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Ind. Eng. Chem. Res. 1993,32, 3112-3116
Acid Properties of Silica-Alumina Catalysts and Catalytic Degradation of Polyethylene Hironobu Ohkita, Ryuji Nishiyama, Yoshihisa Tochihara, Takanori Mizushima, Noriyoshi Kakuta, Yoshio Morioka? Akifumi Ueno,’J Yukihiko Namiki,t Susumu Tanifujif Hiroshi Katohf Hideo Sunazuka,t Reikichi Nakayama) and Takashi Kuroyanagit Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 441, Japan, Shizuoka University, Johoku, Hamamatsu, Shizuoka 432, Japan, and Japan Electric Cable Technology Centre, Hamamatsu, Shizuoka 431 -21, Japan A relationship between the acid strengths and amounts of silica-alumina catalysts and the compositions of products formed by the catalytic degradation of polyethylene a t 673 K was studied. The acid strengths and amounts were varied with SiOdA1203weight ratio in the catalysts. Although the resulting products consisted of gases, oils, and wax, the fraction of gases increased, and, inversely, the fraction of oils decreased, as the acid amounts over the catalysts increased The fraction of aromatics in the oils was enhanced, however, as the acid amounts over the catalysis increased, which was discussed in terms of the acid types: Bronsted and Lewis acids generated on silica-aluminas. Since some inorganic compounds such as MgO, ZnO, TiO2, and carbon are incorporated into plastics, the catalytic activities and selectivities of these additives for polyethylene degradation were also discussed.
Introduction Though several methods have been proposed for recycling waste plastics, Williams (1993) is of a opinion that material recovery will not be a long-term solution to the present problem and that energy or chemical recovery is more attractive. In this method, the waste plastics are thermally or catalytically degraded into gases and oils, which can be utilized as resources of either fuels or chemicals. A wide range distribution of carbon atom numbers has been reported by Murata and Makino (1973, 1975) and Nishizaki et al. (1977) in the gases and oils obtained by thermal degradation of polyethylene and polypropylene. For the gases and oils produced by degradations over solid acid catalysts, relatively sharp distribution curves with peak tops at the lighter hydrocarbons have been reported by Uemichi et al. (1983) and Audisio et al. (1984). It is well-known that the oils produced by catalytic degradation over solid acids contain less olefinic compounds and are rich in the aromatics compared to the oils obtained by thermal degradation. Although the catalysts used in these works were solid acids such as silica-alumina and zeolite,the relationship between the acid amounts and strength of the catalysts and the compositions of the resulting oils is not yet well defined. Consequently, the purpose of this work is to investigate this relationship using silica-alumina catalysts, varying the Si02/A1203 weight ratio and hence the acid amounts and strengths. The results obtained were discussed in terms of the types of the acid sites; Lewis and Bronsted sites, and were compared with those obtained by degradation over HZSM-5 zeolite, since this zeolite is considered to possess large amounts of Bronsted acid sites. Note that the catalytic degradation considered in this work is just the cracking of volatile products of the thermal degradation of polyethylene, as will be seen in the apparatus used in this work. I t is also well-known that several kinds of inorganic additives are incorporated into plastics in order to improve the mechanical strength and/or the thermal resistance. + Shizmoka f
University. Japan Electric Cable Technology Centre.
0888-5885/93/2632-3112$04.00/0
Since in an electric cable, generally consisting of polyethylene, inorganic compounds such as Si02, MgO, TiO2, and ZnO are dispersed as flame retardants, we are pleased’thatthese inorganic additives will play important roles for the degradation of polyethylene so as not torequire additions of other Catalysts. Accordingly, the studies on the roles of Si02, A1203, MgO, TiO2, and ZnO for the polyethylene degradation are another purpose of the present work. Since in some cases carbon powders were also incorporated into the plastics, the roles of carbon powders for polyethylene degradation were also investigated.
