Organic Solvent-Free Olefins and Alcohols (ep)oxidation Using Recoverable Catalysts Based on [PM12O40]3− (M = Mo or W) Ionically Grafted on Amino Functionalized Silica Nanobeads

Catalyzed organic solvent-free (ep)oxidation were achieved using H3PM12O40 (M = Mo or W) complexes ionically grafted on APTES-functionalized nano-silica beads obtained from straightforward method (APTES = aminopropyltriethoxysilane). Those catalysts have been extensively analyzed through morphological studies (Dynamic Light Scattering (DLS), TEM) and several spectroscopic qualitative (IR, multinuclear solid-state NMR) and quantitative (1H and 31P solution NMR) methods. Interesting catalytic results were obtained for the epoxidation of cyclooctene, cyclohexene, limonene and oxidation of cyclohexanol with a lower [POM]/olefin ratio. The catalysts were found to be recyclable and reused during three runs with similar catalytic performances.


Synthesis of the Catalytic Objects.
The synthesis of the catalytic objects was a three-step method (Scheme 1), starting from the synthesis of non-porous SiO 2 beads according to a modified Stöber method using Si(OEt) 4 (TEOS) and ammonia in MeOH [61]. The second step, i.e., the grafting of aminopropyltrietoxysilane (APTES) at the surface of SiO 2 , was performed under classical conditions in toluene [62], the grafted species with pending NH 2 functions (named here SiO 2 @NH 2 ) being isolated as white powder. The final step consisted of the ionic grafting of the POM catalysts on SiO 2 @NH 2 simply by mixing in water SiO 2 @NH 2 beads and the corresponding Keggin heteropolyacids H 3 PM 12 O 40 (M=Mo or W) in water, in a POM/NH 2 functions ratio of 1/3. The final solids SiO 2 @PMo and SiO 2 @PW were isolated as powders. In the case of molybdenum, the powder was slightly blue, indicating an interaction with ammonium [63]. The mixture with the tungsten gave a white powder.

Synthesis of the Catalytic Objects.
The synthesis of the catalytic objects was a three-step method (Scheme 1), starting from the synthesis of non-porous SiO2 beads according to a modified Stöber method using Si(OEt)4 (TEOS) and ammonia in MeOH [61]. The second step, i.e., the grafting of aminopropyltrietoxysilane (APTES) at the surface of SiO2, was performed under classical conditions in toluene [62], the grafted species with pending NH2 functions (named here SiO2@NH2) being isolated as white powder. The final step consisted of the ionic grafting of the POM catalysts on SiO2@NH2 simply by mixing in water SiO2@NH2 beads and the corresponding Keggin heteropolyacids H3PM12O40 (M=Mo or W) in water, in a POM/NH2 functions ratio of 1/3. The final solids SiO2@PMo and SiO2@PW were isolated as powders. In the case of molybdenum, the powder was slightly blue, indicating an interaction with ammonium [63]. The mixture with the tungsten gave a white powder. Amorphous nature, as well as the sizes and morphologies of the isolated objects SiO2, SiO2@NH2, SiO2@PMo and SiO2@PW were analyzed before the catalytic experiments by PXRD, DLS and TEM. Accurate analysis of the functional content has been performed using IR and multinuclear solid-state NMR. Qualitative studies were performed through elemental analysis and 1 H and 31 P solution NMR. Amorphous nature, as well as the sizes and morphologies of the isolated objects SiO 2 , SiO 2 @NH 2 , SiO 2 @PMo and SiO 2 @PW were analyzed before the catalytic experiments by PXRD, DLS and TEM. Accurate analysis of the functional content has been performed using IR and multinuclear solid-state NMR. Qualitative studies were performed through elemental analysis and 1 H and 31 P solution NMR.

Analysis by PXRD
The amorphous state of isolated SiO 2 @PM beads was characterized through powder X-ray diffraction ( Figure S1) and did correspond to what was expected with 2θ = 23 • [49,64,65]. At the difference with POMs grafted in a similar way in mesoporous SBA-15 [66], no diffraction peaks corresponding to the starting heteropolyacids ( Figure S2) could be detected (in comparison with the PXRD spectra of the starting materials), certainly indicating that POMs were grafted but did not remain agglomerated in a crystalline way.

