One-pot sequential oxidation and aldol-condensation reactions of veratryl alcohol catalyzed by the Ru@ZIF-8 + CuO/basic ionic liquid system

Article abstract of DOI:10.1039/c3gc41363b

The development of green and efficient methods to transform lignin into fuels and high value-added chemicals is of great importance. In this work, we studied one-pot sequential oxidation and aldol-condensation reactions of veratryl alcohol in a basic ionic liquid (BIL) 1-butyl-3-methylimidazolium 5-nitrobenzimidazolide, which acted as the solvent and provided the basic conditions required for the reactions. The effects of different factors such as the type of catalyst, reaction time, reaction temperature, and the amount of BIL on the oxidation reaction were investigated. It was demonstrated that the catalytic performance of individual Ru@ZIF-8 (zeolitic imidazolate framework-8) or CuO was very poor for the oxidation of veratryl alcohol to veratryl aldehyde. Interestingly, Ru@ZIF-8 + CuO was very efficient for the oxidation reaction and a high yield of veratryl aldehyde could be obtained, indicating the excellent synergistic effect of the two catalysts in the BIL. The veratryl aldehyde generated by the oxidation of veratryl alcohol could react directly with acetone to form 3,4-dimethoxybenzylideneacetone by aldol-condensation reaction catalyzed by the BIL in high yield.

Full text of DOI:10.1039/c3gc41363b

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One-pot sequential oxidation and aldol-
condensation reactions of veratryl alcohol
catalyzed by the Ru@ZIF-8 + CuO/basic ionic
liquid system†
Cite this: Green Chem., 2014, 16,
600
Received 10th July 2013,
Accepted 30th August 2013
DOI: 10.1039/c3gc41363b
Honglei Fan, Yingying Yang, Jinliang Song, Guodong Ding, Congyi Wu,
Guanying Yang and Buxing Han*
www.rsc.org/greenchem
The development of green and efficient methods to transform reduction reactions, and catalytic oxidation reactions. In the
lignin into fuels and high value-added chemicals is of great impor- oxidation reactions, lignin is converted to aldehydes selec-
tance. In this work, we studied one-pot sequential oxidation and tively,4 in which the number of carbon atoms can be further
aldol-condensation reactions of veratryl alcohol in a basic ionic adjusted for the required fuels by aldol condensation reactions
liquid (BIL) 1-butyl-3-methylimidazolium 5-nitrobenzimidazolide, in the presence of a basic catalyst.
which acted as the solvent and provided the basic conditions
Model compounds are often used to study the reaction
required for the reactions. The effects of different factors such as pathway and mechanism for lignin transformation. As a
the type of catalyst, reaction time, reaction temperature, and the derivative of coniferyl alcohol (a primary monomer of lignin),
amount of BIL on the oxidation reaction were investigated. It veratryl alcohol bearing a 3,4-dimethoxy group has been exten-
was demonstrated that the catalytic performance of individual sively studied for understanding the chemistry of lignin valori-
Ru@ZIF-8 (zeolitic imidazolate framework-8) or CuO was very zation and exploring efficient transformation routes.5 Veratryl
poor for the oxidation of veratryl alcohol to veratryl aldehyde. alcohol can be oxidized into veratryl aldehyde by using
Interestingly, Ru@ZIF-8 + CuO was very efficient for the oxidation different catalysts such as enzymes,6 noble and non-noble
reaction and a high yield of veratryl aldehyde could be obtained, metal porphyrin complexes,7 cobalt(salen) complexes8 and Co-
indicating the excellent synergistic effect of the two catalysts in zeolitic imidazolate frameworks (Co-ZIFs).9 It is reported that
the BIL. The veratryl aldehyde generated by the oxidation of the zeolitic imidazolate frameworks (ZIFs) like ZIF-8, -9, and
veratryl alcohol could react directly with acetone to form 3,4- -10 display excellent chemical and thermal stability, which
dimethoxybenzylideneacetone by aldol-condensation reaction have been developed for various applications such as gas
catalyzed by the BIL in high yield.
storage10 and alcohol oxidation.11 Further, aldol condensation
of aldehyde from lignin can be carried out to adjust the
carbon atoms for obtaining fuels through the hydrodeoxygena-
tion process.12 The aldol condensation reaction can be per-
formed using homogeneous catalysts like NaOH13 or solid
base catalysts14 such as alkali metal and alkaline earth metal
oxides.
