Simplifying levulinic acid conversion towards a sustainable biomass valorisation

Article abstract of DOI:10.1039/c9gc04306c

The demand for sustainable and robust catalysts for the valorisation of biomass is strictly related to the more and more pressing request to not only replace petroleum fuels with eco-friendly alternatives, but also to produce added value chemicals. In this context, the use of noble metals is not practically and economically sustainable and more abundant and stable alternatives are needed. In this contribution we have prepared and tested metal transition based catalysts (namely Ni₃N and Ni nanoparticles) for the hydroconversion of levulinic acid (LA) as a model reaction. LA is useful also to produce valuable N-substituted pyrrolidones. Nanoparticles were prepared via a greener synthesis, using urea and metal salts, and have an average diameter of ～30 nm (as ascertained by XRD and TEM studies). The main product of the levulinic acid hydroconversion was 1-ethyl-5-methylpyrrolidin-2-one. While this product was always the preferred one when Ni was used, Ni₃N favoured the formation of the main product only in a shorter reaction time (below 1 h) with very high selectivity (up to 55% conversion), while a secondary product was formed in a longer time. The stability of the catalysts was also tested. To the best of our knowledge this is the first time that such a reaction is tested using transition metals and metal nitrides, with very promising results.

Full text of DOI:10.1039/c9gc04306c

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ARTICLE
Simplifying Levulinic Acid Conversion Towards a Sustainable
Biomass Valorisation
Chiara Defilippia, Daily Rodríguez-Padrón b, Rafael Luque b,c and Cristina Giordanoa,*
Received 00th January 20xx,
Accepted 00th January 20xx
The demand for sustainable and robust catalysts for the valorisation of biomass is strictly related to the more and more
pressing request to replace petroleum fuels with eco-friendly alternative, but also to produce added value chemicals. In this
contest, the use of noble metals is not practically and economically sustainable and more abundant and stable alternative
are needed. In this contribution we have prepared and tested metal transition based catalysts (namely Ni3N and Ni
nanoparticles) for the hydroconversion of levulinic acid (LA) as model reaction. LA is useful also to produce valuable N-
substituted pyrrolidones. Nanoparticles were prepared via a greener synthesis, using urea and metal salts, and have an
average diameter of ~30 nm (as ascertained by XRD and TEM studies). The main product of the levulinic acid
hydroconversion was 1-ethyl-5-methylpyrrolidin-2-one.While this product was always the preferred one when Ni was used,
Ni3N favoured the formation of the main product only at shorter reaction time (below 1 h) with very high selectivity (up to
55% conversion), while a secondary product was formed for longer time. Stability of the catalysts was also tested. To the
best of our knowledge this is the first time that such reaction is tested using transition metals and metal nitrides, with very
promising results.
DOI: 10.1039/x0xx00000x
solvents and being mostly performed in batch systems,
furthermore these processes are also complex and multistep4.
The moving towards continuous flow reactors and more
Introduction
The continuous population growth and technological
development necessarily lead to a very high energy demand,
rendering of fundamental importance the replacement of fossil
fuels with renewable and cost-effective alternatives that are
also environmentally friendly and produce less waste.
Unfortunately, the production of energy via renewable sources
is still not competitive to petroleum fuels, both in terms of
energy production and cost1,2. Among renewable energies
sources, one of the most promising is surely biomass
valorisation from lignocellulose derivatives to produce the so-
called biofuels that hold the same potential of fossil fuels3 but
also, biomass valorisation can be useful to produce added value
chemicals in a sustainable way. One of the products of the
hydrolysis of lignocellulosic biomass is levulinic acid (LA) that
can be found, for example, in agrochemical product, polymers,
and pharmaceutical compounds, and useful also to produce
valuable N-substituted pyrrolidones.
sustainable and cost-effective catalytic materials, could
certainly paves the way in order to overcome the current
limitations for their direct application at large scale.
