Termination dehydroxylation

Though silica supports are amorphous, the surface may exhibit some local order, such as that of the mineral /3-crystoballite (Fig. 5.23). The surfaces of silica support contain OH groups at densities of between 4 and 5.5 OH per nm that of cristobal-lite is 4.55 OH per nm. Silica surfaces contain only terminal OH groups, i.e. bound to a single Si atom. Heating leads to dehydroxylation, and at high temperatures only the isolated OH groups remain. 

It can be concluded from these observations that whereas benzoate produced by the carboxylation of phenols can be degraded, dehydroxylation with the formation of substituted benzoates may produce stable terminal metabolites. 

Fig. 9. Increasing the calcining temperature, which dehydroxylates the surface, enhances the polymerization activity and especially the termination rate up to 925°C, where sintering destroys the silica base.

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

Fig. 20. After being reduced at 870°C, three series of Cr/silica-titania catalysts yield highest termination rates (RMIP) after reoxidation at 600°C. Catalysts reduced in CS2 display best results because CS2 is the most effective dehydroxylating agent. Carbon monoxide is second best. Trivalent samples calcined in N2 also show the benefit of low-temperature reoxidation, but without the effect of increased dehydroxylation.

However, the strongest arguments against the scheme of Uytterhoeven et al (203) come from NMR work, which is an excellent example of how H and 27A1 MAS NMR can be used in cooperation (168,170). The amounts of terminal and structural hydroxyls were separately measured using lH NMR, while the amount of four-coordinated Al was readily obtained from 27A1 MAS NMR. No three-coordinated Si or Al was observed the amounts of structural hydroxyls and four-coordinated Al were always equal and much less than half of four-coordinated Al was found after complete dehydroxylation. 

The second proposed mechanism for the Fischer-Tropsch reaction is the Pichler-Schulz mechanism (2,16) (Scheme 2), which is characterized by chain growth by insertion of CO. This mechanism implies that the insertion step and subsequent dehydroxylation are fast compared with the chain-termination reaction. We emphasize that, according to this scenario, CO dissociation does not need to proceed rapidly. 

Many metal oxides, such as Ti02 and AI2O3, can be prepared with relatively high surface areas. The exposed surfaces of metal oxides are typically terminated with 0x0 or hydroxy groups. A dehydroxylated surface is one from which most of the surface OH groups have been removed by heating (see below). Full or partial dehydroxylation may expose cation sites. 

Phase diagrams specific to each experimentally observed face have been derived, which show that condihons should exist for which the (0001) surface is dehydroxyl-ated, to give an A1 termination, while other surfaces remain fully hydroxylated. 

Dehydroxylation of decationated mordenite at high temperatures also produces a substantial change in the acidity spectrum as shown in Table VI. Raising the activation temperature of HM zeolites with a Si/AI ratio of about 9.0 from 703 to 1023 K increases the initial differential heat of ammonia adsorption at 573 K from 165 to 180 kJ moP and sharply decreases the concentration of sites near 160-130 kJ mol and the overall acidity (756). IR spectroscopy of molecular hydrogen adsorbed at low temperature showed that mordenite dehydroxylated at 703 K contains only Brpnsted acid sites and nonacidic terminal Si—OH groups, whereas raising the pretreatment temperature decreases the concentration of acidic bridge-type hydroxyl... 

R/R Activation. Figure 5 shows that the enhanced dehydroxyl-ation by carbon monoxide also had a pronounced effect on the termination rate during polymerization. In these experiments, two series of Cr/silica catalyst samples were activated and allowed to polymerize ethylene to a yield of about 5000g PE/g. In one series the catalyst samples were simply calcined five hours in air as usual at the temperatures shown. The relative melt index potential (RMIP) has been plotted against activation temperature and the expected increase up to the point of sintering was observed. 

For metal particles on ionic substrates one can differentiate between interactions with non-polar (stoichiometric) and polar surfaces. The former surfaces are represented by essentially unrelaxed (001) facets of oxides with rock-salt structure, such as MgO(OOl) which is widely utilized as metal support and was addressed in the previous subsection. Oxide supports exhibiting polar surfaces are also common. Clean polar surfaces are unstable and thus difficult to prepare unreconstructed, dehydroxylated, and free of defects [88]. Corundum, a-Al203, is a prototype of such metal oxides. Its most stable Al-terminated (0001) surface... 

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