Catalytic conversion of biomass-derived molecules

Bayahia, Hossein
Catalytic conversion of biomass-derived molecules. PhD thesis, University of Liverpool.

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This study aimed to prepare, characterise and test the performance of heterogeneous catalysts in the conversion of biomass-derived molecules including deoxygenation of propionic acid in the gas phase and the Prins condensation of β-pinene with paraformaldehyde in the liquid phase. The surface area and porosity of catalysts were characterised by BET, water content and thermal stability by TGA, crystallinity by XRD and composition by ICP. The acidity of oxide catalysts was characterised using NH3 adsorption calorimetry and FTIR of adsorbed pyridine. High purity amorphous silicas and crystalline silicalite (MFI structure) were found to be active catalysts of the deoxygenation of propionic acid. Silicalite was prepared by the hydrothermal method. Silica and silicalite were treated with aqueous acidic (HCl) and basic (NH3+NH4NO3 (aq) or NH3 (aq)) solutions in a Teflon-lined autoclave. The reaction was carried out in a fixed-bed continuous flow reactor in the gas phase at 400-500 °C. A preliminary blank reaction showed a small contribution of homogeneous catalysis at 500 °C, with 12% of propionic acid converted to form 3-pentanone. The chemical treatment did not affect silica activity; it showed only 85% selectivity with 39% conversion at 500 °C. HZSM-S zeolite (Si/Al = 180) possessing strong acid sites showed low catalytic activity at 400-500 °C and the main product was ethane. Silicalite had higher activity and selectivity of 3-pentanone than silica. Acidic treatment had little effect on catalyst activity, whereas basic treated silicalite was the most active catalyst in the deoxygenation of propionic acid at 500 °C, because silanol nests formed on the silicalite surface, acting as catalytically active sites for the reaction. Catalyst activity increased with increasing reaction temperature from 400-500 °C. Silicalite performance was stable for at least 28 h time on stream at 500 °C, with 84-92% of 3 pentanone selectivity at 93-80% conversion of acid. Bulk Zn(II)-Cr(III) mixed oxides with a Zn/Cr atomic ratio of 1:1 – 20:1 were found to be active catalysts for the gas-phase ketonisation of carboxylic acids (acetic and propionic) to form acetone and 3-pentanone, respectively, at 300 – 400 oC and ambient pressure. Zn-Cr (10:1) oxide showed the best performance, significantly exceeding that of the parent oxides ZnO and Cr2O3. The catalytic activity was further enhanced by supporting Zn-Cr (10:1) oxide on TiO2 and γ-Al2O3. With 20%Zn-Cr/Al2O3, ketonisation of propionic acid occurred with 97% selectivity to 3-pentanone at 99% conversion at 380 oC, without catalyst deactivation observed during at least 24 h time on stream. Zn-Cr oxides were characterised by BET, XRD, DRIFTS of pyridine and acetic acid adsorption and microcalorimetry of ammonia adsorption. From DRIFTS, carboxylic acid adsorbed dissociatively on Zn-Cr oxide to form a surface metal carboxylate in bidentate bridging bonding mode. A mechanism for ketonisation of carboxylic acids via -ketoacid intermediate route was proposed. Metal oxides such as Nb2O5, Cr2O3, and especially a Zn(II)-Cr(III) mixed oxide were demonstrated to be highly active and recyclable heterogeneous catalysts for Prins condensation, which provides a clean, high-yielding route for the synthesis of nopol through the condensation of biorenewable β-pinene with paraformaldehyde. Zn-Cr mixed oxide with an optimum Zn/Cr atomic ratio of 1:6 gave 100% nopol selectivity at 97% β-pinene conversion, with the catalyst easily recovered and recycled. The acid properties of Nb2O5 and Zn-Cr mixed oxide were characterized by the diffuse reflectance IR Fourier transform spectroscopy of adsorbed pyridine and ammonia adsorption microcalorimetry. An appropriate combination of acid– base properties of Zn-Cr mixed oxide is believed to be responsible for its efficiency.

Item Type: Thesis (PhD)
Additional Information: Date: 2014-10 (completed)
Subjects: ?? QD ??
Depositing User: Symplectic Admin
Date Deposited: 04 Aug 2015 08:52
Last Modified: 17 Dec 2022 01:38
DOI: 10.17638/02004740