Agile Green Process Design for the Intensified KolbeâSchmitt

Article pubs.acs.org/est

Agile Green Process Design for the Intensified Kolbe−Schmitt Synthesis by Accompanying (Simplified) Life Cycle Assessment Sabine Kressirer,† Dana Kralisch,*,‡ Annegret Stark,§ Ulrich Krtschil,∥ and Volker Hessel⊥ †

Institute of Technical Chemistry and Environmental Chemistry, Friedrich-Schiller-University Jena, Lessingstr. 12, D-07743 Jena Institute of Pharmacy, Friedrich-Schiller-University Jena, Otto-Schott-Strasse 41, D-07745 Jena § Institute for Chemical Technology, University of Leipzig, Linnéstr. 3-5, D-04103 Leipzig ∥ Institut für Mikrotechnik Mainz (IMM), Carl-Zeiss-Strasse 18-20, D-55129 Mainz ⊥ Department of Chemistry and Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡

S Supporting Information *

ABSTRACT: In order to investigate the potential for process intensification, various reaction conditions were applied to the Kolbe−Schmitt synthesis starting from resorcinol. Different CO2 precursors such as aqueous potassium hydrogencarbonate, hydrogencarbonate-based ionic liquids, DIMCARB, or sc-CO2, the application of microwave irradiation for fast volumetric heating of the reaction mixture, and the effect of harsh reaction conditions were investigated. The experiments, carried out in conventional batch-wise as well as in continuously operated microstructured reactors, aimed at the development of an environmentally benign process for the preparation of 2,4dihydroxybenzoic acid. To provide decision support toward a green process design, a research-accompanying simplified life cycle assessment (SLCA) was performed throughout the whole investigation. Following this approach, it was found that convective heating methods such as oil bath or electrical heating were more beneficial than the application of microwave irradiation. Furthermore, the consideration of workup procedures was crucial for a holistic view on the environmental burdens.

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in a plant instead of conventional scale up.18 Because of their small dimensions, microreactors offer unique properties for improving reaction performances, e.g., through a high surfaceto-volume ratio, small internal volumes that lead to precisely adaptable reaction times for higher yields, and/or selectivities.19−21 Furthermore, microreactors are usually designed as modular devices that facilitate the exchangeability of the reactors and lead to higher flexibility,22 reduced development times,23,24 and increased process stability due to facile substitution of reaction modules during operation.25 An enhancement of the reaction performance can be achieved by applying alternative forms of energy, e.g., ultrasound26 or microwave irradiation,27 or by implementing novel reaction media. In this respect, ionic liquids (IL)28 as well as supercritical fluids such as supercritical carbon dioxide (scCO2)29 have been beneficially applied in several processes. Furthermore, harsh reaction conditions, e.g., high temperatures, high pressures, or high concentrations, can be handled easier

INTRODUCTION Process intensification (PI) has become an important concept for the development of more sustainable processes.1,2 The principle of PI was first introduced by Ramshaw.3 Stankiewicz and Mouljin4 defined PI as any novel equipment or technique for the significant improvement of production processes or process steps leading not only to smaller plants butmore importantly to lower energy consumption or waste production and, thus, to cheaper and greener processes. Several approaches have been suggested for intensifying processes both in the field of plant design as well as in the reaction performances. In addition to the utilization of multifunctional devices and the combination of process steps,5−7 e.g., reactive distillations or chromatographic reactors,8 PI through innovative plant design can be realized by multiscale processes,9−13 i.e., adaption of channel dimensions to the requirements of the process. Against this background, microreactors became a class of widely recognized reaction devices to be applied beneficially for PI during the last two decades.14−17 Microreactors enable continuously operated processes already on laboratory scale, and large scale production may be realized by numbering up of either the reaction channels or by increasing the number of microreactors © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5362

January 13, 2013 April 8, 2013 April 16, 2013 April 16, 2013 dx.doi.org/10.1021/es400085y | Environ. Sci. Technol. 2013, 47, 5362−5371

Environmental Science & Technology

Article

hydrogencarbonate or hydrogencarbonate-containing ionic liquids, or CO2-amine-adducts (DIMCARB) as reactive media and the application of convective methods for heating such as oil bath and electrical heating as well as the application of microwave irradiation.

