104
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`[260] (a) Hoechst AG (W. A. Herrmann, M. Elison, C. Kocher, J. Fischer, K. Ofele)
`DE 4.447.066 (1955); (b) Hoechst AG (W. A. Herrmann, M. Elison, C. Kocher,
`J. Fischer), DE 4.447.067 (1995); (c) Hoechst AG (W. A. Herrmann, M. Elison,
`C. Kocher, J. Fischer), DE 4.447.068 (1995); (d) Hoechst AG (W. A. Herrmann),
`DE 4.447.070 (1995).
`[261] (a) A. J. Arduengo, R. L. Harlow, M. Kline, f. Am. Chern. Soc. 1991, 113, 361;
`(b) D.A. Dixon, A.J. Arduengo, J. Phys. Chern. 1991, 95, 4180; (c) A.J.
`Arduengo, H. V. Rasika-Diaz, R. L. Harlow, M. Kline, f. Am. Chern. Soc. 1992,
`114, 5530; W. A. Herrmann eta!., Chern. Eur. f. 1996, in press.
`[262] (a) M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M. van Leeu(cid:173)
`wen, Organometallics 1995, 14, 3081; (b) Hoechst AG (W. A. Herrmann, C. W.
`Kohlpaintner, R. Schmid, G. P. Albanese, H. Bahrmann), DE 4.426.577 (1994).
`[263] (a) M. Oguri, T. Noda, JP 06.166.654 (1994); Chern. Abstr. 1994, 121, 230335; (b)
`H. Saito, Y. Tokito, JP 06.135.888 (1994); Chern. Abstr. 1994, 121, 204837.
`[264] (a) I. T. Horvath, J. Rabai, Science 1994, 266, 72; (b) Exxon Res. Eng. Co. (I. T.
`Horvath, J. Rabai), EP 633.062 (1994); (c) J. A. Gladysz, Science 1994, 266, 55.
`[265] Union Carbide Corp. (J. E. Babin, J. M. Maher, E. Billig), US 5.364.950 (1992).
`[266] BP (M. J. Lawrenson), GB 1.197.902 (1967).
`[267] B. L. Moroz, V. A. Likholobov in Handbook of Heterogeneous Catalysis (Eds.: G.
`Ertl, H. Knozinger, J. Weitkamp), VCH, Weinheim, 1997.
`[268] Eastman Kodak Co. (J. Hagemeyer, A. P. Milton), US 2.921.089 (1960).
`[269] B. Cornils, E. Kuntz, f. Organomet. Chern. 1995, 502, 177.
`[270] B. Cornils, E. Wiebus, Rec. Trav. Chim. Pays-Bas, 1996, 115, 211.
`[271] Anon., Chemical Marketing Reporter, Sept. 18, 1995.
`
`2.1.2 Carbonylations
`
`2.1.2.1 Synthesis of Acetic Acid
`and Acetic Acid Anhydride from Methanol
`Michael Gauj3, Andreas Seidel, Paull Torrence, Peter Heymanns
`
`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:
`
`The reaction is catalyzed by metal complexes, the central atoms favorably
`being Co or Rh. Nowadays all other routes to acetic acid (especially via acetal-
`
`(1)
`
`CE Ex. 2034
`Daicel v. Celanese
`IPR2015-00171
`
`001
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`105
`
`dehyde, 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
`complex 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].
`A common catalytic pathway is proposed which involves the nucleophilic
`attack of the active Rh1 catalyst complex, [Rh(CO)z12]-, on methyl iodide (CH31)
`intermediate, [Rh(CH3) (CO)z (1) 3]-. Rapid
`to form a methylrhodium(III)
`methyl migration in this complex generates the acylrhodium(III) intermediate,
`[Rh(CH3CO) (CO)I3]-, which reacts with CO to form [Rh(CH3CO) (CO)zl3](cid:173)
`and subsequently reductively eliminates acetyl iodide and regenerates the
`rhodium(!) anion. The final reaction of acetyl iodide with compounds contain(cid:173)
`ing hydroxyl groups such as water, methanol (CH30H), or acetic acid (eq. (2))
`leads to the formation of hydrogen iodide (HI) and the corresponding acetyl
`derivatives.
