U.S. patent application number 11/006689 was filed with the patent office on 2005-06-09 for process for the separation of organic nitrosonium and/or hydroxylamine compounds by means of cation exchange resins and recovery and oxidation processes based thereon.
This patent application is currently assigned to SCA HYGIENE PRODUCTS AB. Invention is credited to Besemer, Arie, Jetten, Jan.
Application Number | 20050121160 11/006689 |
Document ID | / |
Family ID | 34635884 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050121160 |
Kind Code |
A1 |
Jetten, Jan ; et
al. |
June 9, 2005 |
Process for the separation of organic nitrosonium and/or
hydroxylamine compounds by means of cation exchange resins and
recovery and oxidation processes based thereon
Abstract
A process for the separation of a secondary organic nitrosonium
compound and/or a secondary organic hydroxylamine compound from an
acidic aqueous medium containing them in dissolved form, includes
the step of bringing into contact said acidic aqueous medium with a
cation exchange resin. The process is used in a process for the
recovery of secondary organic nitroxy compounds from an aqueous
medium containing the nitroxy compound and oxidation processes for
hydroxy compounds.
Inventors: |
Jetten, Jan; (Zeist, NL)
; Besemer, Arie; (Amerongen, NL) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET
2ND FLOOR
ARLINGTON
VA
22202
US
|
Assignee: |
SCA HYGIENE PRODUCTS AB
GOTEBORG
SE
|
Family ID: |
34635884 |
Appl. No.: |
11/006689 |
Filed: |
December 8, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60527281 |
Dec 8, 2003 |
|
|
|
Current U.S.
Class: |
162/81 ;
546/184 |
Current CPC
Class: |
D21H 11/20 20130101;
D21C 9/002 20130101 |
Class at
Publication: |
162/081 ;
546/184 |
International
Class: |
D21C 003/16 |
Claims
1. Process for the separation of a secondary organic nitrosonium
compound and/or a secondary organic hydroxylamine compound from an
acidic aqueous medium containing them in dissolved form, said
process comprising the step of bringing into contact said acidic
aqueous medium with a cation exchange resin.
2. Process according to claim 1, wherein the acidic aqueous medium
contains an organic nitrosonium and a protonated organic
hydroxylamine compound.
3. Process according to claim 2, wherein the organic nitrosonium
and hydroxylamine compounds are obtained by disproportionating a
secondary organic nitroxy compound.
4. Process according to claim 1, wherein the acidic aqueous medium
containing the organic nitrosonium and/or hydroxylamine compound is
obtained from a reaction mixture obtained by oxidizing a hydroxy
compound in the presence of a secondary organic nitroxy compound
and/or the corresponding nitrosonium ion.
5. Process according to claim 3, wherein the organic nitroxy
compound has the following formula (V): 4where n=0 or 1 and where
the methylene groups of the ring may carry one or more substituents
selected from alkyl, alkoxy, aryl, aryloxy, amino, amido, oxo,
cyano, hydroxy, carboxyl, phosphonooxy, maleimido, isothiocyanato,
alkyloxy, fluorophosphinyloxy, substituted or unsubstituted
benzoyloxy.
6. Process according to claim 1, wherein the step of contacting
involves passing the aqueous medium over a column filled with a
cation exchange resin.
7. Process according to claim 1, wherein the cation exchange resin
has sulfonate and/or carboxylate anchor groups.
8. Process according to claim 1, wherein the organic nitrosonium
and/or hydroxylamine compounds are recovered from the cation
exchange resin by washing or eluting the resin with an aqueous acid
solution or an aqueous solution of a metal salt.
9. Process according to claim 8, wherein the eluted aqueous medium
contains organic nitrosonium and hydroxylamine, preferably in
equimolar amounts and is neutralized to form the corresponding
nitroxy compound.
10. Process according to claim 8, wherein the eluted aqueous medium
contains at least the hydroxylamine which is oxidized to the
corresponding nitrosonium compound.
11. Process according to claim 4, wherein the organic nitroxy
compound has the following formula (V): 5where n=0 or 1 and where
the methylene groups of the ring may carry one or more substituents
selected from alkyl, alkoxy, aryl, aryloxy, amino, amido, oxo,
cyano, hydroxy, carboxyl, phosphonooxy, maleimido, isothiocyanato,
alkyloxy, fluorophosphinyloxy, substituted or unsubstituted
benzoyloxy.
12. Process for the recovery of secondary organic nitroxy compounds
from an aqueous medium comprising said nitroxy compound, said
process comprising the steps of: (a) acidifying the aqueous medium
to disproportionate the nitroxy compound to obtain a mixture of the
corresponding protonated hydroxylamine compound and nitrosonium
compound, (b) contacting the mixture of protonated hydroxylamine
compound and nitrosonium compound with a cation exchange resin to
retain said mixture, (c) eluting the cation exchange resin with an
aqueous solution of an acid or a metal salt in order to remove the
hydroxylamine and nitrosonium compounds from the cation exchange
resin, and (d) neutralizing the resulting purified mixture of
hydroxylamine and nitrosonium compounds to form the nitroxy
compound.
13. Process according to claim 12, wherein the aqueous medium
comprising said nitroxy compound was obtained from reaction mixture
obtainable by oxidizing a hydroxy compound in the presence of a
secondary organic nitroxy compound and/or the corresponding
nitrosonium ion.
14. Process according to claim 12, wherein the organic nitroxy
compound has the following formula (V): 6where n=0 or 1 and where
the methylene groups of the ring may carry one or more substituents
selected from alkyl, alkoxy, aryl, aryloxy, amino, amido, oxo,
cyano, hydroxy, carboxyl, phosphonooxy, maleimido, isothiocyanato,
alkyloxy, fluorophosphinyloxy, substituted or unsubstituted
benzoyloxy.
15. Process according to claim 12, wherein step (b) involves
passing the aqueous medium over a column filled with a cation
exchange resin.
16. Process according to claim 12, wherein the cation exchange
resin has sulfonate and/or carboxylate anchor groups.
17. Process for the oxidation of a hydroxy compound, said process
comprising the steps of: (i) oxidizing a hydroxy compound in an
aqueous medium in the presence of a secondary organic nitroxy
compound and a primary oxidant, (ii) optionally, acidifying the
resulting reaction mixture to disproportionate unreacted nitroxy
compound to the corresponding protonated hydroxylamine and
nitrosonium ion followed by recovering the nitroxy compound
according to steps (a) to (d) of claim 12, and (iii) recycling the
nitroxy compound recovered thereby to step (i).
