Size-Dependent Catalytic Activity of Oxo-Hydroxo Titanium subnano

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Size-Dependent Catalytic Activity of Oxo-Hydroxo Titanium subnano-islets grafted on Organically Modified Mesoporous Silica Lin Fang, Belén Albela, Boting Yang, Yuting Zheng, Peng Wu, Mingyuan He, and Laurent Bonneviot Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01932 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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Size-Dependent Catalytic Activity of Oxohydroxo Titanium Subnano-Islets Grafted on Organically Modified Mesoporous Silica Lin Fang,a,b Belén Albela, a, b Boting Yang, b Yuting Zheng, a,b Peng Wu, b Mingyuan He b and Laurent Bonneviot a* a

Laboratoire de Chimie, Ecole Normale Supérieure de Lyon; Université de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France; e-mail: [email protected]

b

Shanghai Key Lab of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, China.

Abstract

The reaction between titanium alkoxides, [Ti(OR)4], and surface silanol groups is widely used to generate grafted oxo-hydroxo titanium species, the size of which is difficult to control. Partial capping of the surface silanols in the presence of the masking pattern of self-repelling tetramethylammonium ions allows us to isolate surface silanol islets, on which isolated titanium ions and dimeric oxo titanium species can be generated up to 2 Ti/Si mol%. Above this loading, and up to ~ 8 Ti/Si mol%, higher oligomers (trimers, hexamers, octamers, and so on) are formed, reaching the size obtained at much lower loadings (99%, Merck) as surfactant and sodium hydroxide (>99%, ACROS) to adjust the pH. Typically, the silica sol was prepared by stirring a mixture of Ludox (15.5 g), NaOH (2 g) and H2O (50 ml) at 60 °C (2 h). Then, the sol was added dropwise to a clear solution containing dissolved CTATos (2.5 g) in deionised water (90 ml) and stirred gently at 60 °C. Stirring of the mother liquor was maintained at 60 °C for 2 h. Then, it was transferred and aged in a Teflon-lined autoclave at 130 °C (20 h). The solid was filtered, washed using deionised water and dried at 80 °C overnight.

Cation exchange of TMA+ for CTMA+ leading to LUS-TMA. LUS-CTA (1 g) was stirred in a technical ethanol (30 ml, 96%) solution containing tetramethylammonium bromide (0.45 g, TMABr, 99%, Aldrich) at 40 °C for 15 min, filtrated and washed twice using ethanol and then acetone. After cation exchange and the subsequent steps of the synthesis, the open pores readily adsorbed water that was evacuated by drying overnight at 80°C before storage in a closed vial before the next step of modification.

Molecular stencil of TMA+ ammonium, LUS-T. HCl (1ml of 1 N aq., ACROS, i.e., 1H+/TMA+) was added to an ethanol solution (30 ml of 95% EtOH) containing LUS-TMA (1

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g) to partially extract TMA+ (50%), and stirred at 40 °C (1 h). The filtered solid – washed and dried overnight in air and, denoted as LUS-T (T for TMA), retained the masking molecular stencil of TMA+ partly covering the surface.

Grafting the dipodal EBDMS functions in the presence of TMA+ to yield LUS-T-E and LUS-T-EOH. After cooling down LUS-T to RT under argon, dry cyclohexane (80 ml) and 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopentane (TMDSACP, 95%, ABCR-Roth, 3.0 ml) were added and stirred (1 h) to let the reactant diffuse inside the nano channels of the solid before heating. Then, the mixture was refluxed at 80 °C (16 h), yielding LUS-T-E (E for EBDMS, ethyl 1, 2-bis(dimethylsilyl)). The masking TMA+ was fully extracted using the same concentration of HCl in ethanol (95%, 30 ml) in the same conditions as above, which corresponds at this stage to 2 eq H+ per remaining TMA+. The obtained solid, named LUS-E-

OH, was filtrated and, dried at 130°C. Grafting Titanium on the channels walls of the hexagonal silica, LUS-E-Ti-n. Titanium was grafted either on the organically modified LUS-T-E-OH or on the silica pure LUS, from which CTA+ was fully extracted using aqueous HCl (2 eq H+ per remaining CTA+). The dried solids were reacted with distilled titanium isopropoxide (Ti(O-iPr)4 97%, JANSSEN) in refluxing cyclohexane over night. After filtration and washing as above, the solid was dried at 80 °C in air before storage, and named LUS-E-Ti-n. Characterization of the materials The low angle X-ray powder diffraction (XRD) experiments were carried out using a Bruker (Siemens) D5005 diffractometer with a CuKα monochromatic radiation. Thermogravimetric analyses were performed using a DTA-TG Netzsch STA 409 PC/PG instrument. Samples (10 mg) placed in a 70 µL alumina crucible were heated in air flow up to 1000 °C at a heating rate of 10 °C/min. Nitrogen adsorption-desorption isotherms at 77 K


