Flow hydrogenation of 5-acetoxymethylfurfural over Cu-based catalysts

Article abstract of DOI:10.1016/j.mcat.2020.111132

5-Acetoxymethylfurfural (AMF) is a promising starting material for synthesis of the valuable furan derivatives. The catalytic properties of Cu-based catalysts obtained from layered double hydroxides were studied in flow hydrogenation of AMF to 5-(acetoxymethyl)-2-furanmethanol (AMFM) in the present work. The resulting product can be easily converted to 2,5-bis(hydroxymethyl)furan which has significant potential in the production of polymers and pharmaceuticals. It was found that hydrogenation of AMF gives AMFM with selectivity of 98 percent at the full conversion of the substrate under mild reaction conditions (90°C and 10 bar). The influence of the solvent nature, Cu/Al ratio and calcination temperature on the catalyst performance was investigated.

Full text of DOI:10.1016/j.mcat.2020.111132

MolecularCatalysis494(2020)111132
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Flow hydrogenation of 5-acetoxymethylfurfural over Cu-based catalysts
Alexey L. Nuzhdin*, Marina V. Bukhtiyarova, Olga A. Bulavchenko, Galina A. Bukhtiyarova
Boreskov Institute of Catalysis SB RAS, Ave. Lavrentiev 5, 630090, Novosibirsk Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
5-Acetoxymethylfurfural (AMF) is a promising starting material for synthesis of the valuable furan derivatives.
The catalytic properties of Cu-based catalysts obtained from layered double hydroxides were studied in flow
hydrogenation of AMF to 5-(acetoxymethyl)-2-furanmethanol (AMFM) in the present work. The resulting pro-
duct can be easily converted to 2,5-bis(hydroxymethyl)furan which has significant potential in the production of
polymers and pharmaceuticals. It was found that hydrogenation of AMF gives AMFM with selectivity of 98 % at
the full conversion of the substrate under mild reaction conditions (90°C and 10 bar). The influence of the
solvent nature, Cu/Al ratio and calcination temperature on the catalyst performance was investigated.
5-Acetoxymethylfurfural
Selective hydrogenation
Cu-based catalyst
Layered double hydroxide
Flow reactor
1. Introduction
prepared by calcination of layered double hydroxide (LDH) [17,18] are
capable of catalyzing HMF hydrogenation in BHMF with selectivity
Upcoming depletion of oil reserves requires the development of new
sustainable sources for fuels and bulk chemicals. 5-hydro-
xymethylfurfural (HMF) obtained from the generally available non-
edible lignocellulosic biomass is a versatile platform chemical which
has a great potential for production of the valuable furan derivatives
[1–4]. However, a serious problem for the production, storage and
transportation of HMF is its low stability [5–9]. To overcome the pro-
blem of chemical fragility of HMF, the conversion of carbohydrates to
O-modified HMF derivatives without formation of unstable HMF can be
used [6–11]. In this connection, 5-acetoxymethylfurfural (AMF) is an
important alternative for HMF [7–10].
Several reports on the production of AMF from carbohydrates
without isolating HMF are available [7–9]. For instance, AMF can be
obtained by HCl-catalyzed conversion of lignocellulosic biomass to 5-
chloromethylfurfural followed by reaction with alkylammonium acet-
ates [7]. Gavilà and Esposito [8] reported a 36 % yield of AMF from
cellulose acetate using stoichiometric amount of H2SO4 as the catalyst.
Besides, the Sn-montmorillonite catalysts provide direct conversion of
glucose, sucrose and fructose into AMF by reaction with acetic acid [9].
The reduction of the formyl group of HMF leads to formation of 2,5-
bis-(hydroxymethyl)furan (BHMF). This compound has a great field of
application in the preparation of polymers and pharmaceuticals
[3,12,13]. For this reason, significant efforts have been focused on
finding suitable heterogeneous catalysts which are able to hydrogenate
HMF into BHMF effectively [1–3]. Cu-based materials are the most
attractive catalysts for this transformation due to its high selectivity and
low cost [14–16]. In particular, copper-doped porous metal oxides
higher than 90 % [14]. These materials are considered as promising
hydrogenation catalysts due to the uniform distribution of metal ions
and the small size of Cu° metal particles obtained after reduction
[17–19].
