# Monitoring the hydrothermal carbonization of biomass derived compounds by in-situ high-temperature-high-pressure Raman spectroscopy

Alejandra Palomino, Aneta Slodczyk, Aurélien Canizarès, Rémi Champallier, Stéphane Bostyn, Encarnacion Raymundo-Piñero

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Alejandra Palomino, Aneta Slodczyk, Aurélien Canizarès, Rémi Champallier, Stéphane Bostyn, et al. Monitoring the hydrothermal carbonization of biomass derived compounds by in-situ high-temperature-high-pressure Raman spectroscopy. Carbon, 2026, 251, pp.121360. 10.1016/j.carbon.2026.121360. hal-05528655

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# Monitoring the hydrothermal carbonization of biomass derived compounds by in-situ high-temperature-high-pressure Raman spectroscopy

Alejandra Palomino<sup>a</sup>, Aneta Slodczyk<sup>b,c,*</sup> <sup>ID</sup>, Aurélien Canizarès<sup>c</sup>, Rémi Champallier<sup>b</sup>, Stéphane Bostyn<sup>a</sup>, Encarnacion Raymundo-Piñero<sup>c,**</sup> <sup>ID</sup>

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## ARTICLE INFO

### Keywords:

*In-situ* Raman spectroscopy

Transparent-internally heated pressure vessel

Hydrothermal carbonization

Glucose

Xylose

## ABSTRACT

Hydrothermal carbonization (HTC) is an energy efficient and sustainable method for converting biomass into carbon-based materials at low temperatures. However, the specific reaction conditions, involving the heating of feedstock in a confined environment under autogenous pressure, pose significant challenges for *in-situ* analytical techniques. To date, kinetic studies of HTC have primarily relied on *ex-situ* chemical analysis of samples collected during or after HTC runs. This study presents a novel approach utilizing continuous and rapid *in-situ* Raman spectroscopy to investigate the reaction progress in aqueous media, shedding light on the reaction pathways and kinetics of hydrothermal reactions. A custom-designed heated pressure vessel equipped with sapphire windows enabled the *in-situ* Raman monitoring of hydrothermal carbonization of biomass derivatives, such as glucose and xylose, under various experimental conditions including different heating rates and pressures. The results were compared with Raman data obtained *ex-situ* from a classical batch reactor at different reaction times. The *in-situ* measurements provided valuable insights into the composition of intermediates and products in both the liquid and solid phases simultaneously, yielding information that cannot be obtained through *ex-situ* analysis, such as the temperature of sugar decomposition and carbon precipitation depending on the nature of the sugar, the heating rate or the pressure.

## 1. Introduction

Biomass is a sustainable and renewable resource with the potential to transform the production of high-value products, ranging from platform chemicals such as 5-HMF, alcohols and bio-based polymers to biofuels and advanced carbon materials for energy storage or environmental remediation [1,2].

Hydrothermal carbonization (HTC) has gained attention in recent decades as a sustainable and environmentally friendly method for converting biomass into carbon-based materials [3]. This process can be applied to various feedstocks, including lignocellulosic biomass, which consists mainly of cellulose, hemicellulose, and lignin. During HTC, the biomass is heated in subcritical water at temperatures of around 150-350 °C and autogenous pressure. The reaction mechanisms of biomass conversion to carbon have been studied over the last century due to their complexity and the variety of reactions that take place,

including hydrolysis, dehydration, polymerization, condensation and etherification [4-6]. Under HTC conditions, cellulose and hemicellulose generally decompose by hydrolysis to produce hexoses (e.g. glucose and fructose) and xylose, respectively. This hydrolysis is promoted under acidic conditions. These conditions are produced by the evolution of the water ion product constant within the HTC temperature range [7]. The hexoses undergo further chemical reactions, such as dehydration, to produce furfurals and other compounds. Lignin, which is more stable than cellulose and hemicellulose under HTC conditions, decomposes into phenolic compounds during hydrolysis [8,9]. These compounds, including furfurals and phenolic compounds, can be transformed into intermediate compounds, such as dimers or carboxylic acids. These can then undergo polymerization and condensation reactions to produce carbon, also known as secondary carbon or hydrochar. These degradation products, which will undergo further transformation to produce hydrochar, can be produced in parallel reactions directly from hexoses

<footnote><sup>a</sup> ICARE, CNRS, Université d'Orléans, Orléans, France</footnote>

<footnote><sup>b</sup> Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, Univ Orléans-CNRS-BRGM-OSUC, F-45071, Orléans, France</footnote>

<footnote><sup>c</sup> CNRS, Université d'Orléans, CEMHTI UPR3079, Orléans, France</footnote>

<footnote>* Corresponding author. Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, Université d'Orléans-CNRS-BRGM-OSUC, F-45071, Orléans, France.</footnote>

<footnote>** Corresponding author. CNRS, Univ Orléans, CEMHTI UPR 3079, Orléans, France</footnote>

<footnote>E-mail addresses: aneta.slodczyk@cnrs-orleans.fr (A. Slodczyk), raymundo@cnrs-orleans.fr (E. Raymundo-Piñero).</footnote>
[10,11].

To optimize the HTC process in terms of carbon yield and energetic
consumption, studies have focused on identifying the different reactions
and following the different compounds as a function of reaction time, in
order to determine the Arrhenius kinetic parameters associated with
these chemical transformations [10,12]. Studies of the kinetics of HTC
have mainly relied on ex-situ product chemical analysis of samples taken
during or after HTC runs.

To measure the concentration of species as a function of residence
time different types of reactor has been used as tubular reactors [13],
batch reactors immersed in a salt bath and then rapidly cooled by im-
mersion in a water bath [9] and stirred reactors [10,11]. Furthermore,
the thermal gradients during the heating and cooling phases depend on
the thermal inertia of the reactors in question. Sampling times in a ki-
netic study depend not only on the constraints of the technology used,
but also on the analytical technique employed.

