Proton transfer from alkane radical cations to alkane molecules: selectivity with respect to the site of proton donation and proton acceptance

Information on the donor and acceptor site selectivity in the proton transfer from n-alkane radical cations to n-alkane molecules is gathered from gamma-irradiated frozen CCl(3)F/n-alkane solutions (symmetric transfer) and mixed n-alkane crystals (asymmetric transfer) by EPR spectroscopy at 77 K and gas chromatographic analysis after melting. The proton-donor site is related very strictly to the structure of the semi-occupied molecular orbital of the parent radical cation, with proton transfer taking place from those C-H bonds that carry appreciable unpaired-electron and positive-charge density. Proton acceptance is restricted to C-H bonds at secondary carbon atoms (no proton transfer to C-C bonds nor to C-H bonds at primary carbon atoms), with a preference for the penultimate position and equal (but considerably lower) transfer to the interior sites. In mixed n-alkane crystals, additional selectivity with respect to the site of proton acceptance results from structural factors in combination with the donor site selectivity (structurally determined acceptor site selectivity).     

Adsorption from solution of large alkane and related molecules onto graphitized carbon

A reanalysis of experimental adsorption isotherms and enthalpies of adsorption is made for higher n-alkanes (C₁₆–C₃₂) adsorbed from dilute solutions of nonpolar organic solvents onto graphitized carbon. The submonolayer region of the adsorption isotherms suggests an alignment and close packing of the long-chain alkane molecules parallel to the graphite surface, and the results conform to a simple parallel-layer model with a large Flory-Huggins-type interaction parameter χ^a which is attributed to strong lateral adsorbate-adsorbate interactions. This interpretation of the adsorption isotherms is consistent with the calorimetric data, which yield a pronounced increase of the differential molar enthalpy of displacement with the fraction of surface covered by the long-chain alkane. Preliminary results on competitive adsorption of two solutes are presented in the second part of the paper. Different types of adsorption behaviour (mixed monolayer phase and two-dimensional eutectic) are observed depending on whether the individual solutes form close-packed ordered monolayers at the solution/graphite interface.     

A pillar-layer MOF used as a luminescent probe for detecting small molecules acetone

The reactions of 4,4-bipyridine and 5-Hydroxyisophthalic acid with Zn(NO₃)₂·3H₂O in N,N′-dimethylacetamide lead to the formation of [Zn(5-hip)(bpy)]·2DMA (1). Compound 1 possesses a non-interpenetrating pillar-layer framework with 1D rectangular-shape channels. The experimental results show that the luminescent intensity of 1 highly depends on small solvent molecules, particularly CH₃OH and acetone. Compound 1 can be used as a luminescent probe to detect small molecule acetone.     

小分子烃类蒸汽热裂解自由基机理模型研究方法的探讨

由于热裂解存在反应时间短、自由基数量多、浓度小,且不同原料产生的不同自由基之间、反应深度较大时管壁处于高温和停留时间所生成的不同自由基与主流体间的相互作用会随时改变反应路径,并影响到产物分布,因此造成了用实验方法研究单体烃热裂解反应机理的困难。将Materials Studio软件与Aspen Plus软件相结合来研究单体烃热裂解的自由基反应机理,并通过对乙烷热裂解一次反应机理、乙烷和丙烷混合热裂解相互作用机理、动力学数据准确性对比及正已烷空间位阻的影响,对研究方法进行了论述。结果表明,数值模拟的理论方法与实验方法相比,可以深入了解实验研究不可能达到的一些机理细节问题,如果将实验研究和模拟研究相结合,可避免目前动力学模型研究中的各种假设,提高机理模型研究的准确性,为工业生产预测提供高精度的机理模型。     

Phosphate as promoter of zirconia for alkane isomerization reactions

Phosphate, as catalytic promoter of zirconia, has similar effects as sulfate and tungstate anions, i.e., enhanced thermal stability with high retention of surface area and higher crystallization temperature, in comparison with unpromoted zirconia. $n$ -butane isomerization activity increases and selectivity to isobutane decreases as phosphate content increases. For $n$ -heptane isomerization, activity and selectivity to $i$ -C₇ have maxima.     

Kinetic modeling of thermal cracking reactions

This paper provides a kinetics model for thermal cracking of various oils over time frames which correspond to the long term storage at temperatures up to those experienced in an in situ combustion process. The model describes detailed kinetic mechanisms and concentration changes of individual species during the thermal cracking reactions. Also the modeling results are compared with the experimental data to verify their validity.     

