Palladium catalyzed hydrogenation of biomass derived halogenated furfurals

Article abstract of DOI:10.1039/c6ra21472j

The formation of valuable products and especially fuel candidates from lignocellulosic biomass is highly desirable. Lignocellulose derived halogenated furfurals formation was reported as highly efficient in comparison to the current existing methods of lignocellulose transformation. However, halogenated furfurals are platform chemicals and not end use chemicals, certainly not for fuels. Therefore, transformation methods of halogenated furfurals into fuels, fuels additives, or other valuable compounds are desirable. In this work we present the hydrogenation of halogenated furfurals over carbon supported palladium catalysts. Palladium catalysts showed better performance in the formation of 5-methyl furfural (MF) from halogenated furfurals compared to other catalysts. The reaction products were identified using GC-MS, FT-IR and NMR, and they were quantified using GC analysis. Catalysts were characterized with SEM, BET and pH meter. The role of catalysts properties and reaction parameters in MF preparation, and their effect on MF yields and selectivity were examined. In addition, the catalysts recovery and reuse in subsequent cycles was examined together with the recovery of hydrochloric acid or hydrobromic acid, formed as by products in halogenated furfurals hydrogenation.

Full text of DOI:10.1039/c6ra21472j

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Palladium catalyzed hydrogenation of biomass derived
halogenated furfurals
Elad Meller, Yoel Sasson, Zeev Aizenshtat
The Institute of Chemistry, Casali Institute for applied Chemistry, The Hebrew University
of Jerusalem Edmund Safra campus in Givat ram 91904 Israel
Emails: zeev@ vms.huji.ac.il; ysasson@huji.ac.il;
Elad.meller@mail.huji.ac.il
Corresponding author: Elad Meller
The Hebrew University of Jerusalem. Institute of Chemistry, Casali Bld. room 208 B Edmund
Saphra Campus. Phone 972-2-6585561 Fax 972-2-6585318
Abstract
The formation of valuable products and especially fuel candidates from lignocellulosic
biomass is highly desirable. Lignocellulose derived halogenated furfurals formation was
reported as highly efficient in comparison to the current existing methods of
lignocellulose transformations. However, halogenated furfurals are platform chemicals
and not end use chemicals, certainly not for fuels. Therefore, transformation methods of
halogenated furfurals into fuels, fuels additives, or other valuable compounds are
desirable. In this work we present the hydrogenation of halogenated furfurals over carbon
supported palladium catalysts. Palladium catalysts showed better performance in the
formation of 5-methyl furfural (MF) from halogenated furfurals compared to other
catalysts. The reaction products were identified using GC-MS, FT-IR and NMR, and they
were quantified using GC analysis. Catalysts were characterized with SEM, BET and pH
meter. The role of catalysts properties and reaction parameters in MF preparation, and
their effect on MF yields and selectivity were examined. In addition, the catalysts
recovery and reuse in subsequent cycles was examined together with the recovery of
hydrochloric acid or hydrobromic acid, formed as by products in halogenated furfurals
hydrogenation.

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1
. Introduction
Environmental and political concerns combined with diminishing petroleum resources
have driven our society to look for renewable and sustainable replacements for petroleum
based commodities. For this reason, the bio-refinery concept has attracted much attention
in the last decade. The bio-refinery facility aims to produce chemicals and fuels from
1
biomass components . Biofuel production is a key process for the profitability of the bio-
refinery facility. Today, the transportation sector is strongly dependent on petroleum; in
the U.S. about 92% of the energy used by the transportation sector is sourced from
2
petroleum, while natural gas and biofuels contribute only about 3% and 5%, respectively .
Biomass is attractive for the production of transportation fuels since it can supply organic
3
,4
carbon in liquid form with physical properties similar to petroleum-derived fuels
.
Currently, biodiesel, produced primarily from vegetable oils, and bio-ethanol, produced
from sugar canes and starch, are commercially available and are used mainly as blends
5
,6,7,8
with traditional petro-fuel in existing engines
. The production of fuels from edible
9
,10
resources is limited by agricultural lands shortage . Therefore, non-edible resources
must be considered for biofuel production. The most abundant non-edible material in
nature is lignocellulose. It is estimated that 170 billion tons a year of globally produced
biomass is roughly consist of 75% carbohydrates, 20% lignin and only 5% of other
1
1 , 12
chemicals (oils, fats, proteins, terpenes, alkaloids, etc)
. Lignocellulosic biomass
1
3,14
consists of cellulose (40-50%), hemicellulose (20-40%) and lignin (20-30%)
. While
lignin is a three-dimensional aromatic polymer, cellulose and hemicellulose are
polysaccharides structured from carbohydrates monomers. Since carbohydrates are the
major ingredients of lignocellulose, they are the most interesting lignocellulosic
components for biofuels and value added chemicals production. Cellulose, the major
component in lignocellulose, has a high degree of crystallinity and its bonds are not
accessible to a great extant as a result of its glycoside bonds arrangement, formed by an
1
5 , 16
extensive hydrogen bonding
. Hence, the conversion of cellulose into valuable
products is limited by the high degree of its crystallinity. Promising compounds that are
possible from carbohydrates, and specifically from lignocellulose, in high yields are from
1
7,18
the furfural family
.

