104
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`2.1.2
`
`Carbonylations
`
`2.1.2.1
`
`Synthesis of Acetic Acid and
`Acetic Acid Anhydride from Methanol
`Paull Torrence*
`
`2.1.2.1.1 Basic Catalysis
`
`According to eq. (1) carbonylation of methanol by formal CO insertion into the
`C-0 bond yields acetic acid:
`
`(I)
`
`The reaction is catalyzed by metal complexes, the central atoms favorably being
`Co or Rh. Nowadays all other routes to acetic acid (especially via acetaldehyde, cf.
`Section 2.4.1, and its oxidation, Section 2.4.4) are economically obsolete.
`As far as the central atoms are concerned, in particular the Group VIII metals
`Co, Ni, Ru, Rh, Pd, Ir, and Pt form effective carbonylation catalysts, each metal
`demonstrating a different carbonylation activity. Rh and Ir are the most active and
`preferred catalysts for carbonylation reactions to produce acetic acid or acetic
`anhydride, or for co-production of acetic acid and acetic anhydride [1, 2]. Co is
`only of historical interest.
`The key elements of these carbonylation processes is the ability of a metal com(cid:173)
`plex to undergo facile oxidative addition with methyl halide (especially iodide),
`carbon monoxide (CO) insertion into the methyl-metal bond, and reductive
`elimination of the acetyl group as the acetyl halide [3].
`When Rh is the metal catalyst, a common catalytic pathway is proposed which
`involves the nucleophilic attack of the active Rh1 catalyst complex, [Rh(COhhr,
`on methyl iodide (CH31) to form a methylrhodium(III) intermediate, [Rh(CH3)
`(COh(lhr. Rapid methyl migration in this complex generates the acylrho(cid:173)
`dium(III) intermediate, [Rh(CH3CO)(CO)I3r, which reacts with CO to form
`[Rh(CH3CO)(COhi3r and subsequently reductively eliminates acetyl iodide
`and regenerates the rhodium(!) anion. The final reaction of acetyl iodide with
`compounds containing hydroxyl groups such as water, methanol (CH30H), or
`acetic acid (eq. (2)) leads to the formation of hydrogen iodide (HI) and the corre(cid:173)
`sponding acetyl derivatives.
`
`(2)
`
`R = H, CH3, COCH3
`
`* Based on the contribution to the first edition by Michael GaujJ, Andreas Seidel,
`Paull Torrence, and Peter Heymanns.
`
`CE Ex. 2032
`Daicel v. Celanese
`IPR2015-00171
`
`001
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`105
`
`This final reaction step of the carbonylation mechanism is the primary dis(cid:173)
`tinguishing feature of each carbonylation process. A sufficient concentration of
`water or acetic acid in the reactor is therefore necessary to achieve high acetic
`acid or acetic anhydride formation rates respectively.
`The hydrogen iodide liberated then reacts with methanol, methyl acetate (or di(cid:173)
`methyl ether) to regenerate methyl iodide promoter (eq. (3)):
`
`(3)
`
`For the carbonylation of methyl acetate and the co-carbonylation of methyl
`acetate and methanol, the reaction of acetyl iodide with methyl acetate (or
`dimethyl ether, DME) is a key reaction step also (eq. (4)) [4]:
`
`(4)
`
`Credence for this general carbonylation mechanism is supported by IR model
`studies in various solvents of key steps in the proposed reaction pathway
`[5b, 6-8, 9c,e]. These investigations include isolation and characterization of
`the acyl carbonyl complex as the dimer [10] and most recently spectroscopic
`evidence of the methyl intermediate in the presence of excess CH31 [9c, 9e].
`The carbonylation rate is independent of the type of rhodium compound charged
`to the reaction as long as sufficient CH31 and CO are available. This supports the
`concept of the generation of a common active catalyst under reaction conditions
`[11, 12].
`An overview of Monsanto's catalyst system in comparison with other processes
`is given in Table 1 [23, 80].
`
`Table 1. Catalyst systems for carbonylations of methanol and methyl acetate.
