CEEX PROGRAMME

Polyoxometalates compounds with oxidative catalytic activity for top applications

(POMALB)

INTRODUCTION

          The aim of the project is the study of cellulose bleaching by green processes to eliminate the chlorinated by-products and dioxins from the present technologies used. This green process of cellulose bleaching is based on selective catalytic oxidation of lignin from cellulosic pulp using PolyOxoMetalate (POM) clusters based on Mo, W, V and other transitional metals as catalysts.

          General formula for polyoxometalates is [X_xM_mO_y], where x< m. They are composed of oxide anions held together with metal-oxygen bonds. The principal building blocks of POMs are MO_x (x = 4-6) polyhedra that are linked together by one, two or occasionally three oxygen atoms. X is commonly named central atom or heteroatom and M is the higher oxidation state cation with a cvasioctahedral symmetry; so that is the reason why only a few elements are appropriate (wolfram, molybdenum, vanadium, niobium, tantalum).

          The POM bleaching process is based on the catalysts’ properties to be rapidly reduced and, then, easily re-oxidized. In the bleaching step mixtures of water, pulp and a fully oxidized POM are heated anaerobically. During the reaction the POM is reduced as residual lignin is oxidized (I).

           LigH₂+ POM_ox → Lig_ox + POM_red + 2H⁺                           (I)

After the reaction, the spent bleaching liquor is separated from the pulp and re-oxidized using oxygen to carbon dioxide and water, as represented by equation II.

           LigH_ox+ POM_red + 2H⁺+ O₂ → POM_ox + CO₂+ H₂O            (II)

In order to obtain high quality cellulose (this means high quality paper) the pH of the system must be kept in a range 4 – 8.  

The cycle can be described as followed: