Rate enhancement by Cu in NixCu1-x/ZrO2 bimetallic catalysts for hydrodeoxygenation of stearic acid

Article abstract of DOI:10.1039/c9cy00181f

Hydrodeoxygenation of stearic acid on Ni/ZrO₂ to n-heptadecane occurs via the reduction to 1-octadecanal, followed by decarbonylation of the aldehyde to n-heptadecane. Stearic acid binds stronger than 1-octadecanal on Ni, causing decarbonylation to start only once stearic acid is almost fully converted. This first step is enhanced by addition of Cu either in the form of Cu/ZrO₂ or in the form of a ZrO₂ supported Ni_xCu_1-x nano-alloy. Cu has not only a higher activity for the reduction of stearic acid, it also increases the activity of Ni for decarbonylation of 1-octadecanal by increasing the electron density of Ni in the bimetal catalyst. The combination of these two effects leads to high activity of Ni-Cu bimetallic catalysts.

Full text of DOI:10.1039/c9cy00181f

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DOI: 10.1039/C9CY00181F
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Rate enhancement by Cu in NixCu1-x/ZrO2 bimetallic catalysts for
hydrodeoxygenation of stearic acid
Christoph Denk,a Sebastian Foraita,a Libor Kovarik,b,c Kelsey Stoerzinger,b,d Yue Liu,a
Received 00th January 20xx,
Accepted 00th January 20xx
Eszter Baráth*a and Johannes A. Lercher*a,b
DOI: 10.1039/x0xx00000x
Hydrodeoxygenation of stearic acid on Ni/ZrO2 to n-heptadecane occurs via the reduction to octadecanal, followed by
decarbonylation of the aldehyde to n-heptadecane. Stearic acid binds stronger than 1-octadecanal on Ni, causing
decarbonylation to start only once stearic acid is almost fully converted. This first step is enhanced by addition of Cu either
in the form of Cu/ZrO2 or in the form of a ZrO2 supported NixCu1-x nano-alloy. Cu has not only a higher activity for the
reduction of stearic acid, it also increases the activity of Ni for decarbonylation of 1-octadecanal by increasing the electron
density of Ni in the bimetal catalyst. The combination of these two effects leads to high activity of Ni-Cu bimetallic catalysts.
www.rsc.org/
well as the binding of the acid.9-11 Preliminary experiments have
shown promising properties when Ni was combined with Cu in
the active metal phase.
Introduction
Hydrodeoxygenation (HDO) of fatty acids and their triglyceride
esters is a potentially important route to high quality distillate
fuels.1-3 Such a process needs to saturate double bonds, as well
as to reduce the acid function to the corresponding equilibrated
aldehydes and alcohols, which are decarbonylated and
dehydrated, respectively. The relative abundance of
hydrogenating and acid-base functions determines whether
Ni-Cu alloys are classified as weakly exothermic,12 with both
partners largely retaining their individual electronic
properties.12 Mutual electronic influence is typically attributed
to a reduction of the d-band width and a redistribution of the d-
band electrons. Difference in the oxophilicity of the alloy
constituents; charge differences between them strengthens the
interactions with polar groups.13 It is interesting to note that Cu
supported on transition metal oxides has been reported to be
an efficient catalyst for the selective reduction of acids to
alcohols.14-18
Here we report the combination of Cu and Ni in bimetallic
catalysts supported on ZrO2. The presence of Cu enhances the
stearic acid reduction and the formation of NixCu1-x nano-alloys
increases decarbonylation rate via an electronic promotion of
Ni, leading to higher rates of HDO.
hydrodeoxygenation
decarbonylation.4,5
occurs
via
dehydration4,5
or
Suitable catalysts for HDO of fatty acids include transition
metals, metal phosphides and sulfides. The support also
accelerates reaction rates by introducing redundant pathways
for reduction and deoxygenation steps.1-3,6 ZrO2 has been found
to be a particularly suitable support,5 because oxygen defects
are instrumental for the reactive chemisorption of fatty acids.
The higher defect concentration of monoclinic (m-ZrO2)
compared to tetragonal ZrO2 (t-ZrO2), for example, leads to a
significantly higher catalytic activity.5a Addition of SiO2 to ZrO2
on the other hand creates Brønsted acidity, which in turn
enhances dehydration over decarbonylation.7
Results and Discussion
Catalyst physicochemical properties
ZrO2 supported Ni-Cu bimetallic catalysts with different Ni/Cu
ratios (labeled as NixCu1-x) were prepared by wet impregnation.
The labeling of x represents the atomic fraction of the two
elements. The physicochemical properties of the five catalysts
are compiled in Table 1. All catalysts had the same specific
surface area (84 ± 3) m2 g-1, very similar acid and base site
concentrations of 0.19 ± 0.04 mmol g-1 and 0.27 ± 0.02 mmol g-
1, respectively, indicating that the support19 was hardly
modified by the metal deposition (Table 1). Zirconia was
monoclinic (XRD peaks at 24.5°, 28.3°, 31.6°, 34.5°, 35.3° and
40.7°, JCPDS card No. 37-1484)20 (ESI†, Fig. S1). A small shoulder
at 30.2° suggests a minor tetragonal fraction (JCPDS card No.
