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By correlating structural information with catalytic activity, it is possible to determine the active structure of a catalyst. However, this is far from straightforward and the active structure remains debated even in the well-studied reaction of graphitic carbon formation using noble metal catalysts such as platinum (Pt). One major hindrance is that static observations do not provide access to transitional catalyst states. Here we prove the formation of transitional surface Pt carbide several layers deep as well as a Pt-carbon composite phase during the growth process of carbon nanotubes using atomic-resolution gas in situ transmission electron microscopy combined with density functional theory. Knowledge of the active structure of noble metal Pt is of great interest due to its usage in heterogeneous catalysis. Most importantly, it opens up new avenues to suppress catalyst coking. The unwanted build-up of carbon is the major source of catalyst deactivation in important industrial reactions including propane dehydrogenation, with major financial and environmental consequences. [Display omitted] • Transient Pt carbide phase formation observed during CNT growth. • Pt carbide phase confirmed using in situ TEM, EELS and DFT calculations. • Liquid metal state formed for noble metal Pt during CNT growth. • Growth mechanism of graphitic carbon and coke on Pt catalyst. • Rough Pt surface critical for CNT growth. [ABSTRACT FROM AUTHOR]

Ethylene production by oxidative pyrolysis of natural gas is based on a complex chemical engineering process. One of the main stages of this process is hydrogenation of acetylene in the ethane-ethylene fraction using catalysts that increase the efficiency of the process. Acetylene hydrogenation produces byproducts that cause coking of the catalysts and thereby reduce their activity. This article presents a catalyst coking prevention system based on use of an advanced control system (ACS) and a genetic algorithm. The proposed control system can be used to switch from planned catalyst regeneration to its regeneration based on the actual state of coking. [ABSTRACT FROM AUTHOR]

The main factors affecting the design of a Propane Dehydrogenation Membrane Reactor (PDH MR) are the deactivation of the catalyst and of the membrane due to coking. Both apparently accelerate with increasing temperature or pressure and with depletion of hydrogen; i.e., with conditions that improve conversion in a membrane reactor. Recent studies of this project [Sheintuch et al., 2016; Peters et al., 2016] suggest that pressure should be kept below 5 bar and catalyst temperature should be around 450–500 °C, while the membrane should be kept at 200–250 °C to avoid coking. This favors the distributed reactor design (open architecture) which requires as many as 6 pairs of reactor-separators to achieve the desired 25% conversion with very high sweep to feed ratio (3 for each unit or 18 overall) compared with a single integrated MR that can achieve the same conversion at 450 °C with sweep/feed ratio of 2 or more with counter-current flow. Both designs will yield good selectivity but the catalyst life time is predicted to be ∼2 days while the membrane life time will be shorter in the integrated design as opposed to a stable activity in a cool (250 °C) separator. A new integrated design with an internal gradient is suggested combining the advantages of both approaches. It is based on a three cylindrical zones reactor with catalyst in the outer layer, maintained at 450 °C, permeate in the inner with sweep fed at 250 °C, separated by an inert insolating layer. Initial calculations showed promising results. [ABSTRACT FROM AUTHOR]

Thermodynamic investigation of glycerol reforming has been performed to study hydrogen production, carbon dioxide evolution and coke formation. Gibb's free energy direct minimization procedure was used to calculate the concentration of different components at equilibrium. The analysis was performed at temperatures from 300K to 1000K under unit atmospheric pressure. A comparative study on steam reforming of glycerol (SRG) and glycerol reforming reaction with hydrazine has been conducted in the presence of hydrazine that act as a suitable reducing agent. Incorporation of hydrazine into glycerol reforming system helped in minimizing coke formation, maximizing hydrogen and syn-gas production. A complete conversion of glycerol with coke free products, along with reduced level of carbon dioxide and maximum hydrogen generation was obtained when glycerol steam reforming process (S/G = 1) was combined with higher moles of hydrazine. Reformation at higher temperatures could enhance the hydrogen production and decrease carbon generation due to methanation reaction and hence optimum results were accomplished at 1000K and atmospheric pressure. [ABSTRACT FROM AUTHOR]

