(341e) Reactive Distillation With Biocatalytic Coatings for Structured Packings

Introduction

Reactive distillation is an integrative separation process that is well-established in industry for diverse etherification and esterification reactions (e.g. MTBE, TAME, methyl acetate). The integration of reaction and separation in one unit operation leads to a reduction of equipment and operational costs as well as to an increase in yield of equilibrium-limited reactions. Enzyme-catalyzed reactions often show a thermodynamic and/or kinetic limitation which can be overcome by this kind of integrated reaction/separation setups. In biocatalytic applications there are different examples for integrated processes, e.g. in situ adsorption, in situ extraction and reactive membrane processes [Bechtold et al., 2009[1]]. Due to the thermal sensitivity of the enzymes, up to now the reactive distillation was not considered for enzymatic reactions, although improved immobilization methods and the discovery of more robust enzymes would in principle allow the application in reactive distillation processes.

Presented work

In the current work, a silica-based coating was developed for commercial structured packings Montz A3‑500 in order to immobilize lipase CALB in reactive distillation columns. The advantage of the coating procedure is that the increased surface area of the packing can be used both for a better vapor-liquid mass transfer and as contact area between substrates and biocatalyst. Furthermore, the coating is produced in the sol-gel process which in principle allows the coating of all kind of commercial column internals. By adjusting the sol-gel parameters the chemical and structural properties of the silica matrix can be altered according to the material used as carrier. In addition, the biocatalytic coating can be adjusted to the favored conditions of the enzyme. Lipase CALB showed for example an increased activity in more hydrophobic coatings made of alkoxysilanes [previously shown by Reetz et al. 1997[2]]. The development of the biocatalytic coating is described as well as the influence of certain constituents on the activity and the stability coating.

Subsequently, the new biocatalytic coating was applied in a transesterification reaction of ethyl butyrate with 1-butanol. The withdrawal of one of the products through the distillate stream led to a clear shift in equilibrium to 98% conversion.

This successful introduction of enzymes in a reactive distillation column opened up a new application field for biocatalytic reactions. In the first place stereo- and enantioselective reactions were considered, that cannot be realized with a chemical catalyst or only in several reaction steps. As an example the kinetic resolution of racemic 2-pentanol was performed in the reactive distillation column. For this reaction it was also demonstrated, that withdrawal of one of the products can shift the equilibrium towards almost complete conversion and the residual mixture of (R)-pentanol and (S)-pentyl ester can be separated in the same column. The conversion and the selectivity in the reactive distillation setup is compared with the reaction in a stirred vessel.

For both reaction systems, the stability of the silica-based coating was investigated by monitoring the weight loss of the coating after each reactive distillation experiment. In addition, the thermal and chemical deactivation of CALB was evaluated for the new reaction system at elevated temperatures up to 60 °C.

In summary, the first realization of an enzyme-catalyzed kinetic resolution reaction in a reactive distillation column shows great potential for the enantioselective synthesis. Furthermore, a new immobilization method is presented for the introduction of enzymes in a reactive distillation column, which shows great flexibility for the application of different enzymes.

Outlook

The process of the kinetic resolution of 2-PeOH in the reactive distillation will be optimized in a simulation-based approach. Therefore, the VLE data for the quaternary mixture 2-pentanol‑ethyl butyrate‑2-pentyl butyrate‑ethanol has to be determined since only limited data was available. The enzymatic reaction can be described by a simple Michaelis-Menten two-substrate kinetic with substrate-surplus inhibition.

The project „Enzyme-catalyzed reactive distillation“(SM 82/9-1) is supported by the German Research Association (DFG).