Review of old chemistry and new catalytic advances in the on-purpose synthesis of butadiene.

Increasing demand for renewable feedstock-based chemicals is driving the interest of both academic and industrial research to substitute petrochemicals with renewable chemicals from biomass-derived resources. The search towards novel platform chemicals is challenging and rewarding, but the main research activities are concentrated on finding efficient pathways to produce familiar drop-in chemicals and polymer building blocks. A diversity of industrially important monomers like alkenes, conjugated dienes, unsaturated carboxylic acids and aromatic compounds are thus targeted from renewable feedstock. In this context, on-purpose production of 1,3-butadiene from biomass-derived feedstock is an interesting example as its production is under pressure by uncertainty of the conventional fossil feedstock. Ethanol, obtained via fermentation or (biomass-generated) syngas, can be converted to butadiene, although there is no large commercial activity today. Though practised on a large scale in the beginning of the 20th century, there is a growing worldwide renewed interest in the butadiene-from-ethanol route. An alternative route to produce butadiene from biomass is through direct carbohydrate and gas fermentation or indirectly via the dehydration of butanediols. This review starts with a brief discussion on the different feedstock possibilities to produce butadiene, followed by a comprehensive summary of the current state of knowledge regarding advances and achievements in the field of the chemocatalytic conversion of ethanol and butanediols to butadiene, including thermodynamics and kinetic aspects of the reactions with discussions on the reaction pathways and the type of catalysts developed.

Gallium oxide nanorods: novel, template-free synthesis and high catalytic activity in epoxidation reactions.

Gallium oxide nanorods with unprecedented small dimensions (20-80 nm length and 3-5 nm width) were prepared using a novel, template-free synthesis method. This nanomaterial is an excellent heterogeneous catalyst for the sustainable epoxidation of alkenes with H2 O2 , rivaling the industrial benchmark microporous titanosilicate TS-1 with linear alkenes and being much superior with bulkier substrates. A thorough characterization study elucidated the correlation between the physicochemical properties of the gallium oxide nanorods and their catalytic performance, and underlined the importance of the nanorod morphology for generating a material with high specific surface area and a high number of accessible acid sites.

Fast and selective sugar conversion to alkyl lactate and lactic acid with bifunctional carbon-silica catalysts.

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting.

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity--ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Optical encoding of luminescent carbon nanodots in confined spaces.

Stable emissive carbon nanodots were generated in zeolite crystals using near infrared photon irradiation gradually converting the occluded organic template, originally used to synthesize the zeolite crystals, into discrete luminescent species consisting of nano-sized carbogenic fluorophores, as ascertained using Raman microscopy, and steady-state and time-resolved spectroscopic techniques. Photoactivation in a confocal laser fluorescence microscope allows 3D resolved writing of luminescent carbon nanodot patterns inside zeolites providing a cost-effective and non-toxic alternative to previously reported metal-based nanoclusters confined in zeolites, and opens up opportunities in bio-labelling and sensing applications.

Titanosilicate beads with hierarchical porosity: synthesis and application as epoxidation catalysts.

Porous titanosilicate beads with a diameter of 0.5-1.5 mm (TiSil-HPB-60) were synthesized from a preformed titanosilicate solution with a porous anion-exchange resin as template. The bead format of this material enables its straightforward separation from the reaction mixture in its application as a liquid-phase heterogeneous catalyst. The material displays hierarchical porosity (micro/mesopores) and incipient TS-1 structure building units. The titanium species are predominantly located in tetrahedral framework positions. TiSil-HPB-60 is a highly active catalyst for the epoxidation of cyclohexene with t-butyl hydroperoxide (TBHP) and aqueous H(2)O(2). With both oxidants, TiSil-HPB-60 gave higher epoxide yields than Ti-MCM-41 and TS-1. The improved catalytic performance of TiSil-HPB-60 is mainly ascribed to the large mesopores favoring the diffusion of reagents and products to and from the titanium active sites. The epoxide yield and selectivity could be further improved by silylation of the titanosilicate beads. Importantly, TiSil-HPB-60 is a stable catalyst immune to titanium leaching, and can be easily recovered and reused in successive catalytic cycles without significant loss of activity. Moreover, TiSil-HPB-60 is active and selective in the epoxidation of a wide range of bulky alkenes.

Mapping of the organization of p-nitroaniline in SAPO-5 by second-harmonic generation microscopy.

Second-harmonic generation microscopy (SHGM) has been employed to study crystals of zeolite-like material SAPO-5 filled with p-nitroaniline (PNA). The SHG and 2-photon fluorescence response of PNA in the one-dimensional channels readily reveals the pore accessibility; intergrown crystallites containing hexagonal pyramidal components and internal diffusion barriers are found next to seemingly perfect crystals. The sensitivity of second-harmonic generation to molecular orientation allowed for mapping of the domains of differently organized PNA. Dense domains of highly aligned PNA alternate with dilute zones with loosely aligned PNA.

