Aerogel Catalysts

Aerogels are often largely mesoporous solids, with a porosity which may exceed 90 vol% and a specific surface area up to 1000 m2 g-1. Such materials were first obtained by Kistler in 1932, and designate gels in which the liquid was replaced with a gas without collapsing the gel solid network. Contrary to xerogels dried from wet gels by evaporation with an important shrinkage, the first aerogels were obtained by a “supercritical drying” technique in which the liquid which impregnated the gels was evacuated after being transformed to a supercritical fluid. The diversity in nature of the solid constituting the rigid network is very large. It includes simple oxides, multi oxide compositions, organic and hybrid organic-inorganic polymers and carbon. This diversity as well as the high specific pore volume and surface area make aerogels applicable either as catalysts or as catalyst supports. Besides, molecular catalysts such as transition metal complexes or enzymes can easily be immobilized in aerogels, which opened the road to new supported molecular catalysts and biocatalysts. This communication reviews the synthesis and properties of oxide aerogel catalysts.

Influence of surface modification of Ti-SBA15 catalysts on the epoxidation mechanism for cyclohexene with aqueous hydrogen peroxide

The thermolytic molecular precursor method was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA15 support. The resulting surface Si-OH/Ti-OH sites of the Ti-SBA15 catalysts were modified with a series of (N,N-dimethylamino)trialkylsilanes, Me(2)N-SiMe(2)(R) (where R = Me, (n)Bu, or (n)Oc). Compared with the unmodified catalysts, the surface-modified catalysts are more active in the oxidation of cyclohexene with H(2)O(2) and exhibit a significantly higher selectivity (up to 58%) for cyclohexene oxide formation (vs allylic oxidation products). In situ Fourier transform infrared (FTIR) and diffuse reflectance UV visible (DRUV-vis) spectroscopies were used to probe this phenomenon, and it was determined that active sites with capped titanol centers, (SiO(surface))(3)Ti(OSiR(3)), likely undergo Ti-OOH formation upon addition of H(2)O(2) in a manner analogous to that for active sites of the type (SiO(surface))(3)TiOH. On the basis of the observation of similar Ti-OOH intermediates for both species, the electron-withdrawing effects on the Ti(IV) active site, resulting from the surface modification, are likely responsible for the observed increase in selectivity.

Interaction of Water, Alkyl Hydroperoxide, and Allylic Alcohol with a Single-Site Homogeneous Ti−Si Epoxidation Catalyst: A Spectroscopic and Computational Study

Tetrakis(trimethylsiloxy)titanium (TTMST, Ti(OSiMe3)4) possesses an isolated Ti center and is a highly active homogeneous catalyst in epoxidation of various olefins. The structure of TTMST resembles that of the active sites in some heterogeneous Ti-Si epoxidation catalysts, especially silylated titania-silica mixed oxides. Water cleaves the Ti-O-Si bond and deactivates the catalyst. An alkyl hydroperoxide, TBHP (tert-butyl hydroperoxide), does not cleave the Ti-O-Si bond, but interacts via weak hydrogen-bonding as supported by NMR, DOSY, IR, and computational studies. ATR-IR spectroscopy combined with computational investigations shows that more than one, that is, up to four, TBHP can undergo hydrogen-bonding with TTMST, leading to the activation of the O-O bond of TBHP. The greater the number of TBHP molecules that form hydrogen bonds to TTMST, the more electrophilic the O-O bond becomes, and the more active the complex is for epoxidation. An allylic alcohol, 2-cyclohexen-1-ol, does not interact strongly with TTMST, but the interaction is prominent when it interacts with the TTMST-TBHP complex. On the basis of the experimental and theoretical findings, a hydrogen-bond-assisted epoxidation mechanism of TTMST is suggested.