Poisoning of Ru/C by homogeneous Br?nsted acids in hydrodeoxygenation of 2,5-dimethylfuran via catalytic transfer hydrogenation

Article abstract of DOI:10.1016/j.apcata.2017.06.010

It has been proposed that the combination of metal and acid sites is critical for effective ring opening of biomass-derived furans to linear molecules, a reaction that holds promise for the production of renewable polymer precursors and alkanes. In this work, we use 2,5-dimethylfuran (DMF) as a model compound to investigate hydrogenolysis and hydrogenation pathways using a combination of H₂SO₄ and Ru-mediated catalytic transfer hydrogenation in 2-propanol. Acid-catalyzed hydrolytic ring opening of DMF to 2,5-hexanedione (HDN) occurs readily at 80?°C with a selectivity of 89% in 2-propanol. Over Ru/C, HDN is fully converted after only 2?h at 80?°C, forming a mixture of both ring-closed products (～68% total yield), i.e., 2,5-dimethyltetrahydrofuran (DMTHF) and 2,5-dimethyl-2,3-dihydrofuran (DMDHF), as well as ring opened products (～28% total yield), i.e., 2,5-hexanediol (2,5-HDL) and 2-hexanol (HOL). Rather than observing sequential hydrolysis/hydrogenation reactions, we observe severe suppression of metal chemistry when having both Ru/C and H₂SO₄ in the reaction system. While minor leaching of Ru occurs in the presence of mineral acids, X-ray photoelectron spectroscopy coupled with CO chemisorption studies suggest that the primary cause of the lack of Ru-mediated chemistry is poisoning by strongly adsorbed sulfate species. This hypothesis is supported by the observation of Ru-catalyzed chemistry when replacing H₂SO₄ with Nafion, a solid Br?nsted acid, as sulfonic acid groups tethered to the polymer backbone cannot adsorb on the metal sites.

Full text of DOI:10.1016/j.apcata.2017.06.010

Accepted Manuscript
Title: Poisoning of Ru/C by homogeneous Brønsted acids in
hydrodeoxygenation of 2,5-dimethylfuran via catalytic
transfer hydrogenation
Authors: Matthew J. Gilkey, Dionisios G. Vlachos, Bingjun
Xu
PII:
S0926-860X(17)30258-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.06.010
APCATA 16271
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
2-12-2016
22-4-2017
3-6-2017
Please cite this article as: Matthew J.Gilkey, Dionisios G.Vlachos, Bingjun
Xu, Poisoning of Ru/C by homogeneous Brønsted acids in hydrodeoxygenation
of 2,5-dimethylfuran via catalytic transfer hydrogenation, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.06.010
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Poisoning of Ru/C by Homogeneous Brønsted Acids in Hydrodeoxygenation
of 2,5-Dimethylfuran via Catalytic Transfer Hydrogenation
Matthew J. Gilkey, Dionisios G. Vlachos*, Bingjun Xu*
Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation
University of Delaware, Newark, DE 19711
*Emails: vlachos@udel.edu, bxu@udel.edu
Graphical abstract
Highlights



Homogeneous Brønsted acids are found to severely poison Ru/C mediated hydrodeoxygenation
and ring opening of 2,5-dimethylfuran.
Strong adsorption of sulfates on Ru is confirmed by CO chemisorption, ICP-MS and XPS, and is
identified as the main cause for catalyst deactivation.
Atomically mixed acid and metal sites are likely necessary for synergistic acid/metal bifunctional
catalysis in ring opening chemistry.
1

Abstract
It has been proposed that the combination of metal and acid sites is critical for effective ring opening of
biomass-derived furans to linear molecules, a reaction that holds promise for the production of renewable
polymer precursors and alkanes. In this work, we use 2,5-dimethylfuran (DMF) as a model compound to
investigate hydrogenolysis and hydrogenation pathways using a combination of H2SO4 and Ru-mediated
catalytic transfer hydrogenation in 2-propanol. Acid-catalyzed hydrolytic ring opening of DMF to 2,5-
hexanedione (HDN) occurs readily at 80 °C with a selectivity of 89% in 2-propanol. Over Ru/C, HDN is
fully converted after only 2 h at 80 °C, forming a mixture of both ring-closed products (~68% total yield),
i.e., 2,5-dimethyltetrahydrofuran (DMTHF) and 2,5-dimethyl-2,3-dihydrofuran (DMDHF), as well as ring
opened products (~28% total yield), i.e., 2,5-hexanediol (2,5-HDL) and 2-hexanol (HOL). Rather than
observing sequential hydrolysis/hydrogenation reactions, we observe severe suppression of metal chemistry
when having both Ru/C and H2SO4 in the reaction system. While minor leaching of Ru occurs in the
presence of mineral acids, X-ray photoelectron spectroscopy coupled with CO chemisorption studies
suggest that the primary cause of the lack of Ru-mediated chemistry is poisoning by strongly adsorbed
sulfate species. This hypothesis is supported by the observation of Ru-catalyzed chemistry when replacing
H2SO4 with Nafion, a solid Brønsted acid, as sulfonic acid groups tethered to the polymer backbone cannot
adsorb on the metal sites.
Keywords: Ring opening, catalytic transfer hydrogenation, biomass, catalyst poisoning, dimethylfuran
2