and more safely in microstructured reactors than in conventional reaction equipment due to their small inner dimensions.25,30,31 Hence, reactions can be driven in operation regimes limited only by intrinsic kinetics. These concepts were summarized by Hessel et al.2 under the umbrella term “novel process windows” (NPW) with focus on both chemical intensification using high temperatures, pressures, and/or concentrations (also referred to as NPW conditions) and on process design intensification. The latter takes into account process simplification and integration such as new chemical transformations as well as direct and one-step one-flow syntheses and also syntheses in explosive regimes.24 In addition to the plant design, chemical processes are usually developed with respect to optimizing their yield and the costs of the process. In recent years, sustainability came additionally into focus of process design inspired by an increasing awareness of environmental burdens.32−35 The desire to develop environmentally benign chemical processes leads to an increased demand of evaluation methods in order to quantify the sustainability of the respective process. The sustainability of chemical processes can be assessed through a wide spectrum of different methods. Qualitative evaluations can be performed by using the 12 Principles of Green Chemistry36 or by adopting a solvent selection guide.37,38 Simple environmental metrics, e.g., atom economy,39 atom efficiency,40 E-factor,41 energy loss index,42 and productivity loss index34 or process mass index (PMI)43 as well as the material balance environmental indicator (MBEI)44 enable first qualitative assessments and are often used as screening tools. These metrics frequently take into account only the amount of starting materials, solvents, or waste and require a combination of several metrics for a more comprehensive view. Therefore, more complex methods combine mass and energy balances with toxicity factors and consider also environmental aspects, e.g., the EHS method,45 EATOS,46 or the ECO method.47 All methods mentioned so far have been exclusively developed to be applied in early process design and can be performed even if there is only scarce process information available at this stage. Once process development is finished, the method of life cycle assessment (LCA) standardized in ISO 1404048 and 1404449 is regarded to be more adequate for comprehensive evaluation of environmental burdens. This method gives a holistic view on the environmental impacts caused during the whole life cycle of a process or product from cradle to grave or at least from cradle to gate. Because of the high data requirements of detailed information about the process under investigation, this method is often considered to be overambitious in research and development (R&D) because of the inherent data gaps and evaluation uncertainties.50 However, the environmental burdens of a process are defined by the decisions made during R&D.51−53 Thus, a vertically limited LCA,54 which considers all life cycle stages albeit at a low information depth, is recommended as a suitable screening method to avoid the shifting of problems from the reaction to other life cycle stages and is less time and cost consuming than detailed LCAs.28,51,52 In this article, a method for simplified life cycle assessment (SLCA) was applied to the Kolbe−Schmitt synthesis starting from resorcinol with various CO2 precursors already during process development. Hence, the environmental burdens of several approaches for PI have been studied, e.g., the implementation of sc-CO2, aqueous solutions of potassium

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EXPERIMENTAL SECTION Materials. Experimental procedures as well as chemicals used in this study are described in the Supporting Information. All chemicals were used as purchased without further purification. Both commercially available ionic liquids, 1-ethyl3-methylimidazolium hydrogencarbonate, and 1-butyl-3-methylimidazolium hydrogencarbonate ([EMIM][HCO3] and [BMIM][HCO3], respectively, were received from Sigma Aldrich as mixtures of 50 wt % IL, 20 wt % methanol, and 30 wt % water and were used as received. Methods. Evaluation Approach. We decided to apply a vertically limited SLCA method following the desimplification approach proposed by SETAC Europe.55 Hence, all life cycle stages from cradle to gate were taken into account, including upstream processes as well as workup procedures and disposal of process wastes. The cradle-to-gate approach was chosen because only the syntheses of 2,4-DHBA was varied throughout the various process alternatives, whereas all downstream processes following the isolation and purification of the product were regarded to be the same for every process alternative. All experimental results evaluated by SLCA were obtained on laboratory scale. For modeling the processes, the software Umberto 56 was used in combination with the Ecoinvent 57 database, which enables the consideration of upstream processes in accordance to the requirements of ISO 14040 and 14044. The evaluation method proposed by Guinée et al.58 and the cumulative energy demand (CED),59 which represents the energy consumption of the whole life cycle, were considered to account for a broad spectrum of environmental impacts. We started our investigations with the experimental results available during process development and refined our evaluations as the experiments progressed. Furthermore, we performed scenario and sensitivity analyses by varying significant parameters, e.g., temperature and reaction time as well as the reaction medium, to provide decision support for the developing engineers and chemists and to estimate the robustness of the evaluation results obtained. In order to simplify the screening during process development concerning environmental impacts, at first only the impact categories human toxicity potential (HTP)58 and global warming potential (GWP)58 were used to provide efficient decision support. The GWP accounts for the emission of climate-changing substances such as CO2, CH4, or NOx that commonly occur during the combustion of fossil fuels for the generation of energy. Contrary to this energy-related impact category, the HTP considers the emission of substances that are harmful for human health due to their acute or chronic toxicity or due to their potential to cause cancer. Finally, further impact categories such as CED, abiotic resource depletion potential (ADP),58 ozone depletion potential (ODP),58 photochemical ozone creation potential (POCP),58 acidification potential (AP),58 eutrophication potential (EP),58 and land use and terrestrial ecotoxicity potential (TETP)58 were analyzed in order to determine from selected process alternatives the most environmentally benign option. 5363

