W catalysts

Fox, F.J, and Noren, R.W., Catalyst for condensation of hydrolyzable silanes and storage stable compositions thereof. US Patent 4,101,513, 1978. 

One problem associated with the peroxotungstate-catalyzed epoxidation system described above is the separation of the catalyst after the completed reaction. To overcome this obstacle, efforts to prepare heterogeneous tungstate catalysts have been conducted. De Vos and coworkers employed W catalysts derived from sodium tungstate and layered double hydroxides (LDH - coprecipitated MgCU, AICI3, and NaOH) for the epoxidation of simple olefins and allyl alcohols with... 

Figure 8.3 Wagener s ADMET of 1,9-decadiene using Schrock s [W] catalyst.

At a certain point (0.08 % w/w catalyst to substrate ratio) there ensued a drop in reaction rate, together with a gradual loss of selectivity. Thus, a treadeoff point was chosen so that a minimal load of catalyst could be used in the process, without an undue sacrifice in rate or selectivity. 

Catalyst to substrate ratio (% w/w) Catalyst recycle Time (h) 6(a) HOjC COjH 2(a)... 

Power input furnace max. 185 W Catalyst No. 3 material formation Pt, impregnated... 

Wy Recycling n Sewage/water ta Auxiliaries (isolation) M Solvents W Catalysts rrn Auxiliaries (reaction) CT Substrates Byproducts H Coupled products... 

The reference Pt-Ba/y-Al203 (1/20/100 w/w) catalyst shows surface area values in the range 140-160 m2/g, a pore volume of 0.7-0.8cc/g and an average pore radius close to 100 A (measured by N2 adsorption-desorption at 77 K by using a Micromeritics TriStar 3000 instrument). Slight differences in the characterization data are associated to various batches of the ternary catalyst [24,25],... 

Instrument. Before the H2 chemisorption, each sample was heated in pure H2 at 300°C for 90 min, subsequently it was heated in He at 290°C for 60 min in order to desorb hydrogen from the sample. The chemisorption measurement was performed at 0°C by several H2 pulses with an Ar purge in between, in order to desorb physisorbed hydrogen. A 1/1 H/Pt ratio was used to estimate the Pt dispersion. Values in the range 50-70% have been obtained on the fresh samples Pt—Ba/ y-Al203 (1/20/100 w/w) catalyst. 

Figure 6.2. Storage run in N02 (1000 ppm), He balance (total flow 200Ncc/min, catalyst weight 120 mg) at 350°C over Ba/Al203 (20/100 w/w) catalyst. NO and N02 are outlet concentrations, and N02 is inlet concentration, (b) Results of N02 adsorption using FTIR experiments spectra are reported after 1 and 10 min of exposure to 5 mbar of N02 at 350°C. Each spectrum is reported as difference from the spectrum before N02 admission.

In order to verify if the NO, storage mechanism proposed for BaO sites is operating also in the presence of C02, i.e. under conditions more representative of the real composition of the gas mixture in the engine exhausts, the influence of C02 on the NO storage mechanism over the reference Pt—Ba/Al203 (1/20/100 w/w) catalyst and over the binary Ba/Al203 20/100 w/w reference sample was investigated. As discussed in the first section, primarily BaCOs species are present on the catalyst surface when C02 is contained in the feed stream. 

A typical result obtained during the reduction by H2 at 350°C of NO stored up to saturation at the same temperature over the Pt-Ba/ y-Al203 (1/20/100 w/w) catalyst is presented in Figure 6.10. Upon the stepwise addition of 2000 ppm of H2 at t = 0 s the stored NO was reduced to N2. Indeed H2 was completely consumed while the N2 outlet concentration increased immediately to the level of 360 ppm and then it kept almost constant. Accordingly, at the beginning the reaction is very fast and selective to nitrogen, and limited by the concentration of H2. Small amounts of NO were also observed, immediately after the H2 stepwise addition. 

Figure 6.10. Reduction of stored N( ), with H2 (2000 ppm) balance He (total flow lOONcc/min, catalyst weight 60 mg) at 350°C over Pt—Ba/Al203 (1/20/100 w/w) catalyst after storage at saturation at the same temperature. N2, NH3, NO and H2 are outlet concentrations, and H2 is inlet concentration.

The Pt-free sample [i.e. the binary Ba/y-Al203 (20/100 w/w) catalyst] was considered at first. In this case, the sample was saturated by feeding N02 at 350°C (see Figure 6.2), since it has been shown that the adsorption with N0/02 mixtures did not allow any significant storage of NO species over the Pt-free sample (Section 2 in Chapter 3). 

Pt—Ba/ Y-AljOj (1/5.3/100 w/w) catalyst upon H2 admission (/ = 0 s), the NO, adsorbed species were readily reduced as indicated by the complete H2 consumption and the correspondent evolution of N2. 

Figure 21.9 Extreme cases for FBCR, both requiring relatively large W(catalyst) (a) approach to equilibrium (b) extent of interstage cooling...

Poly(l,4-naphthylenevinylenes) have been prepared by metathesis polymerization of benzobarrelenes [181,182] and the photoluminescence properties of homopolymers and block-copolymers have been studied in some detail [183]. PPV also has been prepared via ROMP of [2.2]paracyclophane-l,9-diene [184] and ROMP of a paracyclophene that contains a solubilizing leaving group [185]. The resulting polymer is converted to PPV upon acid catalysis at room temperature. ADMET of 2,5-dialkyl-l,4-divinylbenzenes using Mo or W catalysts has... 

Figure 8-1 depicts the relationship between the optical purity of the chiral catalyst and the ee of the product. In a simplified case, when two enantiomeric chiral ligands (LR or Ls) are attached to a metal center (M), complexes ML2 may be formed as the reactive species. Three complexes are possible MLRLS, MLrLr, and MLsLs. Supposing that LR is in excess and the stability constant for the muw-complex MLRLS is greater than that of the chiral complexes, if mt, w-MLR Ls is the more active catalyst, a lower than expected ee will be obtained [(—)-NLEs, curve 3 in Fig. 8-7], The ee will be higher than expected if the me.w-catalyst is less reactive than MLRLR or MLSLS [(+)-NLEs, curve 2 in Fig. 8-7],...

Table 19 Estimated rate data for M(CO)6, (M = Cr, W) catalysts after initiation by UV irradiation85...

M/AI2O3 (M = Cr, Mo, W) catalysts were ultra-active for the hydrogenation of ethylene when compared with others prepared by traditional methods from appropriate salts-a low oxidation state of M is of crucial importance for these catalysts to be active in the hydrogenation of olefins. 

OSiHs) [82] found that the unsymmetrical catalysts (X Y) are systematically more efficient for all systems (W, Mo and Re). Overall, Mo and W catalysts are more efficient due to the presence of the imido ligand, because it favors distortion of the initial complexes, except when X Y-in this case, all systems are equally efficient [81],... 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

Figure 13.15 Reduction of stored NO, with H2 (2000 ppm) + He at350°CoverPt-Ba/Al2O3 (l 20 100w/w) catalyst after storage at saturation at the same temperature. 

Figure 13.16 TPD in He and TPSR in H2 (2000ppm) + He after NO-O2 adsorption at 35O C over Pt-Ba/Al2O3 (1 20 100 w/w) catalyst. Adapted from ref. [119].

---