Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst

Article abstract of DOI:10.1039/c4ra09550b

Raney-type metals (Cu, Co and Ni) were employed to catalyze hydrogenation of 5-hydroxymethylfurfural. Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran was achieved with 96% and 88.5% yield respectively over Raney Ni, demonstrating high feasibility for industrialization. The excellent yields can be explained by the fact that Raney Ni facilitates the hydrogenation reaction but has limited deoxygenation ability at low temperature, while high temperature promotes the deoxygenation step. The reaction pathway was analyzed by time course experiments and HMF hydrogenation over model catalysts was performed. The reaction mechanism related to the respective catalytic sites was discussed and proposed, which has great implications in the design of efficient and non-noble metal catalysts.

Full text of DOI:10.1039/c4ra09550b

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Switchable synthesis of 2,5-dimethylfuran and 2,5-
dihydroxymethyltetrahydrofuran from 5-
Cite this: RSC Adv., 2014, 4, 60467
hydroxymethylfurfural over Raney Ni catalyst†
Xiao Kong, Yifeng Zhu, Hongyan Zheng, Fang Dong, Yulei Zhu*ac
ab
ab
c
ab
ac
and Yong-Wang Li
Raney-type metals (Cu, Co and Ni) were employed to catalyze hydrogenation of 5-hydroxymethylfurfural.
Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran was achieved with 96%
and 88.5% yield respectively over Raney Ni, demonstrating high feasibility for industrialization. The excellent
yields can be explained by the fact that Raney Ni facilitates the hydrogenation reaction but has limited
deoxygenation ability at low temperature, while high temperature promotes the deoxygenation step. The
reaction pathway was analyzed by time course experiments and HMF hydrogenation over model
catalysts was performed. The reaction mechanism related to the respective catalytic sites was discussed
and proposed, which has great implications in the design of efficient and non-noble metal catalysts.
Received 31st August 2014
Accepted 6th November 2014
DOI: 10.1039/c4ra09550b
www.rsc.org/advances
hydrogenolysis of C–O bonds need to be proceeded efficiently.
The catalysts with both C]O hydrogenation and deoxygenation
a
promising platform ability (e.g., bifunctional/bimetallic catalysts ) could be good
1
. Introduction
6,12
5-Hydroxymethylfurfural (HMF) is
chemical for future chemical and fuel supply, as it can be candidates. To produce DHMTHF, the furan ring and aldehyde
1,2
obtained from abundant bio-resources. The C]O, C–O and group of HMF need to be saturated without further hydro-
furan ring of HMF make it exible for transformations by genolysis. The catalysts with strong hydrogenation and limited
1
3
various methodologies (e.g., hydrogenation, oxidation, decar- deoxygenation ability could satisfy the needs. Considering the
4
5
bonylation, etherication ). Many useful products including different requirements, a careful development of the catalytic
,5-dimethylfuran (DMF),
2,5-dimethyltetrahydrofuran system is required in order to obtain the desired products
DMTHF), 2,5-dihydroxymethylfuran (DHMF), 1,2,6-hexanetriol selectively.
and 2,5-dihydroxymethyltetrahydrofuran (DHMTHF) could be
Relatively few reports were conducted on the synthesis of
produced by hydrogenation of HMF. Among them, DMF has DHMTHF, for which monometallic catalysts are usually applied
2
(
8
,10,13–16
been identied as a promising liquid fuel candidate with high (e.g., Ru, Ni, Pd) with high efficiency.
energy density (30 kJ cm ) and octane number (RON ¼ 119),
Nevertheless, few
works were further played to investigate DMF synthesis in these
and DHMTHF could be a bulk chemical in the manufacture of systems. Moreover, dehydroxylation reaction is hard to occur in
ꢀ3
6
7–10
ꢁ
10
polyesters and a precursor for 1,6-hexanediol synthesis.
