The synthesis of nitriles from carbon-monoxide, hydrogen and ammonia over iron-based catalysts

Albert Arthur Hummel, Purdue University

Abstract

The nitriles HCN and CH$\sb3$CN can be readily produced from CO, H$\sb2$ and NH$\sb3$ at 700-800 K over a reduced and supported iron catalyst. Catalysts for this reaction were characterized using Mossbauer spectroscopy and NO chemisorption. High dispersion Fe/SiO$\sb2$ catalysts reduced at 673 K have substantial fractions of Fe$\sp{2+}$, which adsorb NO without further significant decomposition of the NO molecule at 473 K. Fe$\sp\circ$ reacts with NO to form N$\sb2$O and then N$\sb2$ with corresponding oxidation of the iron at 473 K. The different responses to NO were used to differentiate surface Fe$\sp{2+}$ and Fe$\sp\circ$ and show that rereduced prenitrided catalysts had a greater fraction of Fe$\sp\circ$, indicating sintering. The reaction in 1:2:1 CO:H$\sb2$:NH$\sb3$ at 723 K over reduced or prenitrided unsupported iron powder is initially active, but deactivates quickly due to extensive carbon deposition. Stable activity is observed over a highly dispersed prereduced Fe/SiO$\sb2$ or Fe on carbon catalyst, however, indicating the importance of the small particle size. The Fe$\sp{2+}$-containing reduced Fe/SiO$\sb2$ catalysts are less active for the nitrile reaction initially than larger particle Fe$\sp\circ$ or Fe$\sb2$N, but increase in activity with time on stream. Mossbauer spectroscopy measurements indicate that the nitrided Fe/SiO$\sb2$ catalyst is converted in minutes to an $\epsilon\sp\prime$-Fe$\sb{2.2}$C carbide during reaction, indicating that little or no bulk nitrogen exists during reaction. Fe$\sp\circ$ in supported or unsupported catalysts quickly converts to a carbide during reaction as well, whereas Fe$\sp{2+}$ only slowly forms carbide, suggesting that the formation of small crystallites of carbide are necessry in the active and stable catalyst. This conclusion is supported by the high and stable activity of small particle Fe on carbon. Transient mass spectrometry was used to follow isotopic labelling experiments using $\sp{15}$NH$\sb3$, $\sp{13}$CO and D$\sb2$. These experiments show that a small pool of active nitrogen and a relatively large pool of carbon exist on the surface. Evidence suggests that this carbon pool leads to the cyanide carbon in acetonitrile. A separate smaller and active carbon pool exists that leads to methane and the methyl carbon in acetonitrile.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

COinS