High Performance Liquid Chromatography (HPLC) is considered as
the reference analytical technique for following HTC reactions mecha-
nisms and kinetics in the liquid phase [14]. However, this conventional
approach requires sampling device from the batch reactor and samples
preparation before analyzing them (filtration, dilution, internal stan-
dard, or even a reagent addition). When analyzing the reaction kinetics
of biomass HTC by removing small amounts of liquid from a batch
reactor at different temperatures, several specific limitations and chal-
lenges emerge. Firstly, removing liquid can induce local cooling, which
may alter reaction rates and product distribution. The solubility of in-
 intermediates, such as furfurals and organic acids, or products, such as
hydrochar precursors, varies at different temperatures. Removing liquid
also reduces the reactor volume, thereby increasing the concentration of
non-volatile species and potentially accelerating or inhibiting reactions
such as polymerization or condensation. Most kinetic models assume
isothermal, constant-volume conditions [15]. However, sampling dis-
rupts these assumptions, necessitating corrections or more complex
modelling. Additionally, due to the unique conditions of HTC, including
autogenous pressure, removing liquid can result in pressure drops. This
can impact the solubility of gases, reaction equilibrium (e.g. decarbox-
ylation or dehydration) and the physicochemical properties of second-
ary carbon as the diameter of carbon spheres [16].

Therefore, there is a need on developing *in-situ* techniques non-
destructive and without sample preparation steps. *In-situ* or online
spectroscopies as Infrared or Raman have been shown as valuable
methods to follow different biomass and sugar treatments for obtaining
chemical or biofuels, as hydrolysis or fermentation by using optical fi-
bers [17–23]. In particular, real-time Raman techniques have been
successfully used to monitor the treatment of biomass to propose better
treatments and maximize yields. This technique can be used to monitor
cellulose and lignin [18,19], quantify glucose and xylose [20], monitor
the fermentation of sugars into ethanol [21–23], and observe the
breakdown curves of aqueous sugar mixtures, including glucose and
xylose with or without ethanol as a co-solvent [17].

However in the particular HTC reaction conditions, which involve
heating the feedstock in a liquid in a confined environment under
autogenous pressure, make it difficult to find analytical techniques
capable of following the dehydration and carboxylation reactions *in-situ*.
There are few reports in the literature on *in-situ* analysis of hydrothermal
carbonization processes, e.g. using high-pressure differential scanning
calorimetry (DSC) [24]. In the field of HTC, Raman has been usually
used just to characterize the hydrochars [25,26].

The present study proposes a continuous and fast analytical in-situ
Raman measurements to advance in the investigations of reaction
progress in aqueous media at high pressure to study hydrothermal re-
action fundamentals. Raman spectroscopy was used to monitor the hy-
drothermal carbonization of biomass derivatives such as glucose and
xylose in-situ during the reaction using an internally heated pressure
vessel (IHPV) equipped with sapphire windows [27]. Glucose and
Xylose have been chosen as model compounds representing the main

intermediates for cellulose and hemicellulose first step, hydrolysis, of
hydrothermal carbonization. Moreover, there are other interest of
monitoring the reactions of those sugars as they are commonly trans-
formed by fermentation into alcohol, such as ethylene glycol from
glucose transformation or furfural obtained from xylose [13,28]. Raman
spectroscopy has already been established as a powerful tool for
analyzing the intermediates and products of biomass reactions [17–23].
Given our objective to demonstrate the potential of the technique for
in-situ monitoring of high-pressure reactions, we have chosen to focus
this study on the dehydration reactions of sugars and the subsequent
formation of secondary carbon. The results obtained by in-situ
high-temperature and high-pressure Raman spectroscopy are used in
conjunction with ex-situ results to gain insights into reaction compounds
and kinetics, highlighting the interest of using in-situ measurements to
obtain information that is not available through ex-situ methods.

## 2. Material and methods

### 2.1. Materials

β(+)-glucose anhydrous (Alpha Aesar, 99%) and Xylose (Sigma Aldrich, 99%) were selected as carbon source for carbonization reactions. 5-hydroxymethyl-furfural (Sigma Aldrich, 99%) and furfural (Sigma Aldrich, 98%) were used as standard.

### 2.2. HTC reactions in batch reactor

The glucose carbonization reactions were carried out in a 300 mL batch stirred reactor (Top Industrie, France). The design of the reactor and reaction procedures have been previously reported [11]. In a typical reaction, the reactor was charged with 5.6 g of glucose and 216 mL of distilled water at specific pH (2.5% total initial solids). Before starting heating, residual air inside the reactor was removed by using a vacuum pump with a pressure lower than 40 mbar. HTC was carried out at three different temperatures of 180 °C (9 bar), 220 °C (21 bar), and 260 °C (49 bar), with an average heating rate of 5.3 ± 0.7 °C min⁻¹. The holding time was 3 h and the stirring speed was 550 rpm. When the target temperature was reached, the holding time started; 11 samples were taken for liquid phase analysis, from initial time (t = 0h) to 3h. The maximum volume of each sample was 2 mL and one purge was taking before each sample (between 1 and 2 mL). The volume withdrawn was less than 35 mL (~15% of the total volume). The liquid samples were stored at 4 °C for further chemical analyses.

### 2.3. Ex-situ analysis by Raman and HPLC

The samples taken from the batch reactor at different temperatures and holding times were directly analyzed by Raman spectroscopy. Ex-situ Raman spectra of the sampled solutions were recorded on a Renishaw InVia Qontor micro-spectrometer. The 15× objective (numerical aperture, N.A., of 0.31), a 355 nm UV laser excitation source and a 3600 l mm⁻¹ grating were used. Note that Raman tests performed with different wavelengths showed important fluorescence in the case of excitation lines from the VIS-electromagnetic range. Each spectrum was recorded in the 180 to 2000 cm⁻¹ spectral range at room temperature and atmospheric pressure, with a 3 cm⁻¹ spectral resolution. The acquisition time was 60s.

The processing of Raman spectra, baseline subtraction and deconvolution, was performed using Origin and Wire software. Baseline subtraction was necessary because of the presence of fluorescence. An increase in fluorescence is systematically observed when the HTC reaction leads to the appearance of the first carbon particles, which are 2-10 nm in size. Several authors have reported that these carbon dots exhibit green luminescence behavior under ultraviolet light [29,30]. The deconvolution of the spectra enables the identification of the main Raman bands, for which hydrochar and glucose, xylose, 5-HMF, and
furfural solutions were used as standards.

