Platinum-catalysed cinnamaldehyde hydrogenation in continuous flow

Article abstract of DOI:10.1039/c5ra14984c

Platinum is one of the most widely used hydrogenation catalysts. Here we describe the translation of batch reactions to continuous flow, affording tunable CO versus CC hydrogenation over a Pt/SiO₂ catalyst, resulting in high steady state activi

Full text of DOI:10.1039/c5ra14984c

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Platinum-catalysed cinnamaldehyde
hydrogenation in continuous flow†
Cite this: RSC Adv., 2015, 5, 80022
*
Lee J. Durndell, Karen Wilson and Adam F. Lee
Platinum is one of the most widely used hydrogenation catalysts. Here we describe the translation of batch
reactions to continuous flow, affording tunable C]O versus C]C hydrogenation over a Pt/SiO2 catalyst,
resulting in high steady state activity and single-pass yields in the selective hydrogenation of
cinnamaldehyde to cinnamyl alcohol under mild conditions. Negligible catalyst deactivation occurs
under extended flow operation due to removal of reactively-formed poisons from the reaction zone.
Process intensification imparts a four-fold enhancement in cinnamyl alcohol productivity.
Received 28th July 2015
Accepted 8th September 2015
DOI: 10.1039/c5ra14984c
www.rsc.org/advances
Chemoselectivity underpins 21st century catalysis,1,2 permitting selectivity, with low coordination sites favoring C]C hydroge-
the targeted modication of specic functional groups within nation,21,22 whereas Zhu and Zaera reported that CinnOH
complex starting materials,3–7 notably from biomass-derived selectivity was insensitive to the size of silica supported Pt
feedstocks.8 Catalytic hydrogenation of organic compounds nanoparticles albeit over a narrow size range.23 Guo et al. have
possessing multiple unsaturated bonds such as a,b-unsaturated shown that connement of Pt nanoclusters within the cavity of
aldehydes is particularly challenging,9–11 necessitating active metal–organic frameworks also promotes CinnOH selectivity;
sites able to discriminate and preferentially activate closely with steric constraints on CinnALD believed to hinder C]C
related moieties. Platinum is widely employed in heteroge- planar adsorption with consequent preferential C]O activa-
neously catalysed hydrogenation of diverse functional groups tion.24 Kinetics of CinnALD hydrogenation are also a function of
including C]C,12 C^C,13 C]O,13 C^N,14 NO2 (ref. 15) and support properties25 and hydrogen pressure. We recently
aromatics.15 The selective hydrogenation of allylic and benzylic reported a detailed mechanistic study of the structural and
aldehydes to unsaturated alcohols is a commercially important electronic factors controlling the liquid phase hydrogenation of
industrial process within the avour, fragrance, agrochemical cinnamaldehyde and related benzylic aldehydes over fumed
and pharmaceutical sectors,9,16 in which active and selective SiO2 and mesoporous SBA-15 supported Pt nanoparticles.26
heterogeneous catalysts for such transformations are essential Kinetic mapping revealed cinnamaldehyde hydrogenation is
to circumvent the greater thermodynamic stability of C]O structure-insensitive over metallic platinum, proceeding with a
relative to C]C bonds.9
common Turnover Frequency (TOF) independent of precursor,
The liquid phase, selective hydrogenation of cinnamalde- particle size or support architecture, while selectivity to CinnOH
hyde (CinnALD) to cinnamyl alcohol (CinnOH) illustrated in is highly structure sensitive. Large nanoparticles and high
Scheme 1 is of signicant interest due to the widespread use of hydrogen pressures favored C]O over C]C hydrogenation due
this allylic alcohol in perfumes and avourants.16–19 Platinum is
a promising catalyst for this challenging reaction, in which
hydrogenation of the C]C bond is both kinetically and ther-
modynamically more favourable than the C]O function,20 and
hence the inuence of the physicochemical properties of plat-
inum nanoparticles is a topic of much intensive recent inves-
tigation in batch reactors. Particle size effects upon CinnOH
selectivity have proved controversial, with oleic acid/oleylamine
stabilised mono- and bimetallic colloidal Pt nanoparticles
reported to exhibit a strong size dependence of CinnOH
European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK.
