A High-Throughput Composite Catalyst based on Nickel Carbon Cubes for the Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran

Article abstract of DOI:10.1002/cctc.201700506

A high-throughput composite catalyst is prepared from porous carbon with an unconventional nanocube morphology decorated with nickel nanoparticles. Owing to the advantageous properties of the designed carbon support, the composite combines a high surface area and a hierarchical pore structure with high functionality. Furthermore, the regularly shaped nanocubes allow for a good packing of a fixed-bed flow reactor, in which the internal transport pores cannot be blocked and stay open for efficient column performance. The composite is employed as a catalyst in the hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF), showing good catalytic performance and overcoming the conventional problem of column blocking.

Full text of DOI:10.1002/cctc.201700506

Accepted Article
Title: A high-throughput composite catalyst based on Nickel carbon
cubes for the hydrogenation of 5-hydroxymethylfurfural to 2,5-
dimethylfuran
Authors: Christian Mbaya Mani, Max Braun, Valerio Molinari, Markus
Antonietti, and Nina Fechler
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201700506
Link to VoR: http://dx.doi.org/10.1002/cctc.201700506
A Journal of
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ChemCatChem
10.1002/cctc.201700506
A high-throughput composite catalyst based on Nickel carbon cubes for the
hydrogenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran
Christian Mbaya Mani, Max Braun, Valerio Molinari, Markus Antonietti, Nina Fechler*
Max Planck Institute of Colloids and Interfaces, Colloid Chemistry, Research Campus Golm, Am Mühlenberg 1, 14476
Potsdam (Germany).
Abstract
A high-throughput composite catalyst is prepared from porous carbon with an unconventional nanocube
morphology decorated by nickel nanoparticles. Due to the advantageous properties of the designed
carbon support, the composite combines a high surface area and a hierarchical pore structure with high
functionality. Furthermore, the regularly shaped nanocubes allow for a good packing of a fixed-bed flow
reactor, while the internal transport pores cannot be blocked and stay open for efficient column
performance. The composite is employed as catalyst in the hydrogenation of 5-hydroxymethylfurfural
(HMF) to 2,5-dimethylfuran (DMF) showing good catalytic performance and overcoming the
conventional problem of column blocking.
Introduction
Creative examples have revealed the powerful trick to combine different materials into a composite
leading to unexpected synergistic properties of the final material.[1-3] This is especially helpful for the
synthesis of catalysts such as metal species immobilized on a porous support as it allows for the
unification of very different properties in one composite.[4] For example, compared to the
corresponding bulk metal the composite possesses better accessible sites and increased diffusion of the
reactants through the pores, hence leading to improved catalytic performance.[5-8] Furthermore, the
composite synthesis enables for the homogeneous dispersion and stabilization of the supported metal
compound (e.g. metal, metal carbide, metal oxide) while ensuring a large active surface area.[9-11]
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ChemCatChem
10.1002/cctc.201700506
Among the various types of support materials, carbons with a hierarchical pore structure have attracted
much attention in heterogeneous catalysis.[6,12-14] This is due to their widely tunable properties,
structuring possibilities and availability.[5, 15-17] With regard to catalytic applications, three-
dimensional (3D) hierarchical pore structures with interconnected micro-, meso- and macropores are
crucial to overcome transport limitations and to ensure good heat and mass transfer throughout the
pore system. This impacts not only the activity but also the selectivity and stability of the composite
catalyst.[18]
With regard to the catalytic setup, so-called fixed-bed reactors are very advantageous as they allow for
reactions in a continuous flow which in turn is beneficial in terms of low operating costs and high
conversion rates per unit mass of catalyst. Nonetheless, most of the current publications and state of the
art research still involve batch mode processes and hence are not appropriate for usage in high
throughput set-ups. The reason for the dominance of batch reactors is the difficulty to fill columns
effectively, and especially column blocking by the debris of catalyst particles is a regular event. Good
packing with high plate numbers is challenging and requires careful design of the packing material. In
industry and on the larger scale, this is commonly approached by the use of catalysts pressed or
2
-1
extruded as pellets. Yet, these catalytic systems often exhibit low surface areas (<150 m g ) which result
2
-1
in lower catalytic activity when compared for instance to lab scale catalysts (>1000 m g ).[19-21]
Moreover, pellets are often brittle and fail over time due to mechanical point stress due to packing,
leading to increased pressure drop.[22] As bead packing and mechanical strength are important
properties for industrial catalysts,[23-26] engineers are tackling this problem by the development of
better palletization techniques, i.e. optimized size and shape, and more resistant materials.[27-30] As
discussed in various reports for inorganic materials, a regularly structured catalyst allows for an
improved packing in fixed-bed reactors and hence a reduced pressure drop.[31-33] For the lab scale and
the involved much smaller reactive columns, a careful design of the catalyst morphology at the
micrometer scale is mandatory. Thereby, the main advantage of pelletized catalyst such as negligible
pressure drop can be combined with the benefits of a powder catalyst, i.e. high surface area, and low
mechanical stress. However, conventional synthesis strategies for hierarchically porous carbon
composite materials are laborious and involve templating, post-synthetic activation steps, and often
subsequent metal loading.
