Zeolite-polymer based Materials for Gas Sensors


Academic Paper, 2016
45 Pages, Grade: 1.1

Excerpt

Content

1. Introduction

2. Scope of the book

3. Experimental
3.1 Synthesis of zeolites
3.1.1 Synthesis of AlMCM41 Zeolite
3.2 The cation exchange process
3.2.1 Cation exchange of zeolite Y
3.2.2 Cation exchange of zeolite L and modernite
3.3 Synthesis of zeolite/polymer composites:
3.3.1 Synthesis of Polyaniline/Cu2+ zeolite composite
3.3.2 Synthesis of D-PDPA/Y_H+ composite
3.3.3 Synthesis of PANI/Clino nanocomposite
3.3.4 Synthesis of PANI/Clino nanocomposite Films
3.3.5 Synthesis of polythiopene/13-X composite
3.4 Instrumentation
3.5 Gas sensing experimental setup

4. Strategies in gas sensing
4.1 Zeolite content
4.2 Zeolite type
4.3 Si/Al ratio
4.4 Cation Type
4.5 Cyclic interval
4.6 Temporal response
4.7 Vapor type

5. Zeolite/polymer composites used for gas sensing to different gases.
5.1 Response of polyaniline/zeolite (Y, 13X, AlMCM41) composite towards CO
5.1.1 Effect of zeolite type
5.1.2 Effect of zeolite content
5.2 Response of Poly (Phenylene-vinylene)/ZeoliteY towards ammonium Nitrate
5.2.1 Effect of NH4NO3
5.3 Response of Poly (p-Phenylene)/Zeolite (ZSM-5) towards Ammonia
5.3.1 Effect of CO and H2
5.3.2 Effect of Cation Type
5.4 Response of Poly (p-phenylenevinylene) Zeolites (ZSM5) towards CO
5.4.1 Effect of Cation Type
5.4.2 Effect of Zeolite Type
5.5 Response of Poly diphenylamine/Zeolite Y towards Halogenated Hydrocarbons
5.5.1 Effect of halogenated Hydrocarbons
5.5.2 Effect of zeolite content
5.5.3 Effect of vapor concentration
5.6 Response of Poly (Para- Phenylene Vinylene)/Zeolite Y towards Ketone Vapor
5.6.1 Effect of Ketone Vapor [119]
5.7 Response of ZSM-5, Y, and mordenite towards ethanol vapor
5.7.1 Effect of ethanol vapor

6. Conclusion

References

This book summarizes various Zeolite/Polymer composites which have been used as a sensor for different gases. In this book; properties of Zeolite/Polymer composites like simple synthesis and good electrical properties, etc. are discussed. Various strategies which affect the sensitivity of the composites are presented here. The sensitivity and response of Zeolite/Polymer composites to different gases is summarized in terms of Zeolite content, zeolite type, concentration of gas, Si/Al ratio and type of cation in the zeolite pores namely, Na+, K+, H+ etc.

