γ-valerolactone ring-opening and decarboxylation over SiO 2/Al2O3 in the presence of water

Article abstract of DOI:10.1021/la101424a

γ-Valerolactone (GVL) has been identified as a promising, sustainable platform molecule that can be produced from lignocellulosic biomass. The chemical flexibility of GVL has allowed the development of a variety of processes to prepare renewable fuels and chemicals. In the present work involving a combination of computational and experimental studies, we explore the factors governing the ring-opening of GVL to produce pentenoic acid isomers, as well as their subsequent decarboxylation over acid catalysts or hydrogenation over metal catalysts. The ring-opening of GVL has shown to be a reversible reaction, while both the decarboxylation and hydrogenation reactions are irreversible and kinetically controlled under the conditions studied (temperatures from about 500 to 650 K). The most significant contributor to lactone reactivity toward ring-opening is the size of the ring, with γ- lactones being more stable and less readily opened than δ- and ε-analogues. We have observed that the presence of either a C=C double bond or a lactone (which opens to form a C=C double bond) is necessary for appreciable rates of decarboxylation to occur. Olefinic acids exhibit higher rates of decarboxylation than the corresponding lactones, suggesting that the decarboxylation of alkene acids provides a lower energy pathway to olefin production than the direct decarboxylation of lactones. We observe lower rates of decarboxylation as the chain length of alkene acids increases; however, acrylic acid (3-carbon atoms) does not undergo decarboxylation at the conditions tested. These observations suggest that particular double bond configurations yield the highest rates of decarboxylation. Specifically, we suggest that the formation of a secondary carbenium ion in the β position leads to high reactivity for decarboxylation. Such an intermediate can be formed from 2- or 3-alkene acids which have at least four carbon atoms.

Full text of DOI:10.1021/la101424a

pubs.acs.org/Langmuir
© 2010 American Chemical Society
γ-Valerolactone Ring-Opening and Decarboxylation over
SiO2/Al2O3 in the Presence of Water†
Jesse Q. Bond, David Martin Alonso, Ryan M. West, and James A. Dumesic*
Department of Chemical and Biological Engineering, University of Wisconsin, Madison, Wisconsin 53706
Received April 9, 2010. Revised Manuscript Received May 10, 2010
γ-Valerolactone (GVL) has been identified as a promising, sustainable platform molecule that can be produced from
lignocellulosic biomass. The chemical flexibility of GVL has allowed the development of a variety of processes to prepare
renewable fuels and chemicals. In the present work involving a combination of computational and experimental studies,
we explore the factors governing the ring-opening of GVL to produce pentenoic acid isomers, as well as their subsequent
decarboxylation over acid catalysts or hydrogenation over metal catalysts. The ring-opening of GVL has shown to be a
reversible reaction, while both the decarboxylation and hydrogenation reactions are irreversible and kinetically
controlled under the conditions studied (temperatures from about 500 to 650 K). The most significant contributor to
lactone reactivity toward ring-opening is the size of the ring, with γ- lactones being more stable and less readily opened
than δ- and ε-analogues. We have observed that the presence of either a CdC double bond or a lactone (which opens to
form a CdC double bond) is necessary for appreciable rates of decarboxylation to occur. Olefinic acids exhibit higher
rates of decarboxylation than the corresponding lactones, suggesting that the decarboxylation of alkene acids provides a
lower energy pathway to olefin production than the direct decarboxylation of lactones. We observe lower rates of
decarboxylation as the chain length of alkene acids increases; however, acrylic acid (3-carbon atoms) does not undergo
decarboxylation at the conditions tested. These observations suggest that particular double bond configurations yield
the highest rates of decarboxylation. Specifically, we suggest that the formation of a secondary carbenium ion in the
β position leads to high reactivity for decarboxylation. Such an intermediate can be formed from 2- or 3-alkene acids
which have at least four carbon atoms.
1. Introduction
applications as a precursor for chemicals and fuels, as illu-
strated in Figure 1. GVL can be produced in high yield
(>95%) by reduction of levulinic acid,9,10 the latter of which
has been produced from waste biomass at the pilot scale
(Biofine process11,12). GVL synthesis can be achieved, typically
in the presence of Ru-based catalysts,13 using the formic acid
coproduct (from levulinic acid production) as an in situ source
of hydrogen.9
GVL can be used directly as a fuel additive in a capacity similar
to ethanol14 or as an environmentally benign solvent.15 Alterna-
tively, it can be hydrogenated to form 1,4-pentanediol,16 which
readily undergoes dehydration to form methyl tetrahydrofuran, a
P-series fuel that can be blended to high levels (70% by volume)
with gasoline for use in the present transportation infrastructure.17
By reaction with formaldehyde, GVL can be converted into
R-methylene-γ-valerolactone, a precursor of acrylic polymers.13
Through transesterification, GVL can be converted into methyl-
pentanoate which can be processed by hydroformylation, hydro-
cyanation, or hydroxycarbonylation to yield caprolactone, capro-
lactam, or adipic acid, respectively.18
Research directed toward the replacement of petroleum with
renewable feedstocks in the production of fuels and chemicals,
such as utilization of lignocellulosic biomass, has received in-
creased attention in the past decade. In 2004, the U.S. Depart-
ment of Energy published a report1 identifying 15 compounds as
those having high potential for application in future biorefineries.
In 2010, this list was revised to include more recent advances in
biomass conversion technologies.2 Among the compounds con-
sidered in the 2010 review, ethanol and glycerol derivatives are
already being produced at a commercial scale. The production of
other interesting platform molecules, for example, furans, xylitol,
sorbitol, lactic acid, and levulinic acid, have been studied exten-
sively at the laboratory scale, and various strategies for their
utilization have been proposed.3-8 In this respect, γ-valerolactone
(GVL) has been identified as a platform molecule with many
† Part of the Molecular Surface Chemistry and Its Applications special
issue.
