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Adsorption of Nitrogen, Neopentane, N-Hexane, Benzene and Methanol for the Evaluation of Pore Sizes in Silica Grades of MCM-41

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Adsorption of Nitrogen, Neopentane, N-Hexane, Benzene and Methanol for the Evaluation of Pore Sizes in Silica Grades of MCM-41 Microporous and Mesoporous Materials 47 >2001) 323±337 www.elsevier.com/locate/micromeso Adsorption of nitrogen, neopentane, n-hexane, benzene and methanol for the evaluation of pore sizes in silica grades of MCM-41 M.M.L. Ribeiro Carrott a,*, A.J.E. Candeias a, P.J.M. Carrott a, P.I. Ravikovitch b, A.V. Neimark b, A.D. Sequeira c a Department of Chemistry, University of Evora, Colegio Luõs Antonio Verney, Rua Roma~o Romalho 59, 7000-671 Evora, Portugal b Center for Modeling and Characterization of Nanoporous Materials, TRI/Princeton, 601 Prospect Avenue, Princeton, NJ 08542-0625, USA c Department of Physics, Nuclear and Technological Institute, 2686-953 Sacavem, Portugal Received 26 February 2001; received in revised form 7 June 2001; accepted 11 June 2001 Abstract Nitrogen, neopentane, n-hexane, benzene and methanol adsorption isotherms were determined on ®ve samples of silica grade MCM-41 with dierent pore sizes and a comparison of dierent methods for evaluating the pore size was carried out. With nitrogen we found a remarkably good agreement between the results obtained from the non-local density functional theory and geometric methods, with corresponding values obtained by the two methods diering by less than 0.05 nm. On the other hand, the results con®rm ®ndings seen before by other workers that, in order to obtain reliable values of pore radii by the hydraulic method, it is necessary to use a cross-sectional area of nitrogen in the monolayer smaller than the normally assumed value of 0.162 nm2. In addition, it was found that the eective pore volume obtained with the four organic adsorptives was almost constant while the value obtained from nitrogen ad- sorption data was always higher. The results indicate that, for the nitrogen data, an average density of 0.84 g cmÀ3 can be used to calculate the pore volume and that in this case the surface area should be calculated by considering a value of 0.131 nm2 for the cross-sectional area. In order to calculate hydraulic pore radii, either this pair of values can be used or if, on the other hand, the usual value of 0.808 g cmÀ3 is considered for the nitrogen density, the value of 0.137 nm2 must be used for the cross-sectional area. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: MCM-41; Nitrogen adsorption; Organic adsorptives; Pore size; Density functional theory 1. Introduction candidates for fundamental studies aimed at testing standard adsorption isotherm analysis The well de®ned pore structure presented by methods >e.g. Brunauer±Emmett±Teller >BET), as MCM-41 materials [1,2] has made them unique and Barrett±Joyner±Halenda >BJH) for pore size distributions) for the characterization of porous solids [3±10] as well as for the veri®cation of recent theoretical models, such as the non-local density * Corresponding author. Fax: +351-66-744971. E-mail address: [email protected] >M.M.L. Ribeiro functional theory >NLDFT) and molecular simu- Carrott). lations, for predicting phase equilibria in con®ned 1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S1387-1811>01)00394-8 324 M.M.L. Ribeiro Carrott et al. / Microporous and Mesoporous Materials 47 72001) 323±337 geometries, adsorption in pores and pore size dis- on the model of the ideal pore structure [5]. How- tributions [3,4,11±19]. ever, it should be noted that the geometric method Several studies have been published [3±10, involves a number of assumptions and can only 12,20±22] in which dierent methods for obtain- give a rigorous estimate for the mean pore radius if ing, in particular, the pore size of these materials, the structure is highly regular >absence of amor- have been compared. These include, on the one phous material) and if the experimental parameters hand, methods based only on the results of low- used, namely, unit cell parameters and, especially, temperature nitrogen adsorption isotherms. For pore volume and silica density, are correct. In instance, methods for calculating pore size distri- many cases, there are doubts about the validity of butions based on the Kelvin equation >such as the assumptions involved and the reliability of the BJH or Broekho-de Boer) [3,5,6,8±10,21,22], experimental parameters. With this in mind, it ap- methods that make use of the concept of hydraulic pears that NLDFT calculations, which are the most pore radius >2V =A, obtained by applying BET or independent and theoretically soundest, should be as) [5,6,9,21±24] and the more recent methods taken as the reference for purposes of comparison. based on NLDFT [3,4,17±19,25,26], have all been Most, by far, of the published characterization used. On