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were determined at BELSORP-max (BEL Japan. INC.). In all cases, samples (50-70 mg) were degassed at 80 °C (12 h) and then at 130 °C (2 h) under vacuum (< 0.01 kPa) before analysis. The specific surface area was calculated according to the BET method in the 0.050.25 range of relative pressure. The mesopore diameter was calculated from the capillary condensation using the equation DBdB(Å)

= 14.60994 +74.67812*x - 81.96198*x2 +

155.8457*x3 where x = P/P0 (0.11 ≤ P/P0 ≤ 0.50).33, 34 Attenuated total reflectance infrared spectra (ATR-IR) were recorded on 1 mg of the solid using a JASCO FT/IR-4200 (JASCO) spectrometer equipped with the ATR PRO470-H accessory. The peak intensities were normalized to compare the evolution of the bands after each step of synthesis. The asymmetric IR bending mode of the [SiO4] tetrahedral units at 460 cm-1 served as an internal reference for normalization. The MAS

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Si NMR spectra were

measured using a Bruker AVANCE III 500 spectrometer at 99.362 MHz and 4 mm zirconia rotors spinning at ca. 5 kHz. Trimethylsilane (TMS) was the external reference for the

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Si

chemical shifts (δ). Solid UV-visible spectra were recorded on the calcined samples using a JASCO V-670 (JASCO) spectrophotometer equipped operated in the diffuse reflectance mode using an integration sphere. Catalytic tests A typical heterogeneous catalytic epoxidation experiment was conducted at 60 °C for 2 h, the catalyst (50 mg) being stirred in a mixture of cyclohexene (>98%, 10 mmol), tert-butyl hydroperoxide (TBHP 30%, 10 mmol), and acetonitrile (10 ml) as a solvent. The catalyst powder was then removed from the reaction by centrifugation. The products were analyzed using a Shimadzu GC-2014 gas chromatograph equipped with a FID detector and cyclohexanone (>99%, ) as internal standard. All the chemicals used in the catalytic reactions were purchased from China National Medicines Corporation Ltd.


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RESULTS AND DISCUSSION The surface was modified according to a stepwise process that progressively built the masking pattern of adsorbed tetramethylammonium ions. Then, we followed up by reacting the exposed silanol groups with the silazane TMDSACP to generate the dipodal EBDMS moieties. The solid was characterized and therefore named at each step of the synthesis. The conditions adopted were very similar to those adopted previously for the monopodal capping agent, trimethylsilyl. We also started from a very similar as-made 2D hexagonal MCM-41 type of mesostructured porous silica named LUS.35 The as-made material still containing the CTA+ surfactant was denoted as LUS-CTA, and was renamed LUS-TMA after exchanging CTA+ for TMA+. The TMA+ surface coverage can be decreased by using a slightly acidic ethanol solution of HCl (1 eq H+, per CTA+ removed), generating the so-called LUS-T.25, 28 Note that TMA+ was preferred to CTA+ as masking agent, since the absence of the C-16 carbon chain produces much less hindrance for the subsequent modifications.28 Then, LUS-T was capped by grafting the dipodal EBDMS moieties, yielding the LUS-T-E material. According to quantitative

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Si NMR (Table 1), ~ 20 % more silicon atoms, called here

organic silicon (Siorg) for convenience, were added to the sample, as they belonged to the grafted EBDMS moieties. This is slightly lower than the Siorg amount necessary to reach full monopodal coverage using TMS, i.e., 23 %.28 To render the unreacted silanol groups accessible again in the material named LUS-E-OH, the masking TMA+ cations were removed at a temperature of 0 °C that minimizes EBDMS degrafting. Finally, twelve different Ti materials were generated from LUS-E-OH treated with various concentrations of Ti(O-iPr)4 in refluxing cyclohexane, yielding LUS-E-Ti-n materials where n is an integer increasing with the nominal T/SiML molar % in the mother liquor (Figure 1 and Table S1). The retention of EBDMS was controlled by elemental analyses, solid

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Si MAS-NMR, and FT-IR

spectroscopies, after extraction of TMA+ ions and Ti grafting (Tables S2 and S3). In the LUS-


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E-Ti-n series, the Ti loading in the final material ranged from 0.6 to 5.5 wt% while this range spanned from 0.2 and 10.6 wt% in absence of EBDMS for the LUS-Ti-n series. To provide a better estimation of the atomic surface occupancy, the Ti loading will be provided respectively in Ti/SiML or Ti/Siinorg molar ratio in solution or in the solid knowing that SiiML and Siinorg refers to silicon atoms of the mother liquor (ML) and those called inorganic for the silica framework (Table S1 for conversion molar ratio to wt%).

