SOLID-STATE NMR STUDIES OF BRONSTED ACID SITES IN ZEOLITES UTILIZING THE PROBE MOLECULE TRIMETHYLPHOSPHINE OXIDE
Open Access
- Author:
- Karra, Murthy Dakshina
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 12, 2002
- Committee Members:
- James David Kubicki, Committee Member
Anne M Andrews, Committee Member
Karl Todd Mueller, Committee Chair/Co-Chair
Alan James Benesi, Committee Member - Keywords:
- zeolite
hy
trimethylphosphine oxide
zsm-5
faujasite
acid sites
nmr - Abstract:
- Zeolites are extremely efficient catalysts for a wide variety of industrial processes such as fluid cracking and xylene isomerization. It is well established that acid forms of zeolites are the most active in these reactions and the excellent catalytic abilities are thought to be due to the presence of Brønsted and Lewis acid centers or sites in the zeolite structure. The purpose of this study is to characterize these acid sites by using the 31P solid-state NMR of the probe molecule trimethylphosphine oxide adsorbed onto these zeolites. The 31P nucleus possesses excellent sensitivity and more importantly a large chemical shift range. This characteristic property of the 31P nucleus is essential to resolve the different acid sites in a zeolite based on their strength or ‘proton donating ability’. When a TMPO molecule is complexed to an acid site, there is a loss of electron density near the oxygen atom, which is relayed back to the 31P nucleus and is seen as a deshielding effect. The NMR resonances of the 31P nucleus are then shifted downfield and there have been suggestions in literature that this shift can be used as an indicator of acid strength. This study undertakes to prove, through the use of double- and triple-resonance NMR techniques that the resonances seen in the 31P spectrum of TMPO complexed to acid zeolites are either due to Brønsted or Lewis sites. The differences in structure between these two complexes are exploited to assign the acid sites. In particular since a Brønsted site is defined by the presence of a proton, dipolar connectivity experiments such as cross-polarization and rotational echo double resonance between protons and 31P nuclei are used to verify the nature of these resonances. Since TMPO contains three methyl groups, it is critical to replace the protons with deuterium hence ensuring that the source of the 1H magnetization is from the zeolite. The deuterated version of the probe molecule, TMPO-d9 has been synthesized and used extensively to study the acid sites in two very important zeolites, HY and HZSM-5 with varying Si/Al ratios. In HY, two types of acid sites have been resolved and identified as Brønsted sites. One resonance in the 31P spectrum is assigned to protons associated with bridging hydroxyl groups pointing into the supercages while the other to protons in the smaller sodalite cages. In the zeolite HZSM-5, three Brønsted resonances have been confirmed and agree with recent reports in literature, which identify three cation positions in this zeolite. As the Si/Al ratio is increased (and the overall acid site concentration decreases), it is found that the relative amounts of the stronger acid sites increases. On comparing the chemical shifts of the probe molecule bound to HY and HZSM-5, it also found that the resonances in HZSM-5 show a greater downfield shift. This result is in qualitative agreement with the general thinking that HZSM-5 contains stronger acid sites. The resonances of the probe molecule bound to zeolite H- appear at positions intermediate to HY and HZSM-5 and is again consistent with literature reports that H-Beta consists of acid sites that are intermediate in strength. The excellent resolution provided by TMPO, the high sensitivity of the 31P nucleus, the ease of the technique and most importantly the information content of the spectra makes TMPO a very useful probe for the characterization of zeolite acidity.