In situ hydrogenation of furfural with additives over a RANEY Ni catalyst

Article abstract of DOI:10.1039/c5ra12844g

The hydrogenation of furfural was studied over a RANEY Ni catalyst (RN) under a N₂ atmosphere in water. Methanol, as a hydrogen donor, was used for hydrogenation via a reforming reaction. Additives, including acetone, acetic acid and phenol, were deliberately added in the in situ hydrogenation process of furfural to investigate the effect and interaction between two kinds of compounds. The results showed that the conversion of furfural decreased to some extent and the product distribution changed a lot because of second additives. Furfuryl alcohol (FA) was detected in the products in the presence of additives and was not detected when furfural was a single reactant. The selectivity of FA reached its highest degree of 19.01% with the addition of acetic acid. The addition of acetone promoted the decarboxylation reaction of furfural, and the selectivity of tetrahydrofurfuryl alcohol increased from 24.53% to 38.07%. The addition of phenol enhanced the rearrangement reaction of furfural and the selectivity of the five-membered ring increased from 47.99% to 88.76%. Compared with the in situ hydrogenation of the single reactant, the conversion of acetone increased and the conversion of acetic acid and phenol decreased in the presence of furfural. The reaction pathway of the hydrogenation of furfural is also discussed in this paper.

Full text of DOI:10.1039/c5ra12844g

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PAPER
In situ hydrogenation of furfural with additives over
a RANEY® Ni catalyst
Cite this: RSC Adv., 2015, 5, 91190
Ying Xu,a Songbai Qiu,a Jinxing Long,a Chenguang Wang,a Jiamin Chang,ab Jin Tan,a
a
a
a
a
*
Qiying Liu, Longlong Ma, Tiejun Wang and Qi Zhang
The hydrogenation of furfural was studied over a RANEY® Ni catalyst (RN) under a N2 atmosphere in water.
Methanol, as a hydrogen donor, was used for hydrogenation via a reforming reaction. Additives, including
acetone, acetic acid and phenol, were deliberately added in the in situ hydrogenation process of furfural to
investigate the effect and interaction between two kinds of compounds. The results showed that the
conversion of furfural decreased to some extent and the product distribution changed a lot because of
second additives. Furfuryl alcohol (FA) was detected in the products in the presence of additives and was
not detected when furfural was a single reactant. The selectivity of FA reached its highest degree of
19.01% with the addition of acetic acid. The addition of acetone promoted the decarboxylation reaction
of furfural, and the selectivity of tetrahydrofurfuryl alcohol increased from 24.53% to 38.07%. The
addition of phenol enhanced the rearrangement reaction of furfural and the selectivity of the five-
membered ring increased from 47.99% to 88.76%. Compared with the in situ hydrogenation of the single
reactant, the conversion of acetone increased and the conversion of acetic acid and phenol decreased
in the presence of furfural. The reaction pathway of the hydrogenation of furfural is also discussed in this
paper.
Received 2nd July 2015
Accepted 15th October 2015
DOI: 10.1039/c5ra12844g
www.rsc.org/advances
cyclopentanol over Ni/CNT catalysts. Over 30 wt% Ni/CNTs, the
conversion of furfural was up to 96.5% with a yield of 83.6%
toward cyclopentanol. Guo15 has developed a new catalytic
Introduction
Fast pyrolysis is a kind of promising technology for the
conversion of biomass into liquid fuels due to its low capital
and operating cost advantages.1,2 But the properties of bio-oil,
the liquid product of biomass fast pyrolysis, result in multiple
signicant problems during the utilization and storage of bio-
oil because it is chemically and thermally unstable.3 There-
fore, the transformations of bio-oil into useful chemicals or
stable compounds have attracted much attention.4 Hydrotreat-
ment in the aqueous phase is one of the potential methods to
upgrade bio-oil.5,6 Acetone,7 acetic acid,6,8 furfural,9,10 and
phenol11 and its derivatives have been chosen as model
compounds to investigate the activity of different catalysts and
the mechanism for upgrading bio-oil. But the mechanism of the
model compounds cannot totally explain the pathway for
compound conversion, and the catalysts are less active in raw
bio-oil. Investigations on the reasons for this are still scarce.
Furfural, as a key platform molecule in biomass conversion,
is one of the main components in bio-oil.12,13 The products of
furfural from the hydrogenation process are complex in nature.
