Crystallization of Caffeine by Supercritical Antisolvent (SAS) Process

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Crystallization of Caffeine by Supercritical Antisolvent (SAS) Process: Analysis of Process Parameters and Control of Polymorphism Gerti Weber Brun,†,‡ Á ngel Martín,*,† Eduardo Cassel,‡ Rubem Mário Figueiró Vargas,‡ and María José Cocero† †

High Pressure Processes Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid, Valladolid, Spain ‡ Department of Chemical Engineering/FENG, Pontifical Catholic University of Rio Grande do Sul, Rio Grande do Sul, Brazil ABSTRACT: The recrystallization of caffeine extracted with dichloromethane from coffee beans by a supercritical antisolvent (SAS) process was studied. Acicular crystalline particles with a particle size down to 2.5 μm were obtained, and product purity was increased as a result of SAS processing. Furthermore, by modification of process conditions, it was possible to vary the proportion between the metastable and stable crystalline polymorphic forms of caffeine, obtaining fractions of stable form ranging from 40% to nearly 100%.

1. INTRODUCTION Caffeine (C8H10N4O2) is an alkaloid present in many drugs, beverages, and plants. It is used as a food additive regulated by the Federal Food and Drug Administration (FDA). In small doses, caffeine is an effective stimulant, but it can be detrimental for health if an excess is consumed. The decaffeination of grains and leaves, and particularly of coffee beans, is economically attractive because it not only produces the decaffeinated version of these products demanded by consumers, but also caffeine as a byproduct, which may be used in soft drinks, medicaments, etc.1−3 The three most extended technologies for coffee bean decaffeination are solvent extraction (particularly, extraction with ethyl acetate or dichloromethane), extraction with water, and supercritical carbon dioxide fluid extraction (SCF).4 By supercritical extraction, a product of high quality may be obtained, because it is not exposed to toxic organic solvents, but since this technology requires a high investment in equipment, many companies still use the extraction with organic solvents. After the extraction with these solvents, a solution of caffeine contaminated with other substances in the organic solvent is obtained that must be further processed in order to make it suitable for food or medical applications. The processing of the extracted caffeine usually involves the purification of the extract and the crystallization of the product. Caffeine can be crystallized by conventional solvent evaporation, but this method offers no possibilities for the purification of the material, and it is hampered by a poor control over the properties of particles. Supercritical antisolvent (SAS) methods5 are an interesting alternative. The principle of the SAS method is to put into contact an organic solution with supercritical carbon dioxide, so precipitation is caused by the simultaneous extraction of the organic solvent to the supercritical fluid and saturation of the organic solution with CO2. Therefore, the solutions obtained © 2012 American Chemical Society

by organic solvent extraction of coffee beans can be directly processed by SAS. Compared to solvent evaporation, SAS precipitation can allow lower concentrations of organic solvents in the final product to be achieved, due to the high solubility of these solvents in supercritical CO2, as well as a more homogeneous product with a smaller particle size due to the high supersaturations and fast crystallization kinetics in supercritical fluids. Thus, SAS processing can be an alternative for improving the quality of the product requiring a smaller investment compared to a full supercritical CO2 extraction of beans, since a smaller amount of product must be processed (only the caffeine extract instead of the whole beans) and a lower pressure is required (approximately 100 bar for SAS precipitation5 compared to approximately 300 bar for supercritical CO2 extraction4). In addition to these advantages, the possibilities of controlling the polymorphism of products by SAS crystallization are of interest for medical applications. Different polymorphs have different physical properties such as solubility, melting point, density, hardness, and crystalline form. The existence of polymorphism may influence the chemical and physical stability of drugs, and therefore its bioavailability.6 The ability of SCF to induce polymorphism by crystallization in drugs is known, due to the possibilities to control the crystallization conditions, and particularly crystallization kinetics, by changes in pressure and temperature.7 Adami et al.8 presented the selective formation of amorphous or crystalline particles of nalmefene HCl by SAS precipitation, both in laboratory and pilot scales. Martiń et al.9 studied the formation of ibuprofen polymorphs, selectively producing different crystalline polymorphs or amorphous particles by modification of the supersaturation. Bouchard et al.10 Received: December 19, 2011 Revised: February 21, 2012 Published: February 22, 2012 1943

