Selective, aerobic oxidation reaction of alcohols by hybrid Pd/ZrO2/PVA catalytic membranes

Article abstract of DOI:10.1016/j.apcata.2016.11.033

Palladium nanoparticles of 2.1 ± 0.9 nm size were generated within a hybrid zirconia/polyvinyl alcohol membrane of 60 ± 5 μm thickness, to afford a bulk 6.7% Pd loading (w/w). The material was used as catalyst for the heterogeneous, batch and continuous flow partial oxidation reaction of functionalized alcohols, using air or oxygen as oxidants and water or methanol as solvents. Good selectivity to the corresponding aldehydes at full conversion and excellent resistance over prolonged time-on-stream under very mild reaction conditions was observed. No metal leaching in solution was detected, as well as no additives nor regeneration steps were needed.

Full text of DOI:10.1016/j.apcata.2016.11.033

Accepted Manuscript
Title: Selective, aerobic oxidation reaction of alcohols by
hybrid Pd/ZrO2/PVA catalytic membranes
Author: Francesca Liguori Carmen Moreno-Marrodan
Pierluigi Barbaro Haruo Sawa
PII:
S0926-860X(16)30582-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.11.033
APCATA 16081
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-9-2016
17-11-2016
27-11-2016
Please cite this article as: Francesca Liguori, Carmen Moreno-Marrodan,
Pierluigi Barbaro, Haruo Sawa, Selective, aerobic oxidation reaction of alcohols
by hybrid Pd/ZrO2/PVA catalytic membranes, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.11.033
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Selective, aerobic oxidation reaction of alcohols by hybrid Pd / ZrO2 /
PVA catalytic membranes
Francesca Liguori a, Carmen Moreno-Marrodan a, Pierluigi Barbaro a,*, Haruo Sawa b
a
Istituto di Chimica dei Composti Organo Metallici - Consiglio Nazionale delle Ricerche (ICCOM-CNR), Via Madonna del Piano
10, 50019 Sesto Fiorentino; Firenze, Italy.
b
Nippon Kodoshi Corporation, 648 Hirookakami, Haruno, Kochi, 781-0395, Japan.
*
Corresponding author: ICCOM-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino; Firenze, Italy. Fax: +39 055 5225203;
Tel: +39 055 5225287.
E-mail address: pierluigi.barbaro@iccom.cnr.it
1

*Graphical Abstract

Highlights




Pd nanoparticles embedded into hybrid zirconia / polyvinyl alcohol membrane
Partial oxidation reactions of alcohols in the liquid phase under continuous flow
Reaction temperature below 100 °C
No metal leaching, stable, use of hydrophilic solvents, no need of regeneration nor additives
ABSTRACT
Palladium nanoparticles of 2.1 ± 0.9 nm size were generated within a hybrid zirconia / polyvinyl alcohol membrane of
60 ± 5 m thickness, to afford a bulk 6.7 % Pd loading (w/w). The material was used as catalyst for the
heterogeneous, batch and continuous flow partial oxidation reaction of functionalized alcohols, using air or oxygen as
oxidants and water or methanol as solvents. Good selectivity to the corresponding aldehydes at full conversion and
excellent resistance over prolonged time-on-stream under very mild reaction conditions was observed. No metal
leaching in solution was detected, as well as no additives nor regeneration steps were needed.
Keywords: Heterogeneous catalysis, palladium, nanoparticles, alcohols oxidation, membranes, flow processes
1. Introduction
The selective oxidation of alcohols is one of the most popular reactions in chemistry because of the
application to the manufacture of several intermediates for the food and pharmaceutical industry [1,2].
Conventional processes usually involve stoichiometric amounts of expensive and toxic oxidants, either
organic or inorganic [3,4]. Therefore, economic and environmental drivers have urged the development of
catalytic methods employing cleaner and cheaper oxidants [5], with the preferred ones based on oxygen,
since water is virtually the only by-product [6,7]. For the synthesis of fine chemicals, such methods shall also
be tolerant to the presence of other oxidation-sensitive functional groups, e.g. C=C double bonds, and/or
allow for the selective production of partial oxidation products, e.g. aldehydes and ketones [8]. A number of
homogeneous catalysts have been described to this purpose employing transition metal species [9,10],
although less explored heterogeneous systems offer unquestionable advantages in terms of handling, catalyst
separation, reuse and integration in existing reactor equipment [11,12]. The most effective solid catalysts so
far reported are based on supported metal nanoparticles (MNP), particularly Au [13,14] and platinum group
2