*To whom correspondence should be addressed. E-mail: dumesic@
engr.wisc.edu. Fax: þ1 608 262 5434.
(1) Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass. Vol. I -
Results of Screening for Potential Candidates from Sugars and Synthesis Gas; NREL
Report DOE/GO-102004-1992; U.S. Department of Energy, 2004. http://www1.eere.
energy.gov/biomass/pdfs/35523.pdf.
(9) Deng, L.; Li, J.; Lai, D. M.; Fu, Y.; Guo, Q. X. Angew. Chem., Int. Ed. 2009,
48, 6529–6532.
(10) Heeres, H.; Handana, R.; Chunai, D.; Rasrendra, C. B.; Girisuta, B.;
Heeres, H. J. Green Chem. 2009, 11, 1247–1255.
(11) U.S. Patent 4897497, 1990
(2) Bozell, J. J. Green Chem. 2010, 12, 525–728.
(3) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. Ed. 2007, 46,
7164–7183.
(12) U.S. Patent 5608105, 1997
(4) Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.;
Fitzpatrick, S. W.; Bilski, R. J.; Jarnefeld, J. L. Resour., Conserv. Recycl. 2000, 28,
227–239.
(5) Roman-Leshkov, Y.; Chheda, J. N.; Dumesic, J. A. Science 2006, 312, 1933–1937.
(6) Blommel, P. G.; Keenan, G. R.; Rozmiarek, R. T.; Cortrigth, R. D. Int.
Sugar J. 2008, 110, 672.
(13) Manzer, L. E. Appl. Catal,. A 2004, 272, 249–256.
(14) Horvath, I. T.; Mehdi, H.; Fabos, V.; Boda, L.; Mika, L. T. Green Chem.
2008, 10, 238–242.
(15) Horvath, I. T. Green Chem. 2008, 10, 1024–1028.
(16) Mehdi, H.; Fabos, V.; Tuba, R.; Bodor, A.; Mika, L. T.; Horvath, I. T. Top.
Catal. 2008, 48, 49–54.
(7) Kunkes, E. L.; Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; Gartner,
C. A.; Dumesic, J. A. Science 2008, 322, 417–421.
(8) Serrano-Ruiz, J. C.; Dumesic, J. A. ChemSusChem 2009, 2, 581–586.
(17) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044–4098.
(18) Lange, J. P.; Vestering, J. Z.; Haan, R. J. Chem. Commun. 2007, 33, 3488–
3490.
Langmuir 2010, 26(21), 16291–16298
Published on Web 06/01/2010
DOI: 10.1021/la101424a 16291

Article
Bond et al.
Figure 1. Reaction pathways for the conversion of γ-valerolactone to fuels and chemicals.
Our group has recently reported two strategies for the produc-
tion of high energy density diesel and jet fuel range hydrocarbons
from GVL. In one processing strategy, GVL is converted to
pentanoic acid, which is used to produce 5-nonanone by ketoni-
zation at high yields (92%).19 5-Nonanone can then be deoxyge-
nated for use in diesel fuel blends or hydrogenated/dehydrated to
produce nonene, which is an appropriate oligomerization feed-
stock for the production of C18 olefins.20 In a second processing
strategy, GVL can undergo decarboxylation to produce butene,
which can be processed directly for the production of branched,
jet fuel range olefins by oligomerization.21 The production of
pentanoic acid takes place over bifunctional metal/acid catalysts
(e.g., Pd-Nb2O5) at 623 K and 35 bar with a hydrogen cofeed.
The production of pentanoic acid occurs through ring-opening
over the acidic niobia support to form a mixture of pentenoic
acids. Then, in the presence of Pd and a hydrogen cofeed, the
CdC double bond is hydrogenated to form pentanoic acid. At
these conditions, the rate of decarboxylation of the intermediate
(pentenoic acid) is slow relative to that of hydrogenation, and
only pentanoic acid and unconverted GVL were observed in the
liquid effluent.19 Over an acidic catalyst, such as SiO2/Al2O3, in
the absence of a metallic hydrogenation component, decarboxy-
lation of GVL is the primary reaction and high yields of butene
and CO2 are achieved from aqueous solutions (30-60 wt %) of
GVL at 648 K and pressures ranging from 1 to 36 bar.21
between GVL and pentenoic acids, and the mechanism of
decarboxylation, both of which occur in the presence of solid
acid catalysts. We first present a computationally derived
overview of the thermodynamics governing the production of
pentenoic acid, butene/CO2, and pentanoic acid from GVL.
Additionally, we have included experimental studies to probe
the extent of equilibrium and kinetic control in the afore-
mentioned reactions. The mechanism of pentenoic acid decar-
boxylation has not been previously described; as such, we have
carried out experimental studies using various feedstocks, and we
present a pathway for the decarboxylation of alkene acids and
lactones, providing insight regarding molecular structures appro-
priate for acid catalyzed decarboxylation. We note that we have
attempted (to the best of our ability) to pattern our approach to
that employed masterfully by Gabor Somorjai throughout his
career: to search for fundamental underlying principles toward
the solution of practical problems faced by society.
2. Material and Methods
2.1. Materials. Amorphous SiO2/Al2O3 obtained from
Grace-Davison/Davicat (SIAL 3113) was used as an acid catalyst
for the decarboxylation of various lactones and organic acids.
