Katia Barbera

Disruptive catalysis by zeolites

The analysis of the new scenario for the industrial production of energy vectors and chemicals evidences the need to foster research in the field of catalysis by zeolites towards a novel, potentially disruptive, type of applications. To stimulate research in this direction, this perspective paper analyses a series of emerging concepts in catalysis by zeolites: i) the role of confinement, ii) the use of the Lewis acidity of zeolites, iii) the new possibilities to extend the concept of confined reactivity, iv) the role of defect sites, and iv) the organo-catalysis by guest species in zeolite cages. Then, two areas of novel possibilities for catalysis by zeolites are discussed more specifically: i) metallo-zeolites for methane conversion and ii) functionalized zeolites for reaction with CO2.

Low-temperature graphitization of amorphous carbon nanospheres

Abstract The investigation by SEM/TEM, porosity, and X-ray diffraction measurements of the graphitization process starting from amorphous carbon nanospheres, prepared by glucose carbonization, is reported. Aspects studied are the annealing temperature in the 750–1000 °C range, the type of inert carrier gas, and time of treatment in the 2–6 h range. It is investigated how these parameters influence the structural and morphological characteristics of the carbon materials obtained as well as their nanostructure. It is shown that it is possible to maintain after graphitization the round-shaped macro morphology, a high surface area and porosity, and especially a large structural disorder in the graphitic layers stacking, with the presence of rather small ordered domains. These are characteristics interesting for various catalytic applications. The key in obtaining these characteristics is the thermal treatment in a flow of N2. It was demonstrated that the use of He rather than N2 does not allow obtaining the same results. The effect is attributed to the presence of traces of oxygen, enough to create the presence of oxygen functional groups on the surface temperatures higher than 750 °C, when graphitization occurs. These oxygen functional groups favor the graphitization process.

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.

HMF etherification using NH4-exchanged zeolites

The properties of BEA, MFI and Silicalite-1 zeolites in the ammonium and protonic forms are studied in the etherification of HMF (5-hydroxymethylfurfural) in anhydrous ethanol and compared with FTIR data on ammonium ion siting and displacement by competitive adsorption, as well as data on ammonium ion dissolution in aqueous solution. For the first time it is demonstrated that ammonium-exchanged zeolites are active and show better performances (particularly for the BEA structure) in the acid-catalyzed etherification reaction. This behavior is associated to a reversible dissociation of NH4+ ions, which is favored by the BEA zeolite structure. A critical condition for enhanced catalytic performances is that dissociated ammonia remains in the zeolite cages, and may be reversibly re-adsorbed. It is thus likely that the dissociated ammonia participates in the reaction or induces a confinement effect.

Onion-Like Graphene Carbon Nanospheres as Stable Catalysts for Carbon Monoxide and Methane Chlorination

Thermal treatment induces a modification in the nanostructure of carbon nanospheres that generates ordered hemi‐fullerene‐type graphene shells arranged in a concentric onion‐type structure. The catalytic reactivity of these structures is studied in comparison with that of the parent carbon material. The change in the surface reactivity induced by the transformation of the nanostructure, characterized by TEM, XRD, X‐ray photoelectron spectroscopy (XPS), Raman, and porosity measurements, is investigated by multipulses of Cl2 in inert gas or in the presence of CH4 or CO. The strained CC bonds (sp2‐type) in the hemi‐fullerene‐type graphene shells induce unusually strong, but reversible, chemisorption of Cl2 in molecular form. The active species in CH4 and CO chlorination is probably in the radical‐like form. Highly strained CC bonds in the parent carbon materials react irreversibly with Cl2, inhibiting further reaction with CO. In addition, the higher presence of sp3‐type defect sites promotes the formation of HCl with deactivation of the reactive CC sites. The nano‐ordering of the hemi‐fullerene‐type graphene thus reduces the presence of defects and transforms strained CC bonds, resulting in irreversible chemisorption of Cl2 to catalytic sites able to perform selective chlorination.