Vapor Phase Hydrogenolysis of Furanics Utilizing Reduced Cobalt Mixed Metal Oxide Catalysts

Article abstract of DOI:10.1002/cctc.201700228

Vapor phase hydrogenolysis of both furfuryl alcohol and furfural are investigated over reduced Co based mixed metal oxides derived from the calcination of a layered double hydroxide precursor. Although a reduced cobalt-aluminum oxide sample displays promising selectivity towards 2-methylfuran (2-MF) production, the addition of an Fe dopant into the oxide matrix significantly enhances the activity and selectivity per gram of catalyst. Approximately 82 % 2-MF yield is achieved at high conversion if furfuryl alcohol is fed into the reactor at 180 °C over the reduced 3Co-0.25Fe-0.75Al catalyst. Based on structural characterization studies including temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption spectroscopy (XAS) it is suggested that Fe facilitates the reduction of Co, allowing for formation of more metallic species. Overall, this study demonstrates that non-precious metal catalysts offer promise for the selective conversion of a key biomass oxygenate to a proposed fuel additive.

Full text of DOI:10.1002/cctc.201700228

Accepted Article
Title: Vapor Phase Hydrogenolysis of Furanics Utilizing Reduced
Cobalt Mixed Metal Oxide Catalysts
Authors: Taylor P Sulmonetti, Bo Hu, Zachary P Ifkovits, Sungsik Lee,
Pradeep K. Agrawal, and Christopher W Jones
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201700228
Link to VoR: http://dx.doi.org/10.1002/cctc.201700228
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Vapor Phase Hydrogenolysis of Furanics Utilizing Reduced Cobalt Mixed
Metal Oxide Catalysts
Taylor P. Sulmonetti,a Bo Hu,a Zachary Ifkovits,a Sungsik Lee,b Pradeep K. Agrawala and
Christopher W. Jonesa*
associated with Cr, development of alternative catalysts is
worthwhile. Many monometallic catalysts, typically supported on
silica, have been investigated for hydrogenation of furanic
compounds. However, these typically suffer from poor selectivities
towards 2-MF.[6,7] Improvements in selectivity for catalytic
conversion of furanics has often been achieved by the incorporation
of multiple metals to form bimetallic or bifunctional catalysts. In this
work, the term bimetallic catalyst will refer to a catalyst that contains
two fully reduced metals forming an alloy, while the term bifunctional
catalyst will refer to catalysts that combine a metallic species and
metal oxide species, which may form acidic or basic sites.[8] In
regards to 2-MF production, prior work involving Ni-Fe/SiO2 has
been conducted to investigate the enhancement in selectivity that
occurs when a secondary metal is added.[9] Through DFT and
kinetic studies, it was suggested that the oxophilicity of Fe facilitated
the adsorption of FA through a η2-(C,O) surface species, which
drove the reaction towards hydrogenolysis of the C=O bond.[9,10]
Additionally, very recent studies conducted with Cu-Fe supported
on carbon and silica also demonstrated a synergistic effect between
the more oxophilic Fe species and the more reduced Cu species for
hydrogenation and hydrogenolysis of FUR.[11,12] Temperature-
programmed desorption (TPD) and high resolution electron energy
loss spectroscopy (HREELS) under ultra-high vacuum conducted
on Pt-Zn catalysts suggested a similar mechanism, where Zn acted
as the oxophilic metal and allowed for a tilted adsorption mode to be
more prevalent on the catalyst surface.[13] Other vapor phase
reactions have been conducted with multiple metal systems
including Cu-Zn-Al and Cu-Zn-Al-Ca-Na.[14,15] Lastly, Mo2C catalysts
have been shown to be selective towards 2-MF due to the creation
of two active sites (bifunctional), a “metal-like site” and a carbidic,
oxycarbidic, or oxide site; however, the type of secondary site
remains unknown based on the evidence presented to date.[16]
Many investigations involving liquid phase reactions, both flow
and batch, for the hydrogenolysis of furanics including FUR to 2-MF
and hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) have
been performed, mainly involving multiple metal catalyst systems.
In regards to HMF to DMF, Ni/Co3O4, Ru/Co3O4, Ni-W2C/C, Cu-
Ru/C, Pt-Co/C, Cu-Co/C and Cu-ZnO have been investigated, and
in regards to FUR to 2-MF, Ru-RuO2/C, Cu-Co/Al2O3, and Pd/TiO2
have been utilized.[4,17–25] Unfortunately, many of these reactions
require high H2 pressures, have low product throughput, and/or use
precious metals, which makes vapor phase flow reactions with non-
precious metals a potentially attractive alternative. Consequently, in
this work, we sought to create a non-precious, multi-metal system
that would be selective towards hydrogenolysis of the C-O bond of
FA in vapor phase flow reactions.
Abstract: Vapor phase hydrogenolysis of both furfuryl alcohol and
furfural were investigated over reduced Co based mixed metal oxides
derived from the calcination of a layered double hydroxide precursor.
Although
a reduced cobalt aluminate sample displays promising
selectivity towards 2-methylfuran (2-MF) production, the addition of an
Fe dopant into the oxide matrix significantly enhances the activity and
selectivity per gram of catalyst. Approximately 82% 2-MF yield is
achieved at high conversion when furfuryl alcohol is fed into the reactor
at 180 °C over the reduced 3Co-0.25Fe-0.75Al catalyst. Based on
structural characterization studies including TPR, XPS, and in-situ XAS
it is suggested that Fe facilitates the reduction of Co, allowing for
formation of more metallic species. Overall, this study demonstrates that
non-precious metal catalysts offer promise for the selective conversion
of a key biomass oxygenate to a proposed fuel additive.
