Supporting Information. Electrochemical Valorization of Furfural to Maleic Acid

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1 Supporting Informtion Electrochemicl Vloriztion of to Mleic Acid Stephen R. Kuot nd Kyoung-Shin Choi * Deprtment of Chemistry, University of Wisconsin Mdison, Mdison, WI 53706, USA. *Correspondence to: kschoi@chem.wisc.edu Numer of Pges: 9 Numer of Figures: 5 Numer of Tles: 5 S1

2 Experimentl Mterils. MnSO 4 monohydrte ( 98%, Sigm-Aldrich), N 2 SO 4 (ACS Grde, DOT Scientific Inc.), P(NO 3 ) 2 (99+%, Sigm-Aldrich), HNO 3 (68-70%, GR ACS), H 2 SO 4 ( %, Sigm- Aldrich), NOH ( 98%, Sigm-Aldrich), KOH ( 85%, Sigm-Aldrich), KH 2 PO 4 (99.0%, Electron Microscopy Sciences), furfurl (99%, Sigm-Aldrich), 2-furoic cid (98%, Aldrich), nd sodium mlete disic nhydrous (100.3%, Chem-Impex Int l Inc.) were ll commercilly purchsed nd used without further purifiction. Electrode preprtion. Fluorine-doped tin oxide (FTO) glss ws purchsed from Hrtford Glss Compny Inc. nd cut into 2 X 2.5 cm pieces nd then clened y sonicting sequentilly in cetone, isopropyl lcohol, nd wter for 15 minutes. Cu tpe ws ttched to the top of the conducting side of the FTO to provide contct for the potentiostt. Teflon tpe ws lso pplied to the top of the electrode to yield working re of 4 cm 2. All Pt electrodes were mde y sputter-coting 20 nm of Ti followed y 100 nm of Pt onto clen glss slides. PO 2 ws electrochemiclly deposited onto FTO in n queous solution composed of 50 mm P(NO 3 ) 2 lowered to ph of 1 with HNO 3. An undivided three-electrode cell ws used. FTO nd Pt were used s the working electrode (WE) nd counter electrode (CE), respectively. A Ag/AgCl (4 M KCl) electrode ws used s the reference electrode (RE). 2.2 V vs Ag/AgCl ws pplied to the WE while pssing 2.5 C/cm 2. P 2+ ions in the plting solution were oxidized to P 4+ ions. Due to the limited soluility of P 4+ ions, upon production, they were deposited s PO 2 film on the working electrode. 1 The s-deposited films were wshed with DI wter, dried in strem of ir, nd then nneled in ir t 500 C for 2 hours, with rmp rte of 2 C/min. MnO 2 ws electrochemiclly deposited onto FTO in n queous solution composed of 50 mm MnSO 4 nd 100 mm N 2 SO 4. An undivided three-electrode cell s descried ove ws used. MnO 2 ws nodiclly deposited y pplying 0.9 V to the WE in the plting solution kept t 60 C while pssing 0.5 C/cm 2. Similr to the PO 2 synthesis, solule Mn 2+ ions where oxidized to Mn 4+ which re insolule nd deposited s MnO 2. The s-deposited films were wshed with DI wter, dried in strem of ir, nd then nneled in ir t 400 C for 2 hours, with rmp rte of 2 C/min. 2 Chrcteriztion. X-ry diffrction (XRD) ws performed using Bruker D8 Advnced PXRD, 298 K, Ni-filtered Cu Kα rdition, λ = Å. Surfce morphology ws exmined with scnning electron microscopy (SEM) using LEO 1530 microscope with 5 KV ccelerting voltge. The s-deposited PO 2 film ws composed of round prticles creting rough surfce (Figure S1). The x-ry diffrction (XRD) pttern of the s-deposited film shows tht it contined mixture of α- nd β-po 2 (Figure S2). The s-deposited film ws nneled t 500 C in ir for 2 h to increse the dhesion etween the PO 2 prticles nd lso etween the prticles nd the sustrte. During nneling PO 2 ws reduced to PO, which ws confirmed y S2

