Aerogel nanoscale magnesium oxides as a destructive ... - Springer Link

CEJC 2(1) 2004 16{33

Aerogel nanoscale magnesium oxides as a destructive sorbent for toxic chemical agents 1¤ , Snejana Bakardjieva1 , Monika Ma· 1, · · Vaclav Stengl r¶³kov¶a1, Jan Subrt Franti·sek Oplu·stil2 , Marcela Ol·sansk¶a2 1

2

Institute of Inorganic Chemistry, AS CR, · z, Czech Republic 250 68 Re· Military Technical Institute of Protection Brno, Vesla·rsk¶a 230, 628 00 Brno Czech Republic

Received 21 August 2003; accepted 16 October 2003 Abstract: An autoclave hypercritical drying procedure has been used to prepare precursors of MgO from Mg(OCH3 )2 . This material was prepared with a speci c surface area of 1200 m2 g 1 . The dehydrated materials consisted of much smaller crystallites than conventionally prepared MgO and were free of OCH3 groups . The precursors and samples of magnesium oxide were taken for experimental evaluation of their reactivity with mustard. The largest percentage of the conversion mustard into non-toxic products after the elapse of the reaction was 77%. ® c Central European Science Journals. All rights reserved. Keywords: Nanostructures, organometallic compounds, chemical synthesis, electron microscopy, surface properties

1

Introduction

¤

Nanosized inorganic oxides are regarded as promising non-aggressive reagents useable for the treatment of sensitive materials contaminated with lethally toxic chemical agents, namely the nerve agents (sarin, soman, VX agent, etc.) as well as blistering agents (e.g. mustard). The detoxi¯cation capability of those highly dispersed oxides (e.g. MgO, CaO, ZnO, AlOx (HO)y , ZrO, TiO2 ) has been extensively studied and reported [1-5]. Products of corresponding reactions have been investigated and the respective detoxi¯cation reaction mechanisms have already been proposed. The detoxi¯cation reactions have been E-mail: [email protected]

µ V. Stengl et al. / Central European Journal of Chemistry 2(1) 2004 16{33

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shown to proceed mainly on the surface of the nanoparticles leading to non-toxic products even at ambient temperature. Reactivity of the nanosized MgO towards the toxic agents has been described more profoundly by Wagner et al [4]. The following basic knowledge has been revealed: { All mentioned toxic agents can be detoxi¯ed, i.e. converted into their non-toxic products, within a reasonable time period, so that the activity of the reagent is versatile. { The chemical activity of the MgO surface cannot be considered uniform but the more active centres are preferentially located at activated sites or defects along the surface of the nanoparticles, e.g. edges, corners, etc. { Heterogeneous reactions of the respective neat toxic agents exhibit two-stage kinetics when proceeded on the MgO nanoparticle surface. The initial fast stage can be attributed to the reaction of a given agent at highly active reaction centres, and the second stage is explained as a process of slower transport of the agent along surface from less active site(s) to more reactive one(s). The process of a secondary redistribution of the neat agent over a surface was convincingly correlated with the respective evaporation rate (agent vapour pressure) as to be a rate-limiting factor. The main aims of the work were: { To investigate the respective method and procedure of preparation of the nanosized magnesium oxide. { Characterise the respective synthesised magnesium oxide samples. { Evaluate the prepared oxides for their respective detoxi¯cation activities. { Assess whether the overall activity of the prepared oxides could be increased utilising an e®ective solvent which is potentially more capable of distributing the toxic agent over the whole surface of the oxide. Magnesium oxide is commonly obtained by thermal decomposition of magnesium hydroxide or carbonate [6], [7] and more recently by a sol-gel process [8], [9]. The oxide morphology, particle size and speci¯c surface area depend on the preparation conditions (pH, gelling agent, calcinations rate and temperature). It has been documented that methoxide or alkoxide-based sol{gel synthesis of metal hydroxides followed by supercritical drying and vacuum dehydration can lead to the formation of nanoparticles of metal oxides [10-15]. In considering the ways in how to additionally facilitate the respective detoxi¯cation reaction, the two following approaches may be proposed to stimulate the overall course of the reaction, namely: { The initial uniform spreading of the agent over the whole surface. { A °ow of the agent along the surface. Since the capillarity of the toxic agents is limited, the agent distribution over the surface of the solid (powdery) reagent could be enhanced utilising a solvent that is capable of both dissolving the toxic agent and wetting the powdery reagent surface, thus penetrating its whole deposit, and spreading there evenly. Moreover, if the chosen solvent is volatile enough it evaporates at the upper layers causing a slow but bene¯cial °ux of the solvent

