Molybdenum Oxide-Modified Iridium Catalysts for Selective Production of Renewable Oils for Jet and Diesel Fuels and Lubricants

Article abstract of DOI:10.1021/acscatal.9b02693

Supported inverse metal-metal oxide catalysts have received significant research interest owing to their effective hydrodeoxygenation (HDO) activity toward biomass substrates, but the high cost of the reported catalysts poses a challenge for commercialization. We present the synthesis of a series of metal-metal oxide catalysts, Ir-MO_x/SiO₂ (M = Re, Mo, W, V, or Nb) and M′-MoO_x/SiO₂ (M = Rh, Ru, Pt, or Pd) and their HDO performance on multifuran (high carbon) substrates to produce renewable jet and diesel fuels and lubricant base oils. A MoO_x-modified Ir/SiO₂ catalyst with a Mo/Ir ratio of 0.13 (Ir-MoO_x/SiO₂) exhibits the highest product yield (78-96%) under mild reaction conditions. Controlled experiments using probe substrates reveal that furan ring hydrogenation and C-O hydrogenolysis of saturated and unsaturated furan rings occur in a sequential manner. The carbon atom adjacent to the furan or saturated furan ring of substrates or intermediate compounds undergoes slow C-C bond scission, resulting in a small fraction of lighter alkanes. Catalyst characterization suggests that Ir is reduced to a fully metallic state to dissociate hydrogen for hydrogenation. Intact MoO_x, partly covering the Ir metal surface, promotes ring opening, hydrogenolysis of etheric and alcoholic C-O bonds, and hydrogenation of Ca? O bonds. This study highlights the potential of low-cost metal-metal oxide catalysts with low loading of oxophilic metals to enable cost-competitive production of bioproducts and demonstrates applicability of these catalysts on other substrates, including fatty acids, fatty esters, and lipids.

Full text of DOI:10.1021/acscatal.9b02693

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Article
Molybdenum Oxide-Modified Iridium Catalysts for Selective
Production of Renewable Oils for Jet and Diesel Fuels and Lubricants
Sibao Liu, Weiqing Zheng, Jiayi Fu, Konstantinos Alexopoulos, Basudeb Saha, and Dionisios G. Vlachos
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02693 • Publication Date (Web): 16 Jul 2019
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Molybdenum Oxide-Modified Iridium Catalysts for Selective Production of Renewable
Oils for Jet and Diesel Fuels and Lubricants
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Sibao Liu,1 Weiqing Zheng,1 Jiayi Fu1,2, Konstantinos Alexopoulos1,2, Basudeb Saha,1* and
Dionisios G. Vlachos1,2*
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1Catalysis Center for Energy Innovation, University of Delaware, Newark, DE 19716, USA
2Department of Chemical and Biomolecular Engineering, University of Delaware, Newark,
DE 19716, USA
*Corresponding authors: vlachos@udel.edu, bsaha@udel.edu
Abstract
Supported inverse metal-metal oxide catalysts have received significant research interest owing to
their effective hydrodeoxygenation (HDO) activity toward biomass substrates but the high cost of
the reported catalysts poses a challenge for commercialization. We present the synthesis of a series
of metal-metal oxide catalysts, Ir-MOx/SiO2 (M=Re, Mo, W, V, or Nb) and M’-MoOx/SiO2 (M=Rh,
Ru, Pt, or Pd), and their HDO performance on multi-furan (high carbon) substrates to produce
renewable jet and diesel fuels and lubricants base oils. A MoOx modified Ir/SiO2 catalyst with
Mo/Ir ratio of 0.13 (Ir-MoOx/SiO2) exhibits the highest product yield (78-96%) under mild reaction
conditions. Controlled experiments using probe substrates reveal that furan ring hydrogenation
and C−O hydrogenolysis of saturated and unsaturated furan rings occur in a sequential manner.
The carbon atom adjacent to the furan or saturated furan ring of substrates or intermediate
compounds undergoes slow C−C bond scission resulting in a small fraction of lighter alkanes.
Catalyst characterization suggests that Ir is reduced to fully metallic state to dissociate hydrogen
for hydrogenation. Intact MoOx, partly covering the Ir metal surface, promotes ring opening,
hydrogenolysis of etheric and alcoholic C−O bonds, and hydrogenation of C=O bonds. This study
highlights the potential of low-cost metal-metal oxide catalysts with low loading of oxophilic
metals to enable cost-competitive production of bioproducts and demonstrates applicability of
these catalysts on other substrates, including fatty acids, fatty esters, and lipids.
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Keywords: Furan, biomass, hydrodeoxygenation, jet fuel, diesel fuel, lubricant base oil, renewable
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fuels, renewable lubricants, metal-metal oxides
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1. Introduction
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Global carbon emissions drive exploration of lignocellulosic biomass as a renewable resource
for chemicals and fuels1-7. Long-chain alkanes, with varying carbon number and structure (linear
or branched), used for jet fuel (C8-C16), diesel fuel (C9-C22), and lubricant base oils (C20-C50),
collectively called oils, are conventionally produced from petroleum. Branched alkanes are
preferred oils for their improved cold flow properties8. Production of high-volume oils from
abundant, inexpensive, and carbon-neutral lignocellulose could mitigate carbon emission
challenges.
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While the synthesis of fuels from furan platforms (e.g., furfural, 2-methylfuran) has been
reported9-25, the synthesis of lubricant base oils is still in its infancy26-29. High carbon alkanes can
be synthesized by carbon-carbon (C−C) coupling of furans with bio-based carbonyl compounds to
obtain high carbon furans (HCFs) followed by their hydrodeoxygenation (HDO) to alkanes. C−C
17,
30
coupling reactions, e.g., hydroxyalkylation/alkylation9-15,
hydroxyalkylation/alkylation31, aldol condensation20, 22-23, alkylation18, benzoin condensation21, 25
,
conjugate addition/
,
can achieve high yields using acid or base catalysts. HDO, on the other hand, requires high reaction
temperatures and pressures14, 24 that lead to high utility and energy costs and undesirable C−C
cracking14-15. We recently reported a supported inverse metal-metal oxide catalyst, partially
reduced rhenium oxide modified Ir/SiO2 catalyst (Ir-ReOx/SiO2), is efficient for the HDO of C12-
C34 HCFs to produce high yield, long-chain branched alkanes under mild reaction conditions16, 29,
31. Replacement of the expensive Re (~7.8 wt%, $1290/lb)32 with a low-cost metal-inexpensive
oxophilic metal oxide (e.g., molybdenum, $12.6/lb)33 will be preferable.
