Acceptor-free dehydrogenation of 4-hydroxy-3-methoxybenzyl alcohol to vanillin over a palladium complex

Article abstract of DOI:10.1039/c6gc01855f

Pd⁰ immobilized on surface-functionalized SBA-15 via in situ H₂-reduction was demonstrated as a highly efficient acceptor-free dehydrogenation catalyst for the oxidation of 4-hydroxy-3-methoxybenzyl alcohol to vanillin in excellent yield without an additional oxidant in the reaction system. Notably, the reaction medium water could be readily reused without further purification, and the catalyst was highly effective for a variety of alcohols. Moreover, a heterogeneous catalyst could be easily recovered and reused without conspicuous loss of activity indicating excellent potential for industrial application.

Full text of DOI:10.1039/c6gc01855f

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Acceptor-free dehydrogenation of 4-hydroxy-3-methoxybenzyl alcohol to vanillin
over palladium complex
Wei Fu, Lumin Yue, Xiaogang Duan, Jun Li*, Guanzhong Lu
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai
201418, PR. China
* Eꢀmail: junliecust0967@sina.com
Abstract
Pd0 immobilized on surfaceꢀfunctionalized SBAꢀ15 via inꢀsitu H2ꢀreduction were
demonstrated as a highly efficient acceptorꢀfree dehydrogenation catalyst for the
oxidation of 4ꢀhydroxyꢀ3ꢀmethoxybenzyl alcohol to vanillin in excellent yield without
additional oxidant in the reaction system. Notably, the reaction medium water could
be readily reused without further purification, and the catalyst was highly effective for
a variety of alcohols. Moreover, the heterogeneous catalyst could be easily recovered
and reused without conspicuous loss of activity indicating excellent potential for
industrial application.
Introduction
Vanillin is an important aroma molecule because of its widespread applications in
food,1 cosmetic,2 fine chemical industries,3 pharmaceutical4 and nutraceutical.5
Currently, more than 99% of vanillin is produced by chemical synthesis especially
through the glyoxylic acid method (Scheme 1),6 however, the toxic oxidants7 and
strongly alkaline were used accompanying complicated separation process for
harvesting high yields. For example, a cobalt schiff base with ionic substituents on the
ligand was used as a catalyst for the oxidation of 4ꢀmethyl guaiacol to vanillin
(Scheme 2) showing good activity (conversion = 100%, Selectivity = 90%), but the
high concentration of NaOH was used, furthermore, the reactions were conducted in
organic solvent and the recyclability of the catalyst was disappointing. 8 Over the past
few years, extensive efforts have been devoted toward the development of new
strategies for the synthesis of vanillin from both economic and green chemistry
viewpoints. Water, in contrast to common organic media has gained special attention

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because it is much cheaper, nontoxic, abundantly available and environmentꢀfriendly.9
Therefore, there is every reason to believe that water as a solvent in organic reactions
is the better options because most of the organic solvents are toxic and volatile.10 As a
matter of fact, in 2012, Augugliaro et al. reported the synthesis of vanillin in water by
using the photocatalytic method for the first time, however, the selectivity ranging
from 1.4 to 21mol% with respect to the converted substrate.11
Nowadays, considerable attention has been paid toward the oxidative
dehydrogenation of alcohols to aldehydes and ketones with palladiumꢀbased
catalysts12 due mainly to the high activity and selectivity even under atmospheric
pressure.13 As we all known, although a remarkable number of reports on the
improvement of the performance of Pd catalysts have been reported, but to our great
pity, until now, most of them suffer from the requirement for acid or base additives,
the toxic organic solvents and the hydrogen acceptor such as quinone, nitrobenzene,
or an olefin. Recently, there have been reports of heterogeneous metal catalysts being
applied to acceptorꢀfree alcohol dehydrogenation without additional oxidant in a
liquid phase.14 Undoubtedly, this would represent a much safer, and environmental
friendly process, which is in line with the concept of green chemistry.
On this basis, it is of particular interest to know whether it is possible to get high
valueꢀadded product vanillin by using heterogeneous palladiumꢀcontaining catalyst in
water medium under mild conditions in the absence of any additives. We report here
that surfaceꢀfunctionalized SBAꢀ15ꢀsupported Pd0 (Scheme 3) acts as an effective
heterogeneous catalyst for acceptorꢀfree dehydrogenation of vanillyl alcohol to
vanillin (Scheme 4) using water as the solvent under an atmospheric pressure of N2.
What is more exhilarating is that the catalyst has good performance in a variety of
alcohols, and show excellent chemical and thermal stability as well as good
recyclability.
Scheme 1 The synthesis of vanillin through the glyoxylic acid method

