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Gas-phase hydrogenation of a wide range of ketones
to alkanes, including hydrogenation of aliphatic ketones and acetophenone,
was investigated using bifunctional metal–acid catalysis. The catalysts were
comprised of a metal (Pt, Ru, Ni, and Cu) supported on acidic caesium salt of
tungstophosphoric heteropoly acid Cs2.5H0.5PW12O40 (CsPW). The reaction
occurred via a sequence of steps involving hydrogenation of ketone to alcohol
on metal sites followed by dehydration of alcohol to alkene on acid sites and
finally hydrogenation of alkene to alkane on metal sites. Catalyst activity
decreased in the order:
Pt > Ru >> Ni > Cu. Pt/CsPW showed
the highest catalytic activity, giving almost 100% alkane yield at
100 °C and 1 bar pressure. Evidence is provided that the reaction
with Pt/CsPW at 100 °C is limited by ketone-to-alcohol hydrogenation,
whereas at lower temperatures (≤60 °C) by alcohol dehydration yielding
alcohol as the main product. The catalyst comprised of a physical mixture of
Pt/C + CsPW was found to be highly efficient as well, which
indicates that the reaction is not limited by migration of intermediates
between metal and acid sites in the bifunctional catalyst.
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موجز عن البحث:
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Deoxygenation
and decomposition of ethers and esters, including anisole, diisopropyl ether
(DPE), and ethyl propanoate (EP), was investigated using bifunctional
metal−acid catalysis at a gas−solid interface in the presence and absence of
hydrogen. The bifunctional catalysts studied comprised Pt, Ru, Ni, and Cu as
the metal components and Cs2.5H0.5PW12O40 (CsPW), an acidic Cs salt of
Keggin-type heteropoly acid (HPA) H3PW12O40, as the acid component, with the
main focus on Pt−CsPW catalyst. It was found that bifunctional metal−acid
catalysis in the presence of H2 is more efficient for ether and ester
deoxygenation than the corresponding monofunctional metal and acid catalysis
and that metal- and acid-catalyzed pathways play different roles in these
reactions. With Pt-CsPW, hydrodeoxygenation of anisole, a model for the
deoxygenation of lignin, occurred with 100% yield of cyclohexane under very
mild conditions (60−100 °C and 1 bar of H2). This catalyst had the highest
activity in anisole deoxygenation for a gas-phase catalyst system reported so
far. The catalyst activity decreased in the order of metals: Pt ≫ Ru > Ni
> Cu. For HPA-catalyzed DPE and EP decomposition, relationships between
the turnover reaction rate (turnover frequency) and the HPA acid strength
were found, which can be used to predict the activity of acid catalysts in
these reactions.
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ملخص المشاركة:
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Methyl isobutyl ketone (MIBK) can be viewed as a key
intermediate for the conversion of biomass derived acetone – the by-product
of biobutanol production – to transportation fuel. Here, we investigated the
hydrogenation of MIBK using heteropoly acids and bifunctional catalysts
comprising Pt, Pd, Ru, or Cu supported on Keggin heteropoly salt
Cs2.5H0.5PW12O40 (CsPW). The effect of
catalyst preparation in hydrogenation of MIBK was also investigated.
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ملخص المشاركة:
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Oxygen-containing organic compounds such as ketones,
carboxylic acids, alcohols, phenols, etc., readily available from natural
resources, are attractive as renewable raw materials for the production of
value-added chemicals and biofuels. Hydrogenation of ketones on supported
metal catalysts (e.g., Pt/C) to form alcohols is feasible, but further
hydrogenation to alkanes is rather difficult to achieve on such catalysts.
The ketone-to-alkane hydrogenation can be achieved much easier using
bifunctional metal-acid catalysts. This process occurs via a sequence of
steps involving hydrogenation of ketone to alcohol on metal sites followed by
dehydration of secondary alcohol to alkene on acid sites and finally
hydrogenation of alkene to alkane on metal sites (eq. 1).
Here, we report on
hydrogenation (hydrodeoxygenation) of a variety of aliphatic and aromatic
ketones in the gas phase using bifunctional metal-acid catalysts comprising a
metal (Pt, Ru, Ni, Cu) supported on acidic caesium salt of tungstophosphoric
heteropoly acid Cs2.5H0.5PW12O40 (CsPW) and Pt supported on zeolites (HZSM-5,
HY, HBeta). It is demonstrated that 0.5%Pt/CsPW is a highly efficient and
versatile catalyst for the ketone-to-alkane hydrogenation, and an insight
into reaction mechanism is gained.
It was found
that the activity of CsPW-supported metal catalysts in ketone hydrohenation
decreased in the order: Pt > Ru >> Ni > Cu. Pt/CsPW showed the
highest catalytic activity, giving in most cases almost 100% alkane yield at
100oC and 1 bar H2 pressure.
Amongst the
Pt/zeolite catalysts studied, Pt/H-ZSM-5 clearly stands out giving >99% selectivity
to methylpentanes at 100% MIBK conversion at 200oC. The Pt/H-ZSM-5 catalyst
showed excellent performance stability. It reached steady state in about 2 h
and operated without deactivation for at least 16 h.2 0.5%Pt/CsPW exhibited
even higher activity, with stable performance for at least 14 hours on
stream, yielding 99% of 2-methylpentane (2MP) at 100oC, with 100% 2MP
selectivity at ≥ 99% MIBK conversion. This is in agreement with the stronger
acidity of CsPW compared to zeolites. Importantly, no 2MP isomerisation was
observed at this temperature, thus allowing synthetically viable complete
transformation of ketone to alkane without carbon backbone alteration.
Evidence was obtained that these
reactions occur via the bifunctional mechanism (eq. 1), with the
rate-limiting step of ketone hydrogenation on metal sites, followed by fast
alcohol dehydration and olefin hydrogenation.
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