UC Berkeley

Light alkanes, namely ethane and propane, derived from shale gas are an important chemical feedstock for the production of a variety of commodity chemicals. One commercial use of light alkanes is their conversion to aromatics, in particular benzene, toluene, and xylene. This reaction, termed dehydroaromatization, is catalyzed using heterogeneous, Lewis acidic, gallium-exchanged H-MFI zeolites (Ga/H-MFI); however, zinc-exchanged H-MFI (Zn/H-MFI) has also attracted attention for this purpose due to its reported high activity and selectivity, and the lower cost of Zn relative to Ga. Additionally, dehydrogenation of light alkanes is used to manufacture alkenes, namely ethene and propene, which also serve as chemical intermediates. Hydroformylation of light alkenes, that is, their reaction with CO and H2, is an important commercial process for the synthesis of aldehydes, and is carried out using homogeneous Rh catalysts. However, heterogeneous single-atom catalysts also show promise as hydroformylation catalysts. Moreover, dealuminated BEA zeolites (DeAlBEA) containing nests of ≡Si-OM-OH groups (M-DeAlBEA), where M denotes a nonprecious metal atom, are effective supports for atomically dispersed noble metal atoms. This dissertation is centered on developing an understanding of the reaction pathway, or of several individual steps, involved in the dehydroaromatization or hydroformylation of alkane or alkene reactants over zeolite catalysts, and elucidating how the identity and structure of the catalytically active metal center, or of extraframework metal sites in the zeolite support, influence the catalyst reactivity. Understanding these relationships will aid in the development of active and selective catalysts for these reactions and for related alkane and alkene transformations. The first three chapters of this dissertation explore individual steps of the process of alkane dehydroaromatization over H-MFI zeolite catalysts. The first step of alkane dehydroaromatization is dehydrogenation to form the corresponding alkene. Lewis acidic metal sites in H-MFI, e.g. Ga and Zn, primarily catalyze the dehydrogenation reaction, and residual Brønsted acid sites catalyze alkane cracking, an undesired competing reaction. Chapter 2 is focused on propane dehydrogenation of over Zn/H-MFI catalysts. While Zn/H-MFI has been identified as an active and selective catalyst for alkane dehydrogenation and dehydroaromatization, the relationships between the Zn loading, the Zn site structure, and catalyst performance are not clear. These relationships were explored for Zn/H-MFI catalysts prepared by solid-state ion exchange (SSIE), also referred to as vapor-phase exchange, of ZnCl2 with dehydrated H-MFI. The activity and selectivity to dehydrogenation of Zn/H-MFI prepared in this manner was also compared to the reported activity of Ga/H-MFI prepared by vapor-phase exchange of GaCl3 (Phadke, N. M. et al. ACS Catal. 2021, 11, 2062–2075). Using complementary techniques including FTIR, XAS, UV-vis spectroscopy, and NMR, it was found that the reaction of ZnCl2 with Brønsted acid protons during SSIE results in the formation of isolated, extraframework Zn species at lower Zn loadings (Zn/Al = 0.06–0.52). These species consist of [ZnCl]+ and [Zn(HCl)]+ cations. At higher Zn loadings, multinuclear ZnAl2O4/ZnAl2O4-xCl2x nanoclusters also form. The Cl and HCl ligands are removed via treatment of the as-prepared Zn/H-MFI in H2 at 773 K, possibly resulting in the formation of [ZnH]+ cations. However, loss of Zn as gas-phase ZnHxCly also occurs as a result of treatment in H2. In the absence of cofed H2, the [ZnH]+ cations are transformed into isolated Zn2+ cations associated with proximate pairs of Al atoms; this transformation is reversible. Reaction rate measurements of propane dehydrogenation and cracking over Zn/H-MFI indicate that Zn2+ cations are more active and selective toward dehydrogenation over cracking relative to [ZnH]+ cations. Furthermore, the dehydrogenation and cracking activity of Zn2+ cations was found to increase with increasing Zn loading, possibly due to the localization of the Zn2+ cations at cations at increasingly distant Al pairs, resulting in decreased stability and increased reactivity. The higher activity might instead result from the localization of an increasing proportion of Zn2+ sites at the MFI channel intersection, which leads to higher entropies of activation. While Zn/H-MFI exhibits a ~53x higher dehydrogenation rates