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Concerning the interpretation of 27Al MAS-NMR spectra of Mo and NiMo catalysts on Al-containing MCM-41 supports
Applied Catalysis A: General 253 (2003) 321–325
Response
Concerning the interpretation of 27Al MAS-NMR spectra of Mo and NiMo catalysts on Al-containing MCM-41 supports A reply to the comment by X. Carrier and M. Che on “Ni and Mo interaction with Al-containing MCM-41 support and its effect on the catalytic behavior in DBT hydrodesulfurization” [Appl. Catal. A 240 (2003) 29–40] Tatiana Klimova∗ Departamento de Ingenier´ıa Qu´ımica, Facultad de Qu´ımica, Universidad Nacional Autónoma de México, Cd. Universitaria, C.P. 04510 Mexico City, D.F., Mexico Received 13 June 2003; accepted 17 June 2003
Abstract A new interpretation for the NMR signal observed at +16 ppm in the 27 Al MAS-NMR spectra of Mo and NiMo/Al-MCM-41 catalysts was done. This signal was assigned to an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− , which can be formed during impregnation step of the catalyst preparation, as well as a result of hydration of Al2 (MoO4 )3 in calcined Mo and NiMo/Al-MCM-41 catalysts. © 2003 Elsevier B.V. All rights reserved. Keywords: 27 Al MAS-NMR; Anderson-type heteropolyanion; Al-MCM-41; Mo and NiMo catalysts
The comment by Carrier and Che [1] deals with the interpretation of 27 Al MAS-NMR spectra presented in our previous paper [2]. It should be mentioned that the problem of the interpretation of the 27 Al MAS-NMR data is not completely solved yet, especially for Mo and NiMo catalysts on different Al-containing supports. This is due, in principle, to the fact that the shape of the spectrum, i.e. the presence of peaks corresponding to Al species in tetrahedral or octahedral coordination, their exact position and sharpness, depend strongly on a series ∗ Tel.: +52-55-5622-53-71; fax: +52-55-5622-53-55. E-mail address: [email protected] (T. Klimova).
of factors. Among them, the particular nature of the material, its hydration state, the presence of additional cationic or anionic species interacting with aluminum ions, pretreatment by drying or calcination can be mentioned. In general, it is well known that two kinds of Al species can be distinguished by solid-state 27 Al MAS-NMR spectroscopy, namely, Al species in tetrahedral coordination (a signal at about 50–60 ppm) and octahedrally coordinated Al atoms (a signal around 0 ppm) [3]. However, the exact position of each of these peaks can be changed depending on the chemical environment of Al atoms. For example, the characteristic signal of aluminum in Al2 (MoO4 )3 appears at −14 ppm, that is far-away from the above-mentioned
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00504-0
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T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
signals at 0 or 50–60 ppm [4,5]. Additionally, it has been found that sometimes the same NMR peak can be attributed to different Al species. For example, the peak at 0 ppm frequently observed in the NMR spectra of acidic forms of zeolites or MCM-41 materials can represent the “real” extra-framework aluminum species or octahedral framework Al that is partially associated to the silica lattice and is the host of two additional water molecules from which one is the hydroxonium ion [6,7]. Also it has been observed that sometimes Al atoms can change reversibly their coordination from octahedral to tetrahedral one as a result of a relatively mild treatment, as it was observed for acidic forms of zeolites or MCM-41 materials after ion exchange with NH4 + or Na+ cations [6]. All these considerations explain why sometimes it is difficult to ascribe unambiguously the signals of the 27 Al MAS-NMR spectra to an exact type of Al species, especially as regards the intermediate peaks appeared in the 27 Al NMR spectrum between 0 and 50–60 ppm. These intermediate signals may be attributed to Al species in intermediate coordination state, like penta-coordinated Al [3,6], the existence of which is still a matter of debate, or to tetrahedral or octahedral Al species, the signals of which are significantly shifted from their “normal” positions due to a specific chemical surrounding. From this point of view, the comment by Carrier and Che [1] gives a new interpretation for the NMR signal observed by us at 15 ppm in the 27 Al MAS-NMR spectra of Mo and NiMo/Al-MCM-41 catalysts relating its appearance with the formation of an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− . The formation of aluminomolybdate anion was first proposed for alumina-supported Mo catalysts [5] and more recently it has been extended to Mo-containing Y-zeolite-supported catalysts [4].
