Selective ketonization of acetic acid over HZSM-5: The importance of acyl species and the influence of water

Article abstract of DOI:10.1016/j.jcat.2016.04.017

The ketonization of acetic acid over HZSM-5 is investigated via a combination of temperature-programmed techniques (TP), IR, and reaction kinetics. TP experiments demonstrate the generation of stable acyl intermediates that require higher temperatures to facilitate C-C coupling with a second acetic acid molecule. Near-exclusive selectivities for the ketonization reaction devoid of significant sequential aldol condensation are observed at temperatures ranging from 270 to 330 °C. The rate-determining step is proposed to involve the interaction of an acyl group with a second acid, with an apparent reaction order of 1.6. This order can be explained by a second-order rate-determining step that is reduced as a result of acid-derived surface species. Isotope labeling experiments reveal that the rate-determining step is the formation of the C-C bond rather than the activation of the second acid. The ketonization reaction exhibits a -0.46 order with respect to water, but this loss in activity is accompanied by a significant increase in catalyst stability.

Full text of DOI:10.1016/j.jcat.2016.04.017

Journal of Catalysis 340 (2016) 76–84
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective ketonization of acetic acid over HZSM-5: The importance
of acyl species and the influence of water
⇑
Abhishek Gumidyala, Tawan Sooknoi, Steven Crossley
School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, OK 73019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The ketonization of acetic acid over HZSM-5 is investigated via a combination of temperature-programmed
techniques (TP), IR, and reaction kinetics. TP experiments demonstrate the generation of stable acyl inter-
mediates that require higher temperatures to facilitate C–C coupling with a second acetic acid molecule.
Near-exclusive selectivities for the ketonization reaction devoid of significant sequential aldol condensa-
tion are observed at temperatures ranging from 270 to 330 °C. The rate-determining step is proposed to
involve the interaction of an acyl group with a second acid, with an apparent reaction order of 1.6. This order
can be explained by a second-order rate-determining step that is reduced as a result of acid-derived surface
species. Isotope labeling experiments reveal that the rate-determining step is the formation of the C–C bond
rather than the activation of the second acid. The ketonization reaction exhibits a ꢀ0.46 order with respect
to water, but this loss in activity is accompanied by a significant increase in catalyst stability.
Ó 2016 Elsevier Inc. All rights reserved.
Received 29 July 2015
Revised 19 April 2016
Accepted 24 April 2016
Keywords:
Ketonization
Zeolites
Acetic acid
IR spectroscopy
Kinetics
Role of water
Bio-oil
1
. Introduction
two carbon atoms in its backbone means that conversion via
1 2
hydrotreating will yield low-value C and C hydrocarbons.
Bio-oil derived from the fast pyrolysis of biomass is a complex
The conversion of acetic acid to a stable, nonacidic molecule
with a longer C–C backbone is highly desirable. The ketonization
reaction is a well-known and effective method of carboxylic acid
upgrading, as it converts two organic acids to a ketone while
mixture of oxygenated compounds typically containing furanics,
phenolics, carboxylic acids, and other small oxygenates [1–13]. In
recent years, upgrading bio-oil has been the subject of significant
interest due to its potential as an alternative energy source and
for mitigating environmental challenges. Two very significant
challenges are associated with the upgrading of bio-oil to fuels
and chemicals. The first is the fact that it is thermally unstable,
with compounds condensing to solids upon aging at room temper-
ature. The second challenge is due to the large amounts of light
2
removing oxygen in the form of water and CO [16–25]. This reac-
tion is very appealing, as the stable ketones formed can be further
converted to produce longer-chain hydrocarbons via aldol conden-
sation/hydrogenation [21–25].
The ketonization of carboxylic acids via the decomposition of
salts [13,26–28], as well as over basic and reducible oxides, has
been extensively studied. While several reactive surface intermedi-
ates have been proposed over high-lattice reducible oxides involv-
ing ketenes, ketene-like species, carboxylates, and even anhydrides
[16–25], consensus is slowly growing on the importance of a
b-ketoacid intermediate. In contrast, the reactive intermediates
responsible for this transformation over acidic zeolites remain
unexplored, while some similar species such as ketenes have been
reported over Brønsted zeolites [29]. Despite the fact that ketones
have been observed as products from the transformation of acetic
acid over zeolites since the early 1980s, specific details of the reac-
tion mechanism still remain elusive [30–34]. While ketones are
reported in several cases, conditions that yield high selectivity to
acetone while minimizing sequential aldol condensation reactions
have yet to be identified. In addition to uncertainty surrounding
2 3
C –C oxygenates that are converted to low-value light gases after
conventional upgrading via hydrotreating, limiting the potential
for incorporation of the biomass-derived species into the gaso-
line/diesel fuel range [14].
