Photocatalytic degradation of acetone and methanol in a flow-through photoreactor with immobilized TiO2

Article abstract of DOI:10.1007/s11164-015-1989-2

Volatile organic compounds must be used in many industrial processes, involving their emission to the environment in gaseous form. This paper describes the degradation of vapors of acetone and methanol in air by photocatalysis. To achieve this, a tubular flow-through reactor with TiO₂ photocatalyst irradiated by either UV-A or UV-C lamps is used. The irradiation wavelength was the main studied parameter. Equal volumetric feed of liquid in the air stream for both pollutants resulted in a concentration of acetone of about 275 ppm and of methanol of about 500 ppm. In separate experiments, using the UV-A lamps, the conversion of acetone was 29 %, while for methanol it was 72 %. The faster oxidation of methanol relative to acetone can be explained by their different molecular structure. For UV-C lamps, the conversion of acetone significantly increased (to 60 %), while that of methanol increased only slightly (to 80 %). The contribution of photolysis to the degradation of both compounds was considered and finally evaluated to be negligible in this setup.

Full text of DOI:10.1007/s11164-015-1989-2

Res Chem Intermed
DOI 10.1007/s11164-015-1989-2
Photocatalytic degradation of acetone and methanol
in a flow-through photoreactor with immobilized TiO2
1
Frantisek Moulis Josef Krysa
1
•
ˇ
´
Received: 24 October 2014 / Accepted: 26 February 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Volatile organic compounds must be used in many industrial processes,
involving their emission to the environment in gaseous form. This paper describes
the degradation of vapors of acetone and methanol in air by photocatalysis. To
achieve this, a tubular flow-through reactor with TiO2 photocatalyst irradiated by
either UV-A or UV-C lamps is used. The irradiation wavelength was the main
studied parameter. Equal volumetric feed of liquid in the air stream for both pol-
lutants resulted in a concentration of acetone of about 275 ppm and of methanol of
about 500 ppm. In separate experiments, using the UV-A lamps, the conversion of
acetone was 29 %, while for methanol it was 72 %. The faster oxidation of
methanol relative to acetone can be explained by their different molecular structure.
For UV-C lamps, the conversion of acetone significantly increased (to 60 %), while
that of methanol increased only slightly (to 80 %). The contribution of photolysis to
the degradation of both compounds was considered and finally evaluated to be
negligible in this setup.
Keywords Gas phase ꢀ Photocatalysis ꢀ Titanium dioxide ꢀ Acetone ꢀ Methanol ꢀ
UV-A ꢀ UV-C
ˇ
Frantisek Moulis
moulisf@vscht.cz
&
&
1
´
Josef Krysa
josef.krysa@vscht.cz
Department of Inorganic Technology, University of Chemistry and Technology, Prague,
´
Technicka 5, 166 28 Prague 6, Czech Republic
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F. Moulis, J. Krysa
Introduction
Volatile organic compounds (VOCs) are liberated to the environment [1] as harmful
agents in many industrial applications [2]. The extent of such pollution differs: very
low levels of pollution on the order of hundreds of ppb or units of ppm can be found
in some residential areas, while on the other hand, exhaust gases from industrial
plants show contamination several orders higher. Presently available and used
purification methods for air decontamination need to be chosen according to the
process, e.g., the pollution level. Furthermore, they have limited efficiency, may
produce secondary waste, or may have other restrictions, e.g., production of
secondary waste in the form of combustion products, spent carbon, waste water or
condensate, which must be treated subsequently [3]. Photocatalytic degradation [4]
is a method that could replace conventional methods and address their constraints.
Its great advantage is the capability to decompose pollutants towards nonharmful
products in one technological step at ambient conditions. This capability has been
known since 1964, when degradation of ethylene and propylene was studied [5].
Interest in photocatalytic technologies started to grow in the 1990s [6]. The majority
of scientific papers on this topic consider photocatalysis in aqueous solution, but
registered patents deal rather with photocatalysis in the gaseous domain, e.g., air
purification [7]. Continuous (flow-through) or batch reactors are used for testing
photocatalytic materials or the behavior of different pollutants under degradation. In
the case of potential industrial applications, continuous reactors are rather used
because of the relatively high air flow rate that must be processed [8]; this type is
used in this work as well. Often-studied pollutants are common solvents such as n-
hexane [9], n-butyl acetate [10] or toluene [11, 12]. Other pollutants are acetone and
methanol, which are used as organic solvents in the field of pharmaceuticals [13].