For the potential application of both products, it would be of switchable synthesis impossible. They considered that the
great convenience for switchable synthesis of both chemicals protic H O leads to the formation DHMF and DHMTHF, while
Pd/C with [BMIm]Cl/H O system even at 200 C, making the
2
2
over the same reactor, because the “real-time” switching could the aprotic solvent tends to form DMF. For HMF hydrogenation
respond more rapidly to the market demand and enhance to DMF, CuRu/C catalyst was rstly employed and exhibited a
11
6
protability, as well as reduce the reactor downtime. It thus yield of ꢂ71%. Most following works were conducted over
1
7,18
19–21
22
23
would be of great importance for tunable synthesis over the noble metal catalysts (e.g. Ru,
same catalyst, especially over the same non-noble catalyst. To metal–acid bifunctional and non-noble Ni–W
produce DMF, the hydrogenation of aldehyde group and reported with 96% DMF yield, where W C played a role of acid
Pd,
Au, Pt ). Recently, a
C/AC catalyst was
2
2
12
site with high deoxygenation ability. Strong acidity of catalyst
would catalyze cleavage of C–O bonds of hydroxymethyl
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese groups24,25 and may not t for DHMTHF synthesis. Moreover,
Academy of Sciences, Taiyuan 030001, P. R. China. E-mail: zhuyulei@sxicc.ac.cn;
the development of efficient and non-noble catalytic system still
remains to be explored, from the aspect of large-scale
industrialization.
Fax: +86 351 7560668; Tel: +86 351 7117097
b
University of Chinese Academy of Sciences, Beijing 100039, P. R. China
c
Synfuels China Co. Ltd, Beijing, 101407, P. R. China
We herein employed the non-noble Raney Ni to catalyze
HMF hydrogenation and achieved tunable selectivity towards
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra09550b
1
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either DHMTHF (96.0%) or DMF (88.5%) over the same catalyst and heated to object temperature within 30 minutes. Aer the
Scheme 1). The catalyst saturated the C]O bonds and furan test, the reactor was quenched in ice-water, and then the liquid
(
ring without broke the C–O bonds at low temperature. Instead, and gas products were analyzed by a GC instrument with a FID
the high temperature compensated the limited hydrogenolysis detector.
ability of catalyst and accelerated the cleavage of C–O bonds. To
our knowledge, this report possesses at least the following
desirable features: (1) highly selective synthesis of DMF and
3
. Results and discussion
DHMTHF over the same commercial non-noble catalyst, 3.1 Catalyst screening
exhibiting high feasibility and protability for industry, (2) the
All the Raney metals have the metal (Cu, Co and Ni) loadings
selective synthesis only needs different reaction temperatures,
of great convenience for “real-time” switching, (3) the discus-
sion and understanding of reaction mechanism related to the
catalytic sites were proposed, which were important for the
rational design of efficient non-noble catalyst for HMF
hydrogenation.
higher than 93%. The fresh Raney Cu, Co and Ni catalysts were
also characterized by XRD measurement (Fig. 1). The XRD
patterns of the Raney Ni and Cu catalysts are consistent with the
typical crystalline Ni (PDF#04-0850) and Cu (PDF#04-0836)
respectively, indicating that both metals are in a complete
crystalline form. Unlike Raney Ni and Cu, Raney Co exhibited
the weak peaks which could be attributed to the characteristics
of crystalline Co (PDF#15-0806). The result illustrated that Co
has amorphous or/and microcrystalline construction and is
highly dispersed. No diffraction peaks of crystalline Al were
observed for all the cases.
2. Experimental section
2.1 Chemicals
5-Hydroxymethylfurfural (Shanghai De-Mo Pharmaceutical
Science and Technology Limited Company, 98%), Raney Ni, Co
and Cu catalysts (Dalian Tongyong Chemical Industry Co., Ltd.),
ZSM-5 (The Catalyst Plant of Nankai University) and 1,4-dioxane
The performance of HMF hydrogenation over these Raney
ꢁ
metals was compared at 180 C (Table 1, entries 1, 2 and 8).
Raney Cu exhibited a low HMF conversion of 25.1% and a DMF
(Sinopharm, AR) were purchased and used as received.
