Enhancing the Acylation Activity of Acetic Acid by Formation of an Intermediate Aromatic Ester

Article abstract of DOI:10.1002/cssc.201700394

Acylation is an effective C?C bond-forming reaction to condense acetic acid and lignin-derived aromatic compounds into acetophenones, valuable precursors to fuels and chemicals. However, acetic acid is intrinsically an ineffective acylating agent. Here, we report that its acylation activity can be greatly enhanced by forming intermediate aromatic esters directly derived from acetic acid and phenolic compounds. Additionally, the acylation reaction was studied in the liquid phase over acid zeolites and was found to happen in two steps: 1) formation of an acylium ion and 2) C?C bond formation between the acylium ion and the aromatic substrate. Each of these steps may be rate-limiting, depending on the type of acylating agent and the aromatic substrate. Oxygen-containing substituents, such as ?OH and ?OCH₃, can activate aromatic substrates for step 2, with ?OH> ?OCH₃, whereas alkyl substituent ?R cannot. At the same time, aromatic esters can rearrange to acetophenones by both an intramolecular pathway and, preferentially, an intermolecular one.

Full text of DOI:10.1002/cssc.201700394

Accepted Article
Title: Enhancing the Acylation Activity of Acetic Acid by Forming an
Intermediate Aromatic Ester
Authors: Nhung Duong, Bin Wang, Tawan Sooknoi, Steven P.
Crossley, and Daniel E. Resasco
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To be cited as: ChemSusChem 10.1002/cssc.201700394
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10.1002/cssc.201700394
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Enhancing the Acylation Activity of Acetic Acid by Forming an
Intermediate Aromatic Ester
Nhung N. Duong, Bin Wang, Tawan Sooknoi, Steven P. Crossleyand Daniel E. Resasco [a]
Abstract: Acylation is an effective C-C bond forming reaction to
condense acetic acid and lignin-derived aromatic compounds into
acetophenones, valuable precursors to fuels and chemicals.
However, acetic acid is intrinsically an ineffective acylating agent.
Here, we report that its acylation activity can be greatly enhanced by
forming intermediate aromatic esters, directly derived from acetic
acid and phenolics. Additionally, the acylation reaction was studied
in the liquid phase over acid zeolites and was found to happen in two
steps, (1) formation of an acylium ion and (2) C-C bond formation
between acylium ion and the aromatic substrate. Each one of these
steps can be rate-limiting, depending on the type of acylating agent
and aromatic substrate. The O-containing substituents such as -OH
and -OCH3 can activate aromatic substrates for step (2), with –OH >
-OCH3, while alkyl substituent –R cannot. At the same time, the
aromatic esters can rearrange to acetophenones via both an
intramolecular pathway and, preferentially, an intermolecular one.
hydrogenation of acetic acid to ethanol followed by alkylation of
ethanol with phenolic compounds. However, the hydrogenation
of acetic acid requires high H2 consumption and ethanol is an
ineffective alkylating agent. Alternatively, acetic acid could be
used directly as an acylating agent for phenolic compounds
producing aromatic ketones that could be directly hydrotreated
or further upgraded to longer chain molecules via aldol
condensation. Since phenolic compounds derived from lignin
also account for a large fraction of bio-oil, this approach appears
promising and might result in very high carbon capture in the
fuel-range liquid.
Friedel-Crafts acylation of aromatics is a well-known and
extensively investigated reaction in organic chemistry that
produces aromatic ketones from an acylating agent and an
aromatic substrate. Different acylating agents such as carboxylic
acids, acyl halides or acyl anhydrides have been investigated,
with acid catalysts including AlCl3, HF, zeolites or
heteropolyacids.[6] It has been recently shown that over H-ZSM5
at 220 C the vapor phase acylation proceeds via dehydration of
acetic acid to form an acyl intermediate, in which acetic acid
may well act as an acylating agent.[7] These intermediates can
desorb as ketenes at elevated temperatures, resulting in catalyst
deactivation. However, by incorporating an effective substrate
such as methylfuran, selective direct acylation may result.[8] The
selective acylation with highly active acyl acceptors have been
proposed as intermediate compounds for the production of
renewable specialty surfactants.[9]
In the condensed liquid phase, especially in the complex
liquid environment of bio-oil, the phenomena involved in
acylation reactions may be more complicated due to solvent
effects, mass transfer limitations, and catalyst deactivation by
heavy products.[10] In the liquid phase, the acyl halides and
anhydrides have shown to be effective acylating agents, in
which the acylation reaction proceeds via formation of an
acylium ion intermediate.[11] Also, it has been reported that
Brønsted acid zeolites are more effective than Lewis acid
zeolites for the acylation of furan by lauric anhydride.[9] However,
with acetic acid as the acylating agent, the reaction mechanism
in liquid phase still remains elusive. For example, it is unclear
whether the reactive intermediate is an acylium ion, as in the
vapor phase reaction[12] or it involves a Lewis-coordinated
carboxylic acid species.[11a] The nature of the rate-limiting step in
the liquid phase is equally debatable. Some studies[10c] have
proposed that the C-C bond formation is rate-limiting, while
others have found significant overall rate differences when
varying the acylating agent, suggesting that formation of the
reactive intermediate is the rate-limiting step.[13] To shed more
light into this important reaction, the present contribution
investigates the nature of the most crucial intermediates and
attempts to determine the rate controlling steps.
