Aluminosilicate structures, stabilization

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

During the first restructuration of the silicate or Si-rich aluminosilicate complexes prior to the zeolitic nucleation, the HM++ entities play a structure stabilizing role. By their particular linear shape, they favour the formation of channel systems. Simultaneously, they can also neutralize one AIO2- negative center of... 

As a conclusion, HM++ ions are not exclusive templates for a given structure but will stabilize an aluminosilicate structure that is preliminarily favoured by other variables such as the Al content in the gel. They will act partly as counterions to the negative framework, partly as pore fillers. Obviously, they exclusively act as pore fillers in the Al free gels, in which alkali cations can be present (this work) or absent (2.). ... 

The stabilizing effect of the aluminosilicate layer of DAY-Saim and DAY-T can be explained by the elimination of the terminal silanol groups and the blocking of the energy-rich Si-O-Si bonds at the crystal surface, where the water molecules attack the framework. In this case, the question of stability of high-silica faujasites is transferred to the question of stability of the aluminosilicate structures and, accordingly, transferred from the alkaline to the acid mechanism of decomposition which is rate-controlling. 

To illustrate the relationship between the two areas we will consider in detail the effect of coordinative unsaturation or underbonding at O in aluminosilicates on their structures, stabilities and properties, using examples from both inorganic chemistry and mineralogy. We use traditional methods of quantum chemistry in order to calculate the properties of such compounds. Similar relationships could be drawm between the bulk and surface structural properties of Fe (oxy)hydroxide) minerals and analog inorganic materials, but we will focus only upon the aluminosilicates, for which the structural and NMR data is most definitive. 

L. Kobera, R. Slavik, D. Kolousek, M. Urbanova, J. Kotek, J. Brus, Structural stability of aluminosilicate inorganic polymers influence of the preparation procedure, Ceramics-Sflikaty 55 (2011) 343—354. 

Pr4N+ cations were recognized to form complexes with silica-te (46) or aluminosilicate (23) species and subsequently to cause replication of the so formed framework structure via stereo-specific interactions (template effect). During this process, they are progressively incorporated and stabilized within the zeolite framework (1 1,47,48). Rollmann (37) has shown that initial Pr +/... 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

The above results show that post synthesis alumination of PSM with AlfNOjfi improves the hydrothermal stability of the resulting AMM material. Similar effect has been observed by Mokaya et al. [12], who reported that the hydrothermal stability of MCM-41 could be enhanced by reaction with chlorohydrate of aluminium. Moreover, from the study of high Si/Al ratio of Y zeolite, Lutz et al. [13] reported that the hydrothermal stability of Y zeolite was enhanced by an external introduction of non-structural aluminum species onto the surface of Y zeolite. The surface layer of Al-rich aluminosilicate or aluminum oxide was suggested to block the terminal OH groups and energy-rich =Si-0-Si= bonds on the surface of Y zeolite, hence minimizing the attack of water molecules on the framework. Due to these properties, the non-structural... 

Layered aluminosilicates catalyze chemical reactions in various ways. They stabilize high-energy intermediates, store energy in their lattice structures and catalyze redox reactions (ref. 1). They often exhibit high surface acidity (ref. 2). 

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

Analysis of vanadium-loaded model materials (such as EuY, amorphous aluminosilicate gels and EuY-gel mixtures) by electron paramagnetic resonance (EPR) has provided information concerning metal oxidation state and stereochemistry (67). EPR data has indicated that when vanadyl cations are introduced in the form of vanadyl naphthenate, they were stabilized in a zeolite with the faujasite structure as pseudo-octahedral V02+ even after calcination at 540°C. Upon steaming, these V02+ cations were then converted almost entirely to V+5 species (67). The formation of EuV04 was verified but the concentration of this vanadate was never proportional to the total rare-earth content of the zeolite. In EuY-gel mixtures the gel preferentially sorbed vanadium where it was stabilized mainly in the form of V205. 

The X-ray structure refinements and spectroscopic measurements described in 6.7 demonstrate that transition metal ions are ordered to varying degrees in the crystal structures of numerous ferromagnesian and aluminosilicate minerals. The enrichment of a transition metal ion relative to Mg2+ or Al3+ in a specific coordination site results from the interplay of several crystal chemical and bond energy factors, including the crystal field stabilization energy. 

Trends in CFSE data derived from the absorption spectral measurements described in chapter 5 may be used to predict or interpret element partitioning involving AT3 cations between coexisting aluminosilicate minerals. In many cases, however, the CFSE data represent average values for cations in several Al3+ sites in individual crystal structures. Nevertheless, the stabilization energies are applicable to interphase partitioning trends based on bulk chemical data for coexisting minerals. 

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