Silicon-enrich zeolites

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]. 

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 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". 

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. 

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... 

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],... 

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. 

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). 

Consistent with the thermal stability results, the silicon-enriched Y zeolite derivatives display greatly enhanced hydrothermal stability, relative to reference low-soda Y zeolite. By comparison, following steam treatment, the low-soda NH Y (content of Na20 = 0.36%) retains only 15% crystallinity, while the silicon-enriched product with Si02 Al203 of 11.7 retains 90% crystallinity. Furthermore, the silicon-enriched product with 6.4 Si02 Al203 ratio already retains 58% crystallinity [9]. 

In addition to the silicon enrichment of zeolites via postsynthesis chemistry, several recent reports by Anderson et al [10], Chang et a] [11,12], and Dessau et a [13] describe the aluminum... 

An important recent catalyst development is related to the nondestructive framework substitution of Si for A1 in the Y zeolite, described above. Using this recently disclosed mild aqueous chemistry, silicon-enriched forms of the Y zeolite structure (called LZ-210) have been synthesized, greatly improving the thermal and hydrothermal stability of the Y crystal. Cracking catalysts prepared from these silicon-rich zeolites show substantially increased cracking activity retention, following severe steaming pretreatments. In addition. 

The 1980s saw major developments in secondary synthesis and modification chemistry of zeolites. SUicon-enriched frameworks of over a dozen zeolites were described using methods of (i) thermochemical modification (prolonged steaming) with or without subsequent acid extraction, (ii) mild aqueous ammonium fluorosilicate chemistry, (iii) high-temperature treatment with silicon tetrachloride and (iv) low-temperature treatment with fluorine gas. Similarly, framework metal substitution using mild aqueous ammonium fluorometaUate chemistry was reported to incorporate iron, titanium, chromium and tin into zeolite frameworks by secondary synthesis techniques. 

The active surface of zeolites is an internal surface that cannot be observed via XPS. The study of these solids thus concerns changes to the external surface of the crystals caused by different treatments. Take the case of the stabilisation of the Y form via thermal treatment. On an as-synthesised product, with a global Si/Al ratio of 2.5, the surface is seen to be enriched in silicon (Si/Al p =4), After hydrothermal treatment, this effect is reversed (Si/Al ps corresponding to the formation of an amorphous alumina layer (visible in transmission electron microscopy) on the surface. Elimination of this extra framework mate rial by add attack can also be followed. 

Removal of framework silicon from Y zeolites (Si/Al = 2.7) upon leaching with alkahne solutions up to pH 12 at 80°C was already discussed in 1988 [230] and coimected with the observed increase of the unit cell size (framework Al/Si ratio). Later, Dessau et al. [231] reported that treatment of ZSM-5 with refluxing 0.5 M Na2C03 solution resulted in partial dissolution of the sample with preferential removal of sihca from the outer shell of the crystals and, hence, in aluminum zoning, with aluminum enriched at the exterior crystal surface. 

Important recent post-synthesis modifications of molecular sieves involve nondestructive changes in zeolite framework Si/Al ratios. Earlier it was reported by Beyer [7] that Y zeolite can be enriched in silicon by reaction with SiCl4 vapor ... 

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