Introduction
Liquid phase upgrading of pyrolysis products is an
important process in the conversion of lignocellulosic biomass to
fuels.[1] The typical bio-oil obtained by condensation of biomass
pyrolysis vapors is an unstable liquid, which cannot be easily
reheated for vapor phase upgrading or fractionation since this
reheating would result in polymerization and significant carbon
losses. The presence of light acids, consisting primarily of acetic
acid, in these streams enhances polymerization reactions and
favors corrosion upon storage. Acetic acid is the most abundant
species in many biomass derived streams, for example, the
liquid product from raw oak after torrefaction at 270 C contains
51% acetic acid on a dry basis.[2] Converting acetic acid to low
value products by conventional hydrotreating would dramatically
reduce the total renewable carbon that can be converted to
gasoline/diesel range transportation fuels. Therefore, to prevent
this loss and stabilize the mixture, different strategies involving
C-C forming reactions have been developed to capture these
reactive light acids into longer and more stable molecules. One
approach that has been widely investigated involves the
ketonization of acetic acid to acetone,[3] followed by aldol
condensation of acetone to form longer chain products[4] or
hydrogenation of acetone to isopropanol followed by alkylation
of isopropanol with phenolic compounds.[5] The major limitation
of these two strategies is that the ketonization step results in a
significant carbon loss due to the formation of one mole of CO2
evolved for every two moles of acetic acid converted. This
results in a maximum theoretical carbon yield of only 75%.
Another alternative that has been considered is the partial
[a]
Nhung N. Duong, Bin Wang, Tawan Sooknoi, Steven P. Crossley
and Daniel E. Resasco
Centre for Interfacial Reaction Engineering (CIRE) and School of
Chemical, Biological and Materials Engineering
University of Oklahoma
100 E. Boyd, Norman OK 73019
E-mail: resasco@ou.edu
Furthermore, since the lignin-derived aromatic compounds
have very diverse structures, which ultimately are combinations
of different substituents on the aromatic ring, the reactivity of
different aromatic substrates has been investigated. It is
Supporting information for this article is given via a link at the end of
the document
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10.1002/cssc.201700394
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expected that the ring substituents should have an important
impact on the electrophilic substitution reaction of the
intermediate species with the aromatic ring. Regardless of the
complexity of the lignin-derived aromatic mixture, in general, the
substituents can be separated into three main groups, including
hydroxyl –OH, alkoxy –OR and alkyl –R substituents on the
aromatic ring. Model compounds with each functional group are
systematically studied here to provide insight into the effects of
said substituents on the activity and selectivity for acylation.
Therefore, by independently changing the nature of the acylating
agent and the aromatic substrate we have attempted to
elucidate the relative importance of the potential rate controlling
step(s), formation of the active intermediate or C-C bond
formation, under different reaction conditions.
It must be noted that when acetic acid is used as a
reactant in the presence of phenol or phenol-derivatives over
zeolite catalysts, it produces aromatic esters. They can undergo
isomerization to a hydroxyaromatic ketone, a reaction known as
Fries rearrangement.[14] A quantitative comparison of the activity
between these esters and acetic acid for the acylation reaction
has not yet been attempted. Furthermore, there are two
possible paths for the Fries rearrangement reaction, one
intramolecular and the other intermolecular. Early work assumed
that ortho-hydroxyaryl ketones are formed via the intramolecular
pathway, while the para isomer is formed via intermolecular
interactions. However, recent studies have shown that the
mechanism may be more involved than this simple
description.[6a] Here, we have investigated the Fries
rearrangement reaction in the liquid phase with both, aromatic
esters and aromatic substrates present in the feed, and have
compared the yields of ortho and para isomers under different
reaction conditions to quantify the contribution of each pathway.
Catalyst Characterization
Table
1:
Conversion
and
yields
to
acylation
(2’-hydroxy-
4’methylacetophenone - HMAP) and esterification (m-tolyl acetate) products
from the reaction of m-cresol and acetic acid over different zeolites. Reaction
conditions: 1g zeolite in 80ml of 1M acetic acid with 8M m-cresol, 250○C, 1hr
Pore Sizea
(nm)
Selectivity of
HMAP
Selectivity of
m-tolyl acetate
Zeolite
H-ZSM5 (Si/Al=25)
HY (Si/Al=30)
0.55-0.56
0.66-0.67
0.74
47%
65%
70%
53%
35%
30%
H-Beta (Si/Al=19)
[a] Systems and processes for catalytic pyrolysis of biomass and
hydrocarbonaceous materials for production of aromatics with optional olefin
recycle, and catalysts having selected particle size for catalytic pyrolysis.
Google Patents, 2011.
In line with previous studies,[15] at similar acetic acid
conversion (50-60%), H-Beta gives somewhat higher selectivity
to the desired acylation product (HMAP) than HY and even
higher than H-ZSM5. The difference in product selectivity might
be ascribed to the pore size of the zeolites. That is, the catalysts
with larger pores (HY and H-Beta) result in higher selectivity
toward the acylation product HMAP, as compared to the one
with smaller pore size (H-ZSM5), which favors the formation of
the ester, an easier reaction that probably can occur on the
external surface, and partly homogeneously in the liquid phase,
as shown below. Due to its higher selectivity to the desirable
product, HMAP, H-Beta was selected as the preferred catalyst to
study the acylation reaction in greater detail.
