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Supercritical extraction of lignin oxidation products in a microfluidic device
Chemical Engineering Science 99 (2013) 177–183
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Supercritical extraction of lignin oxidation products in a microfluidic device Nora Assmann, Holger Werhan, Agnieszka Ładosz, Philipp Rudolf von Rohr n Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland
H I G H L I G H T S
Extraction of lignin oxidation products from the reaction mixture with scCO2. Microfluidic device for continuous supercritical extraction at high pressures. Process for separation and purification of monomeric aromatic chemicals from lignin. Distribution ratios of the main monomers at different pressures and temperatures.
art ic l e i nf o
a b s t r a c t
Article history: Received 27 November 2012 Received in revised form 25 March 2013 Accepted 15 May 2013 Available online 4 June 2013
Lignin represents a promising renewable source of chemicals. Valuable aromatic monomers, such as vanillin and methyl vanillate, can be obtained through its acidic oxidation and subsequently have to be separated and purified for further usage. Extraction directly from the aqueous reaction mixture using supercritical carbon dioxide as solvent was evaluated as a possible separation step. Fast screening of different extraction conditions up to pressures of 121 bar at temperatures from 39.8 to 59.3 1C was enabled using a continuous microfluidic device. Distribution ratios of the main five monomeric products were calculated from concentration measurements. With increasing pressure and decreasing temperature higher quantities of monomers were extracted, thereby decreasing selectivity. Overall, selectivity towards specific monomers was high, especially at conditions close to the critical point of supercritical carbon dioxide, confirming the potential of this green and cheap purification method. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Extraction Lignin Microfluidic device Selectivity Supercritical fluid Vanillin
1. Introduction The search for renewable, non-edible, resources for production of energy and chemicals has generated great interest in the exploitation of lignocellulosic biomass. Two of its main constituents, being cellulose and hemicellulose, can be converted to ethanol in biorefineries, which are expected to considerably grow in the near future. However, this will lead to an excess of the third main constituent, lignin, which is already available in abundance as a side product from the pulping process, and will additionally accrue in large amounts from the lignocellulosic ethanol process (Pandey and Kim, 2011). Lignin is an amorphous and highly branched polymer of phenylpropane units, which can account for up to 40% of the dry biomass weight (Pandey and Kim, 2011). Due to its aromatic structure, lignin offers a great potential to be used as a raw material for fine chemicals production, if it could be broken into n
Corresponding author. Tel.: +41 44 632 24 88; fax: +41 44 632 13 25. E-mail address: [email protected] (P. Rudolf von Rohr). URL: http://www.ipe.ethz.ch/laboratories/ltr.
0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.05.032
single aromatic units. Despite this fact most of it is currently burned for heat and power generation. Only 2% of the lignin available from the pulp and paper industry is commercially used, mostly as polymeric mixtures in low value applications (Zakzeski et al., 2010). Researchers have therefore investigated its depolymerization into low molecular weight products (Pandey and Kim, 2011; Zakzeski et al., 2010). The acidic oxidation of lignin yields high-value monomeric chemicals, mainly vanillin and methyl vanillate, but also polymeric, higher molecular weight products (Werhan et al., 2011). The resulting product mixture has to be separated and purified into valuable products to utilize its full potential and consequently provide an economic, yet environmentally friendly feedstock for chemicals. Liquid–liquid extraction with an organic solvent can recover the reaction products from the aqueous reaction mixture and a following organic solvent nanofiltration step has been demonstrated to separate low molecular weight from high molecular weight products (Werhan et al., 2012). Extraction using supercritical carbon dioxide (scCO2) represents another promising separation step to obtain valuable ligninderived monomeric products. Several patents have demonstrated
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the successful purification of vanillin from the aqueous lignin oxidation product mixture using supercritical carbon dioxide (Coenen and Konrad, 1990; Klemola and Tuovinen, 1989). Compared to organic solvents, the extraction using supercritical fluids offers several advantages. Especially scCO2 is nontoxic, environmentally friendly, and cheap, with rather moderate critical conditions (Tc ¼ 31 1C, pc ¼73.8 bar). Its solvent capacity can be modified by varying pressure and temperature, so that the solute can be easily separated after the extraction by simply reducing pressure. Until now it has been mostly applied in the food and pharmaceutical industry, where it represents a cheap and green alternative solvent to extract a wide range of compounds, like aromas, vitamins and oils. For fast screening of reaction conditions, continuously operated microfluidic devices open new possibilities compared to batchwise operation. Many parameters can be varied in short times and only small amounts of reagent are consumed. Elevated pressures as in supercritical extraction and temperatures can be realized with limited risk due to the small volumes involved. Recently, the acidic oxidation of kraft lignin was studied in a two-phase flow microreactor, enabling fast heating, high temperatures (up to 250 1C) and pressures (up to 100 bar). A microextractor for supercritical extractions has been introduced and demonstrated by our group (Assmann et al., 2012). The silicon/glass device enables the continuous supercritical extraction in microchannels of 300 μm depth and width. The liquid feed and scCO2 are contacted using slug flow and subsequently separated on-chip by capillary forces. This principle has now been applied to investigate the supercritical extraction with CO2 as a possible purification step for lignin oxidation products. Distribution ratios of the main five monomeric products: vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxyvanillate (M-5CV) and methyl dehydroabietate (MDHA) were investigated at pressures ranging from 81 to 121 bar and temperatures from 39.8 to 59.3 1C. The resulting selectivities were used to evaluate the applicability of supercritical extraction as a purification step in the valorization of lignin.
2. Experimental 2.1. Silicon/glass microextractor The microfluidic device was fabricated using standard photolithography, dry etching, and anodic bonding techniques as detailed in (Trachsel et al., 2008). The manufacturing from silicon and glass enables optical access to monitor the flow pattern and resistance towards high pressures and corrosive media. The bottleneck for applications at high pressures and temperatures is the macro-to-micro interface (Fredrickson and Fan, 2004). Coplanar interconnects are known to offer the best high pressure endurance. For this reason fused silica tubing (ID 180 μm, OD 360 μm, Polymicro, Phoenix, AZ, USA) was inserted into the microchannels at one side of the reactor and fixed with Duralco 4703 high temperature glue (Polytec PT, Waldbronn, Germany). Pressures up to 130 bar were realized using these connections. The channel design was adapted from a previous study on the supercritical extraction of vanillin (Assmann et al., 2012) and is depicted in Fig. 1. The two phases were introduced through separate inlets and brought to contact in the microfluidic device. The inlet design promoted slug flow, as the continuous liquid phase flowed through the main channel, whereas the dispersed scCO2 phase was injected against the flow direction through an inlet channel, featuring a high fluidic resistance. Compared to other micro flow patterns, e. g. parallel flow, slug flow is known to enhance mass
Fig. 1. Design of the silicon/glass microextractor with capillary separator.