Introduction
Second generation lignocellulosic biomass, the inedible fraction of
biomass, is a target for conversion into chemicals and fuel additives
because of its perceived likelihood to not interrupt food supplies.[1,2]
Lignocellulose can be broken down into various sugars that can be
further processed through dehydration to create furanic compounds.
Furfural (FUR) and furfuryl alcohol (FA) are major platform
chemicals in biomass processing that can be catalytically converted
into valuable products such as tetrahydrofurfuryl alcohol (THFA), 2-
methylfuran (2-MF), 1,2-pentanediol (1,2-PD), and 1,5-pentanediol
(1,5-PD), among others.[3] Through the hydrogenolysis of FA, 2-MF
is produced, which is a valuable fuel additive. With a research
octane number of 131 and low water solubility, 2-MF is an attractive
replacement for bio-ethanol as a fuel additive.[4] Therefore, recent
research has focused on catalytically converting FUR and/or FA
selectively towards 2-MF.
Industrially, Cu-Cr catalysts have shown versatility in FUR
conversion, and at certain conditions, these catalysts give high
yields towards 2-MF.[5] However, due to environmental concerns
[a]
[b]
T. P. Sulmonetti, Dr. B. Hu, Z. Ifkovitis, Prof. P. K. Agrawal, Prof. C.
W. Jones
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology
311 Ferst Dr., Atlanta, GA 30332, USA
cjones@chbe.gatech.edu
Fax: (+1) 404-894-2866
S. Lee
Advanced Photon Source
Argonne National Laboratory
Lemont, IL 60439, USA
A previous study involving Co-Al mixed metal oxides derived from
layered double hydroxides (LDH) displayed elevated selectivities
Supporting information for this article is given via a link at the end of
the document.
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towards 2-MF in the vapor phase hydrogenation of FUR at 155 °C.[26]
Use of LDH materials as precursors to synthesize mixed metal
oxides is a well-known approach to create well-dispersed and
relatively high porosity materials.[27] The LDH material allows for
high interaction between the various metals in the matrix, and can
lead to smaller metal domains upon reduction. To this end, Co was
chosen as the base metal in the catalyst family explored here, while
various Fe amounts were added into the matrix to tailor the activity
and selectivity. Reduction with hydrogen allowed for the creation of
active metallic species, and through temperature-programmed
reduction (TPR), X-ray absorption spectroscopy (XAS), and X-ray
photoelectron spectroscopy (XPS) it is shown that the incorporation
of Fe facilities reduction at lower temperatures, which in turn
enhances both selectivity and activity towards 2-MF.
prominent (003) and (006) peaks. These peaks represent the
layered hydroxides with intercalating CO3 and H2O to balance
framework charge. The XRD patterns post calcination
-2
Results and Discussion
Our main interest in LDH based materials is that the synthesis
procedure allows for versatility in metal additions while maintaining
similar physical properties. After calcination of these materials, the
intercalated anions are removed, leaving a porous mixed metal
oxide with high dispersion of metal oxide species. This high
dispersion suggests intimate contact of various metal species,
which could improve activity or selectivity of the catalyst. To that end,
Co was chosen as the base metal in the system due to the unique
selectivity properties previously determined in FUR reduction.[26]
Along with Co, small amounts of Fe were added into the mixed metal
oxide matrix to tune the activity and selectivity towards 2-MF. In
Table 1, the physical properties are summarized for various Co/Fe
ratio.
The porosity of the post calcined LDH materials is displayed in
Table 1, which shows that the BET surface area for all materials was
within a narrow range (105 m2/g-150 m2/g), while the pore volumes
were approximately 0.30 cm3/g. ICP analyses were in fairly good
agreement with the targeted molar ratios, with slightly lower Co
amounts than anticipated. For simplicity throughout the manuscript,
the catalysts were labeled with their target molar ratios.
Figure 1: a) XRD patterns of Co-xFe-yAl catalysts after calcination at 400 °C; b)
Enlarged section of major Bragg peak between 35-40°.
demonstrated that a non-crystalline structure was created, as
evidenced by the lack of sharp peaks associated with the oxides
present in Figure 1a. One broad peak is distinguishable between
35-40°, which correlates with the Co3O4 (311) peak at 37.0°.