Dynamic Light Scattering (DLS) Analysis.
The DLS measurements are usually performed to determine the hydrodynamic diameter of colloidal particles. As described previously with the silica particles obtained by the Stöber method [61], we considered the objects as spherical. This technique can give a perfect size (hydrodynamic diameter D h ) of the particles when enough dispersed in the suspension and that no time depending aggregation phenomena do occur [67]. Measurements were performed with SiO 2 , SiO 2 @NH 2 , SiO 2 @PW and SiO 2 @PMo. The results have been graphically indicated in Figure 1. difference with POMs grafted in a similar way in mesoporous SBA-15 [66], no diffraction peaks corresponding to the starting heteropolyacids ( Figure S2) could be detected (in comparison with the PXRD spectra of the starting materials), certainly indicating that POMs were grafted but did not remain agglomerated in a crystalline way.
The DLS measurements are usually performed to determine the hydrodynamic diameter of colloidal particles. As described previously with the silica particles obtained by the Stöber method [61], we considered the objects as spherical. This technique can give a perfect size (hydrodynamic diameter Dh) of the particles when enough dispersed in the suspension and that no time depending aggregation phenomena do occur [67]. Measurements were performed with SiO2, SiO2@NH2, SiO2@PW and SiO2@PMo. The results have been graphically indicated in Figure 1.
The DLS measurements gave stable measurements within the time range for the SiO2 beads only (Figure 1a), the Dh found being around 70 nm in suspension in water. The SiO2@NH2 beads show different behavior (Figure 1b). Within the time, the size distribution was changing and aggregation seemed to occur, the diameter evolving from 190 nm at the first measurement (implicating some small association of the beads under the conditions of the measurements if we considered that the starting beads used for the grafting were the SiO2 presented previously) to even bigger aggregation with higher Rh values, i.e., 340 nm after a longer time. This might be due to the nature of the pending NH2 functions and the possibility of hydrogen bonds. Addition of the POMs to the SiO2@NH2 beads did profound changes to the DLS measurements according to the nature of the POM. With SiO2@PW, the starting measured size was around 190 nm (as for the SiO2@NH2) and evolved to 220 nm (Figure 1c). In the case of the SiO2@PMo beads ( Figure  1d), the starting value was huge from 450 nm to 3900 nm between the different time measurements. This was proof of a time-dependent rearrangement of the beads. DLS did not give information for the hydrodynamic radii of the single particles in this case but pointed out an aggregation phenomenon due to the nature of the surrounding of the silica particles, once the grafting was done. Interactions with the POMs (and protonation of the pending NH2) might change the pH value and favor the aggregation [68]. Such a phenomenon was observed with thiolated silica particles interacting with different concentrations of hydroxyethylcellulose [69].

TEM Analysis
TEM measurements gave the proof of narrow dispersity of the silica beads ( Figure 2). The beads had an average diameter of 75.9 nm for SiO2 and SiO2@NH2 and around 80.6 and 82.9 nm for the SiO2@PW and SiO2@PMo ones respectively, indicating that the structure of the SiO2 core was maintained during the three steps. The coverage of SiO2@NH2 with POMs could be proven in addition by a textural change of the surface of the beads. 10 100 1000 Dh (nm) 10 100 1000 Dh (nm) 10 100 1000 Dh (nm) 100 1000 10000 Dh (nm) The DLS measurements gave stable measurements within the time range for the SiO 2 beads only (Figure 1a), the D h found being around 70 nm in suspension in water. The SiO 2 @NH 2 beads show different behavior (Figure 1b). Within the time, the size distribution was changing and aggregation seemed to occur, the diameter evolving from 190 nm at the first measurement (implicating some small association of the beads under the conditions of the measurements if we considered that the starting beads used for the grafting were the SiO 2 presented previously) to even bigger aggregation with higher Rh values, i.e., 340 nm after a longer time. This might be due to the nature of the pending NH 2 functions and the possibility of hydrogen bonds.
Addition of the POMs to the SiO 2 @NH 2 beads did profound changes to the DLS measurements according to the nature of the POM. With SiO 2 @PW, the starting measured size was around 190 nm (as for the SiO 2 @NH 2 ) and evolved to 220 nm (Figure 1c). In the case of the SiO 2 @PMo beads (Figure 1d), the starting value was huge from 450 nm to 3900 nm between the different time measurements. This was proof of a time-dependent rearrangement of the beads. DLS did not give information for the hydrodynamic radii of the single particles in this case but pointed out an aggregation phenomenon due to the nature of the surrounding of the silica particles, once the grafting was done. Interactions with the POMs (and protonation of the pending NH 2 ) might change the pH value and favor the aggregation [68]. Such a phenomenon was observed with thiolated silica particles interacting with different concentrations of hydroxyethylcellulose [69].