1.1.2. Role of the aromatic ester as an effective acylation
intermediate
To identify the primary and secondary products, we
followed the product distribution as a function of the extent of
reaction in the batch reactor. However, since the catalyst
deactivation was found to be significant (Figure S1) to
accomplish different extents of reaction the activity runs were
carried out for a fixed 1 hour period over varying amounts of
catalyst, as shown in Figure 1.
The Brønsted acid density of the three zeolites was
determined by temperature program desorption (TPD) of
isopropyl amine (IPA). The other physical characteristics of
these catalysts have been studied by XRD, SEM, and DRIFTS.
These characteristics, along with detailed experimental
procedures have been reported elsewhere. [1b, 5, 7]
Results and Discussion
1.
Nature of acylating agent
1.1. Acylation of m-cresol by acetic acid (AA)
1.1.1. Performance of different zeolites
The acylation of m-cresol by acetic acid was tested over
three different zeolites, H-ZSM5, HY and H-Beta. Table S1
reports the measured BET areas and acid site densities for each
sample. As shown in Table 1, only two measurable products, m-
tolyl acetate and 2’-hydroxy-4’methylacetophenone (HMAP),
were obtained from m-cresol and acetic acid. The former is an
esterification product while the latter is an acylation product.
Trace amounts of 2,2-bis(4-hydroxy-2-methylphenyl)propane
were observed, as previously reported by Rohan et al. and Vogt
et al [10a, 10b, 14a]
.
Figure 1: The concentration of products and acetic acid over different
amount of H-Beta. Reaction conditions: 1M acetic acid with 8M m-cresol,
250 C, 1hr
It can be observed that formation of the m-tolyl acetate
ester was observed even without a catalyst, which indicates that
esterification to m-tolyl acetate is self-activated by the acetic
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acid in the liquid phase. However, as the amount of catalyst
increased from zero to 1 g, larger amounts of m-tolyl acetate
were observed, indicating that esterification is catalyzed by the
zeolite. Interestingly, the concentration of the ester gradually
decreased with the extent of reaction, which shows that the
ester is further converted. The desirable product HMAP is seen
to continuously increase from zero without a catalyst, slowly at
low extent of reaction (small amount of catalyst), and more
rapidly at high extent of reaction (larger amount of catalyst). This
trend suggests that HMAP is a secondary product, presumably
involving the m-tolyl acetate ester, initially formed. To further
support this hypothesis, the acylation activity of acetic acid was
compared to that of m-tolyl acetate over a small amount of
zeolite Beta of 0.1g, as shown in Figure 2. It is clearly shown
that, under the same reaction conditions, the yield of HMAP from
m-tolyl acetate (14 %) is significantly higher than that from acetic
acid (3%).
distribution, indicating that the reaction reached equilibrium. To
investigate the reversibility of the reaction, a mixture of acetic
acid, HMAP, and m-tolyl acetate at the same composition of that
reached after 1 h reaction was prepared (0.38 M acetic acid,
0.45 M HMAP and 0.2 M m-tolyl acetate). The only difference
between this prepared mixture and the reaction mixture is that
no water was present, while water is produced during reaction.
Therefore, when this mixture was exposed to 1 g of fresh H-Beta
catalyst, an additional production of HMAP was observed,
showing the role of water in the reverse reaction. Water can
easily react with acetyl ion to form acetic acid and water would
compete with aromatic substrate (m-cresol) to pick up with
acetyl ion, thus inhibit the formation of HMAP. In fact, as shown
in Figure 3, when the same mixture was prepared with added
excess water, no additional HMAP was obtained either, but only
a slight conversion of the m-tolyl acetate to acetic acid.
1.2. Acylation activity of different esters with m-cresol
After showing that aromatic esters are effective acylating
agents, we tested the reactivity of an alkyl ester (methyl acetate)
to assess the importance of the aromatic group in the
effectiveness of the acylating agent. It is possible that the
presence of the aromatic ring affects the ester dissociation
during the formation of the active intermediate species, affecting
the acylating rate. Indeed, as seen in Figure 2, methyl acetate
is a much less effective acylating agent than the aromatic ester,
resulting in only 2% acylation yield (HMAP), compared to m-tolyl
acetate which results in 14% yield.
Interestingly, as shown in Figure 4, the evolution of the
HMAP yield shows an initial slow increase with increasing extent
of reaction, but the variation is more pronounced at higher extent
of reaction, reaching a relatively high concentration (0.27 M).
This behavior indicates that HMAP is not a primary product but
Figure 2: The yield HMAP with different acylating agents. Reaction
conditions: 1M acylating agent in m-cresol, 250 C 0.1g HBeta 1hr
rather
a
secondary product from m-tolyl acetate,
a
transesterification product of methyl acetate and m-cresol. The
concentration of m-tolyl acetate remains constant with
increasing mass of catalyst, which means that it is consumed
and produced at similar rates. This phenomenon is similar to the
case of acetic acid shown above, in which acetic acid was not
active but it underwent esterification with m-cresol to create an
aromatic ester that has high acylation activity.
Figure 3: Concentration of acetic acid, 2'-hydroxy-4'-methylacetophenone
(HMAP) and m-tolyl acetate before and after equilibrium test. Reaction
conditions: 80ml solution of 0.38 M acetic acid, 0.45 M HMAP and 0.2 M
m-tolyl acetate with 1g H-Beta Si/Al=19, 250 C 1hr.