This paper thus describes the oxidative degradation of acetone and methanol in a
flow-through photoreactor with immobilized TiO2, paying special attention to the
influence of the wavelength of UV light.
Some environmental studies for these compounds are available. According to one
of these, acetone concentration can reach 16 ppb in an inhabited industrial area,
being together with acetaldehyde, formaldehyde, propionaldehyde, and butyralde-
hyde one of the most important air pollutants [14]. Another study focused on
working conditions in a furniture store. In this environment, acetone is again among
the most concentrated pollutants, which also include formaldehyde, acetaldehyde,
2-butanone, butyraldehyde, and hexaldehyde; the total concentration of VOCs was
evaluated as 0.8 mg/m3, with the contribution of acetone being 0.03 mg/m3
(13 ppb) [15].
Methanol is an important waste produced by the printed circuit industry. It is one
of the four most polluting compounds in the exhaust air (together with ammonia,
glycol ethers, and formaldehyde); in the waste water, methanol is the foremost
pollutant, followed by copper and its compounds [16]. Overall mineralization
reactions of acetone and methanol are expressed by the equations
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Photocatalytic degradation of acetone and methanol
TiO2;hm
acetone C3H6O þ 4O2 ꢁ! 3CO2 þ 3H2O
TiO2;hm
3
methanol CH4O þ O2 ꢁ! CO2 þ 2H2O
2
To test commercially available photocatalytic filters, a photocatalytic oxidation
(PCO) unit with UV irradiation was developed [17]. Using this unit, degradation of
several VOCs was studied in the concentration range up to 2 ppm with flow rate in
the range of 41–255 Nm3/h. One conclusion of that study was a series of removal
efficiencies, with alcohols being removed most efficiently, followed by ketones,
aromatics, and alkanes. Decrease in relative humidity results in increased
conversion. Similarly, the lower the inlet concentration, the higher the conversion
achieved.
One has to be aware of the role of photolysis, which is important in the case of
some organic compounds when UV-C lamps are used; For example, VUV lamps
(maximum emission at 254 nm and minor emission at 185 nm) have been shown to
improve the degradation efficiency of alkanes, alcohols, and aromatics [17].
Experimental
As described above, a continuous reactor was used in this work. The main reason is
its potential industrial application. This setup is suitable for monitoring outlet
concentrations and thus for verifying apparatus performance stability over time.
Flow rates on the order of Nm3/h can be treated thanks to the relatively large
inserted photocatalytic surface.
Fig. 1 Simplified scheme of the
apparatus with photoreactor
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F. Moulis, J. Krysa
The main parts of the apparatus are (in the flow direction): a membrane pump
supplying flow of up to 2.5 Nm3/h, a float flowmeter, a syringe pump for liquid
dosage (VOC), inlet analysis, a photoreactor, and outlet analysis. A simplified
scheme of the apparatus is shown in Fig. 1.
The photoreactor with height of 90 cm, diameter of 6 cm, and volume of 2.5 dm3
is made of a quartz tube surrounded by eight UV lamps, either UV-A (LT 30 W T8/
073 blacklight blue Narva, 365 nm) or UV-C (Sylvania ultraviolet G30 W,
254 nm). The light intensity in the middle of the photoreactor (where the
photocatalyst support is placed) was measured by a photodiode and evaluated as
10 mW/cm2 for UV-A lamps and 20 mW/cm2 for UV-C lamps. The spectra of the
lamps as measured by a USB4000 spectrophotometer (Ocean Optics Inc.) are given
in Fig. 2. Both types of lamps had equal input power (30 W), so that their efficiency
in the degradation process could be directly compared. An aluminum support
consisting of eight vertical blades was covered by TiO2 photocatalyst and placed in
the center of the photoreactor. TiO2 (P25, Degussa–Evonik) with Brunauer–
Emmett–Teller (BET) specific surface area of about 50 m3/g and anatase-to-rutile
ratio of 70:30 was used as photocatalyst. The total irradiated geometrical area was
34 dm2 and the catalyst mass was 6.9 g, resulting in average coverage of 2 mg/cm2.
The photocatalyst was activated for 1 h before each experiment in a flow of pure
air (2.5 Nm3/h) with the UV light switched on. After that, air (relative humidity
50 10 %) at flow rate of 1.2 (or 2.4) Nm3/h was artificially polluted by liquid
dosed at a flow rate of 1 ml/h. This resulted in inlet concentrations of 275 ppm for
acetone and 500 ppm for methanol.
