Letter to the Editor Comments on ''Vanadium-and chromium-containing mesoporous MCM-41 molecular sieves with hierarchical structure'' [Micropor. Mesopor. Mater. 43 (2001) [227][228][229][230][231][232][233][234][235][236] Tailoring micro-and mesoporous materials by development of bimodal pore size distributions is currently being subject of many publications [1,2]. The presence of regular micropores (<2 nm) and larger mesopores (2-50 nm) in the catalyst leads to the combination of properties like shape selectivity and accessibility to restrict mass transfer limitations in the same material. This has important implications in different catalytic applications of these materials, e.g. in oxidation of light alcohols in the gas phase and large hydrocarbons in the liquid phase [2].
Development of extra porosity is generally achieved by application of different post-synthesis treatments on the as-synthesized material, including steaming at relatively high temperature, acid leaching, or alkaline treatments. The last method removes mainly Si from the zeolite framework, while the former ones lead to dealumination [3]. Pore size distributions (PSDs) derived from gas physisorption measurements can be used as a fingerprint of the resulting pore structure. Yuan et al. [4] recently reported the existence of a hierarchical structure in different mesoporous MCM-41 molecular sieves, consisting of a bimodal PSD centered at 2.5 and 3.9 nm. This was basically concluded from the N 2 adsorption measurements. The purely siliceous material was prepared using cetylpyridiniumbromide, ammonia, tetraethoxysilane and water in the molar ratio 1:18.2:2:300. After stirring and heating this mixture at 353K for 3 days, the resulting material was washed, dried, and finally calcined at 813 K for 5 h. Additionally, vanadium-and chromium-MCM-41 materials were synthesized by adding vanadylacetylacetonate and chromiumacetylacetonate to the synthesis gel, respectively.
In this paper, we would like to comment on the interpretation of the results obtained in the study of Yuan et al. [4] and mainly focus on the mesopore size distributions derived from the N 2 adsorption measurements. The most important conclusion from the work of Yuan et al. [4] is the presence of a bimodal PSD in all calcined MCM-41 materials (purely siliceous, Cr-and V-containing MCM-41), displaying uniform pores at 2.5 and 3.9 nm in the desorption PSD according to the BJH model. Although XRD and TEM were used to characterize the materials, no conclusive evidence for the assignment of the peak at 3.9 nm was provided to complement the results derived from the N 2 adsorption measurements. The two overlapping diffraction peaks in the XRD spectrum can hardly be distinguished due to the low signal-tonoise ratio. The TEM measurements do not show a regular pore arrangement at 3.9 nm, which should correspond to the narrow peak in the pore size distribution. The explanation given by the authors for the existence of this bimodal character was the simultaneous growing of two types of micelles in the same direction and in the same particles, but no detailed mechanism or further explanation were given. The