The effect of Si/Al ratio on local and nanoscale water diffusion in H-ZSM-5: A quasielastic neutron scattering and molecular dynamics simulation study

Mechanistic Insights into the Induction Period of Methanol-to-Olefin Conversion over ZSM-5 Catalysts: A Combined Temperature-Programmed Surface Reaction and Microkinetic Modeling Study

“…In the aforementioned work, we used the reductionist approach to break down the induction period of methanol conversion into adsorption and desorption, competitive adsorption, , diffusion, , and reaction. − In this work, we have used the integrated approach, where we study the induction period of the conversion of methanol to primary olefins over fresh and working catalysts as a whole. Although propylene is still the primary olefin formed during the temperature-programmed surface reaction of methanol, the dominant sites of the induction period are the low-temperature, medium-temperature, and high-temperature binding sites due to water adsorption and desorption.…”

“…The diffusion of methanol, dimethyl ether, and water does not limit the conversion of methanol and dimethyl ether over ZSM-5 catalysts. , At room temperature in ZSM-5 (36) catalysts, isotropic methanol rotation (rotational diffusion coefficient, D R = 2.6 × 10 10 s –1 ) was observed, which is in contrast to diffusion confined to a sphere matching the 5.5 Å channel width in ZSM-5 (135) catalysts, suggesting that motion is more constrained in the lower Si/Al catalyst. At higher temperatures, confined methanol diffusion is exhibited in both ZSM-5 (36) catalysts and ZSM-5 (135) catalysts with self-diffusion coefficients in the range of 8–9 × 10 –10 m 2 s –1 .…”

“…12 wt % of methanol or dimethyl ether . QENS measurements probed water confined in ZSM-5 samples with Si/Al ratios of 15, 40, and 140 at 2.8 wt % of loading . In the lower silica sample, water diffusion was confined to a sphere (with radii ranging from 3.4 to 4.3 Å), suggesting that mobile water was located within the ZSM-5 channels with self-diffusion coefficient of ∼0.9–1.8 × 10 –9 m 2 s –1 .…”

“…43 QENS measurements probed water confined in ZSM-5 samples with Si/Al ratios of 15, 40, and 140 at 2.8 wt % of loading. 44 In the lower silica sample, water diffusion was confined to a sphere (with radii ranging from 3.4 to 4.3 Å), suggesting that mobile water was located within the ZSM-5 channels with self-diffusion coefficient of ∼0.9−1.8 × 10 −9 m 2 s −1 . In the high silica zeolite, the diffusion was observed to be far less confined and more longranged in nature with self-diffusion coefficients of ∼1.8−4.8 × 10 −9 m 2 s −1 .…”

“…This integrated temperature-programmed surface reaction approach complements our previous studies through a reductionist approach that aim to decouple the adsorption, desorption, activity, and diffusion through an individual combination of temperature-programmed desorption, competitive adsorption, temperature-programmed surface reaction, , step response, , and quasi-elastic neutron scattering , studies to probe the formation of primary olefins from methanol over fresh and working ZSM-5 catalysts. The novelty in this work is that we present new data on the temperature-programmed surface reaction of methanol over fresh and working ZSM-5 catalysts of Si/Al ratios of 25, 36, and 135 and additional experimental data on the temperature-programmed surface reaction of dimethyl ether over fresh and working ZSM-5 catalysts of a Si/Al ratio of 25.…”