Diesel and alkane fuels from biomass by organocatalysis and metal-acid tandem catalysis

Article abstract of DOI:10.1002/cssc.201300476

Combo deal: Biomass furaldehydes are upgraded into oxygenated diesel and high-quality C_10-12 linear alkane fuels. The first of two steps involves solvent-free self-condensation (Umpolung) through organocatalysis using an N-heterocyclic carbene

Full text of DOI:10.1002/cssc.201300476

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DOI: 10.1002/cssc.201300476
Diesel and Alkane Fuels From Biomass by Organocatalysis
and Metal–Acid Tandem Catalysis
[a, b]
[a]
Dajiang Liu
and Eugene Y.-X. Chen*
Recognized as a key biorefining building block and
biomass platform chemical, 5-hydroxymethylfurfural
[
1]
(
HMF) has been studied extensively as part of major
efforts in developing technologically and economical-
ly feasible routes for converting nonfood lignocellulo-
sic biomass into feedstock chemicals, sustainable ma-
[
2]
terials, and liquid fuels. Since the important discov-
ery of the CrCl /ionic liquid (IL) catalyst system for
2
the effective conversion of cellulosic glucose to
[
3]
HMF, a large number of other metal and nonmetal
catalyst systems have been developed to promote
the conversion of glucose or, directly, cellulose into
[
4]
HMF. In contrast, research on the upgrading of HMF
or related furaldehydes into higher-molecular-weight
and high-energy-density kerosene/jet (C to C , with
8
16
C12 being the major constituent) or diesel (up to C22)
intermediates or fuels is scarce, and thus much Scheme 1. Four different routes to upgrade HMF into kerosene or diesel intermediates
or fuels.
needed.
Because HMF does not possess a-hydrogen atoms,
it cannot undergo self-aldol condensation. Dumesic
and co-workers reported cross-aldol condensation of HMF with
enolizable organic compounds such as acetone in the pres-
ence of an alkaline catalyst, followed by a dehydration/hydro-
genation processes, to upgrade HMF into C –C liquid alkane
Recent research on HDO has focused on the development
of bifunctional catalysts for upgrading lignin-derived pyrolysis
oils (e.g., phenols, guiacols, and syringols) into hydrocar-
[9,10]
bons.
Water can be used as a suitable solvent for HDO, al-
9
15
[
5]
fuels (Scheme 1, route A). Recently, Corma and co-workers
developed hydroxyalkylation of 2-methylfuran to perform tri-
merization in the presence of an acid catalyst, the product of
which is subject to high-temperature hydrodeoxygenation
lowing for spontaneous separation of hydrocarbons during the
reaction. Supported catalysts such as Ni/HZSM-5, Pd/C+H PO ,
3
4
and Pd/C+HZSM-5 can achieve high to quantitative yields of
cycloalkanes from phenols under 5 MPa H at 2508C within
2
[6]
[10]
(HDO) to produce high-cetane-number 6-alkylundecanes;
2 h. However, for furan compounds derived from cellulosic
HMF can be used to replace one of the 2-methylfuran mole-
cules in the trimerization step (route B). Most recently, Bell and
co-workers reported acid-catalyzed etherification and reductive
etherification of HMF into 5-(alkoxymethyl) furfurals and 2,5-
bis(alkoxymethyl)furans as potential oxygenated biodiesel can-
biomass, HDO products are complicated by furan-ring open-
ing, carbon chain fragmentation, rearrangement, and cycliza-
tion reactions, rendering a wide distribution of hydrocarbons.
Using conditions similar to those employed for the HDO of
lignin-derived pyrolysis oils, the HDO of furfural gave tetrahy-
[
7]
[10a]
didates (route C). This direct HMF etherification route offers
an alternative for producing usable diesel-range fuels to the
etherification of the chloride derivative of HMF, 5-(chlorome-
dropyran in 36% yield besides pentane.
