1. Field of the Invention
The invention relates to a process; for preparing aminopropylalkoxysilanes.
2. Discussion of the Background
Among the group of aminoalkylalkoxysilanes, .gamma.-amino-propyltriethoxysilane in particular finds wide use in industrial applications, for example in the production of glass fibers.
The synthesis of aminopropylalkoxysilanes from allylamine and hydrogensilanes is carried out by known methods in the presence of homogeneously catalyzed systems. Such complex catalysts which are customarily used for carrying out the reaction and are soluble in organic solvents are based on compound of the transition metals cobalt, platinum, rhodium and ruthenium. The reaction product formed contains a regioisomeric mixture of .gamma.-and .beta.-aminopropylalkoxysilanes.
EP-B 0 196 639 teaches a process for preparing aminopropylalkoxysilanes by reaction of a hydrogensilane with allylamine in the presence of a chlorine-free rhodium-triorganophosphorus complex. This homogeneously catalyzed process gives yields of aminopropylalkoxysilanes of from about 60 to 75% and regioisomeric ratios of .gamma. isomer: .beta. isomer of from 6 to 59:1.
Such readily soluble rhodium-organophosphorus complexes, which function as homogeneous catalysts for the preparation of aminopropylalkoxysilanes, do have a high initial activity and selectivity. However, after the reaction is complete and after the distillative separation of the aminosilanes, these catalysts no longer have the activity required for reuse from an economic point of view. The reason for the rapid drop in activity is the dissociation of ligands from the metal complex, often after only a short reaction time. To maintain the activity, considerable excess amounts of the corresponding phosphorus ligands have to be used in the reaction medium.
Other soluble complexes of transition metals of Group VIII of the Periodic Table of the Elements have not achieved any economic importance because of their significantly lower activity and selectivity for the hydrosilylation of allylamines.
A great problem with homogeneously catalyzed processes is the need for maximum recovery of the expensive or environmentally significant transition metals used. The recovery of these metals can often be achieved only by great process effort, otherwise considerable metal losses have to be accepted. Such factors have a considerable influence on the economics and acceptance of a process and the products.
In addition, shaped polymeric complex catalysts of, inter alia, the elements Fe, Co, Ni, Ru, Rh, Pd, Pt, Os and Ir are known. These elements are bridged with one another via phosphorus-containing organosilanes as ligands, with the ligands being coordinately bonded to the respective central atom via strongly bonding phosphorus atoms. The shaped polymeric, in particular spherical, complex rhodium catalysts containing organosiloxanediphenylphosphine or organosiloxanemonophenylphosphine ligands are prepared by a known process. Unlike the SiO.sub.2 -bonded, heterogeneous catalysts, these complex rhodium catalysts are not synthesized by subsequent application of the transition metal to the support, but by direct condensation of the corresponding transition metal complexes with, for example, tetra-ethoxysilane and aminosilanes, followed by hydrolysis. Such shaped polymeric complex catalysts are described in detail in DE-C 40 35 032 and DE-C 40 35 033, with these shaped polymeric complex catalysts being able to be used to carry out hydroformylation, hydrogenation, oligomerization, carbonylation, hydrosilylation, carboxymethylation, isomerization reactions and for reactions of CO or CO.sub.2 with H.sub.2. A need, therefore, continues to exist for a process of preparing aminopropylalkoxysilanes which is economically more attractive.