# From slag to structure: Formation of novel iron oxalate crystals via Cyrene™-driven microwave chemistry

Limor Ben Neon, Aurelien Lebrun, Eddy Petit, Anne Julbe

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Limor Ben Neon, Aurelien Lebrun, Eddy Petit, Anne Julbe. From slag to structure: Formation of novel iron oxalate crystals via Cyrene™-driven microwave chemistry. Materialia, 2025, 41, pp.102443. 10.1016/j.mtla.2025.102443. hal-05090848

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# From slag to structure: Formation of novel iron oxalate crystals via Cyrene™-driven microwave chemistry

Limor Ben Neon<sup>a*</sup>, Aurelien Lebrun<sup>b</sup>, Eddy Petit<sup>a,b</sup>, Anne Julbe<sup>a,*</sup>

a. Institut Européen des Membranes (IEM), Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

b. UAR Plateforme d'Analyses et Caractérisations (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

* Contacting Authors

**Keywords:** Microwaves, Cyrene, size control, green synthesis, waste recycling

## Abstract

Moving towards a circular economy, upgrading industrial waste products rich in metals and minerals into valuable materials essential for digital devices, catalysis, and environmental remediation is a key step. The procedures must be eco-friendly, efficient, and yield high production rates. In this work, we propose a microwave-assisted conversion process that transforms iron-rich oxide products (e.g., Fe₂O₃, Fe₃O₄) into long iron oxalate crystals with mixed oxidation states using a green solvent mixture of water and CyreneMT – a bio-derived solvent. Results show that an equal volume fraction of water and CyreneMT enables uniform crystal growth with an average length of 71.5 ± 10.5 µm while achieving high conversion yields. The procedure is time- and energy-efficient, environmentally friendly, and potentially applicable to a wide range of industrial residues, as demonstrated in this work for steel slag.

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# 1. Introduction

With the increasing energy demand and global population growth natural resources are rapidly depleting [1,2]. Minerals and metals are essential for producing batteries, engines, solar panels, and digital devices [2]. However, their extraction causes environmental degradation and poses health risks. At the same time, industrial waste—such as battery residues and steel slag—is accumulating in large disposal sites, often at high cost. These wastes are rich in valuable transition metals and minerals essential for mobility, renewable energy and digital technologies [2]. Recycling these materials into high-value products is a key step toward a circular economy all while helping to conserve natural resources and reduce environmental impacts.

In this perspective, Carboni et al. demonstrated the possibility to convert battery waste into metal-organic frameworks (MOFs) [3]. Their method involved dissolving transition metals (e.g., Co, Mn, Cu) in strong acids like HCl, $HNO_3$, and $H_2SO_4$, followed by reacting the obtained precipitate with organic ligands such as benzene-1,3,5-tricarboxylic acid (BTC) in organic solvents like dimethylformamide (DMF). Although the process is effective, it has drawbacks: mainly it is time-consuming, relies on hazardous solvents like DMF and uses corrosive acids with significant environmental risks [4,5]. To address these issues, more sustainable alternatives are being explored as for instance the use of milder leaching agents like oxalic acid [5], greener solvents, and faster synthesis methods.

One promising approach is microwave (MW)-assisted solvothermal synthesis. MW heating enables rapid and uniform heating, enhancing interfacial reactions and forming highly reactive microenvironments [6,7]. The combination of microwave irradiation with solvothermal techniques offers several compelling advantages: rapid reaction times, access to novel structures and morphologies, improved phase purity and homogeneity, and enhanced yields and selectivity toward specific crystalline phases. These benefits make it a promising strategy not only for synthesizing advanced materials with original physicochemical properties, e.g. catalysts [8], but also for valorizing industrial residues, as proposed in this study. Indeed, MW techniques have been successfully applied to synthesize MOFs [6,8,9], metallic nanoparticles [10] and for acidic digestion [11]. However, MW-assisted leaching using aqueous oxalic acid remains largely unexplored. Demonstrating its effectiveness for the dissolution of industrial waste could significantly reduce processing time and environmental impact. Additionally, MW processing allows the use of green solvents such as water and Cyrene™.

Introduced in 2014, Cyrene™ (dihydrolevoglucosenone) is a bio-based solvent derived from cellulose waste. Its offers a non-toxic alternative to DMF, with similar physicochemical properties [7,15]. Studies have shown its effectiveness in synthesizing organic compounds [15], metal-organic frameworks (MOFs) [16], and metal nanoparticles [17]. Notably, the solubility of organic compounds in Cyrene™ increases dramatically -up to 100-fold- when water is added, due to the formation of a reversible hydrotropic microenvironment from Cyrene™'s geminal diol [7]. This tunable polarity is expected to be even more effective under MW irradiation. However, few studies have explored Cyrene™ in MW-assisted reactions [18], and its behavior and stability under such conditions remains poorly understood. Additionally, Cyrene™ is unstable in strong acids, leading to the formation of an unidentified degradation product [7], whose role whether as a solvent or reactant is still unclear.

In this study, we developed a MW-assisted strategy to convert steel slag from basic oxygen furnaces into polymerized iron-organic complexes using Cyrene™ and terephthalic acid ($H_2BDC$). The process involves two main steps: (i) MW-assisted solubilization of iron cations in oxalic acid, followed by (ii) MW-assisted solvothermal polymerization of iron oxalate in a Cyrene™/water mixture with $H_2BDC$. To
isolate the specific role of Cyrene™, we first studied reactions using pure iron oxide, thereby eliminating potential interference from other elements present in steel slag. Unexpectedly, instead of yielding iron-terephthalate complexes, our experiments led to the formation of highly polymerized iron oxalate crystals featuring a novel crystalline structure and unprecedented crystal dimensions. These findings suggest that both H₂BDC and a degradation product of Cyrenes™ play a key role in directing specific crystal facet growth and stabilizing mixed-valence iron species.