Prior reports suggest that several non-noble oxophilic metal oxides, including MoOx, WOx,
VOx, or NbOx modified with noble metal are selective for hydrogenation and hydrogenolysis
reactions34, including the ring opening of cyclic ethers with -OH groups to diols35-40, the
hydrogenolysis of polyols to mono-alcohols41-42, the hydrogenation of unsaturated aldehydes to
unsaturated alcohols43-44, and the hydrogenation of amino acids to amino alcohols45-46. The noble
metal and the partially reduced metal oxide cooperate toward enhanced activity and selectivity34.
Some of them, e.g., NbOx, perform even better than ReOx in the selective hydrogenation of
unsaturated aldehydes44. Despite these efforts, complete HDO of furan compounds to alkanes over
such inexpensive catalysts has not been reported.
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In this work, cheap oxophilic metal oxides (MoOx, WOx, VOx, and NbOx) modified Ir/SiO2
catalysts (Ir-MoOx/SiO2, Ir-WOx/SiO2, Ir-VOx/SiO2, and Ir-NbOx/SiO2) are synthesized and tested
for HDO of HCFs to produce renewable oils under mild reaction conditions (Scheme 1). Among
them, Ir-MoOx/SiO2 (Mo/Ir=0.13) catalyst exhibits superior performance (up to 96% yield) than
our previously reported Ir-ReOx/SiO2 with the same low metal loading. Control reactions, using
probe molecules, and catalyst characterization are conducted to elucidate the reaction pathway and
the promoting effect of MoOx.
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A)
B)
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O
O
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O
O
O
OH
Ir-MoOx/SiO2
HDO
O
O
Ir-MoOx/SiO2
HDO
Hydrogenation
Pd/C
C20-HCF
C20-A
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O
O
Ir-MoOx/SiO2
HDO
O
O
O
O
O
C15-A
C15-HCF1
O
O
Ir-MoOx/SiO2
HDO
O
O
C10H21
O
C10H21
C14-HCF
Ir-MoOx/SiO2
HDO
O
C14-A
O
O
O
C30-HCF
C30-A
Ir-MoOx/SiO2
HDO
O
O
C15-A
C15-HCF2
Scheme 1. A): Production of jet and diesel fuels (C15-A and C14-A) from 2-methylfuran with different carbonyl compounds (C15-HCF1, C14-HCF
and C15-HCF2) and their subsequent HDO over Ir-MoOx/SiO2. B): Production of lubricant base oils (C20-A and C30-A) from 2-pentylfuran and
different aldehydes (C20-HCF and C30-HCF) and their subsequent HDO over Ir-MoOx/SiO2.
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2. Experimental
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2.1. Catalyst preparation
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Ir-MOx/SiO2 (M=Re, Mo, W, V or Nb) catalysts were prepared by sequential impregnation as
described previously16, 29. First, Ir/SiO2 was prepared by impregnating SiO2 (Fuji Silysia G-6,
calcined in air at 700 oC for 1 h with a heating rate of 10 oC/min, and BET surface area 535 m2/g)
with an aqueous solution of H2IrCl6 (Sigma-Aldrich). After evaporating the solvent at 75 oC and
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drying at 110 C for 12 h, impregnation happened with an aqueous solution of either NH4ReO4
(Alfa-Aesar), (NH4)6Mo7O24·4H2O (Alfa-Aesar), (NH4)10W12O24·5H2O (Sigma-Aldrich),
NH4VO3 (Sigma-Aldrich), (NH4)NbO(C2O4)2·5H2O (Sigma-Aldrich). Catalysts were dried at 110
oC for 12 h and then calcined in a crucible in air at 500 oC for 3 h by ramping at 10 oC/min. The
loading of Ir was 4.0 wt% and the M/Ir atomic ratio ranged from 0 to 0.5. Other metal impregnated
catalysts, M’-MoOx/SiO2 (M=Rh, Ru, Pt or Pd, Mo/M’=0.25), were prepared following the same
procedure using RhCl3 (Alfa-Aesar), RuCl3 (Sigma-Aldrich), H2PtCl6 (Sigma-Aldrich), and PdCl2
(Sigma-Aldrich). These catalysts are hereafter referred to as M’-MOx/SiO2.
2.2. Synthesis of high carbon furans (HCFs)
HCFs were prepared by HAA of 2-methylfuran (2-MF) or 2-pentylfuran with carbonyl
compounds following our previous work9, 29-30. In brief, the HAA reaction was carried out in a
round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The reaction
temperature was controlled using an oil bath. C14- and C15- HCFs (Scheme 1) were synthesized by
the HAA reaction of 2-MF with furfural and butanal, respectively, over an improved graphene
oxide catalyst30. C15-HCF2 was synthesized by self-condensation of 2-MF over H2SO4.9 C20-HCF
and C30-HCF were prepared by reacting 2-pentylfuran with acetaldehyde and lauraldehyde,
respectively, over a perfluorinated sulfonic acid resin (Aquivion PW98) catalyst29.
2.3. Hydrogenation of C15-HCF1
Hydrogenation of the furan rings of C15-HCF1 was conducted in a 50-ml Parr reactor with an
inserted Teflon liner over a Pd/C (10 wt% Pd, Sigma-Aldrich) catalyst. C15-HCF1 (1 g), Pd/C (0.1
g) and solvent (cyclohexane;10 mL) were added into the Teflon liner together with a magnetic
stirrer. The liner was then placed in the reactor, which was heated at 60 oC with 6 MPa H2 for 4 h.
After reaction, the catalyst was separated by centrifugation. The product was obtained after
removing the solvent by a rotary evaporator.