Recently, we reported a synthesis strategy for the preparation of hierarchically porous carbon materials
derived from the crystals of the coordination complex of zinc (Zn) ions and squaric acid (SA).[34] These
primary crystals determine the structural properties of the final carbon already before the conversion to
carbon providing a porous carbon material with a unique 3D-cube morphology, an inner hierarchical
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ChemCatChem
10.1002/cctc.201700506
pore system and a high specific surface area. Because of these structural features, such carbon
microparticles offer great potential for efficient fixed-bed reactor packing and good catalytic
performance.
In this work, we extended this system towards a composite catalyst composed of a hierarchically porous
carbon support and nickel nanoparticles (Ni-NPs) and show that indeed that pressure drop issues and the
plate number in such a packed bed reactor are favorable for catalysis. The catalytic flow-system is then
exemplified in a reaction of high importance in biorefinery, i.e. the hydrogenation of
5
-hydroxymethylfurfural (HMF) to yield 2,5-dimethylfuran (DMF), which is considered as one of the most
promising biofuels and platform chemical of the near future.[35-36]
Results and Discussion
Squaric acid (SA) is a strong dibasic acid and is predominantly present as a dianion in aqueous solutions.
It contains four electron donating oxygen atoms which are beneficial in the design of 3D-coordination
structures and exhibits non-classical aromaticity of the building block. SA tends to form stable metal-
oxocarbon complexes when coordinated to metal ions.[37] Inspired by this, we recently synthesized
crystals with some unique 3D cube morphologies based on a squaric acid-zinc complex (SAZn).[34] Heat
treatment of SAZn at 900 °C in a nitrogen (N ) atmosphere and subsequent washing in an acid solution
2
yields a porous carbon (PC) material (SAZn_PC) which possesses a hierarchical pore structure under
retention of the cube morphology with inner cavities of fractal-like structure. For the composite
synthesis, this carbon was impregnated with an aqueous solution of Ni(NO ) 6(H O) by using an incipient
3
2
2
wetness impregnation method and subsequent reduction in H /Ar atmosphere.
2
In order to prove the structural retention of the composite after the composite synthesis step, the
morphology of the particles was studied by scanning electron microscopy (SEM, Figure 1). Here, it can be
seen that the 3D cube morphology of the initial porous carbon SAZn_PC was maintained even
throughout the Ni-impregnation process (Figure 1 b). This already hints to the good stability of the
carbon microparticles at elevated temperatures (475 °C) and throughout impregnation treatment.
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ChemCatChem
10.1002/cctc.201700506
Figure 1. SEM images of (a) SAZn_PC and (b)Ni@SAZn_PC.
