- Email: [email protected]
The oxidation of C4 molecules on vanadyl pyrophosphate catalysts
l.W. Hightower. W.N. Delgass. E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary Studies in Surface Science and Catalysis. Vol. 101 © 1996 Elsevier Science B.V. All rights reserved.
991
The Oxidation of C4 Molecules on Vanadyl Pyrophosphate Catalysts V.V. Guliantsa, J.B. Benzigerb, S. Sundaresanb Depannent of Chemistry, Princeton University Department of Chemical Engineering, Princeton University Princeton, NJ 08544-5263, USA a
b
Vanadyl pyrophosphate catalysts displayed a variety of catalytic functionalities in the oxidation of C4 molecules. Hydrocarbons were isomerized and dehydrogenated to olefms, as well as being oxidized. Branched hydrocarbons did not desorb from the pyrophosphate catalyst surface and appeared to undergo total oxidation. Alcohols were dehydrated to olefms and also were isomerized. 1,4 Butanediol was oxidized to maleic anhydride with very high selectivity. An active site for the initial CH bond activation at a cis-(su)peroxo oxovanadium(V) dimer site is proposed. 1. INTRODUCTION
Vanadyl pyrophosphate catalysts, (V0hP2 0 7 , are selective for the partial oxidation of n-butane to maleic anhydride. The best VPO catalysts preferentially expose the (200) planes of (V0hP 20 7 , though the specific termination of the surface is not resolved [I,2l Several mechanisms of n-butane oxidation have been proposed based on hypothetical active sites on the (200) surface shown in Figure I [1-5]. Mechanistic reaction pathways have been proposed generally involving the abstraction of methylene hydrogens leading to the formation of butene followed by butadiene and then a cyclo-addition to form furan [I]. n-Butane does not reversibly chemisorb with hydrogen exchange onto the surface so it is difficult to indentify proposed intermediates during the reaction sequence. Pepera et al. [3] studied n-butane oxidation employing isotopically labelled compounds, and concluded that n-butane oxidation to maleic anhydride or combustion products proceed through the same rate-determining step; selectivity to maleic anhydride is determined by fast reaction steps after the initial C-H bond activation. The kinetic isotope effect for the C-H bond activation (kH/ko =2.18) is consistent with complete breaking ofC-H(D) in the transition state. Centi et al. [1] compared the rate constants for oxidation of the Cr C 7 alkane series and found a correlation with the kinetic constants that consistent with the contemporaneous abstraction of two hydrogen atoms in the 2- and 3- positions by a bridging oxygen and a Lewis acid site. Centi et al. did not provide a complete mechanism from n-butane to maleic anhydride, but they suggested that Bmnsted acid sites may be involved in the reaction steps after the initial activation. The Bmnsted acid sites have been associated with P-OH groups on the surface which have been proposed to
992 (i) facilitate water removal, (ii) stabilize phosphate ester surface intermediates and (iii) facilitate maleic anhydride desorption.
r I
I
/ / / / /
L
Figure 1. Active sites proposed to exist on the vanadyl pyrophosphate surface. (SI) Bmnsted acid site; (S2) Lewis acid site; (S3) terminal oxygen; (S4) bridging oxygen; (Ss) T)2-superoxo or T)l_peroxo site; (S6) VV N 1V redox couple. Schiett and J0rgensen [4] proposed a concerted 4+2 cycloaddition of butadiene to y"+=0 (an S3 site) to yield 2,5 di-hydrofuran. C-H bond activation in dihydrofuran by molecular oxygen adds oxygen in the 2-position forming the 2-hydroxy derivative, which reacts in turn at the 5- position to yield maleic anhydride. This model requires proximity of vanadium atoms in a vanadyl dimer and an T)l-superoxo or T)2_peroxo species. This reaction mechanism is similar to homogeneous vanadium complexes that activate molecular oxygen by a free radical process [7,8]. Agaskar et al. [5] expanded upon the mechanism of Schiett and J0fgensen, proposing an active site for the transformation of n-butane to 1,3 butadiene. They identify a cluster of 4 vanadyl dimers isolated by pyrophosphate groups as key for the selective oxidation. A superoxo Ss species activates n-butane by abstraction of a methylene hydrogen atom resulting in a hydroperoxy group. An adjacent vanadyl group captures the alkyl radical leading to a surface bound alkoxy group. After a second methylene hydrogen abstraction a metal bound Sl-ketaloxy or S2-glycoloxy group is formed that is dehydrated by a Bmnsted acid to butadiene, regenerating the SI surface species. To help clarify the reaction steps and intermediates involved in the selective oxidation of n-butane to maleic anhydride we have studied the reactions of 17 different C4 molecules representing potential reaction intermediates along different reaction pathways. The results of these studies presented here demonstrate that the vanadyl pyrophosphate catalyst is multifunctional, able to react with many different molecules and convert them to maleic anhydride. The reactants yielding maleic anhydride most selectively suggest a different reaction mechanism than has been proposed previously.
