Thermal Decomposition Mechanism of Lead（Ⅱ）3-Nitro-phthalate

WANG Han，FAN Xue-zhong，LI Xiao-jiang，LI Hong-yan，MENG Ling-ling

（Xi′an Modern Chemistry Research Institute，Xi′an 710065，China）

Introduction

Lead 3-nitrophthalate（LNPH）is a new energetic catalyst containing one nitro group（-NO2）and two carboxyl groups［1］.It could be potentially used widespread as a combustion catalyst of solid propellant due to its higher catalytic efficiency［1］.The thermal behavior of LNPH especially decomposition mechanism is fundamentally important for its use in such field.However，a comprehensive study regarding to this issue has never been done before.

The object of the present investigation was to study thermal behavior and thermal decomposition mechanism of LNPH by means of simultaneous DSC-TG-MS［2］and the combination technique of in situ thermolysis cell with rapid-scan Fourier transform infrared spectroscopy（thermolysis／RSFTIR）［3-4］.

1 Experiment

1.1 Material

The involved LNPH material with the purity of more than 99.0%，was prepared at Xi'an Modern Chemistry Research Institute.

1.2 Experimental instruments and conditions

TG，DSC curves as well as MS spectra were obtained by using the synchronous TG-DSC-MS technology including SATA449Csynchronous TG-DSC analysis apparatus（Netzsch Instruments Co.，USA）and QMS 403C mass spectrograph（Netzsch Instruments Co.，USA）in the temperature range of 25-550℃.The operation conditions were as follows：flowing argon gas（purity，99.99%），flowing rate 75mL·min-1；sample mass，about 1.0 mg；heating rates，10℃ ·min-1；aluminum sample sell；the temperature of combined transport tube，180℃.

Thermolysis in situ rapid-scan FT-IR（thermolysis／RSFTIR）measurements were conducted by using a model NEXUS 870FT-IR instrument（Nicolet Instruments Co.，USA）combined with an in-situ thermolysis cell（made in Xiamen University，China），running with a heating rate of 10℃·min-1（temperate range，25-550℃）.The KBr pellet samples，well mixed by about 0.7mg of LNPH and 150mg KBr，were used.IR spectra of LNPH in the range of 4000-400cm-1were obtained by a model DTGS detector at a rate of 11 files·min-1and 8scans·file-1with a resolution of 4cm-1.

2 Results and discussions

2.1 Thermal behavior of LNPH

The TG-DTG and DSC curves of LNPH at the heating rate of 10℃·min-1are shown in Figs.2and 3，respectively.The characteristic parameters of the curves are summarized in Table 1.

Table 1 Characteristic parameters of the TG-DTG and DSC curves

Fig.2 TG-DTG curve of LNPH

Fig.3 DSC curve of LNPH

TG-DTG curve of LNPH（Fig.2 ）suggests that the thermal de-composition of LNPH is a 4-stage process，which corresponds to the temperatures of 299.9，312.8，383.3and 425.4℃in DTG curve，respectively.The mass loss as a result of first stage comes out to about 9.5%，which compares well with the theoretical mass loss（10.5%）considering one mole of LNPH releasing one mole of carbon monoxide in decarboxylation process［5］.The second stage is accompanied with a mass loss of 7.8%，which is in agreement with the theoretical mass loss（6.7%）on the base of releasing one mole of carbon dioxide in decarboxylation process［6］.Sequentially，the mass loss of 6.5%attributed to the third stage is in agreement with the theoretical mass loss（7.2%）of nitric oxide after the isomerization of the nitro（NO2）group to the nitrite（ONO）［7］.The final residue ratio is 60.8%，which is higher than the theoretical value of 53.6%（based on PbO）due to a large amount of carbon existing in the final residue of LNPH decomposition.

It can be seen from the DSC curve of LNPH（Fig.3 ）that there is a significant exothermic peak at 300.1℃with a shoulder peak of 312.9℃and followed by a smaller exothermic peak（384.7℃）corresponding to the frontal three thermal decomposition stages，respectively.Additionally，according to the DSC curve，no obvious thermal exchange event is observed during the forth thermal decomposition stage of LNPH.The thermal decomposition steps mentioned above would be simultaneously studied by using the thermolysis／RS-FTIR and TG-DSC-MS measurements.

2.2 Intermediate condensed products analysis during thermolysis of LNPH

The IR spectra of condensed phase intermediate products of thermal decomposition of LNPH at the temperature of 20，250，350，400，450and 500℃are shown in Fig.4 ，and the IR spectra of the intermediate products in condensed phase at the temperature range of 20-500℃are shown in Fig.5.

Fig.4 IR spectra of condensed products at different temperatures during thermal decomposition process of LNPH

Fig.5 IR spectra of condensed products in the temperature range of 25-500℃during thermal decomposition process of LNPH

A comparison of IR spectra（Fig.4 and Fig.5 ）shows that LNPH is thermally stable up to 250°C as there is no change in absorption bands except some change in their intensity.It can also be seen that，when the temperature exceeds 250℃，the absorption bands at 1 582，1 395，and 751cm-1attributed to the-COO-group begin to weaken rapidly till vanish at approximately 350℃.And then，the absorption bands（vas（NO2andvs（N-O））at 1 520and 1 346cm-1begin to weaken till disappear at approximately 410℃.Thereafter，the absorption bandsvs（C=C）at 1 457cm-1and the C-H in the plane at 705-1 074cm-1for the aromatic ring gradually weaken till disappear.It indicates that the decomposition of LNPH started with a decarboxylation process，which is followed by a disassociation of nitro group and pyrolysis of aromatic ring.