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2.4. HDO of HCFs over metal-metal oxide catalysts
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HDO reaction was performed in a 50-ml Parr reactor with an inserted Teflon liner. The catalyst
was placed into the Teflon liner together with a magnetic stirrer and 10 ml solvent (cyclohexane).
The reactor, equipped with a band heater, was heated at 200 oC with 5 MPa H2 for 1 h to reduce
the catalyst. The stirrer speed was set at 240 rpm. After pre-reduction, the reactor was cooled down,
and the pressure released. The reactor was immediately loaded with 0.3 g HCF substrates or
hydrogenated C15-HCF1 product and sealed. The sealed reactor was purged with H2 (1 MPa) for
three times and pressurized to 5 MPa with H2 and then heated to reaction temperature. The total
time to reach the set temperature was about 25 min. The stirring rate was set at 500 rpm. After a
certain reaction time, the reactor was placed in an ice-water bath and upon cooling, the pressure
released. The reaction solution, along with the spent catalyst, were transferred to a glass vial. The
spent catalyst was separated by centrifugation and filtration. In the catalyst recyclability tests, the
used catalyst after each cycle was washed three times with 10 ml cyclohexane, dried at 110 oC for
12 h, and then regenerated by calcination at 400 oC for 3 h at a heating rate of 10 oC/min.
The HDO reactions of 2,5-dimethylfuran (DMF, Sigma Aldrich), 2,5-dimethyltetrahydrofuran
(DMTHF, Sigma Aldrich), 2-hexanone (TCI America) and 2-hexanol (Sigma Aldrich) were
performed in the similar way. DMF was distilled to remove any impurities before reaction. To
avoid the hydrogenation reaction of furan rings during heating, the reactor was first pressurized
with 0.3 MPa N2 during heating to the set temperature and then pressurized with H2 to a total
pressure of 6 MPa. Control experiments suggested no conversion of the substrate until the
introduction of hydrogen.
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Product analysis was done with a gas chromatograph, GC (Agilent 7890A), equipped with an
HP-1 column and a flame ionization detector using icosane (C20) as an internal standard. DMF,
DMTHF, 2-hexanone, and 2-hexanol hydrotreated products were analysed with a GC (Agilent
7890B) equipped with an HP-INNOWax column and a flame ionization detector using n-decane
as an internal standard. Since a small fraction of cyclohexane (solvent) could be cracked to linear
alkanes during the catalyst pre-reduction and reaction over some of the catalysts, any detected C1-
C6 alkanes in the gas or liquid phase, that could form during HDO of HCFs, were not quantified.
The products from HDO of C15-HCF1 were also analyzed using a Nicolet 8700 FTIR spectrometer
equipped with a DTG detector and a Golden Gate single reflection diamond attenuated total
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reflectance (ATR) attachment. All FTIR spectra were acquired at 32 scans with a resolution of 2
cm−1.
The conversion of furans and the yield of products were calculated on carbon basis and defined
as follows:
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―
mole of initial reactant mole of unreacted reactant
×
Conversion [%] =
100
mole of initial reactant
molproduct C atoms in product
×
×
Yield of detected products [% - C] =
100
mol of total C atoms of initial reactant
2.5. Characterization of Ir-MoOx/SiO2
X-ray diffractograms (XRD) of the pre-reduced and recovered catalysts were recorded by a
diffractometer (Bruker D8) equipped with a Cu Kα radiation source (λ=0.154nm) at 40 kV and 40
mA. X-ray photoelectron spectroscopy (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-MoOx/SiO2 catalyst was pre-reduced as detailed
above and then transferred into the K-Alpha+ Vacuum Transfer Module (VTM, ThermoScientific)
without being exposed to air.
X-ray absorption spectroscopy (XAS) data were collected for the Mo K edge (20000 eV) at
the bending magnet beamline (5BM-D) of the Dow-Northwestern-DuPont Collaborative Access
Team (DND-CAT) of the Advanced Photon Source, Argonne National Laboratory. The
measurements for reference samples (Na2MoO4, MoO3, MoO2 and Mo foil) were taken in
transmission mode. The samples were pelletized using a die of diameter 13mm and then held in a
cell (Linkam stage). The Ir-MoOx/SiO2 and MoOx/SiO2 were in-situ recorded in fluorescence
mode. The catalysts were reduced in flowing N2 (90 ml/min) and H2 (10 ml/min) at 200 oC for 1h
by ramping at 10 oC/min. Spectra were recorded before and after reduction.
The FTIR spectra of pyridine adsorbed on MoOx/SiO2 and Ir-MoOx/SiO2 were collected on a
Thermo Electron Nicolte 8700 FT-IR Spectrometer equipped with an MCT detector (128 scans at
a spectral resolution of 2 cm-1) and an in-house made 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-
made sample holder and the catalyst was reduced at 200 oC under a hydrogen stream (20 ml/min)
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for 1 h. Upon 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 at 150 oC.
The microstructures of the catalysts after pre-reduction were examined using a field emission
transmission electron microscope JEM-2010F FasTEM at 200 kV.
Quantification of leaching from Ir-MoOx/SiO2 in the product solutions was conducted using a
triple quadrupole inductively coupled plasma mass spectrometer (Thermo Scientific iCAP TQ-
ICP-MS).