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Scheme 2 Catalytic oxidation of 4ꢀmethyl guaiacol to vanillin
Scheme 3 Synthesis of heterogeneous palladium catalysts
Scheme 4 Acceptorꢀfree dehydrogenation of vanillyl alcohol to vanillin in water
Experimental
Preparation of SBA-15
The synthesis of SBAꢀ15 has been achieved using known procedure described by
Stucky and his coꢀworkers.15 In a typical preparation procedure, 4.0 g of Pluronic
P123 was dissolved in 30 g of water and 120 g of 2 M HCl solution with stirring at

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35 °C. Then 8.50 g of tetraethoxysilane (TEOS) was added into that solution with
stirring at 35 °C for 20 h. The mixture was aged at 80 °C overnight without stirring.
After completion of the reaction, the solid was filtered off and washed with deionized
water, and dried overnight. The surfactant molecules were removed thoroughly with
hot ethanol/water using a Soxhelet apparatus for 24 h. It was dried in air at 100 ºC for
12h.
Synthesis of surface-functionalized SBA-15 (B)
In order to synthesis surfaceꢀfunctionalized SBAꢀ15, 0.356g (2mmol) of
pyridineꢀ2ꢀcarbonyl
chloride
hydrochloride
and
[3ꢀ(2ꢀaminoethylamino)propyl]trimethoxysilane 0.445 g (2 mmol) were first
suspended in 10 mL of dry CH2Cl2 in a 50 mL roundꢀbottomed flask. The flask was
placed in the iceꢀwater bath under vigorous stirring conditions. After that, the mixture
was stirred at room temperature for another 6 h, and the solvent was removed under
reduced pressure to obtain A (Scheme 3). The resulting SBAꢀ15 (6 g) was allowed to
react with A in refluxing dry toluene (150 mL) under nitrogen for 24 h. The solid was
filtered off and washed thoroughly with toluene and ethanol to obtain B (Scheme 3).
Synthesis of Pd0 catalyst
The catalyst was then prepared by stirring a mixture of B (4 g) and palladium
acetate (0.62 mmol, 0.14 g) in dry acetone (100 mL) at room temperature for 24 h.
The brown solid was filtered, washed with acetone and ethanol. It was then dried at
140 ºC under vacuum overnight. After this process, the resulting solid LnPd(Ⅱ) was
suspended in deionized water and reduced by H2 for 12 h to obtain catalyst in situ.
The powders were filtered, washed with ethanol and deionized water, and dried at 80
ºC overnight to obtain the catalyst LnPd, the loading of Pd in the final catalyst was
quantified by ICP.
General oxidation procedure
Typical procedure given for the oxidation of vanillyl alcohol：A mixture of catalyst
(30 mg catalyst LnPd, 4 ꢁmol of palladium), vanillyl alcohol (0.2 g, 1.3 mmol), and
H2O (20 mL) were added into a 50 mL threeꢀnecked roundꢀbottomed flask, which was
fitted with magnetic stirrer. The reaction was kept at the temperature of 80 °C under

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N2. Aliquots of the sample were taken from the reaction mixtures at different times,
extracted with ethyl acetate and quantified by HPLC. Oxidation of various alcohols
was conducted under the same experimental conditions as above.
Recycling tests were carried out by repeatedly using catalyst LnPd in nine
consecutive reactions. After each reaction, the catalyst was separated by filtration
from the reaction solution, washed with deionized water and ethanol, dried at 353 K
under vacuum for 2 h, and reused in the next run under the same reaction conditions.
Results and discussion
The percentage composition of palladium in the final catalyst is 1.42 wt%
quantified by ICP. The smallꢀangle Xꢀray diffraction patterns of SBAꢀ15 and
palladium catalyst are shown in Fig. 1. The fresh SBAꢀ15 displays a strong diffraction
peak and two small diffraction for the 100, 110 and 200 planes corresponding to a
hexagonal lattice, however, the decrease in peak intensities was observed after
immobilization of the Pd complex onto SBAꢀ15, while the d100 peak was almost
unchanged. These results indicate that periodicity of SBAꢀ15 was maintained well in
the palladium catalysts.
Fig. 2 shows the FTꢀIR spectra of B and catalyst LnPd in the range of 1350ꢀ1800
cmꢀ1. The infrared spectrum of B shows a characteristic band at 1650 cmꢀ1 assigning to
the C=N double bond. After immobilization of Pd on B, this band was shifted to 1635
cmꢀ1, presumably due to the interaction between metal and ligand. Similar shifting of
the band to lower wavenumbers has been also observed in previous studies.16 Fig. 3
depicts the diffuse reflectance UVꢀvis spectra of B and catalyst LnPd, the UVꢀvis
spectra display strong absorption bands at 275 nm for all the samples. Nevertheless,
when palladium catalyst was loaded on B, the presence of a very broad band centered
at 400 nm is also observed, indicating the coordination chemical bonds between
ligand and Pd nanoparticle.
The SEM image of catalyst is similar to SBAꢀ15 raw material (Fig. 4), showing the
characteristic morphology of SBAꢀ15 unchanged, and the TEM image of catalyst is
shown in Fig. 5, The specific surface areas 474 m2⋅gꢀ1 was measured for the catalyst,
which significantly decreased compared to the starting SBAꢀ15 ( 706 m2⋅gꢀ1 ). It can