dx.doi.org/10.1021/es400085y | Environ. Sci. Technol. 2013, 47, 5362−5371


Article

Figure 1. Process conditions applied for the intensification of the Kolbe−Schmitt synthesis.

containing ionic liquid [EMIM][HCO3] and the CO2dimethylamine adduct DIMCARB (N,N-dimethylammonium N, N-dimethylcarbamate).65 Furthermore, the implementation of [EMIM][HCO3] was combined with the application of microwave irradiation as a means for rapid heating of the reaction solution. All experimental details considered for SLCA are summarized in Table 1 in the Supporting Information. Data Background of Batch Reactions Using Reactive solvents. In the experiments using [EMIM][HCO3] combined with the application of microwave irradiation, the molar ratios [HCO3]−:resorcinol (n/n) (5.8:1 to 1.8:1) as well as the temperatures (80−160 °C for a fixed molar ratio [HCO3]−:resorcinol of 2.9:1) were varied (described in more detail in ref 65). The energy consumption for the application of microwave irradiation into the reaction cavity was monitored automatically by the microwave oven SYNTHOS 3000 (Anton Paar). Furthermore, the energy consumption was measured externally for two selected temperatures, and the energy efficiency of the microwave oven was calculated as the ratio of irradiation energy to externally measured energy consumption. This efficiency was used to estimate the energy consumption of all other experiments in which microwave irradiation was applied. Additionally, the consumption of raw materials was extrapolated to the maximum capacity load of eight reaction vessels. As another reactive solvent, DIMCARB at a molar ratio of dimethylamine:CO2 of 1.9:1 was used in an autoclave SPR 16 (Amtec). Reaction times were varied between 31 and 117 min at a constant temperature of 160 °C, and a fixed molar ratio CO2:resorcinol of 3.2:1 was used. The autoclave can hold a maximum of 16 reaction vessels. Thus, the consumption of chemicals was extrapolated again to this maximum capacity. Because the energy required was not measured due to technical hindrances, the theoretical power consumption of 1.2 kW was considered for the estimation of the energy demand. The idea of implementing DIMCARB was the prospect that it can easily be removed from the reaction solution by heating to 70−90 °C where it decomposes to its gaseous components that can be recondensed for further use.