TEM Analysis
TEM measurements gave the proof of narrow dispersity of the silica beads ( Figure 2). The beads had an average diameter of 75.9 nm for SiO 2 and SiO 2 @NH 2 and around 80.6 and 82.9 nm for the SiO 2 @PW and SiO 2 @PMo ones respectively, indicating that the structure of the SiO 2 core was maintained during the three steps. The coverage of SiO 2 @NH 2 with POMs could be proven in addition by a textural change of the surface of the beads. Interesting observations could be done concerning the interactions between particles in the case of SiO2@PW and SiO2@PMo. The contrast at the contact area between particles in TEM pictures ( Figure 3) seem to indicate contacts between the beads, stronger than with SiO2 or SiO2@NH2 with angles that seem to be not due to a simple packing. This feature could be similar to the one observed with Europium-containing POMs entrapped within SiO2 [70] on which fusing could be possible since silica covered the POMs but the situation described herein was reverse since POMs covered the surface of the silica beads. The first possible explanation could be that the surface of the beads was rearranged in the presence of acidic POM (and water media) twinning the silica nanoparticles. The surface of silica could be corroded and reacted again, fusing on contact points. This explanation could be valid since this phenomenon did need the presence of POMs and it was not observed in the case of SiO2@NH2. The fusing was proved by Greasley et al. using SiO2 particles and CaO [71] at temperatures higher than the ones used herein. Another explanation could be that POMs are "agglomerated" between SiO2 beads (but not in a crystalline arrangement since no peaks of the POMs were observed in the XRD spectra of SiO2@PM) and favor a simple ionic contact/fusing between the beads. This plausible explanation looks like silica beads functionalized with β-cyclodextrin and G1 adamantly PPI dendrimers (H-bond interactions) [72] or gold nanoparticles decorated with POMs (ionic interactions) [73]. Due to the nature of the silica part in SiO2@PM beads (positively charged in surface through ammonium functions), a complex association composed of POM/ammonium attractions and ammonium/ammonium repulsions might favor electrostatic interactions with specific angles corresponding to superficial contacts, the closest contact between three beads giving a triangular aspect. Thus, an explanation of the observed phenomenon in TEM seems to be situated Interesting observations could be done concerning the interactions between particles in the case of SiO 2 @PW and SiO 2 @PMo. The contrast at the contact area between particles in TEM pictures ( Figure 3) seem to indicate contacts between the beads, stronger than with SiO 2 or SiO 2 @NH 2 with angles that seem to be not due to a simple packing. This feature could be similar to the one observed with Europium-containing POMs entrapped within SiO 2 [70] on which fusing could be possible since silica covered the POMs but the situation described herein was reverse since POMs covered the surface of the silica beads. The first possible explanation could be that the surface of the beads was rearranged in the presence of acidic POM (and water media) twinning the silica nanoparticles. The surface of silica could be corroded and reacted again, fusing on contact points. This explanation could be valid since this phenomenon did need the presence of POMs and it was not observed in the case of SiO 2 @NH 2 . The fusing was proved by Greasley et al. using SiO 2 particles and CaO [71] at temperatures higher than the ones used herein. Another explanation could be that POMs are "agglomerated" between SiO 2 beads (but not in a crystalline arrangement since no peaks of the POMs were observed in the XRD spectra of SiO 2 @PM) and favor a simple ionic contact/fusing between the beads. This plausible explanation looks like silica beads functionalized with β-cyclodextrin and G1 adamantly PPI dendrimers (H-bond interactions) [72] or gold nanoparticles decorated with POMs (ionic interactions) [73]. Due to the nature of the silica part in SiO 2 @PM beads (positively charged in surface through ammonium functions), a complex association composed of POM/ammonium attractions and ammonium/ammonium repulsions might favor electrostatic interactions with specific angles corresponding to superficial contacts, the closest contact between three beads giving a triangular aspect. Thus, an explanation of the observed phenomenon in TEM seems to be situated between beads fusing and/or strong inter-bead electrostatic interactions. Both phenomena being possible in the aqueous media, it might be concluded that ionic rearrangements can occur when the species are mixed in water. Although TEM did give an image of a dried sample, the time dependent aggregation phenomena observed in solution through DLS seemed to corroborate those assumptions.
At the difference with mesoporous silica based materials [77], the content of APTES and PMs on non-porous silica beads was very low (only onto the surface of non-porous silica beads). Thus, it was not obvious to determine directly through IR if some APTES and PMs were grafted on the surface of SiO2 and SiO2@NH2 respectively. Some shouldering could be seen in SiO2@PMo and SiO2@PW at 950 cm  [66,78]. Those absorptions could correspond to the presence of polyanions. In addition, typical vibrations corresponding to Keggin units are overlapped with the ones of SiO2 [79]. An elegant method to prove the grafting was to do difference spectra between SiO2@NH2 and SiO2 ( Figure S4) or SiO2@PM and SiO2@NH2 (Figure 4). Very small changes could be observed at 2926 and 1450-1700 cm −1 that could give a proof of the presence of the grafted APTES on SiO2 ( Figure S4). After
At the difference with mesoporous silica based materials [77], the content of APTES and PMs on non-porous silica beads was very low (only onto the surface of non-porous silica beads). Thus, it was not obvious to determine directly through IR if some APTES and PMs were grafted on the surface of SiO 2 and SiO 2 @NH 2 respectively. Some shouldering could be seen in SiO 2 @PMo and SiO 2 @PW at 950 cm  [79]. An elegant method to prove the grafting was to do difference spectra between SiO 2 @NH 2 and SiO 2 ( Figure S4) or SiO 2 @PM and SiO 2 @NH 2 ( Figure 4). Very small changes could be observed at 2926 and 1450-1700 cm −1 that could give a proof of the presence of the grafted APTES on SiO 2 ( Figure S4).

