Insights into the activity, selectivity and stability of heterogeneous catalysts in the continuous flow hydroconversion of furfural

Article abstract of DOI:10.1039/c6cy00249h

The continuous flow hydroconversion of furfural to a range of furanic derivatives was carried out using a range of metal-containing heterogeneous catalysts in order to provide insights into the reaction pathways for the hydrogenation of furfural. Most of the catalysts used provided excellent furfural conversion (>60%) and varying selectivities to products at mild temperatures (90 °C), including furfuryl alcohol (FA), 2-methylfuran (2-MF), 2-methyltetrahydrofuran (2-MTHF) and furanic intermediates, depending on the flow conditions and the investigated catalyst.

Full text of DOI:10.1039/c6cy00249h

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Insights into the activity, selectivity and stability of heterogeneous catalysts in the
continuous flow hydroconversion of furfural
Antonio J. GarciaꢀOlmoa, Alfonso Yepeza, Alina M. Balua, Antonio A. Romeroa,
Yingwei Lib, Rafael Luquea*
aDepartamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales,
Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 306, E14014, Cordoba (Spain), e-mail:
q62alsor@uco.es
bState Key Laboratory of Pulp and Paper Engineering, School of Chemistry and
Chemical Engineering, South China University of Technology, Guangzhou 510640,
China.
Abstract
The continuous flow hydroconversion of furfural to a range of furanic derivatives was
carried out using a range of metalꢀcontaining heterogeneous catalysts in order to provide
insights into reaction pathways for the hydrogenation of furfural. Most catalysts
provided excellent furfural conversion (>60%) and varying selectivities to products at
mild temperatures (90ºC) including furfuryl alcohol (FA) 2ꢀmethyl furan (2ꢀMF), 2ꢀ
methyl tetrahydrofuran (2ꢀMTHF) and intermediate furanics depending on the flow
conditions and the investigated catalyst.
Keywords: furfural conversion, flow chemistry, hydrogenation, heterogeneous catalysis

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Introduction
The complexity and recalcitrance of biomass (e.g. lignocellulosics) often entails
a number of challenges to address and solve to facilitate working with such highly
abundant and attractive feedstock in the future. Additional advantages of biomass as
resource for the production of high added value chemicals1 fuels and materials2, 3include
the presence of highly functionalised compounds containing O and N as well as the
potential use of known processing strategies (e.g. hydrodeoxygenation, fermentation,
etc.) for valorisation. To better understand pathways and reactivity of biomass, the
conversion of biomassꢀderived platform molecules to useful end products attracted a
significant deal of research efforts in the past few years. As example, the hemicellulosic
fraction of biomass is enriched in C5 sugars reported to be converted to an useful
platform chemical such as furfural via simple xylose dehydration under mild reaction
conditions4, 5
.
Furfural is a highly useful chemical intermediate that can be further transformed
into several valuable compounds including cyclopentanone (CPO), furfural alcohol
(FA), 2ꢀmethyltetrahydrofuran (2ꢀMTHF), and 2ꢀmethylfuran (2ꢀMF).6 Various reports
have recently investigated a number of homogeneous and heterogeneous catalytic
systems for the hydroconversion of furfural into different high added value chemicals
and fuel precursors7. However, without a few exceptions,8 these generally provided little
insights into reaction pathways and mechanisms and/or a comparison between different
commercial catalytic systems (i.e. noble vs transition metals; homogeneous vs
heterogeneous) to propose some general guidelines on furfural conversion and
selectivities as well as catalyst stability under continuous flow liquid phase conditions
to advance in this important topic in view of the future direct conversion of
hemicellulosic feedstocks to furanic compounds. From various contributions on