Fig. 2. Slug flow of reaction mixture and scCO2 slugs in the rectangular microchannel.
transfer by internal recirculation within the liquid segments (Assmann and Rudolf von Rohr, 2011; Fries et al., 2008). Thus, very short contact times of some seconds suffice to reach equilibrium. A stereomicroscope image of slug flow inside the microchannels is depicted in Fig. 2. Mass transfer from the liquid feed to the scCO2 phase was conducted in the main channel of 336 mm length, 302 μm depth and 300 μm width, after which both phases were separated. Capillary forces were exploited for separation, due to the dominance of surface forces over volume forces, which rendered separation by gravity impossible. The design of the capillary separator had to fulfill two conditions to separate both fluids: firstly, the pressure difference between the two outlets had to be smaller than the capillary pressure difference and secondly, the fluidic resistance of the main channel outlet (scCO2 outlet) should be much higher than that of the raffinate outlet. Thereby, the liquid phase entered the capillaries due to the smaller fluidic resistance, whereas the supercritical phase was detained from entering the capillaries by capillary forces. This principle has first been introduced by Guenther et al. for gas liquid separation (Guenther et al., 2005). The presented capillary separator consists of 50 capillaries of 15 μm width and 302 μm depth and enabled on-chip separation of the two liquids, so that at least one of the phases was obtained in pure form for subsequent analysis. 2.2. Experimental setup For conveying and sampling of the fluids, the microextractor was integrated into a high pressure setup, adapted from previous experiments (Assmann et al., 2012) and depicted in Fig. 3. High pressure syringe pumps (CO2: 260D, Teledyne Isco, Lincoln, NE, USA; Liquid feed: PHD 4400, Harvard Apparatus, Holliston, MS, USA with 8 ml stainless steel syringe) delivered both fluids in liquid state at constant flow rates. The CO2 pump was water cooled to 18.2 1C to obtain a defined density and thus
N. Assmann et al. / Chemical Engineering Science 99 (2013) 177–183
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Sample Loop
N2
Ethanol Ethanol
Back Pressure Regulator CO2
Liquid feed
RaffinateWaste
CO2 + Solutes
Microextractor Fig. 3. Scheme of the high pressure setup, into which the microextractor was integrated.
mass flow. The microextractor was contained within a heated oil bath to generate a constant temperature during the extraction. Due to the large surface-to-volume ratio of the microchannels, heat transfer was virtually instantaneous and no preheating of the liquids was necessary. Two high pressure needle valves (P-470, Upchurch scientific, Oak Harbor, WA, USA) behind each outlet were used to adjust pressure drop to fulfill the working conditions of the capillary separator. Observation of the flow pattern and separation were possible, as the microextractor was partly fabricated from glass and pictures were taken with a stereo microscope (Stemi 2000-C, Carl Zeiss, Oberkochen, Germany). Both outlets of the microextractor led to separate sampling cylinders, which were pressurized with nitrogen. The inflowing liquids displaced the nitrogen which then exited through an automated back pressure regulator (BP-1580-81, JASCO, Tokyo, Japan), maintaining a constant pressure. Connections within the setup consisted of capillary tubing and leak tight connections from stainless steel (1/16" OD, Swagelok) and polyetheretherketone (PEEK, 1/16" OD, Upchurch scientific). A high pressure switching valve with a 2 ml sample loop (RH7040 with RH-7027, Rheodyne, Rohnert Park, CA, USA) was installed directly behind the main channel outlet of the extractor. During the experiment the CO2-rich phase flowed through the loop to the sampling cylinder. To sample a defined volume of the CO2rich phase, the valve was switched and the content of the loop was isolated. Simultaneously, the main channel outlet of the microextractor was directly connected to the sampling cylinder and the experiment could continue without interruption. For analysis, the content of the sample loop was bubbled through 1 ml of ethanol to collect the precipitating solutes when CO2 evaporated. The cylinder was consequently flushed with 1 ml of ethanol to dissolve the remaining solutes, so that the content of the 2 ml loop was finally dissolved in an equal amount of ethanol for further analysis. To guarantee a pure supercritical phase in the sample loop, the complete liquid phase had to enter the capillary separator. With the current device complete separation of both phases was not possible and impurities in the range of 5% had to be expected according to previous experiments with capillary separators (Assmann and Rudolf von Rohr, 2011). By means of the high pressure needle valves, separation could be tuned to achieve purity of either the supercritical or the liquid phase. To collect pure scCO2 phase for analysis, portions of it were tolerated to pass
through the capillary separator together with the liquid phase. This way the complete liquid phase entered the capillary separator and contamination of the sample was prevented.
2.3. Reagents and analysis CO2 (PanGas, 499.995%) was supplied in liquid state from a gas bottle with dip tube. Kraft lignin (Indulin AT, kraft pine lignin, 97% lignin content (dry), 5% moisture, Mead Westvaco, Charleston, USA) was oxidized at 170 1C for 20 min in a 400 ml high-pressure autoclave (Premex Reactor AG, Lengnau, Switzerland). The reaction solvent consisted of an acidic mixture (pH 1) of 80 vol-% methanol and 20 vol-% water. Molecular oxygen served as oxidant and copper chloride was used as homogeneous catalyst. Details of the reaction have been described by Werhan et al. (Werhan et al., 2011). The reaction products were dissolved in methanol and water and filtered (Simplepure Ny 0.45 μm, membrane solutions, Plano, TX, USA) before use to avoid clogging of the microchannels. The structure and molecular weight of the main monomeric products that are regarded in this study are listed in Table 1. After extraction, the solutes from 2 ml of the CO2 rich phase were transferred to an equal amount of ethanol. 0.1 ml of a 0.56 gL−1 solution of Syringaldehyde (98%, Sigma Aldrich, Buchs, Switzerland) was added to 1 ml of ethanol probe as internal standard, using volumetric pipettes. For analysis, splitless injection of 1 μl of the solution into a GC/MS (Trace GC Ultra with Restek RTX-5MS capillary column (30 m 0.25 mm 0.25 mm) and ITQ 900 ion trap (EI), Thermo Scientific, Waltham, MA, USA) using helium as carrier gas was performed. The column was initially kept at 80 1C for 5 min, and then ramped at 10 1C/min to 270 1C, where it was kept constant for two more minutes. The aqueous feed samples had to be prepared by extraction before injection into the GC/MS. 0.5 ml of water were added to 1 ml feed samples and lignin oxidation products were extracted in four steps using 2 ml of chloroform (99.8%, Sigma Aldrich, Buchs, Switzerland) in each step. A preceding experimental study gave evidence to assume complete extraction of all products. The extracts were combined and internal standard was added before injection, analogously to the procedure for ethanol samples. This way, concentrations could be calculated from a calibration.
