Synthesis of Itaconic Acid Ester Analogues via Self-Aldol Condensation of Ethyl Pyruvate Catalyzed by Hafnium BEA Zeolites

Article abstract of DOI:10.1021/acscatal.6b00561

Lewis acidic zeolites are used to synthesize unsaturated dicarboxylic acid esters via aldol condensation of keto esters. Hafnium-containing BEA (Hf-BEA) zeolites catalyze the condensation of ethyl pyruvate into diethyl 2-methyl-4-oxopent-2-enedioate and diethyl 2-methylene-4-oxopentanedioate (an itaconic acid ester analogue) with a selectivity of ca. 80% at ca. 60% conversion in a packed-bed reactor. The catalyst is stable for 132 h on stream, reaching a turnover number of 5110 mol_EP mol_Hf^-1. Analysis of the dynamic behavior of Hf-BEA under flow conditions and studies with Na-exchanged zeolites suggest that Hf(IV) open sites possess dual functionality for Lewis and Br?nsted acid catalysis.

Full text of DOI:10.1021/acscatal.6b00561

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Letter
Synthesis of Itaconic Acid Ester Analogs via Self-aldol
Condensation of Ethyl Pyruvate Catalyzed by Hafnium BEA Zeolites
Yuran Wang, Jennifer D. Lewis, and Yuriy Roman-Leshkov
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00561 • Publication Date (Web): 21 Mar 2016
Downloaded from http://pubs.acs.org on March 28, 2016
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Synthesis of Itaconic Acid Ester Analogs via Self-aldol Condensation
of Ethyl Pyruvate Catalyzed by Hafnium BEA Zeolites
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Yuran Wang, Jennifer D. Lewis and Yuriy Román-Leshkov*
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, US.
KEYWORDS: Itaconic acid ester, aldol condensation, pyruvate, hafnium beta, sodium exchange, open sites, Lewis
acidic zeolites
ABSTRACT: Lewis acidic zeolites are used to synthesize unsaturated dicarboxylic acid esters via aldol condensation of
keto esters. Hafnium-containing BEA (Hf-BEA) zeolites catalyze the condensation of ethyl pyruvate into diethyl 2-methyl-
4-oxopent-2-enedioate and diethyl 2-methylene-4-oxopentanedioate (an itaconic acid ester analog) with a selectivity of
ca. 80% at ca. 60% conversion in a packed-bed reactor. The catalyst is stable for 132 h on stream, reaching a turnover
number of 5110 molEP molHf-1. Analysis of the dynamic behavior of Hf-BEA under flow conditions and studies with Na-
exchanged zeolites suggest that Hf(IV) open sites possess dual functionality for Lewis and Brønsted acid catalysis.
Dicarboxylic acids (diacids) play a central role in the
biobased chemicals portfolio, as evidenced by their preva-
lence in the top 12 chemicals from biomass identified by
the U.S. Department of Energy.1 Diacids are building
blocks in condensation polymerization reactions,2 and
their ester forms (diesters) can serve as lubricants, plasti-
cizers, and polymer intermediates.3 In particular, the un-
saturated diacid itaconic acid is a potential biodegradable
substitute for high-volume petroleum-derived chemicals
such as acrylic acid, maleic anhydride, or acetone cyano-
hydrin.4 It can also be used in the production of super-
absorbent polymers, synthetic latex, and laminating res-
ins.4 Although some diacids (e.g. succinic acid) are al-
ready produced industrially from biomass, most, includ-
ing itaconic acid, suffer from prohibitively high produc-
tion costs.5-8 Current catalytic routes to synthesize diacids
and diesters from biomass-derived molecules, such as
Baeyer-Villiger oxidation,9,10 C-C bond cleavage,11 and no-
ble metal catalyzed aerobic oxidation,12,13 suffer from poor
selectivity,9,10 inefficient carbon utilization,11 and/or cata-
lyst deactivation.12 Flanagan et al. reported a C-C coupling
strategy to produce unsaturated dicarboxylic esters
through the addition dimerization of crotonates;14 howev-
er, the system depends on homogeneous catalysts.
sation are not ideal for the proposed system, as they easily
deactivate in the presence of organic acids.24-26
In this work, we demonstrate a general strategy to syn-
thesize unsaturated dicarboxylic acid esters via the C-C
coupling of keto esters with Lewis acidic zeolites. We em-
phasize the synthesis of itaconic acid ester analogs from
the condensation of ethyl pyruvate (EP, 1) (Scheme 1) due
to the low thermal stability of pyruvic acid.27 Zeolites with
BEA topology containing Lewis acidic framework heteroa-
toms, including Sn(IV), Hf(IV) and Zr(IV), can catalyze C-
C coupling of biomass-derived oxygenates.28-31 These ma-
terials promote aldol condensation via a soft enolization
pathway reminiscent to that observed in type II aldolas-
es.24 Importantly, these zeolites feature remarkable toler-
ance to carboxylic acid groups24 and water,32,33 making
them prime candidates for the biomass-based production
of diacids and diesters.
