Impact of the Silica Support Structure on Liquid-Phase Hydrogenation on Pd Catalysts

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• 作者：Joongjai Panpranot ; Kanda Pattamakomsan ; Piyasan Praserthdam ; and James G. Goodwin ; Jr.
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2004
• 出版时间：September 15, 2004
• 年：2004
• 卷：43
• 期：19
• 页码：6014 - 6020
• 全文大小：115K
• 年卷期：v.43,no.19(September 15, 2004)
• ISSN：1520-5045

文摘

The effect of the silica support structure on the characteristics and catalytic properties in liquid-phase hydrogenation of 1-hexene over two types of silica (amorphous SiO₂ and MCM-41)supported Pd catalysts was studied under mild conditions. The catalysts were characterized byatomic adsorption (AA), N₂ physisorption, X-ray diffraction (XRD), transmission electronmicroscopy (TEM), CO pulse chemisorption, and temperature-programmed reduction (TPR). Thehighest hydrogenation activity was found for the larger pore MCM-41 (d_pore = 9 nm) supportedPd catalysts. Use as a catalyst support of a smaller pore size MCM-41, a small-pore amorphousSiO₂, and a commercial large-pore SiO₂ led to lower Pd dispersions. The results suggest that Pdwas inside the pores of the large-pore MCM-41 and SiO₂, and to a lesser extent for the small-pore MCM-41. However, almost no Pd appears to have been inside the pores of the small-poreSiO₂, the support with the greatest percent of pores <3 nm. There was no evidence of any porediffusion limitations on reaction on any of the catalysts. TPR results showed that the PdO onsmaller pore MCM-41 was not totally reduced at ambient temperature, resulting in a loweramount of active Pd⁰ being available for catalyzing the reaction. Leaching of palladium into thereaction media occurred in all cases, and to a significant degree for several of the catalysts.However, the leaching of the Pd seemed to be primarily a function of Pd particle size, withlarger particles being more susceptible, and probably particle location. This loss of Pd from thecatalyst during reaction was most likely due to formation of palladium hydride, which is knownto form more easily on larger Pd particles. Although activity decreased with every reaction cycle,the concentration of surface Pd metal atoms able to chemisorb CO reached a limiting value.