`
`Company
`
`Product
`
`Central atom
`
`Complex
`
`Co-catalyst
`
`Monsanto
`
`HCC
`
`Eastman
`
`Hoechst
`
`AcOH
`
`AcOH
`
`AczO
`
`Ac 20
`
`BP
`
`BP
`
`Ac20/AcOH
`
`AcOH
`
`Rh
`
`Rh
`
`Rh
`
`Rh
`
`Rh
`
`Ir
`
`[Rh(COhi2rw
`
`[Rh(CO)zlzrLi+
`
`[Rh(COhizrLi+
`
`MeVHI
`
`MeVLii
`
`MeVLii
`
`[Rh(COhizrP(R)4 +
`
`MeVP salts
`
`[Rh(COhizrN(R)4 +
`
`[Ir(COhlz]W
`
`MeVN salts
`(Zr compound)
`
`MeViodide
`salts, metal
`carbonyls
`(i.e., Ru iodide
`carbonyls
`
`002
`
`

`
`106
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`2.1.2.1.2 Acetic Acid
`
`Introduction
`
`The manufacture of acetic acid by the rhodium-catalyzed carbonylation of metha(cid:173)
`nol (eq. (5)) is one of the most important industrial processes.
`
`150-200 oc
`30-60 bar
`
`(5)
`
`Acetic acid is a key commodity building block [1]. Its most important deriva(cid:173)
`tive, vinyl acetate monomer, is the largest and fastest growing outlet for acetic
`acid. It accounts for an estimated 40% of the total global acetic acid consumption.
`The majority of the remaining worldwide acetic acid production is used to man(cid:173)
`ufacture other acetate esters (i.e., cellulose acetates from acetic anhydride and
`ethyl, propyl, and butyl esters) and monochloroacetic acid. Acetic acid is also
`used as a solvent in the manufacture of terephthalic acid [2] (cf. Section
`2.8.1.2). Since Monsanto commercially introduced the rhodium- catalyzed carbo(cid:173)
`nylation process ("Monsanto process") in 1970, over 90% of all new acetic acid
`capacity worldwide is produced by this process [2]. Currently, more than 50% of
`the annual world acetic acid capacity of 7 million metric tons is derived from the
`methanol carbonylation process [2]. The low-pressure reaction conditions, the
`high catalyst activity, and exceptional product selectivity are key factors for the
`success of this process in the acetic acid industry [13].
`Since 1979, numerous reviews have appeared on the kinetics, mechanisms, and
`process chemistry of the metal-catalyzed methanol carbonylation reaction [11,
`14-20], especially the Monsanto rhodium-catalyzed process. In this section, the
`traditional process chemistry as patented by Monsanto is discussed, with emphasis
`on some of the significant improvements that Monsanto's licensee, Celanese
`Chemicals (CC) has contributed to the technology. The iridium-based methanol
`carbonylation process recently commercialized by BP Chemicals Ltd. (BP) will
`be discussed also.
`
`Process History
`
`The low-pressure acetic acid process was developed by Monsanto in the late
`1960s and proved successful with commercialization of a plant producing
`140 X 103 metric tons per year in 1970 at the Texas City (TX, USA) site [21].
`The development of this technology occurred after the commercial implementa(cid:173)
`tion by BASF of the cobalt-catalyzed high-pressure methanol carbonylation
`process [22]. Both carbonylation processes were developed to utilize carbon
`monoxide and methanol as alternative raw materials, derived from synthesis
`gas, to compete with the ethylene-based acetaldehyde oxidation and saturated
`hydrocarbon oxidation processes (cf. Sections 2.4.1 and 2.8.1.1 ). Once the Mon(cid:173)
`santo process was commercialized, the cobalt-catalyzed process became noncom-
`
`003
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`107
`
`petitive. Today over ten companies worldwide practice the methanol carbonyla(cid:173)
`tion technology [2].
`In 1978, Celanese Chemicals (CC) was the first Monsanto licensee to operate
`the Monsanto acetic acid process commercially. Soon after start-up of this unit,
`Celanese implemented several process improvements to expand the unit capacity.
