Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel Ranged Alkanes over a Rhenium-Modified Iridium Catalyst

Article abstract of DOI:10.1002/cssc.201700863

Renewable jet-fuel-range alkanes are synthesized by hydrodeoxygenation of lignocellulose-derived high-carbon furylmethanes over ReO_x-modified Ir/SiO₂ catalysts under mild reaction conditions. Ir?ReO_x/SiO₂ with a Re/Ir molar ratio of 2:1 exhibits the best performance, achieving a combined alkanes yield of 82–99 % from C₁₂–C₁₅ furylmethanes. The catalyst can be regenerated in three consecutive cycles with only about 12 % loss in the combined alkanes yield. Mechanistically, the furan moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high activity of Ir?ReO_x/SiO₂ arises from a synergy between Ir and ReO_x, whereby the acidic sites of partially reduced ReO_x activate the C?O bonds of the saturated furans and alcoholic groups while the Ir sites are responsible for hydrogenation with H₂.

Full text of DOI:10.1002/cssc.201700863

Accepted Article
Title: Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to
Renewable Jet-fuel Ranged Alkanes over a Rhenium Modified
Iridium Catalyst
Authors: Basudeb Saha, Sibao Liu, Saikat Dutta, Weiqing Zheng,
Nicholas S Gould, Ziwei Cheng, Bingjun Xu, and Dionisios G
Vlachos
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To be cited as: ChemSusChem 10.1002/cssc.201700863
Link to VoR: http://dx.doi.org/10.1002/cssc.201700863
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10.1002/cssc.201700863
ChemSusChem
Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel
Ranged Alkanes over a Rhenium Modified Iridium Catalyst
Sibao Liu, Saikat Dutta, Weiqing Zheng, Nicholas S. Gould, Ziwei Cheng, Bingjun Xu, Basudeb
Saha*, and Dionisios G. Vlachos*
Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering,
University of Delaware, Newark, DE 19716, USA
Corresponding authors’ emails: vlachos@udel.edu, bsaha@udel.edu
Abstract
Renewable jet-fuel ranged alkanes are synthesized by hydrodeoxygenation of lignocellulose derived
high carbon furylmethanes over ReOx modified Ir/SiO2 catalysts under mild reaction conditions.
Ir-ReOx/SiO2 with a Re/Ir molar ratio of 2 exhibits the best performance, achieving a combined
alkanes yield of 82-99% from C12-C15 furylmethanes. Catalyst can be regenerated in three
consecutive cycles with only ~12% loss in the combined alkanes yield. Mechanistically, the furan
moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture
of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy
groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high
yield of Ir-ReOx/SiO2 arises from a synergy between Ir and ReOx. The acidic sites of partially
reduced ReOx activate the C-O bonds of the saturated furans and alcoholic groups, while the Ir sites
are responsible for hydrogenation with H2.
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Keywords: Furylmethane, iridium, rhenium oxide, hydrodeoxygenation, jet-fuel, branched alkanes
1. Introduction
Due to recent crude oil depletion and global warming concerns, the conversion of biomass
into fuels is highly desirable[1-10]. Production of jet fuel, a mixture of C8-C16 hydrocarbons, especially
from abundant, inexpensive and carbon-neutral lignocellulose, is of significant interest to the
International Air Transport Association[11]. Jet-fuel ranged branched alkanes can be synthesized from
cellulose or hemicellulose derived furans (e.g., furfural and 2-methylfuran)[12-20] in two steps. High
carbon furylmethanes with branched chain backbone are first synthesized by C-C coupling of furans
with carbonyl functional groups[19], which are then hydrodeoxygenated (HDO) to alkanes[5]. C-C
coupling reactions over acid or base catalysts have been reported, e.g., hydroxyalkylation/alkylation
(HAA)[12-23], aldol condensation[24-25], alkylation[26], benzoic condensation[27-28], giving high yields[47]
In contrast, the HDO step is challenging due to the need of high reaction temperatures and H2
pressure, with concomitant, high energy consumption and undesirable C-C cracking leading to low
yield of desired alkanes[12-18]. Corma et al. conducted HDO of furylmethanes over Pt/C, Pt/C-Al2O3,
.
o
and Pt/C-TiO2 catalysts in a flow reactor[13-15]. At 350 C, these reactions yielded 70-91% jet fuel
ranged alkanes. HDO of furylmethanes has also been performed over Ni-WxC/C, Ni/Hβ, and
o
Ni/HZSM-5 catalysts at high temperatures (>260 C)[16-19]. However, high C-C cracking resulted in
low selectivity to the desired alkanes, especially when 5,5’-(furan-2-ylmethylene) bis(2-methylfuran)
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(trifuran, Scheme 1) and 5,5’-(propane-2,2-diyl)bis(2-methylfuran) (PBM, Scheme 1) were used as
oxygenates. Wang et al. also found that a Ni/SiO2-ZrO2 catalyst is effective for trifuran HDO at 250
oC, achieving 70% C15 alkanes and 10% methane yields[20]. In a batch reactor, a Pd/NbOPO4 catalyst
o
produced about 90% jet-fuel ranged alkanes at 200 C[21], however, C-C cracking remained a
challenge. Thus, development of efficient catalysts to minimize undesirable C-C cracking is
necessary. SiO2 supported partially reduced rhenium oxide modified iridium catalysts (Ir-ReOx/SiO2)
are effective for ring opening of cyclic ethers with -OH groups[29-33], as well as HDO of polyols[34-37]
,
sugars[38], hemicellulose[39], cellulose[40-41], and angelica lactone dimer[42] with little C-C cracking.
