Volatile Organic Compounds (VOCs) are compounds with carbon and hydrogen whose
boiling point ranges from 50-250°C. VOCs include acetone, formaldehyde,
toluene, etc. (Health Canada, 1995). The sources of these
VOCs are biogenic like plants, animals, fungi, etc. and anthropogenic like paints
and coatings and chlorofluorocarbons (Goldstein and Galbally,
2007). These compounds, even in minor quantities, affect the indoor air
quality (Silva et al., 2008). The ill effects
of exposure to these compounds in humans include sick building syndrome (Silva
et al., 2008), respiratory disorders and sensory irritation (Salonen
et al., 2009) and cancer (Guo et al.,
Ethanol in its native form is not a harmful agent but formaldehyde, the oxidation
product of ethanol is toxic. Formaldehyde causes optical and nasal irritation
and sick building syndrome (Takigawa et al., 2010).
Formaldehyde is released into the environment from urea-formaldehyde resins
of plywood in furniture, adhesives and water pre-treatment processes and as
a natural metabolite (Salonen et al., 2009).
Acetone which is harmless at lower concentrations, causes sensory irritations
at very high concentrations.
A method to sense the presence of VOCs in the environment is essential. The
currently available methods for sensing of VOCs are chemiresistive sensing,
optical fibre sensing (Silva et al., 2008) and
potentiometric sensing using NiO thin film electrode (Sato
et al., 2010). Metal Organic Frameworks (MOFs) can also be used for
sensing of VOCs (Zou et al., 2009).
MOFs are porous, crystalline and high surface area coordination compounds with
an inorganic cluster in the vertex and an organic ligand as the spacer in their
unit cell (Rowsell and Yaghi, 2005). MOF-5 is made up
of [Zn4O]6+ in the vertex and terephthalic acid as the
spacer in the unit cell (Civalleri et al., 2006).
MOF sensors can be fabricated by making a self assembled monolayer of MOF on
gold substrates (Shekhah et al., 2011) or by
making pellets (Achmann et al., 2009).
In this study we report the use of MOF-5 pellets for sensing of VOCs like ethanol,
formaldehyde and acetone.
MATERIALS AND METHODS
MOF-5 was synthesized by the second procedure reported in literature (Saha
et al., 2009; Iswarya et al., 2012).
The precursors used for the synthesis of MOF-5 were zinc nitrate hexahydrate,
terephthalic acid, triethyl amine, hydrogen peroxide and chloroform. All the
precursors were procured from Merck, India except terephthalic acid which was
procured from LobaChemie, India. They were used as such without further purification.
Fourier transform infra-red (FTIR) spectroscopy: FTIR spectrum was recorded in FTIR spectrophotometer (Spectrum-100, Perkin-Elmer, USA) to confirm the presence of the required functional groups in the synthesized sample. The sample was scanned over the spectral region of 400 to 4000 cm-1 (10 scans).
X-ray diffraction analysis: X-ray diffraction pattern of the synthesized sample was recorded in a X-ray diffractometer (D8 Focus, Bruker, Germany) with CuK source over the range of 5°<2θ<40°.
Transmission electron microscopy: The sample was imaged in a 200 kV Field Emission Transmission Electron Microscope (JEM 2100F, JEOL, Japan) to confirm its porous nature. The sample was dispersed in ethanol and a drop of the dispersion was placed on a copper grid for imaging.
Micrometrics: Pore size, pore size distribution, pore area and volume analysis were done in a mercury intrusion porosimeter (AutoPore IV 9500 V 1.07, Micromeritics, USA). Analysis was done based on the amount of mercury that intrudes into the pores at pressure in the range of 0-25000 psi.
Sensing studies: Pellets of 6 mm diameter and 3 mm thickness were made
from the synthesized material in a tablet pressing machine (KI356, Khera Instruments,
India). Electrical contacts were made to the pellet and resistance change on
exposure to gases was measured using an electrometer (6517A, Keithley Instruments
Inc., USA). Measurements were made by keeping the pellet in a home made sensing
chamber of 6.4 L capacity with a septum for introducing the gases as described
in our earlier work (Sivalingam et al., 2012).
Sensing studies for ethanol (Merck, India) at 5 ppm, formaldehyde (Merck, India) at 5-100 ppm and acetone (Merck, India) at 5-15 ppm were done in the above described chamber by introducing the gas through the septum.
