Synthesis of acetonitrile over supported iron catalysts

Manish Vrajlal Badani, Purdue University

Abstract

The synthesis of acetonitrile from CO, H$\sb2$ and NH$\sb3$ over iron catalysts has been studied at 673-723 K. The dependence of the turn over frequency (TOF) to acetonitrile on temperature and the partial pressures (pp) of CO, H$\sb2$ and NH$\sb3$ is established for both reduced and nitrided 4% Fe/SiO$\sb2$. The TOF to acetonitrile on reduced catalysts decreases with increase in pp of CO and increases with increase in pp of H$\sb2$ and NH$\sb3$. On the nitrided catalysts, the TOF decreases with increase in pp of CO and goes through a maximum for the increase in pp of H$\sb2$ and NH$\sb3$. The activation energies for acetonitrile and methane, respectively, are in the range of 27-31 kcal/mole and 16-20 kcal/mole. Mossbauer spectroscopic studies after various pretreatments show that the reduced catalysts contain some or no Fe$\sp\circ$ with the balance made up of Fe$\sp{2+}$, while the nitrided catalysts contain varying amounts of $\zeta$-Fe$\sb2$N, Fe(2N) and Fe$\sp{2+}$. The reduced Fe/SiO$\sb2$ catalysts containing only Fe$\sp{2+}$ are less active initially than larger particle Fe$\sp\circ$ or Fe$\sb2$N phases, but increase in activity with time on stream, while the larger particles go through a maximum and then decay in activity. When an Fe$\sp{2+}$-containing catalyst is nitrided and rereduced, it exhibits an activity pattern similar to that observed for Fe$\sp\circ$-containing catalysts. This may be due to the sintering of iron in NH$\sb3$ atmosphere. Mass spectral analysis of transient step changes from pretreatment gases to the reaction mixture indicate that iron carbides form before acetonitrile formation starts. Isotopic $\sp{13}$CO pulse experiments reveal the heterogeneous nature of carbon pool. In situ Mossbauer studies indicate that the small particle $\varepsilon\sp\prime$-Fe$\sb{2.2}$C carbide is the stable active phase of the catalyst. Results obtained on reduced Fe/CSX203, composed of mainly superparamagnetic Fe$\sp\circ$, are consistent with the above suggestion. It appears that the small particle $\varepsilon\sp\prime$-Fe$\sb{2.2}$C formed from Fe$\sp{2+}$ and superparamagnetic Fe$\sp\circ$ gives stable activity, while the larger $\varepsilon\sp\prime$-Fe$\sb{2.2}$C carbide particles formed from Fe$\sp\circ$ and Fe$\sb2$N are thermally unstable and convert to form $\chi$-Fe$\sb{2.5}$C or $\theta$-Fe$\sb3$C. The conversion of $\varepsilon\sp\prime$ to $\chi$ and $\theta$ carbides is accompanied by loss of carbon from the lattice which contributes to the carbon overlayers on the surface of the catalyst, leading to loss in activity.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering|Analytical chemistry|Nuclear physics

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