A Unified Interpretation of High-Temperature Pore Size Expansion

structure, and dynamics of water confined between hydrophobic plates. Pradeep Kumar , Sergey Buldyrev , Francis Starr , Nicolas Giovambattista , H...
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J. Phys. Chem. B 1999, 103, 4590-4598

A Unified Interpretation of High-Temperature Pore Size Expansion Processes in MCM-41 Mesoporous Silicas Michal Kruk and Mietek Jaroniec*,§ Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242

Abdelhamid Sayari* Department of Chemical Engineering and CERPIC, UniVersite LaVal Ste-Foy, Qc, Canada G1K 7P4 ReceiVed: NoVember 13, 1998; In Final Form: March 3, 1999

A unified interpretation is presented for high-temperature (ca. 423 K) unit-cell enlargement of MCM-41 synthesized in the presence of cetyltrimethylammonium (CTMA+) surfactants without auxiliary organics. Such processes during postsynthesis hydrothermal treatments and direct syntheses were reported by several research groups to afford large-unit-cell (d spacing up to 7 nm) and large-pore (up to 7 nm) MCM-41 materials, but their structures were claimed to be different and the proposed mechanisms of the processes were not consistent with each other. The current study demonstrates significant structural similarities of large-pore MCM-41 materials prepared using direct syntheses and postsynthesis treatments, reveals the common mechanism of their formation, and discusses its implications for preparation of large-pore materials, including new synthesis procedures. The proposed comprehensive description of the mechanism of the unit-cellenlargement processes involves the in situ generation of N,N-dimethylhexadecylamine (DMHA) and its swelling action as a driving force of the pore size enlargement. The swelling properties of DMHA were confirmed by a successful synthesis of large-pore MCM-41 in mild conditions in the presence of DMHA and preparation of silicas with pore sizes up to 11 nm and extremely large-pore volumes up to 2.4 cm3/g via restructuring of 3.5 nm MCM-41 in aqueous emulsions of long-chain amines (dimethyldecylamine or DMHA). The neutral amine is formed during the high-temperature processes as a result of decomposition of CTMA+ cations, which is likely to be accompanied by migration of tetramethylammonium cations (TMA+) to the silicasurfactant interface or is possibly related with replacement of CTMA+ by TMA+ at the interface. The reasons for the pore volume increase and surface area decrease during the pore size enlargement are discussed. Moreover, it is suggested that the unit-cell expansion in the direction perpendicular to the pore channels is accompanied by shrinkage of the structure in the direction parallel to the channels.

1. Introduction A remarkable feature of MCM-41 materials1,2 is the possibility of engineering their pore size in a wide range from about 2 to 10 nm.1-7 Numerous studies demonstrated that it is relatively easy to synthesize good-quality MCM-41 with pore diameter below about 4.5 nm using surfactants of different chain lengths.1-7 However, in some cases, MCM-41 with larger pores, i.e., above 4.5 nm in diameter (later referred to as large-pore MCM-41), are highly desirable. For instance, large-pore ordered materials promise to provide a suitable confined environment for immobilized enzymes8 and chemically active colloids.9 The advantage of large-pore MCM-41 silicas over materials with narrower pores has already been demonstrated in some catalytic applications involving large molecules.10,11 In one of the studies,10 asphaltene removal in the presence of NiMo-MCM41 catalyst was shown to increase as the pore size of MCM-41 increased and the catalyst prepared using the support with 8.0 nm pores was found to be more active than a conventional demetalation catalyst. In another study,11 heteropolyacids supported on MCM-41 aluminosilicates were found to exhibit higher catalytic activity, when the support had a pore size of § E-mail: [email protected]. Phone: (330) 672 3790. Fax: (330) 672 3816.

6.6 nm, as compared to 2.9 nm pores. This was attributed to decreased pore blockage during deposition of heteropolyacid aggregates in the mesoporous channels of MCM-41. Moreover, the availability of large-pore MCM-41 materials extends the range of pore sizes attainable for ordered inorganic-organic composite materials that can conveniently be prepared via chemical bonding of various organosilanes.12-15 Custom-tailoring of surface properties of siliceous supports may require introduction of high coverages of relatively large ligands, which results in a substantial decrease in the diameter of pores.14,15 Under such conditions, large-pore supports may be beneficial.15 From the point of view of application of unmodified and organic-modified MCM-41 and related materials as chromatographic packings or liquid-phase adsorbents, pore diameters exceeding 4.5 nm would be highly desirable. For instance, largepore (5.5 nm) MCM-41 with chemically bonded cross-linked mercaptopropylsilane ligands was recently shown to be a highly efficient adsorbent for the removal of heavy metal ions from water.12,13 It is also interesting to note that large-pore MCM-41 samples were recently used as model adsorbents to test fundamental aspects of application of gas adsorption in the characterization of mesoporous solids and to derive equations, which allow for an accurate assessment of pore size distributions for MCM-41 and other mesoporous materials.16

