Comment pubs.acs.org/JPCC
Comment on “Effect of Pore Size and Nickel Content of Ni-MCM-41 on Catalytic Activity for Ethene Dimerization and Local Structures of Nickel Ions” Tino Lehmann*,† and Andreas Seidel-Morgenstern†,‡ †
Institute of Process Engineering, Otto-von-Guericke-University, Universitätsplatz 2, 39106 Magdeburg, Germany Max-Planck-Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany
‡
J. Phys. Chem. C 2012, 116 (9), 5664−5672. DOI: 10.1021/jp2103066 J. Phys. Chem. C 2012, 116. DOI: 10.1021/jp305147e S Supporting Information *
A
membered rings is a problematic one. First of all, an IR band near 550 cm−1 is not a reliable indicator of five-membered rings, even in the case of zeolites (see the discussion given by Astorino et al.8). Furthermore, several groups detected a band between 550 and 580 cm−1 for amorphous SiO2.8−11 In these cases, the respective feature is attributed to an O−Si−O− bending vibration of terminal silanol groups.9−11 This mode is closely related to another vibration between 940 and 960 cm−1, which originates from the corresponding Si−O− stretch.9−12 In fact, we have found that increasing the nickel loading of NiMCM-41 diminishes not only a vibration at 560 cm−1 but also a band at 953 cm−1.3 These effects are caused by the consumption of surface silanols in the hydrolytic adsorption step during nickel phyllosilicate formation.3 The vibration found by Tanaka et al. at 570 cm−1 should therefore be assigned to Si−O− structures taking into account the available experimental facts and the well-known similarities between SiMCM-41 and amorphous silica.13,14 Note that Raman spectroscopy also failed to detect five-membered rings on the MCM-41 surface (see the Supporting Information). Regarding the catalytic experiments, Tanaka et al. have primarily utilized measurements of the total gas-phase pressure in the reaction cell, which is the sum of the partial pressures due to ethene and the reaction products (mainly n-butenes). The latter cannot be neglected here because the changes in total pressure reported in figure 1a of the commented paper suggest significant ethene conversion during the experiments. The evolution of total pressure with time is therefore determined by (see also Figure 1)
recent article by Tanaka et al. reports interesting details of the nature of nickel species in Ni-MCM-41 prepared by a template ion exchange method.1 Here we would like to give an alternative interpretation of some of their results related to both characterization and catalytic aspects. Tanaka et al. have nicely shown on the one hand that the prepared Ni-MCM-41 catalysts contain three- and four-fold coordinated nickel ions (besides octahedral species). These coordinatively unsaturated sites were proposed to be situated within the Si-MCM-41 pores on top of five- and six-membered rings of SiO4-tetrahedra, respectively. FTIR results indicated on the other hand that the nickel ions exist in the form of a 2:1 nickel phyllosilicate (TOT-layer structure2). The latter finding is in accord with our own work.3 Tanaka et al. suggested the layer silicate to be located along the pore walls of the MCM-41. Two points deserve comment concerning these characterization results. First, it is highly unlikely to generate a 2:1 nickel phyllosilicate phase inside the MCM-41 pores with the adopted preparation method. The main reason against an intrapore layer silicate is the chemistry involved in the formation of nickel phyllosilicates, which cannot take place within the templatefilled pores of as-synthesized MCM-413 no matter how the mesoporous silica was prepared. Second, 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 as evidenced by FTIR. A catalyst structure, which is not in conflict with these arguments consists of a phyllosilicate phase situated at the external surface of the Si-MCM-41 particles. We have recently found that such a structure indeed results from the synthesis approach also used by Tanaka et al.3 Note that nickel is in octahedral coordination in an ideal phyllosilicate structure. It has, however, been shown that tetrahedral and trigonal nickel ions additionally exist in both bulk4 and silica-supported5−7 2:1 nickel phyllosilicates. Tanaka et al. also detected an IR band at 570 cm−1, which declined with increasing nickel loading. This finding is consistent with our own studies.3 The effect was rationalized in ref 1 through interaction of the proposed trigonal nickel ions with five-membered SiO4 rings. We have outlined above why the presence of nickel ions within the pores is questionable. Moreover, the assignment of the vibration at 570 cm−1 to five© 2012 American Chemical Society
− ethene adsorption and reaction on nickel sites (phyllosilicate phase) − nonreactive ethene adsorption on MCM-41 sites (see figure 2a in ref 1) − n-butene adsorption (and probably isomerization) on MCM-41 sites (note that butene should adsorb significantly stronger than ethene on the Si-MCM-41 surface, which is moderately acidic3) Received: April 24, 2012 Revised: May 2, 2012 Published: October 2, 2012 22646
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Comment
Tanaka et al. suggested that MCM-41 with smaller pore sizes bears a higher amount of OH groups. This would translate into a larger qSi‑MCM‑41 in eq 2, correspondingly larger olefin adsorption rates with respect to the Si-MCM-41 part of the catalyst surface, and hence a more rapid decline of the total pressure for samples with narrower pores. Note that this explanation is not in conflict with Tanaka et al.’s in situ FTIR investigation of ethene dimerization (figure 2b,c of their paper). The rate constants for different pore diameters were calculated from the changes of spectral intensity attributable to adsorbed butene. This signal intensity is proportional to the number of adsorbed butene molecules, which in turn is positively correlated with the rate of adsorption and thus via eqs 1 and 2 with the available number of adsorption sites. Samples with smaller MCM-41 pores (higher qSi‑MCM‑41) consequentially lead to more intense spectra at specified reference times. The measured spectral changes likely contain a significant contribution due to the rate of butene adsorption on MCM41 sites besides a contribution caused by the rate of butene formation through ethene dimerization. Finally, we would like to point out that a combination of the discussed adsorption arguments also accounts for an effect that went unnoticed in the original paper. Figure 1b of Tanaka et al. illustrates that rate constant variations due to different wNi (determined from total pressure changes) increase with decreasing pore diameters. Whereas the differences at equal pore width are caused by different WSi‑MCM‑41 (vide supra), it is evident that this effect will be more pronounced for larger qSi‑MCM‑41, which is the case for MCM-41 with smaller pores. In summary, we have offered here an alternative interpretation of some characterization results reported in ref 1 without invoking mutually incompatible nickel ion structures. The proposed catalyst structure has the added benefit of avoiding contradiction with the formation chemistry of nickel phyllosilicates. It was furthermore pointed out that the employed reaction setup is not suitable to determine ethene dimerization rates. Some of the observed catalytic effects might be more apparent than real and need verification, preferably in a continuous reactor equipped with species-specific analytics.
