Ti activity

Scheme 1. Mechanism for formation of long-chain branching. Ti = active constrained geometry catalyst.

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Ti = active site generated from TiMe3Cp and B(C6F5)3... 

As a further example of a hydroxyl-assisted epoxidation, geraniol and nerol bearing two isolated C=C double bonds were regioselectively epoxidized with TS-1 at the 2-position (near the OH group), as reported by Kumar et al. (795). On the basis of these results, Kumar et al. (195) proposed that the transition state of the epoxidation of allylic alcohols involves coordination of the alcoholic functional group to the Ti active site and that the double bond interacts with one of the peroxidic oxygen atoms, not with the titanium site (Scheme 9). 

Enteritis necrotians (EN), a spontaneous form of enteric gangrene endemic to the highlands of Papua, New Guinea, is caused by toxins produced when Clostridium perfringens of the gut enter a rapid growth phase (41). It has been postulated that the disease occurs in populations which consume a low protein diet, e.g., sweet potato as the staple food combined with TI activity which... 

Results of oxidation of unsaturated alcohols are shown in Table 3. Both 2-penten-1 -ol and 3-methyl-2-buten-1 -ol exhibited higher reactivity than cyclohexene. A decrease around 20-50% in catalytic activity of organically functionalized samples has been observed. This is probably due to the inhibition of access of the rather hydrophilic substrates to the Ti-active sites surrounded by the organic groups of increased hydrophobicity. It is noteworthy that the epoxidation was favorable for the organically functionalized samples whereas the alcohol oxidation was retarded. 

In summary, the organically functionalized Ti-substituted MCM-41 materials have been successfully synthesized by one-step synthesis method with a varied Ti-incorporation of the Si/Ti ratio from 50 to 600. The hydrothermal treatment resulted in the increase of Ti-incorporation. The epoxidation selectivity was improved by organic functionalization than alcohol oxidation probably due to the increased hydrophobicity nearby the Ti-active sites. 

The catalytic properties of Del-Ti-MWW have been compared with those of other titanosilicates in the epoxidation of cyclic alkenes (Table 4.4). The TON decreased sharply for TS-1, Ti-beta and 3D Ti-MWW with increasing molecular size of cyclic alkenes. Ti-MCM-41 with mesopores, however, showed higher TONs for cyclooctene and cyclododecene. This implies that the reaction space is extremely important for the reactions of bulky molecules. The delamination of Ti-MWW increased the TON greatly for not only cyclopentene but also bulkier cycloalkenes. Especially, the catalytic activity of Del-Ti-MWW was about 6 x higher than that of Ti-MWW for cyclooctene and cyclododecene. Del-Ti-MWW even turned out to be superior to Ti-MCM-41 in the epoxidation of bulky substrates. This should be due to the high accessibility of Ti active sites in Del-Ti-MWW. Thus the delamination was able to change Ti-MWW into an effective catalyst applicable to reactions of bulky substrates. 

Polymer yield (8) Catalyst yield" (kg PE/PP/ mol Ti) Activity / kg PE mol Ti-h-bary Time Relative molecular weight (viscometric) [Ti] (mol) [AH (mol) Remarks... 

The prime requirement of an active metal braze is that it should be able to change the chemistry of the ceramic surface to make it wettable, usually by forming hypostoichiometric TiC, TiN or TiO. This necessitates using alloys with high Ti activities, but alloys with high Ti concentrations are seldom suitable as brazes. Thus, Cu alloyed with 5 or 10 wt.% of Ti wets many ceramics well but... 

It is clear from Table 21.1 that the redox active centers (Mn , Cr [15]) in the AlPO microporons framework (pore diameter 7.3 A) is more effective than the Ti" active center in the microporons silicalite (TS-1 see US 4410501) framework (pore diameter 5.5 A), and that the yield of the desired product with the first of these catalysts are superior to those of the latter. The choice of solvent and substrate oxidant mole ratio has a significant inflnence on the activity and selectivity in these reactions. 