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In 2004 the U.S. department of energy (DOE) released a report reviewing the most
1
9
promising carbohydrates derived chemicals as a platform for bio-refinery processing .
Taking into consideration a number of factors such as known processes, industrial
viability, economical feasibility, the market size and the capacity of the compound to
serve as a platform chemical in bio-refinery, the DOE report has listed 12 chemical
opportunities. The only furanic compound included in this list was 2,5-furan dicarboxylic
acid. In 2010, Bozell and Petersen revised that list, adding and excluding a few sugar
2
0
derived compounds . Most importantly, ethanol, furfural and 5-hydroxymethyl furfural
HMF) were included in the new list. The conversion of lignocellulose to ethanol is the
(
most established technology for carbohydrates derived biofuel production. In this two-
step methodology, cellulose and hemicellulose are hydrolyzed with diluted acid or an
enzyme into sugar monomers followed by fermentation of the mono-sugars to ethanol
2
1
using yeast or bacteria . However, bio-ethanol production from lignocellulose is
expensive and requires a unique pretreatment step in order to achieve high yields.
Furthermore, the process is time consuming and has low atom economy since only two
7
, 22
third of the reactant’s carbon atoms are retained in the product . More appealing
compounds included in Bozell and Pestern report, which retain all of the reactant’s
carbon atoms, are 5-hydroxymethyl furfural (HMF) and furfural. HMF and furfural can
be produced from hexoses and pentoses found in cellulosic and hemicellulosic biomass,
2
3
respectively . Biofuel preparation from furfural and HMF was vastly investigated. Yan
et al. and Nakagawa et al. have reviewed the conversion of furfural and HMF to various
2
4 , 25
valuable compounds and fuel candidates via hydrogenation and hydrogenolysis
.
Catalysts investigated and reported for hydrogenation or hydrogenolysis of furfural or
2
6 , 27 , 28 , 29 , 30 , 31 , 32
HMF include Pd, Ni, Ru, Rh, Cu-Cr, Cu-Fe etc
. In
hydrogenation/hydrogenolysis of furfural and HMF the full or partial hydrogenation of
the aldehyde and aromatic groups can take place. Typical compounds formed by furfural
hydrogenation/hydrogenolysis include furfuryl alcohol, 2-methylfurn, tetrahydrofurfuryl
alcohol and 2-methyltetrahydrofuran. Possible hydrogenation/hydrogenolysis products
that can be formed from HMF include 5-methyl furfural (MF), 5-methyl furfuryl alcohol,
2
,5-dihydroxymethylfuran, 2,5-dimethylfuran or any tetrahydro-form of these compounds.
These furanic compounds are potential biofuels or fuel additives. In addition, diesel range

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fuel can be achieved by esterification, etherification or condensation of furfural or HMF
(cross-condensation or self aldol-condensation of hydrogenated products). These
1
5,25
reactions were reported in reviews published by Nakagawa et al. and James et al
. The
catalytic production of diesel range fuels by C-C bond formation has been reported by
several authors. Liu and Chen demonstrated coupling of HMF and other furanic
3
3
derivatives via Umpolung coupling catalyzed by heterocyclic carbene . Huang et al have
reported C-C bond formation of furfural and other aromatic aldehydes via reductive
3
4
coupling . Furfural is attained from hemicellulose, while cellulose is the major
ingredient of lignocellulos and conversion methods for cellulosic material are highly
significant. HMF can be produced from cellulosic material, but HMF is limited by its
instability in storage and in processing, as reflected by its decomposition to levulinic acid
3
5,36
and polymerization to humic materials
. The formation of halogenated furfurals, 5-
chloromethyl furfural (CMF) and 5-bromomethyl furfural (BMF), from pure cellulose or
3
7,38
from cellulosic waste was proven to be more efficient than HMF formation
. In these
processes more than 80% yield of isolated CMF and BMF could be obtained.
Nevertheless, halogenated furfurals cannot be use as biofuels directly, and therefore they
serve only as platform compounds for biofuel production in a subsequent process.
However, studies regarding subsequent conversions of CMF and BMF into fuel
candidates are rather scarce. Mascal and Nikitin have reported that CMF can react with
ethanol, replacing chloride ions with ethoxide and produce 5-ethoxymethyl furfural in
high yields. Alternatively, CMF can undergo hydrogenation with palladium chloride
catalyst and transform into 5-methyl furfural (MF) in high yields. However, the
3
7
utilization of homogeneous catalyst might limit the applicability of this process . Kumari
et al have also demonstrated the reaction of BMF with ethanol and the formation of 5-
3
8
ethoxymethyl furfural ; however hydrogenation of BMF was not demonstrated in their
report or in any other publication to the best of our knowledge.
In this study we explored the catalytic hydrogenation of CMF and BMF over
heterogeneous palladium catalysts. We found that high yields of MF, that can be
considered as fuel candidate or fuel additive, are obtained in this protocol. In addition, we
observed that in the presence of some types of catalysts, coupling of furfural molecules
proceeds rather than hydrogenation. The products obtained from coupling reactions can