Scheme 1. Self-aldol condensation of ethyl pyruvate
catalyzed by Lewis acidic zeolites with products from
side reactions listed
An alternative route to generate diacids or diesters is
via the C-C coupling of keto acids or esters (Scheme 1),
which are common intermediates in a myriad of metabol-
ic pathways and can be produced through biocatalysis on
a large scale.7,15-20 While this coupling strategy is promis-
ing, only a few homogeneous catalysts have been shown
to activate the carbonyl group adjacent to the ester
group.21-23 However, these catalysts lactonize the aldol
adducts instead of generating linear condensation prod-
ucts. Solid base catalysts typically used for aldol conden-
As depicted in Scheme 1, the self-aldol condensation of
EP 1 followed by dehydration of the aldol addition prod-
uct, diethyl 2-hydroxy-2-methyl-4-oxopentanedioate 2,
results in two diester isomers: diethyl 2-methyl-4-
oxopent-2-enedioate
oxopentanedioate 4. The latter is a functional analog of
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Page 2 of 11
itaconic acid ester. In addition to the main coupling reac- toluene and reusing the material for five consecutive
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tion, several undesired side reactions are triggered by the
presence of acid sites and water generated from the dehy-
dration of aldol adducts. These side reactions (Scheme S1)
include hydrolysis of EP to pyruvic acid and ethanol,
Meerwein−Ponndorf−Verley (MPV) reduction of EP ac-
companied with Oppenauer oxidation of ethanol, and
aldol reaction between EP and these side products. Tolu-
ene was used as the solvent to suppress side reactions that
can occur between EP and alcohols (MPV) or water (hy-
drolysis).
batch reactions at 120 °C. After a gradual conversion de-
crease from 80% (run 1) to 60% (run 4), the catalyst
shows no further conversion drop in run 5 (Figure S8).
Remarkably, calcination of the catalyst under dry air after
run 4 improves the diester selectivity from 68% to 81% in
addition to recovering the initial activity (Table 1, entry
4). This unique behavior prompted us to further investi-
gate the stability and regeneration of Hf-BEA for EP cou-
pling under flow conditions.
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EP conversion
Diester selectivity
Side product selectivity
Table 1. Self-aldol condensation of ethyl pyruvate
catalyzed by Lewis acidic zeolitesa
Selectivity (%)b
EP conversion
(%)
Side
products
Entry
Catalyst
Diesters
1
Hf-BEA
Zr-BEA
Sn-BEA
Hf-BEAc
80
81
68
64
67
81
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6
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19
78
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a Reaction conditions: 3 wt% EP in toluene, EP/metal = 195
N
C,ini
∑
i
Selectivity =
×100%
(mol/mol), 120 oC, 1 h. b
, where
NC,EP
nEP,o − nEP
(
)
NC,i is the number of carbon atoms in compound i, ni is the
number of moles of compound i in the reaction mixture, and
nEP,o is the initial moles of EP added. Diesters: 3 - 5 in Scheme
1. Side products: 6 - 13 in Scheme 1; distribution for a typical
0
20
40
60
80
100
120
140
TOS (h)
o
reaction is shown in Figure S7. c Calcined in dry air at 550 C
Figure 1. EP conversion and selectivity to diesters and side
products as a function of time on stream (TOS) for the flow
reaction with Hf-BEA. Reaction conditions: 120 oC, 12 bar, 320
mg Hf-BEA, 3 wt% EP in toluene, flow rate 0.20 mL min-1,
WHSV 32.2 h-1. Dashed lines represent regeneration of Hf-
BEA: flush with toluene at 120 oC, dry under N2 at 150 oC, and
calcine with dry air at 550 oC for 5 h. Diesters: 3 - 5 in Scheme
1. Side products: 6 - 13 in Scheme 1; distributions are shown
in Figure S9.
for 5 h after four consecutive runs as shown in Figure S8.
