Cascade upgrading of γ-valerolactone to biofuels

Abstract

Cascade upgrading of γ-valerolactone (GVL), produced from renewable cellulosic biomass, can be selectively converted to biofuels pentyl valerate (PV) and pentane in one-pot on the bifunctional Pd/HY catalyst. Excellent catalytic performance (over 99% conversion of GVL, 60.6% yield of PV and 22.9% yield of pentane) were achieved in one-step. These biofuels can be targeted for gasoline and jet fuel applications.

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6984 | Chem. Commun., 2015, 51,6984--6987 This journal is
©
The Royal Society of Chemistry 2015
Cite this: Chem. Commun., 2015,
51,6984
Cascade upgrading of c-valerolactone to biofuels†
Kai Yan,*
ab
Todd Lafleur,
c
Xu Wu,
b
Jiajue Chai,
a
Guosheng Wu
c
and Xianmei Xie*
b
Cascade upgrading of c-valerolactone (GVL), produced from renewable
cellulosic biomass, with selective conversion to biofuels pentyl valerate
(PV) and pentane in one pot using a bifunctional Pd/HY catalyst is
described. Excellent catalytic performance (over 99% conversion of
GVL, 60.6% yield of PV and 22.9% yield of pentane) was achieved in
one step. These biofuels can be targeted for gasoline and jet fuel
applications.
Sustainable production of biofuels from renewable biomass
has been demonstrated as a promising alternative to reduce our
dependence on the dwindling fossil resources.
1
As one of the
major products of plant biomass, renewable g-valerolactone
(GVL) has been suggested as a platform for the production of
biofuels (e.g., valerate esters and alkanes) and fine chemicals.
2
GVL exhibits the most typical characteristics of an ideal sustainable
green solvent for the production of either energy or carbon-based
consumer products.
2b,c
These biof uels deliver the large vo lumes
needed for the transport sector and are potentially more sustainable.
Along the line for the transformation of GVL as shown in Scheme 1,
pentyl valerate (PV) has more appropriate polarities, better volatility,
and higher ignition properties than current and alternative
candidate biofuels (e.g., 2-methyltetrahydrofuran and ethanol).
3
These properties make it compatible for either gasoline or diesel
applications.
3a,4
The synthesis of PV often requires pentanoic
acid (PA) and pentanol, while both of them can be produced
from GVL (Scheme 1). PA, a member of short-chain straight fatty
acids, is widely used as an intermediate in the manufacture of
numerous valuable products,
5
while pentanol is a widely used as
a solvent and a replacement for gasoline.
1c,6
Further dehydration
of pentanol and subsequent hydrogenation would produce
pentane in the line (Scheme 1), wherein pentane is a highly
attractive transportation fuel. The efficient and selective synthesis
of these biofuels (e.g.,PV,pentanolandpentane)inonestep
without the use of extra alcohol reactant under mild conditions
will be ideal for the practical utilization.
Lange et al.
3a
reported the successful synthesis of valerate
biofuels through the integration of several steps and found that
GVL could be directly converted into PV (20–50% selectivity)
using Pt/TiO
2
or Pd/TiO
2
catalysts at 275–300 1C in a fixed-bed
reactor. Besides, they further demonstrated that 90% yield of
PA could be obtained from levulinic acid (LA) using Pt-loaded onto
aSiO
2
-bound H-ZSM-5 catalyst. Through catalyst regeneration by
calcination in air, the catalytic activity in the production of PA can
be maintained. To reduce the high temperature and improve the
yield of PV, Chan-Thaw et al.
7
selectively synthesized PV from GVL
and pentanol using a Cu/SiO
2
–ZrO
2
catalyst in a batch reactor. PV
can be obtained at a conversion of over 90% and selectivity of up to
83% in pentanol at 250 1Cand20h,wherepentanolwasusedas
the reactant and solvent with a GVL versus pentanol molar ratio of
1:10. Several other groups have reported the selective production
of PV through the hydrogenation of LA in ethanol or pentanol
solvent. Good performances have been reported, wherein ethyl
levulinate (EV) and PV were often produced together.
