Acrylate Esters by Ethenolysis of Maleate Esters with Ru Metathesis Catalysts: an HTE and a Technoeconomic Study

Article abstract of DOI:10.1002/hlca.202000035

A high throughput experimentation (HTE) study identified active Ru metathesis catalysts and reaction conditions for the ethenolysis of maleate esters to the respective acrylate esters. Catalysts were tested at various loadings (75–10’000 ppm) and temperatures (30–60 °C) with maleate esters dissolved in toluene (up to ca. 44 wt-%) or neat and at variable partial pressures of ethylene (0.2–10 bar). Ruthenium catalysts containing a PCy₃ ligand, such as 1st or 2nd generation Grubbs catalysts, as well as the state-of-the-art catalysts containing cyclic alkyl amino carbene (CAAC) ligands, are generally inferior to Hoveyda–Grubbs 2nd generation catalyst in ethenolysis of maleates. Productive turnover numbers could exceed 1900 if the ethenolysis reaction is performed at low ethylene pressure (0.2–3 bar) and reach 5200 when a polymeric phenol additive was used. Such catalytic performance falls well within the window practiced in industry. Moreover, a crude technoeconomic analysis finds similar production cost for the ethenolysis route and conventional technology, that is, propene oxidation followed by esterification, justifying research to further improve the ethenolysis route.

Full text of DOI:10.1002/hlca.202000035

HELVETICA
chimica acta
Accepted Article
Title: Acrylate Esters by Ethenolysis of Maleate Esters with Ru
Metathesis Catalysts: an HTE and a Technoeconomic Study
Authors: Alexey Fedorov, Pascal S. Engl, Alexey Tsygankov, Jordan
De Jesus Silva, Jean-Paul Lange, Christophe Copéret, and
Antonio Togni
This manuscript has been accepted after peer review and appears as an
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Helv. Chim. Acta 10.1002/hlca.202000035
Link to VoR: http://dx.doi.org/10.1002/hlca.202000035
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Helvetica Chimica Acta
10.1002/hlca.202000035
Acrylate Esters by Ethenolysis of Maleate Esters with Ru
Metathesis Catalysts: an HTE and a Technoeconomic
Study
a §
a, b §
a
c, d
Pascal S. Engl, Alexey Tsygankov,
Jordan De Jesus Silva, Jean-Paul Lange,
a
a
*a, e
Christophe Copéret, Antonio Togni, Alexey Fedorov,
a
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich,
Switzerland
b
A.N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova str. 28,
19991, Moscow, Russia
Shell Research and Technology Center Amsterdam, Grasweg 31, 1031 HW Amsterdam, The Netherlands
Sustainable Process Technology, University of Twente, Drienerlolaan 5, Enschede 7522 NB, The Netherlands
1
c
d
e
Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, CH-8092 Zürich, Switzerland
E-mail: fedoroal@ethz.ch
§
[
]
These authors contributed equally
Dedicated to our colleague, friend and mentor Prof. Peter Chen on the occasion of his 60th birthday
Abstract: A high throughput experimentation (HTE) study identified active Ru metathesis catalysts and reaction conditions for
the ethenolysis of maleate esters to the respective acrylate esters. Catalysts were tested at various loadings (75–10’000 ppm)
o
and temperatures (30–60 C) with maleate esters dissolved in toluene (up to ca. 44 wt%) or neat and at variable partial pressures
st
nd
3
of ethylene (0.2–10 bar). Ruthenium catalysts containing a PCy ligand, such as 1 or 2 generation Grubbs catalysts, as well
as the state-of-the-art catalysts containing cyclic alkyl amino carbene (CAAC) ligands, are generally inferior to Hoveyda-Grubbs
nd
2
generation catalyst in ethenolysis of maleates. Productive turnover numbers could exceed 1900 if the ethenolysis reaction is
performed at low ethylene pressures (0.2-3 bar) and reach 5200 when a polymeric phenol additive was used. Such catalytic
performance falls well within the window practiced in industry. Moreover, a crude technoeconomic analysis finds similar
production cost for the ethenolysis route and conventional technology, i.e. propene oxidation followed by esterification, justifying
research to further improve the ethenolysis route.