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3. Results and discussion
3.1 Catalysts screening and reaction conditions optimization
First, the catalyst performance was screened for HDO of C15-HCF1 (Scheme 1) over Ir-
MOx/SiO2 catalysts. The first set of experiments used catalysts containing Ir as the metal sites and
various inexpensive oxophilic metals such as MoOx, WOx, VOx and NbOx as oxides. The Ir loading
was the same as in our previously reported Ir-ReOx/SiO2 whereas the atomic ratio of M/Ir was
lower (0.25) (Re/Ir ratio in Ir-ReOx/SiO2 was 2)16. This is significant to preserve metals and reduce
catalysts preparation costs. Ir-ReOx/SiO2 catalyst with Re/Ir atomic ratio of 0.25 was also
synthesized for comparison. Table 1 shows that Ir/SiO2 without any oxophilic metal oxide exhibits
very low activity (Entry 1)16. In contrast, Ir-MOx/SiO2 enhances the HDO activity significantly
(Table 1, Entries 2-4 and 6). The Ir-MoOx/SiO2 (0.25) catalyst exhibits the highest yield to
branched alkanes (Table 1, Entry 2). Ir-ReOx/SiO2 (0.25), Ir-WOx/SiO2 (0.25), and Ir-VOx/SiO2
(0.25) show a moderate catalytic performance and a significant fraction of unconverted oxygenated
species. Ir-NbOx/SiO2 (0.25) is inefficient (Table 1, Entry 5). This result for Ir-NbOx/SiO2 differs
from prior reports in which noble metals (Pd and Pt) supported on Nb-based materials (NbOPO4,
Nb2O5, and Nb2O5/SiO2) were used for HDO of single and multi-ring furan substrates.17, 22, 28, 47-51
This difference in catalytic performance is probably due to the difference in catalysts’ structures
and compositions. Among the M’-MoOx/SiO2 (0.25) catalysts, Rh-MoOx/SiO2 and Pt-MoOx/SiO2
yield only 12-14% long chain alkanes (Table 1, Entries 7 and 8). Ru-MoOx/SiO2 (0.25) and Pd-
MoOx/SiO2 (0.25) yields primarily unconverted oxygenated intermediates (Table 1, Entries 9 and
10). The latter series of catalysts with other hydrogenation metals perform worse than Ir-
MoOx/SiO2 (0.25).
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Table 1. HDO of C15-HCF1 over M’-MOx/SiO2 catalysts.
Yield %
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M/M’
Conv.
%
Entry
Catalyst
C1
Oxygenates
C8
C9
C10
C12
C13
C14
C15
Sum
1
8
9
1
Ir/SiO2
0
99
0.0
0.0
0.0
0.1
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.1
0.3
0.0
0.2
0.2
0.1
0.1
0.0
0.1
0.1
0.2
0.0
0.1
0.8
14
0.9
19
0.2
0.1
2.9
2.3
0.0
1.3
0.0
0.2
0.0
0.0
0.1
0.1
0.1
0.0
0.3
0.6
0.1
0.1
0.1
0.0
0.1
0.0
0.0
0.0
0.0
0.1
0.2
0.1
0.0
0.1
0.1
0.6
0.3
0.3
0.0
0.5
0.1
0.1
0.1
0.0
0.5
0.7
0.6
0.0
0.2
3.3
43
6
47
2.0
30
16
54
21
55
74
72
60
3.5
0.0
1.8
4.0
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Ir-MoOx/SiO2
Ir-WOx/SiO2
Ir-VOx/SiO2
Ir-NbOx/SiO2
Ir-ReOx/SiO2
Rh-MoOx/SiO2
Pt-MoOx/SiO2
Ru-MoOx/SiO2
Pd-MoOx/SiO2
Ir-MoOx/SiO2
Ir-MoOx/SiO2
Ir-MoOx/SiO2
MoOx/SiO2
0.25
100
77
44
50
1.3
52
14
12
0.9
0.0
71
78
69
0.0
40
3
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.05
0.13
0.5
100
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7.8
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4
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5
0.4
9.3
3.1
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0.1
12
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14
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39
-
0.0
8.2
0.0
18
0.0
13
Ir/SiO2
-
100
15a
+MoOx/SiO2
Reaction conditions: catalyst (0.15 g), M’: Ir, Rh, Pt, Ru, Pd (4 wt%), M: Mo, W, V, Nb, Re, C15-HCF1 (0.3 g), cyclohexane (10 mL), initial H2 (5 MPa), 180 oC,
20 h. a: physical mixture of Ir/SiO2 and MoOx/SiO2 with the same metal amount of Ir-MoOx/SiO2 (0.13).
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Next, Ir-MoOx/SiO2 catalysts of Mo/Ir atomic ratio varying between 0.05 to 0.5 were screened.
These catalysts achieve full conversion of C15-HCF1. The yield to long chain alkanes and the
desired C15 alkane (hereinafter referred to as C15-A, Scheme 1) shows a volcano-type behavior vs.
the atomic ratio of Mo/Ir (Table 1, Entries 2 and 11-13) with the Ir-MoOx/SiO2 (0.13) giving the
highest conversion and total yield of C8-C15 alkanes (78%) (Table 1, Entry 12). Control
experiments over either Ir/SiO2 or MoOx/SiO2 exhibit no reaction (Table 1, Entries 1 and 14). A
physical mixture of Ir/SiO2 and MoOx/SiO2 at Mo/Ir=0.13 yields 29% C8-C15 alkanes (Table1,
Entry 15), indicating a synergy. Since Ir-MoOx/SiO2 (0.13) achieves the best performance, it was
chosen for all subsequent experiments.
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The effects of reaction temperature and initial hydrogen pressure on the Ir-MoOx/SiO2 are
shown in Tables S1 and S2. The conversion of C15-HCF1 and the yield to total alkanes, with C15-
A being the major product, increases remarkably with increasing reaction temperature from 160
oC to 170 oC. Higher temperature (≥190 oC) cause a slight decrease in the yield of total alkanes as
well as of C15-A, with a concurrent increase in the yield of C10 and C11 alkanes via C−C cleavage.
Increasing H2 pressure from 2 to 5 MPa exhibits no effect on C15-HCF1 conversion but an
enhanced selectivity to C15-A (Table S2) due to less C−C cracking. Since C−C cleavage mainly
occurs at the tertiary carbon center adjacent to furan moieties9, 14, 16, higher H2 pressure favors
hydrogenation of furan rings and HDO of saturated of furan rings. Further increase in H2 pressure
to 6 MPa shows no pronounced effect on the reaction. Therefore, 180 oC and 5 MPa H2 pressure
were chosen for the time course investigation discussed below.
