2-Hexenal site selectivity

Data obtained in the catalytic epoxidation of 1-hexene over Ox-Ti-P and other samples are summarized in Table 2. Catalytic properties of Ti-P zeolites were studied by Corma et al. [4,10] and Davis et al. [11,12]. Despite some discrepancies, it is agreed that these catalysts are active in the epoxidation of olefins. Our results also indicate that all of our Ox-Ti-P and Ti-P samples are active in the epoxidation of 1-hexene. The selectivity toward epoxide was very low. The major products were ethers, obtained from solvolysis of glycol by methanol which is catalyzed by the zeolite acid sites. It was found that over Ox-Ti-P samples, the reaction takes place slowly, while the hydrogen peroxide is utilized efficiently. Over Ti-P, the reaction takes place very rapidly and is usually finished in less than 1 hour. It was also found that the parent aluminosilicate P (sample 1) was completely inactive in this reaction. Davis et al. [12] demonstrated that framework Ti is the active site in epoxidation reactions, particularly in aqueous media. It is inferred that our catalysis data provide a strong evidence that Ti(IV) species in our Ox-Ti-P samples are present as isolated framework cations. 

Figure 3 shows the initial activities of the isomerization of 1 -butene. The activities were evaluated from the formation of main products of /wwr-2-butene and cis-2-butene. The activities increased with increasing evacuation temperature and showed a maximum after pre-evacuation at 773- 873 K then the activity decreased to almost zero at 973 K. The selectivities of cis-2-butene/ trarw-2-butene on the Ni-481 and Ni-359 catalysts were found to be 0.6 and 0.7, respectively, meaning that the active sites should be solid acid sites[8]. Figure 4 shows the conversion of ethylene oligomerization on the Ni-481 catalysts. The conversions were evaluated from the formation of the main products of butene and hexene at 120 min. The conversion increased with increasing evacuation temperature and showed a maximum after pre-evacuation at 773- 873 K then decreased to almost zero at 973 K. 

The preferential formation of hexenes over SAPO-11 may be consequence of the diffusion patterns within the unidimensional micro pores. Reactants must diffuse into the channel "next to one another" for a bimolecular reaction to take place. Formation of C9 olefins, which would yield C4 and C5 products after cracking, requires either (1) diffusion of three adjacent propylene molecules to an active site, or (2) formation of hexene, followed by diffusion to the end of the micropore and diffusion of adjacent hexene and propylene molecules back towards an acid site. The necessity of hexene transfer to the end of the pore via the second pathway increases the likelihood of C6H12 desorption to the gas phase before further reaction. The selective formation of hexenes over medium-pore SAPO-11, which has a onedimensional micropore system similar to the wide-pore SAPO-5, may be a consequence of steric restrictions, as will be discussed later. 

Fig. 15. Effects of catalyst structural properties and site density on Cs+ selectivity and 1-hexene/n-hexane ratio (experimental Co on Si02, TiOa, and AI2O3 supports 473 K, H2/ CO = 2.1, 2000 kPa, 50-60% Co conversion). Model solid lines.

As noted in Section 6.9, when added to the reactor in trace amounts, some polar compounds can selectively inhibit some sites in preference to others. In addition to affecting the MW distribution, they sometimes have an influence on the branch content and distribution as well. An example of this behavior is shown in Figure 44. Ethylene-hexene copolymers were produced with a Cr/silica-titania catalyst activated at 850 °C. Methanol was added to the polymerization reactor in incrementally greater amounts to "titrate" the activity of the catalyst. As expected, the activity declined as methanol was added, reaching nearly zero at about three CH3OH molecules/Cr atom. The average polymer MW increased as methanol was added. The results imply that some sites, those more... 

The average branch content of these polymers is also shown in Figure 44. Surprisingly, it was found to decline as methanol was added. This is another confirmation that the low-MW producing sites, which were shown to incorporate 1-hexene best, were selectively inhibited by the methanol. 

A similar experiment was conducted without 1-hexene added to the reactor and those results are also listed in Table 36. The same effect on MW distribution was observed. The incorporation of 1-hexene increased significantly when the titania-associated sites were selectively poisoned. This is another indication that titania inhibits branching in the low-MW side. Table 36 shows the drop in the polymer density, and the rise in the amount of 1-hexene incorporated, when the catalyst was treated in CO at only 100 °C. 

This increased efficiency probably derives from two causes. First, as noted above, Cr(II)/silica itself is considerably more efficient than Cr(VI)/silica. (One might say that the copolymerization, which always favors ethylene, is less selective for ethylene.) Second, the sites producing ot-olefin are in close proximity to the sites consuming 1-hexene within the catalyst/polymer pore structure. Therefore, the consuming sites probably experience a higher local concentration of comonomer than exists in the reactor as a whole, and consequently more monomer is incorporated. 

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