Process for selective oxidation of cellulose

Details Process for selective oxidation of cellulose Process for selective oxidation of cellulose

2001-11-21

2004-04-06

Wilson, James O. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carbohydrates or derivatives

C536S018500, C536S056000, C435S101000, C435S028000

Reexamination Certificate

active

06716976

ABSTRACT:

The invention relates to the oxidation of cellulose and cellulose derivatives using nitrosonium ions (oxoammonium ions) obtained by oxidation of nitroxyl radicals, especially 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO).
A process in which TEMPO is reoxidised by chemical means is known from a review by De Nooy in
Synthesis
1996, 1153-1174 and from WO 95/07303.
The oxidation of cellulose to 6-aldehydo-cellulose by photolysis of 6-azido-6-deoxy-celluloses was reported by Horton et al.
Carbohydrate Research,
26 (1973) 1-19.
It was found according to the invention that oxidation of cellulose, especially cellulose fibres, using nitrosonium ions, can be carried out without using chlorine-based oxidising agents and with the use of hydrogen peroxide or oxygen as the ultimate oxidising agent. The oxidation according to the invention is performed using enzymes capable of oxidation or transition complexes of a transition metal and a complexing agent as intermediate oxidants. This oxidation results in the formation of cellulose 6-aldehydes. The aldehydes may be present in the (hemi)acetal form and related structures. The process of the invention is further defined by the characterising features of the appending claims.
In the following description, reference is made to TEMPO only for the sake of simplicity, but it should be understood that other suitable nitroxyls, i.e. organic nitroxyl compounds lacking &agr;-hydrogen atoms, such as 2,2,5,5-tetramethylpyrrolidine-N-oxyl (PROXYL), 4-hydroxy-TEMPO, 4-acetamido-TEMPO and derivatives thereof and those described in WO 95/07303 can be substituted for TEMPO. These di-tert-alkyl nitroxyls are especially suitable for selectively oxidising primary alcohols to aldehyde functions, in particular in the presence of secondary alcohol functions that should not be oxidised. Less sterically hindered nitroxyls, such as 4,4-dimethyloxazolidine-N-oxyl (DOXYL), are suitable for preferentially oxidising secondary alcohols to keto functions, for example in the production of keto cellulose or keto starch. The active oxidising species is the nitrosonium ion (oxoammonium ion>N
+
═O), that is produced in situ by oxidation of the corresponding hydroxylamine and nitroxyl radical. If desired, the reaction can be performed in two steps, the production of the nitrosonium ion being the first and the oxidation of the alcohol function being the second.
A catalytic amount of nitroxyl is preferably 0.1-25% by weight, based on the primary alcohol functions of the cellulose, or 0.1-25 mol % with respect to the primary alcohol. The nitroxyl may also be immobilised, e.g. by coupling of the hydroxyl group of 4-hydroxy-TEMPO to a suitable carrier, or in the form of a polymeric nitroxyl such as: —[(CH
3
)
2
C—NO.—C(CH
3
)
2
—A—]
n
—, wherein A may be an alkylene group and/or a heteroatom, and n is a number form e.g. 10 up to several hundreds.
The process of the invention results in oxidation of cellulosic anhydroglucose units to the corresponding aldehydes. If required the primary products can be further oxidised to the corresponding carboxylic acids by using known oxidising agents such as hypochlorite, chlorite, hydrogen peroxide or by using TEMPO-mediated oxidation under more vigorous conditions such as an increased temperature e.g. from 40-80° C., or for prolonged exposure to the reaction conditions. Alternatively, the aldehyde/carboxylic acid ratio can be increased by using relative low pH's (e.g. pH 3-7), by controlled addition of oxidising agent, by lowering the oxygen concentration, or by first preparing the nitrosonium ion solution (two-step process).
A distinct group of compounds suitable for oxidation with the present process consists of hydroxyalkylated cellulose such as hydroxypropyl cellulose, hydroxyethyl cellulose.
The oxidation of the primary alcohol functions (6-CH
2
