Bordiga Silvia, Regli Laura, Cocina Donato, Lamberti Carlo, Bjørgen Morten, Lillerud Karl Petter
Dipartimento di Chimica IFM and NIS Centre of Excellence, Via P. Giuria 7, I-10125 Torino, Italy.
J Phys Chem B. 2005 Feb 24;109(7):2779-84. doi: 10.1021/jp045498w.
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.
基于菱沸石拓扑结构的沸石材料，如H-SAPO-34，在将甲醇转化为轻质烯烃方面具有独特的形状选择性。除拓扑结构外，沸石酸度与催化剂活性和选择性内在相关。高硅菱沸石（H-SSZ-13）的酸性性质在过去十年中备受关注，因为该材料代表了每个笼中有一个酸性位点的理想化模型体系。迄今为止得出的结论基本上基于量子化学方法。目前尚未有基于实验的H-SSZ-13酸度基准。在这项工作中，透射傅里叶变换红外光谱通过在70 K下使用CO作为分子探针，对H-SSZ-13的不同酸性位点进行了描述。结果表明，H-SSZ-13是一种强布朗斯台德酸性材料，基本上有两个不同的酸性位点家族。与众多先前的报道不同，我们发现考虑所有四个可能位点之间的质子分布至关重要，并且不将解释局限于仅两个位点。目前的数据一致表明，最丰富的质子家族有三个成员，位于通向菱沸石笼的八元环窗口上的不同晶体位置。因此，这些质子暴露于两个相邻的笼中。第二个且数量较少的家族仅由一个位于定义桶状菱沸石笼顶部/底部的六元环上的位点组成。因此，这个质子仅暴露于一个笼中，并且需要更高的CO压力才能形成加合物。对于CO，这两个位点家族具有相同的酸度。还对具有相同酸性位点密度的同构且商业上重要的H-SAPO-34进行了平行实验。这是首次在实验基础上直接比较这两种材料酸度的尝试。
