Reactivity of a Carbon-Supported Single-Site Molybdenum Dioxo Catalyst for Biodiesel Synthesis

Article abstract of DOI:10.1021/acscatal.6b01717

A single-site molybdenum dioxo catalyst, (O_c)₂Mo(=O)₂@C, was prepared via direct grafting of MoO₂Cl₂(dme) (dme = 1,2-dimethoxyethane) on high-surface-area activated carbon. The physicochemical and chemical properties of this catalyst were fully characterized by N₂ physisorption, ICP-AES/OES, PXRD, STEM, XPS, XAS, temperature-programmed reduction with H₂ (TPR-H₂), and temperature-programmed NH₃ desorption (TPD-NH₃). The single-site nature of the Mo species is corroborated by XPS and TPR-H₂ data, and it exhibits the lowest reported MoO_x T_max of reduction reported to date, suggesting a highly reactive Mo^VI center. (O_c)₂Mo(=O)₂@C catalyzes the transesterification of a variety of esters and triglycerides with ethanol, exhibiting high activity at moderate temperatures (60-90 °C) and with negligible deactivation. (O_c)₂Mo(=O)₂@C is resistant to water and can be recycled at least three times with no loss of activity. The transesterification reaction is determined experimentally to be first order in [ethanol] and first order in [Mo] with Δ;H^? = 10.5(8) kcal mol^-1 and Δ;S^? = -32(2) eu. The low energy of activation is consistent with the moderate conditions needed to achieve rapid turnover. This highly active carbon-supported single-site molybdenum dioxo species is thus an efficient, robust, and low-cost catalyst with significant potential for transesterification processes.

Full text of DOI:10.1021/acscatal.6b01717

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Article
Reactivity of a Carbon-Supported Single-Site Dioxo-
Molybdenum Catalyst for Biodiesel Synthesis
Aidan R. Mouat, Tracy L. Lohr, Evan Wegener, Jeffrey Miller,
Massimiliano Delferro, Peter C. Stair, and Tobin J. Marks
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01717 • Publication Date (Web): 23 Aug 2016
Downloaded from http://pubs.acs.org on August 23, 2016
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Page 1 of 12
ACS Catalysis
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Reactivity of a Carbon-Supported Single-Site Dioxo-
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a,‡
a,‡
c
b,c
Aidan R. Mouat, Tracy L. Lohr, Evan C. Wegener, Jeffrey T. Miller, Massimiliano Delfer-
a,b,
a,b,
a,
ro, * Peter C. Stair, * and Tobin J. Marks *
a
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA
b
Chemical Sciences & Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
c
School of Chemical Engineering, Purdue University, West Lafayette, IN, 47907-2100, USA
Supporting Information Placeholder
tion of vegetable oils has proven a viable alternative to envi-
ABSTRACT.
A single-site dioxo-molybdenum catalyst,
ronmentally undesirable transformations requiring strong
(O ) Mo(=O) @C, was prepared via direct grafting of
c
2
2
4
homogeneous bases such as KOH. However, the presence of
MoO Cl (dme) (dme
= 1,2-dimethoxyethane) on high-
2
2
impurities in the feedstock remains a challenge to developing
practical catalytic transesterification processes. Here porous
surface-area activated carbon. The physicochemical and
chemical properties of this catalyst were fully characterized
5
6
-9
by N physisorption, ICP-AES/OES, PXRD, STEM, XPS, XAS,
solid bases have made significant contributions.
2
Temperature-Programmed Reduction with H (TPR-H ), and
2
2
Among the transesterification catalysts reported to date, the
most common classes are: 1) homogeneous strong Brønsted
Temperature-Programmed NH Desorption (TPD-NH ). The
3
3
single-site nature of the Mo species is corroborated by XPS
10
acids, which generate strongly acidic waste and cannot be
and TPR-H data, and it exhibits the lowest reported MoOx
2
recycled, and 2) Lewis acid catalysts, most notably, tin-based
Tmax of reduction reported to date, suggesting a highly reac-
11
systems. Note that the toxicity of tin has driven the search
tive Mo center. (O ) Mo(=O) @C catalyzes the transesterifi-
c
2
2
for cheap, earth-abundant alternatives. Molybdenum-based
catalysts are viable alternatives for transesterification, but
typically suffer from poor recyclability or require high-
cation of a variety of esters and triglycerides with ethanol,
exhibiting high activity at moderate temperatures (60 – 90
12
˚
C) and with negligible deactivation. (O ) Mo(=O) @C is
13
c
2
2
temperatures to achieve useful activity. This Laboratory and
resistant to water and can be recycled at least 3x with no loss
of activity. The transesterification reaction is determined
experimentally to be first-order in [ethanol] and first-order
others have detailed the attractions of grafted single-site
14-24
heterogeneous catalysts,
an approach which permits ra-
tional design of active sites tailored to the reaction of inter-
est. In designing an active site, note that recent catalytic
studies in ethanol implicate in-situ generated molybdenum-
‡
-1
‡
in [Mo] with ꢀH = 10.5 (8) kcalmol and ꢀS = -32 (2) e.u.
