CO2 Based Synthesis of Various Formamides in Miniplant Scale: A

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CO2 based synthesis of various formamides in miniplant scale: a two-step process design Rene Kuhlmann, Kai Kuennemann, Laura Hinderink, A. Behr, and Andreas Vorholt ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05477 • Publication Date (Web): 27 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Sustainable Chemistry & Engineering

CO2 based synthesis of various formamides in miniplant scale: a two-step process design Rene Kuhlmanna, Kai U. Künnemanna, Laura Hinderinka, Arno Behra, Andreas J. Vorholta,b* a

Department of Biochemical and Chemical Engineering, Chair of Technical Chemistry,

Technische Universität Dortmund, Emil-Figgestr. 66, 44227 Dortmund, Germany b Max-Planck-Institute

for chemical energy conversion, Department of molecular catalysis,

Stiftstraße 34-36, 45470 Mülheim a.d.R., Germany, [email protected], tel.:+492083063669

Abstract The utilization of carbon dioxide in the synthesis of valuable chemicals has attained high attention in the last decades. Numerous new syntheses were developed by applying innovative catalysts and demonstrated the versatile application possibilities of the thermodynamic stable molecule. However, only few reaction systems were developed into a technical application. In this work, we present investigations of a homogeneous catalyzed reaction system for the synthesis of formamides in a miniplant scale. The applied catalyst complex Ru-Macho was recycled via immobilization in a nonpolar alcohol phase and showed a high stability within the observed period of 234 hours. The formed products were extracted in-situ into an aqueous phase. An average yield of 48% N,N-dimethylformamide (DMF) proved a good activity of the reaction system. An alteration of the reaction designed into a two-step process allowed an extension of the product range to yield a broad variation of formamides with high yields up to 89%. Keywords: CO2 conversion – miniplant- recycling – hydrogenation- homogenous catalysisprocess deisgn. Introduction Carbon dioxide is labeled as a dangerous, climate threatening greenhouse gas.1,2 Nevertheless, it can also be applied as a versatile C1-building block , which has been demonstrated in a broad variety of research publications in the last decades.3–10 However, only a few new synthesis routes utilizing carbon dioxide as a carbon source attract industrial attentions, such as the production of polyether polycarbonate polyols (PPPs), the chemical precursor for polyurethanes,11 and the synthesis of renewable methanol from carbon dioxide and hydrogen.12,13 A current limitation for carbon dioxide utilized as a raw material is the high activation energy requirement compared to other carbon source.

14,15

Thus, only well1

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developed reaction systems with a sophisticated process concept can compete with established industrial processes in economical as well as ecological matters.16 A promising carbon dioxide application in industrial is the utilization as C1 surrogate for toxic components such as carbon monoxide or phosgene.17 For example, the production of formamides by hydrogenation and amidation of CO2, in industry this is still based on carbon monoxide,18,19 even though alternative syntheses routes with carbon dioxide have been discovered and developed since 1970 by employing molecular catalysis and supercritical fluids.20–23 New Ruthenium based catalyst systems were investigated and showed high activities with TONs up to 1.940.000.

24,25

Besides, the non-noble metals like iron and cobalt

system also showed high activities in laboratory scale.

26–28

However, a suitable concept for

the transition of the investigated homogenous catalysts into a process is not elaborated. We recently described the key effects on the carbon dioxide based synthesis of N,Ndimethylformamide (DMF) catalyzed by the Ruthenium Macho complex (Figure 2). The catalyst was recycled via a water/ethylhexanol two-phase system, in which the catalyst was immobilized in the non-polar phase. (Figure 1).29

Figure 1: Concept of the biphasic reaction system for the synthesis of formamides from CO2

These laboratory investigations revealed that a defined ratio of carbon dioxide to amine in the range of 0.6-0.8:1 showed a significant impact on the basicity of the aqueous phase as well as on the catalytic performance. An equimolar dosage of carbon dioxide resulted in an increase of the yield from 34% up to 81% at 40 bar and 140 °C after 5 hour reaction. At the same time, the applied homogeneous ruthenium catalyst Ru-Macho (Figure 2) showed a high stability and can be recovered up to 99% by immobilization in the nonpolar alcohol 2-ethylhexan-1-ol.

