Christophe Bouchy

Impact of extreme downsizing of *BEA-type zeolite crystals on n-hexadecane hydroisomerization

A series of *BEA-type zeolites with crystal sizes ranging from few nanometers to micrometers were synthesized. These materials were then transformed into bifunctional catalysts by platinum loading and tested in n-hexadecane isomerization. The behavior of Pt/H-*BEA catalysts depends on several parameters such as the balance between the platinum and acid functions, the metal particle size and the zeolite crystal size. In order to design an “ideal” bifunctional catalyst, the metal and acid sites must be well balanced to avoid the cracking of isomer products at low conversion, the metal must be well dispersed in order to inhibit the hydrogenolysis side reactions, and the crystal size must be as small as possible to maximize the isomer yield. But, the extreme decrease of the zeolite crystal thickness to few zeolite unit cells lowers the turnover frequency of acid sites and favors the metal sintering.

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.

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.