These catalysts are also effective for selective hydrogenation of unsaturated aldehydes to unsaturated
alcohols[43-44]. Spectroscopic characterizations of the catalysts indicate that the Ir metal surface is
partially covered with three-dimensional ReOx clusters[45]. It was proposed that the Ir-ReOx/SiO2
catalyst can heterolytically dissociate H2 into H+ and H- at the interface between the Ir metal and
ReOx species. The dissociated hydrogen species on Ir-ReOx/SiO2 is effective for direct, one-step
hydrogenolysis of polyols or esters in water. The Ir-ReOx/SiO2 catalyst also shows high C-O
hydrogenolysis performance in alkane solvent[46], where a two-step acid-catalyzed dehydration of
alcohols and ethers followed by subsequent hydrogenation rather than a direct, one-step
hydrogenolysis is preferred. Given the high performance of Ir-ReOx/SiO2 catalyst, we hypothesize
that it can catalyze HDO of furylmethanes to the corresponding branched chain alkanes with high
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carbon efficiency. Branched chain alkanes with 8-16 carbons are ideal, compared to straight-chain
alkanes, for jet fuel due to improved cold flow properties.
In this paper, we report for the first time that Ir-ReOx/SiO2 is effective for HDO of
furylmethanes (Scheme 1) at low temperature (170 oC), achieving high selectivity for desired alkanes.
High yields (82 -99%) of jet-fuel ranged alkanes are achieved at a Re to Ir molar ratio of 2 at optimal
reaction conditions. A reaction network involving furan ring hydrogenation, ring opening, and C-O
hydrogenolysis is also proposed.
2. Experimental
2.1. Catalyst Preparation
Ir-ReOx/SiO2 catalysts with varying Re to Ir molar ratios (0~2 with 4 wt% Ir) were prepared
by a sequential impregnation method described previously[34-41]. First, Ir/SiO2 was prepared by
impregnating Ir on SiO2 (Fuji Silysia G-6, calcined in air at 700 oC for 1 h with a 10 oC /min heating
rate, BET surface area 535 m2/g) with an aqueous solution of H2IrCl6 (Sigma-Aldrich). After
evaporating the solvent at 75 oC and drying at 110 oC for 12 h, the resulting Ir/SiO2 was impregnated
with an aqueous solution of NH4ReO4 (Alfa-Aesar). The catalysts were calcined in a crucible in air
o
at 500 oC for 3 h using a 10 C /min temperature ramp. The same synthetic procedure was followed
to prepare other M-ReOx/SiO2 catalysts (M=Rh, Ru, Pt and Pd, Re/M=0.5). The metal precursors
were RhCl3 (Alfa-Aesar), RuCl3 (Strem Chemicals), H2PtCl6 (Sigma-Aldrich) and PdCl2
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(Sigma-Aldrich), respectively. The reported metal loadings are based on the theoretical amount of
metals used for impregnation. All the catalysts were used in the powder form with a granule size of <
400 mesh.
2.2. Synthesis of Furylmethanes
Four furylmethanes (C12-C15) were synthesized via HAA coupling of 2-methylfuran (2-MF)
with carbonyl containing compounds over an improved graphene oxide (IGO) catalyst (Scheme 1)
following our previous work[47]. The coupling reactions were carried out in a round-bottom flask
equipped with a reflux condenser and a magnetic stirrer. High yield (85-95%) of furylmethanes was
achieved after 6 h under neat conditions and low temperature (60 C), which was separated by
filtration.
Furylmethanes in this work include 5,5’-(furan-2-ylmethylene)bis(2-methylfuran)
(trifuran), 5,5’-(butane-1,1-diyl)bis(2-methylfuran) (BBM), 5,5’-(propane-2,2-diyl)bis(2-methylfuran)
(PBM), and 5,5’-(ethane-1,1-diyl)bis(2-methylfuran) (EBM).
2.3. Activity Tests
HDO of furylmethanes over Ir-ReOx/SiO2 catalysts was performed in a 50 mL Parr reactor
with an inserted Teflon liner, the catalyst, a magnetic stirrer, and 10 mL solvent (cyclohexane). After
sealing the reactor (equipped with a thermocouple, rupture disk, pressure gauge, and gas release
valve), the mixture was heated at 200 oC and 5 MPa H2 for 1 h at 240 rpm for pretreating the catalyst.
