Hydrothermally treated samples

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

Table 1 lists the BET surface areas and other physical properties of the calcined and hydrothermally treated samples. It can be seen that these samples possess very high surface area in the range of 800-1000 m2/g. Pore wall thickness of the calcined samples was found to be about 19 A and no increase in wall thickness was noticed by adding additional TPA ions However, the wall thickness was found to increase slightly after hydrothermal treatment due to pore wall restructuring during such treatment [12,13],... 

This method of synthesizing MCM-41 with improved hydrothermal stability is applicable with surfactants of different chain length also XRD patterns of the calcined samples synthesized with surfactants Cu and Cm chain length with TPA as additional cation and without any cations are shown in Fig 5. XRD patterns of the hydrothermally treated samples are also included for comparison It can be seen that when no TPA was added to the synthesis... 

The 27A1-MAS-NMR spectra of the hydrothermally treated samples consist of a sharp peak at ca. 61 ppm due to tetrahedrally coordinated Al in the zeolite framework and two signals due to octahe-drally coordinated extra-framework Al, a very broad peak of ca. 

Si-MAS-NMR spectra of the synthetic faujasites indicate a much lower concentration of non-framework Al when dealuminated with SiCl4 than in the case of the hydrothermally treated samples. The difference between the Si/Al ratio as obtained by 29Si-MAS-NMR and elemental analysis is much smaller, indicating that much of the Al is completely removed from the lattice. 

Figure 1. The XRD patterns of (A) NaBir, (B) NiBir, (C) sample obtained by pH titration of NaBir in Ni(N03)2 with NaOH at room temperature, (D) Ni(OH)2-NiBir mixture with Ni/Mn=l/2, and (E) hydrothermally treated sample (D) at 200°C for 2 days,...

Figure 3. XRD patterns of (A) as-prpared Ni(OH)2-manganese oxide precipitate and hydrothermally treated samples for (B) 7h and (C) 4 days at 150 °C.

Figine 4 Pore size distributions of hydrothermally treated samples in comparison to the untreated parent silica. 

The results of porous structure determination of hydrothermally treated samples indicated that there is no visible influence of the presence of cerium oxide on thermal stability. However, in the case of sample calcined at 850 C the tenfold decrease in specific surface area was stated while in the case of sample calcined at 650 "C only fivefold. This observation supports our assumption that the highest temperature of the initial calcination has to be not... 

Figure 5. Product selectivity for n-butane cracking over (a) hydrothermally-treated sample H3, and (b) AHFS-treated sample FS9.363. The reaction products are methane (0), ethane and ethene (A), propane (O), isobutane (b), but-l-ene ( ) and but-2-ene (a).

In-situ FTIR of CO (and other molecules as site specific probes) was studied on the H-form and hydrothermally treated samples of H-ZSM-5. These results have been more thoroughly reported elsewhere [10] but are sununarized here for comparison with experimental data from other techniques. Zeolites have been traditionally examined using infrared spectroscopy of N-containing adsorbates such as ammonia and pyridine to assess Bronsted acidity. The use of weaker Lewis bases allows a more discriminating approach to assessing the strength and quantity of Bronsted sites arising from the partial and sequential dealumination of framework A1 in H-ZSM-5. 

Reichle etal (11), have observed a decrease of surface area from about 120 m for hydrotalcite crystallized at 333 K to 13.9 mV for hydrothermally treated sample. X-ray data of HTl and HT2 are in agreement with their observation. However, the surface area was found to decrease only by half for the hydrothermally treated sample HT2. The hydrothermally treated sample has a surface area of 32 mV The low surface area may be due to a large crystallite size of the hydrotalcite. Surface area of the hydrotalcite prepared at high supersaturation and crystallized at 333 K is 60 mV The surface area was found to be same for the catalysts with different Mg/A Iratio supports. The support with interlayer S04 ion has only 5 mV surface area. 

To further understand the role of strong acid sites in the aromatization activity of the catalysts, time-on-stream (TOS) activity studies were carried out over these three catalysts, and the deactivation patterns of acid sites were studied. It can be seen from fig.4 that, the as-synthesized catalysts T and WT show a constant aromatization activity, where as the hydrothermally treated sample (HT) shows rapid deactivation with in 12 hrs. The highest initial activity in aromatization with the steep fall within 12 hrs TOS observed in case of the catalyst HT, can be understood by considering the high turn over number of super acid sites created during hydrothermal treatment, and their propensity to rapid deactivation (8). 

Activity data of catalysts subjected to high temperature treatments at 700°C and 800°C and that of hydrothermally treated samples are given in Table 2. The percentage loss of catalyst activity during deactivation tabulated in Table 2 is a helpful index to rank catalysts with respect to their resistance to deactivation. 

All of the hydrothermally treated samples were brown in colour indicating the presence of some iron oxide/hydroxide species. The Mossbauer spectra at RT and 78 K of these samples were all quite similar and indicated that essentially all of the iron in these samples was in the ferric state. The spectra consisted of a symmetric doublet with IS of 0.33 mm s indicating iron in octahedral coordination while the QS of 0.84 mm s is close to that for a-Fe203 particles supported on silica [33]. 

The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more ciystalline than by sequential precipitation (SP). Furthermore, the ciystallinity also increases with increase in atomic ratio. Hydrothermal treatments increased the ciystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-COg-HTlcs resulted in the... 

At low temperatures, where mass transport limitation is absent, all samples showed the exponential rise of the activity with temperature associated with the Arrhenius behavior of the catalytic reaction. Ihe activities of the hydrothermally treated samples at low temperatures were equal, indicating that the hydrothermal treatment had not affected the active surface of the copper-on-silica catalyst incorporated in ti e silica matrix. The activation energy was about 95 kj/mol. Above 400 K mass transport limited the activity. The activation energy in the straight part of the curve (500-800 K) was about 6.0 kj/mol. 

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