Multinuclear Solid State NMR
Since IR did not give strongly affirmative answers concerning the nature of the functions surrounding the beads, solid state NMR has been an efficient tool. Indeed, 1 H, 13 C, 29 Si and 31 P are nuclei that can bring several information. All data have been summarized in Table S1.
The 29 Si CP MAS NMR (Table S1 and Figure 5) spectra gave additional information about the grafting on the silica bead itself. In all spectra, the signals at −93, −102 and −111 ppm corresponding to Q2, Q3 and Q4 respectively (Qn = Si(OSi)n(OH)4-n) were in accordance with the SiO2 core [49]. The silane grafting was proved by two signals around −60 and −68 ppm (T2 and T3) [82]. A change of signals proportion was observed from SiO2 to SiO2@NH2 and from SiO2@NH2 to SiO2@PM, the trend being identical when both POMs were added.
Since 29 Si CP MAS NMR could not quantify the Qn, deconvolutions on SiO2 cores only were done on 29 Si MAS NMR spectra, the relative intensity of the signals being indicated in parenthesis in Table  S1. The stronger differences observed in 29 Si CP-MAS (due to cross-polarization) were not so pronounced in 29 Si MAS. ( Figure S6) Those effects could be linked to the interactions between the ionic POMs and the SiO2@NH2 beads, once the proton exchange was performed.
Added POMs covering the SiO2@NH2 beads, different environments could be found, according to ionic interactions between charged POMs and pending ammonium, as well as H-interactions with silanol surfaces [83][84][85][86]. 31 P MAS NMR signals of the grafted ones (Table S1) were shifted comparing to the value of the free POMs (the one used for the grafting) and relatively close to some referenced in the literature in 31 P MAS for "PW12O40" [87] and "PMo12O40" [88] backbones.

Multinuclear Solid State NMR
Since IR did not give strongly affirmative answers concerning the nature of the functions surrounding the beads, solid state NMR has been an efficient tool. Indeed, 1 H, 13 C, 29 Si and 31 P are nuclei that can bring several information. All data have been summarized in Table S1.
The 29 Si CP MAS NMR (Table S1 and Figure 5) spectra gave additional information about the grafting on the silica bead itself. In all spectra, the signals at −93, −102 and −111 ppm corresponding to Q 2 , Q 3 and Q 4 respectively (Q n = Si(Osi) n (OH) 4-n ) were in accordance with the SiO 2 core [49]. The silane grafting was proved by two signals around −60 and −68 ppm (T 2 and T 3 ) [82]. A change of signals proportion was observed from SiO 2 to SiO 2 @NH 2 and from SiO 2 @NH 2 to SiO 2 @PM, the trend being identical when both POMs were added.
Since 29 Si CP MAS NMR could not quantify the Q n , deconvolutions on SiO 2 cores only were done on 29 Si MAS NMR spectra, the relative intensity of the signals being indicated in parenthesis in Table S1. The stronger differences observed in 29 Si CP-MAS (due to cross-polarization) were not so pronounced in 29 Si MAS. ( Figure S6) Those effects could be linked to the interactions between the ionic POMs and the SiO 2 @NH 2 beads, once the proton exchange was performed.
Added POMs covering the SiO 2 @NH 2 beads, different environments could be found, according to ionic interactions between charged POMs and pending ammonium, as well as H-interactions with silanol surfaces [83][84][85][86]. 31 P MAS NMR signals of the grafted ones (Table S1) were shifted comparing to the value of the free POMs (the one used for the grafting) and relatively close to some referenced in the literature in 31

Quantification by Elemental Analysis
From the nitrogen content (%N) found in elemental analysis, it is possible to calculate the number of moles of grafted aminosilane ρ(NH2) and POM grafted (ρ(PM)) per gram of sample S. The values could be compared to the one found using 1 H liquid NMR. The number of moles of nitrogen atoms found in a sample S being equivalent to the number of NH2 fragments, the formula could be defined as follows in Equation (1). According to the Scheme 2, ρ(PM) could be calculated from the elemental analysis since we could postulate that the mixture of SiO2@NH2 with POM did a proton exchange from POM to the pending NH2 functions and no other modifications. ρ(NH2) and ρ(PM) are gathered in Table 1 and Table 2.
In order to find the x value, the important parameter is the mass of the SiO2 core within SiO2@NH2, obtained from %N values that corresponds to the number of grafted APTS fragments. This number is supposed to be unchanged after addition of POM, the variation observed in %N for SiO2@PMo and SiO2@PW will depend on the quantity of POMs retained by the beads (Equation (3)) Figure 5. 29 Si CP MAS NMR spectra of SiO 2 (a), SiO 2 @NH 2 (b), SiO 2 @PW (c) and SiO 2 @PMo (d).

Quantification by Elemental Analysis
From the nitrogen content (%N) found in elemental analysis, it is possible to calculate the number of moles of grafted aminosilane ρ(NH 2 ) and POM grafted (ρ(PM)) per gram of sample S. The values could be compared to the one found using 1 H liquid NMR. The number of moles of nitrogen atoms found in a sample S being equivalent to the number of NH 2 fragments, the formula could be defined as follows in Equation (1). According to the Scheme 2, ρ(PM) could be calculated from the elemental analysis since we could postulate that the mixture of SiO 2 @NH 2 with POM did a proton exchange from POM to the pending NH 2 functions and no other modifications. ρ(NH 2 ) and ρ(PM) are gathered in Tables 1 and 2