The low energy of activation is consistent with the moderate
conditions needed to achieve rapid turnover. This highly
active carbon-supported single-site dioxo-molybdenum spe-
cies is thus an efficient, robust, and low-cost catalyst with
significant potential for transesterification processes.
25
oxo moieties as the catalytically active species. Further sup-
26
porting this, early work by Iwasawa et al., later corroborat-
27,28
ed by Kikutani,
implicates dioxo-molybdenum as the
active species in the oxidation of ethanol by MoO /SiO cata-
x
2
lysts. In homogeneous catalysis, dioxo-molybdenum com-
plexes have demonstrated redox-active catalytic activity in
Keywords: heterogeneous catalysis, molybdenum, support-
ed catalysts, biodiesel, transesterification.
29,30
31
olefin epoxidation
and sulfide oxidation. Very recently,
an MoO (acac)2 catalyst was found to be competent for
2
transesterification in the cross-linking of polymers with tet-
3
2
Introduction
raethylorthosilicate (TEOS), and supported MoO3 was
found to promote transesterification between diethyl oxalate
and phenol.
Lower alcohols – particularly methanol and ethanol – have
attracted recent attention due to their utility in energy pro-
cesses and fine chemical production. Methanol and ethanol
are both bioavailable, making them attractive renewable
feedstocks for upgrading potential fuels. Consequently,
methanol and ethanol have both found use in commercially
important transesterification processes, most notably fatty
acid methyl and ethyl ester synthesis, for converting bioa-
3
3,34
In the past, dioxo-molybdenum complexes
have been successfully heterogenized on polymers and
3
5
3
6
1
,2
mesoporous silicas. We therefore envisioned that a well-
defined dioxo-molybdenum transesterification catalyst with
superior catalytic performance might be prepared by grafting
a molecular dioxo-molybdenum precursor to an appropriate
support.
3
vailable triglycerides into biodiesel fuel. The transesterifica-
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Page 2 of 12
Recently, activated carbon has emerged as an attractive al-
ternative to conventional refractory oxide supports because
has been observed in previous MoO catalysts, and is sug-
x
4
5-50
1
2
3
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9
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gested to be of high catalytic relevance.
XPS analysis re-
3
7
of its high surface area, porous structure, and low cost. Car-
bon materials exhibit tunable surface functionalities and
enhanced hydrothermal stability arising from their hydro-
veals no species other than C, O, and Mo on the catalyst,
although ICP-AES indicates the presence of S, N, and P im-
purities in the starting support (and < 100 ppm Cl). These
impurities in the carbon support were determined to be cata-
lytically insignificant in control experiments (vide infra). A
typical catalyst contains 2.1 wt% Mo. The maximum weight
loading of Mo obtained is 5.0 wt%. The carbon support and
(O ) Mo(=O) @C catalyst were fully characterized by PXRD,
3
8,39
phobicity.
Furthermore, Katz has shown that the aro-
matic character of activated charcoal results in favorable
energetic contributions in the absorption of glucans having
4
0
large hydrophobic backbones. Such interactions should be
of considerable value in biodiesel transformation due to the
hydrophobic character of oil and triglyceride feedstocks.
c
2
2
XPS, XAS, N physisorption, Z-contrast STEM, and ICP-AES
2
0
1
2
3
4
5
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0
(
SI, Figures S2-S4 and Table S2). Grafting of the Mo species
This contribution reports the grafting of isolated well-
defined dioxo-molybdenum sites on a high surface area car-
bon support with known physicochemical properties and
acidity, to create the catalyst (O ) Mo(=O) @C (Scheme 1).
results only in a minor change in surface area (from 1378
2
2
m /g to 1322 m /g) and no change in pore diameter or crys-
tallinity in the catalyst (by BET and BJH). STEM confirms the
amorphous character of the carbon (Figure S2). Furthermore,
images taken in Z-contrast mode reveal no significant Mo
aggregate domains, in support of a catalyst formulation hav-
ing primarily isolated dioxo-molybdenum active sites. As
c
2
2
This product is extensively
further confirmation, no crystalline MoO phase is detected
3
by XPS (Figure 1A), STEM (Figure S2), or PXRD (Figure S4)
before or after catalytic reactions, suggesting the highly dis-
persed dioxo-molybdenum species remains stable under cat-
alytic conditions.