Experimental Section 1. Catalyst Preparation. The catalysts employed in this work are given in Table I with their catalytic performances for polyethylene degradation at 673 K. ,5302, A1203, and Ti02 were prepared from gels, obtained by hydrolysis of the corresponding metal alkoxides such as tetraethoxysilane, aluminum triisopropoxide, and titanium tetraisopropoxide, according to papers by Ueno et al. (19831, Ishikawa et al. (1992), and Nishiwaki et al. (1989), respectively. MgO and ZnO were formed from the aqueous solution of the corresponding metal nitrates using aqueous ammonia as a precipitant. The precipitates obtained were dried in an oven at 383 K for 12 h, followed by calcination at 773 K for 4 h. ZSM-5 zeolite, with a Si/Al ratio of 14, was prepared from aerosil silica, sodium aluminate, and tetra-n-propylammonium hydroxide according to a US. patent (1972) and was cation-exchanged to HZSM-5 by using ammonium hydroxide. Active carbon was obtained from Sumitomo Kagaku Co. and was used as a catalyst without further purification. Silica-aluminas were prepared from gels obtained by hydrolysis of a mixed alkoxide solutions of tetraethoxysilane and aluminium triisopropoxide, varying the SiOd A1203weight ratio from 0.25 to 4.0. The gels were dried and then calcined in the same manner as mentioned above. Unless otherwise specified, silica-alumina with a weight ratio of 4.0 was used in the present work. 2. Catalytic Degradation of Polyethylene. The catalytic degradation was carried out using 15 g of low@ 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3113 Table I. Catalysts Employed and Their Catalytic Performances for the Degradation of Polyethylene at 673 K Fractions of Gases, Oils, and Wax and Expressed in Terms of wt 7 ’ 0 (1.5 g of the Catalyst Was Used for 15 g of Polyethylene) HZSM-5 si02(4)/&03(1) Si02 A1203 ZnO MgO Ti02 carbon thermal oils (wt 5%) 45 52 69 67 70 61 68 60 44 straight (wt %) 19 34 54 75 85 68 70 78 68 13 12 14 14 12 20 gases (wt %) 50 37 16 waxes (wt 5%) 1 4 6 10 5 9 7 7 13 residues (wt % ) trace trace trace trace 3 trace 3 5 17 reaction time (h) c3 4 5 5 7 7 7 7 >7 material balance ( % ) 96 93 91 90 90 84 92 84 94 I4
W 10
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E l e c r r ~ c‘urnace
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T r a ~
Figure 1. Schematic drawing of the apparatus for catalytic degradation of polyethylene.
density polyethylene powder (NUC-9025 from Nippon Unicar Co. Ltd., 10 pm in size, &fn= 16.2 X 103,&fw= 63.5 X lo3) packed at the bottom of a stainless steel reactor (45 mm in diameter) heated at 673 K. In the middle part of the reactor 1.5 g of the catalyst powder was held in a stainless steel gauze, which separates the catalyst from polymer powders so that the vapor of thermally degraded products may pass through the catalyst bed. Nitrogen gas was supplied at the bottom of the reactor with a flow rate of 50 mL/min in order to purge oxygen from the reactor and assist the vapor to be carried into the catalyst bed. Exactly speaking,the catalytic degradation of polyethylene in this work means “the catalytic cracking of the volatile compounds in the thermally degraded polyethylene vapor”, but in the present paper the term “catalytic degradation of polyethylene” was used as the abbreviated form. Note that the reaction temperature was controlled at the bottom of the reactor. The degraded products were introduced into a cooling glass tube, where gases (C1 to C4) were separated from oils (C5 to CZO)to be measured by a gas meter. Low-volatilitycompounds such as waxes deposited on the cooling tube were eliminated by n-hexane. A schematic drawing of the apparatus used is shown in Figure 1.