Quantification by Elemental Analysis
From the nitrogen content (%N) found in elemental analysis, it is possible to calculate the number of moles of grafted aminosilane ρ(NH2) and POM grafted (ρ(PM)) per gram of sample S. The values could be compared to the one found using 1 H liquid NMR. The number of moles of nitrogen atoms found in a sample S being equivalent to the number of NH2 fragments, the formula could be defined as follows in Equation (1). According to the Scheme 2, ρ(PM) could be calculated from the elemental analysis since we could postulate that the mixture of SiO2@NH2 with POM did a proton exchange from POM to the pending NH2 functions and no other modifications. ρ(NH2) and ρ(PM) are gathered in Table 1 and Table 2.
Core Silane x PM In order to find the x value, the important parameter is the mass of the SiO2 core within SiO2@NH2, obtained from %N values that corresponds to the number of grafted APTS fragments. This number is supposed to be unchanged after addition of POM, the variation observed in %N for SiO2@PMo and SiO2@PW will depend on the quantity of POMs retained by the beads (Equation (3)) Scheme 2. Schematic representation of the SiO 2 @NH 2 and SiO 2 @PM beads. In order to find the x value, the important parameter is the mass of the SiO 2 core within SiO 2 @NH 2 , obtained from %N values that corresponds to the number of grafted APTS fragments. This number is supposed to be unchanged after addition of POM, the variation observed in %N for SiO 2 @PMo and SiO 2 @PW will depend on the quantity of POMs retained by the beads (Equation (3)) M(SiO 2 @NH 2 ) can be found using the N% of SiO 2 @NH 2 (Equation (4)) Then, injecting in equation W, the x can be obtained from simple data using Equation (5).
x will give the number of POM vs. N within the sample. According to the equation, the calculated data have been compilated in Table 1.
The ideal x value would have been 0.33, i.e., one POM retained by three NH 2 functions. This could be due to the fact the some NH 2 functions are "free" and the POM in the complete deprotonated form in the case of SiO 2 @PMo while in the case of SiO 2 @PW, the POM was not completely deprotonated.

Quantification of Grafted Functions and Retained POM by Liquid NMR
Multinuclear liquid NMR ( 1 H, and 31 P) was used for this purpose with SiO 2 @NH 2 , SiO 2 @PMo and SiO 2 @PW beads. Silica beads and POMS can be destroyed in alkaline medium, giving silicates for the silica part, and tungstates/molybdates and phosphates for the POMs. It was proved that quantification could be done through the dissolution of the silica in aqueous solution in an alkali medium [89]. The organic backbone was maintained and could be quantified using an internal standard with 1 H NMR. Using this method, the number of APTS fragments ρ(NH 2 ) could be evaluated per gram of sample (Table 2).
Using the same methodology than the 1 H NMR, the SiO 2 @PM was dissolved in very basic solution (pH = 13), in order to isolate the PO 4 3− . The 31 P NMR signals obtained with the beads were quantified using an external calibration curve with different aqueous solutions of H 3 PO 4 at pH = 13. The parameter found through this method (Table 2), ρ(PM), was relatively close to the one found through elemental analysis.

Surface Coverage of the Beads Through EA and NMR.
Considering the number of functions present on one bead and the average size of the beads, using the classical density of SiO 2 , we could evaluate the surface coverage in number of functions per nm 2 . The demonstration of equation 6 is given in Supplementary information (Appendix A1).
Using those calculations, results (Table 2) indicated a relatively constant µ(NH 2 ) value around 6.8 functions per nm 2 . This is in agreement with the hypothesis we assumed for the calculations on 3.4.1.

Catalysis
Homogenous catalysis with commercial POMs (especially H 3 PW 12 O 40 and H 3 PMo 12 O 40 ) has shown excellent activity in oxidation reactions [90][91][92][93][94][95][96][97]. Several examples have proven to be effective and the composition of the POMs can be modified for better selectivity [23,34]. In most of published experiments, organic solvent was needed, and the catalyst could not be recovered. Few examples described grafted POMs using Merrifield resins [34], polymeric quaternary ammonium salts [98], mesoporous supports [99][100][101], MOFs [102,103] or carbonaceous supports [104]. Mesoporous supports are interesting, but POMs entrapped within zeolites might be not all accessible. The advantage of the present process lies on complete accessibility of all POMs since only at the surface of the support. One drawback could be the lack of selectivity, but the reactions studied being quite simple, selectivity is not such a big issue. Following the grafting concept, SiO 2 @PM (M= Mo or W) was studied herein to achieve activity, recovery and reuse. A low ratio of POMs vs. substrate was tested. The SiO 2 @PM objects were recycled and used during three runs, with aqueous TBHP as an oxidant. The activity of the catalysts was tested on four model substrates. Cyclooctene (CO) is known to give essentially the cyclooctene oxide (COE) and few by-products. Cyclohexene (CH) gives cyclohexene oxide (CHO) and more products due to ring opening. Limonene (Lim) is a good biomass-issued substrate to study with different useful by-products. Cyclohexanol (CHol) is also interesting since its oxidation gives normally cyclohexanone, useful for the adipic acid synthesis. Relevant points are the low POM/substrate ratio used in the experiments (see tables) and no added organic solvent. This last point is something important towards the quest of chemical process tending to diminish the Green House effect [105].