Two ZSM-5 catalysts, differing only in their crystal size, viz, nanocrystal at ∼ 100 nm and microcrystal at 13 μm, respectively, were synthesised and tested in methanol to gasoline (MTG) conversion, with a focus on the formation and characteristics of coke deposits. Over time periods when methanol conversion decreased to 50%, herein termed as the service lifespan of the catalyst, the nanocrystal catalyst incurred 31.1 wt% coke deposition, while the microcrystal counterpart had 14.1 wt% coke. The nanocrystal catalyst showed a service lifespan almost seven times longer than the microcrystal catalyst. The difference in the catalytic service lifespans was examined in terms of the rate of formation of internal coke and structural properties of external coke, as determined using nitrogen physisorption, TGA and TEM. It was found that the internal coke was quickly formed in the microcrystal catalyst leading to rapid coverage of the active sites and blockage of the pores, resulting in fast deactivation. In contrast, coke formed preferentially on the external surface in the case of the nanocrystal catalyst. This external coke was of porous graphitic structures, and thus was not detrimental to the catalytic performance. The coke fouled nanocrystal catalyst was regenerated and the activity of the regenerated catalyst was evaluated under the same reaction conditions. An increase in catalytic service lifespan compared to the pristine nanocrystal catalyst was observed, due to the effect of decreased Al concentration on the catalytic performance. [ABSTRACT FROM AUTHOR]

Microwave-assisted heterogeneous catalysis (MHC) is gaining attention due to its exciting prospects related to selective catalyst heating, enhanced energy-efficiency, and partial inhibition of detrimental side gas-phase reactions. The induced temperature difference between the catalyst and the comparatively colder surrounding reactive atmosphere is pointed as the main factor of the process selectivity enhancement towards the products of interest in a number of hydrocarbon conversion processes. However, MHC is traditionally restricted to catalytic reactions in the absence of catalyst coking. As excellent MW-susceptors, carbon deposits represent an enormous drawback of the MHC technology, being main responsible of long-term process malfunctions. This work addresses the potentials and limitations of MHC for such processes affected by coking (MHCC). It also intends to evaluate the use of different catalyst and reactor configurations to overcome heating stability problems derived from the undesired coke deposits. The concept of long-term MHCC operation has been experimentally tested/applied to for the methane non-oxidative coupling reaction at 700 °C on Mo/ZSM-5@SiC structured catalysts. Preliminary process scalability tests suggest that a 6-fold power input increases the processing of methane flow by 150 times under the same controlled temperature and spatial velocity conditions. This finding paves the way for the implementation of high-capacity MHCC processes at up-scaled facilities.

In this study, a thermodynamic equilibrium analysis has been performed on ethanol dry reforming in presence of commonly used reducing agents such as glycine, hydrazine, urea and ammonia. The effect of these agents on hydrogen enrichment and carbon suppression has been investigated at one atmospheric pressure and 300 K–1200 K temperature. Among the investigated additives, hydrazine was found to be the most suitable for maximizing hydrogen production and removing elemental carbon formation. This study was further extended to ethanol steam reforming and similar conclusions were obtained. Industrial significance of this study is associated with reforming reactions where catalyst deactivation due to coking is a major problem. Based on the results obtained, addition of suitable chemical additives in the feed could alleviate this problem to a significant level and help in improving catalyst life cycle. [ABSTRACT FROM AUTHOR]

Catalytic upgrading of raw bio-oil and liquid–liquid extracted bio-oil (high concentrated phenolic with trace acid and acetaldehyde) with methanol over ZSM-5 catalyst had been studied in this work. Temperature played vital function and leaded to increasing gas yield but less catalyst coking. It also changed both chemical distribution and selectivity on both gas and liquid products, with aromatic concentration increasing by 34.42%. Temperature of 400 °C was selected as the optimized reaction conditions with liquid yield of 10.47 wt.% and 75.00% aromatic hydrocarbon in liquid product from 100 g biomass, only with coke yield of 1.42 wt.%. Phenolic-rich extracted bio-oil obtained higher aromatic hydrocarbon yield (7.3 wt.% increased from 1.1 wt.%) and lower coke yield (1.42 wt.% decrease from 15.79 wt.%) than raw bio-oil. Catalyst regenerated from this type of feedstock also achieved higher activity and longer useable running times on aromatic yield compared to fresh catalyst. This result suggested that lignin derived phenolic and guaiacol compounds were not the only reason caused catalyst coking on ZSM-5 catalyst. Small high active molecules of acetic acid and acetaldehyde also acted as important precursors of catalyst coke formation. [ABSTRACT FROM AUTHOR]

