Cellulose DOI 10.1007/s10570-013-9938-y
ORIGINAL PAPER
Preparation of amidated derivatives of monocarboxy cellulose ˇ opı´kova´ Toma´sˇ Taubner • Jana C Pavel Havelka • Andriy Synytsya
•
Received: 10 December 2012 / Accepted: 29 April 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract Amidated derivatives of monocarboxy cellulose (MCC), the product of cellulose oxidation, containing carboxyl groups only at C-6 position, were prepared and characterised. Two-step way of amidation was based on the esterification of C-6 carboxyls in MCC by reaction with methanol at 60 °C for 72 h and further amino-de-alkoxylation (aminolysis) of the obtained methyl ester with n-alkylamines, hydrazine and hydroxylamine in the N,N-dimethylformamide medium. Purity and substitution degree of the products were monitored by vibration spectroscopic methods (FTIR and FT Raman) and organic elemental analysis. Analytical methods confirmed the preparation of highly or moderately substituted N-alkylamides, hydrazide and hydroxamic acid of MCC. Keywords Amidated derivatives of monocarboxy cellulose Amino-de-alkoxylation (aminolysis) Oxidized (monocarboxy) cellulose Abbreviations DA Degree of amidation DM Degree of methylation T. Taubner (&) J. Cˇopı´kova´ A. Synytsya Department of Carbohydrates and Cereals, ICT Prague, Technicka´ 1905, 166 28 Prague 6, Czech Republic e-mail:
[email protected] P. Havelka VUOS a.s., Rybitvı´ 296, 533 54 Pardubice-Rybitvı´, Czech Republic
DMF DO FT FTIR MCC
N,N-Dimethylformamide Degree of C-6 oxidation Fourier-transform Fourier-transform infrared spectroscopy Monocarboxy cellulose
Introduction Cellulose is the most abundant biomass on the surface of the Earth. It is the basic raw material for the preparation of a wide range of products with desired properties for food, cosmetic and biomedical applications. The three hydroxyl groups of a cellulose molecule can undergo chemical reactions common to all primary and secondary alcohols, such as esterification, nitration, etherification and oxidation (Stilwell et al. 1997). The term of oxidized cellulose labels any materials which are prepared by oxidation of this polysaccharide: alcohol groups are thus converted to carbonyl groups, often to carboxyls. Oxidation reduces mechanical properties of cellulose fibre drastically and increases solubility and absorbability by biological tissues (Rysˇava´ et al. 2003). Monocarboxy cellulose (MCC), i.e. oxidized cellulose with uronic carboxyl group at C-6 position (Fig. 1), is the most suitable for various medicine applications due to its biodegradability, biocompatibility and non-toxicity (Gajdziok et al. 2007). Oxidized celluloses containing carboxylic groups represent a new class of biodegradable
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Cellulose
materials. They have been accepted for use in humans to stop bleeding during surgery and to prevent the formation and reformation postsurgical adhesions. Studies also show that they possess antibacterial activities, and may be useful in bone regeneration and periodonal therapy (Kumar and Yang 1999). MCC is material widely applied in pharmaceutical industry, food industry and other fields. Properties of oxidized cellulose depend on the preparation process. Oxidation at neutral or acidic conditions leads to product with strongly reducing properties because of the aldehyde groups; oxidation in alkaline conditions leads to polycarboxylic acids. Oxidized cellulose is insoluble in water, acids and common organic solvents, but partially soluble at aqueous solutions of inorganic and organic bases (alkali hydroxides, ammonium, amines etc.) forming corresponding salts (Gajdziok et al. 2007; El-Sakhawy and Milichovsky 2000). Currently, oxidized cellulose containing 16–24 % (m/m) of carboxylic groups is commercially available in powder, gauze and fabric forms. Considerable efforts are being made to develop new, cost-effective