Converting methane to methanol -- with and without water
Studies of common copper-zinc oxide catalyst suggest strategies for
improving water-free conversion

Date:
November 8, 2021
Source:
DOE/Brookhaven National Laboratory
Summary:
Chemists have been searching for efficient catalysts to convert
methane into methanol. Adding water to the reaction can address
certain challenges, but it also complicates the process. Now
a team has identified a new approach using a common industrial
catalyst that can complete the conversion effectively both with
and without water. The findings suggest strategies for improving
catalysts for the water-free conversion.



FULL STORY ========================================================================== Chemists have been searching for efficient catalysts to convert methane
-- a major component of abundant natural gas -- into methanol, an easily transported liquid fuel and building block for making other valuable
chemicals. Adding water to the reaction can address certain challenges,
but it also complicates the process. Now a team at the U.S. Department
of Energy's Brookhaven National Laboratory has identified a new approach
using a common industrial catalyst that can complete the conversion
effectively both with and without water. The findings, published in
the Journal of the American Chemical Society, suggest strategies for
improving catalysts for the water-free conversion.


========================================================================== "Water is like a trick that people have been using for a long time to
get this reaction going -- and it definitely helps. It improves the
selectivity and it aids the ability to extract the methanol," said Jose' Rodriguez, a leader of Brookhaven Lab's Catalysis Group, who has an
adjunct appointment at Stony Brook University (SBU) in the departments
of Chemistry and Materials Science and Chemical Engineering.

As shown in a recent related study by this group, adding water keeps the reaction from running away to transform the desired product, methanol,
into carbon monoxide (CO) and carbon dioxide (CO2). But adding water also
adds complexity and cost. Plus, at the temperatures and in the amounts
required for this reaction, the water exists as large quantities of steam, which would have to be controlled in an industrial setting.

So, the Brookhaven team set out to explore if they could run the
reaction without water by changing the catalyst -- the substance that
brings the reactants together and helps guide them along a particular
reaction pathway.

Catalytic conversion The new paper describes how a common copper-zinc
oxide catalyst can steer the reaction along different pathways depending
on whether water is present.



========================================================================== "Copper-zinc oxide is a commercial catalyst that is readily
available and not too expensive," said Sanjaya Senanayake, one of
the study co-authors. "We wanted to see whether it might work for methane-to-methanol conversion." According to their study results,
copper-zinc oxide has the best selectivity of any catalyst tested for this reaction without the addition of water -- about 30%. That means methanol,
the desired product (instead of CO or CO2), makes up 30% of the products
of the reaction when it runs without water. (When run with water, the copper-zinc oxide catalyst had 80% selectivity for methanol production.)
For comparison, the team's earlier studies of this reaction using a
cerium oxide catalyst produced almost no methanol without water.

"One of the big challenges of this methanol synthesis reaction in the
presence of just methane and oxygen (and no water) is overoxidation -- the transformation of the methanol into carbon monoxide and carbon dioxide,"
said study co-author Ping Liu. She noted how the earlier studies of the
cerium catalyst revealed how water helped to block that overoxidation
by removing the produced methanol before it could be further transformed.

To find out how the copper-zinc catalyst achieves 80 and 30 percent
specificity with and without water, respectively, the team conducted
detailed studies using a variety of techniques that worked hand-in-hand
with theoretical calculations to reveal crucial details of the reaction mechanism.



========================================================================== X-ray studies "Two of our SBU graduate students, Ivan Orozco and Feng
Zhang, and one of our postdocs, Zongyuan Liu, worked with Slavomi'r
Nemsa'k, a long-time collaborator, at the Advanced Light Source (ALS)
-- a DOE Office of Science user facility at Lawrence Berkeley National Laboratory -- to find evidence of methanol formation on the surface of
the catalyst," Senanayake said. "The technique, called 'ambient-pressure
x-ray photoemission spectroscopy (XPS),' uses ALS's bright beams of x-rays
to detect the carbon, hydrogen, oxygen, and the metal-oxygen combinations
at the active sites of the catalyst as the reaction is taking place.

