Article pubs.acs.org/cm
On the Shelf Life and Aging Stability of Mesoporous Silica: Insights on Thermodynamically Stable MCM-41 Structure from Assessment of 12-Year-Old Samples Beatrice Adeniran and Robert Mokaya* School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom S Supporting Information *
ABSTRACT: The aging characteristics and stability of decade (12 years) old mesoporous silica MCM-41 samples of varying pore size and wall thickness has been tracked and compared with freshly synthesized samples. It was found that calcined forms of hydrothermally synthesized MCM-41 samples are, depending on their initial wall thickness, generally stable to storage under ambient conditions over the 12 year period. The calcined samples retain all (i.e., 96−100%) of their surface area and virtually all (87−96%) their pore volume. Rather surprisingly, calcined MCM-41 with wall thickness of ca. 11 Å exhibit greater apparent aging stability with respect to retention of unit-cell parameter compared to thicker walled (22 Å) analogues. After 12 years of storage, both types of samples exhibit pore wall thickness of 11− 12 Å representing much greater unit cell contraction for the 22 Å sample. Despite varying levels of initial silica condensation and silanol concentration, both types of aged calcined samples exhibit similar 29Si NMR spectra with similar silanol concentration (ca. 6.5 mmol/g) and silanol density (4.5/nm2). The wall thickness, level of silica condensation, silanol loading, and silanol density of the calcined aged samples closely match values that have been shown by molecular dynamics studies to have the highest stabilization energy for MCM-41 structures. The aged calcined samples are thermally stable to further calcination at 800 °C, wherein they retain 90% of their surface area. Aged as-synthesized forms of MCM-41 also have high stability and remain largely unchanged, and on calcination, behave in a manner similar to that of freshly prepared MCM-41. For aged as-synthesized samples, there is an apparent increase in silica condensation and phase separation of template molecules from the silica, and this forms the basis of their aging stability and stability to subsequent calcination. KEYWORDS: mesoporous, MCM-41 silica, shelf life, aging stability, thermal stability, wall thickness
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INTRODUCTION The discovery of mesoporous silica MCM-41 served as a ground breaking solution to the pore size limitations of zeolites. Zeolites, which represent the largest group of ordered porous materials, are widely used in industry, most especially as heterogeneous catalysts and adsorbents.1 Zeolites possess micropores with sizes that are comparable to molecular dimensions that enable adsorption and transformation of small molecular and ionic species. 1 However, zeolite interactions exclude bulkier molecules that require pores in the mesopore (20−500 Å) range. In this regard, mesostructures such as MCM-41 have attracted attention due to their mesoporosity and amenability to pore size control within the mesopore range. Mesoporous silica MCM-41 is thus potentially useful in many industrial processes such as, in heterogeneous catalysis as catalyst or catalyst support,2 adsorbent,3 and ion exchanger.4 It is therefore necessary for the MCM-41 to be stable in a wide range of conditions and environments. Although MCM-41 has been reported to have high thermal stability, it has low hydrothermal stability in aqueous solutions, limiting its suitability for functionalizations in aqueous media.5 This is due to the amorphous pore wall of MCM-41 and the fact that the large mesoporous surface area means that the pore © 2012 American Chemical Society
walls are readily accessible to the detrimental effects of water/ steam, leading to collapse of the walls by silicate hydrolysis and loss of mesoporosity and surface area. Efforts have been directed toward improving the stability of MCM-41 by increasing the wall thickness, for example, via longer synthesis time,6 secondary crystallization,6 use of salt effects during hydrothermal crystallization process7 and via hydrothermal restructuring methods.8 Furthermore, MCM-41 is unstable toward mineralizing agents such as hydroxides and fluorides that dissolve silica,9 and thus has limited use under alkaline conditions. The instability of MCM-41 also raises the question of shelf life. Although much work has been done to improve the thermal and hydrothermal stability of mesoporous silicas,10 there are, as far as we know, no reports that specifically address the question of aging stability, behavior, and shelf life of mesoporous silicas. Burleigh and co-workers observed that mesoporous silica SBA-15 stored under ambient conditions in open vials for 10 months did not undergo any significant Received: September 7, 2012 Revised: October 23, 2012 Published: October 25, 2012 4450
