Introducing Students to Thermogravimetry Coupled with Fourier

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Introducing Students to Thermogravimetry Coupled with Fourier Transform Infrared Spectroscopy Marisol Fernández Rojas, Angélica M. Giorgi Pérez, María F. Agudelo Hernández, and Luz A. Carreño Díaz* School of Chemistry, Universidad Industrial de Santander, Street 9, 27, Bucaramanga 678, Colombia S Supporting Information *

ABSTRACT: Thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR) is a useful technique for the thermal and structural characterization of materials at academic and research levels. This paper describes an experiment for chemistry students to understand the use of TG-FTIR for advanced qualitative and quantitative thermal analysis using a natural compound as a sample. In the classroom, students learned about the fundamental principle of thermal analysis, TG-FTIR instrumentation, and different applications of this coupled technique for materials characterization. In the laboratory, students received a description of each part of the TG-FTIR system, as well as how to tune the system and to develop a method. The primary objective of the experiment was to guide them in the interpretation of results including thermograms, infrared spectra, DSC, and Gram Schmidt (GS) curves. For this, students prepared and analyzed cellulose fibers extracted from pineapple leaves and a mixture of them with sodium hydroxide (NaOH) and monoethanolamine (MEA). Students were initially instructed to study the TG, DSC, and Gram Schmidt curves and to identify the temperatures at which the most significant mass changes occurred. Then, they focused on analyzing the infrared spectra obtained at each temperature range. This practice allowed them to identify the gases emitted during fiber decomposition and the volatile character of the MEA as well as to corroborate the MEA presence in the fiber/MEA mixture. This procedure can be adapted to guide students in the analysis of pure substances and mixtures. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, IR Spectroscopy, Thermal Analysis, Fourier Transform Techniques

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INTRODUCTION The use of thermal analysis techniques such as thermogravimetry (TG) and differential scanning calorimetry (DSC) is essential in the characterization of a wide range of materials. These allow the determination of moisture and volatile content; thermal stability; melting, crystallization, and glass transition temperatures; specific heat and enthalpy; and the composition of a multicomponent system. Besides this, it is possible to study the kinetic decomposition, and the effect of reactive and corrosive atmospheres on materials.1 The thermogravimetric analysis (TGA) system coupled online to a Fourier transform infrared spectroscopy (FTIR) allows for the identification of the evolved gas during the loss of matter. It is a useful complement for a better interpretation of decomposition and volatilization processes and has been reported for the characterization of a wide range of materials including polymers, ceramics, catalyst, drugs, biomass, biofuels, cellulose, and coal, among others.2−4 In this Journal were reported experiments involving an introduction to the TGA5 and TGADSC6 analysis, as well as a combined TGA/FTIR study without direct coupling.7 However, no experiment using a TG-FTIR coupled system has been published. TG-FTIR is one of the © XXXX American Chemical Society and Division of Chemical Education, Inc.

techniques included in the theory and laboratory courses for upper-division undergraduate chemistry students at Universidad Industrial de Santander. This practical session aimed to give the students a hands-on experience in qualitative and quantitative TG-FTIR analysis; likewise, we encouraged students to apply the acquired knowledge about the TG and FTIR techniques and verified the advantages of their coupling. This laboratory experiment reports a two-session learning experience performed by 12 students in the Laboratory of Analytical Chemistry course. In the first 4 h, students extracted the cellulose fibers from pineapple leaves and modified them by alkaline treatment and monoethanolamine immobilization. These fibers were selected as study samples as they decompose almost entirely when heated to 600 °C, generating a sufficient amount of gases to be detected by FTIR. In the second afternoon session, they analyzed the samples. Students received the explanation about the description of the TG-FTIR system and the operation instructions; then, they were guided in the Received: November 9, 2017 Revised: April 12, 2018

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DOI: 10.1021/acs.jchemed.7b00826 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Cellulose fiber extraction from pineapple leaves: (a) pineapple leaves, (b) vegetal material removal, (c) fiber extraction, and (d) raw fibers.

Figure 2. Cellulose fiber treatment: (a) fibers in NaOH solution 5% for 1 h, (b) fibers obtained after alkaline treatment, and (c) fibers after MEA immobilization.

