One-pot hydrodeoxygenation (HDO) of lignin monomers to C9 hydrocarbons co-catalysed by Ru/C and Nb2O5

Article abstract of DOI:10.1039/d0gc01692f

A physical mixture of Ru/C and Nb2O5 is an effective catalyst for upgrading lignin monomers under low H2 pressure at 250 °C to a clean cut of hydrocarbon liquid fuels. The reaction solvent is water with a small amount of methanol additive. Hydrodeoxygenation (HDO) was evaluated using dihydroeugenol (DHE) as an exemplary lignin monomer model. Under optimized conditions, 100% conversion of DHE and very high selectivity to propyl cyclohexane (C9 hydrocarbon) was achieved. Nb2O5 was prepared at a low temperature (450 °C) and was shown to contain acid sites that enhance the production of fully deoxygenated products. The methanol additive serves as a hydrogen source for the Ru/C catalysed reduction of the aromatic ring. In addition, when a substrate mixture of DHE, isoeugenol and 4-allylsyringol simulating lignin products was employed, 100% conversion to propyl cyclohexane (76%) and propyl benzene (24%) was observed, thereby suggesting the general applicability of this catalyst system for funneling lignin monomers into a clean cut of hydrocarbon liquid fuels. This study sheds light on the function of each catalyst component and provides a simple and green utilization of biomass monomers as a feedstock for renewable hydrocarbon fuels. This journal is

Full text of DOI:10.1039/d0gc01692f

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One-pot hydrodeoxygenation (HDO) of lignin
monomers to C9 hydrocarbons co-catalysed
Cite this: DOI: 10.1039/d0gc01692f
by Ru/C and Nb2O5†
a
Simin Li,a,b Baoyuan Liu,a Julianne Truong,a Zhongyang Luo, *b Peter C. Ford
*
a
and Mahdi M. Abu-Omar
*
A physical mixture of Ru/C and Nb2O5 is an effective catalyst for upgrading lignin monomers under low
H2 pressure at 250 °C to a clean cut of hydrocarbon liquid fuels. The reaction solvent is water with a small
amount of methanol additive. Hydrodeoxygenation (HDO) was evaluated using dihydroeugenol (DHE) as
an exemplary lignin monomer model. Under optimized conditions, 100% conversion of DHE and very
high selectivity to propyl cyclohexane (C9 hydrocarbon) was achieved. Nb2O5 was prepared at a low
temperature (450 °C) and was shown to contain acid sites that enhance the production of fully deoxyge-
nated products. The methanol additive serves as a hydrogen source for the Ru/C catalysed reduction of
the aromatic ring. In addition, when a substrate mixture of DHE, isoeugenol and 4-allylsyringol simulating
lignin products was employed, 100% conversion to propyl cyclohexane (76%) and propyl benzene (24%)
was observed, thereby suggesting the general applicability of this catalyst system for funneling lignin
monomers into a clean cut of hydrocarbon liquid fuels. This study sheds light on the function of each
catalyst component and provides a simple and green utilization of biomass monomers as a feedstock for
renewable hydrocarbon fuels.
Received 19th May 2020,
Accepted 13th September 2020
DOI: 10.1039/d0gc01692f
rsc.li/greenchem
Lignin can be converted to bio-oil as a renewable liquid
fuel.11 However, bio-oils cannot be used in conventional gaso-
Introduction
Non-renewable fossil carbon has been the main resource to line and diesel fuel engines due to the high oxygen content
produce most chemicals and fuels over the past century; [up to 60 weight % (wt%)] that makes them immiscible with
however, renewable alternatives are available. Lignocellulose is petroleum-derived fuels.12,13 Thus, oxygen removal while
by far the most abundant renewable source of non-food-based maintaining the carbon structures has the potential to
carbon, and there is considerable interest in upgrading such enhance the utilization of bio-oil as a transportation fuel.
biomass as a sustainable feedstock.1–5 The lignin component
Catalytic hydrodeoxygenation (HDO) can upgrade bio-oils.
of lignocellulose is the second most abundant biopolymer in In most studies, HDO involves treatment at high temperatures
nature (after cellulose), has a high carbon content, and is (250–500 °C) with a high pressure of H2 (50–100 bar). The high
(potentially) the largest renewable resource of aromatic chemi- pressures required may limit the scalability of the reaction and
cals and fuels.5–9 However, it has an irregular structure with is a safety hazard.14 These obstacles encourage research
random linkages between monomeric components having towards alternative approaches for phenolic upgradation
different levels of oxygenation that is challenging to utilize under milder conditions.
chemically.10 Due to this dilemma, pulp and paper industries
and biorefineries typically dispose of lignin by burning it. egies that efficiently funnel lignin disassembly components
The goal of the present study is to develop catalytic strat-
Thus, lignin is an attractive target for biomass valorisation.
into a narrowly defined product stream useful as a feedstock
for the synthesis of aromatic chemicals (C6–C9 aromatic
hydrocarbons) or as drop-in biofuels. Phenolics would be
desirable for the former processes, while oxygen-free hydro-
carbons that can replace fossil-carbon liquid fuels would serve
the latter purpose.15–17 In previous studies, various types of
heterogeneous catalysts,17–20 transition metal compounds
(phosphides,21–26 carbides,24,25 and nitrides25,27), but mainly
noble metals (Pt, Pd, Rh, and Ru),14,15 have been used, but
aDepartment of Chemistry & Biochemistry, University of California, Santa Barbara,
Building 232, Santa Barbara, California, 93106-9510, USA.