Inlet and outlet analysis was carried out using two membrane pumps from which
streams were redirected by a multipath valve to a gas chromatograph, enabling
programmable and fully automatic sampling. Quantitative analysis was carried out
using a Varian CP-3800 gas chromatograph with a CP-Sil 5 CB 15 9 0.25 (0.25)
capillary column and flame ionization detector. Nitrogen was used as carrier gas.
Fig. 2 Emission spectra of used UV lamps
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Photocatalytic degradation of acetone and methanol
Results and discussion
Degradation of acetone and methanol was studied in separate, consecutive
experiments. Photocatalytic experiments with UV-C lamps were done first, then
the lamps were replaced by UV-A ones, and finally the support with photocatalyst
was removed and photolysis was studied.
Photocatalysis with UV-C lamps
The progress of the two subsequent experiments with acetone and methanol is
shown in Fig. 3. The inlet and outlet concentrations were measured alternately.
In the case of acetone, the outlet concentration stabilization was slower, which
may be due to insufficient activated photocatalytic surface in the beginning of the
experiment. In both experiments, when the outlet concentration was stable, the
conversion could be calculated; higher conversion was reached in the case of
methanol (80 %), whereas the acetone degradation was 60 %. Results and
degradation rates are summarized in Table 1.
Photocatalysis with UV-A lamps
After replacement of the lamps, the experiments were repeated with the same inlet
concentrations (Fig. 4). In this setup, a final dark phase was added to check the
system integrity; it could be clearly seen that, after switching off the lamps, the inlet
and outlet concentrations equalized. There is an evident decrease in the conversions
compared with the preceding experiments. This is undoubtedly due to the lower
light intensity of UV-A light. The conversion of acetone halved (from 60 to 29 %),
whereas that of methanol decreased by only one-tenth (from 80 to 72 %). The
calculated degradation rates are again summarized in Table 1.
Fig. 3 Photocatalytic degradation with UV-C lamps
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F. Moulis, J. Krysa
Table 1 Results of photocatalytic degradation
Lamp and
Pollutant Inlet
Outlet
concentration concentration (%)
Conversion Degradation rate, r/Jhm
its Jhm (mol/m2/h) (1 ml/h)
r (mmol/m2/h)
(%)
(ppm)
(ppm)
UV-C 1.53
UV-A 1.10
Acetone
275
110
100
195
140
60
80
29
72
24
58
12
52
1.6
Methanol 500
Acetone 275
Methanol 500
3.8
1.1
4.7
Fig. 4 Photocatalytic degradation with UV-A lamps
Fig. 5 UV–Vis spectra of acetone and methanol in liquid phase (pure compound) and in gaseous phase
(space over liquid in a closed cuvette); UV-C and UV-A maxima at 254 and 365 nm are shown as well
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Photocatalytic degradation of acetone and methanol
The difference between the degradation rates of the two compounds might be
explained by their different molecular structure; decomposition of the larger C3
acetone molecule requires more complex bond breakage in comparison with the
smaller C1 methanol molecule. As a result, we get a several times higher molar
degradation rate for methanol.
UV light absorption of acetone and methanol
There could be other effects than only the energy of UV radiation. UV–Vis spectra
are shown for both compounds in Fig. 5. Their absorption in the gaseous phase
differs. Acetone absorbs significantly at 254 nm (UV-C maximum), whereas
methanol absorbs very little. On the other hand, methanol absorbs significantly only
below 200 nm, as seen from Fig. 5 and reported in [18]. It seems that photolysis at
254 nm is possible in the case of acetone but not in the case of methanol.
Theoretically, acetone may decompose already under radiation at wavelength of
313 nm [19]. The photolysis contribution thus has to be evaluated.
Photolysis contribution
The effect of direct photolysis was studied in the photoreactor (without
photocatalyst support) using UV-C lamps. The phases of the experiment in Fig. 6
are: with photoreactor by-passed, with photoreactor with UV-C light off, and with
UV-C light on. It is clearly shown that the decrease in acetone that could be caused
by photolysis is not measurable. Direct photolysis of acetone in our apparatus was
thus excluded.