Acetic acid is largely responsible for both of these challenges. It
is the most abundant single compound in bio-oil [3,15], and its
acidity is known to present corrosion problems and promote poly-
merization of other species present in the mixture. The resulting
solids are not readily upgraded and they accelerate catalyst
deactivation, which is a major concern for upgrading of pyrolysis
oils [1–13]. In addition, the fact that acetic acid contains only
⇑
Corresponding author.
E-mail address: stevencrossley@ou.edu (S. Crossley).
http://dx.doi.org/10.1016/j.jcat.2016.04.017
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
77
the mechanism, the critical role of water, which is present in signif-
icant quantities in virtually all streams that contain acetic acid, has
not been investigated for this reaction. Zeolites, especially MFI
framework zeolites, are among the most promising and commonly
employed catalysts for the conversion of bio-oil to fuels and chem-
icals. The fact that so little is known about the mechanism of con-
version of the most abundant species in bio-oil over this catalyst is
surprising, and warrants further investigation.
In this contribution, we report the ketonization of acetic acid over
HZSM-5 under conditions that selectively yield acetone. A system-
atic variation of temperature gives insight into the mechanism of
acetone production and the resulting changes in selectivity.
Temperature-programmed desorption and IR studies combined
with reaction kinetics are used to investigate the reaction pathway
and the possible reaction intermediates. The role of water in the
ketonization mechanism and its impact on catalyst deactivation,
competitive adsorption, and interaction with intermediate surface
species are also discussed.
The few studies investigating the ketonization reaction over
zeolites have operated under conditions that also involve a variety
of secondary reactions such as aldol condensation and aromatiza-
tion [34], making it difficult to come to mechanistic conclusions.
In addition to these sequential and parallel products, the intrinsic
ability of a zeolite to activate various oxygenated species, such as
acetic acid, can be nullified by the rapid deactivation that is asso-
ciated with the subsequent conversion of reaction products such
as acetone, as has been illustrated by Kresnawahjuesa et al. [35].
As early as 1983, Chang et al. [32] offered the suggestion that the
ketonization reaction over zeolites may occur via a nucleophilic
attack of an acylium ion on an acetate. Due to the rapid deactiva-
tion observed over zeolites, they proposed that these acetate spe-
cies were formed from the decomposition of acylium species by
dehydroxylation of the zeolite, leading to an inherent irreversible
deactivation of the catalyst. Martens et al. [31] proposed a similar
mechanism involving a surface acyl intermediate similar to that in
Chang et al. Rather than forming an activated acetate via dehy-
droxylation of the zeolite, they speculated that a second acid could
interact directly with the basic site on the zeolite generated via the
formation of a surface acyl group to dissociate the O–H bond on the
carboxylic acid, regenerate the Brønsted site, and form a carboxy-
late anion. The location of the surface acyl species during this pro-
cess is not specified. They proposed that this carboxylate anion
subsequently interacts with the positively charged acyl cation to
form the ketone.
2
. Experimental
2.1. Catalyst preparation
2 2 3
ZSM-5 (CBV-8014) with SiO /Al O purchased from Zeolyst
International in the ammonium form was calcined at 600 °C for
5
h in dry air to obtain the protonated HZSM-5 by thermal decom-
+
+
4 3
position of NH to NH and H . Silicalite-1 was hydrothermally
synthesized from the gel composition of tetrapropylammonium
hydroxide (TPAOH):SiO :H O:EtOH = 9:25:477:100 by following
2
2
the procedure outlined in Ref. [46]. Reagents used for the synthesis
include tetraethyl orthosilicate (TEOS, Sigma Aldrich, reagent
grade, 98%), tetrapropylammonium hydroxide (TPAOH, Alfa Aesar,
4
0% w/w aq. soln.), and deionized water. The gel was stirred at
ambient temperature for 24 h and then hydrothermally treated
for 24 h at 80 °C in an autoclave. The synthesized sample was
washed and filtered by centrifugation, followed by drying over-
night (12 h) at 80 °C. The dried sample was then calcined at
6
00 °C for 5 h to remove the template. Both catalysts were pel-
letized after calcination and subsequently crushed and sieved to
particles with sizes ranging from 90 to 250 m.
l
+
Na exchange was conducted on the samples to titrate a portion
of the Brønsted sites and test for diffusion limitations. A fraction of
the Brønsted acid sites were poisoned with sodium dosing as
reported in the literature [47]. Three different samples (referred
to as samples 1, 2, and 3) were prepared by varying the amount
Other types of reactive intermediates have been proposed for
reactions involving acyl species and acylium ions. Of particular
relevance to this reaction are ketenes. Surface acyl species are
in equilibrium with acylium ions, as well as gas phase ketenes
in low abundance. Ketenes are often proposed as important reac-
tive intermediates for reactions involving carboxylic acids and
acyl groups [29]. Specifically, for the ketonization reaction, it
of sodium loading using 0.5 M NaNO
source. Double-distilled water and commercial NaNO
Aldrich, ReagentPlus, P99.0%) were used to prepare 0.5 M NaNO
3
solution as the sodium
3
(Sigma
3
solution. All samples were characterized by IPA-TPD to quantify
the resulting Brønsted acid site density.