E-mail: a.f.lee@aston.ac.uk; Tel: +44 (0)121 2044036
† Electronic supplementary information (ESI) available: Details of catalyst
synthesis and characterisation, and reaction conditions for batch and
Scheme 1 Principal reaction pathways operating in the Pt-catalysed
continuous ow cinnamaldehyde hydrogenation. See DOI: 10.1039/c5ra14984c
liquid phase hydrogenation of cinnamaldehyde.
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to molecular surface crowding, while in situ ATR-IR highlighted eqn (1), showed a step change, and hence near plug-ow
the role of support polarity in enhancing C]O hydrogenation. behaviour with relatively little broadening from mixing or
Catalytic hydrogenations are traditionally conducted in axial dispersion (Fig. 1). This demonstrates that the catalyst bed
stirred batch reactors, presenting challenges in respect of the was static and lled with well-distributed liquid. At 1 bar,
low hydrogen solubility in most solvents, which results in mass CinnALD hydrogenation was rst order in catalyst charge under
transfer limitations.27,28 These are typically offset through the the conditions employed in this study (Table S2†), conrming
use of high pressures of up to 100 bar, which increases the the absence of external mass-transport limitations.
associated explosion risks. Small-scale microreactors offer a
F(t) ¼ {([CinnALD]t + [Products]t)/[CinnALD]inlet}/s
(1)
safer, alternative approach to high-pressure hydrogenations
with molecular hydrogen, and in conjunction with continuous
ow processing offers signicant benets compared to batch
processes, related to the unique gas–liquid–solid triphasic
reaction conditions present in such transformations, improved
safety, and process intensication.29,30 In recent years the
chemical and pharmaceutical sectors have thus targeted a
switchover from batch to continuous operation to achieve atom
economical, and scalable organic synthesis.31,32 To date, the
continuous, liquid phase selective hydrogenation of cinna-
maldehyde has only been the subject of two peer-reviewed
studies, wherein the focus was evaluating the suitability of
either pellet string33 or high-pressure membrane34,35 reactor
designs for industrial chemical synthesis. In both cases, the
respective Pd/Al2O3 and Pd/MCM-41 catalysts employed were
only selective to hydrocinnamaldehyde, and only studied at
high pressures >10 bar. In contrast, Pt/SiO2 and Pt/SBA-15
catalysts are moderately selective to desirable CinnOH at
lower pressures,26 but are prone to on-stream deactivation in
batch, and exhibit strong time selectivity variations. Here we
demonstrate that continuous ow operation in a packed bed
FlowSyn microreactor with a GAM II coil-in-coil gas pre-satu-
rator36 (Scheme 2); greatly increases the activity and selectivity
of Pt/SiO2 towards the liquid phase hydrogenation of CinnALD
to CinnOH, accompanied by dramatically improved catalyst
stability and ease of separation and exible productivity.
The inuence of residence time on CinnALD conversion and
product selectivity was rst investigated (Fig. 2). At the shortest
residence times (s < 5 min) only trace CinnALD conversion was
observed, consistent with the relatively slow batch kinetics
(initial rates of 500 mmol gꢀ1 Pt hꢀ1). As the residence time
increases, a corresponding monotonic increase in activity was
observed, with single pass conversions reaching 45%, however
CinnALD hydrogenation attained a plateau at longer residence
times above 15 min, likely due to limited hydrogen availability
under these conditions; the reaction order in pH2 was 0.5
(Fig. S3†), in excellent agreement with that reported in batch over
supported Pt nanoparticles. Product selectivity exhibited a more
complex relationship with residence time. CinnOH displayed a
volcano dependence, peaking around 60% for s ¼ 15 min,
coincident with the maximum rate of CinnALD hydrogenation,
and inversely correlated with the formation of undesired 3-phe-
nylpropionaldehyde and ethylbenzene products via competing
Structural properties of the wet-impregnated 2 wt% Pt/SiO2
catalyst are presented in Table S1 and Fig. S1 and S2.† The
liquid residence time s (dened as bed volume/ow rate) was
studied with gas ow through determining the system response
to the introduction of cinnamaldehyde. The resulting residence
time distribution for the normalised response F(t)37 given by
Fig. 1 Residence time distribution curve showing the step change
response associated with the flow of cinnamaldehyde starting at time
t ¼ 0 through the FlowSyn reactor loaded with 2 wt% Pt/SiO2 catalyst.