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structural changes.11 They ascribed this to the fact that SBA-15 silicas have thick pore walls that are thus relatively stable to water induced hydrolysis.11 On the other hand, Broyer and coworkers reported a substantial decrease in total pore volume for calcined MCM-41 materials after aging under ambient conditions for 3 months.12 The difference in the aging stability between SBA-15 and MCM-41 is thought to be due to the thinner walls of the later. In considering the aging stability of MCM-41 silicas it is necessary to take into account the effect of wall thickness and level of silica condensation. While thicker and more condensed walls have been shown to improve the thermal and hydrothermal stability of MCM-41, their effect on aging stability is not known. In particular, it is not known whether MCM-41 materials will age toward a thermodynamically stable structure with a given pore size and wall thickness. Several theoretical studies have hinted at the existence of a size defined thermodynamically stable wall thickness and structure for MCM-41.13−16 On the basis of molecular dynamics simulations, Kleestorfer and co-workers concluded that there exists an optimum range for the pore size and wall thickness for the most thermodynamically stable MCM-41 (i.e., with the lowest lattice energy), and suggested that the optimum wall thickness for MCM-41 with a pore diameter of 35 to 50 Å should be between 8 and 12 Å.13 Similarly, Feuston and Higgins studied model structures for MCM-41 using classical molecular dynamics simulation techniques and concluded that structures with wall thicknesses less than 7 Å are unstable, and that stable materials should have 17−28% of the silicon as silanols.14 Thus despite the known advantages of thicker pore walls (i.e., typically >12 Å) to the thermal and hydrothermal stability of MCM-41, it is of interest to find out whether MCM41 materials that possess walls that are thicker than the theoretically suggested optimum wall thickness are more stable to aging compared to those with optimum sized pores wall. Aging properties are an important consideration with respect to any application of MCM-41 type materials especially in uses where long-time stability is essential. This report presents a study of 12-year-old MCM-41 samples and is aimed at understanding their stability and suitability for use in applications requiring long-term stability or where knowledge of shelf life is necessary. The study has been conducted by comparing freshly prepared as-synthesized and calcined MCM41 with older (12-year-old) samples in order to assess the shelf life and aging stability of MCM-41 under ambient conditions. The old and freshly prepared samples were calcined at a range of temperatures (recalcined in the case of samples stored in their calcined form) to evaluate their thermal stability.
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MCM-41 sample designated as NEWMCM-41. The aged (OLDMCM-41) samples had spent the past 12 years on an open benchtop in conventional sample vials. The full range of samples (freshly prepared and aged) assessed in this study is as follows; NEWMCM-41-as: as-synthesized sample freshly prepared at 150 °C for 48 h. NEWMCM-41: calcined freshly prepared MCM-41 (i.e., NEWMCM-41-as after calcination). OLDMCM-41-1: as-synthesized 12-year-old sample prepared at 150 °C for 48 h. OLDMCM-41-2: calcined 12-year-old sample prepared at 150 °C for 48 h. OLDMCM-41-3: calcined 12-year-old sample prepared at 150 °C for 48 h. OLDMCM-41-4: as-synthesized 12-year-old sample prepared at 150 °C for 96 h. OLDMCM-41-5: calcined 12-year-old sample prepared at 150 °C for 96 h. Stability Assessment. The stability of the samples was assessed in two ways: (i) analysis of the “as-is” old samples and comparison with the freshly prepared samples, and (ii) calcination of the old samples at 500 or 800 °C and comparison with freshly prepared samples calcined at similar temperature. Characterization Techniques. Powder XRD analysis was performed using a PANalytical X’Pert Pro diffractometer with CuKα radiation (λ = 1.5406 Å) and operating at 40 kV and 40 mA, a step size of 0.026° and 48 s per step. To determine porosity, the samples were first heated at 200 °C and degassed under vacuum. Nitrogen sorption isotherms and textural properties were then determined at −196 °C using a Micromeritics ASAP 2020 volumetric sorptometer. The surface area was calculated using the BET method based on adsorption data in the relative pressure (P/Po) range of 0.06−0.22, total pore volume was obtained from the amount of nitrogen adsorbed at P/Po of 0.99 and pore size distribution was assessed via a non-local density functional theory (NLDFT) method using nitrogen adsorption data. Thermogravimetric analysis was performed using a TA Instruments SDT Q 600 analyzer under flowing air conditions. Thermal analysis was carried out by measuring the mass change with increase in temperature (at a heating ramp rate of 10 °C/min.) up to 1000 °C. 29Si MASNMR spectra were acquired with silicon-29 frequency of 79.44 MHz, acquisition time of 20−40 ms, total spectral width of 40 kHz, recyle delay of 30− 60 s, and a MAS rate of 6.8 kHz.
