Catalytic activity of poly[(methacrylato)aluminum(III)] obtained at different gamma-radiation doses

Radiation Physics and Chemistry 80 (2011) 1151–1157

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Catalytic activity of poly[(methacrylato)aluminum(III)] obtained at different gamma-radiation doses ˜ a-Nu´n ˜ ez b,n, V. Sa´nchez-Mendieta a, R. Pe´rez-Herna´ndez b A.R. Vilchis-Nestor a, F. Uren a b

´noma del Estado de Me´xico, Paseo Tollocan and Paseo Colo ´n, A.P. A-20, Toluca, C.P. 50120, Mexico Facultad de Quı´mica, Universidad Auto ´xico-Toluca, Ocoyoacac, Estado de Me ´xico, C.P. 52750, Mexico Instituto Nacional de Investigaciones Nucleares, km 36.5, Carretera Me

a r t i c l e i n f o

abstract

Article history: Received 8 June 2010 Accepted 1 May 2011 Available online 13 May 2011

A novel coordination polymer was obtained throughout the polymerization of aluminum(III) methacrylate at different doses (10, 20, 30, 40, 50 and 80 kGy) using gamma-radiation as initiator. The materials were characterized by electronic paramagnetic resonance (EPR), thermogravimetric analyses (TGA) and scanning electron microscopy/electron dispersive X-ray analyses (SEM/EDAX) techniques; particle size distribution and surface areas were also determined. The samples of poly[(methacrylato)aluminum(III)] obtained at different g-doses were found to catalyze 2-propanol dehydrogenation and dehydration to acetone and propene. A noticeable activity was observed with Poly[(methacrylato)aluminum(III)] obtained at 50 kGy, which has higher conversion of 2-propanol and the highest selectivity to acetone ( 90%). Results suggest that the decomposition of 2-propanol is correlated with the effect of gamma-radiation on the structure and surface area of the catalysts. & 2011 Elsevier Ltd. All rights reserved.

Keywords: g-Radiation polymerization Metal–polymer complexes 2-Propanol dehydration 2-Propanol dehydrogenation

1. Introduction Metal-containing polymers (MCP) acquire specific physical– mechanical features (Pittman et al., 1990), they turned out to be efficient and selective catalysts for various reactions (Guyot, 1988) or they can have biocide activity, among other useful properties (Pomogailo, 1990). Development of MCP as support or catalyst is motivated by two major advantages, first MCPs allow uncomplicated physical separation of the catalyst from the reagents and the products; it is possible to use a large excess of one of the reagent in consequential reactions, without the loss of costly catalysts. The second expected advantage is the ‘‘polymer effect’’, which may enhance activity of the catalysts by site isolation or, on the contrary, by cooperative effects of the neighboring groups. However, industrial applications are not frequent except for the Merrfield synthesis and the acid catalysts (Leadbeater and Marco, 2002; Bergbreiter, 2002). A general approach for the synthesis of MCP consists of synthesizing a metal-containing monomer and then usually polymerizing that monomer in solution with peroxides or azocompounds as initiators (Odian, 1991). Gamma-radiation can also be applied over a metal-containing monomer in solid phase to initiate the polymerization reaction. The efficiency of gammaradiation relies on its capability of penetration, control of the

n

Corresponding author. Tel.: þ52 55 53297200; fax: þ52 55 53297301. ˜ a-Nu´n ˜ ez). E-mail address: [email protected] (F. Uren

0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.05.001

polymerization ratio as a function of the applied dose, high conversions in short reaction times and, in addition, further purification processes are not required because there is no contamination by a catalyst or sub-products (Filardo et al., 2002). The importance of alumina as support has been widely recognized in many catalytic processes of industrial importance such as isomerization, alkylation, catalytic cracking or polymerization. Furthermore, the acid–base properties of alumina play a major role in interfacial reactions and, due to its inherent acidic properties, have been useful in the preparation of a variety of catalysts (Sa´rbu and Beckman, 1999). Dehydration of 2-propanol to propene is an important catalytic test to identify acidic and basic sites in heterogeneous catalyst (Linnekoski et al., 1998; Mourgues et al., 1967). Acidity and basicity are paired concepts usually invoked to explain the catalytic properties of metal oxides (Tananbe et al., 1989). The complete description of surface properties requires the determination of amount, nature (Brønsted or Lewis type) and strength of the acidic and the basic sites. Alcohol decomposition has been carried out in a large number of catalysts showing strong or ¨ medium acidity, like silicoaluminates or alumina (Knozinger ¨ et al., 1972; Knozinger and Schengllia, 1974) in which, the isopropylether selectivity is minor to 10% in these catalysts, and propene is the major product. Since selectivity in alcohol decomposition depends on the strength and distribution of the acid sites, efforts have been recently made in order to design catalysts with controlled acidity (Ai, 1975; Pe´rez-Herna´ndez et al., 2005; Hathaway and Davis, 1976). Although several new systems were

