Calcined silica

One interesting result of stripping off Cr(VI), whether chromate or dichromate, is that it leaves pairs of hydroxyls, even if the catalyst has been calcined previously at 800°C to remove OH pairs. Despite being paired, these hydroxyls do not condense easily when heated a temperature of 800°C is required to remove all of them. However, they do react with chromyl chloride vapor at 200°C to yield a good deal of chromate. (Ordinarily silica calcined at 800°C forms no chromate when it reacts with chromyl chloride.) This suggests that even at 800°C Cr/silica may contain a considerable amount of chromate. 

Silica calcined at 850°C., wet-promoted 2.5 with Cr03, 500°C. activation... 

It should be noted that thermal treatments applied to aluminosilicates can vary significantly with their properties. For instance, silica calcination at temperatures ca. 900°C produces condensation of neighboring silanol groups to form silanoxy bridges (Hozumi et al., 2000 Naono et al., 2000). This process can be schematized as ... 

In view of the feet that complete removal of water vapor cannot be readily achieved, we prepared water-free bulk and silica-supported zirconium sulfate. The bulk anhydrous Zr(S04)2 was obtained by reaction of zirconium tetrachloride with oleum [1]. The silica-supported zirconium sul te resulted from deposition-precipitation of zirconium hydroxide on silica, calcination at 723 K and subsequent reaction with gaseous sulfur trioxide. The catalytic activity of the sul ted zirconia s was measured in the gas-phase /rora-alkylation of benzene (1) with diethylbenzene (2) to ethylbenzene (3, reaction 1) [8,9] and the liquid-phase hydro-acyloxy-addition reaction of acetic acid (4) and camphene (5) to isobomyl acetate (6, reaction 2) [8,10]. With the /roras-alkylation we used an amorphous silica-aliunina catalyst as a reference. 

FIGURE 7 Polymerization kinetics on two catalysts. Above Silica calcined at 400 °C and then reacted with Cr02Cl2 vapor (chromate species only). Below. Silica impregnated with Cr03 and calcined at 400 °C. 

FIGURE 25 Melt index and MW values of polymers made with three series of catalysts, showing the separate responses obtained by calcination of the silica, versus that of calcining the chromium. Series 1 Cr/silica calcined at temperature shown. Series 2 Silica calcined at temperature shown, aqueous impregnation of chromium, air 500 °C. Series 3 Silica calcined at temperature shown, chromium applied anhydrously, air 500 °C. 

FIGURE 90 LCB levels in the polymers made with three series of Cr/silica catalysts, each prepared from the same silica but in a different way. Two independent and additive contributions are evident, one from structural sintering and another from surface dehydroxylation. A Cr/silica activated as shown, B Silica, calcined as shown, aqueous chromium, air 500 °C, C Silica, calcined as shown, anhydrous chromium, air 500 °C. 

FIGURE 118 Melt indices of polymers made with two series of catalysts, showing that the order of impregnation can be important in the two-step activation. Upper line silica-titania calcined in air at 815 °C, anhydrous chromium applied, then air as shown. Lower line Cr/silica calcined in air at 815 °C, anhydrous titanium applied, then air as shown. 

Indeed, one need not necessarily use a chromium alkyl for this purpose, as other organochromium compounds can also be used successfully. The open-ring chromocene, bis(2,4-dimethylpentadienyl) chromium(II), called Cr(DMPD)2, was tested and performed similarly in many respects. Figure 200 presents an example in which this organochromium compound, called Cr(DMPD)2, was added to the reactor along with Cr(VI)/silica (or at the right in the figure, just silica) calcined at 800 °C. 

An experiment is summarized in Table 56 that demonstrates that some even more unusual chromium catalysts can be made by this approach. Bis (f-butyl) chromate, (f-but-0)2CrC>2, is a chromate ester that is soluble in hexane and other hydrocarbons. When deposited onto silica calcined at 600 °C, it has no activity for ethylene polymerization. Dicumenechro-mium(O) is a compound that also dissolves in hydrocarbons, and exhibits no (or marginal) activity when deposited onto calcined silica. However, in the experiments referred to in Table 56, the two compounds were deposited sequentially onto silica, and considerable activity did develop from some unknown redox product formed from the two. In this example, the maximum activity seems to have been obtained when the catalyst contained about 40% Cr(VI) and 60% Cr(0). 

The rehydroxylation of a wide-pore silica gel sample calcined in air at 850 °C and held in water at 100 °C for periods covering 1 to 100 h was found to take 5-10 h for complete rehydroxylation. These and other results indicate that rehydroxylation of dehydroxylated silica (calcined at >400 °C) in the presence of water requires considerable energy to activate the process of dissociative adsorption, Ed. Chemisorption of water appears to take place, resulting in the formation of hydroxyl groups bound through... 

