ACS Catalysis
Research Article
hydroxymethylfurfural (HMF) using formic acid over a Pd
catalyst have been studied with success. Currently, the
capillary column (30 m × 0.25 mm id ×0.50 μm film thickness)
and interfaced directly to the MS (Shimadzu QP2010 Plus).
Identification of the GC/MS spectral features was accom-
plished by comparing the mass fragmentation pattern of the
products with those in the built-in Wiley/NIST library. The
shift in the mass spectrum of each product obtained using
9
mechanism of CTH remains elusive, which prevents the
development of less costly and more efficient catalysts.
Our previous studies established that CTH with an alcohol as
the hydrogen donor is an effective pathway to convert furfural
to 2-methylfuran (MF) over a mildly oxidized Ru/C catalyst
perdeuterated 2-propanol (2-propanol-d ) as the hydrogen
8
14,15
with high yield (76%).
Neither Ru/C nor RuO shows MF
donor was reported relative to that obtained using unlabeled 2-
2
yield nearly as high as the partially oxidized Ru/C (Ru/RuOx/
propanol (2-propanol-d ), primarily focusing on the shifts of
the parent ion of the molecules.
0
C), suggesting that multiple sites are at play on Ru/RuO /C.
x
16−18
The oxidation of Ru/C creates a bifunctional catalyst,
Quantification of the liquid products was achieved using a
gas chromatograph (GC, Agilent 7890A), equipped with an
HP-INNOWax capillary column (30 m × 0.25 mm id ×0.5 μm
film thickness) and an FID detector. Response factors were
determined using self-prepared solutions of known concen-
tration. The kinetic isotope effect (KIE) was calculated from
the ratio of the reaction rate constants obtained from kinetic
data, using both furfural and FA as starting reagents. Rate
constants were estimated by analyzing the experimental data
with a proposed reaction network using a nonlinear least-
consisting of coexisting metal (Ru) and Lewis acid (RuOx)
sites, and this bifunctionality is key to achieving high
performance without hydrogenating the furan ring or cracking
the C−C bonds. Unlike with metal/Brønsted acid catalysts,
fewer side reactions can occur; however, neither the role of
each functionality nor their cooperativity is clear.
Herein, we present a mechanistic study on the reaction
pathways in the CTH of furfural over a mildly oxidized Ru/C
catalyst, referred to as Ru/RuO /C, by detailed mass
x
fragmentation analysis with isotopically labeled chemicals and
rigorous kinetic studies. Furfural is an ideal biomass-derived
“platform” chemical, and its hydrogenolysis product, MF, has a
Computational Methods for Adsorption Energy
high octane number and can be employed for the production of
Calculations. Density functional calculations were performed
19−21
22−25
renewable toluene.
We show that the intermolecular
using VASP software.2
Core electrons were represented
and valence electrons were modeled
6,27
hydride transfer is the dominant pathway for the hydrogenation
of the carbonyl group of furfural to furfuryl alcohol (FA).
Further, hydrogenolysis via ring activation is a major, if not the
dominant, pathway in the C−OH bond cleavage in the
conversion of FA to MF. Our results provide molecular level
understanding of the role of the cooperativity of metal and
Lewis acid sites in mediating the CTH of biomass derivatives,
which can guide the rational design of efficient bifunctional
catalysts by suitable choice and proper atomic level arrange-
ment of Lewis acid and metal sites. Further, we demonstrate
that detailed mass fragmentation analysis is a powerful
technique in mechanistic investigations of complex reactions
involving biomass-derived molecules.
using PAW formalism,
with the exchange-correlation functional of Perdew, Burke, and
28
Ernzerhof (PBE). Ground state energies were calculated self-
consistently using a plane wave basis set with a kinetic energy
cutoff of 400 eV, an electronic energy threshold of 10− eV, and
a force tolerance of 0.05 eV/Å. Lattice constants were
determined in a bulk calculation, employing 3 × 3 × 3 and
6
2
9
15 × 15 × 15 Monkhorst−Pack k-point mesh for RuO and
2
Ru, respectively. The values were found to be a = 4.54 and c =
3.08 Å for the RuO rutile structure and a = 2.70 and c = 4.26 Å
2
for the hcp Ru lattice. The (110) crystal plane of RuO was
2
modeled in a p(3 × 2) supercell with one relaxed and three
unrelaxed layers of O−Ru−O units; the reciprocal space was
sampled at the Γ-point. The p(3 × 3) repetition was employed
for the Ru(0001) surface. Among four layers of atoms, two
bottom layers remained fixed in their bulk coordinates. The
reciprocal space was sampled using the (3 × 3 × 1)
Monkhorst−Pack k-point grid. Adsorption energies were
calculated in a conventional way as
METHODS
■
Materials and Catalyst Preparation. All chemicals were
obtained from Sigma-Aldrich and used without further
purification. The catalyst used was 5 wt % Ru/C (Sigma-
Aldrich), which, prior to each experiment, was treated for 3 h at
3
3
00 °C in H flow of 40 cc/min, followed by mild oxidation for
2
ΔEads = Eads+slab − Eads − Eslab
h at 130 °C in 5% O /He flow (40 cc/min).
2
Here, ΔE , E
, E , and E
are adsorption energy,
slab
Catalyst Evaluation. Catalytic transfer hydrogenation of
ads
ads+slab
ads
energy of a species adsorbed on a slab, energy of a species in
vacuum, and energy of the slab, respectively.
furfural was carried out in a 100 mL Parr batch reactor. In a
typical experiment, the reactor was charged with 24 mL of a 10
vol % 2-propanol (unlabeled (-d ) or perdeuterated(-d ))/
toluene solution of furfural (1 wt %) and 0.1 g of Ru/RuO /C
catalyst, pressurized to 300 psi (2.04 MPa) with N and heated
to a predetermined temperature. It typically takes ∼30 min for
the reactants to reach the desired temperature, which is not
included in the reported reaction time. Reaction was quenched
by soaking the reactor in an ice bath upon reaching the desired
reaction time. The suspension in the reactor was collected after
it reached room temperature, filtered, stored in sealed vials, and
analyzed.
Analytical Methods. Identification of the liquid phase
products was performed on a gas chromatography−mass
spectrometry (GC/MS, Shimadzu QP2010 Plus) system. The
GC (Shimadzu GC2010) is equipped with an HP-INNOWax
0
8
RESULTS AND DISCUSSION
x
■
The hydrodeoxygenation (HDO) of furfural to MF (Scheme 1)
entails, first, the hydrogenation of the carbonyl group of furfural
to form FA. The chemistry can proceed via classic metal-
mediated hydrogenation, that is, the atomic hydrogen adsorbed
on metal sites, formed from the dehydrogenation of alcohols,
adds to the C and O in the carbonyl group. We hypothesize
that an alternative pathway is the Lewis acid-mediated
intermolecular hydride transfer of the β-H in the alcohol to
the carbonyl group, following the Meerwein−Ponndorf−Verley
2
7
,30−33
(MPV) mechanism.
Because of coexistence of metal
(Ru) and Lewis acid (RuO ) sites, both pathways could be at
x
play simultaneously. Upon production of FA, hydrogenolysis
3
989
ACS Catal. 2015, 5, 3988−3994