Tuning And Design Of Active Phase Of Ultra-deep HDS Catalysts For Inferior Diesel

The key to achieve ultra-deep hydrodesulfurization(HDS)of inferior diesels lies in the design and development of high performance HDS catalysts,and the key to solve this problem is the intensive understanding and fine-tuning of the composition and microstructure of the active phase.Based on the features of adsorption and hydroconversion of 4,6-dimethyldibenzothiophene on active phases with different compositions and microstructures,this paper revealed the characteristics of active phases for high-performance HDS catalysts.In turn,a new technical route was proposed to develop a novel W-based precursor hydrothermal deposition method for the preparation of catalysts.In addition,a novel approach to modify the acidity of the catalyst and the decoration degree of promoters by the addition of organic phosphorus compound was proposed,resulting in the technology for the preparation of highly active inferior diesel ultra-deep HDS catalysts.In this thesis,we firstly prepared pre-sulfided catalysts by chemical deposition,and constructed active phases with similar average length and dispersion of slabs but with different decoration degree of promoters and stacking layer numbers by surface atomic engineering.The structure–activity relationships between the active phase composition and stacking and the HDS activity and pathway selectivity of 4,6-DMDBT were investigated.4,6-DMDBT showed high HYDS path selectivity on the active phase with higher stacking number.And the better decoration of the Niatoms at the edge of the active phase formed more coordinatively unsaturated sites(CUS),which not only facilitated the direct desulfurization of 4,6-DMDBT,but also the hydrogenolysis desulfurization of the hydrogenation intermediates of 4,6-DMDBT.Density Functional Theory(DFT)calculations showed that multilayer stacking could provide more accessible chemisorption sites for the adsorption of sterically hindered sulfur-containing compounds.It was established that high active metal dispersion,high sulfidation,multilayer stacking and high auxiliary modification should characterize high performance ultra-deep desulfurization catalysts for inferior diesels.The direction for the catalyst design and preparation was indicated.The idea of preparing alumina-loaded NiW catalysts by surfactant-assisted preparation was proposed to overcome the defects of poor active metal dispersion and low sulfidation degree of catalysts prepared by the conventional impregnation method.In this strategy,conventional precursors W-anions have been converted into dodecyltrimethyldecapolytungstate(DTADT).DTADTs in aqueous solution were successfully deposited onto alumina under mild hydrothermal conditions.The hydrothermal deposition method ensured a uniform distribution of W species.Moreover,long-chain quaternary ammonium cation shell of as-prepared W-precursor not only weakened the metal-support interaction,but also acted as a dispersant to prevent the aggregation of W species after decomposition.The multiple effects facilitated the formation of highly dispersed WS2 slabs with enhanced stacking,and therefore yielded a larger number of accessible NiWS edge sites after Niincorporation.The DTADT-derived NiW catalyst exhibited the highest reaction rate constant of 6.26×10-7 mol·g-1·s-1 and the highest DDS/HYD path selectivity of 4.0.Based on the molecular structure and hydroconversion characteristics of 4,6-DMDBT,surfactant-dispersed W precursors were synthesized with the assistance of quaternary ammonium cations with different alkyl carbon chain sizes,and the structuredirecting effects of the W precursors structures on the microstructure of NiWS active phase were investigated.More residual carbon material formed by the decomposition of longer chains could act as an intermediate carrier to form higher sulfidation(55.6 %)and higher stacking layers(2.9 layers),and could also play the role of dispersants to achieve high dispersion of active metal(?W =0.28).The structure–activity relationships between the microstructure of NiWS active phases and activity and selectivity of 4,6-DMDBT HDS was investigated in conjunction with the analysis of 4,6-DMDBT HDS results.A microstructure-activity correlation indicated that the S vacancies at the NiWS edge sites promoted the π-adsorption of 4,6-DMDBT and C-S bond breaking of hydrogenated derivatives,and the those at the NiWS corner sites facilitated σ-adsorption and direct desulfurization of 4,6-DMDBT.Therefore,NiW catalysts with higher WS2 dispersion(?W= 0.28)could form more brim sites,which facilitated the enhancement of the prehydrogenation activity of 4,6-DMDBT with steric hindrance.The excellent WS2dispersion(0.28)and high average stacking number(2.9 layers)provided more the S vacancies at the NiWS edge sites for the hydrogenolysis of the prehydrogenated products(4,6-THDMDBT and 4,6-HHDMDBT)with weakened steric hindrance,thus exhibiting higher 3,3’-MCHT selectivity.To further enhance the decoration degree of promoter atoms,a novel organic additive,1-hydroxyethylidene-1,1-diphosphonic acid(HEDP),was introduced into the impregnating solution to prepare highly active HDS catalysts.Compared with phosphoric acid,the addition of HEDP improved the availability of surface nickel atoms and weakened the W-Al2O3 interaction,which facilitated a much higher sulfidation degree of W species(50.2 %)and proportion of NiWS phase(51.3 %).The catalytic evaluation results showed that the HEDP-modified NiW catalyst exhibited the highest reaction rate constant and TOF value(6.9×10-7 mol·g-1·s-1,3.8×10-4 s-1).In order to further investigate the role of HEDP,the effects of HEDP and commonly used chelating agents(citric acid,ethylenediaminetetraacetic acid)on the structure of metal precursor,the nature of active phase and catalytic performance of the catalyst were compared.The results showed that HEDP not only complexed with Niatoms and stabilized the metal precursors on the alumina surface,but also modified the acidity of the catalyst.The addition of HEDP facilitated the formation of active phase with finer size(2.5 nm),higher dispersion and stacking(0.34,2.6 layers),and higher proportion of NiMo S phase(65.3 %),which exhibited higher reaction rate constant(3.37×10-7 mol·g-1·s-1)in the DBT HDS.