Electrocatalytic Carbohydrate Oxidation with 4-Benzoyloxy-TEMPO Heterogenised in a Polymer

19th January 2015

Electrocatalytic Carbohydrate Oxidation with 4- Benzoyloxy-TEMPO Heterogenised in a Polymer of Intrinsic Microporosity

Adam Kolodziej 1, Sunyhik D. Ahn 1, Mariolino Carta 2, Richard Malpass-Evans 2, Neil B. McKeown 2, Robert S. L. Chapman 1, Steven D. Bull 1, and Frank Marken 1*

1 Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK 2 School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ, UK

To be submitted to Electrochimica Acta Proofs to F. Marken [email protected]

Abstract The enzymeless and operationally simple electrocatalytic oxidation of carbohydrates by

“heterogenised” 4-benzoyloxy-TEMPO either (i) immobilised as microcrystals at a glassy carbon electrode surface or (ii) embedded into a polymer of intrinsic microporosity (PIM-EA-TB with

1027 m2g-1 BET surface area and highly rigid framework structure) has been studied in aqueous phosphate buffer of pH 12. It is shown that in contrast to microcrystal deposits, 4-benzolyoxy-

TEMPO co-immobilised within PIM-EA-TB give stable catalytic responses for both, stationary and rotating disc electrode systems and for oxidation of glucose, sorbitol, and sucrose. The rigidity and intrinsic microporosity of PIM-EA-TB allow (slow) substrate and product diffusion, whilst maintaining 4-benzoyloxy-TEMPO immobilised in active molecular form.

Keywords: electrocatalysis, fuel cell, carbohydrate, sensor, multi-electron, EC’ mechanism, catalyst heterogenisation.

Graphical Abstract

1. Introduction

Electrocatalytic oxidation of carbohydrates, especially for glucose derivatives, is of considerable interest in conjunction with sensing [1,2], bio-fuel cell development [3,4], and for electrosynthetic transformations [5]. Recent progress for example in multi-catalyst “cascade” catalytic systems offers new promise for better performance in complex multi-electron reactions such as glycerol oxidation

[6]. New and better catalyst systems are still desirable and in particular metal-free and enzymeless processes based on molecular organo-catalysts are attractive [7,8]. The electrocatalytic oxidation of glucose was investigated previously mainly using a range of transition metals [9]. Research was especially focused on nano-structured platinum, gold, iron, nickel, or copper, which all exhibit good catalytic properties in alkaline media [10]. All of these processes are heterogeneous in nature.

Glucose (and glucose derivative) oxidation has also been studied using the homogeneous organocatalyst 2,2,6,6-tetramethyl-1-piperidinyloxy based on a free radical reagent (TEMPO) [11,12].

However, the disadvantage of homogeneous over heterogeneous (electro-)catalysis is the need for separation of products from catalysts and the loss of sometimes expensive catalyst materials. The desirable “heterogenisation” of TEMPO derivatives (for non-electrochemical processes) has been reported, for example in porous silicas [7], as well as for polystyrene supported reactions by

Gilhespy [13] and by Weik [14]. However, there is still a need for improved electrochemical processes based on “heterogenised” TEMPO, for example exploiting novel porous materials.

In order to avoid the use of solution-soluble TEMPO catalysis, immobilisation of water-insoluble

TEMPO derivatives appears possible for example into a porous host material. Interesting new types of porous host materials have been developed, e.g. zeolitic materials [15] or metal-organic frameworks [16]. Recently, polymers of intrinsic microporosity (PIM) have been introduced to electrochemistry [17] and shown to be beneficial, for example as an environment for the glucose oxidation on nano-gold [18]. PIM materials have been applied also in gas storage [19] and separation

[20] with benefits from a very high surface area, a highly rigid molecular structure, and novel properties relying on permanent microporosity. In electrochemical membrane systems, charged PIM poly-electrolyte materials have been shown to introduce ion selectivity, permselectivity, and novel ionic diode effects [21]. The PIM material employed in this study is PIM-EA-TB [22] (see structure in Figure 1). The synthesis route for this PIM material is based on a polymerisation reaction between 2,6(7)-diamino-

