Directing nitrogen-doped carbon support chemistry for improved aqueous phase hydrogenation catalysis

Article abstract of DOI:10.1039/d0cy00391c

Selective hydrogenations in the aqueous phase are an important transformation in the context of developing biorefinery concepts. In this report the application and optimisation of nitrogen-doped carbon (NDC) supported Pd nanoparticles as hydrogenation catalysts is discussed in the context of directing support (e.g. N) chemistry for improved catalytic performance in the aqueous phase. As a demonstrative example, the aqueous phase hydrogenation of phenol to cyclohexanone (e.g. a platform for polyamide production) is utilised. Catalyst supports were prepared based on an initial hydrothermal synthesis to yield NDC xerogels (from biomass precursors), the chemistry of which (e.g. functionality) was directed using a secondary thermal carbonisation (Tc) step at different temperatures (i.e. 350, 550, 750, 900 and 1000 °C). After Pd introduction, it was found that size, dispersion and electronic structure of the formed nanoparticles is affected by the surface chemistry of the NDC. This consequently led to higher turn-over frequency (TOF) and stability of the prepared catalysts compared to a "nitrogen-free"carbon supported Pd and a commercial, carbon supported Pd (Pd/AC) catalyst. Pd/NDC 900 (featuring predominantly quaternary and pyridinic N) catalysed the complete conversion of phenol at 99% selectivity to cyclohexanone, with excellent stability over 11 recycles and no discernible catalyst sintering or leaching (in contrast to the commercial catalyst). High catalytic stability, activity and selectivity make the Pd/NDC 900 catalyst highly applicable for aqueous phase hydrogenation reactions, whilst the general principle opens scope for support tailoring for application (e.g. biorefinery hydrogenations) and the development of structure/activity relationships.

Full text of DOI:10.1039/d0cy00391c

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Directing nitrogen-doped carbon support
chemistry for improved aqueous phase
Cite this: DOI: 10.1039/d0cy00391c
hydrogenation catalysis†
ab
a
c
bcd
Monika Bosilj,
Lina Rustam, Ralf Thomann, Julia Melke,
Anna Fischer *bcd and Robin J. White
*
ae
Selective hydrogenations in the aqueous phase are an important transformation in the context of
developing biorefinery concepts. In this report the application and optimisation of nitrogen-doped carbon
(NDC) supported Pd nanoparticles as hydrogenation catalysts is discussed in the context of directing
support (e.g. N) chemistry for improved catalytic performance in the aqueous phase. As a demonstrative
example, the aqueous phase hydrogenation of phenol to cyclohexanone (e.g. a platform for polyamide
production) is utilised. Catalyst supports were prepared based on an initial hydrothermal synthesis to yield
NDC xerogels (from biomass precursors), the chemistry of which (e.g. functionality) was directed using a
c
secondary thermal carbonisation (T ) step at different temperatures (i.e. 350, 550, 750, 900 and 1000 °C).
After Pd introduction, it was found that size, dispersion and electronic structure of the formed
nanoparticles is affected by the surface chemistry of the NDC. This consequently led to higher turn-over
frequency (TOF) and stability of the prepared catalysts compared to a “nitrogen-free” carbon supported Pd
and a commercial, carbon supported Pd (Pd/AC) catalyst. Pd/NDC 900 (featuring predominantly quaternary
and pyridinic N) catalysed the complete conversion of phenol at 99% selectivity to cyclohexanone, with
excellent stability over 11 recycles and no discernible catalyst sintering or leaching (in contrast to the
commercial catalyst). High catalytic stability, activity and selectivity make the Pd/NDC 900 catalyst highly
applicable for aqueous phase hydrogenation reactions, whilst the general principle opens scope for support
tailoring for application (e.g. biorefinery hydrogenations) and the development of structure/activity relationships.
Received 28th February 2020,
Accepted 16th June 2020
DOI: 10.1039/d0cy00391c
rsc.li/catalysis
appropriate heterogeneous catalysts. In this context,
cyclohexanone is an important base for the synthesis of
Introduction
With
a
view to utilising phenolic-rich streams from
caprolactam (i.e. for nylon-6) and adipic acid (i.e. for nylon-
3
,4
biorefineries (e.g. via lignin deconstruction processes), liquid-
phase hydrogenations should be operated under less
demanding energy conditions. Such transformations will be
performed in the liquid phase, often in polar (e.g. aqueous/
alcoholic) solvents; adding another dimension to designing
6,6). Industrially, cyclohexanone is manufactured based on
5
the oxidation of cyclohexane or the hydrogenation of
1
,2
6
phenol. Through hydrogenation, cyclohexanone is either
synthesised in two steps in the vapour phase or via a
potentially more energy and cost efficient one-step liquid
4
,7–9
phase reaction.
Further, as a more general consideration,
performing liquid phase hydrogenations in environmentally
a
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110
friendly solvents such as water, would also be beneficial as
Freiburg im Breisgau, Germany
b
10,11
the use of hazardous solvents (e.g. dichloromethane
)
Institute for Inorganic and Analytical Chemistry, Albert-Ludwigs-Universität
becomes increasingly regulated.
Freiburg, Albertstraße 21, 79104 Freiburg im Breisgau, Germany.
E-mail: anna.fischer@ac.uni-freiburg.de
The hydrogenation of phenol follows
a
consecutive
c
12
Freiburg Material Research Center, FMF, Albert-Ludwigs-Universität Freiburg,
pathway (Scheme 1).
To synthesize cyclohexanone, the
Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau, Germany
d
Freiburg Center for Interactive Materials and Bioinspired Technologies, FIT,
Albert-Ludwigs-Universität Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg im
Breisgau, Germany
e
Netherlands Organization for Applied Scientific Research, TNO, High Tech
Campus 25, 5656 AE Eindhoven, Netherlands. E-mail: robin.white@tno.nl
†
2
Electronic supplementary information (ESI) available: N physisorption, SEM,
XPS, effects on quaternary N BE, H O adsorption and H-index calculation,
2
Scheme 1 The reaction pathway of phenol hydrogenation to the
Rietveld refinement. See DOI: 10.1039/d0cy00391c
desired cyclohexanone in the liquid phase.
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aromatic ring of phenol must be selectively hydrogenated to
been investigated, using the aqueous phase hydrogenation of
phenol as demonstrative reaction. Properties and catalytic
performance of Pd/NDC catalysts were compared to Pd NPs
supported on a “nitrogen-free” HTC carbon (Pd/HTC) and a
commercial AC supported Pd catalyst (Pd/AC) (matrix
activated carbon, Degussa type E1003 U/W). Besides a
1
-hydroxy-cyclohexene, itself undergoing tautomerization to
13
cyclohexanone. Further hydrogenation leads to the undesired
product, cyclohexanol.
14,15
Therefore, and with particular
reflection on catalyst development, it is important that
cyclohexanone is weakly adsorbed on the catalyst surface and
thus desorbs readily before further hydrogenation proceeds. This
is challenging if the enol is strongly adsorbed on the catalyst,
leading to cyclohexanol formation favoured by associated keto/
superior performance, Pd/NDC 900 (T = 900 °C) was shown
c
to have an improved stability (e.g. inhibition of NP leaching/
sintering) when compared to the commercial Pd/AC catalyst.
4
enol equilibrium thermodynamics and kinetic favorability.
Pd/AC is
a commonly used industrial catalyst in
1
6–18
hydrogenation reactions (e.g. phenol hydrogenation
).
