Robin Kunkel*a,
Volkmar M. Schmidtb,
Carsten Cremersa,
Dominik Müllera,
Detlef Schmiedlc and
Jens Tübkea
aFraunhofer Institute for Chemical Technology ICT, Department of Applied Electrochemistry, Joseph-von-Fraunhofer-Str. 7, D-76327 Pfinztal, Germany. E-mail: robin.kunkel@ict.fraunhofer.de
bMannheim University of Applied Sciences, Institute of Chemical Process Engineering, Paul-Wittsack-Str. 10, D-68163 Mannheim, Germany
cFraunhofer Institute for Chemical Technology ICT, Department of Environmental Engineering, Joseph-von-Fraunhofer-Str. 7, D-76327 Pfinztal, Germany
First published on 1st March 2021
Vanillin, one of the few biobased aromatic compounds available on an industrial level, is an attractive candidate for the synthesis of biobased polymers and polymer building blocks. This study presents a detailed investigation of the reductive electrochemical coupling process by pinacolization of vanillin and divanillin in an electrochemical H-type cell setup to the polymer building block hydrovanilloin and to polyvanillin, respectively. Therein, different cathode materials are screened by linear sweep voltammetry for their capability and activity of hydrodimerization of phenolic aromatic aldehydes in alkaline aqueous media. Product distributions and faradaic efficiencies of the electrochemical vanillin reduction are investigated in bulk electrolysis experiments. Dependencies on electrochemical parameters such as current densities, applied charges and cathode materials are studied. Furthermore, the polyvanillin synthesis from divanillin is also investigated by bulk electrolysis experiments. The effects of selected electrochemical parameters (current density, applied charge and electrode material) on yield and structural features (weight-average molecular weight (MW), number-average molecular weight (MN), polydispersity (MW/MN)) measured by size exclusion chromatography of the obtained polyvanillin were evaluated. Structural features of isolated polyvanillin were determined by 2D-NMR (HSQC, 13C/1H) analyses and by 31P-NMR analyses after in situ labeling with Cl-TMDP and possible pathways for their generation are discussed. These two promising electro-synthetic processes studied are free of hazardous materials and reagents and highlight the contributions of preparative electrochemistry to green chemistry and further pave the way toward the application of electrochemistry in the synthesis of biobased building blocks and polymers.
Electrochemical conversion of vanillin would be an attractive route enabling an overall green process, since electrochemistry is intrinsically fulfilling several important criteria of green chemistry, such as prevention of waste and use of toxic or hazardous reagents as only electrons serve as reactants. Moreover, this leads to a better atom efficiency and an inherently safe process. Also, electrochemical conversions often can be conducted under mild conditions, whereas for comparable conventional syntheses high temperatures and pressures are needed.13,14 Despite the mentioned advantages electrochemistry is nowadays still sparsely used as synthetic process step in the preparative organic chemistry.15,16
A promising first reaction step for the synthesis of biobased thermoplastic, such as polycarbonates and thermosetting resins, is the electrochemical conversion of vanillin to its hydrodimer hydrovanilloin by pinacolization of the aldehyde group. Electrochemical pinacolization of vanillin to hydrovanilloin was first described in 1952 by I. A. Pearl at lead electrodes in aqueous sodium hydroxide solution.17 Jow et al. later investigated product distributions and the kinetics of the cathodic reduction of vanillin at a mercury pool cathode.18 Vanillyl alcohol and hydrovanilloin were found as the only products, in which the latter being the dominant product at lower current densities, higher pH values and high Na+ concentrations. On the basis of their results and published literature of the electroreduction of aromatic aldehydes they proposed the following mechanism, where vanillin is reduced via an adsorbed metal ketyl intermediate (Scheme 1).19,20 Due to the bisphenolic structure of hydrovanilloin the molecule could be used as a renewable drop-in for exemplary bisphenol A. The feasibility of the usage of hydrovanilloin as building block was shown e.g. in the synthesis of a hydrovanilloin-formaldehyde copolymer21 or a hydrovanilloin-diglycidyl ether phenoxy resin.22
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Scheme 1 Electrochemical vanillin reduction in alkaline aqueous sodium hydroxide solution. The covalent bond O–Na of the metal ketyl is only used for convenience.18 |
Another electrochemical approach exploiting the same reaction type of pinacolization is the synthesis of polyvanillin by reductive polymerization of 6,6′-dihydroxy-5,5′-dimethoxy-(1,1′-biphenyl)-3,3′-dicarboxaldehyde, commonly called divanillin or 5,5′-bis-vanillin (Scheme 2). The feasibility of this synthesis was shown by Amarasekara et al. in 2012 at a lead cathode in 1 M sodium hydroxide solution leading to a total vanillin-based polymer.23 The oxidative phenol coupling of vanillin to divanillin can be classically performed by means of Fe(II)-based catalysts with yields of about 50%.24–28 The sustainable synthesis is performed enzymatically employing either laccase from Trametes versicolor in oxygenated solution29 or horse-radish peroxidase type I and hydrogen peroxide30–33 with yields of about 80% enabling a green alternative for this C–C coupling step. Besides for the electrochemical synthesis of polyvanillin divanillin was e.g. used in the preparation of Schiff-base polymers, which were investigated as chelating agents with metal ions.34
Fig. 1 summarizes the biobased value-added chain from lignocellulose to the biobased polymer polyvanillin and the polymer building block hydrovanilloin. As these product lines only employ sustainable process steps and the use of the side stream product lignin as starting material, a total green process would be realized.
