J.H. Bitter

Generation, Characterization, and Impact of Mesopores in Zeolite Catalysts

Amongst the current developments in the field of hierarchical pore structures, the creation of mesopores in zeolite crystals is the most frequently employed way to combine micropores with mesopores in one material. In this review an overview is presented of the different approaches to generate and characterize mesopores in zeolite crystals and establish their impact on the catalytic action. Mesopores can be created via several routes from which steaming and acid leaching are the most frequently applied. Novel approaches using secondary carbon templates that are removed after synthesis have recently been launched. For the characterization of mesopores, nitrogen physisorption and electron microscopy are commonly used. More recently, it was shown that electron tomography, a form of three-dimensional transmission electron microscopy, is able to reveal the three-dimensional shape, size, and connectivity of the mesopores. The effect of the presence of mesopores for catalysis is demonstrated for several industrially applied processes that make use of zeolite catalysts: the cracking of heavy oil fractions over zeolite Y, the production of cumene and hydroisomerization of alkanes over mordenite, and synthesis of fine chemicals over Y, ZSM-5, and Beta. For these processes, the mesopores ensure an optimal accessibility and transport of reactants and products, while the zeolite micropores induce the preferred shape-selective properties.

Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins

From Plant to Plastic Petroleum is primarily used as fuel, but it is also used in the production of plastics. Thus, if biomass were to replace petroleum as societys carbon feedstock, a means of deriving ethylene and propylene—the principal building blocks of todays commodity plastics— would be helpful. Well-known Fischer-Tropsch (FT) catalysts can transform gasified biomass into a range of hydrocarbon derivatives, but ethylene and propylene tend to constitute a small fraction of the overall product distribution. Torres Galvis et al. (p. 835) now demonstrate a class of iron catalysts on relatively passive supports (carbon nanofibers or α-alumina) that robustly directed the FT process toward light olefins. A class of iron catalysts selectively transforms gasified biomass into the building blocks of common plastics. Lower olefins are key building blocks for the manufacture of plastics, cosmetics, and drugs. Traditionally, olefins with two to four carbons are produced by steam cracking of crude oil–derived naphtha, but there is a pressing need for alternative feedstocks and processes in view of supply limitations and of environmental issues. Although the Fischer-Tropsch synthesis has long offered a means to convert coal, biomass, and natural gas into hydrocarbon derivatives through the intermediacy of synthesis gas (a mixture of molecular hydrogen and carbon monoxide), selectivity toward lower olefins tends to be low. We report on the conversion of synthesis gas to C2 through C4 olefins with selectivity up to 60 weight percent, using catalysts that constitute iron nanoparticles (promoted by sulfur plus sodium) homogeneously dispersed on weakly interactive α-alumina or carbon nanofiber supports.

On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis

The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers

Catalytically grown fishbone carbon nanofibers (CNF), are prepared by the decomposition of carbon-containing gases (CH4, CO/H2 or C2H4/H2) over a silica-supported nickel catalyst and an unsupported nickel catalyst at 550 ◦ C. It turns out that both the nickel particle size and the nature of the carbon-containing gas significantly affects the CNF growth process. We demonstrate that at the chosen temperature small supported nickel particles need a carbon-containing gas with a relatively low reactivity, like CH4 or CO/H2, to produce CNF. The resulting fishbone CNF have a uniform and small diameter (25 nm). The CNF thus synthesized hold great potential, e.g. as catalyst support material. However, the large unsupported nickel particles only produce CNF using a reactive carbon-containing gas, like C2H4/H2. The CNF thus obtained show a variety of morphologies with a large range of diameters (50–500 nm). The CNF yield is a subtle interplay between the nickel particle size and consequently the exposed crystal planes on the one hand and the reactivity of the carbon-containing gas on the other.

Iron Particle Size Effects for Direct Production of Lower Olefins from Synthesis Gas

The Fischer-Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of key chemical building blocks from non-petroleum-based sources such as natural gas, coal, or biomass. The influence of the iron carbide particle size of promoted and unpromoted carbon nanofiber supported catalysts on the conversion of synthesis gas has been investigated at 340-350 °C, H(2)/CO = 1, and pressures of 1 and 20 bar. The surface-specific activity (apparent TOF) based on the initial activity of unpromoted catalysts at 1 bar increased 6-8-fold when the average iron carbide size decreased from 7 to 2 nm, while methane and lower olefins selectivity were not affected. The same decrease in particle size for catalysts promoted by Na plus S resulted at 20 bar in a 2-fold increase of the apparent TOF based on initial activity which was mainly caused by a higher yield of methane for the smallest particles. Presumably, methane formation takes place at highly active low coordination sites residing at corners and edges, which are more abundant on small iron carbide particles. Lower olefins are produced at promoted (stepped) terrace sites that are available and active, quite independent of size. These results demonstrate that the iron carbide particle size plays a crucial role in the design of active and selective FTO catalysts.

