Hak Joo Kim

Growing concerns over climate change have accelerated the development of sustainable aviation fuel (SAF). Fischer–Tropsch (FT) synthesis is one of the ASTM-approved pathways for SAF production. However, a substantial fraction of FT products is distributed as heavy wax according to the Anderson–Schulz–Flory distribution. Thus, an additional upgrading process is necessary. In this study, acid/base-treated activated carbon (tAC) was investigated as an alternative catalyst to conventional Y zeolite for selective FT wax upgrading. Activated carbon (AC) was sequentially treated with NaOH and HNO3 to generate mesoporosity and introduce oxygen-containing functional groups (OFGs). The catalysts were characterized by BET, NH3-TPD, FTIR, XPS. Hydrocracking experiments demonstrated that catalyst performance was governed by the combined influence of oxygen-containing surface functionalities and pore structure rather than total acidity alone. Among the prepared catalysts, tAC16H exhibited the highest yield toward SAF-range hydrocarbons. The OFGs introduced by acid treatment on AC likely provided accessible acidic sites for wax cracking, while the mesopores improved molecular transport and suppressed excessive secondary cracking. Compared with conventional Y zeolite catalysts, the tAC catalysts suppressed excessive light hydrocarbon formation and produced paraffin-rich products with lower olefin content. These results suggest that tAC is a promising alternative to conventional zeolite catalysts for the production of SAF-range hydrocarbons.

Conventional homogeneous catalysts exhibit rapid reaction kinetics and affordability but face significant challenges, including the cumbersome separation of catalysts, complexities in product purification, and the generation of wastewater. In this regard, Na-GCN was previously developed as an effective heterogeneous catalyst for transesterification. To advance the industrial application of these catalysts, it is essential to incorporate a continuous production system and high-quality feedstock. Fixed bed reactors offer the advantage of simple operation and have been widely utilized in continuous biodiesel production. In this study, Na-GCN was applied to the transesterification of waste oils in a continuous fixed bed reactor. Various reaction conditions, including temperature, retention time, and the oil-to-methanol ratio, were compared and optimized. The transesterification of waste oil simulant, waste cooking oil, and waste food oil was conducted under the optimized conditions. The FAME content was measured using gas chromatography equipped with a flame ionization detector and HP-INNOWax, following an internal standard method. Notably, FAME yields exceeding 90% were maintained for 100 hours of continuous operation using Na-GCN and waste cooking oil. The reaction system also demonstrated enhanced stability over 100 hours, even when processing waste food oil with high levels of impurities.

Conventional homogeneous catalysts exhibit rapid reaction kinetics and affordability but face challenges including the cumbersome separation of catalysts, product purification complexities, and the generation of wastewater. In response, acid-treated melem and graphitic carbon nitride catalysts have been explored as heterogeneous alternatives for the esterification of oleic acid. Melamine, characterized by its repeating C-N bonds, is synthesized via thermal polymerization, began to condensed into melem and leading to further condensation into carbon nitride. The unique graphitic structure and nitrogen content confer upon carbon nitride enhanced chemical and thermal stability, as well as the potential for chemical modifications. In this investigation, oxygen-containing functional groups were introduced to the carbon nitride framework through a singular acid treatment coupled with sonication. The resulting catalysts were comprehensively characterized employing X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and temperature-programmed desorption/mass spectroscopy (NH3-TPD/MS).

Increasing environmental problems and the energy crisis have led to interest in the development of alternative energy. One of the most promising sustainable alternatives to fossil fuel is biodiesel which is typically produced from the transesterification of refined vegetable oils using a homogeneous base catalyst. However, the current process limitations and steep production costs associated with the use of homogeneous catalysts have limited the global-wide acceptance of biodiesel. Heterogeneous catalysts have been considered suitable alternatives, but they still suffer from low catalytic activity. In this study, by using density functional theory (DFT) calculations, we examined the electronic and catalytic activity of the single Na-doped graphitic carbon nitrides (indicated by Na-doped g-C₃N₄) toward the efficient biodiesel (acetic acid methyl ester) production via the transesterification of triglyceride (triacetin). Our DFT calculation on reaction energetics and barriers revealed the enhancement of biodiesel productivity in the Na-doped catalyst compared to the pristine g-C₃N₄ catalyst. This was related to the large reduction of the barrier in the rate-limiting step. In addition, we investigated the acidity/basicity and electron distribution and density of state for the Na-doped and pristine g-C₃N₄ catalysts to better understand the role of the Na atom in determining the transesterification reaction. This study highlights the importance of the dopant in a g-C₃N₄ catalyst in determining the transesterification reaction, which may open new routes to improve biodiesel production.

