Water Behavior in MCM-41 As a Function of Pore Filling and Temperature Studied by NMR and Molecular Dynamics Simulations

Pajzderska, A.; Ma, González; Mielcarek, Jadwiga; Wąsicki, J.

doi:10.1021/jp505490c

Cited by 29 publications

(39 citation statements)

References 41 publications

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“…Figure 7 (a) shows that the diffusivity of water in C28-saturated pores when capillary condensation has occurred is consistently smaller than the water self-diffusivity in the water condensation reference experiment. There exists both measurement and simulation evidence in support of adsorbate diffusivity varying with the distance from the pore surface, with slower diffusivity closer to the pore surface [47][48][49]; however the data shown in Fig. 7 (a) confirm that the presence of n-C28 within the pore decreases the water diffusivity even more than is found when pure water condenses within an empty pore.…”

Section: Omcts Oil a Thermodynamic Description Of This System Was Dementioning

confidence: 91%

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

Zheng

Brown

Mantle

et al. 2019

Applied Catalysis A: General

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Water is a major product of Fischer-Tropsch synthesis, and hence the behaviour of water within Fischer-Tropsch synthesis catalysts and its potential influence on catalyst rate and selectivity are questions of long-standing interest. The present work applies three different magnetic resonance techniques to study how water interacts with a model wax, n-octacosane, within the pore space of a porous silica of mean pore size ~18 nm. 1 H magnetic resonance spectroscopy, spin-lattice relaxation time and pulsed-field gradient measurements were performed at 195 C, and for water pressure in the range 3-13.6 bar, conditions relevant to low temperature Fischer-Tropsch synthesis. The uptake of water within this system is shown to be very similar to that observed for capillary condensation of water within the empty pore space of the same porous silica under the same experimental conditions; suggesting that capillary condensation of water within the wax-saturated pores is occurring. The behaviour of water is characterised by two regimes. At low water relative pressures of ~0.3 ≤ P/P0  ~0.8 water moves into the pore space, displacing wax from the pore surface and existing as a water-rich layer between the pore surface and an oil-rich phase in the centre of the pore; the strong interaction with the pore surface is evidenced by the short nuclear spin relaxation time values of water at the lowest pressures which then increase slightly as multi-layer adsorption at the pore surface occurs with increase in pressure. In the water relative pressure range ~0.8 ≤ P/P0  ~0.97, condensation of water within the pores is observed, characterised by increases in both spin-lattice relaxation time and molecular diffusivity. Analysis of the data suggests that as much as ~40% of the pore surface is occupied by condensed water after condensation has occurred. It is suggested that these two regimes of water behaviour inside initially wax-filled pores might explain previously reported aspects of the behaviour of Fischer-Tropsch catalyst performance as a function of pore size and amount of co-fed water.

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Section: Omcts Oil a Thermodynamic Description Of This System Was Dementioning

confidence: 91%

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

Zheng

Brown

Mantle

et al. 2019

Applied Catalysis A: General

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“…This decrease in our sample can be interpreted as the result of a gradual freezing of water inside the pores and a reduction of the mobility of the water molecules, which could be due either to a general slowdown of the water dynamics or to a reduction in the amount of water molecules that retain some mobility in the investigated time scale. 24 T 2 * Relaxation. In the temperature region where the FID signal intensity decreases (240−120 K), the apparent spin−spin relaxation time T 2 * was determined experimentally.…”

Section: Experimental Methodsmentioning

confidence: 99%

QENS and NMR Study of Water Dynamics in SBA-15 with a Low Water Content

Kiwilsza¹,

Pajzderska²,

Ma³

et al. 2015

J. Phys. Chem. C

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In this study, motions performed by water molecules adsorbed on the silica surface of SBA-15 material with 6.1% of water content (15% of pore filling) were investigated using NMR and Quasielastic Neutron Scattering (QENS) techniques. The results show no sign of translational diffusion of water, but two types of stochastic localized motions were identified, and both described using a model of proton jumps between two sites. For both motions, the characteristic jump distances and correlation times, as well as activation energies, have been extracted and found to differ significantly. On this basis, the faster motion was ascribed to jumps of water molecules between neighboring positions (d = 2.5 Å, τ = 4 ps at 300 K, and E a = 5.2 ± 0.2 kJ/mol from NMR data, and 5.6 ± 1.1 kJ/mol from QENS), while the slower one exhibits a temperature dependent jump distance and was ascribed to jumps of water molecules between more spatially separated positions (d = 2.9−4.3 Å, τ = 25 ps at 300 K, and E a = 16.1 ± 0.3 kJ/mol from NMR, and 17.3 ± 0.3 kJ/mol from QENS data).

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“…Since the emergence of MCM-41, a large number of adsorption-related material structures and physical and chemical properties have been analyzed [23,26]. Molecular simulations with MCM-41 have been conducted with the Monte Carlo (MC) method to calculate the various gas adsorption conditions [27,28,29,30,31]. The MCM-41 model evolved from earlier simple models [32,33,34,35,36] (e.g., single-layer cylindrical atomic wall, and the rough model of simple potential function), to later all-atom models [37,38] (e.g., carving out models in amorphous silica, and ensuring the bonding relationship between silicon and oxygen atoms), and recent templated synthesis models for restoring real synthetic conditions [39,40].…”

Section: Introductionmentioning

confidence: 99%

“…Ugliengo et al [42] optimized the MCM-41 structure by the density functional method. Pajzderska et al [27] and Coasne et al [43] studied the surface roughness, pore shape, and surface undulation of mesoporous channels, and made corresponding structural improvements. The above works have important significance in the improvement of the overall structure, surface morphology, and adsorption sites of the MCM-41 model.…”

Section: Introductionmentioning

confidence: 99%

Molecular Simulation of Naphthalene, Phenanthrene, and Pyrene Adsorption on MCM-41

Yang

Zhang

Jiang

et al. 2019

IJMS

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The adsorption of three typical polycyclic aromatic hydrocarbons (PAHs), naphthalene, phenanthrene, and pyrene with different ring numbers, on a common mesoporous material (MCM-41) was simulated based on a well-validated model. The adsorption equilibriums (isotherms), states (angle distributions and density profiles), and interactions (radial distribution functions) of three PAHs within the mesopores were studied in detail. The results show that the simulated isotherms agreed with previous experimental results. Each of the PAHs with flat molecules showed an adsorption configuration that was parallel to the surface of the pore, in the following order according to the degree of arrangement: pyrene (Pyr) > phenanthrene (Phe) > naphthalene (Nap). In terms of the interaction forces, there were no hydrogen bonds or other strong polar forces between the PAHs and MCM-41, and the O–H bond on the adsorbent surface had a unique angle in relation to the PAH molecular plane. The polarities of different H atoms on the PAHs were roughly the same, while those of the C atoms on the PAHs decreased from the molecular centers to the edges. The increasing area of the π-electron plane on the PAHs with the increasing ring number could lead to stronger adsorption interactions, and thus a shorter distance between the adsorbate and the adsorbent.

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Water Behavior in MCM-41 As a Function of Pore Filling and Temperature Studied by NMR and Molecular Dynamics Simulations

Cited by 29 publications

References 41 publications

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

QENS and NMR Study of Water Dynamics in SBA-15 with a Low Water Content

Molecular Simulation of Naphthalene, Phenanthrene, and Pyrene Adsorption on MCM-41

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