Intensified Co-Oligomerization of Propylene Oxide and Carbon

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Intensified Co-Oligomerization of Propylene Oxide and Carbon Dioxide in a Continuous Heat Exchanger Loop Reactor at Elevated Pressures Jens Langanke* and Aurel Wolf Bayer Technology Services GmbH, Leverkusen 51368, Germany S Supporting Information *

However, 3 is unwanted since valuable PO (and CO2) as well as polar carbonate moieties (which are essential for the product properties) are lost by its formation in stoichiometric amounts from the targeted molecular structures. Moreover, typical workup by thin film evaporator (TFE) gets more challenging with (increasing amounts of) high-boiling 3 (TB = 242 °C) in the resulting reaction mixtures. Prior to this work the DMC catalyzed co-oligomerization of PO and CO2 to yield 2 has been studied at Bayer Technology Services’ process development department, preferably, as semibatch g/l reaction in lab scale STRs (stirred tank reactor) with 1 L total volume. In principle, the basic process development and the evaluation of zinc hexacyanocobaltatebased DMC catalysts was performed this way. In such a typical process starter alcohol (i.e., oligomeric polyether polyol) containing dispersed fine powdered DMC catalyst was used as reaction template in the reactor. After a catalyst conditioning sequence7 by inertization and confirmation of catalyst activity by pulse-like addition of PO at elevated temperatures the reaction was started by feeding liquid PO and gaseous CO2 continuously into the stirred starter/DMC mixture (while maintaining constant pressure). Afterwards, the ongoing reaction, containing a highly active catalyst system, was typically performed at T ≥ 100 °C and p ≈ 50 bar. However, as a result of its highly exothermic character in combination with significant CO2 pressures PO feed rates had to be limited to obtain stable reaction conditions, adequate (carbonate) selectivities, and good product qualities (e.g., MW(D)), even in small scale synthesis. Thus, long batch times and typical space-time-yields (STYs) of less than 0.1 kg·L−1·h−1 resulted from this kind of semibatch approach.3 This communication reports on the process development to overcome the given limitations by flow processing in combination with intensification of mass and heat transfer. A continuous heat exchanger loop reactor (as shown in Figure 1) is used to realize the intensified PO/CO2 co-oligomerization on mini-plant scale. The chosen combination of flow reactor and heat exchanger enables outstanding performances in isothermal reaction control while homogenizing the bulk reaction mixtures with PO and pressurized CO2. Thus, this straightforward experimental study directly aims for the continuous synthesis of high quality polyol 2 and the significant improvement of reactor

ABSTRACT: We herein report the development of the continuous co-oligomerization of propylene oxide and carbon dioxide to yield high quality polyether carbonate polyols with oligomeric molecular weights (MWs), narrow and monomodal molecular weight distributions (MWDs), and strict OH end group functionality under intensified reaction conditions. A 0.4 L shell-and-tube heat exchanger loop reactor with static CSE-XR mixers was the flow setup of choice for this novel process. The exothermic statistical oligomerization was performed in bulk while CO 2 pressures of up to 100 bar were applied. A parameter study was performed for this propoxylation process: STYs of up to 1.5 kg·L−1·h−1 were realized and the synthesis of more than 75 kg valuable material was demonstrated on a mini-plant rig.

1. INTRODUCTION The continuous synthesis of high quality and custom-made oligo- or polymeric polyether carbonate polyols with molecular weights (MWs) up to a few thousand g/mol, strict hydroxyl end group functionality, and narrow, monomodal molecular weight distributions (MWDs) is rewarding. Especially since these novel CO2-containing polyols were found to be highly interesting for a broad variety of applications including sustainable materials1 and innovative biomedical products.2 As depicted in Scheme 1, these speciality polyols are obtained by the “living” co-oligomerization of propylene oxide (PO) and CO2, while multifunctional alcohols 1 are employed as chain transfer reagents (so-called starters) to obtain strict OH end group functionality and uniform product distribution. Since a balanced, statistical incorporation of both (poly)ether and carbonate moieties is desired, special double metal cyanide (DMC) complexes are suitable catalysts for this type of oligomerization.3 Especially, Co/Zn-DMCs (for example, Zn3[Co(CN)6]2·xZnCl2·y H2O·z L) were proved to be robust and highly productive industrial catalysts in this context.4 A general drawback of this process is the formation of cyclic propylene carbonate (3) as side product. It is believed that 3 is formed via two independent routes: (a) cycloaddition of PO and CO2 catalyzed by (halide) species from DMC and (b) the so-called “backbiting”, which is a thermodynamic favorable ring closure, generally performed by vicinal carbonate and hydroxyl groups resulting in the unintended cleavage of 3.5 Both mechanisms are not fully elucidated for the synthesis of 2. © XXXX American Chemical Society