Fibers were dried at 60 °C for 30 min in an oven and were labeled as raw fibers (RF). Then, the students immersed clean untreated fibers in 5% NaOH solution for 1 h, to remove traces of vegetal material (Figure 2a). They washed the treated fibers with distilled water and dried them for 1 h in an oven at 60 °C. Fibers obtained for each group were labeled as alkali fibers (AF) (Figure 2b). Finally, students distributed AF fibers within a Petri dish to get a film of approximately 2 mm thick and added an aqueous solution of MEA at 40% until all the AF was thoroughly soaked (Figure 2c) (approximately 0.3 mL of solution per gram of fiber). They labeled samples as AF-MEA.

interpretation of TG thermograms and the infrared spectra of evolved gases. They evaluate the thermal behavior of fiber and the influence of their modifications.

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MATERIALS AND METHODS

Materials

We collected pineapple leaves (Ananas comosus MD2) in a marketplace in Bucaramanga, Colombia. León Laboratories Ltd. supplied the sodium hydroxide (NaOH), and Merck supplied the monoethanolamine (MEA) for synthesis. All of the laboratory materials used as well as the reagents and sample amounts are specified in the Supporting Information.

TG-FTIR Analysis

Under lecturer and instrument technician supervision, students performed thermogravimetric analyses of RF, AF, MEA, and AF-MEA using a Netzsch Simultaneous Thermal Analyzer (STA) 449 F5 Jupiter, which can measure TG and DSC simultaneously. Students weighed around 5 mg of each sample in a Pt/Rh (80/20) crucible sample container. Samples were heated from 35 to 600 °C at a heating rate of 20 °C/min, using nitrogen as an inert atmospheric and protection gas with flow rates of 40 and 60 mL/min, respectively. The STA instrument is coupled to a TGA-IR module of a Bruker Tensor II

Pineapple Leaves Fiber Extraction, Alkaline Treatment, and Immobilization of Monoethanolamine (MEA)

Six groups of two students each (12 students) received several pineapple leaves (Figure 1a); they washed and manually removed the covering vegetal material with a knife, as shown in Figure 1b. Once exposed, fibers were obtained by pulling them from the matrix (Figure 1c,d). They were then washed and immersed in distilled water and then sonicated in an ultrasonic bath for 15 min at a frequency of 50 Hz at room temperature. B




Figure 3. TG, DSC thermograms, and Gram Schmidt curves of (a) RF, (b) AF, (c) MEA, and (d) AF-MEA.

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spectrometer through a transfer line at 230 °C. This module has an MCT (mercury−cadmium−telluride) detector and a stainless-steel light pipe gas cell at 200 °C. It has a single-pass beam cell with an optical path length of 123 mm. IR spectra of the evolved gas were recorded with a resolution of 4 cm−1 every 15 s with an average of 32 scans per sample in the range 4000− 650 cm−1 throughout the heating process. Students learned to use Netzsch Proteus-Thermal Analysis 6.1.0 software to analyze curves obtained from the STA. FTIR spectra were analyzed using the Bruker OPUS 7.5 software; the baseline correction was performed using a concave rubber band algorithm computed with 10 iterations on 64 points. A specific TG-IR operative guide, as well as the procedure guide given to students, is included in the Supporting Information. Examples of infrared spectra analysis using libraries are also found there.

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RESULTS AND DISCUSSION

Thermal Analysis

In the second 4 h laboratory session, students analyzed the thermal behavior of samples from thermograms. A data analysis guide given to the students are included in the Supporting Information. The TG-FTIR system provided the TGA, DSC, and Gram Schmidt (GS) curves shown in Figure 3 as an example. Students determined the thermal stability of RF, AF, MEA, and AF-MEA from TGA curves. They associated the stage of highest mass loss with the Gram Schmidt (GS) peak. This peak is related to the amount of evolved gases detected with an FTIR spectrometer since the GS curve reflects the sum of IR absorbance at all wavenumbers as a function of time.2Students spotted endothermic or exothermic events from DSC curves. They found endothermic events associated with a loss of mass. They plotted the first derivative of the TG curve (DTG) for each sample (Figure 4) to identify more efficiently the stages of mass loss as well as the initial and final temperatures of each stage through the DTG peaks.2 Students analyzed TG, DTG, DSC, and GS curves based on previously read TG-IR research articles3,8 and completed Table 1, which summarizes the thermoanalytical data. In this table, students reported three stages of mass loss for RF and AF. An initial mass loss below 120 °C due to water evaporation was always greater for RF than for AF confirming the hydrophobic surface character after alkali treatment. The second stage

