Enhancement of the activity of CaA zeolites as deep oxidation catalysts through transition metal ion exchange

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkov~iand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.

1653

Enhancement of the activity of CaA zeolites as deep oxidation catalysts through transition metal ion exchange E. Diaz a, S. Ord6fiez a*, A. Vega a, J.

Coca a

and A.

Auroux b

Department of Chemical Engineering and Environmental Technology, University of Oviedo, Julifin Claveria s/n, 33006 Oviedo, Spain

a

b Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne Cedex, France The deep oxidation of hexane over CaA zeolites, both protonated and modified by ion exchange with transition metals (Mn 2+, Co 2+, Fe3+), has been studied in this work. Percentage of ion exchange in the zeolites was determined by ICP-MS, whereas surface composition was determined by XPS. Parent and modified zeolites were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, temperature-programmed reduction (TPR), temperatureprogrammed desorption of ammonia (TPD) and inverse gas chromatography (IGC). Catalytic activities of these materials for the deep oxidation of hexane was evaluated by recording the light-off curves. Values of 7"50 were correlated to adsorption enthalpies obtained by IGC. MnCaA showed the best catalytic performance for the studied reaction. 1. INTRODUCTION Volatile Organic Compounds (VOCs) are organic molecules that may undergo photochemical reactions with nitrogen oxides in the presence of sunlight, yielding even more hazardous compounds. They are components of many products associated with the petroleum and gasoline, painting or food industries. From these applications, large amounts of these compounds are released to the environment. Catalytic incineration is a technique in which pollutants, usually diluted in air streams, are oxidized in the presence of a catalyst. The desired reaction is the total oxidation to H20 and CO2 without the formation of by-products. The lower temperature required for the catalytic combustion results in lower fuel demand, thus working at low temperature is important in order to improve the economy of the process. The reactivity of the volatile organic compounds for combustion reactions decreases in the following order: alcohols > aldehydes > aromatics > ketones > alkenes > alkanes [1]. Therefore, an alkane represents a good test for the activity of a given class of catalysts. Zeolites, because of their pore structures, acidic properties, good thermal stability and ion exchange properties, have gained interest as potentially active catalysts for the oxidation of hydrocarbons [2]. In the literature, there are several works about the catalytic role of ion exchanged zeolites in the oxidation of hydrocarbons [3-5]. Likewise, H-zeolites have been considered recently as effective alternative catalysts to noble metal and metal oxide catalysts used in many commercial applications for air pollution control [2, 6, 7].

1654 In our previous works [8,9], adsorption properties of n-alkanes, cyclic hydrocarbons, aromatic hydrocarbons and chlorinated compounds on Co-, Mn- and Fe-exchanged NaX and CaA zeolites were studied by inverse gas chromatography. The influence of surface and chemical properties of these materials on their catalytic behaviour for the decomposition of nalkanes has also been observed. The scope of this work is to evaluate the catalytic behaviour of CaA zeolites (CaA, H-CaA, Co-CaA, Mn-CaA and Fe-CaA) in the combustion of hexane in air, and correlate these results with adsorption properties obtained by inverse gas chromatography. 2. E X P E R I M E N T A L AND METHODS