17-0923).21. For Ni/ZrO2, the average metal particle size was 20
nm, estimated from widths of the Ni(111) at 44.6° and Ni(200)
While earth abundant transition metals, such as Ni, have been
explored as economic and suitable catalysts,8 the parent metal
catalysts require modifications to enhance the catalytic activity,
tailoring both, the specific activity transferring hydrogen, as
a Department of Chemistry and Catalysis Research Center, Technische Universität
München Lichtenbergstraβe 4, 85747 Garching (Germany)
E-mail: eszter.barath@tum.de, johannes.lercher@ch.tum.de
Institute for Integrated Catalysis, Pacific Northwest National Laboratory 902
b
Battelle Boulevard, Richland, WA 99352 (USA)
Environmental Molecular Sciences Laboratory, Pacific Northwest National
c
Laboratory, Richland, WA 99352 (USA)
Physical and Computational Sciences Directorate, Pacific Northwest National
d
Laboratory, Richland, Washington WA 99352 (USA)
† Electronic Supplementary Information (ESI). See DOI: 10.1039/x0xx00000x
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at 51.9° diffraction peak (JCPDS 04-0850) (ESI†, Fig. S1). The 3.5%.
fraction of exposed Ni determined by H2-chemisorption was
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DOI: 10.1039/C9CY00181F
Table 1 Physicochemical properties of bimetallic NixCu1-x/ZrO2 catalysts
c
Catalyst a
SBET
(m2 g-1)
82
81
87
Ni-loading b
Cu-loading b
rex.surf
(%)
3.5
1.5
1.8
2.8
-
Concentration of acid sites d
Concentration of basic sites e
(wt%)
10
7.5
5.6
2.5
0
(wt%)
0
2.1
4.2
6.5
8.7
(mmol g-1)
0.19
(mmol g-1)
0.28
Ni/ZrO2
Ni0.79Cu0.21/ZrO2
Ni0.59Cu0.41/ZrO2
Ni0.29Cu0.71/ZrO2
Cu/ZrO2
0.23
0.23
0.16
0.15
0.27
0.27
0.25
0.29
83
85
a Label according to molar ratio of Ni-Cu.
b Determined by Atomic Absorption Spectroscopy (AAS).
c rex.surf ratio of exposed Ni surface, determined from H2-Chemisorption.
d Determined by TPD of NH3.
e Determined by TPD of CO2.
When adding a low concentration of Cu (Cu:Ni of 0.21:0.79) the
H2-uptake normalized to Ni was lower than for Ni/ZrO2 (Table
1). In agreement with Ponec, the surface concentration of Ni
was lower than the bulk concentration, and remained constant
over a wide range of NixCu1-x compositions, indicating a slightly
over-proportional surface concentration of Cu.12d At higher Cu
concentrations (Cu:Ni of 0.71:0.29), Ni diffraction peaks were
not observed and the fraction of exposed Ni increased to
2.8%,22 while large Cu crystallites of 85 nm were observed as
judged from the widths of the Cu(111) diffraction peak at 43.4°
(JCPDS 04-0836) (ESI†, Fig. S1). It increased to an average
diameter of more than 100 nm on Cu/ZrO2.
On the bimetallic catalysts, Ni-Cu alloys were identified. Fig. 1
shows the XRD pattern of the catalysts in the 2θ range of 42°-
47°. The diffraction peaks of m-ZrO2 support were subtracted to
better visualize the diffraction peaks of the supported metal.
Besides the peaks of Cu(111) at 43.4° (JCPDS 04-0836) and of
Ni(111) at 44.6° (JCPDS 04-0850), a broad and shifted diffraction
peak between them appeared on the Ni-Cu bimetallic samples,
attributed to the NixCu1-x nano-alloy.23 The large width at half
height points to small domains of ordering in the alloyed phase.
Fig. 1 XRD patterns of NixCu1-x/ZrO2 (x = 1, 0.79, 0.59, 0.29, 0) catalysts by
subtraction of the support m-ZrO2 in the 2θ range 42–47°.
The spatially resolved elemental distributions of Ni/ZrO2,
Cu/ZrO2 and NixCu1-x/ZrO2 are compiled in Fig. 2 and Fig. 3. On
the monometallic Ni/ZrO2 and Cu/ZrO2, defined particles of Ni
and Cu are visible, and the average diameter of Ni particles is
smaller than that of Cu particles (Fig.
2). Zirconium is
homogenously distributed over the whole agglomerates for all
materials. The overlaying Ni and Cu distributions for bimetallic
catalysts shows that they both coexist in each particle (Fig. 3).
Small Cu and Ni subdomains were observed.
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Fig. 2 STEM HAADF (scanning transmission electron microscopy high angle
annular dark-field) images of Ni/ZrO2, and Cu/ZrO2 agglomerate used for
energy dispersive spectroscopy (EDS) mapping (oxygen, zirconium and either
nickel or copper).
Fig. 3 STEM HAADF (scanning transmission electron microscopy high angle
annular dark-field) images of NixCu1-x/ZrO2 (x = 0.79, 0.59, 0.29) agglomerate
used for energy dispersive spectroscopy (EDS) mapping zirconium, copper
and nickel. In the composite image zirconium is blue, copper is red and nickel
is green.
