chap17 acetic acid

8/10/2019 Chap17 Acetic Acid

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Monsanto (BP) Acetic Acid & Related Processes

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:

Cobalt Rhodium

oncentration 3 10−1 ( 3 10−4 (

Temperature 3 5406 31/06

Pressure 008700 atm 4080 atm

#electivity 90 ; 99

<5 effect <, <4<,

=t< byproducts

no adverseeffect



Acetic Acid 2

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



Acetic Acid 3

rhodium center. The al>yl species, ?-h%5(e%'4@−

, is e2tremely

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



Acetic Acid

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:



Acetic Acid *

Ir

I

OC CO

I

HI

CH3I

CH3OH

+ H2O

Ir

I

OC CO

I

CH3

I

Ir

I

OC

I

I

CH3

O

+ CO

Ir

I

OC

I

I

CH3

OCO

H3C

O

I

+ H2O

H3C

O

OH

+ HI

The main differences between the 'r and -h catalyst systems:

1% The rate determining step for 'r is the migratory insertion of the 'r8<4 and 'r8 ligands. The (e' o2idative addition step is faster

for 'r due to its lower electronegativity.

5% The stronger 'r8ligand bonds slow down the migratory insertion step

and reductive elimination steps $oster noted this in 1979 paper%.

4% There are considerably fewer side reactions in the 'r system to ma>e

inactive ('''% comple2es soluble or insoluble%. This is also tied

into point E5.

This enables the use of low water conditions that ma>es the !P 'r

system competitive with the elanese i' modified low water high8

activity -h catalyst. Fue to patents issued by elanese and =astman

see ne2t section%, !P could not ma>e use of BnormalC additives to

convert the -h catalyst to a low water system.



Acetic Acid +

!P found, however, that a modifier was needed to remove an iodide

ligand to generate less electron8rich more unsaturated% comple2es that

would favor the 8methyl migratory insertion and the final reductive

elimination of acyl8iodide. They

found that added -u'5%4 would

reversibly abstract an iodide ligand

from the ?'r'4<4%%5@−

comple2 to enhance the rate of the

8methyl migratory insertion.

The enhancing effect is shown in

the graph to the right.

<aynes, JACS , &''(, 126 ,

5/7%,astman Chemical Acetic

Anh%dride Process

" very closely related process is the =astman hemical carbonylation of

methyl acetate to produce acetic anhydride. This was commerciali)ed

in 19/4 and produces over /00 million pounds of acetic anhydride a

year at their Gingston, TH plant.

--

* . - . * 2 -

-

3 -

2 -

. -

# a t e r c o n c / ( " 0 " )

R

a

t e

M

0 h

r

I r ' R u . 1 2 r a t i o

R h ( n o m o d i i e r s )

I r

A c e t i c A c i d C a t a l % s t & t u d i e s



Acetic Acid

Rh

I

OC CO

I

I-

CH3I

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

H3C

O

O

+ I-

H3C

O

OCH3

H3C

O

O

H3C

O

O

O

CH3

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.

chap17 acetic acid

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