Capture of formaldehyde by adsorption on

Journal of Hazardous Materials 300 (2015) 711–717

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Capture of formaldehyde by adsorption on nanoporous materials Jean-Pierre Bellat a,∗ , Igor Bezverkhyy a , Guy Weber b , Sébastien Royer b , Remy Averlant c , Jean-Marc Giraudon c , Jean-François Lamonier c a b c

Université Bourgogne Franche-Comté, ICB UMR 6303 CNRS, 9 Alain Savary BP 47870, 21078 Dijon, France Université de Poitiers, IC2MP UMR 7285 CNRS, 4 Michel Brunet 86022, Poitiers Cedex, France Université de Lille 1 Sciences et Technologies, UCCS UMR 8181 CNRS, Cité Scientifique, 59652 Villeneuve d’Ascq, France

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Adsorption

of pure gaseous formaldehyde is studied on zeolites, silica, carbon and MOF. • High-resolution adsorption–desorption isotherms are measured by TGA. • Na or Cu FAU zeolites have the best adsorption affinities and capacities. • These zeolites are very good candidates to capture or detect formaldehyde.

a r t i c l e

i n f o

Article history: Received 29 May 2015 Received in revised form 20 July 2015 Accepted 30 July 2015 Available online 3 August 2015 Keywords: Formaldehyde Adsorption Zeolites Activated carbon Silica MOF

a b s t r a c t The aim of this work is to assess the capability of a series of nanoporous materials to capture gaseous formaldehyde by adsorption in order to develop air treatment process and gas detection in workspaces or housings. Adsorption–desorption isotherms have been accurately measured at room temperature by TGA under very low pressure (p < 2 hPa) on various adsorbents, such as zeolites, mesoporous silica (SBA15), activated carbon (AC NORIT RB3) and metal organic framework (MOF, Ga-MIL-53), exhibiting a wide range of pore sizes and surface properties. Results reveal that the NaX, NaY and CuX faujasite (FAU) zeolites are materials which show strong adsorption capacity and high affinity toward formaldehyde. In addition, these materials can be completely regenerated by heating at 200 ◦ C under vacuum. These cationic zeolites are therefore promising candidates as adsorbents for the design of air depollution process or gas sensing applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Formaldehyde is a volatile organic compound (VOC) well known in the medical sector for its disinfectant and biocide properties. This chemical is also used in many other industrial applications. For example, formaldehyde is a common precursor for the synthesis of various resins used in the textile industry, the automobile sector and more extensively the wood industry for the manufac-

∗ Corresponding author. E-mail address: [email protected] (J.-P. Bellat). http://dx.doi.org/10.1016/j.jhazmat.2015.07.078 0304-3894/© 2015 Elsevier B.V. All rights reserved.

ture of wood-composites as plywood or chipboard. Formaldehyde is a highly toxic gaseous VOC. It is now admitted by all the medical authorities that the exposure of animals and humans to formaldehyde can lead to cancer [1]. In Europe, formaldehyde is recognized as carcinogen from 2015 and its domestic use is now forbidden. In a near future, this chemical will be probably prohibited in industry. Nevertheless, as no viable alternative to this compound has been found it will be still used in the coming years. Consequently, it is urgent for the safety of humans working with formaldehyde based materials to control the gaseous emission of formaldehyde and to develop gas sensors and air treatment processes able to completely eliminate this toxic chemical. As its occupational exposure limit