In general, the HMF hydrogenation was carried out in batch reactors
[1–3,14–16,20]. Meanwhile, the hydrogenation of furan compounds
under flow conditions is of considerable interest [21,22]. Potential
advantages of continuous flow synthesis over traditional batch proto-
cols include higher efficiency, environmental compatibility, simplified
scaling, and enhanced safety profile [23,24].
In this work, we investigated catalytic properties of Cu-based cat-
alysts obtained from LDHs in flow hydrogenation of AMF to 5-(acet-
oxymethyl)-2-furanmethanol (AMFM) which can be easily converted to
BHMF by hydrolysis [25] (Scheme 1). To the best of our knowledge, the
data concerning catalytic hydrogenation of AMF are absent in open
sources.
2. Experimental part
2.1. Chemicals
5-(Acetoxymethyl)-2-furaldehyde (97 %) from Acros Organics was
used without additional purification. Methanol (99.8 %, J.T. Baker),
isopropanol (≥99.7 %, Sigma-Aldrich), tetrahydrofuran (THF, ≥99.9
%, Sigma-Aldrich), toluene (99.5 %, “ECOS” Russia), ethanol (95.5 %)
and absolute ethanol (0.23 wt% H2O) were employed as solvents.
Absolute ethanol was obtained by drying ethanol containing 4.5 wt% of
⁎ Corresponding author.
E-mail address: anuzhdin@catalysis.ru (A.L. Nuzhdin).
https://doi.org/10.1016/j.mcat.2020.111132
Received 13 May 2020; Received in revised form 16 June 2020; Accepted 20 July 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

A.L. Nuzhdin, et al.
MolecularCatalysis494(2020)111132
2.3. Characterization techniques
The Cu and Al content was determined using atomic absorption
spectroscopy on an Optima 4300 DV instrument (Perkin Elmer). The
water content in the solvents was measured by DL39 Coulometer KF
titrator (Mettler Toledo). Texture properties were derived from low-
temperature (−196 °C) nitrogen adsorption isotherms obtained using
an ASAP 2400 instrument (Micromeritics). Thermogravimetric analysis
(TGA) of the samples was carried out in an air atmosphere in the
temperature range of 40–700 °C on a Netzsch STA 449 thermo-
gravimeter (Netzsch, Germany) with a heating rate of 10 K min−1 (the
sample weight was 50 mg).
X-ray patterns were obtained on a Bruker D8 diffractometer (CuKα-
radiation, 2θ = 5–70° with step of 0.05°, the accumulation time is 3 s).
The multichannel LynxEye detector was used for the signal detection.
Phase analysis was done by comparison of interlayer distances di and
intensities Ii of corresponding reflexes with theoretical values from
ICDD PDF-2 database. The average size of copper crystallites was
evaluated by the Rietveld method (LVol-IB, Bruker TOPAS V4.2).
TPR measurements were carried out in a quartz reactor using 10 %
H2 in Ar at a heating rate of 6 °C/min in the temperature range of
100–600 °C. Before reduction the sample was pretreated at 200 °C for
1.5 h in Ar. The sample mass was about 0.10 g. Thermal conductivity
detector (TCD) was used to determine H2 consumption.