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Table 1 Structure and molecular weight of the main five monomeric reaction products of the catalytic oxidation of lignin.
Vanillin (V) C8H8O3 152 g/mol
Methyl vanillate (MV) C9H10O4 182 g/mol
5-Carbomethoxy-vanillin (5CV) C10H10O5 210 g/mol
In equilibrium, the ratio of the concentration of a solute in the solvent phase csolv to its concentration in the aqueous phase caq equals the distribution ratio D. D ¼ csolv =caq
ð1Þ
This value depends on temperature and, especially in the case of supercritical solvents, also on pressure. If several solutes are present, their distribution coefficients influence each other. The different distribution ratios of various solutes can be utilized for separation. The selectivity αi/j equals the distribution ratio of one solute i divided by the distribution ratio of another solute j and can be used to evaluate the ability to separate two components with a given pair of solvents. αi=j ¼ Di =Dj
ð2Þ
For a useful extraction process, the selectivity has to exceed unity, significantly. A desired purity of a product can be achieved with fewer extraction stages, the higher the selectivity. To calculate D, concentrations in the raffinate phase had to be known. As they were not directly accessible, they were derived from mass balance considerations. The mass balance of a solute i in the extraction process can be written as qfeed ci feed ¼ qraf ci raf þ qsc ci sc
ð3Þ
where q are the volumetric flow rates of the feed (feed), raffinate (raf) and supercritical (sc) solvent phase, and c the concentrations of the solute i in the respective in- or outflowing streams. As CO2 partly dissolved in the liquid phase and also dissolved methanol from the liquid feed, three phase equilibrium data for the mixture of methanol, water and CO2 had to be considered to calculate the volumetric flow rates of the outflowing streams. Experimental data from Yoon et al. (1993) at 40 1C and 100 bar were used to calculate the molar composition of the two phases. Pressure dependence was found to be negligible and temperature dependence very small in the regarded range. For this reason we chose to use these experimental data for the whole experimental series, rather than calculated values, as there is still a lack of accurate models for the supercritical range. Ideal behavior of the mixture was assumed for calculation of the volumetric flow rates. As the real volume of the raffinate phase was smaller than the added volumes of scCO2, water and methanol, the real distribution ratios in the microextractor might be slightly above the ones presented here.
3. Results and discussion 3.1. Residence time On the microscale mass transport is known to be very fast, due to the small diffusion length and large interfacial area. Extraction processes can reach equilibrium within some milliseconds, when
Methyl 5-Carbomethoxy-vanillate (MDHA) C11H12O6 240 g/mol
Methyl dehydroabietate C21H30O2 314 g/mol
Table 2 Concentration of the main monomeric products in the feed for both batches. Component Concentration 1st batch [g/L] Concentration 2nd batch [g/L]
V
MV
5CV
M-5CV
MDHA
0.37 0.22
0.38 0.34
0.15 0.13
0.15 0.13
0.26 0.22
performed in microchannels. To evaluate if equilibrium was reached in the presented microextractor, experiments at different residence times were performed by varying the overall flow rate. The flow rate ratio of CO2 to lignin product mixture was set to 2.4 to obtain stable slug flow. Pressure and temperature were kept constant at 121 bar and 39.8 1C for all experiments. The total flow rate was varied from 51 μl/min to 255 μl/min to achieve residence times of 31 to 6 s, respectively. All flow rates were measured at the pumps. Flow rates in the microfluidic device were higher, due to volume expansion owing to the increased temperature causing transition of the CO2 from liquid to supercritical state. The initial concentration of the main five monomeric products in the feed, being vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxy-vanillate (M-5CV) and methyl dehydroabietate (MDHA), was determined from quantification by GCMS. Two different batches were used for the experimental study and their concentrations of the main monomeric products are presented in Table 2. For the first series of experiments with varying residence times the first batch was used. The extracted mass of the main monomeric reaction products relative to their amount in the feed is plotted versus residence time in Fig. 4. All experiments were repeated twice and the average values of all experiments are plotted together with the single data points as a reference. No significant influence of the residence time was determined within the tested residence time range. Differences were in the same range as deviances of experiments at the same residence time, and did not show a trend. Consequently, concentrations of the solutes in both phases at the outlet could be assumed to be in thermodynamic equilibrium even for the shortest residence time tested. Shorter residence times could not be achieved with the current setup, as the maximum flow rate was limited by the capillary separator. On the other hand, the measurement error increased for lower flow rates, thus limiting the realization of longer residence times. The lowest flow rates applied were 36 μl/ min CO2 and 15 μl/min lignin oxidation products. Fluctuations of the pumps at these low values lead to irregularities in the flow pattern, affecting the overall reproducibility. Therefore, flow rates were fixed to 120 μl/min CO2 and 50 μl/min lignin oxidation products for the following study, yielding a residence time of 9.2 s at 121 bar and 39.8 1C.
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Fig. 4. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different residence times for p ¼121 bar and T ¼ 39.8 1C.
Fig. 5. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different pressures for a temperature of 39.8 1C.
