Developing the synthesis of novel TiO2 and Nb2O5 supports and their applications to the selective oxidation of alkanes

The theme of this project was based on selective alkane oxidation with oxygen as the sole oxidant in a solvent-free system by means of shape selective catalysis. This was to provide a competitor for the application of ‘green’ synthesis of linear primary alcohols, which are exceptionally relevant compounds for the fine chemicals industry from surfactants and coatings to cosmetics. Groundwork for this project was based on the autoxidation of n-decane and subsequent analysis of complex reaction mixtures. Through standard solutions, calculators for conversion and selectivity were developed for 1H-NMR and GC-MS analysis. The oxidation of n-decane is possible with no added initiator or catalyst when operating at temperatures >120 oC, with conversions up to 12% and the major products being (2- to 5-) decanol and decanone, this was ascribed to the presence of autoxidation phenomena. These bench mark studies were then extended to the oxidation of cyclooctane as a model system for cyclic hydrocarbons, and to investigate the effects of a lower bond dissociation energy to the activation of saturated alkanes and the effect that a different steric hindrance can have on the reactivity of micro and meso-porous materials. Heterogeneous catalysts based on commercial bulk supports; like TiO2 and Nb2O5; as well as porous supports like MCM-41 and ZSM-5 were developed, including particular emphasis for the development of micro- and meso- porous TiO2 and Nb2O5. For these materials wetness impregnation was used to add a range of metal nanoparticles, especially focusing on Fe, Mn, and Pd, on supports which were characterized with a range of analytical techniques including: XRPD, ATR-FTIR, TGA, ICP-MS and XPS. Catalytic tests with Fe/TiO2 were used as a benchmark for our studies. It was found bulk Fe/TiO2 could catalyse n-decane oxidation in conditions where no autoxidation had occurred. Using this catalyst optimised parameters for n-decane oxidation were determined (T = 115 oC, PO2 = 1 bar M:S = 1:1000, t = 24 h, stirrer speed = 500-700 rpm). These parameters were then applied to the range of catalysts synthesised. It was found in most cases that iron was the top performer on which ever support was used. Supported noble metals and manganese, despite their literature presence, were found to be poor catalysts at best. This is suspected to be due to the existence of inhibiting metal-support interactions. No clear selectivity for alcohols was observed from the range of catalysts tested. Cyclooctane oxidation was also possible with the catalysts developed. It was found that Fe/ZSM-5 and analogues are capable of very high conversion (>60%) and selectivity for cyclooctanone (>60%) potentially making them candidates to be used in the production of precursors for the fibres industry. Microporous metal oxides with an additional metal within the framework were developed for their potential to create confined metal active sites. Additional dopant metals were chosen based on those found to be active in previous chapters/ those with significant literature presence. Synthesis was conducted via a template-assisted hydrothermal protocol with optimisation developed here. However, literature descriptions of template removal were unable to be replicated (most likely by incorrect synthesis parameters, like the pH, reported in the original references). Instead, several template removal techniques were trialled (calcination, acid washing, template oxidation) as well as an innovative synthesis via a peptization method. This also allowed development of a novel type of analysis where changes in pore structure and thermal stability of these materials were studied in situ using XRPD. We observed that template-free microporous titanium oxide was easily collapsing due to its delicate pore structure upon template removal. Undoped and metal doped template-including and pore-collapsed template-free material were unable to activate alkanes. However, microporous titanium oxide doped with iron nanoparticles was achieved through peptization of anatase followed by wetness impregnation. This material could activate cyclooctane with relatively low conversion (< 10%). Novel microporous template-free iron-doped niobium oxide was synthesised successfully, with the porous framework intact. Again, literature descriptions of template removal were unable to be replicated for the same reasons of TiO2. However, a novel and promising form of template removal with phthalic anhydride was developed. This material was then applied to alkane oxidation, where it was unable to activate n-decane and gave low conversion (< 10%) for cyclooctane oxidation.