Nanocrystals as destructive absorbants for mimcs of chemical warfare agents

Nanocrystals as destructive absorbants for mimcs of chemical warfare agents

NanoSuwhwed Pergamon Materials, Vol. 12, pp. 179-182, 1999 Elsevier Science Ltd Q 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserve...

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NanoSuwhwed

Pergamon

Materials, Vol. 12, pp. 179-182, 1999 Elsevier Science Ltd Q 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-97731991~see front matter

PI1 SO9659773(99)00093-8

NANOCRYSTALS

AS DESTRUCI’IVE ADSORBANTS CHEMICAL WARFARE AGENTS

FOR MIMCS OF

EM.LUCrrs,K.JKlabUIlde Department of Chemistry, Kausas State University Manhattan, KS 66506, USA Abstmct - Hexqtluoropropylene

(HIPIP), is destructively &orbed with specially pteFd nanoscale magnesium oxide powders producing MgF, CO, CO, and graphite. The reaction occurs best at 450°C with a 2:l surface oxide-to-HPT ratio. The maction is monitored using pulsed U-tube rreactors which shows the elution of latge rpnounts of CO and CO, which steadily decmases as the surface oxides m depleted. The elution of small amounts of Cpd is also seen close to the breakthrough point of the reaction. The solid product is partially enclosed in grqnhite and has a 2:I ratio of MgO to h&F* confinned by elemental malysis. Powder XRD shows only the presence of MgFz in the solid indicating the MgO has lost all crystallinity during the reaction. FT-IR of the solid during and ajter reaction indicates the presence of CF, and hexajkoropropylene epoxide as intermediates. 01999 Acta Metallurgica

Inc.

INTRODUCI’ION High surface area metal oxide particles have been shown to destructively adsox% chlorhmted hydrocaxbous.(l,2,3) These reactions produce non-toxic CaCl, and various &n oxides which make these high surface area metal oxides prime candidates for study in the disposal of simple halocaxbons. High surface area MgO has also beeu used to destructively adsoh various organophosphorous mimics of “warfare ageuts.“(4) In this same capacity, hexailuoropropylene (c;F,) is a prime candidate for this type of study. The metal oxide chosen is maguesium oxide. The Lewis base sites on the uanoparticle MgO react well with pertluorotiyl groups, which in this case is further euhauced by the electron withdrawing ef%xts of the lrifluommethyl group. The magnesium oxide is produced by an aerogel method in which the Mg(OH), gel undergoes hypercritical drying aud is theu calcined to 500°C. The resulting nauoparticles have surface areas of approximately 300-400 m’/g and average crystallite sizes of 4 mu.

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SJXI’ION

Gm Chrumatogrcplh Pulse Reactor

Theseexperimentsallowed separationandquantiftcation of gaseousproductsasthey came off the surface of the MgQ. A pyrex U-tube containing MgQ (0.125g) was attachedto a gas chromatography instrument (Gow Mac series 580) such that when the gaseousHFP was injected, it would flow over the powder and then into the cohmm (Alltech PorapakQ 80/100) kept at 75°C for product separation. Injection quantities were 0.5 ml at 20 psi (approx. 2.78 x 10” mol/mj) with the injector at 120°C. The reactor tube was heated to the desired temperatureusing a furnace, the reactantsandproducts were moved by Helium carrier gas (20 ml/mhr). Productswere detectedby a thermal conductivity detectorset at 110 mv and 180°C. Retention times for the varying elutantswere CO: 1:20 mm., CO,: 1:35 min., C-F: 4:12 min., c-p& 6:18 min.

Aerogel prepared MgQ preparation hasbeen describedpreviously (5). Amountsusedfor pulse reactor experiments(0.09g - 0.12g) were adjusted so that the number of surface moles MgO were kept constant(6.95 x lOa moles) which allowed for 25 injections of HFP formiug a 1:l surfaceoxide to HFP mole ratio. The number of surface moles was determined using a previously describedprocedure. (6) lnfimed

Spectroscopy

Studieswere performed using special cell which allowed monitoring of reaction without moving of sample in beam, this way the same point was always being studied. Mg(OH), is pressedon a tungstengrid connectedto a temperature controller then calcinedto 500°C. The grid is then heated to the desired temperature (studiesdone from 25% - 450°C) and exposed to 5 Ton; of gaseousHFP, after which the cell is evacuatedto 10-’ Ton. The instrument is a Mattson R/S-l FT-IR spectrometerwith au MCT liquid nitrogen cooled detector. RESULTS

G.C. Pulse Reactions

Reactions were carried out from room temperature to 500°C. Qptimal results were obtained at 450°Cwhere the break&rough number, the nwnber of injections before I-lFP began appearing in the products,was 12. At this point the surface is consideredalmost depleted of reactive sites. This relatesto approximatelytwo surfaceMgQ’s per mole HFP injected. Figure 1 showsa graphof the cumulative number of moles HFP destroyedper surfacemole of MgQ. The black line indicates the ideal if all moles were destroyed in a 1:1 ratio, from this it is readily obvious where each reaction temperature deviates lium the ideal. For the full 25 injections 83% of the HFP is destroyed at 450°C. The reaction at 500°C does slightly

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L

FIgme 1: Cumulative

moles of HFP destroyed per surface mole of MgO.