## 2. Experimental Part

### 2.1. Synthesis

#### Synthesis of Fe-cₓwᵧ

First, 0.26 g Fe₂O₃ (ROTI®METIC 99.98 %, Carl Roth) was partially solubilized using dihydrated oxalic acid (OA, 99%, Sigma Aldrich). Specifically, 1.26 g of OA was dissolved in different amounts of Milli-Q water (Vwater = 13.7 mL, 20 mL, 33.7 mL, or 40 mL) to create acidic solutions with concentrations of 0.25 M, 0.37 M, 0.5 M, and 0.75 M. These iron oxide-containing solutions were then heated at 100°C for 35 minutes (250 W, 33 °C/min) using a Microsynth Milestone microwave oven equipped with a HPR 1000/10 rotor. After the leaching process, any residual iron oxide was removed by filtration. Afterwards, to avoid the precipitation of iron oxalate, a solution containing 0.45 g of the organic linker terephthalic acid (H₂BDC, 99%, TCI) dissolved in a given amount of CyreneMT (Dihydrolevoglucosenone, 98%, Carl Roth) was added to the Teflon pressure vessel immediately. The volume of CyreneMT (Vc) was adjusted so that the total solvent amount in the vessel equaled 40 mL (Vc = 40 mL – Vw). The Teflon vessel was then heated under MW irradiation for 10 minutes at 150°C (600 W, 33 °C/min). The generated solid was recovered using a Sigma 3-16P centrifuge (8000 rpm, 5 min) and washed with Milli-Q water and ethanol (≥ 99.8%, Sigma Aldrich). Finally, the obtained iron oxalate was dried under vacuum at 65°C overnight. The synthesized samples are referred to as Fe-cₓwᵧ, where x and y correspond to the volume ratio between CyreneMT and water, respectively (x:y = 0:1; 1:2; 1:1; 2:1; 1:0). The synthesis procedure is schematized in Figure 1.

<figure><img src="image_3.png" /><figcaption>Figure 1: Schematic representation of the applied synthesis process.</figcaption></figure>

#### Synthesis of Abs-BDC-Fe-c1w1

To identify the impact and role of H₂BDC, Fe-c1w1 was also synthesized in the absence of H₂BDC. The sample is referred to as Abs-BDC-Fe-c1w1.

#### Application of the synthesis approach to the conversion of industrial iron waste
To validate the efficiency of the strategy in converting iron oxide to iron oxalate, a steel slag (LAFE, average grain size diameter 5 µm) provided by the company Eurogranulats was used as iron precursor instead of Fe₂O₃. The steel slag, originating from a blast oxygen furnace (BOF), contains 30wt% Fe. The same synthesis approach as described for Fe-c1w1 was applied. Specifically, 0.26 g of steel slag was introduced into a Teflon pressure vessel containing 20 mL of Milli-Q water and 1.26 g of OA. The mixture was heated at 100°C for 35 minutes (250 W, 33 °C/min) using a MW oven. Subsequently, 0.45 g of H₂BDC and 20 mL CyreneMT were added to the mixture, which was then heated under MW irradiation for 10 min at 150°C (600 W, 33 °C/min). The generated solid was collected and dried under vacuum at 65°C overnight, as described above. The sample is referred to as BOF-c1w1.

## 2.2. Characterization methods

The surface morphology of the synthetized samples and the elemental distribution of Fe, O and C atoms were examined using a Zeiss Sigma 300 SEM equipped with an Oxford Instruments X-Max N SDD detector for Energy Dispersive X-ray spectroscopy (EDX). The samples were metallized with platinum. To study the impact of CyreneMT and water on the structure of Fe-oxalates, powder X-ray diffractograms (PXRD) were recorded with a Malvern Panalytical X'Pert Powder S at a scan rate of 0.033°/s. The precision in the diffraction peak position was 0.04° (2θ).

To characterize the surface chemistry of samples, infra-red attenuated total reflectance (IR-ATR) spectra were taken using a Nicolet™ iS50 NEXUS FTIR spectrometer, and X-ray photoelectron spectroscopy (XPS) analyses were carried out with an ESCALAB 250 (Thermo Electron). Al Kα (1486.6 eV) was used as a monochromatic excitation source. The analyzed surface area had a diameter of 500 µm. The spectra were calibrated according to the binding energy (BE) of the C-C bond (BE(C1s) = 284.8 eV). Peak fitting was performed with CasaXPS software using a Shirley-type background.

To understand the growth formation of iron oxalate, mass spectrometry and nuclear magnetic resonance spectroscopy were employed to study the chemical composition of synthesis solutions with a CyreneMT/water volume ratio of 1:1. An Orbitrap ID-X spectrometer with an Electro Spray Ionization (ESI) source, coupled with Vanquish LCMS and UHPLC systems (ThermoScientific), was used. In addition, these solutions were analyzed by carbon-13 and proton nuclear magnetic resonance spectroscopy (NMR¹³C, NMR¹H) to determine the presence of organic groups (more information is provided in SI).

To determine the quantity of dissolved Fe from Fe₂O₃ grains (and from BOF steel slag) at different oxalic acid concentrations, an ICP-MS Agilent 7900 equipped with an automatic sample changer Autosample ASX-520 was used. First, different oxalic acid concentrated solutions in Milli-Q water were prepared. Then, 0.26 g of Fe₂O₃ were introduced into 40 mL of each solution (C = 0.25M, 0.37M, 0.5M, 0.75M) and heated for 35 min under MW irradiation at 100°C (33 °C/min, 250 W). During this process, the dissolution of iron is inducing a yellow coloration of the initially colorless solution. Afterwards, 1.5 mL of each sample were collected, filtered with a Millipore filter (0.33 µm, Sigma Aldrich), and diluted with a 2% HCl solution (PlasmaPURE, Analytichem).
## 3. Results

### Morphology and structural analysis of Fe-cₓwᵧ

The MW-assisted reaction described in this work has allowed to successfully transform Fe₂O₃ particles into rhombohedral crystals, as shown in the SEM images (Figure 2). EDX-mapping reveals that these crystals consist of Fe, O and C, which are uniformly distributed across the crystal surface (Figure S1). This suggests the formation of a crystallized polymeric iron-organic complexes.

The results presented in Table 1 indicate that the volume ratio of Cyrene<sup>MT</sup> to water plays a critical role in determining the size and morphology of the crystals. In overall the presence of Cyrene<sup>MT</sup> promotes crystal elongation by approximately 20% while reducing crystal thickness by more than 50%, resulting in thinner and more elongated structures (Table 1). Compared to other environmentally friendly methods, such as the photo-induced formation of iron-oxalate, our approach yields crystals that are six times longer[19]. However, using Cyrene<sup>MT</sup> as the sole solvent results in the formation of large agglomerated particles with a rough surface (Figure 2C).

