J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2821-2826 Adsorption on MCM-41 Mesoporous Molecular Sieves Part 1.-Nitrogen Isotherms and Parameters of the Porous Structure J. Rathousky* and A. Zukal J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 18223 Prague 8,Czech Republic 0.Franke and G. Schulz-Ekloff Institute of Applied and Physical Chemistry, University of Bremen, 28359 Bremen, Germany Nitrogen adsorption isotherms have been measured on a series of aluminosilicate and titanosilicate MCM-41 molecular sieves, whose mean pore radius varied from ca. 0.9 to ca. 2 nm. By means of comparison plots the nature of the adsorption on these materials was found to depend strongly on their pore size. With pores of radius of around 1 nm rnultilayer coverage of the pore walls occurs. If the radius is increased to 1.5-1.8 nm the mechanism of adsorption changes into a two-stage one, i.e.the multilayer coverage of the pore walls is suc-ceeded by the spontaneous filling of the pore volume by capillary condensation without hysteresis. In even greater pores the usual capillary condensation with hysteresis occurs. The estimation of pore structure parameters was based on standard methods of adsorption isotherm pro- cessing, including the calculation of the pore size distribution from the desorption branch of the hysteresis loop. With the smaller pore materials, where the Kelvin equation does not hold, the estimation was based on the cylindrical pore model. A detailed knowledge of the pore structure of MCM-41 materials was thus obtained.The synthesis of a new family of mesoporous molecular sieves using rodlike micelles of cationic surfactant molecules as tem- plates has been reported recently.'g2 These silicate and aluminosilicate materials were designated as MCM-41. Recent results on silicate MCM-41 have revealed that ran- domly oriented rod-like micelles interact with silicate species to yield two or three monolayers of silica encapsulating the external surface of the mi~elles.~.~ Subsequently, these com- posite species spontaneously assemble into the long-range ordered structure characteristic of MCM-41 and then the sili- cate species in the interstitial spaces of the ordered organic- inorganic phase continue to condense.After the removal of the organic part of these organic-inorganic composites uni- formly sized pores are left. This mechanism is most probably operative also in the formation of MCM-41 materials of other chemical composition. MCM-41 molecular sieves have been synthesized with a regular, hexagonal array of uniform channels of approx- imately hexagonal cross-section. If their shape is modelled by a cylinder, their radius varies from 0.5 nm to ca. 5 nm. The most regular arrangement and the best uniformity of pores are usually observed for the smaller pore size materials (r < 2 nm). Those with larger pores display somewhat irregular, yet essentially hexagonal pore arrangements and a slightly deformed pore shape.* MCM-41 molecular sieves are promising materials for both various applications and fundamental ~tudies.~,~ On these molecular sieves capillary condensation without hyster- esis in a system of uniform cylindrical pores was first observed. In two studies'** published so far on this adsorp- tion phenomenon only one sample was investigated with mean pore sizes of 1.46 nm ' and 1.66 nm,' respectively.Data presented in this study were obtained on a series of six samples whose mean pore radius varied from ca. 0.9 to ca. 2 nm. Nitrogen adsorption isotherms were measured because with this adsorbate the methods of analysis of adsorption data and of the determination of parameters of the porous structure of solids are satisfactorily verified.For the analysis of adsorption isotherms comparison were used, i.e. the amount adsorbed on the solid under investigation was plotted against that adsorbed on a reference non-porous adsorbent at the same equilibrium pres- sure. By this method the adsorption on both materials is compared directly without any normalizing factor. The parameters of the porous structure of MCM-41 materials were determined using standard methods and were then used for discussing the observed adsorption processes. Experimental Materials The sources of silica, aluminium and titanium were Ludox AS-40 (Du Pont, 40 wt.