Synthesis of acetonitrile over supported iron catalysts

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