metals [15,16]. However, issues still need to be solved in relation to chemical and thermal resistance, catalyst
deactivation, efficiency under mild conditions, selectivity and/or reproducibility. Aimed at obtaining
catalysts with improved performance, the design of appropriate support materials is therefore of paramount
importance. Both specific support properties (e.g. mass transfer limits [17]), as well as stabilization of the
catalytically active species (e.g. by means of Strong Metal-Support Interactions (SMSI), steric and
electrostatic interactions [18,19]), have to be considered.
For liquid phase heterogeneous aerobic oxidations, choice of gas oxidant is also key for large scale
applications. Advantages to the use of air include reduced operating cost, because neither gas processing nor
the purchase of oxygen is need, superior plant safety and tolerance of metal reactor equipment, because a
mild oxidant is used. Advantages to the use of pure O2 include reduced capital and energy costs and
increased productivity, since the lack of inert gas allows for smaller plants and lower total pressures, while
high productivities at lower reaction temperatures are obtained because of higher reaction rates [20]. On the
other hand, safety is one of the major issues concerning O2-based oxidation process, mostly in relation to
flammability limits and explosion hazards, which requires specially designed apparatuses [21].
An attractive option is provided by catalysis under continuous flow. Packing the catalyst into flow
reactors offers significant advantages compared to conventional batch operations in terms of safety,
purification, catalyst separation, waste emission, automation, energy consumption and space-time-yield
productivity [22,23]. Few flow systems have been reported so far for the heterogeneous aerobic oxidation
reaction of alcohols using MNP catalysts onto either polymeric [24] or inorganic [25,26] supports. Despite
inorganic materials have been mostly employed because of lower volume and porosity changes with swelling
[27,28], use of organic supports may provide significant benefits as a consequence of: i) potential to drive
sorption and permeability of reagents and products, ii) easier incorporation of additives in the catalyst, iii) a
more mature manufacture technology of polymers, that allows for a wider choice of morphologies,
mechanical, chemical and thermal properties, affinity for reagents and catalysts [29,30]. Challenges in the
development of polymeric supports include chemical and mechanical stability, shrinking phenomena, back
pressure evolution, adverse interaction with the catalyst, which negatively affect the efficiency and long-term
productivity of the supported catalysts [31,32]. In order to minimize these drawbacks, various strategies have
been developed, including the addition of inorganic materials to the polymer, e.g. raschig rings [33], glass
[34], and others [35,36], with improved performance in terms of catalyst resistance.
3

In the course of our studies on solid electrolytes, we recently described the synthesis of hybrid inorganic /
polymeric (ZrO2/PVA) membranes for electrochemical [37,38] and fuel cells applications [39,40]. Easy
incorporation of palladium nanoparticles onto these materials [41,42] led to efficient and durable heterogeneous
catalysts for the hydrogenation reactions of various organic substrates in the liquid phase [43]. Both membranes
and spherical beads materials showed to be effective under batch and continuous flow conditions. It was
demonstrated that the addition of zirconia greatly improves the mechanical and chemical resistance of PVA
membranes in hydrophilic solvents, hence, their potential for use under catalytic conditions over prolonged
reaction times. Particularly, the continuous flow hydrogenation reaction of C=C and C≡C bonds and
nitrobenzene was achieved showing remarkable selectivity at full conversion for long time-on-stream [44].
In the present work we expanded the scope of PdNP@ ZrO2 / PVA catalytic membranes, by
demonstrating their usability in the selective, catalytic aerobic oxidation reaction of various alcohols of
industrial interest, both under batch and continuous flow conditions.
2. Experimental
2.1. General information
All reactions and manipulations were routinely performed under nitrogen atmosphere by using standard
Schlenk techniques, unless otherwise stated. The hybrid zirconia / PVA (PVA = polyvinyl alcohol) materials
containing palladium oxide were obtained as previously described [41,42]. Two differently shaped hybrid
materials were used: membranes sheets of 2 x 2 mm surface (60 ± 5 m thickness, composition in weight ratio
normalized to 80 % saponified PVA: ZrO2 0.08, PdO 0.17, polyoxythylene polyglycerol ether 0.35), hereinafter
labelled NKZPDC-1, and 1.7 ± 0.2 mm diameter pellets of similar composition, hereinafter labelled NKZPDB-
5. The Pd0 nanoparticles-containing beads, hereinafter labelled Pd@NKZPDB-5, were prepared as previously
reported [44]. All the other chemicals were reagent grade, commercial products and were used as received
without further purification. Environmental Scanning Electron Microscopy measurements (ESEM) were
performed on a FEI Quanta 200 microscope operating at 25 KeV accelerating voltage in the low-vacuum mode
(1 torr) and equipped with an EDAX Energy Dispersive X-ray Spectrometer (EDS). Samples for Transmission
Electron Microscopy (TEM) analyses were prepared by inclusion of the membranes into Struers EpoFix®
4