OH) results in the corresponding aldehydes and, if desired, to carboxylic acids, with intact ring systems. These products are useful intermediates for functional cellulose derivatives wherein the aldehyde groups are further reacted with e.g. amine compounds and the like. They are also useful intermediates for crosslinked cellulose derivatives, in which the aldehyde groups are further reacted with e.g. diamine reagents.
The catalysts to be used according to the invention are complexes of transition metals, i.e. coordination compounds between a transition metal and an organic molecule as a complexing agent having one or more free electron pairs, especially nitrogen compounds. Suitable nitrogen compounds include amino acids, phenanthrolines and other polyamines. A polyamine, which forms a complex with the transition metal, is understood to refer to compounds which comprise at least two amine nitrogen atoms, separated by at least two carbon atoms. Preferably, the polyamines comprise at least three nitrogen atoms which in each case are separated by two or more, in particular two or three, more in particular two, carbon atoms. The remaining valencies of the nitrogen atoms are preferably bound with small alkyl groups, in particular methyl. It is also possible for the polyamines to have ether or alcohol functions. The polyamines can be linear or cyclic. The polyamines should be alkaline, i.e. should not contain acid functions. Examples of polyamines which can be employed are 2,2′-bipyridyl, 2,2′-bipyrrole, 2-(dimethylaminomethyl)pyridine, tetramethylethylenediamine, pentamethyl-diethylenetriamine, 1,4-dimethylpiperazine, 1,4,7-trimethyl-1,4,7-triazonane (=triaza-cyclonanane), 1,4,7-trimethyl-1,4,7-triazecane, 1,4,7,10-tetramethyl-1,4,7,10-tetraaza-cyclododecane, 1,2-bis(4-methyl-1-piperazinyl)ethane, 1,2-bis(4,7-dimethyl-1,4,7-triazonan-1-yl)ethane, and the corresponding compounds wherein one or more of the said methyl groups have been replaced by, for example, ethyl groups. It is also possible to use porphin and other porphyrins and corresponding macrocyclic polyamine compounds. Histidine and comparable amino acids having an additional nitrogen atom, and their oligopeptides such as histidyl-histidine, are other examples of suitable complexing agents. Preference is given to compounds of the bipyridyl type, triazonane type and to amines whose remaining valencies are linked to methyl groups. The counterions required for neutrality of the complexes may be common, preferably non-toxic counterions such as oxide, halide, perchlorate, acetylacetonate, nitrate, sulphate and the like.
Transition metals to be used in the metal complexes include especially those of the fourth period of the periodic table of elements from vanadium to zinc, preferably manganese, iron, cobalt, nickel and copper, in particular manganese, iron, cobalt and copper. The corresponding metals from the higher periods may also be used, although less preferentially. The metal complexes require hydrogen peroxide, alkyl and ar(alk)yl hydroperoxides (such as tert-butyl hydroperoxide), oxygen or chlorite as an ultimate electron acceptor. About one metal atom to two to four nitrogen atoms of the compelling agent can suitably be used.
The metal complex may be used in a catalytic amount, e.g. in about an equimolar amount with respect to the nitroxyl compound. Suitable amounts of metal complexes are for example 1-25 mol % with respect to the alcohol to be oxidised.
The catalysts to be used according to the invention can also be oxidoreductases or other enzymes that are capable of oxidation in the presence of a suitable redox system. Oxidoreductases, i.e. enzymes capable of oxidation without the presence of further redox systems, to be used in the process of the invention include peroxidases and oxidases, in particular polyphenol oxidases and laccase.
Peroxidases (EC 1.11.1.1-1.11.1.11) that can be used according to the invention include the peroxidases which are cofactor-independent, in particular the classical peroxidases (EC 1.11.1.7). Peroxidases can be derived from any source, including plants, bacteria, filamentous and other fungi and yeasts. Examples are horse-radish peroxidase, soy-hull peroxidase, my

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