2.4. Catalysts performance
The investigation of the catalytic properties was performed using
continuous flow device H-Cube Pro™ (Thalesnano) equipped with
stainless-steel CatCart®30 reactors (catalyst bed length of 24 mm, inner
diameter of 4 mm) [29–32]. Before each catalytic run, the catalyst
(0.165 g, 250–500 μm) was reduced with hydrogen dissolved in iso-
propanol (or another solvent) at T = 120 °C and P = 10 bar for 1 h
(flow rates of solvent and H2 were 0.5 and 30 mL/min, respectively)
[29,30]. Then the inlet was switched to the flask containing 0.05 M
solution of AMF. This moment was chosen as the starting point of the
experiment. Catalytic tests were performed at temperature of 50–90 °C,
pressure of 10 bar, liquid and hydrogen flow rates of 0.5 and 30 mL
min−1, respectively (inlet H2/substrate molar ratio was 54). The per-
formance of catalyst was evaluated by analysis of the samples taken
after 30–35 min of the experiment start.
The reaction mixtures were analyzed by gas chromatography
(Agilent 6890 N Instrument equipped with a capillary HP-1MS column
(30 m × 320 μm × 1.00 μm) and an atomic emission detector JAS)
using n-decane as the internal standard. The error in determining the
conversion and selectivity does not exceed 1 %. Identification of the
products was carried out by GC–MS (Agilent 7000B Triple Quad
System).
Scheme 1. Schematic routes of BHMF production.
water over freshly prepared CaO and then over calcined molecular
sieves (Sigma-Aldrich, 3 Å) with subsequent distillation. THF was dis-
tillated using rotary vacuum evaporator before usage.
2.2. Catalyst preparation
Cu-Al layered double hydroxides with different Cu:Al molar ratio
were prepared by co-precipitation of desired amounts of Cu(NO3)2 and
Al(NO3)3 at pH 9.0
0.1 and temperature of 70 °C using NaOH/
Na2CO3 solution as described earlier [26,27]. The concentrations of
NaOH and Na2CO3 for the precipitant were chosen on the base of data
obtained in [28]. The authors showed that using pure sodium carbonate
solution (1.0 M) or mixture of NaOH and Na2CO3 with high con-
centration of Na2CO3 (0.5–1.0 M) as base solution does not allow
completely removing sodium ions from LDH material by washing pre-
cipitate with distilled water. This is confirmed by our previous work
[27]: the sodium content was 5.74 wt% in the Cu-Al LDH material
obtained by co-precipitation of metal nitrates by solution containing
NaOH and Na2CO3 with concentrations of 0.96 and 0.516 M, respec-
tively. Decreasing sodium carbonate concentration to 0.06 M allows
authors [28] to synthesize Cu-Al LDH sample containing only 0.005 wt
% of Na while complete substitution of sodium carbonate by sodium
hydroxide in the base solution leads to disappearance of sodium ions in
LDH material. However in this case the nitrate ions are present in
layered structure that affects the phase transformation during calcina-
tion. Thus, sodium hydroxide concentration for all samples was 0.96 M
([OH−] = 1.6([Al3+]+[Cu2+])) while the concentration of carbonate
ions was 0.86 M which is needed for LDH synthesis according to [18].
The catalysts were obtained by calcination of as-prepared LDHs at
450 °C or 650 °C for 4 h. The list and physico-chemical properties of the
catalysts, as well as preparation conditions, are given in Table 1.
3. Results and discussion
3.1. Catalyst characterization
The thermal behavior of the CuAl-LDH-1 sample was investigated
by thermal analysis (Fig. 1). The first endothermic step corresponds to
removal of physically adsorbed water. The steps in the temperature
range of 180–250 °C are attributed to dehydroxylation of brucite-like
layers and decarbonization of interlayer carbonate. Additionally, the
small exothermic peak at 568 °C is observed on DTA curve. This peak
can be related to the strongly bonded carbonates [33]. The authors [34]
confirmed this observation for Cu-based catalyst obtained from hy-
drotalcite-like compound using thermal analysis coupled with mass
spectrometer. They showed that there is CO2 formation at temperature
around 600 °C. Thus, the calcination temperature of 650 °C was chosen
to ensure that no carbonate remained in the sample structure. However,
CuAl-LDH-1 was also calcined at 450 °C to investigate the effect of
2

A.L. Nuzhdin, et al.