3.2. Mass balance considerations
weight monomers were extracted in larger amounts than the ones with lower molecular weight. Vanillin was hardly extracted by scCO2, but also displayed slightly better solubility at higher pressures. The molecule with the highest molecular weight, MDHA, was extracted almost completely even at lower pressures. A slight decay at higher pressures might also result from measurement errors and lies within the standard deviation for MDHA. As scCO2 is a non-polar solvent, molecules are extracted better from the polar aqueous feed, the less polar they are. From the molecular structure (Table 1), it becomes clear that the aldehyde group in vanillin and 5CV provides a higher polarity compared to the carbomethoxyl group of methyl vanillate and M-5CV, respectively (Christen 5. Auflage (1982)). 5CV and M-5CV were extracted better with scCO2, suggesting that the additional carbomethoxyl group reduces their polarity. MDHA is a comparably non-polar molecule, featuring few functional groups, matching the fact that almost the complete amount of MDHA was extracted from the polar aqueous feed into scCO2. The distribution ratios of the products were calculated according to Eq. (1) based on the measured concentrations. As almost no MDHA remained in the raffinate phase, the denominator of the ratio was very small, yielding very high distribution ratios for MDHA, but also large errors. Therefore, the distribution ratios of all products, except for MDHA, are plotted in Fig. 6. The distribution ratios displayed the same behavior as the extracted amount, plotted in Fig. 5 and increased with increasing pressure. However, significant differences were present, indicating the possibility to separate the five main monomeric products by supercritical extraction. To quantify the ability to separate the components using extraction with scCO2, the selectivity α was calculated according to Eq. (2). The higher α, the better the components can be separated from each other. The highest α was found for separation of MDHA from any other component. MDHA was almost completely extracted, whereas the other components remained in the raffinate, especially at lower pressures just above the critical pressure. As the distribution ratios of MDHA had a considerable deviation, the values for α were also afflicted with errors. Consequently, the exact values are not shown, ranging from 10 to 250, expressing the possibility to obtain purified MDHA. The selectivity of the other main monomeric products relative to each other at different pressures is presented in Table 3. In general, vanillin was hardly extracted by scCO2, so it could be concentrated in the raffinate phase. Significantly more 5CV and M5CV were extracted than methyl vanillate and vanillin, so these
In a previous study it was demonstrated that the measured concentrations of both phases yielded a complete mass balance for the extraction of vanillin from its aqueous solution with scCO2 using the presented microextractor (Assmann et al., 2012). In order to verify the mass balance of the current study, the raffinate phase was sampled and evaluated for one experiment at 121 bar and 39.8 1C. No scCO2 must enter the capillaries of the separator to achieve a pure raffinate phase in the respective sampling cylinder. Thus, separation was tuned accordingly by means of the high pressure needle valves. After the experiment the system was depressurized and samples could be taken directly from the sampling cylinder. Probes were extracted with chloroform as described for the feed samples before analysis. The average difference to a complete mass balance of all main five monomeric products was less than 5%. Thus, only the supercritical phase was sampled and evaluated in the current study, as only one pure phase could be obtained in each experiment and direct analysis of the raffinate samples was not possible. Concentrations in the raffinate phase were calculated from mass balance considerations as described above. 3.3. Pressure dependence To determine its effect pressure was varied between 66 and 121 bar at a constant temperature of 39.8 1C, using the reaction mixture from the first batch (Table 2). The extracted amount of the main monomeric products relative to their mass in the feed is plotted versus pressure in Fig. 5. All experiments were performed twice, and deviations were below 10% for all products but vanillin. As vanillin concentrations in the extract were close to zero, deviations were as high as 31%, but low in absolute numbers. One control experiment was performed below the critical point of CO2 (pc ¼ 73.8 bar) at 66 bar. No reaction products could be determined in the extract phase, as they do not solve in gaseous CO2. As a conclusion, no solutes were entrained by the CO2 rich phase, which confirms that the measured amounts of monomeric reaction products at higher pressures were solved in scCO2. Above the critical point, the main five monomeric reaction products displayed significant differences as regards their extractability with CO2. In general, solubility in supercritical solvents increases with increasing pressure (Brunner, 1994), which was particularly pronounced for methyl vanillate, 5CV and M-5CV close to the critical pressure. The higher molecular
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Table 3 selectivity α of the main monomeric products. p
α
MV
5CV
M-5CV
81 bar
V MV 5CV V MV 5CV V MV 5CV V MV 5CV V MV 5CV
∞ – – 8.64 – – 6.96 – – 5.22 – – 4.23 – –
∞ 4.02 – 30.72 3.56 – 25.00 3.59 – 17.19 3.29 – 11.62 2.75 –
∞ 7.48 1.86 53.24 6.16 1.73 44.40 6.38 1.78 30.38 5.82 1.77 21.04 4.98 1.81
91 bar
101 bar
111 bar
121 bar
Fig. 6. Distribution ratios of the main monomeric reaction products except MDHA at different pressures for a temperature of 39.8 1C.
component groups could also be separated using supercritical extraction. The lowest α-values of 1.73–1.86 were obtained for separation of 5CV from M-5CV. In general, the highest selectivity was obtained at the lowest pressure above the critical value. As the solubility of all four components was rather low at 81 bar, large amounts of scCO2 would be required for separation. A high volume ratio of scCO2 to feed does not necessarily render the process uneconomical, as CO2 is available in abundance and easily recyclable. 3.4. Temperature dependence The influence of temperature was studied by varying it between 39.8 and 59.3 1C at a constant pressure of 121.1 bar using the reaction mixture of the second batch (Table 2). The extracted weight percentage of the main monomeric products is plotted versus temperature in Fig. 7. Again, the extracted amount of MDHA was around 100% and afflicted with a large error. With rising temperature, the extracted amount of all other products decreased. Especially the amounts of extracted methyl vanillate, 5CV and M-5CV decreased significantly, whereas vanillin was hardly extracted even at 40 1C. The distribution ratios for this experimental series were not calculated, as the equilibrium composition of the two phases is temperature dependent and no experimental data was available for the used conditions. Also, the same effect of an increased temperature can be reached at lower pressures, so that processing at higher temperatures would not be the first choice, especially for temperature sensitive components.
4. Conclusion The supercritical extraction of aromatic products from lignin oxidation was evaluated using a continuous microfluidic device. The microextractor enabled fast screening of different extraction conditions. As it also contained a capillary separator for phase separation, the supercritical phase and the raffinate could be sampled after a well defined residence time. The extracted amounts of the main five monomeric products, namely vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxy-vanillate (M-5CV) and methyl dehydroabietate (MDHA) at pressures from 81 to 121 bar and temperatures from 39.8 to 59.3 1C were measured. Equilibrium concentrations were reached in less than 10 s and only small amounts of the involved chemicals were consumed. It was found that MDHA was almost
Fig. 7. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different temperatures for a pressure of 121 bar.
completely extracted, whereas virtually the entire amount of vanillin remained in the aqueous phase. Generally, the distribution ratio of the lower molecular weight molecules was smaller, which can be linked to their higher polarity. With increasing pressure and decreasing temperature, the distribution ratios of all monomers increased, except for MDHA, which was almost entirely extracted, independently of pressure and temperature. Consequently, selectivity was higher for lower pressures, just above the critical pressure of CO2. The different selectivity of scCO2 towards these products renders separation possible and enables the use of supercritical extraction to remove MDHA and concentrate vanillin and methyl vanillate in the raffinate phase. As a conclusion, supercritical extraction with CO2 can be applied as a purification step in the valorization of lignin. Among other unit operations, like vacuum distillation and crystallization, a multistage process with different pressures and temperatures can yield high purities of single monomeric products. The current study provides important data to realize this economic and green process to exploit lignin as a renewable feedstock for chemicals.
References Assmann, N., Kaiser, S., von Rohr, P.R., 2012. Supercritical extraction of vanillin in a microfluidic device. J. Supercritical Fluids 67, 149–154.
