A covalently anchored 2,4,6-triallyloxy-1,3,5-triazine Pd(ii) complex over a modified surface of SBA-15: Catalytic application in hydrogenation reaction

Article abstract of DOI:10.1039/c4ra10002f

Highly efficient and reusable SBA-15-TAT-Pd(ii) has been synthesized by anchoring a 2,4,6-triallyloxy-1,3,5-triazine (TAT) complex over the organo-functionalized surface of SBA-15. The physiochemical properties of the organo-functionalized catalyst were analyzed by elemental analysis, ICP-OES, XRD, N₂ sorption measurement isotherm, TGA & DTA, solid state ¹³C, ²⁹Si NMR spectra, FT-IR, XPS, DRS UV-visible, SEM and TEM. XRD & N₂ sorption were analyzed to find out the textural properties of the synthesized catalyst and confirm that an ordered mesoporous channel structure was retained even after the multistep synthetic procedures. The electronic environment and oxidation state of Pd in SBA-15-TAT-Pd(ii) were monitored by XPS and DRS UV-visible techniques. The catalytic activity of the synthesized catalyst SBA-15-TAT-Pd(ii) was screened for hydrogenation reactions and shows higher catalytic activity with good turnover numbers (TON) under optimized experimental conditions with maximum conversion (>99%) and selectivity (100%). The anchored solid catalyst can be recycled efficiently and reused upto five times without major loss in reactivity. This journal is

Full text of DOI:10.1039/c4ra10002f

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A covalently anchored 2,4,6-triallyloxy-1,3,5-
triazine Pd(II) complex over a modified surface of
SBA-15: catalytic application in hydrogenation
reaction†
Cite this: RSC Adv., 2014, 4, 58467
*
Priti Sharma and A. P. Singh
Highly efficient and reusable SBA-15–TAT–Pd(II) has been synthesized by anchoring a 2,4,6-triallyloxy-1,3,5-
triazine (TAT) complex over the organo-functionalized surface of SBA-15. The physiochemical properties of
the organo-functionalized catalyst were analyzed by elemental analysis, ICP-OES, XRD, N2 sorption
measurement isotherm, TGA & DTA, solid state 13C, 29Si NMR spectra, FT-IR, XPS, DRS UV-visible, SEM and
TEM. XRD & N2 sorption were analyzed to find out the textural properties of the synthesized catalyst and
confirm that an ordered mesoporous channel structure was retained even after the multistep synthetic
procedures. The electronic environment and oxidation state of Pd in SBA-15–TAT–Pd(II) were monitored by
XPS and DRS UV-visible techniques. The catalytic activity of the synthesized catalyst SBA-15–TAT–Pd(II) was
screened for hydrogenation reactions and shows higher catalytic activity with good turnover numbers (TON)
under optimized experimental conditions with maximum conversion (>99%) and selectivity (100%). The
anchored solid catalyst can be recycled efficiently and reused upto five times without major loss in reactivity.
Received 8th September 2014
Accepted 29th October 2014
DOI: 10.1039/c4ra10002f
www.rsc.org/advances
and subsequent recycling of homogeneous Pd catalysts remain
a scientic challenge. Therefore, developing a facile and expe-
Introduction
dient approach to separate and recycle catalysts is highly
desirable.
Liquid-phase hydrogenation of olens using heterogeneous
catalysts is an industrially relevant process.1–3 Along with
olens, hydrogenation of nitro compounds to amines has
become one of the most important chemical reactions since
organic amines are essential materials for the production of
agrochemicals, dyes, pharmaceuticals, polymers and rubbers.4,5
For more than 100 years chemists have used heterogeneous
catalysts based on noble metals on various supports.6,7 Homo-
geneous hydrogenation catalysis was developed in the 1960's
and is reviewed frequently.8 It is well known that Pd-catalyzed
reactions are of signicant importance in modern chemical
transformations.9–11 There is a growing demand for organo-
metallic reagents that exhibit favorable chemoselectivity and
stereoselectivity in hydrogenation reactions. Generally reaction
proceeds using N-ligand based palladium catalysts. However,
most of these ligands are expensive and signicantly limited to
their industrial applications.12–14 Also some of these new ligands
are not easily available and contain tedious and expensive
synthesis processes and hardly stable in catalytic systems.