Continuous-flow reactors allow testing the catalyst for
prolonged periods of time, thus increasing the productivity and
reducing the energy consumption5. The most employed
catalysts for the conversion of LA to N-heterocycles are those
with excellent π-Lewis acidity like palladium, and transition-
metal complexes3,6, which lead to undesirable drawbacks such
as poor recyclability of catalysts, long reaction times, harsh
reaction conditions, toxic organic solvents, among others. In
particular, for levulinic acid transformation towards N-
substituted pyrrolidones by a catalytic reductive amination
process followed by subsequent cyclization, most of the
catalytic systems used so far are based on noble metal
nanoparticles, including ruthenium, platinum and gold7. The use
of novel catalysts for LA conversion has been also explored by
our research group, although the minimal use of noble metals
was always necessary8. Consequently, finding new synthetic
pathways for N-heterocycles through the development of
sustainable, noble-metal free and cost-effective heterogeneous
catalysts, together with the application of flow chemistry offers
advantages such as facile scale-up, energy safety and cost-
effectiveness.
The reactions currently used for the preparation of N-
heterocycles have several disadvantages, including long
reaction times, harsh reaction conditions, use of toxic organic
a. School of Biological and Chemical Sciences, Chemistry Department, Queen Mary
University of London, Mile End Road, London E1 4NS, United Kingdom.
b. Departmento de Quimica Organica, Facultad de Ciencias, Universidad de
Cordoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km
396, E14014 Cordoba, Spain.
An important factor to consider for the promotion of the
reaction is the preparation of nano-catalysts, due to the higher
surface area and reactive sites compared to bulk, enhancing the
activity of the catalyst. Last but not least, the cost and the
c. 3Peoples Friendship University of Russia (RUDN University), 6 Miklukho Maklaya
str., 117198, Moscow, Russia.
*c.giordano@qmul.ac.uk
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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stability of the catalysts under the demanding reaction Nickel acetate (1 g, Ni(ac)2, Sigma-Aldrich 98%) was mixed with
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DOI: 10.1039/C9GC04306C
conditions is also of key importance to make the process approximately 2.5 mL of ethanol, followed by the addition of
practically and economically feasible.
urea (CO(NH2)2, Sigma-Aldrich 99%) with a molar ratio of 3
Alternative catalysts based on cost-effective transition metals between the urea and Ni-salt as reported in previous work14.
are currently under investigation. For example, Ni-based After ultrasonicating, a homogeneous green solution was
compounds are benchmark catalysts for different organic obtained. Solutions were then thermally treated at 350, 450 or
reactions, such as hydrogenation9,10 and allylic alkylation 500˚C under nitrogen, leading respectively to a grey powder of
reactions11–13. As an alternative, metal nitrides (MN) are a crystalline Ni3N, a black powder of Ni3N+Ni0 or crystalline Ni0.
promising class of catalysts thanks also to their high chemical
and thermal stability, superior to their parent metal14. Within Catalytic experiment
our group, nanoparticles of Ni3N and Ni0, crystalline, phase pure
The catalytic performances were evaluated using the
and well-defined in size, were prepared via a modified sol-gel
conversion reaction of 0.2M levulinic acid (LA) in acetonitrile
route15 and their catalytic activity was investigated for
into N-heterocycles under continuous flow conditions. Around
hydrogenation of different compounds, including nitro-
150 mg of catalysts were packed in ThalesNano CatCarts. The
compounds and alkenes16, showing a catalytic activity for Ni3N
employed CatCarts have a 30 mm length, 24 mm catalyst bed
similar or superior to Ni0, alongside tuneable selectivity, never
length and an internal diameter of 4 mm.
The system was initially washed with methanol and acetonitrile
using a flow of 1 mL/min for 10 min for each solvent. The
solution of LA was then pumped through with reaction
observed for Ni0.
