Surface Effects on Homogeneous Organic Reactions in Microreactors

Abstract

This thesis focuses on microreactors used for single-phase organic reactions and their effect on the chemical transformation. Microreactors are defined as micro-structured flow vessels in which at least one of the geometric dimensions is in micrometer size range. In recent years the area has seen extensive development, especially for studying and performing organic syntheses by both academia and industry. Microreactor technology promises superior control, safety, selectivity and yields in chemical transformations. High surface-to-volume ratio achieved in microreactors enables excellent heat and mass transfer rates by facilitating better transport of reacting species and properties. Although they are relatively expensive to fabricate and have limited capabilities to handle solid reactants, higher yield obtained and minimal waste generation makes the overall chemical synthesis economically viable. One of the striking features of using such reactors for both homogeneous and heterogeneous organic synthesis is dramatic improvement in reaction rates and yields compared to conventional macro sized reaction vessels such as bench-top flask. It is argued that this increase is a direct outcome of enhanced transport properties (heat and mass) realized in microreactors. This enhancement accelerates reaction rates, yield and selectivity by shifting diffusion controlled reaction system to kinetically-controlled reaction regime. The argument is valid for heterogeneous chemical reactions where overall reaction rate is limited by transfer of chemical species across phases, or where the reaction rate is a strong function of temperature. However in principle, factors such as inter-phase heat and mass transfer should not affect course of well-mixed quasi-isothermal homogeneous reactions. Thus, the observed increase in reaction rate for homogeneous chemical reaction in microreactors has sparked a debate regarding their reaction mechanism in the research community. In this work we attempt to analyze this deviation in theoretical and observed experimental reaction parameters by hypothesizing the increase in reaction rate as a direct consequence of appreciable participation of reactor walls (surfaces) in a microreactor. In other words, we hypothesize that homogeneous reactant experiences significant participation of reactor walls due to high surface-to-volume ratios. This leads to higher chemical transformation; in effect `heterogenizing? a homogeneous reaction. The hypothesis is investigated by performing single-phase organic reaction experiments in micro-capillary reactors of different materials and internal cross-sectional areas. We compared the conversion of reactants in microreactors of different materials with same surface-to-volume ratio and vice-versa. The outcome of our study indicates higher conversions in the microreactors as compared to an equivalent synthesis in a macro-scale system with noticeable difference with different material of construction. However a firm conclusion could not be derive due to errors associated with the measurements. Furthermore, we attribute the observed increase in yield is due to participation of reactor surfaces, as in light of similar phenomena observed in `on-water? and `on-surface? reaction studies.