61st Annual Report on Research 2016

We have developed 1-hexyne hydrogenation (a model for ore industrially important acetylene hydrogenation) as a test reaction. We chose hexyne because it offered easier opportunities to change the reactant composition and was more easily integrated into our current reactor system. We now have this reaction up and running, and are using it to compare catalysts. We have tested 1- and 2- hexyne, and see no major differences in reactivity over Au catalysts. To date, we have largely measured light-off curves with this reaction, rather than try to measure rates. We believe that this will be a faster and more useful means of screening catalysts. We are now working to make sure that we understand alkyne hydrogenation over supported Ni catalysts as control materials for Au-Ni nanoparticle catalysts. Interestingly, we have found that Ni/TiO2 catalysts are very selective to alkenes at higher temperatures, provided the space velocity is sufficiently fast. We are working on understanding this chemistry, as we believe that some sort of surface Ni-titanate phase may be prepared in the calcination process.

One of our primary goals is to develop Hammett studies to probe changes in Au surface chemistry. We would like to use hydrogenation reactions to complement our previous work on benzyl alcohol oxidation (a manuscript is in preparation). Because, there is a fair amount of literature on Au catalyzed nitrobenzene hydrogenation, we spent a great deal of time working on this reaction. The reaction is slow at moderate temperatures under one atmosphere of H₂, so our initial studies examined using stronger reducing agents (e.g. NaBH₄). Although these reactions were successful in that the catalysts were active, the strong dependence on the concentration of the reducing agent (greater than 2^nd order), and complicated kinetics (at least 1 intermediate was observed) made it clear that this was not an appropriate reaction for the type of reproducible kinetic tests that we want to develop.

We therefore turned to phenylacetylene reduction, which has the advantage of being more directly comparable to 1-hexyne hydrogenation. After a great deal of testing with multiple reducing agents and reaction conditions, we settled on using hydrogen at ambient pressure and roughly 70 °C . We performed a fairly detailed concentration study to find optimum reaction conditions (see Figure 1); we are also working to interpret this data with a Michaelis-Menten analysis and hope to ultimately compare these results with our Hammett studies.

We have begun Hammett studies with this reaction, with very promising results. Figure 2 shows our preliminary data using Au/TiO2 as the initial catalyst. As expected, r is positive for this reaction, indicating negative charge building on the benzylic carbon in the transition state, which is consistent with a hydride transfer from Au to the triple bond in the rate determining step. The r value for PA hydrogenation is substantially larger than for BA oxidation (1.2 vs. 0.4), suggesting that this reaction will be more sensitive to electronic effects. There are also some important differences in the sterics of the reactions, and strong electron withdrawing groups do not fall on this line. We are currently working to sort out these details before we begin comparing catalysts.

We have also been preparing bimetallic Au-M (M=Ni, Co) nanoparticles and catalysts using PAMAM dendrimer templates. After trying several methods and studying metal uptake by the dendrimer, we have prepared 1:1 NiAu and Co:Au nanoparticles (see Figure 3), and deposited them onto titania and alumina supports. The organic dendrimer was then thermally removed under flowing H2 at 300 C. The metal content of the resulting material, determined by AA spectroscopy, was consistent with 1:1 molar ratios for all the catalysts.

We have since performed 1-hexyne hydrogenation light off curves on these materials, and appropriately prepared Au catalysts. Comparing bimetallic catalysts is not always straightforward; because both metals may be active hydrogenation catalysts, we adjusted the amount of catalyst in the reactor to keep the total moles of metal relatively constant. Figure 4 shows the data for Au, Co-Au, and Ni-Au catalysts. TheCo-Au catalyst is slightly more active than Au, in spite of having less Au. The Ni-Au catalyst is significantly more active, reaching complete conversion at about 150 C. Further, the reaction is essentially 100% selective for 1-hexene, suggesting that the reaction occurs on the Au surface (the lower observed selectivity for the pure Au catalyst is due to the very low conversion, coupled with a small 1-hexene impurity in the feed).

We are extremely excited by these results, and have held off publishing as we consider a possible patent application. Importantly, we have shown a strong proof of concept – that heterometals can drastically improve the reactivity of Au catalysts, while maintaining the desirable selectivity associated with Au and reducing the total amount of Au in the catalyst. We are now working to more carefully measure catalytic activities, perform a complete characterization of the catalysts, and develop preparative routes that will not require the use (and destruction) of expensive dendrimers.

We have continued this work, developing colloidal syntheses to supported Au and NiAu catalysts using oleic acid and oleylamine as capping agents. We have prepared supported Au nanoparticles on 8-10 different supports, and are currently studying them with octyne hydrogenation . The catalysts all have high selectivity to the 1-octene and most have similar reaction profiles. We have identified Au/MgO as being significantly more active than the other catalysts and will study this catalyst further. We have also developed controllable and reproducible colloidal syntheses to Ni (or NiO) nanoparticles that are 3-7 nm in diameter with size distributions of ~10%. We have identified methods for removing the capping agents and have shown that the catalysts behave as expected for supported Ni catalysts.