M. Filipa Ribeiro

Total oxidation of toluene over calcined trimetallic hydrotalcites type catalysts.

Two trimetallic ZnCuAl and MnCuAl hydrotalcites have been successfully synthesized by a co-precipitation method. The manganese based material was identified as a new hydrotalcite phase. Both lamellar precursors were calcined at 450 and 600 degrees C and the resulting catalysts were tested on reaction of total oxidation of toluene. The solids were characterized by X-ray diffraction, thermal analysis, atomic absorption spectroscopy, Fourier transformed infrared spectroscopy, N(2) adsorption and H(2) temperature-programmed reduction. It was found that ZnCuAl materials are composed of copper and zinc oxides supported on alumina; while MnCuAl ones comprise basically spinel phases, which were not completely identified. The catalytic behavior of the calcined samples showed that Mn hydrotalcite calcined at 450 degrees C exhibited the best catalytic performance that corresponds to 100% toluene conversion into CO(2) at about 300 degrees C.

A systematic study on mixtures of Pt/zeolite as hydroisomerization catalysts

Mixtures of bifunctional catalysts were studied in the hydroisomerization of n-hexadecane. The study focused on the impact of the properties of individual catalysts on the hybrid catalysts’ performance. On the one hand, a Pt/HUSY catalyst was mixed with a series of Pt/zeolites with different topologies and acidities, all of the catalysts being individually well-balanced. Despite the diversity of Pt/zeolite catalysts in terms of both activity and isomerization selectivity, the performances of the hybrid catalysts corresponded to the average of individual components. Cooperative effects are proposed to be caused by a great difference in the relative activity of the Pt/zeolite catalysts rather than in selectivity. On the other hand, mixtures of large-pore Pt/HUSY and Pt/HBEA catalysts with different Pt loadings were tested. The performance of the resulting hybrid catalysts was observed to be a function of global metal–acid balance. Mixtures of poorly-balanced and well-balanced catalysts can be at the origin of cooperative effects, as demonstrated experimentally by an improved C16 isomer yield. The use of a dual-function kinetic model to simulate the performance of the hybrid catalysts corroborated these interpretations. This comprehensive work is expected to serve as a guideline for uncovering hybrid catalytic systems with industrial applications such as in the hydroisomerization of long chain n-paraffins.

Cordierite Foam Supports Washcoated with Zeolite-Based Catalysts for Volatile Organic Compounds (VOCs) Combustion

Reticulated cordierite foams produced by a direct foaming method were successfully washcoated with platinum-based zeolite catalysts. For comparison purposes, commercial cordierite monoliths were also washcoated. The effect of the structural properties on the fluid dynamics and catalytic behaviour for the toluene combustion were evaluated. Foam supports revealed highest performances, in terms of conversion into CO2, when compared to conventional honeycomb monoliths. The experimental results suggest that the catalytic behaviour is critically dependent on the fluid dynamics provided by structural characteristics of the supports, such as porosity, density and size of pores. The randomness and tortuosity of foams enhance reactant mixing, as it was evidenced by the higher axial and radial dispersions of the gas flow across the foam structure. This leads to better mass and heat transfers in the reaction system, thus improving the catalytic behaviour.

A recyclable hybrid manganese(III) porphyrin magnetic catalyst for selective olefin epoxidation using molecular oxygen

The synthesis and characterization of a hybrid Mn(III)-porphyrin magnetic nanocomposite is described. Moreover, a sustainable methodology for epoxidation of olefins is reported, using O2 as a green oxidant and the magnetic nanoparticle as a recyclable catalyst. High activity in alkene oxidation was observed, with full selectivity for epoxide formation. The magnetic catalyst presented high stability, being recovered and reused in five consecutive runs without loss of catalytic activity or selectivity in cyclooctene oxidation. Moreover, the catalytic system showed very good reactivity toward epoxidation of a range of terminal, substituted, cyclic or acyclic, aliphatic and aromatic olefins, including terpene and steroid derivatives, affording a range of biologically relevant epoxides in excellent yields. The isobutyric acid, formed as side-product, was recovered with high yield and purity, which provides the potential reutilization of this important industrial product.

NOx SCR with decane using Ag–MFI catalysts: on the effect of silver content and co-cation presence

A series of 4 and 9 wt% Ag/MFI catalysts were prepared using MFI zeolites containing different co-cations (H+ and Na+). The effect of the presence of these co-cations, as well as the silver loading, on the stabilisation of different Ag species was assessed by powder XRD, TEM, H2-TPR, in situ DRS UV-vis and pyridine adsorption followed by FTIR. The results obtained from all techniques allowed the comparison of the catalysts regarding the distribution and nature of the Ag species and their catalytic performance for the selective catalytic reduction (SCR) of NOx, in an excess of oxygen, using n-decane as the reductant. It was observed that with an Ag content of 4 wt%, the presence of Na+ does not affect the activity of the catalyst towards N2 formation, whereas with an Ag content of 9 wt%, it decreases N2 formation. This negative effect observed in the deNOx reaction can be attributed to the decrease in the amount of oxidised Ag species (Ag+ and Agnδ+) and the predominance of Ag0 particles in the samples containing Na. Furthermore, the absence of exchange-sites in the zeolite structure also led to a complete loss of activity. Hence, the exchange-sites are crucial for the stabilisation of the Ag species (e.g. Ag+ and Agnδ+) that are active for the NOx SCR reaction with n-decane.