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containing the dissolved toxic agent, which passes from bulk up to surface of the reagent deposit (batch). The detoxi¯cation activities of the prepared samples of magnesium oxides and magnesium hydroxides (i.e. precursors of ¯nal nanosized MgO syntheses were evaluated using sulphur mustard, bis(2-chloroethyl) sulphide. The agent was chosen deliberately for the experiments because it has been reported to be relatively resistant to detoxi¯cation [4]. When it is brought together with the nanosized MgO it can yield 2-chlorethyl vinyl sulphide and divinyl sulphide, both of which are products of an elimination reaction, and the thiodiglycol, a product of a nucleophilic substitution reaction.

2

Experimental section

2.1 Preparation of nanoscale magnesium oxide A modi¯ed autoclave hypercritical procedure has been developed to prepare nanoscale MgO particles. This method has four steps: preparation of Mg(OCH3 )2 by the reaction of magnesium metal with methanol, hydrolysis of Mg(OCH 3 )2 in the presence of toluene, hypercritical drying in an autoclave and thermal activation. Commercially available Mg(OCH 3 )2 solution (Aldrich, 6 wt % in methanol) were used in lieu of ¯rst step. The experimental conditions are summarised in Table 1. Ultrasound waves [16] were used for hydrolysis of Mg(OCH3 )2 . The hydroxide gel solution was transferred into a 100 ml stainless-steel autoclave. After nitrogen gas °ushing, the autoclave was slowly heated from room temperature to 265± C at a rate of 3± C/min by the PID controller. The temperature was allowed to equilibrate at 265± C for 15 min. The autoclave was vented to release the pressure, which took about 0.5 { 1 min. The autoclave was immediately removed from the oven and cooled to room temperature. The product was removed from the autoclave and dried at 120± C. The ¯nal product was stored in a bottle under normal conditions. On the basis of DTA results (Figure 1) the hydrated MgO precursors obtained from the autoclave were heated under dynamic vacuum by using a furnace, controlled by the PID controller, in a stainless steel tube. The temperature ramp was 1± C/min. After the heat treatment, the sample was allowed to cool to room temperature.

2.2 Characterization methods The speci¯c surface area of the samples was determined via nitrogen adsorption-desorptions isotherms at liquid nitrogen temperature by using a Coulter SA 3100 instrument. TEM micrographs were obtained using Philips 201 transmission electron microscope, SEM photographs were obtained using Philips XL30 CP scanning electron microscope and X-ray powder di®raction was performed with Siemens D5005 di®ractometer. Qualitative analysis was performed with Bede ZDS for Windows, version 1.99 and JCPDS PDF-2database [17]. DTA-TG measurements were carried out using NETZSCH STA 409 apparatus.


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2.3 Methodic of disintegrations mustard Nanosized magnesium samples (precursors, Mg(OH)2 , as well as magnesium oxides) were evaluated for their ability to convert sulphur mustard (hereafter also signed as HD) into non-toxic products. Synthesised powdery samples were dried over 24 hours in a vacuum kiln (at 100 ± C, 400 Pa) before tests. A weighed portion of a given evaluated nanosized sample was put into a glass vial provided with a solid screw cap (Supelco, type CRS-33). The toxic agent in a solution of a chosen solvent was dosed onto the powder reagent layer. The vial was sealed with a cap and placed into the thermostat. All experiments were performed at 25± C, and each run was repeated four to six times. An addition of isopropylalcohol (ca. 2 mL) terminated the reaction. The suspension was vigorously agitated and the liquid fraction was separated from the solid using a centrifuge (9 000 cc. min¡1 for 3 minutes), and subsequently analysed for a residual content of the mustard. The respective detoxi¯cation capabilities of the evaluated nanosized samples were expressed as percentages of mustard elimination from the reaction mixture under given experimental conditions.