2.1. Zeolite preparation Zeolite CaA (Alltech) is available in 40/60 mesh. The protonated form was obtained by calcining the NH4-zeolite in air at 550 ~ for 4 h. The sample in the NH4-form was prepared by ion exchange with a 1 mol/dm 3 NH4NO3 solution at 70 ~ for 12 h. Transition metal solutions (0.25 mol/dm 3) were prepared by dissolving Co(NO3)2"6H20 (Merck), Fe(NO3)3-gH20 (Panreac) or Mn(NO3)2-4H20 (Panreac) in distilled water. Ion exchange between zeolites and Co(If), Fe(III) or Mn(II) solutions was allowed to take place by adding 3 g of zeolite into the metal salt solution under stirring at 70 ~ during 24 h. The synthesis of Fe-CaA has to be carried out at low pH in order to avoid the precipitation of insoluble Fe hydroxide [ 10], thus the pH was adjusted with adding H2SO4. The ion-exchanged zeolite was recovered by filtration and repeatedly washed with distilled water to remove the nitrates completely. The resulting zeolites, were pretreated at 500 ~ in an oven for 4h in order to remove the moisture and other contaminants prior to the experiments. 2.2. Zeolite characterization The chemical composition of all samples was determined by ICP-MS, the zeolitic structure by XRD and the surface composition by XPS. The surface area and pore volume of the zeolites were determined by nitrogen adsorption at-196~ with a Micromeritics ASAP 2000 surface analyser, assuming a value of 0.164 nm 2 for the cross-section of the nitrogen molecule. Acidity strength studies were carried out using a Micromeritics TPD-2900 apparatus connected to a mass spectra analyzer Glaslab 300. For this purpose, 0.25 g adsorbent sample was saturated in ammonia- stream of 10 % NH3/90 % H e - at 50 ~ during 30 min, and then heated from 50 to 950 ~ at 10 ~ in a stream of pure He with a flow rate of 20 cm3/min. Adsorption measurements were carried out in a Varian 3800 gas chromatograph equipped with a thermal conductivity detector (TCD). A loading of 0.6 g from each zeolite was placed into a 27-cm long Supelco Premium grade 304 stainless steel column, with passivated inner walls and an inside diameter of 5.3 mm (o.d. 88 inch). Packing of the zeolite was accomplished with mechanical vibration, and the two ends of the column were plugged with silane-treated glass wool. The columns were then stabilized in the GC system at 300~ overnight under a helium flow rate of 30 cm3/min. In order to avoid detector contamination, the outlet of the column was not connected to the detector during this period. Measurements were carried out in the temperature range of 2 0 0 - 270 ~ Helium was used as carrier gas, and flow rates were measured using a calibrated soap bubble flowmeter. In order to meet the requirement of adsorption at infinite dilution, corresponding to zero coverage and GC linearity [11], amounts injected were in the range of 0.05 to 0.8 pL. For each measurement, at least three repeated injections were performed, obtaining reproducible

1655 results. Air was used as a marker for the retention time correction, and it was used to ensure the absence of dead volume when a new column is placed in the chromatograph. From the evaluated retention time (tR, min) and flow rate (F, cm3/min) of the carrier gas, the retention volume (VR, cm 3) was calculated. The specific retention volume, Vg, in cm3/g, is given as:

Vg = Fj (tR -tM )( P~Po - Pw )I Tmeter T

(1)

where tR is the retention time in min, tM, the retention time of non-adsorbing marker (hold up time), po, the outlet column pressure, pi, the inlet pressure, pw, the vapour pressure of water at the flowmeter temperature in Pa, Tmeter,the room temperature in K, and j, the James-Martin compressibility factor defined as:

3

J:-i

I '2-i1 (pi/Po)3 (p,/po

(2)

-

At low surface coverage, the heat of adsorption is obtained by plotting In Vg against I/T, according to Eq. (3): c3(ln Vg) AHad s - -R

O(1/Z)

(3)

2.2. Reaction studies

Catalytic oxidation reactions were carried out at atmospheric pressure. The reactor was U-shaped and made of quartz and the experimental procedure was described elsewhere [9]. 1 pL of pure n-hexane was injected to the reactor into a continuous flow of synthetic air. Measurements were taken from 50 to 600 ~ Gases at the outlet of the reactor were analyzed on-line using a Glaslab 300 quadrupole mass spectrometer, which used a capillary inlet system for sampling and computer acquisition of multiple mass peaks. The spectrometer was previously calibrated for reaction product response. Conversion was calculated on the basis of both peak areas of CO2 and hexane. Mass balance closures were always higher than 95 %. 3. RESULTS AND DISCUSSION 3.1. Zeolite characterization

The main textural characteristics of the samples are shown in Table 1, where surface areas and pore volumes, measured by N2 physisorption are reported. According to IUPAC recommendations for microporous materials, surface areas and micropore volumes were calculated using the Langmuir and the "t" method of Lippens [ 12, 13]. HA zeolite is less porous and of lower surface area than the original one. However, metal exchanged zeolites show higher mesoporosity although their microporosity decreases. This effect is more pronounced for the iron zeolite, whose microporosity is nearly zero and mesoporosity is the largest.