Scheme 1. Direct grafting of MoO Cl (dme) onto activated
2
2
The activated carbon was determined to be mildly acidic,
possessing 150 ± 15 µmol/g acid sites with pKa between 6.5
carbon to make (O ) Mo(=O) @C.
c
2
2
characterized by ICP-AES/OES, N2 physisorption, PXRD,
TEM, and X-ray Absorption Spectroscopy (XAS). In addition,
the unique chemical character of the isolated Mo site is
probed by XPS, Temperature-Programmed Reduction with
H (TPR-H ), and Temperature-Programmed Desorption of
51
and 10.3, according to Boehm titration. The corresponding
acidity of (O ) Mo(=O) @C could not be determined due to
c
2
2
Mo leaching under alkaline conditions. To determine wheth-
er strong acid sites are generated in grafting of the dioxo-
molybdenum species, Temperature-Programmed Desorption
of NH (TPD-NH ) was measured as shown in Figure 1A. The
2
2
NH (TPD-NH ). It is further demonstrated that this catalyst
3
3
3
3
is competent for liquid-phase transesterification of triglycer-
ides, as well as primary, secondary, and tertiary esters with
lower alcohols under mild conditions (60 – 90 ˚C), displaying
high activity and broad scope. The catalyst operates efficient-
ly in organic solvents and shows no inhibition in the pres-
ence of water. Lastly, the catalyst exhibits no deactivation
and minimal leaching under reaction conditions, as deter-
lack of a significant shift in the Tmax for ammonia desorption
from the (O ) Mo(=O) @C indicates that no strong acid sites
c
2
2
52-54
are present.
Due to the highly absorptive optical nature of activated car-
bon at most spectral wavelengths, and the low loading of Mo,
FTIR, Raman, and UV-vis spectroscopies could not observe
the catalytically-active (O ) Mo(=O) site. However, the
mined by recyclability studies, XPS, PXRD, and H TPR. The
c
2
2
2
catalyst can be characterized with Mo XAS, Mo(3d) XPS and
TPR (Figure 1). Mo K-edge XANES exhibits pre-edge and
edge peaks at 20.0067 and 20.0160 keV respectively, which
results show (O ) Mo(=O) @C to be an economical, effi-
c
2
2
cient, and robust catalyst for transesterification processes.
VI
are indicative of Mo (SI, Figures S10-12 and Table S11 for
IV
VI
Results
Mo and Mo standards comparisons). EXAFS fitting shows
2 Mo=O bonds at a distance of 1.68 Å, 1 Mo-O bond at 1.98 Å
and 1 Mo-O bond at 2.29 Å (Table S11 and Figure S13). XAS
Support and catalyst characterization. The carbon-supported
dioxo-molybdenum catalyst was prepared via direct grafting
41
of MoO Cl (dme) (dme = 1,2-dimethoxyethane) onto an
indicates that (O ) Mo(=O) @C is closely related to MoO ,
c 2 2 3
2
2
activated carbon support in CH Cl at room temperature,
and corroborates the 4-coordinate proposed structure in
2
2
VI
yielding the single-site well-defined (O ) Mo(=O) @C struc-
Scheme 1. As shown in Figure 1B, the binding energy of Mo
c
2
2
ture (Scheme 1; see experimental details in the Supporting
Information). With no post-thermal treatments such as cal-
cination, the four-coordinate tetrahedral dioxo-molybdenum
surface structure (Scheme 1) is proposed. The absence of Cl
atoms in (O ) Mo(=O) @C is confirmed via ICP-OES (Table
in MoO , measured as a reference in the same spectropho-
3
tometer, is found to be 233.3 eV. The binding energy of the
VI
Mo in (O
slightly shifted from the bulk MoO
sistent with literature assignments of hexavalent Mo species
) Mo(=O)
c 2 2
@C catalyst is found to be 232.8 eV,
environment but con-
3
c
2
2
5
5,56
S1) and XPS (Figure S3). There have been previous reports of
and with enhanced hole-screening by the carbon support.
Cl¯ substitution at Mo(O) Cl L complexes indicating that Cl¯
No variation in the Mo binding energy is observed with load-
ing, nor after the catalyst is recovered from catalytic reac-
tions. No MoO , molybdenum carbide, or oxycarbide phase is
3
detected by XPS, arguing that the active site remains as a
highly dispersed dioxo-molybdenum species. The XPS C(1s)
2
2 2
42-44
loss in this system is feasible.
The carbon support shows
XPS evidence of hydroxylated groups (and carboxylic, car-
bonyl, ether, and anhydride functionalities, SI Table S2)
where the Mo can bind with release of HCl. This geometry
2
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ACS Catalysis
peak (Figure S3B) contains three contributions. Bulk graphit-
begins to consume H independently above 450 ˚C, thus the
2
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ic carbon is referenced to 285.0 eV. Additional features ap-
pear at 286.6 eV and 289.6 eV. These species are assignable
to carbon species in –C=O and –COOH environments, as has
analysis was limited to the region below 400 ˚C. The fresh
2.1wt% (O ) Mo(=O) @C catalyst shows a single reduction
c
2
2
event at 218 ˚C, suggesting that grafted dioxo-molybdenum
sites are highly reducible in comparison to the Mo sites on
5
7,58
been previously reported in the literature.
The XPS O(1s)
peak of the support (Figure 1C, Figure S3C) can be deconvo-
luted into three components at 531.2 eV, 533.3 eV, and 536.1
eV. These ionizations can be assigned to carbonyl oxygen
species, alcohol or etheric oxygen species, and adventitious
MoO . The same catalyst was investigated after catalytic
3
turnover (vide infra) and negligible changes are observed in
the TPR profile, indicating that the active site remains in-
tact/unchanged throughout catalysis, in support of the high
activity and recyclability found for these (O ) Mo(=O) @C
5
7,58
water respectively,
in the C(1s) spectrum.
