Freek Kapteijn

EVOLUTION OF NITROGEN FUNCTIONALITIES IN CARBONACEOUS MATERIALS DURING PYROLYSIS

X-ray photoelectron spectroscopy (XPS) was used to investigate the fate of nitrogen functional forms present in a lignite and its chars, chars derived from the model compounds acridine, carbazole and polyacrylonitrile (PAN). Four different peaks have been found in the XPS patterns, corresponding to at least five different nitrogen functional forms, all being aromatic moieties. The XPS patterns of the synthetic chars were recorded for identification purposes. The distribution of nitrogen functional forms changes with increasing severity of the pyrolysis conditions. Under mild pyrolysis conditions, firstly unstable functionalities like pyridones, protonated pyridinic-N and N-oxides of pyridinic-N are converted to pyridinic-N and secondly pyrrolic-N is converted to pyridinic-N during condensation of the carbon matrix. During the condensation process, nitrogen atoms are incorporated in the graphene layers replacing carbon atoms. After severe pyrolysis all nitrogen is eventually present in 6-membered rings located at the edges of the graphene layers as pyridinic-N or in the interior as quaternary-N. Upon exposure to the ambient, N-oxides of pyridinic-N can be formed. During pyrolysis, differences in nitrogen distribution of the char precursors have diminished. It is presumed that the remaining small differences in the nitrogen distribution of the chars cannot significantly influence the formation of nitrogen oxides during combustion of the chars.

An Amine-Functionalized MIL-53 Metal−Organic Framework with Large Separation Power for CO2 and CH4

Functionalizing the well-known MIL-53(Al) metal-organic framework with amino groups increases its selectivity in CO(2)/CH(4) separations by orders of magnitude while maintaining a very high capacity for CO(2) capture.

Heterogeneous catalytic decomposition of nitrous oxide

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N2O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.

Metal–organic framework nanosheets in polymer composite materials for gas separation

Composites incorporating two-dimensional nanostructures within polymeric matrices hold potential as functional components for several technologies, including gas separation. Prospectively, employing metal-organic-frameworks (MOFs) as versatile nanofillers would notably broaden the scope of functionalities. However, synthesizing MOFs in the form of free standing nanosheets has proven challenging. We present a bottom-up synthesis strategy for dispersible copper 1,4-benzenedicarboxylate MOF lamellae of micrometer lateral dimensions and nanometer thickness. Incorporating MOF nanosheets into polymer matrices endows the resultant composites with outstanding CO2 separation performance from CO2/CH4 gas mixtures, together with an unusual and highly desired increment in the separation selectivity with pressure. As revealed by tomographic focused-ion-beam scanning-electron-microscopy, the unique separation behaviour stems from a superior occupation of the membrane cross-section by the MOF nanosheets as compared to isotropic crystals, which improves the efficiency of molecular discrimination and eliminates unselective permeation pathways. This approach opens the door to ultrathin MOF-polymer composites for various applications.

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Catalyst deactivation: is it predictable?: What to do?

Abstract Catalyst deactivation is usually inevitable, although the rate at which it occurs varies greatly. This article discusses the causes of deactivation and the influence on reaction rate. Methods for minimising catalyst deactivation, by tailoring catalyst properties and/or process operations, are presented, as well as reactor configurations suitable for the regeneration of deactivated catalysts. Alkane dehydrogenation is used as an example to demonstrate the variety of engineering solutions possible.

Preparation of monolithic catalysts

Monolithic catalysts can be attractive replacements for conventional catalysts in randomly packed beds or slurry reactors. The conventional procedures for preparing catalysts, however, cannot simply be applied to monolithic catalysts. Different procedures are discussed on how to put a coat layer of a catalyst support material like alumina, silica, or carbon on a monolith body by either filling the pores in that support or by putting a layer on that support. Different methods to apply an active phase to the support are discussed as well. Finally, methods to convert ready-made catalysts into monolithic catalysts are presented.

