,
1599, 1279, 1085, 756, 616.
Fig. 2 Nitrogen sorption isotherm (a) and adsorption pore size distribution
(b) of SnTUD-1.
3. Results and discussion
A broad peak with low intensity approximately at 2θ = 0.5° was
observed to be present in small angle XRD patterns (Fig. 1a),
typical of disordered mesoporous materials such as
TUD-1.24,42 Despite the amorphous silica peak around 2θ of
15–30° observed in high angle powder XRD measurements
(Fig. 1b), the presence of a diffraction line corresponding to
tetragonal SnO2 nanoparticles (rutile structure) was evidenced
from indexed peaks at 2θ (27, 34 and 52°), in good agreement
with literature reports.16,21 Using the Debye–Sherrer equation,
the average crystalline size of SnO2 particles of SnTUD-1 was
calculated to be ca. 20 nm.
N2 sorption isotherms of SnTUD-1 (Fig. 2a) displayed a
Type IV isotherm with a hysteresis loop, typical of TUD-1
type materials.24 SnTUD-1 exhibited an average adsorption
pore size distribution of 7.4 nm (Fig. 2b) and a BET surface
area and pore volume of 555 m2 g−1 and 1.02 cm3 g−1
respectively.
,
Diffuse reflectance UV-Vis spectra of SnTUD-1 (Fig. 3a)
showed two main absorption bands centered at ca. 220 and
280 nm which can be assigned to the presence of isolated four
co-ordinated Sn4+ ions18,22 and hexacoordinated polymeric
Sn–O–Sn type species.12,14 These findings support the presence
of both framework incorporated tin and extraframework SnO2
Fig. 3 Diffuse reflectance UV-Vis (a) and FTIR (b) spectra of SnTUD-1.
species, in good agreement with the XRD results. FTIR spectra
of the calcined SnTUD-1 displayed a broad peak at 3480 cm−1
attributed to the O–H stretching vibration of surface Si–OH
groups as well as residual water present in the materials
(Fig. 3b). Bands at 1230, 1080 and 798 cm−1 could also be
assigned to symmetric and asymmetric stretching of uSi–O–
Siu vibrations, while the weak band at 960 cm−1 can be attri-
buted to Si–O–Sn linkages, as previously reported,12,18 which
Fig. 1 Powder XRD pattern of Sn-TUD-1 in (a) low angle and (b) high angle.
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Fig. 4 SEM (a) and TEM (b and c) images of SnTUD-1.
Fig. 5 XPS spectrum of SnTUD-1: (a) survey, (b) O1s, and (c) Sn3d.
again confirmed the partial incorporation of Sn into the frame- (SnOx). This observation further confirms the presence of tin
work of TUD-1. oxide nanoparticles as observed from XRD, DRS-UV-Vis and
SEM micrographs of Sn-TUD-1 samples depicted TEM studies. However, Sn3d XPS spectra (Fig. 5c) of SnTUD-1
micrometer sized (∼50 μm) irregular and agglomerated stone- provided the most interesting findings with two clearly distin-
like particles (Fig. 4a). The disordered wormhole type mor- guished tin oxide (SnOx) components in Sn3d5/2 (Fig. 5c). A
phology of TUD-1 was also evidenced from TEM micrographs distinction between Sn(IV) oxide and Sn(II) oxide is not possi-
(Fig. 4b and c). Tin oxide nanoparticles (which appeared as ble by XPS as only Mössbauer spectroscopy will be able to
dark spots, Fig. 4b) were found to be present mostly as agglo- provide insightful differentiation between Sn(IV) and lower-
merates at the surface of TUD-1, with nanoparticle sizes valency tin species.43,44 In any case, the major contribution
20–30 nm. These findings are in good agreement with XRD with a large area (ca. 487.6 eV) is, in principle, likely to be
observations. Interestingly, a number of highly dispersed tin related to framework Sn(IV) species in the materials in which
oxide nanoparticles (ca. 6 nm) were also present in the Sn4+ replaced Si4+ atoms. However, the presence of other tin
materials as can be seen in Fig. 4. SEM and TEM-EDAX spectra oxide species as part of this major contribution cannot be
(ESI, S1 and S2†) showed the presence of Si, O, and Sn. The ruled out based only on XPS studies. Comparatively, the
average amount of Sn atoms present in the TUD-1 silica matrix minor contribution at lower binding energy (ca. 484.7 eV),
was estimated to be about 2 wt% (atomic) from TEM-EDAX which is very close to the binding energy of Sn0 (ca. 484
and SEM-EDAX, in good correlation with the theoretical SiO2/ eV),44 indicates the presence of different tin oxide species
SnO2 molar ratio in the synthesis gel.
