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Synthesis and properties of new esters of cellulose and inorganic polyacids containing phosphorus, molybdenum, tungsten and vanadium.


Composite materials based on cellulose, which can combine properties of separate constituent parts, or possess new unique properties not inherent to natural polymers or constituent parts of the composite, assume a gradually increasing importance.

Research on properties and structures of phosphorus-containing heteropolyacids have created an opportunity for their use as catalysts, ion-exchangers, and also compounds for preparative and analytical chemistry (1-4). However, the compounds of phosphoric heteropolyacids are weakly dissociated substances; and consequently the reaction of direct esterification of a natural polymer by their use cannot be carried out. The combination of properties characteristic of the class of phosphorus-containing heteropolyacids and properties inherent to cellulose (of highly advanced fibrous structure and with kinetic capability for chemical interactions) in pure material causes significant practical interest.

Earlier we obtained a new class of derivatives of cellulose and inorganic phosphoric polyacids by direct esterification of cellulose by polyphosphoric acids in carbamide (5, 6). Synthesized derivatives containing up to 100% P and possessing ion-exchange capacity up to 6.5 mmol/g have polyphosphoric chains as (O-P-O). This is favorable for cellulose esterification reaction by inorganic phosphates because each act of interaction of cellulose with polyphosphoric acid introduces not one atom of phosphorus, but a chain consisting of two or three P atoms with the number of mobile protons larger by a factor of 1.5 or 2. Therefore, for direct formation of connections heteropolyacids to ester groups it is more expedient to use acid esters of cellulose and pyro- or trypolyphosphoric acids (7) - as we have done in this work.


Esters of cellulose and simple inorganic acids, such as nitric, sulfuric, and phosphoric, are well known, but there are other acids also used in cellulose esterification. In particular, polyphosphoric acids combine high reactivity, accessibility, and low cost that are of particular interest. These advantages were demonstrated first by Vorobyova, Luneva, and Ermolenko (5-7).

The esters of cellulose and inorganic polyacids were prepared by the phosphorylation in the melt of carbamide and polyphosphoric acids at temperatures in the range 145-160 [degrees] C.

The cellulose phosphates contain up to 10% phosphorus and are multifunctional ion-exchangers with exchange capacity up to 6.5 mmol/g with dissociation pH of 4.0-4.5, 6.0-6.7, and 8.0-9.3.

Lack of a specified method of synthesis with the use of melts of carbamide and inorganic polyacids prevents obtaining cellulose products because of the high consumption of materials as well as difficulties in the realization of the process in the melt. Therefore, we have investigated conditions of esterification of cellulose in water solutions of carbamide mixed with polyphosphoric acids.

It is known that the anionic structure of water solutions of condensed phosphoric acids depends on many factors and changes under the Influence of acid and basic conditions, at high temperatures, by varying concentration of reagents, storage time, etc. (8). Therefore, using the method of ascending chromatography on paper revealed the conditions of forming and conserving condensed phosphoric acids that contain no less than 75% pyro([P.sub.2]) + tripoly([P.sub.3]) + tetrapoly([P.sub.4]) + condensed([P.sub.4]) + initial high-molecular polyphosphates.

It has been established that a decrease of pH of the system from 4.1 to 2.3 causes an increase in the speed of hydrolysis of polyphosphates from 0.22 x [10.sup.-3] to 0.058 x [10.sup.-3] [h.sup.-1]. Introduction of [Cu.sup.2+] and [Sn.sup.2+] ions into the composition catalyzes the process of depolymerization of phosphoric chains. By contrast, adding [Cr.sup.3+] and [Zn.sup.2+] stabilizes the conservation of polyphosphates (9, 10).

It has been established that our selected composition for the esterification of cellulose (pH = 4.1) does not actually change its anionic composition during its storage up to 21 days.

The change of temperature of the composition undergoing esterification substantially influences the anionic composition of phosphates. Temperature elevation up to 120 [degrees] C leads to some increase in [P.sub.2] + [P.sub.3] + [P.sub.4] forms of phosphates (by 8-10%); a further increase of temperature up to 145-150 [degrees] C is accompanied by the appearance of condensed phosphates with low mobility (under the conditions of chromatographic separation).

The realization of esterification of cellulose by phosphorylating composition with the maximum content of polyphosphates (up to 80%) with the use of the process of impregnation (in a foulard-dyeing machine) and thermal treatment at 140-160 [degrees] C produces a cellulosic material containing 4-10% of phosphorus, depending on time of the thermal treatment.

