Amorphous high-alumina silica-aluminas

After the war, industry made a number of advances in cracking catalysts, but all were high surface area, amorphous oxides, like silica-alumina. By the mid-1950s the improvements became smaller and less frequent. Many companies cut out or de-emphasized research in this area because it was commonly believed that cat cracking had gone about as far as it could go. 

High-silica varieties (Si/Al = 10...70) of ferrierite were prepared by conventional crystallization of amorphous alkaline alumina/silica sources in aqueous medium in the presence of various templates as trimethylcetylammonium hydrate [94B2], diethanolamine [95F1], pyrrohdine [95X2, 95Z2], or ethylene diamine [97V1], at temperatures around 420 K. 

Weisz and Miale compared the activity for hexane cracking of a number of zeolites (Table 4.25) with a highly active silica — alumina (10% alumina). The zeolites are at least 10 times as active as amorphous silica —alumina. The catalytic process, however, cannot utilize the activity from a pure zeolite catalyst. The catalyst must be modified to decrease the acid-strength to avoid excessive formation of coke and low molecular weight gases, at the expense of gasoline. Moreover, the catalyst must be able to withstand the thermal and hydrothermal conditions experienced in regeneration. It must also withstand breakup in the mechanical circulation systems. A detailed description of the preparation of industrial catalysts is found in the literature. 

In addition to the crystalline clays described earlier, there are some materials that act like clays but do not have crystalline structure. Amorphous clays do not have a definite X-ray diffraction pattern and are differentiated from the crystalline clays on this basis. They are composed of mixtures of alumina, silica, and other oxides and generally have high sorptive and cation exchange capacities. Few soils contain large amounts of amorphous clays [2],... 

These results show that it is difficult in any of he highly dealuminated samples studied here to prove that the Al observed corresponds only to frame ( k aluminum of the zeolite, since t very different types of Al are observed. On the other hand, Al is observed at 56 ppm in amorphous silica-alumina (7,9). Thus one could safely conelude that during d jlumination by steam and by SiCl, besides the A3. ( 60 ppm), Al ( O ppm), and tetrahedrally... 

In order to study he Lewis acidity of the samples, the intensity of the 1450 cm pyridine band was also measured. Sample HYUS-8 shows a high amount of Lewis centers (Fig. 4d), relative thf HYD-400 sampl (Fig. 5c). This agrees with the absence of A1 as observed by A1 MAS-NMR for HYD samples. However, chemical analysis (Table I) indicates that there is more aluminum in this sample than in that from the unit cell constant m i urements. These differences cculd be explained considering that A1 MAS-NMR does not detect octahedral EFAL because of the low symmetry of its environment (i ). If this is so, it is remarkable that this EFAL does not show Lewis acidity as measured by pyridine ad y ption. On the other hand, if indeed thej is a small amount of A1, then the EFAL should be present as Al" outside the zeolite framework. In this case it should be present as amorphous silica-alumina. 

Table III compares the gasoline composition from three steam deactivated catalyst systems. The first contains 10% rare earth exchanged faujasite (RE FAU) in an inert silica/clay matrix at a cell size of 2.446 nm the second contains 20% of an ultra stable faujasite (Z-14 USY) at a unit cell size of 2.426 nm in inert matrix. The third contains 50% amorphous high surface area silica-alumina (70% AI2O3 30% Si02) and 50% clay the nitrogen BET surface area of this catalyst after steam deactivation is 140 m /g. All three catalysts were deactivated for 4 hrs. at 100% steam and at 816°C.

Isopropylatlon of biphenyl catalyzed by solid acid catalysts gave a mixture of three Isomers of Isopropylbiphenyl (IPBP) and many Isomers of dllsopropylblphenyl (DIBP). The 12-membered zeolites, HM, HY, and HL gave quite different features In selectivity. Isopropylatlon catalyzed by HM was highly selective to give 4-IPBP and 4,4 -DIBP. However, catalyses by HY and HL were nonselectlve similar to the case of amorphous silica-alumina. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

Andreu et ah (11) explained the increased activity (with increasing alumina content of amorphous silica-alumina catalysts) for cracking of sec-butylbenzene by the greater density of acid sites in the high-alumina-content catalysts. Adams et ah (12) proposed that the interaction of several active sites with reactant molecules in mordenite catalysts was partly responsible for the rapid rate of activity loss. 

The stability of MCM-41 is of great interest because, from the practical point of view, it is important to evaluate its potential application as a catalyst or adsorbent. It is known that purely-siliceous MCM-41 (designated here as PSM) has a high thermal stability in air and in oxygen containing low concentration (2.3 kPa) of water vapor at 700 °C for 2 h [1], However, the uniform mesoporous structure of PSM was found to be collapsed in hot water and aqueous solution due to silicate hydrolysis [2], limiting its applications associated with aqueous solutions. After MCM-41 samples were steamed in 100% water vapor at 750°C for 5 h. their surface areas were found to be lower than amorphous silica-alumina and no mesoporous structure could be identified by XRD measurement [3]. In addition, PSM showed poor stability in basic solution [4]. 

There is just a handful of papers in which hydroxyl groups of silica gel are studied directly by high-resolution H NMR techniques (385-387). In particular, Hunger et al. (387) were able to observe two spectral lines in the H MAS NMR spectrum of amorphous silica-alumina gels of different composition The line at 2 ppm from TMS was attributed to nonacidic hydroxyls, since it also occurs in silica and alumina the line at 7 ppm, the... 

---