Propane Molecules Research Articles

Molecular simulation of adsorption sites of light gases in the metal-organic framework IRMOF-1

Molecular simulations are known to be able to reproduce many experimental isotherms of adsorbates in the metal-organic framework IRMOF-1. Here, we show that also the positions and occupations of the adsorption sites of argon and nitrogen match well with experiments. At 30 K the molecules are localized around their crystallographic sites, while at room temperature the molecules are spread throughout the pore volume. The orientations found for individual nitrogen molecules differ from those found experimentally, although the average orientations agree well. We elucidate the pore filling mechanism for both argon and nitrogen. In addition we compute the binding energy at sites for individual argon, nitrogen, carbon dioxide, hydrogen, methane, ethane, and propane molecules and show that for all molecules the preferred site is near the zinc–oxygen cluster in the cavities where the linkers point outward. The second site, higher in energy, is near the zinc–oxygen cluster in the cavities where the linkers point inward, and the least energetically favorable site is above and beneath the linker molecule. We explain why the site in the smaller cavities is only filled up at high loadings.

Dynamics of Propane in Silica Mesopores Formed upon Propylene Hydrogenation over Pt Nanoparticles by Time-Resolved FT-IR Spectroscopy

Propylene hydrogenation over Pt nanoparticles supported onmesoporous silica type SBA-15 was monitored by time-resolved FT-IRspectroscopy at 23 ms resolution using short propylene gas pulses thatjoined a continuous flow of hydrogen in N2 (1 atm total pressure).Experiments were conducted in the temperature range 323-413 K. Propanewas formed within 100 milliseconds or faster. The CH stretching regionrevealed distinct bands for propane molecules emerging inside thenanoscale channels of the silica support. Spectral analysis gave thedistribution of the propane product between support and surrounding gasphase as function of time. Kinetic analysis showed that the escape ofpropane molecules from the channels occurred within hundreds ofmilliseconds (3.1 + 0.4 s-1 at 383 K). A steady state distribution ofpropane between gas phase and mesoporous support is established as theproduct is swept from the catalyst zone by the continuous flow ofhydrogen co-reactant. This is the first direct spectroscopic observationof emerging products of heterogeneous catalysis on nanoporous supportsunder reaction conditions.

Modulation of a fluorescence switch based on photochromic spirooxazine in compositeorganic nanoparticles

We describe a versatile and convenient approach to achieve fluorescence modulationby the preparation of composite nanoparticles (CNPs), based on photochromic5-methoxy-1,3,3-trimethyl-9′-hydroxyspiroindolinenaphthoxazine (SO), fluorescent4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM), andemissive-assistant 1,3-bis(pyrene) propane (BPP) molecules, employing doping techniques.The mechanism of the fluorescence switch is the intermolecular energy transfer assupported by both steady-state and time-resolved spectroscopy results. The addition ofBPP not only enhances the contrast of the fluorescence signal between the ‘ON’ and ‘OFF’state, but also provides a convenient way to tune the excitation wavelength for reading thefluorescence. High-contrast ON/OFF (20:1) fluorescence switching is successfullyimplemented in the CNPs and also in a more practical PVA film loaded with the CNPs.This system may represent an alternative to the covalent system in potentially rewritablehigh-density optical data or image storage utilizing luminescence intensity readout schemes.

Low‐Basicity Oxygen Atoms: A Key in the Search for Propylene Epoxidation Catalysts

Gotta have copper: DFT studies indicate that the performance of heterogeneous epoxidation catalysts depends on the first step in the reaction between oxygen atoms and propylene molecules coadsorbed on the metal surface (see picture; C yellow, H light blue, O red). The high basicity of oxygen atoms on silver favors allylic hydrogen stripping, whereas the low basicity of oxygen atoms on copper favors metallacycle formation and subsequent epoxidation.

Regioselective H/D exchange of propane on Zn/H-MFI zeolite

The hydrogen exchange for propane-d 8 adsorbed on zeolite Zn/H-MFI has been studied by1H MAS NMR spectroscopy in situ within the temperature range of 420–490 K. Kinetic measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that only methyl groups of the alkane are involved in the exchange. Two mechanisms are proposed to rationalize the regioselectivity of the exchange: (i) propane dehydrogenation on Zn-sites followed by protonation of propene by acidic OH groups in accordance to the Markovnikov’s rule and abstraction of deuteride ion from another propane molecule; (ii) the reversible heterolytic dissociative adsorption of propane to form Zn-propyl species and acidic OH groups.

Simplified gauge-cell method and its application to the study of capillary phase transition of propane in carbon nanotubes

A modification of the gauge-cell method for Monte Carlo studies of phase equilibrium in nano-confined systems is presented and employed for studying the capillary phase transition of propane in single-walled carbon nanotubes as a function of tube diameter. It is shown that if an analytical equation of state for the vapor phase is known, the acceptance rule for the trial move of particle exchange can be modified to reference the bulk system through its chemical potential. Under these conditions, the simulation procedure is simplified and acquires many characteristics of a Monte Carlo simulation conducted in the grand canonical ensemble. It is also shown that the critical temperature can be estimated by interpolation of sub- and supercritical values of the slope of the inverse isotherm at the inflection point. To enhance the sampling of propane molecules in the hollow space of the nanotubes the configurational-bias scheme is employed. The simulation results show that the confinement of propane increases its critical density, reduces the critical temperature and narrows the binodal curve with decreasing tube diameter until the system approaches one-dimensional behavior.

On the interaction of magnetic iron clusters with hydrocarbons: Fe 4–propane

The interaction of propane with Fe 4 was studied by means of the BPW91/6-311+G(d,p) method. The two modes Fe 4–C m H 2– (C t H 3) 2 ( I) and Fe 4–C t H 3–C m H 2-C t H 3 ( II) were found. Their spin state is high (2S = 14), and propane interacts with Fe 4 ( t and m are for the terminal and middle C atoms) throughout the formation of two weak C x H–Fe bonds in I ( x = m) and II ( x = t), which bear some resemblance with classical agostic bonds. The vibrational pattern of propane is perturbed significantly, and its dipole moment shows a big increase. Thus, Fe 4 is able to activate the propane molecule.

Co5(µ3-OH)2(btec)2(bpp)]n: a three-dimensional homometallic molecular metamagnet built from the mixed hydroxide/carboxylate-bridged ferrimagnetic-like chains

A three-dimensional homometallic complex [Co(5)(mu(3)-OH)(2)(btec)(2)(bpp)](n) is built from the mixed hydroxide/carboxylate bridged cobalt(ii) chains linked by the 1,2,4,5-benzenetetracarboxylate (btec(4-)) anion and 1,3-bis(4-pyridyl)-propane molecule (bpp). Within each chain, two mu(3)-OH-bridged metal triangles connect to each other by sharing a common vertex to give rise to a bow-tie type Co(5)(mu(3)-OH)(2) subunit, which is joined to adjacent subunits by four mu(1,1)-carboxylate bridges to form a step-like metal-oxygen backbone. The magnetic studies revealed that the coexistence of ferromagnetic and antiferrimagnetic interactions resulted in a ferrimagnetic-like behavior of the homometallic chains. Below a critical temperature (T(N) = 12.5 K), bulk antiferromagnetic ordering was observed at low field due to the weak interchain antiferromagnetic interactions. A metamagnetic transition occurred at a magnetic field of ca. 5 kOe at 2 K.

Molecular fragmentation on collision with protons for freon and propane molecules

In ab initio calculations, we determined the most probable routes of decomposition of the [CF3Cl]+, [CF2Cl2]+, [CFCl3]+, [CCl4]+ molecular ions of freons and [C3H8]+ ions of hydrocarbons formed by collision of neutral molecules with protons with energies of the order of 10 keV. The calculated potential surface sections are compared on a qualitative level with the probability of various ion fragments in experiments on fragmentation. The role of the charge transfer dynamics between the proton and the molecule is discussed.

Two New Hybrid Organic/Inorganic Copper(II)−Oxovanadate(V) Diphosphonates: [Cu2(phen)2(O3PCH2PO3)(V2O5)(H2O)]·H2O and [Cu2(phen)2(O3P(CH2)3PO3)(V2O5)]·C3H8. Synthesis, Structure, and Magnetic Properties

