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Preparation of mesoporous and macroporous materials from rubber of tyre wastes
Microporous and Mesoporous Materials 67 (2004) 35–41 www.elsevier.com/locate/micromeso
Preparation of mesoporous and macroporous materials from rubber of tyre wastes n-Vizuete a, A. Macıas-Garcıa b,*, A. Nadal Gisbert c, E. Mancho C. Fern andez-Gonz alez a, V. G omez-Serrano a,*,1 a Departamento de Quımica Inorg anica, Universidad de Extremadura, 06071 Badajoz, Spain Departamento de Electronica e Ingenierıa Electromecanica, Escuela de Ingenierıas Industriales, Universidad de Extremadura. Avda. de Elvas s/n, 06071 Badajoz, Spain Departamento de Ingenierıa Mecanica y Materiales, Escuela Politecnica Superior de Alcoy, Alicante, Spain b
c
Received 28 July 2003; received in revised form 8 October 2003; accepted 10 October 2003
Abstract From residual rubber (RR) separated from tyre wastes, adsorbent materials were prepared by applying thermal, chemical and combined (thermal and chemical or vice versa) methods. In the preparation of samples, RR was heated at 200–900 C for 2 h in N2 atmosphere. The material was also contacted with H2 SO4 , HNO3 , and H2 SO4 /HNO3 solutions for 24 h. Finally, RR was first heated at 400 C for 2 h in N2 and the resultant product was then treated chemically with an H2 SO4 /HNO3 solution for 24 h, or vice versa. Both RR and the products derived from it were characterised texturally by gas adsorption (N2 , 77 K), mercury porosimetry, and helium and mercury density measurements. Usually, the yield is lower for the heat treatments than for the single chemical treatments (e.g., the smallest yield values are 33.0 and 60.5 wt.%). For the combined treatments, as compared to the single treatments, the yield is higher. RR is a nonporous solid. The treatments of RR as a rule result in a great porosity development mainly in the mesopore and macropore ranges for the heat and chemical treatments, respectively. The presence of HNO3 in the acid solution used in the treatments of RR has proved to be an essential factor for the development of the porosity made up of large pores. When RR is subjected to combined treatments, the predominant effect on the porosity development is that of the treatment effected first. By successive heat and chemical treatments, a product containing the same volumes of mesopores and macropores is obtained. The micropore content is low for all products, but especially for those prepared chemically. 2003 Elsevier Inc. All rights reserved. Keywords: Carbonaceous adsorbents; Tyre wastes; Thermal methods; Chemical methods
1. Introduction Europe, the USA and Japan together generate more than 5 · 106 tonnes of scrap tyres per year [1]. According to recent statistics, the populations of the USA and Canada together dispose of over one tyre per person per year. In Europe, about 1.8–2.0 Mt/a scrap tyres cannot be used any longer for their original purpose [2]. Most of these tyres are simply dumped in the open or in landfills, which is environmentally not acceptable. Tyre piles can be the source of very toxic emissions in the case of a fire *
Corresponding authors. Tel.: +34-924289600; fax: +34-924289395. E-mail addresses: [email protected] (A. Macıas-Garcıa), [email protected] (V. G omez-Serrano). 1 Tel.: +34-24-289300-9040; fax: +34-24-289395. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.10.002
[3] and may act as a breeding ground for mosquitoes [4]. Several recycling methods exist. After retreading of the carcass, the tyres may be reused [5]. Ground tyres can be used in civil engineering applications, for example as an additive in road pavement [6]. Other applications include in playground surface [7], rubber roofs [8], drainage systems [9], and floor mats [10]. A major market for scrap tyres is their utilisation as solid fuels [11], especially in cement kilns [12]. Due to the irreversible chemical structure of the elastomers, however, a primary or secondary recycling is only possible to a limited extent. Therefore, these wastes represent a serious disposal problem. Because in tyre rubber carbon black is one of the most abundant components, its amount (22 wt.%) being only surpassed by that of hydrocarbon (45–48 wt.%) [13],
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and the carbon content is as high as 68–75 wt.% [13], the reuse of this waste as starting material in the preparation of carbonaceous adsorbents was viewed as possible. In this way it should palliate a serious environmental problem, value such a waste, and lower the manufacture cost of the adsorbents. Bearing these facts in mind, it was thought on modifying the chemical composition of tyre rubber to develop the porosity in the residual solid. With such an aim, in this study tyre rubber was first subjected to various heat (pyrolysis), chemical, and combined treatments and the resultant products were characterised texturally. Such treatments were devised to affect not only the organic fraction of tyre rubber, as usual, but also the inorganic one. So far, though the pyrolysis of tyres and the activation of the resultant chars have been studied by a number of researchers [14–19], to the authors knowledge the comparison of the effects of heat and chemical treatments of tyre rubber on the textural properties has not been investigated yet.
2. Experimental 2.1. Starting material and reactants In this study, the starting material was residual rubber (RR, hereafter) separated from tyre wastes, after the process of size reduction and removal of components such as cloth and steel. The material (average particle size between 1 and 3 mm) was supplied by the Escuela Politecnica Superior de Alcoy (Alicante, Spain) and, asreceived, used in these investigations. Commercial sulphuric acid (96 wt.%, 1.835 g cm3 ; Carlo Erba) and nitric acid (65 wt.%, 1.395 g cm3 ; Carlo Erba) were used in the chemical treatments of RR.
2.2. Treatments of RR RR was subjected to various heat, chemical, and combined (i.e., heat and chemical or vice versa) treatments. The heat treatment of RR was intended to remove most of its volatile matter content. As reported earlier [20–23], by pyrolysis the tyre rubber is cracked into oils and gas and the carbon black filler is recovered. Also it has been found in previous studies of tyre pyrolysis [17,24,25] that the quantity of char exceeds the amount of carbon black present in the tyres used. Thus, the solid product may be regarded as a mixture of carbon black and char formed by tyre rubber degradation. In the present study, using a vertical cylindrical furnace and a stainless steel reactor, about 10 g of RR was heated from 25 to 200–900 C with 100 C temperature intervals in N2 atmosphere (flow rate ¼ 225 ml min1 ). Heating rate was 10 C min1 . Isothermal time at maximum heat treatment temperature was 2 h. Such a temperature, yield values, and the notations used for the samples are indicated in Table 1. In the single chemical treatments of RR an aqueous solution of sulphuric acid, nitric acid, or both acids was used. The goal of such treatments was not only to bring about composition changes in the organic and inorganic fractions of RR (as an example, H2 SO4 is a strong dehydrating substance that can take up hydrogen and oxygen from organic matter and cause its carbonisation; HNO3 , however, is a powerful oxidising agent in concentrated solutions) by leaching out some of its components to a certain extent, but also to modify the surface chemistry of the carbonaceous material present in RR by introducing sulphur and nitrogen surface groups, such as HSO3 and NO2 , in the material. These groups, as is well known, may influence the adsorption behaviour of the samples. The heat and chemical treat-
Table 1 Preparation of the samples (Nomenclature and yielda ) Treatment
Temperature (C) or acid solution
Nomenclature
Yield (wt.%)
Heat Heat Heat Heat Heat Heat Heat Heat Acid Acid Acid Acid Acid Heat/chemical Chemical/heat
200 300 400 500 600 700 800 900 H2 SO4 HNO3 H2 SO4 /HNO3 (75%/25%) H2 SO4 /HNO3 (50%/50%) H2 SO4 /HNO3 (25%/75%) 400-H2 SO4 /HNO3 (25%/75%) H2 SO4 /HNO3 (25%/75%)-400
H-200 H-300 H-400 H-500 H-600 H-700 H-800 H-900 C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
100.0 92.0 35.0 39.0 39.5 36.4 33.0 33.0 99.0 62.0 60.5 67.0 61.3 84.0 86.1
a Yield ¼ Mf =Mi 100, where Mi is usually the initial mass of RR and Mf is the final mass of sample. For HC-400-S/N(1:3) and CH-S/N(1:3)-400, Mi is mass of the intermediate product obtained after the first heat or chemical treatment of RR.