The gases were analyzed by GC (Shimadzu GC-8A)using a column packed with VZ-10. The oils were analyzed by GC (HP-5890) using a 10-m DB-2887 capillary column and were identified by comparison of the retention indices with those of authentic compounds (n-alkanes and n-alkenes) in a calibration mixture of a known composition. More precise assignments of aromatics in the oils were carried out, if necessary, by GC-MS (HP-5890) using a 30-m DB-1 capillary column. 3. Measurements of Acid Amounts and Strengths. The acid amounts and strengths of silica-alumina catalysts, heated again at 623 K for 1h in air followed by an evacuation at the same temperature for 30 min, were measured by titration using n-butylamine and various color indicators according to a paper by Hirashima et al. (1988). In order to distinguish Bronsted acid sites from Lewis acid sites generated on the silica-alumina catalysts, infrared spectra of pyridine adsorbed on the catalysts at 473 K were recorded in order to avoid problems caused by the physical adsorption of pyridine. The IR absorption
peaks observed at 1625,1490,and 1455cm-l were assigned to pyridine on the Lewis sites, and the peaks at 1545 and 1490 cm-l were ascribed to pyridine adsorbed on the Bronsted sites, reported by Hughes and White (1967) and by Gates et al. (1979). Change in the fraction of Bronsted sites with the change in SiOz/A1203 weight ratio of silicaaluminas was tentatively determined by measuring the absorption peak intensities at 1625 and 1545 cm-l. The amounts of Bronsted and Lewis acids per gram of the catalysts were estimated from the peak intensity ratios 1iu5/(11545 + 1iszs) and IlSZS/(11545 + 11625) multiplied by the total acid amounts measured by the titration, respectively.
Results 1. Catalytic Performances of Inorganic Additives. The catalytic performances of inorganic additives employed are given in Table I, expressed in terms of the weight percent fractions of the gases, oils, and wax produced at 673 K. In Table I, “straight” means the weight percent fraction of n-alkanes and 1-alkenes in the resulting oils and “residue” means the carbonaceous compounds remaining in the reactor after degradation for 7 h. The time required to complete the degradation of 15 g of polyethylene powder at 673 K is given in Table I. It was assumed in this work that the degradation was completed in the reactor when no more formation of oils was detected in the coolingtube. The materials balance of around 90 % , given at the bottom of the table, was obtained for all the experimental runs. The lack of 10% will be mainly attributed to the experimental errors for a measure of the residue remained in the reactor, since it adhered too tightly on the reactor wall to be thoroughly released. The product distribution curves for the thermal degradation and the catalytic degradations over HZSM-5 and ZnO, as the representatives of solid acid and base catalysts, respectively, are given in Figure 2. In Table I1 are summarized the fractions of 1-olefins, n-paraffins, and aromatics in the oils produced by thermal degradation and by catalytic degradations over ZnO, silica-alumina, and HZSM-5 at 673 K. The compositions of oils produced by the degradation on MgO, TiOz, and A1203 were almost the same as those on ZnO. Others in Table I1 mainly consisted of branched isomers of n-paraffins and 1-olefins. The oils produced on HZSM-5 consisted of significant amounts of aromatics including naphthalene compounds, identified by GC-MS. In Figure 3 are shown the assignments of some of the naphthalene compounds on a GC strip chart. 2. Acid Properties of Catalysts and Product Compositions. Changes in the acid amounts and strengths of silica-aluminas with varied SiOz/A1203ratios are given in Figure 4, where the highest acid strength was HO= -3.0 for all the silica-aluminas employed. The largest acid amount was observed for the silica-alumina possessing the ratio ranging from 4.0 to 1.5. The relationship between the
3114 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
1
2
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10111213141516+
Carbon n u m b e r Figure 2. Product distributions against carbon atom numbers in the PE degradation catalyzed by HZSM-5and ZnO and in the thermal degradation. Table 11. Fractions of 1-Olefins,a-Paraffins, Aromatics, and Others (Mainly Branched Isomers) in the Oils Produced on HZSM-6, Silica-Alumina, and ZnO and by Thermal Degradation at 673 K catalytic degradation (wt % ) SiOA4)I thermal components HZSM-5 AlzOa(1) ZnO degradation (wt %) 34 24 l-olefins 7 4 12 30 51 44 n-paraffins 35 13 0 3 aromatics 15 30 others 46 53
fraction of gases, oils, and wax formed and the SiOdA1203 weight ratio in the catalysts employed is depicted in Figure 5. The changes in the amounts of Brdnsted and Lewis acid sites per gram of the silica-alumina catalysts with the change in the SiO~/A1203weight ratio are shown in Figure 6, together with the change in the amounts of aromatics in the resulting oils.
