Combined catalytic action of supported Cu and Au in imine production from coupled benzyl alcohol and nitrobenzene reactions

Article abstract of DOI:10.1016/j.apcata.2018.03.024

The feasibility of continuous gas phase coupling of benzyl alcohol dehydrogenation with nitrobenzene hydrogenation for imine (N-benzylideneaniline) synthesis over mixtures of Cu/SiO₂ and (TiO₂, MgO) supported Au catalysts has been established. The catalysts were characterised in terms of specific surface area, pore volume, TPR, H₂ chemisorption, STEM, XPS and TGA-DSC analysis. Au/TiO₂ prepared by deposition-precipitation exhibited a narrower particle size distribution (1–6 nm) and smaller mean (3.2 nm) than Au/MgO (1–15 nm, mean = 7.7 nm) prepared by impregnation. Dehydrogenation of benzyl alcohol (to benzaldehyde) over Cu/SiO₂ delivered a (ten-fold) higher rate than supported Au and Au/TiO₂ exhibited a higher H₂ chemisorption capacity and greater (by a factor of ten) nitrobenzene hydrogenation rate than Cu/SiO₂ and Au/MgO. Inefficient hydrogen utilisation is demonstrated for the cross-coupling reaction over Cu/SiO₂ alone and a temporal activity loss is linked to carbonaceous (benzoate) deposition (on the basis of XPS and TGA-DSC analysis). Incorporation of Au/TiO₂ (and Au/MgO) with Cu/SiO₂ increased the imine formation rate and H₂ utilisation efficiency. The combination of Au/TiO₂ with Cu/SiO₂ served to circumvent Cu/SiO₂ deactivation, resulting in enhanced stability with a four-fold increase in imine production and full H₂ utilisation at Cu/Au = 10.

Full text of DOI:10.1016/j.apcata.2018.03.024

Accepted Manuscript
Title: Combined Catalytic Action of Supported Cu and Au in
Imine Production from Coupled Benzyl Alcohol and
Nitrobenzene Reactions
Authors: Maoshuai Li, Fernando C a´ rdenas-Lizana, Mark A.
Keane
PII:
S0926-860X(18)30143-1
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.03.024
APCATA 16598
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
8-2-2018
20-3-2018
22-3-2018
Please cite this article as: Li M, C a´ rdenas-Lizana F, Keane MA, Combined
Catalytic Action of Supported Cu and Au in Imine Production from Coupled
Benzyl Alcohol and Nitrobenzene Reactions, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.03.024
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Combined Catalytic Action of Supported Cu and Au in
Imine Production from Coupled Benzyl Alcohol and
Nitrobenzene Reactions
Maoshuai Li, Fernando Cárdenas-Lizana and Mark A. Keane*
Chemical Engineering, School of Engineering and Physical Sciences,
Heriot-Watt University, Edinburgh EH14 4AS, Scotland
*
corresponding author
Tel: +44(0)131 451 4719, e-mail: M.A.Keane@hw.ac.uk
1

Graphical Abstract
Highlights
1
2
3
4
. Continuous single-pot imine synthesis from benzyl alcohol and nitrobenzene
. Hydrogen ‘‘borrowing’’ strategy for sustainable imine production
. Synergism between Cu and Au for full H utilisation in cross-coupled reaction
. Stable selective N-benzylideneaniline production over Au/TiO +Cu/SiO
2
2
2
Abstract
The feasibility of continuous gas phase coupling of benzyl alcohol dehydrogenation with
nitrobenzene hydrogenation for imine (N-benzylideneaniline) synthesis over mixtures of
Cu/SiO
characterised in terms of specific surface area, pore volume, TPR, H
XPS and TGA-DSC analysis. Au/TiO prepared by deposition-precipitation exhibited a
2
and (TiO
2
, MgO) supported Au catalysts has been established. The catalysts were
2
chemisorption, STEM,
2
narrower particle size distribution (1-6 nm) and smaller mean (3.2 nm) than Au/MgO (1-15
nm, mean = 7.7 nm) prepared by impregnation. Dehydrogenation of benzyl alcohol (to
benzaldehyde) over Cu/SiO
exhibited a higher H chemisorption capacity and greater (by a factor of ten) nitrobenzene
hydrogenation rate than Cu/SiO and Au/MgO. Inefficient hydrogen utilisation is demonstrated
for the cross-coupling reaction over Cu/SiO alone and a temporal activity loss is linked to
carbonaceous (benzoate) deposition (on the basis of XPS and TGA-DSC analysis).
2
delivered a (ten-fold) higher rate than supported Au and Au/TiO
2
2
2
2
Incorporation of Au/TiO
2
(and Au/MgO) with Cu/SiO
2
2
increased the imine formation rate and

H
2
utilisation efficiency. The combination of Au/TiO
Cu/SiO deactivation, resulting in enhanced stability with a four-fold increase in imine
production and full H utilisation at Cu/Au = 10.
2
with Cu/SiO served to circumvent
2
2
2
Keywords: reaction coupling; benzyl alcohol; nitrobenzene; imine; Cu/SiO
2
; Au/TiO .
2
3