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Fig. 1 Sustainable value-added chain of lignocellulose to the biobased polymer polyvanillin and the polymer building block hydrovanilloin. |
The electrochemical reduction of the aldehyde groups in the pinacolization steps compete in aqueous alkaline media with the hydrogen evolution reaction (HER):
2H2O + 2e− → 2OH− + H2 | (1) |
Therefore, cathode materials showing sufficient high overpotentials for the HER are required to achieve adequate faradaic efficiencies (FEs) for the pinacol product. These are often hazardous materials such as lead, mercury and cadmium, which can suffer from cathodic corrosion due to dissolution or can form highly toxic organometallic compounds.35,36 The use of these heavy metals electrode materials, as in the above mentioned earlier studies, should therefore be avoided in future electro-synthetic processes.
To further establish reliable electrochemical processes for the pinacolization C–C coupling steps of vanillin to hydrovanilloin and divanillin to polyvanillin, a more detailed understanding of the underlying reaction mechanisms and the effects of electrochemical parameters, such as current densities, applied charges, on product distributions, yields and structural features are required. The present study addresses these subjects extending the feasibility study of polyvanillin and the state of the art of the electrochemical synthesis of hydrovanilloin as well as further screens for green, sustainable and stable electrode materials as alternatives for the commonly used heavy metals.
4OH− → O2 + 2H2O + 4e− | (2) |
![]() | (3) |
Further, 31P-NMR spectra were recorded of selected isolated polyvanillin samples of the divanillin reduction experiments after in situ labeling for quantification of aliphatic and aromatic OH-groups. Three solvents were used for the preparation of the polyvanillin samples. Solvent I was a homogenous mixture of 16 mL pyridine (water free) and 10 mL CDCl3 (water free). For the preparation of solvent II 10.0 mg of Cr(III)acetyl acetonate were weighed into a HPLC vial and 2 mL of solvent I were added. The vial was caped and the Cr(III) acetyl acetonate was dissolved and homogenized at room temperature by vortex mixing. For the preparation of solvent III 21.92 mg of internal standard cyclohexanol (99%ical) was accurately weighed in a HPLC-vial and exactly 2 mL of solvent I was added. The vial was caped and the solution was homogenized at room temperature by vortex mixing. The freshly prepared solvents were allowed to stay at room temperature. The polyvanillin samples were dried in a vacuum oven at 10 mbar and at a temperature of 50 °C for 48 h. Exactly 20.00 mg of the dried polyvanillin samples were weighed in HPLC vials. Then 800 μL of freshly prepared solvent I, 100 μL of freshly prepared solvent II and 100 μL of freshly prepared solvent III were added to the samples and the vials were caped. The samples were mixed by vortex mixing at room temperature. 10 min before the 31P-NMR-analysis 160 μL of in situ labeling reagent 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (Cl-TMDP) were added to the vials and homogenized again by vortex mixing at room temperature. Only after the addition of Cl-TMDP were the samples completely dissolved and no precipitation was observed. The final sample solution was transferred to an NMR tube. The 31P-NMR analysis was done on a Bruker AVANCE 300 MHz. Chemical shifts are reported relative to the sharp signal (132.2 ppm) originating from the reaction between water and Cl-TMDP. Following NMR parameters were used: scans = 1024, pulse delay = 5 s, 90° pulse and line broadening = 2 and default baseline correction. 31P-NMR measurement is based on the developed method by Granata and Argyropoulos.39 31P-NMR-spectra were processed on ACD Lab 12.0, 1D-processor software. Integration for quantitative calculation was done according to Pu et al.40 Integration limits for aliphatic OH-groups were 150.0–145.40 ppm, for phenolic OH groups 144.50–137.50 ppm and for the internal standard (cyclohexanol) 145.39–144.75 ppm (ESI, Fig. S12†).
