L. Hegedűs et al.
O
H
2
, 5% Ru/C
methanol
N
N
N
OH
N
OCH
3
CH3
MP
CH3
MPD
CH3
CH3
MPOL
MMPA
Scheme 1 Ruthenium-catalysed hydrogenation of 1-methylpyrrole
MP) to 1-methylpyrrolidine (MPD)
(
N
CH3
MP
at relatively high pressures (25–35 bar) and temperatures
130–150 °C). Thus, 2,5-dimethylpyrrole was converted
(
to cis-2,5-dimethylpyrrolidine over 5% Ru/Al O [21] or
2
3
Fig. 1 Structures of pyrrole derivatives hydrogenated previously
unsupported RuO [22], in water, at 130 °C and 28–35 bar
2
with 85–98% conversion and high cis-selectivity (>90%).
hydrogenation due to their unshared pair of electrons. This
effect can be neutralized by conversion of these reactants to
a form in which the nitrogen atom is shielded, for instance,
by adding protic acids [7]. However, this method cannot be
applied if a substrate is sensitive to acids, i.e. a side-reaction
Most recently, 5% Ru/TiO or 5% Ru/TiO catalysts have
2
2−x
been applied in the hydrogenation of MP to MPD in tet-
rahydrofuran, at 90–100 °C and 30 bar obtaining 80–95%
conversion [23].
However, handling the spent catalysts formed during the
hydrogenations is also an important technological aspect of
the heterogeneous catalytic processes in the chemical indus-
try [24–32]. Both precious [25] and base metals [26] on
different supports or in unsupported forms are usually fully
regenerated before their reusing, but the applied regeneration
methods (e.g. incineration or pyrometallurgical processes)
are typically energy-intensive and expensive. In pharmaceu-
tical industry, where carbon supported precious metal cata-
lysts (e.g. Pd/C, Ru/C) are mostly used, the usual method is
applying the completely regenerated spent catalysts due to
the very strict rules of quality assurance (e.g. GMP—Good
Manufacturing Practices [33]). Since their reusing with-
out regeneration has not solved up to now, an easy and less
expensive catalyst recycling process for the heterogenous
catalytic hydrogenations could afford to produce pharma-
ceuticals more economical.
(
[
e.g. polymerisation) takes place in the presence of them
12].
Previously we reported the liquid-phase heterogeneous
catalytic hydrogenations of several pyrrole derivatives, such
as 1-methyl-2-pyrroleethanol (MPOL), methyl 1-methyl-
2
-pyrroleacetate (MMPA) and 1-methylpyrrole (MP)
(
(
Fig. 1), over different, supported precious metal catalysts
Pd/C, Ru/C, Rh/C, Rh/Al O , Pt/C, Ir/C), in non-acidic
2
3
medium [13–16]. The corresponding pyrrolidines, which
are important and valuable pharmaceutical intermediates
[
17–19], were prepared in good yields (80–90%). In these
reductions the light platinum metals (Rh, Ru, Pd) proved
to be the most active catalysts. Although these hydrogena-
tions took place relatively easily under mild reaction condi-
tions (25–80 °C, 6 bar) with complete conversion and high
selectivity, poisoning of the catalysts was observed below
some catalyst/substrate ratios. The values of these ratio lim-
its ranged from 0.03 to 0.2, and they were dependent on
the substrates, catalytic metals and solvents. Furthermore,
the poison sensitivity of these precious metals referring to
nitrogen was also determined which decreased in the follow-
ing sequence: Pd>Ru>>Rh. This order was attributed to
electronic factors [20].
In this paper the effect of reusing the spent, unregener-
ated, carbon supported ruthenium (5% Ru/C) on its activity
and conversion of MP is discussed. An unexpected behav-
iour of this catalyst was examined by X-ray powder diffrac-
tion (XRD) and X-ray photoelectron spectroscopy (XPS)
surface analytical methods.
In this work the poisoning phenomena of heterogene-
ous, supported precious metal catalysts caused by nitrogen
and their reusing without regeneration were investigated in
detail. Based on our previous experience [16], the liquid-
phase hydrogenation of 1-methylpyrrole (MP) to 1-meth-
ylpyrrolidine (MPD) over ruthenium on carbon, in non-
acidic medium (methanol) was chosen as a model reaction
2 Experimental
2.1 Materials
1-Methylpyrrole (99%) was supplied by Merck-Schuchardt
(Hochenbrunn, Germany), while methanol (p. a.) was pur-
chased from Merck (Darmstadt, Germany).
(
Scheme 1).
Apart from our previous investigations [14–16, 20],
The 5% Ru/C catalyst was received from Aldrich (Mil-
waukee, USA), whilst anhydrous ruthenium(IV) oxide
(99.9%) was supplied by Alfa Aesar (Karlsruhe, Germany).
ruthenium was very rarely used in the heterogeneous cata-
lytic hydrogenation of pyrroles [21–23], moreover it acted
1
3