Acidity of Amorphous Silica-Alumina: From Coordination Promotion of Lewis Sites to Proton Transfer
Acidity of Amorphous Silica-Alumina: From Coordination Promotion of Lewis Sites to Proton Transfer
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DOI：
10.1002/cphc.200900797
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发表时间：
2010-01-18
期刊：
影响因子：
2.9
通讯作者：
Raybaud, Pascal
中科院分区：
文献类型：
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作者：
Chizallet, Celine;Raybaud, Pascal
由于其结合的刘易斯和布朗斯特酸性，无定形二氧化硅-氧化铝（阿萨）是精细化学[1]、石油化工精炼[2]和生物质转化[3]中多功能多相催化剂的广泛载体，从实验室规模到工业工厂。[4，5]此外，这些材料被怀疑存在于H-USY沸石中。[6]由于它们的无定形性质，尽管魔角旋转（MAS）NMR实现了改进，[7]酸位点的局部环境仍然存在强烈的争议。沸石类桥连SiO2（OH）SiO2 Al基团有时被引用，[8，9]但受到其他几位作者的质疑。[10通过SiO2-Al桥键合到低配位的铝原子上的硅烷醇已经被提议作为阿萨表面上的大多数酸性布朗斯特位点，这取决于铝原子的数量和配位。[12]Trombetta等人[10]此外，提出在与碱性探针分子相互作用时，在三重配位铝原子附近的硅烷醇可以在硅烷醇的氧和三重配位铝原子之间形成额外的键，这与Crepeau等人相矛盾。[12]在CO吸附实验的基础上，因此，一个公开的辩论仍然对阿萨表面上的Brønsted网站的结构的识别，并在质子transfer step.Earlier期间，他们的行为，我们提出了阿萨表面的第一个模型，从周期性密度泛函理论（DFT）计算结合力场分子动力学。[13]在此，该表面模型上的布朗斯台德酸位点的性质的问题，以及在各种碱性强度的分子（CO，吡啶，二甲基吡啶和氨）的存在下，这些网站的行为被解决，以便解开阿萨的布朗斯台德酸性的起源。
Due to their combined Lewis and Brønsted acidities, amorphous silica–alumina (ASA) are widespread supports for multifunctional heterogeneous catalysts in fine chemistry,[1] petrochemical refining [2] and biomass conversion,[3] from the laboratory scale to the industrial plant.[4, 5] These materials are moreover suspected in H-USY zeolites.[6] Due to their amorphous nature and despite improvement achieved by magic-angle spinning (MAS) NMR,[7] the local environment of the acid sites remains strongly debated. Zeolite-like bridging SiÀ (OH) ÀAl groups are sometimes invoked,[8, 9] but are questioned by several other authors.[10, 11] Silanols bonded to low-coordinated aluminum atoms by a SiÀOÀAl bridge have been proposed as most acidic Brønsted sites on ASA surfaces, depending on the number and coordination of aluminum atoms.[12] Trombetta et al.[10] moreover suggested that upon interaction with a basic probe molecule, silanols in the vicinity of threefold-coordinated aluminum atoms could form an additional bond between the oxygen of the silanol and the threefold-coordinated aluminum atom, which is contradicted by Crepeau et al.[12] on the basis of CO adsorption experiments. An open debate thus remains on the identification of the structure of Brønsted sites on ASA surfaces, and on their behavior during the proton transfer step.Earlier, we proposed the first model for ASA surfaces, from periodic density functional theory (DFT) calculations combined with force-field molecular dynamics.[13] Herein, the question of the nature of the Brønsted acid sites on this surface model and of behavior of such sites in the presence of molecules of various basic strengths (CO, pyridine, lutidine and ammonia) is addressed, so as to unravel the origin of the Brønsted acidity of ASA.