Insights into the formation of the 3D cubes were gained by crystallographic analysis of the SAZn complex
using X-ray diffraction (XRD) (Figure 2). In accordance with crystallographic data from the International
Centre for Diffraction Data (ICDD) (Figure 2 d), the crystalline SAZn units are composed of
Zn(H O) (C O ).[38] Here, the complex is constituted of Zn atoms coordinated by four squarate units and
2
2
4
4
two water molecules in trans-position to each other in a slightly distorted octahedron. The squarate units
are incorporated in a bridging bis-(µ-1,3) coordination motif leading to Zn-squarate chains linked by
strong intermolecular hydrogen bonds which form a 3D Zn-squarate framework.[39-40] The differences
in the relative intensities of the XRD patterns of SAZn compared to the corresponding database is due to
differences in the preferential growth of the crystal.
The diffractogram of SAZn_PC exhibits a weak and broad reflection at 2θ=25° which can be attributed to
the (002) plane of the carbon. This is characteristic for a locally disordered, graphitic carbon and hence
confirms that the crystals were successfully converted to the corresponding carbon material. After the Ni
decoration, XRD of the catalyst Ni@SAZn_PC reveals reflections at 2θ=44.5° and 52°, which are assigned
0
to the (111) and (200) crystalline planes of Ni , respectively.[41] No reflection characteristic of oxides
could be observed from the diffractograms, which confirms the absence of crystalline nickel oxide
species and the successful reduction of Ni(NO ) 6(H O) to metallic Ni. The average crystalline domain
3
2
2
size of the Ni-NPs was calculated from the (111) reflection using the Debye-Scherrer formula whereby a
crystallite size of 11.5 nm was determined.[42]
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ChemCatChem
10.1002/cctc.201700506
Figure 2. XRD patterns of (a) the composite catalyst Ni@SAZn_PC, (b) the carbon support SAZn_PC, (c)
the complex SAZn and (d) the database pattern for SAZn (PDF 04-017-7412).
Elemental Analysis (EA) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) of the
resulting composite catalyst Ni@SAZn_PC reveal a Ni-loading of 15 wt. % (Table S1 in SI). In spite of a
slight decrease of the relative carbon content, this further supports the successful composite formation.
The residual zinc content in the final carbon is 0.1 wt% and together with the applied conditions, this is
negligible with regard to potential influences on the composite catalyst.
In order to obtain more insights on the nanostructure of the composite, Transmission Electron
Microscopy (TEM) images of Ni@SAZn_PC are presented in Figure 3. On the one hand, the images reveal
the carbon support to be constituted of a very light carbon structure including homogeneously
distributed, larger pores. This regular network is attributed to the synthesis in which the crystalline
precursor induces the pre-alignment into a well-distributed pore structure. On the other hand, TEM
images also show the homogeneous loading of the Ni-NPs onto the carbon support with an average size
of 12 nm. This is consistent with the determined nickel crystallite size obtained from XRD pattern and
elemental mapping images revealing the cubes to be constituted by carbon and nickel nanoparticles
(
Figure 3 c, d).
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ChemCatChem
10.1002/cctc.201700506
Figure 3. TEM images of (a,b) the composite catalyst Ni@SAZn_PC and (c,d) EDX elemental mapping of
Ni@SAZn_PC with C and Ni.
Nitrogen sorption experiments were performed in order to determine the influence of nickel
impregnation on the porosity of the final composite, (Figure 4, Table S2 (SI)).
2
-1
Here, the final composite catalyst Ni@SAZn_PC still reveals a high surface area of 1291 m g compared
2
-1
3
-1
to the initial carbon material (1423 m g ) and a total pore volume of 1.55 cm g . The slightly decreased
apparent surface area together with the overall very similar isotherm shape show that almost all original
carbon pores are still accessible. The decreased surface area is therefore attributed to the higher
averaged material density upon nickel decoration. This is further supported by the comparison of the
porosity parameters (Table S2 in SI) which reveal that the hierarchical pore structure of the carbon
support material originally comprised of micro- and mesopores is maintained even after the Ni-loading.
Eventually, the pores of the carbon support and the Ni-NP sites are still accessible for the reactants.
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ChemCatChem
10.1002/cctc.201700506
Figure 4. N -sorption isotherms of Ni@SAZn_PC (black solid line) and SAZn_PC (grey dashed line).