For the HDO of 5-
methylfuran trimer (5,5-bisylvyl-2-pentanone) by Pt/C and Pt/
TiO under 5 MPa H and 4008C, 96% oily products were classi-
2
2
[
8]
thyl)furfural, with alcohol.
fied as C to C hydrocarbons (linear, branched, monocyclic,
9 16
[6]
and bicyclic). In a two-step HDO of furoin consisting of hy-
drogenation by Pd/Al O to render the substrate water soluble
2
3
and the subsequent HDO process with Pt/SiO -Al O , Dumesic
2
2
3
[
a] D. J. Liu, Prof. Dr. E. Y.-X. Chen
Department of Chemistry, Colorado State University
Fort Collins, CO 80523-1872 (USA)
Fax: (+1)970-491-1801
et al. obtained a wide distribution of alkanes, with 34% C se-
10
[5]
lectivity. The HDO of the reductive pinacol coupling products
of furfural and 5-methylfurfural (MF) by Pt/C and solid acid
E-mail: eugene.chen@colostate.edu
[11]
TaOPO afforded high yields of alkanes. Alternatively, open-
4
[b] D. J. Liu
ing the furan rings first under mild conditions (which is appli-
Department of Mechanical Engineering, Colorado State University
Fort Collins, CO 80523-1301 (USA)
[12]
cable only to certain types of furan rings ), followed by HDO,
[13]
produces alkanes more selectively.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300476.
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Ideally, direct coupling of two HMF molecules would make
a C12 jet/kerosene fuel intermediate, which could be catalytical-
ly transformed into liquid fuels. Herein we report the selective
and quantitative coupling of HMF to 5,5’-di(hydroxymethyl)fur-
oin (DHMF) under solvent-free conditions using 1 mol% of an
organic N-heterocyclic carbene (NHC) catalyst, and the subse-
quent transformations of DHMF into oxygenated diesel fuels
through hydrogenation, etherification or esterification; as well
as high-quality kerosene/jet fuels through a highly selective
HDO process that produces linear hydrocarbons (96% C10–12
linear alkanes) nearly quantitatively in the organic phase
coupled using 1 mol% TPT, even at room temperature, into
1,2-di(furan-2-yl)-2-hydroxyethanone (furoin) and 5,5’-dimethyl-
[16]
furoin in 89% and 94% isolated yields, respectively. Umpo-
[14]
lung of aldehydes catalyzed by NHCs is proposed to proceed
through the nucleophilic enaminol or the Breslow intermedi-
[17]
[18]
ate involved in the benzoin reaction. Indeed, we observed
the formation of such an intermediate through the stoichio-
metric reaction of HMF and 1,3-bis(2,4,6-trimethylphenyl)-1,3-
dihydro-2H-imidazol-2-ylidene (IMes) in [D ]DMSO at ambient
6
temperature (Figure S2). This enaminol is the acyl anion equiv-
alent, thus attacking the carbonyl group of a second HMF mol-
ecule to form another tetrahedral intermediate. Collapse of
this tetrahedral intermediate, via proton transfer and elimina-
tion of the NHC, produces DHMF and regenerates the NHC cat-
alyst, thus closing the catalytic cycle and leading to the catalyt-
ic formation of the coupling product DHMF (Scheme 2).
(Scheme 1, route D).
We recently disclosed that, through the NHC-catalyzed Um-
[
14]
polung benzoin condensation mechanism in the presence of
a catalytic acetate ionic liquid (IL), 1-ethyl-3-methylimidazolium
acetate, or a discrete NHC catalyst (5 mol% 1,3,4-triphenyl-4,5-
dihydro-1H-1,2,4-triazol-5-ylidene, TPT), at 608C in THF for 1 h,
HMF can be selectively self-coupled into DHMF, a promising
As the self-condensation products of the furaldehydes are
solids, we investigated three possible routes to transform
them into liquids as potential jet or diesel fuels. We first exam-
ined the hydrogenation route to convert the furoins into their
saturated derivatives using a recyclable Pd/C catalyst. Under
modest hydrogenation conditions (2.7 MPa H2 and 908C) in
the presence of Pd/C, all three furoins could be successfully
converted into liquids (Scheme 3). The liquids were not simply
[
15]
new C12 kerosene/jet fuel intermediate. This coupling reac-
tion is unique to the organically catalyzed Umpolung reaction,
as other types of coupling reactions, such as reductive pinacol
[
11]
coupling, did not work for HMF. In the present study we
found that this catalytic coupling reaction can be carried out
in the absence of any solvent (except for a small amount of an
organic solvent added at the end of the reaction, to remove
traces of the catalyst), although both HMF and the NHC cata-
lyst are solids (Scheme 2). With a TPT loading of 1 mol%, quan-
Scheme 3. Hydrogenation of furoins and etherification and esterification of
DHMF into oxygenated liquid diesel candidates.
