The paper by A. Muiioz-Pbz and M. F. to be a warning in favor of a more rigorous interpretation of XANES spectra by carefully considering the detailed origin of the spectral features in the framework of multiple scattering theory. The authors conclude that, in general, results simply obtained without this analysis could be misleading, and we can of course only agree with this statement.We point out, however, that ab initio simulations on model compounds and the interpretation of experimental XANES spectra arising from Ti atoms absorbing in complex sites (eventually surrounded by adsorbates) are two different things. The complexity of the systems investigated in ref 2 is such that, unlike other authors and as stated clearly as the general philosophy of the paper, we have decided to stay on the more safe and cautious ground of the semiquantitative analysis of the data by considering only intensity and full width at halfmaximum of pre-edge peaks and without pretending to extract more information than actually possible. The correctness of ' And INFN this approach is also recognized by the authors of ref 1 in the last sentence of the first paragraph.With regard to the details of the contribution,' the spectra 1-4 reported in Figure 1A of ref 1 represent the simulation of the rutile XANES spectra performed on clusters of TiiOj of increasing size. This figure clearly shows the importance of long-range effects in the obtained XANES line shape, including the pre-edge features. This data, while interesting on its own account, does not subvert at all the fact that the intense and sharp pre-peak in the Ti XANES spectra is the fingerprint of Ti in tetrahedral symmetry and that a progressive erosion and simultaneous enlargement of the peak indicates (although only in a semiquantitative way) clearly a local change of coordination number and symmetry at the Ti centers. This fact is demonstrated by numerous papers on model compounds quoted in ref 2. Moreover, the high quality of the Ti-silicalite samples investigated in ref 2 (where the Ti/Si ratio is on the order of should guarantee the total incorporation of Ti atoms in the framework, preventing the presence of extraframework clusterized Ti species, as already documented in several by means of numerous and independent characterization techniques like XRD, UV-vis, EPR, IR, Raman, E M S , and XANES. Consequently, Ti atoms being isolated framework species, their second shell environment is formed by Si atoms only. TiiOj clusters are thus unlikely to be present in our samples (with the exception of the trivial TilO4).Finally, we point out that in our previous paper2 the main conclusions about coordination numbers and symmetry, both in the presence and in the absence of adsorbed molecules, were reached only after a detailed comparison with the extended part of the spectrum and with other spectroscopic techniques (IR, Raman, UV-vis, and EXAFS2s3). References and Notes(1) Muhoz-Piez, A.; Ruiz-L6pez, M. F.

~~ 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...

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.

No abstract

In paper 1 we have shown that the structural properties of silicalites depend upon the preparation procedure. In particular Naand Al-free sample (S) prepared following a specifically designed method is characterized by the presence of internal atomic-dimension defects (nanodefects and microcavities). On the basis of spectroscopic data and computer graphic simulations, it is shown that these microcavities derive from one or more missing [Si04] units and can contain up to four OH groups per missing tetrahedron. These OH groups form chains of hydrogen-bonded species which, upon water elimination at high temperature, give Si-O-Si distorted bridges characterized by peculiar vibrational features. It is also shown that internal and external OH groups have distinguishable IR manifestations. Terminal and isolated hydroxyl groups located in internal positions give 1:1 hydrogen-bonded adducts with CO at 77 K, which are associated with red and blue shifts of the OH and CO stretching frequencies, respectively. External OH groups can be perturbed only by dosing CO at more elevated pressures.