Selective Oxidation and Ammoxidation of Olefins by Heterogeneous Catalysis Robert K. Grasselll The Standard Oil Company (Ohio), Research Center, 4440 Warrensville Center Road, Cleveland, OH 44128 Eighty-five percent of industrial organic chemicals are currently produced by catalytic processes from petroleum and natural gas sources. Ahout one-quarter are produced by heterogeneous oxidation catalysis and can be classified by mechanistically related reactions, namely allylic oxidation and ammoxidation, epoxidation, aromatic oxidation, and paraffinic oxidation (Table 1). Commercially, the most important member of allylic oxidation is ammoxidation of propylene, ammonia, and air to acrylonit~ile, CH,=C
a
CH,
+ NH, + 3120,%
CH,=CHCN
+ 3H20
The impact of the Sohio ammoxidation process was an immediate drastic reduction in acrylonitrile price and greatly increasedproduction which led to the discovery of manynew applications in fibers, resins, rubbers and specialty materials (Figs. 1and 2).
(1)
This Sohiordiscovered process ( I ) (Fig. I), accounts for the majority of the 8 billion pounds of acrylonitrile currently produced annually worldwide. I t displaced the more expensive acetylene-HCN-based route in the early 1960's
as well as other obsolete processes which also used expensive reagents (e.g. ethylene oxide
/
CH2-CH2
+ HCN
HOCH&H&N
0 ''
-Ha0 Figure 1. Acrylonihile (Sohio process)--Th
CHz=CHCN (3)
and acetaldehyde) CHzCHO
+ HCN
CH3CHCN OH I
-Ha0
-4
C H y C H C N (4)
m-7W°C
and oxidants (e.g. NO.)
RUBBER
Table 1. Some Heterogeneous Catalytlc Oxldation Processes Oxidation Class ailyllc
Starting Material
Product
End Use
propylene CsHe
acrylonibile C&N
acryilc fibers resins, rubbers. adiponitrile
4.3
ethylene oxide CzH+O
ethylene glycol, antifreeze, polyesters. sufactams
5.8
poiyesters. p l a ~ t i ~ i z efine r~, chemicals
1.7
unsaturated polyester resins. tumaric acid. insecticides. tunnlaide~
0.5
epoxldation ethylene C2H1 aromatic
+xylene phthalic CeH4CHh anhydride C&WPOJ) maleic anhydride C&Wn
216
Journal of Chemical Education
ABSlSA
Million TonslY (World)
Figure 2. World acrylonltrile Consumption by end use. (Reproducedbom ref. 6 with permission. Copyright 1981, Adv. Calal.)
While the acrylonitrile process (eq 1) is the largest commercial application of allylic oxidation, the scope of the reactions includes other ammoxidation, oxidation, and oxydehydrogenation reactions: Ammoxidation
Oxidation
rate-determinine abstraction of an allvlic H hv lattice 0 associated with an a-H abstracting ion i ~ i 3 +sh5+, , Se4+)of the catalvst and formation of a r-allvl intermediate on the 0 or N H inserting site [ ( M o ~ + = ~ ( N H )Sb5+-O(NH), , Te6+-O(NH)], followed by formation of a a - 0 or a-N ally1 species depending on whether or not ammonia is present in the reaction. The reduced active sites are reeenerated bv lattice oxygen of the catalyst which in turn are replenished by gaseous oxygen, generally at a site different from the Development of Selectlve Oxldatlon and Ammoxldatlon Catalysts The development of the Sohio oxidation (acrolein) and ammuxidation (acrylonitrile) processes was hased on the workini! thnt lattice oxveen from a multiralent - hv~othesin .. solid metal oxide would serve as a more selective (2,3) and versatile oxidant than would molecular oxygen. Thus, in the oxidation of propylene to acrolein, lattice oxygen is used to form acrolein in the metal oxide reduction step,
while reoxidation of the reduced metal oxide by gaseous 0 reconstitutes the active oxidant 2M'"-2'+0,., Oxydehydrogenation
+ 0,~,,-2~'"'+0,
The metal oxide becomes a catalyst when the reduction and the oxidation process (i.e., redox process) can be carried out simultaneously in the presence of propylene and molecular oxygen