10.1021/jp9844258 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/04/1999

Pore Size Expansion in Mesoporous Silicas As can be expected from the discussion presented above, the synthesis of large-pore (above 4.5 nm) MCM-41 and other largepore ordered mesoporous materials has attracted considerable and still growing attention.1,2,9,17-34 Already in the initial work of scientists from Mobil,1,2 it was demonstrated that MCM-41 samples with pore sizes up to about 10 nm can be prepared in the presence of 1,3,5-trimethylbenzene (TMB). It was proposed that TMB molecules locate themselves in the hydrophobic interiors of the surfactant micelles, thus increasing their diameter and, consequently, enlarging the pore size of the resulting MCM41 materials. The obtained samples with pore sizes up to 6.5 nm were reported to exhibit 3-4 peaks on their X-ray diffraction (XRD) spectra, which indicated their good structural ordering. XRD patterns for materials with even larger pores featured either a single broad line or an increased background noise at very low angles, suggesting a lower degree of structural uniformity, but transmission electron microscopy (TEM) provided evidence of somewhat irregular hexagonal ordering of these materials.2 Subsequent studies carried out by other research groups demonstrated that it is very difficult to obtain well-ordered MCM-41 using TMB as a swelling agent. Huo et al. reported that pure-silica and large-pore (above 5 nm) aluminosilicate MCM-41 synthesized using alkyltrimethylammonium (ATMA+) surfactants and TMB exhibit only one XRD peak.17,18 Low structural ordering of MCM-41 prepared in the presence of TMB was also confirmed by others19,20 and only in rare cases were the TMB-swollen large-pore materials reported to exhibit more than one XRD peak.21 The applicability of other compounds as expanders was also explored.22 Linear hydrocarbons were shown to be good swelling agents,19,23,24 but the reported materials exhibited low23 or moderate19 structural ordering. Application of trialkylamines (alkyl ) n-octyl,24,25 n-dodecyl25) also made it possible to synthesize large-pore MCM-41, but with low structural uniformity.25 More recently, 1,3,5-triisopropylbenzene (TIPB)20 and dimethylhexadecylamine (DMHA)25 were shown to be promising expanders. TIPB was found to be suitable for preparation of highly ordered large-pore MCM-41, but the pore diameter was rather independent from the amount of TIPB and equal to about 5 nm. An addition of small amounts of TMB led to a further increase in the pore size up to about 6 nm accompanied by some decrease in structural ordering of the obtained MCM41.20 Recently, application of long-chain surfactants afforded MCM-41 materials with pore sizes up to ca. 5.2 nm.18,26 This was accomplished using (i) docosyltrimethylammonium with or without addition of cetyltrimethylammonium surfactants26 as well as (ii) mixtures of alkyltrimethylammonium surfactants and divalent surfactants with eicosyl or docosyl groups.18 In the latter case, it was also possible to further expand the pore size up to about 6 nm by treating as-synthesized materials in water at 373 K.18 Another approach to the synthesis of large-pore materials is the use of proper oligomers or polymers as structure directing agents.9,27-31 However, the obtained samples usually exhibited a relatively low degree of structural ordering9,27-29 and only recently have materials with an appreciable degree of pore structure uniformity and a wide range of pore sizes (5-30 nm) been reported.30,31 All the synthesis procedures presented above appear to have certain disadvantages. Some of them are not suitable for the preparation of good-quality products and may also require introduction of large amounts of auxiliary organics, which is undesirable from the application point of view.7,34 Others require structure directing agents, which are not readily available. Thus,

J. Phys. Chem. B, Vol. 103, No. 22, 1999 4591 the development of synthesis approaches for good-quality largepore MCM-41 using common structure directing agents (e.g., alkyltrimethylammonium surfactants) without addition of expanders is strongly needed.7,34 Synthesis at high temperature (ca. 423 K)32-36 in one or two steps was found to fit this purpose as the pore size increased with time up to 6-7 nm. However, as discussed later, not only did different research groups obtain materials that were claimed to have different porous structures, but the proposed mechanisms for pore size expansion were conflicting. The aim of the current study is 3-fold: (1) to demonstrate that the materials obtained at high temperature by different groups32,34-36 in one- or two-step syntheses are comparable in terms of quality and structure (pore size, wall thickness, etc.), (2) to reveal the common nature of the high-temperature unitcell-enlargement processes reported in the literature for cetyltrimethylammonium-templated MCM-41 materials, and (3) to provide details of the mechanism and structural changes that take place during these processes. 2. Materials and Methods 2.1. Materials. The postsynthesis hydrothermal restructuring of MCM-41 was carried out as described in detail elsewhere.32,33 The synthesis gel composition used was 1 SiO2:0.33 TMAOH: 0.17 CTMABr:0.17 NH4OH:17 H2O, where TMAOH and CTMABr denote tetramethylammonium hydroxide and cetyltrimethylammonium bromide, respectively, and Cab-O-Sil M-5 was the silica source. The synthesis gel was initially heated at 343 K for 3 days and then further heated for a desired period of time at 423 K. The one-step high-temperature synthesis of MCM-41 was very similar to the procedure reported by Luan et al.37 The synthesis gel composition was 1 SiO2:0.27 CTMACl/OH:0.13 Na2O:0.26 TMAOH:60 H2O:x Al2O3, where x ) 0 or 0.02. In a typical synthesis 10 g of a 25% solution of TMAOH was added under vigorous stirring to a solution of 0.128 g of sodium hydroxide and 5.9 g of sodium silicate (27% SiO2, 14% NaOH). Then, 34.2 g of cetyltrimethylammonium chloride (25%) exchanged to a level of 33% with hydroxide (CTMACl/OH) and 6.6 g of silica was added. The obtained mixture was stirred for another 2 h period. The required amount of aluminum sulfate was dissolved in water and added to the synthesis gel, if needed. The pH was then adjusted to 11.5 using a 2 M solution of H2SO4. The gel was transferred to a Teflon-lined autoclave and heated at 423 K for 48 h. The obtained solid materials were filtered, washed, dried, and calcined in dry air at 823 K for 5 h. 2.2. Measurements. Nitrogen adsorption measurements were performed using an ASAP 2010 volumetric adsorption analyzer (Micromeritics, Norcross, GA). Before the adsorption analysis, the samples were outgassed for 2 h at 473 K in the degas port of the adsorption apparatus. XRD spectra were acquired on a Siemens D5000 diffractometer using nickel-filtered KR radiation. Scanning electron microscopy (SEM) images were obtained using a JEOL 840A microscope at an accelerating voltage of 5-15 kV. Transmission electron microscopy (TEM) images were recorded with a Gatan CCD camera on a Philips-CM20 microscope operating at 200 kV. Samples were dispersed ultrasonically in ethanol, and a drop of the suspension was deposited on a holey carbon grid. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a VG Scientific Escalab Mark II system using Mg KR radiation (hν ) 1253.6 eV) as the X-ray source. Thermogravimetric analysis was performed in nitrogen atmosphere using a high-resolution TGA 2950 thermogravimetric analyzer from TA Instruments (New Castle, DE).