Special Issue: Sustainable Chemistry Received: August 19, 2014

A

dx.doi.org/10.1021/op500268r | Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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Scheme 1. Double Metal Cyanide (DMC) Catalyzed Synthesis of Tailored Polyether Carbonate Polyols (2) from Propylene Oxide and CO2 by Employing Alcohol as Chain Transfer Reagent (1) and the Formation of Cyclic Propylene Carbonate (3) as Side Product; in This Study, 2:3 (wt/wt) Ratios of 15.7−45.9 Were Observed

Figure 1. Flowchart of the continuous loop reactor with 0.4 L shell-and-tube heat exchanger including static CSE-XR mixer elements (Fluitec modul)6 as mini-plant setup.

the amount of Co/Zn-DMC catalyst was chosen so that the pure product 2 always contained about 200 ppm remaining catalyst in total (based on estimated PO conversions and carbonate selectivities) to meet internal specifications regarding the Co/Zn content. The density of the reaction mixture was calculated as arithmetic average, while the density of pure CO2 was calculated according to NIST.9 The desired recycle ratio (ν = flowcircuiting/flowin/out) and process pressure were adjusted during the startup procedure as well. Steady State Reaction. After all process parameters were reached and the heat exchanger loop reactor showed a steady behavior, the continuous PO/CO2 co-oligomerization was operated for an additional 8 h. After t/τ = 5, the system was in steady state and constant concentrations, and no toluene from the startup procedure was found. Therefore, samples were withdrawn (cross-checked at integers of t/τ), and the continuously obtained product mixtures were collected. 2.3. Analysis. The reaction mixture was analyzed using 1H NMR and gel permeation chromatography (for details see Supporting Information). Samples were withdrawn at the backpressure valve (Figure 1) using ice chilled vials. 2.4. Downstream Workup. After proper degassing of the obtained reaction mixture a TFE unit was used to remove the side product 3 at 120 °C and 0.1 mbar. In total, more than 75 kg of valuable material 2 was isolated. This CO2-polyol was used for various application tests.

productivity (STY). Besides this, the production of kilogram samples is demonstrated.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were obtained from commercial suppliers and used as received, if not stated otherwise. Propylene oxide (≥99.5%) and toluene (≥99.8%) were obtained from Sigma-Aldrich. Compressed CO 2 (≥99.995%, steel cylinder) was obtained from Linde AG. The employed polyether triol starter A1110N (1, glycerol propoxylate, MW 716 g/mol) was obtained from Bayer MaterialScience AG. The DMC catalyst was prepared following the literature (cf. patent example #6).8 2.2. Typical Procedures. Start-up Process. A typical continuous reaction was started by feeding the starter alcohol A1110N, which contains the dispersed DMC catalyst, PO and liquefied CO2 as shown in Figure 1 into the toluene filled loop reactor at reaction temperature. The toluene solvent template is continuously displaced until the setup only contains the reaction mixture. The feed flows were adjusted in such a way that a target molecular weight of about 3000 g/mol (at full PO conversion) and a residence time (τ) of 30 or 60 min were reached. The amount of CO2 within the loop reactor accounted always for 5% excess to its polyol incorporation (based on retrograde calculation from control experiments). Moreover, B