The effect of photolysis on acetone degradation using the UV-C lamps was
evaluated previously for UV-C lamps with emission maxima at 254 and 185 nm
[17]. In comparison with other studied compounds (ethanol, 1-butanol, hexane,
Fig. 6 Direct photolysis of acetone
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F. Moulis, J. Krysa
octane, toluene, p-xylene, and methylethylketone), the conversion of acetone by
photolysis degradation was very low (below 1 % for inlet concentration of
0.5 ppm), whereas for other compounds the conversion was in the range of 3–17 %.
Those authors explained this by the relatively small electron-withdrawing inductive
effect of the carbonyl group, which is why the contribution of photolysis to ketone
degradation is negligible [17].
Photolysis of acetone in gaseous phase was also studied by other authors at
248 nm and concentration of ca. 212 ppm, but in an apparatus equipped with a
pulsed laser [20]. The measured quantum yield of acetal was close to 1; in other
words, almost complete photolysis of acetone was found.
Effect of flow rate and inlet concentration
Photocatalytic degradation of methanol on TiO2 using UV-A lamps was studied at
two different flow rates (1.2 and 2.4 Nm3/h) and inlet concentrations (ca. 500 and
240 ppm), as shown in Fig. 7. The obtained conversions and degradation rates are
presented in Table 2.
A decrease in conversion with increasing air flow rate is a generally observed
phenomenon, which can be easily explained by the shortened residence time,
disadvantaging the progress of any heterogeneous reaction [17]. It can be seen that
increase in the flow rate (keeping the inlet concentration constant) does not result in
an increase in the degradation rate. This proves that the process is not limited by
mass transfer in the gaseous phase, the main reactor works in the kinetic regime,
there is no concentration gradient perpendicular to the catalytic layer, and the axial
concentration profile is supposed to be linear.
Similar experiments were done by Zhang and Liu [21], who studied the effect of
flow rate on hexane decomposition in a photoreactor with geometry (inner tube
diameter 64 mm) similar to that used in this work, so the hydrodynamics of the
Fig. 7 Photocatalytic degradation of methanol at different flow rates and inlet concentrations
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Photocatalytic degradation of acetone and methanol
Table 2 Results of methanol photocatalytic (UV-A) degradation
Air flow rate
(Nm3/h)
Methanol
(ml/h)
Inlet
concentration
(ppm)
Outlet
concentration
(ppm)
Conversion
(%)
Degradation rate
(mmol/m2/h)
1.2
2.4
2.4
1
1
2
520
240
480
180
100
310
65
58
35
49
40
49
reactor is comparable. They found that, at low flow rates (up to 0.72 Nm3/h), the
reaction was controlled by mass transfer (degradation rate changing with the flow
rate), whereas at higher flow rates (above 0.72 Nm3/h), there was no influence and
thus no mass transfer limitation. Similar results were reported in the case of
trichlorethylene [22].
Increase in the inlet concentration results in an increase of the degradation rate by
about 20 %. This can be explained as follows: The degradation rate is directly
proportional to the concentration of adsorbed pollutant species. If the substance
adsorption obeys the Langmuir isotherm, the surface concentration is a nonde-
creasing function of the bulk concentration, which is in agreement with data in
Table 2, where the degradation rate increases when the inlet concentration
increases. The positive effect of the inlet concentration is most evident at low
pollution levels (ca. 25–125 ppm), where the degradation rate is directly propor-
tional to the bulk concentration and can be described by first-order kinetics [23]. As
the concentration rises, free active sites on the catalyst surface become occupied and
the degradation rate reaches its maximal (limit) value.
Conclusions
The choice of the irradiation source for photocatalytic degradation of organic
pollutants in gaseous phase may play a crucial role in the case of some substances.
In this work, we compared acetone and methanol. At equal input power (30 W
lamps), conversion of acetone could be doubled when UV-A lamps were replaced
by UV-C ones. Methanol seemed to be less sensitive.
The contribution of photolysis to the degradation of both compounds was
considered. Methanol photolysis was excluded directly due to negligible UV-C light
absorption. Even if acetone absorbs UV-C light, the contribution of photolysis (for
UV-C lamps with emission at 254 nm and concentration ca. 250 ppm) was found to
be negligible.
A parametric study with variable flow rate and inlet concentration showed that
the reactor operates in the kinetic regime and the photocatalytic degradation in this
setup is not limited by mass transfer.
Acknowledgments We acknowledge financial support from specific university research (MSMT no.
20/2014) and from the Ministry of Industry and Trade of the Czech Republic (MPO FR-TI3/242).
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