Direct catalytic transformation of biomass derivatives into biofuel component γ-valerolactone with magnetic nickel-zirconium nanoparticles

Article abstract of DOI:10.1002/cplu.201500492

A series of mixed oxide nanoparticles were prepared by a coprecipitation method and characterized by many techniques. Nickel-zirconium oxide catalysts and their partially reduced magnetic counterparts were highly efficient in the direct transformation of biomass derivatives, including ethyl levulinate, fructose, glucose, cellobiose, and carboxymethyl cellulose, into γ-valerolactone (GVL) without the use of an external hydrogen source, producing a maximum GVL yield of 95.2 % at 200 °C for 3 h with hydrogen-reduced magnetic Zr₅Ni₅ nanoparticles (<20 nm). The acid-base bifunctionality of these nanocatalysts plays a synergic role in the synthesis of GVL in alcohols, whereas appropriate control of the nickel/zirconium molar ratio is able to improve the selectivity towards GVL (≈98 %), along with high formation rates (up to 54.9 mmol g^-1 h^-1). Moreover, the magnetic Zr₅Ni₅ nanoparticles were conveniently recovered by means of a magnet for five cycles with almost constant activity. Attractive separation: Acid-base bifunctional NiZr nanocatalysts with strong magnetism show high activity and reusability in the transformation of biomass derivatives, including EL, fructose, glucose, cellobiose, and carboxymethyl cellulose, into γ-valerolactone (GVL) with 95.2 % yield and 98 % selectivity (see figure).

Full text of DOI:10.1002/cplu.201500492

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DOI: 10.1002/cplu.201500492
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Direct Catalytic Transformation of Biomass Derivatives
into Biofuel Component g-Valerolactone with Magnetic
Nickel–Zirconium Nanoparticles
Hu Li,[a, b] Zhen Fang,*[a] and Song Yang[b]
A series of mixed oxide nanoparticles were prepared by a co-
precipitation method and characterized by many techniques.
Nickel–zirconium oxide catalysts and their partially reduced
magnetic counterparts were highly efficient in the direct trans-
formation of biomass derivatives, including ethyl levulinate,
fructose, glucose, cellobiose, and carboxymethyl cellulose, into
g-valerolactone (GVL) without the use of an external hydrogen
source, producing a maximum GVL yield of 95.2% at 2008C for
3 h with hydrogen-reduced magnetic Zr5Ni5 nanoparticles
(<20 nm). The acid–base bifunctionality of these nanocatalysts
plays a synergic role in the synthesis of GVL in alcohols, where-
as appropriate control of the nickel/zirconium molar ratio is
able to improve the selectivity towards GVL (ꢀ98%), along
with high formation rates (up to 54.9 mmolgÀ1 hÀ1). Moreover,
the magnetic Zr5Ni5 nanoparticles were conveniently recovered
by means of a magnet for five cycles with almost constant ac-
tivity.