3.2 Time dependent studies and reaction network
To understand the reaction network for HDO of multi-furan substrates, the time course of C15-
HCF1 was investigated using the Ir-MoOx/SiO2 (0.13) catalyst (Figure 1). The majority of C15-
HCF1 is converted during heating, and conversion is nearly quantitative within 2 h. First,
oxygenated intermediates form as major products by hydrogenation of furan rings. These
oxygenates are then deoxygenated into alkanes. GC-MS and ATR-IR analysis reveals that the
oxygenated intermediates at the beginning of the reaction consist mainly of saturated furan ring,
mono-keto and mono-hydroxyl groups (Figure S1 and S2). Similar functional groups were also
observed over the Ir-ReOx/SiO2 (2) in our earlier work16. Complete deoxygenation of all
oxygenated intermediates to alkanes occurs within 20 h. No C−C cleavage of long chain alkanes
is evidenced when continuing the reaction for a longer time, indicating that Ir-MoOx/SiO2 does not
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catalyze C−C cracking of alkanes after they form; rather, C−C cleavage of C15 oxygenates occurs
during deoxygenation. The highest yield of total alkanes reaches 78% with 43% of it being
branched C15-A.
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C
C
C
C
C
C
C
C
8
alkane
alkane
9
10 alkane
11 alkane
12 alkane
13 alkane
14 alkane
15 alkane
40
20
0
Oxygenates
0
5
10
Time / h
15
20
25
Figure 1. Reaction profile of C15-CF1 HDO over the Ir-MoOx/SiO2 (0.13) catalyst. Reaction conditions: catalyst (0.15
g), C15-CF1 (0.3 g), cyclohexane (10 mL), initial H2 (5 MPa), and 180 oC.
Two other main alkanes, n-decane (13%) and n-undecane (21%), result from the C−C bond
scission adjacent to the tertiary carbon atom. C−C cleavage leads to a C5 or C4 moiety, which likely
deoxygenates to pentane or butane present in the gas phase. Important intermediates detected
include 2-methyl-5-pentyltetrahydrofuran, 2-hexyl-5-methyltetrahydrofuran, 5-decanol, 2-
undecanol and 5-undecanol (Figure S3). These oxygenates were converted to n-decane and n-
undecane during progression of the reaction.
Table 2. HDO of different oxygenated precursors over Ir-MoOx/SiO2 (0.13) catalyst.
Yield %
Conv
Sum of
Entry
Substrate
.
C-C cracked
alkanes
Desired alkane
fuel-ranged
alkanes
%
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7
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C10 12.4
C11 20.8
O
O
1
2
100
100
78
95
43
C15-HCF1
C9 7.7
C11 0.4
O
O
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86
C14-HCF
O
C10 3.8
C11 0.2
O
O
3
4
100
100
89
96
85
C15-HCF2
O
O
O
O
O
OH
C10 3.6
C11 3.5
HO
HO
88
HO
O
O
O
Hydrogenated C15-HCF1
Reaction conditions: catalyst (0.15 g), substrate (0.3 g), cyclohexane (10 mL), initial H2 (5 MPa), 180 oC, 24 h.
To understand the C−C cleavage pathways, model compounds of high carbon furan with alkyl
chain (C14-HCF) and mono-keto chain (C15-HCF2) and hydrogenated C15-HCF1 with saturated
furan rings were exploited (Table 2). HDO of C14-HCF and C15-HCF2 results in no C−C scission
between the tertiary carbon center and the alkyl or mono-keto moiety (Table 2, Entries 2 and 3).
C−C scission of both compounds occurs between the furan moiety and the tertiary carbon atom
(Table 2, Entries 2 and 3). HDO of hydrogenated furan rings of C15-HCF1 yields C10 and C11
alkanes, but their yields are significantly lower than those from the HDO of C15-HCF1 (Table 2,
Entries 1 and 4). This finding indicates that C−C scission occurs at the carbon center adjacent to
the furan rings and saturated furan rings during the hydrogenation/HDO steps. This finding differs
from prior reports9, 14 that hypothesized that C−C scission only occurs at the tertiary carbon center
adjacent to the furans. One-step HDO of C14-HCF achieves 95% fuel-ranged alkanes (Table 2,
Entry 2). 96% yield of alkanes is obtained by hydrogenation at low temperature over Pd/C followed
by HDO of hydrogenated C15-HCF1 at high temperature over Ir-MoOx/SiO2 (0.13) (Table 2, Entry
4). These yields are comparable to those over Ir-ReOx/SiO2 (2)16 and significantly higher than
those over Ni/HBEA and Ni/HZSM-5 catalysts (~80%) reported in the literature14-15. Importantly,
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o
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our reaction temperature (180 C) is lower than that used (230 C) for Ni-based catalysts,
underscoring the effectiveness of Ir-MoOx/SiO2 for HDO of high carbon furan compounds.
The overall reaction pathway for HDO of C15-HCF1 is depicted in Scheme 2. The furan rings
of C15-HCF1 undergo ring saturation and ring opening in parallel to form a mixture of oxygenates
with three different functional branches including saturated furan rings, carbonyl groups, and
hydroxyl groups mainly during heating. The HDO of these oxygenates results in alkanes. C−C
scission occurs at the carbon atom adjacent to the furan or saturated furan rings.
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Hydrogenation
Hydrogenolysis
O
O
O
O
O
O
O
O
O
O
O
O
O
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
HO
HO
O
O
O
OH
O
O
O
O
O
C-C cracking
OH
O
OH
O
OH
Scheme 2. Proposed reaction pathway for the conversion of C15-HCF1 to fuels.
3.3 Application of Ir-MoOx/SiO2 to the production of lubricant base oils
Next, the Ir-MoOx/SiO2 (0.13) catalyst was used for the HDO of high carbon furan compounds
to synthesize C20-C50 alkanes suitable for lubricant base oils, which have higher value than fuels
(Scheme 1). Under the best reaction conditions used for the conversion of C15-HCF1, HDO of C30-
HCF produces C30-A having structural and property similarity (Scheme 1) to commercial
synthetic base-oils (PAO4) and superior properties than regular mineral base-oils29, 52. A maximum
84% yield of C30-A is obtained over Ir-MoOx/SiO2 (0.13) (Table 3, Entry 1). A small fraction (~12%
yield) of C−C cracking products n-nonane and n-heneicosane is detected. These linear alkanes
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could increase the pour point of the resulting base-oil and lower its specification quality. Reaction
o
at lower temperature (170 C) yields 89% C30-A with less yield (6%) to n-nonane and n-
heneicosane (Table 3, Entry 2). Under comparable conditions, HDO of C20-HCF yields 93% C20-
A (Table 3, Entry 3), which has similar structure as the commercial synthetic PAO2 base oil. These
yields are slightly higher than that obtained (86%) over Ir-ReOx/SiO2 (2)29. Therefore, the Ir-
MoOx/SiO2 (0.13) catalyst is promising for the production of lubricant base oils.
