Al-27 NMR measurements in the liquid and the solid state supplemented with chemical analysis of the mixed salts; unambiguously reveal that neither tetrahedral nor octahedral aluminium can be substituted by either iron or chromium in Al-13-Keggin ion salts prepared by conventional methods

Barrault

Bergaya

Bertram

Bradely

Cotton

Imre Kiricsi

István Palinkó

Jean-Christophe Bertrand

János B. Nagy

Kiricsi

Kostapapas

Pálinkó

Singh

Zhao

English

Crossref

On the feasibility of iron or chromium substitution for aluminium in the Al
                    13

Nagy János B.

Bertrand Jean-Christophe

Pálinkó István

Kiricsi Imre

SZTE Publicatio Repozitórium - SZTE - Repository of Publications

On the feasibility of iron or chromium substitution for aluminium in the Al13-Keggin ion

Nagy, J. B.

Bertrand, J. C.

Pálinkó, István

Kiricsi, Imre

Repository of the Academy's Library

J. CHEM. SOC., CHEM. COMMUN., 1995 2269 On the Feasibility of Iron or Chromium Substitution for Aluminium in the AIl3-Keggin Ion Janos B. Nagy," Jean-Christophe Bertrand,a Istvan Palinkob and lmre Kiricsi*c a Unite de Resonance Magnetique Nucleaire, Facultes Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, Namur, 6-5000 Belgium b Department of Organic Chemistry, Jozsef Attila University, Dom ter 8, Szeged, H-6720 Hungary c Applied Chemistry Department, Jozsef Attila University, Rerrich B. ter 1, Szeged, H-6720 Hungary *7AI NMR measurements in the liquid and the solid state supplemented with chemical analysis of the mixed salts unambiguously reveal that neither tetrahedral nor octahedral aluminium can be substituted by either iron or chromium in AIl3-Keggin ion salts prepared by conventional methods. Modification of microporous materials often lead to substances with unusual physical and chemical properties. One such property, the ability to catalyse various transformations, can be drastically influenced by the isomorphous substitution of atoms in the framework or pillars of, for instance, zeolites and pillared layer materials, respectively. Isomorphous substitution changes catalytic properties in various ways, e.g. the size and acidity of the active sites may be altered or elements with redox properties can be incorporated. For pillared layer clays (PILCs), it is hard to tell whether isomorphous substitution does indeed occur, or whether merely co-hydrolysis and then co-pillaring takes place resulting in the formation of mixed pillars. Although ions of virtually every element have been claimed to be introduced into the tetrahedral as well as octahedral positions of the Al13-Keggin ion [A104A112(OH)24(H20)12]7+,1 the highly cited patent did not give any clear-cut evidence for this. Direct proofs for iso- morphous substitution are accumulating slowly and only for ions of certain elements such as germanium2 and gallium.3 Papers have appeared in the literature reporting the success of substituting iron or chromium into the tetrahedral as well as the octahedral positions in the All 3-Keggin i 0 n ; ~ 9  proof, however, is vague, especially since the pillared materials were investi- gated instead of the salt containing the polyhydroxy cations used for pillaring. This is clearly inadequate because the pillaring agents for Fe-PILC or Cr-PILC are anything but ions of Keggin type. In two extreme cases reported by the same authors (one for irons and another for chromiumg), the only proof presented was by citing ref. 1. Thinking over the problem, suspicion should arise at least for incorporating Fe3+ or Cr3+ to any of the positions, but especially the tetrahedral positions of the A113-Keggin ion, since their large size [Cr3+ octahedral (0.76 nm), ionic radius for tetrahedral coordination is not given; Fe3+ tetrahedral (0.63 nm, high-spin complex), octahedral (0.69 nm, low-spin complex, 0.79 nm, high-spin complex) compared to A13+, tetrahedral (0.53 nm), octahedral (0.68 nm)lo] should introduce consider- able strain to the polyhydroxy ion. While successful incorpora- tion of Fe3+ into zeolitic frameworks (isomorphous substitution of iron for aluminium) is well proven, in this instance, the whole crystal can help release any strain. There is another problem with iron substitution. Each study reports the partial hydrolysis of FeIII in the presence of A P .  It has been shown that in such a system the hydrolysis of the iron salt becomes extremely rapid, leading to various forms of iron hydrous oxides.11 Hydrolysis, however, can be slowed down when an iron(@ salt is applied, since FeILFeIII oxidation must precede hydrolysis. Thus, only by use of an iron@) salt can one have a chance to incorporate iron into the A113-Keggin structure. As far as Cr3+ is concerned, octahedral coordination is usually adopted. There are only very few examples of tetrahedrally coordinated Cr3+ and in such cases, the Cr3+ ion is at the centre of complex anion and the tetrahedron is highly distorted. 