methods that would also provide products with different levels of oxidation and degree of polymerization (Kumar and Yang 1999). Oxidized cellulose itself can undergo further chemical modifications that may lead to obtaining of new perspective materials. For example, Follain et al. (2008) modified polyglucuronic acid (oxidized cellulose III) by a reaction with n-butylamine, n-octylamine and 2-methoxy-ethylamine in aqueous media and in the presence of carbodiimide. This work is focused on the preparation of amidated oxidized cellulose derivatives by two-step way including esterification of C-6 carboxyls with methanol and further amino-de-alkoxylation (aminolysis) of the Fig. 1 Preparation of Nalkylamides, hydrazide and hydroxamic acid of MCC
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obtained methyl ester with seven selected primary amines, hydrazine and hydroxylamine. Materials and methods Monocarboxy cellulose (MCC) containing 18.18 % m/m of COOH groups was obtained from VUOS a.s. (Rybitvı´, Czech Republic). It was prepared by selective oxidation of pure a-cellulose with dilute nitric acid mixed with concentrated sulphuric acid and sodium nitrite (Stilwell et al. 1997). MCC was pulverized by ball mill to a white powder. Five n-alkylamines (n-butylamine, n-hexylamine, n-octylamine, n-dodecylamine and n-octadecylamine), ethylendiamine, ethanolamine, hydrazine hydrochloride and hydroxylamine hydrochloride were purchased from Fluka, Germany. N,N-Dimethylformamide, ethanol and acetone were purchased from Lachema, Czech Republic. Cellulose was obtained from Sigma-Aldrich, USA. Preparative procedures Preparation of the methyl ester of MCC Oxidized cellulose 1 (1 g) was suspended in 200 ml of methanol containing 1 ml of sulphuric acid. The reaction was carried out at heterogeneous conditions with stirring at 25 °C or 60 °C for 72 h. The reaction product (2) was filtered, washed with ethanol and acetone, and then dried on air. Preparation of the sodium salts of MCC and its methyl ester Oxidized cellulose 1 or its methyl ester 2 (1 g) was suspended in 200 ml of 80 % aqueous ethanol
Cellulose
containing 3 moll-1 of sodium chloride. The mixture was stirring at 25 °C for 24 h. The products (1a and 2a) were filtered, washed with 50 % aqueous ethanol and acetone, and then dried on air. The sodium salts of MCC 1a and of MCC methyl ester 2a were obtained for analytical purpose.
C-6 oxidation (DO, mol %) was calculated according to the equation: DO ¼
COOH 12 6 100 C 45
where C and COOH are contents of total carbon and COOH groups (% m/m), respectively.
Preparation of amidated derivatives of MCC Organic elemental analysis N-Alkylamides, hydrazide and hydroxamic acid of MCC (3–11) were prepared by the reaction of methyl esterified MCC with seven primary amines (butylamine, hexylamine, octylamine, dodecylamine, octadecylamine, ethylendiamine and ethanolamine), hydrazine and hydroxylamine reagents, respectively. Methyl ester of MCC 2 (0.5 g) was suspended in 50 ml of N,Ndimethylformamide (DMF) by stirring for 30 min and then 10 ml of amidation reagent (butylamine, hexylamine, octylamine, ethylendiamine and ethanolamine) was added. The reaction was carried out at 25 °C for 24 h. Mixture of 50 ml of N,N-dimethylformamide (DMF) with 5 g of amidation reagent (dodecylamine, octadecylamine) was prepared by stirring at 55 °C for 30 min. Then methyl ester of MCC 2 (0.5 g) was added and the reaction was carried out at the same temperature for 72 h. Hydrazide and hydroxamic acid of MCC (10, 11) were prepared by reaction of methyl ester of MCC (0.5 g) with amidation agent (4 g of corresponding hydrochlorides dissolved in 50 ml of 1 moll-1 NaOH by stirring for 30 min) at 25 °C for 24 h. The reactions with dodecylamine, octadecylamine and ethylendiamine were carried out at heterogeneous conditions, so the products 6–8 were obtained in the solid state by filtration. Reaction with