dx.doi.org/10.1021/cg2016758 | Cryst. Growth Des. 2012, 12, 1943−1951

Crystal Growth & Design

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obtained different glycine polymorphs by variation of the solution/CO2 ratio. Rodrigues et al.11 found that a specific polymorph of teophylline was obtained by supercritical processing which was not produced when carbon dioxide was replaced for pressurized nitrogen as a negative control (nonantisolvent processing at high pressures). Caffeine exists as two polymorphs: the stable form II or β under ambient temperature and atmospheric pressure conditions, and the metastable form I or α usually obtained by crystallization of caffeine by solvent evaporation at high temperature by other high temperature treatments.12,13 Park and Yeo studied the recrystallization of caffeine from dichloromethane or chloroform solutions with a batch gas antisolvent (GAS) technique.14 These authors studied the influence of the different parameters such as crystallizing temperature, solvent type, and the carbon dioxide injection rate on the crystal habit and particle size. Acicular particle morphology was obtained in all cases, and sub-micrometer particle size could be achieved using growth retardants. Experimental results also suggested that the crystallinity of particles may be influenced by the use of growth retardants. However, an analysis of the polymorphs obtained by the GAS technique was not presented by these authors. The aim of this work is to study the possible application of SAS precipitation for the crystallization of caffeine obtained by solvent extraction of coffee beans, seeking to achieve a controlled particle size, morphology, and polymorphism. With this aim, a product obtained by dichloromethane extraction of coffee beans in an industrial factory has been used as a starting material. Results are analyzed considering the thermodynamic and kinetic parameters during the crystallization.

Figure 2. SEM image of unprocessed caffeine particles. purity. Dichloromethane (minimum purity of 99.8%) purchased from Sigma, ethanol absolute (minimum purity of 99.5%) from Panreac, and ethyl acetate (minimum purity of 98.9%) from Panreac were used to prepare the caffeine solutions sprayed into the precipitator. Carbon dioxide at 99.95% was delivered by Carburos Metálicos S.A. (Spain). 2.2. SAS Equipment. A schematic diagram of the equipment for the semicontinuous SAS process is presented in Figure 3. The

2. MATERIALS AND METHODS 2.1. Materials. Caffeine obtained by extraction of green coffee beans with dichloromethane by the company of soluble coffee Seda Solubles S.A. (Palencia, Spain) was used in experiments. This caffeine was crystallized by evaporation of dichloromethane at ambient pressure. The purity of the crystallized caffeine was 93%, as determined by chromatographic analysis according to the procedure described in section 2.3, the remaining components present in this material being unidentified. The average particle size (d0.5) determined by dynamic light scattering (DLS) was 13 μm, with a broad particle size distribution as shown in Figure 1. Figure 2 presents a scanning electron

Figure 3. Schematic diagram of SAS apparatus. apparatus mainly consists of a diaphragm pump (Dosapro) with a maximum flow rate of 5 kg/h for CO2, and one chromatograph pump (Jasco) with a maximum flow rate of 10 mL/min for the solution; an isolated and jacketed stainless AISI 316 vessel for the precipitation with 2 L of internal volume and with a porous metallic frit at bottom to collect the particles; and a flask to carry out the separation of CO2 and solvent after depressurization. Heat exchangers are used to cool and condense the carbon dioxide before pumping and to reach the operating conditions of temperature. The instrumentation consists of a Pt-100 thermoresistance with an accuracy of ±0.1 K placed inside the precipitation vessel. For the pressure a DESIN TPR-10 digital pressure meter (DESIN Instruments, Spain), with an accuracy of ±0.05 MPa, was used. CO2 mass flow was measured with a coriolis flow meter (Sensor MICRO Motion Elite CMF010NB, Transmitter MICROMotion Elite RFT91), with an accuracy of ±0.01 kg/h. The injector used to mix CO2 and solution at the precipitator inlet consists of a concentric tube arrangement in which the solution flows through an inner tube with a diameter of the 100 μm and the CO2 flows through an external tube of 1/4 in. 2.3. SAS Precipitation Procedure. Experiments started pumping pure carbon dioxide, preheated at the desired operating temperature, into the precipitation vessel. A fixed CO2 flow rate of 4 kg/h was used in all experiments. Once the operating condition of pressure, temperature, and flow mass of CO2 were achieved and stayed stable,

Figure 1. Particle size distribution of unprocessed caffeine. microscopy (SEM) micrograph of caffeine particles. As shown in this figure, prismatic particle morphology with growth of small needles on the surface of particles was obtained by the solvent evaporation method. Caffeine with a minimum purity of 99.3% purchased from Naturex was used as a standard for the chromatographic determinations of 1944



