Liquid-Phase Catalytic Transfer Hydrogenation of Furfural over Homogeneous Lewis Acid-Ru/C Catalysts

Article abstract of DOI:10.1002/cssc.201500212

The catalytic performance of homogeneous Lewis acid catalysts and their interaction with Ru/C catalyst are studied in the catalytic transfer hydrogenation of furfural by using 2-propanol as a solvent and hydrogen donor. We find that Lewis acid catalysts hydrogenate the furfural to furfuryl alcohol, which is then etherified with 2-propanol. The catalytic activity is correlated with an empirical scale of Lewis acid strength and exhibits a volcano behavior. Lanthanides are the most active, with DyCl₃ giving complete furfural conversion and a 97 % yield of furfuryl alcohol at 180 °C after 3 h. The combination of Lewis acid and Ru/C catalysts results in synergy for the stronger Lewis acid catalysts, with a significant increase in the furfural conversion and methyl furan yield. Optimum results are obtained by using Ru/C combined with VCl₃, AlCl₃, SnCl₄, YbCl₃, and RuCl₃. Our results indicate that the combination of Lewis acid/metal catalysts is a general strategy for performing tandem reactions in the upgrade of furans.

Full text of DOI:10.1002/cssc.201500212

DOI: 10.1002/cssc.201500212
Full Papers
Liquid-Phase Catalytic Transfer Hydrogenation of Furfural
over Homogeneous Lewis Acid–Ru/C Catalysts
[
a]
Paraskevi Panagiotopoulou, Nickolas Martin, and Dionisios G. Vlachos*
The catalytic performance of homogeneous Lewis acid cata-
lysts and their interaction with Ru/C catalyst are studied in the
catalytic transfer hydrogenation of furfural by using 2-propanol
as a solvent and hydrogen donor. We find that Lewis acid cata-
lysts hydrogenate the furfural to furfuryl alcohol, which is then
etherified with 2-propanol. The catalytic activity is correlated
with an empirical scale of Lewis acid strength and exhibits
a volcano behavior. Lanthanides are the most active, with
furfuryl alcohol at 1808C after 3 h. The combination of Lewis
acid and Ru/C catalysts results in synergy for the stronger
Lewis acid catalysts, with a significant increase in the furfural
conversion and methyl furan yield. Optimum results are ob-
tained by using Ru/C combined with VCl , AlCl , SnCl , YbCl ,
3
3
4
3
and RuCl . Our results indicate that the combination of Lewis
3
acid/metal catalysts is a general strategy for performing
tandem reactions in the upgrade of furans.
DyCl giving complete furfural conversion and a 97% yield of
3
Introduction
Hydrogenolysis of biomass-derived furfural to platform mole-
cules, such as 2-methylfuran (MF), is an essential step in the
catalytic conversion of lignocellulosic biomass into transporta-
the reduction of the oxide during the reaction makes funda-
mental studies complicated and the active sites remain elusive.
An alternative to solid Lewis acid catalysts is the use of homo-
geneous Lewis acid catalysts, such as metal halides.
[
1]
tion fuels and chemicals. Furfural hydrogenolysis in the liquid
phase has been studied with various single metal (Pt, Pd, Cu,
The beneficial effect of metal halides has been previously
observed for the conversion of carbohydrates into a variety of
[
2]
[2b,3]
Ru, Ir) and bimetallic (PtÀRu, CuÀCr, CuÀFe, PtÀSn, PtÀGe)
[5]
catalysts on various supports. In most of the studies, high-pres-
versatile intermediate compounds for biofuels and chemicals.
sure H was used, and the MF yield was typically low and was
For example, furfural production from xylose via xylulose can
2
[
3e]
modest (51%) in only one study.
be substantially improved by using CrCl in combination with
3
[
5a]
Despite the low cost of hydrogen due to the increased pro-
duction of shale gas, the use of hydrogen donors for the hy-
drogenolysis of furfural may provide advantages, such as elimi-
nation of high hydrogen pressure and the associated compres-
sion cost, overcoming the unavailability of hydrogen in certain
locations, and the possibility of an entirely green process (for
example, with renewable butanol as a hydrogen donor).
HCl in aqueous media. Transition metal chlorides, such as
CrCl , FeCl , and CuCl , as well as AlCl , were found to be effec-
3
3
2
3
[5c,d]
tive for the conversion of cellulose into levulinic acid.
To
our knowledge, only recently, a related study of HMF hydro-
deoxygenation to DMF reported a synergistic effect with
[6]
a Lewis acidic ZnCl and Pd/C dual catalyst. With consider-
2
ation of the effectiveness of the Ru/RuO /C catalyst but the
2
In our recent study, a high MF yield of 76% was observed
through catalytic transfer hydrogenation (CTH) of furfural by
using secondary alcohols (for example, 2-butanol or 2-penta-
lack of understanding about the active site(s), it would be in-
teresting to explore the synergy of homogeneous Lewis acid
catalysts with heterogeneous metal catalysts for the upgrade
of furfural.
[
2f]
nol) as hydrogen donors. The effective catalyst for the pro-
duction of MF in high yields was found to be a combination of
ruthenium oxide and metallic ruthenium supported on activat-
Herein, we study the interaction of metal chloride Lewis acid
and Ru/C catalysts for the CTH of furfural in the liquid phase
and correlate the performance with an empirical scale of Lewis
acid strength. The effect of reaction temperature, reaction
time, and Al/Ru molar ratio on the furfural conversion and
product distribution is presented. We demonstrate that Lewis
acid catalysts can be very active for the hydrogenation of fur-
fural to furfuryl alcohol and their activity exhibits a volcano-
type behavior with Lewis acid strength. With a single outlier,
hydrogenolysis of furfuryl alcohol to MF requires a metal cata-
lyst. Interestingly, we find primarily synergistic effects between
the metal chloride salts and the Ru/C catalyst but occasionally
also antagonistic interactions.
[
2f,g]
ed carbon.
This is also the case for the hydrogenolysis of 5-
hydroxymethylfurfural (HMF) to give high yields of 2,5-dime-
[
4]
thylfuran (DMF), which indicates that the coexistence of
Lewis acid/metal sites is essential for the production of alkylat-
ed furans. Although the Ru/RuO /C catalyst is fairly effective,
2
[a] Dr. P. Panagiotopoulou, N. Martin, Prof. D. G. Vlachos
Catalysis Center for Energy Innovation
Department of Chemical and Biomolecular Engineering
University of Delaware, Newark, DE, 19716 (USA)http://www.efrc.udel.edu/
E-mail: vlachos@udel.edu
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500212.
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Next, we turn to the product distribution resulting from the
hydrogen donor. Figure S1 in the Supporting Information
shows the 2-propanol conversion and product distribution at
Results and Discussion
Catalysis over homogeneous Lewis acid catalysts
1
808C for all of the Lewis acid catalysts investigated. 2-Propa-
Furfural conversion and product distribution
nol conversion varies from 0.5 to 1.2% (a low conversion given
the excess of the solvent) and follows the same trend as that
of furfural conversion with the various Lewis acid catalysts,
with the acetone yield reaching aprroximately 1.2% with DyCl3
Furfural conversion over Lewis acid catalysts in the absence of
Ru/C was investigated at T=1808C for a short reaction time of
3
h (Figure 1). Furfural conversion depends strongly on the
(
Figure S2 in the Supporting Information). Minor products of
side reactions of 2-propanol include diisopropyl ether (alcohol
self-etherification), propane (product of dehydration of 2-prop-
anol to propylene followed by propylene hydrogenation), and
isopropyl chloride, which is produced from 2-propanol with
the Lewis acid catalyst in accordance with Equation (1).
MCl þ x C H OH ! x C H Cl þ MðOHÞ
ð1Þ
x
3
7
3
7
x
The yield of isopropyl chloride, with respect to the initial
metal chloride, is shown in Figure S3 in the Supporting Infor-
mation.
[5a]
In our previous study, it was found that, when CrCl is dis-
3
Figure 1. Furfural conversion and product yields for the indicated homoge-
neous Lewis acid catalysts. X : Furfural conversion; MF: 2-methylfuran; FA:
furfuryl alcohol; FU: furan; ether: 2-(isopropoxymethyl)furan; dimer: 2-(2-fur-
anylmethyl)-5-methylfuran. Experimental conditions: 1 wt% furfural in 2-
solved in water, the pH value of the solution drops to approxi-
mately 2.1. This is because, in aqueous media, CrCl3 forms
hexa-aqua complexes characterized by ions, such as
F
3
+
[
Cr(H O) ] , which is hydrolyzed and results in Brønsted acidi-
2 6
propanol solution; cLewis acid =3.1 mm; N
t=3 h.
2
atmosphere (2.04 MPa); T=1808C;
ty, in accordance with Equation (2). This Brønsted acidity was
found to catalyze the dehydration of xylulose to furfural.
CrðH OÞ Š $ ½CrðH OÞ OHŠ þ Hþ
3
þ
2þ
½
ð2Þ
Lewis acid catalyst and increases from 34 to 100% following
2
6
2
5
the
order:
AlCl ·6H O<VCl <ZnCl <InCl <MgCl <
3
2
3
2
3
2
CrCl ·6H O<LaCl <SnCl ·5H O<YbCl ꢀDyCl <RuCl .
The
In the present study, the metal chlorides were dissolved in
3
2
3
4
2
3
3
3
main product was furfuryl alcohol (FA). The yield of FA varies
strongly (from 7 to 97%) depending on the Lewis acid catalyst,
2-propanol in the absence of water, and therefore, pH mea-
+
surement and estimation of the H concentration was not
in
the
order
RuCl <SnCl ·5H O<VCl <AlCl ·6H O<
possible. Currently, little is known about the formation of pos-
sible complexes upon dissolving metal halides in organic sol-
vents. Our data clearly show that the Lewis acid catalyzed
Meerwein–Ponndorf–Verley (MPV) reaction is dominant over
homogeneous metal chloride catalysts, and thus, hydrogen
protons are not expected to be present in noticeable concen-
trations to drive Brønsted acidity. To investigate the effect of
the small fractions of water that are encapsulated in hydrated
salts, select experiments were run over nonhydrated metal
chlorides by adding the appropriate amount of water to ach-
ieve the same molar ratio of MCl /H O as that of the hydrated
3
4
2
3
3
2
CrCl ·6H O<ZnCl <InCl <MgCl <LaCl <YbCl <DyCl .
3
2
2
3
2
3
3
3
We previously found that etherification between FA and the
alcohol solvent to form 2-(isopropoxymethyl)furan (hereafter
called the ether; Figure 1) is an important reaction competing
[
2f,g]
with hydrogenolysis over the Ru/RuO /C catalyst.
In the
2
case of the Ru/RuO /C catalyst, etherification is enhanced at
2
short reaction times and/or low temperatures and reverses at
longer reaction times and/or higher temperatures to give FA
and 2-propanol, with the FA converted eventually into MF.
Herein, we find interesting differences in etherification be-
tween different Lewis acid catalysts. In particular, the ether
yield is negligible for ZnCl and MgCl , low for VCl , AlCl ·6H O,
x
2
salt to the anhydrous salt (for example, VCl and VCl /H O=
3
3
2
1:6); the results were similar. This provides strong evidence
that the Lewis acid character and/or catalytic properties of the
metal chlorides are not influenced by the presence of small
2
2
3
3
2
CrCl ·6H O, LaCl , InCl , and DyCl , and high for YbCl ,
SnCl ·5H O, and RuCl .
3
2
3
3
3
3
4
2
3
Furfural decarbonylation to furan (FU) occurs only with RuCl3
fractions of H O contained in the hydrated structures.
2
(
yieldꢀ2%), and the formation of 2-(2-furanylmethyl)-5-meth-
ylfuran (hereafter called the dimer; Figure 1) is negligible for all
Lewis acid catalysts, with yields lower than 1%. These chloride
salts do not catalyze hydrogenolysis to MF with the exception
Mechanistic insights
The hydrogenation of the CHO group of furfural on Lewis acid
catalysts is well known to occur through the MPV interhydride
transfer reaction between the carbonyl group of the com-
of the RuCl catalyst, with which the MF yield is 20%. Traces of
3
MF were also detected for YbCl , SnCl ·5H O, VCl , LaCl , and
3
4
2
3
3
[7]
InCl (yields lower than 1%).
pound and the OH group of 2-propanol. The reaction is ac-
companied by acetone production, with furfural conversion in-
3
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creasing progressively with increasing acetone yield (Figure S2
in the Supporting Information). However, the number of ace-
tone moles produced is, in all cases, higher than the number
of moles of hydrogenation products (FA, MF, the ether, and the
dimer; Figure S4 in the Supporting Information). This provides
evidence that, in addition to the acetone produced through
the MPV reaction, 2-propanol is dehydrogenated to produce
furfural to FA through the MPV reaction followed by etherifica-
tion with the solvent (Scheme 1). In addition, they are reasona-
bly effective catalysts in the dehydrogenation chemistry of the
solvent, a property that is not typically associated with them.
In the presence of furfural, the dehydrogenation activity of 2-
propanol is reduced and furfural hydrogenation proceeds
mainly through the MPV reaction. Finally, these catalysts are
not effective in producing side products of 2-propanol, as ob-
served from the slow alcohol dehydration and hydrogenation
of the resulting propylene and the slow self-etherification.
acetone and H , which leads to hydrogen accumulation in the
2
system. Based on the data (Figure S4 in the Supporting Infor-
mation), we estimate that the fraction of acetone produced
that is consumed in the hydrogenation of known products
varies from approximately 57% (with Zn chloride) to about
9
8% (for example, with Yb chloride).
To explore the dehydrogenation activity of 2-propanol, the
Correlation of performance with Lewis acid strength
reactivity in neat conditions was investigated over AlCl ·6H O
3
2
To understand the effect of the Lewis acid catalysts on furfural
conversion, a descriptor of catalytic activity was exploited.
catalyst. The results showed that 2-propanol is indeed dehy-
drogenated to form acetone (Figure S5 in the Supporting Infor-
mation). However, the amount of acetone produced in the
presence of furfural is significantly higher than that obtained
in neat solvent through 2-propanol dehydrogenation. Byprod-
ucts of 2-propanol side reactions are in small fractions and are
only slightly affected by the presence of furfural. The furfural
interacts more strongly than 2-propanol with the Lewis acid
catalyst, so its presence is expected to retard partially the 2-
propanol dehydrogenation by blocking of the Lewis acid spe-
cies. The enhanced quantities of acetone seen in the furfural
experiments relative to those in the neat solvent experiments
[8]
Zhang proposed that the Lewis acid strength could be com-
posed of some electrostatic and covalent properties. The elec-
trostatic force between a positive charge and a negative
2
charge is approximately proportional to z/r , in which z is the
k
charge number of the atomic core (that is, the number of va-
[8,9]
lence electrons) and r is the ionic radius. With regard to the
k
covalent force, it has been suggested that, because the
s bond is formed by sharing of an electron pair between the
metal ion and the ligand, its strength increases with the ten-
dency of the cation to take on electrons, that is, with the in-
[8]
2
creasing electronegativity of the metal ion. The quantity z/rk
(Figure S5 in the Supporting Information), along with the hy-
was calculated (Table 1) and plotted as a function of the metal
drogen accumulation (Figure S4 in the Supporting Informa-
tion), indicate that most of the 2-propanol reacts through the
MPV reaction but a fraction undergoes dehydrogenation. The
dehydrogenation activity is partially suppressed by furfural rel-
ative to that in neat conditions, and therefore, the fraction of
each pathway contribution to 2-propanol conversion cannot
be accurately estimated.
electronegativity X (Figure S6 in the Supporting Information).
z
The regressed Equation (3) was used to define the equation for
classification, and the function Z, as an empirical metric of the
[8]
Lewis acid strength, is calculated with Equation (4).
2
z=rk ¼ 2:99 Â X À0:67
ð3Þ
ð4Þ
z
In summary, what our results indicate is that these homoge-
neous Lewis acid catalysts are capable of the hydrogenation of
2
Z ¼ z=r À2:99 Â X þ 0:67
k
z
Scheme 1. Scheme of the proposed reaction network in furfural hydrogenolysis with a mixture of Lewis acid/metal catalysts.
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principle, in which the stronger the Lewis acid, the stronger
the adsorption of furfural, the lower the reaction barrier and,
consequently, the higher the conversion of furfural to reaction
products. However, above a certain Lewis acid strength, furfur-
yl alcohol (the product) adsorption becomes so strong that it
may block the catalytically active sites, which results in lower
activity.
2
Table 1. Metal electronegativity, z/r
Z.
k
, and estimated Lewis acid strength
2
k
Metal
Metal electronegativity X
Pauling scale)
z
z/r
Lewis acid strength
(
Z
Zn
In
Mg
Dy
V
La
Cr
Yb
Al
Sn
Ru
1.65
1.78
1.31
1.22
1.63
1.1
1.66
1.1
1.61
1.96
2.2
3.652
4.152
2.704
2.711
5.479
2.184
5.263
2.953
6.584
5.806
5.06
À0.83965
À0.73946
À0.742
The yields of FA and the ether are plotted as a function of
Lewis acid strength in Figure 2B. Although it is expected that
the formation of both FA and the ether takes place on the
same species and that FA is an intermediate in the production
À0.45821
1.048859
À0.61616
0.740596
0.152837
2.215368
0.360967
À1.12314
of the ether, FA etherification is noticeable only on RuCl and
3
SnCl . The rest of the metal chlorides are either not very active
4
or stop the chemistry at furfuryl alcohol (for example, DyCl3).
The activity of a catalyst in the MPV reaction does not correlate
with the ability of the catalyst for etherification. As we recently
[10]
showed theoretically for solid Lewis acid sites of alumina,
the strength of Lewis acids of a certain crystal correlates linear-
ly with the binding energy and also with the dehydration reac-
tion barrier but does not correlate well with the etherification
reaction barrier, probably because of the reaction being bimo-
lecular, that is, as a result of stereochemical hindrance and
modification of the electronic properties from adsorption of
the first molecule.
Interaction of Ru/C and homogeneous Lewis acid catalysts
Furfural conversion and product distribution
The interaction of Ru/C catalyst with different Lewis acids in
the transformation of furfural was investigated at T=1808C for
3
h of reaction time. Figure 3 compares the performance of
Figure 2. A) Furfural conversion and B) FA and ether yields as a function of
the Lewis acid strength in the absence of Ru/C catalyst. Experimental condi-
tions were the same as those given in Figure 1.
