Reactions: Questioning direct air capture’s value, discussing phosphines, and thanking the interim editor in chief

Letters to the editor

Direct air capture

“Can Kenya Become a Direct-Air-Capture Hub?” (Jan. 8, 2024, page 20) shows why direct air capture of carbon dioxide is likely to be a waste of money and comparable to pumping money into the ground. The article says that according to one source “DAC plants will capture 4,000 t per year of CO2 at a cost per plant of as much as $500 million”—that is, $125,000 a metric ton.

The US Department of Agriculture Forest Service will, in contrast, plant about 10 trees for a minimum donation of $10—that is, $1 per tree. When the trees grow to maturity, each will require 48 lb (0.022 t) of carbon dioxide a year (per “The Power of One Tree—the Very Air We Breathe” from the US Department of Agriculture). So the same $125,000 “investment” in 125,000 trees will capture more than 2,700 t a year. The trees, unlike the DAC plants, require no employees, maintenance, or chemicals to absorb CO₂. They also increase their capacity every year and eventually make more of themselves. The Forest Service adds that trees remove genuine pollutants like particulates from the air as well.

This underscores the need to look at the economics of purported solutions to climate change. While nothing in this letter constitutes engineering advice, this assessment suggests that DAC is a waste of money unless the carbon dioxide can be sold for enough money to justify the capital investment and annual outlays.

William A. Levinson
Wilkes-Barre, Pennsylvania

More on phosphenes

I read with attention the article by Brianna Barbu, published on Jan. 29 and titled “Phosphines That Act Like Metal Catalysts,” highlighting a recent paper published in the Journal of the American Chemical Society (JACS) (2024, DOI: 10.1021/jacs.3c10614), along with a mention of a related paper also published in JACS (2023, DOI: 10.1021/jacs.2c13318).

Both of the above papers cite—but only superficially—two publications from my group. The first one, inJACS, was titled “On the Catalytic Hydrodefluorination of Fluoroaromatics Using Nickel Complexes: The True Role of the Phosphine.” In doing the corresponding control experiments, we documented that such catalytic activity leading to C–F activation was made by the used alkylphosphine (100% yield for commercially available PEt₃) (2014, DOI: 10.1021/ja412268y).

We also documented a mechanistic proposal in terms of metallic-like (metallomimetic) reactivity by the phosphine by proposing a nucleophilic attack at the C–F bond, followed by a β elimination and the characterization of the fluorinated phosphorus(V) product that was yielded in the lack of silane and nickel.

We recently published a second paper on this subject in theNew Journal of Chemistry in which we used a closely related fluorinated thiophenol to stabilize the elusive intermediate we missed in the other fluorinated aromatics and postulated a new mechanism discarding the β-elimination step because of the high energy barrier; instead, a direct reaction with a hydrogen source was calculated (2019, DOI: 10.1039/c9nj00721k). Notably, minute amounts of water can do the trick without silanes unless phosphine recycling is a must. However, the cost of silane versus water must be carefully evaluated.

Juventino J. García
Mexico City, Mexico

Interim editor in chief

I could not disagree more with the letter from Landis W. Doner in the Jan. 29, 2024, issue of C&EN (page 3). Michael McCoy’s service as interim editor in chief of C&EN, a publication I have been reading and learning from and have valued highly since 1981, has been outstanding. He has stepped into the role twice over the past year under difficult circumstances each time. He has been transparent about the challenges and plans of C&EN and has provided reassurance about the future of the publication that devoted readers like me have been relieved to hear. I thank him for his long service to C&EN and especially for his leadership of the publication through some difficult times over the past year.

David A. Dzombak
Pittsburgh