Process Parameters in the Purification of Curcumin by Cooling

Article pubs.acs.org/OPRD

Process Parameters in the Purification of Curcumin by Cooling Crystallization Marko Ukrainczyk,* B. Kieran Hodnett, and Åke C. Rasmuson* Department of Chemical and Environmental Science, Synthesis and Solid State Pharmaceutical Centre, Bernal Institute, University of Limerick, Limerick, Ireland S Supporting Information *

ABSTRACT: Purification of crude curcumin by up to four successive cooling crystallizations has been investigated for a wide variety of process conditions. For each crystallization step the influence of various processing conditions on crystal purity, polymorphic outcome, and crystal size and shape is reported. By an extensive number of experiments according to a statistical experimental design, the influence on cooling rate, seeding, seed polymorph, and agitation conditions has been identified. Slow cooling and seeding, particularly with the metastable Form II seed, significantly improves the purification. A correlation between product crystal size and purity is found. By tuning the crystallization parameters the number of recrystallization steps required to reach a certain purity can be reduced, which significantly increased the overall curcumin yield from 28% to 50%.

1. INTRODUCTION Crystallization from solution is one of the most important operations in processing industries (pharmaceutical, food, and chemical). Purification is often the key objective of a crystallization process, but impurities can also exert a significant influence on the process performance and crystalline product quality, such as crystal structure, purity, size, and shape. Structurally related impurities (often byproducts or unreacted reactants) are hard to remove completely by crystallization, because of their structural similarity to solute molecule. Often multistep successive crystallizations are needed to reach a desired purity, significantly lowering the product yield. Many reports in the literature describe the effect of structurally related impurities or additives (so-called tailor-made additives) on the shape and crystallization behavior of organic crystals.1−7 They may influence nucleation8 and growth,1,3 and thus crystal shape,4 size,7,9 and polymorphic outcome6,10,11 and become incorporated into the structure.5,10,12 However, data on the influence of impurities and crystallization parameters on purification of crystalline products together with their influence on crystal size, shape, and structure are limited in the literature.12−14 Curcumin, a major active component of the Indian spice turmeric (Curcuma longa), has a wide range of bioactivities (antimicrobial, anti-inflammatory, and anticancerogenic properties).15,16 Commercially available crude curcumin contains significant amounts of structurally very similar compounds, known as curcuminoids, typically about 17% demethoxycurcumin (DMC) and 3% bisdemethoxycurcumin (BDMC) (Table 1), which makes separation and purification of curcumin quite challenging. Most often chromatographic methods are used at laboratory scale to separate individual curcuminoids, typically using silica gel as stationary phase and different solvent systems with chloroform and methanol giving the best chromatographic separations.17 However, from the industrial production point of view, crystallization processes are often preferred. Curcumin is a trimorphic system, of which Form I is thermodynamically the © 2016 American Chemical Society

most stable, and it crystallizes in monoclinic system; the other two metastable forms (Form II and Form III) are orthorhombic.18,19 In this paper, we report the influence of process conditions on the purification of crude curcumin by successive cooling crystallizations. Statistical experimental design techniques were used to identify the effect of cooling rate, seeding, seed polymorph, and agitation on the yield, crystal purity, particle size, shape, and structure as well as determining the minimum number of crystallization steps needed to arrive at a particular product specification.

2. MATERIALS AND METHODS Isopropanol (99.9%, Fluka), ethanol (99.8%, Sigma-Aldich), and acetonitrile (99.9%, Fluka) were purchased from SigmaAldrich. Crude curcumin of >75% nominal purity (HPLC, based on area fractions of chromatogram peaks, A/A) was purchased from Merck, containing 99.3%) were obtained after four successive crystallizations of crude curcumin at slow cooling (8 °C/h) and using magnetic stirring. Seed crystals of stable Form I exhibited rod-like shape (AR = 4.0) and relatively uniform size (dm = 13.4 μm, CQV = 0.30). For the preparation of curcumin metastable Form II ethanol/water (70% v/v) solvent was used (instead of 2propanol). Form II seed exhibited short rod habits (AR = 2.0) with a narrow size distribution (dm = 14.0 μm, CQV = 0.16). In addition, a second seed material of stable Form I, with a bigger size (dm = 49.3 μm, CQV = 0.40), was prepared using overhead stirring. 2.4. Experimental Design and Statistical Analysis. The experimental design approach was adopted to determine the influence of the crystallization process conditions on the purification of curcumin. The effect of four factors (agitation, cooling rate, seeding, number of crystallization steps) on the crystal properties and purity were identified and quantified by performing factorial design, constructing the matrix with 30 experiments (Table 2). Factorial analysis of variance (ANOVA) was performed to estimate the effect of fixed factors (process parameters) on the crystal properties and yield. To evaluate the relation between the produced particle size and the purity, multiple correlation analysis was performed. In this analysis particle size was treated