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the solution (with a fixed flow rate of 4 mL/min) was fed into the precipitator. When the desired volume of solution (approximately 100 mL) had been injected into the vessel, the solution pump was stopped and only carbon dioxide was pumped during 30 min, a period long enough for the complete removal of residual solvent from the vessel and to dry the formed particles as determined by preliminary experiments. Finally, the system was decompressed and particles retained in the frit at the bottom of the vessel were collected for the posterior analyses. All experiments were made twice. 2.4. Product Analysis. Microphotographs of the particles collected were taken by scanning electron microscopy (SEM). An environmental scanning electron microscope (ESEM) FEI-Quanta 200FEG was used in this study. Samples were gold-sputted before SEM observation. Particle size distributions were measured by dynamic light scattering (DLS) with a laser diffractometer (Mastersizer 2000, Malvern) equipped with a dry powder feeding system (Sirocco, Malvern). Caffeine samples were analyzed by a dry method: during the measurement, the sample powder (1 g) was shaken from a vibrating sample tray where the sample was then blown by compressed air, introduced at a set pressure (2.5 bar), against and around a bend and then across the lens were the particles passed through a focused red laser beam. For each experiment, the measurement of particle size by DLS was made in triplicate. Product purity in caffeine was analyzed in triplicate by HPLC after each experiment. The caffeine particles were diluted in a methanol/ water solution (20/80) with a concentration of sample of 1 ppm. Before analysis, the samples were vacuum filtered with a membrane (0.45 μm) and agitated for 5 min in an ultrasound bath. A Waters 1515 chromatograph equipped with a Simetry C18 column (packing size of 5 μm and dimensions of 4.6 × 150 cm) was used. A methanol/ water solution (40/60) with isocratic flow (1.1 mL/min) was used as the mobile phase, and a Waters 2487 UV detector was set at 273 nm. The precipitation yield was determined by weighing as the relation between the mass of collected particles and the amount of caffeine in the solution sprayed into the SAS precipitator. The thermal phase transitions were determined using a differential scanning calorimetry (DSC) system (Mettler Toledo, model 822e SAE) that was equipped with a FSR detector (temperature range 153− 973 K; resolution < 0.04 μW; sensitivity 15 μV). The samples had a disk shape (sample weight: 2−3 mg), and the DSC measurements were performed by heating from 298 K to 773 K at a rate 10 K/min in an inert atmosphere (N2 flow: 50 mL/min). Particles were also characterized using X-ray powder diffraction analysis (XRPD). XRPD analyses were performed on a diffractometer PHILIPS PW1830 with copper anode (voltage: 40 kV, current: 30 mA). The diffractograms were acquired between 5 and 50° (2θ angle) with a step of 0.02° and an acquisition time of 800 ms.

use a binary interaction parameter kij to calculate parameter a of the mixture, as presented in eqs 2 and 3. a=

i

b=

RT aα − 2 V−b V + 2bV − b2

j

(2)

∑ xibi (3)

i

The Peng−Robinson equation can be used to calculate the fugacity coefficient of components in fluid phases. With the fugacity coefficients, phase equilibrium calculations can be carried out by solving the condition of equality of fugacity of components between phases at equilibrium. However, since the Peng−Robinson equation of state is not able to represent the behavior of solid phases as required for solubility calculations, the fugacity of the solid is obtained from the fugacity of a reference, hypothetical subcooled liquid applying eq 4.16 As an additional simplification, it is considered that the solid is pure solute. With these simplifications, the condition of equality of fugacities between solid and fluid phases is reduced to eq 5. A detailed description of the theoretical framework and procedure for solubility calculations according to this method can be found in previous works.15,16 ⎛ ΔH ⎛ 1 1 ⎞⎞ f φs = φ l⎜⎜ ⎜ − ⎟⎟⎟ T ⎠⎠ ⎝ RT ⎝ Tf

(4)

φιs = φι l ·xi

(5)

The binary interaction parameters kij of eq 2 were obtained by correlation of experimental phase equilibrium data of the corresponding binary systems. The correlation was performed by minimization of the average absolute difference (AAD) between experimental and calculated data according to the objective function presented in eq 6. Phase equilibrium calculations were performed using the computing software Matlab (r) as described in a previous work.17 Pure component properties required for application of the Peng−Robinson equation are presented in Table 1. The correlated binary Table 1. Pure Component Properties Required for Application of the Peng−Robinson Equation carbon dioxide dicloromethane ethyl acetate ethanol caffeine

