Noncatalytic hydrogenation of naphthalene in nanosized membrane reactors with accumulated hydrogen and controlled adjustment of their reaction zone volumes

As part of ongoing studies aimed at designing the next generation of nanosized membrane reactors (NMRs) with accumulated hydrogen, the noncatalytic hydrogenation of naphthalene in pores of ceramic membranes (TRUMEM ultrafiltration membranes with D _av = 50 and 90 nm) is performed for the first time, using hydrogen preadsorbed in a hybrid carbon nanostructure: mono- and multilayered oriented carbon nanotubes with graphene walls (OCNTGs) that form on inner pore surfaces. In this technique, the reaction proceeds in the temperature range of 330–390°C at contact times of 10–16 h. The feedstock is an 8% naphthalene solution in decane. The products are analyzed via chromatography on a quartz capillary column coated with polydimethylsiloxane (SE-30). It is established for the first time that in NMRs, the noncatalytic hydrogenation of naphthalene occurs at 370–390°C, forming 1,2,3,4-tetrahydronaphthalene in amounts of up to 0.61%. The rate constants and activation energy (123.5 kJ/mol) of the noncatalytic hydrogenation reaction are determined for the first time. The possibility of designing an NMR with an adjustable reaction zone volume is explored. Changes in the pore structure of the membranes after their modification with pyrocarbon nanosized crystallites (PNCs) are therefore studied as well. It is shown that lengthening the process time reduces pore size: within 23 h after the deposition of PNCs, the average pore radius (r _av) falls from 25 to 3.1 nm. The proposed approach would allow us to design nanoreactors of molecular size and conduct hydrogenation reactions within certain guidelines to synthesize new chemical compounds.

     

New nanosized catalytic membrane reactors for hydrogenation with stored hydrogen: Prerequisites and the experimental basis for their creation

The prerequisites and prospects for creating a new generation of nanosized membrane reactors are considered. For the first time, hydrogenation reactions take place in ceramic membrane pores with hydrogen adsorbed beforehand in mono- and multilayered oriented carbon nanotubes with graphene walls (OCNTGs) formed on the internal pore surface. It is shown for Trumem microfiltration membranes with D _avg ∼130 nm that oxidation reactions of CO on a Cu_0.03Ti_0.97O_{2 ± δ} catalyst and the oxidative conversion of methane into synthesis gas and light hydrocarbons on La + Ce/MgO are considerably enhanced when they occur in membranes. Regularities of hydrogen adsorption, storage, and desorption in nanosized membrane reactors are investigated through OCNTG formation in Trumem ultrafiltration membrane pores with D _avg = 50 and 90 nm and their saturation with hydrogen at a pressure of 10–13 MPa. It is shown that the amount of adsorbed hydrogen reaches 14.0% of OCNTG mass. Using thermogravimetric analysis in combination with mass-spectrometric analysis, hydrogen adsorption in OCNTG is first determined and its desorption is found to proceed at atmospheric pressure at a temperature of ∼175°C. It is shown that adsorbed hydrogen affects the transport properties of the membranes, reducing their efficiency with respect to liquids by 4–26 times. This is indirect confirmation of its high activity, due apparently the dissociative mechanism of adsorption.     

Nanosized pyrocarbon crystallites, their superposition, and carbon nanotubes of graphenes in the pores of ultrafiltration membranes

The superposition of nanosized pyrocarbon crystallites (NPCs; an NPC superposition is n sequentially formed pyrocarbon nanocrystallites each consisting of m monolayers) was studied. NPCs were obtained by the pyrolysis of methane and deposited on a porous surface of Trumem ultrafiltration membranes with D _pore = 50 and 90 nm. A new uncoupled layer effect was found. After n-fold NPC superposition of an even number of monolayers (the total number throughout the superposition; i.e., m ₁ + m ₂ + ... + m _n is even), further deposition of pyrocarbon nanocrystallites occurs in accordance with the previously determined kinetics of topochemical dehydrogenation of methane. However, if an NPC superposition is formed from an odd number of monolayers, the reaction rate constant decreases by more than one order of magnitude. A comparative analysis of the hydrogen adsorption ability of two carbon structures (NPC superposition and orientated carbon nanotubes of graphenes (OCNTGs)) showed that OCNTGs adsorbed up to ∼14% (relative to their mass) of hydrogen, whereas NPCs did not. It was shown for the first time that hydrogen adsorbed in OCNTG affected the transport properties of membranes, decreasing their performance with respect to liquids by a factor of 4–26.     

