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MCM-69, a novel layered analogue of EU-19
Microporous and Mesoporous Materials 53 (2002) 179–193 www.elsevier.com/locate/micromeso
MCM-69, a novel layered analogue of EU-19 L. Deane Rollmann, John L. Schlenker, Stephen L. Lawton, Carrie L. Kennedy, Gordon J. Kennedy * Exxon Mobil Research and Engineering, Corporate Strategic Research, 1545 Route 22 East, Annandale, NJ 08801, USA Received 3 January 2002; accepted 22 February 2002
Abstract A new form of crystalline silica has been prepared and is designated MCM-69. The material has a crystalline layered structure which can be delaminated and which contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the MCM-69 structure contains a silanol group. To maintain layer separation, considerable care must be taken in the preparation of MCM-69 to avoid dehydration of these silanols and inadvertent crosslinking of the silica layers, which would prevent subsequent delamination. Stoichiometrically, MCM-69 has the idealized formula, H2 O 6SiO2 , and is derived from MCM-69(P), a piperazine (pipz) silicate precursor. MCM-69(P) appears to have the same topology as EU-19. MCM-69(P) differs from EU-19 in its behavior, most notably in the removal of its pipz to yield MCM-69, and its subsequent dispersability. MCM-69 can be swollen and dispersed in aqueous solution by reacting the silanol groups with amines, such as pyrrolidine, n-propylamine, or n-octylamine. The resultant products all show crystallinity as characterized by a strong X-ray diffraction (XRD) peak below 10.5° 2h. The exact position of the low-angle peak indicates the size of the amine controls the interlayer separation. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. The silanol groups react with hydroxide ion and with surface functionalizing agents such as hexamethyldisilazane. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: MCM-69; EU-19; Crystalline silica; Crystalline layered; Delamination
1. Introduction Crystalline layered silicates and aluminosilicates (or, more accurately, polysilicate and polyalumino-silicate hydrates, since the surfaces contain extensive hydroxyl groups) are increasingly recognized, both as they occur in nature and as they * Corresponding author. Tel.: +908-730-2606; fax: 908-7303314. E-mail address: [email protected] (G.J. Kennedy).
are encountered in exploratory zeolite crystallization. The silicates are perhaps best known in the form of kenyaite, magadiite, makatite, octosilicate, kanemite, KHSi2 O5 , EU-19, MCM-20, MCM-25, RUB-18, MCM-47 [1–7]; the latter, for example, as PRE-FER (precursor to ferrierite) and MCM56 [8–11]. Whereas the structure of a crystalline layered aluminosilicate is often inferred from that of the zeolite or family of zeolites to which it is related, the structures of only five of the crystalline layered silicates are known, makatite, KHSi2 O5 , EU-19
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 3 8 - 4
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[1,4], RUB-18 [6], and MCM-47 [7]. The first two are comprised of relatively flexible, single-layer nets of six-membered rings of SiO4 tetrahedra, each Si sharing three O’s with a neighbor and each containing one unshared surface hydroxyl group. The two differ in the ordering of the hydroxyls as they protrude from each side of the net and in the cations which hold the respective layers apart, sodium in the case of makatite vs. potassium in KHSi2 O5 . The third polysilicate, EU-19 (Edinburgh University-19), has a much more robust structure, comprised of double silicate layers based on building blocks containing five- and six-membered rings of SiO4 tetrahedra, so-called 52 61 and 52 62 structural units [12]. In EU-19, one SiO4 in three has an unshared surface oxygen whose negative charge is balanced by a protonated piperazine (pipz) molecule. As depicted in Fig. 1, protonated pipzs hold the layers apart and are presumably the driving force leading to the formation and isolation of this unique polysilicate [4]: Formally, EU-19 should be called a piperazinium silicate. The literature synthesis of EU-19 required 10– 70 days at 150 °C, and numerous attempts to remove the pipz molecules from between the layers without calcination reportedly failed [3]. Calcination of EU-19 yielded a crystalline material of apparently unknown structure, EU-20, which had a complete lack of sorptive properties [3]. In the exploratory zeolite crystallization experiments described below, facile synthesis is reported
Fig. 1. Silicate sheets and intralayer pipz molecules in EU-19/ MCM-69(P).
of a material which appears to be isostructural with EU-19, and which we have called MCM69(Precursor). MCM-69(P) differs from EU-19 most notably in that the pipz molecules can be fully removed, to yield a new, crystalline form of silica, namely, MCM-69 [13]. The structure of MCM-69 is believed to contain individual doublelayer sheets of SiO4 tetrahedra and ordered, reactive surface hydroxyls. Given the presence and importance of the surface hydroxyls, MCM-69 should formally be called a silica hydrate, having the idealized formula 6SiO2 H2 O.
2. Experimental The silica source for all crystallizations was Ultrasil VN3SP (92.4% SiO2 , 0.1% Al2 O3 , and 0.4% Na2 O), available from United Silica, Industrial, Ltd., Taiwan or Degussa. Where Al2 O3 was added to a reaction mixture, the alumina source was 47% NaAlO2 (25.5% Al2 O3 , 19.5% Na2 O). Crystallizations were conducted at 160 °C, usually for 65 h, stirred at 200 rpm. Notably, the pH of successfully crystallized reaction mixtures was typically about 13. Products were washed with water and dried briefly at 120 °C. The washing and drying steps were minimized in order to avoid inadvertent and undesired crosslinking of the silica sheets. Details of some representative crystallizations are in Table 1. NMR spectra (29 Si MAS, 29 Si CP/MAS, and 13 C CP/MAS) were recorded on a Bruker AMX200 spectrometer equipped with a 7.5 mm Chemagnetics Pencilâ probe. 39.75 MHz 29 Si MAS NMR spectra were obtained using 4 kHz sample spinning and a 60 s pulse delay. 39.75 MHz 29 Si CP/MAS NMR spectra were obtained using 4 kHz sample spinning, a contact time of 3.5 ms, and a 1.5 s pulse delay. 50.22 MHz 13 C CP/MAS NMR spectra were obtained using a contact time of 1.5 ms, a 3 s recycle delay, and a spinning rate of 3.5 kHz. TMS was used as an external chemical shift standard. Consistent with data reported for EU-19 [4], the average composition of over 20 MCM-69(P) preparations (see Table 1) was 4.3 (H2 pipz)O 24SiO2 . The low, variable Na content in the
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Table 1 Crystallizations with pipz at 160 °C for 65 h with SiO2 /Al2 O3 ¼ 1000 Run
Na/Si
R/Si
pHi
pHf
OH/Si
Product
Na/uc
R/uc
A B C D E F G H I J K L M N O P Q R S T U V W
0.06 0.06 0.06 0.06 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
12.5 12.6 12.7 12.6 13.3 13.2 13.0 13.1 13.2 13.1 13.2 13.2 13.2 13.2 13.2 13.1 13.3 13.1 12.9 12.9 12.8 13.1 13.4
12.6 12.7 12.5 12.8 13.0 12.9 13.1 13.2 13.1 13.2 13.2 13.1 13.2 13.2 13.2 13.2 13.2 13.2 13.2 13.1 12.9 13.0 12.9
0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P)
0.3 0.3 0.4 0.2 0.1 0.2 0.1 0.3 0.4 0.5
3.9 3.9 5.0 4.9 4.9 4.4 3.7 3.8 3.8 3.9
0.2
4.1
0.4 0.1 0.5 0.4 0.9 1.0 0.2
3.9 3.8 5.0 5.1 4.2 6.7 4.6
Table 1 products indicates that Na can be viewed as a contaminant in these products.