5-Methylfurfural, 2,5-dihydroxymethylfuran and 2,5-dihydroxy-
methyltetrahydrofuran were purchased from T.C.I. Corpora-
tion. The weight percentage of metal and aluminum in the
Raney metals was more than 93% and less than 7%,
respectively.
2.2 Analytical method
XRD patterns were recorded by a X-ray diffractometer (MiniFlex
II, Rigaku) with Cu ka radiation operating at 40 kV, with a rate
ꢁ
ꢀ1
of 4 C min
.
The samples aer reaction were analyzed on GC-920 equip-
ped with a DB-WAXETR capillary column and a FID detector.
The products were identied by GC-MS and comparison of
retention times of pure chemicals.
2.3 Catalytic tests
The tests were performed over a 100 ml tank reactor. For a
typical procedure, the reactor was fed with HMF (1.5 g), Raney
metals (0.5 g) and 1,4-dioxane (35 ml), then sealed and purged
by H (5 times). Aer that, the reactor was lled with 1.5 MPa H
2
2
Scheme 1 Switchable and efficient synthesis of DHMTHF and DMF
from HMF over Raney Ni.
Fig. 1 XRD patterns of fresh Raney Cu, Co and Ni catalysts.
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a
Table 1 Hydrogenation of HMF over Raney metal catalysts
Selectivity/%
ꢁ
Entry
Catalyst
T/ C
Conv./%
DMF
5-MF
MFA
DHMF
DHMTHF
2-Furan
Others
1
2
3
Raney Cu
Raney Co
Raney Ni
Raney Ni
Raney Ni
Raney Ni
Raney Ni
Raney Ni
Raney Ni
Raney Ni
180
180
100
100
120
140
160
180
200
220
25.1
94.3
100
100
100
100
100
100
100
100
42.0
78.5
1.4
1.9
6.0
10.2
46.3
88.5
79.8
69.3
34.5
1.8
0
0
0
0
0
0
0
4.5
2.7
2.0
0
5.1
8.3
8.6
0
0
0
27
0
18.1
13.8
6.1
0
3.0
2.7
11.4
5.9
0
0
0
0
0
0
0
4.6
8.4
2.2
2.1
5.4
7.0
6.4
8.5
17.2
30.7
67.4
96.0
65.4
60.7
32.6
3.0
b
4
5
6
7
8
9
0
0
0
0
3.0
0
1
0
0
0
a
2
Reaction conditions: HMF 1.5 g, catalyst 0.5 g, 1,4-dioxane 35 ml, 1.5 MPa H , and reaction time 15 h at the dened temperatures. 2-Furan means
b
OMBM and A. Others mainly include 2,5-DMTHF, 2-MF, 2-hexanol and C–C cracking products. 30 h was used to optimize the DHMTHF yield.
selectivity of 42.0%. The main byproduct for Raney Cu was hydrogenolysis of hydroxymethyl group was hard to occur. It
5-methylfurfural (5-MF), indicating the weak C]O hydrogena- should be noted that DHMF yield increased until HMF was fully
tion ability of the catalyst. 2,2-Methylenebis(5-methylfuran) converted, revealing that the saturation of furan ring was slow.
0
(
(
OMBM) and 5,5 -(oxybis(methylene))bis(2-methylfuran) (A) The unconverted HMF may inhibit the DHMF conversion to
26,27
2
2-furan, formed by the condensation of 5-MF and MFA
)
DHMTHF due to the fact that h (C,O)-type conguration
were also observed, as a result of the insufficient hydrogenation endows HMF with stronger adsorption than DHMF and the
reactivity of Cu catalyst. Raney Co catalyst exhibited a HMF competitive adsorption on the active site hinders conversion of
conversion of 94.3% and a DMF selectivity of 78.5%, which were
greatly higher than that of Raney Cu.
Raney Ni exhibited the HMF conversion of 100% and a
higher DMF selectivity of 88.5% than other catalysts, indicating
the superior reactivity of Ni catalyst. The Raney Ni catalyst was
then detailed studied at different reaction temperatures. It was
ꢁ
found that DHMTHF could be synthesized at 100 C with a yield
up to 96.0% and a DMF yield of 88.5% could be realized upon
ꢁ
increasing temperature to 180 C (Table 1, entries 4 and 8).
