Heterogeneous addition of H2 to double and triple bonds over supported Pd catalysts: a parahydrogen-induced polarization technique study

Abstract

In this work, the contribution of the pairwise H(2) addition to the overall reaction mechanism was studied under the systematic variation of both the Pd particle size and the properties of the catalyst support using the hydrogenation of propene and propyne over supported Pd catalysts as representative examples. For Pd supported on alumina, silica and zirconia, only propene formed upon hydrogenation of propyne with para-H(2) exhibits hyperpolarization. In contrast, propane formed in hydrogenation of propyne or propene is not hyperpolarized. This demonstrates the existence of different routes of H(2) addition to double and triple bonds on supported Pd catalysts. The unique ability of Pd/TiO(2) catalysts to add H(2) in a pairwise manner not only to the triple but also to the double bond is demonstrated. This finding indicates that the Pd-support interaction is of primary importance in determining not only the magnitude of the hyperpolarization of the NMR lines of the reaction products but even the involvement of the pairwise H(2) addition and hence the mechanism of heterogeneous hydrogenation. The comparative analysis of the selectivities toward pairwise H(2) addition suggested the existence of different surface active sites responsible for all three reaction routes: the direct total hydrogenation of propyne into propane, its selective hydrogenation into propene, and hydrogenation of propene into propane. A reaction scheme which accounts for the formation of the observed hyperpolarized and non-polarized reaction products in propyne and propene hydrogenation with para-H(2) over supported Pd catalysts is suggested. For the first time, application of the PHIP technique allowed us to demonstrate that hydrogenation of propene does not take place in the presence of propyne on supported Pd catalysts.

Strong Metal–Support Interactions for Palladium Supported on TiO2 Catalysts in the Heterogeneous Hydrogenation with Parahydrogen

“…The major differences in the PHIP effects observed for metal nanoparticles supported on titania relative to those previously observed for metals on other supports [11][12][13] were tentatively attributed to low-temperature SMSI effects. Therefore, in this communication the behavior of titania-supported catalysts is …”

“…[9][10][11] Recently,i tw as shown that the nature of the catalyst support can have as ignificant impact on the rate of the pairwise addition of hydrogen. [12,13] Remarkably,t itania-supported metal catalysts were shown to exhibit much higher levels of the pairwise addition of hydrogen (and larger PHIP effects) relative to that exhibited by metals on other supports. [9,14] Therefore, examination of the nature of the active sites for titania-supported nanoparticles in the contexto ft he pairwise addition of hydrogen may open up new possibilities for the rational preparation of catalysts that are able to maximize the observed PHIP effects and for the production of hyperpolarizedm olecular contrast agents potentially suitable for in vivo magnetic resonance imaging investigations.…”

“…[19,20] Therefore, significant alteration in the selectivity of the pairwisea ddition of hydrogen for titania-supportedc atalysts described above is not surprising. [11][12][13] One plausible explanation for the nature of the SMSI effect is the electronic interaction between the metal and the support, which leads to the formation of an electron-rich metal. [21,22] Another possible explanation is migration of the support material and encapsulation of metal nanoparticles.I mportantly,t itania is able to rapidly migrate over considerable distances and to appear on initially pristine metal surfaces.…”

Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: a parahydrogen based investigation

“…68 Interesting, the corresponding 31 P coupling increases to around 70 Hz in related hexadiene based complexes. 69 In the case of related Pd(II) bis phosphine complexes containing H and MeOH ligands couplings of [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Hz are typical and so a virtual triplet would be expected for the alkenic protons in both of these types of product if such large couplings are retained. We therefore suggest in 10 that CO binding to the fourth site occurs, and that the palladium centre is charged, which means that an out-of-plane cis-stilbene ligand is indicated ( Figure 5).…”

“…11,14,15 In addition, as PHIP enables the analogous detection of scalar coupled heteronuclei through polarisation transfer, the reliable characterisation of such species is possible 16 and when this approach is coupled with Exchange Spectroscopy (EXSY) experiments their kinetic significance can be determined. [17][18][19][20] Workers have also used this method to examine not only a range of heterogeneous reactions [21][22][23][24][25] but some that don't involve a metal centre. [26][27][28][29] The PHIP approach has therefore developed substantially from the early starting point of Weitekamp, 30, 31 Eisenberg 32,33 and Bargon, 34,35 as illustrated in several reviews.…”