Debenzylation of vanillic acid over sulfosuccinic acid functionalized mesoporous silica nanocomposites

Article abstract of DOI:10.1016/j.catcom.2014.03.001

Sulfosuccinic acid functionalized mesoporous silica (SBA/SSA) was synthesized and characterized for debenzylation of vanillic acid in liquid phase for the first time. The catalytic activity studies showed high activity (82%) and selectivity (96%) for the desired product vanillic acid over SBA/SSA compared to 74% selectivity over silica/SSA. Synergistic effects of acidic/basic groups responsible for higher activity and selectivity were explained with plausible mechanism.

Full text of DOI:10.1016/j.catcom.2014.03.001

Debenzylation of vanillic acid over sulfosuccinic acid functionalized meso-
porous silica nanocomposites
Divya Sachdev, G. Robin Wilson, Neel Mani Srivastava, Amit Dubey
PII:
DOI:
Reference:
S1566-7367(14)00095-8
doi: 10.1016/j.catcom.2014.03.001
CATCOM 3833
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 December 2013
11 February 2014
2 March 2014
Please cite this article as: Divya Sachdev, G. Robin Wilson, Neel Mani Sri-
vastava, Amit Dubey, Debenzylation of vanillic acid over sulfosuccinic acid func-
tionalized mesoporous silica nanocomposites, Catalysis Communications (2014), doi:
10.1016/j.catcom.2014.03.001
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Debenzylation of vanillic acid over sulfosuccinic acid functionalized
mesoporous silica nanocomposites.
Divya Sachdev1, G.Robin Wilson, Neel Mani Srivastava and Amit Dubey*
*Department of Chemistry Maulana Azad National Institute of Technology (MANIT) - Bhopal
(M.P.)-462051
e-mail: dramit.dubey@gmail.com
1National Physical Laboratory, Council of Industrial and Scientific Research, Dr.K.S.Krishnan
Road, New Delhi-110012, India
Abstract
Sulfosuccinic acid functionalized mesoporous silica (SBA/SSA) was synthesized and
characterized for debenzylation of vanillic acid in liquid phase for the first time. The catalytic
activity studies showed high activity (82%) and selectivity (96%) for the desired product vanillic
acid over SBA/SSA compared to 74% selectivity over silica/SSA. Synergistic effects of
acidic/basic groups responsible for higher activity and selectivity were explained with plausible
mechanism.
Key words: Sulfosuccinic acid, Mesoporous SBA-15, Debenzylation
1. Introduction
The discovery of ordered mesoporous materials (OMS) has opened new avenues to incorporate a
wide variety of functional groups because of their outstanding properties such as high surface
area, uniform and tunable pore diameter, pore volume and easy diffusion. Unique properties of
these OMS have encouraged the organic tailoring of the mesoporous surface for advanced
applications in the field of ion-exchange, adsorption and catalysis [1-2]. Therefore organic
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moieties can easily be functionalized to achieve acidic, basic or bifunctional properties via
different synthetic techniques [3]. Microporous materials such as zeolites [4] and sulphonated
acid catalysts like Amberlyst-15 and Nafion-H were successfully exploited for many significant
catalytic applications[5-6]; however, the smaller size of zeolite materials and diffusional
difficulties limits their practical utilization to incorporate larger organic moieties. The stringent
environmental regulations also forced the modern chemical industry to seek for the development
of new heterogeneous systems for easier product recovery, regeneration of the catalytic active
sites and to overcome effluent treatment problems. One such problem has been associated with
debenzylation of aromatic benzyl ether (especially for de-protection reaction) for which harsh
and toxic chemicals are still in use for many substrates. Benzylation of phenols is required for
selectively protecting the -OH group which is more accessible to react and form side products
during any reaction. However at the end of the reaction debenzylation is done for regenerating
the phenolic group. Although various methods have been employed; catalytic hydrogenolysis (H2
in the presence of Pd/C or Raney Ni) is commonly employed deprotection method wherein the
pyrophoric nature of the reaction limits their versatile use [7-8]. Moreover the procedure is
tedious, costly and the deprotection procedure is not compatible with other functional groups.
