Xu Zhusheng

DYNAMIC PROCESS OF CARBON DEPOSITION ON PT AND PT-SN CATALYSTS FOR ALKANE DEHYDROGENATION

Abstract The process of carbon deposition on Pt/Al 2 O 3 and Pt–Sn/Al 2 O 3 catalysts under different conditions was investigated in a multifunctional apparatus which can be used for hydrogen chemisorption, TPO and TPSR determinations. The results showed that on both Pt and Pt–Sn catalysts, carbon deposition is a dynamic process. The carbonaceous materials which originate from the metal surface may be converted to carbon through successive dehydrogenation on metal sites, or they may continuously migrate to the support with irreversible coke formation through condensation or hydrogen transfer reactions. The results of chemisorption and thermal desorption of ethylene on platinum and platinum-tin catalysts showed that the incorporation of tin reduced the strength of chemisorption of the hydrocarbons on the metal surface, so that the carbon precursors can migrate towards the support more easily. The proportion of carbon deposited on the support of the platinum-tin catalysts was greater than that of the platinum catalysts. With the same level of carbon deposition, 30% of the metal surface on platinum-tin catalyst remained uncovered, whereas only 10% of the metal surface of the platinum catalysts was bare. The high stability of Pt-Sn/Al 2 O 3 catalysts is closely related to the larger bare metal surface under high-temperature conditions.

Studies on the Degradation of Bimetallic Pt-Sn/Al2O3 Catalysts in Regeneration Cycles

Summary The influence of the coking-regeneration cycle (CRC) on the structure and surface properties of bimetallic Pt-Sn catalysts has been studied by micro-reactor tests with n-butane dehydrogenation, as well as by TEM, in-situ Mossbauer spectroscopy and H 2 –chemisorption measurements. Experimental results indicated that the active surface area of the catalyst was diminished with the increase in the amount of the platinum-tin alloy in the course of the CRC operations. It is suggested that the degradation of the bimetallic Pt-Sn catalysts during the CRC treatments was mainly due to the changes in the extent of interactions between the Sn 2+ cations and the Al 2 O 3 support. Modification by the addition of ceria enhanced the stability of the Pt-Sn/Al 2 O 3 catalyst in the regeneration cycles.

Aromatization of methane over different Mo-supported catalysts in the absence of oxygen

Both acidity and structure of the support are important factors in converting methane to aromatics. Lower SiO2/Al2O3 ratio seems to favor the aromatization of methane over the Mo/HZSM-5 catalyst. When Pt is added as a modifier the activity of Mo/HZSM-5 catalyst will decrease slightly, but coke formation will enhanced.

Effect of aluminum on the mechanical stress stability of WOx/ZrO2 superacid

Abstract Tetragonal structure of the ZrO2 support is very important for WOx/ZrO2 catalysts to be highly active. In this study, it was found that mechanical stress treatments (grinding in a mortar or pressing under 50 MPa) resulted in a partial phase transformation of WOx/ZrO2 from tetragonal ZrO2 to monoclinic ZrO2 and also decreased the n-heptane isomerization activity of the Pt/WOx/ZrO2 catalyst. The addition of aluminum greatly improved the resistance of WOx/ZrO2 to mechanical stress and maintained its n-heptane isomerization activity.

Synthesis of SAPO-11 and MgAPO-11 Molecular Sieves in Water–Butanol Biphase Media

SAPO-11 and MgAPO-11 molecular sieves were synthesized in water-butanol biphase media. The water-butanol biphase media favored the incorporation of Si into SAPO-11 and the formation of more acid sites. Therefore, the Pt catalyst supported on the SAPO-11 synthesized in the biphase medium exhibited much higher catalytic activity for the hydroisomerization of n-dodecane than that supported on the SAPO-11 synthesized in an aqueous medium. In contrast, the water-butanol biphase media were unfavorable for the Mg substitution into the framework, and thus the acid sites and the catalytic activity of Pt/MgAPO-11 decreased.

Synthesis of MgAPO-11 Molecular Sieves and the Catalytic Performance of Pt/MgAPO-11 for n-Dodecane Hydroisomerization

Abstract A series of MgAPO-11 molecular sieves have been synthesized by varying the crystallization time, the P/Al molar ratio, and the Mg source. X-ray diffraction, X-ray fluorescence spectroscopy, and temperature-programmed desorption of NH3 have been used to characterize the crystalline phase, the Mg content, and the acidity of MgAPO-11, respectively. The results show that the synthesis conditions have an effect on the crystalline phase, the Mg content, the acidity, and thereby the catalytic performance of Pt/MgAPO-11 in the hydroisomerization of n-dodecane. The reaction data reveal that the activity of Pt/MgAPO-11 is dependent on the number of strong acid sites of MgAPO-11. Short crystallization time, P/Al molar ratio of 1.0, and Mg(NO3)2 as the Mg source favor the increase in the number of the strong acid sites of MgAPO-11 and hence promote the catalytic activity of Pt/MgAPO-11 for the hydroisomerization of n-dodecane.

Role of steam on the reaction performance of methane oxidative coupling over basic catalysts

Abstract The effect of steam on the oxidative coupling of methane over some basic catalysts was investigated. It has been found that the presence of steam in the reactant stream enhanced the methane conversion, C 2 selectivity as well as the C 2 yield. Another effect of the steam was the enhancement of the ethylene to ethane ratio. When the H 2 O CH 4 ratio (mol) was in the range of 0–2, the catalytic performance was improved, and the catalyst was also quite stable during 200 h of operation. The beneficial effect of steam on the C 2 yield was more evident when the reactor was enlarged (the catalyst loading was increased from 1 to 30 ml). Two possible reasons account for this effect, one is the dilution effect of steam on the concentrations of O 2 and CO 2 , another is the uniformity of the temperature distribution along the catalyst bed during the reaction.