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To characterize the mutual electronic influence between Ni and
Cu, IR spectra of adsorbed CO (Fig. 4) and X-ray photoelectron
spectra (XPS, Fig. 5) were used. In the IR spectra, Cu/ZrO2
showed only a minor adsorption of CO, as judged from the very
weak band at 2100 cm-1.22 For Ni/ZrO2, the band at 2038 cm-1 is
attributed to linearly bound CO on Ni0,24 the band at 1921 cm-1
to bridging CO. The intensity of the band of bridging CO
decreased strongly in the presence of Cu, suggesting that Ni is
well interdispersed with Cu.12 Increasing the fraction of Cu led
to a gradual redshift of the band of linearly adsorbed CO from
2019 via 2011 to 2004 cm-1. This redshift indicates a stronger
bond and an increase in the electron back donation for CO,
indicating that Ni increased in electron density as the Cu
concentration increased.22,25,26
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DOI: 10.1039/C9CY00181F
Fig. 4 IR spectra of CO adsorbed on NixCu1-x/ZrO2 (x = 1, 0.79, 0.59, 0.29, 0) at
40°C. The adsorption of CO was performed at 0.3 mbar until equilibrium was
reached, then evacuated (p = 10-7 mbar) for 5 min to remove physisorbed and
gas phase CO. The IR spectra of adsorbed CO were normalized by the weight
of Ni in the respective wafer.
Fig. 5 XPS spectra for NixCu1-x/ZrO2 (x = 0.29; 0.79) showing the binding energy
of Cu and Ni (br. sat./ broad satellite).
Two representative catalysts Ni0.29Cu0.71/ZrO2 (●) and
Ni0.79Cu0.21/ZrO2 (●) were characterized by X-ray photoelectron
spectroscopy (Fig. 5). The Cu2p3/2 binding energy was higher
than for pure Cu (932.47 eV),27 indicating that electron transfer
from Cu to Ni occurred. The Cu2p1/2 and Cu2p3/2 binding energies
were lowered by 0.3 eV with increasing Ni-content (x = 0.29 to
0.79). Thus, it was concluded that the electron density at Cu was
lowered, while that of Ni was increased, in line with the redshift
of the IR bands of adsorbed CO. In contrast to Cu, Ni was not
fully reduced, hence, also binding energies for Ni2+ were found.
The ratio of Ni0 to Ni2+ was higher at higher Ni concentrations.
This suggests that in presence of Cu small Ni particles may exist
in Ni0.29Cu0.71/ZrO2, which are hypothesized to be more easily
oxidized than the Ni-rich alloyed particles. Ni in bimetallic
particles had a Ni2p3/2 binding energy of 852.1 eV, 0.5 eV lower
than bulk Ni,27 suggesting a higher electron density than in
Ni/ZrO2. Both shifts indicate that electron density at Cu was
lower and that of Ni was higher than in monometallic catalysts
(Fig. 5).
Catalytic activity
Previous studies of stearic acid HDO have identified the reaction
network shown in Scheme 1. The critical reaction is the
reductive deoxygenation of the stearic acid to 1-octadecanal
(Scheme 1/ a) followed by the decarbonylation to n-
heptadecane (Scheme 1/ b). Hydrogenation of 1-octadecanal to
1-octadecanol (Scheme 1/ c) is fast and reversible. In presence
of Brønsted acid sites, 1-octadecanol dehydrates to n-
octadecene (Scheme 1/ d), which is in turn hydrogenated to n-
octadecane (Scheme 1/ e). 1-Octadecanol also forms reversibly
stearyl stearate with stearic acid (Scheme 1/ f).4b,5,7 The two
parallel pathways starting from 1-ocatadecanal lead to alkanes
of different size: (i) decarbonylation to n-heptadecane (C17), and
(ii) hydrogenation to 1-octadecanol followed by dehydration
and hydrogenation to n-octadecane (C18). The reductive
deoxygenation to 1-octadecanal is the slowest step on Ni/ZrO2
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(Scheme 1/ step a)4a and a rate increase for this step is critical hypothesize that O-species on Cu are less stableVtihewanArtitchleaOtnloinne
Ni.28 Hence, Cu nanoparticles offer a higher concentration of
oxygen defects than Ni nanoparticles, leading to a higher rate of
acid reduction on Cu. After 2 h on Cu/ZrO2 1-octadecanol and 1-
DOI: 10.1039/C9CY00181F
to achieve higher activity.
(i) Two-step route:
Reductive deoxygenation (a) / Decarbonylation (b)
octadecanal were equilibrated with
a
ratio of 230
(c(C18OH)/c(C17CHO) at 40 bar H2). Only traces of n-heptadecane
and n-octadecane were formed.
H2, [M]/ZrO2
[M]
n-C17H35-COOH
+ CO
n-C17H36
n-heptadecane
(C17
n-C17H35-CHO
- H2O
(b) kb
stearic acid
(C17CO2H)
1-octadecanal
(C17CHO)
(a) ka
)
H2, [M]/ZrO2
kc,r
([M] = metal)
kc,f
(c)
Brønsted
acid site
n-C17H35-CH2OH
n
-C18H36 + H2O
n-octadecene
1-octadecanol
(C18OH)
(d)
=
(C18
)
(f)
H2, [M]
(e)
n-C17H35-COOH
n-C17H35-C(O)O-CH2-n-C17H35 + H2O
n-C18H38
stearyl stearate
n-octadecane
(C18
)
(ii) Multi-step route:
Reductive deoxygenation (a) /
Hydrogenation (c) - (Esterification (f)) / Dehydration (d) - (Hydrogenation (e))
Scheme 1 Reaction network for the hydrodeoxygenation of stearic acid.4b,5,7
ka, kb and kc,f and kc,r are referring to the corresponding rate constant values
determined for step a, b and c (for detailed description see ESI†, Eq.S1-
Eq.S10).
