Gas phase hydrogenation of furaldehydes via coupling with alcohol dehydrogenation over ceria supported Au-Cu

Article abstract of DOI:10.3390/molecules23112905

We have investigated the synthesis and application of Au-Cu/CeO₂ (Cu: Au = 2) in the continuous gas phase (P = 1 atm; T = 498 K) coupled hydrogenation of 5-hydroxymethyl-2-furaldehyde (HMF) with 2-butanol dehydrogenation. STEM-EDX analysis revealed a close surface proximity of both metals in Au-Cu/CeO₂ post-TPR. XPS measurements suggest (support → metal) charge transfer to form Au^δ? and strong metal-support interactions to generate Cu⁰ and Cu⁺. Au-Cu/CeO₂ promoted the sole formation of 2,5-dihydroxymethylfuran (DHMF) and 2-butanone in the HMF/2-butanol coupling with full hydrogen utilisation. Under the same reaction conditions, Au/CeO₂ was fully selective to DHMF in standard HMF hydrogenation (using an external hydrogen supply), but delivered a lower production rate and utilised less than 0.2% of the hydrogen supplied. Exclusive -C=O hydrogenation and -OH dehydrogenation is also demonstrated for the coupling of a series of m-substituted (-CH₃, -CH₂CH₃, -CH₂OH, -CF₃, -N(CH₃)₂, -H) furaldehydes with alcohol (1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol) dehydrogenation over Au-Cu/CeO₂, consistent with a nucleophilic mechanism. In each case, we observed a greater hydrogenation rate and hydrogen utilisation efficiency with a 3–15 times lower E-factor in the coupling process relative to standard hydrogenation. Our results demonstrate the feasibility of using hydrogen generated in situ through alcohol dehydrogenation for the selective hydrogenation of m-furaldehydes with important industrial applications.

Full text of DOI:10.3390/molecules23112905

molecules
Article
Gas Phase Hydrogenation of Furaldehydes via
Coupling with Alcohol Dehydrogenation over Ceria
Supported Au-Cu
Chiara Pischetola , Laura Collado, Mark A. Keane and Fernando Cárdenas-Lizana *
Chemical Engineering, School of Engineering and Physical Sciences, Heriot Watt University,
Edinburgh EH14 4AS, Scotland, UK; cp44@hw.ac.uk (C.P.); laura.lcb84@gmail.com (L.C.);
M.A.Keane@hw.ac.uk (M.A.K.)
*
Correspondence: F.CardenasLizana@hw.ac.uk; Tel.: +44-(0)131-4514115
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 7 October 2018; Accepted: 5 November 2018; Published: 7 November 2018
Abstract: We have investigated the synthesis and application of Au-Cu/CeO2 (Cu: Au
2) in the continuous gas phase (P = 1 atm; T = 498 K) coupled hydrogenation of
-hydroxymethyl-2-furaldehyde (HMF) with 2-butanol dehydrogenation. STEM-EDX analysis
=
5
revealed a close surface proximity of both metals in Au-Cu/CeO post-TPR. XPS measurements
2
δ−
suggest (support
→
metal) charge transfer to form Au and strong metal-support interactions to
generate Cu and Cu . Au-Cu/CeO promoted the sole formation of 2,5-dihydroxymethylfuran
0
+
2
(
DHMF) and 2-butanone in the HMF/2-butanol coupling with full hydrogen utilisation. Under the
same reaction conditions, Au/CeO was fully selective to DHMF in standard HMF hydrogenation
2
(
0
using an external hydrogen supply), but delivered a lower production rate and utilised less than
.2% of the hydrogen supplied. Exclusive -C=O hydrogenation and -OH dehydrogenation is
also demonstrated for the coupling of a series of m-substituted (-CH , -CH CH , -CH OH, -CF ,
3
2
3
2
3
-
N(CH ) , -H) furaldehydes with alcohol (1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol)
3 2
dehydrogenation over Au-Cu/CeO , consistent with a nucleophilic mechanism. In each case, we
2
observed a greater hydrogenation rate and hydrogen utilisation efficiency with a 3–15 times lower
E-factor in the coupling process relative to standard hydrogenation. Our results demonstrate the
feasibility of using hydrogen generated in situ through alcohol dehydrogenation for the selective
hydrogenation of m-furaldehydes with important industrial applications.
Keywords: HMF; coupling hydrogenation/dehydrogenation; m-furaldehydes; alcohols; Au/CeO2;
Au-Cu/CeO2
1
. Introduction
5
-hydroxymethyl-2-furaldehyde (HMF) has emerged as a promising biomass-derived platform
chemical [ ] that can be readily obtained from C carbohydrates (e.g., fructose, glucose) by dehydration
in the presence of acid catalysts (e.g., metal halides, acid metal oxides) [
1
6
2,
3
]. 2,5-Dihydroxymethylfuran
DHMF), a reduction product of HMF, is used as a precursor in the manufacture of polymer plastics
e.g., polyurethane foams, 16M tons per year [ ]) resins and artificial fibres [ ]. Conventional
DHMF reduction involves standard catalytic hydrogenation in batch liquid phase [ ] that
suffers from safety and environmental issues due to the requirement for operating at high pressures
typically 5–100 bar) [ ] under conditions with high excess of hydrogen in order to maximise
H solubility and product yield. Hydrogen production requires non-renewable fossil fuel-based
(
(
4
5,6
HMF
→
7
(
7
2
technologies such as steam and auto-thermal reforming [
as HMF hydrogen treatment can generate several products as shown in Figure 1(IA) [
8
]. Reaction selectivity is also challenging,
11] with
9–
Molecules 2018, 23, 2905; doi:10.3390/molecules23112905
www.mdpi.com/journal/molecules