Fourier transform infra-red spectrum: Figure 1 shows
FTIR spectrum of MOF-5 sample.
|| FTIR spectrum of MOF-5
|| X-ray diffractogram of MOF-5
Two sharp intense peaks were seen at 1581 and 1361 cm-1. Several
small peaks in the range of 1225-950 cm-1 and 800-650 cm-1
and 500 cm-1 were seen. A broad peak at 3403 cm-1 was
X-ray diffractometry: Figure 2 shows the X-ray diffractogram of MOF-5. A sharp intense peak at 2 (theta) = 9.08 was seen.
Transmission electron microscopy: Figure 3 shows the Transmission Electron Micrograph of MOF-5. The image reveals the presence of unordered pores of dimensions below 10 nm.
Micrometric analysis: Figure 4 shows the graph of cumulative pore area (m2 g-1) vs. pore size diameter (μm). BET surface area obtained from porosimeter analysis is 230 m2 g-1. The sample was found to be 66.86% porous and the total pore area was 4.754 m2 g-1.
Sensing studies: Figure 5 shows the graph representing the response of MOF-5 towards acetone, formaldehyde and ethanol at 5 ppm. The response was found to be 0.0872, 0.5216 and 30.36 for acetone, formaldehyde and ethanol, respectively.
Figure 6 shows the graphical representation of response of
MOF-5 on presenting formaldehyde gas at different concentrations.
|| Transmission electron micrograph of MOF-5
||The cumulative pore area at various pore size diameter for
cycle 1 intrusion
||The response of MOF-5 towards acetone, formaldehyde and ethanol
at 5 ppm
||The response of MOF-5 towards formaldehyde at various concentrations
||The response of MOF-5 towards acetone at various concentrations
The response increased linearly and it was 358.73 at 100 ppm of formaldehyde.
Figure 7 shows the graphical representation of response of MOF-5 towards acetone at different concentrations. The response was found to be 0.087, 0.294 and 0.313 for 5, 10 and 15 ppm of acetone, respectively.
FTIR spectrum (Fig. 1) confirms the presence of the required
functional groups present in MOF-5. The peaks at 1581 and 1361 cm-1
are characteristic of the symmetric and asymmetric stretching of C-O bonded
to benzene ring present in the linker (Sabouni et al.,
2010). Several small peaks seen in the region of 1225-950 cm-1
and 800-650 cm-1 are because of the in-plane and out of the plane
stretching of the aromatic C-H groups of the benzene ring present in terephthalic
acid (Coates, 2000). The peaks around 3000 cm-1 are because of the
adsorbed moisture content (Coates, 2000).
The X-ray diffractogram (Fig. 2) is similar to the one obtained
by Saha et al. (2009). The peak at 2θ =
9.08° with d = 9.73 Å is characteristic of MOF-5 and confirms that
it is highly crystalline.
Transmission electron micrograph (Fig. 3) shows that the sample is highly crystalline and porous with unordered pores. The presence of large number of pores contributes to the high surface area of MOF-5.
The porous nature of the sample is confirmed by the total porosity of 66.86% obtained from porosity analysis. BET surface area of 230 m2 g-1 (Fig. 4) is a reasonably good surface area and it makes MOF-5 suitable for sensing experiments.
Figure 6 shows that the sensitivity towards ethanol is better than that of formaldehyde and acetone. So the response of MOF-5 at higher concentrations of acetone and formaldehyde were tested. Sensitivity towards formaldehyde increases linearly as the concentration of formaldehyde increases from 5-100 ppm. There is an increase in the sensitivity towards acetone when the concentration of acetone increases from 5-10 ppm and the increase at 15 ppm is very low.
MOF-5 shows good sensitivity towards ethanol even at concentrations as low as 5 ppm. MOF-5 shows a linear increase in response as concentration of formaldehyde increases. Increase in response at increasing concentrations of acetone stabilizes at 10-15 ppm and the highest concentration at which it can sense acetone is 10 ppm. The material is highly selective towards ethanol. From the results obtained it is evident that MOF-5 is a promising candidate for sensing of VOCs in testing indoor air quality.
This study was supported by (i) PG teaching grant No. SR/NMPG-16/2007 of Nano
Mission Council, Department of Science and Technology (DST), India.