The role of Ru and RuO2 in the catalytic transfer hydrogenation of 5-hydroxymethylfurfural for the production of 2,5-dimethylfuran

Article abstract of DOI:10.1002/cctc.201300945

We have previously shown that 2,5-dimethylfuran (DMF) can be produced selectively from 5-hydroxymethylfurfural in up to 80 % yield via catalytic transfer hydrogenation with 2-propanol as a hydrogen donor and Ru/C as a catalyst. Herein, we investigate the active phase of the Ru/C catalyst by using extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and high-resolution TEM analyses. The results reveal that RuO₂ is the dominant phase in the fresh (active) catalyst and is reduced to metallic Ru during the reaction with the hydrogen produced insitu from 2-propanol. The deactivation of the catalyst is correlated with the reduction of the surface of RuO₂. Reactivity studies of individual phases (bulk RuO₂ and reduced Ru/C catalysts) indicate that RuO₂ mainly catalyzes the Meerwein-Ponndorf-Verley reaction of 5-hydroxymethylfurfural that produces 2,5-bis(hydroxymethyl)furan and the etherification of 2,5-bis(hydroxymethyl) furan or other intermediates with 2-propanol and that the reduced Ru/C catalyst has moderate hydrogenolysis activity for the production of DMF (30 % selectivity) and other intermediates (20 %). In contrast, a physical mixture of the two phases increases the DMF selectivity up to 70 %, which suggests that both metallic Ru and RuO₂ are active phases for the selective production of DMF. The oxidation of the reduced Ru/C catalyst at different temperatures and the insitu hydrogen titration of the oxidized Ru/C catalysts were performed to quantify the bifunctional role of Ru and RuO₂ phases. The mild oxidation treatment of the Ru/C catalyst at 403K could activate the catalyst for the selective production of DMF in up to 72 % yield by generating a partially oxidized Ru catalyst. Double trouble: A selective hydrodeoxygenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran is achieved with a partially oxidized Ru/C as a catalyst and 2-propanol as a hydrogen donor. The oxidized Ru/C catalyst demonstrates bifunctional behavior, in which Ru catalyzes the dehydrogenation of 2-propanol and the hydrogenation-hydrogenolysis of 5-hydroxymethylfurfural and RuO₂ promotes dimethylfuran production via hydrogenolysis.

Full text of DOI:10.1002/cctc.201300945

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DOI: 10.1002/cctc.201300945
The Role of Ru and RuO in the Catalytic Transfer
2
Hydrogenation of 5-Hydroxymethylfurfural for the
Production of 2,5-Dimethylfuran
[
a]
[a]
[b]
[a]
[a]
Jungho Jae, Weiqing Zheng, Ayman M. Karim, Wei Guo, Raul F. Lobo,* and
[a]
Dionisios G. Vlachos*
We have previously shown that 2,5-dimethylfuran (DMF) can
be produced selectively from 5-hydroxymethylfurfural in up to
2,5-bis(hydroxymethyl)furan or other intermediates with
2-propanol and that the reduced Ru/C catalyst has moderate
hydrogenolysis activity for the production of DMF (30% selec-
tivity) and other intermediates (20%). In contrast, a physical
mixture of the two phases increases the DMF selectivity up to
70%, which suggests that both metallic Ru and RuO2 are
active phases for the selective production of DMF. The oxida-
tion of the reduced Ru/C catalyst at different temperatures and
the in situ hydrogen titration of the oxidized Ru/C catalysts
were performed to quantify the bifunctional role of Ru and
80% yield via catalytic transfer hydrogenation with 2-propanol
as a hydrogen donor and Ru/C as a catalyst. Herein, we investi-
gate the active phase of the Ru/C catalyst by using extended
X-ray absorption fine structure, X-ray photoelectron spectros-
copy, and high-resolution TEM analyses. The results reveal that
RuO is the dominant phase in the fresh (active) catalyst and is
2
reduced to metallic Ru during the reaction with the hydrogen
produced in situ from 2-propanol. The deactivation of the cata-
lyst is correlated with the reduction of the surface of RuO . Re-
RuO phases. The mild oxidation treatment of the Ru/C catalyst
2
2
activity studies of individual phases (bulk RuO and reduced
at 403 K could activate the catalyst for the selective production
of DMF in up to 72% yield by generating a partially oxidized
Ru catalyst.