1. Hydration state of the samples and its influence on the 27 Al MAS-NMR spectra First, some additional information must be provided in order to understand better the results of the NMR measurements presented in Fig. 7 and 8 of [2]. Solid-state 27 Al MAS-NMR spectra were obtained with an ASX300 Bruker NMR spectrometer using [Al(H2 O)6 ]3+ as the external 27 Al reference.
The spectra were recorded at room temperature, at a source frequency of 78.210 MHz and a spinning rate of 12 kHz. A 2.0 s pulse was used with a repetition time of 0.5 s between pulses. Four thousand and ninety-six scans were accumulated before Fourier transform. Samples were used after 2-month storage and were equilibrated in ambient air (for 48 h) before the NMR characterization. It should be mentioned then, that the spectra presented in Fig. 7 and 8 were obtained with 3-month interval. Although the sample preparation procedure was the same in both cases, some variation in ambient humidity could have taken place and therefore the degree of hydration of the two series of samples could be different. It can be assumed that the samples in Fig. 7 were more hydrated than those in Fig. 8. This is evidenced from the sharpness of the peaks in Fig. 7. Thus, as has been reported previously, sharp peaks in solid-state 27 Al MAS-NMR spectra of Al-containing materials correspond to well-ordered hydrated samples, whereas broad peak reflect the geometrically more stressed Al species (dry samples) [4,6]. Therefore, the spectra presented in Fig. 7 or 8 can be compared within the same series, however, it is difficult to compare the spectra in Fig. 7 with their counterparts in Fig. 8.
2. Assignment of the peak at 15 ppm Mo/Al-MCM-41 catalysts In the 27 Al MAS-NMR spectra of Mo/Al-MCM-41 catalysts presented in Fig. 7c and 8c of [2], a peak at +15 ppm was observed. A similar peak at +13 ppm has been observed previously by Edwards and Decanio [8] in the 27 Al CP-MAS spectrum of P-Mo/Al2 O3 catalyst, and it was attributed to a “hydrated form of an aluminum molybdate Al2 (MoO4 )” with tentative formula [Al(OH)n (H2 O)6−n O18 ]n (MoO4 ) (n = 1 or 2). More recently, Plazenet et al. [4] and Carrier et al. [5] have assigned the peak at +15 ppm in the 27 Al MAS-NMR spectra of Mo/zeolite or Mo/␥-alumina catalysts to an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− . In both cases, the formation of the aluminomolybdate ions were explained through dissolution of alumina and reaction of dissolved Al3+ with heptamolybdate during impregnation step of the catalyst preparation. Additionally, it was shown that in the case of HY zeolite [4] solely non-framework Al
T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
species could be dissolved giving rise to the formation of [Al(OH)6 Mo6 O18 ]3− . The decomposition of the Anderson-type heteropolyanions upon calcination yields both MoO3 and Al2 (MoO4 )3 . Additionally, it was shown by Plazenet et al. [4] that rehydration of calcined Mo/Y-zeolite catalysts in water-saturated atmosphere for 48 h leads to partial conversion of Al2 (MoO4 )3 into [Al(OH)6 Mo6 O18 ]3− along with solvated Al3+ . Taking all the above-mentioned considerations into account, it can be expected that in the Mo catalysts characterized by 27 Al MAS-NMR spectroscopy in our work an Anderson-type heteropolyanion could also be formed. The peak at +15 ppm corresponding to [Al(OH)6 Mo6 O18 ]3− was detected for calcined and then rehydrated Mo/Al-MCM-41 catalysts. The formation of a heteropolyanion can be attributed therefore to hydration of Al2 (MoO4 )3 according to the reaction proposed by Carrier et al. (reaction (II) [1]) leading to [Al(OH)6 Mo6 O18 ]3− and