Solvent effect in Soxhlet extraction of source rocks

An abundant series of saturated n-C-12 to n-C-32 fatty acid methyl esters (FAMEs) was unexpectedly detected in the aromatic fraction of Maoming oil shale extracts. Since no pretreatments involving saponification, acidification and esterification were used, these saturated n-FAMEs were suspected either originally present in samples or formed by inadventent methylation of n-FAs during Soxhlet extraction. Several carefully designed Soxhlet extraction experiments using a mixture of n-fatty acid standards (MFASTDs) with different volume ratios of dichloromethane (DCM) and methanol (MeOH) as solvents were performed to investigate if the methyl esterification (methylation) could indeed occur in the traditional Soxhlet extraction. The results revealed that when methanol alone was used as the extraction solvent, the esterification of fatty acids was very weak. However, with the addition of a proper amount of dichloromethane, for example, 3:2 or 4:1 vol ratio (v/v) of DCM:MeOH as extraction solvent, strong esterification reactions occurred and the esterification of MFASTDs reached almost 100 %. With 9:1 v/v of DCM:MeOH as used in conventional Soxhlet extraction, MFASTDs at lower loadings were nearly all esterified, while at higher loadings, the esterification of short-chain FAs could reach similar to 80 % while long-chain n-FAs were completely esterified. Therefore, we conclude that n-FAs can be methylated by conventional Soxhlet extraction and DCM plays an important role in esterification. The saturated n-C-12 to n-C-32 FAMEs in Maoming oil shales might be from free n-FAs derived from Botryococcus braunii (B. braunii) since they shared similar carbon number distributions as n-alkanes in the same samples, especially with a striking predominance of n-C-27. Abundant clay minerals existing in the Maoming oil shales are believed responsible for the formation of free n-FAs because they can provide Br & oslash;nsted acidity to accelerate hydrolysis of ester linkage to release the bound fatty acids to free forms in specific environment having suitable low water content.