Propylene Among these are the production of methyacrylonitrile or methacrolein from isobutene, atroponitrile or atropaldehyde from a-methylstyrene, butadiene from n-butenes, and isoprene from isopentenes. Ammoxidation is a 6-electron oxidation, oxidation a 4electron oxidation, and oxydehydrogenation a 2-electron oxidation. From first principles, it is to be expected that ammoxidation is a more difficult and demanding process than oxidation, which is more demanding and difficult than oxydehydrogenation. Although catalysts that catalyze these three reactions are similar in nature, the ammoxidation catalysts are generally the most sophisticated ones among them. Thus a catalyst for the oxydehydrogenation reaction does not necessarily catalyze the ammoxidation reaction: however, ammoxidation catalysts generally catalyze both the oxidation and oxydehydrogenation reactions. Moreover, to achieve the highest yields in any given oxidation class, the individual catalyst must be optimized for the reaction in question. The most effective catalysts for olefin ammoxidation are complex mixed-metal molybdates and antimonates. Such catalysts possess key properties, including optimum metaloxygen bond strengths, isolated active sites, and a facile solid state redox couple. The catalytically active sites perform important steps including the rate-determining allylic hydrogen abstraction, ammonia activation, and 0 or NH insertion into the allvlic moietv. The catalvsts must also contain a redox coupl~consistin~ of a multivaient element in close structural proximity to the active site to dissociativelv chemisorh gaseous oxygen, promote oxygen ion transport, stabilize oxygen deficient structures, and maintain the active sites in a highly oxidized state. The mechanism of selective oxidation and ammoxidation reactions involves the
2
+ Oxygen
Acrolein
+
Water
The involvement of lattice oxygen is readily demonstrated by simply passing propylene in the absence of gaseous oxygen over an oxidant/catalyst such as Bi-molybdate whereby acrolein is formed hy lattice oxygen and the molybdate is reduced in the process. The reduced molyhdate is then readily regenerated to its original state by passing air over it in the absence of propylene. The involvement of lattice oxygen during the catalytic propylene oxidation, where both propylene and gaseous oxygen are simultaneously in contact with the catalyst, was experimentally confined (4) by reacting propylene and 1 8 0 2 over Bi-molybdates resulting in CH2=CHCH1W being the first observed product, followed by CHZ=CHCH'~O. T o accomplish the redox orocess effectivelv. oxidation catalysrs muit be readily red&ble by the h y d k a r b o n s to he oxidized, and must he readilv reoxidizable with gaseous oxygen once they had been reduced by the hydrocarbon (5). Therefore, effective oxidation and ammoxidation catalvsts are multifunctional in nature. They must perform a complex sequence of bond-breaking and bond-making processes (e.g., 32 in ammoxidation) and must provide a facile pathway to the desired intermediates (e.g., acrylonitrile or acrolein) since. thermodvnamicallv. undesirable waste and bv- roduct formation ii.e., CO, CO*, HCN) is more favorable than the formation of the desired products. Effective catalvsts are various molybdates, antirnonates, and tellurates, a i d their respective multicom~onentsvstems. The mechanistic cycle opeiative with such catalysts, e.g., bismuth molvbdates, is composed of a redox cvcle (Fia. 3) wherein propylene and ammonia are activated a t onesite and gaseous oxygen a t another site. The propvlene and am. .. rnonia rt,actionsite is bifunctional in nature and is composed in the rase of bismuth molghdates of Ri-0, which in the u - H
..