the rest reagents (butylamine, hexylamine, octylamine, ethanolamine, hydrazine and hydroxylamine) led to complete dissolving of the polysaccharide, so the conditions were homogeneous; the products were precipitated by distilled water (3–5) or by acetone (9–11) dependently on the reagent used. The solids were filtered, subsequently washed with acidified ethanol (7:1 mixture of ethanol and 4 moll-1 HCl), pure ethanol and acetone, and finally dried on air. Analytical methods Determination of COOH The content of carboxyls in MCC was estimated by titration method (Kumar and Yang 1999). Degree of
Initial MCC 1 and all the reaction products 2–11 were analysed by organic elemental analysis (contents of C, H and N) on Elementar vario EL III (Elementar, Germany) equipment. The degree of amidation (DA, mol %) of the final products was calculated based on the carbon (C) and nitrogen (N) contents (% m/m) according to the formulae: DA ¼
N 12 6 100 C n 14 N c 12
where c and n are the numbers of carbon and nitrogen atoms contents in the amine molecule. The calculations are based on the assumption supported by FTIR analysis that at the end of the reactions all the initial methyl ester groups are hydrolysed or substituted by amine. Microscopic image analysis The samples of cellulose, initial MCC 1 and all the reaction products 2–11 were spread on the sample glass through the special steel tube and put under objective 4 of an optical scanning microscope Nicon SM2-2T (Nicon, Japan) equipped with Intralux 4000-1 lamp, colour camera and digitizer Micro-Movies. Images were focused, digitised and decomposed into pixels using a 256-dot scale. The image processing was made using a LUCIA system. FTIR spectroscopy FTIR spectra of the polysaccharide samples in KBr discs were recorded on FTIR spectrometer Nicolet 6700 (Thermo Scientific, USA); the spectral range of 4000–400 cm-1, 64 scans, spectral resolution 2.0 cm-1. The FTIR spectra were processed using Omnic 7.3 (Thermo Scientific, USA) and Origin 6.0 (Microcal Origin, USA) software. The degree of methylation (DM, mol %) of CMC methyl ester was
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Cellulose
determined by the use of FTIR spectra and organic elemental analysis. Calibration samples (KBr pellets) were prepared by mixing of MCC and its methyl ester (both in the sodium salt form) at various ratios (1:7, 1:3, 1:1, 3:1 and 7:1 m/m). The spectra of MCC sodium salt, MCC methyl ester (acidic and sodium salt forms) and of the mixtures were smoothed (FFT 11 points), baseline corrected and normalised at * 1060 cm-1 (CO and CC stretching vibrations in pyranoid ring). The MCC salt contribution w0 (% m/m) in MCC ester was obtained from the calibration plot {w; H1 - H0}, where w is relative content of MCC salt admixture (% m/m), and H1 - H0 is the absorbance height (H1) at 1611 cm-1 (antisymmetric stretching vibration of COO-) corrected on the contribution of neighbour overlapping band (H0) centred at 1643 cm-1 (in-plane bending vibration of water). The DM value (mol %) was calculated according to the formulae: DM ¼
6 Ce 100 ; 7 Ct Ce
where Ct is carbon content in MCC ester in sodium salt form, and Ce is carbon contribution of the esterified units: DO 7 100 C t Cs Ce ¼ ; DO 6 þ 100 where Cs is carbon contribution of the non-esterified salt units: 0
Cs ¼
w0 Ct DO ; 10000
where Ct0 is carbon content in MCC sodium salt. FT Raman spectroscopy FT Raman spectra of the polysaccharide samples were recorded by using Bruker FT-Raman (FRA 106/S, Equinox 55/S) spectrometer equipped with a quartz beam splitter, a liquid nitrogen cooled germanium detector and excitation at 1064 nm from a Nd:YAG laser. The laser power was set at 100 mW, the spectral range was 3600–150 cm-1, and 256 scans were accumulated with a spectral resolution of 2.0 cm-1. The spectra were exported as text files and processed using Origin 6.0 (Microcal Origin, USA) software.