Ionic liquids (ILs) with negligible vapor pressure, high
thermal stability, and high gas solubility can be used as reac-
tion media for replacing volatile and hazardous organic
solvents.15 Among them, basic ionic liquid (BIL) has been uti-
lized in some base-catalyzed processes such as Heck,16
Suzuki,17 aldol condensation,18 Henry19 and Knoevenagel reac-
tions.20 The main advantage of using BILs is that they can
function as both base and solvent. In addition, recent studies
demonstrated the effectiveness of ILs for the dissolution of
wood and lignocellulosic biomass.21,22
Introduction
Transformation of biomass into high value-added chemicals
and liquid fuels has received much attention in recent years.
With its unique aromatic ring structures and low oxygen
content, lignin is a promising component of biomass for the
production of biofuels and aromatic chemicals.1,2 Currently,
the strategy used to produce biofuel from lignin is typically
based on a two-step process,3 depolymerization followed by
upgrading. The depolymerization reactions can be principally
divided into lignin cracking, hydrolysis reactions, catalytic
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, P.R. China. E-mail: hanbx@iccas.ac.cn;
Fax: (+86) 10 6256 2821; Tel: (+86) 10 6255 9373
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3gc41363b
Herein, we conducted the one-pot reaction for veratryl
alcohol oxidation followed by aldol-condensation in a BIL
1-butyl-3-methylimidazolium 5-nitrobenzimidazolide (the
600 | Green Chem., 2014, 16, 600–604
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structure is shown in the ESI†), and Ru@ZIF-8 + CuO was used
as a catalyst. It was demonstrated that veratryl alcohol could be
oxidized into veratryl aldehyde very efficiently with high yield,
and Ru@ZIF-8 and CuO exhibited an excellent synergistic
effect to catalyze the oxidation reaction. Meanwhile, the BIL
could also promote the following aldol-condensation reaction
effectively. As far as we know, this is the first work on the cata-
lytic oxidative reaction of lignin model compounds using
Ru@ZIF-8 and CuO.
Scheme 1 Sequential oxidation and aldol-condensation reactions of
veratryl alcohol. I: veratryl alcohol; II: veratryl aldehyde; III: 3,4-
dimethoxy-benzylideneacetone.
Experimental
Ethanol, sodium hydroxide, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,
N,N-dimethyl formamide, acetone, diethyl ether, ethyl acetate,
butyl alcohol, chloroform and methanol were obtained
from the Beijing Chemical Reagents Company. Polyethylene
glycol (MW = 200), 2-methyl imidazole, veratryl alcohol, vera-
tryl aldehyde, 3,4-dimethoxybenzylideneacetone, RuCl3·3H2O
and 5-nitrobenzimidazole were provided by Alfa Aesar. 1-Butyl-
3-methylimidazolium chloride was purchased from the
Centre for Green Chemistry and Catalysis, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences.
Carbon dioxide and oxygen were provided by the
Beijing Analytical Instrument Factory and their purity was
>99.99%.
ZIF-8 was prepared according to methods reported in the
literature.23 Procedures to synthesize Ru/ZIF-8, copper oxide,
and BIL as well as catalyst characterization are provided in the
ESI.†
Fig. 1 Oxidation of veratryl alcohol with different catalysts. Reaction
conditions: veratryl alcohol 0.1 g, BIL 1.0 g, O2 0.5 MPa, distilled water
1.8 g, reaction temperature 130 °C, reaction time 55 min, and volume of
the reactor 8 mL. If CuO is used, the amount is 0.02 g. If Ru@ZIF-8
(5 wt%) is used, the amount is 0.007 g.
The X-ray diffraction (XRD) patterns were collected using a
D/Max 2500 V/PC X-ray diffractometer with high intensity
Cu Kα (40 kV, 200 mA) radiation. The high-resolution trans-
mission electron microscopy (HRTEM) observation was carried
out on a JEM 2011 at an accelerating voltage of 200 kV.
The reactions were conducted in an 8 mL reactor made of
316 stainless steel. In a typical experiment, the desired
amounts of veratryl alcohol, catalyst, BIL, and water were
loaded into the reactor. The reactor was placed in the pre-
heated furnace of the desired temperature and air was
removed under vacuum; then O2 was charged into the reactor
and the stirrer was started. After a suitable reaction time, the
Results and discussion
In our work, veratryl alcohol (I) was first oxidized into veratryl
aldehyde (II), which was then reacted with acetone to produce
3,4-dimethoxybenzylideneacetone (III) by the well-known
aldol-condensation reaction (Scheme 1).