9,10-dimethoxyethanoanthracene and dimethoxymethane in trifluoroacetic acid [22]. This process

-1 results in a PIM-EA-TB material with high molecular mass (Mw > 70,000 g mol ) and a high BET surface area of typically 1027 m2 g-1. The intrinsic microporosity allows other types of molecules to be embedded and solvent to penetrate. It has recently been shown that the “heterogenisation” of water-insoluble tetraphenylporphyrinato-iron (FeTPP) redox catalysts for oxygen reduction is readily achieved within PIM-EA-TB and at a glassy carbon electrode [23]. The resulting porous

PIM-FeTPP film effectively immobilised the molecular catalyst (stopping re-distribution and crystallisation) and allowed the aqueous phase to inter-penetrate into the space between the catalyst molecules. A high density of FeTPP catalyst ensured good electronic coupling to the electrode surface.

Figure 1. Molecular structures for PIM-EA-TB and 4-benzoyloxy-TEMPO. Schematic drawing of the mechanism for TEMPO-based oxidation of primary alcohols with a hydroxylamine intermediate and a comproportionation to regenerate the free radical [24,25,26].

In this report the PIM-EA-TB host is employed with a highly water-insoluble 4-benzoyloxy-

TEMPO redox catalyst (see Figure 1). Reactions are investigated at glassy carbon electrodes immersed in aqueous phosphate buffer pH 12. The effect of the presence of the PIM host is studied on stationary electrodes and on rotating disc electrodes. Successful TEMPO “heterogenisation” is demonstrated and implications for new types of electro-catalytic reactions in PIM films are discussed. 2. Experimental

2.1. Chemical Reagents

D(+)glucose, sorbitol, sucrose, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl benzoate (4- benzoyloxy-TEMPO), phosphoric acid (85%), perchloric acid, chloroform, dimethylformamide

(DMF), and sodium hydroxide were purchased from Sigma-Aldrich, TCI Chemicals, or Fisher

Scientific and used without further purification. PIM-EA-TB was prepared following a literature protocol [22]. Solutions were prepared with filtered and deionized water of resistivity 18 MΩ cm from a Thermo Scientific water purification system.

2.2. Instrumentation

A potentiostat system (IVIUM Compactstat or Metrohm micro-Autolab II) was employed with a conventional three-electrode cell configuration: a Pt wire as a counter electrode and a KCl-saturated calomel electrode (SCE, Radiometer, Copenhagen) as a reference. A glassy carbon electrode (BAS) with a diameter of 3 mm was used as a working electrode. For rotating disc voltammetry a Gamry

(RDE710) system was employed with 5 mm diameter glassy carbon disc. All experiments were conducted at a temperature of 293 ± 3K.

2.3. Procedures

A solution of 4-benzoyloxy-TEMPO was prepared by dissolving 10 mg of solid in 1 cm3 of chloroform (or in 1 cm3 DMF). A solution of PIM-EA-TB was prepared by dissolving 10 mg of the

3 3 polymer in 1 cm of chloroform (or in 1 cm DMF acidified with 5 L HClO4). Appropriate volumes of solution were mixed for deposition and stored in the dark at 4oC. In order to modify the glassy carbon electrode, an appropriate amount of 4-benzoyloxy-TEMPO with PIM-EA-TB was pipetted directly on the surface of the glassy carbon electrode followed by drying by a flow of warm air (for chloroform solution) or followed by drying at 90 oC in an oven (for DMF). The procedure based on DMF proved more reproducible. Figure 2 shows typical scanning electron microscopy

(SEM) images for (A,B) the microcrystalline 4-bezoyloxy-TEMPO film on a glassy carbon surface and (C) for a 4-benzoyloxy-TEMPO – PIM-EA-TB film co-deposited onto glassy carbon.

Figure 2. (A,B) Electron microscopy images (backscatter and secondary electron, respectively) for microcrystalline 4-B-TEMPO (30 g deposited from chloroform) on a 3 mm diameter glassy carbon disc). (C) Electron microscopy image of a film of 30 g 4-B-TEMPO/ 30 g PIM-EA-TB codeposited from chloroform onto glassy carbon.