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be noted that all of the samples exhibit type IV isotherms profile (Fig. S1†) with a
sharp hysteresis loop，which is characteristic of the highly ordered mesoporous
materials. 17 The corresponding physical parameters are collected in Table 1, and the
BJH pore size distribution is depicted in Fig. S2.† It showed that the average pore
diameters and specific areas of SBAꢀ15, B and catalyst LnPd decrease in sequence,
which is in good agreement with the fact that the surface modification indeed
happened inside the primary mesopores of the SBAꢀ15. The presence of organic
functional groups was further confirmed by NMR spectroscopy. 29Si NMR spectra for
B and catalyst LnPd are shown in Fig. 6, distinct resonances can be observed for the
siloxane
[
Qn = Siꢀ(OSi)n ꢀ(OH)4 n, n=2ꢀ4; Q4 at
－
111 ppm and Q3 at
－101 ppm]
－
and organosiloxane [Tm= RSi(OSi)mꢀ(OH)3 m, m=1ꢀ3; T3 at
－
65 ppm and T2 at
－57
－
ppm] species.18 The appearance of T3 and T2 peaks confirms a strong covalent linkage
between the organic groups and the silica surface. The obtained materials were also
characterised by means of solid state 13C NMR spectroscopy. In the 13C NMR
spectrum (Fig. 7), the strong signal at 47.8 ppm is attributed to the methylene carbons
(Cc, Cd, Ce), the peaks for Cc, Cd and Ce cannot be distinguished because of
conformational heterogeneity,19 this phenomenon was also observed in previous
studies.20 The signal at 175 ppm dramatically decreases in height indicates strong
chemical bonding between the Pd and amide group, in turn, effects the peak Cf which
attached to the amide group. The continued presence of the peak at 175 ppm suggests
that the amide groups are not yet saturated with Pd. The thermal stability of B,
catalyst LnPd were demonstrated by TGA measurement and it can be observed that
the thermal stability almost invariable (Fig. S3†, Fig. S4†). Three main weight loss
peaks can be seen from the DTG curve. The first weight loss peak observed at about
75 °C is due to physically adsorbed water, and the second decomposition peak
centered at about 337 °C is attributed from the decomposition of incompletely
removed surfactant21 while the third weight loss peak at about 426 °C can be
attributed to the decomposition of the organic Pd complex.
The oxidative dehydrogenation of vanillyl alcohol was studied with 1.42wt % Pd

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catalyst LnPd at 80 °C. As depicted in Table 2, the conversion of vanillyl alcohol is
100 % and the yield of vanillin is 99 % whether it is under the condition of nitrogen
＞
or air environment. It is worth mentioning that the reaction in N2 proceeded at a rate
comparable to that in air, indicating that the reaction is hydrogen acceptorꢀfree
oxidation reaction. With the aforementioned encouraging results, we further explored
the versatility of the catalytic system. As summarized in Table 3, highꢀyield
dehydrogenation was achieved for aromatic alcohols under oxidantꢀfree and anaerobic
conditions, which was comparable with the reported reactions under aerobic and acid
or base additives conditions. The catalytic activity for the oxidation of various
aromatic alcohols is different, especially for diphenylmethanol, which reacted slow
because the adsorption of diphenylmethanol onto catalyst is difficult due to steric
hindrance. A similar phenomenon has also been observed in previous reports.22 It
should be noted that the previous acceptorꢀfree dehydrogenation of alcohols suffer
23, 24
from high reaction temperature (200ꢀ350℃),14b, 23 low yields,
reaction in organic
solvent,14a our results represents a novel nonꢀNaOH, nonꢀorganic, dehydrogenation
oxidation route for the preparation of carbonyl compounds in water, without the
addition of hydrogen acceptor.
It is widely held that metallic state (Pd0) even in the presence of molecular oxygen
could hydrogenolysis of benzylꢀtype alcohols with the coproduct hydrogen,25 but
according to the table 3, the commonly accepted CꢀO bond hydrogenolysis
phenomenon was not happened in this regime. This fact combined with the presence
of amide ligand suggests that catalyst with stronger basicity but no acidity afford

