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Sulfur-tolerance Of Highly Dispersed Pd(-Pt)/H-Y Catalysts For Aromatics Hydrogenation

Posted on:2006-04-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:X C MengFull Text:PDF
GTID:1101360182475030Subject:Industrial Catalysis
Abstract/Summary:PDF Full Text Request
Highly dispersed palladium(-platinum) on modified zeolite Y is prepared by ion exchange method. Its hydrogenation activity in aromatics saturation is investigated in the absence and presence of thiophene. In chapter 2, apparent difference in activity and sulfur-tolerant ability is observed on Pd/Y catalysts, prepared with different ion-exchange procedure for naphthalene hydrogenation. The catalyst is characterized by temperature programmed reduction (TPR), H2 chemisorption and ammonia temperature programmed desorption (NH3-TPD). It is concluded that ion-exchange procedure results in the differences in palladium dispersion state, catalyst acidity, and metal-support interaction, those are responsible for their different catalytic performance. Poisoning effect of thiophene on the consecutive hydrogenation pathway of naphthalene is studied on the basis of kinetic analysis. It is found that the inhibitory effect of thiophene on the second ring saturation step is much more noticeable than that on the first ring hydrogenation step. In chapter 3, a hydrogen type zeolite Y, named HDAY, with improved stability and without mesopores is prepared by silicon tetrachloride treatment. Pd/HDAY is prepared by an ion exchange method with Pd content up to 3 %. HRTEM-EDS, CO chemisorption, XPS and shape-selective hydrogenation of aromatics are employed to investigate the Pd particles dimension, location, and electronic state. Information on the relative contribution of Pd particles on the external surface or inside the supercage to catalytic performance is probed with shape-selective hydrogenation of aromatics. It is shown that Pd is highly dispersed on HDAY with a dispersion up to 74 %. Pd particles are located in various parts of the zeolite crystals as follows. About 70 % Pd atoms is inside the cage of HDAY. In supercages, both Pd clusters confined in one supercage and grape-like Pd clusters formed by filling adjacent supercages in a small volume exist. Larger Pd particles with a dimension of 20-30 nm are observed on the external surface, which are generally composed of aggregates with a diameter of 5-8 nm. The electron-deficient nature of Pd particles is revealed by XPS. The Pd/HDAY catalyst has high activity and interesting sulfur tolerance towards hydrogenation of aromatics of smaller diameter. Pd clusters inside supercages play a predominant role in aromatics hydrogenation. However, Pd/HDAY is not suitable to catalyze hydrogenation of aromatics with larger dimensions such as pyrene. In chapter 4, based on the calculation of molecular dimensions for typical aromatics and sulfur-containing compounds in diesel oil, a pore structure design is proposed for sulfur-tolerant catalysts used in the second stage for aromatics reduction. The suitable pore systems should be composed of smaller pores with diameter less than 7.4 ? and larger pores with diameter above 15 ?. Preferably, the two pore systems inter-connect, or are at least uniformly distributed so that they are in close proximity. To achieve this concept, the pore structure of zeolite Y is tailored by silicon tetrachloride treatment followed by steam dealumination. Changes in framework composition, acidity and pore structure are characterized by XRD, FTIR, N2 adsorption-desorption isotherms and NH3-TPD. The metal dispersion and the electronic state of metal particles are characterized by H2 chemisorption and XPS. Sulfur tolerance of the catalyst is investigated with naphthalene hydrogenation in the presence of thiophene. The ability of the catalyst to catalyze polyaromatics hydrogenation is revealed with pyrene hydrogenation. It is concluded that mesopores surrounded by micropores are generated after the above treatment and the generation of mesopores enhances greatly the ability of the catalyst for hydrogenating polyaromatics. However, sulfur tolerance of the catalyst decreases noticeably, which is related to the weak acidity of the as-prepared support and to the insignificant electron-deficiency of metal particles. In chapter 5, the acid amount of zeolite Y is controlled varying in the range of 1271165 μmol/g by steam dealumination. Changes in framework composition and pore structure are characterized by XRD, FTIR, N2 adsorption-desorption isotherms. The total acid amount and the acid sites distribution are quantified by NH3-TPD (with a titration method) and NH3-STPD. Effects of acidity on naphthalene hydrogenation activity and sulfur tolerance are studied with the steam dealuminated samples as carrier. The electronic state of metal particles is characterized by XPS. At the loading of Pd 1.3 % and Pt 0.6 %, PdPt/USY750 and PdPt/USY550 catalysts, with support acid amounts of 352 μmol/g and 606 μmol/g, respectively, demonstrate excellent sulfur tolerance. The low sulfur tolerance of PdPt/USY825 is related to its weak support acidity and insignificant electron-deficiency of metal particles. The noticeable decrease in sulfur tolerance of PdPt/USY350 and PdPt/USY480 catalysts, in which the support is highly acidic, indicates that another unrevealed factor exists besides support acidity and electron-deficiency, which has also an important effect on the sulfur tolerance of the catalyst.The potential of PdPt/USY550 towards hydrogenating aromatics with larger dimensions is explored by pyrene hydrogenation. It is shown that the catalyst PdPt/USY550 demonstrates high sulfur tolerance and is suitable to hydrogenate aromatics with larger diameter due to its suitable acidity and well-tailored pore structure. In chapter 6, two Y zeolites, with large difference in acid amounts, are used as supports to prepare catalysts of various metal content, to investigate the influence of metal content on catalyst sulfur tolerance normalized to per metal atom. It is concluded that metal-acid balance has a significant effect on the sulfur tolerance of catalyst. For catalysts with metal-acid balance essentially achieved, the contribution of acidity to sulfur tolerance is evaluated by comparing their hydrogenation activity in the presence of thiophene.
Keywords/Search Tags:Pd cluster, PdPt catalysts, Aromatics hydrogenation, Sulfur tolerance, Shape-selective hydrogenation, Acidity, Micropore-mesopore structure
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