JP5867670B2 - Method for decomposing and removing volatile organic compounds - Google Patents

Description

  The present invention relates to a method for decomposing and removing a volatile organic compound (hereinafter sometimes referred to as “VOC”) contained in an air environment such as a working environment or a living environment using ozone and a catalyst.

In recent years, environmental pollution due to organic compounds contained in the exhaust gas from chemical factories has become a problem, and this has been pointed out to adversely affect the human body.
In addition, volatile organic compound emission standards and environmental standards have been set by amending and partially implementing the Air Pollution Control Law, and the enforcement of the PRTR Law imposes an obligation to report exhaust gas from establishments to administrative agencies. Has been.

  Such environmental pollution problems caused by volatile organic compounds have become an urgent social problem, and in the future, development of efficient treatment methods for volatile organic compounds and establishment of technology for treating volatile organic compounds will be continued. Is expected.

  Conventionally, combustion methods, adsorption methods, and the like have been used as methods for removing volatile organic compounds. However, exhaust gas from indoor environments and small and medium-sized establishments is discharged in the region near room temperature under atmospheric pressure, and its concentration is as low as several hundred ppm, so it is a method for removing volatile organic compounds contained in exhaust gas. The combustion method and the adsorption method are not always efficient removal methods.

  On the other hand, a treatment technique for organic compounds in gas using ozone as an oxidizing agent has already been put into practical use as a deodorization technique for refrigerators and the like, and has been reported as a technique for decomposing low-concentration volatile organic compounds in a gas stream.

  As a catalyst material used in such a decomposition and removal method, for example, Patent Document 1 proposes a material in which manganese, iron, nickel, cobalt, chromium, molybdenum, lead, tungsten, copper, and vanadium are supported on activated alumina. . For example, Patent Document 2 proposes a catalyst body containing manganese carbonate in a catalyst composition. For example, Patent Document 3 proposes a method for decomposing and removing volatile organic compounds using a composite oxide of hydrophobic zeolite and manganese oxide as a catalyst. Further, for example, Non-Patent Document 1 proposes a catalyst material in which manganese oxide is supported on a carrier such as silica, alumina, silica, titania, zirconia.

Japanese Patent Laid-Open No. 53-30978 JP-A-5-317717 JP 2007-222697 A

Journal of Physical Chemistry, B 105, 4245-4253, (2001). Journal Of Catalysis, 227, 304-312, (2004). Catalysis Communications, 8 557-560, (2007). Proceedings of the Electrostatic Society of Japan '09, p. 195-198, (2009).

  However, in many cases, it was used for the decomposition of volatile organic compounds at tens to hundreds of ppm, even if the catalyst was thought to be effective in the order of ppm or less of the odor level because it was developed as a deodorization method. If so, generation of by-products such as formic acid is a problem (Non-patent Documents 2 and 3). These cause secondary environmental pollution and odor, and are also causative substances that cause a decrease in catalyst function (Non-patent Document 4).

  On the other hand, in the medium to long term, it is necessary to consider restrictions on the use of toxic heavy metals as materials constituting the catalyst. For example, in the regulations related to specific chemical substances, “(bi) chromic acid and its salts”, “vanadium pentoxide”, “nickel carbonyl”, “manganese and its compounds” and the like are expected. This is the case with some of the active elements of the catalysts produced. Even if the catalyst itself is not toxic, there are concerns about the release into the environment at the time of production and disposal, that is, “environmental risk over the life cycle”, and there is a possibility that these substances will also be asked for “environmental risk” in the future.

  The present invention has been made in order to solve the above-mentioned problems, and the object thereof is to improve the efficiency of volatile organic compounds in gas by ozone and a catalyst with less environmental risk not only at high temperatures but also at low temperatures around room temperature. An object of the present invention is to provide a method for decomposing and removing a volatile organic compound that can be well oxidized and decomposed into carbon dioxide and can suppress the formation of organic by-products such as formic acid.

  As a result of intensive studies on a method for easily ozonolysis-removing volatile organic compounds in a gas stream even at a low temperature around room temperature, the present inventors have found that by using a catalyst having silver nanoparticles supported on zirconium oxide, The present inventors have found a method capable of decomposing and removing organic compounds very efficiently and completed the present invention.