1. Introduction
The conversion of biomass feedstocks to value-added carbon-neutral chemicals and fuels has
demonstrated much promise. Owing to the complexity of biomass-derived substrates, many upgrade
strategies require bifunctional catalysts, i.e., materials with two types of active sites, to achieve satisfactory
activity and selectivity.[1-4] Not only can bifunctional catalysts reduce the overall number of reaction steps
in a chemical process, but in some cases, they can also perform chemistry that cannot otherwise occur over
mono-functional catalysts.[5-7] This is especially true in hydrodeoxygenation (HDO) processes which
typically employ catalysts with both metal and acid/base characters to remove oxygenated functional groups
in biomass-derived feedstocks. While many research groups have successfully implemented materials that
fulfill these traits, fundamental understanding on the interplay between metal and acid sites is still lacking.
Recently, several groups have focused on the production of linear polyols, e.g., 1,6-hexanediol and
1,5-pentanediol, from biomass-derived furans such as 5-hydroxymethylfurfural (HMF) and furfural,
respectively, which requires ring opening/hydrogenolysis of furans or cyclic ethers.[2, 8-11] These
oxygenated furans can be derived from glucose and xylose, respectively, through a series of isomerization
and dehydration steps.[12] However, ring opening typically requires high pressures of hydrogen, which
leaves HMF and furfural susceptible to a variety of side products when exposed to metal catalysts, e.g.
reduction of side groups.[9, 13, 14] Moreover, exposure of oxygenated furans to acids can lead to additional
side products such as polymerization and etherification.[15] The use of model compounds has been proved
helpful in understanding ring-opening chemistry due to the structural complexity of real biomass-derived
substrates. 2,5-dimethylfuran (DMF) is a good substrate choice because it maintains HMF’s furanic
structure while eliminating reactive side groups. DMF is subject to ring opening via hydrolysis in Brønsted
acidic conditions to form 2,5-hexanedione (HDN),[16, 17] although further hydrogenation is necessary to
create linear alcohols or alkanes. Thus, exposing DMF to Brønsted acids, e.g. H2SO4, in the presence of
hydrogenation catalysts, e.g. Ru/C, and a hydrogen source can, in theory, catalyze sequential ring opening
and hydrogenation reactions to form linear alcohols. However, this hinges on the assumption that the two
types of catalysts operate independent of each other, i.e., metal is capable of performing the hydrogenation
3

reaction in the presence of Brønsted acids, or even better synergistically, i.e., they promote each other.
Herein, we report an experimental study on the ring opening of DMF on homogeneous Brønsted
acids and Ru in isolation as well as in combination. 2-propanol is employed as the organic hydrogen source
over Ru/C via catalytic transfer hydrogenation (CTH). We show that sulfuric acid is capable of catalyzing
DMF ring opening facilely to HDN. HDN is susceptible to hydrogenation over Ru/C, leading to the
formation of predominantly ring closed 2,5-dimethyltetrahydrofuran (DMTHF) rather than linear alcohols,
e.g., 2-hexanol (HOL) or 2,5-hexanediol (2,5-HDL). Increasing the concentration of sulfuric acid
drastically suppresses Ru-mediated chemistry, which is attributed to the poisoning by strongly adsorbed
anions based on CO chemisorption and X-ray photoelectron spectroscopy results.
2. Materials and Methods
2.1 Catalytic Activity Evaluations
Activity tests for DMF HDO in this work were conducted in a 50 mL stainless steel pressure vessel
(Parr Instruments). Reaction solutions were made by adding 1 wt% of reactant to 14 mL 2-propanol, where
2-propanol acts as both the solvent and the hydrogen source, as well as 80 mg of reduced Ru/C (5 wt%
loading, Sigma Aldrich). For experiments where Brønsted acid was introduced, standard acid solutions in
H2O were prepared from 18 M H2SO4 (Sigma-Aldrich). Before sealing reactor, 0.5 mL of acid of a desired
concentration was added to the reactor. The reactor was then sealed and purged 3 times with N2 gas before
being pressurized to a final pressure of 300 psig (N2) at room temperature. Temperature of the reactor was
then raised to the desired value and maintained for a predetermined period of time, followed by quenching
the reaction in an ice bath. The catalyst was then filtered, and samples were placed in a vial for analysis in
a gas chromatograph (GC) as well as in a gas chromatograph-mass spectrometer (GCMS). All chemicals
were purchased from Sigma Aldrich and used without further purification.
Time-dependent data was acquired with a 160 mL stirred, pressurized vessel (Parr Instruments),
installed with a dip tube equipped with a 0.20 μm filter. Reaction solutions were made by adding 1 wt% of
4

reactant to 100 mL of pure 2-propanol and loading 500 mg of reduced Ru/C (5 wt% loading, Sigma
Aldrich). Liquid samples were taken over the course of the reaction where volume and pressure changes
were deemed negligible. Samples were subsequently placed in the GC for analysis.
Identification of the liquid phase products was performed on a GCMS (Shimadzu QP2010 Plus)
system. The GC (Shimadzu GC2010) is equipped with an HP-INNOWax capillary column (30 m × 0.25
mm id × 0.50 m film thickness) and interfaced directly to the MS (Shimadzu QP2010 Plus). Identification
of the GCMS spectral features was accomplished by comparing the mass fragmentation pattern of the
products with those in the built-in Wiley/NIST library.
Quantification of the liquid products was achieved using a 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 concentrations. Once response
factors were determined, conversion was calculated based on the formula:
푪
−푪
퐫퐞퐚퐜퐭퐚퐧퐭,ퟎ
푿 =
퐫퐞퐚퐜퐭퐚퐧퐭,퐟퐢퐧퐚퐥 ∙ ퟏퟎퟎ%
[1]
푪
퐫퐞퐚퐜퐭퐚퐧퐭,ퟎ
Yields were calculated by the following formula:
푪
풊,퐟퐢퐧퐚퐥
풀풊 =
∙ ퟏퟎퟎ%
[2]
푪
퐫퐞퐚퐜퐭퐚퐧퐭,ퟎ
2.2 Catalyst Pre-treatment
Reduction of commercial Ru/C catalysts (Sigma-Aldrich, 5 wt% loading) was conducted ex-situ
using a continuous-flow reactor. First, quartz wool was added to a quartz tube prior to the addition of Ru/C
(~2 g) to prevent Ru/C from escaping. The loaded quartz tube was subsequently placed in a furnace and
connected to a combined H2 and He gas line, equipped with mass flow controllers. H2/He flow was held
constant at 20 sccm H2 and 20 sccm He, and the furnace was heated to 300 °C with a temperature ramp of
~5 C min-1. After 3 h of reduction at 300 °C, H2 flow was decreased to 0 sccm and left under He flow (40
sccm) to allow the catalyst bed to cool to room temperature. The Ru/C was then transferred to a vial and
stored in a lab drawer until use in catalytic testing. This procedure was also described elsewhere.[4]
5