the other hand, by assuming an ideal work has been based on nitrogen adsorption data. structure consisting of a hexagonal array of tu- However, it has been found by us and others that bular uniform-sized channels separated by thin adsorptives such as neopentane [24], argon [3,4,26, amorphous walls, it has been possible to estimate a 37±39], carbon dioxide [38,40], sulphur dioxide [38, value of pore size based on geometrical consider- 40], alcohols [38,41], water [24,38,41±43] and benzene ations [5,6,8,9,21,27] and using the pore volume [43] can give lower pore volumes than nitrogen, obtained from nitrogen adsorption, the unit cell when expressed using the normal liquid densities parameter obtained by X-ray diraction >XRD) for the adsorptives. With the particular exception and assuming a constant and normal value for the of water, which alters the pore structure [24,42,44± density of the amorphous silica walls. 46], the results obtained with the other adsorptives The studies so far published show that the four suggest that the mean density of adsorbate in the types of calculations referred to above give dier- pores may be dierent to that of the normal bulk ent values for the pore size. It is now well estab- liquid. However, relatively few studies and using lished that the traditional methods based on the only a limited number of samples have so far been Kelvin equation [28,29] using a standard t-curve published and there is still a need for more detailed [30] considerably underestimate the pore size [31]. work. With this in mind, we present in this paper a For example, the BJH method is in error even in systematic study of the adsorption of several ad- pores as large as 20 nm [25,26]. Even re®ned ver- sorptives in a series of samples with dierent pore sions that take into account the in¯uence of ad- sizes. Analysis of the results indicates that the ap- sorption forces on the curvature of the meniscus parent density of the nitrogen in the pores is higher [8±10,32±35] or make calibrations and empirical than that of the normal liquid. In addition, the results adjustments for the statistical thickness of the ®lm show that in order to calculate the surface area of adsorbed [6] have limited application in the nano- the pore walls, which is needed in order to calculate pore region due to the breakdown of the Kelvin the hydraulic pore width, a value for the cross- equation [30]. With regard to the hydraulic pore sectional area of the nitrogen molecule lower than width approach, it appears that this also gives the normal value of 0.162 nm2 must also be used. lower values than the NLDFT and geometric me- thods, and it has been suggested [6,9,10,36] that this is due to overestimation of the surface area of 2. Theoretical background the pores, whether estimated by the BET or by a comparison method >t or as). NLDFT, on the other NLDFT model provides an independent me- hand, appears to give values slightly higher than thod for evaluating the pore size in nanopores. The those predicted by geometrical considerations based theory predicts the capillary condensation/desorp- M.M.L. Ribeiro Carrott et al. / Microporous and Mesoporous Materials 47 72001) 323±337 325 tion isotherms in cylindrical pores based on the temperature synthesis based on that reported molecular model and given potentials of ¯uid±¯uid previously [47], using as surfactants, respectively, and solid±¯uid interactions. The pore size distri- C10TMABr, C12TMABr, C14TMABr, C16TMABr butions are calculated by numerical deconvolution and C18TMABr >where TMA trimethylammo- of the generalized adsorption isotherm equation, nium), and the molar ratios of reagents used were which represents the experimental isotherm as a 1 >tetraethoxysilane): 0.145 >surfactant):2.96 >NH3): weighted sum of the theoretical isotherms in pores 155>H2O). The surfactants and TEOS were sup- of dierent diameters. To calculate the isotherms plied by Fluka >purity > 98%, except for C18- in individual pores we used the well established TMABr which was >97%) and ammonia by Tarazona's version of the NLDFT, which was de- Riedel±de Haen >25%). All chemicals were used as scribed in details elsewhere [3,4,12,14±16]. Para- received. In all cases the synthesis products were meters of the ¯uid±¯uid potential were chosen to ®t recovered by ®ltration and then washed with de- the bulk equilibrium properties of N2, which in- ionized water, dried at 363 K and ®nally calcined clude the liquid±gas coexistence densities, satura- in air at 823 K >heating rate 5 K/min) for 5 h. tion pressure and the surface tension [12,14±16]. Nitrogen adsorption isotherms at 77 K were Parameters of the solid±¯uid interactions were determined on a CE Instruments Sorptomatic chosen to obtain the best possible description of the 1990, using helium and nitrogen of 99.999% purity standard adsorption isotherm on non-porous sili- supplied by Linde and Air Liquide, respectively.
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