scholarly journals QM/MM Study of the Reactivity of Zeolite Bound Methoxy and Carbene Groups

2021 ◽  
Author(s):  
Andrew Logsdail ◽  
Richard Catlow ◽  
Stefan A. F. Nastase
Keyword(s):  
Acid Site ◽  
Alternative Pathway ◽  
Experimental Studies ◽  
Acid Sites ◽  
Site Formation ◽  
Methanol To Hydrocarbons ◽  
Hydrocarbon Formation ◽  
Autocatalytic Process ◽  
Ft Ir

<div>The conversion of methanol-to-hydrocarbons (MTH) is known to occur via an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produce hydrocarbons by a mechanism involving carbene migration and association. In order to understand these observations, we have performed QM/MM computational investigations on a range of reaction mechanisms for the reaction of zeolite bound methoxy and carbene groups, which are proposed to initiate hydrocarbon formation in the MTH process. Our simulations demonstrate that it is kinetically unfavourable for methyl species to form on the framework away from the zeolite acid site, and both kinetically and thermodynamically unfavourable for methyl groups to migrate through the framework and aggregate around an acid site. Formation of carbene moieties was considered as an alternative pathway to the formation of C-C bonds; however, the reaction energy for conversion of a methyl to a carbene is unfavourable. Metadynamics simulations help confirm further that methyl species at the framework acid sites would be more reactive towards formed C<sub>2+</sub> species, rather than inter-framework migration and that the role of carbenes in the formation of the first –C bond will be via a concerted type of mechanism rather than stepwise. </div>

scholarly journals QM/MM Study of the Reactivity of Zeolite Bound Methoxy and Carbene Groups

2021 ◽  
Author(s):  
Andrew Logsdail ◽  
Richard Catlow ◽  
Stefan A. F. Nastase
Keyword(s):  
Acid Site ◽  
Alternative Pathway ◽  
Experimental Studies ◽  
Acid Sites ◽  
Site Formation ◽  
Methanol To Hydrocarbons ◽  
Hydrocarbon Formation ◽  
Autocatalytic Process ◽  
Ft Ir

<div>The conversion of methanol-to-hydrocarbons (MTH) is known to occur via an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produce hydrocarbons by a mechanism involving carbene migration and association. In order to understand these observations, we have performed QM/MM computational investigations on a range of reaction mechanisms for the reaction of zeolite bound methoxy and carbene groups, which are proposed to initiate hydrocarbon formation in the MTH process. Our simulations demonstrate that it is kinetically unfavourable for methyl species to form on the framework away from the zeolite acid site, and both kinetically and thermodynamically unfavourable for methyl groups to migrate through the framework and aggregate around an acid site. Formation of carbene moieties was considered as an alternative pathway to the formation of C-C bonds; however, the reaction energy for conversion of a methyl to a carbene is unfavourable. Metadynamics simulations help confirm further that methyl species at the framework acid sites would be more reactive towards formed C<sub>2+</sub> species, rather than inter-framework migration and that the role of carbenes in the formation of the first –C bond will be via a concerted type of mechanism rather than stepwise. </div>

scholarly journals Modelling metal centres, acid sites and reaction mechanisms in microporous catalysts

2016 ◽  
Vol 188 ◽  
pp. 235-255 ◽  
Author(s):  
Alexander J. O'Malley ◽  
A. J. Logsdail ◽  
A. A. Sokol ◽  
C. R. A. Catlow
Keyword(s):  
Acid Site ◽  
Reaction Mechanisms ◽  
Acid Sites ◽  
Computational Techniques ◽  
Site Location ◽  
Adsorbed Species ◽  
Methanol To Hydrocarbons ◽  
Acidic Sites ◽  
Reaction Energies

We discuss the role of QM/MM (embedded cluster) computational techniques in catalytic science, in particular their application to microporous catalysis. We describe the methodologies employed and illustrate their utility by briefly summarising work on metal centres in zeolites. We then report a detailed investigation into the behaviour of methanol at acidic sites in zeolites H-ZSM-5 and H-Y in the context of the methanol-to-hydrocarbons/olefins process. Studying key initial steps of the reaction (the adsorption and subsequent methoxylation), we probe the effect of framework topology and Brønsted acid site location on the energetics of these initial processes. We find that although methoxylation is endothermic with respect to the adsorbed system (by 17–56 kJ mol−1 depending on the location), there are intriguing correlations between the adsorption/reaction energies and the geometries of the adsorbed species, of particular significance being the coordination of methyl hydrogens. These observations emphasise the importance of adsorbate coordination with the framework in zeolite catalysed conversions, and how this may vary with framework topology and site location, particularly suited to investigation by QM/MM techniques.