Kumari et al. has compared the acylation activity of acetic
acid vs. acetic anhydride for phenol acylation reaction in the
vapor phase[16] over zeolite Beta, in which acetic anhydride was
shown to be more active than acetic acid. At temperature of
250○C and similar phenol/acylating agent ratio, the yield of
acylation from acetic anhydride is five times higher than that of
acetic acid, which is about the same order of difference as m-
tolyl acetate vs. acetic acid acylation yield as shown in Figure 2.
Figure 4: The concentration of products and methyl acetate over different
amount of H-Beta. Reaction conditions: 1M methyl acetate with 8M m-
cresol 250 C 1hr.
As mentioned above, until now the nature of the
electrophilic intermediate species was unclear, since both an
acylium ion and a Lewis-coordinated acetic acid have been
1.1.3. Reverse reaction and equilibrium limitations
As shown in Figure 1, when the mass of catalyst was larger
than about 1g, there was no further change in product
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proposed in the literature.[12-13] Here, our results clearly show
that acetic acid by itself is not an active acylating agent, but
rather it requires the formation of an aromatic ester, which rules
out the possibility that the Lewis-coordinated acetic acid is the
The simultaneous conversion of acetic acid and anisole on
H-ZSM5 has been previously reported in the literature with p-
MXAP as the dominant product.[17] However, in the present work,
p-MXAP remains almost insignificant (less than 5% yield) across
the broad ranges of temperature (145-250 C), time (1-18 hrs),
and catalyst amount (H-Beta 1-5g) investigated.
active intermediate. Furthermore, being
a
much bulkier
compound, coordination of an aromatic ester on zeolite Lewis
acid to generate an active electrophilic intermediate would be
less likely due to steric hindrance, which would inhibit the
approach of an aromatic substrate. Therefore, a more plausible
pathway is one in which the aromatic ester generates an acylium
ion, which couples with the aromatic substrate. Since both,
acetic acid and aromatic esters can form acylium ions, the much
higher rate observed with the latter would indicate that they can
form acylium ions much more rapidly than the former. These
results agree with DFT-calculated activation barriers for
formation of acylium ion from acetic acid, m-tolyl acetate and
methyl acetate (see section 4, below).
2.
Nature of aromatic substrate
As described above, the acylation reaction follows a two-
step mechanism (1) formation of acylium ion and (2) C-C
coupling between the acylium ion and the aromatic substrate.
The second step is an electrophilic substitution reaction, in
which the substituents in the aromatic ring may have an
activating or a deactivating effect. For example, substituents
such as –OH, –OR, –R are known to be activating, and o-, p-
directing due to their ability to stabilize the carbocation
intermediate.[18] We have investigated the effect of the –OH, –
OCH3 and –CH3 substituents by using phenol, anisole, and
toluene, respectively, and m-cresol to study the combination of –
OH and –CH3 substituents.
1.3. Acylation of anisole by different acylating agents over
H-Beta
Anisole does not have a free phenolic –OH substituent,
thus it is not able to form directly an aromatic ester with acetic
acid. By contrast, when the substrate is m-cresol, acetic acid
and methyl acetate can form an aromatic ester, which has a
higher acylation activity. The comparison of the acylation rate
between anisole and m-cresol with different acylating agents will
help confirm the role of the aromatic ester. The acylation product
of anisole is p-methoxyacetophenone (p-MXAP).
2.1. Comparison acylation rates of anisole (–OCH3) and
toluene (–CH3)
It is clear that aromatic esters are effective acylating agents.
Therefore, in this section, phenyl acetate was used as the
common acylating agent to compare the activity of different
aromatic substrates, i.e. anisole vs. toluene. The product yields
and aromatic ester consumption are compared in the Table 3.
Under identical reaction conditions, the yield of acylated anisole
Indeed, as shown in Table 2, the acylation rate with m-
cresol was much higher than with anisole when acetic acid and
methyl acetate were the acylating agents (40% and 27% vs. 4%). is about twenty times higher than that of acylated toluene, in
This significant difference in activities is attributed to the ability of
m-cresol to undergo esterification with acetic acid and
transesterification with methyl acetate to form m-tolyl acetate, a
more efficient acylating agent. When m-tolyl acetate was used
as the acylating agent, the acylation yield of anisole improved to
30%, compared with 4% in the case of acetic acid and methyl
acetate, which proves again the high acylation activity of the
aromatic ester.
agreement with previous results by Derouane et al.[10c], who
reported the same trend when using acetic anhydride as the
acylating agent. It is interesting that while the acylation rate is
affected by the type of acylating agent, when the acylating agent
is very effective, the rate is also affected by the nature of the
aromatic substrate.
Table 3: Product yield for the acylation of anisole (p-MXAP: p-
methoxyacetophenone) and toluene (MAPs: o- and p-methylacetophenone)
with phenyl acetate ester as acylating agent, along with HAPs (o- and p-
hydroxy acetophenones) and o-AXAP (o-acetoxyacetophenone) arising from
rearrangement of phenyl acetate. Reaction conditions: 80ml 1M phenyl
acetate in anisole or toluene, 0.1g H-beta 250C 1hr
Table 2: Yield of acetylated products of different aromatic substrates by
different acylating agents, p-MXAP from anisole and HMAP from m-cresol.