Cyclooctene (CO) Epoxidation.
CO is interesting because the corresponding epoxide, cyclooctene oxide (COE) is known to be relatively stable towards ring-opening reactions. (Scheme 3) Although not frequent, hydrolysis and subsequent ring-opening might respectively lead to cyclooctanediol and suberic acid [106]. The oxidant (CO/TBHP ratio being 1:1.5) [23] is added once the temperature reached 80 • C. It must be pointed out that no organic solvent was added. The engaged mass of functionalized silica was identical but due to different POM/SiO 2 grafting ratios between Mo and W experiments, the POM/substrate ratio differ. A relatively low POM/substrate ratio (0.070% and 0.058% for H 3

Catalysis
Homogenous catalysis with commercial POMs (especially H3PW12O40 and H3PMo12O40) has shown excellent activity in oxidation reactions [90][91][92][93][94][95][96][97]. Several examples have proven to be effective and the composition of the POMs can be modified for better selectivity [23,34]. In most of published experiments, organic solvent was needed, and the catalyst could not be recovered. Few examples described grafted POMs using Merrifield resins [34], polymeric quaternary ammonium salts [98], mesoporous supports [99][100][101], MOFs [102,103] or carbonaceous supports [104]. Mesoporous supports are interesting, but POMs entrapped within zeolites might be not all accessible. The advantage of the present process lies on complete accessibility of all POMs since only at the surface of the support. One drawback could be the lack of selectivity, but the reactions studied being quite simple, selectivity is not such a big issue. Following the grafting concept, SiO2@PM (M= Mo or W) was studied herein to achieve activity, recovery and reuse. A low ratio of POMs vs. substrate was tested. The SiO2@PM objects were recycled and used during three runs, with aqueous TBHP as an oxidant. The activity of the catalysts was tested on four model substrates. Cyclooctene (CO) is known to give essentially the cyclooctene oxide (COE) and few by-products. Cyclohexene (CH) gives cyclohexene oxide (CHO) and more products due to ring opening. Limonene (Lim) is a good biomassissued substrate to study with different useful by-products. Cyclohexanol (CHol) is also interesting since its oxidation gives normally cyclohexanone, useful for the adipic acid synthesis. Relevant points are the low POM/substrate ratio used in the experiments (see tables) and no added organic solvent. This last point is something important towards the quest of chemical process tending to diminish the Green House effect [105].

Cyclooctene (CO) Epoxidation.
CO is interesting because the corresponding epoxide, cyclooctene oxide (COE) is known to be relatively stable towards ring-opening reactions. (Scheme 3) Although not frequent, hydrolysis and subsequent ring-opening might respectively lead to cyclooctanediol and suberic acid [106]. The oxidant (CO/TBHP ratio being 1:1.5) [23] is added once the temperature reached 80 °C. It must be pointed out that no organic solvent was added. The engaged mass of functionalized silica was identical but due to different POM/SiO2 grafting ratios between Mo and W experiments, the POM/substrate ratio differ. A relatively low POM/substrate ratio (0.070% and 0.058% for H3PW12O40 and H3PMo12O40 respectively) was added, among the lowest observed in the literature. Recovered catalysts were reused under the same experimental conditions to test the activity persistence. A test with non-grafted POMs was performed for comparison. Results have been compiled in Table 3.  Although the activity of SiO 2 @PM was slower during the first 6 h, CO conversions were almost the same than free POMs after 24 h but more selective towards COE in the case of the grafted POMs ( Figure 6). The higher selectivity might be due to less acidic media with SiO 2 @PM. The SiO 2 @PMo catalyst was more active than SiO 2 @PW 12 , giving better CO conversion and higher selectivity towards COE. This trend was also observed with the free POMs. An interesting fact was the reuse of SiO 2 @PM. For both metals, catalytic performances were close during two extra runs ( Figure S7; with average Turn Over Number (TON) values around 950 and 1640 for W and Mo respectively).  Although the activity of SiO2@PM was slower during the first 6 hours, CO conversions were almost the same than free POMs after 24 hours but more selective towards COE in the case of the grafted POMs ( Figure 6). The higher selectivity might be due to less acidic media with SiO2@PM. The SiO2@PMo catalyst was more active than SiO2@PW12, giving better CO conversion and higher selectivity towards COE. This trend was also observed with the free POMs. An interesting fact was the reuse of SiO2@PM. For both metals, catalytic performances were close during two extra runs ( Figure S7; with average Turn Over Number (TON) values around 950 and 1640 for W and Mo respectively).
The mechanism is not straightforward since the POMs are non-substituted and do not act as support of active metal as found in several other articles [107,108]. We suppose the formation of diperoxo on one metal, responsible to the oxygen transfer from perox to olefin [109]. The mechanism is not straightforward since the POMs are non-substituted and do not act as support of active metal as found in several other articles [107,108]. We suppose the formation of diperoxo on one metal, responsible to the oxygen transfer from perox to olefin [109].