993
2. EXPERIMENTAL 2.1 Synthesis Vanadyl pyrophosphate catalysts were prepared from an organic precursor route as reported in the literature [6]. Vanadium pentoxide (Aldrich) was reacted with 100% orthophosphoric acid (Aldrich) (synthesis PN=1.l6) in a 10:1 mixture of i-butanol and benzyl alcohol at reflux for 16 hours. A blue sold was isolated by filtration, washed with i-butanol and acetone and dried in air at 383 K overnight. XRD and Raman spectroscopy confirmed the composition of the precursor to be vanadyl hydrogen phosphate hemihydrate (VOHP0 4 ·O.5H20). The TGA weight loss of the precursor in nitrogen at 873 K was 10.9%, in good agreement with the stoichiometic value of 10.6%. 2.2 Kinetic Measurements The vanadyl hydrogen phosphate hemihydrate precursor was ground and sieved. Approximately 1 g of the 212-235 ~m fraction of the precursor was placed in the catalytic microreactor and activated in situ at 708 K in 1.2 % n-butane in air. The feed flow rate was fixed at 40 ml/min and the effluent was analyzed periodically until the n-butane conversion and selectivity to maleic anhydride had stabilized. It typically took 3 weeks for a catalyst to reach steady state under such conditions, after which the selectivity and conversion were determined at both 708 K and 673 K. The catalyst performance agreed well with that reported in the literature [6]. The reactor temperature was lowered to 633 K and liquid samples of various probe molecules (see Tables I-III) were injected into flowing air at the microreactor inlet by a high precision Harvard syringe pump to give vapor concentrations in air of 1-4 volume percent. Gaseous species were introduced by metering flows of the hydrocarbon (n-butane, isobutane, isobutene, I-butene, cis- and trans-2-butene) and air through mass flow controllers to achieve the desired concentration. The reaction was run for 1-2 hours and the reaction products were analyzed by gas chromatograpy. The reactor was then returned to 673 K in a n-butane/ air feed to check that the steady state performance of the catalyst had not changed. Characterization of the used catalyst by XRD and Raman spectroscopy showed that it contained exclusively vanadyl pyrophosphate. Product analysis was carried out by on-line gas chromatography. A side stream from the reactor effluent ran through a heated line to an HP 5790A gas chromatograph where partial oxidation products were separated on a 2 m Porapak QS column, and detected with a flame ionization detector. The main effluent from the reactor passed through a heated line to a water bubbler, and was then injected into two GC columns in series: a 5 m 30% bis-2-ethoxyethyl sebacate column to separate CO 2 and hydrocarbons, and a 4 m 13X molecular sieve column to separate 02' N 2 and CO. Calibration for maleic anhydride was acheived by periodic acid-base titrations of the bubbler solution with a phenolphthalein indicator. The bubbler solution was sampled after passing the effluent through deionized water at a constant space velocity for 1 hr. A series of calibrations of gas samples were used to calibrate the sensitivities of the gas chromatographs for all the products. After determining conversion and product yields closure on the overall carbon balances were ±5%.
994 3. RESULTS
Table I Transfonnations ofC4 Hydrocarbons over a (VO)2P207 Catalyst
Substrate (Conversion,%)' n-butane (30)
Concentration 6 (mol%) 1.2
Products
Selectivityc (mol%) 21 3 I 73 43 8 6 10 3 I 7 I 23 21 12 21 17
carbon oxides acetic acid acrylic acid maleic anhydride I-butene carbon oxides 1.3 (95) trans-2-butene cis-2-butene 1,3-butadiene furan methylethylketone acetic acid acrylic acid maleic anhydride cis-2-butene carbon oxides 1.3 (95) I-butene trans-2-butene 1,3-butadiene furan 3 methylethylketone I acetic acid 5 acrylic acid I maleic anhydride 21 trans-2-butene 1.3 carbon oxides 43 (95) I-butene 5 cis-2-butene 6 1,3-butadiene 8 acrylic acid 2 maleic anhydride 36 isobutane carbon oxides 68 1.2 (21) acetone I methacrolein 3 methacrylic acid 2 acetic acid 12 acrylic acid 3 II maleic anhydride isobutene 1.3 carbon oxides 60 (89) acetone 2 methacrolein 30 methacrylic acid 2 acetic acid 4 acrylic acid I maleic anhydride 0 a Conversion is defmed as molar ratio of substrate reacted to the total substrate in feed. b Feed flow rate is 40 ml/min with molar % of substrate in feed as indicated. C Selectivity is molar ratio of a particular product to the substrate reacted normalized by the number of carbons in the product.
995
The product distributions and conversions of the linear and branched C4 hydrocarbons tested are summarized in Table I. All the linear C4 hydrocarbons were oxidized to maleic anhydride (MA), although the yields varied dramatically. Apart from MA, only traces of other partial oxidation products (acetic and acrylic acids) resulted from oxidation of nbutane, in agreement with previous observations. Oxidation of 1- and 2- butenes had much lower selectivities to MA, and other oxidation products, such as furan and the products of C-C bond scisson, acetic and acrylic acids, were observed. The vanadyl pyrophosphate catalyst also catalyzed the isomerization and dehydrogenation of butenes in accordance with previous studies [12]. Isomerization of isobutane was observed; maleic anhydride was observed as an oxidation product of isobutane , though the selectivity from isobutane was considerably less than found for n-butane. No maleic anhydride product was observed from isobutene. The selectivity for total oxidation products was much greater for the two branched hydrocarbons (isobutane and isobutene) than observed for any of the linear hydrocarbons. The five-membered heterocycles were all oxidized to maleic anhydride as shown in the results summarized in Table II. Table II Transformations of C4 Heterocycles over a (VO)2P207 Catalyst
Substrate (Conversion,%)' furan (95)
Concentration 6 (mol%) 3.1
