MOF-Based Catalysts for Selective Hydrogenolysis of Carbon-Oxygen Ether Bonds

Article abstract of DOI:10.1021/acscatal.5b02061

We demonstrate that metal-organic frameworks (MOFs) can catalyze hydrogenolysis of aryl ether bonds under mild conditions. Mg-IRMOF-74(I) and Mg-IRMOF-74(II) are stable under reducing conditions and can cleave phenyl ethers containing β-O-4, α-O-4, and 4-O-5 linkages to the corresponding hydrocarbons and phenols. Reaction occurs at 10 bar H₂ and 120 °C without added base. DFT-optimized structures and charge transfer analysis suggest that the MOF orients the substrate near Mg²⁺ ions on the pore walls. Ti and Ni doping further increase conversions to as high as 82% with 96% selectivity for hydrogenolysis versus ring hydrogenation. Repeated cycling induces no loss of activity, making this a promising route for mild aryl-ether bond scission.

Full text of DOI:10.1021/acscatal.5b02061

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Letter
MOF-Based Catalysts for Selective
Hydrogenolysis of Carbon-Oxygen Ether Bonds
Vitalie Stavila, Ramakrishnan Parthasarathi, Ryan W Davis, Farid El Gabaly,
Kenneth L. Sale, Blake A. Simmons, Seema Singh, and Mark D. Allendorf
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02061 • Publication Date (Web): 23 Nov 2015
Downloaded from http://pubs.acs.org on November 25, 2015
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MOF-Based Catalysts for Selective Hydrogenolysis of Carbon-
Oxygen Ether Bonds
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Vitalie Stavila, * Ramakrishnan Parthasarathi,
Ryan W. Davis, Farid El Gabaly, Kenneth L.
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Sale,
Blake A. Simmons, Seema Singh, Mark D. Allendorf *
[
a] Vitalie Stavila, Ramakrishnan Parthasarathi, Ryan W. Davis, Farid El Gabaly, Kenneth L. Sale, Mark D. Allendorf:
Sandia National Laboratories, 7011 East Avenue, Livermore, CA 94550, * Eꢀmail: vnstavi@sandia.gov ;
mdallen@sandia.gov
[
b] Ramakrishnan Parthasarathi, Blake A. Simmons, Seema Singh: Joint BioEnergy Institute, 5885 Hollis Street, Emeryville,
CA 94608; Supporting information for this article is given via a link at the end of the document.
catalyzed reactions are known, including oxidation, silylaꢀ
tion, sulfurization, epoxidation, cycloaddition, and condenꢀ
ABSTRACT: We demonstrate that metalꢀorganic frameꢀ
works (MOFs) can catalyze hydrogenolysis of aryl ether
bonds under mild conditions. MgꢀIRMOFꢀ74(I) and Mgꢀ
IRMOFꢀ74(II) are stable under reducing conditions and
can cleave phenyl ethers containing βꢀOꢀ4, αꢀOꢀ4, and 4ꢀ
Oꢀ5 linkages to the corresponding hydrocarbons and pheꢀ
nols. Reaction occurs at 10 bar H and 120 °C without
added base. DFTꢀoptimized structures and charge transfer
analysis suggest that the MOF orients the substrate near
Mg ions on the pore walls. Ti and Ni doping further
increases conversions to as high as 82% with 96% selecꢀ
tivity for hydrogenolysis vs. ring hydrogenation. Repeatꢀ
ed cycling induces no loss of activity.
1ꢀ3,9,11
sation, to name just a few.
Notably absent from this
list are simple bond cleavage reactions, in particular hydroꢀ
genolysis, which is used extensively industrially to remove
12
sulfur from hydrocarbons and has gained attention for proꢀ
13
duction of valueꢀadded chemicals from biomass. To our
knowledge, there is only one report of MOFꢀcatalyzed hyꢀ
drogenation of an organic molecule (as opposed to organoꢀ
metallic precursors of metal nanoparticles); in this case, the
catalytic reaction occurs in the presence of a sacrificial
2
2+
14
base. Here, we describe experiments and modeling supꢀ
porting the notion that MOFs can be effective catalysts for
the hydrogenolysis of C–O aromatic ether bonds, which are
common linkages in biomass.