% colloidal silica in water), Al(OH), (J. T. Baker, 98%) and tetrabutyl orthotitanate (TBOT, Merck, p.a.). The quaternary ammonium surfactant com-pounds [C,H2,+ ,(CH,),NX, X = OH, C1, Br] were obtained from Fluka (25% water solutions).These compounds are designated as DDTMAX and HDTMAX for n = 12 and 16, respectively. Tetraethylammonium hydroxide solution (TEAOH, Merck, 20 wt.% solution in water), sodium hydrox- ide (Janssen, p.a.), propan-2-01 (Merck, p.a.), toluene (Merck, p.a.) and mesitylene (Merck, p.a.) were also used. All chemi- cals were used as received. Synthesis Procedures Because of the need to obtain materials with pore sizes varying from 1 to 2 nm differing in their chemical composi- tion (aluminosilicates, titanosilicate), three synthesis pro-cedures were adopted, uiz. the procedure I with samples AIMS-1 to -3, the procedure I1 with AIMS-5 and -6, while the titanosilicate TiMS-4 was prepared using procedure 111.All aluminosilicate sieves contained 3.1 mol% of A1,0,, the tita- nosilicate TiMS-4 contained 4.3 mol% of TiO, (determined by AAS). The structure of all sieves was also checked using X-ray diffraction (XRD) and transmission electron micros- copy (TEM). The characteristic feature of their diffractograms is that they exhibit from two to three reflections only at small angles 28 (< 6"). From the comparison with published data',, it follows that they can be indexed on a hexagonal lattice typical of MCM-41. The transmission electron micrograph in Fig. 1 Transmission electron micrograph of AIMS-2 Fig. 1 confirms that the typical example of aluminosilicate MCM-41, AIMS-2, contains an almost regular, hexagonal array of channels with a mean pore diameter of ca.3 nm. Procedure I The reaction mixture was prepared as follows: 0.31 g of Al(OH),, 0.3 g of sodium hydroxide and 1 g of deionized water were put into a 60 ml glass beaker and brought to the boil with stirring until a clear solution resulted. Then 9.26 g of TEAOH solution were added and the solution was cooled. In a separate 400 ml polypropylene beaker 9.26 g of Ludox AS-40 were stirred with a magnetic stirrer (at ca. 600 rpm). These two mixtures (sodium aluminate and the silica suspension) were combined at room temperature by adding the aluminate solution to the silica suspension. The gel was stirred for 5 min (to achieve good homogeneity, an agitation rate of up to 1000 rpm may be needed), mixed with an amount of the 25% aqueous solution of surfactant (see below) corresponding to the composition of the reaction mixture presented below (e.g.10.55 g with HDTMAOH), and stirred for another 5 rnin (500 rpm). Then the gel was reacted with stirring (150 rpm) in a 250 ml polypropylene autoclave at 104 "C for 24 h. The resulting solid product was recovered by filtration, washed with water, extracted with ethanol for 4 h in a Soxhlet apparatus and finally calcined in air at 600°C for 22 h. The reaction mixture of samples corresponded to an oxide molar ratio -of lA1,0, : 31.01Si02 : 2.2(surfactant),O : 3.16(TEA),O : 1.89Na20 : 615-802H20. The content of water depended on the molecular weight of the surfactant used.The following surfactants were used : with AlMS-1, DDTMABr ; with AIMS-2, HDTMAOH; with AIMS-3, HDTM ABr. Procedure I1 The reaction mixture was prepared as follows: 0.62 g of Al(OH),, 0.6 g of sodium hydroxide and 1.5 g of deionized water were put into a 250 ml glass beaker and brought to the boil. After a clear solution resulted, 18.52 g of TEAOH solu- tion were added and the solution was cooled. Then 22.49 g of the solution of HDTMACl were added. Aluminate, which precipitated at first, dissolved again on warming. In a separate 400 ml polypropylene beaker 18.52 g of Ludox AS-40 were agitated with a magnetic stirrer (at ca. 500 rpm). The two mixtures (sodium aluminate and the silica suspension) were combined at room temperature by adding the aluminate solution to the silica suspension.Immediately a gel formed which was stirred for 30 min (at 500 rpm). Then 2.1 g of toluene (with AIMS-5) or 3.2 g of mesitylene (with AIMS-6) was added and the mixture was homogenized for 1 rnin at an agitation rate of up to lo00 rpm. Later the gel was reacted with stirring (150 rpm) in a 250 ml polypropylene J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 autoclave at 104°C for 4 h. The resulting solid product was recovered by filtration and treated as in procedure I. The reaction mixture corresponded to an oxide molar ratio of 1A1,0, : 31.06Si02 :2.21(HDTMA),O : 3.16(TEA),O : 2.20(toluene), or 3.35(mesitylene), :1.89Na20:806H20. Procedure I I I The reaction mixture was prepared as