epoxy resin, followed by lapping. The samples were then cut with a RMC MT-XL ultramicrotome to give a
film thickness of 60 nm. TEM measurements were carried out using a CM12 PHILIPS instrument at 120 KeV
accelerating voltage. Statistical nanoparticle size distribution analysis was typically carried out on 300-400
particles. X-ray Diffraction (XRD) spectra were recorded with a PANanalytical XPERT PRO powder
diffractometer, employing CuK radiation (=1.54187 Å), a parabolic MPD-mirror and a solid state detector
(PIXcel). The samples were subjected to measurement without grinding and prepared on a silicon wafer (zero
background) that was rotating (0.5 rotations per second) during spectra acquisition. All XRD spectra were
acquired at room temperature in a 2 range from 4 to 95º, applying a step size of 0.0263º and a counting time of
77.5 seconds. Reactions under controlled pressure of gas were performed using a non-metallic Büchi
Miniclave® (50 mL internal volume) equipped with a pressure controller and an orbital stirrer set at 150 rpm
rate, or a stainless steel autoclave (25 mL internal volume) constructed at Istituto di Chimica dei Composti
OrganoMetallici-CNR (Firenze, Italy) and equipped with a magnetic stirrer, a Teflon ® inset and a pressure
controller. Catalytic flow oxidations were carried out using a home-made continuous-flow reactor system
constructed at ICCOM-CNR. The system was designed to allow for a concurrent flow of substrate solution and
oxygen gas through a reactor tube containing the heterogeneous catalyst. The reactor was completely inert, as
all wet parts were made of PEEK, PFA or PFTE. The flow of the substrate solution was regulated by an
Alltech® model 426 HPLC pump in PEEK. The concurrent flows of gas and liquid were driven through a T-
shaped PEEK mixer to ensure efficient gas dispersion. A mixed air-substrate solution stream was allowed to
flow in the reactor through a 6-port Rheodyne mod. 9060 switching valve in PEEK. The core of the reactor
consisted in a commercial tubular glass column (Omnifit®, 3 mm i.d. × 25 mm length) equipped with 0.2 μm
PE frits and packed with the solid catalyst. The column ends were equipped with PEEK fittings and 5 m frits
to ensure an optimum flow distribution. The column was heated with a temperature accuracy of ± 0.1 °C by a
home-made oven comprising a Kapton® flexible heating element, a brass and a Teflon holder and a
temperature controller. At the outlet of the reactor, the product solution was collected for GC analysis and the
excess amount of air released to the atmospheric pressure. GC analyses were performed on a Shimadzu GC-
2010 gas chromatograph equipped with a flame ionization detector and 30 m (0.25 mm i.d., 0.25 μm film
thickness) VF-WAXms column or a 30 m (0.25 mm i.d, 0.25 µm f.t.) SPB-1 column. GC-MS analyses were
performed on a Shimadzu QP2010S spectrometer equipped with identical capillary columns. The metal content
5

in the supported catalysts was determined by EDS analysis and by Inductively Coupled Plasma Atomic
Emission Spectroscopy (ICP-OES) using a Varian 720ES instrument at a sensitivity of 500 ppb. Each sample
was treated in a microwave-heated digestion bomb (Milestone, ETHOS LabStation, 20 min.@ 220 °C) with 8
mL aqua regia and 0.5 mL of H2O2 30%. The content of metal leached in the catalytic reaction solutions was
determined by ICP-OES. The Palladium detection limit was 0.006 ppm.
2.2. Synthesis of the catalyst
In a typical procedure, the preformed hybrid inorganic/polymeric 2 x 2 mm NKZPDC-1 membrane (0.40
g, 0.26 mmol Pd) [41,42] was introduced into a round bottom flask equipped with a lateral stopcock
containing nitrogen-degassed water (120 mL). The suspension was cooled to 0 °C and an excess of NaBH4
(400 mg, 10.5 mmol) was added in portions under a stream of nitrogen. The mixture was orbitally stirred
under nitrogen at 0 °C for 30 min, then at room temperature for 24 h. After that time, the water solution was
removed by decantation, and the sold material was carefully washed with degassed water (3 x 50 mL),
methanol (3 x 00 mL) and diethyl ether (3 x 50 mL), then dried under a stream of nitrogen overnight. The
black membrane sheets thus obtained (Pd@NKZPDC-1) were stored under nitrogen before being used in
catalysis. ICP-OES analysis gave a 6.7 % wt Pd bulk loading in the final material.
2.3. Catalytic oxidation reactions by Pd@NKZPDC-1 membranes under batch conditions
In a typical experiment, the Pd@NKZPDC-1 membrane (30 mg, 6.7% wt, Pd, 0.019 mmol Pd), was
placed into 50 mL, two-neck, round-bottomed glass flask equipped with a reflux condenser. A solution of
substrate (1.9 10-2 M, 32.0 mL) was transferred via a Teflon tube into the reactor. The open-air system was
heated at the desired temperature using an oil bath. After stirring at 150 rpm using an orbital stirrer for the
selected time, the solution was completely removed using a gas-tight syringe. In the experiments carried out
under a controlled pressure of gas, the catalyst (30 mg) was placed under nitrogen into a metal-free
autoclave. A solution of substrate (0.5 M, 9.4 mL) was transferred via a Teflon tube into the reactor. The
autoclave was finally charged with the desired pressure of air and stirred at 150 rpm using a orbital stirrer.
The reactor was heated at a given temperature using an oil bath. After the desired time, the reactor was
6

depressurized and the solution was completely removed using a gas-tight syringe. A sample of the reaction
solution was analyzed by GC and GC-MS for product identification and quantification and by ICP-OES for
metal leaching. The reaction products were unequivocally identified by the GC retention times and mass
spectra of those of authentic specimens. Quantitative analysis of the reaction products was carried out via GC
based on calibration curves of the pure compounds. For recycling experiments, a fresh solution of the
substrate was then transferred via a gas-tight syringe into the reactor containing the catalyst recovered. After
the desired time, the mixture was treated as described above. The same recycling procedure was used in the
subsequent oxidation cycles. After use in catalysis, the solid catalyst was washed with methanol (3 x 10 ml)
and diethyl ether (3 x 10 ml), dried in a stream of nitrogen. An analogous procedure was adopted for
oxidation reactions using Pd@NKZPDB-5 catalyst.
2.4. Catalytic oxidation reactions by Pd@NKZPDC-1 membranes under continuous flow conditions
Reactions under continuous flow were carried out using the flow reactor system described above. In a
typical experiment, Pd@NKZPDC-1 (45 mg, 6.7 % wt Pd, 0.028 mmol Pd) was packed into a commercial
glass column (3 mm diameter x 25 mm length, internal volume 176 L). The system was degassed with a N2
flow of 3 mL min-1 for 15 minutes. After that time, substrate solution in methanol (0.1 M) was allowed to
flow through the catalytic bed at a constant 0.1 mL min-1 rate until the catalyst was completely wet. A
concurrent stream of oxygen was then allowed to flow though the packed catalyst at 5.0 mL min-1 rate.
Attainment of steady state conditions, ca. 1 h, was then taken as the reaction start time. The product solution
was periodically analysed for conversion by GC, while aliquots were continuously sampled for Pd leaching
analysis by ICP-OES. The reaction was typically monitored for 8 h time-on-stream, followed by overnight
switch off, and restart the day after for an analogous period. The reaction products were unequivocally
identified by the GC retention times and mass spectra of those of authentic specimens.
3. Results and discussion
3.1. Catalyst preparation and characterization
7

