Rotational dynamics of propylene inside Na-Y zeolite cages

We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.      

Quasi-elastic neutron scattering study of dynamics in condensed matter

Quasi-elastic neutron scattering (QENS) technique, known to study stochastic motions has been successfully used to elucidate the molecular motions and physical properties related to them, in a variety of systems. QENS is a unique technique that provides information on the time-scale of the motion as well as the geometry of the motions. In this paper, results of some of the systems studied using the facility available at Dhruva, Trombay and other mega-facilities are discussed. Emphasis is given on the results obtained from three different systems studied using QENS, namely, (1) alkyl chain motions in monolayer protected metal clusters, (2) molecular motions of propane in Na-Y zeolitic systems and (3) the study of reorientational motions of liquid crystal innO.m series in different mesophases.     

Acetylene diffusion in Na-Y zeolite

Study of diffusivity of acetylene adsorbed in Na-Y zeolite by quasi-elastic neutron scattering (QENS) measurements at temperatures of 300, 325 and 350 K is reported. A model in which the acetylene molecules undergo random-walk diffusion characterized by a Gaussian distribution of jump lengths inside zeolite cages describes the data consistently. The diffusion constant, residence time between jumps and root mean square jump length are determined.     

Hydrophilic and hydrophobic adsorption on Y zeolites

The uniform large micropores of hydrothermally stable Y zeolites are used widely to confine both polar and non-polar molecules. This paper compares the physisorption of water, methanol, cyclohexane, benzene and other adsorbates over various Y zeolites. These adsorbents are commercial products with reproducibly controllable physical and chemical characteristics. Results indicate that the type I isotherms typical for micropore adsorption can turn into type II or type III isotherms depending on either or both the hydrophobicity of the adsorbent and the polarity of the adsorbate. Methanol produced a rare type V isotherm not reported over zeolites before. Canonical and grand canonical Monte Carlo molecular simulations with Metropolis importance sampling reproduced the experimental isotherms and showed characteristic geometric patterns for molecules confined in Na-X, Na-Y, dealuminated Y, and ZSM5 structures. Adsorbate—adsorbate interactions seem to determine the micropore condensation of both polar and non-polar molecules. Exchanged ions and lattice defects play a secondary role in shaping the adsorption isotherms. The force field of hydrophobic Y appears to exert an as yet unexplored sieving effect on adsorbates having different dipole moments and partial charge distributions. This mechanism is apparently different from both the monolayer formation controlled adsorption on hydrophobic mesopores and macropores and the polarizability and small-pore opening controlled micropore confinement in hydrophobic ZSM5.     

Zero modes of various graphene configurations from the index theorem

The diffusion of carbon dioxide in both NaX and NaY Faujasite systems is investigated by combining Quasi-Elastic Neutron Scattering (QENS) and Molecular Dynamics (MD) simulations. The transport diffusivity evaluated experimentally increases with the loading whereas the simulated self diffusivity decreases. This general behaviour is in good agreement with those previously reported in the literature for different gases in similar zeolites systems. It was also shown that the corrected diffusivity exhibits a significant concentration dependence. At low loading, the activation energies for diffusion derived from QENS and MD simulations are in agreement. They increase from NaY to NaX due to a stronger interaction between the CO₂ molecules and the extra-framework cations. The extrapolation of the transport and self diffusivities to zero coverage allowed us to emphasize a good agreement between experiment and simulation.     

Gas sensing performance of ion-exchanged Y zeolites as an impedimetric ammonia sensor

Zeolites are valuable chemical catalysts and excellent sorbents; several reports have used zeolites for chemical gas sensing. This article systematically investigates the gas sensing performance of ion-exchanged Y zeolites. The interactions between zeolites and ammonia can effectively improve their ionic conductivity, and the zeolites are explored as an impedimetric ammonia sensor. The sensor development was supported by a detailed interpretation of the ammonia-supported ionic conductivity, which was deduced from the measurements of temperature-programmed impedance over a wide temperature range and provides an understanding of the sensing parameters (e.g., temperature and concentration of ammonia). The elevated temperature accelerates the kinetics of the ammonia adsorption/desorption, but it reduces the adsorbing capacity of the zeolite. The thickness of the zeolite pellet is a key parameter for determining the detection limit. Ag-Y and H-Y have higher working temperatures than the alkali cation-exchanged Y zeolite. The excellent selectivity of Ag-Y and Na-Y indicates that they are good candidates as practical ammonia sensors.