1. Introduction

The environment is being polluted by the combustion of petroleum products like diesel, heating oil and various fuels due to which a huge amount of toxic gases like CO and SO2 are released in the atmosphere. These gases have various harmful effects. CO leads to chest pain and causes reduced mental alertness. Due to acid rains the chemical composition of soil gets altered and leads to the loss of minerals. In addition to the above mentioned gases present in the atmosphere, fossil fuels also release toxic byproducts like NOX and hydrocarbons. A significant part of NOx emissions originate from motor vehicles. The combination of NOx and CO in presence of sunlight tends to produce O3, which is harmful to plants and also to human beings [1, 2]. Due to the daily exposure of these toxic particulates they get accumulated in the body and hence cause number of diseases [2]. There is a daily use of volatile organic compounds (VOCs) in our life. Nevertheless, the toxicity of these chemical vapors causes serious environmental and human health concerns. These are of two types; non-halogenated hydrocarbons, and halogenated hydrocarbons. Non-halogenated hydrocarbons lack chlorine atom within the molecule and are volatile in nature, found in daily life products such as plastics, cleaning solvents and paints and it can affect human health through the respiratory system. The other type of VOCs is halogenated hydrocarbons which are having a chlorine atom within the molecule. They have their application in dry cleaning, metal cleaning, furniture making, thermoplastic production, degreasing, printing, paper and textile production, and paint removal [3]. They are clear liquids, and vaporize at room temperature [4-6]. The toxicity of these chemical vapors is more severe to human health than non-halogenated hydrocarbons. A safe method is required to detect in advance the presence of these gases in atmosphere prior to their ill effects. Various sensors have been developed for their detection. Polyaniline is a promising polymer used as a sensor which is a good conductor of electricity. Other polymers used are polythiopene, polyfuran etc. Conducting polymers such as poly (p -phenylenevinylene) (PPV) can serve as a potential sensing material because PPV possesses good optical and electrical properties and it can be synthesized by a relative simple technique. Zeolites are aluminosilicates formed from SiO4 and AlO4 tetrahedra building blocks and form a ring structure. They are in the form of a three-dimensional (3D) framework with linked channel systems. They are having well-developed micropores and mesopores that provide an open porosity which gives rise to an exceptionally high surface area. Due to increasing environmental and economic concerns, the need for inexpensive selective gas sensors is of great interest and zeolite based materials used in gas sensing have received a great deal of attention. A zeolite is chosen as a selective microporous adsorbent to be introduced into the polymer matrix in order to increase sensitivity towards the target gas. Zeolites doped with conductive polymers have been used as sensors for the detection of harmful gases in the environment. Here we present a detailed account of Zeolite/Polymer composites used as sensors for different gases.

2. Scope of the book

Our main aim is to highlight the Zeolite/Polymer composites that have been synthesized and used as sensors for different gases. One would get a detailed account on the synthetic procedure of zeolites, cation exchange of zeolites in to different forms and zeolite/polymer composites. Also, one would get a detailed account of the work which has been carried out on gas sensing utilizing zeolite/polymer composites in a concise form.

3. Experimental

3.1 Synthesis of zeolites

3.1.1 Synthesis of AlMCM41 Zeolite

An aqueous solution of HTAB (Aldrich), a 25% concentrated ammonium solution, and a sodium silicate solution with Na/Si ratio of 0.5 (2.4 wt. % SiO2, 9.2 wt. % Na2O, and 88.4 wt. % H2O) are used to synthesize AlMCM-41 zeolite. The mixture is added into a polypropylene bottle to obtain HTA-silicate gel with the molar composition of 4SiO2:1HTAB:1Na2O: 0.15(NH4)2O:200H2O. After the gel is stirred for 1 h, the mixture is heated to 97 ◦C in an oven for 1 day. During this period, the cap is loosened repeatedly in order to reduce the ammonium pressure. The bottle is then cooled to room temperature, followed by drop wise additions of 30% acetic acid to adjust the solution pH value to 10.2. The bottle is then heated to 97 ◦C again in the oven for 24 h and then cooled to room temperature. Subsequently, 3 mol of NaCl/HTAB and sodium aluminate 0.13 mol silicate are added to the mixture and stirred for 30 min. The reaction mixture is adjusted to a pH value of 10.2 and then heated to 97 ◦C twice more. Finally, the precipitate product is filtered, washed with distilled water and dried at 97 ◦C for 12 h. The dried MCM-41 zeolite powder is calcined under an O2 flow with continuously increasing temperature up to 540 ◦C for 10 h and maintained at that temperature for 10 more hours [7]. The procedure for the synthesis of various types of zeolites can be obtained from IZA (International Zeolite Association).

