Comment pubs.acs.org/JPCC
Reply to “Comment on ‘Effect of Pore Size and Nickel Content of NiMCM-41 on Catalytic Activity for Ethene Dimerization and Local Structures of Nickel Ions’” Masashi Tanaka, Yasushige Kuroda, and Masakazu Iwamoto* J. Phys. Chem. C 2012, 116 (9), 5664−5672. DOI: 10.1021/jp2103066 J. Phys. Chem. C 2012, 116. DOI: 10.1021/jp303935b
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n a recent comment,1 Lehmann and Seidel-Morgenstern raised the following four concerns about our recent paper2 in which Ni-MCM-41 samples were characterized by various physicochemical methods and catalysis for the dimerization of ethene. (1) It is highly unlikely that a 2:1 nickel phyllosilicate phase would be generated inside the MCM-41 pores with the adopted preparation method. The main argument against an intrapore layer silicate is the chemistry involved in the formation of nickel phyllosilicates, which cannot take place within the template-filled pores of as-synthesized MCM-41, no matter how the mesoporous silica was prepared. (2) There is a disagreement between the structural features of the postulated tetrahedral and trigonal nickel species and the architecture of the TOT-layered nickel silicate, as evidenced by FTIR. (3) The vibration at 570 cm−1 was not assignable to five-membered Si− O4 rings but to an O−Si−O− bending vibration of terminal silanol groups. (4) Regarding the catalytic experiments, the evolution of the total pressure with time should be analyzed based on ethene adsorption and reaction on nickel sites, nonreactive ethene adsorption on MCM-41 sites, and 1-butene adsorption on MCM-41 sites. In this reply, we will show that the first concern does not have any supporting evidence, that the third concern involves an unfixed assignment of the spectrum, and that the final concern involves a misunderstanding of the reaction kinetics. First, the opinion that the formation of nickel phyllosilicates cannot take place within the template-filled pores of assynthesized MCM-41 is a concept without any scientific evidence. It is widely reported that the surfactants included in the as-synthesized MCM-41 can easily desorb from the MCM41 pores through extraction treatments using various acidic solutions.3 This is an ion exchange of template cations for protons with little destruction of the porous structure. One of the present authors has already described the progress of the template ion-exchange (TIE): template cations → protons → metal cations.4,5 In addition, even the exchange of wall component(s), mainly oxyacid anions constructing the wall of mesostructured materials containing cationic surfactant micelles, for other oxyacid anions has been reported with little collapse of the mesoporous structure.6−9 Apparently the direct analysis of the distribution of nickel species on the Ni-MCM-41 would be the most indubitable evidence to solve the problem. We first tried to use the focused ion beam (FIB) microscopy. Unfortunately, it was impossible for us to prepare the cross section of a Ni-MCM-41 particle for the measurement of the FIB microscopy despite our several efforts. Then, we attempted to determine the distribution of © 2012 American Chemical Society
nickel metal particles, which were produced by hydrogen reduction, using scanning transmission electron microscopy (STEM). We expected the appearance of small nickel particles in the pores of MCM-41, which would be the evidence of the nickel ion-loading in the pores. After the reduction at 773−873 K,10 many nickel particles of 5 to 6 nm in diameter were observed everywhere, and about 5−10% of nickel particles were of ca. 2 nm in diameter and existed along the pore walls of MCM-41. This would result from the easy condensation of nickel metal owing to the high reduction temperature of nickel ion.10 Finally, we observed the calcined Ni-MCM-41 by the field-emission scanning electron microscopy (FE-SEM) and the energy-dispersive X-ray spectroscopy (EDS) to reveal the location of nickel ion. Figure 1 shows the SEM photograph and the EDS mapping images of Si and Ni. The hexagonally ordered pores can be observed in Figure 1a. The EDS mappings indicated clearly no deviation of Si and Ni on this sample, verifying little possibility of uneven distribution of the 2:1 nickel phyllosilicate species only on the outer side of a MCM-41 particle. We believe that the nickel ion was loaded uniformly on our Ni-MCM-41 samples. Of course, we do not deny the possibility that the 2:1 nickel phyllosilicate phase was produced beside the parent MCM-41, not in the pore wall, in the preparation of Ni-MCM-41 by Lehmann et al.11 At present we do not have any answers for the second point. As already mentioned in the conclusions of the report,2 we were aware that there is a disagreement between the structural features of the postulated tetrahedral and trigonal nickel species and the architecture of a TOT-layered nickel silicate. The various Ni 2+ ions with distorted octahedral, distorted tetrahedral, and trigonal structures would be located somewhere in the 2:1 nickel phyllosilicate phase, although the exact nickel silicate structures could not be suggested. Third, Lehmann et al. argued for the assignment of the IR band at 570 cm−1 to an O−Si−O− bending vibration of terminal silanol groups. They cited several studies12−15 to support their assignment. All of the literature,12−15 however, employed only one report12 for the assignment of the O−Si− O− species. In addition, the assignment in the basic report12 was carried out based on a Ph.D. thesis16 and proceedings.17 In contrast, the assignment to five-membered rings consisting of Si−O was widely used in the characterization of many zeolite materials,18−20 and the propriety of the assignment to fiveReceived: May 27, 2012 Revised: October 1, 2012 Published: October 2, 2012 22649
