Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl Alcohol Over Alumina Supported Gold

Article abstract of DOI:10.1007/s10562-019-02944-y

Abstract: We report for the first time 100% benzyl alcohol yield from the liquid phase (T = 353?K, P = 9?bar) hydrogenation of benzaldehyde over Au/Al₂O₃. Under the same reaction conditions, a benchmark Pt/Al₂O₃

Full text of DOI:10.1007/s10562-019-02944-y

Catalysis Letters
https://doi.org/10.1007/s10562-019-02944-y
Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl
Alcohol Over Alumina Supported Gold
1
1
1
1
Yufen Hao · Chiara Pischetola · Fernando Cárdenas‑Lizana · Mark A. Keane
Received: 11 May 2019 / Accepted: 9 August 2019
©
The Author(s) 2019
Abstract
We report for the ꢀrst time 100% benzyl alcohol yield from the liquid phase (T=353 K, P=9 bar) hydrogenation of benzal-
dehyde over Au/Al O . Under the same reaction conditions, a benchmark Pt/Al O catalyst promoted the formation of toluene
2
3
2
3
and benzene as hydrogenolysis by-products. Reaction kinetics was subjected to a Hammett treatment and the reaction constant
(
ρ=0.9) was found to be consistent with a nucleophilic mechanism. A solvent (alcohol, water and alcohol+water) eꢁect is
demonstrated and ascribed to competitive adsorption where solvation by polar (water) facilitates benzaldehyde activation.
Graphic Abstract
Keywords Selective hydrogenation · Benzaldehyde · Benzyl alcohol · Water as solvent · Au/Al O · Pt/Al O
3
2
3
2
1
Introduction
The production of benzyl alcohol by the selective reduction
of benzaldehyde is an important commercial process with
multiple applications as solvent for inks, paints and lacquers
*
Fernando Cárdenas-Lizana
F.CardenasLizana@hw.ac.uk
[
1]. Oxide supported Ru [2], Pd [3, 4], Ni [5, 6] Cu [7] and
1
Pt [8, 9] catalysts have been employed, typically in batch
liquid phase at high pressure (20–40 bar) [1, 2, 5, 8, 10,
Chemical Engineering School of Engineering and Physical
Sciences, Heriot-Watt University, Edinburgh EH14 4AS,
Scotland, UK
Vol.:(0
1
123456789)
3

Y. Hao et al.
1
1]. The target of 100% benzyl alcohol yield (Scheme 1,
protic solvents and 2-butanone by hydrogen bonding that
lowered the activation energy barrier. Drelinkiewicza et al.
[26] concluded that acetophenone hydrogenation was more
path I) remains a challenge as undesired hydrogenolysis (to
toluene and/or benzene, paths II–V [12]) is diꢂcult to cir-
cumvent with reported alcohol yields in the 40–75% range
inꢃuenced by solvent polarity than H solubility. Organic
2
[
2–6, 9, 13]. Application of Au in hydrogenation, though
solvents such as ethanol [3, 4, 9, 10, 13], dioxane [5] and
alkanes (n-octane [3] and dodecane [31]) have been used in
the liquid phase hydrogenation of benzaldehyde. Water as
a benign green reaction medium has not been considered to
any great extent. It is nonetheless worth ꢃagging the work of
Wang and co-workers [32] who demonstrated higher activity
in the chemoselective hydrogenation of α, β-unsaturated car-
less developed than use in oxidation, has been the focus of
recent research [14–19]. In their review, Hari and Yaakob
[
20] established the potential of Au for selective liquid phase
hydrogenation with enhanced alcohol selectivity in the con-
version of aldehydes such as crotonaldehyde [21] and cit-
ral [22]. We provide here the ꢀrst reported application of
(
Al O ) supported Au in the liquid phase hydrogenation of
bonyl compounds over Au/CeO in water relative to organic
2
3
2
benzaldehyde.
solvents (ethanol, isopropanol, dioxane and cyclohexane).
