Efficient hydrogenation of concentrated aqueous furfural solutions into furfuryl alcohol under ambient conditions in presence of PtCo bimetallic catalyst

Article abstract of DOI:10.1039/c6gc03143a

Furfural (FAL), a major biomass-derived chemical, can be hydrogenated to yield the industrially important platform chemical, furfuryl alcohol (FOL). Although heterogeneous catalyst-based methods are known to yield FOL from dilute solutions of FAL, they mainly operate at high temperatures and/or high pressures of hydrogen and in the presence of organic solvents. In this work, we employ bimetallic PtCo/C catalysts with varying metal concentrations to achieve the maximum possible FOL yield (100%) at 35 °C under 0.1 MPa H₂ in water. With concentrated FAL (40 wt%) at 50 °C and under 1 MPa H₂ pressure, 86% yield of FOL was observed. Moreover, efficient catalyst recycling was observed over at least four runs with marginal loss in activity due to handling error and isolation of FOL in pure form confirmed by NMR and HPLC. Characterization of catalysts with several physico-chemical techniques (XRD, TEM, XPS, ICP, TPR) reveals the presence of electron-rich Pt and ionic Co species in proximity with each other and these work synergistically to facilitate maximum possible yield of FOL under ambient conditions and in water medium.

Full text of DOI:10.1039/c6gc03143a

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Received 00th January 20xx,
Efficient hydrogenation of concentrated aqueous
furfural solutions in to furfuryl alcohol under
ambient conditions in presence of PtCo bimetallic
catalyst
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
M. G. Dohade,a P. L. Dhepea,*
One of the biomass derived significant chemical furfural (FAL) can be hydrogenated to yield industrially important platform
chemical, furfuryl alcohol (FOL). Although heterogeneous catalysts based methods are known to yield FOL from dilute
solutions of FAL, those mainly operate at higher temperatures and/or high pressures of hydrogen and in presence of
organic solvents. In this work, we employ bimetallic PtCo/C catalysts with varying metal concentrations to achieve
o
o
maximum possible FOL yield (100%) at 35 C under 0.1 MPa H2 in water. With concentrated FAL (40 wt%) at 50 C and
under 1 MPa H2 pressure, 86% yield for FOL was observed. Moreover, efficient recycling of catalyst in at least 4 runs with
marginal loss in activity due to handling error and isolation of FOL in pure form confirmed by NMR and HPLC is attractive.
The characterization of catalysts with several physico‐chemical techniques (XRD, TEM, XPS, ICP, TPR) reveal the presence
of electron rich Pt and ionic Co species in proximity with each other and those work synergistically to facilitate
accomplishment of maximum possible yield of FOL under ambient conditions and in water medium.
ethyltetrahydrofurfuryl ethers, hydrogenated cyclo products
(cyclopentanol, cyclopentanone) as well as various C10–C15
coupling products can also be derived from FAL and FOL.6 It is
estimated that ca. 60% of global production of FAL is
converted in to FOL.10 Moreover, this produced FOL acts as a
precursor in the preparation of resins, which accounts for
more than 85% of total world FOL production.11, 12
Introduction
To circumvent the drawbacks such as, diminishing reserves,
fluctuating prices and climate change linked with the use of
fossil feedstock; exploitation of non‐edible renewable
feedstock,1
lignocellulosics
consisting
of
cellulose,
hemicelluloses and lignin to produce chemicals is imperative.2
Typically, C5 and C6 based polysaccharide, hemicellulose is
known to produce furfural (FAL) with high yields through in‐
series acid catalyzed hydrolysis (to produce C5 sugars) and
dehydration reactions.3‐5 Further, furfural can be converted
into several industrially important chemicals6‐8 such as furfuryl
alcohol (FOL, hydrogenation of ‐CHO to –CH2OH) and
tetrahydrofurfuryl alcohol (THFA, ring hydrogenation and
hydrogenation of ‐CHO to –CH2OH). Next, hydrodeoxygenation
(HDO) of FOL produces 2‐methylfuran (MF) which has a
potential to be used as fuel additive.6, 9 Besides synthesis of
these chemicals, production of methyltetrahydrofuran, γ‐
In view of the significance of FOL, hydrogenation of FAL in
to FOL is carried out in presence of various organic solvents
like ethanol, methanol, iso‐prapanol (IPA) etc. On industrial
scale hydrogenation of FAL is performed over Cu‐Cr catalyst
(CuCr2O4) at 180 °C under 7‐10 MPa of hydrogen pressure in a
13‐22
batch process.8,
But owing to employment of high
pressures (>7 MPa), toxicity of catalyst (Cr) and deactivation of
catalyst (coke formation, sintering, leaching) in the commercial
process, impelled researchers to develop Cr free catalytic
system for the production of FOL from FAL.15, 23 Considering
this, researchers have studied various other Cu based catalysts
(Cu/Al2O3, Cu‐Mg/Al2O3) to produce FOL under lower hydrogen
pressures (2 MPa) or in presence of proton donating solvent
valerolactone
or
valeric
acid,
ethylfurfuryl
and
with mixed success in achieving high yields for FOL (81‐100%)
24
with stable catalyst.12,
Besides Cu based catalysts,
* Catalysis and Inorganic Chemistry Division, CSIR‐National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411 008, India. Tel. 91‐20‐25902024, E‐mail:
pl.dhepe@ncl.res.in (P. L. Dhepe)
application of Co‐Mo‐B alloy (99% FOL yield),25 10 wt%
Co/SBA‐15 (88% FOL yield)26 and polymer stabilized NiB
catalysts (71% FOL yield)27 are too reported to achieve high
selectivity (>90%) for FOL formation at 1‐2 MPa hydrogen
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
°
pressures and at 100‐180 C. Recently, conversion of FAL on 2
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wt% Pt/Al2O3 is reported for achieving good selectivity (99%) that Pt, which activates hydrogen at lower tempVeierwatAurtricelesOanlninde
for FOL under mild conditions in presence of methanol as a oxophilic Co that polarizes carbonyl bonDdOcIa: 1n0.b10e39c/oCm6GbCin0e3d143toA
solvent.28 In another report use of 10 wt% Pt with same achieve higher yields of FOL under milder reaction conditions.