, Present, for MCM-69(P)––C2/c, a ¼ 13:783 A b ¼ 4:905 A, c ¼ 22:866 A, b ¼ 91:63° (Differences in the c-dimension probably reflect differing degrees of hydration).
3. Results and discussion 3.1. Synthesis and characterization of the precursor, MCM-69(P) Table 1 shows that MCM-69(P) could be synthesized repeatedly and reproducibly. That it is probably isostructural with EU-19 by X-ray diffraction (XRD) analysis is apparent from Fig. 2. The bottom XRD pattern in Fig. 2 was taken from the literature and is that of an EU-19 sample prepared in the absence of Na ion [3], supporting the assertion made above that any Na in these MCM-69(P) products was adventitious. For comparison, the unit cell parameters published for EU-19 and those used in calculating the simulated pattern in Fig. 2 were as follows: , Published, for EU-19 [4]––C2/c, a ¼ 13:57 A b ¼ 4:90 A, c ¼ 22:46 A, b ¼ 91:67°
The 13 C CP/MAS NMR spectrum of as-synthesized MCM-69(P) shows a single resonance at 42.5 ppm corresponding to the chemically equivalent methylenes. For comparison, the spectrum of solid pipz shows a single peak at 44.5 ppm and the spectra of aqueous solutions of neutral and protonated pipz show single peaks at 46.9 and 44.1 ppm, respectively. Thus, the single peak at 42.5 ppm and the 2 ppm upfield shift in the 13 C CP/ MAS NMR spectrum of as-synthesized MCM69(P) suggests that the pipz is intact and protonated. 13 C NMR of the acid-washed material described below, i.e. MCM-69, did not detect any pipz (or any other organic species). 3.2. Synthesis and characterization of MCM-69 The difference between a sample which would be designated MCM-69(P) and one which would
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Fig. 2. XRD patterns of EU-19 (bottom) and MCM-69(P) (experimental in middle and simulated at top).
be EU-19 was in their respective behaviors. As will be shown below, the differences were twofold: (a) Pipz can be substantially removed from MCM-69(P) by a single washing with a variety of acids, such as HCl and HNO3 in concentrations ranging from 0.5 to 6.0 M, to yield MCM-69. By contrast, it was reported that repeated washing of EU-19 with 0.1 M HCl or Al2 (SO4 )3 removes only some of the pipz from EU-19 [3]. (b) After acid-washing of MCM-69(P) and filtration, the wet product, MCM-69, exhibited a defined XRD pattern and was clearly a crystalline material. By contrast, it was reported that acid-washing of EU-19 causes the structure to collapse to a product with a few broad (XRD) peaks [3].
It is speculated that the differing behavior of these two apparently isostructural materials is believed to be a partial crosslinking of the polysilicate layers in EU-19, due either to long crystallization times, to inadequate pipz, or to excessive drying, a distinction which would not be readily apparent in an XRD pattern. The shorter c, relative to dimension of EU-19, 22.46 A , is consistent with partial MCM-69(P), 22.866 A crosslinking in EU-19. Drying detail was absent from the EU-19 literature, but two separate TGA examples were published to affirm that EU-19 does not contain any loosely bound or occluded water. By contrast, the MCM-69(P) samples were intentionally not dried severely, nor were they extensively water-washed. On average, the various MCM-69(P) samples contained 2–3% free water and, as indicated in Section 2, had a pipz content
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per unit cell of 4.3, slightly in excess of the theoretical 4.0. Washing MCM-69(P) with a variety of acids and acid concentrations yielded MCM-69, a crystalline product having a 5–10 lm particle morphology and the XRD pattern shown in Fig. 3. Minor variations in the XRD patterns were observed for different preparations and were attributed to variations in the drying step. Whereas sample K, after pipz removal with 1 M HCl, and sample V, treated with 6 M HCl, could be readily and fully dispersed in aqueous solution, sample W, treated with 1 M HCl, was slightly less readily dispersed. W had been double-water-washed and more extensively dried after crystallization and had apparently undergone some partial crosslink-
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ing of the layers. Among the samples analyzed, treatment with 6 M HCl in one case removed 98% of the pipz; in another, 89%, the difference being attributed to a small degree of inadvertent crosslinking in the latter. Both products showed a strong XRD peak at about 9.5° 2h. By contrast, the 9.5° peak disappeared on simple drying of MCM-69 at 120 °C and did not reappear on treatment of the dried sample with 6 M HCl. The examples in Fig. 3 were prepared with 0.5 M HCl. Other samples of MCM-69 were prepared using 1, 6, and 12 M HCl, as well as 6 M HNO3 , all without difficulty. The structure of MCM-69 is not completely understood. Based on similarity with EU-19 [4] and as discussed in the introduction, it is
Fig. 3. XRD patterns of MCM-69 samples prepared from washing MCM-69(P) samples K (top) and W (bottom) with 0.5 N HCl.
