Met katalytische processen richting duurzame synthese van vertakte, hogere alcoholen vanuit biogebaseerde grondstoffen

The finitude of cheap oil and gas, combined with the incentive to reduce greenhouse gasses directs new developments from bio-based resources, resulting in an increasing availability of bio-alcohol feedstock. The Guerbet reaction is an alcohol coupling reaction that is known for more than a century. Due to the specific branching pattern of the product alcohols, Guerbet alcohols have many interesting applications. In comparison to their linear isomers, branched-chain Guerbet alcohols have extremely low melting points and excellent fluidity. The reaction therefore serves an interesting valorization method for the newly available bio-based alcohols. While the Guerbet reaction of long-chain alcohols is well-established and typically performed in a liquid phase batch process, the condensation of short-chain alcohols proves to be more challenging. Short-chain alcohols, due to their lower boiling temperature, azeotrope composition and higher water solubility are less reactive for Guerbet condensation and facilitate catalytic deactivation in the liquid phase. This reaction is performed with a catalytic combination of an alkaline agent and a transition metal component. Both materials are indispensable to get the reaction started. Reaction water impedes the reaction rate on two levels: due to protonation of the alkoxide, alcohol dehydrogenation is prevented, and by facilitating aldehyde disproportionation to carboxylates, the alkaline catalyst is poisoned. In the scope of reducing this typical carboxylate byproduct formation and catalyst consumption in the Guerbet condensation, the reaction is studied under vapor phase conditions. In a first step, the strongly alkaline catalysts is substituted by metal oxides with balanced acid-base properties, which are studied for their potential to catalyze the vapor phase aldol condensation. Anatase TiO2 and Nb2O5/SiO2 prove to be very active catalysts for the vapor phase aldol condensation of short-chain aldehydes at low temperature and high aldehyde pressure. Branching of the reagent reduces condensation rates notably more on TiO2 than on Nb2O5/SiO2. Condensation rates are also impacted by the presence of difficult to avoid carboxylic acid and alcohol. In comparison to TiO2, Nb2O5/SiO2 shows better stability under these impurities. These elements render Nb2O5/SiO2 to be a promising aldol condensation catalysts within the scope of fusel oil upgrading through the Guerbet reaction. The vapor phase Guerbet condensation of short alcohols is investigated on catalytic mixtures of Cu/SiO2 and TiO2 or Nb2O5/SiO2. The presence of unconverted alcohol on the aldol condensation catalyst strongly reduces the condensation rate in relation to pure aldehyde feed condensation. The aldol condensation is the kinetically relevant step under these conditions. The condensation rates are maximized by elevated reaction temperatures and high copper catalyst fraction, essentially improving alcohol dehydrogenation and reducing its competitive adsorption on the active sites for aldol condensation. Nb2O5/SiO2 appears to be an unselective catalyst, generating significantly more dehydration products compared to TiO2. Conditions and catalyst composition are optimized with a focus on Guerbet dimer components and selectivity. Due to incomplete hydrogenation of the dimer intermediates under these conditions, the selectivity to the final Guerbet alcohol is low and the main product is the saturated dimer aldehyde. From a thermodynamic perspective, it would be more interesting to perform the aldol condensation and hydrogenation steps of the Guerbet reaction separately from the dehydrogenation reaction. Cu/SiO2 is also an active and selective catalysts for the vapor phase hydrogenation of unsaturated aldehydes. The thermodynamic equilibrium of dimer carbonyl hydrogenation is improved by performing the reactions under H2 atmosphere. Combined with anatase TiO2, the conversion of short-chain aldehydes to Guerbet alcohols is investigated. Aldehyde back-hydrogenation poses an important limitation to the reaction rate, again through competitive adsorption of the monomer alcohol. In mixed catalyst systems low copper contents lead to good aldol condensation rates but incomplete dimer hydrogenation whereas higher copper contents cause the formation of alcohol, thereby diminishing the condensation rate. This can be prevented by physically separating the catalytic materials from each other in a dual bed setup. The aldehyde feed is then first in contact with the TiO2 and next with the Cu/SiO2 catalyst. This way high condensation rates are achieved and combined with maximized hydrogenation. A total yield of 45% of Guerbet alcohol and 18% of other Guerbet dimers are obtained at 90% selectivity from the aldehyde. Each reactor setup has its benefits and disadvantages. Good control of reaction water is elementary for liquid phase reactions. An alkaline catalyst is indispensable, especially for short-chain alcohols (C<6), and will be consumed in the reaction. Short-chain alcohols are best reacted under vapor phase conditions, specifically for improved dehydrogenation rates. In the vapor phase, a multiple bed setup can bring improvements to condensation rate and selectivity by optimizing temperature and pressure separately at each catalyst bed and consequently minimizing alcohol adsorption and side reactions on the condensation catalyst. Regardless of the reactor setup, additional hydrogenation is required to obtain a high product purity.