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Assmann, N., Rudolf von Rohr, P., 2011. Extraction in microreactors: intensification by adding an inert gas phase. Chem. Eng. Process. 50, 822–827. Brunner, G., 1994. Gas extraction: an introduction to fundamentals of supercritical fluids and the application to separation processes. Steinkopff, Darmstadt. Springer, New York. Christen, H.R., 1982. Grundlagen der organischen Chemie, 5. Auflage, Otto Salle Verlag GmbH & Co, Frankfurt am Main. Coenen, H., Konrad, R., 1990. Process for the extraction of vanillin. Patent number 4898990. Fredrickson, C.K., Fan, Z.H., 2004. Macro-to-micro interfaces for microfluidic devices. Lab Chip. 4, 526–533. Fries, D.M., Voitl, T., Rudolf von Rohr, P., 2008. Liquid extraction of vanillin in rectangular microreactors. Chem. Eng. Technol. 31, 1182–1187. Guenther, A., Jhunjhunwala, M., Thalmann, M., Schmidt, M.A., Jensen, K.F., 2005. Micromixing of miscible liquids in segmented gas–liquid flow. Langmuir 21, 1547–1555. Klemola, A., Tuovinen, J., 1989. Method for the production of vanillin. Patent number 4847422.
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Supercritical extraction of lignin oxidation products in a microfluidic device Nora Assmann, Holger Werhan, Agnieszka Ładosz, Philipp Rudolf von Rohr n Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland
H I G H L I G H T S
Extraction of lignin oxidation products from the reaction mixture with scCO2. Microfluidic device for continuous supercritical extraction at high pressures. Process for separation and purification of monomeric aromatic chemicals from lignin. Distribution ratios of the main monomers at different pressures and temperatures.
art ic l e i nf o
a b s t r a c t
Article history: Received 27 November 2012 Received in revised form 25 March 2013 Accepted 15 May 2013 Available online 4 June 2013
Lignin represents a promising renewable source of chemicals. Valuable aromatic monomers, such as vanillin and methyl vanillate, can be obtained through its acidic oxidation and subsequently have to be separated and purified for further usage. Extraction directly from the aqueous reaction mixture using supercritical carbon dioxide as solvent was evaluated as a possible separation step. Fast screening of different extraction conditions up to pressures of 121 bar at temperatures from 39.8 to 59.3 1C was enabled using a continuous microfluidic device. Distribution ratios of the main five monomeric products were calculated from concentration measurements. With increasing pressure and decreasing temperature higher quantities of monomers were extracted, thereby decreasing selectivity. Overall, selectivity towards specific monomers was high, especially at conditions close to the critical point of supercritical carbon dioxide, confirming the potential of this green and cheap purification method. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Extraction Lignin Microfluidic device Selectivity Supercritical fluid Vanillin
1. Introduction The search for renewable, non-edible, resources for production of energy and chemicals has generated great interest in the exploitation of lignocellulosic biomass. Two of its main constituents, being cellulose and hemicellulose, can be converted to ethanol in biorefineries, which are expected to considerably grow in the near future. However, this will lead to an excess of the third main constituent, lignin, which is already available in abundance as a side product from the pulping process, and will additionally accrue in large amounts from the lignocellulosic ethanol process (Pandey and Kim, 2011). Lignin is an amorphous and highly branched polymer of phenylpropane units, which can account for up to 40% of the dry biomass weight (Pandey and Kim, 2011). Due to its aromatic structure, lignin offers a great potential to be used as a raw material for fine chemicals production, if it could be broken into n
Corresponding author. Tel.: +41 44 632 24 88; fax: +41 44 632 13 25. E-mail address: [email protected] (P. Rudolf von Rohr). URL: http://www.ipe.ethz.ch/laboratories/ltr.
0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.05.032
single aromatic units. Despite this fact most of it is currently burned for heat and power generation. Only 2% of the lignin available from the pulp and paper industry is commercially used, mostly as polymeric mixtures in low value applications (Zakzeski et al., 2010). Researchers have therefore investigated its depolymerization into low molecular weight products (Pandey and Kim, 2011; Zakzeski et al., 2010). The acidic oxidation of lignin yields high-value monomeric chemicals, mainly vanillin and methyl vanillate, but also polymeric, higher molecular weight products (Werhan et al., 2011). The resulting product mixture has to be separated and purified into valuable products to utilize its full potential and consequently provide an economic, yet environmentally friendly feedstock for chemicals. Liquid–liquid extraction with an organic solvent can recover the reaction products from the aqueous reaction mixture and a following organic solvent nanofiltration step has been demonstrated to separate low molecular weight from high molecular weight products (Werhan et al., 2012). Extraction using supercritical carbon dioxide (scCO2) represents another promising separation step to obtain valuable ligninderived monomeric products. Several patents have demonstrated
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the successful purification of vanillin from the aqueous lignin oxidation product mixture using supercritical carbon dioxide (Coenen and Konrad, 1990; Klemola and Tuovinen, 1989). Compared to organic solvents, the extraction using supercritical fluids offers several advantages. Especially scCO2 is nontoxic, environmentally friendly, and cheap, with rather moderate critical conditions (Tc ¼ 31 1C, pc ¼73.8 bar). Its solvent capacity can be modified by varying pressure and temperature, so that the solute can be easily separated after the extraction by simply reducing pressure. Until now it has been mostly applied in the food and pharmaceutical industry, where it represents a cheap and green alternative solvent to extract a wide range of compounds, like aromas, vitamins and oils. For fast screening of reaction conditions, continuously operated microfluidic devices open new possibilities compared to batchwise operation. Many parameters can be varied in short times and only small amounts of reagent are consumed. Elevated pressures as in supercritical extraction and temperatures can be realized with limited risk due to the small volumes involved. Recently, the acidic oxidation of kraft lignin was studied in a two-phase flow microreactor, enabling fast heating, high temperatures (up to 250 1C) and pressures (up to 100 bar). A microextractor for supercritical extractions has been introduced and demonstrated by our group (Assmann et al., 2012). The silicon/glass device enables the continuous supercritical extraction in microchannels of 300 μm depth and width. The liquid feed and scCO2 are contacted using slug flow and subsequently separated on-chip by capillary forces. This principle has now been applied to investigate the supercritical extraction with CO2 as a possible purification step for lignin oxidation products. Distribution ratios of the main five monomeric products: vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxyvanillate (M-5CV) and methyl dehydroabietate (MDHA) were investigated at pressures ranging from 81 to 121 bar and temperatures from 39.8 to 59.3 1C. The resulting selectivities were used to evaluate the applicability of supercritical extraction as a purification step in the valorization of lignin.
2. Experimental 2.1. Silicon/glass microextractor The microfluidic device was fabricated using standard photolithography, dry etching, and anodic bonding techniques as detailed in (Trachsel et al., 2008). The manufacturing from silicon and glass enables optical access to monitor the flow pattern and resistance towards high pressures and corrosive media. The bottleneck for applications at high pressures and temperatures is the macro-to-micro interface (Fredrickson and Fan, 2004). Coplanar interconnects are known to offer the best high pressure endurance. For this reason fused silica tubing (ID 180 μm, OD 360 μm, Polymicro, Phoenix, AZ, USA) was inserted into the microchannels at one side of the reactor and fixed with Duralco 4703 high temperature glue (Polytec PT, Waldbronn, Germany). Pressures up to 130 bar were realized using these connections. The channel design was adapted from a previous study on the supercritical extraction of vanillin (Assmann et al., 2012) and is depicted in Fig. 1. The two phases were introduced through separate inlets and brought to contact in the microfluidic device. The inlet design promoted slug flow, as the continuous liquid phase flowed through the main channel, whereas the dispersed scCO2 phase was injected against the flow direction through an inlet channel, featuring a high fluidic resistance. Compared to other micro flow patterns, e. g. parallel flow, slug flow is known to enhance mass
Fig. 1. Design of the silicon/glass microextractor with capillary separator.