HAZARDS

The students handled the solutions of NaOH and MEA with personal protective equipment. Before the laboratory session, they were required to consult the risks of exposure to these substances. NaOH causes eye and skin burns, irritation by inhalation, intoxication, and internal burns by ingestion. MEA is harmful by inhalation, causes injuries, and is irritating to eyes, the respiratory system, and skin. C




For all samples, the GS signal peak coincided with the region of highest mass loss, showing a good correlation between this and the evolved gases detected with the FTIR spectrometer. Additionally, AF-MEA exhibited an increase in the GS signal at 158 °C due to MEA evaporation. Evolved Gas Analysis

Once the thermal behavior was understood, students identified more clearly what happens during RF, AF, MEA, and AF-MEA mass loss through the analysis of the evolved gases. Students were instructed to analyze the three-dimensional FTIR plot (as Figure 5) which presents all the FTIR spectra obtained as a

Figure 4. TG and DTG curves of RF, AF, and AF-MEA.

between 250 and 389 °C corresponds to cellulose, hemicellulose, lignin, and noncellulosic polysaccharide decomposition.3 It has been demonstrated that alkaline treatment of natural fibers disrupts hydrogen bonding in the network structure, increases fiber crystallinity degree, and removes noncellulosic constituents in the amorphous region such as lignin, wax, and oils. Consequently, the higher crystalline structure gave AF a better thermal degradation resistance. The last mass loss starting at 390 °C is attributed to the carbonization process.3,8,9 Thermograms of MEA showed one stage of mass loss from the beginning of the heating until 175 °C due to volatilization. This behavior was consistent with what others have reported for this substance.10 For AF-MEA, students observed a mass loss between 140 and 233 °C corresponding to MEA evaporation in addition to the other stages of degradation identified for RF and AF. The AF-MEA thermal stability is decreased slightly relative to AF. In the DSC curves, AF-MEA and MEA exhibited a peak corresponding to an endothermic process in the temperature range of highest mass loss which is attributed to bonds breaking and evaporation, respectively, whereas for RF and AF these peaks were not entirely defined.

Figure 5. Three-dimensional FTIR spectra of evolved gas during AF decomposition.

function of temperature or time. They used these figures to select an FTIR spectrum at a specific temperature or for monitoring a particular signal through the temperature and relate it to the stages of mass loss found in the thermal analysis (see Table 1). For RF and AF, they selected the most visible FTIR spectrum from those collected for the mass loss stage beginning at 295° as well as the spectra recorded at the temperatures of GS maximum, i.e., 350 and 480 °C (Figure 6). For MEA and AF-MEA, the students selected FTIR spectra at which MEA loss mass was observed (Figure 7). For the RF FTIR spectra of evolved gas at 295 °C, the students assigned the signals as follows at υ (cm−1): 3735 and

Table 1. Comparative Thermoanalytical Dataa from TG, DTG, and GS Curves Thermogravimetry Sample

Trange, °C

Raw fibers

60−100 250−389 380−541 Residue, 600 °C 60−100 267−391 391−541 Residue, 600 °C 50−175 60−100 140−233 233−368 368−573 Residue, 600 °C

Alkali fibers

MEAb AF-MEAc

Weight, % 5.67 58.55 5.66 35.41 3.39 57.79 7.59 34.46 100.00 5.95 4.71 46.82 10.45 39.97