Temperature-programmed reduction scans with H (TPR-H )
consistent with the species observed
c
2
2
catalysts (vide infra). H2 consumption by 2.1
%
0
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3
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0
(
O ) Mo(=O) @C during TPR-H was quantified by calibrat-
c
2
2
2
2
2
ing the apparatus with CuO. From this analysis, it was de-
of (O ) Mo(=O) @C and related materials are shown in Fig-
c
2
2
termined that 50% of the Mo sites in (O ) Mo(=O) @C con-
ure 1D. Both the carbon support and reference MoO exhibit
c
2
2
3
sume H2 during TPR, and thus it is reasonable to infer
no reduction events below 400 ˚C, although MoO is ob-
3
served to undergo reduction at > 550 ˚C. The carbon support
Figure 1. (A) TPD-NH traces comparing carbon (i, red) and 2.1 wt% (O ) Mo(=O) @C (ii, orange). (B) XPS spectra of Mo(3d)
3
c
2
2
peaks. (O ) Mo(=O) @C is 2.1 wt% Mo. Black traces are from MoO used as a reference. (C) XPS spectrum of O(1s) peaks.
c
2
2
3
(
O ) Mo(=O) @C is 2.1 wt% Mo. (D) TPR-H traces of reference MoO (i, black), carbon (ii, red), fresh 2.1wt% Mo (iii, green),
c
2
2
2
3
and used 2.1wt% Mo (iv, orange).
that >50% of the dioxo-Mo sites are catalytically significant.
This number is in good agreement with previous reports of
single-site heterogeneous catalysts, which typically have be-
octyl acetate also exhibits a linear relationship in EtOH con-
centration from monitoring the reaction with 0.0 – 4.0 equiv.
of EtOH (Figure S6 and Table S7). Therefore, the overall rate
equation under the present conditions is:
,
15, 59-63,17,20,64,65-70
tween 10 and 70% active sites.
Mo K-edge
XANES of the catalyst at 218 ºC under H flow indicates that
2
the Mo remains in the VI oxidation state (Figure S11 and Ta-
ble S11). EXAFS shows partial reduction to ~ 1.4 M=O bonds
1
1
0
v ~ [Mo] [EtOH] [Ester]
(1)
from 2 (M=O: 1.63 Å) which plausibly results from H addi-
2
tion across a Mo=O bond at ~ 50% of the sites (Table S11 and
The conversion of n-octyl acetate was also monitored over a
temperature range (60 to 90 °C) to generate an Eyring plot
(Figure 2 and Table S3), yielding activation parameters of
Figure S13). At 218°C there is additionally 1 Mo-O bond at 1.98
Å. Reduction by H at 218°C leads to a loss of 0.4 Mo=O
2
‡
-1
‡
bonds at 1.7 Å and 1 Mo-O at 2.29 Å. A XANES combination
ꢀH = 10.5 (8) kcalmol and ꢀS = -32 (2) e.u. (Table S4). The
VI
IV
-1
fit of Mo and Mo (MoO ) is not viable, indicating that the
activation energy was determined to be 11.1 (7) kcalmol (Ta-
2
VI
species at 218 ºC is most likely (O ) Mo (=O)(OH)(H).
ble S5), in accord with previous activation energies for alka-
c
2
-
1 73
line transesterification of 6.2 to 14.7 kcal mol . The very low
Transesterification of lower alcohols and esters. A variety of
esters were screened over (O ) Mo(=O) @C at 90 °C under
‡
ꢀ
H is in line with the low reaction temperatures (60 °C) and
c
2
2
the high activity of the supported Mo-oxo catalyst. This is
200 psi Ar for catalytic transesterification activity (Table
(g)
71,72
corroborated by the unprecedented low temperature (218 °C)
1).
The weakly acidic activated carbon support alone is
‡
of catalyst reduction. The large negative value of ꢀS likely
found to be inactive under these conditions, indicating that
5
reflects the requirement for the incoming substrate to coor-
dinate and react at the supported Mo surface. Conducting
stronger acids are required for this transformation. Primary,
secondary, and tertiary alkyl esters (1) undergo transesterifi-
cation to produce the corresponding alcohols (2) and ethyl
esters (3). Using n-octyl acetate as a model substrate, the
conversion versus time yields a linear relationship, indicating
a rate law that is zero-order in substrate concentration (Fig-
ure S5 and Table S6, see SI pages S4-S7 for kinetic analysis
procedures). The conversion of n-octyl acetate with varying
amounts of the same Mo catalyst (0.0 – 3.0 mol % Mo) yields
a linear relationship (Figure S7 and Table S8) indicating that
the rate law is first-order in Mo content. The conversion of n-
the transesterification in the presence of H O (0 to 100 equiv,
2
see Table S10) did not diminish the catalytic activity at 90 °C,
indicating the (O ) Mo(=O) @C catalyst is not poisoned
c
2
2
under these conditions. No reaction occurs in the absence of
catalyst or with the activated carbon support alone (Table 1,
entries 1 and 2). In a control study with n-octyl acetate at 90
°
C in dry EtOH, MoO is found to exhibit some activity (Ta-
3
ble S6); however, under identical reaction conditions, the
-
1
yield (mmol product/mol Mo h ) is 5.8x higher with
3
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Page 4 of 12
(
O ) Mo(=O) @C, confirming the high activity of the car-
c 2 2
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bon-supported dioxo-Mo species.