Additionally, when that section is enlarged in Figure 1b, there was a
small yet noticeable shift to lower Bragg angles with the increased
addition of Fe. The poor crystallinity along with the shift in the Co3O4
(311) peak towards the Fe2O3 (311) peak at 35.0°, along with no
visible Al2O3 peaks, might suggest good dispersion of the oxide
species, potentially forming a solid solution similar to Ni-Co-Al and
Ni-Mg-Al mixed metal oxides reported in prior literature.[26–28]
The reduction profile of each catalyst was investigated through
TPR to gather information about the catalyst under an H2
atmosphere with varying temperature. Figure 2 shows the reduction
profile of each catalyst in
10% H2/Ar using a 5 °C/min
ramp rate. All five profiles
Table 1: Physical properties of multiple Co based mixed metal oxides.
demonstrate
reduction, one at
temperature
a
two-step
low
between
a
Catalyst
Elemental analysis (Co:Fe:Al)
BET Surface Area
(m2/g)
Pore Volume
(cm3/g)
Average Pore
Diameter (nm)
200 °C- 300 °C and one at
high temperature between
400 °C - 750 °C. The low
temperature peaks may
refer to Co3O4 and Fe2O3
phases reducing to CoO,
and Fe3O4/FeO, while the
broad, high temperature
peak is associated with the
reduction of cobalt and iron
3Co-Al
2.94:0:1.0
129
150
0.35
0.32
11.0
8.6
3Co-0.25Fe-0.75Al
2.83:0.25:0.75
3Co-0.5Fe-0.5Al
3Co-0.75Fe-0.25Al
3Co-Fe
2.83:0.50:0.50
2.92:0.74:0.26
2.93:1.0:0
133
115
107
0.30
0.31
0.33
8.7
10.4
12.1
After calcination at 400 °C, most LDH materials lose their layered
structure to create a more homogeneous mixed metal oxide (MMO).
Through XAS and XPS analysis, it was determined that the majority
of the crystalline species present after calcination were Co3O4,
Fe2O3, and Al2O3, and there was no indication of CoO, FeO, or
significant metal aluminate species. The XRD patterns of the
uncalcined material are shown in Figure S2, which display
oxide species to metallic Co and Fe.[28,29] As expected, the addition
of Fe slightly shifted the peak reduction temperature to lower
temperatures since the reduction potential of Fe oxide was much
greater than Al2O3. Without any Al2O3, the TPR profile became much
sharper, and the H2 uptake began to significantly decline around
500 °C. This indicates that the Al2O3 acts as a stabilizer, even at 6.6
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wt% in the matrix, since it was unable to reduce under these
conditions, and may help prevent complete aggregation of metallic
species.
Initial testing helped distinguish the effects of Fe addition into the
Co-Al matrix. It is clear that the addition of Fe significantly enhances
the activity of the catalysts, while also increasing the selectivity
towards the desired product. In all samples that contained Fe, the
selectivity towards 2-MF hovered around 83% while 2-MF selectivity
for the 3Co-Al was between 53%-57%. The 3Co-Fe catalyst is not
displayed due to its inactivity under the conditions used for these
reactions. A wide variety of other products were obtained due to the
many side reactions and series reactions that are possible (Table
S1). Additional products include furan, 2-MTHF, 1-butanol (1-BOL),
2-pentanone (2-PONE), 1-pentanol, 2-pentanol, etc., the latter of
which are a result of ring-saturation, decarbonylation, and ring-
opening. Interestingly, some hydrocarbons were observed, such as
propane and butane, which indicates minor hydrocracking pathways,
similar to what was previously observed over a Cu/Cr/Ni/Zn/Fe
catalyst in the vapor phase hydrogenation of 2-MF.[30] At similar
WHSV, the Co catalysts with lowest Fe addition displayed the
highest activity, followed by 3Co-0.5Fe-0.5Al and 3Co-0.75Fe-
0.25Al. To assess if even lower Fe loading might further elevate the
catalytic activity, a catalytic test was performed on a lower Fe loaded
catalyst (3Co-0.125Fe-0.875) which demonstrated an incremental
increase in conversion (99% after 40 min on stream) with no
variation in selectivity. This suggested that there may be an
optimum in regards to activity at lower Fe loadings, elevating activity
above that of 3Co-0.25Fe-0.75Al. Since further characterization
studies, such as XAS, at lower Fe loadings would have been difficult,
only the higher loaded catalysts were chosen for further studies.
Figure 2: TPR profile of each Co-xFe-yAl catalyst in a 10% H2 (balance Ar)
atmosphere with a 5 °C/min ramp rate.
Initial reaction studies with FA as the substrate were conducted at
a common weight hourly space velocity (WHSV) to screen the
various catalysts synthesized. Further studies were conducted on
the 3Co-0.25Fe-0.75Al catalyst due to the observed high activity of
this sample during the initial screening. When comparing the
catalysts at similar WHSV, noticeable differences in conversion
were observed.
Figure 4: Recycle tests of 3 Co-0.25Fe-0.75Al displaying conversion and
selectivity towards 2-MF at W/F (gcat*h*mol-1) = 3.63 with FA as a substrate. Data
point taken at approximately 40 min on stream.
Time on stream data indicated deactivation of all the catalysts over
time; however, little to no change in the selectivity occurred. Post
reaction TGA (Figure S12) experiments displayed little to no carbon
deposition, which may suggest that the major deactivation
mechanism was not blocking of sites from carbon deposits.
Additionally, recycle tests were conducted on the 3Co-0.25Fe-
0.75Al catalyst, which included a calcination step at 400 °C in air for
4 h followed by a reduction step outlined in the experimental section.
Figure 3: Time on stream data of FA conversion and 2-MF selectivity over
various 3Co-xFe-yAl catalysts at W/F (gcat*h*mol-1) = 3.63, 180 °C, 1 atm, and FA
conc = 0.0015 mmol/mL; Black lines represent conversion and red lines
represent 2-methylfuran selectivity. a) 3Co-0.25Fe-0.75Al; b) 3Co-0.5Fe-0.5Al; c)
3Co-0.75Fe-0.25Al; d) 3Co-Al; e) 3Co-xFe-yAl; f) 3Co-Al.