Volume 63 Number 3 March 1986
217
~i' 0' = Oxygen responsible for a-H abstraction 0" = Oxygen associated with Mo; responsible for oxygen inser-
tion into the allylic intermediate Proposed center for 0 2 reduction and dissociative chemisorption
o =
Figure 4. Schematic represemion of the catalyticallyactive site of Bi,MoOs. CHFCHCN + 3H20 MI = a-H Abstraction Element MI = Olefin Chemisorptian, 0 - and N-Insertion Element M = Reaxidation Element [ I = Oxygen Vacancy O 2 = Lattice Oxygen Figure 3. Aliylic oxidation mechanism-ammoxidatian mechanism
abstracting site, and Mo=O or MFNH, which is the propylene chemisorbing, ammonia activating, and 0- or N-insetting site. The reduced reaction site is reoxidized by lattice oxygen from the bulk, and the so-created anion vacancies migrate to the surface where they are filled a t the reoxidation site which is capable of dissociatively reducing gaseous oxygen to 02-.In this manner, the catalytic redox cycle is completed. I n situ Raman studies (6) reveal that lattice oxygen8 involved in the a-Habstraction and oxygen-insertion steps are distinct from each other and are located a t different lattice sites. Using Bi2Mo06 as a model catalytic compound, experi-
Figure 5. Molecular probes for 0- and N-Insertion. 218
Journal of Chemical Education
ments were performed with butene-1, propylene, methyl alcohol, and ammonia as reactants and ' 8 0 2 as the reoxidant. The studies show conclusively that the Mo-0 stretching frequencies (844, 803, 725 cm-') clearly shift to lower frequencies by about 14 to 18 cm-' when propylene, methanol, or NHa are used as reactants followed by subsequent 1 8 0 2 reoxidation. while virtuallv no shift is ohserved when the same experiment is performed with hutene-1. This is consistent with our earlier findings that Hi.O isassoriated with a H abstraction while MFO and MFNH are associated with 0 - and N-insertions. Another conclusion of this study is that in BizMoOs the site of gaseous 0 2 uptake and dissociative chemisorption is the locus vicinal to Bi-0-Bi which contains the orbitals of the two lone electron pairs of the two bismuths. A catalyst reaction site consistent with these findings is depicted in Figure 4. Mechanlsilc Aspects The mechanism of selective allylic oxidation and ammoxidation over heterogeneous catalysts, especially bismuth molybdate systems, has been the topic of much study in the
past 30 years and has been extensively reviewed (I, 3, 7). Earlv oublished '3C and D labeling work (8.9)confirmed the production of an allylic intermebiate by rate-limiting nhvdroaen abstraction, a feature of the reaction which has giinedgenera~acceptance. However, the nature of the allylic species (as a cation, radical ( l o ) ,or the less likely anion) as well as details concerning the steps involved in oxygen or nitrogen insertion to form acrolein and acrylonitrile (if ammonia is present), respectively, remained less well defined. In order to gain further insights about these mechanistic auestions. various molecular nrohes were used to nrovide information concerning entry k o different places aiong the' reaction path (Fig. 5) (11). Thus a comparison of oxidation and ammoxidation reactions of propylene, azopropene ( a precursor), and allyl alcohol and allyl amine (a-precursors) provide information concerning these surface intermediates. A detailed general mechanism (Fig. 61, based on the results obtained from studies using va&ous probe molecules (11,12), involves the initial allylic activation via chemisorption on coordinatively unsaturated Mo centers (I), with ahydrogen abstraction by oxygen atoms associated with hismuth to form the s-ally1 Mo complex (8) and, subsequently, the a-0-molyhdate (9) (the acrolein precursor) by C - 0 bond formation. The Mob+ allvl molvbdate (9) . . is analoeous to the Mo" mdolybdate (3) formed f n k the reactionofa~lylalcohol with Bi>Mo?O,?and undergoes a 1.4-hydrogen shift (the second hydrogen absrracrion) tu form acrolrin and a reduced site, a oroc~~sn that is acceltrated hv the oresenceof Hi in the catalyst. When NH3is present, the M-0 groups in the active sites are first converted to MFNH species (11) in a fast step, followed by formation of first the s (13) and then a a-N
allylic Mo species (141, analogous to the a O-species formed in oxidation. The corresponding 1.4-hvdroaen shift oroduces the 3-iminopropene-MOco&plex (i5),Ghich does-not desorb but undergoes another 1,lhydrogen shift after reoxidation of the Mo. This reoxidation step is presumed to occur prior to or simultaneously with the final hydrogen shift since otherwise an overall 5-electron reduction of Mo would be required for the conversion of propylene to acrylonitrile. After acrylonitrile formation, thk catalytic cycle i s completed by replenishment of lattice oxygen, removed during the coreduction cycle, by dissociative chemisorption of gaseous oxygen on reoxidation sites. The position of the equilibrium between the * and a allyl intermediates varies greatly depending on the nature of the catalyst. A sensitive probe to assess this is 1,l-d?propylene (Fig. 7). At 320°C, molybdatexprodure a 62dz:38doacrylonimile mixture indicntive u1rnpidl.v equilibrating allylic complexes a, and a2 which formdz and do acrylon&rile with k~ and k~ respectively (13). This corresponds to a s-u equilibrium constant (K)of about 1. Antimonates produce a 56d2:44do acrylonitrile mixture indicative of less rapidly eouilibratine and a1 soecies. while tellurates oroduce a that thk reaction 4idz:52do a~yl~nitri1e"m~xture'indicating of a, and a2 to form acrylonitrile (d2 and do, respectively) is much faster than the reformation of *-species. Thus, in this latter case the a-to-a allvl reaction is irreversible. i.e.. K is very large (Fig. 7). since q and an are initially formed in equal amounts, a 50:50 mixture of d2:do acrylonitrile is expected. The results are explained on the basis that in the transition series molyhdates highly unsaturated MFO and MFNH (12) surround the Moef oxygen/nitrogen-inserting
Figure 6.Mechanism of selective propyleneammoxldatlon and oxidation over bismuth molybdates.