The catalytic performance of Lewis acidic zeolites for
the self-aldol condensation of EP is shown in Table 1. Hf-
BEA and Zr-BEA generate the highest EP conversions
(>80%) with comparable selectivities (>64%) to diester
products (3 - 5 in Scheme 1) after 1 h at 120 oC (entries 1
and 2). Sn-BEA shows a lower EP conversion of 19% under
identical conditions (entry 3), consistent with previous
studies on aldol reactions.24 Only a ca. 1% conversion is
observed from Si-BEA or metal oxides supported on Si-
BEA, and a 17% conversion (with 12% selectivity) is ob-
tained for Al-BEA (Table S1, entries 1-7). Collectively, the-
se results indicate that the catalytic activity most likely
originates from framework Lewis acidic heteroatoms. In
contrast to previously reported homogeneous catalytic
systems,21-23 the lactonization of 2 is not observed with
these Lewis acidic zeolites. Nonetheless, compound 13 is
likely produced from the lactonization of a C7 molecule
(reaction f in Scheme S1), indicating that although lac-
tonization is feasible, the confining effects of the zeolite
pores may limit this pathway for larger molecules.34 Metal
oxides with Brønsted basicity typically employed for aldol
reactions, including MgO and hydrotalcite,35 show EP
conversions of less than 13% and selectivities of less than
18% (Table S1, entries 8-12). In these systems, pyruvic acid
produced by the hydrolysis of EP is expected to deactivate
the basic sites.24
Figure 1 shows EP conversion and selectivities to
diesters and side products catalyzed by Hf-BEA as a func-
tion of time on steam (TOS) in a packed bed reactor (see
Figure S10 for yields). Hf-BEA was calcined in situ under
dry air at 550 oC for 5 h prior to feeding the reactants. The
catalyst bed was regenerated by calcination under identi-
cal conditions over the course of the experiment; each
cycle is indicated by the dashed lines in Figure 1. The re-
o
actor was operated at 120 C with a weight hourly space
velocity (WHSV) of 32.2 h-1. Steady-state conversions of
ca. 60%, selectivities towards diesters of ca. 80%, and a
total turnover number of 5110 molEP molHf-1 over 132 h on
stream were observed. Elemental analysis of the spent
catalyst showed no detectable change in Hf content (Ta-
ble S3), and powder X-ray diffraction confirmed that the
long-range crystallinity was preserved (Figure S13). N2
adsorption data of the spent catalyst showed a slightly
smaller micropore volume (0.17 cm3 g-1) than that of the
pristine catalyst (0.19 cm3 g-1) (Table S3, Figure S16).
Hf-BEA was investigated in further detail due to its su-
perior diester selectivity (68%) at high conversion (80%).
The recyclability of Hf-BEA was probed by washing with
After each calcination, transient and steady-state re-
gimes are clearly observed in the reactivity data (Figure 1
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ACS Catalysis
The dynamic behavior exhibited by Hf-BEA in flow is
and Figure S10). During the transient periods, the EP con-
1
2
3
4
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version decreases by ca. 10% over 10 h TOS, while the
diester selectivity increases by ca. 20% and the selectivity
to side products decreases by ca. 5%. The decrease in EP
conversion and the increase in diester selectivity appear
to be correlated with a decrease in the rates of side reac-
tions that are initiated by hydrolysis (Scheme S1), indicat-
ing the deactivation of active sites responsible for hydrol-
ysis reactions. The steady-state period, which accounts for
the remaining 14-26 h on stream, is characterized by less
than 5% decrease in EP conversion and stable selectivity
to diesters (Figure 1). Calcination of the catalyst bed re-
sults in a partial recovery of hydrolysis activity, demon-
strating that deactivation of hydrolysis active sites is par-
tially reversible. The regeneration by calcination coupled
with the 5 wt% loss observed by thermogravimetric analy-
sis of the catalyst recovered after 132 h TOS suggests that
interaction of organic molecules with the sites contrib-
utes to the apparent decrease in hydrolysis rates.
