J. Phys. Chem. B 2002, 106, 3333-3334
Reply to the Comment on “Identification of Precursor Species in the Formation of MFI Zeolite in the TPAOH-TEOS-H2O System” Christine E. A. Kirschhock, Raman Ravishankar, Frederik Verspeurt, Piet J. Grobet, Pierre A. Jacobs, and Johan A. Martens* Centrum Voor OpperVlaktechemie en Katalyse, K.U. LeuVen, Kasteelpark Arenberg 23, B-3001 LeuVen, Belgium ReceiVed: December 31, 2001; In Final Form: January 12, 2002 The authors of the comment criticize the content of one particular paper1 out of the series,1-8 in which we traced the molecular steps involved in the formation of MFI- and MELtype zeosils starting from mixtures of TEOS, TPAOH, or TBAOH and water. The criticism is concerned with the following aspects: (i) zeolite crystallization mechanism via selfassembly; (ii) interpretation of FTIR spectra; (iii) experimental conditions and interpretation of 29Si NMR spectra of silicate solutions. Our reply to the comments on these issues is as follows. 1. Evidence for Aggregation of Silicate Oligomers That Are Structurally Related to the MFI Zeolite Framework. We provided direct experimental evidence for an aggregation mechanism, in which the smallest entities selectively condense with each other and systematically form larger and larger units until finally particles displaying Silicalite-1 or -2 type X-ray diffraction are obtained. Such an aggregation mechanism is wellknown in sol-gel science.9 It is referred to as cluster-cluster aggregation and is the alternative for the monomer-cluster mechanism according to which a crystal grows through addition of an always present small molecular species. In silicate systems, both mechanisms may occur depending on conditions.9 In our conditions for Silicalite-1 and Silicalite-2 synthesis, the evidence for the occurrence of an aggregation mechanism was obtained from transmission electron microscopy (TEM), X-ray scattering (XRS), small-angle X-ray scattering (SAXS), and gel permeation chromatography (GPC). These different approaches independently evidenced discrete steps of the following aggregation sequence:
monomer (11 Si atoms (GPC)) f dimer (22 Si atoms (GPC)) f precursor unit (33 Si atoms (GPC, XRS)) f nanoslabs assembled from 2, 6, or 12 precursor units (GPC, SAXS, XRS, TEM) f tablets assembled from nanoslabs (XRS, SAXS, TEM) f intermediates assembled from tablets (XRS) f crystals assembled from intermediates (XRD) (1) All silicon present in the system is engaged in this aggregation process. The nanoslabs are very specific MFI framework fragments. The nature of the precursor unit from which the nanoslabs are assembled is obvious from the nanoslab structure. The structure of the precursor unit was determined without relying on 29Si NMR. * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
3333
The numbers of Si atoms in the species (eq 1) were derived from the relative volumes or weights of the entities in the sequence using the GPC, XRS, SAXS, TEM, AFM, and 29Si MAS NMR approaches. Thus, the number of Si atoms in the different entities were determined with techniques other than 29Si liquid NMR. 29Si liquid NMR was only used to determine the nature of species for which the numbers of Si atoms they were composed of were known. 2. Shifting of the FTIR Frequency of Five Ring Vibrations with Silicalite-1 Particle Size. Micrometer-sized Silicalite-1 and ZSM-5 samples display a framework vibration at 550 cm-1.10,11 So-called colloidal Silicalite-1, with a particle size of 18-100 nm shows a doublet with maxima at 555 and 570 cm-1.2 The obvious difference between these samples is the particle size. When the particle size is reduced further to the Silicalite-1 nanoslabs, measuring 4 × 4 × 1.3 nm, and the precursor, the IR absorption maximum occurs at 570 and 590 cm-1, respectively.1,3 Whereas the shifting of this topologydependent vibration is clearly related to particle size, the fundamental basis for its occurrence is not clear. 3. 29Si NMR Experimentation and Signal Assignment. The argument used by Knight and Kinrade for questioning the 29Si NMR signal assignments is only based on the fact that all species that can exist have already been listed (Figure 1 of the Comment, composed of data from several references), while the ones proposed here do not belong to that list. In this respect, it is noteworthy to reexamine the experimental conditions in which the solutions of the listed as condensed as possible silicate anions have been prepared. It is illustrated by the following statement by Kinrade et al.:12 “The best method for characterizing the formation of cubic octamer and prismatic hexamer turned out to be the simple boil-freeze-thaw technique of Knight and co-workers. In this procedure, the sample is heated to boiling to break down all tetraalkylammonium-stabilized species...” Obviously, in such an experimental procedure, all species relevant to structure direction are destroyed. We took extreme care not to disturb the system, by cooling the samples quickly to 0 °C after the indicated reaction times. Storage and characterization was done at this temperature. The extraction of unconverted TEOS was also performed with precooled octane solvent. Only under such conditions were the samples stable. Our 29Si NMR experiments were performed as follows. The samples were loaded in cylindrical