Research On The Catalysts For High-Efficiency Dehydrogenation Of Ethane

Ethylene is the core of the petrochemical industry and is produced industrially mainly by naphtha steam cracking.With the massive exploration and development of shale gas,the proven reserves of light alkanes such as ethane and propane,which account for up to 20%of the total,have increased greatly.Using ethane as a feedstock to produce ethylene through ethane dehydrogenation is a promising technology route since it is beneficial for decreasing energy consumption,lowering separation costs,and increasing ethylene yields compared to the traditional ethylene production pathway.CO₂-assisted ethane oxidative dehydrogenation can simultaneously convert greenhouse gases and produce high value-added chemicals,i.e.ethylene.However,with high-temperature conditions of reduction and reaction,the catalysts are prone to sintering and accumulating coke depositions,thus leading to severe deactivation.At the same time,the occurrence of side reactions will result in low ethylene selectivity.Therefore,it is of great importance to develop catalysts with high efficiency and stability for direct dehydrogenation of ethane and oxidative dehydrogenation of ethane reactions.Herein,the Pt-based catalysts are used in the ethane dehydrogenation reactions.In order to improve the activity,ethylene selectivity,and catalyst life in the process of ethane dehydrogenation,the effects of different promoters on the performance of ethane direct dehydrogenation reaction are systematically investigated.The efficient and stable Pt-based catalysts for CO₂-assisted oxidative dehydrogenation of ethane are investigated and developed.Moreover,the structure-performance relationship between structural properties and catalytic activities of different Pt-based catalysts are investigated by combining a series of characterization methods and experimental results.The main works are shown as follows:（1）A Zn modified Pt nanoparticles confined within silicalite-1（S-1）zeolite catalyst（Pt Zn@S-1）for ethane direct dehydrogenation（EDH）reaction was synthesized by hydrothermal synthesis using the Pt Zn/Si O₂precursor without any protective ligands,and the catalytic performance and structural properties were characterized and evaluated.The Pt Zn@S-1catalyst achieved extremely high catalytic performance under the reaction conditions of 550°C and 0.3 bar ethane partial pressure(e.g.,15.9%conversion which was close to the thermodynamic equilibrium limitation,99.5%ethylene selectivity,and 0.001 h^-1 the deactivation rate).Combined with the performance evaluation and characterization results,it was revealed that the electron-rich Pt sites with stable electronic environment formed by Zn promoter introduced,and the dilution effect of Zn and the trapping of zinc and platinum species by Si-OH groups in the form of（≡Si-O-Zn）_xPt complex could inhibit migration and aggregation,resulting in smaller metal particles,increased anti-sintering ability and enhanced anti-reduction and coke-resistance abilities,avoiding deep dehydrogenation and the formation of heavy coke depositions.（2）Based on the previous research,the effects of different kinds of promoters（K,Sn,Ga,In）on the performance of Pt-based catalysts for EDH reaction were investigated.It was found that KPt@S-1 had significantly higher initial reaction activity than Pt@S-1 catalyst（11.8%）,with an initial ethane conversion of 15.7%,and a substantially higher reaction stability with a deactivation constant of 0.0026 h^-1.Then,replacing K with Sn,Ga,and In,the initial ethane reaction activity of the Pt Ga@S-1catalyst was reduced compared to that of the monometallic catalysts,which was 5.72%,while the Pt Sn@S-1 and Pt In@S-1 catalysts also showed a obvious increase in ethane initial activity compared to the monometallic Pt@S-1 catalysts,with ethane initial activities of 15.6%and 15.3%,respectively.Furthermore,attempts were made to add dual promoters to the catalytic system to obtain the KPt Sn@S-1,KPt In@S-1 and KPt Ga@S-1 catalysts,respectively.for the KPt Sn@S-1 and KPt In@S-1 catalysts,the introduction of the dual promoters not only maintained a high ethane initial activity,but also the stability of the catalysts was significantly improved with extremely low deactivation constants of 0.0003 h^-1 and 0.0002 h^-1,respectively.The initial ethane conversion and stability of KPt Ga@S-1catalysts were significantly improved compared with that of Pt Ga@S-1catalysts,and the deactivation constants were reduced to 0.0009 h^-1.Through various characterizations,it was found that the higher catalytic stability of the catalysts with the introduction of the Sn and In promoters could be attributed to the fact that Sn had an anchoring effect on the Pt,which avoided the migration and aggregation of the Pt during the course of EDH reaction,and the strong interaction between In and Pt,which can greatly reduce the formation of coke deposits and inhibit the production of graphitic coke.In contrast,the introduction of Ga leads to the formation of the Ga O_x Lewis acid site thus leads to aromatics formed,and the weak dilution and stabilization of Pt macroparticles by Ga is the main reason for the relatively poor stability of KGa Pt@S-1.（3）The catalyst of CoWO₄ supported Pt metal with different contents were prepared and examined in CO₂-assisted oxidative dehydrogenation（CO₂-ODH）of ethane,and the evolution of the metal species during the reaction process was investigated in depth.It was found that the catalyst with 0.5 wt%Pt supported on CoWO₄ had an initial activity of 4.85%for ethane and showed good stability for the CO₂-ODH reaction.When the Pt loading increased to 1.0 wt%,the initial activity of the reaction was increased to 6.47%,and the ethylene selectivity was 53.41%,and the stability of the reaction was maintained well.However,after further increasing the Pt loading to 2.0 wt%,the catalyst stability decreased significantly.Combined with XPS,HRTEM,in situ（C₂H₆+CO₂）-DRIFTS,TG-DTA,and Raman characterizations,it was found that the introduction of CO₂ played a key role on inhibiting the complete reduction of Co species,as well as inhibiting the migration of Pt species and coke deposition,which was ascribed to that the introduced CO₂ combined with reductive H atoms and thus formic acid intermediate formed then effectively avoided over-reduction of metal species,thus high stability was obtained on the 1.0Pt/CoWO₄ catalyst.