5,8
LA is
relatively easily hydrogenated to PA in a relatively high yield, which
undergoes subsequent esterification with pentanol to produce PV.
The above systems are advanced.However,extraalcoholfuels
(pentanol or ethanol) must be also used as reactants and solvents,
which may not follow with the requirements of sustainable develop-
ment. Besides, some of the reaction systems are often operated at
high temperature. The strong corrosiveness of gas-phase LA inputs
Scheme 1 Cascade upgrading of GVL to biofuels.
a
School of Engineering, Brown University, Providence, RI 02906, USA.
E-mail: kai_yan@brown.edu, xxmsxty@sina.com
b
College of Chemistry and Chemical Engineering, Taiyuan University of Technology,
Taiyuan 23001, China
c
Department of Chemistry, Lakehead University, Thunder Bay, ON P7B 5E1,
Canada
† Electronic supplementary information (ESI) available: Experimental details and
characterization information of XRD, EDX, SEM, XPS and NH
3
-TPD analyses. See
DOI: 10.1039/c5cc01463h
Received 17th February 2015,
Accepted 12th March 2015
DOI: 10.1039/c5cc01463h
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special requirements toward the fixtures, which limits the practical
utilization. A more promising route is to efficiently produce PV in a
high yield without the use of extra alcohol reactant in one step under
mild conditions.
So far, the conversion of GVL to liquid hydrocarbon fuels has
been reported to follow two possible routes (Scheme S1, ESI†). One
route (Scheme S1a, ESI†)istoproducepentanoicacidandfurther
upgrade it to butene, which can be subsequently oligomerized to
C12 alkanes.
1a,9
In the other route (Scheme S1b, ESI†)theproduc-
tion of PA occurs with subsequent ketonization to 5-nonanone ,
which can be further upgraded to C9 alkanes or C18 alkanes.
10
So
far, the one-step production of C5 alkanes from GVL has not been
reported. It is highly desirable to develop selective cascade upgrading
of GVL for the efficient production of biofuels (PV, pentanol and
pentane) in one step without the use of the extra alcohol under mild
conditions. Herein, we report a one-step selective and flexible
cascade upgrading of GVL with perfect conversion to biofuels
(60.6% yield of PV and 22.9% yield of pentane) without the use of
additional alcohol reactants in a batch reactor.
Upgrading of GVL to PV theoretically proceeds in four steps:
(i) opening of the GVL ring and further dehydration to pentanoic
acid; (ii) subsequent hydrogenati on of pentanoic acid to PA;
(iii) partial hydrogenation of PA to pentanol; (iv) esterification of
PA and pentanol to PV. However, theconversionofGVLtopentane
precedes the same first three steps (i–iii) and the different step
(iv) composed of the dehydration of pentanol and subsequent
hydrogenation to pentane. In our case, for the selective production
of PV and pentane these four steps occurred in one step using a
bifunctional Pd/HY catalyst. From the initial screening of the five
different supported Pd catalysts (Fig. S1, ESI†), it was clearly seen
that the bifunctional Pd/HY catalyst displayed high selectivity toward
the formation of PV and pentane, where the metal Pd site was
efficient for the hydrogenation and the acidic sites of the HY support
promoted the dehydration and esterification. Besides, there was no
significant difference in terms of the activity at higher loadings of Pd.
Thus, we further concentrated upon the 5% Pd/HY catalyst for
catalytic upgrading of GVL to biofuels (PV and pentane). X-ray
diffraction (XRD) patterns (Fig. S2, ESI†)indexedas2y of (111) at
40.11,(200)at46.31,(220)at67.71,and(311)at81.81 were identified
as a single fcc phase of Pd.
11
The diffraction intensity of fcc Pd
increased with the metal loading from 3% to 5% (Fig. S2b and c,
ESI†). The formation of Pd(0) was further confirmed using X-ray
photoelectron spectroscopy (XPS), as shown in Fig. S3 (ESI†). The
presence of two prominent sets of Pd (3d) peaks, corresponding to
the 3d
3/2
(340.6 eV) and 3d
5/2
(335.3 eV) orbital states , demonst rated
that Pd(0) was present on the surface.
