Selective Hydrogenolysis of Furfural Derivative 2-Methyltetrahydrofuran into Pentanediol Acetate and Pentanol Acetate over Pd/C and Sc(OTf)3 Cocatalytic System

Article abstract of DOI:10.1002/cssc.201702073

It is of great significance to convert platform molecules and their derivatives into high value-added alcohols, which have multitudinous applications. This study concerns systematic conversion of 2-methyltetrahydrofuran (MTHF), which is obtained from furfural, into 1-pentanol acetate (PA) and 1,4-pentanediol acetate (PDA). Reaction parameters, such as the Lewis acid species, reaction temperature, and hydrogen pressure, were investigated in detail. ¹H NMR spectroscopy and reaction dynamics study were also conducted to help clarify the reaction mechanism. Results suggested that cleavage of the primary alcohol acetate was less facile than that of the secondary alcohol acetate, with the main product being PA. A PA yield of 91.8 % (150 °C, 3 MPa H₂, 30 min) was achieved by using Pd/C and Sc(OTf)₃ as a cocatalytic system and an 82 % yield of PDA was achieved (150 °C, 30 min) by using Sc(OTf)₃ catalyst. Simultaneously, the efficient conversion of acetic esters into alcohols by simple saponification was carried out and led to a good yield.

Full text of DOI:10.1002/cssc.201702073

Accepted Article
Title: Selective hydrogenolysis of furfural-derivative 2-
methyltetrahydrofuran into pentanediol acetate and pentanol
acetate over Pd/C and Sc(OTf)3 co-catalytic system
Authors: Kun Zhang, Xing-Long Li, Shi-Yan Chen, Hua-Jian Xu, Jin
Deng, and Yao Fu
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10.1002/cssc.201702073
ChemSusChem
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Selective hydrogenolysis of furfural-derivative 2-
methyltetrahydrofuran into pentanediol acetate and pentanol
acetate over Pd/C and Sc(OTf)3 co-catalytic system
Kun Zhang, §[a, b] Xing-Long Li, §[a] Shi-Yan Chen, [a, b] Hua-Jian Xu, [c] Jin Deng, *[a] and Yao Fu *[a]
Abstract: It is of great significance to convert platform molecules and
their derivatives into high value-added alcohols which had
multitudinous applications. We systematically studied the conversion
of 2-methyl tetrahydrofuran (MTHF), which obtained from furfural, to
1-pentanol acetate (PA) and 1,4-pentandiol acetate (PDA) in this
paper. The reaction parameters such as the species of Lewis acid,
reaction temperature, hydrogen pressure, etc. were investigated and
discussed in detail. 1H-NMR analysis and reaction dynamics were
also conducted to help clarify the reaction mechanism. Results
suggested that the cleavage of primary alcohol acetate was harder to
occur than that of secondary alcohol acetate with the main product
being PA. A yield of 91.8% PA (150 C, 3 MPa H2, for 30 min) was
achieved by using Pd/C + Sc(OTf)3 co-catalytic system and 82% yield
of PDA was achieved (150 C, for 30 min) by using Sc(OTf)3 catalyst,
respectively. Simultaneously, the efficient conversion of acetic esters
to alcohols by simple saponification were carried out and obtained a
good yield.
furfural was used to obtain furfuryl alcohol, which was the main
raw materials for producing phenolic resin. [5] This was very urgent
to develop more downstream industrialized products from furfural.
Scheme 1. Production of 1,4-PDO and 1-PO from 2-MTHF.
Aliphatic alcohols and their derivatives had a variety of
important applications. Pentanediol (PDO) including 1,4-PDO and
1,5-PDO could be used in the production of polyurethanes,
coatings, acrylates, adhesives, polyesters, and plasticizers, etc. [6]
Pentanol can be used as organic solvents, edible spices,
antifoaming agents, and pharmaceutical raw materials, etc.
