Catalytic One-Pot Conversion of Renewable Platform Chemicals to Hydrocarbon and Ether Biofuels through Tandem Hf(OTf)4+Pd/C Catalysis

Article abstract of DOI:10.1002/cssc.201902137

Efficient conversion of renewable biomass platform chemicals into high-quality fuels remains challenging. A one-pot catalytic approach has been developed to synthesize various structurally defined biofuels by using Hf(OTf)₄ and Pd/C for selective tandem catalysis and 2-methylfuran (2-MF) as a renewable feedstock. 2-MF first undergoes Lewis acid-catalyzed hydroxyalkylation/alkylation (HAA) condensation with carbonyl compounds to afford intermediates containing the targeted carbon skeletons of hydrocarbon or ether products, and these intermediates then undergo hydrogenation or hydrodeoxygenation to afford the target products, catalyzed by metal triflate+Pd/C in the same pot. The present process can produce structurally defined alkanes and cyclic ethers under mild conditions.

Full text of DOI:10.1002/cssc.201902137

DOI: 10.1002/cssc.201902137
Communications
Catalytic One-Pot Conversion of Renewable Platform
Chemicals to Hydrocarbon and Ether Biofuels through
Tandem Hf(OTf)4 +Pd/C Catalysis
Dong-Huang Liu,[a, b] Tobin J. Marks,[c] and Zhi Li*[a]
Efficient conversion of renewable biomass platform chemicals
into high-quality fuels remains challenging. A one-pot catalytic
approach has been developed to synthesize various structural-
ly defined biofuels by using Hf(OTf)4 and Pd/C for selective
tandem catalysis and 2-methylfuran (2-MF) as a renewable
feedstock. 2-MF first undergoes Lewis acid-catalyzed hydroxyal-
kylation/alkylation (HAA) condensation with carbonyl com-
pounds to afford intermediates containing the targeted carbon
skeletons of hydrocarbon or ether products, and these inter-
mediates then undergo hydrogenation or hydrodeoxygenation
to afford the target products, catalyzed by metal triflate+Pd/C
in the same pot. The present process can produce structurally
defined alkanes and cyclic ethers under mild conditions.
tion to cyclic ethers, and/or (3) hydrodeoxygenation (HDO) of
ethers to alkanes (Scheme 1a).[4] During HDO, the acid cocata-
lysts[5] may induce undesirable alkane isomerization, thereby
compromising selectivity.[6]
In marked contrast to the above scenario, a variety of alkyl
ethers can be converted into the corresponding alkanes with-
out significant isomerization by using tandem Hf(OTf)4 and Pd/
C catalysis.[7] These catalysts are also highly effective for the se-
lective HDO of ethers with higher order complexity.[5e,8] Fur-
thermore, metal triflates also effectively catalyze hydroxyalkyla-
tion/alkylation (HAA) condensation of 2-MF and aldehydes or
ketones, to produce complex furanoid structures.[9] Upon com-
plete Pd/C-catalyzed hydrogenation, such compounds can
then be converted into cyclic ethers, which undergo catalytic
HDO to alkanes. The efficacy of these separate transformations
raises the intriguing question of whether HAA condensation,
hydrogenation, and HDO can be carried out sequentially in a
single reactor starting from simple platform chemicals.[5c]
Herein we report the realization of such a one-pot,[10] highly se-
lective tandem catalytic strategy to produce hydrocarbons and
cyclic ether biofuels from simple carbonyls and 2-methylfuran
(2-MF) as building blocks (Scheme 1b).