Figure 2: Structure of the precursor Ru-Macho

In this work, the observed effects in previous batch experiments will be examined in a continuously operated miniplant process. The target of these investigations is to identify the 2 ACS Paragon Plus Environment

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fundamental limits to this concept. Challenges are the exact control of ratio of amine and carbon dioxide and the effect on the catalyst performance in a continuously operated system. Furthermore, a synthesis of further formamides was investigated with this process setup in order to show possibilities and limitations of this concept. Finally, a process concept for a versatile synthesis of different formamides is presented based on a two phasic hydrogenation step and a reactive distillation. Experimental section Miniplant setup Long-time investigations were performed in a miniplant that consisted of four main units and is displayed in Figure 3. At first, the solution containing all the substrates and catalyst was conveyed into a continuously stirred tank reactor (R1). There, the biphasic reaction mixture was heated to reaction temperature and intensively stirred so that the reaction could proceed. After the desired residence time, the mixture automatically flowed through an overflow line to the liquid/liquid phase separator (B1). This separator was kept at 30 °C so that the separation could take place under constant conditions. Afterwards, the two separated phases were depressurized and degassed in the liquid/gas phase separators B2 and B3, separately. The product solution was analyzed via an online-GC, whereas the catalyst solution was pumped back into the reactor.

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Figure 3: Flow scheme of the applied miniplant for the conversion of carbon dioxide to formamides

Target values Additional target values were calculated for a better comparison of the reaction results. First, the conversion of an amine is defined as shown in equation (1) for batch and continuous flow investigations. Amine that formed a salt adduct with formate is not available for another salt formation. Xamine[%] =

nformate + nformamide namine,feed

× 100 (continuous flow) or

nformate + nformamide namine,feed

× 100 (batch)

(1)

The productivity of the process can also be described with the space-time-yield (STY) which is defined in equation (2). STYi

[L ×g h] = Vm

i

(2)

liquid

A comparison of the catalytic productivity (eq. 3) and activity (eq. 4) is made by calculating the turnover-number (TON) and the turnover frequency (TOF). TONi [ - ] =

ni

(3)

nruthenium


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TOFi

[1h] =

ni

(4)

nruthenium ∗ 𝑡

Results and discussion First investigations targeted the feasibility of the DMF synthesis in a continuously operated miniplant from CO2 (Scheme 1). O NH

CO2

H2

N

H

H 2O

Scheme 1: Reaction equation of the formamide synthesis from CO2

Furthermore, an evaluation of determined parameters should clarify the impact on the process stability and thus reveal limits. Finally, a transfer to other amines as substrates will show the range of applications. Key parameters in the continuously operated DMF synthesis The formation of the intermediate formate and the subsequent condensation to the desired formamides occur simultaneously in the reactor (R1, Figure 3). Thus, suitable conditions have to be applied for both reaction steps. In order to have comparable catalytic measurements, we kept the initial mass ratio of the polar and the non-polar phase (mDMA-solution:m2-ethylhexan-1-ol), the stirrer speed (rstirrer) and the applied amount of ruthenium precursor (nRu) constant. The average residence time in the reactor (R) was calculated by the liquid volume inside the reactor (Vreactor,liquid) and the volume flows of feed and recycle streams (Vfeed, Vrecycle). Initial investigations were carried out to test the influence of the CO2 to amine ratio since this parameter have showed significant impact on the productivity in batch experiments that an optimized ratio of CO2:dimethylamine of 1.18:1 increased the yield of DMF up to 81% compared to 34% without a defined CO2 addition. 29 The influence was measured by applying different ratios of both components. The mass flow of amine was kept constant while the amount of added carbon dioxide was varied. The results are displayed in Figure 4.