In order to obtain the exact temperature range of characteristic absorption bands change，the characteristic absorption bandsvas（NO2）andvs（NO2）at 1 520and 1 346cm-1，and thevas（COO），vs（COO）and（COO）at 1 582，1 395，and 751cm-1are chosen to make the area vs.temperature curves（Fig.6 ）.It can be easily recognized from Fig.6 that，absorption bands at 1 582，1 395，and 751 cm-1attributed to the-COO-group start to weaken at 264℃till disappear at 349℃.It is shown that the decarboxylation happens in the temperature range of 264-349℃.Similarly，disassociation of nitro group occurs in the range of 349-383℃，and the pyrolysis of aromatic ring above 383℃.

The gas phase products for each stage during the thermal decomposition of LNPH would be analyzed by using simultaneous MS.

Fig.6 Curves of the area of the typical absorption bands vs.temperature

2.3 Gaseous products analysis for thermal decomposition of LNPH

The MS spectra（Fig.7 ）of the ion flow intensity of gas products vs.temperature during the thermal decomposition of LNPH were obtained by using simultaneous QMS 403Cmass spectrograph.

It can be seen from Fig.7 that the MS ion of gas products for its thermal decomposition mainly contains 46，44，30，28，16，and 14（m／zvalue），et al.Therefore，the gas phase products for the thermal decomposition of LNPH are probably composed of CO2，CO，NO and NO2.In the first stage of thermal decomposition，the intensity of MS ion for 28and 16evidently increased.Although the gas withm／zof 44is possible to be CO2or N2O，the major gaseous product during the first-step decomposition is considered to be CO2due to obvious decarboxylation，which has been proved by TG-DTG and Thermolysis／RSFT-IR analysis.Additionally，only the intensity of the ion（m／z=28）come to a maximal value at the second stage of decomposition.The major gas product in this step is considered as CO due to the same reason as for the first stage.

It can also be seen from Fig.7 that the mass spectra of gas products contain a strong peak atm／z30（NO），which might arise from rearrangement of the nitro（NO2）group to the nitrite（ONO）［7］.At the same time，the mass spectra also include a small peak atm／z46（NO2），which might be related to the homolysis of C-NO2.The viewpoint can also be supported by thermal decomposition theory of nitro compound［6-7］.Therefore，it can be concluded that the homol-ysis reaction of C-NO2and rearrangement of the nitro group（NO2）to the nitrite（ONO）are involved in the third stage of thermal decomposition of LNPH.However，it should be noticed that the reaction of such rearrangement is the major reaction due to the fact that the peak intensity form／z30（NO）is stronger than that ofm／z46（NO2）.At last，aromatic ring fission take place in the fourth stage of thermal decomposition of LNPH till most of the attached substituents are removed［8-9］.The aromatic ring fission is accompanied with generations of CO2，CO，carbon and other compounds in that MS ion atm／z44（CO2）and 28（CO）can also be observed from the mass spectra of products.

Fig.7 MS spectra of the intensity for gas products vs.temperature

2.4 Possible decomposition mechanisms for LNPH

According to the results of TG-DSC-MS and thermolysis RSFTIR，it was presumed that the LNPH could undergo a process of decarboxylation at about temperature of 299.9℃，which was accompanied by a mass loss of about 9.5%（theoretically 10.5%），and resulted in formations of PbO，CO2and nitro aromatic ketone radical.Thereafter，the ketone group was eliminated from the nitro aromatic ketone radical at 312.8℃.The process was accompanied by a mass loss of 7.8%（theoretically 6.7%），resulting in generations of CO and nitro aromatic radical.When the aromatic radical was linear heated continuously up to 383.3℃，the homolysis reaction of CNO2would occur with a mass loss of around 6.5%（theoretically 7.2%），generating NO，phenyl and phenol radicals.Meanwhile，the nitro（NO2）group was rearranged to the nitrite（ONO）.At last，the pyrolysis of phenyl and phenol radicals took place，with products of CO2，CO，carbon and other residues.The parameters for thermal decomposition of LNPH and its corresponding intermediate products with theoretical mass loss data are summarized in Table 2.

Table 2 Thermal decomposition processes and main products of LNPH

3 Conclusions

（1）LNPH could firstly undergo a process of decarboxylation at about temperature of 299.9℃，which is accompanied by a mass loss of about 9.5%（theoretically 10.5%），and result in formations of PbO，CO2and nitro aromatic ketone radical.

（2）The intermediate ketone group could be eliminated from the nitro aromatic ketone radical at 312.8℃and this process was accompanied by a mass loss of 7.8%（theoretically 6.7%），resulting in generations of CO and nitro aromatic radical.

（3）When the abovementioned aromatic radical was linear heated continuously up to 383.3℃，the homolysis reaction of C-NO2would occur with a mass loss of around 6.5%（theoretically 7.2%），generating NO，phenyl and phenol radicals.Meanwhile，the nitro（NO2）group was rearranged to the nitrite（ONO）.At last，the pyrolysis of phenyl and phenol radicals took place，with products of CO2，CO，carbon and other residues.

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