Acid-pretreated catalysts were prepared by mixing 14 mL of 2-propanol, 0.5 mL of various
concentrations of acid, and 80 mg of Ru/C catalyst after reduction at 300 C (see procedure in previous
paragraph). After adequate mixing, the slurry was placed over a magnetic stirrer and allowed to stir, covered
at room temperature, for 12 h. The mixture was then filtered, washed several times with 2-propanol, and
allowed to dry. This catalyst was then placed directly into the reactor for catalytic testing.
2.3 X-ray Photoelectron Spectroscopic, Inductively Coupled Plasma Mass Spectrometric, and CO
Chemisorption Measurements
X-ray photoelectron spectroscopic measurements were conducted in a Thermo Scientific K-Alpha+
spectrometer. XPS scans were taken using an accelerating voltage of 15 kV with Al Kα X-rays as the
excitation source (1486.7 eV). Spectra were collected with a 400 µm spot size with a resolution of 0.05 eV.
The XPS data were calibrated to adventitious carbon at 284.6 eV. Peaks were fitted using a blended
exponential and Gaussian-Lorenzian shape profile with a Shirley background. Due to the significant overlap
between C 1s and Ru 3d peaks, the peak fitting of the Ru 3d doublet was conducted as follows. One spectral
band was assigned to Ru 3d5/2 and 3d3/2 each (without deconvoluting Ru with different oxidation states)
with a full width at half maximum (FWHM) of 1.36 eV. Only the relative shift of the binding energy of the
Ru 3d5/2 peak is used for discussion. Acid-treated and reduced catalysts were prepared as described in
Section 2.2 and placed on carbon tape prior to analysis.
Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) measurements were conducted on an
Agilent 7500 Series instrument. Samples were prepared by pre-treating Ru/C in HCl (0.5 mL of 4 M HCl
in 14 mL water) for 2 h at both room temperature and 80 C followed by filtration of the remaining liquid.
HCl, rather than H2SO4 or other sulfate salt, was employed due to incompatibility of sulfates with the
instrument. Ru’s 98 amu through 104 amu isotopes were scanned, where 102Ru gave the greatest sensitivity.
Ru standards in dilute HCl were acquired from VHG Labs and diluted for the ICP-MS calibration.
6

CO chemisorption experiments were conducted on an Altamira Instruments AMI-200ip instrument
equipped with a thermal conductivity detector (TCD). Prior to dosing CO, the catalyst was reduced under
50% hydrogen in helium at 300 °C for 2 h. After reduction, the system was cooled to 30 °C under helium.
Pulses of CO (25% CO in helium) were introduced and TCD spectra were collected and analyzed.
3. Results and Discussion
3.1 Brønsted Acid-catalyzed Hydrolysis of 2,5-dimethylfuran with Sulfuric Acid
Table 1. Product distribution with various reactants and catalysts.
Y (%)
Reactant Catalyst T (°C) t (h) X (%)
Entry
2,5-
HDL balance
C-
HON HOL HDN DMTHF DMDHF
a
1
2
3
4
DMF
HDN
HDN
H2SO4
Ru/C
Ru/C
Ru/C
80
80
55
2
2
2
44
99
89
0
0
0
0
39
0
0
0
89
97
3.9
1.7
N/A
N/A
67
23
1.4
9.7
24
61
107
99
2,5-HDL
(ref. [19])
80
55
80
80
80
80
80
80
80
2
2
2
2
2
2
2
2
2
76
5.5
99
40
99
99
32
96
40
0
0
1.8
0
0
0
74
3.6
33
10
73
10
8.6
59
0
trace
0
N/A
N/A
53
0
5
6
2,5-HDL
(ref. [19])
Ru/C
65
108
72
94
96
88
94
80
73
-
DMF (ref. Ru/C + 0.5
0
22
0
0
0
[19])
mL H2O
7
Ru/C +
DMF
0
14
0
4.6
0
a
H2SO4
8
DMF (ref.
[19])
Ru/C
1.3
0
19
4.1
2.9
0.4
0
0
9
Ru/C + 0.5
mL H2O
HDN
DMF
DMF
DMF
N/A
2.3
0
1.5
2.5
2.2
0
79
12
10
0
10
11
12
13
14
Ru/C +
ZnSO4
0
Ru/C +
Na2SO4
19
0
Ru/C +
NaHSO4
32
Ru/C +
MgSO4
Ru/Cb
DMF
DMF
80
80
2
2
36
0
-
2.7
-
0
-
14
-
5.9
-
3.7
-
NR
a0.5 mL of 2 M H2SO4 was added, bRu/C was pretreated with 5 M H2SO4 or Na2SO4 or ZnSO4, cRu/C was
pretreated with 5 M Na2SO4, dRu/C was pretreated with 5 M MgSO4, eRu/C was pretreated with 5 M ZnSO4.
Entries 1, 2, 7, and 8 were taken from ref. [19]. DMF = 2,5-dimethylfuran, HON = 2-hexanone, HOL = 2-
hexanol, HDN = 2,5-hexanedione, DMTHF = 2,5-dimethyltetrahydrofuran, DMDHF = 2,5-dimethyl-2,3-
dihydrofuran, 2,5-HDL = 2,5-hexanediol.
7

Ring opening of DMF via hydrolysis to HDN occurs readily in the presence of sulfuric acid at 80
°C. After 2 h of reaction, ~44% conversion of DMF was achieved with a ~89% selectivity (~89% carbon
balance) to HDN in 28:1 (vol/vol) mixture of 2-propanol and 2 M H2SO4, i.e., 0.07 M H2SO4 (Figure 1a
and Table 1, Entry 1). This is in agreement with previous studies that report the facile hydrolysis of alkylated
furans under acidic conditions.[16, 17] The high selectivity toward HDN supports the hypothesis that
Brønsted acidity enhances the ability to ring open the furan to form linear alkyl chains. The use of H2SO4
also catalyzed the dehydration of 2-propanol to propylene, which consumed ~5.8% of 2-propanol to give
~5.3% yield to propylene (~91% selectivity). In addition, trace amounts of acetone and diisopropyl ether
(DIIPE) were observed (Figure 1b). Though protons participate in dehydration reactions to convert ~5.8%
of 2-propanol, plenty of free protons are available for DMF hydrolysis, as indicated by Figure 1a and Table
1, Entry 1 (44% conversion, 39% yield).
While Brønsted acidity alone is capable of opening the furan ring through hydrolysis, Brønsted
acids cannot drive HDN hydrogenation to aliphatic alcohols or alkanes due to their inability to hydrogenate
C=O bonds in the absence of Lewis acid or metal sites.[18] Thus, the combination of metal and Brønsted
acid sites is expected to drive the production of alkyl alcohols from oxygenated furanic compounds.
Dumesic et al., among others, proposed that the Brønsted acid/metal combination is responsible for the ring
opening activity of the ReOx/Rh catalyst.[2] 2-propanol and other secondary organic alcohols can function
as effective hydrogen donors over metals such as Ru/C, eliminating the need for high pressure H2.[18] To
understand the interplay between the metal and acid catalysis, we first investigated the reactivity of HDN
over Ru/C.
8