support under 2 MPa hydrogen pressure is shown to
hydrogenate FAL to achieve 62% FOL yield.29 A prevalent
observation with Pt catalysts is that due to its very high activity
and in the aspiration to accomplish complete conversions, Pt
catalysts are prone to catalyze side reactions (hydrogenolysis,
hydrogenation, decarbonylation, ring opening etc.) of FAL and
FOL, thus lowering the yield for FOL. To suppress these side
reactions, Pt only catalysts (46% FOL yield) were decorated
with additives such as Sn (96% FOL yield) and Ge (74% FOL
Scheme 1. Conversion of furfural (FAL) to furfuryl alcohol
(FOL).
o
yield) and improvement in yields was seen at 100 C under 1
31
MPa H2 pressure with iso‐prapanol (IPA) as solvent.30,
Moreover, role of other additives such as, Cu, Fe,
heteropolyacid etc. along with other active transition metals
(Ni, Ru, Pd etc.) to enhance the yields of FOL is also studied.8,
Results and Discussion
10, 25, 32‐40
Although with few catalytic systems high yields of
Catalyst characterization
FOL are achieved but, recyclability of catalysts, use of dilute
substrate solutions and employment of high hydrogen
pressures (2‐10 MPa) and temperatures (>100 oC) still remains
a challenge.13, 24‐26, 29, 41‐43 To surmount the use of molecular
hydrogen, catalytic transfer hydrogenation of FAL is also
performed using IPA and other alcohols as (proton donating)
solvents.24, 44‐46 However, mainly these reactions were carried
out at higher temperatures (>150 °C) so as to activate the
solvent for donation of the proton.46
The synthesized catalysts were analyzed using ICP‐OES
technique to quantify the metal contents and the results
(Table S1, ESI) explain that the theoretical and experimental
values of metal loading are complement with each other very
well.
Nevertheless, it would be desirable to use water as a
solvent and it is shown that Pd‐Cu/MgO catalyzed reactions
can be carried out with molecular hydrogen (0.6 MPa; 98% FOL
yield)47‐49 or along with inorganic acid like H3PO4 (85%) (3 MPa;
27% FOL yield) in presence of water.50 In yet another report,
Ir–ReOx/SiO2 catalyst is shown to convert FAL into FOL under
milder conditions.51 But, it is revealed that FAL in water
undergoes quite a few side reactions such as rearrangement
etc. to produce undesired products and hence the lower yields
for FOL are seen.50, 52 Additionally, leaching and sintering of
metals is another key obstacle in employing these catalysts in
water medium.
In view of this, it is still a challenge to design a water stable
catalyst for the complete conversion of FAL to yield FOL with
high selectivity. The addition of secondary metal to stabilize
primary (active) metal is helpful in this regard but appropriate
choice of these metals is indispensable to attain desired
products with high yields as is known from earlier works.8, 10, 25,
30, 32‐40, 43, 53
Considering the limited literature available on
water based FAL hydrogenation in achieving higher yields of
FOL over stable catalysts, it is requisite to develop an atom
efficient water based stable catalytic system to accomplish
complete and selective conversion of FAL to FOL under mild
reaction conditions (preferably < 50 oC, < 1 MPa H2) with lower
loading of precious metal (Scheme 1). Moreover, to make an
overall method industrially attractive it is essential to employ
concentrated substrate solutions and hence in this work,
reactions are carried out with 40 wt% FAL. Based on the earlier
Figure 1. XRD patterns of mono and bimetallic catalysts. (a) Pt(3)/C and reduced
carbon; (b) Pt(3)Co(3)/C and Co(3)/C.
studies on acetaldehyde hydrogenation54 and reduction of the
carbonyl group in cinnamaldehyde55,
it was hypothesized
56
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The XRD patterns for mono and bimetallic reduced catalyst
samples of Pt and Co supported on carbon are presented in
Figure 1. The XRD pattern for Pt(3)/C shows diffraction peaks
at 2θ of 39°, 46°, 67° and 81°, which are attributed to the Pt
(111), (200), (220) and (311) crystalline planes, respectively of
face‐centered cubic (fcc) structure of Pt with a space group
Fm‐3m (JCPDS File No. 01‐088‐2343).57 The diffraction peaks
at 2θ of 31.47°, 44.24° in Co(3)/C sample indicates the
presence of Co in the form of face‐centered cubic Co3O4 with a
space group Fd‐3m (JCPDS File No. 09‐0418) and peaks at
36.58°, 42.43° specify the occurrence of cubic CoO species
with space group Fm‐3m (JCPDS File No. 09‐0402).58 From XRD
results, it is suggested that Co is present in the mixed oxide
form namely, CoO and Co3O4 with Co in (II) and (III) oxidation
states. In case of diffraction pattern of bimetallic catalyst,
Pt(3)Co(3)/C, absence of peak for Pt(111) and appearance of
new peak at 41.60° discloses the formation of alloy59 with
foremost contribution from tetragonal CoPt species with a
space group P4/mmm (JCPDS File No. 29‐0498). A weak peak
in this sample at 40.5° is attributed to the formation of trace
quantity of cubic CoPt3 species with a space group Pm‐3m
(JCPDS File No. 29‐0499). The non‐appearance of peak(s) for
any of the Co oxide species in bimetallic Pt(3)Co(3)/C catalyst is
because mostly Co is located in the proximity of Pt and is
engaged in the formation of alloy.59 It is also postulated that
due to overall low intensity of peaks and overlapping of many
peaks in the same 2θ range, it is probable that many of the
peaks are masked. Additionally, it is feasible that Co might be
present in the amorphous form and thus is not detected.