Fig. 2. Slug flow of reaction mixture and scCO2 slugs in the rectangular microchannel.
transfer by internal recirculation within the liquid segments (Assmann and Rudolf von Rohr, 2011; Fries et al., 2008). Thus, very short contact times of some seconds suffice to reach equilibrium. A stereomicroscope image of slug flow inside the microchannels is depicted in Fig. 2. Mass transfer from the liquid feed to the scCO2 phase was conducted in the main channel of 336 mm length, 302 μm depth and 300 μm width, after which both phases were separated. Capillary forces were exploited for separation, due to the dominance of surface forces over volume forces, which rendered separation by gravity impossible. The design of the capillary separator had to fulfill two conditions to separate both fluids: firstly, the pressure difference between the two outlets had to be smaller than the capillary pressure difference and secondly, the fluidic resistance of the main channel outlet (scCO2 outlet) should be much higher than that of the raffinate outlet. Thereby, the liquid phase entered the capillaries due to the smaller fluidic resistance, whereas the supercritical phase was detained from entering the capillaries by capillary forces. This principle has first been introduced by Guenther et al. for gas liquid separation (Guenther et al., 2005). The presented capillary separator consists of 50 capillaries of 15 μm width and 302 μm depth and enabled on-chip separation of the two liquids, so that at least one of the phases was obtained in pure form for subsequent analysis. 2.2. Experimental setup For conveying and sampling of the fluids, the microextractor was integrated into a high pressure setup, adapted from previous experiments (Assmann et al., 2012) and depicted in Fig. 3. High pressure syringe pumps (CO2: 260D, Teledyne Isco, Lincoln, NE, USA; Liquid feed: PHD 4400, Harvard Apparatus, Holliston, MS, USA with 8 ml stainless steel syringe) delivered both fluids in liquid state at constant flow rates. The CO2 pump was water cooled to 18.2 1C to obtain a defined density and thus
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Sample Loop
N2
Ethanol Ethanol
Back Pressure Regulator CO2
Liquid feed
RaffinateWaste
CO2 + Solutes
Microextractor Fig. 3. Scheme of the high pressure setup, into which the microextractor was integrated.
mass flow. The microextractor was contained within a heated oil bath to generate a constant temperature during the extraction. Due to the large surface-to-volume ratio of the microchannels, heat transfer was virtually instantaneous and no preheating of the liquids was necessary. Two high pressure needle valves (P-470, Upchurch scientific, Oak Harbor, WA, USA) behind each outlet were used to adjust pressure drop to fulfill the working conditions of the capillary separator. Observation of the flow pattern and separation were possible, as the microextractor was partly fabricated from glass and pictures were taken with a stereo microscope (Stemi 2000-C, Carl Zeiss, Oberkochen, Germany). Both outlets of the microextractor led to separate sampling cylinders, which were pressurized with nitrogen. The inflowing liquids displaced the nitrogen which then exited through an automated back pressure regulator (BP-1580-81, JASCO, Tokyo, Japan), maintaining a constant pressure. Connections within the setup consisted of capillary tubing and leak tight connections from stainless steel (1/16" OD, Swagelok) and polyetheretherketone (PEEK, 1/16" OD, Upchurch scientific). A high pressure switching valve with a 2 ml sample loop (RH7040 with RH-7027, Rheodyne, Rohnert Park, CA, USA) was installed directly behind the main channel outlet of the extractor. During the experiment the CO2-rich phase flowed through the loop to the sampling cylinder. To sample a defined volume of the CO2rich phase, the valve was switched and the content of the loop was isolated. Simultaneously, the main channel outlet of the microextractor was directly connected to the sampling cylinder and the experiment could continue without interruption. For analysis, the content of the sample loop was bubbled through 1 ml of ethanol to collect the precipitating solutes when CO2 evaporated. The cylinder was consequently flushed with 1 ml of ethanol to dissolve the remaining solutes, so that the content of the 2 ml loop was finally dissolved in an equal amount of ethanol for further analysis. To guarantee a pure supercritical phase in the sample loop, the complete liquid phase had to enter the capillary separator. With the current device complete separation of both phases was not possible and impurities in the range of 5% had to be expected according to previous experiments with capillary separators (Assmann and Rudolf von Rohr, 2011). By means of the high pressure needle valves, separation could be tuned to achieve purity of either the supercritical or the liquid phase. To collect pure scCO2 phase for analysis, portions of it were tolerated to pass
through the capillary separator together with the liquid phase. This way the complete liquid phase entered the capillary separator and contamination of the sample was prevented.
2.3. Reagents and analysis CO2 (PanGas, 499.995%) was supplied in liquid state from a gas bottle with dip tube. Kraft lignin (Indulin AT, kraft pine lignin, 97% lignin content (dry), 5% moisture, Mead Westvaco, Charleston, USA) was oxidized at 170 1C for 20 min in a 400 ml high-pressure autoclave (Premex Reactor AG, Lengnau, Switzerland). The reaction solvent consisted of an acidic mixture (pH 1) of 80 vol-% methanol and 20 vol-% water. Molecular oxygen served as oxidant and copper chloride was used as homogeneous catalyst. Details of the reaction have been described by Werhan et al. (Werhan et al., 2011). The reaction products were dissolved in methanol and water and filtered (Simplepure Ny 0.45 μm, membrane solutions, Plano, TX, USA) before use to avoid clogging of the microchannels. The structure and molecular weight of the main monomeric products that are regarded in this study are listed in Table 1. After extraction, the solutes from 2 ml of the CO2 rich phase were transferred to an equal amount of ethanol. 0.1 ml of a 0.56 gL−1 solution of Syringaldehyde (98%, Sigma Aldrich, Buchs, Switzerland) was added to 1 ml of ethanol probe as internal standard, using volumetric pipettes. For analysis, splitless injection of 1 μl of the solution into a GC/MS (Trace GC Ultra with Restek RTX-5MS capillary column (30 m 0.25 mm 0.25 mm) and ITQ 900 ion trap (EI), Thermo Scientific, Waltham, MA, USA) using helium as carrier gas was performed. The column was initially kept at 80 1C for 5 min, and then ramped at 10 1C/min to 270 1C, where it was kept constant for two more minutes. The aqueous feed samples had to be prepared by extraction before injection into the GC/MS. 0.5 ml of water were added to 1 ml feed samples and lignin oxidation products were extracted in four steps using 2 ml of chloroform (99.8%, Sigma Aldrich, Buchs, Switzerland) in each step. A preceding experimental study gave evidence to assume complete extraction of all products. The extracts were combined and internal standard was added before injection, analogously to the procedure for ethanol samples. This way, concentrations could be calculated from a calibration.