Molecules 2018, 23, 2905
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Ru-catalysts delivering high combined activity/selectivity [7]. There is now a pressing demand for a
more sustainable system that satisfies green chemistry principles to promote selective DHMF formation
i.e., #2, 7, 9 and 12 [12]). Several metrics have emerged to quantitatively assess green performance,
(
−
1
most notably the environmental factor (E-factor, kgwaste kgproduct ) [13]. E-factor values within the
–100 range are characteristic of catalytic processes in the manufacture of pharmaceuticals and fine
chemicals industries [13].
5
Figure 1.
frame) and undesired by-products from hydrogenolysis (MF, MFA, DMF), ring reduction (DHMTHF,
MTHFA, DMTHF) and ring opening (2-HXOL) and ( ) 2-butanol dehydrogenation to the target
-butanone (bold arrow, dashed frame) and undesired by-products from dehydration (butene) and
(I) Reaction scheme for (A) HMF hydrogenation to the target DHMF (bold arrow, dashed
B
2
dimerisation (octanone and/or octanol isomers). (II) Schematic representation of the coupling system.
Note: HMF = 5-hydroxymethyl-2-furaldehyde, MF = 5-methyl-2-furaldehyde, 2-HXOL = 2-hexanol, DHMF
=
=
2,5-di-(hydroxymethyl)-furan, MFA = 5-methyl furfuryl alcohol, DMF = 2,5-dimethylfuran, DHMTHF
2,5-di-(hydroxymethyl)-tetrahrydrofuran, MTHFA = 5-methyltetrahydro furfuryl alcohol, DMTHF =
2
,5-dimethyltetrahydrofuran.
Hydrogenation can be effectively carried out in the absence of an external hydrogen supply
through an alternative coupled process in which hydrogen generated in situ via (non-oxidative)
dehydrogenation of alcohols (Figure 1(IB)) is utilised in a hydrogenation process (Figure 1(II)).

Molecules 2018, 23, 2905
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This coupling strategy offers a series of advantages relative to stand-alone hydrogenation and
dehydrogenation reactions including: (i) one-pot simultaneous production of two valuable chemicals
(i.e., improved atom economy), (ii) good thermal efficiency (i.e. heat transfer from exothermic
hydrogenation to endothermic dehydrogenation), (iii) enhanced yields (i.e. consumption in the
hydrogenation step displaces the equilibrium of dehydrogenation increasing hydrogen production) and
(
iv) improved process safety (i.e. no need for pressurised H ) [14,15]. Despite the multiple advantages,
2
it is a challenging system, with only a limited number of published studies on coupled heterogeneous
catalytic systems. This is likely due to several factors, including: (i) a requirement for two active
sites for dehydrogenation and hydrogenation (M and M in Figure 1(II)) that must be separated but
1
2
in close proximity to facilitate hydrogen transport and utilisation, (ii) that sufficient hydrogen must
be generated in the dehydrogenation step for the hydrogenation reaction and (iii) the possibility of
cross reaction between reactants and/or products. It is nonetheless worth mentioning the promising
results reported in the coupling of 1,4-butanediol [16] and n-butanol [17] dehydrogenation with maleic
anhydride hydrogenation, the coupled of cyclohexanol/furfural [15
,18–20] and 1,4-butanediol/furfural
conversion [20 23]. Taking HMF DHMF, the feasibility of coupling HMF with 1,4-butanediol
–
→
over Cu-Al [24] and 2-butanol using zirconium hydroxides [25] has been explored only in batch
liquid systems where the requirement of high pressure (16 bar) or low selectivity to target DHMF
(
SDHMF
≤
90%) are decided drawbacks. We were unable to find any study in the open literature dealing
with DHMF production through a coupling process with HMF in continuous gas-phase operation.
We have recently showed the feasibility of coupling 2-butanol with furfural over a physical
mixture of Au/CeO + Cu/CeO [26]. Minimising the physical separation between Au and Cu
2
2
in bimetallic catalysts should facilitate hydrogen transfer/utilisation. In this work we evaluate the
continuous gas phase dehydrogenation of alcohols (to ketones/aldehydes) coupled with hydrogenation
of HMF to DHMF over Au-Cu supported on (non-toxic, [27]) CeO2. The catalytic action of
Au-Cu/CeO to promote -C=O group reduction for a series of m-substituted furaldehydes has been
2
investigated to understand reaction mechanism and establish the potential of the bimetallic catalyst.
The benefits of the coupled process are examined by comparison with (conventional) stand-alone
hydrogenation using an external hydrogen supply over Au/CeO as a benchmark and employing the
2
E-factor as a measure of environmental performance.
2
. Results and Discussion
2
.1. Catalytic Conversion of 5-Hydroxymethyl-2-Furaldehyde (HMF): Coupling
Dehydrogenation-Hydrogenation vs. Conventional Stand-Alone Hydrogenation Using an External H Supply
2
In the gas-phase coupled 2-butanol dehydrogenation with HMF hydrogenation, Au-Cu/CeO2
was 100% selective towards the production of targets 2-butanone and DHMF (Figure 1(II)). These two
products can be readily separated by distillation due to the (195 K) difference in boiling points [28].
The formation of DHMF demonstrates utilisation of hydrogen generated in the dehydrogenation
step for the continuous conversion of HMF. We could not find any report on DHMF production
through alcohol/HMF coupling dehydrogenation/hydrogenation in continuous gas phase operation.
It is, nonetheless, worth mentioning the recent work of Hu et al. [25], who investigated the coupled
HMF/2-butanol process in batch liquid phase over magnetic zirconium hydroxides although they
reported the undesired formation of 5-methyl-2-furaldehyde (MF) and 5-methylfurfuryl alcohol
(
MFA) with selectivity towards the target DHMF
difficult to fully circumvent also in standard hydrogenation, with unwanted over-hydrogenation
to DHMTHF, Figure 1(IA)) and/or hydrogenolysis (to MFA and DMF) reported in the liquid phase
hydrogen treatment of HMF over supported Ni [10], Pd [11] and Ru [ ] catalysts. In addition to DHMF
≤
90%. Exclusive formation of DHMF has proved
(
9
selectivity, hydrogen utilisation efficiency is a key parameter that must be optimised to guarantee
process sustainability. Full hydrogen utilisation was achieved in the coupled system with all the amount
generated via 2-butanol dehydrogenation being utilised in HMF hydrogenation, i.e., H /HMF = 1.
2
We recorded no conversion in the stand-alone dehydrogenation of 2-butanol (in N ) over Au/CeO
2
2