A diffusion control Ti grafting with an optimum loading. Strikingly, the highest Ti loading was not reached with the highest concentration of titanium alkoxide (Figure 1). In fact, a rather good linear dependence, corresponding to a grafting yield of nearly 100%, was observed from LUS-E-Ti-1 to LUS-E-Ti-7 materials, i. e., up to 0.086 Ti/Si molar ratio. Then, moving from LUS-E-Ti-8 to LUS-E-Ti-12, a drop of Ti uptake from 0.086 down to 0.033 Ti/Si molar ratio was observed, attesting to a drastic drop in the grafting yield. In the

LUS-Ti-n materials having an organic free surface and less hindered channels, the same effect was observed, though it was shifted at much higher Ti molar ratios (insert, Figure 1). The maximum uptake was indeed attained in LUS-Ti-7 for an uptake of 0.216 Ti/Si molar ratio and the drop observed in LUS-Ti-8 for an uptake of 0.150 Ti/Si molar ratio. In these two series, the 2D-hexagonal structure (p6mm point group) was retained, according to X-ray diffraction patterns (Figure S1). Furthermore, nitrogen sorption isotherms were consistent with a preserved channel structure, characterized by a sharp adsorption step typical of such mesoporous silica. Nonetheless, a decrease in pore size was observed after each modification step indicating that the uptake was taking place in the channel of the hexagonal structure (Table 1, Figure S2). Therefore, this striking decrease in the yield of titanium grafting was attributed to a diffusion control process, likely triggered by pore plugging (see discussion below). Note that this plugging was not operating for N2 molecules, as no bottleneck effect was observed on the N2 adsorption isotherms. This indicates that the plugging operates merely


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by decreasing the channel clearance acting preferentially on Ti(OR)4 species that are much larger species than N2 molecules (see below the discussion on oligomer growth).

Consistent pore size and surface polarity evolutions with internal modification. Both pore volume and specific surface area decreased consistently with the partial channel filling due to EBDMS functions (compare LUS and LUS-E-OH entries in Table 1). Accordingly, the pore diameter decreased from 4.0 nm in LUS to 3.3 nm in LUS-E-OH. The CBET parameter, which characterizes the adsorbent-adsorbate affinity, also decreased from 111 ± 3 to 32 ± 1, consistent with a more hydrophobic surface. Titanium incorporation inverted the trend on C that increases with higher metal loadings, as it indeed increases the polarity of the surface adding TiOH functions to the surface (LUS-E-Ti-3, LUS-E-Ti-4 and LUS-E-Ti-8, Table 1). The decrease of the internal surface area at each step of the MSP protocol was also consistent with the decoration of the channel silica walls by both organic EBDMS and inorganic Ti moieties (Table 1, Figure S2).

Progressive internal functionalities monitored by FT-IR spectroscopy. In the as-synthesized LUS-CTA, the presence of CTA+ was attested by the C-H stretching (νC-H) IR modes in the 2850 - 2960 cm-1 range and a set of bending (δC-H) vibrations centered at ca. 1490 cm-1 (Figure 2A-a). These CTA+ bands were replaced by those of TMA+ in LUS-TMA when CTA+ was exchanged for TMA+ (Figure 2A-b).28 The latter bands consistently further decreased upon partial removal of TMA+ (LUS-T, Figure 2A-c), and even further during the incorporation of EBDMS, revealing an unexpected loss of TMA+ ions, as confirmed by elemental analysis (40 ± 5 % loss, LUS-T-E, Figure 2A-d and Table S2). The EBDMS grafting was also revealed by the appearance of novel C-H stretching vibration modes that remained after Ti grafting, attesting to their retention in this last step of the synthesis (Figure 2-A, d-f). The low wavenumber FT-IR region characterizes the silicon environments (Figure 2B). The broad band centered between 964 and 935 cm-1 corresponds mainly to the


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asymmetric νSi-O stretching mode of a variety of silanol and silanolate environments. The vibration modes centered at 964 cm-1 were assigned to silanolate functions, Si-O- (Figure 2B, a-c). The shift of this band after total or partial extraction of CTA+ or TMA+ down to 935 cm-1 accounted for the protonation of the silanolate groups, yielding Si-OH groups (Figure 2B, e).32 This region of the spectra also exhibits a narrow feature at 950 cm-1, when the counterion was either the CTA+ or the TMA+ that is assigned to νC-N vibration.28 Grafting Ti(i-PrO)4 shifts the band at 935 back to 964 cm-1, consistent with the formation of Si-O-Ti bridges (Figure 2B, f).36 The incorporation of EBDMS is characterized by the appearance of two narrow bands at 790 and 833 cm-1, which both overlap a larger one at 800 cm-1, characteristic of the stretching mode of tetrahedral [SiO4] units of the silica matrix. These new featuring bands are assigned to the symmetric νSi-C stretching vibration modes of the [SiOC3] units of the EBDMS moieties, called here “organic silicon”, Siorg. By opposition, the silicon engaged in [SiO4] units belongs to the siliceous wall of the pore and is called here “inorganic silicon, Siinorg. The relatively constant intensity of these characteristic IR bands shows that contrary to the monopodal trimethylsilyl, EBDMS is not degrafted.