PTFE sample containers, in which a smaller cylindrical container with a TMS reference was inserted. In the spectra, the silicate monomer (Q0) was never observed. Knight and Kinrade concentrated their work on the use of TMA.12,13 The formation of cubic octamer in the presence of TMA is obvious from their work. However, the 29Si NMR spectra of solutions prepared with TPA and TBA that they produced (Figure 3 of ref 14) show substantially broadened lines hinting at the presence of particle-like species they even do not discuss. In conclusion, the four bold statements that Knight and Kinrade make on 29Si NMR observations of equilibrated silicate solutions are quite irrelevant to our experimental conditions. Of course, one can always propose alternative assignments by denying the experimental context of the observations. Knight and Kinrade do so based on experience and work with systems and conditions that are different. Their objections in the section
10.1021/jp0146521 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002
3334 J. Phys. Chem. B, Vol. 106, No. 12, 2002 on “known structures” are essentially based on the assumption that their and our systems have to behave identically. With respect to the “unknown structures”, we based our identification on 29Si NMR chemical shifts and relative peak intensities, which is common practice. Given the weakness of the signals at natural 29Si isotope abundance, we performed duplicate experiments (in total, 25 experiments with variation in reaction time and temperature) to detect systematical changes. In this way, we realized that the relative intensities of groups of peaks remained constant in the different spectra. Knight and Kinrade criticize our assignment of specific 29Si NMR peaks to the tetracyclic undecamer, which is the key species in the aggregation sequence (eq 1). In numerous spectra, we did observe this set of Q2 and Q3 signals having constant relative intensities of 2:2:2:2:2:1 and for which in our opinion no other assignment than to the 11 Si atom containing tetracyclic undecamer is possible. The objections against this assignment are again merely based on the presence of species observed under different polymerization conditions. We anticipate that the 29Si NMR spectrum of the precursor should be quite complicated, given the 33 Si atoms present. At this stage, the only conclusion that we derived from the 29Si NMR spectrum was that the distribution of Qn environments was in agreement with that in the precursor structure derived from the structure of the nanoslab. Finally, we assigned 29Si NMR signals to capped double fivering and double five-ring, which been observed in solution before.14 Whereas the chemical shift of prismatic silicate oligomers representing double four-rings, five-rings, or six-rings is expected to be very similar, the double five-ring assignment is retained. Its formation from the tetracyclic undecamer involves a minimal transformation indeed. We agree that on the basis of 29Si chemical shift values alone, it would be very difficult indeed to differentiate the double five-ring from other double ring structures. Addendum In our original publication1, figure legends of Figures 4 and 6 were mixed up. The sample descriptions given in the inserts
Comments of the figures are correct. The caption of Figure 4 should read: “Assigned 29Si NMR spectrum of the sample prepared at room temperature after stirring for 15 min”. The caption of Figure 6 should read: “Assigned 29Si NMR spectrum of the sample prepared at 0 °C after 45 min stirring”. Acknowledgment. The authors acknowledge the Belgian government for financial support through IAP-PAI network. References and Notes (1) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. (2) Ravishankar, R.; Kirschhock, C.; Schoeman, B. J.; Vanoppen, P.; Grobet, P. J.; Storck, S.; Maier, W. F.; Martens, J. A.; De Schryver, F. C.; Jacobs, P. A. J. Phys. Chem. B 1998, 102, 2633. (3) Ravishankar, R.; Kirschhock, C.; Schoeman, B. J.; De Vos, D.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. In Proceedings of the 12th International Zeolite Conference; Treacy, M. M. J., Marcus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrendale, PA, 1998; Vol. III, p 1824. (4) Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P.-P.; Feijen, E. J. P.; Grobet, P. J.; Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4960. (5) Kirschhock, C. E. A.; Ravishankar, R.; Van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. (6) Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021. (7) Kirschhock, C. E. A.; Ravishankar, R.; Truyens, K.; Verspeurt, F.; Jacobs, P. A.; Martens, J. A. Stud. Surf. Sci. Catal. 2000, 129, 139. (8) Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; van Santen, R. A.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. Angew. Chem., Int. Ed. 2001, 40, 2637. (9) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: 1990; p 194. (10) Zecchina, A.; Bordiga, S.; Spoto, G.; Marchese, G.; Petrini, G.; Leofanti, G.; Padovan, M. J. Phys. Chem. 1992, 96, 4985. (11) Coudurier, G.; Naccache, C.; Vedrine, J. C. J. Chem. Soc., Chem. Commun. 1982, 1413. (12) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. (13) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (14) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987; p 80.