Molecules 2018, 23, 2905
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while under similar reaction conditions (P = 1 atm; T = 498 K) Cu/SiO delivers negligible activity
2
in the gas phase hydrogenation of furfural [29]. This suggests that Cu and Au are the active sites for
2-butanol dehydrogenation and HMF hydrogenation, respectively. Alcohol dehydrogenation over
supported copper [30] proceeds through a two-step mechanism that involves sequential H abstraction
from O-H bond and
α
-carbon [26
,
31]. HMF adsorption can proceed via -C=O bond
π
-back donation
δ−
δ+
on Au
[
32] or carbonyl O coordination with Au sites [33]. Spillover hydrogen species migrate
from Cu
→
Au across the CeO surface [34] with auto-transfer in the catalytic conversion of HMF
2
→ DHMF.
We compared the catalytic response in the coupled vs. stand-alone process under the same
reaction conditions of stoichiometric hydrogen supply (i.e. H /HMF = 1). No conversion was detected
2
over Au/CeO or Au-Cu/CeO in standard HMF hydrogenation using an external supply of H .
2
2
2
This lack of activity can be linked to the low capacity of Au for H chemisorption/activation [35
,
36],
2
rate-limiting step in stand-alone hydrogenation [37]. Atomic hydrogen generated in situ in the coupled
process during 2-butanol dehydrogenation is active for hydrogenation [38] and participates in HMF
transformation, circumventing the issue of H activation by gold. An increase in hydrogen supply
2
(
H /HMF = 80) in stand-alone hydrogenation enhanced the available surface H where both catalysts
2
2
promote exclusive DHMF formation at similar production rates, but with less than 0.2% of the hydrogen
supplied being utilised. This result demonstrates a requirement to work under conditions of hydrogen
excess in the stand-alone process. Moreover, same activity/selectivity response over Au/CeO and
2
Au-Cu/CeO further proves that the presence of Cu has no effect on the hydrogenation step which
2
is governed by the gold component. We observed a 3.5-fold greater DHMF production rate in the
coupling process relative to stand-alone hydrogenation over Au-Cu/CeO . This demonstrates that
2
the production of activated hydrogen over Cu by in situ alcohol dehydrogenation is more efficient
than chemisorption of molecular H on Au via standard hydrogenation. Coupled reaction over
2
Au-Cu/CeO was accompanied by lower E-factor (56) relative to standard hydrogenation in gas
2
phase over Au/CeO (260) and batch mode using Ru catalysts (~100) [7]. Our results prove the
2
benefits of the coupled process compared to standard hydrogenation for the transformation of HMF in
terms of greater catalytic activity and full hydrogen utilisation efficiency with exclusive formation of
target DHMF.
2
.2. Catalyst Characterisation: Au-Cu/CeO and Au/CeO
2 2
The critical properties of Au-Cu/CeO and Au/CeO are presented in Table 1. Both catalysts
2
2
exhibit a similar TPR profile (Figure 2) characterised by a broad positive (hydrogen consumption) peak
centred at 427
±
5 K that is within the reported temperature range (408–550 K) for the reduction of
3+
0
2+
Au
→
Au and transformation of Cu to zero valent Cu [39
,
40]. Total H consumption exceeded
2
(by up to a factor of 4) the requirement for complete reduction of the metal precursor(s), indicative
of partial support reduction at the metal-CeO interface by spillover hydrogen [41]. Both catalysts
2
) characteristic of Au3+
(67
→
Au and
0
present peaks with Tmax at 412
±
2 K (
α
) and 426
±
3 K (
β
4+
3+
−1
Ce
and
of CeO support reduction by spillover hydrogen promoted by gold. Limited capacity of Cu for
→
Ce , respectively [42
,
43]. The similar hydrogen consumption for
α
±
2
µ
mol g
)
−
1
β
(233
±
5 µmol g ) in both catalysts suggests same nature of Au and equivalent degree
2
H activation can account for the similar degree of CeO reduction and same catalytic response in
2
2
stand-alone hydrogenation over Au-Cu/CeO and Au/CeO . In addition, Au-Cu/CeO shows peaks
2
2
2
characteristic of Cu2
Cu reduction [44
+
→
Cu (
+
γ
centred at 437 K) and Cu
45], where hydrogen consumption is consistent with a Cu :Cu molar ratio = 3:1.
The presence of cationic copper following TPR to 573 K can be linked to a stabilisation of Cu due
to strong metal support interactions at Ce3+ sites [46
+
→
Cu (
0
δ
at 443 K) indicative of stepwise
+
0
,
+
,
47]. Liu et al. [48], studying methanol steam
+
reforming, showed (by XPS) the presence of Cu after reduction at T ≥ 573 K.