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<caption>Figure 2: SEM observations of (A) pristine Fe₂O₃, (B) Fe-c0w1, (C) Fe-c1w0, (D) Fe-c1w1, (E) Fe-c2w1, and (F) Fe-c1w2.</caption>

<table><caption>Table 1: Average crystal dimensions for iron oxalate analogues determined from SEM observations.</caption><thead><tr><th>Sample</th><th>Cyrene<sup>MT</sup>/water volume ratio</th><th>Crystal length (μm)</th><th>Crystal width (μm)</th><th>Morphology (SEM)</th></tr></thead><tbody><tr><td rowspan="5">Fe-c<sub>x</sub>w<sub>y</sub></td><td>0 : 1</td><td>65.6 ± 16.0 μm</td><td>12.3 ± 3.9 μm</td><td>Rhombohedral</td></tr><tr><td>1 : 0</td><td>99.8 ± 20.3μm* (* diameter)</td><td>-</td><td>Agglomerated granulates</td></tr><tr><td>1 : 1</td><td>71.5 ± 10.5μm</td><td>4.6 ± 0.4 μm</td><td>Rhombohedral</td></tr><tr><td>1 : 2</td><td>91.3 ± 9.0 μm</td><td>7.4 ± 0.8 μm</td><td>Rhombohedral</td></tr><tr><td>2 : 1</td><td>84.2 ± 15.5μm</td><td>4.9 ± 0.9μm</td><td>Rhombohedral</td></tr><tr><td>Abs-BDC-c1w1</td><td>1 : 1</td><td>61.0 ± 11.0 μm</td><td>5.6 ± 0.5 μm</td><td>Rhombohedral</td></tr></tbody></table>

Additionally, incorporating Cyrene<sup>MT</sup> into the aqueous reaction medium enhances control over both the length and width of the crystals, as shown by the distribution diagrams in Figure 3. These observations suggest that uniform crystal sizes are achieved when the volume ratio of Cyrene<sup>MT</sup>/water is 1:1.

<figure><img src="image_10.png" /><figcaption>Figure 3: Distribution of crystal dimensions (A-length and B-width) for Fe-c<sub>x</sub>w<sub>y</sub> and Abs-BDC-c1w1.</figcaption></figure>

The recorded X-ray diffraction (XRD) patterns of the synthesized Fe-c<sub>x</sub>w<sub>y</sub> samples are well-defined and intense, indicating a well crystallized structure that aligns with SEM images. A striking resemblance is observed between the experimental XRD patterns and those reported in the literature for α- and β-iron oxalate polymorphs (Figure 4A) [20,21]. Based on the simulated diffraction pattern from the ICSD database (Figure 4B), it appears that a mixed structure of both polymorphs was preferentially formed. Yet, a direct match with reported crystal structures could not be established due to presence of an atypical diffraction peak at 17.4°. This peak may correspond to the (110) facet of β-Fe(II) oxalate, although the simulated diffractogram suggesting this peak to be less intense. Alternatively, it could be attributed to the (101) facet of β-Fe(III) oxalate, though several diffraction peaks for this phase are missing. Notably, both potential facets overlap with the (202) facet of α-Fe(II) oxalate at 18.6° (Figure 4B). A weak left-shoulder is observed, which strongly aligns with the simulated diffractogram of α-Fe(II).
oxalate, potentially corresponding to the (111) facet. Moreover, a weaker diffraction peak at 18.8° is detected, which could correspond to the (202) facet (Figure 4A, B). Overall, based on the detected peaks, it is not evident whether two phases were formed or whether a new polymorph structure was created.

Nonetheless, it is clear that the addition of Cyrene<sup>MT</sup> and the organic linker H<sub>2</sub>BDC affects diffraction patterns and the resulting (undetermined) crystal phase(s). Specifically, the addition of Cyrene<sup>MT</sup> has caused a downward shift of 0.1° for diffraction peaks located between 15° and 20°, as well as a change in peak intensity. A non-linear trend in intensity is noted at different volume fractions of Cyrene<sup>MT</sup> (Figure 4C). Three main observations can be made: (i) The diffraction peak at 17.4° is absent when Cyrene<sup>MT</sup> is used as the sole solvent. (ii) The peak at 17.4° is at its weakest when the Cyrene<sup>MT</sup>-to-water volume ratio is 2:1. (iii) The presence of water helps maintain an intense diffraction peak at 17.4°. Additionally, the intensity of the right-shoulder peak at 18.8° increases concurrently. This suggests that Cyrene<sup>MT</sup> influences the growth orientation of the iron-based crystals during reaction.

Cyrene<sup>MT</sup> is not the only chemical compound which is affecting the intensity and presence of the diffraction peak around 17.4° during synthesis. As PXRD pattern prove in Figure 5A, the diffraction peak at 17.4° is caused by the presence of the organic linker, terephthalic acid. Yet, on recorded diffractogram of Fe-c1w1, no H<sub>2</sub>BDC diffraction patterns were observed. Additionally, the IR spectra of Fe-c1w1 prepared with or without H<sub>2</sub>BDC are identical, while in the fingerprint region of the IR spectrum absorption bands of H<sub>2</sub>BDC are absent. Subsequently, H<sub>2</sub>BDC is not included in the structure. Nevertheless, rhombohedral crystals are formed in both cases (Figure S2), but differing in their size and homogeneity (Table 1).
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Figure 4: X-ray diffraction patterns of (A.) as prepared Fe-cxwy samples (B.) simulated Fe-oxalate polymorphs, (C.) Calculated intensity ratios of the diffraction peaks at 17.4°, 18.6° and 1.8° for Abs-BDC-c1w1 and Fe-cxwy samples.</figcaption></figure>
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Figure 5: (A.) Comparison between obtained PXRD patterns for iron oxalate prepared without (a.) and with (b.) terephthalic acid, both with a CyreneMT/water volume ratio 1:1. (B.) Simulated diffraction pattern of terephthalic acid. (C.) Comparison between obtained IR-ATR spectra for iron oxalate prepared in without (a.) and with (b.) terephthalic acid, both with a CyreneMT/water volume ratio 1:1
Surface chemistry analysis of Fe-cₓwᵧ