Silica supported sodium hydrogen sulfate[9-10], trifluoroacetic acid [11]and solid supported acid
catalyst[12] have already been reported for debenzyaltion reactions. Several researchers have
incorporated stronger acid ligands such as (propylsulfonic acid, benzenesulfonic acid [13-14] via
co-condensation method into the mesoporous framework but not much efforts were paid to
develop mild and bifunctional acid nanocomposites. Since the synergistic effects (acid-base co-
operation or the bifunctional catalysis) are very common in biological catalytic systems but the
cooperative effect of-NH- and sulfonic groups fused together on mesoporous framework for
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deprotection of benzyl ethers has not been explored. Many heterogeneous catalysts [15] were
reported for the deprotection of benzyl ethers but according to the best of the literature, no report
is available for debenzylation of vanillic acid over functionalized SBAn materials. With this
view, we report synthesis and characterization of bi-functionalized SBA/SSA nanocomposites by
covalently linking carboxylic acid groups into the SBA-15 framework which resulted in a
continuous pattern of their linkage i.e -NH- and sulfonic acid groups for this interesting
conversion.
2. Experimental
2.1 Synthesis of SBA/SSA catalyst
The synthesis of SBA -15 was carried out in accordance to the earlier reports using
Pluronic (P123) (EO20PO70EO20, MW = 5800, Aldrich) as surfactant and TEOS as the silica
source [16]. Typically, to 1g of SBA-15, 10ml of aminopropyltriethoxysilane (APTS) was
stirred in dry toluene (20ml) under inert atmosphere (Scheme 1). The product SBA/APTS
was filtered, washed exhaustively with ethanol in Soxhlet extractor for 24h and dried under
vacuum. For the functionalization of SBA/APTS with SSA, 1.5 ml of sulfosuccinic acid with
1.2 g of HOBt (1-Hydroxybenzotriazole) and 1.2 g of EDCI (1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide) {acts as a carboxyl activating agent for coupling to primary amines) at 00C was
stirred for half an hour. To this mixture, 1g of SBA/APTS was added in stirring conditions at
room temperature for 4 hours. Finally the product was filtered and washed thoroughly with
methanol and water (to remove the urea formed as a byproduct) to generate SBA/SSA
nanocomposites. Ninhydrin test at room temperature (no coloration) confirms the completion of
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functionalization. Silica-SSA was synthesized exactly under the similar reaction conditions (
APTS and SSA)..
3. Results and Discussions
Powder X-ray diffraction pattern (PXRD) of SBA/SSA samples (Fig.1) at low angles showed
peaks similar to the SBA-15 materials. The characteristic diffraction peaks at (100), (110) and
(200) planes can be indexed to reflections comprising of two dimensional hexagonal p6mm
symmetry indicating that the materials possess ordered mesoporous structure[17]. However, a
slight decrease in the intensity of the diffracted peaks for SBA/SSA compared to SBA-15
showed a partial loss of long range ordering due to incorporation of SSA inside the material. The
nitrogen adsorption-desorption isotherm of SBA/SSA , SBA/APTS and SBA materials (Fig S1)
showed Type IV isotherm according to IUPAC classification with an H1 hysteresis loop
indicative of the mesoporous nature of the sample, the hysteresis loop at relative pressure p/p0 in
the range of 0.4-0.5 is consistent and comparable to the original SBA-15 material[17]. Surface
area, pore size and pore volume (Table 1) of SBA/SSA sample decreased after functionalization
confirming the incorporation of sulfosuccinic acid inside the silica framework. The incorporation
of sulfosuccinic acid was further confirmed by 13CCPMAS NMR. The 13C spectrum (Fig2)
showed four types of carbon. The presence of distinguished peak at 177 and 173 ppm for C1 and
C2 carbon respectively indicates the distinct environment of the carbonyl (-CO) group present in
sulfosuccinic acid[18]. Slightly higher δ value for C1 peak is observed due to the shielding effect
of SO3H group present in the vicinity to –COOH groups where as the lower value of C2 may be
attributed to more electron density due to the amide linkage resulting in lowering of the chemical
shift. Peaks at 64 and at 26 ppm are due to the presence of the methylene group (C3 and C4) in
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the SBA/SSA nanocomposites [19]. Three peaks at 41, 20 and 9 ppm indicate the presence
of aminopropyl group attached to sulfosuccinic acid. Additionally the amount of sulfosuccinic
acid within the mesopores of SBA-15 was estimated by EDXRF (Fig3). The amount of sulfur
content (S Kα counts indicates the amount of S present) in the SBA/SSA and silica-SSA ( from
CHN analysis) was found to be 0.218% and 0.27% respectively which is in agreement with
the theoretical calculated values. The SEM images of SBA/SSA and silica–SSA (Fig S3 (A))
showed aggregation compared to SBA/SSA (Fig S3 (B)) wherein the mesoporous material
retained the rod shaped geometry. In order to further confirm the incorporation of sulfosuccinic
acid inside the porous framework, IR frequencies of SSA, SBA/APTS and SBA/SSA (Fig 4)
were compared. Shifting to lower values of OH stretching of SSA (3650 cm-1) compared to
SBA/SSA (3520cm-1) may be due to the intermolecular H-bonding between the groups (-
COOH). Furthermore, broadening of the peak indicates the presence of N-H stretching for amide
linkage in SBA/SSA. Also the N-H frequency is consistent with the SBA/APTS confirming the
covalent binding of SSA to SBA-15 via APTS[20]. The peak at 1728 cm-1 due to -CO- stretching
in SBA/SSA is consistent with the SSA. Peak due to S-O stretching in SSA at 1380 cm-1 has
slightly shifted to lower 1256 cm-1 value in SBA/SSA[21-22] confirming the incorporation of
SSA in SBA-15. Acidity of the SBA/SSA catalyst was measured by TPD (NH3) (FigS2)
method and the number of acidic sites was found to be 25.5 µmol/g. Additionally acidity of
the nanocatalyst (SBA/SSA) was calculated (supplementary information) by titration method.