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Fig. 3 Cathodic LSVs for vanillin reduction at Pb, Zn and GC in static solution. Potential sweep rate is 10 mV s−1. Background currents of neat electrolyte are shown as dashed lines. |
Background currents are neglectable for all three electrode materials at lower overpotentials up to −0.8 V vs. RHE. With increasing overpotential a significant increase of the HER currents are observed for the three investigated cathode materials. Background currents are then in order Zn followed by Pb and GC. For example, at a potential of −1 V vs. RHE currents of −44.3 mA cm−2 for Zn, −19.2 mA cm−2 for Pb and −12.2 mA cm−2 for GC are measured for the HER. At even more negative values background currents of Pb and GC are approaching each other, whereas these negative potentials are hardly achieved at Zn due the higher HER activity depolarizing the Zn cathode. All three cathode materials show activity for the electrochemical reduction of vanillin. Onset potential at Zn and Pb differ in the millivolt range for the electrochemical vanillin reduction at ≈−0.575 V vs. RHE. At GC the onset potential is shifted by 50 mV in negative direction and cathodic currents are inferior to Pb and Zn. At a potential of −0.800 V vs. RHE reductive currents of −24.7 mA cm−2 for Pb, −21.4 mA cm−2 for Zn and −16.1 mA cm−2 for GC are observed in static solution. Therefore, GC shows comparable the worst catalytic properties for the electrochemical vanillin reduction under the three screened electrode materials but exhibits the lowest activity for the competing HER. Almost similar catalytic properties to the commonly used Pb cathodes are observed for the electrochemical vanillin reduction at Zn cathodes, but also showing a higher activity of the competing HER in the higher current density region. To evaluate further the impact of the competing HER quantitatively as well as determine product distributions bulk electrolysis experiments were conducted.
Cathode material | Current density (mA cm−2) | Mass difference cathode (mg) | Vanillin conversion (%) | Yield hydrovanilloin (%) | Yield vanillyl alcohol (%) | Potential at start (mV vs. RHE) | Potential at end (mV vs. RHE) |
---|---|---|---|---|---|---|---|
a Electrolysis was performed under following conditions – catholyte: 0.2 M vanillin in 1 M sodium hydroxide, anolyte: 1 M sodium hydroxide, room temperature, catholyte stirred at 250 rpm, potentials were corrected by electrolyte resistance measured by GEIS. Catholyte was analyzed by CE-UV. | |||||||
Zn | 30 | +0.21 | 99.0 | 97.7 | 1.3 | −720 | −1018 |
60 | +0.00 | 90.7 | 88.9 | 1.8 | −824 | −1112 | |
100 | −0.09 | 80.1 | 78.6 | 1.5 | −1073 | −1134 | |
Pb | 30 | −2.86 | 99.9 | 99.1 | 0.9 | −686 | −1040 |
60 | −5.90 | 95.0 | 93.4 | 1.6 | −803 | −1083 | |
100 | −19.30 | 93.7 | 87.1 | 6.6 | −1266 | −1195 | |
GC | 30 | +0.05 | 97.9 | 95.9 | 2.0 | −750 | −1326 |
60 | +0.12 | 92.9 | 78.6 | 14.3 | −1012 | −1640 | |
100 | +0.13 | 88.0 | 74.9 | 13.1 | −1791 | −1696 |
Throughout the electrolysis at 30 and 60 mA cm−2 the cathode potential drops to more negative values due to decreasing vanillin concentration and increasing conversion resulting in higher overpotentials for the electrochemical reduction of vanillin. For example, at 30 mA cm−2 at an applied charge around 0.60 F mol−1 for GC and 0.75 F mol−1 for Pb and Zn the potential shifts to a new more negative plateau, where preferably the HER is taking place. At this stage the vanillin electrolysis is under mass transfer control. An exemplary progression of the electrochemical vanillin reduction for a Zn cathode at 30 mA cm−2 is shown Fig. 4C and D. A change of the slope of the vanillin conversion is observed transitioning from a linear (kinetically controlled) to an exponential (mass transport controlled) concentration decrease. At 100 mA cm−2 no drop of the cathode's potential is observed suggesting that the reaction is under full mass transport control from start on and fastest conversion rates are achieved under given conditions. Potentials at the start and at the end of the electrolysis decrease with increasing current density due to a stronger cathodic polarization of the electrodes. Similar current–potential–charge behavior was observed by Jow et al. at a mercury pool electrode.18 For example, starting potentials for a 0.033 M vanillin solution in 1 M NaOH decreased from −1.72 V vs. SCE at 30 mA cm−2 to −2.05 V vs. SCE at 100 mA cm−2. Potential shifts to the HER plateau were slightly earlier at applied charges of 0.1–0.5 F mol−1, which may be due to slower mixing of the catholyte and different cell and electrode design. Here, for 30 and 60 mA cm−2 the cathode's potentials for the three materials Zn, GC and Pb at the start of the electrolysis are approximately similar and are in the range between −0.68 and −0.824 V vs. RHE except for GC at 60 mA cm−2, where a potential at the start of −1.01 V vs. RHE is observed. At the highest current density of 100 mA cm−2 and in the mass transport limited region of the electrolysis the potential is dominated by the HER. Due to high HER overpotentials very negative potentials up to −1.79 V vs. RHE are reached either at GC at 30 and 60 mA cm−2 in the mass transport limited regions or at 100 mA cm−2 at Pb and GC throughout the whole electrolysis. Minor HER overpotentials at Zn compared to the other two investigated cathode materials hinder the potential from reaching very negative values being at maximum −1.13 V vs. RHE at 100 mA cm−2.