Therefore, simple easily accessible stable catalyst is desired for
heterogeneous reactions. Unfortunately, the efficient separation
Since the discovery of ordered mesoporous silicas,15 these
materials have attracted wide spread attention in both
academics and industry due to their huge potential applica-
tions.16,17 Today mesoporous materials have been employed
successfully in gas adsorption and storage,18 sensing,19
catalysis,20 optical device21 and ion exchange.22 Since the
synthesis of ordered mesoporous materials in 1992 sparked
worldwide interest in the eld of heterogeneous catalysis and
separation science; SBA-15 has become the most popular
member of the group and possessed extremely high surface
area, easily accessible, uniform pore sizes and stability.23–25
SBA-15 can carry suitable functional groups at their surface,
which can effectively interact with relevant metal ion/molecules,
which makes them a potential candidate for heterogeneous
catalysts. Furthermore, the leaching of the active site can also be
avoided as the organic moieties are covalently attached to the
inorganic support. Various homogenous complexes were widely
used for this transformation via covalent attachment.26–28
Accordingly, in the present chapter we report our attempt to
gra Pd metal complex of 2,4,6-triallyloxy-1,3,5-triazine (TAT),
derivatives over SBA-15 phases and their catalytic properties in
hydrogenation reactions. The immediate goals of our study
were (i) to evaluate the heterogenization method of the
2,4,6-triallyloxy-1,3,5-triazine Pd(II) complex over organic
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411008, India. E-mail:
ap.singh@ncl.res.in; Fax: +91 20 2590 2633; Tel: +91 20 2590 2497
† Electronic supplementary information (ESI) available: Experimental,
Instruments for Characterization. See DOI: 10.1039/c4ra10002f
This journal is © The Royal Society of Chemistry 2014
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modied SBA-15 support, (ii) to characterize the heterogenized
SBA-15–TAT–Pd(II) complexes by various physiochemical tech-
niques, (iii) to measure the catalytic properties in hydrogena-
tion reaction and (iv) to determine the extent stability of the
catalyst as well as their recycling properties.
The uniqueness of mesoporous structure, phase purity,
degree of orderdness and unit cell parameters of all meso-
porous materials were determined by powder X-ray diffraction
(XRD). XRD pattern of as-synthesized SBA-15, calcined SBA-15,
SBA-15–SH, SBA-15–TAT and heterogenized SBA-15–TAT–Pd(II)
complexes are visualized in Fig. 1. The small-angle XRD pattern
in all synthesized samples shows three well-resolved peaks;
indexable as (100), (110), (200) reections associated with p6mm
hexagonal symmetry. Aer calcination, the peak intensity of
(100) plane increases compared to as-synthesized SBA-15 due to
the removal of surfactant molecules (Fig. 1a and b). It is evident
from Fig. 1 that XRD patterns of the modied SBA-15 materials
are almost similar to the parent SBA-15 with small decrease in
overall intensity of (100), (110) and (200) reections as the
organic moieties (–TAT & –SH) get anchored one by one over the
mesoporous SBA-15 wall. Aer organic group modication over
SBA-15 by post graing method, the XRD pattern (Fig. 1b–e)
shows that the p6mm morphology is preserved.29 This perse-
verance of peak positions indicates that partial lling of
2,4,6-triallyloxy-1,3,5-triazine Pd(II) complex inside the meso-
pores is less detrimental to the quality of the SBA-15. The
persistence of the (100), (110) and (200) reections (Fig. 1b–e)
not only proved the structural stability of the support but also
the existence of long range order even aer a number of treat-
ments with organic molecules in solvents. All these observa-
tions clearly show that the ordered mesoporosity and structural
stability retained even aer incorporation of 2,4,6-triallyloxy-
1,3,5-triazine Pd(II) complexes.
Fig. 2 FT-IR spectrum of (a) calcined SBA-15, (b) SBA-15–SH, (c)
SBA-15–TAT and (d) SBA-15–TAT–Pd(II).
symmetric stretching vibrations of the Si–O–Si bonds, while the
band at 1090 cmꢀ1 is assigned to the asymmetric (Si–O–Si)
vibrations.30 In the hydroxyl region (3200–3600 cmꢀ1),31 a broad,
typical peak of Si–O–H and S–H of thiol functionalized SBA-15
were observed at around 3436 cmꢀ1 and 468 cmꢀ1, respectively.