tetrahydrofuran (100)
3.4
2,5-dihydrofuran (100)
4.0
2,3-dihydrofuran (100)
3.1
y-butyrolactone (95)
2.7
Products
carbon oxides acetic acid acrylic acid maleic anhydride carbon oxides ethylene furan acetic acid acrylic acid maleic anhydride carbon oxides furan acetic acid acrylic acid maleic anhydride carbon oxides ethylene furan acetic acid acrylic acid maleic anhydride carbon oxides ethylene propylene acetic acid acrylic acid maleic anhydride
Selectivity" (mol%) 17 7 3 73 14 I I
8 2 74 18 17 2 I
62 49 3 2 5 4 37 32 I I 7 21 38
996 High selectivity to MA was observed in the oxidation of furan, tetrahydrofuran, and 2,5dihydrofuran, although the selectivities for MA did not exceed those for n-butane. Oxidation of 2,3-dihydrofuran was less selective for MA (37% selectivity) and almost 50% of the reaction products were non-selective carbon oxides. y-Butyrolactone was oxidized to MA with suprisingly low selectivity; C2 acids (acrylic and acetic) were significant products from its oxidation. The products from the oxidation of four carbon alcohols and diols over the vanadyl pyrophosphate catalyst are summarized in Table III. Oxidation of linear chain C4 alcohols (1- and 2-butanol) led to a product slate similar to that observed for the linear butenes. Dehydration, as well as oxidation of the linear alcohols was observed. In the case of I-butanol a higher yield of MA was observed. Isobutanol gave a broad product slate that was a blend of products from the oxidation of isobutene and the linear alcohols. Selecitivity for total oxidation of the isobutanol was greater than those for the linear alcohols, similar to what was observed for the hydrocarbons. Different oxidation products and yields to MA were obtained in the case of the four linear chain C 4 diols. The selectivity for oxidation of 1,4 butanediol to maleic anhydride over the vanadyl pyrophosphate catalyst was remarkably high (93 mol%), with only small amounts of furan and complete oxidation products_detected. Considerably lower yields of MA were obtained from 1,2- and 1,3-diols. 1,2-Butanediol gave high yields of acetic and acrylic acids, while the oxidation of 1,3-diol led to appreciable amounts of butadiene and propylene. 4. DISCUSSION The results of this study demonstrate the polyfunctional nature of the vanadyl pyrophosphate catalyst. Almost any C4 molecule can be converted to maleic anhydride over the VPO catalyst, and the product slates show that isomerization, dehydration, C-C bond scisson and oxidation are all catalyzed by the vanadyl pyrophosphate catalyst. The proposed models of n-butane oxidation have been based on the electrophilic vanadyl (IV) oxygen in the edge-sharing trans-oxovanadium(lV) octahedra activating the C-H bond (the S3 site shown in Figure 1). [5] However, the redox chemistry of vanadium reveals only a few examples of oxidation processes involving vanadyl oxygen [7-9]. Rather the generation of highly reactive vanadium(lV) and (V) 1] l_peroxo and 1]2_ superoxo species in the presence of molecular oxygen is well known and widely employed in stoichimetric and catalytic oxidations of organic molecules [10,11]. VPO catalysts are not active when NO is used as an oxotransfer agent in place of molecular oxygen [5], which suggests that 1]1_peroxo and 1]2-superoxo species are the active species on the VPO catalyst surface. The kinetic studies of Centi et al. indicate that the active site is capable of simultaneous abstraction of two methylene hydrogen atoms in n-butane [I,2].
997 Table III Transfonnatinos of C4 Alcohols over a (V0hP207 Catalyst Substrate (Conversion,%)" I-butanol (100)
Concentration 6 (mol%) 2.6
2-butanol (100)
2.7
Products carbon oxides ethylene propylene I-butene trans-2-butene cis-2-butene I,3-butadiene acetone acetic acid acrylic acid maleic anhydride carbon oxides I-butene trans-2-butene cis-2-butene 1,3-butadiene furan
isobutanol (100)
2.5
1,2-butanediol (100)
3.3
acetone acetic acid acrylic acid maleic anhydride carbon oxides propylene isobutene trans-2-butene cis-2-butene 1,3-butadiene methacrolein acetic acid maleic anhydride carbon oxides ethylene propylene furan
I,3-butanediol (100)
2.3
1,4-butanediol (100)
3.2
acetic acid acrylic acid maleic anhydride carbon oxides ethylene propylene I,3-butadiene acetic acid acrylic acid maleic anhydride carbon oxides furan maleic anhydride
Selectivityc (mol%) 28 I 1 2 4 3 5 I 14 6 48 14 9 16 12 13 4 1 7 1 25 45 I 14 3 2 4 2 18 10 17 I I I 25 13 40 22 I 9 8 9 4 50 5 2 93
998 The trans-oxovanadium(IV) dimers on the (200) surface of vanadyl pyrophosphate are arranged is such a way that the vanadium(IV) atoms with vacant apical coordination sites are paired between parallel chains. Given the average 0-0 bond length in vanadium(V) peroxo complexes of 1.5 A [11], the distance between adjacent peroxo species in a transoxovanadium(IV) dimer chain (ca. 2.8- 3.5 A [12]) compares favorably with the methylene H-H distance in the 2- and 3- positions in the eclipsed conformation of nbutane (ca. 3.oA). Adsorption of oxygen molecules onto adjacent transoxovanadium(IV) sites to form the adjacent Th-peroxo oxovanadium(V) dimers is shown in Figure 2. The trends in the reaction products observed in our study can be rationalized on the basis of this "active" site better than sites suggested in previous studies, and this site is more consistent with activation of molecular oxygen by vanadium complexes [10,11]. We suggests that the dirneric site shown in Figure 2, rather than the trans-oxovanadium(IV) dimer may better account for the initial C-H abstraction in n-butane oxidation. ,-----
--
--
----I
I ~O / 0 l O P I I "0" ~ ~/~o,J /,0/' "0, /~«'JI./ 0/ I ~# ~~ 'V~ V~ I I 0 / "0//'1 1"--0 ,OH 0/11', /O'H "--0 I/,6 ,,/ 0 0;', I I ~P o~P ~"I 1 0 I /
0
I
-~~/V'-----~,--~