The scientists studied the samples under different reaction
conditions. They varied the amount of methane, oxygen, and water
(including no water), as well as the pressure and temperature -- tracking
which chemical species were present at different stages of the reaction.

"Each compound has a unique 'chemical fingerprint,' so we can see how
these reactants are transformed into intermediates and final products
under different conditions," Rodriguez said.

The XPS fingerprints clearly showed that methanol was forming. But to find
out exactly which sites on the catalyst were involved in the reactions,
the team turned to theoretical modeling.

Modeling atomic interactions The team used a scanning tunneling microscope
in Brookhaven's Chemistry Department to study the atomic-level structure
of the catalyst, and then used those structural details to build
computational models of the atomic arrangements.

"There are many diverse active sites on the surface of the catalyst,"
said Liu.

To understand those sites and determine whether and how they interacted
with the reactants and products, another SBU graduate student -- Erwei
Huang -- and Liu ran "density functional theory" (DFT) calculations and
kinetic modeling on computing clusters at Brookhaven Lab's Center for Functional Nanomaterials (CFN) and the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory.

DFT calculations identify how the reactants (methane, oxygen, and water)
evolve as they interact with one another and the catalyst, as well
as how much energy it takes to get from one atomic arrangement to the
next. Kinetic modeling tries out all the possible pathways for those transformations to take place under reaction conditions.

This combination of techniques allowed the team to identify the most
energy- efficient (and therefore most likely) path for how methane is transformed into methanol with and without water. The results included
details about which catalytic sites were involved and which intermediates should be present at different stages during the reaction. The team then verified these catalytic interactions and intermediates with "chemical fingerprinting" measurements at ALS.

Pathways to improvement Together, the data indicate that the reaction
proceeds along two different pathways involving two different sites of
the copper-zinc-oxide catalyst -- one for the reaction with water and
one for the reaction without water.

"The particular configuration of active sites for the reaction with water
is different from the configuration without water, and the mechanism is different, too -- it's practically two different processes," Rodriguez
said.

But in both cases, even without water, "the binding between the methanol
and the catalyst is strong enough to allow the methanol to form from
methane, but weak enough to enable the methanol to come off the surface
as a gas before it is further oxidized to CO or CO2," Liu said.

"As soon as the methanol goes into the gas phase you can condense the
whole thing and then separate liquid methanol," Rodriguez said.

That quick "desorption" of methanol from the surface of the catalyst,
which keeps the methanol from reacting further with oxygen, also
eliminates a potentially explosive step.

The team is already using their new knowledge of the reaction mechanisms
to look for ways to further improve the catalyst. Their goal is to
achieve a selectivity of at least 60-70% without water.

"The atomic level understanding is much more advanced than what we've ever
had before. We know really atom by atom that copper zinc oxide is much
better for the preferred no-water reaction condition," Senanayake said.

In the next step, DFT calculations and kinetic modeling will start
to test out other compositions, aiming to further improve the methane conversion and methanol selectivity.

"We'll use the theory to narrow down the candidates based on the
mechanistic understanding acquired from the previous studies,"
Liu said. "Then the experimentalists will do the synthesis and
characterization studies to see if these other compositions will work."
This research was funded by the DOE Office of Science. ALS, CFN, and
NERSC are DOE Office of Science user facilities.

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Erwei Huang, Ivan Orozco, Pedro J. Rami'rez, Zongyuan Liu,
Feng Zhang,
Mausumi Mahapatra, Slavomi'r Nemsa'k, Sanjaya D. Senanayake,
Jose' A.

Rodriguez, Ping Liu. Selective Methane Oxidation to Methanol
on ZnO/Cu2O/ Cu(111) Catalysts: Multiple Site-Dependent
Behaviors. Journal of the American Chemical Society, 2021; DOI:
10.1021/jacs.1c08063 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211108114827.htm

--- up 9 weeks, 4 days, 9 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)