Cyclohexene (CH) (ep)oxidation
(Ep)oxidation of CH, precursor of adipic acid [40] and simplified version of limonene, competes between epoxidation (CHO and the ring opening CHD) and allylic oxidation (CHol and CHone). The studies were done with the same TBHP ratio than for CO but with a five times lower catalyst charge than for CO (i.e., POM/CH ratio of 0.014% and 0.0116% for H 3 PW 12 O 40 and H 3 PMo 12 O 40 respectively) in order to exhibit the high activity of the catalysts. (Scheme 4) As we observed previously with Mo tridentate compounds [30], the epoxidation is the main reaction when TBHP was used as oxidant. (Ep)oxidation of CH, precursor of adipic acid [40] and simplified version of limonene, competes between epoxidation (CHO and the ring opening CHD) and allylic oxidation (CHol and CHone). The studies were done with the same TBHP ratio than for CO but with a five times lower catalyst charge than for CO (i.e., POM/CH ratio of 0.014% and 0.0116% for H3PW12O40 and H3PMo12O40 respectively) in order to exhibit the high activity of the catalysts. (Scheme 4) As we observed previously with Mo tridentate compounds [30], the epoxidation is the main reaction when TBHP was used as oxidant.   H3PW12O40  1  31  <1  4  3  3  11,307   SiO2@PW   1  45  1  2  4  5  21,458  2  43  1  2  2  3  20,649  3  26  3  3  7  7  12,373  H3PMo12O40  1  91  <1  40  3  2  52,728   SiO2@PMo   1  80  13  26  5  2  46,732  2  74  15  22  6  3  42,487  3  60  26  After 48 h at 60 °C, as for CO substrate, SiO2@PMo was much more active than SiO2@PW12. (Table  4) The free H3PMo12O40 favored the epoxidation and the ring opening of the epoxide. The allylic oxidation seemed to be the preferred pathway with W containing species but not with a high difference towards Mo ones, showing that allylic oxidation did work without a catalyst (Figure 7). The reuse of the SiO2@PM exhibited interesting durability during two runs and the third started to show lower activity ( Figure S8). After 48 h at 60 • C, as for CO substrate, SiO 2 @PMo was much more active than SiO 2 @PW 12 . (Table 4) The free H 3 PMo 12 O 40 favored the epoxidation and the ring opening of the epoxide. The allylic oxidation seemed to be the preferred pathway with W containing species but not with a high difference towards Mo ones, showing that allylic oxidation did work without a catalyst (Figure 7). The reuse of the SiO 2 @PM exhibited interesting durability during two runs and the third started to show lower activity ( Figure S8).

Catalyzed Oxidation of Limonene
Oxidation reaction of limonene could lead to several different products corresponding to epoxidation with LO (cis and trans) and epoxide opening LD (ax and eq) and allylic oxidation (carveol C ol , and carvone C one ) [27,29] (Scheme 5).
Under the same conditions than for CO, results have been complied in Table 5. The results were strongly depending to the nature of the catalysts. The major products observed were axand equ-LD for SiO 2 @PMo, and carveol and carvone for SiO 2 @PW 12 (Figure 8). This observation went in a straight line with observations done with CH.
After 48 h at 60 °C, as for CO substrate, SiO2@PMo was much more active than SiO2@PW12. (Table  4) The free H3PMo12O40 favored the epoxidation and the ring opening of the epoxide. The allylic oxidation seemed to be the preferred pathway with W containing species but not with a high difference towards Mo ones, showing that allylic oxidation did work without a catalyst (Figure 7). The reuse of the SiO2@PM exhibited interesting durability during two runs and the third started to show lower activity ( Figure S8).

Catalyzed Oxidation of Limonene
Oxidation reaction of limonene could lead to several different products corresponding to epoxidation with LO (cis and trans) and epoxide opening LD (ax and eq) and allylic oxidation (carveol C ol , and carvone C one ) [27,29] (Scheme 5). Under the same conditions than for CO, results have been complied in Table 5. The results were strongly depending to the nature of the catalysts. The major products observed were ax-and equ-LD for SiO2@PMo, and carveol and carvone for SiO2@PW12 (Figure 8). This observation went in a straight line with observations done with CH.

Catalyzed Oxidation of Limonene
Oxidation reaction of limonene could lead to several different products corresponding to epoxidation with LO (cis and trans) and epoxide opening LD (ax and eq) and allylic oxidation (carveol C ol , and carvone C one ) [27,29] (Scheme 5).

Scheme 5. Oxidation reaction of limonene.
Under the same conditions than for CO, results have been complied in Table 5. The results were strongly depending to the nature of the catalysts. The major products observed were ax-and equ-LD for SiO2@PMo, and carveol and carvone for SiO2@PW12 (Figure 8). This observation went in a straight line with observations done with CH.   Activity of SiO2@PMo was higher than SiO2@PW after each run ( Figure S9). This could be also assumed by the absence of LO with Mo after 24 h, (but present at 6 hours) and the presence of LDs. This was previously observed with other type of complexes [32]. Catalysts have a very weak influence on allylic oxidation, which could explain the similar selectivity of carveol and carvone. Durability of SiO2@PM was also proved by recycling with average TONs of 110 and 228 for SiO2@PW and SiO2@PMo respectively.