Introduction
Efficient transformations of biomass to produce biofuels and
value-added chemicals are deemed as one of promising ways
to alleviate the current reliance on fossil fuel sources.[1] In
recent years, g-valerolactone (GVL) has been identified as
a green and renewable solvent to improve the performance of
biomass conversion and various organic reactions,[2] and as an
additive suitable for liquid fuels, perfumes, and food.[3] More
importantly, GVL can be employed as a precursor to produce
gasoline and diesel fuels (e.g., C8–C18 alkanes and 2-methylte-
trahydrofuran) and valuable chemicals, such as 1,4-pentane-
diol, methyl pentenoate,[4] ionic liquids,[5] and polymers.[6]
over heterogeneous catalysts.[10] In some cases, GVL can be
produced at lower temperatures (1008C), but other parame-
ters, such as high hydrogen partial pressure (e.g., 10 MPa), are
required.[11] Interestingly, remarkably enhanced activity in the
synthesis of GVL (ꢀ90% yields) from LA at relatively low tem-
peratures can be achieved with strongly acidic cocatalysts
(e.g., Amberlyst A15 and A70) or supports, such as DOWEX
50WX2-100, sulfonated polyethersulfone, and acid-functional-
ized mesoporous carbon, in combination with metal particles,
especially ruthenium.[12] It has been speculated that the syner-
gic effect between acids and noble-metal components plays
a significant role in facilitating the conversion of LA into GVL
through either catalytic hydrogenation of dehydrated prod-
ucts, a- and b-angelica lactones, or intramolecular esterification
of in situ generated 4-hydroxypentanoic acid, which is a hydro-
genated intermediate.[13] On the other hand, some non-noble-
metal catalysts (e.g., nickel, copper, iron, and cobalt) have also
been explored in the upgrading of LA to GVL to reduce pro-
duction costs.[14] Unfortunately, few examples mediated by
non-noble-metal catalysts display comparable activities to
those of noble metals in the catalytic transformation of LA and
its esters into GVL.[15]
Levulinic acid (LA) and its esters, which can be synthesized
from lignocellulosic biomass through multiple catalytic steps,
are frequently used as substrates for producing GVL with or
without enantioselectivity.[7] Among various catalytic processes,
noble metals (e.g., gold, platinum, palladium, iridium, and
ruthenium particles) show moderate to excellent activity in the
catalytic conversion of LA into GVL with hydrogen gas as the
hydrogen donor.[8] Metals in the homogeneous phase are
active for this catalytic process,[9] although high reaction tem-
peratures (ꢀ2008C) are normally required to produce GVL
with high selectivity from LA in the absence of acid additives
Both formic acid (FA) and LA can be obtained from sugars in
equal molar ratios, which results in the synthesis of GVL from
LA with FA as a hydrogen donor being highly renewable and
more attractive. Deng et al. reported that the presence of or-
ganic base (e.g., triethylamine and pyridine) could accelerate
the decomposition of FA,[16] which was responsible for the en-
hanced reactivity,[17] but a stronger base (e.g., NaOH and KOH)
resulted in decreased GVL yields because of side reactions
(e.g., condensation). Recently, a Shvo catalyst was demonstrat-
ed to be highly active for producing GVL through solvent-free
transfer hydrogenation of LA after reacting at 1008C for 8 h.[18]
[a] Dr. H. Li, Prof. Z. Fang
Chinese Academy of Sciences, Biomass Group
Key Laboratory of Tropical Plant Resources and Sustainable Use
Xishuangbanna Tropical Botanical Garden
88 Xuefulu, Kunming, Yunnan 650223 (P. R. China)
E-mail: zhenfang@xtbg.ac.cn
[b] Dr. H. Li, Prof. S. Yang
State-Local Joint Engineering Laboratory for Comprehensive
Utilization of Biomass, Center for R&D of Fine Chemicals
Guizhou University, Guiyang 550025 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500492.
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Unlike [RuCl3][16] and [Ru(acac)3] (acac=acetylacetonate),[4a,9b,c]
heterogeneous catalysts, such as silica-immobilized Ru parti-
cles, Au/ZrO2, and Cu/ZrO2 have recently been investigated for
the production of GVL from LA to simplify catalyst recovery.[19]
However, only a very limited number of non-noble metals are
acid resistant and can selectively catalyze the decomposition
of FA into H2 and CO2, rather than CO and H2O.[19c,20]
Results and Discussion
Characterization of catalysts
The XRD patterns of samples of ZrO2 and NiZr (Ni2Zr8, Ni5Zr5,
and Ni8Zr2) are illustrated in Figure 1. Neat ZrO2 calcined at
4508C exhibits a series of wide peaks at 2q=30.5 and 34.68
(tetragonal phase), 28.4 and 31.58 (monoclinic zirconia), and
50.4 and 60.18 (cubic phase). However, only two broad bands,
approximately 31.5 and 50.48, from the characteristic reflec-
tions of ZrO2 are observed in Ni2Zr8 and Ni5Zr5 oxides; this
demonstrates that NiO and ZrO2 powders primarily exist in
noncrystalline phases with small particle sizes. When the molar
ratio of Ni/Zr increases to 8:2, a range of additional diffraction
peaks at 37.2, 43.3, 62.9, 75.4, and 79.28 are detected, which
can be assigned to the reflections of the NiO crystalline struc-
ture from the (111), (200), (220), (311), and (222) planes, respec-
tively. Moreover, the actual compositions of NiZr samples de-
termined by inductively coupled plasma optical emission spec-
troscopy (ICP-OES) are consistent with the controlled Ni/Zr
molar ratios (Table 1).