3

Results and discussion

3.1 Characterization of samples magnesium aerogels Figure 1 shows a typical thermal analysis of the precursors used in the synthesis. Water evolution started at a low temperature (¹ 100± C) with a maximum at 139± C. Carbon dioxide and water from the decomposition of residual {OCH3 groups was detected in the gas phase at ¹ 300± C and peaked at 396± C. The calcination temperature used in the synthesis was 500± C. At this temperature the ¯nal solids obtained were found to consist primarily of particles of MgO without any traces of precursors. The speci¯c surface area and total pore volume are shown in Table 1 and Table 2. The speci¯c surface area decreases with increasing temperature of the annealing of precursors. Figure 2 shows the X-ray di®raction patterns of the precursor Mg15/1 and the samples prepared by their thermal treatment. They are all similar and exhibit three characteristic peaks for MgO (periclas, PDF 45-0496). With increasing annealing temperature the intensity of the peaks has been evolved. No di®raction lines of Mg(OH)2 were detected. Electron micrographs of the selected , samples are shown in Figure 3 (TEM micrographs) Precursor particles (sample Mg15/1) have uniform morphology and particle size as periclase particles (samples Mg15/2-15/6). The particle size from electron micrographs is similar to that of the calculated size from X-ray di®raction (see Table 2). The Scherrer equation was applied to estimate a crystallite size and a programme, WinFit v.1.2, was used for calculation of particle size.

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3.2 The reactivity towards sulphur mustard Two precursors, Mg(OH)2 with high speci¯c surface areas (Mg 15/1 and Mg 25/2) and three di®erent samples of magnesium oxide (Mg 15/1/380, Mg 16/380, Mg 25/2/380), were taken for experimental evaluation of their respective reactivities. The e®ect of the solvent type used on the course of the detoxi¯cation reaction was also tested. Five solvents di®ering in their basic chemical properties (e.g. polarity, proticity, viscosity, capillarity, and volatility) were taken for the experiment as follows: ° petrolether - non-polar, distinct capillarity, highly volatile. ° diethylether - non-polar, aprotic, oxygen-containing, distinct capillarity, highly volatile. ° acetone - weakly polar, aprotic, distinct capillarity, volatile. ° N, N-dimethylpyrrolidone - polar aprotic. ° methylalcohol - protic. The e®ect of the solvent type on detoxi¯cation reaction was only tested with the MgO sample Mg 25/2/380. Mustard was dissolved in the solvents in order to dose approximately the same amounts of the toxic agent onto a layer of the solid tested. The initial concentrations of the mustard in the used solvents are shown in the Table 3. A 30 mg sample of the Mg 25/2/380 was weighed into each lockable glass vial where subsequently 10 ¹L of HD dissolved in the respective solvents was pipetted onto the magnesium sample. The vials were immediately sealed and put into a thermostat for two hours. Stoichiometric ratios of MgO to HD were ca. (20 - 30) to 1, so that the MgO samples were in excess in all experimental cases. Residual content of the HD in the reaction medium was estimated analytically after ¯nishing the reaction. The results are summarised in Table 3. From the obtained data it is obvious that the extent of the HD conversion is appreciably dependent on the kind of the solvent used under the given reaction conditions. Diethylether seems to be the most convenient solvent, which might mainly be attributed to its pronounced capillarity (based on its low surface tension and viscosity). Except for N,N-dimethyl-pyrrolidone, the observed e®ects of the other solvents on the HD distribution and reaction course do not di®er too much, and within an experimental error they can be considered equally e®ective. Since diethylether is not practical for use, since it is narcotic, (as well as its other hazardous properties), it was decided that petrolether was a suitable solvent for all further experimental work. Two common factors should also be considered which can adversely a®ect the reproducibility of the experiments for when HD is dissolved in certain solvents and is dosed onto a layer of a powder reagent: { The structure of the powder layer in a vial can hardly be standardised since the magnesium samples are lumpy (granulated) and the individual lumps do not have a reproducible ¯t, thus, the penetration of the reagent with a solvent can somewhat di®er in the individual experiments. { The extent of wetting the surface of the powder can hardly be controlled or checked