2
has been shown that over oxides such as TiO , gas phase ketenes
are parallel side products that do not lead to ketone formation,
as discussed in depth in a recent review of the topic [21]. Mar-
tinez et al. argued against a sequential ketene-based mechanism,
using a monolith reactor while varying the contact time [11].
Similarly, Pestman et al. [19] used 13C-labeled acids to perform
cross ketonization reactions between a carboxylic acid capable
of forming a ketene and another carboxylic acid lacking an alpha
hydrogen and thus incapable of forming a gas phase ketene.
They showed by labeling the carbonyl carbons of carboxylic
acids that gas phase ketenes are not responsible for the
2.2. Catalyst characterization
The crystallinity of the samples was investigated by X-Ray
diffraction using a Rigaku automatic diffractor (Model D-MAX A)
with a curved crystal monochromator. It had Cu K-alpha as the
radiation source and was operated at 40 kV and 35 mA in the angle
range 5–55°. For analysis, a well-ground catalyst sample was
placed on a plastic slide and spread evenly to obtain a flat surface.
Scanning electron microscopy (SEM) measurements of the
samples were performed by a Zeiss-NEON FEG-SEM instrument
to estimate the particle diameter. To prepare SEM samples, a small
amount of the zeolite aqueous suspension was placed on carbon
tape and dried for 1 h.
2
ketonization reaction over TiO .
Surface-bound acyl species and their corresponding acylium
ions have long been proposed as intermediates for similar C–C cou-
pling reactions as well, such as acylation reactions with aromatics
over zeolites [36–41]. Corma et al. [42] proposed that surface acyl
species and acylium ions are important intermediates involved in
aromatic acylation reactions. This claim was later challenged by
Bonati et al. [43,44], who, upon observing ketenes at trace levels,
argued that ketenes were responsible for these reactions. Deu-
terium labeling studies by the same group later showed that while
ketenes are produced in parallel to acyl groups and acylium ions,
they are not intermediates in the acylation reaction [45], but rather
side products that contribute to catalyst deactivation via coke
formation.
Isopropylamine (IPA) temperature-programmed desorption
(TPD) experiments were conducted on the zeolites used in this
study to investigate the concentration of Brønsted acid sites. A por-
00
tion of 50 mg of catalyst was taken in a quartz reactor (1/4 OD)
and flushed at 300 °C for 1 h with helium as a carrier gas at a flow
rate of 20 ml/min. After pretreatment, the temperature was
reduced to 100 °C and 2 lL pulses of IPA were injected into the
reactor through a septum using a syringe. To ensure saturation of
all the acid sites in the catalyst bed with IPA, the mass-to-charge

7
8
A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
ratios (m/z) of 44 and 58 were tracked at the exit of the reactor
with an MKS Cirrus 200 quadrupole mass spectrometer (MS). IPA
pulses were continued until constant peak heights corresponding
to m/z = 44 and 58 were observed as sequential pulses were
injected. After adsorption of IPA onto the catalyst bed, helium car-
rier gas was flushed over the bed at a rate of 20 ml/min at 100 °C
for 4 h to remove physically adsorbed IPA, after which the temper-
ature was ramped from 100 to 600 °C at a rate of 10 °C/min. The
products desorbing from the reactor during the temperature ramp
were tracked via MS. Products were quantified through calibration
of the MS signal using standards and a sample loop.
dried after flushing with dry helium by tracking water (m/z = 18)
at the exit of the reactor via MS. After pretreatment, the tempera-
ture was reduced to 100 °C and glacial acetic acid was pulsed using
a syringe in 2 lL increments. The catalyst bed was saturated with
acetic acid, as confirmed by analyzing exit gases from the reactor
using MS. m/z = 60 was tracked for acetic acid to ensure a constant
signal per pulse in the exit stream of the reactor. After adsorption
of acetic acid, the catalyst bed was flushed at 100 °C with dry
helium at a flow rate of 50 ml/min for 4 h, after which the temper-
ature was ramped from 100 to 600 °C at 10 °C/minute. The prod-
ucts desorbing from the reactor during the temperature ramp
were tracked by MS and were quantified by injecting standards.