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ments of RR were carried out to ascertain if the previous heat treatment had any effect on the subsequent chemical interaction of the H2 SO4 /HNO3 solution with the resultant intermediate product. Thus, the heat treatment may produce a development of porosity and this increases the degree of chemical interaction of the acid solution. On the other hand, the heat treatment of the product obtained after the chemical treatment was performed to have an idea of whether the surface state arising from the chemical treatment was thermally stable or not. In the chemical treatments, 25 g of RR, 125 ml of distilled water, and 125 ml of acid solution (i.e., the water/acid ratio used was 1:1 by volume) were added to 1 l capacity volumetric flask. Owing to the significant heat release, acid additions were effected slowly and with steady agitation of the reaction flask. One cooled down, the flask was vigorously agitated to get a good mixture of the liquid and solid phases, and sealed. The heterogeneous system was then allowed to react for 24 h. After that, the solid residue was filtered and washed with distilled water until neutral pH in the filtrate, which took a long time (i.e., two weeks, at least). To reduce the water consumption to a significant extent, a glass fritted funnel connected to a water aspirator vacuum was used, the device being activated only from time to time. Moreover, in connection with the possible recovery of the environmentally unfriendly mineral acids from the washed liquid it should be recalled here that, for the moment, it is economically unfeasible for substances such as H2 SO4 because of its low-cost production, the normal practice being then to neutralise the acid with sodium chloride and to drain the residual liquid [13]. Also it should be noted that the determination of whether RR influenced the pH of the washed water or not was not possible as when RR was placed into contact with distilled water in a glass container to effect pH measurements as a function of time a large amount of the material remained without sinking. The solid residue obtained after washing with distilled water was oven-dried in two successive steps at 60 C for 12 h and at 120 C for 24 h. In the combined treatments, RR was first heated at 400 C for 2 h in N2 (flow rate ¼ 225 ml min1 ) and then chemically treated with an H2 SO4 /HNO3 solution, and also vice versa. The H2 SO4 / HNO3 ratios in the solutions used in the chemical treatments, the resultant yield values, and the notations of the samples can be seen in Table 1. 2.3. Textural characterisation The starting rubber and the samples prepared from it were characterised texturally by gas adsorption, mercury porosimetry, and density measurements. The adsorption isotherms for N2 at 77 K were measured using a Quantachrome equipment, Autosorb-1. About 0.15 g of
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sample was first oven-dried at 110 C overnight and then outgassed in the own adsorption equipment at 120 C for 12 h, at a pressure lower than 103 Torr, prior to effecting adsorption measurements. The mercury porosimetries of the samples were carried out with the aid of a porosimeter (Quantachrome), Autoscan-60. After oven-drying at 110 C for 12 h, about 0.5 g of sample was first added to a suitable glass holder. This was then evacuated with an oil pump at 13.33 Pa and filled with mercury in a Quantachrome apparatus. Finally, the holder was placed in the autoclave of the porosimeter and pressurised to 4.135 · 102 MPa. The mercury density was determined when effecting the mercury porosimetry. The volume of sample was obtained by knowing the calibration volume of the sample holder and the volume of mercury present in this container. The latter datum was obtained from the mass of mercury contained in the holder and from the density of mercury at the working temperature. The helium density was measured using a Quantachrome steorepycnometer. About 3 g of sample, after oven drying at 110 C for 12 h, was weighted in the cell holder before carrying out density measurements.
3. Results and discussion 3.1. Preparation of the samples The yield of the preparation of the samples (Table 1) depends on heat treatment temperature and on the composition of the H2 SO4 /HNO3 solution. The yield is much higher at 200 C (100 wt.%) and 300 C (92.0 wt.%) than between 400 and 900 C. In this temperature range the yield varies between 39.5 and 33.0 wt.%, these values being similar to those reported earlier in the literature [17,26–28]. Accordingly, the heat treatment of RR is only effective to remove volatile matter from the material when heating in the 400–900 C temperature range. As a result, the preparation of a large amount of sample for the subsequent textural study was only undertaken at these temperatures. Between 400 and 900 C the yield usually decreases with increasing temperature, though the temperature effect on the mass loss is not very significant in this temperature range. At 400 C, in fact, the release of most volatile matter present in RR occurs. In the single chemical treatments of RR, the yield is quite different according to the composition of the acid solution. When the solution of H2 SO4 alone is used, then the yield is very high (99.0 wt.%), whereas with the rest of the acid solutions the yield is much smaller. In this case it lowers to nearly 60 wt.%. From these results it becomes clear that the presence or not of HNO3 in the acid solution is the determinant factor in
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3.2. Textural characterisation The adsorption isotherms (i.e., the plots of the volume adsorbed, V , versus relative pressure, p=p0 ) for N2 at 77 K measured for all samples are plotted in Figs. 1– 3. The fact that the isotherm determined for RR is situated very closely to the abscissa axis in the whole p=p0 range denotes that RR is an almost nonporous material in the micropore and mesopore ranges. As far as the samples prepared by heat treatment of RR is concerned, the isotherms show the three following features, which enable one to derive information on their textural properties. One of them is the small amount of N2 adsorption at low p=p0 values. Another one is the branch of slight slope that extends up to p=p0 0:9. The third one is the branch of great slope at p=p0 > 0:9. These results are compatible with the presence in the samples of a low micropore volume and of a wide pore
450 400 350 RR H-400 H-500 H-600 H-700 H-800 H-900
3 -1
V (cm g )
300 250 200 150 100 50 0 0.0
0.2
0.4
0.6
0.8
1.0
p/p 0 Fig. 1. Adsorption isotherms (N2 , 77 K) for the samples prepared by heat treatment of RR.
200 RR C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3)
150 3 -1
connection with the mass loss produced in RR. However, the magnitude of the effect does not seem to be conditioned by the amount of HNO3 present in the reaction medium as the yield is similar to C-N, C-S/ N(3:1) and C-S/N(1:3), whereas to C-S/(1:1) it is somewhat higher. In accordance with the results obtained with H2 SO4 and HNO3 it is obvious that the oxidising action of HNO3 on the chemical composition of RR is much more important than the dehydrating one due to H2 SO4 . The yield is strongly dependent on the heat or chemical treatment effected to RR. Thus, as a rule it is lower for the heat treatment. Therefore, heating is more effective than the acid attack to remove matter (i.e., volatile matter when dealing with the first method) from RR. In view of these results it appears likely that both mineral acids act more selectively on the chemical components of RR or that the reactive matter present in this material is not totally accessible to them. In the case of the combined treatments, the yield is usually (except for C-S) rather higher than for the single ones. For the heat and chemical treatments it can be accounted for by bearing in mind that the chemical treatment was carried out using a product that had been heated before at 400 C. Therefore, the content of organic matter was already greatly reduced in such a product and this should restrict the action to the H2 SO4 /HNO3 solution. To the higher mass of sample obtained in the combined treatments may also contribute the formation of a larger amount of surface groups in the intermediate product owing to a greater diffusion of the acid solution in pores of the material. The higher yield also for the chemical and heat treatments is compatible with the fact that an important part of the precursors of volatile matter present in RR was likely leached out in the treatment of RR with such a solution. The results obtained in this combined treatment are of significance as they evidence that H2 SO4 (BP/C, 300, decomposition) and HNO3 (BP/ C, 82.6) were not found as such in the product treated chemically since, otherwise, both acids should be removed by evaporation when heating at temperatures above their boiling points. It was found previously in investigations carried out on the activated carbon/H2 SO4 adsorption system [29]. Although activated carbon was able to uptake a large amount of H2 SO4 , most of this acid was lost from the carbon when heating at temperatures close to its boiling point. Therefore, in the treatment of the product obtained from RR with the H2 SO4 /HNO3 solution these acids either transformed into thermally stable products or the acid excess was removed in the subsequent long washing with distilled water. In fact, as is well known, these acids are readily soluble in water.