4592 J. Phys. Chem. B, Vol. 103, No. 22, 1999 2.3. Calculation Methods. The BET specific surface area38-40 was calculated on the basis of nitrogen adsorption data in the relative pressure range from 0.04 to 0.2. The primary mesopore volume was evaluated using the Rs-plot method as described elsewhere.33 The pore size distributions were obtained from the adsorption branches of isotherms using the BJH method41 with the corrected Kelvin equation and the statistical film thickness curve reported recently.16 3. Results and Discussion 3.1. Background. Khushalani et al. were first to report that MCM-41 silicas synthesized at 343 K using a certain synthesis gel composition with cetyltrimethylammonium (CTMA+) surfactant as template undergo unit-cell expansion accompanied by pore size enlargement up to about 7 nm during the postsynthesis hydrothermal treatment in the mother liquor at 423 K for 2-10 days.32 In this study, the driving force of the pore enlargement process was not identified. However, it was noted that the unitcell enlargement was not likely to involve a significant dissolution of the MCM-41 material, although there was some evidence of dissolution, transport, and redeposition of silica. It was also reported that the silica to surfactant molar ratio was the same for dried samples before and after the unit-cell enlargement and there was no evidence of decomposition of the surfactant. The pore volume increase observed was attributed to the presence of an additional amount of water in the channels of the restructured materials. More recently, Corma et al.34 carried out a one-step synthesis of large-pore (diameters up to 6.6 nm) MCM-41 using synthesis gel compositions and temperatures (usually 423 K) similar to those of Khushalani et al. Corma et al. did not observe any increase in the amount of water in the channels but found that as the pore size expanded, increasing amounts of small cations, such as tetramethylammonium (TMA+), were incorporated into the channels of MCM-41. Thus, the pore size enlargement was related to the replacement of some of the surfactant molecules on the silica/ surfactant interface by these cations. The MCM-41 materials obtained using the two methods described above were found to exhibit good structural ordering and tailored pore dimensions up to about 7 nm.32-34 In addition, a synthesis procedure very similar to that of Corma et al. has been reported earlier by Cheng et al.,35,36 but it was claimed that the observed unit-cell enlargement was due a moderate pore size increase paralleled by a significant pore wall thickening. Interestingly, Cheng et al. found evidence of formation of neutral dimethylhexadecylamine (DMHA) at later stages of unit cell expansion, and DMHA was suggested to play the role of an expander. It should be noted that the one-step procedures of Cheng et al. and Corma et al. were claimed34-36 to afford MCM-41 samples with structures distinctly different from those reported earlier by Khushalani et al.32 Our previous studies demonstrated that the procedure of Khushalani et al., i.e., the postsynthesis hydrothermal restructuring in the mother liquor at 423 K,32 is a convenient way to synthesize good-quality large-pore MCM-41 with pore sizes up to 6.5 nm.33 The pore size enlargement was accompanied by a decrease in the specific surface area and an increase in the primary mesopore volume. The obtained large-pore MCM-41 materials were shown to exhibit remarkable hydrothermal stability.42 After a certain limiting pore size (ca. 6.5 nm) has been achieved, further treatment led to structural degradation, which was manifested in the development of microporosity and constrictions in the mesoporous structure as well as in a significant decrease in the specific surface area and pore volume.

Kruk et al. Hydrothermal restructuring in the mother liquor at 413 K or in water at 423 K did not allow us to synthesize good-quality largepore MCM-41, although unit-cell enlargements and relatively small pore size increases were observed in both cases.42 Moreover, in agreement with Corma et al.’s views regarding the importance of TMA+ cations, a literature survey indicated that all procedures reported so far to afford well-ordered largeunit-cell CTMA+-templated MCM-41 without addition of expanders required the presence of TMA+ cations (or similar cations) in the synthesis gel.42 In another study,25 we found that DMHA forms during the hydrothermal restructuring of MCM41 at 423 K. The role of amine as a swelling agent was confirmed by the successful development of two novel synthesis procedures: (i) direct synthesis of large-pore MCM-41 in the presence of DMHA and (ii) postsynthesis restructuring of MCM41 in aqueous emulsions of amines, which afforded materials (usually disordered) with 2-3 times larger pores (up to 11 nm), narrow pore size distributions, and remarkable primary mesopore volumes up to 2.15 cm3/g. Because of the importance of these two innovative approaches to the pore size engineering, details will be provided a future paper.43 The discussion that follows will attempt to demonstrate (i) the similarity between pore structures of large-unit-cell materials reported by different groups in refs 32-36, (ii) the similarity between synthesis conditions and the common nature of the processes involved in high-temperature syntheses of large-pore MCM-41, and (iii) the occurrence of pore size expansion primarily via the swelling effect of in situ generated DMHA. 3.2. Relations between Structural Parameters of MCM41. The materials synthesized using the three methods of hightemperature synthesis of large-unit-cell MCM-41 in the presence of cetyltrimethylammonium bromide were reported to exhibit XRD patterns characteristic of the honeycomb structure (usually three or more reflections), interplanar spacings d100 up to 5.56.6 nm, and uniform porous structures.32-36 However, the resulting samples were claimed to have markedly different pore sizes and pore wall thicknesses. The work of Khushalani et al.32 and a subsequent detailed study of Sayari et al.33 as well as the results of Corma et al.34 indicated that the pore wall thicknesses were in the range from 0.6 to 1.1 nm and the maximum pore size was about 6.6 nm. In contrast, Cheng et al.35,36 reported pore sizes not exceeding 3.65 nm and exceptionally thick pore walls (from 1.34 to 2.68 nm). Despite these extreme structural differences, the specific surface areas reported in all cases were high (730-1220 m2/g). The hexagonal structure of good-quality MCM-41 is well-defined, so it can be expected that such extreme differences in the pore size and pore wall thickness for similar values of the interplanar spacing should result in significant differences in the pore volume and surface area. Geometrical considerations of an infinite hexagonal structure of uniform pores can be employed to examine this problem and arrive at several interesting and useful relations between structural parameters of MCM-41.44,45 First of all, the pore diameter wd of MCM-41 can be calculated from the (100) interplanar spacing d and the primary mesopore volume Vp using the following simple equation:46,47

(

wd ) cd

FVp 1 + FVp

)

