Surface acidity modifications induced by thermal treatments and acid leaching on microcrystalline H-BEA zeolite. A FTIR, XRD and MAS-NMR study

Abstract

Samples of beta-zeolite thermally treated at different temperatures and acid leached with diluted hydrochloric acid solution have been investigated by XRD, Al-27 MAS-NMR spectroscopy and FTIR spectroscopy. The results have been used to interpret the behaviour of the samples as catalysts for the acylation of 2-methoxy-naphthalene. It has been shown that the as prepared sample presents two types of extraframework species. These are identified as Al hydroxo-ions highly dispersed in the internal zeolite channels and Al oxide nanoparticles. Calcination causes dealumination of the framework and progressive conversion of the Al hydroxo-ions into Al oxide nanoparticles that reduce the zeolite channels practicability, modifying the shape selectivity effect. The Bronsted acid sites present in the sinusoidal channels (nu(OH)=3608 cm(-1)) can be distinguished from those located in the larger ones (nu(OH)=3620-3612 cm(-1)), due to the inability of the bulky probe molecule pivalonitrile to enter the former. However, internal terminal silanols (nu(OH)=3735 cm(-1)) also apparently display a significant Bronsted acidity, definitely higher than the acidity of those absorbing at 3747 cm(-1), thought to be located at the external crystal surface. The strongest Lewis acidity is displayed by the aluminum hydroxo-ions, while that of alumina nanoparticles is a little weaker. Acid-leached beta-zeolite also displays a medium strength Lewis acidity, likely due to framework Al cations. Thus a partial reinterpretation of the real structure of beta-zeolite and a partial reassignment of the bands due to the surface hydroxy groups are proposed

Ethylene production via catalytic dehydration of diluted bioethanol: A step towards an integrated biorefinery

“…broad absorption centred at 3670 cm -1 which could be assigned to another family of Al-OH groups, whose Al is at least partially connected to the BEA framework [46,50]. Isolated Al-OH groups characterized by a FT IR band at 3785 cm -1 and due to extraframework (EF) Al, were detected for sample 1, having the highest Al content.…”

“…FT IR analysis was carried out to investigate the acidic character of the prepared catalysts [46][47][48][49]. In the hydroxyl region of the spectra, recorded after outgassing at 500°C (spectra not reported) several common features appear for samples 1 and 8 (calcined samples):…”

“…2 Weakly H-bound pyridine was also detected, characterized by bands at 1597, 1580 and 1437 cm -1 [46].…”

“…Moreover, the relative intensities of bands typical of pyridinium ion and molecular coordinated pyridine (for instance bands at 1635 and 1620 cm -1 [54]) which is a rough indication of the acidic character of these zeolites, is slightly higher for sample 8 than , readily disappearing after outgassing. These are assigned to weakly bound pyridine, possibly H-bound to silanol, whose stretching peaks appear as negative bands in the subtraction spectra (not reported) [46]. Weak bands above 1600 cm -1 , and around 1450 cm -1 , still detectable after outgassing at increasing temperature are due to pyridine coordinated over Lewis acidic sites corresponding to residual Al.…”

Effect of Silica Alumina Ratio and Thermal Treatment of Beta Zeolites on the Adsorption of Toluene from Aqueous Solutions

“…The three ordered polytypes are designated frameworks A, B, and C [19,20]. Polytype A is tetragonal (space group P4 1 22 or P4 3 It has been reported that thermal and hydrothermal treatments induce chemical and structural modifications in beta zeolites, for instance Trombetta et al [21] observed that thermal treatments can cause dealumination and formation of extraframework aluminium species. The ease of dealumination of beta may be due to the presence of defect sites close to the framework aluminium which promotes bond hydrolysis, nonetheless the microporous structure is not affected by the loss of aluminium [22].…”