The strength Z of the Lewis acids was calculated for all of
the metal chlorides investigated (Table 1) and was correlated
with furfural conversion and the FA and ether yields (Figure 2).
In all cases, the furfural conversion is nearly equal to the
sum of the FA and ether yields (an indication of a lack of any
Figure 3. Effect of the addition of Ru/C catalyst on furfural conversion and
product yield for the indicated homogeneous Lewis acid catalysts. Experi-
mental conditions: 1 wt% furfural in 2-propanol solution; cLewis acid =3.1 mm;
À1
c
Ru/C =4.1 gL ; N
2
atmosphere (2.04 MPa); T=1808C; t=3 h.
major byproducts), with the exception of RuCl , which produ-
3
ces a noticeable yield of MF. It is observed that the catalytic ac-
tivity goes through a maximum (Figure 2A) at around the
strength of DyCl and YbCl ; that is, it exhibits a volcano type
the Ru/C catalyst (in the absence of Lewis acid catalyst) with
those of the Lewis acid/metal mixtures. The catalytic activity of
Ru/C is significantly enhanced with the addition of small
amounts of the Lewis acids. In particular, the furfural conver-
3
3
of behavior. RuCl also exhibits 100% furfural conversion, but
3
a significant fraction of the furfural is converted into MF. The
volcano-type curve can be rationalized by the classic Sabatier
sion increases from 41 to 100% following the order: InCl <Ru/
3
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C<MgCl <ZnCl <LaCl <CrCl ·6H O<RuCl <VCl <
absence of Ru/C (Figure 2B) but reaches higher values in the
presence of Ru/C. The ether yield reaches a plateau above
a certain Lewis acid strength (Figure 4B). This is most possibly
related to the fact that Ru/C itself is able to etherify FA with
the 2-propanol solvent, most probably on the carbon sup-
2
2
3
3
2
3
3
SnCl ·5H O<DyCl <AlCl ·6H O<YbCl .
A
comparison of
4
2
3
3
2
3
Figure 1 and Figure 3 shows that the addition of metallic Ru/C
results in the formation of MF for all Lewis acids examined. In
most cases, the MF yield is higher than that obtained if only
Ru/C catalyst is used (12%). Moderate MF yields of 26–31% are
achieved by using Ru/C combined with VCl , AlCl , SnCl , YbCl ,
[2g]
port. In our previous study, an experiment was conducted
over activated carbon with FA as the reactant and it was found
that FA is mainly etherified with 2-propanol on the surface of
the carbon support; a conversion of 41% was observed, with
a 32% yield of the ether and a 2% yield of MF. The MF yield
(Figure 4C) also exhibits a plateau at high values of Z.
A comparison of Figure 2 and Figure 4 indicates clear syner-
gistic effects of mixtures of strong Lewis acid and Ru/C cata-
lysts. Specifically, physical mixtures of Ru/C and chloride salts
of Al, V, Sn, and Cr exhibit much higher activities and increased
yields of all products with an FA/ether ratio close to 1. Ru and
Yb exhibit a complex interaction with Ru/C to give a higher
yield of MF and a different ether/FA ratio than as standalone
catalysts. An outlier among the strong Lewis acids is CrCl3,
which is very active but exhibits an antagonistic effect with
Ru/C that results in a low MF yield.
3
3
4
3
and RuCl . Significant amounts of FA and the ether are also
3
formed.
The performance of the combined Ru/C and Lewis acid cata-
lysts was correlated with the Lewis acid strength Z described
above (Figure 4). It was found that furfural conversion increas-
es rapidly with increasing Lewis acid strength and reaches
a plateau above a certain value of Z (Figure 4A). The FA yield
goes through a maximum for DyCl with increasing Lewis acid-
3
ity (Figure 4B); it follows a similar trend to that obtained in the
The 2-propanol conversion and acetone yield are higher in
all cases in the presence of Ru/C catalyst, with the exception
of the reaction with DyCl for which the values are comparable
3
(
Figure S7 in the Supporting Information). It has been found
that Ru/C catalyst exhibits high activity for the dehydrogena-
tion of secondary alcohols to produce a ketone and equiva-
[2f]
lents amounts of H2. It can, therefore, be suggested that, in
the presence of both metal (Ru/C) and Lewis acid catalysts,
acetone is produced through MPV and 2-propanol dehydro-
genation reactions. The hydrogenation of the CHO group of
furfural may occur with hydrogen produced through 2-propa-
nol dehydrogenation on metallic Ru sites and the MPV interhy-
dride transfer reaction on Lewis acid sites (Scheme 1). For ex-
ample, in the case of YbCl , although furfural conversion is
3
complete both in the presence and in the absence of Ru/C,
the product distribution differs. The addition of Ru/C results in
the production of MF at the expense of FA. The 2-propanol
conversion and acetone yield are higher in the presence of
both Ru/C+YbCl in the reactant suspension, which indicates
3
that H2 produced from 2-propanol dehydrogenation is con-
sumed for MF production. This is also the case for RuCl , for
3
which both the MF and acetone yields are enhanced with the
addition of Ru/C. As in the absence of Ru/C catalyst, the
number of acetone moles produced is higher than the number
of moles of furfural hydrogenation products (Figure S4 in the
Supporting Information) for all Lewis acids examined. Irrespec-
tive of the reaction by which acetone is produced, the MF
yield increases with the increasing acetone yield (Figure 5), in
[2f,g]
agreement with our previous studies.
