Polymer of Intrinsic Microporosity Induces Host-Guest Substrate Selectivity in Heterogeneous 4-Benzoyloxy-TEMPO-Catalysed Alcohol Oxidations

Abstract

The free radical 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4B-TEMPO) is active as an electrocatalyst for primary alcohol oxidations when immobilised at an electrode surface and immersed into an aqueous carbonate buffer solution. In order to improve the catalytic process, a composite film electrode is developed based on (i) carbon microparticles of 2-12 μm diameter to enhance charge transport and (ii) a polymer of intrinsic microporosity (here PIM-EA-TB with a BET surface area of 1027 m 2 g −1 ). The latter acts as a highly rigid molecular framework for the embedded free radical catalyst with simultaneous access to aqueous phase and substrate. The resulting mechanism for the oxidation of primary alcohols is shown to switch in reaction order from first to zeroth with increasing substrate concentration consistent with a kinetically limited process with competing diffusion of charge at the polymer layer-electrode interface (here the BLEk^case in Albery-Hillman notation). Reactivity optimisation and screening for a wider range of primary alcohols in conjunction with DFT-based relative reactivity correlation reveals substrate hydrophobicity as an important factor for enhancing catalytic currents. The PIM-EA-TB host matrix is proposed to control substrate partitioning and thereby catalyst reactivity and selectivity.

Hydrodynamic Rocking Disc Electrode Study of the TEMPO‐mediated Catalytic Oxidation of Primary Alcohols

“…Recently, some of us investigated by DFT calculations [34] the reaction between various primary alcohols and a derivative of the TEMPO + cation, associated with a NaCO 3 À anion originated from the buffer used experimentally. As expected from a strong oxidant, the oxidation of alcohols to aldehydes by TEMPO + was shown to be thermodynamically favoured (DG À200 kJ mol À1 ).…”

“…TEMPO derivatives alone or combinations of TEMPO with metals [22,23] or with enzymes [24] provide versatile tools in bio-mass conversion [25,26,27] and in electrosynthetic processing [28]. TEMPO usually is employed in aqueous media as a homogeneous catalyst [29], but recently new methods for "heterogenisation" of TEMPO have also been suggested via covalent attachment [30,31,32] or via embedding into a polymer of intrinsic microporosity [33,34]. Mechanistic aspects of electrochemical TEMPO mediated alcohol oxidations have been investigated with the following key conclusions: (i) catalytic currents were affected by the pH [35], (ii) catalytic currents increase non-linearly with the concentration of alcohol to reach a plateau as expected for EC'-type processes, (iii) the rate limiting process at sufficiently high applied potentials is a chemical step (as opposed to electrochemical), (iv) the driving force and rate for the primary alcohol oxidation are linked to the reversible potential for TEMPO derivatives [36], and (v) at high pH (> 11) some TEMPO catalysts are irreversibly degraded by hydroxide ions [28].…”

“…A set of seven primary alcohols are investigated with TEMPO as a homogeneous electrocatalyst. Experimental chemical rate constants are evaluated and compared to calculated DFT data (assuming a concerted hydride transfer transition state as the kinetic limiting step [34]) to rationalise the higher reactivity ob- served for the cases of methanol and aromatic substituents attached to the primary alcohol carbon.…”

Polymer of Intrinsic Microporosity (PIM‐7) Coating Affects Triphasic Palladium Electrocatalysis

“…Polymers of intrinsic microporosity (PIMs) represent a relatively new class of synthetic polymers developed recently based on highly rigid polymeric structures to provide rigid 1–2 nm diameter nanopore channel systems with applications in gas separation, gas storage, and electrochemistry . Relevant for applications in electro‐organic synthesis are the use of PIMs, for example, to immobilise molecular catalysts and to provide a protective film over nano‐metal catalysts . The polymer of intrinsic microporosity PIM‐1 (see molecular structure in Figure ) has recently been shown to bind gases such as molecular hydrogen or oxygen at the electrode surface under “triphasic” conditions (with coexisting solid, liquid, and gaseous phases), thereby affecting electrocatalytic reactions .…”

Ionic Diode Characteristics at a Polymer of Intrinsic Microporosity (PIM) | Nafion “Heterojunction” Deposit on a Microhole Poly(ethylene‐terephthalate) Substrate

“…Electrochemical applications of PIMs have been suggested in fuel cell catalysis , in heterogenisation of catalysts , and in highly ion‐conductive and ion‐selective membranes . PIMs deposited asymmetrically onto a 20 μm diameter microhole in a poly‐ethylene‐terephthalate (PET) film have been shown to give rise to ionic diode behaviour, but also demonstrate “ionic flip‐flop” characteristics when exposed to phytate anions .…”