Low temperature conversion of levulinic acid into γ-valerolactone using Zn to generate hydrogen from water and nickel catalysts supported on sepiolite

Article abstract of DOI:10.1039/d0ra04018e

In the present article, γ-valerolactone has been obtained from levulinic acid with a yield exceeding 25% using very mild conditions without feeding hydrogen (30 °C, atmospheric pressure, water as the hydrogen source). The overall reaction conducted is a two-step process: first, a redox reaction involving the oxidation of metallic Zn to ZnO forin situhydrogen production through the water splitting reaction and, second, a catalytic reaction involving Ni-supported catalysts for the production of γ-valerolactone from levulinic acid. Ni active sites have been supported on sepiolite, an abundant and cheap material. The nickel particle size has been demonstrated to be a parameter of paramount importance determining the catalytic activity, since the best catalytic performance is obtained with the smallest Ni nanoparticles. This combination of Zn and Ni supported on sepiolite shows a good catalytic stability after three catalytic runs.

Full text of DOI:10.1039/d0ra04018e

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Low temperature conversion of levulinic acid into
g-valerolactone using Zn to generate hydrogen
from water and nickel catalysts supported on
sepiolite†
Cite this: RSC Adv., 2020, 10, 20395
a
a
a
a
b
Adrian Garcıa,
Rut Sanchis, Pablo J. Miguel,
Ana M. Dejoz, Marıa Pilar Pico,
´
´
´
c
c
d
Marıa Luisa Lopez, Inmaculada A´ lvarez-Serrano, Tomas Garcıa
and Benjamın Solsona
´
´
´
´
a
*
´
In the present article, g-valerolactone has been obtained from levulinic acid with a yield exceeding 25% using
ꢀ
very mild conditions without feeding hydrogen (30 C, atmospheric pressure, water as the hydrogen source).
The overall reaction conducted is a two-step process: first, a redox reaction involving the oxidation of metallic
Zn to ZnO for in situ hydrogen production through the water splitting reaction and, second, a catalytic reaction
involving Ni-supported catalysts for the production of g-valerolactone from levulinic acid. Ni active sites have
been supported on sepiolite, an abundant and cheap material. The nickel particle size has been demonstrated
to be a parameter of paramount importance determining the catalytic activity, since the best catalytic
performance is obtained with the smallest Ni nanoparticles. This combination of Zn and Ni supported on
sepiolite shows a good catalytic stability after three catalytic runs.
Received 8th April 2020
Accepted 21st May 2020
DOI: 10.1039/d0ra04018e
rsc.li/rsc-advances
on noble metals, ruthenium yields the best results. However, as
noble metals are expensive, different options involving cheaper
Introduction
In an extensive study carried out by the US Department of materials have been proposed. Fortunately, supported catalysts
1
17
18,19
Energy, 12 compounds from sugars were identied as easily of either non-noble metals or mixed metal oxides,
if
removable and with extraordinary transformation potential into properly optimized, have proven to be almost as efficient as
high value compounds. One of these compounds is levulinic catalysts based on noble metals. Nickel is one of the most effi-
acid. As is widely known, biomass can be a sustainable source cient non-noble metals used in the conversion of LA into
2
0–25
for the production of fuels as an alternative to petroleum GVL.
2–5
derivatives. Interestingly, levulinic acid (LA) can be trans-
In the last decades, different metals have been supported on
formed into g-valerolactone (GVL), which is a compound with low-cost minerals and tested in this or in similar catalytic
6–8
high interest as a possible substitute for petroleum fuels, since reactions. One of these materials is sepiolite, which has been
26–28
it can be easily transformed into high-energy density fuels. used as a catalytic support in many reactions.
Sepiolite is
Another of the great advantages of g-valerolactone is its very low a hydrous magnesium silicate which presents many applica-
toxicity. Therefore, it is considered an ideal liquid fuel (or fuel tions. Its main use is as absorbent, although it presents some
9
,10
precursor) to be transported and stored.
The selective conversion of LA into GVL can take place by phytosanitary carriers, animal food. Sepiolite presents
other utilities such as pharmaceutical products, detergents,
29
11
homogeneous and/or heterogeneous catalysis. Typical cata- important advantages over most of the conventional catalytic
lysts are based on noble metals, since these materials achieve supports. Firstly, it is extremely cheap as it can be directly ob-
12–16
the highest productivity to GVL.
Among the catalysts based tained from sepiolite quarries. Second, sepiolite is a brous clay
containing ribbons that presents microcavities in a brous
structure which can be important in catalysis. Accordingly,
a
Departament d’Enginyeria Qu ´ı mica, ETSE, Universitat de Val `e ncia, Av. Universitat,
6100 Burjassot, Valencia, Spain. E-mail: benjamin.solsona@uv.es
Guerrero-Torres et al. studied the incorporation of Ni active
sites on sepiolite support by different preparation methods and
4
b
Sepiolsa, Avda. del Acero, 14-16, Pol. UP-1 (Miralcampo), 19200 Azuqueca de
30
these catalysts were assessed in the hydrogenation of furfural.