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so far, so that the actual extent of loading the surface with toxic agent with mustard is unknown. Also taking into consideration the fact that the observed reactivity of nanosized reagents is proportionally related to the area of their accessible surface (although di®ering in their respective speci¯c reactivity), then such a parameter as \the extent of wetting" should play an important role. The surface of the nanosized reagents are moistened as evenly and reproducibly as possible, however, this may not be attainable in all replicated experiments. Only when the solvent is dosed in excess can it wet the whole surface reliably. However, in such a case the (local) concentration of the dissolved toxic agent by the surface of the powdered reagent should be appreciably diminished, and consequently the appeared reactivity would be lowered. These circumstances may account for the observed di®erences within a series of the replicated experiments. The importance of \the extent of loading the nanosized reagent" with the toxic agent was investigated. Using the same volume of the solvent as well as the charge of powder reagents two extents of loading the nanosized reagent were decided for evaluation, namely the mass ratios of the respective magnesium samples vs. the HD and were chosen as follows: 100/1 or 200/1 (i.e. a stoichiometry where the MgO exceeded the HD by a factor of ca. 50 or 100). The reaction time was 1 hour. The results obtained are summarised in the tables below, (Table 4 and Table 5). At least two conclusions can be made re°ecting these results: a) the yield of the observed reaction can be increased as the loading of the magnesium reagent surface is decreased under given experimental conditions, and b) magnesium oxides commonly exhibit higher reactivity towards the HD compared with the related magnesium hydroxides although their respective dispersities (thus speci¯c surface area) of the hydroxides (precursors) are comparatively greater, namely by a factor of two or three. Therefore, the observed lower capability of magnesium hydroxides to convert the HD into non-toxic products might be attributed either to their lower inherent reactivity (more likely) or lower ability of the solvent to penetrate into a ¯ne-grained structure of those reagents. The kinetic pro¯les that characterise the course of surface heterogeneous reaction of the neat mustard on nanosized magnesium oxides were reported by Wagner et al [4] to have two stages. The reaction is relatively fast during the initial stage while its subsequent course is comparatively slow since being controlled by the redistribution of HD over the surface. It was of interest to investigate the overall course of the reaction if the toxic agent is spread over the nanosized sample being dissolved in petrolether. The Mg 25/2/380 sample was prepared in larger quantity and the current sample was chosen for a kinetic study of the reaction. A sample of 500 mg was weighed and put into six sealable vials. Mustard was dissolved in petrolether to contain 4.2 mg in 1 mL of the solvent. Aliquots of 0.1 ml were pipetted onto a layer of the magnesium sample, the vials were immediately sealed, and put into a thermostat for a certain time. Then the reaction mixtures were analysed for the HD residual content. Each experiment was repeated four times. Results are summarised in Table 6. The results of the kinetics evaluations are illustrated in Figure


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4. The conversion of the HD by the evaluated reagent is not very fast; the conversion can reach up to ca. 40 % during a three-hour reaction. Logarithmic kinetic data gave a smooth curve showing that the course of the observed heterogeneous reaction does not exhibit two stages. Conversely, it’s apparent from the graph that the reaction proceeds quickly at the beginning, and as time of the reaction elapses the reaction rate continuously slows down. This behaviour might be attributed to the gradual depletion of the mustard at the active centers of the magnesium reagent followed by a di®usion-controlled process of the HD redistribution over the reagent surface. Another explanation for the kinetic curve can be based on the concept of a gradual consumption of the magnesium sample since it is present as the stoichiometric reagent, whose surface is considered uniformly (evenly) reactive. In any case the observed course of the reaction can be considered to be faster compared to that of a reaction performed without solvent.

4

Conclusion

The following conclusions can be made from the reactivity experiments: The observed reaction rate of the toxic agent with the powdered nanosized reagent can be accelerated when the agent, dissolved in a suitable solvent, is spread over the powdered reagent making sure that wetting is even over the whole reagent surface. Among the solvents tested, petrolether was the most suitable both for e±ciency and safety reasons.

Acknowledgements This work was supported by the Ministry of Education of the Czech Republic in the frame of the Project No. LN00A028.

