Aldol-condensation of furfural by activated dolomite catalyst

• Mechanism of aldol-condensation of furfural with acetone on dolomite catalyst.
• Catalytic base dolomite produced by calcination and water hydrolysis.
• First condensation via water soluble monomer (4-(furan-2-yl)-4-hydroxybutan-2-one).
• High selectivity to double condensation C13 dimer.
• Deactivation by carbon deposits particularly at high temperatures.

Aldol-condensation of furfural with acetone catalysed by activated dolomite was investigated at temperatures from 306 to 413 K. The process of activation by calcination and hydration produced catalytically active calcium and magnesium hydroxides with improved surface area and surface basicity. The aldol-condensation mechanism began with a deprotonation of acetone forming a carbanion intermediate by hydroxyl ions, which then reacted with the carbonyl group of furfural to form a water soluble C8 monomer (4-(furan-2-yl)-4-hydroxybutan-2-one). This C8 monomer readily dehydrated to form selectively α,β-unsaturated ketone (4-(2-furyl)-3-buten-2-one), which in turn, reacted with furfural forming a C13 dimer (1,4-pentadien-3-one,1,5-di-2-furanyl). Compared with conventional sodium hydroxide catalyst, activated dolomite was less selective towards lumped C8 monomers and C13 dimers owing to carbon losses and deactivation, particularly at high temperatures. Activated dolomite was more selective to C13 dimer owing to higher adsorption enthalpy of C8 monomer compared with acetone competitor. Activated dolomite is therefore a promising catalyst to produce C13 dimers which can be transformed upon hydrogenation and deep hydrodeoxygenation in high-quality diesel fuels. The first-order kinetic model with respect to furfural and acetone fitted well with actual experimental results with an average normalised standard deviation of 6.2%.

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