12 In this contribution we show that, contrary to widespread belief, co-pillaring is a feasible way of modifying A113-pillared layer clays when chromium or iron ions are the modifiers. Also in contrast to widespread practice we provide results obtained mostly on pillar precursors (ions in solution as well as in the crystalline state) instead of for pillared clays. For the preparation of the Al13-Keggin ion partial hydrolysis of AlC13 was applied, while for the synthesis of mixed or isomorphously substituted pillar ions the partial co-hydrolysis of AlC13 and CrC13, or AlC13 and FeC12, of varying composition were applied. The solution 27Al NMR measurements were applied (i) for checking the formation of the Keggin unit after the partial hydrolysis of AlC13 and (ii) studying the composition of the filtrate after treating the solution containing the partially hydrolysed AlCl3 with Na2S04 solution. This treatment led to the precipitation of a white crystalline material which was further studied by 27Al MAS NMR spectroscopy. The sulfate salts of the co-hydrolysed AlC13 + CrC13 or AlC13 + FeC12 mixtures were also studied by 27Al MAS NMR spectroscopy and the ratio of octahedral to tetrahedral aluminium was calculated from the integrated areas of the relevant resonances. Data are listed in Table 1. Data in Table 1 clearly show that the Keggin unit was formed in solution (entry 1) and was precipitated by Na2S04 (entries 2 and 3). Although the solid-state NMR spectra of the 'iso- morphously substituted' Keggin-sulfate salts are complex, resonances belonging to aluminium sitting in octahedral or tetrahedral positions can be easily identified. Table 1 Data obtained by quantitative 27Al NMR measurements Entry Nominal composition 60 (AH/Hz) 6' (AH/Hz) Alo/AlT Samples in the solution phase 1 AlI3-Keggin-C1 (Kl) -0.1 (25) 62.8 (9) 14.0 2 K1 + Na2S04 filtraten 0.0 (25) - - Samples in the solid state 3 A113S04 4.8 61.7 11.9 4 A1 12CrS04b -1.2, -10.1, -64.3, -107.4 61.4 11.5 5 A1 2FeS04c -10.0, -49, -107.2 60.6 8 .O 6 A111Fe2S04d - 12.0, -64.9, - 109.2 61.4 6.5 After partial hydrolysis of the AlC13 solution it was treated with Na2S04 solution resulting in the precipitation of a solid material and the filtrate was subjected to NMR measurement. b Initial A1C13 : CrC13 ratio of 12 : 1. c Initial A1C13 : FeC12 ratio of 12 : 1 d Initial A1C13 : FeC12 ratio of 1 1 : 2. 2270 J. CHEM. SOC., CHEM. COMMUN., 1995 After integration the Alo/AlT ratio decreases, instead of showing the tremendous increase that should occur if substitu- tion in the tetrahedral position really takes place. Incorporation of Cr (entry 4) seems to be negligible by comparison of the Alo/AIT ratio with that for AlI3-S04 (entry 3) assuming that tetrahedral sites are not occupied by Cr. Similar Alo/AlT ratios to those listed in Table 1 (entries 5 and 6) would be obtained for Fe,Al salts if some degree of tetrahedral and significant octahedral substitution occurred. The ionic radius of octahedrally coordinated low-spin Fe3+ indicates that the octahedral positions may be accessible. However, chemical analysis of the Fe,Al salts (dissolving the salt, titrating the iron and aluminium content) gave very low iron to aluminium ratios of 1 : 80-100 instead of 1 : 12 or 2 : 1 1 (entries 5 and 6). FT-IR13 on co-hydrolysed Keggin salts and 57Fe Mossbauer measurements on Fe,Al co-pillared mont- morillonite14 also revealed that overwhelmingly co-precipita- tion occurred instead of tetrahedral or octahedral substitution and that a small amount of iron hydrous oxide covers the AlI3- Keggin salt. This oxide, however, contains the iron in octahedral coordination. This work was supported by the National Science Foundation of Hungary through grant T014275. The financial help is highly appreciated. Received, 9th August 1995; Corn. 5105333A References 1 D. F. W. Vaughan, US Pat., 1987,4,666,877. 2 R. Bertram, S. Schonhen and H. Gorz, Z. Chem., 1984,24,225; 1987, 27, 183. 3 S. M. Bradely and R. A. Kydd, J .  Chem. SOC., Dalton Trans., 1993, 2407. 4 J. Barrault, C. Zivkov, F. Bergaya, L. Gatinau, N. Hassoun, H. van Damme and D. Mari, J.  Chem. SOC., Chem. Commun., 1988, 1403. 5 Y. W. Lee and B. J. Tatarchuk, Hyperfine Interact., 1988,41,661; Y. W. Lee, R. H. Raythatha and B. J. Tatarchuk, J .  Catal., 1989, 115, 159. 6 A. Kostapapas, S. L. Suib, R. W. C o u g h  and M. L. Occelli, Stud. Surf Sci. Catal., 1989, 49, 399. 7 F. Bergaya, N. Hassoun, J. Barrault and L. Gaineau, Clay Miner., 1993, 28, 109. 8 D. Zhao, G. Wang, Y. Yang, X. Guo, Q. Wang and J. Ren, Clays, Clay Miner., 1993,41, 317. 9 D. Zhao, Y. Yang and X. Guo, Zeolites, 1995,15, 58. 10 F. A. Cotton and G. Wilkinson, Advanced Znorganic Chemistry, 5th edn., Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1988, pp. 1387-1388. 11 S. S. Singh and H. Kodama, Clays Clay Miner., 1994,42,606. 12 F. A. Cotton and G. Wilkinson, Advanced Znorganic Chemistry, 5th edn., Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1988, p. 680. 13 I. Kiricsi, A. Molnhr, I. PhlinkB and K. Lhzk, Stud. Surf. Sci. Catal., 1995,94, 63. 14 I. Phlink6, K. Lhzir, I. Hannus and I. Kiricsi, J .  Phys. Chem. Solids, in the press. 

Hyperfine Interact.,

Surf Sci. Catal.,

On the feasibility of iron or chromium substitution for aluminium in the Al13-Keggin ion

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