Preparation And Catalytic Performance Of Micro/Mesoporous Composite Catalyst For N-Alkane Isomerization

Hydroisomerization of n-alkanes is one of the most crucial techniques to upgrade the qualities of oils,and it plays an important role in the fields of fossil energy and renewable resources processing.This process aims to produce high-octane gasoline components and middle distillates with good cold flow properties,replacing some environmental-unfriendly aromatic and olefin compounds in oils.Nowadays,the isomerization technologies of n-alkanes with low carbon numbers(C₅-C₆)have been well established,however,there is no proven isomerization technology for n-alkanes with more than six carbon atoms(?C₇),which is mainly due to their prominent inclinations to crack.The key to the hydroisomerization of heavy n-alkanes is the developments of bifunctional catalysts with high performances.Micro/mesoporous composite molecular sieves can combine the both advantages of microporous zeolites and mesoporous materials,in which the zeolitic intrinsic strong acidity can be partially preserved and concurrently mesoporous frameworks with large-size channels can be introduced.Hence,bifunctional catalysts of noble metal-loaded micro/mesoporous composites are considered to be among the most prospective isomerization catalysts for n-alkanes with more than six carbon atoms.In this study,series of micro/mesoporous composites Y/MCM-41(zeolite Y is the main microporous phase,while MCM-41 is the primary mesoporous phase)were prepared using different synthesis methods.After loading noble metal of platinum,bifunctional catalysts with bi-porous structures(Pt-Y/MCM-41)were obtained.X-ray diffraction,N₂ adsorption,scanning electron microscope,transmission electron microscope,laser particle-size distribution,pyridine FT-IR and H₂ pulse chemisorption were adopted to characterize the physicochemical properties of samples.The hydrogenation/dehydrogenation metal functions of bifunctional catalysts were quantitatively described by the concentration of accessible Pt metal(C_Pt),while the isomerization/cracking acid functions were quantitatively described by the concentration of protonic sites(C_H).Furthermore,C_Pt/C_H was chosen as representative of the balance between metal and acid functions.Hydroisomerization of model compound n-decane(n-C₁₀)was used to evaluate the catalytic performances of bifunctional catalysts.Micro/mesoporous composite YM-c was synthesized without discernable destructions of zeolitic crystal frameworks by method of CTA⁺ion-exchange pretreatment to zeolite Y.As a parallel contrast,micro/mesoporous mechanical mixture YM-m was prepared through the physical pulping.In order to investigate the effect of mesoporous MCM-41 incorporation on isomerization behaviors of n-C₁₀ over bifunctional catalysts with bi-porous structures,catalytic performances of four catalysts,including Pt/MCM-41,Pt/Y,Pt/YM-c and Pt/YM-m,were compared.Results showed that the Pt/MCM-41 catalyst was most selective to isomerized products,followed by Pt/YM-c and Pt/YM-m,while the Pt/Y catalyst had the relatively lower isomerization selectivity than other three.The presence of mesoporous phase MCM-41 contributed to the improvement of systemic metal-acid functional balance(C_Pt/C_H),and meanwhile,mesoscale channels had positive impacts on the rapid diffusion of bulky molecules.In this work,mesoporous composite phase MCM-41 that overgrew along the zeolite outer layer can improve the migration tracks of alkene intermediates over bi-porous catalysts,which contributed to inhibiting secondary reactions.Microporous-mesoporous structural distributions in composites can be regulated through changing the additive amounts of zeolite Y during the CTA⁺ion-exchange pretreatment process.Micro/mesoporous composites with different microporous-mesoporous structural distributions were synthesized,including MY_0.5,MY_1.0,MY_2.0 and MY_3.0,aiming to investigate the effect of structural distribution on the isomerization behavior of n-C₁₀over bi-porous catalysts.Results showed that the concentration of accessible Pt metal(C_Pt)was reduced with the decreasing relative content of mesoporous phase,while the concentration of protonic sites(C_H)gradually increased,which resulted in the downtrend of systemic C_Pt/C_Hvalue.This went against the bifunctional catalytic performance,causing the reduction of catalysts'abilities to inhibit the skeletal multiple branching and cracking reactions,and hence the yields of isomerized products decreased.On the other hand,increasing the relative content of microporous structures was not conducive to the rapid diffusions of bulky molecules,improving the probabilities of undesired secondary reactions.Original zeolite Y was treated using the Na OH alkali solution with a certain concentration,which aimed at the reduction of zeolite crystal particle sizes.The distribution of zeolite crystal particle size can be controlled by changing the alkali-treatment time.Three micro/mesoporous composites(MY-0,MY-2,and MY-5),respectively containing original and alkali-treated zeolite Y as the microporous phase,were prepared.Effects of zeolite crystal particle size distribution on isomerization behaviors of n-C₁₀over Pt-loaded micro/mesoporous composite catalysts were investigated.Results showed that catalysts of Pt/MY-2 and Pt/MY-5,containing alkali-treated zeolite Y as the acidic components,outperformed the Pt/MY-0 catalyst,of which protonic acid function was provided by original zeolite,in terms of isomerization selectivity.For the three micro/mesoporous composites,their average zeolite particle sizes differed from each other,which means that alkene intermediates accessing the zeolite pore mouths of different catalysts would be undergo different diffusion distances inside the microporous frameworks before divorcing from zeolite micropores.Shorter this distance was,less protonic sites intermediates can encounter between two Pt metal sites,which contributed to reducing the probabilities of skeletal multiple branching and cracking reactions.Method of alkali-leaching self-assembly was used to prepare micro/mesoporous composite.Dissolution degrees of zeolite Y crystals were determined by the alkali-treatment time.Three composites(YM-1,YM-2,and YM-3)with different alkali-treatment time were prepared.Effects of the alkali-treatment severity on the formation of mesoporous phase MCM-41 and Pt-loaded composite bifunctional catalytic performances were investigated.Results showed that increasing the alkali-treatment severity would aggravate the desilication occurring on the zeolite crystal surfaces,causing serious destructions of zeolitic microporous frameworks.It means that a lot of tiny silicon-aluminum species were generated,which contributed to the good self-assembly of MCM-41.In the regard of isomerization selectivity,catalysts of Pt/YM-2 and Pt/YM-3 significantly outperformed the Pt/YM-1 catalyst.Pt/YM-2and Pt/YM-3 possessed more potent inhibitions on skeletal multiple branching and cracking reactions.This is because that the higher relative contents of mesoporous MCM-41 in Pt/YM-2 and Pt/YM-3 were beneficial to the improvement of metal-acid functional balance(C_Pt/C_H).Meanwhile,zeolite crystals with small sizes,generated from original zeolite Y alkali-treatment,were in favor of isomerization selectivity.In addition,zeolite intracrystalline channels with large sizes had positive influences on the rapid diffusions of bulky molecules,which potentially decreased the probability of secondary reactions.