with 50% acetonitrile/water. The total chromatographic analysis time was 6 min per sample, with CUR, DMC, and BDMC eluting at retention times of 4.4, 3.9, and 3.4 min, respectively (Figure 1).

Figure 1. Example of HPLC chromatograms of mother liquor sample (ML) and dissolved crude solid material (s) isolated after a first crystallization experiment.

The polymorphic composition of the crystals was determined by Raman spectroscopy (Raman RXN2, Kaiser Optical Systems) and by X-ray powder diffraction (PANalytical Empyrean diffractometer with Cu Kα radiation). The polymorphs were identified according to the Cambridge Structural Database (CSD) files (Form I ref code: BINMEQ05 and Form II ref code: BINMEQ06) and semiquantitatively analyzed by Raman spectroscopy. The characteristic Raman shift at 1118 cm−1 was selected for the semiquantitative analysis (Figure S3). No overlapping (B)DMC Raman bands were detected in this spectral region. Besides visual examination of the spectra, principal component analysis (PCA) was used to evaluate changes in the spectral features (Figure S4). All Raman

Figure 2. Light microscopy photographs (up) and corresponding particle size distributions determined by ImageJ (down) of typical curcumin samples isolated from unseeded (a) and Form II seeded (b) experiments. Scale bars 200 μm. Numbers denote experiments from Table 2. 1595

DOI: 10.1021/acs.oprd.6b00153 Org. Process Res. Dev. 2016, 20, 1593−1602

Organic Process Research & Development

Article

Table 2. Experimental Design Matrix of Selected Crystallization Parameters and Results of Polymorphic Composition, Particle Size and Shape Distribution (dmMean Particle Size, CQVCoefficient of Quartile Variation, ARAspect Ratio), Yield, Purity, and Impurity Profile of Crystallized Curcumin Isolated after Each Successive Crystallizationa crystallization conditions no.

stirring

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

overhead

magnetic

cooling rate (°C/h) 8 8 8 8 8 8 8 8 8 40 40 40 8 8 8 8 8 8 8 8 8 40 40 40 40 40 40 40 40 40

seedingb

I I I II II II

I I I II II II

I I I II II II

results step

phase compositionc

dm (μm)

CQV

first second third first second third first second third first second third first second third first second third first second third first second third first second third first second third

I I I I I I II II 40% II, 60% I I I I I I I I I I II II 95% II, 5% I I I I I I I II II 90% II, 10% I

44.2 48.1 50.1 22.1 42.3 45.8 48.3 49.4 47.6 32.8 39.0 38.3 20.4 26.5 27.2 16.3 15.6 19.1 16.0 18.4 19.8 18.2 23.6 26.2 16.8 21.3 23.7 13.1 17.6 19.5

0.39 0.38 0.34 0.24 0.48 0.42 0.55 0.57 0.53 0.45 0.34 0.36 0.21 0.28 0.41 0.26 0.22 0.25 0.19 0.22 0.21 0.3 0.24 0.29 0.25 0.32 0.32 0.14 0.21 0.26

AR/shaped 3.5 3.9 3.9 3.8 3.4 4.4 2.2 1.9 4.1 3.9 3.3 4.1 2.8 3.7 4.0 3.0 3.1 3.2 2.3 2.2 2.3 3.5 3.2 4.3 2.5 2.2 2.7 1.9 1.7 1.8

(R) (R) (R) (R) (R) (N) (S) (S) (R,S) (R) (R) (N) (R) (R) (N) (R) (R) (R) (R,S) (R,S) (R,S) (R) (R) (N) (R) (R) (R) (R,S) (R,S) (R,S)

yield (%)

CUR (%)

DMC (%)

BDMC (%)

65 73 75 68 74 77 70 75 75 66 74 76 75 78 82 74 80 82 76 79 83 76 78 80 77 79 82 77 80 81