Hydrogen adsorption in the series of carbon nanostructures: Graphenes–graphene nanotubes–nanocrystallites

A comparative analysis of hydrogen absorption capability is performed for the first time for three types of carbon nanostructures: graphenes, oriented carbon nanotubes with graphene walls (OCNTGs), and pyrocarbon nanocrystallites (PCNs) synthesized in the pores of TRUMEM ultrafiltration membranes with mean diameters (D_m) of 50 and 90 nm, using methane as the pyrolized gas. The morphology of the carbon nanostructures is studied by means of powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). Hydrogen adsorption is investigated via thermogravimetric analysis (TGA) in combination with mass-spectrometry. It is shown that only OCNTGs can adsorb and store hydrogen, the desorption of which under atmospheric pressure occurs at a temperature of around 175°C. Hydrogen adsorption by OCNTGs is quantitatively determined and found to be about 1.5% of their mass. Applying certain assumptions, the relationship between the mass of carbon required for the formation of single-wall OCNTGs in membrane pores and the surface area of pores is established. Numerical factor Ψ = m_dep/m_calc, where m_dep is the actual mass of carbon deposited upon the formation of OCNTGs and mcalc is the calculated mass of carbon necessary for the formation of OCNTGs is introduced. It is found that the dependence of specific hydrogen adsorption on the magnitude of the factor has a maximum at Ψ = 1.2, and OCNTGs can adsorb and store hydrogen in the interval 0.4 to 0.6 < Ψ < 1.5 to 1.7. Possible mechanisms of hydrogen adsorption and its relationship to the structure of carbon nanoformations are examined.     

Characterization of Hydrogen Adsorption on Palladium and Pd/c Using a Radiotracer

Hydrogen adsorption on palladium black and palladium supported on activated carbon was characterized by the Temperature Programmed Desorption technique (TPD) using the radiotracer tritium. It is shown that there are five energetic desorbed hydrogens with peak maxima at —50, 35, 115, 350, and 580°C. An additional peak at — 120°C hydrogen desorbed from activated carbon was observed on Pd/C. An isotopic hydrogen separation experiment was designed to demonstrate that hydrogen only dissociatively adsorbed on palladium surfaces whereas it was associatively adsorbed on activated carbon.     

铜锌催化剂上吸附动力学参数的非线性动态分析

本文得到了适应于一氧化碳加氢加压体系非线性动态分析不同吸附物种动力学参数的模型，并优化出工业铜锌催化剂上合成甲醇反应中可逆吸附氢和可逆吸附一氧化碳的吸附速率常数及吸附平衡常数。结果表明：铜锌催化剂上吸附可逆氢比吸附一氧化碳快７倍左右。由于铜锌催化剂上甲醇的生成是可逆吸附氢与可逆吸附一氧化碳共同作用的结果，且铜锌催化剂 可逆吸一氧化碳的表面浓度随气相一氧化碳分压的增加而增加，因而加压将有利于合成甲醇     

Combined hydrogenation of carbon oxides on catalysts bearing iron and nickel nanoparticles

The reaction of the hydrogenation of a mixture of carbon oxides on ultradisperse powder (UDP) catalysts containing Fe and Ni nanoparticles and their bimetallic mechanical mixtures was investigated. It was established that the main reaction product on UDP Ni is methane, while the main products on the bimetallic systems are methane and ethylene. A synergetic effect was observed on the bimetallic catalyst under investigation. It was revealed that the hydrogenation of a mixture of carbon oxides proceeds through the stage of dissociative adsorption of both components, CO and CO₂. The olefin selectivity of the process was explained by the participation of different forms of adsorbed hydrogen (H_I: H_II) at the catalyst surface. It is assumed that the hydrogenation of carbon oxides on iron-nickel catalysts proceeds either through the jumpover effect or via hydrogen spillover.     

