Selective hydrogenolysis of polyols and cyclic ethers over bifunctional surface sites on rhodium-rhenium catalysts

Article abstract of DOI:10.1021/ja2038358

A ReO_x-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C-O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends, NH₃ temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C-O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh-ReO_x/C (1:0.5) was confirmed by NH₃ TPD, and the estimated acid site density and standard enthalpy of NH₃ adsorption were 40 μmol g^-1 and -100 kJ mol^-1, respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside α to carbon in the C-O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C-O hydrogenolysis of biomass-derived feedstocks.

Full text of DOI:10.1021/ja2038358

ARTICLE
pubs.acs.org/JACS
Selective Hydrogenolysis of Polyols and Cyclic Ethers over
Bifunctional Surface Sites on RhodiumꢀRhenium Catalysts
Mei Chia,† Yomaira J. Pagꢀan-Torres,† David Hibbitts,‡ Qiaohua Tan,‡ Hien N. Pham,§ Abhaya K. Datye,§
Matthew Neurock,‡ Robert J. Davis,‡ and James A. Dumesic*,†
†Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin
53706, United States
‡Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, Post Office Box 400741, Charlottesville, Virginia
22904-4741, United States
§Department of Chemical and Nuclear Engineering and Center for Microengineered Materials, University of New Mexico, MSC 01
1120, Albuquerque, New Mexico 87131-0001, United States
S Supporting Information
b
ABSTRACT: A ReOx-promoted Rh/C catalyst is shown to be
selective in the hydrogenolysis of secondary CꢀO bonds for a
broad range of cyclic ethers and polyols, these being important
classes of compounds in biomass-derived feedstocks. Experi-
mentally observed reactivity trends, NH3 temperature-programmed
desorption (TPD) profiles, and results from theoretical calculations
based on density functional theory (DFT) are consistent with the
hypothesis of a bifunctional catalyst that facilitates selective hydro-
genolysis of CꢀO bonds by acid-catalyzed ring-opening and
dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % RhꢀReOx/C
(1:0.5) was confirmed by NH3 TPD, and the estimated acid site density and standard enthalpy of NH3 adsorption were 40 μmol gꢀ1
and ꢀ100 kJ molꢀ1, respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with
rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton
donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with
the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups
that reside R to carbon in the CꢀO bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize
the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new
class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the
selective CꢀO hydrogenolysis of biomass-derived feedstocks.
In this contribution, we first present a catalyst pretreatment
strategy for obtaining a highly active and stable catalyst consisting
of rhodium and rhenium supported on carbon (RhꢀReOx/C).
This catalyst is then demonstrated to be selective for the
hydrogenolysis of not only 1 and 3, but more importantly, a
wide variety of CꢀO hydrogenolysis reactions previously un-
reported in the literature. Mechanistic concepts are then for-
mulated regarding the nature of the active sites for this catalyst
system for the hydrogenolysis of cyclic ethers and polyols. In this
respect, it is shown that the experimental reactivity trends from
this study can be understood in terms of results from density
functional theory (DFT) calculations involving the deprotona-
tion energies (DPEs) of ꢀOH groups associated with RhꢀReOx
clusters, combined with the relative stabilities and reaction
energies of carbenium structures formed by acid-catalyzed reac-
tions and by dehydration and ring-opening activation barriers at
1. INTRODUCTION
A central challenge in biomass conversion lies in developing
catalysts for the selective deoxygenation of highly functionalized
molecules, such as sugars, polyols, and cyclic ethers, often under
conditions where water is used as the solvent.1,2 Catalytic deoxy-
genation may be accomplished through reactions such as decarbox-
ylation, decarbonylation, dehydration, and CꢀO hydrogenolysis. Of
these reactions, selective CꢀO hydrogenolysis of polyols and cyclic
ethers over heterogeneous catalysts represents an important class of
reactions for the production of high value chemicals, such as the
conversion of 2-(hydroxymethyl)tetrahydropyran 1 to 1,6-hexane-
diol 2. Recent experimental work suggests that platinum, ruthenium,
rhodium, and iridium catalysts promoted with rhenium display not
only high activity, but also high selectivity in the hydrogenolysis of
tetrahydrofurfuryl alcohol 3,3 1,4 and glycerol 45ꢀ8 to their corre-
sponding R,ω-diols. However, despite this growing body of experi-
mental work, a systematic understanding remains elusive of the
fundamental reaction mechanisms governing the rate and selectivity
of CꢀO bond scission reactions over these interesting catalysts.
Received: April 26, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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Table 1. Hydrogenolysis of Tetrahydrofurfuryl Alcohol 3 and 2-(Hydroxymethyl)tetrahydropyran 1 over Oxide-Promoted Rh
Catalysts and Monometallic Catalystsa
product selectivity
catalyst:
specific rateb
(μmol gꢀ1minꢀ1
)
catalyst
reactant (g:g)
time (h)
4
reactant
conversion (%)
47.2
compound
(%)
4 wt % RhꢀReOx/C (1:0.5)
1:9
3
1,5-pentanediol
1-pentanol
1,6-hexanediol
1-hexanol
1,5-pentanediol
1-pentanol
1-butanol
othersc
97.2
2.8
180
90
1
3
27.3
51.6
97.0
3.0
4 wt % Rh-MoOx/C (1:0.1)
2:7
4
91.3
3.2
72
0.1
5.4
1
3
25.8
8.5
1,6-hexanediol
1-hexanol
1-pentanol
othersc
88.6
3.5
32
12
7.2
0.7
4 wt % Rh/C
2:7
4
1,5-pentanediol
1,2-pentanediol
1-pentanol
2-pentanol
othersc
59.1
20.7
7.1
10.0
3.1
1
3.3
1,6-hexanediol
1,2-hexanediol
1-hexanol
othersc
43.5
11.1
2.8
4
42.6
ꢀ
3.6 wt % ReOx/C
1.8 wt % MoOx/C
2:7
2:7
12
12
3
1
3
1
NRd
NRd
NRd
NRd
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a Reaction conditions: 393 K, 34 bar H2. Feed mixtures were 5 wt % reactant in water. b Specific rate defined as the moles of 1 or 3 reacted per gram of
catalyst per minute. c Alkanes in gas phase and monoalcohols at trace levels. d NR: no reaction.
