Application of a hydrogenation catalyst modified by formaldehyde

O. G. Degischer1, R. Novi1,2, F. Roessler2, P. Rys1 1ETH Zurich Hoenggerberg, 8093 Zurich, Switzerland. 2Roche Vitamines Ltd, Bldg 214 Room 8, 4303 Kaiseraugst, Switzerland.

Introduction The hydrogenation of nitriles is an important method for the industrial production of primary amines. Normally, the product mixture contains primary, secondary and tertiary amines, as well as traces of amides and alcohols. A general reaction scheme for the hydrogenation of nitriles to the primary and secondary amines is shown in figure 1. The selectivity of the nitrile hydrogenation depends on several factors: the nature of the individual catalyst, the solvent and additives such as ammonia, alkali hydroxides, etc. In addition, the selectivity can also be influenced by conditions such as pressure and temperature. A recently published patent1 describes the modification of Raney catalysts with formaldehyde. Compared to reactions with unmodified catalysts, those run with catalysts modified by formaldehyde show an increased selectivity in favour of the formation of primary amines.

N H2 H2 NH R NH R R 2 A BC

R NH2

NH2

R N R H

D

- NH3

H2 R N R R N R H

E F Figure 1. Reaction scheme for the hydrogenation of nitriles leading to primary and secondary amines.

Results and discussion The hydrogenation of aromatic nitriles was investigated using commercial Raney nickel catalysts, either unmodified or modified by formaldehyde, with benzonitrile as a model compound and methanol as the solvent. After the modification with formaldehyde the activity of the catalyst decreases, whereas the selectivity towards primary amine formation increases, both in the absence as well as the presence of ammonia (table 1).

Table 1. Comparison of untreated and modified Raney nickel. Reaction conditions: 100 ml benzonitrile, 2 l methanol, 5.7 g Raney nickel, temperature 100°C, hydrogen pressure 4 MPa, stirrer speed 1200 rpm.

a a b System NH3 Prim. amine C Sec. amine F r0 /[g] /[%] /[%] /[mmol/(s·g)] untreated catalyst - 65.2 34.5 0.51 modified catalystc - 80.5 14.5 0.26 untreated catalyst 15 80.8 18.4 0.62 modified catalystc 15 92.8 6.5 0.41 a concentration at full conversion of the nitrile as well as of the intermediate imine E (cf. figure 1) b initial hydrogen consumption rate c modified using 1% aqueous formaldehyde solution

In addition, the influence of the formaldehyde concentration applied to modify the Raney nickel catalyst was investigated. With increasing concentration of the formaldehyde solution employed, the activity of the catalyst (monitored by the initial reaction rate) decreases, whereas the selectivity in favour of the formation of the primary amines increases (table 2).

Table 2. Influence of the catalyst modification procedure on the selectivity and the activity of the catalyst. Reaction conditions: 100 ml benzonitrile, 30 g ammonia, 2 l methanol, 5.7 g Raney nickel, temperature 100°C, hydrogen pressure 4 MPa, stirrer speed 1200 rpm.

a a b Catalyst modification Prim. amine C Sec. amine F r0 /[%] /[%] /[mmol/(s·g)] untreated catalyst 89.9 9.1 0.59 1% formaldehyde 94.2 4.9 0.48 2.5% formaldehyde 97.4 1.9 0.20 5% formaldehyde 98.2 0.8 0.08 a concentration at full conversion of the nitrile as well as of the intermediate imine E (cf. figure 1) b initial hydrogen consumption rate

Further research will be necessary to understand why the catalyst properties change after the catalyst has been modified by formaldehyde.

References 1. O. G. Degischer, F. Rössler: US Patent 2001004672, 2001.