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higher selectivity towards benzaldehyde.19 It strongly indicates that the amide ligands
in catalyst not only stabilize the palladium, but also provide alkaline sites for excellent
selectivity towards benzaldehyde. Thus, the present catalytic system makes the
synthesis of carbonyl compounds a high efficient and environmentally benign
chemical process.
To get an insight into the acceptorꢀfree dehydrogenation reaction, special care was
taken to conduct two sets of experiments. First, catalyst LnPd( ) was synthesized at
the same Pd loading (1.42 wt%) with the asꢀprepared catalyst LnPd(0), but without
the inꢀsitu reduction. The oxidation of vanillyl alcohol was then carried out with
catalyst LnPd( ) under the identical conditions. Surprisingly, the conversion of
vanillyl alcohol was approximately zero. Second, we added additive K2CO3 (1.0 equiv)
into this reaction, as can be seen in table 4, the conversion of vanillyl alcohol
dramatically increased and reach the maximum 100% when the reaction time fixed at
17 h. This suggested that Pd( ) species in the catalyst are particularly stable against
reducing reagents (alcohols) without a base (K2CO3) acting as a promoter to cleave
the OꢀH bond to form an alkoxide intermediate, which is in accordance with the
previous reports.26, 27 In comparison with the catalyst LnPd( ), the catalyst LnPd(0)
shows higher activity with a significant decrease in reaction time, implying that when
Pd( ) species were used for the synthesis of vanillin, there must be an induction
period in which a concomitant change of the alcohol to Pdꢀalcoholate, which in turn
triggers the formation of Pd(0) and aldehyde (carbonyl compounds). So, it can be
supposed that basic support is not powerful enough to promote Pd( ) species to
cleave the OꢀH bond of the alcohol to form a Pdꢀalcoholate species, metallic Pd is
more effective than Pd( ) species no matter with or without additives. In other words,
our results demonstrate that Pd( ) exhibit hardly any catalytic activity for alcohol
oxidation in the case of no additives or cocatalysts, whereas reduced Pd catalyst is
none the less active in water.
Upon consideration of the above results and previous reports,28 a plausible reaction

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pathway was illustrated in Scheme 5. For catalyst LnPd(II), oxidation reaction can not
take place without additives, while with the help of K2CO3, the reaction initiated by
formation of a palladium(II)ꢀalkoxide intermediate,29 which is suffered from
βꢀhydride elimination accompanied by the generation of aldehyde and LnPd0 (b). But
the Pdꢀbased catalyst is irreversible due to the oxidantꢀfree system. Then an oxidative
addition of OꢀH band from alcohol (a) to the coordinately unsaturated Pd0 species
affords a Pdꢀalcoholate species, which is transformed into the corresponding carbonyl
compounds and a palladium dihydride (c) with a βꢀhydride elimination. Then,
hydrogen is liberated from c along with the regeneration of Pd0 species. The above
mechanism can also explain why the reaction time is longer with Pd( ) species as
catalyst.
Hotꢀfiltration experiments were carried out with the catalyst LnPd as follows,
dehydrogenation of vanillyl alcohol to vanillin was conducted under the same
experimental conditions as the aforementioned, and the catalyst was hotꢀfiltered off
after 1 h (conversion 18%). The catalystꢀfree solution (the filtrate) was allowed to
continue reaction for another 6 h, but neither further reaction was observed (HPLC
analysis), nor Pd leaching into the solution was detected by ICP. Thus, the present
alcohol oxidation undoubtedly proceeds on the Pd species located on the
functionalized SBAꢀ15 surface, and that the catalytic reaction was truly
heterogeneous under the applied conditions. Furthermore, a leaching experiment of Pd
after 5th cycle was conducted under the same experimental procedure as above, the
catalyst was removed after 1 h (conversion 17.8%), and there was a slight increase in
conversion (18.2% after 6 h). Therefore, the palladium leaching is negligible.
Recycling experiments were carried out with the catalyst LnPd at 80 °C. After
each run, the catalyst was separated by filtration, washed with deionized water and
absolute ethyl alcohol, and then dried at 80 °C under vacuum for 2 h. The filtrate was
extracted with ethyl acetate (4×30 mL), after that, the remaining water and the dried
catalyst were reused in the next run. Varying amounts of water was added into the
reaction system in order to maintain the content of water (20 mL). The results shown
in Table 5 indicate that the catalyst gave the vanillin in >99% yield even after 9 runs,