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mechanistic insights, results provided by Sithisa et al. on the conversion of furfural on
Pd/SiO2 and PdꢀCu/SiO2 catalysts most interestingly pointed to a preferential furfural
decarbonylation on Pd sites while the addition of Cu rendered a most active
hydrogenation catalyst for FA production with a low decarbonylation rate.8a
Nevertheless, most body of work in the field includes both batch and/or catalytic
transfer or vapor phase furfural hydroconversion processes at moderate to high
temperatures of low practicality and applicability.
In this sense, we have previously demonstrated that continuous flow processes
could provide several relevant advantages in studies related to the conversion of
biomassꢀplatform molecules to high added value chemicals (e.g. levulinic acid to
gammaꢀvalerolactone and 2ꢀMTHF)9. These included the possibility to follow reaction
pathways and even fineꢀtuning reactivity for a number of transition and noble metal
heterogeneous catalysts to direct the conversion of levulinic acid to pentanoic acid or
alternatively to 2ꢀMTHF.9 Most importantly, the stability of the catalysts under typical
biomass conversion conditions (i.e. aqueous solution, moderate to high temperatures,
presence of acid, etc.) could be tested as well as metal leaching into the reaction
mixture, one of the most common reasons for catalyst deactivation.10
Based on these premises, this contribution was aimed to provide a number of
insights into reactivity pathways, deactivation and stability of a number of
heterogeneous catalysts in the continuous flow liquid phase hydroconversion of furfural
at low temperatures (< 100oC). Different reaction conditions were screened and
optimised for which times of reaction, selectivity to products and catalyst stability were
tested in view of a potential translation of optimized results to the conversion of furanic
waste streams.

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Experimental
Reactions were conducted in an HꢀCube mini flow hydrogenation instrument
from ThalesNano (Figure 1). A 0.2 M furfural solution in ethyl acetate was prepared
using furfural (0.83 mL furfural in 50 mL ethyl acetate) as starting material. The
solution is filtered off prior to flowing it through the system to avoid the presence of
any undissolved species. The system starts with pumping a solution of pure methanol as
washing solvent (0.3 mL minꢀ1, 15 min) through system. Subsequently, methanol is
changed to the solvent used in the reaction, in this case ethyl acetate (0.3 mL minꢀ1,
10 min) and the system is set to the desired reaction temperature (90 ºC) and hydrogen
pressure (50 bar), as optimum reaction conditions. When the desired parameters are
achieved, a 0.2 M furfural solution is pumped through the system in order to carry out
the continuous flow hydroconversion of furfural. As the reaction is running, several
samples were taken at different times for subsequent analysis and check the catalyst
deactivation in the time under the investigated reaction conditions. At the end of the
continuous flow process, the system is again switched to ethyl acetate (0.3 mL minꢀ1,
10 min) in order to remove unreacted furfural species or different products of reaction
and the reaction parameters (temperature and pressure) are switched off. Eventually, the
system is washed again with methanol (0.3 mL minꢀ1, 20 min). Based on the dead
volume of the reactor, 20 min of reaction is equivalent to approximately 60 seconds (1
min) of residence time of the furfural solution in the catalyst.

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Figure 1. Flow hydroconversion of furfural using the HꢀCube mini system.
The continuous flow hydroconversion of furfural was conducted by passing hydrogen
generated in-situ in the HꢀCube equipment from water electrolysis and the flow solution
through a packed column containing different solid catalysts (CatCart®) as illustrated in
Figure 2.
Figure 2. Continuous flow conversion of furfural (starting material) to products over a
packed bed of heterogeneous catalyst in the HꢀCube mini.
A range of different heterogeneous catalysts were investigated, namely
commercial catalytic noble metal/type systems (5% and 10%Pd/C, 10%Pd/PBSAC,

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5%Ru/C and RuꢀSn/Al2O3) as well as related transition metal catalysts including,
Co/Raney, CuO/Al2O3 and a nickel on silicaꢀalumina (NiSiAl) catalyst.
10% PdꢀPBSAC was provided by SARATECH® Blucher technologies where
PBSAC stands for SATARECH Polymer based Spherical Activated Carbon
(monodispersed spheres, 75 µm average diameter, >95% carbon content, with average
Pd particle sizes < 5 nm). 5 and 10% Pd/C materials as well as 5% Ru/C were obtained
from Alfa Aesar. 5% Pd/C contained 4.93 wt% Pd with a metal area of 20 m2 gꢀ1, while
10% Pd/C contained a 9.87 wt% Pd and a similar 18 m2 gꢀ1 metal area as compared to
5% Ru/C containing a 4.85 wt% Ru. RuꢀSn on alumina (RuꢀSn/Al2O3) powder was a
development catalyst from Johnson Mathey containing ca. 5% Ru and Sn low quantities
of Sn as promoter.
Co/Raney corresponds to Sponge Cobalt A8B46 promoted with Nickel and
Chromium catalyst from Alfa Aesar, containing 1.95% Cr, 2.42% Al, 3.79% Ni and
91.3% Co as measured by ICP (black solid). Cu (II) oxide on alumina is commercial
417971 product (CAS 1317ꢀ38ꢀ0) from Sigma Aldrich, containing ca. 9ꢀ11 wt% Cu,
similar to that of 10%Pd systems. Nickel on silicaꢀalumina catalyst (Alfa Aesar)
contained a 63.3% Ni, with a 41.6% nickel reduction. Comparatively, Ni/Raney was
also commercial from Alfa Aesar with a 92% Ni.
All catalysts were packed by ThalesNano (weight of material packed ca. 0.2 g
per cartdrige) in 30 mm long catcarts compatible with the HꢀCube system. Optimised
reaction conditions included 90ºC, 50 bar hydrogen, flow rate 0.3 mL minꢀ1, 0.2 M
furfural solution in ethyl acetate. The furfural solution (brownish) gradually turns into
colourless in the course of the reaction.