Discussion 1. Roles of Inorganic Additives for Polyethylene Degradation. As is mentioned in the Introduction of this paper, several kinds of inorganic compounds, such as Si02, Al203, ZnO, MgO, TiO2, and carbon, are incorporated in the polymer layer of electric cables as flame retardants in order to improve the thermal properties of the polymer. Although these additives would have been expected to be good catalysts for the polyethylene degradation, no significant differences from the single thermal degradation were observed in the results, shown in Table I, except that the fractions of residue remained in the reactor were much reduced by using these inorganic compounds as catalysts. Since thermal degradation has been considered to pass through hydrocarbon radicals, the residue remaining on the reactor wall might be attributed to the low-volatility compounds, probably formed by recombination of these radicals. One of the roles of inorganic additives is to scavengethese radical species on the surfaces and to crack them into light hydrocarbons depending on the catalytic performances of the additives. The weak acidic and/or basic additives such as SiOz, A1203, ZnO, MgO, and Ti02 seem to scavenge these radicals but do no convert them significantly into the light hydrocarbons. The scavenged radicals were released from these solid surfaces as the corresponding l-olefins, leaving hydrogen atoms on the surfaces. In turn, the hydrogen atoms left on the solid
surfaces reacted with other radicals scavenged on the surfaces to produce n-paraffins. Thus, the oils produced on these solids were rich in l-olefins and n-parafhs, as shown in Table 11,and less residue remained in the reactor compared with the thermal degradation (see Table I). Things were the same when carbon was used as a catalyst. Consequently, similar product distribution curves were obtained for both thermal and catalytic degradations over these inorganic additives, although only the distribution curve obtained for the degradation over ZnO is exhibited. It was concluded that the inorganic additives incorporated into the polyethylene layer of electric cables could not be expected to work as good catalysts for the catalytic degradation of waste polyethylene. 2. Relationship between Acid Properties and Product Compositions. The enhanced production of gases, shown in Table I, is one of the features of the degradation products of plastics over solid acids such as silica-alumina and HZSM-5 zeolite. This is more pronounced in Figure 2, where the gases consisting of C3 and Cq compounds are predominant in the products for the polyethylene degradation over HZSM-5 zealite. In addition, the time required to complete the degradation of 15 g of polyethylene on HZSM-5 was shorter than 3 h, less than one-half of the time required for the single thermal degradation (see Table I). It might be better, however, to use silica-alumina catalysts to study the relationship between acid properties of the catalysts and the composition of the degradation products of polyethylene, since the acid amounts and strengths can be easily controlled by the SiOdA1203ratio in the catalysts. The highest acid strength was HO= -3.0, observed for all the silica-alumina catalysts used, and the largest acid amount was observed for the catalyst possessing the sioz/&o3 ratio ranging from 4.0 to 1.5, as shown in Figure 4. Considering these results together with the results shown in Figure 5, the fraction of oils produced by the degradation on silica-alumina catalysts decreased and, inversely, the fraction of gases increased as the acid amounts on the Catalysts increased. This means that cracking of the higher hydrocarbons takes place on the acid sites of the catalyst surfaces, as has already been accepted. The relationship between the fraction of gases produced and the SiOz/Al203ratio in the catalysts, shown in Figure 5, is similar to the relationship between the amount of Brdnsted acids, but not of Lewis acids, and the SiOdA120 ratio, shown in Figure 6. This suggests that the cracking of olefinic compounds in the thermally degraded polyethylene occurred predominantly on the silica-aluminas, since olefinic compounds have been considered to be more selectively converted into light hydrocarbons on the Bronsted acids, reported by Turkevich and Ono (1969). Consequently, the fraction of olefins in the oils produced over silica-alumina and HZSM-5, possessing a lot of Brdnsted acid sites, are significantly small,as given in Table 11. The amounts of wax and residue in the reactor were negligibly small on the catalyst having a considerable amount of acid. Another feature of the product compositions over solid acids is the formation of aromatics in the oils, which was never observed in the oils produced over solid bases. The change in the fraction of aromatics in the oils with the change in the SiOz/A1203 weight ratio in the catalysts, shown in Figure 6, seems to follow the change in the total acid amounts given in Figure 4. Thus, the aromatizations likely occurred on the acid sites of the catalysts. Two types of acid sites, Lewis and Brdnsted acids, are generated on the silica-alumina catalysts, and they can be distin-
Ind. Eng. Chem. Res., Vol. 32,No. 12, 1993 3115 HaZSM-5
Figure 3. Assignments of aromatic compounds in the oils produced on HZSM-5. 1.o
0 Ho 1-3.0
1.o
0.8
0 Brtlnatsd ac# A Lewk acM 0 Aromatlca
I ,A------&-..