solvated aluminum cations [Al(H2 O)6 ]3+ . The peak at 0 ppm corresponding to the latter species can also be observed in Fig. 7c and 8c [2]. From this point of view, it also becomes clear why the formation of Al2 (MoO4 )3 in the calcined catalysts was clearly documented by XRD and TPR [2], however, the peak at −14 ppm characteristic of aluminum molybdate was not observed in the 27 Al MAS-NMR spectrum of one of the hydrated Mo catalysts (Fig. 7c). The behavior of calcined Mo/Al-MCM-41 catalysts upon rehydration proved to be very similar to that of Mo/Y-zeolite catalysts as described by Plazenet et al. [4]. Hence, it seems logical to assume that [Al(OH)6 Mo6 O18 ]3− was likewise formed during the impregnation of Al-MCM-41 supports with ammonium heptamolybdate solution and it gives rise to the formation of Al2 (MoO4 )3 during calcination. Previously, we have supposed that Al2 (MoO4 )3 is formed as a result of the interaction of Mo species with “highly-reactive” extra-framework Al species. In line with this supposition and the observations of Plazenet et al., now we can assume that this interaction leads to the appearance of an Anderson-type heteropolymolybdate. Hence, it can be concluded that recent publications by Carrier and co-workers [1,5] and Plazenet et al. [4] and our results of the 27 Al MAS-NMR characterization of hydrated Mo/Al-MCM-41 catalysts point
323
to the formation of aluminomolybdate during impregnation, its further evolution to Al2 (MoO4 )3 upon calcination, and its re-appearance upon rehydration, for three different Al-containing supports: ␥-alumina, Y-zeolite and Al-MCM-41. However, it is not clear yet whether this behavior can be generalized to any other Al-containing support.
3. Effect of the hydration on NiMo/Al-MCM-41 catalysts The 27 Al MAS-NMR spectra of hydrated NiMo/ Al-MCM-41 catalysts (Fig. 7d and 8d of [2]) differ from those of the corresponding unpromoted Mo catalysts (Fig. 7c and 8c, respectively). Namely, the peak at 15 ppm is present in both cases (Mo and NiMo catalysts), however, the peak at 0 ppm is clearly observed only for Mo catalysts. Therefore, it was interesting to follow more carefully the evolution of NiMo/Al-MCM-41 catalysts upon hydration. Taking this into account, we repeated the 27 Al MAS-NMR characterization of NiMo/Al-MCM-41 catalysts with a careful control of the sample humidity. In this case, we used NiMo catalysts supported on Al-MCM-41(30) and Al-MCM-41(15) that were prepared following the procedure described in [2]. After calcination (400 ◦ C, 2 h) these catalysts have been stored for 1 month in contact with ambient air (stored samples, NiMo/Al-MCM-41(30)-S and NiMo/Al-MCM-41(15)-S). The 27 Al MAS-NMR spectra of these samples was recorded without additional treatment. After that, each sample was divided into two parts. The first part was dried at 100 ◦ C for 48 h and then transferred into a glovebox filled with dry N2 to prepare the sample for NMR measurement (dried samples, NiMo/Al-MCM-41(30)-D and NiMo/Al-MCM-41(15)-D). The second part was equilibrated for 48 h at room temperature in a desiccator at a relative humidity of 79% generated by a saturated solution of NH4 Cl to measure the 27 Al NMR spectra of this stored and then rehydrated sample (hydrated samples, NiMo/Al-MCM-41(30)-H and NiMo/Al-MCM-41(15)-H). These steps were aimed at evaluating the effect of storage, drying and rehydration of the calcined NiMo/Al-MCM-41 samples.
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T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
Fig. 1. 27 Al MAS-NMR spectra of NiMo/Al-MCM-41(30) catalyst after drying (a), 1-month storage (b) and rehydration (c).