3. THERMODYNAMIC MODELING In this work, the solubility of caffeine in supercritical CO2 + organic solvent mixtures was estimated using the Peng− Robinson equation of state with quadratic mixing rules (eqs 1, 2, and 3). This equation has been frequently used to estimate the phase equilibrium in similar systems.15 The Peng−Robinson equation of state can be expressed as a function of pressure as presented in eq 1. Pure components are characterized in this equation by parameters a and b as well as by a temperature-dependent α function. P=

∑ ∑ xixj(aiaj)0.5(1 − k ij)

Tc (K)

Pc (MPa)

ω

Tf (K)

304.1 510.0 523.2 513.9 780.0a

7.38 6.10 3.83 6.14 4.22a

0.225 0.190 0.363 0.644 0.793a

508b

a

Estimated with the Jobak group contribution method. obtained by DSC analysis of caffeine.12

ΔHf (J/mol)

19871b 18 b

Properties

interaction parameters together with the resulting AAD% and the references from which experimental data was retrieved are reported in Table 2. AAD% =

(1)

When the equation is used to represent mixtures, mixture parameters a and b are calculated applying a mixing rule to combine the parameters of pure components. In this work, conventional quadratic mixing rules have been applied, which

100 ndata

∑

abs(exp − calc) exp

(6)

4. RESULTS AND DISCUSSION 4.1. Crystallization Experiments. Several experiments with three solvents (ethyl acetate, dichloromethane, ethanol, 1945



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This result can be justified analyzing the phase equilibrium conditions during the precipitation, considering that caffeine shows certain solubility in carbon dioxide at 10 MPa. At 313 K and in the pressure range above 9 MPa, carbon dioxide is completely miscible with ethanol, methylene chloride, and ethyl acetate. This is shown in Figure 4, which presents the pressure− composition (P-xy) phase equilibrium diagrams calculated with the Peng−Robinson equation of state for the system CO2 + (dichloromethane, ethyl acetate, or ethanol) at different temperatures. The analysis of solubility of caffeine in this mixture can be therefore done considering a solute in a single fluid phase (organic + CO2) for all the experimental conditions, with the possible exceptions of experiment 13 (carried out at a lower pressure of 8 MPa) and experiment 12 (carried out at a higher temperature of 333 K), which as shown in Figure 4a correspond to near-critical conditions. Considering the solution and CO2 flow rates, the mole fraction of caffeine in the conditions of experiments 2 and 4 can be easily calculated, resulting in 9.8 × 10−5 and 1.1 × 10−4, respectively. On the other hand, the solubility of caffeine in pure carbon dioxide at 10 MPa and 313 K19 is 1.0 × 10−4, and the solubility in the mixture of CO2 + organic solvent is expected to be higher.22 From these results, it can be easily observed that in experiments with ethanol or ethyl acetate, the concentration of caffeine was close to or below the solubility of this compound, even if caffeine-saturated solutions were sprayed into the precipitator, and therefore precipitation did not occur. In contrast, in experiment 6 the concentration of caffeine was 3.6 × 10−4, well above the saturation limit, and therefore precipitation was achieved. It can be concluded that the solubility of caffeine in the organic solvent is an important limiting factor which reduces the possible choices of organic solvents that can be used to crystallize caffeine by the SAS process. Of the three organic solvents tested in this work, only dichloromethane allowed preparing solutions with a concentration of caffeine high enough to allow SAS processing. Therefore, the application of SAS recrystallization to extracts obtained in factories that use ethyl acetate or ethanol as solvent is not possible. The precipitation yield observed in experiments can be justified with solubility calculations. Table 4 presents the estimated solubility of caffeine in the fluid phase (CO2 + organic solvent) in experiments performed with dichloromethane as

