Identification of the Reaction Sequence of the MTO Initiation Mechanism Using Ab Initio-Based Kinetics

Abstract

The initiation of the methanol-to-olefins (MTO) process is investigated using a multiscale modeling approach where more than 100 ab initio computed (MP2:DFT) rate constants for H-SSZ-13 are used in a batch reactor model. The investigated reaction network includes the mechanism for initiation (42 steps) and a representative part of the autocatalytic olefin cycle (63 steps). The simulations unravel the dominant initiation pathway for H-SSZ-13: dehydrogenation of methanol to CO is followed by CO-methylation leading to the formation of the first C–C bond in methyl acetate despite high barriers of >200 kJ/mol. Our multiscale approach is able to shed light on the reaction sequence that ultimately leads to olefin formation and strikingly demonstrates that only with a full reactor model that includes autocatalysis with olefins as cocatalysts is one able to understand the initiation mechanism on the atomic scale. Importantly, the model also shows that autocatalysis takes over long before significant amounts of olefins are formed, thus guiding the interpretation of experimental results.

Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted–Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process

“…In MTH chemistry, an acetal typically symbolizes the presence of dimethoxymethane (DMM, CH 3 O-CH 2 -OCH 3 , an acetal of formaldehyde). 56,58,[73][74][75][76][77][78] Interestingly, methylene (-CH 2 -) in the post-MTH reacted unmodied zeolite ZSM-5 trapped DMM exhibits a twin signal (98.5 and 101.1 ppm), that shows an interchange of cross-peaks with each other. 21 This is a typical signature of exchange between two anomeric DMM structures (trans,trans and gauche,gauche, see Fig.…”

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

“…Theoretical studies have been employed to calculate reaction free energy barriers of possible initiation and C-C coupling steps to shed light on plausible reaction mechanisms [43][44][45][46][47][48][49][50][51][52][53][54][68][69][70]. Using H-SSZ-13 as the catalyst we recently proposed a viable mechanism of the initiation reaction based on DFT calculations that have been corrected using second order Møller Plesset perturbation theory (MP2) [71] thus comprising highly accurate reaction barriers [72].…”

“…Furthermore, the interpretation of activation barriers is particularly complicated in the context of the initiation due to the autocatalytic nature of the olefin cycle. Therefore, we have previously investigated the effect of the above described reaction energetics for H-SSZ-13 on the mechanism of the initiation reaction using a multi-scale modeling approach employing a kinetic batch reactor model comprising both the initiation as well as a substantial part of the autocatalytic olefin cycle [70]. The simulated kinetics showed that the proposed pathway ( Fig.…”

On the Accuracy of Density Functional Theory in Zeolite Catalysis

“…We will use two classes of reactions both employing the chabazite (CHA) zeolite. First, we will investigate Brønsted acid catalyzed reactions related to DME synthesis and the methanol‐to‐olefins (MTO) process in the acidic CHA, H‐SSZ‐13 . The same framework is also used extensively for the selective catalytic reduction (SCR) of NO x emissions.…”

“…First, we will investigate Brønsted acid catalyzed reactions related to DME synthesis and the methanol-to-olefins (MTO) process in the acidic CHA, H-SSZ-13. [33,34,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] The same framework is also used extensively for the selective catalytic reduction (SCR) of NO x emissions. Here, copper-exchanged SSZ-13, Cu-SSZ-13, is employed.…”