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be used as platform for diesel range fuel, while MF is more suitable as a gasoline range
fuel.
2
. Experimental
2
.1 Materials
3
9
CMF and BMF were prepared in our laboratory as published in a previous work . 1,2-
Dichloroethane (DCE) was purchased from Sigma Aldrich and used as a solvent without
any further purification. Several activated carbon supported palladium were examined as
S
catalysts. Commercial 5% palladium on carbon (abbreviated as Pd/C ) was purchased
form Sigma Aldrich. Activated carbon Darco and activated charcoal Norit® (abbreviated
D
N
as Pd/C and Pd/C , respectively) were purchased from Sigma Aldrich and examined as
supports for palladium. Palladium loadings of 5%, 3%, 1% and 0.5% over activated
carbon were prepared in our laboratory according to the following protocol: 1g of
activated carbon (Darco or Norit) was pretreated in an oven at 200°C for 1h. Afterwards,
the treated activated carbon was placed in round bottom flask containing 10ml aqueous
solution of palladium nitrate (0.4686, 0.2816, 0.0937 and 0.0468mmol for loading of 5%,
3
%, 1% and 0.5%, respectively) and the slurry was thoroughly stirred for 1h. Then,
potassium formate, purchased from Sigma Aldrich, was added drop-wise to the slurry
with molar ratio of 20:1 of potassium formate to palladium nitrate), and the slurry was
(
stirred for 24h. At the last step the slurry was filtered and the cake has been washed 3
times with deionized water and dried under air flow.
2
.2 Catalysts characterization
Morphological observations in low and high magnifications of catalysts were performed
with high resolution scanning electron microscope, HR-SEM, Sirion (EFI Company,
USA). The HR-SEM unit is equipped with high stable Schottky field emission source
filament, SE and BSE detectors, and acceleration voltage of 200V to 30kV.
Specific surface area and porous texture characterizations of supported palladium
catalysts were carried out by N adsorption at 77.3K using NOVA-1200e instrument
2
(Quantachrome Instruments, USA). Surface areas were determined using Brunauer-
Emmett-Teller (BET) equation, while pore radiuses and pore volumes were determined
using N t-plot method and Barrett-Joyner-Halenda (BJH) method, respectively. All
2
samples were degassed for 6 hours prior to each experiment under nitrogen flow at 300°C.

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Acidity measurements of the Darco and Norit supports were carried out according to a
4
0
protocol reported by Sugurmaran et al . According to this protocol 1% (w/w) suspension
of the activated carbon in deionized water was heated to 90°C while stirring. Then the
suspension was cooled to room temperature and the pH was measured. The pH values
were measured with a bench top pH meter equipped with glass electrode from MRC
Laboratories.
2
.3 CMF and BMF hydrogenation
Hydrogenation of CMF and BMF was carried out according to the following protocol:
.075mmol of CMF (0.300g) or BMF (0.392g) were dissolved in 40ml 1,2-
2
dichloroethane (DCE) and placed in a 300ml autoclave reactor (Parr Instrument
Company, USA). The reactor has an inner coating of Hastelloy, in order to achieve high
corrosion durability, and it is operating in batch mode. Then 0.030-0.100g of Pd/C
catalyst was added to the CMF or BMF solution. The reactor was sealed and purged 3
times with argon and then 3 times with hydrogen. After purging, the reactor was loaded
with the desired hydrogen pressure (1-20bar). The desired temperature (60-100°C) was
set and heating was executed by a thermocouple connected to controller. At the end of
reaction, after the reactor was cooled down, the excess gas was removed and the reactor
purged 3 times with argon, to remove hydrogen for safety reason. Finally, the solution
was filtered in order to separate and recover the catalyst, and the solvent was evaporated
in order to obtain the crude product.
The identification of products was carried with GC-MS (Thermo Scientific, USA)
constructed from Trace 1300 GC, equipped with 30m Rxi®-1ms non polar column,
connected to a single quadruple mass spectrometer ISQ QD. Assay of reaction products
was performed with gas chromatography (Focus GC, Thermo Scientific, USA) equipped
with a low polarity 30m ZB-5 column. Complementary analyses for products
1
identification was carried out using NMR and FT-IR spectroscopy. H-NMR spectra were
recorded with Bruker DRX-400 instrument (Bruker Corporation, USA). Samples for
6
NMR analyses were prepared by dissolving the crude products in DMSO-d and placing
the solutions in NMR tubes. FT-IR spectra were recorder with FT-IR spectrometer model
Alpha (Bruker Optics, Bruker Corporation, USA). Leaching of palladium from support to
reaction medium was determined using ICP-AES (Perkin-Elmer Corp, Optima 3000).

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ICP-AES calibration was carried out with 10ml solutions of 0.01-10ppm palladium
nitrate dissolved in 0.1M nitric acid. ICP-AES samples were prepared by mixing the
reaction crude products in 5ml 0.1M nitric acid solutions followed by filtering the
solutions with 0.2µm filter.
Hydrochloric acid and hydrobromic acid rejuvenation was carried out by connecting the
reactor to a tube containing 21ml of deionized water and bubbling of the gaseous phase
remained in reactor at the end of reaction, after the reactor was cooled down to room
temperature, into the deionized water. The tube was immersed in an ice bath in order to
cool the water. After bubbling of reaction gaseous phase, the reactor gaseous content was
flushed 3 more times into the collection tube with 10bar N . The collected acid content
2
was determined by titration with 0.1M sodium hydroxide solution.
3
. Results and discussion
3
.1 Catalysts screening and reaction products
Various catalysts were investigated in CMF and BMF hydrogenation. With most of the
examined catalysts the hydrogenation reaction proceeded via hydrodehalogenation,
namely the replacement of halogen atom with hydrogen atom. Then, the hydrogenation
product is 5-methyl furfural (MF), as identified by GC-MS analysis (see supporting
information), and hydrochloric acid or hydrobromic acid are formed as a by-products
from CMF or BMF, respectively (see in Fig. 1).
Fig. 1 A schematic representation of catalytic hydrogenation of (1) CMF and (2) BMF to
MF (with the formation of HCl or HBr as by-products).