     

Effect of guest-host interaction on the dynamics of ethylene glycol in H-ZSM5 zeolite

Dynamics of ethylene glycol (EG) adsorbed in H-ZSM5 zeolite as studied using quasielastic neutron scattering (QENS) technique is reported here. Analysis of QENS data revealed that observed dynamics correspond to isotropic rotational motion of EG. Rotational diffusion coefficient of EG adsorbed in H-ZSM5 zeolite is found to be slower compared to bulk and comparable to that of benzene adsorbed in the same host. Positron Annihilation Spectroscopy (PAS) technique is used to understand the nature of guest-host interaction and the results of both QENS and PAS are found to be consistent with each other.     

Ultrasound-assisted rapid hydrothermal design of efficient nanostructured MFI-Type aluminosilicate catalyst for methanol to propylene reaction

The influence of ultrasound-assisted rapid hydrothermal synthesis of aluminosilicate ZSM-5 catalysts was examined in this work. A series of MFI-type nanostructured materials with sonochemical approach and conventional heating were synthesized and evaluated for conversion of methanol to propylene reaction. The prepared samples were tested by characterization analyses such as XRD, FESEM, BET-BJH, FTIR, TPD-NH₃ and TG/DTG. The obtained results confirmed that ultrasound treatment enhanced the nucleation process and crystal growth for ZSM-5 sample synthesized at moderate temperature of 250 °C. Therefore, it was found the formation of pure MFI zeolite with high crystallinity and improved textural, structural and acidic properties for ZSM-5(UH-250) sample compared with the other zeolites. This observation was attributed to the relationship between the perfect crystallization mechanism and catalytic properties, which led to producing an efficient MFI zeolite toward the optimal catalytic performance. In this manner, the methanol conversion and products selectivity of prepared materials were carried out in MTP reaction at 460 °C and atmospheric pressure. The ZSM-5(UH-250) zeolite with slower deactivation regime exhibited the constant level of methanol conversion (84%) and high propylene selectivity (78%) after 2100 min time on stream. Moreover, the synthesis pathway for MFI zeolite at moderate temperature and also deactivation mechanism of improved sample were proposed.     

Scientific Review: Spectroscopy at IPNS: Recent Instrumental Upgrade and Scientific Highlights

IPNS has four neutron spectrometers: two chopper spectrometers, HRMECS and LRMECS, and two crystal-analyzer spectrometers, QENS and CHEX. At the outset, HRMECS, LRMECS, and QENS (preceded by CAS in the 1980s) were fully users-dedicated instruments. CHEX was an add-on and has been a prototype for pilot experiments and for further development. Over the years, steady incremental improvements have been made on these spectrometers. Recently, QENS and HRMECS have undergone substantial upgrade both in hardware and software. Here, we describe the design and operation of QENS and HRMECS and illustrate their scientific capabilities by selected examples. Other details concerning IPNS' spectrometers are given elsewhere [1].     

Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations. An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.      