IR-ATR and XPS analyses were conducted on prepared Fe-cₓwᵧ samples to examine their surface chemistry. The obtained IR-ATR spectra (Figure 6A) show that the footprint region of the iron-organic polymerized structure changes depending on the utilized Cyrene<sup>MT</sup>/water volume ratio during synthesis. The IR spectra of Fe-c0w1 and Fe-c1w0 are very similar to the one of recovered Fe<sub>2</sub>O<sub>3</sub> after demineralization. The footprint of oxalic acid on iron is detected at 1674, 1423, and 1279 cm<sup>-1</sup>, corresponding to v(COOH), v<sub>s</sub>(C-O)+v<sub>s</sub>(C-C) and v<sub>as</sub>(C-O)+v<sub>as</sub>(C-C), respectively. Additional absorption bands appear strongly at 1585, 1508 and around 1113 cm<sup>-1</sup> for Fe-c0w1 and intensify further when only Cyrene<sup>MT</sup> is used as a solvent. Interestingly, when Cyrene<sup>MT</sup> and water are mixed, we observe the appearance of v(OH) at 3321 cm<sup>-1</sup> and two additional absorption bands at 1360 cm<sup>-1</sup> and 1313 cm<sup>-1</sup> ascribed to v(CO). These three bands, previously observed in vibrational spectroscopic studies of Fe(II)- and Fe(III)-oxalates [22], increase in intensity with higher Cyrene<sup>MT</sup>/water volume ratios (Figure 6B).

<figure><img src="image_17.png" /><figcaption>Figure 6: Generated IR-ATR spectra of Fe-cₓwᵧ samples. (A.) Comparison of the chemical surface of Fe-cxwy when synthesized with Cyrene<sup>MT</sup> (a.) and water (b.), and the spectrum (c.) of Fe<sub>2</sub>O<sub>3</sub> recovered after MW-assisted demineralization of Fe at 100°C for 35 min (250W). (B.) Impact of Cyrene<sup>MT</sup>/water volume ratio used during synthesis on the surface chemistry of Fe-cxwy samples.</figcaption></figure>

In concordance, XPS analysis of Fe-cxwy samples (Figure S3, Figure S4) shows the presence of C-O, C=O, and O-C=O bonds, visible after deconvolution in C1s region at 287.4 eV, 288.9 eV, and 288.8 eV respectively, and in the O1s region at 532.6 eV (C-O) and 533.7 eV (O-C=O) [23]. Additionally, significant variations in the band area for the C-C and O-C=O bonds in the C1s region, detected at 284.8 eV and 289 eV respectively, were observed. The calculated molar ratio between these two bonds is the highest when the Cyrene<sup>MT</sup>/water ratio is 2:1 (Table 2). Moreover, the determined ratio between ferrous and ferric iron is also differing depending on the utilized solvent ratio. The presence of the two different oxidation states of iron, Fe(II) and Fe(III), is observable around 709 eV and 713.0 eV, corresponding to 2p3/2 of Fe(II) and Fe(III), respectively, as well as the large satellite bands of Fe(II) detected around 718 eV (2p3/2) and 728 eV (2p1/2) [24,25]. When the Cyrene<sup>MT</sup> volume fraction is either inferior or superior than 50%, similar Fe<sup>3+</sup>/Fe<sup>2+</sup> ratios are found in the prepared iron oxalate. At 50%, the iron oxalate is rich in ferrous ions. Thus, it can be concluded that Cyrene<sup>MT</sup> and water enable the modulation of Fe(III)/Fe(II) amount in Fe-cₓwᵧ.
<table><caption>Table 2: Molar percentage of C-C and O-C=O bonds as well as Ferrous and ferric ions determined via XPS analysis and their respective calculated ratio.</caption><thead><tr><th>Cyrene<sup>MT</sup>/Water volume ratio</th><th>%mol C-C</th><th>%mol O-C=O</th><th>Ratio (O-C=O/C-C)</th><th>Fe<sup>2+</sup> (709 eV)</th><th>Fe<sup>3+</sup> (713 eV)</th><th>Ratio Fe<sup>3+</sup>/Fe<sup>2+</sup></th></tr></thead><tbody><tr><td>2:1</td><td>29.51</td><td>40.84</td><td>1.38</td><td>27.20</td><td>8.49</td><td>0.31</td></tr><tr><td>1:1</td><td>57.20</td><td>20.96</td><td>0.37</td><td>17.69</td><td>23.59</td><td>1.33</td></tr><tr><td>1:2</td><td>36.99</td><td>15.86</td><td>0.43</td><td>28.16</td><td>11.67</td><td>0.41</td></tr><tr><td>0:1</td><td>67.31</td><td>13.51</td><td>0.20</td><td>36.05</td><td>9.30</td><td>0.26</td></tr><tr><td>1:0</td><td>65.26</td><td>11.66</td><td>0.18</td><td>33.57</td><td>12.67</td><td>0.38</td></tr></tbody></table>

## Chemical stability of Cyrene<sup>MT</sup> in oxalic acid and its reactivity

Since Cyrene<sup>MT</sup> is a newly commercialized green solvent, its properties are not well documented. However, its instability in highly acidic and basic media is well known, though the products formed under these conditions have not been identified. In order to improve our understanding of the reaction, it is crucial to establish whether Cyrene<sup>MT</sup> remains stable in the presence of oxalic acid (OA) when added during the reaction. If it is not stable, it is important to identify the derived chemical compound which acts as the actual solvent.

Herein NMR of proton and carbon-13 were carried out on a recovered solution medium containing a 0.5 M OA dissolved in water and heated at 150°C for 35 min in Cyrene<sup>MT</sup> (Figure 7). This solution is a replica of the reaction medium in the second step without the presence of iron oxide. This solution, referred to as Cyrene<sup>MT</sup>+OA was compared with the NMR spectrum of Cyrene<sup>MT</sup>. The NMR<sup>13</sup>C and NMR<sup>1</sup>H of Cyrene<sup>MT</sup> show the presence of Cyrene<sup>MT</sup> in equilibrium with its germinal diol.