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Figure 4: Impact of the gas dosage ratio on the DMF synthesis in a miniplant scale Reaction conditions: Precursor Ru-Macho, nRu = 0.8 mmol, mDMA-solution:m2-ethylhexan-1-ol = 1:1, Vreactor = 2000 ml, Vreactor,liquid = 310 ml, rstirrer = 750 1/min, Treactor = 140 °C, Tseparator(B1) = 30 °C, ptotal = 40 bar, R = 4 h, Vfeed = Vrecycle, Vfeed = 39.2 ml/h

After a start-up time of approximately 12 hours, the miniplant showed a stable operating behavior with a slight increase in the DMF yield up to 45%. The intermediate formate was significantly present with an average yield of 22%. A decrease of the CO2 ratio to 0.6:1 resulted in a shortage of carbon dioxide whereas a higher ratio of 2.2:1 indicated the beginning of a carbon dioxide saturation since the condensation speed to DMF decreased. Previous investigations with an oversaturation of carbon dioxide (dosing ratio CO2:amine > 5:1) only led to a DMF yield of 14%. This effect is caused by several equilibrium reactions of carbonic acid and bicarbonate with the amine in the aqueous solution. A higher carbon dioxide ratio increases the amount of captured amine in these equilibrium reactions (Scheme 2). Overall, this screening demonstrated that a dosing ratio of 0.9 – 1.6 equivalents CO2 per mole to amine is favorable for a high conversion rate.

Scheme 2: equilibrium reactions of carbon dioxide with amines in aqueous solutions

Besides, the solvent system of water and 2-ethylhexanol was proved to be a very stable and suitable system for the catalyst recycling. Only 0.03 wt.%/h of Ru-Macho were lost in average into the aqueous product solution. Since the reaction system was operating steadily, further 6 ACS Paragon Plus Environment

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investigations into the stability of the process were conducted with a ratio of CO2:amine of 1.1:1 (Figure 5).

Figure 5: Stability investigations of the reaction system in miniplant scale Reaction conditions: Precursor Ru-Macho, nRu = 0.8 mmol, mDMA-solution:m2-ethylhexan-1-ol = 1:1, Vreactor = 2000 ml, Vreactor,liquid = 310 ml, rstirrer = 750 1/min, Treactor = 140 °C, Tseparator(B1) = 30 °C, ptotal = 40 bar, R = 4 h, Vrecycle,1 = 39.2 ml/h, Vfeed,1 = 39.2 ml/h, Vrecycle,2 = 51.6 ml/h, Vfeed,2 = 25.8 ml/h, Vrecycle,3 = 25.8 ml/h, Vfeed,3 = 51.6 ml/h

In order to test the pressure influence, gas supply was stopped after operation time of 143 hours. As shown in Figure 5, the yield of intermediate formate decreased immediately due to the gas shortage. While the influence of pressure drop on the formation of DMC reflected slowly, which occurred 12 hours later. The reason for this mismatch can be explained by the buffering effect of the intermediate that still can be converted to DMF. In contrast to DMF, the formate adduct is nearly insoluble in the organic phase so that changes in productivity are immediately measured in the product stream. Then the gas supply was restarted to desired pressure at 160 h reaction time. The reactivation of the gas stream resulted in a fast regeneration of the reaction system, which reached the similar results as observed before the perturbation. After 10 hours (170 h total reaction time) the formate yield reached the previous level indicating that the catalyst was not affected by this period gas shortage. Then different mass ratios of catalytic phase to aqueous phase in the reactor were investigated. By increasing the mass ratio of non-polar catalytic phase to aqueous phase from 1:1 to 2:1, the yield of DMF raised from 38% to 47%. However, in the case of a 1:2 ratio of non-polar phase to aqueous phase, the yield of DMF dropped a bit from the initial value 47% 7 ACS Paragon Plus Environment


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to 42%. Whereas, increasing the mass of aqueous amine solution has great improvement on the overall space time yield (from 23 to 40 g*L-1*h-1). A further improvement can be achieved by reducing the overall residence time in the reactor, however, this also reduced the average yield in the product stream (see supporting info for details). Nevertheless, the system worked without any complications for over 230 hours even with harsh pertubations. Some characteristic values are summarized in Table 1. Table 1: Results of the miniplant investigations

Time [h]

Changed parameter

YDMF [%]

STYDMF TONDMF TOFDMF [-] [h-1] [g/(L*h)]

41

CO2:amine = 1.6:1

43

33

6108

169

112

CO2:amine = 0.9:1

44

33

17879

171

137

CO2:amine = 2.2:1

41

30

21945

155

180

CO2:amine = 1.1:1 after gas shortage

38

29

29123

146

208

Ratio organic:aqueous = 2:1

47

23

31871

120

234

Ratio organic:aqueous = 1:2

42

40

36119

206

Transfer to other formamides The applied concept for the CO2 conversion to DMF showed promising results since the catalyst system and recycle system was outstanding stable. Further studies focus on the transfer of this reaction system to yield different types of formamides. Different amines (primary, secondary, functionalized and aromatic) were chosen in order to evaluate the limitations of the current concept, which are summarized in Table 2.

