High performance of Au/ZTC based catalysts for the selective oxidation of bio-derivative furfural to 2-furoic acid 1

Article abstract of DOI:10.1016/j.catcom.2020.106234

Furfural is a platform bio-molecule for which is valuable to develop new green upgrading processes in biorefinery. We report here for the first time the high performance of Au/ZTC catalyst for the selective oxidation of furfural to 2-furoic acid, as first step to develop electrodes. The ordered nanostructure and high surface area of BEA structure replica ZTC allows to develop 3D-type electrodes. Au/ZTC catalyst shows higher performance than commercial Vulcan, used as reference conductive carbon in fuel cells. The weak acidity on ZTC avoids decarboxylation and esterification reactions, leading to about 90% of furfural conversion fully selectivity to 2-furoic acid.

Full text of DOI:10.1016/j.catcom.2020.106234

Catalysis Communications 149 (2021) 106234
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
High performance of Au/ZTC based catalysts for the selective oxidation of
bio-derivative furfural to 2-furoic acid1
,
Georgia Papanikolaoua, Paola Lanzafamea *, Siglinda Perathonera, Gabriele Centia,
,
Daniela Cozzab, Gianfranco Giorgiannib *, Massimo Migliorib, Girolamo Giordanob
a Dept. ChiBioFarAm - Chimica Industriale, University of Messina and ERIC Aisbl, Viale F. Stagno D’Alcontres 31, Messina 98165, Italy
b Laboratory of Catalysis and Industrial Chemistry, University of Calabria, Via P. Bucci, I-87036 Rende, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
Furfural is a platform bio-molecule for which is valuable to develop new green upgrading processes in bio-
refinery. We report here for the first time the high performance of Au/ZTC catalyst for the selective oxidation of
furfural to 2-furoic acid, as first step to develop electrodes. The ordered nanostructure and high surface area of
BEA structure replica ZTC allows to develop 3D-type electrodes. Au/ZTC catalyst shows higher performance than
commercial Vulcan, used as reference conductive carbon in fuel cells. The weak acidity on ZTC avoids decar-
boxylation and esterification reactions, leading to about 90% of furfural conversion fully selectivity to 2-furoic
acid.
Zeolite template carbon replica
Selective oxidation
Furfural catalytic upgrading
2-furoic synthesis
Biomass waste valorisation
1. Introduction
increasing [6]. Thus, the selective oxidation of bio-based platform
molecules is an important tool for the development of green biorefinery
Furfural is a bio-derived platform molecules which further valor-
isation is of large interest in biorefineries. The selective oxidation of the
hydroxylic or aldehydic functionalities to the corresponding carboxylic
acids is a very promising way to obtain important building blocks for the
production of fine chemicals, new generation bio-fuels and polymers
[1–3]. 2,5-furandicarboxylic acid (FDCA), obtained by selective oxida-
tion of 5-hydroxymethylfurfural (HMF), is recognized as one of the top
ten value-added products derived from biomass and represents a
promising alternative to terephthalic acid (TPA) for the production of
bio-based polyethylene 2,5-furandicarboxylate (PEF) [1]. Moreover, 5-
hydroxymethyl-2-furancarboxylic acid (HMFCA), another useful build-
ing block for the synthesis of polyesters, can be obtained by the selective
oxidation of the aldehydic group on HMF [4]. Other relevant synthetic
routes involve the conversion of furfural (FF) (Scheme 1). In particular,
the selective oxidation of FF to 2-furoic acid (FA), is a relevant industrial
reaction, because it represents a step for FDCA production via the
Henkel reaction [5] and moreover FA finds high-added-value applica-
tions in the pharmaceutical, agrochemical, flavour and fragrance in-
dustries. Furthermore, by oxidative esterification with alcohols leads to
the formation of green furoate esters which market use is significant
options.
The industrial production of FA from FF is currently performed via
the Cannizzaro disproportionation reaction in the presence of NaOH,
which requires the addition of sulphuric acid to neutralise the solution,
obtaining stoichiometric quantities of furfuryl alcohol (FAOH) and FA,
and sodium bisulfate [7]. Direct selective catalytic oxidation with O2 of
FF to FA represents thus a green alternative. Of recent interest, in
particular, the possibility to realize the process in an electrocatalytic
reactor for the possibility to make the reaction under milder conditions,
and especially to couple in tandem with a reductive reaction on the other
side of the electrocatalytic reactor to obtain process intensification.