`Later, in the early 1980s, CC developed the proprietary, low reaction water tech(cid:173)
`nology which improved the process significantly (known as Acid Optimization
`(AO)). The low-water technology was achieved in part by increasing the rhodium
`catalyst stability by addition of inorganic iodide in high concentrations to the re(cid:173)
`action system [23] above an iodide concentration level not usually thought to be
`effective as a catalyst stabilizer and promoter [15, 24]. This alteration to the cata(cid:173)
`lyst composition allows reactor operation at low water and high methyl acetate
`reaction concentrations to increase reactor productivity, purification capacity,
`and methanol and carbon monoxide efficiency [5, 23]. As a result, the composi(cid:173)
`tion of the catalyst solution used in the low-water technology by Celanese [23] is
`significantly different from the catalyst composition used in the original methanol
`carbonylation process patented by Monsanto [25]. In 1986 BP Chemicals Ltd.
`purchased from Monsanto the technology and licensing rights to the low-pressure
`methanol carbonylation technology which did not include the proprietary technol(cid:173)
`ogy developed by Celanese.
`In 1996 BP announced the commercialization of their version of a low-water
`methanol carbonylation technology named Cativa™ based upon a promoted iri(cid:173)
`dium catalyst. The Cativa™ process replaced the high-water Monsanto process
`which had been used by BP.
`
`Process Chemistry
`
`Monsanto Technology
`
`The reaction chemistry of the rhodium-catalyzed methanol carbonylation process
`under Monsanto conditions has been investigated extensively [6-8, 10, 12, 21,
`26-29] ( cf. Section 2.1.2.1.1 ). The overall reaction kinetics are first order in
`both rhodium catalyst and methyl iodide promoter. The reaction is zero order in
`methanol and zero order in carbon monoxide partial pressure above 2 atm (eq.
`(6)) [27]. The kinetics agree well with the basic mechanism common to the
`three carbonylation reactions (see Section 2.1.2.1.1 and Tables 1 and 2).
`
`-d [~~30H] = k [Rh] [CH3I]
`
`(6)
`
`The reaction medium also plays a key role in the overall activity of the catalyst
`system. The reaction rate is highly dependent on the nature of the medium;
`however, the overall kinetics are unaffected by reaction solvent [5c, 27, 30-32].
`This suggests that the rate dependence of the solvent is not involved in the transi(cid:173)
`tion-state species of the rate-determining step [5c]. Maximum carbonylation rates
`are demonstrated in polar solvents and the additions of protic solvents accelerate
`
`004
`
`

`
`108
`
`2. 1 Carbon Monoxide and Synthesis Gas Chemistry
`
`the reaction rate. In particular, water exhibits a general rate enhancement in
`most reaction solvents [27, 30]. Acetic acid/water is the preferred medium in
`the commercial process for carbonylation reactivity [25]. The dependence on
`water of the reaction rate in acetic acid has been studied [24, 30-32]. The carbo(cid:173)
`nylation reaction rate decreases markedly with a concomitant decrease in water
`concentration (below ca. 10 molar) [30]. The catalyst stability also decreases
`[5c, 23].
`
`Hoechst Celanese Low-Water Acid Optimization (AO) Technology
`
`In the Monsanto process a substantial quantity of water in the reaction system
`is required to maintain catalyst activity, to achieve economically acceptable
`carbonylation rates, and to maintain good catalyst stability [23, 25]. Because
`of the high water concentration in the reactor, the separation of water from
`acetic acid is a major energy cost and unit capacity limitation in this process.
`A considerable saving in operating cost and a low cost expansion potential can
`be realized by operating at a low reaction water concentration if a way can
`be found to compensate for the decrease in the reaction rate and catalyst sta(cid:173)
`bility.