A typical XPS spectrum of the SnTUD-1 sample is shown oxide extraframework tin oxide nanoparticles which could
in Fig. 5. The survey scan (Fig. 5a) of SnTUD-1 showed peaks potentially include contribution of reduced tin oxide
which in principle may be mostly related to the observed tin
a
mainly around 104, 155, 499 and 535 eV that are assigned to species in the systems as these cannot be ruled out by
Si2p, Si2s, Sn3d and O1s, respectively. The core level O1s XPS.43,44
spectrum (Fig. 5b) of SnTUD-1 showed an asymmetric signal,
29Si MAS NMR spectra of SnTUD-1 exhibited clear peaks at
which can be deconvoluted into two peaks. The main peak at −93 ppm (Q2), −103 ppm (Q3) and −110 (Q4), which can be
532.9 eV can be correlated to Si–O bonds from silica (SiO2) as attributed to (SiO)2Si(OH)2, (SiO)3SiOH and (SiO)4 units
compared to the low intense peak (ca. 530.9 eV) which is respectively (Fig. 6). Very similar peak profiles have been pre-
believed to arise from the O1s contribution of tin oxides viously reported in pure Si-TUD-1 materials.23
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Scheme 1 One-pot Mannich reaction of substitued aldehyde, amine and
ketone catalyzed by SnTUD-1.
product. The SnTUD-1 catalysed reaction was found to proceed
effectively in the following solvent order: ethanol > acetonitrile
> dichlomethane > water. Upon optimisation of the systems,
reactions were carried out using ethanol as a benign solvent
Fig. 6 29Si MAS NMR of SnTUD-1.
The surface acidity of the materials was subsequently inves- with substituted benzaldehydes, aniline and acetophenone as
tigated by NH3-TPD and DRIFT studies of adsorbed pyridine. model substrates. β-Amino carbonyl compounds were obtained
Three major bands were observed in the FTIR spectra of in moderate to good yields (60–80%) after 12 h at room
adsorbed pyridine in the region of 1400–1700 cm−1 (Fig. 7a). temperature.
The bands observed at 1592 and 1445 cm−1 can be assigned to
The scope of the reaction was further examined with
pyridine bonded to Lewis acid sites.45 Remarkably no appreci- various ortho- and para-substituted ketones, aldehydes and
able contribution was observed for Brønsted acid sites (band amines under similar conditions (Table 1). Remarkable yields
at ca. ∼1545 cm−1),45 which indicates the exclusive presence of of products (>85%) were achieved using SnTUD-1 as a catalyst
Lewis acid sites in SnTUD-1 materials upon Sn incorporation. (Table 1, entry 1). Substituted benzaldehyde with electron with-
Complementary ammonia TPD results pointed out the pres- drawing or donating groups in para position gave the corres-
ence of Lewis acid sites of varying strength in the material, ponding β-amino carbonyls in good yields (60–85%), whereas
exemplified by two distinct desorption peaks centered at ortho-substituted benzaldehyde such as 2-nitrobenzaldehyde
203 °C and 480 °C (Fig. 7b), with a total material acidity of (Table 1, entry 7) provided no yield of the expected product
0.24 mmol NH3 per g. A majority of weak to moderate strength due to steric effects and imine formation. Electron-deficient
acid sites are evidenced from the large desorption peak in the amines (4-chloroaniline) and ketones (4-chloroacetophenone)
low temperature region. Interestingly, a certain contribution of yielded 70% of the corresponding β-amino compounds
strong acid sites also seems to be present in SnTUD-1 as (Table 1, entries 10 and 12). Interestingly, aliphatic ketones
shown by the desorption peak at a relatively high temperature (e.g. acetone) gave poor yields (Table 1, entry 11) with an impor-
region (480 °C).