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Fig. 4. Si MAS (bottom) and CP/MAS NMR (top) spectra of MCM-69(P) (left) and MCM-69 (right). The MCM-69 was prepared by washing sample K with 0.5 N HCl.
believed to be comprised of a double-layer sheet of SiO4 tetrahedra having ordered, reactive surface hydroxyls and substantial interstitial water. The presence of interstitial water is confirmed by weight difference upon filtration and drying of the wet solid. For example, one test sample was found to contain about 70% water upon filtration––along, possibly, with residual HCl. Isolated as a wet solid, the silica sheets stack to give a lowangle XRD reflection at 9.3–9.6° 2h. The 29 Si MAS and CP/MAS NMR spectra of MCM-69(P) and MCM-69 (i.e. sample K before and after treatment with 0.5 N HCl), shown in Fig. 4, confirm the presence of a regular array of silanols. These spectra consist of resonances centered at 100 and 110 ppm from TMS corresponding to Q3 Si species, containing one silanol, and Q4 Si, fully sharing all four oxygens with other Si atoms. The cross-polarization enhancement of the 100 ppm peak confirms the silanol assignment. The sharpness of these resonances are indicative of a regular,
crystalline environments for all the Si species. Integration of the MAS spectrum of MCM-69 indicates that 32% (one in three) of the SiO4 tetrahedra in MCM-69 were Q3 species and 68% (the other two) were Q4 Si species. A depiction of the MCM-69 sheet and surface is shown in Fig. 5, with one out of every three SiO4 tetrahedra drawn to contain a surface hydroxyl, as indicated by NMR. The distance between the linear silanol oxygens ; that between oxygens in is approximately 4.9 A . adjacent rows, about 7.2 A What is perhaps not apparent from Fig. 5 is the fact that two possibilities exist for the stacking of these MCM-69 sheets––and stacking determines the XRD pattern. Specifically, the sheets (viewed end-on) can stack with an –A–A–A–A– or an –A–B–A–B– projection, as shown in Fig. 6, wherein A and B are mirror images of one another (in this case, sheets rotated 180° one with respect to the other). In EU-19, the published sequence was –A–A–A–A–.
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Fig. 5. Surface silanols in the proposed MCM-69 structure.
The XRD pattern of MCM-69 with the –A–A– sequence is expected to be similar to that of EU-19/MCM-69(P) without the interlayer organic present. Shown in Fig. 7 is the experimental XRD pattern of MCM-69, prepared by washing MCM69(P) sample K with 1.0 N HCl., and the simulated XRD pattern for the –A–B– sequence. Although not a perfect match, the closer similarity of the experimental MCM-69 XRD pattern with the simulated XRD pattern for the –A–B– sequence than with the XRD pattern of EU-19/MCM-69(P) suggests that the stacking sequence in MCM-69 is probably not –A–A– and may be –A–B–. Further
work is necessary to better define the stacking sequence in MCM-69. 3.3. MCM-69 swelling and dispersion with amines If, as proposed, MCM-69 is comprised of silica sheets having regularly spaced surface silanols, it should be possible to exfoliate those sheets in a variety of ways. The simplest would be to replace the bridging pipz molecules with a non-bridging mono-amine, e.g., n-propylamine (nPA), pyrrolidine (pyrr), hexamethyleneimine (hmi), n-octylamine (nOA), or n-dodecylamine (nC12A).
Fig. 6. Possible sheet sequences for the MCM-69 family of structures.
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Fig. 7. Experimental (bottom) and calculated (top) XRD patterns for MCM-69. The MCM-69 sample was prepared from washing MCM-69(P) sample K with 1.0 N HCl.
The first and smallest of the amines, nPA, showed both a remarkable dispersing ability and a clearly enhanced separation of the MCM-69 silanol sheets. Treatment of a suspension of MCM-69 (acid-washed K, about 20 g, or 0.1 mol of hydroxyls) in 100 ml water with an excess of nPA (about 60 g, or 1 mol) yielded a dispersion which passed substantially through a coarse (40– 60 lm) glass filter frit, obvious evidence of the dispersing capability of the amine. Evidence that nPA was intercalated between the layers and presumably bound to the silanol surface was provided by XRD analysis. As shown in Fig. 8, the low-angle reflection in the XRD pattern (the
0 0 2 reflection) was displaced from 9.5 in MCM-69 to 8.2° 2h, corresponding to an increase in d. The shift in this 0 0 2 spacing from 9.3 to 10.7 A reflection to lower angle on addition of nPA directly reflected an increase in the separation between the silica sheets in MCM-69. In brief, nPA replaced the smaller H2 O (i.e., nPAHþ replaced H3 Oþ ). That process was reversible. If nPA molecules were intercalating between and separating the silica sheets, then a larger amine should yield a larger separation and a larger XRD d-spacing (i.e., a shift in the low-angle peak to even lower 2h). That expectation was confirmed, first with pyrr, then with hmi, and finally with
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Fig. 8. XRD patterns for MCM-69 in the Hþ (bottom) and nPAHþ (top) forms.
nOA. The nPA was removed by washing with 0.5 N HCl whereupon the XRD pattern returned to that of MCM-69, and the sample was then slurried overnight in a mixture of 100 g H2 O and 40 g pyrr (about 0.5 mol, pH 13.2). As shown in Fig. 9, the lowest-angle XRD peak with pyrr was at about 4.8° 2h, as compared with 8.2° in the case of nPA. The corresponding d-spacings were 18.4 and 10.7 , respectively. A Analogously, when pyrr was replaced with hmi, the sheet separation was even greater, and the XRD pattern showed an even lower-angle peak, at d-spacing. about 4.4° corresponding to a 20.0 A
To generate the hmi sample shown in Fig. 9, the pyrr was removed with 0.5 N HCl (yielding, once again, the XRD pattern of MCM-69), and the sample was then slurried overnight in a mixture of 100 g H2 O and 50 g hmi (0.5 mol, pH 12.3). The XRD patterns of both the pyrr and the hmi samples in Fig. 9 show at least two low-angle reflections, reflections which are attributed to an excess of amine and to more than one stacking arrangement for the silica sheets. Note in both XRD patterns that there was more than one set of 0 0 2–0 0 4 sequences (e.g., for hmi, respective 0 0 2– 0 0 4 reflections at 4.4° and 8.8°, and at 5.0° and
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Fig. 9. XRD patterns for MCM-69 in the hmiHþ (bottom) and pyrrHþ (top) forms.