V(cm g )
38
100
50
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 2. Adsorption isotherms (N2 , 77 K) for the samples prepared by chemical treatment of RR.
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39
1.0
450 400
3 -1
3 -1
250
CPV (cm g )
300
V (cm g )
0.8
RR H-400 C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
350
200 150
RR H-400 H-500 H-600 H-700 H-800 H-900
0.6
0.4
0.2
100 50
0.0 10
0 0.0
0.2
0.4
0.6
0.8
1.0
100
100000
Fig. 4. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by heat treatment of RR.
1.0
3 -1
CPV (cm g )
0.8
0.6 RR C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3)
0.4
0.2
0.0 10
100
1000
10000
100000
0
R (A )
Fig. 5. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by chemical treatment of RR.
1.0
RR H-400 C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
0.8
3 -1
CPV (cm g )
size distribution in the micropore and mesopore ranges made up mainly of large mesopores. The temperature effect on textural properties of the samples is evident from the higher mesopore volume of products such as H-800 and H-900. For the samples prepared by effecting single chemical treatments to RR as compared to the heat-treated ones, from the adsorption isotherms (Figs. 1 and 2) it follows that the development of porosity in the micropore and mesopore ranges as a rule is smaller for the former products. Thus notice that the isotherms determined for C-S, C-N, C-S/N(1:1), and C-S/N(3:1) are situated close together with the isotherm of RR. In the case of the samples obtained by combined treatments of RR it seems that the predominant effect on the microporous and mesoporous textures is that of the heat or chemical treatment carried out first to RR, as shown by the fact that the isotherms for H-400 and HC-400-S/N(1:3) occupy an almost coincident position in Fig. 3. The plots of the cumulative pore volume (CPV) against pore radius (R), as obtained by mercury porosimetry, for all samples are shown in Figs. 4–6. It is inferred that RR not only is a nonporous solid in the micropore and mesopore ranges but also in the macropore region. For the samples prepared from RR, the pore size distribution in the mesopore and macropore ranges depends on the method used in their preparation. The products obtained by heat treatment of RR possess an uni-modal distribution of pore sizes made up of large mesopores. These results are in good agreement with those obtained by N2 adsorption. The total cumulative pore volume is influenced by maximum heat treatment temperature, being higher for H-400, H-900 and H-800. For the samples prepared by performing single chemical treatments to RR, the plots of mercury intrusion (Fig. 5) show a strong dependence of the pore size distribution in the mesopore and macropore ranges on
10000
0
p/p
Fig. 3. Adsorption isotherms (N2 , 77 K) for the samples prepared by combined treatments of RR.
1000
R(A )
0
0.6
0.4
0.2
0.0 10
100
1000
10000
100000
0
R (A )
Fig. 6. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by combined treatments of RR.
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the HNO3 content in the acid solution used in their preparation. As can be seen, C-S is also a practically nonporous sample in such porosity regions, which contrasts with the wide pore size distributions of C-N, C-S/N(3:1), C-S/N(1:1) and C-S/N(1:3). These results are in accord with the greater mass loss produced in the preparation of this series of samples. Nevertheless, some significant differences are noted in their pore size distribution. This changes usually in the sense of increasing content of wider macropores with increasing HNO3 content in the solution. In the case of C-N and C-S/ N(1:3), both possess an almost identical pore size distribution in the region of larger macropores, however CS/N(1:3) contains narrow macropores and even wide mesopores that are absent from C-N. When comparing with the heat-treated samples (Fig. 4), it is worth noting that for those obtained by chemical methods the pore size distribution is wider and it is made up of larger pores (i.e., macropores, instead of mesopores). With regard to the samples prepared by combined treatments, the pore size distribution is similar to that for the sample obtained by carrying out a single heat or chemical treatment, in special for the couple C-S/N(1:3) and CHS/N(1:3)-400. For H-400 and HC-400-S/N(1:3) (Fig. 6), the only remarkable difference in their mesoporous and macroporous textures is the higher volume of mesopores for the HC-400-S/N(1:3). From the N2 adsorption isotherms, the specific surface area was estimated by applying the Brunauer, Emmett and Teller (BET) equation [30] in the p=p0 2 . From range 0.05–035, taking am to be equal to 16.2 A such isotherms, the micropore volume (Vmi ) and the mesopore volume (Vme ) were obtained simply by reading the volume adsorbed (Vad ) at p=p0 ¼ 0:10 and 0.95. Thus, Vmi ¼ Vadðp=p0 ¼0:10Þ and Vme ¼ Vadðp=p0 ¼0:95Þ Vadðp=p0 ¼0:10Þ . The micropore volume (W0 ) was also derived from the application of the Dubinin–Rad-
ushkevich equation [31]. Vmi , Vme and W0 were expressed as liquid volumes. The values of these pore volumes together with the SBET values are listed in Table 2. From the chart of mercury porosimetry, or from the derived curves of CPV ¼ f(R), the mesopore and macropore volumes ðVme-P , Vma Þ were obtained. Vma was taken as the volume of the pores having a radius greater and Vme-P as the volume of the pores with than 250 A and the lowest radius radii comprised between 250 A (18 A) of mercury intrusion in each porosimetry run. The values of Vme-P and Vma are also compiled in Table 2. From the helium and mercury densities (qHe , qHg ), the total pore volume (VT ) was calculated by making use of the expression: VT ¼ 1=qHg 1=qHe . The values of qHe , qHg and VT are given in Table 2. From data of Table 2 it follows that the samples prepared by heat treatment of RR are chiefly mesoporous solids, though containing relatively high macropore contents; whereas most of those obtained by chemical methods (except for C-S) including the one prepared by the chemical and heat treatments are mainly macroporous solids. Vme-P ranges between 0.34 cm3 g1 for H-500 and H-900 and 0.40 cm3 g1 for H400 and Vma-P between 0.52 cm3 g1 for C-S/N(3:1) and 0.69 cm3 g1 for C-S/N(1:3). For HC-400-S/N(1:3) the effects on both porosity ranges conjugate, then being an equally mesoporous and macroporous solid (Vme-P ¼ Vma-P ¼ 0:24 cm3 g1 ). In accordance with the presence of mostly intermediate and large size pores in the samples, their specific surface area is low. Thus, SBET varies between 13.4 and 47.4 m2 g1 for the heattreated samples and between 2.0 and 7.5 m2 g1 for the chemically treated samples. The somewhat larger values of SBET for the former samples is in line with the greater development of the microporosity in these products, as compared to those prepared by chemical methods. For the products obtained by combined
Table 2 Textural properties of the samples Sample
RR H-400 H-500 H-600 H-700 H-800 H-900 C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
Mercury porosimetry
Density measurements
Vmi (cm3 g1 )
N2 adsorption Vme (cm3 g1 )
W0 (cm3 g1 )
SBET (m2 g1 )
VmeP (cm3 g1 )
Vma (cm3 g1 )
qHg (cm3 g1 )
qHe (cm3 g1 )
VT (cm3 g1 )
0.000 0.002 0.011 0.022 0.022 0.028 0.030 0.000 0.002 0.006 0.004 0.030 0.024 0.006
0.001 0.018 0.117 0.185 0.178 0.182 0.180 0.001 0.015 0.044 0.032 0.037 0.018 0.031
0.001 0.043 0.027 0.048 0.046 0.053 0.045 0.001 0.005 0.012 0.010 0.041 0.050 0.010
0.0 18.1 13.4 21.0 18.2 18.1 47.4 2.3 2.0 5.7 5.0 7.5 14.0 5.0
0.04 0.40 0.34 0.38 0.37 0.37 0.34 0.03 0.08 0.13 0.09 0.07 0.24 0.07
0.03 0.21 0.17 0.20 0.22 0.26 0.31 0.06 0.56 0.52 0.57 0.69 0.24 0.61
1.17 1.97 1.83 1.88 1.97 2.04 2.01 1.25 1.54 1.58 1.56 1.86 2.03 1.75
1.07 0.78 0.86 0.83 0.79 0.77 0.76 1.05 0.69 0.68 0.67 0.65 0.70 0.67
0.08 0.77 0.66 0.67 0.76 0.81 0.82 0.15 0.80 0.84 0.85 1.02 0.94 0.92
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treatments, SBET presents a close value to those for the heat treated samples (HC-400-S/N(1:3), 14.0 m2 g1 ) or for the chemically treated samples (CH-S/ N(1:3)-400, 5.0 m2 g1 ). Finally, the total pore volume is high for all samples, except for C-S (VT ¼ 0:15 cm3 g1 ). Thus, VT ranges between 0.66 cm3 g1 for H-500 and 1.02 cm3 g1 for C-S/N(1:3). This large VT values are compatible with the well developed mesoporosity or macropority in the samples. Nevertheless, the fact that VT > Vmi þ Vme-P þ Vma-P suggests that a fraction of the porosity present in the samples is accessible to helium at room temperature but not to nitrogen at 77 K.
