Kinetic studies of diffusion and oxidative dehydrogenation in microporous materials

Date of Completion

January 1996


Chemistry, Inorganic|Engineering, Chemical




The transient uptake of cyclopropane gas in NaX zeolite under isothermal conditions has been simulated by a mathematical model and curve-fit to the experimental data obtained in a stainless steel microreactor with a small bed of catalyst (NaX zeolite). The response to a switch from a stream of pure non-adsorbing argon to one of 0.5% cyclopropane/argon was simulated by assuming an effective intracrystalline diffusivity and CSTR conditions in the microreactor. The Langmuir adsorption isotherm was used to predict the adsorption of the cyclopropane gas in the active sites of the zeolite catalyst. The effective diffusivity of the cyclopropane in NaX was estimated to be about 2 $\times$ 10$\sp{-11}$ cm$\sp2$/sec from the curve-fit. The diffusivity of cyclopropane in Ni/NaX (11.9% by wt. Ni$\sp{2+}$) was estimated to be about 2 $\times$ 10$\sp{-12}$ cm$\sp2$/sec and 3 $\times$10$\sp{-12}$ cm$\sp2$/sec for Eu/NaX (24.5% by wt Eu$\sp{3+}$) at a temperature of 40$\sp\circ$C.^ Temperature Programmed Diffusion (TPDi) has been used to study the encapsulation of hydrogen in cadmium exchanged rho zeolite. The amount encapsulated after 2 hours is 71 $\mu$mol/g at 50$\sp\circ$C and at 1 atmosphere (over 30 times the amount of hydrogen encapsulated in NaX or NaA at 37$\sp\circ$C, 2 hours, 1 atm). At an encapsulation temperature of 100$\sp\circ$C (1 atm, 2 hours), 161 $\mu$ mol/g (2 hours) and 620 $\mu$ mol/g at 200$\sp\circ$C (1 atm, 2 hours) are observed. After entrapment at 200$\sp\circ$C, three TPDi peaks are observed--at 107$\sp\circ$C, 295$\sp\circ$C and 345$\sp\circ$C, showing the presence of more than one site for encapsulation. With hydrogen exchanged rho zeolite, no uptake of hydrogen was observed, indicating that the presence of the cadmium and/or the cesium is responsible for the hydrogen entrapment.^ Microporous manganese oxides (octahedral molecular sieves, OMS) catalyzed total and partial oxidation of n-butane and l-butene as revealed by steady state kinetic measurements. Selective replacement of the framework manganese cation with Cu$\sp{2+}$, produced a more selective oxidation catalyst. Phase changes in the catalyst, owing to reduction by the hydrocarbon, have been observed by X-ray diffraction (XRD). The availability of oxygen from the catalyst, was observed and measured directly by Temperature Programmed Reduction (TPR). ^