Isolated Pt Species Anchored by Hierarchical-like Heteroatomic Fe-Silicalite-1 Catalyze Propane Dehydrogenation near the Thermodynamic Limit

Abstract

The atomic dispersion of precious metals accompanied by maximum atom utilization can provide specific chemical properties compared to nanoparticles and clusters that have attracted widespread interest. The selection of a suitable carrier to stabilize platinum atoms while maintaining high stability and propylene selectivity is of great challenge for propane dehydrogenation reactions operating at extremely high temperatures. Here, we report a conceptually designed catalyst comprising isolated Pt atoms stably bonded through skeleton O in a hierarchical-like heteroatomic ferrosilicate zeolite (H-Fe-S-1−3; denoted as "Fe-3"), capable of achieving high propane conversions at different temperatures and atmospheres close to the thermodynamic limit. No significant deactivation was observed for 3 days in a pure propane atmosphere at 580 °C, outperforming most of the cuttingedge Pt-based catalysts. The moderate acidity of Fe-3 and anchoring of hydroxyl species other than silanol nests were responsible for maintaining a suitable C−H break rather than an excessive C−C cleavage capacity and a high degree of Pt dispersion, respectively. X-ray absorption spectra and atomically resolved high-angle annular dark-field electron microscopy demonstrated major atomic dispersion of Pt species, along with complementary density functional theory calculations to determine the structure of �Si−O−Pt−O−Fe� corresponding to the T4 location as the key active site. Pt anchoring by sites other than the T4 site with analogous energies, such as T6, could be accountable for the observation that "cluster-like Pt species" are essentially composed of isolated Pt atoms not interacting with each other.

Recent Progress in Metal-Molecular Sieve Catalysts for Propane Dehydrogenation

“…The hierarchical zeolites combine the advantages of fast diffusion of mesopores and the confinement of micropores, which provides a viable pathway to construct efficient PDH catalysts. As summarized in Figure and Table , researchers have developed a set of promising Pt-hierarchical zeolite catalysts which obtain impressive propylene selectivity and catalytic lifetime at thermodynamic conversion. ,,− In the past decade, there is a significant progress of the Pt-free catalysts in PDH performance. Both industrially relevant propylene productivity and selectivity can be obtained using several promising Co/Zn-based molecular sieve systems, − ,, while the typical lifetime of these systems is still inferior to Pt-based systems, especially the Pt-hierarchical zeolite catalysts.…”

“…The diffusion limitations could be evaluated via the Weisz-Parter criterion, intelligent gravimetric analyzer, probe-DRIFT spectra, etc. 61,62,93 The high temperature of the PDH reaction makes the quantitative determination of the diffusion coefficient under reaction conditions difficult. One recent study employs theoretical force-field molecular dynamics (FFMD) simulations to indicate the propane molecules have no obvious diffusion limitations in typical MFI (S-1) and BEA (Si-Beta) topological zeolites.…”

“…146 Similar to the ligand-assisted synthesis of metal-containing bulk zeolites, the addition of metal precursor in the synthesis of hierarchical zeolites also produces encapsulated metal species. Recently, Ji et al 62 reported the incorporation of Fe species within S-1 framework can lead to a hierarchical-like morphology for the Fe-S-1 catalyst. The time-resolved 2,3dimethylpyridine (DMPyr)-DRIFT experiments indicate a faster diffusion efficiency on hierarchical-like catalysts than bulk counterparts.…”

“…Encapsulating of a subnanometric or single-site active center within zeolites is one of the hottest topics in the PDH area because the strong confinement effect prevents metal/oxide species from sintering and leaching. 17,20,21,23,24,37,62,84,103,107 On the other hand, the subnanometric pores of zeolites may cause diffusion limitations especially for the cases with a large granular size and small pore size (≤8-ring, comparable or smaller than the propane molecular size of 0.47 nm 86 ). 148,149 In this context, although several FAU and CHA topologic zeolite-based materials were developed for the PDH reaction, 57,108,109 the diffusion limitations and their effect on activity/selectivity should be carefully checked.…”

“…Encapsulating of a subnanometric or single-site active center within zeolites is one of the hottest topics in the PDH area because the strong confinement effect prevents metal/oxide species from sintering and leaching. ,,,,,,,,, On the other hand, the subnanometric pores of zeolites may cause diffusion limitations especially for the cases with a large granular size and small pore size (≤8-ring, comparable or smaller than the propane molecular size of 0.47 nm). , In this context, although several FAU and CHA topologic zeolite-based materials were developed for the PDH reaction, ,, the diffusion limitations and their effect on activity/selectivity should be carefully checked. The diffusion limitations could be evaluated via the Weisz-Parter criterion, intelligent gravimetric analyzer, probe-DRIFT spectra, etc. ,, The high temperature of the PDH reaction makes the quantitative determination of the diffusion coefficient under reaction conditions difficult. One recent study employs theoretical force-field molecular dynamics (FFMD) simulations to indicate the propane molecules have no obvious diffusion limitations in typical MFI (S-1) and BEA (Si-Beta) topological zeolites .…”