Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 171-178
171
References HO C H 0”-COONGO ~
O ~N - O - F , - N
O
~
N
O
li
I
0
t
HO C N ~ C H ~? N
C HO O N ~ O
It HOGNO
It O
N
~
O
H
/
CROS2LINK
1 HP-Q
/
Alkal, +usion
CHZ
O”*
F i g u r e 14. Mechanism of cross-linking w i t h urethane cross-linking agenta and alkaline fusion.
Guise and Smith,4O and alkali fusion would seem to be applicable as a more detailed method of analysis. Other possible amide type applications concern the analysis of poly(l,4-benzamides) and their chlorine-substituted derivatives41and a wide variety of polyamides that exhibit liquid-crystalline proper tie^.^^ The increasing complexity of polymer produds is further illustrated by the recent commercial development of a transparent engineering nylon and a thermoplastic elastomer based in part on 4,4‘-methylenediphenyl diisocyanate by the Upjohn Co. The analysis of such products is very likely to be successfully conducted by using alkali fusion technique^.^^ A slightly different application has been reportedu with a compounded natural rubber cross-linked with a proprietary polyurethane cross-linking agent, i.e., Novor 924. The polyurethane and its method of cross-linking and subsequent cleavage are shown in Figure 14. Here the aromatic nature of the cross-linker present as a very minor component was readily established. The presence of other minor additives in rubber is possible as should be the recently introduced aliphatic polyurethane cross-linker, Le., Novor 950.
Conclusion The use of alkaline fusion as the preliminary step in the analysis of condensation polymers as conducted in these laboratories is described; other studies are tabulated. Current studi- on other systems are indicated, and further utility of the technique with several additional systems is suggested.
Haken, J. K. Pfog. Org. Coat. 1979, 7 , 209-252. Whltlock, L. R.; Siggla, R. Sep. furif. Methods 1974, 3, 299-337. Smets, G.; De Loecker. W. J. pOrVm. Sci. 1959, 4 1 , 375-380. Siggla, S.; WhRlock, L. R.; Tao, J. C. Anal. Chem. 1989, 4 7 , 1387-1392. ( 5 ) Ettre, K.; Varadl, P. F. Anal. Chem. 1983, 35, 69-73. (6) Frankoskl, S. P.; Siggla, S. Anal. Chem. 1972, 4 4 , 507-511. (7) Frankoskl, S. P.; Slggla, S. Anal. Chem. 1972, 4 4 . 2078-2088. (8) Williams, R. J.; Siggia, S. CRed In Sep. furif. Methods 1974, 3 , 299-337. (9) Rahn, P. C.; Slggia, S. Anal. Chem. 1973, 4 5 , 2336-2341. (10) Schlueter, D. D. Thesis, University of Massachusetts, Amherst, MA, 1976. (11) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 4 9 , 2343-2348. (12) Williams, R. J.; Slggla, S. Anal. Chem. 1977, 4 9 , 2337-2342. (13) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 4 9 , 2349-2353. (14) Gibian, D. 0. Thesis, University of Massachusetts, Amherst, MA, 1979. (15) Sasto, L. G., Jr. Thesis, University of Massachusetts, Amherst, MA. 1982. (16) Anton, A. Anal. Chem. 1988, 4 0 , 1116-1118. (17) Morl, S.; Furusawa, M.; Takeuchl, T. Anal. Chem. 1970, 4 2 , 138- 140. (18) Morl, S.; Furusawa, M.; Takeuchl. T. Anal. Chem. 1970, 42, 959-961. (19) Gladlng, G. J.; Haken, J. K. J. Chromatogr. 1978, 757, 404-409. (20) O’Nelll, L. A.; Christensen, G. J . Oil Colour Chem. Assoc. 