Upon pretreatment, the reactor was cooled and H2 was released. Then, furylmethanes (0.3 g) were
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added and the reactor was immediately closed. The reactor was purged with H2 three times (1 MPa),
pressurized to 5.0 MPa, and heated to the desired temperature with continuous stirring the reaction
mixture at 500 rpm. The heating time to the set temperature was about 25 min. After reaction, the
reactor was immediately transferred to an ice bath. The catalyst was separated from the solution by
centrifugation. Aside from furylmethanes, experiments with DMF and DMTHF, as model
compounds, were conducted. DMF was purified by distillation before use.
High carbon alkane (C8-C15) products and oxygenated intermediates in the solution were
analysed by a gas chromatograph (GC, Agilent 7890A) equipped with an HP-1 column and a flame
ionization detector (FID) using icosane (C20) as an internal standard. The injection and FID
o
temperatures were both set at 300 C. The initial column temperature was held at 40 C for 5 min,
followed by a 10 C/min ramp to 270 C, and a 10 min hold at 270 C. High carbon alkanes and
oxygenated intermediates were identified by a GC (Agilent 7890B) mass spectrometer (Agilent
5977A with a triple-axis detector) equipped with a DB-5 column. The injection and FID
temperatures were set at 250 C and 230 C, respectively. The initial column temperature was held at
40 C for 4 min, followed by a 10 C/min ramp to 315 C, where the temperature was held for 10 min.
An HP-INNOWax column was used for analysis of the HDO products of DMF and DMTHF using
n-nonane as an internal standard. For analysis of DMF/DMTHF HDO products, the GC injector and
FID temperatures were both set at 250 C. The initial column temperature was held at 30 C for 6
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min, followed by a 20 C/min ramp to 250 C, where the temperature was held for 1 min. DMF and
DMTHF HDO products were identified by a Shimadzu GC-MS-QP2010 with the same column and
method. Control experiments showed a small amount of cyclohexane was converted to hexane and
low carbon alkanes over Ir-ReOx/SiO2. Thus, C1-C6 alkanes were not quantified from the HDO of
furylmethanes.
The conversion of furylmethanes, DMF and DMTHF and the yield of all products were
calculated on carbon basis using the following equations:
i i i
C i
i i i
C i
i C
C i i i
o
Spent catalysts were washed with excess methanol, dried in air, and calcined at 500 C for 3 h
before reuse in the next cycle. Control experiments show that a small amount of the solvent,
cyclohexane, is converted to n-hexane on Ir-ReOx/SiO2 under typical reduction and reaction
conditions in the HDO of DMF and 2,5-DMTHF. Thus, n-hexane is not taken into account in the
product distribution in the HDO of DMF and 2,5-DMTHF, which contributes to the relatively low
carbon balances in these experiments.
2.4. Catalyst Characterization
X-ray diffraction (XRD) patterns of the reduced and used catalysts were recorded by a
diffractometer (Bruker D8) equipped with a Cu Kα i i (λ . 54 ) 4 kV and 40
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mA. Thermogravimetric analysis of the reduced and recovered catalysts was performed on a TA
Q600 HT TGA/DSC. ATR-FTIR spectra (4000-400 cm-1) of the products were recorded on a
Thermo Electron Nicolte 8700 FT-IR Spectrometer. XPS experiments were conducted using a
Thermo-Fisher K-alpha+X-ray photoelectron spectrometer equipped with a monochromatic
aluminum K-alpha X-ray source (400 µm). Before analysis, the Ir-ReOx/SiO2 catalyst was reduced in
o
cyclohexane at 200 C for 1 h following the procedure detailed in activity test section. The sample
was further transferred into the K-Alpha+ Vacuum Transfer Module (VTM, ThermoScientific)
without being exposed to air.
The FTIR spectra of pyridine adsorbed on Ir/SiO2, ReOx/SiO2, and Ir-ReOx/SiO2 were
collected on an Agilent Cary 660 FTIR Spectrometer equipped with an MCT detector (128 scans at a
spectral resolution of 2 cm-1) with a homemade in situ transmission cell. The transmission cell was
held at a vacuum level of 0.01 mTorr through a vacuum manifold, which is connected to a
mechanical pump and a diffusion pump. A self-standing catalyst wafer was loaded into a custom
o
made sample holder and reduced under a hydrogen stream (20 ml/min) at 200 C for 1 h. After
cooling to room temperature and evacuation, 100 mTorr of pyridine was introduced to the
transmission cell via the vacuum manifold, and IR spectra were collected from 50 oC to 150 oC.