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proposed for different reactions with Al2O3-base catalysts, no decisive novelty appeared in the last years. In this context, new highly active and selective catalysts are necessary in order to advance new process in the industry. In this paper, we report a solid-state synthesis of the microcrystalline, and submicrometric, novel poly[(methacrylato)aluminum(III)] (PMAAl), which was obtained by gamma-radiation at different doses. The catalytic activity of this coordination polymer over the decomposition of 2-propanol was also studied.

2.3.5. Surface area determination Total surface areas were obtained by the Brunauer, Emmett and Teller (BET) method. A RIG-100 multitask unit ISRI was employed for measuring the total surface area of the catalysts, using N2 adsorption–desorption (30% N2/He gas mixture, 30 cc/min) by the single point method. Samples (0.1 g) were first degassed and preactivated by flowing carrier gas (30% N2 in He) through the cell at 200 1C with a flux of 30 cc/min for one hour. 2.4. Catalytic activity

2. Experimental 2.1. Synthesis of aluminum methacrylate (MAAl) The monomer of aluminum methacrylate was obtained by reacting methacrylic acid (MAc), NaHCO3 and AlCl3  6H20 in deionized water. The method was described previously by Galvan et al. (1999). Formation of the MAAl was confirmed by gravimetric and spectrophotometric analyses (Vilchis-Nestor et al., 2006). 2.2. Polymerization of MAAl Samples of 1 g of monomer were placed in a glass tube, degassed and then vacuum-sealed. Polymerization of MAAl was carried out by irradiation of the samples at room temperature in a gamma-cell unit (Transelektro-LGI-01), provided with a 60Co g-ray source. Doses of 10, 20, 30, 40, 50 and 80 kGy were applied to the monomer at dose rate of 3.5 kGy/h. PMAAl showed insolubility to most common solvents. 2.3. Characterization methods 2.3.1. Electron paramagnetic resonance (EPR) analyses To confirm that the polymerization was via free radicals, EPR analysis was carried out at room temperature immediately after irradiations using a Varian E-15 electron paramagnetic spectrometer operating at a frequency of 9.5 GHz. The instrument settings were as follows: magnetic field 330 mT, scan range 40 mT, scan time 8 min, magnetic field modulation amplitude 0.1 mT, modulation frequency 100 kHz and microwave power 2 mW.

The catalytic activity of the samples was determined by testing the catalyst in the dehydration of 2-propanol. Isopropanol (Aldrich 99.9%) decomposition was determined in a continuous flow reactor. 0.1 g of each sample was re-activated in a stream of He (60 cc/min) from room temperature to 300 1C with a heating rate of 10 1C/min and held at this temperature for 1 h, after that each sample was brought up to the reaction temperature (275 1C) in He (60 cc/min). For isopropanol decomposition He was bubbled through a tank containing 2-propanol; the partial pressure of the isopropanol was 100 torr. The effluent gas of the reactor was analyzed by gas chromatography using a thermal conductivity detector (TCD). A 2 m packed Porapack Q was used at 115 1C to separate the reaction products from the isopropanol decomposition. The activity of the catalyst is defined as C(%), the percent conversion of isopropanol to all products: C ð%Þ ¼ ð½2Pin 2½2Pout Þ=½2Pin  100 The subscriptions in and out indicate the inlet and the outlet concentrations of 2-propanol, respectively. Selectivity towards acetone or propene (%) is given by X % Selectivity ¼ ½ProductðXÞ= ½All products  100 where [Product (X)]¼concentration of the product (acetone or P P propene), and [All products]¼ of the acetoneþ propene. A total of three reaction cycles were tested in order to check the functional life time of the sample with the highest conversion (PMAAl-50). During each cycle, 0.1 g of sample was re-activated at 300 1C for the period of 1 h, after that, the sample was exposed to the same reaction conditions (275 1C in He with flux of 60 cc/min).