Literature also claims that in a CoPt3 alloy system with an
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DOI: 10.1039/C6GC03143A
o
increase in temperature (>500 C) under reducing condition,
sufficient energy to reduce Co is received.60 In this work,
catalysts are reduced at 400 oC and thus it is possible that Co is
present in ionic form. Thus, from XRD analysis it is suggested
that Pt may assist reduce Co to form Co(0) species unlike in
monometallic Co(3)/C catalyst or alloy is formed with electron
rich Pt and electron deficient Co. Based on these postulations,
XRD analysis of bimetallic catalysts indicates that the Co and Pt
interact with each other which may influence the geometric
and/or electronic properties of metals. The diffraction peak at
26.51° (002) in carbon is assigned to the hexagonal graphitic
carbon (JCPDS File No. 41‐1487) and the peak at 44.64° (111) is
assigned to cubic carbon (JCPDS File No. 80‐0017).61
The oxidation states of metals in mono and bimetallic
catalysts were confirmed by XPS studies (Figure 2). In the XPS
spectrum of Co(3)/C catalyst sample, lack of peak for Co metal
at binding energy (B.E) of 778‐779 eV and observance of peaks
for Co 2p1/2 (B.E.=796.2 eV) and Co 2p3/2 (B.E.=781.2 eV) are
attributed to the formation of CoO and Co3O4 species. This
inferred that in monometallic Co(3)/C catalyst, Co is present in
higher (II, III) oxidation states. When monometallic Pt(3)/C
catalyst was examined by XPS, strong peaks for 4f5/2 (74.5 eV)
and 4f7/2 (71.1 eV) levels were observed, which signifies that
the majority of Pt is present as metal [Pt(0)]. However, weak
peaks for 4f5/2 (76 eV) and 4f7/2 (72.2eV) level also shows that
small quantity of Pt is present as Pt(II). It can be said that the Figure 2. XPS spectra of Pt, 4f and Co 2p level (a) Co(3)/C, (b) Pt(3)Co(3)/C, (c)
observations made with XPS analysis for monometallic catalyst
Pt(3)/C, (d) Pt(3)Co(3)/C
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are in line with XRD results. In the XPS spectrum for bimetallic
Pt(3)Co(3)/C catalyst, characteristic peaks for Co (II, III) species each other.
were observed to be retained at 781.2 eV (2p3/2) and 796.7 eV
Journal Name
71‐75
These observations prove that Pt and Co interact with
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DOI: 10.1039/C6GC03143A
Hydrogenation of FAL to FOL
(2p1/2), which confirms that even in the presence of Pt; Co is
not reduced completely.62 The peaks for Pt are shifted to lower
binding energy (74.2 eV for 4f5/2 and 71 eV for 4f7/2) in
bimetallic catalyst compared to monometallic Pt(3)/C catalyst.
In bimetallic catalyst also weak peaks for Pt(II) species were
observed (Figure 2). According to the electronegativity of the
elements, Co (1.88) and Pt (2.28), the charge transfer from Co
to Pt is expected which means that Pt can attract electrons
from Co and thus latter remains in II and III state.63 Generally,
with an increase in electron density on metal, decrease in B.E.
is observed. Considering this, our XPS results corroborate the
fact that Pt becomes electron rich in bimetallic catalyst than in
monometallic catalyst. It is also known fact that the core level
B.E. shift, as for the noble metals, could be relatively weak (< 1
eV).63‐65 Thus from XRD and XPS results it is suggested that
both, electron rich Pt and electron deficient Co are present in
close proximity to each other in case of bimetallic Pt(3)Co(3)/C
Catalysis at 100 °C, 1 MPa H2 pressure
As shown in Figure 3, when FAL hydrogenation reaction was
carried out in iso‐propyl alcohol (IPA) at 100 C under 1 MPa
°
hydrogen pressure with monometallic Pt(3)/C catalyst, 34%
FAL conversion with 33% yield of FOL was observed. However,
with the employment of bimetallic catalyst, Pt(3)Co(3)/C;
substantial increase in FAL conversion (100%) and FOL yield
(100%) was seen. This suggests that with bimetallic catalysts,
good carbon balance was possible to achieve. To check
whether this enhancement in the activity was due to Co alone,
reaction was performed with monometallic Co(3)/C catalyst
and we as expected observed only 3% FOL yield however with
12% conversion of FAL. This suggests that Co alone gives lower
yields than bimetallic catalyst and that particle size of all the
catalysts is roughly similar, it is understood that Pt and Co
metals in a synergetic way facilitate the enhancement of the
activity. In the literature it is acknowledged that in case of
bimetallic catalysts, both the factors; geometric and electronic
participate is deciding the activity of a bimetallic catalyst43 and
accordingly it is assumed that in case of Pt(3)Co(3)/C catalyst
either both of these factors or either of these factors must be
playing a decisive role in deciding the activity. From the XRD
analysis of bimetallic catalyst it is revealed that both Pt and Co
are present in close proximity with each other and from XPS
study it is specified that while Pt is predominantly present in
(0) oxidation state, Co is present in (II, III) oxidation states in
o
catalyst, reduced at 400 C. This proximity can be explained
based on either metallic Co is diluting the Pt particle or by
decoration of Pt surface by Co in higher oxidation state. The
likelihood of decoration is more predominant since XPS results
show presence of Co in higher oxidation states. The carbon
used in this study as a support has a very high BET surface area
(753 m2/g) and it is evident from Table S2 (ESI) that with
increase in metal loading, surface area decreases.
Further, based on the TEM images of Co(3)/C catalyst
(Figure S1, ESI), it can be commented that Co particles (8‐17
nm) are well dispersed on the support. The particle size
distribution and EDX spectra for Pt(3)Co(3)/C, Pt(3)/C and
Co(3)/C catalysts are presented in Figure S1 (ESI). From this, it
can be suggested that in bimetallic catalyst, metal particle size
is average of Pt and Co catalysts.
The temperature program reduction (TPR) study was
carried out to understand the reducible species of Pt and Co
present in mono and bimetallic catalysts (Figure S2, ESI). In all
the samples, a broad peak in the range of 343‐653 °C is
observed which is attributed to the reduction of oxides
present on carbon and burning of carbon (CH4).66 As seen from
TPR profile of Pt(3)/C sample, low temperature peak (180 °C) is
68
observed for the reduction of PtO species.67,
In the TPR
profile of Co(3)/C catalyst, an intense peak with peak maxima
at 306 °C corresponding to reduction of Co (Co2+/3+) oxide
species was seen.69, 70 However in the TPR profile of bimetallic
Pt(3)Co(3)/C catalyst, small broad peak at 265‐388 °C was seen
for reduction of Co. The lower intensity of this peak compared
to monometallic Co(3)/C catalyst is because, in Co‐Pt system,
core is predominantly made up of Co and thus Co is not
available for reduction. In Pt(3)Co(3)/C sample, major peak
with high intensity was observed at 206 °C for reduction of Pt
and Co. It is known that when Pt and Co are present in the
system generally, Pt forms a shell and the peak for reduction
of Pt shifts towards higher temperature. This is probably
because of presence of unreduced Co as the core of particle.68,
Figure 3. Effect of mono and bimetallic catalysts on the hydrogenation of FAL to FOL.
