Silica-alumina framework

Thermal measurements such as DSC and DTA can be used to determine the crystal collapse temperature. The presence of the exothermic peak is associated with the lattice collapse. As shown in Figure 4.44 for a steamed and unsteamed faujasite, the thermal stability improves with increasing silica/alumina framework. 

Decationated zeolites. We start by considering decationated zeolites since they do not contain any metal ions extrinsic to the silica-alumina framework. This type of zeolite is obtained by pretreating, above 350°C, a NH4Y zeolite prepared by exchanging the sodium form of a Y-type zeolite with ammonium ions. Ammonia is evolved leaving a decationated or HY zeolite ... 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

Following calcination, both USY and AFS materials undergo framework changes but continue to show differences. The 29 spectra show an enhancement of the n-0 peak in both USY and AFS spectra but indicate that silicon distributions are different in AFS and USY materials. Silica-alumina framework ratios calculated from... 

The silica-alumina gel is not only made up of a silica gel network in which AIO 4 tetrahedrons are incorporated. Indeed it results on the mixture of a silica-alumina framework partly covered with alumina bonded to silica and free alumina clusters. 

A conventional FCC unit can be an olefin machine with proper operating conditions and hardware. Catalysts with a low unit cell size and a high silica/alumina ratio favor olefins. Additionally, the addition of ZSM-5, with its lower acid site density and very high framework silica-alumina ratio, converts gasoline into olefins. A high reactor temperature and elimination of the post-riser residence time will also produce more olefins. Mechanical modification of the FCC riser for millisecond cracking has shown potential for maximizing olefin yield. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

In conclusion, A1 MAS-NMR and IR results show the formation of amorphous silica-alumina during dealumination. It shows stronger acidity than the hydroxyls attached to zeolite framework aluminums. This silica-alumina can account for the superacidity observed by some authors in steamed zeolite samples (12,13). 

In order to study he Lewis acidity of the samples, the intensity of the 1450 cm pyridine band was also measured. Sample HYUS-8 shows a high amount of Lewis centers (Fig. 4d), relative thf HYD-400 sampl (Fig. 5c). This agrees with the absence of A1 as observed by A1 MAS-NMR for HYD samples. However, chemical analysis (Table I) indicates that there is more aluminum in this sample than in that from the unit cell constant m i urements. These differences cculd be explained considering that A1 MAS-NMR does not detect octahedral EFAL because of the low symmetry of its environment (i ). If this is so, it is remarkable that this EFAL does not show Lewis acidity as measured by pyridine ad y ption. On the other hand, if indeed thej is a small amount of A1, then the EFAL should be present as Al" outside the zeolite framework. In this case it should be present as amorphous silica-alumina. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

Faujasite is a rare aluminosilicate mineral. Its synthetic high-aluminia counterpart, Linde X, was synthesized by Milton (2), and Linde Y, a high-silica/alumina synthetic faujasite, was synthesized by Breck (3). The framework of faujasite-type zeolites has been well established by Bergerhoff et al. (4) and Broussard and Shoemaker ( 5). 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

Zeolites — are any of various natural or synthesized hydrous framework silicates, consisting of interlocked tetrahedrons of SiC>4 and AIO4, with a ratio (Si + Al)/0 equal to 0.5. Z. were firstly discovered and named in 1756, by the Swedish mineralogist Axel Fredrick Cronst-edt, in case of stilbite. Other natural z., as, e.g., analcime, heulandite, natrolite, and stilbite, are often formed near volcanic activity centers, have been formed from rocks by interaction with alkaline groundwaters. Synthetic z., produced since the 1930s, can be made by a slow crystallization process of a silica-alumina gel in the presence... 

XRD measurements show that calcined AFS and USY zeolites have comparable unit cell sizes. Upon steaming, the unit cell sizes for both AFS and USY reduce to identical values. Hence, framework silica-alumina ratios equilibrate to comparable levels independent of the method by which the zeolites were originally dealuminated. 

As-synthesized AFS zeolites do not contain extraframework aluminum as evidenced by Al NMR. As-synthesized USY zeolites contain appreciable amounts of extraframework material as seen by comparing framework and bulk silica-alumina ratios and by examining 27A1 spectra. Upon calcination both AFS and USY materials contain extraframework aluminum. The amount of extraframework aluminum in both AFS and USY materials increases on steaming. 

Acid sites are associated with framework A1 or other trivalent atoms. The number of the acid sites is proportional to the concentration of framework A1 or other trivalent atom. The strength of the acid sites in most zeolites is inversely proportional to the concentration of framework A1 up to about a silica/alumina ratio of 10. The nature of the heteroatom also affects acid strength. A1 zeolites are much more acidic than Ga- or Fe-zeolites. B-zeolites have very weak acidity. ALPO4-S have no exchangeable cations and therefore no acidity. 

Most industrial shape selective catalytic processes today use medium-pore zeolites from the "pentasil" femily. (The name refers to the five-membered rings in their framework and to their high silicon content.) ZSM-5 is by far the most important member of this family. It has high acid catalytic activity and it is very stable The silica/alumina ratio in ZSM-S varies from the teens to the thousands. High silica/alumina ratios give hydrophobidty, high acid strength, and thermal, hydrothermal, and acid stability. 

Easy chemical manipulation of all catalytic sites is an important and widely recognized zeolite characteristic. The near uniformity of the intracrystalline surface (in the absence of protic sites) provides an excellent opportunity to treat active sites uniformly. With protic zeolites, the presence of extra-framework alumina and silica-alumina phases and the need to optimize interaction between protic and Lewis acid sites make chemical manipulation complex, particularly with aluminum-rich zeolites. 

The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1] and the gel pores are the cavities existing between these particles [2], The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. 

The silica hydrogel network constitutes the framework of the silica-alumina gel. From a textural point of wiev, a silica-alumina xerogel seems to be an "image" of the starting silica hydrogel. Depending on the alumina content the silica-alumina gels exhibit predominantly a silica-like texture or an alumina-like texture. 

The framework silica/alumina ratio of several samples was determined by... 

The pH 10 data permit some generalization about the relationship between zeolite composition and stability in HH4NO3. In particular, structural stability decreases as either framework or chemical silica/alumina ratio increases, f hile mechanistic inferences (e gw intracrystalline buffering or "propping" by nonframiework alumina) cannot be drawn from these data, several points are... 

The temperature programmed reduction (TPR) of NiST proceeds in two steps with maxima at around 700 and 815 K (Fig. 2). The observed TPR peaks are indicative for the presence of Ni with different reactivity for hydrogen toward the zero-valent state. Since the non-crystalline silica-alumina phase gave a much poorly resolved TPR profile (Fig. 2), the peaks should be related to the crystal structure of NiST. The nickel cations placed inside the octahedral sheets being coordinated with sbc framework oxygen atoms should be more stable than Ni exposed at the edges of the clay platelets. Consequently, the first reduction step can most likely be attributed to the reduction of the N cations located at those positions. Since the reduction of NiO takes place at approximately 570 K, these data show... 

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