In this work, we examine the role of solvent (n-pentanol,
n-butanol, ethanol, H O and ethanol + H O mixtures) in
Gold delivers lower activity when compared with con-
ventional transition metal catalysts due to a less eꢁective
2
2
activation/dissociation of H [23, 24]. Hydrogenation activ-
determining benzaldehyde hydrogenation over Au/Al O .
2
2 3
ity in liquid phase operation can be inꢃuenced by the sol-
We have targeted 100% benzyl alcohol yield as an objective.
vent [25]. This has been related to diꢁerences in dielectric
The eꢁect of para-positioned substituents (–OCH , –CH ,
3
3
constant (ε) [26–28], bonding capacity (α) [28, 29] and H
–CH CN and –CN) on rate has also been considered. Given
2
2
solubility [26]. Aramendía et al. [30] and Bertero et al. [27]
found that the rate of acetophenone hydrogenation decreased
with increasing ε of C –C alcohols, which they ascribed to
the established application of supported Pt in benzaldehyde
hydrogenation [8, 10, 11], we employed a commercial Pt/
Al O as a suitable benchmark catalyst.
1
3
2
3
solvation eꢁects that inꢃuenced reactant adsorption. On the
other hand, Wan et al. [28] observed an increased hydrogen-
ation rate with increasing α, linked to interaction between
2 Experimental
2.1 Materials
The Al O support and (0.7% w/w) Pt/Al O were pur-
2
3
2
3
chased from Puralox (Condea Vista. Co.) and Alfa Aesar,
respectively. The gold precursor (HAuCl ; 99.999%, Sigma-
4
Aldrich) and urea (≥ 99%; Riedel-de Häen) were used as
received. All the gases used (H , N , O and He) were of
2
2
2
high purity (99.9%, BOC gases). The reactants (p-methoxy-
benzaldehyde, p-tolualdehyde, benzaldehyde, benzyl alcohol
and p-cyanobenzaldehyde; ≥98%, Aldrich; p-formylphenyl
acetonitrile, 96.5%, Apollo Scientiꢀc) and solvents (ethanol,
butanol, pentanol; ≥99.5%, Aldrich) were used as supplied
without further puriꢀcation.
2.2 Catalyst Preparation and Activation
A 1% w/w Au/Al O was prepared by deposition–precipita-
2
3
tion using urea as basiꢀcation agent. An aqueous mixture
3
of urea (tenfold excess) and HAuCl in water (100 cm ,
4
−
3
−3
2
1
2
3
5 × 10 g cm , pH = 2) was added to the support (ca.
0 g). The suspension was stirred and heated to 353 K (at
−
1
K min ) and the pH progressively increased to ca. 7 after
h as a result of thermally induced urea decomposition.
Scheme 1 Reaction pathway associated with benzaldehyde hydro-
genation; pathway to the target alcohol (open arrow), by-products
detected in this work (solid arrows) and reported in the literature [12]
T=353 K
NH − CO − NH + 3H O ���������������������������→�� 2NH + 2OH + CO
+
4
−
2
2
2
2
(
1)
(
dashed arrows)
1
3

Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl Alcohol Over Alumina Supported…
The solid obtained was separated by centrifugation,
washed with deionised water (with centrifugation between
with N . The system was heated to the reaction temperature,
2
pressurised and the stirring engaged (time t=0 for reaction).