5
4 (49)
7
8
16
16
76
(2, 25)
3, 30)
(
2
5 (25)
9
16
16
0
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9
0
44
(
(
)
2, 14)
C=O,8
1
1
0
1
99 (77)
16
16
9
7 (97)
Figure 2. Eyring plot for the conversion of n-octyl acetate to
n-octanol. Conditions: 1 mmol n-octyl acetate, 1.0 mol % Mo,
12
2
5
mL of anhydrous EtOH, under 200 psi Ar(g) and stirring at
00 rpm.
a.
Conditions: 1.0 mmol substrate, 1.0 mol% Mo (0.046 g of 2.1
wt% (O ) Mo(=O) @C, 2.0 mL dry EtOH, 200 psi Ar , 500
rpm, conversion and yields determined by reference to me-
sitylene internal standard by H NMR; products also con-
firmed by GC-MS analysis but not quantified by this method.
Acetaldehyde diethyl acetal observed for reactions with an -
c
2
2
(g)
Transesterification of triglycerides. On a preparative scale, tri-
stearin, a long chain (4, R = n-C H ) triglyceride, was sub-
1
1
7
35
jected to transesterification conditions in EtOH at 90 °C with
mol % Mo metal loading (Scheme 2). After 6 hours, the
1
1
b
OAc group by H NMR but were not quantified no catalyst.
no catalyst. 0.0475 g of activated charcoal as catalyst. 10
reaction mixture was diluted with water and ethyl ester 5 was
extracted with CH Cl . Ester 5 was recovered as a white solid
c
d
2
2
mol % MoO as catalyst.
in 97 % isolated yield after removal of solvent in vacuo.
3
Catalyst recyclability. A recyclability test was performed for
transesterification of n-octyl acetate in neat ethanol at a
loading of 1 mol% Mo under the harshest conditions (90 ˚C,
additional time, for a total of three sequential reactions (Ta-
ble S9). Although a slight decrease in activity is observed
(TOF = 1930 h to TOF = 1800 h , reactions run up to 55 %
conversion are consistent), the catalyst remains highly active
even after a second recycle. After all three reactions, the 2.1
wt% (O ) Mo(=O) @C catalyst was analyzed for Mo content
by ICP-AES) and found to contain 1.7wt% molybdenum,
suggesting that some leaching occurs under the present con-
ditions; nevertheless it does not substantially impact the
-1
-1
200 psi Ar, 500 rpm, 1 – 3 hours). After reaction, the catalyst
was collected by filtration, dried, and added directly to a
second reaction mixture. This procedure was repeated an
c
2
2
a
(
Table 1. Acetate Transesterification Substrate Scope
(O
c
2
) Mo(=O)
2
@C
O
R'
O
R'
R
R
OH
+
Et
90 o
EtOH
C
O
O
O
O
R
O
(O ) Mo(=O) /C
c 2 2
1
2
3
O
OH
R
O
R
HO
OH + 3
O
R
Conv.
Time (Yield)
EtOH, 90 o
C
O
O
200 psi Ar(g)
Substrate, 1
Product, 2/3
n-C H OH
4, R = C17H35
5, R = C17H35 3.63 g
97% isolated yield
(
h)
%
3.57 g
b
0
1
n-C H OAc
2
Scheme 2. Preparative scale triglyceride transesterification.
8
17
8
17
c
0
2
3
n-C H OAc
n-C H OH
2
8
17
8
17
catalytic activity. The additional Mo is present in the solution
SI, Table S9) This is consistent with the TPR-H data, indi-
2
cating that not all Mo species are catalytically active. Pre-
sumably, the non-active species are the only species suscep-
tible to leaching under these conditions.
d
66 (-)
n-C H OAc
n-C H OH
2
8
17
8
17
(
1
9
7 (20)
9 (96)
1
4
n-C H OAc
n-C H OH
8 17
8
17
16
n-C H OH
8
17
4
3
The leached species were determined to be catalytically inac-
tive via a hot filtration experiment under identical condi-
tions. A sample of catalyst was refluxed in ethanol under N2
in a Schlenk flask and the colorless supernatant was cannula-
filtered from the catalyst while still hot. After cooling, the
1
6
5
6
n-C H OC(O)C H
(2, 43)
3, 40)
8
17
7
15
(
4
7 (44)
16
4
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ACS Catalysis
supernatant was transferred to a standard reaction apparatus
(O ) Mo(=O) @C catalysts, Mo K-edge XAS and Mo(3d) XPS
c 2 2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and found to be inactive for transesterification of n-octyl
acetate at 90 ˚C. Further experimental details are available in
the Supporting Information (SI, S7).
provides further evidence for the monomeric character of
IV
(O ) Mo(=O) @C sites. Mo species exhibit no pre-edge
c
2
2
8
2
XANES peak, and edge energies are significantly lower than
VI
IV
IV
for Mo (see Table S11 for Mo and Mo XAS standards). It
is known from the literature that Mo clusters exhibit size-
Discussion
83
dependent binding energy shifts due to final state effects.
