Silica active surface

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

Titanium Alkoxides Silica-supported titanium(IV) alkoxides and Ti-silicalite are industrial epoxidation catalysts [53-56] and have been applied in deperoxidation reactions [57]. Computational and EXAFS data [53, 54] as well as spectroscopic investigations on the surface species [58] have indicated that the dominant active surface species is a four-coordinate trisUoxy complex [(=SiO)3TiOH] [59] whose coordination shell expands to six-coordinate during catalysis [60]. 

We have observed similar IR bands (1520, 1352 and 1295 cm ) on the Pt wire/ FSM-16 sample in an in situ IR study of the PROX reaction. From these results, we propose that the selective CO oxidation in the PROX on Pt wire/FSM-16 proceeds through the reaction of a carboxyl intermediate (COOH) on Pt nanowires (and particles) supported on FSM-16 with active OH groups (Figure 15.26). CO reacts with an active silica surface OH of FSM-16 to convert the HCOO intermediate on Pt wires and particles into CO2, thereby leading to selective CO oxidation. The subsequent H2/O2 chemisorption generates active surface OH groups near the Pt wires and particles on FSM-16. Smaller HCOO intermediates due to the smaller OH interaction on Pt particle/HMM-1 and Pt necklace wire/HMM-1 may reflect in their lower TOFs and lower CO selectivity in the PROX reaction (Figure 15.25a and b). 

Activities of tri-n-butylammonium and tri-n-butylphosphonium ions with two different spacer chain lengths are compared in Table 8 1I8). The greater activity of the phosphonium ions is opposite to what has been reported for analogous soluble phase transfer catalysts119). Activities of the catalysts bound to silica gel were as high as activities of soluble catalysts adsorbed to silica gel118). Without some independent determination of the role of intraparticle diffusion it is not possible to determine whether the reduced activity of the adsorbed catalysts is due to lower intrinsic activity at the silica gel surface or to diffusional limitations. The size selectivity for alkyl bromides suggests that intraparticle diffusion was not a problem. 

The supported chromium complex "CHRISS" was first reported by us in 1997 (Figure 6) [10]. It was shown to be stable to reaction conditions and an active catalyst for the oxidation of methylaromatics with air as the only consumable source of oxygen. The catalyst was prepared by first forming in solution a metal complex with pendant triethoxysilane groups which is then imprinted onto a silica gel surface. 

Silica type Surface area (m2 g ) Pore volume (ccg ) Activity (gg hr-1)... 

Quenching studies have yielded an activation energy for diffusion on the dry silica gel surface of around 4 Kcal/ mol. 

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