Zhou14 investigated the transformation of furfural to
system of CuZnAl for the selective conversion of furfural to
cyclopentanone. CuZnAl-500-0.5 was recycled ve times and
maintained a good activity and stability. Villaverde16 studied the
liquid-phase transfer hydrogenation of furfural on Cu-based
catalysts. With Cu–Mg–Al, aer 6 h at 150 ꢀC, furfural could
be converted to furfuryl alcohol completely. Yang9 pointed
out that in the conversion of furfural into cyclopentanone over
Ni–Cu bimetallic catalysts, furfuryl alcohol, 4-hydroxy-2-
cyclopentenone and 2-cyclopentenone were veried to be
three key intermediates. The rearrangement of the furan ring
was closely related to the attack of a H2O molecule on the 5-
position of furfuryl alcohol. In view of high-pressure H2 being
used in furfural hydrogenolysis, Paraskevi used alcohols as
hydrogen donors in methyl furan production through catalytic
transfer hydrogenation of furfural in the liquid phase.17 From
the reports on furfural hydrogenation, most of the research
performed catalytic hydrogenation to yield different products,
but few studies have focussed on the effect of additives on the
product distribution during the upgrading process of raw bio-
oil.18–21
The present work highlights the in situ hydrogenation of
furfural over RN. In addition, the effect of different kinds of
compounds on the product distribution in bio-oil is also pre-
sented. This study is motivated in part by our recent work.22 The
aKey Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences, CAS, Guangzhou 510640, China. E-mail: zhangqi@
ms.giec.ac.cn
bFaculty of Environmental Science and Engineering & State Key Lab of International
Combustion Engine, Tianjin University, Tianjin 300072, China
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additives included acetone, acetic acid and phenol. The hydrogenation process and the yield of hydrogen generated
comparison of the hydrogenation vs. in situ hydrogenation, and upon the reforming reaction of methanol in the in situ hydro-
the hydrogenation pathway of furfural are also discussed in this genation process depending on the initial pressure of N2,22 it is
paper.
therefore necessary to investigate the inuence of the N2 initial
pressure on the conversion of furfural and the product distri-
bution in the in situ hydrogenation process. Pressure-tuning
Results and discussion
ꢀ
studies were conducted over RN at 180 C for a duration of 4
Effect of the reaction conditions on the conversion of furfural
and product distribution
h. As expected, furfural conversion increased with N2 pressure
and the product distribution changed a lot (Fig. 2).
Furfural conversion increased from 51.04 to 97.79% when
the N2 initial pressure increased from 0.1 MPa to 1 MPa. With
the initial pressure increasing from 1 MPa to 3 MPa, the
conversion of furfural changed little. The product distribution
proles indicated that the THFA and CPO selectivity decreased
with increasing N2 initial pressure. In contrast, the CPL selec-
tivity remained low when the initial pressure was below 1 MPa
and increased signicantly aer the pressure increased to 2
MPa. The formation of THF commenced at the initial pressure
of 1 MPa and the selectivity increased signicantly with
increasing initial pressure. The obvious changes might be
ascribed to the APR of methanol. From our previous research,22
the conversion of methanol and the selectivity of H2 were
affected by the initial pressure. With the initial pressure
increasing, the conversion of methanol and the yield of H2
increased. Meanwhile, the solubility of H2 in the solvent also
increased, which could have promoted the hydrogenation
reaction that happened.
Effect of temperature. The temperature is one of the key
factors for hydrogenation. As inferred from Fig. 1, furfural could
be converted without H2 over RN in water and methanol, which
means that the hydrogenation of furfural could be coupled with
aqueous-phase reforming (APR) of methanol, as for acetone and
phenol.22 The furfural conversion signicantly increased with
increasing reaction temperature and the product distribution
also displayed a noticeable temperature dependence.
When the reaction temperature was 160 ꢀC, the furfural
conversion degree was only 53.11%. But aer the temperature
ꢀ
reached 180 C, the conversion of furfural increased to above
90%. With the temperature increasing, the selectivity of tetra-
hydrofurfuryl alcohol (THFA) and cyclopentanone (CPO)
decreased gradually. Correspondingly, cyclopentanol (CPL) and
tetrahydrofuran (THF) formation commenced at around 180 ꢀC
and the selectivities increased rapidly to 16.96% and 59.03% at
220 ꢀC. More aldehyde groups were cut from furfural molecules
when the reaction temperature increased. The falling off of the
side chain accounted for the decreasing selectivity of CPO and
CPL. On the basis of Fig. 1, it was concluded that the in situ
hydrogenation of furfural was favored at 180 ꢀC, leading to high
furfural conversion and less carbon loss. In summary, a low
temperature was favored for the hydrogenation of C]C and
C]O, leading to the higher THFA and 5-membered carbon ring
compound selectivity, while high temperatures were favored for
the cleavage of the aldehyde group from the furan ring, result-
ing in the higher selectivity of THF.