Role of Zinc in Zn-Loaded ZSM-5 Zeolites in the Aromatization of n-Hexane

1999 ◽  
Vol 64 (1) ◽  
pp. 168-176 ◽  
Author(s):  
Edita Rojasová ◽  
Agáta Smiešková ◽  
Pavol Hudec ◽  
Zdenek Židek
Keyword(s):  
Solid State ◽  
Ion Exchange ◽  
Lewis Acid ◽  
Acid Site ◽  
Acid Sites ◽  
Partial Migration ◽  
Simultaneous Presence ◽  
Lewis Acid Sites ◽  
Strong Lewis Acid

Aromatization of n-hexane over zinc-modified ZSM-5 zeolites was investigated. It was shown that incorporation of zinc by ion exchange into cationic positions of NH4-ZSM-5 zeolite causes acid-site strength redistribution and generation of new relatively strong Lewis acid sites in zeolite increasing the selectivity of n-hexane aromatization in comparison with the parent NH4-ZSM-5 zeolite. Simultaneous presence of Lewis and Broensted acid sites in ZSM-5 zeolite does not affect the strength of Broensted acid sites in zeolite. For the activity/selectivity of aromatization of n-hexane on Zn-modified ZSM-5 zeolites, the amount of Zn and its localization in the cationic positions are decisive. The reaction of n-hexane can be also initiated by the Zn species alone in the cationic positions. ZnO species alone as an extraframework phase was found inactive in the catalyst for aromatization of n-hexane. The influence of ZnO addition on the performance of pure ammonium forms of ZSM-5 zeolites in n-hexane conversion is a result of partial migration of zinc into cationic positions of zeolite by solid-state ion exchange.

scholarly journals Prominent role of mesopore surface area and external acid sites for the synthesis of polyoxymethylene dimethyl ethers (OME) on a hierarchical H-ZSM-5 zeolite

2019 ◽  
Vol 9 (2) ◽  
pp. 366-376 ◽  
Author(s):  
Christophe J. Baranowski ◽  
Ali M. Bahmanpour ◽  
Florent Héroguel ◽  
Jeremy S. Luterbacher ◽  
Oliver Kröcher
Keyword(s):  
Surface Area ◽  
Acid Site ◽  
Acid Sites ◽  
Internal Diffusion ◽  
Mfi Zeolite ◽  
External Acid Sites ◽  
Polyoxymethylene Dimethyl Ethers ◽  
Zeolite P

By varying acid site accessibility, we demonstrated that polyoxymethylene dimethyl ethers suffered from severe internal diffusion inside a MFI zeolite.

scholarly journals Phosphate Enrichment of Niobium-Based Catalytic Surfaces in Relation to Reactions of Carbohydrate Biomass Conversion: The Case Studies of Inulin Hydrolysis and Fructose Dehydration

Catalysts ◽  
2021 ◽  
Vol 11 (9) ◽  
pp. 1077
Author(s):  
Mariana N. Catrinck ◽  
Sebastiano Campisi ◽  
Paolo Carniti ◽  
Reinaldo F. Teófilo ◽  
Filippo Bossola ◽  
...  
Keyword(s):  
Biomass Conversion ◽  
Acid Sites ◽  
Lewis Acidity ◽  
Total Acidity ◽  
Biomass Hydrolysis ◽  
Inulin Hydrolysis ◽  
Ft Ir ◽  
Fructose Dehydration ◽  
Adsorption Desorption

In this work, some physical mixtures of Nb2O5·nH2O and NbOPO4 were prepared to study the role of phosphate groups in the total acidity of samples and in two reactions involving carbohydrate biomass: hydrolysis of polyfructane and dehydration of fructose/glucose to 5-hydroxymethylfurfural (HMF). The acid and catalytic properties of the mixtures were dominated by the phosphate group enrichment. Lewis and Brønsted acid sites were detected by FT-IR experiments with pyridine adsorption/desorption under dry and wet conditions. Lewis acidity decreased with NbP in the composition, while total acidity of the samples, measured by titrations with phenylethylamine in cyclohexane (~3.5 μeq m−2) and water (~2.7 μeq m−2), maintained almost the same values. Inulin conversion took advantage of the presence of surfaces rich in Brønsted sites, and NbOPO4 showed the best hydrolysis activity with glucose/fructose formation. The catalyst with a more phosphated surface showed less deactivation during the dehydration of fructose/glucose into HMF.

scholarly journals Opening up ZSM-5 Hierarchical Zeolite’s Porosity through Sequential Treatments for Improved Low-Density Polyethylene Cracking

Molecules ◽  
2020 ◽  
Vol 25 (12) ◽  
pp. 2878 ◽  
Author(s):  
Karolina A. Tarach ◽  
Kamila Pyra ◽  
Kinga Góra-Marek
Keyword(s):  
Catalytic Cracking ◽  
Product Distribution ◽  
Acid Sites ◽  
Limiting Factors ◽  
Low Density Polyethylene ◽  
Low Density ◽  
Ft Ir ◽  
Sequential Treatments ◽  
Opening Up