The method for decomposing and removing a volatile organic compound according to the present invention for solving the above-mentioned problem is that 1.0% by weight to 20% by weight of silver nanoparticles having a number average particle size of 0.7 nm to 11.5 nm with respect to zirconium oxide. It is characterized by decomposing and removing volatile organic compounds by using a catalyst and ozone supported by weight%.

  According to the present invention, since silver nanoparticles are supported in an amount of 1.0% to 20% by weight with respect to zirconium oxide as a carrier, stable and regular regeneration treatment is possible, and volatile organic compounds are added. It can be efficiently converted to carbon dioxide by oxidative decomposition with ozone. Further, an expensive platinum group element, which is an element required for the combustion catalyst, is not required, and a cheaper material can be used.

  In the method for decomposing and removing a volatile organic compound according to the present invention, the volatile organic compound is preferably decomposed and removed at a temperature of 80 ° C to 150 ° C.

  According to this invention, volatile organic compounds in a gas stream can be efficiently decomposed and removed using ozone at a temperature of 150 ° C. or lower, which could not be realized with a conventional combustion catalyst.

  In the method for decomposing and removing a volatile organic compound according to the present invention, the volatile organic compound is preferably 1 ppm to 500 ppm.

  According to the present invention, 1 ppm to 500 ppm of volatile organic compounds can be decomposed and removed, so that the present invention can be applied not only to improvement of the working environment but also to countermeasures for odors in a general living environment or a medical site where a comfortable space is required.

  According to the present invention, since silver nanoparticles having a number average particle size of 0.7 nm to 11.5 nm are supported on zirconium oxide, not only decomposition and removal of volatile organic substances but also low concentration even without ozone prior to decomposition and removal. Volatile organic substances can be removed by adsorption.

  In the method for decomposing and removing a volatile organic compound according to the present invention, the volatile organic compound may be one or more selected from benzene, toluene, xylene, ethylbenzene, ethylene oxide, acetaldehyde, formaldehyde, and dichloromethane. preferable.

  According to this invention, other aromatic, oxygen-containing, chlorinated hydrocarbons can be efficiently decomposed and removed.

  According to the method for decomposing and removing a volatile organic compound according to the present invention, by using a composite oxide in which silver nanoparticles are supported on zirconium oxide as a catalyst, the volatile organic compound in the gas stream can be oxidized very efficiently. While being able to convert into carbon, the production | generation of organic by-products, such as formic acid, can be suppressed.

It is a reaction system figure used in the Example of this invention. It is a graph of the toluene decomposition performance which concerns on the Example of this invention. It is a graph of the toluene decomposition performance which concerns on the Example of this invention. It is a graph of toluene decomposition performance and adsorption performance concerning the example of the present invention. It is a graph of formic acid generation | occurrence | production suppression which concerns on the Example of this invention. It is a graph which shows the amount of ozone required for decomposition | disassembly of toluene which concerns on the Example of this invention. It is an electron micrograph of the catalyst surface which concerns on the Example of this invention.

  Hereinafter, a method for decomposing and removing a volatile organic compound according to the present invention will be described with reference to the drawings. The technical scope of the present invention is not limited to the following embodiments.

  The catalyst used in the method for decomposing and removing volatile organic compounds according to the present invention is a composite oxide obtained by using zirconium oxide as a carrier and supporting silver nanoparticles on this. A composite oxide is not a compound in which zirconium oxide and silver nanoparticles are merely in a physical mixed state but a compound having a chemically bonded state / interaction.

  Zirconium oxide is preferably one having a high adsorption capacity for volatile organic compounds at the temperature at which the catalyst is actually used, specifically at 25 to 150 ° C.

Zirconium oxide may be any thermally stable material having a composition of ZrO ₂ . The specific surface area of the zirconium ^{oxide, 10m 3 / g~100m 3 / g} , preferably from ^{35m 3} / ^g~100m 3 / g. When the specific surface area is within this range, when silver particles are supported on zirconium oxide as a carrier, the silver particles can be maintained in a nano-sized size.

  Silver in the silver nanoparticle takes 0-valent and + 1-valent oxidation numbers. In the present invention, since oxidation and reduction easily occur in the reaction atmosphere, silver of any valence can be used at the start of the reaction.