dx.doi.org/10.1021/jp305147e | J. Phys. Chem. C 2012, 116, 22649−22651
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Comment
nickel ions. Note that the high surface areas of (Ni-)MCM-41 were maintained even after the loading of nickel ions. Our model provides a reasonable explanation. The fourth point suggested by Lehmann et al. is a misunderstanding. We estimated the pseudo rate constants of ethene dimerization on Ni-MCM-41 based on the total pressure change at 2−20 min (the initial stage), as described in the Results and Discussion.2 As shown in Figure 2 of the paper,2 the adsorption of ethene on the parent MCM-41 was finalized within 2 min; therefore, the measurement of the reaction rate on Ni-MCM-41 was started 2 min after the introduction of ethene. In contrast, it is easily assumed that the formation of butenes would affect the pressure change in the gas phase. The data included in the calculation of the rate constants, therefore, were limited to those within the initial 20 min. One can recognize the small changes in the partial pressures within this range in Figure 1 of our recent paper.2 We can suggest that the pseudo rate constants estimated by our method indicated the catalytic activity of nickel species for ethene dimerization with minimum experimental errors. In general, the assignment and the interpretation of the spectra are dependent on the prepared materials. We believe that we can answer most of the questions raised in the comment.1 In our previous work,2 we noted in the conclusions that the following points remained unclear: why the fivemembered Si−O rings were preferentially produced in small pore M41, how the layered nickel silicate structure is related to the active nickel species, and how the Ni2+ ions catalyze the ethene dimerization. We still believe these problems remain to be addressed.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aids from the Japan Society for Promotion of Science (JSPS, METI), the New Energy and Industrial Technology Development Organization (NEDO, MITI), and the Advanced Low Carbon Technology Research and Development Program (ALCA, JST).
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REFERENCES
(1) Lehmann, T.; Seidel-Morgenstern, A. J. Phys. Chem. C 2012, 116. 10.1021/jp303935b. (2) Tanaka, M.; Itadani, A.; Kuroda, Y.; Iwamoto, M. J. Phys. Chem. C 2012, 116, 5664. (3) For example, Hitz, S.; Prins, R. J. Catal. 1997, 168, 194. (4) Yonemitsu, M.; Tanaka, Y.; Iwamoto, M. Chem. Mater. 1997, 9, 2679. (5) Iwamoto, M.; Tanaka, Y. Catal. Surv. Jpn. 2001, 5, 25. (6) Wu, P.; Iwamoto, M. Chem. Lett. 1998, 1213. (7) Iwamoto, M.; Kitagawa, H.; Watanabe, Y. Chem. Lett. 2002, 814. (8) Takada, H.; Watanabe, Y.; Iwamoto, M. Chem. Lett. 2004, 62. (9) Wu, P.; Liu, Y.; He, M.; Iwamoto, M. Chem. Mater. 2005, 17, 3921. (10) Ikeda, K.; Kawamura, Y.; Yamamoto, T.; Iwamoto, M. Catal. Commun. 2008, 9, 106. (11) Lehmann, T.; Wolff, T.; Hamel, C.; Veit, P.; Garke, B.; SeidelMorgenstern, A. Microporous Mesoporous Mater. 2012, 151, 113. (12) Decottignies, M.; Phalippou, J.; Zarzycki, J. J. Mater. Sci. 1978, 13, 2605.
Figure 1. FE-SEM photograph (a) and EDS images of silicon (b) and nickel (c) of calcined Ni-MCM-41 prepared with C10H21(CH3)3NBr. The Si/Ni ratio of the sample was 80. All pictures were collected on a Hitachi HR-S5500 with a Horiba EMAX detector at acceleration voltage of 30 kV.
membered rings has been previously discussed.21 We believe that the 570 cm−1 band should be assigned to five-membered rings and that nickel ions were loaded on the sites;2 this assignment reasonably explained the following experimental findings. Our paper2 and Lehmann’s paper11 both showed the phenomenon that the band at 570 cm−1 decreased significantly with increasing nickel contents. In Lehmann’s model11 in which the nickel phyllosilicate phase was produced beside the original MCM-41 structure, it is very difficult to explain why the band assigned to an O−Si−O− bending vibration of terminal silanol groups in the MCM-41 structure decreased with the loading of 22650
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Comment
(13) Duran, A.; Serna, C.; Fornes, V.; Fernandez Navarro, J. M. J. Non-Cryst. Solids 1986, 82, 69. (14) Ocaña, M.; Fornés, V.; Serna, C. J. J. Non-Cryst. Solids 1989, 107, 187. (15) Camblor, M. A.; Corma, A.; Perez-Pariente, J. J. Chem. Soc., Chem. Commun. 1993, 557. (16) Wilmot, G. B. Ph.D. Thesis, Massachusetts Institute of Technology, 1954. (17) Kamiya, K.; Sakka, S.; Yamanaka, I. C. R. Trav., Congr. Int. Verre 1974, 13, 44. (18) Jansen, J. C.; van der Gaag, F. J.; van Bekkum, H. Zeolites 1984, 4, 369−372. (19) Wang, H.; Liu, Y.; Pinnavaia, T. J. J. Phys. Chem. B 2006, 110, 4524−4526. (20) Sidhpuria, K. B.; Parikh, P. A.; Bahadur, P.; Jasra, R. V. J. Porous Mater. 2008, 15, 481−489. (21) Astorino, E.; Peri, J. B.; Willey, R. J.; Busca, G. J. Catal. 1995, 157, 482.
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