Quantification of Brønsted Acid Sites of Grafted Amorphous Silica–Alumina Compounds and their Turnover Frequency in m‐Xylene Isomerization

Abstract

The number and types of acid sites and the catalytic activity of amorphous silica–alumina, obtained by grafting silicon species to the surface of γ‐alumina, varies with the synthesis conditions and the amount of grafted silicon. IR spectroscopy of the OH region proves that deposition occurs selectively, first on the (1 0 0) and then on the (1 1 0) facets. Grafting onto the (1 0 0) surface lowers the number of active sites in ethanol dehydration. Grafting onto the (1 1 0) surface yields Brønsted acid sites that are active in the dehydration of ethanol and that catalyze the isomerization of m‐xylene. Strong Lewis acid sites, or “defect sites”, as detected by CO adsorption, also appear, although they are absent on the parent alumina. The stoichiometric dehydration of ethanol on Brønsted sites, monitored by using thermogravimetric analysis, enables the calculation of the turnover frequency of these sites in m‐xylene isomerization (1.4×10−3 s−1 site−1 at 350 °C, atmospheric pressure, 0.6 cm3 h−1 of m‐xylene, and 0.5 g of catalyst). This number is 22 times lower than on a USY zeolite without extraframework aluminum.

Behaviour of Amorphous Aluminosilicates towards Ageing by High‐Temperature Steaming

“…The amount of ethanol dehydrated (Table 2) is calculated on the basis of the surface area of each sample (column 5) and the percentage of surface aluminium species (column 6). Maximum amounts are obtained on pure alumina, which has LAS as the dehydration sites 5. Moreover, alumina has the highest percentage of ethanol dehydrated of the total ethanol remaining after purging with helium (column 7).…”

“…The evolution of the CO stretching region (2100–2250 cm −1 ) and the OH stretching region of Si/Al 2 O 3 materials after greater coverage of the surface with CO is illustrated in Figure 7 A and B, respectively. The CO region of alumina (Table 1, entry 1) is characterised by two main peaks at approximately 2175–2215 and 2160 cm −1 corresponding to CO adsorption on weak/medium LAS and weak OH groups, respectively 5. CO adsorption on these OH groups leads to negative bands in the OH region: These weak OH groups are mainly Al VI OH of the (1 0 0) surface (3770 cm −1 ) and Al V OH of the (1 1 0) surface (3730 cm −1 ).…”

“…CO adsorption on these OH groups leads to negative bands in the OH region: These weak OH groups are mainly Al VI OH of the (1 0 0) surface (3770 cm −1 ) and Al V OH of the (1 1 0) surface (3730 cm −1 ). The grafting of silicon species on SA(12/anh) (entry 3) causes the appearance of strong LAS (2230 cm −1 ) and a decrease in the number of weak/medium LAS 5. Silanol groups appear, as indicated by a new band at approximately 2135 cm −1 corresponding to CO physisorption on silanol groups.…”

“…Herein, we compare the effect of steaming on Si/Al 2 O 3 and Al/SiO 2 materials prepared by grafting. We select a Si/Al 2 O 3 sample with only partial coverage of the γ‐alumina facets, especially the (1 1 0) surface on which BAS are created by the deposition of silicon species 5. Si/Al 2 O 3 demonstrates better resistance to steaming than Al/SiO 2 : The porosity is marginally affected, but the rearrangement of surface silicon species leads to a different distribution of species on the facets of γ‐alumina, and part of the BAS disappear.…”

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

“…Indeed, the intimate chemical mixing of silica with alumina in ASA materials is empirically known to promote an enhanced surface Brønsted acidity with respect to the reference alumina or silica solids themselves. At the same time, this acidity is milder than the one observed in microporous aluminosilicates such as zeolites [8,9]. This milder acidity is targeted in industrial processes such as hydrocracking (HCK) in order to reach optimal cracking selectivity into middle distillate products [10,11].…”