`Low-water operation can be accomplished with modifications to the process
`which include significant changes in the catalyst system [23]. The main catalytic
`cycle for high-water methanol carbonylation is still operative in the low-water
`process (see Section 2.1.2.1.1 ), but at low water concentration two other catalytic
`cycles influence the carbonylation rate. The incorporation of an inorganic or or(cid:173)
`ganic iodide as a catalyst co-promoter and stabilizer allows operation at optimum
`methyl acetate and water concentrations in the reactor. Carbonylation rates com(cid:173)
`parable with those realized previously at high water concentration (ca. 10 molar)
`are demonstrated at low reaction water concentrations (less than ca. 4 molar) in
`laboratory, pilot plant, and commercial units, with beneficial catalyst stability
`and product selectivity [23]. With this proprietary AO technology, the methanol
`carbonylation unit capacity at the Celanese Clear Lake (TX) facility has increased
`from 270 X 103 metric tons per year since start-up in 1978 to 1200 X 103 metric
`tons acetic acid per year in 2001 with very low capital investment [33]. This unit
`capacity includes a methanol-carbonylation acetic acid expansion of 200 X 103
`metric tons per year in 2000 [33].
`Recently start-up of a new 500 X 103 metric tons per year acetic acid unit at the
`Celanese Singapore facility was successful using AO technology.
`
`Promotion by Methyl Acetate
`
`In the low-water AO technology [23], the major function of the iodide salts is to
`stabilize the rhodium carbonyl catalyst complexes from precipitation as insoluble
`rhodium triiodide (Rhi3) [5c]. Lithium iodide (Lii) is the preferred salt. The iodide
`salts also promote catalyst activity (see below). However, the key factor that con-
`
`005
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`109
`
`tributes most significantly to carbonylation rate enhancement at low water is the
`methyl acetate (CH30Ac) concentration [5, 23] (CH30H fed to the reactor exists
`mainly as CH30Ac in an acetic acid catalyst solution).
`Monsanto investigators demonstrated that the methanol carbonylation is zero
`order in CH30H even at low CH30H concentration [21]. This is true as long as
`the concentration of the active catalytic species, [Rh(CO)zi2r, does not vary
`with CH30H (CH30Ac) concentration, which is probably the case under the
`high water concentrations of the Monsanto process. For the low-water/high(cid:173)
`iodide-promoted catalyst system, increasing the CH30Ac concentration over a
`range (ca. 0-1 molar) affords an increase of the carbonylation rate by raising
`the proportion of total rhodium in the catalyst solution as [Rh(CO)zi2r, the active
`catalyst species [23, Sc]. This shift in the concentration of [Rh(CO)zi2r results
`from the direct effect of CH30Ac concentration on the rhodium-catalyzed
`water-gas shift reaction (WGSR; see Section 3.2.11) [5c]. The rhodium-catalyzed
`WGSR produces carbon dioxide and hydrogen (eq. (7)), the major inefficiency of
`the methanol carbonylation technology.
`
`(7)
`
`This reaction is inherent to the process and plays an integral role in the activity
`of the carbonylation reaction. It has been well studied by two different research
`groups [15, 34, 35]. The WGSR consists of an oxidation and reduction process
`as represented in eqs. (8) and (9) and shown in more detail in Scheme 1.
`
`The steady-state concentration of [Rh(CO)zi2r which affects the carbonylation
`rate depends on whether the reduction or the oxidation process is rate-limiting in
`the WGSR catalytic cycle (Scheme 1 ). The CH30Ac concentration determines
`which reaction is the rate-determining step of the WGSR by influencing the
`hydriodic acid concentration in the catalyst solution. The CH30Ac concentration
`affects the equilibrium concentration of HI due to the equilibrium represented in
`eq. (10) [23].
`
`(10)
`
`In the low-water/high-Lii catalyst system at high CH30Ac, the HI concentra(cid:173)
`tion is very low ( <0.004 molar
`limit of detection). This is also indicated by
`the presence of lithium acetate (LiOAc) (ca. 0.3 molar) in the catalyst solution
`from equilibrium with Lii (eqs. (11) and (12)) [23].