2.3. Continuous-flow reactions
2
.5. IR experiments
Flow reaction studies were performed in a quartz tube reactor
1/4 OD) at atmospheric pressure and temperatures varying from
50 to 340 °C. The catalyst sample was packed into the reactor
0
0
(
2
Infrared spectroscopic study of acetic acid over CBV8014 was
conducted in a PerkinElmer Spectrum 100 FT-IR Spectrometer
equipped with a Harrick Praying Mantis chamber. After CBV8014
was pretreated at 300 °C in helium flow of 50 ml/min for 1 h, the
temperature was reduced to 50 °C and a blank spectrum of the zeo-
lite surface was taken as a reference. Acetic acid was then adsorbed
onto the surface for 30 min at 50 °C through a sample bubbler
maintained at ꢀ7 °C while helium was flowed at a rate of 50 ml/
min. This was followed by flushing for 2 h at 50 °C under a flow
of 50 ml/min to remove physically adsorbed acetic acid. After
flushing, the spectrum of the CBV8014 surface adsorbed with
acetic acid was collected at 50 °C and then the temperature was
raised at a steady rate of 10 °C/min. Analysis at each desired tem-
perature was conducted by stopping the ramp, holding at the
desired temperature, and performing 64 scans to obtain the spec-
trum prior to continuing the temperature ramp.
between quartz wool and the reactor was filled with acid-
washed glass beads with a diameter of 1 mm from the inlet of
the reactor to the catalyst bed. The inlet of the reactor was heated
to create a vaporization zone and the outlet stream of the reactor,
which was connected to a six-port valve for sampling, was heated
to 250 °C to prevent condensation. The temperature of the catalyst
bed was controlled by a thermocouple attached to the outer wall of
the reactor. The catalyst was preheated and flushed with helium
flowing at 50 ml/min for 1 h at 300 °C before acetic acid (Sigma
Aldrich, ACS reagent, P99.7%) was introduced via a syringe pump.
The results and product distribution at the desired time on stream
(
TOS) were analyzed using a Hewlett Packard 6890 gas chro-
matograph equipped with a flame ionization detector and an Inno-
wax column of dimensions 30 m and 0.25 m. The vapors
l
downstream of the six-port valve were condensed in a sample bub-
bler using ice and water as a coolant medium.
External mass transfer limitations in the flow reactions were
tested at 300 °C by varying the helium carrier gas flow rate from
3. Results and discussion
5
0 to 125 ml/min. It is important to mention that the W/F (weight
3.1. Reaction mechanism and evidence for stable acyl species
of catalyst in g/feed flow rate in g/h) and the partial pressure of
acetic acid were held constant during this study. When measured
rates are controlled by external mass transport limitations, the rate
increases linearly with increasing (carrier gas flow/particle diame-
ter (dp)) [48]. This test was used to ensure that data used to
obtain kinetic parameters were not corrupted by diffusion
limitations.
The kinetic isotope effect on ketonization of acetic acid over
HZSM-5 was studied in this flow reactor at a reaction temperature
of 300 °C under atmospheric pressure. Two different isotopically
labeled acetic acid probe molecules were tested, acetic acid-d
A simple analysis of the Brønsted acid sites post-reaction pro-
vides insight into the nature of activation of the carboxylic acids
before the ketonization reaction. Fig. 1 shows that the Brønsted
sites are clearly recovered after desorption of acetic acid and pro-
duce ketones at 300 °C, demonstrating that irreversible dehydrox-
ylation of the zeolite is not a prerequisite for activation of the acids
for this reaction. The peak at 3605 cm corresponding to Brønsted
acid sites disappears after acetic acid is adsorbed. A subsequent
temperature ramp to 300 °C is sufficient to desorb the ketones that
were produced. The reappearance of the Brønsted sites indicates
the absence of zeolite dehydroxylation to form an acetate as a nec-
essary step for the ketonization reaction. A similar reappearance of
IR bands corresponding to Brønsted sites was reported by Martens
0
.5
ꢀ
1
(
4
Aldrich, 99 at.% D) and acetic acid-d (Aldrich, 99.5 at.% D). As with
the previous runs, the catalyst bed was preheated and flushed with
helium at 300 °C for 1 h before the feeding of the reactant. Acetic
acid was fed to the reactor through a syringe pump with a rate
of 0.0000728 mol/min at a reactant partial pressure of 10.67 Torr.
A catalyst weight of 0.025 g was used for these experiments. Prod-
ucts at the desired time on stream (TOS) were analyzed using the
Hewlett Packard 6890 gas chromatograph described above.
2.4. Acetic acid TPRx experiments
The temperature-programmed reaction of acetic acid over
Brønsted acid sites of CBV8014 was studied in a reactor setup sim-
ilar to that used for IPA TPD, explained in Section 2.2. A portion of
00
5
0 mg of catalyst was packed into a quartz reactor (1/4 OD) and
flushed at 300 °C for 1 h with helium at a flow rate of 50 ml/min.