2
Ru/C catalysts) indicate that RuO mainly catalyzes the Meer-
2
wein–Ponndorf–Verley reaction of 5-hydroxymethylfurfural that
produces 2,5-bis(hydroxymethyl)furan and the etherification of
Introduction
Lignocellulosic biomass has been investigated worldwide as
metallic catalysts (CuÀRu and NiÀFe) supported on carbon and
[
1]
[3]
a renewable liquid fuel feedstock. The hydrogenolysis of bio-
mass-derived furan compounds, such as furfural and 5-hydrox-
ymethylfurfural (HMF), to platform molecules, such as 2-meth-
ylfuran (MF) and 2,5-dimethylfuran (DMF), is an essential step
in the catalytic conversion of biomass to chemicals and trans-
silica or reducible oxides, such as CoO . The general conclu-
x
sion is that bimetallic (CuRu/C and Ni–Fe/SiO ) and bifunctional
2
(Ru/Co O ) catalysts are more effective than monometallic cata-
3
4
lysts for hydrogenolysis. For example, CuRu/C and Ru/CO O
3
4
give DMF yields of 71 and 93%, respectively, under mild reac-
tion conditions (temperature of 403-493 K and hydrogen pres-
sure of 6–7 bar (1 bar=100 kPa). It has been proposed that
the role of bimetallic catalysts (NiÀFe) and reducible support
[
2]
portation fuels. Several groups have studied the hydrogena-
tion of furfurals to furans with reduced side groups (MF and
DMF) by using various singe metals (Cu, Pd, Ni, and Ru) and bi-
[
3d,f]
(
CoO ) is to promote the hydrogenolysis of the CÀO bond.
x
In contrast, we recently reported the selective production of
DMF from HMF in 80% yield via catalytic transfer hydrogena-
tion (CTH) with alcohols as hydrogen donors and monometallic
+
++
++
[a] Dr. J. Jae,
Dr. W. Zheng, Dr. W. Guo, Prof. R. F. Lobo, Prof. D. G. Vlachos
Catalysis Center for Energy Innovation
Department of Chemical and Biomolecular Engineering
University of Delaware
Newark, DE 19716 (USA)
Fax: (+1)302-831-1048
[4]
Ru/C as catalysts. The process is extendible to furfural to pro-
[5]
duce MF in high yields. During CTH, the hydrogen required
for the hydrogenation of HMF or furfural is produced in situ
from the dehydrogenation of alcohols, such as 2-propanol. The
required hydrogenation for the production of DMF or MF (hy-
E-mail: lobo@udel.edu
vlachos@udel.edu
[b] Dr. A. M. Karim
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
PO Box 999, Richland WA 99352 (USA)
drogenation of C=O to CH ÀOH) and the hydrogenolysis of
2
CH ÀOH to CH are solely catalyzed by the Ru/C catalyst. The
2
3
reaction network of the CTH of HMF to DMF is shown in
Scheme 1. Reaction evolution studies show that HMF is mainly
hydrogenated to 2,5-bis(hydroxymethyl)furan (BHMF), which
undergoes a sequential hydrogenolysis of the alcohol groups
to produce DMF with the hydrogen produced from the dehy-
drogenation of 2-propanol. The etherification of HMF or other
+
[
] Current address:
Korea Institute of Science and Technology
Seoul 136-791 (Republic of Korea)
+
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300945.
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Scheme 1. Reaction network of the hydrogenation of HMF to DMF with 2-propanol and Ru/C. Compound 1: 5-[(1-methylethoxy)methyl] furfural, compound
2
: 5-hydroxymethyl-2-[(1-methylethoxy)methyl] furan, compound 3: 2-methyl-5-[(1-methylethoxy)methyl] furan, compound 4: 2,5-[bis(1-methylethoxy)methyl]
[
4]
furan. Adapted from Jae et al.
intermediates, such as BHMF, with 2-propanol is also observed Results and Discussion
to produce ethers such as 2-methyl-5-[(1-methylethoxy)methyl]
Characterization of activated carbon-supported Ru catalyst
furan (3). However, these ethers are converted slowly into DMF
through de-etherification.
[4]
In a previous report, we described the preparation of DMF
from HMF in approximately 80% yield. The commercial Ru/C
catalyst was not activated before the reaction, because the cat-
alyst can be reduced in situ under our reaction temperature of
463 K. The independent TPR of the catalyst indicated that the
Ru/C catalyst was reduced at 403 K. Although the use of the
fresh Ru/C catalyst gave high DMF yield, the catalyst was deac-
tivated significantly after one reaction cycle. For instance, with
the use of the spent catalyst after the reaction, HMF conver-
sion and DMF selectivity decreased to 47 and 13%, respective-
ly. The inductively coupled plasma optical emission spectrome-
try analysis of the product solution and CO chemisorption of
spent catalysts suggested that Ru leaching is not the cause of
deactivation. It is thus hypothesized that the change in the oxi-
dation state of Ru is a main source of catalyst deactivation.