Preparation of hydrogen for feeding fuel cells by low-temperature conversion of ethanol on Ni/ZnO and Ni-Cu/ZnO catalysts

Preparation of hydrogen by low-temperature steam conversion of ethanol on nickel and binary nickel-copper catalysts supported on zinc oxide was studied experimentally in the temperature interval 200–450°C. High efficiency of hydrogen evolution in the course of ethanol conversion on these catalysts was demonstrated. At temperatures lower than 350°C, the main conversion products are hydrogen, methane, carbon monoxide, and carbon dioxide. At 400°C, the conversion products contain no carbon monoxide, which allows the mixture obtained to be used for feeding fuel cells with proton-conducting membranes.     

氢在多壁碳纳米管上吸附行为研究

根据热力学平衡原理推导了通用吸附等温方程.通过比较氢在碳纳米管和炭狭缝孔上的高阶维里吸附系数，分析了77～297 K温度区间，温度、管径(孔宽)对碳纳米管、炭狭缝孔吸附空间储氢容量的影响，并由氢在石墨平面上的最大吸附容量计算了本次试验多壁碳纳米管(MWCNTs)在各平衡温度时的最大氢吸附容量.运用确定参数后的吸附等温方程，线性回归分析了氢在本次试验MWCNTs上的吸附数据.结果表明，在160～180 K温度区间，管内被吸附氢分子之间由于吸附受压产生的排斥能出现极大值；随着温度升高，氢分子之间以吸引力为主，提高氢气压力后才发生明显吸附.     

Calculation of the characteristics of ethanol and methanol adsorbed in a pore of activated carbon by the density functional method

Density functional theory was used to perform quantum-chemical calculations of changes in the energy and structure characteristics of methanol and ethanol molecules caused by their adsorption in model slitlike pores of activated carbon. The conclusion was made that changes in these characteristics (bond lengths, angles, charges on atoms, and harmonic vibration frequencies) is additional evidence of the validity of the Tolmachev thermodynamic model, in which adsorption is treated as a quasi-chemical reaction of the addition of adsorbate molecules to adsorption centers of an adsorbent. It was shown that the arrangement of alcohol molecules, when the C-O and C-C bonds were approximately parallel to pore walls and the hydrogen atom was directed toward a nearer pore wall, was most favorable energetically. Two adsorbed alcohol molecules are also arranged parallel to pore walls and form a hydrogen bond.     

Adsorption of hydrogen peroxide on functionalized mesoporous silica surfaces

MCM-41 and MSU-H mesoporous silicas were successfully functionalized with hydrogen bonds forming organic moieties, which have been proven by elemental analysis. Both moieties, based on oxygen and nitrogen containing groups, were introduced with high efficiency—the amount of carbon in all cases exceeded 10 % and the elemental ratios suggest binding to the surface through two or three Si–O–Si bonds. Hydrogen peroxide adsorption was conducted in its aqueous solutions and the amount adsorbed was determined using the ferric thiocyanate method. Results are presented as a function of hydrogen peroxide concentration in aqueous solution from 5 to 30 %. Both functionalized silicas show increased adsorption capacity when compared with that of their unfunctionalized analogues. The surface modified with nitrogen-based organic moiety revealed better adsorption properties as well as higher resistance against oxidation. MSU-H silica, due to its larger pore diameter, provides more space to bind hydrogen peroxide molecules and thus was found to have higher adsorption capacity: it adsorbed up to four times more hydrogen peroxide than MCM-41.     

Dependence of the thermodynamic characteristics of forms of hydrogen adsorbed on a porous nickel surface on the degree of deactivation

Thermodynamic characteristics of the adsorption equilibria of individual forms of adsorbed hydrogen are calculated using data from adsorption-calorimetric studies of liquid-phase hydrogenation in a model of a surface with discrete nonuniformity in the ideal adsorption layer approximation under conditions of liquid-phase hydrogenation. It is shown that partial deactivation raises the fraction of the molecular α form via the atomic β form of adsorbed hydrogen and increases the adsorbate-adsorbent binding energy.     

Initial hydrogenations of pyridine on MoP(001): a density functional study

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(β),C(β),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(β) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.     