Reaction condition: FAL, 0.35 g; catalyst, 0.078 g; iso‐propyl alcohol, 35 mL; 100 °C; 5h;
H2, 1 MPa at room temperature; 900 rpm. FAL: Furfural, FOL: Furfuryl alcohol.
the Pt(3)Co(3)/C catalyst. Hence it is believed that electron
deficient Co may act as Lewis acid site for the adsorption of
carbonyl group through oxygen atom (electron pair) of
substrate, which essentially polarizes the C=O bond. At the
same time, electron rich Pt(0) species would promote the
heterolytic dissociation of hydrogen molecules and thus
hydrogenation of the carbonyl group is facilitated.
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Additionally, in monometallic Pt(3)/C catalyst, as known from reaction was carried out for 10 h and with bimetallic,
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the XRD and XPS studies, Pt is predominantly present in Pt(3)Co(3)/C catalyst high yield (91%) was observed. The gas
metallic (0) state and hence is electron rich however, due to phase of reactions was analyzed by GC‐TCD but no detectable
this, it is difficult for FAL to interact with Pt and eventually peak for any gaseous products was observed. As Pt(3)Co(3)/C
o
adsorb on the Pt surface. When under similar conditions, catalyst showed very high activity even at 50 C with 1 MPa
reaction was carried out in the absence of a catalyst, <2% FAL hydrogen pressure, it was decided to carry out reactions at
conversion along with <2% FOL yield was seen. To understand lower hydrogen pressure. The results imply that the activity of
the influence of carbon on the catalytic activity, reaction was the catalyst is proportional to the hydrogen pressure (Figure
carried out with only carbon and we observed 8% FAL S5, ESI).26 Generally, with increased pressures, solubility of
conversion with 2% FOL yield. The slight increase in FAL hydrogen is increased in solvents and it was believed that this
conversion with carbon as a catalyst compared to non‐catalytic increase in solubility might play a role in enhancing the
reaction was because of adsorption of 4% FAL on carbon activity.76 However, to check the effect of overall pressure on
(Table S3, ESI) (checked by adsorption study carried out at the activity, reaction was carried out with 1 MPa N2 pressure
room temperature) as well as formation of FAL condensation (instead of hydrogen) and mere 10% conversion (10 h) without
product on carbon surface in absence of active metal site as it formation of FOL was observed as compared with 92%
is known from literature that FAL is unstable and undergo conversion obtained with 1 MPa hydrogen. Further to check
polymerisation reaction.10
the role of IPA at higher temperature, reactions were carried
To avoid the use of higher Pt concentration in the out at 100 and 150 °C with 1 MPa N2 pressure for 5 h. When
bimetallic catalyst, series of catalysts with different Pt loadings reaction was done at 100 °C, 7% FAL conversion with no FOL
(0.5‐3 wt%) by keeping Co concentration same (3 wt%) were formation was achieved. However when reaction was
synthesized. Amongst all these catalysts, when reactions were performed at 150 °C, 8% FAL conversion with 8% FOL yield was
conducted in presence of IPA at 100 oC under 1 MPa hydrogen seen. The conversion at 100 °C could be due to the undesired
pressure for 5 h, Pt(3)Co(3)/C catalyst showed maximum side reaction in absence of H2. It is also reported that at 150 °C
activity 100% yield for FOL) with turnover number (TON) of IPA can act as proton donor.24 This outcome implied that
2582 (Figure S3, ESI). However, under similar conditions with under our reaction conditions (<100oC), i) instead of total
Pt(2)Co(3)/C catalyst also near maximum FOL yield (92%) was pressure, hydrogen pressure is essential and ii) source of
obtained with TON of 4275. The higher TON with Pt(2)Co(3)/C hydrogen is hydrogen gas and not the solvent. However, when
catalyst indicates that it is more efficient catalyst than the reaction was carried out at 0.1 MPa hydrogen pressure
Pt(3)Co(3)/C. When reactions were performed with lower with increased catalyst concentration (S/C = 2.12 wt/wt),
loading of Pt (0.5 and 1 wt%), lower selectivity for FOL was improvement in the FOL yield to 57% was seen compared with
observed as found with Co(3)/C catalyst (Figure S3, ESI, Figure 12% obtained with lower (S/C = 4.35 wt/wt) catalyst
3). This is because with the decrease in Pt concentration, Co concentration at 0.1 MPa (Figure S6, ESI). After obtaining these
catalyzed side reactions are predominant (Figure 3). Next, encouraging results with 0.1 MPa hydrogen pressure, reaction
reactions were also performed with the catalysts having was carried out for longer time (15 h) instead of 10 h and 80%
varying concentrations of Co (0.25, 0.50, 1, 2, 3, 4, 5 wt%) by FOL yield was achieved. Nevertheless, further increase in
keeping Pt concentration constant (3 wt%) as shown in Figure reaction time (20 h) did not yield much improvement in FOL
S4 (ESI). In all these catalysts, Pt(3)Co(3)/C catalyst showed yield (84%). It is known that alcohols are used as proton
o
best activity (100% FOL yield) and all other catalysts showed donating solvents at higher temperatures (typically >150 C)
lower yields in the range of 35‐90%. With higher Co and thus when reactions were carried out at 50 °C in this work
concentration of 5 wt%, selectivity for FOL was decreased to using IPA, it was apparent that IPA does not contribute any
90% because with excess of Co in catalyst, initiation of side protons which means that solvent is not contributing (as
reactions of FAL is possible as evident from monometallic catalytic transfer hydrogenation) towards the reaction at 50
Co(3)/C catalyst result (Figure 3). Moreover, with higher °C. Considering this, it was decided to study the effect of
concentration of Co, burial of active Pt sites is possible as it is solvent on the reaction. The reactions were performed at 50 °C
suggested that Co may decorate Pt particles. The analogous for 1 h under 1 MPa hydrogen pressure with different solvents.
observation is as well made when Pt concentration was As shown in Table 1, compared to any organic solvent, with
altered. Thus, from these results it is inferred that Pt(3)Co(3)/C water as a solvent highest FOL yield (95%) with (95%)
catalyst is the best catalyst to achieve highest possible FOL conversion of FAL was achieved within 1 h (Entry 8, Table 1).
yield and the second best catalyst is Pt(2)Co(3)/C in terms of Primarily, this difference in activity is believed to be associated
TON.
with the various properties of solvents such as polarity,
Hansen solubility parameter(HSP), reactivity of FAL with
solvent and capacity to dissolve hydrogen (Table S4, ESI).