It is important to mention that a moisture trap was installed in
the helium line to avoid the introduction of moisture with the
gas stream. It was ensured that the catalyst bed was completely
Fig. 1. Acetic acid IR on CBV8014 shows the regeneration of Brønsted acid sites.

A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
79
et al. upon exposure of HT zeolite to butyric acid followed by evac-
uation at 300 °C [31].
A temperature-programmed desorption (TPD) experiment with
acetic acid over CBV8014 was conducted to probe the reaction
2
mechanism. Water (m/z = 18), CO (m/z = 44), acetone (m/z = 58),
and m/z = 43 (a fragment of both acetic acid and acetone) corre-
spond to the most abundant species produced in this experiment.
This profile, shown in Fig. 2, clearly indicates that an initial dehy-
dration step occurs on the reaction pathway. This would likely
result in the formation of an acyl intermediate, as had been specu-
lated in previous studies [30–32]. It is important to note that CO
2
has a lower heat of adsorption on HZSM-5 as compared with that
of water [49], so readsorption or bed effects could not explain
the evolution of water prior to CO
produced during the dehydration step amounts to 16.75
which corresponds to the dehydration of an acid on >82% of the
0.3 moles of Brønsted acid sites present in the catalyst bed.
FTIR analysis of the catalyst after exposure to acetic acid and
2
. Quantification of the water
l
moles,
Fig. 3. Acetic acid IR on CBV8014 after adsorption of acetic acid at 50 °C and
subsequent heating to 150 °C. The IR spectra are offset for clarity.
2
l
subsequent heating provides better understanding of the surface
species formed during the course of the reaction. The IR spectrum
of the catalyst surface after pretreatment is shown in Fig. 3. After
acetic acid is introduced to the catalyst at 50 °C via a saturated
ꢀ
1
vapor stream, a significant peak is observed at ꢁ1720 cm , as
indicated by the solid vertical line. This peak is associated with
surface-adsorbed acetic acid [50–52]. After the temperature is
ꢀ1
ramped to 150 °C at a steady rate, the peak at ꢁ1720 cm shifts
to lower wavenumber along with another peak appearing at
ꢀ1
1
755 cm , as indicated by the two dashed lines, which have been
attributed to acetyl species [53]. It is important to note that the
Brønsted acid sites are not recovered in this temperature range,
as indicated by the lack of reappearance of the band at
ꢀ
1
3
605 cm after heating to 150 °C, as can be seen in Fig. S3 in
the Supplemental Information. This indicates that the acyl species
remain bound to the zeolites under these conditions.
The IR results presented in Fig. 3 support the presence of a
stable acyl intermediate, and the TPD results in Fig. 2 suggest that
the dehydration step is not the rate-determining step for the over-
all coupling reaction. A reaction mechanism can be proposed as
shown in Fig. 4. The coupling step on the left may be a combination
of several steps in series. Dashed bonds are not meant to represent
a proposed transition state, but simply to illustrate the bonds
involved in a possible route to the formation of a b-keto acid,
which can subsequently decompose to form acetone. A rate-
determining step involving a surface acyl group and another acid
will yield a second-order kinetic dependence with respect to acetic
acid, as will be discussed in Section 3.4. This is true if the rate-
determining step involves (a) the tautomerization or activation of
Fig. 4. Acetic acid TPD on CBV8014 showing an initial dehydration step involved in
the reaction pathway followed the decomposition of the intermediate to acetone
and CO .
2
the second acid or (b) the formation of a C–C bond with the acyl.
Further insight may be obtained through kinetic fitting to deter-
mine the reaction order and deduce a rate-determining step.
3.2. Identification of a selective ketonization regime under continuous
flow
Subsequent aldol condensation and aromatization of acetone
often complicates the kinetic analysis of the ketonization reaction.
The conversion of acetic acid in a flow reactor over CBV8014 at
Water (m/e=18)
Acetone (m/e=43)
CO₂ (m/e=44)
300 °C yields nearly 100% selectivity to acetone and CO
2
as shown
in Fig. 5. After the CO generated during the ketonization reaction
2
for each mole of acetone produced is accounted for [16–25,30–34],
over a 95% carbon balance was observed for each reaction unless
otherwise noted. The fact that most studies report a variety of sub-
sequent aldol condensation products can clearly be mitigated by
operating at milder temperatures under the conditions used in this
study.