Almost no mass difference for the Zn and GC electrodes are found after electrolysis for all investigated current densities deducing stable behavior. Mass differences range from −0.09 to + 0.21 mg and + 0.05 to + 0.13 mg for Zn and GC, respectively. Small positive changes of the cathode's mass may be due to small deposits of vanillin or reaction products on the electrode surface and measurement errors. However, for reproducible potential courses at GC intensive polishing of the electrode surface is necessary showing some change of the morphology under given conditions. This agrees well with observations by Chandrasekaran et al., who found a sensitive behavior to the pre-treatment by means of polishing of GC cathodes for the reduction of vanillin in aprotic and aqueous media.42 For Pb significant loss of electrode material was observed. Mass differences were in the range between −2.86 to −19.30 mg per electrolysis run. Similar cathodic corrosion of lead was observed e.g. by Gütz et al., who used leaded bronzes as alternative cathode material for electro-reduction such as deoxygenation, dehalogenations and hydrodimerization.36
Nearly full conversion of vanillin was achieved after an applied charge of 2 F mol−1 at all three electrode materials at 30 mA cm−2. Hydrovanilloin is the main product in all cases and vanillyl alcohol formation is neglectable with a maximum yield of 2% at GC. With increasing current density vanillin conversion is decreasing at the fixed applied charge of 2 F mol−1, because the current is consumed by the competing HER. At 100 mA cm−2 the lowest vanillin conversion of 80.1% is observed for Zn. At 100 mA cm−2, however, the highest selectivity for the pinacolization product hydrovanilloin is achieved for Zn among the three investigated cathode materials with neglectable yields of vanillyl alcohol of 1.5%. In comparison significant vanillyl alcohol yields of 13–14% are observed for GC at 60 and 100 mA cm−2 as well as 6.6% for Pb at 100 mA cm−2.
Analogously to Jow et al. FEs of the reduction products hydrovanilloin and vanillyl alcohol after an applied charge of 0.6 F mol−1 were calculated.18 At this stage the reaction is still kinetically controlled at low current densities and FEs stay constant (see Fig. 4D).43 For proving reproducibility, the experiments for the FE evaluation were conducted two times and the resulting mean values are presented in Fig. 5. At the lowest current density of 30 mA cm−2 FEs for hydrovanilloin formation are in the range of 85–88% for Pb and Zn, whereas for GC a FE of ≈98% is obtained due to its higher overpotential for the competing HER. FEs for vanillyl alcohol are below 0.5% for Pb and GC. A slightly higher FE for vanillyl alcohol formation at Zn cathodes of 2.3% is observed. Comparatively, no vanillyl alcohol formation was reported by Jow et al. for a Hg cathode in the current density range of 15–100 mA cm−2. FEs for hydrovanilloin decreased from 80% to 45% when increasing the current density from 15 mA cm−2 to 100 mA cm−2. But it should be noted that the starting vanillin concentration was lower being 33 mM of vanillin in 1 M NaOH.18 Here, total FEs of vanillin reduction products decrease gradually to 71–75% at 100 mA cm−2 for Pb or GC and to 55% for Zn. However, comparable FEs for hydrovanilloin formation of 55–62% are achieved among the three investigated cathode materials, since the FE for vanillyl alcohol formation stays at low percentages for Zn, whereas FEs of 12–13% for vanillyl alcohol are obtained for GC and Pb.
According to the published literature the dissociated hydroxyl group in basic aqueous media leads to a decrease of the stability of the metal ketyl intermediate due to electron release to the benzene ring and therefore to a preferably dimerization reaction pathway (see Scheme 1). Significant vanillyl alcohol formation is only observed at very negative potentials, where kinetics for the second electron transfer to the metal ketyl intermediate would be fast enough compared to the dimerization pathway.18 These negative potentials are either reached herein at Pb and GC electrodes at higher current densities or in mass transport controlled region of the electrolysis. For example, significant vanillyl alcohol formation at GC at 60 mA cm−2 is only observed after an applied charge of 0.5 F mol−1, when the potential drops from −1 V vs. RHE to the HER plateau of ≈−1.6 V vs. RHE (ESI, Fig. S3†). Therefore, FEs at 0.6 F mol−1 for GC at 60 mA cm−2 are still low, although a high vanillyl alcohol yields is observed after an applied charge of 2 F mol−1. It is assumed that the comparable lower HER overpotential depolarizes the Zn electrode hindering the potential of reaching very negative values and therefore inhibiting the second electron being transferred to the metal ketyl intermediate. As a result, the highest selectivity for hydrovanilloin at higher current densities is obtained for Zn and side product formation of vanillyl alcohol is neglectable. The negative impact of the minor HER overpotential at Zn on the FE of hydrovanilloin is low showing comparable FEs of ≈55% to Pb and GC up to high current densities. Furthermore, it is interestingly to note that the viable potential window of Zn for electroreductions in aqueous alkaline media is very limited. At potentials higher and equal to the open circuit potential of −0.450 V vs. RHE corrosion occurs and at potentials more negative than −0.800 V vs. RHE a significant increase of the competing HER is observed. The electroreduction of vanillin with an onset potential of −0.575 V vs. RHE at Zn seems to fit perfectly in the viable potential window.