Considering the multitudinous applications of alcohol compounds
and their derivatives, the development of green and efficient
synthetic methods became a research issue. Pentanol and
pentandiol were more difficult to be obtained from the petroleum
raw materials, mainly due to the multistep procedure containing
the separation of petroleum-based raw materials, oxidation and
reduction. The complicated operation steps and the lower
economic value could not to be ignored. [7a] Alcohol compounds
were mainly obtained from the biological fermentation, but
enzymes demanded high requirements for living environment and
were easy to deactivate. [7b-7e] Cellulose and its derivatives, such
as sugar alcohols, have also been used to produce C3~C7
alcohols. [8a-8i] Cellulose were the polysaccharide polymer, and the
selectivity of alcohol products were difficult to control during the
reaction. Meanwhile, sugar alcohols, as derivatived of cellulose,
could be converted with high selectivity to alcohol products. But it
is itself the high value-added chemicals, which reduced the
economic advantages of the process. With a view to the
importance of furfural and the lack of its downriver industrial
products, the development of furfural and its derivatives to the
production of alcohols was of great significance. In this case, the
selective hydrogenolysis of the C-O bonds in cyclic ethers was
considered to be the most challenge for the synthesis of useful
alcohols.
Introduction
The increasing consumption of petrochemical resources brought
serious environmental problems and energy crisis. The
production of high-value-added fuels and chemicals from
renewable biomass resources had attracted more and more
attentions. [1]
Furfural, obtained from the hydrolysis of hemicellulose, is an
[2]
important platform molecule.
The current global production
[3]
scale of furfural has reached about 300,000 tons annually.
Furfural could be converted into a variety of high-value-added
fuels and chemicals, including furfuryl alcohol (FOL),
tetrahydrofurfuryl alcohol (THFOL), 2-methyltetrahydrofuran
(MTHF), γ-valerolactone (GVL), pentanol, pentanoate, isoprene,
pentanediol and levulinic acid, etc. [4] At present, about 60% of
[a]
Hefei National Laboratory for Physical Sciences at the Microscale,
iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui
Province Key Laboratory of Biomass Clean Energy, Department of
Chemistry, University of Science and Technology of China.
Hefei 230026, China. Fax: (+86) -551-63606689;
E-mail: dengjin@ustc.edu.cn
E-mail: fuyao@ustc.edu.cn
[b]
[c]
§
Nano Science and Technology Institute, University of Science and
Technology of China, Suzhou 215123, P. R. China.
School of Biological and Medical Engineering, Hefei University of
Technology, Hefei 230009, China.
These authors contributed to the work equally and should be
regarded as co-first authors.
At present, pentanediol or pentanol was obtained by
hydroconversion of furfural and its derivatives such as furfuryl
alcohol, tetrahydrofurfuryl alcohol, LA and GVL in the presence of
[8j]
bifunctional catalyst.
Furfural, furfuryl alcohol, and
tetrahydrofurfuryl alcohol were mainly converted to 1,5-PDO and
1,2-PDO. LA and GVL were mainly converted to obtain 1,4-PDO.
1,5-PDO was prepared initially from tetrahydrofurfuryl alcohol by
using copper chromite catalyst, but the reaction took place under
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cssc.201702073
ChemSusChem
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high temperature and high pressure with a lower product yield. [9]
Noble metal catalysts have been extensively studied for their
excellent activity in this reaction process. Pt/Co2AlO4 catalyst was
used to convert furfural to alcoholic mixtures, but the selectivity of
products was very poor. [10] Chatterjee et al. used Rh-MCM-41 as
the catalyst to convert tetrahydrofurfuryl alcohol to 1,5-PDO in
scCO2, and found that the co-existence of Rh2O3 and Rh0 species
discussed in detail. The possible reaction mechanism was
discussed according to 1H-NMR and GC analyses.
Simultaneously, 1,4-PDO and 1-PO were obtained in good yield
by simple saponification of PDA and PA, respectively.
Results and Discussion
on the catalyst were important to obtain high selectivity of 1,5-
Due to the complicated conversion of furfural, we used 2-
MTHF as the substrate to screen the reaction conditions. In view
of our previous work, we found that metal triflate played an
[11]
PDO.