3. Results and Discussion
3.1 Optimization of Catalyst Composition
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Catalyst activity was assessed with different Re/Ir molar ratios in the range of 0 to 2 (Table
1). The yields of C8-C15 alkanes are insignificant over the catalysts containing only Ir or ReOx
(Ir/SiO2 and ReOx/SiO2; Entries 1 and 6 of Table 1). Over Ir/SiO2, the majority of detectable
products are oxygenates (47%). ReOx/SiO2 results in 45.9% conversion. Coke formation could be the
reason for carbon loss over Ir/SiO2 and ReOx/SiO2 catalyzed reactions. In addition, there could be
other unidentified oxygenated intermediates. Clearly, single component catalysts are not effective for
deoxygenation of the intermediates. In contrast, the yields of C8-C15 alkanes are high using the
Ir-ReOx/SiO2 catalysts. As the Re/Ir molar ratio increases, the C15 alkane yield increases due to
reduced C-C cracking (Entries 2-5; Table 1). Since a Re/Ir molar ratio of 2 results in optimal
selectivity in C15 alkane, it was chosen for subsequent experiments. Similar trend in catalytic
activities with varying Re/Ir molar ratios has been reported previously for hydrogenolysis of
glycerol[35], tetrahydrofurfuryl alcohol[29], and cellulose[41]. A physical mixture of Ir/SiO2 and
ReOx/SiO2 at the same Re/Ir molar ratio as Ir-ReOx/SiO2 gives lower yields of alkanes than the
Ir-ReOx/SiO2 catalyst (Entry 7; Table 1). This indicates a clear synergy between the Ir and the
neighboring ReOx. Other rhenium modified noble metals (M) catalysts: Rh-ReOx/SiO2,
Ru-ReOx/SiO2, Pt-ReOx/SiO2 and Pd-ReOx/SiO2 with a Re/M ratio of 0.5 were synthesized and
evaluated for the HDO of trifuran. Among these, Ru-ReOx/SiO2 exhibits the best performance, with
a combined C8-C15 alkanes yield of 36% (Table 1, Entry 8), which is only approximately half of that
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for Ir-ReOx/SiO2 at the same Re/M value (Table 1, Entry 3). Rh-ReOx/SiO2, Pt-ReOx/SiO2 and
Pd-ReOx/SiO2 are ineffective for this reaction, with the combined C8-C15 alkanes yield below 6%.
This comparison indicates that Ir-ReOx/SiO2 is uniquely effective for the HDO of trifuran.
3.2 Time Profile of Reaction, reaction network and proposed reaction mechanism
To understand the reaction network, we investigated the product distribution at various
reaction times using the Ir-ReOx/SiO2 catalyst with a Re/Ir molar ratio of 2. Figure 1 shows that the
complete conversion of trifuran occurs during heating the reaction mixture from room temperature to
o
180 C. Almost no alkane was formed and a large amount of oxygenated intermediates mainly with
three oxygen atoms were formed. The products at this stage, identified by GC-MS, are C15H28O3,
C15H26O3, C15H28O2 and C15H30O and with m/z values of parent ions at 256, 254, 240, and 226,
respectively. The main functional groups in these oxygenated intermediates are saturated furan rings,
carbonyls, and hydroxyls. Based on the MS and FTIR spectroscopic data (Figure S1), we propose
that C15H28O3 has two saturated rings with one mono-hydroxyl chain (Compounds 1 and 2, Figure 2).
The presence of a hydroxyl group is confirmed by the hydroxyl stretching band at 3592-3206 cm-1
observed in ATR-FTIR (Figure S1). Several oxygenates could correspond to an m/z of 254
(C15H26O3), where all the furan rings of trifuran are fully hydrogenated (Compound 3, Figure 2) or
two of them are hydrogenated with one mono-keto chain (Compounds 4 and 5, Figure 2). The
presence of a carbonyl group is confirmed by the C=O stretching peak at 1715 cm-1 in ATR-FTIR
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(Figure S1). These oxygenates (Compounds 1-5) are the hydrogenated products from trifuran.
Intermediate C15H28O2 with an m/z of 240 amu could contain two saturated furan rings with one
alkyl branch (Compounds 6 and 7, Figure 2) or one saturated furan ring, with one mono-keto chain
and one alkyl branch (Compounds 8 and 9, Figure 2). Compounds 3-5 (Figure 2) are likely formed
by deoxygenation of Compounds 1 and 2. There are two intermediates with the parent ion of 226
amu (C15H30O), which could be the stereoisomers of the structure with one hydrogenated furan ring
and two alkyl branches (Compounds 10, Figure 2). Compound 10 could result from the ring opening
of compounds with two saturated furan-rings, e.g., Compound 3, followed by the partial
deoxygenation of the two oxygen atoms. As the reaction progresses for 1 h at 180 °C, the amount of
oxygenates containing three oxygen atoms decreases with a concomitant increase in the
concentration of oxygenates with one or two oxygen atoms. Figure 2 shows that the oxygen content
of the oxygenates decreases gradually with time as the amount of C8-C15 alkanes increases. The
oxygenates are fully converted to alkanes within 12 h.
