Decarbonylation of heptanoic acid over carbon-supported platinum nanoparticles

Article abstract of DOI:10.1039/c3gc41287c

The decarbonylation and decarboxylation of heptanoic acid over carbon-supported Pt nanoparticles were studied in a continuous flow fixed bed reactor at 573 K and 37 bar for liquid-phase operation and 1 bar for gas-phase operation. Under liquid-phase conditions, the TOF over Pt supported on Norit carbon was 0.0052 s^-1 and independent of Pt loading. At very low conversions, approaching zero, the product selectivity was consistent with decarbonylation as the primary reaction, producing mostly hexenes and CO. As conversion increased from 1% to 5% at 37 bar, substantial amounts of hexane and CO₂ were observed, presumably from secondary side reactions such as water-gas shift (WGS) and hydrogenation instead of direct decarboxylation. The terminal olefin was observed with high selectivity (57%) only during gas-phase operation (1 bar) which facilitated transport of the olefin away from the Pt that also catalyzed double bond isomerization. Some sintering of the Pt metal particles during reaction of heptanoic acid was observed by X-ray diffraction analysis of the spent catalyst. Catalyst regeneration studies were performed over spent catalyst but they failed to restore any catalytic activity.

Full text of DOI:10.1039/c3gc41287c

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Decarbonylation of heptanoic acid over
carbon-supported platinum nanoparticles
Cite this: Green Chem., 2014, 16,
683
Juan A. Lopez-Ruiz and Robert J. Davis*
The decarbonylation and decarboxylation of heptanoic acid over carbon-supported Pt nanoparticles
were studied in a continuous flow fixed bed reactor at 573 K and 37 bar for liquid-phase operation and
1 bar for gas-phase operation. Under liquid-phase conditions, the TOF over Pt supported on Norit carbon
was 0.0052 s−1 and independent of Pt loading. At very low conversions, approaching zero, the product
selectivity was consistent with decarbonylation as the primary reaction, producing mostly hexenes and
CO. As conversion increased from 1% to 5% at 37 bar, substantial amounts of hexane and CO2 were
observed, presumably from secondary side reactions such as water-gas shift (WGS) and hydrogenation
instead of direct decarboxylation. The terminal olefin was observed with high selectivity (57%) only during
gas-phase operation (1 bar) which facilitated transport of the olefin away from the Pt that also catalyzed
double bond isomerization. Some sintering of the Pt metal particles during reaction of heptanoic acid was
observed by X-ray diffraction analysis of the spent catalyst. Catalyst regeneration studies were performed
over spent catalyst but they failed to restore any catalytic activity.
Received 2nd July 2013,
Accepted 12th August 2013
DOI: 10.1039/c3gc41287c
www.rsc.org/greenchem
1. Introduction
Deoxygenation refers to a broad class of chemical reactions
involving the removal of oxygen from organic compounds that
can be accomplished by reactions such as dehydration, decar-
bonylation and decarboxylation. Carboxylic acids found in
vegetable oils and animal fats can be readily converted to fuels
and chemicals by decarbonylation and decarboxylation reac-
tions. For example, linear organic acids are converted to linear
alkenes and linear alkanes by decarbonylation and decarboxy-
lation, respectively, as shown for the reaction of heptanoic acid
in Scheme 1. Recent work on decarbonylation and decarboxy-
lation of carboxylic acids over transition metal catalysts is
often performed in the presence of dihydrogen to inhibit cata-
lyst deactivation, but the reactions are highly selective towards
the production of paraffins because of the rapid hydrogenation
of any alkenes formed in the process.1,2 Terminal alkenes
formed by decarbonylation are appealing organic materials
because of their application in the polymer industry. There-
fore, an effective catalyst for decarbonylation prevents both
hydrogenation of product α-olefin as well as the double bond
isomerization to form internal olefins.
Scheme 1 Decarbonylation and decarboxylation of heptanoic acid.
terms of catalyst efficiency and selectivity towards terminal
olefin formation in the product.1,3 In 1982, Maier et al. studied
the decarboxylation of carboxylic acids for production of fuel-
like hydrocarbons.4 In their work, they showed that supported
Pd and Ni catalysts were highly selective towards deoxygena-
tion products. In 2006, Murzin and coworkers studied several
other supported metals such as Ru, Pt, Ir, Os, and Rh, and
they concluded that the most promising of the transition
metals are Pd and Pt, which achieved more than 90% selecti-
vity towards alkane and alkene production when the metals
were supported on carbon.1 Nickel has also been extensively
studied for deoxygenation of fatty acids and triglycerides, but
is not an attractive metal because it favors alkane production
and cracking.1,5,6 Furthermore, Maier et al. showed that a
variety of fatty acids can be converted over Ni and Pd catalysts
via decarboxylation, however, the use of H2 in the reaction was
essential.4
Relatively little has been reported on the conversion of fatty
acids to olefins, with generally poor results being presented in
Department of Chemical Engineering, University of Virginia, 102 Engineer’s Way,
P.O. Box 400741, Charlottesville, VA 22904-4741, USA. E-mail: rjd4f@virginia.edu;
Fax: +1 4349822658; Tel: +1 4349246284
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C and SiC support particles were crushed and sieved between
180 μm and 425 μm. Vulcan C powder was used as received.
After impregnation of the desired Pt precursor solution, the
catalyst was dried in air at 393 K overnight. The dried solids
were then reduced in H2 (GT&S 99.999%) flowing at 100 cm3
Scheme 2 Ketonization of heptanoic acid.
min−1
. The temperature of reduction was increased at
5 K min−1 from room temperature to the desired value,
which ranged from 623 to 873 K, and held isothermally for
3 h. The catalyst was then cooled, exposed to air, and stored in
a vial.
2.2. Dihydrogen chemisorption
Scheme 3 Hydrogenation of 1-hexene.
The metal dispersion was measured by H2 chemisorption
performed on a Micromeritics ASAP 2020 adsorption system.