Propyl thiol-functionalized SBA-15 shows a characteristic peak at
2934 cmꢀ1 corresponds to C–H stretching vibrations. The weak
bands at 1340 cmꢀ1, 1381 cmꢀ1, 1468 cmꢀ1 are assigned to
bending vibrations of methylene groups. Whereas, the band at
705 cmꢀ1 relates to the C–S stretching vibrations.32
Except the calcined SBA-15, other samples show distinct C–H
vibrations at 2934 cmꢀ1, corresponding to C–H bond in the
modied samples (Fig. 2b–d). Furthermore, SBA-15 modied
samples (Fig. 2b–d) exhibit strong and broad absorption band
at 3435 cmꢀ1, due to N–H stretching vibration which overlap
with the –OH stretching frequency. All the modied SBA-15–SH,
SBA-15–SH–TAT and SBA-15–SH–TAT–Pd(II) exhibit strong band
at 1571 cmꢀ1, due to ring >C–N stretching vibration and addi-
tional peak at 1168 cmꢀ1 (C–N) gets overlap with the symmetric
stretching vibrations of the Si–O–Si bonds (1088 cmꢀ1).33 The
above mode of vibrations clearly indicate that 2,4,6-triallyloxy-
1,3,5-triazine Pd(II) complex and organic modiers were incor-
porated successfully over the surface modied SBA-15.
The presence of isolated surface silanols, hydrogen bonded
hydroxyl group and anchored complex, 2,4,6-triallyloxy-
1,3,5-triazine Pd(II) are evident from the IR spectrum of calcined
SBA-15, SBA-15–SH, SBA-15–TAT and SBA-15–TAT–Pd(II) in Fig. 2.
The bands observed in the range of 850–770 cmꢀ1 account for the
The diffuse reectance spectra (200–800 nm) of calcined
SBA-15 and SBA-15–TAT–Pd(II) show the characteristic absorp-
tion at 254 nm, corresponds to typical siliceous material
(Fig. 3a).34 SBA-15–TAT–Pd(II) shows characteristic absorption
band at 260 nm corresponding to a metal–ligand charge
transfer (MLCT). The shi towards higher energy values result
from the metallation by Pd acetate. The presence of high
coordinating ligand like 2,4,6-triallyloxy-1,3,5-triazine (TAT),
the Pd(II) shis the absorbance band towards the higher
frequency region resulting in a slight deviation from theoretical
values. Another strong absorbance at 380 nm i.e. in the visible
region may be due to a change in p / p* and at 682 nm
corresponds to n / p* transitions aer Pd graing over
organo-modied SBA-15–TAT complex32 (Fig. 3b). These results
are in agreement with XPS results.
Fig. 1 XRD pattern of (a) as-synthesized SBA-15, (b) calcined SBA-15,
(c) SBA-15–SH, (d) SBA-15–TAT, (e) SBA-15–TAT–Pd(II).
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intensity of the un-reacted carbon peak is lower, it is conrmed
that the maximum amount of anchoring of TAT over 3-MPTMS
modied SBA-15. All the resonance peaks of 13C NMR support
the successful anchoring of 2,4,6-triallyloxy-1,3,5-triazine over
the 3-MPTMS modied SBA-15 surface.
The degree of functionalization of surface silanol groups of
SBA-15 with organic moiety can be monitored by 29Si CP MAS
NMR spectroscopy. 29Si CP–MAS NMR spectra of calcined SBA-
15 and SBA-15–TAT–Pd(II) are depicted in Fig. 5. The spectrum
shows broad resonance peaks from ꢀ90 to ꢀ120 ppm, indica-
tive for a range of Si–O–Si bond and the bands are centered at
ꢀ93 ppm, ꢀ102 ppm and ꢀ111 ppm assigned to Q2 [germinal
silanol, (SiO)2Si(OH)2], Q3 [single silanol, (SiO)3Si(OH)] and Q4
[siloxane, (SiO)4Si] sites of the silica framework, respectively
(Fig. 5a and b). The calcined SBA-15 shows the presence of
broad resonance peaks from ꢀ98 to ꢀ126 ppm, indicative for a
range of Si–O–Si bond, and it is noteworthy that the sample
contains large amounts of Q4 sites showing a high framework
cross-linking. In general, the Q3 sites are considered to be rich
with isolated Si–OH groups, which may be free or hydrogen-
bonded. Spectrum of SBA-15–TAT–Pd(II) show two peaks at
ꢀ67 ppm and at ꢀ60 ppm which are assigned to T3 [SiR(OSi)3]
and T2 [Si(OH)R(OSi)2], respectively. The presence of major T3
peak indicate that the proper heterogenization of 3-mercapto-
propyltrimethoxy silane over SBA-15.35 29Si CP MAS NMR
spectra provide direct evidence for the formation of a highly
Fig. 3 UV-vis spectra of (a) calcined SBA-15 and (b) SBA-15–TAT–Pd(II).