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In order to find the most active and selective catalyst for CMF and BMF hydrogenation a
catalysts screening has been performed and the results are summarized in Table 1. The
S
N
D
abbreviations Pd/C , Pd/C and Pd/C are assigned for commercial activated carbon
supported Pd (purchased from Sigma), Norit type activated carbon supported Pd and
Darco type activated carbon supported Pd, respectively (the chemical and textural
differences between these catalysts will be elaborated in the following sections).
Table 1. Summary of CMF and BMF hydrogenation experiments with different catalysts.
All experiments were carried out with 0.300g CMF or 0.393g BMF as substrate, 40ml
1
,2-dichloroethane (DCE) as reaction solvent, 11bar H and reaction time of 5h.
2
Substrate Temperature
Catalyst
5% Ru/C
Conversion
0%
MF yield Others yield
CMF
CMF
CMF
CMF
CMF
50°C
50°C
80°C
50°C
50°C
70°C
50°C
50°C
50°C
50°C
50°C
-
-
-
5% Rh/C
9.1%
9.1%
18%
14.5%
6.2%
99%
5.1%
53%
-
65% Ni/SiAl
18.5%
24.7%
100%
100%
5.1%
0.5%
10.2%
93.8%
1%
5% Pt/C
S
5% Pd/C
a
N
CMF
BMF
BMF
BMF
BMF
BMF
BMF
0.5% Pd/C
5% Ru/C
5% Rh/C
-
53%
65% Ni/SiAl
5% Pt/C
0%
-
85.5%
2%
-
86.7%
100%
31.9%
1.2%
98%
31.9%
S
5% Pd/C
N
50°C
0.5% Pd/C
a
N
With 0.5% Pd/C catalyst, 70°C and 0.1g catalyst were needed in order to achieve full
conversion of CMF to MF.
Catalysts screening results reveal that Ru/C, Rh/C and Ni/SiAl are poor catalysts for
CMF and BMF hydrogenation, and they result in low conversions under the reaction
conditions specified in Table 1 (although Rh/C shows a medium activity in
hydrogenation of BMF with 53% conversion to MF). Pt/C catalyst brings to CMF and
BMF conversions of 24.7% and 86.7%, respectively. However, the selectivity of this
catalyst towards MF is poor since only 14.5% and 1.2% yields of MF from CMF and

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BMF, respectively, have been obtained. The other products formed were identified as 5-
methylfurfuryl alcohol and condensation products (53.3% yield of 5-methylfurfuryl
alcohol and 32.2% yield of condensation products from BMF). Pd/C catalysts were found
to be the most active catalysts for the conversion of halogenated furfurals into MF. 100%
conversion of BMF and 98% yield of MF were obtained with commercial 5% palladium
S
on active carbon (Pd/C ), and 100% conversion of CMF and 98% yield of MF were
obtained with 0.5% palladium on active carbon (Pd/CN), which was prepared in our lab.
Surprisingly, the commercial palladium catalyst was also very active for CMF
hydrogenation, but it yielded mainly a complex products mixture comprised from
condensation products rather than MF. The reaction products formed with commercial
S
Pd/C catalyst, Pd/C , are depicted in Fig. 2.
S
Fig. 2 Identified reaction products obtained in CMF hydrogenation catalyzed by Pd/C .
S
When CMF was hydrotreated in the presence of Pd/C catalyst, only 6.2% yield of MF
has been formed. In addition to condensation products, a small amount of non-

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condensation products could be identified. These include 5-methyltetrahydrofurfuryl
alcohol (yield of 5.9%) and traces of saturated and partially saturated chlorinated
S
compounds (total yield of 1.5%). The main products formed with Pd/C catalyst were
condensation reaction products, with varying degrees of saturation. The identified
compounds are presented in Fig. 2 and their total yield was 69.5%. Analysis of products
structures was carried with GC-MS and supported by complementary information
obtained by H-NMR spectroscopy and FT-IR spectroscopy (see supporting information).
The proton NMR spectrum indicates the absence of aldehyde hydrogen and aromatic
hydrogen in the products mixture, while the FT-IR spectrum indicates the existence of
carbonylic and hydroxyl groups.
3
.2 Catalysts characterization
3
.2.1 The acidity of carbon supports
S
The acidities of the commercial palladium catalyst, 5% Pd/C , and the different carbon
supports, Norit and Darco, were determined by pH measurements according to the
protocol detailed in section 2.2. Norit activated carbon was found to be slightly basic with
pH of 8.88, while the Darco activated carbon was determined as acidic with pH value of
S
3
.15. A pH value of 8.39 has been recorded for the commercial 5% Pd/C catalyst. The
acidic or basic characteristics of the supports influence, as we shall see in the following
sections, on the dispersion of palladium particles, and as a consequence on the yields of
MF obtained from CMF and BMF.
3
.2.2 SEM characterization of catalysts
The different catalysts tested in this work were examined with scanning electron
microscopy for the characterization of morphology and dispersion of palladium particles
over supports. SEM images are presented in Fig. 3.

