Enhanced production of benzyl alcohol in the gas phase continuous hydrogenation of benzaldehyde over Au/Al2O3

Article abstract of DOI:10.1016/j.catcom.2013.12.024

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393-413 K, 1 atm) was achieved over Au/Al ₂O₃, Au/TiO₂ and Au/ZrO₂. Synthesis of Au/Al₂O₃ by deposition-precipitation generated a narrower distribution (2-8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1-21 nm, mean = 7.9 nm) with increased H₂ uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al₂O₃ with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO₂ and Au/ZrO₂.

Full text of DOI:10.1016/j.catcom.2013.12.024

Catalysis Communications 46 (2014) 187–191
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Enhanced production of benzyl alcohol in the gas phase continuous
hydrogenation of benzaldehyde over Au/Al O
2
3
Maoshuai Li, Xiaodong Wang, Noémie Perret, Mark A. Keane ⁎
Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 October 2013
Received in revised form 12 December 2013
Accepted 18 December 2013
Available online 24 December 2013
Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K,
1
atm) was achieved over Au/Al
generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation
1–21 nm, mean = 7.9 nm) with increased H uptake under reaction conditions and higher benzaldehyde
turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective
hydrogenation rate over Au/Al with 100% benzyl alcohol yield, attributed to increased available reactive
hydrogen. This response extends to reaction over Au/TiO and Au/ZrO
2 3 2 2 2 3
O , Au/TiO and Au/ZrO . Synthesis of Au/Al O by deposition–precipitation
(
2
2 3
O
Keywords:
Benzaldehyde
Selective hydrogenation
Benzyl alcohol
2
2
.
© 2013 Elsevier B.V. All rights reserved.
2 3
Au/Al O
Water as carrier
1
. Introduction
2 3 2 2 2
no detectable alcohol formation over Cu on Al O , TiO , CeO or ZrO ir-
respective of temperature [13]. Exclusive C_O reduction, circumventing
hydrogenolysis and phenyl ring reduction at high conversions remains
challenging [14].
Oxide supported Au has exhibited enhanced chemoselectivity
in the hydrogenation of aldehydes and ketones relative to standard
transition metal catalysts [15,16]. This is offset somewhat by lower
Benzyl alcohol is widely utilised as a solvent for inks, paints and
lacquers and as precursor to a range of esters in the cosmetics and
flavouring industries [1]. Industrial production via benzyl chloride
hydrolysis releases chlorine with significant by-product (benzyl ether
and NaCl) formation while toluene oxidation consumes large quantities
of solvents with the inclusion of toxic bromides to enhance alcohol
yield [2,3]. As an alternative, benzaldehyde hydrogenation has been
promoted over Rh and Ru complexes using alcohols and organic acids
as hydrogen donors [4,5]. Product separation and catalyst reuse are fa-
cilitated by switching to heterogeneous systems. Application of solid
catalysts has focused on liquid phase reaction over Ni, Ru, Pt and Pd
reaction rates due to the lesser capacity of Au for H
previous work, we reported exclusive hydrogenation of benzaldehyde
to benzyl alcohol over Au/Al (prepared by impregnation) under con-
ditions where Ni/Al and Pd/Al were non-selective [17]. Complete
selectivity over Au/Al was obtained at low hydrogenation rates
(conversion ≤ 15%). As an extension to that work, we have evaluated
the catalytic action of Au/Al prepared by deposition–precipitation
2
activation [16]. In
2 3
O
O
2 3
2 3
O
2 3
O
on a range of supports (monolith, Al
6–9]; pertinent conversion/selectivity and associated reaction condi-
tions are compiled in Table 1. Okamoto and co-workers [9] recorded
00% alcohol selectivity at partial aldehyde conversion (84%) over poly-
ethylene glycol (PEG) modified Pd/SiO . Zhao et al. [10] achieved near
complete benzaldehyde conversion and full selectivity to benzyl alcohol
over Pt/C at elevated H pressure (4 MPa). Gas phase continuous oper-
ation has employed a series of metal oxides (ZnO, Fe , Al , ZrO
CeO and TiO ) [11] and oxide supported Pt, Cu and Ni [12–14].
Supported Pt exhibited superior performance with full selectivity to
the alcohol at conversions up to 80% over Pt/TiO (Table 1), attributed
2
O
3
, activated carbon and SiO
2
)
2 3
O
[
to generate smaller Au size. Moreover, we demonstrate for the first
time the use of water as an environmentally benign aldehyde carrier
to elevate catalytic productivity in continuous gas phase operation
(at 0.1 MPa) with 100% alcohol yield. We also assess the catalytic
1
2
2 2
performance of Au/TiO and Au/ZrO to validate the promotional effect
2
of water.