  As the silver nanoparticles, it is preferable to use nanoparticles having a number average particle diameter of 0.7 nm to 11.5 nm, preferably 1.0 nm to 7.5. When the number average particle size is in this range, not only the volatile organic substances are decomposed and removed, but also the effect that the low concentration volatile organic substances can be adsorbed and removed without ozone prior to the decomposition and removal. The number average particle diameter was obtained by measuring an arbitrary number of electron micrographs, enlarging and copying the diameter of each silver nanoparticle with a vernier caliper (the geometric mean value of the major axis and the minor axis) and tabulating.

  The proportion of zirconium oxide and silver nanoparticles used is 1.0% to 20% by weight, preferably 3.0% to 20% by weight, based on zirconium oxide. If the silver nanoparticles are less than 1.0% by weight based on zirconium oxide, they may not exist as silver nanoparticles on the catalyst surface, and at least the catalytic action cannot be sufficiently exhibited. On the other hand, when the amount exceeds 20% by weight, a lump of micron-order silver nanoparticles is formed and has no catalytic action, so that inefficient silver increases.

As a method for producing ozone, a discharge method, a light emission method, a water decomposition method, or the like is generally used. It is preferable that ozone is allowed to coexist with 5 to 15 mol, preferably 6 to 12 mol with respect to the target volatile organic compound. Within this range, CO and HCOOH as partial oxidation products can be completely oxidized to CO ₂ .

  It is preferable to decompose and remove the volatile organic compound at a reaction temperature of 80 ° C to 200 ° C, preferably 80 ° C to 150 ° C. If the temperature is less than 80 ° C., surface adsorbed species that reduce the catalytic activity accumulate, so periodic regeneration treatment is required. However, if the temperature exceeds 200 ° C., ozone itself begins to decompose, Thermal catalytic reaction also begins, and the catalytic effect is reduced. Therefore, a reaction temperature of 150 ° C. or lower is preferable from the viewpoint of efficiently using ozone. Extremely effective as a method for decomposing and removing volatile organic compounds in gas, such as exhaust gas from offices, at such a reaction temperature, by oxidizing and removing them in the presence of ozone to quickly convert them into carbon dioxide. It is.

Moreover, a volatile organic compound is 1 ppm-500 ppm, Preferably it is 1 ppm-200 ppm or less. If the volatile organic compound is less than 1 ppm, ozone can be easily removed even if it is not the catalyst (for example, only ZrO _{2 of the} carrier), so that the effect of the present invention is diminished. Reaction may occur and the effect of coexisting ozone is lost.

  The volatile organic compound contained in the gas is preferably one or more selected from benzene, toluene, xylene, ethylbenzene, ethylene oxide, acetaldehyde, formaldehyde, dichloromethane and the like. Specifically, volatile organic compounds such as aromatic, oxygen-containing and chlorinated hydrocarbons can be decomposed and removed.

  In order to prepare a composite oxide in which silver nanoparticles are supported on zirconium oxide, a silver complex as a precursor is dissolved in water or an organic solvent such as alcohol, ketone or carboxylic acid or a mixed solvent system thereof in advance. Then, a method of impregnating and supporting zirconium oxide may be employed. Thereafter, the catalyst is dried and calcined in an oxidizing atmosphere at a temperature of 200 to 500 ° C. to obtain a catalyst as a composite oxide according to the present invention. The shape of the obtained catalyst may be any of powder, pellet, gel, honeycomb structure and the like.

  The thus obtained zirconium oxide catalyst carrying silver nanoparticles is put into a cylindrical reactor, and a gas stream containing volatile organic compounds and ozone is introduced into the reactor. Although ozone itself is harmful to the human body, residual ozone is completely decomposed and converted into molecular oxygen by adjusting the amount of catalyst.

  Hereinafter, the present invention will be described in detail with reference to examples and comparative examples. These descriptions do not limit the present invention.