Volume 63 Number 3 March 1986
219
m
-
-
K(rr=o)
(reversible Very large (Irreversible) Bismuth Molybdates Antimonates Selenium Tellurates 1.0
Figure 7. Comparison of isotope effectfor second hydrogen abstraction.
element, while in the main group antimonates and tellurates highly coordinatedly saturated bridging oxygenlnitrogen Sb--0-Sh, Sh-NH-Sh; Te-0-Te, Te-NH-Te, environments are located around the Sh5+andTe6+ oxygenhitrogen inserting elements, respectively. A consequence is reversible C-N bond formation which characterizes nitrogen insertion into the coordinately unsaturated transitionmetal catalysts, e.g., Mo6+=NH in the molyhdates which results in the 62d2:38domixture cited ahove, an irreversible C-N bond formation and mechanistically irreversible formation of a 50:50 mixture of the two a-species from the rcomplex for the tellurates, and an intermediate situation between these two extremes for the antimonates (Fig. 7). Electronic Structure Requirements of Selective Catalysts The most active and selective heteroeeneous catalvsts for the oxidation and ammoxidation of olefins to their corresounding aldehvdes and nitriles contain a t least three essential components ( 1 3 . The cntalyrically active sites are rornoosed of elements u,hich rhernisort, the olefins, ahlitract '\hydrogens from olefins to generate allylic species and selectively insert oxygen or nitrogen into the allylic surface intermediates. The active sites' immediate surroundings must provide a means for the facile transport of electrons, oxygen ions and vacancies. The most effective elements for these catalytic functions are listed in Table 2. T h e thusobtained emoirical correlation corroborates molecular orobe findings that the n-H abstracting component of the active site requires some electron density on the metal cation in the oxide. This imparts a partial radical character to the oxygen honded to the metal in question and thereby facilitates the abstraction of a hydrogen radical from the olefin. Except for uranium, the electronic structure of the cations listed is characterized by a large overlap between their occupied s Table 2.