High-Performance Liquid Chromatography (HPLC) analysis has been
performed over the aqueous phase after separation from the solid phase
by filtration using a Whatman filter paper with a pore size of 8 µm.
Samples were diluted 5-50 times and filtered with a non-sterile nylon
syringe filter (0.2 µm). A Varian HPLC system with a SPECTRA SYSTEM
UV2000 UV–VIS detector (λ = 275 nm) and LiChrospher® RP–18
column (5 µm) was used to separate the components. The temperature of
the column was held at 30 °C. The injection volume was 50 µL. The
mobile phase consisted of methanol and water acidified (0.5% formic
acid) at a flow rate of 0.7 mL min⁻¹. The initial methanol: water acid-
ified ratio was maintained at 10:90 (v/v) for 8 min, then the ratio
increased at a ramp of 2%·min⁻¹ to 30:70 (v/v) and was maintained at
30:70 (v/v) for 22 min.

Concentration profiles were simulated for comparison purposes
using the kinetic parameters from the literature [10] and the COMSOL
v6.2 software.

## 2.4. *In-situ* analysis of the HTC reactions by Raman spectroscopy

*In-situ* Raman measurements were performed in the Earth Science Institute of Orleans, using an experimental facility of the Planex platform [27]. The experimental setup (Fig. 1a), based on the coupling between the Raman spectrometer and the so-called transparent Internally Heated Pressure Vessel (t-IHPV) [31–33], allows to record the Raman spectra of various samples heated (up to 1200 °C) under high pressure (up to 2000 bar). The t-IHPV is equipped with three sapphire windows and sapphire internal sample container (Fig. 1b). The use of high chemical purity, c-crystallographic axis oriented and optically polished sapphire (Al₂O₃) makes this device transparent for the UV and VIS radiation and thus enables *in-situ* Raman monitoring. As shown in Fig. 1b the sample container is equipped with two sapphire pistons in direct contact with the solution enabling continuous pressure and density/volume changes during the measurements. The compressed He gas is used to pressurize the vessel. The pressure and temperature can be changed independently [31,32]. The homogenous temperature zone, of about h ~10 mm, is achieved due to efficient Mo-based resistive furnace with ceramic isolation. The pressure transmitter in the outlet of the t-IHPV and thermocouple placed inside the furnace very close to the sample, allow the pressure and temperature to be controlled with high accuracy [31–33].

Prior to *in-situ* Raman measurements the glucose and xylose solutions
were introduce to the sapphire cell: the mass of each solution was ~0.1g
and h ~ 3 mm.

As optimized for the *ex-situ* Raman studies, the 355 nm wavelength

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** a) Experimental setup for *in-situ* Raman measurements: Raman probe coupled to transparent - Internally Heated Pressure Vessel; b) sapphire sample container: sapphire cell and two pistons equipped with the O-rings.</figcaption></figure>

(UV) was employed for the *in-situ* measurements.

As shown in Fig. 1a, the 355 nm laser (Coherent Genesis CX355-100 SLM) beam was steered toward the homemade Raman probe designed and developed by CEMHTI laboratory [27]. This Raman probe (80 mm working distance, numerical aperture, N.A., of 0.3) offers a laser spot with diameter of ~150 µm and a focus depth of 1.5 mm, the perfect conditions in the case of hydrous solutions that undergo various chemical and kinetic transformations. The laser was focused through a sapphire window on the solution placed in the sapphire container inside the t-IHPV. The back-scattered light was collected by the same optics including Edge filter (Semrock) and injected to the optical fibre (N.A. of 0.22, diameter of 105 µm). This fiber transports the signal to the 500 mm focal length spectrometer (Andor Shamrock 500i) equipped with a back-illuminated iDus CCD detector (Andor), cooled by Peltier effect down to -70 °C. In order to obtain a good spectral resolution, the 2400 l mm⁻¹ grating was employed (2 cm⁻¹ spectral resolution).

Once the solution placed in the t-IHPV, a room temperature, room pressure spectrum was recorded first. Then the pressure was increased up to 100 bar (or 250 bar) with a constant rate of 30 bar min⁻¹. At the reached pressure value, the temperature was increased up to 180 °C (or 220 °C) with a heating rate of 2 °C min⁻¹ (or 5 °C min⁻¹). During the temperature ramp, the Raman spectra were recorded continuously. Since the Raman signals of glucose/xylose solutions are very weak, and further attenuated by the presence of sapphire window (4 mm thick) and sapphire container (0.5 mm thick), each spectrum was recorded during 5 min (i.e. 5 accumulations of 60s). These recording conditions are sufficient to collect temporally resolved Raman spectra with a good signal-to-noise ratio, suitable for the kinetics of the targeted reactions, which were previously analyzed using *ex-situ* Raman measurements (see Section 3.2 and Fig. 4). All of these experimental conditions were first optimized using a glucose solution at a heating rate of 5 °C min⁻¹ and a pressure of 100 bar. Several experiments were performed to ensure a high signal-to-noise ratio and the reproducibility of the spectral data. Subsequently, the measurement of each solution under given conditions of pressure, temperature and heating rate was repeated at least twice to enable us to accurately determine the pressure-temperature-induced modifications of hydrothermal carbonization of glucose and xylose. Data analysis of *in-situ* Raman spectra has been discussed as specified in section 2.3, i.e. deconvolution of the spectra was done using Origin and Wire software whereas peak assignment was proposed on the basis of *ex-situ* Raman results obtained previously, as well as the reference Raman spectra of individual sugars or product solutions.

## 3. Results and discussion

Fig. 2 shows a simplified schema of the reactions arising during the
hydrothermal carbonization of biomass compounds (cellulose, hemi-
cellulose, and lignin) to give secondary carbon or hydrochar, as
described in the introduction section [4–6,34]. The reactions that will be
monitored here are those involving glucose (R2, R3, R4, and R10 in
Fig. 2) and xylose (R6, R7, and R10).

### 3.1. Raman spectra of single solutions

Raman spectra of aqueous solutions of glucose and xylose are shown
in Fig. 3a and b, respectively. In the spectral range of interest, 900-2000
cm⁻¹, the spectrum of 5 w% glucose solution shows clearly an important
band at 1630 cm⁻¹ corresponding to the bending of H₂O; its high in-
tensity with respect to the other peaks can be attributed to the low
concentration of glucose. The Raman bands detected at 1458 cm⁻¹,
1370 cm⁻¹, 1332 cm⁻¹ and 1260 cm⁻¹ are assigned to: the: bending
mode, wagging (α and β isomers), and twisting, vibrations of CH₂
respectively. The strong band at 1125 cm⁻¹ corresponds to the bending
of C–O–H, while the modes at 1060 cm⁻¹ and 1035 cm⁻¹ can be assigned
to the stretching of C–O for α and β of D–glucose [35,36]. Similar Raman
bands can be found for xylose, although there are some differences in
<figure><img src="image_5.png" /><figcaption>Fig. 2. Simplified schema of reactions of lignocellulosic biomass main components under HTC conditions.</figcaption></figure>

intensity and wavenumber shifts. For example, the fingerprint Raman band is detected at 1120 cm⁻¹. Moreover, it has been recently shown [17] that glucose and xylose can be clearly differentiated by Raman spectroscopy of a mixture of sugars by a simple data treatment methodology.

