Nature of the framework hydroxyl groups in steamed H-RHO and H-erionite zeolites

Shannon, R. D.; Staley, Ralph H.; Vega, Alexander J.; Fischer, R. X.; Baur, W. H.; Auroux, A.

doi:10.1021/j100342a062

Cited by 18 publications

(6 citation statements)

References 3 publications

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“…The sideband manifolds observed in the 1 H/ 27 Al TRAPDOR NMR difference spectra of the resonance at 2.8 ppm (Figure b) are slightly larger than those observed for the Brønsted acid site. The sideband manifold from the resonance at 2.7 ppm, observed in the difference spectrum at 4MMA-DY, at a 27 Al offset frequency of 1.0 MHz (also assigned to an Al−OH species), can be fit with a dipolar coupling of 2.2 (±0.2) kHz, which is again slightly larger than the dipolar coupling constants of 2.0 kHz, typically observed for the Brønsted acid sites but is in agreement with observation of a larger static line width (8 kHz, full width at half-height) for the resonance at 2.4 ppm in steamed HRHO . As discussed in our earlier paper, the TRAPDOR experiment will not affect all parts of the powder pattern equally: different 1 H spins in the powder pattern are coupled to 27 Al spins in different Zeeman levels and consequently have different dipolar couplings.…”

Section: Discussionsupporting

confidence: 86%

“…The sideband manifold from the resonance at 2.7 ppm, observed in the difference spectrum at 4MMA-DY, at a 27 Al offset frequency of 1.0 MHz (also assigned to an Al-OH species), can be fit with a dipolar coupling of 2.2 ((0.2) kHz, which is again slightly larger than the dipolar coupling constants of 2.0 kHz, typically observed for the Brønsted acid sites but is in agreement with observation of a larger static line width (8 kHz, full width at half-height) for the resonance at 2.4 ppm in steamed HRHO. 41 As discussed in our earlier paper, 18 the TRAPDOR experiment will not affect all parts of the powder pattern equally: different 1 H spins in the powder pattern are coupled to 27 annihilation of the 1 H spectrum, the sideband manifolds in the difference spectrum can be assumed to be representative of those of the true spectrum. The Al-H dipolar coupling is not consistent with a proton coupled to more than one 27 Al spin, e.g., in bridging Al-O(H)-Al groups, such as those proposed to occur on alumina surfaces, 30 where a significantly larger dipolar coupling is expected.…”

Section: Discussionmentioning

confidence: 99%

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Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Kao

Grey

1996

J. Phys. Chem.

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A combination of variable temperature 1 H, 15 N, and 27 Al MAS NMR and 1 H/ 27 Al and 15 N/ 27 Al TRAPDOR NMR has been used to study the adsorption of monomethylamine (MMA) on dehydrated zeolite HY. Monomethylammonium cations (MMAH + ) are formed, which at temperatures below -40°C are rigidly bound to the zeolite framework. At loading levels of MMA that exceed the number of Brønsted acid protons, 1 H resonances intermediate in chemical shift between those of MMA and MMAH + are observed from species undergoing rapid proton transfer reactions between MMA and MMAH + . These exchange processes are frozen out at -140°C, and MMAH + and physisorbed MMA are observed. The deprotonation of the Brønsted acid SiO(H)Al site, and the consequent formation of MMAH + , results in a reduction of the 27 Al quadrupole coupling constant (QCC) of the nearby aluminum atom by more than 12 MHz, and a QCC for this site of 3.0 ((0.3) MHz is determined. Rehydration of HY and then subsequent calcination at 400°C results in the formation of silanols, two aluminum hydroxide species, and Lewis acid sites. The aluminum hydroxide species with associated 1 H resonances at 2.8 and 0.9 ppm have 27 Al QCCs of g11.7 and 10.0 ((0.8) MHz, respectively. MMA bound to the Lewis acid site was observed with 15 N and 1 H MAS NMR. Significant mobility of the MMA species, and proton transfer reactions between MMAH + and MMA coordinated to a Lewis acid site, was detected in these samples at ambient temperatures; these processes were frozen out at -150°C, and discrete resonances from MMAH + , a Lewis acid-MMA complex, and physisorbed MMA were observed. 15 N/ 27 Al TRAPDOR NMR demonstrates that the MMA in the Lewis acid-MMA complex is directly bound to an aluminum atom at the Lewis acid site, with a 27 Al QCC of 8.3 ((0.3) MHz. The Al-N internuclear distance was estimated to be shorter than 2.7 Å.