Cu and Ni have different catalytic activities for reductive
deoxygenation of stearic acid (Scheme 1/ a) and
decarbonylation of 1-octadecanal (Scheme 1/ b). Ni/ZrO2 shows
moderate activity for both reactions, while in contrast, Cu/ZrO2
is highly active in the reductive acid deoxygenation, but hardly
shows activity for decarbonylation (Fig. 6).
Fig. 7 Product distribution for the hydrodeoxygenation of stearic acid on
Ni/ZrO2 (A) and Cu/ZrO2 (B) as a function of time. Conversion of stearic acid (
■), yield of 1-octadecanol (●, ●), n-heptadecane (▼, ▼), n-octadecane (♦, ♦),
1-octadecanal (50 fold) (×, ×) and stearyl stearate (*, *). Reaction conditions:
Stearic acid (0.5 g), Ni/ZrO2 or Cu/ZrO2 (0.2 g), n-dodecane (100 mL), 260 °C,
p(H2) = 40 bar, 600 rpm.
A Ni-Cu bimetallic catalyst is, thus, hypothesized to combine the
high activity of Cu for the reduction of stearic acid to 1-
octadecanal (Scheme 1/ a) and the ability of Ni for
decarbonylation of 1-octadecanal to n-heptadecane (Scheme 1/
b). Let us compare now the NixCu1-x/ZrO2 and a physical mixture
of Ni/ZrO2 and Cu/ZrO2 as the reference.
Fig. 6 Initial reaction rate for (a) the reductive deoxygenation of stearic acid
on Ni/ZrO2 and Cu/ZrO2 and (b + c) decarbonylation reaction of 1-octadecanal.
Reaction conditions: Stearic acid or 1-octadecanol (0.5 g), Ni/ZrO2 or Cu/ZrO2
(0.05 g), n-dodecane (100 mL), 260 °C, p(H2) = 40 bar, 600 rpm.
Indeed, HDO of stearic acid on Ni/ZrO2 (Fig. 7A) showed that, 1-
octadecanol and 1-octadecanal were formed as (pseudo)
primary intermediates and n-heptadecane appeared as
secondary product (Fig. 7A). On Cu/ZrO2 stearic acid was rapidly
reduced to 1-octadecanol (Fig. 7B). As the metal-oxygen bond
dissociation enthalpy for Ni is higher than for Cu, we
HDO of stearic acid on Ni-Cu nano-alloys
Using the physical mixture of 0.79 Ni/ZrO2 and 0.21 Cu/ZrO2
(Fig. 8A) and the respective alloy Ni0.79Cu0.21/ZrO2 (Fig. 8B)
resulted in the rapid conversion of stearic acid to 1-octadecanol
as primary product for both catalysts, independently if the
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Ni0.79Cu0.21 alloy or the physical mixture was used (Fig. 8B). lower rate to n-heptadecane compared to a Ni rich alloy
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Compared to the monometallic catalysts (Ni/ZrO2 or Cu/ZrO2), (Ni0.79Cu0.21/ZrO2, Fig. 8B). Full conversionDtOoI:h1y0d.1r0o3c9a/Crb9CoYn0s0w18a1Fs
the overall formation of n-heptadecane was higher on both the achieved on all three bimetallic catalysts. Ni0.59Cu0.41/ZrO2
0.79 Ni/ZrO2 + 0.21 Cu/ZrO2 mixture and Ni0.79Cu0.21/ZrO2. While combines a moderate acid reduction rate with the highest
n-heptadecane yield on Ni/ZrO2 was only 15% and it was hardly decarbonylation rate, thus, the fastest production of n-
converted on Cu/ZrO2 after 8 h, the yield of n-heptadecane heptadecane. This catalyst can be fully recycled, reaction rates
reached 82% after 8 h on 0.79 Ni/ZrO2 + 0.21 Cu/ZrO2 mixture declined only slightly over three cycles and the selectivity
and 100% readily after 4h on Ni0.79Cu0.21/ZrO2. Analyzing the changed only marginally (ESI†, Fig. S2).
individual reaction steps, the reductive dehydrogenation rate of
stearic acid on Ni0.79Cu0.21/ZrO2 was similar to that on the
physical mixture: stearic acid conversion of 85% after 2 h on the
former and 80% on the later. In contrast, the decarbonylation
of 1-octadecanal, represented by the formation rate of n-
heptadecane, was much more efficient on Ni0.79Cu0.21/ZrO2 than
on the physical mixture.
Fig. 9 Product distribution for the hydrodeoxygenation of stearic acid on
Ni0.29Cu0.71/ZrO2, (A) and Ni0.59Cu0.41/ZrO2 (B) as a function of time. Conversion
of stearic acid (■), yield of 1-octadecanol (●, ●), n-heptadecane (▼, ▼), n-
octadecane (♦, ♦), 1-octadecanal (50 fold) (×, ×) and stearyl stearate (*, *).