Figure 1. Schematic illustration of olefin adsorption and reaction on Ni-MCM-41 composed of separate nickel phyllosilicate and Si-MCM41 phases.
The adsorption processes on the Si-MCM-41 part of the catalyst surface would be irrelevant for continuous reactors operated under steady-state conditions. They do, however, have a significant impact when ethene dimerization rates are to be extracted from batch experiments like in ref 1. Measuring solely the temporal change of total pressure without any information about gas-phase compositions is not sufficient to determine the consumption of gaseous ethene in the nickel-catalyzed dimerization. The foregoing reasoning also provides alternative explanations for some of the observed catalytic effects. Rates of adsorption (extensive definition: moles adsorbed per unit time) can be described in a general manner as
rads = f (L , p)
(1)
where L is the total number of adsorption sites and the vector p comprises all relevant partial pressures. For a specified adsorbent (catalyst), L is given by L = W ·q (2) with W being the mass of adsorbent and q the specific number (moles per unit mass) of adsorption sites. Moreover, we note that the structural model discussed above (separate 2:1 phyllosilicate and Si-MCM-41 phases, see Figure 1) yields
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WNi ‐ MCM ‐ 41 =W2:1 ‐ PS + WSi ‐ MCM ‐ 41
ASSOCIATED CONTENT
S Supporting Information *
= w2:1 ‐ PSWNi ‐ MCM ‐ 41 + wSi ‐ MCM ‐ 41WNi ‐ MCM ‐ 41
Raman spectroscopic analysis of ring structures at the MCM-41 surface. This material is available free of charge via the Internet at http://pubs.acs.org.
(3)
■
where w represents mass fractions and w2:1‑PS is proportional to the nickel content wNi. Tanaka et al. found a more rapid decrease in total pressure for catalysts with lower wNi. They assumed faster dimerization and higher rate constants for these catalysts (see figure 1b of the original paper). However, WNi‑MCM‑41 was varied in these experiments to ensure the same total amount of nickel ions (equal W2:1‑PS) in each run. Such a procedure leads to a larger WSi‑MCM‑41 in experiments with catalysts exhibiting lower wNi ∝ w2:1‑PS (see eq 3). Consequently, the total adsorptive capacity (Ltotal = L2:1‑PS + LSi‑MCM‑41) is higher in these experiments compared with those carried out with samples having higher wNi. This results in a faster decline of total pressure in the former case. The observed trend might thus be traced back to differences in the adsorption rates. Adsorption effects instead of different dimerization rates might likewise be responsible for the faster decrease in total pressure with smaller pore diameters. In this case, wNi and therefore WNi‑MCM‑41 were roughly the same in all experiments.
AUTHOR INFORMATION
Corresponding Author
*Phone: +49 391 6751685. Fax: +49 391 6712028. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Jacqueline Kaufmann for collecting the Raman spectra. REFERENCES
(1) Tanaka, M.; Itadani, A.; Kuroda, Y.; Iwamoto, M. J. Phys. Chem. C 2012, 116, 5664. (2) Burattin, P.; Che, M.; Louis, C. J. Phys. Chem. B 1998, 102, 2722. (3) Lehmann, T.; Wolff, T.; Hamel, C.; Veit, P.; Garke, B.; SeidelMorgenstern, A. Microporous Mesoporous Mater. 2012, 151, 113.
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(4) Tejedor-Tejedor, M. I.; Anderson, M. A.; Herbillon, A. J. J. Solid State Chem. 1983, 50, 153. (5) Wendt, G.; Fritsch, E.; Schöllner, R.; Siegel, H. Z. Anorg. Allg. Chem. 1980, 467, 51. (6) Wendt, G.; Gottschling, J.; Staudte, B.; Schöllner, R. Z. Anorg. Allg. Chem. 1983, 500, 215. (7) Wendt, G.; Mörke, W.; Schöllner, R.; Siegel, H. Z. Anorg. Allg. Chem. 1980, 467, 43. (8) Astorino, E.; Peri, J. B.; Willey, R. J.; Busca, G. J. Catal. 1995, 157, 482. (9) Decottignies, M.; Phalippou, J.; Zarzycki, J. J. Mater. Sci. 1978, 13, 2605. (10) Duran, A.; Serna, C.; Fornes, V.; Fernandez Navarro, J. M. J. Non-Cryst. Solids 1986, 82, 69. (11) Ocaña, M.; Fornés, V.; Serna, C. J. J. Non-Cryst. Solids 1989, 107, 187. (12) Camblor, M. A.; Corma, A.; Perez-Pariente, J. J. Chem. Soc., Chem. Commun. 1993, 557. (13) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (14) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525.
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