Low-temperature Fourier-transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite

Abstract

~~ Nanosized ZSM5 zeolites with microcrystal dimensions in the 20-120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 addducts with silanols (both internal and external), Br~nsted-acid groups (both framework and extraframework), Na+ ions, and Lewis A13+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.Silicon-rich pentasil zeolites represent a series of mediumpore molecular sieves that have been extensively used as adsorbents and catalysts. ZSM5 (a representative member of the series) has a three-dimensional channel system where straight channels run along the [OlO] direction and have openings of 0.54 nm x 0.56 nm, defined by 10-membered rings of tetrahedra. Intersecting these channels, at right angles, there is a second type of sinusoidal channels running along [loo] direction with openings of 0.51 nm x 0.55 nm.'72 The basic features of this structure are independent of the Si : A1 ratio and are also shown by silicalite, the silica endmember of the ZSM5 compositional series.3P5The combination of high acidity, shape selectivity, thermal stability and low coke formation and ageing rate are the main merits of pentasil zeolites in catalytic ~s a g e .~,~-" Some of these features (e.g. the acidic behaviour and catalytic activity) rely on the nature and distribution of protonic and electronaccepting acid sites.The protonic structure of H-ZSM5 and related zeolites has been the subject of a number of studies using mainly IR spectroscopy,20-28 MAS NMR,29-32 adsorption ~a l o r i m e t r y ,~' .~~ temperature-programmed desorption and thermogravimetric a n a l y~i s .~*~~-~~ Two types of framework hydroxy groups showing IR stretching frequencies at 3720-3750 and 3600-3620 cm-' are generally observed and assigned to silanol groups and to bridged Si(0H)Al hydroxy groups, respectively. 2 3-2 8 . 3 7 -3 8 Th e latter are responsible for the highly acidic properties (Brarnsted sites) of the pentasil zeolites. In addition, a weaker absorption band is sometimes observed at ca. 3670 cm-' and tentatively assigned to O H groups located on extraframework aluminium species.26 However, in spite of the amount of work already done, the nature of the hydroxy groups in H-ZSM5 and the corres...

Direct dimethyl ether synthesis from syngas on copper–zeolite hybrid catalysts with a wide range of zeolite particle sizes

“…is also observed which can assigned to the bridged Si-OH-Al groups which correspond to the 12 zeolite Brönsted acid sites [59]. The intensity of the FTIR band at 3620 cm -1 assigned to the 13 bridged hydroxyl groups increases with the increase in zeolite crystallite sizes from ZSM-14 5@65 to ZSM-5@110.…”

Utilizing full-exchange capacity of zeolites by alkaline leaching: Preparation of Fe-ZSM5 and application in N2O decomposition

“…As shown in Fig. 5, the following events occur: disappearance of the adsorption band at 3610 cm −1 , characteristic of Brønsted acid sites [49] on alkaline treatment due to the full exchange with Na + (B); full recovery of the band on ion exchange with NH 4 NO 3 and subsequent calcination (C); and final disappearance of the band on Fe III exchange (D). This proves that indeed Fe is responsible for the disappearance of the absorption band at 3610 cm −1 .…”

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

“…They are assigned respectively to Brønsted acid groups (associated with framework aluminum (Si(OH)Al)) and to terminal silanol groups (located inside the zeolite channels and on the external surface) [46][47][48], respectively. A small shoulder detected at 3650 cm −1 is attributed to hydroxyl groups of extra-framework aluminum species [49]. Following FeCl 3 CVD and washing (Fe/ZSM5-17-H 2 O, Fig.…”