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D
D
Fig. 3 Scanning electron microscopy images of (a) 0.5% Pd/C , (b) 5% Pd/C , (c) 0.5%
N
N
N
N
S
Pd/C , (d) 1% Pd/C , (e) 3% Pd/C , (f) 5% Pd/C and (g) 5% Pd/C .
The SEM images in Fig. 3 (c-f) reveal that for the Norit support (the basic activated
carbon), as palladium loading is higher, larger and poorly dispersed agglomerated
particles were formed. In all samples, polydispesed particles with size range of 3-475nm
were found. A higher magnification reveals that all aggregates are built from small
particles with size range of 1.75-4.75nm (see supporting information). It also appears that
in higher palladium loadings over support the proximity between particles is higher, i.e.
the particles are dispersed in a denser manner. In lower palladium loadings, smaller and
less frequent agglomerates were formed, and the distances between particles are large.
D
From the SEM images in Fig. 3(a) and 3(b), for 0.5% and 5% of Pd/C , respectively, we
can conclude that also for the Darco support as palladium loadings are higher larger
agglomerates were formed. However, in the case of the acidic Darco support, palladium
particles are dispersed less densely than palladium particles supported over the basic
Norit activated carbon. In addition, as shown in Fig. 3 (a) and (b), the Darco supported
palladium particles are distant regardless of the metal loading. The differences between
Norit and Darco supports in palladium particles agglomeration and dispersions might be
4
1,42
attributed to the acidity of the supports
. The acidic Darco support has a very low pH,
and densely assembles acidic surface groups that interact strongly with palladium
precursors prevent the formation of highly dispersed palladium particles. On the other
hand, the formation of highly dispersed palladium particles is possible with the slightly
basic Norit activated carbon since the interactions between precursor and surface groups
S
of the support are not too strong. In the case of Pd/C , in Fig 3(g), the occurrence of large
agglomerated particles seems to be more frequent than the other catalysts. Higher
S
magnification SEM image of Pd/C reveals also the presence of dense highly dispersed
small sized palladium particles, even more densely dispersed than the other examined
S
catalysts (see supporting information). It is not likely that the differences between Pd/C
N
and Pd/C are caused by different surface acidity since they both possess a week basic
surface (see section 3.2.1). Therefore, differences might derive from several factors,
which are unknown for the commercial catalyst, such as the treatment of the active
carbon, reduction temperature, differences in metal precursors etc.

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3
.2.3 Surface area and pores size measurements
2
The surface area (m /g), pore radius (Å) and pore volume (cc/g) were determined for all
activated carbon supported palladium catalysts. The results are summarized in Table 2.
Table 2. Summary of surface area, pore radius and pore volume of palladium on active
carbon catalysts with different palladium loadings and different activated carbon type.
2
Catalyst
Surface area (m /g)
860.409
Pore radius (Å)
15.536
Pore volume (cc/g)
0.170
N
0
.5%Pd/C
N
N
N
1
%Pd/C
%Pd/C
%Pd/C
659.473
24.742
0.177
3
5
645.811
24.709
0.168
614.820
24.764
0.149
D
0
.5%Pd/C
1241.495
1149.588
847.629
15.396
0.320
D
5
%Pd/C
17.291
0.290
S
5
%Pd/C
24.768
0.257
In general, we can conclude from Table 2 that the acidic Darco supported catalysts have
larger surface areas than the basic Norit supported catalysts. The higher tendency of
palladium particles to agglomerate over the acidic Darco support, in comparison to Norit
support, might contribute to the higher surface areas of Darco supported palladium
catalysts as a result of lower occupancy of surface and pores by palladium particles. The
lower surface and pores occupancy is also consistent with the larger pore volumes of
Darco supported catalysts in comparison with the pore volumes of Norit supported
S
catalysts. The surface area and pore volume values determined for 5% Pd/C are similar
N
D
to the values determined for 0.5% Pd/C and 5% Pd/C , respectively. In addition, as the
support is loaded with larger amount of palladium the surface area become smaller. The
same phenomenon was observed for the pore volume, as palladium loading is higher the
N
pore volume is smaller (although 1% Pd/C behaves exceptionally and it has larger pore
N
volume than 0.5% Pd/C ). These phenomena are reasonable since higher palladium
loading will bring to higher surface coverage and higher occupancy of pore volume. The
pore radius values also seem to be related to the palladium loading; in general as the
N
palladium loading is higher the pore radius is larger (with the exception of 3%Pd/C ,

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N
which shows a smaller pore radius than 1%Pd/C ). The increase of pore radiuses in
catalysts with higher palladium loadings might caused by increased number of particles
occupying pores forcing the carbon pores to be broadened. Generally, Darco supported
palladium catalysts show smaller pore radiuses than Norit supported palladium catalysts.
N
D
0
.5% Pd/C and 0.5%Pd/C have the smallest pore radiuses, 15.536Å and 15.396Å,
S
respectively. The commercial catalyst, 5% Pd/C , exhibits the largest pore radius,
2
3
3
4.768Å.
.3 Catalysts and reaction parameters
.3.1 Effect of palladium loading
Palladium loadings of 0.5-5% over Norit support were tested in CMF and BMF
hydrogenation. The results are presented in Fig. 4.
Fig. 4 Effect of palladium loading (%wt) on hydrogenation products yields from (a) BMF
and from (b) CMF. All reactions were carried out with 2.075mmol of reactant (0.300g
CMF or 0.392g BMF), 0.030g catalyst, 40ml DCE, 11bar H , reaction time of 5h and
2
temperature of 50°C or 70°C for BMF and CMF, respectively.
The results in Fig. 4(a) indicate that for BMF conversion to MF, as palladium loading
over the carbon support is higher the yields of MF are higher. The maximal yield of MF,
N
9
3.3%, is obtained with 5% Pd/C catalyst. Then, BMF conversion is also about 93.3%
and only traces of humic materials were formed. The correlation between catalyst loading
and BMF conversion is reasonable since higher loadings bring to larger amounts of
catalytic sites. In the case of CMF conversion, in Fig. 4(b), higher loadings also bring to
higher CMF conversions (here too, the total yields of products are identical to

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conversions). However, as Norit support is loaded with larger amounts of palladium, the
yields of the other products (mostly condensation products) are increased in place of MF
yields. The reason for this phenomenon is not well understood, but several explanations
derived from the morphological and textural characters of the supported catalysts are
4
3,44
possible. First, selectivity and activity of catalysts can be influenced by particles sizes
.
As we saw in section 3.2.2, increasing palladium loading over Norit support result in
larger agglomerated palladium particles. In larger palladium agglomerates, the active
sites might be blocked for CMF hydrodehalogenation and the competitive slower
condensation reactions are taking place more rapidly. Another possible explanation might
relate to the fact that larger palladium particles have higher activity towards
4
5
hydrogenation of the aromatic ring . Ring hydrogenation is required in order to enable
condensation reactions. As observed by SEM images, in Fig. 3, higher palladium
loadings lead to the formation of larger Pd particles. As a result, condensations reactions
can proceed over catalysts with higher loadings over Norit support. A third hypothesis is
related to the proximity of the dispersed Pd particles. The condensation of two
halogenated furfurals molecules can take place only if they are in close proximity, as
4
6
required in bimolecular reactions . A more densely packaging of particles over support
has been observed for higher palladium loadings. The close proximity of palladium
particles enables the contact of reactants and the formation of condensation products.
Oppositely, when particles are distant from each other in lower palladium loadings, as
N
observed for 0.5% Pd/C , the contact of two halogenated furfural molecules is less likely.
Finally, the texture of support might also influence on the occurrence of condensation
reactions. BET results in Table 2 reveal that pore radiuses are larger for higher palladium
N
loadings (although 3% Pd/C behaves exceptionally). As pore radius is larger, the
accessibility of two reactants to the active catalytic site is more feasible. Therefore,
N
condensation products could not be detected when 0.5% Pd/C was used since it has the
smallest pore radius, 15.536Å. It should be pointed that the formation of condensation
products is reasonably influenced by two, or more, of the above factors simultaneously. It
is also clear from Fig. 4 that condensation products were formed from CMF and not from
N
BMF over Pd/C . Bromine atom is a better leaving group than chlorine atom and thus
BMF reacts faster than CMF with hydrogen. Therefore, hydrogenation of BMF over the

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catalytic sites and MF desorption from the catalytic sites are too fast for the competitive
condensation reactions to occur. On the other hand, in the case of CMF hydrogenation,
the chlorine atom is substituted with hydrogen slowly enough for the competitive
condensation reactions to occur.
3
.3.2 Effect of catalyst type
Three types of palladium supported catalysts were examined in the hydrogenation of
D
CMF and BMF: Darco supported Pd catalyst (Pd/C ), Norit supported Pd catalyst
N
S
(
Pd/C ) and commercial 5% Pd/C catalyst (Pd/C ). The activities of Darco and Norit
activated carbon supports were tested with palladium loadings of 0.5% and 5%. The
results are summarized in Fig. 5.
Fig. 5 Effect of catalyst type on hydrogenation products yields from (a) BMF and from
(b) CMF. All reactions were carried out with 2.075mmol of reactant (0.300g CMF or
0
.392g BMF), 0.030g catalyst, 40ml DCE, 11bar H , reaction time of 5h and temperature
2
of 50°C or 70°C for BMF and CMF, respectively.
The results in Fig. 5(a) indicate that for palladium loading of 0.5%, the Darco support has
a little higher activity than the basic Norit support towards BMF hydrogenation to MF.
However, with higher palladium loading of 5%, Norit support is more active than Darco
support towards the formation of MF from BMF, with MF yields of 93.3% and 77.6%,
respectively. In the case of CMF hydrogenation, in Fig. 5 (b), it is clear that Norit support
has higher activity than Darco support with both lower and higher palladium loadings. A
N
9
9% conversion of CMF was recorded with 5% Pd/C catalyst while the CMF

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D
conversion obtained with 5% Pd/C catalyst was 92.8%. However, the acidic Darco
support has higher selectivity towards MF than the basic Norit support. MF yield of
7
8.9% and other products yield (mostly condensation products) of 13.9% have been
D
N
recorded with 5% Pd/C catalyst, while with 5% Pd/C catalyst only 42% yield of MF
has been obtained and the other products yield was 56.5%. The difference in activity
towards CMF hydrogenation was more markedly with palladium loading of 0.5%.
N
Utilization of 0.5% Pd/C brought to full selectivity of MF under the specified reaction
D
conditions with 72.8% yield of MF, while utilization of 0.5% Pd/C brought to 15.8%
yield of MF and 1.2% yield of other compounds In general, Norit support result in more
active catalysts than Darco support, probably as a result of the highly dispersed Pd
particles over Norit support. In addition, it is clear from Fig. 5 that CMF is more prone
for condensation over Norit supported catalysts than over Darco supported catalyst. The
lower yields of condensation products formed over Darco supported catalysts might
result from differences in supports acidities, differences in Pd dispersion over supports
and different metal-support interactions. In addition, geometrical limitation imposed by
the low pore radius of the Darco carbon support might also contribute to the lower yields
of condensation products formed with Darco supported catalyst. The commercial catalyst,
S
5
% Pd/C , shows high activity toward BMF and CMF conversion. It is the most active
catalyst for BMF hydrogenation into MF, with 98.6% yield of MF. This catalyst is also
highly active for CMF conversion. However, only to 6.2% yield of MF has been obtained
S
with 5% Pd/C catalyst, while the rest of the products are mainly condensation products.
The enhanced activity of this catalyst is probably derived from a combination of large
pore radius and pore volume and close proximity of highly dispersed particles over
support. It should be pointed that the differences in activities of the acidic Darco
supported catalysts and the basic Norit supported catalysts, as well as the commercial
catalyst, might also derive from the hydrophilic or hydrophobic character of the supports
4
7
surfaces . Catalysts with acidic surface are more hydrophilic than the catalysts with the
basic surface and therefore wetting of the catalyst acidic surface with the organic reactant
is not as efficient as wetting with the basic catalysts.
S
In conclusion 0.03g of 5% Pd/C shows the best performance in selective BMF
N
hydrogenation to MF. On the other hand, 0.5% Pd/C seems to be the most selective