3 XRD (Figure S2). The decomposition of PO 2 to more reduced PO x oxidtion sttes culminting finlly in PO nd ½ O 2 over temperture rnge of C is well-known phenomenon. 3-4 Due to the loss of oxygen nd the resulting volume contrction, the prticles in the PO film otined fter nneling looked smller thn those in the s-deposited PO 2 film (Figure S1). When the nneled film ws used for furfurl oxidtion, the surfce of the PO film ws re-oxidized to PO 2 while the core of the prticles remined s PO, which ws confirmed y XRD (Figure S2c) nd liner sweep voltmmetry (LSV) of the PO film (Figure S3). We use PO 2 to denote the led oxide electrode used in this study for furfurl oxidtion tht contins PO in the core nd PO 2 t the surfce ecuse PO 2 is the ctlyticlly ctive lyer. The detiled chrcteriztion of MnO 2 used in this study cn e found elsewhere. 2 Electrochemicl furfurl oxidtion. The ctlytic ctivity of electrodes for furfurl oxidtion ws first exmined using liner sweep voltmmetry (LSV). LSVs were performed in ph 1 H 2 SO 4 solutions without nd with 10 mm furfurl in n undivided three-electrode cell s descried ove with MnO 2, PO 2, or Pt s the WE. The potentil ws swept in the positive direction from the open circuit potentil t rte of 5 mv/s. Potentils were recorded versus the Ag/AgCl RE nd then converted to e versus the reversile hydrogen electrode (RHE) with the following eqution. E (vs. RHE) = E (vs. Ag/AgCl) + E Ag/AgCl (reference) V ph (E Ag/AgCl (reference) = V vs. NHE t 25 C) Constnt-potentil electrolysis experiments were performed to determine rection products, yields, nd Frdic efficiency. Electrolyses were performed in n H-shped glss cell divided y glss frit. The MnO 2, PO 2, or Pt WE nd the Ag/AgCl RE were immersed in the nodic comprtment contining 15 ml of ph 1 H 2 SO 4 solution with 10 mm furfurl. The Pt counter electrode ws immersed in the cthodic comprtment with 15 ml of ph 1 H 2 SO 4 solution. Electrolyses were performed y pplying constnt potentil to the WE while stirring with mgnetic stir r t 1400 rpm. Product nlysis. After electrolysis, products were identified using Bruker Avnce III 400 nd 500 MHz nucler mgnetic resonnce (NMR) spectrometers. conversions nd product yields were determined with Shimdzu Prominence-i LC-2030C 3D hi-performnce liquid chromtogrphy (HPLC) system contining photodiode rry (PDA) detector. During the course of electrolysis, 50 µl liquots were tken from the solution of the WE comprtment nd were diluted to 200 µl with wter. 10 µl smple liquots of the diluted WE solution were injected directly onto 300 mm 7.8 mm ICSep ICE-Coregel 87H3 column (Trnsgenomic), nd the column oven temperture ws mintined t 65 C. An queous moile phse composed of 0.1% sulfuric cid ws used with flow rte of 0.5 ml min -1 in isocrtic mode. The identifiction of the products nd clcultion of their concentrtions were determined from stndrd solutions of known concentrtion for, furoic cid, nd furfurl. Due to the S3

4 extrordinry expense of 2-furnol, stndrd solutions could not e prepred for HPLC. Therefore, the 2-furnol concentrtion in the HPLC result ws pproximted sed on the concentrtion rtio of 2-furnol to determined y NMR nd the concentrtion of determined y the HPLC. Retention times of 11, 19, 33, nd 48 minutes correlte to, 2- furnol, furoic cid, nd furfurl, respectively. Becuse ech compound exhiits different light sorption profile, detection wvelengths of 203, 195, 252, nd 276 nm were chosen s λ mx for, 2-furnol, furoic cid, nd furfurl, respectively. References (1) Li, X.; Pletcher, D.; Wlsh, F. C., Electrodeposited led dioxide cotings. Chem. Soc. Rev. 2011, 40, (2) Kuot, S. R.; Choi, K.-S., Electrochemicl oxidtion of 5-hydroxymethylfurfurl to 2,5- furndicroxylic cid (FDCA) in cidic medi enling spontneous FDCA seprtion. ChemSusChem 2018, DOI: /cssc (3) White, W. B.; Roy, R., Phse reltions in the system led-oxygen. J. Am. Cerm. Soc. 1964, 47, (4) Poll, C. G.; Pyne, D. J., Electrochemicl synthesis of PO 2, P 3 O 4 nd PO films on trnsprent conducting sustrte. Electrochim. Act 2015, 156, S4