We recently reported the large-scale manufacture of hybrid PdO/ ZrO2 / PVA materials at competitive
costs by a straightforward procedure [41,42]. In agreement with previous investigations using palladium
oxide [45,46], the native materials showed to be catalytically inactive prior of palladium reduction both in
hydrogenation and aerobic oxidation reactions (see below). However, catalytically active Pd0 nanoparticles
(PdNP) could be generated within the solid matrix by conventional borohydride reduction in water [44]. This
procedure was adopted in the present work to generate PdNP onto both spherical beads (NKZPDB-5, 1.7
mm diameter) and membrane (NKZPDC-1, 2 x 2 mm, 60 ± 5 m thickness) hybrid supports to afford the
corresponding catalytic species, namely Pd@NKZPDB-5 and Pd@NKZPDC-1 respectively (Fig. 1).
The as-prepared catalysts showed neither significant physical or morphological changes, nor signs of
breaking with respect to the native materials by ESEM analysis and could be handled and used with ease,
provided that they were stored under nitrogen for long periods. The palladium loading determined by ICP-OES
analysis was 1.6% wt and 6.7% wt for Pd@NKZPDB-5 and Pd@NKZPDC-1, respectively. Comparable values
were obtained from EDS analysis. Irrespective of the morphology, TEM investigations showed the catalysts to
contain evenly dispersed, spheroidal PdNP, whose mean diameter was 3.2 ± 0.9 nm for Pd@NKZPDB-5 [44]
and 2.1 ± 0.9 nm for Pd@NKZPDC-1, respectively. A representative TEM image and the corresponding Pd
particles size distribution for Pd@NKZPDC-1 are reported in Fig. 2. X-ray diffraction data were in line with
those obtained from TEM in both cases. According to previous observations, the different size of embedded
PdNP in the two materials can be tentatively attributed to the different metal loading in the original hybrids
[47]. Importantly, the size of the supported PdNP was significantly lower than that reported for PVA-stabilized
palladium colloids (8-50 nm) [48] for both catalysts, that can be safely attributed to the steric stabilization of
the solid support [19,49]. Chemisorption experiments were hampered by the limited accessibility of gas
reactants to the PVA hybrids in the dry state [42,50].
3.2. Selective aerobic oxidation reaction of alcohols by Pd@NKZPD catalysts
8

3.2.1.
Catalysis under batch conditions
A preliminary investigation of the suitability of PdNP / ZrO2 / PVA hybrids as heterogeneous catalysts
for the aerobic oxidation of alcohols was carried out using Pd@NKZPDB-5 and Pd@NKZPDC-1 materials
and trans-cinnamyl alcohol (1) as probe substrate (Fig. 3). Catalytic benchmark tests were performed with
stirring in water under 90 °C, 1 bar air pressure and batch setups. Both materials were catalytically active
under these conditions, showing complete conversion within reasonable timeframes and full selectivity to the
partial oxidation product trans-cinnamaldehyde (1a). Representative results are reported in graphical format
in Fig. 4 for Pd@NKZPDB-5. Turnover frequencies (TOF = molsubstrate converted / molPd x h) were in the order 3
h-1 at full substrate conversion (Table 1). Neither induction periods nor catalytic activity of the hybrid
materials prior of Pd reduction were noticed under the above experimental conditions.
The catalysts were truly heterogeneous, since the contribution of homogeneous species to the conversion
could be ruled out by the absence of catalytic activity of the recovered reaction solutions [51] and by the
leaching of palladium below the ICP-EOS detection limit in any case. Catalyst reuse was possible by simple
decantation of the reaction solution, showing minor activity decay (< 5%) over three consecutive runs.
The above findings are in line with those previously reported for the use of Pd@NKZPD-type catalysts in
hydrogenation reactions [43] and demonstrate the strong palladium anchoring and the high resistance of the
catalysts also under the conditions of catalytic oxidation. Deactivation of polymer-supported PdNP catalysts
was previously attributed to the dissolution of Pd crystallites, to unfavourable interactions with the support
and to the oxidative degradation of the polymer under aerobic conditions [52,53]. In the case in our hands,
these effects may be prevented by the steric stabilization of PdNP due the microporous polymer matrix
[19,41] and by the mechanical strengthening and SMSI effect of ZrO2 [42].
Trans-cinnamyl alcohol, besides being used as probe substrate to ascertain the resistance of C=C groups
in alcohol oxidation reactions [54], is the precursor for trans-cinnamaldehyde, an important market product
largely used in agrochemistry [55,56] and in the food and perfumery industry, because of the cinnamon
flavour and fragrance [57]. The mechanism of cinnamyl alcohol oxidation by O2 using heterogeneous Pd
catalysts was investigated in detail by Baiker et al. [58,59]. The liquid phase, selective aerobic oxidation of 1
to 1a has been previously described using a number of supported catalysts [5]. The vast majority of these are
9