Results and discussion
Despite its wide applications in industrial processes, such as
in the hydrogenation of organic fine chemicals, aromatic
hydrogenations etc., the morphological changes of Pd entities
and consequently a decrease of activity down to uneconomic
levels are typical reasons for the deactivation of palladium
catalysts supported on carbons. With reference to the
successful implementation of biorefinery concepts, catalytic
materials will be required that possess improved
hydrothermal stability under aqueous operating conditions
and tailored surface chemistry (e.g. appropriate wettability) to
NDC xerogels were carbonised at increasing temperatures (Tc
= 350, 550, 750, 900, 1000 °C) under N flow. The recovered
2
porous carbons were impregnated with tetraaminepalladium-
IJII) nitrate via incipient wetness impregnation. An un-doped
“nitrogen-free” HTC supported Pd catalyst was also prepared
(denoted as Pd/HTC 550). CHNS elemental analysis (EA) of
the NDCs showed decreasing nitrogen amounts with
1
9
c
increasing T (Table 1). The O content was calculated from
the CHNS elemental analysis by assuming that the residual
wt% after substraction of the C, H, N, S wt% is ascribed to
oxygen. The N content decreased from 4.3 wt% (NDC 350) to
2.1 wt% (NDC 1000), with an O content reducing from 16.2
control product/reactant adsorption. With
a
view to
improving on commercial activated carbon, heteroatom
doping, most commonly with “N”, is known to positively
influence the interaction between support and nanoparticle
c
to 8.3 wt% over the same T range. The pristine NDC product,
(
NP) (i.e. metal support interaction), often leading to
i.e. directly after the HTC synthesis without further
carbonisation (denoted as NDC 180) contained 5.6 wt%
nitrogen. The trends observed from this bulk EA analysis are
consistent with the results obtained from surface XPS
analysis, confirming the uniform distribution of C, N and O
through the structure of the NDC supports.
improved catalytic activity and stability in hydrogenation
reactions.
on a porous carbon structure is potentially advantageous with
regard to NP size/distribution, NP support attachment as well
as NP electronic structure.
also lead to improved oxidation and catalyst stability. Using
HTC to produce carbon materials is also attractive with
respect to directing material properties (e.g. surface
2
0,21
The ability to direct N-containing functionality
2
2–26
Furthermore, N-doping can
2
7
Regarding porosity, NDC 180 presented a surface area (SBET)
2
−1
3
−1
and a pore volume (Vpore) of 138 m
g
and 0.20 cm g ,
respectively. Thermal carbonisation at T
increase in SBET and Vpore, as HTC decomposition products
c
≤ 550 °C, led to an
2
8
chemistry, porosity etc.) through sol–gel-like syntheses.
Combining this with T selection opens scope to develop first
trapped in the NDC 180 pores are thermally removed (Table 1).
c
principle structure–activity relationships. Moreover, using
renewable feedstocks for synthesis of N-doped carbon
materials is another advantage (e.g. vs. use of toxic or
As T
c
further increased (T
c
≥ 750 °C), the porosity and specific
surface area was found to decrease, as a result of further carbon
structure alterations (pore closure and material contraction). All
NDC supports presented type IV/H3 reversible N adsorption
2
2
9
hazardous chemicals or processes to introduce N groups).
3
0–32
Based on this idea and on previous reports,
the use of
isotherms with a pore size distribution reflecting the expected
xerogel structure, featuring partially defined maxima in the
micropore range (pore diameter ∼1 nm) and lower mesopore
region (pore diameter ∼3 nm) (Fig. S1† – calculated using a
quenched solid density functional theory (QSDFT) method).
SEM images of the prepared NDC materials demonstrated
a coral-like continuous porous structure (Fig. S2†); consistent
with similar aerogel materials reported previously by White
nitrogen-doped carbons (NDC) (as prepared through the
hydrothermal carbonisation (HTC) of biomass-derived
precursors) were investigated in the present manuscript as
“tunable” support media for hydrogenation catalysts. The
chemistry of these NDC xerogels was then directed through a
thermal carbonisation step at a defined temperature (denoted
3
2,33
as T ) (e.g. to direct N bonding motif).
Following an
c
3
0
incipient wetness impregnation, the resulting NDC-supported
Pd NP catalysts were investigated for the aqueous phase
hydrogenation of phenol to cyclohexanone, with the aim of
demonstrating the impact of subtle variations in support
chemistry on catalytic performance and stability.
et al.
c
At T > 550 °C, the NDC structure became
increasingly finer due to the carbonisation processIJes) and
network contraction, which is in good agreement with the N2
sorption results. The structure of the HTC 550 support was
by contrast composed of aggregated spherical NPs with
diameters ≤20 nm (in agreement with morphologies
In this report, the impact of altering NDC support
chemistry on the catalytic activity of deposited Pd NPs has
3
3,34
discussed in an earlier report).
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Table 1 Support material properties
a
2
−1
b
3
−1
b
3
−1
Support
S
BET [m g
]
V
pore [cm g
]
V
meso [cm g
]
C/N/H/O [wt%]EA
C/N/O [atom%]XPS
NDC 350
NDC 550
NDC 750
NDC 900
NDC 1000
HTC 550
293
462
344
250
173
476
0.36
0.35
0.21
0.19
0.16
0.43
0.28
0.22
0.12
0.16
0.14
0.29
75.1/4.3/4.4/16.2
83.6/4.3/2.5/9.6
86.1/3.6/1.7/8.6
87.3/2.9/1.4/8.4
88.3/2.1/1.3/8.3
83.0/—/2.7/14.3
84.9/3.6/11.5
88.8/3.2/8.0
93.4/2.8/3.8
94.8/2.4/2.8
95.6/1.8/2.6
95.1/—/4.9
a
b
Specific surface area from BET method. Pore size characteristics obtained via the QSDFT model based on N
2
adsorption data.
Table 2 Supported Pd catalyst properties
a
b
b
d
S
BET
V
pore
V
meso
Average Pd crystallite size
[nm]XRD
Pd
[wt%]ICP
C/N/Pd
[atom%]XPS
Hydrophilicity
index
Pd dispersion
[%]
2
−1
3
−1
3
−1
c
Catalyst
[m g
]
[cm g
]
[cm g
]
Pd/NDC 350
Pd/NDC 550
Pd/NDC 750
Pd/NDC 900
Pd/NDC 1000
Pd/HTC 550
Pd/AC
287
410
334
224
182
432
835
0.18
0.13
0.21
0.20
0.15
0.31
0.57
0.17
0.05
0.13
0.16
0.14
0.18
0.34
3.3/10.3
5.5/21.6
4.6
4.4
4.8
4.3
4.1
4.7
4.4
4.4
4.1
4.2
84.4/4.1/0.4
90.4/3.6/0.4
89.8/3.2/1.1
91.7/2.7/0.8
94.0/1.4/0.6
93.6/—/1.0
79.7/—/4.2
0.36
0.39
0.24
0.24
0.04
0.33
0.62
7.9
4.5
14.2
16.4
11.1
9.4
1.4
1.6/10.9
60.0
a
b
c
Specific surface area from BET method. Pore size characteristics obtained via the QSDFT model. Hydrophilicity index calculated: amount
d
2 2 0 2
of adsorbed H O/amount of adsorbed N at P/P = 0.93. Pd dispersion calculated from H pulsed chemisorption for a Pd/H ratio of 1 : 1.
Inductively coupled plasma optical emission spectroscopy
ICP-OES, Table 2), indicated a relatively consistent Pd
loading (4.5 ± 0.3 wt%), despite different SBET and pore
volumes, approaching the theoretical target of 5.0 wt%. For
Pd/AC a similar Pd loading was determined (4.2 wt%).
To investigate the influence of different support properties
on Pd crystallite size, XRD patterns were acquired and
analysed by Rietveld refinement (Fig. 1 measured in Bragg–
less oxygen in comparison to the NDC supports (with the
exception of the NDC 350 support). Therefore different
interactions between Pd and the HTC support in the absence
of N doping are expected than for the NDC supports (as
discussed in more detail below with regard to XPS analysis).
For the commercial Pd/AC the Pd crystallite size was found to
be bimodal – 1.6 nm for the smaller and 10.9 nm for the
larger size ranges.
(
Brentano geometry). The broadened reflection with
maximum at 2θ = 24.2° is associated with the carbon support
NDC, HTC, AC). The higher the T , the more defined this
a
TEM images reveal a broad Pd NP size range on the AC
support, with an average size of 5.4 ± 3.5 nm (Fig. 3). This
broad particle size distribution presumably arises due to
interactions between the AC support and the Pd NPs, which
are not strong enough to prevent aggregation or sintering,
and with the porous structure not providing any confinement
benefits (i.e. predominantly microporous). TEM images of
Pd/HTC 550 reveal the smallest NPs with a narrow particle
size distribution; in agreement with XRD results (Table 2).