MolecularCatalysis494(2020)111132
Table 1
List of the catalysts.
Catalyst
LDH precursor
[Cu2+]/[Al3+
]
Tcal, °Ca
Cu, wt%
Al, wt%
SBET, m2 g−1
Vpore, cm3 g−1
CuAl-1
CuAl-2
CuAl-3
CuAl-LDH-1
CuAl-LDH-1
CuAl-LDH-2
1
1
2
650
450
650
47.2
40.7
60.6
19.9
17.2
12.8
68
87
47
0.15
0.20
0.12
a
Calcination temperature.
calcination temperature on the catalytic activity.
The XRD pattern of the Cu-Al LDHs dried at 110 °C shows the
characteristic peaks corresponding to the hydrotalcite phase (not pre-
sented) [26,27]. The calcination of the sample at 650 °C results in de-
struction of the layered structure and formation of copper (II) oxide
phase (Fig. 2) [26]. There are no reflexes associated with any crystalline
form of alumina, probably, due to its amorphous nature. Copper (II)
aluminate starts to form at calcination temperatures of ∼750–850 °C
[35,36] that is much higher than 650 °C which was used in this study. In
situ XRD of the CuAl-1 sample was carried out in hydrogen atmosphere
to determine the Cu° particle size after reduction (Fig. 2). According to
the obtained results, the reduction of CuO phase occurs in the range of
210–230 °C. The average size of copper crystallites in the reduced
sample was 7 nm that is much smaller than in case of the supported Cu/
Al2O3 catalysts with high copper content. The similar value of particle
size is possible to achieve for the 5% Cu/Al2O3 catalyst [30], but this
sample is rapidly deactivated in hydrogenation reactions [29].
The investigation of textural properties showed that an increase in
the calcination temperature of CuAl-LDH-1 from 450 to 650 °C pro-
motes decline of specific surface area (SBET) and pore volume (Vpore) due
to oxides sintering. The CuAl-3 sample with the Cu:Al molar ratio of 2
Fig. 2. In situ XRD of the CuAl-1 sample in hydrogen atmosphere.
has the lowest SBET and Vpore which are 47 m2 g−1 and 0.12 cm3 g−1
,
hinders the copper oxide reduction [37]. The reduction of the CuAl-2
sample can be considered as a two-stage process due to the shoulder
presence on the TPR curve. The first peak at 253 °C can be related to
reduction of CuO phase to Cu2O. The second stage at 312 °C corre-
sponds to the following reduction of Cu2O to Cu°. This assumption
correlates with data [34] which show formation of Cu2O in the course
of reduction by NEXAFS spectroscopy. The CuAl-1 sample calcined at
650 °C has one TPR maximum at the temperature of 240 °C that cor-
responds to reduction of CuO to Cu°.
respectively (Table 1).
H2-TPR of the samples with Cu:Al ratio of 1:1 was performed to
explore the reduction behavior of copper oxides (Fig. 3). A change in
the thermal conductivity of the flowing gas (proportional to the hy-
drogen consumption) during H2-TPR was compared. TPR experiment of
the CuAl-1 catalyst calcined at 650 °C shows completely different shape
of the reduction profile in comparison to that of the CuAl-2 sample
calcined at 450 °C meaning that the residual carbonate species affect
the reduction mechanism of copper oxide. The onset and end peak
temperatures of the carbonate-containing sample are shifted to the
higher temperature values indicating that the presence of carbonate
Fig. 1. Thermal analysis of the CuAl-LDH-1 sample.
3

A.L. Nuzhdin, et al.
MolecularCatalysis494(2020)111132
Fig. 4. Effect of hydrogen flow rate on the AMF conversion over the CuAl-1
catalyst in isopropanol (60 °C, 10 bar H2, liquid flow rate of 0.5 mL min−1).