The catalyst was evacuated at 573 K for 10 h under vacuum
followed by heating to 623 K at a rate of 5 K min−1 after which
it was reduced with flowing H2 (GT&S 99.999%) at 623 K
for 2 h.
Following reduction, the catalyst sample was evacuated at
623 K and cooled to 308 K in vacuum. The chemisorption of
H2 was performed at 308 K over the pressure range of 40 to
450 Torr. The amount of metal on the surface was evaluated by
the total amount of H2 adsorbed extrapolated to zero pressure,
assuming a stoichiometric relationship between H2 to metal
(H2 : Pt) of 1 : 2 (i.e. H/Pt = 1).
A recent study showed that ketonization can play an
important role in upgrading fatty acids into fine chemicals or
fuels.7 This reaction involves the coupling of two carboxylic
acids to produce a symmetrical ketone with the simul-
taneous removal of oxygen in the form of CO2 and H2O, as
depicted in Scheme 2. Gaertner et al.8 theorized that ketoniza-
tion might be an intermediate step in the deoxygenation reac-
tion and, in the presence of a transition metal, the ketone
could further convert to olefins (mostly α-olefins) and CO.
The ketonization reaction is also referred to as ketonic
decarboxylation.
As mentioned above, hydrogenation is an important side
reaction that can also take place over supported metal catalysts
(see Scheme 3). Because hydrogenation of α-olefins forms less
valuable paraffins, sources of hydrogen in the reaction need to
be minimized. Some studies on decarboxylation and decarbo-
nylation include H2 not only to maximize the selectivity
towards the formation of paraffins (typically for fuel com-
ponents), but also to influence the deoxygenation rate.9,10
Snåre et al. showed that reaction under low H2 conditions
formed coke.10 Olefins can also be formed from the decarboxy-
lation of unsaturated fatty acids that had been dehydrogenated
earlier in the reactor.9
2.3. Dinitrogen physisorption
The specific surface area and pore size were measured by N2
physisorption using a Micromeritics ASAP 2020. The samples
were evacuated for 180 min at 373 K prior to the N2 physisorp-
tion, after which adsorption isotherms were measured at 77 K.
The Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–
Halenda (BJH) methods were used to determine the specific
surface area and the pore size, respectively.
2.4. X-ray diffraction
In this study, we studied systematically the decarbonylation
and decarboxylation of heptanoic acid over a wide variety of
carbon-supported Pt catalysts in a steady-state, fixed bed flow
reactor to elucidate the reaction path and evaluate the turnover
frequency of the reaction. In particular, the influences of Pt
dispersion, Pt loading, carbon support and reaction phase on
the conversion of heptanoic acid and selectivity to various
products were explored.
Powder X-ray diffraction (XRD) analysis was carried out on a
X’Pert Pro MPD with monochromatic Cu Kα-radiation (λ =
1.54056 Å) using a step size of 1/4° and a time step of
1 s. Diffraction angles were measured from 2θ = 20° to 100°.
The Scherrer equation was used to estimate the particle size as
described in Patterson et al.11
2.5. Electron microscopy
Transmission electron microscopy (TEM) was performed on a
FEI Titan operated at 300 keV in EFTEM mode. Catalyst
samples were deposited on a SPI lacey-carbon coated Cu grid
after being dispersed in methanol.12 Fresh (after catalyst syn-
2. Experimental methods
2.1. Catalyst synthesis
The catalysts were prepared by an incipient wetness impreg- thesis) and spent (after exposure to reaction conditions) cata-
nation method using aqueous solutions of tetraammineplati- lyst samples were imaged to determine the effect of reaction
num(II) nitrate, (NH3)4Pt(NO3)2 (Sigma Aldrich) to obtain 1, 3, conditions on particle morphology. The spent samples were
5, and 10 wt% Pt loadings. Norit activated carbon (ROX 0.9), recovered from the reactor, washed with 25 mL of methanol
SiC (SICAT β-SiC), and Vulcan carbon (VXC72R) were used as and sonicated in a methanol solution for 30 min to remove
supports for the metal particles. Prior to impregnation, Norit reactant and products from the surface of the catalyst. After
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sonication, the solids were recovered by filtration and dried
overnight in air at 400 K.
2.6. Catalytic conversion of heptanoic acid
The decarbonylation/decarboxylation of fatty acids can be a
challenging reaction, mainly because of low reaction rates and
rapid catalyst deactivation.2 Furthermore, saturated fatty acids
with more than 9 carbons in the aliphatic chain have a high
melting point. This physical property makes operation in a
continuous mode particularly challenging since the entire
pumping system as well as the reactor must be heated. In this
study, heptanoic acid was selected as the reactant because it is
a liquid at room temperature, therefore eliminating the need
to pre-heat the lines and the pumping system.
Fig. 1 Schematic diagram of reactor system. TC represents a thermo-
Decarbonylation/decarboxylation reactions are often per- couple, GC represents a gas chromatograph, PG represents a pressure
gauge, and PC represents a back pressure regulator.
formed in a solvent with H2 simultaneously fed to the reactor
to extend catalyst life. Under these conditions, the fatty acid
could be hydrogenated before deoxygenation, leading to the
The liquid sample was analyzed by a gas chromatograph
formation of alcohols and other products, which can react
(Agilent 7890A) equipped with a ZB-FFAP column (length 45 m
with the acid substrate to form esters.2–4,7–9,13,14 To avoid
× 0.538 mm) and a flame ionization detector. The presence of
undesirable side reactions, neither solvent nor H2 was cofed
dodecane as an internal standard allowed for quantification of
with the heptanoic acid. Dodecane was used as an internal
the products.
standard because it has a similar boiling point to heptanoic
The product gas was continuously removed from the condenser
acid and is thermally stable. The amount of dodecane was
by an N2 (GT&S 99.999%) purge gas, which was fed to an on-line
minimized (5 wt%) to avoid hydrogen transfer from it to the
gas chromatograph (HP 5890 Series II) equipped with a packed
column, ShinCarbon ST 80/100, and a thermal conductivity
products.9
The main organic products from converting heptanoic acid
detector (TCD) for detection of N2, CO, CO2, and light hydrocar-
were expected to be 6 carbon hydrocarbons, hexane, 1, 2, and
bons (if any). The N2 purge gas was also used to maintain pressure
3-hexene, and the ketonization product 7-dihexyl ketone (a
in the system and was the internal standard for GC analysis.
symmetrical ketone). These compounds were therefore moni-
A series of control experiments was performed to determine
the background conversion of the system at our typical reac-
tored by gas chromatography.