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Fig. 2 Microscopic images of the particle ensembles of cellulose (A), MCC 1 (B) and MCC methyl ester 2 (C)
Results and discussion Preparation of methyl ester and amidated derivatives of MCC Esterification of MCC with methanol was carried out at heterogeneous conditions using acidic catalysis
Cellulose Fig. 3 Microscopic images of the particle ensembles of amidated MCC derivatives 3–11 (A–I)
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Cellulose Table 1 Organic elemental analysis of MCC and its derivatives Sample
Reagent
Content (% m/m) N
C
Substitution (mol %) H
DO
DM
DA
–
–
1
HNO3/H2SO4/NaNO2
0
37.09
4.99
1a
NaCl
0
33.98
5.06
2
Methanol
0
40.51
5.59
2a
NaCl
0
38.94
5.51
3
n-Butylamine
0.85
41.96
6.32
–
–
11.25
4
n-Hexylamine
1.77
44.73
6.77
–
–
25.57
5
n-Octylamine
1.48
44.51
6.72
–
–
22.21
6
n-Dodecylamine
1.67
48.88
7.69
–
–
27.17
7
n-Octadecylamine
2.58
60.07
10.72
–
–
65.19
8
Ethylendiamine
5.83
36.78
6.07
–
–
47.15
9
Ethanolamine
3.67
39.86
6.07
–
–
56.22
10
Hydrazine
2.90
36.66
6.21
–
–
20.35
11
Hydroxylamine
2.48
36.25
5.79
–
–
35.21
(Eq. 1), so this process depended on the surface interaction between solid phase (polysaccharide suspension) and liquid medium (methanol/H?). MCCCOOH þ CH3 OH ! MCCCOOCH3
ð1Þ
It was important to estimate the structural changes of solid polysaccharide phase via esterification. The microscopic images of commercial cellulose, MCC 1 and MCC methyl ester 2 are demonstrated in Fig. 2A–C. The powder of 1 consists of roundish, or oblong or shapeless particles (Fig. 2B). By contrast, the particles of 2 (Fig. 2C) are fibrous and very similar to cellulose (Fig. 2A). Cellulose oxidation at mild conditions leads to the formation of carboxylic groups at C-6 position. These carboxyls are involved into the system of intraand intermolecular hydrogen bonds. In addition, modified macromolecules became more hydrophilic and flexible. As a result, clustering of the polysaccharide chains into globular structures may occur. Methyl esterification of carboxylic groups in oxidized cellulose leads to some changes in physical properties of the polysaccharide: improves thermal stability (Fukuzumi et al. 2010), reduces swelling and ion conductivity of the layers in polysaccharidic films (Mu¨ller et al. 2010). Subsequent esterification of oxidized cellulose transformed carboxyl groups into methyl esters that significantly weakened their participation in hydrogen bonds. Therefore, esterification of 1 restored fibrilar structure of solid particles probably
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78.43 –
61.33
–
due to weakness of polar interactions between polysaccharide strains. Reagent molecules not only interacted on the surface of solid particles, but also penetrate into the particle and thus unroll a ball consisting of irregularly wound fibrilar structures. Amidation of MCC methyl ester 2 was carried out in N,N-dimethylformamide (DMF) using pure alkylamines or alkaline reagents, i.e. hydrazine HCl/NaOH (Eq. 2) and hydroxylamine HCl/NaOH (Eq. 3) in active free base form R–NH2 (Eq. 4). N2 H2þ 6 þ OH ! N2 H4 þ H2 O
ð2Þ
HONHþ 3 þ OH ! NH2 OH þ H2 O
ð3Þ
MCCCOOCH3 þ RNH2 ! MCCCONHR ð4Þ An excess of the reagent (up to 1:10 m/m) is necessary to move the equilibrium of whole process towards the amidated product (Sinitsya et al. 2000). DMF is reaction medium suitable for preparation of gentle polysaccharide suspension (Yevdakov et al. 1972) and has been previously used for amidation of HM citrus pectin with n-alkylamines at heterogeneous conditions (Synytsya et al. 2003a; 2004). For most of the reagents, however, complete dissolving of solid phase occurred during the reaction, so amidated products were then precipitated from the medium. Water and acetone were used for the precipitation of less (3–5) or more (9–11) hydrophilic derivatives,
Cellulose