Laser-induced desorption of small molecules from oxide surfaces: A first-principles study

Recent efforts in the theoretical simulation of laser-induced desorption of small molecules from surfaces are summarized. As a representative example, photodesorption of CO molecules from a Cr₂O₃(0001) surface is investigated since detailed quantum state resolved experimental results are available for this system. In particular, vectorial properties such as the alignment of the desorbing species are considered. Furthermore, the influence of surface temperature as a control parameter is investigated, and lateral velocity distributions are calculated and compared with experimental results. All simulations presented in the present study are based on ab initio potential energy surfaces (PESs) for the electronic ground state as well as electronically excited states involved in the desorption process. These PESs provide the prerequisite for extensive high-dimensional quantum mechanical simulations of the dynamics of nuclear motion based on a stochastic wave packet scheme. These wave packet calculations allow for a detailed microscopic understanding of experimental results and provide a perspective for future experiments.     

A luminescent europium metal-organic framework probe for selective sensing of pollutant small organic molecules in high sensitivity

A luminescent europium metal-organic framework[Eu(Hbptc)(H₂O)₃]_n (1) has been successfully prepared by the facile hydrothermal reaction of Eu(NO₃)₃ ⋅ 6H₂O and the π-conjugated ligand H₄bptc (H₄bptc = biphenyl-2,3,3′,5′-tetracarboxylic acid). Single crystal structure analysis reveals that 1 exhibits a 2D layered structure. The luminescence studies revealed that this material can be selectively and sensitively quenched or enhanced by some small organic molecule solvents, showing potential as fluorescence probe.     

Modelling the kinetics of fast catalytic cracking reactions

Instantaneous kinetic constants and gasoline selectivities have been determined for catalytic cracking of n-hexadecane. The pulse technique was used in order to model the sequential build-up of coke which occurs on cracking catalyst within a riser transport-line reactor. The total amount of hydrocarbon injected per unit weight of catalyst was between 0 and 10. The mathematical model used to analyze the data was based on the unsteady state mass balance of the microcatalytic reactor with the assumption of plug flow. Results suggest a fast deactivation process during the run with fresh catalyst, while regenerated catalyst showed a slower deactivation. The catalyst regenerated three times evidenced a low apparent activation energy when temperature was increased from 500°C to 550°C.     

Scanning probe microscopy of atoms and molecules on insulating films: from imaging to molecular manipulation

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) of single atoms and molecules on ultrathin insulating films have led to a wealth of novel observations and insights. Based on the reduced electronic coupling to the metallic substrate, these techniques allow the charge state of individual atoms to be controlled, orbitals of individual molecules to be imaged and metal-molecule complexes to be built up. Near-contact AFM adds the unique capabilities of imaging and probing the chemical structure of single molecules with atomic resolution. With the help of atomic/molecular manipulation techniques, chemical binding processes and molecular switches can be studied in detail.     

Catalytic hydrogenation of alkenes on acidic zeolites: Mechanistic connections to monomolecular alkane dehydrogenation reactions

Brønsted acid sites in zeolites (H-FER, H-MFI, H-MOR) selectively hydrogenate alkenes in excess H₂ at high temperatures (>700 K) and at rates proportional to alkene and H₂ pressures. This kinetic behavior and the De Donder equations for non-equilibrium thermodynamics show that, even away from equilibrium, alkene hydrogenation and monomolecular alkane dehydrogenation occur on predominantly uncovered surfaces via microscopically reverse elementary steps, which involve kinetically-relevant (C–H–H)⁺ carbonium-ion-like transition states in both directions. As a result, rate constants, activation energies and activation entropies for these two reactions are related by the thermodynamics of the overall stoichiometric gas-phase reaction. The ratios of rate constants for hydrogenation and dehydrogenation reactions do not depend on the identity or reactivity of active sites; thus, sites within different zeolite structures (or at different locations within a given zeolite) that favor alkane dehydrogenation reactions, because of their ability to stabilize the required transition states, also favor alkene hydrogenation reactions to the exact same extent. These concepts and conclusions also apply to monomolecular alkane cracking and bimolecular alkane–alkene reaction paths on Brønsted acids and, more generally, to any forward and reverse reactions that proceed via the same kinetically-relevant step on vacant surfaces in the two directions, even away from equilibrium. The evidence shown here for the sole involvement of Brønsted acids in the hydrogenation of alkoxides with H₂ is unprecedented in its mechanistic clarity and thermodynamic rigor. The scavenging of alkoxides via direct H-transfer from H₂ indicates that H₂ can be used to control the growth of chains and the formation of unreactive deposits in alkylation, oligomerization, cracking and other acid-catalyzed reactions.     