based on Pt-group metals or Ru onto inorganic materials, with best efficiencies observed for Al2O3 [60], SiO2
[61], hydroxyapatite [62], hydrotalcite [63,64], TiO2 [65,66], MOFs [67] and MgO [68]. TOFs in the range
1-70 h-1 and selectivity around 80-99 % were commonly obtained at full conversion. However, most systems
performed satisfactorily using pure oxygen as oxidant in the temperature range 60-90 °C, in conjunction with
organic solvents (toluene, xylene, α,α,α-trifluorotoluene, acetic acid). Use of air as oxidant was successfully
reported over modified Pd@SiO2 catalysts, however under 110 °C in xylene [69]. A polymeric (Amberlyst)-
supported perruthenate catalyst was also described using O2 as co-oxidant in toluene, although with limited
reusability due to degradation of the polystyrene matrix [70]. Given the lower reaction rates and the higher
temperatures usually required compared to oxygen, the fact that Pd@NKZPD catalysts allows for the
oxidation of cinnamyl alcohol to take place with comparable efficiencies in water using only 1 bar air and 90
°C must be underlined. As a general rule, use of operating conditions below 100 °C and 10 bar gas pressure
is an important target for sustainable industrial processes [71], as well as for improved catalyst stability and
selectivity. Aerobic oxidation of 1 to 1a by O2 at low temperature (30 °C) was previously described,
although in the presence of large excess of base additives [72].
Having established that Pd@NKZPD materials are reliable catalysts for the selective, aerobic oxidation of
alcohols to aldehydes, we extended the scope of the catalysts by investigating the oxidation of other alcohol
substrates (Fig. 3). Furfuryl (2) and benzyl (3) alcohols were selected to this aim. The partial oxidation of 2
produces furfural (2a), a renewable chemical used as platform for the production of several key furanic
intermediates (e.g. methylfuran, acetylfuran, furfurylamine) and as solvent for aromatics and other
unsaturated olefins, particularly for use in the refining of lubricating oils [73]. The product of further
oxidation furoic acid (2c) is used as bactericide, fungicide and flavouring agent [74]. On the other hand,
benzyl alcohol represents a model substrate for the oxidation of activated aromatic alcohols wherein the
partial oxidation product benzaldehyde (3a) is a valuable chemical with widespread applications in
perfumery, dyestuff and in agrochemical industries [75].
Due to the larger exposed surface area and the closer packing into tubular reactors (vide infra), we
focused our attention on Pd@NKZPDC-1 membrane type catalyst. Furfuryl and benzyl alcohols are
sparingly insoluble in hot water, therefore methanol was used as solvent in this case. As a results, the
aldehyde partial oxidation products were partially converted to the corresponding dimethylacetals (Fig. 3), as
indicated by GC-Ms analysis. The finding can be attributed to the residual acidic catalytic activity of the
10

hybrid Kodoshi materials [43] or to the contribution to catalysis by accidental formic acid, although, in line
with the recalcitrance of methanol to aerobic oxidation [76,77], neither traces of formic acid nor formiates
could be detected under our experimental conditions. All reaction products were carefully identified and
quantified by GC, GC-Ms and NMR. Representative results are summarized in Table 2.
The oxidation of 2 by Pd@NKZPDC-1 in methanol under 70 °C and 10 bar air, resulted in an overall >
99% selectivity to the partial oxidation aldehyde product at 60% conversion (24 h), 75.4 % of which in the
form of dimethylacetal 2b (Table 2, entry 2). No traces of over oxidation products furoic acid (2c) or methyl
furanoate (2d) were detected. Full conversion could be achieved for longer reactions times, with no
significant selectivity decay. The selectivity of the oxidation reaction was confirmed by a countercheck
experiments carried out on furfural (2a) as substrate, which resulted in full conversion to 2b with no
formation of 2c under the same reaction conditions (Table 2, entry 3). Similar findings were obtained for the
oxidation reaction of 3. A > 90% selectivity to aldehyde, 43% as dimethylacetal 3b, was observed at 89%
conversion. Minor amounts of benzoic acid (3d, 2% yield), methyl benzoate (3e, 4%) and benzyl methyl
ether (3c, 2%) were detected. The oxidation reactions proceeded at reasonable rates with productivities in the
range 6-9 h-1. Irrespective of the substrate, Pd leaching in solution was below the ICP-OES detection limit in
each experiment. The oxidation reactions were explored in other solvents other than water or methanol
(toluene, n-hexane, tetrahydrofuran), however with much lower catalytic activity. A conversion < 10% was
observed in toluene under the same experimental conditions. This finding can be tentatively attributed to the
better wettability and permeability of the hybrid PVA membrane in polar protic solvents [42].
The selective, aerobic oxidation reaction of 2 was previously described using heterogeneous catalysts. As
other heterocyclic alcohols, which are considered to be difficult substrates for oxidation reaction, harder
reaction conditions were usually required compared to 1. Representative literature examples are summarized
in Table 3. Best results were obtained for Pd [78] or Pt [79] NP onto zeolite-type metal-organic frameworks
under 100 °C and 1 bar O2 (Table 3, entry 2, 3), wherein an active role of Cr sites was demonstrated, and
molybdenyl acetylacetonate onto MgO (Table 3, entry 4) [80]. Use of air oxidant or lower temperatures was
possible using polyoxometalate [81] (Table 3, entry 5) or hydrotalcite-supported gold catalysts (Table 3,
entry 6), respectively, although the latter required visible light irradiation to achieve modest selectivity [82].
An heterogenized homogeneous complex onto amine-modified silica was also reported in water phase with
11

