Zeolite silicon/aluminium ratio

Figure 2.12 Plot of the area per T-atom vertex SI) versus the average ring size, n, for a variety of zeolites, silica clathrasils and dense silicates. (All zeolites have a silicon aluminium ratio exceeding three, so that the approximate stoichiometry of all these frameworks is Si02). Zeolite and clathrasil frameworks are labelled by the code adopted by the International Zeolite Association [18]). The shaded domain indicates the window of geometrically accessible values of as a function of the ring size. Despite the allowed geometric variability, the value of D is close to 12.2A2 for all these "silicates", regardless of the ring size and consequent intrinsic curvature.

Another zeolite parameter which may have a strong effect on the conversion, is the silicon to aluminium ratio. Some ZSM-5 samples with approximately the same crystal size, but different silicon to aluminium ratios were tested. It appeared, that with increasing silicon to aluminium ratio, the yield of aminopicoline decreased probably because of the decreasing number of active sites (Figure 3). 

The yield in the rearrangement of m-phenylenediamine is directly proportional to the number of active sites, which corresponds to the silicon to aluminium ratio of the zeolitic framework. The major part of the reaction, however, takes place on the outer surface of the zeolite. After passivating the active sites of the outer surface with a silylchloride, the conversion decreases, but the selectivity increases. 

Zeolites.- Zeolites with high silicon to aluminium ratio such as H-mordenite or H-ZSM-5 are sometimes considered as superacids. The reason for such classification is that the BreSnsted centres of the zeolites act in a similar way to protons in superacid solution. It is however, necessary to point out that such centres, in spite of certain similarity to superacid protons, are less active. n-Alkane reaction takes place in the presence of zeolites at temperatures above 523K. Hydroxyl groups interacting with aluminium polymeric compounds (AlO) are responsible for... 

The low activity of these zeolite catalysts is connected with their highly hydrophility as result of low silicon to aluminum ration. The deactivation by sorption of polar products and solvent on pores of zeolite still remained a serious issue for oxidation of alkanes (with low polarity). Even dealumination of the structure up to silicon to aluminium ratio above 100, increased the activity only twice [48], The creation of a hydrophobic environment around the active site was required to circumvent the activity and sorption problems. In the case of the reaction... 

Camblor MA, Corma A, Martinez A, Mocholf FA, Perez Pariente J. Catalytic cracking of gasoil Benefits in activity and selectivity of small Y zeolite crystallites stabilized by a higher silicon-to-aluminium ratio by synthesis. Appl Catal 1989 55 65-74. 

The assignment given is such (see Table III) that the peak at -88ppm is associated with silicon linked (within the tetrahedral manifold) via oxygen to 2 silicons and 1 aluminium whilst the peak at -93ppm is associated with silica linked exclusively to other silicons (25-27) Using an analysis based on Loewenstein s rule and widely used in zeolite struct jral analyses (28) we have shown elsewhere (20) that for this particular synthetic heidellite the (Si/Al). ratio is 11.5 ... 

Aluminium analysis in zeolites in order to establish the Si/Al ratio is conducted using atomic spectrometry when the detection sensitivity of XRF is insufficient, i.e. when A1 < 1%. The sample is converted to a solution via an HF-HNO3 attack, then dried and the dry extract recovered using a hydrochloric acid solution. The standard solutions contain the same level of silicon and have the same acidity as the sample solutions. Analysis is carried out using an internal standard. 

In a zeolite, where the framework comprises TO4 tetrahedra (T silicon or aluminium), the chemical shift of the silicon, compared to tetramethylsilane adopted as reference, varies with the number of aluminium atoms located in the second co-ordination sphere (cf. Fig. 13.6). From the NMR spectrum, it is possible to obtain the concentration of each of these configurations and, by applying Lowenstein s rule (no Al-O-AI links), calculate the atomic Si/Al ratio of the framework using the formula ... 

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. 

The synthetic conditions, compositions and properties of the starting and dealuminated zeolite samples are presented in Table 1. The method of dealumination of the NH4 -form of Y zeolite with a solution of (NH4)2Sip6 is based on isomorphous substitution of aluminium in the zeolite structure by silicon. As the eoncentration of (NH4)2Sip6 increases, the overall Si/Al ratio in zeolite (determined by chemical analysis) increases. The IR spectra of the modified samples on various steps of synthesis were used to determine the Si/Al ratio in the zeolite framework. 

Si Al ratio of the outer layers (ca. 10 A) of the zeolite crystals. The extent to which the surface composition differs from the bulk composition appears to depend on preparation conditions, and all three possible situations (silicon rich surface, silicon deficient surface and surface composition equal to bulk composition) have been reported (refs. 12-14). Variations in aluminium distribution have also been probed by high resolution scanning electron microscopy (ref. 15) and energy dispersive X-ray analysis (ref. 16). 

Alkaline treatment of Fe-Z15, its Si/Al ratio of 16 being outside the optimal range for mesopore formation as shown in Fig. 1, leads to a significantly lower Si extraction of 290 mg F as compared to the iron-free system (570 mg f ), while a similar (limited) mesoporosity (50 m g ) has been measured (Table 1). The slight increase of 10 m g" in mesopore surface area must be attributed to the relatively high framework aluminium concentration in Z15, which stabilizes the zeolite framework against desilication and suppresses silicon extraction [18]. Similarly to s-FeS-at, the presence of non-framework iron does not facilitate sihcon dissolution. 

Figure 3.20 The MASNMR spectra (A-J) of faujasitic zeolites X and Y with framework Si/Al ratios from 1.19 to 2.75. Deconvolutions of the spectra in terms of resonances due to silicon surrounded by 0 to 4 second nearest aluminium neighbours are given to the right of each spectrum. [Reproduced from reference 105 with permission. Copyright 1982 Royal Society of Chemistry.]...

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