Marks et al. used Pd/Al2O3 and Yb(OTf)3 co-catalyst
system to convert a variety of ether compounds to alcohol
products in ionic liquid [EMIM] [OTf] under high temperature and
[32a]
important role on product distribution in the reaction system.
high pressure, but the ionic liquid was difficult to recover and
It was pointed out that Lewis acid and solvents had important
[12]
recycle.
Meanwhile, the fracture mechanism of ether bond
effect on the acetylation of cyclic ethers, and the common
[34]
under Pd-based catalyst and M(OTf)x was explained, suggesting
acetylation solvent was anhydride.
Since acetic acid was
that M(OTf)x which had a high charge density and a low ionic
derived from biomass resources and could be more readily
prepared, we investigated the effect of different catalysts on
product distribution using 2-MTHF as substrate and acetic acid as
solvent. (Table 1)
[13]
radius was beneficial to the reaction.
Tomishige et al.
developed a series of catalysts such as (Ir, Rh, Pt, Pd, Ru)-(ReOx,
MoOx, WOx, NbO x, FeOx, CoOx, VOx, MnOx)/SiO2 for effective
[14]
conversion of furfural,
furfuryl alcohol and tetrahydrofurfuryl
alcohol [15] to pentanediol. Huber et al. used Rh-Re/C catalyst to
catalyze the conversion of furfural to 1,5-PDO through multi-step
reactions including dehydration, hydration, isomerization, and
hydrogenation reactions. Subsequently, cost accounting and
process analysis were carried out. [16] Chen et al. obtained 85.8%
conversion of furfuryl alcohol and 70.3% selectivity of 1,2-PDO
and 1,5-PDO mixtures using Cu-Al2O3 catalyst. It was found that
the ion size and surface acidity of Cu catalyst were important to
the production of PDO. [17] Ru/MnOx catalyst could not only inhibit
the oligomerization of furfuryl alcohol, but also enhance the
selectivity of 1,2-PDO. Low hydrogen pressure, high temperature
and the presence of water could promote the reaction, and 42.1%
Table 1. The screening of metal triflate in the conversion of 2-MTHF. [a]
Yield /%
Metal
Entry
Conversion /%
triflate
PDA
PA
pentane
1
2
3
4
5
6
7
8
La(OTf)3
52.2
100.0
100.0
89.8
37.7
14.0
0.1
Hf(OTf)4
Zn(OTf)2
Cu(OTf)2
Ni(OTf)2
Ce(OTf)3
Sc(OTf)3
Al(OTf)3
1.2
12.5
10.4
35.9
53.4
4.3
79.5
80.2
68.9
30.0
29.4
90.4
83.0
17.8
5.6
6.0
yield of 1,2-PDO was obtained. [18] 1,4-PDO was mainly obtained
68.5
1.2
[19]
from the hydrogenation of LA and GVL.
Heterogeneous
100.0
100.0
100.0
2.4
catalyst systems mainly included noble catalysts like Pt–Mo/HAP,
[20]
4.8
Ru–Re/C, [21] Rh–Mo /SiO2, [22] Ir-MoOx/SiO2, [23] Ru/C, [24] and
non-noble metal catalysts like Cu/ZrO2-OG, [25] Cu-TiO2 /ZrO2-CP-
600, [26] Ni/TiO2. [27] Homogeneous catalysts included Ru-based
catalysts, [28] Co-based catalysts, [29] and Fe-based catalysts. [30]
Dumesic et al. reported the conversion of methyltetrahydrofuran
2.3
14.1
[a] Reaction conditions: 1.2 mmol 2-MTHF, 0.8 mol% 10% Pd/C, 4 mol%
metal triflate, 2 MPa H2, 150 oC, 5 mL acetic acid, 30 min. PDA: 1, 4-
pentanediol acetate; PA: pentanol acetate.
using Rh-ReOx/C to 2-pentanol, but the conversion rate was low
[31]
under this reaction conditions.
Currently, reported reaction
processes still exist problems including high hydrogen pressure,
long reaction time, and poor product selectivity, etc. It is very
challenging to develop a green and efficient catalytic system for
the conversion of biomass substrate to pentanol or pentanediol.
