(349s) Understanding the Influence of Chain Length, Branching, and Void Environment on the Stability of Surface-Bound Alkyl Intermediates in Zeolites Using DFT

Surface-bound alkyls, also referred to as alkoxy species or alkoxides, are prevalent intermediates in several zeolite-catalyzed processes. Alkene methylation, alkene coupling, and cracking reactions form alkenes with different chain lengths and branching during processes such as methanol-to-olefins (MTO) and oligomerization. The presence of surface-bound alkyls has been experimentally confirmed during these processes; however, it is difficult to isolate and study the multitude of factors governing alkyl stability at reaction conditions. Here, we use density functional theory (DFT) to expand on the limited scope of prior computational studies examining surface-bound alkyls. Fully periodic DFT is used to examine all structural isomers of C₁–C₉ surface-bound alkyls at the single crystallographically unique T-site in CHA and at all 12 unique T-sites in MFI. Internal C–C bonds were reoriented for each alkyl to study an array of conformational isomers, and alkyls were spatially reoriented around the acid site to maximize dispersive framework interactions. The thermodynamic favorability of increasing an alkyl chain length was calculated as ΔG_methylation, the free energy to form an alkyl of a given size (C_nH_2n+1–Z) by methylating the next smallest alkyl (C_n−1H_2n−1–Z) (Fig. 1). In CHA, ΔG_methylation suggests that increasing the chain length of an existing alkyl is more thermodynamically favorable than methylating a new acid site (Fig. 1). Forming larger alkyls from an existing alkyl depends on the structure of the alkyl isomer formed, but the ΔG_methylation to form the most stable isomer does not change (up to C₉), indicating that while some alkyl structures are sterically hindered in CHA, even a C₉ isomer can find a stable conformation. This data, along with similar data for all 12 T-sites in MFI, is used here to predict abundant surface intermediates relevant to the methanol-to-olefins and oligomerization chemistry as a function of temperature, pressure, framework, and T-site location.