The effects of external surface barriers on diffusion and reaction in zeolite catalysts

Zeolites are attractive heterogeneous catalysts due to their crystalline structure, high surface area and thermal stability. However, conventional zeolites display diffusion limitations in many relevant catalytic processes. The slow diffusion rate through the extended network of micropores leads to low catalyst utilization and can, furthermore, lead to reduced selectivity and catalyst lifetime [1]. One nature-inspired approach to decrease diffusion limitations is to use hierarchically structured porous materials with an optimized network of broad and narrow pores. Observations in nature can help to provide a mechanistic basis to design the optimal pore network, since the architecture of nature is dominated by hierarchical structures. Hierarchical transport networks are indeed common in many natural systems, such as the respiratory and circulatory systems, as well as in leaves. At large length scales transport is dominated by convective flow, while at smaller length scales transport is dominated by diffusion [2]. Thus, the incorporation of hierarchical porosity can enhance the diffusion and reduce or even eliminate diffusion limitations in zeolite catalysts [3]. In addition to enhanced catalyst utilization, increasing the external surface area and maximizing the rate of intracrystalline diffusion could also lead to improved catalyst life times. Nevertheless, recent experimental and computational work suggests that external surface barriers in zeolite-based, hierarchical catalysts might play a significant role in affecting overall transport and reaction rates in such catalysts [4]. Rao et al. [5] demonstrated the existence and impact of surface barriers on the alkylation of benzene with ethylene by comparing reactor simulations with experimental results. In recent work in our group, ZSM-5 zeolites with similar bulk properties were prepared with different external surface properties, using different synthesis methods and conditions. The synthesized materials were studied extensively using different characterization techniques to determine their chemical, structural and textural properties. This set of catalysts was then used for appropriate catalytic experiments to investigate the impact of surface barriers on the catalytic properties of zeolites. This knowledge will be important to understand how surface barriers can be either avoided or exploited. References [1] M. Hartmann, A.G. Machoke, W. Schwieger: Catalytic test reactions for the evaluation of hierarchical zeolites. Chemical Society Reviews 45, 3313-3330 (2016). [2] P. Trogadas, M. Nigra, M. O. Coppens: Nature-inspired optimization of hierarchical porous media for catalytic and separation processes. New Journal of Chemistry 40, 4016-4026 (2016). [3] D. Mehlhorn, R. Valiullin, J. Kärger, K. Cho, R. Ryoo: Exploring the hierarchy of transport phenomena in hierarchical pore systems by NMR diffusion measurement. Microporous and Mesoporous Materials 164, 273-279 (2012). [4] G. Ye, Y. Sun, Z. Guo, K. Zhu, H. Liu, X. Zhou, M. O. Coppens: Effects of zeolite particle size and internal grain boundaries on Pt/Beta catalyzed isomerization of n-pentane. Journal of Catalysis 360, 152-159 (2018). [5] S. M. Rao, E. Saraçi, R. Gläser, M. O. Coppens: Surface barriers as dominant mechanism to transport limitations in hierarchically structured catalysts – Application to the zeolite-catalyzed alkylation of benzene with ethylene. Chemical Engineering Journal 329, 45-55 (2017)

Synthesis and catalytic properties of hierarchically structured zeolite catalysts with intracrystalline macropores

Zeolites belong to the most important heterogeneous catalysts. They are widely applied in crude oil refining, petrochemistry, fine chemistry, as well as in environmental applications. A unique feature of zeolites is their well-ordered micropore system with pore diameters similar to the dimensions of molecules. These small pores give rise to the shape selective properties of zeolite catalysts. However, the diffusion of molecules to and from the active sites confined within the micropores is very slow, which often leads to diffusion limitations. These diffusion limitations result in reduced utilization of the zeolite crystal and can also lead to reduced selectivity or lifetime of zeolite catalysts. A nature-inspired approach to overcome such diffusion restrictions is the utilization of catalysts with an optimally designed, hierarchical structure. In nature, mass transport systems, such as trees or lungs, possess an optimized hierarchical architecture to reduce transport limitations across a wide range of length scales.1 Adapting this approach to zeolites can be realized by including at least one additional system of larger pores interconnected to the zeolitic micropores. Hereby, hierarchical zeolites coul already demonstrate enhanced diffusion properties and, consequently, better catalytic performance.3 In order to prepare a truly nature inspired catalyst, a guided material design is crucial. Therefore, the transport pore system must exhibit an optimal porosity and the zeolitic domains in between the transport pores need to be small enough to eliminate local diffusion limitations. The pore size can be neglected, if it is larger than a certain minimum pore size, usually in the range of macropores or very large mesopores.3 However, preparation approaches for hierarchical zeolites are often unguided and result mostly in materials containing relative small mesopores. In this contribution we introduce a synthesis approach for zeolite single crystals with intracrystalline macropores by a so-called inverse crystallization, which allows control over the porosity, pore size and wall thickness of the hierarchical zeolite (see Figure 1 b). This synthesis approach utilizes mesoporous spherical silica particles as a sacrificial template for the macropore formation during zeolite synthesis by steam-assisted crystallization. Furthermore, we show the effect of these additional intracrystalline macropores on the catalytic performance for the direct conversion of methanol to short chain olefins (MTO), with focus on coke formation and catalyst lifetime. Please click Additional Files below to see the full abstract