L Figure 2. Proposed cis-peroxo oxovanadium(V) dimeric active site for partial oxidation of hydrocarbons to maleic anhydride on the vanadyl pyrophosphate catalyst. Unsaturated hydrocarbons were much more reactive than n-butane. Both 1- and 2butene underwent isomerization, probably catalyzed by surface HPOt or H2P zOt groups acting as Bmnsted acids (the SI sites shown in Figure 1). The formation of 1,3butadiene may be accounted for by radical abstraction of two allylic H atoms in 2-butene by surface peroxo oxovanadium(V) dimer, followed by intramolecular electron transfer and formation of a stable system of conjugated double bonds. The peroxo oxovanadium species would be converted to oxovanadium (V) species with the liberation of water. Butadiene could then undergo the [4+2] cycloaddition with the surface oxovanadium(V) oxyradical Vv_O_ to yield 2,5 dihydrofuran and the original reduced V IV site [13]. Furans are well-known to undergo cycloaddition with singlet oxygen [15]. Two consecutive abstractions of hydrogen atoms in the 5- position by VV -0- yields maleic anhydride and IV the V site. An alternative path for the reaction of cis-2-butene is the dihydroxylation of a 1,4 alkenyl diradical, similar to the VzOs-catalyzed oxidation of olefms by peroxide [14] or the mechanism proposed by Mimoun et al. for oxidation of aromatics [15]. Such a diradical may be stabilized by delocalization of the unpaired electrons through 1t-
999
conjugation [16]. This reaction path is consistent with the very selective oxidation observed for 1,4-butanediol. Other partially oxidized species may results from (i) hydration of an olefin followed by oxidation of the alcohol into methylethylketone, and (ii) addition of the peroxo group to the C=C double bond to yield acrylic and acetic acids, respectively, via decomposition of the peroxymetallocycle with C-C bond scission and further oxidation of the molecular fragments. The simple alcohols appear to first dehydrate via the formation of a carbocation to form olefins. Formation of a linear chain species from isobutanol is indicative of the skeletal rearrangement of the carbocations. Oxidation of I-butanol resulted in a higher yield of maleic anhydride as compared to 2-butanol. This may be explained by an access to a "shorter" oxidation path, similar to intramolecular cyclization of 13- and o-unbranched primary alcohols [14]. Oxidation of 1,4 butanediol on the vanadyl pyrophosphate catalyst was highly selective. Several possibilities exist for 1,4 diol cyclization. The diol may be oxidized to y-butyrolactone via 4-hydroxyaldehyde. A second possibility is the Paal-Knorr synthesis [17], involving the cyclizing dehydration of 1,4 dicarbonyl compounds, which is widely used to synthesize furans. An acid catalyzed partial dehydration with cyclization is a third alternative for the reaction of the 1,4 butanediol. Dehydration of diols is evident in the formation of 1,3-butadiene from the 1,3 diol. The formation of maleic anhydride from isobutane clearly shows the vanadyl pyrophosphate catalyzed skeletal rearrangements. This could occur at Lewis acid sites on the surface via carbocation rearrangement. A 1,3 diradical formed by activation of two C-H methyl bonds may also undergo skeletal rearrangement via methyl group migration with a cyclic intermediate [18]. 5. CONCLUSION The oxidation of 17 C4 molecules to maleic anhydride over a vanadyl pyrophosphate catalyst was studied. The product distributions indicate that the pyrophosphate catalysts catalyze isomerization, dehydrogenation, dehydration and oxidation reactions. Branched molecules either isomerize to straight chain species or are totally oxidized. Alcohols can be selectively oxidized to maleic anhydride. The oxidation of 1,4-butanediol to maleic anhydride is remarkably selective. The wide variety of reactions catalyzed by the vanadyl pyrophosphate catalyst demonstrate the polyfunctional nature of the catalyst surface, which makes it very difficult to identify a single active catalyst site. We have proposed an active site for the initial activation of n-butane consisting of a cis-(su)peroxo oxovanadium(V) dimer present of the (200) plane of (VO)2PZ07' This active "site" is consistent with activation of molecular oxygen by vanadium complexes and provides a rational explanation for the reaction products observed.
1()()()
ACKNOWLEDGEMENT
This work was supported by the Amoco Chemical Corporation and the National Science Foundation Grant CTS-9100BO. REFERENCES 1. Centi, G.; Trifiro, F.; Ebner, J.R.; Franchetti, V.M.; Chem Rev. 88 (1988) 55. 2. Catalysis Today (proceedings, Vanadyl Pyrophosphate Catalysts), Centi, G., Ed. Elsevier: Amsterdam 16 (1993) 1-154 and refs. therein. 3. Pepera, M.A; Callahan, J.L.; Desmond, MJ.; Milberger, E.C.; Blum, P.R., Bremer, N.J.; J. Am. Chern. Soc 107 (1985) 4883. 4. Schimt, B; Jmgensen, K.A.; Hoffman, R; J. Phys. Chem. 95 (1991) 2297. 5. Agaskar, P.A; DeCaul, L.; Grasselli, RK.; Catal. Lett. 23 (1994) 339. 6. Bergna, H.E.; US Patent 4,769,477, 1988. Assigned to E.!. Du Pont de Nemours and Co. 7. Butler A; Carrano, CJ.; Coord. Chern. Rev. 109 (1991) 61. 8. Butler, A.; in Vanadium in Biological Systems, Chasteen, N.D., ed.: Kluwer, Dordrecht, The Netherlands, 1990, 25. 9. Holm, R.H.; Chem Rev 87 (1987) 1401. 10. Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L.; J. Am. Chern. Soc. 108 (1986) 3711. 11. Butler, A; Clague, MJ.; Meister, G.E.; Chern. Rev. 94 (1994) 625, and refs. therein. 12. Middlemiss, N.E.; Ph.D. thesis McMaster University 1978. 13. Gorman, AA; Lovering, G.; Rodgers, M.A.; J. Am. Chern. Soc. 101 (1979) 3050. 14. Organic Synthesis by Oxidation with Metal Compounds; Mijs, WJ.; de .Tonge, C.R.H.; Eds., Plenum Press: New York 1986. 15. Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fischer, 1.; Weiss, R; J. Am. Chem. Soc. 105 (1983) 3101. 16. Rajca, A; Chem. Rev. 94 (1994) 871. 17. Scott, L.T.; Naples, 1.0.; Synthesis (973) 209. 18. March, 1.; Advanced Organic Chemistry; Wiley: New York, 1992.