To understand the ring hydrogenation, ring opening and subsequent deoxygenation of
trifuran and their oxygenated intermediates, we studied the conversion of DMF and DMTHF as
model compounds (Table S1 and S2). In case of DMF conversion, both ring opening products
(2-hexanone and 2-hexanol) and ring saturation product (DMTHF) were observed during heating the
o
reaction mixture to 140 C. The dominant product is DMTHF. In the hydrogenolysis of DMTHF,
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o
almost no conversion of DMTHF was observed during heating to 140 C, which indicates that the
formation of 2-hexanol is proceeded by the hydrogenation of 2-hexanone. Therefore, the ring
opening of DMF to 2-hexanone and 2-hexanol and the ring-saturation of DMF to DMTHF occur in
parallel over the Ir-ReOx/SiO2 catalyst (Scheme S1). This is consistent with the proposed mechanism
that hydrogenation of trifruan occurs prior to hydrogenolysis during the heating period. The
ring-saturated product (DMTHF) is the dominant product in the conversion of DMF, indicating that
the reaction involving trifuran predominantly begins via hydrogenation of unsaturated furan rings.
2-hexanone is quickly hydrogenated to 2-hexanol, which then undergoes hydrogenolysis to
n-hexane[48]. The HDO of DMTHF to n-hexane likely proceeds through the hydrogenolysis of the
C-O ether bond, with 2-hexanol as the intermediate. 2-Hexanone is not detected, which indicates the
mono-keto group of oxygenates 4, 5, 8 and 9 (Figure 2) was formed from the direct ring opening of
trifuran without prior ring saturation. This finding differs C ’ w k, in which
tetrahydrofurans were the intermediates of ketones[15], likely due to the comparatively low reaction
temperature used here. HDO of DMF on Ir-ReOx/SiO2 yields primarily ring hydrogenated DMTHF,
while ring-opened products, i.e., 2-hexanone and 2-hexanol, are relatively minor (Table S1). This is
due to the fact that the formation of DMTHF is more thermodynamically favorable[49], and thus the
ring-opening of DMF occurs via partially and/or fully hydrogenated ring intermediates. This is
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consistent with previous work that the ring-opening of DMTHF is more difficult than DMF[50], and
thus requires higher temperatures.
Combining these results, we propose an overall reaction pathway for the HDO of trifuran
into jet-fuel ranged alkanes (Scheme 2). In the hydrogenation step (occurred during heating period to
the reaction temperature), the furan rings of trifuran underwent ring saturation and ring opening in
parallel to form a mixture of complex oxygenates with three different functional branches including
saturated furan ring, carbonyl groups and hydroxyl groups. The ring saturation reaction was the main
route. Then, carbonyl groups were hydrogenated to the corresponding hydroxyl groups, followed by
dehydration and hydrogenation steps to yield alkyl groups. In addition, ring opening, dehydration
and hydrogenation of saturated furan rings also led to alkyl branches. Thus, the formation of desired
alkanes involved several cascading steps in a complex reaction network over Ir-ReOx/SiO2.
To elucidate the relationship between the structure of the catalyst and its performance in the
hydrogenolysis of C-O bonds during HDO of trifuran, we characterized Ir-ReOx/SiO2 after reduction
by XPS (Figure 3), XRD (Figure S2) and FT-IR spectroscopy of pyridine adsorption (Figure 4). XPS
was carried out to determine the oxidation states of Ir and Re after the reduction procedure. To avoid
air exposure, reduced catalyst sample was transferred in a glove box via Vacuum Transfer Model to
the XPS chamber. Both Ir 4f and Re 4f core level spectra of the reduced Ir-ReOx/SiO2 are
summarized in Figure 3. The Ir 4f spectrum was deconvoluted with Doniach-Š ji (DS) y i
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line shape and Gaussian-Lorentzian sum form (SGL)[51], which has the binding energies for 4f7/2 and
4f5/2 peaks of ~60.8 and 63.8 eV, respectively, indicating that Ir exists in the metallic state. The
deconvoluted Re 4f spectrum shows peaks for Re0, Re2+ and Re4+, which were assigned according to
the literature values[52]. The XPS analysis suggests a significant portion of Re is partially reduced to
Re2+ and Re4+ (41% and 11%, respectively), which is consistent with prior reports of the
Ir-ReOx/SiO2 catalyst for the hydrogenolysis of low carbon molecules[45]
.
XRD patterns (Figure S2) showed that the peak position of Ir in Ir-ReOx/SiO2 (Re/Ir=2)
after reduction was similar to that of Ir/SiO2, which indicated that not an appreciable level of
alloying between Ir and Re formed in the Ir-ReOx/SiOx catalyst, as alloying will change the binding
energies of Ir and Re[53]. In addition, peaks for Re species were not detected at all in the
Ir-ReOx/SiO2 (Re/Ir=2) catalyst, suggesting that the Re species are likely highly dispersed.