± ± ± ± ± ± ± ±

0.17 1.76 0.17 1.06 0.10 1.74 0.23 1.03

± ± ± ± ±

0.18 0.14 1.40 0.31 1.20

DSC Endothermic Effect,d °C

DTGe Peak, °C

Gram Schmidt Maximum, °C

347 ± 3.20

347 ± 3.20

342 ± 3.70

356 ± 4.34

349 ± 3.14

350 ± 3.14

173 ± 2.08 320 ± 4.16

173 ± 2.08 315 ± 2.84

173 ± 2.08 158 ± 1.27 320 ± 2.56

Data reported in this table correspond to the average obtained by all groups. bMEA: monoethanolamine. cAF-MEA: alkali fibers treated with an aqueous solution of MEA. dEndothermic events in the differential scanning calorimetry curves associated with a loss of mass. eFirst derivative of the thermogravimetric curve. a

D




Figure 6. FTIR spectra of evolved gas from (a) RF and (b) AF at noncellulosic (295 °C) and cellulose decomposition (350 °C), and carbonization (480 °C). Variation of the main infrared bands with temperature during (c) RF and (d) AF decomposition.

cellulose, and other noncellulosic polysaccharide decomposition, between 200 and 300 °C.3,11,12 When these spectra were compared with those obtained from the evolved gases from AF at the same temperature, a decrease was observed in the intensity of the signals corresponding to acetic and formic acids. Signals corresponding to H2O and products of lignin decomposition also decreased. This indicated xylan and lignin removal due to alkali treatment. Spectra recorded at 350 °C for RF and AF were similar, and students assigned the observed signals to CO2, CO, H2O, formaldehyde, methanol, and formic acid. It has been reported that these gases are emitted mainly during the decomposition of cellulose between 300 and 400 °C.3 This assignment was in agreement with the GS peak observed at around 350 °C since cellulose is the main component of RF and AF and was detected at a higher amount in the infrared spectrometer. At 480 °C during the carbonization process, the CO2 peak for AF showed less intensity than that for RF, whereas the peak at 3015 cm−1 corresponding to CH4 showed a higher intensity for AF than for RF. Students selected some of the most important signals of evolved gas infrared spectra of the RF and AF, to see their evolution with temperature (Figure 6c,d). The absorbance of AF signals between 200 and 300 °C was lower than for RF. This decrease allowed them to confirm the removal of noncellulosic compounds due to alkali treatment. Between 300 and 400 °C, an opposite behavior was observed, and they related it to the higher cellulose content of AF.

Figure 7. FTIR spectra of evolved gases from MEA at several temperatures and for AF-MEA at the first GS maximum.

3649 (antisymmetric (asy) and symmetric (sy) stretch (s) H2O), 3567 (OH s of acetic (aa) and formic acids (fa)), 2828 (CH2 s formaldehyde), 2359 (asy s CO2), 2178 and 2110 (CO), 1796 (CO s aa), 1774 (CO s fa), 1750 (CO s formaldehyde), around 1500 (products of lignin decomposition), 1269 (OH bend aa), 1177 (CO s aa), 1105 (CO s fa), 1033 (CO s methanol).3,11 These gases have been reported to be emitted mainly during xylan, lignin, hemiE