MFI Type Zeolite Aggregates with Nanosized Particles via a Combination of Spray Drying and Steam-Assisted Crystallization (SAC) Techniques

This article belongs to the Special Issue Catalysis on Zeolites and Zeolite-Like Materials II

Effect of the Active Metal on the NO_x Formation during Catalytic Combustion of Ammonia SOFC Off-Gas

Catalytic combustion of hydrogen and ammonia containing off-gas surrogate from an ammonia solid oxide fuel cell (SOFC) was studied with a focus on nitrogen oxides (NOx) mitigation. Noble and transition metals (Pt, Pd, Ir, Ru, Rh, Cu, Fe, Ni) supported on Al2O3 were tested in the range of 100 to 800 °C. The tested catalysts were able to completely convert hydrogen and ammonia present in the off-gas. The selectivity to NOx increased with reaction temperature and stagnated at temperatures of 600 °C and higher. At low temperatures, the formation of N2O was evident, which declined with increasing temperature until no N2O was observed at temperatures exceeding 400 °C. Over nickel and iridium-based catalysts, the NOx formation was reduced drastically, especially at 300 to 400 °C. To the best knowledge of the authors, the current paper is the first study about catalytic combustion of hydrogen-ammonia mixtures as a surrogate of an ammonia-fed SOFC off-gas

Aggressive and challenging behavior in emergency medicine

Mitarbeiter*innen in der präklinischen und klinischen Notfallmedizin sind regelmäßig mit gewalttätigem Verhalten konfrontiert. Auch wenn Definitionen und Studiendesigns sehr heterogen sind, handelt es sich um ein relativ gut im Ausmaß beschriebenes internationales Problem. Viele Mitarbeiter*innen akzeptieren gemachte Gewalterlebnisse, welche überwiegend verbaler Natur sind, als „Teil des Jobs“, obwohl sich diese gravierend auf ihre Arbeit und ihr Leben auswirken können. Der intoxikierte oder psychiatrisch erkrankte männliche Patient steht hierbei ursächlich im Vordergrund. Leider stellt das in der Notfallmedizin tätige Personal durch Unerfahrenheit im Erkennen und in der Interpretation konfrontativer Situationen sowie einen Mangel an kommunikativen Fähigkeiten oft unwissentlich einen Teil der Eskalationsspirale dar.Staff in pre- and in-hospital emergency medicine are regularly exposed to aggressive and challenging behavior, which is a relatively well-described international problem, although definitions and study design are heterogeneous. Many colleagues accept violence that is predominantly of verbal nature as “part of the job”, although it can seriously affect their work and private lives. Male patients who are intoxicated/under the influence of drugs or mentally ill are typically the cause. Unfortunately, due to inexperience in recognizing and interpretation of confrontational situations and paired with a lack of communicative skills, staff in emergency medicine themselves often unknowingly become a part in its escalation

Effect of the Active Metal on the NOx Formation during Catalytic Combustion of Ammonia SOFC Off-Gas

Catalytic combustion of hydrogen and ammonia containing off-gas surrogate from an ammonia solid oxide fuel cell (SOFC) was studied with a focus on nitrogen oxides (NOx) mitigation. Noble and transition metals (Pt, Pd, Ir, Ru, Rh, Cu, Fe, Ni) supported on Al2O3 were tested in the range of 100 to 800 °C. The tested catalysts were able to completely convert hydrogen and ammonia present in the off-gas. The selectivity to NOx increased with reaction temperature and stagnated at temperatures of 600 °C and higher. At low temperatures, the formation of N2O was evident, which declined with increasing temperature until no N2O was observed at temperatures exceeding 400 °C. Over nickel and iridium-based catalysts, the NOx formation was reduced drastically, especially at 300 to 400 °C. To the best knowledge of the authors, the current paper is the first study about catalytic combustion of hydrogen-ammonia mixtures as a surrogate of an ammonia-fed SOFC off-gas

Catalyst Coatings for Ammonia Decomposition in Microchannels at High Temperature and Elevated Pressure for Use in Decentralized and Mobile Hydrogen Generation

We report an investigation of catalyst performance for the decomposition of ammonia under industrially relevant conditions (high temperatures of up to 800 °C and an elevated pressure of 5 bar) with further emphasis on their stability at high reaction temperatures. The catalysts were applied and tested as coatings in 500 µm wide channels of microreactors. Nickel-based catalysts were studied and compared to a ruthenium-based catalyst supported on SiO2. The effect of the support on the catalytic performance was investigated, and CeO2-supported nickel catalysts were found to exhibit the highest activity. Promoters were applied to increase the NH3 decomposition activity of the Ni/CeO2 catalysts. The addition of cesium led to a slight reduction in activity, while lanthanum, calcium, and barium doping resulted in increased activity. In particular, the barium-doped Ni/CeO2 catalyst showed very high ammonia conversion and closed the activity gap with respect to ruthenium catalysts at reactor temperatures of 650 °C and higher. The hydrogen production rates achieved in this work were compared to values in the literature and were shown to exceed values found earlier for both nickel- and ruthenium-based catalysts. Furthermore, the ruthenium-based catalysts under investigation were rapidly deactivated at 700 °C, while the nickel-based catalysts did not show deactivation after 220 h on time on stream at 700 °C