ReOH acid sites on a model 201-atom RhRe cluster. More
generally, experimental and theoretical results from this work can
be extended to provide a consistent description of a class of
bifunctional heterogeneous catalysts, based on the combination
of a highly reducible metal with an oxophilic metal, for the
selective CꢀO hydrogenolysis of biomass-derived feedstocks.
hydrogenolysis rate of 3 was increased 15-fold over 4 wt %
RhꢀReOx/C (1:0.5) compared to the monometallic 4 wt % Rh/
C catalyst. The promoting effect of MoOx is not as pronounced
as for ReOx: only a 6-fold increase in the hydrogenolysis rate of 3
was observed for 4 wt % Rh-MoOx/C (1:0.1). Importantly, the
ReOx or MoOx promoters lead to a remarkable enhancement of
selectivities to the R,ω-diols for both cyclic ethers: scission of the
CꢀO bond was observed to occur primarily at the more sterically
hindered secondary carbonꢀoxygen bond for the promoted Rh
catalysts, resulting in high selectivities to the respective R,ω-
diols. This behavior is in contrast to nonselective CꢀO hydro-
genolysis observed over monometallic 4 wt % Rh/C, as well as
metal-catalyzed ring-opening of substituted cyclopentane and
cyclohexane derivatives over Pt and other transition metals which
preferentially occur at the least-substituted carbonꢀcarbon
bonds.9 The increases in hydrogenolysis activity and selectivity
for ReOx-promoted Rh/C are in agreement with results reported
by Chen, et al.4 Experiments using monometallic 3.6 wt %
ReOx/C, and 1.8 wt % MoOx/C catalysts indicate that the Re
and Mo promoters in the absence of a highly reducible metal,
such as Rh, do not possess hydrogenolysis activity, and this result
is consistent with the literature.3,4,10
2. RESULTS AND DISCUSSION
2.1. Catalyst Formulation and Initial Reactivity Studies.
A carbon-supported Rh catalyst was modified with two different
oxophilic promoters, namely ReOx and MoOx, and initial reac-
tion kinetics studies were carried out using 1 as the reactant
(Table S1, Supporting Information [SI]) to identify the amount
of promoter required to maximize catalytic activity. For both
promoters used, the specific activity of the promoted Rh catalysts
passes through a maximum as the amount of promoter is
increased. Catalysts consisting of 4 wt % RhꢀReOx/C (1:0.5
atomic ratio) and 4 wt % Rh-MoOx/C (1:0.1 atomic ratio) were
found to be the most active materials, and these catalysts were
selected for use in subsequent reaction kinetics studies.
The effect of oxophilic promoters, such as ReOx or MoOx, on
the CꢀO hydrogenolysis activities of Rh catalysts is evident in
experiments with reactants 1 and 3 (Table 1). Specifically, the
2.2. Catalyst Characterization. Temperature-programmed
reduction (TPR) profiles of dried, unreduced catalysts suggest
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that contact between Rh and the Re and Mo promoters is
established during catalyst preparation (Figure 1), in view of
the observation that the H2 consumption peaks for 4 wt %
RhꢀReOx/C (1:0.5) and 4 wt % Rh-MoOx/C (1:0.5) coincide
with the H2 consumption peak of monometallic 4 wt % Rh/C, in
agreement with results from the literature.11ꢀ13 These TPR
profiles show that the presence of Rh lowers the reduction
temperature of the Re and Mo precursors, suggesting the
formation of metal alloys.14,15 Quantitative analysis of H2 con-
sumption by 4 wt % RhꢀReOx/C (1:0.5) reveals that Re is
present in a highly reduced state after reduction at temperatures
above 600 K: approximately 0.24 mmol H2 was consumed
compared to 0.25 mmol H2 required to completely reduce the
Rh and Re to the metallic state. The low valence of Re in 4 wt %
RhꢀReOx/C (1:0.5) after reduction with H2 is consistent with
the literature.4,16
metallic nanoparticles typically contained both Rh and Re, as
seen in electron dispersive X-ray spectroscopy (EDS) micro-
analyses of Figure 2. The average metallic particle size of the 4 wt
% RhꢀReOx/C (1:0.5) catalyst before and after reaction was
estimated to be 2.1 nm from HAADF-STEM analysis of over
700ꢀ900 randomly selected particles, as displayed in Figure 3.
The extent of irreversible CO adsorption at 298 K on the
various supported carbon catalysts was used to estimate the
dispersions of the metallic particles (where dispersion is defined
as the fraction of the metallic atoms that are on the surface), and
these results are shown in Table 2. The metal dispersion was
found to be approximately 50% for both monometallic 4 wt %
Rh/C and 4 wt % RhꢀReOx/C (1:0.5). The particle size of the
as-prepared 4 wt % RhꢀReOx/C (1:0.5) catalyst determined
from metal dispersion values was found to be in the range of
2.2ꢀ2.3 nm (Table 2).
High angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) studies of the 4 wt % RhꢀReOx/C
(1:0.5) catalyst before and after reaction revealed that the
In general, estimates of the metal particle sizes obtained from
HAADF-STEM and metal dispersion values compare well with
one another, and indicate that no significant changes in particle
size take place upon exposure to reaction conditions.
Temperature-programmed desorption (TPD) profiles for
NH3 from reduced catalysts indicate that acid sites are present
on 4 wt % RhꢀReOx/C (1:0.5) (Figure 4). All catalyst samples
were pretreated in flowing H2 at 523 K for 4 h prior to the
adsorption of NH3. As shown in Figure 5, the NH3 desorption
profile for 4 wt % RhꢀReOx/C (1:0.5) displays a peak with
maximum at 498 K. The number of acid sites on 4 wt %
RhꢀReOx/C (1:0.5) was 40 μmol gꢀ1, and number ratio of
acid to metal sites was equal to 0.28. Assuming that the standard
entropy change of adsorption (ΔS°ads) is approximately ꢀ150 J
Kꢀ1 molꢀ1 20,21
which corresponds to loss of the gas-phase
,
translational entropy, and that first-order desorption and rapid
readsorption occurs, the heat of adsorption of NH3 (ΔH°ads) on
4 wt % RhꢀReOx/C (1:0.5) can be estimated22 to be ꢀ100 kJ
molꢀ1. In contrast, no discernible NH3 desorption peaks were
observed for monometallic 4 wt % Rh/C and 3.6 wt % Re/C
under the conditions employed. The lack of acidity of Re/C here
could be due to a combination of low Re loading and poor metal
dispersion.
Figure 1. Temperature-programmed reduction profiles for (a) 4 wt %
Rh/C, (b) 3.6 wt % Re/C, (c) 1.8 wt % Mo/C, (d) 4 wt % RhꢀRe/C
(1:0.5), (e) 4 wt % RhꢀMo/C (1:0.5).
Figure 2. Representative HAADF STEM images and EDS spot-beam analysis results of (a) as-prepared and (b) spent 4 wt % RhꢀReOx/C (1:0.5)
catalysts.
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Figure 3. Representative HAADF-STEM images and particle size distribution histograms for (a, c) as-prepared and (b, d) spent 4 wt % RhꢀReOx/C
(1:0.5) catalysts. Number-averaged particle sizes are presented in the figures.