KEYWORDS : MetalꢀOrganic Frameworks, Catalysis, CꢀO
Bond Cleavage, Hydrogenolysis, Aromatic Ethers
MOFꢀ74 was selected as a starting point for catalyst develꢀ
opment because this topology has several advantages. First,
the recently reported isoreticular IRMOFꢀ74(n) series proꢀ
15
vides hexagonal 1ꢀD channels with diameters between 1.2
nm and 9.8 nm that can accommodate a range of substrate
sizes. Second, the density of open metal sites (OMS) in these
MOFs, which can behave as Lewis acids to activate CꢀO
bonds, is the highest known for this class of materials. Third,
IRMOFꢀ74(n) can be synthesized with a wide range of metꢀ
als, (Mg, Mn, Fe, Co, Ni, Cu, and Zn), as well as mixedꢀ
The catalytic properties of nanoporous materials known as
Metal–Organic Frameworks (MOFs) are attracting considerꢀ
able attention as a result of their exceptional chemical and
structural versatility. MOFs are crystalline materials conꢀ
sisting of metal ions coordinated to bridging organic linkers,
forming a microꢀ or mesoporous structure. These materials
offer an exceptionally high degree of synthetic versatility,
enabling rational design of pore dimensions and chemistry to
achieve product selectivity with high turnover rates. MOFs
have remarkably high thermal and chemical stability (some
are stable to temperatures as high as 500 °C). Water stabilꢀ
ity, an issue with some frameworks, is no longer a limiting
factor; a large number of waterꢀstable MOFs are now
known, some of which are unaffected by boiling in acidic or
basic solution, and strategies for improving water stability in
1,2
16
metal compounds of up to ten different metals, allowing
the reactivity of the OMS to be readily tuned. Finally, we
recently demonstrated that MgꢀMOFꢀ74 has exceptional
thermal and chemical stability, withstanding meltꢀinfiltration
3,4
10
with the highly reactive metal hydride NaAlH4. Moreover,
when doped with a titanium halide, this MOF reversibly
catalyzes the rehydrogenation of the NaH and Al products
into NaAlH , possibly by activating hydrogen or generating
4
5
mobile reactive species. The detailed reaction data, catalyst
characterization, and mechanism discussion presented here
follow a limited and preliminary report by our team.
those that are not are now available. MOFs can also serve as
hosts for metal nanoparticles (NPs) known to catalyze hyꢀ
17
6ꢀ8
drogenation reactions. Finally, compared with catalysts
supported on amorphous substrates, MOFs possess uniform
cavities and a high density of reactive centers, which should
contribute to increased turnover rates and selectivities. As
a result of these attractive properties, many types of MOFꢀ
We selected phenylethylphenyl ether (PPE), benzylphenyl
ether (BPE), and diphenyl ether (DPE) (Scheme 1) as repreꢀ
sentative substrates which incorporate the βꢀOꢀ4, αꢀOꢀ4, and
9,10
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4
ꢀOꢀ5 (or 4,4′) linkages. In addition to the pure MOFs, we
absence of the MOF catalyst (Table S1, Supp. Info). When
compared to commercial Raney Ni and TiCl (entries 25 and
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also prepared IRMOFꢀ74(I) and IRMOFꢀ74(II) samples inꢀ
filtrated with TiCl and Ni NPs, which were previously
shown to catalyze aryl ether hydrogenolysis.
tion of Ti species was performed using our previously deꢀ
3
26, Table 1), the Ni@IRMOF and Ti@IRMOF catalysts
display similar conversion efficiency, but significantly better
selectivity. In addition, the products of the reactions cataꢀ
lyzed by Ni Raney include substantial quantities of cycloꢀ
hexanol, whereas no products of ring hydrogenation or ring
opening were detected in any of the reactions catalyzed by
the MOFs. PXRD data (Fig. 1) indicate that the MOF strucꢀ
ture is unchanged by the hydrogenolysis reaction. Moreover,
elemental analysis of the supernatant following the reaction
yielded no evidence of Mg, Ti, or Ni, confirming that the
MOF itself is the active catalyst and not a solubilized metalꢀ
component. Finally, repeated cycling of the catalysts did not
affect their performance; for example, the recovered
Ni@IRMOFꢀ74(I) catalyst was reused for up to five tests
without any significant loss in catalytic activity (Table S1,
Supp. Info).