respectively. The solubility of subsequently modified polysaccharide in the reaction medium depends on the relationship between polar and non-polar interactions. The morphology of resulting solid particles of 3–11 (Fig. 3A–I) were significantly influenced by the structure of reagent R–NH2, reaction conditions (homo- or heterogeneous) and solvent for precipitation of the product (water or acetone). However, summary effect of these factors was very complex and no trends were observed. Products 3–7, 9 and 10 had roundish and compact particles of variable size (Fig. 3A–E, G, H). The largest particles were observed for products 6, 7 and 9 (Fig. 3D, E, G) that could be explained by increased adhesion of polysaccharide chains due to hydrophobic (6, 7) or hydrophilic (9) interactions between the substituents. By contrast, despite of high substitution degrees, initial fibrous structure of solid particles persisted to a large extent for derivatives 8 and 11 (Fig. 3F, I). Similarly as it has been earlier
reported for amidation of HM citrus pectin (Sinitsya et al. 2000), several side reactions (Eqs. 5–8) leading to the formation of the N-alkylammonium, hydrazonium or hydroxylammonium salts of MCC may occur when admixture of water is present in the reaction mixture: RNH2 þ H2 O ! RNHþ 3 þ OH
ð5Þ
MCCCOOH þ OH ! MCCCOO þ H2 O ð6Þ MCCCOOCH3 þ OH ! MCCCOO þ CH3 OH
ð7Þ
MCCCOO þ RNHþ 3 ! MCCCOO þ H3 NR ð8Þ Subsequent washing of the solid products with acidified ethanol (7:1 mixture of ethanol and 4 moll-1 HCl), pure ethanol and acetone is necessary to remove
Fig. 4 FTIR (A) and FT Raman (B, C) spectra of MCC 1 and its methyl ester 2
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Cellulose
the excess of amidation reagent, convert free carboxylic groups into the protonated form by cation exchange between R–NH3? and H? on the polysaccharide matrix and thus remove the rest of amidation reagent bound as MCC salt (Eq. 9). MCCCOOþ H3 NR þ Hþ ! MCCCOOH þ RNHþ 3
ð9Þ
This conversion is important for DA estimation based on the results of elemental analysis and was previously confirmed by FTIR spectra. Organic elemental analysis Results of organic elemental analysis of MCC (1), MCC methyl ester (2), sodium salt of MCC (1a), sodium salt of MCC methyl ester (2a) and amidated derivatives 3–11 as well as the values of substitution degrees are summarised in Table 1. The degree of methylation (DM) of 2 (2a) was 61.33 %; the degrees of amidation (DA) of the final products were in the range of 11.25–65.19 %.
Fig. 5 A FTIR spectra (1900–1500 cm-1) of 2 (solid), 2a (dash), 1a/2a mixtures (1:7—dot; 1:3— dash dot; 1:1—dash double dot; 3:1—short dash; 7:1— short dot) and 1a (short dash dot). The arrows demonstrate determination of H0 and H1 values. B The plot {w; H1–H0} demonstrating graphical determination of w0
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FTIR and FT Raman analysis FTIR (Fig. 4A) and FT Raman (Fig. 4B, C) spectra of MCC (1) and its methyl ester (2) confirmed that the reaction of MCC with methanol led to the highly esterified product. Significant IR absorption around 2620 cm-1 (OH stretching of water and hydroxyls) and at 1420 cm-1 (COH bending in carboxyls) decreased after esterification. New IR and Raman bands of CH3 vibrations at 2957–2959 cm-1 (antisymmetric stretching), 1445 cm-1 (antisymmetric bending) and 915 cm-1 (rocking) in the corresponding spectra of 2 confirmed the presence of methyl esters (Sinitsya et al. 2000; Synytsya et al. 2003b). The IR/ Raman band of MCC at 1736–1738 cm-1 assigned to stretching vibration of C=O bonds (Saito et al. 2006; Zhang et al. 2012) was narrowed and shifted to 1740–1749 cm-1 for 2. These changes are also indicative to the conversion of free carboxyls COOH into esters COOCH3. FTIR spectra (1825–1525 cm-1) of the sodium salt of MCC (1a), MCC methyl ester in acidic (2) and
Cellulose
sodium salt (2a) forms and the 1a/2a mixtures are shown in Fig. 5A. The bands centred at 1740 and 1611 cm-1 were assigned to the C=O and antisymmetric COO- stretching vibrations, respectively. The smaller band of water in-plane bending vibration near 1643 cm-1 is well resolved in the spectrum of 2, but overlapped by the carboxylate band in the spectrum of 2a. The plot {w; H1 - H0} demonstrate graphical determination of w0 (Fig. 5B). Calculation of DM was complicated by the presence of three types of units in 2a, i.e. methyl ester of b-D-glucuronic acid, sodium b-D-glucuronate and b-D-glucose. The carbon contribution of each unit type was taken into the account. Obtained DM value for 2a was 61.33 mol % (Table 1) corresponding to molar reaction yield of 78.20 mol %. FTIR (Fig. 6A) and FT Raman (Fig. 6B, C) spectra of the products of amino-de-alkoxylation with five