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and the slight increase in reaction time is probably due to the minimal lose of the Pd
species during the recovery. TEM image of the catalyst LnPd after multiple uses
showed in Fig. 5, compared with the fresh catalyst, the mesoporous structure of the
used catalyst almost constant. A comparison of IR spectra of catalyst LnPd before and
after several reaction runs did not reveal significant differences (Fig. S5†) , no peak at
1650 cmꢀ1 was observed, indicating that immobilized Pd has always been there.
Moreover, as can be observed in Fig. S6†, compared to the catalyst LnPd, the UVꢀvis
absorption spectra of the recycled catalyst LnPd has almost unchanged. Therefore, the
catalyst can be promising catalytic materials in dehydrogenation of vanillyl alcohol
to vanillin reaction with high activity and stability.
Conclusion
In summary, we have developed a highly recoverable and efficient palladiumꢀbased
catalyst for acceptorꢀfree dehydrogenation of vanillyl alcohol to vanillin in water
under mild conditions. Furthermore, the recycled water can be successfully reꢀused in
the next round without any bad effect on product properties. This approach may
possess potential for industrial applications in the manufacture of vanillin. Moreover,
we have also demonstrated that the catalyst is suitable for various alcohols, therefore
meeting the increasing demands for environmentally friendly chemical processes.
Acknowledgements
The authors acknowledge the authorities of Shanghai Institute of Technology for their
constant encouragement.
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List of tables and figures
Fig. 1 XRD patterns of SBAꢀ15 and the catalyst LnPd.
Fig. 2 IR spectra of B and the catalyst LnPd.

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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Catalyst LnPd
B
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3 UVꢀvis absorption spectra of B and the catalyst LnPd.
Fig. 4 SEM images of the SBAꢀ15 raw material (left) and catalyst LnPd (right).

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Fig. 5 TEM images of catalyst LnPd (a) and recycled catalyst (b).
Fig. 6 29Si NMR spectrum of the B (left) and Catalyst LnPd (right).
Fig. 7 13C NMR spectrum of the B (left) and Catalyst LnPd (right).

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Table 1 Porosity parameters of SBAꢀ15, B, the Catalyst LnPd
Sample
SBAꢀ15
B
SBET (m2gꢀ1)
Pore Volume (cm³/g)
706
1.08
0.89
0.86
488
Catalyst
474
Table 2 Influence of reaction atmosphere in the oxidation of vanillyl alcohol a
reaction
Entry
Time (h)
Yield (%)
atmosphere
1
2
air
7
7
＞99
＞99
N2
a
Reaction conditions: vanillyl alcohol (0.2 g), temperature 80 , catalyst LnPd 30 mg, water (20
mL).
Table 3 Catalytic dehydrogenation of various alcohols over the catalyst LnPd a
Entry
1
Substrate
Product
Time (h)
2
Yield (%)
＞99
O
2
3
5
＞99
＞99
OH
17
a
Reaction conditions: alcohol (0.2 g), temperature 80 , catalyst 30 mg, water (20 mL), N2
balloon.

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a
Table 4 The oxidation of vanillyl alcohol with different catalysts
Entry
Catalyst
LnPd( )
LnPd( ) b
LnPd(0)
Time (h)
Conv. (%)
Sel. (%)
1
2
3
20
17
7
0
0
100
100
＞99
＞99
a Reaction conditions: vanillyl alcohol (0.2 g), temperature 80 , catalyst 30 mg, water (20 mL), b
LnPd(Ⅱ) with K2CO3 (1.0 equiv.) in the reaction system.
Table 5 The recycling study of the catalyst and water in the oxidation of vanillyl alcohol a
Cycle
Time (h)
Conv. (%)
100
Yield (%)
＞99
＞99
＞99
＞99
＞99
＞99
＞99
＞99
＞99
1
2
3
4
5
6
7
8
9
7
7
100
7
100
8
100
8
100
8.5
9
100
100
9.2
11
100
100
a
Reaction conditions: vanillyl alcohol (0.2 g), temperature 80 ℃, catalyst LnPd 30 mg, water
(20mL), N2 balloon.

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RCHO + 2H+
H2
RCH2OH
K2CO3
Oxidantꢀfree
H
H
LnPd(II)
LnPd
LnPd(0)
Water
Water
RCHO + 2H+
RCH2OH
RCH2OH
RCHO
OCH3
OH
Scheme 5 Indicates the reaction pathways for vanillin.
R=

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Graphic Abstract:
Acceptor-free dehydrogenation of 4-hydroxy-3-methoxybenzyl alcohol to vanillin
over palladium complex without additional oxidant in water