`
`HI + LiOAc
`
`Lil + HOAc
`
`(11)
`
`(12)
`
`006
`
`

`
`110
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`H20
`
`METHANOL
`CARBONYLATION
`CYCLES
`
`[ )LRh(CO)I3]-
`
`" [CH3-Rh(C0)2I3r
`
`Scheme 1. Interrelated reaction paths for the rhodium-catalyzed methanol carbonylation
`process [5c]; Me= CH3•
`
`At this low HI concentration, the rate-determining step of the WGSR is
`probably the oxidative addition of HI to [Rh(CO)zi2r to form [HRh(COhhr
`(Scheme 1). The rate of oxidation of [Rh(CO)zi2t is very low. The [Rh(CO)zi2r
`complex is therefore the predominant species in solution which affords a maximum
`methanol carbonylation rate for each level of water concentration. Increasing the
`water concentration within the low-water regime, the rate-limiting step of the
`WGSR remains the oxidation of [Rh(COhlzr, so the C02 production is still
`very low; but the [Rh(CO)zi2r concentration increases since the rate of reduction
`of [Rh(C0)2I4r increases (Scheme l).
`Another effect of the low-water/high-Lil catalyst system at high CH30Ac is the
`suppression of the overall WGSR (ca. 10-fold) with the reduction of HI
`concentration [23]. This decrease in HI concentration leads to a marked
`improvement in the CO efficiency of the process.
`
`Promotion by Iodide and Acetate
`
`Though the primary effect of the addition of iodide salts at low water concentra(cid:173)
`tion is catalyst stabilization, high iodide salt concentration and the low concentra-
`
`007
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`111
`
`tion of acetate salt generated by the iodide salt under process conditions ( eq. (11))
`affords a moderate promotional effect on the carbonylation reaction. This promo(cid:173)
`tional effect has been proposed by others, however, under significantly different
`reaction conditions [36-38]. This rate effect has been demonstrated by room-tem(cid:173)
`perature IR model studies of the oxidative addition of CH31 on [Rh(C0)2I2r and
`kinetic studies of the reaction in batch and continuous experimental units [24].
`Under process conditions the rate enhancement by these salts is lower than the
`rate enhancement by CH30Ac of the carbonylation rate.
`Prior to these investigations by HCC the promotional effect of iodide on the
`oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demon(cid:173)
`strated that the rate enhancement of this reaction in anhydrous medium was
`attributable to increased nucleophilicity of the rhodium catalyst with added iodide.
`The rationale for this observation was the generation of an anionic rhodium
`carbonyl complex, [Rh(COhi(L)r. Generation of this species was observed
`only with iodide added to certain neutral Rh1 species. No rate enhancement
`occurred with iodide added to the anionic complex, [Rh(C0)2IX [39]. Similarly,
`in solvents with a high water concentration, iodide salts exhibited no rate enhance(cid:173)
`ment in the presence of [Rh(C0hi2r [11]. Maitlis and co-workers, in more recent
`investigations, reported a promotional effect of iodide in aprotic solvents on the
`oxidative addition of CH31 on [Rh(COhhr [9a, 9c].
`The promotion by iodide or acetate salt of methanol carbonylation at a low
`water concentration is truly unique. Based on the currently available evidence,
`the overall carbonylation rate increase is presumably due in part to the formation
`of a strong nucleophilic five-coordinate dianionic intermediate [Rhi2(C0h(L)2
`-
`(L = 0 or OAc)] which is more active toward oxidative addition of Mel. This re(cid:173)
`action pathway is described in Scheme 1, together with the traditional proposed
`rate-determining step. In related nucleophilic reactions, a five-coordinate dianion
`is proposed for the promotion by halide salt in anhydrous solvents of the oxidative
`addition of CH31 to [Rh(COhi2r in the carbonylation of methyl acetate to acetic
`anhydride [9a]. Also, the Rh1 dianion, [Rh(C0hl3f-, is postulated for the reaction
`in the rhodium-catalyzed WGSR [35]. Though
`of HI with [Rh(COhi2r
`[Rh(C0)212(L)f- has not been detected spectroscopically under ambient condi(cid:173)
`tions in model studies, it cannot be ruled out at higher temperatures [5, 9c].
`Most likely, the more nucleophilic dianion is present in much lower concentration
`relative to the monoanion catalyst, so detection is very difficult.