However, one of the aspects necessary in this directly is the development
of novel 3D-type electrodes to obtain higher accessibility and production
rate per unit area of electrode. It is necessary for this to develop 3D type
conductive carbon materials which can eventually host metal
nanoparticles.
Zeolite-templated carbons (ZTCs), synthesised by using a zeolite as
template, are attractive materials for several applications [8] and thanks
to their ordered microporous structure and the extremely high surface
area, can be considered an interesting support to realize a stable catalyst
* Corresponding author.
E-mail addresses: planzafame@unime.it (P. Lanzafame), gianfranco.giorgianni@unical.it (G. Giorgianni).
1
This paper in honor of Professor James G. Goodwin, Jr., in the occasion of his 75th birthday, to celebrate his outstanding contribution to catalysis sciences and
technology.
https://doi.org/10.1016/j.catcom.2020.106234
Received 9 July 2020; Received in revised form 30 October 2020; Accepted 8 November 2020
Available online 10 November 2020
1566-7367/© 2020 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

G. Papanikolaou et al.
Catalysis Communications 149 (2021) 106234
with a high dispersion of noble metal NPs [9]. Furthermore, the pres-
ence of oxygenated groups on the ZTCs surface could play an additional
role in the activation of molecular oxygen, as also reported for the
oxidation of benzyl alcohol [10]. Their conductive characteristics and
the presence of an ordered porous structure is ideal to develop these
novel 3D type of electrodes. However, a first step to use them in these
electrocatalytic applications is to show how they behaves under thermal
catalytic conditions. This is the aim of this study.
to the use of a reference materials such as Au NPs supported on a
commercial carbon (Vulcan) support. The selection of Vulcan as the
carbon support is motivated from being a conductive carbon widely used
in fuel cell electrodes, and thus a good reference in view of the devel-
opment of electrodes for the electrocatalytic selective oxidation of
furfurals.
2. Experimental
Several catalysts based on noble metals supported on different metal
oxides were studied in the attempt to develop efficient thermal catalytic
systems for the selective oxidation of FF [11]. Quantitative yields to FA
were obtained by using supported AuPd/Mg(OH)2 in the presence of
NaOH [12], although the use of a base still represents a drawback for the
environmental sustainability of the process. Moreover, an additional
acidification step is required to convert the obtained furoate to FA.
Analogous catalytic systems based on Au nanoparticles (NPs) supported
on acidic metals oxides (particularly ZrO2) were efficiently used for the
oxidative esterification of FF with methanol to methyl furoate (MF) in
the absence of bases [13,14], because the formation of the esters sta-
bilizes the product and inhibits side reactions. However, the formation
of FA rather than MF is preferable from the industrial process perspec-
tive, being recovery easier and offering FA a higher variety of
utilizations.
2.1. Catalysts synthesis and characterization
β-ZTC was synthesised using Na-BEA (Si/Al = 25) as template, via
chemical vapour deposition technique [16]. 1.5 g of zeolite was load in a
tubular quartz-reactor inserted in a lab-scale furnace (Carbolite GHA
12/1050). Ethylene was used as a carbon source feeding a mixture of
◦
this hydrocarbon with a nitrogen-carrier for 4 h at 700 C (ethylene:
0.03 mol/mol, total flow rate: 500 NmL/min). After carbon deposition,
the system underwent a venting step by feeding nitrogen at 700 ◦C for 1
h. After cooling, β-ZTC was purified [17] by removing the zeolite matrix
in a room temperature double step acid-washing procedure: contacting
hydrofluoric acid (48% vol, Merck) for 12 h, washing the sample with
ultrapure water, then contacting with hydrochloric acid (37% vol, Carlo
Erba Reagenti) for 6 h and rinsing the sample again with distilled water.
The average yield of ZTC was above 0.4 g for any synthesis.
