Mechanistic Insights into Microwave-Assisted Hydrogen Generation from Crude Oils: Influence of Polarity, Dielectric Properties, and Clay Minerals

This study aims to evaluate the efficiency of microwave-assisted combustion for in-situ hydrogen production from crude oils with varying compositions, focusing on the role of crude oil polarity, particularly resins-to-asphaltenes ratios, and lithological interactions. Special emphasis is placed on understanding how dielectric properties and the presence of mineral phases, such as clays, influence hydrogen yields, gas composition, and thermal behavior compared to conventional combustion. Four crude oils spanning a wide viscosity range were characterized via SARA (Saturates, Aromatics, Resins, Asphaltenes) analysis and dielectric property measurements. Pseudo-reservoir systems were prepared by mixing oil, water (50:50), and crushed Permian Basin reservoir rock. Microwave heating (2.45 GHz, 1200 W) was applied, and gas compositions were continuously monitored by gas chromatography. Control experiments with conventional heating were conducted for comparison. Additional runs incorporating 5 wt% clay into Oil 1 and Oil 4 were performed to evaluate mineral effects on hydrogen generation and reaction pathways. Microwave-assisted combustion produced measurable hydrogen in all crude oil systems, whereas conventional heating yielded no detectable hydrogen despite higher overall oil-to-gas conversion. Hydrogen yield trends were: Oil 3 (Waxy Oil) > Oil 4 (Light Oil) > Oil 1 (Bitumen) > Oil 2 (Extra-Heavy Oil), aligning with higher polar content and favorable resins-to-asphaltenes ratios. Oils with strong resin–asphaltene interactions exhibited reduced microwave coupling efficiency despite high polar content. Dielectric measurements revealed that crude oil loss tangent correlated strongly with resins-to-asphaltenes ratios (R2 = 0.9949), indicating that polarity balance, not just magnitude, governs microwave absorption. When mixed with rock and water, loss tangent values converged (~0.29), but dielectric constant differences still influenced thermal and gas production performance. Clay addition significantly enhanced hydrogen yields and total gas conversion for Oil 1 and Oil 4, even when maximum temperatures were lower than clay-free cases. This enhancement is attributed to a secondary hydrogen generation pathway, potentially involving microwave-activated clay–bound water–methane reforming. Correlation analysis showed CO generation tracked with hydrogen yield (R2 = 0.8709), likely via reforming and water–gas shift reactions, while CH4 and C2H4 production were closely linked (R2 = 0.9204), consistent with parallel cracking pathways. Clay reduced CH4 and C2H4 production at high hydrogen yields, suggesting pathway selectivity toward reforming over cracking. This work provides the first integrated mechanistic assessment of microwave-assisted hydrogen generation from crude oils, linking molecular polarity, dielectric properties, and mineral-phase effects. By demonstrating that clay can simultaneously enhance hydrogen yields and suppress undesired hydrocarbon cracking products, the study identifies a tunable pathway for selective hydrogen production. The findings offer a scalable, low-emission strategy for in-situ hydrogen generation from petroleum reservoirs, with implications for clean energy recovery in both conventional and unconventional systems.