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Collected samples were analyzed by GC and GC MS using an Agilent 5890N
fitted with a SUPELCO EQUITY TMꢀ1 fused silica capillary column (60 m × 0.25 mm
× 0.25 µm) and a flame ionisation detector (FID). The identity of the products was
confirmed by GC MS at the SCAI of Universidad de Cordoba. The experimental error
in all data provided is within typical GC errors (ca. 5ꢀ10%) and carbon balances of all
reactions conducted in this work are > 95%.
Results and discussion
Continuous flow hydrogenations were conducted under optimised reaction
conditions for the selective catalytic systems. These reaction conditions were selected as
generally moderate to low conversions were obtained at lower temperatures (<90ºC),
higher flow rates (>0.5 mL minꢀ1) and/or reduced hydrogen pressures (below
30ꢀ40 bar). Figure 3 demonstrates that good to excellent catalytic activities were
achieved for all systems at 90oC and very short residence times (typically 20 min
reaction, residence time ca. 1 min). Most interestingly, the continuous flow
methodology could provide relevant insights into the activity, selectivity and stability of
the different investigated systems.
To begin with, a remarkable difference in terms of activity was observed for
noble metal systems as compared to that of transition metal catalysts (Figure 3). As
expected, all transition metal systems could provide maximum furfural conversions of
ca. 60ꢀ70%, with a very low catalytic activity observed for CuO/Al2O3. Comparably,
quantitative conversion (>97%) was obtained for Pd and Ru systems with the exception
of RuꢀSn/Al2O3 which will be discussed separately. Generally speaking, selectivities
ranged from good to excellent to a single product which varied depending on the

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catalyst. Four main products were exclusively observed in all catalytic runs under the
investigated conditions, namely furfuryl alcohol (FA), 2ꢀmethylfuran (2ꢀMF), 2ꢀMTHF
and pentꢀ3ꢀenꢀ1ꢀol (PENTENOL), an intermediate from 2ꢀMF hydroconversion
(Scheme 1). The reaction pathways identified for the different catalytic systems in the
continuous flow hydroconversion of furfural under the investigated conditions are
highlighted in Scheme 1.
Starting from noble metal systems, a generally high FA selectivity was observed
for 5%Ru/C catalysts, in good agreement with previous literature reports.11 Further
hydroconverted products were observed in low selectivities in the reaction using these
catalysts including 2ꢀMF and even 2ꢀMTHF. Interestingly, Pd/C systems provided a
remarkably different product selectivity under the investigated conditions, with 2ꢀMF
and interestingly pentꢀ3ꢀenꢀ1ꢀol (only for high loaded 10%Pd/C) as major products at
quantitative conversion. Comparably, FA was mostly obtained for 10%Pd/PBSAC, with
some quantities of linear enols as major side products.

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Figure 3. Comparative performance of noble metal (top plot) and transition metal
(bottom plot) heterogeneous catalysts in the conversion of furfural under continuous
flow. Reaction conditions: 0.2 M furfural in ethyl acetate, 90oC, 50 bar hydrogen,
HꢀCubeꢀmini, 0.3 mL minꢀ1 flow rate, 0.2 g packed catalyst, 20 min on stream (ca.
1 min residence time).
With regards to Pd catalysts, a strong adsorption of furfural on Pd surfaces has
been previously 8a. Such adsorption normally takes place through the aldehyde group
which leads to a stepwise hydrogenation/hydrogenolytic conversion of furfural into FA
and eventually 2ꢀMF, proposed to be related to an interaction of Pd with both C and O
atoms of the carbonyl group which generate stable acyl surface species as precursor of
decarbonylation.8a Our results are in good agreement with such data despite the
relatively low temperature of our studies. The presence of the methyl group in 2ꢀMF
may favor cleavage through the CꢀO bond giving rise to linear dienes as illustrated in
Scheme 1.