Table 2. Binary Interaction Parameters for Application of the Peng-Robinson Equation to the CO2 + Dichloromethane + Caffeine System T (K)

kij

AAD (%)

property correlated

CO2−caffeine

313−353

0.058

9.6

CO2− dicloromethane caffeine− dicloromethane

291−311

0.0809

3.3

caffeine solubility bubble point pressure caffeine solubility

system

298

−0.028

0

ref 19 20 21

chosen for being the three most used organic solvents in industrial decaffeination plants by organic solvent extraction) and different conditions (initial concentration of caffeine solution, temperature and pressure) have been performed in order to study the influence of these parameters in the characteristics of the particles. Table 3 presents the operating temperature and pressure (T and P), the initial concentration of caffeine in solution (c0), the purity of the particles in caffeine determined by HPLC, the mean particle size (d0.5), and the precipitation yield (Y). As previously discussed, all experiments were performed in duplicate, and particle size and HPLC analyses were repeated three times for each experiment. Estimated experimental errors reported in Table 3 are calculated from the variation of results on these replications of experiments using standard statistical methods. On the other hand, the precipitation yield reported in Table 3 is the average value of the results obtained with the duplicate experiments at each experimental condition. For example, for experiments 6, 8, and 12, the range of values for the yield was 6.5−5.7%, 78.1− 77.5%, and 93.0−94.5%, respectively, demonstrating that this experimental result also showed a good reproducibility. As shown in Table 3, in all experiments performed with ethanol and ethyl acetate as solvents, no particles were obtained. To ensure that this result was not due to loss of product through the filter, experiments 2 and 4 were repeated a third time using an external filter (pore size: 1−2 μm) in addition to the porous metallic frit (pore size: 10−15 μm) introduced in the precipitator (the external filter was not used in the remaining experiments in order to avoid possible blocking problems), resulting again in no collection of particles in any of the two filters. Table 3. Summary of Experiments Performeda

a

exp

solvent

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

CCb ethanol ethanolc ethyl ac ethyl acc DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM

P (MPa) 10 10 10 10 10 10 10 10 10 10 10 10 8 12

T (K) 313 313 313 313 313 313 313 313 313 313 303 333 313 313

c0 (g/mL) 0.0067 0.0076 0.0067 0.0081 0.0067 0.0270 0.0400 0.0540 0.0670 0.0800 0.0540 0.0540 0.0540 0.0540

purity (%)

d0.5 (μm)

92.96 ± 0.10

12.2 ± 0.9

95.8 96.1 97.53 96.1 95.1 93.56 97.20 96.7 96.1

± ± ± ± ± ± ± ± ±

0.2 0.3 0.14 0.5 0.3 0.10 0.09 0.5 0.2

6.37 3.1 2.6 2.7 2.5 58.7 640 4.4 6.47

± ± ± ± ± ± ± ± ±

0.18 0.3 0.3 0.3 0.3 1.6 30 0.3 0.09

Y (wt%)

6.1 41.5 77.8 72.1 78.2 7.3 93.8 77.6 9.2

CO2 flow rate: 4 kg/h, solution flow rate: 4 mL/min. bUnprocessed caffeine. cSaturated solution. 1946



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Figure 4. Pressure−composition P-xy phase equilibrium diagrams calculated with the Peng−Robinson equation of state for the system CO2 + dichloromethane at T = 303 K, 313 and 333 K (a) and for the systems CO2 + dichloromethane, CO2 + ethyl acetate, and CO2+ ethanol at T = 313 K (b).

organic solvent (xcafeq), compared to the concentration of caffeine in the fluid phase in the corresponding experiment (xcaf). The solubility has been estimated with the Peng−Robinson equation, as described in section 3, and considering that CO2 and organic solvent form an homogeneous fluid phase. From these solubility calculations, Table 4 also presents the theoretical yield of the

aspect ratios (length/width of needles), with needle lengths in the range 10−20 μm, and needle widths of 1−2 μm in most cases. Compared with the morphology of unprocessed particles (Figure 2), the recrystallized particles showed smoother surfaces, single-crystal morphologies, as well as a considerable reduction of particle size. Acicular morphology was also observed by Park and Yeo14 in their experiments of caffeine recrystallization by batch GAS process. Figure 6 presents particle size distributions of recrystallized caffeine measured by DLS. It must be noted that this technique yields volume averaged particle sizes (i.e., assimilating the acicular particles observed in the SEM pictures to spheres of equivalent volume). From the particle size distribution measurements, it can be observed that, compared to the particles obtained by solvent evaporation (Figures 1 and 2), SAS processing yields a narrower particle size distribution in addition to the reduction of particle size. Indeed, in Figure 6 it can be observed that 85% of particles obtained in experiments 7−10 had a particle size ranging from 1 to 10 μm, while in the case of unprocessed caffeine (Figure 1), 85% of particles had a size ranging from 1 to 40 μm. With respect to the variation of particle size and morphology with process conditions, analyzing the results of experiments 6−10 it can be observed that an increase in the initial solution concentration led to a decrease of particle size, until a limit value of around 2.5 μm was obtained with initial caffeine concentrations above 0.054 g/mL. This result can be correlated with the variation of supersaturation, which as shown in Table 4, increases as the initial concentration is increased. As the supersaturation is increased, the formation of new particles is faster (with nucleation kinetics proportional to exp (S) if only homogeneous nucleation is considered), while the growth of particles is a mass-transfer controlled process whose rate only varies linearly with the concentration of solute in the fluid.23 Therefore, a reduction of particle size is expected as supersaturation is increased. On the other hand, and as shown in Figure 5, the initial concentration did not influence the crystal habit, and needle-like particles with smooth surfaces were obtained in experiments 6−10. With experiments 11−14, the influence of variations of pressure and temperature conditions with a constant concentration of solute was studied. Compared with the results at T = 313 K, a noticeable increase in particle size was observed both in the experiment performed at a lower temperature of