Catalysis at 50 °C, 1 MPa H2 pressure
Since, selective formation of FOL was achieved at 100 oC under Previously, it is reported that, owing to the preferential
1 MPa hydrogen pressure in presence of IPA as a solvent adsorption of water compared to FAL on the catalyst surface of
within 5 h reaction time; reaction was performed at 50 oC for 5 (hydrophilic) SiO2 supported Cu catalyst, lower conversions are
h under 1 MPa hydrogen pressure. In this reaction, 52% FOL possible.77 It is also demonstrated that due to strong
yield was achieved with 100% selectivity. Considering this, next adsorption of FAL on PtSn/SiO2 catalyst in non ‐polar solvents,
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decrease in the activity of catalyst is possible.43 In this study, δ/MPa and FOL has HSP value of 25.6 δ/MPa. A careful look at
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owing to presence of hydrophobic carbon as a support it was HSP values for all the solvents divulge the fact that water is
feasible to prevent the adsorption of polar solvents (water) on superior solvent than organic solvents for carrying out this
catalyst and this ultimately prevents the poisoning of catalyst reaction. Furthermore, amongst all organic solvents, methanol
by water and allows FAL to adsorb and interact with the is better solvent to achieve good yields compared with other
catalytically active sites on the catalyst surface.
solvents due to its higher HSP value (29.7 δ/MPa) compared
with FAL (22.9 δ/MPa) and FOL (25.6 δ/MPa) and other
organic solvents.82, 83
Catalysis at 35 oC, 1 MPa, water as solvent and catalyst recycle
study
Table 1. Solvent study for hydrogenation of FAL to FOL.
Entry
Solvent
FAL conversion
FOL yield
When reaction was carried out in IPA for 10 h at 50 °C under 1
MPa pressure with Pt(3)Co(3)/C catalyst, 91% yield of FOL was
observed (Entry 2, Table 2) and hence further reaction was
performed at 35 oC for 10 h under 1 MPa hydrogen pressure.
In this reaction, only 41% FOL yield over Pt(3)Co(3)/C catalyst
was observed (Entry 3, Table 2). Because, earlier it was seen
that compared with organic solvents, better results can be
obtained with water, same reaction was performed in water
no.
1
2
3
4
5
6
7
8
(%)
60
48
18
17
17
17
7
(%)
33
40
16
15
13
11
6
Methanol
Ethanol
1‐Propanol
2‐Propanol
1‐Butanol
2‐Butanol
Toluene
Water
95
95
Reaction condition: FAL, 0.35 g; Pt(3)Co(3)/C, 0.078 g; solvent, 35 mL; 50 °C, 1 h;
H2, 1 MPa at room temperature; 900 rpm. FAL: Furfural, FOL: Furfuryl alcohol.
Acetal formation is seen when methanol and ethanol are used as solvents.
Table 2. Hydrogenation of FAL to FOL under 1 MPa H2 in IPA and water solvents.
Entry no.
Time
(h)
5
10
10
FAL conversion (%)
FOL yield
(%)
The other reason for improvement of yield in water might be
due to increase in concentration of H2 in water since water has
a KH value of 7.45 x 10‐3 MPa, means it has a high capacity to
dissolve H2 than for instance compared to ethanol (KH = 0.44 x
1[a]
2[b]
3
100
92
43
100
91
41
100
4[c]
10
100
o
10‐3 MPa) when measured at 50 C.78, 79 All these factors are
Reaction condition: FAL, 0.35 g; Pt(3)Co(3)/C, 0.078 g; iso‐propyl alcohol, 35
mL; 35 °C; 10 h; H2, 1 MPa pressure at room temperature; 900 rpm; [a] 100 °C;
[b] 50 °C; [c] water 35 mL. FAL: Furfural, FOL: Furfuryl alcohol.
thus responsible for observing increased activity of a catalyst
in presence of water, toluene being the least polar solvent
amongst all, showed lowest activity. Moreover, various
alcohols (except methanol) having a polarity index between
3.9 to 4.3 showed almost similar activity. However it is well
reported in the litrature that during furfural hydrogenation
reaction in presence of methanol as a solvent, formation of 2‐
hydroxy, 1‐methoxy ethyl furan can be obtained and in
presence of ethanol 2‐furaldehyde diethyl acetal formation is
observed.28, 31, 80, 81 Similar results were observed in this work
when reactions are done in presence of methanol and ethanol.
In presence of methanol solvent the conversion of FAL was
60% but the FOL yield was only 33% (Entry 1, Table 1). This
lower selectivity for FOL is due to the formation of 2‐hydroxy,
1‐methoxy ethyl furan as side product as identified by GC‐MS
analysis (m/z, 142, 111, 95, 53). When reaction was carried out
in presence of ethanol, the conversion of FAL was 48% and
40% yield of FOL was achieved (Entry 2, Table 1). In this case
too, formation of side product, 2‐furaldehyde diethyl acetal
(m/z 170, 125, 97, 95) was seen. However, when reactions
were carried out in presence of propanol and butanol,
formation of acetal products was not observed. Also decrease
in selectivity for FOL with most organic solvents was due to
adsorption of FAL on catalyst surface as mentioned earlier. Yet
another property of solvents namely, Hansen Solubility
Parameter (HSP) also showed correlation with the activity. As
summarized in Table S4 (ESI), FAL has a HSP value of 22.9
and 100% FAL conversion with 100% FOL yield was achieved
(Entry 4, Table 2). Hence the reactions with Pt(3)/C, Co(3)/C
and Pt(3)Co(3)/C catalysts were carried out at 35 °C for 10 min.
in water and results are summarized in Figure S7 (ESI). It was
observed that with Pt(3)/C catalyst, 10% FAL conversion with
7% yield of FOL was obtained (161.3 min‐1 TOF), but Co(3)/C
Table 3. Hydrogenation of FAL to FOL in water solvent.