3.2 The cation exchange process

3.2.1 Cation exchange of zeolite Y

Zeolite can be ion exchanged in to different forms by the following procedure. The solution of MgCl2, CaCal2 and KCl at 0.5 M is added in to 5 g of Na-form of the zeolite and stirred at 25 °C for 24 h. To synthesize zeolite Y (Si/Al 5.1, 50 % mole of Na+ and 50 % mole of Mg2+) or 50MgNaY, zeolite Y (Si/Al 5.1, 50 % mole of Na+ and 50 % mole of Ca2+) or 50CaNaY, zeolite Y (Si/Al 5.1, 80 % mole of Na+ and 30 % mole of K+) or 30 KNaY, zeolite Y (Si/Al 5.1, 50 % mole of Na+ and 50 % mole of K+) or 50 KNaY and zeolite Y (Si/Al 5.1, 80 % mole of Na+ and 20 % mole of K+) or 80KNaY are to be taken. Then zeolite samples are filtered and washed with deionised-water for 5 times [8 ].

3.2.2 Cation exchange of zeolite L and modernite

Zeolites L and mordenite can be ion-exchanged with different cations in order to investigate the effects of cation type on the sensitivity of the zeolite/polymer composites. Zeolite L (parent form) is stirred in 1M solution of NaCl at 80 ◦C for 2 h, at the ratio of 1 g: 70 ml. Zeolite mordenite is stirred in 1M of LiCl and KCl solutions using the same procedure. This process is repeated many times until each zeolite contained the required amount of cation. After filtering and washing with distilled water for several times, the ion exchanged zeolites are dried and calcined at 500 ◦C for 5 h [9].

3.3 Synthesis of zeolite/polymer composites:

3.3.1 Synthesis of Polyaniline/Cu2+ zeolite composite

Prior to composite formation, zeolites Y and 13X are calcined under a N2 flow at 200 ◦C for 2 hrs. Zeolites are converted into Cu2+ form by stirring 1 g of zeolite in 200 ml 2.1×10−4 M CuCl2 for 12 h at room temperature. The precipitate is then filtered and washed with distilled water and dried at 80 ◦C for 2 hrs in an oven. Polyaniline/zeolite composites are formed by mixing PANI powder with the zeolites. The composites are compressed into pellets by a hydraulic press at a pressure equal to 5 t [10].

3.3.2 Synthesis of D-PDPA/Y_H+ composite

The D-PDPA and zeolite Y_H+ powder is ground prior to pellet formation. The powder is then pressed into pellets for measurement of electrical conductivity and sensitivity with a hydraulic press machine (GRASEBY SPECAC) applying a 4-5 ton load. Pellets are prepared in disc form using a 1 cm stainless steel die. The thickness of the pellets can be measured by a digital thickness gauge (PEAACOCK, dial stand type model PDN-20) [11].

3.3.3 Synthesis of PANI/Clino nanocomposite

Ammonium persulfate is used as an oxidant for the polymerization of aniline monomer in the clinoptilolite framework. It is observed that the aniline monomer finds it difficult to penetrate into the clinoptilolite channels than anilinium cation. Hence, a reaction between H+ present in clinoptilolite structure and anilinium cation in solution is carried out. Therefore, clinoptilolite at weight ratios of 0.25, 0.5, 1, 2, 3, 4 and 5% w/w per aniline monomer is mixed in 200 ml of HCl aqueous solution (1 M) already containing 4 ml aniline monomer. Stir the mixture for 48 hours at room temperature. Dissolve 8.09 g of ammonium persulphate in 200 ml of deionized water and add drop wise for a period of 6 hours to the above mixture. Meanwhile the reaction mixture is stirred at -2 to -5 °C. Wash the obtained composite repeatedly with 80/20 water/methanol solution unless the under washing solution becomes colorless [12].

