Preparation Of Pt-Al/MCM-41Catalysts And Its Hydrodearomatization Performance

Supported noble-metal catalysts are industrial applied for hydrodearomatization of dieselfuel. However, these catalysts are poor sulfur-tolerance. The catalytic activity andsulfur-tolerance of these catalysts can be talored by adjusting the acidic property of thesupport. In this study, AlCl3, Al(NO3)3and Al(CH3)3promoted Pt-Al/MCM-41catalystswith different Al-Pt interaction manners were synthesized for tetralin hydrogenation. Theproperties of the catalysts were analyzed with XRD, TEM, NH3-TPD, Py-FTIR, CO-FTIRand TG. The catalytic activity and sulfur-tolerance of the catalysts were evaluated by tetralinand benzothiophene hydrogenation. This study was focused on the mechanism of synergistic-effect of acidic sites-noble metal and its roles in promoting the catalytic activity andsulfur-tolerance.1. Coupled with phase equilibria modeling, the effects of reactant’s phase-transition onthe activity and selectivity of hydrodearomatization catalyst are investigated by tetralinhydrogenation with different solvents (n-tetradecane, n-decane and n-octane). It was foundthat catalytic activity decreases rapidly when the phase state of feedstock transferred fromgas-liquid to single gas phase. It was suggested that the evaluation of hydrodearomatizationcatalysts should be performed under gas-liquid-solid reaction condition as well as with modelfuel composed of aromatic compound and solvent (generally, alkanes are choosed) withsuitable high boiling point.2. The effects of Al-Pt interaction sequences on the property and performance of catalystwere investigated with AlCl3as promoter. It was found that the Al-Pt interaction increases theacid amount of catalysts, improves the dispersion of Pt and decreases the electron density ofPt. The catalyst which Al-grafting is after Pt-supporting has the best platinum dispersion andsuitable electron density of Pt particles. This catalyst also shows good catalytic activity intetralin and benzothiophene hydrogenation as well as good sulfer-tolerance. It was suggestedthat the competitive adsorption of benzothiophene, which hinders the adsorption andhydrogenation of tetralin, is the main cause of catalyst deactivation.3. The effects of Al-promoter types on the property and performance of catalyst wasinvestigated with AlCl3, Al(NO3)3and Al(CH3)3as promoter. It was found that AlCl3bringsin more Lewis acid sites and Al(CH3)3brings in more hydroxy. The promotion effect onplatinum dispersion is Al(CH3)3>AlCl3>Al(NO3)3≈Al-free. The electron density of Ptparticle is Al(CH3)3≈Al-free>AlCl3>Al(NO3)3. The Al(CH3)3-promoted catalyst, which has the best platinum dispersion, also shows the best catalytic activity in tetralin andbenzothiophene hydrogenation as well as the best sulfer-tolerance.4. The effects of Al/Pt mole ratios on the property and performance of catalyst wereinvestigated with AlCl3and Al(CH3)3as promoter. With the increase of Al/Pt ratio, the acidamount and acid intensity of the catalysts increases, the electron-density of Pt particlesdecreases, and the platinum dispersion increases at first and then decreases. The optimumAl/Pt is2and10, respectively, for AlCl3and Al(CH3)3promoter to achieve the best catalyticperformance (tetralin and benzothiophene hydrogenation activity and sulfur-tolerance). TheAl(CH3)3-promoted catalyst with an Al/Pt=10increases the specific first-order reaction rateconstant of tetralin hydrogenaiton to9.9(sulfur-free) and14.27times (sulfur-containing) ashigh as the Al-free one.5. The mechanism of Al-Pt interaction and its effect on the performance of catalyst werediscussed. The Al-promoter, which is grafted on the support surface, improves the platinumdispersion via isolation effect and anchor effect, and leads to the formation ofelectron-deficient Ptδ+via electron-withdrawing effect. The Al-promoter also provides acidsites, improves hydrogen-spillover and enhances the competitive adsorption ofbenzothiophene. The hydrogenation activity and sulfur-tolerance of the catalysts are mainlydepended on the dispersion and electron density of Pt nano-particles. High dispersion andhigh electron density of Pt nano-particles benefit tetralin hydrogenation, while high dispersionand low electron density of Pt nano-particles benefit the sulfur-tolerance and benzothiophenehydrogenation.