Catalysis Oxidation of Cyclohexanol
Cyclohexanol (CYol), as a precursor of adipid acid, i.e., one component of KA oil. Cyclohexanone (CYone) was the only oxidation product that was observed (Scheme 6). Under the same catalytic conditions than for CO and Lim, results have been compiled in Table 6.  y Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Powder Xdiagrams of SiO2 (blue), SiO2@PW (orange) and SiO2@PMo (grey) particles. Figure S2: f Powder X-ray diffractions of (a) H3PMo12O40 (orange) and SiO2@PMo (blue) and (b) H3PW12O40 iO2@PW (blue). The intensities of SiO2@PMo and SiO2@PW were magnified 10 times. Figure S3: wn: Relevant IR vibration zones for SiO2, SiO2@NH2, SiO2@PW, SiO2@PMo. Figure S4: Difference NH2 -SiO2) on specific ranges (in blue). The spectrum of APTES is indicated in orange Table S1: -state NMR data. Figure S5: 13 C MAS NMR spectra of SiO2@PW (up), SiO2@PMo (middle) and wn). Figure S6: 29   Activity of SiO 2 @PMo was higher than SiO 2 @PW after each run ( Figure S9). This could be also assumed by the absence of LO with Mo after 24 h, (but present at 6 h) and the presence of LDs. This was previously observed with other type of complexes [32]. Catalysts have a very weak influence on allylic oxidation, which could explain the similar selectivity of carveol and carvone. Durability of SiO 2 @PM was also proved by recycling with average TONs of 110 and 228 for SiO 2 @PW and SiO 2 @PMo respectively.

Catalysis Oxidation of Cyclohexanol
Cyclohexanol (CYol), as a precursor of adipid acid, i.e., one component of KA oil. Cyclohexanone (CYone) was the only oxidation product that was observed (Scheme 6). Under the same catalytic conditions than for CO and Lim, results have been compiled in Table 6. Both grafted catalysts had low conversion (Figure 9), SiO2@PMo being more active than SiO2@PW12 (with average conversion of 10% and 18% respectively) but with moderate activity compared to the free POMs. The mechanism might imply the formation of the diperoxo compound [110]. Although slow, the processes were more selective when grafted, certainly due to less acidic conditions. At the difference with other efficient processes using non-grafted compounds but microwave activation [111], recycling of the catalyst was possible ( Figure S10) and interesting with similar conversions within the time.

Conclusions
Ionic immobilization of POMs on silica beads functionalized by APTES led to the development of new catalytic materials (SiO2@PM) used for organic solvent-free (ep)oxidation reactions, showing with the studied substrates better selectivity than the corresponding free POMs. Morphological (DLS and TEM) studies of the SiO2@PM objects exhibited interesting behavior with ionic interactions going to dynamic particles agglomeration. This phenomenon seems to ensure the stability of nonmonolithic recoverable catalytic objects, interesting in terms of potential industrial use. This methodology of catalysis uses organic solvent-free process, smoother oxidant, minimal catalyst loading and catalyst recyclability in a straight line with some principles of green chemistry. This environmentally benign protocol with a new type of catalytic materials can be easily modulated (other functionalization on beads, other grafted catalysts and other catalyzed reactions) and does open a new field of future investigations. Both grafted catalysts had low conversion (Figure 9), SiO 2 @PMo being more active than SiO 2 @PW 12 (with average conversion of 10% and 18% respectively) but with moderate activity compared to the free POMs. The mechanism might imply the formation of the diperoxo compound [110]. Although slow, the processes were more selective when grafted, certainly due to less acidic conditions. At the difference with other efficient processes using non-grafted compounds but microwave activation [111], recycling of the catalyst was possible ( Figure S10) and interesting with similar conversions within the time. Both grafted catalysts had low conversion (Figure 9), SiO2@PMo being more active than SiO2@PW12 (with average conversion of 10% and 18% respectively) but with moderate activity compared to the free POMs. The mechanism might imply the formation of the diperoxo compound [110]. Although slow, the processes were more selective when grafted, certainly due to less acidic conditions. At the difference with other efficient processes using non-grafted compounds but microwave activation [111], recycling of the catalyst was possible ( Figure S10) and interesting with similar conversions within the time.

Conclusions
Ionic immobilization of POMs on silica beads functionalized by APTES led to the development of new catalytic materials (SiO2@PM) used for organic solvent-free (ep)oxidation reactions, showing with the studied substrates better selectivity than the corresponding free POMs. Morphological (DLS and TEM) studies of the SiO2@PM objects exhibited interesting behavior with ionic interactions going to dynamic particles agglomeration. This phenomenon seems to ensure the stability of nonmonolithic recoverable catalytic objects, interesting in terms of potential industrial use. This methodology of catalysis uses organic solvent-free process, smoother oxidant, minimal catalyst loading and catalyst recyclability in a straight line with some principles of green chemistry. This environmentally benign protocol with a new type of catalytic materials can be easily modulated (other functionalization on beads, other grafted catalysts and other catalyzed reactions) and does open a new field of future investigations.

Conclusions
Ionic immobilization of POMs on silica beads functionalized by APTES led to the development of new catalytic materials (SiO 2 @PM) used for organic solvent-free (ep)oxidation reactions, showing with the studied substrates better selectivity than the corresponding free POMs. Morphological (DLS and TEM) studies of the SiO 2 @PM objects exhibited interesting behavior with ionic interactions going to dynamic particles agglomeration. This phenomenon seems to ensure the stability of non-monolithic recoverable catalytic objects, interesting in terms of potential industrial use. This methodology of catalysis uses organic solvent-free process, smoother oxidant, minimal catalyst loading and catalyst recyclability in a straight line with some principles of green chemistry. This environmentally benign protocol with a new type of catalytic materials can be easily modulated (other functionalization on beads, other grafted catalysts and other catalyzed reactions) and does open a new field of future investigations.