Scheme 2. Solvent-free self-condensation of furaldehydes to furoins cata-
lyzed by an NHC catalyst, and catalytic cycle for the Umpolung condensa-
tion of HMF to DHMF.
fully hydrogenated products, but accompanied by some hy-
drogenolysis products, as suggested by the elemental results
that showed C and H contents higher than the theoretical
values of the fully hydrogenated products (Table S1). The heat-
ing values of the hydrogenated products from furoin and 5,5’-
titative HMF conversion was observed at 608C after 1 h and
DHMF was formed quantitatively (by NMR) with a high isolated
yield of 95%. Using a lower catalyst loading of 0.5 mol%, an
isolated yield of 87% could still be achieved. This coupling re-
action was also performed in toluene, where DHMF precipitat-
ed out of the solution and the TPT catalyst remained in solu-
tion, allowing for convenient product separation/purification
and catalyst recovery. Likewise, furfural and MF could also be
À1
dimethylfuroin were measured to be 32.7 and 33.3 MJkg , re-
spectively, which are noticeably higher than that for ethanol
À1
(28.6 MJkg ), approaching to the value for dimethylfuran
À1
(33.7 MJkg ). These results indicate the potential use of furoin
and 5,5’-dimethylfuroin as oxygenated liquid fuels after simple
hydrogenation.
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Next, following the procedures established for etherification
tained by Pt/CsH PW O (2.6 mol% Pt loading relative to
2
12 40
[7]
of HMF with alcohols to form 5-(alkoxymethyl)furfurals and
acetylation of the hydrogenated acetal derived from furfural
DHMF), consisting of 11.3% C11 and 40.6% C12 alkanes (Fig-
ure S3). We also compared the performance of two different
heteropolyacids (CsH PW O and Cs H PW O ), revealing
[
19]
and glycerol, we investigated etherification and esterification
routes to convert DHMF into its corresponding ether, with eth-
anol, and ester, with propionic anhydride. Indeed, HMF, em-
ployed as a control run, was quantitatively converted to 5-
ethoxymethylfurfural by NMR in excess ethanol at 758C for
2
12 40
2.5 0.5
12 40
that Pt/Cs H PW O only converted DHMF to a trace
2.5 0.5
12 40
amount of alkanes. This result shows that the stronger poly-
acid CsH PW O is needed to promote the furan ring open-
2
12 40
ing. Varying HDO conditions, including a biphasic system of
hexane/water and higher temperature at 3008C (Table S2), ac-
tually lowered the alkane selectivity to 24.7% and 39.7%, re-
spectively. The isolated liquid fuels after the HDO process con-
tain noticeably higher carbon ratios (75–80%) than those by
hydrogenation (60-66%). Most excitingly, utilizing the [Pt/C+
2
4 h using Dowex G-26 (H-form) resin as solid acid catalyst.
Under similar conditions, DHMF was also quantitatively con-
verted to the corresponding liquid ether, 5,5’-di(ethoxymethyl)-
furoin (DEMF), with the secondary alcohol remaining intact.
On the other hand, esterification of DHMF using excess pro-
pionic anhydride esterified all three hydroxyl groups, thus
forming DHMF-tripropionic ester (Scheme 3). Hence, both the
etherification and esterification routes can serve as alternative
strategies for liquefying DHMF into diesel fuels.