`In low-water conditions, it is proposed that the promotional effects of iodide
`and acetate involve two competitive pathways between four-coordinate and
`five-coordinate nucleophilic intermediates for rate-determining reactions with
`CH3I (Scheme 2).
`The rate law from the steady-state derivation of the proposed reaction paths is
`consistent with kinetic studies under process conditions of the overall reaction and
`with room-temperature model studies for the rate-determining oxidative addition
`step (eq. (13)) [5].
`
`-d [Rh(COhl2-]
`dt
`
`(13)
`
`008
`
`

`
`112
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`[Me-Rh(C0)213]2-
`~ (fast)
`
`0
`II
`[Me-C-Rh(CO)I2Lr
`
`Scheme 2. Pathways to [AcORh(CO)IzLr.
`
`These kinetic and model studies support a promotional effect contributed
`primarily by the formation of acetate salts [5]. The iodide functions as a catalyst
`stabilizer to preclude the formation of insoluble rhodium iodides [5c, 23]. Under
`process conditions the majority of the inorganic salt is in the Lii form (eq. (11))
`[23].
`Other explanations have been proposed for the carbonylation rate enhance(cid:173)
`ment at low water concentration with iodide and acetate salts; e. g., contact
`ion-pairing or "general salt effect" [9a, 9c] or formation of CH3I from the re(cid:173)
`action of iodide with CH30Ac (eq. 11) [9c, 19]. Ion-pairing effects cannot be
`ruled out but are highly unlikely. Model studies of the rate of oxidative addition
`of CH3I on [Rh(CO)zi2r with Lii and LiOAc demonstrate a very small effect
`of polar solvents on the reaction rate or the IR spectra of [Rh(COhizr [5b].
`In comparison, poorly coordinating salts such as LiBF4 and LiCF3S03 have
`no effect on the rate of carbonylation or on the IR spectra of the rhodium
`am on.
`It is speculated also that the promotional effect observed for LiOAc in the
`model studies is rather a "general salt effect" from the formation of Lil [9c].
`This is clearly not the case. In model studies, no CH30Ac is detected when
`LiOAc is added to CH31 under the IR conditions. The reaction rate to Lil from
`LiOAc and CH31 under these conditions is very slow relative to the oxidative
`addition reaction.
`Formation of additional CH31 [9c] from Lil is not sufficient in the low-water
`AO process to affect the carbonylation rate. In model studies, the generation of
`CH31 is not possible since CH30Ac is not present in any of the experiments. In
`continuous carbonylation experiments under commercial reaction conditions,
`the reactor operation is controlled to minimize variations in the CH3I concentra(cid:173)
`tion in the catalyst solution so that the CH31 is kept constant [5c].
`
`009
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`113
`
`In anhydrous conditions for the carbonylation of CH30Ac to acetic anhydride,
`CH3I is regenerated from Lii; this is considered an important step in the reaction
`and can become rate-determining [41 ]. In the low-water methanol carbonylation
`process, Lii has little effect on the regeneration of CH3I. Instead, the CH3I is
`regenerated from a faster and irreversible hydrolysis of acetyl iodide (see
`Scheme 1).
`The process chemistry of the methanol carbonylation reaction is summarized
`in Scheme 1. This catalytic reaction scheme depicts the balanced relationship
`between the methanol carbonylation, the WGSR and the iodide cycles under
`both regimes of water concentration. Within the scope of methanol carbony(cid:173)
`lation in an aqueous/acetic acid medium, the overall reaction rate depends
`not only on the nature of the rate-determining step(s), but also on reaction con(cid:173)
`ditions influencing the steady-state concentration of the active Rh1 species,
`[Rh(COhlzr.
`
`BP Low-Water Technology (Cativa™ Process)
`
`In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation
`chemistry first discovered by Paulik and Roth and later defined in more detail
`by Forster [20]. The thrust of this research was to identify an improved methanol
`carbonylation process using Ir as an alternative to Rh. This re-examination by BP
`led to the development of a low-water iridium-catalyzed process called Cativa™
`[20]. Several advantages were identified in this process over the Rh-catalyzed
`high-water Monsanto technology. In particular, the lr catalyst provides high car(cid:173)
`bonylation rates at low water concentrations with excellent catalyst stability
`(less prone to precipitation). The catalyst system does not require high levels of
`iodide salts to stabilize the catalyst. Fewer by-products are formed, such as
`propionic acid and acetaldehyde condensation products which can lead to low
`levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is
`improved.