The catalytic conversion of biomass-derived platform chemicals
has emerged as a sustainable approach to renewable fuels and
value-added commodity chemicals.[1] Highly selective catalytic
transformations may enable the production of single-compo-
nent hydrocarbon fuel molecules with diversified molecular
structures from simple biomass platform chemicals. If success-
ful, these would greatly expand the scope for precisely opti-
mizing “tailor-made” fuel performance by blending.[2] Recent
studies have shown that, based on current production technol-
ogies, 6-pentylundecane, 2-methyltetrahydrofuran, and ethyl
levulinate are promising next-generation biofuels that could
exceed the performance of bioethanol in both cost and envi-
ronmental impact.[3] Furthermore, efficient and precise new
ways to produce hydrocarbon, cyclic ether, and levulinate bio-
fuels from simple biomass platform chemicals may drive break-
throughs to next-generation biofuels. Upgrading strategies to
convert platform chemicals into higher molecular weight fuels
typically consist of two or three independent steps: (1) acid- or
base-catalyzed condensation of simple fuel precursors into
carbon-chain-extended complex fuel precursors, (2) hydrogena-
HAA condensation of 2-MF and 5-methylfurfural is efficiently
catalyzed by metal triflates under neat conditions (see the Sup-
porting Information, Table S1). Hf(OTf)4 is found to be the most
active catalyst. Raising the reaction temperature to 458C af-
fords 96% yield of the isolated condensation product in 2.5 h.
The same conditions can also be applied to the HAA reactions
of 2-MF with many aldehydes, as well as Michael reactions
with many enals (Scheme S1). Typically, 0.5 mol% of catalyst is
sufficient for aldehydes, whereas 1 mol% suffices for less reac-
tive ketones or enones. Many of these aldehydes and ketones
can be produced from biomass, such as furfural, valeraldehyde,
levulinic acid, levulinate, 5-hydroxymethylfurfural (HMF), 5-
methylfurfural, and furan-2,5-dicarbaldehyde from C5 and C6
sugars;[1g,11] acetone, 1-butanal, and aromatic aldehydes from
fermentation;[12] butanone from levulinic acid decarboxyla-
tion;[13] cyclopentanone from furfural or furancarbinol isomeri-
zation;[14] and cyclohexanone from lignocellulosic biomass.[15]
In general, more electrophilic aldehydes such as aromatic alde-
hydes bearing electron-withdrawing groups afford higher reac-
tivity and higher yields. Nevertheless, extended reaction times
may result in product decomposition, and sterically hindered
aldehydes and ketones are less reactive.
[a] D.-H. Liu, Prof. Z. Li
School of Physical Science and Technology, ShanghaiTech University
393 Middle Huaxia Road, Pudong District, Shanghai 201210 (China)
E-mail: lizhi@shanghaitech.edu.cn
[b] D.-H. Liu
University of Chinese Academy of Sciences
Beijing 100049 (China)
[c] Prof. T. J. Marks
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL, 60208 (USA)
In the same reactor, the above condensation products were
next subjected to HDO in the presence of a hydrogenation cat-
alyst and H2 (Scheme 1b). The products obtained from alde-
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cssc.201902137.
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Scheme 1. Strategies for catalytic biofuel production from platform chemicals: a) Conventional stepwise biofuel production strategies. b) This work: One-pot
conversion.
hydes and ketones diverge at this point: those from aldehydes
undergo complete HDO to alkanes, whereas those from ke-
tones are converted into cyclic ethers. The one-pot reaction
between 2-MF and aldehydes was optimized first using 1-hexa-
nal and Hf(OTf)4 as the catalyst because the desired product
4a is a valuable biodiesel fuel.[3,16] Various hydrogenation cata-
lysts were surveyed and Hf(OTf)4 +Pd/C was found to be the
most effective combination (Table S2). Note that HDO requires
higher loadings of Hf(OTf)4 than does HAA; thus extra Hf(OTf)4
is added along with Pd/C and solvent for the second reaction
stage. Solvent screening showed that ethyl acetate is superior
to other solvents (Table S3), possibly because it stabilizes cat-
ionic intermediates during HDO while not being so Lewis basic
as to deactivate the metal triflate.[17] In addition, ethyl acetate
is inert towards HDO at 1508C but capable of removing water
by sacrificial hydrolysis to EtOH, ether, and AcOH (Scheme S2),
which likely prevents inferior reaction pathways (Table S3, en-
tries 7–10).