The reaction was monitored by GC, with F(t) indicating the fraction of
total [cinnamaldehyde + all products] detected from the reactor outlet
as a function of time, and s corresponding to the mean residence time
(13.86 min). Reaction conditions: 200 mg of Pt/SiO2, 0.08 to 2.4
Scheme
2
Schematic of FlowSyn continuous flow reactor for cm3 minꢀ1 liquid flow (84 mM in anisole), 40 cm3 hydrogen gas flow at
heterogeneously catalysed cinnamaldehyde hydrogenation.
5 bar and 90 ꢁC.
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over the course of 7 h, occurring at an approximately exponential
rate which can be extrapolated back to a similar maximum TOF ¼
1300 hꢀ1 at zero time as observed in ow; exponential activity
decay due to catalyst poisoning by strongly bound products has
been previously observed in the (semi-)batch hydrogenation of
a-methylethylbenzene39 and sitosterol40 over commercial Pd/C
catalysts. In contrast, negligible deactivation of the Pt/SiO2 cata-
lyst was observed in ow, consistent with the rapid removal of
reactively-formed products and low attendant concentration of
potential poisons. The different time-on-stream activity proles
in ow versus batch are mirrored by their corresponding product
distributions (Fig. 3b and c). In ow, the desired CinnOH is the
major product throughout the course of reaction, with 3-phe-
nylpropionaldehyde and ethylbenzene formed in equal concen-
trations along with trace and 3-phenylpropan-1-ol, evidencing
C]O hydrogenation as the dominant primary reaction of Cin-
nALD. Strong time dependent selectivity variations are observed
under batch conditions, with the fresh catalyst strongly favouring
C]C hydrogenation of CinnALD to 3-phenylpropionaldehyde
and 3-phenylpropan-1-ol; we have previously shown the latter
secondary product only arises via hydrogenation of reactively-
formed CinnOH. On-stream catalyst deactivation coincides with
a dramatic loss in 3-phenylpropan-1-ol and concomitant rise in
selectivity to CinnOH, conrming their interrelationship, and
small decrease in 3-phenylpropionaldehyde formation.
The coincidence of deactivation and suppression of C]C
hydrogenation pathways in batch, strongly implicates a unique
Pt active site responsible for C]C hydrogenation, discrete
from that catalysing C]O hydrogenation. This hypothesis
meshes perfectly with our previous studies on size-controlled
Pt nanoparticles over silica supports, wherein low coordina-
tion sites prevalent on smaller particles were observed to
promote C]C hydrogenation of CinnALD, while larger parti-
cles and associated Pt terrace promoted C]O hydrogenation.
It is well-established in heterogeneous catalysts that edge and
vertex sites of transition metal particles are highly reactive due
to their undercoordination, and (consequentially) prone to self-
poisoning due to the higher adsorption energies of atomic and
molecular reactants/products at these sites relative to close-
Fig. 2 The influence of residence time on (a) CinnALD hydrogenation,
and (b) selectivity over 2 wt% Pt/SiO2. Reaction conditions: 200 mg of
Pt/SiO2, liquid flow 0.08 to 2.4 cm3 minꢀ1 (84 mM in anisole), 40 cm3
hydrogen gas flow at 5 bar and 90 ꢁC catalyst.
C]C hydrogenation and decarbonylation of the resulting satu-
rated aldehyde. Small quantities (3–8%) of 3-phenyl propanol
and cinnamyl cinnamate were also formed, mirroring the vari-
ation in 3-phenylpropionaldehyde (Fig. S4†). These trends are
similar to those seen in batch, wherein CinnALD hydrogenation
to 3-phenylpropionaldehyde and 3-phenylpropan-1-ol is favoured
at conversions <5 % (the latter a secondary product from rapid
hydrogenation of reactively-formed CinnOH over SiO2 supported
platinum), with selectivity towards CinnOH increasing to 45% at
40% conversion. The fall in CinnOH selectivity and concomitant
rise in 3-phenylpropionaldehyde, and ethylbenzene and
3-phenylpropan-1-ol (latter shown in Fig. S4†) secondary prod-
ucts at longer residence times (high conversion) is attributed to a
combination of hydrogen starvation and thermodynamically
favoured C]C hydrogenation. It is immediately apparent that
tuning the residence time affords a simple means to regulate
C]O versus C]C hydrogenation (Fig. S5†) and hence optimise
the CinnOH productivity at constant pH2 and temperature, and
that good conversion and selectivity are achievable even for
comparatively short reaction times. The sigmoidal increase in
cinnamaldehyde conversion (and hence total product yield) with
¨
residence time contrasts with the report by Noel et al. for the
metal-free peruoroalkylation of N-methylpyrrole with CF3I,
wherein a surprising decrease in peruoroalkylation rate was
observed with increasing residence time, attributed to poorer
mixing of the gas and liquid phases,38 highlighting the superior
mixing characteristics of the GAM II gas pre-saturator in the
present work which ensured efficient hydrogen dissolution even
at low ow rates.
