Site densities calcination

In agreement with the TPR results, the hydrogen chemisorption/pulse reoxidation data provided in Table 8.3 indicate that, indeed, the extents of reduction for the air calcined samples are -20% higher upon standard reduction at 350°C (compare 02 uptake values). Yet in spite of the higher extent of reduction, the H2 desorption amounts, which probe the active site densities (assume H Co = 1 1), indicate that the activated nitric oxide calcined samples have higher site densities on a per gram of catalyst basis. This is due to the much smaller crystallite that is formed. The estimated diameters of the activated air calcined samples are between 27 and 40 nm, while the H2-reduced nitric oxide calcined catalysts result in clusters between 10 and 20 nm, as measured by chemisorption/pulse reoxidation. 

Microcalorimetry experiments with NH3 and pyridine as probe molecules indicated that insertion of Ga into the offretite aluminosilicate structure increased the overall acid sites strength of the crystals while decreasing its acid sites density [255], The observed heterogeneity of acid site strength distribution of H,Ga,Al-offretites was attributed to some extra-framework Al(Vl) and Ga(Vl) species generated during the ion exchange and calcination procedures used to prepare H-offretite crystals. 

Zakharov et al. have used a radio tagging technique to measure the active site density in which polymerization is killed with labeled methanol (72, 73). They found only about 1 % or less of the chromium to be active, or about one tenth of Hogan s number. But because they calcined Cr/silica at only 400-500°C, their catalyst was probably only one-tenth as active. So the two studies are not necessarily in conflict. As expected, the active site density found by tagging increases with time during a polymerization run. 

Because ZSM-5 normally may exhibit high selectivities to aromatics and paraffins, the zeolite and the process conditions have been modified to improve selectivity to light olefins. The effective shape selectivity was increased by the deposition of silica in the channel system [175]. Phosphorus modification [121] with trimethylphosphite (PCOCHj) ) followed by air calcinations improved olefin selectivity by lowering acid site strength and probably also by reducing the effective pore size. Ion exchange with + [176], and higher SiO/Al Oj ratios (lower site density) [39] were all reported to improve olefin selectivities. 

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

Coke selectivity directly influences the rate of catalyst deactivation as seen by comparing coke selectivities in Tables VI and VII with observed rate constants in Table V. Our data indicate calcined AFS zeolites show higher coke selectivities than USY zeolites when compared at similar unit cell sizes. This result suggests that distribution of framework acid sites(as reflected by the distribution of framework silicon) has a strong impact on coke selectivity. In addition, coke selectivity has been shown to correlate with the density of strong acid sites in the framework(20). Our data confirm this and show that steaming decreases the density of such sites which, in turn, leads to decreased coke selectivities. 

For relatively small particles of PdO or CuO and high proton concentration in their vicinity, the reaction given by Eq. (12) is virtually complete at 400°C (76,205). The reaction is driven by the formation of water and the migration of the ions to sites with high electronegative charge density. The ions then have the same oxidation states and locations as they had after original calcination. For Cu/NaY this has been demonstrated by electron... 

Bronsted acidity is increased only for alumina-bound Y and does not change for alumina-bound ZSM-5, as shown in Fig. 2. The reaction of AI2O3 from the binder with Si02 in the zeolite at high temperature such as at calcination, which results in new Bronsted acid sites[5,6], is responsible for the increase in Bronsted acid density. The reason that the alumina-bound Y samples were with increased Bronsted acid sites may be due to free Si02 (non-frame silica) present in the zeolite or easier insertion of alumina into Y zeolite than into ZSM-5. 

When zeolites are dealuminated by steam-calcination part of the framework A1 is extracted and generates extra-framework species (EFAL) that can be cationic, anionic or neutral. Some of these EFAL species can act as Lewis acid sites [19] or can influence the Brpnsted acidity, by either neutralizing Brpnsted acid sites by cation exchange, or by increasing the acidity by a polarization effect and/or by withdrawing electron density from lattice oxygens [20-22]. However, under mild steaming the A1 can also become partially, and reversibly, disconnected from the lattice [23]. This opens the way to Lewis acid catalysis by the A1 [24]. 

Table IV shows the cation distributions estimated for several forms of X dehydrated under various conditions. For trivalent cations (.—30 per unit cell), sites I and II are most suitable electrostatically, but the sample of La-X calcined at unspecified temperature (probably 350°C) showed all the La atoms on I, while site II was occupied by electron density explainable by 32 water molecules. This remarkable distribution yields a very stable chemical complex with each La bonded to 3 03 and 3 HoO at 2.5A and with each H20 bonded to either 2 or 3 La atoms. The H20 also is bonded weakly to 3 02 atoms of a free 6-ring.

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