When OA is mixed with Cyrene<sup>MT</sup>, we observe the appearance of numerous additional peaks. With the help of advanced NMR sequences (i.e., DOSY, COSY) we determined that the epoxide bridge opens to form a six-membered ring, while the ketone transforms into an α,β-unsaturated aldehyde, clearly visible at δ<sub>H</sub> = 9.2 ppm and δ<sub>C</sub> = 189.8 ppm. Due to the presence of an α,β unsaturated organic group and an alcohol group on the derived product from Cyrene<sup>MT</sup> (called 3-hydroxy-3,4-dihydro-2H pyran-6-carbaldehyde, abbreviated HDPC), the reactivity with H<sub>2</sub>BDC can be favored. This can occur firstly through an acid-base reaction between the alcohol and the carboxylic acid, and secondly, the formed deprotonated terephthalate can react with the β-carbon of HDPC. Either way the formed molecule as well as HDPC are able to intervene in the formation of a complex with Fe. Besides, it was also demonstrated by Hernández-Pagánet *et al.* that Cyrene<sup>MT</sup> can act as both a solvent and a reducing agent for the synthesis metallic particles due to the formation of an aldehyde [17]. The undetermined structure corresponds to the elucidated structure of HDPC. The formation of a mixed Fe(III)/Fe(II) compound could potentially have been favored by the presence of HDPC.
<figure><img src="image_18.png" /></figure>

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<figure><img src="image_21.png" /><figcaption>Figure 7: NMR<sup>1</sup>H and NMR<sup>13</sup>C spectra of Cyrene<sup>MT</sup> in the presence (A, C) and absence of OA (B, D) showing the formation of HDPC.</figcaption></figure>
4. Discussion

Effect of microwave heating on the conversion reaction

In this work, we present a green conversion approach that first involves the extraction of Fe cations from Fe₂O₃ using a concentrated aqueous solution of oxalic acid (OA), followed by their transformation into long iron oxalate crystals. By applying MW heating, the process is time- and energy-efficient for both steps. This method avoids the use of strong, environmentally hazardous concentrated acids like H₂SO₄ or HCl, which are typically employed in the leaching process of Fe from iron-containing materials [5,26]. The typical amount of dissolved Fe from Fe₂O₃ after 35 minutes under MW irradiation at 100°C is shown in Figure 8, as a function of OA concentration. This MW-assisted leaching process is considerably more efficient than conventional heating, exhibiting, on average 20% lower iron dissolution rates at 35 minutes [26].

<figure><img src="image_22.png" /><figcaption>Figure 8: Amount (wt. %) of dissolved Fe from Fe₂O₃ as a function of oxalic acid (OA) concentration, after 35 minutes under MW irradiation at 100°C (250W).</figcaption></figure>

The implementation of MW irradiation during synthesis impacts the growth kinetics of iron oxalate crystals and the nature of the formed polymorph. This observation is consistent with other MW-assisted synthesis approaches of different chemical compounds where a clear impact on crystal growth was observed [6,9]. Most synthesis procedures of iron oxalate reported in the literature follow the preparation method reported by Angermann and Töpfer, which consists in precipitating iron chloride salt with iron oxalate under continuous stirring for 1.5 hours to form β-iron oxalate. To generate α-Fe oxalate, the precipitation solution needs to be heated to 90°C and then cooled down for 16 hours [21]. Other procedures using solvothermal approaches under conventional heating have demonstrated to be effective in altering the morphology of α-Fe oxalate and in producing mixed α/β phases [27].

Yet, none of the previous methods have generated such long crystals, with an average length of 65.6 ± 16.0 µm in water. This indicates that MW irradiation enhances crystal growth. When CyreneMT is added, even longer crystals are formed. However, using CyreneMT as the sole solvent results in the formation of microaggregates which is consistent with reported observations of nanoparticle synthesis [17]. The aggregation of nanoparticles into large microaggregates can be explained by the similar thermal behavior of CyreneMT to ethylene glycol [15]. Thus, it can be inferred that, similarly to ethylene glycol, CyreneMT acts as a highly reactive medium under MW irradiation, favoring particle aggregation over crystal growth.
Additionally, we determined that Cyrene<sup>MT</sup> is not stable in OA and forms HDPC, a protic, polar molecule composed of an ether, aldehyde and alcohol group, as evidenced by NMR (Figure 7). During the reaction, HDPC can react with H<sub>2</sub>BDC and polymerize, potentially inducing the aggregation of iron oxalate complexes into microaggregates. However, when Cyrene<sup>MT</sup> is mixed with water, these aggregates disappear, resulting in higher rhombohedral crystal yields. This can be explained by the existence of two liquid phases —organic and aqueous— forming a microenvironment which allows to tune the polarity of the solvent mixture and subsequently control solubilization properties of Cyrene<sup>MT</sup>, as demonstrated by De Bruyn *et al.* [7]. Thus, certain compounds will preferentially dissolve in one phase over the other. For instance, Hernández-Pagánet *et al.* established that inorganic metal salts are insoluble in Cyrene<sup>MT</sup> contrarily to water [17]. Conversely, terephthalic acid is highly soluble in organic solvents as HDPC/Cyrene<sup>MT</sup>. The formation of a liquid interphase with different concentration gradients creates a polarized liquid/liquid interface promoting reaction energy efficiency and crystal growth of iron oxalate.

## Influence of terephthalic acid on the structure of Fe-oxalate

Terephthalic acid is usually known for its chelating behavior and is often used as an organic linker for MOF synthesis [6,8]. In this work, we observe that H<sub>2</sub>BDC can also act as a directing agent. By comparing the obtained crystals sizes of Fe-c1w1 and its counterpart prepared without H<sub>2</sub>BDC (Abs-BDC-c1w1), we found rhombohedral crystals differing significantly in their length (Δ = 17%) and in their thickness (Δ = 22%) (Table 1). For Abs-BDC-c1w1, the average crystal length is around 61.0 ± 11.0 µm. This value is identical to the crystal length of Fe-c0w1, for which only water was used as a solvent in the presence of iron cations, oxalic acid and H<sub>2</sub>BDC. Subsequently, we conclude that H<sub>2</sub>BDC promotes the growth in the length of iron oxalate crystals. Moreover, with a thickness difference of 22% between Fe-c1w1 and Abs-BCD-c1w1, we can also conclude that H<sub>2</sub>BDC helps in preventing the growth of thick crystals. Distribution diagrams of crystal length and width are evidencing also that in the presence of H<sub>2</sub>BDC, the control over length growth is significantly enhanced, leading to the formation of uniform crystal sizes (Figure 3). The effect on crystal width is less pronounced, making H<sub>2</sub>BDC control over thickness less distinctive.