3.3.4 Synthesis of PANI/Clino nanocomposite Films

One gram of the nanocomposite as synthesized by 2.3.3 procedure is added slowly (over 5 hours) to the 40 ml of N-methyl pyrrolidone (NMP) solvent and stirred with a magnetic stirrer at room temperature. It is observed that if the polyaniline powder is added rapidly to the NMP solvent, it will be aggregate. The resulting viscous solution is filtered to remove any undissolved particles. The free standing films of polyaniline or PANI/Clino nanocomposite are prepared by casting the viscous solution of nanocomposite in NMP on the surface of glass plate (2×8 cm) which is then dried in an oven at 50 °C for 48 hours. The dried films with 20 μm thickness are removed from the glass surface by immersing in distilled water. Before conductivity measurement the prepared films are doped by immersion in aqueous solution of hydrochloric acid (1 M) for 24 hours followed by washing with excess deionized water and drying at 25 °C for 5 hours [12].

3.3.5 Synthesis of polythiopene/13-X composite

A particular quantity of 13X-zeolite powder (1.0 g) in a conical flask containing 25 ml of CHCl3 is sonicated for15 min. Add a known volume of TP (0.5 g) to it and stir magnetically for 15 min. Thereafter, add a particular quantity of FeCl3 (4.0 g) to this solution at a time under continued stirring for 1 h at room temperature. To stop the polymerization and to remove excess of FeCl3, add 20 ml of acetone. Then filter and wash the mixture with MeOH (10 ml) followed by acetone (5 ml). Finally, dry the black residue at 50 ◦C under vacuum for 1 h [13].

3.4 Instrumentation

A four-point probe is used to measure the electrical conductivity of nanocomposites. A galvanostat/potentiostat SAMA 500 (Iran) and A three-electrode electrochemical cell system consisting a gold coated electrode as working electrode, a platinum gauze as counter electrode and an Ag/AgCl as reference electrode are used for electrochemical experiments. A micrometer SM 1201 Teclock Corporation (Japan) is used to measure the thicknesses of polyaniline and nanocomposite films or nanocomoposite pellets. A Scanning Electron Microscope (SEM LEO440 i-England) is used to investigate the surface morphology of polyaniline and nanocomposite. A Fourier Transform Infra-Red spectroscopy (FTIR, Bruker Tensor 27-Germany) is used to investigate the physicochemical interactions between organic and inorganic phases. An X-ray diffractometer D500 Siemense (Germany) is used to study the crystallinity of the nanocomposite [12].

3.5 Gas sensing experimental setup

The gas sensing experiments are carried out in a setup given below [14]. The gas chamber has one temperature controlled heater for quick evaporation of the liquid organic when they are injected into the gas chamber using a micro syringe, a small 12 V DC fan is placed inside the gas chamber for quick circulation and homogeneous spreading of chemical vapors. The chamber is connected to nitrogen gas cylinder to flush out the exposed chemical vapors after sensing. This process is necessary to allow the sensor to achieve the base line resistance prior to the next vapor exposure for sensing. Petroleum jelly is used to seal the chamber. This type of gas handling system is called as static gas generation system. For ammonia vapor sensing, the sensors are mounted on the sample holder and the electrical connections are taken by using silver paint and is connected to the LCR meter for data acquisition.

illustration not visible in this excerpt

The response of the composite towards the gas is determined by the following equation.

illustration not visible in this excerpt

Where,

σ CO = conductivity of the composite in presence of the target gas; CO.

σ N2= conductivity of the composite in presence of nitrogen.

The sensitivity of the composite is determined by the following equation:

illustration not visible in this excerpt

The gas sensing measurements are performed on the prepared samples/pellets/films at different concentrations of gas vapors by using the above setup.