1/2

(1)

where c is a constant and F is the density of the pore walls. The value of c is dependent on the pore geometry and is equal to 1.213 for cylindrical pores.46 Only cylindrical pores are considered in the current study, but the assumption of a different

Pore Size Expansion in Mesoporous Silicas

J. Phys. Chem. B, Vol. 103, No. 22, 1999 4593

pore geometry (e.g., hexagonal) does not result in any significant changes in the values of the pore size and other quantities calculated on the basis of geometrical considerations.45 Numerous studies of silica-based ordered mesoporous materials showed that their pore walls are amorphous2-5,7 and therefore it is reasonable to assume F equal to that of amorphous silica, which is known48 to be about 2.2 g/cm3, especially as several measurements of pore wall density of MCM-41 provided such pore wall density values.47,49,50 The pore wall thickness of MCM-41 (denoted here as bd) can be calculated as the difference between the pore-center distance a ) 2/31/2d and the pore diameter wd:46,51

(

[

bd ) a - wd ) d

)]

FVp 2 -c 1/2 1 + FVp 3

1/2

(2)

Equation 2 can easily be rearranged in order to obtain the expression for the primary mesopore volume Vp as a function of the interplanar spacing and the pore wall thickness:44,45

( [(

Vp ) F

1/2

2/3

) ])

2 c -1 - bd/d

-1

(3)

The primary mesopore surface area Sp can also be expressed as a function of the same variables:44,45

Sp )

(

[(

) ])

4Vp 2 c ) 4 (2/31/2d - bd)F -1 1/2 wd 2/3 - bd/d

-1

(4)

Using eqs 3 and 4, one can investigate the influence of (i) the interplanar spacing and (ii) the pore wall thickness on the pore volume and surface area of MCM-41. Let us assume that the interplanar spacing and the primary mesopore volume for a given sample are equal to 5.50 nm and 1.00 cm3/g, respectively, which are close to those reported for MCM-41 obtained using the hydrothermal restructuring in the mother liquor at 423 K for 24 h.33 On the basis of eqs 1, 2, and 4, one can determine the pore diameter, pore wall thickness, and primary mesopore surface area to be 5.53 nm, 0.82 nm, and 723 m2/g. As shown in our previous experimental study,16 the surface areas of primary mesopores obtained from the geometrical considerations (i.e., on the basis of experimental Vp and d values using eq 4) were about 10-15% lower than the primary mesopore surface areas estimated using the Rs-plot method (equivalent to the BET specific surface area). In addition, MCM-41 samples have certain external surface areas, but the latter is usually quite low (below 100 m2/g) for materials obtained in high-temperature conditions.33 Therefore, one could expect SBET of about 800900 m2/g for a sample with the structural properties described above. This value is quite typical for MCM-41 materials with a similar d spacing.16,33,34 Let us assume now that the interplanar spacing is equal to 5.5 nm, but the pore wall thickness is 2.68 nm, which is the value reported by Cheng et al. for an MCM-41 material with a similar d spacing.35,36 Using eqs 3 and 4, Vp ) 0.2 cm3/g and Sp ) 194 m2/g (the expected SBET is in the range from 210 to 320 m2/g) are obtained, which are exceptionally low. Even in the case of a wall thickness of 1.5 nm, Vp and Sp would be equal to only 0.51 cm3/g and 421 m2/g, and the expected SBET would be 460-580 m2/g. The considerations presented above strongly indicate that since such low surface areas were not reported for MCM-41 samples described in refs 35 and 36, it is very likely that the pore wall thicknesses were significantly overestimated, the pore sizes were underestimated, and the actual

properties of the obtained MCM-41 materials were quite similar to those obtained from other high-temperature synthesis methods.32-34 It should be noted that Cheng et al. calculated the pore sizes from desorption branches of nitrogen adsorption isotherms using the standard form of the Kelvin equation, which significantly underestimates sizes of small mesopores (see ref 16 and references therein). 3.3. Similarity of synthesis conditions for large-unit-cell MCM-41. As already discussed in our previous work,42 the three reported methods for the synthesis of large-unit-cell MCM-41 in the presence of cetyltrimethylammonium surfactants and without addition of swelling agents have several common features. In each case, good-quality materials were obtained at 423 K or higher using synthesis gels containing tetramethylammonium ions (TMA+), the presence of which was found by Corma et al.34 to be essential. The synthesis of large-unit-cell materials was also possible in the presence of TEA+ or Na+, but it was noted that the use of Na+ led to less stable MCM41. Other researchers also obtained MCM-41 with enlarged unit cells under high-temperature conditions (heating at 413 or 423 K for 2 days) in the presence of Na+ and other cations, but the observed unit-cell enlargement was rather small (d ) 4.6 and 4.85 nm).52 In several other cases, high-temperature (above 423 K) syntheses or postsynthesis treatments of CTMA+-templated MCM-41 prepared without TMA+ also did not give rise to highquality large-pore materials. Depending on their conditions, these experiments led to (i) retention of MCM-41 structure without appreciable unit-cell enlargement,53 (ii) formation of large-unit-cell materials with low degrees of structural ordering,54 or (iii) transition to MCM-4855 or to lamellar structures and, subsequently, to zeolitic phases.56 Hydrothermal restructuring in water at 423 K was also shown to be unsuitable for the preparation of good-quality large-pore MCM-41 despite the fact that the unit-cell expansion was actually observed.42 In contrast, a literature survey showed that MCM-41 silicas prepared at high temperature in the presence of CTMA+ and TMA+ often exhibited high structural ordering1,2,37,57-59 and relatively large interplanar spacings (values up to 5.0 nm were reported in refs. 57-59). Comparison of the results of different studies reported so far suggests that the rate of unit-cell expansion increases as the TMA+/CTMA+ molar ratio in the synthesis gel increases.42 Moreover, it was demonstrated33-36 that an increase in temperature accelerates the unit-cell enlargement. It can be concluded that the presence of TMA+, its relative amount and temperature appear to have a crucial influence on the formation of largepore MCM-41. It is important to mention here that as early as in 1995, a high-temperature (423 K) procedure was reported for the synthesis of MCM-41 silicas and aluminosilicates using the CTMA+ surfactant and an appreciable amount of TMA+ (TMA+/CTMA+ molar ratio of about 1).37 The purely siliceous MCM-41 prepared using this procedure was reported to exhibit the pore size of about 4 nm and its XRD spectrum indicated the (100) interplanar spacing of about 4-4.5 nm. XRD spectra for the aluminosilicate materials had (100) peaks at similar 2Q angles, suggesting similar unit-cell sizes. However, soon after, this synthesis procedure was found by one of us60 to often afford good-quality MCM-41 materials with significantly larger unit cells than those reported in ref 37. Shown in Figure 1 (upper frame) is a TEM image of one of such pure-silica MCM-41 samples (denoted here as Si-MCM-41). A clear honeycomb structure can be seen with distance between the pore centers of about 6 nm. The electron diffraction pattern shown in Figure 1 (lower frame) confirms the occurrence of a well-ordered