Table 2. Quantification of CO Adsorption on Monometallic and Promoted Rh Catalysts at 298 K
Rh loading Re loading
pretreatment
irreversible CO uptake
dispersion particle size
catalyst
(wt%)
(wt%)
Rh:Re (mol:mol) temperature (K)
(μmol gꢀ1
)
CO:Rh (mol:mol)
(%)a
(nm)b
Rh/C
Re/C
4
ꢀ
4
ꢀ
ꢀ
ꢀ
393
723
393
523
723
393
723
151
29
0.39
0.15c
0.38
0.39
0.36
0.02
0.20
52
ꢀ
2.1
ꢀ
3.6
3.6
RhꢀRe/C (as-prepared)
1:0.5
143
145
135
7
51
51
48
ꢀ
2.2
2.2
2.3
ꢀ
RhꢀRe/C (spent)
4
3.6
1:0.5
74
ꢀ
ꢀ
a Dispersion calculated with respect to total Rh loading and assuming 0.75 ML coverage of CO at full saturation. b Particle size = 1.1/dispersion; factor of
1.1 calculated from the literature.17ꢀ19 c CO:Re.
2.3. Catalyst Pretreatment and stability. Rhenium is ox-
ophilic and some of its oxides (e.g., Re2O7 and ReO3) are soluble
in water, leading potentially to the leaching of rhenium from
the catalyst under aqueous reaction environments and to possi-
ble homogeneous catalysis. To optimize catalyst pretreatment
conditions for maximum activity and stability of the 4 wt %
RhꢀReOx/C (1:0.5) catalyst, the effect of catalyst reduction
temperature on the rate of hydrogenolysis for reactant 1 was
first determined, and the extent of rhenium leaching monitored
(Table S2, SI). These experiments were conducted in a batch
reactor, and pretreatment of the dry catalyst was performed
under H2 atmosphere at temperatures of 393 and 523 K prior
to addition of the aqueous feed. After reaction for 4 h at
393 K, the reactor was cooled to 298 K, the vessel was
depressurized, and the reaction solution was filtered to remove
the catalyst. Elemental analysis of the filtrates was performed
by inductively coupled plasma atomic emission spectroscopy
(ICP-AES). The results in Table S2 (SI) show that the catalytic
activity and the extent of rhenium leached into the reaction
solution were highest without pretreatment of the catalyst (2%)
or with a low temperature pretreatment (393 K, 1%). The extent
of Re leaching was substantially lower (<0.5%, which is at the
detection limit of our system) after pretreatment at 523 K, and
therefore, this condition was chosen for use in continuous flow
experiments to further study catalyst stability versus time-on-
stream.
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7 > tetrahydrofuran 8. For both classes of cyclic ethers, the
absence of the hydroxyl group leads to a significant decrease in
CꢀO hydrogenolysis activity. These results suggest that the
hydroxyl groups in 1 and 3 are responsible for the high CꢀO
hydrogenolysis rates and selectivities to their respective R,ω-
diols over 4 wt % RhꢀReOx/C (1:0.5). This selectivity toward
ring-opening at the more substituted carbon center is not
consistent with the preferential ring-opening of the less sterically
hindered sites of substituted cyclohexane and cyclopentane
intermediates over metal surfaces.9 As is described below in
regard to the ring-opening of compounds 5 and 7, metal-
catalyzed ring-opening first occurs through dehydrogenation,
causing the unsubstituted sites to be preferentially activated.
We suggest instead that the experimentally observed hydro-
genolysis selectivity trends are consistent with a bifunctional
mechanism in which the catalyst facilitates initial acid-catalyzed
ring-opening at the more substituted carbon center followed by
metal-catalyzed hydrogenation. To elucidate the observed differ-
ences in reactivity and product selectivity, a quantitative measure
of the reactivity of these different molecules over solid acids is
needed. Accordingly, a Born cycle analysis can be used to
establish a relationship between the changes in the activation
barrier and changes in the catalyst or the molecules reacted. More
specifically, the activation barrier (Ea) for a reaction can be
written in terms of the deprotonation energy (DPE) of the acid,
which provides a measure of solid acidity, the gas-phase carbe-
nium ion reaction or formation energies (ΔHrxn(R+)), and the
interaction energy (ΔEint) between the gas-phase carbenium ion
and the negative charge of the conjugate base that forms in the
transition state:23,24
Figure 4. NH3 temperature-programmed desorption profiles for (a)
3.6 wt % Re/C, (b) 4 wt % Rh/C, and (c) 4 wt % RhꢀRe/C (1:0.5).
Catalyst samples were pretreated in flowing H2 (100 cm3 (STP) minꢀ1
)
at 523 K for 4 h prior to dosing of NH3.
Ea ¼ DPE þ ΔHrxnðRþÞ þ ΔEint
ð1Þ
As the catalyst is identical for all of the reactions considered in
Table 3, the reactions can therefore be described by the changes
in the gas-phase carbenium ion reaction energies, ΔHrxn(R+), and
changes in the interaction energies, ΔEint. If the catalyst is the
same, the changes in the interaction energies that result from
different reactants are often linearly related to the changes in the
proton affinity of the reactant or the gas-phase carbenium ion
reaction energies.25 The acid-catalyzed CꢀO activation is
thought to proceed via the simultaneous protonation of the
oxygen atom in the ring and ring-opening, thus resulting in the
formation of a secondary carbenium which can be stabilized
through a Coulombic interaction between the positive charge on
the carbenium ion center and the neighboring oxygen on the
hydroxyl group through a back-side interaction as shown in
Figure 6. This secondary carbenium ion intermediate can rapidly
undergo a hydride transfer with the neighboring ꢀCHꢀOH
intermediate to form the more stable RCH2ꢀCdO(+)H oxo-
carbenium intermediate depicted in Figure 6 which more effec-
tively stabilizes the charge on the oxygen atom. Alternatively, the
ring-opening can proceed directly with a concerted hydride
transfer from the neighboring CHꢀOH group to form the more
stable RCH2ꢀCdO(+)H intermediate. While we have calcu-
lated the reaction energies to form the oxirane (3-membered
ring) and oxetene (4- membered ring) as well as the RCH2ꢀ
CdO(+)H oxocarbenium ions that result from the stabilization
of the OH intermediates (see Table 4), we herein discuss
only the results for the energetically more stable oxocarbenium
ion intermediates. To further demonstrate and ensure the
validity of gas-phase carbenium ion calculation results, more
Figure 5. Results for the hydrogenolysis of 2-(hydroxymethyl)
tetrahydropyran 1 over 4 wt % RhꢀReOx/C (1:0.5) in a continu-
ous flow system. Conversion of 1 (9), selectivities to 1,6-hexanediol
2 (O) and 1-hexanol (2), and reaction rates ( ꢁ ) at 393 K, 34 bar H2,
WHSV = 0.52 hꢀ1, feed: 5 wt % 1 in water. The catalyst was pretreated in
flowing H2 (60 cm3 (STP) minꢀ1) at 523 K for 4 h and cooled to the
reaction temperature prior to initiation of liquid feed flow.
Figure 5 shows results for the hydrogenolysis of 1 over 4 wt %
RhꢀReOx/C (1:0.5) in a continuous packed-bed reactor system.
The catalyst was pretreated under flowing H2 for 4 h at 523 K
prior to initiating the flow of liquid feed. Under these continuous
flow conditions, the performance of the catalyst stabilized at
conversion level of 32% and a selectivity to 2 of 90%. After 120 h
time-on-stream, the activity of the catalyst declined by only 16%.