Within this contribution, hydroconversion of levulinic acid was
tested for the first time using nanosized Ni3N and Ni0 catalysts
as replacement for more expensive classic catalysts such as Ru
conditions of 100 °C, 50 bar and 0.3 mL/min, following the
or Pd. Nanoparticles were prepared via a greener synthesis,
procedure previously reported4. During the reaction, hydrogen
using urea as N-source for the preparation of the nitride phase.
was generated in situ by water electrolysis in the H-Cube
Urea also provides the necessary reducing atmosphere for the
equipment. The continuous flow process was followed up to
preparation of metallic Ni. In both cases, urea also plays the role
three hours, with sample collection every 10 min for the first
of stabilising agent during nanoparticles formation, as also
our and then each hour for further analysis by gas
previously observed17,18
,
urea hinders nanoparticles
chromatography (GC) in an Agilent 6890N gas chromatograph
(60 mL min−1 N2 carrier flow, 20 psi column top head pressure)
using a flame ionization detector (FID). The capillary column HP-
5 (30 m × 0.32 mm × 0.25 mm) was employed. In addition, the
collected liquid fractions were analysed by GC-MS using the
5977B High Efficiency Source (HES) GC/MSD (Gas
Chromatograph/Mass Selective Detector), in order to identify
the obtained products. To recover the catalyst, the system was
washed after the reaction with acetonitrile. The catalysts can be
easily separated from the reaction media by centrifugation and
decantation.
agglomerations and sintering by forming a protective C/N rich
matrix, which however decomposes before the end of the
reaction, avoiding any post-synthesis purification of the final
product. Notably, the synthesis takes place under nitrogen, a
much safer alternative to the more classically used ammonia.
On the other hand, the use of biomass as feedstock is becoming
essential in view of the limited reserves of unrenewable crude
oil. Levulinic acid from biomass valorisation could give rise to a
wide range of valuable compounds in a sustainable way. These
compounds include: -Valerolactone (GVL), which could be
employed as solvent, fuel additive, and as precursor for the
synthesis of jet fuel and polymer monomers; valeric acid, which
can be employed for the preparation of alkyl valerates as
potential biofuels; and N-heterocycles, which synthesis from
levulinic acid has been comparatively less explored. Thus, N-
heterocycles represent an attractive family of organic
compounds which are present in many of the most demanded
chemicals in modern society19. Their unique ability to be used
as bio-mimetic as well as active pharmacophores makes them
valuable compounds towards the design of a myriad of
chemicals such as dyes, agrochemicals, polymers and
pharmaceutical compounds20.
The products were investigated alongside conversion and
selectivity of the reaction by GC-MS using the 5977B High
Efficiency Source (HES) GC/MSD. Levulinic acid calibration was
carried out in the 2.00–50.00 gL-1 range (R2=0.98).
The conversion and selectivity were calculated from the
chromatograms by:
퐶표푛푣푒푟푠푖표푛(%) = [
] × 100
퐶퐼푛푖푡푖푎푙 ― 퐶퐹푖푛푎푙
Equation 1
Equation 2
퐶퐼푛푖푡푖푎푙
퐶푃푟표푑푢푐푡
푆푒푙푒푐푡푖푣푖푡푦 (%) = [
] × 100
퐶퐼푛푖푡푖푎푙 ― 퐶퐹푖푛푎푙
Where CInitial and CFinal are the concentrations of the reagents
before and after the reaction, respectively and the CProduct is the
concentration of the product, as determined by Gas
Chromatography (GC).
A schematic representation of the reaction is reported in Figure
1.
To the best of our knowledge this is the first time that levulinic
acid hydroconversion is tested using transition metals and
metal nitrides, with very promising results alongside a high
stability.
Experimental section
Catalyst preparation
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DOI: 10.1039/C9GC04306C
Figure 2. XRD patterns of the Ni-based catalysts prepared. The red and the black vertical
lines are the expected patterns for pure Ni3N (ICCD: 00-010-0280) and pure Ni0 (ICCD:
03-065-0380), respectively.
Nanoparticles nominal diameter was calculated for the pure
samples from XRD patterns by the Scherrer equation and
measured from the TEM images, obtaining a diameter of 20-30
nm for Ni0 (d=32 nm by XRD) and 10-30 nm for Ni3N (d=33 nm
by XRD).