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J.-P. Bellat et al. / Journal of Hazardous Materials 300 (2015) 711–717

value is fixed at 0.5 ppm per 8 h [2], one needs to design materials able to trap this molecule at very low concentration levels. Though formaldehyde is one of the most widespread VOCs, its removal by adsorption on solids has not been extensively studied. Most of studies quoted in the literature concern the adsorption of formaldehyde on activated carbons [3–10]. These works showed that the adsorption of formaldehyde depends on pore geometry and surface chemistry of the adsorbents. As we could expect, the adsorption of formaldehyde is favored by a small pore size, a high specific surface area and a high microporous volume of the adsorbent. Nevertheless, the chemical properties of the adsorbent surface are the most important parameters. The presence of functional groups such as hydroxyl groups, nitrogen oxides, pyridonic and pyridinic structures as well as aluminum-, silver- or copper coordinated metallic centers, enhances the adsorption affinity of the material for formaldehyde [11–14]. In addition, some experiences performed on activated carbons containing surface amino groups indicate that chemisorption of formaldehyde occurs on the surface [15]. Moreover, the adsorption properties of activated carbons for formaldehyde can be affected by the presence of water. Thus, Li et al. [16] have observed that the adsorbed amounts of formaldehyde significantly decreased in the presence of humidity. The adsorption of formaldehyde was also studied on various inorganic porous materials including silicas, phosphates and aluminosilicates. Furthermore, Srisuda and Virote [17] have investigated adsorption properties of amino group functionalized mesoporous silicas for formaldehyde. These materials have a high adsorption capacity (up to 1.2 g/g under 80 000 ppm of formaldehyde), which is significantly higher than those of activated carbons. As observed for activated carbons, the presence of surface amino groups on mesoporous silicas improves the adsorption capacity of formaldehyde. Nakayama et al. [18] and more recently Zhang et al. [19] have studied the adsorption of formaldehyde on zirconium phosphates. These materials can adsorb formaldehyde with adsorption capacity of about 0.20 g/g. However, adsorption kinetics seems to be extremely low (several days to reach the equilibrium). Moreover, a self-catalytic oxidation–reduction reaction of formaldehyde can occur, leading to the production of formic acid and methanol. Only a few works on the adsorption of formaldehyde on zeo-

lites can be found in the literature [20–22]. These microporous solids in particular the cationic forms of LTA zeolites and faujasites (FAU) which have a strong adsorption affinity, are evidenced as promising adsorbents to capture formaldehyde at very low concentrations. However, the data published on all these systems are not detailed enough to draw solid conclusions about the real potentialities of these materials. In particular, no high-resolution adsorption–desorption isotherms are reported in the literature allowing to obtain a detailed understanding of the adsorption process of formaldehyde, and comparison with the properties of adsorbents of different nature is not available. The objective of this work is to study different nanoporous adsorbents for the detection or capture of indoor formaldehyde by adsorption. In both applications, the role of the adsorbent that we look for is to concentrate formaldehyde, present in air at extremely low concentration, in order to allow its detection by gas sensors [23] or its efficient elimination by usual catalytic oxidation processes [24,25]. Thus, reversibility of the adsorption process, during the desorption step, is crucial to achieve a gas phase concentration of formaldehyde sufficiently high for carrying out a precise quantification or a complete oxidation. Consequently, only physisorption must take place in order to have a reversible adsorption–desorption process. This is the reason why a special attention has been paid to the measurements of high-resolution adsorption–desorption isotherms of pure gaseous formaldehyde, especially in the very low pressure range. 2. Experimental 2.1. Materials Adsorption of formaldehyde was studied on four types of adsorbents: FAU and LTA zeolites, a mesoporous silica, an activated carbon and a metal organic framework. Their microporous volume (Vmicro ), mesoporous volume (Vmeso ) and specific surface area (SBET ) were determined by nitrogen adsorption at −196 ◦ C, except for the LTA zeolite because nitrogen cannot enter the cavities of 3A zeolite. For this material the microporous volume has been determined by water adsorption at 25 ◦ C and the specific surface

Fig. 1. Scheme of the McBain thermobalance used for measuring the adsorption isotherms of gaseous formaldehyde.


