Characterization and Acidic Properties of Aluminum-Exchanged Zeolites X and Y

Abstract

Na-Y with defined numbers of extraframework aluminum cations were prepared by exchange in an aqueous solution of aluminum nitrate. A maximum concentration of Brønsted acidic bridging OH groups in supercages (SiOH sup Al) was reached upon dehydration of zeolites Al,Na-X and Al,Na-Y at 423 K. Further raising of the dehydration temperature led to a dehydroxylation of zeolites due to the recombination of aluminum hydroxyl groups with hydroxyl protons of bridging OH groups. High-field 27 Al multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy was utilized to study zeolites Al,Na-X/61 and Al,Na-Y/63 dehydrated at 423 K. Second-order quadrupolar effect parameters of 10.1-11.0 MHz for tetrahedrally coordinated framework aluminum atoms, compensated in their negative charge by hydroxyl protons (Al IV /H + ) and aluminum cations (Al IV /Al x+ ), 3.6-4.4 MHz for tetrahedrally coordinated framework aluminum atoms compensated by sodium cations (Al IV /Na + ), and 5.6-7.6 MHz for pentacoordinated extraframework aluminum cations (Al x+ cat.) were obtained. Comparison of the number of AlOH groups with the number of pentacoordinated extraframework aluminum cations determined by one-dimensional highfield 27 Al MAS NMR spectroscopy gave a ratio near 1:1. This finding and the five-fold coordination of the cationic extraframework aluminum species hint to the presence of HO-Al + -O-Al + -OH compounds, but also a minor number of Al(OH) 2 + and AlO + species could exist. The enhanced acid strength of bridging OH groups in zeolites Al,Na-X and Al,Na-Y in comparison with zeolites H,Na-X and H,Na-Y, as found by adsorption of acetonitrile, may be due to a polarizing effect of cationic extraframework aluminum species in the vicinity of Brønsted acid sites.

Effect of Phosphate Modification on the Brønsted Acidity and Methanol‐to‐Olefin Conversion Activity of Zeolite ZSM‐5

“…With increasing impregnation degree, the occurrence and increase of the signal at -17 ppm hints at the formation of octahedrally coordinated aluminum species in aluminophosphate complexes [21,22]. The very broad signal at about 35 ppm indicates the presence of pentacoordinated aluminum species, which can be explained by aluminum species on extra-framework cation positions [23,24]. Both these types of signals increase with rising phosphate contents and decrease again upon washing with water (compare spectra of samples without and with 'W').…”

“…Upon rehydration, on the other hand, cationic extra-framework aluminum species can form Brønsted acid sites via the Hirschler-Plank mechanism, i.e. by dissociative adsorption of water [24].…”

Extraction of arabinoxylans from wheat bran using hydrothermal processes assisted by heterogeneous catalysts

“…This proton results in a Brønsted acid site, which is actually a strong acid site. Therefore, when the Al/Si ratio increases, the number of strong acid sites is higher if all the cations are replaced by protons (Huang et al, 2008;Kao, Chang, Liao, Lee, & Chien, 2008;Krithiga et al, 2005;Ribeiro Carrott et al, 2006). As mentioned before, the catalyst precursor (RuCl3) used for the synthesis may the responsible for the high acidity of the RuCl3-based catalysts and therefore, for their better performance (Guisnet et al, 1993).…”

General aspects of zeolite acidity characterization

“…Signals at even higher ppm (up to 16 ppm) have been observed, probably as a consequence of strong hydrogen bridging with nearby silanols [63,64]. Spectra of dealuminated zeolites show signals at 2.4e3.6 ppm originated by hydroxyl groups bound to extra-framework species created during the removal of structural aluminum [65]. This absorption at ppm higher than expected in hydroxyls found in isolated aluminum centers indicates the location of extra-framework species in narrow pores or small cavities, as supported by several researches [66e69].…”