The controlled synthesis of metal-acid bifunctional catalysts: The effect of metal:acid ratio and metal-acid proximity in Pt silica-alumina catalysts for n-heptane isomerization

Abstract

Several series of metal-acid bifunctional catalysts with controlled metal:acid ratios and metal site-acid site proximity were evaluated for n-heptane isomerization. Through the study of metal deposition reported in the companion paper, proximity could be achieved at four distinct scales; atomic, nano-, micro-, and millimeter scales. In the first two catalyst series, atomic and nanometer scale intimacy was obtained by depositing Pt onto acidic silica-alumina supports (Pt/Al-Si). It is demonstrated that poorly dispersed Pt/silica-alumina catalysts with nanometerscale proximity displayed a greater degree of bifunctionality than highly dispersed catalysts with atomic-scale proximity. In the latter two series of catalysts the scale of intimacy was stretched to micrometers using physical mixtures, and to millimeters by separating layers of nonacidic Pt/silica and metal free silica-alumina. Good bifunctionality is maintained at micrometer-scale intimacy and breaks down only at the millimeter scale. The best bifunctionality is achieved at very high acid to metal site ratios; results indicated that a single metal site can supply several hundred acid sites. Optimized bifunctional performance of Pt/silica-alumina catalysts for n-heptane isomerization requires a high acid-to-metal site ratio with nanometer to micrometer scale site proximity. The control of the numbers and proximities of metal and acid sites achieved in this work can be extended to many other metal-acid bifunctional reactions.

Nickel–zeolite composite catalysts with metal nanoparticles selectively encapsulated in the zeolite micropores

“…Metal-zeolite catalysts often exhibit complex behaviour, because of their multicomponent composition and structure. The concentrations and intrinsic activity of the metal and acid sites, and in particular, their localization are of utmost importance for the catalyst activity, selectivity and stability [20][21][22][23][24][25][26]. The reaction selectivity is often affected by the so called "site intimacy" [21].…”

Versatile Roles of Metal Species in Carbon Nanotube Templates for the Synthesis of Metal–Zeolite Nanocomposite Catalysts

“…Adsorption of nitrogen in the range of P/P0 = 0.5 -1.0 and relevant hysteresis can be explained by mesoporosity developed in the presence of cobalt, nickel or magnesium oxides supported on CNT. Note that almost no increase in the zeolite mesoporosity has been observed when metal-free CNT (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) or CNT were used as mesoporous hard templates (Table 1). The presence of metal species on CNT is therefore templates.…”

“…The distribution of metal component within the zeolite is of paramount importance for the resulting catalytic performance, which can be strongly affected by the distance between metal nanoparticles and acid sites, the so-called "site intimacy" 4 and by diffusion of the intermediates during their transport from one site to another. 9,10,11,12 The very small pore size of zeolites (~1 nm) imposes diffusional limitations for many catalytic reactions, in particular, for those involving bulky molecules. In order to overcome these, numerous efforts have made to synthesize nano-sized zeolite crystals 13 , extra-large pore zeolites 14 , or mesoporous zeolites 15,16 , the latter are often referred to as hierarchical zeolites.…”

Platinum‐promoted fibrous silica Y zeolite with enhanced mass transfer as a highly selective catalyst for n ‐dodecane hydroisomerization

“…Conversely, hydroisomerization of n‐ alkanes can also proceed via hydrogen spillover mechanism, with the metallic sites only involved in dissociative hydrogen adsorption . A concerted effort has been made to enhance the isomer yield of C 4 ‐C 7 hydroisomerization for improving the octane number of gasoline . This is basically due to increasingly stringent environmental protection regulations with regards to the use of aromatics and olefins in gasoline for octane number enhancement.…”

“…4,5 A concerted effort has been made to enhance the isomer yield of C 4 -C 7 hydroisomerization for improving the octane number of gasoline. [6][7][8] This is basically due to increasingly stringent environmental protection regulations with regards to the use of aromatics and olefins in gasoline for octane number enhancement. Additionally, production of high cetane diesel fuel 9 and improved cold flow properties, such as freezing point, pour point, and viscosity, requires an efficient catalyst to facilitate hydroisomerization of C 7 -C 15 n-paraffins.…”