To identify the active phases of the commercial Ru/C catalyst
for this reaction, a series of pre- and postcharacterization stud-
ies have been performed. The TEM images of fresh and spent
This approach is promising for upgrading HMF with an inex-
pensive medium. However, we lack understanding of the struc-
ture and composition of the active sites for the CTH of HMF. In
addition, catalyst deactivation and regeneration must be un-
derstood for the practical application of this process. The ob-
jective of this study is to investigate the active phase of the
catalyst for the CTH of HMF by using a battery of catalyst char-
acterization methods and the catalyst activation/regeneration
method. Specifically, extended X-ray absorption fine structure
(
EXAFS), X-ray photoelectron spectroscopy (XPS), high-resolu-
tion TEM (HRTEM), and temperature-programmed reduction
TPR) analyses of fresh and spent Ru/C catalysts were per-
(
formed to determine their oxidation state. In addition, the
fresh Ru/C catalyst was subjected to different pretreatments to
control its oxidation state. To elucidate the role of RuO , a pure
2
RuO catalyst was prepared and tested for the CTH of HMF. We
2
show that the active Ru/C catalyst demonstrates bifunctional
behavior, in which metallic Ru possibly catalyzes the dehydro-
genation of 2-propanol and the hydrogenation–hydrogenolysis
[4]
Ru/C catalysts in our previous work suggest highly dispersed
Ru particles on the carbon support. The surface properties of
fresh and spent Ru/C samples were analyzed by XPS, which
could identify the oxidation state of Ru and the chemical state
of HMF and RuO promotes DMF production via hydrogenoly-
2
sis.
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(
Aldrich) are shown in Figure 2a.
The XANES spectra demonstrate
that the fresh catalyst looks simi-
lar to bulk RuO2 whereas both
the fresh and spent catalysts re-
duced in situ look similar to the
Ru foil. The XANES spectrum of
the spent catalyst appears to
reveal a mixture of Ru metal and
RuO ; however, quite unexpect-
2
edly, no isosbestic points were
observed. In contrast to the
spent catalyst, the XANES spec-
tra of the reduced fresh and re-
duced spent catalysts exposed
to air were found to form iso-
sbestic points with the spectra
for the reduced catalyst and
RuO (Figure S2). This is an im-
2
portant result because it sug-
gests that RuO2 formed under
reaction conditions either is
a strained tetragonal RuO2 or
has a different structure. The
Fourier transform of the EXAFS
spectra is shown in Figure 2b,
and the fit results are summar-
ized in Table 1. The fresh catalyst
Figure 1. Core-level photoelectron spectra of C1s, Ru3d5/2, and O 1s of fresh and spent Ru/C catalysts.
BE=binding energy.
of the carbon surface. The C1s, Ru3d5/2, and O1s XPS spectra
of fresh and spent samples are shown in Figure 1. Before the
reduced in situ and RuO were used as standards to calculate
2
0
4+
the percentages of Ru and Ru , which are listed in Table 1.
0
reaction, RuO dominates on the surface of the Ru/C catalyst
The %Ru and coordination numbers for RuÀRu and RuÀO
2
(Figure 1a). A shift to lower binding energy over the spent Ru/
listed in Table 1 suggest that the fresh catalyst was mostly
0
C sample (by ꢀ1 eV) indicates a partial/complete reduction of
RuO with a small percentage of Ru and the spent catalyst
2
RuO during the reaction, even though both fresh and spent
was a mixture of Ru metal and RuO . The RuÀO bond length
2
2
catalysts are exposed to air. Both C1s and O1s spectra show
the typical CÀO and C=O features of activated carbon along
with high oxygen content owing to the low graphitic and dis-
ordered structure of carbon. The small changes in the peak in-
tensities of oxygen species (Figure 1b) are likely due to humin
formation during the reaction, which was confirmed by induc-
for the spent catalyst (2.02Æ0.02 ꢁ) was longer than that of
the fresh catalyst (1.97Æ0.01 ꢁ). This result suggests that the
structure of RuO in the spent catalyst is strained or could be
2
different from the tetragonal structure present in the fresh cat-
alyst, which is in agreement with the results of XANES analysis.
[
4]
tively coupled plasma analysis in our previous report.
The XPS analysis of the catalyst collected after 0.5 and 2 h
demonstrate that all the Ru3d5/2 peaks of the spent catalyst
shift to lower binding energies compared with those of the
fresh Ru/C catalyst (Figure S1). The fresh Ru/C catalyst shows
a HMF conversion of 60% after 0.5 h, whereas the spent Ru/C
catalyst collected after 0.5 h shows a HMF conversion of only
[a]
Table 1. Ru K-edge EXAFS fitting results.
