Valeric biofuels: A platform of cellulosic transportation fuels

Article abstract of DOI:10.1002/anie.201000655

(Chemical equation presented) Fueling the future: Valeric esters can be produced by acid hydrolysis of lignocellulose to levulinic acid, followed by hydrogenation to valeric acid and its subsequent esterification (see scheme). Valeric biofuels are fully compatible for blending with gasoline or diesel, and have passed a road trial of 250 000 km.

Full text of DOI:10.1002/anie.201000655

Angewandte
Chemie
DOI: 10.1002/anie.201000655
Cellulosic Biofuels
Valeric Biofuels: A Platform of Cellulosic Transportation Fuels**
Jean-Paul Lange,* Richard Price, Paul M. Ayoub, Jurgen Louis, Leo Petrus†, Lionel Clarke, and
Hans Gosselink
At the beginning of the 21st century mankind is facing an
energy challenge as a consequence of the worldꢀs increasing
energy demand, the depletion of “easy” oil and gas fields, and
the impact of CO2 emissions on the Earthꢀs climate (“three
hard truths”).[1] Much research is therefore being devoted to
the exploration and development of new, carbon-lean energy
sources. These include biofuels, which are the most promising
option for the transportation sector in the coming decades.[2]
The first generation of biofuels is presently produced from
sugars, starches, and vegetable oil. Although instrumental in
developing the market, these biofuels are not likely to deliver
the large volumes needed for the transport sector because
they directly compete with food for their feedstock. A more
promising feedstock is lignocellulosic material, which is more
abundant, has a lower cost, and is potentially more sustain-
able.[3]
developing since 2004 and which can deliver both gasoline
and diesel components that are fully compatible with trans-
portation fuels.
The manufacture of valeric biofuels (Scheme 1) consists
of the acid hydrolysis of lignocellulosic materials to LA, the
hydrogenation of the acid to gVL and valeric acid (VA), and
Lignocellulose is recalcitrant and, therefore, requires
complex and expensive processes for upgrading to biofuels.[4]
Interestingly, it has been claimed that levulinic acid (LA) can
be easily and cheaply produced from lignocellulosic materials
by using a simple and robust hydrolysis process.[5] Several LA
derivatives have been proposed for fuel applications, for
instance ethyl levulinate (EL), g-valerolactone (gVL), and
methyl tetrahydrofuran (MTHF).[5,6] However, these compo-
nents do not exhibit satisfactory properties when blended in
current fuels. Herein, we present a new platform of LA
derivatives, the “valeric biofuels”, which we have been
[*] Dr. J.-P. Lange, Dr. P. M. Ayoub, Dr. L. Petrus, Dr. H. Gosselink
Shell Global Solutions International B.V.
Shell Technology Centre Amsterdam
Scheme 1. Platform of valeric biofuels: reaction scheme and key
performance factors for the individualÀp1 rocess steps (selectivity
[mol%], productivity [tproduct mÀ3reactor h ], and concentration [wt %]).
EV: ethyl valerate; EG: ethylene glycol; PG: propylene glycol; IER:
acidic ion-exchange resin.
Grasweg 31, 1031 HW Amsterdam (The Netherlands)
Fax: (+31)20-630-3010
E-mail: jean-paul.lange@shell.com
Dr. R. Price, Dr. L. Clarke
Shell Global Solutions (UK)
Shell Technology Centre Thornton
Pool Lane, Ince, Cheshire CH2 4NU (UK)
finally esterification to alkyl (mono/di)valerate esters. One of
these steps, the hydrogenation of gVL to VA (Scheme 1,
step 3), has not been reported in the literature and was
developed in our laboratory. All the other steps are known
but were nevertheless revisited and, wherever possible,
improved. This holds for the acid-catalyzed hydrolysis of
lignocellulose to LA,[5,7] the hydrogenation of LA to gVL with
the use of supported metal catalysts,[8] as well as the familiar
esterification of carboxylic acids. Herein, we present the main
results of the hydrogenation of gVL to VA (Scheme 1, step 3),
key improvements in the hydrogenation of LA to gVL
(step 2), options for integrating steps 2–4, and finally a
thorough evaluation of the fuel performance of the resulting
Dr. J. Louis
Shell Global Solutions Germany
Produkte, Anwendung und Entwicklung
Hohe Schaar Strasse 36, 21107 Hamburg (Germany)
[†] Deceased
[**] We are grateful to numerous colleagues for their invaluable
contributions: K. L. von Hebel, R. Blessing, R. J. Haan, P. van den
Brink, M. Nijkamp, and L. Domokos for their assistance in catalyst
preparation and testing; J. Smith, A. Felix-Moore, and M. Beutler for
fuel evaluation. We also wish to thank Z. Li and K. Self (Accelergy
Corp.) for their assistance in catalyst screening.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000655.