92.57 96.67 98.34 93.38 97.21 98.74 93.64 97.71 98.86 91.46 95.89 97.75 93.07 97.08 98.4 93.49 97.6 98.99 94.63 98.41 99.85 92.3 96.49 98.1 92.54 96.76 98.2 93.41 97.33 98.77

7.11 3.33 1.66 6.34 2.79 1.26 6.21 2.29 1.14 8.24 4.11 2.25 6.65 2.92 1.60 6.26 2.40 1.01 5.23 1.59 0.15 7.40 3.51 1.90 7.21 3.24 1.80 6.44 2.67 1.23

0.32

0.28

0.15

0.30

0.28

0.25

0.14

0.30

0.25

0.15

a

Starting crude curcumin material: 17.7% DMC and 3.6% BDMC. bUnseeded (−); stable Form I (I) and metastable Form II (II) seeds (dm = 14 μm). cDetermined by semiquantitative Raman analysis. dShape description is given in parentheses: Rrod, Nneedle, Sspherulite.

Figure 3. Characteristic X-ray powder diffraction pattern (a) and Raman spectra (b) of curcumin samples isolated from unseeded (I) and Form II seeded (II) experiments. Calculated X-ray patterns from Cambridge Structural Database (CSD) of Form I (black) and Form II (blue) are shown as references. Raman shift at 1118 cm−1, used for semiquantitative analysis, is indicated with arrows.

as a continuous input variable (covariate), process parameters as categorical inputs (fixed factors), and crystal purity as an output variable. The effect is considered to be significant if the ANOVA p-value is less than 0.05 (95% level of significance).

The residual analysis of the ANOVA results was performed to support the validity of statistical ANOVA assumptions, such as normality and homogeneity of variance (Table S1, Figure S5). The experimental design and statistical analysis were performed 1596



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Figure 4. Light microscopy photographs of curcumin samples (Form I, identified by XRD and Raman) isolated from unseeded experiments at different crystallization steps (first and third step) and applying different stirring (overhead and magnetic). Scale bars 50 μm. Numbers denote experiments from Table 2.

Figure 5. Light microscopy photographs of curcumin samples isolated from seeded experiments using metastable From II seed at different crystallization steps (first and third step) and applying different stirring (overhead and magnetic). Scale bars 50 μm. Phase composition determined by semiquantitative Raman analysis is shown in Table 2 (numbers denote experiments from Table 2).

efficiency, and (4) yield. Three consecutive crystallizations were sufficient to produce a curcumin purity of >98%. The linear cooling rate, the seeding, and seed polymorph were found to be the most relevant for determining purity and physical properties of the curcumin crystals. 3.1. Polymorphic Composition. The results of structural analyses (Raman and X-ray diffraction) of the curcumin crystals obtained revealed that Form I was the only polymorph obtained at the end of each unseeded crystallization experiment

using MATLAB (R2015b, MathWorks) and Statistics and Machine Learning Toolbox.

3. RESULTS AND DISCUSSION The experimental conditions and the results of the purification of crude curcumin by cooling crystallizations are given in Table 2. Below these results will be analyzed in terms of the influence of the processing conditions on (1) polymorphic composition, (2) particles size and shape distributions, (3) purification 1597



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first and second successive crystallizations, respectively. In the third crystallization, the concentration of DMC is reduced to c(DMC) = 0.13 g/L; BDMC was not detected. Impurities can influence the relative growth rates of different faces,1,22,23 which leads to changes in crystal shape. Impurity molecules can preferentially adsorb onto specific rapidly growing faces, inhibiting their growth and consequently modifying crystal shape.3,23,24 The size distribution of particles is clearly influenced by stirring conditions. Typical particle size distributions for different stirring conditions are shown in Figure 6. The