Reaction conditions: Stearic acid (0.5 g), NixCu1-x/ZrO2 catalyst (x = 0.59, 0.29;
0.2 g) (atomic ratio Cu:Ni) n-dodecane (100 mL), 260 °C, p(H2) = 40 bar, 600
rpm.
Fig. 8 Product distribution for the hydrodeoxygenation of stearic acid on 0.79
Ni/ZrO2 + 0.21 Cu/ZrO2 catalyst (0.2 g), (A) and Ni0.79Cu0.21/ZrO2 (B) as a
function of time. Conversion of stearic acid (■), yield of 1-octadecanol (●), n-
heptadecane (▼), n-octadecane (♦), 1-octadecanal (50 fold) (×) and stearyl
stearate (*). Reaction conditions: Stearic acid (0.5 g), NixCu1-x/ZrO2 catalyst (x
= 0.79, and respective physical mixture; 0.2 g) (atomic ratio Cu:Ni) n-
dodecane (100 mL), 260 °C, p(H2) = 40 bar, 600 rpm.
The decarbonylation rate depends strongly on the conversion
level of stearic acid. Up to 50% of stearic acid conversion, n-
heptadecane was not formed (Fig. 8 and Fig. 9), although the
concentration of 1-octadecanal readily reached the steady state
concentration. In this period, the production of 1-octadecanol
(Scheme 1/ a and c) (ESI†, Fig. S3) was very selective. Above 50%
conversion of stearic acid, the decarbonylation rate increased
strongly (Fig. 10). The inhibition of decarbonylation by stearic
acid (Fig. 10) indicates stronger adsorption of stearic acid than
of 1-octadecanal on Ni. With pure Ni/ZrO2 the decarbonylation
By varying the composition of the alloy, different catalytic
activities were obtained. High Cu contents (Ni0.29Cu0.71/ZrO2, Fig.
9
A) induced higher activity in the reductive deoxygenation, but
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rate was low even up to 80% conversion. This is indicative of the increased linearly with the Cu content. With incorporation of
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DOI: 10.1039/C9CY00181F
competitive adsorption of stearic acid, 1-octadecanol and 1- Cu, the Ni0.79Cu0.21/ZrO2 and Ni0.59Cu0.41/ZrO2 had 3.5-fold
octadecanal. The coverage of each substance was given in Eq. higher rates than Ni/ZrO2 and also higher activation energy of
S1 to S2 based on Langmuir type adsorption.
135 kJ mol-1. The results imply a change of acid reduction
The reaction order with respect to stearic acid reduction was pathways. The OH group of the fatty acid is abstracted by the
5a
zero (ESI†, Fig. S4), the decarbonylation reaction was also zero oxygen defects of ZrO2 on Ni/ZrO2 while this reaction step
order in 1-octadecanal (ESI†, Fig. S5), but a reaction order of rather occurs on Cu sites on the bimetallic catalysts. The
one was observed for conversion of 1-octadecanol (ESI†, Fig. similarity in rates for Ni0.79Cu0.21/ZrO2 and Ni0.59Cu0.41/ZrO2
S6). Thus, we conclude that the adsorption strength declines alloys is speculated to be caused by a similar surface
from stearic acid via 1-octadecanal to 1-octadecanol. In contrast concentration of Cu and Ni in both materials,12d implying that,
to Cu/ZrO2 the Ni containing catalysts did not reach equilibrium the physical mixtures do not match perfectly with their
between 1-octadecanol and 1-octadecanal (ESI†, Fig. S7).
respective alloy. The rates on the Cu rich alloy Ni0.29Cu0.71/ZrO2
-1
and Cu/ZrO2 were further doubled to ~12.5 mmol gcat h-1,
accompanied by a lower activation energy of 107 kJ mol-1,
showing that the decreased electron density on Cu lowered the
energy barrier of OH abstraction on fatty acid.
Fig. 10 Decarbonylation reaction rate versus the conversion of stearic acid on
NixCu1-x/ZrO2 (x = 1, 0.79, 0.59, 0.29, 0) and 0.29 Ni/ZrO2 + 0.71 Cu/ZrO2 as a
representative physical mixture. Reaction conditions: Stearic acid (0.5 g), n-
dodecane (100 mL), 260 °C, p(H2) = 40 bar, 600 rpm.
Fig. 11 Initial reaction rate for the reductive deoxygenation of stearic acid on
NixCu1-x/ZrO2 (x = 1, 0.79, 0.59, 0.29, 0) and physical mixtures of Ni/ZrO2 and
Cu/ZrO2 (empty symbols) as a function of Ni content. Reaction conditions:
Stearic acid (0.5 g), NixCu1-x/ZrO2 catalyst (x = 1, 0.79, 0.59, 0.29, 0; 0.05 g)
(atomic ratio Cu:Ni), n-dodecane (100 mL), 260 °C, p(H2) = 40 bar, 600rpm.
Reduction of stearic acid and decarbonylation of 1-
octadecanal on Ni-Cu nano-alloys
To decipher the exceptional high activity of Ni-Cu alloys in the
HDO of stearic acid, we next analyze and compare the reaction
rate of each individual step leading to n-heptadecane, namely
The decarbonylation rate is expressed as n-heptadecane
formation rate (see Eq. S7). When at zero order with respect to
1-octadecanal, the rate can be expressed by (Eq.2):
reductive deoxygenation of acid (Scheme
1 / a) and
decarbonylation of 1-octadecanal (Scheme 1 / b). The former
determines the conversion rate of the fatty acid feedstock while
the latter determines the formation rate of the target alkane
product. The respective rate equation are formally given by Eq.