The 27 Al MAS-NMR spectra of stored, dried and rehydrated NiMo/Al-MCM-41 catalysts are presented in Fig. 1 and 2. In the 27 Al MAS-NMR spectra of dried NiMo/ Al-MCM-41 catalysts (Fig. 1a and 2a), two peaks are observed. In addition to the line at 50 ppm assigned to tetrahedral framework Al species, a signal at about −14 ppm characteristic of Al2 (MoO4 )3 can be detected. In NiMo catalysts, the presence of aluminum molybdate can be expected judging
Fig. 2. 27 Al MAS-NMR spectra of NiMo/Al-MCM-41(15) catalyst after drying (a), 1-month storage (b) and rehydration (c).
from TPR results where high-temperature (at about 700 ◦ C) H2 consumption was observed. However, the formation of Al2 (MoO4 )3 was not detected by XRD, probably, due to its higher dispersion in the presence of Ni. The intensity of the peak at −14 ppm is higher in the spectrum of NiMo catalyst supported on MCM-41 with higher Al loading (Al-MCM-41(15)) and therefore higher proportion of extra-framework Al in the support (Fig. 2a). It confirms that Al2 (MoO4 )3 forms due to the interaction of Mo species with “highly-reactive” extra-framework Al of the support. The spectra of the same NiMo catalysts but after 1-month storage in contact with ambient air were different (Fig. 1b and 2b). The intensity of the peak at −14 ppm substantially decreased and a new peak at about +16 ppm appeared. Hydrated samples, NiMo/ Al-MCM-41(30)-H and NiMo/Al-MCM-41(15)-H, preserved in a closed water-saturated atmosphere for 48 h, showed only two peaks (at 53 and +16 ppm) in the solid-state 27 Al MAS-NMR spectra (Fig. 1c and 2c). The signal corresponding to aluminum molybdate (−14 ppm) was completely absent. This illustrates how drastically the hydration state of the sample can change its 27 Al MAS-NMR spectrum. The opposite change in the intensities of the peaks at −14 and +16 ppm, i.e. its decrease for the former peak and simultaneous increase for the latter, point to the formation of aluminomolybdate species (the peak at +16 ppm) from aluminum molybdate (the peak at −14 ppm) as a result of hydration. The previous results allow one to mention some interesting points making difference between Mo and NiMo catalysts. For both cases of Mo and NiMo catalysts, the formation of Anderson-type heteropolyanions can be related with hydration of Al2 (MoO4 )3 . However, if for Mo catalysts this hydration leads to the formation of both the Anderson-type anions (+15 ppm) and solvated Al3+ (0 ppm), for NiMo catalysts only the peak at +16 ppm is clearly observed. Solvated partially mobile octahedral Al (0 ppm) is absent. Therefore, it is necessary to clear up whether it is not formed during hydration of the NiMo catalysts or it changes the octahedral coordination to tetrahedral one as a result of the interaction with Ni2+ . Previously, it has been reported that the Andersontype heteropolyanion is destroyed between 230 and 250 ◦ C [9]. In good agreement with this, Plazenet
T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
et al. [4] observed that the aluminomolybdate anion obtained in Mo/Y-zeolite catalysts was preserved after drying at 100 ◦ C overnight. However, our results show that in the case of NiMo/Al-MCM-41 systems drying at 100 ◦ C for 48 h easily yields Al2 (MoO4 )3 (peak at −14 ppm, Fig. 1a and 2a). These results show that the behavior of the Anderson-type heteropolyanions can be different depending on whether they were formed during the catalyst preparation, namely, Mo impregnation stage through alumina dissolution in the presence of molybdate, or are “reformed” upon rehydration of the pre-calcined catalyst. There is no information which allows explanation of this difference. More complete characterization of NiMo/Al-MCM-41 systems is needed to clarify these points.