Adsorption of gas-water binary systems in carbon micropores: Computer simulation

The adsorption of gas-water mixture in micropores of carbon materials at 298 K has been studied using computer simulation. Methane, nitrogen, ammonia, carbon dioxide, and hydrogen sulfide were considered as gas components. In the grand canonical ensemble Monte-Carlo simulation of adsorption, the displacement of a gas component from a pore as a result of the formation of water microclusters was observed for all systems studied. Cluster growth conditions on graphite-like and activated surfaces differ significantly. The comparative stability of adsorbed gas-water mixtures has been determined for all gases.     

Modification of ceramic membranes by alcohol adsorption

This study aims at modifying ceramic membranes by means of alcohol chemisorption. Composite ceramic membranes with a skin layer of γ-alumina were used. First, the adsorption of several alcohol on powdered γ-alumina was investigated emphasising the thermal stability of the adsorbed compounds. Later, a commercial γ-alumina membrane was modified by alcohol adsorption. The permeability of water and several organic compounds was obtained for both the non-modified and modified ceramic membrane. Also, its isoelectric point was determined. The results prove that all the alcohol were readily adsorbed on powdered γ-alumina not only physically but also chemically forming an alkoxide. The chemisorbed alcohol was stable up to 200°C. Beyond this temperature, the alkoxide breaks up releasing the alcohol although the alkoxide also can react yielding an olefin or ether. The ceramic membrane was also successfully modified by alcohol adsorption. The layer of chemisorbed alcohol imparts hydrophobic characteristics to the membrane surface, so water permeability decreases significantly. This cannot be merely explained by pore size reduction due to the adsorbed layer. Thermal treatment at 250°C recovered original permeability with only minor damage to the membrane.     

Biomaterial immobilization in nanoporous carbon molecular sieves: influence of solution pH, pore volume, and pore diameter

The adsorption of lysozyme (Lz) onto nanoporous carbon molecular sieves with various pore diameters has been studied at different solution pH values. All the adsorption isotherms have successfully been correlated by the Langmuir equation. The amount of adsorbed Lz depends on the solution pH as well as on the specific pore volume and pore diameter of the adsorbents. The maximum adsorption was observed near the isoelectric point of the Lz (pI approximately 11), suggesting that suppression of electric repulsion between the enzymes plays an important role in the adsorption process. Moreover, the amount adsorbed depends on the pore size and pore volume of the nanoporous carbon adsorbents, indicating that the Lz molecules are adsorbed inside the mesopores. CMK-3-150 shows a larger amount of Lz adsorption as compared to CMK-3. The increased Lz adsorption capacity of CMK-3-150 may be due to the larger pore volume and pore diameter as compared to that of CMK-3. The unaltered structural order of the nanoporous adsorbents after the adsorption has been confirmed by the physicochemical characterization techniques such as XRD and N(2) adsorption. In addition, FT-IR spectroscopic studies confirm that the Lz used in this study is stable even after the adsorption on the nanoporous carbon. These results indicate that nanoporous carbon has superior water stability and thus is a more appropriate adsorbent for biomaterials than nanoporous silica.     

Thermodynamics of hydrogen adsorption in slit-like carbon nanopores at 77 K. Classical versus path-integral Monte Carlo simulations

Hydrogen in slit-like carbon nanopores at 77 K represents a quantum fluid in strong confinement. We have used path-integral grand canonical Monte Carlo and classical grand canonical Monte Carlo simulations for the investigation of the "quantumness" of hydrogen at 77 K adsorbed in slit-like carbon nanopores up to 1 MPa. We find that classical simulations overpredict the hydrogen uptake in carbon nanopores due to neglect of the quantum delocalization. Such disagreement of both simulation methods depends on the slit-like carbon pore size. However, the differences between the final uptakes of hydrogen computed from both classical and quantum simulations are not large due to a similar effective size of quantum/classical hydrogen molecules in carbon nanospaces. For both types of molecular simulations, the volumetric density of stored energy in optimal carbon nanopores exceeds 6.4 MJ dm(-3) (i.e., 45 kg m(-3); Department of Energy target for 2010). In contrast to the hydrogen adsorption isotherms, we found a large reduction of isosteric enthalpy of adsorption computed from the quantum Feynman's path-integral simulations in comparison to the classical values at 77 K and pressures up to 1 MPa. Depression of the quantum isosteric enthalpy of adsorption depends on the slit-like carbon pore size. For the narrow pores (pore width H in [0.59-0.7] nm), the reduction of the quantum isosteric enthalpy of adsorption at zero coverage is around 50% in comparison to the classical one. We observed new phenomena called, by us, the quantum confinement-inducing polymer shrinking. In carbon nanospaces, the quantum cyclic polymers shrink, in comparison to its bulk-phase counterpart, due to a strong confinement effect. At considered storage conditions, this complex phenomenon depends on the size of the slit-like carbon nanopore and the density of hydrogen volumetric energy. For the smallest nanopores and a low density of hydrogen volumetric energy, the reduction of the polymer effective size is the highest, whereas an increase of the pore size and the density of hydrogen volumetric energy causes the polymer swelling up to a value slightly below the one computed from the bulk phase. Quantum confinement-inducing polymer shrinking is of great importance for realizing the potential of quantum molecular sieves.     