Prior to reaction kinetics studies, the SiO2/Al2O3 catalyst was
dried and calcined in situ under flowing air at 723 K. A 1 wt %
Pd/C catalyst was prepared by incipient wetness impregnation of
a carbon support (Norit) with a 10 wt % tetraamine palladium(II)
nitrate solution in 10 wt % nitric acid (Aldrich). After impregna-
tion, the catalyst was dried overnight at 393 K and calcined for 3 h
in flowing 20% O2 in He gas (250 cm3(STP)/min) at 533 K.
γ-Valerolactone (GVL) (Sigma-Aldrich, >98%), δ-valerolac-
tone (DVL) (Sigma-Aldrich, tech. grade), γ-caprolactone (GCL)
(Sigma-Aldrich, 98%), ε-caprolactone (ECL) (Sigma-Aldrich,
97%), 2-pentenoic acid (2PEA) (Sigma-Aldrich, 97%), 3-pente-
noic acid(3PEA) (Sigma-Aldrich, 95%), 4-pentenoic acid (4PEA)
(Sigma-Aldrich, 97%), 3-butenoic acid (BEA) (Sigma-Aldrich,
In the present paper, we address in greater detail the various
chemical transformations of GVL in the presence of metal or acid
catalysts. Of particular interest is the reversible conversion
(19) Serrano-Ruiz, J. C.; Wang, D.; Dumesic, J. A. Green Chem. 2010, 12, 574–
577.
(20) Alonso, D. M.; Bond J. Q.; Serrano-Ruiz, J. C.; Dumesic, J. A. Green
Chem. 2010, published online April 26, http://dx.doi.org/10.1039/C001899F.
(21) Bond, J. Q.; Alonso, D. M.; Wang, D.; West, R. M.; Dumesic, J. A. Science
2010, 327, 1110–1114.
16292 DOI: 10.1021/la101424a
Langmuir 2010, 26(21), 16291–16298

Bond et al.
Article
Figure 2. Reaction pathways for ring-opening of γ-valerolactone to pentenoic acids and subsequent decarboxylation to butene, or
hydrogenation to pentanoic acid. ΔG° and ΔH° (in brackets) are included in kJ/mol for each reaction illustrated at standard conditions.
97%), 2-hexenoic acid (2HEA) (Sigma-Aldrich, >98%), penta-
noic acid (PAA) (Sigma-Aldrich >99%), propanoic acid (Sigma-
Aldrich >99.5%), acrylic acid (Sigma-Aldrich, 99%), 5,6-dihydro-
2H-pyran-2-one (DHP) (Sigma-Aldrich, tech. grade), R-angelica
lactone (AAL) (Sigma-Aldrich, 98%), and decane (Sigma-aldrich,
>99%) were used as purchased without further purification.
2.2. Computational Studies. Gaussian 0322 software was
used for simulation of the thermodynamic properties of the
molecules illustrated in Figure 2. Geometry optimizations and
subsequent frequency calculations were performed using B3LYP/
6-311þG(2d,p). Frequency calculations provided estimates for
standard changes of enthalpy, entropy, and Gibbs free energy,
which were used in estimating thermodynamic properties over the
range of temperatures relevant to experimental studies.
reactor was controlled using Brooks mass flow controllers (model
5850S). Upon reaching the desired reaction conditions, an aqu-
eous solution of GVL (10 wt %) was prepared and fed to the
packed-bed tubular reactor using a HPLC pump (Lab Alliance
SeriesI). The liquideffluent wascollected for quantitative analysis
in a separator (Jerguson Gage and Valve) at ambient temp-
erature and analyzed by GC (Shimadzu GC-2010 with FID
detector). Unknown product peaks were identified using GC-
MS (Shimadzu GCMS-QP2010S). CO and CO2 inthe gas effluent
were quantified using a Shimadzu GC-8A instrument equipped
with a TCD detector, and gas phase alkenes and alkanes were
quantified using a Varian GC (Star 3400 CX) apparatus equipped
with an FID detector. Butene carbon yield is reported as a percent
of theoretical yield of butene (C4) from GVL (C5). A continuous
hydrogen sweep was adjusted to keep the partial pressure of all
species inthe reactor constant equal to0.016, 0.164, and 0.820 atm
for GVL, H2, and water, respectively, for all temperatures and
weight-hourly space velocities (WHSV, defined as the mass of
GVL per mass of catalyst per hour). The hydrogen flow was
chosen such that the molar ratio of H2/GVL was equal to 10.
Total carbon balances typically closed to within 10%.