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References [1] X. Carrier, M. Che, Appl. Catal. A: Gen. 240 (2003) 29–40. [2] T. Klimova, M. Calderon, J. Ram´ırez, Appl. Catal. A: Gen. 240 (2003) 29. [3] J.P. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie, M.P. Shatlock, J. Chem. Soc., Chem. Commun. (1987) 91. [4] G. Plazenet, E. Payen, J. Lynch, B. Rebours, J. Phys. Chem. B 106 (2002) 7013. [5] X. Carrier, J.F. Lambert, M. Che, J. Am. Chem. Soc. 119 (1997) 10137. [6] S. Hitz, R. Prins, J. Catal. 168 (1997) 194. [7] B.H. Wouters, T.H. Chen, P.J. Grobet, J. Am. Chem. Soc. 120 (1998) 11419. [8] J.C. Edwards, E.C. Decanio, Catal. Lett. 19 (1993) 121. [9] X. Carrier, J.F. Lambert, S. Kuba, H. Knözinger, M. Che, J. Mol. Struct., in press.
Response
Concerning the interpretation of 27Al MAS-NMR spectra of Mo and NiMo catalysts on Al-containing MCM-41 supports A reply to the comment by X. Carrier and M. Che on “Ni and Mo interaction with Al-containing MCM-41 support and its effect on the catalytic behavior in DBT hydrodesulfurization” [Appl. Catal. A 240 (2003) 29–40] Tatiana Klimova∗ Departamento de Ingenier´ıa Qu´ımica, Facultad de Qu´ımica, Universidad Nacional Autónoma de México, Cd. Universitaria, C.P. 04510 Mexico City, D.F., Mexico Received 13 June 2003; accepted 17 June 2003
Abstract A new interpretation for the NMR signal observed at +16 ppm in the 27 Al MAS-NMR spectra of Mo and NiMo/Al-MCM-41 catalysts was done. This signal was assigned to an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− , which can be formed during impregnation step of the catalyst preparation, as well as a result of hydration of Al2 (MoO4 )3 in calcined Mo and NiMo/Al-MCM-41 catalysts. © 2003 Elsevier B.V. All rights reserved. Keywords: 27 Al MAS-NMR; Anderson-type heteropolyanion; Al-MCM-41; Mo and NiMo catalysts
The comment by Carrier and Che [1] deals with the interpretation of 27 Al MAS-NMR spectra presented in our previous paper [2]. It should be mentioned that the problem of the interpretation of the 27 Al MAS-NMR data is not completely solved yet, especially for Mo and NiMo catalysts on different Al-containing supports. This is due, in principle, to the fact that the shape of the spectrum, i.e. the presence of peaks corresponding to Al species in tetrahedral or octahedral coordination, their exact position and sharpness, depend strongly on a series ∗ Tel.: +52-55-5622-53-71; fax: +52-55-5622-53-55. E-mail address: [email protected] (T. Klimova).
of factors. Among them, the particular nature of the material, its hydration state, the presence of additional cationic or anionic species interacting with aluminum ions, pretreatment by drying or calcination can be mentioned. In general, it is well known that two kinds of Al species can be distinguished by solid-state 27 Al MAS-NMR spectroscopy, namely, Al species in tetrahedral coordination (a signal at about 50–60 ppm) and octahedrally coordinated Al atoms (a signal around 0 ppm) [3]. However, the exact position of each of these peaks can be changed depending on the chemical environment of Al atoms. For example, the characteristic signal of aluminum in Al2 (MoO4 )3 appears at −14 ppm, that is far-away from the above-mentioned
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00504-0
322
T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
signals at 0 or 50–60 ppm [4,5]. Additionally, it has been found that sometimes the same NMR peak can be attributed to different Al species. For example, the peak at 0 ppm frequently observed in the NMR spectra of acidic forms of zeolites or MCM-41 materials can represent the “real” extra-framework aluminum species or octahedral framework Al that is partially associated to the silica lattice and is the host of two additional water molecules from which one is the hydroxonium ion [6,7]. Also it has been observed that sometimes Al atoms can change reversibly their coordination from octahedral to tetrahedral one as a result of a relatively mild treatment, as it was observed for acidic forms of zeolites or MCM-41 materials after ion exchange with NH4 + or Na+ cations [6]. All these considerations explain why sometimes it is difficult to ascribe unambiguously the signals of the 27 Al MAS-NMR spectra to an exact type of Al species, especially as regards the intermediate peaks appeared in the 27 Al NMR spectrum between 0 and 50–60 ppm. These intermediate signals may be attributed to Al species in intermediate coordination state, like penta-coordinated Al [3,6], the existence of which is still a matter of debate, or to tetrahedral or octahedral Al species, the signals of which are significantly shifted from their “normal” positions due to a specific chemical surrounding. From this point of view, the comment by Carrier and Che [1] gives a new interpretation for the NMR signal observed by us at 15 ppm in the 27 Al MAS-NMR spectra of Mo and NiMo/Al-MCM-41 catalysts relating its appearance with the formation of an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− . The formation of aluminomolybdate anion was first proposed for alumina-supported Mo catalysts [5] and more recently it has been extended to Mo-containing Y-zeolite-supported catalysts [4].