Molecules 2018, 23, 2905
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Table 1. Physico-chemical characteristics of CeO supported Au-Cu and Au catalysts.
2
Au-Cu/CeO2
Au/CeO2
Metal loading (mol%)
TPR
1 (Au)/2 (Cu)
1
α
414
429
437
443
410
423
-
β
γ
δ
Tmax (K)
-
H2 consumption (µmol g 1)
−
a
b
c
307 /69 b
a
340 /63 /61
4
dSTEM (nm) d
3
0
Au (%)
83.6 (100)
928.7 (35)
932.0 (65)
83.5 (100)
0
Binding energies (eV)
Ce3+ atomic ratio e
Cu (%)
XPS
+
Cu (%)
0.24
0.29
a
experimental value; theoretical hydrogen requirement for Au3+
b
Au0; c theoretical amount of hydrogen for
→
2
+
+
0 d
Cu
→
Cu
→
Cu ; mean metal particle size from scanning transmission electron microscopy (STEM) analysis
e
3+
3+
4+
3+
(Equation (1)); Ce atomic ratio = Ce /(Ce + Ce ) from XPS measurements (see Figure S1 in Supplementary
3
+
Materials). Note:
Au , Ce → Ce Cu → Cu and Cu → Cu , respectively.
α
3+,
,
β
,
γ
2+
and
δ
+
represent hydrogen consumption peaks during TPR associated with transition of Au
→
0
4+
+
0
Figure 2. H -Temperature programmed reduction (TPR) profiles for (I) Au-Cu/CeO and (II) Au/CeO2.
2
2
Note: Raw data is shown as open symbols
and dashed lines, respectively.
ꢀ
while curve fitted and envelope is represented by solid
Scanning transmission electron microscopy (STEM) measurements (Figure 3(I-II)) revealed a
similar metal dispersion and mean size for both catalysts characterised by pseudo-spherical nano
particles with an associated surface area weighted mean diameter of 3.5
±
0.5 nm. The surface
composition of the Au-Cu/CeO catalyst was probed by STEM/EDX elemental analysis. A detailed
2
elemental analysis over small areas was carried out and the EDX spectra of three isolated metal
particles (IIIAa–IIIAc) in Au-Cu/CeO are shown in Figure 3. The EDX spectra exhibit peaks due to
2
nickel in the grid (7.5 and 8.3 keV) [49] and the CeO support (4.9–5.8 keV) [50]. The signals at 2.2, 9.7
2
and 11.5 keV in the EDX of particles a–b are characteristic of Au [50]. In contrast, particle c (IIIAc)
shows only a peak at 8 keV consistent with the presence of Cu [50]. The detailed EDX analysis over
single metal nanoparticles reveals a close proximity of Au and Cu on the surface. In the case of the
Au-Cu system, the miscibility gap is such that bulk alloy formation is possible at T
is a dearth of literature dealing with Au-Cu systems; however, metal segregation for Au-Cu/CeO
≥
490 K [51]. There
2
catalysts post-thermal treatment at T
and attributed to strong interactions at the metal-CeO interphase. Zhang et al. [52] concluded that
≤
573 has been proven theoretically [52] and experimentally [53]
2
CeO support induces segregation for the Au-Cu system based on the CeO -induced preferential
2
2
segregation energy (Eseg,CeO2-Au), a parameter indicating the strength of the Cu-CeO vs. Cu-Au bond
2
energy. Ta and co-workers [54] showed by atomic resolution environmental transmission electron