loading (Table 1, entry 5) results in lower overall yield, whereas
lower concentrations of reagents and catalysts do not effect
significant improvement (Table 1, entry 6). The conditions of
Table 1, entry 2 were denoted as Modality A for further scope
studies, for it provides a maximum combined yield of C16 hy-
drocarbons 4a+5a under the lowest loading of catalysts and
solvent. A 4a stability test under Hf(OTf)4 +Pd/C catalysis
(Table S5) shows no isomerization, indicating that isomeric
products are probably generated concurrently with 4a during
the HDO step. Carbocation and olefin intermediates originating
from CÀO bond cleavage[7b] might yield products 5a and 6a,
especially when an adjacent tertiary or quaternary carbon
center is present (Scheme S3).[18] When the reagent concentra-
tion is decreased while that of the catalysts is held constant
(Table 1, entries 7–9), the selectivity to product 4a is signifi-
cantly enhanced. Meanwhile, reducing the Hf(OTf)4 loading fur-
ther increases selectivity to 4a, but at the expense of extended
reaction times (Table 1, entries 9–16). Note that in these exam-
ples all Hf(OTf)4 can be added in one portion along with the
solvent in reaction stage 1 without diminishing yield and selec-
tivity (Table 1, entries 13–16). In fact, the reaction time of
stage 1 can be reduced to 40 min while achieving full conver-
sion. The conditions for Table 1, entry 13 were denoted as Mo-
dality B in further studies, aiming to optimize selectivity to 4a
with high catalyst loadings. A comparison of stepwise reaction
selectivity to 4a, 5a, and 6a in Modalities A and B (Scheme S3)
The one-pot conditions were thus optimized using ethyl
acetate as the solvent (Table 1). The desired product C16H34
(4a) always comes with a CÀC bond cleavage product C11H24
(6a) and many C16Hx (x<34) isomers (5a), which seemed to be
saturated monocyclic alkanes but could not be unambiguously
identified. At high reagent concentrations and low catalyst
loadings, the Hf/Pd ratio does not significantly impact the se-
lectivity (Table 1, entries 1–4). Further lowering the catalyst
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C13–C17 range as Cn and isomeric Cn mixtures along with small
amounts of lower carbon number components (Table 2, en-
tries 1–9). Modality B is capable of precise production of target
Cn hydrocarbons (e.g., 4a) in the C13–C28 jet fuel and diesel
range (Table 2, entries 1–12). Note that for entry 7, the reaction
requires higher temperature and longer time to enable HDO of
a primary CÀO bond. EtOAc also underwent HDO to AcOH in
this case. When starting from enals, significant isomerization
products are still observed, even with Modality B (Table 2, en-
tries 13–19). For entries 15 and 17, target Cn could not be dis-
tinguished from their isomers by GC. As a comparison, 5-meth-
ylfurfuryl alcohol achieves high selectivity to linear alkanes
with Modality A (Table 2, entry 20) without isomerization.
In contrast to the above results, the fuel precursors obtained
from ketones and 2-MF are not suitable for alkane production.
Substantial CÀC bond cleavage occurred in several examples
at high temperature under either Modality A or B, probably
due to the quaternary carbon center (Scheme S4).[6d] Neverthe-
less, these are excellent precursors for the production of cyclic
ethers, which are valuable fuel additives,[2] and oxygen-contain-
ing diesel fuels.[21] In contrast to alkane biofuels, ethers con-
taining more oxygen atoms should provide better performance
as fuels.[22] Moreover, cyclic and polycyclic ethers are excellent
solvents for chemical reactions and lithium battery electro-
lytes.[23] Many bioactive natural products and antibodies con-
tain ether rings as well.[24] Nevertheless, selective production of
renewable cyclic ethers from biomass-derived platform chemi-
cals remains a challenge. Hydrogenation of furan precursors
often results in unwanted CÀO bond hydrogenolysis to alco-
hols, even if the Pd hydrogenation catalyst is optimized and
acid catalysts from previous steps removed.[21] New strategies
for selective production of cyclic ethers are therefore highly
desirable.