2
As the composite is a promising candidate for catalytic applications, Ni@SAZn_PC was exemplarily
applied as catalyst in the selective hydrogenation of HMF for the production of DMF. For this, the
composite catalyst was loaded in a lab-scale hydrogenation flow reactor called H-Cube Pro™ and the
pressure drop of the system was investigated at various parameters (Table 1).
Table 1: Pressure studies on Ni@SAZn_PC (159 mg, 15 wt% Ni) at different conditions in the flow system
without using hydrogen; Pinlet: pressure at the entrance of the column; P : pressure adjusted for the
set
column; all measurements were performed using ethanol.
-
1
T [°C]
Pset [bar]
F [ml min ]
0.50
Pinlet [bar]
6-7
6
0
6
6
6
6
6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
00
20
40
50
50
50
50
50
50
50
50
50
50
50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
6-7
6
6-7
6-7
6
6-7
10
14
18
30
6
6
6
6
6
10-11
14-15
18-19
30
6
6-7
6-7
7
9-10
0.50
0.25
0.50
0.75
1.00
2.00
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ChemCatChem
10.1002/cctc.201700506
Here, the monitoring of the back pressure applying different temperatures (60 °C to 150 °C), pressures
-
1
(
6 bar to 30 bar) and flow rates (0.25 to 2.0 ml min ) revealed negligible pressure drops at all conditions
0 to 1 bar). Compared to pressure drops of about 100 bar which are often observed with the
(
H-Cube Pro™ using fine powder catalysts in such a flow system, the highest back pressure (3-4 bar) which
-
1
was measured with 2 mL min flow rate at 150 °C and 6 bar is negligible.[43] This promising result is
attributed to the combination of a high pore volume and the presence of also larger pores of the
catalyst, but also its homogeneous micrometric morphology. Besides the backpressure, the theoretical
plate model gives further information on the processes in a packed column. Following the principles of
column chromatography, the number of theoretical plates N in a column is a measure of the column
efficiency which, among others, is a function of the materials properties, of the column packing, and of
the column dimensions.[44-46] Here, high numbers of theoretical plates are associated with high column
efficiency. Exemplarily, N was determined for the column packed with the Ni@SAZn_PC based on the
retention time of toluene (equation 1), where t is the retention time of toluene and w is the peak
R
H
width at half maximum.[46]
2
)
푡푅
푁 = 5.545 × (
(1)
푤퐻
The composite material revealed a high N value of 751 compared to the N value of 9 for a commercial
carbon black under the same conditions. For the composite, a narrow Gaussian peak and a retention
time of approximately 2 min were observed with toluene (Figure.5). In contrast, the column packed with
commercial carbon black revealed a broad and asymmetrical peak with a retention time of 36 sec
(
Figure 5).
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ChemCatChem
10.1002/cctc.201700506
Figure 5. Chromatogram of toluene in a column packed with carbon black (dashed line) and the
composite Ni@SAZn_PC (solid line).
These initial tests support the hypothesis that the herein presented composite cubes are superior to
amorphous powders when used as a column material in a flow reactor.
The catalytic column was then employed for the conversion of HMF to DMF (Table 2 and Figure 6) as an
example for an important reaction in biorefinery, i.e. the upgrade of carbohydrates to fuels (DMF) and
platform chemicals. The deposited nickel nanoparticles are an excellent candidate for hydrogenation
reactions due to its abundance and low cost when compared to noble metals such as Pt, Pd, Ru etc.
Table 2: Overview of applied parameters and results for the conversion of HMF (0.05 M in Ethanol) to
DMF using Ni@SAZn_PC (159 mg, 0.4 mmol Ni) as catalyst. The pressure of the system was kept at 6 bar
(Δp [bar] back pressure).
HMF
Ni-CUBES
Entry
Productivity
-
1
X [%]
YDMF [%]
SDMF[%]
Δp [bar]
-1
-1
F [mL min ]
[
mmolDMF h gcat ]
1
2
3
4
0.25
0.5
100
79
41
68
50
33
41
86
89
94
0
1.9
6.3
0-1
0-1
1
0.75
1.0
56
7.08
6.14
35
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ChemCatChem
10.1002/cctc.201700506
Figure 6: Dependency of conversion over residence time at 150 °C and 6 bar converting HMF to DMF
over 159 mg Ni@SAZn_PC.