Figure 1. Product distribution of (a) DMF hydrolysis and (b) 2-propanol dehydration in the presence of
homogeneous Brønsted acid. Reaction conditions: 1 wt% DMF in 14 mL 2-propanol with 0.5 mL of 5 M H2SO4,
T = 80 °C, t = 2 h, PN2 = 300 psig.
3.2 2,5-hexanedione (HDN) Hydrogenation over Ru/C
Reduction of the carbonyl groups in HDN ensues readily over Ru/C. After 2 h of reaction at 80 °C,
complete conversion of HDN was reached in the presence of 2-propanol with a 67% yield to 2,5-
dimethyltetrahydrofuran (DMTHF), a 24% yield to 2,5-hexanediol (2,5-HDL), and a 4% yield to 2-hexanol
(HOL) via CTH (Table 1, Entry 2). Thus, hydrogenation of HDN predominantly resulted in the ring closed
DMTHF, rather than a linear molecule. In addition, high carbon balance was achieved after 5 h (>90%),
suggesting that very few, if any, undetected byproducts formed. Interestingly, the time-dependent data
collected over 24 h indicate that 2,5-dimethyl-2,3-dihydrofuran (DMDHF) is a reactive intermediate in
HDN hydrogenation, peaking after only 20 min at 55 °C (Figure 2, purple trace). DMDHF formation from
HDN could occur via several possible pathways. One such pathway involves the partial hydrogenation of
one C=O bond in HDN to form 5-hydroxy-2-hexanone over Ru/C, which can then undergo ring closure
and dehydration to DMDHF via Lewis acidic RuOx sites. Although Ru/C catalysts were reduced at 300 C
prior to reaction, reduction was not conducted in situ. After reduction, 80 mg Ru/C was transferred to the
reactor, which could introduce some oxide sites upon exposure to air during the transfer. However, based
9

on the experimental data, neither the precise catalytic sites for HDN ring closure to DMDHF nor the
molecular-level reaction pathway can be determined. Yield of 2,5-HDL (Figure 2, green trace), via
hydrogenation of both carbonyl groups of HDN, increased in the first 5 h to ~70% before its slow decline.
Because DMDHF is the first intermediate to be observed in the time-resolved experiment, the formation of
2,5-HDL could also stem from the ring opening of DMDHF and subsequent hydrogenation. Although this
pathway could not be determined experimentally, theoretical studies suggest that partially saturated furan
rings can be ring-opened over metallic surfaces,[19] and thus it is a potential reaction pathway. DMTHF
could form via two possible pathways (Figure 2, red trace): 1) ring hydrogenation of DMDHF, and 2)
etherification of 2,5-HDL. Etherification reactions are typically catalyzed by acid sites, suggesting that 2,5-
HDL cyclization to DMTHF might be facilitated by Lewis acidic sites on Ru in the form of RuOx (Table 1,
entries 4 and 5),[3, 4, 18, 20] which likely exist due exposure to air after pre-treatment of Ru/C. Trace
amounts of HOL were also observed, likely due to either hydrogenolysis of 2,5-HDL or hydrogenolytic
ring opening of DMDHF.
Figure 2. Product distribution of CTH of 2,5 hexanedione (HDN) as a function of
time at 55 °C. Conditions: 1 wt% HDN in 100 mL 2-propanol, T = 55 °C, mcat
500 mg Ru/C, PN2 = 300 psig. Error bars included at t = 4 h to indicate
representative uncertainty.
=
-
10

3.3 Combined Metal and Homogeneous Brønsted Acid-catalyzed DMF Hydrogenation/Hydrogenolysis
Scheme 1. Reaction network from DMF (left) and 2-propanol (right) in the presence of both Ru/C and H2SO4,
showing the formation of hydrolytic ring opening to HDN followed by hydrogenation in the DMF pathway and
the emergence of propylene and sulfurous acid, bis(1-methylethyl)ether (SAE) in the 2-propanol pathway.
The combination of metal and homogeneous Brønsted acid, i.e., H2SO4, in the reaction mixture with DMF
creates a complex reaction network (Scheme 1), whereby metal-catalyzed hydrogenation and
hydrogenolysis occur in parallel with Brønsted acid-catalyzed hydrolysis. When 0 M H2SO4, i.e., pure
water, was added to the reaction, full conversion of DMF was achieved, where DMTHF, HOL, and 2,5-
HDL were formed with yields of 33%, 22%, and 53%, respectively (Table 1, Entry 6). At low acid
concentrations (0.5 mL 1 M H2SO4 in 14 mL 2-propanol, final concentration of 0.36 M H2SO4) the
combination of metal and acid catalysis did not enhance ring opening toward aliphatic alcohols, i.e., 2,5-
HDL or HOL; instead, lower activity was observed after 2 h (~75% vs. 100% conversion on Ru/C) under
otherwise identical conditions (Figure 3). In addition, DMTHF formed as the primary product (~31% yield),
while yields toward HOL, 2,5-HDL, and HDN remained below 5% (Figure 3). This could be due to The
ratio between stable products DMTHF and HOL is helpful in assessing the interplay between metal and
acid catalysis because HOL production occurs solely over metal sites[13, 20, 21] while DMTHF production
11