The ketonization reaction over zeolites is surprisingly selective
over a broad range of temperatures. Fig. 6 shows the flow reaction
of acetic acid on CBV8014 at various temperatures, with W/F being
constant for all of the runs. These experiments were performed at
temperatures ranging from 270 to 340 °C and, as expected, the
yield of acetone increased with increasing temperature. High car-
bon balances were observed only up to a temperature of 330 °C,
1
00
200
300
400
500
600
Temperature (°C)
Fig. 2. Acetic acid TPD on CBV8014, showing an initial dehydration step involved in
the reaction pathway followed by the decomposition of the intermediate to acetone
2
and CO .

8
0
A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
beyond which a significant amount of carbon was lost due to the
formation of heavier species that do not desorb from the catalyst.
Vervecken et al. [30] and later Martens et al. [30,31] reported a
similar effect, with higher yields to aldol condensation products
at elevated temperatures. Though no condensation products were
observed in our study, the drop in carbon balance at 330 °C indi-
cates their formation. Higher temperatures may be required to des-
orb these species from the active site so that they leave the catalyst
particle at a rate higher than subsequent condensation reactions
that ultimately lead to coke formation. The lack of aldol condensa-
tion products observed at lower temperatures could be due to a
combination of the reduced rate for this reaction and a lower pop-
ulation of vacant sites for acetone readsorption at lower reaction
temperatures.
Figs. 5 and 6 show that stable conversion may be achieved under
more moderate conditions, with deactivation resulting from consec-
utive reactions, as well as potentially through parallel reactions
involving ketene species. This loss of activity as a function of time
on stream (TOS) can be recovered by a simple calcination. Fig. S4
in the Supplemental Information highlights the reaction rate as a
function of TOS across both the fresh and regenerated catalyst. After
the catalyst has been exposed to acetone at 300 °C for 105 min, the
carbonaceous deposits are removed by temperature-programmed
oxidation. The catalyst is then placed back in the reactor and the ini-
tial catalyst activity is recovered. This provides further support for
the result that irreversible dehydroxylation of the zeolite does not
occur significantly under these conditions.
Fig. 6. Flow reaction studies of acetic acid on CBV8014 showing yields of acetone
and carbon balance as a function of temperature. The acetic acid partial pressure
was maintained at 10.67 Torr. Rates at each temperature were taken at 30 min TOS.
3.3. Effect of water on catalyst deactivation rates
Most streams that contain acetic acid are also accompanied by
large amounts of water. Bio-oil produced from the pyrolysis of
lignocellulosic biomass contains approximately 60% water by
weight; thus understanding the role of water in the ketonization
reaction is critical for any practical application utilizing these
renewable streams. The mechanism outlined in Fig. 4 implies that
water may inhibit the formation of acylium species. However, a
second-order step may be rate-determining, as will be discussed
in more detail below. The role of water in this reaction can be two-
fold, interacting with surface acyl species or desorbed ketenes to
reform acids and by competing for adsorption sites.
Fig. 7 shows a flow reaction conducted at 300 °C during which
water co-fed at a 1:1 molar ratio with acetic acid is introduced
after a TOS of 100 min. A sudden drop in conversion is observed
upon the introduction of water. When water co-feeding is stopped,
the activity is recovered to a level equal to that observed prior to
Fig. 7. Acetic acid flow reactions at 300 and 320 °C, showing a sudden drop in
conversion when water is introduced. Catalyst weights 0.025 and 0.014 g were used
for reactions at 300 and 320 °C, respectively, to match the initial rates, with an
ꢀ
5
acetic acid flow of 7.28 ꢂ 10 mol/min at a partial pressure acetic acid of 10.8 Torr.
Water was introduced at a 1:1 molar ratio.
water introduction into the system, showing no measureable irre-
versible effect of water on the catalyst. The drop in the activity
with water co-feeding could be due to either (i) the reversible reac-
tion of acylium formation, i.e., the surface acyl interacting with
water to form acetic acid, or (ii) competition between water and
acetic acid for the active sites on the catalyst. The same experiment
is repeated at a higher temperature of 320 °C with a reduced W/F
to maintain the same conversion levels as those observed at
3
00 °C. When water is added at the higher temperature, the impact
of water on the rate of ketonization is less pronounced.
Any influence of competitive adsorption for active sites will
become less pronounced at higher temperatures; thus the role of
water in competition for active sites cannot be ruled out. Similarly,
if the dehydration step to form an surface acyl species is endother-
mic, as was shown to be the case by Boronat et al. [54] studying the
Koch reaction over H-MOR zeolites, higher temperatures will favor
a greater equilibrium concentration of acyl groups on the surface. A
more detailed analysis of water partial pressure’s effect on the
reaction rate is presented in the next section.
The rate of acetone formation after the catalyst has been
exposed to water co-feeding, shown in Fig. 7, indicates that water
inhibits deactivation of the catalyst. While water decreases the rate
of the reaction, the stability of the catalyst may actually be
improved. The positive influence of water on inhibition of the
deactivation rate is more pronounced when the reaction order
with respect to water is evaluated as a function of time on stream.