good yields after workup [83](Table 3, entry 7). To the best of our knowledge, Pd@NKZPDC-1 is the first
polymer-based heterogeneous catalyst reported for the aerobic oxidation of 2. Compared to the known
systems, slightly slower reaction rates were observed, however with higher selectivity under friendlier
reaction conditions (70 °C, air).
Several types of heterogeneous catalysts were described for the batch aerobic oxidation of 3 in the liquid
phase [5]. Among them, the most successful examples of solid-supported metal particles are based on
inorganic materials, particularly Pd@hydroxyapatite [62], Ru onto Al2O3 [60], zeolites [84], hydrotalcite
[85] or MgO [68], Ir@TiO2 [86], Pt@MOFs [79] and Au onto ionic liquid-modified graphene [20]. Typical
reaction conditions involve use of toluene, trifluorotoluene or DMSO solvents and temperatures in the range
60-90 °C, to result in > 92% aldehyde yields and TOF in the range 2-100 h-1. Best results were obtained for
Pd onto Al2O3 in scCO2 (TOF > 1000 h-1) [87]. The polymer-supported perruthenate catalysts
RuO4@Amberlyst also gave aldehyde 3a in 95% yield at 75 °C, however with scarce reusability [70]. The
performance of the Pd@NKZPDC-1 catalyst is in line with that of the above mentioned systems, taking into
account a generally lower reaction rate due to the use of air instead of O2 oxidant.
3.2.2.
Catalysis under continuous flow conditions
Prompted by the above favourable results, we decided to investigate the viability of the Pd@NKZPDC-1
oxidation catalyst for 2 and 3 under continuous flow conditions. To this purpose, we engineered a pressure
and temperature resistant reactor system allowing for concurrent, controlled flows of substrate solution and
air through the catalytic reactor. The catalyst was packed into a commercial glass column resistant up to
1000 psi (176 L inner volume, Fig. 5) and placed in a tubular heater equipped with a temperature controller
(± 1 °C). Attainment of steady state conditions, ca. 1 h, was taken as the reaction start time. At the outlet of
the reactor, the solution was collected for GC and GC-Ms analysis for product determination and ICP-OES
for Pd leaching, while air was released at atmospheric pressure. A scheme of the system is shown in Fig. 6.
The system was completely inert since all wet parts were made of glass, PEEK or Teflon®.
12

The conversion, selectivity and Pd leaching during oxidation reactions were examined under various
combination of air and solution flow rates. Experimental conditions as similar as possible to those of batch
operations were adopted for comparative purpose. As general observation, conversions increased either by
decreasing the solution flow rate (i.e. by increasing the residence time ) or by increasing the air flow rate
(i.e. by increasing the air / substrate molar ratio), while a slight selectivity decrease was observed upon
increasing conversions, as expected for such a flow system [88]. Since conversions were unsatisfactory (i.e.
< 25%) by using air as oxidant up to 90 °C temperature, we then decided to use pure oxygen in this case.
Selected, best compromise results between conversion and selectivity are summarized in Table 4.
Productivities are reported both in terms of TOF (h-1) and space-time-yield (STY = kg product / litre reactor volume
x h).
Using the above setup, oxidations were successfully achieved under mild conditions, i.e. oxygen pressure
1 bar, residence times and temperatures below 240 s and 90 °C, respectively, irrespective of the substrate.
The oxidation of 2 resulted in 40% conversion with full selectivity to aldehyde (70% of which 2a and 30%
the acetal 2b) under 80 °C and  = 212 s. Under similar conditions ( = 106 s), the oxidation of 3 resulted in
77% conversion, with 77% selectivity to aldehyde (93% of which 3a and 3% the acetal 3b), with minor
amounts of 3d (3%), 3e (16%) and 3c (3%). Compared to the batch processes, better acetal / free aldehyde
selectivities were observed at similar conversion levels, although a larger amount of over oxidation products
was obtained in the oxidation reaction of 3. This finding can be attributed both to the use of O2, instead of
air, as oxidant and to the lower contact time of the intermediated products with the catalyst, compared to the
batch setup. For continuous processes, productivities were similar to those of batch arrangement in terms of
TOF, but ca. 102 times higher in terms of STY.
All flow processes were typically monitored for 8 h continuous time-on-stream, followed by overnight
switch-off and restart the day after for an analogous period. Noteworthy, no significant activity nor
selectivity decay were detected under these conditions. Data for the continuous flow aerobic oxidation
reaction of 3 are graphically reported in Fig. 7, as representative example. A mean conversion value of
76.7% ± 0.9 was observed over 16 h overall time-on-stream with no Pd leached in solution detected by ICP-
OES analysis.
13