Henares, Spain
c
As expected, it was found that the dispersion and size of the Ni
metal particles was depending on the preparation method,
Instituto de Carboqu ´ı mica (CSIC), C/Miguel Luesma Cast a´ n, 50018 Zaragoza, Spain modifying the interaction between metal particles and sepiolite
Departamento de Qu ´ı mica Inorg a´ nica, Facultad de Ciencias Qu ´ı micas, Universidad
Complutense de Madrid, 28040 Madrid, Spain
d
†
Electronic supplementary information (ESI) available. See DOI:
support. Ni supported on sepiolite, if properly synthesized,
1
0.1039/d0ra04018e
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achieved high performance for the decarbonylation of furfural
to furan, being favored when small Ni nanocrystals were
synthetized.
The conversion of LA into GVL presents some similarities
with the furfural hydrogenation as both involve a hydrogenation
step. However, an additional dehydration step is required for
GVL production. In any case, these reactions necessarily require
a hydrogen source. Different solutions are proposed in the
literature, being most of the studies performed with high
pressure H2 in batch reactors, although feeding of high-
pressure H in continuous gas-phase systems has also been
2
31
reported. However, the storage and handling of high-pressure
hydrogen involves a series of hazards due to its high amma-
bility and low energy ignition. For these reasons, several alter-
natives have been studied in order to generate in situ hydrogen,
Scheme 1 Scheme showing the transformation of levulinic acid into
g-valerolactone using Ni catalysts and Zn.
2
avoiding the use of molecular H . One of these solutions is the
metals that in the presence of water could lead to the formation
use of formic acid as a hydrogen donor. Formic acid can be
obtained from sugar transformation together with levulinic
acid. Then, depending on the process conditions, formic acid
decomposes to give CO and H and this H can be later used for
of H
2
, because its evolution is a non-violent reaction, in contrast
with the use of some alkali metals such as potassium or sodium.
In the Zn–water reaction media, H
2
generation is carried out
2
2
32–36
according to the following reaction:
the hydrogenation of LA to GVL
avoiding the supply of
external hydrogen. Unfortunately, harsh reaction conditions are
usually required.
; DH ¼ ꢁ63 kJ molꢁ (298 K) (1)
1
Zn + H
2
O / ZnO + H
2
The LA to GVL transformation can also take place without
the addition of molecular hydrogen by catalytic transfer
hydrogenation employing different types of alcohols as
hydrogen donors, such as 2-propanol, 2-butanol or isopropyl
From a green chemistry point of view, the main problem
associated with the use of Zn as reactant could be the fact that
aer reaction (1) Zn is almost completely oxidized to ZnO.
Hence, a reduction regeneration step needs to be carried out for
its use in a cyclic operation. An interesting green solution would
be the use of solar chemical reactors where Zn yields from ZnO
37–40
alcohol.
Then, catalytic transfer hydrogenation via the
Meerwein–Ponndorf–Verley presents, selecting the reaction
conditions, good chemo-selectivity for the carbonyl groups
reduction and this allows this reaction can take place with
reasonably high efficiency.
44
greater than 90% has been reported. Thus, a cyclic operation
could be proposed where, rstly, the hydrogenation of levulinic
acid to gamma-valerolactone is carried out with in situ H
2
2
Another interesting and simple option to obtain in situ H is
generated throughout the Zn-water redox system and, secondly,
Zn is regenerated from ZnO using solar energy. To our knowl-
edge, although this zinc reuse strategy would not sort out the
energy problem of the global process, it could be an interesting
green alternative that deserves to be studied.
the addition of a certain amount of a suitable metal to the
reaction media containing water (for example, alkaline and
alkaline-earth metals or some transition elements). Then, H2
production is accomplished through a redox reaction involving
metal oxidation and water splitting reaction, Me + H
2
O / MeO
The transformation of LA into GVL at low temperatures has
1
21,41
+
H (or Me + H O / MeOH + H ).
2 2 2
This reaction, which is
2
been scarcely reported and always with direct supply of
thermodynamically favored, can even take place at room
temperature. It must be indicated that the metal added to water
does not act as a catalyst in obtaining hydrogen but simply as
a reagent. Therefore, the amount of hydrogen produced will
depend on the amount of metal used.
45,46
hydrogen.
In the present article, we have newly proposed
a green alternative involving the use of Zn to obtain hydrogen
from water and non-noble metal catalysts based on Ni
(Scheme 1) in order to transform LA into GVL. Reaction
conditions were very mild using reaction temperatures close to
room temperature, atmospheric pressure and aqueous media.
Therefore, the main novelty of this article lies in the use of low
temperature and the absence of pressurized molecular
hydrogen in the catalytic conversion of levulinic acid into g-
valerolactone. Moreover, the active sites of the catalysts
employed are not noble metals and the support used is sepio-
lite, an abundant, cheap and natural clay.