4. Conclusions From the above results the following conclusions may be drawn: • The resultant yield in the preparation of the samples is significantly lower for the heat treatments of RR than for the chemical treatments. The maximum yield (i.e., 99.0 wt.%) corresponds to the treatment with the H2 SO4 solution, whereas the minimum one (i.e., 33.0 wt.%) is for the heat treatment at 800 and 900 C. Also, the combined treatments as a rule results in higher yields than the single treatments. • RR is a nonporous material in all porosity ranges. • Usually, the main porosity development occurs in the mesopore range for the heat treatments and in the macropore range for the chemical treatments. The presence of HNO3 in the acid solution used in the treatments of RR is determinant in order to create large pores in the material. Only the single treatment with the H2 SO4 solution does not affect the porosity of RR. • In the combined treatments of RR, the predominant effect on textural properties of the samples is the one due to the treatment performed first. Nevertheless, the product obtained by successive heat and chemical treatments of RR possesses an equal volume of mesopores and macropores. • By treatment of RR, the tailoring of the pore size distribution in the mesopore and macropore ranges is possible. This is a remarkable finding allowing for that nowadays there is a need of large pores containing materials to be used in the chemical industry, as an example, in catalysis processes. Thus, the aforesaid materials can accommodate a great variety of host molecules, e.g., substances of pharmacological interest, which are much larger than those that can adsorb the traditional microporous zeolites [32]. Furthermore, the adsorbents would be prepared from a cheap starting material, which would surely lower their production cost.
41
References [1] G. San Miguel, G.D. Fowler, C.J. Sollars, Ind. Eng. Chem. Res. 37 (1998) 2430–2435. [2] G. Capelle, Gummi Fasern Kunststoffe 50 (4) (1997) 297. [3] P.J. Steer, C.H.M. Tashiro, W.D. McIlivenn, R.E. Clement, Water Air Soil Pollunt. 82 (1995) 659. [4] P.J. Lampman, S. Hansonand, R. Novak, J. Am. Mosquito Control Assoc. 13 (1997) 193. [5] G. Ferrer, Resour. Conserv. Recyc. 19 (1997) 221. [6] B.Z. Savas, Sh. Ahmad, D. Fedroff, Transportation Research Record no. 1574, November 1996, National Research Council, Washington, D.C., USA, 1996, pp. 80–88. [7] B. Siurn, Scrap Tire News 12 (1998) 8. [8] B. Siurn, Scrap Tire News 12 (1998) 1. [9] B. Siurn, Scrap Tire News 11 (1997) 1. [10] B. Siurn, Scrap Tire News 12 (1998) 5. [11] M. Blumenthal, in: Advances in Instrumentation and Control; International Conference and Exhibition Proceedings of the 1996 International Conference on Advances in Instrumentation and Control. ISA/96. Part 1 (of 2), 51 (1) 1996, USA 2. Instrument Society of America Research Triangle Park, N.C., USA, 1996, pp. 405–414. [12] B. Siurn, Scrap Tire News 11 (1997) 10. [13] X. Elias Castells, Reciclaje de residuos industriales, Ed. Dıaz de Santos, Madrid, 2000, p. 495. [14] A.M. Cunliffe, P.T. Williams, Energy Fuels 13 (1) (1999) 166–175. [15] S. Ogasawara, M. Kuroda, N. Wakao, Ing. Eng. Chem. Res. 26 (12) (1987) 2552–2556. [16] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (12) (1990) 1474–1482. [17] A.A. Merchant, M.A. Petrich, AIChE J. 39 (8) (1993) 1370–1376. [18] H. Teng, M.A. Serio, M.A. Wojtowicz, R. Bassilakis, P.R. Solomon, Ing. Eng. Chem. Res. 34 (9) (1995) 3102–3111. [19] S.H. Lin, J. Chem. Technol. Biotechnol. 57 (4) (1993) 387–391. [20] C. Roy, B. Labrecque, B. Caumia, Resour. Conserv. Recyc. 4 (1990) 203. [21] W. Kaminsky, H. Sinn, in: J.L. Jones, S.B. Radding (Eds.), Termal Conversi on of Solid Wastes and Biomass, ACS Symposium Series 130, ACS Publishers, Washington, D.C., 1980, p. 423. [22] S. Kawakami, K. Inonue, H. Tanaka, T. Sakai, in: J.L. Jones, S.B. Radding (Eds.), Termal Conversi on of Solid Wastes and Biomass, ACS Symposium Series 130, Acs Publishers, Washington, D.C., 1980, p. 557. [23] P.T. Williams, S. Besler, D.T. Taylor, R.P. Bottrill, J. Inst. Energy 68 (1995) 11. [24] B.L. Schulman, P.A. White, in: J.L. Jones, S.B. Radding (Eds.), Solid Wastes and Residues: Conversi on by Advanced Thermal Processes, American Chemical Society, Washington, D.C., 1978. [25] W. Kaminsky, H. Rossler, Chemtech 22 (1992) 108. [26] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (12) (1990) 1474– 1482. [27] H. Teng, M.A. Serio, M.A. Wojtowicz, R. Bassilakis, P.R. Solomon, Ind. Eng. Chem. Res. 34 (9) (1995) 3102–3111. [28] A. Ariyadejwanich, W. Tanthapanichakoon, K. Nakagawa, S.R. Mukai, H. Tamon, Carbon 41 (2003) 157–164. [29] V. G omez-Serrano, M. Acedo-Ramos, C. ValenzuelaCalahorro, A.J. L opez-Peinado, J. Chem. Technol. Biotechnol. 75 (2000) 835– 839. [30] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [31] M.M. Dubinin, in: J.F. Danielli, M.D. Rosenberg, D.A. Cadenhead (Eds.), Progr. Surf. Membr. Sci., 9, Academic Press, New York, 1975, p. 1. [32] J. Perez-Pariente, A.L. Doadrio, J.C. Doadrio, P. Horcajada, I. Izquierdo-Barba, A. Martınez, B. Mu~ noz, A. Ramila, M. ValletRegı, in: Proccedings of the XXIX Reuni on Bienal de la Real Sociedad Espa~ nola de Quımica. S3O4, 2003, Madrid.