Pd/C and metal triflate co-catalyst system had a high activity
and selectivity to break the C-O bond in cyclic ether and obtained
alkanes by hydrodeoxygenation of condensation products. [32a, 32b]
La(OTf)3 was adopted firstly because it had better catalytic activity
according to a document. [35] The experimental results showed
that the conversion of 2-MTHF was only 52.2% by using La (OTf)3
catalyst at 2 MPa H2 for 30 min (Table 1, Entry 1). At this point,
the products were mainly PDA and PA with almost no pentane
being produced. The yield of PDA and PA were 37.7% and 14%,
respectively. 2-MTHF could be converted completely by using
Hf(OTf)4 (Table 1, Entry 2), with main products of PA (yield of
79.5%) and pentane (yield of 17.8%). The residual 1.2% yield of
PDA proved that Hf(OTf)4 had stronger Lewis acidity than that of
La(OTf)3. The main product of alkanes have been reported by
using Hf(OTf)4 as acid catalyst at higher temperature and
This co-catalyst system was also used to selectively break the
[12]
ether bond to obtain alcohol products in ionic liquid.
And we
also found straight chain acetate polymers with Pd/C and W(OTf)6
co-catalyst system can be converted middle-chain carboxylic acid.
[32c, 32d]
Herein, in this work, we used 2-MTHF which obtained from
furfural, [33] as the substrate to produce PDA and PA. The effect of
reaction parameters such as reaction temperature, hydrogen
pressure and catalyst ratio, etc. on product distribution were
[32]
hydrogen pressure in our previous work and literatures.
Contrarily, in this work, PA was obtained as the main product. This
indicated that our catalytic system had a certain stability to
primary alcohol acetate. Subsequently, we screened several
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10.1002/cssc.201702073
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other metal triflate (Table 1, Entry 3-8) and found that the highest
yield (90.4%) of PA was obtained using Sc(OTf)3 as a catalyst
(Table 1, Entry 7). It might be due to the suitable effective charge
density and Lewis acidity of Sc(OTf)3.
In order to further investigate the effect of reaction conditions
on product distribution, we scaned other reaction conditions as
follows.
of pentane was produced. The activity of the catalytic system
increased with the increase of temperature. Further increased the
reaction temperature, the yield of PA and PDA increased first and
then decreased, and the highest yield of PA was obtained at 150
C. The yield of pentane gradually increased with the increased
temperature. Marks et al. reported that the cleavage capacity of
acetate was related to the stability of the carbocation formed
during the reaction, and the cleavage activity trend was tertiary >
Effect of hydrogen pressure
[36]
secondary > primary.
PDA, which containing primary acetate
bond and secondary acetate bond, was obtained by the ring-
opening of 2-MTHF with acetic acid in the presence of Lewis acid
catalyst. The cleavage of secondary acetate was carried out more
easily than that of primary acetate, and no 2-pentanol acetate was
observed in reaction system. This was consistent with the results
[36]
reported in the literature.
Further increased the reaction
temperature from 150 C to 180 C, the yield of PA decreased
slightly. This showed that PA had better stability in reaction
system in this temperature range. The yield of PA decreased
obviously along with the increased yield of pentane at 200 C.
Through the investigation of reaction temperature, the suitable
reaction temperature could obtain higher yield of PA.
Figure 1. Reaction conditions: 1.2 mmol 2-MTHF, 0.8 mol% 10% Pd/C, 1.8
mol% Sc(OTf)3, 5 mL acetic acid, 150 C, 30 min. PA: pentanol acetate; PDA:
1, 4-pentanediol acetate.