E-mail: mabuomar@ucsb.edu
bState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou,
310027, PR China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc01692f
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these require high temperature and high pressure conditions. co-reactant has a significant influence on the product distri-
For example, Lu et al. showed that Pd/TiO2 catalyses the HDO/ bution and provides evidence-based mechanistic insight.
hydrogenation of guaiacol to cyclohexane, but this process Furthermore, this system can be tuned to give high selectivity
required 30 bar H2 at 260 °C.28 Kim et al. reported that ruthe- towards hydrocarbon products that can be employed as drop-
nium on carbon (Ru/C) converted guaiacol to cyclohexanol in fuels.
with 60% selectivity using the H-donor, 2-propanol without
adding H2 at 200 °C for 5 h, but there was only partial HDO.29
The use of niobium oxide has primarily been studied to
show its specific role involved in HDO. As shown in the studies
of Wang et al., Nb2O5 was primarily used as a catalyst support
Experimental
Regents and feedstocks
All commercial chemicals were purchased and used as
for ruthenium to form indane with and without the addition
received. 2-Methoxy-4-propylphenol (≥99%), isoeugenol (98%),
2,2-biphenol (99%), and niobium(V) oxide (325 mesh, 99.9%)
were purchased from Sigma-Aldrich. Dichloromethane (ACS
reagent grade), methanol (ACS reagent grade), ethanol (200
Proof), and ethyl acetate (ACS reagent grade) were purchased
from Fisher Chemical. Cetyltrimethylammonium bromide
(CTAB, 98%) and n-dodecane (99%) were purchased from Alfa
Aesar. Deuterium oxide (D, 99.9%) and methanol-d4 (D,
99.8%) were purchased from Cambridge Isotope Laboratories
Inc. Propylbenzene (98%) was purchased from Frontier
Scientific. para-Cresol (cresylic acid) was purchased from
Hercules Powder Company. Hydrochloric acid (GR ACS) was
purchased from EMD Millipore Corporation. Niobium(V) chlor-
ide (≥99%) was purchased from Strem Chemicals. Hydrogen
gas (5.0 grade) and nitrogen (99.998%) were purchased from
Praxair. Water used for reaction and sample preparation was
obtained from an A10 Milli-Q water purification system by
Millipore.
of CH2Cl2 as its primary solvent. The main function Nb2O5
partook in was catalysing the intramolecular cyclization and
hydrogenation of lignin oil.7 Jiang et al. utilized the Ni/Nb2O5
catalyst to produce value-added alcohols from lignin-derived
phenols which exhibited selective HDO to give a total alcohol
yield of 74%, but requires higher pressure (25 bar H2).8
Puurunen et al. used Pt/Nb2O5 to perform HDO on the lignin
monomer 4-propylphenol under harsh temperature and
pressure (350 °C, 20 bar H2) conditions to give propylbenzene
with a selectivity of 77%.9 Rinaldi et al. studied Ni/Nb2O5 by
tuning the acidic and hydrogenating properties of the catalyst
to convert lignin to hydrocarbons in 91% yield under 15%
catalyst loading and harsh conditions (200 °C, 40 bar H2).30
Yang et al. reported Nb2O5 as a catalyst support for Pd and Pt
wherein they concluded HDO conversion of lignin to C7–C9
products in 42 and 64% yields, respectively.18
Described here is the hydroprocessing of dihydroeugenol
(DHE) in an aqueous medium using a physical mixture of two
catalysts acting synergistically, one is Ru/C and the other is
niobium pentoxide (Nb2O5). DHE was used as the primary
model compound for testing the present catalytic system,
since it includes the methoxy, hydroxy, and propyl groups,
characteristic of the lignin monomers present in bio-oils
(Fig. 1). Nb2O5 is an air stable, water insoluble white solid that
exhibits both strong Lewis and Brønsted acid sites.31 It has
proved to be an effective catalyst for hydration/dehydration,
cracking, condensation, isomerization, and alkylation32–35 as
well as for HDO.32–34,36
Catalyst preparation
Ru/C was obtained from Sigma-Aldrich with 5 wt% Ru loading
and used as received. Nb2O5 was synthesized using a hydro-
thermal method according to a modified literature pro-
cedure.35 Typically, a 20 mmol portion of the precursor NbCl5
was dissolved in 20 mL ethanol with rigorous stirring for
10 min, and then the solution was added to a water solution of
CTAB (1 g in 15 mL distilled water) dropwise. The mixed solu-
tion was then stirred for 0.5 h, followed by adding 20 mL of
aqueous HCl (pH 1) that was previously prepared by dissolving
a specific amount of hydrochloric acid in water and stirring for
another 1.5 h. The resulting solution was then placed into a
Teflon-lined autoclave and aged at 160 °C for 24 h.
Subsequently, the solid was separated and washed with dis-
tilled water and dried at 60 °C overnight. After this, the sample
was ground and packed for calcination in air. A Thermolyne
F6020 1200 C Muffle furnace was used to calcine the niobia
The dual catalyst system is effective in hydroprocessing
DHE and several other lignin monomers under relatively mild
conditions to produce hydrocarbons in high yields. We find
that the addition of small quantities of methanol (MeOH) as a
sample. Ramping rate of the furnace was pre-set to 1 °C min−1
.
After 6 h of calcination at 450 °C, the active Nb2O5 catalyst was
collected at room temperature.
Catalyst characterization
NH3-temperature programmed desorption (TPD). To evalu-
ate the acid sites on Nb2O5, NH3-TPD was performed on a
Micromeritics AutoChem 2920 instrument. A 200 mg sample
of Nb2O5 was placed into a U-shaped quartz tube. This
Fig. 1 Structures of lignin monomer compounds: (A) 4-allyl-2,6-
dimethoxyphenol (4-allysyringol), (B) isoeugenol, and (C) 2-methoxy-4-
propylphenol (dihydroeugenol, DHE).