Grafting mode of the capping EBDMS function monitored by

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Si MAS NMR. The

question as to whether EBDMS is mono- or di-grafted, and whether it pairs onto the silica surface was resolved using 29Si MAS NMR. Indeed, three signals were observed at 6.7, 12.8 and 18.6 ppm, and were assigned to C3-Si-OH (M0), C3-Si-O-Si-O3 (M1, M1’, M1”) and C3-SiO-Si-C3 (M2) types of silicon atoms, respectively. The first one is the fingerprint of monografted EBDMS moieties with a non-grafted silyl end-group, M0. Conversely, M2 characterizes dimers (Scheme 1). Note that the presence of higher oligomers is improbable, as no loss of Siorg was observed after the cleaving of the Siorg-O-Siorg bridges (see below and Table S3). The unresolved M1, M1’ and M1” signals characterize the Siorg of the anchoring organosilyl end-groups, belonging to bi-grafted, mono-grafted EBDMS species or bi-grafted


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EBDMS pairs, respectively (Scheme 1).37 The signal of grafted Siorg atoms at 12.8 ppm, which was much broader than the other two signals, became narrower after removal of the TMA+ ions (Figure 3). This narrowing effect, also observed on monopodal trimethyl silyl, is likely due to the suppression of the ring current produced by the neighboring TMA+ ions moving in the strong field imposed during the NMR measurement.38 The relative concentration of each species was obtained from the

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Si NMR peaks

according to a formula provided in the caption of Table S3. We found that the LUS-T-E material mainly possesses di-grafted EBDMS moieties (85%), no monografted EBDMS species (0%), and di-grafted dimeric EBDMS species (15%) (Tables 1 and S3). When TMA+ was removed, monografted monomers were generated (ca. 10%) at the expense of the digrafted species (75%), while the dimeric species remained intact. More monografted EBDMS were generated during the incorporation of Ti(IV) ion, mainly at the expense of the digrafted EBDMS pairs. Indeed, in LUS-E-Ti-4, about one third of the EBDMS pairs disappeared, while this reduction was about two-thirds in LUS-E-Ti-8. In LUS-E-Ti-11, the trend was consistently less pronounced, and the particular case of high Ti loadings is discussed below. The easier cleavage of Siorg-O-Siorg bridges that are pairing grafted EBDMS pairs in comparison to Siorg-O-Siinorg bridges during the last step of the synthesis is attributed to their protruding position. This cleavage likely proceeds via a nucleophilic attack assisted by Ti isopropoxide, being bulky Lewis acids. The oxo-titanium adduct thus obtained hangs from one end of the grafted EBDMS moieties and occupies a favorable position to react further with underlying surface silanol groups. The formation of this second Ti-O-Si linkage stabilizes the Ti-adduct at the surface by the chelation effect (Scheme 1). The

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Si chemical

shift of this second C3-Si-O-Ti environment is likely similar to that of the C3-Si-OH environment, as both Ti and H are less electronegative than Si atoms. Assuming that both signals are not resolved, this may explain the broadening of the M0 signal and its increasing


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intensity at low Ti loading in the LUS-E-Ti-n series (Scheme 1, Figure 3). In fact, the intensity increases up to a plateau corresponding to ~ 0.02 mol of EBDMS per mol of Siinorg. The signification of the plateau and the relation to the nuclearity of the titanium oligomers is discussed below, together with the UV band shift.

Shift of the charge-transfer band and size of the grafted TiOX species. The UV-visible investigation concerns only the calcined samples where the deposits of the isopropoxide moieties were transformed into [Tin(OSi)u(OTi)v(OH)w] grafted oxo-hydroxotitanium species (Scheme 1, Figure 4). When the titanium loading was decreased from 0.1 to 0.01 molar ratio, the charge-transfer band was shifted slightly from 264 to 247 nm for EBDMS free surfaces. By contrast, this variation is much stronger, i. e. 259 to 217 nm, for EBDMS capped surfaces (Figure 5A and 3S, Table S1). This trend is depicted on Figure 4 comparing samples LUS-ETi-1 and LUS-Ti-2 that contain a similar Ti/Si molar ratio (~ 0.007) or comparing samples LUS-E-Ti-2 and LUS-Ti-9 that exhibits a similar LMCT bands for different Ti/Si molar ratios (0.007 and 0.046, respectively). Therefore, capping the surface by EBDMS moieties allowed us to produce a much stronger blue-shift for a given Ti molar ratio. In fact, it is known that the blue-shift of the LMCT band of Ti is correlated to downsizing TiO2 clusters according to the quantum size effects (Figure 5). It is observed for nano TiOx clusters entrapped in the micropores of titanium borosilicalite TBS-1 and, also for silica-supported TiOx clusters.39, 40, 41, 42