Table 4. Concentration of Caffeine in the Fluid Phase (xcaf) Compared to the Solubility of Caffeine in the Fluid (xcafeq), Precipitation Yield (Ycalc), and Supersaturation (S) Estimated with the Peng−Robinson Equation of State exp

xcaf

xcafeq

Ycalc (wt%)

S

5 6 7 8 9 10 11 12a 13a 14

0.000087 0.000353 0.000522 0.000705 0.000875 0.001045 0.000705 0.000705 0.000705 0.000705

0.000102 0.000102 0.000102 0.000102 0.000102 0.000102 0.000283 0.000012 0.000026 0.000173

0 71 80 86 88 90 60 98 96 76

0.9 3.5 5.1 6.9 8.6 10.2 2.5 57.3 27.1 4.1

a

Calculations do not consider the possible formation of two phases (liquid + gas).

precipitation (Ycalc), as well as the estimated supersaturation (S), calculated according to eq 7. Comparing the results of experimental and calculated yields, it can be seen that, according to calculations, the initial concentration of caffeine in experiment 5 was not high enough to achieve supersaturation, in agreement with experimental results that showed no formation of particles in this experiment (Table 3). In the remaining experiments, the theoretical yield is always higher than the experimental yield, which can be justified by the loss of product during the manual recollection of particles. Higher discrepancies can be observed in the experiments with lower experimental and calculated yields (experiments 6, 11, and 14) x S = caf eq xcaf (7) Figure 5 presents SEM pictures of particles obtained in different experiments. It can be observed that the recrystallized caffeine particles exhibited acicular morphology, with high 1947



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Figure 5. SEM micrographs of particles obtained in SAS experiments.

T = 303 K, and at a higher temperature of T = 333 K (experiments 11 and 12 of Table 3), even if the estimated supersaturation for the latter experiment was higher than in all remaining experiments (S = 57.3 as shown in Table 4). This experimental result can be due to the operation in the vicinity or inside of a two fluid phase region, and the consequent formation of particles from a liquid phase instead of a supercritical fluid (liquid CO2 in the case of the experiment

at 303 K, and liquid CO2 + dichloromethane phase in the experiment at 333 K, as shown in Figure 4). The operation of SAS crystallization in a two-phase region usually led to an increase in particle size due to worse transport properties and lower CO2 concentration in the liquid compared to the supercritical fluid, as well as to the influence of the liquid−gas phase boundary in the process.24 Moreover, SEM micrographs of particles obtained in experiment 12 show a lack of 1948



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Figure 6. Cumulative particle size distribution of SAS processed caffeine.