As-prepared samples were tested for the reductive amination
of levulinic acid towards N heterocycles, as described in the
experimental part, and their performances were compared to
study the influence of the composition on the catalytic activity
of the materials. A blank measurement was run for comparison,
by performing the reaction under the same experimental
conditions, but replacing in this case the catalytic material by an
inert sample, namely SiC. It is worth to highlight that by using
the hydrogenation reactor, acetonitrile, used as solvent and
reactant at the same time, was in-situ reduced to the
corresponding amine, in this case ethylamine. The latest
compound acts as nucleophile, attacking, in a first step, the
carbonyl carbon of the ketone group and in a second step, the
carbonyl carbon of the acid group through a cyclization
reaction. Subsequently, the dehydration of the alcohol took
place, giving rise to the corresponding alkene, 1-ethyl-5-methyl-
1,3-dihydro-2H-pyrrol-2-one (C7H11NO, 125.08 g/mol), referred
as P1 in the manuscript. As well, under the employed
conditions, the aforementioned double bond could get easily
hydrogenated, obtaining 1-ethyl-5-methylpyrrolidin-2-one,
C7H13NO (127.10 g/mol), called P2, as the major product of the
reaction. GC-MS analysis confirmed the formation of both
products in all cases, as revealed from the molecular weights
and the fragmentation pattern, in each case. MS fragmentation
pattern for the two products are reported in the supporting
information (figure SI.1). No products were found in the blank
experiment. A schematic representation of the reaction is
reported in Figure 3.
Figure 1. Schematic reaction scheme for the hydroconversion of levulinic acid.
Techniques
X-Ray powder diffraction (XRD) measurements were performed
both on a Panalytical X'Pert Pro diffractometer (Cu K radiation,
λ = 0.154 nm, at 45kV and 40mA) equipped with a X'Celerator
detector.
To observe size and shape of the particles, transmission
electron microscopy (TEM) was performed on a JEOL 2010
operated at an acceleration voltage of 200 kV and with a FEI
Tecnai G2 system, equipped with a CCD (“charge coupling
device”) camera. Samples were ground and then suspended in
ethanol. One drop of this suspension was put on a holey-carbon
coated copper grid of 300 mesh and left to air to dry.
XPS experiments were accomplished in an ultrahigh vacuum
multipurpose surface analysis instrument SpecsTM. The
samples were evacuated overnight under vacuum (10-6 Torr)
and measurements were performed at room temperature using
a conventional X-ray source with a Phoibos 150-MCD energy
detector. XPS spectra were analysed employing the XPS CASA
software.
Results and discussion
Phase attribution on the prepared catalysts was made via XRD
analysis, which confirmed the formation of Ni3N (hexagonal,
ICCD: 00-010-0280) and Ni0 (cubic, ICCD: 03-065-0380). Figure 2
shows the experimental XRD patterns of Ni3N, Ni and Ni3N+Ni.
From figure 2, the XRD pattern of Ni3N shows diffraction peaks
at 39.4°, 41.8°, 44.7° and 58.6°, attributed to the (110), (002),
(111) and (112) crystallographic planes, respectively. In the
same figure, the peaks of metallic nickel (at 44.6° and 52°) are
also observed and attributed to the (111) and (200) planes of
the metal. In addition, XRD pattern of Ni3N+Ni sample (blue line,
Figure 2), showed a mixture of both crystalline phases.
Figure 3. Reaction scheme of the conversion of levulinic acid towards N-heterocycles,
using Ni-based catalyst.
GC-MS analysis was conducted on all fraction collected and
conversion and selectivity calculated as reported in the catalytic
experiment part. Plot of the conversion and selectivity over
time for the catalysts tested are reported in Figure 4 and values
are reported in Table 1. After 1h of reaction it was observed a
levulinic acid conversion of 20%, 8% and 36%, for the Ni0, Ni3N
and Ni3N+Ni materials, respectively. Even if from these results,
it could be assumed that Ni3N+Ni displayed the best catalytic
performance, such sample showed a considerable deactivation
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J. Name., 2013, 00, 1-3 | 3
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after 2h of reaction, with a decreased conversion of 8%. It can
be concluded from these data that Ni0 exhibited the best
balance between activity, selectivity and stability. In terms of
selectivity, the main product was P2 for all the samples, but in
the case of Ni3N the formation of P1 was higher compared to Ni.