Then, the effect of hydrogen pressure on product distribution were
investigated at different reaction temperatures (Figure 1). The
raw material could be converted completely at a relatively low
reaction hydrogen pressure (Figure 1, 1MPa H2). The results
showed that the reaction temperature had a direct effect on the
ring opening of tetrahydrofuran. The main product is PA (yield of
68.4%), but 23.3% yield of PDA was still obtained indicating that
the hydrogenolysis rate of esters was relatively slower at a lower
hydrogen pressure. As the hydrogen pressure increased to 2 MPa,
the yield of PA had been greatly increased with the yield of PDA
decreased sharply. Meanwhile, the yield of pentane increased
slightly. Such results indicated that the increased hydrogen
pressure was beneficial for the cleavage of ester bonds. Further
increased the hydrogen pressure to 3 MPa and 4 MPa, the yield
of PA declined slightly, with the yield of pentane increased slightly.
It showed that further increased hydrogen pressure had little
effect on product distribution and the optimum hydrogen pressure
was 2 MPa H2. The highest yield of PA (91.8%) was obtained at
150 C,3 MPa H2, 0.5 h.
Figure 2. Reaction conditions: 1.2 mmol 2-MTHF, 0.8 mol% 10% Pd/C, 1.8
mol% Sc(OTf)3, 2 MPa H2, 5 mL acetic acid, 30 min. PA: pentanol acetate; PDA:
1, 4-pentanediol acetate.
Effect of catalyst ratio
Considering that the catalytic system containing both
hydrogenation catalyst and Lewis acid catalyst, the impact on
ratio of hydrogenation catalyst and Lewis acid catalyst (Pd/C: Sc
(OTf)3) was investigated (Figure 3). It was found that the
conversion of 2-MTHF was 83% and obtained 82% yield of PDA
in the presence of only Sc(OTf)3. It was reported that the ring
opening of tetrahydrofuran ring carried out more easily to obtain
acetate products in acetic anhydride solvent under Lewis acid
catalyst. [34] The anhydride was a strong acylating agent and had
a stronger nucleophilicity than that of acetic acid. Due to the use
of acetic acid solvent with weak nucleophilicity, the rate of reaction
was slower and resulted in the non-completed conversion of 2-
MTHF in the reaction.
Effect of reaction temperature
The ring-opening of tetrahydrofuran ring generally had a great
relationship with reaction temperature. The conversion of 2-MTHF
was only about 15%, with PDA and a little of PA obtained at 100
C (Figure 2). This indicated that the catalyst activity was poor at
a lower temperature. With the increase of reaction temperature to
120 C, the conversion of raw materials increased significantly.
Higher yields of PA and PDA were achieved and a small amount
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The solvent-free condition was beneficial to obtain
cyclohexene and acetic acid according to Marks' work (Scheme
2). [36] Differently, the deacetylation of PDA was inhibited and PDA
was maintained using acetic acid as a solvent. A slight conversion
of MTHF obtained using only Pd/C as a catalyst indicating that
metal triflate was beneficial to the ring-opening of cyclic ethers.
The conversion of 2-MTHF was improved and the main products
were PDA and PA with the ratio of Pd/C:Sc(OTf)3 at 0.8:0.9
(mol%). The yield of PA improved obviously, which indicated that
the increased amount of metal triflate was beneficial to the
cleavage of secondary ester bond when the catalyst ratio of Pd/C:
Sc(OTf)3 is 0.4:1.8 (mol%). But still more PDA were obtained at
this ratio. The reduced Pd/C catalyst might undertake this
responsibility, which probably slowed the hydrogenation rate of
intermediate. In order to verify this statement, we increased the
amount of Pd/C to 0.8 mol%, and found that the yield of PA was
further increased with the yield of PDA decreased. Further
increased the catalyst ratio of Pd/C: Sc(OTf)3 to 0.8:3.6 (mol%),
the yield of PA essentially remained stable, and the yield of
pentane slightly increased. It was shown that further increasing
the amount of Sc(OTf)3 had no obvious effect on product
distribution. The optimal ratio of Pd/C: Sc(OTf)3 was 0.8:1.8
(mol%).
the Figure 4, the position of 0 min belonged to the beginning time
when the temperature was raised to 150 C (namely heated for
30 min). 52% yield of PDA and about 17% yield of PA were
obtained along with the 72% conversion of 2-MTHF in the
constant heating time for 0 min. This indicated that the ring-
opening reaction of tetrahydrofuran to acetate esters was
occurred even in a shorter reaction time. Sc(OTf)3 , a strong Lewis
acid catalyst, could promote ring-opening reaction of
tetrahydrofuran carried out more easily. However, the product
with cleavage of secondary ester bond did not obtain in a large
amount due to the short reaction time (just heating for 30 min).