3. Results 2.3.2. Thermogravimetric analyses Thermal degradation of monomer and polymer samples was performed using a TA Instruments TGA-51 apparatus, calibrated with a standard of calcium oxide. Experiments were conducted at 20–800 1C, with a heating rate of 10 1C/min, under nitrogen atmosphere; sample weight was around 10 mg. 2.3.3. Scanning electron microscopy and electron dispersive X-ray analysis Polymer (PMAAl-50) morphologies, before and after catalytic activity, were analyzed and imaged with a PHILIPS XL-30 scanning electron microscope (SEM) at 25 kV coupled to a detector for energy dispersive X-ray analysis (EDAX). Samples were fixed on a copper film support and coated with gold by sputtering. 2.3.4. Particle size distribution The particle size distribution of the polymers and the monomer was measured using a light disperser Coulter 230. Ten mg of sample dispersed in 10 ml of water was used for each analysis. Frequency of the number of particles as a function of its grain size was obtained by the software LS32 version 3.01.

3.1. Synthesis of monomer (MAAl) Aluminum(III) methacrylate (MAAl) was synthesized with a yield of 89.1% . Scheme 1 shows that the reaction proceeds in two steps; first, sodium methacrylate is formed, followed by the exchange of Na þ by Al3 þ to form aluminum(III) methacrylate. The reaction of aluminum(III) with the carboxylic group of the sodium methacrylate results in the formation of [Al(OH)x(OH2)y(O2C(CH3)C¼CH2)z]. This product was determined by X-ray photoelectron (XPs), infrared (FTIR) and Raman analyses and it was reported elsewhere (Vilchis-Nestor et al., 2006). This structure is closely related to the carboxylate-alumoxanes [Al(O)x (OH)y(O2CR)z]n where 2xþyþz ¼3, which have been synthesized and studied previously by Landry et al. (1995). 3.2. Polymerization of the monomer (PMAAl) Formation of polymers was carried out by the gamma irradiation of the monomer at different doses: 10, 20, 30, 40, 50 and 80 kGy. In Scheme 2, the formation of poly[aluminum(III) methacrylato] via free radicals is shown.

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Reaction 1 CH2=C(CH3)COO-Na+ + H2O + CO2(g)

CH2=C(CH3)COOH(liq) + NaHCO3(aq)

Reaction 2 CH2=C(CH3)COO-Na+ + AlCl3(aq)

[Al(OH)x(OH2)y(O2C(CH3)C=CH2)z] + 3NaCl

Scheme 1. Schematic representation of the reactions involved in MAAl synthesis.

Reaction 3

Table 1 Main decomposition temperatures and percentage of residue at 800 1C for MAAl and PMAAls.

[Al(OH)x(OH2)y(O2C(CH3)C=CH2)z](s) + γ-radiation {Al(OH)x(OH2)y[O2C(·C(CH 3)H2C·)z]}2

n

{Al(OH)x(OH2)y(O2C(-(CH3)H2C-)z]}n (s) Scheme 2. Schematic representation of the polymerization, where (S)¼ solid phase.

Sample

Temperature (1C)

Weight loss (%)

Residue at 800 1C (%)

MAAl

319.2 450.0 136.5 326.9 453.8 134.6 334.6 453.8 209.6 319.2 451.9 226.9 319.2 453.8 226.9 319.2 453.8

13.62 40.79 4.28 17.68 57.10 5.60 17.57 50.23 6.00 16.94 57.71 6.28 16.39 55.56 5.14 16.74 52.12

42.2

PMAAl-10

PMAAl-20

PMAAl-30

PMAAl-40

PMAAl-50

315

320

325 330 335 Magnetic field (mT)

340

345

Fig. 1. EPR spectra of PMAAl obtained at 40 and 80 kGy.

