Amorphous aluminosilicates

In principle these compounds offer access to materials with AliCh-SiCL and Al203 2Si02 stoichiometries. The latter stoichiometry is equivalent to the Al[OSi(OBu-t)3 (OBu-t)] precursor. The major drawbacks with these materials are their air and moisture sensitivity, and the cost of the starting materials. Although the idealized stoichiometries of the above ceramics products are not those of crystalline aluminosilicates, amorphous aluminosilicate glasses are often important in optical applications or in scratch-resistant coatings. Furthermore, they may offer potential for CVD-type applications. There still remains considerable need for simple precursors to crystalline aluminosilicates, especially for structural applications. Dense, phase pure crystalline ceramic materials are desired for optimal mechanical properties, e.g. ceramic fibers for composite manufacture. 

Synthesis of zeolite ZSM-35 bulk material from aluminosilicate amorphous sinter... 

The Chemical Abstract Service has defined these materials under the CAS number 142844-00-6 as Refractories, fibers, aluminosilicates. Amorphous man-made fibers produced from melting, blowing or spinning of calcinated kaolin clay or a combination of alumina (AI2O3) and silica (SiOa). Oxides such as zirconia, ferric oxide, magnesium oxide, calcium oxide and alkalines may also be added. 

X-ray amorphous clays + H2Si04 + cations + HCOf -> cation-rich aluminosilicates + CO2 + H2O... 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

Netter, P., Steinmetz, J., Gillet, P., Kessler, M., Bardin, T., Fener, P., Burnel, D., Gaucher, A., Pourel, J. and Bann-warth, B. (1991). Amorphous aluminosilicates in synovial fluid in dialysis-associated arthropathy. Lancet 337, 554-555. 

Many different zeolite structures are already known, but there is permanent need for new or improved ones to satisfy novel and specific industrial and technological applications. To successfully accomplish this task, a deeper understanding of the zeolite crystallization process is certainly needed. One of the important parts in that study is the structural investigations of their amorphous aluminosilicate precursors (gels). 

Preparation of the composite materials with the FAU and BEA nanodomains has been reported previously [3,4]. The procedure was based on the impregnation of the parent materials with concentrated template solutions and subsequent recrystallization in the hydrothermal conditions. Amorphous aluminosilicates of chemical composition 13%Al20387%Si02 (13A187Si) and 6%Al20394%Si02 (6A194Si) were used as the parent materials for the FAU and BEA composites, respectively. 

The formation of the microporous phase dispersed in porous, amorphous matrices was followed by XRD and TEM. The XRD patterns of the BEA-composite prepared using the Al-poor aluminosilicate (6A187Si) are shown in Figure 1. BEA was the sole... 

Recrystallization procedure applied to the amorphous aluminosilicates of different chemical composition resulted in the formation of the dispersed zeolitic domains of the FAU and BEA structure in porous matrices. The structural transformation into the composite material was proved with TEM, XRD and 27Al and 29Si MAS NMR spectroscopies. The IR data revealed that strong Bronsted acid centers were main active sites generated in the composite materials, irrespectively of the Al content. 

Figure 4 shows TEM images of CNT synthesized by using aluminosilicates. The results show that A1 incorporated in mesoporous silica reduces considerably the quantity of amorphous carbon, increasing the catalyst selectivity. The Fe/Al-MCM41 (10) shows the MWCNTs with the highest purity (98%), an average diameter of 40 nm and the lowest quantity of amorphous carbon. 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

Catalytic cracking is a process that is currently performed exclusively over fluidized catalyst beds. The fluid catalytic cracking (FCC) process was introduced in 1942 and at that time replaced the conventional moving bed processes. These early processes were based on acid-treated clays as acidic catalysts. The replacement of the amorphous aluminosilicate catalysts by Faujasite-type zeolites in the early-1960s is regarded as a major improvement in FCC performance. The new acidic catalysts had a remarkable activity and produced substantially higher yields than the old ones. 

The EDX spectrum (Fig. 11.8) shows the main surface scale impurity peaks of silica, aluminium, sodium, chloride and iron. If this EDX is compared to that of a new, clean membrane surface (Fig. 11.9), the clean surface shows sulphur, carbon and oxygen, which is typical of a porous polysulphone support. It was concluded that the scale is amorphous, composed of aluminosilicate and silicate. These compounds are normally found in trace amounts in brine solutions. Analysis showed that the surface could be cleaned with hydrochloric acid and analysis of the dissolved scale was similar to the EDX spectrum analysis. Review of the plant operation determined that the precipitation was the result of high pH in combination with high silica concentrations in the brine. 

---