The fraction of H produced that is consumed in the hydro-
2
genation of known products was estimated based on the data
of Figure S4 in the Supporting Information, and it was found
to vary from approximately 43% for the less active catalysts
(
such as InCl ) to about 96% (for example, with LaCl ).
3
3
Figure 4. A) Furfural conversion, B) FA and ether yields, and C) MF yield as
a function of the Lewis acid strength with a mixture of Lewis acid and Ru/C
catalysts. Experimental conditions were the same as those in Figure 3.
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The results are in general agreement with recent work on
the hydrodeoxygenation of HMF to DMF with tetrahydrofuran
as the solvent and external hydrogen as the hydrogen
[6]
source. It was found that the combination of Lewis acidic
ZnCl2 and Pd/C completely converted HMF and exhibited
a high DMF selectivity of 85%. Under comparable reaction
conditions, Pd/C alone produced only 27% DMF, whereas
ZnCl alone did not catalyze deoxygenation or hydrogenation
2
of HMF, which suggests a strong synergistic effect between
the two catalysts.
The results of the present study agree with and provide fur-
ther support for our previous findings, in which it was demon-
strated that the effective catalyst for high MF yields is a combi-
[2f,g]
.
nation of Ru (metal) and RuO (Lewis acid)
Whereas the ori-
2
Figure 5. MF yield as a function of acetone yield for mixtures of Lewis acid
and Ru/C catalysts. Experimental conditions were the same as those given in
Figure 3.
gins of the synergy of these homogeneous Lewis acid/metal
catalysts are currently unknown, our results provide insights
into the development of heterogeneous catalysts for donor-
based hydrogenolysis. They underscore the fact that not all
Lewis acid catalysts combined with a metal are effective in car-
rying out hydrogenolysis. Rather, strong Lewis acid catalysts
are essential for this. At the same time, the results in Figure 4
provide evidence that, above a certain Lewis acid strength, MF
production cannot be further improved, at least under the
present experimental conditions; thus, a minimum Lewis acid
strength is essential but hydrogenolysis activity does not scale
up with the Lewis acid strength. Higher reaction temperature
or longer reaction time, as well as different Lewis acid/metal
ratios, may be required; thus, the effect of these parameters is
investigated in the following sections.
Mechanistic insights
The results in Figure 1 and Figure 3 clearly indicate that MF
production requires a combination of metal/Lewis acid cata-
lyst. This is more pronounced in the cases of AlCl and VCl , for
3
3
which neither the metallic Ru alone nor the Lewis acid alone
are capable of catalyzing effectively the production of MF from
furfural (Figure 6), but in coexistence, they are capable of carry-
ing out the hydrogenolysis.
Effects of operating conditions and metal/Lewis acid cata-
lyst ratio
Effect of reaction time
The effect of the reaction time on the catalytic activity and
product distribution has been investigated at T=1808C over
AlCl , both in the absence and in the presence of Ru/C catalyst.
3
In the former case (Figure 7A), furfural conversion increases
from 31 to 97% with an increase in the reaction time from
1
to 9 h. The main reaction products detected were FA and the
ether.
The addition of Ru/C catalyst (Figure 7B) results in an almost
complete furfural conversion already after 3 h and is accompa-
nied by MF production, with its yield increasing from 13 to
45% with an increase in the reaction time from 1 to 9 h. The
yield of the ether increases up to 32% at t=9 h, which is
lower than that observed with AlCl catalyst alone, whereas
3
the opposite is observed for shorter reaction times. This is
most probably related to the enhancement of FA hydrogenoly-
sis to form MF at longer times by the synergism of the Ru/C
and AlCl catalysts. Both the ether and MF are produced from
3
FA, the yield of which decreases as a function of time. FU and
the dimer were also detected, but their yields are low at all
[2g]
times. The 2-propanol conversion and the acetone yield in-
crease with the reaction time, and both are enhanced with the
addition of Ru/C (Figure S8 in the Supporting Information).