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DOI: 10.1039/C9GC04306C
100
100
100
90
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
B
A
80
70
60
50
40
30
20
10
C
20
40
60
80
20
40
60
80
20
40
60
80
Time (min)
Time (min)
Time (min)
Figure 4. Catalytic performances of A) Ni0, B) Ni3N and C) Ni3N+Ni0 in the conversion of
LA to N heterocycles. Black line: conversion of LA, red line: selectivity towards P1 and
blue line: selectivity towards P2.
Figure 6. TEM images of the catalysts before A) Ni0, C) Ni3N and E) Ni3N+Ni0 and after B)
Ni0, D) Ni3N and F) Ni3N+Ni0 the reaction.
Table 1. Conversion of LA and selectivity over time for Ni0, Ni3N and Ni3N+Ni0 catalysts.
However, a closer look at the Ni3N nanoparticles after testing,
shows the presence of a shell around the nanoparticles not
observed in the as-prepared sample (figure 6D). The shell
formation has no influence on the catalytic activity of the nitride
phase (LA conversion is not changed) but has a drastic effect on
its selectivity.
The shell is amorphous and are not visible via XRD analysis but
its formation was confirmed by XPS analysis, where a set of new
bands are observed in the tested sample. XPS results on all
samples are reported in Table 2 and Figure 7.
Catalyst
Conversion LA
%
Selectivity P1 %
Selectivity P2 %
t=1h
t=2h
t=1h
t=2h
t=1h
t=2h
Ni0
Ni3N
Ni3N+Ni0
Blank
20
8
36
0
25
20
8
17
54
13
0
19
25
12
0
83
46
87
0
81
75
88
0
0
From figure 4, it is interesting to observe that the Ni3N favours
the formation of the P1 product at reaction time of 60 mins
(with a selectivity of 55%), while P2 is more selectively formed
after 60 mins. In any case, Ni3N displayed considerable
conversion just after 50 mins of reaction, suggesting that the
material requires certain activation time under the investigated
experimental conditions. For the pure Ni catalyst and for the
nitride/metal mixture, the preferred product is always the P2.
To check the stability of the catalysts at the reaction conditions,
XRD and TEM investigation were performed on the sample after
the reaction. A comparison of the XRD pattern of the catalysts
before and after testing is reported in Figure 5. For all samples,
no phase changes were observed, confirming the stability of the
prepared catalysts under the reaction conditions and thus their
reusability.
Table 2. XPS binding energies for the samples Ni0, Ni3N and Ni3N+Ni0 before and after the
reaction.
Sample
Ni 2p3/2 (eV)
Ni0/Ni2+
C 1s (eV)
O 1s
(eV)
N 1s
(eV)
Ni0
852.5/855.2
852.3/855.5
852.5/855.4
852.8/856.1
852.6/855.6
852.1/855.6
284.6
284.6
284.6
284.6
284.6
284.6
531.0
530.8
531.0
531.5
531.1
531.2
-
Ni3N
398.4
398.7
-
Ni3N+Ni0
Ni0-tested
Ni3N-tested
Ni3N+Ni0-
tested
399.0
398.7
A
C
B
30
40
50
60
70
30
40
50
60
70
30
40
50
60
70
2 
2 
2 
Figure 5. XRD comparison of A) Ni0, B) Ni3N and C) Ni3N+Ni0, the catalysts before (black
lines) and after (red lines) the reaction.
TEM images of the samples before and after the reaction are
reported in Figure 6 and also in this case no significant changes
can be observed. Remarkably, no further nanoparticles
agglomeration or sintering, and no morphological or
dimensional changes can be observed, further confirming the
stability of the prepared catalysts at the reaction conditions (50
bars and 100 °C).
