Dehydration Reactions in Lewis Acidic Zeolites
Ryan E. Patet, Paraskevi Panagiotopoulou, Stavros Caratzoulas, Dionisios G. Vlachos
Center for Catalysis and Energy Innovation (CCEI), Department of Chemical and Biomolecular Engineering, University of Delaware, 221 Academy Street, Newark, DE 19716
Zeolites have long been used in the petroleum industry in the refining of crude oil to fuels and chemicals.1 Their utility is derived from the solid acid characteristics of their active sites confined within the nanoscale pores of the larger zeolite structure.1 In the classical application of zeolites, Si atoms are exchanged with Al atoms within the silica framework of the zeolite, creating an inherent negative charge imbalance within the zeolite structure.2 To balance the charge, H+ can be incorporated within the zeolite pores, creating a Brønsted acidic solid acid catalyst.2
It is possible to form Lewis acidity within these zeolites, as well. Rather than exchanging H+ into the zeolite active sites, other cations such as Li+, Na+, or K+ can be exchanged within the zeolite pores.3 Additionally, other methods exist whereby rather than exchanging a Si atom with an Al atom, other atoms can be homomorphically substituted into the zeolite framework itself, such as Sn, Ti, or Zr.4 In these cases, the incorporation of these framework substituents do not create the same negative charge imbalance within the zeolite as Al. They are by themselves, however, able to act as Lewis acid sites within the zeolite framework. Additionally, in a mechanistic study of an independent reaction, it was shown that the closed framework substituted Lewis acid zeolites can be hydrolyzed by water to form an open structure.5 It was shown that the open form of the Lewis acid zeolite facilitated the isomerization of glucose, rather than the non-hydrolyzed closed structure.5 Applications for ion-exchanged and homomorphically substituted Lewis acid catalysts have found utility both within and outside the petroleum industry.1,6
In this study, we use ONIOM models to investigate both forms of Lewis acid zeolites. We analyze the inherent zeolite properties and geometries to investigate how they affect the adsorption of relevant probe molecules at their active sites. Hard and soft acid and base (HSAB) theory is used to investigate how the properties of the zeolite cation and adsorbate affect their interaction in these systems. Through these adsorption studies, we gain insight into the factors that affect the Lewis acid strength within these different systems and how they interact with adsorbent molecules of varying properties.
With this understanding of the Lewis acid properties of the zeolite, we investigate the ability of the homomorphically substituted zeolites to catalyze the dehydration of alcohols to olefins. For the dehydration of these alcohols, the open and closed forms of the homomorphically substituted zeolites are compared. Multiple mechanisms are investigated in the open and closed systems to understand how the different zeolite substituents and structures affect these reactions.
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