2.3. GVL Ring-Opening and Hydrogenation/Decarboxy-
lation Studies. Ring-opening of GVL and subsequent hydro-
genation or decarboxylation were studied in an up-flow reactor at
atmospheric pressure and temperatures from 498 to 623 K. The
catalyst (SiO2/Al2O3 or a physical mixture of SiO2/Al2O3 and Pd/
C) was loaded into a 1/4” tubular stainless steel reactor. When
necessary, the catalyst was mixed with crushed granules of fused
silica (Sigma-Aldrich) to fill the reactor volume. The catalyst bed
was held in place by two plugs of quartz wool, and the reactor was
mounted inside an aluminum block within a well insulated
furnace (Applied Test Systems). Reactor pressure was controlled
with a backpressureregulator (GOBP-60). Reactiontemperature
was monitored at the reactor wall by a Type K thermocouple
(Omega) mounted within the aluminum block and controlled by
using a Series 16 temperature controller (Love Controls). Prior to
introduction of feed, the desired reaction temperature and pres-
sure were achieved under flowing inert gas (He). Gas flow to the
2.4. Decarboxylation Studies of Various Lactone/Acid
Feedstocks. The production of alkenes from aqueous solutions
of alcohols, organic acids, and lactones through acid catalysis was
investigated in a tubular, fixed bed catalytic reactor identical to
that described above. Commercial SiO2/Al2O3 was used for each
of the experiments, and it was conditioned by calcination (4 h at
723 K inflowing air at 50 cm3(STP)/min) prior to the introduction
of a new feed. In the case of highly water-soluble species (GVL,
DVL, ECL, GCL, BEA, acrylic acid, propanoic acid), the feed was
introduced to the reactor as a 20 wt % solution using an HPLC
pump (Lab Alliance, Series I). For sparingly soluble reagents
(organic acids, AAL, DHP), the organic feed was delivered using
a syringe pump (Harvard Apparatus) and diluted to 20 wt % by
cofeeding deionized water using a HPLC pump (Lab Alliance,
Series I). Because hexenoic acid is a solid at room temperature, it
was dissolved in decane and introduced using a syringe pump
(Harvard Apparatus) and diluted as described above. The value of
WHSVfortheseexperimentsisdefinedasthemassoffeedmolecule
per mass of catalyst per hour. The liquid effluent was collected for
quantitative analysis in a separator (Jerguson Gage and Valve) at
ambient temperature and analyzed by GC (Shimadzu GC-2010
with FID detector). Unknown product peaks were identified using
GC-MS (Shimadzu GCMS-QP2010S). Gas phase products were
purged from the separator by flowing He at 50 cm3 (STP) min-1
using a mass flow controller (Brooks model 5850). CO and CO2 in
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.;
Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
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Bond et al.
the gas effluent were quantified using a Shimadzu GC-8A instru-
ment equipped with a TCD detector, and gas phase alkenes
and alkanes were quantified using a Varian GC (Star 3400 CX)
apparatus equipped with an FID detector.
3. Results and Discussion
3.1. Ring-Opening Thermodynamics. The initial step in
GVL conversion is acid-catalyzed ring-opening to form an iso-
meric mixture of pentenoic acids, the latter of which can be
hydrogenated to form pentanoic acid (metal catalyst) or undergo
decarboxylation to form butene and CO2 (acid catalyst). The
related pathways for the production of pentanoic acid and butene
are shown in Figure 2. Previous studies using a SiO2/Al2O3
catalyst and aqueous solutions of GVL (30-60 wt %) showed
high conversions of GVL at 648 K and pressures from 1 to 35 bar
at space velocities near 1 h-1 21
however, low selectivities to
;
decarboxylation products were observed. The primary products
observed were 2-, 3-, and 4-pentenoic acids. At low space
velocities (0.18 h-1), nearly quantitiative yields of butene/CO2
were observed without substantial byproduct formation. These
results indicate that pentenoic acid is an intermediate in the
decarboxylation reaction (and thus also an appropriate decar-
boxylation feedstock), or that it is produced in parallel with
butene from GVL. Additionally, initial studies suggest that the
interconversion between GVL and pentenoic acid isomers is
reversible and occurs rapidly relative to the rate of decarboxylation.
The pathways for which we have carried out thermodynamic
calculations are shown in Figure 2. Computational calculations at
standard conditions indicate that thermodynamics favor the
formation of GVL over PEA isomers. The enthalpy change of
reaction for the ring-opening of GVL to 4-pentenoic acid was
estimated to be 36 kJ/mol at 298 K, which compares favorably
with a previously reported value of 38-40 kJ/mol.23 Calculated
enthalpy changes associated with the production of other pente-
noic acid isomers are lower than the estimate included for
4-pentenoic acid. The enthalpy changes for the production of
2-trans, 3-trans, 2-cis, and 3-cis pentenoic acids are estimated
to be 17, 25, 26, 32 kJ/mol, respectively. Similarly, changes in
Gibbs free energy at 298 K are estimated to be 25, 8, 14, 16, and
19 kJ/mol for 4-, 2-trans, 3-trans, 2-cis, and 3-cis pentenoic acids,
respectively. At these conditions, the formation of pentenoic acid
through opening the GVL ring is endothermic and entropically
favored; thus, it is expected that the ratio of pentenoic acid to
GVL will increase with reaction temperature. Experimental
results showed that the concentration of the thermodynamically
most stable isomer, 2-trans pentenoic acid, was in all cases the
highest, while the concentration of the least stable isomer,
4-pentenoic acid, was always the lowest. This isomer distribution
differs from prior results reported for a similar reaction, the
production of methyl pentanoate from GVL. Lange et al.18 found
that the distribution of methyl pentenoates was 25-35% pent-
4-enoate, 65-75% cis/trans pent-3-enoates, and only 1-5% cis/
tras pent-2-enoates. In this case, however, pentenoic acid reacts
rapidly with the alcohol and is removed by distillation, and thus,
the concentration predicted by thermodynamic equilibrium is not
achieved. Thus, in the system we have studied, it is likely that the
less stable 4, 3-cis, and 3-trans pentenoic acid isomers are produced
by GVL ring-opening and subsequently isomerize in the presence
of acid sites to form the more stable 2-cis and 2-trans isomers.
Using similar estimations for the decarboxylation of pentenoic
acids, at standard conditions and considering an average distribu-
Figure 3. Ratio of outlet pentenoic acid to γ-valerolactone at
increasing WHSV on SiO2/Al2O at different temperatures. (9)
623 K; (O) 598 K; (2) 573 K; (b) 548 K); (0) 523 K. Solid lines are
included to highlight the trend observed in data.