The organic moiety 2,4,6-triallyloxy-1,3,5-triazine anchored
over the modied (–SH) surface of SBA-15 is further demon-
strated by 13C NMR techniques. 13C Liquid NMR of
2,4,6-triallyloxy-1,3,5-triazine (TAT) and 13C solid state NMR
spectrum of SBA-15–TAT–Pd(II) are depicted in Fig. 4a and b.
The peaks at 8, 20 and 34 ppm, present in SBA-15–TAT–Pd(II)
catalyst are assigned to the carbon atoms (C1–C3) of the propyl
chain in organic modier (3-MPTMS), indicate the successful 3-
mercaptopropyl trimethoxysilane functionalization over SBA-15
(Fig. 4b). The presence of new resonance in the 13C CP MAS
NMR spectrum and also in 13C Liquid NMR of 2,4,6-triallyloxy-
1,3,5-triazine (TAT), at 72 ppm corresponds to carbon attached
to the oxygen atom in the allylic chain of triazene ligand (Fig. 4a
and b). Furthermore, peak present in both TAT and
SBA-15–TAT–Pd(II) at 169 ppm corresponds to the carbon
present in the ring of triazene ligand (TAT). In addition, low
intensity extra peaks in the range of 103–130 ppm corresponds
to the unmodied allylic group of triazene ligand (Fig. 4a and
b), which might be due to the unreacted portion of the allylic
group of 2,4,6-triallyloxy-1,3,5-triazine ligand. Since; the
Fig. 5 Solid state 29Si CP MAS NMR spectrum of (a) calcined SBA-15
and (b) SBA-15–TAT–Pd(II).
Fig. 4 Liquid 13C NMR of (a) 2,4,6-triallyloxy-1,3,5-triazine (TAT) and
(b) solid state 13C CP MAS NMR of SBA-15–TAT–Pd(II).
Fig. 6 TEM Images (A) calcined SBA-15 and (B) SBA-15–TAT–Pd(II).
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Thermal stability of all the synthesized materials were
studied by thermo gravimetric analysis (TGA) under air atmo-
ꢁ
sphere from ambient temperature to 1000 C with a tempera-
ture increment of 10 ꢁC minꢀ1
. TGA and DTA of (a)
As-synthesized SBA-15, (b) calcined SBA-15, (c) SBA-15–SH and
(d) SBA-15–TAT are shown in the Fig. 8A and B. TGA plots of
calcined SBA-15 and modied SBA-15 samples show approxi-
ꢁ
mately 5% weight loss below 120 C caused due to the desorp-
tion of physiosorbed water molecules (Figꢁ. 8A-a–d).38–40 A loss of
ꢁ
ꢂ56 weight% was observed between 132 C and 400 C for the
as-synthesized SBA-15 corresponds to the removal of trapped
surfactant within closed pores (Fig. 8A-a). Whereas, nearly no
weight loss in TGA was observed in the calcined SBA-15 between
132 ꢁC and 400 ꢁC indicates the complete removal of surfactant
from SBA-15. Moreover a total weight loss of ꢂ7% was noticed
for calcined SBA-15 and this may be due to removal of surface
silanol groups at very high temperature (Fig. 8A-b). TGA result of
SBA-15–SH samples shows weight loss in two steps. In the rst
step, weight loss was occurred between 70 ꢁC and 150 ꢁC
corresponds to the loss of loosely bounded water and adsorbed
moisture. In the second step, weight loss was observed in the
Fig. 7 SEM images of (A) calcined SBA-15 and (B) SBA-15–TAT–Pd(II).