Frequently occurring data gaps, which we had to deal with during the process design, were as follows: (1) The energy consumption for heating of the reaction was not measured for all heating devices due to technical hurdles. (2) Workup procedures were only carried out for some selected experiments. To overcome these data gaps, either the energy consumption measured in similar experiments was adapted or the required energy was estimated by the theoretical power consumption. Furthermore, the workup procedure of the aqueous solutions was adapted to a hypothetical workup of experiments using the reactive solvents, too. More details about these considerations are given in the next section. Data Background. As stated before, the experimental investigations targeted the PI of the Kolbe−Schmitt synthesis starting from resorcinol with solutions containing various CO2 precursors. The desired product of this reaction is the kinetically preferred 2,4-dihydroxybenzoic acid (2,4-DHBA). 2,6-Dihydroxybenzoic acid (2,6-DHBA) is formed as a byproduct especially at long reaction times and high reaction temperatures.53 The target product 2,4-DHBA is industrially used as a dyestuff additive or pharmaceutical intermediate.60 Conventionally, this reaction is carried out in aqueous solutions of sodium or potassium hydrogencarbonate under reflux conditions, i.e., 100 °C and ambient pressure. A high molar excess of sodium or potassium hydrogencarbonate (typically 5:1) and long reaction times (at least 2 h) are required, and only moderate reaction yields (40−60%) are obtained.61,62 To overcome these drawbacks, a broad variety of PI options (Figure 1) have been tested in this cooperation project simultaneously. The experimental investigations at the Institut fü r Mikrotechnik Mainz (IMM) focused on the setup of a continuously operating apparatus using high pressure and high temperature (NPW conditions) that targeted the development of a new reactor design that can easily be applied to pilot scale and finally also to production scale.63,64 In parallel, Stark and co-workers concentrated on the implementation of reactive solvents in batch experiments. These reactive solvents included both the hydrogencarbonate 5364



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Figure 2. GWP of the Kolbe−Schmitt synthesis starting from resorcinol using either [EMIM][HCO3] combined with microwave irradiation (reaction time 60 min, reaction temperature 80−160 °C, molar ratio resorcinol:IL 1.8−5.8:1; left) or DIMCARB in an autoclave (reaction temperature 160 °C, molar ratio resorcinol:DIMCARB 3.2: ; right).

Data Background of Continuously Operating Processes. Simultaneously to the batch experiments, continuously operating processes were investigated using first a capillary microreactor. A conventional oil bath was used for heating of the reaction mixture, and the energy consumption was measured. Various reaction conditions were investigated, such as the reaction temperature, pressure, and reaction medium. Either aqueous solutions of KHCO3 or the aqueous/ methanolic solutions of the ionic liquids [BMIM][HCO3] or [EMIM][HCO3] were chosen as reaction media. In one experiment, sc-CO2 was additionally used with the aqueous solution of KHCO3 (entry 13) to investigate the influence of pressurized CO2 in the reaction system. The molar ratio [HCO3]−:resorcinol was held constant at 3:1. Additionally, a batch synthesis according to the reaction protocol given in a student’s textbook66 was performed to establish an experimental benchmark (entry 10). For even better heat transfer to the reaction mixture, an electrically heated microreactor (THTMD, tube heat transfer micro device) with equally distributed microchannels around a heating cartridge was developed at the IMM.64 In this microreactor, exclusively aqueous solutions of KHCO3 were reacted with resorcinol at a pressure of 70 bar under varying flow rates (0.7−5.4 L/h) and different temperatures (160−220 °C) as a first step to scale up to pilot scale. Workup Considerations. The experimental investigations focused on the development of a reaction protocol. Thus, reaction yields were mainly analyzed directly from the reaction mixture, and the solutions were disposed of without further product isolation or solvent/starting material recycling.

However, because different reaction media were investigated, the downstream processes were regarded to be very important with respect to a green process design. Thus, some investigations were performed addressing suitable procedures for product isolation, too. During LCA, occurring data gaps were filled by the expertise of the practitioners in laboratory. In the case of the application of aqueous KHCO3 solutions, the crude product was precipitated from the collected reaction solution by dropping the reaction mixture into diluted hydrochloric acid. The precipitate was filtered and recrystallized from deionized water.67 Because the product is initially formed as negatively charged carboxylate of the respective cation (i.e., 1-ethyl-3-methylimidazolium 2,4-dihydroxybenzoate or N,Ndimethylammonium 2,4-dihydroxybenzoate), this procedure was also adapted to the hypothetical workup estimations of the processes implementing either [EMIM][HCO3] or DIMCARB to obtain 2,4-DHBA as free acid. Prior to acidification and only in the case of DIMCARB, the reaction mixture was heated to 90 °C to decompose DIMCARB, and dimethylamine and CO2 were condensed in a round-bottomed flask cooled with an ice−salt mixture. Hence, about 95% of DIMCARB can be recovered for reuse.68 CO2 was injected into the mixture when necessary to regain the desired properties (viscosity, aggregate state). For LCA analysis, product isolation from the reaction mixture was assumed by precipitation with concentrated hydrochloric acid in accordance to the procedure applied for aqueous solutions. For the ease of analysis, it was supposed that the desired product 2,4-DHBA was obtained without precipitation of 2,65365