Entry no.
Catalyst
FAL conversion
(%)
FOL yield
(%)
1
2
3
5
6
7
8
Pt(2)/C
22
52
100
47
100
100
100
16
48
95
40
95
Pt(2)Co(0.25)/C
Pt(2)Co(1)/C
Pt(1)Co(3)/C
Pt(2)Co(3)/C
Pt(2)Co(5)/C
Pt(3)Co(3)/C
95
100
Reaction condition: FAL, 0.35 g; catalyst, 0.078 g; water, 35 mL; 35 °C; 10 h; H2,
1 MPa at room temperature; 900 rpm. FAL: Furfural, FOL: Furfuryl alcohol.
6 | J. Name., 2012, 00, 1‐3
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catalyst showed only 1% conversion. On the other hand, done at 35 °C, the 81% conversion with 100% seleVciteiwvAitryticolefOFnOlinLe
DOI: 10.1039/C6GC03143A
bimetallic catalyst showed significant increase in yield of FOL could be achieved (Entry 1, Table 4). However reaction carried
(27%) with 28% conversion of FAL (TOF to 753.1 min‐1). Since out at 50 °C showed 95% conversion with 100% selectivity
o
at 35 C under 1 MPa pressure, better results were obtained, (Entry 2, Table 4) and at 100 °C, complete conversion with
subsequently, all the mono and bimetallic catalysts were 100% selectivity of FOL was observed (Entry 3, Table 4). This
evaluated under similar conditions and the results are shows that with an increase in temperature, rate of conversion
summarized in Table 3. As summarized in Table 3, with most of of FAL increases. This can be because of increase in solubility
o
the catalysts, good selectivity for FOL was achieved at 35 C. of FAL in water.
Earlier it is reported that when reactions are conducted at 25
oC over monometallic 10 wt% Pt/Al2O3 catalysts, very high H2
pressures (6 MPa) are required to achieve 100% selectivity for
Table 4. Effect of reaction temperature on the hydrogenation of FAL to FOL in
FOL formation (65% yield) though with only ca. 65% FAL
water as a reaction medium.
conversion.29 Compared to earlier result in this work, using
bimetallic PtCo catalyst with 3 wt% Pt loading, 100%
conversion of FAL with 100% selectivity to FOL under 1 MPa H2
pressure could be achieved. This indicates that use of
bimetallic catalysts decreases the H2 pressure to achieve high
yields of FOL.
Entry
no.
Temperature
( °C)
FAL
conversion
(%)
FOL yield (%)
1
2
3
35
50
81
95
81
95
100
100
100
Reaction condition: FAL, 0.35 g, Pt(3)Co(3)/C, 0.078 g; water, 35 mL; 1 h; H2,
1 MPa pressure at room temperature, 900 rpm. FAL: Furfural, FOL: Furfuryl
alcohol.
Catalysis at 35‐100 °C, 1 MPa, water as solvent with 5‐40 wt% FAL
Literature suggests that mostly these reactions are carried out
with <10 wt% FAL solutions, however as mentioned earlier,
industries would prefer to use concentrated substrate
solutions to achieve better economics. Considering this,
reactions were carried out with concentrated FAL solutions
and the results are summarized in Table 5. When the reaction
was carried out with 5 wt% FAL solution, 98% conversion with
98% yield of FOL was observed (Entry 1, Table 5). Later,
Figure 4. Recycle study of Pt(3)Co(3)/C catalyst with substrate to catalyst ratio
constant. Reaction condition: FAL, 1 wt%; S/C wt ratio, 4.45; water; 35 °C; 10h; H2, 1
MPa at room temperature; 900 rpm. FAL: Furfural, FOL: Furfuryl alcohol, S/C:
substrate/catalyst
Table 5. Effect of FAL concentration on the hydrogenation in water as a reaction
medium.
Entry
no.
FAL
(wt%)
Temperature Time
FAL
conversion
(%)
FOL yield
(%)
The catalyst recycle study was performed with Pt(3)Co(3)/C
catalyst after separating catalyst from reaction mixture by
centrifugation and again charging the wet catalyst (without
weighing) in the next run. The results indicated that after each
cycle, activity was dropped by ca. 10% mostly due to handling
loss of catalyst which changed the S/C ratio (Figure S8, ESI) and
also due to leaching of Co metal. However, catalyst was active
at least until 4th runs and showed >99% selectivity for FOL
formation. Later, catalyst recycle study was performed after
drying the recovered catalyst. Due to this, it was possible to
maintain the S/C ratio constant in all the reactions. The results
obtained in these reactions are presented in Figure 4 and as
seen, catalyst showed similar activity until 3rd run.
( °C)
(h)
1[a]
2
5
20
35
35
10
10
98
50
98
49
3
20
40
40
50
50
100
6
6
3
100
100
83
100
86
28
4
5
Reaction condition: FAL; Pt(3)Co(3)/C; Substrate/Catalyst ratio, 4.45 (wt/wt);
water, 35 mL; H2, 1 MPa pressure was maintained during the reaction by
charging H2 intermittently; 900 rpm; FAL: Furfural, FOL: Furfuryl alcohol. [a] H2,
1 MPa was charged at room temperature and during reaction no additional H2
was charged in the reactor.
