Hysteresis in Physical Sorption for MCM

MESOPOROUS MOLECULARSIEVES 1998 Studies in Surface Science and Catalysis, Vol. 117 L. Bonneviot, F. B61and,C. Danumah, S. Giasson and S. Kaliaguine(Editors) ~ 1998 Elsevier Science B.V. All rights reserved.

575

H y s t e r e s i s in P h y s i c a l S o r p t i o n f o r M C M . W.C. Conner b, M. A. Springuel-Huet', J. Fraissard a, J. Bonardet', T. McMahon b , L. Boudreaub, and J. Masciadrelli b a Laboratoire de Chemie des Surfaces, Universit~ P. et M. Curie, 4 Place Jussieu, Paris, FRANCE b Chemical Engineering, University of Massachusetts, Amherst, MA. 01003 USA

A variety of samples of mesoporous solids were studied as prepared by the techniques giving rise to the MCM-class of materials. These were studied by physical sorption and X-ray diffraction. We found that there are significant differences between the sorption isotherms which may or may not exhibit sorption hysteresis. A correlation between X-ray and sorption analyses was not evident. Moreover, conventional analyses of desorption is not appropriate.

A combination of sorption and X-ray analyses are required to

characterize MC M materials. 1. INTRODUCTION The discovery of the MCM family of solid opened up several new dimensions in the synthesis of unique materials and the characterization of mesoporous solids [1 ]. First a new class of materials was discovered which were, for the most part, amorphous and, yet, commonly possessed X-ray diffraction patterns characteristic of an hexagonal array of solid spacing greater than 2.0 nm. Electron microscopy confirmed that a mesoporous parallel network of (hexagonal) pores had been created. Sorption analyses initially supported the picture that the pores were uniform in dimension and parallel, i.e., there was no hysteresis between adsorption and desorption. Further, conventional analyses showed adsorption and desorption at relative pressures (P/Po, where Po is the saturation pressure of the adsorbing gas) between 0.3 and 0.6. The pore dimensions which conventionally correspond to adsorption in this range or relative pressures, span the range from 1-6 nm in diameter; however, it is believed that conventional analyses for mesoporous solids (>5 nm) should not be extrapolated this low and adsorption in microporous solids apparently occurs by a different mechanism. Modifications to the original recipe for MCM synthesis soon gave rise to the synthesis of a large class of related mesoporous materials. As more analyses became available [2], it became evident that the MCM materials were more complex than originally perceived. Hysteresis between adsorption and desorption of N2 at 77K was sometimes evident, and was initially believed to be due to quadrupolar interactions between N2 and the MCM surface [3]. Other studies confirmed that a lack in hysteresis in sorption was the rule [4], not an exception. Variations in X-ray diffraction spectra confLrmed the complexity.

576 2. EXPERIMENTAL The MCM-41 samples were prepared from sodium silicate which is added to a basic solution (NaOH) containing the template, tetradecyltrimethyl (denoted Cl4) or hexadeeyltrimethyl (denoted Ci6) ammonium bromide, and heated to 40 ~ By addition of a 1M HCI solution, the pH is decreased from 11.4 to about 9. The mixaae is stirred 1 hour at 40 ~ then filtered, washed with distilled water, and heated at 40 ~ calcination was performed with air at 600 ~ during 24 h (heating rate 24 ~

overnight. The

For several of the samples, mesitylene has been added to the initial solution (containing the template and the silicate) at 60 ~ with different template/mesitylene ratios: 1/5, 1/4, 1/3 (see Table 1) in order to obtain larger pore sizes. The solution is stirred 2 hours at 60~ after the addition of HCI to adjust the pH to 9-9.5. The sample is then filtered and washed with distilled water and is finally heated at 60 ~ overnight. The calcination was performed under the same conditions as above. N2 adsorption-desorption isotherms were measured at 77 K with a Micromeretics instrument (ASAP 2010) using conventional volumetric techniques. The isotherms for most of these samples are discussed and shown below in Figures 2-4. 3. RESULTS & DISCUSSION 3.1 X-ray Analyses The samples were analyzed by X-ray powder diffraction using a Phillips X-ray defractometer. These results are shown in Figure 1.

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2O Figure 1" X-ray diffraction for samples which exhibit peaks characteristic of M CM type materials. Note that M05 and M06 showed no diffraction peaks for 2 < 20 < 10.

