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Selective hydrogenation of acetylene in ethylene rich feed streams at high pressure over ligand modified Pd/TiO2.

The removal of trace acetylene from ethylene streams is a key step in obtaining commercially valuable stocks of the ethylene for polymerization. In this work, the selective hydrogenation of acetylene from ethylene rich streams was conducted at high pressure and in the presence of CO over Pd/TiO2 catalysts. Modification of the catalysts by exposure to triphenyl phosphine or diphenyl sulfide and subsequent reduction at 393 K led to improved performance.

 

Figure 1 shows the dramatic improvement in acetylene conversion for the modified catalysts, especially when modified by diphenyl sulfide.

Figure 1.  Acetylene conversion for catalysts pre-reduced at 393 K in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 H2[thin space (1/6-em)]:[thin space (1/6-em)]N2 for 1 h, then in reaction at 10 bar and 323 K in a gas mixture comprising 6000 ppm C2H2, 30% C2H4, 15% H2 (25[thin space (1/6-em)]:[thin space (1/6-em)]1 H2[thin space (1/6-em)]:[thin space (1/6-em)]C2H2), 100 ppm CO, N2 balance.

 

As shown in Figure 2, both of the ligand modified catalysts produced a net gain of ethylene with triphenyl phosphine modification providing the greater benefit. However, this modified catalyst failed to reduce acetylene levels to values which would be industrially acceptable. In contrast, the diphenyl sulfide modified catalyst led to a net gain in ethylene and also led to the conversion of acetylene to below detectable limits

 

Figure 2. Remaining acetylene, loss/gain of ethylene and by-product formation after 5 h on stream over (a) Pd(1.7)/TiO2, (b)

PPh3–Pd(1.7)/TiO2 (c) Ph2S–Pd(1.7)/TiO2. Reaction conditions: 30 mg catalyst, pre-reduced at 393 K 1[thin space (1/6-em)]:[thin space (1/6-em)]1 H2[thin space (1/6-em)]:[thin space (1/6-em)]N2 1 h, P = 10 bar,

323 K. Reactant gas contained 6000 ppm C2H2, 30% C2H4, 15% H2 (25[thin space (1/6-em)]:[thin space (1/6-em)]1 H2[thin space (1/6-em)]:[thin space (1/6-em)]C2H2), 100 ppm CO, N2 balance.

 

Overall, the presence of the modifier and the CO in the feed both result in significant changes to the activity-selectivity profile as well as to the extent of deactivation and by-product formation. Performance could also be optimised by tuning the CO feed to map the deactivation profile rather than the constant feed employed here. This complex interaction between modifier, CO and deactivation results in changes in adsorption energy as well as surface coverage, the predominant factors which determine the catalytic behaviour. However, it is clear that modification of the palladium surface with sulphides and phosphines provides a method by which the selectivity and deactivation of the catalysts may be controlled under realistic industrial reaction conditions.

 

Reference: McKenna, F.M., Mantarosie, L., Wells, R.P.K., Hardacre, C. and Anderson, J.A., Catalysis Science and Technology 2 (2012), 632-638.