Design And Performance Of Pt/ZSM-22 Catalysts For Hydroisomerizarion Of Long-chain N-paraffins

Hydroisomerization of long-chain n-paraffins has been proved with application value in oil upgrading&dewaxing.The design&synthesis of bi-functional catalysts with high performance in hydroisomerization remains challenging.The main side reaction in hydroisomerization,cracking,could be effectively restrained by well-balanced metal&acid function as well as proper shape-selectivity of the catalyst.This work focuses on preparing feasible Pt/ZSM-22 catalyst for hydroisomerization of long-chain n-paraffins.The crystallization process&post-synthesis process of ZSM-22supporter has been studied and tailored.Pt loading method has also been explored.The shape-selectivity,acid function&metal function of Pt/ZSM-22 catalyst has been related to its performance in hydroisomerization.This work aims at providing effective methodology for designing catalyst for hydroisomerization of long-chain n-paraffins.Firstly,a two-step crystallization process is proposed,in order to produce ZSM-22 of shorter rod-like nano crystals with lower acidity,compared with conventional hydrothermal crystallization process.The growth of ZSM-22 rod-like nano crystals is restrained by shortening high temperature hydrothermal crystallization.The ZSM-22nano crystals are covered by a mesoporous silica shell formed in low temperature hydrothermal crystallization.As a result,the size of as-prepared ZSM-22 crystals is reduced,new pore-mouths were created,and the external acidity was lowered.However,the aggregation of ZSM-22 nano crystals reduces pore-mouths,increases microporous acidity and facilitates cracking.In order to restrain the aggregation,a seed silanization process is proposed.The pre-crystallization,silanization and hydrothermal crystallization process could effectively restrain the growth&aggregation of as-formed ZSM-22 nano crystals.The external surface area of as-prepared ZSM-22 could be as high as 117 m²/g.As a result,more pore-mouths are created and acidity is tailored.After loaded with Pt,the Pt/ZSM-22 achieved higher activity&selectivity in n-dodecane hydroisomerization.Total isomer yield could be as high as 72.3%,and the mono-/multi-isomer yield ratio is doubled.Secondly,a duel-protected alkali treatment process is proposed for higher ZSM-22 yield and tailored mesopore size,compared with conventional post-synthesis process.Under the protection of in-situ microporous structure directing agent,as well as the drilling effect caused by mesoporous structure directing agent,the desilication efficiency is increased by 5 times.As-formed mesopores center at 3.2-3.5 nm.As a result,more pore-mouths are created.After loaded with Pt,the catalyst achieves much higher mono-branched isomer yield,and total isomer yield is increased to 73.4%.A controllable NH₄⁺exchange method under the protection of microporous structure directing agent is proposed to selectively tailor the acidity distribution and microporous acidity.As a result,the Br（?）nsted acidity is effectively exchanged at pore-mouths and on external surfaces of ZSM-22.The isomer production distribution over Pt loaded catalysts indicates that,the microporous Br（?）nsted acidity near pore-mouths enhances the formation of multi-branched isomers through chemisorption,and the reduction of microporous Br（?）nsted acidity deep inside micropores substantially reduces the secondary cracking through（s,s）β-scission.Thirdly,competitive ion-exchange and consecutive incipient wetness impregnation are proposed to load Pt onto ZSM-22.The effect of alkali metal ion on metal-acidity balance of Pt/ZSM-22 is explored.Through competitive ion-exchange,alkali metal ion could effectively increase the dispersion and lower the loading amount of Pt,while retaining the hydrogenation/dehydrogenation balance of Pt/ZSM-22catalyst.