Since in this study we are interested in following the reaction paths of glucose to 5-HMF and xylose to furfural transformations (Dehydration reactions R3 and R6 in Fig. 2), Fig. 3 shows the reference Raman spectra of aqueous solutions of 5-HMF and furfural.

5-HMF solution (Fig. 3a) reveals three characteristic Raman bands at 1525 cm⁻¹, 1580 cm⁻¹ and 1664 cm⁻¹, corresponding to the C=C symmetric stretching, C=C asymmetric stretching and C=O stretching, respectively. This is worth noting that the 1525 cm⁻¹ mode allowing clearly distinguish it from the glucose will be considered further as its fingerprint in the interpretation of ex-situ/in-situ measurements. The C–H bands are observed at 1197 cm⁻¹ (rocking), 1370 cm⁻¹ (wagging) and 1404 cm⁻¹ (bending mode) whereas the C–O stretching ones appear at 965 cm⁻¹, 995 cm⁻¹ and 1028 cm⁻¹ [37].

Although some wavenumber shifts and intensity differences, the spectrum of furfural solution (Fig. 3b) is similar to the one of 5-HMF (Fig. 3a). The strongest band observed at 1668 cm⁻¹ corresponds to the C=O stretching and together to the modes at 1158 cm⁻¹ and 1370 cm⁻¹ assigned to C–O–H bending and the H–C–C/O₂ bending respectively [37], can be considered as its fingerprint that allow to distinguish it from xylose.

## 3.2. Ex-situ monitoring of HTC reactions

Fig. 4a compares the Raman spectra of the solutions extracted from the HTC batch reactor at 180 °C, 220 °C, and 260 °C together to the Raman spectra of the glucose solution at room temperature and a glucose derived hydrochar. By monitoring the 5-HMF fingerprint band at 1525 cm⁻¹ and the glucose one at 1125 cm⁻¹, it is possible to track the dehydration reaction of glucose to form 5-HMF (R3 Reaction in Fig. 2). The figure reveals the presence of 5-HMF at all three temperatures. In the case of reaction carried out at 180 °C the 5-HMF is a minor component relative to glucose. For higher temperature reactions, the bands related to 5-HMF become prominent at the same time that the glucose ones decrease on intensity and disappear. As the solutions were not filtered for Raman analysis, the bands corresponding to the formation of the first hydrochar particles, particularly the most intense carbon G band at 1609 cm⁻¹, are visible from 220 °C. In this sense, Fig. 4a shows the Raman spectra of the hydrochar filtered from the solution after reaction at 260 °C, revealing the characteristic weak band D at 1384 cm⁻¹ and the strong band G at 1609 cm⁻¹, which correspond to the typical Raman peaks of carbon materials and differ entirely from glucose

[25,26].

More interestingly, Fig. 4b, c, and d show the evolution of the Raman spectra as a function of holding time after solution extraction from the reactor at temperatures of 180 °C (b), 220 °C (c), and 260 °C (d).

At 180 °C (Fig. 4b), the main glucose Raman band at 1125 cm⁻¹ decreases significantly after 30 min and is almost invisible after 60 min of holding time. At the same time, the 1525 cm⁻¹ and 1580 cm⁻¹ peaks characteristic of 5-HMF, become increasingly visible, indicating a complete dehydration of the glucose. From 30 min, a new band at 1609 cm⁻¹ appears and gains intensity from 60 min of holding time. As mentioned above, this peak corresponds to the typical G band of carbon produced by in-plane vibrations of sp²-bonded carbons. This indicates that hydrochar particles formed within 30 min of holding time at 180 °C, either through the reaction of 5-HMF (R4 in Fig. 2) or the direct reaction of glucose (R2 in Fig. 2).

In the case of the glucose solution heated up to 220 °C, we obtain a completely different picture, as presented in Fig. 4c. At this temperature, 5-HMF is already present at the beginning of the holding time (0 min spectrum), indicating that all the glucose has been already dehydrated. Moreover, the signature of carbon particles at 1609 cm⁻¹ is also detected. The intensity of the 5-HMF C=C peaks at 1525 cm⁻¹ and 1580 cm⁻¹ increases with increasing holding time up to 30 min and then starts to decrease, eventually disappearing at 120 min. At the same time, the relative intensity between the C=C asymmetric band of 5-HMF and the G-band of carbon at 1609 cm⁻¹ decreases, indicating the progressive formation of carbon over time. After 120 min of reaction, only the D and G bands of carbon at 1384 cm⁻¹ and 1609 cm⁻¹, respectively, are visible, indicating the complete conversion of 5-HMF to secondary carbon through polymerization and condensation.

Fig. 4d illustrates the impact of holding time at a reaction temperature of 260 °C. At this temperature, 5-HMF reacts very rapidly, and the characteristic peaks at 1525 cm⁻¹ and 1580 cm⁻¹ are barely visible after 30 min. Only 45 min are needed at that temperature to transform all the glucose into carbon, with only the D and G bands appearing in the Raman spectra. Fig. S1 shows the Raman spectra of carbon materials obtained after 180 min at 180 °C, 220 °C and 260 °C, and highlights that the I<sub>D</sub>/I<sub>G</sub> ratio increases with temperature. This suggests that carbon obtained at 260 °C has a higher degree of structural order.

