Arcolein adsorption on ~- and r-polymorphous modifications of CoMoO 4 has been ... a- and r-phases of CoMoO4 to the coordination of molybdenum ions.
React. Kinet. Catal. Lett., Vol. 19, No. 3 - - 4 , 3 6 7 - 3 7 1 (1982)
E F F E C T OF MOLYBDENUM ION C O O R D I N A T I O N ON A C R O L E I N A D S O R P T I O N ON a- AND r-COBALT MOLYBDATE I. I. Zakharov, G. Ya. Popova and T. V. Andrushkevich Institute of Catalysis, Novosibirsk, USSR Received September 30, 1981 Accepted November 27, 1981 Arcolein adsorption on ~- and r-polymorphous modifications of CoMoO 4 has been studied by the thermal resorption technique. The effect of molybdenum coordination on the bond strength of surface acrolein compounds has been treated in terms of the interaction between frontier molecular orbitals of the active center and acrolein. MeTO~OM TepMo,aecop6thaH HCClIeROBaHbI~OpMbI a~cop6mm aKponeaHa Ha a rl fl-NOYIHMOpd~bHl,lX MO~I,Id~bHKalIK~X CoMoO~ n 13 p-,.:~,ax B3aHMO~eI~CTBI,t~ rpa_rlHqHblX MOYleKyJIapHbIX op6aTa.rlefi aKT~rBHoro tteHTpa C aKpoJIeaHOM pacCMOTpeHo Bnn~inHe goop~HHaI.[HH HOHOB MO/Ia6~eI-la Ha IIpOqI-IOC'r/, CB~I3H o6pa3yIoiIBIxc/I noBepXHOCTHLIX coetlHHeH~41~aKponenHa c KaTaJIH3aTOpOM.
It has been shown previously /1[ that the catalytic properties of the C o M o O 4 system in acrolein oxidation are determined not only by the chemical but to a great extent by the phase composition. Unlike the o~-phase of CoMo04, its r-phase is a highly selective catalyst. The main difference betwen a- and r-modifications is the coordination of molybdenum ions/2, 3/. One of the essential factors determining the reaction route is the character of the interaction between the oxidizable substance and the catalyst surface /4/. According to thermal desorption studies on acrolein on a- and r-modifications of CoMoO4, we have made an attempt to attribute the bond strength of the surface acrolein compounds and the difference betwen the catalytic properties of the a- and r-phases of CoMoO4 to the coordination of molybdenum ions. RESULTS AND DISCUSSION Catalyst preparation and the thermal desorption technique were described elsewhere /1, and 5, respectively/. Figure 1 illustrates the dependence of the total yield of acrolein ~desorption products from the surface of a- and r-phases of CoMoO4 (Curves 1 and 2, respectively) 367
ZAKHAROVet al." ACROLEINADSORPTION
150 I/,0 130 120 110 100
"6 90 E 80 i 70 6O .~
5(1 30 20
,,
373
,I
, ,,['q~,
473
,,,l',,"tO,
573
673
1 , ,,
773
,I
883
T(K) Fig. 1. Chromatographic analysis of the products of acrolein desorption from the surface of a- (Curve 1) and /3- (Curve 2) phases of CoMoO,
on the desorption temperature. As is seen, acrolein on both modifications is adsorbed in two (weak and strong) forms desorbed at temperatures of 293-473 and 573-773 K, respectively. Qualitative composition of desorption products from the surface of both modifications is approximately the same, whereas their quantitative compositions differ significantly. On fl-CoMoO4 and o~-CoMoO4 the quantity of adsorbed acrolein is about 54 and 103 % monolayer, respectively. On fl-CoMoO4 acrolein is adsorbed mainly in a weak form removed from the surface when the temperature increases to 473 K as acrolein and acrylic acid, 90 % of adsorbed ac. rolein being desorbed in unchanged form. The strong adsorption form accounts for only 7 % and is removed as carbon oxides and acetic acid. On a-CoMoO4 a significant portion of acrolein ("~ 40 %) is adsorbed in the strong form removed at desorptJon temperatures above 573 K as products of deep and destructive oxidation (carbon oxides, acetic acid, acetaldehyde, formaldehyde and water). In this case the desorption products contain acrylic acid as traces. By analogy with V-Mo-Si oxide catalysts /5/, we believe the acrolein adsorption centers on cobalt molybdate are the molybdenum ions. We have made an attempt to treat the difference in acrolein adsorption forms on a- and fl-phases of 368
ZAKHAROV et al.: ACROLEIN ADSORPTION
CoMo04 in terms of interaction between the oxidizable molecule and the molybdenum cation in a tetrahedral (/~-CoMo04) or octahedral (a-CoMoO4) environment. The structure of active centers in the Co-Mo oxide catalyst can be schematically represented as:
o\ i/o
o\j/
[]
[]
Me
r
o/\o
/!\o b)
a)
Assuming the point group of Td and C4v for these active centers, the splitting of d-levels of the cation can be represented schematically as:
xy xz yz
xZ-y 2 .,, ~t22
// "~
x 2_ y2
/,
9
(r
./
/./
~..