consistent with prior studies of Lewis acidic zeolites that
suggest amorphization and site restructuring are respon-
sible for changes in activity and selectivity after extended
TOS.30,40,41 Specifically, for aqueous dihydroxyacetone
isomerization, Lari et al. observed an 18% loss in crystal-
linity for Sn-BEA and a 16% decrease in tetrahedral coor-
dination for Sn-MFI after 24 h TOS.41 Lewis et al. demon-
strated with 119Sn MAS and 119Sn{1H} CP/MAS NMR that
open and distorted framework Sn sites were generated in
119Sn-BEA during the transfer hydrogenation and etherifi-
cation of 5-(hydroxymethyl)furfural with ethanol, while
only closed sites were observed in the pristine catalyst.40
Studies by Boronat et al. and Bermejo-Deval et al. also
suggest that the distribution of open and closed sites in
Sn-BEA changes with post-synthesis treatment, e.g. calci-
nation or exposure to reaction conditions.42,43
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The molecular connectivity of active sites, i.e. open ver-
sus closed, has been suggested to affect catalytic activity
and selectivity due to differences in Lewis acidity, flexibil-
ity, and functionality of the neighboring silanol group.42-46
Bermejo-Deval et al. demonstrated with Na-treated Sn-
BEA that the cations likely exchange onto silanol groups
proximal to Sn(IV) open sites.45 The presence of Na+ alters
the glucose isomerization reaction pathway from a fruc-
tose-producing 1,2-hydride shift to a mannose-producing
1,2-carbon shift, and the effect can be reversed by washing
with a H2SO4 solution to remove the Na+ cations.45 Com-
putational studies indicate that the change in product
distribution is a consequence of the electrostatic stabiliza-
tion of the carbon shift relative to hydride shift caused by
the presence of Na+ ions at the open Sn center.47 To probe
the nature of the active sites in Hf-BEA, we conducted a
preliminary study with Na-exchanged Hf-BEA. Specifical-
ly, two Na-exchanged Hf-BEA catalysts were prepared
using methods adapted from the work of Bermejo-Deval
et al. (see Section S1).45 These samples were denominated
as Hf-BEA-Na-1 and Hf-BEA-Na-2, with Na/Hf molar rati-
os of 0.69 and 0.02, respectively (Table 2). Batch reactions
catalyzed by Hf-BEA-Na-1 resulted in 15% EP conversion
and ca. 34% selectivity to diesters (Table 2 and S2, entry
2). This significant reduction in activity and selectivity is
unlikely a result of pore blockage (Section S2). Despite its
low Na content, Hf-BEA-Na-2 resulted in a diminished EP
conversion of 59% when compared to the 77% conversion
generated by pristine Hf-BEA (Tables 2 and S2, entry 3).
The catalyst performance and pore volume of both Na-
exchanged zeolites were recovered by washing with
H2SO4 (Tables 2 and S2, entries 4-5). This reversible activ-
ity drop is likely caused by a strong interaction between
Na+ and the active sites, which we investigate further us-
ing Fourier transform infrared (FTIR) spectroscopy with
deuterated acetonitrile (CD3CN) adsorption.
The increased diester yield coupled with the concurrent
decrease of hydrolysis activity in the transient regimes
suggests that different moieties in the active site ensem-
ble catalyze the aldol condensation and hydrolysis reac-
tions. Control batch reactions with Si-BEA (Table S1, en-
tries 4-5) showed no activity, thus pointing to the in-
volvement of Hf in generating these sites. In addition to
Lewis acidity, spectroscopic evidence36 and computational
studies37-39 suggest the presence of weak Brønsted acidity
associated with the silanol moiety of monohydrolyzed
framework heteroatom sites (i.e. open sites) in Lewis
acidic zeolites. Thus, these weak Brønsted acid sites are
likely responsible for hydrolysis, and selective deactiva-
tion of the acidic proton due to alkoxide formation could
explain the transient deactivation of hydrolysis. Indeed,
similar deactivation behavior has been observed in the
flow studies of Lewis acidic zeolites.40,41 Lewis et al.
showed through Sn- or Hf-BEA catalyzed tandem reac-
tions between 5-(hydroxymethyl)furfural and alcohols
that Lewis acid-catalyzed transfer hydrogenation activity
remained unchanged, while Brønsted acid-catalyzed
etherification activity decreased over time and could be
partially recovered by calcination.40 Importantly, 13C{1H}
CP MAS NMR of the spent catalyst showed alkoxide spe-
cies formation on silanol groups in the zeolite.40
An important feature demonstrated in the flow data is
the increase in selectivity toward diesters after the first
regeneration by calcination (Figure 1, after 24 h TOS) that
is consistent with the increase in selectivity observed after
calcination in the batch studies. Specifically, the steady-
state diester selectivity increases from 65% to 79% (yield:
42% to 48%), and the steady-state selectivity to side
products decreases from 17% to 10% (yield: 11% to 5%).