Preparation of mesoporous and macroporous materials from rubber of tyre wastes n-Vizuete a, A. Macıas-Garcıa b,*, A. Nadal Gisbert c, E. Mancho C. Fern andez-Gonz alez a, V. G omez-Serrano a,*,1 a Departamento de Quımica Inorg anica, Universidad de Extremadura, 06071 Badajoz, Spain Departamento de Electronica e Ingenierıa Electromecanica, Escuela de Ingenierıas Industriales, Universidad de Extremadura. Avda. de Elvas s/n, 06071 Badajoz, Spain Departamento de Ingenierıa Mecanica y Materiales, Escuela Politecnica Superior de Alcoy, Alicante, Spain b
c
Received 28 July 2003; received in revised form 8 October 2003; accepted 10 October 2003
Abstract From residual rubber (RR) separated from tyre wastes, adsorbent materials were prepared by applying thermal, chemical and combined (thermal and chemical or vice versa) methods. In the preparation of samples, RR was heated at 200–900 C for 2 h in N2 atmosphere. The material was also contacted with H2 SO4 , HNO3 , and H2 SO4 /HNO3 solutions for 24 h. Finally, RR was first heated at 400 C for 2 h in N2 and the resultant product was then treated chemically with an H2 SO4 /HNO3 solution for 24 h, or vice versa. Both RR and the products derived from it were characterised texturally by gas adsorption (N2 , 77 K), mercury porosimetry, and helium and mercury density measurements. Usually, the yield is lower for the heat treatments than for the single chemical treatments (e.g., the smallest yield values are 33.0 and 60.5 wt.%). For the combined treatments, as compared to the single treatments, the yield is higher. RR is a nonporous solid. The treatments of RR as a rule result in a great porosity development mainly in the mesopore and macropore ranges for the heat and chemical treatments, respectively. The presence of HNO3 in the acid solution used in the treatments of RR has proved to be an essential factor for the development of the porosity made up of large pores. When RR is subjected to combined treatments, the predominant effect on the porosity development is that of the treatment effected first. By successive heat and chemical treatments, a product containing the same volumes of mesopores and macropores is obtained. The micropore content is low for all products, but especially for those prepared chemically. 2003 Elsevier Inc. All rights reserved. Keywords: Carbonaceous adsorbents; Tyre wastes; Thermal methods; Chemical methods
1. Introduction Europe, the USA and Japan together generate more than 5 · 106 tonnes of scrap tyres per year [1]. According to recent statistics, the populations of the USA and Canada together dispose of over one tyre per person per year. In Europe, about 1.8–2.0 Mt/a scrap tyres cannot be used any longer for their original purpose [2]. Most of these tyres are simply dumped in the open or in landfills, which is environmentally not acceptable. Tyre piles can be the source of very toxic emissions in the case of a fire *
Corresponding authors. Tel.: +34-924289600; fax: +34-924289395. E-mail addresses: [email protected] (A. Macıas-Garcıa), [email protected] (V. G omez-Serrano). 1 Tel.: +34-24-289300-9040; fax: +34-24-289395. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.10.002
[3] and may act as a breeding ground for mosquitoes [4]. Several recycling methods exist. After retreading of the carcass, the tyres may be reused [5]. Ground tyres can be used in civil engineering applications, for example as an additive in road pavement [6]. Other applications include in playground surface [7], rubber roofs [8], drainage systems [9], and floor mats [10]. A major market for scrap tyres is their utilisation as solid fuels [11], especially in cement kilns [12]. Due to the irreversible chemical structure of the elastomers, however, a primary or secondary recycling is only possible to a limited extent. Therefore, these wastes represent a serious disposal problem. Because in tyre rubber carbon black is one of the most abundant components, its amount (22 wt.%) being only surpassed by that of hydrocarbon (45–48 wt.%) [13],
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and the carbon content is as high as 68–75 wt.% [13], the reuse of this waste as starting material in the preparation of carbonaceous adsorbents was viewed as possible. In this way it should palliate a serious environmental problem, value such a waste, and lower the manufacture cost of the adsorbents. Bearing these facts in mind, it was thought on modifying the chemical composition of tyre rubber to develop the porosity in the residual solid. With such an aim, in this study tyre rubber was first subjected to various heat (pyrolysis), chemical, and combined treatments and the resultant products were characterised texturally. Such treatments were devised to affect not only the organic fraction of tyre rubber, as usual, but also the inorganic one. So far, though the pyrolysis of tyres and the activation of the resultant chars have been studied by a number of researchers [14–19], to the authors knowledge the comparison of the effects of heat and chemical treatments of tyre rubber on the textural properties has not been investigated yet.
2. Experimental 2.1. Starting material and reactants In this study, the starting material was residual rubber (RR, hereafter) separated from tyre wastes, after the process of size reduction and removal of components such as cloth and steel. The material (average particle size between 1 and 3 mm) was supplied by the Escuela Politecnica Superior de Alcoy (Alicante, Spain) and, asreceived, used in these investigations. Commercial sulphuric acid (96 wt.%, 1.835 g cm3 ; Carlo Erba) and nitric acid (65 wt.%, 1.395 g cm3 ; Carlo Erba) were used in the chemical treatments of RR.
2.2. Treatments of RR RR was subjected to various heat, chemical, and combined (i.e., heat and chemical or vice versa) treatments. The heat treatment of RR was intended to remove most of its volatile matter content. As reported earlier [20–23], by pyrolysis the tyre rubber is cracked into oils and gas and the carbon black filler is recovered. Also it has been found in previous studies of tyre pyrolysis [17,24,25] that the quantity of char exceeds the amount of carbon black present in the tyres used. Thus, the solid product may be regarded as a mixture of carbon black and char formed by tyre rubber degradation. In the present study, using a vertical cylindrical furnace and a stainless steel reactor, about 10 g of RR was heated from 25 to 200–900 C with 100 C temperature intervals in N2 atmosphere (flow rate ¼ 225 ml min1 ). Heating rate was 10 C min1 . Isothermal time at maximum heat treatment temperature was 2 h. Such a temperature, yield values, and the notations used for the samples are indicated in Table 1. In the single chemical treatments of RR an aqueous solution of sulphuric acid, nitric acid, or both acids was used. The goal of such treatments was not only to bring about composition changes in the organic and inorganic fractions of RR (as an example, H2 SO4 is a strong dehydrating substance that can take up hydrogen and oxygen from organic matter and cause its carbonisation; HNO3 , however, is a powerful oxidising agent in concentrated solutions) by leaching out some of its components to a certain extent, but also to modify the surface chemistry of the carbonaceous material present in RR by introducing sulphur and nitrogen surface groups, such as HSO3 and NO2 , in the material. These groups, as is well known, may influence the adsorption behaviour of the samples. The heat and chemical treat-
Table 1 Preparation of the samples (Nomenclature and yielda ) Treatment
Temperature (C) or acid solution
Nomenclature
Yield (wt.%)
Heat Heat Heat Heat Heat Heat Heat Heat Acid Acid Acid Acid Acid Heat/chemical Chemical/heat
200 300 400 500 600 700 800 900 H2 SO4 HNO3 H2 SO4 /HNO3 (75%/25%) H2 SO4 /HNO3 (50%/50%) H2 SO4 /HNO3 (25%/75%) 400-H2 SO4 /HNO3 (25%/75%) H2 SO4 /HNO3 (25%/75%)-400
H-200 H-300 H-400 H-500 H-600 H-700 H-800 H-900 C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
100.0 92.0 35.0 39.0 39.5 36.4 33.0 33.0 99.0 62.0 60.5 67.0 61.3 84.0 86.1
a Yield ¼ Mf =Mi 100, where Mi is usually the initial mass of RR and Mf is the final mass of sample. For HC-400-S/N(1:3) and CH-S/N(1:3)-400, Mi is mass of the intermediate product obtained after the first heat or chemical treatment of RR.