follows: 19.26 g of Ludox AS-40 were put into a 400 ml polypropylene beaker and stirred by a magnetic stirrer at an agitation speed of ca.1200 rpm. Afterwards, 18.52 g of TEAOH solution and sub- sequently 16 g of HDTMACl (1/3 of the total amount) were added. To the gel formed the other 2/3 of the surfactant and 1.8 g of TBOT, diluted with 1.9 g of propan-2-01, were added simultaneously. All vessels, except that with TBOT, were washed with 10 g of deionized water to achieve a quantitative transfer of reagents. As the vessel with TBOT could not be washed with water because of the hydrolysis, an extra 0.05 g of TBOT was always added to make up for the losses. All components were cooled down to 10°C in an ice bath and also the gel was formed at the same temperature.The gel was agitated for 1 rnin and then reacted with stir- ring (150 rpm) in a 250 ml polypropylene autoclave at 104 "C for 24 h. The resulting solid product was recovered by fil- tration and treated as in procedure I. The reaction mixture corresponded to an oxide molar ratio of 31.01Si02 :0.64(TBOT), : 4.53(HDTMA),O : 3.04(TEA),O :4.50(C3H,),0 :972H20. Preparation of Reference Adsorbents As the nature of the surface of the reference adsorbents should be identical or at least similar to that of the invest- igated solid, they were prepared by the thermal destruction of appropriate aluminosilicate or titanosilicate MCM-41 materials at lo00 "C for 2 h. Methods The adsorption isotherms of nitrogen (Linde, purity 99.9996%) at -196°C were measured with an Accusorb 2100E instrument (Micromeritics).The temperature of the liquid-nitrogen bath was measured by a thermistor probe. Each sample was degassed at 330°C for at least 20 h until a pressure of Pa was attained. With TiMS-4, AIMS-5 and AIMS-6 the scanning behaviour was investigated, i.e. desorp-tion steps were started before the saturation pressure p" was reached. Powder X-ray diffraction data were obtained on a Seifert 3000 P diffractometer in the Bragg-Brentano geometry arrangement using Co-Kor radiation with a graphite mono- chromator and a scintillation detector. A TEM image was obtained on a Phillips EM 420 trans- mission electron microscope, operated at 120 kV, equipped with an LaB, cathode.Results Reference Adsorbents When the adsorption on reference aluminosilicate adsorbents was related to the unit surface area all isotherms were practically identical. For the analysis of adsorption data the isotherms on reference adsorbents prepared from AIMS-2 (available in the largest amount) and TiMS-4 were used. The BET surface area of the aluminosilicate reference adsorbent was 25.5 m2 g-' and the constant, c = 45.8. With the titan- osilicate reference adsorbent the respective values were 6.1 m2 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 40 P 0 0.2 0.4 0.6 0.8 1 PIP" Fig. 2 Adsorption isotherms of nitrogen at -196°C on AlMS-1 (a) and AlMS-6 (b): (0)adsorption, (m) desorption g-' and 16.3.The BET equation was valid for p/p" from ca. 0.05 to 0.40, and less precisely up to about 0.5-0.6. MCM-41 Materials With the exception of AlMS-1 (Fig. 2), the adsorption iso- therms on all alumino- and titano-silicate sieves were ana- logous. Their shape is unusual with a sudden increase in adsorption occurring at p/p" =-0.25 (Fig. 2 where AlMS-6 was shown as a typical example). The position of this increase can be characterized by which is the relative pressure corresponding to the point of inflection (Table 1). Adsorption isotherms can be approximated by the BET equation from p/po z0.1 (with all sieves) to an upper limit which increased with the serial number of the sample from ca. 0.20 (AlMS-1) to ca. 0.35 (AlMS-6). Monolayer capacities, n,, c constants and surface areas, ABET,are summarized in Table 1.Discussion The regular porous structure, which is created using rod-like micelles as templates, is termed the primary structure. In the later stages of crystal growth, a secondary mesoporous struc- Table 1 Adsorption of nitrogen at -196 "C on MCM-41 molecular sieves (the BET equation) sample n,,,/mmol g - c ABET/mZg-' (p/p")inf AlMS- 1 8.377 78.3 817.5 - AlMS-2 10.982 76.0 1071.7 0.29 AlMS-3 9.953 55.3 971.3 0.33 TiMS-4 8.474 59.4 826.9 0.34 AIMS-5 8.416 62.9 821.3 0.39 AlMS-6 10.403 61.3 1015.2 0.43 n,, monolayer capacity; c, constant in the BET equation; ABET, BET surface area; (~/p')~,,~,relative pressure at the point of inflection in the region of capillary condensation.ture is formed