3 Continuous Catalytic Processes with Supported Ionic Liquid …
63
of the ionic liquid [C 4 C 1 im][I] and the dimeric precursor species [Rh(CO) 2 I] 2 as
depicted in Scheme 3.5. Under reaction conditions resembling typical industrial
conditions (T = 180 °C, p = 20 bar), the water-free gas-phase carbonylation of
methanol was carried out at full conversion of methanol reaching a TOF for the
combined acetyl products (acetic acid plus methyl acetate) of 76.5 h
−1 and production
rate of 21.0 mol L
−1 h
−1 . This productivity was comparable to that obtained with
an analogous bubble-column reaction system containing about 100-fold volume of
ionic liquid catalyst solution, clearly indicating the high efficiency of the designated
SILP system. One drawback, however, was found to be the relatively low selectivity
toward acetic acid (21% instead of 96% as in the large-volume reaction system).
The Riisager group recently reported the application of phosphine-modified
palladium catalysts for the continuous gas-phase methoxycarbonylation of ethylene to methylpropanoate with promising results (see Scheme 3.6) [41]. The
SILP catalyst was prepared by impregnation of a silica support with a solution containing a Brønsted acid IL, Pd(OAc) 2 , and the ligand (1,2-bis(di-tertbutylphosphinomethane)benzene dtbpmb (see Scheme 3.6) that is well known to
yield methylpropionate with 100% selectivity in the studied reaction.
Different reaction parameters were investigated, such as IL loading, Pd loading,
and dtbpmb/Pd ratio. It was found that high IL loadings had a negligible effect on the
catalytic activity probably due to mass transfer limitations in a thicker IL layer. On
the other hand, the reaction seems to be nearly first order with respect to palladium
at the tested reaction conditions; the higher the palladium content, the higher the
conversion. Interestingly, a strong correlation was found between the dtbpmb to Pd
ratio and the stability of the catalytic system (up to 50 h on stream) by increasing the
dtbpmb to Pd ratio from 5 to 30. Further increase led to a very low conversion due
to the excess of ligand, which prevents the substrates from approaching the metal
center.
Scheme 3.5 Preparation of SILP [C 4 C 1 im][Rh(CO) 2 I 2 ]–[C 4 C 1 im][I]–SiO 2 catalyst
CO
MeOH
MeO
O
+
+
P(t-Bu)2
P(t-Bu)2
1,2-bis(di-tert-butylphosphino
methane)benzene dtbpmb
Scheme 3.6 Pd-catalyzed methoxycarbonylation of ethylene and dtbpmb ligand structure
63
of the ionic liquid [C 4 C 1 im][I] and the dimeric precursor species [Rh(CO) 2 I] 2 as
depicted in Scheme 3.5. Under reaction conditions resembling typical industrial
conditions (T = 180 °C, p = 20 bar), the water-free gas-phase carbonylation of
methanol was carried out at full conversion of methanol reaching a TOF for the
combined acetyl products (acetic acid plus methyl acetate) of 76.5 h
−1 and production
rate of 21.0 mol L
−1 h
−1 . This productivity was comparable to that obtained with
an analogous bubble-column reaction system containing about 100-fold volume of
ionic liquid catalyst solution, clearly indicating the high efficiency of the designated
SILP system. One drawback, however, was found to be the relatively low selectivity
toward acetic acid (21% instead of 96% as in the large-volume reaction system).
The Riisager group recently reported the application of phosphine-modified
palladium catalysts for the continuous gas-phase methoxycarbonylation of ethylene to methylpropanoate with promising results (see Scheme 3.6) [41]. The
SILP catalyst was prepared by impregnation of a silica support with a solution containing a Brønsted acid IL, Pd(OAc) 2 , and the ligand (1,2-bis(di-tertbutylphosphinomethane)benzene dtbpmb (see Scheme 3.6) that is well known to
yield methylpropionate with 100% selectivity in the studied reaction.
Different reaction parameters were investigated, such as IL loading, Pd loading,
and dtbpmb/Pd ratio. It was found that high IL loadings had a negligible effect on the
catalytic activity probably due to mass transfer limitations in a thicker IL layer. On
the other hand, the reaction seems to be nearly first order with respect to palladium
at the tested reaction conditions; the higher the palladium content, the higher the
conversion. Interestingly, a strong correlation was found between the dtbpmb to Pd
ratio and the stability of the catalytic system (up to 50 h on stream) by increasing the
dtbpmb to Pd ratio from 5 to 30. Further increase led to a very low conversion due
to the excess of ligand, which prevents the substrates from approaching the metal
center.
Scheme 3.5 Preparation of SILP [C 4 C 1 im][Rh(CO) 2 I 2 ]–[C 4 C 1 im][I]–SiO 2 catalyst
CO
MeOH
MeO
O
+
+
P(t-Bu)2
P(t-Bu)2
1,2-bis(di-tert-butylphosphino
methane)benzene dtbpmb
Scheme 3.6 Pd-catalyzed methoxycarbonylation of ethylene and dtbpmb ligand structure