5 Figure S1. SEM imges of PO 2 film () s-deposited nd () fter nneling t 500 C for 2 hours. Figure S2. XRD of () s-deposited PO 2 which cn e indexed s mixture of α- PO 2 (red, JCPDS ) nd β-po 2 (lue, JCPDS ), () fter nneling t 500 C which mtches pure PO (red, JCPDS ), nd (c) overly of PO 2 s deposited (lck), fter nneling (red), nd fter furfurl oxidtion (lue). After furfurl oxidtion, the min crystlline peks still mtch with PO, ut roder peks tht mtch with s-deposited PO 2 peks pper gin, indicting the formtion of PO 2 on the electrode surfce under nodic conditions. * mrks FTO sustrte pek. S5

6 Figure S3. The first (green) nd second (lck) LSV of PO 2 node fter nneling in ph 1 H 2 SO 4 solution t scn rte of 5 mv s 1. The first scn shows two nodic peks efore wter oxidtion due to the oxidtion P(II) in PO to P(III) nd P(IV) on the electrode surfce. The second scn shows only nodic current due to wter oxidtion on the PO 2 surfce. Figure S4. LSVs of PO 2 node otined in ph 1 H 2 SO 4 solution without (lck) nd with 10 mm furfurl (red), nd in ph 1 H 2 SO 4 solution with 10 mm furoic cid (lue) t scn rte of 5 mv s 1. S6

7 Figure S5. LSVs of PO 2 nodes without (dotted line) nd with (solid line) 10 mm furfurl in ph 1 H 2 SO 4 solution (red), ph 7 phosphte uffer solution (lue), nd ph 13 NOH solution (green) t scn rte of 5 mv s 1. Tle S1. Results of furfurl oxidtion y PO 2 t 2.0 V vs. RHE. Chrge Pssed (C) Furoic cid 2-Furnol Missing > > > missing represents the difference etween furfurl conversion nd the sum of the product yields (yields of nd 2-furnol). FE shows percent of totl chrge used for the production of nd 2-furnol. S7

8 Tle S2. Results of furfurl oxidtion y MnO 2 t 1.8 V vs. RHE. Chrge Pssed (C) Furoic cid 2-Furnol Missing missing represents the difference etween furfurl conversion nd the sum of the product yields (yields of, 2-furnol, nd furoic cid). FE shows percent of chrge used for the production of, 2-furnol, nd furoic cid. Tle S3. Results of furfurl oxidtion y Pt t 2.2 V vs. RHE. Chrge Pssed (C) Furoic cid 2-Furnol Missing > missing represents the difference etween furfurl conversion nd the sum of the product yields (yields of, 2-furnol, nd furoic cid). FE shows percent of chrge used for the production of, 2-furnol, nd furoic cid. S8

9 Tle S4. Results of furoic cid oxidtion y PO 2 t 2.0 V vs. RHE. Chrge Pssed (C) Furoic cid 2-Furnol Furoic cid Missing > Furoic cid missing represents the difference etween furoic cid conversion nd the sum of the product yields (yields of nd 2-furnol). FE shows percent of chrge used for the production of, 2-furnol. Tle S5. Results of furfurl oxidtion y PO 2 t 2.2 V vs RHE in ph 7 nd 13 conditions fter 100 C chrge pssed. Solution Furoic cid 2-Furnol Missing ph 7 H 2 PO 2 4 /HPO ph 13 NOH missing represents the difference etween furfurl conversion nd the sum of the product yields (yields of, 2-furnol, nd furoic cid). FE shows percent of chrge used for the production of, 2-furnol, nd furoic cid. S9