Variable
Fresh
Spent
2.8 (1.0)
Fresh reduced Spent reduced
N
N
RuÀRu
7.3 (7)
7.9 (8)
RuÀO
6.1 (8)
3.9 (7)
2.67 (1)
2.02 (2)
0.009 (4)
0.007 (3)
À1.3 (1.0)
2.5 (2.0)
41
R
R
s
s
RuÀRu [ꢁ]
R2 uÀO [ꢁ]
2.649 (4)
0.0063 (5)
0.3 (2)
2.654 (4)
0.0061 (6)
0.2 (2)
1.97 (1)
2
16.4% when reused for the HMF reaction (Table S1). The activi-
[ꢁ ]
RuÀRu
2
2
ty of the spent Ru/C catalyst decreases only slightly with the
increase in the reaction time from 0.5 to 6 h. These results, in
conjunction with XPS analysis results, suggest that the deacti-
vation of the fresh Ru/C catalyst is likely due to the reduction
RuÀO [ꢁ ]
0.008 (2)
E
E
0
(RuÀRu) [eV]
0
(RuÀO) [eV]
À2.0 (1.4)
0
Ru (XANES) [%]
Ru (XANES) [%] 90
10
100
100
4
+
59
of the surface RuO rather than the deposition of humin on
2
[
a] Numbers in parentheses indicate the statistical error in the last signifi-
the catalyst surface.
cant digit obtained from the fit in the Artemis program, for example,
0
4+
The XANES and EXAFS spectra of fresh and spent Ru/C cata-
lysts before and after in situ reduction are shown in Figure 2.
6.1(8)=6.1Æ0.8. The percentages of Ru and Ru were calculated from
XANES data by the linear combination fits by using fresh Ru/C reduced
2
in situ and RuO as standards.
The XANES spectra of the Ru foil and the bulk RuO powder
2
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cause of the knock-on effect of the high-intensity electron
beam, because RuO can easily be reduced by carbon if the
2
temperature is high enough, and because Ru is an effective
catalyst for the methanation of carbon. However, we observed
the surface oxidation of the Ru particle (Figure 3b), in which
a larger atomic distance of the top layers of the Ru particle is
detected. This spacing was further confirmed by using DFT cal-
culations (Figures 3c and S3). Specifically, the surface layer dis-
tance in the vertical direction of Ru(0001) is slightly com-
pressed, as expected; however, for 1 monolayer (ML) of O/Ru,
the surface vertical lattice expands by 8% because the RuÀRu
metal bond is weakened by the OÀRu interaction. For the
RuO /Ru(0001) structure, simulated with surface and subsur-
2
face oxygen MLs, the lattice expands by 59% compared with
that of bulk Ru. The RuO /Ru lattice agrees with the TEM mea-
2
surement, which confirms that a RuO layer is formed. TEM
2
and DFT results are consistent with XPS and EXAFS data.
Catalytic properties of bulk RuO2
To identify the catalytic roles of Ru and RuO , bulk RuO was
2
2
used for the liquid phase HMF conversion. The reactivity of
bulk RuO2 for the CTH of HMF at 453 K for 6 h for different
RuO loadings is depicted in Figure 4. For a HMF/RuO molar
2
2
Figure 2. a) Ru K-edge XANES spectra and b) magnitude of Fourier trans-
2
formed k -weighted EXAFS spectra of fresh Ru/C (fresh), spent Ru/C (spent),
fresh Ru/C reduced in situ (fresh reduced), and spent Ru/C reduced in situ
(spent reduced) catalysts.
After in situ reduction, the fresh and spent catalysts showed
similar average RuÀRu first shell coordination numbers, 7.3Æ
0
.7 and 7.9Æ0.8, respectively. The slightly higher RuÀRu coor-
dination number for the spent catalyst could be due to a small
extent of sintering during the reaction.
Figure 4. Effect of RuO
toward different products. Reaction conditions: 1.2 wt% HMF in 2-propanol
2
on the conversion of HMF and the selectivity
The HRTEM images of a representative Ru particle over fresh
and spent Ru/C catalysts are shown in Figure 3; however,
solution, RuO catalyst at various loadings, T=463 K, P=20.4 bar N , t=6 h.
2
2
Hydrogenated intermediates include 5-MF, FA, and MFA.
a complete RuO particle has not been observed, probably be-
2
ratio of 30 (loading similar to
that of the supported Ru/C cata-
lyst), HMF conversion is 70%
and the major product is BHMF
(
70% selectivity) with the con-
comitant production of ethers
10%) and hydrogenated inter-
(
mediates (10%). With the in-
crease in RuO loading, the se-
2
lectivity toward ethers increases
from 10 to 65% and the selectiv-
ity toward BHMF decreases from
Figure 3. HRTEM images of Ru particles with fast Fourier transform images in insets: a) Ru/C-fresh and b) Ru/C-
used catalysts. c) DFT calculation of Ru(0001) with two O MLs. Spheres with crosses were fixed during structure
relaxations (see also Figure S2).
70 to 0%. Some DMF is also pro-
duced at high RuO2 loadings.