4. Strategies in gas sensing

4.1 Zeolite content

The sensitivity of zeolite/polymer composites to various gases has been found to increase initially up to certain increase in the zeolite content. Above certain zeolite content; there has been found no increase in the sensitivity of the composite. There are several evidences of this fact. The sensitivity of the composites fabricated from zeolite 13X of 3 weight ratios—PANI-10MA/10zeolite13X, PANI-10MA/20 zeolite 13X, PANI-10MA/40 zeolite13X shows a decrease from 1.02 to 0.103 S/cm as we increase the zeolite content from 0 to 40 wt %. A similar trend is observed in the presence of N2. But they show a reverse trend on exposure to CO. An increase (from 0.157 to 0.950) in the sensitivity of the above composites is found on increasing the zeolite content from 0.004 to 0.138 at 1000 ppm of CO. This increase in the sensitivity of the composites on exposure to CO is because of the fact that CO molecules get adsorbed physically into the zeolite 13X frame work by dipole-dipole interaction between Cu2+ and CO molecules. Hence, as we increase zeolite content more and more zeolite pores/unit surface area are available for CO molecules to interact [10].

4.2 Zeolite type

Within a first approximation, it can be said that zeolites having a higher ion exchange capacity and favorable ion position are found to increase the electronic conductivity sensitivity of the composite to a particular gas. The sensitivity of Y, 13X and AlMCM41 zeolites having a pore size of 7A◦, 10A◦, 36A◦ and cation exchange capacities of 0.161, 0.086, 0.044 mol/g against CO were studied and it was observed that a channel system is more interactive in enhancing sensitivity than a cage system. Even if zeolite Y and 13X have comparable pore size of 10A◦ and 7A◦, yet they differ in their sensitivities. This is due to their different ion exchange capacities of 0.086 (zeolite Y) and 0.161 (zeolite13X) mol/g. Due to higher amount of Cu2+ in zeolite Y, there is a small space available for CO molecules. Therefore a small interaction occurs. Further, there is a favorable location of Cu2+ ions in zeolite 13X, which enhances the interaction and hence increase the sensitivity [10].

4.3 Si/Al ratio

A lower Si/Al ratio of composite results in lower sensitivity. On increasing Si/Al ratio of the zeolite/polymer composite; an increase in the electrical conductivity sensitivity is found. The effect of Si/Al ratio (Si/Al = 5.1, 30, 60, 80, H+) for zeolite Y against NH4NO3 has been studied. Further the different Si/Al ratios of zeolite Y possess nearly same pore size, surface area and densities. There has been found an increase by one order of magnitude in the electrical conductivity sensitivity of the zeolite Y on exposure to NH4NO3 at 377 ppm in comparison to the exposure of N2 [15]. On increasing the Si/Al ratio, a gradual increase in the electrical conductivity sensitivity occurs. A higher Si content present in zeolite Y also means higher cations present. They can enhance the interaction between oxygen on the Si of zeolite and NH4NO3 molecules [16, 17].

4.4 Cation Type

To understand the effect of type of cation present in the zeolite/polymer composite on the electrical conductivity sensitivity a general understanding of desorption of CO on a metal ion present in the zeolite is necessary. In order to understand this, desorption of CO on Na+ and Cu+ is described here. Generally, it is observed that CO molecules get adsorbed more strongly on Na+ ions than Cu+ ions. There has been found more strong interaction between CO molecules and Cu+ ions. This is because CO molecules are lewis bases. They interact with Cu+ ions and form coordinate bonds. Hence, Cu+ ions act as active sites in the zeolite for CO adsorption. Therefore it is clear that CO molecules form stable carbonyls with Cu+ and does not interact with Na+ ions. Hence, in such a situation CO molecules get adsorbed on Cu+ ions and do not interact with the polymer chains. Thus, resulting in a weak sensitivity of the composite. This is contrary to Na+ ions, in which the CO molecules do not interact with the cation rather than the polymer chains and hence result in a strong sensitivity. N. Thongchai et al [18] reported positive (1.48) electrical conductivity sensitivity value of dPPV/Na+-ZSM5 (Si/Al=23) and a negative value (-0.154) for Cu+ form of the composite dPPV/Na+-ZSM5 (Si/Al=23). This is in support of the above fact.