x
18ꢀ20
The infiltraꢀ
10
scribed procedure, in which TiCl is vaporꢀinfiltrated into
4
the pores of the activated MOF, followed by treatment with
gaseous hydrogen at 90 °C. The insertion of Ni was achieved
using methods pioneered by Fischer’s group to create metal
NPs in MOFs. The asꢀsynthesized MOFꢀbased catalysts
were evaluated for their catalytic activity in hydrogenolysis
reactions with the PPE, BPE and DPE substrates (Scheme
1), results of which are given in Table 1. The reactions were
performed in stainless steel cells loaded with the substrate
compound in pꢀxylene and the MOF catalyst. Mixtures were
pressurized with H and heated to 90 – 120 ºC, well within
the stability range for the IRMOFꢀ74(I,II) materials. In all
cases the reactor pressure was 10 bar H ; such pressures are
commonly used in industrial hydrogenation reactions, inꢀ
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cluding many heterogeneous catalysis processes.
Table 1. Catalytic effect of tested MOFs on hydrogenolysis
of arylꢀether compounds.
Experiments using pure IRMOFꢀ74(I,II) catalysts revealed
that all three ethers react with H to generate small amounts
of phenol and the corresponding aromatic hydrocarbon
Selectivity
Subꢀ
strate
Time Conv.,
2
Entry
Catalyst
T, °C
hours
%
1
2
3
4
1
2
3
4
5
6
7
8
9
IRMOFꢀ74(I)
Ti@IRMOFꢀ74(I) PPE
Ni@IRMOFꢀ74(I) PPE
IRMOFꢀ74(II)
Ti@IRMOFꢀ74(II) PPE
Ni@IRMOFꢀ74(II) PPE
IRMOFꢀ74(I)
Ti@IRMOFꢀ74(I) BPE
Ni@IRMOFꢀ74(I) BPE
IRMOFꢀ74(II)
Ti@IRMOFꢀ74(II) BPE
PPE
120 16
120 16
120 16
120 16
120 16
120 16
120 16
120 16
120 16
120 16
120 16
120 16
12 87
51 89
68 91
39 83
60 79
82 96
10
33
57
17
42
91
90
94
87
83
98
84
79
85
78
89
95
(
Scheme 1). In all cases, higher conversions were obtained
using the IRMOFꢀ74(II) catalyst. The difference can be quite
substantial; for example, the conversion of PPE increased
from 12% to 39% by using IRMOFꢀ74(II) as the catalyst
instead of IRMOFꢀ74(I) (Table 1). Optimized geometries
computed using density functional theory (DFT) indicate
that all three substrates will fit within the pores of either
MOF (see Supp. Info), suggesting that other factors, such as
access to the OMS, the transition state geometry, or the reacꢀ
tant and/or product diffusion rates within the pore, are reꢀ
sponsible for this trend.
PPE
BPE
78
75
82
73
84
91
10
BPE
1
1
1
2
Ni@IRMOFꢀ74(II) BPE
76
13
IRMOFꢀ74(I)
DPE 120 16
4
19
29
9
20
34
51 86
56 76
67 83
39 86
47 92
56 93
76 81
39 74
79 82
81 85
80 87
75 78
77 80
85 87
92
14 Ti@IRMOFꢀ74(I) DPE 120 16
1
1
1
5
6
7
Ni@IRMOFꢀ74(I) DPE 120 16
IRMOFꢀ74(II) DPE 120 16
Ti@IRMOFꢀ74(II) DPE 120 16
18 Ni@IRMOFꢀ74(II) DPE 120 16
19 Ni@IRMOFꢀ74(II) PPE 90 16
2
2
2
0
1
2
Ni@IRMOFꢀ74(II) PPE
Ni@IRMOFꢀ74(II) PPE
Ni@IRMOFꢀ74(II) PPE
100 16
110 16
79
82
91
95
97
75
73
120
120
120
120 16
120 16
1
2
4
23 Ni@IRMOFꢀ74(II) PPE
24 Ni@IRMOFꢀ74(II) PPE
2
2
5
6
Raney Ni
TiCl
PPE
PPE
3
Powder XRD indicates that both IRMOFꢀ74(I) and IRMOFꢀ
4(II) showed no signs of structural degradation or loss of
7
crystallinity upon infiltration with guest species or after cataꢀ
lytic tests (Fig. 1). The metal loadings, determined by eleꢀ
mental analysis, are 1.52 wt% Ti and 2.91 wt% Ni in
IRMOFꢀ74(I) and 1.75 wt% Ti and 3.07 wt% Ni in IRMOFꢀ
74(II). The Ti:Cl ratio in the titaniumꢀ infiltrated samples
Scheme 1. Reactions catalyzed by IRMOFꢀ74(I, II).