Run | Current density (mA cm−2) | Applied charge (F mol−1) | Divanillin concentration (mol L−1) | Isolated yield (%) | MWb (g mol−1) | MNb (g mol−1) | MW/MN |
---|---|---|---|---|---|---|---|
a Electrolysis was performed under following conditions – cathode: Zn (5 cm2), anode: Ni-foam, catholyte: 0.1 M divanillin in 1 M sodium hydroxide (first batch of divanillin containing 4.9 wt% vanillin, if not stated otherwise), anolyte: 1 M sodium hydroxide, room temperature, catholyte stirred at 250 rpm, isolated products were analyzed by SEC.b Calibrated versus pullulan standard.c Second batch of divanillin containing no vanillin. | |||||||
1 | 15 | 2 | 0.1 | 57.9 | 1771 | 1360 | 1.30 |
2 | 15 | 4 | 0.1 | 66.2 | 3132 | 2321 | 1.35 |
3 | 15 | 8 | 0.1 | 64.1 | 3171 | 2362 | 1.34 |
4 | 30 | 2 | 0.1 | 74.6 | 2095 | 1464 | 1.43 |
5 | 30 | 4 | 0.1 | 64.2 | 3103 | 2174 | 1.42 |
6 | 30 | 8 | 0.1 | 69.9 | 3023 | 2186 | 1.38 |
7 | 60 | 2 | 0.1 | 71.2 | 1687 | 1239 | 1.36 |
8 | 60 | 4 | 0.1 | 64.8 | 2864 | 1748 | 1.64 |
9 | 60 | 8 | 0.1 | 69.7 | 3016 | 2201 | 1.37 |
10 | 60 | 8 | 0.2c | 72.8 | 3082 | 2260 | 1.36 |
11 | 60 | 8 | 0.3c | 81.1 | 3208 | 2344 | 1.37 |
At the lowest applied current density of 15 mA cm−2 the reaction is kinetically controlled showing a drop of the cathode potential from −800 mV vs. RHE to −900 mV vs. RHE at an applied charge of ≈2 F mol−1 due to decreasing divanillin concentration (Fig. 8A). In the latter part of the reaction the divanillin reduction is under mass transport control and, as in the electrochemical vanillin reduction, the competing HER is the dominating reaction. No potential drop at a current density of 60 mA cm−2 corresponding to a cathode potential of ≈−1 V vs. RHE is observed indicating that the reaction is under mass transport control respectively highest possible conversion rate from start on. The slight increase of the cathodic potential with increasing applied charge in the mass transport-controlled region is explained by the decreasing resistance of the electrolyte in the cathodic chamber, which was typically 0.5 Ω at an applied charge of 8 F mol−1. On the basis of charge neutrality Na+-ions migrate through the ion selective membrane from the anodic to the cathodic chamber leading to a lower resistance between the working and the reference electrode on the cathode side and therefore to a lower recorded potential with increasing reaction time.
An exemplary molecular weight build-up from vanillin to polyvanillin at the Zn cathode for a current density of 30 mA cm−2 in the reductive polymerization step of divanillin is shown in Fig. 8B. Since in the first batch of the enzymatic conversion of vanillin to divanillin a remaining vanillin fraction of 4.9 wt% was found, a small pre-peak in the SEC spectra of divanillin is observed. It should be noted that vanillin and divanillin are no polymeric compounds and have an exactly weight of 152 g mol−1 for vanillin and 302 g mol−1 for divanillin. Due to their low molecular weight such substances are usually not appropriate for SEC analysis and the broad molecular weight distributions (MWDs) do not correspond to their actual molecular weight. However, these compounds are shown in Fig. 8B for a better visualization of the process. The actual polymerization of divanillin to polyvanillin shows a bimodal frequency distribution of molar mass under given electrochemical process conditions. A first local maximum is observed at a molecular weight of ≈1000 g mol−1 and a second local maximum at ≈3200 g mol−1. With increasing reaction time or applied charge the first local maximum decreases and the second local maximum increases until a final state of the MWD shape is reached. The first local maximum becomes more and more a shoulder. This result is observed for all three investigated current densities of 15, 30 and 60 mA cm−2 (ESI, Fig. S7 and S8†). Similar bimodal molecular weight distributions were e.g. observed for the emulsion polymerization of ethylene47 or isotactic polystyrene.48 No further change of the shape of the MWD is observed after an applied charge of 4 F mol−1 (double theoretical charge for full divanillin pinacolization) for 15 and 30 mA cm−2, whereas this shape is achieved after 8 F mol−1 (quadruple theoretical charge for full divanillin pinacolization) at the highest current density of 60 mA cm−2. The most energy efficient polymerization is therefore conducted at low current densities, where the competing HER is still slow, whereas fastest conversion is conducted at 60 mA cm−2 under mass transport control for the given conditions. The formation of the bimodal frequency distribution of molar mass was at first suggested due to remaining vanillin in the divanillin, since vanillin showed a higher activity compared to divanillin. This would lead to fast formation of small pre-polymers followed by the actually main polymerization of divanillin. However, polyvanillin synthesis experiments with a second divanillin batch containing no remaining vanillin resulted in a similar shape and similar polymerization degrees (see Fig. 8D, run 10–11). Therefore, the influence of the remaining vanillin fraction of 4.9 wt% in the first batch can be neglected and the bimodal frequency distribution of molar mass still remains subject of further studies.