Table 2: The properties of different amines for further studies

entry amine

solubility in water

boiling point

[g/l]

[°C]

characteristic

pKB


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1

H 2N N H

2 3 4

H 2N

OH NH2

primary

miscible

78

3,36

secondary

30

105

3,09

functionalized

miscible

172

4,50

aromatic

36

184

9,40

The properties of targeted amines are quite different. The boiling point is a key factor for the isolation via distillation. Besides, the basicity and functionality should have a high impact on the catalytic conversion. First investigations were performed in batch experiments, and the results are displayed in Figure 6.

Figure 6: Results of batch experiments with different amines as substrates. Left: conversion-time plot; right: product distribution. Reaction conditions: Precursor Ru-Macho, cRu = 1 mmol/l, mamine-solution:m2-ethylhexan-1-ol = 1:1, 40 wt.% amine in water, Vreactor = 300 ml, Vreactor,liquid = 75 ml, rstirrer = 750 1/min, Treactor = 140 °C, Tseparation = 30 °C, ptotal = 40 bar

Compared to dimethylamine, all other amines were converted considerably slower which matches the literature known observations.21 The highest yield of 22% could be obtained with N-n-butylformamide after 5 hours. Whereas only traces of formanilide could be observed, which might be due to the non-basic properties of aromatic amine. Furthermore, the applied solvent system gave poor extraction degrees for the nonpolar formamides limiting this concept for polar amines only. Two-step process concept for the formamide synthesis The poor conversion rates and low extraction selectivities with other amines led to the development of a new process concept. Distillation experiments for the DMF isolation revealed that left over formate adducts condensed in the thermal separation process.30 This observation 9 ACS Paragon Plus Environment


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would allow a two-step process where the carbon dioxide could be hydrogenated to a formate adduct in the first reaction step under high pressure followed by the second reaction step the reactive distillation process in which the formamide is the highest boiling compound. This concept is displayed in Figure 7.

Figure 7: Concept of the two-step process

In order to prove the feasibility of this new concept, the formate synthesis (step 1) has to be investigated at first. Numerous reaction systems are described in literatures, and here an efficient formate synthesis was chosen so that the key parameters could be adopted. The ternary amine triethylamine (TEA) was applied as a base in order to investigate the hydrogenation reaction solely. According to the literature, temperature is the one of the key parameters for the formatter synthesis that the exothermic reaction was faster at temperatures of 80 °C.29 Hence, the initial experiments were carried out at 80 °C. In order to prove the feasibility of the first step (Figure 7), miniplant investigations were performed. As the general design of the catalyst separation process did not change, so that the miniplant setup keep the same. The results for the first step are displayed in Figure 8.


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Figure 8: Synthesis of triethylammonium formate in a miniplant scale with different residence times () Reaction conditions: Precursor Ru-Macho, nRu = 0.8 mmol, wTEA = 40 wt.% in water mTEA-solution:m2-ethylhexan-1-ol = 1:1, Vreactor = 2000 ml, Vreactor,liquid = 310 ml, rstirrer = 750 1/min, Treactor = 80 °C, Tseparator(B1) = 30 °C, ptotal = 40 bar, nCO2:nTEA = 1:1, Vfeed = Vrecycle, Vfeed = 39.2 ml/h,  = residence time in reactor (R1)

Firstly, different residence times were investigated. As shown in Figure 8, a formate equilibrium could be achieved quite fast with a yield of approximately 60% in the biphasic mixture. Residence times up to 3 h lead to minor improvements up to yields of 65%. The high viscosity of the product solution lead to fluctuation in the online GC measurements so that an average value was determined with the help of HPLC measurements. Noteworthy, with a residence time of 30 minutes the miniplant was at its production capacity limit. The ruthenium leaching was measured to equal 0.05 wt.%/h. This was slightly higher than in the DMF synthesis but still in a very low range. A statistical experiment design was performed for temperature and pressure screenings. The results are summarized in Figure 9.