577 The X-ray diffraction patterns conf'Lrmed the formation of a hexagonal structure for several of the samples as seen in Figure 1. The position of the primary peaks give lattice spacings of I = 29 - 35A for the primary peaks. There is evidence for the secondary hexagonal peaks (at 1/~3 and I/2 ) for each of the samples shown in Figure 1. There were, however, no peaks in the diffraction patterns for samples M05 and M06. As far as X-ray could detect, these were amorphous samples. 3.2. Adsorption Data

Adsorption/desorption isotherms for nitrogen adsorption at 77K are shown in Figures 2-4.

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Relative Pressure (P/Po) Figure 2: Sorption isotherms for nitrogen adsorption at 77K on M01 and M03. The adsorption isotherms (a) are below the desorption isotherms (d) in each case. Comparisons between these data show that there is considerable variation in the types of isotherms found for these samples. Hysteresis in adsorption is often but not universally found. Further, the nature of the hysteresis differs significantly. As examples, the hysteresis for sample MO 1 occurs during desorption while the corresponding adsorption branch of the isotherm only increases slowly while the hysteresis for M03 represents a steep adsorption and only a small decrease in the desorption isotherm until P/Po goes below 0.5 whence it desorbs rapidly with a decrease in relative pressure.

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Relative Pressure (P/Po) Figure 3" Sorption isotherms for nitrogen adsorption at 77K on M04 and M05. The adsorption isotherms are below the desorption isotherms in each case. 60

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Relative Pressure (P/Po) Figure 4: Sorption isotherms for nitrogen adsorption at 77K on M01 and M06. The adsorption isotherms are below the desorption isotherms in each case.

579 Figure 3 compares two samples with seemingly similar isotherms, M04 and M05. There is evidence for pore filling for each adsorption isotherm commencing at P/Po = 0.5 for M04 and at P/Po = 0.6 for M05. Both desorption branches desorb only slightly until P/Po < 0.6 whence considerable desorption is evident with small reduction in pressure. Whereas M04 is apparently crystalline, M05 is amorphous. Similarly, Figure 4 compares the sorption isotherms for samples M01 and M06. Without the X-ray analyses, one would conclude that the pore structures of these two samples are essentially identical. Minor hysteresis is evident at P/Po = 0.4-0.45 during desorption for each sample and there is also evidence for pore filling and emptying centered at P/Po -- 0.17 for M01 and at P/Po -~ 0.25 for M06. These pore fillings and emptyings take place without any evidence for sorption hysteresis. Again, although the isotherms of M01 and M06 are apparently quite similar, only one sample (M01) exhibits the X-ray diffraction patterns for a "crystalline " MCM-type void structure. The sorption analyses is apparently more sensitive to the void structures than are the X-ray analyses. It should be noted that hysteresis at P/Po ~ 0.4-0.45 is often evident for these samples. In some cases (M01 and M06), the hysteresis is minor while in other samples (M03, M04 and M05) this would appear to reflect the dominant void structure. In each ease, automated analyses of desorption will show a peak in the pore size distribution curve reflecting pores 18-20A. However, analyses of the adsorption will not always show a corresponding pore dimension (shifted up in size when hysteresis is evident). Only samples M04 and M05 exhibit peaks in the adsorption analyses which correspond to the hysteresis in desorption at P/Po ~ 0.4-0.45. We are drawn to conclude that peaks in desorption analyses which have no corresponding peak at the same (without hysteresis) or larger pore dimension (with hysteresis) may be due to the common tensile strength artifact [5].

3.3 Analyses Pore structure analyses can be done on both adsorption and desorption isotherms. FHH analyses [5] could be performed employing the conventional values of the constants in the commonly assumed "Halsey equation" for t (the thickness of the adsorbed layer) and P/Po (the relative pressure): t = 6.04 [p/po]l/3

(1)

However, the steepness of the adsorption isotherms below the region of pore filling suggests that this "conventional" equation will over estimate the smaller pores [6]. A more appropriate relationship between the relative pressure and the thickness of the adsorbed layer was estimated by fitting the data in this region of data, below pore filling. The following relationship reflects Halsey's original concepts [7] and was fit to the data: t = C [P/PoJ l/uc