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material was first pretreated by heating under flowing helium heated to 250 °C and held for 16 h with magnetic stirring at
(25 cm3 min−1) at 300 °C for 0.5 h. A mixture of NH3 in He 700 rpm. After the reaction, products in the liquid phase were
(1 : 9 v/v) was then passed through the tube at a flow rate of extracted using ethyl acetate. The catalyst was washed using
15 cm3 min−1 at 25 °C for 1 h. After this, the sample was ethanol and collected by centrifugation, and then dried in a
flushed with He (25 cm3 min−1) at 100 °C for another hour. vacuum chamber for 24 h at room temperature. Prior to the
TPD measurements were carried out over the temperature next recycling run, the catalyst mixture was heated in an oven
range 100–500 °C at a ramp rate of 10 °C min−1 and the at 120 °C for 1 h. The following runs were performed with the
ammonia concentration in the effluent was monitored using a same portion of this catalyst mixture collected from the pre-
filament thermal conductivity detector (TCD). The amount of vious run. Turnover number (TON) of each catalyst was calcu-
desorbed ammonia was determined based on the integrated lated based on the total amount of C9 hydrocarbons, i.e. pro-
peak area.
pylbenzene and propyl cyclohexane, produced after the fifth
X-ray diffraction (XRD). The phase structure of Nb2O5 was run to show the productivity of each catalyst. The TON was
analyzed by powder X-ray diffraction in the diffraction angle 2θ defined and calculated as follows:
between 10° and 80° on a PANalytical X’Pert Pro X-ray diffract-
total moles of C9 hydrocarbons by 5 runs
ometer with Cu Kα1 radiation (45 kV and 40 mA, k = 1.5406 Å).
TON ¼
moles of catalyst
Scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM). The particle size and micro mor-
GC-MS analysis. A Hewlett-Packard 5890A gas chromato-
phology of Nb2O5 were characterized by scanning electron graph (GC) coupled to a Hewlett-Packard 5970B Mass Selective
microscopy (Hitachi SU-8010) at an acceleration voltage of 15 Detector (MSD) was used to identify the products qualitatively.
kV. The pore structure of Nb2O5 was examined using high- A J&W DB-5 capillary column (30 m × 0.250 mm I.D. × 0.25 μm
resolution transmission electron microscopy (Tecnai G2 F20 film thickness) was installed for analyte separation. Prior to
S-TWIN) with an acceleration voltage of 200 kV.
the injection, the liquid sample was dissolved in ethyl acetate
X-ray photoelectron spectroscopy (XPS). The niobium oxi- and filtered through a 0.2 micron PTFE syringe filter. The GC
dation state in the synthesized Nb2O5 catalyst was analyzed by injector inlet was set to 280 °C. The oven temperature was held
X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha+, at 50 °C for 2 min. Then the oven was heated to 300 °C at a
USA) with a monochromatic radiation source Al Kα (12 kV, rate of 20 °C per min and held for 10 min. The MSD had a
6 mA, 72 W). Wide scans were performed with 100 eV pass dedicated electron ionization (EI) source and a quadrupole
energy and 1 eV energy step, and high-resolution scans were mass analyzer. The mass range of detection was 40 to 550 m/z
performed with 30 eV pass energy and 0.1 eV step size. The C at a rate of 1.6 scans per second.
1s signal of adventitious carbon (284.8 eV) was used for energy
calibration.
GC-FID analysis. An Agilent 6890N gas chromatograph
equipped with a flame ionization detector (FID) was used to
quantify the reaction mixtures. A J&W DB-5 GC column (30 m
× 0.250 mm I.D. × 0.25 μm film thickness) was selectively used
Catalytic reaction and product analysis
Reactions in a Parr reactor. Batch reactions were carried out for separation. The liquid product sample was first passed
in a stainless steel 75 mL 6-series pressure reactor (Parr through a 0.2 micron PTFE syringe filter to remove solid par-
Instrument Company, 5000 series). The reactor vessel was ticles, and then diluted to 25 mL in a volumetric flask. A
equipped with a magnetic stirring system. For a typical reac- 10 mM n-dodecane solution was pre-made as the internal stan-
tion, 0.1 g Ru/C and 0.2 g Nb2O5 were physically mixed in the dard for GC quantification. The sample solution was mixed
vessel with 12 mL distilled water as the solvent. To this were with internal standard (1 : 1 v/v) in a 2 mL Agilent GC vial. The
added the substrate (0.2 mL) and MeOH (0.8 mL). The reactor sample was injected using an autosampler. The inlet tempera-
was then sealed and purged with H2 three times. Then, the ture was kept at 280 °C, while the detector temperature was
reactor was filled with H2 (6 bar). The reactor was heated to 310 °C. The initial temperature of the oven was 40 °C and held
250 °C and held at that temperature for a defined time (typi- for 7 min. Then the oven was heated to 250 °C at a ramp rate
cally for 12 h). The stirring rate was kept at 700 rpm during the of 10 °C min−1 and kept at the final temperature for 5 min.
whole reaction period. Subsequently, the reactor was cooled to The split mode was used with a split ratio of 10 : 1. Helium
room temperature. The products in the liquid phase were was used as a carrier gas at a flow rate of 14 mL min−1. The
extracted using ethyl acetate and the gas phase products were instrument was calibrated using the known samples of the
collected in a sealed gasbag for further analysis.