TEM of Pd/NDC 350 and Pd/NDC 550 (Fig. 2) confirms the
bimodal particle size distribution found by XRD (Fig. 1a).
The impact of surface chemistry specifically of nitrogen-
doped carbons (e.g. N-species) on Pd NPs size and
(
c
reflection appears in line with increasing ordering and
stacking of the carbon phase. For Pd/AC, reflections with a
maximum at 2θ = 34.1°, 43.1°, 55.7°, 61.0° and 72.9°
correspond to the (002/101), (110), (112), (103) and (202)
3
5,36
reflections of PdO, respectively.
All other catalysts show
the typical (111), (200), (220), (311) and (222) reflections of
face-centred cubic (fcc) palladium nano-crystallites.
The average Pd crystallite size was determined through
Rietveld refinement (see ESI,† Fig. S7 and S8 measured in
Debye–Scherrer geometry). Bimodal particle size distribution
was revealed for Pd/NDC 350, Pd/NDC 550 and Pd/AC, whilst
for Pd/NDC 750, Pd/NDC 900 and Pd/NDC 1000 monomodal
distributions were observed (Table 2). For the NDC supports
carbonised at higher Tc (≥750 °C) smaller Pd nano-
crystallites were obtained than for NDCs carbonised at lower
Tc. In addition, very small Pd nano-crystallites were found on
HTC 550 with a size of circa 1.4 nm with a monomodal size
distribution. Smaller Pd size may be explained by different
support morphologies (spherical NPs) and different surface
chemistries. The HTC 550 support is “N-free” and contains
2
3,37,38
distribution has been reported in the literature
and will
be discussed in more detail below with regard to XPS
analysis.
In order to confirm that Pd is present in the form of
δ+
nanoparticles and not as single molecular Pd
sites,
scanning electron microscopy in transmission mode (TE-
SEM) coupled with energy dispersive X-ray spectroscopy
(EDX) analysis of Pd/NDC 900 was further performed (Fig. 4).
The local TE-SEM-EDX analysis revealed a homogeneous
distribution of N, C and O in the system. The comparison of
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Fig. 2 TEM images of the investigated supported Pd catalysts (5 wt%
Pd loading) and their corresponding particle size distributions.
X-ray photoelectron spectroscopy (XPS) analysis, which
confirms a decrease in N amount with increasing Tc.
However, the determination of the O amount on the surface
Fig. 1 XRD patterns for a) Pd/NDC catalysts and b) Pd/AC and Pd/HTC
550 catalysts. Measured in Bragg–Brentano geometry.
the EDX spectra of “Area 1” and “Area 3” with visible Pd NPs
in comparison to “Area 2” with no visible Pd particles,
indicates that Pd is predominantly (in line with XRD) present
as NPs and the presence of single molecular sites on the
NDC 900 support can largely be excluded.
H O adsorption analysis were performed and
2
a
hydrophilicity index was calculated for all investigated
samples according to the description given in ref. 39 in order
to compare the hydrophilicity between the catalysts with
distinct morphologies (Table 2, ESI† Fig. S5). The
hydrophilicity of carbon materials can be in part influenced
by the presence of oxygen as well as nitrogen functional
4
0,41
groups on the carbon material surface.
Pd catalysts
supported on NDC carbonised at low T ≤ 550 °C show a
c
higher hydrophilicity index than Pd/NDC catalysts carbonised
c c
at higher T , suggesting that T controls the hydrophilicity of
the Pd/NDC catalysts. These results are in line with the
amount of O and N functional groups in/on the carbons. Fig.
c
S6 in ESI† shows the correlation between the T and the N
and O surface amount on NDC. The amount of N functional
groups on the Pd/NDC catalysts was determined by surface
Fig. 3 TEM image of commercial Pd/AC catalyst (5 wt% Pd loading).
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48
in phenols and ethers and occluded CO or CO (O4). It is
2
observed that with higher T ≥ 550 °C the amount of oxygen
c
singly bonded to carbon decreases. A significant difference
was observed for the “N-free” HTC 550 material, which
exhibited a higher amount of oxygen in a singly bonded state
to carbon and a lower amount of carbonyl functional groups.
This confirms large differences in the oxygenated surface
chemistry of the HTC 550 material compared to the NDC
material carbonised at the same temperature.
High-resolution XPS analysis of the C 1s photoelectron
envelopes of the investigated catalysts was also conducted
(
Fig. 5a). The C 1s envelope is characterised by five main
contributions (ESI,† Table S1 and S2) with BEs of 284.8 eV
2
(
C1, CC, sp ), 285.7 eV (C2, C–C and C–H ), 286.5–286.7 eV
x
(
2
C3, C–N/C–O), 287.7–287.9 eV (C4, CO/CN) and 289.1–
3
0,49,50
89.4 eV (C5, O–CO) respectively.
In addition, BEs at
290.8–291.2 eV (C6) is attributed to π–π* shake up satellites
and the presence of pre-graphitic polyaromatic domains. As
expected, Pd/NDC 350 (with the lowest T ) has the largest
c
x
contribution from C–C/C–H (34.7 at%), C–N/C–O (24.5 at%)
and of CO/CN (10.5 at%) in comparison to the other Pd/
NDC catalysts.
Fig.
4
TE-SEM image of the Pd/NDC 900 catalyst (a), and the
The reduction in N-content for the NDC materials as
characterised by EA (Table 1) was also observed and further
confirmed via XPS (Table 1). These results indicate a
homogeneous distribution of nitrogen at the surface and in
the bulk of the NDC carbon materials (e.g. decomposition
proceeds uniformly as a consequence of the thin branched
corresponding EDX elemental mapping images (b–e) and local EDX
spectrum (f).
of the Pd catalysts is not straightforward by XPS due to the
overlap between the O 1s and the Pd 3p3/2 core levels (which
will be discussed in more detail below with regard to XPS
analysis). The “N-free” Pd/HTC 550 catalyst shows a slightly
lower hydrophilicity index than the Pd/NDC 550 catalyst,
c
porous structure). As expected, increasing T from 350 to
1000 °C, results in a decrease in N-content in Pd/NDC
catalysts at the surface, from 4.1 at% to 1.4 at%, respectively
(Table 2). Regarding the corresponding high resolution N 1s
photoelectron envelopes (Fig. 5b), peak fitting revealed for
Pd/NDC 350 mainly thermally unstable species such as
pyrrolic (N4; 400.4 eV) and amine (N1; 399.2 eV) which are
c
which was carbonised at the same T . Commercial Pd/AC
catalyst obtained the highest hydrophilicity index indicating
the highest degree of hydrophilicity in line with the highly
oxidised surface of the activated carbon as a consequence of
the activation process.
controls the hydrophilicity of the Pd/NDC catalysts, which
may greatly improve the catalyst dispersibility in aqueous
5
1
present at the surface. For T
c
≥ 550 °C, pyridinic N (N2;
4
2
These results indicate that Tc
398.3–398.6 eV) and quaternary N or graphitic N (N5; 401.0–
401.1 eV) become increasingly prevalent, alongside
contributions from pyridinic-N-oxide (N6; 402.7–402.9 eV)
media and in turn contribute to
performance.