713

Table 1 Characterization of the porosity of adsorbents determined by nitrogen adsorption at −196 ◦ C or by water adsorption at 25 ◦ C (*). Chemical formula

Vmicro (cm3 g−1 )

Vmeso (cm3 g−1 )

SBET (m2 g−1 )

DaY NaY KY NaX CuX

Na2 [Al2 Si190 O384 ] Na52 [Al52 Si140 O384 ] K52 [Al52 Si140 O384 ] Na86 [Al86 Si106 O384 ] Cu43 [Al86 Si106 O384 ]

0.293 0.325 0.304 0.348 0.317

– – – – –

717 749 703 690 653

LTA

3A

K96 [Al96 Si96 O384 ]

0.230*

–

497

Silica

SBA-15

SiO2 (amorphous)

0.085

0.422

595

Carbon

AC NORIT RB3

C

0.358

0.101

942

MOF

MIL-53 (Ga) IM-19

Ga(OH)(O2 C C6 H4

0.470

–

560

Type

Adsorbent

ZeoliteFAU

Scheme of porous structure

area has been calculated from that one of 5A zeolite assuming that the exchange of calcium by potassium does not modify the molar surface area. Corresponding values are reported in Table 1. The dealuminated Wessalith DaY faujasite zeolite was purchased from Degussa. The NaY, NaX and 3A zeolites were supplied by Union Carbide. KY and CuX were prepared by successive cation exchange of NaY and NaX, respectively, with potassium nitrate solution under reflux. The mesoporous silica SBA-15 was synthesized using classical acidic conditions as previously reported [24]. The activated carbon AC NORIT RB3 was purchased from Sigma–Aldrich. The gallium based-MOF material Ga-MIL-53, so called IM-19, was provided by G. Chaplais of the Institute of Materials Science of Mulhouse, IS2M-LCR 7228, in France, and the procedure of synthesis can be found in Ref. [26]. 2.2. Adsorption–desorption isotherms Adsorption–desorption of formaldehyde was studied by TGA under controlled vapor pressure using a homemade McBain thermobalance equipped with the high vacuum technology (Fig. 1). All pieces of this apparatus were either in stainless steel or in glass in order to avoid any corrosion by formaldehyde. The sample is placed in a platinum crucible, which is hanged on a quartz spring. The elongation of the spring, which is proportional to the sample weight variation, is measured with an optical displacement sensor composed of a light source and a CDD camera. The gaseous formaldehyde was generated by thermal dissociation of solid paraformaldehyde. For this purpose a metallic bulb containing a few grams of paraformaldehyde was connected to the balance, put under vacuum, heated at 200 ◦ C for 1 h and then cooled down at 25 ◦ C. Thus, we obtained a reserve of gaseous formaldehyde,

CO2 )

which can be maintained in the gas phase at room temperature several days before to polymerize again in paraformaldehyde. It has been controlled by mass spectrometry that the gas phase is well composed of formaldehyde. The weight of adsorbent was around 10 mg. Before adsorption, the sample was outgassed under dynamic vacuum (10−5 hPa) overnight at 400 ◦ C, except for the MOF sample which was activated at a lower temperature 200 ◦ C because of its lower thermal stability compared to the other adsorbents. Adsorption–desorption isotherms were measured at 25 ◦ C using a static method by increasing or decreasing the pressure step by step once a mass equilibrium plateau was reached. The pressure range investigated was 10−5 –2 hPa (0.01–2000 ppm) with a relative precision of around ±0.01%. The weight sensitivity was ±0.05%. The adsorption temperature was maintained at 25 ◦ C within a tolerance of ±1 ◦ C. The adsorbed amounts are expressed in percentage of weight of formaldehyde adsorbed by weight of anhydrous adsorbent (ma wt% or g of adsorbate/100 g of adsorbent). 3. Results and discussion 3.1. Zeolites The adsorption–desorption isotherms of formaldehyde on the FAU and LTA zeolites are given in Figs. 2 and 3. For the non-cationic DaY zeolite, the adsorption isotherm is of type III according to the IUPAC classification with a very low slope at low pressure, indicating a weak interaction between formaldehyde and DaY surface. The adsorption capacity at 2 hPa does not exceed 10 wt% (Fig. 2). On the contrary, the adsorption isotherms for the cationic forms are of type I with a sharp increase of the adsorbed amount at low pressure (Figs. 2 and 3). The presence of cations as Na+ , K+ or Cu2+ con-

