Article

Mutable Lewis and Bronsted Acidity of Aluminated SBA-15 as Revealed by NMR of Adsorbed Pyridine-N-15

Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany.
Langmuir (Impact Factor: 4.38). 08/2011; 27(19):12115-23. DOI: 10.1021/la2017566
Source: PubMed

ABSTRACT (1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Brønsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Brønsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.

0 Followers
 · 
139 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Two-step hydrodeoxygenation of benzyl phenyl ether (BPE), a lignin-derived phenolic dimer containing an alpha-O-4 linkage, was performed to produce high carbon number saturated hydrocarbons. The ether linkage of BPE was first isomerized to alcohols of benzylphenols on the solid acid catalysts of silica (SA), alumina (AA), and silica-alumina aerogels (SAAs), which were further hydrodeoxygenated to saturated cyclic hydrocarbons on a silica-alumina-supported Ru catalyst. During the isomerization of BPE, noble-metal-free catalysts suppressed the formation of phenyl monomers but produced the phenolic dimers. SA, AA, and SAA-73 (Al/(Si + Al)=0.73) exhibited negligible activity. However, SAA-38 and SAA-57 containing Al/(Si + Al) contents of 0.38 and 0.57, respectively, exhibited high catalytic activity among the prepared aerogel catalysts. The BPE conversion on SAA-38 reached 100% at a temperature range of 100-150 degrees C. The Bronsted acid sites appear to be catalytic active sites. On the basis of the predominant isomerization of phenyl ether to phenols over ether decomposition on the SAAs, the following second step of hydrodeoxygenation (H DO) after the first step of isomerization of BPE produced deoxygenated C13-19 cyclic hydrocarbons, as opposed to the saturated deoxygenated cyclic hydrocarbons produced trhough a one-step reaction process with silica-alumina-supported Ru catalysts, demonstrating this to be a promising process for producing high carbon number hydrocarbons from lignin dimers and oligomers. (c) 2013 Published by Elsevier B.V.
    Applied Catalysis B Environmental 10/2013; 142–143:668-676. DOI:10.1016/j.apcatb.2013.05.039 · 6.01 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Previous work demonstrated the ability of a trace amount of pyridine to stabilize Covalent Organic Frameworks (COF)-5 and -10 in humid air. Pyridine was found to form a mixture of Lewis and Brønsted B[4] Py–B complexes in addition to the un-complexed B[3] sites in the framework structures. Further research has shown that higher doses of pyridine convert all remaining B[3] in COF-5/-10 to Lewis B[4] and bring about the total and irreversible structural decomposition of COF-5 and COF-10. The results suggest that the accumulated strain in the five-member rings of COF-5/-10 resulting from the formation of tetrahedrally-distorted B[4] sites at high pyridine loadings, may explain the decomposition of these structures. Alternatively, COF-1 is unstable to exposure to humid air at all pyridine loadings tried, but is not unstable to high doses of pyridine. Whereas the same tetrahedrally-distorted B[4] sites are formed in COF-1, in this case the six-membered B3O3 ring can accommodate the accumulated ring strain and retain an ordered structure. A thorough solid state NMR and molecular dynamics investigation has led to a new proposed stabilization mechanism in humid air based on the formation of Brønsted B[4].
    10/2013; 1(42). DOI:10.1039/C3TA12515G
  • [Show abstract] [Hide abstract]
    ABSTRACT: Quantitative determination of acidic surface sites is highly important for the characterization of solid acids because the activity of a catalyst is often related to the concentration of these sites. A recently developed method using 15N Nuclear Magnetic Resonance spectroscopy (NMR) for the quantification of acidic Lewis and Brønsted sites has been tested for a series of nanoscopic aluminum hydroxide fluorides. Comparison with other methods for the quantitative determination of acidic sites shows that this 15N NMR quantification method is a promising technique for the comprehensive investigation of acidic sites. Three different acidic sites, one Brønsted and two Lewis sites, can be distinguished by their 15N chemical shifts of pyridine and simultaneously quantified under conditions corresponding to catalytic reaction conditions. Determination of the individual concentrations of acidic sites allows further insight into the catalytic process. It was found that the concentration of Brønsted sites correlates with catalyzed conversion of citronellal to isopulegol in the investigated series of catalysts. Additionally, investigations indicate that one of the Lewis sites become blocked during the reaction of citronellal.
    RSC Advances 10/2014; 4(100). DOI:10.1039/C4RA09477H · 3.71 Impact Factor