a
better catalytic
attributed to chemisorption processes (e.g. during sample
handling in air). It was observed that with increasing T (T
c c
4
3–45
52
The chemical environments of C and N in the NDC
supports and the oxidation state of the investigated Pd/NDC,
Pd/HTC and Pd/AC catalysts were examined using X-ray
photoelectron spectroscopy (XPS) (Table 2 and Fig. 5). The
core level peaks were fitted after a Shirley background
subtraction and core level spectra were calibrated vs. the C 1s
peak (binding energy (BE) = 284.8 eV). The main problem in
the XPS analysis of the oxygen/palladium system is the
≥ 750 °C) a small shift of ca. 0.1–0.2 eV to higher BE is
detectable for pyridinic N (N2) species in Pd/NDC 750/900/
1000 catalysts in comparison to Pd free NDC supports. An
additional component (N3) is required for the NDCs catalysts
with T ≥ 750 °C; component, which is attributed to the N–
c
Pd interaction. The N 1s core level spectra of the Pd/NDC
catalysts were compared to the N 1s of the NDC support
materials before Pd impregnation and a clear contribution of
an N–Pd interaction peak is observed for Pd/NDC catalysts
4
6,47
overlap between the O 1s and Pd 3p3/2 core level,
therefore the O 1s contribution of the Pd catalysts was not
analysed. The O 1s core level peak fitting of the NDC and
HTC 550 support materials without Pd is presented in ESI†
with T
assigned to a N–Pd interaction is found to be in the same
range as reported previously for metal–nitrogen
interactions. Table 3 summarises all N 1s BEs and at% of
the respective N species for the Pd/NDC catalysts and
corresponding Pd-free NDC supports.
c
≥ 750 °C (Fig. 6 and Table 3). The binding energy
5
3
(Tables S3 and S4 and Fig. S3). Four main oxygen functional
groups were revealed, namely carbonyls (O1), carboxylic (O2),
oxygen singly bonded to carbon in aromatic rings (O3), like
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Fig. 5 High-resolution XPS scans of (a) C 1s, (b) N 1s, (c) Pd 3d core spectra for Pd catalysts supported on HTC and NDC supports and the
0
δ+
δ+
2+
4+
commercial Pd/AC catalyst, (d) Pd /Pd ratio where Pd represents sum of Pd and Pd at%.
The fits of the Pd 3d photoelectron envelopes consist of
surface chemistry of the support significantly changes with
increasing T , which might influence the electronic properties
three doublet components (Fig. 5c, Table 4 and in ESI† Table
c
0
54
2+
S6), in Pd 3d namely Pd (335.5–335.8 eV), Pd (336.4–
of the Pd NPs and consequently contribute to a shift to
higher binding energies of the Pd 3d BE. Shifting of the Pd
3d BE to higher BE was reported in the literature as a result
of charge transfer from Pd NPs to N-species (e.g. pyridinic N)
5
/2
5
5
4+
56
3
37.2 eV) and Pd (338.0–338.8 eV). Table 4 summarises
all the BEs and respective at% found in the Pd 3d high
resolution spectra of the Pd/NDC catalysts in comparison to
the Pd/AC and the Pd/HTC 550 catalysts as determined by
4
7,57,58
on the surface of carbon supports.
XPS Pd 3d core level peak fitting. BEs for Pd 3d5/2 and Pd
In order to investigate possible interactions between the
Pd NPs and the NDC supports, the BEs of the pyridinic N in
the NDC supports and in the Pd/NDC catalysts were plotted
0
3
d3/2 electronic states of Pd were observed to shift towards
higher values with increasing T . As shown previously, the
c
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Fig. 6 High-resolution XPS scans of N 1s region of NDC supports in comparison to the NDC supports impregnated with Pd.
Table 3 Binding Energy (B.E; eV) and at% of N 1s core level of Pd/NDC catalysts in comparison to NDC supports
N1 (amine,
R-NH
N2 (pyridine,
N)
N4 (pyrrole,
N)
N5 (quaternary
N6 (pyr-N-oxide,
C H N O )
5 5
+
+ −
2
)
C
6
H
5
N3 (N–Pd)
C
4
H
5
N, [N-R
4
] )
Catalyst/support
B.E. (eV)
at%
B.E. (eV)
at%
B.E. (eV)
at%
B.E. (eV)
at%
B.E. (eV)
at%
B.E. (eV)
at%
Pd/NDC 350
NDC 350
Pd/NDC 550
NDC 550
Pd/NDC 750
NDC 750
Pd/NDC 900
NDC 900
399.2
399.1
—
399.2
—
—
—
—
—
44.8
37.8
—
17.5
—
—
—
—
—
—
—
—
—
—
—
399.6
—
399.8
—
399.7
—
—
—
—
—
15.2
—
15.6
—
11.4
—
400.4
400.4
400.4
400.5
—
—
—
—
—
21.7
37.8
39.5
29.9
—
—
—
—
—
401.1
401.0
401.0
401.1
401.0
401.0
401.1
401.1
401.1
401.1
19.4
17.9
15.7
18.0
43.6
50.6
41.0
49.6
48.8
49.1
402.8
402.8
402.9
402.8
402.7
402.7
402.8
402.9
402.8
402.8
14.0
6.5
6.6
398.6
398.6
398.5
398.5
398.4
398.5
398.3
398.5
38.2
24.1
28.9
33.2
27.1
29.0
18.7
24.2
9.7
12.2
16.2
16.3
21.0
21.2
26.8
Pd/NDC 1000
NDC 1000
—
—
—
—
with the Pd 3d5/2 BE (Fig. 7). An increase towards higher BE
the Pd/NDC catalysts carbonised at T ≥ 750 °C with the
c
0
0
(from 335.5 eV to 335.8 eV) of the Pd 3d5/2 BE is observed
shift of the Pd 3d5/2 BE to higher BE indicate an electron
for increasing T . With T ≥ 750 °C predominantly pyridinic
donation from the Pd to the pyridinic N on the support.
For Pd/NDC 350 and Pd/NDC 550 a lower Pd 3d5/2 BEs of
c
c
0
and quaternary N are present on the surface of the carbon
support. In line with the shift towards higher BE (0.1–0.3
eV) with increasing Tc of the Pd 3d5/2 BE, the BE of the
pyridinic N in the Pd/NDC catalysts decreases by ca. 0.1–0.2
c
eV with increasing T when compared to the BE of the
pyridinic N in the Pd-free NDC support. The concomitant
shift of BE towards lower BE of the pyridinic N groups in
335.5 eV was observed, indicating no charge transfer
between Pd and the respective support. For the quaternary
N in the NDC support, no significant differences between
the BE before and after Pd impregnation were observed
(ESI,† Fig. S4) suggesting no charge transfer between
0
2
2
quaternary N and Pd.
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Table 4 Binding Energies (B.E.; eV) and at% of Pd 3d core level of Pd/NDC catalysts in comparison to Pd/HTC 550 and Pd/AC
Pd 3d5/2
Pd 3d3/2
0
2+
4+
0
δ+
0
2+
4+
0
δ+
Catalyst
B.E. Pd at% B.E. Pd
at% B.E. Pd
at% Ratio Pd /Pd
B.E. Pd at% B.E. Pd
at% B.E. Pd
at% Ratio Pd /Pd
Pd/NDC 350 335.5
Pd/NDC 550 335.5
Pd/HTC 550 335.5
Pd/NDC 750 335.7
Pd/NDC 900 335.7
Pd/NDC 1000 335.8
12.0 336.5
27.4 336.4
42.7 336.5
34.2 336.7
38.4 336.6
36.2 336.8
31.9 337.2
26.7 338.1
16.8 338.0
10.9 338.0
16.9 338.4
12.3 338.2
15.4 338.4
21.7 338.8
19.7 0.3
13.6 0.9
4.9 2.7
7.4 1.4
8.5 1.8
8.4 1.5
6.6 1.1
340.7
340.7
340.7
341.0
341.0
341.1
341.0
8.0 341.8
18.2 341.5
28.5 341.7
22.8 341.8
25.6 341.8
24.1 342.0
21.3 342.3
20.8 343.6
11.7 343.1
8.9 343.0
10.4 343.2
10.0 343.3
10.8 343.3
10.1 342.3
13.0 0.2
12.4 0.8
4.0 2.2
8.4 1.5
5.7 1.6
5.0 1.5
10.1 1.1
Pd/AC
335.8
Regarding the “N-free” Pd/HTC 550 and Pd/AC catalysts,
presented in Table 2 and reveal a higher Pd dispersion for
the Pd catalysts supported on NDC supports carbonised at Tc
≥ 750 °C than for Pd/NDC 550 and Pd/NDC 350 catalysts.
