Gas phase catalytic hydrogenation of alkynols over palladium and nickel catalysts
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The focus of this thesis is the development of catalytic systems (i.e. supported Pd- and Ni-based catalysts and bulk Mo2N) for partial -C≡C- bond hydrogenation in the transformation of functionalised alkynes directed at sustainable chemical processing. The continuous gas phase hydrogenation of 2-methyl-3-butyn-2-ol (to 2-methyl-3-buten-2-ol), 3-butyn-2-ol (to 3-buten-2-ol, 3BE), 3-butyn-1-ol (to 3-buten-1-ol, 3BEOL), acetylene (to ethylene) and phenylacetylene (to styrene) has been performed at atmospheric pressure at 373-453 K. A range of characterisation techniques (AAS, H2-TPR, H2-TPD, H2 chemisorption, SSA, XPS, SEM, TEM, STEM, EDX) have been used to unravel the bulk and surface catalytic properties. Thermodynamic and mechanistic kinetic analyses have been employed to evaluate catalytic performance. In the hydrogenation of 3-butyn-1-ol , 3-butyn-2-ol and 2-methyl-3-butyn-2-ol over Pd/Al2O3 the reaction follows an electrophilic mechanism where the triple bond electron density is affected by the number of electron donating methyl groups and the position of the hydroxyl functionality, i.e. increase activity increasing the number of -CH3 groups. Thermal treatment of Pd/ZnO results in the formation of a β-phase PdZn alloy that promotes partial - C≡C- → -C=C- bond hydrogenation but at low conversions, attributed to metal particle encapsulation by the ZnO support. The synthesis of Al2O3 supported colloidal Pd-Zn catalysts serves to control alloy composition and avoid metal encapsulation while attaining high selectivity to target alkenol. We probe a direct correlation between alkynol hydrogenation rate and surface hydrogen where the nature of the carrier (carbon, Al2O3, MgO, CeO2) affects the electronic properties of the palladium phase but does not alter significantly catalytic performance. Undesired isomerisation (to 2-butanone) is promoted in the hydrogenation of 3-butyn-2-ol over Pd but not over Ni, although the latter delivers low activity due to limited hydrogen uptake capacity. A Pd-Ni/Al2O3 catalyst (Pd: Ni = 1:1) delivers increase activity and 3-buten-2-ol (selectivity, which we attribute to formation of PdNi nanoparticles. We have also considered the catalytic response over bulk NiZn alloys and Mo nitride catalysts. Variations in Zn content results in the generation of several NiZn alloys with different crystallographic phase (i.e. α-NiZn, β-NiZn and δ-NiZn) with α-NiZn delivering the highest alkenol production in the hydrogenation of 3-butyn-2-ol. We also demonstrate that catalytic activity can be enhanced by spillover hydrogen for reaction over α-NiZn/ZnO. y-Mo2N promotes the partial hydrogenation of acetylene, phenylacetylene and 2-methyl-3-butyn-2-ol where incorporation of palladium in trace amounts serves to elevate surface hydrogen concentration and catalytic activity.