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Figure 3. GWP of the Kolbe−Schmitt synthesis starting from resorcinol in a batch process (100 °C) and a continuously operating capillary microreactor using either aqueous KHCO3 solutions or ionic liquids ([EMIM][HCO3], [BMIM][HCO3], constant molar ratio resorcinol:[HCO3]− of 1: , constant flow rate of 0.2 L/h, oil bath heating).

category GWP. Additionally, the HTP for selected aspects is described in the Supporting Information to give a deeper insight into the environmental aspects. Because the purpose of this study was to investigate and compare various methods of PI to identify the most preferable process conditions in view of green process design, all experiments were scaled to entry 15 as a mutual basis. This link between the investigation of reactive solvents in batch reactors and the continuously operating processes ensures the comparability between the respective figures of this study even if not all scenarios can be presented in a single chart. Results of Batch Experiments Using Reactive Solvents. The performance of the Kolbe−Schmitt synthesis in reactive solvents was investigated in batch experiments using [EMIM][HCO3] or DIMCARB. In the reactions carried out in [EMIM][HCO3], the influence of different reaction temperatures as well as the dependence on the molar ratio of resorcinol to ionic liquid at a constant reaction time of 60 min was investigated (Figure 2, left). The molar ratio considerably influences the environmental impact of the process because the GWP was the highest for a molar ratio resorcinol:[HCO3]− of 5.8:1 (entry 1) although it led to the highest reaction yield of 62%. This is due to the strong impact of the provision of ionic liquid on this category. Thus, the lowest GWP was found for the reaction with the lowest molar ratio of 1.8:1 (entry 5). Further, [EMIM][HCO3] was substituted by DIMCARB as another potential reactive solvent. In these experiments, the reaction times were varied, and the energy was applied by placing the autoclaves into a heating block. The GWP of these

DHBA (if present). Finally, the recycling of DIMCARB was considered by the experimental method already described. The workup procedure for the implementation of [EMIM][HCO3] was not performed in any experiments and was therefore accounted for based on theoretical assumptions. Thus, the precipitation of 2,4-DHBA was presumed by adding concentrated hydrochloric acid to the reaction mixture. This leads to the transformation of any unreacted [EMIM][HCO3] to [EMIM]Cl. The regeneration of the ionic liquid was assumed by an anion exchange between the intermediate [EMIM]Cl and KHCO3 following the patented production procedure.69 For this recycling method, a loss of IL of 5% was assumed for each recycling cycle, which is a recycling rate often found for this type of reaction with ionic liquids.70 However, due to the lack of appropriate life cycle inventory data on anion exchangers necessary for this exchange, the exchanger part of the life cycle contribution was neglected. Although these assumptions are in parts vague due to the little experimental data available, this approach was considered helpful to get a first impression of the influence of workup procedures and recycling of reaction media on the overall life cycle impacts of this Kolbe−Schmitt synthesis. However, further investigations should focus on the development of suitable workup strategies.

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RESULTS AND DISCUSSION In the following sections, the results of the broad screening of parameter changes and process alternatives of the Kolbe− Schmitt synthesis are discussed for the example of the impact 5366



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Figure 4. Environmental screening of selected results to obtain a more holistic view on the overall environmental impacts of the processes (CED, cumulative energy demand; HTP, human toxicity potential; GWP, global warming potential; TETP, terrestrial eco toxicity potential; Low-NOx POCP, photo oxidant creation potential caused by NOx emissions at ground level; ADP, abiotic resource depletion potential; ODP, ozone depletion potential; AP, acidification potential; E, eutrophication potential; 100 a/25 a, average period of residence time of harmful substances in the atmosphere, overlap of aqueous reactions due to similar yields).