Example 1
100 g of an aqueous solution of silver nitrate in which silver ions are dissolved so that the supported amount of silver is 5% by weight is added to 3.0 g of zirconium oxide (ZrO ₂ ), and after sufficient stirring, the mixture is allowed to stand overnight. Thereafter, the water vapor is removed at 40 ° C. using a rotary evaporator, and the dryer sample at 120 ° C. is sufficiently dried. Thereafter, the temperature was raised to 500 ° C. at a rate of 10 ° C. per minute in the air and calcined for 10 hours to obtain a 5 wt% zirconium oxide (Ag / ZrO ₂ ) catalyst.

(Comparative Example 1)
Except for changing the zirconium oxide aluminum oxide _(Al 2 _{O 3),} Example 1 aluminum oxide carrying 5 wt% silver in the same manner as (Ag / _Al 2 _{O 3)} to prepare a catalyst.

(Comparative Example 2)
A titanium oxide (Ag / TiO ₂ ) catalyst supporting 5% by weight of silver was prepared in the same manner as in Example 1 except that the zirconium oxide was changed to titanium oxide (TiO ₂ ).

(Comparative Example 3)
A silicon oxide (Ag / SiO ₂ ) catalyst supporting 5% by weight of silver was prepared in the same manner as in Example 1 except that zirconium oxide was changed to silicon oxide (SiO ₂ ).

(Comparative Example 4)
A magnesium oxide (Ag / MgO) catalyst supporting 5% by weight of silver was prepared in the same manner as in Example 1 except that zirconium oxide was changed to magnesium oxide (MgO).

[Measurement results and evaluation]
(Decomposition of toluene)
Using the catalysts obtained in Example 1 and Comparative Examples 1 to 4, the decomposition reaction of toluene using ozone as an oxidizing agent was performed by a fixed bed flow system. A schematic diagram of the reaction system is shown in FIG.
Nitrogen gas containing 200 ppm of toluene, pure nitrogen gas, and pure oxygen gas were mixed to prepare a reaction gas, and each gas flow rate was controlled by a thermal mass flow controller (TH3610, manufactured by Honma Riken).

  Ozone was synthesized with a creeping discharge type ozone generator (quartz tube type internal coil discharge electrode type) using pure oxygen as a raw material. The ozone concentration was measured with an ozone monitor (EG550, manufactured by Sugawara Jitsugyo Co., Ltd.).

  The catalyst was pretreated in advance in an oxygen stream for 1 hour (500 ° C.) to pretreat the catalyst. The analysis of the reaction gas was performed by an infrared spectrophotometer (FTS-135, manufactured by Bio-Rad) equipped with a gas cell having a long optical path (2.4 m). The reaction conditions were a toluene concentration of 200 ppm, an ozone concentration of 1000 ppm, an oxygen concentration of 20%, a gas flow rate of 500 ml / min, a catalyst amount of 0.2 g, and a reaction temperature of 100 ° C.

  FIG. 2 shows the removal rate (%) when toluene was decomposed using the catalysts of Example 1 and Comparative Examples 1-4. The condition is an initial ozone concentration of 250 to 2000 ppm. In the figure, the black triangle is Example 1, the white circle is Comparative Example 1, the black circle is Comparative Example 2, the white triangle is Comparative Example 3, and the white inverted triangle is Comparative Example 4.

Table 1 shows toluene decomposition rate (%), CO ₂ selectivity (%), and carbon material balance (%) when toluene was decomposed under the conditions of initial ozone concentration of 1000 ppm for Example 1 and Comparative Examples 1 to 4. , Ozone consumption rate (%), formic acid (HCOOH) production (ppm).

  As shown in FIG. 2, the toluene decomposition rate improved with the increase in the initial ozone concentration. Even at a reaction temperature of 100 ° C., only carbon dioxide and carbon monoxide were observed as products in the gas stream, and no formic acid was produced. Comparing Comparative Examples 1 to 4 and Example 1, it can be seen that Example 1 exhibits a high toluene removal rate in the range of the initial ozone concentration of 1000 to 2000 ppm. Here, the carbon balance is defined by equation (1).

[Chemical 1]
Carbon balance = (carbon dioxide + carbon monoxide production + formic acid production) / (toluene decomposition) x 7 (1)

  From the measurement results in Table 1, it was found that in Example 1, the carbon balance was 97% or more, and toluene was quantitatively converted to carbon dioxide and carbon monoxide. These also show similar reactivity with benzene and xylene. Moreover, although several tens ppm of formic acid was produced | generated with the catalyst of the comparative example 1 and the comparative example 2, it was proved that the production | generation of formic acid can be suppressed completely with the catalyst of Example 1.