By using inexpensive and abundant alcohols as hydrogen
donors, catalytic transfer hydrogenation (CTH) of LA and its
esters to form GVL over non-noble metals and oxides has at-
tracted much attention.[21] Three different reaction routes
mediated by acid and/or base sites are generally proposed for
the catalytic transformation of ethyl levulinate (EL) into GVL:
1) Lewis acids are able to initially catalyze the hydrogenation
of levulinates into 4-hydroxypentanoates, followed by lactoni-
zation, to yield GVL.[22] 2) In a basic alcoholic solution with
metal particles, GVL is most likely to be produced through the
cyclization of LA to form the hydroxy lactone, followed by hy-
drogen transfer reduction of a-angelica lactone, which is a de-
hydration product of hydroxy lactone.[23] 3) Basic sites (O2À),
with the aid of Lewis acidic sites, have recently been proposed
to synergistically activate the dissociation of hydroxyl groups
in alcohols for CTH reaction to give 4-hydroxypentanoate,
which is subjected to intramolecular esterification or transes-
terification to produce GVL.[24] Nevertheless, the one-pot trans-
formation of more accessible biomass derivatives, such as
sugars, into GVL over a single catalyst is hard to realize be-
cause of the need for multiple sequential reaction steps, which
involve the formation of hydroxymethylfurfural (HMF), LA, and
other intermediates or byproducts,[25] largely restricts its cata-
lytic activity.[26]
The textural properties of samples of ZrO2 and NiZr (Ni2Zr8,
Ni5Zr5, and Ni8Zr2) were examined by N2 adsorption–desorption
isotherms and pore size distributions (Figure 2 and Figure 1S in
the Supporting Information), and results of BET surface areas
and average pore sizes of these samples are listed in Table 1.
For samples of Ni2Zr8 and Ni5Zr5, the type IV hysteresis loops of
Herein, a series of magnetic nickel–zirconium nanoparticles
were prepared through a facile and cheap method that in-
volved coprecipitation and partial reduction. These nanoparti-
cles were tested for GVL production and easily separated from
the products by means of a magnet. Several important param-
eters, such as acid–base bifunctionality, nickel/zirconium molar
ratio, reaction time, and temperature were optimized to direct-
ly convert sugars (e.g., fructose, glucose, cellobiose and car-
boxymethyl cellulose) and EL into GVL in alcohols without the
use of an external hydrogen source.
Figure 1. XRD patterns of ZrO2 and NiZr oxides with different Ni/Zr molar
ratios.
Table 1. Physicochemical properties of ZrO2 and NiZr nanoparticles with different Ni/Zr molar ratios.
[a]
[c]
[d]
[e]
Sample
NNi/Zr
–
BET surface area [m2 gÀ1
]
Pore size [nm][b]
Acidity [mmolgÀ1
]
Basicity [mmolgÀ1
]
Ms [Am2 kgÀ1
]
Particle size [nm][f]
ZrO2
84.8
123.7
147.2
91.6
7.4
5.9
5.5
0.35
0.59
0.72
0.87
0.46
0.62
0.53
0.49
–
2.2
10.7
22.6
82.3
12.5
19.6
55.4
Ni2Zr8
Ni5Zr5
Ni8Zr2
1:4.18
1:1.27
3.68:1
14.1
[a] The Ni/Zr molar ratio (NNi/Zr) was determined by ICP-OES. [b] Average pore size was calculated by using the Barrett–Joyner–Halenda (BJH) method.
