Flame-Made Solid Acid Catalysts and Their Applications in Sustainable Fuels Production

Abstract

In this Thesis, amorphous silica-alumina (a-SA) and amorphous silica-alumina-phosphate (a-SAPO) were developed and synthesized via the one-step flame spray pyrolysis (FSP). The work begins with the demonstration of the effect of substitutional doping of Al³⁺ cations in Si⁴⁺ of silica or Si⁴⁺ in P⁵⁺ sites in alumina-phosphate (AlPO₄). Introducing of different metals has generated acid sites on a-SA and a-SAPO. A comparative study was carried out by benchmarking with well recognized strong acidic zeolitic material, ZSM-5. Although the amount of acid sites in a-SA and a-SAPO is low, both have acid strengths up to or even stronger than ZSM-5. Upon identifying the optimum composition (Al/(Al+Si) = 0.4) to afford high acidity, the effect of particle size on acidity was examined. Smaller a-SA with same composition shows lower acidity and this is attributed to the decrease of Al³⁺ dispersion as a result of clustering of Al₂O₃ to compensate high surface energy which increases with specific surface area.

In the second part of the thesis, a-SA and a-SAPO were applied to catalyse dehydration of methanol (MeOH) to dimethyl ether (DME), a sustainable fuel. The trend of DME yield coincides with the increasing trend of weak acid densities of solid acid. Given DME formation is bimolecular reaction, close proximity between two acid sites with suitable strength is required. The presence of strong acid sites facilitates formation of long chain and branched saturated hydrocarbons that deactivates the catalysts. This has resulted decrease in MeOH conversion and carbon balance for catalysts populated with high amount of strong acid sites. In comparison with reaction catalysed by conventional FER and γ-alumina, a-SA (Al/(Al+Si) = 0.4) offers a more efficient MeOH-to-DME pathway and gives lower activation energy of reaction (16 kJ mol^-1). Meanwhile, surface chemistry study shows that the MeOH dehydration to DME is an acid catalysed reaction and formation of DME proceeds via Langmuir-Hinselwood reaction mechanism.

The last part of the work involves synthesis of robust platform chemical from glucose decomposition over a-SA and a-SAPO. The reaction consists of three key steps, namely, isomerisation of glucose to fructose, dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) and rehydration of 5-HMF to levulinic acid (LA). Thus, different types of active sites are required. a-SA (Al/(Al+Si) = 0.4) and a-SAPO (Si/(P+Si) = 0.25) with combination of Brønsted and Lewis acid shows high LA yield. Glucose preferentially isomerizes over Lewis acid sites while rehydration of 5-HMF takes place exclusively at Brønsted acid sites. In addition, kinetic studies show that there is possibility of additional Brønsted acid generated by organic acid from decomposition of glucose and/or fructose taking part in catalysing formation of LA and thus enhances LA yield.

Date of Award	6 Sept 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jin SHANG (Supervisor) & Wey Yang TEOH (External Co-Supervisor)