relative to H-MFI at a Zn/Al ratio of 0.24 (in the spent catalyst), the rate of propane cracking is ~8x higher relative to H-MFI. The dehydrogenation rate, and the dehydrogenation to cracking rate ratio, was found to be lower than that reported for Ga/H-MFI. Chapters 3 and 4 of the dissertation focus on the second step of alkane dehydroaromatization over Lewis acid-exchanged zeolites, namely, the oligomerization and concurrent β-scission of alkenes. While it is understood that [GaH]2+ and [Ga(H)2]+ cations in Ga/H-MFI catalyze the initial dehydrogenation step of the dehydroaromatization process, there is controversy regarding the role of Ga species in alkene oligomerization and β-scission. Specifically, it is not well understood whether oligomerization is catalyzed solely by Brønsted acid sites, or also by Ga species. This question was addressed in Chapter 3. The kinetics of ethene oligomerization were investigated over H-MFI and Ga/H-MFI catalysts of varying Ga loadings that contain well-defined, isolated Ga species as a result of their preparation via vapor-phase exchange of GaCl3. Rates of ethene oligomerization were found to be an order of magnitude higher over Ga/H-MFI catalysts (Ga/Al = 0.1–0.3) relative to H-MFI. Furthermore, the selectivity to C-C bond formation over β-scission reactions is enhanced for Ga/H-MFI compared to H-MFI. However, selective titration of Brønsted acid protons in Ga/H-MFI with NH3 indicated that Ga/H-MFI exhibits low ethene oligomerization activity in the absence of Brønsted acid protons. These results, in conjunction with in-situ FTIR experiments that demonstrate the formation of alkoxide surface intermediates during oligomerization over both H-MFI and Ga/H-MFI, indicate that Brønsted acid protons catalyze ethene oligomerization over Ga/H-MFI, but cooperative effects between Brønsted acid protons and [GaH]2+ and [Ga(H)2]+ cations serve to enhance oligomerization rates. In Chapter 4, the mechanism of ethene oligomerization over H-MFI was examined in greater detail to address how odd-carbon number oligomers form as primary products of this reaction. The product distribution of ethene oligomerization over H-MFI was found to consist primarily of propene and butene isomers, and also pentene and hexene isomers, albeit in lower quantities. The formation of propene and pentenes as primary products can be rationalized by the β-scission of higher-molecular weight alkoxide oligomers prior to their desorption. The measured trends in ratio of the rate of propene formation to the rate of butene formation can be explained by the involvement of two (or more) distinctly different types of Brønsted acid protons in the oligomerization and β-scission reactions, and the reaction rate data was fitted to a proposed two-site mechanism. It was hypothesized that the Brønsted acid protons involved in the oligomerization and β-scission reactions may differ with respect to their locations within the H-MFI channel system. Chapter 5 is focused on ethene hydroformylation over zeolite-supported Rh sites. Iron-exchanged dealuminated BEA zeolite (Fe-DeAlBEA) was used as a support for the preparation of atomically dispersed of Rh atoms. It is understood that the interaction of Rh sites with atoms of another metal influences the catalyst hydroformylation performance. Chapter 5 examines the mechanism and kinetics of ethene hydroformylation over Rh-Fe-DeAlBEA, and how the identity of the metal promoter influences ethene hydroformylation activity. Ethene hydroformylation is understood to involve the partial hydrogenation of ethene to form ethyl species, CO insertion to form acyl (propionyl species), and hydrogenation of propionyl species to form propanal. Reaction rate measurements, including measurements of the H/D kinetic isotope effect, indicate that the rate-limiting step of ethene hydroformylation over Rh-Fe-DeAlBEA is hydrogenation of the propionyl surface intermediate. The ethene hydroformylation rate, and selectivity to propanal over ethane, was found to be lower over Rh-Fe-DeAlBEA relative to Rh-Co-DeAlBEA and Rh-Zn-DeAlBEA (Liang Qi, Private communication). This was attributed to the presence of multiple different Rh species in Rh-Fe-DeAlBEA, namely Rh0 clusters and two types of isolated Rh sites varying with respect to formal oxidation state or ligation, evidenced by FTIR of adsorbed CO. It was proposed that the more cationic Rh sites are less active for ethene hydroformylation relative to the more cationic Rh sites.