n-alkylamines confirmed the presence of N-alkylamide substituents in the derivatives. The band at 1730–1738 cm-1 corresponding to C=O bonds of the reminder carboxyls. This peak is not so intense like in the spectrum of 2 and its position corresponds to nonesterified free COOH groups. Two IR bands at 1654–1657 and 1550–1554 cm-1 (amide I and amide II vibrations, respectively) appeared in the spectra of the products 3–5 (Sinitsya et al. 2000). Corresponding IR (Raman) bands of products 6 and 7 were intense, narrow and found at 1648 (1684) cm-1 and 1514 (1550) cm-1, respectively. In addition, a new weak and narrow IR/Raman band near 3310 cm-1 (NH stretching) was also found in the spectra of these two products. These vibration features point on the conformation change of modified cellulose to more ordered form after amidation. The presence of N-alkyl
Fig. 6 FTIR (A) and FT Raman (B, C) spectra of N-alkylamides of MCC (3–7) obtained by amino-de-alkoxylation with five n-alkylamines
123
Cellulose
Fig. 7 FTIR (A) and FT Raman (B, C) spectra of N-alkylamides (8, 9), hydrazide (10) and hydroxamic acid (11) of MCC obtained by amino-de-alkoxylation with ethanolamine, ethylendiamine, hydrazine and hydroxylamine
groups in the products was confirmed by new IR bands near 2920, 2850, 1467–1471, 719 and 535 cm-1 (antisymmetric stretching, symmetric stretching, inplane bending and rocking vibrations of CH2, and skeletal CCC bending, respectively), and by new Raman bands at 2883, 2850, 1441, 1296, 1132 and 1063 cm-1 (antisymmetric stretching, symmetric stretching, in-plane bending and waging/twist vibrations of CH2, and skeletal CC stretching, respectively) pronounced for highly substituted products of reaction with n-alkylamines with long alkyl chains (6 and 7) (Sinitsya et al. 2000; Synytsya et al. 2003a, 2004). FTIR (Fig. 7A) and FT Raman (Fig. 7B, C) of the other four products (8–11) prepared by amino-dealkoxylation with ethylendiamine, ethanolamine, hydrazine and hydroxylamine also confirmed the substitution. The IR/Raman band of C=O stretching vibration of reminder carboxyls was found at 1720–1740 cm-1. Two IR bands of products 8 and 9 at 1659–1667 cm-1 (1665–1670 cm-1 in Raman) and 1550 cm-1 (non-pronounced in Raman) were assigned
123
to amide I and amide II vibrations, respectively (Sinitsya et al. 2000). IR bands at 1721 cm-1 (amide I), 1638 cm-1 (scissoring of NH2) and 1530 cm-1 (amide II) of 10 were assigned to the hydrazide group (Tripathi and Katon 1979), while those at 1680, 1630 cm-1 (amide I), 1520 cm-1 (amide II) and 977 cm-1 (NO stretching) indicate hydroxamic acid in 11 (Higgins et al. 2006; Hope et al. 2010, 2011). Raman band or shoulder near 1600 cm-1 (products 8 and 10) was assigned to scissoring of NH2 in the substituents.
Conclusion Methyl ester and amidated derivatives of MCC were prepared and characterised by vibration spectroscopy and organic elemental analysis. The amidation degrees of the final products were in the range of 11.25–65.19 mol %. Administration of N-alkylamide, hydrazide or hydroxamic acid groups onto the cellulose macromolecule may significantly change physical and
Cellulose
chemical properties of the derivatives in comparison with cellulose or MCC. Furthermore, modified polysaccharides carrying additional reactive groups in the substituents may undergo further chemical modifications. Like MCC itself, its derivatives described here could be interesting for some medicinal applications. Acknowledgments This work was supported by the Ministry of Industry and Trade (project No 2A-1TP1/041), Ministry of Education, Youth and Sport (project No CEZ MSM 6046137305), Czech Science Foundation (project No 503/11/2479) and IGA (project No A1_FPBT_2013_010, A2_FPBT_2013_004, financial support from specific university research MSMT No 20/2013).
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