S4 to S6). Due to the observed reaction order of zero, the rate
equation of stearic acid reduction simplifies to Eq.1.
dcHept
dt
dcHept
1
 ncatkb  kb 
ncat dt
Eq.2
This rate increased with Ni content (Fig. 12), because Cu was
nearly inactive (Fig. 6). The nano-alloy NixCu1-x/ZrO2 catalysts
had much higher rates than Ni/ZrO2, Cu/ZrO2 and their mixture.
The TOFs for decarbonylation of 1-octadecanal (TOF, rate
normalized to surface Ni) are compiled in Fig. 13. Ni/ZrO2 had
the lowest TOF values (90 molC17 molNi-1h-1), while for NixCu1-
x/ZrO2 TOFs were higher than 460 molC17 molNi-1h-1 (Fig. 13). The
activation energy decreased from 139 kJ mol-1 on Ni/ZrO2 to 130
kJ mol-1 on Ni0.79Cu0.21/ZrO2 and to 121 kJ mol-1 over
Ni0.29Cu0.71/ZrO2 (ESI†, Fig. S9). The decline in activation energy
dcAcid
dt
1 dcAcid
ncat dt
 kancat  ka  
Eq.1
These zero-order reaction rates of fatty acid reduction on
NixCu1-x/ZrO2 are compared in Fig. 11, with those on physical
mixtures of Ni/ZrO2 and Cu/ZrO2 as reference. The rates on
Ni/ZrO2 were the lowest, having an activation energy (Ea) of 125
kJ mol-1 (ESI†, Fig. S8). When mixing with Cu/ZrO2, this rate
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Catalysis Science & Technology
is attributed to the increased electronic density at Ni in the The reduction of the acid to the aldehyde is VfieowllAortwicleedOnlbinye
alloy, which we hypothesize to strengthen the interaction with decarbonylation. Stearic acid binds strongDeOr It:h1a0n.1013-9o/cCt9aCdYe0c0a1n81aFl
the carbon atom of the carbonyl group and in turn to facilitate on Ni, inhibiting the adsorption of 1-octadecanal and the
the activation of the carbonyl group.12,13
subsequent decarbonylation. By forming an alloy with Cu the
electron density of Ni is increased. This has been unequivocally
deduced from the lower binding energy on XPS, and the redshift
of the vibration band of linearly adsorbed CO. The increase in
electron density strengthened the interaction with the carbonyl
group and enhanced the decarbonylation rate. Having both, the
high rates of the acid reduction and of the aldehyde
decarbonylation, Ni-Cu alloy is a promising catalyst for HDO of
stearic acid to alkanes.
Experimental section – Materials and methods
Chemicals All chemicals, i.e., Zr(OH)4 × H2O (XZO 1247/01, MEL
Chemicals), Ni(NO3)2 × 6H2O (Acros Organics, ≥98.5%), Cu(NO3)2 ×
H2O (Sigma-Aldrich, 99.999%), synthetic air (20.5% O2/ 79.5% N2,
Westfalen), hydrogen (Westfalen, 99.999%), argon (Westfalen,
99.999%), stearic acid (Sigma-Aldrich, ≥99.5% analytical standard),
1-octadecanol (Sigma-Aldrich, ≥ 99.5% SelectophoreTM), n-
octadecane (Sigma-Aldrich, 99%), n-heptadecane (Sigma-Aldrich,
99%), n-dodecane (Sigma-Aldrich, ≥ 99%, ReagentPlus), were
purchased commercially and were not further purified.
Fig. 12 Decarbonylation reaction rate constant on NixCu1-x/ZrO2 (x = 1, 0.79,
0.59, 0.29, 0) and physical mixtures of Ni/ZrO2 and Cu/ZrO2 (empty symbols)
as a function of Ni content. Reaction conditions: Stearic acid (0.5 g), NixCu1-
x/ZrO2 or x Ni/ZrO2 1-x Cu/ZrO2 (x = 1, 0.79, 0.59, 0.29, 0; 0.2 g) n-dodecane
(100 mL), 260 °C, p(H2) = 40 bar, 600 rpm.
Catalyst preparation The support ZrO2 material was prepared by
calcination of Zr(OH)4 × H2O at 400 °C in ambient air for 4 h (heating
rate: 10 °C min-1). Bimetallic NixCu1-x/ZrO2 catalysts with five different
NixCu1-x-ratios (x = 0, 0.29, 0.59, 0.79, 0) and a total metal loading of
8 - 10 wt.% were prepared by wet impregnation. Ni(NO3)2 × 6H2O and
Cu(NO3)2 × xH2O (Table S1, see SI) were dissolved in deionized H2O
(5.0 g). The resulting solution was added dropwise within half an
hour to the support under stirring in ambient air. The slurry was
further stirred for 4 h, followed by drying at 110 °C overnight.
Subsequently, the ground solid was calcined in synthetic air (flow
rate: 100 mL min-1) at 450 °C for 4 h (heating rate: 2 °C min-1) and
reduced in H2 flow (flow rate: 100 mL min-1) at 500 °C for 4 h (heating
rate: 2 °C min-1).