18. Process for the oxidation of a hydroxy compound in the presence
of an organic nitrosonium compound comprising the steps of: (i')
oxidizing in an aqueous medium a hydroxy compound in the presence
of a secondary organic nitrosonium compound while the nitrosonium
ion compound is reduced to the corresponding protonated
hydroxylamine, (ii') optionally, acidifying the resulting oxidation
mixture, followed by separating unreacted nitrosonium compound, if
present, and hydroxylamine formed, according to the process of
claim 1 which comprises contacting them with a cation exchange
resin, (iii') eluting the unreacted nitroxy compound, if present,
and the protonated hydroxylamine from the resin, (iv') adding a
primary oxidant in order to convert the protonated hydroxylamine
compound to the corresponding nitrosonium compound, and (v')
recycling the nitrosonium compound to step (i').
19. Process according to claim 18, wherein the cation exchange
resins have sulfonate and/or carboxylate anchor groups, and the
organic nitrosonium and/or hydroxylamine compounds are recovered
from the cation exchange resin by washing or eluting the resin with
an aqueous acid solution or an aqueous solution of a metal
salt.
20. Process according to claim 17, wherein the hydroxy compound to
be oxidized is cellulose present in cellulosic pulp.
21. Process according to claim 20, wherein the oxidation is
conducted in the absence of chlorine-containing oxidants.
22. Process according to claim 18, wherein the hydroxy compound to
be oxidized is cellulose present in cellulosic pulp.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the 35 USC 119(e) benefit of prior
U.S. Provisional application Ser. No. 60/527,281 filed on Dec. 8,
2003.
FIELD OF THE INVENTION
[0002] The present invention relates to a process for the
separation of organic nitrosonium and/or hydroxylamine compounds by
means of a cation exchange resin, a recovery process for the
corresponding nitroxy compounds based thereon as well as a process
for the nitroxy-mediated oxidation of hydroxy compounds that also
makes use of this separation process.
BACKGROUND OF THE INVENTION
[0003] Nitroxy-mediated oxidations are a useful tool to convert
primary alcohols to aldehydes and/or carboxylic acids or secondary
alcohols to ketones. Organic nitroxy compounds suitable for this
purpose must be capable of forming stable radicals and therefore,
as a rule, are secondary nitroxy compounds sterically shielded by
at least one bulky group having a quaternary carbon atom in
.alpha.-position to the nitrogen atom. Especially TEMPO
(2,2,6,6-tetramethylpiperidine-N-oxyl formula I in the scheme
below) and its derivatives have attracted a lot of interest over
the last years since they allow the selective introduction of
aldehyde and/or carboxylic acids into alcohol substrates such as
polysaccharides.
[0004] The reaction can be conducted in three ways. First, a
nitrosonium ion (III) which is the actual active species can be
generated catalytically in situ from a nitroxide in the presence of
a primary oxidant, such as hypochlorous acid/hypochlorite as shown
in the scheme below. Depending on the process conditions, the
nitrosonium ion will oxidize the alcohol, i.e. the hydroxy compound
to the corresponding aldehyde (as shown in the scheme) or carboxy
compound while being reduced itself to the hydroxylamine. Equimolar
amounts of the resulting hydroxylamine and nitrosonium ion will
then recombine (synproportionate) under suitable reaction
conditions to form the nitroxy compound. Then, the catalytic cycle
newly starts. 1
[0005] Second, the nitrosonium ion can be generated in situ by a
disproportionation reaction of the nitroxy compound (I) to the
corresponding hydroxylamine (II) and the nitrosonium ion (III) as
shown in the following scheme. As the disproportionation reaction
must be conducted under strongly acidic conditions, the
hydroxylamine (II) is typically present in its protonated form
(IV). 2
[0006] Third, it is also possible to prepare a salt of the
nitrosonium ion, which then can be used in stoichiometric
amounts.
[0007] Moreover, a relatively new technique involves the enzymatic
conversion of nitroxy compounds as described in WO 00/50463.
[0008] There is extensive patent and scientific literature dealing
with the synthesis and the use of nitroxy compounds, mostly TEMPO
and its derivatives in oxidation reactions.
[0009] Reviews were published for instance by J. M. Bobbitt et al.
in Heterocycles, vol. 27, No. 2, 1988 "Organic nitrosonium salts as
oxidants in organic chemistry" and by A. E. J. De Nooy et al. in J.
Synthetic Org. Chem 1996 (10), 1153-1174 "on the use of stable
organic nitroxyl radicals for the oxidation of primary and
secondary alcohols".
[0010] If TEMPO and related nitroxy compounds are used as a
catalyst in the oxidation of alcohols, for instance carbohydrates,
the nitroxy compound is often lost during the isolation of the
reaction product due to the relatively high volatility of nitroxy
compounds such as TEMPO. This is an undesired situation for an
economical process. Very few documents deal with technical
solutions for overcoming this problem or the recovery of TEMPO in
general.
[0011] WO 96/36621 proposes in this regard a method for obtaining a
catalytically active mixture based on stable nitroxide radicals
wherein at least a part of the reaction mixture is subjected to an
azeotropic distillation. During the azeotropic distillation step
volatile nitroxide radicals are at least partially distilled over
with the liquid medium. However, this method requires distillation
devices and creates high costs due to its energy consumption.
Further, this method is not applicable to less volatile nitroxy
compounds.
[0012] Another technique for avoiding the loss of TEMPO as a
catalyst involves affixing the same to a polymer solid phase which
then can be easily recovered from the reaction medium (T. Miyazawa,
T. Endo et al. in J. Polymer Sci., Polymer Chem. Ed. 23 (1985)
1527-1535 and 2487-2494). Similarly, DE 42 09 869 A describes a
method for immobilizing 4-hydroxy-TEMPO on polyvinyl-benzyl.
[0013] U.S. 2003/0073871 pertains to the continuous oxidation of
alcohol substrates to aldehydes in the presence of nitroxy
compounds in an alkaline 2-phase system. The nitroxy compound can
be present in the organic phase, in the aqueous phase or bound to
solid supports.
[0014] However, the reaction rate of immobilized organic nitroxy
compounds is typically lower than in homogeneous systems.
[0015] WO 02/59064 A1 relates to various recovery processes for
TEMPO and related nitroxy compounds based on hydrophobic
interactions. One option involves bringing the nitroxy compound
into contact with hydrophobic resins such as XAD resins, for
instance by passing the reaction mixture over a chromatographic
column filled with one of these resins. The hydrophobic character
of TEMPO and other organic nitroxy compounds leads to a strong
affinity to hydrophobic resins allowing the separation from the
reaction mixture. Nevertheless, this recovery process creates new
problems insofar as, after eluting the organic nitroxy compound
from the column, the organic solvent (eluent) has to be removed by
evaporation. At this point in time, the high volatility of some
organic nitroxy compounds such as TEMPO or 3,4-dehydro-TEMPO leads
again to an undesired loss of this costly material. Furthermore, it
would be desirable to recycle the organic nitroxy compound in
aqueous media. Aqueous media are preferred for many
nitroxy-mediated reactions and less hazardous than organic
solvents.