Enzymatic hydrogelation of small molecules

Enzymes, a class of highly efficient and specific catalysts in Nature, dictate a myriad of reactions that constitute various cascades in biological systems. Self-assembly, a process prevalent in Nature, also plays important roles in biology, from maintaining the integrity of cells to performing cellular functions and inducing abnormalities that cause disease. To explore enzyme-regulated molecular self-assembly in an aqueous medium will help to understand and control those important biological processes. On the other hand, certain small organic molecules self-assemble in water to form molecular nanofibers and result in a hydrogel, which is referred to as a "supramolecular hydrogel" (and the small molecules are referred to as "supramolecular hydrogelators"). Supramolecular hydrogelators share common features, such as amphiphilicity and supramolecular interactions (pi-pi interactions, hydrogen bonding, and charge interactions among the molecules, among others) that result in nanostructures and form the three-dimensional networks as the matrices of hydrogels. In this Account, we discuss the use of enzymes to trigger and control the self-assembly of small molecules for hydrogelation, which takes place in vitro or in vivo, extra- or intracellularly. Using phosphatase, thermolysin, beta-lactamase, and phosphatase/kinase as examples, we illustrate the design and application of enzyme-catalyzed or -regulated formation of supramolecular hydrogels that offer a new strategy for detecting the activity of enzymes, screening for enzyme inhibitors, typing bacteria, drug delivery systems, and controlling the fate of cells. Since the expression and distribution of enzymes differ by the types and states of cells, tissues, and organs, using an enzymatic reaction to convert precursors into hydrogelators that self-assemble into nanofibers as the matrices of the hydrogel, one can control the delivery, function, and response of a hydrogel according to a specific biological condition or environment, thus providing an accessible route to create sophisticated materials for biomedicine. Particularly, intracellular enzymatic hydrogelation of small molecules offers a unique means for scientists to integrate molecular self-assembly with inherent enzymatic reactions inside cells for developing new biomaterials and therapeutics at the supramolecular level and improving the basic understanding of dynamic molecular self-assembly in water.     

Relative reactivities of alkanes in multi-component catalytic cracking reactions

An experimental technique is discussed for measuring relative reactivities of alkanes in the catalytic cracking of multi-component hydrocarbon mixtures over a heterogeneous, Y-zeolitebased catalyst at 250–350 °C. With the technique, ca. 0.1 l of an alkane mixture is evaporated and contacted with a catalyst, after which the mixture of reaction products and the unreacted feed enters the chromatographic column and is immediately analyzed. The technique is used to measure relative reactivities of 21 alkanes in a single experiment. The principal results of these experiments are similar to the results of single-component cracking: alkane reactivity rapidly increases with the increase of the carbon number, and methyl-branched alkanes are more reactive than linear alkanes. However, the variations in alkane reactivities as a function of their molecular weight and skeleton structure differ very significantly between single- and multicomponent experiments.     

Quantum mechanical computations and spectroscopy: from small rigid molecules in the gas phase to large flexible molecules in solution

Interpretation of structural properties and dynamic behavior of molecules in solution is of fundamental importance to understand their stability, chemical reactivity, and catalytic action. While information can be gained, in principle, by a variety of spectroscopic techniques, the interpretation of the rich indirect information that can be inferred from the analysis of experimental spectra is seldom straightforward because of the subtle interplay of several different effects, whose specific role is not easy to separate and evaluate. In such a complex scenario, theoretical studies can be very helpful at two different levels: (i) supporting and complementing experimental results to determine the structure of the target molecule starting from its spectral properties; (ii) dissecting and evaluating the role of different effects in determining the observed spectroscopic properties. This is the reason why computational spectroscopy is rapidly evolving from a highly specialized research field into a versatile and widespread tool for the assignment of experimental spectra and their interpretation in terms of chemical physical effects. In such a situation, it becomes important that both computationally and experimentally oriented chemists are aware that new methodological advances and integrated computational strategies are available, providing reliable estimates of fundamental spectral parameters not only for relatively small molecules in the gas phase but also for large and flexible molecules in condensed phases. In this Account, we review the most significant methodological contributions from our research group in this field, and by exploiting some recent results of their application to the computation of IR, UV-vis, NMR, and EPR spectral parameters, we discuss the microscopic mechanisms underlying solvent and vibrational effects on the spectral parameters. After reporting some recent achievements for the study of excited states by first principle quantum mechanical approaches, we focus on the treatment of environmental effects by means of mixed discrete-continuum solvent models and on effective methods for computing vibronic contributions to the spectra. We then discuss some new developments, mainly based on time-dependent approaches, allowing us to go beyond the determination of spectroscopic parameters toward the simulation of line widths and shapes. Although further developments are surely needed to improve the accuracy and effectiveness of several items in the proposed approach, we try to show that the first important steps toward a direct comparison between the results obtained in vitro and those obtained in silico have been made, making easier fruitful crossovers among experiments, computations and theoretical models, which would be decisive for a deeper understanding of the spectral behavior associated with complex systems and processes.     