991
The Oxidation of C4 Molecules on Vanadyl Pyrophosphate Catalysts V.V. Guliantsa, J.B. Benzigerb, S. Sundaresanb Depannent of Chemistry, Princeton University Department of Chemical Engineering, Princeton University Princeton, NJ 08544-5263, USA a
b
Vanadyl pyrophosphate catalysts displayed a variety of catalytic functionalities in the oxidation of C4 molecules. Hydrocarbons were isomerized and dehydrogenated to olefms, as well as being oxidized. Branched hydrocarbons did not desorb from the pyrophosphate catalyst surface and appeared to undergo total oxidation. Alcohols were dehydrated to olefms and also were isomerized. 1,4 Butanediol was oxidized to maleic anhydride with very high selectivity. An active site for the initial CH bond activation at a cis-(su)peroxo oxovanadium(V) dimer site is proposed. 1. INTRODUCTION
Vanadyl pyrophosphate catalysts, (V0hP2 0 7 , are selective for the partial oxidation of n-butane to maleic anhydride. The best VPO catalysts preferentially expose the (200) planes of (V0hP 20 7 , though the specific termination of the surface is not resolved [I,2l Several mechanisms of n-butane oxidation have been proposed based on hypothetical active sites on the (200) surface shown in Figure I [1-5]. Mechanistic reaction pathways have been proposed generally involving the abstraction of methylene hydrogens leading to the formation of butene followed by butadiene and then a cyclo-addition to form furan [I]. n-Butane does not reversibly chemisorb with hydrogen exchange onto the surface so it is difficult to indentify proposed intermediates during the reaction sequence. Pepera et al. [3] studied n-butane oxidation employing isotopically labelled compounds, and concluded that n-butane oxidation to maleic anhydride or combustion products proceed through the same rate-determining step; selectivity to maleic anhydride is determined by fast reaction steps after the initial C-H bond activation. The kinetic isotope effect for the C-H bond activation (kH/ko =2.18) is consistent with complete breaking ofC-H(D) in the transition state. Centi et al. [1] compared the rate constants for oxidation of the Cr C 7 alkane series and found a correlation with the kinetic constants that consistent with the contemporaneous abstraction of two hydrogen atoms in the 2- and 3- positions by a bridging oxygen and a Lewis acid site. Centi et al. did not provide a complete mechanism from n-butane to maleic anhydride, but they suggested that Bmnsted acid sites may be involved in the reaction steps after the initial activation. The Bmnsted acid sites have been associated with P-OH groups on the surface which have been proposed to
992 (i) facilitate water removal, (ii) stabilize phosphate ester surface intermediates and (iii) facilitate maleic anhydride desorption.
r I
I
/ / / / /
L
Figure 1. Active sites proposed to exist on the vanadyl pyrophosphate surface. (SI) Bmnsted acid site; (S2) Lewis acid site; (S3) terminal oxygen; (S4) bridging oxygen; (Ss) T)2-superoxo or T)l_peroxo site; (S6) VV N 1V redox couple. Schiett and J0rgensen [4] proposed a concerted 4+2 cycloaddition of butadiene to y"+=0 (an S3 site) to yield 2,5 di-hydrofuran. C-H bond activation in dihydrofuran by molecular oxygen adds oxygen in the 2-position forming the 2-hydroxy derivative, which reacts in turn at the 5- position to yield maleic anhydride. This model requires proximity of vanadium atoms in a vanadyl dimer and an T)l-superoxo or T)2_peroxo species. This reaction mechanism is similar to homogeneous vanadium complexes that activate molecular oxygen by a free radical process [7,8]. Agaskar et al. [5] expanded upon the mechanism of Schiett and J0fgensen, proposing an active site for the transformation of n-butane to 1,3 butadiene. They identify a cluster of 4 vanadyl dimers isolated by pyrophosphate groups as key for the selective oxidation. A superoxo Ss species activates n-butane by abstraction of a methylene hydrogen atom resulting in a hydroperoxy group. An adjacent vanadyl group captures the alkyl radical leading to a surface bound alkoxy group. After a second methylene hydrogen abstraction a metal bound Sl-ketaloxy or S2-glycoloxy group is formed that is dehydrated by a Bmnsted acid to butadiene, regenerating the SI surface species. To help clarify the reaction steps and intermediates involved in the selective oxidation of n-butane to maleic anhydride we have studied the reactions of 17 different C4 molecules representing potential reaction intermediates along different reaction pathways. The results of these studies presented here demonstrate that the vanadyl pyrophosphate catalyst is multifunctional, able to react with many different molecules and convert them to maleic anhydride. The reactants yielding maleic anhydride most selectively suggest a different reaction mechanism than has been proposed previously.