Higher Pd dispersions are in line with the smaller and
monomodal Pd NPs size distributions obtained for the Pd/
different interactions between the Pd and the supports are
0
expected. The Pd 3d5/2 BE of the Pd/AC catalyst is at higher
0
BE (335.8 eV) when compared to the Pd 3d5 BE of the Pd/
/2
HTC 550 catalysts (335.5 eV). The higher BE in the case of
the Pd/AC catalyst might be attributed to the highly
NDC catalysts carbonised at T
c
≥ 750 °C, while lower Pd
oxygenated surface (as indirectly proved by the H O
dispersions for Pd/NDC 350 and Pd/NDC 550 are in line with
their larger and bimodal particle size distributions. These
results might be attributed to the stabilizing effect of
pyridinic nitrogen species on the Pd NPs as pyridinic
2
adsorption measurements, Table 2, hydrophilicity index).
Indeed, it has been reported in the literature that electron-
depleted Pd atoms may be a result of a charge transfer
between Pd and oxygenated surface functionalities of the
nitrogen groups were described in the literature as possible
5
7,59
60
carbon support.
Contrary, Pd/HTC 550 exhibits a more
anchoring sites for metal NPs.
The pyridinic N–Pd
hydrophobic surface (and hence should expose less
oxygenated groups at its surface) and consequently a lower
interactions described before support these findings. The
N-free Pd/HTC 550 catalyst has a lower Pd dispersion than
the Pd/NDC catalysts, with the exception of the Pd/NDC 350
catalyst. The highest Pd dispersion of 60% was obtained by
the commercial Pd/AC catalyst amongst all investigated
sample.
0
BE of Pd 3d5/2 is found.
0
δ+
The Pd 3d state analysis indicated lower Pd /Pd ratios
5
/2
δ+
2+
4+
(where Pd is sum of Pd and Pd at%) for Pd/NDC 350
and Pd/NDC 550 (Fig. 5d). This suggests the presence of
higher amounts of PdOx surface oxides on the Pd/NDC
Combining these results, we can summarize that
catalysts carbonised at T
c
≤ 550 °C. Considering the “N-free”
N-dopants present at T
pyridinic and quaternary
c
≥ 750 °C, represented mainly by
and sitting on rather
0
δ+
catalysts, a significantly higher Pd /Pd ratio was obtained
for the Pd/HTC 550 catalyst when compared to the Pd/AC
catalyst.
N
a
hydrophobic surface, act as electron acceptor resulting in
electron-deficient Pd NPs. Therefore, the Pd 3d5/2 BE of these
catalysts shifts by about 0.1–0.3 eV to higher BE values. These
findings are supported by the N–Pd interactions, which are
observed in the N 1s spectra of the Pd/NDC 750, Pd/NDC 900
and Pd/NDC 1000 catalysts, as compared to the bare NDC
support (Fig. 6). In addition, the Pd catalyst supported on
The Pd dispersion was investigated by means of pulsed H
chemisorption. The ratio between the number of Pd atoms
available for H adsorption at the nanoparticle surface to the
2
2
total number of Pd atoms is used in this paper as a measure
of palladium dispersion. A H to Pd stoichiometry of 1 was
assumed to determine the Pd dispersion. The data are
0
δ+
NDC carbonised at T
c
≥ 750 °C showed higher Pd /Pd
ratios, higher Pd dispersion and smaller Pd NPs sizes
compared to the Pd catalysts supported on NDC T ≤ 550 °C.
c
It is to be expected that the modulation of the size,
dispersion, surface chemistry and metal–support interactions
will have an impact on the catalytic performance. Thus, the
performance of the prepared Pd/NDC catalysts was
investigated for phenol hydrogenation to cyclohexanone in
aqueous phase and compared to “N-free” Pd/HTC 550 and
Pd/AC reference catalysts.
As a demonstrative reaction and also with reference to
emerging valorisation of bio-aromatics and the further
elaboration of biorefinery concepts, the performance of the
described catalysts was evaluated for the hydrogenation of
0
phenol in the aqueous phase; a challenging reaction
Fig. 7 Effects of electron transfer from Pd 3d to pyridinic N in Pd/
NDC 750, Pd/NDC 900 and Pd/NDC 1000 catalyst.
(Table 5). As discussed above, a (“one-step”) hydrogenation of
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Table 5 Comparison of investigated Pd catalysts for the aqueous phase hydrogenation of phenol to cyclohexanone
a
−1
Catalyst
Conversion [%]
Selectivity cyclohexanone [%]
Selectivity cyclohexanol [%]
TOF [h
]
Blank
1.3
5.9
100
—
—
Pd/NDC 350
Pd/NDC 550
Pd/NDC 750
Pd/NDC 900
Pd/NDC 1000
Pd/HTC 550
Pd/AC
93.3
97.0
96.5
98.2
93.8
97.4
97.8
6.7
3.0
3.5
1.8
6.2
2.6
2.2
29.2
82.4
143.6
157.6
99.5
79.0
53.0
10.1
57.7
67.5
28.8
18.9
84.9
a
2
Mole of cyclohexanone produced per mole of active Pd per hour. Active Pd calculated based on pulsed H chemisorption. Reaction
conditions: t = 4 h, using 50 mg Pd catalyst (5 wt% Pd loading) and 3.2 mmol phenol at 100 °C and 6 bar of H
2
.
−
1
phenol to cyclohexanone (an important precursor for
polyamide monomers) would be a more efficient and greener
process over cyclohexane oxidation and a “two-step” phenol
hydrogenation.
obtained TOF value of 157.6 h from Pd/NDC 900 catalyst
and the dispersion is normalised by the highest calculated
dispersion of 60% from Pd/AC, therefore both representing a
maximum of 100%. For the Pd/NDC carbonised at T
c
≥ 550
Among the Pd/NDC catalysts, the phenol conversion
increases with increasing T
phenol conversion for Pd/NDC 350, 10.1% for Pd/NDC 550,
7.7% for Pd/NDC 750 to 67.5% for Pd/NDC 900 (Table 5). For
c
of the NDC support, from 5.9%
5
Pd/NDC 1000 catalyst the trend is broken as phenol conversion
decreases to 28.8%. As such, significantly higher phenol
conversions were obtained for Pd/NDC catalysts carbonised at Tc
≥
750 °C. As well the selectivity to cyclohexanone is following a
quite similar trend. The highest phenol conversion of 84.9%
was reached by the commercial Pd/AC catalyst, with 97.8%
selectivity to cyclohexanone. Significantly lower conversion of
18.9% was obtained by the Pd/HTC 550 catalyst with 97.4%
selectivity towards cyclohexanone.
It is important to note that the elucidation of a key factor
driving the catalytic performance in the Pd/NDC system is
very challenging with NP size, dispersion and indeed
electronic structure as well as support material properties
(e.g. hydrophilicity) all playing a role.
Therefore, the catalyst activity for phenol hydrogenation
was also expressed in terms of turn-over frequency (TOF)
Table 5). TOF for cyclohexanone formation is defined as the
moles of cyclohexanone produced per mole of active Pd (i.e.
surface active Pd atoms determined by pulsed
(
H
2
chemisorption) per hour. For the Pd/NDC catalysts, the TOF
follows the same trend as observed for the phenol
conversion. The TOF increases with increasing T of NDC
c
support from Pd/NDC 350 to Pd/NDC 900 and decreases for
Pd/NDC 1000. For the N-free Pd/HTC 550 catalyst and all Pd/
NDC catalysts, with the exception of Pd/NDC 350, equal or
significantly higher TOF were observed compared to the Pd/
AC reference catalyst, which obtained the highest phenol
conversion (Table 5). These results encouraged us to try to
identify the catalysts parameters governing their activity.
Therefore, we investigated if a correlation between the TOF
0
δ+
and the Pd dispersion, the Pd speciation in terms of Pd /Pd
ratio and the catalyst hydrophilicity exists.
Fig. 8a shows the correlation of the Pd dispersion and Pd
size with the TOF. TOF values are normalised by the highest
Fig.