can occur via a combination of metal and acid sites. Conversion of DMF on Ru/C resulted in roughly a 4:1
mixture of DMTHF to HOL at 80 °C in 2 h (Table 1),[20] whereas the ratio of DMTHF to HOL increased
to 15:1 when the reaction was catalyzed by Ru/C with diluted H2SO4 (0.5 mL 1 M H2SO4 added in the
reaction mixture) due to the HDN hydrogenation reaction to DMTHF (Figure 3). The enhanced selectivity
for DMTHF indicates sequential acid and metal catalyzed reactions, i.e., HDN, formed from the hydrolysis
of DMF by H2SO4, was subsequently converted to DMTHF on Ru/C. When 0.5 mL of 2 M H2SO4 was
added (final concentration of 0.72 M H2SO4), both DMF conversion and DMTHF yield dropped
considerably, resulting in a 40% conversion, a 10% DMTHF yield under otherwise identical conditions.
Meanwhile, no HOL was observed, providing evidence of Ru’s deactivation. In this case, the conversion is
very similar to that when no Ru/C was added (~44%), which suggests that in the presence of both H2SO4
and Ru/C much of the initial DMF conversion proceeds via acid-mediated hydrolysis to HDN followed by
partial hydrogenation and ring closure. It further suggests that the surface is only partially poisoned,
inhibiting Ru’s ability to catalyze hydrogenation reactions directly from DMF. An alternative explanation
is that DMF and sulfate molecules might compete for sites on the Ru surface, making DMF’s ability to
access active Ru sites difficult. Meanwhile, protons in solution are available to perform hydrolysis to HDN.
As evidenced by the product distribution, HDN can still be hydrogenated, followed by ring-closure to
DMTHF, which suggests that HDN and sulfates do not appear to compete for the same type of Ru sites
while DMF and sulfates do. HDN’s ability to hydrogenate, however, suggest that the surface is only
partially poisoned when 2 M H2SO4 (0.5 mL) was added, where some Ru sites are still catalytically active.
Once the concentration of the added H2SO4 reached 5 M (final concentration of 1.78 M H2SO4), Ru-
mediated catalysis was completely shut off, producing neither DMTHF nor HOL. Only acid-catalyzed
hydrolysis occurred to form 31% yield of HDN.
12

Figure 3. Effect of the concentration of added sulfuric acid on DMF product distribution.
Conditions: 1 wt% DMF in 14 mL 2-propanol with 0.5 mL of varying concentrations of H2SO4, T =
80 °C, t = 2 h, mcat = 80 mg Ru/C, PN2 = 300 psig. Final concentration of H2SO4 in the mixture: 0.36
M (0.5 mL 1 M H2SO4), 0.72 M (0.5 mL 2 M H2SO4), 1.78 M (0.5 mL 5 M H2SO4)
Although water is added with varying concentrations of H2SO4, it does not contribute to the
poisoning of Ru sites. Water, however, does play a role in tuning product selectivity of 2,5-HDL vs.
DMTHF. In particular, when water is added (0 M H2SO4, Figure 3), yield of 2,5-HDL is 53% and yield of
DMTHF is 33%, which is in stark contrast to that when no water is added (0% and 73%, respectively, Table
1, Entries 6 and 8). This could be due to one of two possibilities: (1) RuOx phases form Ru(OH)x species
on the surface and contribute to 2,5-HDL formation,[20] or (2) increasing water concentration shifts
equilibrium to favor 2,5-HDL over DMTHF, where water is a product of cyclization of 2,5-HDL to
DMTHF. If the latter explanation holds, it is expected that using HDN as a starting material in a mixture of
2-propanol/water will result in a larger 2,5-HDL to DMTHF ratio, while yield to other hydrogenation
products should remain similar. After reaction of HDN at 80 °C, full conversion of HDN was reached, and
a 79% yield to 2,5-HDL and 10% yield to DMTHF was observed (Table 1, Entry 9). This 2,5-HDL to
DMTHF ratio of 7.9 is drastically increased from 0.36 when no water was added (Table 1, entry 2).
However, the sum of 2,5-HDL and DMTHF yields are very similar in both cases (89% vs. 91%), suggesting
there is a similar degree of hydrogenation. In addition, HOL and DMDHF yields remained similar to that
13

when no water was used (4.1% and 1.5% versus 3.9% and 1.4%, respectively). These data suggest that
water plays a negligible role in affecting Ru’s hydrogenation ability, and likely only plays a role in affecting
the equilibrium between DMTHF and 2,5-HDL.
3.4. Deactivation of Ru/C in Homogeneous Brønsted Acids
The deactivation of Ru/C in the presence of sulfuric acid could be attributed to site-blocking by
adsorbed sulfate or bisulfate anions, which could potentially be facilitated by the reaction between protons
and oxide phases, e.g., RuOx, on the surface. When replacing H2SO4 with a sulfate salt soluble in 2-
propanol, e.g., ZnSO4, in the identical concentration to that of H2SO4, significant decrease in DMF
conversion was observed in comparison to that when no additive was added while maintaining a high carbon
balance (~88%) (Figure 4a and Table 1, Entry 10). In contrast to using H2SO4, only partial loss in metal
activity was observed in the presence of sulfates, likely due to the incomplete blocking of metal sites by
adsorbed sulfate. While lower yields for DMTHF and HOL were observed (8.6% and 2.6%, respectively –
Table 1, Entry 10), the ratio between the two products remained similar to that when H2SO4 was absent
(~3:1 vs. ~4:1), indicating that site blocking is likely the primary cause of deactivation. When ZnSO4 was
replaced with a salt that is insoluble in 2-propanol, e.g., Na2SO4, no appreciable change in product
distribution is observed after reaction (Table 1, Entry 11), most likely due to the lack of sulfate ions in
solution. Replacing H2SO4 with an equimolar concentration of NaHSO4 resulted in a very similar product
distribution to that of H2SO4, i.e., high selectivity for hydrolysis product (HDN) but no hydrogenation
products (Figure 4a and Table 1, Entry 12). Thus, protons likely facilitate site blocking by interactions with
Ru sites. Alternatively, it is possible that protons increase Ru’s electron density near the surface, hindering
its catalytic performance. Such a phenomenon has been observed by Karim et al. when Pt was used for
aqueous-phase reforming of glycerol in acidic conditions.[22]
While a significant amount of propylene (0.18 mol) was detected in the presence of H2SO4, its
participation in Ru/C’s deactivation through proton-mediated oligomerization reactions is likely minimal.
In the post-reaction mixture when both 80 mg Ru/C and 0.5 mL of 2 M H2SO4 was added, propylene (5.3%
14