The continuous flow aerobic oxidation reaction of benzyl alcohol to benzaldehyde by heterogeneous
catalysts based on supported MNP has been previously reported. Representative, liquid-phase literature
examples are summarized in Table 5. Inorganic materials have been mostly used as support [89,90], despite a
bimetallic Au-Pd capillary reactor coated with an amine-functionalized polysiloxane was also described [24].
Best results in terms of turnover frequency and aldehyde yield were reported for Au-Pd@TiO2 catalyst using
neat alcohol as substrate [91] and for a multiphase benzotrifluoride / water reactor system using bimetallic
Au-Pt@C catalysts [92], respectively. Good yields were also reported for a recirculating Ru@Al2O3 reactor
[93]. The hybrid Pd@NKZPDC-1 catalyst ranks halfway, however under generally milder reaction
conditions. With regard to the aerobic oxidation of furfuryl alcohol, to the best of our knowledge no previous
examples under continuous flow conditions have been reported in the literature.
The above obtained results indicate that Pd@NKZPD-type catalysts are featured by a remarkable
mechanical and chemical resistance, which allows for their use under oxidation conditions both in batch and
continuous flow operations. Particularly, a contactor type catalytic membrane reactor [94,95] could be
engineered by packing of the hybrid Pd@NKZPDC-1 material, showing good productivity and selectivity in
the aerobic oxidation of functionalized primary alcohols under very mild flow conditions and remarkable
resistance for prolonged reaction times, with no need of regeneration treatments. PVA-stabilized PdNP have
a number of precedents in heterogeneous catalysis. PdNP @ polyvinyl alcohol films were used for the
Suzuki–Miyaura catalytic reaction of p-iodonitrobenzene with phenylboronic acid. However, the combined
action of oxygen and iodide ions resulted in extensive palladium leaching and in limited catalyst reuse [96].
PdNP onto crosslinked polyvinyl alcohol / polyacrylamide particles were used as catalyst for Heck-type
reactions [97]. The catalyst was not truly heterogeneous in this case, since a homogeneous mechanism
involving the participation of dissolved Pd atoms was highlighted. The catalytic reduction of Cr(VI) to
Cr(III) was achieved using PdNP onto polyethyleneimine/PVA nanofibers [98]. The superior performance of
the hybrid Pd@NKZPD catalysts can be attributed to the favourable combination of several factors,
including the lack of strong coordination interaction between Pd and functional groups from the support
hampering the effective replacement of reactants around the active site [99], the steric stabilization of PdNP
by the porous polymer network [19], the mechanical strengthening and SMSI effect of ZrO2 [18], the good
swelling propensity of PVA.
14

4. Conclusions
Due to environmental and economic constraints, the chemical industry strongly awaits novel, more
sustainable and intensive processes for the selective oxidation of alcohols, that replace conventional methods
based on stoichiometric (hazardous) reagents or homogeneous catalysts. The challenge ahead requires the
design of efficient heterogeneous catalysts using molecular oxygen under mild reaction conditions. We have
shown that materials comprising Pd nanoparticles onto a hybrid zirconia / PVA matrix act as versatile
catalysts for both hydrogenation and oxidation processes. Particularly, heterocyclic and substituted primary
alcohols could be converted to the corresponding aldehyde partial oxidation products in polar, protic
solvents, with good selectivity and productivity under smooth oxidation conditions (temperature  90 °C, air
/ 1 bar O2 pressure). Compared to the known catalysts, the reported system provides multiple advantages in
terms of cost and availability, milder operating conditions, better stability, no need of stabilizers or (basic)
additives [5]. These results can be ascribed to the peculiar features and the optimal permeability in water or
methanol of the hybrid support material. In addition, the engineering of the catalyst into a continuous flow,
contactor type catalytic membrane reactor has been demonstrated for the oxidation reaction of furfuryl and
benzyl alcohol. The reactor showed no significant loss of catalytic performance over prolonged time-on-
stream, with no need of intermediate regeneration steps. The applications of catalytic membranes are often
hampered by issues related to their manufacturing costs and limited durability. Key points to address are the
development of methods for the synthesis of membranes that are stable in a wide range of solvents and
conditions and that show high and reproducible performance over long times [100]. The results herein
obtained are therefore of relevance to elaboration of green methods for the long-term, sustainable production
of fine-chemicals and, particularly, to the engineering of new reactors for continuous processing
applications.
Acknowledgments
Thanks are due to the European Research Institute of Catalysis a.i.s.b.l. and to Centro Microscopie
Elettroniche (CNR, Firenze) for technical support.
15

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Fig. 1. Sketch of the synthesis of the hybrid catalytic
membrane with labelling adopted (top). Optical image of
Pd@NKZPDC-1 catalysts (bottom).
Fig. 2. Left: TEM image of Pd@NKZPDC-1. Right: size
distribution of PdNP.
18

Fig. 3. Sketch of substrates investigated and products detected with labelling scheme adopted.
100
80
60
40
conversion
20
0
selectivity 1a
1
2
3
4
5
6
7
Time (h)
Fig. 4. Batch aerobic oxidation reaction of
1 by
Pd@NKZPDB-5 catalyst. Experimental conditions: water
solution, 90 °C, 1 bar air.
19

Fig. 5. Column glass reactor packed with Pd@NKZPDC-1 catalyst.
A
B
C
E
F
- Pressure regulator
- 3 way switching valve
- Shutoff ball valve
- Flow controller
- Flow meter
G
H
L
M
N
- Check valve
- Mixing tee
- HPLC pump
- Reactor
F
G
- By-pass
A
M
C
O2
H
brass holder
B
B
kapton heating
element
N
C
L
teflon holder
Substrate
solution
Products
controller
Fig. 6. Scheme of the continuous flow reactor system and tubular heater
80
75
70
65
conv.
sel.
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
time-on-stream (h)
Fig. 7. Continuous flow aerobic oxidation reaction of 3 over of Pd@NKZPDC-1 catalyst. Reaction conditions: methanol solution 0.1
M, 80 °C, solution 0.1 mL min-1, O2 5.0 mL min-1. Reactor volume 176 l. Catalyst 45 mg, 6.7% wt Pd. Start time: attainment of
steady state conditions, 1h. Line breaks represent overnight switch-offs.
20