Table 1. Conditions optimization for one-pot production of hydrocarbon-
s.[a]
Entry
Conc. (1a/Hf/Pd) [mm]
t [h]
Yields[b] (4a/5a/6a) [%]
1
2000:40:12
2000:40:16
2000:40:20
2000:40:24
2000:20:16
1000:20:8
1000:40:16
500:40:16
200:40:16
200:20:16
200:20:16
200:10:16
200:10:16
200:10:24
200:6:16
5
5
5
5
5
5
5
5
59:31:2
69:27:1
58:28:1
58:25:1
40:20:1
57:26:1
66:21:2
63:27:2
77:17:2
60% alkanes[d]
76: 10:1
84:10:1
87:8:1
2[c]
3
4
5
6
7
8
9
10
11
12
13[e,f]
14[e]
15[e]
16[e]
5
5
20
20
20
20
20
20
87:6:1
6% alkanes[d]
ethers[d]
200:4:16
[a] Reaction conditions, unless otherwise stated: 1) 1a (1.0 mmol),
2
(2.0 mmol), and first 0.5 mol% Hf(OTf)4 were stirred at 458C in a high-
pressure reactor for 5 h; 2) remaining Hf(OTf)4, Pd/C (5% Pd on activated
carbon), ethyl acetate were added and heated at 1508C for a specified
time under 40 bar of H2. [b] Yields of 4a and 6a determined by GC-FID
calibrated by authentic samples. 5a calibrated with respect to 4a.
[c] Conditions employed as Modality A. [d] Incomplete conversion of
cyclic ethers observed. [e] At stage 1, all Hf(OTf)4 added along with sol-
vent and stirred, for 40 min. Stage 2 remains the same. [f] Conditions em-
ployed as Modality B.
Since cyclic ethers are intermediates in alkane production,
the same Hf+Pd catalyst system and condensation–hydroge-
nation multistep strategy should also apply in these cases. The
reaction between 2-butanone and 2-MF was chosen to opti-
mize the reaction conditions (Table 3). Surprisingly, unlike other
reported catalytic protocols,[21] alcohol byproducts do not form
under the current conditions. When ethyl acetate is used as
the solvent in the one-pot procedure, the CÀC bond cleavage
byproduct 9a is generated in 24% yield (Table 3, entry 1). This
product is not observed if Hf(OTf)4 is removed before hydroge-
nation (Table 3, entry 7), indicating that 9a formation is likely
due to the Lewis acid. Hence, when water (a stronger Lewis
base than EtOAc) is used as the solvent, generation of 9a is
less significant (Table 3, entries 2–6), but hydrolysis product 8a
is formed in 11% yield (Table 3, entry 3). In temperature screen-
ing within the range 608C–1008C, the best selectivity to 7a is
obtained at 808C (Table 3, entries 2–4). Although the reaction
at 1008C provides far more 8a and 9a byproducts than that at
808C, we found that two-stage heating—first at 808C and
then at 1208C—can effectively transform 8a into the desired
product 7a (Table 3, entries 5 and 6, and Scheme S5).
indicates that Modality B prevents generation of 5a from cyclic
ethers.
At this stage, TfOH was investigated as a catalyst to deter-
mine whether the observed catalytic reactions were the result
of metal triflate hydrolysis.[19] Although results comparable to
those with 0.5 mol% Hf(OTf)4 were obtained in HAA using 1–
2 mol % TfOH (Table S1, entries 11–13), significantly lower se-
lectivity and carbon balance were observed in the one-pot re-
actions catalyzed by a series of TfOH concentrations following
either Modality A or B, mimicking partial hydrolysis scenarios
(Table S4).[8b] These results demonstrate the distinctive role of
Hf(OTf)4 beyond that of a mere Brønsted acid precursor.[20] Nev-
ertheless, the true catalytic species remains elusive in that the
reaction system is a dynamic mixture and any component may
influence the activity/selectivity of the catalyst. This report fo-
cuses on overall strategy and scope, leaving detailed mecha-
nistic studies to future efforts.