In this zeolite and the boron free isostructural TS-1, the apex of the band varies from 315

nm for 3D TiOx clusters of anatase like structure down to 210 and 230 nm for isolated framework Ti.39, 40 With a wavelength of at most ca. 259 nm, the species investigated here are likely 2D TiO2 single layer rafts though 3D stacking may not be excluded because of the quantum size effect (see discussion below). Taking into account the size of the cavities in zeolite that limits the size of the cluster, it is clear that we are dealing with sub-nano oxohydroxo TiOx species.42 As an indication, an alkoxide Ti hexamer calcined on the surface of


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mesoporous silica exhibits a LMCT at 235 nm, which is a value corresponding to the middle of the scale of variation observed in the LUS-E-Ti-n series.23

Correlation between TiOX nuclearity and Ti loading in EBDMS patterned surfaces. It is worth noting that LUS-E-Ti-1 is characterized by a LMCT pointing at 217 nm and consistent with isolated Ti species. It compares well with the LMCT band spiking at 215 nm reported by Capel-Sanchez et al. for Ti single site obtained by grafting Ti(atran)isopropoxide precursor on fumed silica.21 In comparison, LUS-E-Ti-2 and -3 match better with dimeric Ti species with an apex of the LMCT band at ~222 nm. In parallel, these first three samples of the EBDMS series are characterized by an increasing number of C3-Si-O-Ti moieties correlated to a larger number of half-degrafted EBDMS species during Ti grafting. The disappearance of this correlation at higher Ti loadings suggests a change of grafting mode where the additional Ti ions are grafted without the assistance of pre-grafted organosilanes (Scheme 1). Hence, the first grafted titanium ions accounting for ~ 0.02 Ti/Si molar ratio are seeding the growth of the larger oligomers, the size of which is then correlated to the Ti loading. The absorption edge is steep for all the samples in the LUS-E-Ti-n series, which strongly support the assumption that the species distribution is narrow and the growth homogeneous (Figure 4). Accordingly, LUS-E-Ti-4 containing 0.033 Ti/Si molar ratio corresponds mainly to trimers for a band maximum at 235 nm. Comparatively, with 0.065 and 0.078 Ti/Si molar ratios in

LUS-E-Ti-5 and -7, this is the hexamers and the octamers that predominate with an apex at ~ 250 and 255 nm, respectively. Strikingly, this is at 235 nm and not at 250 nm that the maximum of the LMCT band is reported for calcined Ti alkoxide hexamers.22 This apparent discrepancy might be reasonably explained assuming that these molecular Ti hexamers are indeed split apart into fragments, the distribution of which is mainly centered on trimers.

Channel plugging, Ti uptake limitation and LMCT blue-shift. It is worth reminding that the maximum of grafted Ti species occurs at somewhat different Ti loadings (0.086 and 0.200


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Ti/Si molar ratio, respectively) in each LUS-E-Ti-n and LUS-Ti-n series. Nevertheless, it affords very similar LMCT band with both apex arising at ca. 255 nm in both series. The loading difference is of course related to the higher number of silanol available on the organic free surface that is accordingly 2.5 times larger than on the capped surface, consistent with a EBDMS coverage of ~ 70% (Figure 1, insert and Table S1). The position of the apex indicates that, in both cases, the TiOx species have similar sizes and/or possess number of layers (see below). Adding more titanium precursor in the grafting solution and arriving just above the limit of incorporation, leads to an additional redshift. Then, the apex is pointing at 259 and 265 nm in each LUS-E-Ti-n and LUS-Ti-n series, respectively. The drastic drop of the yield of grafting for higher Ti concentration in solution is the fingerprint of another regime of growth, moving from 2D raft to 3D multilayers and to channel plugging. The plugging effect is related also to the channel diameter and the kinetic diameter of the oxohydroxotitanium precursor, i. e., titanium isopropoxide. The latter can be calculated using the van der Waals size of the predominant monomeric species, which is ca. 0.8 nm.43 Besides, the channel diameter is ~ 4.0 and 3.2 nm before and after capping using EBDMS, therefore a single layer accounting for ~ 0. 4 nm (Table 1). From the [O-Ti-O-H] sequence of bondings a TiOx monolayer can be estimated to be ca. 0.45 nm thick and slightly thicker than the EBDMS monolayer. This is the reason why the channel diameter decreases by only - 0.31 nm after grafting Ti species. Then, each additional layer deposited on the 2D TiOX islet would decrease the diameter by twice the monolayer thickness, i. e., - 0.9 nm leaving a clearance at the channel entrance of 0.22, 0.13 nm and 0.04 nm with one, two and three additional layer(s). Accordingly, TiOx oligomers grown up to four atomic layers will suffice to plug the channel entrance. During titanium grafting, the concentration of Ti precursors being higher at the entrance of the channels, this is where the rate of growth is faster and, consequently, the number of multilayered oligomers the larger. Along the same reasoning, grafting with a


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higher Ti precursor concentration accelerates the plugging process. This is in full agreement our experimental data (Figure 1).