2
H production from the redox reaction of metal and water is
41–43
not a new concept.
In fact, metal–water reactions have been
proposed to be used for fuel cell devices in portable applica-
tions, such as computers and vehicles. Unfortunately, these
applications do not seem to be economically protable because
of the large amount of metals required for hydrogen produc-
tion. Positively, aluminum–water interaction for H production
2
has shown to be reasonably interesting for some specic non-
43
portable applications, such as electronic devices. Herein, we
have proposed the formation of in situ hydrogen for its use in
hydrogenation processes, preventing the inconveniences
Results and discussion
arising from the direct use of high-pressure molecular Initially, commercial Zn was tested at very mild conditions:
ꢀ
hydrogen. Specically, we have selected Zn among the different 30 C, atmospheric pressure and levulinic acid solution in
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Table 1 Physico-chemical characteristics of the Ni/sepiolite catalysts synthesized
2
ꢁ1
a
3
ꢁ1
D
p
( A˚ ) (4V/A)b
Catalyst
Sepiolite
Preparation method
Ni wt%
S
BET (m g
)
V (cm g
)
Support
0
1
2
5
10
100
2
242
202
202
210
218
7
0.392
Not determined
0.447
Not determined
0.480
—
98.9
Not determined
118.3
Not determined
120.6
—
1
2
5
1
1
2
Ni/sep-ox
Ni/sep-ox
Ni/sep-ox
0Ni/sep-ox
00Ni-ox
Oxalic acid in ethanol
Oxalic acid in ethanol
Oxalic acid in ethanol
Oxalic acid in ethanol
Oxalic acid in ethanol
Simple impregnation
Ni/sep
204
Not determined
Not determined
a
0
b
Single point adsorption total pore volume at p/p ¼ 0.99. Desorption average pore width.
water. Transformation of LA into GVL was observed although well as on the productivity per Ni site. Interestingly, the
with very low conversion. Interestingly, the GC analysis of the maximum yield was achieved at Ni-contents between 2 and
gas evolved showed hydrogen formation. Subsequently, pure 5 wt%, whilst the highest productivity per Ni site was attained
bulk metallic nickel catalyst (commercial) was tested in the by those catalysts with the lowest Ni-loading (1 and 2 wt% Ni). It
same experimental conditions. Again, the transformation of is worth commenting that catalytic activity was clearly related to
levulinic acid was marginal, being the formation of valer- the synergetic effect between the Ni supported catalysts and the
olactone negligible. Moreover, gas analysis did not show any Zn–water redox system since the sum of the GVL yields obtained
hydrogen formation. Then, Zn and metallic nickel were simul- in individual control experiments using Zn and Ni/sepiolite
taneously incorporated to the reaction media. Interestingly, hardly reached 2%. Additionally, a control reaction using the
some GVL conversion was observed, being higher than that bare sepiolite support with Zn showed a comparable GVL yield
theoretically obtained from the sum of the experiments with Zn to the Zn control reaction, proving the inert role of sepiolite in
and Ni, separately. Therefore, it could be assumed that this the reaction. Remarkably, the use of both Zn and Ni/sepiolite
reaction system could produce GVL from LA at mild conditions. led to conversions higher than 25% for Ni loadings of 2 and
However, catalyst optimization would be needed since the 5 wt%. These excellent results can be linked to the presence of
number of available Ni sites in bulk catalysts was rather limited. a cascade-type reaction where, rst, the in situ H
Accordingly, Ni nanoparticles were nely dispersed in a low-cost throughout the redox Zn–water system and, second, the cata-
support by different preparation methods. lytic hydrogenation/dehydration of LA to GVL over metallic Ni
2
generation
As support we selected sepiolite, a natural but complex sites, are produced. The use of oxalic acid in the preparation
material with formula Mg Si O (OH) $6H O. High availability, procedure was demonstrated to be benecial for the production
4
6
15
2
2
low cost and relatively high surface area, which allows a proper of GVL. Then, the catalyst prepared in the absence of oxalic acid
dispersion of metals on its surface, are the main advantages of 2Ni/sep obtained GVL yield ca. 1/3 of that obtained by the
this support. The most relevant compositional and structural analogue catalyst prepared with oxalic acid (Table 2).
characteristics of the provided sepiolite were obtained from
previous XRD, TGA, Si-NMR, SEM and EDS data. Thus, low temperatures (in the 30 to 50 C range) on the 2Ni/sep-ox +
sepiolite showed an orthorhombic structure with cell parame- Zn combination. An increase of the temperature involves
The inuence of the reaction temperature was studied for
2
9
47
ꢀ
˚
ters a ¼ 13.400, b ¼ 26.964 and c ¼ 5.2626 A, and did not present a faster reaction between Zn and water and, also, for the LA to
any secondary phase or impurities. Besides, this support had GVL transformation. Then, the amount of available hydrogen in
2
ꢁ1
a surface area of 250 m g , which was slightly decreased aer the media could vary in each experiment and then the effect of
2
ꢁ1
reduction (242 m g ).
the temperature on the overall reaction could be uncertain.