Figure 6. Synergism of the Lewis acid and Ru/C combination relative to re-
3 3
duced Ru/C catalyst with A) AlCl and B) VCl catalysts. Experimental condi-
tions were the same as those given in Figure 3.
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Figure 7. Effect of reaction time on furfural conversion and product yields
over A) AlCl and B) AlCl and Ru/C catalysts. Experimental conditions:
wt% furfural in 2-propanol solution; cLewis acid =3.1 mm; cRu/C =0 (A) or
Figure 8. Effect of reaction temperature on furfural conversion and product
yields over A) AlCl and B) AlCl and Ru/C catalysts. Experimental conditions:
3 3
3
3
1
4
À1
1 wt% furfural in 2-propanol solution; cLewis acid =3.1 mm; cRu/C =0 (A) or
2
.1 gL (B); N atmosphere (2.04 MPa); T=1808C.
À1
4
.1 gL (B); N
2
atmosphere (2.04 MPa); t=3 h.
Effect of reaction temperature
varying the AlCl concentration (1–10.5 mm) while keeping the
3
The effect of the reaction temperature was investigated in the
Ru/C concentration (0.4%w/v) constant (Figure 9A) or by vary-
ing the Ru/C concentration (0.2–1.65%w/v) while keeping the
AlCl3 concentration (3.1 mm) constant (Figure 9B). In both
cases, the furfural conversion increases with an increase in the
Al/Ru molar ratio, whereas the product distribution depends
range of 120–1958C over AlCl , both in the absence (Fig-
3
ure 8A) and in the presence (Figure 8B) of Ru/C catalyst. In the
former case, the furfural conversion and FA yield increase with
an increase in the temperature and reach 70 and 51%, respec-
tively, at T=1958C. The yield of the ether takes comparable
values with those of FA up to T=1608C but is not further in-
creased at higher reaction temperatures. The MF yield is only
on whether the concentration of AlCl or Ru/C is being varied.
3
In the former case, the FA yield decreases from 53 to 34% with
an increase in the Al/Ru molar ratio in the range of 0.5–5.0,
whereas the opposite is observed in the latter case. The yield
of the ether increases from 18 to 49% with an increase in the
2.5% at T=1958C.
The addition of Ru/C results in significantly higher furfural
conversions over the entire temperature range examined and
is accompanied by MF production at temperatures higher than
T=1308C, with a yield of 35% achieved at T=1958C. The
yield of FA increases up to 53% at T=1608C and then decreas-
es to 28%, as a result of its consumption in the production of
MF and the ether through hydrogenolysis and etherification
with 2-propanol, respectively. The yield of the ether is higher
than that obtained in the absence of Ru/C catalyst. As in the
case of the time-dependent experiments, the 2-propanol con-
version and acetone yield increase with reaction temperature
and are enhanced with the addition of Ru/C (Figure S9 in the
Supporting Information).
concentration of AlCl while the Ru/C concentration is kept
3
constant. In contrast, the yield of the ether varies slightly with
the increase in the concentration of Ru/C while the AlCl con-
3
centration is kept constant, which indicates that FA etherifica-
tion with 2-propanol is favored on Lewis acid sites relative to
metal sites. On the other hand, the MF yield increases moder-
ately (from 19 to 28%) with an increase in the AlCl concentra-
3
tion (Figure 9A) but decreases considerably (from 44 to 18%)
with a decrease in the Ru/C concentration (Figure 9B), provid-
ing evidence that metallic Ru contributes significantly on FA
hydrogenolysis to MF. However, the MF yield is only 12% for
an Al/Ru molar ratio of 0, which indicates that it is the synergy
of Lewis acid and metal sites that is responsible for the high
MF yield. Although the results of the present study cannot elu-
cidate the details of the interplay between metal/Lewis acid
sites, such a synergy possibly arises from a molecular interac-
tion between the two catalysts as a result of adsorption of the
Effect of the ratio of Lewis acid/metal catalysts
The effect of the Al/Ru molar ratio on the furfural hydrogenoly-
sis activity was investigated at T=1808C for t=3 h, either by
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Figure 10. Furfural conversion and product yields over the fresh, spent, and
regenerated catalysts. Experimental conditions are the same as those given
in Figure 3.
catalyst and may be a result of residual Cl ions left on the Ru
catalyst. Importantly, the product distribution is completely re-
covered after catalyst regeneration (regenerated catalyst) fol-
lowing the same pretreatment used for the fresh catalyst (re-
duction for 3 h at 3008C in H flow). The results indicate that
2
the Ru/C catalyst deactivation is reversible and the catalyst is
recyclable after regeneration.