3
−1
−1
each washing) and dried in He (45 cm min ) at 2 K min
In a series of blank tests, there was negligible conversion in
to 373 K, which was maintained for 5 h. Au/Al O and Pt/
the absence of catalyst or without H . The initial molar reac-
2
3
2
3
3
Al O were crushed and sieved to 75 μm average particle
tant to metal ratio spanned the range 2×10 –11×10 . A liq-
2
3
3
diameter (ATM ꢀne test sieves) and activated in 60 cm
uid sampling system via syringe with in-line ꢀlters allowed a
−
1
−1
3
min H at 2 K min to 523 K (Au/Al O [33]) and 773 K
controlled withdrawal of aliquots (≤1.0 cm ) from the reac-
2
2
3
(
Pt/Al O ) [34]). Post activation, samples were cooled to
tor. The concentration of reactant/product was determined
from the total mass balance in the reaction mixture. Frac-
2
3
2
98 K and passivated in 1% v/v O /He for ex situ analysis.
2
tional conversion of benzaldehyde (X
) is deꢀned as
Benzaldehyde
2.3 Catalyst Characterisation
C
− CBenzaldehyde
Benzaldehyde,0
X
=
(3)
Benzaldehyde (−)
C
Benzaldehyde,0
The metal (Au and Pt) content was measured by inductively
coupled plasma-optical emission spectrometry (ICP-OES,
Vista-PRO, Varian Inc.) from the (ꢀvefold) diluted extract
where subindex ‘0’ refers to initial concentration. Benzalde-
−1
−1
3
hyde consumption rate (R, mol mol
h ) was determined
metal
in HF (10 cm , 0.1 M). Catalyst activation and chemisorp-
from a linear regression of the temporal benzaldehyde con-
tion (at 353 K) measurements were conducted using the
commercial CHEM-BET 3000 (Quantachrome) unit. The
sample was loaded into a U-shaped quartz cell (3.76 mm
centration proꢀles at X
<0.35 [35] according to:
Benzaldehyde
ꢀΔC
ꢁ ꢀ
ꢁ
3
−1
Benzaldehyde
1
−1
metal
−1
h ) =
i.d.), heated in 17 cm min (Brooks mass ꢃow controlled),
R (mol mol
⋅
(4)
−
1
Δt
n
metal
5
% v/v H /N at 2 K min to 523–773 K and maintained
2
2
until the signal returned to baseline. Samples were swept
−
3
3
−1
where n
is the metal concentration (mol
cm ). Turn-
metal
metal
with 65 cm min N for 1.5 h, cooled to 353 K and sub-
2
−
1
over frequency (TOF, s ) was calculated using metal dis-
jected to H chemisorption using a pulse (10–50 µl) titration
2
persion measurements from STEM as described elsewhere
procedure. Hydrogen pulse introduction was repeated until
the signal area was constant, indicating surface saturation.
Metal particle morphology (size and shape) was determined
by scanning transmission (JEOL 2200FS ꢀeld emission
gun-equipped TEM unit) electron microscopy (STEM),
employing Gatan DigitalMicrograph 1.82 for data acquisi-
tion/manipulation. Samples were dispersed in acetone and
deposited on a holey carbon/Cu grid (300 Mesh). Up to 1000
individual particles were counted and the surface area mean
[
36]. Selectivity to benzyl alcohol (S
) is given by
Benzyl alcohol
C
Benzyl alcohol
S
(%) =
× 100 (5)
Benzyl alcohol
C
− CBenzaldehyde
Benzaldehyde,0
Product composition was determined by gas chroma-
tography using a Perkin-Elmer AutoSystem XL chromato-
graph equipped with a programmed split/splitless injector
and a ꢃame ionisation detector, employing a DB-1 capil-
lary column (i.d. = 0.33 mm, length = 50 m, film thick-
ness = 0.20 μm). Data acquisition and manipulation were
performed using the TotalChrom Workstation (Version 6.3.2
for Windows) chromatography data system. Repeated reac-
tion runs with the same batch of catalyst delivered raw data
reproducibility and carbon mass balances within± 5%.
metal (Au, Pt) diameter (d
) calculated from
STEM
∑
nidi
i
d
= ∑
(2)
STEM
ni
i
where n is the number of particles of diameter d .
i
i
2.4 Catalytic System
3 Results and Discussion
The liquid phase hydrogenation reactions (T = 353 K;
P=9.0 bar) were carried out in a commercial batch stirred
3.1 Catalyst Characterisation
3
stainless steel reactor (100 cm autoclave, Parr reactor)
The metal loading, particle size (from STEM analysis) and
equipped with a H supply system (GCE-Druva). The tem-
H chemisorption values are given in Table 1. Both sup-
2
2
perature was maintained at 353± 1 K using a process con-
ported catalysts exhibit similar loading where the metal
phase takes the form of pseudo-spherical particles (Fig. 1I)
over the 1–8 nm size range (Fig. 1II). This equivalence
permits a direct comparison of catalytic response. Hydro-
gen uptake on supported Au is sensitive to metal size and
troller (Scientiꢀc & Medicine Products Ltd). At the begin-
ning of each run, the aldehyde (or benzyl alcohol) solution
3
(
40 cm of pentanol, butanol, ethanol, H O or ethanol+H O;
2
2
0
.05 M) and catalyst were charged and ꢃushed three times
1
3