993
2. EXPERIMENTAL 2.1 Synthesis Vanadyl pyrophosphate catalysts were prepared from an organic precursor route as reported in the literature [6]. Vanadium pentoxide (Aldrich) was reacted with 100% orthophosphoric acid (Aldrich) (synthesis PN=1.l6) in a 10:1 mixture of i-butanol and benzyl alcohol at reflux for 16 hours. A blue sold was isolated by filtration, washed with i-butanol and acetone and dried in air at 383 K overnight. XRD and Raman spectroscopy confirmed the composition of the precursor to be vanadyl hydrogen phosphate hemihydrate (VOHP0 4 ·O.5H20). The TGA weight loss of the precursor in nitrogen at 873 K was 10.9%, in good agreement with the stoichiometic value of 10.6%. 2.2 Kinetic Measurements The vanadyl hydrogen phosphate hemihydrate precursor was ground and sieved. Approximately 1 g of the 212-235 ~m fraction of the precursor was placed in the catalytic microreactor and activated in situ at 708 K in 1.2 % n-butane in air. The feed flow rate was fixed at 40 ml/min and the effluent was analyzed periodically until the n-butane conversion and selectivity to maleic anhydride had stabilized. It typically took 3 weeks for a catalyst to reach steady state under such conditions, after which the selectivity and conversion were determined at both 708 K and 673 K. The catalyst performance agreed well with that reported in the literature [6]. The reactor temperature was lowered to 633 K and liquid samples of various probe molecules (see Tables I-III) were injected into flowing air at the microreactor inlet by a high precision Harvard syringe pump to give vapor concentrations in air of 1-4 volume percent. Gaseous species were introduced by metering flows of the hydrocarbon (n-butane, isobutane, isobutene, I-butene, cis- and trans-2-butene) and air through mass flow controllers to achieve the desired concentration. The reaction was run for 1-2 hours and the reaction products were analyzed by gas chromatograpy. The reactor was then returned to 673 K in a n-butane/ air feed to check that the steady state performance of the catalyst had not changed. Characterization of the used catalyst by XRD and Raman spectroscopy showed that it contained exclusively vanadyl pyrophosphate. Product analysis was carried out by on-line gas chromatography. A side stream from the reactor effluent ran through a heated line to an HP 5790A gas chromatograph where partial oxidation products were separated on a 2 m Porapak QS column, and detected with a flame ionization detector. The main effluent from the reactor passed through a heated line to a water bubbler, and was then injected into two GC columns in series: a 5 m 30% bis-2-ethoxyethyl sebacate column to separate CO 2 and hydrocarbons, and a 4 m 13X molecular sieve column to separate 02' N 2 and CO. Calibration for maleic anhydride was acheived by periodic acid-base titrations of the bubbler solution with a phenolphthalein indicator. The bubbler solution was sampled after passing the effluent through deionized water at a constant space velocity for 1 hr. A series of calibrations of gas samples were used to calibrate the sensitivities of the gas chromatographs for all the products. After determining conversion and product yields closure on the overall carbon balances were ±5%.
994 3. RESULTS
Table I Transfonnations ofC4 Hydrocarbons over a (VO)2P207 Catalyst
Substrate (Conversion,%)' n-butane (30)
Concentration 6 (mol%) 1.2
Products
Selectivityc (mol%) 21 3 I 73 43 8 6 10 3 I 7 I 23 21 12 21 17
carbon oxides acetic acid acrylic acid maleic anhydride I-butene carbon oxides 1.3 (95) trans-2-butene cis-2-butene 1,3-butadiene furan methylethylketone acetic acid acrylic acid maleic anhydride cis-2-butene carbon oxides 1.3 (95) I-butene trans-2-butene 1,3-butadiene furan 3 methylethylketone I acetic acid 5 acrylic acid I maleic anhydride 21 trans-2-butene 1.3 carbon oxides 43 (95) I-butene 5 cis-2-butene 6 1,3-butadiene 8 acrylic acid 2 maleic anhydride 36 isobutane carbon oxides 68 1.2 (21) acetone I methacrolein 3 methacrylic acid 2 acetic acid 12 acrylic acid 3 II maleic anhydride isobutene 1.3 carbon oxides 60 (89) acetone 2 methacrolein 30 methacrylic acid 2 acetic acid 4 acrylic acid I maleic anhydride 0 a Conversion is defmed as molar ratio of substrate reacted to the total substrate in feed. b Feed flow rate is 40 ml/min with molar % of substrate in feed as indicated. C Selectivity is molar ratio of a particular product to the substrate reacted normalized by the number of carbons in the product.
995
The product distributions and conversions of the linear and branched C4 hydrocarbons tested are summarized in Table I. All the linear C4 hydrocarbons were oxidized to maleic anhydride (MA), although the yields varied dramatically. Apart from MA, only traces of other partial oxidation products (acetic and acrylic acids) resulted from oxidation of nbutane, in agreement with previous observations. Oxidation of 1- and 2- butenes had much lower selectivities to MA, and other oxidation products, such as furan and the products of C-C bond scisson, acetic and acrylic acids, were observed. The vanadyl pyrophosphate catalyst also catalyzed the isomerization and dehydrogenation of butenes in accordance with previous studies [12]. Isomerization of isobutane was observed; maleic anhydride was observed as an oxidation product of isobutane , though the selectivity from isobutane was considerably less than found for n-butane. No maleic anhydride product was observed from isobutene. The selectivity for total oxidation products was much greater for the two branched hydrocarbons (isobutane and isobutene) than observed for any of the linear hydrocarbons. The five-membered heterocycles were all oxidized to maleic anhydride as shown in the results summarized in Table II. Table II Transformations of C4 Heterocycles over a (VO)2P207 Catalyst
Substrate (Conversion,%)' furan (95)
Concentration 6 (mol%) 3.1