yield), acetone (0.1% yield) and DIIPE (0.1% yield) were formed from 2-propanol (5.8% conversion),
resulting in a very high carbon balance (95% with respect to converted 2-propanol), where the majority of
propylene was detected in the liquid phase. Therefore, it is possible that the liquid-phase propylene could
react with protons in the solvent to form polypropylene or oligomers. However, such undetected products
formed from these reactions, if any, make up only a small portion of the carbon balance and are unlikely to
be the main cause of catalyst deactivation. In addition, the acid sites which can catalyze propylene
oligomerization exist homogeneously in solution, rather than on the catalyst (Ru/C) surface.[23] Thus,
longer alkyl chains that form in solution are unlikely to subsequently adsorb on Ru sites and inhibit
hydrogenation reactions. Experiments performed using sulfate salts in place of H2SO4 provide further
evidence that propylene, along with oligomers derived from it, do not contribute significantly to catalyst
poisoning. When alcohol-soluble sulfate salts, e.g., ZnSO4 and MgSO4, are co-fed with Ru/C, negligible
propylene is observed due to a lack of homogeneous Brønsted acid sites; moreover, severe deactivation of
Ru sites is still observed (Figure 4a and Table 1, entries 10 and 13), which subsequently leads to the
conclusion that sulfates, rather than propylene-based oligomers, play a key role in catalyst poisoning.
Halides also showed similar deactivating effect on Ru-mediated hydrogenation, evidenced by
minimal hydrogenation products when introducing halide acids and halide salts (Figure 4b). Adding 0.5
mL of 4 M of HCl, HBr, or HI as the homogeneous Brønsted acid produced >90% selectivity to HDN and
no conversion of DMF to hydrogenation or hydrogenolysis products, suggesting that halides are similarly
capable of poisoning Ru sites. Similar deactivation by halides was observed by Karim et al. due to strong
binding of the anion to metallic surfaces.[22] Halide salts, i.e., NaCl, LiBr, etc., were more effective in
deactivating the Ru sites than sulfates at similar concentrations, which is likely due to the strong binding of
halides to the metal.
15

Figure 4. (a) Effect of acids and salts on DMF conversion and product distribution. (b) Effect of halide-based
salts and acids on DMF conversion and product distribution, illustrating very pronounced loss in activity.
Conditions: 1 wt% DMF in 14 mL 2-propanol with 0.5 mL of 5 M acid or salt (in water), T = 80 °C, t = 2 h,
mcat = 80 mg reduced Ru/C, PN2 = 300 psig.
When Ru/C catalysts were pre-treated ex-situ by sulfuric acid or sulfate salts (as described in the
Experimental Section), similar deactivation toward DMF hydrogenation was observed, further supporting
the hypothesis of deactivation of Ru by site blocking. Pretreatment of Ru/C catalysts was conducted by
soaking the catalyst in a mixture of 14 mL 2-propanol and 0.5 mL 5 M acid or salt solution at room
temperature for 2 h. Following filtering, washing, and drying of the H2SO4, Na2SO4, or ZnSO4 treated
catalyst, minimal DMF conversion and no products were detected (Table 1, Entry 14). This suggests that
16

upon drying of the catalysts, sulfuric acid or sulfate salt is able to irreversibly block or remove the active
sites on Ru/C. However, over these treated catalysts, HDN can still be converted (>99%) to form DMTHF
(~23% yield), 2,5-HDL (~38%), and DMDHF (~6%), although carbon balance is significantly lower
(~67%). This suggests that available Ru sites are significantly poisoned and cannot hydrogenate DMF
effectively. However, hydrogenation of HDN to HDL can still occur, followed by ring-closure. Pulse CO
chemisorption experiments indicate that the treatment of Ru/C with both sulfate salt and acid pre-treatments
drastically reduces the availability of metal sites. Reduced Ru/C catalysts adsorbed 73 μmolCO/g, which
was in excellent agreement with Ru/C catalysts pre-treated with only 2-propanol (70 μmolCO/g). However,
Ru/C catalysts that were pre-treated with ZnSO4, Na2SO4, and H2SO4 adsorbed only trace amounts of CO
(<1 μmolCO/g), which indicates that either: (a) sulfates are strongly bound to the surface, covering metal
sites irreversibly, or (b) sulfate salts and acids cause severe leaching of Ru from the support. The latter
possibility was tested via inductively coupled plasma mass spectrometry (ICP-MS). Analysis of the filtrate
in ICP-MS showed that only ~1.4% Ru is removed by acid treatment at room temperature, and ~6% of Ru
is removed at 80 °C. Thus, leaching of Ru is unlikely the main cause of the complete deactivationThis is
supported by CO chemisorption experiments where negligible CO was adsorbed on pre-treated catalysts
even after the catalysts were reduced at 300 °C for 3 h in-situ prior to pulsing CO. It should be noted that
300 °C is a high enough temperature to completely reduce RuOx in H2.[24] Leaching of Ru to the solution
could lead to Ru-phase redistribution and morphology changes on the catalyst surface, subsequently
deactivating the catalyst. However, this possibility cannot explain the complete loss of CO chemisorption
onto the surface (<1 μmolCO/g) and complete loss of Ru-mediated activity (Figure 3), although some losses
could be expected. In addition, this possibility can be ruled out by experiments where sulfate/halide salts
were employed. In these cases, no protons are available to drive leaching and reassembly of Ru molecules
on the surface, and yet strong deactivation of metal sites was observed (Figure 4). Thus, it is most likely
that sulfates/halides are bound strongly to the surface after being pre-treated with acid and dried.
Ru that is leached into solution by interactions with protons is only mildly active for hydrogenation
reactions. This is evidenced by reactions conducted with supernatant solutions created by refluxing Ru/C
17