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Section: Discussionsupporting

confidence: 86%

Section: Discussionmentioning

confidence: 99%

Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Kao

Grey

1996

J. Phys. Chem.

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“…From various MC measurements an average adsorption energy of 146 f 23 kJ/mol on different types of H and H/Na forms of zeolites is found.6.2IJ6. [29][30][31][32][33][34] If the activation energy for desorption is interpreted as the heat of adsorption there is an ambiguity in the choice of the method tocalculate the Mac' from theTPD spectrum. Different methods can give results that are quite different.…”

Section: Survey Of Experimental Datamentioning

confidence: 99%

Ammonium in zeolites: coordination and solvation effects

Teunissen¹,

Santen²,

Jansen³

et al. 1993

J. Phys. Chem.

172

115

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Proton transfer from a zeolitic cluster to NH3 and subsequent coordination of the ammonium cation onto the zeolitic cluster are studied by using a b initio quantum chemical cluster calculations. Proton transfer from the zeolite cluster to NH3 is favorable if, after proton transfer, the resulting NH4+ cation is coordinated to the zeolitic cluster with two or three hydrogen bonds. These structures are referred to as 2H and 3H, respectively. Their adsorption energies the energy needed for the process of proton transfer followed by the binding of the NH4+ cation, are calculated to be -1 14 and -1 13 kJ/mol, respectively. The geometries were optimized at the S C F level and the adsorption energies were calculated at the second-order Mprller-Plesset perturbation theory level (MP2), using the counterpoise correction (CPC) to avoid the basis set superposition error (BSSE). The basis set is the 6-3 1 l+G(d,p)/STO-3G one, which has previously been shown to give proper binding and proton transfer energies. The calculated heats of adsorption compare well with experimental heats of desorption. Proton transfer also occurs when another NH3 molecule is coadsorbed. However, the process of coadsorption is energetically less favorable than the 2H and 3 H structures: the adsorption energy per NHs molecule is only -30 kJ/mol. For the clusters the N-H stretching frequencies have been calculated at the SCF level in the harmonic approach. They have been compared with experimental spectra of the NH4+ forms of some zeolites.The N-H stretching region of these spectra can be explained as a superposition of the spectra of the 2H and 3H structures. By comparison of the adsorption energy on a geometry optimized cluster and on a fixed geometry cluster, it was found that the choice of the geometry is important. On enlarging the fixed geometry cluster the adsorption energy remained constant.

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“…Although the existence of water is unlikely from the preparation conditions, water could have diffused through small leaks of the sample can. Another possible interpretation is the assignment to n.f.a, condensed to form a neutral A1203 or AIOOH complex, as discussed by Shannon, Staley, Vega, Fischer, Baur & Auroux (1989) for zeolite rho. Strong evidence for such n.f.a, species is given by the 29Si and 27A1 MAS NMR results of the former studies (Baur et al, 1987) of the same sample in its unloaded hydrogen (deuterium) form.…”

Section: Rho-i-5mma Nmentioning

confidence: 99%

Zeolite Rho Loaded with Methylamines. I. Monomethylamine Loadings

Weidenthaler¹,

Fischer²,

Abrams³

et al. 1997

Acta Crystallogr Sect B

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Samples of two differently prepared zeolite rho loaded with different amounts of monomethylamine (MMA) were studied in their hydrated and dehydrated forms by X-ray and neutron diffraction. Both zeolites are partially dealuminated, as indicated by nonframework alumina, which is assumed to be A1203 or A1OOH. Series I was prepared from dry-calcined NHa-rho at 873K, series II from steam-calcined NHn-rho at 773K. The samples were loaded with different amounts of deuterated MMA, Rietveld refinements yielded the following results for series I (dry): (4), anhydrous and deuterated, neutron data collected at 5 K, disproportionation into two phases in 12~3m with a = 14.9151 (2) and 14.6475 (8)/~, Rwp = 0.031. In the hydrated samples MMA resides on the center axis in the c~-cage with the N atoms pointing to the single eight-ring; upon dehydration it migrates into the double eight-rings.

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Nature of the framework hydroxyl groups in steamed H-RHO and H-erionite zeolites

Cited by 18 publications

References 3 publications

Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Ammonium in zeolites: coordination and solvation effects

Zeolite Rho Loaded with Methylamines. I. Monomethylamine Loadings

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Nature of the framework hydroxyl groups in steamed H-RHO and H-erionite zeolites

Cited by 18 publications

References 3 publications

Probing the Brønsted and Lewis Acidity of Zeolite HY: A 1H/27Al and 15N/27Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Probing the Brønsted and Lewis Acidity of Zeolite HY: A 1H/27Al and 15N/27Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Ammonium in zeolites: coordination and solvation effects

Zeolite Rho Loaded with Methylamines. I. Monomethylamine Loadings

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Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY

Probing the Brønsted and Lewis Acidity of Zeolite HY: A ¹H/²⁷Al and ¹⁵N/²⁷Al TRAPDOR NMR Study of Monomethylamine Adsorbed on HY