(experiments 1−3, 10−12, 13−15, and 22−24, Table 2), as well as in systems where stable Form I seed was introduced (experiments 4−6, 16−18, and 25−27, Table 2). When using metastable Form II seed the polymorphic composition depends on the number of crystallization steps. After the first and second consecutive steps, pure Form II is obtained at the end of each crystallization experiment (experiments 7−9, 19−22, and 28−30, Table 2). However, in overhead stirring experiments the solid phase contains about 40% Form II/60% Form I at the end of the third crystallization in spite of seeding by Form II seed, and in magnetically stirred experiments the solid phase contains about 90% Form II/10% Form I. The typical diffractogram and Raman spectra of the dried curcumin samples, obtained after unseeded and Form II seeded crystallization experiments, are shown in Figure 3. The polymorphic forms obtained differ in their appearance; Form I crystals exhibit a yellowish-orange color, whereas metastable Form II crystals are reddish-orange. The differences in polymorphic outcome can be attributed to the solution impurity concentration at each consecutive crystallization step. At the first two consecutive crystallizations, high concentration of DMC (0.30 g/L < c(DMC) < 1.08 g/L) favors the formation of the metastable Form II, possibly due to inhibition of nucleation and/or crystal growth of the stable form, as demonstrated for other organic systems6,21 crystallizing in the presence of structurally related impurities. However, in the third successive crystallization at significantly lower DMC concentrations (0.09 g/L < c(DMC) < 0.13 g/L), the metastable polymorph is initially formed at first but partially transforms to the thermodynamically most stable Form I, within the experimental time. 3.2. Particle Size and Shape Distributions. Crystal shape was affected by the crystallization conditions as well as by the number of consecutive crystallizations (Table 2). Stable Form I presented a rod-like/needle-like habit, with a typical AR of about 3. The results indicate that the AR of Form I crystals increases with increasing number of successive crystallizations. The highest values are in the range from 3.9 to 4.4 and can be observed after the third crystallization (experiments 3 and 6, Table 2). Figure 4 shows typical light microscopy images of stable Form I crystals isolated from unseeded experiment at different crystallization steps (first and third step), where apparent changes of crystal shape occurred; curcumin crystals become less elongated. Metastable Form II mainly appears as spherulitic particles or a mixture of rod-like crystals and spherulites, depending on stirring conditions (Figure 5, Table 2). The spherulitic particles dominate in overhead stirring systems in the product from the first and second crystallization steps (experiments 7 and 8, Table 2), whereas a major habit change occurred in the third step (experiment 9, Table 2). However, mixtures of the two forms are obtained at the third step, which consequently increased the mean AR from about 2 to 4 (Form I exhibited elongated rods), since the particle AR distribution was determined without consideration of polymorphic form. Typical AR values of spherulite Form II particles varied in the range from 1.9 to 2.3 following the first and second crystallizations. Changes in AR of Form I can be related to impurity concentrations present in the crystallization system at different steps. Impurity concentrations are significantly reduced in each consecutive crystallization step. Typical impurity concentrations present in the unseeded crystallization systems is c(DMC) = 1.05 and 0.33 g/L and c(BDMC) = 0.25 and 0.02 g/L in the

Figure 6. Particle size distribution of curcumin samples isolated from magnetic (blue) and overhead (red) stirring systems (experiments 1 and 13 in Table 2, respectively); curves are fitted according to lognormal distribution. Inset shows stirring effects on mean size, dm (open dots) and coefficient of quartile variation, CQV (full dots); estimated marginal means (conf. limits 95%) are indicated.

calculated mean size and coefficient of quartile variation (CQV) for all investigated conditions are summarized in Table 2. The particles obtained using magnetic stirring are smaller with narrower distributions (lower CQV), than those obtained using overhead stirring. The mean size of particles varies from about 22 to 50 μm for experiments in which overhead stirring was used, and from about 13 to 27 μm for experiments which employed magnetic stirring. A significantly higher CQV value was found using overhead stirring (average value of 0.5) by comparison with magnetic stirring (average CQV values around 0.3). From the estimated marginal means (Figure 6, inset: according to an ANOVA model) none of the confidence intervals overlap, which means that marginal means differ significantly. The adjusted coefficient of multiple determinations was R2adj = 0.83, indicating that the ANOVA model was adequate to explain the variability in the experimental data. Besides the effect of different stirring conditions, the crystallization step number also shows significant effect on the mean size (p = 0.050). The mean size increases with increasing number of successive crystallizations. The observed effect of agitation on the particle size is probably a consequence of higher nucleation rates resulting in an increase of the total number of particles and decrease in their mean size. The two stirring systems have different hydrodynamic conditions which does affect the primary nucleation behavior25 as well as secondary nucleation and crystal growth. Regardless of agitation technique used, the mean particle size increases with decreasing impurity concentration. This suggests that the crystal growth rate of 1598