3.3. Samples characterization 3.3.1. Electron paramagnetic resonance Fig. 1 shows the EPR spectra of PMAAl obtained at 40 and 80 kGy. A well-defined signal centered at around 330 mT can be observed for PMAAl samples with different intensities as a function of the received dose; these signals are due to the presence of free radicals. It must be said that no signal was observed for the monomer. 3.3.2. Thermogravimetric analysis Thermograms of monomer and polymers, at different doses, are listed in Table 1. A first weight loss, at about 135 1C, can be attributed to the elimination of adsorbed water, which is in agreement to the TGA analyses results obtained by McNeill and Zulfiqar (1978) and Rufino and Monteiro (2000) for metal salts of poly(methacrylic acid). Thermogravimetric analysis showed that the catalysts have high thermal stability ( 4400 1C). The temperatures of maximum decomposition rate obtained for the monomer and the polymers were 450 and 453.8, respectively. It is interesting to note that X-ray diffraction of the powered residue at 800 1C exhibits the peaks for -alumina(JCPDS 12-0539) and d-alumina ( JCPDS 16-0394) for MAAl and PMAAls, respectively. 3.3.3. Scanning electron microscopy Fig. 2A and B shows SEM images of the polymer at 50 kGy of dose (PMAAl-50) before and after catalytic activity, and their

31.5

32.0

31.7

32.0

32.0

respective EDAX analysis. The PMAAl-50 exhibits morphologies constituted by micro-spheroid particles. Filament type needles compose these particles. The micro-spheroid particles size is near to 0.6 mm and they form agglomerates of  4 mm. Chemical analysis by EDAX shows only carbon, oxygen and aluminum in PMAAl-50, before and after the reaction of 2-propanol decomposition; nevertheless, after the catalyst activity, the morphology of the catalyst particles has changed, from micro-spheroids constituted by filaments to agglomerates of about 6mm formed by amorphous particles covered by small granules. 3.3.4. Particle size distribution Particle size distribution of the monomer and polymers are shown in Fig. 3. MAAl, PMAAl-20 and PMAAl-30 have an average grain size of  0.5 mm; in PMAAl-10, PMAAl-50 and PMAAl-80 the average grain size lies around 3.5 mm and PMAAl-40 showed the highest average grain size, which is close to 6 mm. 3.3.5. Specific surface area analysis (BET) Table 2 summarizes the BET surface area measurements of the samples at different g-radiation doses. The specific surface of the new material (MAAl) was near to 6 m2/g. Irradiation of MAAl with g-radiation caused an increment in the surface area up to 20 kGy; with the applied doses of 30 and 40 kGy, the surface area diminished. The largest surface area ( 37 m2/g) was found for the sample irradiated to 50 kGy. However, a dose of 80 kGy also caused a decrease in the surface area of the material to 12 m2/g. 3.4. Catalytic activity The total conversion of 2-propanol was determined by gas chromatography. Fig. 4 shows the total conversion of the isopropanol as a function of time. At the beginning of the reaction,

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Fig. 2. SEM images showing the chemical composition and the morphologies of (A) PMAAl-50 and (B) R-PMAAl-50, before and after the catalytic reaction, respectively.

100

% CONVERSION

80 MAAl PMAAl10 PMAAl20 PMAAl30 PMAAl40 PMAAl50

60

40

20

0 20

Fig. 3. Variation of catalysts particle size with respect to the applied doses after polymerization.

Table 2 Effect of g-radiation dose on surface area of the catalysts. Sample

Dose (kGy)

Surface area (m2/g)

MAAl PMAAl-10 PMAAl-20 PMAAl-30 PMAAl-40 PMAAl-50 PMAAl-80

0 10 20 30 40 50 80

6.4 20.0 23.1 17.0 17.8 36.8 12.2

the maximum conversion was observed using the PMAAl-50 catalyst. A moderate deactivation was observed for all the samples under these reaction conditions, except for the PMAAl50 that showed a higher deactivation after 20 min of reaction;

40

60

100 120 80 TIME (min)

140

160

180

Fig. 4. Variation of the catalytic activity of PMAAl and MAAl versus the applied doses on the conversion of 2-propanol at 275 1C.

furthermore, PMAAl-50 exhibited a larger conversion toward decomposition of 2-propanol than the other catalysts. Two main products were formed during the decomposition reaction of 2-propanol using the MAAl and PMAAl as catalysts, acetone as product of the dehydrogenation and propene as product of dehydration of isopropanol. Table 3 shows the selectivity of the catalysts to produce acetone; again the PMAAl-50 shows a major selectivity to acetone ( 90%); meanwhile, for other catalysts the selectivity was around 50/50% in the formation of acetone/propene, with a pronounced decay during the reaction time. The catalytic activity of the PMAAl-50, after 3 cycles of reaction, is showed in Fig. 5. After the first cycle, the activity decreases during the first 150 min of reaction; however after the second and third cycles of reaction, the activity is almost constant. From Fig. 5, it is clear that the final activity profile