This study investigates the decomposition of methane using solar thermal energy as a heat source. Instead of the direct thermal decomposition of the methane at a temperature of 1200 °C or higher, a catalyst coated with carbon black on a metal foam was used to lower the temperature and activation energy required for the reaction, and to increase the yield. To supply solar heat during the reaction, a reactor suitable for a solar concentrating system was developed. In this process, a direct heating type reactor with quartz was initially applied, and a number of problems were identified. An indirect heating type reactor with an insulated cavity and a rotating part was subsequently developed, followed by a thermal barrier coating application. Methane decomposition experiments were conducted in a 40 kW solar furnace at the Korea Institute of Energy Research. Conversion rates of 96.7% and 82.6% were achieved when the methane flow rate was 20 L/min and 40 L/min, respectively.

Supercritical methylcyclohexane cracking of NiAl₂O₄ spinel-based catalysts with varying Ni/Al deficiencies was investigated. Thus, catalysts with Ni content of 10–50 wt.% were prepared by typical co-precipitation methods. The calcined, reduced, and spent catalysts were characterized by X-ray diffraction, O₂ temperature-programmed oxidation, NH₃ temperature-programmed desorption, N₂ physisorption, O₂ chemisorption, scanning and transmission electron microscopy, and X-ray fluorescence. The performance and physicochemical properties of the reference stoichiometric Ni₃Al₇ catalyst differed significantly from those of the other catalysts. Indeed, the Ni-deficient Ni₁Al₉ catalyst led to the formation of large Ni particles (diameter: 20 nm) and abundant strong acid sites, without spinel structure formation, owing to the excess Al. These acted with sufficient environment and structure to form the coke precursor nickel carbide, resulting in a pressure drop within 17 min. On the other hand, the additional NiO linked to the NiAl₂O₄ spinel structure of the Al-deficient Ni₅Al₅ catalyst formed small crystals (10 nm), owing to the excess Ni, and displayed improved Ni dispersion. Thus, dehydrogenation proceeded effectively, thereby improving the resistance to coke formation. This catalytic behavior further demonstrated the remarkable activity and stability of this catalyst under mild conditions (450 °C and 4 Mpa).

Hypersonic aircrafts suffer from heat management problems caused by the air friction produced at high speeds. The supercritical catalytic cracking of fuel is endothermic and can be exploited to remove heat from the aircraft surfaces using specially designed heat management systems. Here, we report that an acid/base-treated activated carbon (AC) catalyst shows superior performance to the conventional ZSM-5 catalyst at 4 MPa and 450 °C. Further, under these conditions, coke formation is thermodynamically avoided. Of the prepared catalysts, the AC catalyst treated with NaOH and subsequently with HNO₃ (denoted AC-3Na-N) was the most active catalyst, showing the highest selectivity toward light olefins and best heat sink capacity. The acid/base-treated ACs and ZSM-5 catalysts were characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, NH₃ temperature-programmed desorption, and Fourier-transform infrared spectroscopy measurements. Characterization reveals the importance of acid strength and density in promoting the cracking reaction pathway to light olefins observed over the acid/base-treated AC catalysts, which show comparable activity at 450 °C to that of the ZSM-5 catalyst operated above 550 °C. The low-temperature activity suppressed coke and aromatic compound (coke precursors) formation. The stability of the acid/base-treated activated carbon catalysts was confirmed over a time-on-stream of 30 min.