Analysis methods
X-Ray powder diffraction (XRD) was performed on a Philips X’Pert
Pro and a PANalytical Empyrean System equipped with a Cu Kα
radiation source (40 kV/45 mA) with a step size of 0.01711° and a
scan rate of 1.08° min-1 in the 2θ range of 5−70°; with a step size of
0.0131303° and a scan rate of 0.002205° min-1 in the 2θ range of
42−47° using Kα1). Deconvoluted diffraction patterns resulted by
subtracting the diffractogram of the ZrO2-support normalized to the
maximum intensity of the m-ZrO2 peak from the diffractogram of
NixCu1-x/ZrO2 catalyst normalized to the maximum intensity of the m-
ZrO2 peak.
Fig. 13 Turnover frequency of the conversion of 1-octadecanol on NixCu1-
x/ZrO2 (x = 1, 0.79, 0.59, 0.29)as a function of Ni content. Reaction conditions:
1-octadecanol (0.5 g), NixCu1-x/ZrO2 (x = 1, 0.79, 0.59, 0.29; sum 0.05 g), n-
dodecane (100 mL), 260 °C, p(H2) = 40 bar, stirring at 600 rpm.
Conclusions
The rate of hydrodeoxygenation of stearic acid to n-
heptadecane was increased by combining Cu and Ni in a
bimetallic catalyst. While Cu/ZrO2 itself is highly active in the
reduction of stearic acid, it hardly catalyzes decarbonylation. In
contrast, Ni catalyzes both reactions, albeit with rather low
rates.
N2-sorption For measurement of the BET surface area the sample
was activated in vacuum at 250 °C for 2 h before measurement. The
adsorption of N2 was performed at –196 °C by using the Sorptiomatic
1990 series instrument.
8 | Catal. Sci. Technol., 2019, 00, 1-3
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ARTICLE
H2-chemisorption After reduction of the Ni based catalyst samples in coated with lacey carbon support films, and then immediately
View Article Online
H2-flow at 450 °C for 1 h, they were evacuated at 300 °C for 1 h. The loading them into the STEM airlock to minDiOmI:iz1e0.1a0n39e/Cxp9CoYsu0r0e18t1oF
H2 adsorption isotherms accounting for both chemisorption and atmospheric O2.
physisorption were measured on a Thermo Scientific’s Surfer
instrument at a pressure ranging from 9 to 400 mbar at 25 °C. For XPS analysis The respective material was pressed to achieve a self-
removing physisorbed H2 the system was evacuated for 20 min supporting pellet. These were reduced (10°C min-1, 100 mL min-1
5
afterwards. By extrapolating the isotherm to zero H2 pressure the vol.% H2 in N2, 500°C, 1 h), cooled to ambient temperature and were
concentration of chemisorbed hydrogen on the metal was transferred to a glove box, placed in a pressurized vessel and loaded
determined. The Ni dispersion was derived by assuming an average into the XPS under N2 atmosphere. XPS was measured with a
surface Ni to H ratio of 1. Furthermore, it is assumed that Cu does monochromated Al Kα source using a low energy flood gun for charge
not chemisorb H2.22
compensation. The binding energy scale was calibrated to the Zr3d5/2
peak.29 Ni metal peaks were fit with an asymmetric line shape, while
Temperature programmed desorption (TPD) of ammonia and Cu metal peaks and Ni-O peaks were fit using a Gaussian-Lorentzian
carbon dioxide was carried out in a parallel reactor system (six fold). line shape using CasaXPS.
A prior activation of the pressed samples (500–710 µm) in He at
500 °C for 1 h was conducted. Consequently, the sample was Catalyst activity and kinetic measurement
evacuated at 10-2 mbar the adsorbent NH3 or CO2 was loaded at a For catalytic reactions an autoclave (Parr, 300 mL) was used. Stearic
partial pressure of 1 mbar and 100 °C or 40 °C, respectively. The acid or 1-octdadecanol, catalyst, and 100 mL n-dodecane were
sample was then purged with He for 1 h in order to remove loaded into the autoclave, and pressurized with H2 (3 × 20 bar).
physisorbed molecules. After activation, the six samples were heated Typically, the reaction was carried out at 260 °C in the presence of 40
from 100 to 770 °C with a rate of 10 °C min-1 to desorb NH3 and from bar H2 at a stirring speed of 600 rpm. In situ sampling was performed
40 to 700 °C to desorb CO2. The signals were detected by a Balzer during the reaction. Typically, each sample of max. 0.5 mL was
QME 200 mass spectrometer. For calibration purposes NH3 was withdrawn from the reaction slurry and filtered through a 2 µm filter
desorbed from a HMFI-90 standard and CO2 generation from NaHCO3 at the bottom of the reactor in order to make sure that the sample is
decomposition was used for CO2 calibration.
free of catalyst, that the reaction in the sample vial is stopped and
that the mass of catalyst in the reactor stays constant. The dead
Atomic absorption spectroscopy (AAS) was used to determine the volume between filter and end of the sampling tube (0.05 mL) was
Ni and Cu content of the catalysts with a ThermoFisher Solaar M5 flushed and discarded prior to every sampling. The liquid samples
AA-Spectrometer. Prior to Ni and Cu determination, the catalysts were analyzed by a Agilent 7890B GC system, equipped with a flame
were dissolved in a mixture of HF, HNO3 and boiling concentrated ionization detector (FID) and Agilent 5977 MS detector, using a HP-5
H2SO4.
capillary column (30 m, 0.32 mm inner diameter, 0.25 µm film).