Characterization of fluorescence in heat-treated silver-exchanged zeolites.

Thermal treatment of Ag(+)-exchanged zeolites yields discrete highly photostable luminescent clusters without formation of metallic nanoparticles. Different types of emitters with characteristic luminescence colors are observed, depending on the nature of the cocation, the amount of exchanged silver, and the host topology. The dominant emission bands in LTA samples are situated around 550 and 690 nm for the samples with, respectively, low and high silver content, while in FAU-type materials only a broad band around 550 nm is observed, regardless of the degree of exchange. Analysis of the fluorescent properties in combination with ESR spectroscopy suggests that a Ag(6)(+) cluster with doublet electronic ground state is associated with the appearance of the 690-nm emitter, having a decay of a few hundred microseconds. Tentatively, the nanosecond-decaying 550-nm emitter is assigned to the Ag(3)(+) cluster. This new class of photostable luminescent particles with tunable emission colors offers interesting perspectives for various applications such as biocompatible labels for intracellular imaging.

Pore-filling-dependent selectivity effects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47.

Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal-organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 degrees C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 degrees C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70-150 degrees C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.

Selective adsorption and separation of ortho-substituted alkylaromatics with the microporous aluminum terephthalate MIL-53.

The metal-organic framework MIL-53(Al) was tested for selective adsorption and separation of xylenes and ethylbenzene, ethyltoluenes, and cymenes using batch, pulse chromatographic, and breakthrough experiments. In all conditions tested, MIL-53 has the largest affinity for the ortho-isomer among each group of alkylaromatic compounds. Separations of the ortho-compounds from the other isomers can be realized using a column packed with MIL-53 crystallites. As evidenced by Rietveld refinements, specific interactions of the xylenes with the pore walls of MIL-53 determine selectivity. In comparison with the structurally similar metal-organic framework MIL-47, the selectivities among alkylaromatics found for MIL-53 are different. Separation of ethyltoluene and cymene isomers is more effective on MIL-53 than on MIL-47; the pores of MIL-53 seem to be a more suitable environment for hosting the larger ethyltoluene and cymene isomers than those of MIL-47.

Morphology of large ZSM-5 crystals unraveled by fluorescence microscopy.

Understanding the internal structure of ZSM-5 crystallites is essential for improving catalyst performance. In this work, a combination of fluorescence microscopy, AFM, SEM, and optical observations is employed to study intergrowth phenomena and pore accessibility in a set of five ZSM-5 samples with different crystal morphologies. An amine-functionalized perylene dye is used to probe acid sites on the external crystal surface, while DAMPI (4-(4-diethylaminostyryl)- N-methylpyridinium iodide) is used to map access to the straight channels in MFI from the outer surface. The use of these dyes is validated by studying the well-understood rounded-boat type ZSM-5 crystals. Next coffin-shaped ZSM-5 crystals are considered; we critically evaluate the seemingly conflicting 2-component and 3-component models that have been proposed to account for the hourglass structure in these crystals. The data prove that observation of an hourglass structure is essentially unrelated to a 90 degree rotation of the pyramidal crystal components under the (010) face. Hence, in perfectly formed coffin-shaped crystals, the straight channels can be accessed from (010). However, in other crystal batches, sections with a 90 degrees rotation can be found; they are indeed located inside the crystal sections under (010) but often only partially occupy these pyramidal components. In such a case, both straight and sinusoidal pores surface at the hexagonal face. The results largely support the 3-component model, but with the added notion that 90 degree rotated sections (as proposed in the 2-component model) are most likely to be formed inside the defect-rich, pyramidal crystal sections under the (010) faces.

Study of the adsorption of organic molecules on transition metal exchanged zeolites via solid state NMR. Part 1: theoretical aspects.

A theoretical study is performed of the (1)H relaxation behavior of an organic molecule that is adsorbed in a zeolite exchanged with transition metal ions. An expression is derived for the spin-lattice and spin-spin relaxation rate constant of a uniform proton ensemble, while the occurrence of molecular exchange is taken into account. The influence is analyzed of an approach of a uniform proton ensemble toward the nearest neighboring paramagnetic ion system. It is shown that there exists a signal extinction effect, for which the critical distance was estimated.

Study of the adsorption of organic molecules on transition metal exchanged zeolites via solid state NMR. Part 2: adsorption of organic molecules on zeolite NaX, CaX, and CaCoX.