In order to provide a clearer representation of the kinetics of 5-HMF production and its transformation into secondary carbon as a function of temperature, the deconvolution of the Raman spectra presented in Fig. 4 was performed. Fig. 5 displays the evolution of the 5-HMF 1525 cm⁻¹ peak area during the isothermal holding period at 180 °C, 220 °C, and 260 °C (Fig. 5a), as well as the ratio of the areas of the carbon 1609 cm⁻¹ and 5-HMF 1525 cm⁻¹ peaks (Fig. 5b) obtained after the deconvolution of the Raman spectra presented in Fig. 4b, c and d. It is shown that at
<figure><img src="image_6.png" /><figcaption>Fig. 3. Raman spectra of (a) 5 wt% solution in water of glucose and 2.5 wt% solution in water of 5-HMF and (b) 5 wt% solution in water of xylose and 2.5 wt % solution in water of Furfural. Fingerprint distinctive bands are color shaded. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

180 °C, the extensive production of 5-HMF starts after 30 min of reaction time, reaching a plateau that can be attributed to a balance between production and transformation into carbon, as shown in Fig. 5b. At 220 °C, the production of 5-HMF is higher, but with time, after reaching

a maximum, it decreases. In contrast, at 260 °C, the 5-HMF amount decreases very rapidly. As shown in Fig. 5b, for the two highest temperatures, the production of carbon after 3 h of holding time will be similar and higher than at 180 °C, but the reaction kinetics are completely different. Specifically, at 220 °C, there is a plateau up to 45 min, followed by a further increase with time, whereas at 260 °C, there is a faster increase to reach the maximum value after 45 min.

These results are consistent with those reported in the literature when developing kinetic models based on the analysis of the composition of solutions during carbohydrate extraction from batch reactors using HPLC. For example, Dos Santos et al. [10] used these experimental values to determine the temperature dependence of the kinetic parameters using an Arrhenius relationship (Eq. (1)):

$$k_i(T) = A_i \exp(Ea_i/RT) \quad (1)$$

where k is the reaction rate constant, min⁻¹; A is the pre-exponential factor, s⁻¹; Ea is the activation energy, J mol⁻¹; R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T (K) is the temperature. The kinetic parameters obtained are presented in Table S1.

Although the test conditions in Ref. [10] are not identical, we can still compare the results in terms of trends. Therefore, the concentration profiles of glucose, 5-HMF and degraded products at a given temperature and as a function of holding time were obtained using the kinetic parameters in Table S1, with first-order reactions employed to describe reactions R2, R3 and R4, as:

$$R2: -dC_{GLU}/dt = k_2C_{GLU} \quad (2)$$

$$R3: -\frac{dC_{GLU}}{dt} = \frac{dC_{HMF}}{dt} = k_3C_{GLU} \quad (3)$$

$$R4: -dC_{HMF}/dt = k_4C_{HMF} \quad (4)$$

where C is the molar concentrations in mol m⁻³ of the different species.

Fig. S2 presents the concentration profiles at 140 °C, 160 °C, 180 °C, 210 °C, 220 °C and 260 °C obtained using those parameters over our solution containing 5 wt% of glucose. Fig. 5c compiles the 5-HMF profiles for comparison with the Raman experimental results presented in Fig. 5a. A comparison of the two figures shows that, overall, the experimental data follow similar trends to the simulated ones even if a simplified approach assuming that all the reactions are of fist order was used [10,12]. At 180 °C, 5-HMF increases steadily before reaching a plateau. By contrast, at temperatures between 210 °C and 220 °C, the concentration is at its maximum in the first few minutes, after which it decreases. At 260 °C, a very rapid decrease in 5-HMF concentration is seen during the initial moments of the reaction. The concordance between the experimental and simulated results provides further evidence of the reliability of using Raman spectroscopy to study HTC reaction kinetics.

In order to validate the approach of using Raman spectroscopy, the solutions sampled at different temperatures and holding times were filtered, and the concentration of 5-HMF was measured by HPLC (Fig. 5d). The results show that even without calibrating the Raman signals with solutions of known concentration of 5-HMF, the kinetics profiles of 5-HMF obtained by HPLC (Fig. 5d) are remarkably similar to those obtained by Raman measurements (Fig. 5a). Therefore, Raman spectroscopy is a valuable technique for monitoring HTC reactions, offering the advantage of simultaneously monitoring both the liquid and solid phases. However, as has been demonstrated for other biomass and sugar processing applications [17–22], on-line or in-situ Raman spectroscopy could be a real breakthrough in HTC research.

### 3.3. In-situ Raman monitoring of HTC reactions

As mentioned in Section 2.4, prior to in-situ Raman measurements, the sugar solutions (5 wt% in water) are placed in the sapphire cell
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Ex-situ Raman spectra of hydrothermal carbonization of 2.5% glucose: (a) Effect of temperature: 25 °C (glucose), 180 °C, 220 °C and 260 °C at 0 min and spectra of hydrochar carbon obtained at 260 °C; effect of holding time at: (b) 180 °C, (c) 220 °C and (d) 260 °C. Shaded bands corresponds to glucose (blue), 5-HMF (purple) and carbon (green) fingerprints bands. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

(Fig. 1b) inside the t-IHPV. The *in-situ* experiment provides real-time information about the reaction as a function of temperature, pressure or time, with one spectrum acquired every 5 min (the time set for one spectrum acquisition), without the need to withdraw any solution. Therefore, analysis can be performed without varying the reaction media volume and with a considerable time gain, as no preparation is required. Moreover, in contrast to *ex-situ* analysis, where the requirement for solution extraction results in a cooldown of the temperature and a return to ambient pressure during preparation and analysis of the sample, *in-situ* analysis can be performed at the actual temperature and pressure. Therefore, as a difference from the *ex-situ* analysis, where sampling during constant-temperature steps is privileged, we were able to collect a significant amount of information during the heating step with the *in-situ* measurements.