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Each of the catalysts in the recycle test was run for approximately
150 min. The recycle tests demonstrated the catalysts regained
their initial activity upon reuse over 3 recycles, as shown in Figure
4. This may indicate the mixed metal oxide matrix prepared through
the co-precipitation method does not facilitate irreversible sintering
after reduction and the mildly deactivated sample can be easily
regenerated. A more likely deactivation mechanism could be
through the oxidation of metallic sites, since 2-MF production
creates H2O, potentially causing surface oxidation similar to what
Resasco et al. observed over Ni-Fe/SiO2.[6] However, operando
spectroscopic studies would need to be conducted to further verify
the major deactivation mechanism.
be conducted to determine the root cause of ring-saturation
propensity to THFA only at lower conversions.
Investigations into the reactivity of FUR over the Co-Fe-Al catalysts
were conducted to verify reaction pathways and to determine if
additional side products emerged. Various weight hourly space
velocities were tested utilizing the 3Co-0.25Fe-0.75Al catalyst due
its greater activity for FA conversion. As expected, the data for FUR
conversion in Table 2 demonstrate an increase in conversion with
an increase in catalysts loading, and an increase in 2-MF selectivity
as the conversion increases. Similarly, the yield to FA seems to
decrease linearly with the increase in 2-MF, confirming the series
reaction of FUR to FA to 2-MF observed in prior literature.[9]
Further investigations of the 3Co-0.25Fe-0.75Al catalyst were
conducted as a function of the weight hourly space velocity, W/F.
Figure 5 shows the conversion and selectivity of 2-methylfuran
along with significant minor products while the temperature and
pressure remained constant at 180 °C and 1 atm. The values were
obtained at approximately 40 min on stream after the system
reached a pseudo steady-state.
A combined selectivity towards 2-MF and FA of 75-79% was
achieved at a variety of conversion levels. An important distinction
between reactions between FA and FUR was the slight decrease in
carbon balance when FUR was fed into the reactor, and significantly
more unknown peaks were visible at high retention times, indicating
more oligomer formation from FUR. Though the carbon balance
closed within 91% for each FUR run, there was a clear indication of
the increased propensity of FUR oligomerization with itself or other
unidentifiable side products.[31,32] Deactivation was also observed
during continuous flow similar to the deactivation with FA as the
substrate. Ultimately, a yield of 60% towards 2-MF was achieved
when feeding FUR, which is equivalent to or better than most prior
literature using vapor phase reactions.[9,15,16,33] Though a very
recently study of imbedded Cu in SiO2 demonstrated higher yields
towards 2-MF in the vapor phase when FUR was fed as the reactant,
the reactions were conducted mainly above 220 °C.[34,35]
Further characterization of these complex mixed metal oxides was
carried out to better understand the electronic environment of the
catalyst before reaction. X-ray photoelectron spectroscopy was
utilized to probe the catalytic surface after reduction; however, due
to pyrophoric nature of the reduced catalyst, a 1 h passivation step
at room temperature in 1% O2 was conducted prior to employing the
characterization technique. As shown in Figure S5, there is a clear
change in the Co species before and after reduction/passivation of
the 3Co-0.25Fe-0.75Al sample. Prior to reaction, the spectrum
mimics Co3O4 due to the lack of a 2p3/2 satellite peak and the peak
maximum of 2p3/2 at approximately 780.5 eV.[26,36–38] The
emergence of a shoulder after reduction at approximately at 778 eV,
and the presences of a satellite peak, indicates the reduction of
Co3O4 to CoO and Co0.[36] With regards to the Fe species, very little
change is indicated before and after reduction/passivation, with the
peak maximum around 712 eV. This maximum is located at higher
eV than standard Fe oxides (FeO, Fe3O4, and Fe2O3), which may
further support the strong interaction between Fe and Co, causing
Figure 5: Varying W/F of the 3Co-0.25Fe-0.75Al catalyst at 180 °C and 1 atm
with a FA flow rate varying between 5.5-12 mmol/h and H2 flow rate varying
between 60-130 mL/min while keeping the FA concentration at 0.0015 mmol/mL.
Inset graph displays the selectivity of significant minor products over varying
W/F. Values taken at approximately 40 min on stream.
As expected, conversion increases with an
increase in residence time, but interestingly, at
Table 2: Reaction data of FUR over 3Co-0.25Fe-0.75Al at 180 °C at various W/F
high W/F, there is noticeably little difference in
the selectivity towards 2-MF (80-85%). On the
contrary, when lower residence times were
used there was an incremental decrease in 2-
MF selectivity, and an increase in ring
saturation products, THFA, and FUR. These
results were reproducible yet unexpected
since THFA and FUR are not intermediates to
2-MF. The observation is likely a result of
differing surface coverages of various species,
but more mechanistic studies and density
functional theory calculations would need to
W/F
Conversion (%)
2-MF
(%)
FA
(%)
Furan
(%)
1-BOL
(%)
2-PONE
(%)
Others
(gcat*h*mol-1)
3.63
7.27
45.0
83.0
90.0
43.7
57.1
66.6
35.0
19.2
8.5
3.9
4.5
4.0
2.6
3.8
3.9
1.4
1.5
2.4
13.4
13.9
14.6
10.91
Values obtained at approximately 40 min on stream. 1-BOL: 1-butanol, 2-PONE: 2-pentanone,
others include hydrocarbons, MTHF, THFA, 1,2-PD, 1-pentanol, and 2-pentanol.