Molecules 2018, 23, 2905
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microscopy that gold nanoparticles of 2–4 nm strongly anchored onto CeO and did not sinter after
2
reduction at T ≤ 573 K.
Figure 3.
(I
) Representative STEM images with (II) associated metal size distribution histogram for (
A)
Au-Cu/CeO (solid bars) and (
B
) Au/CeO (hatched bars). Note: STEM-EDX analyses of (a–c) isolated
2
2
metal particles in (IA) are shown in (IIIAa–IIIAc).
XPS spectra of in situ activated Au-Cu/CeO (I) and Au/CeO (II) over the Au 4f (A) and Cu
2
2
2
p3/2 (B) binding energy (BE) regions are represented in Figure 4, and the results after deconvolution
given in Table 1. The Au 4f profiles for Au-Cu/CeO and Au/CeO are equivalent suggesting same
2
2
electronic properties for the gold phase with no measurable electron transfer in Au-Cu/CeO relative
2
to Au/CeO . The XPS spectrum presents two peaks with associated binding energies (BE) at 83.5
2

Molecules 2018, 23, 2905
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and 87.2 eV corresponding to 4f7 and 4f5/2 levels, respectively [55]. The BE values are lower than
/2
0
those reported for Au (Au 4f7 = 84.0 eV and Au 4f5/2 = 87.7 eV [55
,56]), indicative of Au-support
/2
interactions that impact on Au electronic character [57]. A similar 0.4-1.0 eV downshift in BE for Au
nanoparticles
≤
6 nm on CeO was reported by Lai et al. [58] and attributed to electron transfer from the
2
δ−
support. The presence of Au nanoparticles is consistent with the adsorption/activation of the -C=O
functionality in HMF via
π
-back donation [32]. The XPS spectrum for Au-Cu/CeO over the Cu 2p
2
0
region (Figure 4(IB)) is characterised by a Cu 2p3/2 contribution at BE = 928.7 eV, corresponding to Cu
(
(
35%) [59]. A signal at higher BE (= 932.0 eV) with greater intensity can be linked to the presence of Cu+
65%) [59]. The surface composition and oxidation state of copper from XPS measurements is consistent
with TPR results. The XPS spectra over the Ce 3d region for CeO , Au-Cu/CeO and Au/CeO are
2
2
2
presented in Figure S1, Supplementary Materials. The as-received and H -treated (to 573 K) CeO
2
2
3+
samples show an equivalent response characterised by a Ce atomic ratio = 0.12
±
0.01, indicative
of a higher temperature requirement for Ce4+
→
Ce reduction (>625 K) [42]. A similar higher Ce
3+
3+
atomic ratio (0.26
±
0.03) was recorded for both supported catalysts (Table 1). The presence of Au
43], in
on CeO surface can induce a partial reduction of the support, due to hydrogen spillover [42
,
2
agreement with H consumption during TPR analysis (Table 1).
2
Figure 4. XPS spectra over the (
A
) Au 4f and (
B
) Cu 2p3/2 regions for (
I) Au-Cu/CeO and (II)
2
Au/CeO . Note: Raw data is shown as open symbols
by solid and dashed lines, respectively.
ꢀ
while curve fitted and envelope is represented
2
2
.3. Dehydrogenation of Alcohols Coupled with Hydrogenation of meta-Substituted Furaldehydes
We examined the potential of the Au-Cu/CeO catalyst for coupling a range of alcohols with
2
m-substituted furaldehydes and the results are presented in Figure 5. In each case, we achieved full
selectivity to the target aldehyde/ketone (from dehydrogenation) and alcohol (from hydrogenation)
with no evidence of undesired hydrogenolysis, ring reduction, dehydration or dimerisation. We
first evaluated the effect of changing the nature of the alcohol in the dehydrogenation step and
the results for the coupled process with HMF are shown in Figure 5(I). In each case, the rate of
DHMF production was up to a 4-fold greater than that recorded over Au/CeO using an external
2
hydrogen supply. The modified hydrogenation activity for the different alcohols (cyclohexanol <
1-propanol < 1-butanol < 2-propanol < 2-butanol) demonstrates that hydrogen generation (from
dehydrogenation) is rate limiting in the coupling system. The greater hydrogenation rate for secondary
vs. primary alcohols (2-butanol vs. 1-butanol and 2-propanol vs. 1-propanol) observed is in line
with reports in the literature [60,61] and consistent with reaction thermodynamics where hydrogen
o
−1
[62]) is
extraction from 2-butanol (to methyl elthyl ketone; reduction potential ∆Hf = 69 kJ
·
mol
more favourable than 1-butanol conversion (to butyl aldehyde, ∆H of = 80 kJ
·
mol ) [62]) as well as
−1

