The chemistry at the heart of the work is deceptively simple. When nitric acid attacks kukersite kerogen, it preferentially cleaves oxidation-labile groups in the macromolecular network, chiefly resorcinol subunits described in the Lille-Blokker structural model, releasing a mixture of dicarboxylic acids with chain lengths typically spanning C4 to C10, along with mono- and tricarboxylic acids. Historical Estonian pilot operations in the 1960s to 1980s demonstrated yields of up to 26 percent dicarboxylic acids after vacuum distillation, but the process demanded extensive beneficiation to produce a 90 percent kerogen concentrate and severely corroded stainless-steel reactors. The new study shows that much of that complexity can be engineered away.

Under atmospheric pressure, the team screened the reaction in round-bottom flasks using oil shale concentrates containing 45, 70 and 90 percent kerogen. Temperature emerged as the single strongest lever on yield, particularly at short reaction times of three to six hours, because higher temperatures accelerate the thermal decomposition of nitric acid into oxidation-initiating species such as nitrous acid and nitrogen dioxide. These species trigger radical-mediated oxidation that chews through the kerogen matrix. At 120 degrees Celsius with 65 weight percent nitric acid, yields climbed rapidly, while at 80 degrees Celsius even 48-hour reactions produced lower overall yields, underscoring the kinetic importance of heat.

Concentration proved equally decisive. Reactions with 65 weight percent acid outperformed 30 weight percent acid at identical molar oxidant-to-carbon ratios, indicating that DCA formation is primarily kinetically controlled rather than limited by total oxidant stoichiometry. Concentrated nitric acid is less thermally stable and more prone to decomposing into the reactive nitrogen species that initiate oxidation. Yet the data also revealed a safety-relevant sweet spot: once sufficient oxidant was present, roughly an HNO3-to-TOC molar ratio of 0.67 to 1, further excess provided diminishing returns. In one 48-hour experiment at 80 degrees Celsius, mass ratios of 1:10 and 1:5 oil shale to acid delivered nearly identical yields of 24.2 and 23.5 percent, while leaner 1:3 and 1:2 ratios fell to 16.1 and 12.4 percent.

Product distribution responded to reaction severity in a way that gives process designers a tunable dial. Harsher conditions, higher temperatures, longer exposure and stronger acid, pushed overall yields upward but promoted the formation of shorter-chain dicarboxylic acids in the C4 to C6 range, and over extended times began to decompose the longer-chain C7 to C11 acids that carry the highest value. Milder conditions favored the longer-chain fraction, which plateaued at roughly 9.5 percent yield regardless of whether the reaction ran hot for hours or cool for days. Notably, the kerogen concentration of the feedstock had little effect on yield, meaning that extensively beneficiated concentrate is unnecessary and lower-grade oil shale can serve as a viable feedstock, a finding with major implications for resource efficiency.

The real breakthrough came when the team moved the reaction into pressurized Hastelloy-lined Parr reactors. Elevated pressure improves the solubility of nitric oxide and nitrogen dioxide in the aqueous phase, raising the effective concentration of the oxidation-initiating species and accelerating radical formation. In a designed experiment at 5 bar with 30 weight percent acid, yields of around 21 percent were reached in just two hours, compared with less than 10 percent after three hours at atmospheric pressure under comparable conditions. Statistical modeling confirmed that temperature dominated the response, with a regression coefficient of 0.585 versus 0.069 for time, and the model explained 98.3 percent of the variance in yield.

Pushing pressure higher delivered dramatic gains. At 140 degrees Celsius with a 30-minute reaction time and an HNO3-to-TOC ratio of 3, increasing pressure from 5 to 15 bar raised the DCA yield from 19.8 to a maximum of 30.1 percent, while pressures above 15 bar offered no further benefit. Gas analysis showed deeper oxidation at higher pressure, with rising carbon monoxide, carbon dioxide and nitrous oxide and falling nitrogen oxides, consistent with greater availability of dissolved reactive nitrogen species. Pre-pressurizing the reactor with nitrogen or a nitrogen-oxygen mixture had no significant effect on yield, and excess oxidant beyond a molar ratio of 3 provided no additional yield, further confirming that concentration and temperature, not stoichiometric excess, govern the reaction.

Timing proved critical for preserving product quality. Carbon conversion from the solid kerogen approached completion after roughly 40 minutes, with DCA yield peaking at 30 to 40 minutes before longer-chain acids began to degrade. A carbon mass balance for a 40-minute reaction showed 56.2 percent of carbon recovered in the liquid phase, including 14.7 percent as dicarboxylic acids, 43.0 percent in gaseous products and only 0.9 percent remaining in the solid residue. Compared with the previously investigated two-step wet air oxidation route, which left 16 to 29 percent of carbon in the solid residue, direct nitric acid oxidation achieved far greater solubilization with lower solid waste. The team also found that 30 weight percent acid delivered high yields, meaning higher concentrations provide limited benefit relative to increased oxidant consumption and safety considerations.

Material selection, a historical Achilles heel of the process, was addressed directly. PTFE and Hastelloy linings performed equivalently, and intriguingly, the presence of a titanium piece in the reactor appeared to boost yields, reaching 30.9 percent at 140 degrees Celsius, possibly through catalytic or surface-mediated effects that the authors flag for further study. Historical work identified titanium as the most durable material under nitric acid oxidation conditions, and the new results suggest it may serve as a reactor lining in future designs. Scale-up will demand careful heat management because the reaction is strongly exothermic, and the team argues that a continuous-flow reactor without mechanical agitation, the basis of the Kerox process, offers superior heat and mass transfer with fewer hot spots than batch processing.

The environmental and safety profile of the revived process looks manageable. No volatile nitrated or nitrosated compounds were detected in the product fraction, and the oxidized solid residue contained just 0.37 weight percent nitrogen after 40 minutes, indicating that oxidative degradation rather than nitration dominates under the employed conditions. Off-gases containing nitrogen oxides can be scrubbed to recover nitric acid for recycle, though nitrous oxide formation, roughly 2.5 percent of the oxidant, must be minimized because it cannot be recycled and carries a global warming potential 273 times that of carbon dioxide. With downstream recovery strategies such as liquid-liquid extraction or esterification under investigation, the findings demonstrate a viable and scalable route for transforming a local energy resource into chemical intermediates, exactly the kind of shorter value chain that European material supply security strategies now demand.

Subject of Research: Chemical Engineering

Article Title: Nitric acid oxidation of kerogen in atmospheric and pressurized systems – basis for the Kerox process

Article References: Reinaas, M., Kaldas, K., Muldma, K., Mets, B., Uustalu, J. M., Koern, V. Ö., & Lopp, M. (2026). Nitric acid oxidation of kerogen in atmospheric and pressurized systems – basis for the Kerox process. Frontiers in Chemical Engineering, 8, Article 1880713. https://doi.org/10.3389/fceng.2026.1880713

Image Credits: AI Generated

DOI: 10.3389/fceng.2026.1880713

Keywords: kerogen, nitric acid oxidation, dicarboxylic acids, oil shale, Kerox process, kukersite, chemical valorization, supply security, Nitric, acid, oxidation, atmospheric