The stability of the catalyst under continuous liquid flow condi-
tions provides definitive evidence that we have established
catalyst pretreatment and reaction conditions under which 4
wt % RhꢀReOx/C (1:0.5) functions fully as a heterogeneous
catalyst.
2.4. Hydrogenolysis of Cyclic Ethers. To better understand
the high hydrogenolysis selectivities of 1 to 2 over 4 wt %
RhꢀReOx/C (1:0.5), the hydrogenolysis rates of cyclic ethers
with different functional groups were studied in a batch reactor,
and the results are shown in Table 3. The reactivity trend for the
six-membered cyclic ethers was observed to be 1 . 2-methylte-
trahydropyran 5 > tetrahydropyran 6. The reactivity trend of the
five-membered ring cyclic ethers is analogous to that of the six-
membered ring molecules, where 3 . 2-methyltetrahydrofuran
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Table 3. Hydrogenolysis of Cyclic Ethers and Polyols over 4 wt % RhꢀReOx/C (1:0.5)a
a Reaction conditions: 393 K, 34 bar H2, mass ratio of catalyst:reactant = 1:9. b Specific rate defined as the moles of reactant consumed per gram of
catalyst per minute. Total specific rates are divided by two to account for symmetry in molecules 11, 12, 15, 16, and ethylene glycol. c NR: no reaction.
d Mass ratio of catalyst:reactant = 2:7. e Alkanes in gas phase and monoalcohols at trace levels. f Ring-closing rate denoted in parentheses. g Catalyst used
was 4 wt % Rh/C.
detailed analyses of the reaction paths and corresponding
activation barriers were performed for the five-membered
ring structures 3 and 7 over a model Rh200Re1OH cluster, as
outlined below.
The results from DFT calculations for simple gas-phase
carbenium ion reactionenergies presented in Figure6 andTable 4
show that the high selectivities for ring-opening 1 and 3 to the
R,ω-diol can be attributed to the greater stability (30 kJ molꢀ1) of
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Figure 6. DFT-calculated carbenium and oxocarbenium ion formation or reaction energies for 2,3-butanediol 16, 2,4-pentanediol 15, and
tetrahydrofurfuryl alcohol 3. The dotted lines refer to sequential paths which proceed via the formation of the OH-stabilized three (oxirane) and
four (oxetene) ring intermediates that subsequently form the corresponding oxocarbenium ion intermediates, whereas the solid lines refer to concerted
protonation/hydride transfer steps that result in the direct formation of the oxycarbenium ion intermediates.
the secondary carbenium ion structures leading to the R,ω-diol,
versus the primary carbenium ion structures which result in the
formation of the 1,2-diol. Furthermore, the presence of a hydroxyl
group at the R or β position allows for the concerted protonation
and hydride transfer to form the more stable oxocarbenium
(RCH2ꢀCH=O(+)H) ion intermediate. For cyclic ethers, the
presence of the hydroxyl substituent R to the dehydration center
is shown to increase the stability of the carbenium ion, such that
the carbenium ion formation energies for 1 and 3 are ꢀ845 and
ꢀ852 kJ molꢀ1, respectively, while the corresponding structures
without the RꢀOH substituent, 5 and 7, are significantly less
stable at ꢀ742 and ꢀ743 kJ molꢀ1, respectively (Figure 6 and
Table 4). This trend was confirmed by more rigorous transition
state simulations over model Rh200Re1OH clusters, displayed in
Figure 7, which show that the activation barrier to ring-open 3
(Figure 7A) is lower by 14 kJ molꢀ1 compared to the barrier to
open 7 (Figure 7C) as a result of the back-side stabilization of the
positive charge in the carbenium ion transition state by the OH
group and by stabilization with water. The concerted protonation
and hydride transfer reaction to form the oxocarbenium ion
transition state lowers the activation barrier by an additional
16ꢀ84 kJ molꢀ1 as shown in the Results displayed in Figure 8.
While these calculations explain the selectivities reported in
Table 3, the absolute values of the activation barriers are likely
overpredicted due to the simplicity of the model system; both the
presence of water and acid sites stronger than those in the
modeled system would lead to lower activation barriers.
The ring-opening of cyclic ethers 5 and 7 is characteristically
different from the functionalized cyclic structures discussed
above, in that the CꢀO activation occurs at the sterically less-
hindered primary carbonꢀoxygen bond, leading to the forma-
tion of the corresponding secondary alcohols (i.e., 2-hexanol 9
and 2-pentanol 10, respectively). This preferential hydrogeno-
lysis of 7 at the primary carbonꢀoxygen bond is similar to that
observed by Gennari, et al. who performed gas-phase reactions
over unpromoted Pt catalysts.26,27 The activation of the ring for
these cyclic ethers, however, cannot be explained through acid
chemistry, as it would result in the formation of a highly unstable
primary carbenium ion. Instead, these experimental trends are
consistent with DFT results for ring-opening of cyclic ethers via a
solely metal-catalyzed route over Rh (Figure 9), showing that
metal-catalyzed insertion is favored at the less substituted CꢀO
bond by 30 kJ molꢀ1 due to steric hindrance at the substituted
CꢀO bond, preventing the initial dehydrogenation which leads
to CꢀO cleavage.
2.5. Hydrogenolysis of Polyols. To study further the role of
hydroxyl groups in the hydrogenolysis of neighboring CꢀO
bonds, the reactivity profiles for several straight-chain polyols
over the 4 wt % RhꢀReOx/C catalyst were examined, and these
results are shown in Table 3. For all diols studied, it is apparent
that 1,2-diols are more reactive than R,ω-diols. Furthermore, the
selectivity is significantly higher toward hydrogenolysis of the
hydroxyl group at the secondary carbon atom for all 1,2-diols,
leading to the formation of the corresponding primary alcohol as
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Table 4. DFT Calculated Gas-Phase Reaction Energies for the Formation of Carbenium and Oxocarbenium Ion Intermediates
Involved in the Ring-Opening of Cyclic Ethers and Dehydration of Polyols
the predominant product. The high reactivity for hydrogenolysis
of a secondary hydroxyl group adjacent to a primary hydroxyl
group is also exhibited in the reactivity trends of the triols. In
addition, a secondary hydroxyl group in close proximity to
another secondary hydroxyl group shows high reactivity for
undergoing hydrogenolysis over 4 wt % RhꢀReOx/C (1:0.5).
These reactivity trends show that the position of hydroxyl groups
in a reactant molecule is instrumental in dictating its hydro-
genolysis reactivity over 4 wt % RhꢀReOx/C (1:0.5).
reactants. The higher reactivity and selective hydrogenolysis of
1,2-diols, such as 1,2-pentanediol 13 and 1,2-butanediol 14, at
the secondary CꢀO bond can be explained by the formation of
the more stable RCH2ꢀCH=O(+)H oxocarbenium ion inter-
mediates resulting from the concerted elimination of water and
hydride transfer from the primary ꢀCH2OH group, as reported
in Table 4. Additionally, the higher reactivities of β-diols such as
2,4-pentanediol 15 and 2,3-butanediol 16 over that for 1,2-diols
can be similarly explained by the formation of the secondary
oxocarbenium ions in 15b and 16b which are more than 100 kJ
molꢀ1 more stable than the secondary carbenium ions in 15a
and 16a.