Two new hybrid organic/inorganic copper oxovanadium diphosphonates [Cu2(phen)2(O3PCH2PO3)(V2O5)(H2O)] x H2O (1) and [(Cu2(phen)2(O3P(CH2)3PO3)(V2O5)] x C3H8 (2) have been obtained by hydrothermal synthesis. The compounds are monoclinic, and they crystallize in the space group P2(1)/n with cell parameters of a = 11.788(2) A, b = 17.887(3) A, c = 14.158(2) A, and beta = 93.99(0) degrees and in the space group C2/c with cell parameters of a = 11.025(1) A, b = 18.664(2) A, c = 15.054(2) A, and beta = 90.06(0) degrees, respectively. Both compounds present two-dimensional frameworks built up from infinite chains of corner-sharing vanadium tetrahedra and diphosphonate groups connected by copper tetramers for (1) and copper dimers for (2). The remarkable feature of (2) is the encapsulation of propane molecules, stabilized by strong hydrogen bonding between the layers. The magnetic properties of the compounds have been investigated showing antiferromagnetic coupling with Tmax = 64 K for (1) and Curie-like paramagnetic behavior for (2).

Associative Charge Transfer Reactions. Temperature Effects and Mechanism of the Gas-Phase Polymerization of Propene Initiated by a Benzene Radical Cation

In associative charge transfer (ACT) reactions, a core ion activates ligand molecules by partial charge transfer. The activated ligand polymerizes, and the product oligomer takes up the full charge from the core ion. In the present system, benzene(+*) (Bz(+*)) reacts with two propene (Pr) molecules to form a covalently bonded ion, C(6)H(6)(+*) + 2 C(3)H(6) --> C(6)H(12)(+*) + C(6)H(6). The ACT reaction is activated by a partial charge transfer from Bz(+*) to Pr in the complex, and driven to completion by the formation of a covalent bond in the polymerized product. An alternative channel forms a stable association product (Bz.Pr)(+*), with an ACT/association product ratio of 60:40% that is independent of pressure and temperature. In contrast to the Bz(+*)/propene system, ACT polymerization is not observed in the Bz(+*)/ethylene (Et) system since charge transfer in the Bz(+*)(Et) complex is inefficient to activate the reaction. The roles of charge transfer in these complexes are verified by ab initio calculations. The overall reaction of Bz(+*) with Pr follows second-order kinetics with a rate constant of k (304 K) = 2.1 x 10(-12) cm(3) s(-1) and a negative temperature coefficient of k = aT(-5.9) (or an activation energy of -3 kcal/mol). The kinetic behavior is similar to sterically hindered reactions and suggests a [Bz(+*) (Pr)]* activated complex that proceeds to products through a low-entropy transition state. The temperature dependence shows that ACT reactions can reach a unit collision efficiency below 100 K, suggesting that ACT can initiate polymerization in cold astrochemical environments.

Characterization of tanshinones in the roots of Salvia miltiorrhiza (Dan‐shen) by high‐performance liquid chromatography with electrospray ionization tandem mass spectrometry

The qualitative analysis of tanshinones in the roots of Salvia miltiorrhiza (Dan-shen in Chinese) was performed using high-performance liquid chromatography with electrospray ionization tandem mass spectrometry (ESI-MS(n)). Tanshinones are the major bioactive constituents of Dan-shen, which is used in China for the treatment of haematological abnormalities and cardiovascular diseases. The ESI-MS(n) fragmentation behavior of tanshinones was investigated. For tanshinones with the tanshinone I nucleus, the fragmentation was triggered by loss of a molecule of CO except bearing a substituent at C17 or C18, followed by sequential eliminations of CO. If C(15-16) was a saturated bond, the fragmentation was triggered by elimination of a molecule of H2O. For tanshinones with the tanshinone IIA nucleus, the fragmentation was triggered by loss of a molecule of H2O, followed by successive eliminations of CO. Ions corresponding to loss of a molecule of propylene (Delta m = 42) were also observed. Moreover, when C(15-16) was a saturated bond, ions corresponding to losses of CH3, H2O and propylene were more abundant. If no D-ring existed, the presence of isopropyl resulted in an elimination of a molecule of H2O with an adjacent CO or OH. In addition, the extension of the pi-conjugation in the A-ring (especially at C(1-2)) induced the fragmentation by loss of a molecule of CO. These fragmentation rules were applied to the identification of tanshinones in a chloroform/methanol (3:7) extract of Dan-shen, which was separated on a C18 column with gradient elution. A total of 27 tanshinones were identified, including five new constituents. The established method could be used for the sensitive and rapid identification of tanshinones in the Dan-shen drug and its pharmaceutical preparations.