Molecules 2018, 23, 2905
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o
−1
[62]) and 1-propanol (formation of
in case of 2-propanol (formation of acetone, ∆H = 70 kJ
·
mol
−1
f
propanal, ∆H of = 86 kJ
·
mol
−1
[
62]). Greater ∆H of (77 kJ
·
mol
[62]) and increased steric hindrance [63
]
can account for the lower dehydrogenation rate of (cyclic) cyclohexanol (to cyclohexanone) relative to
secondary aliphatic alcohol conversions.
In the dehydrogenation of 2-butanol coupled with hydrogenation of substituted furaldehydes
bearing -CH , -CH CH , -CH OH, -CF , -N(CH ) and -H substituents in the meta-position,
3
2
3
2
3
3 2
Au-Cu/CeO was again fully selective in generating 2-butanone and the corresponding alcohol
2
product. The higher hydrogenation activity in the coupling process over Au-Cu/CeO relative to
2
stand-alone hydrogenation using Au/CeO observed in the case of HMF extended to all the substituted
2
furaldehydes (Figure 5(II)). The following activity sequence was established in the coupling process:
5
<
-dimethylamino-2-furaldehyde < 5-ethyl-2-furaldehyde < 5-methyl-2-furaldehyde < furfural < HMF
5-trifluoromethyl-2-furaldehyde. The hydrogenation of the carbonyl group has been proposed to
occur through a nucleophilic mechanism [64] with formation of a negatively charged hydroxyalkyl
intermediate [65] that results from the attack of the carbonyl oxygen by hydrogen that acts as a
weak nucleophilic agent [66]. Electron withdrawing functionalities in meta-position of aromatic
systems favour the delocalisation of the negative charge in the hydroxyalkyl intermediate lowering
the activation energy barrier [65] of the first reaction step (i.e. addition of the first hydrogen), which, in
turns increases hydrogenation rate. The σm factor (Hammet constant) is an empirical parameter that
provides a measure of the electron donating/acceptor character of a functionality in meta-position on
aromatic systems [67,68]. In a nucleophilic attack, reaction rate is increased by electron-withdrawing
groups with σm > 0. The higher activity with increasing σm (from -N(CH3)2 σm = −0.16 to CF3
σm = 0.43) [63] (see Figure 5(II)) is consistent with a nucleophilic mechanism.
The hydrogen utilisation efficiency (see Material and Methods) in the conversion of the different
m-furaldehydes by stand-alone hydrogenation (using conventional pressurised H ) and coupling is
2
presented in Figure 5(III). The hydrogen utilisation for stand-alone hydrogenation of all the tested
furaldehydes was 200–900 times greater than the stoichiometric (=1) requirements, a result that
represents high inefficiency. This issue was circumvented in the coupled process where, in each case,
hydrogen utilisation efficiency was close or equal to stoichiometry with an associated E-factor 3–15
times lower relative to stand alone hydrogenation.

Molecules 2018, 23, 2905
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Figure 5. Comparison of hydrogenation performance over Au-Cu/CeO in the coupled process vs.
2
stand-alone reaction using Au/CeO : Hydrogenation rate in the coupling reaction (r , bars) and rate
2
C
enhancement (in the coupling vs. stand-alone hydrogenation given as r /rSA,
3
) for the conversion
C
of (I
) a series of alcohols + HMF and (II) 2-butanol + m-substituted furaldehydes; (III) H utilisation
2
efficiency in coupling system ( ) and in stand-alone hydrogenation using an external H supply
ꢀ
2
Note: horizontal dashed line in (III) represents full H utilisation under stoichiometric conditions.
2
Reaction conditions: P = 1 atm, T = 498 K.