3. Results and Discussion

3.1. Voltammetric Characterisation of 4-Benzoyloxy-TEMPO Deposits at Glassy Carbon

Figure 3A shows a typical set of cyclic voltammograms for the oxidation and back-reduction of 4- benzoyloxy-TEMPO immobilised in the form of microcrystals at a glassy carbon electrode surface

(compare Figure 2). Given the known pH-dependent reactivity of TEMPO derivatives [27,28], the study was conducted in 0.1 M phosphate buffer (pH 12). In initial experiments it was observed that peak currents in cyclic voltammograms “develop” (the currents increase with each potential cycle) over the first few cycles presumably due to some redistribution/activation of material on the electrode surface. Therefore, before performing the experiment electrodes were pretreated with 10 potential cycles at a scan rate of 200 mVs-1 and in potential range of 0.3 to 0.9 V vs. SCE. In the case of rotating disc experiments, electrodes were pretreated with rotation speed of 2000 rpm.

Data in Figure 3A show an oxidation peak at 0.7 V vs. SCE and a corresponding reduction peak at

0.6 V vs. SCE indicative of a chemically reversible oxidation with both the starting material and the products immobilised at the electrode surface (no stripping process). This suggests a mechanism where an anion from the solution phase (e.g. phosphate) forms a solid product with the oxidised form of 4-benzoyloxy-TEMPO. When changing the scan rate there is clearly an increase in the peak currents for both oxidation and reduction. However, due to the complexity of the process further analysis is not warranted (vide infra).

Figure 3. (A) Cyclic voltammogram (scan rate (i) 200, (ii) 50, (iii) 10 mVs-1) for a 30 g deposit of 4-benzoyloxy-TEMPO on a 3 mm diameter glassy carbon electrode immersed in 0.1 M phosphate buffer pH 12 (pretreatment: 10 potential cycles with 200 mVs-1). (B) As before, with 90 g 4- benzoyloxy-TEMPO. (C) As before, with 150 g 4-benzoyloxy-TEMPO. (D) Rotating disc voltammetry data (scan rate 10 mVs-1) for 120 g 4-benzoyloxy-TEMPO on a 5 mm diameter glassy carbon electrode with (i) 100, (ii) 650, (iii) 1250, (iv) 2000 rpm rotation speed (pretreatment: 10 cycles with 200 mVs-1 at 2000 rpm).

When changing the amount of 4-benzoyloxy-TEMPO microcrystalline deposit on the electrode surface, there is initially a clear increase in current for higher amounts of deposit (see Figure 3A,B), but also a decrease for even higher amounts of deposit (Figure 3C). The amount of “accessible” redox active 4-benzoyloxy-TEMPO at the electrode surface is likely to depend on the distribution of microcrystalline material over the electrode surface and the resulting triple phase boundary reaction zone [29]. Excess of redox active material is likely to “block” the reaction.

Rotating disc voltammetry experiments were performed with a similar amount of 4-benzoyloxy-

TEMPO deposit per area (120 g 4-benzoyloxy-TEMPO was deposited instead of 30 g for the smaller diameter electrode). The voltammetric responses shown in Figure 3D are composed of (i) peak-shaped oxidation-reduction responses similar to those observed for stationary electrodes and

(ii) a hydrodynamic “tail” of exponentially increasing current indicating an underlying process possibly associated with loss of catalyst from the electrode surface. Perhaps surprisingly, the magnitude of the hydrodynamic current component decreases with higher rotation rate, which could be indicative for a “self-mediation effect”, where oxidised 4-B-TEMPO+ (if not removed by convection) transfers an electron to the solid to extend the reaction zone under less vigorous convection conditions. 3.2. Voltammetric Characterisation of 4-Benzoyloxy-TEMPO Deposits at Glassy Carbon in the Presence of Carbohydrates

The catalytic ability of TEMPO derivatives in alkaline electrolyte solution is well known [25,27] and in particular primary alcohols are readily oxidised to aldehydes or further to carboxylates [30,31].

