Assessing the Acidity of High Silica Chabazite H−SSZ-13 by FTIR Using CO as Molecular Probe: Comparison with H−SAPO-34
- 27 January 2005
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry B
- Vol. 109 (7), 2779-2784
- https://doi.org/10.1021/jp045498w
Abstract
Zeolitic materials based on the chabazite topology, such as H-SAPO-34, possess unique shape-selectivity properties for converting methanol into light olefins. In addition to the topology, zeolite acidity is inherently linked to catalyst activity and selectivity. The acidic properties of high silica chabazite (H-SSZ-13) have attracted much attention in the past decade because the material represents an idealized model system having one acidic site per cage. Conclusions drawn so far have essentially been founded on quantum chemical methods. An experimentally based benchmark of the acidity of H-SSZ-13 has hitherto not been available. In this work, transmission FTIR spectroscopy provides a description of the different acidic sites of H-SSZ-13 by using CO as molecular probe at 70 K. The results demonstrate that H-SSZ-13 is a strongly Brønsted acidic material, essentially having two distinct families of acidic sites. In contrast to numerous preceding reports, we find it fundamental to consider proton distributions among all four possible sites, and do not delimit the interpretations to only two sites. The present data consistently suggest the most abundant family of protons to have three members being located on different crystalline positions on the eight-membered-ring window giving access to the chabazite cage. Consequently, these protons are exposed to two neighboring cages. The second, and less abundant family, is constituted by only one site that is situated on the six-membered ring defining the top/bottom of the barrel-shaped chabazite cage. This proton is therefore only exposed to one cage and requires a higher CO pressure to form adducts. Toward CO, both families of sites possess the same acidity. Parallel experiments were also carried out for the isostructural and commercially important H-SAPO-34 having an equal density of acidic sites. This is the first attempt to directly compare, on an experimental basis, the acidity of these two materials.Keywords
This publication has 26 references indexed in Scilit:
- Vibrational and Thermodynamic Properties of H2 Adsorbed on MgO in the 300−20 K IntervalThe Journal of Physical Chemistry B, 2004
- Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of waterThe Journal of Chemical Physics, 2004
- Spectroscopic Characterization of Acidity in ChabaziteThe Journal of Physical Chemistry B, 2003
- The location of fluoride and organic guests in ‘as-made’ pure silica zeolites FER and CHAJournal of Materials Chemistry, 2003
- Spectroscopic characterisation of microporous aluminophosphate materials with potential application in environmental catalysisCatalysis Today, 2003
- Vibrational Spectroscopy of ZeolitesPublished by Wiley ,2001
- Methanol-to-hydrocarbons: catalytic materials and their behaviorMicroporous and Mesoporous Materials, 1999
- Fourier-Transform Infrared Study of CO Adsorbed at 77 K on H-Mordenite and Alkali-Metal-Exchanged MordenitesLangmuir, 1995
- Low-Temperature Fourier Transform Infrared Study of the Interaction of CO with Cations in Alkali-Metal Exchanged ZSM-5 ZeolitesThe Journal of Physical Chemistry, 1994
- Adsorption of Gases in Multimolecular LayersJournal of the American Chemical Society, 1938