Catalyst characteristics and nature of the active site. In con-
Mo species in small oligomeric or polymeric clusters shift the
VI
trast to traditional preparative methods such as incipient
Mo (3d) binding peak to higher energies than observed for
,25,74
VI
84,85
wetness impregnation with ammonium hepta-molybdate
Mo in MoO3.
In contrast, for Mo species constrained
8
6
which yields supported Mo catalysts with up to 30 wt% Mo in
from polymerizing (e.g., when stabilized in Y zeolite ) this
shift is not observed. The carbon support employed for
(O ) Mo(=O) @C is more polarizable than MoO , so that
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
25,74
previous studies,
the preparation of the present
(
O ) Mo(=O) @C catalysts by grafting reaches a maximum
c 2 2
c
2
2
3
loading of 5 wt% Mo. The maximum surface density of Mo-
final state effects would be expected to shift isolated Mo-
species binding energies to lower values than for MoO3.
2
sites is thus 0.22 Mo/nm , although most reactions reported
2
here were carried out at 0.10 Mo/nm . No significant differ-
Comparison to reference MoO in our spectrometer, as well
3
ence in activity per Mo was observed at either loading, sug-
gesting the sites remain uniformly dispersed up to the maxi-
as literature values, indicates that the predominant
(
O ) Mo(=O) @C species is indeed hexavalent with no de-
c 2 2
2
V
IV
87,88
mum loading investigated of 0.22 Mo/nm . Dispersions of
tectable Mo or Mo species
, and that the shift of the
conventional MoO catalysts supported on refractory metal
x
Mo(3d5/2) peak to the low binding energy of 232.8 eV is indic-
2
VI
oxides are typically in the range of 0.15 – 5.0 Mo/nm (e.g.,
the preparations in reference
usually obtained at loadings closer to a monolayer
ative of monomeric Mo species.
75-77
), but maximum activity is
7
5,78-80
Temperature-programmed reduction (TPR-H
)
2
of the
,
(
O ) Mo(=O) @C species confirms the XPS analysis (Figure
c
2
2
suggesting substantial chemical differences between tradi-
1D). (O ) Mo(=O) @C reveals a single low-temperature re-
c
2
2
tional MoO preparations and the present (O ) Mo(=O) @C
x
c
2
2
duction event at 218 ˚C, suggesting the Mo active site to be
both highly uniform and highly reducible. Indeed, at least
50% of the Mo sites present in (O ) Mo(=O) @C consume
catalyst. From these results, it can be inferred that the use of
a discrete, monomeric Mo precursor in a controlled grafting
reaction enables superior activity of the resulting single-site
species and a greater profusion of active sites (in this case, ca.
c
2
2
H in this experiment. To the best of our knowledge, 218 ˚C is
2
the lowest reported reduction temperature of a supported
5
0%).
MoO species. For example, Wang reported reductions of
x
2
BJH N physisorption analysis of the (O ) Mo(=O) @C cata-
2
c
2
2
MoO /SBA-15 at surface densities from 0.25 – 10.24 Mo/nm
x
37
8
9
lysts indicates both meso- and microporosity. Since the
porosity does not change with Mo grafting, it is inferred that
the Mo sites do not locally block carbon micropores. XPS is
capable of detecting Mo species in the catalyst (Figure 1B)
with a sampling depth of 10’s of nms. The high chemical sur-
face area of the carbon results from its mesoporous nature. It
and reported no reduction events below 400 ˚C. In addi-
tion, reduction temperatures were found to decrease with
increasing MoOx oligomerization, suggesting monomeric
species are the least reducible under the reported conditions,
in direct contrast to the present (O ) Mo(=O) @C catalyst.
c
2
2
Miranda studied MoOx supported on a variety of silica-
90
is therefore most likely that the (O ) Mo(=O) @C species is
c
2
2
aluminas and observed no reduction below 350 ˚C. Feng
well-dispersed throughout the meso- and micropores of the
carbon, rather than localized on the carbon surface. This
interpretation is consistent with a lack of pore blockage by
Mo species and supported by the lack of change in BET and
supported ammonium heptamolybdate on activated carbon
and found multiple reducible species with reduction temper-
atures decreasing with increasing loading. For example,
2
Mo/C with 0.86 Mo/nm showed a reduction event at 341 ˚C.
