Ton V.W. Janssens

Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

Reflection absorption infrared spectroscopy and kinetic studies of the reactivity of ethylene on Pt(111) surfaces

Abstract The chemistry of ethylene on Pt(111) single-crystal surfaces has proven quite complex because it involves the simultaneous occurrence of several reactions, namely molecular desorption, dehydrogenation to ethylidyne, HD exchange within the adsorbed molecules, and hydrogenation to ethane. Reflection absorption infrared spectroscopy (RAIRS) has been used here in conjunction with isothermal kinetic measurements to identify the possible intermediates involved in each of those reactions, and to follow their thermal chemistry on the platinum surface. All vinyl, ethyl and ethylidene moieties were prepared by thermal decomposition of their corresponding iodides and characterized by RAIRS. The experimental data available to date favors the formation of ethylidene as an intermediate in the conversion of ethylene to ethylidyne, but the complexity of the kinetics of that reaction, which changes significantly with changing surface coverages, makes the final proof of this mechanism quite difficult. In addition, a side ethylene-ethyl equilibrium which starts at temperatures below those required for the formation of ethylidyne is responsible for HD exchange in ethylene. Finally, the hydrogenation of ethylene to ethane also involves an ethyl intermediate, but only occurs at the ethylene high coverages needed for the transition of the di-σ strongly bonded species to a weak π configuration. The relevance of the reactions seen under vacuum to the high-pressure catalytic hydrogenation of ethylene is briefly discussed.

The role of hydrogen-deuterium exchange reactions in the conversion of ethylene to ethylidyne on Pt(111)

Abstract Reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to study H/D scrambling reactions during the conversion of ethylene to ethylidyne in coadsorbed layers of C 2 H 4 and C 2 D 4 , C 2 H 4 and D 2 and C 2 D 4 and H 2 on Pt(111). The infrared spectra obtained after coadsorbing C 2 H 4 and C 2 D 4 at 330 K reveal the formation of partially deuterated ethylidyne. Extensive isotope scrambling was also observed when coadsorbing C 2 D 4 with H 2 , indicating that the exchange most likely involves surface hydrogen. Additional evidence points to the idea that the observed H/D exchange is not necessarily connected with the formation of ethylidyne, but due to other reactions occurring simultaneously on the surface. In particular, TPD experiments with C 2 H 4 coadsorbed with either D 2 or C 2 D 4 display significant desorption signals for partially substituted ethylenes; it is that exchanged ethylene the most likely source of the partially deuterated ethylidyne observed in the RAIRS experiments. Arguments are presented here in favor of a mechanism for the ethylene H/D exchange involving the formation of an ethyl intermediate. H/D exchange between ethylidyne and surface hydrogen or deuterium does occur too, but at a much slower rate than ethylidyne formation.

New insights into catalyst deactivation and product distribution of zeolites in the methanol-to-hydrocarbons (MTH) reaction with methanol and dimethyl ether feeds

Methanol (MeOH) and dimethyl ether (DME) have been compared as feedstock for the methanol-to-hydrocarbons (MTH) reaction over H-ZSM-5 (MFI), H-SSZ-24 (AFI) and H-SAPO-5 (AFI) catalysts at 350 and 450 °C. Several clear observations were made. First, the MeOH–DME equilibrium is not always established in the MTH reaction, because the rate of MeOH dehydration to DME is similar to the rates of the methylation reactions over strong Bronsted acid sites. In the presence of weak acid sites (i.e. the AlPO framework of SAPO-5), which are nearly inactive to hydrocarbons formation, the MeOH–DME equilibrium can be reached. Second, the MTH activity is ostensibly higher for DME compared to MeOH. Third, the carbon conversion capacity of the catalysts is generally higher (up to 16 times higher under the conditions used in this work) with a DME feed compared to a MeOH feed. Incorporation of AlPO-5 as dehydration catalyst before or mixed with a H-SSZ-24 catalyst for MTH, leads to lower MeOH concentrations in the reaction mixture, and a significant increase of the conversion capacity. Finally, a MeOH feed results in a higher selectivity for aromatic products and ethylene, pointing to a larger contribution of the arene cycle, compared to a DME feed. We hypothesize, that MeOH causes formation of formaldehyde, while DME does not. Formaldehyde is a known coke precursor, which provides an explanation for the faster deactivation of zeolites in a MeOH feed.