Transmission FTIR spectroscopy of pyridine adsorbed on the in-situ reduced catalysts
(Ir/SiO2, ReOx/SiO2 and Ir-ReOx/SiO2) was used to evaluate their acidic properties. An IR band at
1450 cm-1 is assigned to pyridine adsorbed on Lewis acid sites (Figure 3), whereas a band at 1540
cm-1 emerges from pyridine interacting with the Brønsted acidic sites. The peak at 1490 cm-1 can
contain contributions from both Lewis and Brønsted acid sites[54]. The IR spectrum of pyridine
adsorbed on Ir-ReOx/SiO2 (Figure 3) clearly exhibits the presence of Lewis acid sites, which could
be due to the partially reduced ReOx species. A weak but distinctive band at 1540 cm-1 was also
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observed, indicating the presence of Brønsted acid sites. This is consistent with a prior report, in
which Zhang et al. observed both Lewis and Brønsted acid sites on silica supported Ir-ReOx catalyst
after reduction [33]. In both the Ir/SiO2 and the ReOx/SiO2 catalysts, a significant peak at 1445 cm-1
o
was observed at 50 C but pyridine adsorbed on Brønsted acid sites was absent. The peak at 1445
cm-1 could be associated with physically adsorbed pyridine on the silica support[54]; however, the
presence of Lewis acid sites could not be excluded, as the peak at 1445 cm-1 may overlap with the
peak at 1450 cm-1. Previously, Davis et al. observed Lewis acid sites on ReOx supported on silica
after reduction in H2 at 473 K[52]. When the temperature was increased to 100 oC, surface adsorbed
pyridine on Ir/SiO2 and ReOx/SiO2 was fully desorbed. In contrast, Ir-ReOx/SiO2 showed peaks
corresponding to Brønsted and Lewis acid sites at 150 oC, indicating that an interaction between Ir
and ReOx not only creates Brønsted acid sites absent on individual components, but also enhances
the acidic strength of both types of sites. However, we are unable to establish the correspondence
between specific oxidation states of Re and acid sites based on the results presented in this work, and
thus, no attempt has been made to correlate deconvoluated XPS bands with acid site densities.
The hydrogenolysis of etheric and alcoholic C-O bonds is the final steps in the formation of
alkanes over the Ir-ReOx/SiO2 catalyst. Tomishige and co-workers have proposed that the
hydrogenolysis of secondary mono-alcohols was likely assisted by the Lewis or Brǿnsted acid sites
of Ir-ReOx/SiO2[46], however, they did not characterize acid sites on the catalyst. Dumesic et al.
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studied the selective hydrogenolysis of C-O bonds of a wide range cyclic ethers and polyols over a
Rh-ReOx/C catalyst in water solvent[55-56], and proposed Brønsted acidic sites on ReOx, associated
with metallic Rh, participated in the ring-opening of tetrahydrofuran. Marks et al. reported that high
valent metal triflate Lewis acid sites, e.g., Hf(OTf)4, exhibit high activity in the hydrogenolysis of
etheric and alcoholic C-O bonds[57-58] in an alkane solvent. Taken all together, it is reasonable to infer
that the acid sites on the partially reduced ReOx of the Ir-ReOx/SiO2 catalyst could play an important
role in activating the C-O bonds of saturated furan ring and promoting the ring-opening to form
alcohols as intermediates followed by their dehydration in cyclohexane solvent. Finally, Ir metal
catalyzes the hydrogenation of alkenes to produce alkanes.
Two other alkane molecules that form in considerable yields are n-decane and n-undecane.
These may form via C-C bond scission adjacent to the tertiary carbon atom in the middle of trifuran,
resulting in the removal of a C5 or C4 moiety. Examination of the reaction time profile (Figure 1)
reveals that n-decane and n-undecane formed at the beginning of the reaction. The amount of
n-decane and n-undecane increases only slightly with a proportional decrease in the yield of desired
alkane at longer reaction time. This indicates that the fragmentation mainly occurs in the initial
stages (Scheme 3), i i wi h C ’ w k[15], which reported that the C-C bond scission of
furylmethane occurs preferentially between a furan moiety and a tertiary carbon atom, catalyzed by
acid
sites[15]
.
Similarly,
we
observed
2-methyl-5-pentyltetrahydrofuran
and
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10.1002/cssc.201700863
ChemSusChem
2-hexyl-5-methyltetrahydrofuran intermediates at the beginning of the reaction, which could form
via C-C cleavage by surface acid sites on the Ir-ReOx/SiO2 catalyst. However, the C-C cleavage of
the C15 alkane could not be completely ruled out based on results in this work (Scheme 3).
3.3 Optimizing Reaction Temperature and H2 Pressure
The effect of the reaction temperature on the HDO of trifuran is shown in Figure 5(a). The
yields of C8-C15 alkanes, with C15 being the major product (41-74%), increase with increasing
o
o
o
reaction temperature from 150 C to 170 C. Higher temperature (>160 C) causes a slight decrease
in the yield of C15 alkane, owing to forming C-C cleavage products (mainly C10 and C11). The
o
highest yield of C8-C15 alkanes reaches 91% at 170 C, with a C15 alkane yield of 75%. Our
maximum alkane yield is comparable to a previous report over a Pd/NbOPO4 catalyst (91%)[21] and
higher than those Ni/Hβ Ni/HZSM-5 catalysts (~70%)[16-18]. Notably, our reaction
o
o
temperature (170 C) i w h i w k (≥200 C, with several works employing much
higher temperatures), underscoring the effectiveness of Ir-ReOx/SiO2 for the HDO of trifuran.