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Figure 7. Effect of crystallization step (a), cooling rate (b), and seeding (c) on curcumin purity. Correlation between the purity and produced particle size, dm, is plotted in (d) keeping crystallization number, seed polymorph, and cooling rate fixed to second step, Form I seeding, and slow cooling rate (8 °C/h). Estimated marginal means (conf. limits 95%) are indicated.

therefore the crystallization rate. If the crystals are grown slowly, at conditions not far from equilibrium, impurities are more easily rejected by the growing crystals. Moreover, it was found that crystallization of Form II improves the purification of curcumin significantly, perhaps indicating weaker affinity of the impurity molecules for the metastable Form II crystal structure. Incorporation of additives/impurities into organic crystals have been studied by several authors6,10,11,26 which showed that the mode and extent of interactions between impurity molecules and the host crystal lattice play a dominant role. For example, Simone et al.10 demonstrated that benzoic acid impurity has higher affinity for metastable polymorphs of aminobenzoic acid (Form II and III) by comparison with the thermodynamically most stable Form I, due to a specific system of hydrogen bonding in the crystal structure which prevents the inclusion of impurity molecules. Besides the experiments reported in Table 2, we have also made a few experiments using a larger seed size. When bigger seed crystals of stable Form I (dm = 49 μm, CQV = 0.4) were used in experiments with overhead stirring and slow cooling, at the same seed loading (5% w/w of the total dissolved curcumin), the purity was 92.60%, 96.76%, and 98.42% after the first, second, and third crystallizations, respectively. In comparison, with smaller seed crystals (dm = 14 μm, CQV = 0.3) as reported in Table 2 (experiments 4−6), the purity was higher especially for the first two crystallization steps. The detailed impurity profile present in curcumin samples isolated after consecutive crystallizations are presented in Table 2. DMC is the dominating impurity in the starting crude curcumin material (17.7% DMC and 3.6% BDMC) with BDMC/DMC ratio of 0.20. After the first crystallization the BDMC/DMC ratio decreases significantly, with typical ratio now being in the range 0.03−0.04. Obviously, BDMC is easier to remove than DMC; 60% of DMC is typically removed in first step and more than three steps are required for efficient removal (>91%), whereas in the case of BDMC 92% is removed immediately in first step, followed by complete removal in second step. The fact that DMC is harder to remove can be explained by differences in molecular structure. The DMC molecule is more similar to the curcumin molecule in

curcumin might be reduced by the presence of impurities, which consequently affect the size of crystals. This finding is in general agreement with the results of other authors which pointed out that size of the crystals increases during consecutive crystallizations as a result of less inhibited growth at lower impurity concentrations.12 3.3. Purification Efficiency. The results of the HPLC analysis of the crystallized curcumin samples are shown in Table 2. The results indicate that higher purity can be reached if slow cooling is applied. Also seeding contributes to the purification efficiency. The type of seed introduced has a pronounced effect on the purification. The highest purity is reached in the seeded experiments using metastable Form II seed under magnetic stirring and slow cooling (experiments 19−21, Table 2): 94.63%, 98.41%, and 99.85% curcumin after the first, second, and third crystallization steps, respectively. However, in the overhead stirring systems, a clearly lower purity is obtained. The final purity of the curcumin samples varied from about 93.6% to 98.9% (experiments 7−9, Table 2). The lowest purity is reached in unseeded crystallization experiments with overhead stirring and a high cooling rate (experiments 10−12, Table 2): 91.46% pure curcumin after the first crystallization step and the final purity of 97.75% obtained after 3 successive crystallizations. Figure 7 shows the dominant effect of crystallization step, cooling rate, and seeding on purity. Estimated marginal means are indicated, according to a statistical ANOVA model, R2adj = 0.993. By comparing the magnitude of the standardized effects it is evident that the effect of cooling rate is slightly stronger that the effect of seeding. A correlation between purity and product particle size was found (slope of the correlation, β = −0.098, p = 0.038) indicating that higher crystal purity is reached under conditions where smaller crystals are produced. The correlation between the purity and produced particle size is shown in Figure 7d under fixed parameters of second step, Form I seeding, and slow cooling rate. The observed effect of slow cooling and seeding is in general agreement with the literature results which show that highquality crystals (better purity and well-defined shape) can be produced at low supersaturations and low growth rates.12,26,27 The cooling rate influences the level of supersaturation and 1599



