Effect of segmented fluid flow, sonication and phase transfer catalysis on biphasic reactions in capillary microreactors

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Abstract

The contact between immiscible liquids in a microfluidic system creating segmented flow offers great potential in the study of biphasic reactions in organic chemistry with significant advantages with respect to conventional flask techniques.

For a simple biphasic hydrolysis, we show that the application of various reaction conditions in microreactors using segmented flow can dramatically increase the reaction rate, especially when sonication and phase transfer catalysis are combined with segmentation.

Introduction

Microreaction technology can offer significant advantages in chemical synthesis over existing conventional techniques. In reactions involving liquids a high surface-to-volume ratio can be easily achieved in microreactor systems. This offers a fast and efficient heat and mass transfer which can lead, together with short diffusion distances, to increased reaction rates. As a result, there is a growing interest in microreactor applications in organic chemistry [1].

In fluidic biphasic systems, an efficient mixing between the two immiscible phases is a challenge because of the interfacial tension caused by differences in the physical properties of both phases. Reactions in such systems can either occur at the interface or in one of the phases in which one reactant must be transferred from one phase into the other [2].

Depending on the microchannel's geometry and material as well as the physical properties of solvents along with other parameters, the contact between two immiscible liquids can create different flow patterns. The most common form of biphasic flow pattern is known as parallel flow in which the respective fluid phases align side-by-side as shown in Fig. 2. Another biphasic form, known as segmented flow, is a flow system characterized by a series of regular liquid segments of one phase separated by the segments of the other phase. High interfacial tension between the phases and a T-shaped geometry of the inlet junction is required to cause the segmented flow as shown in Fig. 1. Once these fluid segments are formed, an internal fluid vortex is generated because of the interaction of the liquid with the channel wall, causing rapid mixing within each segment while continuously refreshing the interface. In the last few years, an increasing interest has emerged in the utilization of multiphase flow microreactors for the development of chemical systems, especially when it comes to segmented flow [3].

Section snippets

Results and discussion

Following on previous results, we now report new results to further increase the rate of hydrolysis of p-nitrophenyl acetate in the microreactor under a variety of new conditions. We investigated the hydrolysis of p-nitrophenyl acetate 1 in toluene with 0.5 M aqueous sodium hydroxide as a biphasic system [4]. Under these conditions, the hydrolysis of p-nitrophenyl acetate involves a nucleophilic attack by the hydroxide at the carbonyl carbon atom to displace the p-nitrophenyl moiety. Once the

Conclusion

In conclusion, by utilizing the large specific interfacial area provided by the microreactor under segmented flow, the hydrolysis reaction of p-nitrophenyl acetate 1 was found to be much more efficient than parallel flow and flask method.

The combination of sonochemistry and segmented flow was found to enhance the rate more than just segmented flow and phase transfer catalyst combined. Further enhancement was obtained when segmented flow was combined with both, sonochemistry and phase transfer

Experimental

  • General method for the biphasic parallel flow using V-inlet junction (width 150 μm and depth 300 μm)/segmented flow using T-inlet junction (width 300 μm and depth 300 μm) hydrolysis of p-nitrophenyl acetate 1 in a PMMA microreactor at 20 °C (microchannel: 300 μm × 300 μm, 400 mm): a solution of p-nitrophenyl acetate 1 in a 1:1 mixture of toluene and acetonitrile (10 ml, 0.05 M) was loaded into syringe A and an aqueous solution of sodium hydroxide (10 ml, 0.5 M) loaded in syringe B. Syringes A and B were

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    Citation Excerpt :

    Exothermic liquid–liquid reactions can also be handled safely in micro reactors as segmented flow promotes isothermal conditions [15] by faster heat removal. Omer et al. [2] combined segmentation, phase transfer catalysis and sonication to obtain superior reaction rate. Apart from enhancement of biphasic reactions, segmented flow is also employed to increase conversion in homogeneous (single phase) reactions.

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