714


Table 2 Amounts of formaldehyde and paraformaldehyde adsorbed at 25 ◦ C under 2 hPa. Zeolite

Mass of formaldehyde adsorbed ma (wt%)

NaYKYNaXCuX3A 32.78 23.66 30.32 28.31 16.14

Amount of formaldehyde adsorbed per surface unity Na (␮mol m−2 )

Volume of liquid formaldehyde adsorbed a VFOR (cm3 g−1 )

Volume of solid paraformaldehyde adsorbed a (cm3 g−1 ) VPARA

14.6 11.2 14.6 14.4 10.8

0.402 0.290 0.372 0.347 0.198

0.230 0.167 0.213 0.199 0.114

35

0.341 0.324 0.335 0.379 0.185

4.00 3.30 8.30 4.90 2.12

2925

Absorbance / u.a.

a

m / wt %

volume of the zeolite VCrystal (cm3 g−1 )

After one adsorption-desorption cycle

NaY

30 25

Retention capacity (wt%)

Crystallographic

KY

20 15 10

2958

2853

DaY

Before adsorption

5 0

0

0.5

1

1.5

p / hPa

2

3000

2.5

2950

2900 2850 Wave number / cm -1

2800

Fig. 2. Adsorption isotherms of formaldehyde on the NaY, KY and DaY zeolites at 25 ◦ C (the desorption branches of the isotherms are not represented for sake of clarity).

Fig. 4. FTIR spectra measured by transmission through wafers of NaY supported in KBr before adsorption and after one adsorption–desorption cycle of formaldehyde at room temperature.

siderably enhances the adsorption affinity and capacity of zeolites for formaldehyde. For all the zeolites, the adsorption–desorption isotherms show a hysteresis loop and the desorption is not complete even after pumping under dynamic vacuum for several days. It remains always a small amount of adsorbed formaldehyde, which varies from about 2% for the 3A zeolite up to more than 8% for the NaX zeolite (Table 2). Zeolites have been also characterized by infrared spectroscopy after one adsorption–desorption cycle. In comparison with the zeolite free of adsorbate, the IR spectrum obtained after desorption under vacuum shows three additional vibrational bands at 2853, 2925 and 2958 cm−1 , which are attributed to paraformaldehyde (Fig. 4). The adsorption capacities determined at the plateau of the adsorption isotherms under the pressure of 2 hPa are given in Table 2. The volumes occupied by the adsorbed phase are also reported. They are calculated assuming either liquid formaldehyde or solid paraformaldehyde is adsorbed. The densities used are 0.8153 g cm−3 for formaldehyde

and 1.420 g cm−3 for paraformaldehyde. The microporous volumes which were calculated from crystallographic data are also reported for comparison. The analysis of the data indicates that the volume of formaldehyde (Va FOR ) largely exceeds the crystallographic micropore volume (V Cryst ) for the NaY and NaX zeolites, while the volume of paraformaldehyde is always lower for all the zeolites studied. Based on these results and on the infrared spectra obtained after desorption, it is reasonable to assume that the adsorbed phase is a mixture of formaldehyde and paraformaldehyde. Indeed, it is well known that formaldehyde is not stable in the gas phase. So, when formaldehyde is confined in the zeolite supercages it inevitably polymerizes in paraformaldehyde. Then, this latter cannot be desorbed by pumping under vacuum at room temperature, because of the size of the molecule. Only a heating of the sample under dynamic vacuum at high temperature that is 400 ◦ C allows to dissociate paraformaldehyde and thereafter ensures its complete desorption, as shown in Fig. 5. The regeneration is more difficult for NaX than for NaY. This is attributed to the distribution of sodium

30

NaX

25

CuX

8 7

20

a

3A

15 10

5 4 3 NaY

2

5 0

NaX

6

m / wt %

a

m / wt %

35

1

0

0.5

1

1.5

2

2.5

p / hPa Fig. 3. Adsorption–desorption isotherms of formaldehyde on the NaX, CuX and KA (3A) zeolites at 25 ◦ C.