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RESULTS AND DISCUSSION Structural Ordering and Stability of Calcined Samples. The powder XRD patterns of calcined forms of the freshly prepared and old MCM-41 samples are shown in Figure 1. The XRD pattern of the freshly prepared MCM-41 (NEWMCM-41) is typical of well-ordered hexagonal MCM41, showing an intense (100) peak corresponding to basal (d100) spacing of 39.3 Å as given in Table 1. The XRD pattern of sample NEWMCM-41 also shows higher order (110), (200), and (210) peaks. Calcined forms of 12-year-old MCM-41 samples (OLDMCM-41-2 and OLDMCM-41-3), which were previously prepared at 150 °C and 48 h (i.e., conditions identical to those of NEWMCM-41), exhibit XRD patterns that are similar to that of sample NEWMCM-41. The XRD patterns of old samples OLDMCM-41-2 and OLDMCM-41-3 show a high-intensity basal (100) peak and good resolution of the (110) and (200) peaks, suggesting that long-range order in these calcined MCM-41 samples is retained over a 12 year storage period. The old samples, OLDMCM-41-2 and OLDMCM-41-3, have d100 spacing of 39.6 and 39.8 Å, respectively (Table 1), which is slightly smaller (by ca. 7%) than that observed (i.e., 42.8 Å) when they were freshly prepared.6 This suggests that the mesostructural ordering of
EXPERIMENTAL SECTION
Material Synthesis. The freshly prepared samples were synthesized using a procedure similar to that used for the old samples.6 In brief, tetramethylammonium hydroxide, TMAOH (2.49 g, 0.027 mol), and cetyltrimethylammonium bromide, CTAB (6.16 g, 0.017 mol) were mixed in distilled water at 35 °C until a clear homogeneous solution was formed. Fumed silica (4 g, 0.067 mol) was added portion wise to the solution while stirring. Upon complete addition (ca. 1.5 h), the mixture was left stirring at room temperature for a further 1 h. The resulting gel was aged for 20 h at room temperature, and then transferred to a Teflon-lined autoclave and heated at 150 °C for 48 h. The solid obtained was recovered by filtration, washed with distilled water, and air-dried at 60 °C for 2 days to give as-synthesized MCM-41 designated as NEWMCM-41-as. Some of the freshly prepared as-synthesized (NEWMCM-41-as) sample was calcined in air at 550 °C for 6 h to yield the calcined 4451
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On the other hand, OLDMCM-41-5, which was prepared via crystallization for 96 h rather than 48 h, exhibits a d100 peak at a lower 2θ value corresponding to d100 spacing of 48 Å, which is significantly lower than that observed (60 Å) for the sample 12 years ago.6 It therefore appears that although the calcined samples retain mesostructural integrity, there is an apparent lattice contraction of varying magnitude during the 12 years of storage. Nevertheless, after 12 years, samples synthesized for 48 h