10.1° 2h). Moreover, as might be expected with these relatively volatile amines, the XRD patterns of these samples changed on standing open to the air or on treatment with a less concentrated amine solution. After standing open to the air for four days to allow excess amine to evaporate, the pyrr sample
showed a new XRD pattern, shown in Fig. 10. The pattern was now characterized by a single lowangle reflection at 7.5° corresponding to d-spacing . Similarly, with hmi, an XRD pattern of 11.8 A containing only a single low-angle reflection was obtained, in this case by using a lower concentration of amine. When a sample of MCM-69 was
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Fig. 10. XRD patterns for MCM-69 in the pyrrHþ form before (top) and after (bottom) standing open in the air for 4 days.
treated with 1 M hmi solution vs. the 4 M solution used to prepare the sample described earlier, the XRD pattern shown at the bottom of Fig. 11 was obtained. In comparison with the earlier XRD pattern, only the lowest-angle reflection, at 4.5° d-spacing), was seen. Elemental analysis (19.6 A showed 0.8 hmi molecules for every three Si atoms, quite close to the 1.0 expected for an idealized MCM-69. 13 C CP/MAS NMR showed the hmi to be protonated.
MCM-69 dispersions with surfactant-molecular-weight amines were also possible, as shown with nOA and nC12A. A 52 g sample of MCM-69(P) (sample V, 0.2 mol of eventual silanol) was converted to the H-form, (i.e., to MCM-69) using 6 M HCl. It was filtered, combined with an excess (0.4 mol) of 3 M nOA in water, and left over a weekend. After filtering, it was combined with 1 l of 1 M nOA. After exhaustive filtration, the product exhibited the XRD patterns shown in Fig. 12, which
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Fig. 11. XRD patterns for MCM-69 in the hmiHþ prepared using 1 M solution (bottom) and excess hmi (top).
were characterized by two strong low-angle reflections, at 5.8°and 8.6° 2h. It analyzed 1.4 nOA molecules per 3 Si’s, vs. the theoretical 1.0 silanols per 3 Si’s. After washing with n-hexane and airdrying, it analyzed 0.8 nOA’s per 3 Si’s. The hexane-washed sample showed two low-angle XRD reflections, at 6.0° and 9.0° 2h. Similarly a sample of MCM-69 was contacted with nC12A in 1:1 ethanol:water. The final, dried product showed
strong XRD peaks at 2.7°, 3.4°, 3.9°, and 6.5° 2h. It analyzed 0.4 nC12A’s per 3 Si’s. 3.4. Ionization of MCM-69 If, as proposed, MCM-69 is comprised of silica sheets having regularly spaced surface silanols, it should be possible to react the silanol protons with hydroxide ion without dissolving the silica. That
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Fig. 12. XRD patterns for MCM-69 in the nOAHþ forms: (a) Initial, (b) after two weeks, and (c) after washing with n-hexane and air drying.
reaction was demonstrated using tetrapropylammonium (TPA) hydroxide. A 25 g sample of wet MCM-69 was slurried with 100 ml water and the pH adjusted to 10.9 by titration with 40% TPAOH. Since the sample contained residual HCl, no stoichiometry calculation was possible. After stirring
for 3 h, the solid was filtered. Its XRD pattern showed a new low-angle reflection, at about 6.2° 2h. When the procedure was repeated, this time with the initial pH adjusted to 12.0 and the stirring time increased to 16 h, the XRD peak at 6.2° in the filtered solid more than doubled in intensity.
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3.5. Surface functionalization of MCM-69 If surface silanols exist as proposed, they should be functionalizable, either to make the silica sheets hydrophobic and dispersible in non-aqueous solvents or to provide for bonding, e.g., to metal ions. The reaction between a silanol group and hexamethyldisilazane (HMDS) is said to be both facile and clean [14] 2Si–OH þ ðCH3 Þ3 Si–NH–SiðCH3 Þ3 ! 2Si–O–SiðCH3 Þ3 þ NH3 Moreover, any water present in the sample would react to yield hexamethyldisiloxane, a liq-
uid, which could be removed by filtration. Since amine should not interfere with the reaction, a preswelled, hmi-intercalated sample of MCM-69 was used. A 20 g sample of wet hmi-MCM-69 (prepared from K) was stirred with a large excess (60 g, about 0.4 mol) of HMDS for 3 days at room temperature, with Drierite protection from the atmosphere. NH3 was evolved. The sample, filtered, washed with acetone, and air-dried, exhibited an XRD pattern (Fig. 13) having low-angle reflections at 5.1°, 6.1°, and 7.7° 2h. The material was strongly hydrophobic, as evidenced by its ready dispersion in n-hexane and toluene and its complete lack of wetting on contact with water.
Fig. 13. XRD pattern for MCM-69 after surface functionalizing with HMDS.
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4. Conclusions A new form of crystalline silica, designated MCM-69, has been prepared that is unique among crystalline layered silica hydrate structures, both in its relative robustness and in the regularity of its silanol surface. MCM-69 is derived from a pipz silicate precursor, denoted MCM-69(P), MCM-69 can be delaminated and contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the structure contains a silanol group. MCM-69 can be exfoliated in aqueous solution by reacting the silanol groups with amines. The resultant products all show crystallinity and are all characterized by a strong XRDS peak below 10.5° 2h. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. Initial experiments suggest that silanol groups react with hydroxide ion and with surface functionalizing agents such as HMDS.
Acknowledgements Thanks for excellent technical assistance in zeolite synthesis and characterization go to J. Sockwell, E.A. Moy, and D. Colmyer. Helpful input on various aspects of this work from D.C. Calabro, A.W. Chester, W.S. Borghard, J.S. Beck, J.C.