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Figure 9: Results of pressure and temperature screening for the formate synthesis Reaction conditions: Precursor Ru-Macho, nRu = 0.8 mmol, mTEA-solution:m2-ethylhexan-1-ol = 1:1, Vreactor = 2000 ml, Vreactor,liquid = 310 ml, rstirrer = 750 1/min, Tseparator(B1) = 30 °C, R = 2 h, nCO2:nTEA = 1:1, Vfeed = Vrecycle, Vfeed = 39.2 ml/h

The pressure had a significantly higher impact on the equilibrium yield than the temperature. Even at 65 °C, reasonable amounts of formate were synthesized. The highest yield of formate (70%) was generated at 45.8 bar and 95 °C, which demonstrated a good feasibility of the first reaction step in this miniplant scale. Noteworthy, formic acid could be produced with this reaction setup, if an amine was applied, that would allow the separation of formate by destillation.31 Investigations on the condensation step (step 2, Figure 7) to yield formamides were conducted in batch distillation experiments. At first, water was distilled from the product solution leading to the formation of a high boiling azeotrope of formic acid and triethylamine with a ratio of 5:2 mol/mol.32 This formate adduct acts as an ionic liquid since it is a viscous and is composed of ionic compounds.33 The addition of desired amine in an equimolar ratio to formic acid led to an instantaneous base exchange, which released heat and triethylamine. At the same time, the formation of a second or third phase could be observed. Only aniline did not perform an initial base exchange due to its low basicity. Then the catalyst-free aqueous phase mixtures was heated to 140 °C, where the formamide as a high boiling condensation product will be isolated and stay in the reactor. An exemplary concentration-time plot of the distillation process of N-butylformamide is displayed in Figure 10.


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Figure 10: Concentration-time plot of the reactive distillation (batch) of n-butylamine with triethylammonium formate to N-butylformamide (bottom composition) Reaction conditions: nformate:nn-butylamine = 1:1, Theater = 140 °C, p = 1 atm

The formation of N-butylformamide occurred smoothly and resulted in a high product purity over 95 wt.%. Even though, n-butylamine is very volatile, most of the head product consisted of triethylamine and water. This indicated that the primary amine is well bound to the formate. All other amines proceeded similar except that the formation of formanilide required a longer distillation time due to its reduced nucleophilicity. Since no other byproducts could be observed, the condensation process has very high selectivity. The results of all investigated amines are summarized in Figure 11.

Figure 11: Results of the condensation step for the formamide synthesis

Since all individual reaction steps were successfully investigated and optimized, a new process concept for the synthesis of formamides was developed (Figure 12). 13 ACS Paragon Plus Environment


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Figure 12: Process concept for the carbon dioxide based synthesis of formamides

Conclusion The reaction system with Ru-Macho as catalyst complex in a biphasic solvent system could be successfully transferred into a miniplant scale. The application of an equimolar carbon dioxide to amine ratio resulted in highe conversion rates and yields of DMF up to 47%. The catalyst complex behaved very stable over 230 hours with minor ruthenium losses of approximately 0.03 %/h in the Miniplant experiment. Other amines were investigated, however limited to their high molecular weight the formate intermediates cannot condensate efficiently to the desired products, especially for the non-basic aromatic amines. A two-step process was developed to improve the condensation and following product separation, which are the hydrogenation of carbon dioxide to formate in the miniplant reactor and the subsequent condensation of the amine adduct in the distillation purification process. The new generation of formamide synthesis process resulted in a fast formate formation with a yield up to 70% followed by a selective condensation to the desired formamides nearly independent of the amine source.

Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Supporting information   

additional data on miniplant runs distillation data analytical data 14 ACS Paragon Plus Environment

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

16 pages, 16 figures, 2 tables

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Table of Contents graphic

Select your amine- this contribution shows the feasibility of a continuous operated process to convert different amines with CO2 to various formamides.


CO2 Based Synthesis of Various Formamides in Miniplant Scale: A

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