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Table 1 Structure and molecular weight of the main five monomeric reaction products of the catalytic oxidation of lignin.
Vanillin (V) C8H8O3 152 g/mol
Methyl vanillate (MV) C9H10O4 182 g/mol
5-Carbomethoxy-vanillin (5CV) C10H10O5 210 g/mol
In equilibrium, the ratio of the concentration of a solute in the solvent phase csolv to its concentration in the aqueous phase caq equals the distribution ratio D. D ¼ csolv =caq
ð1Þ
This value depends on temperature and, especially in the case of supercritical solvents, also on pressure. If several solutes are present, their distribution coefficients influence each other. The different distribution ratios of various solutes can be utilized for separation. The selectivity αi/j equals the distribution ratio of one solute i divided by the distribution ratio of another solute j and can be used to evaluate the ability to separate two components with a given pair of solvents. αi=j ¼ Di =Dj
ð2Þ
For a useful extraction process, the selectivity has to exceed unity, significantly. A desired purity of a product can be achieved with fewer extraction stages, the higher the selectivity. To calculate D, concentrations in the raffinate phase had to be known. As they were not directly accessible, they were derived from mass balance considerations. The mass balance of a solute i in the extraction process can be written as qfeed ci feed ¼ qraf ci raf þ qsc ci sc
ð3Þ
where q are the volumetric flow rates of the feed (feed), raffinate (raf) and supercritical (sc) solvent phase, and c the concentrations of the solute i in the respective in- or outflowing streams. As CO2 partly dissolved in the liquid phase and also dissolved methanol from the liquid feed, three phase equilibrium data for the mixture of methanol, water and CO2 had to be considered to calculate the volumetric flow rates of the outflowing streams. Experimental data from Yoon et al. (1993) at 40 1C and 100 bar were used to calculate the molar composition of the two phases. Pressure dependence was found to be negligible and temperature dependence very small in the regarded range. For this reason we chose to use these experimental data for the whole experimental series, rather than calculated values, as there is still a lack of accurate models for the supercritical range. Ideal behavior of the mixture was assumed for calculation of the volumetric flow rates. As the real volume of the raffinate phase was smaller than the added volumes of scCO2, water and methanol, the real distribution ratios in the microextractor might be slightly above the ones presented here.
3. Results and discussion 3.1. Residence time On the microscale mass transport is known to be very fast, due to the small diffusion length and large interfacial area. Extraction processes can reach equilibrium within some milliseconds, when
Methyl 5-Carbomethoxy-vanillate (MDHA) C11H12O6 240 g/mol
Methyl dehydroabietate C21H30O2 314 g/mol
Table 2 Concentration of the main monomeric products in the feed for both batches. Component Concentration 1st batch [g/L] Concentration 2nd batch [g/L]
V
MV
5CV
M-5CV
MDHA
0.37 0.22
0.38 0.34
0.15 0.13
0.15 0.13
0.26 0.22
performed in microchannels. To evaluate if equilibrium was reached in the presented microextractor, experiments at different residence times were performed by varying the overall flow rate. The flow rate ratio of CO2 to lignin product mixture was set to 2.4 to obtain stable slug flow. Pressure and temperature were kept constant at 121 bar and 39.8 1C for all experiments. The total flow rate was varied from 51 μl/min to 255 μl/min to achieve residence times of 31 to 6 s, respectively. All flow rates were measured at the pumps. Flow rates in the microfluidic device were higher, due to volume expansion owing to the increased temperature causing transition of the CO2 from liquid to supercritical state. The initial concentration of the main five monomeric products in the feed, being vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxy-vanillate (M-5CV) and methyl dehydroabietate (MDHA), was determined from quantification by GCMS. Two different batches were used for the experimental study and their concentrations of the main monomeric products are presented in Table 2. For the first series of experiments with varying residence times the first batch was used. The extracted mass of the main monomeric reaction products relative to their amount in the feed is plotted versus residence time in Fig. 4. All experiments were repeated twice and the average values of all experiments are plotted together with the single data points as a reference. No significant influence of the residence time was determined within the tested residence time range. Differences were in the same range as deviances of experiments at the same residence time, and did not show a trend. Consequently, concentrations of the solutes in both phases at the outlet could be assumed to be in thermodynamic equilibrium even for the shortest residence time tested. Shorter residence times could not be achieved with the current setup, as the maximum flow rate was limited by the capillary separator. On the other hand, the measurement error increased for lower flow rates, thus limiting the realization of longer residence times. The lowest flow rates applied were 36 μl/ min CO2 and 15 μl/min lignin oxidation products. Fluctuations of the pumps at these low values lead to irregularities in the flow pattern, affecting the overall reproducibility. Therefore, flow rates were fixed to 120 μl/min CO2 and 50 μl/min lignin oxidation products for the following study, yielding a residence time of 9.2 s at 121 bar and 39.8 1C.
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Fig. 4. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different residence times for p ¼121 bar and T ¼ 39.8 1C.
Fig. 5. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different pressures for a temperature of 39.8 1C.