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ments of RR were carried out to ascertain if the previous heat treatment had any effect on the subsequent chemical interaction of the H2 SO4 /HNO3 solution with the resultant intermediate product. Thus, the heat treatment may produce a development of porosity and this increases the degree of chemical interaction of the acid solution. On the other hand, the heat treatment of the product obtained after the chemical treatment was performed to have an idea of whether the surface state arising from the chemical treatment was thermally stable or not. In the chemical treatments, 25 g of RR, 125 ml of distilled water, and 125 ml of acid solution (i.e., the water/acid ratio used was 1:1 by volume) were added to 1 l capacity volumetric flask. Owing to the significant heat release, acid additions were effected slowly and with steady agitation of the reaction flask. One cooled down, the flask was vigorously agitated to get a good mixture of the liquid and solid phases, and sealed. The heterogeneous system was then allowed to react for 24 h. After that, the solid residue was filtered and washed with distilled water until neutral pH in the filtrate, which took a long time (i.e., two weeks, at least). To reduce the water consumption to a significant extent, a glass fritted funnel connected to a water aspirator vacuum was used, the device being activated only from time to time. Moreover, in connection with the possible recovery of the environmentally unfriendly mineral acids from the washed liquid it should be recalled here that, for the moment, it is economically unfeasible for substances such as H2 SO4 because of its low-cost production, the normal practice being then to neutralise the acid with sodium chloride and to drain the residual liquid [13]. Also it should be noted that the determination of whether RR influenced the pH of the washed water or not was not possible as when RR was placed into contact with distilled water in a glass container to effect pH measurements as a function of time a large amount of the material remained without sinking. The solid residue obtained after washing with distilled water was oven-dried in two successive steps at 60 C for 12 h and at 120 C for 24 h. In the combined treatments, RR was first heated at 400 C for 2 h in N2 (flow rate ¼ 225 ml min1 ) and then chemically treated with an H2 SO4 /HNO3 solution, and also vice versa. The H2 SO4 / HNO3 ratios in the solutions used in the chemical treatments, the resultant yield values, and the notations of the samples can be seen in Table 1. 2.3. Textural characterisation The starting rubber and the samples prepared from it were characterised texturally by gas adsorption, mercury porosimetry, and density measurements. The adsorption isotherms for N2 at 77 K were measured using a Quantachrome equipment, Autosorb-1. About 0.15 g of
37
sample was first oven-dried at 110 C overnight and then outgassed in the own adsorption equipment at 120 C for 12 h, at a pressure lower than 103 Torr, prior to effecting adsorption measurements. The mercury porosimetries of the samples were carried out with the aid of a porosimeter (Quantachrome), Autoscan-60. After oven-drying at 110 C for 12 h, about 0.5 g of sample was first added to a suitable glass holder. This was then evacuated with an oil pump at 13.33 Pa and filled with mercury in a Quantachrome apparatus. Finally, the holder was placed in the autoclave of the porosimeter and pressurised to 4.135 · 102 MPa. The mercury density was determined when effecting the mercury porosimetry. The volume of sample was obtained by knowing the calibration volume of the sample holder and the volume of mercury present in this container. The latter datum was obtained from the mass of mercury contained in the holder and from the density of mercury at the working temperature. The helium density was measured using a Quantachrome steorepycnometer. About 3 g of sample, after oven drying at 110 C for 12 h, was weighted in the cell holder before carrying out density measurements.
3. Results and discussion 3.1. Preparation of the samples The yield of the preparation of the samples (Table 1) depends on heat treatment temperature and on the composition of the H2 SO4 /HNO3 solution. The yield is much higher at 200 C (100 wt.%) and 300 C (92.0 wt.%) than between 400 and 900 C. In this temperature range the yield varies between 39.5 and 33.0 wt.%, these values being similar to those reported earlier in the literature [17,26–28]. Accordingly, the heat treatment of RR is only effective to remove volatile matter from the material when heating in the 400–900 C temperature range. As a result, the preparation of a large amount of sample for the subsequent textural study was only undertaken at these temperatures. Between 400 and 900 C the yield usually decreases with increasing temperature, though the temperature effect on the mass loss is not very significant in this temperature range. At 400 C, in fact, the release of most volatile matter present in RR occurs. In the single chemical treatments of RR, the yield is quite different according to the composition of the acid solution. When the solution of H2 SO4 alone is used, then the yield is very high (99.0 wt.%), whereas with the rest of the acid solutions the yield is much smaller. In this case it lowers to nearly 60 wt.%. From these results it becomes clear that the presence or not of HNO3 in the acid solution is the determinant factor in
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3.2. Textural characterisation The adsorption isotherms (i.e., the plots of the volume adsorbed, V , versus relative pressure, p=p0 ) for N2 at 77 K measured for all samples are plotted in Figs. 1– 3. The fact that the isotherm determined for RR is situated very closely to the abscissa axis in the whole p=p0 range denotes that RR is an almost nonporous material in the micropore and mesopore ranges. As far as the samples prepared by heat treatment of RR is concerned, the isotherms show the three following features, which enable one to derive information on their textural properties. One of them is the small amount of N2 adsorption at low p=p0 values. Another one is the branch of slight slope that extends up to p=p0 0:9. The third one is the branch of great slope at p=p0 > 0:9. These results are compatible with the presence in the samples of a low micropore volume and of a wide pore
450 400 350 RR H-400 H-500 H-600 H-700 H-800 H-900
3 -1
V (cm g )
300 250 200 150 100 50 0 0.0
0.2
0.4
0.6
0.8
1.0
p/p 0 Fig. 1. Adsorption isotherms (N2 , 77 K) for the samples prepared by heat treatment of RR.
200 RR C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3)
150 3 -1
connection with the mass loss produced in RR. However, the magnitude of the effect does not seem to be conditioned by the amount of HNO3 present in the reaction medium as the yield is similar to C-N, C-S/ N(3:1) and C-S/N(1:3), whereas to C-S/(1:1) it is somewhat higher. In accordance with the results obtained with H2 SO4 and HNO3 it is obvious that the oxidising action of HNO3 on the chemical composition of RR is much more important than the dehydrating one due to H2 SO4 . The yield is strongly dependent on the heat or chemical treatment effected to RR. Thus, as a rule it is lower for the heat treatment. Therefore, heating is more effective than the acid attack to remove matter (i.e., volatile matter when dealing with the first method) from RR. In view of these results it appears likely that both mineral acids act more selectively on the chemical components of RR or that the reactive matter present in this material is not totally accessible to them. In the case of the combined treatments, the yield is usually (except for C-S) rather higher than for the single ones. For the heat and chemical treatments it can be accounted for by bearing in mind that the chemical treatment was carried out using a product that had been heated before at 400 C. Therefore, the content of organic matter was already greatly reduced in such a product and this should restrict the action to the H2 SO4 /HNO3 solution. To the higher mass of sample obtained in the combined treatments may also contribute the formation of a larger amount of surface groups in the intermediate product owing to a greater diffusion of the acid solution in pores of the material. The higher yield also for the chemical and heat treatments is compatible with the fact that an important part of the precursors of volatile matter present in RR was likely leached out in the treatment of RR with such a solution. The results obtained in this combined treatment are of significance as they evidence that H2 SO4 (BP/C, 300, decomposition) and HNO3 (BP/ C, 82.6) were not found as such in the product treated chemically since, otherwise, both acids should be removed by evaporation when heating at temperatures above their boiling points. It was found previously in investigations carried out on the activated carbon/H2 SO4 adsorption system [29]. Although activated carbon was able to uptake a large amount of H2 SO4 , most of this acid was lost from the carbon when heating at temperatures close to its boiling point. Therefore, in the treatment of the product obtained from RR with the H2 SO4 /HNO3 solution these acids either transformed into thermally stable products or the acid excess was removed in the subsequent long washing with distilled water. In fact, as is well known, these acids are readily soluble in water.