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Table 3. Production of lubricant base oils by HDO of C30-HCF and C20-HCF over the Ir-MoOx/SiO2 (0.13) catalyst.
T
Yield %
Conv.
%
Entry
Substrate
/ oC
Target alkane
C-C cracked alkanes
1
2
3
C30-HCF
C30-HCF
C20-HCF
180
170
170
100
100
100
C30 84
C30 89
C20 93
C21 7.5
C21 4.2
C11 1.2
C9 4.2
C9 2.1
C9 1.5
Reaction conditions: catalyst (0.15 g), substrate (0.3 g), cyclohexane (10 mL), initial H2 (5 MPa), 20 h.
3.4 Catalyst characterization results
We characterized pre-reduced Ir-MoOx/SiO2 by XRD, TEM, CO chemisorption, XPS, in-situ
XAS, and Py-FT-IR techniques to elucidate the relationship between the properties of the catalysts
and their performance. Figure 2 shows XRD patterns of pre-reduced Ir-MoOx/SiO2 (0.05-0.5),
Ir/SiO2, and MoOx/SiO2. The signals assigned to Ir metal are observed in all of the Ir-catalysts
while no peak for MoOx is present, indicating that Ir is in metallic state, and MoOx species are
likely highly dispersed or amorphous. The peak intensity of Ir metal slightly decreases with
increasing Mo/Ir atomic ratio, suggesting a decrease in average particle size of Ir metal likely
caused by the increasing amount of Mo. TEM images (Figure 3) show the average particle size
(Table 4) of Ir metal in all the Ir-MoOx/SiO2 catalysts is smaller (3.4-3.8 nm) than those of Ir/SiO2
(4.0 nm), consistent with the XRD data.
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Ir (200)
Ir (111)
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a
30
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50
60
70
2 Theta / degree
Figure 2. XRD patterns of the catalysts after reduction. a: MoOx/SiO2, b: Ir/SiO2, c: Ir-MoOx/SiO2 (0.13), d: Ir-
MoOx/SiO2 (0.25), e: Ir-MoOx/SiO2 (0.5)
Figure 3. TEM images of the pre-reduced catalysts. a: Ir/SiO2, b: Ir-MoOx/SiO2 (0.13), c: Ir-MoOx/SiO2 (0.25), d: Ir-
MoOx/SiO2 (0.5).
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XPS profiles of pre-reduced Ir/SiO2 and Ir-MoOx/SiO2 (0.13-0.5) in Ir 4f and Mo 3d regions
are shown in Figure 4. Both Ir/SiO2 and Ir-MoOx/SiO2 (0.13-0.5) samples show two peaks around
61 eV and 64 eV, which can be assigned to Ir0 4f7/2 and Ir0 4f5/2, respectively.53 The result suggests
that Ir is present in metallic state, consistent with the XRD analysis. The Mo 3d spectra show a
peak shift of Mo species of pre-reduced Ir-MoOx/SiO2 (0.13-0.5) to a lower binding energy when
compared with those of fresh Ir-MoOx/SiO2 obtained after calcination. The signals are mainly
distributed between Mo0 and Mo6+, indicating partially reduced Mo species.
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In situ XAS measurements at Mo k-edge were conducted to probe the bulk oxidation state of
Mo species upon reduction in H2 and compared with standards MoO3, MoO2, Mo foil and
Na2MoO4 (Figure 5 and Figure S4). Prior to reduction, all catalysts (Ir-MoOx/SiO2 and MoOx/SiO2)
showed the Mo 1s-4d pre-edge peaks at around 19997 eV 54, which are similar to those of MoO3.
o
Upon reducing in H2 at 200 C, the pre-edge peak of Ir-MoOx/SiO2 catalysts disappear and the
edge energy of Mo shifts close to MoO2. This result supports the XPS findings that Mo species are
partially reduced at 200 oC in H2. In contrast, the XANES spectra of MoOx/SiO2 without Ir remains
o
constant in H2 at 200 C, while the reduction of supported MoO3 clusters requires much higher
temperature (>350 oC)55. These results indicate a strong interaction between Ir and MoOx, where
Ir significantly promotes the reduction of MoOx at mildly reducing condition, probably due to the
H2 spillover.
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Mo/Ir=0.5 reduced
Mo/Ir=0.5 fresh
Mo/Ir=0.25 reduced
Mo/Ir=0.25 fresh
Mo/Ir=0.13 reduced
Mo/Ir=0.13 fresh
Mo/Ir=0 reduced
Mo/Ir=0 fresh
72 70 68 66 64 62 60 58
240 236 232 228 224
Binding Energy / eV
Binding Energy / eV
Figure 4. Normalized core-level Ir 4f and Mo 3d XPS spectra of Ir/SiO2 and Ir-MoOx/SiO2 catalysts before reduction
(fresh) and after reduction (reduced).
1.2
1.0
0.8
0.6
MoO2
0.4
MoO3
Ir-MoOx/SiO2 reduced
Ir-MoOx/SiO2 fresh
MoOx/SiO2 reduced
MoOx/SiO2 fresh
0.2
0.0
19980
20000
20020
20040
Energy (eV)
Figure 5. Mo K edge XANES spectra of Ir-MoOx/SiO2 (0.13) and MoOx/SiO2 before reduction (fresh) and after
reduction (reduced) and corresponding standards (MoO2 and MoO3).