Y. Hao et al.
Table 1 Metal content, mean metal particle size from STEM analysis
Scheme 1. Step I represents the target carbonyl group reduc-
tion. Subsequent hydrogenolysis results in the formation of
benzene (step II) and toluene (step III) [41, 42]. Benzene
and toluene can also be formed directly from benzaldehyde
(
d
), H chemisorption (at 353 K) and catalytic activity expressed
STEM 2
as reaction rate (R) and turnover frequency (TOF) in the liquid phase
hydrogenation of benzaldehyde (using water as solvent): T=353 K;
P=9 bar
(
steps IV and V) [41]. Further hydrogenation of benzyl alco-
Au/Al O
Pt/Al O
2
3
2
3
hol to cyclohexylmethanol (step VI) has been reported for
reaction over Ru/C [43] with toluene reduction to methyl-
clohexane (step VII) over Ni/Al O [6]. Under all reaction
Metal content w/w (%)
1.1
3.6
2.8
78
0.7
1.7
22.7
446
186
dSTEM (nm)
2
3
H uptake (μmol g−1)
conditions used in this study, benzaldehyde was converted
solely to the target benzyl alcohol over Au/Al O .
2
−
1
−1
R (mol mol
h )
metal
2
3
TOF×10 (s−1)
−
3
65
3
.2.1 Reaction Under Kinetic Control
dissociative adsorption is favoured on smaller (1–10 nm)
particles [37]. The eightfold lower H uptake on Au/Al O
Benzaldehyde hydrogenation rate was determined from
the linear variation of concentration with time, as shown
in Fig. 2I. We carried out a series of tests in order to mini-
mise diꢁusion and mass transfer limitations and ensure the
reaction was operated under kinetic control. The eꢁect of
variations in agitation speed (Fig. 2IA) and catalyst mass
(Fig. 2IB) on reaction rate was examined as two well-estab-
lished diagnostic tests to assess external and internal transfer
constraints [44]. An increase in stirring speed from 300 to
700 rpm was accompanied by a proportional increase in rate.
2
2
3
relative to Pt/Al O (Table 1) can be linked to the high
2
3
activation barrier for H adsorption on Au [38] and greater
2
capacity of Pt to dissociate H [39, 40].
2
3.2 Liquid Phase Hydrogenation of Benzaldehyde
Reaction selectivity is challenging in benzaldehyde hydro-
genation [1–6, 9, 13] with possible by-products, as shown in
Fig. 1 I Representative STEM
images with II associated parti-
cle size distribution for (A) Au/
Al O and (B) Pt/Al O
2
3
2
3
1
3

Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl Alcohol Over Alumina Supported…
Fig. 3 Hammett plot for the selective –C=O group reduction of para-
Fig. 2 I Variation of benzaldehyde concentration in water
substituted benzaldehydes over Au/Al O at T=353 K and P=9 bar.
2
3
−3
(
C
, mol cm ) with time; dependence of benzaldehyde
Benzaldehyde
−1 −1
Note: Solid line represents ꢀt to Eq. (6)
consumption rate (R, mol mol
h ) with (IA) stirring speed and
Au
(
IB) mass of catalyst (W, g) for reaction over Au/Al O . Note: Solid
2 3
lines provide a guide to aid visual assessment. Reaction conditions:
T=353 K, P=9 bar
linear correlation is close to that (0.7) reported for liq-
uid phase hydrogenation of acetophenones over Pd/C [49]
and suggests a partial charge in the transition state [7]. A
positive reaction constant is consistent with nucleophilic
addition [50]. Hydrogen can undergo heterolytic disso-
Rate remained constant > 700 rpm, indicative of minimal
gas–liquid/liquid–solid mass transfer contributions to overall
rate [45]. Based on these results, the stirring speed was set
at 900 rpm for subsequent tests. Rate was invariant over the
range of catalyst masses considered in this work, conꢀrm-
ing chemical control where external or internal transport
constraints do not contribute to the catalytic response [45].