The catalytic decarbonylation/decarboxylation of heptanoic
tion conditions (573 K, 37 or 1 bar, and 0.01 cm3 min−1 of
acid (Sigma Aldrich 99.0%) was performed in a 1/4″ OD × 8″
liquid feed composed of 95 wt% heptanoic acid and 5 wt%
dodecane). In the absence of catalyst, no conversion of hepta-
noic acid was observed. However, when Norit carbon and
Vulcan carbon supports were used, a background conversion
length × 0.035″ wall thickness stainless steel fixed bed reactor.
After loading the catalyst in the reactor, the system was first
flushed for 30 min with 100 cm3 min−1 of N2 (GT&S 99.999%)
at 40 bar to remove any dioxygen from the system and to
of 2–3% was detected. No conversion was observed over SiC.
perform a leak test. Then, the temperature was increased to
the desired reduction temperature, 623–873 K, under 100 cm3
2.7. Turnover frequency and selectivity calculations
min−1 of N2 at 5 K min−1 at 1 bar. The temperature was held
isothermally for 2 h as the catalyst was reduced in H2 (GT&S The turnover frequency (TOF) was defined by Boudart as the
99.999%) flowing at 100 cm3 min−1 at atmospheric pressure. number of revolutions of the catalytic cycle per unit time.15 In
The reactor was finally cooled to the reaction temperature, this work the TOF was calculated as the rate of formation of
573 K, and pressurized to 37 bar in 100 cm3 min−1 of N2 the products referred to the number of surface Pt atoms. The
(GT&S 99.999%). The feed mixture was composed of 95 wt% main components in the liquid products were hexane (C6),
heptanoic acid (Sigma Aldrich 99.00%) and 5 wt% dodecane 1-hexene (α-C6), 2 and 3-hexenes (i-hexenes, i-C6), 7-tridecanone
(Sigma Aldrich anhydrous 99.00%). The feed was pumped into (ketone), and minor amounts of heavier unknown products
the reactor containing 0.25 g of catalyst using a syringe pump (which accounted for ≈3% of the total detected liquid
(ISCO 500D) operating at different liquid flow rates (0.005, product). The TOF was therefore calculated as [(rate of pro-
0.01, or 0.05 cm3 min−1). The acid feed passed through a duction (mol s−1) of C6 + α-C6 + i-C6 + 2× moles of ketone +
heated zone before entering the catalytic reactor. The reactor others)/(mol of surface Pt counted by H2 chemisorption)]. The
was operated in an upflow mode because gases were generated major gas products observed were CO and CO2. The product
in the reaction. Fig. 1 shows a schematic of the reactor system.
selectivity was defined as the moles of a product formed
The outlet of the reactor was connected to a condenser, divided by the total moles of all identified products present in
maintained at room temperature, and samples of the liquid the same phase (liquid or gas), i.e. selectivity of 1-hexene [(rate
and gas products were collected approximately every 150 min. of α-C6)/(rate of all C6 products + moles of 7-tridecanone +
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moles of unknowns)] and selectivity of CO [(rate of CO)/(rate of pelletize, which may account for why so many of the earlier
CO + rate of CO2)]. Selectivities of liquid and gas products were studies in the field were performed in batch systems. Oper-
calculated independently.
ation of a fixed bed reactor system with a fine powder can
The TOF associated with the Pt was based on measured develop a very large pressure drop across the catalyst bed.
rates after subtracting the background conversion from the Therefore, we chose to explore some of the pelletized carbons
support and normalized by the dispersion of Pt evaluated on a as supports for Pt nanoparticles.
freshly prepared catalyst.
Three different carbonaceous supports were tested in this
study, each having very different physical properties. Norit
carbon is a high surface area support that can be purchased in
pellet form, which can be easily crushed to the desired particle
size. However, it has many small pores that might influence
the transport rates of products out of the pellets and therefore
affect the side reactions. Vulcan carbon is nonporous, but has
a much smaller surface area than Norit carbon. Furthermore,
the as-received Vulcan carbon could not be easily pelletized.
Preliminary tests of Vulcan carbon powder in our fixed bed
reactor revealed a very large pressure drop when we tried to
pump liquid reactant through the bed. Therefore, Vulcan
carbon was only used as a support for Pt in experiments with
gaseous heptanoic acid. Silicon carbide is a macroporous
support that was obtained in pellet form. However, it has a
much lower surface area than Norit carbon.
Catalysts composed of 5 wt% Pt supported on the various
carbon supports were characterized using N2 physisorption, H2
chemisorption, and XRD, and the results are summarized in
Table 1. While a high Pt dispersion of 31% was obtained on
Norit carbon, very large Pt particles were formed on Vulcan
carbon and SiC. The diffraction peaks in Fig. 2 observed at
about 39.5°, 45.5°, 57°, and 79.5° are attributed to the (111),
(200), (220), and (311) reflections of Pt.25 Analysis of the peak
width by the Scherrer equation gave an average Pt crystallite
size of 21 and 40 nm for Pt supported on Vulcan C and SiC,
respectively. The lack of a Pt diffraction pattern associated
with Pt/Norit C was consistent with the high metal dispersion
revealed by H2 chemisorption.
