Synthesis And Application Of Phosphorus Doped Boron Nitride Catalyst For Dehydrogenation Of Ethylbenzene To Styrene

As one of the most important basic raw chemicals in modern petrochemical industry,styrene has been extensively used as a monomer for various macromolecular organic polymer syntheses such as rubber and resin,which could create huge economic and social benefits.The annual output of styrene in China is close to 10 million tons.The industrial production of styrene includes direct dehydrogenation of ethylbenzene,oxidative dehydrogenation of ethylbenzene as well as the propylene oxide and styrene monomer co-production process.Among them,the direct dehydrogenation of ethylbenzene is the primary scaled technology for industrial production of styrene because of its high selectivity.However,the potassium-containing iron oxide（Fe-K）catalyst used in this process is very easy to be deactivated by carbon deposition,thus excess superheated steam is inevitably co-fed with ethylbenzene to maintain its stability.In addition,the use of heavy metal auxiliaries in Fe-K catalyst might also causes some environmental hazards.Therefore,the development of novel catalysts and reaction process with satisfactory activity,selectivity and environmental friendliness is the key to improve the production level of styrene industry in China.In recent years,metal-free catalysis has shown great potential in a wide range of reactions including dehydrogenation of alkanes.Hexagonal boron nitride（h-BN）,due to its stable physical and chemical properties and high thermal stability,as well as the stronger polarity and adjustability of B-N bond in the six-membered ring frameworks than C-C and C-N bond,has shown a stronger potential for catalytic applications.In this thesis,h-BN with stable physical and chemical properties was chosen as the research focus.Through functionalization and molecular local structure recombination,a novel robust phosphorus-doped h-boron nitride（PBN-x）catalyst with bifunctional function groups containing N₂P=O and N₃P-OH was constructed,and the catalytic performance of PBN-x was studied both in the oxidative dehydrogenation（ODH）and direct dehydrogenation（DDH）of ethylbenzene.In addition,the reaction mechanism of PBN-x catalyst in DDH of ethylbenzene was revealed by the combination of characterization and density functional theory（DFT）calculation.Moreover,aiming to expand its industrial application,the structure,stability and regeneration of structured PBN-x catalyst were further studied.The main contents and conclusions are as follows:（1）A novel PBN-x catalyst was farbricated by urea,boric acid and etidronic acid monohydrate（HEDP）through a recrystallization-calcination two-step process.Followed,the structure and surface species of PBN-x catalysts were analyzed by Brunauer-Emmet-Teller（BET）,Atomic force microscope（AFM）,X-ray photoelectron spectroscopy（XPS）,Nuclear magnetic resonance（NMR）,etc.The results show that PBN-x delivered a two-dimensional nano-lamellar structure similar to h-BN.The thickness of single PBN-x layer is 1.2～1.5 nm and the specific surface area is more than 300 m²g^-1.Besides,the characterization of XPS,¹¹B NMR and ³¹P NMR indicated that the B atoms at Bayer and Corner positions in h-BN were substituted by P atoms,forming N₃P-OH species as the Br（?）nsted acid active sites and N₂P=O species as the Br（?）nsted base active site in PBN-x catalyst,respectively.The number of these two species is related to the ratio of P to B in the precursor.Except for the P species,B-OH,B-NH₃and a small amount of C=O,C-O species were also formed on the surface of PBN-x catalyst.（2）The catalytic performance of PBN-x catalyst in ODH process of ethylbenzene was studied.The results showed that the initial ethylbenzene conversion(x _EB)and styrene selectivity(S_ST)of PBN-1.5catalyst could reach 87.01%and 66.11%under a ethylbenzene/oxygen（EB/O₂）molar ratio of 1:3 and an reaction temperature of 500°C.Temperature programmed desorption of NH₃and CO₂atmosphere（NH₃-TPD and CO₂-TPD）further confirmed that the initial catalytic performance was related to the amount of acid-base activity sites on the surface of PBN-x catalyst.The more amount of the acid-base sites,the higher the initial catalytic performance thereon.In addition,it was found during