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valeric biofuels. Details of the experimental procedures and
(Scheme 2). The production of VA requires a balancing of
the acidic and hydrogenation functionalities of the catalyst:
changing the metal/zeolite ratio either increases the co-
secondary results are available in the Supporting Information.
gVL is a relatively stable product under hydrogenation
conditions. It was, nevertheless, hydrogenated to VA in the
presence of bifunctional catalysts that contain both hydro-
genation and acidic functions. An evaluation of about 150
catalysts in a continuous high-pressure plug-flow reactor
identified Pt-loaded SiO2-bound H-ZSM-5 as a very effective
catalyst (Figure 1a). However, good yields were also achieved
Scheme 2. Probable reaction mechanism for the conversion of gVL to
VA over bifunctional catalysts.
production of pentenoic acid (low metal loading) or favors the
formation of MTHF, pentanal/pentanol, and/or pentane/
butane (high metal loading). Pentyl valerate (PV) was
observed as a minor co-product (Figure 1a) and is likely to
have formed by the esterification of VA with an over-
hydrogenation product, such as 1-pentanol or MTHF. For
instance, co-feeding MTHF to the gVL feed resulted in a
significant increase in PV production.
Catalyst extrudates of Pt/ZSM-5 bound with SiO2 (1.6 mm
diameter) were operated with high activity (differential VA
productivity of approximately 2 gVA gcatÀ1 hÀ1) and high selec-
tivity (> 90 mol%). This performance could be maintained
for more than 1500 h with intermittent catalyst regeneration
under hot H2 and/or airflow at 4008C (Figure 1b). Once
unloaded, the spent catalyst showed marginal loss of Pt and
Al (from the zeolite framework), marginal decrease in
support surface area, and no measurable loss of mechanical
strength of the catalyst extrudates. Pt/ASA catalysts were also
very promising candidates. Although they had a lower initial
activity, they showed no sign of deactivation over runs of 200–
300 h. Their stability is tentatively attributed to their weaker
acidity, which may facilitate the desorption of reactive
intermediates, such as pentenoic acid, and thereby depress
their tendency to form oligomeric deposits that poison the
catalyst surface.
Figure 1. Conversion of gVL to VA over Pt/H-ZSM-5/SiO2 catalysts.
a) Conversion and selectivity; b) long-term operation with multiple
regeneration by hot H2 strips at 10 bar H2 and 4008C (0.7% metal
loading; run conditions: 2508C, 10 bar, H2/gVL molar ratio 9:1, weight
hourly space velocity (WHSV)=2 h). C5À: C1–C4 hydrocarbons, PV:
pentyl valerate, PeOH: 1-pentanol.
with other zeolites and hydrogenation metals. Promising
zeolites included the small-pore TON, the medium-pore PSH-
3 (also called MCM-22), and the large-pore mordenite and
Beta. The acidic zeolites can be replaced by other strong or
weak solid acids, such as W/ZrO2 and amorphous silica–
alumina (ASA). Clearly, the conversion of gVL to VA is not
very demanding in terms of “shape selectivity” or acid
strength. Pt, Pd, and Rh from among the noble metals are all
particularly active; however, Rh was not desirable because it
co-produced significant amounts of gas. Alloying Pt or Pd
with other noble metals did not deliver measurable improve-
ments.
