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Table 1
tivity for the 3-phenyl-1-propanethiol coated catalyst (Fig. 2 (c)).
Such a decrease in selectivity is consistent with the hypothesis that
a ligand-specific interaction is necessary to achieve high selectivity,
in the monolayer structure [12]. As the specific interaction effect
of the thiol is compromised, the selectivity will decrease to parity
with the non-specific selectivity improvement of the C18 coating.
As seen in Fig. 2, over the course of sequential recycle reactions,
the selectivity of each catalyst converged towards 50% selectivity.
This is consistent with the hypothesis that there is degradation of
the ligand specific 3-phenyl-1-propanethiol interaction, but also
that the selective poisoning of sulfur on the surface still results in
selectivity improvement over that of the uncoated catalyst.
We hypothesize that a critical factor to maintaining the per-
formance of thiol-coated catalysts in solution is maintaining an
adequate surface coverage of the thiolate under reaction con-
ditions. Therefore, we used an alternative recycle method of
regenerating the SAM-coated catalysts before reusing them in a
subsequent reaction, referred to as the “regeneration–recycle” pro-
cedure. Here, the catalysts were dried and decanted after reaction
and then immersed in a thiol solution similar to that used for initial
deposition, but at a lower concentration (methods section) to limit
physisorption of thiols on the catalyst surface. This procedure was
expected to replenish the thiols that had desorbed into the solution
As shown in Fig. 3(a), rinsing of the uncoated catalyst with
ethanol after recycle resulted in near-constant selectivity for all
runs, in contrast to the procedure of simply re-using the cata-
lyst without an ethanol rinse (Fig. 2 (a)). This result suggested
that the rinsing step removed adsorbed organic molecules pro-
duced during the reaction, and that the removal of these molecules
affected selectivity. The C18-coated catalyst was greatly influenced
by this recycle-regeneration procedure, which saw the selectivity
increase from 50% to 79% during the 3rd reaction. The selectivity of
somewhat from 93% in the first reaction to 82% by the third reac-
tion, but the decrease was much less significant than the decrease
observed for the simple recycle experiment (without regeneration)
discussed in Fig. 2.
Another method of maintaining the thiolate surface coverage,
and thus the efficacy, of a thiol-coated catalyst was to add thiol
technique has already been described in ref. [12] but is treated
in further detail here with rate data and surface characteriza-
tion to complement the discussion. The technique of adding thiols
epoxybutene in ethanol [19], a system where addition of thiol was
hypothesized to maintain a critical equilibrium coverage of thiol
in the reaction solvent. One of the key effects observed in this
method was a sharp decrease in the rate, likely due to thiol out-
competing the reactant for surface sites. Shown in Fig. 4, very dilute
concentrations of thiols, 0.005 mM C18 and 0.3 mM 3-phenyl-1-
propanethiol, were used in the reaction mixture to minimize the
decrease in reaction rate while still allowing the study of reaction
selectivity.
Here, the C18-coated catalyst showed a higher selectivity for
the first reaction than a C18-coated catalyst run in a pure solvent;
however, this increase was observed only for the first reaction, after
for a fresh C18-coated catalyst in a reaction mixture without thiol.
The 3-phenyl-1-propanthiol-coated catalyst retained high
selectivity when thiols were added to the reaction mixture, as
shown in shown in Fig. 4 (b). In contrast to a basic recycle,
this method of introducing thiols to the reaction mixture shows
Recycle procedures used for regenerating catalysts between reactions. For the
recycle method of adding thiols to the reaction solution, the “Recycle with no regen-
eration” technique was used with thiol added to the reaction mixture.
Recycle with no regeneration
Recycle with catalyst regeneration
1 Run reaction
1 Run reaction
2 Allow catalyst to settle
3 Decant reactant supernatant solution
2 Allow catalyst to settle
3 Decant reactant supernatant
solution
4 Dry in desiccator under vacuum
5Run subsequent reaction
4 Dry in desiccator under vacuum
5 Deposit catalyst overnight in thiol
solution
6 Decant and rinse for 4 h in ethanol
7 Decant and dry catalyst in desiccator
under vacuum
8 Run subsequent reaction
Such modification of the near surface environment by thiol
modifiers has been shown to be effective, but preliminary studies
showed the layer was susceptible to degradation [12]. The primary
focus of the following sections is to understand both how non-
specific and specific thiolate–reactant interactions are affected by
recycling- and aging-induced structural changes in the monolayer.
3.2. Recyclability of thiol SAM-coated catalysts
The effects of recycling were investigated for three types of
catalysts: an uncoated Pt/Al2O3, an octadecanethiol (C18) coated
Pt/Al2O3 catalyst, and a 3-phenyl-1-propanethiol-coated Pt/A2O3
catalyst. As discussed above, the C18 modifier has been asso-
ciated with selectivity improvement due to the influence of
sulfur on surface reactivity, while the 3-phenyl-1-propoanethiol
coating exhibits additional enhancements in selectivity due to non-
covalent near-surface interactions. It is important to note that C18
and 3-phenyl-1-propanethiol modifiers produce monolayer sur-
catalyst in air. In the most basic method of catalyst recycling, the
catalyst was used in a fresh reaction mixture immediately follow-
ing the drying step (Table 1). Cinnamyl alcohol selectivity versus
conversion plots for these catalysts are shown in Fig. 2.
Fig. 2 shows the cinnamyl alcohol selectivity as a function
of conversion for the uncoated, C18-coated, and 3-pheny-1-
propanethiol-coated catalysts. Even on the uncoated catalysts,
sion. The weak dependence of selectivity on conversion until very
high conversion has been previously observed and attributed to
the suppression of cinnamyl alcohol hydrogenation by the pres-
ence of hydrocinnamaldehyde [2]. For simplicity, below we will
generally compare selectivity measured at 50% conversion. The
uncoated Pt/Al2O3 catalyst selectivity at 50% conversion improved
after recycling from 28% to 45%. This improvement was attributed
to the deposition of carbonaceous species during the initial reaction
period, which may selectively poison the undesired reactions. CO-
in Section 3.3 below) were consistent with the presence of carbona-
ceous deposits on the surface following reaction. Partial coverage
of the catalyst surface by spectator species may reduce the num-
ber of binding sites and/or the binding affinity of the double bond
in ␣,-unsaturated aldehydes [2,21–24]. The C18-coated catalyst
selectivity showed little dependence on recycling, with selectiv-
ity remaining within experimental error during the three recycle
reactions. This was also supported by Fig. S1, which showed that the
nature of the available sites on a C18-coated catalyst were constant
over multiple recycle reactions.