J

Figure 2: Hexafluoropropylene adsorbed on MgO [email protected] 450°C. HFP 5 Torr l-10 min.

outperform the 450°C reaction, break&tough number = 14,85% destruction, but not efftciency is gahxd a this temperature. At 450°C large amounts of CO and CO, began to elute off immediately, approximately 1.5 moles CO and 2.0 moles CO, per mole of HFP injected. This amount stayed constant until past the breakthrough point when the amotmts began to steadily decrease. At the break&rough point, small amounts of c;F, begin to elute off (1 mole for every 7.5 moles of HFP injected) at all temperatures. The powdered solid after reaction was carbon black at high temperatures, MgO statted out white, indicating the breakdown of organics on the surface. Powder XRD showed only the presence of MgF* at high temperatures, low temperature reactions showed mainly MgO. Elemental analysis (Galbraith Inc.) gave Mg - 35.62%, Fluorine - 17.12%, Carbon - 5.55%. This shows two MgO’s for every MgF, present. IT-IR

Studies

Studies were performed at 25”C, 15O”C, 400°C and 450°C. Major peaks appearing, outside of hexafiuoropropylene peaks, while the cell is held at 5 Totr HFP are at 1744 cm-‘, 1660 cm-‘, 1592 cm-’ and 1074 cm-‘. The HFP peaks essentially disappear upon evacuation of cell showing the compound to be only physisorbed. The peak at 1744 cm-’ is barely perceptible at room temperature, but steadily increases in intensity as the temperature is mcreased. This peak is possibly due to an ~(,a dihalogen substituted aldehyde. This is reinforced by small peaks near 2800. When the cell is evacuated at lower temperatures this peak is much more visible, at higher temperatures this peak disappears during evacuation. The peak at 1660 cm“ is most likely due to a C=C bond, again this one gains intensity as temperature is increased and does not disappear during evacuation. The peak at 1590 does not appear till 45O”C, but does stay on during evacuation of cell. This peak relates well to the presence of a hexatluoropropyl epoxide. (7) The peak at 1074 cni’ does not appear till 450°C as well, and also stays strong during evacuation. This peak is indicative of a CF, stretch either related to the epoxide recently

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formed or to :CF, bound to the acidic Mgz’ sites. The presence of GFd in the products as the sites begin to fall short lends credence to the idea that CF, is being forced off the surface, and forming tetrafluoroethylene @I&,= -596 kJ/mol), by incoming HFP’s.

DISCUSSION Elemental analysis as well as quantification

of elutants at 450°C gives a chemical reaction

Of

6 MgO + 3 C,F, -6

MgFz + 1 CO + 2.5 CO* + 2.5 C + 3 CF,

This relates to a 2 surface oxide to 1 hexatluoropropylene ratio. The calculated AI&,= -1168 kJ/mol, which means there must be a very high activation energy. The fact that the HFP is only physisorbed shows that, until the surface sites are depleted, all the HFP is destroyed and converted into non-toxic compounds. The IR indicates that the hexatluoropropylene adsotbs to the surface through one of two arrangements. At low temperature the reaction favors rearrangement of the fluorines to produce a fluoroaldehyde, which readily desorbs during evacuation. While being held at the surface, the fluorines bind with the Mgz’ and replace the @ forming MgF* and CO or CO,. The c&on bound to the oxygen likely stays bound to the surface as CF, which is later seen eluting off as C$.,. At high temperatures the reaction also uses breaking of the double bond to form an epoxide. The high efficiency of destruction of the hexafluoropropylene, shows that this procedure could be quite effective for destruction of fluorocarbons as well as the “warfare agent” the HFP is mimicking. While only the surface of the particles reacted with the adsorbent, the small particle size and high surface area gave an overall bulk MgO-to-HFP mole ratio of 9: 1.

ACKNOWLEDGICMEN’IS The authors wish to thank the U.S. Army Research Office for their support.

REFERENCES

(1) Koper, 0.; Li, X.L.; Klabunde, K.J. Chem. Muter. 1993,& 500. (2)

(3) (4) (5) (6)

(7)

Koper, O.B.; Wovchko, E.A.; Glass, J.A.; Yates, J.T. Jr.; Klabunde, K.J. Ltmgmuir 1995, l-l, 2054. Koper, 0.; Lagadic, I.; Klabunde, K.J. Chem. Muter. 1997,9,838. Li, Y.X.; Schlup, J.R.; Klabunde, K.J. Langmuir 1991,1, 1394. Utamapanya, S.; Klabunde, K.J.; Schhrp, J.R. Chem. Mder. 1991,& 175. Li, X.Y.; Klabunde, K.J. Lungmuir 1991,2, 1388. U.S. Pat. 3,358,003 (Dec. 12, 1967), Eleuterio, H.S.; Meschke, R.W. (to E. I. du Pont de Nemours & Co., Inc.)