homogeneity in the size and morphology of the product, which includes small acicular particles as well as larger particles with polyhedral shape and unsmooth surface. Similarly, operating at a lower pressure of 8 MPa (experiment 13 of Table 4), an increase of particle size is observed with respect to the experiment at 10 MPa, even if the calculated supersaturation is considerably higher at 8 MPa (S = 27.1) than at 10 MPa (S = 6.9). The particles obtained in the experiment at 8 MPa and 303 K experiment presents a similar morphology as the product obtained at 10 MPa and 333 K, showing large polyhedral particles with attached or independent smaller acicular particles, which may be due to the operation at near-critical or two phase conditions in both experiments (Figure 4). As presented in Table 4, if the precipitation is carried out at a higher pressure of 12 MPa, particle size increases. This is in agreement with the reduction in the supersaturation caused by the increased caffeine solubility in supercritical CO2 as pressure is increased (Table 4). Moreover, the particles formed show more irregular shapes (Figure 5). It is also noticeable that at this pressure, hollow particles with narrow parallel pores were formed, a morphology that was not observed in the experiments at lower pressures. 4.2. Chromatographic Analyses. Table 3 shows the results for caffeine quantification by high performance liquid chromatography (HPLC). In general, by SAS processing the purity was increased comparatively with the unprocessed caffeine, from 93% in the unprocessed caffeine, up to 97.5% in SAS-processed particles. Thus it can be concluded that after the SAS precipitation, the caffeine does not suffer chemical degradations and the product was more pure than the unprocessed material. The increase in purity can be justified by a partial extraction of contaminants from the product by supercritical CO2 during SAS processing. A similar purification of caffeine was observed in all experiments, except in experiment 11, carried out at the lower temperature of T = 303 K, in which the purity of the crystallized caffeine (93.6%) was similar to the purity of the unprocessed material. 4.3. Characterization of the Crystalline Structure of Particles. The crystalline structure of the produced particles was analyzed by differential scanning calorimetry (DSC). The results obtained are presented in Figure 7 and Table 5. The DSC curves of caffeine show three endothermic peaks: the first associated with the phase transition of caffeine from the crystal form β or II (stable) to the form α or I (metastable); the second one corresponding to the fusion of α form of the caffeine; and the third one associated with the evaporation of

Figure 7. DSC scans of caffeine samples recrystallized at 303, 313, and 333 K.

liquid caffeine (not shown in the results presented in this work). The temperatures of the peaks of transition and fusion identified for all experiments show a good agreement with literature data.12,25−27 The heat of transition associated with each endothermic peak was different for the unprocessed caffeine and for the SAS-recrystallized caffeine depending on the processing conditions. Furthermore, in order to test the reproducibility of the results of SAS processing with respect to the crystalline structure, particles obtained in several duplicates of experiments were independently analyzed by DSC. As shown in Table 5 (experiments 7a/7b and 12a/12b), the results of DSC analyses of independent experiments show good agreement, confirming that the results obtained with respect to this parameter are reproducible. In the results presented in Table 5, a “relative crystallinity” is presented, calculated as the ratio between the transition and melting enthalpy observed in samples, and the corresponding values reported in the literature for crystalline, anhydrous caffeine.12 The relative crystallinity can be used to quantify the proportion between crystalline and amorphous material (from the ratio between fusion enthalpies) as well as the proportion between polymorphic forms I and II (from the ratio between transition enthalpies). From the results of fusion enthalpies and the corresponding relative crystallinities, it can be observed that, in general, SASprocessed samples presented a higher degree of crystallinity (90−95% in most cases) than the unprocessed particles, obtained by solvent evaporation (88%). Analyzing the results of the experiments where the temperature was increased (exp. 8, 11, and 12), the heat of fusion was highest for the experiment at low temperature indicating a more orderly arrangement of molecules and consequently a higher degree of crystallinity of 1949



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Table 5. Experimental Data of (Solid−Solid) Phase Transition and Fusion Behaviors of Caffeine Obtained by DSCa exp

ΔHt (J/g)

Tt (°C)

rel crys (%)

ΔHf (J/g)

Tf (°C)

rel crys (%)

7a 7b 8 9 10 11 12a 12b 13 14 caf unprocessed caf anhydrous caf anhydrous caf anhydrous

7.42 7.81 14.44 18.72 14.10 14.23 15.22 15.08 11.33 9.57 9.78 17.56 16.50 16.47

155 155 152 151 155 155 155 155 150 156 153 148 130−135 153

42 44 82 106 80 81 87 86 65 55 56

95.69 96.70 90.33 85.43 94.38 97.37 91.73 92.75 96.17 92.36 89.93 102.33

235 235 235 236 235 235 236 236 235 236 233 236

93 94 88 83 92 95 90 91 94 90 88

104.07

239

comment

Dong et al.12 Lehto and Laine25 Cesaro and Starec26

ΔHt is the transition enthalpy of the polymorphic transformation (form II (stable) to form I (metastable)); Tt is the peak transition temperature; ΔHf is the fusion enthalpy; Tf is the peak fusion temperature.