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These results suggest that RuO behaves as Lewis acid that can
Both XRD and TEM results reveal that the RuO catalyst re-
2
2
hydrogenate HMF to BHMF, probably via interhydride transfer,
duces under our reaction conditions. RuO is an outstanding
2
[6]
according to the Meerwein–Ponndorf–Verley reaction, from
-propanol to HMF, converting CHO to CH OH, and subse-
oxidation catalyst for reactions such as CO oxidation below
[7]
2
room temperature. RuO is also an efficient catalyst in many
2
2
[8]
quently etherifying BHMF or other hydrogenated intermediates
methanol oxidation reactions, involving direct H elimination
and/or oxidative dehydrogenation reactions. Because
a common intermediate step in all surface reactions involving
with 2-propanol. The hydrogenolysis activity of RuO may be
2
due to the metallic Ru generated by the in situ reduction of
RuO with 2-propanol.
CH OH is the scission of the OÀH bond yielding adsorbed H
2
3
[9]
The XRD patterns of the commercial hydrous RuO2
and methoxy, CH O, it is not surprising that the redox cycle
3
(
(
RuO ·xH O), pretreated RuO (RuO -fresh), and spent RuO2
of RuÀRuO terminates under our reaction conditions owing to
2
2
2
2
2
RuO -used) samples are shown in Figure 5. The XRD pattern of
the absence of oxygen and water.
2
the RuO -fresh sample indicates
2
a tetragonal structure of RuO2,
which was compared with the ref-
Catalytic properties of the reduced Ru/C catalyst
erence pattern of RuO [JCPDS 00-
2
0
4
18-1139; 28.1 (110), 35.0 (101),
0.4 (200), 40.8 (111), 54.2 (211),
Ru/C catalysts were reduced in flowing hydrogen at tempera-
tures between 373 and 573 K. Reduction temperatures lower
than 573 K were used to minimize sintering. HMF conversion
and product selectivity as a function of reduction temperature
and 69.58 (301)]. The XRD pattern
of the RuO -used sample indicates
2
a hexagonal metallic Ru structure,
if compared with the reference
Figure 5. Powder XRD pat-
terns of RuO
catalysts collect- pattern of Ru [JCPDS 00-001-1253;
2
ed before and after the reac-
tion.
3
5
8.6 (100), 42.4 (002), 44.4 (101),
8.8 (102), 69.6 (110), 79.1 (103),
and 85.08 (112)].
The morphology of fresh and spent RuO samples was inves-
2
tigated by using TEM. The HRTEM images of fresh and spent
RuO2 catalysts revealing the polycrystalline structure of the
two samples are shown in Figure 6c and d. The analysis of var-
ious lattice fringes and their characteristic acute angles con-
firmed that the fresh sample demonstrates the tetragonal
Figure 7. Catalytic performance of the Ru/C catalyst pre-reduced at different
temperatures. Reaction conditions: 1.2 wt% HMF in 2-propanol solution, Ru/
structure of RuO and the spent sample a metallic Ru structure.
2
C catalyst (HMF/Ru molar ratio of 35), T=463 K, P=20.4 bar N
2
, t= 6 h.
Ethers 1–4 and other intermediates FA and MFA are observed.
are plotted in Figure 7. HMF conversion increases from 74.8 to
8
5
3.4% with the increase in reduction temperature from 373 to
73 K. The major products produced over the reduced Ru/C
catalysts are DMF, ethers, 5-methylfurfural (5-MF), and BHMF.
With the increase in reduction temperature, the selectivity
changes slightly: from 27.8 to 30.4% for DMF, from 31.8 to
28.0% for ethers, from 12.6 to 7.2% for 5-MF, and from 1.4%
to 7.2% for BHMF. These results suggest that the Ru metal can
catalyze the hydrogenolysis of HMF to DMF but is not as
active and selective as the fresh (unreduced) Ru/C catalyst,
which consists of both Ru and RuO2.
Because neither bulk RuO nor the reduced Ru catalyst can
2
produce DMF selectively, a physical mixture of bulk RuO and
2
reduced Ru/C for HMF conversion was investigated to emulate
the conditions of the fresh Ru/C catalyst. HMF conversion and
product selectivity over RuO , reduced Ru/C, and a physical
2
mixture are shown in Figure 8. The amount of reduced Ru/C
and RuO in the physical mixture was chosen to be about half
2
of the RuO loading and the reduced Ru/C loading, respective-
2
ly. Although RuO produces ethers as major products and re-
Figure 6. Representative TEM images of a) RuO
2
-fresh and b) RuO
2
-used cata-
2
lysts. HRTEM of c) RuO -fresh and d) RuO -used catalysts.
2
2
duced Ru/C produces some DMF along with other products
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4
8%. Given the performance of bulk RuO (Figure 4), this result
2
indicates that the oxidation treatment of Ru/C at temperatures
higher than 473 K could form bulk RuO . The oxidation treat-
2
ment of Ru/C at mild temperatures (<403 K) possibly results in
a partially oxidized Ru phase (i.e., similar to a Ru and RuO mix-
2
ture), which is the active phase for DMF formation.
