Amorphous silica alumina

Amorphous Silica—Alumina Based Processes. Amorphous siHca—alumina catalysts had been used for many years for xylene isomerization. Examples ate the Chevron (130), Mamzen (131), and ICI (132—135). The primary advantage of these processes was their simpHcity. No hydrogen was requited and the only side reaction of significance was disproportionation. However, in the absence of H2, catalyst deactivation via coking... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

Compared to amorphous silica-alumina catalysts, the zeolite catalysts are more active and more selective. The higher activity and selectivity translate to more profitable liquid product yields and additional cracking capacity. To take full advantage of the zeolite catalyst, refiners have revamped older units to crack more of the heavier, lower-value feedstocks. 

The breakthrough in FCC catalyst was the use of X and Y zeolites during the early 1960s. The addition of these zeolites substantially increased catalyst activity and selectivity. Product distribution with a zeolite-containing catalyst is different from the distribution with an amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are 1,000 times more active than the amorphous silica alumina catalysts. 

In view of the accessibility of zeolite A (only linear molecules adsorb) the coupling will take place at the outer surface of the zeolite crystals. Indeed, Ag-Y and especially a Ag-loaded amorphous silica-alumina, containing a spectrum of wider pores, mrned out to be much better promoter-agents (ref. 28). The silica-alumina is etched with aqueous NaOH and subsequently exchanged with Ag(I). 

Weight percent profiles through first-stage (left) and second stage reactor of a) alkanes (full fines) and cycloalkanes (dashed fines) and b) aromatic components. Thick lines correspond to C23 finctions, thin lines to 23 fractions. Operating conditions p, 17.5 MPa LHSV 1.67 niL (nv hf molar H2/HC 18 Tmiei 661 K (reactor 1) 622 K (reactor 2). Catalyst NiMo on amorphous silica-alumina. 

Table 9.5. Approximate product distributions of fluid catalytic cracking for amorphous silica-alumina and zeolite catalysts.

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Silica-aluminas contain acid sites stronger than those found in most halide-treated aluminas. We have attempted to tailor the acidity of amorphous silica-alumina by adding varying levels of "permanent" inorganic basic titrants. Such titrants were chosen in order to meet the following four criteria ... 

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

De Klerk, A. 2007. Effect of oxygenates on the oligomerization of Fischer-Tropsch olefins over amorphous silica-alumina. Energy Fuels 21 625-32. 

Fig. 28. The cumulative COj/CO ratio for coke bum-off on spherical catalyst beads versus combustion temperature in air (O) white amorphous silica-alumina ( ) green Cr02-containing amorphous silica-alumina (M) macroporous white catalyst. The weight (mg) of the bead tested is denoted by the numerals adjacent to the respective symbol. Dashed line represents intrinsic ratios from carbon combustion research. From Weisz (1966).

Omegna, A., van Bokhoven, J.A., and Prins, R. (2003) Flexible aluminum coordinabon in alumino-sihcates. Structure of zeolite H-USY and amorphous silica-alumina. J. Phys. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

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 conclusion, A1 MAS-NMR and IR results show the formation of amorphous silica-alumina during dealumination. It shows stronger acidity than the hydroxyls attached to zeolite framework aluminums. This silica-alumina can account for the superacidity observed by some authors in steamed zeolite samples (12,13). 

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. 

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