Sodium Alanate Nanoparticles − Linking Size to Hydrogen Storage Properties

Important limitations in the application of light metal hydrides for hydrogen storage are slow kinetics and poor reversibility. To alleviate these problems doping and ball-milling are commonly applied, for NaAlH 4 leading to particle sizes down to 150 nm. By wet-chemical synthesis we have prepared carbon nanofiber-supported NaAlH 4 with discrete particle size ranges of 1-10 microm, 19-30 nm, and 2-10 nm. The hydrogen desorption temperatures and activation energies decreased from 186 degrees C and 116 kJ.mol (-1) for the largest particles to 70 degrees C and 58 kJ.mol (-1) for the smallest particles. In addition, decreasing particle sizes lowered the pressures needed for reloading. This reported size-performance correlation for NaAlH 4 may guide hydrogen storage research for a wide range of nanostructured light (metal) hydrides.

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Reaction Pathways for the Deoxygenation of Vegetable Oils and Related Model Compounds

Vegetable oil-based feeds are regarded as an alternative source for the production of fuels and chemicals. Paraffins and olefins can be produced from these feeds through catalytic deoxygenation. The fundamentals of this process are mostly studied by using model compounds such as fatty acids, fatty acid esters, and specific triglycerides because of their structural similarity to vegetable oils. In this Review we discuss the impact of feedstock, reaction conditions, and nature of the catalyst on the reaction pathways of the deoxygenation of vegetable oils and its derivatives. As such, we conclude on the suitability of model compounds for this reaction. It is shown that the type of catalyst has a significant effect on the deoxygenation pathway, that is, group 10 metal catalysts are active in decarbonylation/decarboxylation whereas metal sulfide catalysts are more selective to hydrodeoxygenation. Deoxygenation studies performed under H2 showed similar pathways for fatty acids, fatty acid esters, triglycerides, and vegetable oils, as mostly deoxygenation occurs indirectly via the formation of fatty acids. Deoxygenation in the absence of H2 results in significant differences in reaction pathways and selectivities depending on the feedstock. Additionally, using unsaturated feedstocks under inert gas results in a high selectivity to undesired reactions such as cracking and the formation of heavies. Therefore, addition of H2 is proposed to be essential for the catalytic deoxygenation of vegetable oil feeds.

Deactivation of solid acid catalysts for butene skeletal isomerisation : on the beneficial and harmful effects of carbonaceous deposits

Skeletal isomerisation of n-butene to isobutene is mainly controlled by catalyst pore topology, acid strength, acid site density and location of the acid sites. It is established that the pore structure of the catalyst is the most important feature with regard to isobutene selectivity and stability. The most favourable activity versus selectivity and stability characteristics are displayed by the zeolite ferrierite, for which the presence of carbonaceous deposits coincides with the selective performance in butene skeletal isomerisation. By-product formation, mainly propene, pentenes and octenes, as well as isobutene production initially takes place via oligomerisation and cracking throughout the ferrierite crystals. After some time-on-stream the pore system of ferrierite is largely filled by aliphatic carbonaceous deposits, with catalysis primarily occurring at the pore mouths of the channels. At this stage cracking of these aliphatic deposits is the origin of small amounts of by-products. Slowly the deposits are converted into aromatic coke, thus, further reducing reactivity and concomitant formation of non-selective products. It is emphasised that the observed increase of isobutene selectivity with time-on-stream is a consequence of the decrease of cracking reactions. Final deactivation of the catalyst is due to blockage of the pore mouth inlets by poly-aromatic compounds formed after a prolonged time-on-stream.

Carbon Nanofiber Supported Transition‐Metal Carbide Catalysts for the Hydrodeoxygenation of Guaiacol

Hydrodeoxygenation (HDO) studies over carbon nanofiber‐supported (CNF) W2C and Mo2C catalysts were performed on guaiacol, a prototypical substrate to evaluate the potential of a catalyst for valorization of depolymerized lignin streams. Typical reactions were executed at 55 bar hydrogen pressure over a temperature range of 300–375 °C for 4 h in dodecane, using a batch autoclave system. Combined selectivities of up to 87 and 69 % to phenol and methylated phenolics were obtained at 375 °C for W2C/CNF and Mo2C/CNF at >99 % conversion, respectively. The molybdenum carbide‐based catalyst showed a higher activity than W2C/CNF and yielded more completely deoxygenated aromatic products, such as benzene and toluene. Catalyst recycling experiments, performed with and without regeneration of the carbide phase, showed that the Mo2C/CNF catalyst was stable during reusability experiments. The most promising results were obtained with the Mo2C/CNF catalyst, as it showed a much higher activity and higher selectivity to phenolics compared to W2C/CNF.