Fundamental Catalysis and Engineering Challenges in Energy Harvesting

This commentary aims to stimulate further thoughts on pyrolysis technology by highlighting some aspects that are not yet fully resolved. Biocrude pyrolysis oil looks like petroleum oil in appearance, but the composition, energy content, and production process are quite different from those in petroleum oil. Therefore, the question arises on what can be done to make the pyrolysis oil technology-process and product-more attractive for commercial use. In the author's view, the perception created has been that it is a ready-to-use technology, only waiting for a political or business decision to progress in the implementation. There are however many research-related questions not yet answered. From this perspective, three areas involving catalysis and engineering aspects are discussed. First, the energy balance in the reactor is a poorly understood area. In other words, where does the energy required to maintain an endothermal reaction come from, especially in locations where utilities are scarce? Then, options to create additional value to make the technology more competitive and commercially viable are discussed. Finally, one of the least investigated topics, the performance of biocrude pyrolysis oil mixtures and its acid-catalyzed upgraded counterparts, is addressed

Coke formation in the oxidative dehydrogenation of ethylbenzene to styrene by TEOM

A packed bed microbalance reactor setup (TEOM-GC) is used to investigate the formation of coke as a function of time-on-stream on gamma-Al2O3 and 3P/SiO2 catalyst samples under different conditions for the ODH reaction of ethylbenzene to styrene. All samples show a linear correlation of the styrene selectivity and yield with the initial coverage of coke. The COX production increases with the coverage of coke. On the 3 wt% P/SiO2 sample, the initial coke build-up is slow and the coke deposition rate increases with time. On alumina-based catalyst samples, a fast initial coke build-up takes place, decreasing with time-on-stream, but the amount of coke does not stabilize. A higher O-2 : EB feed ratio results in more coke, and a higher temperature results in less coke. This coking behaviour of Al2O3 can be described by existing "monolayer-multilayer" models. Further, the coverage of coke on the catalyst varies with the position in the bed. For maximal styrene selectivity, the optimal coverage of coke should be sufficient to convert all O-2, but as low as possible to prevent selectivity loss by COX production. This is in favour of high temperature and low O-2 : EB feed ratios. The optimal coke coverage depends in a complex way on all the parameters: temperature, the O-2 : EB feed ratio, reactant concentrations, and the type of starting material.</p

Crotonaldehyde hydrogenation on Rh/TiO2 catalysts. In situ DRIFTS studies

The surface and catalytic properties in the vapor-phase hydrogenation of crotonaldehyde on Rh/TiO2 has been studied. It was found that a partial reduction of the support produces a surface decoration of the metal component. Thus, interfacial sites are created, which are responsible of an increase in the selectivity to crotyl alcohol, via enhancement of the polarization of the C=O bond. Photoelectron spectra revelead that rhodium is in different oxidation states, with a contribution of ca. 20 % Rhd + and 80 % Rhº species for LTR catalyst and only a slight increase of Rhd + for HTR catalyst. TEM studies revelead that Rh has metal particle size close 3 nm with small increases in the catalyst reduced at high temperature. DRIFTS essayed carried out under reaction conditions allowed to identify crotonaldehyde species strongly adsorbed through the C=C bond and weakly coordinated through both the C=C and C=O bonds. After reduction at 723 K an increase in the peak at 1660 cm-1 ascribed to an interaction between the carbonyl group and the surface, was observed. This peak seems to be stabilized at interfacial Rh/TiOx sites The deactivation in crotyl alcohol formation can be ascribed to the generation of strongly chemisorbed asymmetric carboxylate species detected by band at 1740 cm-1. This band grows at expense of crotonaldehyde O s - bonded intermediate chemisorbed on coordinatively unsaturated sites (Lewis acid sites) responsible of the crotyl alcohol obtaintion (detected by a band at 1653 cm-1). Additionally, a small band at 2068 cm-1 assigned to CO adsorbed on transition metals, which increases with time on-stream may explain the deactivation of the catalysts in flow systems

Molecular-level understanding of interfacial carbonates in stabilizing CuO-ZnO(Al2O3) catalysts

A descriptor of active CuO-ZnO(Al2O3) methanol-synthesis and water–gas-shift catalysts is the presence of high-temperature carbonates (HT-CO3) in the oxidic catalyst precursor. Previous reports have shown that such HT-CO3 lead to an appropriate interaction between the oxides; as a result, a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) can be achieved. Yet, their nature is not well understood. In this study, the nature of these carbonates was investigated by experimental and theoretical methods in the oxidic precatalyst. A calcined Cu-Zn-Al hydrotalcite model compound revealed to have well-dispersed ZnO and CuO phases, together with highly stable HT-CO3. It was hypothesized that these HT-CO3 groups may be placed at critical locations at nano-scale as a glue, thus avoiding the growth of the oxide crystallites during calcination. This is an excellent pre-condition to achieve a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) upon reduction, and therefore a high activity. To prove that, first-principles calculations were carried out based on the density functional theory (DFT); alumina was not considered in the model as the experimental data showed it to be amorphous but it may still have an effect. Comprehensive calculations provided evidence that such carbonate groups favourably bind the CuO and ZnO together at the interface, rather than being isolated on the individual oxide surfaces. The results strongly suggest that the HT-CO3 groups are part of the structure, in the calcined precatalyst, where they would prevent thermal sintering through a bonding mechanism between CuO and ZnO particles, which is a novel interpretation of this important catalyst descriptor

