Designing Catalytic Desulfurization Processes to Prepare Clean Fuels—2nd Edition

Following the success of the first edition, this second volume of the Special Issue “Designing Catalytic Desulfurization Processes to Prepare Clean Fuels” brings more advanced and innovative strategies for the removal of sulfur compounds from fuels. Sulfur-containing fuels are major contributors to the occurrence of acid rain and environmental pollution, largely due to the release of sulfur dioxide (SO₂) and fine particulate matter containing metal sulfates during fuel combustion. These emissions are extremely toxic and corrosive, being harmful to both human health and ecosystem equilibrium and safety. In response to these challenges, increasingly strict regulations worldwide have pushed the refining industry to adopt more rigorous and advanced desulfurization technologies. While hydrodesulfurization (HDS) remains the predominant industrial method, its limitations—including the need for high temperatures, high pressures, and high hydrogen consumption—pose economic and environmental concerns. Furthermore, HDS is not well suited to processing complex or heavy feedstocks such as heavy fuel oils, which require alternative or complementary solutions.

This Special Issue aims to highlight recent advances in catalytic desulfurization approaches that offer sustainable, cost-effective, and versatile alternatives to traditional HDS. Contributions include, but are not limited to, oxidative, extractive, adsorptive, and biocatalytic desulfurization processes, with a strong emphasis on those with potential for industrial application. The selected articles reflect the multidisciplinary nature of this field and demonstrate ongoing global efforts to meet environmental targets through cleaner fuel technologies, adapted to different kinds of fuels.

By gathering original research papers and critical reviews, this edition not only showcases the breadth of scientific innovation in catalytic desulfurization but also underscores the importance of continuous development in pursuit of greener and more efficient fuel processing solutions.

One study, “Methyl Mercaptan Removal from Methane Using Metal-Oxides and Aluminosilicate Materials”, addresses the challenge of removing methyl mercaptan, a sulfur compound commonly found in natural gas that poses corrosion risks in infrastructure, from natural gas. By evaluating the catalytic activity of various metal oxides and aluminum silicates using a fixed-bed reactor, the study reveals that combining manganese, copper, and zinc oxides on aluminum silicate significantly enhances sulfur capture—reaching a capacity of 1226 mg S/g. This work highlights the potential of synergistic metal oxide formulations to serve as high-performance desulfurization catalysts while minimizing harmful byproducts such as dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). These findings contribute to the growing field of low-temperature, low-energy sulfur removal technologies suitable for gas processing industries.

In the article “Deactivation and Regeneration Studies of Molybdenum-Based Catalysts in the Oxidative Desulfurization of Marine Fuel Oil”, the researchers investigated catalyst deactivation mechanisms during the oxidative desulfurization (ODS) of heavy fuel oil (HFO), a key process for producing cleaner marine fuels. Mo-based alumina-supported catalysts were analyzed before and after the reaction using techniques like XRD, Raman, XRF, and TGA. The authors’ findings show that catalyst activity and its regeneration depend on molybdenum surface speciation—well-dispersed Mo species are active and regenerable, while crystalline MoO₃ reduces performance and hinders regeneration. Sulfone adsorption had little effect on deactivation; instead, non-sulfur compounds were the main contributors. Regeneration success also varied with Mo content and oxidant type—organic oxidants like tBHP improved catalyst recovery. This work provides critical insight into deactivation in real HFO ODS systems and offers strategies to improve catalyst durability.

Another notable contribution is the “Nano-TiO₂-Enhanced Surface Functionalization of Recycled Concrete Aggregates for Improved Degradation Efficiency of Low-Concentration Sulfur Dioxide”. This innovative work utilizes the functionalization of recycled concrete aggregates (RCAs) with nano-TiO₂ to enhance the photocatalytic degradation of low-concentration sulfur dioxide (SO₂). The nano-TiO₂ coatings significantly improved SO₂ removal under light irradiation, especially by UV light, due to photo-induced oxidation. The coated RCAs demonstrated strong durability, retaining 85% of their photocatalytic activity after five reuse cycles. A mathematical model was used to illustrate how environmental variables, like SO₂ concentration and flow rate, influence degradation efficiency. This research highlights the potential of nano-TiO₂-modified RCAs for sustainable construction and air pollution control, offering an innovative approach to environmental remediation and the circular use of construction waste.

The paper entitled “Production of Green Fuel Using a New Synthetic Magnetite Mesoporous Nano-Silica Composite Catalyst for Oxidative Desulfurization: Experiments and Process Modeling” explores the use of eco-friendly oxidative desulfurization catalysts using bentonite-derived nano-silica combined with HY zeolite for sulfur removal from kerosene. Two Fe–silica composite catalysts were prepared—CAT-1 (100% nano-silica) and CAT-2 (80% nano-silica, 20% HY-zeolite)—and catalyst performance was tested under various temperatures, durations, and airflow conditions. CAT-2 demonstrated significantly higher sulfur removal (87.88%) compared to CAT-1 (50%), highlighting the enhancing effect of HY zeolite. A mathematical model was developed and validated to optimize reaction conditions, predicting over 99% of the sulfur removal. Characterization techniques confirmed the structural and surface properties, supporting the catalyst’s efficiency.

Lastly, the review “Advanced Technologies Conciliating Desulfurization and Denitrogenation to Prepare Clean Fuels” highlights the urgent need for the effective removal of both sulfur- and nitrogen-containing compounds from fuels to prepare more sustainable fuels that can easily meet strict environmental regulations. While traditional hydrodesulfurization and hydrodenitrogenation processes are limited by harsh conditions and poor simultaneous removal efficiency, non-hydrogen alternatives offer promising results under milder conditions. Despite decades of research on desulfurization and, more recently, denitrogenation, few studies have addressed their simultaneous elimination. The paper critically evaluates various non-hydrogen methods—such as adsorption, extraction, (photo)catalytic oxidation, and ultrasound-assisted oxidation—and emphasizes the importance of integrated strategies to develop cost-effective, sustainable technologies for clean fuel production.

Collectively, the contributions to this Special Issue reflect the diverse and dynamic landscape of desulfurization research, showcasing global scientific collaboration aimed at developing cleaner and more sustainable fuel solutions. Most of the papers in this collection demonstrate that the design of suitable materials is the key to this dynamic topic. The knowledge shared through these studies lays a strong foundation for continued innovation, guiding the advancement of more efficient and environmentally friendly desulfurization technologies.

We sincerely thank all of the authors for their valuable contributions, the reviewers for their insightful evaluations, and the readers for their engagement. Your commitment and expertise are vital to ongoing progress in the pursuit of clean energy. We eagerly anticipate future developments and discoveries that will shape the future of catalytic desulfurization.