[0016] WO 02/58844 describes a special group of nitroxy compounds
wherein 4 TEMPO molecules are integrated into a heterocyclic
molecule and their use in the oxidation of alcohol substrates.
These nitroxy compounds can be recovered by filtration or water
evaporation steps. One method for the recovery of the nitroxy
compound catalyst involves the filtration at pH values above 8.
According to another embodiment, the organic phase containing the
catalyst is stirred with concentrated hydrochloric acid (HCl) to
transfer the catalyst into the aqueous phase followed by
evaporating the water under formation of the catalyst salt.
[0017] Further, it is known in the art that
N-methylmorpholine-N-oxide (NMMO), i.e. a tertiary nitroxide, which
is used as solvent in the Lyocell spinning process can be recovered
and purified by means of anion exchange resins (H. Mertel et al.,
Papier (Darmstadt), 46 (3), 101-5, 1992, "Purification, recovery
and methods for quantitative determination of
N-methylmorpholine-N-oxide". The separation of tertiary amine
oxides, such as NMMO from aqueous solutions by means of cationic
exchange resins having carboxylate anchor groups is further known
from EP 0 468 951 A1.
[0018] In view of the above, there is a strong need for an
efficient recovery process for TEMPO, related organic nitroxy
compounds and/or the catalytically active species thereof, i.e. the
corresponding nitrosonium ion.
OBJECTS OF THE INVENTION
[0019] Therefore, it is one object of the present invention to
provide a separation process for an organic hydroxylamine and/or
organic nitrosonium ion compound which can be used in a recovery
process for organic nitroxy compounds such as TEMPO and the
corresponding nitroxy-mediated oxidation of hydroxy compounds.
[0020] It is one further object of the present invention to provide
a separation process for organic nitrosonium and/or hydroxylamine
compounds wherein organic solvents can be avoided.
[0021] It is a further object of the present invention to provide a
process for the recovery of organic nitroxy compounds, such as
TEMPO wherein the volatility of some nitroxy compounds does not
cause any problems.
[0022] It is one further object of the present invention to provide
a recovery process for organic nitroxy compounds with which the use
of organic solvents can be avoided.
[0023] It is a further object of the present invention to provide a
process for the oxidation of hydroxy compounds wherein the reaction
mixture can be easily purified and the nitroxy mediator and/or the
corresponding nitrosonium ion is easily recovered.
[0024] Finally, it is an object of the present invention to provide
a process for the oxidation of a hydroxy compound that facilitates
the recycling of various starting materials if these are not fully
reacted.
[0025] Moreover, it is an object of the present invention to
provide a closed cycle oxidation process for hydroxy compounds
wherein the loss of organic nitroxy compound due to its volatility
is minimised.
SUMMARY OF THE INVENTION
[0026] The above technical objects are solved by the following
processes:
[0027] 1. Process for the separation of a secondary organic
nitrosonium compound and/or a secondary organic hydroxylamine
compound from an acidic aqueous medium containing them in dissolved
form, said process comprising the step of bringing into contact
said acidic aqueous medium with a cation exchange resin.
[0028] 2. Process for the recovery of secondary organic nitroxy
compounds from an aqueous medium comprising said nitroxy compound,
said process comprising the steps of:
[0029] (a) acidifying the aqueous medium to disproportionate the
nitroxy compound to the corresponding protonated hydroxylamine
compound and nitrosonium compound,
[0030] (b) contacting the mixture of protonated hydroxylamine
compound and nitrosonium compound with a cation exchange resin to
retain this mixture,
[0031] (c) eluting the cation exchange resin with an aqueous
solution of an acid or a metal salt in order to remove the
hydroxylamine and nitrosonium compounds from the cation exchange
resin,
[0032] (d) neutralizing the resulting purified mixture of
hydroxylamine and nitrosonium compounds to form the nitroxy
compound.
[0033] 3. Process for the oxidation of a hydroxy compound., said
process comprising the steps of:
[0034] (i) oxidizing a hydroxy compound in an aqueous medium in the
presence of a secondary organic nitroxy compound and a primary
oxidant,
[0035] (ii) if necessary, acidifying the resulting reaction mixture
to disproportionate unreacted nitroxy compound to the corresponding
protonated hydroxylamine and nitrosonium ion followed by recovering
the nitroxy compound in line with the above recovery process, which
comprises contacting the mixture with a cation exchange resin,
[0036] (iii) recycling the nitroxy compound recovered thereby to
step (i).
[0037] 4. Process for the oxidation of a hydroxy compound in the
presence of an organic nitrosonium compound comprising the steps
of:
[0038] (i') oxidizing in an aqueous medium a hydroxy compound in
the presence of a secondary organic nitrosonium compound while the
nitrosonium ion compound is reduced to the corresponding protonated
hydroxylamine,
[0039] (ii') if necessary, acidifying the resulting oxidation
mixture, followed by separating unreacted nitrosonium compound, if
present, and hydroxylamine formed, in line with the above
separation process which comprises contacting them with a cation
exchange resin,
[0040] (iii') eluting the unreacted nitroxy compound, if present,
and the protonated hydroxylamine from the resin,
[0041] (iv') adding a primary oxidant in order to convert the
protonated hydroxylamine compound to the corresponding nitrosonium
compound, and
[0042] (v') recycling the nitrosonium compound to step (i').
DETAILED DESCRIPTION OF THE INVENTION
[0043] According to the claimed separation process, a secondary
organic nitrosonium compound and/or a secondary organic
hydroxylamine compound are brought into contact with a cation
exchange resin retaining them. By virtue of the resulting
interaction with the cation exchange resin, the nitrosonium and/or
hydroxylamine compound which is protonated under acidic conditions
can be separated from the acidic aqueous solution (e.g. reaction
mixture) containing them and purified. As will be shown later, it
is a major advantage of this separation process that the species to
be separated are ionic (positively charged), in contrast to their
precursor, i.e. the nitroxy compound, since ionic compounds forming
a salt show a strong interaction with cation exchange resins and
furthermore almost no volatility. This allows removing the aqueous
medium while minimising the material loss.
[0044] It should be emphasized that the present invention is not
restricted to particularly volatile nitroxy compounds such as TEMPO
or 3,4-dehydro-TEMPO but can be equally used for less volatile
species.
[0045] The separation process of the invention is conducted in an
acidic aqueous medium (also referred to as "solution").
[0046] The pH value of this solution is preferably less than 4,
more preferably less than 3, even more preferably less than 2
(throughout the specification and the claims pH values refer to a
measurement at 20.degree. C.). Under these conditions, the existing
equilibrium between organic nitroxy compound and
hydroxylamine/nitrosonium ion will be shifted to the side of
nitrosonium ion and hydroxylamine. Furthermore, the hydroxylamine
is typical protonated under these acidic conditions, which entails
a reduced volatility.
[0047] Accordingly, it is preferred that the acidic aqueous
solution contains an organic nitrosonium and a protonated organic
hydroxylamine compound, more preferably in equimolar amounts.