Further it is found that the maximum degree of polymerization is independent of the applied current density at a Zn cathode up to potentials of ≈−1 V vs. RHE, which corresponds to a current density of 60 mA cm−2 (Fig. 8C). Weight average molecular weights MW up to ≈3200 g mol−1 are achieved. The polydispersity MW/MN is relatively low showing a narrow distribution. Values for the polydispersity lie in the range between 1.30 and 1.43 excluding run 8, where a polydispersity of 1.64 is determined. This higher value, however, is calculated for a partly polymerized sample at a current density of 60 mA cm−2 and an applied charge of 4 F mol−1 showing almost two equally high peaks in the bimodal frequency distribution of molar mass (ESI, Fig. S8†).
Since higher concentrations of divanillin might lead to an increase of the polymerization degree, polyvanillin synthesis was performed for higher concentration of divanillin up to 0.3 M (run 10–11). A higher divanillin concentration would lead to an increased radical intermediate concentration at the electrode's surface, thus, might increase pinacol formation (radical combination) instead of alcohol formation terminating polymer chain growth. Similar observation were shown for the electroreduction of vanillin to hydrovanilloin.18 However, no increase of the polymerization degree was found when tripling the concentration from 0.1 M to 0.3 M divanillin (Fig. 8D).
Isolated yields ranged between 58% and 74% at a divanillin concentration of 0.1 M showing no discernible trend when varying the current density and the applied charge. The results suggest that reaction products are in solubility equilibrium when titrating the alkaline catholyte to pH = 2 for product precipitation also showing potential for improvement in the isolation step as in the hydrovanilloin synthesis. However, an increase of the isolated yields is observed for higher concentrations reaching values of 81% at a divanillin concentration of 0.3 M. Isolated polyvanillin yields by Amarasekara et al. for a 0.175 M divanillin solution were slightly higher being 91% after an applied charge of 35.2 F mol−1.23
Finally, it is interestingly to note, that the reductive pinacolization forms soluble polymers. Since on the other hand, the oxidative polymerization for example of pyrrole, aniline or thiophene lead to the formation of electronic conducting polymer films on the anode.49,50 No deposition of polymers on Zn cathodes was observed throughout all polyvanillin electrolysis experiments.
As Pb cathodes were used in the original synthesis of polyvanillin by Amarasekara et al.23 and GC cathodes also showed reasonable activity in the electrochemical vanillin reduction experiments, the impact of different cathode materials were further investigated. The applied charge was selected to be 8 F mol−1, since full polymerization was achieved at Zn cathodes at this amount of applied charge. The molecular weight data of the polyvanillin samples synthesized at GC and Pb cathodes are summarized in Table 3. Potential–charge plots and MWDs are shown in the (ESI, Fig. S9–S11†).
Cathode material | Current density (mA cm−2) | Applied charge (F mol−1) | MWb (g mol−1) | MNb (g mol−1) | MW/MN |
---|---|---|---|---|---|
a Electrolysis was performed under following conditions – anode: Ni-foam, catholyte: 0.1 M divanillin in 1 M sodium hydroxide, anolyte: 1 M sodium hydroxide, room temperature, catholyte stirred at 250 rpm, isolated products were analyzed by SEC.b Calibrated versus pullulan standard. | |||||
Pb | 15 | 8 | 2444 | 2023 | 1.21 |
Pb | 30 | 8 | 2958 | 2285 | 1.29 |
Pb | 60 | 8 | 1898 | 1501 | 1.26 |
GC | 15 | 8 | 2986 | 2384 | 1.25 |
GC | 30 | 8 | 3382 | 2631 | 1.29 |
GC | 60 | 8 | 1264 | 1017 | 1.24 |
Bimodal frequency distributions of molar mass are also found for GC and Pb cathodes in all cases. Weight-average molecular weights MW of ≈3000 g mol−1 are approximately similar to polyvanillin synthesized at Zn cathodes for the current densities of 15 and 30 mA cm−2. A significant drop of the weight-average molecular weight is observed at Pb and GC at the highest applied current density of 60 mA cm−2 to 1898 g mol−1 and 1264 g mol−1, respectively. A high amount of a low molecular weight fraction still exists in the polyvanillin sample synthesized at the GC cathode showing a high first local maximum in the bimodal frequency distributions of molar mass at ≈800 g mol−1. Low polydispersities MW/MN ranging between 1.21 and 1.29 are found for synthesized polyvanillin at GC and Pb cathodes also showing very narrow MWDs. Cathode potentials are significant more negative compared to Zn reaching values of −1.30 V vs. RHE and −1.75 V vs. RHE for Pb and GC at a current density of 60 mA cm−2, respectively. This agrees well with obtained results in the electrochemical vanillin reduction experiments. A decrease of the cathode's potential is observed at 60 mA cm−2 for Pb and GC at the start of the electrolysis indicating that the reaction is not under full mass transport control from begin on, which is due to higher overpotentials of the competing HER.