3.1 Catalytic Studies
The catalytic activity results for the debenzylation of vanillic acid (Scheme 2) showed 82%
conversion with 96% selectivity of the desired product (A). The activity of SBA/SSA was
compared with homogeneous sulfosuccinic acid (SSA) and silica gel supported SSA (solid
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support) synthesized by impregnation method under similar reaction conditions (details of
method in supplementary information). The results obtained were quite interesting as highest
selectivity of the desired product was achieved on SBA/SSA nanocomposites (Table2) indicating
the advantage of ordered mesoporous solid support. This difference in the activity and
selectivity is attributed to the high surface area of the SBA framework and better dispersion of
the active sites during grafting of SSA on SBA/SSA compared to silica/SSA (also shown by
SEM images in FigS3) and homogeneous conditions. No conversion was obtained without
catalyst indicating the necessity of the acidic centers for this interesting reaction. Furthermore,
decrease in the selectivity of the product (A) was observed under homogeneous conditions due to
the rearrangement of the benzyl ether through formation of benzylic cation (Scheme 2 and 3)
resulting in C-benzylated product (B). The presence of amide group quenches the charge density
of the transition state of SBA/SSA catalyst during the reaction, forming an adduct with benzyl
cation (Scheme3). In addition, amide group hindered the free attack of benzyl cation at the ortho
position to form C-benzylated product (B) while facilitating the selective synthesis of the product
A. Screening of different solvents such as toluene, ethylacetate, methanol, acetonitrile, and
ethanol indicates that the highest activity (83%) and selectivity can be achieved with ethanol.
This may be due to the higher solubility of the product vanillic acid (V.A) in polar solvent than
the reactant benzylated vanillic acid (OBn V.A) and the stability of the charged transition state
(Scheme3). These results have advantages over previous results wherein additives were added
with solvents to suppress the formation of undesired C-benzylated product (B) which are
difficult to separate from the products [12]. In the present studies, the cooperative effect of
bifunctional moieties (-NH- and -SO3H- groups) over SBA-15 framework enhances the rate of
the reaction and also retards the formation of benzyl cation. The conversion of the product
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increased with increase in the catalyst weight (shown in supplementary information, Fig.S4). The
product (V.A) was extracted easily from the reaction mixture by simple acidification since, the
product is water soluble.
3.2 Effect of the reaction time
The effect of the reaction time (Fig.5) indicates that the maximum conversion can be achieved
with in 8h of reaction even though the reaction was allowed to proceed for 24h to see the
stability of the product and variation in product selectivity. However no difference in the total
conversion and selectivity of the debenzylated product (A) was observed with time indicating
that the interesting conversion can be scaled up industrially. The catalyst was washed with
acetone and water to remove the undesired products or substrate on the surface of the catalyst
and was tested for recyclability. Slight decrease in conversion (5-10%) was noted up the fourth
recycles suggesting their promise use industrially and this decrease may be due to the reduction
in the catalytic active sites up to the fourth cycle.
3.3 Plausible mechanism
The cooperative reaction mechanism involves (step B) the formation of benzyl electrophile due
to the protonation of oxygen by SBA/SSA. The benzyl electrophile generated in step B is
stabilized (via neighboring group participation) by nucleophile –NH– of amide group (step C)
[23] responsible for high selectivity (debenzylated vanillic acid ;step D). However the reaction
conducted in SSA and silica gel/SSA resulted in low selectivity (step C1) because the benzyl
cation generated is free in nature which is responsible for C-benzylated side product. This
distinct feature of SBA/SSA nanocomposites confirms the profound role of continuous range of
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acidic/ basic sites together in this catalytic system which is otherwise often difficult to achieve
[24].