Preparation of Glycerol Carbonate Esters by using Hybrid Nafion-Silica Catalyst

Glycerol carbonate esters (GCEs), which are valuable biomass-deriv. compds., have been prepd. through the direct esterification of glycerol carbonate and long org. acids with different chain lengths, in the absence of solvent, and with heterogeneous catalysts, including acidic-org. resins, zeolites, and hybrid org.-inorg. acids. The best results, in terms of activity and selectivity towards GCEs, were obtained using a Nafion-silica composite. A full reaction scheme has been established, and it has been demonstrated that an undesired competing reaction results in the generation of glycerol and esters derived from a secondary hydrolysis of the endocyclic ester group, which is attributed to water formed during the esterification reaction. The influence of temp., substrate ratio, catalyst-to-substrate ratio, and the use of solvent has been studied and, under optimized reaction conditions and with the adequate catalyst, it was possible to achieve 95 % selectivity for the desired product at 98 % conversion. It was demonstrated that the reaction rate decreased as the no. of carbon atoms in the linear alkyl chain of the carboxylic acid increased for both p-toluenesulfonic acid and Nafion-silica nanocomposite (Nafion SAC-13) catalysts. After fitting the exptl. data to a mechanistically based kinetic model, the reaction kinetic parameters for Nafion SAC-13 catalysis were detd. and compared for reactions involving different carboxylic acids. A kinetic study showed that the reduced reactivity of carboxylic acids with increasing chain lengths could be explained by inductive as well as steric effects.The authors wish to acknowledge the Spanish Science and Innovation Ministry (Consolider Ingenio 2010, CTQ-2011-27550 and MULTICAT CSD2009-00050 projects) and the Generalitat Valenciana (Prometeo program) for their financial support. S.M. thanks the Ministerio de Educacion for a FPI fellowship.Climent Olmedo, MJ.; Corma Canós, A.; Iborra Chornet, S.; Martínez Silvestre, S.; Velty ., A. (2013). Preparation of Glycerol Carbonate Esters by using Hybrid Nafion-Silica Catalyst. ChemSusChem. 6(7):1224-1234. doi:10.1002/cssc.201300146S1224123467BUDRONI, G., & CORMA, A. (2008). Gold and gold–platinum as active and selective catalyst for biomass conversion: Synthesis of γ-butyrolactone and one-pot synthesis of pyrrolidone. Journal of Catalysis, 257(2), 403-408. doi:10.1016/j.jcat.2008.05.031Climent, M. J., Corma, A., & Iborra, S. (2011). 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Innovations in the synthesis of Fe-(exchanged)-zeolites

Several aspects on the preparation of Fe-zeolites are discussed. In contrast to the many studies highlighting the characterisation of the active sites, new approaches for incorporation of Fe are presented.Full utilization of exchange capacity of zeolites has been achieved by a controlled alkaline treatment of the parent sample. With this method, iron can be fully exchanged by liquid phase ion-exchange on ZSM5 without the formation of inactive Fe-oxides.The second topic is the-use of a mild oxidant (H2O2, and peroxides in general) to break down strong complexating equilibria during ion-exchange by controlled redox titration of the ligands. Hydrogen peroxide oxidizes effectively chelating groups releasing Fe species at a controlled rate. The method is demonstrated for the preparation of Fe-FER through Ferric-citrate.The final concept discussed is the detemplating of the zeolite with the simultaneous incorporation of the iron (combined detemplation and ionexchange). This one-pot preparation minimizes the number of steps considerably. To realize this, a strong oxidant is necessary to remove the organic template, and Fe-cations for exchange. Both requirements are met with the Fenton's-chemistry (Fe3+/2+/H2O2 mixtures) involving radical chemistry. (c) 2005 Elsevier B.V. All rights reserved.</p

Innovations in the synthesis of Fe-(exchanged)-zeolites

Several aspects on the preparation of Fe-zeolites are discussed. In contrast to the many studies highlighting the characterisation of the active sites, new approaches for incorporation of Fe are presented. Full utilization of exchange capacity of zeolites has been achieved by a controlled alkaline treatment of the parent sample. With this method, iron can be fully exchanged by liquid phase ion-exchange on ZSM5 without the formation of inactive Fe-oxides. The second topic is the-use of a mild oxidant (H2O2, and peroxides in general) to break down strong complexating equilibria during ion-exchange by controlled redox titration of the ligands. Hydrogen peroxide oxidizes effectively chelating groups releasing Fe species at a controlled rate. The method is demonstrated for the preparation of Fe-FER through Ferric-citrate. The final concept discussed is the detemplating of the zeolite with the simultaneous incorporation of the iron (combined detemplation and ionexchange). This one-pot preparation minimizes the number of steps considerably. To realize this, a strong oxidant is necessary to remove the organic template, and Fe-cations for exchange. Both requirements are met with the Fenton's-chemistry (Fe3+/2+/H2O2 mixtures) involving radical chemistry. (c) 2005 Elsevier B.V. All rights reserved