Full Papers
HON are the final products observed following the reduction
of the carbonyl group in MFL (Figure 5b). Following hydroge-
nation to form MFA, hydrogenolysis to DMF occurs, from
which either ring-opening to HOL or ring-saturation to DMTHF
proceeds. The DMTHF and HOL selectivities are nearly identical
to those if MFA is used as the starting material (~2.5), which is
consistent with the observation that MFA is a common inter-
mediate for all products from MFL. The carbon balance in the
reaction with MFL decreases steadily throughout the course of
the reaction. This might, again, be attributed to oligomeriza-
tion or polymerization reactions from MFA,[24] DMF,[27] or both.
A higher carbon balance throughout the course of the reaction
from MFL might be attributed to the propensity of furanic
compounds with a ÀCH2OH side group to polymerize com-
pared to those with a carbonyl group. For example, furfuryl al-
cohol is more prone to polymerization than furfural.[28]
product. Another proposed role of the oxide species in the
ring-opening chemistry is to provide binding sites for oxygen-
containing side groups, and the hydrogenated furan ring is
opened on adjacent metal sites.[18,35–37] Therefore, results from
the current study, along with previous work, suggest that the
selective ring-opening of furanic compounds is unlikely to
occur on monometallic surfaces and requires more than multi-
ple types of active sites.
Conclusions
A combined experimental and computational investigation of
the ring-opening of 2,5-dimethylfuran (DMF) and oxygenated
furanic compounds on Ru catalysts was employed to map out
the reaction network and energetics. With 2-propanol as the
hydrogen source, ring-opening to 2-hexanone and 2-hexanol
and ring-saturation to 2,5-dimethyltetrahydrofuran from DMF
occur in parallel. No interconversion between the ring-opening
and ring-saturation products is observed. Computations indi-
cate that DMF adsorbs on Ru in an open-ring configuration,
which is the common intermediate for the ring-opening and
ring-saturation pathways. The ring-closing of partially hydro-
genated open-ring species is thermodynamically favored and
is aided by steric interactions with co-adsorbed 2-propoxy de-
rived from the solvent. Experiments that started from oxygen-
ated furanics 5-methylfurfural and (5-methyl-2-furyl)methanol
show that DMF is a common reaction intermediate, which indi-
cates that the reduction of the oxygenated substitutional
groups is preferred to furan ring-opening on Ru. Our findings
combined with previous results suggest that bi- or multifunc-
tional catalysts are needed to facilitate selective ring-opening.
DMF is the common intermediate in the HDO of oxygenated
furanic compounds over Ru/C, which could be attributed to
the oxophilic nature of Ru. This is consistent with previous
studies in which high yields to DMF from HMF and to MF from
furfural were achieved on catalysts with oxophilic
metals.[6,9,29,30] The preferential reduction of side groups, rather
than the furan ring, is likely because of the strong interaction
of oxygen-containing groups with the oxophilic surface. Alky-
lated furans, for example, MF and DMF, can be further reduced
through ring-opening or ring-saturation pathways, which is
shown in this work and elsewhere.[30–33] In this work, the use of
CTH at 808C resulted in a ~4:1 ratio of DMTHF to HOL at full
conversion. The incorporation of molecular H2 in place of CTH
could drive changes in product selectivity, for which overhy-
drogenation to alkanes, for example, n-hexane, or CÀC bond
cracking to lighter hydrocarbons may occur if high-pressure H2
is used. In addition, Wang et al. proposed that a higher surface
coverage of H2 favors ring-saturation over ring-opening be-
cause of the change of the adsorption configuration of the re-
action intermediates, which suggests that the ratio of DMTHF
and HOL might be influenced by H2 pressure.[34] The preserva-
tion of oxygen-containing side groups is key in the production
of valuable linear oxygenates, for example, 1,6-hexanediol, for
which the selective cleavage of CÀO bonds in the ring is
needed. In this regard, Ru/C is a poor catalyst. Driving selectivi-
ty toward ring-opening requires bi- or multifunctional catalysts
able to adsorb furanic molecules selectively through surface–
ring interactions and stabilize oxygen-containing side groups.
Dumesic et al.[17] reported that Rh-ReOx catalysts can ring-open
oxygenated tetrahydrofurans selectively, for example, tetrahy-
drofurfuryl alcohol (THFA). The ring-opening of alkylated tetra-
hydrofurans, for example, 2-methyltetrahydrofuran, over these
bimetallic catalysts proceeds more than 20 times slower. This is
in agreement with our findings that DMTHF is inactive toward
ring-opening over Ru/C because of the endothermicity and
energy barriers affiliated with the dehydrogenation (adsorp-
tion) and ring-opening reaction. Moreover, Brønsted acidity, in-
troduced by ReOx, was proposed to play a key role to proton-
ate the oxygen in the fully hydrogenated furan ring to lead to
an open-ring adsorbed intermediate. Adsorbed hydrogen on
metal sites then hydrogenates the intermediate to the final
Experimental Section
Catalytic activity evaluation
Reactions were conducted by using a 50 mL stainless-steel pres-
sure vessel (Parr Instruments) with magnetic stirring, and the reac-
tion temperature was controlled by using a proportional–integral–
derivative (PID) controller. Reaction solutions were made by adding
1 wt% of reactant to 14 mL of pure 2-propanol, in which 2-propa-
nol acts as both the solvent and the hydrogen source, along with
80 mg of reduced Ru/C (5 wt% loading, Sigma Aldrich). The reactor
was then sealed and purged three times with N2 before it was
pressurized to a final pressure of 300 psig (N2; 1 psig=68.9 mbar).
The temperature of the reactor was then increased to the desired
temperature and maintained for a predetermined period of time.
After the reaction, the reactor was quenched in an ice bath. The
catalyst was then removed by filtration, and samples were placed
in a vial and stored for further analysis. Reactions with intermittent
sampling were conducted by using a mechanically stirred reaction
vessel (Parr Instruments, 5500 Series; 160 mL) equipped with a dip
tube and a 0.20 mm filter. Solutions of 100 mL of 1 wt% of reactant
were added along with 500 mg of reduced Ru/C. The reactor was
sealed and pressurized as discussed above. Once the desired tem-
perature was reached, liquid samples were collected at intermedi-
ate times over the course of the reaction, and pressure and
volume changes caused by sampling were deemed negligible.
Each sample was stored in a vial for further analysis. All chemicals
ChemSusChem 2016, 9, 1 – 10
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