The higher DMF selectivity after mild oxidation treatment
could be due to a change in the Ru particle size and metal dis-
persion rather than due to the formation of partially oxidized
Ru. To exclude this possibility, the Ru/C catalyst after oxidation
treatment at 403 K was resubjected to reduction at 573 K for
3 h and tested for the reaction. The selectivity toward DMF de-
creased, and the overall catalytic activity was similar to that of
the reduced Ru/C catalyst. This result suggests that the role of
mild oxidation treatment is to produce a partially oxidized Ru
catalyst.
Figure 8. HMF conversion and product selectivity obtained with reduced Ru/
C, bulk RuO
conditions: 1.2 wt% HMF in 2-propanol solution, 100 mg of reduced Ru/C,
2 mg of RuO , a physical mixture of 70 mg of reduced Ru/C and 16 mg of
RuO , T=463 K, P=20.4 bar N , t=6 h. Ethers 1–4 and hydrogenated inter-
mediates (BHMF, 5-MF, FA, and MFA) are observed.
2 2
, and a physical mixture of RuO and reduced Ru/C. Reaction
3
2
2
2
To elucidate the relation between catalytic activity and oxi-
dation temperature of the Ru/C catalyst, we performed in situ
(ethers and hydrogenated intermediates such as BHMF and
hydrogen titration of the oxidized Ru/C catalyst. The H -TPR
2
5
-MF), the physical mixture selectively produces DMF in 70%
profiles of Ru/C catalysts treated at different oxidation temper-
atures are shown in Figure 10. Two major peaks were obtained
yield, which is close to the value obtained with the fresh Ru/C
catalyst. These results suggest that the synergy of Ru and RuO2
is responsible for the selective production of DMF from HMF,
but a proximity of active sites does not appear to be critical.
Oxidation pretreatment of the Ru/C catalyst
Motivated by the high DMF selectivity obtained with a physical
mixture of reduced Ru/C and RuO catalysts, we investigated
2
the oxidation treatment of Ru/C catalysts. The catalyst was re-
duced at 573 K for 3 h in flowing hydrogen and then oxidized
at different temperatures as described in the Experimental Sec-
tion. HMF conversion and product selectivity as a function of
oxidation temperature of the reduced Ru/C catalysts are plot-
ted in Figure 9. The selectivity toward DMF varies significantly
with catalyst pretreatment and goes through a maximum
2
Figure 10. H -TPR profiles of Ru/C catalysts after reduction at 573 K for 3 h
followed by oxidation for 3 h at different temperatures. TCD=thermal con-
ductivity detector.
(
72%) at an oxidation temperature of 403 K. For oxidation tem-
for each catalyst: a sharp peak at 320–450 K and a broad peak
at 610–950 K. The broad peak at 610–950 K is related to the re-
duction of the surface functional groups of the carbon sup-
port. The sharp peak at 320–450 K is related to the reduction
peratures higher than 473 K, the major products are ethers
and 5-MF. The selectivity toward ether 1 (5-[(1-methylethoxy)-
methyl] furfural) increases significantly up to approximately
[10]
of RuO2. The peak temperatures for the reduction of RuO2
depend on the oxidation temperature of Ru/C catalysts. The
peak temperatures of Ru/C catalysts oxidized at a temperature
below 403 K appear at 363 K and shift toward 423 K if the
Ru/C catalysts are oxidized at temperatures above 473 K. These
results indicate that there are structural changes in RuO with
2
the increase in oxidation temperature from 403 to 473 K. Given
the high ether formation rates after the oxidation of Ru cata-
lysts at 473 K (Figure 9), it is likely that bulk RuO forms at oxi-
2
dation temperatures above 473 K. For oxidation temperatures
below 403 K, no distinguishable changes in the sample TPR
peak temperatures were observed, even though the catalytic
activity does change. The ratio of peak areas at oxidation tem-
peratures of 303, 373, and 403 K is 1:1.5:1.5, which indicates
that the degree of oxidation increases with the increase in oxi-
dation temperature up to 403 K, as indicated by previous re-
Figure 9. Effect of oxidation temperature on the catalytic performance of
the Ru/C catalyst. Reaction conditions: 1.2 wt% HMF in 2-propanol solution,
Ru/C catalyst (HMF/Ru molar ratio of 35), T=463 K, P=20.4 bar N
2
, t=6 h.
Ethers 2–4 and other intermediates BHFM, FA, and MFA are observed.
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[
11]
ports. In addition, shoulders can be detected at 413, 433,
and 483 K, which corresponds to the Ru/C catalysts oxidized at
tion of methyl furans from furfurals needs two types of catalyt-
ic sites and provide insight into the design of bifunctional cata-
lysts for reductive upgrading of biomass-derived oxygenates to
fuels and chemicals.
