(93c) Impact of Intermetallic Ni-Ga Phase on the Selective Hydrogenation of Acetylene in Excess Ethylene

Intermetallic compounds form an exciting subset of heterogeneous catalysts because these materials usually have catalytic properties distinct from their constituent elements and often outperform traditional single metal catalysts in terms of activity and selectivity for industrially relevant reactions. One of the most studied reactions on intermetallics in recent times is acetylene semi-hydrogenation in presence of excess ethylene and hydrogen.

Acetylene is present as a trace impurity (1%) in the ethylene feed destined for polyethylene manufacture. Its concentration must be reduced to ppm level upstream of the polymerization reactor because it is a poison for the Zeigler-Natta polymerization catalyst. The most environment-friendly approach is to convert the acetylene selectively to desirable ethylene. However, traditional hydrogenation catalysts like Pd and Pt is unselective for this semi-hydrogenation reaction and not only over-hydrogenates the acetylene but also depletes feed ethylene by converting all unsaturated C₂ to low value ethane. In contrast, Pd-M (M = Ag, Zn, Ga, In) alloy catalysts are highly selective for this reaction because of the ability of inert Ag to break up Pd active sites into small clusters which reduces ethylene binding strength on the catalyst surface, thereby boosting selectivity.

At present, the focus has shifted from Pd towards base metal intermetallic catalysts using Ni as the active element. Although, the main by-product on Ni is oligomers (rather than ethane) semi-hydrogenation selectivity can be increased by reducing Ni-Ni coordination through introduction of inert spacer atoms because oligomerization requires larger active ensembles compared to acetylene semi-hydrogenation. Previous work from our group has shown Zn effectively introduce such a geometric effect in Ni-Zn intermetallics and increase semi-hydrogenation selectivity compared to Ni. Ni-Ga intermetallics represent and interesting alternative since the Ga-H interactions may enable Ga to serve as a co-catalyst, potentially impacting the oligomerization selectivity.

Ga is relatively inert compared to Ni; however, it has higher hydrogen dissociation ability than other inert metals commonly used in intermetallic selective hydrogenation catalysts such as Zn, In, Sn, Pb. We aim to identify if Ga impacts the performance of Ni-Ga intermetallic catalysts for acetylene semi-hydrogenation due to a secondary Ga-mediated hydrogenation pathway which was not observed in Ni-Zn. We test three different phase pure Ni-Ga intermetallic compounds (NiGa, Ni₅Ga₃ and Ni₃Ga) for acetylene semi-hydrogenation under large excess of ethylene and hydrogen and at two different temperatures, 85 and 160°C. We use unsupported bulk materials to avoid complication and artifacts arising from support interaction and diffusion limitations which have been previously shown to play a major role in this chemistry.

Our selectivity results show all Ni-Ga intermetallics perform reasonably well with net ethylene selectivity of ~60%. Most notably, of the four catalysts tested, the NiGa catalyst had the highest selectivity at 85°C but the lowest selectivity at 160°C (Figure 1), contrary to observations on other bimetallics where increased inert concentration invariably increased selectivity. Typically, high (greater than unity) hydrogen rate orders in Ni catalysts are attributed to carbon deposition, and since Ga is expected to inhibit coking, catalyst with increased Ga content is expected to have hydrogen rate order closer to unity if the active site is purely Ni ensembles. However, preliminary rate order data shows that hydrogen order on NiGa (2.1) is higher than Ni₃Ga (1.8), suggesting an additional hydrogen dissociation pathway may be available in the Ga rich intermetallic which becomes more favorable with increasing hydrogen partial pressure. Further, Arrhenius plot on NiGa shows two distinct and extended regions of linearity which is not seen for the other Ni-rich catalysts and may be indicative of a change in the reaction pathway (or availability of alternate pathways) on NiGa at elevated temperature. H₂-D₂ exchange as well as comparison of rate order plots with NiZn are also employed to investigate the presence of a weak catalytic hydrogenation tendency of Ga which has not been previously reported in literature in the context of selective hydrogenation on intermetallic catalysts. This behavior may be important in understanding the temperature-dependent behavior of Ni-Ga intermetallic catalysts for other conversion reactions involving hydrogen.