still have a smaller unit-cell parameter, ao (where ao = 2d100√3), of ca. 46 Å compared to that crystallized for 96 h (unit-cell parameter of ca. 56 Å), which is in agreement with previous reports on the effects of longer synthesis time.6,17 The basal spacing of the old samples (OLDMCM-41-2 and OLDMCM-41-3) is ca. 7% lower than when they were freshly prepared 12 years ago,6 whereas that of OLDMCM-41-5 is 20% lower. This apparent contraction in unit-cell parameter, which occurs over a long period of time, may be caused by reorganization of the silica walls to generate a thermodynamically stable structure. In pure silica MCM-41, both internal and external surfaces of the amorphous silica walls possess silanol (Si−OH) groups as revealed by NMR and IR studies of calcined MCM-41 samples.18−20 It is also known that calcined MCM-41 samples stored at room temperature can interact with and retain water. Indeed, it has been shown that calcined MCM-41 evacuated at room temperature exhibits an IR band at 1685 cm−1, which is ascribed to adsorbed water.19 Silanol groups can undergo self-condensation to give siloxanes,18−20 and therefore, because the old samples were not kept under vacuum over the 12 year period, it is possible that their silica pore walls have reorganized to generate intrinsically stable framework that retains mesostructural ordering over the long aging period. It is likely that the rearrangement of the silica framework causes the varying lattice contraction (7% for OLDMCM-41-2 and OLDMCM-41-3, and 20% for OLDMCM-41-5) and also apparently generates more uniformly ordered MCM-41 samples according to the increased intensity of the higher order (110), (200), and (210) peaks (Figure 1). The nitrogen sorption isotherms of calcined forms of the freshly prepared (NEWMCM-41) and 12-year-old MCM-41
Figure 1. Powder XRD patterns of calcined freshly prepared (NEWMCM-41) and old MCM-41 samples. See Experimental Section for sample designation.
Table 1. Textural Properties of Calcined Forms of Freshly Prepared (NEWMCM-41), 12-Year-Old MCM-41 Samplesa sample
d100 spacing (Å)
surface area (m2g−1)
pore volume (cm3/g)
pore sizeb (Å)
wall thicknessc (Å)
NEWMCM-41 FMCM-41-2/3 OLDMCM-41-2 OLDMCM-41-3 FMCM-41-5 OLDMCM-41-5
39.3 42.8 39.6 39.8 60.0 48.0
1270 918 914 887 700 855
1.17 0.89 0.77 0.85 0.79 1.01
34.5 38.0 34.5 35.0 47.0 43.5
10.9 11.4 11.2 11.0 22.3 11.9
a
For comparison we have included data obtained 12 years ago when the samples (designated as F-MCM-41-X) were freshly prepared. b Pore size obtained from NLDFT pore size analysis. cWall thickness is calculated from the equation, wall thickness = ao − pore Size(DFT) (where ao = the unit-cell parameter obtained from formula, ao = 2d100/√3)
these calcined old MCM-41 samples remained generally stable during storage at ambient conditions over a period of 12 years.