2.8. Mass transfer limitations
The mass transfer characteristics were estimated using the
Weisz–Prater criterion for internal transport limitations (ΦI),
eqn (1),
robsRp2ρp
ΦI ¼
, 1
ð1Þ
DEACAS
and the analogous expression for external transport limitations
(ΦE), eqn (2),
robsRpρp
ΦE ¼
, 0:15
ð2Þ
kACAB
assuming first order reaction.16 In eqn (1) and (2), robs is the
observed rate, Rp is the radius of the support particle, ρp is the
density of the catalyst pellet, DEA is the effective diffusivity, kA
is the external mass transfer coefficient, CAS is the surface con-
centration, and CAB is the bulk concentration of heptanoic
acid.16,17
The effective diffusivity (DEA) was estimated using a porosity
(εP) of 0.5 and a tortuosity (τ) of 3 as recommended in Davis
and Davis.17 The self-diffusion coefficient (DAA) for heptanoic
acid was calculated using the Stokes–Einstein equation for
analysis of the liquid-phase and using gas kinetic theory for
analysis of the gas-phase reaction. Because the average Pt par-
ticle size was ≈2 nm or greater, it was assumed for this analy-
sis that the size of the pore in which the catalytic reaction
takes place is at least 2 nm, which was used to calculate the
Knudsen diffusivity (DKA) for the gas-phase operation. The
external mass transfer coefficient (kA) was obtained using a
mass transfer correlation for fluid–solid interface for packed
bed reactors obtained from Cussler.18
The three catalysts were tested under the same reaction con-
ditions (573 K, 0.25 g of catalyst, and a flow rate of 0.01 cm3
min−1) and their performance is summarized in Table 2.
The most obvious conclusion from Table 2 is the highest
conversion of heptanoic acid over Pt/Norit C, presumably
because of the highest metal dispersion. The results in Table 2
also show an effect of the reaction phase on the product
selectivity. For Pt/Norit C and Pt/SiC, the selectivity towards
the formation of α-olefin was increased in gas-phase reaction
Using the highest observed rates, ΦI was calculated to be
much less than 1 for the liquid and gas phase reactions (0.005
and 0.012, respectively) and ΦE was much less than 0.15 for
the liquid and gas phase reactions (0.004 and 0.001, respect-
ively) for a catalyst particle size of 425 μm. The criteria suggest
that the system was not mass transfer limited with respect to
the reactant.
Table 1 Summary of the characterization results for 5 wt% Pt on
different supports
Dispersion
(%)
Surface area
Pore size
(nm)
Pt particle
size (nm)
3. Results and discussion
3.1. Effect of carbonaceous support
Support
(m2 g−1
)
Norit carbon
Vulcan carbon
SiC
31a
—
—
1428
217
26
N.D.b
7.3
20
3.7a
21c
Carbon is a widely used support for transition metals in deoxy-
genation reactions because of its high surface area
(700–1500 m2 g−1) and relative inertness.1,2,19–24 Carbon sup-
ports are available in different particle sizes, surface areas, and
40c
a Result estimated from H2 chemisorption. b Not determined, both
micropores and mesopores are present on this sample. c Result
pore size distributions. Most carbon supports are difficult to obtained using XRD patterns and Scherrer equation.
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Table 3 Result of the characterization of 3 wt% Pt/Norit C reduced at
four different temperatures
Metal
dispersion
temperature (K) (%)
Pt particle size (nm)
Surface
area
Reduction
Chemisorption TEM
(m2 g−1
)
623
673
773
873
49
21
13
13
2.3
5.3
8.9
8.4
1.9 0.2 1477
2.1 0.5 1348
2.5 0.8 1495
3.1 1.0 1374
heptanoic acid. Table 3 summarizes the results of the catalyst
characterization by chemisorption, physisorption and electron
microscopy.
Although the dihydrogen uptake decreased on the samples
reduced at elevated temperatures, electron microscopy revealed
very little growth of the small particles (from 2 to 3 nm). These
contrasting findings could be the result of some Pt migrating
outside of the Norit C pores to form large particles on the
external surface. It should be noted that a wider Pt particle
size distribution was observed by microscopy, which is consist-
ent with Pt migration on the sample. Fig. 3 shows the images
of the catalysts obtained by TEM.
Conversion of heptanoic acid and product distribution over
the catalyst reduced at different temperatures were monitored
for 35 h, and the results are presented in Fig. 4 and 5. The cata-
lytic activity and product distribution of the four catalysts did
not reach steady state until about 15 to 20 h of reaction.
Because of this induction period, the cause of which is
unknown, the results in all of the subsequent tables are
reported after at least 15 to 20 h of reaction.
Fig. 2 Comparison of the XRD patterns of 5 wt% Pt supported on Norit
carbon, Vulcan carbon and SiC reduced at 623 K under 100 cm3 min−1
of H2 at a heating rate of 5 K min−1
.
Represents features of SiC
support. The patterns were offset for clarity.
at 1 bar compared to the liquid-phase reaction at 37 bar. On
Pt/Norit C, internal olefins were the majority product at both
liquid-phase and gas-phase conditions. However, the product
distribution on Pt/SiC shifted from mostly paraffin (hexane) to
mostly ketone after moving from the liquid-phase conditions
into the gas-phase.
Given the lack of success with preparing highly dispersed
Pt on Vulcan C and SiC after reduction at 623 K, and the un-
favorable product distributions on Pt/Vulcan C and Pt/SiC, we
decided to focus our efforts on studying the behavior of Pt/
Norit C.