3
03, 373, and 403 K. These shoulders can be assigned to the
reduction of Ru (possibly) in a different oxidation state. Inter-
estingly, peak temperatures shift to a higher temperature as
the oxidation temperature increases from 303 to 403 K, which
is in good agreement with the increasing DMF selectivity in
Figure 9. In summary, TPR studies of the Ru/C catalyst oxidized
at different temperatures suggest that changes in catalytic ac-
tivity and selectivity can be linked to changes in the degree of
oxidation of the Ru/C catalyst. Specifically, the mild oxidation
treatment of the Ru/C catalyst at 403 K and below produces
the partially oxidized Ru species, which are the active phase
for the selective production of DMF from HMF via CTH.
Experimental Section
Materials and pretreatment of catalysts
HMF (99%), DMF (99%), 5-MF (99%), furfuryl alcohol (FA, 99%),
and 2-methylfuran (MF, 99%) were purchased from Sigma–Aldrich.
BHMF (98%) was purchased from Penn A Kem. 5-Methyl furfuryl al-
cohol (MFA, 95–100%) was purchased from Parchem. Ru on the
carbon catalyst (5 wt% loading) was purchased from Sigma–Al-
drich. The catalyst was stored in a glove box under Ar atmosphere
to avoid exposure to air.
The fresh Ru/C catalyst was loaded in the glass tube and pretreat-
ed at various conditions before catalytic testing. For reduction, the
fresh Ru/C sample (0.5 g) was reduced for 3 h at 373–573 K in
Conclusions
We investigated the structure–property relations of the Ru/C
catalyst in the catalytic transfer hydrogenation (CTH) of 5-hy-
droxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) with
a flowing mixture of He (50%) and H (50%) at a rate of
2
3
À1
40 cm min . The catalyst samples were heated to the desired
temperatures (heating rate: 5 Kmin ), held for 3 h in a flowing
À1
2-propanol as a hydrogen donor. The samples were analyzed
mixture of He and H , and cooled down to RT in flowing He. After
2
by using X-ray photoelectron spectroscopy, extended X-ray ab-
sorption fine structure, high-resolution TEM, and temperature-
programmed reduction, and a kinetic evaluation of pretreated
catalysts and single phase catalysts (reduced Ru and bulk
reduction, the glass tube was dissembled from the furnace and
transferred to a glove box. The open ends of the tube were plug-
ged with rubber caps to prevent air exposure during the transfer.
For reduction and oxidation treatments, the fresh Ru/C sample
(0.5 g) was first reduced for 3 h at 573 K in a flowing mixture of He
RuO ) was performed. We found that partially oxidized Ru/C
2
3
À1
catalysts are effective for the selective production of DMF from
HMF via CTH. The surface of fresh Ru/C catalysts is mainly
(50%) and H (50%) at a rate of 40 cm min . After cooling down
2
to RT in flowing He, the samples were subjected to oxidation for
3
À1
3
(
h at 293–473 K in flowing O (flow rate: 5 cm min ) and He
2
RuO , as confirmed by X-ray photoelectron spectroscopy and
2
3
À1
flow rate: 35 cm min ). After oxidation treatment, the samples
extended X-ray absorption fine structure and quickly catalyzes
DMF production. However, these surface RuO2 species are
were transferred to the glove box under Ar atmosphere and stored
there.
easily reduced in situ during the CTH reaction. Pure RuO cata-
2
The pure RuO2 sample was obtained through the calcination of
lyzes the hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan
via interhydride transfer from 2-propanol to HMF as well as the
etherification of 2,5-bis(hydroxymethyl)furan and other inter-
mediates with 2-propanol, presumably acting as a Lewis acid
RuO ·xH O (purchased from Alfa Aesar) with flowing inert gas (He)
2
2
at 573 K for 3 h, and it was labeled as RuO -fresh for further use.
2
catalyst. Bulk RuO is also reduced during the reaction, pre-
2
Characterization methods
sumably by the hydrogen produced from the dehydrogenation
of 2-propanol. The reduced Ru/C catalysts show some activity
toward the production of DMF (up to 30% selectivity); howev-
er, they are less active than the fresh (partially oxidized) Ru/C
catalysts. The use of a physical mixture of reduced Ru/C and
The powder XRD measurements were performed with a Philips
Norelco powder diffractometer using CuK radiation and a Ni filter
a
(lK =1.540 ꢁ). The HRTEM images were obtained with a JEOL JEM-
2
010F transmission electron microscope equipped with a field
emission gun emitter. The XPS measurements were performed
with a Physical Electronics 5600 series XPS system equipped with
an Al anode X-ray source and a multichannel hemispherical ana-
lyzer. The binding energy scale was calibrated by comparing the
location of the primary photoelectron peak for Ag, Au, and Cu ref-
bulk RuO as catalysts significantly increases the DMF yield up
2
to 70%, which suggests that both Ru metal and RuO act in
2
synergy for the production of DMF. The mild oxidation treat-
ment of the reduced Ru/C catalyst at 403 K activates the cata-
lyst, which achieves a selective conversion of HMF to DMF in
up to 72% yield. This oxidation treatment generates a partially
oxidized Ru surface (suboxide layer on Ru metals), which is
necessary for the hydrogenolysis reaction. In contrast, oxida-
[12]
erence foils with the values published in the literature. Powder
samples were mounted by pressing onto an In foil.
