Cellulose: chemical modification CHEM-E2140 Eero Kontturi 3 rd November 2015
Outline (1) Chemical modification of cellulose motivation (2) Background: terminology, challenges (3) Esterification of cellulose Inorganic esters: - xanthogenate - carbamate Organic esters: - formate - acetate (4) Etherification of cellulose: - alkyl ethers - hydroxyalkyl ethers - carboxymethyl cellulose (5) Regioselectivity in chemical modification of cellulose
Motivation for cellulose modification Preparation of substances that have different properties from cellulose, yet they are of renewable origin and biodegradable - One of the most important properties is that most cellulose esters and ethers are thermoplastic (cellulose is not) Modified cellulose, i.e., cellulose derivatives often possess properties that are not easily achieved with totally synthetic polymers (With nanocellulose) modify the surface of nanocelluose to achieve better compatibility with its environment (composites etc.)
Basic concepts The idea of chemical modification of cellulose is to introduce functional groups in the cellulose backbone OH OR O HO O OH n O O RO O OR n O Usually achieved by substituting the protons in the hydroxyl groups of cellulose to a varying extent
Basic concepts Degree of substitution (DS) Quality which measures the average amount of substituted hydroxyl groups in an anhydroglucose unit n/2 Cellulose acetate with DS=2 Carboxymethyl cellulose with DS=0.5
Basic concepts Regioselectivity: which OH group/groups are modified
Basic concepts (1) Homogeneous modification Cellulose is dissolved and individualized cellulose chains are modified in a homogeneous solution Uniform, homogeneous modification (2) Heterogeneous modification Fibres, microfibrils, nanocrystals etc. are modified in a heterogeneous suspension Usually results in surface modification (not necessarily)
Challenges The fundamental challenge in chemical modification of cellulose is that cellulose is relatively inert and does not automatically obey the common rules of organic chemistry (For example, cellulose hydroxyl groups are alcohols but they do not form esters with carboxylic acids under normal conditions) Regioselectivity is often difficult to achieve
Esterification of cellulose
Commercial cellulose esters Klemm et al. Angew. Chem. Int. Ed. 2005, 44, 3358.
Esterification of cellulose Inorganic esters: - cellulose xanthogenate - cellulose carbamate - cellulose sulphate - cellulose nitrate
Cellulose xanthogenate Cell-OH Cell-O - Cell-O - + CS 2 Cell-O-CSS - Na + Half ester, achieved by a reaction between cellulose alkoxy ion (Cell-O - ) and carbon disulfide (CS 2 ) in alkaline medium Cellulose xanthogenate is water-soluble because of the polyelectrolyte (charged) nature it introduces to the cellulose chain The reaction is important in practice because of its use in viscose process - cellulose xanthogenate is produced from native cellulose - cellulose xanthogenate is dissolved - the dissolved xanthogenate is regenerated in acid solution, enabling controlled regenaration of cellulose into fibres and films
Cellulose xanthogenate Cell-OH Cell-O - Cell-O - + CS 2 Cell-O-CSS - Na + Chemistry of xanthogenation is relatively complex: ca. 70% of CS 2 input is converted into cellulose xanthogenate the rest is consumed by mainly formation of inorganic sulfidic products
Cellulose xanthogenate Cell-OH Cell-O - Cell-O - + CS 2 Cell-O-CSS - Na + Xanthogenated cellulose material DS at C-2/C-3 DS at C-6 Fiber xanthogenate (DS 0.61) 0.38 0.17 Viscose, non-ripened (DS 0.58) 0.34 0.24 Viscose, moderately ripened (DS 0.49) 0.16 0.32 Viscose, extensively ripened (DS 0.28) 0 0.32 Ripening: phase of the viscose process where the DS of xanthogenate is reduced to the level that is desired for the fibre spinning process. Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose carbamate High temperature reaction (~140 ): processed above the melting point of urea Catalyzed by metal salts, particularly zinc sulphate is used Urea forms isocyanic acid which is the actual reagent with cellulose: O 140 C NH C O + NH 3 H 2 N NH 2
Cellulose carbamate Cellulose carbamate with DS 0.2-0.3 can be dissolved in aqueous NaOH Basis for the CarbaCell process: Aimed at substituting the environmentally hazardous viscose process Enabled by the effortless conversion of carbamate into cellulose in alkali No commercial applications as of yet
Cellulose sulphate Can be prepared in a large variety of systems, usually containing either SO 3 or sulphuric acid Water soluble at above DS 0.2-0.3 Preparation is generally accompanied by severe chain degradation due to acid hydrolysis No commercial applications as of yet