TaOPO ] catalyst system (3.2 mol% Pt loading relative to
4
DHMF, Table S2), the highest alkane selectivity of 96% was ach-
ieved at 3008C for 3 h, producing 27.0% C10 (n-decane), 22.9%
C11 (n-undecane) and 45.6% C12 (n-dodecane) (Figure 1). The
The third route utilized the HDO process through metal-acid
tandem catalysis. The overall HDO process of DHMF to linear
alkanes (Scheme 1) can be reasoned to proceed through
metal-catalyzed hydrogenation to give the saturated polyol,
acid-catalyzed ring-opening/hydrolysis of furan rings in aque-
ous solution to yield a straight-chain polyol, and acid-catalyzed
dehydration, followed by metal-catalyzed hydrogenation to
afford the final saturated linear C12 alkane, n-dodecane, ideally
with minimum fragmentation, branching, or cyclization. This
overall picture calls for a bifunctional catalyst with both metal
and acid sites (e.g., noble metal on acidic support, Pt/
[
20]
CsH PW O
) to promote this HDO process, comprising hy-
2
12 40
drogenation–ring-opening/hydrolysis–dehydration–hydrogena-
tion cascade reactions. This picture is consistent with our re-
sults obtained from the above hydrogenation over Pd/C that
produces the furan-containing polyol without ring-opening
Figure 1. GC-MS chromatogram of the organic-phase products produced by
HDO of DHMF with Pt/C+TaOPO .
4
(vide supra) and the observation by Dumesic et al. that hydro-
genation, but not ring-opening, of the furan ring was the pri-
mary reaction for the furan-containing compounds when sub-
jected to HDO conditions using metal-acid bifunctional cata-
clean formation of three linear C10–12 alkanes through this
highly effective HDO process is remarkable. Moreover, both Pt/
[
5]
lysts.
C and TaOPO can be readily recycled by simple filtration.
4
To generate hydrocarbon premium liquid fuels by the HDO
process, we investigated HDO of DHMF under moderate condi-
When compared to current methods for upgrading biomass
furan compounds into biofuels, the DHMF route reported
herein possesses the following four potential advantages:
1) DHMF is obtained from self-coupling of HMF, without the
need for cross condensation with other petrochemicals;
2) HMF self-coupling is catalyzed by the organic NHC catalyst,
which can be carried out under solvent-free conditions (neat)
at 608C and 1 h, affording DHMF in near quantitative isolated
yield; 3) owing to its solubility in water, the HDO of DHMF can
be carried out directly in water, allowing for spontaneous sepa-
ration of hydrocarbons from the aqueous phase; and 4) DHMF
hydrodeoxygenation achieves high conversion and near quan-
titative selectivity towards linear C –C alkanes with a narrow
tions (250–3008C and 3.5 MPa H pressure) with a number of
2
bifunctional catalyst systems. Furoin was reported to be con-
verted to alkanes by a two-step process, with the first step
being hydrogenation to make it soluble in water, followed by
[
5]
subsequent HDO to avoid choking problems. As DHMF is
water-soluble, its HDO process can be carried out directly in
water without prior hydrogenation. After initial catalyst screen-
ing, we identified the following three bifunctional catalyst sys-
tems that worked well for DHMF conversion to alkanes:
a) acidic solution (H PO ) and Pd/C; b) heteropoly acid
3
4
(
(
CsH PW O )-supported Pt; and c) acidic solid catalyst
2 12 40
10
12
TaOPO ) and Pt/C. In all cases, DHMF was completely convert-
distribution of alkanes.
4
ed and no or a negligible amount of alkanes below C10 were
In summary, we report a highly effective new strategy for
upgrading biomass furaldehydes to liquid fuels. This strategy
consists of organocatalytic self-condensation (Umpolung) of
biomass furaldehydes into C10–12 furoin intermediates, followed
by hydrogenation, etherification, or esterification into oxygen-
observed. For the Pd/C+H PO system (5.9 mol% Pd loading
3
4
relative to DHMF, Table S2), the alkane selectivity in the organic
phase was 38%, consisting of 8.6% C , 17.6% C and 12.2%
10
11
C12 alkanes. A relatively higher alkane selectivity (52%) was ob-
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ated biodiesel, or hydrodeoxygenation by metal–acid tandem
catalysis into premium alkane jet fuels. The Umpolung cou-
pling step is carried out under solvent-free conditions, cata-
lyzed by an organic NHC, and quantitatively selective and
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