1978, 5 9 , 285-290. (21) Haken, J. K.; Obita, J. A. J. Oil Colow Chem. Assoc. 1980, 6 3 , 200-209. (22) Haken, J. K.; Ob&, J. A. J. Chromatogr. 1981, 213, 55-62. (23) Lee, H.; Stoffey, D.; Neville, K. ”New Linear Polymers”; McGraw-HIII, New York, 1967; Chapter 6-6. (24) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 265-270. (25) Haken, J. K.; Rohanna, M. A. J. Chromatogr. 1984, 2 9 8 , 263-272. (26) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 259-263. (27) Preston, J. US. Patent 3376269, 1966. (28) Preston, J. US. Patent 3484407, 1969. (29) Haken, J. K.; Oblta, J. A. J. Chromatogr. 1982, 2 3 9 , 377-384. (30) Haken, J. K. “The Gas ChromatoaraDhy of Coating Materials”; Dekker. New York, 1974. (31) Vlmaiaslrl, P. A. D. T.; Haken, J. K.; Burford, R. P. J . Chromatogr. 1985, 379, 121-130. (32) Matuszak, M. L.; Frisch, K. C.; Reegen, S. L. J. f o k m . Sci. 1973, 1 7 , 1683- 1690. (33) Barringer, C. M. Teracel30 Polyalkylene Ether Glycol Bulletin No. 11R1-1956, Du Pont, Wllmlngton, DE, 1956. (34) Haken, J. K.; Burford. R. P.; Vlmalaslrl, P. A. D. T. Advances in Chromatography; Elsevier: Amsterdam, 1985; pp 347-356. (35) McFadden, J.; Scheulng, J. Chromatcgr. Sci. 1984, 2 2 , 310-312. (36) Haken, J. K., unpubllshed results, 1984. (37) Hercules Inc. U.S. Patent 2926 154, 1960. (38) Earle, R. H., Jr.; Saunders, R. H.; Kangas, L. R. Appi. folym. Sci. Symp. 1971, 18, 707-714. (39) Smith, P.; Mills, J. H. CH€M€CH 1973, 3 , 748-755. (40) Guise, 0. B.; Smith. G. C. J. Chromatogr. 1982, 235, 365-376. (41) Morgan, P. W. U.S. Patent 3943 110, March 9 1976. (42) Morgan, P. W. CH€MECH 1979, 9 , 316-326. (43) Chem. Eng. News July 9, 1984, 62(28), I O . (44) Burford, R. P.; Haken, J. K.; Obita, J. A. J. Chromatogr. 1983, 268, 515-521,
(1) (2) (3) (4)
Receiued for review
December 31, 1984 A c c e p t e d December 27, 1985
Desirable Catalyst Properties In Selective Oxidation Reactions Harold H. Kung Chemical Engineering Department and the Ipatieff Laboratoty, Northwestern University, Evanston, Illinois 6020 1
Heterogeneous oxide-catalyzed selective oxidation reactions can be classified into dehydrogenationand dehydrogenationwith oxygen insertion. The oxide properties that are important In each of the steps in these reactions are discussed. The breaking of the C-H bonds in alkanes is facilitated by weakly adsorbed oxygen. The C-H bond breaking,of alkenes is enhanced by strongly basic surface lattice oxygen and cations that 01 96-4321/86/1225-017 1$01.50/0
are soft acid and undergo redox readily. Desorption of alkenes and dienes is enhanced by cations that are hard acid. The selective CO bond formation Is controlled by the number and the ease of removal of the available lattice oxygen, while the combustion reaction can be minimized by shortening the residence time of the surface intermediates, weakening the adsorption of the desired products and minimizing the amount of
0 1986 American Chemical Society
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weakly adsorbed oxygen or the density of combustion sites. The function of a promoter is to enhance the rate of the rateand selectivitydeterminingsteps. Thus the desirable influence of a promoter on the soli depends on the nature of the critical step.