4 | J. Name., 2012, 00, 1-3
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A
E
I
M
(rather than usually used toxic ammonia) as N-source for the
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nitride formation and to provide thDeOIn: e10c.e10s3s9a/rCy9GrCe0d4u3c0i6nCg
atmosphere. The nanoparticles were tested as prepared (no
need of post-synthesis purification or activation step were
needed), without the addition of a co-catalyst or any noble
metals. The prepared nanoparticles were shown to be
crystalline, with a mean diameter of around 30nm, both for Ni0
and Ni3N, and no crystalline side products were formed .
Catalysts performances were compared to study the influence
of the composition on the activity of the materials for the
reductive amination of levulinic acid towards N heterocycles.
The main product was in every case the 1-ethyl-5-
methylpyrrolidin-2-one, C7H13NO (P2), while the 1-ethyl-5-
methyl-1,3-dihydro-2H-pyrrol-2-one (P1) was found as
secondary product. Interesting, Ni3N favours the formation of
the P1 product at reaction time of 60 min with a selectivity of
55%, while P2 is more selectively formed after 60 min.
Characterization of the catalysts made before and after testing,
showed no significant difference in size, shape or composition
of the nanoparticles. To the best of our knowledge this is the
first time that such reaction is tested comparing the
performance of transition metals and metal nitrides, in absence
of any noble metals, showing very promising results.
B
F
J
N
C
D
K
O
G
P
H
L
Figure 7. XPS deconvoluted spectra for Ni0 sample in the C1s XPS region A: before and B:
after the reaction and in the Ni2p XPS region C: before and D: after the reaction. XPS
deconvoluted spectra for Ni3N sample in the C1s XPS region E: before and F: after the
reaction, in the Ni2p XPS region G: before and H: after the reaction, and in the N1s XPS
region I: before and J: after the reaction. XPS deconvoluted spectra for Ni3N+Ni0 sample
in the C1s XPS region K: before and L: after the reaction, in the Ni2p XPS region M: before
and N: after the reaction, and in the N1s XPS region O: before and P: after the reaction.
Conflicts of interest
There are no conflicts to declare.
XPS results of Ni0 sample revealed the presence of Ni and C,
while for the samples Ni3N and Ni3N+Ni, Ni, C and N were
detected. Carbon signal for the three sample was deconvoluted
in three peaks, respectively, associated with C-C/Cꢀ=ꢀC (ca. 284.6
eV), C=O (ca. 285.5 eV) and COO- (ca. 288.5 eV) bonds. No Acknowledgements
significant differences were found in the carbon signal of the
material after the reaction, in comparison with the fresh
Cristina Giordano and Chiara Defilippi acknowledge QMUL for
catalyst. Nonetheless, for the Ni0 sample, it was found an
increment of the carbon peak around 288.5 eV, most likely due
to the adsorption of levulinic acid in the surface of the material
after the reaction. Ni3N and Ni3N+Ni0 XPS spectra exhibited the
presence of a band around 399.5 eV, which confirmed the
presence of nitrogen in both samples. Even if in the reaction N-
containing compounds are present, the reused Ni3N and
Ni3N+Ni0 samples did not exhibit considerable modifications in
the N1s region, suggesting that no adsorption of such
substances occurs. In addition, Ni XPS region was acquired in
the three cases. Interestingly, it was found that the surface of
Ni sample was partially oxidized. Deconvolution of the Ni 2p3/2
XPS region gave rise to two main signals located at ca. 852.3 eV
and 855.2 eV, related to metallic nickel and Ni2+ species,
respectively21,22. The latest contribution increases, in the three
materials, after the performed reaction. The peak assignments
for each atom are reported in Tables SI 1-4.
financial support and RSC for the researcher mobility grant,
RM1802-1846. Rafael Luque gratefully acknowledges MINECO
for funding under project CTQ2016-78289-P, co-financed with
FEDER funds and the contract for Daily Rodriguez-Padrón
associated to this project. The publication has been prepared
with support from RUDN University program 5-100.
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4
Conclusions
The hydroconversion of levulinic acid towards N-heterocycles
was tested here using noble metal free catalysts and in
particular nanosized Ni3N and Ni0. The catalytic nanoparticles
were prepared via a green sol-gel based process, using urea
5
6
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