H<sub>2</sub>BDC affects also the crystal structure and phase nature of synthetized iron oxalate, as evidenced by the notable absence of the diffraction peak at 17.4° (Figure 5). In the absence of this peak, assuming the co-existence of an α/β mixed phase, pure α-Fe oxalate is formed with large exposed (111)α facets, smaller exposed (202)α facets and absent (200)α facets. Comparing the intensity ratio between (111)α facets and (202)α facets at 18.6° and 18.8° respectively, between Fe-c1w1 and Abs-BDC-c1w1, H<sub>2</sub>BDC appears to favor the growth of (111)α facets over (202)α facets by 15% (Figure 4C). This observation concords with the 17% increase in crystal length in the presence of H<sub>2</sub>BDC. Subsequently, it is highly probable that H<sub>2</sub>BDC promotes the homogeneous growth of (111)α facets of the α-polymorph, resulting in longer crystals. These observations are illustrated in Figure 9.
<figure><img src="image_23.png" /><figcaption>Figure 9: Schematic summary illustrating the effect of terephthalic acid and Cyrene<sup>MT</sup> addition (translated by its derived compounds HDPC) on the growth of iron oxalate crystals.</figcaption></figure>

## Effect of Cyrene<sup>MT</sup> addition on the structure of Fe-oxalate

- **Effect of Cyrene<sup>MT</sup> as the sole solvent**

When Cyrene<sup>MT</sup> is used as the sole solvent during both synthesis steps (leaching of iron and crystal growth) a change in morphology of iron oxalate crystals is observed. Large, agglomerated particles with rough surface are formed. These particles, averaging 100 µm in size, exhibit a different diffraction pattern compared to Fe-c0w1, where water was the sole solvent. The main difference is the absence of diffraction peaks in between 18° and 19°. Based on simulated diffraction peaks of terephthalic acid and α/β-iron oxalate, no distinctive phase match could be distinguished. However, a noticeable resemblance between the diffractogram of Fe-c1w0 and those of terephthalic acid and β-iron oxalate was observed. Specifically, the diffraction peak at 17.4° appears to correspond to the (110) facet of the β polymorph. Given the particular morphology of the nanoparticles, it is highly plausible that the (200) facet, detectable around 18.3° is nearly non-existent and thus not visible in PXRD. XPS and IR spectra do not show any conclusive differences between iron oxalate prepared in water or Cyrene<sup>MT</sup>. Therefore, from a chemical perspective, Fe-c1w0 and Fe-c0w1 are equivalent but differ significantly in their crystalline structure, facet exposure, and morphology. The observed agglomeration of nanoparticles in the presence of Cyrene<sup>MT</sup> has been reported in other studies for the synthesis of metal and polymer nanoparticles, and is subsequently not surprising for Fe-c1w0 [17].

- **Effect of Cyrene<sup>MT</sup> mixed with water at various volume ratios**

When Cyrene<sup>MT</sup> is mixed with water, large, well-defined rhombohedral iron oxalate crystals are formed. These crystals are significantly longer and thinner than those prepared in pure water. By varying the volume ratio of Cyrene<sup>MT</sup>-to-water, the length and thickness of the crystals can be adjusted. We note that the amount of Cyrene<sup>MT</sup> relative to water does not significantly affect the crystal length, as the values remain approximately within the same range. However, it has a clear impact on the crystal thickness: the lower the amount of Cyrene<sup>MT</sup> is during synthesis, the thicker are crystals. This observation is consistent with the fact that thick crystals are synthesized in pure water. Additionally,
in the presence of a mixed solvent solution, the control over both length and thickness is significantly
enhanced compared to Fe-c0w1.

The volume fraction of Cyrene<sup>MT</sup> during synthesis also impacts the structure and facet exposure of iron oxalate. Figure 4C shows a clear variation in the intensity ratios of the main exposed facets (<math><mrow><mtext>(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;/(110)</mtext></mrow><mrow><mtext>&beta;</mtext></mrow></math>) and (<math><mrow><mtext>(20&#x0304;2)</mtext></mrow><mrow><mtext>&alpha;/(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>). Notably, the ratio of (<math><mrow><mtext>(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;/(110)</mtext></mrow><mrow><mtext>&beta;</mtext></mrow></math>) is significantly high when the Cyrene<sup>MT</sup>/water volume ratio is 2:1. This suggests that a 67% volume fraction of Cyrene<sup>MT</sup> favors the growth of (<math><mrow><mtext>(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>) facet and reduces the facet area of (<math><mrow><mtext>(110)</mtext></mrow><mrow><mtext>&beta;</mtext></mrow></math>). Moreover, as for Abs-BDC-c1w1 the diffraction peak at 17.4° on the diffractogram of Fe-c2w1 is absent. This indicates that the <math><mrow><mtext>&beta;-Fe</mtext></mrow><mrow><mtext>oxalate</mtext></mrow></math> component is nearly absent, and a major <math><mrow><mtext>&alpha;-Fe</mtext></mrow><mrow><mtext>oxalate</mtext></mrow></math> phase is present in Fe-c2w1. In regard to the impact of the volume ratios between Cyrene<sup>MT</sup> and water on the intensity ratio between (<math><mrow><mtext>(20&#x0304;2)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>) and (<math><mrow><mtext>(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>), a similar observation is made. The (<math><mrow><mtext>(20&#x0304;2)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>) facet is highly exposed compared to (<math><mrow><mtext>(11&#x0304;1)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>) when Cyrenes<sup>MT</sup> volume fraction equals 67%. When its volume fraction is lower, the diffraction peak intensity ratios are similar to samples prepared solely in water. Consequently, it can be concluded that the addition of Cyrene<sup>MT</sup> to water, as H<sub>2</sub>BDC, directs the growth of iron oxalate by favoring the growth of specific crystal facets, particularly (<math><mrow><mtext>(20&#x0304;2)</mtext></mrow><mrow><mtext>&alpha;</mtext></mrow></math>) (Figure 9).