Size constraint, 2D and 3D growth in the organically patterned surface. Patterning the surface using TMA+ quaternary ammonium as masking agent defines the surface that will be protected from capping and used subsequently for grafting Ti ions. This surface likely corresponds to the projected diameter of the TMA+ cations. The van der Waals diameter is estimated at ca. 0.45 nm, though a better choice is the hydrated diameter of 0.69 nm given by electrochemical measurements.44, 45,46 In fact, according to molecular dynamic calculation, this value corresponds to the sphere containing the first layer of water molecules surrounding TMA+ ions.47 This value taken as the size of the grafting spot provides the maximum number of four Ti4+ ions in a [Tin(OSi)u(OTi)v(OH)w] oxo-hydroxotitanium monolayer. In fact, LUS-E-

Ti-1 contains mostly Ti single site while LUS-E-Ti-2, -3, -4 contains mixtures of dimers, trimer and tetramers with all Ti ions linked to the support via Ti-O-Si bridges and exposed to the liquid or gas phase. In comparison, there are inevitably double-deck and maybe some triple-deck oligomers in LUS-E-Ti-5, -6 and -7 with dispersions centered around hexamers and octamers, respectively and some loss of Ti ion exposure. It is in LUS-E-Ti-8 that overgrown 4-layered TiOx aggregates likely appear and plug the channel entrance. Accordingly, one may reasonably assign the apex of the LMCT band at 217, 222, 235, 250, 255 and 259 nm to the dominant presence of monomers, dimers, trimers, 2-layered-hexamers, 2-layered-octamers and 3-layered TiOx species, respectively. The energy gap often related to the reactivity was calculated from Tauc plots and provided for monomers and octamers (Figure 3S).

Catalytic activity in relation to Ti loading and, site nuclearity and exposure. To probe Ti exposure, the presence of isolated titanium sites and, check whether Ti oligomers could exhibit some catalytic activity, both series of materials were tested in the epoxidation of


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cyclohexene (Scheme 2). The reaction was performed using acetonitrile as a solvent and tbutyl hydroperoxide (TBHP 30 wt% in water) as oxidant. TBHP and acetonitrile were preferred as an oxidant and as a solvent, since the series of materials with the organically modified surface were poorly active in aqueous hydrogen peroxide solutions. Of course, the selectivity was very different on both types of surfaces since epoxidation without hydrolysis was favored on the apolar surface and in acetonitrile, while hydrolysis of the epoxide into diol was favored on the polar surface even in acetonitrile. Therefore, we focused exclusively on the evolution of activity in function of Ti loading, as shown in Figure 5B. The highest activity, expressed as the turnover number (TON) per Ti site, was observed at the lowest titanium content. This trend shifted to higher loadings when the surface was organically modified, followed the redshift of the titanium LMCT band on both series of samples. A closer look on the catalytic performances of the LUS-E-Ti-n series revealed that the activity decreased more rapidly for the lowest titanium loadings (Ti/Si molar ratios < 0.04) than at higher loadings between 0.04 to 0.08 Ti/Si molar ratios. Above, The reactivity was negligible. In principle, the relation between reactivity and structure is strictly valid for catalytic reactions running under kinetic conditions. Since the Koros-Nowak test was not performed we could not a priori discard the possibility of some partial diffusion control in the present conditions. Nonetheless, the conversion remains modest even for the fastest catalysts

LUS-E-Ti-n compared at similar conversion (15, 23, 23, 26, 11 % for n = 1, 3, 4, 5 and 7, respectively). Furthermore, the distribution of the catalytic sites is by design very homogeneous inside the channel of the silica. If any, the diffusion control would minimize the effect of the loss of active sites. Therefore, the strong decrease of conversion at increasing loading up to 7 Ti/Si mol %, where plugging was absent, is clearly structure-dependent. In addition, the external surface being negligible (30 to 40 m2/g) in comparison to the internal surface (600 m2/g), the reactivity is mostly due to the titanium grafted inside the nanopores of


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the materials. The change of reaction rate per site at intermediate loading revealed that the higher oligomers (trimers and above) are less active than monomers and dimers. This latter observation is consistent with previous observations that oligomers participates also to the catalytic activity.23, 48 To summarize, the reactivity fully supports the idea that pre-capping the silanol surface favors the stabilization of monomers and oligomers of very small size. They present potential catalytic activity, and must be at an ideal concentration for catalytic applications. Though isolated species appears the most useful to catalyze epoxidation reaction, the higher oligomers may find there application in other reaction or other properties. For instance, hybrid TiOx/SiO2 support based mainly on [Ti3(OSi)u(OTi)v(OH)w] grafted cyclic trimers like LUS-E-

Ti-4 instead of layers of TiO2 would present a very interesting alternative to anchor monomeric or oligomeric species of vanadium, tungsten or molybdenum ions.49, 50 Our ongoing investigations show that these titanium subnano-islets are indeed suitable anchors to control the dispersion of such ions at very high exposure.