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Vartuli, A.Malek, L.S. Kennedy, T.F. Degnan, and B.G. Silbernagel was much appreciated.
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MCM-69, a novel layered analogue of EU-19 L. Deane Rollmann, John L. Schlenker, Stephen L. Lawton, Carrie L. Kennedy, Gordon J. Kennedy * Exxon Mobil Research and Engineering, Corporate Strategic Research, 1545 Route 22 East, Annandale, NJ 08801, USA Received 3 January 2002; accepted 22 February 2002
Abstract A new form of crystalline silica has been prepared and is designated MCM-69. The material has a crystalline layered structure which can be delaminated and which contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the MCM-69 structure contains a silanol group. To maintain layer separation, considerable care must be taken in the preparation of MCM-69 to avoid dehydration of these silanols and inadvertent crosslinking of the silica layers, which would prevent subsequent delamination. Stoichiometrically, MCM-69 has the idealized formula, H2 O 6SiO2 , and is derived from MCM-69(P), a piperazine (pipz) silicate precursor. MCM-69(P) appears to have the same topology as EU-19. MCM-69(P) differs from EU-19 in its behavior, most notably in the removal of its pipz to yield MCM-69, and its subsequent dispersability. MCM-69 can be swollen and dispersed in aqueous solution by reacting the silanol groups with amines, such as pyrrolidine, n-propylamine, or n-octylamine. The resultant products all show crystallinity as characterized by a strong X-ray diffraction (XRD) peak below 10.5° 2h. The exact position of the low-angle peak indicates the size of the amine controls the interlayer separation. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. The silanol groups react with hydroxide ion and with surface functionalizing agents such as hexamethyldisilazane. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: MCM-69; EU-19; Crystalline silica; Crystalline layered; Delamination
1. Introduction Crystalline layered silicates and aluminosilicates (or, more accurately, polysilicate and polyalumino-silicate hydrates, since the surfaces contain extensive hydroxyl groups) are increasingly recognized, both as they occur in nature and as they * Corresponding author. Tel.: +908-730-2606; fax: 908-7303314. E-mail address: [email protected] (G.J. Kennedy).
are encountered in exploratory zeolite crystallization. The silicates are perhaps best known in the form of kenyaite, magadiite, makatite, octosilicate, kanemite, KHSi2 O5 , EU-19, MCM-20, MCM-25, RUB-18, MCM-47 [1–7]; the latter, for example, as PRE-FER (precursor to ferrierite) and MCM56 [8–11]. Whereas the structure of a crystalline layered aluminosilicate is often inferred from that of the zeolite or family of zeolites to which it is related, the structures of only five of the crystalline layered silicates are known, makatite, KHSi2 O5 , EU-19
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 3 8 - 4
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[1,4], RUB-18 [6], and MCM-47 [7]. The first two are comprised of relatively flexible, single-layer nets of six-membered rings of SiO4 tetrahedra, each Si sharing three O’s with a neighbor and each containing one unshared surface hydroxyl group. The two differ in the ordering of the hydroxyls as they protrude from each side of the net and in the cations which hold the respective layers apart, sodium in the case of makatite vs. potassium in KHSi2 O5 . The third polysilicate, EU-19 (Edinburgh University-19), has a much more robust structure, comprised of double silicate layers based on building blocks containing five- and six-membered rings of SiO4 tetrahedra, so-called 52 61 and 52 62 structural units [12]. In EU-19, one SiO4 in three has an unshared surface oxygen whose negative charge is balanced by a protonated piperazine (pipz) molecule. As depicted in Fig. 1, protonated pipzs hold the layers apart and are presumably the driving force leading to the formation and isolation of this unique polysilicate [4]: Formally, EU-19 should be called a piperazinium silicate. The literature synthesis of EU-19 required 10– 70 days at 150 °C, and numerous attempts to remove the pipz molecules from between the layers without calcination reportedly failed [3]. Calcination of EU-19 yielded a crystalline material of apparently unknown structure, EU-20, which had a complete lack of sorptive properties [3]. In the exploratory zeolite crystallization experiments described below, facile synthesis is reported
Fig. 1. Silicate sheets and intralayer pipz molecules in EU-19/ MCM-69(P).
of a material which appears to be isostructural with EU-19, and which we have called MCM69(Precursor). MCM-69(P) differs from EU-19 most notably in that the pipz molecules can be fully removed, to yield a new, crystalline form of silica, namely, MCM-69 [13]. The structure of MCM-69 is believed to contain individual doublelayer sheets of SiO4 tetrahedra and ordered, reactive surface hydroxyls. Given the presence and importance of the surface hydroxyls, MCM-69 should formally be called a silica hydrate, having the idealized formula 6SiO2 H2 O.
2. Experimental The silica source for all crystallizations was Ultrasil VN3SP (92.4% SiO2 , 0.1% Al2 O3 , and 0.4% Na2 O), available from United Silica, Industrial, Ltd., Taiwan or Degussa. Where Al2 O3 was added to a reaction mixture, the alumina source was 47% NaAlO2 (25.5% Al2 O3 , 19.5% Na2 O). Crystallizations were conducted at 160 °C, usually for 65 h, stirred at 200 rpm. Notably, the pH of successfully crystallized reaction mixtures was typically about 13. Products were washed with water and dried briefly at 120 °C. The washing and drying steps were minimized in order to avoid inadvertent and undesired crosslinking of the silica sheets. Details of some representative crystallizations are in Table 1. NMR spectra (29 Si MAS, 29 Si CP/MAS, and 13 C CP/MAS) were recorded on a Bruker AMX200 spectrometer equipped with a 7.5 mm Chemagnetics Pencilâ probe. 39.75 MHz 29 Si MAS NMR spectra were obtained using 4 kHz sample spinning and a 60 s pulse delay. 39.75 MHz 29 Si CP/MAS NMR spectra were obtained using 4 kHz sample spinning, a contact time of 3.5 ms, and a 1.5 s pulse delay. 50.22 MHz 13 C CP/MAS NMR spectra were obtained using a contact time of 1.5 ms, a 3 s recycle delay, and a spinning rate of 3.5 kHz. TMS was used as an external chemical shift standard. Consistent with data reported for EU-19 [4], the average composition of over 20 MCM-69(P) preparations (see Table 1) was 4.3 (H2 pipz)O 24SiO2 . The low, variable Na content in the
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Table 1 Crystallizations with pipz at 160 °C for 65 h with SiO2 /Al2 O3 ¼ 1000 Run
Na/Si
R/Si
pHi
pHf
OH/Si
Product
Na/uc
R/uc
A B C D E F G H I J K L M N O P Q R S T U V W
0.06 0.06 0.06 0.06 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
12.5 12.6 12.7 12.6 13.3 13.2 13.0 13.1 13.2 13.1 13.2 13.2 13.2 13.2 13.2 13.1 13.3 13.1 12.9 12.9 12.8 13.1 13.4
12.6 12.7 12.5 12.8 13.0 12.9 13.1 13.2 13.1 13.2 13.2 13.1 13.2 13.2 13.2 13.2 13.2 13.2 13.2 13.1 12.9 13.0 12.9
0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P) MCM-69(P)
0.3 0.3 0.4 0.2 0.1 0.2 0.1 0.3 0.4 0.5
3.9 3.9 5.0 4.9 4.9 4.4 3.7 3.8 3.8 3.9
0.2
4.1
0.4 0.1 0.5 0.4 0.9 1.0 0.2
3.9 3.8 5.0 5.1 4.2 6.7 4.6
Table 1 products indicates that Na can be viewed as a contaminant in these products.
