Heterogeneous catalysis in an oscillatory baffled flow reactor

  • Eze V
  • Phan A
  • Pirez C
 et al. 
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Abstract

The first demonstration of heterogeneous catalysis within an oscillatory baffled flow reactor (OBR) is reported, exemplified by the solid acid catalysed esterification of organic acids, an important prototypical reaction for fine chemicals and biofuel synthesis. Suspension of a PrSO 3 H-SBA-15 catalyst powder is readily achieved within the OBR under an oscillatory flow, facilitating the continuous esterification of hexanoic acid. Excellent semi-quantitative agreement is obtained between OBR and conventional stirred batch reaction kinetics, demonstrating efficient mixing, and highlighting the potential of OBRs for continuous, heterogeneously catalysed liquid phase transformations. Kinetic analysis highlights acid chain length (i.e. steric factors) as a key predictor of activity. Continuous esterification offers improved ester yields compared with batch operation, due to the removal of water by-product from the catalyst, evidencing the versatility of the OBR for heterogeneous flow chemistry and potential role as a new clean catalytic technology. A Introduction The application of heterogeneous catalysis 1 and process intensifi-cation 2 in chemical synthesis are cornerstones of Green Chemistry. Together, these offer improved use of raw materials and energy efficiency, as well as safer operation through smaller system footprints 3 and reductions in work-up steps and associated waste production. A particular challenge for industrial chemists is the replacement of mineral and inorganic acids widely used in fine, speciality and bulk chemical manufacturing processes, ranging from alkylation, acylation, and dehydration to condensation and esterification reactions. The esterification of organic acids is a simple reaction, but one of great significance due to its widespread application in the production of fine and speciality chemicals, including the manufacture of intermediates for fragrances, lubricants, fabrics and polymers. 4 Esterification is also a key step in biofuels production, with the acid catalysed conversion of fatty acids to fatty acid alkyl esters integral to current biodiesel technologies. 5 Furthermore, esterification pre-treatments of bio-oils derived from fast pyrolysis of biomass are a potential route to improved oil stability via elimination of short chain acids and deoxygenation. 6 Tightening legislation on the emission of hazardous pollutants produced through existing homogeneously catalysed processes is stimulating the quest for alternative heterogeneous routes, which offer several intrinsic advantages, most notably ease of product separation and catalyst re-use, and continuous flow versus batch operation. 7 Various process engineering solutions have been considered for continuous esterification, including the use of fixed bed processes for fatty acids, 8 microchannel-flow reactors, 9 pervaporation methods, 10,11 or reactive distillation 12–14 for acetic acid esterifica-tion with butanol or ethanol. Continuous reactors must be carefully designed to utilise the full potential of the associated hetero-geneous catalyst; plug flow is a desirable reactor characteristic, as it allows tighter control of product composition and thereby reduces downstream separation processes and capital and running costs. However, conventional plug flow reactors are unsuitable for slow reactions as they necessitate very high length:diameter ratios to achieve the required mixing. Such designs are problematic due to their large footprints and pumping duties, and control difficulties. The Oscillatory Baffled Reactor (OBR) circumvents these problems, offering good mixing and plug flow by oscillating the reaction fluid through orifice plate baffles. 15 In an OBR, mixing is decoupled

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