3.4 Effect of different substituent(s)
In order to see the influence of different substituent(s) attached to the aromatic moiety on the
yield of the phenols, series of experiments were conducted (Table 3). However no difference in
the activity and selectivity was observed. It is important to mention here that formation of
complexes due to friedel craft acylation (with electron releasing substituent) was not observed
under heterogeneous conditions which is more common in homogeneous catalysis [11]. This is
an advantage of heterogeneous support which restricts the degrees of freedom of benzyl cation
and thereby hinders the formation of acylated products. Electron withdrawing groups
comparatively gives higher yield compared to electron releasing groups because of the better
ease of formation and stability of benzyl carbocation.
4. Conclusions
In conclusion, SBA/SSA nanocatalyst provides an efficient and co-operative catalytic method for
deprotection of benzylated vanillic acid in liquid phase. The detailed catalytic activity studies for
deprotection of different groups and substituent’s are under investigation.
Acknowledgments
AD thanks MPCST-Bhopal and CSIR for research funding.
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SBA-15
SBA/SSA
1
2
3
4
5
2 Theta ( degrees)
Fig 1. PXRD pattern of SBA-15 and SBA/SSA catalysts.
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Fig2. C13 NMR of SBA/SSA catalyst
Fig3. EDXRF spectrum of SBA/SSA catalyst.
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SSA
SBA-APTS
SBA-SSA
160
140
120
100
80
-OH-
SO3H
-CO-
60
O-H and N-H
40
4000
3500
3000
2500
2000
1500
1000
500
wavenumber (cm-1)
Fig 4 FT-IR of SSA (neat), SBA-APTS and SBA/SSA catalysts.
100
80
60
Conversion SBA/SSA
Selectivity SBA/SSA
40
Conversion Silica-SSA
Selectivity Silica-SSA
20
0
0
5
10
15
20
25
Time (h)
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Fig 5 the reaction time on the conversion of product (A) for SBA/SSA and Silica gel/SSA
catalyst(conditions as given in Table-2).
Table 1. Structural parameters of different catalysts.
Materials
Surface area
Pore Volume
(cm3/g)
0.93
Pore size
(nm)
8.4
(m2/g)
SBA-15
SBA-APTS
SBA/SSA
653
152
0.29
6.2
80
0.168
5.4
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Table2. Effect of different catalysts on the conversion and selectivity of debenzylation of
vanillic acid
Catalyst
%Conversiona % Selectivityb
%yieldc
SBA-15
SSA
0
0
0
65
83
70
72
96
74
55
70
60
SBA/SSA
Silica-SSA
a Benzylated vanillic acid (1mmol), catalyst (100mg), ethanol (10ml) and Temp-100°C
b selectivity of the product (A)
c calculated on the basis of vanillic acid (V.A) removed from OBn V.A
Table 3. Effect of substituent(s) on the conversion and selectivity of desired product.
Benzylated reactanta
Conversion(%) c
Selectivityb
OBn
80
98
O2N
NO 2
OBn
83
77
98
97
O2N
OBn
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OMe
OBn
OBn
82
97
CHO
OMe
72
95
96
OBn
74
Br
OBn
65
75
97
95
H3C
OBn
MeO
OMe
OBn
83
96
COOH
a Reaction Conditions: Benzylated reactant (1mmol), catalyst (100mg), ethanol (10ml) and
Temp-100°C
b selectivity of the product (A)
c Conversion as detected by the GC.
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s
Scheme 1. Schematic presentation of SBA/SSA synthesis
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HO
COOH
BnO
MeO
COOH
HO
COOH
SBA/SSA
+
MeO
MeO
Bn
C-benzylated vanillic acid
B
Vanillic acid
Benzylated Vanillic Acid
A
Scheme 2 Debenzylation of vanillic acid
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Scheme 3 Plausible Mechanism ( refers to deactivation of catalyst)
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Graphical Abstract
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Highlight
 Sulfosuccinic acid functionalized ordered mesoporous silica was synthesized and
characterized for first time.
 Liquid phase debenzylation of vanillic acid showed very high activity (82%) and (96%)
selectivity.
 Plausible mechanism suggests the synergism of acidic/basic groups responsible for higher
activity and selectivity.
20