4.5 Cyclic interval

As a common sense fact, the effect of cyclic interval on the sensitivity can be said that sensitivity will go on decreasing as we go on increasing the no. of intervals. There will be a difference between the sensitivity of a composite during the 1st run, 2nd run and so on. The electrical conductivity sensitivity value of 50 KNaY was found to be: 3.42×10−01±4.67×10−04, 2.09×10−01±6.27×10−04, 1.81×10−01±6.12×10−04 and 1.28×10−01±9.12×10−04 for the 1st, 2nd, 3rd and 4th interval respectively. This decreasing trend in the sensitivity after repeating the cyclic after 2 to 4 times is because of the fact that during 1st interval the gas molecules which get adsorbed do not get desorbed quickly i.e. it is not a reversible process. They decrease the no. of active sites. Hence, a decrease in the sensitivity value after repeating the cycle [19].

4.6 Temporal response

A zeolite having more Cu2+ ion exchange capacity will have a small temporal response. Therefore a composite from such a zeolite would show a quick or more response to the gas vapors. The effect of temporal response on the sensitivity of composites: PANI-10MA/10Zeolite-Y, PANI-10MA/10Zeolite-13X and PANI-10MA/10Zeolite-AlMCM41 were studied. They obtained a temporal response of 169, 250, and 365 min with Cu2+ exchange capacity of 0.161, 0.086 and 0.044 mol/g, respectively. From this study it is clear that the response time of the composites is inversely proportional to the Cu2+ ion exchange capacity [10].

4.7 Vapor type

The effect of vapor type on the zeolite/polymer composite can be expressed in terms of the electrophilic and nucleophilic nature of the target gas. Electrophilic gas molecules withdraw electrons from the polymer chains; resulting in increase in the electrical conductivity of p –type doped polymer which possess polarons and bipolarons. On the other hand; a nucleophilic gas molecules give electrons to the polymer chains. Hence, there occurs a decrease in the no. of charge carriers, polarons and bipolarons, and hence the decrease in electrical conductivity. P. Phumman et al [20] studied the effect of vapor type in terms of three vapors CO, H2, and 10% NH3 for dPPP. On exposing the sample of dPPP to CO a small positive response with a sensitivity of 2.1×10−2 is obtained. Since, CO molecules are electrophilic in nature; they are expected to withdraw electrons from dPPP. Therefore, the p-type doped conductive polymer possessing polarons and bipolarons cause an increase in electrical conductivity [21].

5. Zeolite/polymer composites used for gas sensing to different gases.

5.1 Response of polyaniline/zeolite (Y, 13X, AlMCM41) composite towards CO

The response and sensitivity of polyaniline/zeolite (Y, 13X, and AlMCM41) to CO in terms of zeolite content, pore size and ion exchange capacity are described here. Since CO leads to inhibited transportation of blood supply in humans [22]. A no. of sensors have been developed for the detection of CO which are having high selectivity, long life times and low manufacturing times [23]. Conductive polymers are used as sensors because they are having advantages over conventional metal sensors because they are lighter, less expensive and can be applied at low voltage and temperature [24]. A no. of conducting polymers have been used as sensors for many gases [25]. Polyaniline (PANI) is most used conductive polymer [23]. Polyaniline has also been reported as gas sensor for the detection of methanol vapor [26, 27], H2 [28], SO2 [29], NH3 [30], CO2, NO2 and SO2 [31].

[...]

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Title
Zeolite-polymer based Materials for Gas Sensors
Grade
1.1
Authors
Year
2016
Pages
45
Catalog Number
V415666
ISBN (eBook)
9783668662407
ISBN (Book)
9783668662414
File size
1242 KB
Language
English
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nil
Quote paper
Muzzaffar Ahmad Mir (Author)Sheikh Abdul Majid (Author)Muzzaffar Ahmad Bhat (Author), 2016, Zeolite-polymer based Materials for Gas Sensors, Munich, GRIN Verlag, https://www.grin.com/document/415666

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