NMR investigations of nanostructured ceramics

Edgar, Mark; Schubert, Markus; Limbach, H. H.; Göltner, Christine G.

doi:10.1002/bbpc.19971011147

Cited by 7 publications

(7 citation statements)

References 11 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We would like to mention that a similar effect of a “low-dimensional” motion of pyridine was previously found in a comparative study of a H I and a MCM-41 silica sample. In these experiments very weak, anisotropic signals were registered for the MCM-41 silica . Our experiments show that the discussed effect has the same nature for the both H I and MCM-41 silica and the difference has been caused, probably, because of a relatively high surface roughness of the MCM-41 sample, employed in the former study.…”

Section: Discussionmentioning

confidence: 47%

“…By crystallizing a number of 1:1 complexes of 2,4,6-trimethylpyridine (collidine)- 15 N with a number of different acids a correlation of the 15 N NMR isotropic and anisotropic chemical shifts with the dipolar 2 H− 15 N couplings and hence the nitrogen−hydrogen distances and p K a values has been observed using solid state NMR . These correlations allow us now to explore the nitrogen−hydrogen distance distribution function of pyridine and its derivatives in mesoporous solids, as some of us have shown in a preliminary communication, that it is possible to obtain solid state 15 N NMR spectra of pyridine- 15 N in mesoporous solids . On the other hand, the diffusion coefficient of pyridine in these materials was found to be strongly reduced as compared to bulk pyridine, which was attributed to hydrogen bonding to hydroxyl groups of the inner surfaces …”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

et al. 2003

View full text Add to dashboard Cite

The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 15N CPMAS and MAS experiments (CP ≡ cross-polarization) performed on pyridine-15 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be n OH ≈ 3 nm-2 for MCM-41 and ≈3.7 nm-2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers n OH. Using a correlation established previously between the 15N and 1H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine−hydroxyl pairs were found to be about r HN = 1.68 Å, r OH = 1.01 Å, and r ON = 2.69 Å. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a of about 4. Room-temperature 15N experiments on static samples of pyridine-15 N in MCM-41 at low coverage show a residual 15N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C 2 axis is suppressed even at room temperature, and that the angle between the Si−O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2H NMR experiments performed on pyridine-4-d 1. In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 29Si CPMAS of a substantial amount of Si(OH)2 groups in SBA-15, in contrast to the MCM-41 materials. The Si(OH)2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including t...

show abstract

Section: Discussionmentioning

confidence: 47%

Section: Introductionmentioning

confidence: 99%

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

et al. 2003

View full text Add to dashboard Cite

show abstract

“…Total surface areas calculated by the BET algorithm can be as high as 1000 m 2 g 21 while a mesopore system is capable of providing just 200-300 m 2 g 21 surface area, which means the surface area is dominated by micropores (70-80%), and adsorption does not correlate to the symmetry of the mesopores, which explains a number of the NMR observations on so-called mesoporous silica. 47,48 It is obvious from these comments that no single analytic techniques reveals all the characteristics of a porous sample.…”

Section: Analysis Of Porous Materials: Problems and Developmentsmentioning

confidence: 99%

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

2002

View full text Add to dashboard Cite

Nanocasting, the 3D-transformation of self-assembled organic nanostructures into hollow inorganic replicas under preservation of fine structural details has recently turned out to be a versatile tool, both for the synthesis of porous media with new pore topology as well as for the characterization of the assembled structures themselves. This review gives a review on recent work describing the potential and restrictions of nanocasting using surfactants, polymers, colloids as well as supramolecular tectons as porogens.

show abstract

“…The potential for catalytic application has triggered several recent studies of the dynamics of guest molecules in mesoporous silica. 1,14,17,20,23,26,31,34 A direct consequence of the solid-liquid interaction at the pore surface is the perturbation of the three-dimensional ordering of the guest molecules during formation of crystal structures when freezing the guest phase. Figures 1c-f schematically display possible scenarios of the arrangement of guest molecules in the pores and the resulting surface and pore-volume phases.…”

Section: Introductionmentioning

confidence: 99%

“…For possible catalytic applications the study of transport phenomena in the silica channels is of great interest, because the guest molecules need efficient transport channels to and from the inner surfaces of the pores, such that they can react, and afterward leave the pores and thus the catalytic power of the silica material is closely related to the transport process of the guest molecules through the pores. The potential for catalytic application has triggered several recent studies of the dynamics of guest molecules in mesoporous silica. ,,,,,,, …”

Section: Introductionmentioning

confidence: 99%

²H-Solid-State NMR Study of Benzene-d₆ Confined in Mesoporous Silica SBA-15

et al. 2002

View full text Add to dashboard Cite

Benzene-d 6 confined in the hexagonal ordered cylindrical pores of mesoporous silica SBA-15 (pore diameter 8.0 nm) was studied by low-temperature 2H-solid-state NMR spectroscopy in the temperature range between 236 and 19 K and compared to bulk benzene-d 6. The solid-state spectra of the bulk benzene-d 6 exhibit quadrupolar Pake patterns at high and low temperatures, and in the intermediate temperature regime the typical line shape changes caused by rotational jumps around the 6-fold axis. At all temperatures the benzene molecules are characterized by a single rotational correlation time. For benzene-d 6 confined in SBA-15, however, these exchange dominated line shapes are not found. At all temperatures below the freezing point the spectra of benzene in the silica show the coexistence of two states with temperature-dependent intensity ratios. This behavior is the result of a Gaussian distributions of activation energies for the rotational jumps inside the pores. For the solid I−solid II (fast 6-fold jump to slow 6-fold jump) transition the center of the distribution is at 40 K (6.0 kJ/mol) with a width of 19.5 K (2.9 kJ/mol). For the liquid−solid I (liquidlike to fast 6-fold jump) transition the center of the distribution is at 204 K (30.6 kJ/mol) and the width is 15 K (2.2 kJ/mol). From the pore volume and the filling factor, a thickness of four molecular layers of this surface phase is estimated.

show abstract

NMR investigations of nanostructured ceramics

Cited by 7 publications

References 11 publications

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

²H-Solid-State NMR Study of Benzene-d₆ Confined in Mesoporous Silica SBA-15

Contact Info

Product

Resources

About

NMR investigations of nanostructured ceramics

Cited by 7 publications

References 11 publications

Pyridine-15N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

Pyridine-15N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

2H-Solid-State NMR Study of Benzene-d6 Confined in Mesoporous Silica SBA-15

Contact Info

Product

Resources

About

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

Pyridine-¹⁵N A Mobile NMR Sensor for Surface Acidity and Surface Defects of Mesoporous Silica

²H-Solid-State NMR Study of Benzene-d₆ Confined in Mesoporous Silica SBA-15