1. Hydration state of the samples and its influence on the 27 Al MAS-NMR spectra First, some additional information must be provided in order to understand better the results of the NMR measurements presented in Fig. 7 and 8 of [2]. Solid-state 27 Al MAS-NMR spectra were obtained with an ASX300 Bruker NMR spectrometer using [Al(H2 O)6 ]3+ as the external 27 Al reference.
The spectra were recorded at room temperature, at a source frequency of 78.210 MHz and a spinning rate of 12 kHz. A 2.0 s pulse was used with a repetition time of 0.5 s between pulses. Four thousand and ninety-six scans were accumulated before Fourier transform. Samples were used after 2-month storage and were equilibrated in ambient air (for 48 h) before the NMR characterization. It should be mentioned then, that the spectra presented in Fig. 7 and 8 were obtained with 3-month interval. Although the sample preparation procedure was the same in both cases, some variation in ambient humidity could have taken place and therefore the degree of hydration of the two series of samples could be different. It can be assumed that the samples in Fig. 7 were more hydrated than those in Fig. 8. This is evidenced from the sharpness of the peaks in Fig. 7. Thus, as has been reported previously, sharp peaks in solid-state 27 Al MAS-NMR spectra of Al-containing materials correspond to well-ordered hydrated samples, whereas broad peak reflect the geometrically more stressed Al species (dry samples) [4,6]. Therefore, the spectra presented in Fig. 7 or 8 can be compared within the same series, however, it is difficult to compare the spectra in Fig. 7 with their counterparts in Fig. 8.
2. Assignment of the peak at 15 ppm Mo/Al-MCM-41 catalysts In the 27 Al MAS-NMR spectra of Mo/Al-MCM-41 catalysts presented in Fig. 7c and 8c of [2], a peak at +15 ppm was observed. A similar peak at +13 ppm has been observed previously by Edwards and Decanio [8] in the 27 Al CP-MAS spectrum of P-Mo/Al2 O3 catalyst, and it was attributed to a “hydrated form of an aluminum molybdate Al2 (MoO4 )” with tentative formula [Al(OH)n (H2 O)6−n O18 ]n (MoO4 ) (n = 1 or 2). More recently, Plazenet et al. [4] and Carrier et al. [5] have assigned the peak at +15 ppm in the 27 Al MAS-NMR spectra of Mo/zeolite or Mo/␥-alumina catalysts to an Anderson-type heteropolymolybdate [Al(OH)6 Mo6 O18 ]3− . In both cases, the formation of the aluminomolybdate ions were explained through dissolution of alumina and reaction of dissolved Al3+ with heptamolybdate during impregnation step of the catalyst preparation. Additionally, it was shown that in the case of HY zeolite [4] solely non-framework Al
T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
species could be dissolved giving rise to the formation of [Al(OH)6 Mo6 O18 ]3− . The decomposition of the Anderson-type heteropolyanions upon calcination yields both MoO3 and Al2 (MoO4 )3 . Additionally, it was shown by Plazenet et al. [4] that rehydration of calcined Mo/Y-zeolite catalysts in water-saturated atmosphere for 48 h leads to partial conversion of Al2 (MoO4 )3 into [Al(OH)6 Mo6 O18 ]3− along with solvated Al3+ . Taking all the above-mentioned considerations into account, it can be expected that in the Mo catalysts characterized by 27 Al MAS-NMR spectroscopy in our work an Anderson-type heteropolyanion could also be formed. The peak at +15 ppm corresponding to [Al(OH)6 Mo6 O18 ]3− was detected for calcined and then rehydrated Mo/Al-MCM-41 catalysts. The formation of a heteropolyanion can be attributed therefore to hydration of Al2 (MoO4 )3 according to the reaction proposed by Carrier et al. (reaction (II) [1]) leading to [Al(OH)6 Mo6 O18 ]3− and solvated aluminum cations [Al(H2 O)6 ]3+ . The peak at 0 ppm corresponding to the latter species can also be observed in Fig. 7c and 8c [2]. From this point of view, it also becomes clear why the formation of Al2 (MoO4 )3 in the calcined catalysts was clearly documented by XRD and TPR [2], however, the peak at −14 ppm characteristic of aluminum molybdate was not observed in the 27 Al MAS-NMR spectrum of one of the hydrated Mo catalysts (Fig. 7c). The behavior of calcined Mo/Al-MCM-41 catalysts upon rehydration proved to be very similar to that of Mo/Y-zeolite catalysts as described by Plazenet et al. [4]. Hence, it seems logical to assume that [Al(OH)6 Mo6 O18 ]3− was likewise formed during the impregnation of Al-MCM-41 supports with ammonium heptamolybdate solution and it gives rise to the formation of Al2 (MoO4 )3 during calcination. Previously, we have supposed that Al2 (MoO4 )3 is formed as a result of the interaction of Mo species with “highly-reactive” extra-framework Al species. In line with this supposition and the observations of Plazenet et al., now we can assume that this interaction leads to the appearance of an Anderson-type heteropolymolybdate. Hence, it can be concluded that recent publications by Carrier and co-workers [1,5] and Plazenet et al. [4] and our results of the 27 Al MAS-NMR characterization of hydrated Mo/Al-MCM-41 catalysts point
323
to the formation of aluminomolybdate during impregnation, its further evolution to Al2 (MoO4 )3 upon calcination, and its re-appearance upon rehydration, for three different Al-containing supports: ␥-alumina, Y-zeolite and Al-MCM-41. However, it is not clear yet whether this behavior can be generalized to any other Al-containing support.
3. Effect of the hydration on NiMo/Al-MCM-41 catalysts The 27 Al MAS-NMR spectra of hydrated NiMo/ Al-MCM-41 catalysts (Fig. 7d and 8d of [2]) differ from those of the corresponding unpromoted Mo catalysts (Fig. 7c and 8c, respectively). Namely, the peak at 15 ppm is present in both cases (Mo and NiMo catalysts), however, the peak at 0 ppm is clearly observed only for Mo catalysts. Therefore, it was interesting to follow more carefully the evolution of NiMo/Al-MCM-41 catalysts upon hydration. Taking this into account, we repeated the 27 Al MAS-NMR characterization of NiMo/Al-MCM-41 catalysts with a careful control of the sample humidity. In this case, we used NiMo catalysts supported on Al-MCM-41(30) and Al-MCM-41(15) that were prepared following the procedure described in [2]. After calcination (400 ◦ C, 2 h) these catalysts have been stored for 1 month in contact with ambient air (stored samples, NiMo/Al-MCM-41(30)-S and NiMo/Al-MCM-41(15)-S). The 27 Al MAS-NMR spectra of these samples was recorded without additional treatment. After that, each sample was divided into two parts. The first part was dried at 100 ◦ C for 48 h and then transferred into a glovebox filled with dry N2 to prepare the sample for NMR measurement (dried samples, NiMo/Al-MCM-41(30)-D and NiMo/Al-MCM-41(15)-D). The second part was equilibrated for 48 h at room temperature in a desiccator at a relative humidity of 79% generated by a saturated solution of NH4 Cl to measure the 27 Al NMR spectra of this stored and then rehydrated sample (hydrated samples, NiMo/Al-MCM-41(30)-H and NiMo/Al-MCM-41(15)-H). These steps were aimed at evaluating the effect of storage, drying and rehydration of the calcined NiMo/Al-MCM-41 samples.