As expected, increasing the reduction temperature
decreased the Pt dispersion, which negatively affected the con-
version. However, the TOF associated with the four catalysts
was relatively constant at 0.0052 s−1 (Table 4). The constancy of
the TOF suggests that the system was not mass transfer
3.2. Effect of reduction temperature
Simakova et al.26 previously showed that Pd catalysts with limited since the Koros–Nowak (or Madon–Boudart) criterion
different but high metal dispersions can exhibit different is obeyed at this one reaction temperature, which is consistent
activities and product distributions. However, those catalysts with the conclusion from mass transfer criteria discussed
with high dispersions approached similar activity after pro- earlier.27
longed reaction times.26
Table 4 shows that the product distributions in the liquid
To obtain different Pt metal dispersions on 3 wt% Pt/Norit (olefins and paraffin) and gas (CO and CO2) over the four cata-
C, we reduced the same catalyst at four different temperatures lysts were different. Although this finding differs from the lit-
(625, 673, 773 and 873 K) and tested them in the reaction of erature, it could be a result of the different conversion levels
Table 2 Effect of support on the reaction of heptanoic acid over Pt nanoparticlesa
Selectivity (%)
Support
Pressure (bar)
Conversion (%)
α-Olefin
i-Olefin
Paraffin
Ketone
Others
Norit carbon
37
1
1
37
1
10.3
11.1
3.5
1.6
0.5
5.65
26.4
14.1
2.5
61.0
55.6
64.7
19.9
0.0
28.4
15.3
18.5
54.7
18.6
0.6
3.4
0.2
3.5
58.7
4.3
0.4
2.6
19.5
0.0
Vulcan carbon
SiC
22.7
a The reaction was run at 573 K, 37 bar for liquid-phase experiments and 1 bar for gas-phase experiments, with 0.25 g of catalyst and a flow rate
of 0.01 cm3 min−1 composed of 95 wt% heptanoic acid and 5 wt% dodecane.
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Fig. 4 Effect of reduction temperature ( represents 623 K,
rep-
resents 673 K, represents 773 K, and represents 873 K) on heptanoic
acid conversion over 3 wt% Pt/Norit C. The reactions conditions were
573 K, 37 bar, and 0.01 cm3 min−1 of feed composed of 95 wt% hepta-
noic acid and 5 wt% dodecane.
Fig. 3 TEM images of fresh 3 wt% Pt/Norit C catalyst reduced at four
different temperatures. (A) was reduced at 623 K, (B) was reduced at
673 K, (C) was reduced at 773 K, and (D) was reduced at 873 K.
As expected, the conversion of heptanoic acid increased
with the amount of surface Pt in the reactor. The TOF at 553
and 573 K was constant at each temperature over the four
different metal loadings tested with an average value of 0.0022
, respectively. Furthermore, the TOF
obtained at 573 K matched the one obtained over 3 wt%
Pt/Norit C reduced at different temperatures (Table 4).
Increasing the Pt loading did not considerably increase the Pt
particle size but instead increased the number of particles per
unit of area, as shown by the TEM images in Fig. 7. The average
Pt particle size obtained by TEM was relatively constant at a value
of about 2 nm for all the samples, while the value estimated by
H2 chemisorption varied between 2.3 and 4.0 nm (Table 5).
Again, this difference might be due to some large Pt particles on
the external surface of the pellets that were not observed by
microscopy but were accounted for by H2 chemisorption.
achieved by each catalyst.26 Furthermore, the selectivity
towards the formation of α-olefin was very low and was most
likely due to rapid double bond isomerization over the Pt at
the reaction temperature.
s−1 and 0.0052 s−1
3.3. Effect of metal loading
Given the non-uniformity of the Pt particle size distribution
formed by varying the reduction rate, we decided to test the
performance of heptanoic acid conversion over catalysts with
different Pt loadings but reduced at the same temperature.
Therefore, four catalysts with different metal loadings of 1, 3,
5, and 10 wt% Pt were synthesized and tested at two different
reaction temperatures. The results from characterization and
reaction testing are summarized in Table 5 and Fig. 6.
Fig. 5 Effect of reduction temperature ( represents 623 K, represents 673 K, represents 773 K, and represents 873 K) on the olefin (left) and
CO (right) selectivity over time during the conversion of heptanoic acid over 3 wt% Pt/Norit C. The reactions conditions were 573 K, 37 bar, and
0.01 cm3 min−1 of feed composed of 95 wt% heptanoic acid and 5 wt% dodecane. Olefin selectivity is relative to liquid products only and CO selecti-
vity is relative to CO + CO2.
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Table 4 Effect of metal dispersion on the decarbonylation/decarboxylation of heptanoic acid over 3 wt% Pt/Norit Ca
Product selectivity (%)
Liquid
Gas
CO
Reduction temperature (K)
Dispersion (%)
TOF (s−1
)
Conversionb (%)
α-Olefin
i-Olefins
Paraffin
Ketone
Others
623
673
773
873
49
21
13
13
0.0052
0.0051
0.0052
0.0051
9.1
3.8
2.4
2.5
5.2
5.0
4.8
5.0
63.2
62.9
61.1
54.7
27.7
28.2
31.5
34.0
1.2
1.4
0.5
4.0
2.7
2.5
1.6
2.3
57.7
69.1
66.6
55.4
a The reactions conditions were 573 K, 37 bar, 0.01 cm3 min−1 of feed composed of 95 wt% heptanoic acid and 5 wt% dodecane. Results were
recorded after 20 h of reaction. b Background conversion has been removed.
Table 5 Effect of Pt loading on heptanoic acid reaction over Pt/Norit C at 553 K and 573 Ka
Pt particle size (nm)
Chemisorption
Conversionb (%)
TOFb (s−1
)
Loading (wt% Pt/C)
Dispersion (%)
TEM
553 K
573 K
553 K
573 K
1.0
3.0
5.0
10.0
28
49
31
39
4.0
2.3
3.7
2.9
1.8 0.4
1.9 0.2
1.9 0.3
2.1 0.5
0.7
—
4.4
1.7
9.1
8.2
0.0022
—
0.0024
0.0021
0.0052
0.0052
0.0045
0.0051
10.1
24.3
a The catalysts were tested at 37 bar and 0.01 cm3 min−1 of feed composed of 95 wt% heptanoic acid and 5 wt% dodecane. Results were recorded
after 20 h of reaction. b Background conversion has been removed.