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Fig. 10 Proposed structural features of polyvanillin based on the results of 2D-NMR (HSQC) analysis. |
Cross-coupling signals δ 13C/δ 1H in ppm | Assessment | Note |
---|---|---|
≈55.60/3.7 | (CH3–O-Ar) | Methoxyl group |
64.9/5.0 | R–CH2-OH | Linked to primary aliphatic OH-group, sometimes weak signal |
79.5/5.0 & 78.9/5.4 | Cα′–Hα′, Cα′′–Hα′′ | Vicinal OH groups in aliphatic region |
109.0/7.7 | C2–H2 | C5-substituted guaiacyl, cross-coupling δ 13C/δ 1H aromatic ring |
123.6/7.7 | C6′–H6′ | Cross-coupling δ 13C/δ 1H aromatic ring |
126.6–125.6/6.7 | Cα′–Hα′ & Cα′′–Hα′′ | Stilbene like double bond instead of vicinal OH groups |
130.0/7.9 | C6–H6 | C5-substituted guaiacyl, cross-coupling δ 13C/δ 1H aromatic ring |
191.4/10.1 | Cα–Hα | Aldehyde end group, sometimes weak signal |
123.87/7.220; 135.91/7.580 & 150.35/8.740 | C–H cross-coupling in pyridine | Pyridine-d5, 99%ical |
111.5/7.5; 116.4/7.2; 121.5/7.3 | Unknown | Could be cross-coupling signals of 13C/1H in aromatic rings, differing in chemical environment |
The existents of structural features of polyvanillin were confirmed in all cases. Cross-coupling signals of the methoxy groups coupled to the aromatic rings at δH = 3.7 ppm/δC = 55.60 ppm were found in every sample. The vicinal hydroxy groups in the aliphatic region generated by the pinacol formation (see Fig. 10, brown motif) were assigned to the cross-coupling signals at δH = 5.0 ppm/δC = 79.5 ppm and δH = 5.4 ppm/δC = 78.9 ppm and cross-coupling signals from the aromatic ring of the C5-substituted guaiacol to δH = 7.7 ppm/δC = 109.0 ppm, δH = 7.7 ppm/δC = 123.6 ppm and δH = 7.9 ppm/δC = 130.0 ppm (Table 4). Very broad peaks of the methoxy groups signals in the 1H-spectra confirm the high molecular weight of the polyvanillin samples (Fig. 9B–E). In the two samples with lower molecular weight (Fig. 9A and F) 1H-signals from the methoxy group are split into a high sharp peak and a lower broader peak verifying the existence of a very low molecular fraction. Moreover, very sharp peaks are also observed in the aromatic region in the 1H-spectra between 6.5 ppm and 8.5 ppm further confirming the existence of a very low molecular fraction. A cross-coupling signal observed at δH = 10.1 ppm/δC = 191.4 ppm was assigned to the unreacted aldehyde end groups (see Fig. 10, green motif). Only very weak to none signals at all from the remaining unreacted aldehyde end groups can be seen in the spectra of the high molecular weight samples (Fig. 9B–E) confirming full conversion divanillin for the Zn cathode after an applied charge of 8 F mol−1 at the three current densities 15, 30 and 60 mA cm−2 and for the GC cathode at the current density of 15 mA cm−2. As expected a strong signal of unreacted aldehyde groups is found in the 1H-NMR spectra for the party polymerized sample at the Zn cathode at 30 mA cm−2 when only applying a charge of 2 F mol−1 (Fig. 9A). Surprisingly, unreacted aldehyde groups are also found in the polyvanillin sample synthesized at the GC cathode at 60 mA cm−2 after an applied charge of 8 F mol−1 (Fig. 9F). This explains the low molecular weight analyzed in the SEC analysis. Since a drop of the cathode's potential at the start of the electrolysis was observed indicating some conversion of divanillin, it is assumed that the main reaction in the latter period of the reaction is the HER leading to no further divanillin reduction. At the moment this phenomenon cannot be explained and is subject of further studies. Besides undergoing pinacolization the aldehyde end groups can be reduced by a 2e− reduction to the corresponding alcohol terminating the polymer chain growth (see Fig. 10, red motif). As shown in the electrochemical vanillin reduction experiments and by Jow et al. the ratio between alcohol formation and pinacolization increases with decreasing potential respectively under more severe conditions in favor of alcohol formation.18 The cross-coupling signal of the primary, terminal alcohol group was assigned to the peak at δH = 5.0 ppm/δC ≈ 64.9 ppm. For the polyvanillin samples synthesized at Zn cathodes the amount of terminal alcohol groups slightly increases with increasing current density respectively decreasing cathode potential and increasing applied charge (Fig. 9A–D). At GC comparably, a higher amount of these terminal alcohol groups is already observed at 15 mA cm−2 due to more negative potentials. In the sample synthesized at a current density of 60 