Silico-aluminates

Many complex silico-aluminates or aluminosilicates are formed in nature. Of these, clay in more or less pure form (pure clay, kaolinite kaolin, china clay. HrjSiiA Oc) or AI2O3 2SiC>2 2H2O) is of great importance. Clay is formed by the weathering of igneous rocks, and is used in the manufacture of bricks, pottery, porcelain, Portland cement. Sodium aluminosilicate is used in water purification to remove dissolved calcium compounds. 

Table 2.4 Comparison of product composition obtained during isobutane/2-butene alkylation on H2S04 and a solid silico-aluminate catalyst...

For a naphtha feedstock,. the catalyst is based on nickel deposited on calcium or magnesium silico-aluminate incorporating potasskun, or nkkel on an alumina support with uranium promoter. Nickel on calcium aluminate is generally used for post-combustion. 

Encompassing the mantle lies the outermost—and thinnest—layer called the crust. It is only about 8 km beneath the oceans (oceanic crust) and an average of about 40 km under the continents (continental crust). Its mass is estimated at 2.4 x 1022 kg, and is constituted primarily of (oxygenated) compounds of Si (approximately 60%) and A1 (approximately 15%) silica, quartz, silicates, silico-aluminates, and others (e.g., metal oxides). As can... 

The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects the change of the pH and of the relative amount of TPAOH to silico-aluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of OH" availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. 

At high TPAOH content both the high availability of OH and the formation mechanism do not allow formation of large, ramified silico-aluminate oligomers favoring the growth in sheets, i.e. ERS-8 type silica-aluminas. 

In the sol-gel synthesis of silica-aluminas the TPAOH amount affects the textural properties of the final material modifying the pH and the relative amount of TPAOH with respect to silico-aluminate oligomers. 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

As alternative matrix additives for FCC catalysts, several silico-alinninates with controlled structural properties were prepared and characterized. These silico-aluminates have the potential to crack high molecular weight hydrocarbons without a loss of selectivity. In this... 

Table 2 shows the results obtained from the base material and kaolinite-derived silico-aluminates having promise in the formulation of FCC matrices for primary cracking of heavy hydrocarbons. The starting material was a kaolinite with low porosity, low area and, consequently, low heavy hydrocarbon cracking capability. As a result of the treatments to which kaolinite was subjected, its textural properties were substantially modified, increasing surface area up to ten-fold and pore volume up to four-fold. 

Table 2. Controlled Pore Distribution of experimental silico-aluminates...

Figure 2 shows pore-size distributiona of silico-aluminates having promise for FCC matrix application. For comparison purposes, the initial kaolinite is included. Modification of the kaolinite increases porosity, and, consequently, pore volume and surface area are increased. A bi-modal pore distribution is created, generating large fractions of pores in the 20 - 70 A and... 

Hydrothermal stability is a critically important property of the constituents of FCC catalysts accordingly prototype and reference materials were subjected to a hydrothermal deactivation treatment at a temperature of 788°C, 4 and 8 hours and 100% steam. Results are summarized in Table 3. Synthetic silico-aluminate (MX-0994) and alumina/kaolin (MM-0894) base matrices, which are used in the preparation of commercialFCC catalysts designed to crack heavy feedstocks, were used as references. 

Data in Table 3 show that after high temperature treatment the alumina/kaolin base matrix retains, 32 % of its original surface area, while the silico-aluminate of reference retains 58 %. The experimental prototypes treated under the same conditions, in addition to having a considerably greater area, after being deactivated, appear to be more stable, since they retain 78% on the average of their original surface area. 

An additional advantage is that hydrothermal treatment of experimental silico-aluminates does not modify substantially their pore distribution (Figures 2 and 3). Although the reference silico-aluminate, does not contain macropores, it retains its mesopores, while the alumina-based matrix, undergoes area reduction accompanied by a loss of porosity. 