Introduction Selective oxidation of hydrocarbons is the largest class of reactions catalyzed by transition-metal oxides. It is also among the most important and most studied catalytic reactions. From the industrial point of view, a desirable process must produce the desired products in high selectivity and high yield. The requirement of high selectivity is extremely challenging in partial oxidation processes where the desired products are produced instead of carbon monoxide and dioxide, which are the combustion products that are thermodynamically much more favorable, even though the chemical transformations involved in partial or total oxidation share many common features. In this paper, the oxide properties that are known to affect selectivity will be discussed, using as examples some of the systems that have been relatively well studied. A number of excellent recent review articles have addressed the question of selectivity from various points of view.l-14 A monograph summarizing most of the literature before 1974 is also available.1s I t is clear that the overall activity and selectivity depend on the chemical nature of the catalyst, especially the composition and distribution of the components; the physical nature of the catalyst such as texture, pore size, and pore volume; and the process variables such as conversion, temperature, hydrocarbonto-oxygen ratio, and the presence of water. It is also evident that many of these factors are interrelated. For example, the gas-phase hydrocarbon-to-oxygenratio affects the oxygen content of the catalyst. At any particular instance, the steady-state composition of the catalyst depends on the relative rates of reduction of the oxide by the hydrocarbon and reoxidation by the gaseous oxygen. The long-term stability of the catalyst depends on the fugacity of the oxygen in the gas phase, since the oxide must gradually attain a state that is in a thermodynamically stable point with the gas. For a given oxygen fugacity, the steady-state oxygen content of the solid also depends on the temperature. It is common that products of partial oxidation are intermediates in the combustion process that produces carbon oxides. These intermediates can undergo secondary reactions for further oxidation. The extent of such secondary reactions increases with increasing conversion and contact time of the intermediate with the catalyst. It increases if the desorbed partial oxidation products need to diffuse in the long and narrow pores of the catalyst before leaving the catalyst. Thus the selectivity depends on the texture of the catalyst. In addition, the texture of the catalyst also affects the heat-transfer characteristics, which determines the temperature profile of the catalyst. Water is often added to the feed in partial oxidation processes. It could have two functions. Often it is added to aid in the removal of the heat released in the oxidation reaction. Its presence may also affect the acidity and basicity of the catalyst by changing the degree of surface hydroxylation, which may be important in the reaction. In addition to these physical and process variables, the selectivity critically depends on the chemical composition of the catalyst. It is often found that selective partial oxidation catalysts are multicomponent oxides that are based on a surprisingly few number of oxides as basic components. These basic components include oxides of
Table I. Common Oxide-Catalyzed Selective Oxidation Reactions reaction catalyst Dehydrogenation ethylbenzene styrene Fe-Cr-K-0 isopentane, isopentene Sn-Sb-0 isoprene butane, butene butadiene Bi-Mo-0, promoted Fe-0, promoted V-0 methanol formaldehyde Fe-Mo-0, MOO,
- -
Dehydrogenation and butane, butene maleic anhydride propene acrolein (propene and NH, acetonitrile) propene acrolein, acrylic acid, acetaldehyde benzene maleic anhydride o-xylene, naphthalene phthalic anhydride methane methanol, formaldehyde ethylene ethylene oxide
-
-
-
-
--
methyl ethyl ketone biacetyl methyl ethyl ketone acetaldehyde, acetic acid
Oxygen Insertion
v-P-0 Bi-Mo-0 Bi-Mo-0, U-Sb-0, Fe-Sb-0, Bi-Sb-Mo-0 Co-Mo-Te-0, Sb-V-Mo-0 V-P-0, V-Sb-P-0 promoted V-0 Mo-0, V-0 Fe-Mo-0 (also catalyzed by promoted Ag) Co-0 (promoted by Ni, Cu) V-Mo-0
molybdenum, vanadium, copper, iron, antimony, tin, and uranium. To these basic components other components, often referred to as promoters, are added to result in enhanced activity and selectivity. The exact chemical interaction of the promoters with the basic components and among themselves is a subject of strong research interest. Several types of interactions are identified: formation of new compounds, increase in acidity or basicity, and generation of defect sites for hydrocarbon or oxygen activation. Categorically, a promoter enhances catalytic activity by facilitating processes involved in the rate-determining step. It enhances selectivity by facilitating processes leading to the desired product and/or inhibiting processes leading to the undesired product. Therefore, to better understand catalytic selective oxidation, it appears helpful to consider each of the chemical steps involved in the reaction as to what chemical properties would facilitate that step. When the rate-determining step or the critical branching step that determines selectivity is known, such a consideration could lead to more systematic search for promoters and better understanding of why some oxides are better basic components than others. This is the purpose of this paper. Here, we first discuss classification of selective oxidation reactions. This is followed by considerations of the chemical interactions involved in each of the steps of the reactions. Types of Selective Oxidation Reactions Selective oxidation reactions can be classified into two types: one involves only dehydrogenation, the other involves both dehydrogenation and oxygen insertion into the hydrocarbon molecule. Table I summarizes the common oxide-catalyzed selective oxidation reactions and the catalysts. 1. Dehydrogenation Reactions. These are reactions in which a hydrocarbon molecule is converted into a more unsaturated hydrocarbon by breaking carbon-hydrogen bonds and forming C=C bonds. In the absence of oxidants, hydrogen is a byproduct. In such cases, the reactions are run at rather high temperatures (above 500 O C ) because the thermodynamic equilibrium normally favors the reactants at low temperatures. At these high temperatures, undesired coking takes place readily, and water