products. The analytes were then identified according to their
Catalyst recyclability test. Recycling experiments were per- retention time. The quantification of each analyte was
formed in five successive runs with 1 mL DHE loading in each acquired from a calibration curve which represented the
run. A physical mixture of fresh Ru/C (0.1 g) and Nb2O5 (0.2 g) relationship between concentration versus the ratio of peak
was employed in the first run. MeOH (1 mL) was then added area over internal standard.
with 12 mL distilled water as the solvent to the reaction
GC-TCD analysis. Gas phase products were analyzed by
mixture in a 75 mL reactor vessel. The reactor was sealed and GC-TCD. An Agilent 6890N (G1530N) gas chromatograph
purged with H2 three times. Then, the reactor was filled with equipped with a thermal conductivity detector (TCD) and a
11 bar H2 at room temperature. After this, the reactor was 30 m × 0.53 mm Fused Silica Carboxen 1010 capillary column
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was used. The detector was set to 250 °C with H2 flow at 7 mL benzene (2) made up 64% of the product mixture. The rest was
min−1 and air flow at 8 mL min−1. The gas phase products mostly the partially deoxygenated product 4-propylcyclohexa-
were collected in a RESTEK polypropylene combo valve gas nol (3). Thus, this catalyst mixture is a promising HDO system.
sampling bag. For each measurement, the 50 μL gas sample The studies described here were designed to examine the
was manually injected using a gastight syringe into the GC effects of key variables such as MeOH concentration, H2
inlet at 245 °C. The carrier gas, He, was set to 7 mL min−1. The pressure, and catalyst loading in order to define those features
column was pre-heated to 35 °C and held for 5 min. Then the that may give the optimum selectivity toward the desired
temperature ramped to 245 °C at a rate of 10 °C min−1 and product streams.
held for 10 min. Each gas analyte was identified by its reten-
tion time.
Table 1 illustrates the remarkable sensitivity of this system
to the amount of methanol added as well as the cooperative
NMR analysis. 1H NMR was obtained using a Varian Unity requirement for both MeOH and H2 to obtain the desired
Inova 600 MHz spectrometer. The analyte was extracted using HDO products (1) and (2). For example, the reaction with no
2
700 μL CDCl3 and packed in a glass NMR tube for analysis. H MeOH but with P(H2) = 6 bar (RT) (entry 1, Table 1) gave sub-
NMR was performed using an Agilent 400-MR DDR2 400 MHz stantial conversion of DHE, but only ∼7% of the fully deoxyge-
spectrometer. CHCl3 with 10% CDCl3 internal standard was nated hydrocarbons. In the absence of both H2 and MeOH, no
used as the solvent for 2H NMR analysis.
conversion was observed, and no products were detected (entry
5, Table 1).
Entries 2–4 in Table 1 show the comparison of the effect of
changing the amount of MeOH added while holding P(H2)
constant at 1 bar (RT). Under these conditions, the optimum
amount of MeOH proved to be 0.8 mL. Surprisingly, increasing
the amount of MeOH to 4 mL, only about one-third the quan-
tity of the aqueous co-solvent, suppressed both the conversion
of DHE and the relative amount of HDO products.
Table 2 summarizes the product distributions found for
DHE reactions for various P(H2) (RT) under typical conditions
with 0.8 mL MeOH. In the absence of any externally added H2
(entry 1, Table 2), there was about 64% conversion of DHE, but
the only products were phenol and cyclohexanol derivatives (4)
and (3). The amount of conversion increases upon increasing
H2 and is 100% when P(H2) is 6 bar or greater (entries 4 and 5,
Table 2). More importantly, the yield of propyl cyclohexane (1)
is 100%. Thus, the latter result requires the addition of both
MeOH and H2 and the distribution of HDO products is a func-
tion of P(H2) with high selectivity occurring at a relatively low
P(H2).
Results
Optimization of dihydroeugenol (DHE) hydrodeoxygenation
A typical HDO run involved heating a mixture of the model
substrate DHE (0.20 mL) with the catalysts Ru/C (100 mg) and
Nb2O5 (200 mg) added separately, water (12 mL) and a small
amount of MeOH (0.8 mL) in a closed Parr® high pressure
reactor that had been flushed with H2 (P(H2) = 1 atm at room
temperature, RT) and then sealed. After a 12 h reaction at
250 °C, the conversion of DHE was 95% (entry 3, Table 1), and
of the five potential products shown in Fig. 2, the fully deoxy-
genated hydrocarbons propyl cyclohexane (1) and propyl
Table 1 Performance comparison with different amounts of methanol
Product distribution (%)
Entry
MeOH (mL)
Conv. (%)
1
2
3
4
5
Table 3 summarizes experiments with different catalyst mix-
tures. Interestingly, when the reaction was examined with the
addition of Nb2O5 alone, the material prepared in this labora-
tory proved to be much more active than that purchased from
1a
2b
3b
4b
5c
0
82
80
95
36
0
3.3
3.6
42
8
3.5
9.5
22
10
—
80
42
34
—
—
13
45
2
24
—
—
—
—
58
—
0.4
0.8
4
0
—
Common conditions for each reaction: DHE 0.2 mL, Ru/C 100 mg,
Nb2O5 200 mg, H2O 12 mL, 250 °C, 12 h. Unless noted otherwise, the
reactor was first flushed with H2. a Initial P(H2) at RT = 6 bar (5 bar,
gauge). b Initial P(H2) at RT = 1 bar. c Purged with nitrogen, no added
H2.