In addition to its impact on crystal structure, the chemical surface composition of Fe-cxwy is also affected when Cyrene<sup>MT</sup> and water are used simultaneously as solvents. On IR spectra we observe that the absorption bands at 1316 cm<sup>-1</sup> and 1360 cm<sup>-1</sup> increase in intensity with higher Cyrene<sup>MT</sup> ratios. It should be emphasized, that the presence of mixed-valence iron oxalate could also explain the unique diffraction pattern and the different colored Fe-cxwy samples. Indeed, Ahouari *et al.* have determined that Fe(III)- humboldtine oxalate (yellow) crystalizes differently from Fe(II)-oxalate (green) [28], having a triclinic unit cell (Figure 10). The integration of ferric ions into the iron oxalate structure could distort the unit cell and explain the diffraction peak below 18°. This hypothesis is supported by the fact that similar diffraction patterns were obtained when Fe<sub>3</sub>O<sub>4</sub> was used as the iron source for iron oxalate formation in the presence of Cyrene<sup>MT</sup> and water (1:1) (cf. SI, FigureS8, Figure S9). Fe<sub>3</sub>O<sub>4</sub> contains Fe(II) and Fe(III). Once converted to iron oxalate, a green powder containing both Fe(II) and Fe(III) is formed, as confirmed by XPS (Figure S8).

<figure><img src="image_24.png" /><figcaption>Figure 10: Unit cell structures of (A.) Fe(III)-oxalate, (B.) β-ferrous (II) oxalate, (C.) α-ferrous (II) oxalate.</figcaption></figure>
## 5. Application: valorization of steel slags

The green process developed in this work for converting iron oxides into iron oxalate crystals aims to valorize iron-rich industrial residues. To validate the protocol's applicability to industrial wastes, steel slag generated from a blast oxygen furnace (BOF) was used as a precursor for iron cations. Since a Cyrene<sup>MT</sup>/water ratio of 1:1 was found to be the most efficient in obtaining uniform iron oxalate crystals with high yields, we applied the same synthesis protocol as for the sample Fe-c1w1.

EDX analysis revealed the presence of various chemical elements in BOF steel slag (Figure 11A), with iron being the most abundant at 25 wt.%, followed by high a concentration of Ca (17 wt.%) and Al (5 wt.%). When BOF steel slag was treated with MW heating in 0.5 M oxalic acid, iron and aluminum were selectively dissolved in majority (Figure 11B). The high concentration of dissolved iron allowed it to complex with oxalic acid, leading to the formation of iron oxalate, which polymerized into large, square-shaped crystals exhibiting different sizes (Figure 12A).

<figure><img src="image_25.png" /><figcaption>Figure 11: Quantification of chemical elements (A.) by EDX (wt.%) in BOF steel slag. (B.) by ICP-MS in the 0.5M OA solution after 35 min MW heating at 100°C (250W).</figcaption></figure>

As shown by EDX mapping (Figure 12B), the crystals are composed of C, O, and Fe, forming the fundamental skeleton of iron oxalate. We also observe the agglomerated presence of calcium, which forms specific square-shaped crystals, suggesting different facet exposures. The PXRD pattern of BOF-c1w1 indicates that the used steel slag as iron source is no longer visible after the reaction, meaning it was completely transformed in the presence of OA, H₂BDC, water and Cyrene<sup>MT</sup> (Figure 13). The characteristic diffraction peaks of Fe-c1w1 are detected on the BOF-c1w1 diffractogram. Besides, additional diffraction peaks between 15° to 15.5° are visible, likely due to the introduction of Ca into the structure as a dopant. This assumption is supported by the similarity of the diffraction pattern to those of Li-doped iron oxalate [14].

The UV-Vis spectrum of this compound show several absorption bands between 200 and 600 nm, indicating that the material is photo-absorbent (Figure S10). Using Tauc-plot, two significant energy gaps were determined at 1.85 eV and 3.45 eV. Based on these results, we can conclude that the developed green procedure is effective in transforming steel slag, an industrial residue, into an advanced material exhibiting an interesting structure and chemical characteristics.
<figure><img src="image_26.png" /><figcaption>Figure 12: SEM images (A) and EDX mapping (B) of BOF-c1w1 sample.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>Figure 13: PXRD patterns of steel slag (BOF) converted to iron oxalate analogue (BOF-c1w1), compared to Fe-c1w1 obtained from Fe₂O₃.</figcaption></figure>

## 6. Conclusion

In this work, we have developed a new green procedure to transform iron-rich materials, such as iron oxides and steel slag into mixed Fe(II)/Fe(III) iron oxalate complexes within an hour. The synthesis approach avoids the use of iron salts and hazardous acids, while promoting the use of industrial residues with little commercial value. Additionally, it economizes time and energy and utilizes green solvents as water and Cyrene<sup>MT</sup>, a bio-derived solvent produced from plant waste. The strategy has enabled the formation of mixed-valance iron oxalate with large crystal sizes in between 71.5 and 91.3 µm depending on the volume ratio between both solvents. Indeed, the results show that at a Cyrene<sup>MT</sup>/water volume ratio of 1:1 high conversion yields with uniform crystal sizes and chemical composition are achieved. The successful application of this strategy to steel slag originating from basic oxygen furnace demonstrates its feasibility for other iron-rich residues.

The obtained iron oxalate from steel waste is photoactive and retains the crystalline structure of iron oxalate crystals generated from Fe₂O₃. However, the exact structure of the iron oxalate is still under investigation. Further structural analysis is being conducted to elucidate whether the α and β polymorphs were formed, leading to a stacked α/β plane structure, or whether the observed contribution of β-Fe oxalate is due to Fe(III) causing a distortion in the unit cell.

Although the precise structure of the resulting iron oxalate remains to be fully elucidated, our findings highlight the significant influence of H₂BDC and HDPC—the degradation product of Cyrene™ under strongly acidic conditions, identified here for the first time—on crystal orientation and size. Importantly, the degradation of Cyrene™ does not hinder the reaction. On the contrary, HDPC
continues to function effectively as both a solvent and a modulator. Thus, the inherent instability of Cyrene™ in acidic media can be harnessed to control crystal growth and stabilize mixed-valence iron species. Building on these insights, we aim to extend this strategy to other metal-rich industrial wastes (i.e. precious metals or rare earths), to synthesize high-value metal-organic complexes for applications in environmental remediation and for the energy sector.

## Author Contributions

Conceptualization and methodology, L.B.N.; validation, A.J.

Investigation, L.B.N., A.L., E.P.; data curation, L.B.N.;

Writing—original draft preparation, L.B.N.; Review and editing, L.B.N. and A.J.

Supervision, project administration and funding acquisition, A.J.

All authors have read and agreed on the published version of the manuscript. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This research was funded by the French National Research Agency (ANR project PEHY-0016-BHyoLOHC). The authors thank Bertrand Rebiere (UAR Plateforme d'Analyses et Caractérisations (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM) for SEM-EDX analyses.