Keywords: ethenolysis, high throughput experimentation, acrylates, Ru metathesis catalysts, technoeconomic analysis
Introduction
Ethenolysis, that involves the cross-metathesis reaction between ethylene and another olefin, has recently emerged as a
powerful methodology for the production of a-olefins and in particular the catalytic upgrading of biomass-derived
feedstocks, e.g. the conversion of long-chain esters such as oleates to the corresponding terminal olefins.[1-8] The research
focus has revolved around oleates and related esters owing to their abundance in vegetable oils and the high value of
terminal olefins produced from those substrates and ethylene. Currently, ethenolysis of oleates serves as a benchmark
reaction for the search of improved metathesis catalysts, often including screening of large catalyst libraries.[9-15] Achieving
high productivity in ethenolysis can be hampered by deactivation of intermediate Ru methylidene species,[16, 17] [Ru=CH
],
2
by a bimolecular coupling[18-21] or methylidene abstraction,[22] both processes being facilitated by the small size and the
high reactivity of the methylidene ligand.[23] In addition, a b-hydrogen elimination pathway contributes significantly to
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Helvetica Chimica Acta
10.1002/hlca.202000035
decomposition of unsubstituted ruthenacyclobutane intermediates.[24, 25] Ethenolysis of functionalized olefins such as
esters is further exacerbated by the presence of functional groups and impurities in ester-containing feeds.[26] Catalyst
decomposition is unfavorable also for reaction selectivity, as the decomposed catalyst could promote side reactions.[1, 27]
In contrast to long-chain esters, ethenolysis of a,b-conjugated olefins, e.g. maleates, remains largely underexplored. Such
substrates are challenging for olefin metathesis since nucleophiles (for instance, phosphine ligands of the catalyst initiator)
can add to those Michael acceptors forming reactive basic species, that in turn decompose the catalyst or key
metallacyclobutane intermediates of the Chauvin mechanism.[28-31] In addition, alkylidene intermediates with a carboxyl
group, [Ru=CHCOOR], formed with maleate and acrylate substrates, are known to be highly reactive and unstable.[32]
That said, ethenolysis of cheap and abundant maleate esters could represent an attractive strategy to produce industrially
important acrylate monomers, provided efficient catalysts and reaction conditions are identified. A computational study
predicted that ethenolysis of dimethyl maleate is viable both kinetically and thermodynamically, with the overall reaction
being slightly endothermic.[33]
The use of a ruthenium catalyst previously allowed converting dimethyl maleate to methyl acrylate by the cross-metathesis
with methyl 10-undecenoate.[34] Notably, ethenolysis of ethyl crotonate to ethyl acrylate and propene using the second
generation Hoveyda-Grubbs (HG-2) catalyst gave turnover numbers (TON) around 350 under ca. 18 bar of ethylene, and
decreasing ethylene pressure was reported to lower the TON.[35] This study also demonstrated that ethyl acrylate
undergoes self-metathesis to diethyl fumarate, suggesting that ethenolysis of ethyl crotonate is equilibrium limited. In
addition, the catalyst stability was identified as a key factor to achieve high TON values.[35] However, conversion of
cinnamic acid or ethyl cinnamate in the ethenolysis with HG-2 catalyst to respective acrylates and styrene was shown to
depend critically on ethylene pressure, and highest conversion of substrates were achieved using ca. 1 bar ethylene while
decreasing sharply at higher ethylene pressure.[36] These contrasting results point at an entangled dependence of yield
of acrylates produced via ethenolysis on the catalyst stability, including at high ethylene pressures and high product
concentrations, and position of the thermodynamic ethenolysis equilibrium.