condensed siloxane network with organic group covalently
bonded to the mesoporous silica.36
TEM images of calcined SBA-15 and SBA-15–TAT–Pd(II)
provide structural evidence that the material is organized into
ordered arrays of two-dimensional hexagonal mesopores
(Fig. 6A and B). The signicant difference of the TEM patterns
was not observed between the two Fig. 6A and B. However, aer
the anchoring of TAT–Pd(II) complex inside the mesoporous
channels of SBA-15, the image had shown distinct deep con-
trasting meso parallel channels with respect to the light shaded
surface. This might be interpreted as due to the presence of the
TAT–Pd(II) complex inside the mesopores of the SBA-15, but not
on the surface. If the TAT–Pd(II) complex was anchored on the
surface of the functionalized SBA-15, then the high-contrast
dark meso parallel channels would have appeared along the
boundary of the visualized SBA-15 and not inside the porous
body as observed previously by Shephard et al.37 From this
results it is very clear that the Pd-complex was immobilized
inside the pore channels of modied SBA-15. SEM image of the
calcined SBA-15 and SBA-15–TAT–Pd(II) are shown in Fig. 7A
and B, respectively. Calcined SBA-15 shows uniform arrays of
small agglomerate particle meso channels arrangement and
clear molecular-scale periodicity in the SEM images. Further-
more, SBA-15–TAT–Pd(II) demonstrates molecular-based mate-
rials; the particle morphology in the large molecular system
does not change and becomes denser aer TAT–Pd(II) complex
anchoring over the mesoporous surface in comparison to the
calcined SBA-15.
ꢁ
region of 245–385 C in TGA analysis and a sharp exothermic
peak visible in DTA analysis is attributed to decomposition of
3-MPTMS (Fig. 8A and B-c).
TGA result of SBA-15–SH material shows quantitatively
14.27% weight loss, which is greater than calcined SBA-15;
strongly supports successful anchoring of 3-MPTMS over
SBA-15 (Fig. 8A-b and c). In the case of SBA-15–TAT samples, a
total weight loss of ꢂ35% was noticed, is due to the complete
decomposition of organic moieties heterogenized over SBA-15,
which is supported by the exothermic peak in the DTA spec-
trum of the same sample (Fig. 8B-d). All the TGA and DTA
results clearly indicate that 2,4,6-triallyloxy-1,3,5-triazine is well
anchored over the modied surface of calcined SBA-15.
The nitrogen adsorption–desorption results of calcined
SBA-15, SBA-15–TAT–Pd(II) and their corresponding pore size
distribution curves are plotted in Fig. 9. The surface area,
average pore diameter and pore volume determined for the
calcined SBA-15 and SBA-15–TAT–Pd(II) are summarized in the
Table 1. All the samples show type IV adsorption isotherms,
according to the IUPAC classication, indicating mesopores
Fig. 8 (A) TGA, (B) DTA of (a) as-synthesized SBA-15, (b) calcined SBA-15, (c) SBA-15–SH and (d) SBA-15–TAT.
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X-ray photoelectron spectroscopy (XPS) is the powerful tool
to investigate the electronic properties of the species formed on
the surface of the materials. XPS is highly sensitive to electronic
environment, e.g. oxidation state and or multiplicity inuences
in the binding energy of the core electron of the metal. The
synthesized material SBA-15–TAT–Pd(II) was characterized by
X-ray photoelectron spectroscopy (XPS) to ascertain the oxida-
tion state of Pd species. In Fig. 10 the Pd binding energy of
SBA-15–TAT–Pd(II) exhibits two strong peaks centered at
337.9 eV and 344 eV, respectively, which are assigned to the Pd
3d5/2 and Pd 3d3/2, respectively. The observed peaks indicate
that palladium is present in the Pd2+ oxidation state in the
synthesized SBA-15–TAT–Pd(II). According to literature report
pure Pd acetate metal salt binding energy for Pd 3d5/2 and Pd
3d3/2 orbital appears at 338.2 eV and 345.3 eV, respectively.41 In
Fig. 9 Nitrogen adsorption–desorption isotherm and inset (BJH) pore
size distribution of (a) calcined SBA-15 and (b) SBA-15–TAT–Pd(II).