Fig. 3. H2-TPR of the catalysts with Cu:Al ratio of 1.
increases with increasing gas velocity. In this case, hydrogen mass
transfer affects the course of the reaction. However, at higher flow rates
of H2 (≥30 mL min−1), the AMF conversion is independent of gas ve-
locity that indicates the absence of external mass transfer limitations
under these conditions [31,40]. The apparent activation energy (Ea) for
AMF hydrogenation over the CuAl-1 catalyst (within the temperature
range from 50 to 70 °C) estimated from the slope of the Arrhenius plot is
about 81 kJ mol−1 (Supplementary Material). This value indicates that
the reaction occurs in the kinetic regime [31] and, therefore, sub-
sequent catalytic tests were performed at a H2 flow rate of 30 mL
3.2. AMF hydrogenation
The catalytic properties of Cu-Al mixed oxides were studied in AMF
hydrogenation under continuous-flow conditions in the temperature
range of 50–90 °C and hydrogen pressure of 10 bar. Prior to the cata-
lytic test, CuO phase of catalysts was reduced in hydrogen flow at
120 °C to obtain Cu° metal particles [29,30]. It was found that hydro-
genation of AMF in the presence of the CuAl-1 catalyst leads to for-
mation of 5-(acetoxymethyl)-2-furanmethanol (AMFM) with selectivity
of 98 % at the full conversion of the substrate (Table 2). In addition to
AMFM, the small amounts of undesired 5-methylfurfural, 5-methyl-
furfuryl alcohol and acetic acid were observed among the reaction
products.
Liquid-phase hydrogenation over solid catalysts under continuous
flow conditions can be limited by external transfer of reactants to the
catalyst surface rather than by reaction kinetics [31,38,39]. Taking this
into account, a series of experiments was carried out to study the effect
of hydrogen flow rate on the conversion of AMF over the CuAl-1 cat-
alyst by increasing the flow rate of H2 from 5 to 45 mL min−1 at con-
stant liquid velocity, temperature and hydrogen pressure (Fig. 4). At
hydrogen flow rate from 5 to 30 mL min−1, the AMF conversion
min−1
.
The solvent effect on AMF hydrogenation over CuAl-1 catalyst was
studied. As shown in Table 2, there is no influence of the solvent nature
on the AMFM selectivity (Entries 1–6). However, it affects the AMF
conversion which decreases in the following order: absolute ethanol ∼
MeOH > i-PrOH > EtOH > > THF > toluene (Table 2, Entries
7–12). The protonic solvents, such as methanol, ethanol and iso-
propanol, are likely to activate the carbonyl group through the for-
mation of hydrogen bonds promoting the hydrogenation of AMF to
AMFM [41]. Thus, excellent yield of AMFM is obtained by AMF hy-
drogenation in alcohols including ethanol, which are considered as
“green” solvent [42]. It should be noted that the reduction of the CuO
phase of the catalysts does not occur in a mixture of hydrogen/ethanol
due to the presence of a large amount of water (∼4.5 %) that shifts the
reaction equilibrium towards copper oxides. Therefore, to carry out the
reaction in ethanol, the catalyst must be reduced using a low water
alcohol (Table 2, Entries 3 and 9).
Table 2
Hydrogenation of AMF over Cu-based catalysts in a flow reactor.
The CuAl-1 catalyst calcined at 650 °C provides a significantly
higher AMF hydrogenation rate than the CuAl-2 sample calcined at
450 °C (Table 2, Entries 7 and 13). It can be explained by incomplete
decomposition of carbonate species in the LDH structure at 450 °C that
hinders copper oxide reduction as was shown by H2-TPR (Fig. 3). The
CuAl-3 sample with the Cu:Al ratio of 2 reveals lower catalytic activity
(Table 2, Entries 1, 7, 14 and 15) in comparison to CuAl-1, probably,
due to its lower specific surface area (Table 1).
The time-dependent study of the CuAl-1 catalyst in AMF hydro-
genation showed only a slight decrease in AMF conversion during 2.5 h
of experiment, while AMFM selectivity remained unchanged (Fig. 5).