**Fig. 6a** shows the *in-situ* Raman monitoring of the HTC of a 5 wt% glucose solution at a pressure of 100 bar. The spectrum at 0 min was recorded at room temperature and ambient pressure. Then, the pressure

was increased to 100 bar. At that constant pressure value the temperature was raised to 180 °C at a heating rate of 5 °C min⁻¹. The figure demonstrates that we can clearly follow the early stages of glucose dehydration and the onset of 5-HMF formation. As previously in the case of *ex-situ* Raman measurements, the reaction can be easily monitored by tracking the behavior of fingerprint Raman bands: the 1125 cm⁻¹ of glucose and the 1525 cm⁻¹ of 5-HMF (see Fig. 3). Moreover, the formation of carbon can be monitored by following the G-band at 1609 cm⁻¹, which is included in a broad band together to the H₂O bending band at 1630 cm⁻¹. The latter can be considered constant, since the water solutions used are poorly concentrated, so an increase in intensity in that region will correspond to hydrochar formation. Fig. S3 provides an example of the deconvolution of several Raman spectra at varying temperatures, thus illustrating the capacity to distinctly attribute the contributions of 5-HMF, H₂O and the G-band of carbon, facilitating the discernment of the onset of 5-HMF formation or carbon precipitation. Moreover, it is noteworthy that the area or intensity of the H₂O peak
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Kinetics of hydrothermal carbonization of 2.5% glucose during holding time at 180 °C, 220 °C and 260 °C based on *Ex-situ* Raman spectroscopy, simulation and HPLC: (a) Peak area evolution of 5-HMF band at 1525 cm⁻¹ from the deconvolution of Raman spectra in Fig. 4b, c and d; (b) Area ratio between G carbon band at 1609 cm⁻¹ and 5-HMF band at 1525 cm⁻¹ from the deconvolution of Raman spectra in Fig. 4b, c and d; (c) Simulation of 5-HMF concentration at 140 °C, 160 °C, 180 °C, 210 °C, 220 °C and 260 °C using kinetic parameters in Ref. [8]; (d) 5-HMF concentration obtained by HPLC after filtering solutions.</figcaption></figure>

remains unaltered and can be regarded as constant. Fig. 6a shows that above 172 °C, the intensity of the glucose band at 1125 cm⁻¹ decreases simultaneously with the appearance of the 5-HMF band at 1525 cm⁻¹. At the same time, the band at 1609 cm⁻¹, corresponding to the polymerization and condensation of 5-HMF into carbon, starts to emerge, following the same trend than spectra obtained at 180 °C and 220 °C *ex-situ* (Fig. 4a). The band corresponding to glucose appears to decrease slightly around 100 °C. Its intensity rapidly decreases around 170 °C, followed with a rapid increase in the intensity of the band corresponding to 5-HMF. The band corresponding to carbon also appears to increase slightly from 180 °C, with a more rapid increase from 200 °C. These results demonstrate the proof-of-concept that *in-situ* Raman spectroscopy is a powerful analytical chemistry tool that enables the identification of the components of the aqueous phase, along with real-time data on the reaction pathways under the specific experimental conditions of hydrothermal carbonization. To the best of our knowledge, this is the first time that Raman spectroscopy has been used to monitor aqueous phase reactions at high pressure in general and hydrothermal transformation of biomass derived compounds in particular.

In order to gain more detailed information about the HTC reaction of glucose, Fig. 6b shows the same experiment, but with the temperature increased at a rate of 2 °C min⁻¹, resulting in one spectrum every 10 °C (the spectrum acquisition time is 5 min), as compared to the 5 °C min⁻¹ heating-rate presented in Fig. 6a, which implies one spectrum every 25 °C. After increasing the pressure to 100 bar, the glucose solution was heated to 180 °C, with a short holding time included. Fig. S4a shows the totality of the recorded Raman spectra, illustrating the increase in fluorescence attributed, as previously indicated in section 2.3, to the

formation of the first carbon particles, which increases with the extent of carbon precipitation. However, Fig. S4b shows that, after background subtraction, this has little impact on the Raman intensities. For clarity, Fig. 6b only includes the spectrum recorded at room temperature and pressure, and those recorded under 100 bar at temperatures above 100 °C. Fig. 6b shows that under these conditions, the dehydration of glucose into 5-HMF and its subsequent polymerization/condensation into carbon can be detected at lower temperatures than in the previous experiment using a heating rate of 5 °C min⁻¹ (see Fig. 6a). Indeed, when heating at 2 °C min⁻¹, the 5-HMF C≡C band at 1525 cm⁻¹ can be already detected at 162 °C, and the G-band of precipitation of carbon particles at 1609 cm⁻¹ is clearly visible at 172 °C (see deconvoluted spectra in Fig. S3). The intensities of the 5-HMF and hydrochar bands continue to increase at 180 °C, and even more so when the temperature is held at 180 °C for 5 min.

To better estimate the effect of heating rate on HTC reactions at 100 bar, Fig. 7 presents the evolution of glucose, 5-HMF, and carbon amounts as a function of temperature, obtained by reporting the 1125 cm⁻¹, 1525 cm⁻¹, and 1609 cm⁻¹ areas over the 1630 cm⁻¹ one, assuming that the latter remains constant during the reaction as presented above (Fig. S3).

By comparing the results at a heating rate of 5 °C min⁻¹ (Figs. 7a) and 2 °C min⁻¹ (Fig. 7b), the dehydration of glucose and the formation of 5-HMF are delayed by more than 20 °C for the higher heating rate. Fig. 7a shows that at a heating rate of 5 °C min⁻¹, at 172 °C, the dehydration of glucose has just started, with no 5-HMF formed and no carbon precipitation yet. At 196 °C, the glucose dehydration is extensive and 5-HMF is formed. However, at such a temperature, the 5-HMF is
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** *In-situ* Raman spectroscopy monitoring of hydrothermal carbonization: (a) of 5.0% glucose at 100 bar and heating rate 5 °C min⁻¹ up to 220 °C; (b) of 5.0% glucose at 100 bar and heating rate 2 °C min⁻¹ up to 180 °C, including 5 min holding temperature at 180 °C; (c) of 5.0% glucose at 250 bar and heating rate 2 °C min⁻¹ up to 180 °C including 5 min holding temperature at 180 °C; and (d) of 5.0% xylose at 100 bar and heating rate 2 °C min⁻¹ up to 180 °C. In (a), (b), (c) shaded bands correspond to glucose (blue), 5-HMF (purple), carbon (green); in (d) shaded bands correspond to xylose (blue), furfural (purple) and carbon (green) fingerprints. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

easily transformed into hydrochar (R3 in Fig. 2) or the glucose directly react to produce carbon (R2 in Fig. 2), as denoted by the high area of the carbon G band. By increasing the temperature up to 220 °C, the glucose has been almost completely dehydrated, and the amount of 5-HMF continues to increase, leading to the precipitation of higher amounts of carbon through its polymerization and condensation. This result is consistent with the *ex-situ* results presented in Fig. 4a, since the heating rate in those experiments was similar (around 5.3 °C min⁻¹), and it was observed that the glucose signature disappeared between 180 °C and 220 °C, as the bands for 5-HMF and carbon were visibly higher.

