Structural Information from the Selective Methylation of Acidic O-H and C-H Sites in an Argonne Premium Coal Sample

摘要

It has been demonstrated that important coal structural information can be established by the application of selective chemical modification reactions to derivatize independently the acidic O-H and C-H sites in coal. For example, it is possible to 0-alkylate the phenols and carboxylic acids in bituminous and subbituminous coals by reaction with hydroxide and methyl iodide or methyl tosylate. We have found that is is possible to 'fine tune' this 0-methylation reaction in favor of carboxyl vs phenolic 0-methylation by limiting the amount of hydroxide base and replacing the methyl iodide or methyl tosylate with dimethyl sulfate. Specifically, the coal is treated with an excess of TBAH. After a specified time period (16 h), the excess base is neutralized with HCl. This 'back titration' with acid is monitored, a glass electrode being used to evaluate the pH of the solution. After equilibration, the resulting coal anions are quenched with C-13, C-14 double labelled dimethyl sulfate. The pH dependency of the reaction is evaluated by measuring the total number of introduced methyl groups using C-14 combustion analysis. The number of methyl esters in the alkylated coals is defined as the number of hydroxide labile methyl groups measured by hydrolysis. The reaction selectivity can be optimized by careful control of the reaction pH. The 'site specificity' of the reaction can be evaluated quantitatively by measuring the ratio: base labile methyls to total methyls. An important step in the selective methylation of the acidic C-H sites in coal is the conversion of the phenols and carboxylic acids to their methyl ether and methyl ester derivatives. This is accomplished by the preparation of a pH 12 0-methyl coal using natural abundance dimethyl sulfate as the methylating agent. (ERA citation 12:036297)