tetrahydrofuran (100)
3.4
2,5-dihydrofuran (100)
4.0
2,3-dihydrofuran (100)
3.1
y-butyrolactone (95)
2.7
Products
carbon oxides acetic acid acrylic acid maleic anhydride carbon oxides ethylene furan acetic acid acrylic acid maleic anhydride carbon oxides furan acetic acid acrylic acid maleic anhydride carbon oxides ethylene furan acetic acid acrylic acid maleic anhydride carbon oxides ethylene propylene acetic acid acrylic acid maleic anhydride
Selectivity" (mol%) 17 7 3 73 14 I I
8 2 74 18 17 2 I
62 49 3 2 5 4 37 32 I I 7 21 38
996 High selectivity to MA was observed in the oxidation of furan, tetrahydrofuran, and 2,5dihydrofuran, although the selectivities for MA did not exceed those for n-butane. Oxidation of 2,3-dihydrofuran was less selective for MA (37% selectivity) and almost 50% of the reaction products were non-selective carbon oxides. y-Butyrolactone was oxidized to MA with suprisingly low selectivity; C2 acids (acrylic and acetic) were significant products from its oxidation. The products from the oxidation of four carbon alcohols and diols over the vanadyl pyrophosphate catalyst are summarized in Table III. Oxidation of linear chain C4 alcohols (1- and 2-butanol) led to a product slate similar to that observed for the linear butenes. Dehydration, as well as oxidation of the linear alcohols was observed. In the case of I-butanol a higher yield of MA was observed. Isobutanol gave a broad product slate that was a blend of products from the oxidation of isobutene and the linear alcohols. Selecitivity for total oxidation of the isobutanol was greater than those for the linear alcohols, similar to what was observed for the hydrocarbons. Different oxidation products and yields to MA were obtained in the case of the four linear chain C 4 diols. The selectivity for oxidation of 1,4 butanediol to maleic anhydride over the vanadyl pyrophosphate catalyst was remarkably high (93 mol%), with only small amounts of furan and complete oxidation products_detected. Considerably lower yields of MA were obtained from 1,2- and 1,3-diols. 1,2-Butanediol gave high yields of acetic and acrylic acids, while the oxidation of 1,3-diol led to appreciable amounts of butadiene and propylene. 4. DISCUSSION The results of this study demonstrate the polyfunctional nature of the vanadyl pyrophosphate catalyst. Almost any C4 molecule can be converted to maleic anhydride over the VPO catalyst, and the product slates show that isomerization, dehydration, C-C bond scisson and oxidation are all catalyzed by the vanadyl pyrophosphate catalyst. The proposed models of n-butane oxidation have been based on the electrophilic vanadyl (IV) oxygen in the edge-sharing trans-oxovanadium(lV) octahedra activating the C-H bond (the S3 site shown in Figure 1). [5] However, the redox chemistry of vanadium reveals only a few examples of oxidation processes involving vanadyl oxygen [7-9]. Rather the generation of highly reactive vanadium(lV) and (V) 1] l_peroxo and 1]2_ superoxo species in the presence of molecular oxygen is well known and widely employed in stoichimetric and catalytic oxidations of organic molecules [10,11]. VPO catalysts are not active when NO is used as an oxotransfer agent in place of molecular oxygen [5], which suggests that 1]1_peroxo and 1]2-superoxo species are the active species on the VPO catalyst surface. The kinetic studies of Centi et al. indicate that the active site is capable of simultaneous abstraction of two methylene hydrogen atoms in n-butane [I,2].
997 Table III Transfonnatinos of C4 Alcohols over a (V0hP207 Catalyst Substrate (Conversion,%)" I-butanol (100)
Concentration 6 (mol%) 2.6
2-butanol (100)
2.7
Products carbon oxides ethylene propylene I-butene trans-2-butene cis-2-butene I,3-butadiene acetone acetic acid acrylic acid maleic anhydride carbon oxides I-butene trans-2-butene cis-2-butene 1,3-butadiene furan
isobutanol (100)
2.5
1,2-butanediol (100)
3.3
acetone acetic acid acrylic acid maleic anhydride carbon oxides propylene isobutene trans-2-butene cis-2-butene 1,3-butadiene methacrolein acetic acid maleic anhydride carbon oxides ethylene propylene furan
I,3-butanediol (100)
2.3
1,4-butanediol (100)
3.2
acetic acid acrylic acid maleic anhydride carbon oxides ethylene propylene I,3-butadiene acetic acid acrylic acid maleic anhydride carbon oxides furan maleic anhydride
Selectivityc (mol%) 28 I 1 2 4 3 5 I 14 6 48 14 9 16 12 13 4 1 7 1 25 45 I 14 3 2 4 2 18 10 17 I I I 25 13 40 22 I 9 8 9 4 50 5 2 93
998 The trans-oxovanadium(IV) dimers on the (200) surface of vanadyl pyrophosphate are arranged is such a way that the vanadium(IV) atoms with vacant apical coordination sites are paired between parallel chains. Given the average 0-0 bond length in vanadium(V) peroxo complexes of 1.5 A [11], the distance between adjacent peroxo species in a transoxovanadium(IV) dimer chain (ca. 2.8- 3.5 A [12]) compares favorably with the methylene H-H distance in the 2- and 3- positions in the eclipsed conformation of nbutane (ca. 3.oA). Adsorption of oxygen molecules onto adjacent transoxovanadium(IV) sites to form the adjacent Th-peroxo oxovanadium(V) dimers is shown in Figure 2. The trends in the reaction products observed in our study can be rationalized on the basis of this "active" site better than sites suggested in previous studies, and this site is more consistent with activation of molecular oxygen by vanadium complexes [10,11]. We suggests that the dirneric site shown in Figure 2, rather than the trans-oxovanadium(IV) dimer may better account for the initial C-H abstraction in n-butane oxidation. ,-----
--
--
----I
I ~O / 0 l O P I I "0" ~ ~/~o,J /,0/' "0, /~«'JI./ 0/ I ~# ~~ 'V~ V~ I I 0 / "0//'1 1"--0 ,OH 0/11', /O'H "--0 I/,6 ,,/ 0 0;', I I ~P o~P ~"I 1 0 I /
0
I
-~~/V'-----~,--~