References [1] O. Koper, E. Lucas and K.J. Klabunde: "Development of reactive topical skin protectants against sulfur mustard and nerve agents", Journal of Applied Toxicology, Vol. 19, (1999), pp. 59 { 70. [2] O. Koper, E. Lucas and K.J. Klabunde: "Oxide Nano particles as Countermeasures against Chemical and Biological Threats", In: Proceedings of the Joint Service Chemical and Biological Decontamination Conference, Salt Lake City (USA), May 2000. [3] G.W. Wagner and O.W. Bartram: "Reactions of the nerve agent simulant diisopropyl °uorophosphate with self-decontaminating adsorbents. A P-31 MAS NMR study" Journal of Molecular Catalysis A: Chemical, Vol. 144, (1999), pp. 419 { 424. [4] G.W. Wagner, O.W. Bartram, O. Koper and K.J. Klabunde: \Reactions of VX, GD, and HD with nanosize Mgo", Journal Phys. Chem. B, Vol. 103, (1999), pp. 3225 { 3228. [5] G.W.Wagner, O. Koper, E. Lucas, S. Decker and K.J. Klabunde: "Reactions of VX, GD, and HD with nanosize CaO: Autocatalytic dehydrohalogenation of HD." , Journal of Physical Chemistry B, Vol. 104, (2000), pp. 5118 { 5123.


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[6] D.R. Lide (Ed.): CRC Handbook of Chemistry and Physics. A Ready-Reference Book of Chemical and Physical Data, 77th Edition, CRC Press, Boca Raton { New York { London { Tokyo. [7] M.A. Aramendia, V. Borau and C.Jimenez: \Synthesis and characterization of various mgo and related systems", J. Mater. Chem., Vol. 6(12), (1996), pp. 1943 { 1949. [8] B.Q. Xu, J.M. Wei and H.Y. Wang: \Nano-MgO: Novel preparation and application as support of Ni catalyst for CO2 reforming of methane", Catalysis Today, Vol. 68(13), (2001), pp. 217 { 225. [9] H.S. Choi and S.T. Hwang : \Sol-gel-derived magnesium oxide precursor for thin-¯lm fabrication", J. Mat. Res, Vol. 15, (2000), pp. 842 { 845. [10] T. Lopez, R.Gomez, J. Navarrete and E. Lopez-Salinas: \Evidence for Lewis and Bronsted acid sites on MgO obtained by sol-gel", Journal of Sol-Gel Science and Technology, Vol. 13, (1998), pp. 1043 { 1047. [11] S. Utamapanya, K.J. Klabunde and J.R. Schlup: "Nanoscale metal-oxide particles clusters as chemical reagents - synthesis and properties of ultrahigh surface-area magnesium-hydroxide and magnesium-oxide\, Chem.Mater, Vol. 3, (1991), pp. 175 { 181. [12] J.V. Stark and K.J. Klabunde: "Nanoscale metal oxide particles/clusters as chemical reagents. Adsorption of hydrogen halides, nitric oxide, and sulfur trioxide on magnesium oxide nanocrystals and compared with microcrystals", Chem. Mater, Vol. 8(8), (1996), pp.1913 { 1918. [13] J.V. Stark, D.G. Park and I. Lagadic: "Nanoscale metal oxide particles/clusters as chemical reagents. Unique surface chemistry on magnesium oxide as shown by enhanced adsorption of acid gases (sulfur dioxide and carbon dioxide) and pressure dependence", Chem. Mater., Vol. 8(8), (1996), pp. 1904 {1912. [14] L. Znaidi, K.Chhor and C. Pommier: "Batch and semi-continuous synthesis of magnesium oxide powders from hydrolysis and supercritical treatment of Mg(OCH 3 )2 ", Mat.Res.Bull, Vol. 31, (1996), pp. 1527 { 1535. [15] O.B. Koper, I. Lagadic, A.Volodin and K.J. Klabunde: "Alkaline-earth oxide nanoparticles obtained by aerogel methods. Characterization and rational for unexpectedly high surface chemical reactivities\, Chem. Mater, Vol. 9, (1997), pp. 2468 { 2480. · · [16] V. Stengl, S. Bakardjieva, M. Ma·r¶³kov¶a, P.Bezdi·cka and J. Subrt: "Magnesium oxide nanoparticles prepared by ultrasound enhanced hydrolysis of Mg-alkoxides", Mat Lett., Vol. 57(24-25), (2003), pp. 3998 { 4003. [17] "JCPDS PDF 2 database", Release 2001, International Centre for Di®raction Data, Newton Square PA, USA.