Fig. 6 Effect of Pt weight loading supported on Norit C on product
selectivity and conversion at 573 K, 37 bar, and 0.01 cm3 min−1 of feed
composed of 95 wt% heptanoic acid and 5 wt% dodecane. The product
selectivity (%) is represented with color bars and can be read on the left
□
axis. Represents 1-hexene, represents 2 and 3-hexenes, represents
Fig. 7 TEM images of fresh Pt/Norit C catalysts reduced at identical
conditions but with four different Pt loadings. (A) has 1 wt% loading, (B)
has 3 wt% loading, (C) has 5 wt% loading, and (D) has 10 wt% loading.
■
hexane, represents 7-tridecanone, and represents the unknown pro-
ducts. The black squares represent conversion (%) and can be read on the
right axis. These results were recorded after 20 h of reaction.
Due to a lack of reported TOF values for the decarbonyla-
tion/decarboxylation reactions over Pt catalysts, we compared
our TOF values to those reported for Pd. In general, the TOF
obtained with Pt in this study was lower than the TOF reported
in the literature for Pd. For example, TOF values over Pd
ranged from 0.01 to 0.96 s−1 for batch reaction at 573 K22–24,28
and from 0.027 to 0.004 s−1 for fixed-bed operation at
As shown in Fig. 6, the product selectivity changed as the
metal loading and therefore the conversion increased. For
example, the overall selectivity towards the formation of
olefins decreased at higher conversion; interestingly the rela-
tively low selectivity of α-olefin relative to all olefins (1, 2, and
3-hexenes) remained constant (Fig. 6).
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543 K.2,13 As mentioned in the Experimental methods section,
Norit C revealed a low level of background conversion (≈2%)
at our reaction conditions (573 K, and 0.01 cm3 min−1 of feed),
which contributed significantly to the observed conversion at
low Pt loadings or low Pt dispersions. If the background con-
version were not accounted for properly, the TOF for Pt would
be artificially inflated. Likewise, for Pd catalysts with low load-
ings or low dispersions, it is possible that some background
conversion on the support could contribute to the high TOF
reported in the literature.22,26 Furthermore, most of the exper-
iments with Pd involved co-feeding H2 and diluting the fatty
acid with long chain hydrocarbons, which have been reported
to enhance the TOF and the stability of the catalyst.9,10
3.4. Effect of flow rate
To minimize the impact of support activity on the results, we
chose to study a 10 wt% Pt/Norit C catalyst at various flow
rates of heptanoic acid to explore the influence of conversion
on the product distribution. The feed flow rate was varied from
0.005 to 0.05 cm3 min−1 over a period of 120 h (≈2200 turn-
overs) to obtain a range of conversion values. Fig. 8 shows the
relationship between space time and conversion, which was
fairly linear at low conversion. At higher conversions, however,
the catalyst appeared to be less active than expected, which
might be evidence for inhibition by product CO on the Pt
sites. The conversion at very short space time was reproduced
after 120 h of reaction, confirming that a long term de-
activation process was not significant during the course of this
particular experiment. If the Pt metal nanoparticles sintered
under reaction conditions, particle growth must have occurred
during the initial induction period.
Fig. 9 Dependence of decarbonylation product selectivity on conver-
■
sion over 10 wt% Pt/Norit C at 573 K and 37 bar.
Represents the
⧫
selectivity to decarbonylation products, olefins, in the liquid product.
Represents the selectivity to decarbonylation product, CO, in the gas
▲
product.
Represents the selectivity to α-olefins in the liquid product.
These results were recorded after 20 h of reaction.
Fig. 9 shows a very strong correlation of the product selecti-
vity to conversion. Extrapolation of the product selectivity for
gaseous products (CO + CO2) to zero conversion indicates 80%
of the gas product is CO. Likewise, extrapolation of the liquid-
phase products to zero conversion results in 80% selectivity to
olefins. Evidently, the initial reaction of heptanoic acid under
these conditions is mainly decarbonylation. The very low
selectivity to α-olefins was the result of rapid isomerization of
the double bond in 1-hexene over Pt to form 2 and 3 hexenes.
With increasing conversion, the selectivity for CO decreased
more extensively than the selectivity for olefins, suggesting
there is a side reaction that converts CO into CO2. A likely side
reaction is the water–gas shift (WGS) reaction shown in
Scheme 4, which is believed to play an important role in the
decarbonylation/decarboxylation reactions, not only for con-
verting CO into CO2, but for providing a source of H2 that can
hydrogenate the olefins into paraffin.23,24 In contrast, Lestari
et al. theorized that CO is actually formed from CO2 in the
reverse water–gas shift reaction, rather than by direct decarbo-
nylation reaction of fatty acids.29 Our results in Fig. 9 clearly
support the idea that decarbonylation was the primary reaction
on Pt metal nanoparticles under our experimental conditions.
The fact that the fraction of olefins in the liquid products
exceeded the fraction of CO in the gaseous products at increas-
ing conversions (Fig. 9) suggests that the WGS reaction occurs
more readily than the hydrogenation of olefins using H2 pro-
duced by WGS. Fig. 9 also shows that selectivity of α-olefins
(1-hexene) in the liquid product was always less than about 10%,
Fig. 8 Dependence of conversion of heptanoic acid at different flow
rates over 10 wt% Pt/Norit C at 573 K and 37 bar. These results were
recorded after 20 h of reaction. The W/F was calculated as the weight of
catalyst used divided by the mass flow rate.
Scheme 4 Water–gas shift (WGS) reaction.
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even at very low conversions. Evidently, the double bond iso- α-olefin to the internal olefins, even at low pressure.
merization of 1-hexene to 2 and 3-hexenes was rapid over Pt Fig. 10 also shows that lowering the conversion by increas-
catalysts in the liquid-phase conversion of heptanoic acid at ing the reactant flow rate improved selectivity to α-olefin,
573 K.
which is expected when α-olefin is a primary product of
decarbonylation.