Different amounts of nickel (1, 2, 5 and 10 wt%) were sup- However, the trend observed is an increase in the GVL forma-
ported on the sepiolite support (Table 1) by a wet impregnation tion with the reaction temperature, as it could be expected.
ꢀ
ꢀ
method using oxalic acid (Ni/sep-ox series). Oxalic acid was Then, at 30 C the GVL yield was 26.9%, at 40 C 34.7% and at
ꢀ
added since, as reported, it favors the creation of small Ni 50 C 40.5%.
nanoparticles nely dispersed on the support. Again, marginal
GVL formation was observed for these materials and the anal- under reaction conditions was also studied. For that, aer
ysis of the gas did not show any hydrogen formation.
a standard experiment with 2Ni/sep-ox + Zn (GVL yield ¼
The possible leaching of nickel in the Ni/sepiolite catalysts
Then, Zn and Ni/sep-ox catalysts were simultaneously tested 26.1%), the solids were removed by ltration and the liquid with
for LA hydrogenation. Interestingly, a notorious transformation the GVL obtained in the previous experiment was collected.
of levulinic acid was always observed, being the selectivity to Then, this liquid together with fresh Zn was le for two more
GVL higher than 96%. The GC-MS analysis mainly detected GVL hours in typical reaction conditions. The analysis of the solu-
as the reaction product (see ESI, Fig. S1†), although traces of tion showed a GVL yield of 28.7%. This result tted with the
other compounds were also identied (see ESI, Fig. S2†). Table 2 expected slight increase in the GVL due to the addition of fresh
shows the inuence of the nickel content on the yield to GVL as
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a
Table 2 Catalytic performance of Ni and sepiolite catalysts with the assistance or not of Zn
b
c
d
Catalyst
LA conversion (mol%)
Yield to GVL (mol%)
GVL productivity
GVL productivity per Ni
Zn
Ni
Zn + Ni
Sep
2.1
<1
3.5
0
1.98
Traces
3.5
0.07
—
0.12
0
—
—
0.12
—
0
2
Ni/sep-ox
<1
Traces
2.1
—
—
—
Zn + sep
2.1
17.5
26.9
26.1
18.2
7.0
10.0
0.08
0.59
0.91
0.88
0.62
0.23
0.33
Zn + 1Ni/sep-ox
Zn + 2Ni/sep-ox
Zn + 5Ni/sep-ox
Zn + 10Ni/sep-ox
Zn + 100Ni-ox
Zn + 2Ni/sep
17.0
26.1
25.3
17.8
6.5
60.0
46.0
17.9
6.3
0.23
16.9
9.6
a
b
c
Reaction conditions in text. Standard conditions. 2 h experiment. Yield to GVL. The selectivity to GVL exceeds 95% in all cases. Per mass of Ni-
ꢁ1
d
ꢁ1
catalyst as molGVL (kgcat h)
.
Per mass of Ni in the Ni-catalyst as molGVL (kgNi h) .
Zn. Then, it could be concluded that either there was no nickel the surface area for the catalysts compared to the pure support.
2
ꢁ1
leaching or dissolved nickel did not produce GVL.
The surface area of the support was 242 m g whereas that of
2
ꢁ1
Ni/sepiolite catalysts were characterized by several tech- the supported nickel was between 202 and 218 m g . Finally,
niques in order to see the reason for the different reactivity of the sepiolite free Ni catalyst had a low surface area of only 7 m
2
ꢁ1
Ni/sepiolite catalysts. N
the support and all the catalysts exhibited a type IV isotherm, bulk Ni catalysts.
related to mesoporous materials (Fig. 1). Hysteresis cycles due Fig. 2 shows XRD patterns of the Ni-based catalysts and their
2
adsorption isotherms pointed out that
g , being this value close to that obtained by others authors in
48,49
to capillary condensation phenomena were clearly observed in NiO precursors before and aer the reduction step. As can be
all catalysts. The obtained pore distribution curve for the sepi- observed in Fig. 2A, the NiO/sepiolite precursors presented the
olite (Fig. 1C) showed that the average pore diameter was diffraction peaks typical of sepiolite. Interestingly, the thermal
around 4 nm, which was coherent with previously reported data treatment did not modify the initial sepiolite structure. Thus,
48,49
for this type of clay.
No appreciable differences were the only loss of water corresponded to zeolitic water, i.e. there
observed in the prole of the support and the supported nickel was not a folding of sepiolite aer the reduction process.
catalysts. Regarding to the pore distribution of the Ni supported Moreover, wide reections from cubic NiO (NiO, JCPDS: 47-
catalysts (Fig. 1C), maxima at ca. 3.9 nm were observed in all the 1049) could be observed in all cases, suggesting the small size of
Ni/sepiolite samples, suggesting that nickel was not blocking NiO crystals. The intensity of these NiO peaks enhanced when
the sepiolite pores. The addition of nickel led to a slight drop in the nickel loading increased. In Fig. 2B, the diffractograms of
Fig. 1
2
N isotherms patterns for sepiolite and Ni/sepiolite catalysts (A) and their corresponding pore distributions (B).