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catalyst towards MF formation from CMF, but a larger quantity of 0.1g catalyst is needed
to achieve full conversion of CMF.
3
.3.3 Effect of reaction time
Reduction of reaction time has a great significance for industrial application. The optimal
time of reaction was determined for the formation of MF from CMF and BMF. The
results are presented in Fig. 6.
Fig. 6 MF yields as a function of reaction time (h) from CMF and from BMF. All
reactions were carried out with 2.075mmol of reactant (0.300g CMF or 0.392g BMF),
4
0ml DCE and 11bar H . For BMF hydrogenation temperature of 50°C and 0.030g 5%
2
S
Pd/C catalyst were used. For CMF hydrogenation temperature of 70° and 0.100g 0.5%
Pd/C catalyst were used.
N
The optimal reaction time for the conversion of BMF into MF is 4h, and then MF yield is
9
8.6%. In the case of CMF conversion into MF the optimal reaction is 5h, and then MF
yield is 99.2%. In both cases 100% conversion was recorded and traces of humic
materials were formed. The reaction time of BMF is shorter than the reaction time of
CMF since bromine atom is a better leaving group than chlorine atom and it will be
replaced with hydrogen atom more easily (and thus more rapidly). It should be pointed
that shorter and longer reaction time lead to lower MF yields. In longer reaction times
MF yields are lower as a result of increased condensation products formation, this time
from MF. The condensation products were identified to be the same as detailed in
previous sections as confirmed by GC analysis.
It should also be pointed that the effects of the hydrogen pressure, 1-20bar, and
temperature, 60-100°C, were also examined. However, these parameters have no impact

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or a negative impact on MF yields. Investigating the effect of hydrogen pressure revealed
that higher hydrogen pressures did not bring to subsequent hydrogenation of MF, since
the aromatic bonds or the aldehyde group were not hydrogenated under the reaction
conditions. However, lower MF yields have been obtained with lower hydrogen pressures.
Temperature examination showed that lower MF yields have been obtained when higher
and lower temperatures were applied. Higher temperatures bring to larger amount of
condensation products, and even increased formation of humic materials, and lower
temperatures bring to lower MF yields as a result of insufficient energy to promote
reaction.
Therefore, the optimal hydrogen pressure for BMF and CMF conversion to MF is 11bar
and the optimal temperatures of reaction are 50°C and 70°C for BMF and CMF
conversion to MF, respectively.
3
.4 Catalyst and acid recycling
N
S
The most active catalysts in CMF and BMF hydrogenation, 0.5% Pd/C and 5% Pd/C ,
were tested in sequential reaction cycles. The catalysts were separated after each cycle
and returned into the reactor for another cycle without any further treatment. The
catalytic performance of the recovered catalysts in CMF and BMF hydrogenation is
presented in Fig. 7.
Fig. 7 MF yields from CMF and BMF hydrogenation in four sequential cycles. All
reactions were carried out with 2.075mmol of reactant (0.300g CMF or 0.392g BMF),
4
0ml DCE, 11bar H . For BMF hydrogenation a reaction time of 4h, temperature of 50°C
2
S
and 0.030g 5% Pd/C were used. For CMF hydrogenation a reaction time of 5h,
temperature of 70°C and 0.100g 0.5% Pd/C were used.
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It can be observed from Fig. 7 that in the second and third cycles of both catalysts MF
yields are slightly decreasing. In the case of CMF hydrogenation, MF yields of 99.2%,
9
7.3% and 94% have been obtained in the first, second and the third cycles, respectively.
N
However, in the fourth repetition of reaction with 0.5% Pd/C catalyst a substantial lost
of activity was observed and the yield of MF was dropped to 61.6%. The same
S
phenomenon has been observed in BMF hydrogenation with 5% Pd/C catalyst. MF
yields of 98.6%, 98.5% and 91.52% were obtained in the first, second and third catalytic
cycles, respectively. However, in the fourth cycle of the recovered catalyst, a significant
lost of activity has been observed and a yield of 63.6% of MF was recorded. The trend of
activity decrease must be a consequence of a slow deactivation of catalytic sites followed
by a rapid deactivation that proceed when a critical point had been reached. Several
factors might contribute to activity loss. Sintering of palladium particles and loss of
surface area is a known factor to promote deactivation. This phenomenon occurs more
markedly with activated carbon supported palladium catalysts than metal oxide supported
4
8
palladium catalysts as a result of weak metal-support interactions . However,
deactivation of Pd/C as a result of particles sintering is driven from the presence of
hydrogen and elevated temperatures. In our study, at low temperatures, palladium
particles are stable to sintering, as supported by SEM analyses performed on catalysts
after reaction (see supporting information). The stability of Pd particles on carbon
4
9
supports in low temperature hydrodechlorination was also reported by Janiak and Okal .
Leaching of palladium from supports might explain deactivation and thus leaching tests
S
for samples collected from the three first cycles of both 5% Pd/C in BMF hydrogenation
N
S
and 0.5% Pd/C in CMF hydrogenation. For 5% Pd/C catalyst, a palladium leaching of
.216ppm, which corresponds to 0.144% of the palladium content, was determined in the
0
first reaction cycle. In the second and third cycles no palladium content could be detected
in samples and therefore we can conclude that or no leaching of palladium did occur or
the palladium content in samples was under the detection limit of the ICP instrument (and
N
thus leaching was taking place only in very small amounts). The results for 0.5% Pd/C
in CMF hydrogenation showed similar behavior. At the first cycle palladium leaching of
0
.04ppm could be detected (inaccurate result because of the value's closeness to the
detection limit) but in second and third cycles no palladium could be detected in samples.