The reaction conditions given for Table 3, entries 6 (one-pot
in water) and 7 (stepwise), were denoted as Modalities C and
D, respectively, for further investigation of cyclic ether produc-
Both Modalities A and B were investigated in the aldehyde
scope study. Modality A produces jet fuel hydrocarbons in the
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Table 2. One-pot production of designer fuels from aldehydes and 2-MF.
Entry
Structures
target Cn
Product distribution[b] [%]
aldehyde
Modality A[b]
target Cn
Modality B[c]
[d]
[d]
isomeric
CÀC cleaved target Cn
isomeric
CÀC cleavage
[e]
[e]
Cn mixture[e] Cn-5
Cn mixture[e] Cn-5
1
2
3
51
60
75
26
22
15
ND
78
93
92
4
5
3
ND
ND
ND
ND
ND
4
63
23
ND
76
10
ND
5
69
42
46
27
21
30
1
4
87
86
82
8
1
6
1
5
6[f]
7[g]
ND
ND
8
37
20
9
90
4
2
9
62
–
11
–
7
–
–
90
42
40
4
8
8
4
10[h]
11[i]
7[k]
–
–
22[k]
12[j]
–
–
–
55
6
38[k]
13
14
15
16
–
–
–
–
–
–
–
–
–
–
–
–
15[l]
26[l]
35
56
1
3
2
4
97[m]
36[l]
44
17
–
–
–
59[m]
12
18
19
20
–
–
–
–
0
–
–
23[l]
26[l]
60
55
4
6[k]
–
94
ND
–
[c] Yields determined by GC-FID calibrated with 6-pentylhexadecane (for C19–C28 products), 6-pentylundecane (for C14–C18 products), and n-undecane (for
C9–C13 products). ND=not determined. [b] Reaction conditions according to Modality A. Catalyst loading: 1 mol% of Hf(OTf)4 and 0.4 mol% of Pd/C per
MF unit. Reaction time for stage 1 varies depending on aldehyde. See the Supporting Information for details. [c]Reaction conditions according to Modali-
ty B. Catalyst loading: 5 mol% of Hf(OTf)4 per carbonyl, and 4 mol% of Pd/C per MF unit, unless otherwise stated. Reaction time for stage 1 was 40 min for
all aldehydes. [d] Structures identified by NMR spectroscopy and GCMS unless otherwise stated. [e] Components identified by NMR spectroscopy, GCMS,
and comparison with authenticated samples; target molecules excluded from isomeric Cn mixtures, unless otherwise stated. [f] Modified procedures and
loadings of catalysts applied for both Modalities A and B: stage 1: 0.5 mol% Hf(OTf)4, 458C, 2.5 h; stage 2: ethyl acetate along with 24 mm Pd/C (1.2 mol%
for Modality A, 12 mol% for Modality B), 808C, 40 bar H2, 3 h; stage 3: Hf(OTf)4 added up to 60 mm (3 mol%) for Modality A or 10 mm (5 mol%) for Modali-
ty B; then 1508C, 40 bar H2, 5 h for Modality A or 20 h for Modality B. [g] 1808C, 80 h for stage 2 of both Modalities A and B. [h] 6 mol% of Hf(OTf)4 used.
[i] 8 mol% of Hf(OTf)4 used. [j] 5 mol% of Hf(OTf)4 used. [k] CnÀ5 +CnÀ10. [l] GC yield of the suspected target Cn. Structure not identifiable by NMR spectros-
copy. [m] All Cn products combined.
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Additionally, 2-cyclohexenone is converted in high selectivity
and efficiency in tandem Michael–HAA–hydrogenation reac-
tions with 3 equivalents of 2-MF into cyclic ether 7h when Mo-
dality D is applied (Scheme 3). Interestingly, cyclopentenone
only participates in the Michael reaction stage with 1 equiva-
lent of 2-MF. When carrying out hydrogenation/HDO, Modali-
ty A affords alkane 7i in high selectivity, which is derived from
extrusion of oxygen atoms of both the furan ring and the Mi-
chael product carbonyl.[5k] In contrast, enals give mixtures of
cyclic ether 7j and byproducts under Modality D. Note from
Table 2 that they also provide alkane mixtures under Modali-
ty B.
