Aluminosilicates acid strength

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

The synthetic boron substituted mordenltes have alpha values similar to the aluminosilicate analogues. This Ls not surprising since boron replaces only 10% of the aluminum In these materials. For boron to affect the alpha values of these samples, the acid strength of a B-OH proton would have to be much greater than for a A1-0H proton, inihlch Ls clearly not the case. 

Indeed, NMR data indicates that the OH groups of amorphous aluminosilicates are primarily terminal whereas those of zeolites are primarily bridging, the interaction of O with Al weakening the OH bond and increasing the acid strength (11). 

Upon adsorption of benzophenone on oxides with strongly acidic properties, the phosphorescence spectrum exhibits a structureless band with a Atnax at about 490 nm in addition to the normal phosphorescence of benzophenone. The A max of the excitation spectrum of this band was observed at approximately 380 nm, and its intensity increased in the order of the aluminosilicate, H-mordenite, and HY zeolite. In the spectrum of HY zeolite containing benzophenone, only one structureless phosphorescence band could be observed. A similar phosphorescence band could be observed for benzophenone dissolved in CHCI3, which also involves dry HCl. We can therefore assign phosphorescence at about 490 nm to the protonated form ofbenzophenone. These findings correspond with studies of the photoluminescence of benzophenone dissolved in various concentrated acidic solutions (277). Consequently, since the presence of a phosphorescence spectrum at about 490 nm with benzophenone adsorbed on the aluminosilicate, H-mordenite, or HY zeolite is associated with the presence of the protonated form of benzophenone, the data indicate the existence of proton-donor centers on these oxides with acid strengths < for benzophenone (about 5.6) (216). On HY zeolite, almost all the adsorbed benzophenone changes into protonated benzophenone. On aluminosilicate surfaces, the relative intensities of the phosphorescence spectra attributed to the protonated and unprotonated forms are approximately the same. 

As described above for the amorphous aluminosilicates the acid strength of a zeolite increases with increasing Si Al ratios. Species with particularly high ratios have been prepared by removing some of the aluminum by reaction with materials such as silicon tetrachloride. On the other hand, the basic character of these materials increases with increasing numbers of A104 species so aluminum-rich zeolites are better bases, particularly when the Na" " is replaced with a large cation such as Cs. 52... 

Substitution of either A1 or Si with various heteroatoms changes acid strength from the extremely weak acidity of borosilicates to the superacid-like strength of certain aluminosilicates. The acid sites of Ga- and Fe-silicates are weaker than those of their Al-analogs [35]. Several shape selective commercial processes use hetwoatom substituted molecular sieve catalysts. Iron-substituted pentasils (Encilite) are used for xylene isomerization and for producing ethylbenzene fi om benzene and ethanol [36,37]. 

Indeed, crystallography provides a less complete information in the case of the mesoporous materials than in the ease of zeolites. At the nanometer scale, the mesoporous materials can present an ordered pore system with a defined space group. However, at the Angstrom scale, the walls between the mesopores are amorphous and diffraction methods are unable to define the position of each atom, as in the case of microporous zeolites. The organisation of the walls has important consequences on the properties of the materials. The inner surface of the pores corresponds to an interrupted framework and mesoporous aluminosilicates present a much smaller acid strength than aluminosilicate zeolites [66], This situation logically prompted studies on the incorporation of zeolite-like entities inside the walls of ordered mesoporous materials [67-70]. 

The protonic sites at the surface of amorphous silica are weakly acidic compared to the acid centers in crystalline aluminosilicates (e.g., zeolites). On ZSM-5, Bronsted sites are formed by protons adjacent to aluminum atoms in the tetrahedral framework. The concentration of acid sites increases with the aluminum content. The total acidity as well as the acid strength distribution can be determined by using n-butylamine and Hammet or arylmethanol indicators (25). Depending on the pKa of the indicator, a relative scale of the strength distribution is obtained (54). Results for a series of amorphous porous silicas of graduated pore size are shown in Figure 10. The acidity varies between pKa -1 and +9 and is nearly the same for all silicas studied. The results are specifically valid for this method only and cannot be compared with those derived from other methods. 

MCM-41 is an ordered mesoporous aluminosilicate of the M41-S family which can be synthesized with pore dimensions in the range 20-100 A and with Si/Al ratios between 10 and infinity [58,59]. It has been established that its acid strength is similar to that of amorphous silica-alumina and its acidity is always much weaker than that of zeolites [60]. 

As Shown in Figure 4, catalyst deactivation is an important factor in the hydroxy-alkylation reactions. This makes quantitative comparison difficult, as each material has a balance between activity and deactivation. The main conclusion from the results is that materials having a low acidity, ALPO-5 (vide supra) and 7-alumina, give best results, because of their low rate of deactivation. Already when SAPO-5, having an acid strength between ALPO-5 and aluminosilicates, is used, a rapid deactivation was observed in the phenol/formaldehyde reaction. 

Strong acidic and high temperature hydrothermally stable mesoporous aluminosilicates with well-ordered hexagonal structure (MAS-5) have been successfully synthesized from assembly of pre-formed aluminosilicate precursors with cetyltrimethylammonium bromide (CTAB) surfactant. The MAS-5 shows extraordinary stability both in boiling water (over 300 h) and in steam (800°C for 2 h). Temperature programmed desorption of NH3 shows that acidic strength of MAS-5 is much higher than that of MCM-41. 

Al-rich aluminosilicate Hydrophilic Polar High-cation exchange capacity Relatively low chemical, thermal and hydrothermal stability Catalytic activity Relatively low acid strength in H form Moderately high basic strength in cationic form (Cs+)... 

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