New Chemistry, Including Pulp Bleaching Processes, of the Golden-Aged, Water-Soluble Compound, Tris(hydroxymethyl)phosphine

Collaborative studies with the Pulp and Paper Institute of Canada (Paprican) have led serendipitously to development of a pulp bleaching process using tris(hydroxymethyl)phosphine, P(CH2OH)(3) (abbreviated THP), and this on-going work is reviewed. Work on hydrogenation of aromatic residues in lignin, using transition metal catalysts under homogeneous, aqueous-phase conditions, led to the discovery that commercially available THP, in the absence of any metal, bleached selected pulps. Use of THP or a related diphosphine 2 is competitive with the currently used bleaching agents, hydrosulfite (S2O42-) and alkaline peroxide. (Hydroxymethyl)phosphines react as strong nucleophiles and/or reductants toward unsaturated chromophore moieties of lignin, decreasing the degree of conjugation, and hence leading to bleaching. The resulting phosphorus-containing moieties in the lignin also give a more permanent brightening of the pulp. The organo-phosphorus chemistry realized is relevant to other processes catalyzed homogeneously by transition metal-phosphine systems. In situ metal-THP species can catalyze hydrogenation of C=C and/or C=O bonds in lignin chromophores, and investigations on such systems have unearthed new THP-promoted transition metal chemistry.