1
. Introduction
Higher substituted imines are extensively used as intermediates in the manufacture of fine
chemicals, pharmaceuticals and agrochemicals [1]. Conventional production draws on
reductive amination (alkylation) of aldehydes/ketones with amines that are derived from the
reduction of nitro-compounds at elevated H
naturally occurring feedstock and current production (ca. 95%) is fossil fuel based, notably by
methane steam reforming and coal gasification [5]. Issues linked to sustainable H production
2
pressures (4-12 bar) [2-4]. Hydrogen is not a
2
are now the driver for the search of more efficient routes to imines. Work to date has focused
on batch liquid phase catalytic oxidation of amines [6] (e.g. benzylamine over Au [7] and Cu
[
8]) and oxidative condensation of alcohols with amines [9] (e.g. benzyl alcohol and aniline
over Cu [10], Pd [11] and Au [12]) at (O /air) pressures up to 5 atm. The multi-step nature of
the oxidative route where the amine reactant must be generated first is a decided drawback
13]. We propose to integrate the nitro- → amine reduction step (Fig. 1, step (I)) and reaction
2
[
with aldehyde (step (III)) in a “one pot” imine production that circumvents multiple reactor
operation. The alcohol dehydrogenation (step (II)) is used to generate aldehyde and hydrogen
that is utilised in –NO
2
hydrogenation. This removes the requirement for an external supply of
pressurised H and can be step-changing in terms of sustainability and process efficiency.
2
Synthesis of imines via coupling of alcohol dehydrogenation with nitro-compound
reduction has not been studied to any significant extent. The viability of batch liquid phase
coupling of benzyl alcohol dehydrogenation (as hydrogen donor) with nitrobenzene
hydrogenation has been demonstrated for reaction over Mg
amine (>7% yield) was generated as by-product [14]. Xiang et al. [15] have reported imine
synthesis from nitrobenzene and furfural over Pd-Au/Al (with MeOH as hydrogen source).
6
Al
2
(OH)16CO
3
supported Pd but
2
O
3
We can also flag work dealing with batch conversion of nitroarenes and benzaldehyde over
iron powder (with HCl as hydrogen donor) [16] and nitrobenzene with benzyl alcohol over
4

TiO supported Au-Pd and Ru-Pd [17]. A move from batch to continuous processing has
2
significant benefits in terms of economics, high throughput and quality control. This has been
identified as a priority for sustainable production in the pharmaceutical and fine chemical
sectors [18]. Studies of continuous gas phase operation employing a hydrogen ‘‘borrowing’’
strategy have predominantly dealt with coupling of dehydrogenation and hydrogenation [19-
2
2] with no report of cross-coupling of alcohols with nitro- reactants for imine synthesis. Baiker
et al. [23,24] studied the formation of dimethyldodecylamine by gas phase conversion of
dimethylamine and 1-dodecanol over Cu/Al and observed loss of activity and selectivity in
2
O
3
the absence of an external hydrogen supply. Deactivation has been a feature of coupling
reactions involving alcohols and amines over supported Cu and Ni catalysts and attributed to
nitride formation, coking and amine deposition [25].
We have previously established the viability of hydrogen transfer (from 2-butanol
dehydrogenation) for nitrobenzene reduction over Cu/SiO
work to evaluate feasible imine production in the absence of external H
of Cu/SiO and supported Au catalysts with benzyl alcohol and nitrobenzene as reactants (Fig.
). Nitroarene hydrogenation over supported Au shows a dependency on support reducibility
27], where Au on TiO and MgO exhibited differences in chemoselective amine production
rates [28]. We evaluate H utilisation and catalyst stability as two critical considerations for
sustainable imine production.
2
[26]. In this study, we extend that
2
using a combination
2
1
[
2
2
2
2
. Experimental
.1 Catalyst Preparation and Activation
Cu/SiO was prepared by deposition-precipitation with NaOH (Riedel-de Haën, 99%) as
2
precipitation agent. Silica powder (Sigma-Aldrich, 20 g) was dispersed in an aqueous solution
3
of Cu(NO
3
)
2
(Sigma-Aldrich, 99%, 0.03 M, 200 cm ). The suspension was stirred (600 rpm) at
ambient temperature for 1 h and NaOH (2 M) added dropwise until pH >10 to ensure full
5

deposition as Cu(OH) [29]. The mixture was heated in air to 353 K with subsequent ageing
2
for 4 h to facilitate homogeneous deposition [29]. The solid obtained was separated by
filtration, washed with distilled water until pH = 7 and dried at 393 K. The dried sample was
-1
calcined in air (at 10 K min to 723 K for 4 h) to generate a supported copper oxide (CuO)
[29]. Gold on MgO (Sigma-Aldrich, 10 g) was prepared by impregnation with aqueous HAuCl
4
-2
3
-1
(
Sigma-Aldrich, 99%, 5  10 M, 50 cm ). The slurry was heated (at 2 K min ) to 353 K at
00 rpm and maintained in a He purge for 5 h. Gold on TiO (P25, Degussa) was prepared by
deposition-precipitation using urea (Riedel-de Haën, 99%) as basification agent. An aqueous
6
2
-3
3
solution of urea (100 fold excess) and HAuCl
15 g) and the suspension stirred and heated to 353 K under reflux (2 K min ). The pH
progressively increased to reach ca. 7 after 3-4 h as a result of thermal decomposition of urea.
4
(5  10 M, 400 cm ) was added to the support
-1
(
3
-1
The solid was separated by filtration, washed with distilled water and dried in 45 cm min He
at 373 K for 5 h. The catalyst precursors were sieved (ATM fine test sieves) to mean particle
diameter = 75 μm. Prior to ex situ characterisation and catalytic testing, samples were activated
3
-1
-1
in 60 cm min H
2
at 2-4 K min to 523-545 K, held for 1-2 h and passivated at ambient
temperature in 1% v/v O
2
/He.
2
.2 Catalyst Characterisation
Metal loading was determined by atomic absorption spectroscopy (Shimadzu AA-6650
spectrometer with an air-acetylene flame) analysis of the diluted extract in aqua regia (25% v/v
HNO /HCl). Temperature programmed reduction (TPR) and H chemisorption were conducted
3
2
on the CHEM-BET 3000 (Quantachrome Instrument) unit with data acquisition/ manipulation
using the TPR WinTM software. The sample was loaded into a U-shaped Pyrex glass cell (3.76
3
-1
at 2-4 K min-
mm i.d.) and heated in 17 cm min (Brooks mass flow controlled) 5% v/v H
2
/N
2
1
3
-1
to 523-545 K. The activated samples were swept with 65 cm min N
2
for 1.5 h, cooled to
6