Figure 4. Rate of production of CO2 at increasing WHSV on
SiO2/Al2O3 at different temperatures. (9) 623 K; (O) 598 K;
(2) 573 K; (b) 548 K; (0) 523 K; (1) 498 K. The thick solid line
corresponds to the inlet flow of γ-valerolactone ( μmol min-1
gcat-1). Solid lines are included to highlight the trend observed in
data.
tion of isomers, the production of butene and CO2 is predicted to
proceed with changes in enthalpy, entropy, and Gibbs free energy
of -49 kJ/mol, 148 J/mol/K, and -93 kJ/mol, respectively. Thus,
the conversion of GVL to butene and CO2 proceeds with overall
changes in enthalpy and Gibbs free energy equal to 22 and
-77 kJ/mol, respectively. Because the decarboxylation leads to
a large increase in entropy, the production of butene and CO2
from pentenoic acid or GVL is irreversible and favored under all
relevant reaction conditions.
The hydrogenation of pentenoic acid to produce pentanoic
acid is exothermic (-111 kJ/mol) and results in a net decrease in
entropy (-120 J/mol/K). At 298 K, this reaction leads to a Gibbs
free energy change of -75 kJ/mol and is favorable at all relevant
conditions. The overall changes in enthalpy and Gibbs free energy
(23) Emel’yanenko, V. N.; Verevkin, S. P.; Burakova, E. N.; Roganov, G. N.;
Georgieva, M. K. Russ. J. Phys. Chem. A 2008, 82, 1521–1526.
16294 DOI: 10.1021/la101424a
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Bond et al.
Article
Table 1. Reaction rates and product selectivities for various lactones (20 wt % in Water) at 648 K, 1 bar over SiO2/Al2O3
product selectivity (%)
feed rate
(μmol min-1 g-1
conversion rate
(μmol min-1 g-1
decarboxylation rate
(μmol min-1 g-1
feed
)
)
)
olefin
alkene acid
γ-lactone
other
GVL
DVL
GCL
ECL
32
30
26
28
20
28
19
28
9
7
5
5
46
25
26
18
54
35
38
20
<1
<1
36
40
23
37
for the production of pentanoic acid from GVL are estimated to
be -84 and -59 kJ/mol, respectively.
3.2. Ring-Opening and Decarboxylation of GVL. Experi-
ments to study the ring-opening of GVL were carried out in a
fixed bed flow reactor over SiO2/Al2O3. The relative outlet
concentrations of PEA to GVL at different temperatures between
523 and 623 K are plotted against WHSV in Figure 3. We
observed that, as the temperature increases, the PEA/GVL ratio
increases from an average value of 0.13 at 523 K to 0.88 at 623 K,
a result which agrees with trends expected based on computa-
tional estimates. For each temperature studied, we observe a
similar trend in the ratio of PEA to GVL. Over a range of low to
moderate space velocities, the ratio observed is constant, which
corresponds to an equilibrium controlled reaction. At high space
velocities, the observed ratio begins to decrease with increasing
WHSV. Most likely, this regime corresponds to a range of space
velocities in which the rate of ring-opening is slow relative to the
inlet feed rate. Thus, pentenoic acid is not produced quickly
enough to achieve equilibrium. At low space velocities and high
temperatures (623 K), the pentenoic acid to GVL ratio (data not
included in the figure) is lower than the predicted value, which can
be attributed to the onset of decarboxylation. Figure 4 shows the
rate of CO2 production versus WHSV at the same conditions
Figure 5. Chemical structure of the lactones used in this study.
decarboxylation, we have considered the catalytic activity for
decarboxylation of other analogous lactones such as δ-valerolac-
tone (DVL), ε-caprolactone (ECL), and γ-caprolactone (GCL) as
well as various alkene acids such as acrylic acid, butenoic acid
(BEA), pentenoic acid (PEA), and hexenoic acid (HEA). Addi-
tionally, based on the initial results of GVL decarboxylation, we
have measured the catalytic activity of broadly related lactones
that are ostensibly appropriate for decarboxylation such as R-angeli-
calactone (AAL) and 5,6-dihydro-2H-pyran-2-one (DHP).
3.4. Saturated Lactones. The structural attributes that we
have studied in various lactone feedstocks are (i) the length of the
carbon chain in the lactone and (ii) the size of the ring comprising
the lactone. Accordingly, we have studied analogous 5- and
6- carbon lactones to determine if the length of the carbon chain
has a significant effect on the reactivity of the lactone or the rate of
decarboxylation. Experimental conditions and production rates
of observed products are summarized in Table 1, while the
different structures are illustrated in Figure 5. The results given
inTable 1 suggest that ringsize contributessignificantly to lactone
reactivity. γ-lactones (5-member rings) appear to be the more
stable than δ- (6-member rings) and ε- (7-member rings) lactones.
as Figure 3. At 498 K, the average rate of CO2 production was
-1
0.07 μmol min
g
cat
-1, while at 623 K and high WHSV the rate
of CO2 production was 95 μmol min-1 gcat-1. The results in
Figures 3 and 4 indicate, that under typical operating conditions,
the interconversion of GVL and pentenoic acid isomers is quasi-
equilibrated. Thus, for a given inlet concentration and tempera-
ture, the partial pressures of GVL and pentenoic acid will remain
in approximately the same ratio. The decarboxylation step is
kinetically controlledand, as thepartial pressure of pentenoic acid
is roughly constant, the production rate of CO2 does not reveal an
effect of space velocity. At low space velocities and 623 K, the rate
of CO2 production is lower because it is controlled by the rate at
which GVL is introduced to the reactor and not by the equilib-
rium partial pressure of pentenoic acid.
