The classification of adsorption hysteresis loops recommended by the IUPAC in 1984 was based on experimental observations and the application of classical principles of pore filling (notably the use of the Kelvin equation for mesopore analysis). Recent molecular simulation and density functional (DFT) studies of the physisorption of gases by model pore structures have greatly improved our understanding of the mechanisms of hysteresis and it is therefore timely to revisit the IUPAC recommendations. In this review, we conclude that there is no immediate need to change the IUPAC classification of physisorption isotherms and hysteresis loops. However, in the light of recent advances, we are able to offer a revised checklist for the analysis of nitrogen isotherms on nanoporous solids: this includes a carefully regulated application of DFT in place of a classical procedure such as the well-known Barrett-Joyner-Halenda (BJH) method.

Molecular probes can be employed in three different ways for the characterization of nanoporous adsorbents. The simplest approach is to use a range of globular nonpolar molecules of different diameter in order to assess the effective size of the pore entrances in a molecular sieve. For this purpose, conventional gas adsorption (static or dynamic measurements) or gas chromatographic techniques can be used. The aim of the second approach is to characterize the nanopore structure with the aid of nitrogen adsorption at 77 K together with isotherm and adsorption energy measurements with a variety of adsorptives of different molecular size, shape and polarity. The third approach is to make use of polar probe molecules (notably water and alcohols) to investigate the surface chemistry and stability of the adsorbent.

We present a study of the use of amine-functionalized, mesoporous silicas as supports for nanoparticulate palladium, and the use of the composite materials as heterogeneous catalysts for the Suzuki coupling of aryl bromides. Upon modification of the silica, via attachment of N-functionalized aminopropylsilyl ethers to surface silanol groups, only a small reduction in surface area and average pore diameter is observed. The mesoporosity and high surface area are also maintained after introduction of nanoparticulate palladium, as evidenced by the measurement of BET nitrogen sorption isotherms. Electron microscopy shows that the palladium particles are well-dispersed and of typical diameter 3-6 nm. Catalysis was initially tested using the coupling of phenylboronic acid with 4-bromoanisole in the presence of K 2 CO 3 and with toluene as solvent. This revealed that the choice of organic modification has a crucial role in determining the activity and recyclability of the catalyst: optimum behavior was found for diamine-and triamine-containing systems, while quaternary alkylammonium salts showed poor activities. The optimized catalysts are also active in the coupling of a range of aryl bromides and phenylboronic acids, and after three catalytic runs they show virtually no drop in activity. Upon further cycling, however, and after six catalytic runs, we do observe a drop in activity, and this is accompanied by some leaching of palladium and pore-blocking by reaction products and byproducts.

No abstract

Empirical methods, which can be used for the analysis of physisorption isotherms, involve the input of adsorption data on reference adsorbents. In particular, the αS-plot method can be used for the characterisation of a porous material when the corresponding standard isotherm has been determined on a well-defined non-porous solid of similar surface structure. The interpretation of αS-plots is based on established principles underlying the mechanisms of physisorption: monolayer–multilayer adsorption, preferential filling of narrow micropores (ultramicropores), monolayer adsorption followed by the filling of wider micropores (supermicropores) and capillary condensation in mesopores. Various distinctive features of αS-plots allow these different processes to be identified and the internal and external areas and effective pore volumes to be quantified. The validity of BET-nitrogen areas can be checked and also, by using probe molecules of different sizes, one can obtain a semi-quantitative evaluation of the micropore-size distribution. The comparison plot is a simple way of comparing the shapes of two related isotherms. This approach has been found useful for investigating textural changes such as those accompanying the thermal decomposition of hydroxides or the activation of carbons.