Activity and hence conversion of substrates always depend on the residence time when using a flow
system. With the nickel based catalyst (Ni@SAZn_PC) this dependency is shown for 150 °C in Figure 6,
where the reaction is so fast that it complies with our flow set-up (1/e conversion below 2 min). Longer
residence times come with increasing conversion. Practically full conversion of HMF was measured at a
-
1
flow rate of 0.25 mL min , corresponding to a contact time with the catalytic system of 3.2 minutes.
-
1
Additionally yield and selectivity are given. With higher flow rates between 0.5 and 1.0 mL min the
reaction reached around 90 % selectivity towards DMF. With an increased flow, the conversion dropped
to 35 % due to a reduced residence time in the column, whereas the selectivity stayed high and no
significant back pressure was observed. The reaction looks like a typical first order kinetics, i.e. the
reaction runs rather ideally. Besides the conversion, the productivity is another figure of merit and our
-
1
-1
-1
setup showed a maximum of 7.08 mmol gcat h at around 0.75 mL min , calculated on the base of the
full mass of the catalyst, i.e. the weight of the carbon as well as of the employed nickel. In order to
compare the results of the presented catalyst and to classify the results in a broader context, the
material was listed against different other catalysts from literature (Table 3).
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ChemCatChem
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Table 3: Overview and comparison of hydrogenolysis of HMF to DMF with different catalysts.
Productivity
-1
[mmolDMF h gcat ]
H2
Loading
T
[°C]
conv
[%]
Time
[min]
-1
Entry
Catalyst
Mode
Ref
[
mg]
[
bar]
1
2
PtCo@HCS
Pd/C/Zn
50
10
8
160
150
100
120
8h
batch
batch
19.20
8.40
[47]
[48]
50
˃99
Ni@SAZn_
PC
this
work
3
159
6
150
56
~ 1
conti
7.08
Ni-W C/AC
4
5
6
120
155
100
40
40
10
180
180
150
100
100
˃99
180
180
24 h
batch
batch
batch
2.50
0.97
0.60
[49]
[49]
[50]
2
Ni/AC
Ru/Co O4
3
Most research on the production of DMF published until now presents only data of batch reactions. To
make our continuous system comparable to the state of the art, the productivity was calculated referring
the product to reaction time and mass of catalyst. Noble metals such as platinum-cobalt or palladium
show naturally the highest productivity due to the high reactivity of these expensive noble metals.
Notably, in spite of using the non-precious nickel in this work, Ni@SAZn_PC reveals a comparable
performance even to Pd-based catalysts. Furthermore, the composite catalyst exhibited the highest
-
1
-1
productivity among other nickel based catalysts (7.08 vs. maximum 2.5 mmol h g ) even when more
cat
strict reaction conditions are used (longer reaction time, higher temperature and pressure).The
comparably low costs together with the low back pressure in the flow reactor make indeed the
presented nickel composite a promising candidate for continuous set-ups for biomass upgrade.
The composite Ni@SAZn_PC was used over 15 hours of reaction under variable conditions without
showing any significant decrease in catalytic activity, neither an increase of back pressure, what was the
main focus of this work. The porous cube structure guarantee a continuous flow over all this time what
-
1
was further approved with an additional 21 h experiment using 150 °C, 6 bar as well as 0.25 mL min and
-
1
a hydrogen flow of 30 mL min . After the first long time experiment, the reactor column was opened in
order to recover the catalyst and investigate the influence of the reaction on the cubes. Most part of the
cubes retained their initial cube morphology (Figure S4 in SI), while some “sharp fragments” were found.