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the Fe spectra to shift as a consequence of the more electronegative
Co oxide.[39] XPS suggests that the Fe oxide species hardly reduced
or that the passivation had enough of an impact to bring it close to
its original state after initial reduction.
Co species was indicated by the major shift in the white line XANES
of each catalyst and the emergence of a new peak in EXAFS
analysis. Near edge analysis did indicate that the majority of the Co
species were in a metallic state (Table S3). EXAFS analysis of the
catalyst post reduction displayed one major feature at the 1st shell,
which corresponds to Co-Co (Fe) scattering peak associated to a
FCC structure, similar to that of Co foil and stable metallic Co.[42]
Fitting of each catalyst using Co foil as a reference was conducted
to determine the coordination number or Co-Co (Fe) species, since
the magnitude of the peaks are significantly lower than the foil. The
parameters of the fit are given in Table 3, and example fits are
shown in Figure S10 and S11.
To further probe the catalyst structure, in situ X-ray absorption
spectroscopy was used to investigate the electronic environment of
these materials before and after reduction without the need to
passivate the catalyst. The same catalyst pretreatment conditions
were implemented at ANL, and scans were taken at room
temperature to reduce the effect of the Debye-Waller factor, a
correction for thermal vibration effects.[40,41] After investigating the
Co k-edge before reduction and matching each spectrum to
standards, it was clear by X-ray absorption near-edge structure
(XANES, Figure S6) and extended X-ray absorption fine structure
(EXAFS) analysis that the major species was Co3O4, which matches
XPS results (Figure S5). There was no indication of additional spinel
structures formed including CoFe2O4 or CoAl2O4, though there may
still be strong interactions between the various oxide species. Once
the samples were reduced for an hour in H2, a clear change in the
All of the R factors are <0.03, indicating good agreement with the
experimental data and the curve fits. Upon fitting of each spectrum,
it became clear that there was no significant contribution of a Co-O
scattering peak, indicating that the majority if not all the Co species
were reduced to a metallic state. Additionally, even though there
were Fe species most likely intercalated into the Co species, the Co-
Co(Fe) bond distance was within error of the metallic standard. This
may be due to the high amount of Co in the sample as well as Co
and Fe atomic radius being very similar. The most notable
difference between the standard and the fitted catalysts after
treatment was the clear reduction in coordination number of each of
the samples tested, indicating the formation of particles and not a
bulk-like and significantly aggregated metallic structure. Compared
to the 3Co-Al catalyst, all of the 3Co-xFe-yAl catalysts had a minor
increase in coordination number, which may be due to the reduction
of Fe that becomes coordinated with Co or increased ease of
reduction, supported by TPR, causing increased aggregation.
Further investigation of the Fe k-edge provided more evidence of a
change in the electronic environment after reduction. The Fourier
transformed EXAFS data are displayed in Figure 8, and they show
a significant difference between each 3Co-xFe-yAl catalyst. After
reduction, there was clear increase in the peak height of the 1st shell.
Fitting of the Fe EXAFS spectra proved to be more complicated
than for the Co region since the Co k-edge is approximately 600 eV
after Fe k-edge. This limits the EXAFS range of Fe, causing fewer
independent variables available for the fitting. Additionally, the
complexity and potential inhomogeneity of each species in the
sample results in an increase in constraints. XANES analysis of the
Fe k-edge in Figure S9 indicates qualitatively that there may be a
significant presence of FeO for a few samples, which may require a
Fe-O scattering peak within the EXAFS fit. Additionally, fits without
the inclusion of Fe-O proved to be poor for the 3Co-0.25Fe-0.75Al
Figure 6: Fourier transformed EXAFS of Co k-edge post reduction at 500 ºC.
Scans were taken at room temperature, and the k range displayed is 2.7-12 Å-1
with a k weight of k2
Table 3: EXAFS results for the Analysis of Co k-edge post reduction[a]
Sample
Shell
CN
r (Å)
∆σ (10-3 Å2)
∆E0 (eV)
R factor
Co foil[b]
3Co-Al
Co-Co
Co-Co
12
2.50 ± 0.01
2.49 ± 0.01
6.4 ± 0.4
6.5 ± 0.9
7.8 ± 0.5
8.6 ± 1.1
0.008
0.015
5.7 ± 0.6
3Co-0.25Fe-0.75Al
3Co-0.5Fe-0.5Al
Co-Co(Fe)
Co-Co(Fe)
Co-Co(Fe)
6.9 ± 0.6
7.8 ± 0.4
7.1 ± 0.3
2.49 ± 0.01
2.49 ± 0.01
2.47 ± 0.01
6.5 ± 0.7
6.9 ± 0.4
6.5[c]
7.7 ± 0.8
7.1 ± 0.5
7.3 ± 1.0
0.007
0.003
0.013
3Co-0.75Fe-0.25Al
[a]Fitting parameters: Fourier transform range, ∆k, 2.7-12 Å-1 with weighting k2. [b]Fourier transform range, ∆k, 2.7-14 Å-1 with weighting k2. Coordination
number assigned from standard FCC Co0 structure; S02 (Co-Co) = 0.806 determined from Co foil fitting. [c]The Debye-Waller factor was fixed to the average to
keep consistency with the other samples/model.