We observe that the rate of ring-opening for δ-valerolactone
(DVL) is significantly higher (28 μmol min
either γ-valerolactone (GVL) (20 μmol min
g
g
-1) than that of
-1
cat
-1) or γ-capro-
-1
3.3. Study of the Reactivity of Different Feedstocks.
Experiments similar to those described in the previous section
were carried out using a physical mixture of Pd/C and SiO2/Al2O3
and a H2 cofeed. In these experiments, it was observed that the
rate of CO2 production decreases by 2 orders of magnitude upon
the introduction of a metal catalyst. At these conditions, pente-
noic acid is quantitatively converted to pentanoic acid, which does
not readily undergo decarboxylation. Therefore, GVL decarboxy-
lation likely proceeds through alkene acid intermediates. It has
been previously reported that the decarboxylation of primary
aliphatic acids (e.g., heptanoic acid) using a supported Ni catalyst
is more difficult than the decarboxylation of tertiary aliphatic and
aromatic acids.24 It is thus apparent that the presence of CdC
functionality greatly decreases the barrier for decarboxylation of
organic acids. To determine the parameters governing facile
cat
lactone (GCL) (19 μmol min-1 gcat-1). The rate of cleavage of
ε-caprolactone (ECL) is comparable to that of δ-valerolactone
(DVL). (In fact, the rates of ring-opening for DVL and ECL are
sufficiently fast that these lactones undergo complete conversion
at the conditions of this study.) We also observe that 40% and
23% of δ-valerolactone and ε-caprolactone react to form γ-lac-
tones, suggesting that larger, less stable lactones undergo acid-
catalyzed ring-opening to form an alkene acid, which can iso-
merize and subsequently close, forming the more stable γ-lactone.
A significant difference in selectivity toward the formation of
olefins is observed between DVL and GVL, with 25% of DVL
consumed to produce the olefin product, incomparison to 46% of
GVL converted to olefin at the same conditions. Importantly,
however, the product distribution in the conversion of DVL is
limited exclusively to alkene acids and more stable lactones, which
will both ultimately undergo decarboxylation to form olefins.
Thus, DVL is consumed by ring-opening to form pentenoic acids,
(24) Maier, W. F.; Roth, W.; Thies, I.; Schleyer, P. V. Chem. Ber./Recl. 1982,
115, 808–812.
Langmuir 2010, 26(21), 16291–16298
DOI: 10.1021/la101424a 16295

Article
Bond et al.
Table 2. Reaction Rates and Product Selectivities for Various Organic Acids (20 wt % in Water) at 648 K, 1 bar over SiO2/Al2O3
product selectivity (%)
feed rate
(μmol min-1 g-1
conversion rate
(μmol min-1 g-1
decarboxylation rate
(μmol min-1 g-1
feed
)
)
)
olefin
γ-lactone
other
acrylic acid
3BEA
2PEA
3PEA
4PEA
2HEA
45
37
30
30
28
26
1
21
21
20
19
14
<1
21
13
13
12
6
11
>99
61
62
68
89
0
0
0
0
17
0
39
38
31
38
42
which rapidly react to form GVL before they can undergo
decarboxylation. Alternatively, it is possible that the direct
decarboxylation of GVL is possible, whereas DVL cannot under-
go direct decarboxylation (i.e., without forming the pentenoic
acid intermediate).
The above observations are supported by prior studies regard-
ing the chemistry of lactones. For example, δ-valerolactone will
readily polymerize while the similarly nonsubstituted 4-carbon
γ-butyrolactone will not. This difference indicates that the num-
ber of C atoms of the lactone ring has a substantial influence on
the reactivity of lactones,25 and these authors have attributed trends
in activity to the extent of distortion of bond and tetrahedral angles
vicinal to the ester moiety. γ-Butyrolactone (5-member ring) shows
less geometric distortion than DVL (6-member ring); consequently,
it has less ring strain and is a more stable molecule. Generalizing
these results to the 5- and 6-carbon lactones studied here, it would be
expected that δ- and ε-lactones would be more susceptible to ring-
opening than γ-lactones, as observed experimentally in this study.
When examining the results of larger, 6-carbon lactones, we observe
that the rate of conversion by ring-opening does not change signifi-
cantly from GVL to GCL, indicating that they are equally suscep-
tible to cleavage by acid hydrolysis despite a difference in chain
length. This result supports the previous claim that the primary
driver for lactone reactivity (toward ring-opening) is the degree of
ring strain and lactone instability. Interestingly, although chain
length has little effect on lactone reactivity, we observe a significant
loss of selectivity to desired products when testing 6-carbon lactones.
In the case of GCL and ECL, we observe selectivities of 36% and
37%, respectively, to byproducts, primarily cyclohexenone and
methylcyclohexenone which form in approximately equimolar
quantities. The increased rate of byproduct formation could account
for the lower rates of olefin production from larger (6-carbon)
lactones when compared to 5-carbon lactones.
3.5. Organic Acids. We have outlined above that the de-
carboxylation of GVL can occur through the production of
pentenoic acid intermediates, and that there is a rapid intercon-
version between GVL and pentenoic acid. Accordingly, we have
investigated the behavior of various organic acids under condi-
tions appropriate for decarboxylation. The results are summari-
zed in Table 2. In this study, we have considered differences in
molecular structure such as the position of the double bond in the
starting material, the presence of a CdC double bond versus a
saturated acid, and the chain length of the acid. The structure of
each acid studied is illustrated in Figure 6. First, we have assessed
the activity of propanoic and pentanoic acid to discern whether
the production of alkanes and CO2 is achievable from saturated
organic acids. With each of these probe molecules, decarboxyla-
tion was not observed to an appreciable extent, which is in accord
with previously discussed results regarding the production of
pentanoic acid from GVL. In contrast, significant quantities of
Figure 6. Chemical structure of the acids used in this study.
olefins and CO2 were measured from unsaturated acids. We are
thus able to conclude that the barrier to decarboxylation is greatly
reduced by the presence of a CdC double bond. Examining the
results reported in Table 2, we can conclude, based on the results
obtained for the various isomers of pentenoic acid, that the initial
position of the double bond in the carbon chain has little effect on
either the reactivity of the acid tested or the selectivity toward the
production of butene. Given the relative ease of alkene isomeriza-
tion in the presence of solid acid catalysts,26,27 it is likely that the
isomers of pentenoic acid rapidly equilibrate such that there is little
difference in any of the pentenoic acids under reaction conditions.