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T. Klimova / Applied Catalysis A: General 253 (2003) 321–325
Fig. 1. 27 Al MAS-NMR spectra of NiMo/Al-MCM-41(30) catalyst after drying (a), 1-month storage (b) and rehydration (c).
The 27 Al MAS-NMR spectra of stored, dried and rehydrated NiMo/Al-MCM-41 catalysts are presented in Fig. 1 and 2. In the 27 Al MAS-NMR spectra of dried NiMo/ Al-MCM-41 catalysts (Fig. 1a and 2a), two peaks are observed. In addition to the line at 50 ppm assigned to tetrahedral framework Al species, a signal at about −14 ppm characteristic of Al2 (MoO4 )3 can be detected. In NiMo catalysts, the presence of aluminum molybdate can be expected judging
Fig. 2. 27 Al MAS-NMR spectra of NiMo/Al-MCM-41(15) catalyst after drying (a), 1-month storage (b) and rehydration (c).
from TPR results where high-temperature (at about 700 ◦ C) H2 consumption was observed. However, the formation of Al2 (MoO4 )3 was not detected by XRD, probably, due to its higher dispersion in the presence of Ni. The intensity of the peak at −14 ppm is higher in the spectrum of NiMo catalyst supported on MCM-41 with higher Al loading (Al-MCM-41(15)) and therefore higher proportion of extra-framework Al in the support (Fig. 2a). It confirms that Al2 (MoO4 )3 forms due to the interaction of Mo species with “highly-reactive” extra-framework Al of the support. The spectra of the same NiMo catalysts but after 1-month storage in contact with ambient air were different (Fig. 1b and 2b). The intensity of the peak at −14 ppm substantially decreased and a new peak at about +16 ppm appeared. Hydrated samples, NiMo/ Al-MCM-41(30)-H and NiMo/Al-MCM-41(15)-H, preserved in a closed water-saturated atmosphere for 48 h, showed only two peaks (at 53 and +16 ppm) in the solid-state 27 Al MAS-NMR spectra (Fig. 1c and 2c). The signal corresponding to aluminum molybdate (−14 ppm) was completely absent. This illustrates how drastically the hydration state of the sample can change its 27 Al MAS-NMR spectrum. The opposite change in the intensities of the peaks at −14 and +16 ppm, i.e. its decrease for the former peak and simultaneous increase for the latter, point to the formation of aluminomolybdate species (the peak at +16 ppm) from aluminum molybdate (the peak at −14 ppm) as a result of hydration. The previous results allow one to mention some interesting points making difference between Mo and NiMo catalysts. For both cases of Mo and NiMo catalysts, the formation of Anderson-type heteropolyanions can be related with hydration of Al2 (MoO4 )3 . However, if for Mo catalysts this hydration leads to the formation of both the Anderson-type anions (+15 ppm) and solvated Al3+ (0 ppm), for NiMo catalysts only the peak at +16 ppm is clearly observed. Solvated partially mobile octahedral Al (0 ppm) is absent. Therefore, it is necessary to clear up whether it is not formed during hydration of the NiMo catalysts or it changes the octahedral coordination to tetrahedral one as a result of the interaction with Ni2+ . Previously, it has been reported that the Andersontype heteropolyanion is destroyed between 230 and 250 ◦ C [9]. In good agreement with this, Plazenet
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et al. [4] observed that the aluminomolybdate anion obtained in Mo/Y-zeolite catalysts was preserved after drying at 100 ◦ C overnight. However, our results show that in the case of NiMo/Al-MCM-41 systems drying at 100 ◦ C for 48 h easily yields Al2 (MoO4 )3 (peak at −14 ppm, Fig. 1a and 2a). These results show that the behavior of the Anderson-type heteropolyanions can be different depending on whether they were formed during the catalyst preparation, namely, Mo impregnation stage through alumina dissolution in the presence of molybdate, or are “reformed” upon rehydration of the pre-calcined catalyst. There is no information which allows explanation of this difference. More complete characterization of NiMo/Al-MCM-41 systems is needed to clarify these points.
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