## Additional information

Supplementary material is supplied with this manuscript.

## Data availability

Additional data will be made available on request.

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# Supplementary Information

## From slag to structure: Formation of novel iron oxalate crystals via Cyrene™-driven microwave chemistry

Limor Ben Neon<sup>a</sup>, Aurelien Lebrun<sup>b</sup>, Eddy Petit<sup>a,b</sup>, Anne Julbe<sup>a,*</sup>

a. Institut Européen des Membranes (IEM), Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

b. UAR Plateforme d'Analyses et Caractérisations (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

## Additional experimental information

### NMR analysis

NMR spectra were recorded at 298 K on a Bruker Avance III 600 MHz NMR spectrometer, using TCI Prodigy® Nitrogen Cryoprobe. 13C UDEFT sequence was employed with a spectral width of 36 000 Hz and 512 scans. 2D homonuclear spectra ¹H-¹H g-COSY spectra were recorded using a data matrix of 800 real (t1) × 2048 (t2) complex data points, with 8 scans per t1 increment and a spectral width of 6000 Hz in both dimensions. In addition, 2D heteronuclear ¹³C-¹H g-edited HSQC and ¹³C-¹H g-HMBC spectra were acquired for compound assignment (16-48 scans, 512 real (t1) × 2048 (t2) complex data points). Chemical shift data are reported in δ ppm, calibrated with residual protic solvent (e.g., D₂O: 4.79 ppm). Spectra were processed and visualized using Topspin 4.4.0 (Bruker Biospin) on a Windows 11 station. D₂O was purchased from Innovachem, France.

<footnote>* Contacting Author</footnote>
# Additional characterisation data

## SEM/EDX analysis

<figure><img src="image_28.png" /><figcaption>Figure S1: EDX-mapping showing the elemental distribution of Fe, O and C for Fe-cₓwᵧ samples, prepared with various Cyrene<sup>MT</sup>/water volume ratios.</figcaption></figure>

<table><caption>Table S1: Molar percentage of C, Fe and O determined via EDX for Fe-cₓwᵧ samples.</caption><thead><tr><th>Reference</th><th>C</th><th>O</th><th>Fe</th><th>C/Fe</th><th>O/Fe</th></tr></thead><tbody><tr><td>Fe-c1w1</td><td>29.5</td><td>62.5</td><td>7.9</td><td>3.7</td><td>7.9</td></tr><tr><td>Fe-c0w1</td><td>37.5</td><td>66.8</td><td>19.0*</td><td>2.0</td><td>3.5</td></tr><tr><td>Fe-c2w1</td><td>31.9</td><td>60.8</td><td>7.3</td><td>4.4</td><td>8.4</td></tr><tr><td>Fe-c1w2</td><td>32.9</td><td>56.4</td><td>10.7</td><td>3.1</td><td>5.3</td></tr><tr><td>Fe-c1w0</td><td>24.0</td><td>47.4</td><td>28.5*</td><td>0.8</td><td>1.7</td></tr></tbody></table>
<figure><img src="image_29.png" /><figcaption>Figure S2: SEM images of iron oxalate analogue derived from Fe₂O₃ with (A.) and without (B.) terephthalic acid, and with equal volume ratio (1:1) of CyreneMT and water.</figcaption></figure>

XPS analysis

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>Figure S3: XPS deconvolution spectra of Fe-cxwy iron oxalate samples prepared from Fe₂O₃ in water and CyreneMT exclusively.</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>Fe-c1w1</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>Fe-c1w2</figcaption></figure>

<figure><img src="image_34.png" /></figure>

Figure S4: XPS deconvolution spectra of Fe-cxwy iron oxalate samples prepared from Fe₂O₃ in CyreneMT/water mixture at various volume ratios (2:1, 1:1; 1:2).
Infra-Red spectroscopy

<figure><img src="image_35.png" /><figcaption>Figure S5: IR-ATR spectra of Cyrene<sup>MT</sup>, Cyrene<sup>MT</sup> heated with oxalic acid (OA) at 150°C for 10 min using MW irradiation, and a dissolved solution of terephthalic acid (H<sub>2</sub>BDC) in Cyrene<sup>MT</sup>.</figcaption></figure>

Mass spectrometry

<figure><img src="image_36.png" /><figcaption>Figure S6: Mass spectrometry sequencing of the derived form of Cyrene<sup>MT</sup> in the presence of oxalic acid (OA), labeled HDPC.</figcaption></figure>
Conversion of Fe<sub>3</sub>O<sub>4</sub> to iron oxalate

The same synthesis approach as described for Fe-c1w1 was applied. Briefly, 0.26 g of Fe<sub>3</sub>O<sub>4</sub> (Sigma Aldrich) were introduced into a Teflon pressure vessel containing 20 mL of Milli-Q water and 1.26 g of oxalic acid (OA). The mixture was heated at 100°C for 35 min (250 W, 33 °C/min) using a microwave (MW) oven. Afterwards, 0.45 g of H<sub>2</sub>BDC and 20 mL Cyrene<sup>MT</sup> were added to the mixture and heated under MW irradiation for 10 min at 150°C (600 W, 33 °C/min). The resulting solid was collected and dried under vacuum at 65°C overnight, yielding a green powder.

<figure><img src="image_37.png" /><figcaption>Figure S7: SEM observations of Fe<sub>3</sub>O<sub>4</sub> (A.) and the derived iron oxalate crystals (B.) obtained by MW-assisted conversion of A. in an equal mixture of Cyrene<sup>MT</sup>/water (1:1) and in the presence of H<sub>2</sub>BDC.</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>Figure S8: XPS deconvolution spectra of the O1s, C1s, Fe2p regions on green iron oxalate powder obtained from Fe<sub>3</sub>O<sub>4</sub>.</figcaption></figure>
<figure><img src="image_39.png" /><figcaption>Figure S9: PXRD patterns (A.) and ATR-IR spectrum (B.) of iron oxalate obtained from Fe₃O₄.</figcaption></figure>

UV-Vis analysis was carried out on BOF-c1w1 with a UV-Visible-PIR CARY 5000 (Agilent).

<figure><img src="image_40.png" /><figcaption>Figure S10: Kubelka-Munk spectrum of BOF-c1w1, along with its Tauc-plot, highlighting notable adsorption bands between 200 nm and 600 nm, and two significant energy bands at 1.85 eV and 3.45 eV.</figcaption></figure>