The literature available on the hydrogenation of LA to
gVL (Scheme 1, step 2) provides no information on the long-
term stability of the catalysts or their resistance to leaching
when operating in liquid LA.[8] These issues were addressed
through leaching tests of various supports, catalyst evaluation
over > 100 h, and analysis of spent catalyst samples. Carbon
supports are known to resist aggressive aqueous media but do
not survive frequent regeneration by coke burn-off. Prefer-
ence was therefore given to SiO2, TiO2, and ZrO2 supports,
which are stable to “decoking” conditions and appeared to
retain their integrity after a weekꢀs exposure to hot carboxylic
acid (LA or VA). This contrasts with other oxidic materials
(e.g., alumina, silica–alumina, and oxides of magnesium,
barium, and antimony) that are leached or even dissolve
under these conditions.
The reaction mechanism of step 3 (Scheme 1) is believed
to proceed by acid-catalyzed ring opening of gVL to
pentenoic acid and subsequent hydrogenation to VA
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Evaluation of some 50 catalysts showed the best perfor-
mance was with Pt supported on TiO2 or ZrO2: LA was
hydrogenated with high activity (differential productivity
10 ggVL gcatÀ1 hÀ1), high selectivity to gVL (> 95 mol%), and
marginal deactivation over 100 h (Figure 2). The main by-
compatibility with current fuels. The components that fail
on these criteria would require modifications of vehicles and/
or the distribution network and would, therefore, suffer from
a slow and costly deployment. Fuel compatibility was assessed
against a few basic properties such as polarity, (volumetric)
energy content, boiling point, and ignition indices, for
example octane or cetane number (CN) for gasoline and
diesel, respectively (Figure 3). The components that success-
Figure 2. Hydrogenation of LA to gVL over Pt/TiO2 (1 wt% metal,
2008C, 40 bar H2, H2/LA molar ratio 5:1, WHSV=9 h).
products, VA and MTHF, were formed with < 0.5 mol%
selectivity. Carbon and SiO2 supports provided a tenth of the
activity observed with TiO2 and ZrO2 supports (see the
Supporting Information). Pd-based catalysts also showed a
much lower activity and selectivity than their Pt counterparts.
Alloying the Pt with other noble metals did not improve
catalytic performance (see the Supporting Information).
Finally, the resistance to leaching was further confirmed by
X-ray photoelectron spectroscopy analyses of spent Pt/TiO2
and PtRe/ZrO2 catalysts, which showed no significant
decrease of Pt/Ti or Pt/Re/Zr ratios, and thereby no signifi-
cant loss or sintering of active metal.
The four-step process discussed so far provides flexibility
and robustness, which are invaluable in work toward deploy-
ing a novel technology. However, options for future cost
reductions through process integrations have additionally
been identified; for example, combining the LA and VA
hydrogenation steps with the possibility of even integrating
the esterification step. These schemes are described in the
Supporting Information; however, one of them is worth
presenting here. This is the single-step conversion of gVL to
PV, which is a promising diesel component (see below). PV
was indeed produced with 20–50% selectivity upon passing
gVL over Pt or Pd/TiO2 catalysts at 275–3008C (see the
Supporting Information). Pt-based catalysts tend to provide
higher PV/VA ratios but also produce more undesired light
hydrocarbons. VA can be recycled to the reactor for further
conversion to PV. Note that PV can also be co-produced over
bifunctional VA catalysts (e.g., Pt/ZSM-5) upon increasing
the hydrogenation activity to produce more MTHF and/or
pentanal, and recycling these co-products over the reactor for
upgrading to PV.
Figure 3. Screening parameters for fuel performance (MV, EV, PrV, and
PV: methyl, ethyl, propyl, and pentyl valerates, respectively). The
blending research octane number (BRON) values of PrV, PV, and fatty
acid methyl ester (FAME) are estimated from a CN–RON correla-
tion;[10] gray shading represents the property windows of hydrocarbon
fuels.
fully passed this screening were then evaluated against
additional properties, such as oxidation stability, fouling
tendency, corrosion, lubricity, water affinity, and response to
conventional fuel additives.
Valeric biofuels passed all these tests (Figure 3). They
have acceptable energy densities and more appropriate
polarities than current and alternative candidate biofuels
(ethanol, n-butanol, EL, gVL, and MTHF). Their volatility–
ignition properties make them compatible for either gasoline
or diesel applications, depending on their alkyl chain length.