• Periodic process of propane dehydrogenation regarding catalyst coking is studied • Modeling approaches of different complexity are developed and parametrized • Systematic model discrimination provides most suitable deactivation model • Overall periodic production process optimized with respect to space time yield • Considering catalyst activity function intensifies overall reactor performance Direct dehydrogenation is one well-established process for the provision of short chain alkenes and suffers from rapid catalyst coking. Kinetic modeling of coke built-up and catalyst deactivation due to coking is essential to optimize the total production process consisting of alternating deactivation and regeneration cycles. In this contribution two different deactivation approaches will by studied and evaluated. The first one, introduced by Janssens, describes deactivation as a loss of active catalyst mass depending on the conversion of the reactant. The second, an advanced microkinetic approach, was suggested by Dumez and Froment and explains the loss of activity by coke formation during the production process. Thus, a time dependent activity function is derived and correlated with the reaction rate. Results of experiments performed in a lab scale tubular reactor on a VO x catalyst provided the basis for extending and parametrizing models for reaction kinetics, coke built-up and deactivation, respectively. Temperature and oxygen concentration have been varied to describe the activity dependence on time and reactor position. Finally, modeling the cyclic operation of deactivation and regeneration was possible and allowed optimizing the individual time intervals to maximize total space time yield. A first proposal of an optimized cyclic process regime is presented. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] Staged feeding is very advantageous in the ODH reaction of EB to ST. [•] Conversion of EB and ST selectivity are a function of the overall O2 to EB molar feed ratio. [•] Maintaining a low (local) O2 partial pressures is essential for high ST selectivity. [•] The lower partial pressure of O2 and EB will result in a lower catalyst activity. [ABSTRACT FROM AUTHOR]

Isomorphously substituted Fe-MFI zeolite catalysts with various Si/Al and/or Si/Fe ratios were synthesized and characterized by many different techniques, such as ICP, XRD, SEM, TPR, microcalorimetry, FTIR, and EPR. Under standard reaction conditions the best catalyst gave 20% benzene conversion and over 90% selectivity to phenol. For Fe-ZSM5 catalysts, addition of steam to the feed improved catalyst activity, selectivity, and durability. Phenol formed onto Fe-based sites only. Active sites could very likely be composed of oxygen-bridged, extraframework binuclear Fe redox species, charge-compensating the framework Fe3+ or Al3+ ions. Surface acidity was not responsible for activity in the main reaction, but it was heavily involved in catalyst deactivation by coking. Catalyst deactivation derived mainly from the decomposition-condensation of phenol onto acid sites; the stronger the latter, the quicker was the coking rate. [Copyright &y& Elsevier]

Microwave-assisted heterogeneous catalysis (MHC) is gaining attention due to its exciting prospects related to selective catalyst heating, enhanced energy-efficiency, and partial inhibition of detrimental side gas-phase reactions. The induced temperature difference between the catalyst and the comparatively colder surrounding reactive atmosphere is pointed as the main factor of the process selectivity enhancement towards the products of interest in a number of hydrocarbon conversion processes. However, MHC is traditionally restricted to catalytic reactions in the absence of catalyst coking. As excellent MW-susceptors, carbon deposits represent an enormous drawback of the MHC technology, being main responsible of long-term process malfunctions. This work addresses the potentials and limitations of MHC for such processes affected by coking (MHCC). It also intends to evaluate the use of different catalyst and reactor configurations to overcome heating stability problems derived from the undesired coke deposits. The concept of long-term MHCC operation has been experimentally tested/applied to for the methane non-oxidative coupling reaction at 700 °C on Mo/ZSM-5@SiC structured catalysts. Preliminary process scalability tests suggest that a 6-fold power input increases the processing of methane flow by 150 times under the same controlled temperature and spatial velocity conditions. This finding paves the way for the implementation of high-capacity MHCC processes at up-scaled facilities. [ABSTRACT FROM AUTHOR]