Reducing the heating rate of the HTC reaction to 2 °C min⁻¹ allows for the collection of a spectrum at every 10 °C, enabling a more detailed follow-up of the HTC glucose reactions within a given temperature range. Fig. 6b shows a selection of spectra. By representing the data as relative area versus the H₂O nearly constant band, Fig. 7b can be directly compared to Fig. 7a to study the effect of the heating rate on the HTC

glucose reaction. Fig. 7b indicates that at 162 °C, there is the onset of glucose dehydration, with a very small amount of 5-HMF formed. At 172 °C, the glucose dehydration is more extensive, leading to 5-HMF formation that is simultaneously polymerized/condensed to form a hydrochar. At 180 °C, there is a high degree of glucose dehydration, and the amount of 5-HMF detected reaches a plateau as the precipitation of carbon becomes more visible. Therefore, comparing these results with those shown in Fig. 7a for a 5 °C min⁻¹ heating rate, it can be concluded that using Raman *in-situ* and a heating rate of 2 °C min⁻¹, it is possible to assess with precision the temperature of glucose decomposition into 5-HMF and carbon precipitation, i.e., 160-162 °C and 170-172 °C, respectively. In such conditions, Raman *in-situ* can provide a level of information that other *ex-situ* techniques cannot access, and this can be achieved in a single, fast experiment.

Knowing the temperature at which an intermediate or product appears can provide direct access to activation energies (Equation (1)).
<figure><img src="image_10.png" /><figcaption>Fig. 7. Evolution of liquid/solid components monitored by Raman in-situ during hydrothermal carbonization. Relative areas of bands 1125 cm⁻¹ for glucose, 1525 cm⁻¹ for 5-HMF, 1609 cm⁻¹ for carbon, 1120 cm⁻¹ for xylose and 1668 cm⁻¹ for furfural versus 1630 cm⁻¹ band of H₂O (a) of 5.0% glucose at 100 bar and heating rate 5 °C min⁻¹ up to 220 °C; (b) of 5.0% glucose at 100 bar and heating rate 2 °C min⁻¹ up to 180 °C; (c) of 5.0% glucose at 250 bar and heating rate 2 °C min⁻¹ up to 180 °C and (d) of 5.0% xylose at 100 bar and heating rate 2 °C min⁻¹ up to 180 °C.</figcaption></figure>

Moreover, determining the temperatures at which sugar decomposes or carbon precipitates is crucial for optimizing processes, particularly those targeting high-value intermediates such as 5-HMF or those aiming to produce carbon. Accurate temperature data enables synthesis parameters to be fine-tuned, allowing optimal temperatures to be selected to stabilize intermediate phases or achieve the desired product, thereby maximizing yield and purity.

Moreover, the results obtained are in line at the trend level with the

Arrhenius kinetic data presented in the literature [10,12] obtained from equation (1). Fig. 8 shows the evolution of the kinetic constants for reactions R2 (the direct decomposition of glucose into intermediate compounds resulting in hydrochar), R3 (glucose to 5-HMF) and R4 (5-HMF decomposition). In Fig. 8a, it can be clearly observed that the Arrhenius constant k₂ increases with temperature more rapidly than k₃ and k₄, in line with the observations in Raman spectra relatively quantified in Fig. 7a and b, where the formation of carbon occurs

<figure><img src="image_11.png" /><figcaption>Fig. 8. Evolution of Arrhenius kinetic constants for reactions R2 (the direct decomposition of glucose into intermediate compounds resulting in hydrochar), R3 (glucose to 5-HMF) and R4 (5-HMF decomposition) with temperature (a), magnification of low region y-axe (b).</figcaption></figure>
simultaneously with the decrease in glucose amount. Fig. 8b, presenting a magnification of Fig. 8a to better distinguish k₃ and k₄, indicates that from a temperature of around 150-160 °C, k₄ becomes greater than k₃, contributing to the decrease of 5-HMF formation and the acceleration of carbon production observed in Fig. 7b.

Other effect that has been studied here is the role of pressure during the HTC reaction. Indeed, it has been demonstrated that carrying out HTC reactions under high pressure enhances the energy efficiency of transforming biomass into carbon [16]. Fig. 6c shows a selection of Raman spectra obtained every 10 °C during the heating of a glucose solution at 2 °C min⁻¹ at a pressure of 250 bar, instead of the 100 bar used in Fig. 6b. From Fig. 6c, it appears that the evolution of the glucose band at 1125 cm⁻¹, the 5-HMF band at 1525 cm⁻¹, and the carbon band at 1609 cm⁻¹ is similar to that obtained at 100 bar (compare with Fig. 6b). Indeed, the quantitative results presented in Fig. 7c confirm that 5-HMF starts to appear at the same temperature and carbon precipitates at the same temperature as when using 100 bar. However, it seems that the kinetic of the reactions are promoted under increased pressure. Indeed, analysis of the Raman spectra enables the effect of pressure on the kinetics of glucose conversion to 5-HMF or hydrochar to be tracked by monitoring the ratio of their characteristic peaks. Fig. 9a illustrates the evolution of the area ratio of the glucose to 5-HMF peaks over time (reaction R3 in Fig. 2), with t = 0 defined as the moment when 5-HMF first appears. Similarly, Fig. 9b and c shows the evolution of the

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Experimental reactions kinetics of reactions R3 (a), R4 (b) and R2 (c) from HTC reaction of glucose at 100 bar and 250 bar obtained from the area ratio of fingerprints bands of glucose, 5-HMF and carbon obtained by *in-situ* Raman. Initial time (t = 0 min) has been fixed at the temperature where reaction products are first observed in the Raman spectra.</figcaption></figure>

area ratio of the 5-HMF or glucose peaks over hydrochar, which is used to monitor the kinetics of the 5-HMF-to-hydrochar reaction (reaction R4 in Fig. 2) or the glucose-to-hydrochar reaction (reaction R2 in Fig. 2), with t = 0 defined as the onset of hydrochar formation. Fig. 9a shows that increasing pressure positively impacts the kinetics of 5-HMF formation, as well as accelerating the transformation of 5-HMF into hydrochar and the direct transformation of glucose into carbon at higher pressures. These results confirm the findings of Yu et al., who showed that high-pressure water can cleave hydrogen bonds in polymers such as cellulose, facilitating dehydration reactions [16].

