Chemistry of the Oxychlorination Catalyst: An In situ, Time-resolved, Dispersive XANES Study

Nowadays, almost all the world's production of PVC (poly vinyl chloride) is obtained by the polymerisation of vinyl chloride (VCM), which is produced using C₂H₄, Cl₂ and O₂ as reagents, following the scheme represented in Figure 43. Production of VCM is based on the cracking of C₂H₄Cl₂, which in its turn is produced by two parallel processes, direct chlorination:

C₂H₄ + Cl₂ C₂H₄Cl₂ (1)

and oxychlorination:

C₂H₄ + 2HCl + 1/2O₂ C₂H₄Cl₂ + H₂O (2)

The oxychlorination reaction (2) allows the HCl produced by the cracking of C₂H₄Cl₂ (EDC) to be recycled, representing a large scale reaction specifically developed to reduce the raw material consumption (Cl₂) and to avoid the removal from the cycle of unused reagents (HCl), in agreement with the modern requests of chemical industry. Commercial catalysts are produced by impregnation of -alumina with CuCl₂ (4-8 wt% Cu) following the incipient wetness method [1].

Fig. 43: Schematic representation of the industrial production of vinyl chloride (VCM).

Despite abundant literature on the subject, a significant improvement in the knowledge of the system has been gained only recently [1-3]. By separately feeding in the three reagents, it was shown that the overall ethylene oxychlorination reaction (2) is catalysed by a highly-dispersed CuCl₂ phase following a three steps redox mechanism: (i) chlorination of ethylene by reduction of CuCl₂ to CuCl, (ii) oxidation of CuCl to an oxychloride and (iii) re-chlorination with HCl (closure of the catalytic cycle) [2,3]:

2CuCl₂ + C₂H₄ C₂H₄Cl₂ + 2CuCl (3)

2CuCl + 1/2O₂ Cu₂OCl₂ (4)

Cu₂OCl₂ + 2HCl 2CuCl₂ + H₂O (5)

An in situ, time-resolved, XANES study, performed on ID24 has allowed us to determine the Cu²⁺ Cu⁺ transformation occurring on the CuCl₂/-Al₂O₃ catalyst in the ethylene oxychlorination reaction atmosphere within the 373-623 K range. This is shown by the derivative spectra reported in Figure 44a and quantified in the red line of Figure 44b). The simultaneous determination of the catalysts activity (blue stars in Figure 44b), has demonstrated that the catalyst works in the reduced state, and that the rate determining step of the reaction (2) is the oxidation of CuCl according to Equation (4).

Fig. 44: Part (a): derivative XANES spectra (collected each 30 s) of the catalyst during the heating ramp from 373 to 623 K by 12 K min­1. The presence of two isosbestic points at 8984 and 8995 eV are noticeable. Part (b) Cu2+ fraction (red full line, left axis) determined from the spectra reported in part (a) and simultaneous O2 conversion (blue stars, right axis).

This study has also determined the key role of the potassium chloride dopant, added to the industrial catalysts for fixed-bed reactors, by proving that KCl decreases the rate of the reduction step (3), which becomes the rate-determining step for the KCl/CuClC₂/-Al₂O₃ catalyst. This dispersive XANES study, supported by parallel IR investigation supports the thesis that the active phase of the industrial catalyst is a non-stoichiometric mixed chloride K_XCuCl_2+X.

References
[1] G. Leofanti, M. Padovan, M. Garilli, D. Carmello, A. Zecchina, G. Spoto, S. Bordiga, G. Turnes Palomino and C. Lamberti, J. Catal. 189, 91-104 (2000).
[2] G. Leofanti, A. Marsella, B. Cremaschi, M. Garilli, A. Zecchina, G. Spoto, S. Bordiga, P. Fisicaro, G. Berlier, C. Prestipino, G. Casali, and C. Lamberti, J. Catal. 202, 279-295 (2001).
[3] G. Leofanti, A. Marsella, B. Cremaschi, M. Garilli, A. Zecchina, G. Spoto, S. Bordiga, P. Fisicaro, C. Prestipino, F. Villain and C. Lamberti, J. Catal. 205, 275-381 (2002).

Principal publication and Authors
C. Lamberti (a,b), C. Prestipino (a,c), F. Bonino (a), L. Capello (a,b), S. Bordiga (a,b), G. Spoto (a), A. Zecchina (b), S. Diaz Moreno (c), B. Cremaschi (d), M. Grilli (d), A. Marsella (d), D. Carmello (d), S. Vidotto (d) and G. Leofanti (d,e), Angew. Chem. Int. Ed. 41, 2341-2344 (2002).
(a) University of Turin (Italy)
(b) INFM UdR of Turin University (Italy)
(c) ESRF
(d) EVC Technological Centre, Porto Marghera, Venezia (Italy)