, Present, for MCM-69(P)––C2/c, a ¼ 13:783 A b ¼ 4:905 A, c ¼ 22:866 A, b ¼ 91:63° (Differences in the c-dimension probably reflect differing degrees of hydration).
3. Results and discussion 3.1. Synthesis and characterization of the precursor, MCM-69(P) Table 1 shows that MCM-69(P) could be synthesized repeatedly and reproducibly. That it is probably isostructural with EU-19 by X-ray diffraction (XRD) analysis is apparent from Fig. 2. The bottom XRD pattern in Fig. 2 was taken from the literature and is that of an EU-19 sample prepared in the absence of Na ion [3], supporting the assertion made above that any Na in these MCM-69(P) products was adventitious. For comparison, the unit cell parameters published for EU-19 and those used in calculating the simulated pattern in Fig. 2 were as follows: , Published, for EU-19 [4]––C2/c, a ¼ 13:57 A b ¼ 4:90 A, c ¼ 22:46 A, b ¼ 91:67°
The 13 C CP/MAS NMR spectrum of as-synthesized MCM-69(P) shows a single resonance at 42.5 ppm corresponding to the chemically equivalent methylenes. For comparison, the spectrum of solid pipz shows a single peak at 44.5 ppm and the spectra of aqueous solutions of neutral and protonated pipz show single peaks at 46.9 and 44.1 ppm, respectively. Thus, the single peak at 42.5 ppm and the 2 ppm upfield shift in the 13 C CP/ MAS NMR spectrum of as-synthesized MCM69(P) suggests that the pipz is intact and protonated. 13 C NMR of the acid-washed material described below, i.e. MCM-69, did not detect any pipz (or any other organic species). 3.2. Synthesis and characterization of MCM-69 The difference between a sample which would be designated MCM-69(P) and one which would
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Fig. 2. XRD patterns of EU-19 (bottom) and MCM-69(P) (experimental in middle and simulated at top).
be EU-19 was in their respective behaviors. As will be shown below, the differences were twofold: (a) Pipz can be substantially removed from MCM-69(P) by a single washing with a variety of acids, such as HCl and HNO3 in concentrations ranging from 0.5 to 6.0 M, to yield MCM-69. By contrast, it was reported that repeated washing of EU-19 with 0.1 M HCl or Al2 (SO4 )3 removes only some of the pipz from EU-19 [3]. (b) After acid-washing of MCM-69(P) and filtration, the wet product, MCM-69, exhibited a defined XRD pattern and was clearly a crystalline material. By contrast, it was reported that acid-washing of EU-19 causes the structure to collapse to a product with a few broad (XRD) peaks [3].
It is speculated that the differing behavior of these two apparently isostructural materials is believed to be a partial crosslinking of the polysilicate layers in EU-19, due either to long crystallization times, to inadequate pipz, or to excessive drying, a distinction which would not be readily apparent in an XRD pattern. The shorter c, relative to dimension of EU-19, 22.46 A , is consistent with partial MCM-69(P), 22.866 A crosslinking in EU-19. Drying detail was absent from the EU-19 literature, but two separate TGA examples were published to affirm that EU-19 does not contain any loosely bound or occluded water. By contrast, the MCM-69(P) samples were intentionally not dried severely, nor were they extensively water-washed. On average, the various MCM-69(P) samples contained 2–3% free water and, as indicated in Section 2, had a pipz content
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per unit cell of 4.3, slightly in excess of the theoretical 4.0. Washing MCM-69(P) with a variety of acids and acid concentrations yielded MCM-69, a crystalline product having a 5–10 lm particle morphology and the XRD pattern shown in Fig. 3. Minor variations in the XRD patterns were observed for different preparations and were attributed to variations in the drying step. Whereas sample K, after pipz removal with 1 M HCl, and sample V, treated with 6 M HCl, could be readily and fully dispersed in aqueous solution, sample W, treated with 1 M HCl, was slightly less readily dispersed. W had been double-water-washed and more extensively dried after crystallization and had apparently undergone some partial crosslink-
183
ing of the layers. Among the samples analyzed, treatment with 6 M HCl in one case removed 98% of the pipz; in another, 89%, the difference being attributed to a small degree of inadvertent crosslinking in the latter. Both products showed a strong XRD peak at about 9.5° 2h. By contrast, the 9.5° peak disappeared on simple drying of MCM-69 at 120 °C and did not reappear on treatment of the dried sample with 6 M HCl. The examples in Fig. 3 were prepared with 0.5 M HCl. Other samples of MCM-69 were prepared using 1, 6, and 12 M HCl, as well as 6 M HNO3 , all without difficulty. The structure of MCM-69 is not completely understood. Based on similarity with EU-19 [4] and as discussed in the introduction, it is
Fig. 3. XRD patterns of MCM-69 samples prepared from washing MCM-69(P) samples K (top) and W (bottom) with 0.5 N HCl.