O
2 3
2
O
3
2
,
2
2
2. Experimental
2
2.1. Catalyst preparation and activation
to C_O activation at the metal-support interface [12]. Product distribu-
tion over supported Cu was dependent on support and temperature
The γ-Al
ports were used as received. The ZrO
ZrOCl (0.1 M) precipitation with aqueous NH
stirring (600 rpm) at pH 9.4–11.8. The hydrogel was dried at 373 K
2
O
3
(Puralox, Condea Vista) and TiO
carrier was synthesised by
(2.5 M) under vigorous
2
(P25, Degussa) sup-
where Cu/SiO
2
delivered high selectivity (83% at 373 K) but there was
2
2
3
⁎
Corresponding author. Tel.: +44 131 451 4719.
E-mail address: M.A.Keane@hw.ac.uk (M.A. Keane).
(24 h) and calcined (1 K min ) in 60 cm3 min air at 673 K (5 h).
−
1
−1
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.12.024

188
M. Li et al. / Catalysis Communications 46 (2014) 187–191
Table 1
Conversions (X) and selectivities (S) with associated temperatures and pressures reported
for benzaldehyde hydrogenation.
450 K
Catalyst
Phase
T (K)
PH2 (MPa)
X (%)
S (%)
Ref.
PEG-Pd/SiO
Pt/C
2
Liquid
Liquid
Gas
Gas
Gas
453
323
453
373
393
3
4
0.1
0.1
0.1
84
98
80
68
15
100
100
100
83
[9]
[10]
[12]
[13]
[17]
Pt/TiO
Cu/SiO
Au/Al
2
2
2
O
3
100
(
A)
2 3
Alumina supported Au prepared by impregnation (Au/Al O -IMP) is de-
415 K
scribed in detail elsewhere [17]. Synthesis by deposition–precipitation
(
Au/Al
2
O
3
-DP, Au/ZrO
2
and Au/TiO
2
) employed an aqueous solution of
(
B)
−
4
urea (100 fold excess) and HAuCl
heating the suspension to 353 K (2 K min ), which was maintained
4
(5 × 10
M) with support addition,
−
1
for 3 h with a pH increase to ca. 7. The solid was washed and dried in
He (45 cm min ) at 373 K (2 K min ) for 5 h. Catalyst precursors
3
00
350
400
450
500
550
600
3
−1
−1
T (K)
were sieved into a batch of 75 μm average diameter and activated at
−
1
3
−1
2
K min to 473–603 K in 60 cm min
2
H .
Fig. 1. TPR profiles for (A) Au/Al
O
2 3
-DP and (B) Au/Al
2 3
O -IMP.
2
.2. Catalyst characterisation
5
4
.8 × 10− mol h−1. The molar Au to benzaldehyde feed rate (n/F)
−
2
−2
Gold content was determined by atomic absorption spectroscopy
Shimadzu AA-6650 spectrometer). Temperature programmed reduc-
tion (TPR) and H chemisorption were recorded on the commercial
spanned the range 1.2 × 10 –9 × 10
h. The reactor effluent was
(
condensed in liquid nitrogen for analysis by capillary GC (Perkin-
Elmer Auto System XL). Benzaldehyde (Fluka, ≥98%), benzyl alcohol
(Riedel-de Haën, ≥99%) and ethanol (Sigma Aldrich, ≥99%) were
used as received. Repeated reaction with different samples from the
same batch of catalyst delivered conversion/selectivity values that
were reproducible to better than ± 5%.
2
CHEM-BET 3000 (Quantachrome) unit, as described previously [17].
Nitrogen adsorption–desorption isotherms were obtained using the
Micromeritics Gemini 2390 system where total surface area was deter-
mined using the standard BET method and cumulative pore volumes by
BJH analysis. Powder X-ray diffractograms (XRD) were recorded on a
Bruker/Siemens D500 incident X-ray diffractometer. Diffractograms
were identified using JCPDS-ICDD reference standards, i.e. Au (Card
3. Results and discussion
No. 04-0784) and γ-Al
O
2 3
(10-0425). Gold particle size and morpholo-
3.1. Catalyst characterisation
gy were determined by transmission (TEM, JEOL JEM 2011) and scan-
ning transmission (STEM, JEOL 2200FS) electron microscopy. Samples
for analysis were prepared by dispersion in acetone and deposited on
a holey carbon/Cu grid (300 Mesh). Surface area weighted mean Au
size (dTEM) was obtained from a count of at least 300 particles.