Total ionization cross-sections of CH 4 and C 3H 8 molecules for impact of 10–28 keV electrons

The total ionization cross-sections of methane and propane molecules are measured for impact of 10–28 keV electrons. The cross-sections are found to follow the variation shown by Bethe formula as a function of electron impact energy. Also, the present results compare well with the available data in the literature. Further, the measured cross-sections of the above hydrocarbon molecules are found to correlate strongly to the total number of molecular electrons and to the molecular dipole polarizability.

Living propene polymerization with Bis(phenoxy-imine) group 4 metal catalysts: A theoretical study

Bis(phenoxy-imine)Ti catalysts with ortho-F-substituted phenyl rings on the N can promote living propene polymerization. On the basis of DFT calculations, it has been proposed that the “living” behavior originates from an unprecedented attractive interaction between the aforesaid ortho-F atoms and a β-H of the growing polymer chain, which would render the latter less prone to being transferred to the metal and/or to the monomer. In this paper, we report on a thorough full-QM and combined QM/MM investigation of representative model catalysts, demonstrating that the key factor is instead the repulsive nonbonded contact of the F-substituted rings with the growing polymer chain and an incoming propene molecule, which destabilizes the sterically demanding 6-center transition structure for chain transfer to the monomer. A conceptually similar substituent effect has been reported before for several metallocene and nonmetallocene catalysts.

Kinetic modeling of slurry propylene polymerization using rac‐ET(Ind)2ZrCl2/MAO

AbstractThe slurry homopolymerization of propylene catalyzed by the isospecific metallocene rac‐Et(Ind)2ZrCl2/MAO was investigated using a semi‐batch reactor. A full factorial design with three temperatures (50, 65 and 75°C) and four monomer partial pressures (1.5, 2.5, 3.2, and 3.8 atm) was performed. Analysis by 1H NMR revealed the formation of vinylidene, cis‐2‐butenyl and 4‐butenyl end‐groups. A kinetic model based on a coordination‐insertion mechanism was developed to predict instantaneous reaction rate, molecular weights and polymer chain ends. The kinetic rate constants were estimated using a systematic optimization strategy. The model predicts that the insertion of the first propylene molecule is rate limiting with respect to propagation. Molecular weight decreases with temperature due to the high activation energy of the main chain transfer reaction (101 kJ/mol) relative to propagation (52 kJ/mol). The percentage of butenyl end‐groups with respect to vinylidene decreases with temperature, thus predicting a lower activation energy for the β‐hydride elimination after secondary insertion (27 kJ/mol). © 2006 American Institute of Chemical Engineers AIChE J, 2006

TiO2 Nanoparticles in Mesoporous TUD‐1: Synthesis, Characterization and Photocatalytic Performance in Propane Oxidation

A series of TiO2-TUD-1 samples was synthesized with a variable Ti loading in the range Si/Ti = 100, 20, 2.5, and 1.6, by using a one-pot surfactant-free procedure. The materials obtained were characterized by elemental analysis; X-ray diffraction (XRD); N2 sorption measurements; high-resolution TEM (HR-TEM); 29Si NMR, UV-visible and Raman spectroscopy. As a function of increasing metal loading either isolated Ti atoms, or (above a Ti loading of approximately 2.5 wt- %) combinations of isolated Ti atoms and anatase (TiO2) nanoparticles were obtained; both were incorporated in the highly porous siliceous matrix. The photocatalytic performance of these materials was tested by studying the propane oxidation process following irradiation at lambda = 365 nm, selectively activating the anatase nanoparticles. In comparison to commercial anatase powder, TiO2 nanoparticles in TUD-1 showed high photochemical selectivity towards acetone, the sample with a Si/Ti ratio of 1.6 being the most selective. Size and confinement effects are consistent with the difference in performance of the TUD-1 materials and TiO2, limiting the number of electron transfers available for each propane molecule.

Stereodynamic Effects in the Adsorption of Propylene Molecules on Ag(001)

We report on the experimental evidence of the role of rotational alignment of the gas-phase molecules in the interaction of propylene with Ag(001). Molecular alignment has been controlled by a velocity selection of the impinging molecules, flying in a supersonic seeded molecular beam. The experimental findings indicate that at low surface coverage the sticking probability is independent of molecular alignment, while when coverage exceeds few percent of a monolayer, molecules impinging rotating parallel to the surface (helicopter-like configuration) achieve a higher chance to be trapped than those which impinge rotating perpendicularly (cartwheels). The sudden appearance of a large stereodynamic effect suggests that the adsorption proceeds via a mobile precursor state and is tentatively correlated with a change in the configuration of the added propylene molecules, which adsorb tilted rather than flat at the surface.

