Water–active site interactions in zeolites and their relevance in catalysis

Stančiaková, Katarína; Weckhuysen, Bert M.

doi:10.1016/j.trechm.2021.03.004

Cited by 40 publications

(31 citation statements)

References 108 publications

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“…Furthermore, it should be noted that water molecules can occupy BASs/form hydronium ions, thus there might be changes in the extent of BAS-assisted dimerization/ cyclization steps and consequent hydrocarbon pool mechanism. 38 Effect of Water Co-feeding on Hydrocarbon Formation Mechanism. In order to provide more in-depth mechanistic information, advanced "mobility-dependent" magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy was performed on post-reacted 2 wt % Mo/H-ZSM-5 material after the MDA reaction at 725 °C, using fully 13 C-enriched methane ( 13 CH 4 ) reactant (please see section S2.10.1 in the Supporting Information for detailed discussion).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Illuminating the Intrinsic Effect of Water Co-feeding on Methane Dehydroaromatization: A Comprehensive Study

et al. 2021

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Among all catalytic natural gas valorization processes, methane dehydroaromatization (MDA) still has a great potential to be utilized at an industrial level. Although the use of Mo/H-ZSM-5 as an MDA catalyst was first reported almost three decades ago, the process is yet to be industrialized, because of its inherent challenges. In order to improve the overall catalytic performance and lifetime, the co-feeding of water constitutes a promising option, because of its abundance and nontoxicity. Although water's (limited) positive influence on catalyst lifetime has earlier been exhibited, the exact course of action (like mechanism or the water effect on active sites) is yet to be established. To bridge this knowledge gap, in this work, we have performed an in-depth investigation to elucidate the effects of water cofeeding over a well-dispersed Mo/H-ZSM-5 catalyst by using an array of advanced characterization techniques (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry−temperature-programmed oxidation/mass spectroscopy (TG-TPO/MS), scanning transmission electron microscopy (STEM), N 2 physisorption, Raman spectroscopy, inductively coupled plasma−optical emission spectroscopy (ICP-OES)). Our results demonstrate that the addition of water results in the occurrence of steam reforming (of both coke and methane) in parallel to MDA. Moreover, the presence of water affects the reducibility of Mo sites, as corroborated with computational analysis to examine the state and locality of Mo sites under various water levels and transformation of the catalyst structure during deactivation. We anticipate that our comprehensive study of the structure−function relationship on Mo/H-ZSM-5 under humid MDA conditions will be beneficial for the development of future methane valorization technologies.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Illuminating the Intrinsic Effect of Water Co-feeding on Methane Dehydroaromatization: A Comprehensive Study

et al. 2021

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“…The adsorption of a molecule such as fructose on the solid is rather on the BAS acid sites due to the higher Gibbs free energy, which is also the strongest sites on the protonic zeolites [46,48]. Accordingly, water molecules inside pores of a zeolitic structure are not homogeneously distributed among Al sites, demonstrated by theoretical and experimental data [54]. Thus, both molecules (fructose and water) will compete for the same adsorption site on the zeolite channels, which will lead to decrease the conversion and selectivity for this dehydration reaction.…”

Section: Crystalline and Amorphous Aluminosilicate Materialsmentioning

confidence: 99%

Dehydration of Fructose to 5-Hydroxymethylfurfural: Effects of Acidity and Porosity of Different Catalysts in the Conversion, Selectivity, and Yield

et al. 2021

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There is a demand for renewable resources, such as biomass, to produce compounds considered as platform molecules. This study deals with dehydration of fructose for the formation of 5-hydroxymethylfurfural (HMF), a feedstock molecule. Different catalysts (aluminosilicates, niobic acid, 12-tungstophosphoric acid—HPW, and supported HPW/Niobia) were studied for this reaction in an aqueous medium. The catalysts were characterized by XRD, FT-IR, N2 sorption at −196 °C and pyridine adsorption. It was evident that the nature of the sites (Brønsted and Lewis), strength, quantity and accessibility to the acidic sites are critical to the conversion and yield results. A synergic effect of acidity and mesoporous area are key factors affecting the activity and selectivity of the solid acids. Niobic acid (Nb2O5·nH2O) revealed the best efficiency (highest TON, yield, selectivity and conversion). It was determined that the optimum acidity strength of catalysts should be between 80 to 100 kJ mol−1, with about 0.20 to 0.30 mmol g−1 of acid sites, density about 1 site nm−2 and mesoporous area about 100 m2 g−1. These values fit well within the general order of the observed selectivity (i.e., Nb2O5 > HZSM-5 > 20%HPW/Nb2O5 > SiO2-Al2O3 > HY > HBEA).