comparison
to
literature
value,
the
synthesized
SBA-15–TAT–Pd(II) show shi in binding energy of Pd towards
lower value viz. 337.9 eV Pd 3d5/2 and 344 eV Pd 3d3/2, respec-
tively. The decrease of Pd binding energy in 2,4,6-triallyloxy-
1,3,5-triazine complex functionalized SBA-15 implies that
there is a strong coordination interaction between Pd metal
species and 2,4,6-triallyloxy-1,3,5-triazine ligand.42 These results
are in agreement with the UV-vis observations.
with completely reversible nature, uniformly sized, with a
capillary condensation step at P/P0 ¼ 0.3–0.4. The total surface
area, average pore diameter and pore volume observed for the
calcined SBA-15 and SBA-15–TAT–Pd(II), were found to be 830
3
3
ꢀ1
m2 gꢀ1, 65 A, 1.13 cm gꢀ1 and 285 m2 gꢀ1, 55 A, 0.46 cm g
,
˚
˚
respectively. The decrease in total mesoporous surface area
(65%), pore diameter (15%) and pore volume (59%) aer 2,4,6-
triallyloxy-1,3,5-triazine Pd(II) complex immobilization over
organo-modied SBA-15 is indicative of the graing of
TAT–Pd(II) complex over the mesoporous SBA-15 (Table 1).
It is clear from Table 1 that even though silylation proce-
dures changed the textural properties of the mesoporous
material, the decrease is more prominent aer 2,4,6-triallyloxy-
1,3,5-triazine Pd(II) complex immobilization over SBA-15 since
the bulkier organic moieties inside the pore channels occupy
more area of the void space (Fig. 9 inset a and b). The capillary
condensation steps of SBA-15–TAT–Pd(II) get reduced to lower
P/P0 values compared to calcined SBA-15 (Fig. 9a and b). This
shi towards slightly lower partial pressure shows a possible
reduction in the pore size and a partial distortion in pore
arrangement, consistent with the XRD results. It is also known
that the inection position in N2 sorption isotherms depends
on the diameter of the mesopores and the sharpness usually
indicates the uniformity of the mesopores, due to capillary
condensation of nitrogen molecule. These results directly sup-
porting the previous results of proper heterogenization of metal
complex inside the mesoporous channels of the SBA-15.
Catalyst SBA-15–TAT–Pd(II) screening for hydrogenation
reactions
Hydrogenation is one of the most basic chemical trans-
formations of organic substances. Olen and nitro compounds
Fig. 10 XPS spectra of SBA-15–TAT–Pd(II).
Table 1 Textural properties of mesoporous calcined SBA-15 & SBA-15–TAT–Pd(II)
Loading of Pdb (wt%)
Na (wt%)
1.85
Input
Output
S.A (m2 gꢀ1
)
DP (A)
Vp (cm3 gꢀ1
)
˚
S. no.
Sample
1
2
Calcined SBA-15
SBA-15–TAT–Pd(II)
830
285
65
55
1.13
0.46
4
2.2
a
Calculated based on elemental (nitrogen) analysis value. b Input is based on the amount of Pd during synthesis reaction; output is based on the
ICP-OES analysis. S.A: BET surface area (DP): average pore diameter, (Vp): pore volume.
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hydrogenation is an industrially important reaction. The cata- negligible styrene conversion was observed (ꢂ7%). It is note-
lytic activity of SBA-15–TAT–Pd(II) was tested in the hydrogena- worthy to mention that no ring hydrogenated products were
tion of a variety of unsaturated olen and aromatic nitro observed with styrene, and a-methyl styrene.
compounds to their corresponding products. The catalytic
Non-aromatic cyclic olen group containing substrates
performance of the heterogenized catalyst SBA-15–TAT–Pd(II) in cyclopentene, cyclohexene and cyclooctene gave hydrogenated
olen hydrogenation was initially studied with styrene as the products in 30 min, 60 min, 90 min with 2320, 2320, 1850 TON,
substrate under mild reaction conditions (Pressure H2 ¼ 150 respectively (Table 2, entries 7, 2 and 3). The increase in reaction
psi, T ¼ 40 ꢁC). The reactions were carried out in methanol time for non-aromatic cyclic olen containing substrates may
solvent at room temperature and under 100 psi hydrogen be due to the increase in size of ring in term of carbon atoms.
