The aim of this work is to investigate homogeneous and heterogeneous catalysis by HPAs for the conversion of renewable feedstocks. This includes the preparation, characterisation and testing of a range of acidic solid materials such as bulk HPAs, CS2.sHo.sPW12040and the acidic composites comprising H3PW12040 (HPW) supported on Nb20s, Zr02 and Ti02. The HPW supported on Ti02, Zr02 and Nb20s were prepared by impregnation method and then characterised regarding their acid properties and the chemical structure of HPA on the catalyst surface, compared to "standard" HPA catalysts such as bulk and silica-supported H3PW12040 and CS2.5HO.SPW12040In. contrast to the parent acid H3PW12040 and CS2.sHo.sPWI2040, possessing strong Brensted acid sites, the catalysts supported on Ti02, Zr02 and Nb20s possess both Brensted and Lewis acid sites, with the latter mainly originating from the oxide support. These catalysts possess weaker acid sites than H3PW 12040and CS2.sHo.sPW12040,with their acid strength being similar to that of acidic zeolites. The catalytic activity (turnover frequency) in gasphase isopropanol dehydration decreases in the order: H3PW12040> CS2.sHo.sPW12040> 15%H3PW12040/Si02 > 15%H3PW120401Ti02 » 15%H3PW120401Nb20S > 15%H3PWI2040/Zr02, which is in line with the acid strength determined by NH3 adsorption calorimetry. 31p MAS NMR and FTIR indicate an increasing interaction between support and HPA in the following order: Si02 < Ti02 < Nb20s, Zr02, whilst the strength of acid sites decreases in that order. Esterification of hexanoic acid and transesterification of ethyl propanoate and ethyl hexanoate with excess methanol (1:20 molar ratio) were tested at 60 QC and ambient pressure with a range of HPA catalysts in homogeneous and heterogeneous systems in comparison with conventional homogeneous and solid acid catalysts such as H2S04, Amberlyst-15 and zeolites HY and H-Beta. In these reactions, the intrinsic catalytic activity (turnover frequency, TOF) of HPA catalysts is significantly higher than that of the conventional acid catalysts. The TOF values decrease with decreasing catalyst acid strength in the order: H3PW12040~ CS2.sHo.5PW12040> ~SiW 12040> 15%H3PW12040INb205, 15%H3PWI2040/Zr02, 15%H3PWI2040/Ti02> H2S04> HY, H-Beta> Amberlyst-15. Supported HPA catalysts suffer from leaching and exhibit significant contribution of homogeneous catalysis by the leached HPA. The isomerisation of a-pinene is studied in the gas phase over solid HPA catalysts in a fixed bed continuous flow reactor at 200°C and ambient pressure. The reaction yields camphene as the main product in a mixture with monoterpene byproducts such as limonene, terpinolenes, terpinenes, p-pinene, p-cymene and others. The strong Brensted solid acids exhibit high initial activities, but suffer from catalyst deactivation, resulting in low camphene yields. Conversely, the HPA catalysts supported on Nb20S, Zr02 and Ti02, although weaker acids, show more stable performance in a-pinene isomerisation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:539488 |
| Date | January 2010 |
| Creators | Alsalme, Ali Mohammed |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0019 seconds