Constant heating for 10 min at 150 C, the conversion of 2-MTHF
was 94%. The yield of PA increased along with the decrease of
PDA yield. It was reported that similar catalysts could be used to
achieve rapid hydrogenolysis of the secondary ester bond. The
highest yield of PA was 90.5% in constant heating time for 30 min
at 150 C. At this time, MTHF converted completely and a small
amount of PDA and pentane observed. Further prolongated the
constant heating time, PA decreased slightlyand was converted
to pentane. This might be that PA could be stable for a long time
under the current reaction conditions according the previous
document. [36]
Figure 4. Reaction conditions: 1.2 mmol 2-MTHF, 0.8 mol% 10% Pd/C, 1.8
mol% Sc(OTf)3, 2 MPa H2, 5 mL acetic acid, 150 C. PA: pentanol acetate; PDA:
1, 4-pentanediol acetate.
Figure 3. Reaction conditions: 1.2 mmol 2-MTHF, 2 MPa H2, 5 mL acetic acid,
150 C, 30 min. PA: pentanol acetate; PDA: 1, 4-pentanediol acetate.
Scheme 2. Different reaction pathways under similar reaction conditions.
Effect of reaction time
Figure 5. 1H-NMR spectra of intermediates and products.
Our reaction initial temperature was 30 C, increased the reaction
temperature from 30 C to 150 C over 30 min (heating time) and
then the temperature was maintained for a certain time at 150 C
until the reaction reached completion (constant heating time). In
The product distribution of intermediates and products during
the reaction were also analyzed by 1H-NMR analysis. We initially
assigned the characteristic peaks of the possible intermediates in
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1H-NMR analysis. The 1H-NMR spectrum of MTHF, PDA, and PA
were shown in Figure 5. We did not analyze pentane because of
its lower boiling point. The characteristic peaks of each material
were marked in red outlines. The characteristic peaks of MTHF,
PDA, and PA were 3.7 ppm, 0.9 ppm and 4.9 ppm, respectively.
Then the solution with different reaction times were analysed by
1H-NMR spectrum and the results were listed in Figure 6. The
treatment procedure of reaction solution with different times were
listed in Supporting Information. The peaks of the MTHF and
pentane have not been detected in all reaction solutions, which
might be related to the removal during the treatments (the lower
boiling point). With the prolonging of the constant heating time,
the peaks belonged to PDA gradually decreased (4.9 ppm), while
the peaks belonged to PA gradually increased (0.9 ppm). Only the
peak belonged to PA could be observed at the constant heating
time for 30 min, which was consistent with the results of GC
analysis.
Figure 7. Reaction conditions: 1.2 mmol 2-MTHF, 0.8 mol% 10%Pd/C, 1.8 mol%
Sc(OTf)3, 2 MPa H2, 5 mL acetic acid, 30 min, 150 C. PA: pentanol acetate;
PDA: 1, 4-pentanediol acetate.
The study indicated that the product distribution had not
changed after cycling twice times which showed a certain stability.
The yield of PA and pentane decreased along with the yield of
PDA increased after cycling three times. It might be due to the
partial hydrolysis of the Lewis acid catalyst Sc(OTf)3 similar to our
[32a]
previous work.
We also found that the Pd / C structure was
partially altered. There was almost no difference between fresh
and used Pd/C catalysts according to the XRD analysis (Figure
8). However, the XPS analysis showed that the proportion of Pd0
and Pd2+ species in Pd/C catalyst were changed (Figure 9). The
increased proportion of Pd2+ species and the operating losses
during the catalyst recovery process might lead to a reduction of
the yields. And the hydrolysis reaction of metal triflate was easy
to occur because of the strong Lewis acidity of metal triflate.
Figure 6. 1H-NMR analysis of the reaction solution with different reaction times.