Oxidation of veratryl alcohol
reactor was cooled in ice water. The products were extracted The performances of different catalysts were studied and the
with ethyl acetate and analyzed by a gas chromatograph (GC) results are given in Fig. 1. As expected, the yield of veratryl
using an HP 4890 GC equipped with a flame ionization detec- aldehyde was very low without using the catalyst. When CuO
tor (FID), and butyl alcohol was used as the internal standard. or Ru@ZIF-8 was used individually, the yield of the product
To conduct the aldol-condensation reaction, acetone was was also very low. Interestingly, when the two catalysts were
added into the reactor after the oxidation reaction described used together (Ru@ZIF-8 + CuO), the yield could reach 94.9%,
above, and finally the amount of condensation product was indicating an excellent synergistic effect of the two catalysts.
determined quantitatively by GC. Identification of the products
It was reported that in the presence of Ru nanocatalysts, the
and reactant was done using a GC-MS (SHIMADZU-QP2010) as alcohol first combined with Ru to form Ru-alcoholate species
well as by comparing the retention times to respective stan- and then β-hydride was eliminated to produce the corres-
dards in GC traces. The yields are calculated as the mass of the ponding aldehyde and a Ru hydride species. Subsequently, the
products formed divided by the mass of the reactant initially Ru hydride species reacted with O2 to produce water and com-
loaded into the reactor.
plete the cycle.24 For the CuO catalyst, alcohols were converted
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Fig. 2 Effect of reaction time on the oxidation of veratryl alcohol. Reac-
tion conditions: veratryl alcohol 0.1 g, BIL 1.0 g, CuO 0.02 g, Ru@ZIF-8
(5 wt%) 0.007 g, O2 0.5 MPa, distilled water 1.8 g, reaction temperature
130 °C, and volume of the reactor 8 mL.
Fig. 3 Effect of loading amount of Ru in the Ru@ZIF-8 on the oxidation
of veratryl alcohol. Reaction conditions: veratryl alcohol 0.1 g, BIL 1.0 g,
CuO 0.02 g, Ru@ZIF-8 0.007 g, O2 0.5 MPa, distilled water 1.8 g, reac-
tion temperature 130 °C, reaction time 55 min, and volume of the
reactor 8 mL.
into aldehydes or ketones by the dehydrogenation process.25
The mechanism of the synergistic effect between Ru@ZIF-8
and CuO is very interesting and needs to be studied further.
The XRD patterns of the CuO particles are consistent with
that in the JCPDS database, and the XRD patterns of the ZIF-8
are the same as that reported in the literature23 (Fig. S1†). The
TEM images of the Ru@ZIF-8 and CuO particles are shown in
Fig. S2.† The diameter of Ru particles in the catalyst Ru@ZIF-8
was in the range of 3 to 5 nm, and the size of CuO particles
was in the micron scale.
In the following, the effects of reaction parameters were
further studied using combined catalysts. The dependence of
the transformation of veratryl alcohol on reaction time is
shown in Fig. 2. The conversion increased with reaction time
and reached a plateau after 70 minutes. The yield increased
Fig. 4 Effect of reaction temperature on the oxidation of veratryl
with reaction time and reached a maximum at 55 minutes. alcohol. Reaction conditions: veratryl alcohol 0.1 g, BIL 1.0 g, CuO
0.02 g, Ru@ZIF-8 (5 wt%) 0.007 g, O2 0.5 MPa, distilled water 1.8 g, reac-
tion time 55 min, and volume of the reactor 8 mL.
The yield of veratryl aldehyde could reach 94.9%. As we know,
oxidation of an alcohol is a cascade reaction, i.e., the produced
aldehyde can be further oxidized. In the meantime, for the
aldehydes without active hydrogen at the α site, disproportio-
of the produced compounds became dominant after the
loading amount of Ru exceeded 5 wt%, leading to a decrease
in yield.
nation reaction can occur to produce equimolar alcohol and
acid under the basic conditions. Therefore, oxidation of
veratryl alcohol and disproportionation of veratryl aldehyde
occurred in the reaction system. After optimum reaction time,
more aldehyde was converted by disproportionation reaction
and overoxidation reaction. So, the yield of veratryl aldehyde
decreased with reaction time after the maximum.
Ru@ZIF-8 catalysts containing different amounts of Ru
were used to catalyze the reaction. The results are shown in
Fig. 3. The conversion increased as the loading amount of Ru
was changed from 2.5 to 5.5%. The yield of veratryl aldehyde
increased first and decreased after the loading amount of Ru
exceeded 5%. Generally, increasing the amount of active com-
ponents is favorable for the conversion of reactants. But for
cascade reactions like the oxidation of alcohol, catalytic
sites also promote further transformation of the produced pro-
ducts. For the reaction studied in this work, transformation
The effects of reaction temperature on the formation of
veratryl aldehyde were studied in the range of 90 to 170 °C,
and the results are shown in Fig. 4. The conversion of veratryl
alcohol increased in the range of 90 to 130 °C, and remained
unchanged with further increasing the temperature. In the
temperature range of 90 to 130 °C, the effect of temperature on
yield is significant, and the maximum yield was obtained at
130 °C. The temperature affected the yield in two opposite
ways. On the one hand, the conversion of the reactant
increased with increasing temperature, which was favorable
for enhancing the yield of the product. On the other hand,
elevating the temperature also promoted the occurrence of
side reactions. The competition of the two factors resulted in
the maximum of the yield.