Quantification error is less than 5% for all experiments.
IR spectroscopy of adsorbed CO was performed on a Bruker VERTEX Conversion = (weight of converted reactant / weight of the starting
70 spectrometer at a resolution of 2 cm-1 with 128 scans in the range reactant) × 100%. Yield (C%) = (C atoms in each product / C atoms in
of 1000-4000 cm-1. For the measurements, the samples were pressed the starting reactant) × 100%. Selectivity (C%) = (C atoms in each
in to self-supporting wafers and mounted in the sample holder. The product/sum of C atoms in all the products) × 100%. The initial
Ni-Cu/ZrO2 catalysts were activated in H2-flow at 450 °C for 1 h, and reaction rate was deduced from the slope of the linear fit to the
then subsequently outgassed under vacuum (p = 10-7 mbar) to conversion versus time plot in the linear region at low conversions
remove H2 while cooling to 40 °C. The adsorption of CO was (<20%). Rate = mole of converted reactant / reaction time. TOF = rate
performed at 0.3 mbar until equilibrium was reached, then / mole of accessible Ni on the catalyst’s surface = mole of converted
evacuated (p = 10-7 mbar) for 5 min to remove physisorbed and gas reactant / mole of accessible Ni on the catalyst’s surface / reaction
phase CO. The IR spectra of adsorbed CO were obtained by time. The apparent activation barrier Ea(app) was determined
subtracting the activated sample, and then were normalized by the regarding an Arrhenius plot for the reaction of stearic acid to 1-
weight of the Ni in the respective wafer.
octadecanol. The enthalpy of activation and entropy of activation
for the decarbonylation is determined by the Eyring equation. Only
EDS analysis Imaging and EDS mapping was performed with Scanning in the latter case, the determination of the TOF was possible, since
Transmission Electron Microscope (STEM) (JEOL JEM-ARM200F) this reaction is only occurring on Ni sites. The respective figures are
operated at 200 kV. The microscope houses aberration corrector for given in (ESI†, Fig. S9) and (ESI†, Fig. S10). The decarbonylation
the probe forming lens (CEOS GmbH double-hexapole aberration reaction rate of 1-octadecanal is calculated based on the increase of
corrector), which allows imaging with sub angstrom resolution. The heptadecane yield within a specific time. Rate(decarbonylation) =
presented images were acquired on HAADF detector, with beam ΔYield(heptadecane) / ΔTime.
convergence of 27.5 mrad and collection angle of 68-280 mrad.
Elemental analysis was performed using Energy Dispersive X-ray
Spectroscopy (EDS) using high collection angle SSD. (~0.7 srad, JEOL
Conflicts of interest
Centurio). Acquisition and evaluation of the spectra was performed
by NSS Thermo Scientific software package. The STEM sample
preparation involved mounting of the powder samples on Au grids
There are no conflicts to declare.
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Catal. Sci. Technol., 2019, 00, 1-3 | 9
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11 (a) X. R. Li, C. Zhang, H. Y. Cheng, L. M. He, W. W. Lin, Y. C. Yu and
F. Y. Zhao, J. Mol. Catal. A Chem., 2014, D3O95I:,110–.160;3V(9ieb/wC) A9XrC.ticYRle0.0OL1ni,8lin1CFe.
Zhang, H. Y. Cheng, W. W. Lin, P. J. Chang, B. Zhang, Q. F. Wu, Y.
C. Yu and F. Y. Zhao, ChemCatChem, 2015, 7, 1322–1328; (c) B.
Mallesham, P. Sudarsanam and B. M. Reddy, Ind. Eng. Chem. Res.,
2014, 53, 18775–18785; (d) W. Sun, Y. Song, X. Q. Gong, L. M. Cao
and J. Yang, Chem. Sci., 2015, 6, 4993–4999; (e) R. Ding, Y. Wu, Y.
Chen, H. Chen, J. Wang, Y. Shi and M. Yang, Catal. Sci. Technol.,
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Acknowledgements
The financial support is highly appreciated of the AlgenFlugKraft
(FKZ LaBay74) project, sponsored by Bavarian Ministry of
Economic Affairs and Media, Energy and Technology
(“Bayerisches Staatsministerium für Wirtschaft und Medien,
Energie und Technologie”) and Bavarian State Ministry of
Education,
Science
and
the
Arts
(“Bayerisches
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Staatsministerium für Bildung und Kultus, Wissenschaft und
Kunst”). C. D., L. K., K. S., and J. A. L. were supported by the US
Department of Energy (DOE), Office of Basic Energy Sciences
(BES), Division of Chemical Sciences, Geosciences & Biosciences.
A portion of the research was performed using EMSL, a national
scientific user facility sponsored by the Department of Energy's
Office of Biological and Environmental Research and located at
Pacific Northwest National Laboratory. We thank Franz-Xaver
Hecht for help with N2-sorption and H2-chemisorption, Martin
Neukamm for AAS measurements.
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DOI: 10.1039/C9CY00181F
TOC – For graphical use only
The combination of Cu and Ni in bimetallic catalysts on ZrO2, lead to a more efficient HDO of stearic
acid.