mA cm−2 the absolute peak height of the terminal alcohol groups is lower than at a current density of 15 mA cm−2. However, in this partly polymerized sample the 1H-NMR signal of the primary, terminal alcohol group is still relatively high compared to the 1H-NMR signal of the pinacol groups. Moreover, an increasing signal at δH = 6.7 ppm/δC ≈ 126.0 ppm is observed with decreasing cathode potential respectively under more severe conditions. This signal was assigned to stilbene-like double bond systems, since the signal is located in the unsaturated and aliphatic region of the 1H/13C-NMR spectra (see Fig. 10, blue motif). Unfortunately, it cannot be distinguished whether these stilbene-like double bond systems are created during the isolation process by titrating with concentrated HCl or in the actual electrolysis. However, a clear trend of increasing double bond systems with harsher electrolysis conditions is observed favoring the case of stilbene formation in the electrolysis. It is suspected that the generated vicinal alcohol groups in the aliphatic region generated by pinacolization are converted to these stilbene-like double bond systems by elimination of water followed by a further reduction to the corresponding double bond. Similar hetero-catalyzed reductions have been shown for the conversion of acetylated hydrovanilloin, which is the pinacol product of vanillin, to the stilbene and stilbene epoxides to alkenes.44,52,53
Furthermore, 31P-NMR analyses of selected polyvanillin samples were conducted for quantification of phenolic and aliphatic OH groups. In theory, the polyvanillin molecule should be characterized by having more or less the same molar number of phenolic and aliphatic OH groups. By means of 31P-NMR analysis, after in situ labeling Cl-TMDP, this should be illustratable. An exemplary 31P-NMR spectrum is shown in the (ESI, Fig. S12†) and quantitative calculated results are summarized in Table 5. Contrary to theoretical considerations, the polyvanillin samples prepared in this study often exhibit a molar ratio of aliphatic to phenolic OH groups of approximately 0.5. This finding can of course be discussed speculatively with respect to a steric hindrance of the in situ labeling process or an uncontrolled water separation during the isolation from the electrolyte. As no conclusive answer could be given for this yet, further studies will get to the bottom of this preliminary result. However, the low observed degree of polymerization in the SEC analysis for the polyvanillin sample synthesized at GC at a current density of 60 mA cm−2 is confirmed by the 31P-NMR results showing almost no generated aliphatic hydroxy groups by pinacolization.
Cathode material | Current density (mA cm−2) | Applied charge (F mol−1) | Phenolic OH (mmol g−1) | Aliphatic OH (mmol g−1) | Ratio aliph. OH/phen. OH |
---|---|---|---|---|---|
Zn | 30 | 2 | 6.630 | 3.401 | 0.513 |
Zn | 30 | 8 | 6.236 | 2.730 | 0.438 |
Zn | 60 | 8 | 8.775 | 4.362 | 0.497 |
GC | 60 | 8 | 8.234 | 0.513 | 0.067 |
Finally, it should be noted that in comparison higher weight-average molecular weights MW up to 17000 g mol−1 of polyvanillin synthesized at a Pb cathode with a divanillin concentration of 0.175 M were reported in the preliminary feasibility study by Amarasekara et al., which could not be reproduced.23 However, exact current densities, SEC calibration standards and columns were not stated and DMF was used as solvent for SEC analysis. Therefore, the SEC molecular weight data of polyvanillin is hardly comparable. However, the published 1H-NMR spectra of the polyvanillin sample showed unreacted aldehyde end groups and a sharp methoxy group peak indicating not full conversion of divanillin and existence of a high amount of a very low molecular fraction. Therefore, relatively higher molecular weights should have been achieved in this presented study.
For further scalability of these promising electrochemical processes a transfer to an electrochemical flow reactor is required, as batch cells are limited in production rate due to low electrode area to electrolyte ratios and may suffer from poor undefined hydrodynamics.54 Thus, the transfer to an electrochemical flow reactor of these processes including a detailed investigation of the mass transport behavior will be addressed in future investigations overcoming the mentioned limitations of batch cells.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra00649e |
This journal is © The Royal Society of Chemistry 2021 |