Titania reaction selectivity with

Interactions with other molecules can affect the ultimate reaction selectivity of the process. For example, the catalytic dehydration of formic acid on TiO2(001) occurred as a unimolecular process at high temperatures and low formate coverage, while a bimolecular dehydrogenation process dominated at near-saturation coverage of the titania crystal. 

These mesoporous mixed titania-silica oxides are hydrophilic materials and are excellent catalysts for epoxidations of olefins, allylic alcohols and a,jff-unsaturated ketones with alkyl hydroperoxides in non-aqueous media [37]. Their performance can be improved even further by adding organic or inorganic bases to neutralize acid sites present on the surface [38,39], The latter cause side-reactions, especially with acid sensitive epoxides. Amine addition was particularly effective and led to the development of a mesoporous Ti-Si mixed oxide containing surface-tethered tertiary amino groups as an active, selective, and recyclable catalyst for the epoxidation of allylic alcohols [38]. 

Process of selective catalytic reduction of nitrogen oxides by ammonia (SCR) involves injection of ammonia into a gas stream containing nitrogen oxides, then reduction of NOx by ammonia on the surface of a catalyst typically containing vanadium oxide on titania. The reactions involved are mildly exothermic (additional heat is required in most cases). Limits of the optimal process temperature, usually from 200 to 350°C, are dictated by catalyst activity at low temperatures and by the reaction selectivity at high temperatures. The NOj-containing gas flows often have low temperature and variable flow rates and concentrations. This combination of factors makes application of an RFR to NO reduction advantageous. One industrial unit for NO selective catalytic reduction was reported to operate in Russia [44], with ammonia water injection between two catalyst beds. 

Despite the importance of solid-state nuclear magnetic resonance (NMR) spectroscopy for the characterization of solid catalysts, in situ studies related to photocatalysts are rare. Mills and O Rourke [59] monitored the selective photooxidation of toluene by in situ NMR using an NMR tube as the photoreactor. A Ti02 precursor paste was prepared by hydrolysis of titanium propoxide and following treatment at 228 °C. The obtained anatase-type titania was mixed with poly(vinyl alcohol). The obtained paste was coated on the walls of the NMR tube, rotated over night and calcined. In parallel, batch experiments were carried out. The reaction mixture containing the catalyst was directly placed into the tube, which was irradiated outside the spectrometer and then inserted into the NMR spectrometer. 

Previous studies have concluded that 4-, 5-, and 6-coordinate W species are present on AI2O3 and Ti02 supports [17,30] depending on surface W density and on hydration state. The present study has detected W03-like distorted octahedral domains at all surface densities and irrespective of hydration on Zr02. These species catalyze alkane isomerization reactions with much higher turnover rate and selectivity than dispersed WOx moieties on alumina or titania. 

Although the mesoporous materials, such as Ti-MCM-41, have lower intrinsic epoxidation selectivity than TS-1 and Ti-beta, they must nevertheless be used as catalysts for reactions of large molecules typical in the fine chemicals industry. It is, therefore, interesting to elucidate how these ordered mesoporous materials compare with the earlier generation of amorphous titania-silica catalysts. Guidotti et al (189) recently compared Ti-MCM-41 with a series of amorphous titania-silica catalysts for the epoxidation of six terpene molecules of interest in the perfumery industry (Scheme 6). Anhydrous TBHP was used as the oxidant because the catalytic materials are unstable in water. The physical characteristics of these catalysts are compared in Table XII. 

The gas-phase selective oxidation of o-xylene to phthalic anhydride is performed industrially over vanadia-titania-based catalysts ("7-5). The process operates in the temperature range 620-670 K with 60-70 g/Nm of xylene in air and 0.15 to 0.6 sec. contact times. It allows near 80 % yield in phthalic anhydride. The main by-products are maleic anhydride, that is recovered with yields near 4 %, and carbon oxides. Minor by-products are o-tolualdehyde, o-toluic acid, phthalide, benzoic acid, toluene, benzene, citraconic anhydride. The kinetics and the mechanism of this reaction have been theobjectof a number of studies ( 2-7). Reaction schemes have been proposed for the selective pathways, but much less is known about by-product formation. 

In this work, the performance of two green acid catalysts, a Ti02 synthesized by the sol-gel method and sulfated in situ was compared with a traditional NiY zeolite in the trimerization of isobutene. The reaction was carried out at mild conditions atmospheric pressure and 40°C of temperature. The results obtained in the catalytic evaluation showed higher conversion and stability as well as a better selectivity to tri-isobutylene for the sulfated titania catalyst with respect to the NiY zeolite. 

The reaction of propylene on ZrC>2 exhibits the same characteristics as on other oxides. Propane-d2, for example, is selectively formed in the deuteration process, with no hydrogen exchange in propylene215. New features appear, however, when zirconia is dispersed on other oxides (alumina, silica, titania)215,216. A considerable rate increase is observed and exchange in propylene proceeds simultaneously with addition via the associative mechanism through the common intermediate n-propyl and s-propyl species. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Apparently at a temperature above 300 C, the oxidation kinetics of NOx and ammonia gas is so fast that slip of the reactants, when fed from the opposite sides of an alumina or alumina-titania membrane, can be avoided. Vanadium oxide, used as the catalyst for the reaction, is impregnated onto the membrane pore surface. The conversion of NOx reach 70% with the selectivity for nitrogen up to 75% in the temperature range of 300 to 350 C [Zaspalis etal., I991d]. 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

Industrial application of the Au catalysts would be feasible if 10% conversion of propene could be achieved. However, there still remains a number of serious problems low PO 4elds (<2%), low selectivities at high reaction temperature, deactivation with time-on-stream and low H2 efficiency (<30%). The first two problems could be somewhat improved when titania-modified silica and titanosilicates were used as supports for Au nanoparticles. The initial conversion of propene was increased to ca. 5% from 2% or below, when the reaction temperature was raised to 423 K from 353 K or below without considerable loss in PO selectivity [40,402]. To date, only small improvements in the deactivation of Au catalysts with time-on-stream have been reported,... 

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