The length of the carbon chain for various organic acids has a
more significant effect. In the case of acrylic acid, we observe an
insignificant extent of decarboxylation (<1 μmol min-1 gcat-1).
Most likely, ethylene production is not observed as, in acrylic acid,
the CdC double bond is necessarily R- to the acid, and its
position does not allow for the formation of a stable, protonated
intermediate. In the case of acrylic acid, protonation of the
R-carbon would result in the formation of a primary carbenium
ion which is not energetically favorable. Beginning with bute-
noic acid, we observed significant decarboxylation for all of the
feeds studied, suggesting that, for decarboxylation to occur, the
possibility must exist for a CdC double bond which can be
protonated to form a secondary carbenium ion β- to the acid.
We would expect that such an intermediate could be formed
from either 2- or 3-alkene acids having no fewer than four
carbon atoms. Additionally, as the length of the unsaturated
acid increases, we observe a decrease in the production rate of
(and selectivity to) olefins. At 648 K, we observe that butenoic
-1
-1
acid undergoes decarboxylation at a rate of 21 μmol min
g
cat
.
-1
This rate decreases to 12-13 μmol min-1 gcat for pentenoic
acid isomers and decreases again to 6 μmol min
g
cat
-1 for hexe-
-1
noic acid. In addition to a decreased rate of decarboxylation,
(26) Quann, R. J.; Green, L. A.; Tabak, S. A.; Krambeck, F. J. Ind. Eng. Chem.
Res. 1988, 27, 565–570.
(27) Zhou, Y. B.; Woo, L. K.; Angelici, R. J. Appl. Catal., A 2007, 333, 238–244.
(25) Houk, K. N.; Jabbari, A.; Hall, H. K.; Aleman, C. J. Org. Chem. 2008, 73,
2674–2678.
16296 DOI: 10.1021/la101424a
Langmuir 2010, 26(21), 16291–16298

Bond et al.
Article
Table 3. Reaction Rates and Product Selectivities for 5,6-Dihydro-2H-pyran-2-one (DHP) and r-Angelicalactone (AAL) (20 wt % in Water)
at 648 K, 1 bar over SiO2/Al2O3
product selectivity (%)
feed rate
(μmol min-1 g-1
conversion rate
(μmol min-1 g-1
decarboxylation rate
(μmol min-1 g-1
decarbonylation rate
(μmol min-1 g-1
feed
)
)
)
)
CO2
CO
BALa
LAb
other
DHP
AAL
31
30
31
23
16
1
2
5
52
5
6
22
0
17
0
53
41
2
abeta-angelicalactone (BAL). bLevulinic acid (LA).
we observe a loss of selectivity, as larger acids allow for the
formation of a broader range of byproduct. For butenoic acid,
we observe complete selectivity to propylene. With pentenoic
acids, product selectivity is distributed between butene (60-
70%) and γ-valerolactone (30-40%). From hexenoic acid,
more products are observed. In addition to pentene (42%
selectivity), we observe the formation of γ-caprolactone (38%)
and various unsaturated, cyclic C6 ketones (17%). As the chain
length increases, it becomes increasingly likely that, in the
presence of an acid catalyst and water, the organic acid will
undergo ring-closure to form the lactone, and we observe
significant quantities of the most stable γ-lactone when starting with
both 5- and 6-carbon organic acids. In addition, as the chain length
increases from 5 to 6 carbon atoms, we begin to see the occurrence of
unsaturated, cyclic C6 ketones (as in the case reported for γ- and
ε-caprolactones). Importantly, however, the lactones formed from
5- and 6-carbon acids exist in equilibrium with the acids. Thus, as
the acid is consumed by the irreversible decarboxylation reaction
(i.e., at lower space velocities), the lactones will undergo ring-
opening and decarboxylation. Accordingly, we expect that quanti-
tative yields of butene are possible from pentenoic acid; however, it
appears that byproduct formation will prevent high olefin yields in
the decarboxylation of 6-carbon alkene acids. In addition, as
discussed below, the lower decarboxylation rate observed for alkene
acids of increasing chain length is also caused by an increased
number of isomers related to the location of the CdC double bond,
decreasing the probability of having the CdC double bond at the
highly reactive 2- or 3-position.
3.6. Unsaturated Lactones. In addition to unsaturated
organic acids and lactones, we studied the behavior of related
feedstocks based on their chemical similarity to molecules for
which acid catalyzed decarboxylation had been previously de-
monstrated (GVL, alkene acids). In this study, we have measured
the rates of conversion of R-angelicalactone (AAL, a GVL
precursor) and 5,6-dihydro-2H-pyran-2-one (DHP, pyran analo-
gue of AAL). The structures of AAL and DHP are illustrated in
Figure 5, and catalytic activity toward decarboxylation is sum-
marized in Table 3. In the case of DHP, trace amounts of carbon
were detected in the aqueous product; however, even after several
hours on stream, no organic product had accumulated. The
entirety of measurable products was detected as MVK, butadiene,
CO, and CO2 in the gas phase, and we were only able to account
for 60% of the carbon introduced to the system in the products.
