Effects of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis on HSAPO-34

Hwang, Andrew; Le, Thuy T.; Shi, Zhichen; Dai, Heng; Rimer, Jeffrey D.; Bhan, Aditya

doi:10.1016/j.jcat.2018.10.031

Cited by 41 publications

(18 citation statements)

References 83 publications

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“…Diffusion coefficients can be derived from the free energy profiles according to ar andom walk model and transition state theory as outlined in Section S1.8 of the SI. Alkane diffusion in SAPO-34 is independent on the presence of acid sites and can take place through each of the six 8-ring windows with equal probability.H owever,a lkenes will show aclear preference to escape through windows containing acid sites.Incommon H-SAPO-34 catalysts-characterized by an (P + Al)/Si ratio ranging between 5a nd 20, [3,6,57] corresponding on average to adensity of 1-2 acid sites per cage-there is av ery high probability that the majority of the cages will contain at least one 8-ring decorated with an acid site. Consequently,the alkene diffusion probability is non-uniform in the six directions and diffusion coefficients thus can only be estimated in the limit of av ery low or high acid site density.…”

Section: Resultsmentioning

confidence: 99%

“…[1][2][3] Once formed, the products need to diffuse away from the active centers.Q uantifying adsorption and diffusion in the zeolite nanopores is very challenging due to the simultaneous occurrence of various phenomena at different length and time scales.A ni nteresting case study where diffusion limitations were suggested to have as trong correlation with the ultimate product distribution, is the methanol-to-hydrocarbons (MTH) process on small-pore zeolites like SAPO-34. [3][4][5][6][7][8][9][10] TheS APO-34 zeolite (CHA topology) consists of large elliptical cages that are connected by two double 6-ring (d6r) units and six 8-ring windows,through which diffusion of alkenes and alkanes can take place,a ss hown in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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“…28−31 Bhan and co-workers recently developed a method for analyzing transport phenomena in complex processes and discovered that diffusional constraints had the largest effect on some particular steps of the MTO reaction cycle. 31 Hereijgers et al performed seminal work to understand the selectivity and deactivation on H-SAPO-34. 9 They showed that the product distribution is controlled by product shape selectivity.…”

Section: Introductionmentioning

confidence: 99%

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors

et al. 2020

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The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host–guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.

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“…This gives rise to coke formation through the degradation of hydrocarbon pool species into bulky aromatics constraining the diffusion of methanol and olefins. [22][23][24][25][26][27][28] The insitu or ex-situ analyses of retained species reveal the presence of mono-, bi-, tri-and tetracyclic aromatics remaining in the catalyst during or after the MTO reaction (which are soluble in organic solvents and therefore these species are often referred to as soluble species). [6,17,24,[29][30][31][32][33][34][35] There are many research works on the MTO reaction using in-situ or operando spectroscopies to study the retained species in the catalyst, particularly ultraviolet-visible (UV-vis) spectroscopy [24,33,34,[36][37][38][39] and Fourier transform infrared (FTIR) spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

“…In SAPO‐18 and ‐34 catalysts, the aromatic pool species remain trapped in the cavities (as the openings are smaller than the benzene molecule, being this the smallest aromatic species). This gives rise to coke formation through the degradation of hydrocarbon pool species into bulky aromatics constraining the diffusion of methanol and olefins [22–28] . The in‐situ or ex‐situ analyses of retained species reveal the presence of mono‐, bi‐, tri‐ and tetracyclic aromatics remaining in the catalyst during or after the MTO reaction (which are soluble in organic solvents and therefore these species are often referred to as soluble species) [6,17,24,29–35] .…”

Section: Introductionmentioning

confidence: 99%

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

et al. 2021

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The dynamics of retained and deactivating species in a SAPO-18 catalyst during the methanol-to-olefins reaction have been followed using a combination of ex-situ and in-situ techniques in differential and integral reactors.The retained species were analyzed using extraction, in-situ FTIR and in-situ UV-vis spectroscopies combined with online product analysis (gas chromatography and mass spectrometry). The composition of the extracted soluble species was determined using gas chromatography-mass spectrometry and that of the insoluble species using high-resolution mass spectrometry. We observe a decrease in the formation and degradation rates of retained species when co-feeding water, whereas the extent of the decreases is the same across the entire spectrum of retained molecules. This indicates that co-feeding water unselectively quenches the formation of active and deactivating species. At the same time, the catalyst has an extended lifetime when cofeeding water due to the diffusion of species (particularly olefins) out of the SAPO-18 crystals, and subsequent growth of heavy polycyclic aromatic structures that imply less deactivation. These conclusions can be extrapolated to other MTO catalysts with relatively similar pore topology such as SAPO-34 or SSZ-13 structures.

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Effects of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis on HSAPO-34

Cited by 41 publications

References 83 publications

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

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

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

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