4594 J. Phys. Chem. B, Vol. 103, No. 22, 1999

Kruk et al.

Figure 2. Nitrogen adsorption isotherms and pore size distributions for Si-MCM-41 and Al-MCM-41.

Figure 1. Transmission electron micrograph of Si-MCM-41 (upper frame) and the corresponding electron diffraction pattern (lower frame).

hexagonal structure. In agreement with TEM data, the nitrogen adsorption isotherm featured a pronounced hysteresis loop at relative pressures above 0.4 (see Figure 2), which is characteristic of large-pore MCM-41.16,18,33 Similarly, an aluminosilicate sample with a Si/Al molar ratio of 25 (denoted as Al-MCM41) exhibited a hysteresis loop at somewhat lower relative pressure. Si-MCM-41 and Al-MCM-41 were found to exhibit the following characteristics: (100) interplanar spacings of 5.7 and 5.0 nm, respectively; primary mesopore diameters wd (calculated using eq 1) equal to 5.7 and 4.9 nm; pore wall thicknesses of about 0.85 nm (eq 2); BET specific surface areas of about 900 m2/g; and primary mesopore volumes of 0.97 and 0.87 cm3/g, respectively. Both samples had narrow pore size distributions (Figure 2) and their low-pressure adsorption data indicated the absence of any detectable amount of micropores. Good-quality MCM-41 materials with enlarged pores were also obtained using dodecyl- and tetradecyltrimethylammonium surfactants.60 The structural properties described above are similar to those of MCM-41 synthesized using either one-step

procedure34 or the postsynthesis hydrothermal restructuring in the mother liquor.33 Based on our current knowledge, this is no longer surprising, since the synthesis conditions of ref 37 (high temperature and high TMA+/CTMA+ ratio) were favorable for the formation of large-pore materials. It is also interesting to note that the structural parameters presented here for our SiMCM-41 and Al-MCM-41 were obtained for the samples, which have been stored in the calcined form for about 3 years. During such a storage, some of our MCM-41 materials made under more common conditions (343-373 K) suffered considerable loss of structural uniformity, which manifested itself in a decrease in specific surface areas, pore sizes, and pore volumes, broadening of pore size distributions and enhancement of lowpressure adsorption. The lack of structural degradation during storage of the samples synthesized via the one-step hightemperature procedure indicates their facile hydrothermal stability, which is likely to be similar to MCM-41 materials synthesized using the hydrothermal restructuring in the mother liquor at 423 K. The latter were shown to exhibit remarkable stability during treatment in water at 373 K42 and can be kept in calcined form for at least 2 years without any noticeable changes in their structures, as inferred from detailed nitrogen adsorption studies. It should be noted that it is not clear why, in contrast to the original study, the synthesis procedure of Luan et al.37 led in our hands to large-pore MCM-41 materials. The only difference in preparation appears to be the degree of ion-exchange of CTMACl to CTMAOH, which was not specified by these authors. Nevertheless, we demonstrated that large-pore MCM41 materials very similar to those obtained using the hydrothermal restructuring in the mother liquor can be synthesized using a one-step high-temperature procedure. 3.4. Influence of DMHA Formation and TMA+ Incorporation on the Formation of Large-Pore MCM-41. As was already discussed, in the case of the postsynthesis hydrothermal restructuring32,33 and the one-step syntheses (refs 34-36 and the current study), the formation of large-unit-cell MCM-41 using CTMA+ takes place under similar conditions (temperature, composition of the synthesis gel) and the obtained materials exhibit very similar structures. In both procedures, the formation of the large-pore materials is kinetically controlled as small pores are initially obtained and then a gradual unit-cell enlargement takes place, paralleled by a pore size increase.32-36 On the basis of all these similarities, it is likely that the mechanism of the unit-cell expansion is essentially the same for all these processes, despite the striking differences in the mechanisms of formation