It was reported that a two-step reaction with furan ring hydrogenation at low temperature
followed by HDO at high temperature was effective for both the conversion of furfural to
[30, 31]
1,5-pentendiol over Pd- or Rh-Ir-ReOx/SiO2
and the synthesis of hydrocarbons from aldol
condensation product of HMF and acetone over a Pd/C-Hf(OTf)4 catalyst [59]. To explore the
effectiveness of this strategy, we performed a two-step reaction in which the reaction mixture was
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10.1002/cssc.201700863
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o
o
heated at 50 C for 4 h, followed by elevating the reaction temperature to 170 C to continue the
reaction for another 12 h. The yield of C8-C15 alkanes from the two-step reaction is slightly lower
(84%) than that obtained from one-step reaction (91%, Table S3).
The effect of initial hydrogen pressure on the HDO of trifuran is shown in Figure 5(b). The
yields of C8-C15 alkanes increase remarkably as the H2 pressure increases from 2 to 5 MPa, with C15
exhibiting the highest yield (41-75%). This can be explained by the positive reaction order with
respect to H2 pressure in the HDO over Ir-ReOx/SiO2, consistent with prior reports[29, 35]. Higher H2
pressure (6 MPa) did not affect the yields of C8-C15 alkanes. The C-C cracking products (mainly C10
and C11) decrease significantly with increasing H2 pressure. Since C-C cleavage mainly occurs at the
beginning of the reaction, higher H2 pressure favors hydrogenation and saturation of the furan ring
and thus suppresses C-C bond scission in the unsaturated ring trifuran molecule.
3.4 Catalyst Stability
Catalyst reusability is important in determining its potential for practical applications. The
results of the reused Ir-ReOx/SiO2 (Re/Ir=2) are shown in Table 2. After the 1st cycle, the catalyst is
separated from the products by centrifugation, washed with methanol, and dried at 110 C for 10 h
before reuse in the next cycle. Due to a small mass loss of catalyst (7~11 wt%) during recovery, we
decreased the scale of each subsequent recycling experiment in which the ratios of catalyst, substrate
and solvent were similar to the 1st run. A reduction pretreatment of the catalyst was performed before
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10.1002/cssc.201700863
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every cycle, as discussed in the experimental section. Reusing the catalyst without calcination results
in a significant decrease in the yields of C8-C15 alkanes, with the majority of unconverted
intermediates being oxygenates (Entry 2; Table 2). The result of TG-DTA analysis of the recovered
catalyst after 1st cycle show ~5 wt% carbon deposition on the catalyst surface (Figure S3) that causes
catalyst deactivation. Thus, after drying in air, we calcined the spent catalyst at 500 oC in air for 3 h.
After calcination, the recovered catalyst regained comparable activity to that of the fresh catalyst
(Entry 3; Table 2). The yields of C8-C15 alkanes in the 3rd cycle is ~12% lower (80%) than the 1st
cycle (91%), suggesting that modest catalyst deactivation does occur. The XRD peaks of the
recovered catalyst assigned to the metallic Ir phase became sharper (Figure S4). Calculations using
the Scherrer equation indicate an increase of the average crystalline size of Ir particles from 2.5 nm
(fresh catalyst) to 3.2 nm after three cycles[60], consistent with previous reports[30-31, 39-41]. Therefore,
observed activity loss in the 3rd cycle is likely caused by sintering of the catalyst during reaction and
regeneration.
3.5 HDO of Other Furylmethanes
Three other furylmethanes of carbon numbers C12-C14 were used for the HDO over the
Ir-ReOx/SiO2 catalyst at the optimal reaction conditions developed for trifuran (Table 3). GC-MS
results show complete conversion of these furylmethanes to alkanes under optimized reaction
condition. The yields of the corresponding alkanes with the same number of carbon atoms as the feed
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varies between 61 and 96%, while the maximum yield of total jet-fuel ranged alkanes varies between
82 and 99%. C12 furylmethane (EBM) forms C12 alkane with the highest selectivity (99%). The
selectivity of other alkanes from their respective furylmethanes i h EBM ≈
BBM > trifuran > PBM. The results suggest that the Ir-ReOx/SiO2 catalyst is effective for the HDO
of all furylmethanes used in this work.
The selectivity of alkane products from different furylmethanes can be explained by the
relative stability of their respective carbocations. The stability of the carbocations increases in the
order of primary < secondary < tertiary < quaternary carbons. The C-C fragmentation at the
quaternary carbon adjacent to the furan moiety occurs preferentially. Thus, PBM yields the least
amount (61%) of alkanes with its corresponding maximum carbon number (C13) due to the formation
of a stable tertiary carbocation by C-C scission. C-C cracking of trifuran, EBM, and BBM involves
secondary carbocations which are less stable than the tertiary ones. Trifuran has one additional furan
ring compared to EBM and BBM, which can stabilize the adjacent carbocation by conjugation.