The final test that determines if experimental silico-aluminates can be applied in the formulation of FCC catalysts is the evaluation of their catalytic behavior. Materials deactivated for 4 and 8 hours at 788°C in 100% steam were evaluated in a microreactor at 520°C, with a cat/oil ratio of 5, WHSV of 15.82 h using a modified gas oil feedstock with 5 wt. % of atmospheric residue. The material of reference in this study was the kaolinite with no heat nor chemical treatments coded as Filsttq, which typically is used in the formulation of FCC catalysts. Two experimental silico-aluminates, CARB-11 and Filttq, which are the result of thermal and chemical treatments of the Filsttq kaolinite, were evaluated. 

Figure 3. Pore-size distributions of steamed experimental silico-aluminates.

Modified silico-aluminate materials were investigated as candidates for inclusion in mat-ricies of FCC catalysts for cracking heavy hydrocarbons. Through thermal and chemical treatments of kaolinite is possible to gain at least 40 conversion units with respect to the base material, as well as a reduction of 75 wt.% of heavy cycle oil and a 500 % increase in high-olefinic LPG yield. 

Due their enhanced textural properties, hydrothermal-stability, and catalytic properties, the experimental silico-aluminates are promising alternative candidates for application as matrix additives to FCC catalysts for the efficient upgrading of bottoms into higher quality gasoline and LPG with high olefinicity. 

The specific surface areas vary between 0.6 and 11.7 m /g. The main impurities of the AI2O3 samples are CaO, Si02, Na20 and MgO. The eventual combination of these elements on the alumina surface is not known. The values of 7, determined at 100°C are in the 90-100 mj/m range and there is no evident correlation between 7 whatever the impurities contents. However, it is known, particularly in catalysis, that the association of AI2O3 and Si02 leads to silico-aluminates of increased Lewis acidity. To check this, we evaluated an acid base parameter, which is simply the ratio of the retention volumes of a basic probe, i.e. diethyl ether, and of an acidic probe, i.e. dichloroethane ... 

Aluminophosphate zeolites with interesting molecular sieve properties have also been synthesized (69) but the pentavalency and trivalency of P and Al respectively do not result in a supplementary negative charge as observed in silico-aluminate zeolite, and subsequently no acidity is expected. However, alumino-silico-phosphate zeolites with variable Al and P contents may well be of interest in the future. 

The mimic is prepared by sequential ion-exchanges with iron(ll) and Pd(ll) tetrammine cations followed by calcinations and reduction of the Pd(ll) to Pd(0) as previously described(14). A material with 2wt% Fe(ll) and 1wt% Pd(0) is used by immersing the dry zeolite solid in neat substrate alkane and then pressuring the reaction vessel with a 3 1 mixture of oxygemhydrogen. After shaking this mixture at room temperature for 4 hours the products are analyzed by capillary GC. As a control to assess the intrinsic selectivity of such a Pd/Fe system in the absence of steric effects of the zeolite, catalysts prepared with amorphous silico-aluminate supports were run for comparison. In these cases all reactions must take place at the particle surface since there is no interior pore structure available. In addition, comparison of reaction selectivities of this catalysts with our zeolite materials allows us to ascertain that the Fe active sites must be actually inside (and not on the exterior surface) of the zeolite crystallites. 

We gratefully acknowledge B. Morrow for helpful discussions and financial help from CNRS (Action de Recherche Coordonnee Hydroxyles Superfi-ciels des Silices, Alumines, et Silico-Aluminates ). 

Zeoiex . [JAl. Huber] Sodium silico-aluminate reinforcing filler, conditioner, anticaking agent absorbent to... 

These techniques operate in accordance with similar principles (Fig. 3.7). The preheated feed, normally heated by heat exchange with the reactor effluent, is introduced in the reactor in a downflow stream. Conversion takes place in the presence of acid catalysts such as phosphoric arid deposited on silica or silico-aluminates. placed in several fixed beds. This arrangement allows effective control of the temperature rise dne to the exothermic nature of the reaction, by the injection between the beds of.a quench liquid consisting of unconverted Ct. The operating conditions are moderate (temperatures average 120°C, limits 50 to 200 C, pressure 1 to 4.10s Pa absolute). The reactor effluent... 

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