12
Besides, very weak peaks
were observed at 341.7 eV and 336.5 eV, which was possible due to
the interaction between the Pd and the oxygen from the HY
support.
11b,12a
Additional evidence for the existence of each element
(Si, Al, and Pd) was provided using energy-dispersive X-ray spectro-
scopy (EDX) (Fig. S4, ESI†), the peaks appeared where the element
should be.
To improve the yields of biofuels (PV and pentane), we further
investigated the crucial reaction parameters (Fig. 1). It was found
that lower temperature (260 1C), higher pressure (80 bar H
2
) and
longer reaction time (24 h) are better for the production of PV.
50.5% yield of PV and 13.1% yield of pentanol were obtained at
260 1C, 24 h and 80 bar H
2
.AsshowninFig.1A,thereactiontime
and hydrogen pressure have a crucial influence on the yield of PV.
Upon prolonging the reaction time to 30 h at higher pressure
(80 bar), the yield of PV increased significantly and 60.6% yield of
PV was achieved. So far, this is the highest yield of PV reported for
heterogeneous catalysts in a batchreactorsystem.TheGCspectrum
for this case is shown in Fig. S5a (ESI†). Trace amounts of side
products (e.g.,1,1-oxybispentaneether,1,4-pentanediolandMTHF)
were also detected. Further prolonging the reaction time to 36 h, the
side reactions were clearly promoted. For practical utilization, both
high yields and low costs are required. In our case, the GVL/catalyst
weight ratio was 22, which is 2-fold higher than the previously
reported value of 10.
7
For the selective production of pentane, it was observed that
the yield was increased at higher temperature (280 1C) and
longer reaction time (24 h), which was due to the step (iii) being
enhanced under these conditions as suggested by the increased
amount of the pentanol intermediate. The highest yield of
pentane of 22.9% was obtained at 280 1C, 24 h and 40 bar
H
2
. To the best of our knowledge, this is the first time that
production of pentane from GVL in such a high yield has been
reported. The GC spectrum for this case is shown in Fig. S5b
(ESI†). Traces of butene and 5-nonanone were not found in the
gas product from our GC results. We attributed the formation
of small quantities of MTHF and 1,1-oxybispentane ether
generated through the deep hydrogenation of GVL and
potential dehydration of pentanol to 1,1-oxybispentane ether.
At 280 1C, more hydrogen (e.g., 80 bar) was disadvantageous for
Fig. 1 Cascade upgrading of GVL to PV and pentane. Reaction condi-
tions: 0.10 g catalyst, 5 mL octane and a stirring speed of 1000 rpm; conv.:
the conversion of GVL; Y(PV): the yield of PV; PA: pentanoic acid, PL:
pentanol.
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the selective production of pentane because the side products
MTHF, 1,4-pentanediol and 1,1-oxybispentane ether were
clearly increased. Besides, it was found that a high amount of
PA was produced under the conditions of low temperature
(260 1C), low pressure (40 bar H
2
) and short reaction time
(12 h), as shown in Fig. 1A. For comparison, we further reduced
the reaction temperature and it was clearly found to have a
crucial influence on the yield of PA (Fig. 1B). The highest PA yield
(over 73%) was obtained at a low temperature of 240 1C, for 12 h
and 40 bar H
2
. This performance is much higher than that
obtained in a batch reactor, as reported in a previous report.
5a
To understand the reaction pathway, we tested different
intermediates and the catalytic results are compared in Fig. S6
(ESI†). When PA was chosen as the reactant, a low conversion of
37.5% was observed, which was possible due to the strong coke
formation when the catalyst was exposed to a pure acidic environ-
ment, as previously reported by Luo et al.