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Scheme 1. Reaction products and intermediates obtained in the continuous flow
hydroconversion of furfural using heterogeneous catalysts.
Most importantly, the nature of Pd species found in both the fresh and used Pd/C
commercial catalysts seem to support these findings as shown in Figure 4. XPS studies
of fresh Pd/C (both 5 and 10%) indicated a majority of Pd (II) content in the materials,
with a minor proportion (<20%) of Pd (0) as depicted in Figure 4, top spectra. The
presence of Pd (II) species can promote hydrogenolytic cleavage at low temperatures12
(FA to 2ꢀMF and further to pentenols), with the comparably low Pd (0) species
12
promoting the subsequent partial hydrogenation of the diene upon formation.8,
.
Interestingly, hydroxyꢀcontaining dienes were also detected in traces in all Pd
containing systems (also for 5%Pd/C) which seem to correlate well with the proposed
reaction mechanism and current findings. To the best of our knowledge, this is the first
example of a Pdꢀcatalysed flow hydroconversion of furfural able to provide quantitative
yields of pentenols (>99% conversion, >95% selectivity) under mild reaction
temperatures.

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Pd3d
Pd (II)
Pd (II)
Pd (0)
Pd (0)
Pd (0)
Pd (0)
Pd (II)
Pd (II)
360
355
350
345
340
335
330
325
Binding Energy (eV)
Figure 4. XPS data of 5%Pd/C fresh (before reaction, top spectra) vs 5%Pd/C used
(after 2 h on stream, bottom spectra). Clear differences in Pd (II) vs Pd (0) species can
be observed for both catalysts.
An interesting behavior was also observed for 5%Pd/C with time on stream. A
slight deactivation (>99% to 77ꢀ80%) was observed with a remarkable change in
selectivity from 2ꢀMF as main product to FA (Figure 5). The deactivation may be due to
some nanoparticle sintering as clearly visualized for the case of 5% Pd/C (Figure 6)
since negligible Pd leaching (ca. 2% of total Pd content) as measured by XPS (surface
Pd) and repeated ICPꢀMS could be observed in the material with time on stream.
Surface areas did not significantly change also in fresh vs deactivated catalysts.
Remarkably, the change in selectivity was in fact due to a change in the oxidation state
of Pd species with time on stream (Figure 4, bottom spectra). XPS data clearly pointed
out that the high hydrogen pressures employed under optimum conditions (50 bar) lead

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to a gradual reduction of Pd (II) species to Pd (0), with a majority of Pd (0) present in
5%Pd/C after 2ꢀ4 h on stream (Figure 4 bottom vs Fig 4 top spectra). In the used
5%Pd/C system, Pd (0) species favoured the hydrogenation of the carbonyl group to FA
at low temperatures, supporting the reduction in the hydrogenolytic capacity (FA to 2ꢀ
MF) of the system due to a reduction of Pd (II) to Pd (0) with time of stream under the
investigated flow conditions. A similar behavior was also observed for some transition
metal systems and even for Ru catalysts, although this selectivity switch was not so
evident.
Figure 5. Selectivity evolution with time of reaction for 5%Pd/C in the continuous flow
hydroconversion of furfural. Reaction conditions: 0.2 M furfural in ethyl acetate, 90oC,
50 bar hydrogen, HꢀCubeꢀmini, 0.3 mL minꢀ1 flow rate, 0.2 g packed 5% Pd/C; >99%
conversion (20 min reaction, ca. 1 min residence time); 77% conv. (60 min reaction, ca.
3 min residence time); 80% conv. (120 min reaction, ca. 6 min residence time).

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Figure 6. TEM images of 5%Pd/C fresh (top images) and used (after 2 h on stream,
bottom images). The presence of large Pd aggregates (>30 nm) can be visualised in the
used catalyst, with an also large increase in particle size diameter (average ca.15 nm in
size, counting 50 nanoparticles) as compared to the fresh 5%Pd/C (average ca.13 nm in
size, counting 50 nanoparticles).