For the removal of the surfactants from organically modified mesostructured silicas, calcination at elevated temperatures is only an option in rare special cases. Instead, extraction of the surfactant is the method of choice. For the extraction, however, the inorganic pore structure has to be stable towards the removal of the surfactant and the chemicals involved in this process, without the stabilization provided by further condensation within the silica walls, which takes place during the high-temperature calcination process, but will not occur during extraction at low temperatures. 

Figure 11.2. Pore size distributions calculated from gas adsorption data for SB A-16 silicas calcined at different temperatures.

A thermally induced pore closure process was also observed for Fm3m structure of large-pore FDU-12 (LP-FDU-12) silica synthesized without an acid treatment," but in this case, closed-pore silicas were obtained at much lower temperatures (550-640°C). For the LP-FDU-12 silica calcined at temperatures from 450 to 640°C, SAXS patterns featured several peaks whose relative positions... 

Figure 11.5. (a) Nitrogen adsorption isotherms and (b) pore size distributions of LP-FDU-12 silica (calcined at 450 °C) before and after surface modification with trimethylsilyl (TMS) and butyldimethylsilyl (BDMS) groups. The pictures of models of the TMS and BDMS groups are included to illustrate the relative size of these two kinds of surface groups. The adsorption data for unmodified LP-FDU-12 were taken from our earlier publication. ... 

The IR spectra of Pt/silica calcined at 400°C (sample 4, in Table 17.1) were also obtained after exposure of CO/O2 at different temperatures. This sample, according to the EXAFS results, initially contains no metallic Pt. The IR results (Figure 17.13) accordingly show no CO adsorption at room temperature. At 100°C, however, there is a small, broad band at 2105 cm-. As the temperature increases, however, a band at 2077 cm-, corresponding to linearly adsorbed CO on metallic Pt, begins to increase while the band at 2105 cm- decreases. 

This patent teaches the importance of preparing the same catalysts discussed in patent (VIII) above on silica calcined at a temperature at or above 800°C in order to manufacture ethylene/1-hexene copolymers with a very narrow molecular weight distribution. The catalysts shown in Table 2.6 were evaluated in a continuous gas-phase fluidized-bed reactor with triethylaluminum as cocatalyst to produce LLDPE polyethylene samples with a density of 0.918 g/cc. 

Catalyst Silica Calcination Temperature °C TEOS Loading (mmol/g silica) MFR... 

The infrared spectra of the silicas that were calcined at 300, 600 and 800°C before and after treatment with DBM are shown in Figure 2.12. The silicas calcined at 600 and 800°C exhibit a single peak at 3746 cm L which has been assigned to isolated hydroxyl groups on these silicas. The infrared spectrum of the silica calcined at 300°C contains both a singlet peak due... 

Silica calcination Temperature Hydroxyl content mmol OH/g silica DBM fixed mmol Mg/g silica TEAL fixed Mmol Al/g silica... 

Figure 2.11 Fixation of Mg from Dibutylmagnesium or Al from triethylaluminum to silica calcined at 300, 600 or 800°C [50].

The data in Table 2.8 show, as expected, the higher the silica calcination temperature, the lower the surface hydroxyl concentration and the Mg or Al content of the sihca/metal alkyl reaction product. But the data also indicate that the amount of DBM or TEAL that can react with sihca greatly exceeds the siuTace hydroxyl concentration on each of the silicas. Hence, additional sites on the silica smface are available for reaction with metal alkyls other than siuface hydroxyl sites. Most likely the metal alkyl compounds coordinate to the silica surface through an electron pair on the oxygen atom of a siloxane (Si-O-Si) linkage. In addition, DBM reacts with each silica to a greater extent than does the TEAL. 

The Mg content from the interaction of silylated treated silicas, calcined at 300, 600 and 800°C, reacted with an excess of DBM as described above is shown in Table 2.9. 

The silicas calcined at 300, 600 and 800°C and then reacted with Me SiNMe also exhibited no residual hydroxyl groups, as indicated by their IR spectrum [50]. However, when these silylated silicas were reacted with DBM, the silylated silicas had a Mg concentration of 0.65, 0.69 and 0.85 mmol/g of silylated silica, respectively, demonstrating the presence of a second site of attack for the fixation of the DBM molecule to the silica surface. The interaction of the silylated silica/magnesium intermediates (Catalysts 1 and 3 in Table 2.10) with TiCl provides catalyst precursors with the higher activity. 

Data Point Silica Calcination Temp. (°C) Silylation step Metal Alkyl Ti wt% Productivity kg PE/g Ti/hr/100 psi... 

Nonetheless, Karol et al. [25] and Lunsford et al. [27]postulate that the chromium species in Figure 3.17 is the predominant structure on silica calcined at 800°C, because silica calcined at 800 C is believed to contain only isolated silica-hydroxyl groups, therefore the deposition reaction shown in Figure 3.18 is not possible. 

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