reaction was carried out with 20 wt% FAL solution and 50%
conversion with 49% yield of FOL was achieved (Entry 2, Table
5). Since in this reaction, selectivity was good but conversion
was less, further reaction was carried out at 50 °C with 20 wt%
FAL solution for 6 h. In this reaction, 100% conversion with
Catalysis at 35‐100 °C, 1 MPa, water as solvent with 1 wt% FAL
solutions
In order to study the effect of temperature on the conversion
of FAL to FOL, reactions were carried out under different
temperatures for 1 h. The result shows that when reaction was
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100% FOL yield was obtained (Entry 3, Table 5). Further FOL was extracted from water using ethyl acetaVteie.w LAartitcelerOtnhlinise
13
1
reaction was carried out with 40 wt% FAL (8 g FAL in 20 mL recovered FOL was dissolved in CDCl3 aDnOdI:t1h0e.103C9/C&6GCH03N1M43RA
water) at 50 °C under 1 MPa H2 pressure for 6 h. In this spectra were recorded to check its purity. The chemical shift
reaction, 86% yield of FOL with 100% conversion of FAL was values observed in the NMR spectra (Figures S9 and S10, ESI)
observed. It is known from the literature that at 25 °C, the matched well with the literature.86, 87 These results reveal that
solubility of FAL in water is only 8% and that of FOL is infinite.84 FOL is obtained in pure form which again indicates that 100%
However, it is also reported that solubility of FAL increases to selectivity was obtained in these reactions. Further, in HPLC
infinite above 120 °C.85 Since, in this work reactions were profile (Figure S11, ESI) single peak for FOL was seen which
carried out at 50 °C, solubility study of 40 wt% FAL and FOL again indicates that FOL is obtained in pure form.
was carried out in water at room temperature and similar
Proposed reaction mechanism
results as that with literature could be seen (Table S4, ESI).
This may create a biphasic system in the reactor when From the study of FAL conversion to FOL in presence of
reactions are done with higher concentrations of FAL. bimetallic catalyst it is proposed that Pt(0) facilitate the
Considering this limitation of lower solubility of FAL in water, it heterolytic splitting of the H2 molecule into H+ and H‐. The
was thought that reactions can be done at higher temperature oxophilic Co(II) ion attracts the carbonyl oxygen and polarizes
to increase the solubility of FAL in water which would in turn the carbonyl group of FAL which increases the interaction
improve the yield and reaction rate. Hence, reaction was between catalyst and FAL. The proposed reaction mechanism
carried out at 100 °C for 3 h with 40 wt% FAL solution. It was is shown in Figure S12 (ESI).
observed that although conversion of FAL reached up to 83%
under reaction conditions but, only 28% yield of FOL could be
Characterization of spent catalyst
achieved. This suggests that when higher concentration Pt(3)Co(3)/C spent catalyst obtained from the reaction (35 °C,
solutions are used, it is better to carry out reactions at lower 0.1 MPa H2, 10 h, water) was characterized by XRD, XPS, TEM,
temperatures or else side reactions become predominant. ICP and BET surface area techniques to check the morphology.
These results (Entries 3 and 4, Table 5) also revealed that even XRD pattern of Pt(3)Co(3)/C spent catalyst as shown in Figure
if at lower reaction temperatures biphasic system is formed, it S13, (ESI) shows the peak for PtCo alloy species at the same
does not have any substantial effect on the product formation
position as observed in fresh catalyst. The defractogram of
fresh catalyst is matching with the spent catalyst. From the XPS
analysis (Figure S14, ESI) shift of 781 eV peak of Co (II) to 782
Catalysis at 35 oC, 0.1 MPa and water as solvent
The ultimate aim of carrying out any chemical reaction is to eV position for Co 2p level was seen, this might be due to
perform the reaction under ambient conditions and hence oxidation of Co in water medium, The deconvolution spectrum
reaction with optimized catalyst, Pt(3)Co(3)/C was carried out of Pt 4f level shows that major amount of Pt is still present in
in water as a solvent at 35 oC under 0.1 MPa hydrogen (0) oxidation state with small amount of Pt(II). TEM image of
pressure for 10 h and a very promising result of 100% FAL spent catalyst given in (Figure S15, ESI) shows well dispersion
conversion with 100% yield of FOL was obtained as shown in of particle with particle size distribution between 7 to 13 nm as
Table 6. In order to reduce the reaction time under lower H2 observed in fresh catalyst without sintering of particles. The
pressure (0.1 MPa H2) reaction was carried out at higher BET surface area of spent catalyst was found to be (635 m2/g)
temperature (100 °C) for 4 h and 100% conversion of FAL with which is similar to the fresh catalyst (630 m2/g). An ICP analysis
100% yield of FOL was achieved (Entry 4, Table 6). After the of spent catalyst showed absence of any leaching of Pt metal
reaction was carried out under optimized reaction condition however, slight loss of Co metal (1.94 ppm) was observed.
[Pt(3)Co(3)/C, 35 °C, 0.1 MPa hydrogen, 10 h, water] product, Almost similar amount of Co was quantified in the reaction
solution.
Table 6. Hydrogenation of FAL to FOL in water as a reaction medium and under
Experimental Section
ambient conditions (35 oC, 0.1 MPa H2 pressure).
Catalyst synthesis
Entry no.
Time (h)
FAL conversion
(%)
FOL yield (%)
Supported mono (Pt/C, Co/C) and bimetallic (PtCo/C) catalysts
were prepared by wet‐impregnation method. Before synthesis
of catalyst, carbon (s. d. fine chemicals, having surface area
753 m2/g) was activated at 150 °C for 6 h. In a typical
procedure, 1 g of activated carbon was immersed in 10 mL of
water and the mixture was kept under stirring for 30 min. To
this, 2.50 mL aqueous solution of Pt(NH3)4(NO3)2 (Alfa Aesar
99.99% purity) containing 30 mg of Pt was drop wise added
and the resultant mixture was stirred for 16 h at room
temperature. This is followed by evaporation of water at an
elevated temperature 60 °C using rotary evaporator. The
1
2[a]
3
5
5
65
83
63
72
10
4
100
100
100
100
4[b]
Reaction condition: FAL, 0.35 g; Pt(3)Co(3)/C, 0.078 g; water, 35 mL; 35 °C, H2,
0.1 MPa at room temperature, 900 rpm. [a] catalyst, 0.16 g; [b] 100 °C; FAL:
Furfural, FOL: Furfuryl alcohol.