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3. RESULTS AND DISCUSSION 3.1. Characterization of Flow Regimes. First, the 0.4 L shell-and-tube heat exchanger reactor with static CSE-XR mixer elements (Figure 1) was analyzed to get an idea about the flow and phase conditions in this solvent-free reaction system. Therefore, Reynolds (Re*) as well as Bodenstein (Bo*) numbers were estimated using adapted equations6a for the given case as well as standard operation parameters (see Section 3.2): Re* equals approximately 15 when the shortest residence time (τ) of 30 min and the highest recycle ratio (ν) of 80 were taken into account under otherwise typical conditions (e.g., 100 bar). Thus, even under the most intense flow conditions applied in this study a completely laminar flow regime can be assumed in the main part of this continuous loop reactor setup. Moreover, from the residence time distribution of the used heat exchanger (data provided by the manufacturer Fluitec AG, Switzerland) it was possible to estimate Bo* ≈ 120, which indicates an internal plug flow character with strong axial mixing. At the same time, the loop design of the entire apparatus guarantees for full (back) mixing and, thus, a continuous stirred-tank reactor (CSTR) characteristic regarding τ and substrate/product/catalyst concentrations of this miniplant rig is considered. The mixing performance of the static CSE-XR mixer geometries (for cross sectional mixing) was also quantified in situ. The pressure drop across the heat exchanger, which corresponds to power input and mixing quality, was measured during 100 bar experiments at ν = 20−80 and accounted for 0.5−0.6 bar. Beside these estimations, the phase behavior was investigated by observation under flow reaction conditions. Therefore, a pressure-resistant sight glass was installed behind the heat exchanger. The reaction mixture was analyzed on stream by visual inspection (digital images) at 130 °C, τ = 30 min, and ν = 20. Starting from 100 bar, p was reduced stepwise revealing a bubble-free, slightly hazy, but quasi-homogeneous phase down to 67 bar. By further reducing the total pressure, vigorous gas evolution was observed, while at 65 bar and below a stable biphasic reaction mixture existed (for images see Supporting Information). The g/l separation at p < 67 bar exhibited again the outstanding mixing power of the employed equipment. In the solvent-free and thus slightly viscous reaction medium, submillimeter bubble sizes were observed, while moderate flow conditions (ν = 20) were applied. Since the described flow oligomerization is a complex multicomponent system, which includes PO, CO2, starter 1, and polyols 2 and 3 (and of course a very small fraction of the heterogeneous DMC catalyst), the visual inspection to determine the phase behavior under continuous operation conditions is an easy but effective way to assess the overall situation. Moreover, these reported observations are in good accordance to sophisticated phase equilibrium data, which were previously gained within a related process development project.10 3.2. Oligomerization: Parameter Study. The catalytic oligomerization itself was investigated using this loop setup, which is able to provide full homogenization of reaction mixtures in combination with isothermal reaction control by a high heat transfer surface area per volume ratio. For the first proof-of-principle conti experiments conditions derived from previous semibatch experiments were used as starting points. Nevertheless, the initial catalyst conditioning sequence, which is

performed (in batch) to transform complex DMC-type catalysts into an active working mode had to be integrated into the flow synthesis itself. Thus, in conti mode the essential catalyst conditioning sequence is performed in situ. Therefore, a slightly elevated reaction temperature of 130 °C was employed in the first set of experiments while comparably short τs were selected. In general, circuiting flows with ν in the range of 20 to 80 were applied. The influence of process pressure was analyzed first. The CO2 incorporation in the polyol structure showed an almost linear increase from 22 to 76 bar (Figure 2) and reached a

Figure 2. CO2 incorporation and c/l carbonate ratio (selectivity) as a function of process pressure.

plateau at 76 and 100 bar with 12.1 and 12.5 wt % CO2 in formed polyol, respectively (Table 1, entries 1−7). These results are in very good accordance to the observed phase behavior. Under monophasic conditions (i.e., > 76 bar) the mass transport reaches a maximum. Above this point, an increase in pressure results in marginal effects on CO2 incorporation. At the same time the pressure exhibited no significant influence on PO conversion at the given τ of 30 min. The selectivity in terms of the cyclic (3) to linear (in 2) carbonate ratio (c/l) was hardly affected by pressure. Values close to 0.2 were observed for 32−100 bar. Only at the lowest applied pressure (22 bar) c/l = 0.28 was found. Thus, the content of carbonate moieties within the molecule chain is proportional to the formation of 3, at least in the range of 32 to 100 bar and under the chosen flow conditions. Moreover, the addition of up to 40% excess CO2, which represents a typical amount of CO2 in semibatch experiments did neither improve its incorporation nor did it alter the c/l ratio at otherwise fixed conditions in flow synthesis (no detailed data given). It appears that even a small excess of CO2 led to the formation of cooligomer 2 with the desired amount of carbonate moieties. In addition, very narrow and monomodal MWDs from 1.08 to 1.15 were obtained as a benefit of the relatively high reaction temperature of 130 °C (e.g., Figure 3). DMC-catalyzed propoxylations are “living” type reaction systems,11 and the statistical PO/CO2 co-oligomerization is believed to be of the same type. A diffusion controlled reaction mechanism in which the fast interaction of terminal OH groups on growing oligo chains and active catalyst sites enables a uniform product distribution even though the synthesis is performed under CSTR residence time conditions (i.e., heat exchanger loop reactor) is here at work. In an attempt to further increase the carbonate content of 2 and at the same time diminish the formation of 3, the C