Study of ethylene adsorption on zeolite NaY modified with group I metal ions

The adsorption of ethylene by zeolite NaY and zeolite NaY modified by cation exchange with potassium, rubidium, and cesium ions was studied. Cation exchanges were carried out using KNO₃, RbNO₃, and CsNO₃ in the concentration ranges of 0.2-10 mM. XRD patterns and specific surface areas illustrated that modification of NaY zeolite by very dilute solutions containing K⁺, Rb⁺ and Cs⁺ did not lead to significant changes in the crystallinity. Analysis of metals content (ICP-OES) showed that Cs⁺ can replace Na⁺ better than Rb⁺ and K⁺. Particle analysis indicated slight decreases in surface area but pore volumes and pore diameters remained unchanged. Ethylene adsorption isotherms indicated that Na-Y zeolite which was modified by 5.0 mM KNO₃, 0.5 mM RbNO₃ and 1.0 mM CsNO₃ could adsorb ethylene better than zeolite Na-Y. K-NaY zeolite adsorbed up to 102.45 cm³/g ethylene, while Rb-NaY and Cs-NaY zeolites adsorbed up to 98.50 cm³/g and 90.15 cm³/g ethylene, respectively. Ethylene adsorption capacities depended on number of adsorption sites and surface interactions.     

Diffusion of water in nano-porous polyamide membranes: Quasielastic neutron scattering study

Dynamics of water sorbed in a reverse osmosis polyamide membrane (ROPM) as studied by quasielastic neutron scattering (QENS) is reported here. The trimesoylchloride-m-phenylene diamine based ROPM is synthesized by interfacial polymerization technique. QENS data indicates that translational motion of water confined in ROPM gets modified compared to bulk water whereas rotational motion remains unaltered. Translational motion of water in ROPM is found to follow random jump diffusion with lower diffusivity compared to bulk water. Translational diffusivity does not show the Arrhenius behaviour.     

Experimental evidence of selective heating of molecules adsorbed in nanopores under microwave radiation

We have performed in situ quasielastic neutron scattering (QENS) measurements on zeolite-guest systems under microwave irradiation, for comparison with corresponding simulations. Both experiment and simulation reveal selective heating of methanol in silicalite, but little to no heating of benzene in silicalite. Effective translational and rotational temperatures extracted from QENS data under microwave heating were found to depend on microwave power. In agreement with simulation, QENS rotational temperatures significantly exceed translational ones at high microwave power, thus providing the first microscopic proof for athermal effects in microwave-driven nanopores.     

Hydration properties and compressive strength development of Low Heat Cement

For Low Heat Cement (LHC), its hydration reaction properties and compressive strength development were studied by quasi-elastic neutron scattering (QENS) and compressive strength test. In the QENS experiments, the amount of hydration products was estimated and the hydration behavior of LHC was revealed in the early hydration times of 7 days. Simultaneously, we compared the compressive strength of a mortar specimen made from LHC with the amount of hydration products. It was found that the compressive strength is strictly proportional to the amount of hydration products in the range from 15 h to 7 days of hydration.     

Investigating one-dimensional diffusion by quasielastic neutron scattering: a theoretical approach

Scattering functions and full widths at half maximum for quasielastic neutron scattering (QENS) are calculated for diffusion in systems of one-dimensional channels. The self-correlation function for diffusion in isotropically oriented channels is given and it is found that this function diverges at the origin. The calculations are carried out for both normal and single-file diffusion and the influence of the ballistic phase is investigated. It is found that the ballistic phase influences the scattering functions very strongly for large diffusion coefficients. QENS data from the literature are analyzed with respect to this influence. The influence of three different resolution functions (triangular, Gaussian, and Lorentzian) is considered.     

水泥老化过程中水动态的准弹性中子散射(QENS)谱分析

准弹性中子散射(quasi-elastic neutron scattering, QENS)实验是研究水泥老化过程中水动态的一种新颖的实验方法.本文利用老化时间分别为7, 14和30 d水泥样品的QENS谱实验数据, 通过应用四个高斯项的和的能量分辨函数R(Q, E)代替一个高斯项的能量分辨函数来改进经验扩散模型(empirical diffusion model, EDM), 再进行非线性最小二乘拟合.由此导出水泥样品中水动态的相关物理参数: 不动水数密度A, 自由水指数FWI=B₁/(A+B₁+B₂), 洛伦兹函数的半高宽Γ, 移动水跳跃之间的平均停留时间τ ₀及自扩散系数D_t, 而且可得出更为精准的QENS谱拟合曲线.拟合得到的物理参数可定量描述水泥老化过程中水动态过程, 从而为QENS实验在水泥老化过程中水动态研究的应用提供一种合理实用的谱分析方法.     