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and 3Co-0.5Fe-0.5Al samples. Furthermore, in both 3Co-0.25Fe-
0.75Al and 3Co-0.5Fe-0.5Al, the Fe-Fe(Co) peaks for metallic Fe
were modelled as a FCC structure while for 3Co-0.75Fe-0.25Al the
side note, Fe-Al bonds were neglected since there was no evidence
of alloying of Al species, and there was only existence of an Al3+
oxide structure. Table 4 displays the best fit and the structural
parameters associated with each.
Analysis of the Fe k-edge proved to have a more significant
difference than the Co k-edge for each catalyst. Even though the
majority of the Co species were in metallic form, the Fe species still
maintained some Fe oxide species in the lower Fe loaded samples.
Additionally, through various model tests, EXAFS suggested that
the 0.25 and 0.5 Fe loaded samples contained Fe0 species that
were substituted into the lattice of the FCC Co0. This supports the
hypothesis of good dispersity of each species resulting from the
starting LDH precursor, and may suggest the formation of a
bimetallic particle upon reduction. As the Fe loading increases,
EXAFS analysis suggests a BCC structure similar to bulk metallic
Fe begins to emerge.[44] In regards to the Fe-O scattering path fitted
in the two samples, the bond distance was determined to be
approximately 2.00 Å, which is lower than a standard Fe-O in an
FeO structure, 2.17 Å.[45] However, for an Fe-O bond incorporated
into an FeAl2O4 matrix the bond distance is approximately 1.95 Å,
which suggests that Al influences the Fe-O bond distance,
decreasing the overall average bond distance.[46]
Unlike the Co EXAFS analysis, there is a clear trend in which there
was an increase in metal-metal coordination as the amount of Fe
increased. This correlated well with TPR, which indicated that the
higher Fe loading materials were easier to reduce overall. The
increase in metal-metal coordination was indicative of aggregation,
which could result in larger particle sizes. This larger particle size
and loss in metallic surface area supported the loss in activity per
gram of catalyst for higher Fe loading materials, as shown in Figure
3. Additionally, at the higher Fe loading it may have be possible that
the metallic Fe phases are beginning to separate from Co due to the
formation of a BCC structure. Though reactivity studies showed
Figure 7: Fourier transformed EXAFS of Fe k-edge post reduction at 500 ºC.
Scans were taken at room temperature, and the k range displayed is 2.7-10.5 Å-1
at a k weight of k2.
Fe-Fe(Co) peak was modelled after a BCC crystal structure, similar
to that of Fe foil. This became necessary after attempts at fitting a
BCC structure for all catalysts, and poor fits were obtained for the
lower Fe loaded samples. Lastly, a qualitative evaluation of the 2nd
shell (4-5 Å) shows the emergence of a major peak for 3Co-0.75Fe-
Table 4: EXAFS results for the analysis of Fe k-edge post reduction[a]
Sample
Fe Foil[b]
Shell
CN
r (Å)
∆σ (10-3 Å2)
∆E0 (eV)
R factor
0.004
Fe-Feedge
Fe-Fecenter
8
6
2.46 ± 0.01
2.85 ± 0.01
5.0 ± 0.5
7.5 ± 1.1
5.2 ± 1.3
5.2 ± 1.3
Fe(III)acac[c]
Fe-O
6
1.93 ± 0.01
10.3 ± 0.8
-0.5 ± 1.2
0.012
0.027
3Co-0.25Fe-0.75Al
Fe-O
2.1 ± 0.6
3.6 ± 0.3
2.01 ± 0.02
2.49[f]
9.5[e]
6.5[f]
-5.9 ± 1.0
-5.9 ± 1.0
Fe-Co(Fe)[d]
3Co-0.5Fe-0.5Al
Fe-O
1.4 ± 0.5
5.1 ± 0.3
1.99 ± 0.03
2.49[f]
9.9[e]
-3.6 ± 0.6
-3.6 ± 0.6
0.013
0.006
Fe-Co(Fe)[d]
6.9[f]
3Co-0.75Fe-0.25Al
Fe-Feedge (Co)
Fe-Fecenter (Co)
5.6 ± 0.7
4.2 ± 0.5
2.44 ± 0.01
2.82 ± 0.01
5.3 ± 0.1
7.7 ± 0.2
-3.7 ± 2.7
-3.7 ± 2.7
[a]Fitting parameters: Fourier transform range, ∆k, 2.7-10.5 Å-1 and R range, ∆R, of 1.2-3.0 Å with weighting k2 unless stated otherwise. [b]Fourier transform
2
range, ∆k, 2.3-13 Å-1 with weighting k2; Coordination number assigned from standard BCC Fe0 structure; S0 (Fe-Fe) = 0.733 determined from Fe foil fitting.
2
[c]Coordination number assigned from standard Fe(III) acetylacetonate structure; R range, ∆R, of 1.1-2.2 Å and S0 (Fe-O) = 0.721 determined from fitting.
[d]Modelled after Co0 FCC structure where Fe0 replaces Co0 atoms. [e]Manually optimized Debye-Waller factor. [f]Constrained values to corresponding Co-Co
from Co EXAFS fitting since the model is Fe0 replacing Co0 in the lattice.