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Table 3 Selectivity data obtained from catalytic testing of PMAAl and MAAl on 2-propanol decomposition at 275 1C. Sample

Time (min)

MAAl

propagation of ion radical is as follows:

Products

2 39 57.5 94.5 2 20.5 57.5 94.5 2 20.5 57.5 94.5 2 20.5 57.5 94.5 2 20.5 57.5 94.5 2 20.5 57.5 94.5 2 20.5 57.5 94.5

Acetone (%)

Propane (%)

19.8 29.6 39.4 43.3 13.3 21.3 29.3 21.5 26.2 32.9 42.8 50.9 35.6 36.3 35.2 31.3 47.8 50.9 39.1 30.6 93.7 48.3 33.1 21.8 8.2 24.5 52.8 67.3

86.2 84.7 60.6 56.7 86.7 78.7 70.7 78.5 73.8 67.1 57.2 49.1 64.4 63.7 64.8 68.7 52.2 49.1 60.9 69.3 6.3 51.7 66.9 78.2 91.8 75.5 47.5 32.4

50

Fig. 1 shows the first derivatives of the paramagnetic absorption spectra of two PMAL samples irradiated at dose levels 40 and 80 kGy. Each EPR spectrum consists of three lines caused mainly by the hyperfine interaction of the unpaired electron with H-nuclei at the b position. However it must be mentioned that poor resolution of EPR spectra is normally found for radicals in many organic materials, whose electron paramagnetic signals are broadened for diverse reasons, e.g. poly-orientation, spin–lattice interaction and overlapping of spectra from different types of radicals (Regulla and Deffner, 1982). The differences in EPR signal intensities of the spectra in Fig. 1 are associated to the amount of radicals produced for each applied dose; also, the EPR signal intensity increases as a function of the received dose. It must be pointed out that EPR measurements were carried out immediately after irradiations and that no EPR signal was found after  40 h of irradiation, which means that the polymerization process had been finished. Thermal gravimetric analysis of the compounds shows that they decompose in several steps (Table 1). At the first stage, 4–6% of the initial weight is lost between 135 and 220 1C. Then, a weight loss of 13–18% occurs between 310 and 340 1C; however, the main weight loss occurs above 400 1C for all the polymers, with the exception of the monomer that occurs at 450 1C. The decomposition pathway of these kinds of compounds has been discussed in the literature by McNeill and Zulfiqar (1978) and Zulfiqar and Masud (2002). Furthermore, the weight loss of 13–15% for PMAAl is in concordance with results reported by Landry et al. (1995) concerning the dehydration of bohemita. Landry et al. (1995) and Rufino and Monteiro (2000) reported on their thermal studies on bohemita and metallic salts of poly(methacrylic acid), respectively, that the first weight loss is due to absorbed water, the second is due to the dehydration of the aluminum(III) complex, as shown below:

40

2[Al(O)(OH)]-Al2O3 þH2O

PMAAl-10

PMAAl-20

PMAAl-30

PMAAl-40

PMAAl-50

PMAAl-80

100 1st CYCLE

90

2nd CYCLE

80 % CONVERSION

1155

3rd CYCLE

70 60

30 20 10 0 0

20

40

60

100 80 TIME (min)

120

140

160

Fig. 5. Catalytic activity of PMAAl-50 after 1, 2 and 3 cycles of reaction.

was similar during the three cycles, which means that the same active sites remains at the end of the three cycles.