In order to quantitatively benchmark the performance of our
2 wt% Pt/SiO2 catalyst in ow versus batch CinnALD hydrogena-
tion, we subsequently conducted extended testing over 7 h to
evaluate catalyst stability and net CinnOH productivity. Fig. 3a
reveals striking differences in catalyst activity, with TOFs at least
50% higher in ow than batch, even during the early stages of
reaction. This suggests either that catalyst deactivation is
extremely rapid in batch, or that CinnALD hydrogenation in our
batch reactor is rate-limited by mass-transport (presumably of
hydrogen). The former appears more plausible, since mixing in
stirred batch reactors is known to be highly efficient, and
furthermore signicant deactivation was subsequently observed
¨
packed facets. Extended Huckel calculations by Delbecq and
Sautet have shown that stepped surfaces favour pC]C CinnALD
adsorption modes and C]C hydrogenation relative to Pt(111)
terraces which favour di-sCO CinnALD adsorption and C]O
hydrogenation.41 We therefore propose that during batch Cin-
nALD hydrogenation, one or more reactively-formed products
associated with C]C hydrogenation at reactive, low coordi-
nation sites is strongly adsorbed over the sites responsible for
its formation, resulting in self-poisoning of C]C hydrogena-
tion pathways. Comparison of the ow versus batch product
distributions suggests ethylbenzene as the frontrunner candi-
date poison, it being formed at low concentrations in ow, but
rapidly swept away from the catalyst bed minimising the time
for its re-adsorption/equilibration. The absence of ethyl-
benzene within the bulk reaction media during batch, over
the same catalyst to that which forms trace ethylbenzene under
identical reaction conditions of temperature, pH2 and [Cin-
nALD] in ow, supports the proposal that catalyst poisoning in
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Fig. 3 (a) Activity towards CinnALD hydrogenation, and product distributions in (b) flow and (c) batch as a function of time-on-stream over 2 wt%
Pt/SiO2. Reaction conditions: 200 mg of Pt/SiO2, 5 bar H2 and 90 ꢁC (batch and flow); liquid flow 0.11 cm3 minꢀ1 (84 mM in anisole), 40 cm3
hydrogen (flow).
batch may reect adsorption of reactively-formed ethylbenzene
over long contact times. This hypothesis was tested by spiking
Conclusions
the reaction solution with a constant ow of either 5% or 10%
ethylbenzene (Fig. S6†), which resulted in an immediate 14%
or 32% decrease in catalyst activity. Cinnamaldehyde conver-
sion rapidly returned to its initial value upon removal of eth-
ylbenzene poison from the reactant stream, evidencing fully
reversible deactivation due to site-blocking by strongly-bound
ethylbenzene. The absolute CinnOH productivity of our 2
wt% Pt/SiO2 catalyst in ow versus batch is compared in Fig. 4,
which highlights an approximately constant four-fold yield
enhancement for CinnALD hydrogenation under ow.
The transition from liquid phase batch to continuous ow
cinnamaldehyde hydrogenation over a Pt/SiO2 heterogeneous
catalysts affords signicant enhancements in activity and
selectivity towards C]O activation under optimised (mild)
reaction conditions of 5 bar and 90 ꢁC. Superior catalytic
performance in ow is attributed to removal of the reactively-
formed cinnamyl alcohol and 3-phenylpropionaldehyde
primary products from the reaction zone, limiting their over-
hydrogenation and/or hydrogenolysis to 3-phenylpropan-1-ol
and ethylbenzene which are implicated as strongly adsorbing
catalyst poisons under batch operation. The combination of
excellent catalyst activity, stability and good selectivity in
continuous ow deliver (predictable) cumulative cinnamyl
alcohol productivities four times greater than possible under
comparable conditions in batch.
Acknowledgements
We thank the EPSRC for nancial support (EP/G007594/4 and
EP/K014749/1) and the award of a Leadership Fellowship to
AFL. KW thanks the Royal Society for an Industry Fellowship.
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