Table 2 Performance with different hydrogen pressures
Product distribution (%)
Entry
P(H2) in bara
Conv. (%)
1
2
3
4
5
1b
2c
3
4
5
0
1
2
6
64
95
98
100
100
—
42
53
100
100
—
22
17
—
—
31
34
27
—
—
69
2
3
—
—
—
—
—
—
—
11
Common conditions for each reaction: DHE 0.2 mL, Ru/C 100 mg,
Nb2O5 200 mg, H2O 12 mL, MeOH 0.8 mL, 250 °C, 12 h. Unless noted
otherwise, the reactor was first flushed with H2. a Initial P(H2) at RT.
b Reactor was purged with N2 only, P(N2) = 1 atm at RT. c The same
experiment as entry 3 in Table 1.
Fig. 2 DHE conversion into five main products: propyl cyclohexane (1),
propyl benzene (2), 4-propylcyclohexan-1-ol (3), 4-propylpheol (4), and
4-propylcatechol (5).
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Table 3 Performance with different catalyst mixtures
Product distribution (%)
Entry
Ru/C (mg)
Nb2O5 (mg)
Conv. (%)
1
2
3
4
5
Pfinal (bar)
1a
2b
3
0
0
100
100
100
200
200
0
0
200
80
—
96
100
100
—
—
7.7
72
100
—
—
8.2
—
—
—
—
82
28
—
4
96
—
—
—
—
6
6
14.5
11.6
11
—
2.1
—
—
4c
5a,d
Reaction conditions: DHE 0.2 mL, H2O 12 mL, methanol 0.8 mL, P(H2) = 6 bar, 250 °C, 12 h. a Nb2O5 was custom prepared in this laboratory as
described in the Experimental section. b Commercial Nb2O5 calcined at 1000 °C. c Ru/C was pre-treated in 12 mL H2O under 10 bar H2 at 200 °C
for 2 h. d The same reaction as entry 4 in Table 2.
a commercial source. The XRD pattern, XPS analysis, SEM and
The observation that the Ru/C catalyst is more active after
TEM images of the synthesized Nb2O5 showed it to be an pre-treatment suggested that the early stages of the reactions
amorphous, mesoporous catalyst with niobium in the +5 oxi- with the physically mixed catalyst would show an induction
dation state (ESI Fig. S1 and S2†). The principal reaction in the period while Ru/C was being activated. This suggestion led to
former case was hydrolysis of the DHE methoxy group to give the experiments reported in Fig. 3 and ESI Table S1† to
catechol (5) (entry 1, Table 3), while little activity was seen with examine the products formed by different catalyst combi-
the commercial Nb2O5 (entry 2, Table 3). One possible expla- nations over an initial period of 2 h. The take-home lesson
nation may lie in the manner in which the two Nb2O5 samples from Fig. 3 is that, on this time scale, Ru/C alone displays
were processed. The commercial sample had been calcined at modest activity (14% conversion), but the only product
1000 °C, while the Nb2O5 sample prepared in our laboratories observed was phenol (4), the result of HDO removal of the
and used for catalysis was calcined at 450 °C. Accordingly, we DHE methoxy group. With Nb2O5 alone, conversion was con-
speculated that the higher temperature calcination may have siderably higher, but the primary product was catechol (5). The
diminished the number of acid sites on Nb2O5. This idea was result with both catalysts illustrates the synergy of this
tested by TPD studies of Nb2O5 samples that had been first mixture. Although the conversion was about the same as seen
dried and then exposed to ammonia. The amount of NH3 with Nb2O5 alone, products (1), (2) and (3) were also observed.
absorbed by the Nb2O5 sample calcined at 450 °C was Scheme 1 suggests a possible sequence of these transform-
28 mmol per gram of Nb2O5, while an analogous sample pre- ations, wherein the principal role of Nb2O5 is to catalyse the
pared in this laboratory but calcined at 600 °C only absorbed hydrolysis of the DHE methoxy group to form catechol (5),
and released about 2 mmol NH3 per gram of the sample (ESI although, surprisingly, a small amount of phenol (4) was still
Fig. S3†). Furthermore, the commercial sample absorbed and formed with Nb2O5 alone.
desorbed essentially no NH3. Thus, it is clear that higher
Fig. 4 illustrates the behaviour of the mixed catalyst system
temperature calcination strongly diminishes the number of as a function of reaction time. These data confirm the
acid sites on Nb2O5.
When Ru/C alone was used as a catalyst without pretreat-
ment, the major product was propylcyclohexanol (3), although
about only 8% each of the hydrocarbons (1) and (2) were
formed (entry 3, Table 3). When Ru/C was first pre-treated by
heating under hydrogen to 200 °C, the observed reactivity
changed. The major product was propylcyclohexane (1) (72%)
with cyclohexanol (3) being the remainder (28%) (entry 4,
Table 3). Under analogous conditions, the system to which
both Ru/C (not activated with H2) and Nb2O5 had been added
gave 100% conversion to the propylcyclohexane product exclu-
sively (entry 5, Table 3). Notably, when either form of Ru/C was
present, the final pressure in the Parr reactor (after cooling to
RT) was significantly higher than the initial P(H2). This is
apparently due to the catalytic reforming of MeOH.
Thus, H2-pretreated Ru/C can catalyse the HDO/hydrogen-
ation of DHE, although the reaction is more efficient when
Nb2O5 is present. On the other hand, low T calcined Nb2O5
Fig. 3 Graphic showing conversion (vertical axis) and product distri-
bution after a 2 h reaction for a mixture of DHE (0.2 mL), H2O (12 mL),
alone only catalysed the conversion of DHE to catechol (5), pre-
sumably the product of hydrolysis of the methoxy group.