reaction temperature (498 K) and subjected to H
effluent gas passed through a liquid N trap and H
conductivity detector (TCD). In blank tests, there was no measurable H
TiO or MgO) supports. Nitrogen adsorption-desorption isotherms were obtained using the
Micromeritics Gemini 2390p system. Prior to analysis, the samples were outgassed at 423 K
for 1 h in N . Total specific surface area (SSA) and cumulative pore volumes were obtained
2
chemisorption by pulse (10 μl) titration. The
consumption was monitored by a thermal
uptake on the (SiO ,
2
2
2
2
2
2
using standard BET and BJH analysis. Metal particle morphology (size and shape) was
examined by scanning transmission electron microscopy (STEM, JEOL 2200FS field emission
gun-equipped unit), employing Gatan Digital Micrograph 1.82 for data
acquisition/manipulation. Samples for analysis were prepared by dispersion in acetone and
deposited on a holey carbon/Cu grid (300 Mesh). Surface area weighted mean metal size (d)
was based on a count of up to 800 nanoparticles according to
3
i nidi
d 
(1)
2
i nidi
where n
i
is the number of particles of diameter d . Thermogravimetric analysis (TGA) and
i
differential scanning calorimetry (DSC) of the samples pre- and/or post-reaction were
performed on a SDT Q600 simultaneous TGA/DSC analyser (TA Instrument) by monitoring
temporal mass and heat flow changes with temperature. The samples (ca. 10 mg) were heated
3
-1
-1
in 100 cm min air at 373 K for 1 h and ramped to 1073 K (at 20 K min ). Elemental (C, H
and N) analysis for samples post-reaction was carried out on an Exeter CE-400 elemental
analyser. X-ray photoelectron spectroscopic (XPS) analysis was conducted on an Axis Ultra
-8
instrument (Kratos Analytical) under ultra-high vacuum conditions (<10 Torr) with a
monochromatic Al Kα radiation (1486 eV). The C1s, Cu 2p3/2, Au 4f5/2 and 4f7/2, N1s and Si
2
p spectra were collected. Characteristic Cu 2p3/2 binding energies (BE) for metallic Cu and
CuO are 932.8 eV [30] and 934.0 eV [31], respectively. Values in the 83.6-84.2 eV and 87.2-
7

8
8.4 eV ranges for 4f7/2 and 4f5/2 levels have been reported elsewhere [32] for metallic Au. We
define adventitious carbon as any carbon associated with air exposed samples [33]. The C1s
profile for this adventitious carbon is characterised by a principal peak at 284.5 eV, indicative
of alkyl species (C–C, C–H) [34], and was used as an internal standard to compensate for
charging effects. The spectra were calibrated against the Si 2p (of SiO , BE = 103.5 eV) [35]
2
and fitted with abstraction of the Shirley background using the Gaussian-Lorentzian function
in XPSPEAK 41.
2
.3 Catalytic Procedure
Reactions (stand-alone benzyl alcohol dehydrogenation and nitrobenzene hydrogenation
and cross-coupling) were carried out at ambient pressure in situ after activation in a continuous
flow fixed bed vertical glass reactor (i.d. = 15 mm) at 423-523 K. A layer of borosilicate glass
beads served as preheating zone, ensuring that the reactant(s) (benzyl alcohol and/or
nitrobenzene) was (were) vaporised and reached reaction temperature before contacting the
catalyst bed. Isothermal conditions (±1 K) were maintained by diluting the catalyst with ground
glass (75 µm) and continuously monitored by a 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). Dehydrogenation of benzyl alcohol and hydrogenation of nitrobenzene were tested
4
-1
separately in a co-current flow of N
2
with benzyl alcohol (GHSV = 1.0 × 10 h , molar metal
-3
-2
to reactant feed rate nmetal/F = 3.8 × 10 – 5.0 × 10 h) and H
2
with nitrobenzene (GHSV = 1.0
4
-1
-3
-2
×
10 h , nmetal/F = 8.0 × 10 – 1.2 × 10 h), respectively. The coupled reaction was carried
3
-1
-1
-1
out in N
2
(GHSV = 2.0 × 10 h , nmetal/F = 1.2 × 10 – 3.6 × 10 h). In blank tests, passage of
each reactant through the empty reactor or over the (SiO
2
, TiO or MgO) support alone did not
2
result in any detectable conversion. The reactor effluent was condensed in a liquid nitrogen trap
8