(Example 2)
An aluminum oxide (0.5-Ag / ZrO ₂ ) catalyst supporting silver was prepared in the same manner as in Example 1 except that the supported amount of silver was changed to 0.5% by weight.

(Example 3)
An aluminum oxide (1.0-Ag / ZrO ₂ ) catalyst supporting silver was prepared in the same manner as in Example 1 except that the amount of silver supported was changed to 1.0% by weight.

Example 4
An aluminum oxide (3.0-Ag / ZrO ₂ ) catalyst supporting silver was prepared in the same manner as in Example 1 except that the amount of silver supported was changed to 3.0% by weight.

(Example 5)
An aluminum oxide (10-Ag / ZrO ₂ ) catalyst supporting silver was prepared in the same manner as in Example 1 except that the amount of silver supported was changed to 10% by weight.

(Example 6)
An aluminum oxide (20-Ag / ZrO ₂ ) catalyst supporting silver was prepared in the same manner as in Example 1 except that the amount of silver supported was changed to 20% by weight.

(Comparative Example 5)
As the zirconium oxide (ZrO ₂ ), a commercially available reagent (trade name: RSC-Hi, manufactured by Daiichi Rare Element Co., Ltd.) was used.

[Measurement results and evaluation]
(Toluene decomposition again)
Using the catalysts obtained in Example 1 and Comparative Examples 1 to 4, the decomposition reaction of toluene using ozone as an oxidizing agent was performed in the same manner as described above.

FIG. 3 shows the removal rate (%) when toluene was decomposed using the catalysts of Examples 1 to 6 and Comparative Example 5. The condition is an initial ozone concentration of 250 ppm to 2000 ppm. In the figure, the white square is Example 1, the black circle is Example 2, the white triangle is Example 3, the black triangle is Example 4, the black square is Example 5, and the white inverted triangle is Example 6.

  From FIG. 3, it can be seen that a high toluene removal ability is exhibited when the supported amount of silver is 1.0% by weight or more, and a very high removal ability of toluene is obtained when the supported amount of silver is 3.0% by weight or more.

  FIG. 4 shows the toluene removal rate (%) at a reaction temperature of 100 ° C. and an initial ozone concentration of 1000 ppm and the toluene adsorption amount (mole) on 0.2 g of various catalysts at 100 ° C. measured before the start of the reaction. In the figure, white circles indicate the toluene removal rate (%), and black circles indicate the toluene adsorption amount (mol).

FIG. 4 shows that when the loading amount is low, the toluene conversion rate is low and the toluene adsorption amount is not increased, and the characteristics of the ZrO ₂ surface are strongly reflected. It can be seen that when the supported amount of silver is 3.0% by weight or more and 20% by weight or less, the toluene conversion rate reaches 50% or more, and the amount of toluene adsorbed is 6 times or more of the unsupported amount, and the silver effect can be expected.

  From these measurement results, the loading of silver nanoparticles dramatically increases the adsorption ability of toluene possessed by zirconium oxide, and the reaction field of reactive oxygen species generated from toluene and ozone, which are reaction substrates, on the catalyst surface is improved. It can be seen that can be supplied.

(Measurement of formic acid content)
The amount of formic acid was measured for the catalysts of Examples 1-6. For the measurement, an infrared spectrophotometer (FTS-135, manufactured by Bio-Rad) equipped with a gas cell having a long optical path (2.4 m) was used.

  FIG. 5 shows the amount of formic acid (ppm) observed with the catalysts of Examples 1-6. In the figure, the white square is Example 1, the black circle is Example 2, the white triangle is Example 3, the black triangle is Example 4, the black square is Example 5, and the white inverted triangle is Example 6.

  FIG. 5 shows that formic acid is generated as a by-product with the initial ozone concentration in the catalyst having a silver loading of 0.5% by weight. A catalyst having a silver loading of 1.0% by weight shows a tendency to decrease when the initial ozone amount is increased. With the catalysts having a silver loading of 3.0 wt%, 5.0 wt%, 10 wt%, and 20 wt%, the formation of formic acid could be completely suppressed.