Therefore, HDO of trifuran yields less alkane of maximum carbon number (C15) than the
corresponding alkanes of EBM and BBM.
4. Conclusions
In this work, Ir-ReOx/SiO2 catalysts with different Re/Ir molar ratio were used for the production
of renewable jet-fuel ranged alkanes (C8-C15) by HDO of C12-C15 furylmethanes under mild reaction
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conditions. High yield (82-99%) of C8-C15 alkanes is achieved at an optimal Re/Ir molar ratio of 2.
The main pathway involves hydrogenation of the furan rings to saturated rings, followed by ring
opening to hydroxyl containing chains. A minor parallel pathway involves ring opening of the
unsaturated furans to mono-keto and mono-hydroxy units. These parallel pathways lead to the
formation of several oxygenated intermediates. Sequential hydrogenolysis of these oxygenates over
Ir-ReOx/SiO2 forms alkanes via a cascade pathway. Controlled experiments and characterization
illustrate a strong synergy between Ir and ReOx of the catalyst in its high activity for HDO of high
carbon furylmethanes. The Ir metal initiates the hydrogenation steps via H2 dissociation and the acid
sites of partially reduced ReOx activate both etheric and alcoholic C-O bonds.
Acknowledgements
This work was supported as part of the Catalysis Center for Energy Innovation, an Energy
Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of
Basic Energy Sciences under Award Number DE-SC0001004. The authors acknowledge the
Advanced Materials Characterization Laboratory at the University of Delaware for providing XRD
and TG-DTA facilities and Jeffrey Everhart for helping with GC/GCMS analysis of the reaction
products. We also acknowledge instrumentation support for XPS (funded by NSF award number
142149).
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Table 1. HDO of trifuran over the Ir-ReOx/SiO2 catalyst.
Yield / %
Re/M
Conv.
Entry
Catalysts
molar ratio
/ %
98.5
100
100
100
100
45.9
100
100
100
100
100
C8
C9
C10
0.8
9.3
9.0
8.6
6.4
0.3
7.0
8.9
0.8
1.2
0.0
C11
0.9
C12
0.2
1.3
0.2
0.2
0.3
0.6
5.1
2.5
0.1
0.2
0.0
C13
0.6
0.1
0.2
0.3
0.5
0.2
0.1
1.6
0.1
0.3
0.1
C14
0.1
0.5
0.8
1.0
1.3
0.0
0.2
2.2
1.0
0.3
0.0
C15
3.3
Sum
6
Oxygenates
1
2
Ir/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
ReOx/SiO2
0
0.25
0.5
1
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.5
0.0
0.0
0.0
0.0
0.2
0.2
0.2
0.2
0.0
0.1
2.2
0.1
0.1
0.0
47
21
9
14.0
12.7
11.2
7.2
26.6
42.3
53.0
60.6
0.4
52
65
75
77
2
3
4
1
5
2
0.0
0.0
22
22
70
57
52
6
7a
-
0.3
Ir/SiO2 +ReOx/SiO2
Ru-ReOx/SiO2
Rh-ReOx/SiO2
Pt-ReOx/SiO2
Pd-ReOx/SiO2
-
13.4
9.9
14.2
8.3
40
36
4
8
0.5
0.5
0.5
0.5
9
0.8
1.1
10
2.1
1.4
6
11
0.0
0.1
0.2
Reaction condition: catalyst (0.15 g), M: Ir, Rh, Ru, Pt and Pd (4 wt%), trifuran (0.3 g), cyclohexane (10 ml), initial H2 (5 MPa), 180 oC, 20 h. aphysical mixture of Ir/SiO2 and
ReOx/SiO2 with the same metal amount of Ir-ReOx/SiO2 (Re/Ir=2).
28
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Table 2. Reusability study of the Ir-ReOx/SiO2 (Re/Ir=2) catalyst for the HDO of trifuran.
Yield / %
Conv.
Entry
Catalyst
Cycle
/ %
100
100
100
100
C8
0.0
0.1
0.0
0.0
C9
0.1
0.2
0.1
0.1
C10
6.4
9.3
5.6
5.5
C11
8.2
8.4
7.7
8.5
C12
0.3
0.3
0.4
0.4
C13
0.5
0.3
0.5
0.4
C14
1.1
0.5
1.0
0.9
C15
Sum
91
Oxygenates
1
2
3
4
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
1
2a
2
74.6
15.9
71.3
64.6
0.0
30
35
87
0.0
3
80
6.3
Reaction condition: catalyst (0.15 g), Ir (4wt%), trifuran (0.3 g), cyclohexane (10 ml), initial H2 (5 MPa), 170 oC, 12 h. a: without calcination. Some catalyst mass was lost
during recovery. The amounts of fresh or recovered catalysts used in the 1st, 2nd and 3rd cycles were 0.15 g, 0.137 g and 0.128 g, respectively. The weight ratio of catalyst,
trifuran and cyclohexane was kept the same in each cycle.
29
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