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Another plausible reason for the catalysts activity lost in CMF and BMF hydrogenation is
the formation of coke on the catalytic sites from traces of humic materials formed during
reaction. Humic materials can adsorb and accumulate on the catalytic sites after each
cycle until a critical amount of coke form over the catalytic sites and MF yields drop
sharply. Additionally, acid by-products can also adsorb and accumulate on the catalytic
sites and inhibit halogenated furfurals hydrogenation. In order to recover the catalysts,
they were washed 3 times with 1M NaOH solutions followed by washing twice with
deionized water (in addition to washing 3 times with DCE). The basic solution can
remove humic materials (especially if humic acids were formed) and HCl or HBr and
keep the catalytic sites free and available for the following cycles. Catalytic performance
S
N
results of 5% Pd/C and 0.5% Pd/C catalysts in four sequential cycles of CMF and BMF
hydrogenation to MF when the catalysts were treated with basic solution after each cycle
are presented in Fig. 8.
Fig. 8 MF yields from CMF and BMF hydrogenation in four sequential cycles when
catalysts treated with 1M NaOH solution after each cycle. All reactions were carried out
with 2.075mmol of reactant (0.300g CMF or 0.392g BMF), 40ml DCE, 11bar H . For
2
S
BMF hydrogenation a reaction time of 4h, temperature of 50°C and 0.030g 5% Pd/C
were used. For CMF hydrogenation a reaction time of 5h, temperature of 70°C and
N
0
.100g 0.5% Pd/C were used.
We can conclude from Fig. 8 that after the catalysts were washed after each cycle with
M NaOH solution their activities in the fourth reaction cycles were improved compared
to the untreated catalysts. Now in the fourth cycles of the catalysts, MF yields of 83.8%
1

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(instead of 63.4% without treatment) and 81.6% (instead of 61.6% without treatment)
S
N
were recorder with 5% Pd/C and 0.5% Pd/C catalysts, respectively.
CMF and BMF are formed by treatment of carbohydrates with concentrated hydrochloric
or hydrobromic acid, respectively. In CMF and BMF hydrogenation HCl and HBr are
regenerated as side products, and their recycling (and reusing in halogenated furfurals
preparation) is economically significant for the overall process. Therefore, experiments of
HCl and HBr recovery have been carried out as described in section 2.3. For HBr, 5.1ml
of 0.1M NaOH solution was needed for titration of the acid collected in the water tube,
and calculation shows that it equals to 24.57% of the theoretical hydrobromic acid
content. In the case of HCl, 10ml of 0.1M NaOH was needed for titrattion, that imply on
recovery of 48.07% of the hydrochloric acid content.
4
. Conclusions
In this work the catalytic hydrogenation of biomass derived halogenated furfurals, CMF
and BMF, into 5-methyl furfural (MF) over active carbon supported palladium catalysts
is demonstrated. MF itself can be used as fuel additive or fuel candidate, or it can be
further convert into other valuable products. It should be pointed that in the current
technological status, the utilization of MF as a fuel additive is more likely. Active carbon
supported palladium catalysts are the most active catalysts towards MF formation via
CMF or BMF hydrogenation, and MF yields of 99.2% and 98.6% can be achieved from
CMF and BMF, respectively. Several types of Pd/C catalysts were examined in this
research: acidic type Darco supported catalyst, basic type Norit supported catalyst and
commercial Pd/C catalyst. It turns out that the catalysts characters, including their
palladium loading and the support type can determine not only MF yield but also the
selectivity towards MF production compared to other products, identified mostly as
condensation products, which can form. The most active and selective catalysts towards
S
N
MF formation from BMF and CMF are 5%Pd/C and 0.5% Pd/C , respectively. In
addition to the catalytic parameters, the optimal reaction parameters were also determined.
Finally, the catalyst and the acid by-product (HCl or HBr) recovery were examined. The
catalysts performance in four reaction cycles was demonstrated, and they showed a high
deactivation in the fourth cycle. The catalysts could be partially rejuvenated by treatment
with 1M sodium hydroxide solution.

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Acknowledgments
We thank the Israel Strategic Alternative Energy Fund (I-SAEF) for financial support of
this research. Special thanks for supporting in part the Ph.D. scholarship of E.M. We also
like to thanks Ms. Evgenia Blayvas from the unit for nanoscopic characterization at The
Hebrew University Center For Nanoscience And Nanotechnology for their contribution
in the catalyst characterization using SEM.
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Graphical abstract
Highly efficient catalytic hydrogenation of halogenated furfurals to 5-methyl furfural
over activated carbon supported palladium catalysts is demonstrated.