It can be seen that, under similar conditions, the reactivity of the dialkyl sulfides is directly linked to their molecular size Et2 S > Pr2 S > Bu2 S, and saturated sulfides are more reactive than allyl or aryl sulfides Pr2 S > Me S Allyl > Allyb S > Me S Ph > Ph2 S. These results can be explained, first, if we take into account the relative easiness of thioethers accessibility to the Ti active sites of the catalytic species located in the zeolite framework. The diffusion of the bulkier molecules, such as Ph2S is very difficult even inside the large pores of Ti-beta zeolite. Secondly, the reactivity of thioethers is in agreement with the nucleophilicity of the sulfur atom, so that alkyl sulfides are more easily oxidized than allyl or aryl sulfides by H2O2 (an electrophilic oxidant) in agreement with reported results [1-9]. It must be pointed out, that in the case of allyl methyl sulfide and di-allylsulfide, the epoxidation of the allyl system is not observed under our experimental conditions. 

FIGURE 114 MW and branch distributions of polymers made with Cr/silica-titania catalyst (5 wt% Ti) activated at 700 °C (parent) followed by CO at 350 °C (CO-reduced). 

In that Table it can be seen that TS-1 is much more active than Ti-Beta for the oxidation of n-hexane, the activity per Ti site being about 20 times higher in the former. However, this difference in activity is strongly reduced in the case of cyclohexane, provided this molecule has serious problems to diffuse inside the pores of TS-1, and hence, to reach the Ti active sites. It has also to be noticed that Ti-Beta shows a higher selectivity to cyclohexanone than TS-1 at a similar conversion level. 

Protease inhibitors in soybeans, known as trypsin inhibitors (TIs), play important roles in nutritional properties of soybeans and soy protein products. Two types of TI are the Kunitz inhibitor and the Bowman-Birk inhibitor. The Kunitz inhibitor has a MW of 21,500 with two disulfide bonds, while the Bowman-Birk inhibitor has a MW of 7,900 with seven disulfide bonds (Wolf, 1977). The large ratio of disulfide bonds to MW in the Bowman-Birk inhibitor stabilizes protein conformation and makes the Bowman-Birk inhibitor highly resistant to heat denaturation and inactivation. The Kunitz inhibitor inhibits trypsin, while the Bowman-Birk inhibitor inhibits both trypsin and chymotrypsin. The kinetics of TI inactivation when heating at high water activity were determined by Johnson et al. (1980) they estimated that 83-91% of the TI activity in soybeans is due to the Kunitz inhibitor. Recently, the Bowman-Birk inhibitor was attributed cancer-protecting qualities and also interest exists in using purified soy TI to treat AIDS patients (Kennedy, 1995, 1998 Kennedy Szuhaj, 1994). 

Only 40% of growth inhibition in test animals, however, is related to TI activ-... 

Relationships between heat processing and nutritional indicators of SFs are shown in Table 19.3. Increased heating reduces TI activity, which is reflected in lessened enlargement of the pancreas in test rats. Although minimum protein denaturation is desired to maximize SF solubility and functionality, the fabricated product must be adequately cooked before consumption (Fulmer, 1989a). 

In the raw state, mature soybeans and many other plant foodstuffs contain protease Inhibitors that diminish the proteolytic activities of trypsin and chymotrypsln in the Intestinal tract, cause pancreatic hypertrophy and suppress growth. Trypsin Inhibitors (TI) account for about 40% of the pancreatic hypertrophic effect and growth-inhibitory capacity of raw soy proteins. The resistance of the raw undenatured protein to tryptic digestion accounts for the remaining 60%. The practical significance of residual TI activity In heat-processed soy protein products and the biochemical effects of other protease Inhibitors have been reviewed (40). 

Trypsin Inhibitors. TI activity in 108 varieties and strains of soybeans ranged from 66 to 233 trypsin unlts/mg protein (41). Mean TI values of 57 specific activity units per mg protein were reported for 16 field-type soybeans grown In India, whereas in 8 vegetable-type soybeans, the mean TI value was 41 units/mg protein (42). 

As shown In Table V, TI activity generally Increased during maturation. The Dare (field-type) variety had the greatest Increase In activity. Dehulled immature beans held 2,5 minutes in boiling water had 97-98% of the extractable TI activity destroyed regardless of variety. 

Germination for 3 days, after a 24-hour soaking period, did not cause TI activity to change appreciably for the four soybean varieties studied (43). On the other hand. Bates et al. (33) reported that TI activity In water extracts of soybeans germinated 4 days decreased about 70% (Table VI). Autoclaving for... 

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