8 a) Correlation between the dispersion and TOF (both
0
δ+
normalised), b) correlation between the Pd /Pd ratio and TOF (both
normalised) and c) correlation between the hydrophilicity (H-index)
and TOF (both normalised).
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C, some correlation trends between the increase of Pd
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°
900 and Pd/NDC 750, despite its relatively high Pd dispersion
0
δ+
dispersion and the increase of TOF are observed. However
there is no clear correlation between the TOF and the Pd
dispersion, when one compares all Pd/NDC catalysts with
each other or when one compares all catalysts including the
N-free Pd/AC and Pd/HTC 550 catalysts. This clearly suggests
that beside Pd dispersion some other factors affect the Pd
activity of the Pd/NDC and Pd/HTC 550 catalysts. Indeed,
despite the significantly lower Pd dispersion found for Pd/
NDC 900 and Pd/NDC 750 in comparison to the Pd/AC
catalyst, very high TOF compared to the TOF of the Pd/AC
catalysts are found, indicating a higher activity of the Pd
surface sites in case of the NDC system.
and Pd /Pd ratio, may be due to the significantly less
hydrophilicity of the support (normalised hydrophilicity
index <10%), which hinder the catalyst dispersion in the
aqueous phase and the adsorption of hydrophilic phenol to
4
4
the active sites.
This is suggesting an optimum
hydrophilicity index (hydrophilicity index = 0.24, Table 2) of
Pd/NDC catalysts in order to obtain high TOF. In the case of
the “N-free” catalysts, lower hydrophilicity index is correlated
to the higher TOF, again suggesting an optimum
hydrophilicity of N-free catalysts for high activity in aqueous
phase phenol hydrogenation. Therefore, it can be concluded
that an optimum degree of surface hydrophilicity for phenol
hydrogenation in aqueous phase is desired; however the
hydrophilicity does not directly affect the catalyst activity.
It is reasonable to conclude that the catalytic activity of
the reported Pd catalysts for aqueous phase phenol
In a next step, the TOF was tentatively correlated with the
0
δ+
Pd speciation in terms of Pd /Pd ratio. Fig. 8b shows the
0
δ+
impact of the normalised Pd /Pd ratio on the normalised
0
δ+
TOF values. Pd /Pd ratio was normalised by the highest
0
δ+
calculated Pd /Pd ratio of 2.7 reached from the Pd/HTC 550
hydrogenation is presumably
a
complex synergistic
0
δ+
catalyst. As one can see from Fig. 8b normalised Pd /Pd
combination of different factors including Pd dispersion, Pd
0
δ+
ratio correlates well with the normalised TOF within the Pd/
NDC system, with the exception of Pd/NDC 1000. Hence for
speciation (Pd /Pd ratio), as well as catalyst hydrophilicity/
hydrophobicity. However in case of the Pd/NDC catalysts, we
can establish that the catalytic activities are a result of the
0
δ+
T < 1000 °C, a higher Pd /Pd ratio correlates with a higher
c
0
0
δ+
TOF, revealing Pd as catalytically active site for the phenol
high Pd /Pd ratio in combination with a good Pd dispersion
and an optimum degree of hydrophilicity of the catalyst.
Interactions between pyridinic N and Pd might in addition
stabilise metallic Pd NPs on the NDC support and improve
their catalytic activity. This is in the case of Pd catalysts
supported on the NDC carbonised at Tc ≤ 550 °C not
observed due to the lack of pyridinic N–Pd interactions.
Consequently lower Pd 3d5/2 BEs for Pd/NDC 350 and Pd/
hydrogenation. Looking at the N-free catalysts, Pd/AC and Pd/
0
δ+
HTC 550, again an increase in Pd /Pd ratio correlates with
an increase in TOF, even if it is clear from the relative
0
increase of both factors, that other parameters than the Pd /
δ+
Pd must also influence the catalytic activity. Keeping the
0
δ+
low Pd /Pd ratio of the Pd/AC catalyst in mind, this might
actually be the reason for the low TOF values of the Pd/AC
catalyst, despite its high Pd dispersion.
0
δ+
NDC 550 were observed and low Pd /Pd ratio as well as
decreased Pd dispersion due to the larger Pd NPs. Because of
these reasons Pd/NDC 350 and Pd/NDC 550 are resulting in
reduced catalytic activities compared to the Pd catalysts
0
δ+
While a certain correlation between the Pd /Pd ratio and
the TOF can be observed within the respective support
systems (i.e. N-containing and N-free), it is clear that the
correlation breaks when comparing both support systems
with each other. This is again a clear indication, that beyond
Pd dispersion and Pd speciation, also the type of support
with its respective surface chemistry will influence the
catalytic activity. Indeed, the surface chemistry of the support
in terms of surface functional groups, might, as suggested
supported on the NDC carbonised at T
c
≥ 750 °C. In the case
of “N-free” catalysts, no clear parameter was revealed as the
key factor influencing the catalytic activity, suggesting that
the “N-free” support with its respective surface chemistry will
influence the catalytic activity differently than the NDC
support.
3
8,58
already in the literature,
strongly influence Pd–support
As the most active catalysts, Pd/NDC 900 and Pd/AC were
tested further to investigate the conversion of phenol and
selectivity to cyclohexanone/cyclohexanol as a function of
reaction time (Fig. 9). Under identical reaction conditions, a
interactions as well as support or catalyst hydrophilicity/
2
5
wettability and hence catalyst–phenol interactions.
Besides Pd dispersion and Pd speciation, hydrophilicity/
hydrophobicity of the Pd catalysts needs to be considered,
since the catalysts were applied in the aqueous phase phenol
hydrogenation. Indeed, the impact of catalyst hydrophilicity
on the reaction activity was evidenced by many other
7
,44,61
studies.
hydrophilicity index of the catalysts on the normalised TOF
both normalised by the highest value). As can be seen, there
Fig. 8c shows the correlation of the normalised
(
is no clear trend between the TOF and the hydrophilicity
index of the catalysts. One can however say, that higher
normalised TOFs (>60%) are observed for lower normalised
hydrophilicity indexes (<50%). However, the lower catalytic
activity of the Pd/NDC 1000 catalyst compared to the Pd/NDC
Fig. 9 Reaction time profiles for a) Pd/NDC 900 and b) Pd/AC
catalysts.
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maximum phenol conversion of 100% with a 99% selectivity
to cyclohexanone was obtained over Pd/NDC 900 after 8 h. In
contrast, the maximum phenol conversion of 84% was
achieved after 3 h over the Pd/AC catalyst, with a 98%
selectivity to cyclohexanone. After this point, the conversion
of phenol and the selectivity to cyclohexanone does not
change anymore between 3 h and 5 h for the Pd/AC catalyst.
Comparison of powder XRD patterns of a fresh and reused
Pd/AC catalyst revealed the presence of PdO and Pd in the
fresh catalysts and the absence of PdO (002/101; reflection at
34.1°) and presence of newly formed shoulders at ca. 38.6°,
44.9°, 65.4° and 79.0° in the spend catalyst. These shoulders
6
4
correspond to palladium hydride (PdH
x
)
(which is stable
under ambient conditions) formed in situ during the
6
5
This might be explained by a preferentially adsorbed H O on
hydrogenation reaction.
Pd interactions with hydrogen
2
the hydrophilic Pd/AC catalyst, while inhibiting adsorption of
situated in octahedral sites of the fcc Pd lattice are already
6
1
18,19
49,66
phenol or by a catalyst deactivation after t > 3 h.
At
well known.
These interactions depend on the exposed
6
7
reaction times >5 h, the selectivity to cyclohexanone begins
to decrease (from 98 to 92%), with a corresponding increase
in selectivity to cyclohexanol (i.e. the undesired
hydrogenation product), indicating further hydrogenation of
Pd surface of the catalyst, reaction temperature, amount of
hydrogen and may influence the catalytic activity. The
6
8
formation of the PdH phase and the disappearance of the
x
PdO on the Pd/AC was observed already after the 1st reaction
cycle, with the PdH phase formation increasing over
x
6
2,63
cyclohexanone to cyclohexanol.
In contrast, selectivity of
Pd/NDC 900 towards cyclohexanone after longer reaction
times stayed stable (Fig. 9a).
repetitive reaction cycles. Pd/AC showed after the 2nd
reaction cycle the highest phenol conversion (Fig. 10a), which
was found to continuously decrease thereafter. ICP-OES
analysis revealed decreased Pd amount after the 5th reaction
cycle from 4.2 to 3.8 wt%, and after 11 cycles to 3.6 wt%,
suggesting that lower Pd/AC activity is caused due to Pd
leaching (Table 6).
Conversely, the XRD pattern of the spent Pd/NDC 900
catalyst (Fig. 11, 2b) showed significantly smaller shoulders
attributed to the PdHx phase, indicating lower amounts of
As next step, the stability and reusability of the Pd/NDC
900 and the Pd/AC catalysts were also investigated (Fig. 10).
The reusability test was performed based on 11 repeated
hydrogenation reaction cycles (i.e. under identical reaction
conditions as shown in Table 5). Catalysts were separated
from the reaction mixture via hot filtration after each run,
followed by thorough washing with water and drying in a
vacuum oven at 80 °C for 12 h, prior to the next run. The Pd/
AC activity was found to be slightly increased after the 1st
cycle but then it continuously decreased with every run
x
PdH formed in the Pd/NDC 900 catalyst. In addition, TEM
images of Pd/NDC 900 show minor changes in the particle
size distribution (Fig. 12b).
(
Fig. 10a). Conversely the catalytic activity of the Pd/NDC 900
catalyst remained stable throughout the repeated runs
Fig. 10b). After 11 reaction repeats, Pd/NDC 900 and Pd/AC
XPS analysis of spent Pd/AC after 11 recycles revealed a
0
δ+
(
significant increase in the Pd /Pd
ratio (Fig. 13a),
were characterised by ICP-OES, XRD and XPS. ICP-OES
analysis showed a decrease in Pd loading for the Pd/AC
catalyst over 11 reaction cycles (i.e. a reduction from 4.2 to
supporting the XRD findings that indicate the absence of a
0
δ+
separate PdO phase for this catalyst (Fig. 11). The Pd /Pd
0
ratio for the Pd/AC increased from 1.1 to 4.8 whereas the Pd /
δ+
3.6 wt%) (Table 6). In contrast, Pd/NDC 900 did not show any
Pd ratio of Pd/NDC 900 stayed unchanged (Fig. 13b and
0
significant loss in Pd loading.
ESI† Tables S7 and S8). The Pd content in the Pd/AC catalyst
δ+
0
increased due to a reduction of Pd species to Pd by
hydrogen during the reaction. In addition, the XPS survey
scan confirms a lower total Pd amount on the surface of the
spent Pd/AC catalyst after 11 recycles in comparison to the
fresh catalyst, which is in line with ICP-OES analysis
(
Table 6). XPS further confirmed PdH
x
formation in the spent
δ+
0
catalysts (e.g. formed along with the reduction of Pd to Pd
during the hydrogenation reaction). A positive shift in BE
of 0.20 eV for Pd 3d5/2 and Pd 3d3/2 AC/AC catalyst and a
very small shift (<0.1 eV) AC/NDC 900 after 11 recycles is
observed (i.e. in comparison to their fresh counterparts). This
6
9
0
0
x
was attributed to the formation of the PdH phase. Previous
studies have described such a chemical shift in Pd 3d core
levels due to charge transfer in both directions between Pd
7
0,71
and H.
Despite a decreased Pd loading in the Pd/AC catalyst, its
phenol conversion after 11 reaction cycles is still slightly
higher than of Pd/NDC 900, namely 67% phenol conversion
for Pd/AC with 98.5% selectivity to cyclohexanone and
Fig. 10 a) Pd/AC, b) Pd/NDC 900 catalyst activity after 11 repeated
reaction cycles and c) Pd/AC catalyst activity after 15 repeated
reaction cycles.
66% phenol conversion for Pd/NDC 900 with 98.5%
selectivity to cyclohexanone. XPS analysis revealed an
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Table 6 Pd loading after 5th and 11th reaction cycle, determined by ICP-OES analysis
Catalyst
Pd [wt%]ICP fresh
Pd [wt%]ICP spent 5th cycles
Pd [wt%]ICP spent 11th cycles
Pd/AC
Pd/NDC 900
4.2
4.4
3.8
4.3
3.6
4.3
0
δ+
increased Pd /Pd ratio for the Pd/AC after 11 cycles,
which was discussed as an important factor for improving
the catalytic activity of Pd catalysts for phenol hydrogenation.
Therefore, it is assumed that the spent Pd/AC catalyst after
catalytic performance and indeed stability, attributed to
synergetic effects between the Pd NPs and the NDC support,
leading to strong anchoring of the metal nanoparticles.
1
1
reaction cycles, generated slightly higher phenol
Conclusion
conversions than the Pd/NDC 900 catalyst despite its loss in
Pd loading due to the significantly increased Pd /Pd ratio in
combination with its very high Pd dispersion.
For the spent Pd/AC catalyst an additional 4 reaction
cycles (i.e. after the first 11 reaction cycles) were performed to
investigate the changes in catalyst composition and structure
0
δ+
In conclusion the synthesis of N-doped carbon xerogel
supported hydrogenation (Pd) catalysts has been discussed in
the context of directing support material chemistry for
improved
performance
and
stability.
Using
aqueous-phase
the
demonstrative,
biorefinery-related,
(
e.g. decreased Pd amount, absence of PdO and presence of
PdH phase). It was observed that the catalytic activity between
the 11th and 15th cycles continued to decrease (from 67% to
7% conversion of phenol with decreasing cyclohexanone
hydrogenation of phenol as an example, the effect of altering
the NDC support properties (e.g. N-bonding motifs) on
catalytic performance were investigated. Based on the initial
hydrothermal carbonisation of biomass-derived precursors,
NDC xerogels were subsequently thermally carbonised at
x
5
selectivity from 98.5% to 98.2%), demonstrating further
deactivation and loss in activity (e.g. due to continued Pd
loss). These results suggest that, in contrast to the NDC 900
support, the AC support does not sufficiently stabilise the Pd
NPs for use under the applied reaction conditions. In
contrast, the Pd/NDC 900 catalyst showed a very good
c
increasing temperature (T ) and Pd NPs were deposited via
incipient wetness impregnation. Through this approach the
physicochemical properties of the NDC support materials
could be directed, in turn influencing the (e.g. electronic,
geometric) properties of the deposited Pd NPs.
NDC-supported Pd catalysts provided high catalytic
performance in the aqueous phase hydrogenation of phenol
with excellent selectivity to cyclohexanone (i.e. an important
precursor for nylon synthesis). Improved catalytic
performance and indeed stability was found and could be
attributed to N-containing functional groups (e.g. pyridinic)
on these xerogel supports acting as strong anchoring points
for Pd NPs. At T
c
≥ 750 °C, it is proposed that N–Pd
Fig. 11 XRD patterns for 1a) fresh Pd/AC, 1b) spent Pd/AC after 1st
reaction cycle, 1c) spent Pd/AC after 5th reaction cycle and 1d) spent
Pd/AC after 11th reaction cycle. 2a) fresh and 2b) spent Pd/NDC 900
Fig. 12 TEM images and Pd amount of spent a) Pd/AC and b) Pd/NDC
after 11th reaction cycle. Orange lines represent PdH
x
reflections.
900 catalyst after 11 reaction cycles.
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Material and catalyst characterisation
The prepared supports and catalyst were characterised
through N2 adsorption, H O adsorption, pulsed H2
2
chemisorption, SEM, TEM, XPS, ICP-OES and XRD in Bragg–
Brentano and capillary geometry as described in the ESI†
document in more detail. XRD patterns obtained in
transmission in capillary geometry are presented in Fig. S7†
as well as Rietveld refinement pattern fittings (Fig. S8†).
