Using Trimethylphosphine as a Probe Molecule to Study the Acid Sites in Al−MCM-41 Materials by Solid-State NMR Spectroscopy

Abstract

Using trimethylphosphine (TMP) as a probe molecule, acid sites in a series of Al-MCM-41 materials with Si/Al ratios ranging from 16 to 80 were investigated by various solid-state NMR techniques including 29 Si, 31 P, 27 Al, 1 H magic-angle spinning (MAS), and some double-or triple-resonance methods such as 31 P f 1 H cross polarization (CP), 1 H/ 27 Al, and 1 H/ 31 P/ 27 Al TRAPDOR (transfer of population in double resonance). By means of the 1 H/ 27 Al TRAPDOR technique, which has the ability to establish a correlation between 1 H and 27 Al, the hydroxyl groups associated with Al could be discriminated from the silanol groups. Two signals at 3.5 and 1.9 ppm were observed in the 1 H/ 27 Al TRAPDOR spectra; they are likely due to the bridging hydroxyl (SiOHAl) and aluminum hydroxyl groups, respectively. 31 P MAS NMR strongly supports the formation of zeolitelike Bro ¨nsted acid sites in the mesoporous material after the incorporation of aluminum. The concentration of the Bro ¨nsted sites determined from 31 P MAS NMR is about 2 orders of magnitude lower than that of total SiOH groups, which renders their direct observation by 1 H MAS NMR spectroscopy difficult. However, these sites can be well studied by 31 P f 1 H CP/MAS and various triple-resonance TRAPDOR NMR techniques, which provide a more detailed description of the interactions between TMP and the different hydroxyl groups in the Al-MCM-41 materials. Although the acid strength of the Bro ¨nsted sites in the Al-MCM-41 materials is lower than that of microporous zeolites such as HY, these sites can protonate TMP molecules. Some of the framework T-site aluminum atoms are not associated with the bridging hydroxyl groups and are probably present as Al-OH. No Lewis acid sites were found in our samples, though the 27 Al MAS spectra show the existence of five-coordinate extraframework aluminum.

In situsolid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

“…After that, it has been extensively used in acidity investigations of zeolites [79][80][81][82][83][84][85][86][87] and mesoporous materials. 88,89 The interaction of TMP with bridging hydroxyl groups in zeolites also leads to protonation of the probe molecules. 31 P MAS NMR investigations show that the protonated TMP occurs at about À3 to À5 ppm corresponding to the Brønsted acidic sites while the chemical shift of Lewis-bound TMP is in the range from about À30 to À60 ppm.…”

Application of Advanced Solid‐State NMR Techniques to the Characterization of Nanomaterials: A Focus on Interfaces and Structure

“…Solid-state NMR has been extensively used in the characterization of MMs and some recent review articles have already dedicated some relative attention to this topic [14,[107][108][109]. Generally speaking, the domain of MM is very broad and many NMR studies have investigated specific aspects such as structure [110][111][112][113][114][115][116][117][118][119][120][121][122][123][124], interactions at interfaces [8,[125][126][127][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][143][144], confinement of organic molecules within nanopores [145][146][147][148][149][150][151]…”

“…Shenderovich et al [157] outlined the existence of surface defects in MCM-41 and SBA-15 and this was related to the amount of Q 2 species, as observed after pyridine adsorption; trimethylphosphine (TMP) was instead used by Luo et al [148] to probe the surface of Al-MCM-41. 31 P, 1 H, and 27 Al NMR responses were investigated via CP-MAS and TRAPDOR experiments to probe the proximity of TMP to acidic surface sites.…”

Molecular dynamics simulation of liquid trimethylphosphine

“…[1][2][3][4][5] The first of such investigations was reported by Lunsford et al, 1 who described the role of Brönsted and Lewis acidity in governing catalytic activity in zeolites and catalytic surfaces, and proposed a method to evaluate the nature, strength, and number of acidic sites in a catalyst. According to Lunsford et al, 1 the 31 P nucleus is a more attractive candidate for surface characterization than 13 C and 15 N because 31 P possesses a large chemical-shift range and is more easily detected by NMR spectroscopy.…”

“…Since then, several works have been concerned on the use of TMP or TMPO as probe molecules of acidic sites. [3][4][5] For instance, Kao et al 2 indicated the advantage of TMP over other NMR probes, such as pyridine or methylamine, proper to high sensitivity of the 31 P nucleus due to its high magnetogyric ratio and natural abundance. They also performed ab initio calculations in order to investigate the structure of the TMP -Bronsted acid site complex in zeolite.…”