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Table 1. Selected Results from the Parameter Study on the Intensified PO/CO2 Co-Oligomerization in a Continuous Heat Exchanger Loop Reactora #

T (°C)

p (bar)

ν

τ (min)

conversion of PO (%)b

CO2 in formed polyol (wt %)b,c

cycl./linear carbonate ratiob,d

MWDe

1 2 3 4 5 6 7f 8 9f 10 11 12 13 14

130 130 130 130 130 130 130 105 115 130 130 105 105 105

22 32 41 53 62 76 100 100 100 100 100 100 100 100

20 20 20 20 20 20 20 20 20 50 80 20 50 80

30 30 30 30 30 30 30 30 30 30 30 60 60 60

99.8 99.1 100 99.0 99.0 99.0 99.9 ± 0.1 68.9 93.4 ± 0.7 98.4 98.6 92.4 88.2 86.7

4.1 7.5 8.1 9.6 10.3 12.1 12.5 ± 0.3 18.8 18.4 ± 0.4 11.4 16.7 14.3 14.4 14.4

0.28 0.19 0.21 0.21 0.20 0.19 0.22 ± 0.01 0.05 0.09 ± 0.01 0.15 0.14 0.09 0.09 0.08

1.11 1.12 1.08 1.15 1.12 1.13 1.16 ± 0.01 1.64 1.34 ± 0.15 1.12 1.17 1.45 1.27 1.48

a

All data points were obtained at t/τ = 5 and steady state conditions. The concentration of DMC catalyst was approximately 200 ppm in pure 2. bVia H NMR analyses (all mass balances are closed (PO, 2 and 3) within the limits of NMR accuracy). cMass fraction of C(O)O in the formed polyol structure. dRatio of 3 in proportion to −(CH2−CH(CH3)−OC(O)O)− units. eVia GPC analyses. fWith average values ± standard deviation. 1

Figure 4. Influence of recycle ratio (ν) on c/l carbonate ratio for two operation points.

Figure 3. Gel permeation chromatogram (GPC) of typical product (cf. Table 1, entry 3) with Mn = 3326 g/mol and 1.08 MWD. Following the general formula given in Scheme 1 with 1 = polyether triol (with MW 716 g/mol and x = 3), the observed number-average oligo structure of 2 refers to repeating unit m ≈ 7.9 and n ≈ 1.6 (with an MH factor of 112).

h−1 were easily realized under these exothermic flow synthesis conditions, and the product, high quality polyol 2, was obtained on kilogram scale.

temperature was reduced. The other parameters (ν, τ, and p) were kept constant. As expected, the PO conversion dropped form 99.9 ± 0.1% at 130 °C to 93.4 ± 0.7% at 115 °C, and finally, a value as low as 68.9% was observed for a reaction temperature of 105 °C. Although, a remarkable increase in CO2 incorporation (up to 18.8 wt %) and strongly improved c/l ratios (down to 0.05) were observed for process temperatures of 115 °C or even 105 °C (Table 1, entries 7−9). However, the incomplete PO conversion and the relative high MWD up to 1.64 render operation conditions with strongly reduced reaction temperatures and τ = 30 min unattractive. The influence of flow and mixing in terms of the recycle ratio was also investigated (Table 1, entries 7 and 10−14). The resulting c/l selectivities are presented in Figure 4 as a column plot for two distinct operation points. For severe reaction conditions, such as high T and short τ, and hence high PO feed rates, ν has a strong influence on c/l. Mixing and isothermal reaction control is provided by intense (internal) flow and results in reduced c/l (e.g., 0.14 at ν = 80). Under milder reaction conditions the influence of ν on the carbonate selectivity seems negligible. In conclusion, STYs of 1.5 kg·L−1·

4. CONCLUSIONS In summary, we reported on the development of the directed PO/CO2 co-oligomerization to yield high quality polyether carbonate polyols in a continuous manner. This novel process was performed under intensified mass and heat transfer conditions by employing a shell-and-tube heat exchanger with static mixer geometries as continuous loop reactor. Because of the “living” type character of this directed, double metal cyanide catalyzed oligomerization, products with very narrow MWDs were obtained on the kilogram mini-plant scale. Moreover, clearly improved selectivities and CO2 incorporations were achieved by the rational choice of reaction parameters. In total, more than 75 kg of valuable product were synthesized within this study, while STYs of 1.5 kg·L−1·h−1 were easily realized under these exothermic synthesis conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Details about 1H NMR and GPC analysis as well as images of the phase behavior of the reaction mixture under typical D