[c] Determined by NH3 temperature-programmed desorption (TPD). [d] Determined by CO2-TPD. [e] Saturation magnetization (Ms) was determined from
the corresponding H2-reduced NiZr oxides. [f] The crystallite sizes of ZrO2 and H2-reduced NiZr samples were estimated from TEM images.
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H2-reduced Ni2Zr8, Ni5Zr5, and Ni8Zr2 correspondingly increased
and was 2.2, 10.7, and 22.6 Am2 kgÀ1, respectively (Table 1); this
means the resulting magnetic materials can be separated from
solution with a magnet (Figure 4S in the Supporting Informa-
tion). The presence of Ni0 species in reduced NiZr samples can
be confirmed by X-ray photoelectron spectroscopy (XPS) with
a binding energy of 852.8 eV (Figures 5S and 6S in the Sup-
porting Information). All reduced NiZr samples are nanosized
particles (Table 1), which can be estimated from a TEM image
by taking Ni2Zr8, for example (Figure 4a). In addition, the
HRTEM (high-resolution TEM) image of H2-reduced Ni2Zr8 nano-
particles shows a series of typical lattice planes for NiO and
ZrO2 species (Figure 4b), which are consistent with the XRD
pattern (Figure 7S in the Supporting Information).
Figure 2. BET analyses of ZrO2 and NiZr oxides with different Ni/Zr molar
ratios.
isotherms suggest the presence of mesoporous structures.
With respect to the sample of Ni8Zr2, the relative pressure of
the loop significantly shifted to 0.8–1.0, which was concomi-
tant with the decreased BET surface area, but increased aver-
age pore size, compared with ZrO2, Ni2Zr8, and Ni5Zr5 (Table 1).
Furthermore, the acid/base strength and contents of the ZrO2
and NiZr samples were measured by CO2-/NH3-TPD, respective-
ly (Figures 2S and 3S, respectively, in the Supporting Informa-
tion). Two CO2 desorption bands at approximately 150 and
4508C for all tested samples are observed (Figure 2S in the
Supporting Information) from the weak and moderate base
strengths, respectively. Interestingly, the base density is closely
correlated with ZrO2 content and surface area (Table 1). On the
other hand, the temperature for NH3 desorption gradually
shifts from approximately 130 to 1508C, related to weak acidi-
ty, as the molar ratio of Ni/Zr increases from 0 to 8:2 (Figure 3S
in the Supporting Information).
Saturation magnetization (Ms) of H2-reduced NiZr samples
was also investigated by vibrating sample magnetometry
(VSM; Figure 3). With increase Ni species content, the Ms for
Figure 4. a) TEM and b) high-resolution (HR) TEM images of H2-reduced
Ni2Zr8 catalyst.
Conversion of EL into GVL
Initial experiments on the synthesis of GVL from EL in 2-propa-
nol catalyzed by different mixed metal oxides were performed
at 2308C for 0.5 h. As observed in Figure 5, a moderate yield of
GVL (65.5%) from EL (79.5% conversion) at a GVL formation
rate of 39.4 mmolgÀ1 hÀ1 was achieved over ZrO2. The catalytic
activity was further improved after introducing an additional
metal oxide (e.g., CuO, ZnO, Al2O3, Fe2O3, and NiO) in combina-
tion with ZrO2, as a result of increased acidity. In particular, the
Figure 3. VSM curves of H2-reduced NiZr catalysts with different Ni/Zr molar
ratios.