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Fig. 2 XRD patterns of NiO/sepiolite precursors (A) and final reduced Ni/sepiolite catalysts (B) prepared by impregnation using ethanol.
the nal reduced catalysts showed that nickel phase was only width was in the 8 to 50 nm range. Nickel was in all cases
apparent in the catalysts with 10 wt% Ni, where a wide and low detected as metallic nickel although some differences were
ꢀ
intensity peak at ca. 44 corresponding to metallic nickel (Ni, observed among these catalysts. TEM images evidenced the
JCPDS: 01-1260) could be observed. This suggested that nickel crystallinity of the Ni nanoparticles, as indicated by the
was present in these catalysts in the form of very tiny particles. observed periodical contrasts of 0.20 nm in the images, which
XRD peaks related to the support did not show any signicant were assigned to (111) interplanar distances of cubic Ni (see
difference compared to pure sepiolite, pointing out that no inset of Fig. 3). Thus, they were well stabilized as metallic Ni,
important modication of the crystalline structure of the sepi- since they are not easily oxidized under air exposure, once
olite support took place in Ni/sepiolite samples aer the supported on the sepiolite. Overall, using oxalic acid in the
reduction step.
preparation method the obtained Ni nanoparticles were smaller
High resolution transmission electron microscopy (HRTEM) in particle size compared to previously reported Ni–clay sup-
50,51
study was carried out to provide a more accurate view of the ported catalysts.
The catalyst with 2 wt% Ni presented zones
location and size of the Ni particles on the sepiolite support of the support with low concentration of Ni particles and other
prepared with oxalic acid. Fig. 3 shows the HRTEM images of Ni areas with high concentration. This lack of uniformity could be
catalysts with 2, 5 and 10 wt% Ni. Large rods of sepiolite support due to the preparation method followed, as wet impregnation
with variable size were apparent in these samples. The length of methods oen lead to heterogeneous distributions of nano-
these rods largely varied from 50 nm to several mm whereas the particles. However, it must be noted that Ni nanoparticles were
Fig. 3 TEM images of Ni/sepiolite catalysts prepared by wet impregnation method.
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in all cases over the support either directly or in a few cases
forming a couple of layers, probably due to their magnetic
nature. As shown in Fig. 4, particle size distribution was quite
homogeneous, presenting a small average size, ca. 3 nm, with
80% of the particles being smaller than 4 nm. Some agglom-
erations of free Ni particles were also apparent, leading to
aggregates of 5–6 nm. In the catalyst with 5 wt% Ni, a higher
amount of Ni nanoparticles was observed on the support
surface. Additionally, more agglomerations of Ni nanoparticles
were also visible. Finally, a higher density of nanoparticles was
apparent in the 10 wt% Ni catalyst. In addition, a higher degree
of agglomeration was attained although some isolated nano-
particles supported on sepiolite were also observed. Therefore,
we can conclude from detailed microstructural analysis that the
key parameters governing the catalytic performance of LA
hydrogenation seem to be the size of the Ni nanoparticles and
ꢀ
Fig. 5 Re-use testing of 2Ni/sep-ox and Zn at 30 C. Symbols: GVL
ꢁ1
productivity in molGVL (kgcat h) ( ), yield to GVL ( ), weight of solid
loaded, Ni-catalyst plus Zn recovered, in g ( ). Remaining reaction
its dispersion, being preferred those small Ni nanoparticles conditions in Experimental section.

nely dispersed on the whole external surface of the sepiolite
bers.

GVL was obtained from LA at very mild conditions
temperature close to room temperature (30 C), atmospheric
It was appreciated that the catalytic performance was main-
tained regardless of the excess amount of fresh Zn employed,
probing that the amount of released hydrogen was not the rate
limiting parameter in the LA hydrogenation.
ꢀ
(
pressure and water solution) using Zn as non-catalytic in situ
generator of hydrogen from water and supported Ni materials
as active catalysts. This combination led to an excellent
performance in terms of LA conversion and GVL selectivity.
However, catalyst stability is another important parameter that
needs to be studied. Re-use testing experiments were under-
taken using 2Ni/sep-ox sample as catalyst and Zn (Fig. 5). Aer
a standard reaction, the Ni catalyst and Zn were collected aer
For a standard experiment using 171.6 mg of Zn, 2.63 ꢂ
ꢁ3
10
mol of H would be produced in the case of a full Zn
2
transformation into ZnO, according to reaction (1). On the other
hand, the hydrogen released in a standard experiment with 2Ni/
sep-ox and Zn was calculated. For this, the amount of hydrogen
required for the transformation of LA into GVL, the amount of
hydrogen released to the gas phase (which can be analyzed by
GC and quantied knowing the free room of the ask) and the
amount of hydrogen in the liquid phase (estimated by the
Henry's law equation) were considered. Accordingly, a theoret-
ical transformation of Zn into ZnO about 16% was determined,
pointing out that a partial oxidation of Zn into ZnO is accom-
plished. In Fig. 6 we can see the TEM images of the 2Ni/sep-ox +
Zn mixtures aer use. Long sepiolite bers with small nickel
particles were found together with large Zn/ZnO particles of
variable size (typically 50–100 nm wide and 100–500 nm long).
