Reactions

Letters to the editor

Greener rockets

Having worked for the past 2 decades under Office of Naval Research sponsorship on green replacements for ammonium perchlorate (AP) in rocket propellants, I was very pleased by Mark Peplow’s recent highlight, “An Oxidant for Greener Rockets,” in the Science Concentrates section of the Nov. 23, 2020, issue of C&EN (page 7). Unfortunately, this highlight contains numerous inaccuracies and omissions.

The highlight reports on the discovery of nitraminodiacetic acid bis(2,2,2-trinitroethyl ester) (NABTNE) by a group of researchers at the Ludwig Maximilian University Munich as a promising candidate for the green replacement of AP (Energy Fuels 2020, DOI: 10.1021/acs.energyfuels.0c02910).

The Munich group used a two-step synthesis for NABTNE from iminodiacetic acid and trinitroethanol, with yields of 22% and 20 or 13%, respectively, for an overall yield of 4.4 or 2.86%. For practical purposes, these low yields preclude its usefulness and even do not include any yield for the trinitroethanol, for which the Munich authors give neither a commercial source nor a synthetic procedure.

The performance calculations in the original paper are flawed. For NABTNE, a highly energetic binder—glycidyl azide polymer (GAP) or 3,3-bis(azidomethyl)oxetane (BAMO)—was used, whereas for AP, the nonenergetic binder hydroxy-terminated polybutadiene (HTPB) was used. When HTPB is used for both systems, the maximized specific impulse (I_sp) for the NABTNE formulations is only 240 s, which is 25 s lower than that of 265 s, generally used for the AP-based system. Since for solid propellants, a 5 s increase in I_sp roughly doubles the payload, a 25 s decrease in I_sp is clearly unacceptable.

In the original Munich paper, our previous work on a superior system, based on ammonium tetrakis-(3,5-dinitro-1H-1,2,4-triazolyl)borate (ADNTB) (US patent 9,309,266 and Inorg. Chem. 2013, DOI: 10.1021/ic400919n and 10.1021/ic400504h), has been ignored. Contrary to NABTNE, the I_sp of our ADNTB-based system closely matches that of the AP-based system.

Some minor points: the impact sensitivity of NABTNE (6 J) is higher than those of ADNTB (13 J) and AP (20 J), and the thermal stability of NABTNE is closer to 160 °C than 180 °C, based on the onsets of its differential scanning calorimetry and derivative thermogravimetric curves.

Karl O. Christe
Calabasas, California

The following is a response from the authors of the paper referenced above:

We appreciate the comments made by Dr. Christe in his letter. Aside to the many positive letters we received personally, critical voices are important for a scientific discussion. We are aware that the synthesis should be improved according to the yields and chemicals used. This has already been pointed out by Niklas Wingborg in the well-researched article from Mark Peplow. Constant improvement is an ongoing process in our research group.

Concerning the performance calculations, it is correct that for AP-based systems, HTPB is used as one potential binder. The specific impulses for the HTPB-based systems of NABTNE are shown in the supporting information, along with the specific impulses resulting from calculations using poly(butadiene-acrylic acid-acrylonitrile) (PBAN), GAP, and BAMO as binders. Moreover, as newer oxidizers have already shown some incompatibilities with HTPB, we did not want to exclude this binder but add more possibilities based on current research. For practical purposes it is necessary to evaluate the compatibility with the most suitable binder. It also needs to get pointed out that the oxygen balance (Ω_CO) for NABTNE is +19.1%, whereas for ADNTB it is +10.9%.

Cornelia C. Unger, Marcel Holler, Burkhard Krumm, and Thomas M. Klapötke
Munich

Homochirality on planets

Chirality is noted in the article on searching for signs of life (C&EN, Nov. 30, 2020, page 30), which could remind us of an idea discussed by George Wald (1967 Nobel Prize), summed up in his words in “The Origin of Optical Activity”: “If the choice of optical isomers is as arbitrary as proposed one should expect that a survey of life throughout the universe would reveal approximately equal numbers of planetary populations in which the choice of metabolically connected series of dissymmetric molecules came out l- or d-; roughly equal numbers in which life is based upon l- and upon d-amino acids, and similarly, for the other molecules” (Ann. N. Y. Acad. Sci. 1957, DOI: 10.1111/j.1749-6632.1957.tb49671.x).

Research on the stereochemistry of helical polymers supports Wald’s hypothesis (Origins Life Evol. Biospheres 2009, DOI: 10.1007/s11084-009-9180-7).

Mark M. Green
Brooklyn, New York