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Peculiar Diffusion Mechanisms within Micropores of Zeolite Catalysts

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Title: Peculiar Diffusion Mechanisms within Micropores of Zeolite Catalysts
Other Titles: ゼオライト触媒のミクロ孔内における特異な拡散現象
Authors: MASUDA, Takao1 Browse this author →KAKEN DB
Authors(alt): 増田, 隆夫1
Keywords: Diffusivity
MFI zeolite
Adsorption controlled diffusion
Multi-conponent system
Configurational diffusion
Issue Date: 2003
Publisher: 石油学会
Journal Title: Journal of the Japan Petroleum Institute
Volume: 46
Issue: 5
Start Page: 281
End Page: 294
Publisher DOI: 10.1627/jpi.46.281
Abstract: Diffusion mechanisms of hydrocarbons within zeolites, especially MFI-type zeolite, investigated by discriminating intracrystalline diffusivity from effective diffusivity. Intracrystalline diffusivity directly represents the mobility of molecules within pores. Effective diffusivity is obtained by multiplying the intracrystalline diffusivity by a partition factor given by the ration of the concentrations of molecules in zeolite crystals to that in gas phase. Intracrystalline diffusivity was the main subject of this study. Diffusion within MFI-type zeolite is dominated by the following mechanisms: (1) configurational diffusion, (2) resistance to mass transfer at pre mouths, (3) adsorption-controlled diffusion and (4) co-existing molecules with slow diffusing molecules. Intracrystalline diffusivity in the adsorption process is lower than in desorption process for (1) and (2) if the minimum molecular size is larger than the pore diameter, such as ortho- and meta-xylenes. The resistance to mass transfer at pore mouths becomes dominant in the adsorption process. This tendency is also observed for paraffins. Model equations were proposed for evaluating intracrystalline diffusivity based on the molecular size, molecular weight and pore diameter. Overall intracrystalline diffusivity for (3) is correlated with the configurational diffusion and the trapping effect on acid sites. This effect disturbs the mass transfer, especially at temperatures below 573 K, for aromatics. Similar effects are observed for lighter paraffins and olefins within MFI-type zeolite with metal cations. Intracrystalline diffusivities for (4) for silicalite-1 in a multicomponent system were measured using a new desorption under reduced pressure method. The diffusivity of the slow component in the multicomponent system agreed well for that of a single component. However, the diffusivity of the fast component was largely decreased by co-existing slow molecules. A random walk simulation and an empirical equation could explain this tendency. Effective diffusivity was calculated from the intracrystalline diffusivity and the partition factor, which was obtained from the adsorption isotherm. The partition factor suggested a marked condensation effect for MFI-type zeolites. Intracrystalline and effective diffusivities for beta- and Y-type zeolites, and mordenite were also investigated.
ゼオライトの拡散機構について, MFI型ゼオライトを中心に紹介した。拡散には結晶内拡散係数と有効拡散係数がある。結晶内拡散係数はゼオライト結晶内のmobilityを表し, その値に分配係数 (=拡散分子の結晶内濃度/気相濃度) を乗ずることで有効拡散係数が得られる。そのため, 主に結晶内拡散係数を取り扱った。MFI型ゼオライトの拡散は主に, (1) 形状拡散, (2) 細孔入口部の拡散抵抗, (3) 吸着阻害の拡散, (4) 共存物質の影響, に支配される。 (1) と (2) に関して, まず吸着過程と脱着過程の結晶内拡散係数を測定した。両過程の拡散係数を比較したところ, 拡散分子と細孔内および細孔入口部との立体障害によって拡散が支配されており, その寄与が吸着と脱着過程で異なることが分かった。また, 金属格子内の拡散現象を基に, 結晶内拡散係数を推算するモデル式を導出した。 (3) に関しては, 低温領域では拡散分子の酸点上での滞留が拡散を支配することが分かった。また, 酸点および金属カチオンを含むMFI型ゼオライトへの低級パラフィンやオレフィンの拡散でも, 同様に吸着阻害の拡散現象が観察された。 (4) に関しては, 二成分のうち拡散が速い成分だけが共存分子の影響を受け, 拡散係数は単成分系での値よりも著しく減少した。また, 二成分系と単成分系の結晶内拡散係数の比をRandom walk simulationで予測するとともに, 計算する実験式を提案した。さらに, 吸着等温線から求めた分配係数を結晶内拡散係数に乗ずることで有効拡散係数を求めた。その値は異なる結晶サイズのゼオライトを用いた反応実験で推算した値と良好に一致した。また, 分配係数の値より, ゼオライト中の分子の濃度は気相の100倍以上となっており, 濃縮効果を見い出した。
Type: article
Appears in Collections:工学院・工学研究院 (Graduate School of Engineering / Faculty of Engineering) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

Submitter: 増田 隆夫

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