0.25Al that mimics a BCC structure formation, while the other
catalysts still mimic scattering peaks of the 2nd shell associated with
FCC.[43] Therefore, for the two lower Fe loading catalysts, a model
where Fe0 atom is substituted into the FCC lattice of Co0 and Fe-O
scattering path was more consistent with the data collected. As a
there was no indication of a change in active sites since the
selectivities do not vary, the loss in activity may be due to the loss
in the amount of active sites due to metal aggregation. The highly
dispersed Fe species for each catalyst may provide an electronic
effect on the majority metallic Co species present in the catalysts,
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Co-Fe-Al mixed metal oxides with varying Co:Fe ratios of 12:1, 6:1, 4:1, 3:1,
and no Fe addition (Co/(Fe+Al) always kept at 0.25) were prepared in the
following manner, similar to prior LDH syntheses, with minor
which may lead to minimal change in selectivtiy over the different
Fe/Al ratios. Lastly, the small amount of Fe oxide present in the
samples with lower Fe loadings may facilitate the adsorption of the
highly oxygenated compound allowing for an increase in activity per
gram of catalyst, similar to what was observed with a partially
reduced Fe species mixed in a Cu-Fe/SiO2 catalyst.[11] Unfortunately,
due to the complexity of these catalysts and the highly oxophilic
nature after reduction (discussed above with XPS characterization)
it proved difficult to measure the active site density on the catalyst
post reduction, and thus TOFs are not reported.
modifications.[26,47]
A 0.6 M metal nitrate solution, Solution A, with
corresponding metal cations ratios above, and a 1.0 M NaOH solution,
Solution B, were created. A 0.3 M solution of Na2CO3, Solution C, was placed
in a 3-neck, 1 L flask, and using a peristaltic pump Solution A and Solution B
were added dropwise into Solution C at approximately 5 mL/min under
vigorous stirring, at room temperature, with N2 bubbling. The pH was
maintained at 10 by adjusting Solution B. The pump was shut off once
Solution A ran out, and the final solution was heated to 60 °C, and maintained
under vigorous stirring and N2 bubbling for 24 h. Once complete, the final
solution was filtered and washed until the filtered solution had a pH of ~7
(about 1.5 L of DI water was used) to obtain the LDH precipitant. The
recovered solid was dried in an oven at 105°C overnight. To create the mixed
metal oxide, the dried LDH material was calcined at 400°C with 50 mL/min
flow of air with a ramp rate of 10°C/min, being held at 400°C for 4 hours.
Reaction Studies
Vapor phase reactions were performed in a ¼” tubular stainless steel
reactor located inside a furnace. The catalyst was pelletized (>270 mesh),
diluted with SiC (200-400 mesh), and loaded into the reactor between layers
of SiC (46 mesh) and quartz wool. The catalysts were reduced in situ under
60 mL/min H2 (Airgas, UHP) to 500˚C with a ramp rate of 5°C/min, then
subsequently held for one hour. Once the reduction was completed, the bed
was cooled under H2 to reaction temperature, which was typically conducted
at 180 °C. Unless stated otherwise, reactions were conducted in a flow of 60
mL/min of H2, while FA was pumped into the vaporization zone at a rate of
5.5 mmol/h (with ~4 mol% ethylene glycol diethyl ether as internal standard).
The FA flow met heated H2 in the vaporization zone, which was heated to
approximately 180 °C. The vapor flowed through the catalyst bed, and then
it flowed into an online Agilent 7890A GC through lines heated to
approximately 185°C to mitigate reactant and product condensation.
Selectivity and conversion were determined by the internal standard method,
and carbon balances were within 95% typically due to small amounts of
unknown products, unless stated otherwise. After the reaction or reduction
was completed, the catalyst was passivated for one hour at room
temperature under 1% O2/N2 (Airgas), separated form SiC, and stored under
argon for further characterization studies.
Conclusion
Co based mixed metal oxides were derived from the calcination of
LDH materials, yielding porous, thermally stable, non-precious
metal catalysts for the hydrogenolysis of FA to 2-methylfuran, a
promising biomass derived fuel additive. By utilizing the LDH
synthesis method, a well-dispersed solid solution between each
oxide domain was formed, which was supported by the weak/broad
diffraction peaks in each XRD pattern. Additionally, the method
allowed for the simple addition of other metal components such as
Fe, which drastically impacted the electronic environment of the
catalyst without significant deviations in physical properties (N2
physisorption and XRD). An array of Co-Fe-Al catalysts with varying
Co:Fe ratios were investigated in a vapor phase flow reactor, and
prior to reaction the catalysts were reduced in situ in an H2
atmosphere. Through XAS analysis, it was evident that the
reduction parameters conducted allowed for the almost full
reduction of Co3O4 for each catalyst, with varying degrees of
reduction for the Fe species. As the loading of Fe increased, there
was an increase in Fe reduction due to the ease of Fe oxide
reduction over Al2O3, which may contribute to increased
aggregation of metallic species and a loss in surface area. This was
evident during reactivity studies, which showed that the activities for
each Co-Fe-Al catalyst followed as such: 0.25Fe > 0.5Fe > 0.75Fe.
Despite differences in activity among the Fe loaded samples, it was
evident that the addition of Fe enhanced both the activity per gram
and selectivity towards 2-MF compared to the 3Co-Al catalyst.
Furthermore, the 3Co-0.25Fe-0.75Al catalyst exhibited high
selectivities towards 2-MF over a range of conversions (75%-84%).