Unsaturated lactones have been cited previously to serve as coke
precursors,28 and it is possible that the loss of carbon in the case of
DHP occurred through polymerization side reactions that re-
sulted in coke formation within the catalyst bed. The carbon
balance for AAL closed to within 10%. The two lactones follow
Figure 7. Proposed mechanism for decarboxylation of 3-pente-
noic acid to 1-butene and CO2 (pathway 1) and ring-opening and
direct decarboxylation of γ-valerolactone to 1-butene and CO2
(pathway 2).
selectivities toward decarboxylation (suggesting the production of
butadiene) and decarbonylation (suggesting the production of methyl-
vinylketone). It appears that DHP primarily underegoes decarboxyla-
tion, whereas AAL primarily undergoes decarbonylation.
3.7. Mechanism of Decarboxylation. We have observed in
this study that lactones, such as GVL and ECL, can undergo
decarboxylation to produce olefins and CO2. In addition, un-
saturated acids undergo decarboxylation to produce the corre-
sponding olefin and CO2, while saturated organic acids, such as
pentanoic orpropanoic acid, donot undergo decarboxylation to a
measurable extent. These results suggest that substrates appro-
priate for decarboxylation should possess a CdC double bond
(unsaturated acids) or exist as cyclic esters (lactones) that can be
opened through acid catalysis to produce unsaturated acids. The
mechanism that we propose for decarboxylation begins with
protonation of the unsaturated acid/cyclic esters as illustrated in
Figure 7. A series of subsequent proton transfer steps follow the
initial protonation and ultimately result in decarboxylation to
produce CO2 and the corresponding alkene. The appearance of
GVL as a product in the study of pentenoic acids supports the
claim of reversibility for the ring-opening of GVL. Because the
rates of olefin and CO2 production from organic acids are higher
than those from the corresponding lactones, we speculate that
pathway 1 is the lower energy route and that GVL decarboxyla-
tion proceeds more readily through an unsaturated acid inter-
mediate than directly from the lactone as shown in pathway 2.
Additionally, although the position of the CdC double bond in
the probe molecule did not play an important role in the final
reactivity (attributed to rapid pentenoic acid isomerization), it is
likely that the preferred configuration for decarboxylation in-
volves the location of the CdC double bond in the 2- or 3-position
of the en-acid, which could be protonated in the R- or γ- positions
respectively to form a secondary β-carbenium ion. The lower
decarboxylation rate observed for C6 acids and lactones can
potentially be attributed to an increased number of isomers,
the previously observed trend of general reactivity, with DHP
being completely converted (31 μmol min
g
cat
-1) and the rate of
-1
-1
AAL conversion being significantly lower (23 μmol min
g
cat
-1).
Interestingly, the two unsaturated lactones, which differ only in
ring size as illustrated in Figure 5, show remarkably different
(28) World Patent WO/2008/142127, 2008.
Langmuir 2010, 26(21), 16291–16298
DOI: 10.1021/la101424a 16297

Article
Bond et al.
decreasing the probability of having the CdC double bond at the
2- or 3-position.
lation and selectivity to decarboxylation products; however, we
have found that at least four carbon atoms are required for
appreciable rates of decarboxylation. The loss of selectivity
observed with larger lactones can be attributed to increased
incidence of byproduct formation, particularly with 6 carbon
species. We expect that quantitative olefin yields can be
achieved from 4- and 5-carbon acids and lactones, as the only
nonalkene products observed are themselves appropriate dec-
arboxylation feedstocks. In the case of 6-carbon feeds, the
overall yield to alkenes is limited by the formation of stable
byproducts such as unsaturated, 6-carbon, cyclic ketones.
Finally, we have observed that unsaturated lactones do not
undergo decarboxylation as readily as saturated lactones;
instead, decarbonylation pathways to produce unsaturated
ketones become more prevalent.
4. Conclusions
We have examined in this study the thermodynamics for ring-
opening of GVL as well as subsequent reactions such as de-
carboxylation and hydrogenation. Experimental results obtained
for the ring-opening of GVL suggest that rapid interconversion
exists between the lactone and pentenoic acids. The production of
either pentanoic acid or butene/CO2 from GVL is thermodyna-
mically favorable and essentially irreversible under our reaction
conditions. We have compared the decarboxylation activity of
lactones and organic acids of varying chain length. In general, the
rate of alkene acid decarboxylation was higher than that observed
for analogous lactones, suggesting either that lactone decarboxy-
lation proceeds through an alkene acid intermediate, or that
alkene acids produced by ring-opening of the lactone provide a
lower energy, parallel pathway for the production of olefins and
CO2. The observation of a significant quantity of lactones in the
product distribution of experiments beginning with pentenoic
acids supports the reversibility of GVL ring-opening and rein-
forces the involvement of pentenoic acid in the mechanism of
GVL decarboxylation. We have found that lactone ring size (γ, δ,
ε) has a significant effect on the stability of the lactone, with the
γ-lactone (5 member ring) being the most favorable configuration
and larger 6- and 7-member rings (δ- and ε-lactones) being less
stable and more susceptible to ring-opening. In general, smaller
lactones and acids appear to offer the highest rates of decarboxy-
Acknowledgment. This work was supported through funding
from the Defense Advanced Research Projects Agency (Surf-cat:
Catalysts for Production of JP-8 range molecules from Ligno-
cellulosic Biomass). The views, opinions, and/or findings
contained in this article/presentation are those of the author/
presenter and should not be interpreted as representing the
official views or policies, either expressed or implied, of the
Defense Advanced Research Projects Agency or the Department
of Defense. In addition, this work was supported in part by the
U.S. Department of Energy Office of Basic Energy Sciences. One
of us (J.A.D.) also thanks Professor Gabor Somorjai for con-
tinued inspiration and encouragement over the years.
16298 DOI: 10.1021/la101424a
Langmuir 2010, 26(21), 16291–16298