in 14 mL 2-propanol solutions containing 0.5 mL of 5 M H2SO4 for 2 h at 80 °C. After filtering the Ru/C
from the resulting mixture, DMF was added to the liquid to make the concentration 1 wt%, and the mixture
subsequently was added to the reactor vessel, pressurized with N2 (300 psi), and heated to 80 °C for 2 h.
Post-reaction analysis revealed that DMF conversion was 45%, with 32% yield to HDN, 6% yield to
DMTHF, and 5% yield to HDL. The DMF conversion is similar to that when no leached Ru is available in
solution, which suggests that the leached Ru does not effectively hydrogenate DMF. Instead, it is likely that
HDN is first formed via homogeneous Brønsted acid sites, and homogeneous Ru sites can hydrogenate
C=O bonds to form HDL. HDL can then undergo proton-mediated ring closure to form DMTHF. In
reactions where DMF was left to react in the presence of both Ru/C and 0.5 mL of 5 M H2SO4, no
hydrogenation products were observed (Figure 3). This can be rationalized by the low concentration of
homogeneous Ru sites at the start of the reaction when both Ru/C and H2SO4 were co-fed with DMF. Over
the course of the reaction, a small amount of Ru is gradually leached into solution. Thus, only at longer
reaction times are there sufficient Ru sites to perform hydrogenation reactions, at which point the reaction
is quenched. When supernatant solutions are used, the reaction begins with a relatively high Ru
concentration in solution, where both hydrolysis and hydrogenation reactions can occur at a faster rate than
the cases not using supernatant solutions.
X-ray photoelectron (XPS) spectral features of H2SO4- or sulfate salt-treated catalysts strongly
indicate that sulfate molecules are bound to the surface after pre-treatment. The XP spectra of each of the
pre-treated Ru/C catalysts showed a prominent band at 169.8 eV (Figure S1), which is characteristic of the
S 2p3/2 band of sulfate.[25-27] In addition, the cations of the salts used to treat Ru/C catalysts were also
observed in the XP spectra. This is evidenced by a doublet feature in the XP spectra of ZnSO4-treated
catalysts located at 1022.1 and 1045.2 eV, respectively assigned to Zn 2p3/2 and 2p1/2 (Figure S2a).[28, 29]
Similarly, a band corresponding to Na 1s was observed at 1071.0 eV in the Na2SO4-treated catalyst, which
is in good agreement with reports in literature (Figure S2b).[30, 31]
18

Chemical shifts in the Ru 3d bands were observed in the XP spectra of pre-treated Ru/C catalysts,
which could be explained by chemical shifts induced by chemically bonded sulfate. To diagnose the impact
sulfate deposits on Ru, the Ru 3d and C 1s spectral features were investigated. The Ru 3d region overlaps
significantly with that of C 1s (Ru0 3d5/2 peak at ~280 eV[32] and the C 1s peak of graphitic carbon at 284-
285 eV[33, 34]),[24, 35] which introduces uncertainty in the peak deconvolution and quantitative analysis.
Rather than quantitative assessment of atomic ratios, only the general envelope of the Ru 3d peaks was fit
and the binding energy (BE) of Ru 3d5/2 was used in the comparison of oxidation state of Ru in various
catalysts.[24] The reduced Ru/C sample shows a 3d5/2 band at 281.0 eV, which indicates Ru is at least
partially oxidized,[24, 32, 35] (Figure 5a). This is likely due to the oxidation of the reduced Ru catalyst
upon air exposure when transferring the sample to the XPS chamber for the ex situ characterization. In
Ru/C catalysts pre-treated with sulfuric acid or sulfate salts, a ~0.5 eV shift of the Ru 3d5/2 peak to higher
Figure 5. XPS spectra of Ru 3d binding region for (a) untreated, reduced Ru/C, (b) ZnSO4-treated Ru/C, (c)
Na2SO4-treated Ru/C, (d) H2SO4-treated Ru/C, where (e) shows a schematic of catalysts with sulfate deposits
onto the surface.
19

BE in was observed as compared to the reduced Ru/C sample (Figure 5b-d), which is likely caused by the
adsorption of sulfate on Ru. The downshift of the O 1s peak is also consistent with the chemical shifts in
the Ru 3d5/2 peak (Figure S3). Prior to the sulfate pre-treatment, the most prominent peak in the O 1s region
appeared at 533.0 eV, which was assigned to C-O bonds in the carbon support.[36, 37] A downshift of the
O 1s peak to ~532.0 eV was observed after pre-treatment with the sulfate, which was attributed to the
addition of oxygen from adsorbed sulfate.[38, 39] No noticeable changes are observed in the C 1s envelope,
indicating that most sulfate adsorbs on the metal, rather than the carbon support (Figure S4).
Reactivity studies coupled with ex-situ characterization techniques suggest that the deactivation of
Ru/C proceeds through the blocking of Ru sites by strongly adsorbed, negatively charged sulfate species.
While fresh Ru exists primarily as an oxide in the form of RuO2 (Figure 5a), Ru can be reduced by CTH
under reaction conditions[24] and is therefore likely to exhibit both metallic and oxide phases. The use of
sulfuric acid or sodium bisulfate strongly deactivated Ru/C (Figure 4a). We hypothesize that when these
acidic species are used as co-catalysts, protons facilitate site blocking by removing oxygen from RuO2 via
removal of water (Scheme 2), leaving a site available for a sulfate molecule to bind. By extension,
increasing the concentration of protons, and therefore sulfate anions, should result in a more pronounced
loss in catalytic activity, which is consistent with the reactivity data shown in Figure 3. Meanwhile, when
sulfate salts, in the absence of proton, are used in identical concentration to sulfuric acid, a milder loss in
activity was observed. Rather than removal of water through proton-mediated dehydration of the surface,
we propose that sulfate salts can only interact with metallic Ru after partial reduction of the surface by
CTH. In this case, we attribute the site blocking to negatively charged sulfate ions, rather than to the
adsorption of propylene-derived oligomers. When using sulfate salts, propylene formation from 2-propanol
cannot occur due to the lack of proton in solution, and thus propylene oligomers are unlikely to play any
major role in site blocking. In addition, ex-situ characterizations by pulsing CO onto a sulfate salt-treated
Ru/C catalyst after reduction at 300 °C reveal that sulfate is bound very strongly to the Ru surface. Thus,
we propose that at least two mechanisms could be at play: (1) proton-mediated water removal and
subsequent covalent sulfate bonding, and (2) CTH-mediated reduction of Ru followed by non-covalent
20