Molecules 2018, 23, 2905
10 of 16
3
. Materials and Methods
3
.1. Catalyst Preparation and Activation
The CeO support was purchased from Sigma-Aldrich and used as received. 1 mol% Au/CeO
2
2
was prepared by deposition–precipitation using urea (Riedel-de Haën, 99%) as basification agent.
-
4
3
An aqueous solution of urea (3 M) and HAuCl4
·
H O (6
2
×
10 M, 300 cm , Sigma-Aldrich, 99.995%)
was added to the support (5 g) and the suspension was stirred (600 rpm) and heated to 353 K. The pH
progressively increased to reach 7 as a result of thermal decomposition of urea with consequent
formation of Au(III) surface complexes [69]. The (1 mol% Au and 2 mol% Cu) Au-Cu/CeO catalyst
2
was prepared by stepwise deposition–precipitation of Cu followed by Au. NaOH (2 M, Fisher Scientific,
≥
97%) as basification agent was added to an aqueous solution of the metal precursor (Cu(NO ) 3H O
·
3
2 2
−
2
3
(1
×
10 M, 200 cm Sigma-Aldrich, 99%)) containing the support (5 g) until pH = 10, heated to
353 K and aged under vigorous stirring for 4 h to ensure homogeneous deposition of Cu(OH) [70].
2
Au incorporation to the Cu/CeO catalyst was performed by deposition–precipitation, method as
2
above. The solid obtained was filtered, washed with distilled water until pH = 7 and dried at 393 K
overnight. Prior to use in catalysis, the samples (sieved into a batch of 75 µm average diameter) were
3
−1
−1
activated in 60 cm min H at 2 K min to 573 K, which was maintained for 1 h. The samples were
2
cooled to ambient temperature and passivated in 1% v/v O /N at 298 K for ex situ characterisation.
2
2
3
.2. Catalyst Characterisation
The Au and Cu loading was measured by atomic absorption spectroscopy (AAS) using a Shimadzu
AA-6650 spectrometer with an air-acetylene flame from the diluted extract in aqua regia (25% v/v
HNO /HCl). Temperature programmed reduction (TPR) was conducted on the CHEM-BET 3000
3
(
Quantachrome Instrument) unit with data acquisition/manipulation using the TPR WinTM software.
3
−1
Samples were loaded into a U-shaped Pyrex glass cell (3.76 mm i.d.) and heated in 17 cm min
−
Brooks mass flow controlled) 5% v/v H /N to 573 K at 2 K min . The effluent gas passed through a
2 2
1
(
liquid N trap and H consumption was monitored by a thermal conductivity detector (TCD). Curve
2
2
fitting TPR profile (CasaXPS 2.3.17 software) was employed to analyse the reduction step(s) during
thermal treatment [71]. Metal particle morphology (size and shape) was examined by STEM probe
corrected on a JEOL ARM 200CF with an energy dispersive X-ray (EDX) detector operated at an
accelerating voltage of 200 kV. The scanned images were collected using either Gatan 806 High Angle
Annular Dark Field, Gatan 805 Annular Dark Field/Bright field or JEOL ADF1detectors under the
control of a Gatan DigiScan II, employing Gatan DigitalMicrograph software (version 2.31) for data
acquisition/manipulation. Samples were prepared for analysis by dry deposition on a holey carbon/Ni
grid (300 Mesh). At least 250 individual metal nanoparticles were counted for each catalyst and the
surface area-weighted mean metal particle size (dSTEM) was calculated from:
nid3i
nid2i
∑
i
dSTEM
=
(1)
∑
i
where n is the number of particles of diameter d . X-ray photoelectron spectroscopy (XPS)
i
i
measurements were performed using a monochromatised Al anode (K
α
1486.6 eV, 10 kV, 20 mA). Prior
2
to analysis, the samples were activated in situ in a pre-chamber under H atmosphere (10 mbar) at
2
−
1
3
2
K min to 573 K. The source power was maintained at 3.9
×
10 W and the emitted photoelectrons
2
◦
were sampled from an area of 13 mm ; the photoelectron take-off angle was normal emission (0 ).
The analyser pass energy was 150 eV for survey (0–1100 eV) and high-resolution spectra (over the Au
4
f, Cu 2p and Ce 3d core levels). The C 1s peak was calibrated at 284.5 eV and used as internal standard
to compensate for charging effects. Spectra curve fitting and quantification were performed with the
CasaXPS software, using relative sensitivity factors provided by Kratos.

Molecules 2018, 23, 2905
11 of 16
3
.3. Catalytic Procedure
Reactions (independent hydrogenation of m-furaldehyde in H , alcohol dehydrogenation in
2
N and coupled alcohol dehydrogenation/m-furaldehyde hydrogenation in N ) were carried out
2
2
under atmospheric pressure, in situ immediately after activation, in a fixed bed vertical continuous
flow glass reactor (i.d. = 15 mm) at 498 K. Operating conditions ensured negligible internal/external
mass and heat transfer limitations [72]. A layer of borosilicate glass beads served as preheating
zone where the organic reactant(s) was(were) vaporised and reached reaction temperature before
contacting the catalyst bed. Isothermal conditions (
±
1 K) were maintained by thoroughly mixing
the catalyst with ground glass (75 m). Reaction temperature was continuously monitored by a
µ
thermocouple inserted in a thermowell within the catalyst bed. The reactant(s) was(were) delivered
to the reactor via a glass/Teflon air-tight syringe and Teflon line using a microprocessor-controlled
infusion pump (Model 100 kd Scientific). Stand-alone hydrogenation and dehydrogenation were
3
− 1
carried out in a co-current flow of H with m-furaldehyde (GHSV = 3.4
×
10 h ; molar Au to reactant
2
−
3
3
−1
feed rate (nAu/F
m
-furaldehyde) = 1
×
10 h) or N with alcohol (GHSV = 3.4
×
10 h , nAu/Falcohol
=
=
2
−
2
3
− 1
3
2
×
10 h). The coupled reaction was carried out in N (GHSV = 3.4
×
10 h , nAu/F
m
2
-furaldehyde
−
4
− 3
×
10 – 1
×
10 h). The catalytic response over Au-Cu/CeO vs. Au/CeO in the stand-alone
hydrogenation of HMF (using an external hydrogen supply) was examined by using an aprotic solvent
2
2
(
anisole) to avoid any contribution to hydrogenation from in situ hydrogen production via alcohol
dehydrogenation over Cu. In a series of blank tests, passage of each reactant in a stream of H2
or N through the empty reactor or over the (CeO ) support alone did not result in any detectable
2
2
conversion. The reactor effluent was condensed in a liquid nitrogen trap for subsequent analysis using a
Perkin-Elmer Auto System XL gas chromatograph equipped with a programmed split/splitless injector
and a flame ionisation detector, employing a DB-1 (50 m
column (J&W Scientific). Data acquisition and manipulation were performed using the TurboChrom
Workstation Version 6.3.2 (for Windows) chromatography data system. Furfural (Sigma-Aldrich,
×
0.33 mm i.d., 0.20 µm film thickness) capillary
9
9
9%), 5-methyl-2-furaldehyde (Sigma-Aldrich, >98%), 5-ethyl-2-furaldehyde (Sigma-Aldrich,
8%), 5-hydroxymethyl-2-furaldehyde HMF (Sigma-Aldrich, 99%), 5-trifluoromethyl-2-furaldehyde
(
2
>
Sigma-Aldrich), 5-dimethylamino-2-furaldehyde (Sigma-Aldrich), 1-butanol (Sigma-Aldrich, 99.8%),
-butanol (Sigma-Aldrich, 99.5%) 1-propanol (Fisher Scientific, >99%), 2-propanol (Fisher Scientific,
99.5%), cyclohexanol (Fisher Scientific, >98%) and anisole (Sigma-Aldrich, 99.7%) were used as
supplied without further purification. All gases (O , H , N and He) were of ultra-high purity
2
2
2
(
>99.99%, BOC). Reactant (i) fractional conversion (X ) is defined by:
i
[
reactant]i,in − [reactant]i,out
X =
i
(2)
[
reactant]
i,in
while selectivity to product j (S ) is defined as:
j
[
product]j,out
Sj(%) =
× 100
(3)
[
reactant]i,in − [reactant]i,out
where subscripts in and out refer to the inlet and outlet streams. The catalysts exhibited a
decline in conversion to reach a pseudo-steady state after 2 h on-stream. Catalytic activity is also
quantified in terms initial rate (r = hydrogenation in coupling process and r = hydrogenation in
C
SA
−
1
−1
h ), determined from time on-stream measurements as described
stand-alone reaction; mol molAu
elsewhere [72] according to:
F
× Xi
m−furaldehyde
nAu
rC or SA
=
(4)