`
`CH3COI + HOR
`
`-
`
`CH3COOR + Ht
`
`(2)
`
`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
`dimethyl ether) to regenerate methyl iodide promoter (eq. (3)):
`
`HI + CH30R
`
`-
`
`CH3I + HOR
`
`(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]:
`
`CH3COI + CH30R
`
`-
`
`CH3I + CH3COOR
`
`(4)
`
`002
`
`

`
`106
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`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 CH3I [9c, 9e].
`The carbonylation rate is independent of the type of rhodium compound
`charged to the reaction as long as sufficient CH3I and CO are available. This
`supports the concept of the generation of a common active catalyst under reac(cid:173)
`tion conditions [11, 12].
`An overview of Monsanto's catalyst system in comparison with other pro(cid:173)
`cesses 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
`
`[Rh(C0) 2I2]-H+
`[Rh(CO)zlz]-u+
`
`Mel/HI
`Mei/Lil
`Mei/Lil
`[Rh(CO)zlzru+
`[Rh(CO)zlzrP(R)4 + Mei/P salts
`[Rh(CO)zlzrN(R)4 + MeliN salts,
`(Zr compound)
`
`Monsanto AcOH
`AcOH
`HCC
`Eastman
`Ac20
`Ac20
`Hoechst
`BP
`Ac20/AcOH
`
`Rh
`Rh
`Rh
`Rh
`Rh
`
`2.1.2.1.2 Acetic Acid
`
`Introduction
`
`The manufacture of acetic acid by the rhodium-catalyzed carbonylation of
`methanol (eq. (5)) is one of the most important industrial processes.
`
`CH30H + CO
`
`150-200 oc
`30-60 bar
`
`CH3COOH
`
`(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 consump(cid:173)
`tion. The majority of the remaining worldwide acetic acid production is used
`to manufacture other acetate esters (i.e., cellulose acetates from acetic anhy(cid:173)
`dride 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-
`
`003
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`107
`
`catalyzed carbonylation 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 selecti(cid:173)
`vity 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 sec(cid:173)
`tion, the traditional process chemistry as patented by Monsanto is discussed,
`with emphasis on some of the significant improvements that Monsanto's licen(cid:173)
`cee, Hoechst Celanese, has contributed to the technology. Discussions also
`incorporate recent investigations toward a further understanding of the reac(cid:173)
`tion mechanism.
`
`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 implemen(cid:173)
`tation 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
`Monsanto process was commercialized, the cobalt-catalyzed process became
`noncompetitive. Today over ten companies worldwide practice the methanol
`carbonylation technology [2].
`In 1978, Hoechst Celanese (HCC) was the first Monsanto licencee to
`operate the Monsanto acetic acid process commercially. Soon after start-up of
`this unit, HCC implemented several process improvements to expand the unit
`capacity. Later, in the early 1980s, HCC developed the proprietary, low reac(cid:173)
`tion water, technology which improved the process significantly. The low-water
`technology was achieved in part by increasing the rhodium catalyst stability by
`addition of inorganic iodide in high concentrations to the reaction 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 catalyst compo(cid:173)
`sition 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 composition
`of the catalyst solution used in the low-water technology by HCC [23] is signifi(cid:173)
`cantly different from the catalyst composition used in the original methanol
`carbonylation process patented by Monsanto [25]. In 1986 BP Chemicals Ltd.
`
`004
`
`

`
`108
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`purchased from Monsanto the technology and licencing rights to the low-pres(cid:173)
`sure methanol carbonylation technology which did not include the proprietary
`technology developed by HCC.
`
`Process Chemistry
`
`Monsanto Technology
`
`The reaction chemistry of the rhodium-catalyzed methanol carbonylation pro(cid:173)
`cess 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 [Cd~30H] = k [Rh] [CH3I]
`
`(6)
`
`The reaction medium also plays a key role in the overall activity of the cata(cid:173)
`lyst 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 in(cid:173)
`volved in the transition-state species of the rate-determining step [5c]. Maxi(cid:173)
`mum carbonylation rates are demonstrated in polar solvents and the additions
`of protic solvents accelerate 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 carbonylation 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 Technology
`
`In the Monsanto process a substantial quantity of water in the reaction system
`is required to maintain catalyst activity, to achieve economically acceptable car(cid:173)
`bonylation 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 con(cid:173)
`siderable 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 stability.
