Rates of Reaction of Wyodak Rawhide Coal Hydroxyl Groups with Tetra-alkylammonium Hydroxides and Rates of the Concomitant Swelling

Reaction rates of coals are often limited by the mass transfer rates of reagents into coals. The reaction of tetralkylammonium hydroxides with coals giving coal polyanions is much more rapid than any other derivatization of coal's hydroxyl groups. A calorimetric technique for measuring these rates was developed. The rates of coal swelling in the base solutions were measured independently. The following observations were made for Wyodak Rawhide coal: (1) NaOH and KOH do not rapidly diffuse into coals, (2) R 4 N + Br - does not swell coals, (3) R 4 N + OH - reacts rapidly with coals, the rate of heat liberation being first order and independent of the size of R in methanol and slightly dependent on the size of R in water, and (4) R 4 N + OH - swells coals, the swelling rate being proportional to √t, independent of R, and slower than the rate of heat liberation. The neutralization of the hydroxyls and the swelling of the coal are kinetically independent, although the former provides the driving force for the latter. The hydroxyl's reaction is diffusion controlled and the slow step may be (1) diffusion of the hydroxyl anion or (2) the desolvation of the basic anion as it enters the hydrophobic coal, or (3) self-diffusion of coal molecular segments. The R 4 N + ions can penetrate the coal because they are poorly solvated in water and methanol making desolvation easy and because they interact favorably with aromatic structures. The swelling rate-determining step appears to be the diffusion of the coal macromolecular segments through and past each other and is dominated by coal-coal interactions.