`The Ir-catalyzed methanol carbonylation reaction has been studied extensively
`by several groups 9f-h. The mechanism for the reaction is more complex than for
`the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The
`extent of participation by each cycle depends on the reaction conditions. The
`anionic carbonylation pathway predominates in the Cativa™ process. The active
`Ir catalyst species is the iridium carbonyl iodide complex, [Ir(COhi2r. The carbo(cid:173)
`nylation reaction proceeds through a series of reaction steps similar to the Rh
`catalyst process shown in Figure 1; however, the kinetics involve a different
`rate determining step.
`the
`The proposed rate-determining step in Ir-catalyzed carbonylation is
`formation of the acyl complex, [Ir(CH 3CO)(CO)zi3] -. via methyl migration of
`the methyliridium(III) intermediate, [Ir(CH3)(CO)z(lh] -. This step involves the
`elimination of iodide and the subsequent addition of CO. This pathway is con(cid:173)
`sistent with the direct dependence of CO and the inverse dependence of iodide
`on the observed reaction rate.
`
`010
`
`

`
`114
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`BP extended this Ir carbonylation chemistry with the discovery of pro(cid:173)
`prietary promoters to achieve commercially viable high reaction rates at low
`reaction water conditions with essentially no dependence of CO partial
`pressure on the reaction rate [20]. These promoters can be categorized in two
`groups: simple iodide complexes of Zn, Cd, Hg, Ga, and In or carbonyl
`complexes of Re, Ru, Os, or W. It is believed these promoters participate in the
`to abstract
`iodide
`from
`thus
`[Ir(CH3)(C0h(lh] -,
`rate-determining
`step
`to
`form
`the corresponding acyl complex,
`facilitating methyl migration
`[Ir(CH3)(COh (Ih] -.
`Since the development of Cativa™, BP has converted three world-scale acetic
`acid plants from the old Rh-based high-water Monsanto technology to theIr-based
`low-water process. Significant capital and operating cost savings were achieved
`from the conversion of a Rh-based process to an Ir-based process. Also, the
`start-up in 2000 of a 500 X 103 metric ton per year acetic acid plant in Malaysia
`uses the Cativa™ process [20d].
`
`Process Technology
`
`The continuous rhodium-catalyzed methanol carbonylation process consists of
`three major areas: the reaction, flasher, and purification sections, as represented
`in Figure 1 [15].
`
`reactor
`
`flasher
`
`light ends
`column
`
`dehydration
`column
`
`heavy ends
`column
`
`to vent
`recovery
`
`to vent
`recovery
`
`,--------------------.
`
`product
`acetic acid
`
`mixed acid
`by-products
`
`purification column recycle
`
`Figure 1. Rhodium-catalyzed methanol carbonylation commercial process flow scheme [15].
`
`011
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`115
`
`Reaction Section
`Acetic acid is manufactured in a liquid-phase reaction at ca. 150-200 oc and
`3-6 MPa [15, 20]. Carbon monoxide and methanol are introduced continuously
`into a back-mixed reactor. Carbon monoxide mass transfer into the reactor liquid
`phase is maximized with adequate mixing at a high carbon monoxide partial
`pressure. The noncondensable by-products are vented from the reactor to maintain
`an optimum carbon monoxide partial pressure in the reactor. The reactor off-gas is
`treated to recover reactor condensables (i.e., CH31) before flaring. Methanol and
`carbon monoxide efficiencies are greater than 98% and 90% respectively [15].
`Major inefficiencies of the process are the concurrent manufacture of carbon di(cid:173)
`oxide and hydrogen from the WGSR, and methane from the hydrogenolysis of
`methanol. Both reactions are catalyzed by the same rhodium/iodide catalyst
`system as is methanol carbonylation. Propionic acid is the major liquid ineffi(cid:173)
`ciency [ 42-44]; however, higher-boiling carboxylic acids are also formed.