Only two experimental reports on the ethenolysis of maleates have appeared to this date, both in the patent literature and
exploiting the second generation Hoveyda-Grubbs (HG-2) catalyst. These patents present diverging results regarding
catalytic performance: a more recent patent described only several productive metathesis turnovers from a maleate to
the acrylate ester using HG-2 catalyst,[37] while in the earlier patent, the activity of HG-2 catalyst was orders of magnitude
higher, reaching up to ca. 3600 turnovers in the presence of p-cresol or benzoquinone,[38] and under 1 bar of ethylene.[39]
Note, however, that those high turnover numbers were achieved at yields of ethyl acrylate not exceeding ca. 30 %.[39]
Here, we report the investigation utilizing high throughput catalytic testing of several Ru-based metathesis catalysts that
confirm high activity of the HG-2 catalyst. However, this activity is only attainable at low partial pressure of ethylene (0.2–
3
bar). Turnover numbers exceeding 1900 could be achieved when converting dimethyl maleate to methyl acrylate in
toluene or neat and using the HG-2 catalyst. Turnover numbers can be pushed to ca. 5200 with a polymeric phenol resin
additive that is found to stabilize the HG-2 catalyst al low loadings. Conversion of dimethyl maleate appears to be limited
by catalyst decomposition at high yields of methyl acrylate (ca. 65-70%). Technoeconomic analysis reveals that the
ethenolysis pathway to acrylates by ethenolysis of maleates merits further consideration for possible industrial utilization.
Results and Discussion
Primary catalyst screening. We examined the activity of a library of Ru metathesis catalysts in the ethenolysis of maleate
esters,[40] and report the key results below. Widely-used 2nd generation Grubbs catalysts with SIMes and SIPr ligands
(
1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene and 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-
ylidene, respectively, Ru-1 and Ru-2),[41, 42] the so-called third generation Grubbs catalyst (Ru-3),[43] 2nd generation Piers
Ru-4),[44] and 2nd generation Hoveyda-Grubbs catalysts (Ru-5, Scheme 1, top),[45, 46] were tested in the ethenolysis of
(
methyl, iso-propyl and n-butyl maleate esters (1-OMe, 1-OiPr, 1-OnBu, respectively, Scheme 1, bottom). The catalysts
were injected to a solution of esters in toluene after equilibration to the desired temperature and ethylene pressure (10
bar, quasi-isobaric conditions), remembering that high ethylene pressure was reported to increase the TON values in
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Helvetica Chimica Acta
10.1002/hlca.202000035
ethenolysis of ethyl crotonate.[35] Table 1 presents results of the initial screening of the parameter space including 3
substrates and 5 catalysts tested at 10 bar ethylene pressure and variable temperature, catalyst loading and reaction
time. Results reveal that the least sterically bulky ester, dimethyl maleate (1-OMe), typically yields the respective acrylate
(
2-OMe) with a higher selectivity (>90%) compared to 1-OiPr (<12%) and 1-OnBu (<23 %). Conversion of maleate esters
in these experiments was low, i.e. ca. 3-7 % (Table 1, entries 1-5). Noteworthy, acrylates can be separated from maleates
o
by distillation owing to a large difference in boiling points (204 and 80 C for 1-OMe and 2-OMe, respectively). While the
first generation Grubbs and Hoveyda-Grubbs catalysts were inactive in ethenolysis of esters 1 in these conditions (data
not shown), catalysts Ru-1 and Ru-2 gave turnover numbers (TONs) based on the yield of the desired acrylate ester
reaching ca. 300, which is 2-3 times higher than for Ru-3 and Ru-4 catalysts (Table 1, entries 1-6). Gas uptake curves
recorded during the ethenolysis reactions at 35-60 °C revealed that catalysts Ru-1-5 deactivate rapidly, within first ca. 10-
o
1
5 min after their injection. Catalyst Ru-5 performs better at 50-60 C compared to catalysts Ru-1-4 and increasing its
loading to 0.5 and 1 mol% (5000 and 10000 ppm) allows for a higher conversion of 1-OMe, reaching 39 and 59 %,
however accompanied by a decrease in selectivity to 68 and 60% respectively (Table 1, entries 7-9). To summarize, our
preliminary screening identified 2nd generation Hoveyda-Grubbs catalyst Ru-5 and 1-OMe as preferred catalyst and
maleate ester for further optimization studies owing to the higher stability and selectivity to the ethenolysis product 2-OMe,
with product yields reaching 35%.
iPr
iPr
N
N
N
N
N
N
N
N
N
N
iPr
iPr
Cl
Cl
Cl
Cl
Cl
Ru
N
Ru
N
Ru
Ru
O
Ru
Cl
Ph
Ph
Br
Cl
Ph
Cl
PCy3
Cl
Cl
PCy3
PCy3
BF
4
iPr
Br
Ru-1
Ru-2
Ru-3
Ru-4
Ru-5
O
[Ru]
OR
OR
OR
+
2
O
O
R
Me
iPr
nBu
1-OMe
1-OiPr
1-OnBu
2-OMe
2-OiPr
2-OnBu
Scheme 1. Selected Ru metathesis catalysts (top) and synthesis of acrylate esters by ethenolysis of maleate esters (bottom).