Molecules 2018, 23, 2905
12 of 16
Hydrogen utilisation efficiency in the stand-alone hydrogenation vs. coupled process was
assessed by:
H supply
H consumed
2
2
H utilisation e f f iciency =
2
(5)
where H supply is the molar hydrogen provided ((i) from an external gas cylinder supply or
2
(
ii) via alcohol dehydrogenation) while H consumed is the amount utilised in the conversion of
2
m-furaldehyde. Repeated reactions with different samples from the same batch of catalyst delivered
raw data reproducibility and carbon mass balance within ± 5%.
4
. Conclusions
Activation in hydrogen of Au-Cu/CeO2 (Cu/Au mol ratio = 2) prepared by stepwise
deposition–precipitation generated metal nanoparticles in the range 1–7 nm (mean = 3.5 nm).
STEM-EDX analysis of Au-Cu/CeO has revealed that Au and Cu nanoparticles are in close proximity
2
δ−
0
+
on the surface while XPS results are consistent with formation of Au , Cu and Cu . Au/CeO
2
δ−
(
bearing Au nanoparticles with mean size = 3 nm) promoted the gas phase continuous hydrogenation
of HMF (P = 1 atm, T = 498 K) exclusively to DHMF with only a small fraction of the hydrogen supplied
being utilised. Under the same reaction conditions, Au-Cu/CeO delivered a higher DHMF production
2
rate and full hydrogen utilisation in the coupled hydrogenation/dehydrogenation of HMF/2-butanol
with a lower E-factor. Exclusive carbonyl-group hydrogenation and hydroxyl-group dehydrogenation
with (up to a 6-fold) increase hydrogenation rate and hydrogen utilisation efficiency with a lower
E-factor (relative to conventional stand-alone hydrogenation) extends to the coupling of a series
of m-substituted (-CH , -CH CH , -CH OH, -CF , -N(CH ) and -H) furaldehydes with alcohol
3
2
3
2
3
3 2
(1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol) dehydrogenation over Au-Cu/CeO2.
Hydrogen generation is rate limiting and furaldehyde hydrogenation proceeds via a nucleophilic
mechanism where the presence of electron withdrawing substituents (in the meta-position) is shown
to increase hydrogenation rate. Our results open new possibilities for the application of bimetallic
Au-Cu catalysts for sustainable hydrogenation/dehydrogenation coupling directed at production of
high value chemicals from furaldehydes.
Supplementary Materials: The following are available online, Figure S1: XPS spectra over the Ce 3d region for (I)
fresh CeO , (II) CeO thermally treated in H to 573 K, (III) Au-Cu/CeO and (IV) Au/CeO . Note: Raw data are
2
2
2
2
2
shown as open symbols () while curve fitted and envelope is represented by solid and dashed lines.
Author Contributions: M.A.K. and F.C.L. conceived the idea and were in charge of overall direction and planning
of the project; L.C. and C.P. synthesised and characterised the samples; C.P. carried out the experiments and wrote
the manuscript with input from all the authors.
Funding: This research was funded by the Engineering and Physical Sciences Research Council EPRSC grant
number EP/L016419/1 [Ph.D. studentship to Chiara Pischetola, CRITICAT program].
Acknowledgments: We acknowledge C. Baddeley, F. Grillo and S. Francis for assistance with XPS measurements
and are grateful to N. Tomás Arias for her contribution to the work.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds ...... are available from the authors.
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