CONCLUSIONS In contrast to non-modified mesoporous silicas, organically modified mesoporous silicas favor not only the formation of isolated Ti species, but also the formation of small oligomeric Ti sites that are highly exposed and active in catalytic cyclohexene epoxidation. The first requirement was the pre-capping of the surface silanol in the presence of adsorbed TMA+ ions, forming a masking molecular pattern at the sub-nano scale. The second was the control over the oxo-hydroxotitanium nuclearity by the Ti loading. Indeed, the change of grafting mode above a Ti/Si molar ratio of 0.02 (~1.3 wt%) and the steep edge of the LMCT bands advocated for a homogeneous growth up to a Ti/Si molar ratio of 0.08 (~5 wt%) and for the successive formation of monomers, dimers, and so on, all the way up to octamers (Figure 3S).


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The estimated size of the uncapped spot left by the sub-nano patterned capping (Ø < 0.7 nm) allows us to anticipate a limitation of four Ti per [Tin(OTi)u(OSi)v(OH)w] oxo-hydroxotitanium monolayer layer. Above this nuclearity of four, one anticipates a loss of exposure by the formation of a second layer up to the formation of octamers that grow further in size provoking channel plugging. These assumptions are supported by thorough cross-analyses of UV-visible, FT-IR, pseudo-quantitative 29Si NMR, and elemental analysis investigations. The catalytic reactivity demonstrates that monomeric and dimeric titanium sites are more active than oligomers of higher nuclearity sites. The present investigation paves the way for alternative routes to isolate metal ions and oligomers of low nuclearity, varying in a range of about 1 to 8. This proves also to be a useful technique in designing MOx-SiO2 hybrid supports with the size of the MOx sub-monolayers controlled at the sub-nano scale.

ACKNOWLEDGMENTS Financial supports from NSFC of China (21533002, 21373089) and from the Joint Research Institute for Science and Society (JORISS) of ECNU and ENS de Lyon are gratefully acknowledged. ECNU is gratefully acknowledged for the visiting professorship position of LB as Higher End Foreign Expert.

PRESENT ADDRESS

Yang Boting: Department of chemistry and biology, Beihua University, 15 Jilin Road, Jilin 132013; [email protected] Fang Lin: Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS, Solvay, 3966 Jin Du Road, Shanghai 201108, China; [email protected] Yuting Zheng: Sinopharm Steriguard Medical Service Co.,Ltd, 608 Soho Plaza A, 1055 West Zhongshan Road, Shanghai, P.R.China 200051; [email protected]


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TOC of Table Table 1 page 21

TOC of Graphics Scheme 1

page 22

Scheme 1

page 22

Figure 1

page 23

Figure 2 & 3

page 24

Figure 4 & 5

page 25

Figure captions page 26 - 27 ORCID numbers ORCID page 27 Supplementary information page 27 References pages 28-30


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TABLE Table 1. Textural characteristics of some samples and distribution of the different grafting modes of EBDMS.

Sample

Ti/Sia

SBETb Vp c DBdBd monograftedf bigraftedg dimerh Siorg/Siinorge 2 -1 3 -1 m g cm g nm % % %

LUS

—

1030

1.01

4.0

0.0

LUS-T-E

—

—

—

—

0.21

LUS-E-OH

—

644

0.51

3.3

LUS-E-Ti-3 0.018

621

0.48

LUS-E-Ti-4 0.033

600

LUS-E-Ti-8 0.083

582

—

—

—

0

70

30

0.20

20

50

30

3.2

—

—

—

—

0.46

3.1

0.20

30

50

20

0.43

3.1

0.20

33

56

11

a, Ti molar ratio reported to Si in the SiO2 support (called Siinorg) from elemental analysis, accuracy ± 0.001; b, accuracy ± 5%; c, total pore volume measured at P/P0 = 0.92, accuracy ± 0.01; d, Y = 14,60994 + 74,67812X + 81,96198X2 + 155,8457X3, X = P/P0 (0.11 ≤ P/P0 ≤ 0.50), Y = pore diameter (in Å), accuracy ± 0.1; e, Siorg/SiInorg = ΣMi / ΣQi , ΣMi = M0 + M1 + M1’+ M1’’ + M2, ΣQi = Q2 + Q3 + Q4, determined by 29Si MAS NMR, accuracy ± 0.02; f, %mono = 100 x M0 / [(ΣMi + ΣQi) x ΣEBDMS], accuracy ± 5; g, %bi = 100 x [(M1+ M1’+M1”) – M2] / [(ΣMi + ΣQi) x ΣEBDMS], ΣEBDMS = mono + bi + 2 dimer; h, %dimer = 100 x M1’’ / [2 x (ΣMi + ΣQi) x ΣEBDMS].