Table 1. Ethenolysis of maleate esters with catalysts Ru-1-5 in toluene (ca. 44 wt%). Conversions and selectivities were determined by GC and
1
confirmed by H NMR. Ru-4 was added to reaction mixtures before pressurizing with ethylene. The ethylene pressure was set to 10 bar. Conversion,
yield, selectivity, and productive turnover number (TON) were calculated as follows: Conv. = 1 − n(1-OR)final / n(1-OR)in, Yield = n(2-OR)final / 2 ´ n(1-OR)in
,
Sel. = Yield / Conv. and TON = n(1-OR)in ´ Yield(2-OR)final / n(Ru), where n is expressed in moles.
Loading,
Time,
[min]
T,
Conv.
[%]
Sel.
[%]
Yield
[%]
Entry
Substrate
1-OMe
1-OMe
1-OMe
1-OMe
1-OMe
1-OMe
Catalyst
Ru-1
Ru-1
Ru-2
Ru-2
Ru-3
Ru-4
TON
330
100
190
60
[
Ru, ppm]
[°C]
1
2
3
4
5
6
210
300
30
35
50
35
50
60
60
7
5
>99
>99
>99
>99
99
7
5
500
210
300
30
4
4
500
3
3
1’000
2000
30
5
5
50
30
21
96
20
100
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Helvetica Chimica Acta
10.1002/hlca.202000035
7
8
9
1-OMe
1-OMe
1-OMe
1-OiPr
Ru-5
Ru-5
Ru-5
Ru-1
Ru-4
Ru-5
Ru-1
Ru-1
Ru-4
Ru-5
Ru-5
500
5’000
10’000
500
30
30
30
30
30
30
30
30
30
30
30
50
60
60
35
60
35
35
50
60
35
50
8
>99
68
60
<1
12
7
8
27
35
<1
5
160
60
30
<1
30
60
40
60
60
60
100
39
59
41
41
41
42
42
54
44
46
1
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1-OiPr
2000
500
1-OiPr
3
1-OnBu
1-OnBu
1-OnBu
1-OnBu
1-OnBu
500
4
2
500
8
3
2’000
500
23
8
12
3
500
10
5
Solvent and ethylene pressure effects. Further optimization of reaction conditions using Ru-5 and 1-OMe revealed that
decreasing the ethylene pressure from 10 bar to 1-3 bar drastically improves conversion of 1-OMe and selectivity to
2
-OMe at high conversions, resulting in productive TON of ca. 1400 and 920 in toluene and THF, respectively with yields
of 2-OMe equal to 36 and 23%, respectively (Table 2, entries 1-2). These data suggest that catalyst instability at 10 bar
of ethylene was responsible for the observed lower TON values (Table 1). Decreasing the reaction temperature from 60
to 35 °C does not improve conversion or selectivity (Table S2). Conversion and selectivity are generally higher in toluene
or chlorobenzene than in benzene, THF, or iPrOH and the resulting yield of 2-OMe reaches 67% (Table 2, entries 1-7).[47]
Notably, the reaction proceeded in neat 1-OMe, although it was accompanied by the formation of dimethyl fumarate
(
Table 2, entry 8-10).
Conversion of 1-OMe did not improve when Ru-5 was added in three injections such that additional two injections of Ru-5
were made after ethylene consumption ceased (Table 2, entry 11). This result may be explained by the presence of
thermodynamic equilibrium in the ethenolysis of dimethyl maleate. To verify this hypothesis, we have conducted a reverse
6 6
reaction, viz. the self-metathesis of methyl acrylate. We observe that at room temperature in C D solution (ca. 28 wt %
of 2-OMe), 540 ppm of Ru-5 provides no detectable 1-OMe but delivers 10 % yield of its thermodynamically favored
isomer, dimethyl fumarate, after 90 min reaction time under Ar atmosphere (1 bar). Moreover, the yield of dimethyl
fumarate increases to 18 % when the reaction mixture is diluted 10-fold. Therefore, the expected equilibrium between
maleate and acrylate should not be limiting as formed acrylate will be consumed to dimethyl fumarate. Because dimethyl
fumarate is not observed in significant amounts (>1%) in most of our experiments (with an exception of reactions
conducted in neat 1-OMe), we suggest that the thermodynamic equilibrium does not limit our catalytic optimization results.
Consistent with this inference, no ethylene uptake was observed when additional 1-OMe was injected into the reaction
mixture that had reached ca. 66 % conversion of 1-OMe, suggesting that the deactivation of Ru-5 at these high levels of
1
-OMe conversion plays a role. To summarize, control experiments indicate that 1) the catalytic ethenolysis reaction
between 1-OMe and 2-OMe does not appear to be limited by the thermodynamic equilibrium under the tested conditions,
2
3
) high concentrations of 2-OMe are likely responsible for deactivation Ru-5 at low ethylene pressures (1-3 bar), and that
) Ru-5 deactivates in the presence of 1-OMe at high ethylene pressure (10 bar) even at low concentrations of 2-OMe.
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Helvetica Chimica Acta
10.1002/hlca.202000035
The presented data indicates two regimes of catalyst’ instability: at 10 bar ethylene pressure, Ru-5 provides low TON to
acrylates disregard of the maleate conversion, while at 1-3 bar ethylene pressure, high TON values can be achieved, yet
the yield of 2-OMe does not exceed ca. 67%, pointing at the product-induced decomposition of Ru-5 at those high yields
of 2-OMe.
o
Table 2. Solvent screen for the ethenolysis of dimethyl maleate (1-OMe) with Ru-5 at 60 C. Conversion and selectivity values were determined by
GC-FID. Except for entries 1-3, experiments were performed on the 96-parallel autoclave (ILS GmbH).[48]
Loading,
Time,
[min]
P,
Conv.
[%]
Sel.
[%]
Yield
[%]
Entry
Solvent
toluene
THF
TON
1440
920
[
Ru, ppm]
[bar]
1
2
3
4
5
6
7
8
9
250
120
120
120
70
3
3
3
1
1
1
1
3
3
1
3
67
24
9
53
98
36
23
7
250
250
500
500
500
250
125
250
500
3´500
iPrOH
toluene
75
280
66
44
67
24
21
45
41
56
99
66
44
67
24
20
36
41
56
1320
880
C
6
D
6
70
99
C
C
6
H
H
5
Cl
Cl
70
99
1340
960
6
5
70
99
neat
neat
120
120
70
96[a]
81[a]
99[a]
99
1600
1440
820
1
1
0
1
neat
toluene
90
370
[
a] Formation of methyl fumarate was observed.
Catalyst decomposition studies. Further insight into the reactivity and stability of Ru-5 in ethenolysis of maleates was
1
obtained from in situ H-NMR experiments. While Ru-5 is stable for at least 1 hour at 23 °C in the presence of 10 equiv.
of 1-OMe, keeping this solution at 60 °C for 1 hour induces decomposition of ca. 30 % of Ru-5, as indicated by the
1
decrease of the benzylidene H-NMR resonance signal, as well as the formation of metathesis products, 1-isopropoxy-2-
vinylbenzene and methyl (E)-3-(2-isopropoxyphenyl)acrylate (Figure S3). The rate of Ru-5 decomposition increases when
ethylene is introduced to the Young tube. For instance, in the presence of 10 equiv. 1-OMe and 0.7 bar of ethylene, ca.
5
0 % of Ru-5 decomposes in 2 hours at 23 °C (Figure S4). After 72 h, complete decomposition of Ru-5 is observed
accompanied by formation of methyl fumarate, likely owing to the isomerization of 1-OMe by the decomposition products
of Ru-5. However, these catalyst life times are still substantially longer than the duration of the ethylene uptake after
o
injection of 500 ppm of Ru-5 to 1-OMe at 3 bar ethylene pressure and 60 C, which proceeds only for ca. 15-20 min under
these conditions (Figure S1). This suggests that at low ethylene pressure (1-3 bar), methyl acrylate strongly undermines
catalyst stability.