As previously shown for cyclic ethers, the above experimental
trends for polyols are also consistent with a bifunctional
catalyst which facilitates initial acid-catalyzed carbenium ion
chemistry followed by metal-catalyzed hydrogenation. The low
hydrogenolysis rates of R,ω-diols 2, 1,5-pentanediol 11, and 1,4-
butanediol 12 are in agreement with the lower stability of primary
carbenium ions which would form upon dehydration of these
2.6. Governing Principles of Substrate Reactivity and
Selectivity. The experimental reactivity and selectivity trends
shown in Table 3 are in agreement with the carbenium ion
formation energies presented in Table 4. The reactants studied
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Figure 7. DFT-calculated reactant and transition state structures and corresponding activation barriers for the acid catalyzed ring-opening of: (a) water-
stabilized tetrahydrofurfuryl alcohol 3, (b) tetrahydrofurfuryl alcohol 3 and (c) 2-methyltetrahydrofuran 7 on a model Rh200Re1OH cluster.
Figure 8. DFT-calculated reactant and transition state structures for the concerted protonation, hydride transfer and ring-opening of the water-
stabilized tetrahydrofurfuryl alcohol 3 over a model Rh(111) surface with well dispersed ReꢀOH sites.
are ranked from the most to the least reactive in Table S3 (SI),
along with their corresponding carbenium ion reaction energies.
The results indicate that the most reactive molecules are the
cyclic ethers which contain hydroxyl substituents that are located
R to the carbon atom in the CꢀO bond where the ring is opened.
These reactants can undergo concerted ring-opening and hy-
dride transfer from the neighboring ꢀCH2OH group to form
stable primary oxocarbenium ion transition state structures
which are further stabilized by the OH groups that result upon
protonation of the ring. As such, structures 1 and 3 are two of the
most reactive structures examined with specific reaction rates of
90 and 180 μmol gꢀ1 minꢀ1, respectively.
β-Diols (15 and 16) also lead to stable oxocarbenium ion
structures that can effectively stabilize the carbenium ion charge
that results upon dehydration. These structures appear to be just as
reactive as those for 1 and 3 with specific rates of 58 (15) and 78
(16) μmol gꢀ1 minꢀ1. In agreement with this experimental
finding, the theoretical results show similar carbenium ion forma-
tion energies. The structures of carbenium ions from linear polyols
with R,βꢀOH groups are less stable, as the oxocarbenium ions
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that form are now delocalized over primary carbon atoms in the
oxocarbenium ion structure without the additional OH stabiliza-
tion in the transition state; the reactivities of these reactants
lie between 23 and 62 μmol gꢀ1 minꢀ1. Linear polyols that do
not contain R,β or β,βꢀOH substituents are unable to form
oxocarbenium ion structures and must effectively stabilize the
charge on the secondary carbon centers. As such the reactivities
of these reactants are significantly lower and found to be in the
range of 10 μmol gꢀ1 minꢀ1. Finally, all of the primary carbenium
ion centers from linear and cyclic polyols are unstable, and as such
these reactants do not show significant reactivity.
The results from Table S3 (SI) (with the exception of the data
for the ring-opening of 5 and 7 which are metal-catalyzed
reactions, and data from the unreactive R,ω-polyols) are plotted
in Figure 10, showing a correlation between the carbenium ion
formation energy and the logarithm of the normalized rate
expressed as the ratio between specific reaction rate (μmol gꢀ1
minꢀ1) and initial reactant concentration (μmol mLꢀ1). This
ratio is employed to reflect an appropriately normalized reaction
rate which is independent of the minor differences in the initial
molar concentrations of reactants (see below). It is also notable
that specific rate values were divided by two to account for
molecules with a plane of symmetry, namely, 11, 12, 15, 16, and
ethylene glycol. The differences in reactivity between unsubsti-
tuted (primary carbenium ions), substituted (secondary), and
substituted and OH-stabilized cyclic ethers and polyols are
highlighted by different colored regions in Figure 10.
We note here that we have included glycerol in the correlation
of Figure 10, showing that the overall reactivity for conversion of
this molecule is controlled by acid catalysis, consistent with the
high selectivity for the formation of 1,3-propanediol 17, com-
pared with the metal-catalyzed route over Rh/C. It is interesting
that the rate of formation of 1,2-propandiol over the RhꢀReOx/
C catalyst is also much higher than the corresponding reaction
over Rh/C, suggesting that an acid-catalyzed pathway exists
involving a secondary carbenium ion along the way to form the
1,2 oxirane that can subsequently react to form both 1,2 and 1,3
propanediol products close to the relative percentages that are
seen experimentally.
2.7. Reaction Kinetics. Although it was impractical to evaluate
the complete reaction rate expression for all 15 of the different
substrates in Table 4, we evaluated the reaction kinetics for our
prototypical cyclic ether, 2-(hydroxymethyl)tetrahydropyran
(compound 1), and made the reasonable assumption that other
substrates in this study will behave similarly. Importantly, we
used a continuous flow reactor for this study to eliminate any
possible complications that could be caused by initial transient
phenomena (e.g., catalyst deactivation, initial leaching of rhenium)
that could take place in a batch reactor. The results of these
flow reactor studies, presented in Table S4 (SI), show that the
reaction is approximately first order with respect to species 1 and
slightly less than first order (i.e., approximately equal to 0.9) with
Figure 9. DFT-calculated reaction path and energies for the metal-
catalyzed ring-opening of 2-methyltetrahydrofuran 7 at the substituted
and unsubstituted carbon centers over Rh.
Figure 10. Comparison of the normalized rate, defined as the ratio between specific reaction rate (μmol gꢀ1 minꢀ1) and initial reactant concentration
(μmol mLꢀ1) (on a logarithmic scale), and DFT-calculated carbenium and oxocarbenium ion energies for selected cyclic ethers and linear polyols. Each
region shows distinct shifts in activity due to the stability of the carbenium and oxocarbenium ions as discussed in Table S3 (SI).