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SCHEMES

Scheme 1

Scheme 2


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FIGURES Figure 1


23

Langmuir

Figure 2

Figure 3

(A) 2850~2960

f

1490

Q4

(a)

Q3

e Q2

d Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

c

50

0

-50

-100

-150

-200

-150

-200

-150

-200

-150

-200

Q4

b

(b) M1+M1' Q3

a

M1"

4000

3000

1500

1000

-1

50

Wavenumber (cm )

0

-50

-100

Q4

(c)

964 950

(B)

M1+M1'

833 M0

935

Q3

M1"

Q2

f 0

50

-50

-100

e d

Q4

(d) M1+M1' Q3

c

M0

M1"

b 50

0

a 1000

900

800 Wavenumber (cm-1)

-50

-100

Chemical shift (ppm)

700


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Figure 4

Figure 5


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CAPTIONS Scheme 1 Various grafted modes of dipodal ethylene-bis-dimethylsilyl moieties (EBDMS, figure top), some possible titanium isopropoxide grafting modes on surface silanol groups in between EBDMS capped silanol (middle) including or not direct interaction with neighboring EBDMS moieties and formation of oxide oligomers (bottom) upon further reaction with other titanium isopropoxide molecules; M1 ( ), M1’( ), M1”( ), M2 ( ), and M0 ( ) species refer to different types of silicon atoms characterized by a different

29

Si-NMR shift (see text and

figure 1c); EBDMS capped zone of the surface (dashed in blue) and meso pore wall (dashed in grey); After calcination, all the organic ligands and capping functions are removed yielding [Tin(OSi)xOy(OH)z] grafted oxo-hydroxotitanium islet, noted TiOx. Scheme 2. Epoxydation of cyclohexane inside the nanochannel of LUS-E-Ti: blue rectangle for the area of the surface capped by ethane-1,2-bisdimethylsilyl moieties, red triangles for isolated titanium ions and clustered triangles for titanium oxide oligomers. Note that the reaction may produce either cyclopentane-1,2-epoxyde or cyclohexane-1,2-diol. Table 1. Titanium and “organic” silicon content and textural analysis of materials LUS, LUSE-OH and LUS-E-Ti-n. Figure 1. Correlation between Ti added in the mother liquor, Ti/SiML, and Ti/Siinorg molar ratio in LUS-Ti-n (black circle) and in LUS-E-Ti-n (green and red circles) materials; numbers refer to n of the sample name. Inset for full span of Ti concentrations. The (blue) straight line stands for 100% yield of Ti incorporation.

Figure 2. FT-IR spectra in the 1000-4000 (A) and 700-1050 cm-1 (B) ranges at different steps of the materials elaboration (a) LUS-CTA, (b) LUS-TMA, (c) LUS-T, (d) LUS-T-E, (e) LUSE-OH, (f) LUS-E-Ti-11; intensity of 833 cm-1 νSi-C peak in spectrum relative to the intensity of the peak at 450 cm-1 used as reference (d) 0.31 ± 0.01, (e) 0.28 ± 0.01, (f) 0.31 ± 0.01.


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Figure 3. 29Si MAS NMR spectra of the materials (a) LUS-T, (b) LUS-T-E, (c) LUS-E-OH, and (d) LUS-E-Ti-11. Peak position (ppm): Q2 = -91.8 ± 0.4, Q3 = -101.9 ± 0.3 and Q4 = 111.0 ± 0.3; M0 = 18.6 ± 0.2, M1 + M1’ + M1” = 12.8 ± 0.3 and M2 = 6.7 ± 0.1. Linewidths for Q species were 8.07 ppm in (a), 8.71 ppm in (b), 8.63 ppm in (c), and 8.45 ppm in (d) and for M species 5.63 ppm in (b), 4.06 ppm in (c), and 4.64 ppm in (d). Analytical integration accounts for the same Gaussian/Lorentzian shape ratio of 1.00.

Figure 4. Typical blueshifts observed on the UV spectra of titanium in LUS-Ti-n and the in LUS-E-Ti-n (horizontal arrow pointing the blue shift, ∆σB); on the left (green line) standing for the most blue-shifted samples LUS-E-Ti-1 (Ti/Si=0.008); in the middle (red line) for LUS-E-Ti-9 (Ti/Si=0.046) and on the right (black line) for the less blue-shifted sample LUSTi-2 (Ti/Si=0.007) of this particular series.

Figure 5. Comparison of the charge transfer blue shift with the catalytic reactivity in LUSTi-n and the in LUS-E-Ti-n: A) Shift of the charge-transfer band and B) TON of epoxidation of cyclohexene: A) Half-filled circles (black) for LUS-Ti-n, Half-filled circles with numbers (green and red) for LUS-E-Ti-n); B) Upper and lower horizontal dash-lines for the less and most blueshifted LMCT covered in this study, the full (black) and empty (green) circles for LUS-Ti-n and LUS-E-Ti-n series, respectively.

ORCID number Laurent Bonneviot: 0000-0002-9092-8966; Belén Albela: 0000-0002-2182-2375 Supplementary Information: UV data, epoxide selectivity, elemental analyses, NMR raw quantitative data XRD diagrams and N2 sorption isotherms. ACS Paragon Plus Environment

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Size-Dependent Catalytic Activity of Oxo-Hydroxo Titanium subnano

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