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synthesis conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. C. Gürtler (Bayer MaterialScience AG) for supporting this project and the fruitful collaboration. We thank Dr. J. Hofmann (Bayer MaterialScience AG) for providing the DMC catalyst and consulting with regard to its handling. Dr. J. Hofmann and Dr. M. Wohak (Bayer MaterialScience AG) are also acknowledged for valuable discussions and reading of the manuscript. Partial funding was provided by the German Federal Ministry of Education and Research (BMBF).

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REFERENCES

(1) (a) Bayer Research 2008, 12, 74−78. (b) Bayer Research 2012, 23, 80−85. (c) Gürtler, C. From Dream Reaction to Dream Production: CO2-based Polyols, UTECH Europe, Exhibition Program, April 17, 2012. (d) Pirkl, H.-G.; Klesczewski, B. G. G. CO2-Based Polyols: A Future New Product Class for the Flexible Foam Market?, UTECH Europe, Exhibition Program, April 17, 2012. (2) (a) Luinstra, G. A. Polym. Rev. 2008, 48, 192−219. (b) Qin, Y.; Wang, X. Biotechnol. J. 2010, 5, 1164−1180. (c) Luinstra, G. A.; Borchardt, E. Adv. Polym. Sci. 2012, 245, 29−48. (3) Langanke, J.; Wolf, A.; Hofmann, J.; Böhm, K.; Subhani, M. A.; Müller, M. A.; Leitner, W.; Gürtler, C. Green Chem. 2014, 16, 1865− 1870. (4) (a) Mleczko, L.; Wolf, A.; Grosse Böwing, A. Polycarbonates. In Applied Homogeneous Catalysis with Organometallic Compounds: a Comprehensive Handbook in Three Volumes; Cornils, B., Herrmann, W. A., Beller, M., Ed.; Wiley-VCH: Berlin, Germany, in press. (b) Langanke, J.; Wolf, A.; Peters, M. Polymers from CO2: An Industrial Perspective. In Carbon Dioxide Utilization: Closing the Carbon Cycle; Styring, P., Quadrelli, A., Amstrong, K., Ed.; Elsevier: New York, in press. (5) (a) Darensbourg, D. J.; Holtkamp, M. W. Coord. Chem. Rev. 1996, 153, 155−174. (b) Luinstra, G. A.; Haas, G. R.; Molnar, F.; Bernhart, V.; Eberhardt, R.; Rieger, B. Chem.Eur. J. 2005, 11, 6298− 6314. (c) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618−6639. (d) Eberhardt, R.; Allmendinger, M.; Rieger, B. Macromol. Rapid Commun. 2003, 24, 194−196. (6) (a) Georg, A.; Däscher, M. B. Chem. Ing. Technol. 2005, 77, 681− 693. (b) Däscher, M. B.; Georg, A. Chem. Tech. 2009, 124−126. (7) Gürtler, C.; Grasser, S.; Hofmann, J.; Wolf, A. WO Patent 2011/ 089120 A1. (8) Hofmann, J.; Ehlers, S.; Klinksiek, B.; Fechtel, T.; Ruhland, M.; Scholz, J.; Foehles, F.; Esser, U. WO Patent 2001/080994 A1. (9) NIST Chemistry WebBook. Thermophysical Properties of Carbon Dioxide. http://webbook.nist.gov/cgi/fluid.cgi?ID= C124389&Action=Page (accessed June 1, 2014). (10) Fonseca, J. M. S.; Dohrn, R.; Wolf, A.; Bachmann, R. Fluid Phase Equilib. 2012, 318, 83−88. (11) (a) Kim, I.; Byun, S. H.; Ha, C.-S. J. Polym. Sci., A: Polym. Chem. 2005, 43, 4393−4404. (b) Pazos, J.; Browne, E. Abstracts of Papers, 235th ACS National Meeting, New Orleans, LA, United States, April 6−10, 2008; POLY-680. (12) The exact Mark−Houwink (MH) GPC correction factor has not been determined here. Therefore, the information about the number average oligo structure of this product (Table 1, entry 3) is estimated with MH = 1. A typical MH factor for 2 is believed to be close to 1. E


Intensified Co-Oligomerization of Propylene Oxide and Carbon

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