Role of internal coke for deactivation of ZSM-5 catalysts after low temperature removal of coke with NO2

By treating a deactivated ZSM-5 catalyst for the conversion of methanol to hydrocarbons with NO2, coke deposits can be removed at around 350 °C, which potentially enables catalyst regeneration at 350–400 °C, which is about 200 °C lower compared to a conventional regeneration in oxygen. To evaluate the regeneration with NO2 at 350 °C, the activity of a used ZSM-5 catalyst was measured after treatment with 1% NO2/He and 0.7% NO2/7% O2/He at 350 °C, and 2% O2/He at 550 °C. After the treatments with NO2 at 350 °C, some activity was restored, but the catalysts showed a fast deactivation. Temperature programmed desorption of ammonia and 27Al MAS NMR measurements indicate that the amount of framework aluminium in the regenerated catalysts is about 60% of that in the fresh catalysts, and some redistribution of the aluminium takes place. Gravimetric temperature programmed oxidation showed that the catalysts still contain 0.3–0.6 wt% coke. GC-MS analysis of the retained species and very high-speed 1H MAS NMR revealed that the remaining coke species are methyl benzenes, which are located inside the micropores of the ZSM-5 zeolite. It is concluded that the deactivation not only depends on the amount of coke, but also on the location of the coke in the catalyst.

Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH3-SCR

The equilibrium reaction between NO and Cu-nitrate, Cu(II)-NO3− + NO(g) ⇌ Cu(II)-NO2− + NO2(g), has been proposed to be a key step in the selective catalytic reduction of NO by ammonia (NH3-SCR) over Cu-CHA catalysts and points to the presence of Cu-nitrites. Whereas the formation of gaseous NO2 has been observed, a direct observation of Cu-nitrite groups under conditions relevant to NH3-SCR has been so far unsuccessful. In an effort to identify and characterize Cu-nitrites, the reaction between Cu-nitrates hosted in the CHA zeolite and NO is investigated by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis) and X-ray absorption spectroscopy (XAS). We find that NO reacts with Cu-nitrates and that about half of the Cu-nitrate species are converted. After the reaction, the Cu(II) state is different from the original oxidized state. Analysis of XAS data indicates that the final state of the Cu-CHA catalyst is consistent with the partial conversion of the Cu-nitrate species to a bidentate Cu-nitrite configuration.

Synthesis and characterization of mesoporous ZSM-5 core-shell particles for improved catalytic properties

Abstract HZSM-5 is a unique catalyst for the conversion of methanol, dimethyl ether and other oxygenates into gasoline. During this process, catalyst deactivation by coking requires frequent regeneration and the improvement of catalyst life time is one of the challenges in catalyst development. In this study, a series of mesoporous samples consisting of a ZSM-5 core and a silicalite shell have been synthesized and characterized by XRD, N2-sorption, IR spectroscopy and electron microscopy techniques. Additionally, desilicated conventional and mesoporous ZSM-5-type samples were investigated. All samples were tested in the MTG reaction, and the results showed that both the shell-coated and the desilicated zeolites are significantly more resistant to coke formation. These results are ascribed to the effect of the removal of structural defects rather than to an improvement of the diffusion properties due to the formation of mesopores.

Hierarchical Vanadia Model Catalysts for Ammonia Selective Catalytic Reduction

There is an ongoing debate on the structure of titania supported vanadia used for catalytic partial oxidation reactions as well as for abatement of nitrogen oxide (NOx) by ammonia selective catalytic reduction (NH3-SCR). To gain further insight into the essential features of the catalyst composition of the surface we introduce a new class of vanadia model catalysts based on monodispersed silica particles as a platform. The evaluation of the influence of different catalyst components (TiO2, WO3) and different vanadia synthesis approaches (incipient wetness, ion-exchange) is facilitated by the hierarchical structure of the catalysts as well as the controlled synthesis approach. Catalysts tested under industrial ammonia SCR conditions show a strong dependence on the catalyst composition. In particular, the presence of crystalline titania and tungsta leads to a strong increase in NO conversion. Detailed catalyst characterization by X-ray diffraction and by visible Raman, UV Raman, UV–Vis and X-ray photoelectron spectroscopies provides insight into the bulk structure, the surface composition as well as the vanadia surface structure of the catalysts. The potential of UV resonance Raman spectroscopy for structural analysis of vanadia model catalysts for SCR is highlighted. Differences in catalytic activity are discussed in light of the structural results and the available literature.

A molecular dance to cleaner air

Pairs of mobile copper ions intercept oxygen molecules in diesel-exhaust catalysts The emission of nitric and nitrous oxides (NOx) is a major cause of unhealthful air quality and is strictly regulated in many places. To meet regulations, exhaust gases from power plants, ships, trucks, and passenger cars are passed through catalytic cleaning systems designed to remove the harmful carbon monoxide, unburnt hydrocarbons, soot, and NOx before emission to the atmosphere. For gasoline engines, a “three-way” catalyst removes all harmful compounds, but such catalysts cannot be applied in the oxygen-rich environment in diesel exhausts. A class of catalysts for NOx reduction in diesel exhausts are copper (Cu) zeolites, which reduce the NOx with added ammonia and oxygen, so as to form unharmful nitrogen and water through selective catalytic reduction (SCR) (1, 2). Such catalysts are already deployed in diesel engine exhaust systems. On page 898 of this issue, Paolucci et al. (3) show that the traditional view of active sites in Cu zeolites, a central concept in catalysis, is not sufficient to understand how they truly function.