The ability to recycle these catalysts was shown after calcining and
re-reducing the catalysts. Through this study, it is evident that mixed
metal oxides derived from LDH materials can yield multi-metal
systems where there is good interaction between each component,
which can produce selective catalysts for FA and FUR conversion.
Further investigations on various additives into an LDH matrix may
allow for additional tuning to reduce catalyst deactivation or
enhance the yield to 2-MF even further.
Characterization Techniques
Nitrogen physisorption was performed in a Micromeritics Tristar II at -196°C
after the samples were pretreated at 200°C under vacuum for 12 h.
Chemisorption and TPR experiments were conducted in a Micromeritics
AutoChem II 2920. For each experiment, approximately 50 mg of sample was
placed on top of a small bed of quartz wool in a quartz U-tube. In regards to
the TPR experiment, the mixed metal oxide was pretreated in 20 mL/min of
He (Airgas, UHP) at 200°C for 2 h to remove pre-adsorbed species. The
sample was then cooled to 50°C and 20 mL/min of 10% H2 (balance Ar) was
passed through the sample. The furnace was heated to 800°C at 5°C/min
while under the flow of 10% H2/Ar. The outlet gas passed through a liquid
acetone/nitrogen trap, and then was passed through a thermal conductivity
detector (TCD). Elemental analysis for each catalyst was conducted at the
Georgia Tech Renewable Bioproducts Institute on an inductively coupled
plasma optical emission spectroscopy (ICP-OES) instrument, a PerkinElmer
OPTIMA 7300 DV, after dissolving the metal oxides in dilute H2SO4 solution.
Powder X-Ray diffraction patterns were collected using a Philips X-pert
diffractometer using Cu Kα radiation. X-Ray Photoemission Spectroscopy
(XPS) analysis was performed using a Thermo K-Alpha spectrometer
employing a monochromatic Al Kα. The pressures inside the analytical
chamber was approximately 1x10-7Torr. The binding energies (BE) of all
elements were tuned to the Ag 3d peak (368.2 eV) with an uncertainty of ±
0.2 eV.
Experimental Section
Materials and Chemicals
Furfural (99% purity, ACS Grade) and Furfuryl Alcohol (98% purity) were
purchased from Sigma Aldrich, vacuum distilled, and then stored in inert
atmosphere before use in reactions. Co(NO3)2ˑ6H2O (99% purity), Na2CO3
(99.5% purity), and ethylene glycol diethyl ether (98% purity) were purchased
from Sigma Aldrich. Fe(NO3)3ˑ9H2O, and Al(NO3)3ˑ9H2O were all purchased
from Alfa Aesar (98%-102%). NaOH (97% purity) was purchased from EMD.
Lastly, unless noted, all chemicals were used as is without any further
purification.
In situ X-ray absorbtion spectroscopy (XANES and EXAFS) were
conducted at the Advanced Photon Source (APS), Argonne National Lab
(ANL) at beamline 12-BM. The data were obtained in transmission mode at
the Fe K-edge (7112 eV) in the range of 6910-7910 eV, and the Co K-edge
(7712 eV) in the range of 7510-8712 eV, both with a spot size of 0.5 mm x
1.2 mm. To perform the in situ reduction XAS, the catalyst sample was diluted
and ground with boron nitride (5:1 BN: catalyst), and then ~8 mg of the
Catalyst Synthesis
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10.1002/cctc.201700228
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mixture was loaded and pressed into a “six shooter” holder (multiple samples
could be pressed into the holder). This holder was then placed into a quartz
tube with Kapton windows at each end to allow X-rays to pass through, and
a gas line was attached to the tube. A scan was collected prior to reduction
at room temperature. Next, the catalyst was heated to 500°C at 5°C/min and
held for 1 h, all while under a 60 mL/min flow of pure H2. After the reduction
was complete, the sample was cooled down in H2, and a final scan was
collected at room temperature. Various standards were used to verify the
species present in the catalyst sample. The XAS data were processed and
analyzed with Athena software including background removal, edge-step
normalization, and Fourier transform. Artemis software was utilized to fit the
Fourier transformed EXAFS data with a model. Co-Co, Fe-Fe, Fe-O
reference parameters were calculated using Co foil (FCC), Fe foil (BCC), and
Fe(III) acetylacetonate as standards. Multiple scattering paths through
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Acknowledgements
This work was supported as part of the Catalysis Center for Energy
Innovation, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science under grant DE-
SC0001004. We would also like to thank Dr. Faisal Alamgir for a
discussion on EXAFS fitting.
XAS studies were conducted using resources at the Advanced
Photon Source, a U.S. Department of Energy (DOE) Office of
Science User Facility operated for the DOE Office of Science by
Argonne National Laboratory under Contract No. DE-AC02-
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Keywords: biomass • furanics • hydrogenation • cobalt-iron
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Co-Fe-Al oxides derived from
layered double hydroxides can be
reduced to form a catalytically active
material for hydrogenolysis of
furanics to 2-methylfuran
Taylor P. Sulmonetti,a Bo Hu,a Zachary
Ifkovits,a Sungsik Lee,b Pradeep K.
Agrawala and Christopher W. Jonesa*
Page No. – Page No.
Vapor Phase Hydrogenolysis of
Furanics Utilizing Reduced Cobalt
Mixed Metal Oxide Catalysts
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