For example, regular gasoline splash blended with ethyl
valerate (EV) at 10 and 20 vol% still meets the research
(RON) and motor octane number (MON) specification for
European gasoline (EN 228; see the Supporting Informa-
tion). The relatively low polarity of EV makes it less sensitive
to elastomer swell or water pickup than EtOH or EL
(Figure 4). EV also offers the advantages of a higher energy
density and lower blending volatility (dry vapor pressure
equivalent, DVPE) than EtOH. This eliminates the need to
Beyond developing the manufacturing route, we also
carried out a thorough study of the fuel properties of the
“valeric biofuels”. In a first step, we focused on their
Angew. Chem. Int. Ed. 2010, 49, 4479 –4483
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is simple and robust. However, it is the advance in the
conversion of LA to VA that has opened up the complete
manufacturing process. The valeric platform potentially offers
cellulosic biofuels that can be used as components in both
gasoline and diesel up to high blend ratios.
Note added upon revision: The potential of LA and gVL
as intermediates for biofuel manufacture is further confirmed
by a paper that appeared during revision of this communi-
cation. It reports the conversion of gVL to kerosene- and
diesel-range hydrocarbons through decarboxylation to
butenes and subsequent butene oligomerization.[11]
Figure 4. Fuel performance of EV, ethanol (EtOH), and EL blended at
5% in gasoline (the water affinity is measured for neat biofuel). RVP:
Reid vapor pressure.
Experimental Section
The catalysts were prepared by incipient wetness impregnation of
various supports with soluble salts of noble metals, followed by drying
at 1208C and calcination at approximately 4508C. The supports were
commercial extrudates, where available, or based on commercial
powders that were extruded in our laboratory.
The catalytic tests were carried out in high-pressure steel or
Hastelloy reactors equipped with liquid feed pumps, gas manifold,
and cold gas–liquid product separators. The catalysts were loaded
either as full extrudates or as 0.2–0.5 mm crushed particles, diluted
with inert particles of SiC. The catalysts were reduced under H2 flow
at atmospheric pressure and 3008C prior to operation. The liquid
product was collected and analyzed off-line by means of gas
chromatography. The gaseous products were analyzed on-line by
gas chromatography. More details of the procedures are reported in
the Supporting Information.
remove light hydrocarbons from the base fuel prior to
introducing the biocomponent. Interestingly, ethyl penten-
oate (EP), which is readily produced from gVL,[9] is also a
promising gasoline component; it presented better octane
properties than its saturated analogue EV without showing
detrimental effects on other properties.
Heavier esters, such as butyl and pentyl valerates, showed
polarity, volatility, and ignition properties that are suitable for
diesel (Figure 3). PV has better volatility and cold-flow
property match with diesel than FAME. However, this is at
the cost of a lower energy density. Di- and trivalerates, which
can be produced by esterifying VA with ethylene and
propylene glycols as well as glycerol, are compatible with
diesel with respect to solubility and volatility. However, their
modest cetane properties become limiting to the blend ratio
at which they can be used in diesel. All these heavy valerate
esters are soluble in diesel to high concentrations, a feature
that does not apply to heavy levulinates (e.g., pentyl
levulinate, PL). Valerate esters, such as FAME, provide
lubricity benefits to diesel.
Received: February 3, 2010
Published online: May 5, 2010
Keywords: biofuels · biomass · heterogeneous catalysis ·
.
lignocellulose · sustainable chemistry
The fuel evaluation was complemented by a road trial run
on a blend of 15 vol% EV in regular gasoline. The trial was
based on ten vehicles (both new and used cars) that are
representative of current market technologies. Mileage was
accumulated by contract drivers who followed a mixed
driving pattern (500 kmdayÀ1) for a cumulative distance of
250000 km. Attention was paid to exhaust emissions, perfor-
mance, drivability, oil quality, status of engine and fuel lines,
and information from the engine management system (see the
Supporting Information). The presence of EV in gasoline
showed no measurable impact on engine wear, oil degrada-
tion, vehicle durability, engine deposits, or regulated tailpipe
emissions (EURO 4 and 5 specifications). Some power
benefits were realized as a result of the good octane proper-
ties of EV. However, the lower energy density did result in a
small loss in volumetric fuel economy compared to non-
oxygenated gasoline. The 15 vol% EV blend was stable over
the four-month period of the test and had no negative impact
on the fuel storage and dispensing equipment (tanks, pipes,
pumps, and filters).
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