Infrared spectroscopy investigation of cellulose shows vibrational spectral peaks of the introduced phosphates: 470-550 ([Delta] O-P-O), 800-950 ([Delta] P-OH), 1200-1300 ([Delta] P-OH + vP = O) [cm.sup.-1] (11,12) [ILLUSTRATION FOR FIGURE 1, CURVES 1-3 OMITTED]. However, it is impossible to identify the form of phosphate introduced into the cellulose derivative using this method because of superposition of absorption bands of phosphates with the absorption of cellulose. Therefore, the differential record of IR-spectra of cellulose phosphates in relation to initial cellulose was used [ILLUSTRATION FOR FIGURE 1, CURVES 4-6 OMITTED].

By analyzing character of absorption of synthesized products in the spectral interval 400-1500 [cm.sup.-1] it is possible to note three areas of significant change in the intensity bands: 400-550, 800-1100, 1100-1400 [cm.sup.-1]. Fluctuations of the polyphosphoric structures are displayed as follows: 480 ([Delta]P[O.sub.3]), 925, 970 ([] P-O-P), and 1080-1120 [cm.sup.-1] ([v.sub.s] and []P[O.sub.2]).

To establish the degree of condensation of phosphate substituents in cellulose by chromatographic separation, one needs phosphates soluble in water or other solvents. Conversion of cellulose phosphates into water-soluble compounds was carried out by a method of soft hydrolytic splitting on 1-4 glucoside bond with use the cellulase ferment (13).

The fermentative hydrolysis was carried out by adding 1 ml of solution (80 ml of ferment in 12 ml of water) to 50 ml of cellulose phosphate and 1 ml of a buffer solution (1, 2% solution of succinic acid) and kept at pH 4.5-4.8 during 18 hours at 37 [degrees] C. The solution of monose and cellulose was used for the analysis of anionic structure of phosphates using paper chromatography according to a procedure described by Karl-Kroupa (14).

We find that the esterified cellulose contains up to 85% of phosphates (in pyro- and tripoly forms). Determination of structural orientation of cellulose esterification by condensed phosphates using the method of mesylation (i.e., interaction with C[H.sub.3]S[O.sub.2]Cl) of groups free from phosphorylation with further interaction with potassium iodide showed that the process proceeds in primary alcoholic groups (13); the degree of substitution of hydroxyl groups on the residues of polyphosphoric acids reaches 0.3-0.6.

Thus, the reaction of esterification of cellulose by condensed polyphosphoric acids produces the derivatives of cellulose and pyro- and tripolyphosphoric acids:

R-OH + [H.sub.4][P.sub.2][O.sub.7] [equilibrium] R-O-P-O-P[O.sub.2][(OH).sub.3] + [H.sub.2]O

R-OH + [H.sub.5][P.sub.3][O.sub.10] [equilibrium] R-O-P-O-P-O-P[O.sub.3][(OH).sub.4] + [H.sub.2]O,

where R-OH-cellulose

The estimation of the fire-protection properties of phosphorus-containing derivatives of cellulose (GOST 12.1044-89, ISO 6940-84) has shown that the content of phosphorus necessary for significant if not for complete elimination of inflammation in a polymer depends on the structure of ester groups introduced. Also in the samples with equal content of phosphorus, a higher degree of condensation of ester groups appearing in the structure of the polymer enhances the fire resistance.

Cellulose phosphates containing 2-10% of phosphates are classified as materials that are barely combustable; the fire-resistance of the materials remains unchanged during multiple alkaline treatments and washing (15).

The use of cellulose phosphates for obtaining hydrated cellulose fibers produces viscose rayon with an improved ability to absorb a coloring agent (2-9 times) and a high coefficient of homogeneity of distribution of the coloring agent and low combustibility.

An investigation of ion-exchange properties of the products of esterification has shown that the enrichment of ester groups of cellulose by condensed phosphates results in an increase of the ion-exchange capacity at the expense of capability of polyacid protons to participate in reactions of ion exchanger [ILLUSTRATION FOR FIGURE 2 OMITTED]. Earlier we established that cellulose polyphosphates effectively absorb the ions of calcium, cadmium, lead, uranium, molybdenum, tungsten, and other metals (16-19). Moreover, we developed methods for stabilizing donor blood, a method of detecting ore deposits, and a method of radiation monitoring (20-23).

The esters of cellulose and phosphorus-molybdenum or phosphorus-tungsten acids were obtained by the sorption of molybdenum and tungsten. Sorption curves of tungsten (1, 2), molybdenum (3-6), and vanadium (7) ions depending on concentration of absorbed salts are shown in Fig. 3.

The sorption of molybdenum, tungsten, and vanadium by cellulose phosphates was carried out in static conditions with variable initial concentration of ammonium molybdate and sodium tungstenate (0.002-0.04 M, 0.01-0.1 M, and 0.008-0.004 M correspondingly) in an interval of pH 1-3 during 0.4-24 hours.