Literature [Denayer et al. Microporous Mesoporous Mater. 2007, 103, 1 and Denayer et al. Microporous Mesoporous Mater. 2007, 103, 11] shows that zeolite NaX exchanged with Ca(2+) and Co(2+) ions is able to remove cyclopentadiene (CPD) impurities from a 1-octene feed with high selectivity. In the present work, the adsorption of dicyclopentadiene (DCPD), CPD, 1-octene, and n-octane on zeolite X, exchanged with Ca(2+) and/or Co(2+) ions, has been investigated via (1)H magic-angle spinning (MAS) NMR spectroscopy. The liquid adsorbate was dosed under inert atmosphere in an MAS rotor filled with dry adsorbent, at a pore filling degree of 70%. Next, the evolution in time was recorded of the (1)H MAS NMR spectrum and the (1)H spin-lattice and spin-spin relaxation times of the adsorbed components. For the various adsorbate-adsorbent systems, a plot is made of the signal intensity versus the square root of the contact time. It is found that, over the considered time interval, Fickian diffusion takes place. On the basis of the change in time of the spin-lattice relaxation time, a transport diffusion coefficient ranging between 1 and 2 x 10(-15) m(2) x s(-1) is calculated. Moreover, there appear to be two sorption regimes, with different diffusivities. A comparison is made between the (1)H spin-lattice relaxation behavior of DCPD, 1-octene, and n-octane, indicating that 1-octene and n-octane are located closer to the paramagnetic ions than DCPD. The average distance between the adsorbate molecules and the paramagnetic ions is derived from relaxometric data. By analyzing the chemical shifts of the resonance lines, it is found that the pi-interaction of CPD and 1-octene is stronger than that of DCPD.

Catalytic glycerol conversion into 1,2-propanediol in absence of added hydrogen.

Conversion of glycerol into high yields of 1,2-propanediol in absence of added hydrogen is possible with Pt impregnated NaY zeolite characterized by extra-zeolitic metal particles combined with zeolite Brønsted acidity.

Recent progress in the immobilization of catalysts for selective oxidation in the liquid phase.

Reliable strategies are presented for the immobilization of molecular catalysts for selective oxidation in the liquid phase. Besides classical strategies such as ion exchange or covalent anchoring, new approaches are emerging, e.g. based on supported ionic-liquid phases or on incorporation of the active centre in a coordination polymer or a metal-organic framework.

Origin of byproducts during the catalytic autoxidation of cyclohexane.

The formation of byproducts during the Co(acac)2 and Cr(acac)3-catalyzed cyclohexane autoxidation is compared with the noncatalyzed thermal process. CoII ions seem to cause only a moderate perturbation of the reaction mechanism, causing a fast conversion of the cyclohexyl hydroperoxide via a redox cycle, rather than via abstraction of the alphaH-atom by chain carrying peroxyl radicals. Nevertheless, both the radical propagation and the CoII-induced decomposition of the hydroperoxide cause the formation of cyclohexoxy radicals that are partially transformed to 6-hydroxyhexanoic acid, the major primary byproduct for these systems. However, during the CoII-catalyzed reaction, the concentration of cyclohexanone increases much faster than that of the hydroperoxide, causing the ketone to take over the role of dominant byproduct source. A mechanism for the conversion of cyclohexanone to ring-opened byproducts is put forward. Cr(acac)3 seems to catalyze additional reactions, some of them probably leading directly to byproducts. Indeed, the evolution of (by)products is significantly different from the CoII-catalyzed and the thermal systems, in the sense that they all seem to be primary in origin.

Mechanism of thermal toluene autoxidation.

Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain-carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The "hot" oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical-chain reactions. The very fast benzyl peroxyl-initiated co-oxidation of benzyl alcohol generates HO2* radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain-terminating reaction of HO2*with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2* right harpoon over left harpoonPhCH2OO* + H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co-oxidation of the alcohol means that HO2* gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.

The Chemical Route to a Carbon Dioxide Neutral World.

Excessive CO2 emissions in the atmosphere from anthropogenic activity can be divided into point sources and diffuse sources. The capture of CO2 from flue gases of large industrial installations and its conversion into fuels and chemicals with fast catalytic processes seems technically possible. Some emerging technologies are already being demonstrated on an industrial scale. Others are still being tested on a laboratory or pilot scale. These emerging chemical technologies can be implemented in a time window ranging from 5 to 20 years. The massive amounts of energy needed for capturing processes and the conversion of CO2 should come from low-carbon energy sources, such as tidal, geothermal, and nuclear energy, but also, mainly, from the sun. Synthetic methane gas that can be formed from CO2 and hydrogen gas is an attractive renewable energy carrier with an existing distribution system. Methanol offers advantages as a liquid fuel and is also a building block for the chemical industry. CO2 emissions from diffuse sources is a difficult problem to solve, particularly for CO2 emissions from road, water, and air transport, but steady progress in the development of technology for capturing CO2 from air is being made. It is impossible to ban carbon from the entire energy supply of mankind with the current technological knowledge, but a transition to a mixed carbon-hydrogen economy can reduce net CO2 emissions and ultimately lead to a CO2 -neutral world.