In order to determine the surface sites of Ir-MoOx/SiO2, CO chemisorption was conducted for Ir-
MoOx/SiO2 (0.13-0.05), Ir/SiO2, and MoOx/SiO2. The CO adsorption amount and the atomic ratio
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of the surface Ir to the total Ir (CO/Ir) are shown in Table 3. The CO adsorption on the Ir metal
sites of Ir/SiO2 was calculated to be 0.041 mmol·g-1, and the CO/Ir was estimated to be ~19.7%.
The CO adsorption amount on Ir-MoOx/SiO2 (0.13-0.05) gradually decreases with increasing
atomic ratio of Mo/Ir, suggesting less exposed Ir metal atoms, inconsistent with the decreasing
particle size of Ir-MoOx/SiO2 (0.13-0.05). Given that MoOx/SiO2 does not adsorb CO, our
hypothesis is that the Ir metal particle surface of Ir-MoOx/SiO2 is partially covered with MoOx
species as suggested in other oxophilic metal oxides modified with noble metal catalysts, such as
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Ir-ReOx/SiO2 56, Rh-ReOx/SiO2 41, Rh-MoOx/SiO2 38 and Pt-MoOx/C 35.
Table 4. Summary of characterization results.
CO chemisorption
TEM
db / nm
4.0
Entry
Catalyst
CO amount / mmol g-1
CO/Ir ratioa / %
1
2
3
4
5
Ir/SiO2
0.041
0.037
0.022
0.019
0
19.7
17.8
10.5
9.2
-
Ir-MoOx/SiO2 (0.13)
Ir-MoOx/SiO2 (0.25)
Ir-MoOx/SiO2 (0.5)
MoOx/SiO2
3.8
3.4
3.5
-
a: Atomic ratio of the Ir on the surface to the total Ir, assuming the molar ratio of CO to surface Ir is equal to 1. b: The
average particle size of Ir
Transmission FTIR spectroscopy of pyridine adsorbed on the in-situ reduced catalysts (Ir/SiO2,
MoOx/SiO2 and Ir-MoOx/SiO2 (0.13)) was used to evaluate their acidic properties. An IR band at
1450 cm-1 is assigned to pyridine adsorbed on Lewis acid sites (Figure S5), whereas a band at 1540
cm-1 emerges from pyridine interacting with Brønsted acidic sites57. In our earlier work, almost no
adsorbed pyridine was observed on Ir/SiO2 above 100 oC. In contrast, a band at ~1450 cm-1 was
observed on both Ir-MoOx/SiO2 and MoOx/SiO2 at 150 oC, indicating the presence of Lewis acid
sites from the partially reduced MoOx species. A peak corresponding to the Brønsted acid sites is
16
also observed on Ir-MoOx/SiO2, consistent with our prior observation on Ir-ReOx/SiO2 .
3.5 Ir-MoOx synergy revealed using probe-molecule chemistry
To understand the role of MoOx in the Ir-MoOx/SiO2 catalysts, hydrogenation and HDO of
model compounds, such as DMF, DMTHF, 2-hexanone and 2-hexanol, were conducted at 120 oC.
The turnover frequencies (TOFs) and product distribution over Ir/SiO2 and Ir-MoOx/SiO2 (0.13)
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are shown in Table 5. Both saturated furan ring (DMTHF) and ring opening products (2-hexanone
and 2-hexanol) form during DMF conversion, in line with intermediates observed at the beginning
of HDO of C15-HCF1. This is consistent with prior reports16, 58 that furan ring-opening of DMF to
2-hexanone and 2-hexanol and ring-saturation to DMTHF occur in parallel (Scheme 3). The
reaction rate for hydrogenation of DMF over both catalysts is similar, suggesting the
hydrogenation of furan rings mainly occurs over Ir sites (Table 5, Entry 1). This finding is
consistent with the emerging picture is that most metals adsorb furans strongly in a flat
conformation and carry out ring hydrogenation and ring opening. Bell and co-workers proposed
that the interaction between the furan rings of the substrates and the metal surface is crucial in
weakening the C-O bond of the furan rings, resulting in cleavage of the C-O bond.59 DFT
calculations and microkinetic modeling on the hydrogenation of DMF over Ru/C indicated that
DMF absorbs on Ru in a flat configuration, which is potentially a common surface intermediate
that leads to both ring-opening and ring-saturation products58. Although the activation barriers for
the two pathways are comparable, formation of DMTHF is more thermodynamically favorable58.
These are consistent with our finding that the selectivity to ring-saturation-product, DMTHF, is
much higher than that to ring-opening-products, 2-hexanone and 2-hexanol. HDO of DMTHF to
n-hexane over Ir-MoOx/SiO2 is more selectivity than over Ir/SiO2 (Table 5, Entry 2). The reaction
rates for the ring-opening of DMTHF and HDO of 2-hexanol over Ir-MoOx/SiO2 are ~2 and ~6
fold higher than those over Ir/SiO2, respectively (Table 5, Entries 2 and 4). This finding suggests
that MoOx in Ir-MoOx/SiO2 promotes ring opening of DMTHF as well as enhances the C−O
hydrogenolysis of alcoholic groups. This finding could be due to the oxophilicity of MoOx and the
acidic sites of Ir-MoOx/SiO2, which facilitate surface adsorption of the cyclic ether rings of
DMTHF via its electronegative oxygen atoms and dehydration of alcohol (2-hexanol), derived
from ring opening. Prior studies suggest that the acid sites on the partially reduced oxophilic metal
oxides (e.g., ReOx and NbOx) are critical for C−O hydrogenolysis of cyclic ethers and alcohols47,
60. Control experiments using 2-hexanone hydrogenation to 2-hexanol indicate that the rate over
the Ir-MoOx/SiO2 catalyst is ~65 times higher than that over Ir/SiO2 (Table 5, Entry 3). Thus,
MoOx sites enhance C=O hydrogenation over metallic Ir. We attribute this enhanced rate to an
interaction between the Lewis acidic sites on the partially reduced MoOx and the oxygen atom of
the carbonyl group. Such interactions between partially reduced metals of NbOx and ReOx and the
carbonyl group of unsaturated aldehydes have been demonstrated by inꢀsitu FTIR studies43-44. The
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adjacent Ir metal dissociates hydrogen and spills this over for hydrogenation of ketone to an
alcohol. Given the superior C−O hydrogenolysis and C=O hydrogenation activities of Ir-
MoOx/SiO2, it is rational to apply this catalyst for the HDO of fatty acids, fatty esters and lipids to
produce diesel-ranged alkanes (see examples for stearic acid, methyl stearate, palm oil, and
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soybean oil in Figure S6). Nearly quantitative yields of C9-C18 alkanes can be obtained.