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Fig. 6 Effect of reaction time of the aldol-condensation on the yield of
3,4-dimethoxybenzylideneacetone for the one-pot sequential oxidation
and aldol-condensation reactions of veratryl alcohol (Scheme 1). Reac-
tion conditions for the oxidation of veratryl alcohol to form veratryl
aldehyde were the same as those given in Fig. 1 using Ru@ZIF-8 + CuO
as the catalyst. Reaction conditions for aldol-condensation: acetone
0.35 g, reaction temperature 85 °C, and volume of the reactor 8 mL.
Fig. 5 Effect of added amount of BIL on the oxidation of veratryl
alcohol. Reaction conditions: veratryl alcohol 0.1 g, CuO 0.02 g,
Ru@ZIF-8 (5 wt%) 0.007 g, O2 0.5 MPa, distilled water 1.8 g, reaction
temperature 130 °C, reaction time 55 min, and volume of the reactor
8 mL.
Fig. 5 shows the dependence of the conversion of the reac-
tant and the yield of the product on the amount of BIL used in
the reaction. When the amount of BIL increased from 0.55 to
1.54 g, the conversion increased at first and then reached a
plateau. The maximum yield could be observed when the
amount of BIL was 1.0 g. The BIL in our work contains the imi-
dazole anion, which can combine with protons in water, and
hydroxide anions are formed. It is known that the hydroxide
anion is helpful for the deprotonation of veratryl alcohol,
thereby promoting the formation of veratryl aldehyde.26 So,
yield increased with the addition of the IL continuously at the
beginning. At the same time, the degree of overoxidation and
disproportionation reactions became larger with increasing
the amount of BIL, leading to a decrease of the yield of veratryl
aldehyde.
dehydration.27 For base catalyzed aldol condensation, deproto-
nation of acetone is the rate determining step and an enolate
anion is formed. With a long reaction time, the target product
3,4-dimethoxybenzylideneacetone could react with another
molecular veratryl aldehyde to produce the further conden-
sation product. Thus, the yield of product III decreased due to
the occurrence of a side reaction.
Conclusions
We have studied the one-pot sequential oxidation and aldol-
condensation reactions of veratryl alcohol in a BIL, which func-
tions as both base and solvent for the reactions. It is found
that Ru@ZIF-8 or CuO cannot catalyze the oxidation of veratryl
alcohol to veratryl aldehyde effectively. However, Ru@ZIF-8
and CuO show an excellent synergistic effect to catalyze
the oxidation reaction, and the reaction can proceed
very efficiently in the BIL using a small amount of Ru@ZIF-8 +
CuO catalyst. There is a maximum in the yield vs. reaction
time curve, the yield vs. reaction temperature curve, and
the yield vs. the amount of IL curve because the product
We carried out the experiment using 1.0 g of BIL without
water. It was found that the yield of veratryl aldehyde was
only 2.8%. This showed that water in the reaction system was
necessary to guarantee that there is sufficient amount of
hydroxide anions in the reaction system.
Sequential oxidation and aldol-condensation reactions of
veratryl alcohol
We also studied the one-pot sequential oxidation and aldol- veratryl aldehyde can be further converted. Under the opti-
condensation reactions of veratryl alcohol to produce III mized reaction conditions, the yield of veratryl aldehyde
(Scheme 1). The oxidation of veratryl alcohol to form veratryl can be as high as 94.9%. Meanwhile, the BIL can promote an
aldehyde was first conducted using Ru@ZIF-8 + CuO as the aldol-condensation reaction between veratryl aldehyde and
catalyst under the conditions given in Fig. 1. After oxidation to acetone to produce 3,4-dimethoxybenzylideneacetone. The
form veratryl aldehyde, the oxygen in the reactor was removed. Ru@ZIF-8 + CuO/BIL is an excellent catalytic system for the
Then acetone was added into the reactor, allowing the aldol- one-pot sequential oxidation, and the yield of 3,4-dimethoxy-
condensation reaction to occur. Fig. 6 shows the conversion of benzylideneacetone can reach 86.4%. We believe that the com-
veratryl alcohol and the yield of product III as a function bination of BILs and suitable catalysts opens the way for
of reaction time. There was also a maximum in the yield vs. developing greener and more efficient methods to transform
reaction time curve. It is known that the aldol-condensation lignin-related compounds as well as the catalytic valorization
reaction involves three steps: enolization, addition, and of lignin.
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13 C. J. Barrett, J. N. Chheda, G. W. Huber and J. A. Dumesic,
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