Optimization of the catalytic performance. We tested Ru-5 in the ethenolysis of 1-OMe in toluene (44 wt% solution) at
a decreased partial pressure of ethylene, comparing results at 0.2, 1 and 3 bar and at 500 ppm catalyst loading at 60 °C.
There is a modest increase of TON from 920 to ca. 1100 with decreasing ethylene pressure from 3 and to 1 bar, but no
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Helvetica Chimica Acta
10.1002/hlca.202000035
further notable increase under 0.2 bar of ethylene (Table 3, entries 1-3). These relatively stable TON values at variable
low ethylene pressures are consistent with no catalyst deactivation by ethylene at 1-3 bar (in the time scale of the catalytic
reaction). In addition, it shows that catalyst productivity (assessed by TON) is not limited by ethylene concentration. Thus,
1
bar ethylene pressure was chosen for testing 3 lower temperatures (50, 40, 30 °C) that gave similar turnover numbers
ranging from 1260 at 50 °C and 1020 at 30 °C (Table 3, entries 4-6). Notably, using twice less catalyst (250 ppm) and 0.2
bar ethylene partial pressure allowed for nearly doubling turnover number to 1960, which was achieved at identical
conversion (56%) but slightly lower selectivity (99% vs. 87%, Table 3, entries 1 and 7). Interestingly, substitution of
chlorine by bromine ligands in Ru-6 slightly increases TON when compared to Ru-5 (Scheme 2 and Table 3, entries 2,
8
).[49] However, at 1 bar ethylene decreasing the loading of Ru-6 to 250 ppm decreases TON from 1380 to 1000, in
contrast to what was described above for Ru-5 at 0.2 bar ethylene partial pressure. Ruthenium metathesis catalysts
featuring cyclic alkyl amino carbene (CAAC) ligands, such as Ru-7-8 (Scheme 2), have previously been shown to exhibit
exceptionally high turnover numbers in ethenolysis of linear olefins.[10] To our surprise, catalysts Ru-7-8 demonstrated
little or no activity in ethenolysis of 1-OMe (Table 3, entries 10-11).
We have further screened various additives (alcohols,[50] o-phenylenediamine, 1,2-benzenedithiol or 1,5-hexadiene,[7] see
Table S3) with the objective of finding conditions to lower the loading of Ru-5, or increase yield of 2-OMe. Notably, a
polymeric phenol resin that was previously identified as an effective additive for cross-metathesis of acrylate esters using
phosphine-based metathesis catalysts such as Ru-2,[30] also increases TONs of phosphine-free Ru-5. For instance, with
5
2
000 ppm of the commercial polymeric phenol additive, TON at 3 bar of ethylene and 250, 125 and 75 ppm or Ru-5 are
320, 3520 and 5200, respectively (Table 3, entries 12-14). These increasing TON values with decreasing loadings are
consistent with no deactivation of Ru-5 at low concentration of 1-OMe. An alternative explanation is that catalyst
deactivation occurs via dimerization of Ru species, which is slowed downs at low concentrations of Ru-5. From a practical
standpoint, this data suggests that polymeric phenol additive can quench the decomposition of Ru-5 by Brønsted bases
such as enolates, which can be formed by a nucleophilic addition to 2-OMe. Yet this polymeric phenol additive does not
increase the yield of 2-OMe at conversions of 1-OMe around 65-70%.
iPr
iPr
N
N
N
N
Br
iPr
Cl
Cl
Ru
O
Ru
O
Ru
O
Br
Cl
Cl
iPr
iPr
iPr
Ru-6
Scheme 2. Additional tested Ru metathesis catalysts.
Ru-7
Ru-8
Table 3. Ethenolysis of dimethyl maleate (ca. 44 wt% in toluene). Conversions and selectivities were determined by GC-FID and turnovers
calculated based on GC yields of methyl acrylate.
Loading,
Time,
[min]
P,
T,
Conv.
[%]
Sel.