Pore Size Expansion in Mesoporous Silicas proposed in the original papers and in the structural properties reported therein.32,34-36 The hypothesis about the common nature of the unit-cell-enlargement processes allowed us to reexamine the data reported by others and greatly facilitated our studies, providing new insights into these fascinating phenomena. On the basis of our results and the literature data, we suggest that there are two crucial factors, which influence the final structure of MCM-41: (i) partial decomposition of CTMA+ leading to the formation of DMHA and (ii) incorporation of TMA+ cations, probably accompanied by counterions. 3.4.1. Amine Formation and Its Swelling Action. In their onestep synthesis of large-unit-cell MCM-41, Cheng et al. reported the formation of DMHA and suggested that the amine works as an expander.35,36 In contrast, Khushalani et al.32 used solution 1H NMR after solvent extraction of the occluded organic material and found no evidence of surfactant decomposition. Using 13C CP MAS NMR, we confirmed the occurrence of DMHA in our noncalcined large-pore MCM-41 materials obtained using the postsynthesis hydrothermal treatment.25 In addition, two independent experimental approaches were designed to provide straightforward evidence that DMHA is a highly efficient expander.25 A brief account is reported here for illustration, while detailed results will be published in a future paper.43 The first approach was based on application of different amines as swelling agents during direct synthesis of large-pore materials at low temperature, e.g., 343 K.25 The same gel composition as in the postsynthesis hydrothermal restructuring method (1 SiO2:0.33 TMAOH:0.17 CTMABr:0.17 NH4OH:17 H2O), was used, but different amines, such as DMHA, trioctylamine (TOA), and tridodecylamine (TDDA), were also added. The amine/CTAB ratio was varied from 0 to 1 for DMHA and was equal to 0.5 for TOA and TDDA. The synthesis gel was heated at 343 K for 3 days. The obtained materials had pore sizes up to 7.7 nm and pore volumes up to 1.7 cm3/g. It is worth noting that the high temperature needed to generate DMHA via partial decomposition of CTMA+ was no longer required. The second approach demonstrated that amine is capable of penetrating the surfactant-filled channels of a preformed MCM-41 material and expanding them.25 Thus a standard noncalcined MCM-41 sample with 3.5 nm pores was treated at temperatures between 343 and 403 K for different periods of time in the presence of an emulsion of 0.5 g of amine in 30 g of water. Depending on the conditions used and the nature of the amine, materials with pore sizes up to as high as 11 nm and exceptional pore volumes up to 2.4 cm3/g were obtained. Since DMHA was proven to be a powerful expander and it was shown to be formed during the high-temperature syntheses, it is very likely that the formation of amine is a crucial factor in the synthesis of CTMA+-templated large-pore MCM-41 materials. The mechanism of CTMA+ decomposition and DMHA formation is not fully understood, but it most likely involves a direct or indirect reaction of CTMA+ with OH- ions present in the mother liquor, finally producing DMHA and CH3OH. The rate of formation of DMHA is probably very low at temperatures below about 423 K and dramatically increases at temperatures above this limiting value. This would explain the strong influence of temperature on the unit-cell enlargement. It is interesting to note that under our experimental conditions, partial decomposition of CTMA+ gave rise to DMHA, while thermal decomposition of the template, for instance during the thermogravimetric analysis of as-synthesized MCM-41, was usually attributed to base-catalyzed Hoffmann elimination that would lead to the formation of trimethylamine and hexa-

J. Phys. Chem. B, Vol. 103, No. 22, 1999 4595 decene.61,62 If hexadecene is formed in addition to DMHA, it is also expected to be an expander, since linear hydrocarbons are known to be good swelling agents.23 3.4.2. Incorporation of TMA+. As was already discussed, the high-temperature unit-cell-enlargement processes have usually been observed in the synthesis gels containing TMA+. A small amount of TMA+ was found even in the micelles of MCM-41 material, which did not undergo any appreciable unit-cell expansion and the content of TMA+ was shown to increase during the unit-cell enlargement in the high-temperature direct synthesis procedure.34 The elemental analysis data42 for the hydrothermal restructuring in the mother liquor32,33 indicated that the carbon/nitrogen molar ratio (denoted as C/N) decreased as the unit-cell expansion proceeded. For example, C/N was 18 for MCM-41 with 3.4 nm pores prepared at 343 K without further treatment, while for MCM-41 with 6.5 nm pores obtained via hydrothermal treatment at 423 K for 2 days C/N decreased to 16.4. It should be noted that these results were very similar to those reported by Corma et al.34 for the one-step synthesis, since the CTMA+ and TMA+ contents determined in their work allow one to estimate C/N of 18 and 16.2 for MCM-41 materials with pore sizes of 3.9 and 6.6 nm, respectively. The observed variations of C/N during the hydrothermal restructuring can partially be related to decomposition of CTMA+ (C/N ) 19) to DMHA (C/N ) 18), if the other decomposition product, most likely CH3OH, leaves the surfactant micelles. However, as the C/N decrease is larger than that expected in the latter case and it is known that the presence of TMA+ significantly affects the course of the process, one can expect that TMA+ is incorporated into the micelles during the hydrothermal restructuring in the mother liquor,42 similarly to the high-temperature synthesis of Corma et al.34 Thus, TMA+ incorporation into the MCM-41 channels appears to be a common feature for high-temperature unit-cell-enlargement processes in the presence of TMA+. A complete explanation of the reasons of TMA+ incorporation and the role of these ions in the unit-cell enlargement will require further studies, but two major possibilities can be envisioned, which are not mutually exclusive and either or both of them may account for the observed phenomenon. First, TMA+ may migrate into the channels to accompany OH- counterions, which participate in the surfactant decomposition to DMHA, as described above. In such a case, TMA+ would be retained in the channels to provide the charge compensation for the framework tSisO- species, which once interacted with already decomposed CTMA+ ions. Second, TMA+ may replace CTMA+ on the silica-surfactant interface, as suggested by Corma et al.,34 either before or after CTMA+ decomposition. The first scenario implies the incorporation of TMA+ with OHcounterions, and the second scenario is also likely to involve the incorporation of counterions in the micelles for the purpose of charge compensation. To this end, Khushalani et al.32 detected bromine in hydrothermally restructured materials using the EDX technique. This finding was not supported by our semiquantitative analysis of as-synthesized, washed, and dried materials using X-ray photoelectron spectroscopy (XPS). Only Si, O, C, and N species were detected. Since the depth of analysis for XPS is of the same order of magnitude as the d100 distance, the composition calculated from XPS data was, within (10%, identical to the bulk composition determined by elemental analysis. It is thus inferred that our samples were actually bromine free, provided bromine was not removed during filtration or washing of the sample. It should be noted that the absence of bromine for as-synthesized nonrestructured MCM41 is consistent with the S-I+ mechanism for the formation of