Figure 2. Nitrogen sorption isotherms (left) and pore size distribution (right) of calcined freshly prepared (NEWMCM-41) and old MCM-41 samples. For clarity the isotherms are offset (y-axis) by 150 (OLDMCM-41-3), 400 (OLDMCM-41-5), and 550 (NEWMCM-41). 4452
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OLDMCM-41-3 is ca. 11 Å, which is similar (within experimental error) to when the samples were fresh.6 Overall, therefore, the change in wall thickness and pore size for OLDMCM-41-2 and OLDMCM-41-3 are small and may be considered as being within experimental error. Nevertheless, the fact that the pore walls are still ca. 11 Å thick after 12 years hints at the possibility that this value represents the wall thickness of an inherently stable MCM-41 framework.13−16 On the other hand, for sample OLDMCM-41-5 the decrease in the lattice parameter is much larger at 20%, but again with only a small decrease in pore size. Thus the decrease in lattice parameter is mainly due to contraction of the pore wall thickness from ca. 22 Å6 to 11.9 Å. This observation is surprising since based on current understanding of the stability of MCM-41, the thicker walled sample would have been expected to be more stable and therefore to undergo lesser contraction. The fact that the aged samples with vastly different initial wall thickness have, after 12 years, settled at a pore wall thickness of ca. 11 Å suggests that such a pore wall is representative of a thermodynamically stable value for MCM41 structures. It is important to note that wall thickness of 11 Å agrees well with pore wall thickness of energetically stable MCM-41 structures obtained from molecular dynamics simulations.13−16 The rearrangement of the silica walls in the aged samples to generate MCM-41 structures that possess pore wall thickness of ca. 11 Å is likely caused by interaction with water wherein eventually an equilibrated (energetically stable) framework is reached with respect to wall thickness, amount of water and level of silica condensation. Thermal analysis of the three aged samples (see the Supporting Information, Figure S3) indicated (from the mass loss up to 120 °C) that the aged samples contained similar amounts of water at 7−8 wt %, whereas a freshly prepared sample contains only 2%. Clearly, the calcined samples took up, interacted with and equilibrated with water during the 12 year period. To assess the extent of silica condensation in the aged calcined samples we performed Si magic angle spinning (MAS) nuclear magnetic resonance (NMR). The 29MASNMR spectra, shown in Figure 3, exhibit three resonances at −90 ppm (Q2), −100 ppm (Q3), and −109 ppm (Q4) with the later two being the main resonances. The resonances are due to silicon in Si(OSi)2(OH)2 (Q2), Si(OSi)3OH (Q3), or Si(OSi)4 (Q4)
(OLDMCM-41-2 and OLDMCM-41-3) samples are shown in Figure 2. All the samples exhibit an isotherm that is typical for well ordered MCM-41. The sorption isotherms of the 12-yearold samples are remarkably similar to that of the freshly prepared NEWMCM-41 sample. More remarkable, however, is the fact that the isotherms of samples OLDMCM-41-2 and OLDMCM-41-3 are virtually identical to the isotherm taken 12 years ago when the samples (designated as FMCM-41-2/3) were freshly prepared (see the Supporting Information, Figure S1).6 Thus the old samples appear to have retained their mesoporosity as indicated by the presence of the characteristic “mesoporous” step in the relative pressure (P/Po) range of 0.35−0.5 (Figure 2 and the Supporting Information, Figure S1). This adsorption step indicates nitrogen pore filling into a uniform mesopore system and shows that any structural change to the calcined MCM-41 samples over the 12 year period was not at the expense of pore size uniformity. As shown in Table 1, the old samples retain high surface area (914 m2/g for OLDMCM-41-2 and 887 m2/g for OLDMCM-41-3) and high pore volume (0.77 cm3/g for OLDMCM-41-2 and 0.85 cm3/g for OLDMCM-41-3), which is typical of well-ordered MCM-41 and very close to the textural values (918 m2/g and 0.89 cm3/ g)6 when the samples were freshly prepared 12 years ago. It is thus clear that 12 year storage has not had any detrimental effect on the overall surface area and pore volume of the calcined MCM-41 samples. The pore size distribution (PSD) curves of the samples are given in Figure 2 and confirm retention of narrow PSD. The pore size distribution of the 12year-old samples (OLDMCM-41-2 and OLDMCM-41-3) is identical to that of the similarly prepared fresh NEWMCM-41. It is particularly noteworthy that the pore size of OLDMCM41-2 and OLDMCM-41-3 (i.e., ca. 35 Å) is only slightly smaller than what it was 12 years ago (see the Supporting Information, Figure S1, and Table 1).6 The isotherm of OLDMCM-41-5 is also characteristic of a well ordered sample and exhibits hysteresis which is typical of MCM-41 samples with pores larger than 40 Å.21 Furthermore, the isotherm of the OLDMCM-41-5 sample is very similar to the isotherm taken 12 years ago when the sample (designated as FMCM-41-5) were freshly prepared (see the Supporting Information, Figure S2).6 The larger pores in the OLDMCM-41-5 sample are a result