Performing the hydrogenolysis reactions in the presence of
the TiCl and Niꢀinfiltrated MOF catalysts leads to a signifiꢀ
x
cant increase in conversion efficiency (Table 1). GCMS and
1
+3
H NMR analysis (Figs. S1 and S2) of the reaction products
annealed under hydrogen is close to 1:3, suggesting that Ti
10
of all three substrate molecules clearly indicate new peaks
corresponding to the monomeric products (1–4). The conꢀ
versions obtained using both MOFs follow the trend PPE >
BPE > DPE (Fig. S3). In all cases (with or without TiCl or
Ni dopants), it is clear that the MOFꢀbased catalysts faciliꢀ
tate the reaction; no substrate conversion occurred in the
species are present in the final product. The metal distribuꢀ
tion in the asꢀsynthesized powders was determined through
scanning electron microscopy (SEM) measurements using
energyꢀdispersive spectroscopy (EDS). The elemental maps
confirm that both Ti and Ni species are present and are well
dispersed within the MOF particles (Figure 2 and Fig. S4,
x
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denced by the Ni 2p3/2 peaks located at 852.6 and 856.3 eV,
Supp. Info). Since the penetration depth of 15 keV electrons
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used in EDS measurements is about 5 ꢁm, this method
probes the entire catalyst particle and not only its surface.
respectively (Fig. 2, right bottom panel). The Ni(OH) speꢀ
2
cies likely result from the short exposure to air during the
transfer of the sample into the XPS chamber (see Fig. S4,
Supp. Info). Nitrogen BET analysis indicates that the surface
2
area of IRMOFꢀ74(I) is reduced from 1627 m /g for the actiꢀ
2
2
vated material to 431 m /g and 459 m /g upon infiltration
with Ti and Ni species, respectively. The surface area reducꢀ
2
tion is somewhat less for IRMOFꢀ74(II), from 1736 m /g for
2
the activated IRMOFꢀ74(II) to 672 and 591 m /g for
0
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Ti@IRMOFꢀ74(II) and Ni@IRMOFꢀ74(II). As MgꢀIRMOFꢀ
74 is a robust framework (TGA shows no decomposition
23
below 350 °C and it exhibits a surface area near the theoꢀ
retical value after degassing under vacuum for 16 hours at
Figure 1. Powder XRD patterns of the asꢀsynthesized, infilꢀ
trated, and cycled IRMOFꢀ74(I) (left) and IRMOFꢀ74(II)
24
225 °C ), we believe that partial blockage of the 1D chanꢀ
nels by guest species is the likely cause of the decreased
surface area. Some pore collapse leading to amorphous doꢀ
mains undetectable by XRD cannot be fully ruled out, but if
this occurs, these regions would most likely be inaccessible
to guest molecules. The fact that the BET data show accessiꢀ
ble pore volume remains after infiltration with Ti and Ni and
that, in all cases, conversions are higher for the largerꢀpore
IRMOFꢀ74(II), points to reactions occurring within the MOF
pores and not processes on the MOF surface.
(right) catalysts.
PPE
PPE + H
2
2 3
PhCH CH +PhOH
Figure 3. Optimized gasꢀphase geometries for adsorbed PPE,
PPE+H , and the hydrogenolysis products, obtained using the
2
QM/QM method. Substrate compounds and OMS on the MOF
are shown as spheres vs. the stick model of the cluster.