However, no further improvement in diester selectivity is
observed after successive calcinations. This irreversible
shift in selectivity after the first regeneration is indicative
of permanent changes to the catalyst active sites that con-
tribute to the decrease in hydrolysis activity, the increase
in aldol condensation activity, or a combination of both.
FTIR spectra of CD3CN adsorbed on pristine Hf-BEA
show the characteristic bands seen with M-BEA zeolites
(Figure 2a).42,45,48 The C≡N stretching vibrations of CD3CN
at 2310 cm-1 2275 cm-1 and 2268 cm-1 are representative of
,
,
CD3CN that is strongly bound to Lewis acid sites (i.e. tet-
rahedral Hf(IV) centers), coordinated to silanol groups,
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Table 2. Aldol condensation of ethyl pyruvate catalyzed by Na-exchanged and acid-washed Hf-BEAa
1
2
Selectivity (%)b
3
4
Si/Hf
ratioc
Na/Hf
Micropore
EP conv.
Side
products
Entry Cat.
Treatment
Diesters
molar ratiod volume (cm3 g-1) (%)
5
6
7
8
1
Hf-BEA
None
115
116
n/a
0.20
0.17
0.20
0.20
0.19
77
15
69
34
70
69
79
7
2
3
4
5
Hf-BEA-Na-1e
Hf-BEA-Na-2f
Hf-BEA-AW-1g
NaNO3 (1 M)
0.69
0.02
n.d.
n.d.
10
7
NaOH (pH = 10) 113
59
74
77
H2SO4 (1 M)
115
12
Hf-BEA-AW-2g H2SO4 (1 M)
119
7
9
a Reaction conditions same as in Table 1, but with a different batch of Hf-BEA zeolite. b Selectivity same as defined in Table 1.
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c No significant changes in Si/Hf ratios after different treatments, indicating the treatments do not cause Hf leaching. n/a,
not applicable. n.d., not detected. Hf-BEA after Na exchange treatment 1 with NaNO3 (1 M). Hf-BEA after Na exchange
d
e
f
g
treatment 2 with NaOH (pH =10). Hf-BEA-Na-1 or Hf-BEA-Na-2 washed with H2SO4 (1 M) at room temperature for 1 h, re-
covered and then calcined at 550 oC. Full details on material synthesis available in Section S1.
and physisorbed, respectively.42,45 The band at 2310 cm-1
appears to be a single peak, which differs from previous
reports of two bands corresponding to open and closed
sites for Sn-BEA.42,45 However, these data are consistent
with studies of Zr-BEA that show only one band in this
region.36 Na exchange results in a decrease in the band at
2310 cm-1 and the appearance of a new band at 2284 cm-1,
which has previously been assigned as CD3CN adsorbed
on Na-exchanged open sites for Sn-BEA.45 This change is
more prominent for the material with a higher degree of
Na exchange (Hf-BEA-Na-1, Figure 2b) than for the mate-
rial with low Na loading (Hf-BEA-Na-2, Figure S18a). Con-
trols with Na-exchanged Si-BEA (Si-BEA-Na) confirm that
framework Hf(IV) is required to observe this feature (Fig-
ure S18b).49 After acid wash, the FTIR spectra resemble
again those of the pristine material, demonstrating the
reversibility of the Na exchange (Figure 2c).
Assuming that Na+ interacts with the open framework
Hf(IV) site in a similar manner to that proposed by Ber-
mejo-Deval et al. for Sn-BEA,45 we hypothesize that the
open sites are the main active sites for aldol condensa-
tion. Thus, open site ensembles likely have twofold func-
tionality: the Lewis acidic character of the heteroatom
catalyzes aldol reaction, while the Brønsted acidic silanol
group catalyzes hydrolysis. The change in the chemical
environment of the open site upon Na exchange—
evidenced by the change in the FTIR band corresponding
to CD3CN adsorbed on Hf sites from 2310 to 2284 cm-1—
may explain the lower reactivity of Na-exchanged Hf-BEA
for aldol condensation. However, the presence and cata-
lytic contribution of Hf(IV) closed sites cannot be exclud-
ed. Detailed characterization and kinetics studies are cur-
rently underway to confirm these hypotheses.
indicating a strong interaction between Na+ and the ac-
tive sites. The open framework Hf(IV) site is hypothesized
to possess strong Lewis acidity and weak Brønsted acidity
that are responsible for the dual functionality of the cata-
lyst. However, further investigation is needed to fully elu-
cidate the nature of the active site and guide catalyst and
reaction optimization.