Catalyst recycling
The recycling of catalyst system was carried out subsequently
(Figure 7). The catalyst circulation experiment was carried out in
the following way: after the reaction, the Pd/C catalyst was
centrifuged and washed with acetic acid for two times. The
recovered Pd/C catalyst was put into the next cycle directly after
centrifugation. Add tridecane as an internal standard for
quantitative GC analysis of the products in the reaction solution.
Then, the solvent and the internal standard were removed by
vacuum evaporation. Then the residue containing recovered
Sc(OTf)3 was dissolved with acetic acid and transferred to the
autoclave for the next cycle.
Figure 8. The XRD spectra of fresh Pd/C catalyst and the used catalyst.
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Application
Scheme 4. The preparation of PDO and PO from MTHF.
Because of the difficult cyclization of PDA and PA, high yield of 1,
4-pentanediol (1,4-PDO) and n-pentanol (1-PO) could be
achieved from 2-MTHF by using PDA and PA as the
intermediates (Scheme 4). We first synthesized PDA and PA by
our developed reaction system. The yield of PDA and PA were
82% and 90%, respectively. Subsequently the saponification was
carried out. 1,4-PDO and 1-PO were obtained with the yield of 95%
and 93%, respectively. And the total yield of 1,4-PDO and 1-PO
obtained from 2-MTHF were 77.9% and 83.7%, respectively. The
detailed treatments were listed in the Support Information.
Figure 9. The XPS spectra of Pd3d for catalysts. (a) Fresh Pd/C catalyst. (b)
Pd/C catalyst after recycle four times.
Reaction mechanism
Conclusions
The selective hydrogenolysis of 2-MTHF to PDA and PA was
carried out over Pd/C and Sc(OTf)3 co-catalytic system. A yield of
91.8% PA (150 C, 3 MPa H2, 30 min) was achieved using Pd/C
and Sc(OTf)3 co-catalyst system and 82% yield of PDA was
achieved (150 C, 30 min) in the absence of Pd/C catalyst,
respectively. It was found that the cleavage of primary alcohol
ester bond was difficult to carry out than that of the secondary
alcohol ester bond and the main product was the PA. The yield of
PA and pentane decreased along with the yield of PDA increased
after cycled three times. Simultaneously, the total yield of 1,4-
PDO and 1-PO obtained from 2-MTHF by using PDA and PA as
the intermediates were 77.9% and 83.7%, respectively. It
provides a simple and efficient method for the synthesis of
pentanol and pentandiol from biomass substrates.
Scheme 3. Possible reaction pathways.
According to the previous document and our experiment results,
a possible pathway for hydrogenolysis of 2-MTHF was shown in
Scheme 3. 2-MTHF was first converted to PDA in the presence
of Sc(OTf)3 and acetic acid. The primary alcohol ester bond and
the secondary alcohol ester bond both existed in PDA. And the
secondary alcohol ester bond was easy to cleavage. Therefore,
only the product of 1-PA was obtained and no product of 2-PA
was detected. 1-PA was further converted to pentane under the
catalytic system. According to the GC analysis, we found that 2-
MTHF could be quickly converted to PDA. The literature has
reported that the saturated cyclic ether compounds could be
rapidly acetylated to acetate products with acetic anhydride and
[34]
Lewis acid catalysts.
Acetic acid was difficult to undergo
Experimental Section
acetylation because of its weak acylation activity. However, the
acetylation of 2-MTHF could be performed efficiently under mild
conditions in the presence of Sc(OTf)3. The primary alcohol ester
bond and the secondary alcohol ester bond both existed in PDA
and the cleavage rate of ester bond in different positions was
Materials and reagents
All the chemicals used in the experiments were commercially available.
Hf(OTf)4, Pd/C (10%) were purchase from Alfa Aesar (China) Chemicals
Co., Ltd. La(OTf)3, Ce(OTf)3, Sc(OTf)3, Al(OTf)3 Ni(OTf)2, Zn(OTf)2,
Cu(OTf)2 were purchased from Adamas Reagent Co., Ltd. Phenylbenzene,
2-methyltetrahydrofuran, acetic acid, dichloromethane were purchased
from Sinopharm Chemical Reagent Co., Ltd. Pure water was bought from
Hangzhou Wahaha. The preparation methods of the possible
intermediates and products and the NMR spectrum datas were listed in
Supporting Information.