The activity of the catalysts follows the trend Ni@IRMOFꢀ
74 > Ti@IRMOFꢀ74 > IRMOFꢀ74, regardless of substrate,
with the highest conversions obtained for the βꢀOꢀ4 and αꢀ
Oꢀ4 linkages (82% and 76%, respectively) using
Ni@IRMOFꢀ74(II). More remarkably, all three catalysts
display very good selectivity for hydrogenolysis vs. hydroꢀ
genation, with selectivity towards ethylbenzene and phenol
formation from PPE as high as 98%. Based on the stoichiꢀ
ometry of the reactions in Scheme 1, equal amounts of the
two products should be obtained. We find that slightly lower
amounts of 1, 2, or 3 are obtained relative to phenol (Table
1
). This is likely due to the higher volatility of the comꢀ
pounds, leading to finite concentrations in the head space
above the reaction that are not fully recovered when the hyꢀ
drogen pressure is released. The residual gas analysis (RGA)
measurements of the volatiles present in the reaction vessel
indicate that, in addition to hydrogen (m/z=2), fragments of
aromatic hydrocarbons (m/z=78, 91) (Figure S6, Supp. info)
from the solvent and/or reaction products are present as well.
Figure 2. SEM, EDS and XPS analysis of Tiꢀ and Niꢀdoped
IRMOFꢀ74(I). The scale bar in all EDS maps is 500 nm.
XPS measurements (Figure 2, bottom panels) indicate that
the Ti(III) is present as TiCl , based on its characteristic
3
peaks centered at 457.5 (Ti 2p ) and 463.0 eV (Ti 2p )
3
/2
1/2
(
Fig. 2, left bottom panel). In the case of Niꢀdoped samples,
That substrate confinement within the MOF pore plays a key
role in establishing the efficiency and selectivity of these
0
the species detected are 45% Ni and 55% Ni(OH) , as eviꢀ
2
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catalysts is supported by DFT calculations, which indicate tions of reactive Hꢀspecies. Both titanium and nickel are well
27
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that the OMS in the IRMOFꢀ74 pores bind and orient the
substrate within the pore. We used a cluster model (Figs. 3,
known to activate the dihydrogen molecule and it is likely
that they perform a similar role here. Our hypothesis sugꢀ
gests an additional, synergistic role for the MOF, namely, to
increase the local concentration of active hydrogen, thereby
accelerating the reaction. This is consistent with our prior
study of Tiꢀdoped MgꢀIRMOFꢀ74(I), which showed that the
2
+
S7, S8) consisting of four fiveꢀcoordinate Mg ions, each
connected to five oxygen donor atoms, approximating one
wall of the hexagonal pore. Computed binding energies (see
details in the Supp. Info) indicate that all three substrates
interact strongly with the cluster, even in the presence of the
pꢀxylene solvent (Tables 2 and S3). These energies follow
the trend PPE > BPE > DPE (Table 2), consistent with the
experimental conversions. The involvement of the OMS is
indicated by the observation that, for all three substrates, the
relaxed clusterꢀsubstrate geometries position one of the subꢀ
reversibility of NaAlH decomposition in the pores is draꢀ
4
10
matically increased by the presence of the dopant.
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In summary, MgꢀIRMOFꢀ74(I) and MgꢀIRMOFꢀ74(II) seꢀ
lectively catalyze CꢀO arylꢀether bond cleavage, which is a
new reaction category for MOFs. The results indicate that
the MOF itself actively participates in the reaction, most
likely through the interaction of the OMS with the substrate.
Substrate conversion is enhanced by confinement of transiꢀ
tion metal dopants within the MOF pores; additional mechaꢀ
nistic studies are required, however, to determine the extent
to which these effects are synergistic. Although the observed
catalytic activity is slightly lower compared to the best
2
+
strate aromatic rings directly over a Mg ion (shown in Fig.
for PPE and Fig. S9 for BPE and DPE). Global charge
3
14
transfer analysis (Table S4) indicates that in the MOFꢀ
substrate complex there is charge transfer from the model
compound to the MOF, with the MOF acting as an electron
acceptor and the aromatic ether molecules as electron doꢀ
nors. In addition, the computed free energies of hydrogenolꢀ
ysis (ꢁG°) on the cluster are more negative than the purely
gasꢀphase reaction (Table 2). Examination of the predicted
18ꢀ20,20,28
known CꢀO hydrogenolysis catalysts,
a significant
advantage of these MOFꢀbased catalysts is that their activity
is achieved without the addition of a base, such as
binding energies ꢁH° for the reactants and products chemiꢀ
B
t
20,28
NaO Bu.