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25,
is often added to reduce coking. The more common dehydrogenation processes are conducted with oxygen as oxidant to yield water as a byproduct. Sometimes iodine is used instead of oxygen. The formation of water provides the thermodynamic driving force for the reaction. Thus the reaction can be conducted at a lower temperature than without oxygen, and deactivation due to coking is less severe. In these dehydrogenation reactions, the carbon skeletons of the hydrocarbon molecules remain intact. 2. Dehydrogenation and Oxygen Insertion. There are many examples of this type of oxidation reaction. Oxygen is needed as oxidant both for incorporation into the hydrocarbon molecules and in the formation of water in the dehydrogenation steps. The general features of these reactions are that C-H bonds are broken and C-0 bonds are formed. Exceptions to these are the oxidation of ethylene to ethylene oxide, in which no C-H bonds are broken, and the ammoxidation reactions such as propene to acrylonitrile, in which C-N bonds are formed. In some cases, such as the oxidation of benzene to maleic anhydride, the carbon skeleton is broken. In others, the carbon skeleton remains intact. The selectivity is determined in part by the ability of the oxide to catalyze the formation of C-O bonds without breaking (or breaking only a desired number) of C-C bonds. Excessive breaking of the C-C bonds, of course, leads to combustion.
Chemical Factors Affecting Selectivity A large variety of factors have been investigated in attempts to correlate changes in catalytic activity and selectivity with properties of the oxide. Factors such as the presence of cation vacancies, the nature of the metal-oxygen (M-0) bond, the strength of the M-0 bond, the crystal structure, and the surface acidity and basicity have been investigated, and successful correlations with limited groups of reactions have been obtained. Suffice to say, however, that there is not yet one single correlation that is successful for the entire class of selective oxidation reactions. This is not unexpected because different chemical processes are involved in different types of oxidation reactions, such as those classified in Table I. The oxidation of butane to maleic anhydride can be used as a generic illustrative example to discuss the oxide properties important in selective oxidation. This reaction can be expressed in a stepwise manner: 0
C,H,
- I
C,H8
2
9 II
0
C4H6
CO. CO,
H20
The first step is the activation of alkane, and the second step is the activation of alkene that leads to the formation of butadiene. They involve only oxidative dehydrogenation. The third and fourth steps lead to the formation of maleic anhydride. They involve both oxidative dehydrogenation and oxygen insertion. Thus this overall reaction includes both common types of selective oxidation. From it, it is also easy to see why there are catalysts that only catalyze dehydrogenation to produce dienes, while there are catalysts that when used under different conditions catalyze either oxygen insertion to produce oxygenates or dehydrogenation to produce dienes. There had been a common perception that the oxygen in the formation of water during dehydrogenation and in
No. 2, 1986 173
the insertion into the hydrocarbon are the same type of oxygen. A recent report by Ueda et al. shows that this is false.16 They find that on a Bi2Mo06catalyst at 400 "C the conversion of propene pulses decreases rapidly in the absence of gaseous oxygen. However, the conversion of butene pulses decreases much more slowly. From the total amount of propene reacted, the extent of reduction of the catalyst that corresponds to the deactivated state with respect to propene oxidation can be calculated. If the catalyst is first prereduced to the same extent by the butene dehydrogenation reaction or by the oxidation of hydrogen before the propene pulses are administered, the conversion of propene is found to be essentially the same as for a fresh catalyst. Thus the reduction by butene or H2does not remove the oxygen important in the oxidation of propene. This conclusion is further confirmed by l80labeling experiments. In these experiments, Ueda et al. first reduce the catalyst to a certain extent either by butene pulses or by propene pulses. The catalyst is then reoxidized with 1802. The reoxidized catalyst is then used for propene oxidation, and the l80content in acrolein is monitored. If the reduction of the catalyst is achieved with propene, 180-labeledacrolein appears immediately in the first propene pulse. The amount of labeled acrolein produced steadily decreases for subsequent propene pulses. If the reduction is achieved using butene, the acrolein formed initially contains little 180-labeling. The degree of labeling increases steadily with pulse number before falling again. Thus there must be two types of oxygen: one for water formation in dehydrogenation and one for insertion into the hydrocarbon. It is possible (and likely) that the optimal properties for the two processes are different. Reduction of Bi2Mo06followed by reoxidation with 1802 also causes shifts in Raman bands related to the Mo=O bonds at 725,803,and 844 cm-'. The magnitude of the band shifts are much larger if the reduction is with propene rather than with l-b~tene,'~J' consistent with the above conclusion that different oxygens are involved in insertion and in water formation.