3.5. Effect of pressure
It should be noted that the conversion of heptanoic acid
over the three different Pt catalysts did not correlate with the
surface Pt when operated in the gas-phase, which contrasts the
direct relation observed in the liquid-phase, as reported in
Fig. 10 and Table 5. We attribute the observed behavior of Pt
catalysts in gas-phase operation to significant deactivation,
which is discussed in the next section.
Previous studies showed that the formation rate of alkene did
not match that of CO.2,22 Our results clearly show that reac-
tions of the primary products, α-olefin, CO (and presumably
H2O), proceed at rates that are quite rapid. The formation of
CO2 and H2 via the water–gas shift reaction, which is then fol-
lowed by hydrogenation of olefin to paraffin, might account
for the product distribution at higher conversion. Although
the conversion of heptanoic acid was not mass transfer limited
with respect to the acid, it is possible that rapid conversion of
3.6. Catalyst deactivation
the primary products in the catalyst pores might be limited by Catalyst deactivation during the deoxygenation of carboxylic
mass transfer rates out of the catalyst particles. To increase the acids has been widely reported in the literature. Snåre et al.10
effective diffusivity and external mass transfer coefficient of and Mäki-Arvela et al.13 suggested that deactivation originated
the molecules, we performed exactly the same reaction experi- from poisoning of the metal by the reactants, products
ments at 1 bar pressure, which allows the major products and (especially CO), and coking. Furthermore, Simakova et al.
heptanoic acid to remain in the gas phase.
reported that deactivating aromatics are formed in reactions
Fig. 10 compares the performance of Pt/Norit C with performed under H2-scarce conditions.30 The initial concen-
different metal loadings and system pressures. For operation tration of carboxylic acid is also proposed to influence catalyst
with liquid-phase reactant (37 bar), the main products at high deactivation. Mäki-Arvela et al. observed that deactivation was
conversions (>20%) were hexane and CO2. At lower conver- more extensive with higher initial concentration of carboxylic
sions, mostly olefins were produced, although they were acid.13
mainly 2 and 3 hexenes formed from double bond isomeriza-
Ping et al. imaged Pd nanoparticles supported on silica
tion of 1-hexene. When the reaction was run at low pressure with TEM before and after reaction.31 The catalyst did not
(1 bar) resulting in gas-phase conditions in the reactor, the show a significant increase in metal particle size after reaction
selectivity towards the formation of α-olefin was higher than indicating negligible metal sintering occurred. Since most of
that in the liquid phase. Moreover, higher metal loadings in the catalyst activity was regained after hot extraction with
the catalyst pores facilitated subsequent conversion of the solvent and subsequent reduction in H2 at 573 K, the authors
Fig. 10 Effect of reaction pressure and feed flow rate on catalyst activity and product distribution over Pt/Norit C with different loadings of Pt
during the conversion of heptanoic acid at 573 K. The feed composition was 95 wt% heptanoic acid and 5 wt% dodecane. The color bars can be
□
read on the left axis and represents the product distribution. Represents 1-hexene, represents 2–3-hexenes, represents hexane, represents
■
7-tridecanone, and represents the unknown products. The black squares represent conversion (%) and can be read on the right axis. These results
were recorded after 20 h of reaction. (A) was run in liquid-phase (37 bar) conditions and flow rate of 0.01 cm3 min−1. (B) was run in gas-phase (1 bar)
conditions and a flow rate of 0. 01 cm3 min−1. (C) was run in gas-phase (1 bar) conditions and a flow rate of 0.05 cm3 min−1
.
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Fig. 11 Effect of deactivation on catalyst activity and product distribution over 10 wt% Pt/Norit C during the conversion of heptanoic acid at 573 K
and a feed flow rate of 0.01 cm3 min−1 composed of 95 wt% heptanoic acid and 5 wt% dodecane. The bars with patterns represents the product dis-
□
tribution and can be read on the left axis. Represents 1-hexene, represents 2–3-hexenes, represents hexane, represents 7-tridecanone, and
■
represents the unknown products. The black squares represent conversion (%) and can be read on the right axis. These results were recorded after
20 h of reaction.
concluded that the observed deactivation resulted from depo-
sition of organic molecules onto the active sites.31
Fig. 11 summarized catalyst performance after several
changes in reaction conditions to study the deactivation
phenomenon. Although our studies revealed stable operation
of Pt/Norit C up to 120 h of operation with the liquid-phase
reactant, we observed significant deactivation during gas-
phase operation at 1 bar. For this series of experiments, the
catalyst was assumed to be at steady state after prolonged
liquid-phase reaction. At this point, the pressure was lowered
to 1 bar to begin operation in the gas phase. Although the con-
version decreased after the phase change, catalyst deactivation
continued to lower conversion with time. After 40 h of gas-
phase operation, the catalyst deactivated to half its activity at
the time of the phase change. The pressure was then increased
to return to liquid-phase operation. After 20 h of steady state
operation in liquid-phase conditions, the catalyst activity
remained low, indicating the liquid-phase flow was unable to
recover the original activity.
The deactivated catalyst was finally exposed to flowing hep-
tanoic acid at 350 K at 1 bar for 24 h to remove possible pro-
ducts from the surface. The reactor was then flushed with N2
and heated at 5 K min−1 to reaction temperature. Since the cata-
lyst activity was not regained by this treatment, the catalyst was
also re-reduced in flowing H2 at 623 K and 1 bar for 4 h, but
again the activity was not recovered. A mild oxidation treatment
of the catalyst in air at 623 K and 1 bar for 4 h followed by a re-
reduction in flowing H2 at 623 K and 1 bar for 4 h also did not
re-activated the catalyst. Clearly, the mode of deactivation was
not easily reversed by standard regeneration methods.