3.2. Mass balance considerations
weight monomers were extracted in larger amounts than the ones with lower molecular weight. Vanillin was hardly extracted by scCO2, but also displayed slightly better solubility at higher pressures. The molecule with the highest molecular weight, MDHA, was extracted almost completely even at lower pressures. A slight decay at higher pressures might also result from measurement errors and lies within the standard deviation for MDHA. As scCO2 is a non-polar solvent, molecules are extracted better from the polar aqueous feed, the less polar they are. From the molecular structure (Table 1), it becomes clear that the aldehyde group in vanillin and 5CV provides a higher polarity compared to the carbomethoxyl group of methyl vanillate and M-5CV, respectively (Christen 5. Auflage (1982)). 5CV and M-5CV were extracted better with scCO2, suggesting that the additional carbomethoxyl group reduces their polarity. MDHA is a comparably non-polar molecule, featuring few functional groups, matching the fact that almost the complete amount of MDHA was extracted from the polar aqueous feed into scCO2. The distribution ratios of the products were calculated according to Eq. (1) based on the measured concentrations. As almost no MDHA remained in the raffinate phase, the denominator of the ratio was very small, yielding very high distribution ratios for MDHA, but also large errors. Therefore, the distribution ratios of all products, except for MDHA, are plotted in Fig. 6. The distribution ratios displayed the same behavior as the extracted amount, plotted in Fig. 5 and increased with increasing pressure. However, significant differences were present, indicating the possibility to separate the five main monomeric products by supercritical extraction. To quantify the ability to separate the components using extraction with scCO2, the selectivity α was calculated according to Eq. (2). The higher α, the better the components can be separated from each other. The highest α was found for separation of MDHA from any other component. MDHA was almost completely extracted, whereas the other components remained in the raffinate, especially at lower pressures just above the critical pressure. As the distribution ratios of MDHA had a considerable deviation, the values for α were also afflicted with errors. Consequently, the exact values are not shown, ranging from 10 to 250, expressing the possibility to obtain purified MDHA. The selectivity of the other main monomeric products relative to each other at different pressures is presented in Table 3. In general, vanillin was hardly extracted by scCO2, so it could be concentrated in the raffinate phase. Significantly more 5CV and M5CV were extracted than methyl vanillate and vanillin, so these
In a previous study it was demonstrated that the measured concentrations of both phases yielded a complete mass balance for the extraction of vanillin from its aqueous solution with scCO2 using the presented microextractor (Assmann et al., 2012). In order to verify the mass balance of the current study, the raffinate phase was sampled and evaluated for one experiment at 121 bar and 39.8 1C. No scCO2 must enter the capillaries of the separator to achieve a pure raffinate phase in the respective sampling cylinder. Thus, separation was tuned accordingly by means of the high pressure needle valves. After the experiment the system was depressurized and samples could be taken directly from the sampling cylinder. Probes were extracted with chloroform as described for the feed samples before analysis. The average difference to a complete mass balance of all main five monomeric products was less than 5%. Thus, only the supercritical phase was sampled and evaluated in the current study, as only one pure phase could be obtained in each experiment and direct analysis of the raffinate samples was not possible. Concentrations in the raffinate phase were calculated from mass balance considerations as described above. 3.3. Pressure dependence To determine its effect pressure was varied between 66 and 121 bar at a constant temperature of 39.8 1C, using the reaction mixture from the first batch (Table 2). The extracted amount of the main monomeric products relative to their mass in the feed is plotted versus pressure in Fig. 5. All experiments were performed twice, and deviations were below 10% for all products but vanillin. As vanillin concentrations in the extract were close to zero, deviations were as high as 31%, but low in absolute numbers. One control experiment was performed below the critical point of CO2 (pc ¼ 73.8 bar) at 66 bar. No reaction products could be determined in the extract phase, as they do not solve in gaseous CO2. As a conclusion, no solutes were entrained by the CO2 rich phase, which confirms that the measured amounts of monomeric reaction products at higher pressures were solved in scCO2. Above the critical point, the main five monomeric reaction products displayed significant differences as regards their extractability with CO2. In general, solubility in supercritical solvents increases with increasing pressure (Brunner, 1994), which was particularly pronounced for methyl vanillate, 5CV and M-5CV close to the critical pressure. The higher molecular
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Table 3 selectivity α of the main monomeric products. p
α
MV
5CV
M-5CV
81 bar
V MV 5CV V MV 5CV V MV 5CV V MV 5CV V MV 5CV
∞ – – 8.64 – – 6.96 – – 5.22 – – 4.23 – –
∞ 4.02 – 30.72 3.56 – 25.00 3.59 – 17.19 3.29 – 11.62 2.75 –
∞ 7.48 1.86 53.24 6.16 1.73 44.40 6.38 1.78 30.38 5.82 1.77 21.04 4.98 1.81
91 bar
101 bar
111 bar
121 bar
Fig. 6. Distribution ratios of the main monomeric reaction products except MDHA at different pressures for a temperature of 39.8 1C.
component groups could also be separated using supercritical extraction. The lowest α-values of 1.73–1.86 were obtained for separation of 5CV from M-5CV. In general, the highest selectivity was obtained at the lowest pressure above the critical value. As the solubility of all four components was rather low at 81 bar, large amounts of scCO2 would be required for separation. A high volume ratio of scCO2 to feed does not necessarily render the process uneconomical, as CO2 is available in abundance and easily recyclable. 3.4. Temperature dependence The influence of temperature was studied by varying it between 39.8 and 59.3 1C at a constant pressure of 121.1 bar using the reaction mixture of the second batch (Table 2). The extracted weight percentage of the main monomeric products is plotted versus temperature in Fig. 7. Again, the extracted amount of MDHA was around 100% and afflicted with a large error. With rising temperature, the extracted amount of all other products decreased. Especially the amounts of extracted methyl vanillate, 5CV and M-5CV decreased significantly, whereas vanillin was hardly extracted even at 40 1C. The distribution ratios for this experimental series were not calculated, as the equilibrium composition of the two phases is temperature dependent and no experimental data was available for the used conditions. Also, the same effect of an increased temperature can be reached at lower pressures, so that processing at higher temperatures would not be the first choice, especially for temperature sensitive components.
4. Conclusion The supercritical extraction of aromatic products from lignin oxidation was evaluated using a continuous microfluidic device. The microextractor enabled fast screening of different extraction conditions. As it also contained a capillary separator for phase separation, the supercritical phase and the raffinate could be sampled after a well defined residence time. The extracted amounts of the main five monomeric products, namely vanillin (V), methyl vanillate (MV), 5-carbomethoxy-vanillin (5CV), methyl 5-carbomethoxy-vanillate (M-5CV) and methyl dehydroabietate (MDHA) at pressures from 81 to 121 bar and temperatures from 39.8 to 59.3 1C were measured. Equilibrium concentrations were reached in less than 10 s and only small amounts of the involved chemicals were consumed. It was found that MDHA was almost
Fig. 7. Extracted weight percentage of the main monomeric reaction products relative to their amount in the feed at different temperatures for a pressure of 121 bar.
completely extracted, whereas virtually the entire amount of vanillin remained in the aqueous phase. Generally, the distribution ratio of the lower molecular weight molecules was smaller, which can be linked to their higher polarity. With increasing pressure and decreasing temperature, the distribution ratios of all monomers increased, except for MDHA, which was almost entirely extracted, independently of pressure and temperature. Consequently, selectivity was higher for lower pressures, just above the critical pressure of CO2. The different selectivity of scCO2 towards these products renders separation possible and enables the use of supercritical extraction to remove MDHA and concentrate vanillin and methyl vanillate in the raffinate phase. As a conclusion, supercritical extraction with CO2 can be applied as a purification step in the valorization of lignin. Among other unit operations, like vacuum distillation and crystallization, a multistage process with different pressures and temperatures can yield high purities of single monomeric products. The current study provides important data to realize this economic and green process to exploit lignin as a renewable feedstock for chemicals.
References Assmann, N., Kaiser, S., von Rohr, P.R., 2012. Supercritical extraction of vanillin in a microfluidic device. J. Supercritical Fluids 67, 149–154.
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