While P-methoxyacetophenone (p-MXAP) and its isomer (o-MXAP) were
observed in the acylation of anisole, o-MXAP only appeared in negligible
amounts. Reaction conditions: 1M acylating agent with 8M of aromatic
substrate, 1g HBeta Si/Al=19, 2500C 1hr.
Anisole
19%
-
Toluene
-
p-MXAP (from anisole)
MAPs (from toluene)
Phenolic
substrate
1%
Acylating
agent
HMAP
p-MXAP
HAPs and AXAP
4%
4%
40%
52%
27%
4%
Unbalanced phenyl acetate (%)
Total Yield
0
6%
23%
23%
5%
30%
4%
Conversion of phenyl acetate (%)
11%
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In addition to the acylation, other products have been
observed, including o-HAP, p-HAP and o-AXAP, which can
result from the Fries rearrangement of phenyl acetate. They had
similar total yield in both cases (anisole and toluene), which
suggests that these products do not derive from acylation of
toluene or anisole feed. In the case of anisole, the conversion of
phenyl acetate is equal to the yield of acylated products, i.e. the
total carbon balance is 100%. By contrast, in the case of toluene,
more than half of the phenyl acetate converted was missing from
the measurable products. This low carbon balance suggests that
a significant fraction of the acetyl ions formed from the aromatic
ester cannot be efficiently picked up by toluene and they may be
further converted to ketene (via dehydration) and subsequently
to coke, which lowers the total liquid yield. It worth to mention
that the carbon balance for all of the other results that are
reported in this study is as high as more than 90%.
2.2. Comparison of anisole (–OCH3), phenol (–OH) and
m-cresol (–OH + –CH3) as aromatic substrates with single and
double substituents
Figure 5: Product distribution vs. fraction of phenol in the feed. Reaction
conditions: 1M phenyl acetate in mixture of phenol and anisole, 0.1g H-Beta
with 80ml feed, 1hr 250℃
To compare the activity of phenol, anisole and m-cresol, we
had to use two aromatic esters. One might presume that the
most straightforward way to compare the activity of different
substrates would be using the same aromatic ester under the
same reaction conditions, as we have done in the previous
section. However, the higher complexity of the reactions
involving these three aromatic substrates makes the comparison
more problematic. For example, phenol and m-cresol can
undergo transesterification with any aromatic ester and form
phenyl and m-tolyl aromatic esters, respectively, through their -
OH group. Consequently, depending on the aromatic substrate
and acylating agent, a mixture of two aromatic esters will be
generated in the reaction mixture. For example, if the aromatic
substrate is m-cresol and the acylating agent is phenyl acetate,
m-tolyl acetate will be produced. Likewise, if phenol is the
aromatic substrate and the acylating agent is m-tolyl acetate,
phenyl acetate will be generated. Thus, in both cases, an
uncertainty due to the potentially different activity of the two
aromatic esters would be created in the rate of the first step
(acylium ion forming), which would make the comparison invalid.
In order to avoid this complexity, we have used mixtures of
phenol and anisole with phenyl acetate (which has no methyl
group) as the acylating agent. Likewise, we have used mixtures
of m-cresol and anisole with m-tolyl acetate (which has one
methyl group in the m-position) as the acylating agent. In this
way, the impact of transesterification will be eliminated.
Reactions of pure anisole with the two aromatic esters
reveal that phenyl acetate results in approximately a factor of
two higher yield of acylated anisole than m-tolyl acetate. Then,
to compare the relative activity of the different aromatic
substrates, anisole was used as the reference since it was run
simultaneously with phenol or with m-cresol. In Figure 5, the
yield of acylation products obtained by using phenyl acetate as
acylating agent was plotted versus fraction of phenol in the
aromatic substrates feed. As shown in the scheme below, the
acetylated products from anisole is p-methoxyacetophenone (p-
MXAP) and the ones from phenol are o-hydroxyacetophenone
(o-HAP) and p-hydroxyacetophenone (p-HAP).
Figure 6: Product distribution vs. fraction of m-cresol in the feed. Reaction
conditions: 1M m-tolyl acetate feed with mixture of m-cresol and anisole at
different fraction of m-cresol, 0.1g H-Beta with 80ml solution, 1hr 250℃
and acetyl ion. It has been previously proposed[19] that o-HAP is
a primary product from the intramolecular rearrangement of
phenyl acetate, and that p-HAP is a secondary product formed
via hydrolysis of p-AXAP. However, our results show that the
amount of p-HAP increases proportionally with the phenol in the
feed, while p-AXAP is independent of the phenol concentration.
Therefore, the formation of p-HAP cannot be directly linked to
the hydrolysis of p-AXAP.
In Figure 6, the yield of acylation products obtained by
using m-tolyl acetate as acylating agent was plotted versus
fraction of m-cresol in the aromatic substrates feed. As shown
in the scheme below, the acetylated product from anisole is p-
methoxyacetophenone (p-MXAP) and the one from m-cresol is
2’-hydroxy-4’-methylacetophenone (HMAP).