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that only one methoxy group is absent, while BDMC is lacking both (Table 1). Figure 8a shows the typical progress of the DMC concentration in the solid phase along successive crystalliza-

Table 3. DMC Impurity Decay Constant (ε) and Fractional Decrease Factor (n) Obtained for Different Process Conditions Using Crude Commercial Curcumin (xDMCo = 17.7%) as well as Recovered Crude Curcumin (xDMCo = 22.4%, xBDMCo = 1.9%)a from Collected Mother Liquors x0/%

stirring

cooling rate (°C/h)

17.7

overhead overhead overhead overhead magnetic magnetic magnetic magnetic magnetic magnetic magnetic

8 8 8 40 8 8 8 40 40 40 40

22.4a a

Figure 8. DMC mass fraction in solid samples, xDMC, as a function of crystallization step number, N, for two different experiments (slow cooling): using Form I seed, magnetic stirring (full dots) and unseeded, overhead stirring (open dots). Dashed curves are fitted according to eq 1. Inset shows the fractional decrease per crystallization step for the unseeded experiment (n = 0.87).

I II

I II I II

ε

n

0.91 1.03 1.05 0.77 0.94 1.04 1.22 0.87 0.90 1.01 0.89

0.87 0.86 0.92 0.91 0.89 0.93 0.98 0.87 0.87 0.89 0.92

Unseeded (−), stable Form I (I), and metastable Form II (II) seeds.

than 1 (n = 0.89 ± 0.03) which indicates that impurity fractional decrease per step reduces with each successive crystallization step (Figure 8, inset, n = 0.87). This means that the purification becomes gradually more difficult. On the other hand, it is also worthwhile to note that both ε and n are close to unity and accordingly the relation is close to a simple exponential decay where the fractional decay in impurity concentration is simply inversely proportional to the number of consecutive crystallization steps. The proposed model (eq 1) is useful for prediction purposes. By having only two experimental points, the composition of the initial crude material and after the first crystallization, the crystal purity in further steps can be effectively estimated. The same model was successfully applied to data gathered for another crude curcumin material, having 22.4% of DMC impurity. This material was obtained by recovering the mother liquor from the crystallization experiments and evaporation of the solvent, and the results are reported in Table 3. 3.4. Yield. Often the curcumin concentration in solution at the end of the experiments is higher than expected. The concentration decays to a value in the range of 1.5−2.1 g/L being clearly higher than the curcumin solubility in pure solvent = 0.9 g/L. Hence, the yield of curcumin is lower (65% < yield 99.1% curcumin. However, at more optimized process conditions: using metastable Form II seed, magnetic stirring and slow cooling (experiments 19−21, Table 2), the purity of about 99.8% is achieved by three steps and 98.4% already by second step. The dashed lines drawn through the experimental points (Figure 8) are obtained by fitting a generalized exponential decay function, eq 1: x DMC = x0 exp( −εN n)

seedinga

(1)

where x0 is the initial impurity fraction and N is the crystallization step number. It was found that the proposed model fits the experimental data well. Similar fittings were obtained for all experimental conditions investigated, and the values of parameters ε and n, estimated by Levenberg− Marquardt method of optimization, are listed in Table 3. It should be noted that parameter interaction was found to be reasonably low: the interdependency of ε and n, calculated according to the procedure as described elsewere,28 is below 0.49 (less than 49% of the variance in ε and n is due to its mutual interaction). With this model efficiency of purification can be described in a simple way by two parameters. The values of the decay constant (ε) are in the range 0.77 > ε > 1.22 and vary significantly depending on the conditions. Higher ε indicates a higher efficiency of purification. The highest values, ε > 1.0, are obtained under seeding and slow cooling rate conditions. Seeding (p = 0.009) and cooling rate (p = 0.006) significantly influence the ε parameter, whereas agitation has a relatively small effect (p = 0.075). On the other hand, no systematic effect of crystallization conditions on the fractional decrease factor (n) was found. The average value of n is lower 1600