In Situ 1H and 13C MAS NMR Kinetic Study of the Mechanism of H/D Exchange for Propane on Zeolite H−ZSM-5

The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and variously deuterated propanes (propane-d(8), propane-1,1,1,3,3,3-d(6), propane-2,2-d(2)) have been monitored in situ by (1)H MAS NMR spectroscopy within the temperature range of 503-556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of (13)C-labeled carbon scrambling in propane-2-(13)C in situ with (13)C MAS NMR at 543-573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time. The main route of the exchange consists of a direct exchange of the acidic OH groups of the zeolite with either the methyl groups or the methylene group presumably with a pentacoordinated carbonium ion intermediate. The assumption that the intramolecular H scrambling between the methyl groups and the methylene group of propane via carbenium-ion-type intermediates is the fastest process among the other possible routes does not account for the experimental kinetics of H/D exchange for propanes with different initial contents and locations of deuterium in a propane molecule. The rate constant (k(3)) for intramolecular H/D exchange between the methyl and the methylene groups is 4-5 times lower compared to those of the direct exchange of both the methyl (k(1)) and the methylene (k(2)) groups with Brønsted acid sites of the zeolite, the k(1) being ca. 1.5 times higher than k(2). At lower temperature (473 K), the exchange is slower, and the expected difference between k(1) and k(2) is more essential, k(1) = 3k(2). This accounts for earlier observed regioselectivity of the exchange for propane on H-ZSM-5 at 473 K. Faster direct exchange with the methyl groups compared to that with the methylene groups was attributed to a possible, more spatial accessibility of the methyl groups for the exchange. Similar activation energies for H and C scramblings with a 2 times more rapid rate of H scrambling was rationalization by the proceeding of these two processes through an isopropyl cation intermediate, as in classical carbenium ion chemistry.

Study on alkylation of benzene with propylene over MCM-22 zeolite catalyst by in situ IR

For the alkylation reaction of benzene with propylene over MCM-22 zeolite catalyst, two completely different results can be obtained for two different operating procedures. If the benzene is pumped into a reactor then followed by propylene (operation 1), the alkylation reaction proceeds normally, while the reaction can not occur if the propylene is introduced first followed by benzene (operation 2). In situ IR technology was used to investigate two modes designed to simulate the above operating processes. The mechanisms of the two operations are as follows: in operation 1, benzene molecules first fully were adsorbed on the acidic sites of MCM-22 zeolite, when propylene was introduced, propylene molecules seized the acidic sites by repelling benzene, at the same times propylene molecules were polarized and reacted with absorbed benzene molecules. This is synchronous reaction mechanism; in operation 2, propylene introduced was molecularly absorbed on acidic sites strongly resulting in producing carbonium ions of CH 3–C +H–CH 3, then the carbonium ions reacts with other propylene molecules further to form polymeric species. The polymers blocked the pores and covered the acidic sites so that the alkylation reaction with benzene can not take place. This process is carbonium ions reaction mechanism of propylene itself.

In situ 1H and 13C MAS NMR study of the mechanism of H/D exchange for deuterated propane adsorbed on H-ZSM-5

The kinetics of hydrogen exchange between the molecules of propane- d 8, propane-1,1,1,3,3,3- d 6, propane-2,2- d 2, and Brönsted acid sites of the zeolite H-ZSM-5 have been monitored in situ by 1H MAS NMR spectroscopy in the temperature range of 230–280 °C. The intramolecular hydrogen transfer was estimated by in situ 13C MAS NMR spectroscopy of the kinetics of 13C-label scrambling in adsorbed propane-2- 13C. The hydrogen exchange was found to occur directly between methyl or methylene groups of the alkane and the zeolite acid sites via a pentacoordinated carbonium ion. The exchange with the methyl groups is faster than that with the methylene group. This accounts for the earlier observed regioselectivity of the hydrogen exchange for propane on acidic zeolites (J. Am. Chem. Soc. 117 (1995) 1135). The intramolecular hydrogen transfer between methyl and methylene groups is one order of magnitude slower than the hydrogen exchange of the both groups with the zeolite acid sites.