5a
In this case, the main
product was a PV ester and 1,1-oxybispentane ether, which suggested
that two reactions occurred. One was the esterificati on of PA and the
produced pentanol to the PV ester. The other was the dehydration of
pentanol that occurred on acidic sites to 1,1-oxybispentane ether. In
the case of pentanol as the reactant, a large amount of ether and a
small amount of PV were produced.Asmallamountofpentanewas
also traced in this case. When pentanol and PA were selectively
chosen as substrates, the PV yield increased clearly to 53.5% in 12 h.
Based on these data, the probable reaction pathway is proposed in
Scheme S2 (ESI†).
To check the recyclability of the 5% Pd/HY catalyst, recycling
experiments were performed under the test conditions 260 1C,
12 h and 80 bar H
2
. The catalyst can be easily separated from
the reaction solution by simple centrifugation. The catalyst was
then dried at 90 1 C for 12 h and then used for the GVL
upgrading. The catalytic results are shown in Fig. 2a. 41.5%
yield of PV and 29.3% yield of PA at a conversion of 93.6% GVL
were obtained, which were close to the catalytic performance on
the fresh 5% Pd/HY catalyst under these conditions. A good
performance was possible due to the beneficial H
2
and the
hydrophobic character of the HY zeolite in octane solvent that
reduced the coke formation.
14
However, in the second and the
third cycles, a decrease in performance was observed. In the
third cycle, a poor performance was observed with 12.9% PV
and 10.5% PA at 67.2% conversion of GVL, which was possible
due to the coke formation or the carbon deposit on the surface
over several cycles.
3a,14
XRD was used to explore the structural stability of the spent
5% Pd/HY catalyst. As shown in Fig. S7 (ESI†), the resulting
metal catalysts (a and b) display similar phase structures, which
confirmed that the catalyst was stable under the reaction
conditions. Transmission electron microscopy (TEM) images
of the fresh 5% Pd/HY (Fig. 2b) and the spent 5% Pd/HY
(Fig. 2c) catalyst recycled after the third run, demonstrated no
clear crystalline structure change, which is in good agreement
with XRD analysis (Fig. S7, ESI†). However, clear aggregation
and bigger particle sizes of Pd in the spent 5% Pd/HY catalyst
after the third run (Fig. 2c) were observed, which was possible
due to exposure of Pd nanoparticles to the reaction environment
at 260 1C for long reaction times over several recycles. XPS
(Fig. S8, ESI†) was further used to understand the Pd change
on the surface and displayed two prominent sets of Pd (3d
3/2
and
3d
5/2
) peaks at 342.8 eV (3d
3/2
) and 337.1 eV (3d
5/2
), confirming
the visible Pd oxide state in the spent case.
11b
Temperature-
programmed desorption of ammonia (NH
3
-TPD) analysis (Fig. S9,
ESI†)wasusedtocomparetheaciditychangebetweenthefreshand
spent catalysts; it was clearly found that that the peaks have clear
shifts, which indicated that the acidic sites were possibl y covered.
5a,13
Additional evidence was revealed by N
2
adsorption–desorptio n (BET)
analysis. The N
2
-isotherms of the fresh 5% Pd/HY catalyst displays
the classic type II curve of microporous materials (Fig. 2d, (i)) and the
spent 5% Pd/HY catalyst exhibits the typical IV isotherm of meso-
porous structures (Fig. 2d, (ii)). It indicated increased particle sizes
and expanded pores in the spent catalyst, which matched well with
the above TEM analysis (Fig. 2b and c). The surface areas, pore sizes
and the pore volume of the HY support and fresh 5% Pd/HY and
spent 5% Pd/HY catalysts are given in Table S1 (ESI†). The details
of BET plots and pore size distribution are shown in Fig. S10 (ESI†).
