Towards catalytic synthesis of fuels and useful chemicals from low-value feedstocks

Mensah, Jim Kabenla Miezakyi

Title: Towards catalytic synthesis of fuels and useful chemicals from low-value feedstocks
Creator: Mensah, Jim Kabenla Miezakyi
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2021
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: The primary challenge in the conversion of feedstocks such as plastics, oil sands and tars in the refining process is the difficulty of enabling the heavy/bulky reactants to access the microporous channels of conventional zeolites such as Y, BEA, MWW and ZSM-5. In principle, crystalline zeolitic materials exhibit a well-defined pore system with pore sizes in the micropore regime and, thus, do not feature hierarchical porosity. To introduce an additional pore system into zeolitic materials to create hierarchical porosity, several preparation strategies were explored. These strategies can be divided into two main categories; introduction of additional porosity into the zeolitic materials via demetallation and/or intercalation of inorganic moieties and/or exfoliation; and combination of the inherent microporous zeolitic material with the porosity of a second material of a different length scale via seed-assisted crystallization or mechanical mixing to form composite materials. A laboratory scale flow reactor was constructed for the catalytic cracking of sterically challenging feedstocks over a wide range of zeolite materials of microporous, mesoporous, and hierarchical nature. Investigation of the role of zeolitic porosity and acidity in influencing the catalytic activity, selectivity and deactivation of plastics, paraffin wax, 1,3,5-triisopropylbenzene, and n-hexadecane cracking was conducted under differential reactor conditions (T = 280 – 350°C, WHSV = 0.7 min-1). The cracking rates, mechanism and TOF calculations were conducted under differential reactor conditions (determined at < 15% conversion). The enhanced textural properties and accessibility of the catalytic active sites played very crucial roles in enhancing the catalytic performance of the hierarchical zeolites. In all cases, the hierarchical zeolites displayed a much-improved catalytic performance and deactivation resistance to coke deposition as compared to the conventional counterpart. To overcome diffusional limitations associated with the catalytic cracking of large hydrocarbons, a novel core-shell hierarchical zeolite was developed and catalytically evaluated. The novel hierarchical composite was synthesized by preliminary seeding of the core BEA crystals and subsequent growth under hydrothermal conditions leading to the formation of an intergrown and distinctive nanocrystalline ZSM-5 shell zeolite. Large, branched hydrocarbons encounter diffusional limitations in micropores of zeolites for cracking reactions, a limitation overcome by improved textural properties of hierarchical zeolites, leading to enhanced cracking activity. The impact of the hydrocarbon chain length involving long chain paraffin (hexadecane) and a highly branched aromatic (1,3,5-TIPB) was also investigated to study the influence of the presence of the micro-mesopore network in overcoming diffusion limitations. 1,3,5-Triisopropylbenzene cracking over MFI with different mesoporosities and Si/Al ratios (15, 20, 140) generated by hierarchization of MFI zeolites were investigated to elucidate the role of catalyst acidity and mesoporosity on the reaction. To do this, a considered selection of MFI (microporous, nanocrystalline, hierarchical) and SBA-15 (mesoporous) zeolites with distinctive porosity and range of acid site density and strength variations were synthesized. Furthermore, we demonstrate that the passivation of ZSM-5 via the epitaxial growth of silicalite-1 (HZSM-5@Silicalite-1) provides unhindered access, passivates the external acid sites and causes pore-blockage which significantly reduces the 1,3,5-TIPB cracking activity. Additionally, nanocrystalline ZSM-5 with low acid site concentration and enhanced mesoporosity was characterised by a relatively low catalytic cracking activity. The cracking results suggest that the cracking activity and product selectivity is mostly influenced by the extent of mesoporosity and acidity. The activity and product selectivity of LDPE and 1,3,5-TIPB cracking over hierarchical Y zeolite and mesostructured (SBA-15) zeolites were studied in a continuous fixed bed flow reactor. The influence of acid site speciation and mesoporosity was studied using single step and sequential demetallation strategies involving dealumination, desilication and acid wash to create hierarchical structures in Y zeolite. Hierarchical Y zeolite, Na-Y and SBA-15 zeolites were used as reference and benchmark materials for acidity and porosity respectively. The catalytic studies suggests that the enhanced strength of the Brønsted acid site influenced by the presence of extraframework aluminium enhances the activity of the hierarchical zeolites whilst the secondary mesoporosity promotes resistance to catalyst deactivation. Hence, the conversion, yield and selectivity were significantly influenced by the synergy of enhanced mesopore surface area (Smeso) and strength of the accessible Brønsted acid sites (BAS). The major findings depict that dealuminated Y zeolite was the most effective catalyst with the highest catalytic activity in 1,3,5-TIPB cracking of approximately 10-times increase over parent zeolite Y, attributed to the optimized balance between mesoporosity and acid sites. Finally, this study investigated the potential of delaminated and pillared lamellar zeolites for large hydrocarbon cracking. Two dimensional lamellar precursors possess an array of sheets which can be modified into swollen, exfoliated, and pillared materials with virtually unrestricted accessibility and significant catalytic potential. The introduction of secondary porosity into lamellar structures provides extra advantages such as superior catalytic activity, selectivity and life-time to hierarchical zeolite compared to conventional 3D frameworks. The catalytic cracking of 1,3,5-TIPB over solid acid catalysts with unrestricted accessibility to strong acid sites offers a reliable pathway for the synthesis of fuels and petrochemical feedstocks. To address the limitation of 3D analogues of zeolites in the cracking of bulky molecules; delaminated MWW and pillared MWW were prepared from an as-synthesized MCM-22 precursor. The catalytic performance of MCM-22 prepared with different Si/Al ratios were compared to delaminated and pillared forms with dual micro-mesoporosity and purely mesoporous Al-MCM-41 as benchmark and reference in a continuous fixed bed flow reactor.
Subject: hierarchical zeolite; cracking; heterogenous catalysis; energy; plastics; thesis by publication
Identifier: http://hdl.handle.net/1959.13/1513883
Identifier: uon:56777
Language: eng
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