pressure. Detailed observations of all the reactions are given in Whereas 1-hexene affords 100% conversion to the correspond-
Table 2. It is evident from the Table
2
that the ing hydrogenated product hexane in 15 min with 2320 TON. It is
SBA-15–TAT–Pd(II) is quite active for the hydrogenation reaction clear from the Table 2 that acyclic olens afford higher rate of
under mild reaction conditions. In the case of styrene and reaction in comparison to the cyclic olen in presence of
a-methyl styrene, the hydrogenated products (ethylbenzene & SBA-15–TAT–Pd(II); the reason might be that the olen group
cumene) were obtained within 15 min, 45 min with 2320, 2320 faces steric hindrance in cyclic structure of aromatic or non-
TON, respectively (Table 2, entries 1 and 5). The reason for aromatic compounds. Furthermore, the SBA-15–TAT–Pd(II)
longer reaction time period for a-methyl styrene might be due to catalyst affords 100% conversion of the aromatic nitro (nitro-
the steric hindrance produced by the methyl group next to the benzene) in 1 h with 2200 TON without the ring hydrogenation
olen group. In the absence of SBA-15–TAT–Pd(II) catalyst, (Table 2, entry 4). The trans-stilbene, did not give any product
Table 2 SBA-15–TAT–Pd(II) catalyst for hydrogenation reactiona
S. no.
1
Reactant
Product
Time (min)
15
Yield (%)
100
TON
2320
2
3
60
90
100
80
2320
1850
4
60
95
2200
5
6
45
100
2320
480
—
—
7
8
30
15
96
2230
2320
100
9b
60
100
—
a
Reaction conditions: of substrate (10 mmol), SBA-15–TAT–Pd(II) catalyst (0.025 g), H2 Pressure ¼ 150 psi, reaction temperature ¼ 40 ꢁC, solvent ¼
50 mL methanol. b Reaction carried out in presence of 10 wt% Pd/C.
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under the specied time period. This is probably due to the
relatively large steric hindrance (Table 2, entry 6) of the reactant
molecule.
In contrast, commercial 10% Pd/C was used to hydrogenate
styrene; it takes 60 min to complete the reaction, whereas the
SBA-15–TAT–Pd(II) completed the reaction within 15 min under
similar experimental conditions (Table 2, entry 9).
Heterogeneity and recycling studies of catalyst
SBA-15–TAT–Pd(II)
In order to understand whether the observed catalytic activity
arises from truly heterogeneous catalyst systems, a series of
leaching experiments was conducted. To test if metal is leached
out from the solid catalyst during a reaction; the hot ltration test
and recycling study of heterogeneous catalyst SBA-15–TAT–Pd(II)
under the reaction conditions; 10 mmol of styrene, 150 psi of H2
pressure, 0.025 g of catalyst SBA-15–TAT–Pd(II), 50 mL methanol,
Fig. 11 Recycling study of heterogeneous catalyst SBA-15–TAT–Pd(II).
Reaction conditions: 10 mmol of styrene, 150 psi of H2 pressure, 0.025
g of catalyst SBA-15–TAT–Pd(II), 50 mL methanol, T ¼ 40 ꢁC.
ꢁ
T ¼ 40 C was performed. Hydrogenation reaction mixture was
collected by ltration at temperature (40 ꢁC) aer a reaction time
of 5 min which give 58% conversions of styrene. The residual
activity of the supernatant solution was studied. It was observed
that aer ltration of SBA-15–TAT–Pd(II) catalyst from the reaction
the recycled catalyst clearly shows the presence of peaks at 337.5
eV and 342.6, corresponding to 3d5/2 and 3d3/2 respectively, of
Pd(II) species the absence of peaks at 335.6 eV and 340.7 eV,
corresponding to Pd(0) moiety. Even aer repeated cycles of the
hydrogenation reaction, catalyst SBA-15–TAT–Pd(II) shows the
same Pd 3d peaks (Fig. S1†) (spent and fresh), suggesting that
the oxidation state of the immobilized Pd did not change in
repeated hydrogenation reactions.
ꢁ
mixture at the elevated reaction temperature (40 C) (in order to
avoid possible re-coordination or precipitation of soluble palla-
dium upon cooling) hydrogenation reactions did not proceed
further. Thus, results of the hot ltration test suggest that Pd was
not being leached out from the solid catalyst during the reaction.