with irregular pores of practically all sizes. It is closely related to the crystal morphology. Application of the Comparison Plots The surface areas of both reference adsorbents were small, which clearly shows that the porous structure of the original molecular sieves was totally destroyed and the thermal treat- ment caused the sintering of the material. Therefore, adsorp- tion isotherms on them can be used for the construction of comparison plots, in which the adsorption in the porous system of MCM-41 materials is compared with that on the flat surface. The treatment used is analogous with that of ref. 10. Fig. 3 shows the dependence of n us. nref (where n is the adsorption on the adsorbent studied, while nrefis that on the reference adsorbent at the same equilibrium pressure) obtained by the transformation of the adsorption branches of isotherms up to p/p" = 0.8.The low-pressure part of all plots of n us. nref can be approximated by a straight line going through the origin. With AlMS-1, a sharp knee and a plateau with a small slope follow. In contrast, both AlMS-2 and AlMS-6 are characterized by a steep upward swing passing gradually into a plateau. With AlMS-2 this steep increase, occurring in the reversible part of the isotherm, has been already explained as a consequence of capillary condensation without hysteresis in primary me sop ore^.^ With AlMS-6, in contrast, it occurs in the hysteresis region.By scanning the hysteresis loop (Fig. 4) it was found that the adsorption hys- teresis occurs during the filling of primary mesopores. The Kelvin capillary condensation mechanism is obviously oper- ative here. The direct proportionality indicates that in the first stages of adsorption the formation of adsorbed layers on MCM-41 is the same as that on a non- porous adsorbent. The slope B, (Table 2) determines the surface area of the studied solid A = B1Aref, where Arefis the surface area of the reference adsorbent. As the ratio ABET/B1 (Table 2) is very close to Aref,the similarity of the mechanism 30 A7.. . ..... -. . ._.. ... .. -20.-I -0 E E--. 0 0.2 0.4 0.6 0.8 nre,/mmol g -' Fig. 3 Comparison plot of the adsorption of nitrogen on AIMS-1 (a),AlMS-2 (b) and AlMS-6 (c) 2824 30 25 r I (5, E --.E C 20 15 I I I 0.35 0.45 0.55 0.65 0.75 P/P" Fig.4 Scanning of the hysteresis loop of the adsorption isotherm on AIMS-6: (0)adsorption, (m) desorption of adsorption on MCM-41 and the reference adsorbents is confirmed. By a detailed inspection of the n us. nref plots, small but systematic deviations from direct proportionality were found to occur. From this it follows that the surfaces of MCM-41 materials and reference adsorbents are not totally identical as regards the nature of the adsorbent-adsorbate interactions. This fact is apparent from the variation of c (from the BET equation, Table l), which for MCM-41 materials varies between 55.3 and 78.3, while for the reference absorbents it decreases to 45.8 (aluminosilicate) or as low as 16.3 (ti tanosilica te).J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The plateau observed in n us. nref plots of AlMS-1, AlMS-2 and AlMS-3 at around p/p" = 0.4 obviously formed after the primary mesopore structure had been totally filled. The small slope of this plateau is caused by the multilayer coverage of the surface of the secondary mesopores. If this part of the n us. nrefplot is approximated by the straight line n = C, + Clnref (2) then C, gives the adsorption capacity of primary pores and C, the surface area of secondary mesopores, Ame2= C,A,,,. The values of C,, C, and Am,* together with the volumes of the primary pores Vmel= Coum0,(umol being the molar volume of nitrogen in the liquid state) are given in Table 2.With TiMS-4, AlMS-5 and AlMS-6, eqn. (2) cannot be applied because primary mesopores are filled by Kelvin capil- lary condensation, while on the reference adsorbent multi- layer coverage of the surface occurs. On the basis of the parameters Vmel and Ame2, the mean radius of the primary mesopores, rmel,can be simply estim- ated. For a cylindrical pore: The calculated values of rmel are presented in Table 2. Determination of the Parameters of the Porous Structure of MCM-41 Molecular Sieves As the method of comparison plots cannot be used with all samples for the determination of the volume of primary pores and the surface area of secondary ones, the distribution of the volume and the surface area of mesopores was calculated from the desorption