Table 5. Conversion of different model compounds over Ir-MoOx/SiO2 (0.13) catalyst.
Conv
Selectivity / %
TOF1
Entry Substrate
Catalyst
.
O
O
OH
/ h-1
/%
Ir/SiO2
9.4
8.4
1.5
3.0
0.4
26
48.7
38.7
12.5
28.0
24.7
2.5
100
97.3
-
0.2
2.0
75.3
97.5
0
13796
13401
18
O
1
Ir-
58.8
11.2
DMF
MoOx/SiO2
Ir/SiO2
-
-
-
-
-
-
-
-
-
-
-
-
O
2
Ir-
41
DMTHF
MoOx/SiO2
Ir/SiO2
1209
79428
37
O
3
Ir-
2.7
100
100
2-Hexanone
MoOx/SiO2
Ir/SiO2
0.6
8.8
OH
4
Ir-
-
227
2-Hexanol
MoOx/SiO2
Reaction condition: catalyst (0.01-0.1g), substrate (1 g), cyclohexane (10 ml), initial N2 (0.3 MPa at R. T.), N2+H2
(6MPa at 120 oC), 120 oC, 5 min-2 h. 1: TOF were calculated based on the exposed Ir metal.
O
Ring
-openin
enation
x/SiO2
Ir-MoO
x/SiO
g
Hydrog
Ir-MoO
in
2
Ir
HDO
Ir-MoOx/SiO2
+H2
+2H2
OH
O
DMTHF
+
H2O
+ H2
+2H
+H2
2
n-Hexane
2-Hexanol
DMF
Ring
in
O
-openin
Ir-MoO
x/SiO
Ir
enation
/SiO2
g
Hydrog
Ir-MoOx
2
2-Hexanone
Scheme 3. Proposed reaction pathway for the conversion of DMF to n-hexane over the Ir-MoOx/SiO2 catalyst.
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3.6 Catalyst reusability
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Catalyst reusability is key for potential use of a catalytic process. We found the Ir-MoOx/SiO2
(0.13) catalyst is stable for the HDO of C30-HCF; only 2% loss in the yield of C30-A is observed
after five consecutive runs (Figure 6). ICP-MS, XRD and TEM characterizations for both the fresh
and used catalysts elucidate a slight deactivation of the catalyst. ICP-MS analysis of the reaction
solution, after filtering off the catalyst, shows a negligible amount of Ir (0.0045 wt%, based on
total Ir amount) and very slight leaching of Mo (0.2062 wt%, based on the total Mo amount).The
XRD patterns of Ir-MoOx/SiO2 (0.13) after the 5th cycle show that the peaks assigned to Ir metal
became sharper (Figure S7). The TEM images (Figure S8) show the particle size after the 5th cycle
is 5.4 nm, which is larger than that of the fresh catalyst (3.8 nm), possibly due to sintering during
reaction and/or recalcination. Therefore, 2% yield loss of C30-A in the 5th cycle could be due to the
combined effect of small amounts of metal leaching, especially Mo, and sintering of the catalyst.
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Oxygenates
C30-A
0
1
2
3
4
5
Cycle
Figure 6. Recyclability the Ir-MoOx/SiO2 (0.13) catalyst. Reaction conditions: catalyst (0.15 g), C30-CF1 (0.3 g),
cyclohexane (10 mL), initial H2 (5 MPa), 170 oC, 20 h.
4. Conclusions
We have synthesized Ir-MOx/SiO2 (M=Re, Mo, W, V, or Nb) and M’-MoOx/SiO2 (M=Rh, Ru,
Pt, or Pd) catalysts and screened them for the production of renewable fuel and lubricant base oils
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from high carbon furan substrates under mild reaction conditions. MoOx, WOx, and VOx oxophilic
metal oxides improve the HDO activity of Ir/SiO2, while NbOx is ineffective. Among catalysts,
MoOx modified Ir/SiO2 (Ir-MoOx/SiO2; Mo/Ir=0.13) is the most effective, and is superior to Ir-
ReOx/SiO2 reported previously at low metal loading amount. High yields (78-96%) of fuels (C8-
C18) and lubricant base oils (C20-C50) are achieved. Control experiments using HCFs with different
functional groups suggest that the reaction progresses via hydrogenation and parallel ring opening
of saturated and unsaturated furan rings to mono-keto and mono-hydroxy chains. These
intermediates then undergo hydrogenolysis in a cascade sequence to alkanes. Modest C−C bond
scission at the carbon atom adjacent to a furan or a saturated furan ring results in lower carbon
alkanes. Detailed characterization of the Ir-MoOx/SiO2 catalyst suggests that MoOx sites partially
cover the Ir metallic particles and that the MoOx sites facilitate ring opening, C−O hydrogenolysis
of cyclic ethers and alcohols, and C=O hydrogenation. These results highlight the potential of low-
cost oxophilic metal oxides modified noble metals and specifically of Ir-MoOx/SiO2 to enable cost-
competitive production of bioproducts.
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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 ICP-MS, and C. Wang (U. Penn) for measuring CO chemisorption of the catalysts. We also
acknowledge instrumentation support for XPS (NSF award number 142149). The XAS work was
performed at the DuPont-Northwestern-Dow Collaborative Access Team (DNDCAT) located at
Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern
University, E. I. DuPont de Nemours & Co., and the Dow Chemical Company. This research used
resources of the APS, a U.S. Department of Energy (DOE) Office of Science User Facility operated
for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-
06CH11357.
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Table of Contents
R2
R
O
O
Ir-MoOx/SiO2
HDO
R1
R1
R1
R1
High carbon furans
from biomass
Jet/diesel fuels
& lubricants
27
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