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However, Pd (II) to Pd (0) reduction also evidenced in the case of 10%Pd/C
(results not shown) was not driven to a major Pd (0) presence in such commercial
catalysts, perhaps due to the higher Pd content, leaving the right balance between Pd (II)
species to promote hydrogenolytic cleavage of FA and 2ꢀMF to pentenols and Pd (0)
species to promote the subsequent hydrogenation steps.
Last but not least, the stability of the catalysts was investigated under the
optimized reaction conditions, pointing to very different trends between noble and
transition metal catalysts (Figures 7 to 9). All Pd systems were highly stable under the
investigated reaction conditions, with only minor leaching (maximum 5 ppm, 1ꢀ5% of
total Pd content) observed for all catalysts including 5%Pd/C for which a slight decrease
in activity was observed (>99 to 80%, Figure 7). The conversion for 5% Ru/C slightly
decreased with time on stream (from >98 to 85%, due to metal sintering as in the case
of Pd) but no significant leaching was also detected.
Figure 7. Evolution of conversion of noble metalꢀbased catalyst with time on stream.
Reaction conditions: 0.2 M furfural in ethyl acetate, 90oC, 50 bar hydrogen,

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HꢀCubeꢀmini, 0.3 mL minꢀ1 flow rate, 0.2 g packed catalyst. 20 min time on stream is
equivalent to ca. 1 min residence time, hence residence times ranged from 1 to 12
minutes.
Comparably, all transition metal systems significantly deactivated under the
investigated reaction conditions, with the exception of CuO/Al2O3 which in any case
provided low conversion (15ꢀ20%) in the flow experiments. No further
leaching/stability studies were conducted with such catalysts due to their low stability
under the investigated continuous flow conditions but preliminary tests indicated a
combination of both metal sintering and leaching. High selectivities to FA were
obtained for all transition metal catalysts regardless of deactivation issues. Of interest
was the high selectivity of CuO/Al2O3 to 2ꢀMF and especially 2ꢀMTHF in time despite
the mild reaction conditions (Figure 7). These findings were in good agreement with
previous work of the group in the hydroconversion of levulinic acid for which
CuO/Al2O3 provided excellent activities and selectivity to 2ꢀMTHF.9 A further
investigation on the Cu catalyst indicated the presence of Cu (II) species as pointed out
in XPS experiments might be responsible of the observed behavior (results not shown)
but further investigations are needed in this point. Literature data seem to contradict
these findings, with CꢀO hydrogenolysis dominating at high temperatures (due to high
activation required) as opposed to furfural hydrogenation taking place at low
13
temperatures.8a,
No further studies were performed to ascertain such different
behavior due to the low conversion and stability observed for Cu systems under
continuous flow liquid phase conditions.

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Figure 8. Evolution of conversion of transition metalꢀbased catalyst with time of
reaction. Reaction conditions: 0.2 M furfural in ethyl acetate, 90oC, 50 bar hydrogen,
HꢀCubeꢀmini, 0.3 mL minꢀ1 flow rate, 0.2 g packed catalyst. 20 min time on stream is
equivalent to ca. 1 min residence time, hence residence times ranged from 1 to 12
minutes.

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Figure 9. Evolution of selectivity of CuO/Al2O3 with time on stream in the
hydroconversion of furfural. Reaction conditions: 0.2 M furfural in ethyl acetate, 90oC,
50 bar hydrogen, HꢀCubeꢀmini, 0.3 mL minꢀ1 flow rate, 0.2 g packed CuO/Al2O3. 20
min time on stream is equivalent to ca. 1 min residence time, hence residence times
ranged from 1 to 12 minutes.
Conclusions
The proposed studies clearly demonstrated the significance and potential of continuous
flow processes for the conversion of platform molecules to useful chemicals, providing
the additional possibility to find out stable and highly active heterogeneous catalytic
systems for further work with actual biomass feedstocks (i.e. hemicelluloses). Pd
catalysts provided optimum results in the flow conversion of furfural to 2ꢀMF and even
pentenols, with promising insights into mechanistic aspects and deactivation pathways
depending on Pd loading (5ꢀ10%) and essentially oxidation states of Pd species in the
catalyst, with respect to 5%Ru/C as optimum alternative for high yields of FA.
Comparably, transition metal catalysts were proved of low significance in the flow
hydroconversion of furfural under the investigated conditions, with moderate yields and
varying selectivity to products (mostly to FA) and prone to deactivation with time on
stream. We envisage the current findings to be able to pave the way to the design of
more stable designer catalytic systems for the continuous flow hydroconversion of other
platform chemicals (e.g. succinic and lactic acids, sorbitol) which will be reported in
due course.
Acknowledgments

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R.L. gratefully acknowledges support from Consejeria de Ciencia e Innovacion, Junta
de Andalucia under project P10ꢀFQMꢀ6711. R.L. is also very grateful and thanks deeply
both reviewers of this manuscript for their time spent in their reviews and most valuable
comments to improve the quality of this work.
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