2
BJH N physisorption after the grafting reaction. Thus, the
2
At 2.3 Mo/nm , however, a lower-temperature reduction
(
O ) Mo(=O) species must then be well-dispersed through-
c 2 2
peak near 300 ˚C was also observed. Chary reported on
MoO supported on Nb O -TiO and showed that at the low-
out the bulk of the carbon support, an interpretation con-
x
2
5
2
sistent with the lack of change in BET and BJH N physisorp-
2
est MoO loading, the lowest-temperature reduction was at
x
24c
tion after the grafting reaction. In contrast, while ICP-AES
indicates the presence of sulfur, nitrogen, and phosphorous
impurities (SI, Table S1), XPS does not detect these heteroa-
toms, suggesting that they are localized deep within the sup-
port and are not uniformly dispersed throughout. Therefore,
it is proposed that the (O ) Mo(=O) @C sites are grafted
4
14 ˚C.
Previous studies indicated that highly dispersed MoO spe-
x
75
cies generate Brønsted-acidic sites, however, the relation-
ship between Mo loading, acid site density, and the strength
52
of the acid sites remains ill-defined. Typically, assignment
c
2
2
of Brønsted acid strength can be established by the Tmax peak
onto carbon through the C-O functionalities detected by XPS
Figure 1B). The relatively low saturation point for surface
position in NH TPD; temperature ranges are designated as
3
(
2
weak acid (150 – 300 ˚C), moderate acid (300 – 500 ˚C), and
Mo (0.22 Mo/nm ) suggests that the grafting reaction is sur-
face-controlled and appropriate reactive surface sites are in
low density.
53
strong acid (> 500 ˚C). Commonly, dispersing MoOx spe-
cies increases the density of weak and moderate acid sites
52
while consuming strong acid sites. While (O ) Mo(=O) @C
c
2
2
Previous studies of supported MoO catalysts indicate that at
x
is not stable above 450 ˚C, based on literature precedent,
high dispersions, the dominant Mo surface structure is a
dispersed MoO species do not produce strong Brønsted-acid
x
45-47,49,50,81
1
3,52-54,75
tetrahedral dioxo species.
In the present
sites,
so signals > 500 ˚C are not expected. Note that
5
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Page 6 of 12
no new acid sites appear in (O ) Mo(=O) @C relative to the
the ethanoate product (5) is isolated as a white solid in 97 %
c
2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
carbon support, and the Tmax of the largest ammonia peak
shifts down from 280 ˚C (carbon) to 250 ˚C (2.1wt%
(O ) Mo(=O) @C). No new peaks corresponding to moder-
yield, indicating that this catalyst and methodology is a via-
ble alternative to harsh alkaline and/or Brønsted acidic pro-
cess for the production of biodiesel.
c
2
2
ate acid sites are observed for (O ) Mo(=O) @C although
c
2
2
A proposed catalytic cycle for the transesterification of esters
with ethanol is shown in Scheme 3. The catalytic cycle fol-
desorption peaks from 2.1wt% (O ) Mo(=O) @C are consid-
c
2
2
3
2
erably sharper than for the parent carbon; this is consistent
lows those previously suggested in the literature and bears
similarities to that of the well-studied mechanism for trans-
esterification by oxo-tin compounds such as di-n-butyltin
75
with previous reports. From these data, it appears that the
present (O ) Mo(=O sites do not operate as classic
c
2
)2
98
+
Brønsted-acidic catalysts, suggesting an alternative catalytic
oxide (DBTO). Activation of ethanol occurs via H abstrac-
tion by the Mo=O fragment (Scheme 3, species a) and con-
comitant formation of a Mo-ethoxide species (Scheme 3,
species b). Molybdenum oxo-hydroxo-alkoxides are known
3
2
mechanism proposed in this work (vide infra) and others.
The stability of highly-dispersed dioxo-molybdenum sites in
O ) Mo(=O) @C is confirmed via recyclability studies and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
c
2
2
99
further supported by spectroscopy and H -TPR measure-
in the literature and the resulting structures exhibit catalyt-
2
1
00
ments on expended catalysts. Mo(3d) XPS indicates no bind-
ing energy shift in the Mo species after catalysis. Similarly,
ic activity and/or have been implicated in catalytic mecha-
1
01-104
nisms.
Since the present kinetic studies show the reac-
H -TPR indicates that the Mo species remains uniform and
tion to be first-order in EtOH concentration, it is reasonable
to propose addition of one equiv. EtOH to the pre-catalytic
molybdenum-dioxo site to form the active species (Scheme 3,
b). When the activated Mo-ethoxide species coordinates an
ester (as in Scheme 3, i), the activated ester carbonyl
2
highly reducible. Lastly, the activity of the (O ) Mo(=O) @C
c
2
2
catalyst does not diminish significantly even after repeated
use.