[%]
Yield
[%]
Entry
Catalyst
Ru-5
Ru-5
Ru-5
Ru-5
Ru-5
Ru-5
TON
1120
1100
920
[
Ru, ppm]
[bar]
[ °C]
1
2
3
4
5
6
500
60
90
90
90
90
90
0.2
1
60
60
60
50
40
30
56
65
65
63
57
56
99
85
71
99
93
93
56
55
46
63
53
51
500
500
500
500
500
3
1
1260
1060
1020
1
1
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Helvetica Chimica Acta
10.1002/hlca.202000035
7
8
9
Ru-5
Ru-6
Ru-6
Ru-7
Ru-8
Ru-5
Ru-5
Ru-5
250
500
250
500
500
250
125
75
60
90
90
90
90
60
60
60
0.2
1
60
60
60
60
60
60
60
60
56
73
40
23
6
87
94
63
33
–
49
69
25
8
1960
1380
1000
160
1
1
0
1
1
1
1
–
–
1 [
2
a]
a]
a]
3
64
50
44
92
87
89
58
44
39
2320
3520
5200
1 [
3
3
1 [
4
3
[a]
In the presence of 5000 ppm of a polymeric phenol resin.
Industrial potential. The technical progress reported above and summarized for a representative experimental result in
Table 4 warrants a first evaluation of the industrial potential of the maleate-to-acrylate ethenolysis reaction. First, the
catalytic performance achieved here was compared to general industrial targets proposed in the literature.[51] According
to Table 4, the ethenolysis of dimethyl maleate meets the general selectivity (up to 90%), productivity (up to 1.1 kg L−1 h−1)
and product concentration (up to 40 wt%) criteria for industrial operation. The obtained catalyst life-time productivity of
6
00-800 g product (g catalyst)−1 is also close to industrial requirements, making this reaction a viable target for further
development, in particular in terms of compatibilities of all criteria under the same conditions.
Table 4. Comparison of industrial targets and obtained catalytic performance.
Selectivity,
%
Product concentration,
wt%
Product formation rate,
g L−1 h−1
Product / Catalyst ratio,
g (product) g (Ru)−1
Target / Experiment
Industrial target
Table 3, entry 3
> 90
71
> 10
~22
> 100
1000
~880 [a]
~1570
[
a] Taking into account that ethylene uptake terminates in ca. 15 min (Figure S1).
An initial assessment of the economic viability of the ethenolysis route to the state-of-the-art technology, i.e. the oxidation
of propylene via acrolein, was performed. To this end, we estimated the raw material cost by assuming a stoichiometric
consumption of the various reactants and using a consistent set of market prices from commercial data bases (available
at Shell Research and Technology Center). The remaining cost contribution, i.e. investment, fixed costs, catalysts and
energy costs, etc., were lumped into a ‘conversion cost’ and assumed to amount to $200 t−1 of total feed consumed, as
proposed elsewhere.[51] The conversion of maleic anhydride to 2-OMe was assumed to proceed in two steps, i.e.
esterification and ethenolysis steps. The propene route consisted of 2.5 steps, i.e. the two-step oxidation, first to acrolein
then to acrylic acid, which was counted as 1.5 step owing to the absence of the product workup between the steps,
followed by an esterification step. Next, we considered basic economic sensitivities by varying the feed prices by +/− 10%
and by varying the conversion cost by +/− 25% for all steps. Notice that a penalty of +10% on the feed price corresponds
to a penalty of −10% of overall product selectivity.
The ethenolysis route falls in the same band of manufacturing cost as the propylene oxidation route (the diagonal band
−1
of $1250-1500 t ), i.e. within the uncertainty band discussed above and represented by the size of the bubbles in Figure
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Helvetica Chimica Acta
10.1002/hlca.202000035
1
. The ethenolysis route shows a higher raw material cost, mainly owing to the price of maleic anhydride. However, this
additional cost is largely offset by lower conversion cost, resulting from a lower number of process steps (2 instead of
.5), combined with a lower overall feed intake (1.1 t feed/t product instead of 1.42 t/t). The cost of the catalysts in the
2
ethenolysis route is assumed to be moderate, at this stage falling within the inaccuracy of the method.
We also calculated the production cost when starting from butane and propane, i.e. by including the oxidation of butane
to maleic anhydride or the oxidative dehydrogenation of propane to propene into the overall manufacturing process. This
does not dramatically change the conclusions since both routes still falls within the band of $1250-1500 t−1 production
cost, although with a larger uncertainty that is due to the higher capex that is needed (Figure 1).