MeOH (0.8 mL) under H2 (6 bar) with Ru/C (100 mg), Nb2O5 (200 mg) or
both at 250 °C.
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Scheme 1 Possible sequence of reactions leading from DHE to (1).
production of (1), (2) and (3) with cyclohexanol (3) and phenol
(4) now being the major products (entry 4, Table 4). The
pattern was again different when the solvent mixture was D2O
with perprotio MeOH (entry 5, Table 4). The conversion was
greater (81%) than when methanol-d4 was used (54%), and
measurable amounts of (1), (2) and (3) were observed, but the
primary product was (4). Thus, isotope effects are observed in
the product distributions from the deuteration of each solvent.
The DHE reaction with the two catalysts was also run in the
D2O/methanol-d4 solvent with a higher P(H2) of 6 bar (entry 7,
Table 4). As observed above (entry 6, Table 4), increasing the
hydrogen pressure substantially accelerated the reaction and
improved the selectivity toward (1) to 94% in the deuterated
medium.
1
Fig. 5 displays the aromatic regions of the H NMR spectra
of products isolated after reactions in H2O/MeOH (entry 1,
Table 4), D2O/methanol-d4 (entry 4, Table 4) and D2O/MeOH
(entry 5, Table 4). ESI Fig. S4† displays the aliphatic regions of
Fig. 4 Graphic showing conversion (vertical axis) and product distri-
bution after 2 h, 4 h, 8 h, and 12 h reaction for a mixture of DHE
(0.2 mL), H2O (12 mL), MeOH (0.8 mL) under H2 (6 bar) with Ru/C
(100 mg) and Nb2O5 (200 mg) at 250 °C.
these spectra.
A particularly meaningful comparison is
between the spectra of entries 1 and 4 in Table 4 since the
product distributions for these are very similar to (4) being the
primary aromatic product in each case. A simple inspection of
these spectra shows that considerable exchange of the aro-
matic protons with the solvents accompanies the transform-
ation of DHE into phenolic product (4). Their similar patterns
in the aliphatic region indicate that the deuterated reactants
and solvents have little influence on the exchange of alkyl
suspected induction period, during which the Ru/C catalyst is
activated. After 4 h, all DHE had been converted to products
with 2/3 of the total being hydrocarbons (1) and (2). By 8 h, all
the products were (1) and (2) with propyl cyclohexane (1) repre-
senting an impressive 94%, while after 12 h only a single
product, (1), was observed (ESI Table S1†). Formation of (1)
requires HDO of both oxygen-containing functional groups of
DHE and the hydrogenation of the aromatic ring. Notably, cyclo-
hexanol (3) is observed as an intermediate that presumably
undergoes HDO, but the direct product of DHE ring hydrogen-
ation, 4-propyl-2-methoxy-cyclohexanol, was not observed.
2
protons. The H NMR spectrum of the product mixture from
entry 4 in Table 4 confirms the introduction of both aromatic
and aliphatic hydrogens from the deuterated solvent (ESI
Fig. S5†).
The same conclusion can be drawn from the mass spec-
trum (obtained by GC-MS analysis) of product (2) formed by a
reaction in D2O/methanol-d4 (entry 4, Table 4). The parent MS
Reactions of DHE with deuterated reactants
Table 4 summarizes the conversion and products from the peak for propylbenzene should appear at mass 120, but the
analogous reactions of DHE in deuterated solvents. Notably, major peak in this region of the MS spectrum (SI Fig. S6†)
when both D2O and methanol-d4 were used instead of the per- appeared at M/e 123, again indicating that H/D exchange of
protio analogues, there was a significant suppression both of the aromatic ring occurred prior to ring hydrogenation.
the conversion and of the formation of the more reduced pro- Scheme 2 shows a plausible pathway for such exchange via tau-
ducts (1), (2) and (3) with only (4) and (5) being found (entry 3, tomerization of catechol product (4) formed by hydrolysis of
Table 4). Extending the reaction time to 24 h did increase the the DHE methoxy group.
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Table 4 Reaction performance in different isotopically labelled reactants
Product distribution (%)
Entry
Water
Methanol
Hydrogen
Conv. (%)
1
2
3
4
5
Pfinal (bar)
1a,b
2b
3
H2O
H2O
D2O
D2O
D2O
H2O
D2O
H2O
MeOH
MeOH
Methanol-d4
Methanol-d4
MeOH
MeOH
Methanol-d4
MeOH
H2 (1 bar)
H2 (1 bar)
H2 (1 bar)
H2 (1 bar)
H2 (1 bar)
H2 (6 bar)
H2 (6 bar)
D2 (6 bar)
80
95
54
85
3.6
42
—
2.5
1
100
94
61
9.5
22
—
5.4
3.1
—
0.1
21
42
34
—
48
11
—
6.2
16
45
2
—
—
34
—
2.3
—
—
—
8
11
6
9.7
9.1
11
8.9
9.6
66
44
82
—
—
2
4c
5
81
6b
7d
8
100
100
100
Conditions: DHE 0.2 mL, Ru/C 100 mg, Nb2O5 200 mg, water 12 mL, methanol 0.8 mL unless noted, purged with H2 and vented to give P(H2) = 1
bar, unless noted, 250 °C, 12 h reaction time, unless noted. a Methanol 0.4 mL. b Entries 1 and 2 are the same reactions as entries 2 and 3 in
Table 1; entry 6 is the same reaction as entry 4 in Table 2. c Reaction time 24 h. d P(H2) = 6 bar.