V(cm g )
38
100
50
0 0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Fig. 2. Adsorption isotherms (N2 , 77 K) for the samples prepared by chemical treatment of RR.
E. Manchon-Vizuete et al. / Microporous and Mesoporous Materials 67 (2004) 35–41
39
1.0
450 400
3 -1
3 -1
250
CPV (cm g )
300
V (cm g )
0.8
RR H-400 C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
350
200 150
RR H-400 H-500 H-600 H-700 H-800 H-900
0.6
0.4
0.2
100 50
0.0 10
0 0.0
0.2
0.4
0.6
0.8
1.0
100
100000
Fig. 4. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by heat treatment of RR.
1.0
3 -1
CPV (cm g )
0.8
0.6 RR C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3)
0.4
0.2
0.0 10
100
1000
10000
100000
0
R (A )
Fig. 5. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by chemical treatment of RR.
1.0
RR H-400 C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
0.8
3 -1
CPV (cm g )
size distribution in the micropore and mesopore ranges made up mainly of large mesopores. The temperature effect on textural properties of the samples is evident from the higher mesopore volume of products such as H-800 and H-900. For the samples prepared by effecting single chemical treatments to RR as compared to the heat-treated ones, from the adsorption isotherms (Figs. 1 and 2) it follows that the development of porosity in the micropore and mesopore ranges as a rule is smaller for the former products. Thus notice that the isotherms determined for C-S, C-N, C-S/N(1:1), and C-S/N(3:1) are situated close together with the isotherm of RR. In the case of the samples obtained by combined treatments of RR it seems that the predominant effect on the microporous and mesoporous textures is that of the heat or chemical treatment carried out first to RR, as shown by the fact that the isotherms for H-400 and HC-400-S/N(1:3) occupy an almost coincident position in Fig. 3. The plots of the cumulative pore volume (CPV) against pore radius (R), as obtained by mercury porosimetry, for all samples are shown in Figs. 4–6. It is inferred that RR not only is a nonporous solid in the micropore and mesopore ranges but also in the macropore region. For the samples prepared from RR, the pore size distribution in the mesopore and macropore ranges depends on the method used in their preparation. The products obtained by heat treatment of RR possess an uni-modal distribution of pore sizes made up of large mesopores. These results are in good agreement with those obtained by N2 adsorption. The total cumulative pore volume is influenced by maximum heat treatment temperature, being higher for H-400, H-900 and H-800. For the samples prepared by performing single chemical treatments to RR, the plots of mercury intrusion (Fig. 5) show a strong dependence of the pore size distribution in the mesopore and macropore ranges on
10000
0
p/p
Fig. 3. Adsorption isotherms (N2 , 77 K) for the samples prepared by combined treatments of RR.
1000
R(A )
0
0.6
0.4
0.2
0.0 10
100
1000
10000
100000
0
R (A )
Fig. 6. Curves of the cumulative pore volume (mercury porosimetry) against pore radius for the samples prepared by combined treatments of RR.
40
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the HNO3 content in the acid solution used in their preparation. As can be seen, C-S is also a practically nonporous sample in such porosity regions, which contrasts with the wide pore size distributions of C-N, C-S/N(3:1), C-S/N(1:1) and C-S/N(1:3). These results are in accord with the greater mass loss produced in the preparation of this series of samples. Nevertheless, some significant differences are noted in their pore size distribution. This changes usually in the sense of increasing content of wider macropores with increasing HNO3 content in the solution. In the case of C-N and C-S/ N(1:3), both possess an almost identical pore size distribution in the region of larger macropores, however CS/N(1:3) contains narrow macropores and even wide mesopores that are absent from C-N. When comparing with the heat-treated samples (Fig. 4), it is worth noting that for those obtained by chemical methods the pore size distribution is wider and it is made up of larger pores (i.e., macropores, instead of mesopores). With regard to the samples prepared by combined treatments, the pore size distribution is similar to that for the sample obtained by carrying out a single heat or chemical treatment, in special for the couple C-S/N(1:3) and CHS/N(1:3)-400. For H-400 and HC-400-S/N(1:3) (Fig. 6), the only remarkable difference in their mesoporous and macroporous textures is the higher volume of mesopores for the HC-400-S/N(1:3). From the N2 adsorption isotherms, the specific surface area was estimated by applying the Brunauer, Emmett and Teller (BET) equation [30] in the p=p0 2 . From range 0.05–035, taking am to be equal to 16.2 A such isotherms, the micropore volume (Vmi ) and the mesopore volume (Vme ) were obtained simply by reading the volume adsorbed (Vad ) at p=p0 ¼ 0:10 and 0.95. Thus, Vmi ¼ Vadðp=p0 ¼0:10Þ and Vme ¼ Vadðp=p0 ¼0:95Þ Vadðp=p0 ¼0:10Þ . The micropore volume (W0 ) was also derived from the application of the Dubinin–Rad-
ushkevich equation [31]. Vmi , Vme and W0 were expressed as liquid volumes. The values of these pore volumes together with the SBET values are listed in Table 2. From the chart of mercury porosimetry, or from the derived curves of CPV ¼ f(R), the mesopore and macropore volumes ðVme-P , Vma Þ were obtained. Vma was taken as the volume of the pores having a radius greater and Vme-P as the volume of the pores with than 250 A and the lowest radius radii comprised between 250 A (18 A) of mercury intrusion in each porosimetry run. The values of Vme-P and Vma are also compiled in Table 2. From the helium and mercury densities (qHe , qHg ), the total pore volume (VT ) was calculated by making use of the expression: VT ¼ 1=qHg 1=qHe . The values of qHe , qHg and VT are given in Table 2. From data of Table 2 it follows that the samples prepared by heat treatment of RR are chiefly mesoporous solids, though containing relatively high macropore contents; whereas most of those obtained by chemical methods (except for C-S) including the one prepared by the chemical and heat treatments are mainly macroporous solids. Vme-P ranges between 0.34 cm3 g1 for H-500 and H-900 and 0.40 cm3 g1 for H400 and Vma-P between 0.52 cm3 g1 for C-S/N(3:1) and 0.69 cm3 g1 for C-S/N(1:3). For HC-400-S/N(1:3) the effects on both porosity ranges conjugate, then being an equally mesoporous and macroporous solid (Vme-P ¼ Vma-P ¼ 0:24 cm3 g1 ). In accordance with the presence of mostly intermediate and large size pores in the samples, their specific surface area is low. Thus, SBET varies between 13.4 and 47.4 m2 g1 for the heattreated samples and between 2.0 and 7.5 m2 g1 for the chemically treated samples. The somewhat larger values of SBET for the former samples is in line with the greater development of the microporosity in these products, as compared to those prepared by chemical methods. For the products obtained by combined
Table 2 Textural properties of the samples Sample
RR H-400 H-500 H-600 H-700 H-800 H-900 C-S C-N C-S/N(3:1) C-S/N(1:1) C-S/N(1:3) HC-400-S/N(1:3) CH-S/N(1:3)-400
Mercury porosimetry
Density measurements
Vmi (cm3 g1 )
N2 adsorption Vme (cm3 g1 )
W0 (cm3 g1 )
SBET (m2 g1 )
VmeP (cm3 g1 )
Vma (cm3 g1 )
qHg (cm3 g1 )
qHe (cm3 g1 )
VT (cm3 g1 )
0.000 0.002 0.011 0.022 0.022 0.028 0.030 0.000 0.002 0.006 0.004 0.030 0.024 0.006
0.001 0.018 0.117 0.185 0.178 0.182 0.180 0.001 0.015 0.044 0.032 0.037 0.018 0.031
0.001 0.043 0.027 0.048 0.046 0.053 0.045 0.001 0.005 0.012 0.010 0.041 0.050 0.010
0.0 18.1 13.4 21.0 18.2 18.1 47.4 2.3 2.0 5.7 5.0 7.5 14.0 5.0
0.04 0.40 0.34 0.38 0.37 0.37 0.34 0.03 0.08 0.13 0.09 0.07 0.24 0.07
0.03 0.21 0.17 0.20 0.22 0.26 0.31 0.06 0.56 0.52 0.57 0.69 0.24 0.61
1.17 1.97 1.83 1.88 1.97 2.04 2.01 1.25 1.54 1.58 1.56 1.86 2.03 1.75
1.07 0.78 0.86 0.83 0.79 0.77 0.76 1.05 0.69 0.68 0.67 0.65 0.70 0.67
0.08 0.77 0.66 0.67 0.76 0.81 0.82 0.15 0.80 0.84 0.85 1.02 0.94 0.92
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treatments, SBET presents a close value to those for the heat treated samples (HC-400-S/N(1:3), 14.0 m2 g1 ) or for the chemically treated samples (CH-S/ N(1:3)-400, 5.0 m2 g1 ). Finally, the total pore volume is high for all samples, except for C-S (VT ¼ 0:15 cm3 g1 ). Thus, VT ranges between 0.66 cm3 g1 for H-500 and 1.02 cm3 g1 for C-S/N(1:3). This large VT values are compatible with the well developed mesoporosity or macropority in the samples. Nevertheless, the fact that VT > Vmi þ Vme-P þ Vma-P suggests that a fraction of the porosity present in the samples is accessible to helium at room temperature but not to nitrogen at 77 K.