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Figure 11. DFT-calculated deprotonation energies for various surfaces and atom clusters.
respect to hydrogen pressure. Because the reactivity trends of this
paper indicate that acid sites must be involved in a kinetically
significant part of the overall reaction scheme, one might have
expected the reaction to be zero order in hydrogen. However, the
observed positive order is consistent with the bifunctional nature
of the catalyst and the complexity of the mechanism, as illustrated
by Boudart and Djega-Mariadassou in studies of the influence of
hydrogen on the kinetics for isomerization of n-pentane to
isopentane over a bifunctional Pt/alumina catalyst (containing
metal sites and acid sites), even though hydrogen does not appear
in the overall stoichiometric equation of alkane isomerization.28
In that example, the rate determining step is the acid-catalyzed
isomerization of n-pentene to i-pentene, and the observed
effect of the hydrogen pressure is caused by the equilibrium of
n-pentane with n-pentene on the metal sites prior to the rate
determining acid-catalyzed isomerization step. In our case, we
observed positive order in hydrogen resulting from the required
hydrogenation of an unsaturated intermediate after the acid-
catalyzed ring-opening step. In addition, it is possible that a
positive effect of the hydrogen pressure is also caused by the
hydrogenation of RedO species to form the acidic ReꢀOH
groups that are required for the overall reaction.
In summary, a simple first order reaction rate constant cannot
be calculated for the reactions in this paper since they occur over
a bifunctional catalyst containing acid sites and metal sites, both
of which contribute to the overall reaction. However, because the
reaction appears to be approximately first order with respect to
the concentration of the organic reactant, we have instead chosen
to correlate observed rates divided by the reactant concentration,
to approximate the rate at nearly identical experimental condi-
tions. The trend in Figure 10 of this normalized rate versus the
calculated carbenium ion formation energies is surprisingly good,
given the fact that rates vary by more than an order of magnitude
across the series of compounds.
2.7. Role of Rhenium and Nature of the Active Site. We
suggest that the importance of acid chemistry over the rho-
diumꢀrhenium catalyst originates from the Brønsted acidity
of hydroxylated Re atoms on RhꢀRe nanoparticles, arising from
strong ReꢀO bonds, resulting in a weak OꢀH bond as well
as high electron affinity for the conjugate base. Other solid
acids such as zeolites23 and heteropolyacids (HPAs)29 achieve this
high electron affinity through the distribution of the negative
charge across multiple oxygen atoms. As shown in Figure 11,
hydroxylated rhenium species on RhꢀRe nanoparticles demon-
strate deprotonation energies at the corner, edge and terrace sites
of 1140 kJ molꢀ1, as compared to values of 1050ꢀ1200 kJ molꢀ1
for well-known solid acids such as zeolites23 and HPAs.29
A more detailed analysis of the electronic structure shows that
there is electron transfer from Re to the oxygen. Also, it is evident
from Figure 11 that the DFT-predicted acidities of hydroxylated
molybdenum species on RhꢀMo nanoparticles are markedly
lower than that for RhꢀRe, consistent with the lower hydro-
genolysis rates observed of 1 over the 4 wt % Rh-MoOx (1:0.1)
catalyst (Table 1). Further experimental evidence of the presence
of surface acid sites on the RhꢀRe catalyst is provided by
results from NH3 TPD studies (Figure 4) which show that the
acid site density over this catalyst is approximately 40 μmol gꢀ1
.
Additionally, we found that the hydrogenolysis of 1 over
monometallic Rh/C in the presence of 0.1 M H2SO4 or HCl
proceeds at rates and selectivities to 2 which are significantly
lower than that over RhReOx/C (Table 5). The above results
thus suggest that the active sites for RhReOx/C are located at the
catalyst surface in the form of hydroxylated rhenium near
rhodium.
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Table 5. Effect of Homogeneous Acid and Base on Hydrogenolysis Rate and Product Selectivity of 2-(Hydroxymethyl)-
tetrahydropyran 1 over Rh-Containing Catalystsa
selectivity to
selectivity to
specific rateb
(μmol gꢀ1 minꢀ1
)
catalyst
time (h)
conversion (%)
1,6-hexanediol 2 (%)
1,2-hexanediol (%)
4 wt % RhꢀReOx/C (1:0.5)
4 wt % RhꢀReOx/C (1:0.5) and 0.1 M NaOH
4 wt % Rh/C
4
4
27
NRc
NRc
13
97
ꢀ
0
ꢀ
ꢀ
1
90
ꢀ
ꢀ
9
20
20
20
20
ꢀ
4 wt % Rh/C and 0.1 M H2SO4
4 wt % Rh/C and 0.1 M HCl
4 wt % Rh/C and 0.1 M NaOH
41
32
ꢀ
6
NRc
7
4
ꢀ
ꢀ
a Reaction conditions: 393 K, 34 bar H2, mass ratio of catalyst: 1 = 1:9. Reactant mixtures were 5 wt % 1 in water. b Specific rate defined as the moles of 1
reacted per gram of catalyst per minute. c NR: no reaction
We propose that the bifunctional nature of the RhReOx/C
catalyst described above can be generalized to other transition
metal catalysts containing rhenium. For example, we have
recently reported the promotion of the hydrogenolysis of 4 on
Re-promoted Pt catalysts.8 In that work, addition of Re to the Pt
catalyst increased the turnover frequency of the hydrogenolysis
of 4 by a factor of 20, with 17 now appearing in the product
mixture. Indeed, the selectivity of the hydrogenolysis reaction to
17 over PtꢀRe was 34% at 443 K, a temperature at which Pt
alone exhibited only trace conversion of glycerol.
activity of RhꢀReOx/C decreases when the catalyst is pretreated
in flowing H2 at increasing temperatures. These results again
indicate that the active sites contain oxygen species associated
with the oxophilic Re promoter of the RhꢀReOx/C catalyst. We
note here that we have chosen a pretreatment temperature in H2
of 523 K for the RhꢀReOx/C catalyst in our studies (see
Figure 5), as a compromise between retaining oxygen species
associated with Re and also preventing leaching of highly
oxidized Re species into the aqueous solution during reaction.
Similarly, we observe here that the hydrogenolysis of 4 over 4 wt
% RhꢀRe/C (1:0.5) resulted in a marked increase in hydrogeno-
lysis rates and selectivities to 17 compared to the performance of
monometallic 4 wt % Rh catalyst (Table 3). The higher selectiv-
ities to 17 in the presence of rhenium are consistent with the
premise that monometallic Pt or Rh are only able to facilitate
metal-catalyzed CꢀO scission, while the addition of rhenium
results in a bifunctional catalyst which is also capable of acid
chemistry. King, et al.30 suggested the possible formation of acid
sites associated with the presence of ReOx clusters in PtRe/C
catalysts, and they suggested the possible influence of these species
in the aqueous phase reforming of glycerol. The basis of this
proposed acidity over PtꢀRe catalysts is clearly demonstrated by
the DFT data presented in Figure 11, wherein PtꢀRe nanopar-
ticles are shown to display acidities comparable to that for RhꢀRe
nanoparticles, suggesting that similar acid-catalyzed chemistry
would be displayed in both systems.