Therefore, the heating rate and pressure are important parameters in HTC reactions, influencing the temperature and kinetics of sugar conversion and carbon precipitation in different degrees.

Finally, to demonstrate the reliability of using *in-situ* Raman measurements to monitor biomass-related HTC reactions, xylose was used as a precursor. As explained earlier, xylose is one of the main intermediates in HTC reactions of hemicellulose [12,38]. Based on the previous experiences with glucose, the conditions chosen for the HTC reaction were 2 °C min⁻¹ and 100 bars. Fig. 6d shows some of the Raman spectra recorded during heating up to 180 °C. From the Figure, it can be observed that as the temperature increases, intensity of the characteristic band of xylose at 1120 cm⁻¹ decreases. At the same time, the characteristic band of furfural as the 1668 cm⁻¹ together to the modes at 1158 cm⁻¹ and 1370 cm⁻¹ (Fig. 3b) appear as a result of the dehydration of xylose (Reaction R6 in Fig. 2). This confirms that it is also possible to follow the dehydration of xylose leading to the formation of furfural using Raman spectroscopy. Moreover, the spectra in Fig. 6d show that the band comprising the constant contribution of water at 1630 cm⁻¹ and the contribution of the G-band of carbon at 1609 cm⁻¹ increases in intensity at temperatures above 172 °C indicating the precipitation of the hydrochar.

Fig. 7 presents the results of Raman band area ratio analysis allowing a semi-quantitative comparison of the behavior of HTC reactions of xylose (Fig. 7d) and glucose (Fig. 7b) under identical experimental conditions. For building Fig. 7d, the band at 1668 cm⁻¹ in Fig. 6d was used to monitor furfural. Fig. 7d indicates that xylose dehydration to produce furfural occurs at temperatures as low as 142 °C, but carbon precipitation is not observed up to 162 °C. Furthermore, from 172 °C onwards, the amount of furfural decreases, while the hydrochar amount increases, indicating that furfural conversion into hydrochar is nearly complete at 182 °C. Comparing the results in Fig. 7b and d shows that xylose decomposes to furfural at temperatures between 140 and 142 °C, while glucose decomposes to 5-HMF at temperatures between 160 and 162 °C. However, it has been found that carbon precipitation from furfural polymerization/condensation does not occur immediately alongside furfural formation, as has been observed for 5-HMF (see Fig. 7a and b). Therefore, these results are confirmation that the most abundant xylose-based intermediate, furfural, does not produce hydrochar as readily as the more reactive 5-HMF, the most abundant intermediate of glucose. Furfural is less reactive with respect to polycondensation due to its higher molecular structural stability, which is attributed to the stabilization of the aldehyde moiety and the furan ring [39]. Thus, the kinetics of the reaction depends on the nature of the sugar.

Those results indicate that *in-situ* Raman spectroscopy can be effectively used to monitor the reaction pathways and kinetics of different sugars undergoing hydrothermal carbonization (HTC). Further developments are aimed at decreasing the time required for spectra acquisition while maintaining a high signal-to-noise ratio, in order to overcome the possible temporal resolution limitations of the technique and enable its application to the monitoring of faster kinetic reaction steps. The 5-min acquisition time used in this study was chosen to compromise between achieving an acceptable signal-to-noise ratio and capturing the kinetic evolution of the reaction. However, it is well established that alternative approaches, such as using higher-concentration solutions, more powerful laser sources, enhanced
detectors or additional signal-enhancement techniques, could significantly increase the intensity of the Raman signal [2,40–42] for allowing high-quality spectral data to be acquired in shorter time periods and thus faster kinetics.

As *in-situ* Raman spectroscopy has been demonstrated to be a reliable tool for following biomass-related reactions, including those involving mixtures of sugars [17], at atmospheric pressure, our development of this technique for high-pressure applications can open up new possibilities in the field of hydrothermal processing of biomass.

## 4. Conclusions

This work demonstrates the development of *in-situ* high-temperature-high-pressure Raman spectroscopy as a powerful analytical chemistry tool. This tool has enabled components in the aqueous phase of hydrothermal carbonization to be identified and has provided real-time data on reaction pathways in a single, fast experiment. In addition to providing information on the chemical nature of reaction intermediates, *in-situ* Raman measurements have revealed data that cannot be obtained through *ex-situ* methods. For instance, they have revealed the precise temperature at which sugars decompose. *In-situ* measurements have also made it possible to determine the temperature at which solid carbon precipitates. Therefore, this method has the additional advantage over traditional *ex-situ* methods of enabling the liquid and solid phases to be monitored simultaneously.

This work demonstrates the effectiveness of the developed methodology when applied to two different sugars, glucose and xylose, under distinct reaction conditions. *In-situ* Raman spectroscopy technology operating at high temperatures and pressures can be employed to examine the impact of various reaction parameters, including temperature, pressure, and reactant concentrations, on the HTC reaction pathways and kinetics. This can provide significant insights into the fundamental processes of the HTC process, helping to enhance HTC processes for producing sustainable carbon materials from biomass with a high yield, while also directing the design of more effective and selective HTC reactors.

Furthermore, the developed technology is not limited to HTC reactions. It can be used to monitor any liquid-phase reaction that occurs under high-pressure and high-temperature conditions, including chemical and pharmaceutical synthesis or organic geochemistry. Monitoring these reactions in real time under such conditions can provide valuable insights into reaction mechanisms and enable process conditions to be optimized for improved efficiency, selectivity and yield.

### CRediT authorship contribution statement

**Alejandra Palomino:** Investigation, Formal analysis. **Aneta Slodczyk:** Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Aurélien Canizarès:** Methodology, Investigation, Formal analysis. **Rémi Champallier:** Methodology. **Stéphane Bostyn:** Validation, Supervision, Methodology, Investigation, Conceptualization. **Encarnacion Raymundo-Piñero:** Writing – review & editing, Writing – original draft, Validation, Project administration, Investigation, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: RAYMUNDO PINERO Encarnacion reports financial support was provided by Centre-Val de Loire Region. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors thank the Région Centre val de Loire (Project APR-IA PRESERVE convention n°2019 00134933) for financial support. The Equipex PLANEX project (ANR-11-EQPX-0036), which allowed to develop *in-situ* HP-HT Raman platform is greatly acknowledged.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2026.121360.

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