a

particles. A similar trend was achieved for caffeine recrystallized from chloroform with carbon dioxide by Park and Yeo14 and for sulfabenzamide recrystallized from acetone at various temperatures using water as an antisolvent by Park et al.28 With respect to the results regarding the proportion between polymorphic forms I and II, it can be observed that drastic differences were observed depending on the process conditions. While the unprocessed caffeine, crystallized by solvent evaporation contained a 56% of the stable polymorphic from II, the proportion of form II in SAS-recrystallized particles ranged from 40% to nearly 100%. In order to confirm this result, powder X-ray diffraction (PXRD) analyses were carried out. Figure 8 presents the diffractograms corresponding to experiment

Comparing these trends with the results discussed in section 4.1 for particle size and morphology, it can be observed that, in general, the proportion of stable form II increased in experiments that also led to the formation of smaller particles (with the only exception of the transition from T = 313 K to T = 333 K, where bigger particles were obtained, yet a higher proportion of stable form II was observed). This indicates that, as particle size, the proportion between polymorphs is governed by the supersaturation and the corresponding particle formation kinetics: smaller particles with a higher proportion of polymorph II are obtained applying process conditions which increase the supersaturation and therefore accelerate the nucleation kinetics. In agreement with this conclusion, in Table 5 it can also be observed that the proportion of polymorph II obtained by solvent evaporation was only 56%, while proportions of 80−100% were obtained in several SAS experiments. Indeed, very high supersaturations can be achieved by SAS processing compared to a solvent evaporation method,23 thus allowing acceleration of the nucleation kinetics and production of a high proportion of polymorph II.

5. CONCLUSIONS Caffeine particles were micronized from dichloromethane solutions with a semicontinuous SAS process. Acicular particles with volumetric mean particle sizes down to 2.5 μm were obtained depending on process conditions. In particular, the mean particle size of particles was strongly influenced by temperature. The process did not cause degradation of the product; on the contrary, it allowed purification of the caffeine from 92% to up to 97%. Moreover, it has been shown that with an adequate selection of process parameters such as temperature, initial concentration, and pressure, it is possible to selectively produce caffeine particles with different degrees of crystallinity, as well as with different proportions of polymorphic forms. In general, smaller particles as well as higher proportions of the polymorphic form II were obtained applying process conditions that increased the supersaturation and therefore accelerated particle formation kinetics. These results show that the SAS process can be an advantageous alternative for the direct processing of solutions obtained by solvent extraction of caffeine from coffee beans.

Figure 8. PXRD patterns of particles from experiments 7 and 10.

7 (42−44% of form II according to DSC results) and experiment 10 (80% of form II). It can be seen that the results for experiment 7 show only one well-defined peak in the range 25° < 2θ < 28°, characteristic of the metastable form I of caffeine,13,25 while in the results of experiment 10 there are two peaks in the same range, which are characteristic of the stable form II.29,30 Analyzing the influence of process conditions, at 10 MPa the proportion of form II increased as the initial concentration of caffeine was increased (from 40% at the lowest concentration to 80−100% at the highest concentration), and when temperature was increased (from 80% at 303 K to nearly 90% at 333 K). The results obtained both at lower and higher pressures (8 and 12 MPa) show a smaller proportion of form II (55−65%). 1950



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AUTHOR INFORMATION

Corresponding Author

*Address: School of Engineering, Doctor Mergelina s/n 47011 Valladolid, Spain. Tel: +34 983423147. Fax: +34 983423013. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partially financed by Capes Foundation, Ministry of Education ́ and by the Spanish Ministry of Brazil BEX 4987/10-1, Brasilia, of Economy and Competitiveness through Project ENE-201124547. G.W.B. is grateful for financial support from Pontifical Catholic University of Rio Grande do Sul, Brazil, and from Universidad de Valladolid, Spain. A.M. thanks the Spanish Ministry of Economy and Competitiveness for a Ramon y Cajal research fellowship. The authors thank Seda Solubles (Palencia, Spain) for providing the caffeine extract used in this work.

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LIST OF SYMBOLS c0 Caffeine initial concentration (g/mL) d50 Mean diameter (μm) ΔHf Fusion enthalpy (J·mol−1) ΔHt Transition enthalpy (J·mol−1) P Pressure (Pa) Pc Critical pressure (Pa) R Gas constant (J mol−1 K−1) S Supersaturation T Temperature (K) Tc Critical temperature (K) Tf Fusion temperature (K) Tt Transition temperature (K) V Molar volume (L·mol−1) xi Mole fraction composition of component i Y Precipitation yield φ Fugacity coefficient ω Acentric factor

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Crystallization of Caffeine by Supercritical Antisolvent (SAS) Process

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