The study of the spent catalyst by elemental analysis allows to conclude
that the leaching of Cu and Al from the catalyst do not occur during the
reaction (Supplementary Material).
The thermogravimetric analysis under air was performed to esti-
mate the amount of carbonaceous deposits formed on the spent CuAl-1
catalyst (Fig. 6). The DSC curve of the spent catalyst showed two en-
dothermic peaks at 107 and 166 °C and one exothermic peak at 247 °C.
The peaks at 107 and 166 °C were attributed to desorption of physically
adsorbed water and dehydroxylation of the brucite layers, respectively
[43]. The exothermic peak at 247 °C can be associated with two
Entry
Catalyst
Solvent
T, °C
Conversion, %
Selectivity, %
1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-1
CuAl-2
CuAl-3
CuAl-3
Isopropanol
Methanol
Ethanol
Absolute ethanol
THF
90
90
90
90
90
90
60
60
60
60
60
60
60
90
60
> 99.9
> 99.9
> 99
> 99.9
99
91
85
87
80
88
28
13
34
95
55
98
98
98
98
98
98
99
99
99
99
99
99
99
98
99
2
3a
4
5
6
7
8
Toluene
Isopropanol
Methanol
Ethanol
Absolute ethanol
THF
9a
10
11
12
13
14
15
Toluene
Isopropanol
Isopropanol
Isopropanol
Reaction conditions: AMF (0.05 M), catalyst (165 mg), 10 bar H2, liquid flow
rate of 0.5 mL min−1, H2 flow rate of 30 mL min−1, reaction time 30–35 min.
a
Absolute ethanol was used in the course of the catalyst’s reduction.
4

A.L. Nuzhdin, et al.
MolecularCatalysis494(2020)111132
following
order:
absolute
ethanol
∼
MeOH > i-
PrOH > EtOH > > THF > toluene. The use of ethanol as a solvent
provides the sustainability of the reaction.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
This work was conducted within the framework of the budget
projects
#
АААА-А17-117041710083-5
and
АААА-А17-
Fig. 5. Time dependence of conversion and reaction selectivity during hydro-
genation of AMF (0.05 M) in isopropanol over the CuAl-1 catalyst at 90 °C.
117041710090-3 for Boreskov Institute of Catalysis. The studies are
conducted using the equipment of the Center of collective use “National
Center of Catalyst’s Research”. The authors are grateful to N.P. Yatsko,
T.Ya. Efimenko, and O.A. Nikolaeva for their help in carrying out this
work.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111132.
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4. Conclusions
The catalytic properties of the Cu-based catalysts prepared by cal-
cination of layered double hydroxides was studied in flow hydrogena-
tion of 5-acetoxymethylfurfural (AMF) to 5-(acetoxymethyl)-2-fur-
anmethanol (AMFM). It was found that the Cu/Al ratio and calcination
temperature affect the catalyst performance in AMF hydrogenation. The
increase of calcination temperature of Cu-Al layered double hydroxide
(Cu/Al of 1) from 450 to 650 °C was shown to enhance significantly
conversion of AMF over Cu-based catalyst. It was concluded that in-
complete decomposition of the carbonate species in the LDH structure
after calcination at 450 °C retards copper species reduction that is a
reason of lower activity of the CuAl-2 sample (calcined at 450 °C).
Increase of the Cu/Al molar ratio to 2 leads to decrease of AMF con-
version, probably, due to low specific surface area and total pore vo-
lume. Thus, the best hydrogenation activity is obtained over the CuAl-1
sample with the Cu/Al ratio of 1 prepared by calcination at 650 °C.
Flow hydrogenation of AMF over CuAl-1 catalyst gives the AMFM
selectivity of 98 % at the full conversion of AMF under mild reaction
conditions (90 °C and 10 bar). Solvent nature has no noticeable effect
on the AMFM selectivity while AMF conversion decreases in the
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