the investigation of reaction temperature and EB/O₂molar ratio that the higher the reaction temperature and oxygen concentration in the feed,the higher the initial x _EBof PBN-x catalyst,but the faster its deactivation rate.Instead,the performance of PBN-1.5 catalyst remained basically stable when the oxygen content was very low,such as the EB/O₂molar ratio was 2:1.Moreover,it was inferred from inductive coupled plasma emission spectrometer（ICP）,thermogravimetric analysis（TG）,and XPS characterization of fresh and used PBN-1.5 catalysts that the P species introduced into the PBN-x material could be easily removed from the h-BN matrix under ethylbenzene ODH conditions due to the large difference between catalyst preparation and reaction atmosphere,which would change the main framework of the catalyst into a carbon doped h-BN（BCN）like structure,resulting in a rapid decrease in catalytic performance.The kinetic parameters of the stable stage of PBN-1.5 catalyst showed that the activation energy（E_a）is 83.04 k J mol^-1,which was very close to that of BCN in ODH of ethylbenzene.It also proved that the structure of PBN-1.5 has changed at the later stage.（3）Based on the results of ethylbenzene ODH process,we further tested the catalytic performance of PBN-x in oxygen-and steam-free DDH process of ethylbenzene.The results showed that P species could exist stably on h-BN substrate in the absence of oxygen,thus the PBN-1.2catalyst prepared with HEDP as P source obtained x _EBof 62.35%and S_STof 92.74%under 600°C DDH process,with an the styrene formation rate(r _ST)of 17.45 mmol _STg^-1h^-1,much higher than that of carbon based catalysts reported in the literature.Further,it can still maintain more than6.0 mmol _STg^-1h^-1after 100 h reaction.The kinetic calculation showed that the E_aof PBN-1.2 in DDH of ethylbenzene was 125.04 k J mol^-1.In addition,according to the relationship between oxygen concentration and catalyst deactivation rate in ODH process,we carefully designed a regeneration process with low oxygen concentration,which realized a better regeneration of PBN-1.2 catalyst for ethylbenzene DDH.（4）The characterization of fresh and used PBN-x catalysts as well as the DFT calculation indicated that the surface species such as B-OH,B-NH₃and C=O in ethylbenzene DDH process were almost negligible,while N₂P=O and N₃P-OH were proved to be the active sites.The specific catalytic mechanism was as follows:firstly,the N₂P=O species catalyzed the breaking of C-H（α-H）bond of ethyl into vinyl,while N₂P=O itself snatched the broken H atom and converted into N₃P-OH.Subsequently,N₃P-OH reacted with vinyl at high temperature to promoted the rupture of vinyl’sβ-H,and then released the capturedα-H to desorbed into H₂,and itself returned to N₂P=O thereon.During the reaction process,N₂P=O and N₃P-OH converted into each other,and synergistically catalyzed the Dehydrogenation reaction.What’s more,the characterization of fresh and used PBN-x catalysts also showed that the N₃P-OH was much more prone to coke accumulation,thus the rate-determining step of ethylbenzene DDH was considered to be the H-H association-desorption process.（5）To further promote the industrial application of PBN-x catalyst,a structured catalyst PBN@Al₂O₃（N）with commercial Al₂O₃as support was synthesized via an“impregnation-calcination”two-step technique.The long-term performance showed that PBN@Al₂O₃（N）could run stably for at least 800 h under 600°C with a stable x _EBof 93.51%and S_STover 95%,respectively.Combined with the characterization results,the excellent performance of PBN@Al₂O₃（N）could be attributed to the synergistic effect of in situ grown thin PBN layers and the B@Al₂O₃（N）support,due to the fully-exposed N₂P=O and N₃P-OH active sites as well as the optimum acid-base properties of the BN_xO_1-x/Al₂O₃species.In addition,the excellent mass transfer and heat transfer properties of structured catalyst also promoted the excellent performance of PBN@Al₂O₃（N）.Moreover,compared with other metal-free catalysts,PBN@Al₂O₃（N）also has the advantages of simple preparation,low cost,strong ability to strain complex reaction conditions and satisfying regeneration performance.Therefore,it can be said that PBN@Al₂O₃（N）structured catalyst has a very broad industrial application prospects.