0

0

100

200

300

400

500

T / °C Fig. 5. Regeneration curves during the heating of the NaX and NaY zeolites under dynamic secondary vacuum (10−5 hPa).


715

4

7

10

MIL-53

1000

-1

5

SBA-15

K / wt %.hPa

4 3 AC

2

NaX CuX 1720 1400

NaY 1428

KY 235

100

3A 183

DaY 47

H

a

m / wt %

6

SBA 34

10

AC 23

1 1

0.5

1

1.5

p / hPa

2

2.5

Fig. 6. Adsorption isotherms of formaldehyde on activated carbon (AC), SBA-15 and Ga-MIL-53 at 25 ◦ C.

cations in NaX, which is different from that of NaY. In NaX there are additional cations in site III located at the entrance of supercages, which can create more favorable interactions with formaldehyde and reduce its diffusion. The adsorption–desorption hysteresis loop is more pronounced for the 3A zeolite than for the others zeolites. Moreover it is worth noting that adsorption kinetics is much lower. This is probably due to the small size of pore entrance of this zeolite (0.3 nm), which is of the same order as the size of the formaldehyde molecule (0.25 nm). The diffusion of formaldehyde in this material is extremely slow and consequently adsorption data reported on the adsorption branch are not true adsorption equilibriums.

3.2. Mesoporous silica, activated carbon and MOF As for DaY the adsorption isotherm of formaldehyde on mesoporous silica SBA-15 exhibits a low slope in the low pressure range characteristic of weak adsorbate–adsorbent interactions (Fig. 6). Such a result is not surprising since both materials have very similar siliceous surfaces with only a few sodium cations or hydroxyl groups as potential sites for adsorption. Under the pressure of 2 hPa the adsorption capacity of SBA-15 is only around 7 wt%, which is much lower than that of cationic zeolites. A hysteresis loop is also observed upon desorption (not shown in Fig. 6) suggesting that paraformaldehyde again is formed inside the main mesopores or inside the secondary micropores of the porous structure. The efficiency of activated carbon is also limited (Fig. 6). The adsorption affinity is much lower than on silica and the adsorption capacity under 2 hPa does not exceed 2 wt%. Adsorption–desorption is not also reversible. Concerning the MOF material Ga-MIL-53, the adsorption isotherm is of type IV in accordance with what is currently observed with this kind of material for the adsorption of VOCs (Fig. 6). At low pressure the adsorbed amount is the same as for the activated carbon, indicating once again a weak adsorption affinity for formaldehyde. However, the adsorbed amount increases sharply between 0.4 and 0.7 hPa to reach around 7 wt% under 2 hPa. The same shape of adsorption isotherm is obtained when Ga-MIL-53 is exposed to water vapor [27]. This sharp increase in the adsorbed amount results from the flexibility of the framework, so-called “breathing effect”, well-known in this type of MOF. During the adsorption of formaldehyde the structure of Ga-MIL-53 activated at high temperature, probably undergoes, as we observed for water vapor adsorption [28], a phase transition from a narrow pore form to a large pore form. In this case, it is worth noting that the adsorption–desorption process is perfectly reversible, contrary to what was observed with the other adsorbents.

Adsorbent Fig. 7. Ranking of a series of adsorbents according to their Henry constants at 25 ◦ C, which are related to their adsorption affinities.