In the oxidative esterification, Au NPs promotes the dissociation of
O2 to atomic oxygen able to activate methanol leading to the selective
formation of MF through the acetal pathway [13,14]. This acetal
pathway, however, is less efficient with respect to the direct oxidation
path [2]. The use of Au based catalysts supported on carbon materials,
without acidic functionalities on the surface, could hinder the acetals
pathway, thus leading to a more efficient catalytic system able to favour
the direct oxidation path and the formation of esters. The use of Au/
carbon catalysts for the selective oxidation has been scarcely investi-
gated in literature. Only de Jongh and coworkers [15] investigated the
oxidation of HMF to HMFCA in aqueous solution and in the presence of a
base.
Gold metal NPs were deposited on β- ZTC and commercial Vulcan
(CB Vulcan XC-72 from Cabot Corporation USA) using the sol immobi-
lization method, by suspending 250 mg of carbon support into an
aqueous solution containing HAuCl4⋅3H2O (0.013 mmol, Sigma-Aldrich)
and polyvinyl alcohol (PVA, Sigma-Aldrich) (Au/PVA = 1:1 wt/wt) as
well NaBH4, under continuous stirring for 2 h. The Au-based catalysts
(indicated hereinafter as Au/ZTC and Au/Vulcan) were washed, filtered,
dried at 80 ◦C for 12 h and calcined at 200 ◦C for 2 h in air (heating ramp
of 2 ◦C/min). The final Au-metal loading was 1 wt% [18].
The crystalline structure and of β-ZTC and Au deposited on both
supports were analysed by powder XRD (D2 Phaser diffractometer,
Bruker) equipped with a Cu tube (λ = 1.54056 Å) and the data recorded
in the 2θ range of 5–50◦ (15◦–100◦ in case of metal deposited samples),
with an angular step size of 0.02◦.
In this communication, we report for the first time the use of ZTC
replica of BETA zeolite (β-ZTC) as support for Au NPs, showing high
activity and selectivity in the selective oxidation of FF to FA, without
using added bases, which can be obtained using this catalyst, compared
Textural properties were determined by adsorption/desorption
Scheme 1. Furfural conversion pathways
2

G. Papanikolaou et al.
Catalysis Communications 149 (2021) 106234
isotherms of N2 at ꢀ 196 ◦C (ASAP2020, Micromeritics). Before each
diffraction patterns for Au supported on Vulcan and ZTC catalysts are
reported in Fig. 1 revealing the diffraction peaks for metallic cubic gold
at 2θ = 38◦, 44◦, 64◦ and 78◦ which corresponds to the crystal planes of
(111,200,220) and (311), respectively (JCPDS no.04–0784) [23].
β-ZTC SEM micrographs (Fig. 2) revealed the presence of micron-
sized particles, reproducing the crystal morphology of the β-Zeolite
scaffold (not shown), while TEM micrographs show the presence of an
ordered carbon structures having characteristic dimension of about 0.5
nm (Supplementary Info). SEM micrographs show the presence of a
homogeneous dispersion of round-shaped particles of around 100–200
nm, with EDX mapping showing the presence of well dispersed Au in the
carbon ZTC matrix (Supplementary Info). No residual Si could be
detected, while the presence of O could be revealed, also resulting well
dispersed by EDX mapping. TEM images of Au/ZTC and Au/Vulcan
(Supplementary Info) show a well dispersion of round-shaped Au
nanoparticles with an average size of about 5 nm and comparable in the
two samples.
measurement, sample was degassed at room temperature till 50 μmHg,
and then, the temperature increased to 350 ◦C and hold at the end for 6
h. Finally, each sample was cooled to r.t. and backfilled with He. The
specific surface area (SBET) was calculated by the Brunauer, Emmett and
Teller (BET) method, using the Rouquerol method [19]. The total pore
volume was evaluated at p/p◦ 0.96. The microporous volume and
microporous surface area were evaluated by using the t-plot (Harkins
and Jura). The pore size distributions (PSD) were calculated by using the
HS-2D-NLDFT method, designed for N2 adsorption on slit-like pores of
infinite size present on carbon-based materials (regularization 0.01)
[20].
