Palladium catalysts supported on nanoparticle alumina [nano-Al₂O₃(+), surface area (SA): 695 m²/g] have been prepared using two different methods of depositing the active metal onto the support, namely wet impregnation and precipitation.  The prepared nanoparticle-supported catalysts have been tested for activity in the coupling reaction of 4-methylpyridine to 4,4'-dimethyl-2,2'-bipyridine and their activities compared with two commercially available catalysts (Pd/C and Pd/Al₂O₃), as well as catalysts prepared using precipitation onto a conventional alumina support and a second nanoparticle alumina support [nano-Al₂O₃(-), SA: 275 m²/g].  Previous results have indicated that Pd/C is one of the most efficient catalysts, even though the yields may vary somewhat, and that palladium supported on alumina is a poor catalyst in this reaction.  This is indeed the behavior observed in Table 1, Rows 1-3.  Our results indicate that the palladium supported on the nanoparticle alumina is in fact a very active catalyst, but the activity is dependent on the preparation method.  While the impregnated catalyst performs poorly, the catalyst prepared via precipitation gives the highest yield observed to date for this reaction system (Table 1, Rows 4 and 5).  Despite the fact that Pd/Al₂O₃ catalysts are known to be poor catalysts in this reaction the precipitated Pd/nano-Al₂O₃(+) performs better than the commonly used commercial Pd/C catalyst (compare Rows 5 and 2).  However, the catalytic activity is very dependent on the nature of the alumina support.  While a catalyst prepared using another nanoparticle alumina with larger particle sizes [nano-Al₂O₃(-), surface area 275 m²/g] did not have a significant activity, a commercial bimodal gamma-Al₂O₃ support (surface area: 260 m²/g) can result in an active catalyst if prepared via the precipitation method (Table 1).  However, the yields obtained from Pd/bimodal-gamma-Al₂O₃ are lower and less reproducible compared to the best performing catalyst, Pd/nano-Al₂O₃(+).  Palladium surface area measurements indicate that while the catalytic activity appears to correlate with the palladium surface area on Pd/C and the precipitated Pd/nano-Al₂O₃(+), this is not the case for the impregnated Pd/nano-Al₂O₃(+) and the commercial Pd/Al₂O₃ (see Table 1).  The activities of the impregnated Pd/nano-Al₂O₃(+) and the commercial Pd/Al₂O₃ catalysts are lower than what would be expected for the corresponding palladium surface areas.  Preliminary X-ray photoelectron spectroscopy (XPS) measurements indicate that the fresh commercial Pd/Al₂O₃ is not active due to a lower PdO content compared with fresh Pd/C.  The XPS corroborate a higher palladium dispersion of the precipitated versus the impregnated Pd/nano-Al₂O₃(+) catalysts.  However, apart from this and the presence of residual Na (from the precipitation base), there are no other evident differences between the two catalysts in the XPS spectra.  A more detailed investigation is necessary to explain the observed differences between the catalyst activities. 

Although it is possible that a palladium species in solution is the catalytically active species, our experiments indicate that presence of a heterogeneous catalyst is necessary for the reaction to proceed.  A reaction mixture that was filtered hot after 24 hours and then brought back to reflux without the solid catalyst for an additional 72 hours did not form any more product than what is expected from a 24-hour run.  The recovered catalyst (after the 24-hour experiment) is active, but the activity is considerably lower than that of the original catalyst.  The experiments also indicated that the quality of the reactant, the 4-methylpyridine, is important for the reaction.  It is necessary to distil the 4-methylpyridine over KOH in order to obtain the highest yields.

Initial tests have also been performed on a few other nanoparticle oxide supports, TiO₂, ZnO, Al(OH)₃, CaO, and Ce-doped ZrO₂ [ZrO₂(Ce)].  The results are summarized in Table 1. 

                                                            Raw              Pd                 Yield

Catalyst Description ^a                         Product        Surface         [g/g

                                                            Yield             Area             Surface

                                                            [g/g Pd] ^b        [m²/g]           Pd]

5% Pd/C Commercial                                16              5.2             71

5% Pd/C Commercial                                36              5.2             155

5% Pd/Al₂O₃ Commercial                          2.5 ^c           2.2             (25)

5% Pd Imp, on nano-Al₂O₃(+)                   4                3.0             29

5% Pd Prec. on nano-Al₂O₃(+)                   53              7.6             152

5% Pd Prec. on nano-Al₂O₃(+)                   52              -                -

5% Pd Prec. on nano-Al₂O₃(-)                   6                0.6             230

5% Pd Prec. on com-Al₂O₃(BM)                24              1.3             399

5% Pd Prec. on com-Al₂O₃(BM)                34              -                -

5% Pd Prec. on nano-TiO₂                         31              -                -

5% Pd Prec. on nano-ZnO                         40              -                -

5% Pd Prec. on nano-Al(OH)₃                   26              -                -

5% Pd Prec. on nano-CaO                         3                -                -

5% Pd Prec. on nano-ZrO₂(Ce)                  58              -                -

^a   Imp.: Impregnated, Prec. Precipitated, nano-Al₂O₃(+): nanoparticle alumina (NanoActive Aluminum Oxide Plus), nano-Al₂O₃(-): nanoparticle alumina (NanoActive Aluminum Oxide), com-Al₂O₃(BM): commercial bimodal gamma-Al₂O₃. 

^b  Product: 4,4'-dimethyl-2,2'-bipyridine.  Raw yield contains a small amount of the terpyridine byproduct: 4,4',4”-trimethyl-2,2',6'2”-terpyridine as the only byproduct.  

^c  There was very little 4,4'-dimethyl-2,2'-bipyridine product in this run.  The solids recovered appear to be mostly catalyst and organic byproducts.