1656 XRD studies showed that the starting CaA zeolite contained no amorphous phase and had a well-crystallised framework, as the high intensities of the main peaks and the uniform low background indicate. The crystallinity of the exchanged samples, measured according to the procedure proposed by L6pez-Fonseca et al. [6] (based on the determining of the intensities of the main diffraction peaks and assuming 100 % crystallinity for the starting material), is also shown in Table 1. A loss of crystallinity has been observed in all the cases, the manganese-exchanged zeolite retaining the largest degree of crystallinity, even more than the protonated one. Complete crystallographic degradation was noted for the iron-exchanged sample, since it decreased to nearly 0 % as the result of exchange. This decrease of the crystallinity in Fe-exchanged zeolites is reported in the in the literature for zeolite HSM [ 14], NaY [15] and FSM and NaY [16]. Table 1 Crystallinity, surface area, pore volume, micropore volume and mesopore volume data for the zeolites Crystallinity (%) CaA HA Co-CaA Mn-CaA Fe-CaA

100 74 16 82 -~0

SLangmuir(m2/g)

Vmesopores(BJH)

549 391 422 553 301

(cm3/g)

Vmicropores (t-Lippens) (cm3/g)

0.062 0.041 0.138 0.071 0.333

0.176 0.092 0.093 0.167 0.004

The elemental analysis of the samples expressed both in weight percent and as atomic ratio Si/Me, as well as elemental ratios in the XPS sampling region, are shown in Table 2. The binding energies of the spectrometers of XPS are 780.51 eV for Co 2p3/2, 642.30 eV for Mn 2p3 and 711.30 eV for Fe 2p3/2. The signal shapes indicated Mn 4+ and Fe 3+ as the main oxidation states of the metals for the Mn-CaA and Fe-CaA zeolites, respectively, however, in zeolite Co-CaA, CoO and Co304 may coexist, since these two species are difficult to distinguish using XPS. In all the exchanged zeolites, a slight dealumination is noted (see, the Si/A1 atomic ratios), being this phenomena more important for Fe-exchanged zeolite. Table 2 Characteristics of the samples studied (bulk composition by ICP elemental analysis, surface composition by XPS) Samples

% weight

CaA HA Co-CaA Mn-CaA Fe-CaA

Si/AI 1 1.07 1.13 1.14 1.27

atomic ratio Me 8.0 16.7 18.0

(Si/A1)b 0.96 1.03 1.09 1.10 1.22

(Si/A1)s 1.88 1.46 3.59

(Si/Me)b 5.19 2.26 2.12

(Si/Me)s 1.11 14.28 2.04

1657 The Si/Me atomic ratio for Fe-zeolite derived from XPS, it is noticeable that it was significantly lower than the bulk ratio for Co-CaA, so the ion exchange takes place mainly on the surface. The opposite behaviour was observed for the zeolite Mn-CaA whereas in the case of Fe-CaA, no differences were noticed between the bulk and the surface concentration of metal. These results suggest different mechanisms for the ion exchange of these metals. The NH3-TPD spectra of zeolites CaA and derivates show similar behaviour for CaA and Mn-CaA. The curve corresponding to zeolite HA displayed a major desorption peak at relatively low temperature. As regards to metal-exchanged zeolites, Co-CaA presents a sharp peak around 470 ~ with higher intensity than the parent material. The Fe3+-exchanged zeolite desorbed NH3 at lower temperature and Co-exchanged zeolite releases ammonia at very high temperatures, higher than for CaA, suggesting the formation of new NH3 adsorbing sites due to Co oxides.

3.2. Catalytic activity results for n-hexane oxidation The parent CaA as well as the metal-exchanged and protonated zeolites were evaluated in the catalytic oxidation of n-hexane. The light-off curves (i.e. the evolution of conversion with reaction temperature) for the oxidation of hexane over zeolites are shown in Fig. 1. The selectivity for CO2 was 100 % and no intermediates, such as CO or other hydrocarbons, were detected. In a previous work, it has been shown that the homogeneous reaction occurred with conversion of only 13 % at 600 ~ [9].

100 80-

,~

60

>r

40-

g

/

2

/ I

2 /!