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Fig. 4. Si MAS (bottom) and CP/MAS NMR (top) spectra of MCM-69(P) (left) and MCM-69 (right). The MCM-69 was prepared by washing sample K with 0.5 N HCl.
believed to be comprised of a double-layer sheet of SiO4 tetrahedra having ordered, reactive surface hydroxyls and substantial interstitial water. The presence of interstitial water is confirmed by weight difference upon filtration and drying of the wet solid. For example, one test sample was found to contain about 70% water upon filtration––along, possibly, with residual HCl. Isolated as a wet solid, the silica sheets stack to give a lowangle XRD reflection at 9.3–9.6° 2h. The 29 Si MAS and CP/MAS NMR spectra of MCM-69(P) and MCM-69 (i.e. sample K before and after treatment with 0.5 N HCl), shown in Fig. 4, confirm the presence of a regular array of silanols. These spectra consist of resonances centered at 100 and 110 ppm from TMS corresponding to Q3 Si species, containing one silanol, and Q4 Si, fully sharing all four oxygens with other Si atoms. The cross-polarization enhancement of the 100 ppm peak confirms the silanol assignment. The sharpness of these resonances are indicative of a regular,
crystalline environments for all the Si species. Integration of the MAS spectrum of MCM-69 indicates that 32% (one in three) of the SiO4 tetrahedra in MCM-69 were Q3 species and 68% (the other two) were Q4 Si species. A depiction of the MCM-69 sheet and surface is shown in Fig. 5, with one out of every three SiO4 tetrahedra drawn to contain a surface hydroxyl, as indicated by NMR. The distance between the linear silanol oxygens ; that between oxygens in is approximately 4.9 A . adjacent rows, about 7.2 A What is perhaps not apparent from Fig. 5 is the fact that two possibilities exist for the stacking of these MCM-69 sheets––and stacking determines the XRD pattern. Specifically, the sheets (viewed end-on) can stack with an –A–A–A–A– or an –A–B–A–B– projection, as shown in Fig. 6, wherein A and B are mirror images of one another (in this case, sheets rotated 180° one with respect to the other). In EU-19, the published sequence was –A–A–A–A–.
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Fig. 5. Surface silanols in the proposed MCM-69 structure.
The XRD pattern of MCM-69 with the –A–A– sequence is expected to be similar to that of EU-19/MCM-69(P) without the interlayer organic present. Shown in Fig. 7 is the experimental XRD pattern of MCM-69, prepared by washing MCM69(P) sample K with 1.0 N HCl., and the simulated XRD pattern for the –A–B– sequence. Although not a perfect match, the closer similarity of the experimental MCM-69 XRD pattern with the simulated XRD pattern for the –A–B– sequence than with the XRD pattern of EU-19/MCM-69(P) suggests that the stacking sequence in MCM-69 is probably not –A–A– and may be –A–B–. Further
work is necessary to better define the stacking sequence in MCM-69. 3.3. MCM-69 swelling and dispersion with amines If, as proposed, MCM-69 is comprised of silica sheets having regularly spaced surface silanols, it should be possible to exfoliate those sheets in a variety of ways. The simplest would be to replace the bridging pipz molecules with a non-bridging mono-amine, e.g., n-propylamine (nPA), pyrrolidine (pyrr), hexamethyleneimine (hmi), n-octylamine (nOA), or n-dodecylamine (nC12A).
Fig. 6. Possible sheet sequences for the MCM-69 family of structures.
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Fig. 7. Experimental (bottom) and calculated (top) XRD patterns for MCM-69. The MCM-69 sample was prepared from washing MCM-69(P) sample K with 1.0 N HCl.
The first and smallest of the amines, nPA, showed both a remarkable dispersing ability and a clearly enhanced separation of the MCM-69 silanol sheets. Treatment of a suspension of MCM-69 (acid-washed K, about 20 g, or 0.1 mol of hydroxyls) in 100 ml water with an excess of nPA (about 60 g, or 1 mol) yielded a dispersion which passed substantially through a coarse (40– 60 lm) glass filter frit, obvious evidence of the dispersing capability of the amine. Evidence that nPA was intercalated between the layers and presumably bound to the silanol surface was provided by XRD analysis. As shown in Fig. 8, the low-angle reflection in the XRD pattern (the
0 0 2 reflection) was displaced from 9.5 in MCM-69 to 8.2° 2h, corresponding to an increase in d. The shift in this 0 0 2 spacing from 9.3 to 10.7 A reflection to lower angle on addition of nPA directly reflected an increase in the separation between the silica sheets in MCM-69. In brief, nPA replaced the smaller H2 O (i.e., nPAHþ replaced H3 Oþ ). That process was reversible. If nPA molecules were intercalating between and separating the silica sheets, then a larger amine should yield a larger separation and a larger XRD d-spacing (i.e., a shift in the low-angle peak to even lower 2h). That expectation was confirmed, first with pyrr, then with hmi, and finally with
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Fig. 8. XRD patterns for MCM-69 in the Hþ (bottom) and nPAHþ (top) forms.
nOA. The nPA was removed by washing with 0.5 N HCl whereupon the XRD pattern returned to that of MCM-69, and the sample was then slurried overnight in a mixture of 100 g H2 O and 40 g pyrr (about 0.5 mol, pH 13.2). As shown in Fig. 9, the lowest-angle XRD peak with pyrr was at about 4.8° 2h, as compared with 8.2° in the case of nPA. The corresponding d-spacings were 18.4 and 10.7 , respectively. A Analogously, when pyrr was replaced with hmi, the sheet separation was even greater, and the XRD pattern showed an even lower-angle peak, at d-spacing. about 4.4° corresponding to a 20.0 A
To generate the hmi sample shown in Fig. 9, the pyrr was removed with 0.5 N HCl (yielding, once again, the XRD pattern of MCM-69), and the sample was then slurried overnight in a mixture of 100 g H2 O and 50 g hmi (0.5 mol, pH 12.3). The XRD patterns of both the pyrr and the hmi samples in Fig. 9 show at least two low-angle reflections, reflections which are attributed to an excess of amine and to more than one stacking arrangement for the silica sheets. Note in both XRD patterns that there was more than one set of 0 0 2–0 0 4 sequences (e.g., for hmi, respective 0 0 2– 0 0 4 reflections at 4.4° and 8.8°, and at 5.0° and
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Fig. 9. XRD patterns for MCM-69 in the hmiHþ (bottom) and pyrrHþ (top) forms.