,, z2
-~.
xy
"~
xz
z2
(t2~)
yz -
Td,
C4v
(O h )
The splitting of d-levels of the cation in a ligand field of C4v symmetry is similar to that in the Oh symmetry. (For simplicity, in what follows, active centers of types(a) and (b) will be referred to as tetrahedral (ACtetr) and octahedral (ACoct), respectively). Note that in both cases the dxy, dxz and dyz orbitals of the AC cation will participate in rr-back-donation, whereas dx~ _y2 and dz~ take part in a-donor interaction with a molecule coordinated along the Z axis. Energy levels of the molecular orbitals of ACtetr and ACoct with the M o 6+ cation (d ~ can be schematically represented similarly to MeL4 and MeL6 complexes with occupied a- and zr-orbitals of the ligands: 369
Z A K H A R O V et at.': A C R O L E I N A D S O R P T I O N
t~
t 2g ~
/ II
/ ,./
Ld / / cation ~\ "~",,'
\ e*
/ \
-\ \
.///
//
\~ _~4f / " / ~":~) - - ~ cation '~\'~,
"N~X \ N
\XXX / ' /X~ /
~\\ t2 9 j / ' / /
\
/ \ \ t 2 //
Mo of tetrahedral cornptex
1'4oof octahedral complex
Hence, the electronic structure of ACtetr and ACoet differ tadicaUy in the nature of the frontier MO. The highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of ACtetr have the. symmetry of dx2_y~ and dz2 orbitals of the cation. They are capable only of o.donor interaction with the substrate. In ACoct the HOMO has the symmetry of dxy, dxz and dyz, which are in a position for n-back-donor interaction with the rr*-orbitals of the substrate and the LUMO has the symmetry of dx2_y2 and dz2-orbitals capable of o-donor interaction with the g-orbitals of the oxidizable substance. The electronic structure of acrolein /5/indicates that the HOMO is the lone o-electron pair of oxygen, whereas LUMO is the antibonding rr*-orbital of the molecule. Assuming that the most effective interaction of the active center with the substrate proceeds through the frontier MO, it should be noted that a o-donor interaction requires the presence of unoccupied o-orbitals (dx2_y~ and dz~ symmetry) and that of the ~r-back-donor interaction presupposes the presence of occupied rr-orbitals (dxy , dyz and dxz-Symmetry) of AC. Therefore, the contributions of the o-donor and rr-back-donor interactions in the active center-substrate bond formation will radically depend on the nature of frontier orbitals of the active center and substrate. Studies of the adsorption forms of olefins on oxide catalysts/6/ indicate that in all steps of the partial oxidation weakly bonded labile surface compounds of both the oxidizable substance and the oxidation products should be formed. The formation of strongly bonded adsorbed forms leads to deep oxidation products. Proceeding from the above nature of the active center-substrate interaction in terms of the Dewar.Chatt-Duncanson model /7/, the formation of weakly bonded surface compounds can be attributed to either the o-donor or the n.back-donor interaction. For example, Cu(I), Ag(I) and Au(I) complexes in interaction with olefins give labile and thermodynamically unstable compounds/8/(with occupied d 1o, only 370
ZAKHAROV et al.: ACROLEIN ADSORPTION 7r-back-donation by d-orbitals is possible). Similar situation should also be predicted for the existence of o-donor interaction, e.g. for Ti complexes in the polymerization of olefins/9/. When both types of interaction coexist with opposite electron transfer between active center and substrate, the formation of strongly bonded surface compounds should be predicted. For example, the most stable olefin complexes are formed for the transition metals having both occupied and unoccupied d-orbitals /8/ responsible for both back-donation and donor interactions. For Oh symmetry the t2g HOMO of the active center realize back-donation interaction with the zr* LUMO of acrolein along with the donor interaction between the o HOMO of acrolein and the e* LUMO of ACoet. According to our assumptions, in this case the formation of strongly adsorbed forms of acrolein should be predicted. In the case of Td symmetry the main contribution to bonding with the catalyst is provided only by the donor interaction of the o HOMO of acrolein with the e* LUMO of the ACtetr (occupied t2-1evels are located lower and do not interact effectively with the zr* level of acrolein). Hence, we believe that the existence of the tetrahedral coordination of molybdenum in/3-CoMoO4 favors the formation of weakly bonded labile surface complexes of acrolein, which are reactive in acrolein oxidation to acrylic acid.
REFERENCES 1. T. V. Andrushkevich, G. Ya Popova, G. K. Boreskov, L. M. Plyasova, N. P. Boronina, G. K. Kustova: Kinet. Katal., 19, 184 (1978). 2. G. W. Smith, I. A. lberes: Aeta Crystallogr., 19, 269 (1965). 3. L. M. Plyasova, V. I. Zharkov, G. N, Kustova, L. G. Karakchiev, M. M, Andrushkevich: Izv. Akad. Nauk SSSR, Set. Neorg. Mater., 9, 519 (1973). 4. G. K. Boreskov: Kinet. Katal., 14, 7 (1973). 5. G. Ya. Popova, A. A. Davydov, I. I. Zakharov, T. V. Andrushkevich: Kinet. KataL (in press). 6. V. G. Mikhalchenko, V. D. Sokolovskii, G. K. Boreskov: Kinet. Katal., 14, 698 (1973). 7. M. J. S. Dewar: Bull. Soc. Chim. Ft., 18, C17 (1951); J. Chart, L. A. Duneanson: J. Chem. Soe., 2939 (1953). 8. M. Harberhold: ~r-Complexesof Metals. Mix, Moskva 1975. 9. V. I. Avdeev, I. I. Zakharov, V. A. Zakharov, G. D. Bukatov, Yu. I. Yerrnakov: Zh. Strukt. Khim., 18, 525 (1977).
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