0 20C 0

200

400

600

800

Temperature (~C) Fig. 1. Light-off curves of n-hexane decomposition over CaA ( 0 ), HA (n), Co-zeolite ( A ), Mn-zeolite (o) and Fe-zeolite (X)

1658 70 60 0

E v cat) "O

-1<1

Mn-CaA.

H A , / CaA~ ~ ~, / Co-CaA

50 40 30

I

Fe-CaA

20 10 0.9

!

!

|

a

1.9

2.9

3.9

4.9

5.9

surface acid sites concentration (a.u.) Fig. 2. Relationship between the surface acid sites concentration and the adsorption enthalpy of hexane (the amount of surface acid sites was obtained by integration of the area under the NH3-TPD

curve) Mn-CaA zeolite was found to be the most active. The decomposition of n-hexane started at about 150 ~ for all the zeolites. The following order of activity was observed (values in brackets corresponding the light-off temperature or Tso, temperature at which 50% conversion was attained): MnCaA (281 ~ > CoCaA (333 ~ > FeCaA (360 ~ > HA (398 ~ > CaA (457 ~ Spinicci et al. [17] also found the Co and Mn were more reactive than Fe in the oxidation of hexane, claiming that the low activity of Fe-perovskite among the perovskites formed was due to his lower surface area. Likewise, in our case, the Mn-zeolite which presents the highest surface area, shows also the highest catalytic activity. The correlation between acidity and the catalytic activity is not clear in the literature [4]. In our case, it is observed that although there is a good correlation between the amount of acid sites and the adsorption enthalpy of the hexane (Fig. 2), there is not any clear relationship between the acidity and the activity for hexane oxidation. So, although the acidity of the catalysts affect their adsorption properties, the activity of the selected zeolites does not depends only on this aspect. The catalytic activity could be due to effects other than changes in the chemical or morphological properties of the zeolites, such as the adsorption. Thus, if a reactivity parameter (such as 7'5o) is plotted vs. the adsorption heat (obtained by IGC, using eq. 3), a so-called Volcano plot is obtained (Fig. 3), having an optimum value of AHad~.. Lower and higher values lead to poorer catalytic performance. In a previous work of our research group [9], it was observed that chemisorbed oxygen plays an important role in this reaction over these materials. O2-TPD experiments over the metal ion-exchanged zeolites revealed that Mn-CaA zeolite, which is the most active, is able to adsorb oxygen, which could be released at moderate temperatures (in the TPD experiment, the main release peak takes place at 115 ~

1659

Ca,,]

500

\

400 -

Fe-CaA

HA j~

!

0 o v

0 laO

I--

300 Mn-CaA 200 0

l

I

I

20

40

60

80

-AHads (kJ/mol) Fig. 3. Volcano plot for hexane oxidation over the studied zeolites By contrast, Fe-CaA zeolite, can also adsorb oxygen, but it interacts strongly with the catalyst (the main release peak takes place at 440 ~ and is not available for the reaction. On the other hand, both parent and protonated zeolites do not adsorb oxygen significantly. This could be one of the causes for the poorer performance of these zeolites, being the Mnexchanged zeolite the only one that performs clearly better than the parent zeolite. It could be inferred that the best performance of the protonated zeolite, if compared with the parent CaA zeolite, is due to the higher acidity of the first one, which leads to strong interaction with the alkane. 4. CONCLUSIONS Transition metal (Co 2+, Mn 2+, Fe 3+) modified CaA zeolites and the protonated one were prepared by ion exchange. Their properties were characterized using different physicochemical techniques. The applicability of these materials to the catalytic combustion of hexane, a typical VOC encountered in many industrial emissions, has been studied in a pulse microreactor. Considering all the characterisation and reaction data reported in this paper, it is obvious that introduction of the studied transition metal ions into the zeolites modifies both its physical and chemical nature. These modifications depend on the ion-exchanged metal ion. Mn-CaA proved to be the most active among the catalysts of this study. As a final conclusion, these materials (specially Mn-exchanged CaA zeolites) seem to be promising catalysts for the abatement of VOCs.

1660 ACKNOWLEDGEMENTS This work was supported by the Research I+D+I Plan of Asturias within Research Project PR-01-GE-17. One author (E.D.) acknowledges a personal grant to Asturias Research Foundation (FICYT).

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