Precise Construction And Regulation Of Silanol-induced Cobalt Sites In Molecular Sieves For Propane Dehydrogenatio

Propane dehydrogenation（PDH）is a cost-effective and environment friendly process for producing propylene,and the byproduct H₂ can also be used in hydrogen energy projects,which is of major strategic importance to green and low-carbon transformation of national energy.Nevertheless,the technology is generally relied on the catalysts of precious Pt-based and toxic Cr-based.However,their scale-up applications are constrained by the complex and diverse Co speciations on the surface of supports,which adversely affects the catalytic activity and propylene selectivity.In contrast,Co-based catalysts are thought to be one of the most promising alternative catalytic materials for research and development due to their low cost,non-toxic,and comparable C-H bond activation ability with noble metals.As a result,this paper chooses siliceous zeolite as the research object,adopting a synergistic induction strategy of ligand protection and silanols to explore the interaction mode between Co species and silanols,constructing homogeneous and stable Co active sites to improve propane dehydrogenation performance,and further clarifying the dehydrogenation mechanism.This work can offer a new suggestion for the design of effective non-precious metal PDH catalysts.In this study,uniformly stable hydroxylated cobalt（II）silicate species active sites（defined as{OH-Si-（OH）-Co-O-Si-（OH）₃}）embedded within the silicalite-1（S-1）zeolite framework were successfully fabricated by the assistance of Co precursors and silanols via a facile hydrothermal synthesis.Under comparable conditions,a superior catalytic PDH performance with attractive propylene formation rate of 14.6 mmol·g_cat^-1·h^-1,propylene selectivity higher than 93%and very low deactivation rate of 0.0206 h^-1 after7 h is achieved over the Co@S-1（EDA）catalyst.Furthermore,the catalytic performance can be restored to the same as that of the fresh catalyst after 3 successive regenerations.In situ FTIR spectra analysis further reveals the dynamic changes in the characteristic absorption bands associated with the hydroxylated Co²⁺silicate sites and the intermediate product（2-propyl）during the PDH reaction and a reaction mechanism is proposed accordingly.In view of the complex diversity of silanols on the surface of siliceous zeolite,clarifying the interaction between different silanol types and Co species is a key challenge to realize the precise and controllable construction of Co species active sites,which can provide an important support to improve the PDH performance of catalysts.Given this,Beta zeolite is selected as the research object in this paper,and successfully achieve the quantitative control of geminal silanols and silanol nests through the dual strategies of Si/Al ratio modulation and acid treatment.Subsequently,a series of Co/Si-Beta zeolite catalysts were prepared by solid state grinding synthesis using acetylacetone Co precursors.The results show that the Co O nanoclusters bound to the silanols of the siliceous zeolite enhance the PDH performance,and the prepared 1Co/Si-Beta（500-12）samples form homogeneous dispersed Co O nanoclusters with the sizes of about 4.95 nm,which exhibits a more excellent propylene formation rate(25.0 mmol·g_cat^-1·h^-1)and propylene selectivity（ca.96%）.In contrast,the catalytic stability of the Co O clusters confined in the silanol nests is poor,with deactivation rate about 0.1551 h^-1 after 8 h.This is mainly due to the rapid deactivation of the catalyst during the PDH reaction when the Co O species are aggregated into large particles leading to the occurrence of side reactions,which produce coke blocking the pores or attaching to the active sites.Further studies confirm that the Co O nanoclusters interacted with geminal silanols are more favorable for PDH reaction,and the silanol nests simultaneously produce a synergistic effect to jointly anchor the Co species and stabilize their distribution within the zeolites,which further promote the catalyst stability and propylene selectivity.