The coupling of APR of methanol and in situ hydrogenation of
furfural
In view of hydrogen being produced from the APR of methanol,
the coupling of APR of methanol and in situ hydrogenation of
furfural was investigated. The conversion of methanol and the
product distribution under the optimum conditions with and
without furfural are shown in Table 1. From Table 1, the
hydrogenation of the substrate could promote the APR of
methanol, and the conversion of methanol increased from
18.18% to 25.38%. In the gas product from furfural in situ
Effects of the N2 initial pressure. In view of hydrogen being
produced upon the reforming reaction of methanol in the in situ
Fig. 1 Conversion of furfural and product distribution at different Fig. 2 Conversion of furfural and product distribution at different
temperatures. Reaction conditions: 0.5 g of RN, 30 mL of water, 0.2 initial pressures of N2. Reaction conditions: 0.5 g of RN, 30 mL of
mol of methanol, 0.04 mol of furfural, N2 pressure of 1 MPa, and batch water, 0.2 mol of methanol, 0.04 mol of furfural, temperature of
reaction time of 4 h.
180 ꢀC, and batch reaction time of 4 h.
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step hydrogenation–esterication of furfural and acetic acid.23
Because of the abundance of methanol as the solvent and
hydrogen donor at the beginning of the reaction, the esteri-
cation reaction between acetic acid and methanol was the
dominant reaction. As second additives, acetic acid and phenol
showed conversions of 40.84% and 36.50%, while, as sole
reactants, their conversions were 52.35% and 46.01%. The
conversions of acetic acid and phenol were suppressed in the
presence of furfural in the mixed feed.24 There was no reaction
happening between the second additives and furfural in the in
situ hydrogenation system.
Table 1 Conversion of methanol APR and product distribution with
and without furfural over RANEY® Ni catalystc
Content (%)
C (%)
H2
CnHm
CO2
CO
N2
18.18a
25.38b
42.05
2.90
5.81
9.44
21.78
29.40
—
0.11
30.36
58.15
a
b
APR of methanol without furfural. APR of methanol with furfural.
Reaction conditions: 0.5 g of RN, 30 mL of water, 0.2 mol of
c
methanol, 0.04 mol of furfural, temperature of 180 ꢀC, N2 pressure of
1 MPa, and batch reaction time of 4 h.
Conversion of furfural and product distribution with second
additive
hydrogenation under the optimum conditions, 2.90% H2 was
le over.
As the single reactant, furfural displayed a conversion as high as
97.79%, while in the mixed feed systems, all of the furfural
conversions decreased to some extent (Table 3).
Hydrogenation of a mixed feed of furfural and a second
additive
It is interesting to note that, with acetone, acetic acid and
phenol addition to furfural, the product distribution changed
a lot. FA, which could not be detected in the product from
furfural as single reactant, was in the products of furfural in
presence of second addition. The selectivity of FA reached the
highest point (19.01%) with acetic acid as the coreactant. The
CPL selectivity increased to some extent in the presence of
acetone and acetic acid. Meanwhile, the rearrangement of the
ꢀ
Experiments were performed over RN at 180 C and 1 MPa to
investigate the in situ hydrogenation of a mixed feed of furfural
and acetone, furfural and acetic acid, and furfural and phenol.
For comparison, the reactions with furfural, acetone, acetic acid
and phenol as the single reactants were conducted under the
same identical reaction conditions.
Conversion of the second additives and product distribution
The conversion of the second additives and the product distri-
bution were different from those obtained with a single reactant
(Table 2). As a single reactant, acetone showed a conversion of
42.50%, while the conversion degree increased to 72.68% upon
hydrogenation with furfural. Isopropanol was the main product
whose selectivity was above 96% in both of the in situ hydro-
genation processes. The improvement of the conversion of
acetone might be attributed to the addition of furfural. In our
previous work,22 acetone was not found and isopropanol was
detected in the upgraded bio-oil upon in situ hydrogenation.