A minor product of this reaction is p-acetoxyacetophenone
(p-AXAP), which is the coupling product between phenyl acetate
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In Figures 5 and 6, it is seen that the total yield of
acetophenones is approximately constant at 22% and 15%,
respectively, with varying phenol/anisole and m-cresol/anisole
ratios ranging from 0-1. This remarkable result indicates that the
total acylation yield does not depend on the composition of the
aromatic substrate mixture, but only on the nature of the
aromatic ester (acylating agent). The total carbon balance was
always higher than 90% in all cases. Furthermore, as the
fraction of phenol or m-cresol in the feed increases, the
selectivity toward their corresponding acylation products also
increases.
One additional reaction that may complicate the analysis of
the products in this case is the Fries rearrangement, which is the
isomerization of an aromatic ester to a hydroxyaryl ketone. To
compare the acylation products of phenol, anisole and m-cresol,
it is needed to differentiate the acylated products arising from
the direct acylation of the aromatic substrates in the initial feed
from those resulting from the aromatic esters via Fries
rearrangement. Since the methoxy group in anisole is stable
under reaction conditions, and the only acylation product arising
from anisole is p-MXAP, it can be concluded that all the
hydroxyaryl ketones obtained when using only anisole as the
substrate arise from Fries rearrangement. Furthermore, the
yield of Fries rearrangement products only depends on the
nature and initial concentration of the aromatic esters, and not
on the composition of the aromatic substrates feed, therefore,
we can safely subtract them from the product distribution in the
same amounts observed when feeding only anisole. In Figs 5
and 6, the initial concentration of esters is kept constant at 1M,
the concentrations of Fries rearrangement products are 0.02M
o-HAP and 0.012M p-HAP for phenyl acetate, and 0.05M HMAP
for m-tolyl acetate (as observed when the fraction of phenol and
m-cresol is 0). Then, the acylated products of the aromatic feed
are calculated by subtracting these amounts from the total
acetophenone products. While these amounts are smaller than
the acylated products, it is important to conduct this correction to
obtain a precise analysis for the discussion below.
Figure 7: Fraction of acylated anisole (p-MXAP) in the acylated products
from aromatic substrate feed vs. fraction of anisole in the aromatic substrate
feed, based on figure 5 and 6
the selectivity of the aromatic substrate. However, the methyl
group alone is not effective enough to activate the aromatic ring
for efficient C-C coupling activity, as proven in the case of
toluene.
3.
Inter and intra molecular rearrangement of phenyl
acetate
Fries rearrangement of aromatic esters, particularly phenyl
acetate, has been reported and investigated extensively in the
literature[6a, 20]. In the case of phenyl acetate, both o- and p-HAP
are products of this Fries rearrangement reaction. It could
proceed via either inter or the intramolecular mechanism. In the
intramolecular mechanism, the ester rearranges itself into o-
HAP. While in the intermolecular mechanism, the ester
decomposes into phenol and acylium ion, and the phenol
molecule couples with acylium ion to form both o- and p-HAP.
Accordingly, Figure 7 has been constructed from Figs. 5
and 6. In this figure, it shows the fraction of acylated anisole (p-
MXAP) in the corrected acylated products vs. the fraction of
anisole in the feed. The solid line with a slope of 1 would
represent equal acylation selectivity for anisole in comparison
with m-cresol or phenol. Yields observed above this line would
indicate that anisole is more selective than either phenol or m-
cresol, since a small increase in the anisole fraction in the feed
leads to a great increase in the fraction of acylated anisole in the
products. By contrast, the yield observed below that line mean
that anisole is less selective than either phenol or cresol. Figure
7 shows that indeed, this is the case, anisole is less selective
than both m-cresol and phenol. That is, the selectivity of the
aromatic substrates decreases in the order of m-cresol > phenol
> anisole. It shows that the hydroxyl –OH and methoxy –OCH3
substituents can activate the aromatic ring, and the presence of
a methyl group –CH3 at the meta position may further enhance
Figure 8: Ratio of o: p HAP product vs. fraction of phenol in the feed of
phenol and anisole Reaction conditions: 1M phenyl acetate in mixture of
phenol and anisole, 0.1g H-Beta with 80ml feed, 1hr 250℃
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As mentioned above, the amount of HAPs obtained when
phenyl acetate is fed with pure anisole corresponds to Fries
rearrangement products of phenyl acetate. If this Fries
rearrangement reaction had happened via an intermolecular
mechanism exclusively, when phenol is introduced into the
aromatic substrate feed, the ratio between o- and p- HAP would
have been constant. In the intermolecular mechanism, phenyl
acetate decomposes to acetyl ion and phenol. This phenol
should behave analogously to the phenol molecule present as
the aromatic substrate feed, which in turns has the same
preference to pick up an acylium ion at an o- or a p- position,
resulting in a constant ratio of o- to p- products. In Figure 8, the
ratio of o-HAP to p-HAP has been plotted vs. the fraction of
phenol in the feed when phenyl acetate was fed with different
mixtures of anisole and phenol. It is observed that the o- to p-
ratio first decreases and then reaches a plateau at increasing
fraction of phenol in the feed. This trend indicates that the Fries
rearrangement of phenyl acetate does not only occur
intermolecularly only, but also intramolecularly. When fraction of
phenol in the feed is zero, the total yield of HAPs is small and
the intramolecular o-HAP product dominates, making a high o-
to p- ratio. As the fraction of phenol in the feed increases, phenol
couples with the acylium ion to form HAPs. Thus, the fraction of
intramolecular o-HAP products starts decreasing and the o- to p-
ratio approaches the value that represents the reference of a
phenol molecule to pick up the acylium ion at o- and p- positions.