8 | J. Name., 2012, 00, 1‐3
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powder obtained was dried at 60 °C in oven for 16 h followed °C/min) until desired temperature (35‐100 °C) is reached under
View Article Online
by vacuum drying at 150 °C for 6 h. Further this dry catalyst slow stirring (100 rpm). After attDaOinI:in10g.103th9/eC6GrCe0a3c1t4io3nA
powder was reduced in a flow of hydrogen (H2, 10 mL/min) for temperature, stirring was increased to 900 rpm and this time
2 h at 400 °C to obtain Pt(3)/C catalyst. A monometallic was considered as starting time of the reaction. Sample was
Co(3)/C catalyst was prepared using aqueous solution of withdrawn and quantification of furfural was done to ascertain
Co(NO3)2∙3H2O (Thomas Baker, 97% purity) following same the conversions and yields.
procedure as mentioned above. Bimetallic PtCo/C catalysts
containing 0.5, 1, 2 and 3 wt% and 2 wt% Pt and 0.25, 0.5, 1, 2,
Recycle experiments
3, 4, 5 wt% of Co were prepared by co‐impregnation method. After completion of a reaction, the catalyst (solid) was
A catalyst with 3 wt% Pt and 0.25 wt% Co loading was named recovered by centrifugation of reaction mixture. The
as, Pt(3)Co(0.25)/C. Similarly, all other catalysts were named recovered wet catalyst was used in the next reaction without
based on their loadings.
any pre‐treatment. Also in order to check the activity of
catalyst by maintaining substrate to catalyst ratio constant the
moisture of recovered wet catalyst was removed by drying at
Characterization of catalyst
The monometallic Pt/C, Co/C and bimetallic PtCo/C catalysts 60 °C for 12 h followed by 150 °C for 3 h after each recycle run.
were characterized by XRD, N2 sorption, TEM, ICP‐OES and The dried catalyst was used to carry out next reaction without
XPS, TPR techniques. XRD spectra were obtained using further treatment.
PANanalytical X’pert Pro, Netherlands; with duel goniometer
diffractor equipped with a Cu Kα (1.5418 Å) radiation source
Analysis of the reaction mixture
with Ni filter. Data were collected in the 2θ range of 5–90° After completion of reaction, reactor was allowed to cool to
with a step of 4°/min and 30‐50° with a step of 1°/min. N2 room temperature under flow of air. Later reactor was
physisorption experiments were done at ‐196 °C in a Autosorb immersed in ice bath for (30 min). The reaction mixture was
1C Quantachrome instrument, USA. Prior to analysis, samples collected and centrifuged to separate solid and liquid. The
were activated under vacuum at 200 °C for 3 h. The specific liquid was filtered through 0.22 µm syringe filter. Analysis of
surface area was determined by BET method and pore size was organic reaction mixture was done using gas chromatography
calculated by BJH method. The electronic states of metals (Varian GC) attached with FID for calculation of FAL
were analyzed by X‐ray photoelectron spectroscopy (XPS, conversion, and selectivity of FOL. The conditions used for GC
Perkin‐Elmer PH 15000C) having x‐ray source Al Kα radiation. analysis were as follows: HP‐5 capillary column (0.25 µm x 0.32
The mode of lens was LAXPS and Energy Step Size 0.1 eV. Peak mm x 30 m), a FID detector (30 mL/min N2 flow as a carrier
in the spectrum were deconvoluted by XPSPEAK‐41. All binding gas, 250 °C), and the oven temperature programme was as
energy values were calibrated using the value of carbon (C1S = follows: 50 °C (hold time 1 min) to 200 °C (hold time 10 min) at
284.4 eV) as a reference. TEM images were obtained using FEI the rate of 15 °C/min ramp rate. Injector temperature used
TECNAI T20 model instrument working at an accelerating was 200 °C. The GC‐MS (Agilent‐7890 GC and Agilent MS‐
voltage of 200 kV. Samples were dispersed in iso‐propyl 5977A MSD) equipped with DB‐MS column (0.25 µm X 0.25
alcohol (IPA) by sonication and the drop casted on carbon mm X 30 m) was used for the analysis. The temperature
coated copper grid. Metal contents in the catalysts were programs used were similar to that for GC‐FID. NIST library was
determined by ICP‐OES analysis using ‘SPECTRO ARCOS used for the identification of products. The HPLC (Agilent
Germany, FHS 12’ instrument. Samples were prepared by Technologies, 1200 infinity series, USA) equipped with an HC‐
calcining catalyst at 600 °C (5 °C/min ramping rate) for 6 h in 75 Pb2+ column (Hamilton, 7.8 mm × 300 mm, 80 °C) and
air atmosphere and solid residue after calcination was refractive index detector (Agilent Technologies, 1200 infinity
dissolved in aquaregia. All ICP‐OES samples were diluted with series, 40 °C) was used to analyze aqueous reaction mixture.
deionized water and filtered (through 0.22 μm syringe filter) HPLC grade water was used as an eluent, at a flow rate of 0.5
before analysis. The TPR studies were done by Micromeritics mL/min. FAL conversion and product FOL yields were
Autochem‐2920 instrument in the temperature range 50−800 calculated on mole basis using the following formulae.
°C in presence of 5% H2 in He with a ramping rate of 5 °C/min.
The catalyst was treated at 300 °C for 1 h in presence of He gas
before the TPR analysis. The measurement of H2 consumption
in the TPR analysis was done by a thermal conductivity
detector (TCD) instrument.
Catalytic runs
Hydrogenation of furfural (FAL) was performed in a 50 mL
stainless steel high pressure high temperature Amar make
reactor by charging 35 mL solvent, 0.35 g FAL and 0.078 g
catalyst. The reactor was flushed with H2 for four times to
remove any air. Then, the reactor was pressurised with H2 up
TOF an TON are calculated by using following formula
to desired pressure (0.1‐1 MPa) and was heated slowly (3
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DOI: 10.1039/C6GC03143A
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o
16.
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17.
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Acknowledgements
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Keywords: furfural • furfuryl alcohol • co-impregnation •
hydrogenation • aqueous phase
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Table of Content
Efficient hydrogenation of concentrated aqueous furfural solutions in to furfuryl alcohol under ambient
conditions in presence of PtCo bimetallic catalysts