Selective radical depolymerization of cellulose to glucose induced by high frequency ultrasound†

The depolymerization of cellulose to glucose is a challenging reaction and often constitutes a scientific obstacle in the synthesis of downstream bio-based products. Here, we show that cellulose can be selectively depolymerized to glucose by ultrasonic irradiation in water at a high frequency (525 kHz). The concept of this work is based on the generation of H˙ and ˙OH radicals, formed by homolytic dissociation of water inside the cavitation bubbles, which induce the cleavage of the glycosidic bonds. The transfer of radicals on the cellulose particle surfaces prevents the side degradation of released glucose into the bulk solution, allowing maintaining the selectivity to glucose close to 100%. This work is distinguished from previous technologies in that (i) no catalyst is needed, (ii) no external source of heating is required, and (iii) the complete depolymerization of cellulose is achieved in a selective fashion. The addition of specific radical scavengers coupled to different gaseous atmospheres and ˙OH radical dosimetry experiments suggested that H˙ radicals are more likely to be responsible for the depolymerisation of cellulose.


S3
resulting from the recombination of •OH radicals, as a function of the ultrasonic irradiation time (without cellulose). We observed in the graph S1b, a linear and constant increase of H2O2, indicating that the homolytic dissociation of water in the cavitation bubbles takes place as soon as the ultrasound starts (no induction period). This result is in line with Scheme 3 on dosimetry experiments. Note that we also performed a reaction by initially co-adding H2O2 with cellulose, but it had no impact on the induction period, ruling out a possible reaction of cellulose with accumulated H2O2 inside the reactor. However, we noticed that the induction period varies according to the nature of the gas bubbled. For instance, when air was replaced by Ar/H2 or H2 the induction period was decreased from 3 h to less than 1 h. Under bubbling of O2, this induction period is 2 h. Hence, we suspect that this induction period may correspond to problems of mass transfer (i.e. dissolution and diffusion of the gas into water), homogenization of the cavitation bubble cloud, etc. However, these are only hypotheses and, so far, our results do not let us to rationalize this induction period and we are currently working on this aspect with expert of ultrasound (T. Chave's group in the author list), mainly because we observe the same trend on other HFUS-mediated reactions we are currently investigating in the lab.

Characterization of the HFUS by titration of H2O2 without cellulose
H2O2 was titrated by UV-visible spectroscopy (Thermo Fisher Evolution 60S) using TiOSO4. This latter reacts with H2O2 to form a yellow-colored Ti(IV)-H2O2 complex with a typical adsorption at 412 nm.
TiO 2+ + H2O2 → TiO(O2) + 2H + In a typical procedure, 553 mg of TiOSO4 was dissolved in 2.8 mL of H2SO4 (96%) and water was added at 50°C up to a total volume of 100 mL was reached.

High Performance Liquid chromatography (HPLC)
The reaction media was analyzed by a Shimadzu HPLC equipped with a ZORBAX NH2 apolar type column, a pump (LC-20AT), a thermostated autosampler (SIL-10A) and an oven heated at 40 °C (CTO-20AC). The eluted compound were detected and quantified using a refractive index detector (RID-10A). The mobile phase consists of water and acetonitrile (20:80) injected at a flow rate of 0.8 mL.min -1 . Standard solutions (glucose, fructose) were prepared from commercial products purchased from Sigma-Aldrich. The retention times, mathematic expression of the calibration curve and R 2 are summarized in Table S1.

Matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF)
The unreacted cellulose remaining after ultrasonic irradiation was removed by centrifugation.
The as-obtained solution was freeze-dried and then analysed by MALDI-TOF. An ionic preparation comprised of 2,5-dihydroxybenzoic acid (DHB) and N,N-dimethylaniline (DMA) was used as the MALDI matrix, as described by Ropartz et al [1]. Zooming around the expected masses, we observe, a peak corresponding to a sodium adduct of glucose (m/z = 203, Figure S1b) and no other peak that could correspond to glucose oxidation or oligosaccharides (dimer and trimer) were observed, as shown on the zooms around DP2
For this analysis, the residual MCC after ultrasonic irradiation was removed by centrifugation.
The as-obtained solution was then freeze-dried and solubilized in D2O.

Size Exclusion Chromatography coupled to Multi-Angle Laser Light Scattering and Refractive Index (SEC-MALLS-RI) detection
The

FT-IR characterization
FT-IR analysis was carried out using a Perkin Elmer Spectrum One FT-IR Spectrometer infrared spectrometer coupled with an ATR module (Perkin Elmer Universal ATR sampling accessory). The scans were recorded between 4000 cm -1 and 650 cm -1 (Figure S5).

X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) of cellulose before and after ultrasonic irradiation was performed on a Kratos Axis Ultra DLD apparatus equipped with a hemispherical analyzer and a delay line detector. The spectra were recorded using an Al monochromated X-ray source (10 kV, 15 mA) with a pass energy of 40 eV (0.1 eV per step) for high resolution spectra, and a pass energy of 160 eV (1 eV per step) for survey spectrum in hybrid mode and slot lens mode, respectively. XPS spectra were calibrated with respect to the C 1s orbital at 284.8 eV.
According to the XPS results, no surface oxidation of cellulose after ultrasonic irradiation was observed. Figure S6: XPS spectra of cellulose before (a) and after (b) ultrasonic irradiation under air

X-ray diffraction (XRD)
The X-ray diffractometer used is an "EMPYREAN" (PANalytical) equipped with a copper tube (characteristic wavelength: λ (Kα1) = 0.1540562 nm), a "fast" linear detector, called "X'Celerator", and a platinum (or "spinner") allowing a rotation of the sample. The measurement are made between 5° and 50° in 2Thétas, and the displacement was fixed at 0.1° for an accumulation of 600 s per step.
The crystallinity index (ICR) of the samples was calculated as in Langford et Wilson [6] from the XRD spectra using the following equation Where I200 is the intensity of the crystal peak located at 22.6° corresponding to the plane (200) and IAM is the intensity of the valley situated between the two peaks located at 22.6° and 15.5° corresponding to the amorphous intensity.  Ar-H2 S15 Figure S11: 13 C NMR spectrum of commercial D-fructose in D2O. Assignment was done on the basis of a previous work reported on ref [8]