However, the separation process of the present invention can also
be successfully applied to acidic aqueous solutions containing only
one of these species. Furthermore, according to one preferred
embodiment, the mixture of nitrosonium and hydroxylamine compounds
is obtained by disproportionating a secondary organic nitroxy
compound under the above-explained acidic conditions.
[0048] To achieve this disproportionation reaction, any acid can be
used (e.g. perchloric acid), although it is generally preferred to
employ non-oxidising and non-reducing organic or inorganic acids
for adjusting the pH to the above values. It is generally
advantageous to use stronger acids, for instance those having pK
values (at 25.degree. C. in an aqueous solution) of less than 5,
preferably less than 3, for instance less than 1 or less than -1.
Very strong acids fulfilling even the latter requirement are
sulfuric acid or hydrochloric acid.
[0049] If recovery and oxidation processes, including the
separation process of the invention, are to be conducted under TCF
(total chlorine free) conditions, it is preferred to use
chlorine-free acids, such as organic sulfonic acids (e.g.
benzenesulfonic acid, o- or p-toluenesulfonic acid), sulfuric acid,
phosphoric acid or nitric acid.
[0050] The type of acid used will also determine the counterion of
nitrosonium and protonated hydroxylamine compound.
[0051] The phrase "aqueous solution" or "aqueous medium" as used in
the specification and the claims refers preferably to water.
However, it is also conceivable to use mixtures of water and a
water-miscible organic solvent as a major and minor component,
respectively. The amount of water-miscible organic solvent ranges
preferably from 0 to 40 vol. %, e.g. 0 to 20 vol. %. Examples of
suitable water-miscible organic solvents are alcoholic solvents
such as ethanol, 1- or 2-propanol or t-butanol and in particular
non-alcohol solvents such as ethers (e.g. dioxane or THF) or
ketones (e.g. acetone) or nitriles (e.g. CH.sub.3CN).
[0052] The acidic aqueous solution to be contacted with the cation
exchange resin may also stem from a reaction mixture obtained by
oxidising a hydroxy compound in the presence of a secondary organic
nitroxy compound and/or the corresponding nitrosonium ion. The
reaction can be subjected to filtration steps to remove insoluble
matter before it is further purified by means of the cation
exchange resin. The reaction mixture, as explained below in further
detail, may contain unreacted hydroxy compound, oxidation products
such as the resulting aldehyde and/or carboxy compound, unreacted
nitrosonium ion, its reduced (consumed) form, i.e. the
hydroxylamine, and/or nitroxy compounds. If this reaction mixture
is brought into contact with a cation exchange resin, the organic
nitrosonium and/or protonated hydroxylamine compounds will be
subjected to a cation exchange reaction wherein the cation
(typically hydrogen or an alkali metal ion, such as Na.sup.+)
loosely bound to the cation exchange resin is displaced by the
stronger binding organic nitrosonium and/or hydroxylamine compound.
This exchange reaction allows removing the organic nitrosonium
and/or hydroxylamine compounds from the reaction mixture.
[0053] As already indicated, the organic nitrosonium and/or
hydroxylamine compound is preferably produced from a secondary
organic nitroxy compound.
[0054] The term "secondary" means that the corresponding nitroxy
compound is disubstituted by organic residues (This, as well as the
following description of structural features, also naturally
applies to the corresponding nitrosonium and hydroxylamine
compounds). The organic nitroxy compound preferably has at least
one quaternary carbon atom in .alpha.-position. The quaternary
carbon atom causes sterical hindrance of the NO group thereby
stabilizing the same. Preferably the organic nitroxy compound has
two bulky groups (quaternary carbon atoms) in .alpha.-position. The
NO functionality may be bound to optionally substituted phenol or
aliphatic substituents that are linked to the nitrogen atom of the
NO functionality by a quaternary carbon atom, e.g. tert-butyl.
Moreover, it is preferred that both .alpha.-positions of the
organic nitroxy compound are dimethyl-substituted.
[0055] It is similarly preferred that the nitroxy compound lacks
.alpha.-hydrogen atoms. Two substituents can also be combined into
an alkylen unit optionally interrupted by a hetero-atom (e.g. O,N)
(to form an alicyclic or heterocyclic ring).
[0056] The total carbon number of the organic nitroxy compound is
preferably less than 40, more preferably less than 30, in
particular less than 20. The molecular weight is preferably less
than 500.
[0057] It is preferred that the nitrogen atom of the NO group is
linked to only two organic residues and not three as for instance
in NMMO (N-methyl morpholin-N-oxide).
[0058] Preferred oxidation systems can be represented by the
following formula (V) 3
[0059] where n=0 or 1 and where the methylene groups of the ring
may carry one or more substituents selected from alkyl, alkoxy,
aryl, aryloxy, amino, amido (e.g. acetamido, 2-bromacetamido and
2-iodacetamido), oxo, cyano, hydroxy, carboxyl, phosphonooxy,
maleimido, isothiocyanato, alkyloxy, fluorophosphinyloxy
(particularly ethoxyfluorophosphinyloxy), substituted or
unsubstituted benzoyloxy, e.g. 4-nitrobenzoyloxy. If n=1 (i.e. the
ring represents a piperidine), these groups preferably substitute
the 4-position of the piperidine. Examples are 4-acetamido-TEMPO,
4-acetoxy-TEMPO or 4-hydroxy-TEMPO. The di-tert.-alkyl nitroxy unit
can also be present as part of a polymer structure such as
{(CH.sub.3).sub.2--C--(CH.sub.2).sub.2-3--(CH.sub.3).sub.2--C--NO--}.sub.-
m--. Hydroxy, amino and amido are preferred among these
substituents on account of the stability of the nitroxy compound
under acidic conditions. The ring may also be partially unsaturated
as in 3,4-dehydro-TEMPO.
[0060] An example of n=O is PROXYL
(2,2,5,5-tetramethylpyrrolidine-N-oxyl)- .
[0061] Among the formula V compounds, the case of n=1 is preferred.
This leads to the optionally substituted TEMPO compounds
(2,2,6,6-tetramethyl-4-piperidine-N-oxide) that can selectively
oxidize the primary hydroxy group at C(6) of the glucose unit of
the cellulose into aldehyde and/or carboxyl groups.
[0062] DOXYL (4,4-dimethyloxazolidine-N-oxyl) can also be used as
nitroxy compound in the present invention.
[0063] According to a preferred embodiment, the acidic aqueous
solution containing the nitrosonium and/or hydroxylamine compound
is contacted with the cation exchange resin by passing the aqueous
solution over a chromatographic column filled with the resin. It is
however also possible, to add the cation exchange resin to the
acidic aqueous solution, for instance under stirring, followed by
filtrating the resulting slurry. In this manner, the cation
exchange resin, on which nitrosonium and/or hydroxylamine compounds
are adsorbed, can be separated.