Textural characteristics of the four catalysts considered in this study
are given in Table 2. TPR of Au/Al
profiles shown in Fig. 1 with temperature maxima (Tmax) in H
consumption at 450 K and 415 K. A single TPR peak over 434–490 K
has been reported elsewhere for Au/Al and ascribed to the reduction
consumed coincided with that
required for precursor reduction (theoretical values recorded in
Table 2). The reduction profile for Au/Al -IMP spanned a lower tem-
perature range than Au/Al -DP, suggesting weaker precursor–
support interactions that result in more facile reduction. Activation
2 3 2 3
O -DP and Au/Al O -IMP generated
2
2 3
O
of Au3 species to Au0 [18]. The H
+
2
.3. Catalytic procedure
2
2 3
O
Catalyst testing was carried out in situ after activation in a continu-
2 3
O
ous flow fixed-bed tubular reactor (i.d. = 15 mm, l = 30 cm). Isother-
mal conditions (393–423 ± 1 K) were maintained by diluting the
catalyst with ground glass (75 μm). Benzaldehyde was delivered as
ethanolic or aqueous solutions via a glass/teflon air-tight syringe and
teflon line using a microprocessor controlled infusion pump (Model
1
00 kd Scientific). A co-current flow of benzaldehyde and H
2
was main-
4
−1
tained at GHSV = 2 × 10
h
with an inlet benzaldehyde flow (F) of
(
(
A)
B)
Table 2
Physico-chemical characteristics of the supported Au catalysts.
Catalyst
Au/Al
2
O
3
-DP Au/Al
2
O
3
-IMP Au/TiO
2
Au/ZrO
2
Au loading (%w/w)
1.1
1.1
1.2
48
0.12
364
126
0.8
93
0.13
452
56
(
(
C)
D)
2
−1
BET area (m
g
)
166
0.36
450
87
161
0.36
415
79
Pore volume (cm g−1
3
)
TPR Tmax (K)
Measured TPR H
2
consumption
−
1
(
μmol g
)
a
Theoretical TPR H
2
consumption 84
80
45
91
61
−
1
(
μmol g
chemisorption (μmol g A− u1)
at 413 K
Au particle size range (nm)
)
H
2
318
162
137
20
30
40
50
60
70
80
2
θ (°)
2–8
1–21
2–9
3–12
d
TEM (nm)
4.3
7.9
4.5
7.0
Fig. 2. XRD patterns for (A) Au/Al
2 3 2 3
O -DP and (B) Au/Al O -IMP and JCPDS-ICDD reference
a
Amount required for Au precursor reduction.
for (C) γ-Al (10-0425) and (D) Au (04-0784).
2 3
O

M. Li et al. / Catalysis Communications 46 (2014) 187–191
189
4
0
of Au/ZrO
matched the requirement for Au → Au (Table 2). The Tmax recorded
for Au/TiO reduction was lower than that for Au/Al -DP. Excess H
consumption during the TPR of Au/TiO can be attributed to partial
support reduction [19]. Delannoy et al. [20] considered the possible
role of the carrier to modify Au precursor reducibility and proposed an
2 2 3
exhibited a similar Tmax (452 K) to Au/Al O -DP and
1
7
5
2
0
00
5
3
+
0
2
O
2 3
2
30
20
2
2 2 2 3
order of decreasing reducibility (TiO N CeO N Al O ) that is consistent
0
with our results.
Analysis by XRD generated the diffractograms presented in Fig. 2.
Au/Al -DP exhibited diffraction peaks at 2θ = 37.6°, 39.5°, 45.9°
and 67.0° corresponding to (311), (222), (400) and (440) planes of
γ-Al . There was no detectable signal due to Au, suggesting a well
dispersed Au phase. In contrast, signals at 38.1°, 44.3°, 64.6° and 77.5°
for Au/Al -IMP can be assigned to the (111), (200), (220) and (311)
2 3
O
1
0
5
2 3
O
0
O
2 3
0.5
1.0
1.5
2.0
2.5
3.0
planes of Au, diagnostic of larger Au particles. This was confirmed
by TEM/STEM analysis and representative images are given in Fig. 3.
The Au particles exhibited a quasi-spherical morphology with a
Time on-stream (h)
Fig. 4. Temporal variations of conversion (X) and benzyl alcohol selectivity (S) over
Au/Al O -DP (■ and ×) and Au/Al O -IMP (● and ○): T = 413 K; n/F = 1.2 × 10 h.