Catalytic scope and mechanism. Primary esters such as n-
octyl acetate (Table 1, entry 4) and n-octyl octanoate (Table 1,
entry 5) react to form n-octanol, ethyl acetate, and ethyl oc-
tanoate respectively. N-octyl octanoate reacts at roughly half
the rate (43 % conversion at 16 h) as n-octyl acetate (99%
conversion at 16 h), due most likely to increased steric hin-
(
Scheme 3, c) undergoes nucleophilic attack by the ethoxide
to yield the ethyl ester product (Scheme 3, ii) and a new
91
drance. In n-octyl octanoate the fall in rate is more pro-
nounced than in the case of the very sterically challenged
and electronically different 1-methylcyclohexyl acetate (Table
1, entry 12), which reacts at a rate comparable to n-octyl ace-
tate (97% conversion in 16 h). The secondary esters 2-octyl
acetate and cyclohexyl acetate (Table 1, entries 6 and 7) react
more slowly than the primary n-octyl acetate, excluding sub-
stitution at the alkyl position as a factor in rate. The lack of a
distinct trend in degree of substitution implies that the
mechanism does not proceed through a carbocation inter-
92,93
mediate, excluding C-O cleavage or deacylation
as a reac-
tion pathway and suggesting instead the scenario presented
in Scheme 3. Note that replacing the acyl group with an aryl
moiety increases the rate (Table 1, entry 8). In the case of
entry 8, although overall conversion is high (76%), the yield
of recovered product is reduced (25% for the alcohol, 30% for
the ethyl ester). This is likely a result of adsorption to the
catalyst surface, although whether it is of the substrate or an
94-96
aldehyde reaction intermediate is unclear.
Subjecting γ-
valerolactone (Table 1, entry 9) to transesterification condi-
tions in EtOH results in 25 % conversion to ethyl butyrate.
Benzylic 1-phenethyl acetate undergoes 44 % conversion in
16 hours producing the corresponding alcohol as well as the
ketone in 8 % yield (Table 1, entry 10). Again, the final yield
of the aromatic product is low. The ketone presumably forms
Scheme 3. Proposed catalytic cycle for the transesterification
of esters with ethanol by (O ) Mo(=O) @C
97
from the dehydrogenation of the alcohol. In contrast to
other aromatic substrates, entry 11 (Table 1) is recovered in
good yield (77%), suggesting that adsorption of the substrate
to the catalyst surface does not occur with all substrates and
may require an α,β-unsaturated carbonyl functionality.
.
c
2
2
Mo-OR fragment (Scheme 3, d). Further exchange of the Mo-
OR fragment with EtOH (Scheme 3, iii) regenerates the acti-
vated Mo-OEt fragment (Scheme 3, c) and enables catalytic
turnover. Since both stability and recyclability of the catalyst
has been established, it is proposed that the
The transesterification of the long chain (n-C H ) triglycer-
1
7
35
ide tristearin with ethanol to produce the corresponding
ethanoate and glycerol is performed on a large scale (Scheme
(
O ) Mo(=O) @C fragment (Scheme 3, a) can be recovered
c 2 2
2
). After aqueous workup followed by extraction with CH Cl ,
2 2
6
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ACS Catalysis
via elimination of an equivalent of EtOH to retain its chemi-
tions. ECW and JTM were supported by start-up funding
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
cal identity.
from Purdue University and the School of Chemical Engi-
neering.
Conclusions
REFERENCES
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and shown to have unique chemical characteristics among
known supported MoO materials. Under mild conditions,
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of
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c
2
2
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2
3
esterification procedure, preparative scale triglyceride trans-
esterification, n-octyl acetate conversion, n-octyl acetate
conversion with varying [EtOH], n-octyl acetate conversion
with varying [Mo], H NMR of tristearin (triglyceride), H
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AUTHOR INFORMATION
Stalzer, M. M.; Miller, J. T.; Gagliardi, L.; Hupp, J. T.; Marks,
T. J.; Cramer, C. J.; Delferro, M.; Farha, O. K. J. Am. Chem.
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T.; Pruski, M.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc.
Corresponding Author
pstair@northwestern.edu (P. C. S)
t-marks@northwestern.edu (T. J. M.)
delferro@anl.gov (M.D.)
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Author Contributions
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These authors contributed equally.
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2
(
ACKNOWLEDGMENT
Financial support by the Chemical Sciences, Geosciences,
and Biosciences Division, U.S. Department of Energy
through grant DE FG02-03ER15457 to the Institute of Cataly-
sis in Energy Processes (ICEP) at Northwestern University.
NSF grant CHE-1213235 supported T.L.L. and provided reac-
tor equipment. We gratefully acknowledge Northwestern
Clean Cat facilities. Use of the Advanced Photon Source is
supported by the U.S. Department of Energy, Office of Sci-
ence, and Office of Basic Energy Sciences, under Contract
DE-AC02-06CH11357. MRCAT operations are supported by
the Department of Energy and the MRCAT member institu-
Thivolle, C. J. Acc. Chem. Res. 2010, 43, 323.
(22) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.;
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(
interaction between hydrophobic ester substrates and the
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y = -5258.4x + 7.4255
R² = 0.9851
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DH = 10.5 (8) kcal/mol
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DS = - 32 (2) e.u.
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.0027 0.0028 0.0029 0.003 0.0031
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