Cellulose nitrate Traditionally produced in a ternary system: HNO 3 /H 2 SO 4 /H 2 O Nitrogen contents of commercial cellulose nitrates range from 10.5-13.6% Nitrogen content vs. DS Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose nitrate The chemistry of traditional nitration is relatively complex: Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose nitrate Nitrogen content vs. reaction time on nitration of spruce sulphite pulp Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose nitrate Celluloid (combs, hair ornaments, ping pong balls) Explosives (nitrogen content above 12.6%) Filters, membranes Component in lacquers
Esterification of cellulose Organic esters: - Cellulose formate - Cellulose acetate
Cellulose formate O HO OH O OH O HCOOH H 2 O O O O O O O O Among the only organic esterifications that proceed spontaneously with the free acid itself Cellulose formate is unstable: cellulose formate with DS 2.0-2.5 is decomposed to cellulose and formic acid in 10 h in boiling water
Cellulose formate Effect of catalyst on the formylation of cellulose Very high DS values of cellulose formate can be achieved Philipp et al. Cellulose Chem. Technol. 1990, 24, 667.
Cellulose acetate Cellulose acetylation proceeds with acetic anhydride and a suitable catalyst in water-free conditions
Cellulose acetate preparation Heterogeneous acetylation (2 different methods): (1) Fibrous acetylation : native cellulosic fibre is acetylated and no clear changes in gross morphology of the fibres are observed (2) Solution process: native cellulosic fibre is acetylated and as the acetylation proceeds, the acetylated cellulose chains on the surface are solubilized into the solution NOTE: The solution process (2) is occasionally called homogeneous acetylation, but in reality it is a heterogeneous process that may eventually lead into a homogeneous product (cellulose acetate solution). NOTE: All industrial procedures are initially heterogeneous processes.
Cellulose acetate preparation Schematic view on the solution process: acetylated cellulose chains are released to the solution from the cellulose microfibril surface.
Cellulose acetate preparation Acetylation Cross sectional views of Valonia microfibrils upon heterogeneous solution acetylation. Electron microscopy and schematic images. Sassi and Chanzy Cellulose 1995, 2, 111.
Cellulose acetate First, cellulose triacetate (CTA) is formed after which it is deacetylated in homogeneous conditions.
Cellulose acetate deacetylation Medium: DMSO/Water/aliphatic diamine Hexamethylenediamine Dimethylamine Aliphatic amines are selective towards C-2 /C-3 in deacetylation. Regioselectivity of deacetylation depends on the reaction medium Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose acetate solubility Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Cellulose acetate applications
Cellulose ethers - methyl cellulose - carboxymethyl cellulose - hydroxyethyl cellulose - hydroxypropyl cellulose
Commercial cellulose ethers
Methyl cellulose preparation Conventional preparation by Williamson reaction with gaseous or liquid chloroform (S N 2 type nucleophilic substitution) 40% NaOH used in the industrial procedure (heterogeneous reaction) DS 1.5-2.0 are produced commercially
Methyl cellulose preparation Heterogeneous methylation of alkali cellulose with excess CH 3 Cl 90 C 80 C 70 C 90 C 80 C 70 C Fast initial stage in methylation Levelling off at just below DS 2 Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Methyl cellulose solubility DS dependence of solubility of methyl cellulose (and ethyl cellulose) Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Methyl cellulose solubility At DS 1.7-2.3 the solubility of methyl cellulose is temperature sensitive at ca. 50-60 C: it forms gels above a critical temperature and the gelation is reversible along the hysteresis loop. Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Methyl cellulose applications Some applications of methyl cellulose
Carboxymethyl cellulose The most important ionic cellulose derivative Generally produced by a substitution reaction of monochloroacetic acid to alkoxy cellulose
Carboxymethyl cellulose - preparation Requires 20-30% NaOH concentration 1 mol NaOH consumed per 1 mol CMC substitution (main reaction) 2 mol NaOH consumed per 1 mol sodium glycolate (side reaction) Temperature 50-70 C Exothermic process Heterogeneous process in water/isopropanol (or water/t-butanol)
Carboxymethyl cellulose - preparation Commercial grades possess DS values 0.4-0.8 CMC is water-soluble when DS>0.4
Carboxymethyl cellulose - solubility Aqueous CMC solution does not usually represent a complete dissolution down to the molecular level Considerable part of the solution could be separated from the solution by centrifugation, indicating poorly dissolved aggregates etc. Higher DS generally less insoluble fragments However, depends on starting material and preparation process Klemm et al. Comprehensive Cellulose Chemistry Vol. 2, Wiley-VCH, 1998.