−
ciation on the surface of Au/Al O to generate H that
2
3
−
bonds to Au [16]. The hydride ion (H ) supplied by Au
acts as a nucleophile that attacks the positively charged
carbon in the –C=O group, activated on support Lewis
3
+
acid sites (Al ) at the interface with the metal [51]. An
electron-donating substituent (in the para-position) low-
ers the reactivity of the carbonyl-carbon which, in turns,
lowers the rate of reaction.
3.2.2 Reaction Mechanism and Solvent Eꢀects
The solvent serves to dissolve the reactant and/or prod-
ucts but can also interact with the catalyst and inꢃuence
performance [45]. Benzaldehyde hydrogenation rate was
The eꢁect of ring substituents was considered by testing
the hydrogenation of substituted benzaldehydes bearing
−
1
−1
electron donating and withdrawing substituents (–OCH ,
3
lower in ethanol (23 mol mol h ) relative to water (78 mol
Au
−
1
−1
–
CH , –CH CN, –CN) in the para position. In each case,
3 2
mol h ). The higher rate in aqueous solution can not be
Au
Au/Al O showed full selectivity to the corresponding alco-
2
3
due to H solubility as under reaction conditions (T=353 K,
2
−
3
hol with decreasing activity in the order: p-cyanobenzalde-
hyde>p-formylphenyl acetonitrile>benzaldehyde>p-tol-
ualdehyde > p-methoxybenzaldehyde. The rate data were
subjected to a Hammett correlation in order to probe reac-
tion mechanism [46]. Rate for the substituted benzaldehyde
P = 9 bar) solubility is higher in ethanol (3.7 μmol cm )
−
3
than water (0.8 μmol cm ) [25]. The dielectric constant
ε) provides a measure of solvent capacity to interact with
(
charged surface sites [52]; ε for water + ethanol mixtures
was calculated following the approach described previously
[25]. An increase in rate with solvent dielectric constant (ε)
is shown in Fig. 4. Competition of solvent and benzalde-
hyde for adsorption sites [53] can impact on hydrogenation
rate where increased surface aꢂnity for the solvent inhibits
benzaldehyde activation, resulting in lower reaction rate.
Water [54] and alcohols [55] can adsorb on Lewis acid sites
through the oxygen of the hydroxyl group and the greater
electron density of the hydroxyl oxygen in alcohols must
result in a stronger interaction that increases with increasing
alcohol chain length [56].
(
R ) is related to benzaldehyde as reference (R) according to
i
ꢀ
ꢁ
Ri
log
= ꢀ × ꢁp
(6)
R
The reaction constant (ρ) provides a measure of the
susceptibility to substituent electronic eꢁects while the
σ factor is an empirical parameter (values taken from
p
[
46]) that reꢃects substituent electron donating/acceptor
character [47, 48]. The ꢀt of the experimental rate data
is presented in Fig. 3. The low ρ (0.9) extracted from the
1
3

Y. Hao et al.
3
.2.3 Au/Al O versus Pt/Al O
2 3 2 3
The reaction rate (and TOF) recorded for Pt/Al O was
2
3
signiꢀcantly higher (Table 1), which can be attributed to
the greater H chemisorption capacity. Reaction over Au/
2
Al O exhibited full selectivity to the target alcohol at all
2
3
levels of benzaldehyde conversion (Fig. 5I) to deliver 100%
benzyl alcohol yield. This response was probed further by
considering benzyl alcohol hydrogenation over Au/Al O ;
2
3
no conversion was detected. The benchmark Pt/Al O pro-
2
3
moted hydrogenolysis to toluene and benzene with a lower
yield (95%) to benzyl alcohol at full benzaldehyde conver-
sion. The reaction was prolonged for a further 15 h after
complete benzaldehyde conversion (Fig. 5II). Conversion
of benzyl alcohol (to toluene and benzene) over Pt/Al O
Fig. 4 Benzaldehyde hydrogenation rate (R, mol mol−
1
−1
2
3
h
) over Au/
Au
resulted in a temporal decline in selectivity to the alcohol.