4. Discussion To produce the monomer of aluminum(III) methacrylate it was necessary to form the intermediate of sodium methacrylate; this was made to favor the interchange between the Na þ and Al3 þ ions, in this way, the monomer was obtained in a high yield. Polymerization of the monomer was made via free radicals induced by gamma-radiation. EPR analysis proved that the

And the third weight loss corresponds to decomposition of the polymer structure. Following the TGA results, the catalysts can be used at temperatures under 300 1C; this was the reason to select 275 1C as the temperature to carry out the catalytic studies. Scanning electron microscopy images (Fig. 2) show that there is a clear morphological change in PMAAl-50 before and after the catalytic activity. It can be observed that the morphologies of PMAAl-50 (Fig. 2A) are closer to a spherical shape that resembles fibers hanks. Similar fiber-type morphology has been observed in scanning electron microscopy studies of poly[(methacrylato)zinc(II)] (Vilchis-Nestor, 2000). The morphology of PMAAl-50, after the reaction, changes to form small agglomerates, Fig. 2B, which are believed to come from the degradation of the polymer, and consequent oligomer formation, due to the action of the temperature of reaction. EDAX studies, as mentioned above, showed that only carbon ( 67%), oxygen (  26%) and aluminum ( 7%) are present in PMAAl samples. The maximum degree of polymerization occurs at a dose of 40 kGy of gamma-radiation, which corresponds to the highest particle average size of  6 mm. Galvan et al. (1999) reported that the maximum degree of polymerization for iron(III) polymethacrylato occurs at 25 kGy and at a dose of 80 kGy the degradation of the polymer (PMAAl) occurs as a consequence of the gamma-radiation effect.

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4.1. Catalyst activity Dehydration of 2-propanol to propene is widely used to characterize the acidity of some metallic oxides used as catalysts (Pe´rez-Herna´ndez et al., 2005; Aramendı´a et al., 1996); also the dehydrogenation of 2-propanol to acetone can reveal basicity (Pe´rez-Herna´ndez et al., 2005; Hathaway and Davis, 1976); however, this point remains unclear since acetone formation can occur by a redox mechanism, particularly at low temperature (Orita et al., 1991). Under the conditions used for 2-propanol decomposition in the samples that were subjected to doses of 10–40 kGy, including the monomer, the percentages of conversion of isopropanol, after 2 min of reaction, followed the order: PMAAl-304PMAAl80 4PMAAl-204PMAAl-404MAAl4PMAAl-10. However, for the catalyst obtained at 50 kGy the conversion was higher than the others samples, about 77%. After 20 min of reaction the conversion of all samples falls and remains practically unaffected. Table 3 showed the selectivity towards acetone and propene at 90 min of reaction. 2-Propanol dehydrates on PMAAl-50 showing its more pronounced basic character, while in the other samples, the acid character is predominant. It must be pointed out that when the dose was increased at 80 kGy the reaction rate was lower than the samples irradiated at doses smaller than 50 kGy, notwithstanding the selectivity increase with the reaction time producing 67.4% of acetone in 76 min. The catalyst PMAAl-50 showed the highest selectivity to produce acetone and hydrogen. This is important, because hydrogen can be used on the polymer electrolyte fuel cell (PEFC) systems as an energy source. While, in the other catalysts, the formation of propene (about 60%) is also observed. The results obtained in the decomposition of 2-propanol as model reaction to determine the acid–base properties in the catalysts, allowed to establish that PMAAl-50 exhibits important acid–base properties, while in MAAl, PMAAl-10, PMAAl-20, PMAAl-30, PMAAl40 and PMAAl-80 samples the acid character is predominant. This outcome could be attributed to the crosslinking process occurring during the polymerization of PMAA1 with a 60Co g-ray source at different doses. Crosslinking makes the active sites of alumina more available for the isopropanol transformation as the dose increases up to 50 kGy. After this dose value, a total crosslinking of the polymer or its decomposition may take place. This might inhibit the active sites of alumina, and therefore the catalyst is deactivated. On the other hand, we have previously proposed that carboxylate groups exist as bridging ligands on the aluminum(III) ions (Vilchis-Nestor et al., 2006). However, the formation of carboxylate-aluminum(III) mono-dentate complex could be rationalized as a part of the MAAl and PMAAl submicrometric particle surface. The bands at 1590 and 1455 cm  1 in the IR spectra confirm the presence of bridging carboxylate moieties (Nakamoto, 1997). In addition, the formation of an aluminum(III) carboxylate and new hydroxide moieties, via the protonation of an oxo ligand, has been previously postulated by Barron and Koide (1995):