tant number of unidentifiable products observed after reaction
Having proved the nature and type of acid sites and tin workup, indicating an unselective reaction of acetone with
species in the material, the activity of SnTUD-1 in the one-pot aniline and benzaldehyde. The reactivities of 4-methyl aceto-
Mannich reaction of substituted aldehydes, amines and phenone and cyclohexanone were comparably lower than that
ketones was subsequently investigated (Scheme 1). A blank of acetophenone; only trace amounts of products were
reaction with siliceous TUD-1 did not yield any desirable obtained for –CH3 substituted acetophenone (Table 1, entries
Fig. 7 NH3-TPD profile (a) and FTIR spectra of adsorbed pyridine (b) of SnTUD-1.
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Fig. 8 Reusability cycles of SnTUD-1 for benzaldehyde, acetophenone and
aniline coupling reactions.
Scheme 2 Proposed reaction pathway for Mannich reaction catalyzed by
gratifyingly observed without any subsequent formation of the
desired Mannich product. Comparatively, the reaction with the
recovered SnTUD-1 containing a reduced ca. 1.3 wt% Sn (after
leaching) progressed under typical reported yields at room
temperature (>80%, 12 h reaction, similar to that of cycle 1 in
Fig. 8). These findings pointed out the need for Lewis acidic
Sn framework species in Sn-TUD-1 to specifically promote step
2 (Scheme 2) leading to the final β-amino carbonyl com-
pounds, in good agreement with our proposed mechanism.
Importantly, ICP-OES analysis of the filtrate from subsequent
reuses of the catalyst (cycles 2, 3 and 4, Fig. 8) showed no
leaching of Sn species into solution, indicating the high stabi-
lity of SnTUD-1 after the first use under the investigated reac-
tion conditions. The catalyst was stable and highly active upon
reuse. Results also confirmed the truly heterogeneous nature
of the reaction and pointed out the presence of weakly incor-
porated Sn species on SnTUD-1 which could have been simply
removed prior to the first run by a thorough conditioning step
under ethanol reflux.
SnTUD-1.
8 and 9). Apart from the desired product, nearly 10–20% for-
mation of the corresponding imines (Schiff base; their for-
mation was observed within 1 h of reaction) was noticed in the
investigated reactions which did not react further to yield the
Mannich product even after 48 h.
Since the Schiff base formation was observed within 1 h of
reaction, a plausible mechanistic pathway proposed here is the
formation of an iminium ion from aldehydes and amines
(Scheme 2, step 1). The formation of an enol takes place in a
subsequently slow expected step (Scheme 2, step 2) to then
lead to β-amino carbonyl compounds. This step can be accele-
rated on the coordinatively available Lewis acidic sites of
SnTUD-1.41,46 After reaction, SnTUD-1 was filtered, and reused
for up to four cycles (Fig. 8). The material was found to pre-
serve over 90% of its initial activity after 4 cycles, indicating
the stability of the catalyst under the investigated reaction con-
ditions. Studies on Sn leaching under the investigated reaction
conditions were subsequently conducted to ascertain the hetero-
geneous nature of the protocol. For this purpose, the cata-
lyst was filtered off half-way through the reaction (after 6 h)
and the filtrate with potentially Sn leached species was left to
react for a longer period of time (12 h). The filtrate was also
analyzed by ICP-OES for Sn content. In parallel, the recovered
catalyst from the reaction was washed thoroughly and con-
Further studies are currently ongoing to expand the appli-
cation of SnTUD-1 in related acid-catalysed chemistries includ-
ing alkylations and acylations as well as biomass-related
valorisation processes (e.g. isomerisations of sugars).