material) intensive manufacturing process. In this comparison, no workup procedure or solvent recycling was considered. Thus, the disposal of the excess reactive solvent contributed considerably to the GWP of entries 14−16. Comparison of reactions performed in [EMIM][HCO3] in continuously operating processes (entries 14, 15) to reactions performed in batch mode using microwave irradiation (Figure 2) leads to the conclusion that microwave irradiation does not lead to the expected benefits compared to conventional oil bath heating. This is due to the low energy efficiency of 16−20% derived from an external measurement of the energy consumption of the microwave oven. Thus, the microwave irradiation was abandoned, and convective methods of energy application were utilized in all further experiments. However, convective heating methods required a microheat exchange for reasonable conversions within short residence times, either by small heat transfer dimensions in a microcapillary or by a tailored electrically heated microreactor. Thus, in order to adapt the laboratory scale processes to pilot scale, an electrically heated THTMD microreactor was designed at the IMM.69 It was observed that at any given temperature the reaction yields decreased with increasing volume flows (up to 5.4 L/h, Supporting Information). For example, at a reaction temperature of 220 °C, the reaction yields decreased moderately from 36% to 30% leading to an increase in the GWP of only 20% compared to an increase in GWP of 186% at 180 °C. Hence, using harsher reaction conditions proved to be successful to increase the productivity of the aqueous Kolbe− Schmitt synthesis from laboratory scale to pilot scale. Product flows of up to 0.2 kg 2,4-DHBA/h were obtained by applying high volume flows. Combination of Results for Batchwise and Continuously Operated Processes. In general, reactions in aqueous

reactions was dominated by the energy consumption as shownin the right portion of Figure 2. This was due to the comparatively low yields of 2,4-DHBA in DIMCARB, which decreased further with increasing reaction time on the cost of the formation of 2,6-DHBA as a side product. The lowest GWP was obtained for a reaction time of 31 min at which a yield of 15% 2,4-DHBA was obtained (entry 6), which might compete with the configurations employing [EMIM][HCO3]. However, the highest yield of 15.5% was received at a reaction time of 46 min (entry 7), but the slightly higher yield of 2,4-DHBA was overcompensated by a significantly higher energy consumption. Results Obtained in Continuously Operating Processes. Simultaneously, reactions performed in a continuously operating capillary microreactor were found to considerably reduce the reaction times due to the efficient application of NPW conditions. To establish an internal benchmark for the process development and for quantifying the benefits of the chosen PI concepts, a batch process using aqueous KHCO3 was additionally performed under conventional conditions (entry 10). From the GWP (Figure 3), it can be concluded that the reaction yield influences the environmental performance of the aqueous Kolbe−Schmitt synthesis to a great extent. Because the reaction yields differed only slightly in aqueous media, the GWP of these processes were very similar. Even the additional pressurization with sc-CO2 did not lead to increased reaction yields and, thus, did not bring any benefits. This result is in compliance with the investigations of Dessimoz et al.,71 who found that the partial pressure of CO2 does not influence the kinetics of the Kolbe−Schmitt synthesis in aqueous KHCO3 solutions. Furthermore, it is obvious, that the application of ionic liquids leads to increased reaction yields, but on the other hand, this advantage is overcompensated by their energy (and 5367



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Figure 5. GWP of selected results of the Kolbe−Schmitt synthesis using various reaction conditions in batchwise and continuously operated processes: comparison of processes without consideration of workup procedures (left) to processes with consideration of workup procedures (right).

media were more beneficial than those involving either ionic liquids or DIMCARB as reactive solvents. Because of the formation of the byproduct 2,6-DHBA, DIMCARB can be only used at short reaction times. Furthermore, the molar ratio of [HCO3]−:resorcinol very much influenced the GWP of [EMIM][HCO3]-containing experiments. Here, the smaller the excess of ionic liquid compared to resorcinol, the smaller was the environmental burden of the process because less ionic liquid was needed in the process (entry 5). The reaction temperature seems to have a smaller influence than the choice of media. Once a certain threshold temperature is reached to obtain acceptable reaction yields at a given reaction time, higher temperatures most often led to higher energy consumption without further increase in reaction yields. In their extensive kinetic and thermodynamic study, Dessimoz et al.71 found that the reaction is exothermic, and therefore, reaction temperatures should be kept as low as possible. Additionally, in continuous processes, the residence time revealed a varying influence on the environmental impacts depending on the reaction temperature applied. A comprehensive figure depicting these trends is given in the Supporting Information. Similar trends were observed in other environmental impact categories (Figure 4). Except for AP and ODP, the Kolbe− Schmitt synthesis in DIMCARB (entry 6) showed the highest environmental impacts of all reaction media. This resulted mainly from the low reaction yields obtained in DIMCARB and, thus, from the provision of resorcinol. The ODP as well as the AP was considerably affected by the provision of the ionic liquid. Therefore, entry 15 was chosen as a basis for all comparisons, which showed higher environmental impacts in these two impact categories due to the amount of ionic liquid