An adequately tuned acid wash of hierarchical ZSM-5 zeolites offers a levelling up in the catalytic cracking of low-density polyethylene. Identification of crucial and limiting factors governing the activity of the zeolite was extended with studies about the accessibility of acid sites, nature of the realuminated layer and role of Lewis acid sites. The sequential treatment of a ZSM-5 zeolite offered enhanced activity in low-density polyethylene (LDPE) cracking at low and high conversions, as confirmed by a decrease in the temperatures needed to reach 20% and 80% conversion (T20 and T80, respectively). A linear dependence of the T80 on the coupled IHF (indexed hierarchy factor) and AFB (accessibility factor) highlighted the importance of the textural and acidic parameters in the catalytic cracking of LDPE. Operando FT-IR-GC studies confirmed a higher fraction of short-chain hydrocarbons (C3–C5) in the product distribution of hierarchical catalysts resulting from the effective polymer cracking in easily accessible pores.

Role of the nature of the acid sites in the oxydehydrogenation of propane on a VPO/TiO2 catalyst. An in situ FT-IR spectroscopy investigation

1996 ◽  
Vol 38 (3-4) ◽  
pp. 197-201 ◽  
Author(s):  
L. Savary ◽  
J. Saussey ◽  
G. Costentin ◽  
M. M. Bettahar ◽  
J. C. Lavalley ◽  
...  
Keyword(s):  
Ir Spectroscopy ◽  
Acid Sites ◽  
Tio2 Catalyst ◽  
Ft Ir ◽  
Ft Ir Spectroscopy

The bifunctional Lewis acid site improved reactive oxygen species production: a detailed study of surface acid site modulation of TiO2 by ethanol and Br-

2021 ◽  
Author(s):  
Yi Zheng ◽  
Junqing Shi ◽  
Haiming Xu ◽  
Xingzhi Jin ◽  
Yujing Ou ◽  
...  
Keyword(s):  
Reactive Oxygen Species ◽  
Acid Site ◽  
Reactive Oxygen Species Production ◽  
Reactive Oxygen ◽  
Lewis Acid Site ◽  
Acid Sites ◽  
Ros Production ◽  
Oxygen Species ◽  
Species Production

Modulation of surface acid sites (SAS) can effectively enhance the efficiency of reactive oxygen species (ROS) production in recent. However, the role of SAS has been neglected for photo-reduction reactions....

Calculation-and-experimental estimation of the role of the frequency ratio in changing the endurance at two-frequency deformation modes

2019 ◽  
Vol 85 (1(I)) ◽  
pp. 64-71 ◽  
Author(s):  
M. M. Gadenin
Keyword(s):  
High Frequency ◽  
Experimental Studies ◽  
Low Frequency ◽  
Reduction Factor ◽  
Single Frequency ◽  
Cyclic Loads ◽  
Small Ratio ◽  
Harmonic Components ◽  
Deformation Modes

The cycle configuration at two-frequency loading regimes depends on the number of parameters including the absolute values of the frequencies and amplitudes of the low-frequency and high-frequency loads added during this mode, the ratio of their frequencies and amplitudes, as well as the phase shift between these harmonic components, the latter having a significant effect only with a small ratio of frequencies. Presence of such two-frequency regimes or service loading conditions for parts of machines and structures schematized by them can significantly reduce their endurance. Using the results of experimental studies of changes in the endurance of a two-frequency loading of specimens of cyclically stable, cyclically softened and cyclically hardened steels under rigid conditions we have shown that decrease in the endurance under the aforementioned conditions depends on the ratio of frequencies and amplitudes of operation low-frequency low-cycle and high-frequency vibration stresses, and, moreover, the higher the level of the ratios of amplitudes and frequencies of those stacked harmonic processes of loading the greater the effect. It is shown that estimation of such a decrease in the endurance compared to a single frequency loading equal in the total stress (strains) amplitudes can be carried out using an exponential expression coupling those endurances through a parameter (reduction factor) containing the ratio of frequencies and amplitudes of operation cyclic loads and characteristic of the material. The reduction is illustrated by a set of calculation-experimental curves on the corresponding diagrams for each of the considered types of materials and compared with the experimental data.

Helping when desire is low: The role of Expectancy in helping

2020 ◽  
Author(s):  
Małgorzata Kossowska
Keyword(s):  
Experimental Studies ◽  
Helping Behavior ◽  
Perceived Effectiveness ◽  
Do So

One might assume that the desire to help (here described as Want) is the essential driver of helping declarations and/or behaviors. However, even if desire to help is low, helping behavior may still occur if the expectancy regarding the perceived effectiveness of helping is high. We tested these predictions in a set of three experimental studies. In all three, we measured the desire to help (Want) and the Expectancy that the aid would be impactful for the victim; in addition, we manipulated Expectancy in Study 3. In Studies 1 and 3, we measured the participants’ declaration to help while in Study 2, their helping behavior was examined. In all three studies, we used variations of the same story about a victim. The results supported our hypothesis. Thus, the studies help to tease apart the determinants of helping behavior under conditions of lowered desire to do so, an issue of great importance in public policymaking.

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