Zeolite type molar ratio

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. 

The experiments were carried out in a small flow type fixed bed reactor which has been described in a recent publication (9) along with the methods of analysis by capillary gas-liquid chromatography. Results are reported that were gained with all pure n-alkanes ranging from n-hexane to n-dodecane. Feed hydrocarbons were delivered from Fluka, Buchs, Switzerland (purum). Purity exceeded 99. 5 wt. -% in any case. The Pt/Ca-Y-zeolite catalyst (0. 5 wt. -% Pt, SK 200, Union Carbide, Linde Division volume of catalyst bed 2 cm3 particle size 0. 2 - 0. 3 mm) was calcined in a dried stream of Ng and activated in a dried stream of at atmospheric pressure prior to use. The mass of dry catalyst was 1.0 g. The total pressure and molar ratio hydrogen n-alkane were kept constant at 39 bar and 17 1, respectively, whereas the reaction temperatures and space velocities were varied. 

This relationship between the water to sodium oxide molar ratio and the type of zeolite formed was not previously known. For example, U.S. patents issued to Milton (JL, 7), show a water to sodium oxide molar ratio of from 35 to 200 for the production of zeolite A and a water to sodium oxide molar ratio of from 35 to 60 for the production of zeolite X. If anything, this would imply that the reaction mixture for preparing zeolite A should have a higher water to sodium oxide molar ratio than the reaction mixture for preparing zeolite X. In U.S. patent 3,119,659, the water to sodium oxide molar ratio for the production of zeolite A is from 30 to 60. None of the above examples show that the water to sodium oxide molar ratio should be higher for making zeolite X than for making zeolite A. 

Sodium, potassium LSX (NaKLSX) was synthesized following the procedure reported by Kuhl [8]. On the basis of Kuhl s work, the synthesis conditions such as the ageing temperature, reactants composition etc. were optimized further so that it was easy to repeatedly prepare NaKLSX with crystallinity near 100% in our lab [9]. The product was characterize by XRD and elemental analysis. XRD pattern indicated that the synthesized NaKLSX was pure ujasite-type zeolite without detectable crystalline impurities or amorphous materials. Elemental analysis showed that the synthesized NaKLSX had Si/Al molar ratio close to 1.0. 

Zeolite Type Unit Cell Composition Si/AI Molar Ratio Unit Cell Volume (nm ) Void Volume " (cm /cm ) Pore Openings (A) Supercage Diameter (A) Kinetic Diameter (nm) tc ( C)... 

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. 

One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralizing role of water, which is greatly cissisted by the free OH concentration in the solution / hydrogel. Apart from this basic requirement of mineralizability, other factors like. Si / A1 molar ratio, pH of the gel, aging at lower temperature, crystallization temperature and time etc., influence the type and quality of the crystalline material in rather specific ways [1]. It is clear that the enhancement of the crystallisation rate is not much dependent of the choice of counter cation (H, Na or K) of a particular oxyanion promoter, at least for high silica... 

Tvaruzkova et al. used zeolite type HLZ-132 with a Si/Al molar ratio of 33 as a catalyst for methanol transformation. This zeolite is topologically related to levynite with an effective pore size of 0.43 nm. The experiments were carried out in the range of 350 to 500 °C and with a feed containing methanol 17%, water 68% and nitrogen 15%. The total pressure was 10 kPa. At 400 C and after 1 h on stream, the product distribution was as follows C2 hydrocarbons 51 wt% (with less than 1% of ethane), propane 3.5 wt%, propylene... 

Adsorption of dibenzothiophene (DBT) over FAU zeolites exchanged with alkali cations has been studied. Cristallinity (by XRD and IR), exchange level (XRF) and basic properties (CO2 TPD) of different adsorbents used have been determined. The influence of Si/Al molar ratio and type of cation exchanged in the zeolite as well as the presence of toluene in feedstock mixture on DBT adsorption capacity and selectivity of adsorbent has been also determined. Thermogravimetric analysis showed a stronger DBT adsorption over X zeolites. 

ZSM-5 type medium pore zeolites, with a Si02/Al20j molar ratio of 70 and extruded with 35 % alumin (1) In distillate mode (fixed bed, WHSV 0.5-1,4-10 MPa, 190-310 °C (2) in gasoline mode (0.4-3 MPa, 285-385 °C) (1) 80 % of the product was distillate fuels Cl 1-C20 with a cetane number after hydrogenation of over 50 (2) C5+ olefinic gasoline with octane no. 92 65 g P... 

Low-silica zeolites such as sodium type A, having a molar ratio of Si Al near unity, contain the maximum number of cation exchange sites that balance the aluminum in the structure and thus have the highest possible cation exchange capacities [104,105,120]. Intermediate-silica zeolites, for example, of the faujasite type, have ratios of 2-5 and high silica zeolites, for example, ZSM types, have ratios of 10-50, respectively [104]. 

Another type of the encapsulated catalyst with the H-P zeolite shell over Co/Al Oj pellets was described in [106]. The core-shell structure without defects was produced by the hydrothermal synthesis. The molar ratio of C /C in the products increased by 64% for the encapsulated catalyst compared with the physically mixed components... 

Si/Al molar ratio in the activated fly ash, zeolites can be classified/graded as low silica zeolites , intermediate silica zeolites and high silica zeolites , as listed in Table 2.2. In general, for zeolites, an increase in this parameter (i.e., Si/Al from 0.5 to infinity) [5] can significantly result in the increase in various other parameters (viz., acid resistivity, thermal stability and hydrophobicity) except few parameters (viz., hydrophilicity, acid site density and cation concentration) which get decreased [5, 8, 10,40, 41]. In general, synthetic zeolites hold some key advantages over their counterparts i.e. natural zeolites. Zeolites type A, X, Y, P and Na-Pl are well known synthetic zeolites synthesized from fly ash which have a wider range of industrial applications than the natural zeolites [1, 8, 20, 22, 36, 42, 43]. 

The Si/Al ratio of the solution can only be influenced by the synthesis conditions (viz., temperature, liquid to solid (L/S) ratio, molarity of the alkali, types of the alkali and fly ash) employed, which can significantly affect the chemical and mineralogical compositions of the synthesized zeolites [3, 5, 13, 22-24]. A summary of the detailed study of the available literature is presented in Table 4.2 to exhibit the effect of variation in L/S ratio, temperature, activation time and molarity of the alkali solution on the final yield (i.e., in terms of quantity of the product as weight % of fly ash) of zeolitization of the fly ash. 

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