No abstract

A systematic investigation of the IR and Raman spectra of pure and Al-, B-, Ti-and Fe-substituted silicalites in the SiO stretching region (1500-700 cm-') is presented. As well as t h e characteristic stretching modes of the skeleton, silicalites containing hydroxylated nests show also broad bands at ca. 960 cm-' (IR) and at ca. 976 cm-' (Raman), associated with 0,Si-OH group modes, with prevailing SSi-OH stretching character. The replacement of Si with heavier elements (like Ti or Fe) causes the appearance of new IR-and Raman-active modes (i) at 960 cm-' (IR and Raman) and at 1127 cm-' (Raman) in Ti silicalite; (ii) at 1015 cm-' (IR) and at 1020 cm-' (Raman) in Fe silicalite. Neither t h e Raman nor the IR spectra of the skeletal modes are substantially modified by t h e introduction of Al (ZSM5). The presence of boron induces t h e appearance in the IR spectra of a complex absorption at 1380 cm-' and at 960-930 cm-', which corresponds to the absorptions at 1417 and 976 cm-' in the Raman spectra. The assignment of the absorptions associated with heteroatoms and hydroxylated nests is discussed in detail.

~~ 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...

No abstract

The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through : (i) formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Brsnsted sites, (ii) a protonation step and (iii) a chain-growth step. The relative strength of the hydrogen bonds in the e t h e n M H and p r o p e n M H n-complexes (precursors) is estimated on the basis of the downward shift of both the v(0H) and v(C=C) frequencies (-389 and -11 cm-' for ethene and -539 and -19 cm-' for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear or low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework.