Solvent effect in Soxhlet extraction of source rocks

IF 2.6 3区地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS

Organic Geochemistry Pub Date : 2025-02-01 DOI:10.1016/j.orggeochem.2024.104917

Jing Liao , Juncang Wang , Hong Lu , Guoying Sheng , Ping’an Peng , Chang Samuel Hsu

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引用次数: 0

Abstract

An abundant series of saturated n-C₁₂ to n-C₃₂ 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 ∼ 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-C₁₂ to n-C₃₂ 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-C₂₇. Abundant clay minerals existing in the Maoming oil shales are believed responsible for the formation of free n-FAs because they can provide Brønsted acidity to accelerate hydrolysis of ester linkage to release the bound fatty acids to free forms in specific environment having suitable low water content.

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来源期刊

Organic Geochemistry 地学-地球化学与地球物理

CiteScore

5.50

自引率

6.70%

发文量

100

审稿时长

61 days

期刊介绍： Organic Geochemistry serves as the only dedicated medium for the publication of peer-reviewed research on all phases of geochemistry in which organic compounds play a major role. The Editors welcome contributions covering a wide spectrum of subjects in the geosciences broadly based on organic chemistry (including molecular and isotopic geochemistry), and involving geology, biogeochemistry, environmental geochemistry, chemical oceanography and hydrology. The scope of the journal includes research involving petroleum (including natural gas), coal, organic matter in the aqueous environment and recent sediments, organic-rich rocks and soils and the role of organics in the geochemical cycling of the elements. Sedimentological, paleontological and organic petrographic studies will also be considered for publication, provided that they are geochemically oriented. Papers cover the full range of research activities in organic geochemistry, and include comprehensive review articles, technical communications, discussion/reply correspondence and short technical notes. Peer-reviews organised through three Chief Editors and a staff of Associate Editors, are conducted by well known, respected scientists from academia, government and industry. The journal also publishes reviews of books, announcements of important conferences and meetings and other matters of direct interest to the organic geochemical community.