ACS Catalysis
Research Article
it is evident that Pt itself under N2 or H2 is not active in the
decomposition of EP in the temperature range studied, but
addition of Pt to CsPW in the presence of H2 improves catalyst
resistance to deactivation. The latter can be explained by
reduction of catalyst coking due to hydrogenation of alkene
coke precursors.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
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S
Thermodynamic calculations and reaction time courses
4. CONCLUSION
AUTHOR INFORMATION
Corresponding Author
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Here, we have investigated the deoxygenation and decom-
position of ethers and esters, including the aromatic ether
anisole, the aliphatic diisopropyl ether (DPE), and the aliphatic
ester ethyl propanoate (EP), using bifunctional metal−acid
catalysis at a gas−solid interface in the presence and absence of
hydrogen. The bifunctional catalysts comprise Pt, Ru, Ni, and
Cu as the metal components and Cs2.5H0.5PW12O40 (CsPW) as
the acid component, with the main focus on the Pt−CsPW
catalyst. It has been demonstrated that bifunctional metal−acid
catalysis in the presence of H2 is more efficient for ether and
ester deoxygenation in comparison to the corresponding
monofunctional metal and acid catalysis. In addition, it has
been found that metal- and acid-catalyzed pathways play
different roles in these reactions. Hydrodeoxygenation of
anisole is a model for the deoxygenation of lignin; with Pt-
CsPW, it occurs with an almost 100% yield of cyclohexane
under very mild conditions at 60−100 °C and 1 bar of H2
pressure. In this reaction, Pt-catalyzed hydrogenation plays the
key role, with a relatively moderate assistance of acid catalysis,
further increasing the cyclohexane selectivity. The preferred
catalyst formulation is a uniform physical mixture of Pt/C or
Pt/SiO2 with excess CsPW, with a Pt content of 0.1−0.5%,
which provides much higher activity and better catalyst stability
to deactivation in comparison to the Pt/CsPW catalyst
prepared by impregnation of platinum onto CsPW. The Pt/C
+ CsPW mixed catalyst has the highest activity in anisole
deoxygenation for a gas-phase catalyst system reported so far.
On the other hand, the aliphatic ether DPE decomposes readily
over CsPW via an acid-catalyzed pathway (E1 mechanism)
without metal assistance to give propene and isopropyl alcohol,
with propene selectivity increasing with reaction temperature at
the expense of isopropyl alcohol. Platinum alone (Pt/C), in the
absence of CsPW, is inactive in this reaction, under either H2 or
N2. However, in the presence of Pt-CsPW under H2, DPE
decomposition is significantly accelerated, yielding the more
thermodynamically favorable product propane instead of
propene. Decomposition of the EP aliphatic ester is also very
efficient via an acid-catalyzed pathway without metal assistance
to yield ethene and propanoic acid. Addition of Pt to CsPW
under H2 causes hydrogenation of ethene to ethane but does
not affect the rate of EP decomposition. Nevertheless, in EP
decomposition, the Pt-CsPW bifunctional catalyst under H2
shows much better performance stability in comparison to the
CsPW acid catalyst, which can be attributed to reduction of
catalyst coking in the presence of Pt and H2. The kinetics of the
acid-catalyzed decomposition of DPE and EP has been studied
with a wide range of tungsten HPA catalysts. Good linear
relationships between the logarithm of the turnover reaction
rate and the HPA catalyst acid strength represented by
ammonia adsorption enthalpies have been demonstrated,
which can be used to predict the activity of other Brønsted
acid catalysts in these reactions.
Notes
The authors declare no competing financial interest.
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