(Measurement of ozone requirement)
The ozone molecular weight necessary for removing one molecule of toluene when toluene was decomposed was measured. This was determined by dividing the amount of ozone consumed before and after the reaction by the amount of toluene removed as in equation (2).
[Chemical 2]
Required ozone amount = (Amount of ozone consumed) / (Toluene decomposition amount) (2)

  FIG. 6 shows the amount of ozone molecules (ozone required amount) necessary for removing one molecule of toluene when toluene is decomposed using the catalysts of Examples 1 to 6. The conditions were an initial ozone concentration of 250 ppm to 2000 ppm. In the figure, the white square is Example 1, the black circle is Example 2, the white triangle is Example 3, the black triangle is Example 4, the black square is Example 5, and the white inverted triangle is Example 6.

  FIG. 6 shows that a catalyst having a silver loading of 1.0% by weight or more requires 7 to 10 molecules relative to toluene. However, when the amount of silver supported is 1.0% by weight, the same amount of ozone is consumed, but the generation of formic acid cannot be suppressed. Therefore, the activity required for decomposing from formic acid to carbon dioxide without by-producting it. It can be seen that in order to generate oxygen species from ozone, it is necessary to support 3.0% by weight or more of silver nanoparticles on the zirconium oxide support.

  As for the amount of ozone necessary for decomposing volatile organic substances (VOC), about 10 molecules of ozone are necessary for decomposing one molecule of VOC in benzene, xylene and dichloromethane.

  The catalyst used in the catalytic oxidation method using volatile organic substance (VOC) ozone according to the present invention shows that the combination of mere silver and zirconium oxide cannot suppress the generation of secondary organic by-products. ing. It is necessary to prepare a material in which silver nanoparticles of several nanometers having a strong interaction with zirconium oxide are supported on the oxide with a high degree of dispersion and a certain amount.

(Measurement by electron microscope)
Silver nanoparticles were observed with an electron microscope (EM-002B, manufactured by TOPCON). The result is shown in FIG. FIG. 7 is a photograph showing the state (existence state) of silver on the catalyst surface taken with an electron microscope. From FIG. 7, it was confirmed that silver nanoparticles of 2 nm to 3 nm were dispersed and supported on the surface of zirconia oxide. A catalyst having a silver loading of 0.5% by weight could not confirm silver nanoparticles in the field of view because the loading was small. On the other hand, when the loading is increased, extremely coarse silver nanoparticles (silver lump) may be observed in a small part, but the presence of silver nanoparticles of several nanometers is confirmed in a wide range in any sample. It was. However, it is considered that the coarsened silver nanoparticles have a low interaction with zirconium oxide as a carrier and have no catalytic activity or are extremely low.

In addition, since silver nanoparticles of the same size are observed on Ag / TiO _{2 which} is less active than Ag / ZrO ₂ catalyst, not only simple particle size but also strong interaction with ZrO ₂ support It is considered that the silver nanoparticles having the above-mentioned properties give these characteristics.

DESCRIPTION OF SYMBOLS 1 Nitrogen gas 2 Oxygen gas 3 Flow control apparatus 4 Toluene 5 Ozone generator 6 Electric furnace 7 Catalyst 8 Long light path gas cell 9 FT-IR spectrophotometer 10 Ozone concentration meter

Claims (4)

Volatile organic compounds are decomposed and removed using a catalyst carrying 1.0 to 20% by weight of silver nanoparticles having a number average particle size of 0.7 to 11.5 nm with respect to zirconium oxide and ozone. A method for decomposing and removing volatile organic compounds.   The method for decomposing and removing a volatile organic compound according to claim 1, wherein the volatile organic compound is decomposed and removed at a temperature of 80C to 150C.   The method for decomposing and removing a volatile organic compound according to claim 1 or 2, wherein the volatile organic compound is 1 ppm to 500 ppm. The volatile organic compound according to any one of claims 1 to 3 , wherein the volatile organic compound is one or more selected from benzene, toluene, xylene, ethylbenzene, ethylene oxide, acetaldehyde, formaldehyde, and dichloromethane. Of removing organic organic compounds.