4. Conclusions
ditioned in ethanol and water prior to another reaction run A novel Sn-incorporated TUD-1 material has been successfully
with fresh reagents. Results from the filtrate continued reac- synthesised by a direct hydrothermal method using inexpen-
tion in the first run indicated that the reaction proceeded sive TEA as a structure directing agent. UV-Vis, FTIR and XPS
somehow in the absence of catalyst due to Sn leaching. confirmed the presence of framework Sn4+ in tetrahedral
ICP-OES results confirmed significant Sn leaching into solu- coordination in SnTUD-1, with the presence of tin oxide nano-
tion (ca. 0.71 wt%, 0.14 mg L−1 Sn), accounting for over a third particles on the surface as evidenced from UV-Vis, XRD and
of the total Sn content in SnTUD-1. However, only the for- TEM. SnTUD-1 featured a high surface area and an interesting
mation of the iminium ion (from aldehyde and amine) was Lewis acidity, with acid sites of varying strength from Sn
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incorporation, which conferred excellent activities on 14 T. R. Gaydhankar, P. N. Joshi, P. Kalita and R. Kumar,
Mannich-type three component coupling reactions at room J. Mol. Catal. A: Chem., 2007, 265, 306–315.
temperature. A range of β-aminocarbonyl compounds could be 15 U. S. Taralkar, P. Kalita, R. Kumar and P. N. Joshi, Appl.
synthesized in moderate to very good yields under very mild Catal., A, 2009, 358, 88–94.
reaction conditions. The proposed Lewis-acidic material is 16 P. Shah, A. V. Ramaswamy, K. Lazar and V. Ramaswamy,
envisaged to have multiple applications in acid catalysed pro- Microporous Mesoporous Mater., 2007, 100, 210–226.
cesses including biomass valorisation chemistries currently 17 S. Y. Chen, H. D. Tsai, W. T. Chuang, J. J. Lee, C. Y. Tang,
under investigation in our laboratories.
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Acknowledgements
The authors are thankful to Prof. B. Viswanathan, NCCR, IIT-- 19 J. S. Lee, G. B. Han and M. Kang, Energy, 2012, 44, 248–256.
Madras for providing laboratory facilities to carry out this 20 T. M. AbdelFattah and T. J. Pinnavaia, Chem. Commun.,
work. M.P.P. is thankful to DST-FIST and UGC – Research
Fellowship scheme. R.L. gratefully acknowledges Ministerio de 21 K. Bachari and O. Cherifi, Appl. Catal., A, 2007, 319, 259–
Ciencia e Innovacion, Gobierno de España for the concession 266.
of a Ramon y Cajal contract (ref. RYC-2009-04199) and Conse- 22 S. Sisodiya, S. Shylesh and A. P. Singh, Catal. Commun.,
jeria de Ciencia e Innovacion, Junta de Andalucia for funding 2011, 12, 629–633.
under the project P10-FQM-6711. Funding from projects 23 J. C. Jansen, Z. Shan, L. Marchese, W. Zhou, N. von der Puil
CTQ-2010-18126 (MICINN) and P09-FQM-4781 (Consejeria de and T. Maschmeyer, Chem. Commun., 2001, 713–714.
Ciencia e Innovacion, Junta de Andalucia) is also gratefully 24 S. Telalovic, A. Ramanathan, G. Mul and U. Hanefeld,
acknowledged. Revisions suggested by reviewers along the J. Mater. Chem., 2010, 20, 642–658.
lines of Sn leaching and stability studies are kindly acknow- 25 M. J. Climent, A. Corma and S. Iborra, Chem. Rev., 2011,
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27 R. Muller, H. Goesmann and H. Waldmann, Angew. Chem.,
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