Effect of zeolite topology and reactor configuration on the direct conversion of CO2 to light olefins and aromatics

The direct transformation of CO2 into high-value-added hydrocarbons (i.e., olefins and aromatics) has the potential to make a decisive impact in our society. However, despite the efforts of the scientific community, no direct synthetic route exists today to synthesize olefins and aromatics from CO2 with high productivities and low undesired CO selectivity. Herein, we report the combination of a series of catalysts comprising potassium superoxide doped iron oxide and a highly acidic zeolite (ZSM-5 and MOR) that directly convert CO2 to either light olefins (in MOR) or aromatics (in ZSM-5) with high space–time yields (STYC2-C4= = 11.4 mmol·g–1·h–1; STYAROM = 9.2 mmol·g–1·h–1) at CO selectivities as low as 12.8% and a CO2 conversion of 49.8% (reaction conditions: T = 375 °C, P = 30 bar, H2/CO2 = 3, and 5000 mL·g–1·h–1). Comprehensive solid-state nuclear magnetic resonance characterization of the zeolite component reveals that the key for the low CO selectivity is the formation of surface formate species on the zeolite framework. The remarkable difference in selectivity between the two zeolites is further rationalized by first-principles simulations, which show a difference in reactivity for crucial carbenium ion intermediates in MOR and ZSM-5

A supramolecular view on the cooperative role of Brønsted and Lewis acid sites in zeolites for methanol conversion

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Bronsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Bronsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH](+) and double protonated binuclear metal clusters [M(mu-OH)(2)M](2+) (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Bronsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Bronsted and Lewis acid sites

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

The combination of well-defined acid sites, shape-selective properties and outstanding stability places zeolites among the most practically relevant heterogeneous catalysts. The development of structure-performance descriptors for processes that they catalyse has been a matter of intense debate, both in industry and academia, and the direct conversion of methanol to olefins is a prototypical system in which various catalytic functions contribute to the overall performance. Propylene selectivity and resistance to coking are the two most important parameters in developing new methanol-to-olefin catalysts. Here, we present a systematic investigation on the effect of acidity on the performance of the zeolite 'ZSM-5' for the production of propylene. Our results demonstrate that the isolation of Bronsted acid sites is key to the selective formation of propylene. Also, the introduction of Lewis acid sites prevents the formation of coke, hence drastically increasing catalyst lifetime

Propriétés Magnétiques, Electroniques et Dynamiques des nanostructures à base de nanotubes de carbone (Etude RMN@)

Le but poursuivi dans ce travail est l'analyse des propriétés physiques des nanotubes de carbone et certains de ses dérivées, à l'aide des techniques RMN. Premièrement, nous avons étudié les propriétés structurales des nanotubes. D'une part une relation entre les diamètres des tubes et les déplacements chimiques est proposée et les largeurs des raies et les nombres des feuilles des tubes d'autre part. Deuxièmement, nous avons étudié la fonctionalisation des nanotubes de carbone. Nous avons observé des réponses précises sur la nature des liaisons chimiques formées, leur réversibilité et la présence des défauts structuraux. Nous avons apporté la confirmation de l'interaction diamagnétique entre les nanotubes et des chaînes polymères (SPEEK). Troisièment, nous avons mis en évidence les propriétes magnétiques et dynamiques des "peapods". Nous avons proposé un mécanisme à l'origine de l'écrantage diamagnétique à l'intérieur des nanotubes, lié aux courants de cycle à la surface de nanotubes qui entraine des déplacements diamagnetiques. Les défauts à la surface des tubes éliminent cet effet pour créer un déplacement paramagnétique. Nous avons également déterminé les dynamiques de réorientations des molécules C60 à l'intérieur des nanotubes. Nous avons montré que la plupart des molécules C60 subissent une transition de phase d'une rotation isotropique à une rotation entravée vers 100 K et un blocage complet vers 30K. Quatrièment, nous avons révélé les détails des propriétes physiques et chimiques associés à des molécules C60 hydrogénées à l' intérieur des nanotubes. Nous avons observé des déplacements diamagnétiques et paramagnétiques des signatures RMN et des carbones de type sp3. Enfin, nous avons étudié les propriétés électromagnétiques des nanotubes bifeuillets. Nous avons trouvé deux types des nanotubes, métalique de type 1-D et semiconducteurThis work deals with physical properties of carbon nanotubes and some derivatives, using NMR spectroscopy. Firstly, we studied the structural properties of nanotubes. According to the size and shape, we established the dependence of their NMR signatures and find two empiric expressions for the line shifts and line widths. Secondly, we reported a detailed analysis based on high resolution 13 C NMR of functionalized nanotubes. We identified the chemical groups or defects present in functionalized carbon nanotubes. We confirmed the magnetic interaction between carbon nanotubes functionalized with polymer chains (SPEEK). Thirdly, we investigated the local magnetic properties and the 1-D dynamics of C60 molecules inside carbon nanotubes (peapods). We modelized the origine of the diamagnetic shielding inside carbon nanotubes. We reported this diamagnetic shift to the ring-currents at the nanotube surface. Moreover, defects like C-vacancies, pentagons and chemical functionalization of the outer nanotube quench this diamagnetic effect and restore NMR signatures to slightly paramagnetic shifts. Equally, we determined the rotational dynamics of the 1-D C60 molecules inside carbon nanotubes. We have shown that the majority of C60 molecules undergo dynamical phase transitions, from free rotations to hindered rotations at 100 K and to frozen states at 30 K. Forthly, we studied the chemical and physical modifications of hydrogenated C60 molecules inside nanotubes. We observed the characterestic diamagnetic and paramagnetic shifts of the NMR lines and the appearance of sp3 carbon resonances. Finally, se studied the electomagnetic properties of inner nanotubes in double-walled carbon nanotubes. We found two kinds of nanotubes, 1-D metal and semiconductingMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Catalyst switch strategy enabled a single polymer with five different crystalline phases