(2)

580 The values of the relationship between the thickening of the adsorbed layer and pressure in the regions where pores are neither filled or emptied does vary for these matc~als. The values of the "Halsey constant" (HC in equation 2) was found to vary from ca. 1.4 to 2.3 as shown in Table 1. This is a significant variation and is also substantially different from the value of 3 used in the "conventional" [3, 8] analyses employed in all automated sorption analyses systems. It should be noted that if a larger value of this exponent is assumed in the analyses of pore structure, smaller pores will "found" in the analyses that do not exist. This is a common artifact in conventional analyses of MCM-type solids. We have never found that the appropriate exponent between the relative pressure and the thickness of the adsorbed layer is near 3 for M CM solids. The results from these analyses are summarized in Table 1 which specifies the methods of preparation as detailed in the experimental section, above. The BET surface areas and the mean diameters determined using density functional theory (with cylinder geometry and Harkins & Jura equation) are summarized in columns 4 and 6. Column 5 shows the value of the C constant from the BET analyses. The characteristic dimension from X-ray analyses is shown in the last column. Table 1 Measured properties of MCM Samples. Sample Temp./Add. Halsey Exp. m-mesitylene ,,

,,

Surface Area C Constant DP (desorp.) X-ray M2/g from BET A A

,

,,,

,,

,,

MOI C14 1.44 (0.I->0.4) 1354 17 18 29 1159 63 21 34 M03 C16 1.80 (0.I->0.4) 990 44 51 35 M 0 4 Cl6&I/4m 2.27(0.I->0.4) 68 32 amorp.* M05 Cl6&I/3m 2.02(0.I->0.4) 1073 M06 C!6&l/5m 1.4 (0.1-0.2) 1358 29 46 amorp.* The template employed (C14 or C16) with or without mesitylene are listed in column 2. The Halsey constants (fit over the range of relative pressures) are shown in column 3. The BET surface areas and resulting C constants are shown in column 4. The maximum in the pore diameters from desorption analyses is shown in column 5. The lattice dimension of the largest peak in the X-ray diffraction is shown in column 6. *The last two samples showed no evidence of X-ray diffraction, appearing amorphous. .

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581 4. ANALYSES & CONCLUSIONS Several conclusions can be drawn from these data. First, there is no common isotherm for adsorption and desorption for MCM-based materials. Hysteresis in pore filling and emptying is sometimes evident but most ot~en it is not. Second, it is necessary to properly analyze both the adsorption and the desorption branches of the isotherms. Conventional sorption analyses employing a "standard" isotherm (or Halsey relationship) will result in estimates of pores which do not exist. Finally, both X-ray and sorption analyses are required to characterize the void morphology of MCM-type materials. Samples which exhibit similar isotherms can have different X-ray diffraction patterns and samples with similar diffraction patterns can have significantly different adsorption isotherms. The new class of mesoporous materials requires that the analyses of their morphology are modified. Neither X-ray nor sorption analyses alone give a complete def'mitive analysis, both must be employed. Further, both adsorption and desorption analyses are required and conventional approached to determine pore dimensions should not be used without modification. As for any new class of materials, care must be exercised to characterize these unique mesoporous solids. 5. REFERENCES [ 1] J. Beck, J. Vartuli, W. Roth, M. Leonowicz, C. Kresge, K. Schmitt, C. Chu, D. Olson, E. Sheppard, S. McCullen, J. Higgins and J. Schlenker, J. Am. Chem. Soc. vol 114, 1992, 10834-10843 [2] Characte~tion of Po~us Solids. IV, The Royal Society of Chemistry, pbs., 1997. [3]. P. Branton, P. Hall, and K. Sing, J. Chem. Soc. Chem. Commun. 1993, 1257-1258 [4]. A. Corma, Chem. Reviews, vol 97 n ~ 6, 1997, 2373-2419 [5]. S. J. Gregg and K. S. W. Sing, "A.dsorption, Surface Area and Porosity." Academic Press, London, 1982. [6]. W. C. Conner, Journal of Porous Materials, 3-2, 1996, 1 [7]. G. Halsey, J. Chem. Physics, 16, 931, 1948 [8] E. Barrett, L. Joyner, and P. Halenda, J. Amer. Chem. So~, 73, 1951,373