3.3. Comparison of the adsorbents A ranking of these adsorbents has been performed by using the following two relevant criteria: (i) the Henry constant which is related to the adsorption affinity, i.e., the adsorption Gibbs energy; this constant is determined from the slope of the adsorption isotherm dm/dp at the limit zero pressure, (ii) the adsorption capacity under the pressure of 0.01 hPa, which corresponds to a gas phase concentration of 10 ppm. Figs. 7 and 8 show the histograms of the values of the Henry constant and of the adsorption capacity for each of the adsorbents, respectively. The NaY, NaX and CuX zeolites are the adsorbents that exhibit the highest adsorption affinities and capacities at 0.01 hPa. NaX is the most efficient adsorbent for trapping formaldehyde but NaY and CuX have also very interesting adsorption properties for this molecule. In Table 2 are also given the amounts of formaldehyde adsorbed per square meter of adsorbent. It may be worth noting that the FAU zeolites exchanged with sodium or copper have surface adsorption capacities identical (around 14.6 ␮mol m−2 ) and higher than those of FAU and LTA zeolites exchanged with potassium (around 11 ␮mol m−2 ). This result suggests that the nature of the compensation cation is a key parameter in the adsorption process of formaldehyde. This one is probably more important than the geometry of the nanoporous structure. Formaldehyde probably interacts stronger with sodium and copper than with potassium. Moreover due the bigger size of the potassium cation (138 pm) compared to those of sodium (102 pm) and copper (73 pm) cations, it is right to obtain a lower surface adsorption capacity with zeolites containing potassium.

100

a

0

m at 10 ppm / wt %

0

MIL53 1.6

10

NaX 12

NaY 9 KY 2.4

1

0,1

CuX 9

DaY 0.37

3A 0.7

SBA 0.33

AC 0.20 MIL53 0.02

0,01 Adsorbent Fig. 8. Ranking of a series of adsorbents according to their adsorption capacity at 25 ◦ C under the pressure of 0.01 hPa, which corresponds to a gas phase concentration of 10 ppm.

716


Table 3 Parameters of thermodynamic models used for modeling the adsorption isotherms of formaldehyde on cationic FAU zeolites at 25 ◦ C. R is the correlation factor. Model

Langmuir

Equation

m =

Parameters

maL /wt%

NaX

29.460 R = 0.99721 26.631 R = 0.98846 31.534 R = 0.99017 24.121 R = 0.99962

a

CuX NaY KY

SIPS

K p maL 1+KL p L

m = a

−1

Toth (K p)1/n maLF LF 1/n 1+(KLFp )

KL /hPa

KLF /hPa

n

75.445

29.166 R = 0.99752 30.042 R = 0.99890 35.079 R = 0.99886 23.925 R = 0.99966

76.883

0.9238

22.102

1.4989

22.604

1.4775

12.494

0.9723

29.147 R = 0.99743 31.857 R = 0.99891 36.528 R = 0.99933 23.912 R = 0.99962

34.013 30.683 12.190

30

a

m / wt %

25 20 15 10 5 0.001

KT p

0.01 0.1 p / hPa

1

10

Fig. 9. Modeling of the adsorption isotherm of formaldehyde on NaX at 25 ◦ C (o: experiment; ×: Langmuir; +: Toth; solid line: SIPS).

3.4. Modeling of adsorption isotherms Modeling of formaldehyde adsorption isotherms has been performed by using the classical adsorption thermodynamic Langmuir model and others derived from this one, such as the Toth and SIPS (Langmuir–Freundlich) models. The equations of these models and the values of the corresponding characteristic parameters giving the best fit of our experimental data are reported in Table 3. As we could expect the best fit is obtained with the SIPS and Toth models, which take into account the energetic heterogeneity of the adsorbent surface (Fig. 9). The Langmuir model, which assumes that the adsorption sites are isoenergetic and that there are no molecular interactions in the adsorbed phase, is not applicable for heterogeneous adsorbents as zeolites. However, the fit with this model is found to be quite satisfactory and it can be used with a reasonable precision if one wants to use a simple equation for the simulation of industrial processes. 4. Conclusion A selection of adsorbents has been performed with the aim to trap or detect formaldehyde in the gas phase by adsorption. Several materials, such as alumino–silicates, a silica, an activated carbon and an organic–inorganic hybrid material showing different porous structures and surface chemistries have been tested in this study. Sodium- and copper-faujasites (NaX, NaY, CuX) are the most efficient adsorbents. They exhibit the highest adsorption affinities and capacities, which make them suitable for capturing formaldehyde even at very low concentration (