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4. CONCLUSIONS Cooling crystallization of curcumin is sensitive to process parameters including seed polymorph, stirring method, temperature, cooling rate, and the number of crystallization steps, the latter being closely related to impurity concentrations. Manipulation of these process parameters allows for polymorph selection, particle habit and size control, purity, and yield control. This work demonstrates that impurities had an impact on crystal shape, polymorphic outcome, and yield of curcumin. It was found that the removal efficiency of demethoxycurcumin, the major impurity present in crude curcumin, exponentially decreases as a function of the number of successive crystallizations. Seeded crystallization and slow cooling are conditions that increase the purity of the curcumin crystals obtained in each step. In particular, using seeds of the metastable Form II significantly improves the purification and crystallization performance of curcumin. A multiple correlation analysis of the data suggests that higher crystal purity is reached under crystallization conditions where smaller crystals are produced. By carefully tuning the crystallization conditions, it is possible to improve the yield and the purification in each step. The number of successive crystallizations can be reduced, which further increase the overall yield. At substantially optimal crystallization conditions, the high-purity curcumin (>99.1%) can be obtained with an overall yield of 50%. The results of this research provide a better understanding of the influence of structurally related impurities on crystallization performance and final crystal properties and contribute to the development and optimization of high yield crystallization/ purification.

The decreasing yield at increasing impurity concentration can be due to an impurity effect on the solubility and/or on the kinetics of nucleation and crystal growth.5 Impurities incorporated into the crystal lattice can induce lattice strain leading to an increase in the solid−liquid solubility.1,29 However, the influence of impurities on the thermodynamics has not been investigated to a greater extent and is expected to only be of significant importance at fairly high impurity concentrations. On the other hand, it is well-known that impurities can have a very significant effect on crystal growth also at quite low impurity concentrations. The existence of a supersaturation zone where no effective crystallization occurs could point to growth inhibition by impurities adsorbed on the crystal surface blocking the growth sites.1,3,7,22,30 The overall yield of purified curcumin, obtained after multiple successive crystallizations, strongly depended on the crystallization conditions and the number of the steps needed to reach a desired purity. Figure 9 shows the overall yield as a

■ Figure 9. Overall yield of purified curcumin as a function of the number crystallization steps, N, for two different experiments (slow cooling): (open dots) using Form II seed, magnetic stirring and (full dots) unseeded crystallization, overhead stirring. Inserted numbers denote the purity of curcumin at corresponding points. Dashed curves are fitted according to eq 2.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.6b00153. HPLC calibration curves, crystallizer setup, Raman calibration curves for semiquantitative determination of polymorph composition, Raman spectra evaluated by PCA method, residual analysis of ANOVA model (DOC)

■

function of the number of crystallization steps, for two different process conditions of stirring and seeding. The lowest overall yield of 28% (per mass of curcumin initially dissolved) is achieved in unseeded crystallization systems using overhead stirring, where four successive crystallizations were performed to obtain 99.10% pure curcumin. However, in the seeded crystallization using metastable Form II seed and magnetic stirring (experiments 19−21, Table 2) the overall yield after three successive crystallizations was 50%, reaching even higher purity of 99.85%. If a target of the desired purity is set to about 98.4% the yield increases from 47% to 60%, for the same mentioned crystallization systems at third (98.34% pure) and second (98.41% pure) crystallization step, respectively. A similar expression, as in the case of purity (eq 1), can be applied in order to describe the overall yield along successive crystallizations (Figure 9): yield tot = 100 exp(−kN n)

ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by Science Foundation Ireland (Grant No. 12/RC/2275).

■

REFERENCES

(1) Sangwal, K. Additives and Crystallization Processes: From Fundamentals to Applications; John Wiley & Sons: Chichester, 2007. (2) Fiebig, A.; Jones, M.; Ulrich, J. Cryst. Growth Des. 2007, 7, 1623− 1627. (3) Salvalaglio, M.; Vetter, T.; Giberti, F.; Mazzotti, M.; Parrinello, M. J. Am. Chem. Soc. 2012, 134, 17221−17233. (4) Kuvadia, Z.; Doherty, M. Cryst. Growth Des. 2013, 13, 1412− 1428.

