Monsanto (BP) Acetic Acid & Related Processes The carbonylation of methanol produces acetic acid: H 3 C OH H 3 C OH O +CO This is the second largest industrial homogeneous carbonylation process with over 7 billion pounds of acetic acid produced each year using this technology. Prior to 1970, acetic acid was ma de using cobalt cat alysts !"#$ process % re&uiring rather severe conditions. 'n 1970 (onsanto commerciali)ed a rhodium carbonyl iodide catalyst that is commonly called the (onsanto "cetic "cid Process developed in the late *0+s by ames -oth and his research team at the corporate research center in #t. ouis%. 'n 19/* (onsanto sold the acetic acid plant and t echnology to !ritish Petroleum !P%, but it is still commonly referred to as the (onsanto "cetic "cid process. "s with hydroformylation catalysis, rhodium is 10 3 to 10 4 times more active than the corresponding cobalt catalyst, which means that much lower pressures and moderately lower temperatures are re&uired. (ost importantly, the rhodium catalyst gives e2tremely hig h selectivities to acetic acid: Cobalt Rhodium oncentration 3 10 −1 ( 3 10 −4 ( Te mperature 3 5406 31/06 Pressure 008700 atm 4080 atm #electivity 90; 99< 5 effect < , < 4 <, =t< byproducts no adverse effect
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The carbonylation of methanol produces acetic acid:
H3C OHH3C OH
O
+ CO
This is the second largest industrial homogeneous carbonylation processwith over 7 billion pounds of acetic acid produced each year using thistechnology. Prior to 1970, acetic acid was made using cobalt catalysts!"#$ process% re&uiring rather severe conditions. 'n 1970 (onsantocommerciali)ed a rhodium carbonyl iodide catalyst that is commonly
called the (onsanto "cetic "cid Process developed in the late *0+s byames -oth and his research team at the corporate research center in #t.ouis%. 'n 19/* (onsanto sold the acetic acid plant and technology to!ritish Petroleum !P%, but it is still commonly referred to as the(onsanto "cetic "cid process.
"s with hydroformylation catalysis, rhodium is 103 to 104 times moreactive than the corresponding cobalt catalyst, which means that much
lower pressures and moderately lower temperatures are re&uired.(ost importantly, the rhodium catalyst gives e2tremely high selectivitiesto acetic acid:
The mechanism has been e2tensively studied by $orster and cowor>ersat (onsanto and is shown below. This catalytic reaction is an unusual
dual cycle system involving <' as one catalyst and ?-h'5%5@− as the
transition metal component. <' cataly)es the conversion of (e< to
(e' and <5 at the beginning of the -h8cataly)ed carbonylationreaction, followed by regeneration of <' at the end of the -h8cycle byhydrolysis of the acyl8iodide. The -h catalyst carbonylates the (e' to produce the acyl8iodide.
Rh
I
OC CO
I
HI
CH3I
CH3OH
+ H2O
Rh
I
OC CO
I
CH3
I
Rh
I
OC
I
I
CH3
O
+ CO
Rh
I
OC
I
I
CH3
OCO
H3C
O
I
+ H2O
H3C
O
OH
+ HI
The reaction is independent of pressure, and first order in bothrhodium and (e'. The rate determining step is the o2idative addition of
(e' to the ?-h%5'5@−
catalyst. Thus, the production of (e' from
methanol, cataly)ed by <', is critically important. 'odide ligands areconsidered to be &uite important in this reaction due to the <' cataly)edconversion of (e< to (e' and their relatively good donor abilities on
the -h center. The negative charge on the ?-h%5'5@−
catalyst is
believed to be critical in assisting the o2idative addition of (e' to the
reactive towards insertion to form the acyl comple2.
Celanese LiI Modiied !Lo" #ater$ Catal%st %stem
ne problem with the original (onsanto process is that moderately highamounts of <5 are needed to produce <5 in the reactor via the water-
gas shift rxn A <5 5 A <5%. The water and <5 were
needed to react with precipitated -h'4 and BinactiveC ?-h'%5@−,
which formed from side reactions, to regenerate the active -h'%
catalyst. The reaction of water with inactive ?-h'%5@− to generate
active -h'% catalyst, ?-h'5%5@−
, is shown below.
Rh
OC
OC I
I
I
I
H
O
H
Rh
OC
I
I
I
I
O
O
H
- H+ - I'
Rh
OC
I
I
I
O
O
H
Rh
OC
I
I
H
I
- CO 2
- HI
+ CO Rh
OC
I
CO
I
2'
The relatively high amounts of <5 needed also increases the amount ofthe highly corrosive <' present leading to engineering problems. Thewater reactivation process is also not especially efficient leading to moreinactive -h present in the reactor.
'n the late 70+s elanese developed a maDor improvement on the(onsanto technology not covered by their narrow patent that involvedthe simple addition of i' and other proprietary modifiers% to reduce the
amount of <5 and <'% used in the catalysis. "dded i' increased the
catalyst stability by minimi)ing the side reactions that produced inactive-h'''% species. 't also increased the amount of the more reactive
dianionic ?-h'4%5@2− catalyst species. This considerably increased
the catalyst activity, throughput, and efficiency.
Problem: Increasing the iodide concentration increases the amount
of the following 18e- complex. Why is this more reactive and why
doesnt this saturated 18e- complex slow down the subse!uent
oxidative addition reaction with "eI#
Rh
OC
OC I
I
I 2-
+
CH3I Rh
OC
OC I
I
I
+ I-
CH3
BP Ir'Based Catia %stem
The ativa 'r8based acetic acid catalyst system was announced withmuch fanfare in 1999. <owever, much of this catalytic chemistry was part of the original (onsanto "cetic "cid patent. The 'r cycle wasoriginally studied in considerable detail by $orster along with the -hsystem% in 1979 JCS Dalton, 1$%$, 1*49%. The fundamentalmechanism is essentially the same as the -h cycle:
Hote that this is essentially the same as the (onsantoIelanese processes, only the initial reactant has changed. =astman also wor>edout that adding e2cess iacetate%, much li>e elanese adds i',
eliminates the need for water completely, which is important if onewants to ma>e water sensitive acetic anhydride.
#ince this wasn+t covered by the (onsanto or elanese% patents,=astman hemical didn+t have to pay any royalty or licensing fee,saving them a bunch of JJ.