L Figure 2. Proposed cis-peroxo oxovanadium(V) dimeric active site for partial oxidation of hydrocarbons to maleic anhydride on the vanadyl pyrophosphate catalyst. Unsaturated hydrocarbons were much more reactive than n-butane. Both 1- and 2butene underwent isomerization, probably catalyzed by surface HPOt or H2P zOt groups acting as Bmnsted acids (the SI sites shown in Figure 1). The formation of 1,3butadiene may be accounted for by radical abstraction of two allylic H atoms in 2-butene by surface peroxo oxovanadium(V) dimer, followed by intramolecular electron transfer and formation of a stable system of conjugated double bonds. The peroxo oxovanadium species would be converted to oxovanadium (V) species with the liberation of water. Butadiene could then undergo the [4+2] cycloaddition with the surface oxovanadium(V) oxyradical Vv_O_ to yield 2,5 dihydrofuran and the original reduced V IV site [13]. Furans are well-known to undergo cycloaddition with singlet oxygen [15]. Two consecutive abstractions of hydrogen atoms in the 5- position by VV -0- yields maleic anhydride and IV the V site. An alternative path for the reaction of cis-2-butene is the dihydroxylation of a 1,4 alkenyl diradical, similar to the VzOs-catalyzed oxidation of olefms by peroxide [14] or the mechanism proposed by Mimoun et al. for oxidation of aromatics [15]. Such a diradical may be stabilized by delocalization of the unpaired electrons through 1t-
999
conjugation [16]. This reaction path is consistent with the very selective oxidation observed for 1,4-butanediol. Other partially oxidized species may results from (i) hydration of an olefin followed by oxidation of the alcohol into methylethylketone, and (ii) addition of the peroxo group to the C=C double bond to yield acrylic and acetic acids, respectively, via decomposition of the peroxymetallocycle with C-C bond scission and further oxidation of the molecular fragments. The simple alcohols appear to first dehydrate via the formation of a carbocation to form olefins. Formation of a linear chain species from isobutanol is indicative of the skeletal rearrangement of the carbocations. Oxidation of I-butanol resulted in a higher yield of maleic anhydride as compared to 2-butanol. This may be explained by an access to a "shorter" oxidation path, similar to intramolecular cyclization of 13- and o-unbranched primary alcohols [14]. Oxidation of 1,4 butanediol on the vanadyl pyrophosphate catalyst was highly selective. Several possibilities exist for 1,4 diol cyclization. The diol may be oxidized to y-butyrolactone via 4-hydroxyaldehyde. A second possibility is the Paal-Knorr synthesis [17], involving the cyclizing dehydration of 1,4 dicarbonyl compounds, which is widely used to synthesize furans. An acid catalyzed partial dehydration with cyclization is a third alternative for the reaction of the 1,4 butanediol. Dehydration of diols is evident in the formation of 1,3-butadiene from the 1,3 diol. The formation of maleic anhydride from isobutane clearly shows the vanadyl pyrophosphate catalyzed skeletal rearrangements. This could occur at Lewis acid sites on the surface via carbocation rearrangement. A 1,3 diradical formed by activation of two C-H methyl bonds may also undergo skeletal rearrangement via methyl group migration with a cyclic intermediate [18]. 5. CONCLUSION The oxidation of 17 C4 molecules to maleic anhydride over a vanadyl pyrophosphate catalyst was studied. The product distributions indicate that the pyrophosphate catalysts catalyze isomerization, dehydrogenation, dehydration and oxidation reactions. Branched molecules either isomerize to straight chain species or are totally oxidized. Alcohols can be selectively oxidized to maleic anhydride. The oxidation of 1,4-butanediol to maleic anhydride is remarkably selective. The wide variety of reactions catalyzed by the vanadyl pyrophosphate catalyst demonstrate the polyfunctional nature of the catalyst surface, which makes it very difficult to identify a single active catalyst site. We have proposed an active site for the initial activation of n-butane consisting of a cis-(su)peroxo oxovanadium(V) dimer present of the (200) plane of (VO)2PZ07' This active "site" is consistent with activation of molecular oxygen by vanadium complexes and provides a rational explanation for the reaction products observed.
1()()()
ACKNOWLEDGEMENT
This work was supported by the Amoco Chemical Corporation and the National Science Foundation Grant CTS-9100BO. REFERENCES 1. Centi, G.; Trifiro, F.; Ebner, J.R.; Franchetti, V.M.; Chem Rev. 88 (1988) 55. 2. Catalysis Today (proceedings, Vanadyl Pyrophosphate Catalysts), Centi, G., Ed. Elsevier: Amsterdam 16 (1993) 1-154 and refs. therein. 3. Pepera, M.A; Callahan, J.L.; Desmond, MJ.; Milberger, E.C.; Blum, P.R., Bremer, N.J.; J. Am. Chern. Soc 107 (1985) 4883. 4. Schimt, B; Jmgensen, K.A.; Hoffman, R; J. Phys. Chem. 95 (1991) 2297. 5. Agaskar, P.A; DeCaul, L.; Grasselli, RK.; Catal. Lett. 23 (1994) 339. 6. Bergna, H.E.; US Patent 4,769,477, 1988. Assigned to E.!. Du Pont de Nemours and Co. 7. Butler A; Carrano, CJ.; Coord. Chern. Rev. 109 (1991) 61. 8. Butler, A.; in Vanadium in Biological Systems, Chasteen, N.D., ed.: Kluwer, Dordrecht, The Netherlands, 1990, 25. 9. Holm, R.H.; Chem Rev 87 (1987) 1401. 10. Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L.; J. Am. Chern. Soc. 108 (1986) 3711. 11. Butler, A; Clague, MJ.; Meister, G.E.; Chern. Rev. 94 (1994) 625, and refs. therein. 12. Middlemiss, N.E.; Ph.D. thesis McMaster University 1978. 13. Gorman, AA; Lovering, G.; Rodgers, M.A.; J. Am. Chern. Soc. 101 (1979) 3050. 14. Organic Synthesis by Oxidation with Metal Compounds; Mijs, WJ.; de .Tonge, C.R.H.; Eds., Plenum Press: New York 1986. 15. Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fischer, 1.; Weiss, R; J. Am. Chem. Soc. 105 (1983) 3101. 16. Rajca, A; Chem. Rev. 94 (1994) 871. 17. Scott, L.T.; Naples, 1.0.; Synthesis (973) 209. 18. March, 1.; Advanced Organic Chemistry; Wiley: New York, 1992.