R

Al

O Al

O

+ RCO 2H Al

Al O H

Such studies reveal different coordination geometries and electronic environments for the aluminum(III) ions, as confirmed by XPS studies (Amor et al., 2000), which can be correlated to the acidic or basic behavior during the catalytic test. The PMAAl surface can be considered to consist of three different types of sites. The O anions can act as electron-donating Lewis base sites and incompletely coordinated cations as electron-accepting Lewis acid sites, and the OH anion can act as either a Lewis acid or base, ¨ but also as a proton-exchanging Bronsted acid–base site. However, recent XPS measures on aluminum oxides of Al 2p binding energies and the binding energies of the resolved OH and O components in the O 1s peak showed that the aluminum oxides ¨ have OH sites with the same Bronsted/Lewis acid–base properties, O sites with the same Lewis base properties, and Al sites with very similar Lewis acid properties (Van den Brand et al., 2004). That means that if we have this kind activity sites, the selectivity of the 2-propanol decomposition should be oriented to the dehydrogenation to propene predominantly on acid sites and dehydrogenation to acetone on basic or concerted acid–base pair sites, as occurs in our catalyst. Additionally, the g-radiation doses used to obtain the PMAAls modified their surface groups and as a result the selectivity to propene/acetone also changes . Furthermore the variation specific surface area of the catalysts is the result of changes in the polymer structures as an effect of the g-radiation dose employed for the polymerization. In fact the high conversion and selectivity to acetone of PMAAl-50 can also be correlated with the higher specific surface area of this sample (Fig. 4 and Table 2).

5. Conclusions

C

O

From our previous spectroscopy results (Vilchis-Nestor et al., 2006), the bridging complexes in MAAl, where a carboxylate bridges two adjacent aluminum ions, can be represented as follows:

In conclusion, we have used different gamma-radiation dose to obtain aluminum catalyst supported on polymeric material. This functional material was used as catalyst on 2-propanol decomposition. Acetone and propene were the main products from the reaction as a result of the dehydrogenation and dehydration processes, respectively. The coordination polymer obtained with a dose of 50 kGy showed the better performance to produce acetone and hydrogen. Whereas the others catalysts showed better behavior toward 2-propanol dehydration. Finally we conclude that it is very important to control the synthetic process,

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to select and manipulate the sought-after products. This is currently a challenge and a very important research topic.

Acknowledgments We want to thank Cristino Rodriguez and Leticia Carapia from ININ for their help on TGA and SEM studies, respectively. References Ai, M., 1975. The oxidation activity and acid–base properties of SnO2-based binary catalysts: II. The SnO2–MoO3 and SnO2–P2O5 systems. J. Catal. 40, 327–333. Amor, S.B., Baud, G., Jacquet, M., Nanase´, G., Fioux, P., Nardin, M., 2000. XPS characterization of plasma-treated and alumina-coated PMMA. Appl. Surf. Sci. 153, 172–183. Aramendı´a, M.A., Borau, V., Jime´nez, C., Marinas, J.M., Porras, A., Urbano, F.J., 1996. Magnesium oxides as basic catalysts for organic processes: study of the dehydrogenation–dehydration of 2-propanol. J. Catal. 161, 829–838. Barron, A.R., Koide, Y., 1995. A model for the interaction of carboxylic acids with boehmite. Organometallics 14, 4026–4029. Bergbreiter, D.E., 2002. Using soluble polymers to recover catalysts and ligands. Chem. Rev. 102, 3345–3384. Filardo, G., Caputo, G., Galia, A., Calderaro, E., Spadaro, G., 2002. Polymerization of methyl methacrylate trough ionizing radiation in CO2-based dense systems. Macromolecules 33, 278–283. ˜ a, F., Flores, H., Lo´pez, R., 1999. Determination of the crystallinity Galvan, A., Uren index of iron polymethacrylate. J. Appl. Polym. Sci. 74, 995–1002. Guyot, A., 1988. Polymer supports with high accessibility. Pure Appl. Chem. 60, 365–376. Hathaway, P.E., Davis, M.E., 1976. Base catalysis by alkali-modified zeolites: II. Nature of the active site. J. Catal. 116, 279–284. ¨ ¨ H., Kochloefl, K., 1972. The dehydration of alcohols on alumina: Knozinger, H., Bul, reactivity and mechanism. J. Catal. 24, 57–68. ¨ Knozinger, H., Schengllia, A., 1974. Influence of steric and inductive effects on product distributions in the dehydration of secondary alcohols on alumina. J. Catal. 33, 142–144. Landry, C., Pappe´, N., Mason, M., Apblett, A., Tyler, A., Maclness, A., Barron, A.R., 1995. From minerals to materials: synthesis of alumoxanos from the reaction of boehmite with carboxylic acid. J. Mater. Chem. 5, 331–341. Leadbeater, N.E., Marco, M., 2002. Preparation of polymer-supported ligands and metal complexes for use in catalysis. Chem. Rev. 102, 3217–3274.