o-2f122.si 20 20
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00 0
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Figure4. Changesin the acid amount and strength of silica-aluminas with the SiOZ/Al208weight ratio.
0
"
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1.6
I
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100
Ahmine content
Figure 6. Relationship between the amount of aromatics in the oile produced on silica-aluminas and the amounts of BrBnsted and Lewis acid sites detected on the catalysts.
of hydrogen atoms on the surface of solid acids, and these hydrogen atoms were consumed for the hydrogenation of olefins, as reported by Ayame et al. (1979). Thus, the fraction of 1-olefinsin the oils produced on the solid acids was significantly small (see Table 11). Since plenty of C3 and C4 compounds were produced over solid acids, the precursors of CSand C4 species on the catalyst surfacesmight be released from the surfaceseither as CBand C4 gases or as aromatic compounds depending on the interactions between the precursors and the catalyst surfaces, modified by the acid properies of the catalysts employed.
gases
Literature Cited 0
20
40
60
80
100
Alumha content I wt% Figure 5. Changes in the fractions of gases, oils,and wax produced on silica-aluminas with the SiOZ/Al203weight ratio.
guished by measuring infrared spectra of pyridine adsorbed. The changes in the amounts of Briinsted and Lewis acid sites with the change in the Si02/A1203weight ratio are also given in Figure 6, suggesting that the formation of aromatics could not be specified to occur selectively either on the Brdnsted acids or on the Lewis acids. However, considering that significant amounts of aromatics were produced over HZSM-5 possessing a lot of Brdnsted acids (see Table 111, aromatization seems to be favorable on the Briinsted acids. Aromatization left a lot
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Hughes, T. R.;White, H. M. A Study of the Surface Structure of Decationized Y Zeolite by Quantitative Infrared Spectroscopy. J. Phys. Chem. 1967, 71,2192.
3116 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 Ishikawa, T.; Ohashi, R.; Nakabayashi, H.; Kakuta, N.; Ueno, A.; Furuta, A. Thermally Stabilized Transitional Alumina Prepared by Fume Pyrolysis of Boehmite Sols. J. Catal. 1992, 134, 87. Murata, K.; Makino, T. Thermal Degradation of High Density Polyethylene. Nippon Kagaku Kaishi 1973, 2414. Murata, K.; Makino, T. Thermal Degradation of Polypropylene. Nippon Kagaku Kaishi 1975, 192. Nishiwaki, K.; Kakuta, N.; Ueno, A. Generation of Acid Sites on Finely-divided TiOz. J. Catal. 1989, 118, 498. Nishizaki, H.; Sakakibara, M.; Yoshida, K.; Endoh, K. Oil Recovery from Atactic Polypropylene by Fluidized-bed Pyrolysis. Nippon Kagaku Kaishi 1977, 1899. Turkevich, J.; Ono, Y. Catalytic Research on Zeolites. Adv. Catal. 1969,20, 135.
Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Product Distribution in Degradation of Polypropylene over Silica-
alumina and CaX Zeolite Catalysts. Bull. Chem. SOC. Jpn. 1983, 56, 2768. Ueno, A.; Suzuki, H.; Kotera, Y. Particle-size Distributing of Nickel Dispersed on Silica and its Effects on hydrogenation of Propionaldehyde. J. Chem. SOC.,Faraday Trans. Z 1983, 79, 127. U.S. Patent 3,702,886;1972,Mobile Oil Co. Williams, V. Preprint of Symposium of Waste Plastic Recycle; 1993, Tokyo.
Received for review April 29, 1993 Revised manuscript received August 25, 1993 Accepted August 30, 19930
* Abstract published in Advance ACS Abstracts, October 15, 1993.