Acetic acid, as a single reactant or as a second additive to
furfural, was reacted with methanol via esterication, and was
then hydrogenated to ethanol over RN. The ester product was
only methyl acetate which was different from the results of one-
Table 3 Conversion of furfural and product distribution with second
additivesa
Selectivity (%)
Additive
Con. (%)
—
Acetone
Acetic acid 86.94
Phenol
97.79
87.63
8.98
19.66
16.73
79.75
39.01
14.96
21.19
9.21
24.53 27.48
38.07 18.88
27.29 15.78
—
8.43
19.01
5.41
94.40
0
5.63
a
Reaction conditions: 0.5 g of RNs, 0.04 mol of furfural, 0.02 mol of
second additive, 30 mL of water, 0.2 mol of methanol, temperature of
180 ꢀC, N2 pressure of 1 MPa, and batch reaction time of 4 h.
Table 2 Conversion of the second additives and product distribution with and without furfuralc
Selectivity (%)
Reactant
Con. (%)
Acetonea
Acetoneb
Acetic acida
Acetic acidb
Phenola
42.50
72.68
52.35
40.84
46.01
36.50
98.23
96.75
—
—
—
1.77
3.25
—
—
—
—
—
—
—
46.67
82.65
—
—
—
—
53.33
17.35
—
—
83.01
89.11
—
—
—
16.99
10.89
—
Phenolb
—
—
—
—
a
c
Single reactant. b As a second additive added to furfural. Reaction conditions: 0.5 g of RNs, 30 mL of water, 0.2 mol of methanol, 0.04 mol of
furfural, 0.02 mol of second additive, temperature of 180 ꢀC, N2 pressure of 1 MPa, and batch reaction time of 4 h.
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ve-membered carbon ring of furfural was signicantly through ltration and washed with methanol. 70–80% catalyst
improved in the presence of phenol. The total selectivity of CPO could be recycled for the next use, but the activity of the catalyst
and CPL increased from 47.99% to 88.96%, and the selectivity of is a little lower than the fresh one.
CPL increased from 8.98% to 79.75%. The THFA selectivity
decreased from 27.48% to 18.88% and 15.78% in the presence
of acetone and acetic acid, and could barely be detected when
the coreactant was phenol.
Reaction pathway of the in situ hydrogenation of furfural
For a better understanding of the reaction pathway, the in situ
hydrogenation of FA and THFA were conducted under identical
reaction conditions (Table 4).
Comparison of furfural in situ hydrogenation with
hydrogenation in a hydrogen atmosphere
As shown in Table 4, when FA was the reactant, CPO, CPL
and THFA were detected in the products but not THF. When
THFA was chosen as the reactant, there was no reaction
happening over THFA. This suggested that CPO, CPL and THFA
were all formed from FA. When the ve-membered carbon ring
was formed from furfural and FA, a rearrangement reaction was
inevitable during the in situ hydrogenation process. That means
that FA was one of the key intermediates to form ve-membered
carbon ring products.10 And also, once the hydrogenation of
C]O happened, leading to C–O, furan was not easy to form,
which showed that the formation of FA and furan were parallel
reactions. In summary, in the in situ hydrogenation system,
there were several parallel and cascade reactions, including
decarboxylation, hydrogenation of C]O/C]C bonds and ring
opening/closing (Fig. 4).
The comparison between the in situ hydrogenation and hydro-
genation in a H2 atmosphere was conducted under identical
reaction conditions (Fig. 3).
In the in situ hydrogenation, the conversion of furfural was
97.79%, while in water in a H2 atmosphere, a nearly total
conversion (98.51%) was also observed. In water and methanol,
the products of the in situ hydrogenation of furfural included
CPO, CPL, THFA and THF and there was no THF detected in the
H2 atmosphere. Meanwhile, the selectivities of THFA and CPO
in the hydrogenation were higher than those in the in situ
hydrogenation. The selectivity of THFA decreased from 27.48%
to 10.26%. The total selectivity of CPL and CPO in the hydro-
genation increased from 47.99% to 89.74% compared with in
the in situ hydrogenation, which means that in the absence of
methanol, the main reaction pathway is the rearrangement of
furfural and furfuryl alcohol to cyclopentanone in water as the
solvent under a H2 atmosphere.9 The effect on the rearrange-
ment of the furan ring in the presence of methanol needs
further investigation.
