Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small‐Pore Zeolites

Abstract

The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolitecatalyzedprocesses.T ransport of small alkenes in the confined zeolite pores can become hindered, resulting in as ignificant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8-ring windows of zeolite SAPO-34 is characterized by ac omplementary set of first-principle molecular dynamics simulations,PFG-NMR experiments,and pulse-response temporal analysis of products measurements,yielding information at different length and time scales.O ur results clearly show ap romotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene,whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable p-H interactions with acid protons,asc onfirmed by IR spectroscopymeasurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations.

Aromatics Production via Methanol-Mediated Transformation Routes

“…The recent advances in modeling/imaging techniques also allows one to obtain a better understanding of a heterogeneous catalytic process, from different length and time scales. ,,− For example, the use of APT clearly reveals the relationship between coke species and Brønsted acid sites distribution, and it is found that the aluminum-rich surface of ZSM-5 crystals accommodate more coke species . As to the MTA catalyst, which has both Lewis acidic metals and Brønsted acid sites, however, the spatial relationship between different acid sites and coke is still unknown.…”

“…Therefore, it is essential to apply advanced spatial-resolved imaging techniques to disclose such a relation, which facilitates designing better coking-resistant MTA catalysts. Besides, the simulation advances also provide in-depth theoretical understandings for the MTH chemistry. − Based on small-pore zeolite, enhanced sampling molecular dynamics simulations indicate that the diffusion of light olefins is manipulated by many factors, among which the presence of Brønsted acid sites in the 8-ring enhances the diffusion process . As for the MTA catalysts, the factors affecting the diffusion properties of various species such as the light olefins and aromatics, still need to be illuminated theoretically.…”

A quantitative multiscale perspective on primary olefin formation from methanol

“…To uncover the effect of acidity and adsorbate loading on the transport mechanism behind the diffusion of paraffins and olefins, the dynamic properties of ethene, ethane, propene and propane inserted in H-SAPO-34 have been investigated using Pulsed-field gradient (PFG) NMR analysis together with ab initio MD (AIMD) and ab initio umbrella sampling (AI-US) modelling techniques. 216 The umbrella sampling technique was employed to determine the free energy profile of diffusion through the 8 membered ring (8MR) windows of the H-SAPO-34 zeolite, having 1-3 adsorbates and 0-2 Brønsted acid sites per unit cell. The increase in adsorbate loading from one to three molecules per cage led to a slightly enhanced self-diffusion rate of ethene, while ethane mobility decreased.…”

“…1 H PFG NMR analysis at room temperature provided further evidence that the diffusion coefficient (D) of ethene is directly proportional to Brønsted acid site (BAS) density, as D E 10 À11 m 2 s À1 at BAS = 0.65 mmol g À1 goes to D E 2 Â 10 À11 m 2 s À1 at BAS = 1.13 mmol g À1 . 216 The improved diffusion profile of alkenes at higher BAS density is attributed to the polarization induced by the BAS on the adsorbed olefins creating an electrostatic field in the proximity of the 8MR, which leads to a flatter free energy surface that facilitates the transition from one cage to another. Supportive evidence has been found in the formation of alkene p-H complexes, detected at 3307 cm À1 by FT-IR measurements.…”

“…Supportive evidence has been found in the formation of alkene p-H complexes, detected at 3307 cm À1 by FT-IR measurements. 216 To further determine the effect of product distribution on olefins diffusion, additional computational analysis replicating the conditions closer to experimental work has been conducted by employing a series of techniques such as force-field MD and MTD, together with AIMD and AI-US, to model ethene and propene mobility in H-SAPO-34, considering the transfer from one CHA cage to another cage via an 8MR window. 217 Several simulations determining the mobility of propene next to or with additional propene molecules, from one cage to another was modelled.…”