The Ru/C catalyst was also characterized at the Ru K-edge by using
an in-house built, in situ X-ray absorption spectroscopy cell with
[13]
a glassy carbon tube (internal diameter=4 mm).
The catalyst
tion at temperatures above 473 K produces bulk RuO , as con-
2
samples were packed in the glassy carbon tube between quartz
wool plugs. The XANES and EXAFS measurements were performed
at a beamline X18A at the National Synchrotron Light Source oper-
ated by the Synchrotron Catalysis Consortium at Brookhaven Na-
tional Laboratory. The fresh and spent catalysts were characterized
firmed by H -temperature-programmed reduction analysis, and
2
is therefore unsuitable as a catalyst pretreatment. The charac-
terization and pretreatment studies of the Ru/C catalysts reveal
that the catalyst for the CTH of HMF to DMF is a bifunctional
material consisting of both Ru and RuO . Our studies verify the
ex situ (in air) and in flowing H (100%) after in situ reduction at
573 K for 30 min (ramp rate: 10 Kmin ). Three scans were collect-
2
2
À1
findings of previous reports showing that the efficient produc-
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ed and merged after they were aligned by using a Ru foil spectrum
collected simultaneously for each scan. XANES and EXAFS data
processing and analysis were performed with the Athena and Arte-
mis programs of the IFEFFIT data analysis package. After the nor-
malization of the absorption coefficient, the smooth atomic back-
ed Ru(0001), 1 ML of O on Ru, and RuO /Ru(0001) (ML of a surface
2
[17]
and a subsurface O) by using the RPBE
1 k mesh, and 350 eV cutoff.
functional, 4ꢂ4ꢂ
[14]
ground was subtracted by using the AUTOBKG code to obtain c(k) Acknowledgements
in which k is the photoelectron wave number). The theoretical
(
EXAFS signals for RuÀRu and RuÀO scattering paths were con-
This work was supported in part by the Catalysis Center for
Energy Innovation, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences (award no. DE-SC0001004). The use of the Na-
tional Synchrotron Light Source, Brookhaven National Laboratory,
for the EXAFS experiments was supported by the US Department
of Energy, Office of Basic Energy Sciences (grant no. DE-FG02-
[15]
structed with the FEFF6 code. For RuÀRu, we used a hexagonal
close-packed crystal structure with lattice constants a=b=2.706 ꢁ
and c=4.281 ꢁ. The RuÀO scattering path was calculated from the
RuO tetragonal structure with lattice constants a=b=4.51 ꢁ and
2
c=3.11 ꢁ. Because for both Ru metal and RuO catalysts there are
2
two scatterings paths for RuÀRu and RuÀO that are separated by
only 0.05 and 0.1 ꢁ, respectively, we used one path for RuÀRu and
RuÀO. The theoretical EXAFS signals were fitted to the data in r
space with the Artemis program of the IFEFFIT data analysis pack-
age. The spectra were fitted by varying the coordination number
05ER15688). Beamline X18A was supported, in part, by the Syn-
chrotron Catalysis Consortium. The DFT calculations were carried
out at the Center for Functional Nanomaterials, Brookhaven Na-
tional Laboratory (supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, under Contract DE-AC02-
of the single scattering paths for RuÀRu and RuÀO (N
and
and
R2 uÀRu
RuÀRu
N
), the bond length disorder (Debye–Waller factors; s
RuÀO
2
s
), the effective RuÀRu and RuÀO scattering lengths (RRuÀRu and
RuÀO
RuÀO
98CH10886) and the TeraGrid provided by Texas Advanced Com-
R
), and the correction to the threshold energy for RuÀRu and
2
>
RuÀO (DE ). The passive electron reduction factor (S ) was ob-
puting Center (TACC) of the University of Texas at Austin.
0
0
tained by analyzing the spectrum for a Ru foil, and the best fit
value (0.82) was fixed during the fitting. The k range used for the
Fourier transform of c(k) was 2.5–13.5 ꢁ , and the r range for fit-
ting was 1.1–3.1 ꢁ.
À1
Keywords: bifunctional catalyst · biomass · hydrogenation ·
ruthenium · ruthenium oxide
The H -TPR of the samples was performed by using an AMI-200ip
2
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Published online on February 20, 2014
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