Figure 4. (A) Cyclic voltammograms (scan rate 10 mVs-1) for 90 g of 4-benzoyloxy-TEMPO immobilised onto a 3 mm diameter glassy carbon electrode and immersed into 0.1 M phosphate buffer pH 12 with additions of 0 to 10 mM glucose. (B) As before, but with sorbitol. (C) As before, but with 0 to 7 mM sucrose.

Data in Figure 4 demonstrate the electrocatalytic oxidation of glucose (Figure 4A), sorbitol (Figure

4B), and sucrose (Figure 4C), all of which show concentration dependent current responses. The increase in current with concentration is consistent with an EC’-type mechanism with a kinetically controlled limiting current (there are no classical peak features in Figure 4). Glucose gives a current response that increases proportional to concentration. Oxidation of sorbitol appears to result in approximately twice as high currents (which may be linked to sorbitol providing two reactive primary alcohol groups compared to only one for glucose). Sucrose also shows an increased current when compared to glucose but the increase in the number of active primary alcohol functionalities appears to be balanced by a more complex structure slowing down the reaction. It is interesting to note the low potential onset (compared to Emid) of the electrocatalytic oxidation for all three carbohydrates due to fast oxidation kinetics. Additional rotating disc voltammetry experiments were performed but current responses proved unstable (reagent is lost from the electrode surface) and difficult to quantify. For reactions under hydrodynamic conditions to be feasible, the 4-benzoyloxy-

TEMPO catalyst needs to be immobilised more permanently.

3.3. Voltammetric Characterisation of 4-Benzoyloxy-TEMPO – PIM-EA-TB Codeposits at

Glassy Carbon

In order to more effectively “trap” the 4-benzoyloxy-TEMPO electrocatalyst at the electrode surface, co-immobilisation with the polymer of intrinsic microporosity, PIM-EA-TB (see Figure 1), is introduced. It has recently been shown that highly water-insoluble electrocatalysts such as FeTPP can be successfully immobilised into the rigid PIM-EA-TB framework and used as catalyst in aqueous media [23]. The relatively open but rigid PIM-EA-TB structure allows water and substrate in without loss of the catalyst out of the host structure. A similar process is demonstrated here for 4- benzoyloxy-TEMPO. Figure 5A shows voltammetric responses for a codeposit of 30 g 4- benzoyloxy-TEMPO and 30 g PIM-EA-TB.

Figure 5. (A) Cyclic voltammograms (scan rate (i) 10, (ii) 50, (iii) 200 mVs-1) for 30 g 4- benzoyloxy-TEMPO and 30 g PIM-EA-TB codeposited onto a 3 mm diameter glassy carbon electrode and immersed in 0.1 M phosphate buffer pH 12 (pretreatment: 10 potential cycles with 200 mVs-1). (B) Rotating disc voltammograms (rotation speed 100/ 650/ 1250/ 2000 rpm; scan rate 10 mVs-1) for 120 g 4-benzoyloxy-TEMPO and 120 g PIM-EA-TB codeposited onto a 5 mm diameter glassy carbon electrode (pretreatment: 10 potential cycles with 200 mVs-1 at 2000 rpm). (C) Plot of currents at 0.65 V vs. SCE versus square root of rotation speed contrasting 120 g TEMPO and 120/120 g TEMPO/PIM codeposit.

Current responses for the oxidation of 4-benzoyloxy-TEMPO embedded in PIM-EA-TB generally are an order of magnitude lower compared to 4-benzoyloxy-TEMPO only. In the SEM image in

Figure 2C it can be seen that a uniform film deposit is formed, which is not significantly affected when performing electrochemical measurements. The catalyst-PIM-EA-TB codeposit is much less open to diffusion from solution to the reaction zone. Therefore, slower transport of reagents through the PIM-EA-TB film is likely to be the main factor in lowering the observed currents. Figure 5A shows that small peak responses for oxidation and back-reduction are observed with the same Emid potential and therefore associated with the one-electron 4-benzoyloxy-TEMPO redox process.