a-H
Electronlc Structure of Some Catalytically Actlve Elements
Abstraction
Olefin Chemisorptian/ 0 insertion
Bi3+5d06P0p0
MoBt4858
Te4+4d05P5p0
Mo6+485P
220
Redox Couple
Example
Ce3+/Ce" Bi203.nMoOs Fe2+/Fe3+ Ma2+M,JiBiflo,O, Ce3+/Ce++ TezMoO,
Journal of Chemical Education
levels and the oxygen 2p electronic states in the solid state. The unoccupied p levels of the metal lie above the 2p states of oxygen, thereby constituting the conduction band for the solid. The olefin chemisorbing and 0-or N-inserting component is a fully oxidized, coordinatively unsaturated cation with the proper metal-oxygen bond strength for partial oxidation of the allylic intermediate. Another common feature of the oxygen-inserting component is its variable valence with a t least two stable oxidation states in the solid state. The redox comoonent is a variable valence metal with a reduction potential greater than that of the oxygenlnitrogen-insertion element. In this manner, the redox component maintains the activity of the catalyst by promoting rapid reoxidation and reconstruction of the surface active sites of the catalyst. The redox element may also serve as the site for catalyst reoxidation by dioxygen. Of course, it is not sufficient for an effective catalyst to fulfill only these stated electronic requirements; the elements of choice must also be contained in the appropriate catalytic structure and matrix. Summary and Conclusions Chances - in the industrial utilization of chemical feedstocks in the last century indicate that stoichiometric reactions are beine- reolaced bv catalvtic reactions.. exoensive . . oxidants by air (oxygen), mktistep processes by single-step processes, expensive reactive feeds by cheaper, more available and less reactive feeds, and wasteful nonselective catalysts by highly selective catalysts. The impact of the development of selective oxidation and ammoxidation catalysts can be summarized by the following major points:
cals produced by catalytic routes. 2) Heterogeneous catalytic ammoxidation of propylene accounts for virtually all of the 8 billion pounds of acrylonitrile pro-
duced annually worldwide. 3) The introduction of the Sohio ammoxidation process in 1960 resulted in a substantial price drop and increased production and use of acrylonitrile by displacing the more expensive acetylene1HCN-based route. 4) The development of selective oxidation and ammoxidation catalysts is based on the theories of active site isolation, utilization of lattice oxvcen from a multivalent metal oxide, and kinetic isolation of disired selective products. 5) Active and selective catalysts for the ammaridation or oxidation of olefins require several key properties: an a-H abstracting site (Bi3+-0,Sb3+-0, Te4+-O), a N(0)-insertion site
Molybdstpr
Figure 8. Mechanistic comparisons.
[Mo6+=NH(0), Sh5+-NH(O),Te6+-NH(O)],a redox component (Fe2+I3+,Ce3+i4+,US+"+), and a solid matrix which integrates the key catalytic components, cocatalysts, and phases, and allows rapid OZ-diffusion to reconstitute the catalytic active surface sites. The first two properties are necessary for selective conversion of olefin to selective product via ratedetermining a- and suhsequent o-ally1 formation, while the second two permit rapid reconstruction of reduced sites to maintain the optimum oxidation state of the active and seleetive surface sites (Fig. 8) (14). Acknowledgmenl I should like t o acknowledee .. m v. earlv mentors Franklin Veatch and lame^ L. Callahan and my closest colleagues and , James D. collaborators 1)ev 1). Suresh..lames F. B r a ~ d i land Burrington without whom this contribution t o science a n d technology would n o t have been possible.
Literature CRed
1970.9, 13" (31 G~ssselli,R. K.: Burrington, J. D.; Brazdil. J. F. Forodoy Disc"& Chrm. Soe. 1982. 72. 203. . (41 ~ e u l h s ,W. ~ .~ . C o r 1970,19,232. (5) Brazdil, J. F.;Suresh,D. D.:Gra~selli,R.K.J.Cot.1. 1980.66.347. (6) Glsener. L. C.;Brazdil, J. F.; H a & M. A. S.; Mehick M.; Grasselli. R. K. Famdoy Tmnmrfions, Dollon SDI~PB, io press. (7) Keulk8.G. W.; Krenzke,L.D.;Notormsnn,T.M.Adu.Cntal. 1978,27,183. (8) Sachtler, W.H.:deBocc, N. H.Proc.3rdlnf. Congr. Cofol..Amalerdom 1965.3.252. 191 Adems, C. R.: Jennings, T. J. J. Cofol. 1964,3.549; 1962,2,63. (10) (a) Haber, J.: Grrybonske, B. J. Carol. 1973. 28, 489 (1973): (b) Grzybonski, B.; Haber,J.: Jsnas,J. J. Colol. 1977.49, 150. (11) Graaselli, R. K.; Burrineon, J. D. I & EC Proc. Rea. and Dso. 1984,23,393. (12) Burrin@on,J. D.:Karti~k,C.T.;Grasselli,R.K. J. Cofol. L983,81,489. (13) Grasselli. R. K.; Braldil. J. F.: Burrington, J. D. Proc.8fhInt. Conpr. Cotal., Berlin (West) 1984. V-369. 114) (sl Gcassclli.R. K.:Bmzdil. J. F.:Bunineon, J. D. Ploc.3rdInt. Symp Ind. Use~af Sdanium end Teilurium, (Stockholm. Sweden) 1984. 18% lbl Graraslli. R. K.: Bmzdil. J. F.;B"rrington. J. D., in prea..
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