4596 J. Phys. Chem. B, Vol. 103, No. 22, 1999 silica-surfactant mesophase under basic conditions using a cationic surfactant. The increase in the content of water in as-synthesized hydrothermally restructured materials was reported by Khushalani et al.32 However, this increase was supported neither by TG-MS studies of Corma et al.,34 nor by our high-resolution TGA data. It also needs to be noted that a decrease in the amount of CTMA+ per gram of noncalcined material was observed during the one-step synthesis of Corma et al.,34 but it was relatively small when TMA+ was used in the synthesis gel and may be mainly caused by an increase in the mass of the silicasurfactant phase due to the incorporation of TMA+, possibly accompanied by counterions. So, the depletion of CTMA+ from the micelles is likely to be negligible in this case. 3.5. Structural Changes during the Unit-Cell Expansion. 3.5.1. Silica-Surfactant Ratio during the Unit-Cell Enlargement. The formation of DMHA and its action as an expander appears to be a crucial factor in the unit-cell-expansion process.25 Let us assume at this point that there is no appreciable solubilization of the silica framework and the silica/surfactant molar ratio (or more precisely, silica/(CTMA+ and DMHA) molar ratio) remains constant during the process. This assumption is well-supported by several previous studies. The silica/ surfactant ratio was shown to be approximately constant in studies of Khushalani et al.32 Almost constant silica/surfactant ratio can also be inferred from data of Corma et al.34 for the one-step synthesis in the presence of TMA+. Moreover, Zhou and Klinowski63 made a similar assumption in their mechanistic considerations. Although dissolution/redeposition of silica may take place to some extent on the external surface of the MCM41 particles,33 there are several reasons why the unit-cellenlargement process is unlikely to involve an extensive dissolution/redeposition. First, the unit-cell expansion is gradual,32-36 so the possibility of formation of a hexagonal phase with larger d spacing by dissolution of the initially present hexagonal phase with lower d spacing can be ruled out as noted elsewhere.32 Second, it is difficult to envision the pore size enlargement via dissolution/redeposition in an array of hexagonal pores. Note that in such a system, dissolution may increase the pore size but will not increase the interplanar spacing. Similarly, redeposition of silica inside the cylindrical channels would lead to lowering of the pore diameter and would not increase the d spacing, unless the redeposition is accompanied by some complicated changes in the structure of silicate species which already form the framework. Finally, thermogravimetric studies of MCM-41 samples at different stages of the hydrothermal restructuring in the mother liquor indicate that the surfactant/ silica mass ratio is relatively constant during the unit-cell enlargement. For instance, the ratio of the weight loss at temperatures between 373 and 623 K (related mainly to the loss of the surfactant and its decomposition products, if present) to the residue at 1273 K (mostly SiO2) was found to be 0.97, 0.99, and 0.92 for washed and dried, but noncalcined 3.9, 4.3, and 5.5 nm MCM-41 materials prepared via the hydrothermal restructuring in the mother liquor for the same MCM-41 sample synthesized at 343 K. Assuming the constancy of the silica/ surfactant ratio and no significant dissolution/redeposition of silicate species,32 one can arrive at several interesting conclusions regarding structural changes of MCM-41 materials during the unit-cell enlargement. These conclusions are outlined below. 3.5.2. Pore Volume Increase and Its Origin. The unit-cell enlargement during the hydrothermal restructuring process is relatively large (up to about 60% for noncalcined materials).33 Thus, one can expect a similar increase in the diameter of the

Kruk et al. surfactant micelles. The corresponding increase in the volume of the micelles appears to be much smaller for the following reason. The experimentally observed increase in the volume of mesoporous channels of MCM-41 does not exceed about 50%, as can be inferred from detailed adsorption studies.16,33 Most of this increase can be attributed to decreased shrinkage of the structure during calcination, which was observed in several studies.32,33,35,36 The shrinkage is typically about 15% before and may be less than 5% after the restructuring in the mother liquor MCM-41 (see, for instance, ref 33). The lower degree of shrinkage during calcination was attributed to an increased degree of framework condensation after the high-temperature synthesis/restructuring,32,35,36 although in one of the studies the degree of framework condensation was reported to be approximately constant.34 If one assumes that the shrinkage is the same in directions parallel and perpendicular to the channels and the difference in decrease in d spacing after calcination is about 10%, the corresponding pore volume increase would be about 33%. So, the actual increase in the volume of the micelles during the restructuring may be fairly small (most likely below 20%) and can probably be attributed to (i) incorporation of TMA+ and its counterions, if the latter actually enter the micelles; and (ii) thermal expansion (increase in volume) of the liquid crystalline surfactant phase resulting from a considerable difference between the temperature of preparation of the mother samples (343 K) and that of the hydrothermal treatment (423 K). 3.5.3. Surface Area Decrease and Its Origin. Let us assume that the original shape of the cross-section of the micelles (probably circular) is retained during the restructuring process (this assumption is not crucial but will facilitate the discussion). Due to the increase in the diameter of the micelles, the area of their cross-section in the direction perpendicular to the channels increases by a factor up to 2.5. The corresponding increase in the volume of the micelles does not exceed 20%, as discussed above. So, such a significant increase in the cross-sectional area and a rather small increase in the volume of the micelles can be accomplished if the length of the micelles decreases during the process. Likewise, it can be shown that the outer surface area of the micelles should decrease to some extent during such changes. This in turn is likely to lead to a decrease in the area of the siliceous walls, which confine the micelles. The decrease in the primary mesopore surface area was actually reported in studies of the hydrothermal restructuring of MCM-41 in the mother liquor.33 Considerable lowering of the specific surface area was also observed during the one-step high-temperature syntheses.34-36 It is interesting to note here that decomposition of CTMA+ and the resulting depletion of headgroups from the surface of the micelles is expected to result in lowering of the outer surface of micelles. Since the charged headgroups interact with the silica surface, the decrease in the outer surface of micelles would most likely be accompanied by a decrease in the surface area of the MCM-41 channels. The decrease in the outer surface area may be less pronounced, if CTMA+ cations are to some extent substituted at the surfactant-silica interface by TMA+ cations. As was discussed above, the decrease in the surface area of MCM-41 pores was actually observed to accompany the pore size enlargement during the postsynthesis hydrothermal restructuring in the mother liquor.33 In contrast, the treatments of MCM-41 materials in aqueous emulsions of amines usually involved a much smaller decrease in the primary mesopore surface area, despite much larger pore size enlargement.25 This may be due to the fact that in the course of the amine treatment,