`
`005
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`109
`
`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 cata(cid:173)
`lytic cycles influence the carbonylation rate. The incorporation of an inorganic
`or organic iodide as a catalyst co-promoter and stabilizer allows operation at
`optimum methyl acetate and water concentrations in the reactor. Carbon(cid:173)
`ylation rates comparable with those realized previously at high water concen(cid:173)
`tration (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
`technology, the methanol carbonylation unit capacity at the HCC Clear Lake
`(TX) facility will triple from 270 X 103 metric tons per year since start-up in
`1978 to 820 x 103 metric tons acetic acid per year with very low capital invest(cid:173)
`ment [33]. This unit capacity includes a methanol-carbonylation acetic acid
`expansion of 250 X 103 metric tons per year in 1995 [33].
`
`Promotion by Methyl Acetate
`
`In the low-water HCC technology [23], the major function of the iodide salts is
`to stabilize the rhodium carbonyl catalyst complexes from precipitation as
`insoluble rhodium triiodide (Rhl3) [5c]. Lithium iodide (Lii) is the preferred
`salt. The iodide salts also promote catalyst activity (see below). However, the
`key factor that contributes most significantly to carbonylation rate enhance(cid:173)
`ment 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(C0)2I2]-, 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(cid:173)
`high-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(C0)2I2]-,
`the active catalyst species [23, 5c]. This shift
`in
`the concentration of
`[Rh(C0)2I2]- 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)
`
`006
`
`

`
`110
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`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.
`
`Scheme 1. Interrelated reaction paths for the rhodium-catalyzed methanol carbonylation
`process [Sc). Me = CH3•
`
`The steady-state concentration of [Rh(C0)2I2]- which affects the carbonyla(cid:173)
`tion rate depends on whether the reduction or the oxidation process is rate(cid:173)
`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].
`
`007
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`111
`
`In the low-water/high-Lil 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 Lil (eqs. (11) and (12)) [23).
`
`HI + LiOAc
`
`-
`
`Lil + HOAc
`
`(11)
`
`(12)
`
`At this low HI concentration, the rate-determining step of the WGSR is
`probably the oxidative addition of HI to [Rh(C0) 2I2)- to form [HRh(C0) 2I3)(cid:173)
`(Scheme 1). The rate of oxidation of [Rh(C0)2I2)- is very low. The [Rh(C0) 2I2)(cid:173)
`complex is therefore the predominant species in solution which affords a maxi(cid:173)
`mum methanol carbonylation rate for each level of water concentration. In(cid:173)
`creasing the water concentration within the low-water regime, the rate-limiting
`step of the WGSR remains the oxidation of [Rh(CO)zl2)-, so the C02 produc(cid:173)
`tion is still very low; but the [Rh(C0) 2I2)- concentration increases since the
`rate of reduction of [Rh(CO)zi4)- increases (Scheme 1).
`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 concen(cid:173)
`tration is catalyst stabilization, high iodide salt concentration and the low con(cid:173)
`centration of acetate salt generated by the iodide salt under process conditions
`(eq. (11)) affords a moderate promotional effect on the carbonylation reaction.
`This promotional effect has been proposed by others, however, under signifi(cid:173)
`cantly different reaction conditions [36-38). This rate effect has been demon(cid:173)
`strated by room-temperature IR model studies of the oxidative addition of
`CH3I on [Rh(C0) 2I2]- and kinetic studies of the reaction in batch and contin(cid:173)
`uous experimental units [24]. Under process conditions the rate enhancement
`by these salts is lower than the rate enhancement by CH30Ac of the car(cid:173)
`bonylation 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 rho-
`
`008
`
`

`
`112
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`dium carbonyl complex, [Rh(CO)zi(L)]-. Generation of this species was ob(cid:173)
`served only with iodide added to certain neutral Rh1 species. No rate enhance(cid:173)
`ment occurred with iodide added to the anionic complex, [Rh(C0)2I2]- [39].
