Browsing by Author "Mendes, Pedro S. F."
Now showing 1 - 6 of 6
Results Per Page
Sort Options
- Bridging the gap between academic and industrial hydrocracking: on catalyst and operating conditions' effectsPublication . Mendes, Pedro S. F.; Silva, João M; Ribeiro, M Filipa; Daudin, Antoine; Bouchy, ChristopheThis work aims at bridging the knowledge gap between the well-studied Pt/zeolite catalysts and the industrially-employed NiMoS/(Al2O3+ zeolite) ones. To do so, catalyst series based on HUSY zeolite were evaluated in the hydroconversion of n-hexadecane under similar operating conditions, but the industrially-relevant H2S and NH(3)contaminants were added when evaluating the latter. The intrinsic performance of Pt/HUSY and NiMoS/(Al2O3+ HUSY) catalysts was noticeably similar, when well-balanced, with the catalytic activity being however much lower in the latter (temperature gap of 125 K). Ammonia inhibition of more than 99% of the protonic sites was revealed to be the origin of such low activity. On the other hand, the metal-acid balance was only reached at sufficiently low reaction temperatures (ca.593 K),i.e.when the NH(3)inhibiting effect increased the metal to acid site ratio in NiMoS/(Al2O3+ HUSY) to values five orders of magnitude larger than those in the Pt/HUSY one. In addition to the understanding achieved on the role of metal-acid balance in industrial-like catalysts and its key controlling parameters, these findings also point to the need to develop better hydrogenating functions to improve the efficiency, and consequently the sustainability of liquid fuel production.
- Hydrogenating activity of Pt/zeolite catalysts focusing acid support and metal dispersion influencePublication . Mendes, Pedro S. F.; Lapisardi, Grégory; Bouchy, Christophe; Rivallan, Mickaël; Silva, João; Ribeiro, M. FilipaToluene hydrogenation was studied over catalysts based on Pt supported on large pore zeolites (HUSY and HBEA) with different metal/acid ratios. Acidity of zeolites was assessed by pyridine adsorption followed by FTIR showing only small changes before and after Pt introduction. Metal dispersion was determined by H2–O2 titration and verified by a linear correlation with the intensity of Pt0–CO band obtained by in situ FTIR. It was also observed that the electronic properties of Pt0 clusters were similar for the different catalysts. Catalytic tests showed rapid catalyst deactivation with an activity loss of 80–95% after 60 min of reaction. The turnover frequency of fresh catalysts depended both on metal dispersion and the support. For the same support, it changed by a 1.7-fold (HBEA) and 4.0-fold (HUSY) showing that toluene hydrogenation is structure-sensitive, i.e. hydrogenating activity is not a unique function of accessible metal. This was proposed to be due to the contribution to the overall activity of the hydrogenation of adsorbed toluene on acid sites via hydrogen spillover. Taking into account the role of zeolite acidity, the catalysts series were compared by the activity per total adsorbing sites which was observed to increase steadily with nPt/(nPt + nA). An increase of the accessible Pt atoms leads to an increase on the amount of spilled over hydrogen available in acid sites therefore increasing the overall activity. Pt/HBEA catalysts were found to be more active per total adsorbing site than Pt/HUSY which is proposed to be due to an augmentation in the efficiency of spilled over hydrogen diffusion related to the proximity between Pt clusters and acid sites. The intervention of Lewis acid sites in a greater extent than that measured by pyridine adsorption may also contribute to this higher activity of Pt/HBEA catalysts. These results reinforce the importance of model reactions as a closer perspective to the relevant catalyst properties in reaction conditions.
- Interplay of the adsorption of light and heavy paraffins in hydroisomerization over H-beta zeolitePublication . Mendes, Pedro S. F.; Chizallet, Céline; Pérez-Pellitero, Javier; Raybaud, Pascal; Silva, João; Ribeiro, M Filipa; Daudin, Antoine; Bouchy, ChristopheThe effect of pressure on the activity and selectivity of well-balanced Pt/H-beta zeolite bifunctional catalysts in n-hexadecane (n-C16) hydroisomerization was studied. The turnover frequency per Brønsted acid site of the catalyst decreased when the total pressure was increased due to the lower concentration of olefins at equilibrium, in line with the classical bifunctional mechanism. Conversely, when the total pressure was increased, the C16 isomer yield unexpectedly decreased in contradiction with the pure kinetic/ thermodynamic effect of olefin pressure on the reaction rates. Thanks to grand canonical Monte Carlo (GCMC) calculations, non-idealities in adsorption behavior in the zeolite micropores were revealed when a representative reaction medium was considered. Via mechanistic kinetic simulations combined with GCMC simulations for the relevant intermediate concentrations, the pressure effect on catalyst selectivity is proposed to be due to the interplay between the (light) cracked products and the (heavy) hexadecanes in the pores of beta zeolite which leads to a pressure dependency on the adsorption behavior.
- Nanoscale insights into Pt-impregnated mixtures of zeolitesPublication . Mendes, Pedro S. F.; Taleb, Anne-Lise; Gay, Anne-sophie; Daudin, Antoine; Bouchy, Christophe; Silva, João; Ribeiro, M. FilipaA 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.
- Quantification of metal-acid balance in hydroisomerization catalysts: a step further toward catalyst designPublication . Mendes, Pedro S. F.; Silva, João; Ribeiro, M. Filipa; Duchene, Pascal; Daudin, Antoine; Bouchy, ChristopheA methodology was developed to interpret the results of n-.paraffins hydroisomerization over bifunctional catalysts based on two simple kinetic models used consecutively. First, a macrokinetic model was used to obtain the corresponding turnover frequency over the acid sites and the maximum of C-16 isomer yield. Second, a dual-function model was used to correlate these catalytic descriptors to the ratio of metal to acid sites of the catalyst. To illustrate the methodology, Pt/HBEA and Pt/HUSY catalysts with different Pt loadings were evaluated. The impact of metal-acid balance on the catalytic turnover frequency and the maximal C-16 isomer yield were adequately captured for the bifunctional HUSY and HBEA catalysts. Moreover, the parameters of the dual-function model revealed to be intrinsic to the catalytic properties of the zeolite under the scope. This methodology is believed to be of interest for information-driven catalyst design for the hydroisomerization of n-paraffins.
- Synergies, cooperation and other effects: a review for hydroconversion catalystsPublication . Mendes, Pedro S. F.; Silva, João M; Ribeiro, M Filipa; Daudin, Antoine; Bouchy, ChristopheMixing different components is the basis of chemistry. In heterogeneous catalysis, combinations of different active phases are frequently used, but involved in a mysterious way similar to alchemy. The language employed to describe such systems is typically imprecise due to the lack of clear definitions and efforts to quantify the observed performances. In this review, focused in the particular case of bifunctional catalysis, a set of definitions is provided aiming at an analytical and coherent examination of the literature. The works on hybrid zeolite based catalysts for hydroconversion processes are reviewed, and an attempt to classify the various origin of potential cooperation effects is provided. By such means, we hope to provide not only a clear view on the current state-of-the-art on hydroconversion catalysts but also a common framework for a more rigorous investigation of possible synergies of hybrid catalysts over all catalysed reactions.