The conversion of light hydrocarbons to value-added chemicals has emerged as a pivotal domain within the broader field of sustainable and green chemistry. This transformation process, often carried out over a variety of specialized catalysts, offers a strategic approach to harnessing the untapped potential of abundant light hydrocarbon feedstocks such as methane, ethane, and propane.
Light hydrocarbons, generally derived from natural gas, refinery gases, and shale gas, are primarily composed of carbon and hydrogen. These low-cost, abundant, and energy-dense resources have traditionally been used as fuels. However, their transformation into value-added chemicals, including methanol, dimethyl ether (DME), benzene, toluene, and xylene (BTX), can significantly augment their economic value and contribute to the diversification of chemical feedstocks.
This transformation process is a key element of modern chemical engineering and represents a sustainable path towards a circular carbon economy. It not only provides a high-value outlet for under-utilized resources but also contributes to the reduction of greenhouse gas emissions. Furthermore, the chemicals derived from light hydrocarbons are key intermediates for a myriad of industrial applications, including the synthesis of plastics, solvents, and fuels, thereby extending the value chain of these feedstocks.
However, the conversion of light hydrocarbons to value-added chemicals is not without challenges. The reactions often involve complex kinetics, catalyst deactivation issues, and sensitivity to reaction conditions. It is, therefore, imperative to develop effective and efficient catalysts and to gain a deeper understanding of the reaction mechanisms. This will enable the design of more robust and efficient processes and thus contribute to the advancement of this important area of research.
This article aims to provide a comprehensive overview of the recent developments in the field of light hydrocarbon conversion to value-added chemicals, with a particular emphasis on the underlying reaction mechanisms, the role of catalysts, and strategies for addressing the associated challenges. We hope that this review will stimulate further innovation and research in this promising and rapidly evolving field. This doctoral thesis combines the development of two comprehensive microkinetic models for chemical reactions of industrial importance: the conversion of syngas to dimethyl ether (DME) over a bifunctional catalyst (CZA/FER) and the synthesis of aromatic compounds (BTX) from ethane/propane over Ga/ZSM-5 with a focus on catalyst deactivation due to coking.
In the first part, a novel microkinetic model for DME synthesis from syngas via methanol over a CZA/FER hybrid catalyst is established. The model examines detailed reaction rates and site fractions, accounting for 28 reactions over CZA and nine reactions over FER, and reveals the dominance of the associative pathway for DME synthesis. Reaction parameters are determined using advanced theoretical approaches, and the pre-exponential factors of Arrhenius rate constants are estimated with experimental data. This model delineates a nuanced understanding of the catalytic reaction system, differing from previous research, and provides a viable operating condition range for CO2 conversion in the feed.
The second part presents a detailed kinetic model for the synthesis of BTX from ethane and propane over a Ga/ZSM-5 catalyst, considering both Langmuir adsorption kinetics and lumped oligomerization kinetics. The model includes the dynamics of coke accumulation, which leads to catalyst deactivation. Through a series of adsorption-driven reactions and oligomerization reactions, intermediates are formed, leading to BTX synthesis and coke formation. Experimental data over a wide temperature range and feed compositions validate the model predictions for the conversion of reactants and the yield of products.
Collectively, this thesis presents a comprehensive understanding of the catalyst role in these industrially significant reactions and the deactivation effects due to coke accumulation, thus enhancing our knowledge of the reaction kinetics and providing a theoretical foundation for optimizing these processes and designing more effective catalysts.