for subsequent analysis using a Perkin-Elmer Auto System XL gas chromatograph equipped with
a programmed split/splitless injector and a flame ionization detector, employing a DB-1 (50 m ×
0.33 mm i.d., 0.20 μm film thickness) capillary column (J&W Scientific). Data acquisition and
manipulation were performed using the TurboChrom Workstation Version 6.3.2 (for Windows)
chromatography data system. Benzyl alcohol (Riedel-de Haën, ≥99%), benzaldehyde (Fluka,
≥
98%), nitrobenzene (Riedel-de Haën, ≥99%), aniline (Sigma-Aldrich, ≥99%), toluene (Sigma-
Aldrich, 99%), N-benzylideneaniline (Sigma-Aldrich, 99%) and N-benzylaniline (Sigma-
Aldrich, 99%) were used as received without further purification. The (H , N , O and He) gases
were of high purity (BOC, >99.98 %). Reactant (i) fractional conversion (X ) is defined by
reactant]i, in [reactant]i, out
2
2
2
i
[
X 
i
(2)
[
reactant]i, in
j
and composition (N ) with respect to product j by
[
Product]j,out
Nj
(%) 
100
n
(3)
[
Product]j,out

j1
where subscripts “in” and “out” refer to the inlet and outlet gas streams, respectively. Activity is
quantified in terms of reaction rate (R, mmol molmetal h-1), determined from time on-stream
measurements as described elsewhere [28]. Repeated reaction with different samples from the
same batch of catalyst delivered raw data reproducibility and mass balances within ±5%.
-1
9

3
3
. Results and Discussion
.1 Cu/SiO : Characterisation (Pre-reaction) and Catalytic Response in Stand-Alone and
2
Cross-Coupling Reactions.
The physicochemical characteristics of Cu/SiO given in Table 1 reveal a lower SSA than
2
2
-1
3
-1
the silica support (223 m g ) but a comparable pore volume (SiO
by TPR (Fig. 2(AI)) generated a positive (H consumption) peak at the final isothermal hold
545 K) where the H consumed matched the requirements for Cu → Cu reduction (Table
2
= 0.35 cm g ). Activation
2
2
+
0
(
2
1
). Hydrogen chemisorption capacity is a critical catalyst characteristic for hydrogenation
(25 µmol gCu-1) at ambient
applications. We have noted previously low H
2
uptake on Cu/SiO
2
temperature [26] that can be linked to the filled d band [36]. Hydrogen adsorption on Cu is an
activated process, where Cu/SiO showed higher uptake at 498 K (Table 2). This value is
2
appreciably lower than that recorded for silica supported hydrogenation metals such as Pd
1650 µmol gPd-1) [37] and Ni (614 µmol gNi-1) [38]. STEM analysis (Fig. 2(AII)) revealed the
(
presence of Cu particles at the nano-scale (1-6 nm, Fig. 2(AIII)) with a surface area weighted
mean of 3.1 nm.
The coupled dehydrogenation-hydrogenation system requires a higher dehydrogenation
rate relative to hydrogenation to ensure sufficient H
separately the dehydrogenation of benzyl alcohol (in N
using an external H supply) over Cu/SiO . Benzyl alcohol dehydrogenation generated
2
supply for reaction. We first examined
2
) and hydrogenation of nitrobenzene
(
2
2
benzaldehyde (Fig. 1, step (II)) as the sole product over the 423-498 K temperature range.
Toluene formation (≤10% selectivity, step (V) in Fig. 1) was observed for reaction at 523 K.
Full selectivity to the target benzaldehyde at T < 498 K is consistent with that reported for K-
Cu/TiO
2
[39]. Benzyl alcohol hydrogenolysis over Cu/MgO required temperatures ≥513 K
40]. We have previously established pseudo-first order kinetics for catalytic dehydrogenation
of 2-butanol) [26] and nitroarene hydrogenation [41]. The associated Arrhenius plots (423-
[
(
1
0

4
98 K) using the extracted pseudo-first order rate constants (k) are presented in Fig. 3 with an
-1
associated apparent activation energy for the dehydrogenation reaction of 37 kJ mol . We
could not find any reported activation energy for benzyl alcohol conversion to benzaldehyde.
-1
The value (20 kJ mol ) recorded for n-octyl alcohol dehydrogenation (over Cu/Al
is lower [42]. Nitrobenzene hydrogenation (Fig. 1, step (I)) over Cu/SiO resulted in exclusive
production of aniline with no hydrodenitrogenation or aromatic ring reduction. The apparent
2
O -MgO)
3
2
-1
-1
activation energy (32 kJ mol , Fig. 3) falls within the range (12-54 kJ mol ) reported for
reaction over supported Ag [43], Pd [44] and Ni [45] catalysts.
The prerequisite for our coupled system to work in the absence of an external H
i.e. in N ) is that the quantity of hydrogen generated in the benzyl alcohol → benzaldehyde
step meets the stoichiometry (=3) for the nitrobenzene → aniline transformation (Fig. 1, steps
I-III)). A four-fold higher benzyl alcohol dehydrogenation rate than nitrobenzene
2
supply
(
2
(
hydrogenation satisfies this requirement. Temporal variations in conversion for reaction at 498
K (where the highest selective benzyl alcohol dehydrogenation rate was achieved) are shown
in Fig. 4. The coupled reaction generated imine and benzaldehyde with no detectable
hydrogenolysis to toluene (step (V), Fig. 1) and/or amine from further hydrogenation of imine
(
step (IV)). This represents the first reported continuous imine synthesis via coupled alcohol
dehydrogenation and nitroarene hydrogenation without external H . Imine generation in N
demonstrates that the catalyst “borrows” hydrogen generated from the alcohol dehydrogenation
2
2
(
step (II), Fig. 1) with an “auto-transfer” in the catalytic reduction of nitrobenzene to aniline
step (I)). As aniline is a strong nucleophile, it undergoes facile condensation with
(
benzaldehyde to the target imine (step (III)) [46]. This represents innovation in process
intensification and enhanced sustainability over conventional multi-stepped methodologies that
require an external H
2
supply [2,3,8,10,47]. A temporal decline in conversion was observed
(
Fig. 4). Baiker [23] studying the gas phase (T = 510 K, P = 1 atm) cross-coupling alkylation
1
1

