The use of bimetallics to control the selectivity for the upgrading of lignin-derived oxygenates: Reaction of anisole on Pt and PtZn catalysts

Abstract

a b s t r a c tThe adsorption and reaction of anisole on Pt and PtZn catalysts were investigated using both model single crystal and high surface area supported metal catalysts. Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) studies of the interaction of anisole with Pt (1 1 1) demonstrated that there is a strong interaction between the phenyl ring of anisole and the surface, resulting in CAO and CAH bond scission at relatively low temperatures. In contrast, anisole was observed to bond to a Zn-modified Pt(1 1 1) surface primarily via the oxygen at Zn sites or possibly adjacent Pt sites, with the phenyl ring tilted away from the surface. Such bonding configuration facilitated selective CAO bond cleavage producing phenyl groups and methoxide groups with the latter being bonded to the Zn sites. These results suggested that PtZn may be an effective catalyst for hydrodeoxygenation (HDO) of lignin-derived aromatic oxygenates with low activity for ring hydrogenation. This hypothesis was then tested and verified by investigating the reaction of anisole and H 2 over high surface area carbonsupported Pt and PtZn catalysts.

Mechanistic study of the hydrodeoxygenation of lignin-derived oxygenates on a CoPt bimetallic catalyst: reaction of anisole on Co-modified Pt(111)

“…Benzene was observed in a broad peak between 300-550 K, with the peak center near 430 K. Other than benzene, the only reaction products detected were CO and H 2 . The H 2 was produced in two distinct peaks centered at 360 and 490 K, with a much broader feature spanning from 540-750 K. CO desorbed in two peaks centered at 430 and 490 K. With the exception that benzene was observed, these data are in agreement with that reported in our previous study [17]. These results are also similar to those reported previously by Réocreux et al [25] who also observed benzene as a product, although they reported only a single CO peak centered at 440 K. This difference may be related to the different anisole doses used in the two studies.…”

“…We postulate that this change in bonding configuration is a result of the oxygen in the carbonyl group interacting with an oxyphilic Co adatom on the Pt(111) surface as shown in figure 6. This conclusion is consistent with what we have observed previously for Pt(111) surfaces modified with oxyphilic Zn adatoms [17,31] where the oxygens in both alcohols and carbonyl compounds preferentially bond to the Zn sites on these surfaces.…”

“…In our previous study [17] we concluded based on HREELS data that anisole undergoes O-CH 3 bond cleavage on Pt(111) at temperatures as low as 250 K to form an oxocyclohexadienyl-structured intermediate (C 6 H 6 O) that bonds to the surface in an η 5 -π configuration. While the reader is referred to this previous study for a detailed analysis of the HREELS data that led to this conclusion, the most salient aspects of the analysis can be gleaned from the data in figure 2, which includes HREEL spectra for anisole-dosed Pt(111) at 115 and 300 K. The spectrum at 115 K is consistent with that expected for molecular anisole (see peak assignments in table 1).…”

“…The reaction pathways for anisole on Zn/Pt(111), however, are substantially different from those observed here for Co/Pt(111). In our previous studies we showed that, in addition to providing an adsorption site for the oxygen functionalities, Zn also has a strong electronic effect which destabilizes the bonding of aromatic rings with the Pt(111) surface [16][17][18]27]. This destabilization is sufficient to change the most stable bonding configuration for anisole from one where the aromatic ring is situated parallel to the surface to one where the ring is tilted away from the surface.…”

“…Alloying a group 10 metal with a second oxyphilic metal such as Fe or Zn [12][13][14][15] has emerged as a promising approach to overcome this problem and such catalysts show higher HDO selectivity to the desired aromatic products. In order to understand the mechanism by which the addition of the oxyphilic metal affects the overall catalyst HDO activity and selectivity our group has undertaken a series of mechanistic studies of the reaction of lignin-derived aromatic oxygenates, including benzaldehyde, anisole, and guaiacol, on ZnPt model catalysts consisting of a Pt(111) surface decorated with Zn adatoms [16][17][18]. These studies have revealed that the Zn additive has multiple effects including providing a binding site for the oxygen atoms in the reactant molecules which helps facilitate selective C-O bond cleavage.…”