Interestingly, Ni@IRMOFꢀ74 has higher reacꢀ
sorbed to the cluster indicates that, in addition to orienting
the substrate, the cluster facilitates the reaction by stabilizing
the products relative to the reactants; product binding enerꢀ
gies are higher than those of the reactants (Table S3). (Note
that ꢁG°(gas phase) represents the overall thermodynamic
change for the hydrogenolysis reaction, whether or not the
cluster is present). Significantly, ꢂH°B is considerably
stronger and ꢁG° is more negative for PPE and BPE than for
DPE. Moreover, the interaction of DPE with the cluster has
very little effect on the predicted ꢁG° (ꢀ81.8 kJ/mol on the
cluster vs. ꢀ80.3 kJ/mol in the gas phase). These results are
consistent with the experiments, which show much higher
conversions for PPE and BPE compared to DPE for all cataꢀ
lysts and conditions tested. We also note that the MOF OMS
tivity with PPE than DPE, whereas the ligandless Ni catalyst
reacts with these at comparable rates, suggesting the possiꢀ
bility of selective βꢀOꢀ4 hydrogenolysis using the MOF. The
Ni@IRMOFꢀ74 catalysts also exhibit higher stability when
recycled than Ni NPs stabilized on MILꢀ120, which exhibit
9,29
consistent degradation of activity with reuse. Finally, in a
few limited tests in which we suspended these catalysts in an
ionic liquid (see Supporting Information), no structural degꢀ
radation or dissolution was observed after 16 hours, indicatꢀ
ing that these MOFs are compatible with some novel bioꢀ
30
mass pretreatment methods under development.
ASSOCIATED CONTENT
Supporting Information. Additional details on experiments
and calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
may play a role in activating or at least orienting H in the
2
2
5
pores; it is established by both neutron scattering studies
2
6
2+
and DFT calculations that the Mg OMS in MgꢀIRMOFꢀ
4(I) are the strongest H binding sites.
7
AUTHOR INFORMATION
2
Table 2. Computed substrate binding energies (ꢂH°) (to the
IRMOFꢀ74 cluster model) and change in Gibbs free energy
(ꢂG°) upon hydrogenolysis at 393 K.
Corresponding Authors
Vitalie Stavila, vnstavi@sandia.gov; Mark D. Allendorf,
mdallen@sandia.gov
∆
G°, kJ/mol
Substrate + H
Hydrocarbon + PhOH
Funding Sources
∆H°
kJ/mol
B
2
→
No competing financial interests have been declared. This work
was supported by the Sandia Laboratory Directed Research and
Development Program. Sandia National Laboratories is a multiꢀ
program laboratory managed and operated by Sandia Corporaꢀ
tion, a wholly owned subsidiary of Lockheed Martin Corporaꢀ
tion, for the U.S. Department of Energy’s National Nuclear
Security Administration under contract DEꢀAC04ꢀ94AL85000).
Substrate
No
Solvent
With
Solvent
Gas
phase
On MOF
cluster
PPE
BPE
DPE
135.1
124.5
44.9
81.9
75.2
37.2
ꢀ103.5
ꢀ106.6
ꢀ80.3
ꢀ142.3
ꢀ129.1
ꢀ81.8
Clearly, the Ti and Ni dopants have an important, but differꢀ
ent, role from the MOF itself, one that is not directly adꢀ
dressed by our DFT calculations. However, a reasonable
hypothesis is that the transition metal species inside the
MOF pores activate dihydrogen molecules more efficiently
than the pure MOFs, reducing the activation energy of the
hydrogenolysis reaction and generating higher concentraꢀ
ACKNOWLEDGMENT
The authors gratefully acknowledge Joanne Volponi and Jeffrey
Chames for help with porosimetry and SEM/EDS experiments,
and Drs. Michael S. Kent and Jonathan Brown for helpful disꢀ
cussions.
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