Surface passivated zeolites: synthesis and catalytic contribution in de fossilization strategy

In recent years, great interest has been focused on the search for strategies aimed at tuning the structural and acidity properties of zeolites to improve catalytic performances. The acid sites located inside the crystals catalyse reactions exploiting the internal channel shapeselectivity different depending on the zeolitic structure employed. On the contrary, acid sites located on the external surface did not offer the possibility to control the size of molecules involved in reactions. This aspect generally leads to a loss of selectivity towards desired products and to the uncontrolled production of coke. Passivating surface acidity is a promising way to overcome deactivation issues and to enhance zeolites catalytic performances. In the literature, different techniques have been studied with the aim of neutralising external acid sites such as chemical liquid and vapour deposition of tetraethyl orthosilicate. Moreover, other post-synthetic treatments with HNO3 or with NaOH, deposition of MgO or impregnation with phosphorus have also been investigated. Recently, the synthesis of zeolitic core-shell materials attracted great attention among researchers. Particularly, this technique could be used to passivate external acid sites through the growth of a layer of aluminium-free zeolite, e.g. Silicalite-1, over the surface of a starting acidic zeolite meanwhile preserving the catalytic activity maintaining the accessibility of the internal acid sites. Furthermore, the synthesis of core-shell type catalysts coupling different zeolitic structures could also be useful to enhance their performances in catalytic processes. Zeolite composites synthesis through the integration of shell zeolites with smaller pores and core zeolites with larger pores is a rational strategy. The shell's structure is designed for separation tasks, but it also allows for additional catalytic activities to take place in the core. Meanwhile, the shell could also be used to neutralize the core external acidity, thereby preventing the occurrence of unwanted side reactions. This PhD work can be divided into two main parts. In the first part of this PhD thesis, the synthesis of zeolite catalysts with different structures, the passivation of their external acid sites through the employment of different techniques and the characterization of their main properties have been reported. This section in turn includes three steps. The first step is focused on the employment of the epitaxial growth technique to synthesise a shell of Silicalite-1 over the surface of ZSM-5 zeolites with the aim of neutralizing external acid sites. Firstly, an acidic ZSM-5 core with a high aluminium content (Si/Al ratio equal to 11) has been synthesized and coated with Silicalite-1. Then, a novel strategy for the growth of Silicalite-1 has been proposed; particularly, the coating of Silicalite-1 has been performed using an as-made form core (with the organic template still inside the channels) to prevent the partial blockage of the pores due to the growth of Silicalite-1 crystals. Moreover, ZSM-5 zeolites with different aluminium content have been synthesised and passivated employing the epitaxial growth technique in order to study the effect of acid sites distribution on the properties of resulting composite ZSM5@Silicalite1 catalysts. The second step of this work relates to the synthesis of BEA@Silicalite-1 samples via a steam-assisted crystallization technique. The third step has been focused on the use of chemical vapour deposition of TEOS to passivate external acid sites of ZSM-5 zeolite (with a Si/Al ratio equal to 25) in order to study the properties of passivated catalysts and to compare them with results obtained using ZSM5@Silicalite-1 composite materials. All the obtained catalysts have been characterised by classical analytic techniques such as XRD, porosimetry, TG/DTA, SEM-EDX, NH3-TPD and FT-IR. The main results are summarized in Chapter 4. The second part of this PhD thesis mainly focuses on the application of catalysts synthesized for different purposes. External acidity passivation could be useful to prevent the formation of undesired products during reaction and the deactivation of catalysts. For this reason, parent zeolites and passivated samples have been tested for two different catalytic applications. In a period in which accelerating the pathway toward decarbonization and reducing the dependence on fossil fuels became essential for governments, the synthesis of dimethyl-ether and its employment as a chemical intermediate has attracted great attention. For this reason, methanol to DME (MTD) and DME to olefins (DTO) reactions have been chosen to test synthesised catalysts and to analyse the effect of the passivation via the epitaxial growth of Silicalite-1 over samples catalytic behaviour. Interesting and challenging results, reported in Chapters 5 and 6, have been obtained for both applications. Finally, also a new application of BEA@Silicalite-1 materials is shown: the synthesis of Zeolitetemplated carbons (ZTCs) employing the composite as a scaffold. In this part of the work, the “gate effect” due to the narrower pores of the Silicalite-1 layer coating the BEA surface on ZTCs final properties has been evaluated. Samples have been characterised through XRD, TG/DTG, SEM, FT-IR and Raman Spectroscopy. Obtained ZTC materials have been employed as adsorbents in Carbon Capture and Storage (CCS) processes and interesting differences have been found between samples obtained starting from a simple BEA zeolite and samples obtained from BEA@Silicalite-1 composite materials. The main results of this part of the work are summarized in Chapter 7.