To quantify the fraction of products obtained via the
intramolecular mechanism, the o-HAP was plotted vs. p-HAP, as
shown in Figure 9. A linear relationship between o- and p-HAP
is clearly apparent. The intercept of this line is the concentration
of intramolecular o-HAP product, which is 0.01M, and the slope
means that when a phenol molecule couples with acetyl ion, the
chance of forming o- or p- isomer is 0.8. The total Fries
rearrangement product yield of phenyl acetate is 0.032M and the
intramolecular product yield is 0.01M. Therefore, the amount of
intermolecular HAPs is 0.032M minus 0.01M, which is 0.022M.
Based on these results, we can calculate the selectivity of
intra vs. inter-molecular Fries rearrangement products, as
reported in Table 4
Table 4: Selectivity of intra vs. inter-molecular mechanism for Fries
rearrangement of phenyl acetate
Intramolecular Mechanism
Intermolecular mechanism
30%
o-HAP only
70%
Selectivity
o-HAP : p-HAP=0.8
Figure 9: Concentration of o-HAP vs. p- HAP product. Reaction conditions:
1M phenyl acetate in mixture of phenol and anisole, 0.1g H-Beta with 80ml
feed, 1hr 250℃
In previous studies, intramolecular pathway was identified
as the main contributor to the production of o-HAP from phenyl
acetate. Simultaneously, in the Fries rearrangement, o-HAP is
Figure 10: DFT calculation for the acetyl ion formation for acetic acid (left), m-tolyl acetate (middle) and methyl acetate (right) in HBeta. The initial and
final structures are shown at the bottom. The values calculated using the PBE and HSE functionals are shown in black and red, respectively.
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the primary product while p-HAP is secondary.[21] However, this
study has shown that while the intramolecular pathway does
occur, the intermolecular one is dominant. Additionally, while the
intramolecular o-HAP product is primary, the intermolecular o-
and p-HAP products are clearly secondary.
acylation yield than toluene, as seen in Table 3. It can be
concluded that, in the C-C bond formation with the acylium ion,
aromatic substrates that have oxygen-contained functional
substituents (e.g., phenol, anisole, m-cresol) are effective while
alkylated aromatic substrates such as toluene are not. Therefore,
one could argue that, in the last case, step (2) is the rate-limiting
step. This is in agreement with recent reports investigating the
mechanism of C-C bond formation over methylfuran, in which
the ability of the aromatic compound to stabilize the positive
charge of the acylium ion was found to lower the activation
barrier for C-C coupling to the point where it was no longer rate-
controlling.[8] Based on these observations, it can be concluded
that in this reaction there is not a definite rate-limiting step, but it
depends on the relative energy barriers for steps (1) and (2).
When a less effective acylating agent, such as acetic acid or
methyl acetate is used, the rate-limiting step is the formation of
acylium ion (1). However, when an effective acylating agent (e.g.
aromatic ester) and a poor aromatic substrate (e.g. toluene) are
used, the rate-limiting step becomes the C-C bond formation (2).
4.
DFT calculations of the acylium ion formation
Regarding the nature of the acylating agent, experimental
results have shown that acetic acid and methyl acetate have low
acylation activity, while m-tolyl acetate is effective. To further
investigate the meaning of these results, DFT calculations were
conducted to calculate the energy barriers for the formation of
acetyl ions from these three different acylating agents: acetic
acid, methyl acetate and m-tolyl acetate, as shown in Figure 10.
From the results in Table 5, the lowest intrinsic activation
energy for acetyl ion formation is from m-tolyl acetate (83
kJ/mol), followed by acetic acid (102 kJ/mol) and finally methyl
acetate (118 kJ/mol). Furthermore, due to the strong adsorption
of m-tolyl acetate, the apparent activation energy required for
the formation of acetyl ion of m-tolyl acetate is significantly lower, Therefore, a high acylation yield is only obtained when both the
compared to the other two acylating agents. Interaction between
the bulky aromatic moiety and the zeolite framework stabilizes
both the initial adsorption of the acetate and the formed acetyl
ion. For both acetic acid and methyl acetate, the apparent
activation barrier is about 30 kJ/mol in the DFT-PBE calculations
and about 60 kJ/mol in the DFT hybrid calculations. In the latter
case, the calculation of the activation barrier is improved by
using the HSE hybrid functional to reduce the delocalization
error[22] (see methods). In the case of methyl acetate, the
reverse reaction to form methyl acetate from acetyl ion and
methanol is barrier-less, indicating that this reverse reaction
would be easy. These DFT results suggest that m-tolyl acetate
will be the most active acylating agent, in good agreement with
the experimental results.
acylating agent and aromatic substrate are effective.
Interestingly, when an aromatic ester is used as the acylating
agent with aromatic substrates that have oxygen-contained
functional substituents such as phenol, anisole and m-cresol,
step (1) formation of acylium ion is the rate-limiting step and the
total acylation yield depends on the type of aromatic ester, as
seen in section 2.2. Phenyl acetate has a higher acylation yield
than m-tolyl acetate, suggesting that the nature of the exiting
group is important for the formation of acylium ion. Phenyl is less
bulky than tolyl group and maybe easier to leave, therefore the
acylation yield of phenyl acetate is higher compared with m-tolyl
acetate.