2 3 2 3
−
2
2 3
narrower distribution of smaller Au particles on Au/Al O -DP (2–8 nm,
mean = 4.3 nm) relative to Au/Al -IMP (1–21 nm, mean = 7.9 nm).
2 3
O
(b1 μmol g− ) was appreciably lower than that reported for oxide
1
This agrees with the literature where catalyst preparation by DP
generated smaller Au particles and the residual chloride associated
with IMP samples resulted in mobility and agglomeration of Au spe-
cies during thermal treatment [21]. Gold particle size distribution
−
1
supported transition metals, e.g. Ni/Al O (4 μmol g ) and Pd/Al O3
2
3
2
−
1
(24 μmol g ). Hydrogen chemisorption on supported Au is an
activated process with increased uptake under reaction conditions
(45–318 μmol g
−
1
(
2–9 nm, mean = 4.5 nm) on TiO
2
was close to that determined for
Au
at 413 K, Table 2). Bus et al. [22] have shown
Au/Al -DP, whereas Au/ZrO exhibited larger Au size (3–12 nm,
mean = 7.0 nm). Hydrogen uptake/activation is a critical catalyst
property in hydrogenation applications. We have noted previously
2
O
3
2
that H2 chemisorption is favoured on smaller Au particles (supported
on Al O ) that possess a higher fraction of low coordinated Au atoms
2
3
at corners and edge sites. This agrees with the higher uptake that we
have recorded for Au/Al O -DP relative to Au/Al O -IMP.
[
17] that H
2
2 3
chemisorption on Au/Al O -IMP at ambient temperature
2
3
2 3
A
5
4
3
2
1
0
0
0
0
0
0
1
2
3
4
5
6
7
8
20 nm
d (nm)
B
40
3
2
1
0
0
0
0
<
4
5
6
7
8
9
10 > 11
d (nm)
2 3 2 3
Fig. 3. Representative STEM and TEM images for (A) Au/Al O -DP and (B) Au/Al O -IMP with Au size distribution histograms.

190
M. Li et al. / Catalysis Communications 46 (2014) 187–191
Table 3
Table 4
Initial conversion (X
0
) and selectivity (S
0
) for benzaldehyde hydrogenation over
Initial turnover frequency (TOF
using ethanol and water as carrier: T = 413 K; n/F = 1.2 × 10 h.
0 0
) and selectivity (S ) for benzaldehyde hydrogenation
−
2
Au/Al
2
O
3
-DP as a function of temperature, contact time and carrier.
T (K)
Carrier
n/F × 102 (h)
X
0
(%)
S
0
(%) benzyl alcohol
S
0
(%) toluene
Catalyst
Au/Al
Carrier
TOF
0
(h−1
)
0
S (%) benzyl alcohol
3
4
4
4
4
93
13
23
13
13
Ethanol
Ethanol
Ethanol
Water
1.2
1.2
1.2
1.2
9
19
27
53
58
100
100
100
92
100
100
–
–
8
–
–
2 3
O -DP
Ethanol
Water
Ethanol
Water
Ethanol
Water
86
188
52
98
37
100
100
100
100
100
100
Au/TiO
2
Water
Au/ZrO
2
46
3
.2. Catalyst activity and selectivity
interactions [25]. In gas phase applications, the solvent serves as a car-
rier and possible contributions to catalyst performance have not been
studied. Green chemistry principles highlight the use of innocuous
(non-toxic) solvents where water can serve as an inexpensive and be-
nign polar solvent. In this study, use of water as carrier resulted in a dra-
matic increase in conversion, while retaining full selectivity to benzyl
alcohol (Table 3). Moreover, an increase in the n/F parameter, which
equates to contact time, resulted in the complete conversion of the
inlet benzaldehyde and 100% selectivity to the alcohol. This represents
unprecedented selective hydrogenation efficiency with respect to the
existing literature (Table 1) for both liquid and gas phase operations.