The electron diffraction analyses showed that Zn was present as
both metallic Zn and oxidized ZnO, in their respective hexag-
onal crystalline structures. The SAED results (Table S1†) cannot
quantify accurately the content in Zn and ZnO but we observed
some areas rich in Zn and others rich in ZnO. Therefore, these
data corroborated the partial oxidation of Zn to ZnO during our
experiment at standard conditions.
ꢀ
washing with deionised water, ltering and drying at 120 C.
Not all the solid could be recovered as it remained in the lter
paper and the reactor walls, so that a loss of weight of ca. 20%
was observed. We assume that the loss of weight is equal for
both Zn and the Ni-catalyst. This procedure was carried out for
three times. This way, aer 3 uses previous reduction, the GVL
productivity observed decreased only ca. 20% compared to the
productivity obtained with the fresh material.
To get deep into the role of Zn in the whole process, several
experiments were carried out with Ni catalyst and adding
different amounts of Zn, varying from 130 to 285 mg (Fig. S3†).
The separation of Ni/sepiolite catalysts and Zn could be
possible by considering the different physical and magnetic
properties of them but it would be easier if during the process
Ni/sepiolite catalysts and Zn were not together. Moreover, we
would like to highlight the paramount importance of in situ
generated hydrogen. This issue was demonstrated in a nal
experiment using two interconnected three neck asks where
H
2
generation through the redox Zn–water reaction and LA
hydrogenation were separately performed. Accordingly, metal
Fig. 4 Particle distribution showing the Ni particle size distribution of
Ni/sepiolite catalysts prepared using oxalic acid. Symbols: ( ) 2Ni/sep- Zn particles were suspended in a water solution under stirring
ox, ( ) 5Ni/sep-ox, ( ) 10Ni/sep-ox.
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Fig. 6 TEM images of 2Ni/sep-ox + Zn after use at different magnifications. General view (a), image focused into a particle rich in Zn (b) and
image focused into sepiolite fibers (c). The SAED pattern of (b) is included.
Fig. 7 Scheme of the different experiments conducted. Note: Ni-catalyst corresponds to 2Ni/sep-ox.
in the rst ask, whilst a LA aqueous solution with Ni/sepiolite not in contact with the support. In the catalyst with 2 wt% Ni the
was set up in the second ask (see Fig. 7). The results obtained lower size of nanoparticles compared to the catalyst with 5 wt%
showed that although formation of GVL was possible, GVL yield leads to a higher amount of available active sites normalized per
(3.3%) was remarkably lower than that using the mixture Zn–Ni/ Ni loading in the catalyst with 2 wt%. However, only this factor
sep-ox in a single ask (yield ¼ 26.1%). Probably, atmospheric cannot justify the higher activity normalized per Ni loading
pressure hydrogen in the gas phase generated in one of the observed. Thus, it must be taken into account that the sepiolite
interconnected asks only allowed a small amount of hydrogen free Ni catalyst presents very low activity per Ni loading.
dissolved into the water/LA/Ni/sepiolite mixture then prevent- Therefore, the optimal Ni loading is 1–2 wt% and this is due not
ing the hydrogenation of LA into GVL.
only to the higher amount of active sites per Ni loading but also
The relative reaction rate between the in situ generated likely to the higher reactivity of nanoparticles in close contact
hydrogen and the capacity of Ni-catalyst for the hydrogenation with the sepiolite.
of levulinic acid should be controlled for an optimization of the
The approach followed in the present article allows the use of
combined system. Zn can alone exert both functionalities remarkable low temperatures and the avoidance of the addition
although a high excess does not lead to a signicant improve- of pressurized H because of the hydrogen generator role of Zn.
2
ment in the GVL formation (Fig. S3†). On the other hand, the It has been shown an example of the use of metals to react with
loading of nickel in the catalyst seem to be of paramount water at very low temperature to generate H , this hydrogen
2
importance to reach high GVL production and low nickel being used for a subsequent transformation, and this strategy
loadings are required. Catalysts with low Ni-loading present could be followed for some other reactions involving
small Ni nanoparticles whereas with high Ni-loading the size of hydrogenation.
nanoparticles is higher and, in some cases, these particles are
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Teller (BET) specic surface area (SBET) was determined through
the relative pressure range from 0.05 to 0.25. The pore size
Materials and methods
Commercial Ni supplied by Alfa-Aesar-150–200 mesh was used distribution was analyzed using the Barrett–Joyner–Halenda
for comparison. This sample was called as Ni.
(BJH) method by analyzing the adsorption branch of the N
2
Natural sepiolite was collected from Toledo (Castilla La adsorption isotherms.