of dimethylamine with 1-dodecanol over Cu/Al
2
O
3
reported a severe loss of activity that was
N) from NH generated by the
ascribed to the formation of copper nitride (Cu
disproportionation of dimethylamine.
3
3
Temperature is a critical process variable that influences reaction rate and product
distribution [48]. Imine formation rate (R), molar (benzaldehyde/imine) product ratio (N-C=O/N
C=N) and hydrogen utilisation efficiency, expressed as the ratio of mol H generated from
benzyl alcohol dehydrogenation to mol imine produced (H /–C=N) are presented as a function
of temperature (448-498 K) in Table 2. An increase in temperature at the reaction
stoichiometry (inlet alcohol/nitroarene (–OH/–NO ) = 3) resulted in a greater imine formation
rate (by a factor of 50) with a higher imine content in the product and improved H utilisation.
The increased reaction rate can be attributed to greater hydrogen availability (from the
Arrhenius plot in Fig. 3) in tandem with improved H chemisorption capacity of Cu/SiO at
higher temperatures as determined from H titration measurements. It should however be noted
that product ratio (N-C=O/N-C=N) was greater than that (= 2) shown in the reaction scheme (Fig.
) and H /–C=N exceeded the stoichiometry requirement (= 3) for full H utilisation,
-
2
2
2
2
2
2
2
1
2
2
necessitating further process optimisation.
It has been demonstrated that –OH/–NO
2
is critical in determining product distribution in
served to increase the imine formation
liquid phase cross-coupling [14]. A higher –OH/–NO
2
rate (Table 2). This suggests competition between nitrobenzene and benzyl alcohol for surface
sites where higher alcohol content in the feed favours alcohol dehydrogenation with greater
hydrogen release that contributed to the hydrogenation step. Shimizu et al. [49] proposed that
dehydrogenation was rate determining in the coupled reaction of aniline with benzyl alcohol
in N
and product ratio (N-C=O/N-C=N) were largely insensitive to variations in –OH/–NO
Incomplete H utilisation can be linked to the low hydrogenation activity of Cu catalysts.
2
for amine (N-benzylaniline) synthesis over Ni/Al
2
O
3
. Hydrogen utilisation (H
2
/–C=N)
2
(Table 2).
2
1
2

Incorporation of a second metal with greater hydrogenation capability can serve as a means of
increasing imine production. The second metal must not promote side-reactions (benzyl
alcohol hydrogenolysis and imine hydrogenation). Supported Pd, Pt and Ni exhibit high
activity in the reduction of unsaturated functions (e.g. –NO
hydrogenolysis, generating a range of by-products (e.g. toluene) [50,51]. In contrast, nano-
scale Au has displayed unique chemoselectivity for hydrogenation of –NO in the presence of
2
and –C=O) but promote
2
other reactive groups (e.g. –C=O) [3]. Supported Au was chosen in this study as a suitable
candidate to improve hydrogen utilisation directed at enhanced imine production rates.
3
.2 Supported Au: Characterisation (Pre-reaction) and Catalytic Response in Stand-
alone Dehydrogenation and Hydrogenation Reactions.
It has been shown that –NO
2
adsorption/activation at TiO
2
and Au-TiO interface sites
2
deliver enhanced selective nitroarene reduction rates [52]. In order to probe possible support
effects, we also examined the performance of Au/MgO as an additive in the cross-coupled
reaction. The TPR profile of Au/TiO
2
(Fig. 2(BI)) exhibits a single H
2
consumption peak (Tmax
3
+
0
=
378 K) that exceeded the amount needed for Au reduction to Au (Table 1), suggesting
partial support reduction [53]. Reduction of Au/MgO (Fig. 2(CI)) generated a weak and broad
higher temperature (Tmax = 510 K) signal, where H consumption (Table 1) was appreciably
2
lower than that required for precursor reduction. This can be ascribed to reduction pre-TPR,
during catalyst synthesis and drying [54]. Representative STEM images (BII, CII) and particle
size distributions (BIII, CIII) are provided in Fig. 2. Au/TiO
2
is characterised by a narrow
. In contrast, Au/MgO
particle size range (1-6 nm) and a mean (3.2 nm) close to that of Cu/SiO
2
bore larger Au particles (1-15 nm, mean = 7.7 nm) that is a consequence of synthesis by
impregnation, which results in weaker metal-support interaction and agglomeration during
thermal treatment [55]. Gold particle size <10 nm has been identified as critical for activity in
hydrogenation applications [36]. A significantly higher H chemisorption capacity was
2
1
3