When employing rotating disc voltammetry (see Figure 5B) very similar features are observed with or without convection in addition to a rising current at more positive potentials. The stability of the catalyst signal is improved for both stationary conditions and voltammetric signals under rotating disc conditions. Therefore it appears likely that a codeposit of 4-benzyloxy-TEMPO and PIM-EA-

TB could provide a stable catalyst system with the homogeneous catalyst effectively heterogenised and access of the water-soluble substrate to the catalyst through the porous host.

3.4. Voltammetric Characterisation of 4-Benzoyloxy-TEMPO – PIM-EA-TB Codeposits at

Glassy Carbon in the Presence of Carbohydrates

In order to explore electrocatalysis applications for 4-benzoyloxy-TEMPO immobilised into a PIM-

EA-TB film, experiments were performed in 2 mM glucose in 0.1 M phosphate buffer pH 12.

Figure 6A and 6B contrast the behaviour of the 4-benzoyloxy-TEMPO modified electrode with and without PIM-EA-TB. In both cases currents associated with catalytic glucose oxidation are observed, but currents in the presence of PIM-EA-TB are significantly lower.

Figure 6. (A) Cyclic voltammograms (scan rate (i) 10, (ii) 50, and (iii) 200 mVs -1) for 30 g 4- benzoyloxy-TEMPO deposited onto a 3 mm diameter glassy carbon electrode immersed in 0.1 M phosphate buffer pH 12 with 2 mM glucose (pretreatment: 10 potential cycles at 200 mVs-1). (B) As above, but for 30 g 4-benzoyloxy-TEMPO with 30 g PIM-EA-TB. (C) Rotating disc voltamemtry (rotation speeds from (i) 100 to (ix) 2000 rpm, scan rate 10 mVs-1) for 120 g 4- benzoyloxy-TEMPO on a 5 mm diameter glassy carbon disc electrode immersed in 0.1 M phosphate buffer pH 12 with 2 mM glucose (pretreatment: 10 potential cycles at 2000 rpm). (D) As before, but with 120 g 4-benzoyloxy-TEMPO and 120 g PIM-EA-TB. (E) Levich plots for TEMPO and TEMPO/PIM-EA-TB.

This is confirmed in rotating disc electrode measurements (Figure 6C and 6D). In both cases

(with/without PIM-EA-TB) pretreatments were applied and measurements were then performed from lower (100 rpm) to higher (2000 rpm) rotation speeds. For 4-benzoyloxy-TEMPO catalytic currents are observed but they rapidly decrease mainly due to the electrode being unstable. Loss of

4-benzoyloxy-TEMPO is most likely the cause. For 4-benzoyloxy-TEMPO in PIM-EA-TB, currents are stable but also lower. The inset Figure 6E summarises the current responses with PIM- embedded catalyst giving rotation speed independent responses (transport within the PIM-EA-TB polymer appears to be rate limiting under these conditions). Although the catalytic currents generated within the PIM-EA-TB film appear low, they can be further improved and optimised.

Figure 7. (A) Cyclic voltammograms (scan rate 10 mVs-1) for 1.5 g 4-benzoyloxy-TEMPO with 15 g PIM-EA-TB deposited onto a 3 mm diameter glassy carbon electrode immersed in 0.1 M phosphate buffer pH 12 with (i) 0, (ii) 1, (iii) 2, (iv) 4, (v) 8, and (vi) 16 mM glucose (pretreatment: 10 potential cycles at 200 mVs-1). (B) As above, but for sorbitol. (C) As above, but for sucrose. (D) Plot of estimated catalytic limiting currents for glucose, sorbitol, and sucrose versus concentration. (E) Plot of the estimated catalytic currents for glucose versus concentration and for (i) 3 g 4- benzoyloxy-TEMPO – 15 g PIM-EA-TB, (ii) 1.5 g 4-benzoyloxy-TEMPO – 15 g PIM-EA-TB, and (iii) 1.5 g 4-benzotloxy-TEMPO – 30 g PIM-EA-TB. Data in Figure 7A to 7C show voltammetric responses for thinner films of PIM-EA-TB with less