The surface acidity of the samples was determined by NH3-TPD
measurements (TPRO 1000, Thermofisher) by modifying a procedure
reported elsewhere [21]. In order to avoid the release of undesired
species during NH3 desorption, 50 mg of sample were loaded in a linear
quartz micro-reactor and pre-treated (i.d. 4 mm; length 200 mm) in
Helium flow (20 STPmL/min) at 150 ◦C for 2 h and, then, treated in the
The β-ZTC and Au/ZTC samples present Type Ib/IV N2 physisorption
isotherms (Fig. 3) typical of micro/mesoporous materials, and a hys-
teresis loop closing above p/p◦ 0.9, and, apparently, of type H4, typical
of particles containing narrow slit-shaped pores [24], due to partial
disorder induced in the carbon structure that replicates the full-space of
the Na+-β scaffold [17]. The Au/ZTC sample, with respect to the parent
β-ZTC, presents a significant reduction of N2 adsorption capacity.
The Vulcan XC-72 present a Type Ib/IV isotherm with an H4 and
textural features in line with the literature [25]. Among the prepared
samples, the β-ZTC presents the largest specific surface area, total pore
volume, and micropore volume (Table 1). The latter, in line with the
shape of the isotherm, decreased by 28%, 33%, and 30%, respectively,
◦
◦
range 100–500 C (ramp 10 C/min, holding time 30 min) to remove
any volatile species from the samples. After that, each sample was
◦
treated at 150 C by a 20 STPmL/min flow of NH3/He mixture (NH3:
0.10 mol/mol) for 2 h. Physically adsorbed ammonia was removed by
purging in helium at 150 ◦C until TCD baseline stabilization. Ammonia
desorption measurements were then performed in the range 100–700 ◦C
(ramp 10 ◦C/min, holding time 30 min).
The morphology of the prepared materials was investigated via
Scanning Electron Microscopy (SEM) (FEI 200, Quanta) and Trans-
mission Electron Microscopy (TEM) (JEM 1400 Plus, Jeol).
after the deposition of gold, while the external surface area (SBET- Smic
)
2.2. Catalytic activity assessment
decreased only by 15%. The isotherms do not provide indication about
changes in the structure induced by the deposit of Au NPs. The micro-
pore volume is due to a partial pore blocking in the micropore region
(Table 1).
The oxidation of furfural was tested in a 250 mL Teflon-lined stain-
less-steel autoclave (Parr Instrument Company, 4560 Series) operating
in batch mode. 100 mg of catalyst was suspended in a 150 mL methanol
solution of furfural (0.02 M). Before each test, the reactor was purged
three times by using O2. Then, at room temperature, the O2 pressure was
increased to 6 barg, the stirring set to 1000 rpm, and, finally, the tem-
perature was increased to 120 ◦C (15 min). The measured autogenous
pressure at 120 ◦C was 12 barg. The reaction progress was monitored for
6 h by collecting and analysing liquid samples. The collected samples
were filtered, and the performance of the reaction measured by GC-FID
and GC–MS. Reactant conversion was measured by GC-FID (7890, Agi-
The Vulcan support, with respect to the β-ZTC and Au/ZTC samples,
shows only a modest microporosity. At the same time, no significant
effect on textural properties was observed after Au deposition (Table 1).
The pore size distribution for the β-ZTC and Au/ZTC samples (Supple-
mentary Info) show the prevalence of two major families of micropores
centered at 9 and 13 Å, respectively, in line with pore openings given by
the ideal crystallographic structure of β-ZTC samples [26]. A small
amount of irregular mesopores was observed between 50 and 500 Å.
Conversely, the Vulcan support presents an irregular pore network with
a significant contribution of mesopores centered at 70, 100, and 155 Å,
respectively.
lent) equipped with an auto-sampling device (injected volume 1
μL),
DB1 column (Agilent J&W 125–1032, length 30 m, φ = 0.53 mm, film
thickness 1.50 μm)). Helium was used as carrier gas (column flow 1.6
mL/min). The oven temperature was increased from 100 to 240 ◦C
(heating rate 25 ◦C/min), holding the final temperature for 10 min.