1157

˚ M., Moljord, K., Holmen, A., 1998. EtherificaLinnekoski, J.A., Krause, A.O.I., Kjetsa, tion of isobutene with 1-propanol and 2-propanol. Appl. Catal. A: Gen. 174, 1–11. McNeill, I.C., Zulfiqar, M., 1978. Thermal degradation of ammonium polymethacrylate and polymethacrylamide. J. Polym. Sci. Polym. Chem. Ed. 16, 2465–2474. Mourgues, L., Peyron, F., Trambouze, Y., Prettre, M., 1967. Kinetics of the catalytic dehydration of 2-propanol. J. Catal. 7, 117–125. Nakamoto, K., 1997. IR and Raman Spectra of Inorganic Coordination Compounds, Applications in Coordination, Organometallic and Bioinorganic Chemistry Part B. John-Wiley & Sons, New York. Odian, G., 1991. Principles of Polymerization. Wiley & Sons, New York (pp. 721–738). Orita, H., Hayakawa, T., Shimizu, M., Takemira, K., 1991. Reaction of 2-propanol over heteropolyacids after pre-treatment in air. Appl. Catal. 77, 133–140. Pe´rez-Herna´ndez, R., Aguilar, F., Go´mez-Corte´s, A., Dı´az, G., 2005. NO reduction with CH4 or CO on Pt/ZrO2–CeO2 catalysts. Catal. Today 107, 175–180. Pittman, C., Carraher, C., Sheats, J., Tinken, M., Zeldin, M., 1990. Inorganic and metal-containing polymers, an overview. In: Sheats, J., Carraher, C., Pittman, C., Zeldin, M., Currel, B. (Eds.), Inorganic and Metal-containing Polymeric Materials. Plenum Press, New York, pp. 1–27. Pomogalio, A.D., 1990. Metal-containing polymers in the soviet union. In: Sheats, J., Carraher, C., Pittman, C., Zeldin, M., Currel, B. (Eds.), Inorganic and Metalcontaining Polymeric Materials. Plenum Press, New York, pp. 29–60. Regulla, D.F., Deffner, U., 1982. Dosimetry by ESR spectroscopy of alanine. J. Appl. Radiat. Isot. 33, 1101–1114. Rufino, E.S., Monteiro, E.C., 2000. Characterization of lithium and sodium salts of poly(methacrylic acid) by FTIR and thermal analyses. Polymer 41, 4213–4222. Sa´rbu, T., Beckman, E.J., 1999. Homopolymerization and copolymerization of cyclohexane oxide with carbon dioxide using zinc and aluminum catalysts. Macromolecules 32, 6904–6912. Tananbe, K., Misono, M., Ono, Y., Hattori, H., 1989. New solid acids and bases. In: Tananbe, K., Misono, M., Ono, Y., Hattori, H. (Eds.), Studies in Surface Science and Catalysis. Elsevier, New York, pp. 142. Van den Brand, J., Snijders, P.C., Sloof, W.G., Terryn, H., De Wit, H.W., 2004. Acid– base Characterization of aluminum oxide surfaces with XPS. J Phys. Chem. B 108, 6017–6024. Vilchis-Nestor, A.R., 2000. Synthesis by gamma radiation and characterization of poly[(methacrylato)zinc(II)]. BSc. Thesis. Facultad de Quı´mica, Universidad Auto´noma del Estado de Me´xico, Me´xico. ˜ a-Nun ˜ ez, F., Lo´pez-Castan ˜ ares, R., Vilchis-Nestor, A.R., Sa´nchez-Mendieta, V., Uren Ascencio, J.A., 2006. X-ray photoelectron, infrared, and Raman spectroscopy studies of novel sub-micrometric poly [(methacrylato)aluminum(III)]. J. Appl. Polym. Sci. 102–6, 5212–5223. Zulfiqar, S., Masud, K., 2002. Thermal degradation of blends of allyl methacrylate– methyl methacrylate copolymers with aluminum isopropoxide. Polym. Degrad. Stab. 78, 305–313.