1
discrepant. According to the GC analysis and H-NMR analysis,
only the product of 1-PA was obtained and no product of 2-PA
was detected. This might be due to the high reaction activity of
secondary alcohol ester bond under the catalytic system. The
literature also reported that the hydrogenolysis of secondary
alcohol ester bond could occur more easily than the
hydrogenolysis of primary alcohol ester bond.
[36]
So the main
product we obtained was 1-PA under the co-catalytic system.
Further prolonged the reaction time, primary alcohol ester bond
on 1-PA was hydrogenated to obtain pentane.
Catalyst characterization
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Catalysts were characterized by X-ray photoelectron spectroscopy (XPS),
X-ray power diffraction (XRD). X-ray photoelectron spectroscopy (XPS)
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data were obtained with
a Thermo Scientific Escalab 250-X-ray
photoelectron spectrometer which equipped a hemispherical electron
analyzer and an Al Kα X-ray source. All binding energies were referenced
to C 1s line at 284.7 eV. X-ray power diffraction (XRD) patterns were
obtained on X’ pert (PANalytical) diffractometer with Ni-filtered Cu-Kα
radiation, at 40 kV and 40 mA. 2θ range was 20°~80°.
Typical experiment and product analysis
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If not stated otherwise, the reaction was carried out in an autoclave
sponsored by Anhui Kemi Machinery Technology Co., Ltd. Usually, the
initial temperature was 30 C, then heating to the required reaction
temperature through intelligent temperature controller in 30 min. After that
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The conversion of 2-methyltetrahydrofuran was carried out in a 25 mL
Parr autoclave equipped with magnetic stirring, temperature probe and
heating controller. 1.2 mmol of substrate, 0.8 mol% of Pd/C catalyst, 4
mol% of metal triflate, and 10 mL of acetic acid were added to the
autoclave. Tighten the screws and replace the gas with hydrogen gas four
times. Then filling out the autoclave with a certain hydrogen pressure at
room temperature. Heating to the desired temperature under stirring in 500
rpm/min. After a certain period of heat preservation reaction, cool down,
release pressure, and transferred the reactant out from the reactor with 20
mL dichloromethane. A certain amount of phenyl benzene was added as
the internal standard to the solution. Then the solution was analyzed with
Gas Chromatography equipped with a DM-Wax capillary column (30 m ×
0.32 mm × 0.25 µm, SHIMADZU, GC-2014C. The conversion of 2-
MTHFand the yield of products were calculated according to the following
formula:
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n
MTHF : mol of 2-MTHF before reaction
n’MTHF : mol of 2-MTHF after reaction
Acknowledgements
This work was supported by the NSFC (21572212, 21402181,
and 21325208), MOST (2017YFA0303502), CAS (YZ201563),
FRFCU and PCSIRT. The authors thank Anhui Kemi Machinery
Technology Co., Ltd. for free equipment that benefited our ability
to conduct this study.
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Keywords: 2-methyltetrahydrofuran · pentanediol diacetate ·
pentanol acetate · hydrogenolysis · metal triflate
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Entry for the Table of Contents (Please choose one layout)
Kun Zhang, §[a, b] Xing-Long Li, §[a] Shi-
Yan Chen, [a, b] Hua-Jian Xu, [c] Jin Deng,
*[a] and Yao Fu *[a]
Page No. – Page No.
Selective hydrogenolysis of furfural-
derivative 2-methyltetrahydrofuran
into pentanediol acetate and pentanol
acetate over Pd/C and Sc(OTf)3 co-
catalytic system
Selective hydrogenolysis of 2-methyltetrahydrofuran to 1, 4-pentandiol and 1-
pentanol with pentanediol acetate and pentanol acetate as the intermediates over
Pd/C and Sc(OTf)3 co-catalytic system.
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