Silica-alumina molar ratio

The reaction time necessary for crystallization at a given reaction temperature can be controlled in a variety of ways, but the major way of controlling reaction time is by adjusting the water to sodium oxide molar ratio of the reaction mixture. The reaction time necessary to form these zeolites is directly proportional to the water to sodium oxide molar ratio used. For example, when the synthesis conditions indicate that the water to sodium oxide molar ratio for making zeolite A is between 15.1 and 20.1 for making a combination of zeolite X and zeolite A it should be between 25 1 and 35 1. Therefore, to synthesize zeolite A in a reaction mixture having a sodium oxide to silica molar ratio and a silica to alumina molar ratio where normally zeolite X would be formed, one would decrease the water to sodium oxide ratio. 

Adjusting the silica to alumina molar ratio of the reaction mixture also affects the reaction time necessary for crystallization at a given reaction temperature, but this effect is less than the effect of sodium oxide to silica molar ratio, which in turn is much less than the effect of water to sodium oxide molar ratio. 

The mordenite zeolites used in this study were purchased from both PQ Corporation (CBV-20A, silica/alumina molar ratio 20, Na20 content 0.02 wt%, surface area 550 m2/g, in ca. 1.5 mm extruded form) and from Union Carbide Corporation (LZM-8, silica/alumina molar ratio 17, Na20 content 0.02%, surface area 517 m2/g in powder form). All samples were calcined at 540 °C prior to use. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

Table II. Hydroisomerization of n-Pentane Influence of Silica-Alumina Molar Ratio of Activity of Pd-H-Mordenite Catalysts...

The reaction time necessary to form these zeolites is directly proportional to the silica to alumina molar ratio. 

For each family of silica-aluminas several synthesis parameters can be identified and applied to control the textural properties of final products. For example the role of the type and the amount of gelling agent (8,9), the solvent role (10), the silica/alumina molar ratio (9) have been discussed for MSA and ERS-8 formation. 

In particular in this work the contribution of TPAOH to the alkalinity of the reagent mixture during the preparation of silica-aluminas is discussed. Several molar ratios TPAOH/S102 (0.050, 0.075, 0.100, 0.125, 0.150, 0.175 and 0.200) have been considered and their effect on porosity and surface area has been examined. 

The synthesized materials were compared to a commercial silica-alumina gel, having 100 as Si02/Al203 molar ratio, delivered by Grace (Grade J639)... 

Typical catalysts of this type contain 60-80% of silica alumina, with the remainder being the hydrogenation component. The compositions of these catalysts are closely held secrets. Over the years, broad ranges of silica/alumina molar ratios have been used in various cracking applications, but silica is almost always in excess for high acidity and stability. A typical level might be 25% alumina (AI2O3). 

Pask [29] reports that mullites with higher molar ratios of alumina to silica (i.e., >3 1) have been prepared by homogenous melting of the constituents above the liquids and subsequent quenching. As a note, mullites prepared by fusion are generally weaker than those produced by sintering [33]. 

The conventional amorphous silica-alumina catalysts have been substituted here by zeolites, especially of the H-ZSM-5 type [49J. Higher yields and higher pyridinc/p-picoline ratios arc obtained with zeolite catalysis. The micropores will reduce the formation of higher alkylated pyridines. The zeolites can be further improved by incorporating metal oxides (e.g. Pb, Tl, Co) or noble metals or by applying both types of promoters. As an example, a Pb-MFI catalyst, operated at 450 °C in a fixed bed reactor and fed with CH2 O/CH3CHO/NH3 in a 1.0 2.0 4.0 molar ratio gave 79 % total pyridines with a pyridine/p-picoline ratio of 7.5. Also zeolites MCM-22 and Beta [50] perform well in combined pyridine/p-picoline synthesis. 

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

Reaction over Base Catalysts. - Masada et al. reported a detailed study on the reaction with methyl propionate over silica-supported various base catalysts. The reaction is conducted in the presence of methanol but in the absence of water vapor. HCHO free from water is obtained by the thermal decomposition of cyclohexanol-hemiformal which was previously prepared from formalin and cyclohexanol. The performances of catalysts are summarized in Table 8. The KOH catalysts are more active than the CsOH catalysts, although the selectivity to methyl methacrylate is lower. Incorporation of halides of alkali metal into the KOH catalyst improves the yield of methyl methacrylate, though the halides by themselves are inactive for the reaction. The best results are obtained with a KOH (1.5 percent) + Csl (0.5 percent) on silica catalyst. The single-pass yield of methyl methacrylate reaches about 59 mol% based on the charged HCHO with a methyl propionate/HCHO/ methanol molar ratio of 10/1/10. It is also found that the selectivity of methyl propionate to methyl methacrylate is very high, nearly 100 mol%. The best support is found to be silica gel. No catalytic activity is observed with the alumina-supported catalysts. 

Amorphous silica-aluminas with controlled pore size distribution in the micro and mesoporous region (i.e. ERS-8 and MSA respectively) have been synthesised by sol-gel route, changing the Si02/Al203 and tetrapropylammonium hydroxide/Si02 molar ratios and the solvent (e.g. methanol, ethanol and 1-propanol). By a proper control of such parameters it is possible to tailor the desired textural properties. 

Amorphous micro/mesoporous silica-alumina gel having surface area of 500 m g with a molar ratio of SiOj/AlaOj >30 1... 

Silica-alumina-nickel oxide gel Si02/ AI2O3 molar ratio of 30-500 1 and NiO/ Si02 molar ratio from 0.001-0.01 1... 

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