Dynamic study of N’N-dimethylparanitroaniline encapsulated in silicalite-1 matrix using neutron spin-echo spectroscopy

The present work focuses on the dynamic studies of N’N-dimethyl-paranitroaniline (dmpNA) encapsulated in silicalite zeolite. Quasielastic neutron scattering (QENS) experiments are carried out using neutron spin-echo technique. Polarisation of the scattered neutron beam is measured at carefully chosen values of Q = 0.35, 0.9, 1.1 and 1.45  ?⁻¹ at fixed T = 298  K and at fixed Q = 0.9  ?⁻¹ at 150, 200, 250 and 298 K. This gives insight into the motion and the related activation energy of the guest dmpNA molecule. The quasielastic signal observed in the present system within the time range considered is due to fast local rotational motions of protons of the end methyl groups. The results are in good agreement with the dynamics of methyl group rotations reported in the literature by back-scattering QENS technique.     

Dynamic crossover in hydration water of curing cement paste: the effect of superplasticizer

The influence of a new comb-shaped polycarboxylate-based superplasticizer (CSSP) on the hydration kinetics and transport properties of aged cement pastes has been investigated by high-resolution quasi-elastic neutron scattering (QENS) and low temperature differential scanning calorimetry (LT-DSC). A new method of analysis of QENS spectra is proposed. By applying the refined method we were able to access to four independent physical parameters including the self-diffusion coefficient of the hydration water confined in the cement paste. Mean squared displacement (MSD) of the hydrogen atom for mobile water molecules displays a dynamic crossover temperature in agreement with DSC data. The experimental results indicate that CSSP polymer added into cement paste moderates the hydration process and decreases the dynamic crossover temperature of the hydration water.     

AC impedance, FTIR and XRD investigations of poly acrylonitrile complexed with LiCF3SO3

Solid polymer electrolytes of high ionic conductivity are prepared using poly acrylonitrile (PAN), propylene carbonate (PC), ethylene carbonate (EC) and LiCF₃SO₃. The polymer films are characterised by X-ray diffraction, FTIR and a.c. impedance spectroscopic techniques. The conductivity studies of PAN-LiCF₃SO₃-PC-EC polymer electrolyte systems are carried out in the temperature range 301–373 K. The temperature dependence of the conductivity of the polymer films obeys the VTF relation. The conductivity values are presented and the results are discussed.     

基于准弹性中子散射谱分析水化硅酸钙(C-S-H)中受限水的动态

研究水化硅酸钙(C-S-H)中受限水动态的一种重要手段是准弹性中子散射(quasi-elastic neutron scattering, QENS)实验. C-S-H样品的QENS谱数据可通过跳跃扩散和转动扩散模型进行分析拟合, 进而导出C-S-H样品微纳孔中水动态的相关物理参数: 不动水指数C、转动扩散系数D_r、均方位移< u² >、自扩散系数D_t及平均停留时间τ₀. 本文对水与水泥质量比为30%的C-S-H样品, 测量温度为230-280 K的QENS谱进行了分析, 得到的拟合参数可定量描述C-S-H样品内不同尺度的微纳孔中受限水随温度变化的动态过程. 转动扩散系数D_r随散射矢量Q的变化可知, Q值较大时, 水分子的转动对QENS谱影响较大. 均方位移 < u² > 随Q值的不同而变化, 其拟合值可区分C-S-H样品中的不动水、强受限水和受限水. 在Q较小时, D_t 和τ₀ 的拟合值随温度而变化, 并分别在230和240 K突然增大, 由此揭示温度为230-240 K 时, C-S-H 样品中受限水分子的动态特性发生了转变.