Ethane/Ethene Separation Turned on Its Head: Selective Ethane Adsorption on the Metal−Organic Framework ZIF-7 through a Gate-Opening Mechanism

Ethane is selectively adsorbed over ethylene in their mixtures on the zeolite imidazolate framework ZIF-7. In packed columns, this results in the direct production of pure ethylene. This gas-phase separation is attributed to a gate-opening effect in which specific threshold pressures control the uptake and release of individual molecules. These threshold pressures differ for the different molecules, leaving a window of selective uptake operation. This phenomenon makes ZIF-7 a perfect candidate for the separation of olefins from paraffins, since in contrast to most microporous materials, the paraffin is selectively adsorbed. Mixture adsorption, as studied by breakthrough experiments, demonstrates that gate-opening effects can be effectively used to separate molecules of very similar size.

The development of nitrogen functionality in model chars during gasification in CO2 and O2

Abstract The development of the nitrogen functionality of model chars as a function of burn-off for gasification in CO2 or in O2 has been studied by X-ray photoelectron spectroscopy (XPS). The type of carbon precursor (sucrose or phenolformaldehyde resin) and of nitrogen precursor (uracil, aniline or 3-hydroxypyridine), to synthesise the nitrogen doped model chars, did not have an influence on this development. The high-temperature chars (1373 K) exhibit N-functionalities attributed to pyridinic nitrogen, pyridones, and oxidic nitrogen species at the edges of the graphene structures and quaternary nitrogen incorporated in the graphene structure. With increasing burn-off levels, nitrogen accumulates in the char, especially during O2 gasification. A gradual transition from quaternary nitrogen to pyridone and pyridinic nitrogen is observed, due to the removal of surrounding carbon. In O2 this phenomenon is more pronounced than in CO2 gasification, and more pyridone is formed due to its association with carbon-oxygen functionalities. A schematic model is presented that accounts for the development of the nitrogen functionalities and the nitrogen retention.

Permeation characteristics of a metal-supported silicalite-1 zeolite membrane

Abstract Permeation data are presented which give an overview of the permeation and separation characteristics of a metal supported silicalite-1 zeolite membrane over a broad temperature and pressure range. Methane, ethane, ethene, propane, propene, n -butane, i -butane, carbon dioxide, hydrogen, and i -octane are used as probe molecules and helium is used as sweep gas. One-component and binary systems are studied in a temperature range of 193–673 K and a pressure range of 0.05–500 kPa. Large differences have been found between the different one-component permeation fluxes, which amounts up to a factor of about 500 between methane and the bulky iso -butane. The permeation fluxes at 295 K, generally decrease with increasing molecular size. The alkenes permeate faster than their corresponding alkanes. Diffusion coefficients calculated with the Maxwell-Stefan equations are in accordance with the literature. A remarkable temperature dependency has been observed. For the bulky i -butane the permeance increases steadily with temperature. For methane, ethane, and n -butane a maximum in permeation is observed and for methane and ethane also a minimum. This maximum can be explained by the combined temperature dependency of diffusion and adsorption in configurational mass transport. The minimum is explained by the occurrence of a Knudsen-like mass transport at low occupancy and high temperature. In many cases the separation selectivity of a mixture does not reflect the one-component permeation ratio. Besides molecular sieving and difference in diffusivity, difference in adsorption appears to be a key factor in separation selectivity. The permeation of weakly adsorbing molecules (e.g. hydrogen at 295 K) can drop over two orders of magnitude in the presence of strongly adsorbing molecules (e.g. n -butane at 295 K). This results in high separation selectivities favouring the strongest adsorbing component. Typical separation selectivities for hydrogen/ n -butane (at 295 K, 95 kPa/5 kPa), n -butane/ i -butane (at 295 K, 50 kPa/50 kPa), and methane/ i -octane (at 423 K, 25 kPa/5 kPa) mixtures, are 125, 27 and > 300, respectively. An inversion in separation selectivity is observed during a temperature programmed permeation which is explained from the temperature dependence of adsorption. The membrane appears to be very stable upon thermal cycling (193–673 K) and the permeation characteristics have changed less than 10% over the testing period of 1.5 year.