When further increasing temperature, C–C cracking products
and DMTHF were generated and DMF selectivity was decreased.
Herein, this is the rst report of highly selective synthesis of
DHMTHF and DMF over the same non-noble catalyst, tuned
simply by reaction temperatures.
3.2 Reaction mechanism
The reaction pathways were investigated by time courses at
ꢁ
ꢁ
1
00 C and 180 C to understand the tunable synthesis of DMF
and DHMTHF (Fig. 2). Typical GC charts of the products at both
ꢁ ꢁ
00 C and 180 C were shown in Fig. S1 and S2† respectively. In
1
both cases, HMF was totally converted within 10 h, indicating
the strong hydrogenation ability of Raney Ni. The absence of
5-hydroxymethyltetrahydrofurfural
and
5-methyltetrahy-
drofurfural for both temperatures could also indicate that
hydrogenation of aldehyde group is a fast reaction among the
three functional groups (C]O, C]C, C–O), which is accordance
14,15
ꢁ
with previous reports.
At 100 C, DHMF yield reached a
plateau at 5 h and then decreased, whereas DHMTHF yield
increased monotonically with increase of reaction time.
The results revealed a step-wise reaction sequence (Scheme 2,
HMF / DHMF / DHMTHF). No 5-methyltetrahydrofurfuryl
Fig. 2 HMF conversion and product yield versus reaction time at
ꢁ ꢁ
1
00 C (a) and 180 C (b). Reaction conditions: HMF 1.5 g, Raney Ni 0.5 g,
2
alcohol (MTHFA) or DMTHF was detected, indicating that 1,4-dioxane 35 ml, 1.5 MPa H .
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ꢁ
in Table 2 (see below). Different from reactivity at 100 C,
hydrogenolysis of hydroxymethyl group is greatly promoted at
ꢁ
1
80 C. The slow ring hydrogenation and rapid hydrogenolysis
of hydroxymethyl group contributed to the high DMF yield and
trace amount of DHMTHF. It is also interesting that it was hard
ꢁ
to stop the reaction in DHMF at 100 C, while the ring reserved
ꢁ
at 180 C. According to the previous report, the polar CH
2
OH
groups in the DHMF molecule have more affinity to the catalyst
surface and can assist in the adsorption of DHMF onto the Ni
28
metal surface. However, the methyl group of DMF forces the
aromatic ring further away from the catalytic surface, thereby
31
reducing the probability of a reaction. They also reported that
Scheme
2 Proposed reaction pathway for HMF hydrogenation/
hydrogenolysis (MTHFA was presented only when ZSM-5 was added). the protective methyl group contributes to a less selectivity
toward cracking in DMF hydrogenation than in 2-MF hydroge-
nation. Thus, DHMF is relatively more reactive than DMF and
DHMF. On the other hand, the C]C bonds on the ring are susceptible to hydrogenation. When DMF was formed, hydro-
conjugated and not easily to be hydrogenated. Thus, HMF genation of furan ring became slower. Briey, (1) the C]O bond
28
29
converted to DHMF rapidly and high yield of DHMTHF could be of HMF could occupy active sites and hinder DHMF adsorption,
ꢁ
obtained at long reaction time. At 180 C, trace amount of (2) the C]C bonds are conjugated and not easily to be hydro-
DHMTHF was detected, revealing that hydrogenation of furan genated, (3) the C–O could assist in DHMF adsorption and be
ring is not preferential. A series of C–O cleavage products (MFA, cleaved at high temperature. In all, the different adsorption
5
-MF, DMF, DMTHF) were obtained, indicating that hydro- geometry and strength of functional groups (C]O, C]C and C–
genolysis reaction could be promoted at high temperature for O) offer a degree of exibility to tunable DMF and DHMTHF
3
0
Raney Ni catalyst, which is in accordance with previous report.
synthesis.