This suggests sequential hydrogenation (step I, Scheme 1)
and hydrogenolysis (steps II and III) steps. In contrast, full
selectivity to benzyl alcohol was maintained with Au/Al O
Al O as a function of solvent dielectric constant (ε) for reaction in
2
3
(
(
1) pentanol; (2) butanol; (3) ethanol; ethanol:H O=(4) 5:1; (5) 3:1;
2
6) 1:1; (7) 0.8:1; (8) 0.6:1; (9) H O. Note: Solid line provides a guide
2
to aid visual assessment. Reaction conditions: T=353 K, P=9 bar
2
3
over extended reaction times with no measurable hydrogen-
olysis. The results from this work illustrate the potential of
supported Au for the selective transformation of substituted
benzaldehydes with 100% yield of the target alcohol over
prolonged reaction times.
The solvent can also influence reactant activation
through hydrogen bonding with the –C=O function [28,
2
9]. This results in charge transfer from the lone pair of
the oxygen atom in the carbonyl group to the O–H bond
in protic solvents [57], the carbonyl-carbon becomes more
electrophilic and the rate of reaction increases. Akpa et al.
4 Conclusions
[
58] employed DFT calculations to investigate 2-butanone
hydrogenation in water over Ru/SiO and concluded that
The liquid phase (T=353 K, P=9.0 bar) hydrogenation of
benzaldehyde over (1% w/w) Au (mean size=3.6 nm) on
Al O was fully selective with 100% yield of the target ben-
2
strong interaction between water and 2-butanone lowered
the activation energy barrier and enhanced reaction rate.
Catalyst (Ru/C) testing has also shown higher 2-butanone
hydrogenation rate in solvents with higher ε [28]. In con-
trast, Bertero et al. [27] and Aramendía et al. [30] using
Ni/SiO and Pd/AlPO , respectively, reported a decrease
2
3
zyl alcohol. Exclusive –C=O reduction extended to a range of
p-substituted (–OCH , –CH , –CH CN, –CN) benzaldehydes.
3
3
2
The reaction proceeds via a nucleophilic mechanism where
electron withdrawing ring substituents elevated rate as dem-
onstrated by the linear Hammett relationship. Selective hydro-
genation rate increased with increasing solvent dielectric con-
stant (ε), which is accounted for in terms of: (i) competition
2
4
in acetophenone hydrogenation rate with increasing ε due
to solvation that inhibited the adsorption step.
Fig. 5 Selectivity (S , %) as
i
a function of (I) benzalde-
hyde fractional conversion
(
X
) and (II) time, after
Benzaldehyde
1
00% conversion of benzal-
dehyde (in water) had been
reached over Au/Al O (solid
2
3
symbols) and Pt/Al O (open
2
3
symbols); benzyl alcohol (open
square, ꢀlled square), benzene
(
(
open triangle) and toluene
open circle). Reaction condi-
tions: T=353 K, P=9 bar
1
3

Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl Alcohol Over Alumina Supported…
for surface adsorption sites that is more pronounced for
alcohols with lower ε; (ii) reactant solvation by solvents with
higher ε that activates –C=O for nucleophilic attack. Reaction
over a Pt/Al O benchmark (mean Pt size=1.7 nm) was non-
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Acknowledgements The authors are grateful to Dr. Maoshuai Li for
his contribution to the project. This work received support from the
Overseas Research Students Award Scheme (ORSAS) to Dr. Y. Hao
and the Engineering and Physical Sciences Research Council EPRSC
2
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Grant Number EP/L016419/1 to Chiara Pischetola through CRITICAT
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Compliance with Ethical Standards
33. Mustaꢀn K, Cárdenas-Lizana F, Keane MA (2017) J Chem Tech-
nol Biotechnol 92:2221
Conflict of interest The authors declare no conꢃicts of interest.
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tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
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