(1.5 g) 4-benzoyloxy-TEMPO immobilised. The catalytic currents are clearly substrate

concentration dependent and also the changes in currents when going from glucose to sorbitol and

to sucrose can be observed (similar to data in Figure 4, but with an order of magnitude lower

currents). The plot in Figure 7D summarises the data with a plateau-effect at higher carbohydrate

concentration indicative of the possibility of a slower catalytic process within the porous host

environment. The plot in Figure 7E highlights the effects of (i) doubling the amount of 4-

benzoyloxy-TEMPO catalyst and (iii) doubling the amount of PIM-EA-TB host polymer

(essentially halving the concentration of the catalyst in the film). Although external mass transport

effects for these currents remain insignificant (transport within the PIM-EA-TB film and/or slower

reaction kinetics appear rate limiting) there are clearly opportunities to increase the catalytic

response. It can therefore be concluded that PIM-EA-TB host films with 4-benzoyloxy-TEMPO

catalyst immobilised in the porous structure are feasible for electro-catalysis and the

“heterogenisation” of the usually homogeneous TEMPO catalyst has been successful. The system is

readily scaled up onto higher electrode surfaces and it should find many new application for

example in flow electrosynthesis. An additional future benefit from PIM-EA-TB and similar PIM-

host materials will be pore-size selectivity and new environmental control for the kinetics of

catalytic process. This may obviously include future processes based on chiral TEMPO derivatives

[32] or chiral PIM environments.

4. Conclusion

It has been shown that versatile homogeneous catalyst systems such as TEMPO can be

“heterogenised” by co-deposition with a polymer of intrinsic microporosity, PIM-EA-TB. The rigid host environment allows electron transfer from the glassy carbon substrate (probably via hopping and formation of TEMPO+ cations) and the intrinsic porosity (typical pore sizes are 1-2 nm [21]) allows substrate molecules (here glucose, sorbitol, and sucrose) into contact with the catalyst. In part, diffusion in and out appear rate limiting with some evidence for “saturation” of the catalyst at higher glucose concentrations. More interestingly, the kinetics of this catalytic reaction appears changed and in future it will be very important to explore PIM-pore-geometry effects on catalytic reaction in more detail. Overall, this study opens the doors for new applications of “heterogenised

TEMPOs” and, more importantly, for a new generation of PIM-type materials tailored to control catalyst reactivity (e.g. introduce chirality) and to change processes that are classically “batch-type”

(with separation of catalyst) into “flow-type” or continuous process (without the need for separation of catalyst).

Acknowledgement

SDA gratefully acknowledges support from the University of Bath and Inochem Ltd. RSLC thanks the Centre for Sustainable Chemical Technologies for a PhD stipend.

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Figure 1. Molecular structures for PIM-EA-TB and 4-benzoyloxy-TEMPO. Schematic drawing of the mechanism for TEMPO-based oxidation of primary alcohols with a hydroxylamine intermediate and a comproportionation to regenerate the free radical [24,25,26].

Figure 2. (A,B) Electron microscopy images (backscatter and secondary electron, respectively) for microcrystalline 4-B-TEMPO (30 g deposited from chloroform) on a 3 mm diameter glassy carbon disc). (C) Electron microscopy image of a film of 30 g 4-B-TEMPO/ 30 g PIM-EA-TB codeposited from chloroform onto glassy carbon.

Figure 3. (A) Cyclic voltammogram (scan rate (i) 200, (ii) 50, (iii) 10 mVs-1) for a 30 g deposit of 4-benzoyloxy-TEMPO on a 3 mm diameter glassy carbon electrode immersed in 0.1 M phosphate buffer pH 12 (pretreatment: 10 potential cycles with 200 mVs-1). (B) As before, with 90 g 4- benzoyloxy-TEMPO. (C) As before, with 150 g 4-benzoyloxy-TEMPO. (D) Rotating disc voltammetry data (scan rate 10 mVs-1) for 120 g 4-benzoyloxy-TEMPO on a 5 mm diameter glassy carbon electrode with (i) 100, (ii) 650, (iii) 1250, (iv) 2000 rpm rotation speed (pretreatment: 10 cycles with 200 mVs-1 at 2000 rpm).