[0064] There are no specific limitations regarding the cation
exchange resin to be used in the present invention. Commercially
available cation exchange resins having anionic anchor groups such
as sulfonate or carboxylate can be used. Suitable cation exchange
resins have for instance an optionally crosslinked
polystyrene/divinylbenzene-matrix or (meth)acrylic acid matrix to
which the anionic anchor groups are linked. The cation exchange
resin is preferably available in the form of small beads having an
average particle size in the range of 30 .mu.m to 1200 .mu.m,
preferably 35 to 300 .mu.m.
[0065] Suitable, commercially available cation exchange resins
involve strong acidic cation resin with 8% crosslinked gel cation,
such as Dow 50 WX, Dow HCR-S; Rohm & Haas Amberlite IRP-120;
Sybron C-249, C-267; Purolite C-100; Resintech CG8; IWT C-211; 10%
crosslinked gel cation, such as Dow HGR; Rohm & Haas IR-122;
Sybron C250, Purolite C-100x10; Resintech CG10; IWT C-361; or
condensate polisher gel cation, such as Dow HCR-W2, HGR-W2; Rohm
& Haas IR-130C, IR-132C; Sybron C-298, C-299; Purolite
SGC-100x10 Dow HCR-W2, HGR-W2; Rohm & Hass IR-132C; Sybron
C-298, C-299; purolite SGC-100x10; macroporous cation resin, such
as Dow MSC-1, MAC-3, Rohm & Haas Amberlite 200, Amberlite 252;
Sybron CFP-110; Purolite C-150; Resintech SACMP; IWT C-381; weak
acid cation resin, such as Dow CCR-2, MWC-2; Rohm & Haas
IRC-84, IRC-50, Sybron CCP; Purolite C-105, C-106; Resintech WACMP;
IWT C-271, C-281; or uniform particle size cation resins, such as
Dow Monosphere 650C or 750C, Marathon C and C-10; Rohm & Haas
Amberjet 1200 and 1500; Sybron Impact CS-299 and CS-399 UPS Grades;
Purolite Picopure 100 and 650C.
[0066] The products available from Dow Chemical are marketed under
the trade name Dowex.RTM.. Dowex.RTM. 50WX has a
styrene/divinylbenzene matrix and sulfonic acid groups. Dowex.RTM.
IDA-1 is characterized by a carboxylate-linked
styrene/divinylbenzen matrix. Dowex.RTM. monosphere 650C und 750C
are styrene/divinylbenzene-based sulfonic acid resins. Dowex.RTM.
MAC-3 is macroporous polyacrylic acid based carboxy resin. Dow.RTM.
Marathon C represents again a styrene/divinylbenzene-based sulfonic
acid resin.
[0067] Among these resins, strong acid cation resins, such as DOWEX
50WX (e.g. 50WX8) and Amberlite IRP-120 are preferred.
[0068] After being adsorbed on the cation exchange resin, the
organic nitrosonium and/or hydroxylamine compound can be washed off
or eluted, respectively, by means of suitable
electrolyte-containing solutions. In this respect, it is
recommendable to follow the instructions of resin manufacturer.
Typically, an aqueous acid solution or the aqueous solution of a
suitable metal salt can be used for this purpose. Suitable aqueous
acids involve for instance 1 to 5 wt % HCl in water or 0.5 to 0.8
wt % H.sub.2SO.sub.4 in water.
[0069] Preferably, metal salts showing a low affinity to the anion
exchange resin, more preferably salts of a monovalent metal, in
particular alkaline metal salts, such as sodium or potassium salts
(e.g. their chlorides) are used. Typical and preferred
concentrations of these salts range from 1 to 30 wt %. Higher
concentrations of H.sup.+ or metal ions, as a rule, enhance the
desorption from the column.
[0070] There are various manners of using the nitrosonium and/or
hydroxylamine compounds desorbed from the cation exchange
resin.
[0071] According to one preferred embodiment referring to mixtures
of nitrosonium and hydroxylamine compounds, the eluted aqueous
solution is neutralized, if necessary, to form the corresponding
nitroxy compound by synproportionation. This synproportionation
reaction starts to take place at pH values of at least 3,
preferably at least 4, more preferably at least 5. The most
preferred pH range for conducting this synproportionation reaction
is 5 to 11. For this purpose any suitable, preferably non-oxidising
and non-reducing base can be used, for instance monovalent metal
bases, such as alkali metal bases or basic alkali metal salts such
as NaOH, KOH, sodium or potassium carbonate, sodium or potassium
hydrogen carbonate, sodium or potassium acetate. In this manner,
the nitroxy compound, i.e. the starting material of organic
nitrosonium and/or hydroxylamine is recovered and can be further
used.
[0072] According to a second preferred embodiment, the
hydroxylamine is directly oxidized to the corresponding nitrosonium
ion, which represents the actual oxidizing species in
nitroxy-mediated reactions. The resulting process variant allows
generating the nitrosonium ion separately from the reaction
mixture. This is advantageous insofar as a contact between the
primary oxidant and the hydroxy compound substrate can be avoided
if the latter is relatively sensitive to oxidation and tends to
form undesired side products in the presence of a primary oxidant.
The direct oxidation from the hydroxylamine to the corresponding
nitrosonium ion can be achieved by strong oxidants, such as
hypochlorous acid under acidic conditions where little or no
synproportionation occurs (at a pH below 5, below 4 or below 3).
Further suitable process conditions will be explained below, in the
context of step (iv') of the second oxidation process of the
invention.
[0073] The claimed process for the recovery of secondary organic
nitroxy compounds comprises the following four steps:
[0074] (a) acidifying an aqueous solution containing the nitroxy
compound to disproportionate the same to the corresponding
protonated hydroxylamine compound and nitrosonium ion compound,
[0075] (b) contacting the mixture of protonated hydroxylamine
compound and nitrosonium ion compound with a cation exchange resin
to retain this mixture,
[0076] (c) eluting the cation exchange resin with an aqueous
solution of an acid or a metal salt in order to remove the
hydroxylamine and nitrosonium ion compounds from the cation
exchange resin,
[0077] (d) neutralizing the resulting purified mixture of
hydroxylamine and nitrosonium ion compounds until the nitroxy
compound has formed.
[0078] As to the chemical structure of this nitroxy compound,
reference can be made to the above description.
[0079] This applies as well to step (a), as far as the conditions
for acidifying the aqueous solution are concerned. The term
"aqueous solution" is also understood in the same manner as
above.
[0080] Step (b) of the recovery process according to the present
invention involves contacting the mixture of protonated
hydroxylamine and nitrosonium ion with a cation exchange resin
under conditions that were already described in the context of the
claimed separation process. The purpose of this step is to retain
the mixture of hydroxylamine and nitrosonium ion on the cation
exchange resin.