Carboxymethyl cellulose applications 99.5 % Information from CPKelco
Hydroxyethyl cellulose Hydroxyethyl cellulose (HEC) A range of reactions occurs when HEC is prepared with ethylene oxide in water The hydroxyalkyl side chain can grow further from the first substitution
Hydroxyethyl cellulose Quantification of substitution: DS: as usual, i.e., number of substituted hydroxyl groups in cellulose per anhydroglucose unit MS: molecular substitution, denoting the average number of alkylene oxide molecules added per anhydroglucose unit
Hydroxyethyl cellulose MS/DS ratio represents the average length of the HEC side chains With high substitutions, MS/DS reaches a plateau Arisz et al. Cellulose 1996, 3, 45.
Hydroxyethyl cellulose preparation Heterogeneous system NaOH/cellulose between 0.3:1 to 1:1 H 2 O/cellulose between 1.2:1 to 3.5:1 Temperature: 30-80 C MS (DS) determined by ethylene oxide input
Hydroxyethyl cellulose Not thermoplastic: decomposes below 100 C Soluble in water above MS 1 Major applications:
Hydroxypropyl cellulose Thermoplastic (can be extruded at 160 ) Soluble in cold water at MS 4 Gelling in water at 40 C, precipitation at 45 C
Regioselective cellulose modification
Background Generally, cellulose hydroxyl groups react in the order O6>O2>O3 Reactivity of different hydroxyl groups can be tuned by reaction conditions but they are rarely exclusive Regioselective synthesis applies various pathways to achieve nearly complete regioselectivity of certain OH group / groups Regioselectively prepared cellulose derivatives yield information on the structure-property relationship of polysaccharides and the function of the different hydroxyls on cellulose
Background Terminology
Pathways to regioselectivity Regioselective reactions with cellulose nearly always require homogeneous reaction conditions where cellulose is completely dissolved Solvents applied: dimethylacetamide/licl, tert-butyl ammonium fluoride / dimethylsulfoxide (TBAF/DMSO)
Pathways to regioselectivity Koschella et al. Macromol. Symp. 2006, 244, 59.
Protecting group: trityl One of the most popular protective groups for regioselective modification is triphenylmethyl (trityl):
6-mono-O-methyl cellulose Two different protecting groups: trityl and 1-propenyl Kondo et al. Carbohydr. Res. 1993, 238, 231.
Protecting group: TDMS = Thexyldimethylsilyl Highly swollen yet heterogeneous medium exclusive for C-6 Homogeneous 2,6 selective Koschella et al. Macromol. Symp. 2006, 244, 59. Fox et al. Biomacromolecules 2011, 12, 1956.
3-mono-O-ethyl cellulose Soluble in water and DMSO Koschella et al. Polym. Bull. 2006, 57, 33.
Summary Important cellulose esters: cellulose xanthogenate, cellulose nitrate, cellulose acetate Important cellulose ethers: methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose Regioselective cellulose modification is a modern trend; it is an important scientific advance in cellulose modification