In comparison with the HY support (Table S1, no. 1, ESI†), a little
larger pore size of 2.12 nm, a lower BET surface area of 451 m
2
g
!1
with the micropore volume of 0.1850 cm
3
g
!1
(no. 2) were obtained
for the fresh 5% Pd/HY catalyst. It indicated that the Pd nano-
particles were successfully introduced on the porous HY zeolite
support. However, in the case of the spent 5% Pd/HY catalyst
recycled after the third cycle, the pore size was increased largely to
5nm,thesurfaceareawasreducedto135m
2
g
!1
and the
microporous volume was decreased to 0.0201 cm
3
g
!1
(no. 3). These
data clearly confirmed the increased aggregation of metal nano-
particles and more pore blocking over several recycles.
Fig. 2 (a) Catalytic activity of the spent and recycled 5% Pd/HY catalyst
under the reaction conditions of 260 1C, 12 h, 80 bar H
2
, 0.10 g catalyst,
5 mL octane and a stirring speed of 1000 rpm. Other products
1,1-oxybispentane, methyl valerate, 1,4-pentanediol and MTHF were
observed. Cycle 1 to Cycle 3: the catalyst was recycled and used for the
next run without reduction; (b) the TEM image of the fresh 5% Pd/HY
catalyst; (c) the TEM image of the spent 5% Pd/HY catalyst after the third
cycle; (d) N
2
-isotherm curves of the fresh 5% Pd/HY (i) and the spent 5%
Pd/HY (ii) catalyst after three cycles.
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To get rid of the possible carbon deposit or coke on the
catalyst surface, simple calcination and further reduction of
the spent catalyst was performed (more details are given in the
ESI†) and the resulting catalyst was used for GVL upgrading.
The TEM image (Fig. 3a) of the reduced 5% Pd/HY catalyst in
the fourth run confirmed no structure change. N
2
-sorption
analysis (Fig. 3b) further suggested a stable porous structure
as indicated by the similar values of the surface area, pore size
as well as pore volume (Table S1, no. 4, ESI†) to those of the
fresh 5% Pd/HY catalyst (Table S1, no. 2, ESI†). Thereby, a
better distribution with a lower degree of pore-blocking was
conceivable. XPS (Fig. 3c) further confirmed the successful
reduction and the absence of the oxidation state of PdO. We
further employed the reduced catalyst in the catalytic GVL
upgrading and stable performance was obtained over several
runs (Fig. 3d). Even in the fourth run, 45.7% PV and 41.0% PA
were produced at 98.7% conversion of GVL. Analysis of the
reaction solution showed that no detectable leaching of Pd was
found. The high stability was also possible due to the increased
hydrophobic character of the HY zeolite in octane solvent.
14
In conclusion, a one-step selective cascade upgrading of GVL
to biofuels pentyl valerate and pentane has been successfully
achieved using a bifunctional 5% Pd/HY catalyst. 60.6% yield of
pentyl valerate and 22.9% yield of pentane were flexibly pro-
duced upon the perfect conversion of GVL in a batch reactor
system without the use of an additional pentanol reactant and/
or a solvent under relatively mild conditions. The bifunctional
Pd/HY catalyst maintained stable performance over several
runs. The catalytic system and the bifunctional catalyst developed
are promising for general biofuel upgrading processes.
K. Yan is thankful for the help from Prof. Dr A. Peterson at
Brown University, and the kind introduction to this area by PhD
supervisor Prof. Dr W. Leitner (RWTH Aachen University) and
Dr N. Theyssen (Max-Planck-Institute for Coal Research). The
authors acknowledge the fund from the National Natural
Science Foundation of China (Grant No. 50872086) and the
Science Technology key projects in Shanxi Province (Grant No.
20130321033-02).
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Fig. 3 (a) TEM image of the reduced 5% Pd/HY catalyst in the fourth
run; (b) N
2
-isotherm curves of the reduced 5% Pd/HY catalyst in the fourth
run; (c) XPS analysis of the reduced 5% Pd/HY catalyst in the fourth run;
(d) catalytic activity of the reduced 5% Pd/HY catalyst under the reaction
conditions of 260 1C, 12 h, 80 bar H
2
,0.10gcatalyst,5mLoctaneandastirring
speed of 1000 rpm. Other products observed were 1,1-oxybispentane, methyl
valerate, 1,4-pentanediol and MTHF.
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