implemented. Nevertheless, these impacts can be reduced by the use of less ionic liquid (entry 5). Still, the aqueous Kolbe− Schmitt synthesis protocols crowded in the center of this chart represent the least environmental harmful reaction conditions. Because of similar yields and reaction conditions, the chosen aqueous reaction conditions can hardly be distinguished (compare also Figure 3) and are overlapped by the turquoise line representing the aqueous Kolbe−Schmitt synthesis in a conventional batch reactor (entry 10). The investigations have shown that there is not one best process option that can satisfy both sustainability and productivity issues. From the comparison of experiments carried out in batch mode as well as in continuously operated reactions, no clear advantage of the processing in microstructured reactors can be identified by means of LCA. The advantage of continuously operated syntheses in microstructured reactors may lie in the ease of handling of the applied process conditions and the easier scale-up to pilot/ industrial scale by applying higher volume flows or by increasing the number of microreactors utilized in the process, which leads to considerably reduced development times for the establishment of industrial processes and development costs.23,24 Consideration of Workup Procedures. So far, no product isolation or workup procedures had been considered in the SLCA. However, the consideration of workup procedures as well as the recycling of reactive solvents is essential to establish a holistic environmentally benign process design. Thus, some experiments were carried out to determine suitable product isolation from aqueous reaction solutions and recycling of the reactive solvent DIMCARB. In the case of [EMIM][HCO3], the environmental impact of the product isolation and 5368



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solvent recycling could only be examined in a theoretical way as described above. In the case of the aqueous Kolbe−Schmitt syntheses, an increase in the GWP was observed when the workup (Figure 5) is included into the assessment. This is due to the product loss during the precipitation in diluted HCl (partial solubility of 2,4DHBA). Because the reaction yield using DIMCARB was very low, the product isolation (including the associated substance loss) and recycling of DIMCARB led to an increase in the GWP particularly with regard to the energy consumption during the reaction. The contrary was observed for the ionic liquid. Here, the recycling of [EMIM][HCO3] would lead to a significant decrease in the respective GWP contributions of the ionic liquid in entries 5 and 15. In combination with the high yields obtained in this reaction medium, the overall environmental burden of both processes would become comparable to that of the aqueous Kolbe−Schmitt syntheses when including product isolation. Thus, an efficient workup procedure of the continuously operated reaction in [EMIM][HCO3] with oil bath heating (entry 15) might lead to the most environmentally benign process of all reaction alternatives compared in Figure 5. For this reaction, a reduction of the GWP of about 22% compared to the Kolbe−Schmitt syntheses in aqueous solutions (entries 10, 11, and 29) can be achieved. Although these statements depend highly on the efficiency of the recycling method applied and the assumptions made for LCA analysis, this study clearly demonstrated the influence of various recycling procedures on the overall environmental burdens of different process set-ups. Thus, from an environmental point of view, the choice of the reaction medium and establishment of an efficient recycling procedure for the solvent and non-reacted resorcinol, as well as suitable product isolation with only negligible substance losses, revealed the highest potential for low environmental burdens of the Kolbe−Schmitt synthesis examined in this study. These aspects were found to be more effective for this specific reaction than the transfer to a continuous microreaction process.

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ASSOCIATED CONTENT

Tables, figures, and data described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Phone: +49.3641.9499-51, fax: +49.3641.9499-42, e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

The financial support of the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt, DBU) for the joint project “PIKOS” (DBU-AZ 25715) is gratefully acknowledged. The authors also thank Dorothee Reinhard (IMM), Ronald Trotzki, and Steffen Jupe (both Friedrich-SchillerUniversity Jena) for the technical support as well as Sigma Aldrich for the provision of ionic liquids. 5369



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