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longing the reaction time to 1 h, a slight increase in
GVL yield from 91.2 to 92.6% is observed, whereas
the selectivity towards GVL decreases to 95.4%. As
the reaction time reaches 2 h, both the GVL yield and
selectivity decrease, owing to the formation of poly-
meric compounds (PC1–4, 2% yield; identified by
GC-MS in Figure 8S in the Supporting Information)
derived from either GVL or EL (Scheme 1S in the Sup-
porting Information, pathways 1 and 2).[21d] The con-
tinuous decrease in GVL yield to 85.8% after 5 h
proves that high temperatures are prone to result in
the degradation of GVL through condensation reac-
tions (7% yield of condensed products). However, no
detectable ring-opening products of GVL or valerate
were observed in the reaction mixtures; this may be
attributed to the lack of strong Brønsted acid sites
provided by zeolites[29] or residual aqueous acids[21e]
for GVL ring opening or deoxygenation (removal of
hydroxyl groups). At 2008C (Figure 6b), the highest
GVL selectivity of 97.8%, along with a yield of 93.8%,
was achieved after 3 h. Interestingly,
a certain
amount of IPL (3.2–17.7% yield) was detected in
a short reaction time of 0.5–2 h; this demonstrates
that the key intermediate towards GVL is most likely
to be IPL, despite intermediate compounds (ICs),
such as isopropyl and ethyl 4-isopropoxypentanoate,
also being identified by GC-MS in trace amounts
(Scheme 1S, pathway 3, and Figure 8S in the Support-
ing Information).[21d] More IPL (up to 38.7% yield) is
formed at a low temperature of 1708C (Figure 6c),
which supports the reaction pathway 3 illustrated in
Scheme 1S in the Supporting Information. However,
it is almost impossible to avoid side reactions (<10%
yields), which lead to a decrease in GVL yield and se-
lectivity at even lower temperatures (e.g., 170 and
2008C). On the contrary, a much longer time is re-
quired to obtain high yields of GVL.
Figure 5. Catalytic conversion of EL into GVL in 2-propanol over mixed metal oxides (EL
(0.65 g, 5.5 wt%), 2-propanol (11.8 g), catalyst (0.15 g), at 2308C and 0.5 h). a) Ni5Zr5 was
treated with tetraethoxysilane (0.1 g) to cover the Lewis acid sites;[27a] b) Ni5Zr5 was titrat-
ed with benzoic acid (0.1 g) to poison the Lewis base sites.[27b]
Ni5Zr5 catalyst exhibits the highest GVL yield (91.2%), selectivity
Direct conversion of sugars into GVL
(97.5%), and formation rate (54.9 mmolgÀ1 hÀ1), possibly owing
to its moderate pore size (5.5 nm), surface area (147.2 m2 gÀ1),
and acid/base contents (0.72/0.53 mmolgÀ1), compared with
its counterparts Ni2Zr8 and Ni8Zr2 (Table 1). Moreover, poisoning
either the Lewis acid or base sites of the Ni5Zr5 catalyst[27] re-
sults in a significant decrease in its catalytic performance,
which indicates that Lewis acid and base sites appear to play
a synergic role in Meerwein–Ponndorf–Verley or CTH reduc-
tion,[28] as well as subsequent lactonization to boost the cata-
lytic transformation of EL into GVL.
The one-pot synthesis of GVL directly from biomass-derived
carbohydrates, including fructose, glucose, cellobiose, and car-
boxymethyl cellulose, was subsequently examined, and all ex-
periments were performed in an alcoholic solvent at 2008C. In
Table 2, both Ni5Zr5 oxide and H2-reduced Ni5Zr5 catalyzed the
conversion of EL into GVL in high yields, as determined by GC
(93.8 and 95.2%, respectively; Table 2, entries 1 and 2), as well
as those isolated (88.2 and 91.9%, respectively; 1H and
13C NMR spectra provided in Figure 9S in the Supporting Infor-
mation) by column chromatography (ethyl acetate/hexane=
1:5). Apart from GVL yields, the mass balance and GVL forma-
tion rates of NiZr samples are also superior to previously re-
ported catalytic systems (Table 2, entries 3–8). Both of them
are almost inactive for the synthesis of GVL from fructose after
3 h, except for a small amount of IPL generated (Table 2, en-
tries 9 and 10). Instead of 2-propanol, ethanol as a solvent and
hydrogen donor is able to somehow improve the conversion
Effect of reaction temperature and time on EL-to-GVL
conversion in ethanol
Based on the above experiments, Ni5Zr5 was used as a catalyst
to study the influence of temperature and time on the produc-
tion of GVL from EL (Figure 6). At 2308C, a high GVL selectivity
(97.5%) is obtained in a short time of 0.5 h (Figure 6a). By pro-
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