`Similarly, in solvents with a high water concentration, iodide salts exhibited no
`rate enhancement in the presence of [Rh(CO)zi2]- [11]. Maitlis and co-workers,
`in more recent investigations, reported a promotional effect of iodide in apro(cid:173)
`tic solvents on the oxidative addition of CH3I on [Rh(CO)zi2]- [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 for(cid:173)
`mation of a strong nucleophilic five-coordinate dianionic intermediate [Rhi2
`
`(COML)2- (L=O or OAc) which is more active toward oxidative addition of
`Mel. This reaction pathway is described in Scheme 1, together with the tradi(cid:173)
`tional proposed rate-determining step. In related nucleophilic reactions, a five(cid:173)
`coordinate dianion is proposed for the promotion by halide salt in anhydrous
`solvents of the oxidative addition of CH3I to [Rh(C0)2I2]- in the carbonylation
`of methyl acetate to acetic anhydride [9a]. Also, the Rh1 dianion, [Rh(CO)zi3]2-,
`is postulated for the reaction of HI with [Rh(C0)2I2]- in the rhodium-catalyzed
`WGSR [35]. Though [Rh(C0)2I2(L)]2- has not been detected spectroscopically
`under ambient conditions 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).
`
`[Me-Rh(C0)213]2-
`~ (fast)
`
`0
`II
`Me-C-Rh(CO)I2L]-
`
`Scheme 2. Pathways to [Ac0Rh(CO)I2L]-.
`
`009
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`113
`
`The rate law from the steady-state derivation of the proposed reaction paths
`is consistent with kinetic studies under process conditions of the overall reac(cid:173)
`tion and with room-temperature model studies for the rate-determining oxida(cid:173)
`tive addition step (eq. (13)) [5].
`
`(13)
`
`These kinetic and model studies support a promotional effect contributed
`primarily by the formation of acetate salts [5]. The iodide functions as a cata(cid:173)
`lyst 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 reac(cid:173)
`tion 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 addi(cid:173)
`tion of CH3I on [Rh(C0)2I2]- with Lii and LiOAc demonstrate a very small
`effect of polar solvents on the reaction rate or the IR spectra of [Rh(C0)2I2](cid:173)
`[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 rho(cid:173)
`dium anion.
`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 Lii [9c].
`This is clearly not the case. In model studies, no CH30Ac is detected when
`LiOAc is added to CH3I under the IR conditions. The reaction rate to Lii
`from LiOAc and CH3I under these conditions is very slow relative to the oxi(cid:173)
`dative addition reaction.
`Formation of additional CH3I [9c] from Lil is not sufficient in the low-water
`process to affect the carbonylation rate. In model studies, the generation of
`CH3I 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 concen(cid:173)
`tration in the catalyst solution so that the CH3I is kept constant [5c].
`In anhydrous conditions for the carbonylation of CH30Ac to acetic anhy(cid:173)
`dride, 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 car(cid:173)
`bonylation process, Lil has little effect on the regeneration of CH31. 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 carbon-
`
`010
`
`

`
`114
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`ylation 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
`conditions influencing the steady-state concentration of the active Rh 1 species,
`[Rh(C0) 212]-.
`
`Process Technology
`
`The continuous rhodium-catalyzed methanol carbonylation process consists of
`three major areas: the reaction, flasher, and purification sections, as represent(cid:173)
`ed in Figure 1 [15].
`
`Reaction Section
`Acetic acid is manufactured in a liquid-phase reaction at ca. 150-200 oc and
`30-60 bar [15, 20]. Carbon monoxide and methanol are introduced contin(cid:173)
`uously 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
`
`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
`
`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.,
`CH3I) 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 dioxide and hydrogen from the WGSR,
`and methane from the hydrogenolysis of methanol. Both reactions are cata(cid:173)
`lyzed by the same rhodium/iodide catalyst system as is methanol carbonylation.