Abstract Well-defined multicrystalline multiblock polymers are essential model polymers for advancing crystallization physics, phase separation, self-assembly, and improving the mechanical properties of materials. However, due to different chain properties and incompatible synthetic methodologies, multicrystalline multiblock polymers with more than two crystallites are rarely reported. Herein, by combining polyhomologation, ring-opening polymerization, and catalyst switch strategy, we synthesized a pentacrystalline pentablock quintopolymer, polyethylene-b-poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(L-lactide)-b-polyglycolide (PE-b-PEO-b-PCL-b-PLLA-b-PGA). The fluoroalcohol-assisted catalyst switch enables the successful incorporation of a high melting point polyglycolide block into the complex multiblock structure. Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and differential scanning calorimetry revealed the existence of five different crystalline phases

Quantifying the impact of dispersion, acidity and porosity of Mo/HZSM-5 on the performance in methane dehydroaromatization

The catalytic performance of the bifunctional catalyst Mo/HZSM-5 for methane dehydroaromatization (MDA) depends on the Mo dispersion and on zeolite acidity. Here we separately quantify the effect of dispersion and the effect of acidity on aromatic yields and coke selectivity. Also, the effect of porosity on the same is quantitatively assessed. For that, a suite of 17 samples with varying Mo dispersion were synthesized by means of several methods, including chemical vapor deposition with MoCl 5 , MoO 2 Cl 2 and Mo(CO) 6 as precursors and the conventional methods, incipient wetness impregnation and solid ion exchange. These catalysts were characterized with pyridine IR-spectroscopy, XPS, UV–vis spectroscopy, N 2 adsorption, XRD, TGA and 27 Al MAS NMR. The combined results yielded a measure of how much Mo is anchored to the zeolite as well-defined cationic species and how much is present as bigger clusters on the outer surface of the zeolite. Through relating these characterization results to the catalytic behavior of the catalysts, it was found that the maximum instantaneous benzene and naphthalene yields as well as the integral selectivities during methane dehydroaromatization linearly increase with the amount of Mo present as mono- or dimeric species. At the same time, the selectivity to coke increases with the amount of Mo present as bigger clusters or nanoparticles on the outer surface of the zeolite. The number of Mo cationic sites is the most important factor determining the activity of Mo/HZSM-5 for low loadings of Mo. But at higher loadings, the high rate of aromatics formation requires an easily accessible pore structure as well. </p

Alkane Metathesis with the Tantalum Methylidene [( SiO)Ta(=CH2)Me-2]/[( SiO)(2)Ta(=CH2)Me] Generated from Well-Defined Surface Organometallic Complex [( SiO)(TaMe4)-Me-V]

By grafting TaMe5 on Aerosil(700), a stable, well-defined, silica-supported tetramethyl tantalum(V) complex, [( SiO)TaMe4], is obtained on the silica surface. After thermal treatment at 150 degrees C, the complex is transformed into two surface tantalum methylidenes, [( SiO)(2)Ta(-CH2)Me] and [( SiO)Ta(-CH2)Me-2], which are active in alkane metathesis and comparable to the previously reported [( SiO)(2)TaHx]. Here we present the first experimental study to isolate and identify a surface tantalum carbene as the intermediate in alkane metathesis. A systematic experimental study reveals a new reasonable pathway for this reaction

A Silica-Supported Double-Decker Silsesquioxane Provides a Second Skin for the Selective Generation of Bipodal Surface Organometallic Complexes

A well-defined silica-based material with a homogeneous nanolayer presenting identical pairs of vicinal silanols has been prepared by reaction of the surface organometallic species[ SiOZr(CH2CMe3)(3)], obtained on a silica dehydroxylated at 900 degrees C, with the double-decker-shaped silsesquioxane (OH)(2)DD(OH)(2). The surface structure has been established using extensive NMR characterization (H-1, C-13, Si-29, HETCOR, double-quantum, triple-quantum). Treatment with Zr(CH2CMe3)(4) leads to the first well-defined single-site bipodal grafted bis-neopentyl zirconium complex

A well-defined mesoporous amine silica surface via a selective treatment of SBA-15 with ammonia

2D double-quantum H-1-H-1 NMR unambiguously shows that the ``isolated'' Si-OH surface silanols of dehydroxylated SBA-15 are converted upon treatment with ammonia into single silylamine surface site Si-NH2. The ``gem'' di-silanols (= Si(OH)(2)) remain intact. Treatment using HMDS produces (= Si(OSiMe3)(2)) but leaves Si-NH2 untouched. The resulting surface is hydrophobic and stable