Figure 9. Effect of Al/Ru molar ratio on furfural conversion and product
yields by varying the concentration of A) AlCl and B) Ru/C catalysts. Experi-
mental conditions: 1 wt% furfural in 2-propanol solution; N atmosphere
2.04 MPa); T=1808C; t=3 h.
Conclusions
3
2
We have studied the effect of homogeneous chloride-based
Lewis acid catalysts, as well as the interaction of these Lewis
acid catalysts with Ru/C catalyst, on the hydrodeoxygenation
of furfural in the liquid phase through catalytic transfer of hy-
drogen (CTH) with 2-propanol as the solvent and hydrogen
donor. We have shown that homogeneous Lewis acid catalysts
effectively convert furfural into furfuryl alcohol (FA) and its
ether with 2-propanol. These catalysts are ineffective in the hy-
drogenolysis of FA to form 2-methylfuran (MF) except for
(
Lewis acid on the metal. Further work will be needed to under-
stand the molecular origin of this interaction.
The 2-propanol conversion and acetone yields are higher
with higher Ru/C concentrations, most probably as a result of
the parallel occurrence of the MPV and 2-propanol dehydro-
genation reactions (Figure S10B in the Supporting Informa-
tion). As expected, isopropyl chloride formation is favored for
RuCl . The catalyst activity exhibits a volcano-like behavior
3
higher AlCl concentrations (Figure S10A in the Supporting In-
versus Lewis acid strength. The most active catalysts are salts
of Ru, Sn, Yb, and Dy. Hydrogenation of furfural proceeds pri-
marily through the MPV interhydride transfer. However, direct
dehydrogenation of 2-propanol leading to the production of
3
formation).
Catalyst stability
gaseous H occurs in parallel and could result in some hydro-
2
Catalyst stability has been examined by conducting catalyst re-
genation of the CHO side group of furfural.
cycling experiments over Ru/C+AlCl catalysts at T=1808C for
Neither the metallic Ru/C alone nor a Lewis acid alone is ca-
pable of effectively catalyzing the hydrodeoxygenation of fur-
fural to MF, which is consistent with our previous results dem-
onstrating that the coexistence of both catalysts is essen-
3
3
h of reaction time. The catalyst was recovered after the reac-
tion by filtration, washed with 2-propanol several times, and
reused after drying at T=1008C (hereafter called the spent cat-
alyst). Fresh Ru/C catalyst (<10 wt% of total catalyst) was
added to replenish the catalyst mass lost during recovery.
[2f,g,4]
tial.
The synergy of Lewis acid and metal catalysts signifi-
cantly increases both the furfural conversion and the MF yield,
with VCl , AlCl , SnCl , YbCl , and RuCl exhibiting better re-
Fresh AlCl was then added, and the experiment was repeated.
3
3
3
4
3
3
The results are summarized in Figure 10. On the spent catalyst,
the furfural conversion remains practically unchanged (ꢀ99%),
whereas the MF yield decreases from 28 to 12.5%. The de-
crease in the MF yield is accompanied by an increase in the FA
yield from 36 to 48% and a slight increase in the ether yield
from 32 to 34%. Our results demonstrate that, although the
furfural conversion over the spent catalyst does not decrease,
FA production increases at the expense of MF. We hypothesize
that this change in selectivity is due to modification of the Ru
sults. The MF yield increases with an increase in the reaction
time or temperature and can be further optimized by selecting
the molar ratio of Al/Ru. The highest MF yield was approxi-
mately 45%.
Catalyst recycling experiments showed that furfural conver-
sion remains complete over the spent catalyst and the FA yield
increases at the expense of the MF yield. However, the catalyst
performance is regained completely after catalyst regenera-
tion.
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Our results support a picture whereby the Lewis acid cata-
lyst is mainly responsible for the hydrogenation of furfural to
FA, along with the etherification (a side and reversible reaction
of FA and 2-propanol), and the Lewis acid/metal catalyst syner-
gy drives the hydrogenolysis of FA to MF. This synergy extends
beyond that of heterogeneous catalysts that we discovered
before to homogeneous Lewis acid/heterogeneous metal cata-
lysts and, as such, opens up new possibilities for carrying out
this cascade of reactions in a single pot. Our results reveal for
the first time that good catalysts require a strong Lewis acid
functionality and optimization of the ratio of the two function-
alities.
the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Award Number DE-SC0001004.
Keywords: homogeneous catalysis · hydrogenation · lewis
acids · oxygen heterocycles · ruthenium
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The work was financially supported from the Catalysis Center for
Energy Innovation, an Energy Frontier Research Center funded by
Received: February 8, 2015
Published online on May 26, 2015
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