‡ ETH
Zurich, Institute for Chemistry and Bioengineering, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland
§ Paul
Scherrer Institute, 5232 Villigen, Switzerland
Acc. Chem. Res., 2017, 50 (2), pp 418–425
DOI: 10.1021/acs.accounts.6b00534
Publication Date (Web): February 2, 2017
Abstract
Conspectus
In
the recent years methane has become increasingly abundant. However,
transportation costs are high and methane recovered as side product is
often flared rather than valorized. The chemical utilization of methane
is highly challenging and currently mainly based on the cost-intensive
production of synthesis gas and its conversion. Alternative routes have
been discovered in academia, though high temperatures are mostly
required. However, the direct conversion of methane to methanol is an
exception. It can already be carried out at comparably low temperatures.
It is challenging that methanol is more prone to oxidation than
methane, which makes high selectivities at moderate conversions
difficult to reach. Decades of research for the direct reaction of
methane and oxygen did not yield a satisfactory solution for the direct
partial oxidation toward methanol. When changing the oxidant from oxygen
to hydrogen peroxide, high selectivities can be reached at rather low
conversions, but the cost of hydrogen peroxide is comparably high.
However, major advancements in the field were introduced by converting
methane to a more stable methanol precursor. Most notable is the
conversion of methane to methyl bisulfate in the presence of a platinum
catalyst. The reaction is carried out in 102% sulfuric acid using SO3
as the oxidant. This allows for oxidation of the platinum catalyst and
prevents the in situ hydrolysis of methyl bisulfate toward the less
stable methanol. With a slightly different motif, the stepped conversion
of methane to methanol over copper-zeolites was developed a decade ago.
The copper-zeolite is first activated in oxygen at 450 °C, and then
cooled to 200 °C and reacts with methane in the absence of oxygen, thus
protecting a methanol precursor from overoxidation. Subsequently
methanol can be extracted with water. Several active copper-zeolites
were found, and the active sites were identified and discussed. For a
long time, the process was almost unchanged. Lately, we implemented
online steam extraction rather than off-line extraction with liquid
water, which enables execution of successive cycles. While recently we
reported the isothermal conversion by employing higher methane
pressures, carrying out the process according to prior art only yielded
neglectable amounts of methane. Using a pressure <40 bar methane gave
higher yields under isothermal conditions at 200 °C than most yields in
prior reports. The yield, both after high temperature activation and
under isothermal conditions at 200 °C, increased monotonously with the
pressure. With this account we show that the trend can be represented by
a Langmuir model. Thus, the pressure dependence is governed by methane
adsorption. We show that the isothermal and the high temperature
activated processes have different properties and should be treated
independently, from both an experimental and a mechanistic point of
view.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.