recorded for Au/TiO
particle size [56] and a contribution due to the Au-TiO
activation/dissociation. The performance of Au/TiO and Au/MgO can be assessed against
Cu/SiO in stand-alone benzyl alcohol dehydrogenation and nitrobenzene hydrogenation in
2
relative to Au/MgO (Table 1). This can be linked to the smaller Au
2
interface [57] that facilitates H
2
2
2
Fig. 5. The results establish lower (by an order of magnitude) dehydrogenation rates for the
two Au catalysts (Fig. 5(A)). Negligible activity for gas phase alcohol dehydrogenation over
supported Au is consistent with results for ethanol dehydrogenation over Au/SiO
K [58]. Au/TiO
and Au/MgO (Fig. 5(B)), which correlates with H
2
at T < 523
2
delivered a significantly higher nitrobenzene hydrogenation rate than Cu/SiO
2
2
chemisorption capacity (Table 1).
3
.3 Cu +Au Physical Mixtures: Catalytic Response in Cross-Coupled Reactions.
We first assessed the catalytic action of mixed (Cu + Au) catalysts, varying the Cu/Au
molar ratio (10, 20 and 80). The results are presented in Fig. 6 and Table 3 where benzaldehyde
and the target N-benzylideneaniline were the only detected products. Imine production was
enhanced on incorporating Au/TiO
2
with Cu/SiO
Cu/Au from 80 to 10). This demonstrates the contribution of Au/TiO
generated from dehydrogenation, Fig. 1, step (II)) and promote nitroarene reduction (step (I))
2
with a rate increase at higher Au content
(
(
2
to harness the hydrogen
with subsequent condensation (aniline + aldehyde) to imine (step (III)). We envision a
synergism between Au and Cu that is illustrated in Fig. 7. The supported Cu phase promotes
alcohol dehydrogenation with hydrogen abstraction (step (I)). Fridman et al. [59] have
proposed a two-step H abstraction mechanism for Cu catalysed cyclohexanol dehydrogenation.
Interaction between metal and abstracted hydrogen results in the formation of transient (Ru-H
[
60], Ag-H [46] and Ni-H [49]) species. Hydrogen on Cu sites (Cu-H) can “spillover” across
the Cu-SiO interface, the physical interface with Au/TiO and onto Au sites (step (II)) [61]
with auto-transfer in the catalytic conversion of nitrobenzene to aniline (step (III)) and
2
2
1
4

subsequent condensation (step (IV)). Further hydrogenation of imine to N-benzylaniline was
not observed. This may be due to competition with nitrobenzene for surface hydrogen.
We obtained full hydrogen utilisation (H
2
/–C=N = 3) and imine formation (N-C=O/N-C=N =
2
) over Cu/SiO +Au/TiO at Cu/Au = 10. This represents unprecedented performance in terms
2
2
of atom efficiency and hydrogen usage when compared with conventional pressurised batch
liquid phase reactions. Activity and selectivity in the coupled reactions over the physical
mixture were invariant (at Cu/Au = 10) for up to 6 h on-stream (Fig. 6(A)), which is a marked
improvement in stability relative to Cu/SiO (Fig. 4). We could find no published study of
2
catalytic coupled reactions over physical combinations of Cu+Au catalysts. We should note
work where mixtures of (graphite) supported Pd (or Pt) with supported bimetallic Fe-Ce
exhibited higher stability and activity relative to the individual catalysts in the hydrogenation-
isomerisation of 1,3-butadiene to butane [62]. Incorporation of Au/MgO with Cu/SiO
, Fig. 6(B)) also served to enhance the imine production rate with increasing Au content
Cu/Au = 20 → 10). This is again indicative of a cooperative effect in terms of alcohol
dehydrogenation at Cu sites with hydrogen release and use in–NO reduction on Au/MgO. The
2
(Table
3
(
2
promotional effect was not as significant as that observed with Au/TiO , which exhibited
2
higher nitrobenzene hydrogenation activity (Fig. 5(B)).
3
.4 Cu/SiO
Given the decline in activity with time on-stream for Cu/SiO
spent sample to characterisation measurements in order to identify the possible cause for
deactivation. We also carried through the same analyses for a spent Au/TiO +Cu/SiO (Cu/Au
10) physical mixture that delivered full conversion/hydrogen utilisation over 6 h on-stream
Fig. 6(A)). Catalyst deactivation in reactions involving alcohols and amines (or nitro-
compounds) has been ascribed to active site occlusion/poisoning (nitride formation [23], coke
2
vs. Cu/SiO
2
+ Au/TiO
2
: Catalyst Characterisation (Post-reaction)
2
(Fig. 4), we subjected a
2
2
=
(
1
5

[63] and amine/nitro-compound deposition [25]) and/or changes to the oxidation state of the
active metal [64]. Analysis by XPS provides information on surface composition and the
chemical/electronic state of the supported metal phase. Spectra over the C1s (A) and Cu 2p3/2
(
B) binding energy (BE) regions are shown in Fig. 8. The C1s profile for unused Cu/SiO
exhibited a contribution due to adventitious carbon (BE = 284.5 eV) with a secondary peak
BE = 286.1 eV) corresponding to C–OH or C–O–C [34]. An additional peak at 288.7 eV for
spent Cu/SiO (AII) can be tentatively linked to the carboxyl functionality in benzoate species
BE = 288.6-289.1 eV) [65]. The Cu 2p3/2 spectrum for the passivated unused Cu/SiO
2
(AI)
(
2
(
2
(BI)
δ+
presents a principal peak at BE = 933.2 eV characteristic of electron deficient copper (Cu =
9
33.0-933.6 eV) [66]. The spectrum also shows a secondary peak at higher BE (934.1 eV) that
corresponds to CuO (BE = 934.0 eV [31]) and may be due to surface Cu oxidation during
passivation [67]. Post-reaction (BII), a peak at higher BE (936.5 eV) suggests a surface ionic
Cu species [67,68] that can be attributed to copper benzoate formation. The formation of a
surface benzoate has been proposed elsewhere [69] for benzaldehyde hydrogenation over oxide
(
SiO
The occurrence of Cu benzoate resulting from surface reaction of benzaldehyde with water
release from –NO reduction) can reduce the number of available active sites and inhibit
benzyl alcohol dehydrogenation, limiting hydrogen generation and subsequent nitrobenzene
reduction/imine formation. The spectrum for used Cu/SiO over the N1s region (396-400 eV,
2
, TiO
2
, ZrO
2
and CeO
2
) supported Cu under similar reaction conditions (T = 373-623 K).
(
2
2
not shown) did not present any detectable signals. We have no evidence for the formation of
copper nitride and/or surface nitrogenous species (T = 510 K, P = 1 atm) as was proposed
elsewhere [23].
The XPS profile of the (Au + Cu) physical mixture post-reaction (Fig. 8(AIII)) exhibits a
C1s peak at 287.8 eV that falls within the BE range (287.3-287.9 eV) reported for –C=O species
[70-72] and can be assigned to surface benzaldehyde. Liu and co-authors [71] attributed a C1s
1
6