of a longer (96 h) crystallization time, and have been retained after 12 years of storage. The 43.5 Å pore size of OLDMCM-41−5 is only slightly lower than when the sample was freshly prepared (see the Supporting Information, Figure S2, and Table 1).6 Thus the aged calcined samples, regardless of the initial pore size, generally retain their pore size over the 12 year period, undergoing only very small reductions. If one considers that the small reductions are within experimental error, then at best the aged calcined samples retain their pore size very well. It is noteworthy that rather than decreasing, the surface area and pore volume of OLDMCM-41-5 increase over the 12 year period; surface area by 22% from 700 to 855 m2/g, and pore volume by 28% from 0.79 to 1.01 cm3/g.6 The likely reasons for these increase in textural properties are discussed later. It is interesting that retention (or increase) in surface area and pore volume in the aged calcined samples is accompanied by hardly any decrease in pore size along with more significant decreases in basal (d100) spacing and unit-cell parameter (ao). A decrease in lattice parameter for MCM-41 materials can be due to pore size reduction, pore wall contraction, or both. As shown in Table 1, the pore wall thickness of OLDMCM-41-2 and
Figure 3. 29Si MAS NMR spectra of 12-year-old OLDMCM-41-2 and OLDMCM-41-3 (prepared at 150 °C for 48 h) and OLDMCM-41-5 (prepared at 150 °C for 96 h). Samples were stored in calcined form. 4453
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environments. The 29Si MAS NMR spectra indicated a similar level of silica condensation for all three aged calcined samples despite the fact that OLDMCM-41−5 was initially much more highly condensed.6 It is likely that the NMR spectra indicate an equilibrated level of silica condensation for MCM-41. Furthermore, the silanol concentration (in mmol/g) calculated from the NMR spectra22 was comparable at 6.8, 7.3, and 6.1 for OLDMCM-41-2, OLDMCM-41-3, and OLDMCM-41-5, respectively. We note that such a silanol loading (6.1−7.3 mmol/ g), when associated with pore size of 35−42 Å and wall thickness of 11 Å has been shown by molecular dynamics studies to have the highest stabilization energy for MCM-41 structures.13 Clearly the silanol loading of the aged samples is higher than that normally observed for dehydrated calcined MCM-4123 due to equilibration with water that occurs over the 12 year period. The silanol density of the three samples, i.e., 4.5/nm2 for OLDMCM-41-2, 4.9/nm2 for OLDMCM-41-3 and 4.3/nm2 for OLDMCM-41-5 is virtually identical and is in the expected range for highly stable MCM-41 structures predicted by molecular dynamics studies.14 We note that the silanol concentration of the aged samples is similar to that of vitreous or glassy (amorphous) silica. The overall scenario suggested by our data is that, during the prolonged 12-year storage, the calcined MCM-41 samples approach a stabilized structure with optimized wall thickness and level of silica condensation. The changes that sample OLDMCM-41−5 undergoes are remarkable as there is a significant decrease in pore wall thickness and level of silica condensation, and an associated increase in silanol concentration. We speculate that the age induced changes involve a rearrangement of the MCM41 walls into a glassy silica-like structure in which the gaps and imperfections known to exist in the walls16,26 are healed/ removed thus generating apparently thinner walls. The contraction of the pore wall, whereas the pore size is largely unchanged may be the genesis of the increase in textural properties for OLDMCM-41−5. To assess the thermal stability of the aged calcined samples, we recalcined OLDMCM-41−3 at 800 °C (see the Supporting Information, Figure S3). We found that even after calcination at 800 °C, the 12-year-old calcined (OLDMCM-41−3) sample still exhibited an XRD pattern with a well-defined basal (100) peak and higher order at (110), (200), and (210) peaks, in a manner similar to that of the freshly prepared (NEWMCM-41) sample (see the Supporting Information, Figure S4). The intensity of the peaks, after calcination at 800 °C, is lower than that of the “as-is” materials. The changes in basal spacing of the two samples (see the Supporting Information, Table 1) are also similar with both samples undergoing a small lattice contraction of ca. 7%. Both samples retain their mesoporosity after calcination at 800 °C, which is testament to the stability of the 12-year-old sample. The nitrogen sorption isotherms of the 800 °C calcined samples (see the Supporting Information, Figure S5), in both cases exhibit a well-developed mesopore filling step. Indeed, the 12-year-old sample is no less stable in this regard as both samples retain a high proportion of their surface area and pore volume after calcination at 800 °C (see the Supporting Information, Table 2). The retention of a uniform pore size distribution (see the Supporting Information, Figure S6) after further calcination at 800 °C is comparable for the new and old MCM-41; the pore size of the NEWMCM-41 reduces from 37 to 31.5 Å, whereas that of the OLDMCM-41-3 reduces from 38 to 30 Å.