Scheme 2. Proposed mechanisms for deactivation where (a) protons can remove water to allow for sulfate
adsorption, and (b) reduction of RuOx phases by CTH precedes weak sulfate binding.
sulfate-Ru interactions. When acids are used, both mechanisms can occur, resulting in a more pronounced
loss in activity; salts, on the other hand, can only proceed by the second mechanism due to the lack of
proton, leading to a milder loss in activity.
Our work reveals that the presence of mineral acids has an inhibitive effect on Ru/C, wherein strong
adsorption of sulfate or halide anions block Ru sites such that DMF hydrogenolysis cannot occur
effectively. Mechanisms previously proposed for ring opening have suggested that bifunctional catalysis is
necessary, where Brønsted acidity facilitates either the binding of the substrate[40-42] or the protonation
of the furan ring.[5] In our system, enhancement of ring opening activity was not observed in the presence
of both metal and a mineral acid. While both were added to the reaction mixture, coexistence of both kinds
of sites did not exist under reaction conditions; instead, Ru sites were eliminated by adsorbed anions, such
as sulfate. This hypothesis is supported by employment of a solid acid with tethered sulfonic acid groups
that cannot come in direct contact with the Ru surface, i.e., Nafion. A physical mixture of Nafion beads and
Ru/C should in principle provide both metal and acid sites and eliminate interaction between sulfonic
groups and metal. Thus, both acid and metal chemistry is expected to occur. Adding 2.5 g Nafion NR50,
which corresponds to ~2 mmol H+ based on the ion exchange capacity (IEC), resulted in a 18% conversion
and a 13% HDN yield in the absence of Ru/C (Figure 6, dark blue bars). While the total number of protons
was identical to that of 0.5 mL of 2 M H2SO4, a considerable drop in conversion and yield is observed
21

compared to that of 2 M H2SO4 (Figure 6, dark red bars). This difference in activity is likely due to either
the reduced acid strength of the tethered sulfonic groups, as compared to H2SO4, or the slow diffusion of
DMF into the pores of Nafion beads. Nonetheless, combining both 80 mg Ru/C and 2.5 g Nafion NR50 in
one pot resulted in a ~42% combined yield of ring opened products (21% yield to HOL, 24% yield to 2,5-
HDL) after 2 h of reaction, maintaining full DMF conversion (Figure 6, light blue bars). In this case, DMF
conversion can proceed either via direct Ru-mediated hydrogenation/hydrogenolysis to form DMTHF,
DMDHF, and HOL, or via sequential reactions consisting of proton-mediated hydrolysis to HDN and Ru-
mediated hydrogenation to 2,5-HDL, DMDHF, and DMTHF. Thus, the rather high yield (21%) to HOL
provides strong evidence that the Ru/C surface remains fully active because this yield is very similar to that
observed when no acid (solid or homogeneous) was added (Table 1, entry 8). In addition, HDN’s
hydrogenation/ring closure to DMDHF and DMTHF can be attributed to participation from Ru or RuOx
sites (Figure 2), although the mechanistic details of this transformation cannot be distinguished based on
our experimental data. The presence of 2,5-HDL is likely due to the hydrogenation of HDN, produced by
acid-catalyzed hydrolysis, on Ru, providing evidence that both acid and metal catalysis proceed unimpeded.
However, the desired enhancement of ring opening was not observed; instead, serial reactions (DMF
hydrolysis to HDN, followed by HDN hydrogenation) occur.
Bimetallic catalysts which contain both acid and metal sites co-impregnated on a common support,
e.g., Rh-Re or Ir-Re supported on SiO2, exhibit synergy between both phases, likely due to the close
proximity of these two types of sites. Mechanisms proposed by Dumesic et al.[5], Tomishige et al.[40] and
others,[41] suggest that acid sites such as ReOx provide protons to activate the cyclic ether in
tetrahydrofuran while metal sites perform the necessary hydrogenation steps, resulting in α,ω-diols.
22

Therefore, it is likely the proximity of the metal and acid sites on the molecular level is needed to enable
the operative chemistry favoring the ring opening pathway. In our system, the solution-phase protons can
exist near the Ru sites to perform such chemistry; however, the anion component of the homogeneous acid
severely inhibited Ru sites from conducting the necessary hydrogenation. Alternatively, using Nafion
instead of sulfuric acid prevented sulfate-mediated site-blocking, as well as the proximity between acid and
metal sites, which leads to sequential reactions rather than the desired synergistic effect. Thus, our results
highlight the importance of synthesizing bifunctional catalysts with atomically mixed metal and acid sites
for selective ring opening catalysis.
4. Conclusions
In this study, we show that mineral acids combined with Ru/C do not act synergistically in
facilitating the ring opening of DMF. High selectivity toward a ring closed product, e.g. DMTHF, was
observed, rather than enhanced selectivity toward alcohols in the presence of dilute sulfuric acid and Ru/C.
Severe deactivation of Ru-mediated hydrogenation and hydrogenolysis chemistry was observed in the
Figure 6. Reactivity of DMF in both homogeneous and heterogeneous acids in the presence (or absence) of
Ru/C. Conditions: 1 wt% DMF in 2-propanol at 100 °C for 2 h, PN2 = 300 psig, mRu/C = 80 mg, mnafion = 80 mg.
23

presence of high concentrations of mineral acids. Metal sulfates and halides are also able to poison Ru sites,
however, not as efficiently as the corresponding acids. XPS and CO chemisorption results indicate that site
blocking by sulfate and halide anions on Ru sites is likely the main cause of poisoning. This hypothesis is
supported by the lack of catalyst poisoning when replacing mineral acids with a solid acid, i.e., Nafion,
because the tethered sulfonic groups cannot adsorb on Ru sites. However, the lack of the proximity between
the acid and metal sites in this case leads to sequential acid and metal mediated reactions, rather than
promoting the ring opening chemistry.
Acknowledgements
We acknowledge support from 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.
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