Experimental
Materials
Methanol ($99.5%, analytical reagent), furfural ($99.5%,
analytical reagent) and furfuryl alcohol ($99.5%, analytical
reagent) were purchased from Beijing Hengzhang Chemical Co
Ltd. Acetone ($99.5%, analytical reagent), acetic acid ($99.5%,
analytical reagent), phenol ($99.5%, analytical reagent) and
tetrahydrofurfuryl alcohol ($99.5%, analytical reagent) were
purchased from Tianjin Fuchen Chemical Reagents Factory.
In situ hydrogenation of furfural with a recycled RANEY® Ni
catalyst
The comparison between the in situ hydrogenation and hydro-
genation in a H2 atmosphere was conducted under identical
reaction conditions. The RANEY® Ni catalyst can be recycled
Preparation of catalysts22
The Ni–Al alloy powder, a commercial product supplied by
Dalian Toyounger Chemical Co. Ltd, was slowly added to a 20%
aqueous solution of NaOH at 50(ꢁ2) ꢀC. The solution was
precipitated aer 1.5 h of magnetic stirring at a temperature of
Table 4 In situ hydrogenation of FA and THFAa
Selectivity (%)
Reactant
Con. (%)
64.18
27.07
25.57
47.36
Fig. 3 Conversion of furfural and product distribution in the in situ
hydrogenation and hydrogenation. In situ hydrogenation: 0.5 g of RN,
0.04 mol of furfural, 30 mL of water, 0.2 mol of methanol, temperature
of 180 ꢀC, N2 pressure of 1 MPa, and batch reaction time of 4 h.
Hydrogenation: 0.5 g of RN, 0.04 mol of furfural, 30 mL of water,
temperature of 180 ꢀC, H2 pressure of 3 MPa, and batch reaction time
of 4 h.
0
—
—
—
a
0.5 g of RN, 0.04 mol of FA, 30 mL of water, 0.2 mol of methanol,
temperature of 180 ꢀC, N2 pressure of 1 MPa, and batch reaction time
of 4 h.
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Acetone, acetic acid and phenol had a signicant effect on the
product distribution from furfural. In the presence of additives,
FA was found in the products and FA was not detected when
furfural was a single reactant. In the presence of phenol, the
main reaction pathway was the rearrangement of furfural to
CPL & CPO and the selectivity of ve-membered carbon rings
was 88.96%. (3) In the in situ hydrogenation of furfural process,
there were several parallel and cascade reactions, including
decarboxylation, hydrogenation of C]O/C]C bonds and ring
opening/closing. (4) In view of the high content of oxygen and
water, converting raw bio-oil into stable oxygenated fuel might
be one of the potential methods to replace petroleum in the
production of fuels for the transportation sector.
Fig. 4 The reaction pathway during in situ hydrogenation of furfural.
50 ꢀC. Actually, Al is oxidized and removed from the alloy; then
almost pure Ni is obtained. The solid phase was washed with
distilled water until the pH reached 8–9 and then with ethanol 6
times. The prepared RN was stored in ethanol.
Acknowledgements
This project was supported by the Science and Technology Plan
Key Project in Guangdong Province of China (Project No.
2014A010106019), Chinese Academy of Sciences (Project No.
KGZD-EW-304-3), Science and Technology Plan Key Project in
Dongguan City of China (Project No. 201208101005) and
Foundation of Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences (Project No. y307r91001).
In situ hydrogenation process
In situ hydrogenation of furfural was performed in a 100 mL
stainless autoclave in the presence of 0.5 g of RN. The experi-
mental apparatus is similar to the one previously reported.22 In
a typical experiment, water (30 mL), methanol (0.2 mol) and
furfural (0.04 mol) were loaded into the reactor, which was sealed
and purged 3 times with N2 to exclude air. An automatic
controller was used to control the temperature and the revolution
of the stirrer. The pressure was raised to 0.1–3 MPa and the
reaction temperature was set at 160–220 ꢀC for 4 h. Aer reaction,
the autoclave was cooled down to room temperature and then the
liquid products were sampled for GC analysis. The second
additive (0.02 mol), including acetone, acetic acid and phenol,
was added in the in situ hydrogenation process separately.
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The liquid phase samples were analyzed using a Shimadzu
model GC-2014 using a HP-INNOWAX column (30 m ꢂ 0.25
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ꢀ
ꢁ
moles of reactants
Conversion ¼ 1 ꢃ
moles of reactants loaded initially
ꢂ 100%
(1)
ꢀ
ꢁ
moles of product
P
Selectivity of product ¼
ꢂ 100% (2)
moles of product
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