[0081] Step (c) of the recovery process involves eluting the cation
exchange column with an aqueous solution of an acid or a metal salt
in order to remove hydroxylamine and nitrosonium ion from the
resin. A description of preferred embodiments of this eluent was
already given in the preceding sections.
[0082] According to step (d) of the claimed recovery process, the
resulting purified mixture of hydroxylamine and nitrosonium ion is
neutralized until they recombine to the nitroxy compound by means
of a synproportionation reaction. This synproportionation reaction
starts to take place at pH values of at least 3, although it is
preferred to adjust the pH to values of at least 4, preferably at
least 5, in particular 5-11 in order to shift the equilibrium as
far as possible to the nitroxy compound. Bases being suitable for
neutralization were already mentioned.
[0083] According to one preferred embodiment of the claimed
recovery process for nitroxy compounds, the aqueous solution
subjected to steps (a)-(d) represents a reaction mixture obtained
by oxidizing a hydroxy compound in the presence of a secondary
organic nitroxy compound and/or the corresponding nitrosonium ion.
As previously explained, this synthesis mixture may contain, apart
from the nitroxy compound, primary oxidant and/or the reduced form
thereof, nitrosonium and/or hydroxylamine compounds, unreacted
hydroxy compound and its oxidation products as well as various side
products which may for instance form by oxidative cleavage
reactions. The recovery process of the present invention enables an
efficient purification of this reaction mixture.
[0084] It is a specific merit of the recovery process according to
the invention that the recovery steps for the nitroxy compound are
performed with ionic compounds, i.e. the corresponding nitrosonium
ion and protonated hydroxylamine salts. In contrast to the organic
nitroxy compound, these are not volatile. Therefore, the nitroxy
compound can be recovered without major material losses.
[0085] This recovery process can be used in a closed cycle
oxidation process for a hydroxy compound in the presence of a
secondary organic nitroxy compound and/or the corresponding
nitrosonium ion. A first embodiment of the resulting oxidation
process comprises the steps of:
[0086] (i) oxidizing a hydroxy compound in an aqueous medium in the
presence of a secondary organic nitroxy compound and a primary
oxidant,
[0087] (ii) if necessary, acidifying the resulting reaction mixture
to disproportionate unreacted nitroxy compound to the corresponding
protonated hydroxylamine and nitrosonium ion followed by recovering
the nitroxy compound in line with previously described steps (a) to
(d),
[0088] (iii) recycling the nitroxy compound recovered thereby to
step (i).
[0089] As to step (i), i.e., there is no specific limitation with
respect to the type of hydroxy compound, primary oxidant and
reaction conditions to be used.
[0090] It is one feature of nitroxy-mediated oxidations that they
are compatible with a huge variety of different substrate hydroxy
compounds. Thus, the hydroxy compound can comprise either a primary
or secondary hydroxy functionality. The oxidation of a primary
hydroxy compound is however preferred since techniques known in the
art allow conducting the nitroxy-mediated oxidation of primary
hydroxy compound with particularly high selectivity. The primary
hydroxy compound is oxidized to the corresponding aldehyde and/or
carboxylic acid. The interruption of the oxidation process and/or a
suitable reaction conditions enable the isolation of the
intermediate aldehyde. Interrupting the oxidation process is
particularly suited for the oxidation of polysaccharides.
[0091] Examples for suitable hydroxy compound substrates are low
molecular weight (up to MW 1000) aliphatic or aromatic-aliphatic
hydroxy compounds, such as oligosaccharides as well as hydroxy
compounds having a higher molecular weight including polymeric
hydroxy compounds such as polysaccharides. The oxidation of oligo-
and polysaccharides, such as starch or cellulose typically leads to
the corresponding C6-aldehyde and/or carboxy derivates. The
oxidation of cellulose and cellulose-containing materials, such as
pulp is particularly preferred.
[0092] The starting pulps which may be used for oxidation may
relate to primary fibrous materials (raw pulps) or to secondary
fibrous materials, whereby a secondary fibrous material is defined
as a fibrous raw material recovered from a recycling process. The
primary fibrous materials may relate both to a chemically digested
pulp (e.g. Kraft or sulfite pulp) and to mechanical pulp such as
thermorefiner mechanical pulp (TMP), chemothermorefiner mechanical
pulp (CTMP) or high temperature chemithermomechanical pulp
(HTCTMP). Synthetic cellulose-containing fibers can also be used.
Preference is nevertheless given to the use of pulp from plant
material, particularly wood-forming plants. Fibers of softwood
(usually originating from conifers), hardwood (usually originating
from deciduous trees) or from cotton linters can be used for
example. Fibers from esparto (alfa) grass, bagasse (cereal straw,
rice straw, bamboo, hemp), kemp fibers, flax and other woody and
cellulosic fiber sources can also be used as raw materials. The
corresponding fiber source is chosen in accordance with the desired
properties of the end product in a manner known in the art.
[0093] The oxidized pulps can be used for the manufacture of paper,
in particular tissue paper having improved strength properties.
Prior to or after oxidation the starting pulps can be beaten
(refined) with the aim of further enhancing paper strength. In the
manufacture of oxidized pulps it is further preferred to carry out
all oxidation steps in the absence of chlorine-containing oxidants
as basis for the production of TCF or ECF paper.
[0094] The hydroxy compound to be oxidized is preferably dissolved
or dispersed in a suitable aqueous medium as defined above.
[0095] A list of suitable reaction conditions and compounds is
found for instance in A. E. J. De Nooy, Synthesis 1996, 1153-1174).
The pH range of the oxidation reaction generally varies between 1
and 14, preferably 2 and 12, particularly 4 and 11. The reaction
temperature is preferably between 2.degree. C. and 80.degree. C.
The nitroxy compound may be added to the hydroxy compound as a
solid (also as a pasty substance) or as a solution (usually as an
aqueous solution). For mechanistical reasons, as known in the art,
oxidation reactions performed at lower pH tend to proceed
slower.
[0096] Preferably first the nitroxy compound and then the primary
oxidizing agent is added. Suitable oxidants can be selected among
primary oxidants typically used together with organic nitroxy
compounds such as chlorine, bromine, iodine, hypochlorite, chlorite
(in combination with hypochlorite), hypobromite, iodite,
Fe(CN).sub.6.sup.3-, transition metals of periods Va to VIIIa in
the oxidation state of at least +3, oxidases, ozone, hydrogen
peroxide, peroxosulfate and/or peracids. For each of the primary
oxidants suitable reaction conditions, as known in the art, are to
be selected. Thus, it can for instance be undesired to use
hypchlorite at pH<2 since then chlorine is formed, and tends to
escape from the reaction mixture.