Conclusions
Table 5: Heat of adsorption and true activation energy of different acylating
agents
In this study, liquid phase acylation over zeolite Beta has
been investigated and proven to occur via
a two-step
∆Hadsorption (kJ/mol)
∆Htrue activation(kJ/mol)
mechanism, (1) formation of acylium ion followed by (2) C-C
bond formation between aromatic substrate and acylium ion.
Experimental results and DFT calculations have shown that
aromatic esters are effective in generating acylium ions while
acetic acid and alkyl acetate such as methyl acetate are not.
However, if acetic acid or methyl acetate is fed with aromatic
substrates that contain -OH substituent, they can undergo fast
esterification or transesterification to create aromatic esters,
which are more active acylating agents, resulting in high yield of
the acylation products. This finding has important consequences
for bio-oil upgrading since acylation could become an effective
way to enhance the utilization of acetic acid without loss of
carbon, as would occur with the alternative C-C bond forming
ketonization. One could pretreat the lignin-derived aromatic
streams to convert the abundant alkoxy substituent compounds
(Ar-OR) to phenolic derivatives (Ar-OH). This pretreatment can
easily be done via transalkylation by feeding the mixture over
zeolite to convert the alkoxy –OR to hydroxyl –OH and alkyl –R
functional groups.[23] Therefore, even though acetic acid is not a
good acylating agent, it can undergo esterification with these
phenolic compounds (Ar-OH) to produce aromatic esters, and
further creates the desired acetophenones, versatile molecules
that can be directly hydrotreated[24] to drop-in fuels or further
Acetic acid
Methyl acetate
m-Tolyl acetate
-65
-89
102
118
83
-126
5.
Rate-limiting step
From the discussion in the previous sections, it has been
demonstrated that the acylation reaction occurs in two-steps, (1)
formation of acylium ion and (2) C-C bond formation between
aromatic substrate and acylium ion. What is not clear from this
analysis is whether one of the two steps can be considered rate-
limiting. In fact, as observed in Table 2, the nature of the
acylating agent used has a significant impact on the reaction
yield. When the aromatic substrate is m-cresol or anisole,
aromatic esters are proved to be effective acylating agents,
while acetic acid and methyl acetate are not. Therefore, one
could argue that step (1), which is the formation of the acylium
ion, is the rate-limiting step. However, when the same aromatic
ester was used as the acylating agent the nature of the aromatic
substrate also affected greatly the observed yield. That is, with
phenyl acetate as acylating agent anisole gave a much higher
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transition state and calculate reaction barriers. The transition states have
been further verified by vibrational calculations. An HSE (Heyd-Scuseria-
Ernzerhof) hybrid functional[32] was used to calculate the total energy of
the initial, transition, and final states that had already been optimized by
DFT-D3 calculations to reduce the underestimation of the reaction
barriers caused by the charge delocalization error of the local and semi-
local exchange-correlation functionals.[22]
upgraded to longer chain molecules via different chemistries
such as aldol condensation due to the ketone functionality.[1a, 25]
Furthermore, in this reaction, once the acylium ions are
generated, the aromatic compounds with oxygen-containing
functionalities, such as hydroxyl –OH and alkoxy – OCH3, are all
active to pick them up with the activity of this C-C coupling step
decreasing in the order of m-cresol > phenol > anisole. Alkyl
aromatic compounds such as toluene are not as effective for
picking up the acylium ions, which leads to a low acylation yield
and high loss of acylium ion to coke. The presence of oxygen-
contained functional groups in the lignin derived aromatic
compounds was not considered favorable due to the
requirement for intensive hydrotreating step. However, as shown
in this study, these functional groups have the advantage of
activating the aromatic ring and make them easier in the
upgrading process, particularly for C-C bond formation reactions.
It is thus very important to perform C-C coupling reaction via
acylation with the available functionalities in bio-oil before the
hydrotreating process.
Acknowledgements
This work was supported by the U.S. Department of Energy,
DOE/EPSCOR (Grant DESC0004600). The computational
research used the supercomputer resources of the National
Energy Research Scientific Computing Centre (NERSC) and the
OU Supercomputing Centre for Education & Research (OSCER)
at the University of Oklahoma.
In the last part of the study, the Fries rearrangement of
phenyl acetate to HAPs was investigated to differentiate
between the intermolecular and intramolecular reaction pathway.
The reaction was found to happen via both mechanisms, in
which the intermolecular one is more dominant.
Keywords: acylation • acetic acid • aromatic ester • alkyl ester •
aromatic substrate
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Entry for the Table of Contents
FULL PAPER
In the acylation reaction of aromatic
compounds, acetic acid is not an effective
acylating agent. We have proposed a
novel upgrading strategy that converts
acetic acid to aromatic esters, more
Nhung N. Duong, Bin Wang, Tawan
Sooknoi, Steven P. Crossley and
Daniel E. Resasco*
Page No.1 – Page No.9
effective
acylating
agents,
via
Enhancing the Acylation Activity
of Acetic Acid by Forming an
Intermediate Aromatic Ester
esterification with Ar-OH. Furthermore, as
shown in the figure, the rate limiting step
of this reaction depends on both acylating
agent and aromatic substrate. The
phenolic derivatives are active to pick up
the
acylium
ion,
with
m-
cresol>phenol>anisole, while the alkylated
aromatics are not active.
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