Vanadium-Substituted Phosphomolybdic Acids for the Aerobic Cleavage of Lignin Models—Mechanistic Aspect and Extension to Lignin

This work deals with the aerobic oxidative cleavage of C-C and C-O bonds catalyzed by the Keggin-type phosphovanadomolybdic acid (H6[PMo9V3O40], noted H6PV3). The latter was synthesized by an adapted hydrothermal procedure classically used for lower vanadium content and was tested as a catalyst for the aerobic cleavage of 2-phenoxyacetophenone (noted K1HH) and 1-phenyl-2-phenoxyethanol (A1HH) used as two lignin models. The operative conditions (solvent, catalytic loading, etc.) were adjusted on K1HH and extrapolated to A1HH. The cleavage of the alcohol model required more drastic conditions and therefore further optimization. Preliminary attempts on an Organosolv wheat straw lignin were performed too. From the kinetic study, high performance liquid chromatography (HPLC) and gas chromatography–mass spectrometry (GC-MS) data, a mechanism of the cleavage of both models was proposed.

Yield calculated at 254 nm.  Table S2a. Gas chromatography-mass spectrometry (GC-MS) identification of products arising from C-O bond cleavage.   Figure S3. X-ray diffraction (XRD) profile of H6PV3 (main characteristic peaks marked by * symbol) vs. starting MoO3 and V2O5 and reference H3PMo12.

M U L T I --P A T T E R N
-Choice of the 31 P NMR relaxation delay for P quantification in H6PV3 Figure S5. Spectrum of H6PV3.

NB:
The pH is more acidic as 300 mg of H6PV3 was used instead of 30 mg. Since the position of the peaks depends on the pH, the phosphorous center in H6PV3 [S5] in those conditions was more deshielded (vs Figure 4).
The peak at 0 ppm corresponds to H3PO4. The peak n 6 corresponds to H[PMo9V3O40] 5-and the peaks 2-5 correspond to [PMo9V3O40] 6− and other H3+xPVx (x < 3) [S5]. The order of the peaks is unchanged compared to typical 31 P NMR analysis (30 mg H3+xPVx). The longest relaxation delay is 1.02 s. Thus, the minimal relaxation time should be 6.1 s. So, as the relaxation time in typical 31 P NMR analysis was 32 s, the integration is quantitative.
(2) Therefore, the equation (Equation S3) is obtained from the application of (Equation S2)  The predicted evolution of O2 solubility determined from (Equation S5) and (Equation S6) in function of the volumic fraction of acetic acid is plotted on Figure S6.

Mechanistic studies
As mentioned in Table S2c, the structure proposed by the NIST Library (X') was not satisfying. Further investigations had to be done by comparing in particular the masses of the different fragments of X with those of K1HH (Table 4). Indeed, the molecular peak (m/z = 240) of the reference compound X' proposed by the NIST Library was absent in the mass spectrum of the compound X whereas the peaks at m/z = 94 (PhOH .+ ) and m/z = 136 (PhCOCHO .+ , may be obtained from the cleavage of O-Ac and C-OPh bonds) were only observed for X and not X'. Besides, the peak at m/z = 227 may correspond to an oxyradical from hydroxylated K1HH. The peak at m/z = 121 (HOPhCHO + ) was not observed for X unlike the compound W whereas the peak at 94 was observed unlike the compound W'. Moreover, the peak at m/z = 136 (PhCH2OH + ) is observed only for X only. As an intense peak is observed at m/z = 43 is present, it is probable the hydroxyl group is acetylated. Figure S10. Hypothetic structure of the intermediate X.

Lignin phosphorylation
The phosphorylation was carried out according to the method of GRANATA and ARGYROPOULOS with slight modifications [S7,S8] using pyridine-CDCl3 1.6-1 as the solvent.
Before the reaction starts, the purified lignin sample is not totally soluble in the solvent but usually the reaction leads to the dissolution of the solid. An example of the obtained 31 P NMR spectra is given on Fig. S7. This was not observed for oxidized lignin samples. So, the total content of OH functions could not be determined in these conditions. Figure S12a. 31 P NMR spectrum of phosphorylated non-oxidized lignin. Figure S12b. Division of the interesting region of the 31P NMR spectrum of phosphorylated lignin for the quantification of the different types of OH groups (The internal standard was N-hydroxy-6norbornene-2,3-dicarboximide (151.9 ppm)).