Inhibition of galectins with small molecules

Evidence that the galectin family of proteins plays crucial roles in cancer, inflammation, and immunity has accumulated over the last decade. The galectins have consequently emerged as interesting drug targets. A majority of galectin functions occurs by means of cross-linking glycoproteins and by doing so controlling glycoprotein cellular localization and residence times. The glycoprotein cross-linking occurs when galectin dimers or multimers, or galectins with two binding sites, bind galactose-containing glycans of the glycoproteins. Such galectin-glycan interactions have been successfully blocked with compounds having multivalent presentation of galactose, lactose, or N-acetyllactosamine, with peptides, and with small carbohydrate (galactose) derivatives. This review summarizes and analyzes attempts to develop efficient and selective small-molecule galectin inhibitors through derivatization of monosaccharides, mainly galactosides, with non-carbohydrate structures that protrude into subsites adjacent to the core-conserved galactose-recognizing site of the galectins.     

Reactions of small covalent molecules with coal: trifluoroacetic acid

Anhydrous ammonia, methanol, and other protonic reagents have been utilized for the chemical comminution of coal. In this investigation, subbituminous, low-volatile bituminous and high-volatile bituminouscoals have been comminuted readily by trifluoroacetic acid. Approximately 5% of the original coal dissolves during the trifluoroacetic acid treatment leaving a material which has a high calorific value. The pyritic and organic sulphur contents and the ash yields are greatly reduced. Micrographs after treatment show the layered structure of the coal.     

重油催化裂化沉降器结焦历程分析

沉降器结焦需具备2个条件:存在易结焦物质,存在结焦的环境和条件。在重油催化裂化沉降器中结焦物质包括油气组分、油液滴和催化剂颗粒,结焦的环境和条件指沉降器内的物料流场、温度场和浓度分布情况。分析了沉降器中气相组分、油液滴和催化剂颗粒等结焦物质的结焦历程,提出为了定量掌握结焦历程中各个过程的细节有待于进一步进行研究的问题。     

Single-event microkinetics for coke formation during the catalytic cracking of (cyclo)alkane/1-octene mixtures

A single-event microkinetic model (SEMK) is applied to model initial coking rates during the catalytic cracking of (cyclo)alkane/1-octene mixtures at 693–753 K and (cyclo)alkane and 1-octene inlet partial pressures of 26.6 and 4.8 kPa on a REUSY equilibrium catalyst. Three types of irreversible alkylations involving both gas phase and surface coke precursors, viz., alkylation of phenyl substituted carbenium ions with C₃–C₅ alkenes, alkylation of the nucleus of monoaromatics with C₃–C₅ alkylcarbenium ions, and alkylation of C₈–C₁₀ alkylcarbenium ions with C₃–C₅ alkenes, have been considered as rate-determining steps in coke formation. The bulky alkylated species formed out of these alkylations are considered as coke. The activation energies for these alkylations obtained via non-isothermal regression are independent of the feedstock within the parameters confidence limits reflecting the fundamental character of the SEMK. The negative effect of temperature on the experimentally observed coking rates is qualitatively described and is explained in terms of an overcompensation of the increase of the rate coefficient by a lower surface coke precursor concentration.     

Kinetics of the thermo-oxidative and thermal cracking reactions of Athabasca bitumen

The thermo-oxidative and thermal cracking reactions of Athabasca bitumen were examined qualitatively and quantitatively using differential thermal analysis (DTA). Reaction kinetics of low temperature oxidation (LTO) and high temperature cracking (HTC) were determined. The rate of the LTO reaction was found to be first order with respect to oxygen concentration. The activation energy and the Arrhenius pre-exponential factor were 64 MJ kg^?1 mol^?1 and 10^5.4 s^?1, respectively. The effects of atmosphere, pressure, heating rate and support material on the thermal reactions of bitumen were studied. In general, it was found that partial pressures of oxygen > 10% O₂ favoured exothermic oxidation reactions. High pressure increased the rates of LTO and HTC as well as the exothermicity of these reactions. A major contribution of this study to thermal in-situ processes is that heating rate can be used effectively to control the extent of low temperature oxidation and hence fuel availability during in-situ combustion. Low linear heating rates (2.8 °C min^?1) favoured low temperature oxidative addition and fission reactions. The reaction products readily formed coke and combusted upon heating. High linear heating rates (24.5 °C min ^?1) led to rapid oxidation reactions in the high temperature zone; the high temperature and the energy released during oxidation appeared to promote combustion. Finally, when sand was used as the support material there appeared to be a catalytic effect in both LTO and HTC reactions.