branch of the hysteresis loop by the method of Dollimore and Heal.' The calculation directly provided V,, =f(rme) and A,, = g(rme), where V,, and A,, are the volume and surface area of pores of radius r 2 r,,,,.Table 2 Adsorption of nitrogen at -196"Con MCM-41 molecular sieves (comparison plots) AlMS- 1 33.76 24.7 9.102 3.46 0.317 88.2 0.87 AIMS-2 44.99 23.8 21.337 2.58 0.743 65.8 1.48 AlMS-3 3 8.47 25.2 19.103 4.54 0.665 115.8 1.55 TiMS-4 163.42 5.06 - - - - - AlMS-5 33.26 24.7 AlMS-6 40.37 25.2 ~~~~ ~ B,, C,and C, are constants of the linear parts of the comparison plot; V,,, and rmelare the volume and radius of primary mesopores; Arne, is the surface area of secondary mesopores.Table 3 ~~ AIMS-1 ~ 0.344 124.4 AlMS-2 0.578 168.9 AIMS-3 0.525 163.7 TiMS-4 0.345 286.0 AIMS-5 0.646 480.1 AlMS-6 1.056 864.4 Porous structure parameters of MCM-41 molecular sieves 74.5 743.0 0.368 51.1 1020.6 0.768 74.6 896.7 0.686 35.1 791.8 0.704 41.4 779.9 0.716 94.8 920.4 0.895 (v,,),,,and (A,,),,, are the volume and surface area of all pores with r 2 1.8 nm; (A*),,', (I/*),,', (n*),,, volume, capacity and mean radius of pores with r -= 2.5 nm; (A*),,, ,surface area of pores with r 2 2.5 nm. 10.565 0.99 22.049 1.50 19.695 1.53 20.21 1 1.78 20.556 1.84 25.695 1.94 and (r*),,' are the surfacea area, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 These distribution functions end at I,, = 1.8 nm (the lower closure point of the hysteresis loop), the corresponding values of V,, and A,, being equal to the total surface area (A,,),,, and volume (V,,),,, . At the lower closure point of the hysteresis loop all pores with r < 1.8 nm are filled by the adsorbate. Their volume V,, .8 can be calculated according to where no.4 is the adsorption at the lower closure point of the hysteresis loop and (nief)0.4is the adsorption of the reference adsorbent at p/p" = 0.4 related to the unit surface area. The dependence of the mesopore volume on their radius V,, = Vme(rme)calculated by the method of Dollimore and Heal'' can be converted to an increasing function of pore radius using which gives the volume of pores whose radius lies between 1.8 nm and T,,.Then the total volume of pores of r d r,, equals The dependence of V us. rmeis shown in Fig. 5. With TiMS-4, AlMS-5 and AlMS-6 the distribution of primary mesopores is partly included. It must be considered with caution as the 1.o 0.8 0.6 01 $ 0.4I-0.2 0.0 1.5 2.0 2.5 3.0 3.5 4.0 r/nm Fig. 5 Pore size distribution I/ vs. rme: (a) AIMS-1, (b) AIMS-2, (c) AIMS-3, (d)TiMS-4, (e) AIMS-5, cf)AIMS-6 Table 4 Adsorption of nitrogen at application of the Kelvin equation is not straightforward at p/po +0.4.' Nevertheless, the conclusion can be reached that with AlMS-5 and AIMS-6 primary mesopores are not uniform, their radius lying in an interval of the width of ca.0.5 nm. This conclusion is also supported by TEM (Fig. 1). With all of these samples r,, = 2.5 nm can be chosen as the upper limit of the pore size of primary mesopores. Based on a chosen limiting pore size, a set of porous structure parameters can be obtained (Table 3). The volumes of primary mesopores (V*),, were determined directly from the plot of V us. rme [eqn. (6),Fig. 51. With AlMS-1, -2 and -3, (V*),,, is rather larger than Vmeldetermined using com- parison plots. It is clear why: the volumes determined using comparison plots relate only to the primary mesoporous structure, while (V*),,, relates to all pores of T < 2.5 nm.(For this reason all parameters based on the limit of 2.5 nm are designated by an asterisk). Using (V*),,', the adsorption capacity of primary mesopores can be simply calculated according to (n*),,' = (V*)mel/~mo,.The surface area of primary mesopores, (A*)mel,is given by the difference A,,, -(A*),,*, where the surface area of secondary mesopores, (A*),,, ,was directly determined from the distribution curve, A,, = g(rme),as the surface area of all pores of r > 2.5 nm. The mean radius of the primary pores (r*),,' was estimated analogously to rmelaccording to eqn. (3). All parameters of the porous structure are summarized in Table 3. With AlMS-1, -2 and -3, rmel and (r*)mel are in rea- sonable agreement; the values of rme1calculated using com- parison plots are obviously nearer the reality.The surface area of secondary mesopores is relatively small with all samples varying between 50 and 100 m2 g-'. With increasing radius of the primary mesopores, however, the total surface area of the