These results conrm that the palladium catalyst remains on the
support. This clearly demonstrate that the reaction ceased aer
the removal of SBA-15–TAT–Pd(II) catalyst from the reaction
mixture.
Conclusions
In summary, highly stable and recyclable SBA-15–TAT–Pd(II)
Reusability is an important feature to be monitored for catalyst was synthesized by anchoring 2,4,6-triallyloxy-1,3,5-
application of heterogenized single-site catalysts. Reusability of triazine complex over the surface of organo-functionalized
heterogeneous catalyst SBA-15–TAT–Pd(II) was performed under SBA-15–SH and subsequent complexation with Pd acetate.
the reaction conditions; 10 mmol of styrene, 150 psi of H2 Modied SBA-15 and SBA-15–TAT–Pd(II) were analyzed by
pressure, 0.025 g of catalyst SBA-15–TAT–Pd(II), 50 mL methanol elemental analysis, ICP-OES, XRD, N2 sorption measurement
solvent, T ¼ 40 ꢁC. Hydrogenation reaction mixture was ltered isotherm, TGA & DTA, solid state 13C & 29Si NMR spectra FT-IR,
using a sintered glass funnel, and the residue extensively XPS, DRS UV-vis, SEM and TEM. Textural properties of synthe-
washed with, 1,2 dichloromethane (DCM) (2–5 mL) and further sized catalyst and conrmation of 2,4,6-triallyloxy-1,3,5-triazine
with methanol. The gas chromatography analysis of the ltrate Pd(II) complex attached to SBA-15 are revealed by XRD and N2
shows no detectable amounts of reagents and products. The sorption analyses. The (100), (110) and (200) reections in
catalyst was dried under vacuum overnight before performing SBA-15 show the structural stability. The thermal stability of
the reusability test. The catalyst could be reused directly without synthesized SBA-15–TAT–Pd(II) was monitored by TGA-DTA.
further purication and was reused up to ve cycles (Fig. 11). Solid state 13C NMR spectra and FT-IR spectroscopy evidently
The amounts of Pd leaching into solution for the hydrogenation support the organic moieties anchored over the surface of
reactions were detected through ICP. The loss of Pd amount for SBA-15 and inside the pore wall. Furthermore, solid state
the hydrogenation reaction was less than 1.0 weight percentage 29Si NMR spectroscopy provides the information about degree
of total Pd content. Even though a small amount of Pd loss of silylation. The electronic environment and oxidation state of
could be detected, catalyst still shows high reusability and Pd in SBA-15–TAT–Pd(II) were conrmed by XPS, DRS UV-vis
stability for hydrogenation reactions. Nearly similar conversion techniques. Subsequently, the morphologies information of
was found even aer ve cycles by using SBA-15–TAT–Pd(II) synthesized catalyst was monitored by SEM and TEM analysis.
catalyst in the hydrogenation reaction.
The catalytic activity of newly synthesized heterogeneous cata-
XPS analyses of fresh SBA-15–TAT–Pd(II) and spent lyst SBA-15–TAT–Pd(II) was screened in the hydrogenation
SBA-15–TAT–Pd(II) catalyst (ve recycle) have been carried out to reaction and found highly active in comparison to the
probe the change in the oxidation state of palladium in its commercial catalyst (10 wt% Pd/C) within specic reaction time
coordination sphere (Fig. S1, in the ESI†). The XPS spectrum of period under similar optimized reaction conditions. The
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Scheme 1 Schematic diagram of heterogeneous SBA-15–TAT–Pd(II) synthesis. Surface Modification via 3-MPTMS (3-mercaptopropyl trimethoxy-
silane) of SBA-15, (B). Anchoring of 2,4,6-triallyloxy-1,3,5-triazine (TAT) complex over modified surface of SBA-15, (C). Metalation by Pd acetate.
present SBA-15–TAT–Pd(II) catalytic system tolerates a broad adsorption measurements (surface area) and Dr Vinod for the
range of olen compounds in hydrogenation reaction. The XPS studies.
heterogenized solid catalyst SBA-15–TAT–Pd(II) can be recycled
efficiently and reused several time (Five times) without major
loss in activity.
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In a typical synthesis, 1 g of organofunctionalized SBA-15,
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