Strength Improvement of Cordierite Foams by a Dip Coating Method

The structure and morphological aspects of highly porous (higher than 90%) cordierite (Mg2Al4Si5O18) foams, prepared by a direct foaming method, have been evaluated by Scanning Electron Microscopy analysis. The resulting ceramic foams consisted of a three-dimensional array of struts forming a well-defined open-cell structure. This type of structure seems very attractive for catalyst support purposes. Attempts have been made in order to control the pore structure since it directly affects the physical properties, namely the mechanical strength. In this respect, the use of a dip coating method to improve the strength of the resulting foams was found to be effective in reducing defects (e.g. pores, flaws) in the struts. Based on image analysis, estimated mean cell sizes were about 550 µm whereas strut thicknesses varied in the range of 60-70 µm. The compressive strength of the developed foams increased by one order of magnitude (0.1 to 1 MPa) by increasing the relative density from 0.06 to 0.18.

Selective Catalytic Reduction of NOx over Zeolite-Coated Cordierite-Based Ceramic Foams: Water Deactivation

The reactors used for Selective Catalytic Reduction (SCR) of NOx require low pressure drop structured catalyst packing. Structured packings, such as ceramic foams, are gaining increasing interest for application in low pressure drop reactors, membrane reactors and catalytic distillation units. In this work, cobalt ion exchanged mordenite (Co-HMOR)-coated cordierite-based foams produced by the replication method were evaluated for catalytic reduction of NOx with methane. The addition of 0.3 wt.% Pd to 2 wt.% Co-HMOR leads to a material that can convert 50 % NOx to N2 at 450 °C in a reaction mixture containing 2000 ppm CH4, 1000 ppm NOx, 5 % O2 and balance helium, at GHSV=17000 h-1. Although in an early stage of development, an efficient coating procedure was explored and different ways of exchange of Co and Pd cations into mordenite (Si/Al=10) were studied. Additions of 2 wt.% fumed silica enhanced adhesion of the zeolite onto the ceramic foam. Pd-exchanged Co-HMOR showed to be very sensitive to steam. A 50 % decrease in NOx conversion to N2 was observed after Pd/Co-HMOR samples were exposed at 450 °C to a reaction mixture containing 2 vol% H2O. Although further research is needed to ascertain the mechanism of this deactivation behaviour, agglomeration of Pd forming PdO particles is envisaged.

Influence of Si/Al ratio on the catalytic properties of NiH mordenite in the disproportionation of toluene

Abstract The disproportionation of toluene on dealuminated NiH mordenites has been studied in order to understand the influence of the Si/Al ratio on the activity, selectivity and stability of these catalysts. Four dealuminated H mordenites were prepared by combined treatments of calcination under wet air and boiling HC1 solution; the atomic Si/Al ratio of the framework, determined by 29Si NMR spectroscopy, varied from 5 to 29.3. A maximum of activity was obtained for a Si/Al ratio of ~12.6. This catalytic behaviour is explained by the influence of the Si/Al ratio on the acidity and also by strong diffusional limitations for the non-dealuminated catalyst. The most active catalyst is also the most selective in aromatic compounds. On the other hand, the non-dealuminated catalyst is the least selective, as explained by a strong hydrogenolysis activity. The decrease of selectivity in aromatics of the dealuminated NiH mordenites after ageing can be explained by the activation of some of the nickel which was not previously reduced, and also by the higher temperatures of the catalytic tests, which favour the hydrogenolysis and hydrocracking reactions.

Nanoscale insights into Pt-impregnated mixtures of zeolites

A series of catalysts prepared by Pt deposition over intimate mixtures of HUSY and HBEA zeolites was investigated for the first time and compared to individual Pt/zeolite solids. The samples were characterized through macroscopic techniques and at the nanoscale by electron microscopy and electron tomography, in order to evaluate both the acid and the hydrogenation functions. Comparable macroscopic properties were observed between Pt-impregnated individuals and zeolite mixtures. In contrast, at the nanoscale, differences were evident. When mixtures of zeolites were impregnated, Pt was not homogeneously distributed over the sample. In fact, Pt was located mainly on the HBEA zeolite rather than on HUSY, as confirmed by TEM-EDS semi-quantification. This selectivity in Pt location was tentatively explained by the higher amount of terminal silanol and EFAL species in the HBEA sample. The preferential Pt location at the nanoscale may open up new possibilities in the design and application of hybrid catalysts.