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Sample

Mg-methoxide

Methanol+water

Toluene

SBET

Vp

[mL]

[mL]

[mL]

[m2 g¡1 ]

[ccg¡1 ]

Mg15/1

83,4*

0 + 2,1

416,6

1047

1,41

Mg16

102*

0 + 2,1

510

754

0,78

Mg25/1

9,4

4,2 + 0,28

68

1099

1,38

Mg25/2

9,4

4,2 + 0,28

68

1236

1,42

* 6% solution of magnesium methoxide, other 8,68% solution Table 1 Experimental conditions and resulting speci c surface areas, SBET , and total pore volume, V p , of Mg{aerogels prepared from magnesium methoxide.


Sample

25

T [ C]

SBET [m2 g¡1 ]

Vp [ccg¡1 ]

Phase identi ed by XRD

L44;2µ [nm]

Mg15/1/360

360

537

0,35

Periclase

1,7

Mg15/2/380

380

497

0,38

Periclase

2,0

Mg15/3/400

400

377

0,42

Periclase

2,9

Mg15/4/450

450

328

0,47

Periclase

3,6

Mg15/5/500

500

288

0,56

Periclase

4,2

Mg16/380

380

327

0,39

Periclase

1,9

Mg25/1/380

380

246

0,38

Periclase

2,1

Mg25/2/380

380

358

0,39

Periclase

2,0

±

Table 2 Speci c surface areas and crystallite sizes of nanoscale dehydrated MgO samples.


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Characteristics

Solvent N,NPetrolDimethylether pyrrolidone

Diethylether

Acetone

Methyl alcohol

Surface tension at 25± C (® ) [mN.m¡1 ]

16.7

23.5

22.0

-

17.7

Viscosity at 25± C (² ) [mPa.s]

0.2

0.3

0.8

-

0.3

Initial content of the HD (mg HD per 30 mg charge of Mg reagent)

0.511

0.392

0.471

0.487

0.530

Average residual content of the HD (mg HD per 30 mg charge of Mg reagent)

0.207

0.260

0.298

0.420

0.295

Standard deviation of the average

152.3

65.9

136.7

33.6

45.8

Percentage of the HD conversion after the reaction [%]

59.5

33.6

36.6

13.8

44.2

Table 3 Residual content of the HD after reaction on the magnesium sample Mg 15/1/380 (reaction time 2 h, temperature 25¯ C, charge of the reagent 30 mg, the HD dissolved in 0,02ml of solvent).


Characteristics

Precursors

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Mg15/1

Mg25/2

Mg15/1/ 380

MgO Mg16/ 380


0.303

0.290

0.235

0.156

0.286


1.1

1.6

8.4

2.7

1.6

Percentage of the HD conversion after the elapse of the reaction [%]

39.2

41.9

52.9

68.8

42.7

Mg25/2/ 380

Table 4 Residual content of HD after reaction on the magnesium samples (reaction time 1 h, temperature 25¯ C, charge of the respective samples in the reaction 30 mg, 0.5 mg of HD dissolved in 0.1 ml of petrolether).


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Characteristics

Precursors Mg15/1

Mg25/2

Mg15/1/ 380

MgO Mg16/ 380


0.132

0.144

0.100

0.057

0.132


3.6

2.6

0.2

1.7

4.3


46.9

42.4

59.9

77.0

47.1

Mg25/2/ 380

Table 5 The residual content of HD after its reaction on the magnesium samples (reaction time 1 hour, temperature 25¯ C, charge of the respective samples in the reaction 30 mg, 0,25mg HD dissolved in 0.1 ml petrolether).


Time of reaction, (i.e. the HD conversion), [min]

29

0

15

30

60

120

180

Average residual content of HD (mg HD per 500 mg charge of Mg reagent)

0.422

0.363

0.327

0.299

0.272

0.261


9.4

34.9

1.7

7.1

6.1

5.0


0.0

13.9

22.4

29.1

35.5

38.2

Table 6 Time dependence of the mustard conversion on Mg 25/2/380 sample (the Mg reagent charge of 500 mg, 0.42 mg of HD in 0.1 ml of petrolether, temperature of 25¯ C).

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Fig. 1 DTA - TG of sample Mg15/1.


Fig. 2 XRD spectra sample Mg15 /1 and samples Mg15/1/360 - Mg15/1/500.

31

32


Fig. 3 TEM micrographs of precursor Mg15 and heated samples Mg15/1/360-15/1/500.


Fig. 4 Time dependence of the HD conversion on the Mg 25/2/380 reagent.

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