Table 1
Catalytic aerobic oxidation reactions of trans-cinnamyl
alcohol (1) under batch conditions.a
Catalyst
Time
(h)
Conversion Selectivity to 1a
(%)
(%) b
Pd@NKZPDB-5 20.5
99.9
99.9
> 99
> 99
Pd@NKZPDC-1 21.0
a
Reaction conditions: substrate water solution 0.02 M, 90
°C, air 1 bar, Pd@NKZPDB-5 100 mg, Pd@NKZPDC-1 30
mg. Data from GC analysis.
b Selectivity 1a = 1a/(1a+1b).
Table 2
Representative aerobic oxidation reactions of alcohols by Pd@NKZPDC-1 catalyst under batch conditions.a
Entry
Substrate
Solvent
Conversion
(%)
TOF b
(h-1)
Selectivity to aldehyde (%)
total aldehyde c
1a > 99
dimethylacetal d
1
2
3
4
H2O e
99.9
60.6
73.6
88.9
3
6
8
9
-
1
CH3OH
CH3OH
CH3OH
2a + 2b > 99.9
2a + 2b > 99.9
3a + 3b 90.4
2b 75.4
2b 100.0
3b 42.9
2
2a
3
a Reaction conditions: substrate solution 0.5 M, 70 °C, air 10 bar, Pd@NKZPDC-1 30 mg. Data from GC analysis.
b Calculated on overall conversion.
c Es. selectivity 2a + 2b = (2a + 2b)/(2a + 2b + 2c + 2d).
d Es. selectivity 2b = 2b / (2a + 2b).
e Reaction conditions: substrate solution 0.02 M, 90 °C, air 1 bar.
Table 3
Representative heterogeneous catalysts for the aerobic oxidation reaction of 2.
Entry Catalyst
Solvent
Oxidant
Temperature Conversion
Selectivity a
(%)
Ref.
(°C)
(%)
1
2
3
4
5
6
7
Pd@NKZPDC-1
CH3OH
toluene
toluene
toluene
toluene
1,4-dioxane
H2O
air, 10 bar
O2, 1 bar
O2, 1 bar
O2, 1 bar
air, 1 bar
O2, 1 bar
O2, 1 bar
70
60.6
99
99.9
98
this work
[78]
Pd@MOF
100
100
110
80
Pt@MOF
99
85 b
96
[79]
MoO2(acac)n@MgO
Na6.3Fe0.9[AlMo11O39]
Au@hydrotalcite
FeCl3-imine@SiO2
[80]
98
99
[81]
40
77.1
78
25.2
100
[82]
80
[83]
a Selectivity in partial oxidation to aldehyde.
b Isolated yield.
21

Table 4.
Selected data for continuous flow oxidation reactions by Pd@NKZPDC-1 catalyst a
Reaction conditions
Conv.d
(%)
Sel.e
(%) f
TOF f STY f
Solution
O2
Substrate
Temp.
(°C)
(h-1)
(kg l-1 h-1)
Res. time b
Flow rate Pressure c
(mL min-1) (bar)
Solvent
Flow rate
(mL min-1) (s)
0.05
0.10
212
106
5.0
5.0
1.0
1.0
80
80
CH3OH
CH3OH
40 ± 1
100
4.2
0.05
0.20
2
3
76.7 ± 0.9 77
16.2
a Reactor volume 176 L. Solution 0.1 M methanol. Catalyst 45 mg, 6.7% wt Pd. Data from GC analysis. Start time: attainment of
steady state conditions, ca. 1h.
b Residence time ().
c Pressure at the reactor inlet.
d Average conversion value over 16 h time-on-stream.
e Selectivity in partial oxidation to aldehyde.
f Calculated on aldehyde product and bulk Pd content.
Table 5
Continuous flow catalytic aerobic oxidations of benzyl alcohol to benzaldehyde.
Catalyst
Reactor
type
Solvent
Temp.
(°C)
O2
Conversion
(%)
Selectivity TOF
Ref.
(bar)
(%)
(h-1)
6.7% Pd@NKZPDC-1 packed-bed
methanol
toluene
80
1.0
8.0
5.0
7.0
5.0
1.0
77
77
99
78
98
99
99
92
16.2
270
this work
[89]
0.9% Ru@Al2O3
1% Au-Pd@TiO2
2% Au@Fe3O4-SiO2
5% Ru@Al2O3
packed-bed
micro
115
120
150
90
20
neat
95
> 1000 [91]
inductive heating
recirculating
multiphase
capillary
benzene
100
99
7.7
-
[90]
[93]
[92]
[24]
toluene
BTF/water b
DCE/water d 50
3% PI-Au-Pt@C a
60
100
99
3
c
e
e
Au-Pd@Si-NH2
-
-
a
Polymer incarcerated catalysts.
b BTF = benzotrifluoride.
c Amine-functionalized polysiloxane.
d DCE = dichloroethane.
e Not available.
22