signal at 287.8 eV to residual aldehyde on surface copper during furfural hydrogenation. In the
δ+
Cu 2p3/2 BE range (Fig. 8(BIII)), peaks characteristic of Cu and CuO were observed with no
signal at BE > 936 eV for Au/TiO
Au/TiO + Cu/SiO and Cu/SiO suggests the occurrence of surface benzaldehyde and benzoate
species, respectively. Suppressed benzaldehyde → benzoate transformation results from the
addition of Au/TiO . The incorporation of Au/TiO with Cu/SiO provides a more efficient site
for nitrobenzene hydrogenation and aniline condensation with benzaldehyde (Fig. 7). This can
ameliorate Cu deactivation as benzaldehyde reaction and –NO reduction (with H
2
+ Cu/SiO
2
. The comparison of XPS profiles for the spent
2
2
2
2
2
2
2
2
O
generation) proceeds at Au sites. The BE for Au 4f7/2 (83.2 ± 0.2 eV) and Au 4f5/2 (86.7 ± 0.2
0
eV) were lower than the reference Au (84.0 eV and 87.8 eV) [73], suggesting the formation
δ-
of an electron rich (Au ) phase as a result of electron donation from TiO
2
[74]. There was no
measurable difference in Au BE pre- and post-reaction (Fig. S1, Supplementary Material).
The possible formation of carbonaceous deposits was further evaluated by TGA-DSC and
elemental (C, H and N) analysis; the results are presented in Fig. 9. Pre-reaction, Cu/SiO
2
(I)
and Cu/SiO + Au/TiO (II) exhibited a similar (2-3% w/w) mass loss at T ≤ 373 K due to water
2
2
release (Fig. 9(A)). Both spent samples displayed an additional higher temperature mass loss
that can be attributed to combustion of carbonaceous species [63]. Elemental analysis (see inset
Table in Fig. 9(A)) revealed a higher carbon content on the spent Cu/SiO
physical mixture. There was no measurable nitrogen, consistent with XPS results. The DSC
profile of the spent Cu/SiO (Fig. 9(B)) exhibited two exothermic mass losses (at 602 K and
02 K) during temperature programmed oxidation, indicative of different structural carbon
2
compared with the
2
7
deposits. The lower temperature exothermic mass loss matches that (553-613 K) corresponding
to combustion of amorphous aromatic carbonaceous species [75,76]. Based on the DSC work
of Siqueira et al. [77], the exothermic mass loss at higher temperature can be attributed to
combustion of (copper) benzoate species. The DSC profile for the used Cu/SiO
2
+ Au/TiO
2
1
7

mixture presents a principal mass loss at a lower temperature (582 K) with a secondary shoulder
at 690 K. This is indicative of limited deposition of surface benzoate as suggested by XPS.
4
. Conclusions
We have established the viability of ‘‘hydrogen-free’’ imine (N-benzylideneaniline)
production via continuous gas phase cross-coupling of benzyl alcohol dehydrogenation with
nitrobenzene hydrogenation and condensation over physical mixtures of supported Cu and Au
catalysts. Dehydrogenation of benzyl alcohol to benzaldehyde over Cu/SiO
.1 nm) delivered a ten-fold higher rate than Au/TiO (mean Au size = 3.2 nm). The latter
exhibited significantly greater H chemisorption capacity and nitrobenzene hydrogenation rate.
Reaction over Cu/SiO promoted cross-coupling to imine where an increase in temperature and
inlet reactant alcohol/nitroarene ratio (–OH/–NO ) served to enhance imine formation. The Cu
2
(mean Cu size =
3
2
2
2
2
catalyst suffered a severe temporal decline in activity linked to carbon (benzoate species from
XPS and TGA-DSC measurements) deposition. Low hydrogen utilisation is due to the limited
hydrogenation capacity of Cu. Incorporation of Au/TiO
2
with Cu/SiO resulted in (4-fold)
2
higher imine production rate with full hydrogen utilisation at Cu/Au = 10. This is the result of
synergism between Cu and Au where hydrogen release from benzyl alcohol dehydrogenation
on Cu is transferred to Au for nitrobenzene reduction (to aniline) and subsequent condensation
with benzaldehyde to the imine. The promotional effect of Au/TiO
observed with the addition of Au/MgO due to the greater H chemisorption capability and –
NO reduction capacity of Au/TiO . Coupled reaction over Cu+Au exhibited stable
2
was greater than that
2
2
2
performance with time on-stream where hydrogenation and condensation at Au sites
circumvents Cu deactivation. Our results establish the feasibility of continuous stable gas phase
imine synthesis using a ‘‘hydrogen borrowing’’ strategy over combined supported Cu and Au
catalysts.
1
8

Acknowledgements
This work was financially supported by the Overseas Research Students Award Scheme (ORSAS)
and in part by a grant from EPSRC (EP/M029476/1). We also acknowledge EPSRC support for free
access to the XPS facility at the University of Newcastle and are grateful to J. F. Portoles and Dr. Y.
Hao for their contributions to XPS analysis and catalyst preparation.
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2
3

