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Cyclic voltammograms for a range of TEMPO derivatives with different structures (R groups) illustrating the relationship between the differences in R groups, oxidizing activity, and reversibility. (A) acetamido-TEMPO (ACT, green traces), (B) methoxy-TEMPO (MT, orange traces), (C) TEMPO (T, yellow traces), (D) hydroxy-TEMPO (HT, blue traces), (E) oxo-TEMPO (OT, purple traces), and (F) amino-TEMPO (AT, red traces). CVs obtained in a 0.5 M borate buffer solution (pH 9.2) containing 1 mM of varying TEMPO catalysts at scan rates 10 mV/s−1 V/s.
Source publication
Biomass is an abundantly available, underutilized feedstock for the production of bulk and fine chemicals, polymers, and sustainable and biodegradable plastics that are traditionally sourced from petrochemicals. Among potential feedstocks, 2,5-furan dicarboxylic acid (FDCA) stands out for its potential to be converted to higher-value polymeric mate...
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... rate dependence of an electroactive molecule can give insight into the stability and reversibility of a molecule as well as the redox potential at which it is oxidized. Cyclic voltammograms for all six TEMPO catalysts are shown in Figure 3 at varying scan rates to demonstrate their electrochemical behavior. The oxidation potentials for each TEMPO derivative are as follows: ACT was 0.62 V, MT was 0.62 V, T was 0.53 V, HT was 0.62 V, OT was 0.715 V, and AT was 0.53 V. Three out of six catalysts, ACT (peak ratio of 0.90), MT (peak ratio of 0.87), and T (peak ratio of 0.97) display electrochemically reversible behavior in basic solvent conditions, as shown by equal magnitude anodic and cathodic peak currents. ...
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... chemical decay of the oxidized intermediate prevents the TEMPO species from being reduced back to its original state which results in the disappearance of the reverse peak and gives rise to peak current ratios <1.0. The voltammetric scan rate data in Figure 3 can be used to understand the reversibility of each catalyst and, in turn, predict the efficacy of the catalyst in an electrosynthetic reaction. CV can be a diagnostic technique and predictive model for the outcome of a subsequent bulk electrolysis experiment based on observations in mechanism and reversibility. ...
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... substrate titrations were completed at a range of scan rates from 10 mV/s to 1000 mV/s. A selection of faster scan CVs can be found in SI Figure S3. Specifically, the faster scan rate of 1000 mV/s in Figure S3 shows there is not a significant increase in the current with substrate titrations, indicating that at higher scan rates, the scanning time scale is faster than the chemical step between the TEMPO oxoammonium cation and BHMF. ...
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... selection of faster scan CVs can be found in SI Figure S3. Specifically, the faster scan rate of 1000 mV/s in Figure S3 shows there is not a significant increase in the current with substrate titrations, indicating that at higher scan rates, the scanning time scale is faster than the chemical step between the TEMPO oxoammonium cation and BHMF. ...
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... data was collected on each TEMPO species, evaluating pH dependence on scan rate and substrate additions. In the case of ACT, MT, and T, all three catalysts displayed electrochemically reversible voltammograms over the entire pH range (see SI Figure S31). The redox potentials of these species were also independent of pH. ...
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... demonstrate this, a substrate addition of 1 mM BHMF was added to each catalyst solution immediately following the scan rate study (Figure 9). Cyclic voltammetry data taken with substrate in solution only shows catalytic behavior at pHs ≥ 8 for ACT, MT, and T. At solution conditions more acidic than pH 7, no catalytic behavior is observed, despite the TEMPO displaying perfectly reversible behavior (see SI Figures S26−S30). Thus, it is important to consider optimal solution conditions for catalyst and substrate. ...
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... HT, OT, and AT have been labeled quasi-or irreversible thus far in this study and display interesting pH dependence. HT, OT, and AT all display electrochemically reversible voltammograms between pH 3−7 but start to lose reversibility in basic conditions around pH 8 and completely lack a reverse peak by pH 10 (see SI Figure S31). Upon addition of substrate, no catalytic behavior was observed in neutral or acidic conditions (< pH 8) and minimal catalytic behavior was revealed in basic conditions (pH ≥ 8), as described in previous sections. ...
Citations
... The reliance on fossil resources to produce fuels and chemicals is a concern that highlights the necessity for more environmentally friendly chemistry based on innovative and sustainable production methods using renewable biobased resources [1][2][3]. Biomass, as a renewable and abundant carbon-containing resource, is expected to play a crucial role in providing a sustainable source of chemicals and fuels for the world [4][5][6][7][8]. Based on this background, growing interest is being devoted on the catalytic transformation of biomass into valuable chemicals [9,10]. ...
The conversion of 5-hydroxymethylfurfural (HMF) and carbohydrates to 5-ethoxymethylfurfural (EMF) is an attractive biomass transformation owing to the potential application of the resulting product as a fuel additive. In this work, the imidazolium hydrogen sulfate ionic liquid was grafted on the surface of silica-coated iron oxide magnetic nanoparticles using cyanuric chloride as linker, and such a magnetically separable solid acid catalyst is referred to as HSO4/SMNPs. The catalytic activity of HSO4/SMNPs was evaluated for the etherification of biomass-derived HMF with ethanol and sequential dehydration/etherification of sugars to produce EMF. The impact of various reaction parameters was studied to achieve the optimal yield of EMF. HSO4/SMNPs provided EMF yields of 94.3%, 68.2%, and 53.3% from HMF, fructose, and inulin in ethanol, respectively, without the need for high boiling point co-solvents. Extraordinarily, the catalyst can be readily separated from the reaction mixture by applying a permanent magnet and reused for five times without substantial activity decrease.
... TEMPOH may be oxidized at the electrode to TEMPO or engage in comproportionating with TEMPO + to complete the catalytic cycle. 38,[52][53][54] The use of the 16-port fluidic switching valve enabled us to screen 6 substrates molecules in one experimental run with no human intervention. We examined a substrate scope consisting of isopropanol, ethanol, acetaldehyde, trifluoroethanol, ethylene glycol, and glycerol (Scheme 1). ...
Comprehensive studies of molecular electrocatalysis require tedious titration-type experiments that slow down manual experimentation. We present eLab as an automated electrochemical platform designed for molecular electrochemistry that uses opensource software to modularly interconnect various commercial instruments, enabling users to chain together multiple instruments for complex electrochemical operations. We benchmarked the solution handling performance of our platform through gravimetric calibration, acid–base titrations, and voltammetric diffusion coefficient measurements. We then used the platform to explore the TEMPO-catalyzed electrooxidation of alcohols, demonstrating our platforms capabilities for pH-dependent molecular electrocatalysis. We performed combined acid–base titrations and cyclic voltammetry on six different alcohol substrates, collecting 684 voltammograms with 171 different solution conditions over the course of 16 hours, demonstrating high throughput in an unsupervised experiment. The high versatility, transferability, and ease of implementation of eLab promises the rapid discovery and characterization of pH-dependent processes, including mediated electrocatalysis for energy conversion, fuel valorization, and bioelectrochemical sensing, among many applications.