Fig. 5. Flow reaction studies of acetic acid on CBV8014 at 300 °C showing very high
selectivity to acetone and CO
2
. The flow rate of acetic acid is maintained at
0
.0000728 mol/min with a catalyst weight of 25 mg.

A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
81
Fig. 8 shows that the reaction order increases when measured at
later TOS values. This indicates that, in extrapolation to 0 TOS,
the ꢀ0.46 order with respect to water is a measure of the true
impact that water has on the reaction rate. At later TOS, as the cat-
alyst begins to deactivate, the overall order is shifted by the influ-
ence that water has on the catalyst decay rate, with the apparent
order approaching ꢀ0.2. This can be observed more directly by
evaluating the exponential dependence of the rate as a function
of time. By considering the exponential dependence of rate in
d
terms of a deactivation coefficient, k , it can clearly be observed
that water inclusion positively impacts the catalyst stability. The
rates fit the decay curves; the deactivation coefficients as a func-
tion of water partial pressure can be found in the Supplemental
Information in Fig. S8. This positive influence of water on catalyst
stability has very promising practical implications for real renew-
able streams, which are often accompanied by significant amounts
of water.
Fig. 9. Dependence of acetic acid reaction rate over carrier gas flow rate at constant
W/F = 0.095 h and reactant concentration over CBV8014 at 310 °C and 30 min TOS.
3.4. Reaction kinetics and identification of the rate-determining step
Tests were conducted to ensure that the kinetic data are not
limited by mass transfer. The reaction rate at 310 °C as a function
of helium carrier gas flow rate is shown in Fig. 9. At low flow rates,
the rate increases with increasing carrier gas flow, indicating that
the rate is controlled by external mass transport to the catalyst sur-
face. This effect disappears above 100 ml/min of gas flow, suggest-
ing that the reaction is devoid of external diffusion corruptions
above this flow rate. All kinetic experiments were therefore con-
ducted with a carrier gas flow rate of 125 ml/min. This includes
the experiments reporting reaction orders vs. TOS as described in
the previous section.
The necessity of Brønsted acid sites for this reaction to proceed
was confirmed by passing acetic acid over silicalite-1 at 300 °C,
with no measurable conversion observed over silicalite-1, as
+
shown in Fig. S5. This was further confirmed by Na exchange of
Fig. 10. Rate of acetic acid conversion as a function of the density of Brønsted acid
sites at a constant W/F = 0.095 h over CBV8014 at 300 °C and 30 min TOS. The
concentration of Brønsted acid sites was varied via Na exchange of CBV8014.
the HZSM-5 (samples 1, 2, 3 in Table S1 in the Supplemental Infor-
mation). The activity dropped in proportion to the concentration of
+
+
Brønsted sites passivated by Na , as shown in Fig. 10, with a con-
ꢀ1
stant TOF of 1.03 ± 0.09 min per Brønsted site over all of the cat-
alysts tested. This confirms that the measured rates are not limited
by internal or external mass transfer under these conditions. This
result also demonstrates that any Lewis acidity introduced by the
incorporation of Na cations does not accelerate the rate-
determining step.
By varying the concentration of acetic acid fed, we observe that
the order of the ketonization reaction is greater than one. Rates
were found by extrapolating the conversion to initial TOS values
Fig. 11. Natural logarithm of the rate of reaction vs. acetic acid concentration over
CBV8014 at 300 °C, 0.025 g catalyst, with rates extrapolated to 0 min TOS.
Conditions are identical to those in Fig. 8.
to eliminate effects of catalyst deactivation, as described in the
previous section. The fact that the reaction appears to be greater
than first order, as shown in Fig. 11 implies a rate-determining step
involving two acid-derived species. The reason for an observed
order less than two with respect to acetic acid could be due to
the coverage of acetic acid and acid-derived surface species. This
behavior is very similar to that observed when the apparent
Fig. 8. Order of reaction rate with respect to water as measured at varying TOS
values. Conditions are vs. water concentration over CBV8014 at 300 °C, 0.025 g
catalyst, with initial rates extrapolated to 0 min TOS. Error bars represent ±1
standard error value of the reaction order coefficient.

8
2
A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
reaction order is measured for coupling of acids over reducible oxi-
des, where the second-order coupling exhibits a fractional appar-
ent order [21,22].
