Catalytic materials for hydrogenation processes in refinery and biorefinery applications: catalytic study in the o-xylene hydrogenation process

Prompted by the recent climate change, air quality has become, in the recent years, one of main issues of the international scientific community. In particular, sulphur dioxide it is recognized as the main responsible for acid rains and its abatement is a major environmental problem. While the SO2 level in the atmosphere is still very high, increasingly strict standards for control have prompted the development of a number of options and techniques for reducing SO2 emissions, such as the use of low-sulphur fuels and the introduction of desulfuration technologies. Since 2009, in fact, the European Union fully introduced the “zero-sulphur” legislation which limits the sulphur content in gasoline and diesel less than 10 mg/kg (ppm), thus requiring the development of new processing technologies and improved HDS catalysts for ULSD (ultra-low sulphur diesel) applications. Commercial hydrotreating catalysts usually consist of molybdenum (Mo) supported on an alumina carrier with promoters such as cobalt (Co) or nickel (Ni). These types of catalysts are affected by several drawbacks that limit their performance. In particular, the acidic properties of HDS catalyst can promote undesired side reactions such as cracking, polymerization and isomerization, depending on reaction feedstock, leading to catalyst deactivation by coke formation. Despite intensive efforts made by catalyst scientists to study deactivation phenomena, the nature of acid sites on the catalyst and their role in deactivation by coking is still unclear. On this account, this Ph.D. Thesis entitled “Catalytic materials for hydrogenation processes in refinery and biorefinery applications: Catalytic study in the o-xylene hydrogenation process” aims to shed light on the effects of the acidity of HDS/HDT catalyst on the catalytic pathway in the o-xylene hydrogenation process, used as model reaction, disclosing, at the same time, the key factors affecting catalysts stability. In this respect, the research activity has been mainly devoted to the synthesis and characterization of several CoMo or NiMo based catalysts and to the evaluation of their catalytic performance in the oxylene hydrogenation reaction. The obtained results showed a clear a straight relationship between the dispersion of active site and the reaction rate. Then, as a rule, the sulfiding procedure affects the active nature of "active site" and the extension of active phase on the catalyst surface, resulting the catalytic behaviour strongly dependent by: • the nature of the catalytic element; i.e. Co or Ni as promoters of MoSx active species; • the loading of the metals; • the acidic character of the material; • the activation protocol. In particular, the acidic character of the catalyst has a strong influence on both the catalytic pathway and the stability of the material during the reaction. In fact, an excessive number of strong Lewis's acid sites on the catalyst surface leads to a “super-reactivity” character of the catalyst in the o-xylene hydrogenation process, but they also promote several side reactions, such as isomerization and polycondensation, which rapidly drives to the catalyst deactivation by carbon deposition (coking phenomena). Therefore, it has been proved that two of the key factors controlling the catalyst deactivation are the diffusion of reagents on the “active sites” and the rate of formation of carbon deposits on the catalyst surface, which in turn affect the activity and stability of the material. Then, new synthesis procedure have been evaluated in order to reduce the acid strength of the catalysts and therefore their tendency to coke formation, using weak inhibitors of Lewis acid sites, which are able to slow the formation of carbon deposits without permanently blocking the catalyst active sites. Moreover, "mass transport reducing" agents were also investigated for a fine tuning and precise control of the accessibility of the reagentson the active surface.