Products determination was performed by GC–MS (Agilent 7820A,
MSD 5977E), equipped with an auto-sampling device (injected volume
NH3 TPD results (see also Supplementary Info) show that Au/Vulcan
has about 70% more acidic sites than Au/ZTC (Table 1), 29.5 with
1 μL), HP-5 Column (Agilent, L = 30 m, φ = 0.25 mm, δ = 0.25 μm, Split
ratio = 100. Helium was used as carrier gas (column flow 1 mlL/min),
and the oven temperature kept at 40 ◦C for 6 min and then increased to
300 ◦C (heating rate 10 ◦C/min) and holding the final isothermal con-
dition for 5 min.
3. Results and discussion
3.1. Characterization
XRD patterns of the synthesised β-ZTC (Supplementary Info) shows a
basal peak around 2θ = 7◦ in the same position as the diffraction of the
β-Zeolite. This indicates the presence of regular pores within the struc-
ture of carbon like that of the zeolite with a “basal spacing” or period-
icity of ≈1.2 nm in the carbon structure [22]. The XRD patterns exhibit a
further peak at 2θ = 15◦, a position as in β-Zeolite, coherent with a good
level of replication of the structure in ordered carbon [22]. The
Fig. 1. XRD patterns of the investigated samples after Au deposition.
3

G. Papanikolaou et al.
Catalysis Communications 149 (2021) 106234
acidity, being oxygen typically located at edge sites to form -OH or
carboxylic groups. Although some acidity has been detected in ZTC, this
is not correspondent to the presence of significant amount of oxygen
(about 8–9% wt) which remains as a consequence of the elimination of
zeolite template. This indicates that most of this oxygen in not located at
accessible edge sites and thus able to give rise to surface acidity.
3.2. Catalytic tests
The catalytic performance of Au/ZTC catalyst was studied in the
selective oxidation with O2 of FF using methanol as solvent and in the
absence of added bases. While typically an aqueous medium is used to
have a high solubility of the substrate, we instead selected methanol as
the solvent to achieve a higher solubility of oxygen in the reaction me-
dium [27]. The catalytic behaviour of Au/ZTC was compared with those
of a reference Au/Vulcan catalysts (Fig. 4). As commented in the
introduction, we used Vulcan as comparative example, because the aim
is to compare the thermal catalytic behaviour of samples which can be
used then as catalytic electrodes for the oxidation of bio-based platform
molecules. There is thus the need of conductive substrates and Vulcan is
the most used carbon substrate for the preparation of electrodes for fuels
cells. In addition, Au/Vulcan catalyst behaves better as catalyst for the
target reaction with respect other nanocarbon materials, as shown in the
Supplementary Info by comparison with the behaviour of Au/CNT, e.g. a
sample where Au NPs are deposited on carbon nanotubes.
Fig. 2. SEM image of β-ZTC.
Data indicates that the ZTC support significantly increases the sub-
strate conversion also at early stages of the reaction, as furfural con-
version increases up to 3 h reaching a plateau value of 0.89, due to
limitations in oxygen availability. A similar increasing trend over time
was observed also by Au/Vulcan even though the maximum conversion
was lower than that of Au/ZTC (being present side reactions consuming
more oxygen) and the plateau value (0.84) was reached after 5 h of
reaction, indicating the slower reaction rate. With respect to selectivity,
it is worth to note that GC–MS spectra indicate the presence of only 2-
furoic acid as product of reaction at all reaction times. Other products
Fig. 3. Porosimetry curve of support and Au-supported samples.
respect to 8.1 mol/g of acidic sites (Au/Vulcan vs Au/ZTC). The acidity
μ
is related to the support and no significant changes are detected after
gold loading. NH3-TPD curves (Supplementary Info) evidence the pres-
ence of only weak acidity differently from Vulcan showing stronger
acidity. Vulcan acidity is related to the presence of -SO3H groups in
commercial Vulcan, while is surprising the low acidity present in ZTC.
Note that SEM data (Supplementary Data) indicate the significant
presence of oxygen in the ZTC carbon sample (as well as in the sample
before gold loading), differently from the Vulcan sample. Usually the
presence of oxygen as heteroatom in carbon samples in an index of
Fig. 4. Furfural conversion evolution over time: effect of carbon support and
Au/ZTC catalyst amount.