Three‐Dimensional Ultralarge‐Pore Ia3d Mesoporous Silica with Various Pore Diameters and Their Application in Biomolecule Immobilization

Highly ordered mesoporous three‐dimensional Ia3d silica (KIT‐6) with different pore diameters has been synthesized by using pluronic P123 as surfactant template and n‐butanol as cosolvent at different synthesis temperatures in a highly acidic medium. The materials were characterized by XRD and N₂ adsorption. The synthesis temperature plays a significant role in controlling the pore diameter, surface area, and pore volume of the materials. The material prepared at 150 °C, KIT‐6‐150, has a large pore diameter (11.3 nm) and a high specific pore volume (1.53 cm³ g^?1). We also demonstrate immobilization of lysozyme, which is a stable and hard protein, on KIT‐6 materials with different pore diameters. The amount of lysozyme adsorbed on large‐pore KIT‐6 is extremely large (57.2 μmol g^?1) and is much higher than that observed for mesoporous silicas MCM‐41, SBA‐15, and KIT‐5, mesoporous carbons, and carbon nanocages. The effect of various parameters such as buffer concentration, adsorption temperature, concentration of the lysozyme, and the textural parameter of the adsorbent on the lysozyme adsorption capacity of KIT‐6 was studied. The amount adsorbed mainly depends on solution pH, ionic strength, adsorption temperature, and pore volume and pore diameter of the adsorbent. The mechanism of adsorption on KIT‐6 under different adsorption conditions is discussed. In addition, the structural stability of lysozyme molecules and the KIT‐6 adsorbent before and after adsorption were investigated by XRD, nitrogen adsorption, and FTIR spectroscopy.     

Nanosized nickel oxide particles and modification with poly(methyl methacrylate)

In recent years, there is an increasing interest in the fabrication of inorganic–organic materials considering the remarkable change and improvement in properties. In this investigation, nanosized nickel oxide (NiO) particles were first prepared by calcination of nickel hydroxide precursor obtained by a simple liquid‐phase process. Mixed phases of NiO and nickel hydroxide were present as the calcination temperature was lower than 250°C. Non‐stoichiometric NiO was formed between 250°C and 350°C, and a pure NiO was obtained as the temperature reached 500°C. The surface characteristics of NiO particles were evaluated by measuring the adsorption behavior of anionic and cationic surfactants and some biomolecules. NiO/poly(methyl methacrylate) composite particles were then prepared using variable NiO/methyl methacrylate (MMA) ratio by seeded emulsion polymerization. The efficiency of NiO incorporation in the composite increased as the MMA content was increased in the recipe. The composite particles were colloidally stable, and the obtained particles were characterized by Fourier transform infrared, scanning electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

A Density Functional Theory Study on the Effect of Zero‐Point Energy Corrections on the Methanation Profile on Fe(100)

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4 H?CH+3 H?CH₂+2 H?CH₃+H?CH₄, are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero‐point energy (ZPE) corrections are included. The C, CH and CH₂ species are most stable at the fourfold hollow site, while CH₃ prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero‐point energy corrections are rarely provided in the literature. Since they are derived from C? H bonds with characteristic vibrations on the order of 2500–3000 cm^?1, the equivalent ZPE of 1/2 hν is on the order of 0.2–0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH_x and H, the ZPE correction is expected to be significant, as additional C? H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen‐containing species.