10.1° 2h). Moreover, as might be expected with these relatively volatile amines, the XRD patterns of these samples changed on standing open to the air or on treatment with a less concentrated amine solution. After standing open to the air for four days to allow excess amine to evaporate, the pyrr sample
showed a new XRD pattern, shown in Fig. 10. The pattern was now characterized by a single lowangle reflection at 7.5° corresponding to d-spacing . Similarly, with hmi, an XRD pattern of 11.8 A containing only a single low-angle reflection was obtained, in this case by using a lower concentration of amine. When a sample of MCM-69 was
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Fig. 10. XRD patterns for MCM-69 in the pyrrHþ form before (top) and after (bottom) standing open in the air for 4 days.
treated with 1 M hmi solution vs. the 4 M solution used to prepare the sample described earlier, the XRD pattern shown at the bottom of Fig. 11 was obtained. In comparison with the earlier XRD pattern, only the lowest-angle reflection, at 4.5° d-spacing), was seen. Elemental analysis (19.6 A showed 0.8 hmi molecules for every three Si atoms, quite close to the 1.0 expected for an idealized MCM-69. 13 C CP/MAS NMR showed the hmi to be protonated.
MCM-69 dispersions with surfactant-molecular-weight amines were also possible, as shown with nOA and nC12A. A 52 g sample of MCM-69(P) (sample V, 0.2 mol of eventual silanol) was converted to the H-form, (i.e., to MCM-69) using 6 M HCl. It was filtered, combined with an excess (0.4 mol) of 3 M nOA in water, and left over a weekend. After filtering, it was combined with 1 l of 1 M nOA. After exhaustive filtration, the product exhibited the XRD patterns shown in Fig. 12, which
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Fig. 11. XRD patterns for MCM-69 in the hmiHþ prepared using 1 M solution (bottom) and excess hmi (top).
were characterized by two strong low-angle reflections, at 5.8°and 8.6° 2h. It analyzed 1.4 nOA molecules per 3 Si’s, vs. the theoretical 1.0 silanols per 3 Si’s. After washing with n-hexane and airdrying, it analyzed 0.8 nOA’s per 3 Si’s. The hexane-washed sample showed two low-angle XRD reflections, at 6.0° and 9.0° 2h. Similarly a sample of MCM-69 was contacted with nC12A in 1:1 ethanol:water. The final, dried product showed
strong XRD peaks at 2.7°, 3.4°, 3.9°, and 6.5° 2h. It analyzed 0.4 nC12A’s per 3 Si’s. 3.4. Ionization of MCM-69 If, as proposed, MCM-69 is comprised of silica sheets having regularly spaced surface silanols, it should be possible to react the silanol protons with hydroxide ion without dissolving the silica. That
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Fig. 12. XRD patterns for MCM-69 in the nOAHþ forms: (a) Initial, (b) after two weeks, and (c) after washing with n-hexane and air drying.
reaction was demonstrated using tetrapropylammonium (TPA) hydroxide. A 25 g sample of wet MCM-69 was slurried with 100 ml water and the pH adjusted to 10.9 by titration with 40% TPAOH. Since the sample contained residual HCl, no stoichiometry calculation was possible. After stirring
for 3 h, the solid was filtered. Its XRD pattern showed a new low-angle reflection, at about 6.2° 2h. When the procedure was repeated, this time with the initial pH adjusted to 12.0 and the stirring time increased to 16 h, the XRD peak at 6.2° in the filtered solid more than doubled in intensity.
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3.5. Surface functionalization of MCM-69 If surface silanols exist as proposed, they should be functionalizable, either to make the silica sheets hydrophobic and dispersible in non-aqueous solvents or to provide for bonding, e.g., to metal ions. The reaction between a silanol group and hexamethyldisilazane (HMDS) is said to be both facile and clean [14] 2Si–OH þ ðCH3 Þ3 Si–NH–SiðCH3 Þ3 ! 2Si–O–SiðCH3 Þ3 þ NH3 Moreover, any water present in the sample would react to yield hexamethyldisiloxane, a liq-
uid, which could be removed by filtration. Since amine should not interfere with the reaction, a preswelled, hmi-intercalated sample of MCM-69 was used. A 20 g sample of wet hmi-MCM-69 (prepared from K) was stirred with a large excess (60 g, about 0.4 mol) of HMDS for 3 days at room temperature, with Drierite protection from the atmosphere. NH3 was evolved. The sample, filtered, washed with acetone, and air-dried, exhibited an XRD pattern (Fig. 13) having low-angle reflections at 5.1°, 6.1°, and 7.7° 2h. The material was strongly hydrophobic, as evidenced by its ready dispersion in n-hexane and toluene and its complete lack of wetting on contact with water.
Fig. 13. XRD pattern for MCM-69 after surface functionalizing with HMDS.
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4. Conclusions A new form of crystalline silica, designated MCM-69, has been prepared that is unique among crystalline layered silica hydrate structures, both in its relative robustness and in the regularity of its silanol surface. MCM-69 is derived from a pipz silicate precursor, denoted MCM-69(P), MCM-69 can be delaminated and contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the structure contains a silanol group. MCM-69 can be exfoliated in aqueous solution by reacting the silanol groups with amines. The resultant products all show crystallinity and are all characterized by a strong XRDS peak below 10.5° 2h. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. Initial experiments suggest that silanol groups react with hydroxide ion and with surface functionalizing agents such as HMDS.
Acknowledgements Thanks for excellent technical assistance in zeolite synthesis and characterization go to J. Sockwell, E.A. Moy, and D. Colmyer. Helpful input on various aspects of this work from D.C. Calabro, A.W. Chester, W.S. Borghard, J.S. Beck, J.C.
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Vartuli, A.Malek, L.S. Kennedy, T.F. Degnan, and B.G. Silbernagel was much appreciated.
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