2
4

Table 1: Physicochemical characteristics of Cu/SiO
2
, Au/TiO and Au/MgO.
2
Cu/SiO
2
Au/TiO
2
Au/MgO
3.4
Metal loading (%w/w)
Specific surface area (SSA, m g )
1.8
1.9
2
-1
192
52
24
3
-1
Pore volume (cm g )
0.34
0.07
0.04
510
T
max (K)
545
378
TPR
-
1
281 /281b
a
170 /143b
a
5a/256b
19
H
2
consumption (μmol g )
H )
chemisorption (μmol gmetal-
1 c
37
185
2
d (nm)d
3.1
3.2
7.7
Particle size range (nm)d
1-6
1-6
2+
1-15
a
experimental value obtained from TPR analysis; calculated value for Cu → Cu and Au → Au0;
b
0
3+
c
d
measurement at reaction temperature (498 K); from STEM analysis.
2
5

Table 2: Effect of reaction temperature (T) and inlet (alcohol/nitroarene) molar
reactant ratio (–OH/–NO2) on imine formation rate (R), molar
(
benzaldehyde/imine) product ratio (N-C=O/N-C=N) and hydrogen utilisation
efficiency (mol H generated from alcohol dehydrogenation per mol imine product
/–C=N)) in coupled benzyl alcohol and nitrobenzene reactions over Cu/SiO
Reaction conditions: T = 448-498 K, P = 1 atm.
2
(
H
2
2
;
T
K)
R
–
OH/–NO2
N-C=O /N-C=N
H
2
/–C=N
-1
-1
(
(mmol molCu h )
4
4
4
4
4
48
73
98
98
98
3
3
3
1
9
7
20.3
6.8
3.8
3.7
4.4
40
10
6
66
358
67
6
468
8
2
6

Table 3: Imine formation rate (R), molar (benzaldehyde/imine) product ratio
-C=O/N-C=N) and hydrogen utilisation efficiency (mol H generated from alcohol
dehydrogenation per mol imine produced (H /–C=N)) as a function of Cu/Au molar
ratio for coupled benzyl alcohol and nitrobenzene reactions over Cu/SiO and oxide
(
N
2
2
2
supported Au + Cu physical mixtures. Reaction conditions: T = 498 K, P = 1 atm,
inlet alcohol/nitroarene molar ratio (–OH/–NO ) = 3.
2
Cu/Au
R
N
-C=O /N-C=N
H
2
/–C=N
Catalyst
Cu/SiO
-1
-1
(
mmol molCu h )
2
-
358
609
3.8
4.4
2.6
2.0
4.9
2.9
6
6
4
3
6
4
Cu/SiO
Cu/SiO
Cu/SiO
2
2
2
+Au/TiO
+Au/TiO
+Au/TiO
2
2
2
80
20
10
20
10
1219
1407
368
Cu/SiO
2
+Au/MgO
+Au/MgO
Cu/SiO
2
961
2
7

Figure captions
Fig. 1: Reaction network in coupled benzyl alcohol and nitrobenzene reactions. Note: dashed
arrow represents hydrogen transfer; target imine product is framed.
Fig. 2: (I) Temperature programmed reduction (TPR) profiles, (II) representative STEM
images and (III) associated metal particle size distribution histograms for (A) Cu/SiO
2
, (B)
Au/TiO and (C) Au/MgO.
2
Fig. 3: Arrhenius plots for stand-alone dehydrogenation of benzyl alcohol () and
hydrogenation of nitrobenzene () over Cu/SiO . Reaction conditions: T = 423-498 K, P = 1
2
atm.
Fig. 4: Variation of fractional conversion (X
on-stream for the coupled benzyl alcohol and nitrobenzene reactions over Cu/SiO
conditions: T = 498 K, –OH/–NO = 3, P = 1 atm.
i
, : benzyl alcohol, : nitrobenzene) with time
2
. Reaction
2
Fig. 5: Reaction rates for (A) dehydrogenation of benzyl alcohol and (B) hydrogenation of
nitrobenzene over Cu/SiO , Au/TiO and Au/MgO. Reaction conditions: T = 498 K, P = 1 atm.
2
2
Fig. 6: Variation of fractional conversion (X
stream for the coupled benzyl alcohol and nitrobenzene reactions as a function of Cu/Au ratio
: 10, : 20 and : 80) over (A) Cu/SiO + Au/TiO and (B) Cu/SiO + Au/MgO physical
mixtures. Reaction conditions: T = 498 K, –OH/–NO = 3, P = 1 atm.
i
) and product composition (N
j
) with time on-
(
2
2
2
2
Fig. 7: Schematic illustrating Cu-Au cooperation effect in imine production.
Fig. 8: XPS spectra: (A) C1s and (B) Cu 2p3/2 (I) pre- and (II) post-reaction over Cu/SiO and
2
(III) after reaction over Cu/SiO
2
+ Au/TiO ; experimental data are given by  where lines
2
represent the fits with peak deconvolution.
2
8

Fig. 9: (A) TGA profiles for catalysts pre-reaction (solid lines: (I) Cu/SiO
Au/TiO ) and post-reaction (dashed lines: (III) Cu/SiO ; (IV) Cu/SiO + Au/TiO
elemental (C and H) analysis. (B) DSC analysis of Cu/SiO (solid line) and Cu/SiO
dashed line) post-reaction.
2
; (II) Cu/SiO
). Inset Table:
+ Au/TiO
2
+
2
2
2
2
2
2
2
(
2
9