Support synthesis
NDC materials were synthesised based on a previously
3
0
reported synthesis with glucose, ovalbumin and de-ionised
water, however using a different mass ratio of 1 : 0.15 : 4.5 (w/
w/w). Hydrothermal carbonisation was performed at 180 °C
for 5.5 h in a pre-heated laboratory oven. HTC experiments
were performed in a Teflon lined laboratory autoclave, with
the precursor solutions placed in a borosilicate glass insert
Fig. 13 High-resolution XPS scans of Pd 3d region of fresh catalyst in
comparison to the spent catalyst after 11 reaction cycles for a) Pd/AC
and b) Pd/NDC 900 catalyst.
(
30 mL volume). After 5.5 h, the autoclave was removed and
cooled in a water bath to room temperature. The recovered
gel was washed extensively with water (3 × 100 mL) and
ethanol (3 × 100 mL) until washings were clear. Drying
overnight (vacuum oven at 80 °C for 12 h) yielded the NDC
carbogels.
interactions become increasingly beneficial with regard to
catalytic performance and stability under the applied
conditions. Pd catalysts supported on NDCs where T ≥ 750
c
°C showed higher phenol conversion than Pd/NDC 350 and
Pd/NDC 550 or “nitrogen-free” Pd/HTC 550.
The nitrogen-free HTC baseline support was prepared in a
similar way as the NDC, based on HTC approach using the
With regard to reusability, the Pd/NDC 900 catalyst
presented excellent activity and selectivity for phenol
hydrogenation to cyclohexanone when compared with a
commercial Pd/AC catalyst (over 11 repeated reaction cycles).
The Pd/NDC 900 catalyst maintained its activity and
selectivity with an unchanged NP size distribution and Pd
loading. In contrast, the Pd/AC catalyst was observed to
continuously lose activity (as a result of Pd leaching –
confirmed by ICP-OES and XPS analysis) with increasing
recycle number.
Based on the interactions between the Pd and
N-containing support functionalities, heterogeneous catalysts
with still higher performance may be designed and
synthesized based on the controlled integration of nitrogen
into these carbon support structures. The presented work
and indeed the approach to catalyst synthesis will be of
interest to a wide range of applications in biorefinery-related
catalysis, particularly where aqueous-phase friendly materials
are needed in context with good stability.
3
3
previously reported procedure,
borate and de-ionised water in a mass ratio of 1 : 0.015 : 2.3
w/w/w). HTC was performed at 180 °C for 8 h in a pre-heated
using glucose, sodium
(
laboratory oven. HTC experiments were performed in a
Teflon lined laboratory autoclave, with the precursor
solutions placed in a borosilicate glass insert (30 mL volume).
After 8 h, the autoclave was removed and cooled in a water
bath to room temperature. The recovered gel was washed
extensively with water (3 × 100 mL) and ethanol (3 × 100 mL)
(
e.g. until washings were clear). Drying overnight (vacuum
oven at 80 °C for 12 h) yielded the base HTC carbogels.
The secondary thermal carbonisation step of NDC and
HTC carbogels was performed in a tube furnace (quartz tube;
−
1
2
D = 5 cm/L = 90 cm) under flowing N (100 mL min , 5.0 gas
−
1
purity). The heating rate was 7 K min to the desired
temperature (350, 550, 750, 900 or 1000 °C), followed by an
isothermal period of 4 h. HTC carbogel was thermally treated
only at 550 °C. Samples were carbonised in ceramic
carbonisation oven boats.
Experimental
Materials
Catalyst synthesis
Ovalbumin grade V, 98% purity, tetraaminepalladiumIJII)
nitrate (Pd/(NO ) × 4NH ) solution 10 wt% in H O, 99.99%
Impregnation of the prepared NDC and HTC supports was
performed using an incipient wetness impregnation
approach with water as solvent. 10 wt% PdIJNH ) IJNO )
3 2
solution was used as the Pd NP precursor. The target loading
of Pd on support material was 5 wt%. Adsorption capacity of
1.0 g support material, which was carbonised at various
3
2
3
2
purity and sodium borate (Na B O ), ≥99% purity were
2
4
7
3 4
purchased from Sigma Aldrich, Germany. The commercial 5
wt% Pd/AC catalyst (dry basis) was provided by Sigma Aldrich
(matrix activated carbon, wet support, Degussa type E1003 U/
W). D-Glucose (C
6
H
12
O
6
) ≥ 99.5 purity, ethanol ≥99.8% and
2
temperatures, was before impregnation tested with H O. After
phenol (C H OH) ≥ 99.5% were purchased from Carl Roth
the H O adsorption capacity test, 1.0 g of the support was
6
5
2
(Germany).
dried in a vacuum oven for 12 h at 80 °C. As next step, 1.0 g
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of the support material was impregnated with appropriate
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concentration of PdIJNH ) IJNO ) dissolved in an a volume of
3
4
3 2
water equal to the pore volume of the support material
chosen. Following impregnation, the recovered materials
were dried under vacuum at 40 °C for 2 h, then 60 °C for 2 h
and finally at 90 °C for the last 2 h. Prior to testing, Pd was
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1
reduced under flowing H (100 mL min ) at a ramp rate of 4
2
−
1
K min to 250 °C, followed by an isothermal period of 2 h.
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J.-F. Zhu, G.-H. Tao, H.-Y. Liu, L. He, Q.-H. Sun and H.-C.
Catalyst testing
Liu, Green Chem., 2014, 16, 2664.
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flushing with H (2 bar) three times, the reactor was charged
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2
2
with stirring at 280 rpm. The reactor was cooled to a room
temperature in cold water after completed reaction. The
catalyst was recovered by filtration with recovered filtrate
analysed by a gas chromatograph (Agilent 7890) equipped
with an FID detector and a capillary column HP-5 (30 m ×
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Recovery and reuse of catalyst
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After each reaction recycle, Pd/NDC 900 and Pd/AC catalysts
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filtration followed by thorough washing with deionised water.
The catalysts were then dried in a vacuum oven (80 °C for 12
h), followed by subsequent further testing in the next
hydrogenation reaction.
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1
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Conflicts of interest
There are no conflicts to declare.
2
2 R. Arrigo, M. E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L. L.
Sun, K. E. Hermann, M. Friedrich, P. Kast, M. Hävecker, A.
Knop-Gericke and R. Schlögl, ACS Catal., 2015, 5, 2740.
3 S. Mao, C. Wang and Y. Wang, J. Catal., 2019, 375, 456.
4 W. Shi, B. Zhang, Y. Lin, Q. Wang, Q. Zhang and D. S. Su,
ACS Catal., 2016, 6, 7844.
Acknowledgements
2
2
This work was supported by an “Attract” award for RJW (as
financed by the Fraunhofer Society and Fraunhofer Institute
for Solar Energy Systems ISE) and by the Fraunhofer Internal
Programs under Grant No. 035-600564. AF thanks the BMBF
for generous funding (FKZ 01FP13033F, FKZ 03X5524
2
5 X. Ning, Y. Li, B. Dong, H. Wang, H. Yu, F. Peng and Y.
Yang, J. Catal., 2017, 348, 100.
2
2
6 I. C. Gerber and P. Serp, Chem. Rev., 2020, 120, 1250.
7 W. Dong, P. Chen, W. Xia, P. Weide, H. Ruland, A. Kostka, K.
Köhler and M. Muhler, ChemCatChem, 2016, 8, 1269.
8 M.-M. Titirici, R. J. White, C. Falco and M. Sevilla, Energy
Environ. Sci., 2012, 5, 6796.
(
EDELKAT), and FKZ 03SF0531D (DEKADE)). The authors
would like to thank A. Siegel for CHNS elemental analysis, A.
Becherer for SEM support, P. Hügenell for N and H
2
2
O
2
2
adsorption analysis, S. Fehr for pulsed H2 chemisorption
analysis and J. Schmidt for XPS analysis.
9 M.-M. Titirici, R. J. White, N. Brun, V. L. Budarin, D. S. Su, F.
del Monte, J. H. Clark and M. J. MacLachlan, Chem. Soc.
Rev., 2015, 44, 250.
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