Cellulose materials after the sorption of metals were washed carefully to negate the reaction to phosphorus and the element Introduced according to Feigl and Anger (24). Then samples were analyzed with respect to the content of phosphorus, molybdenum, tungsten, and vanadium by a spectral method. For this purpose, samples were subjected to wet acidic degradation in a chloric acid or mixed with sulfuric and nitric acid (in the ratio 100:1) and analyzed under established procedures (25).


We have established, in the entire interval of concentrations used for sorption of salts, that molybdenum, tungsten, and vanadium appear in the cellulose.

The maximum absorption of metals occurs at pH in the range 2.6-4.5 (for molybdenum), at pH between 1-2.5 (for tungsten), and 1.5-2.0 (for vanadium). Thus, the peculiarities of the process of sorption testify to absorption of the metals through electrostatic interactions and by formation of complexes.

For cellulose phosphates with elements named above, connections are formed in ester groups in which the atomic ratio of phosphorus to heteroatom is []:[C.sub.p] = 1:10 for molybdenum, [C.sub.w]:[C.sub.p] = 1:20 for tungsten, and [C.sub.w]:[C.sub.p] = 1:2 for vanadium.

The formation of phosphorus-containing heteropolyacids in the structures of ester groups is confirmed by an IR-spectroscopy investigation. The complexity of structures of heteropolycompounds causes large difficulties in establishing the bands of absorption in IR-spectra in the literature. There are many articles devoted to IR-spectroscopy of molybdenum- and tungsten-containing compounds; but few of these deal with phosphorus-vanadium structures. The bands of absorption found in the range of 850-960 [cm.sup.-1] for molybdenum- and tungsten-containing structures can be assigned to fluctuations of Mo = O groups in the cation structure [Mathematical Expression Omitted] and [Mathematical Expression Omitted] (26, 27), and to fluctuations of end and bridge bonds of tungsten-oxygen (28).

As to cellulose phosphates containing vanadium, the complexity of the problem in assigning the bands results from the fact that the bands are obscured by the absorption of cellulose itself and by the phosphorus-containing structural elements. The intervals of the appearance of fluctuations of vanadium structures: 785, 1010, and 1100 [cm.sup.-1], are in the literature assigned to fluctuations of bridge groups V-O-V (29, 30). Identification of fluctuations of vanadium-containing groups in derivatives of cellulose has appeared possible only after recording IR-spectra in the differential form in relation to cellulose phosphates.

Potentiometric titration curves of the esters of cellulose and phosphorus-molybdenum and phosphorus-tungsten acids are shown in Figs. 2 and 4. As is visible, the share of acidic functional groups in the esters of cellulose and heteropolyacids dissociating in the interval of pH 2.5-5.0 has been substantially increased.

The fact that ion-exchange capacity of the materials obtained significant increases which testifies to the benefit of formation of phosphates of new phosphorus-molybdenum structural elements in the structure of cellulose. Interactions of cellulose phosphate with tungsten salts proceeds differently: after the treatment of cellulose phosphate by sodium tungstate, a significant decrease of ion-exchange capacity is observed.

The materials obtained are of interest for use as effective highly-selective sorbents and ion-selective membranes (16). Moreover, these products are light-sensitive, forming visible images with high optical density during exposure and can be used for recording of the information in real time (31).

Phosphorus-molybdenum and phosphorus-tungsten esters of cellulose are effective sorbents which absorb radionuclei of [Cs.sup.137], [Cs.sup.134], [Sr.sup.90] [Y.sup.90], [Eu.sup.152], [Eu.sup.154], [Mn.sup.54], and [Co.sup.60] from solutions with specific activity of 5.4 x [10.sup.-5] Ci/1 with value of sorption 99.99%.


The results of this study indicate that the modification of cellulose by phosphoric acids leads to acidic esters of cellulose and polyophosphoric acids.

The cellulose derivatives synthesized actively absorbed molybdenum, tungsten, and vanadium forming heteropolyacids (phosphorus-molybdenum, phosphorus-tungsten, and phosphorus-vanadium) in the ester groups. The new materials are highly effective sorbents for the removal of radionuclides from solutions.


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Title Annotation:International Forum on Polymers - 1996
Author:Luneva, N.K.; Oputina, A.G.
Publication:Polymer Engineering and Science
Date:Jun 1, 1997
Previous Article:Co-condensates of resorcinols and methylol compounds for adhesive resins.
Next Article:Novel atropisomeric binaphthol containing comb-shaped copolymers forming chiral nematic phases.

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