Abstract
Isotopic enrichment of 29Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt %) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP, and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense 1H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si–O–Si linkages at the surface (identified as Q4(3Al)–Q4(3Al)) and confirm that the anchoring of the surface overlayer with the alumina occurs through AlIV and AlV species only. This suggests the presence of Q3/Q4 Si at the surface affects the neighboring Al species, modifying the surface structure and making it less likely AlVI environments are in close spatial proximity. In contrast, Q1/Q2 species, bonded to the surface by fewer covalent bonds, have less of an effect on the surface, and more AlVI species are consequently found nearby. The combination of isotropic enrichment and DNP provides a definitive and fully quantitative description of the Si-modified alumina surface, and we demonstrate that almost one-third of the silicon at the surface is connected to another Si species, even at the low level of coverage used, lowering the propensity for the formation of Brønsted acid sites. This suggests that a variation in the synthetic procedure might be required to obtain a more even coverage for optimum performance. The work here will allow for more rigorous future investigations of structure–function relationships in these complex materials.
Highlights
Silicated aluminas are commonly employed as solid acid catalysts, with applications in a number of processes ranging from ethanol dehydration to hydrocarbon cracking and skeletal isomerization.[1−4] The presence of both Si and Al at the surface generates the mild acidity that is essential to catalytic behavior,[1] but the exact structure of these acidic environments is still debated.[2−4] Early studies of catalytic cracking postulated that Brønsted acidity is attributable to aluminol groups in close proximity to silanols[5] or protons that compensate for the negative charge at the surface.[6,7]
The direct polarization (DP) and CP spectra exhibit better resolution than the dynamic nuclear polarization (DNP) spectrum, as a result either of increased relaxation arising from the presence of the radical or, more likely, of the lower temperature of the experiments, leading to a broader distribution of shifts
In contrast to the materials studied in ref 24, there are Article transfer to all Si species regardless of their chemical nature and OH functionality
Summary
Silicated aluminas are commonly employed as solid acid catalysts, with applications in a number of processes ranging from ethanol dehydration to hydrocarbon cracking and skeletal isomerization.[1−4] The presence of both Si and Al at the surface generates the mild acidity that is essential to catalytic behavior,[1] but the exact structure of these acidic environments is still debated.[2−4] Early studies of catalytic cracking postulated that Brønsted acidity is attributable to aluminol groups in close proximity to silanols[5] or protons that compensate for the negative charge at the surface.[6,7] More recent investigations (primarily using IR spectroscopy, probe molecule adsorption, and 1H MAS NMR spectra) propose that the catalytic acid sites are bridging Si−OH−Al groups[8−10] or silanols in the vicinity of AlIII, AlIV, or AlV.[2,3] Identifying the true origins of the catalytic response demands an atomic-level description of the reactive surface, but this is far from trivial. We exploit both isotopic enrichment of 29Si and DNP enhancement to determine the detailed surface structure of a Si-γ-Al2O3 material with 1.5 wt % Si doping This combined strategy enables us to obtain the quantitative NMR spectra that are so key to understanding surface structure, and to exploit the significant sensitivity advantages offered by DNP and acquire multidimensional experiments that would not be possible otherwise. Line shape fitting was carried out using dmfit.[45]
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