Mancha, Spain) being supplied by Sepiolsa and was used as
a support without previous treatment. The chemical analysis Catalytic tests and analyses
indicates that the support consists of 97.0% sepiolite, 1.9%
The catalytic conversion of LA into GVL was carried out using
a 74 mL three neck ask. In standard experiments the ask was
fed with 1.24 mmol of LA, 3.5 mL of water, 175.6 mg of catalysts
and 171.6 mg of Zn.
The study of the inuence of the Zn-loading was undertaken
using 175.6 mg of catalyst and amounts of Zn varying from 0 to
dolomite and 1.1% other clays. Different Ni solutions were
prepared by solving Ni(NO ) $6H O (Sigma-Aldrich) in ethanol
3
2
2
with oxalic acid (H
2
C
2
O
4
$2H
2
O, Sigma-Aldrich), always being
the molar Ni : oxalic ratio equal to 1 : 3. Sepiolite was added to
ꢀ
the solution and evaporation of ethanol was carried out at 60 C
in a hotplate stirrer under vigorous stirring. These catalysts
were labeled as xNi/sep-ox, x being the Ni content (1, 2, 5 and 10
wt% were tested), and ox indicates that the catalysts were
prepared by the impregnation method with oxalic acid.
For comparison, a simple 2 wt% Ni-loaded Ni/sepiolite
catalyst obtained by wet impregnation was prepared similarly
to Ni/sep-ox but in the absence of oxalic acid during the
synthesis procedure. This catalyst was named as 2Ni/sep.
2
85 mg. The asks also contained 1.24 mmol of LA and 3.5 mL
of water.
Another experiment was undertaken using two three neck
asks connected. One ask contained Zn (171.6 mg) and water
3.5 mL) and the other ask was lled with 1.24 mmol of LA,

(
3
.5 mL of water and 175.6 mg of Ni-catalyst.
For all the experiments, the asks were previously purged
three times with N
tion and later were sealed and introduced into a 30 C water
bath and stirred for 2 h. Aer that, the mixture was immediately

2
in order to avoid the possible metal oxida-
ꢀ
Finally, all samples were dried in a furnace at 100 C for 6 h,
ꢀ
ꢀ
and then heat-treated in static air for 2 h at 500 C. Before
reaction, catalysts were heat-treated in owing H for 2 h at
2
ltered with the appropriate membrane to obtain the liquid
ꢀ
4
00 C in order to reduce the metal oxide precursors.
phase containing the reaction product. Blanks in the absence of
catalysts were carried out to check that GVL was not produced.
Metallic Zn supplied by Alfa Aesar, CAS no.: 7440-66-6 has
been used. To avoid oxidation, it has been kept under a nitrogen
atmosphere and has been used in reaction as it is without
subsequent transformation.
Analytical method
LA, GVL and the remaining reaction products were analyzed, as
described in ref. 21 and 41.
Characterization techniques
The liquid samples were identied by gas chromatography,
Catalysts were analyzed by high resolution transmission electron using the GC instrument Hewlett Packard 5890 (Palo Alto,
microscopy (HRTEM), using a eld emission gun TECNAI G2 F20 California, USA), equipped with an Agilent HP-1 column (30 m
microscope (FEI Company, Hillsboro, OR, USA), operated at 200 ꢂ 0.32 mm ꢂ 0.25 mm). The detector employed was a FID
ꢀ
kV in order to analyze their structure and morphology. This detector working at 240 C, and an injection port, working at
ꢀ
equipment was also used for undertaking selected area electron 220 C. The temperature program for a typical run was as
ꢀ
ꢀ
diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). follows: (i) 35 C isothermal for 6 min, (ii) from 35 to 230 C with
ꢀ
ꢁ1
ꢀ
Catalysts samples for TEM were sonicated in absolute ethanol for a heating rate of 20 C min , and (iii) isothermal at 230 C for
0 minutes and placed on a holey carbon lm, which was sup- 26 min. The retention times for GVL and LA were 5.4 and
2
ported on a copper grid. Finally, the copper grid was dried. TEM 9.5 min, respectively. Controls without LA and without catalysts
photographs were used to calculate the average size of the nickel were also analyzed to compare with the other reaction samples.
particles. High resolution transmission electron microscopy Finally, liquid samples were also analyzed by gas chromatog-
(
HRTEM) results were also obtained using a JEOL JEM 3000F (300 raphy mass spectrometer (GC-MS 5977A MSD-7890A, Agilent,
kV) microscope. Samples were prepared by crushing the powders Santa Clara, CA, USA) to identify other reaction byproducts. In
under n-butanol and dispersing them over copper grids covered this case, the retention times for GVL and LA were 12.4 and
with a holey carbon lm.
16.4 min, respectively.