Activation of Alkane The first step in activating alkane is to break a C-H bond. (In catalysis by superacids, the first step is to protonate the hydrocarbon, followed by release of a hydrogen molecule to produce a carbenium ion; this will not be discussed.) This step can be expressed as RCHZR'
+ M-0
+
RCHR'-M-OH
(1)
In considering the rate of this step, it is relevant to question whether the transition state more closely resembles a situation where the dissociation of the C-H bond is accompanied by real charge separation to form H+ and RCH,- or it is without charge separation. The heats of reaction, AHo of the C-H bond dissociation process with and without charge separation in the gas phase have been determined for some hydrocarbons. These values are listed in Table 11. Roughly speaking, the heats of reaction of the homolytic processes are about 400 kJ/mol, while those of the heterolytic processes are about 1600 kJ/mol. The much larger value for the heterolytic processes is due to the energy required for charge separation. The catalytic processes would require less energy than the gas-phase AH" values because the hydrogen atom or proton would form 0-H bonds with the surface oxygen and the hydrocarbon fragment would interact with a surface cation (or site). These interactions should be so strong that step 1 eventually becomes thermodynamically favorable.
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Table 11. A H o of Breaking C-H Bonds in Hydrocarbons dissociation AHo, kJ/mol Homolytic" 435 CH, CH3 + H 410 C2H6 CZH5 + H 410 C3Hs n-C3H7 + H 395 C3H8 s - C ~ H+~H 410 n-CIH,o n-C4Hg + H 395 n-CIH,o S-CqHg + H 405 l'-C4H1,3 i-C,Hg + H t-C,H,o t-C4H9 + H 375
-
-+
+
+
-
+
+
+
--
Heterolyticb CzH2 CZH- + H+ C3H6 C3Hc + H+ C2H4 -+ CpHC + H+ CHI CH3- + H+
1580 1640 1690 1690
" Calculated from AHof of the alkanes and the radicals, the latter from ref 86. bFrom ref 87. Because saturated hydrocarbons are rather inert and they interact relatively weakly with other species, it is most likely that for this reaction step the transition state is reactant-like. In that case, the activation energy would parallel the AHo values of the gas-phase process, unless the species in the charge separation process is stabilized substantially by electrostatic interaction with the solid. Since the electrostatic potential above an ionic solid is less than 20% of the electrostatic potential at a bulk ion stabilization by electrostatic interaction with the solid is less than 3 eV (or 300 kJ/mol). Thus this interaction is insufficient to substantially stabilize the charge separation process. I t is concluded that for saturated hydrocarbons the first step in activating the molecule is the dissociation of the C-H bond in a manner similar to the production of hydrocarbon free radicals. Since a tertiary C-H bond is weaker than a secondary C-H bond, which is in turn weaker than a primary C-H bond, it further suggests that the probability of bond dissociation also follows this order. Nonetheless, this step must be thermodynamically favorable for the reaction to proceed at any reasonable rate. The AH" of reaction 1 is given by AHo = EC-H - ( E M 4 - EM-L) - ( E 0 - H - E 0 - L )
(2)
In this equation, EC-H is the C-H bond energy. It is about 380 kJ/mol. EM