Figure 2. FTIR spectra with decreasing CD3CN coverage on a)
Hf-BEA, b) Hf-BEA-Na-1, and c) Hf-BEA-AW-1. Signals are
referenced to the bare material and normalized by the com-
bination and overtone modes of zeolite Si-O-Si stretches
(1750–2100 cm-1). Reference lines are for physisorbed CD3CN
(2268 cm-1) and CD3CN adsorbed on silanols (2275 cm-1), Na-
exchanged open Hf(IV) sites (tentative, 2284 cm-1), and Lewis
acidic Hf sites (2310 cm-1).
In summary, we have developed a new approach to syn-
thesize unsaturated dicarboxylic acid esters from keto
esters via aldol condensation catalyzed by Lewis acidic
zeolites. In particular, we demonstrate that Hf-BEA can
catalyze the self-aldol condensation of EP in toluene to
produce itaconic acid ester analogs. The catalyst is stable
for 132 h TOS in a packed-bed reactor with a diester selec-
tivity of ca. 80% at 120 C. Na exchange substantially de-
creases Hf-BEA activity and significantly alters the FTIR
band for CDCN3 adsorbed on the framework Hf(IV) sites,
ASSOCIATED CONTENT
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Supporting Information
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Liu, J.; Du, Z.; Lu, T.; Xu, J. ChemSusChem 2013, 6,
The supporting information includes experimental proce-
1
2
3
4
5
6
7
dures for catalyst synthesis, characterization, and test reac-
tions, side reaction scheme, a typical GC chromatogram for
reaction mixtures, MS data for reaction products, additional
reaction data for batch studies, flow studies and Na exchange
studies, and catalyst characterization data (ICP-MS, PXRD,
N2 adsorption-desorption and FTIR). This material is availa-
ble free of charge via the Internet at http://pubs.acs.org.
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Wang, Y. R.; Van de Vyver, S.; Sharma, K. K.; Roman-
Leshkov, Y. Green Chem. 2014, 16, 719-726.
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AUTHOR INFORMATION
Corresponding Author
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Schwartz, T. J.; Shanks, B. H.; Dumesic, J. A. Curr.
*Email: yroman@mit.edu.
Notes
The authors declare no competing financial interest.
Opin. Biotechnol. 2016, 38, 54-62.
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ACKNOWLEDGMENT
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This work was sponsored by the Chemical Sciences, Geosci-
ences and Biosciences Division, Office of Basic Energy Sci-
ences, Office of Science, U.S. Department of Energy, under
Award No. DE-FG0212ER16352. J.D.L. was partially supported
by the National Science Foundation Graduate Research Fel-
lowship under Grant No. 122374. Any opinion, findings, and
conclusions or recommendations expressed in this material
are those of the authors and do not necessarily reflect the
views of the National Science Foundation.
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ABBREVIATIONS
Diacids, dicarboxylic acids; diesters, dicarboxylic acid esters;
C-C, carbon-carbon; EP, ethyl pyruvate; MPV reduction,
Meerwein−Ponndorf−Verley reduction; TOS, time on stream;
WHSV, weight hourly space velocity; FTIR, Fourier transform
infrared spectroscopy; CP, cross polarization; MAS, magic-
angle spinning; NMR, nuclear magnetic resonance.
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Catal. 2015, 5, 6946-6955.
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Int. Ed. 2015, 54, 12554-12561.
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Scheme 1. Self-aldol condensation of ethyl pyruvate catalyzed by Lewis acidic zeolites with products from
side reactions listed
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Figure 1. EP conversion and selectivity to diesters and side products as a function of time on stream (TOS)
for the flow reaction with Hf-BEA. Reaction conditions: 3 wt% EP in toluene, 120 oC, 12 bar. Flow reactor
operating conditions: 320 mg Hf-BEA, flow rate 0.20 mL min-1, WHSV 32.2 h-1. Dashed lines represent
regeneration of Hf-BEA: flush with toluene at 120 oC, dry under N2 at 150 oC and calcine with dry air at 550
oC for 5 h. Diesters: 3 - 5 in Scheme 1. Side products: 6 - 13 in Scheme 1; distributions are shown in Figure
S9.
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b
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coverage
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