The proposed catechol tautomerization mechanism was
tested by carrying out the reaction of p-cresol with Nb2O5 and
D2O under 6 bar H2 at 250 °C for 12 h. No HDO of this sub-
strate was observed, but the inspection of the aromatic region
2H NMR and mass spectrum of the recovered p-cresol (ESI
Fig. S7†) showed that there had been considerable hydrogen
exchange with the solvent, as would be expected via tautomeri-
zation as illustrated in Scheme 2.
Table 4 also lists the DHE reaction run with the two cata-
lysts in the H2O/MeOH solvent with D2 at a pressure of 6 bar
(entry 8, Table 4). In this case, the yield of (1) was only 61% as
opposed to the 100% selectivity seen under comparable con-
ditions with H2 (entry 6, Table 4). Products (2) (21%) and (3)
(16%) made up the bulk of the other products. Thus, there is a
substantial kinetic isotope effect on HDO and aromatic ring
Fig. 5 1H NMR in the aromatic region of the products obtained after
DHE reactions in D2O/methanol-d4 (top, entry 4 in Table 4), D2O/MeOH
(middle, entry 5 in Table 4) and H2O/MeOH (bottom, entry 1 in Table 4).
Scheme 2 Hypothetical pathways for the exchange of aromatic protons with the solvent. H–D exchange reaction is very fast in water.
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Fig. 7 The reaction with a mixture of monolignols: dihydroeugenol
(DHE), isoeugenol and 4-allylsyringol. Conditions: 70 μL of each sub-
strate, Ru/C (100 mg), Nb2O5 (200 mg), H2O (12 mL), MeOH (1.2 mL),
P(H2) = 11 bar, 250 °C, 12 h reaction time.
Scheme 3 The apparent roles of different hydrogen sources.
Fig. 6 and ESI Table S2†). The conversion of DHE decreased
significantly to 78.6%, for run 3. Attempts to reactivate the
catalyst mixture by calcining at 450 °C (run 4) or heating under
H2 (run 5) were unsuccessful with conversion decreasing to
56% and 47%, respectively. Thus, the catalyst mixture became
less active toward HDO reactions after the second recycling
run given that propyl phenol (4) was the main product in the
last three runs. Overall, the catalyst mixture was able to cata-
lyse 15.06 mmol DHE to fully deoxygenated products (1) and
(2) during the first five runs. Notably, the addition of fresh
Nb2O5 (0.1 g) led to a marked increase in conversion (80%)
and yield of hydrocarbon products. For the six runs, the
material balance of the recovered reactant and products aver-
aged 91 3 mol%.
2
reduction reactions upon replacing H2 with D2. The H NMR
spectrum of the product mixture shows deuterium in the ali-
phatic region corresponding to (1) and (3) and no deuterium
in the aromatic region (ESI Fig. S8†), consistent with dihydro-
gen (H2/D2) as the source of aromatic hydrogenation.
Scheme 3 outlines the various pathways indicated by these
isotope effects.
Catalysis recyclability and stability test
The recyclability of the Ru/C and Nb2O5 mixture for DHE con-
version was tested in five successive runs (Fig. 6 and ESI
Table S2†). An optimized reaction condition with a higher sub-
strate loading (1 mL DHE) and a longer reaction time (16 h)
was applied to understand the performance of this catalyst
mixture. Within the first two runs, 12.5 mmol DHE was fully
converted to products (1) (81%) and (2) (19%) (runs 1 and 2,
The calculated TON for hydrocarbon production for the
first 5 runs was 301 based on ruthenium (0.05 mmol) or 20
based on Nb2O5 (0.75 mmol) initially present.
Application to a mixture of lignin monomers
DHE, isoeugenol, and 4-allylsyringol are among the most
common lignin monomers. When a mixture of these three
substrates (70 μL of each) was subjected to the standard pro-
cedure for 12 h, the only products were hydrocarbons (1)
(76%) and (2) (24%) according to the GC analysis (Fig. 7).
Since a 100% yield of (1) can be achieved with an extended
reaction time, this system offers a viable new strategy for fun-
nelling the multiple monolignols from lignin disassembly into
a much simpler mixture of C9 alkanes.
Discussion
Reported are batch reactor studies addressing the hydrodeoxy-
genation (HDO) of the lignin monomer dihydroeugenol to the
hydrocarbons propyl cyclohexane (1) and propyl benzene (2).
This transformation is affected by a Ru/C and Nb2O5 co-cata-
lyst mixture under H2 in an aqueous medium containing a
Fig. 6 Performance of catalyst reusability. The catalyst mixture was
washed twice using ethanol, dried in a vacuum chamber at RT for 24 h,
and heated in an oven at 120 °C for 1 h prior to the next run. For run 4,
the catalyst was calcined under N2 at 450 °C for another 45 min before small amount of methanol additive. The key observations are:
use. For run 5, the catalyst was reduced under H2 (5%)/Ar at 350 °C for
3 h before use. For run 6, fresh Nb2O5 0.1 g was added to the recovered
catalyst (0.2 g remained after run 5). The height of the bar represents
the amount of conversion, while the small circle represents the material
(a) MeOH serves as a secondary hydrogen donor that pro-
motes HDO under lower H2 pressures compared to current
methods.37–41 The overall reaction appears optimal with a
H2O/MeOH ratio ∼15 and H2 pressures of 1–6 bar. The
balance of the recovered reactant and products which averaged ∼91%
after workup.
increased pressure after the reaction and the presence of CO2
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in the gas phase (ESI Fig. S9†) are consistent with methanol
The reactions labelled with dashed lines and blue colour in
being reformed in the presence of Ru/C.