Structural Ordering and Stability of As-Synthesized Samples. The XRD patterns of old as-synthesized samples (OLDMCM-41-1 and OLDMCM-41-4) are compared with that of the freshly prepared as-synthesized sample (NEWMCM-41-as) in Figure 4. The freshly prepared
Figure 4. Powder XRD patterns of as-synthesized freshly prepared (NEWMCM-41-as) and old MCM-41 samples. See Experimental Section for sample designation.
NEWMCM-41-as and OLDMCM-41−1 samples, which were prepared at crystallization temperature of 150 °C for 48 h, show similar XRD patterns except for the appearance of a new peak at 2θ ∼ 7 for the later. The samples also exhibit relatively similar basal spacing of 42.7 Å for NEWMCM-41-as and 43.2 Å for OLDMCM-41-1. On the other hand, OLDMCM-41-4 (which was prepared at 150 °C for 96 h) exhibits a pattern typical of a well-ordered MCM-41 but with a peak at lower 2θ value (corresponding to basal spacing of 54.5 Å). The XRD patterns in Figure 4 indicate that as-synthesized MCM-41 samples retain their structural ordering over a 12 year period. This is the first time that such long-term stability has been documented. Furthermore, the higher basal spacing of the sample prepared for 96 h (compared to samples synthesized for 48 h) is retained after storage for 12 years.6 In an attempt to identify the origin of the XRD peak at ∼7° 2θ for sample OLDMCM-41-1, we performed thermogravimetric analysis (TGA) of the as-synthesized samples. The TGA curves shown in Figure 5 indicate a mass loss between 150 and 400 °C, which is due to the burn off of surfactant molecules. The residual mass (%) at ca. 800 °C is 38, 53, and 68 for OLDMCM-41-1, NEWMCM-41-as, and OLDMCM-41-4, respectively. The OLDMCM-41-1 sample therefore appears to have higher “template” content than the similarly synthesized freshly prepared NEWMCM-41-as sample. The ratio between the occluded template (i.e., mass loss between 120 and 400 °C) and residual silica mass at 800 °C, expressed as a CTMA/SiO2 molar ratio (where CTMA is cetyltrimethylammonium), was found to be 0.288, 0.152, and 0.083 for OLDMCM-41-1, NEWMCM-41-as, and OLDMCM-41-4 respectively. The CTMA/SiO2 molar ratios imply that sample OLDMCM-41-1 contains more template molecules per unit weight of assynthesized material than NEWMCM-41-as or when it was newly synthesized more than 12 years ago.6 On the other hand, the CTMA/SiO2 molar ratio of the sample prepared for 96 h 4454
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NEWMCM-41-as sample. The overall compositional effect of such a scenario is an increase in the CTMA/SiO2 ratio as observed. However, as shown in Figure 4, such compositional change does not significantly affect that overall structural integrity of the as-synthesied MCM-41 sample over the 12 year period. Further evidence for some disengagement of CTMA from the silica walls in aged OLDMCM-41−1 was provided by nitrogen sorption data (see the Supporting Information, Figure S8); the aged as-synthesized sample was found to have a surface area on ca. 150 m2/g compared to