4. Conclusions From the above results the following conclusions may be drawn: • The resultant yield in the preparation of the samples is significantly lower for the heat treatments of RR than for the chemical treatments. The maximum yield (i.e., 99.0 wt.%) corresponds to the treatment with the H2 SO4 solution, whereas the minimum one (i.e., 33.0 wt.%) is for the heat treatment at 800 and 900 C. Also, the combined treatments as a rule results in higher yields than the single treatments. • RR is a nonporous material in all porosity ranges. • Usually, the main porosity development occurs in the mesopore range for the heat treatments and in the macropore range for the chemical treatments. The presence of HNO3 in the acid solution used in the treatments of RR is determinant in order to create large pores in the material. Only the single treatment with the H2 SO4 solution does not affect the porosity of RR. • In the combined treatments of RR, the predominant effect on textural properties of the samples is the one due to the treatment performed first. Nevertheless, the product obtained by successive heat and chemical treatments of RR possesses an equal volume of mesopores and macropores. • By treatment of RR, the tailoring of the pore size distribution in the mesopore and macropore ranges is possible. This is a remarkable finding allowing for that nowadays there is a need of large pores containing materials to be used in the chemical industry, as an example, in catalysis processes. Thus, the aforesaid materials can accommodate a great variety of host molecules, e.g., substances of pharmacological interest, which are much larger than those that can adsorb the traditional microporous zeolites [32]. Furthermore, the adsorbents would be prepared from a cheap starting material, which would surely lower their production cost.
41
References [1] G. San Miguel, G.D. Fowler, C.J. Sollars, Ind. Eng. Chem. Res. 37 (1998) 2430–2435. [2] G. Capelle, Gummi Fasern Kunststoffe 50 (4) (1997) 297. [3] P.J. Steer, C.H.M. Tashiro, W.D. McIlivenn, R.E. Clement, Water Air Soil Pollunt. 82 (1995) 659. [4] P.J. Lampman, S. Hansonand, R. Novak, J. Am. Mosquito Control Assoc. 13 (1997) 193. [5] G. Ferrer, Resour. Conserv. Recyc. 19 (1997) 221. [6] B.Z. Savas, Sh. Ahmad, D. Fedroff, Transportation Research Record no. 1574, November 1996, National Research Council, Washington, D.C., USA, 1996, pp. 80–88. [7] B. Siurn, Scrap Tire News 12 (1998) 8. [8] B. Siurn, Scrap Tire News 12 (1998) 1. [9] B. Siurn, Scrap Tire News 11 (1997) 1. [10] B. Siurn, Scrap Tire News 12 (1998) 5. [11] M. Blumenthal, in: Advances in Instrumentation and Control; International Conference and Exhibition Proceedings of the 1996 International Conference on Advances in Instrumentation and Control. ISA/96. Part 1 (of 2), 51 (1) 1996, USA 2. Instrument Society of America Research Triangle Park, N.C., USA, 1996, pp. 405–414. [12] B. Siurn, Scrap Tire News 11 (1997) 10. [13] X. Elias Castells, Reciclaje de residuos industriales, Ed. Dıaz de Santos, Madrid, 2000, p. 495. [14] A.M. Cunliffe, P.T. Williams, Energy Fuels 13 (1) (1999) 166–175. [15] S. Ogasawara, M. Kuroda, N. Wakao, Ing. Eng. Chem. Res. 26 (12) (1987) 2552–2556. [16] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (12) (1990) 1474–1482. [17] A.A. Merchant, M.A. Petrich, AIChE J. 39 (8) (1993) 1370–1376. [18] H. Teng, M.A. Serio, M.A. Wojtowicz, R. Bassilakis, P.R. Solomon, Ing. Eng. Chem. Res. 34 (9) (1995) 3102–3111. [19] S.H. Lin, J. Chem. Technol. Biotechnol. 57 (4) (1993) 387–391. [20] C. Roy, B. Labrecque, B. Caumia, Resour. Conserv. Recyc. 4 (1990) 203. [21] W. Kaminsky, H. Sinn, in: J.L. Jones, S.B. Radding (Eds.), Termal Conversi on of Solid Wastes and Biomass, ACS Symposium Series 130, ACS Publishers, Washington, D.C., 1980, p. 423. [22] S. Kawakami, K. Inonue, H. Tanaka, T. Sakai, in: J.L. Jones, S.B. Radding (Eds.), Termal Conversi on of Solid Wastes and Biomass, ACS Symposium Series 130, Acs Publishers, Washington, D.C., 1980, p. 557. [23] P.T. Williams, S. Besler, D.T. Taylor, R.P. Bottrill, J. Inst. Energy 68 (1995) 11. [24] B.L. Schulman, P.A. White, in: J.L. Jones, S.B. Radding (Eds.), Solid Wastes and Residues: Conversi on by Advanced Thermal Processes, American Chemical Society, Washington, D.C., 1978. [25] W. Kaminsky, H. Rossler, Chemtech 22 (1992) 108. [26] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (12) (1990) 1474– 1482. [27] H. Teng, M.A. Serio, M.A. Wojtowicz, R. Bassilakis, P.R. Solomon, Ind. Eng. Chem. Res. 34 (9) (1995) 3102–3111. [28] A. Ariyadejwanich, W. Tanthapanichakoon, K. Nakagawa, S.R. Mukai, H. Tamon, Carbon 41 (2003) 157–164. [29] V. G omez-Serrano, M. Acedo-Ramos, C. ValenzuelaCalahorro, A.J. L opez-Peinado, J. Chem. Technol. Biotechnol. 75 (2000) 835– 839. [30] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [31] M.M. Dubinin, in: J.F. Danielli, M.D. Rosenberg, D.A. Cadenhead (Eds.), Progr. Surf. Membr. Sci., 9, Academic Press, New York, 1975, p. 1. [32] J. Perez-Pariente, A.L. Doadrio, J.C. Doadrio, P. Horcajada, I. Izquierdo-Barba, A. Martınez, B. Mu~ noz, A. Ramila, M. ValletRegı, in: Proccedings of the XXIX Reuni on Bienal de la Real Sociedad Espa~ nola de Quımica. S3O4, 2003, Madrid.