ꢁ
DHMF yield exhibited a similar prole to that of 100 C, passing
To support the above mechanism, hydrogenation/hydro-
through the maximum at 2 h, indicating that it was a common genolysis of intermediates (DHMF and 5-MF) was performed
intermediate for both cases. 5-MF was also observed at the under the working conditions (Scheme S1–S3†). DHMF is a
initial stage, revealing that hydrogenolysis of alcohol group of common intermediate for both DMF and DHMTHF synthesis
HMF occurs in parallel to the hydrogenation of aldehyde group (Scheme S1 and S2†). The reaction temperature has great effects
at high temperature. Based on the above results and previous on the reaction pathway. At low temperature, hydrogenation of
29
reports, pathways of HMF hydrogenation at high temperature furan ring occurred and DHMTHF was formed. At high
are summarized (Scheme 2). In all, DHMF is the main inter- temperature, hydrogenolysis of the side –CH OH was promoted
2
mediate for both cases due to the fast hydrogenation of alde- and DMF was achieved. The hydrogenation of 5-MF (Scheme
ꢁ
hyde group. At 100 C, hydrogenolysis of –CH OH was slow and S3†) revealed that MFA can be obtained from 5-MF hydroge-
2
little hydrogenolysis products were detected. With increase of nation (the hydrogenation of C]O bonds) and DMF can be
ꢁ
temperature to 180 C, the hydrogenolysis of –CH
2
OH is greatly obtained from MFA hydrogenolysis (the hydrogenolysis of
promoted and 5 MF is observed. However, due to the weak CH
2
OH group).
deoxygenating ability of Raney Ni catalyst, DHMF is still the
Pathways of HMF hydrogenation are summarized (Scheme 2).
main intermediate. The behavior is dramatically different with For Raney Cu, Co catalysts, some condensation intermediates
Ni–W
C/AC catalyst which was designed with Lewis acid sites (OMBM, A) were detected due to the slow conversion of 5-MF
and strong deoxygenating ability.
2
12
and MFA. For Ni catalyst with superior ability, reaction pathways
At 100 C, DHMTHF was achieved with high yield and were highly depended on reaction temperatures. The tempera-
hydrogenolysis products were not detected. We ascribed the ture inuenced the occurrence of CH –OH hydrogenolysis and
ꢁ
2
high yield to the weak acidity of Raney Ni catalyst and proved it determined the product distributions. In our case, Raney Ni
a
Table 2 Hydrogenation of HMF over model catalysts
Yield/%
Entry
Catalyst
DMF
DMTHF
DHMTHF
MTHFA
DHMF
Others
1
2
3
4
Raney Ni
1.4
1.6
1.9
0.8
0
0.8
0
67.4
86.5
96.0
85.9
0
5.1
0
27
2.4
0
4.2
3.6
2.1
5.9
Raney Ni + ZSM-5
b
Raney Ni
b
Raney Ni + ZSM-5
2.9
4.5
0
a
ꢁ
Reaction conditions: Raney Ni: Raney Ni (0.5 g); Raney Ni + ZSM-5: Raney Ni (0.5 g) and ZSM-5 (0.1 g), HMF 1.5 g, 1,4-dioxane 35 ml, 100 C 1.5 MPa
b
H
2
, reaction time ¼ 15 h. Reaction time ¼ 30 h.
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catalyst plays a role in the hydrogenation step but has limited
deoxygenation ability at low temperature, while high tempera-
ture promotes the deoxygenation step. Therefore, a tunable
synthesis of products was achieved. The key to our success could
be attributed to the nature of Raney Ni and avoidance of addi-
tional acid sites, considering that a metal catalyst with high
12
acidity could catalyze hydrogenolysis reaction more effectively.
To obtain DMF, hydrogenolysis of hydroxymethyl group is
needed. It is generally concluded that metal–acid bifunctional
catalysts are good candidates in hydrogenolysis reaction, where
acid promotes dehydration of CH –OH and metal acts as active
2
32,33
phase to saturate/hydrogenate intermediates.