and lignin. TG-FTIR analysis of volatile products. Thermochim. Acta 2014, 581, 70−86. (4) Risoluti, R.; Fabiano, M. A.; Gullifa, G.; Vecchio Ciprioti, S.; Materazzi, S. FTIR-evolved gas analysis in recent thermoanalytical investigations. Appl. Spectrosc. Rev. 2017, 52 (1), 39−72. (5) Burrows, H. D.; Ellis, H. a; Odilora, C. a. The Dehydrochlorination of PVC: An Introductory Experiment in Gravimetric Analysis. J. Chem. Educ. 1995, 72 (5), 448. (6) Harris, J. D.; Rusch, A. W. Identifying Hydrated Salts Using Simultaneous Thermogravimetric Analysis and Differential Scanning Calorimetry. J. Chem. Educ. 2013, 90 (2), 235−238. (7) Williams, K. R. Analysis of Ethylene-Vinyl Acetate Copolymers: A Combined TGA/FTIR Experiment. J. Chem. Educ. 1994, 71 (8), A195. (8) Wang, S.; Liu, Q.; Luo, Z.; Wen, L.; Cen, K. Mechanism study on cellulose pyrolysis using thermogravimetric analysis coupled with infrared spectroscopy. Front. Energy Power Eng. China 2007, 1 (4), 413−419. (9) Li, X.; Tabil, L. G.; Panigrahi, S. Chemical Treatments of Natural Fiber for Use in Natural Fiber-Reinforced Composites: A Review. J. Polym. Environ. 2007, 15 (1), 25−33. (10) De Á vila, S. G.; Logli, M. A.; Matos, J. R. Kinetic study of the thermal decomposition of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA) and methyldiethanolamine (MDEA). Int. J. Greenhouse Gas Control 2015, 42, 666−671. (11) Shimanouchi, T. Tables of molecular vibrational frequencies. Consolidated volume II. J. Phys. Chem. Ref. Data 1977, 6 (3), 993− 1102. (12) Shimanouchi, T. Tables of Molecular Vibrational Frequencies: Consolidated Vol. I; National Standard Reference Data System, National Bureau of Standards, Series 39; U.S. National Bureau of Standards: Washington, DC, 1972. https://nvlpubs.nist.gov/nistpubs/ Legacy/NSRDS/nbsnsrds39.pdf (accessed Apr 2018). (13) National Institute of Standards and Technology, U.S. Department of Commerce. NIST Chemistry WebBook, SRD 69, entry for “Monoethanolamine”. https://webbook.nist.gov/cgi/cbook.cgi?ID= C141435&Type=IR-SPEC&Index=0# (accessed Apr 2018).

Students analyzed spectra recorded for evolved gases from MEA between 60 and 150 °C (see Figure 7). These are very similar; however, the spectrum obtained at 120 °C looks clearer. Students identified and assigned the signals at υ (cm−1): 3671 and 3580 (OH s), 3420 (NH2), 2934 and 2882 (CH2 asy and sy s), 1623 (NH2 scissors), 1387 (C−H deformation vibration), 1231 (CH2 wagging), 1035 (CO s), and 781 (NH2 wagging).11 These spectra corresponded to characteristic gas phase spectrum of MEA in the NIST database library;13 therefore, this was a confirmation that mass loss was due to MEA volatilization. For AF-MEA at 160 °C (Figure 7), students observed only the signals of higher intensity of MEA, due to the low amount of MEA that was actually immobilized (approximately 4% according to TG). AF-MEA and AF spectra recorded at higher temperatures were very similar.

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CONCLUSION Chemistry students learned about TG-FTIR analysis, identifying mass loss stages and products of decomposition or volatilization. Students recognized the importance of the thermal characterization of materials and how the coupled system allowed a more detailed characterization of the material. This experiment applies primarily to students interested in developing their careers in applied engineering or materials science and technology. With this kind of practical experiment, the students acquired a better understanding of the analytical techniques and a better understanding of the results.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00826. Instructor notes (PDF, DOC) Student handout (PDF, DOC)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marisol Fernández Rojas: 0000-0003-0219-4227 Luz A. Carreño Díaz: 0000-0002-4952-0053 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Universidad Industrial de Santander, the Vice-rectorate for Research and Extension, and the Laboratory of Instrumental Chemical Analysis of Chemistry School, for making the resources and the equipment available for the analyses.

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REFERENCES

(1) Xie, W.; Pan, P. Thermal characterization of materials using evolved gas analysis. J. Therm. Anal. Calorim. 2001, 65, 669−685. (2) Schindler, A.; Neumann, G.; Rager, A.; Füglein, E.; Blumm, J.; Denner, T. A novel direct coupling of simultaneous thermal analysis (STA) and Fourier transform-infrared (FT-IR) spectroscopy. J. Therm. Anal. Calorim. 2013, 113 (3), 1091−1102. (3) Benítez-Guerrero, M.; López-Beceiro, J.; Sánchez-Jiménez, P. E.; Pascual-Cosp, J. Comparison of thermal behavior of natural and hotwashed sisal fibers based on their main components: Cellulose, xylan, F


Introducing Students to Thermogravimetry Coupled with Fourier

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