Silicon enrichment

The reaction of zeolites with an aqueous fluorosilicate solution under relatively mild conditions has been shown to yield zeolites with silicon enriched frameworks which are essentially free of structural defects (1). As a result of the treatment, the framework topologies of the respective zeolites are relatively unchanged, but the zeolite compositions which are produced either do not occur naturally or are not synthesized directly. The fluorosi1icate treatment process has been termed "Secondary Synthesis". 

In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-ffamework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. 

High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/Al ratios. 7a1 MAS NMR appears to show the presence of aluminium species other than those teU a- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondary mesopores occurs. 

ECN [40] developed silicon-enriched SiAlON tape-cast ceramic substrates using a plasma-sprayed Si seeding layer. Closed layers of 1 cm2 have been achieved with grain sizes of 10-100 pm when using a melt of In with 1%A1 at 960° C. 

The HY sample (Linde LZY-82) had a bulk Si02/Al203 ratio of 5.4 and BET surface area of 761 m /g. After calcination at 540°C in air, the two silicon-enriched HY used (Linde LZ-210 type crystals) had BET surface area of 625 m /g and 629 m /g and bulk Si02/Al203 ratio of 10.1 and 13.2, respectively. 

Figure 5. Laser Raman spectra of steam aged crystals containing 5% V (A) HY (Linde s LZY-82), (B) Silicon-Enriched HY (Linde s LZ-210) with Si/Al = 6.1 and Na O = 0.18 wt%. ...

The effect of steam-aging on the surface area of V-loaded, silicon-enriched, HY crystals. 

Since the Si—O bond length (1.66 A) is shorter than that (1.75 A) of Al—O, the crystal lattice of zeolites is shrunken and the structure is stabilized after dealumination and silicon enrichment, as confirmed as follows. Of course, unavoidably there exist silicon-deficient hydroxyl nests after both dealumination and ultra-stabilization, and some mesopores will be generated in the USY framework. 

Chemical Dealumination and Silicon Enrichment of Zeolites Liquid Phase Dealumination and Silicon Enrichment... 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

The modern clearcutting of Tropical Forest ecosystems leads to soil erosion and destruction of silicon enriched horizons. The deeper and more ancient soil layers with Fe and A1 high contents, so called laterite and hardpan, become common for many hilly tropical and subtropical agricultural areas. 

The A1 centers can be replaced by trivalent atoms such as B, Fe, Cr, Sb, As, and Ga, and the Si centers by tetravalent atoms such as Ge, Ti, Zr and Hf. Silicon enrichment up to a pure Si02 pentasil zeolite (sUicaUte) is also possible [4],... 

Chem. Descrip. Silicone-enriched fatty acid-amine condensate Uses Fabric softener base for brushed or raised fabrics, elastomeric... 

Huss, A. and Schwartz, A.B., "Coke reduction in catalytic cracking using silicon-enriched ZSM 20", U.S. Patent 4,933,069 June 12 (1990). 

Y zeolite, the re-insertion mechanism involving the refilling of lattice vacancies was again confirmed. The samples dealuminated by exctraction with both EDTA and (NH4)2[SiFg] were foimd to be practically free of extra-framework aluminum. Nevertheless, a significant increase in the framework aluminum concentration was observed upon treatment of both samples with KOH solutions. Mainly based on the treatment effects on crystal morphology and concentration of silanol groups, this phenomenon was attributed to the dissolution of the outer silicon-enriched layer in the case of the EDTA-treated sample and to the removal of framework silicon in the zeolite dealuminated with (NH,USi ,l... 

Catalysis with silicon-enriched Y zeolites and with aluminophosphate-based molecular sieves provides new technological opportunities in catalytic cracking and in shape-selective catalysis, including conversion of methanol to small alkenes, oligomerization of alkenes, and synthesis of -xylene. 

The treatment with SiCl readily results in high Si/Al ratios, yielding products showing hydrophobic character. The silicon-enriched products display enhanced thermal stability, reflecting the higher stability of the Si-0 versus the Al-0 linkage. 

More recently, a new method was described for the nondestructive silicon enrichment of zeolites. According to Skeels and Breck [8], the replacement of zeolite framework A1 atoms by Si atoms can be achieved by the application of aqueous solutions of fluorosilicate reagents at mild reaction temperatures. A favored reaction scheme applied to Y zeolite is as follows ... 

The overall reaction seems to proceed through two consecutive steps. It is conceived that first framework A1 atoms are removed from the Y zeolite by the hydrolysed fluorosilicate solution. Subsequently, Si atoms are inserted in a slower reaction step. Rapid dealxjunination without silicon insertion may cause crystal collapse while the silicon-enriched products become more stable relative to the starting Y zeolite. 

Silicon enrichment of the Y zeolite results in major increase in thermal stability. At 60% replacement of the aluminum by silicon, changing the Si02 Al203 ratio from 5 to 14.8, causes the DTA crystal collapse temperature to increase from 860 to 1128°C (Table V). 

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