Powder X-ray diffraction (XRD) was employed in order to
Gas analysis was undertaken to observe possible gas phase
determine the crystalline phases of the samples. An Enraf products and quantify the amount of hydrogen. This analysis
Nonius FR590 sealed tube diffractometer (Bruker, Del, The was done utilizing a Hewlett Packard 5890 (Palo Alto, California,
Netherlands) with a monochromatic Cu Ka
1
source operating at USA) gas chromatograph equipped with two columns: (i)
4
0 kV and 30 mA was used.
Catalysts were submitted to N
Carbosieve-S and (ii) Porapak QS.
ꢀ
2
adsorption at ꢁ196 C, using
a Micromeritics ASAP 2460 apparatus (Norcross, GA, USA).
Catalysts were degassed at 150 C before the analysis. Total pore
Conclusions
ꢀ
volumes were determined, employing the adsorbed volume at A novel green process has been envisaged for the hydrogenation
a relative pressure of 0.99. A multipoint Brunauer–Emmet– of levulinic acid (LA) into g-valerolactone (GVL) at very mild
20402 | RSC Adv., 2020, 10, 20395–20404
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ꢀ
conditions (30 C, atmospheric pressure and LA water solution)
using Ni-based catalysts supported on sepiolite without the
external addition of hydrogen. Signicantly, a redox reaction
involving the oxidation of metallic Zn to ZnO for in situ
8 J. Cui, J. Tan, T. Deng, X. Cui, Y. Zhu and Y. Li, Green Chem.,
2016, 18, 1619–1624.
9 J. Zhang, S. Wu, B. Li and H. Zhang, ChemCatChem, 2012, 4,
1230–1237.
hydrogen production throughout the water splitting reaction, 10 J. Q. Bond, D. M. Alonso, D. Wang, R. M. West and
instead of using high pressure molecular hydrogen, is J. A. Dumesic, Science, 2010, 327, 1110–1114.
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J. Sherwood and J. H. Clark, Chem. Eng. J., 2019, 372, 992–
1006.
smaller than 4 nm, which are homogeneously distributed on 12 P. P. Upare, J. M. Lee, D. W. Hwang, S. B. Halligudi,
the external surface of the support. Accordingly, the presence of
very small Ni nanoparticles nely dispersed on the sepiolite
Y. K. Hwang and J. S. Chang, J. Ind. Eng. Chem., 2011, 17,
287–289.
support seems to be the key parameter leading to remarkable 13 W. Luo, M. Sankar, A. M. Beale, Q. He, C. J. Kiely,
productivities per Ni site. Regarding the role of Zn, in situ
generated hydrogen demonstrates to be of paramount impor-
P. C. A. Bruijnincx and B. M. Weckhuysen, Nat. Commun.,
2015, 6, 6540–6550.
tance facilitating the hydrogenation of LA into GVL, since 14 S. Cao, J. R. Monnier, C. T. Williams, W. Diao and
marginal GVL yield is achieved when gas phase hydrogen at
atmospheric pressure is used in the reaction media.
J. R. Regalbuto, J. Catal., 2015, 326, 69–81.
15 W. Luo, U. Deka, A. M. Beale, E. R. H. van Eck,
P. C. A. Bruijnincx and B. M. Weckhuysen, J. Catal., 2013,
3
01, 175–186.
Conflicts of interest
1
1
1
6 S. Cao, J. R. Monnier and J. R. Regalbuto, J. Catal., 2017, 347,
72–78.
7 K. Shimizu, S. Kannoa and K. Kona, Green Chem., 2014, 16,
3
8 S. Ishikawa, D. R. Jones, S. Iqbal, C. Reece, D. J. Morgan,
D. J. Willock, P. J. Miedziak, J. K. Bartley, J. K. Edwards,
T. Murayama, W. Ueda and G. J. Hutchings, Green Chem.,
2017, 19, 225–236.
There are no conicts to declare.
899–3903.
Acknowledgements
Authors from UCM thank MINECO (MAT2017-84118-C2-2-R
project) and UCM CAI center of EM. Authors from UV thank
MINECO (MAT2017-84118-C2-1-R project) and FEDER for
funding. A. G. thanks MINECO for the pre-doctoral grant. SCSIE 19 I. Orlowski, M. Douthwaite, S. Iqbal, J. S. Hayward,
from UV is also acknowledged for characterization of the
materials employed and the GC-MS analyses. Said Agouram is
acknowledged for assistance in microscopy experiments.
T. E. Davies, J. K. Bartley, P. J. Miedziak, J. Hirayama,
D. J. Morgan, D. J. Willock and G. J. Hutchings, J. Energy
Chem., 2019, 36, 15–24.
Miranda S ´a nchez is acknowledged for assistance in catalytic 20 K. Sakakibara, K. Endo and T. Osawa, Catal. Commun., 2019,
work.
125, 52–55.
2
2
1 R. Sanchis, T. Garc ´ı a, A. Dejoz, I. V ´a zquez, F. Llopis and
B. E. Solsona, Materials, 2019, 12, 2918–2934.
2 K. Shimizu, S. Kanno and K. Kon, Green Chem., 2014, 16,
3899–3903.
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