Pore Size Expansion in Mesoporous Silicas amine molecules needed for the pore size expansion are introduced externally and the decomposition of CTMA+ is not required to generate them. Hence, the amount of CTMA+ ions and consequently the area of the outer surface of the micelles may not change appreciably despite a considerable increase in the diameter of the micelles. 3.5.4. Hypothesis of Widening and Thinning Process. It is also important to note that the cross-sectional area (in the plane perpendicular to the channels) of the walls of MCM-41 particles increases during the unit-cell enlargement, especially as the process is accompanied by some thickening of the pore walls, as shown elsewhere.33-36 If one assumes that the density of the siliceous framework of noncalcined materials does not decrease during the unit-cell enlargement (decreased shrinkage during calcination would suggest that the framework density for noncalcined samples actually increases), the volume of the framework is not expected to increase. Thus, because of mass balance requirements, the expansion of the framework in the plane perpendicular to the channels would have to result in shrinkage of the structure in the direction parallel to these channels. As was already discussed, the pore volume changes during the unit-cell expansion indicated a decrease in the length of micelles present in the MCM-41. Obviously, due to charge balance requirements, these two processes must take place simultaneously, since the channels are expected to have essentially the same length as the micelles confined in them. So, regarding processes considered here, the unit-cell enlargement is suggested to lead to widening and thinning of MCM-41 particles. Scanning electron micrographs provided some evidence that particle thinning takes place during hydrothermal restructuring in the mother liquor. However, due to strong effects of many experimental parameters on particle morphology, these structural changes are difficult to analyze on the basis of SEM data. 3.6. Proposed Mechanism of the Unit-Cell Enlargement. On the basis of the discussion presented above, the following mechanism of the high-temperature unit-cell enlargement is proposed. The main driving force of the process is the in situ generation of DMHA, which acts as an expander and is primarily responsible for the formation of large-pore materials. This is accompanied by migration of TMA+ (probably with OHcounterions) to the channels of MCM-41. The swelling action of the amine leads to an increase in diameter of the micelles, thus increasing the pore size of the resulting materials. The expansion process does not involve any appreciable solubilization of the silicate framework of MCM-41. The framework itself transforms in accord with the increased diameter of the micelles, which is likely to proceed via widening and thinning of particles of the material. As discussed in our previous study,33 at a certain stage, the unit cell ceases to expand and the structure of MCM-41 begins to deteriorate. Microporosity develops,33 which is likely to be related to breakage of parts of pore walls.32 The pore volume dramatically drops, the constrictions in the porous structure develop, and the surface area decreases considerably. The XRD spectra become more and more featureless, indicating loss of structural ordering of the treated materials. It can be concluded that the formation of good-quality largepore MCM-41 requires changes in the diameter of the micelles, but at the same time the silicate framework must be flexible enough to undergo the expansion process. It seems that high temperature (about 423 K) makes the framework more prone to the structural changes, which is also clearly illustrated by the postsynthesis pore size enlargement using amines.25 At the

J. Phys. Chem. B, Vol. 103, No. 22, 1999 4597 same time, the elevated temperature is required to generate the expander DMHA, responsible for the pore size enlargement. 3.7. Implications for the Mechanism of the Postsynthesis Unit-Cell Enlargement in Water at 373 K. Our study may provide some insight into unit-cell-expansion processes, which take place in water at 373 K for MCM-41 synthesized using surfactant mixtures.18 The mechanism of the pore size enlargement in this case was also not fully understood and we suggest that it might be related to the decomposition of a certain fraction of molecules of one or both of the surfactants used to the corresponding amines. Alternatively, since the unit-cell enlargement in such conditions was shown to take place only in the presence of geminal or divalent surfactants,18 some of the molecules of these surfactants may undergo a transformation of one of their headgroups from ammonium ion to the corresponding amine group, which in turn would lead to the formation of surfactant molecules with one headgroup and a single very long chain ([CnH2n+1N(CH3)2C3H6](CH3)3N+) or bichain surfactant molecules with one very long chain ([CnH2n+1N(CH3)2CsH2s](CnH2n+1)(CH3)2N+). So, the decomposition of one or both of the surfactants to the corresponding amines and their subsequent swelling action or the formation of one-headgroup surfactant molecules with extremely long chains might be responsible for the pore size enlargement observed during the hydrothermal restructuring in water at 373 K. 4. Conclusions Due to the ordered structure of MCM-41, the volume and surface area of ordered pores are expected to be functions of the unit-cell size and the pore size. Therefore, it is highly unlikely that MCM-41 samples with similar unit-cell parameters, surface areas, and pore volumes would have significantly different pore sizes and pore wall thicknesses. It is thus inferred that the structures of large-unit-cell materials obtained at high temperature32-36 are likely to be similar. This was confirmed by experimental studies of large-pore materials synthesized both directly or via the postsynthesis hydrothermal restructuring. The hypothesis about the common nature of the unit-cell-expansion processes for CTMA+-templated MCM-41 materials allowed us to reexamine the previous, often contradictory, results reported in the literature and helped us develop a comprehensive picture of these fascinating and potentially very useful phenomena. The in situ generation of DMHA was suggested to be the driving force of these unit-cell-enlargement processes. The latter were also related to the presence of TMA+ cations, which are likely to penetrate the channels of MCM-41 or even replace some of the CTMA+ at the silica-surfactant interface. The decreased shrinkage during calcination is likely to be primarily responsible for pore volume increase and to contribute to the pore size increase. There is also evidence that during the pore size expansion process, the MCM-41 particles simultaneously widen in the plane perpendicular to the pore axis and shrink along this axis. The hypothesis about the amine involvement in the pore size enlargement processes was directly confirmed by the successful synthesis of large-pore MCM-41 under mild conditions (343 K) using DMHA and other amines as expanders. Moreover, a new synthesis procedure based on the postsynthesis treatment of as-synthesized MCM-41 in aqueous emulsions of amines afforded large-pore materials with narrow pore size distributions, pore sizes up to 11 nm, mesopore volumes up to as high as 2.4 cm3/g, and usually disordered structures. Acknowledgment. The authors are grateful to Dr. A. Adnot (U. Laval) for XPS data and Mr. Y. Yang (U. Laval) for XRD

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