Table 1
Textural properties of the catalysts.
SAMPLE
Specific Surface Areaa (m2/g)
Specific Micropore Surface Areab (m2/g)
Micropore Volumed (cm3/g)
Total Pore Volumed (cm3/g)
Acidityc (
μmol/g)
Vulcan
235
92
0.04
0.04
1.15
0.79
0.54
0.53
1.90
1.26
29.3
29.5
7.9
Au/Vulcan
β-ZTC
213
86
3400
2433
2884
1998
Au/ZTC
8.1
a
B.E.T. specific surface area.
b
c
Micropore volume calculated by the t-plot method.
From NH3-TPD profiles.
4

G. Papanikolaou et al.
Catalysis Communications 149 (2021) 106234
were not detected. Note that the dimeric acid form cannot be excluded
from GC–MS, but the formation of this product seems unlikely.
novel class of carbocatalysts for other applications.
Another relevant evidence of Fig. 4 is the initial activity of the Au/
ZTC samples showing ca. 0.5 conversion at reaction close to zero. Zero is
the time when the autoclave reaches the reaction temperature of 120 ◦C
± 2 ◦C (about 15 min). Thus, in Au/ZTC already during this step, con-
version of FF begins, whilst for Au/Vulcan the initial conversion results
close to zero. Looking for another proof of the initial Au/ZTC activity, a
test was also performed by loading 25 mg of catalyst. Note that the
catalyst to substrate (FF) ratio in these conditions decreases from about
0.35 to 0.09 wt. With respect to the total volume of reaction, the catalyst
amount passes from 1185 to 4740 times lower. Thus, the amount of
catalyst is quite low in these experiments, confirming the high activity.
Au/ZTC catalyst presents high stability in the investigate conditions and
no evidence of reactivity loss was observed (Supplementary Info).
By using 25 mg of catalyst (Fig. 4), the conversion increases slowly
with respect to the test with 100 mg of Au/ZTC, with a 0.06 conversion
at reaction time = 0 and a conversion plateau (0.86 close to that reached
with higher catalyst amount) achieved after 4 h of reaction. These re-
sults confirm that Au/ZTC is very active and shows a promising activity
in furfural oxidation toward 2-furoic acid. Note that we have used here a
Among the main possible side products, we can indicate furan, as the
product of decarboxylation. The side carboxylic acid is easy to be
eliminated in the presence of acid sites [28]. The absence of detection of
furan, differently from Au/Vulcan case, confirms the presence of weak
acid sites in Au/ZTC. No products deriving from the addition of meth-
anol to the acid (esters), neither of other products deriving from the
base-catalysed opening of the aldehydic group in the presence of an
alcohol [2], were detected. In addition, humins are a typical by-product
observed in furfural oxidation, although often not properly reported.
However, this is a main industrial problem, in the conversion of HMF
and FF [29]. The absence of humins in Au/ZTC is another advantage of
thus catalyst. Furfural derivatives could generate chain α‑carbonyl al-
dehydes through hydrolytic ring opening reaction [30]. This type of
products was also not detected, conforming the good properties of Au/
ZTC.
The Au/ZTC catalyst show a stable behaviour, at least on a lab-scale
experimentations, as shown in the data reported in the Supplementary
Info.
◦
Therefore, despite the presence of different side pathways, as further
illustrated in Scheme 1, the formation of only FA as product of reaction
even using an alcohol as solvent, without thus the typical products
observed in the case of oxidative esterification, reveals the unique cat-
alytic properties of Au/ZTC. To note that from an industrial perspective,
the use of an alcohol rather than water is highly preferable because
significantly lower the energy intensity of the downstream separation
unit. Realization of a full selective synthesis of an acid by selective
catalytic oxidation in an alcohol solvent is for the first time reported
here, as far as we known. The acid could be easy recovered by membrane
crystallization [31], which is a low energy separation process, where the
use of an alcohol rather than water significantly further improves the
energy saving and allow a continuous recirculation of the alcohol sol-
vent. Thus, the innovation introduced using Au/ZTC as a catalyst would
reflect also in a significant improvement in terms of green processes for
biorefinery.
reaction temperature of 120 C to have comparable data with earlier
studies on Au/ZrO2 catalysts [5,6,13,14,34]. The comparison with
literature data using oxide supports for Au confirm the higher perfor-
mances (activity, selectivity) of Au NPs supported on ZTC with respect to
other supports.