Fig. 12 TEM images of spent Pt/Norit C catalysts. The catalysts were
prepared by IWI and reduced with H2 at 623 K at a heating rate of 5 K
min−1. (A) represents a spent sample of 1 wt% Pt, (B) represents a spent
sample of 5 wt% Pt, and (C) represent a spent sample of 10 wt% Pt. The
spent samples were first run in liquid-phase operation until steady-state
operation and then they were run in gas-phase operation until de-
activation. At that point in time, the spent samples were recovered from
the reactor and treated as described in the Experimental methods
section. The spent catalysts were collected after a total of 80 h under
reaction conditions.
samples shown in Fig. 7. In contrast, XRD patterns of the cata-
lyst before and after reaction presented in Fig. 13 suggested
the presence of some large Pt particles on the spent catalysts.
The resulting average particle size of the spent catalyst was
Fig. 12 shows high magnification TEM images of three Pt/
Norit C catalysts with different weight loadings after 80 h of
reaction in gas-phase operation conditions, and they did not
reveal substantial particle growth compared to the fresh
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TOF reported here for Pt (which is based on dispersion of a
freshly prepared catalyst), the constancy of TOF in liquid-phase
operation with metal loading suggests that most of the Pt
remained highly dispersed. Nevertheless a difference in metal
particle size between the fresh catalyst and the spent catalyst
might explain the rather low TOF reported in this work, which
is based on initial Pt dispersion.
4. Conclusions
Platinum nanoparticles were supported on Norit carbon,
Vulcan carbon and silicon carbide, characterized by chemi-
sorption, X-ray diffraction and electron microscopy, and evalu-
ated in the conversion of heptanoic acid at 573 K. Results from
characterization and reactivity confirmed that highly dispersed
Pt was formed in Norit carbon (dispersion ≈49%) after a mild
reduction treatment at 623 K. Similar treatment of Pt/Vulcan
carbon and Pt/SiC resulted in the formation of much larger
particles of Pt. Variations of Pt dispersion, Pt loading and
reactor space time were used to explore the influence of con-
version on product distribution during heptanoic acid reac-
tion over Pt/Norit C. In summary, decarbonylation of
heptanoic acid was the primary reaction to produce olefin, CO
and water. Rapid conversion of α-olefin to internal olefin was
observed in liquid-phase operation, even at low conversion.
Higher selectivity to α-olefin product was achieved by oper-
ation in the gas phase, presumably because of the greater rate
of mass transfer of product molecules out of the catalyst pores.
Although various criteria indicated the rate of heptanoic acid
conversion was not limited by mass transfer, subsequent reac-
tions of the products could be. Higher conversion of heptanoic
acid in liquid-phase operation also was accompanied by
sequential conversion of CO to CO2, presumably via water–gas
shift reaction. In addition, hydrogenation of olefin was
observed at higher conversion as expected from H2 generated
by the WGS reaction. A summary of the reaction network
observed here is provided in Fig. 15.
Fig. 13 Comparison of the XRD patterns of Norit C support, fresh 10 wt%
Pt/Norit C after reduction, and spent 10 wt% Pt/Norit C after reaction.
The spent sample was first run in liquid-phase operation until steady-
state operation and then it was run in gas-phase operation until de-
activation. At that point, the spent sample was recovered from the
reactor and treated as described in the Experimental methods section.
The spent catalyst was collected after a total of 80 h under reaction
conditions. The patterns were offset for clarity.
estimated with the Scherrer equation to be 19 nm. The
migration and sintering of Pt would explain why the various
attempts at catalyst regeneration were unsuccessful. The sinter-
ing of Pt particles was not observed in Fig. 12 because the
images were associated with very thin areas of the sample to
obtain good contrast for imaging. We speculate that the
large Pt particles reside outside the small pores of the
carbon support. Indeed, the small Pt particles of 2 to 3 nm
observed in Fig. 12 probably reside inside the highly porous
structure Norit C which likely inhibited particle growth inside
the pores.
Fig. 14 presents a schematic diagram of our working hypoth-
esis for deactivation of the Pt particles during reaction. What is
apparently unknown at this point is why gas-phase operation
facilitates deactivation compared to liquid-phase operation. We
hypothesize that some organic species are strongly absorbed on
the Pt particles after gas-phase operation.
Although metal sintering observed by XRD might occur very
early during the reaction, which might account for the lower
Fig. 14 Schematic depiction of working hypothesis for catalyst
Fig. 15 Reaction network for the decarbonylation/decarboxylation of
sintering.
heptanoic acid over supported Pt at low conversion levels.
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13 P. Mäki-Arvela, M. Snåre, K. Eranen, J. Myllyoja and
Acknowledgements
D. Murzin, Fuel, 2008, 87, 3543–3549.
This material is based upon work supported by the National 14 C. A. Gaertner, J. C. Serrano-Ruiz, D. J. Braden and
Science Foundation (NSF) under Award no. EEC-0813570. Any J. A. Dumesic, Ind. Eng. Chem. Res., 2010, 49, 6027–6033.
opinions, findings, and conclusions or recommendations 15 M. Boudart, Chem. Rev., 1995, 95, 661–666.
expressed in this material are those of the author(s) and do 16 P. B. Weisz and C. D. Prater, Adv. Catal., 1954, 6, 143–196.
not necessarily reflect the views of the NSF. We acknowledge 17 M. E. Davis and R. J. Davis, Fundamentals of Chemical Reac-
discussions with Professor Matthew Neurock at the University
of Virginia, Dr. Hien Pham and Professor Abhaya Datye at the
tion Engineering, Dover Publications, Mineola, New York,
1st edn, 2012.
University of New Mexico. We also gratefully acknowledge the 18 E. L. Cussler, Diffusion: Mass Transfer in Fluid Systems, Cam-
help of Mr Richard White and Mr Matthew Schneider at the
University of Virginia for their help with the TEM.
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