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“…8,9 Novel zeolite applications, such as zeolite- catalyzed biomass conversion, involve water formation or require an aqueous environment. 10 In these cases, the presence of water alters zeolite catalytic activity and stability. 11−15 A dramatic structural collapse of the zeolite framework has been observed for zeolites in the presence of supercritical water; however, this phenomenon is still poorly understood.…”

Section: Introductionmentioning

confidence: 99%

“…Characterization of water–zeolite interactions has, therefore, been an area of intense investigation. Water adsorption on the BAS and formation of protonated water clusters, such as hydronium ions, have been studied using many spectroscopic techniques, in particular, Fourier transform infrared (FT-IR) and NMR spectroscopy. , Studies on large zeolite ZSM-5 crystals provided detailed insights into the impact of water on the zeolite integrity, which can be significantly impaired at elevated temperatures due to water-induced zeolite dealumination. , Novel zeolite applications, such as zeolite-catalyzed biomass conversion, involve water formation or require an aqueous environment . In these cases, the presence of water alters zeolite catalytic activity and stability. − A dramatic structural collapse of the zeolite framework has been observed for zeolites in the presence of supercritical water; however, this phenomenon is still poorly understood. , …”

Section: Introductionmentioning

confidence: 99%

Understanding Water–Zeolite Interactions: On the Accuracy of Density Functionals

et al. 2021

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Water is ubiquitous in zeolite catalysis, and electronic structure calculations play a crucial role in arriving at an atomistic understanding of water–zeolite interactions. However, a critical evaluation of the performance of different electronic structure methods in describing the interactions between water and zeolites is still missing. Here, we model the adsorption of one water molecule in all-silica chabazite (CHA) and of one and two water molecules in the acidic zeolite SSZ-13 using different electronic structure methods, which include 11 density functional theory (DFT)-based methods and two post Hartree–Fock (HF) methods, namely, the random phase approximation (RPA) and second-order Møller–Plesset (MP2) perturbation theory. We find that all DFT functionals lead to similar structures as long as water is strongly coordinated to the adsorption site, but adsorption energies vary in a range of 50 kJ/mol between the used methods. Subsequently, we use ab initio molecular dynamics calculations to show that all methods reproduce the experimentally observed hydrophobicity of purely siliceous zeolites. Comparing DFT energetics with RPA and MP2 calculations shows that PBE and revPBE-D3 adsorption energies show the best agreement with RPA, while BEEF–vdW agrees the best with MP2 results. At the same time, the performance of PBE functional without any dispersion correction is less consistent with respect to different adsorption sites (BAS, LAS, or the zeolite wall of all-silica CHA) and the BEEF–vdW functional fails to reproduce relative stabilities of the protonation sites. For the adsorption of two water molecules, most methods agree on the formation of a protonated water dimer, and only vdW-DF, vdW-DF2, and BEEF–vdW prefer the formation of a neutral complex. Based on these results, we suggest using the revPBE-D3 functional model water adsorption in purely siliceous or protonated zeolites since it can correctly capture covalent and dispersion interactions, is computationally efficient, correctly predicts the formation of a positively charged water dimer, and is able to closely reproduce adsorption energies calculated at the RPA or MP2 level of theory.

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Water–active site interactions in zeolites and their relevance in catalysis

Cited by 40 publications

References 108 publications

Illuminating the Intrinsic Effect of Water Co-feeding on Methane Dehydroaromatization: A Comprehensive Study

Illuminating the Intrinsic Effect of Water Co-feeding on Methane Dehydroaromatization: A Comprehensive Study

Dehydration of Fructose to 5-Hydroxymethylfurfural: Effects of Acidity and Porosity of Different Catalysts in the Conversion, Selectivity, and Yield

Understanding Water–Zeolite Interactions: On the Accuracy of Density Functionals

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