Indeed, W2C
acts as acid to catalyze deoxygenation reaction for DMF synthesis
12
has been reported with high efficiency. However, the acid sites
may not t for DHMTHF synthesis, in which C]O and C]C
bonds are hydrogenated without further hydrogenolysis. To
examine the roles of acid sites for DHMTHF synthesis, a solid
Fig. 4 XRD patterns of fresh and used Raney Ni catalyst.
acid (ZSM-5) was physically mixed with Raney Ni and employed 3.3 Catalyst stability
in HMF hydrogenation (Table 2). HMF was fully converted in all
Reusability of Raney Ni was also investigated under relatively
severe condition (180 C) (Fig. 3). Aer each test, the catalyst
cases. ZSM-5 could promote DHMTHF yield from 67.4 to 86.5%
when the intermediate DHMF was not totally converted (15 h,
Table 2, entry 1 and 2). However, MTHFA was formed, indicating
that hydrogenolysis of hydroxymethyl group was also promoted.
Besides, no obvious change was observed for the mechanical-
mixed catalyst aer extending the reaction time from 15 to 30 h.
It might indicate the deactivation of the catalyst. This result
might be also caused by the fact that acid could promote DHMF
conversion to DHMTHF, but have little effect on DHMTHF
conversion, considering that DHMTHF was more stable than
ꢁ
was washed with solvent (3 times) and recycled. Aer 5 runs, the
conversion of HMF only decreased to 92.1% compared to 100%
conversion of the fresh catalyst, demonstrating the good
hydrogenation ability of metallic Ni. Nevertheless, DMF selec-
tivity decreased from 88.5% to 74%. The deactivated Raney Ni
catalyst exhibited a similar catalytic performance with Raney Cu
and Co catalysts (presence of 2-furan and 5-MF), indicating a
loss of hydrogenation ability. No evident growth of Ni particles
was observed aer 5 runs (Fig. 4). The loss of DMF yield was
thus probably caused by the mass loss of catalyst during
washing process and the formation of high molecular weight
34
DHMF. The results clearly suggested that critical roles of acid
sites were to promote both hydrogenation of furan ring and
2
hydrogenolysis of CH –OH group. The results also suggested that
DMF could be formed via dehydration of DHMF, which is similar
byproducts which adsorbed on the metal sites, as conrmed in
12,18
previous reports.
Although the switchable synthesis of DMF
with 2-methylfuran formation from furfuryl alcohol hydro-
and DHMTHF was achieved with high yields and a systematical
reaction mechanism was proposed, the low reactivity and
stability would hinder the applications. Thus, future work will
be needed for exploring efficient and especially stable catalysts
for selective hydrogenation of HMF.
35
genolysis. Thus, when the target compound involves C–O
hydrogenolysis, control of the acidity is essential. The Raney Ni
system facilitated the hydrogenation reaction at low temperature
and the hydrogenolysis reaction at high temperature, achieving a
switchable synthesis of DHMTHF and DMF.
4. Conclusion
We have demonstrated a highly efficient and selective non-
noble catalyst that can be used for hydrogenation of HMF.
Notably, the reaction temperature offers a degree of exibility to
the process such that either DMF or DHMTHF could be
obtained as the major product, due to the sufficient hydroge-
nation ability and weak hydrogenolysis ability of the Raney Ni
catalyst. The detailed reaction mechanism and catalyst reus-
ability were also studied. The results here demonstrated the
potential of non-noble metal catalysts for HMF hydrogenation.
Acknowledgements
This work was supported by the Major State Basic Research
Development Program of China (973 Program, no.
2012CB215305).
Fig. 3 Recycle experiments of the Raney Ni catalyst. Conditions: HMF
ꢁ
1
.5 g, Raney Ni 0.5 g, 1,4-dioxane 35 ml, 1.5 MPa H
2
, 180 C, 15 h.
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1
7 Y. Zu, P. Yang, J. Wang, X. Liu, J. Ren, G. Lu and Y. Wang,
Appl. Catal., B, 2014, 146, 244–248.
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