(2)

where N is crystallization step number, with the determined parameters being n = 0.80 and 0.84 and k = 0.43 and 0.28 in the case of the unseeded (overhead stirring, slow cooling) and the seeded experiment (experiments 19−21, Table 2), respectively. 1601



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(5) Shtukenberg, A.; Lee, S.; Kahr, B.; Ward, M. Annu. Rev. Chem. Biomol. Eng. 2014, 5, 77−96. (6) Mukuta, T.; Lee, A. Y.; Kawakami, T.; Myerson, A. S. Cryst. Growth Des. 2005, 5, 1429−1436. (7) Ottens, M.; Lebreton, B.; Zomerdijk, M.; Rijkers, M. P. W. M.; Bruinsma, O. S.; van der Wielen, L. A. Ind. Eng. Chem. Res. 2004, 43, 7932−7938. (8) Pino-Garcia, O.; Rasmuson, A. Cryst. Growth Des. 2004, 4, 1025− 1037. (9) Saleemi, A.; Onyemelukwe, I.; Nagy, Z. Front. Chem. Sci. Eng. 2013, 7, 79−87. (10) Simone, E.; Steele, G.; Nagy, Z. CrystEngComm 2015, 17, 9370−9379. (11) Cashell, C.; Corcoran, D.; Hodnett, B. K. Cryst. Growth Des. 2005, 5, 593−597. (12) Simone, E.; Zhang, W.; Nagy, Z. J. Chem. Technol. Biotechnol. 2016, 91, 1461−1470. (13) Borsos, A.; Majumder, A.; Nagy, Z. Cryst. Growth Des. 2016, 16, 555−568. (14) Hogan, D. E.; Crowley, L. M.; Stokes, S. P.; Lawrence, S. E.; Moynihan, H. A. Impurity Exclusion and Retention during Crystallisation and Recrystallisation  The Phenacetin by Ethylation of Paracetamol Process. In Advanced Topics in Crystallization; Mastai, Y., Ed.; InTech: Rijeka, 2015; Chapter 3, http://dx.doi.org/10.5772/ 59715. (15) Pandey, A.; Gupta, R. K.; Srivastava, R. Asian J. Appl. Sci. 2011, 4, 343−354. (16) Hatcher, H.; Planalp, R.; Cho, J.; Torti, F. M.; Torti, S. V. Cell. Mol. Life Sci. 2008, 65, 1631−1652. (17) Revanthy, S.; Elumalai, S.; Benny, M.; Benny, A. J. Exp. Sci. 2011, 2, 21−25. (18) Liu, J.; Svärd, M.; Hippen, P.; Rasmuson, A. C. J. Pharm. Sci. 2015, 104, 2183−2189. (19) Sanphui, P.; Rajesh, G. N.; Khandavilli, U. B. R.; Bhanoth, S.; Nangia, A. Chem. Commun. 2011, 47, 5013−5015. (20) Wichitnithad, W.; Jongaroonngamsang, N.; Pummangura, S.; Rojsitthisak, P. Phytochem. Anal. 2009, 20, 314−319. (21) Gu, C.; Chatterjee, K.; Young, V.; Grant, D. J. Cryst. Growth 2002, 235, 471−481. (22) Yang, X.; Qian, G.; Duan, X.; Zhou, X. Ind. Eng. Chem. Res. 2012, 51, 14845−14849. (23) Ukrainczyk, M.; Stelling, J.; Vucak, M.; Neumann, T. J. Cryst. Growth 2013, 369, 21−31. (24) Ukrainczyk, M.; Greiner, M.; Elts, E.; Briesen, H. CrystEngComm 2015, 17, 149−159. (25) Liu, J.; Rasmuson, A. C. Cryst. Growth Des. 2013, 13, 4385− 4394. (26) Kurihara, K.; Miyashita, S.; Sazaki, G.; Nakada, T.; Durbin, S.; Komatsu, H.; Ohba, T.; Ohki, K. J. Cryst. Growth 1999, 196, 285−290. (27) Adawy, A.; van der Heijden, E. G.; Hekelaar, J.; van Enckevort, W. J. P.; de Grip, W. J.; Vlieg, El. Cryst. Growth Des. 2015, 15, 1150− 1159. (28) Ukrainczyk, N. Chem. Eng. Sci. 2010, 65, 5605−5614. (29) Heberling, F.; Vinograd, V. L.; Polly, R.; Gale, J. D.; Heck, S.; Rothe, J.; Bosbach, D.; Geckeis, H.; Winkler, B. Geochim. Cosmochim. Acta 2014, 134, 16−38. (30) Ukrainczyk, M.; Gredicak, M.; Jeric, I.; Kralj, D. Cryst. Growth Des. 2014, 14, 4335−4346.

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Process Parameters in the Purification of Curcumin by Cooling

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