[0097] The oxidizing agent can be added all at once or distributed
over the duration of the reaction (e.g. by evenly adding it in
drops). The primary oxidizing agent (e.g. peracetic acid, ozone,
hypohalite, the above metal compounds, enzyme) is preferably used
in an amount of 1 to 100 mol %, in relation to the hydroxy
functionality to be oxidized. The catalytic quantity of the nitroxy
compound is preferably 0,01 to 5 mol % relative to the hydroxy
functionality to be oxidized.
[0098] One version of performing the oxidation with the formula (V)
nitroxy compound is described in WO 95/07393 which teaches the
oxidation with a catalytic amount of the nitroxy compound and a
hypohalite (e.g. NaOCl) as a primary oxidizing agent in an aqueous
reaction medium at a pH between 9 and 13. Under these conditions, a
primary hydroxy group (e.g. of cellulosic pulp) is oxidized via the
corresponding aldehyde group into the carboxyl group.
[0099] An alternative version lies in the oxidation with a peracid,
a precursor or a salt thereof as a primary oxidizing agent in the
presence of a catalytic amount of the nitroxy compound
(particularly optionally substituted TEMPO) and a catalytic amount
of a halide (e.g. NaBr), preferably in the pH range of 2 to 11,
particularly 2.5 to 3.5. The halide is preferably used in a
quantity of 0.1 to 40, particularly 0.5 to 10 mol % relative to the
hydroxy groups, e.g. those of cellulosic pulp. In the latter case,
the nitroxy compound is preferably used in a quantity of 0.1 to 2.5
wt. %, relative to the dry weight (oven-dried) of the cellulosic
pulp.
[0100] The peracid is preferably a peralkanoic acid, particularly
peracetic acid, or persulfuric acid. Depending on the reaction
duration, this embodiment of oxidation leads to aldehydes and/or
carboxyl groups, for instance at C(6) of the glucose unit of the
cellulose. It is, however, also possible to perform oxidation just
using the nitroxy compound (particularly the formula (V) compound)
as a mediator and peracid (e.g. persulfuric acid) as an oxidizing
agent without halide, particularly bromide. More, preferred pH
ranges are 6 to 8 for peracetic acid/bromide and 7 to 9 for
persulfuric acid, respectively.
[0101] Another version lies in the combination of a catalytic
amount of the nitroxy compound (particularly optionally substituted
TEMPO) and a suitable oxidic compound of a metal of the transition
metals of periods Va to VIIIa in the oxidation state of at least
+3, e.g. oxides and oxygen-containing ions of manganese, chromium,
iron, nickel, ruthenium and vanadium, e.g. vanadium pentoxide, iron
oxide, chromium (VI) oxide, chromates and particularly manganese
(IV) oxide and salts of permanganic acid. The reaction is
preferably conducted at a pH between 2 and 8. The nitroxy compound
is preferably used in a molar amount of 0,01 to 10%, preferably
0,05 to 5% based on the hydroxy functionality to be oxidized. As to
pulp oxidation, an amount of 0.1 to 2.5 wt. %, relative to the dry
weight (oven-dried) of the fibrous material is preferred. The
reaction temperature is preferably less than 80.degree. C.,
particularly 30 to 60.degree. C. Depending on the duration of the
reaction, this embodiment of the oxidation leads to aldehydes
and/or carboxyl groups, for instance at C(6) of cellulosic
pulp.
[0102] A further version lies in the oxidation of the cellulose
with a catalytic amount of the formula (V) nitroxy compound that is
hydroxy-, amino- or amido-substituted (e.g. 4-hydroxy TEMPO) at a
pH between 1 and 7, particularly 2 to 6. In this version, a
hypohalite (e.g. NaOCl) or ozone is particularly suitable as a
primary oxidizing agent. The nitroxy compound is preferably used
here in an amount of 4 to 20 mol % with respect to the hydroxy
functionality to be oxidized. In the oxidation of pulp, the use of
0.05 to 15 wt. % nitroxy compound and 0.1 to 20 wt. % primary
oxidizing agent, each relative to the dry weight (oven-dried) of
the fibrous material is preferred. The reaction temperature is
preferably 5 to 50.degree. C. Depending on the reaction duration,
this embodiment of oxidation results in aldehydes and/or carboxyl
groups, e.g. at C(6) of the glucose unit of the cellulose.
Halogen-free acids, such as sulfuric acid or toluenesulfonic acid,
are particularly suitable for setting the pH.
[0103] According to a preferred embodiment, the nitroxy compound
can also be used with enzymes and/or metal complexes. In this
embodiment the catalytic amount of the nitroxyl compound is
preferably 0,1-2,5 mol % with respect to the C(6)-alcohol to be
oxidized.
[0104] The catalysts to be used according to this embodiment are
oxidoreductases or other enzymes that are capable of oxidation in
the presence of a suitable redox system. Oxido-reductases, i.e.
enzymes capable of oxidation without the presence of further redox
systems, to be used include peroxidases and oxidases, in particular
polyphenol oxidases and laccase. Certain hydrolases, such as
phytase, can be used when a further redox system is present such as
a metal complex, e.g. vanadate. Metal complexes as such, without an
enzyme protein, can also be used; examples include copper and iron
complexes with porphyrins, phenanthrolins, polyamines such as EDTA,
EGTA and the like. The metal-assisted enzymes and metal complexes
require hydrogen peroxide, alkyl and ar(alk)yl hydroperoxides (such
as tert-butyl hydroperoxide) or chlorite as an ultimate electron
acceptor. 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 horseradish peroxidase, soy-hull peroxidase, myelo peroxidase,
lactoperoxidase, Arthromyces and Coprinus peroxidases. Several
peroxidases are commercially available. The peroxidases require
hydrogen peroxide as an electron acceptor. Polyphenol oxidases (EC
1.10.3.1.) include tyrosinases and catechol oxidases such as
lignine peroxidase. Suitable polyphenol oxidases may be obtained
from fungi, plants or animals. The polyphenol oxidases require
oxygen as an electron acceptor. Laccases (EC 1.10.3.2) are
sometimes grouped under the polyphenol oxidases, but they can also
be classified as a distinct group, sometimes referred to as
p-diphenol oxidases. The laccases can be derived from plant sources
or from microbial, especially fungal, sources. The laccases also
require oxygen as an electron acceptor. The process for producing
the aldehydes according to this embodiment can be performed under
relatively mild conditions, e.g. at a pH between 2 and 10, and at a
temperature between 15 and 60.degree. C. (both depending on the
particular enzyme or metal complex). The reaction medium can be an
aqueous medium, or a homogeneous mixed medium, e.g. a mixture of a
water and a water-immiscible organic solvent such as a hydrophobic
ether, a hydrocarbon or a halogenated hydrocarbon. In the latter
case, the enzyme and/or the nitroxyl and the oxidising agent may be
present in the aqueous phase and the hydroxy compound substrate and
the aldehyde or ketone product may be present in the organic phase.