The beneficial effect of water as carrier in terms of elevated selective
Time on-stream conversion (X) and benzyl alcohol selectivity
S) over Au/Al -DP and Au/Al -IMP exhibit a temporal decline
Fig. 4). Initial conversion (X ) can be extracted from fitting the data to
(
(
2
O
3
2 3
O
0
ðX−X Þ
Δt
0
¼
0
ðX −X Þ ðβ þ ΔtÞ
3
h
where β is a time scale fitting parameter and X3 h is the conversion after
h on-stream. Initial conversions were used to determine turnover fre-
quency (TOF , calculated using Au dispersion obtained from TEM analy-
sis) where Au/Al
than Au/Al -IMP (30 h ). We attribute this to the occurrence of
smaller nano-scale Au particles on Au/Al -DP and greater H uptake
Table 2). Mohr et al. [23] demonstrated structure sensitivity in the
3
0
−
1
2 3
O
-DP delivered a significantly higher value (86 h
)
−
1
2 3
O
2 2
hydrogenation rate extended to Au/TiO and Au/ZrO (Table 4). The
O
2 3
2
promotional role of water on C_O reduction in the liquid phase hydro-
genation of crotonaldehyde was attributed to facilitated interaction
of the hydrophilic C_O moiety with surface catalytic sites [26]. In gas
phase hydrogenation over supported Au, activity is limited by available
surface reactive hydrogen and any additional hydrogen supply should
elevate rate. The generation of surface hydrogen (protons) from
(
2
hydrogenation of acrolein over Au/ZrO where TOF increased with de-
creasing particle diameter (from 8 to 4 nm). However, there is insuffi-
cient published data to establish any clear consensus regarding Au size
effects in C_O reduction.
2 3
The influence of temperature on Au/Al O -DP catalytic response
water dissociation is promoted by Lewis acid sites on Al
O
2 3
[27] and ox-
with ethanol as carrier can be assessed from Table 3 where conversion
was elevated at higher temperatures with the formation of toluene as
by-product (at 423 K). We note that Saadi et al. [13] have reported pref-
erential production of benzyl alcohol over supported Ni and Cu at low
reaction temperature (b373 K) with the occurrence of toluene at tem-
peratures N383 K. Exclusivity to the alcohol is challenging as illustrated
by the reaction pathways in Fig. 5, where toluene formation occurs via
consecutive hydrogenolysis of benzyl alcohol or direct conversion of
benzaldehyde. Benzene results from scission of the aldehydic C\H
bond [24]; there was no detectable benzene formation in this study.
The solvent can influence activity and selectivity, notably in batch liquid
operation. Solvent effects in the hydrogenation of unsaturated alde-
ygen vacancies on TiO
coordinate O sites on the support to form bridging hydroxyl groups.
Buchanan and Web [29] have demonstrated that surface hydroxyl
2
[28]. The abstracted protons can bond with two-
2−
2 3
groups on Al O act as a source of atomic ‘hydrogen’ and dehydroxylated
Au/Al was inactive in butadiene hydrogenation. Theoretical calcula-
2 3
O
tions have established that the dissociation energy of water on Al and
Au sites is in the ranges 0.83–2.12 eV [30] and 0.61–2.2 eV [31,32], re-
spectively that are of the same magnitude as the dissociative adsorption
of hydrogen on Au (0.16–1.4 eV) [33,34]. Activity of Au/ZrO
drogenation of 1,3-butadiene has been correlated with surface hydroxyl
group density where dehydroxylated Au/ZrO was inactive and activity
was partially recovered by water treatment [35]. We therefore attribute
the beneficial effect of water as carrier to a facilitated surface dissocia-
tion that generates reactive hydrogen.
2
in the hy-
2
2
hydes/ketones are associated with polarity, H solubility and surface
4. Conclusion
Gas phase hydrogenation of benzaldehyde over Au supported
on Al
the target product. Increased benzaldehyde TOF over Au/Al
prepared by deposition–precipitation is associated with increased H
2 3 2 2
O , ZrO and TiO was fully selective to benzyl alcohol as
2
O
3
2
uptake on well dispersed Au (mean particle size = 4.3 nm) relative
to synthesis by impregnation (mean = 7.9 nm). Use of an aqueous
rather than ethanolic benzaldehyde feed delivered appreciably higher
chemoselective rates with 100% benzyl alcohol yield. This is attributed
to water dissociation on the catalyst surface, generating reactive hydro-
gen that compensates for the limited capability of Au to dissociate H
2
,
with an overall increased hydrogenation rate. This promotional effect
extends to benzaldehyde conversion over Au/ZrO and Au/TiO .
2 2
Acknowledgements
We acknowledge Dr. F. Cárdenas-Lizana and Yufen Hao for contribu-
tions to this work and financial support to M.L. and X.W. through the
Overseas Research Students Award Scheme.
Fig. 5. Reaction pathways in the hydrogenation of benzaldehyde.

M. Li et al. / Catalysis Communications 46 (2014) 187–191
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