`Propionic acid is the major liquid inefficiency [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].
`
`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
`conditions 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
`aqueous acetic acid side draw-off from the light ends column feeds the dehy(cid:173)
`dration 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 column 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 acid by-products, primarily propionic acid, are removed as bot(cid:173)
`toms from this column.
`With continuing refinements to the rhodium-catalyzed, liquid-phase, metha(cid:173)
`nol 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.
`
`012
`
`

`
`116
`
`2.1 Carbon Monoxide and Synthesis Gas Chemistry
`
`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 acetal(cid:173)
`dehyde. The carbonylation 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 manu(cid:173)
`facture 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 feed(cid:173)
`stocks for synthesis gas.
`The broad development of homogeneously catalysed syntheses of commer(cid:173)
`cial anhydride manufacturing is directly related to the process developments of
`several companies. In particular, Tennesee Eastman developed a rhodium-cata(cid:173)
`lyzed process based on syngas [47].
`On the basis of the carbonylation of methyl acetate using Co, Ni or Fe cata(cid:173)
`lysts by BASF [48] in the 1950s and of the initial results from the Rh catalyzed
`carbonylation of methanol by Monsanto [21, 49] in the early 1970s, Hakon
`[49, 50], Eastman [41b, 51], Ajinamoto [52], Showa Denko [53], BP [2, 20, 54,
`55], and Hoechst [56] worked on substantial developments for the Group VIII
`metal-catalyzed manufacture of acetic anhydride. Promising catalyst metals are
`Rh, Pd, Ni, and Co; among these, Rh has an essential position due to its
`exceptional carbonylation activity [20].
`The preference for rhodium was known from the investigations conducted
`by Monsanto. Diversifying from these patents, the available low-cost catalyst
`metals were studied which have catalytic properties comparable with those of
`rhodium. In the presence of alkali metal salts and Group VIB metal carbonyls,
`Ni-catalyzed carbonylation operates under mild conditions of nearly 190°C and
`40 bar [57]. Nevertheless, only limited success was found with other catalyst
`metals, which mostly showed decreasing selectivity, lower yields and higher
`energy consumption.
`
`Process Chemistry
`
`Through the investigation of different carbonylation raw materials for manufac(cid:173)
`ture of acetic anhydride, carbonylation of methyl acetate (eq. (15)) was found
`to be the method of choice.
`
`CH3COOCH3 + CO -
`
`(CH3C0)20
`
`(15)
`
`013
`
`

`
`2.1.2.1 Synthesis of Acetic Acid and Acetic Acid Anhydride from Methanol
`
`117
`
`Using a Monsanto catalyst system (Rh/Mei/CO) the carbonylation of methyl
`acetate has a long induction period in the absence of water and a low reaction
`rate in comparison with the synthesis of acetic acid under typical reaction con(cid:173)
`ditions.
`As a result, two significant developments contributed to increased carbon(cid:173)
`ylation rates and reduced induction periods. The first development was the
`introduction of hydrogen with the carbon monoxide feed to reduce Rhm to the
`activated Rh1 carbonylation complex (eq. (16)).
`
`This reduction reaction minimizes the formation of unreactive Rhm-species
`that form insoluble compounds such as rhodium triiodide (Scheme 3).
`
`(16)
`
`Yeo = 2075 em·1
`
`v,, = 2040 em·'
`
`v,, = 2120; 2090 em- 1
`
`v,, = 2060; 1990 em-1
`
`co 11 -co
`~ -n
`21-
`
`Rhl3
`
`Scheme 3. Rhm-complexes through formal addition of iodine to the active Rh1
`-
`complex.
`
`This catalyst deactivation results from the oxidation of the Rh1 complex by
`formal addition of iodine (eqs. (18)-(20)). This form of deactivation in partic(cid:173)
`ular occurs at high temperatures under CO-deficient conditions [58]. The state
`of activity of the rhodium complex system can be observed by IR spectroscopy
`(Figure 2). Inactive Rhm species are transformed to the Rh1 complex by reduc(cid:173)
`tion with hydrogen.
`The second developm