The latter is very unusual for fractured carbon materials as they usually possess rather random
morphologies. Eventually, this hints to the partial fracture of the carbon cubes into pieces throughout
column packing which, however, retain main characteristics of the cube-shaped carbons thus keeping
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ChemCatChem
10.1002/cctc.201700506
the catalyst performance. Additionally, as catalysts are dynamic systems during catalysis, it is also
possible that partial rearrangement of the Ni particles affected the stability of the carbon. The energy
input in form of temperature, pressure or convection also leads to the growth of bigger Ni- particles on
the expense of the smaller ones first seen at the outer surface of the cubes and ascribed to Ostwald
ripening (Figure S5 in SI). The TEM image of the recovered composite catalyst shows also the inner parts
of the catalyst and reveals an increased average particles size of 22 nm in diameter (Figure 7b). Yet, in
contrast to the outer species, these nanoparticles are still homogeneously distributed and thus possess
good contact to the carbon support. However, the initial metallic Ni phase was observed also after the
reaction as detected by XRD (Figure 7c). A crystallite size of 25 nm in diameter was calculated from the
(111) reflection at 2θ=44.5° by applying the Debye-Scherrer equation which is in good agreement with
the particle size determined by TEM.
Nitrogen sorption analysis of the recovered composite catalyst revealed a decreased surface area and
2
-1
3
-1
2
-1
3 -1
total pore volume, accounting for 580 m g and 0.92 cm g compared to 1291 m g and 1.55 cm g of
the initial composite catalyst. However, the nitrogen sorption isotherm of the used catalyst confirms the
retention of meso- to macropores (Figure 7d). This speaks for partial pore blocking, most presumably
due to a side reaction, i.e. HMF condensation, which forms humins in all analyzed cases. [51]
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ChemCatChem
10.1002/cctc.201700506
Figure 7: (a) SEM image of the composite catalyst Ni@SAZn_PC recovered after 15 hours of reaction, (b)
corresponding TEM image (c) its diffractogram and (d) nitrogen sorption isotherm.
It is the micropore contribution which is blocked, presumably first by HMF adsorption, then by the
byproducts of the reaction, which is common for other flow systems.[52] This is supported by the
isotherm shapes where the closure of desorption branch is shifted to lower relative pressures for the
used composite compared to the initial material, as well as the reduction of the “micropore intercept” by
about 50 %. Interestingly, this loss of micropores is usually not coupled to a significant decrease of
performance of the catalyst, as fluid reactions usually do not rely on those pores (too small for fluid
transport, in contrary to gases). The nickel content quantified in the recovered catalyst after 15 hours did
not change significantly (14.5 wt% compared to the initial 15 wt%) which further explains the constant
activity of the catalyst and the weakness of nickel leaching even under the applied high temperature
conditions. Thereby, the nickel carbon cubes are able to efficiently serve for the hydrogenation of HMF
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ChemCatChem
10.1002/cctc.201700506
to DMF within a flow reactor setup even over extended operation times without significant pressure
drop and materials failure. This is attributed to the special cube morphology including a special internal
micro- as well as mesoporosity as well as a homogeneous nickel particle distribution.
Summary
A functional nickel-carbon composite with an unusual cube-shaped morphology and high porosity was
synthesized from rather cheap, readily available and safe precursor squaric acid followed by nickel
immobilization. These cubes possess a high specific surface area, a high total pore volume and a
hierarchically porous structure consisting of interconnected micro-, meso- and macropores.
The composite cubes were then tested for the hydrogenation of HMF to DMF within a continuous flow
setup. Due to the special morphology, the composite cubes revealed promising performance in terms of
packing and maximal productivity, which was found to be even comparable to precious metal catalysts.
Especially the negligible pressure drop and the hierarchical porosity allow for the performance of
hydrogenation reactions over longer time, indicating the usability in upgrading biomass derived
compounds in continuous flow reactors. Taking into account the variety of metal species that can be
deposited onto the fundamental carbon cube tectons, the herein presented synthesis approach offers a
promising route also for other catalytic reactions and opens the access to other flow conversion
reactions.
Acknowledgement
The authors wish to thank UNICAT, the German Excellence Cluster for Catalysis, as well as the Max
Planck-Fraunhofer cooperation Initiative “Dendrorefining” for financial support.
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10.1002/cctc.201700506
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