If necessary, a phase transfer catalyst may be used. The reaction
medium can also be solid/liquid mixture, in particular when the
enzyme of the nitroxyl are immobilised on solid carrier. A
heterogeneous reaction medium may be advantageous when the
substrate or the product is relatively sensitive or when separation
of the product from the other reagents may present
difficulties.
[0105] According to the present invention it is preferred to
conduct step (i), i.e. the oxidation of a hydroxy compound, with
cellulosic pulp a hydroxy compound substrate in the presence of the
above enzyme as primary oxidant and catalytic amounts of a nitroxy
mediator, preferably those of formula (V), such as TEMPO or
4-acetamido TEMPO.
[0106] Regarding suitable reaction conditions for conducting the
nitroxy-mediated oxidation of a hydroxy compound, reference can
also be made to WO 95/07303, WO 99/57158, WO 99/58574, WO 00/26257,
WO 00/50621, WO 00/50388, WO 00/50463, WO 01/00681, WO 00/50462, WO
01/34656, WO 01/34903, WO 01/34657, WO 01/83887, EP 1,149,846, EP
1,215,217 A and EP 1,251,140 A in the name of TNO and SCA Hygiene
Products GmbH (AB, Zeist B. V).
[0107] Steps (ii) and (iii) are preferably conducted under the same
conditions as outlined before. This extends as well to the term
"aqueous" used to define the medium of step (i).
[0108] According to a second embodiment of the claimed oxidation
process, the already formed nitrosonium ion compound is reacted
with the hydroxy compound, rather than a mixture of nitroxy
compound in catalytic amounts and primary oxidant. Thus, the
generation of the nitrosonium ion via the oxidation by a primary
oxidant takes place separately from the hydroxy compound substrate.
This is of particular advantage if the hydroxy compound substrate
tends to form undesired side products in the presence of a primary
oxidant. This alternative embodiment comprises the following steps
(i') to (v'):
[0109] (i') oxidizing in an aqueous medium a hydroxy compound in
the presence of a secondary organic nitrosonium compound while the
nitrosonium ion compound is reduced to the corresponding protonated
hydroxylamine,
[0110] (ii') if necessary, acidifying the resulting oxidation
mixture, and separating unreacted nitrosonium compound, if present,
and hydroxylamine formed, in line with the above-described
separation process which comprises contacting them with a cation
exchange resin,
[0111] (iii') eluting the unreacted nitroxy compound, if present,
and the protonated hydroxylamine from the resin,
[0112] (iv') adding a primary oxidant in order to convert the
protonated hydroxylamine compound to the corresponding nitrosonium
compound, and
[0113] (v') recycling the nitrosonium compound to step (i').
[0114] As to the hydroxy compound, the aqueous medium and the
nitrosonium ion compound defined in step (i'), reference can be
made to the above description. Moreover, there is no specific
limitation regarding the pH to be used. It may however be of
advantage, if the reaction is conducted under acidic conditions
(e.g. pH below 4), since then no acidification is necessary in step
(ii'), although the reaction then proceeds more slowly.
[0115] It should be added that the resulting hydroxylamine easily
synproportionates with equimolar amounts of nitrosonium compound
under weakly acidic, neutral or alkaline conditions thereby forming
the nitroxy compound.
[0116] Further, in particular amido-substituted nitroxy compounds
(e.g. 4-acetamido TEMPO) of the aforementioned formula (V) are
suitable for step (i'). In this reaction, halogen-free acids such
as sulfuric acid or toluenesulfonic acid are particularly suitable
for adjusting the pH, preferably to values of 2 to 3. After the
reaction, the consumed (reduced) form of the nitrosonium compound
can be regenerated with ozone or another oxidizing agent,
preferably in separate process step (iv'). An important advantage
of the oxidation version discussed here is the ability to use the
choice of a suitable pulp (TCF) to conduct the entire pulp or paper
production method without any halogen-containing chemicals.
[0117] The nitrosonium compound is preferably used in an amount of
4 to 440 mol %, relative to the hydroxy functionality to be
oxidized. This also includes the possibility of partially oxidizing
polyhydroxy compounds such as cellulose present in pulp.
[0118] Regarding steps (ii') and (iii') the above-described
conditions are fully applicable.
[0119] Primary oxidants suitable for step (iv') can be selected
among primary oxidants typically used together with organic nitroxy
compounds such as chlorine, bromine, iodine, hypochlorite, chlorite
(in combination with hypochlorite), hypobromite, iodite,
Fe(CN).sub.6.sup.3-, transition metals of periods Va to VIIIa in
the oxidation state of at least +3, oxidases, ozone, hydrogen
peroxide, peroxosulfate and/or peracids.
[0120] The oxidation is preferably conducted in an aqueous medium
as afore-mentioned under conditions as explained for the first
embodiment of the claimed oxidation process. It is however
preferred to use acidic conditions involving a pH of preferably
less than 4, more preferably less than 3, even more preferably less
than 2 to suppress the synproportionation reaction between (not yet
oxidized) hydroxylamine and already formed nitrosonium ion.
[0121] Step (v') of the second oxidation process of the invention
closes the cycle by feeding the recovered nitrosonium compound back
to step (i').
[0122] The present invention is now explained by means of the
following example:
EXAMPLE 1
[0123] To 1 L of a solution containing 4 g TEMPO, 15 ml of
concentrated hydrochloric acid (37%) was added. The mixture was
allowed to react for 30 minutes at room temperature. From this
solution the absorbance at .lambda.=430 mm (characteristic for
TEMPO) and at .lambda.=480 mm (characteristic for the corresponding
nitrosonium salt) were measured and compared with the absorbance of
the original solution. From the spectra it appeared that the
mixture was disproportionated. 600 ml of the solution containing
the protonated hydroxylamine (15 mmol) and nitrosnonium ion (15
mmol) was poured over a column containing 30 g cation exchanger
(Dowex 50WX8). In the eluate the extinction of both TEMPO and its
nitrosonium salt were measured (see Table 1). As can be seen, the
eluted liquid contains only a limited amount of nitrosonium ion. To
release the charged intermediates, the ion exchanger was washed
with about 600 ml 1M hydrochloric acid. From this solution the
absorbance at .lambda.=430 and at 480 nm were measured.
1TABLE 1 Extinctions of TEMPO and its nitrosonium salt Sample
Absorbance (430 nm) * Absorbance (480 nm) Acidified TEMPO 0.127
0.159 solution prior to separation Eluate from column 0.024 0.005
Eluate after washing 0.078 0.101 with 1 M HCl * This absorbance can
be partly attributed to the presence of nitrosonium salt.
[0124] From the data expressed in Table 1 it follows that the
positively charged intermediates (protonated hydroxylamine and
nitrosonium ion) initially bound to the cation exchanger material
can be released from the column by elution with a high
concentration of H.sup.+. After neutralization of eluted liquid,
TEMPO was formed. Experiments in which the ion exchange material is
eluted with Na.sup.+ gave the same results.
* * * * *