Our proposed reaction scheme involving acid-catalyzed
dehydration at the secondary carbon of a polyol, followed by hydro-
genation to form the R,ω-diol, is consistent with the exper-
imental result that the presence of NaOH completely suppressed
the hydrogenolysis activity of 1 over 4 wt % RhꢀReOx/C (1:0.5)
(Table 5). Also, previous work involving the hydrogenolysis of
4 over PtRe showed that addition of NaOH led to suppression of
20 formation.8 Other investigators have also shown that promo-
tion of Rh31 or Ru32 with Re enhances the formation of 17 from
4 in neutral water. Moreover, examination of the LIII edge of Re
in PtRe8 as well as RhRe31 bimetallic catalysts for the hydro-
genolysis of 4 confirmed that Re is not reduced entirely to the
metallic state after mild reduction in H2, but instead remains in a
partially oxidized state.
3. CONCLUSIONS
We have demonstrated in this work that a ReOx-promoted
Rh/C catalyst is selective for the hydrogenolysis of secondary
CꢀO bonds for a broad range cyclic ethers and polyols. It is also
shown that a catalyst pretreatment strategy of reducing the
catalyst in H2 at 523 K prior to contact with the aqueous reactant
results in the attainment of high catalytic activity and stability in a
continuous flow reaction system. Importantly, results from
experimentally observed reactivity trends, NH3 TPD, and DFT
calculations are consistent in supporting the hypothesis of a
bifunctional catalyst which facilitates acid catalyzed ring-opening
and dehydration coupled with metal-catalyzed hydrogenation.
The presence of surface acid sites on 4 wt % RhꢀReOx/C (1:0.5)
has been confirmed by NH3 TPD, and indicates that the acid site
density and standard enthalpy of NH3 adsorption are approxi-
mately 40 μmol gꢀ1 and ꢀ100 kJ molꢀ1, respectively. Results
from DFT calculations show that hydroxyl groups on rhenium
associated with rhodium are acidic, thus enabling proton dona-
tion to reactant molecules and formation of carbenium ion
transition states. The observed reactivity trends and DFT
calculations are also consistent with the enhancement in stability
of carbenium and oxocarbenium ion structures that form as a
result of protonation and concerted protonation and hydride
transfer steps, respectively. The experimentally obtained rates of
hydrogenolysis can be correlated with DFT-calculated carbe-
nium ion formation energies. Results from this study provide the
first consistent account for the bifunctional nature of active sites
in metal catalysts promoted with oxophilic additives for hydro-
genolysis reactions, providing guidance for the use of this class of
heterogeneous catalysts for the selective deoxygenation of bio-
mass to fuels and chemicals.
Finally, we note that it is possible to destroy the active sites for
RhꢀReOx/C catalysts by reduction in H2 at elevated tempera-
tures. In particular, the results in Figure 1 from temperature
programmed reduction studies indicate that the Rh and Re in the
RhꢀReOx/C catalyst are both highly reduced at temperatures
above about 600 K. In addition, we have found that the catalytic
4. METHODS AND MATERIALS
Catalyst Preparation. Catalysts were prepared by incipient wet-
ness impregnation of Vulcan XC-72 with aqueous solutions of RhCl3,
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NH4ReO4, and (NH4)6Mo7O24 4H2O. Rh/C catalysts with oxophilic
(i.e., 393 K), the aqueous reactant mixture pumped into the vessel using
a HPLC pump (LabAlliance, Series 1), and mechanical stirring of the
reaction mixture initiated.
3
additives were obtained by successive impregnation of dried, unreduced
Rh/C with the corresponding precursor of the oxophilic additive. The
loading amount of the second metal is stated as a atomic ratio (e.g., 4 wt
% RhꢀReOx/C (1:0.5) catalyst designates a metal atomic ratio of Rh:
Re = 1:0.5). Prior to use in experiments, catalysts were dried in air (393 K),
reduced in flowing H2 (723 K), and passivated with flowing 2% O2 in He
(298 K). Reduced and passivated catalysts are described as “as-prepared”
catalysts in this paper.
CO Adsorption. Prior to CO adsorption measurements, catalysts
were pretreated in 150 cm3(STP) minꢀ1 flowing H2 at 393, 523, or 723
K for 4 h. The CO adsorption uptakes at 298 K were measured on a
standard gas adsorption apparatus described elsewhere.33
Continuous Flow Reaction Experiments. The flow reactor was
a 6.4 mm (0.25 in.) outer diameter stainless steel tube with wall thickness
of 0.7 mm (0.028 in.). The catalyst bed consisted of reduced and
passivated catalyst (0.35 g) loaded between a quartz wool plug and fused
SiO2 granules (ꢀ4 + 16 mesh, Sigma-Aldrich). The reactor was heated
with a furnace composed of close-fitting aluminum blocks externally
heated by an insulated furnace (1450 W/115 V, Applied Test Systems
series 3210), and temperature was monitored using a K type thermo-
couple (Omega) placed outside the reactor immediately next to the
position of catalyst bed. Temperature was controlled with a PID
temperature controller (Love Controls, series 16A). Liquid feed solution
was introduced into the reactor with cofeeding of H2 in an up-flow
configuration. A HPLC pump (LabAlliance, series 1) was used to
control the liquid feed rate. Gas flow was controlled with a mass-flow
controller (Brooks 5850 model), and the system pressure was main-
tained at 34 bar by passing the effluent gas stream through a back-
pressure regulator (GO Regulator, model BP-60). The effluent liquid
was collected in a gasꢀliquid separator and drained periodically for
analysis by HPLC. Carbon balances closed within 5% for all data points.
Analytical Methods. Quantitative analyses were performed using
a Waters 2695 separations module high performance liquid chromatog-
raphy instrument equipped with a differential refractometer (Waters
410), a photodiode array detector (Waters 996), and an ion-exclusion
column (Aminex HPX-87H, 300 mm ꢁ 7.8 mm). 0.005 M H2SO4 at a
flow-rate of 0.6 mL minꢀ1 was used as the mobile phase. Gas-phase
products from batch reactions were collected in a gas bag and analyzed
using a Varian GC (Saturn 3) equipped with a FID detector and a GS-Q
column (J&W Scientific). The presence of rhenium in solution was
detected using inductively coupled plasma atomic emission spectro-
metry (ICP-AES).
Temperature-Programmed Reduction and NH3 Desorp-
tion. Temperatureꢀprogrammed experiments were carried out using
an apparatus comprising of a tube furnace connected to a variable power-
supply and PID temperature controller (Love Controls) with a K-type
thermocouple (Omega). For temperature-programmed reductions, the
reducing gas consisted of 5% H2 in He, at a flow-rate of approximately
15 cm3(STP) minꢀ1. Dried, unreduced catalyst samples (0.2 g) were
loaded into a 12.6 mm outer diameter, fritted quartz tube, purged with
the reducing gas for 30 min, followed by initiation of a temperature ramp
at 10 K minꢀ1 to 973 K. For NH3 temperature-programmed desorption
studies, reduced and passivated catalyst samples (0.4 g) were loaded into
the fritted quartz tube, pretreated in flowing H2 (100 cm3(STP) minꢀ1
)
at 523 K for 4 h, and purged with He (200 cm3(STP) minꢀ1) at 523 K
for 90 min. NH3 adsorption was performed at 298 K using 1% NH3 in
He at a flow-rate of 100 cm3(STP) minꢀ1. After NH3 adsorption, the
sample was purged with flowing He (200 cm3(STP) minꢀ1) at 298 K for
90 min. The He flow-rate adjusted to approximately 30 cm3(STP)
minꢀ1 followed by initiation of a temperature ramp at 10 K minꢀ1 from
298 to 1073 K. The effluent from the reactor tube was introduced into a
vacuum chamber (5 ꢁ 10ꢀ5 Torr) via a constricted quartz capillary, and
the composition of the gas was monitored by a mass spectrometer
system (quadruple residual gas analyzer, Stanford Instruments RGA
200). Vacuum for the mass spectrometer was provided by a diffusion
pump connected in series to a rotary pump. The NH3 desorption
profiles reported are based on the mass 15 (NH) signal which was
verified to be directly indicative of NH3 desorption alone and did not
experience any interference from the fragmentation of contaminants
such as H2O and CO2.
