Unconventional sustainable catalytic processes for the direct upgrading of methane

The need to move away from fossil fuels towards more sustainable and greener energy sources is becoming more important over the course of time. Since the development of the green chemistry pillars, the chemical industry is slowly shifting towards more sustainable and less energy-intensive processes. By speeding up and decreasing the energy cost of reactions and being already present in almost any reaction, catalysis is one of those pillars that has a pivotal role to play in the chemical industry evolution. The well-established thermo-catalytic processes are, for some, extremely energy-intensive and operate in harsh conditions. The conversion of small molecules, such as N2, CO2, and CH4, in a more energy-efficient way, through alternative and more sustainable processes, such as electro-, photo-, or plasma-catalysis, provides an opportunity to efficiently exploit sustainable resources and decrease the global energetic costs, while offering the foundations for a carbon-neutral chemical production. In the framework of the European Research Council SCOPE project (Surface-COnfined fast-modulated Plasma for process and Energy intensification in small molecules conversion), this work presents the development of alternative sustainable technologies, specifically focusing on photo- and plasma-catalysis, for the direct conversion of methane to hydrocarbons, via a non-oxidative route, which is the most atomically efficient method for methane transformation. Historically, indirect routes for methane transformation have been preferred by industries, due to the thermodynamic and economical barrier associated with the selective activation of the C-H bond. However, the rise of alternative sustainable technologies provides alternative pathways and shortcuts for mild, direct and selective methane upgrading. The majority of the research activities have been conducted at the laboratory CASPE/INSTM (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina (Dept. Chibiofaram). During the third year, six months were spent at the Interface Science department of the Fritz-Haber Institute of the Max-Planck Society (FHI), in Berlin, under the supervision of Dr. Shamil Shaikhutdinov and Prof. Beatriz Roldán Cuenya, in the research group of surface and reactivity. The Ph.D. thesis is organised into five main chapters plus the general conclusions. Chapter 1 introduces the main topic of the thesis, by briefly presenting catalysis and exposing the main current and future challenges for the conversion of important small molecules. Strong emphasis is given to methane upgrading. The traditional industrial processes for activating the C-H bond and the different methane utilisation strategies are presented. Particular attention is given to the direct methane transformation through the Non-Oxidative Coupling of Methane reaction (NOCM). The reasons that have prevented its industrial development and the need for alternative paths for methane conversion are also described. Chapter 2 is divided into three sections. The first section introduces photocatalysis, explaining the general theoretical concepts underlying photocatalytic processes. The second section presents the development of photocatalytic materials, detailing their conception for controlling charge-carrier generation, separation, and surface reactions. Several innovative concepts are discussed to provide insights for developing a highly active photocatalyst that maximises solar-to-fuel efficiency. In the third part, the literature reporting the application of photocatalytic processes for the NOCM reaction is briefly described to present the state-of-the-art. Chapter 3 focuses specifically on the light-assisted NOCM reaction. The results reported in this chapter, demonstrate that, starting from the theoretical concepts presented in Chapter 2, one of the most basic and economic photocatalysts, TiO2, can yield outstanding productivity and selectivity results. Modification with metal nanoparticles (Au, Ag, and Pd) has been carried out, and the roles of size and oxidation state of Pd species have been elucidated, through advanced optical and structural characterisation techniques. In addition, a photo-active membrane was developed using the synthesised catalysts and tested in an innovative flow-through photo reactor. Finally, stability tests (24h of continuous irradiation) were performed and allowed the analysis of the hydrocarbon selectivity. A mild oxidative regeneration treatment was also successfully applied to extend the catalyst’s lifetime. Chapter 4 presents the use of plasma technology, an alternative process for the conversion of small molecules, that represents an ideal gate for the electrification of the chemical industry. The chapter discusses non-thermal plasmas, covering the underlying chemical and physical principles and their application for gas conversion. Particular emphasis is given to the catalyst introduction within the plasma zone, a relatively recent field still possessing many “unknowns”. The complexity of plasma catalysts surpasses that of traditional thermal catalysis and other electrocatalytic or photocatalytic processes, due to the reciprocal relationship between the catalyst and the plasma. Hence, a significant part of the catalysis science may not apply to plasma systems, and the wide range of possible parameters for a plasma-catalytic system, makes the extrapolation of results from one system to another nearly impossible. Therefore, the knowledge gained exclusively on the plasma-assisted NOCM reaction is highlighted, both with and without catalysts. Furthermore, the possible synergy between photocatalysis and plasma technologies is introduced. Chapter 5 presents the development of two innovative Dielectric Barrier discharge (DBD) reactors for the plasma-assisted NOCM reaction, allowing the light irradiation of the catalyst in the plasma zone. First, a planar reactor has been developed through various successive modifications, to employ an electrode that serves also as a photocatalyst. The anodisation process of a Ti gauze produces a hierarchical 3D macro-mesoporous structure, based on highly ordered photoactive TiO2 nanotubes arrays. The as-prepared catalysts have been successfully tested in the designed reactor, to show promising synergistic effects between photo- and plasma-catalysis. Second, a tubular reactor employing water as the ground electrode and two quartz dielectric barriers is proposed and applied for the first time to the NOCM reaction. This concept of a water electrode reactor is compared with a traditional DBD, showing a greater power-to-conversion but more importantly a better control of selectivity to prevent the formation of black carbon. The effect of variation in the water temperature and the gap size is investigated, and, if the water temperature of the electrode has very little influence on performance, narrowing the gap to the micrometric range (0.5 mm) dramatically changes almost every property of the plasma. In this case, due to controlled C-H activation, an unprecedented alkane selectivity of 97% among all produced hydrocarbons (at conv. ca. 10%), is obtained while preventing the formation of carbon deposits. The effect of packing TiO2 or Pd-TiO2 catalysts of various sizes is also investigated, indicating that loading Pd significantly increases the alkane selectivity and promotes the formation of heavier hydrocarbons, likely due to its ability to facilitate C-C coupling reactions.