Assessing the Acidity of High Silica Chabazite H−SSZ-13 by FTIR Using CO as Molecular Probe:  Comparison with H−SAPO-34

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.

Methanol to olefins: activity and stability of nanosized SAPO-34 molecular sieves and control of selectivity by silicon distribution

“…In the case of nano-SAPO-34 ( Fig. 8), the bands corresponding to HF and LF brönsted acid sites are shifted after adsorption and interaction of bridging hydroxyl groups with CO to a broad band around 3200-3500 cm -1 which is in agreement with that reported in the literature 28,41 . , this band at 3280 cm -1 has been assigned to CO interacting with acid sites corresponding to Si in the borders of the silicon islands, and which have a stronger acidity.…”

Methylation of phenol over high-silica beta zeolite: Effect of zeolite acidity and crystal size on catalyst behaviour

“…From a theoretical point of view, the phenomenon is called Fermi Resonance and the effect on the spectra is the appearance of three bands identified as A, B and C, separated by two Evan's windows: the presence and the relative intensity of these three bands indicate the entity of the interaction and consequently the acidity of the material [44][45][46][47][48].…”

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

“…This is a widely used strategy for measuring the acidity of solid acids, including zeolites. 28,29 Figure 4c shows the difference spectrum for CO adsorbed on H-2dH, where a red shift of 379 cm -1 is observed.…”