mesopores, (A,,),,,, calculated from the desorp- tion branch of the hysteresis loop increases and approaches the total surface area, ABET. Mechanism of Adsorption in Primary Mesopores It can be shown that trends of the values of parameters derived from adsorption isotherms and comparison plots are reasonable, being in agreement with the concepts of observed adsorption phenomena. The values of the relative pressure, (pip"),,and the adsorp- tion, ne, corresponding to the end of the direct proportion- ality between n and nref were determined from comparison plots (Table 4).In order to compare individual samples the quotients n,/n, were calculated, expressing n, in terms of the monolayer capacity, n, . Analogously, the mean pore radius of the primary mesopores was expressed as the quotient (r*)mel/~m, where the mean statistical thickness of the nitro- gen monolayer, t, = 0.354 nm. It is evident from Table 4 that (pip"), and n,/n, increase with increasing (~*),,~/t,. The region of equilibrium pressure -196°C on MCM-41 molecular sieves AIMS- 1 2.8 0.19 9.708 1.16 0.83 AIMS-2 4.2 0.26 15.256 1.39 0.66 AIMS-3 4.3 0.30 14.429 1.45 0.68 TiMS-4 5.0 0.30 13.059 1.54 0.62 AlMS-5 5.2 0.34 13.151 1.56 0.6 1 AIMS-6 5.5 0.38 16.861 1.62 0.59 (r*)mel, mean radius of pores with r -= 2.5 nm; t,, mean statistical thickness of nitrogen monolayer; (plp"),, n, and relative pressure, adsorption and adsorption in primary mesopores at the end of the direct proportionality between n and nref;n,, monolayer capacity; (n*)mel, capacity of pores with r < 2.5 nm.where nitrogen molecules are adsorbed similarly on both the surface of MCM-41 sieves and the flat surface widens with increasing radius of the primary pores. The extent of the filling of primary pores by multilayer coverage of their walls is expressed by the ratio (ne)mel/(n*)mel(Table 4), while the difference 1 -(ne)mel/(n*)melgives the proportion that is filled by the capillary condensation.[In contrast to n, ,the amount of (ne)mel= relates only to the surface of primary pores.] From the first row of Table 4 it follows that with AIMS-1, primary pores are filled almost entirely by the formation of the monolayer. In contrast, with AIMS-2 and -3, both containing larger pores, the cooperative adsorbate-adsorbate interactions significantly manifest themselves. They cause the capillary condensation by which the pores fill spon- taneously in the reversible part of the isotherm. With TiMS-4, AIMS-5 and AIMS-6 the number of mesopores that are large enough for Kelvin capillary condensation with hys- teresis, gradually increases. Conclusions The nature of the adsorption in mesopores of MCM-41 molecular sieves strongly depends on their pore size._While with pores of radius of around 1 nm the multilayer coverage of the surface is effective, it turns into a two-stage process when the radius is increased to 1.5-1.8 nm. In these pores multilayer coverage is followed by capillary condensation without hysteresis. In even larger pores (r > 1.8 nm) the usual Kelvin capillary condensation takes place. For estimation of porous structure parameters, both the nature of the adsorption and the limits of the methods used J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 must be taken into account. The values of the pore structure parameters obtained provide detailed knowledge of these new materials. The authors are grateful to the Volkswagen Foundation for financial support (Grant 1/69 159). References 1 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature (London), 1992,359, 710. 2 J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T-W. Chu, D. H. Olson, E. W. Schep-pard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. SOC., 1992,114, 10834. 3 C-Y. Chen, H-X. Li and M. E. Davis, Microporous Muter., 1993, 2, 17. 4 C-Y. Chen, S. L. Burkett, H-X. Li and M. E. Davis, Microporous Muter., 1993, 2, 27. 5 P. Behrens, Adu. Mater., 1993,5, 127. 6 P. Behrens and G. D. Stucky, Angew. Chem., 1993,105,729. 7 0. Franke, G. Schulz-Ekloff, J. Rathousky, J. Starek and A. Zukal, J. Chem. Soc., Chem. Commun., 1993,724. 8 P. J. Branton, P. G. Hall and K. S. W. Sing, J. Chem. SOC., Chem. Commun., 1993,1257. 9 S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, pp. 94, 84, 153. 10 M. Ribeiro Carrott, P. Carrott, M. B. Carvalho and K. S. W. Sing, J. Chem. SOC., Faraday Trans., 1991,87, 185. 11 D. Dollimore and G. R. Heal, J. Appl. Chem., 1964,14,109. Paper 4/014971; Received 14th March, 1994