CEEX PROGRAMME

Polyoxometalates compounds with oxidative catalytic activity for top applications

(POMALB)

The activities during this project were focused on two main directions:

(a)    study of lignin oxidation reaction using POM in order to determine the optimum reaction parameters (the involved partners were CO-INCDTIM Cluj and P1-SC JIALEX SRL);

(b)   characterization of natural fibers and wood pulp respectively, submitted to bleaching processes by means of POM (the involved partners were P2-Technical University “Gheorghe Asachi” Iaşi, and P3-University “Aurel Vlaicu”, Arad).

Parameters determination of lignin selective catalytic oxidation reaction by means of POM catalysts was performed using wood pulp samples (CO) and flax and hemp natural fibers respectively, prior mechanically cottonized by P1. Two polyoxometalate catalysts with combined addenda and saturated Keggin structure were tested, that is K₅H₄PMo₆V₆O₄₀x10H₂O (POM1), and Na₃H₂PMo₁₀V₂O₄₀ (POM2) respectively.

            In case of the present study the parameters of lignin selective oxidation from Kraft pulp samples are:

-   Kraft pulp consistency of 8%;

-   dried pulp-POM ratio of 1:1.25 g/mmole;

-   temperature of 90^°C;

-   time of first step bleaching process: 90 minutes;

-   conditions of catalyst POM_red regeneration with O₂: t > 90^oC, continuous flow of O₂;

-   tracing and control method of POM bleaching process – potentiometric titration.

Optimum parameters of the catalytic oxidation reaction of lignin from flax and hemp natural fibers are:

-   flax/hemp natural fiber and POM solution optimum ratio of 1/10;

-   POM catalyst optimum concentration of 3.3 mM for POM1 and 3.5 mM for POM2 respectively;

-   optimum reaction temperature for the selective oxidation of natural fiber lignin of 90 – 95^°C;

-   reaction time of 2 hours;

-   pH of the natural fiber suspension during lignin oxidation reaction: 3.5-5.5, interval for which lignin is selectively oxidized, while the natural fibers remain unaffected.

Samples obtained by partner P1 from natural fibers of flax and hemp, treated with POM in different concentrations (12.5 and 5 g/l respectively) were sent to partner P3 for the determination of natural fibers specific characteristics: degree of white, lignin content and smoothness.

For these samples, a good correlation between degree of white, K number and fiber thickness was determined. Thus, the larger the lignin content in a sample, the lower the degree of white and the more reduced the smoothness of the obtained fibers.

There are no important differences in the properties of fibers treated with various POM concentrations. As for their wetting capacity, the treated samples are very similar, that means there is no significant difference between them. Natural fibers of flax and hemp bleached with POM catalyst present good physico-chemical characteristics, due to a low influence of the bleaching working conditions on the cellulose fiber. POM bleaching leads to a good separation of the elementary natural fibers. Thus, the cleaning treatment becomes milder and with little effect on the cellulosic matrix, after the mechanical operations in the spinning and weaving processes. The obtained fiber could be suited for the spinning technology of cotton.

            Two experimental methods were tested for wood pulp bleaching (humidity of 25%): the first one using peroxide and the second one a mixture of peroxide and POM compounds. In case of the latter experimental method (mixture of peroxide and POM) the tests run showed that both POM catalysts used improve the degree of delignification compared to the use of peroxide only. The obtained degree of white is superior in case of the second experimental technique. Experimental results show that selectivity is comparable to the case of lignin delignification using oxygen.

            A mixture of POM and peroxide is recommended to be used for cellulose bleaching, as the selectivity and degree of white of cellulose are significantly improved.