Scheme 4 describe the proposed pathways via intermediates
(b) Nb2O5 is primarily active in the conversion of DHE to (IM1)–(IM4) facilitated by activated Nb2O5 and based on the
4-propylcatechol (5), presumably by hydrolysis of the methoxy observations of 4-propylcatechol (5) formation in the initial
group.42 Nb2O5 prepared in this laboratory and calcined at stage (entry 1, Table 3, and Fig. 3). Key steps in this mecha-
450 °C was much more active than a commercial sample that nism are Nb2O5 catalysed tautomerizations of (4) and (5) to
was apparently calcined at a much higher temperature.43 The give carbonyl species (IM1 and IM3), followed by activated Ru/
difference was attributed to the much greater number of acid C catalysed hydrogenation to give cyclic diene alcohols (IM2
sites on the former material as shown by the NH3-TPD test and IM4), which upon dehydration are rearomatized to (2) and
(Fig. S1†).44
(4), respectively.47 Support for the tautomerization steps was
(c) Ru/C catalyses the HDO of (5) and the subsequently the observation that Nb2O5 alone catalyses the deuteration of
formed intermediates 4-propylcyclohexan-1-ol (3) and 4-propyl- the aromatic C–H groups with solvent D2O as demonstrated by
phenol (4), and hydrogenation of the aromatic ring.45,46 Ru/C 2H NMR and MS experiments (Fig. S5 and Scheme S1†).
is more effective after pre-treatment by heating under H2. Notably, even though this reaction was run under H2, no HDO
While Ru/C alone can catalyse the HDO of DHE to products of p-cresol occurred in the absence of Ru/C. Therefore, all the
(1)–(4), the synergistic activity of the two-catalyst system is dehydration reactions should be primarily catalysed by Ru/C.
more efficient and selective, toward as much as 100% conver- This point was verified by examining the reactions of a mixture
sion and selectivity to propylcyclohexane (1).
of propyl cyclohexanol (3) and 2-methoxy-4-propylcyclohexanol
Scheme 4 outlines the likely sequence of catalysed reactions with either Ru/C or Nb2O5 alone under the standard con-
leading to the potential products. Conversion of DHE is ditions (ESI Table S3†). In the former case, nearly 100% con-
initiated into two pathways by each catalyst. The proposed version to propyl cyclohexane (1) was achieved; however, with
mechanism of Ru/C catalysed HDO is highlighted in green Nb2O5 alone, conversion was 62% with only 14.5% selectivity
colour with solid arrows in Scheme 4. This is based on the towards (1) and the remaining products were not identified.
observed production of (1)–(4) with Ru/C alone (entries 3 and
The reactions with deuterated solvents (D2O, methanol-d4)
4, Table 3, and Fig. 4). The deoxygenated products can be and with D2 summarized in Table 4 provide further insight
obtained through hydrogenolysis, dehydration, and hydrogen- into the HDO/hydrogenation mechanism(s). Strong isotope
ation reactions.
effects on the product are observed upon using D2O and
methanol-d4. These are particularly observed in the Ru/C cata-
lysed HDO steps of (5) → (4) as well as (4 and 3) → (1 and 2).
What’s more, the final pressure in the reactor is lower when
using methanol-d4. An isotope effect was also observed when
D2 (6 bar) replaced H2 (6 bar) in a reaction with the H2O/
MeOH medium. In this case, the principal isotope effect was
some decrease in ring hydrogenation, further supporting the
view that MeOH and H2 supply reducing equivalents at dis-
tinctly different stages of the reaction.
The stability and reusability tests show this co-catalyst
system to be recyclable and stable during the first 32 h in pro-
ducing a clean cut of C9 hydrocarbons and excellent material
balance of isolated products >90%. The catalyst system dis-
played decreasing activity in the second, third and fourth re-
cycling runs (runs 3, 4, and 5 in Fig. 6), although propyl cyclo-
hexane was consistently produced in each case. Notably,
adding fresh Nb2O5 to the system (run 6, Fig. 6) partially
restored the activity in terms of the production of (1), although
the product distribution did not match that of a completely
fresh catalyst. Thus, a continuous feeding of fresh Nb2O5 after
the first recycling run could improve the total yield of hydro-
carbons and maximize the usability of the Ru/C catalyst.
Ongoing studies will address the strategies to minimize cata-
lyst deactivation pathways.
Lastly, subjecting a simulated bio-oil mixture of lignin
monomers to the catalyst system optimal for the conversion of
DHE gave a mixture of just hydrocarbons (1) and (2). Thus,
this system provides the opportunity to funnel complex bio-oil
mixtures primarily composed of oxygenated lignins to simple
Scheme 4 A proposed sequence of mechanistic steps for the conver-
sion of DHE by the catalyst mixture of Ru/C and Nb2O5 in a H2O/MeOH
medium under H2. Hydrogenation is abbreviated as “HYD”.
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C9 hydrocarbons more compatible with applications as liquid 10 H. Kim and J. Ralph, Solution-state 2D NMR of ball-milled
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Conflicts of interest
The authors have no conflicts to declare.
12 D. C. Elliott, Historical developments in hydroprocessing
bio-oils, Energy Fuels, 2007, 21(3), 1792–1815.
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This work was supported by the US Department of
Energy, Office of Science, Basic Energy Science, award no.
DE-SC0019161 (M. M. A.-O.), the Department of Chemistry and
Biochemistry, the National Key Technologies R&D Program
of China no. 2018YFB1501405, and the Mellichamp
Sustainability Initiative at UCSB through a summer fellowship
to J. T.
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