Synthesis and catalytic activity of organic–inorganic hybrid Ti-SBA-15 materials

Juan A. Melero, et. al., J. Mater. Chem., 2007, 17, 377-385.

Introduction :

Organic–inorganic hybrid mesoporous materials have received much attention during recent years because of their potential applications in many fields of science.

Different synthetic pathways have been applied in order to obtain organically functionalized mesoporous silicas, viz., grafting, co- condensation of alkyltrialkoxysilanes with tetraalkoxysilanes, PMO’s.

PMO materials has been carried out using cationic , anionic, neutral and non – ionic oligomeric surfactants and under acidic and basic conditions.

Titainium species have also been incorparated into the structures of ethane based PMO materials templated with cationic surfactants under basic conditions.

In these works it was observed that the Ti species is hydrophobic and very efficient in of alpha- pinene, propene, etc

In the present paper, the synthesis of titanium functionalized periodic mesoporous organosilica through a direct synthesis method using block-copolymer surfactants and titanocene dichloride as the titanium precursor.

Ti – PMO is synthesised under acidic conditions and non – ionic surfactants

The use of them as catalyst for the epoxidation of unfunctionalized olefins is tested.

Titanium functionalized periodic mesoporous organosilicas (Ti-PMO)

Titanium functionalized methylated-SBA-15 (Ti-Met-SBA) ( S -- 3)

Similar procedure to that used in the synthesis of Ti-PMO. the methyl functionality was added through the incorporation in the synthesis of MTES after a prehydrolysis step of the TEOS for 45 min. The reactants were used in the molar ratio MTES : TEOS = 10 : 90.

4 g of P123 + 125ml of 0.5 M HCl Heated to 40 oC

Cl2TiCp2 is added Hydrolysed for 3 hrs

BTSE : TEOS In the varying ratios 0 :100 to 100: 0 1. Stirred at 40 oC fro 20 hours2. Aged at 100 oC for 24 hours

Solid product filtered and air - dried

Washed with ethanol under reflux for 24 hrs vacuum dried at 150oC over night

S - 4

Synthesis

Titanium incorporation by grafting procedure S - 6

stirred Toulene + HMDS was added

1. Stirred for 1 h2. Refluxed for 24 h3. Filtered4. Dried under N2 atm For 4 h at 200 oC temp

S – 2, S - 5

Out gassed material + 200 ml dry toulene

Silylation procedure S – 2, S - 5

TEOS : BTSE = 66 :33 In Toulene + Cl2TiCp2 Reflux for 24h Filtered

Refluxed in ethanol for 24 h

Dried under N2 atm for 24 h S-6

Low amount of titanium incorporated – due to strong acidic hydrothermal conditionsAmong these PMO samples still low - 1. leeching in template extraction,

2. decrease in the reac. points between Ti & silicon precursorUse of titanocene dichlorideThe hydrophobic nature of Cp ligands allows deep interactions of the titanocene species with micelles, which results in them acting as protecting agents against the strong acidic medium responsible for the dissociation of Ti –O –SiAlso due to greater stability of Cp groups prevents the homo condensation of titanium species to yield titanium oxide domains

Fig. 1 29Si MAS-NMR spectra of Ti-containing SBA-15 type materials

Q4 δ –110 ppmQ3 δ –100 ppmQ2 δ –90 ppm

T3 δ – 60 to 70 ppmT2 δ – 45 to 60 ppm

TMS δ – 10 ppm

Q – Si- O- SiT – Si- C

Spectra deconvoluted for quantification (T + TMS )/ ( Q+T+TMS) proportion of organosilicon groups proportion to hydrophobic character

S-3 sample exception since organically bonded silonol groups are lower.

T2 , Q2 , Q3 , indicates presence of –OH groups i.e., hydrophilic character.

Intensity of these peaks is lower in S-2, S – 5 samples

Fig. 2 (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions in the mesoscopic range for different Ti-containing SBA-15 materials.

From S-1 to S-6 clear transformation of loop from type –IV H1 to a mixture of H1 +H3

H3 loop gives the presence of slit shaped pores, may be due to interparticular adsorption

Surface area values are comparable, S-2 shows much lower surface area than S-1 due to partial blocking during the silylation step

pore size distributions confirm the loss of mesostructured porosity for Ti – PMO materials

with increasing organic content pore volumes decreases and pore size distributions become wider

Fig. 3 SEM and TEM images of Ti-containing mesostructured materials synthesized with different organic contents.

SEMS-1 form fibrous aggregatesS-6 bean shaped particles

TEM S-1 ordered hexagonal pores

ordering decreases from S-1 S-4 to S-6

Fig. 4 Schematic illustration of the growth of the corona area when increasing the organosilicon content during the synthesis.

Fig. 5 XRD patterns of Ti-containing SBA-15 type materials.

All samples show d100 peak representing hexagonal structure

Fig a shows narrow peaks, May be due to loss of mesoporosity or hexagonal structure

Constant d100 spacing indicatesincrease in porewall thickness as the organic content increases

Table 2 Microporous textural parameters and TG results for different titanium -containing mesoporous materials.

Fig. 6 DR-UV-Vis spectra of Ti-SBA-15 type materials before (black) and after calcination (grey): (a) silica based samples and (b) Ti-PMO samples.

2 peaks1.At 210nm due to Ti4+2.310nm due to the Cp rings Coordinated to the Ti centre

Confirmed by FTIR 1380 – 1420cm-1 C = C.

Fig. 7 Activity per titanium site for the different Ti-SBA-15 materials in the epoxidation of 1-octene with TBHP.

Table 3 Oxidation of 1-octene with TBHP over titanium-containing SBA-15 type materials

Lower TOF values for S-3 suggests that methyl groups do not influence significantly the hydrophobic surface properties due to high hydrophilic behaviour of the –OH groups

In S-5 sample the silylation step combined with the hydrophobicity induced by the organic content Increases the TOF.

Conclusions Ti-functionalized mesostructured materials were synthesized under acidic conditions

using triblock copolymers as structure directing agents and Cl2TiCp2. with different organic contents to give oxidation catalysts with different hydrophobic surface properties.

The conventional surface hydrophobization through post-synthesis silylation procedures enhances the catalytic activity of these materials in the epoxidation of 1-octene with hydroperoxides.

The synthesis of the titanium-containing mesostructured samples as periodic mesoporous organosilicas (Ti-PMOs) leads to materials with low metal content but a higher catalytic activity per titanium site than silica-based mesostructured samples.

Moreover, the removal of highly hydrophilic silanol groups in Ti-PMO materials

causes an outstanding improvement of the catalytic activity.

In conclusion, we have presented the benefits of coupling internal (incorporating organic moieties within the silica framework) and external (removing highly hydrophilic silanol groups) hydrophobization of titanium containing materials for the enhancement of their catalytic performance in epoxidation reactions.