Combining the observance of water evolution in Fig. 2 with the
observation of acyl surface species at 150 °C in Fig. 3 implies that
the dehydration of an acetic acid molecule to form a surface acy-
lium species is not the rate-determining step. The further obser-
Further insight may be gained by investigating the dependence
of water on the rate expression. By varying the water partial
pressure and extrapolating data to initial TOS values, the rate of
acetone production exhibits a ꢀ0.46 order with respect to water
as shown in Fig. 12. Based on rate Eq. (2), competitive adsorption
of water alone would not be able to account for the ꢀ0.46 order
observed. Further simplification of expression (2), where the sur-
face is completely saturated with adsorbed water species, would
yield
vance of CO
50 °C in Fig. 2 indicates that an acylium ion and an activated car-
boxylic acid can couple to form an intermediate and subsequently
decompose to yield CO and acetone. The kinetic order greater than
2
(m/z = 44) and acetone (m/z = 43) evolution at
2
2
2
kKCom
K
equilibriumK
Acid Acid
P
rate ¼
ꢀ
ꢁ
:
ð3Þ
one further supports a second-order rate-determining step. This
second-order dependence, which is manifested as an apparent
2
K
H2
O
P
H
2
O
1
.6-order rate dependence on acetic acid partial pressure, can be
Rate Eq. (3) does not agree with the observed experimental rate.
Lower coverage of water, without considering the possibility of
surface acyl species, would yield an order with respect to water
that varies from ꢀ1 to ꢀ2, as discussed in the Supplemental
Information. Similarly, if the surface were completely saturated
with surface acyl and acid/acyl species, the order with respect to
acetic acid would range from 0 to 1. An order of 1.6 with respect
to acetic acid indicates that sites are not all occupied by acyl
groups or acyl/acid complexes, but clearly a significant fraction
of the sites must be occupied by these species to justify an order
with respect to water that is between 0 and ꢀ1.
explained by examination of a rate law derived assuming that an
adsorbed acid and an adsorbed acyl interact during the rate-
determining step to form a b-keto acid that rapidly decomposes
2
to yield acetone and CO . The left hand side of Eq. (1) represents
an acetic acid molecule in the proximity of an acyl group with
the right hand side representing a b-keto acid. The nature of the
transition state of the rate determining step could involve either
the activation of the second acetic acid or C–C bond formation as
discussed in section 3.1. We will investigate the kinetic relevance
of these two possibilities later in this section.
Ison and Gorte [49] measured a heat of adsorption of water over
HZSM-5 as 51 kJ/mol at low coverage, while Kresnawahjuesa et al.
of the same group [55] later reported the heat of adsorption of
acetic acid over HZSM-5 as 139 kJ/mol. This indicates that while
water could potentially compete for adsorption sites at high
enough concentrations, excluding entropic contributions, water is
þ
k
þ
RCOOH=RCO ! ½RCORCOOHꢃ
ð1Þ
An expression highlighting the second-order dependence of this
rate-determining step is derived in the Supplemental Information,
resulting in equation (2) below:
2
kKCom
equilibrium Acid
K K P
Acid
2
rate ¼
ð2Þ
K
equilibrium
K
Acid
P
Acid
K
Com
K
equilibrium
H2
K
Acid
P
Acid
P
H2
O
ð1 þ KAcid
P
Acid
þ
þ
þ KKetone
P
Ketone þ KH2
O
P
H2
O
þ KCO2
P
CO2
Þ
P
H2
O
P
O
Several scenarios depicting variants of most abundant surface inter-
mediates (MASI) are highlighted in the Supplemental Information.
One can clearly see by inspection of (2) that a fractional order of
not expected to occupy the majority of Brønsted sites under these
conditions. While we cannot exclude the possibility that a signifi-
cant portion of the negative order attributed to water in the reac-
tion rate is due to competition for active sites, it is a reasonable
assumption that a greater proportion of Brønsted sites are occu-
pied by acyl species than by water under these conditions, based
on the proposed rate expression. While we have eliminated the
influence of water on catalyst deactivation rates with this analysis,
the possibility of water interacting with transition states and facil-
itating proton transfer has not been investigated. If, for example,
the tautomerization of the carboxylic acid in proximity to an acyl
1
.6 with respect to acetic acid can be attributed to contributions
to the rate equation by a combination of surface acetic acid, acetic
acid/acyl complex, and surface acyl groups (corresponding to the
three terms in the denominator that increase with the partial pres-
sure of acetic acid).
Fig. 12. Natural logarithm of the rate of reaction vs. water concentration over
CBV8014 at 300 °C, 0.025 g catalyst, with initial rates extrapolated to 0 min TOS.
Acetic acid partial pressure is maintained at 10.8 Torr.
Fig. 13. TOF values for acetic acid and its deuterated counterparts in the OD
position (D1) and fully deuterated (D4). Reaction conditions are described in
Section 2.3.

A. Gumidyala et al. / Journal of Catalysis 340 (2016) 76–84
83
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Financial support from the American Chemical Society (Grant
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