Density Functional Theory Calculations. All gas-phase ener-
gies were calculated using gradient-corrected DFT calculations as
implemented in DMol.3,14,34,35 Wave functions were represented by
numerical basis sets of double numerical quality (DNP) with d-type
polarization functions on each atom and expanded out to a 3.5 Å cutoff.
The PerdewꢀWang 9136(PW91) form of generalized gradient approx-
imation (GGA) was used to model gradient corrections to the correla-
tion and exchange energies. The electronic density for each self-
consistent iteration converged to within 1 ꢁ 10ꢀ5 au. The energy in
each geometry optimization cycle was converged to within 2 ꢁ 10ꢀ5
Hartree with a maximum displacement and force of 4 ꢁ 10ꢀ3 Å and
3 ꢁ 10ꢀ3 Hartree/Å, respectively.
Electron Microscopy. Catalysts were dispersed in ethanol and
mounted on holey carbon grids for examination in a JEOL 2010F 200 kV
transmission electron microscope equipped with Oxford Energy Dis-
persive Spectroscopy (EDS) system for elemental analysis. Images were
recorded in high angle annular dark field (HAADF) mode with a probe
size of 1.0 nm.
The gas-phase carbenium ion reaction or formation energies were
calculated for ring structures as:
Spent Catalyst Samples. All spent catalyst samples were obtained
from batch reactor measurements for the hydrogenolysis of 1 (393 K, 34
bar H2, 4 h, mass ratio of catalyst:1 = 1:9). Reaction mixtures were
filtered immediately after cooling and depressurization of the reactor.
Catalyst samples were vacuum-dried at room temperature prior to use.
Batch Reactor Studies. Reactions were carried out using a 50 mL
pressure vessel (Hastelloy C-276, model 4792, Parr Instrument). After
loading the reactant solution, catalyst, and magnetic stirrer bar into the
reactor, the vessel was sealed, purged with 34 bar He, and pressurized
with H2 (34 bar). Mechanical stirring was maintained using a magnetic
stirrer plate (500 rpm).
Catalyst Pretreatment in Batch Reaction Studies. Pretreat-
ment of the dry catalyst with H2 was carried out at temperatures of 393
and 523 K for 12 h. Reduced and passivated dry catalyst was pretreated in
a 50 mL pressure vessel (Hastelloy C-276, Model 4792, Parr In-
strument) under H2 atmosphere (34 bar) at the stipulated pretreatment
temperature. The reactor was then cooled to the reaction temperature
ROR þ Hþ f RORHþ
where ROR and RORH+ refer to the initial ring structure and the ring-
opened carbenium ion. The energies for the linear polyols, however,
result in dehydration and, as such, were calculated as:
ROH þ Hþ f RHþ þ H2O
where ROH and RH+ refer to the initial polyol and the resulting
dehydrated carbenium ion. Molecules which contained neighboring R
or β OH substituents were found to preferentially undergo a concerted
protonation and hydride transfer to stabilize the resulting product:
RꢀCH2OH ꢀ CH2OH f ½RꢀCHðþÞꢀCH2OHꢂ
f RꢀCH2ꢀCHdOðþÞH
The gas-phase carbenium ion energies are used herein only as an
initial probe of the activity and selectivity of the different molecules
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examined experimentally. This approach oversimplifies the environment
of the metal in solution. While the presence of the aqueous solution will
clearly lower the overall reaction energies and activation barriers, we do
not believe that selectivity patterns reported here will be altered.
All calculations on the metal surfaces and metal clusters were
performed using gradient-corrected periodic plane wave DFT calcula-
tions as implemented in the Vienna Ab Initio Simulation Program
(VASP).37ꢀ39 The PW91 form of the GGA was used to determine
gradient corrections to the exchange and correlation energies. Wave
functions were constructed by expanding a series of plane waves within a
cutoff energy of 400 eV where interactions between the core and valence
electrons were modeled using Vanderbilt ultrasoft pseudopotentials.40
For periodic calculations, a 4 ꢁ 4 ꢁ 1 MonkhorstꢀPack k-point mesh41
was used to sample the first Brillouin zone, for the calculations on the
metal cluster, only the Γ-point was used. The electronic energies
converged to within 1 ꢁ 10ꢀ4 eV and forces on each atom were
optimized to within 0.05 eV Åꢀ1. The 111 surfaces were modeled using a
3 ꢁ 3 unit cell with four metal layers and 16 Å vacuum region between
each slab where the top two layers were allowed to relax and the bottom two
held fixed to their lattice constant for Rh (3.8034 Å). The calculations on
RhRe201 clusters were carried out in a 30.4352 Å cubic unit cell to ensure a
sufficient vacuum region to separate clusters in neighboring cells; all cluster
atoms were allowed to relax in energy calculations and transition state
searches. Transition state searches were performed using the dimer42
method with the initial guesses for the transition state structure and the
reaction trajectory obtained through the nudged elastic band method.43
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’ ASSOCIATED CONTENT
S
Supporting Information. Table of the effect of metal-
b
loading ratio on catalytic activity and selectivity, table of the effect
of pretreatment conditions on hydrogenolysis activity and Re
leaching, table of hydrogenolysis rates and DFT-calculated
carbenium and oxocarbenium ion energies, table of hydrogeno-
lysis rates of 1 over 4 wt % RhꢀReOx/C (1:0.5) in a continuous
flow reaction system with varying reactant concentrations and
PH2, figure of DFT-calculated metal-catalyzed ring-opening of 3,
figure of DFT-optimized structure for water-stabilized tetrahy-
drofurfuryl carbenium ion transition state. This material is
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’ AUTHOR INFORMATION
Corresponding Author
dumesic@engr.wisc.edu
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’ ACKNOWLEDGMENT
This material is based upon work supported by the National
Science Foundation under Award No. EEC-0813570 and the
National Science Foundation Partnership in International Re-
search and Education (PIRE) under NSF grant OISE 0730277.
We also kindly acknowledge the computational time at the
National Center for Computational Sciences at Oak Ridge
National Laboratory which was used to carry out the simulations
on the 201 atom RhRe particles.
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