Table 3. Optimization of conditions for one-pot production of cyclic
ethers.[a]
Pd/C
Solvent
T [8C]
t [h]
Yields[b]
Lastly, we investigated the reaction of 2-MF with ethyl levuli-
nate. Modalities A, B, and C all afford complex mixtures of
products, whereas Modality D gives about 50% yield of the de-
sired cyclic ether (Scheme 4).
[mol%]
(7a/8a/9a) [%]
1
2
3
4
5
0.2
0.2
0.2
0.2
0.2
EtOAc
H2O
H2O
H2O
H2O
80
60
80
100
80
120
80
3
10
3
3
3
+20
3
+20
3
20:26:24
65:13:2
66:11:2
27:36:18
72:4:5
In summary, we have developed a facile, efficient, and selec-
tive strategy for the one-pot catalytic conversion of biomass-
derived platform chemicals into well-designed biofuels. Four
optimized reaction modalities were developed to meet the re-
quirements of producing either hydrocarbons or cyclic ethers
of desired quality.
6[c]
7[d]
0.3
0.2
H2O
78:2:trace
120
80
EtOAc
80:trace:trace
[a] Reaction conditions: 1) 3a (1.0 mmol), 2 (2.0 mmol), Hf(OTf)4 (1 mol%),
were stirred under 458C in a high pressure reactor for 6 h (without H2);
2) Pd/C (5% Pd on activated carbon) and specified solvent (0.5 mL)
added and stirred at indicated temperature under 30 bar H2 following in-
dicated temperature program. [b] Total yield from isolated product mix-
ture. Selectivity determined by GC-FID. [c] Conditions employed as Mo-
dality C. [d] Hf(OTf)4 removed by silica gel column before stage 2. Condi-
tions employed as Modality D.
Acknowledgements
Financial support for this work was generously provided by the
Basic Research Funding of the Science and Technology Commis-
sion of Shanghai Municipality (Grant No. 17JC1404000) and
ShanghaiTech University start-up funding. Z.L. and T.J.M. con-
ducted initial investigations of this work, first under the U.S. De-
partment of Energy contract DE-AC0206CH11357 for the Institute
of Atom-Efficient Chemical Transformation (IACT), an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Sciences, and Office of Basic Energy Sciences,
followed by support under NSF homogeneous catalysis grant
tion from biomass-derived ketones (Scheme 2). Compound-
s 7a–d are obtained by Modality C in good yields, among
which 7b–d are obtained with nearly 100% selectivity. Howev-
er, Modality C performs poorly when applied to com-
pounds 7e–g, whereas stepwise Modality D affords satisfactory
overall yields and selectivities.
Scheme 2. Cyclic ether production from ketones and 2-MF.
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Scheme 4. Conversion of ethyl levulinate and 2-MF.
CHE-1464488. We thank the Shanghai Advanced Research Insti-
tute, CAS for supporting lab space. We thank Dr. Min Yang and
Dr. Huawei Liu of the Analytical Instrumentation Center of
ShanghaiTech University for MS and NMR expertise, respectively.
Conflict of interest
The authors declare no conflict of interest.
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Version of record online: && &&, 0000
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COMMUNICATIONS
D.-H. Liu, T. J. Marks, Z. Li*
&& – &&
Catalytic One-Pot Conversion of
Renewable Platform Chemicals to
Hydrocarbon and Ether Biofuels
through Tandem Hf(OTf)4 +Pd/C
Catalysis
Fueling good: A one-pot catalytic ap-
proach has been developed to synthe-
size biofuels by using Hf(OTf)4 and Pd/C
for selective tandem catalysis and 2-
methylfuran (2-MF) as a renewable feed-
stock. 2-MF undergoes Lewis acid-cata-
lyzed hydroxyalkylation/alkylation con-
densation and the intermediates then
undergo hydrogenation or hydrodeoxy-
genation to afford structurally defined
alkanes and cyclic ethers under mild
conditions.
&
ChemSusChem 2019, 12, 1 – 8
www.chemsuschem.org
8
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!