4. Conclusions
We report here for the first time the very high performances of Au/
ZTC catalysts in the selective oxidation of FF in the absence of an added
base, and the peculiar ability to form the acid rather than the ester using
methanol as a solvent. This has many positive implications in terms of
industrial development of this green chemical synthesis of interest for
biorefinery areas.
A highly performing catalyst for the selective oxidation of furfural to
furoic acid was obtained by depositing Au nanoparticles on β-ZTC car-
bon prepared by carbon replica of BETA zeolite. The comparison of the
catalytic performance of Au/Vulcan and Au/ZTC catalysts shows that
the use of ZTC support significantly increases the conversion of furfural
also at early stages of the reaction, with an increase of FF conversion up
to 3 h reaching a plateau value of 0.89, with total selectivity toward the
formation of FA. The presence of only weak acid sites on the supports
hinders the acetalization and esterification pathways, favouring the se-
lective oxidation of the aldehydic functionality on the furan ring.
Further studies are in progress to assess the exact role of the ZTC support
on the overall catalytic performance of the Au/ZTC catalyst and their
use also as metal-free carbocatalyst, with the first results giving in-
dications of an active role in the mechanism in addition to the role of
stabilization of well dispersed Au NPs and the possibility to develop
novel electrocatalysts for bio-based platform molecules conversion. The
relatively large amount of oxygen on the external surface could play a
key role in the mechanism of conversion, confirming the peculiar
properties of these carbon materials with respect to other type of
nanocarbon materials [32].
The absence of furfuryl alcohol supports the point that Cannizzaro
reaction (requiring base catalyst) [7] does not takes place in the inves-
tigated conditions. As commented before, this is the current industrial
process to produce FA, but having various drawbacks both in terms of
coproduction and in terms of environmental impact. The absence of
furan (FU) in the reaction products remarks the absence of decarbox-
ylation/decarbonylation reactions.
Since esterification or acetalization reactions require acid sites to
take place [13,14], the acid obtained as solely product confirms the
presence of only weak acidity on Au/ZTC samples, and that FA itself
does not act as organocatalyst.
Likely ZTC is not playing only a role as conductive carbon substrate
which can host well dispersed Au nanoparticles due to the presence of a
well regular nanostructure due to the template synthesis from a zeolite.
The presence of large amount of oxygen (deriving from the preparation
procedure), but not associated to acid sites such as phenolic or carbox-
ylic sites at edges, indicate the presence of a graphene-oxide surface in
–
the ZTC, with a large presence of C O or C-O-C non acidic) oxygen sites
–
which are known are responsible in various carbocatalytic reactions
[32]. This suggests the interest of metal-free ZTC as novel carbocatalyst.
The presence of large amount of oxygen (about 6–9 wt%) with
minimal surface acidity could be attributed to the specific structure of
this support. The pore connectivity of the zeolite support, imposes a
curvature of the graphene sheet, leading to the formation of carbon edge
sites, highly unstable which can react with water during the template
removal process [33]. Epoxy, carbonyl and lactone groups are thus
predominantly present on the ZTC support in a higher amount with
respect to other nanocarbon materials. Although the analysis and
demonstration of these aspects is not part of this communication, we
could anticipate as preliminary indication that ZTC are an interesting
Funding
No funding was received for this work.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
5

G. Papanikolaou et al.
Catalysis Communications 149 (2021) 106234
oxidation, Catal. Today 203 (2013) 196–201, https://doi.org/10.1016/j.
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Prof. Lorenzo Caputi (University of Calabria, Italy) and CNR-IMM
(Catania, Italy) are kindly acknowledged for HR-TEM image.
The authors also thank the support of PRIN2017 project CO2 ONLY
(project nr. 2017WR2LRS) and part also the support of the EU project
PERFORM (project 820723).
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