Finally, we tested the robustness of this analysis by evaluating an earlier concept to produce methyl acrylate from maleic
anhydride, namely the selective decarboxylation of maleic acid.[52] This methodology has not been commercialised and
is therefore expected to be more expensive than the routes discussed above. We assessed the decarboxylation route by
assuming that it also proceeds with a quantitative yield and requires two process steps, i.e. esterification of maleic
anhydride to its mono-methyl ester and decarboxylation to methyl acrylate. We find that this route is uncompetitive by
about $1000 t−1 because of the unaffordable raw material cost and slightly disadvantaged conversion cost (Figure 1).
Despite its intrinsic inaccuracy, our approach is apparently capable of identifying uncompetitive processes.
This analysis has so far focused on the manufacture of methyl acrylate. But one may wish to produce acrylic acid or
another acrylate ester (e.g. butyl acrylate). When manufacturing acrylic acid, we need to remove the cost of MeOH for
both routes, remove the esterification step for the propene route and add a hydrolysis step for the maleic anhydride route.
This would lower the cost of the propene route to ca. $950 t−1 and raise that of the maleic anhydride route to ca. $1650 t−1,
which is a notable penalty for the maleic anhydride route. When producing another ester, the relative cost of both routes
would be affected if we need to use an alcohol that is incompatible with the ethenolysis step. In such case, the maleic
anhydride route would come with a modest economic disadvantage of ca. $200 t−1 by requiring an additional
transesterification step, while the propene route would simply switch its esterification step to using an alcohol other than
MeOH. Such $200 t−1 penalty is significant, but it still falls largely within the uncertainty band. A more detailed evaluation
would be needed to reliably label the ethenolysis route as uncompetitive. Alternatively, this penalty would justify further
study to improve the ethenolysis step and make it compatible with the desired alcohol.
Overall, the present methodology does not reveal major differences between those routes to produce methyl acrylate or
any acrylate ester that is compatible with the ethenolysis step. The ethenolysis route shows neither obvious cost
advantage nor economic flaws. It is thereby worthy of further economic analysis, considering experimental yields, precise
energy demands and evaluation of major equipment requirements.
Figure 1. Preliminary economic screening of the production of methyl acrylate from various feedstock (NB: The diagonal
lines represent line of identical total manufacturing cost; the size of the bubbles represents the uncertainties of +/−10%
on raw material and +/−25% on conversion cost).
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Helvetica Chimica Acta
10.1002/hlca.202000035
Conclusions
A screening of a library of Ru metathesis catalysts for the ethenolysis of dimethyl maleate to methyl acrylate yielded
productive turnover numbers approaching 2000 using the second generation Hoveyda-Grubbs-type catalyst, with
selectivity in several cases exceeding 80% and conversion reaching 70%, likely limited by the product-induced catalyst
decomposition. Addition of a polymeric phenol resin was found to stabilize Hoveyda-Grubbs-2 catalyst at low loadings
(
75 ppm), allowing for higher turnover numbers of ca. 5200. Based on these experimental results, a first technoeconomic
analysis was performed that showed no major differences between the ethenolysis route to methyl acrylate compared to
other conceivable routes. The ethenolysis route is therefore poised for further development and optimization, with
prospects of industrial implementation.
Supplementary Material
Supporting information for this article is given via a link at the end of the document.
Acknowledgements
The authors are grateful to the Scientific Equipment Program of ETH Zürich and the SNSF (R’Equip grant
2
06021_150709/1) for financial support of the high throughput catalyst screening facility (HTE@ETH). J.D.J.S. was
supported by the Luxembourg National Research Fund (AFR Individual Ph.D. Grant 12516655).
Author Contribution Statement
J.P.L., A.F. and C.C. designed research; P.E., A.Ts., J.D.J.S. performed catalytic and NMR tests; J.P.L. performed the
technoeconomic analysis; all authors analyzed the data; A.F. and J.P.L. wrote the paper with contributions from all
authors.
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TOC
HTE Screening: Ru catalysts and conditions
TON up to 5200
Methyl acrylate yield up to 69%
O
HG-II catalyst
OMe
OMe
+
2
OMe
O
O
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