`These heavy ends are derived from methanol homologation reactions (eq. (14);
`cf. Section 3.2.7) [45].
`
`(14)
`
`Flash Section
`
`The product acetic acid and a majority of the light ends (methyl iodide, methyl
`acetate, water) are separated from the reactor catalyst solution and forwarded
`with dissolved gases to the distillation section by an adiabatic single-stage
`flash. This crude separation also functions to remove the exothermal heat of
`reaction. The catalyst solution is recycled to the reactor. Under the process condi(cid:173)
`tions of the flash, the rhodium catalyst is susceptible to deactivation at the low CO
`partial pressure of the flash vessel [46].
`
`Purification Section
`
`The purification of acetic acid requires distillation in a three-column process
`[15]. The vapor product from the flasher overhead feeds a light ends column.
`Methyl iodide, methyl acetate, and a portion of the water condense overhead
`in the light ends column to form two phases (organic and aqueous). Both over(cid:173)
`head phases return to the reaction section. The dissolved gases from the light
`ends column feed vent through the distillation section. Before this vent stream
`is flared, residual light ends are scrubbed and recycled to the process. The aque(cid:173)
`ous acetic acid side draw-off from the light ends column feeds the dehydration
`column. Water and some acetic acid from this column separate and recycle to
`the reaction section. The dry crude acetic acid is a residue stream from this col(cid:173)
`umn which feeds the heavy ends column. Product acetic acid is afforded as a
`vapor side draw-off of the heavy ends column. A mixture of high-boiling
`
`012
`
`

`
`116
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`acid by-products, primarily propionic acid, are removed as bottoms from this
`column.
`The corresponding Ir-catalyzed process consists of the same sections as
`described in Figure 1 for the Rh version except for a few improvements made
`to the reaction and purification section. In the reaction section, an agitator is
`not required to stir the reaction solution. Instead the reactor mixing is provided
`by the jet mixing effect of a reaction cooling loop. In the purification, the light
`ends and dehydration columns of the Rh-catalyzed process are combined in one
`distillation column, the "drying column" [20d].
`With continuing refinements to the rhodium-catalyzed, liquid-phase, methanol
`carbonylation technology (see Section 2.1.2.1.5), this industrial process will
`remain the most competitive route to acetic acid, well into the 21st century.
`
`2.1.2.1.3 Acetic Anhydride
`
`Introduction
`
`The processes for the manufacture of acetic anhydride have included, initially, the
`distillation of wood pulp, which was followed by the ketene route from acetic acid
`or acetone and finally the ethylene based oxidation of acetaldehyde. The carbony(cid:173)
`lation of CH30Ac to acetic anhydride has in part replaced anhydride capacity
`from the more expensive processes.
`The intensive investigation of new metal-catalyzed processes for the manufac(cid:173)
`ture of acetic anhydride and acetic acid was driven by the high cost of petroleum
`and raw materials in the 1970s. As a result, synthesis gas-based technologies were
`introduced. The major sources of syn gas are coal and heavy petroleum residues.
`Natural gas or naphtha fractions were also used as feedstocks for synthesis gas.
`The broad development of homogeneously catalysed syntheses of commercial
`anhydride manufacturing is directly related to the process developments of several
`companies. In particular, Tennesee Eastman developed a rhodium-catalyzed
`process based on syngas [47].
`On the basis of the carbonylation of methyl acetate using Co, Ni or Fe catalysts
`by BASF [ 48] in the 1950s and of the initial results from the Rh catalyzed carbo(cid:173)
`nylation of methanol by Monsanto [21, 49] in the early 1970s, Hakon [49, 50],
`Eastman [41 b, 51], Ajinamoto [52], Showa Denko [53], BP [2, 20, 54, 55], and
`Hoechst [56] worked on substantial developments for the Group VIII metal-cata(cid:173)
`lyzed manufacture of acetic anhydride. Promisi