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Catalytic properties of MCM-41 for the feedstock recycling of plastic and lubricating oil wastes
MESOPOROUS MOLECULARSIEVES 1998 Studies in Surface Science and Catalysis, Vol. 117 L. Bonneviot, F. B61and,C. Danumah, S. Giasson and S. Kaliaguine (Editors) 9 1998 Elsevier Science B.V. All rights reserved.
437
Catalytic properties of MCM-41 for the feedstock recycling of plastic and lubricating oil wastes D.P. Serrano, J. Aguado, J.L. Sotelo, R. Van Grieken, J.M. Escola and J.M. Mendndez Chemical Engineering Department, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain
The potential application of Al-containing MCM-41 for the catalytic degradation of polyolefinic plastics and used lubricating oils has been investigated and compared with the behavior of ZSM-5 zeolite and commercial amorphous SiO:AlzO3. For all the studied raw plastics, MCM-41 presents significant higher activity than the amorphous catalyst, and even superior to that of ZSM-5 for the degradation of pure polypropylene, a mixture of three polyolefins (high density polyethylene + low density polyethylene + polypropylene) and a lube distillate. On the contrary, the ZSM-5 zeolite leads to higher conversions in the degradation of pure polyethylenic plastics and shows a slower deactivation during the lube oil cracking. Regarding the product distribution, while ZSM-5 leads mainly to light hydrocarbons (C_,-C5), MCM-41 cracks these wastes into liquid fractions (gasoline and middle distillates), which suggests the cracking pathway is not the same with these two materials, being governed by their pore size and/or their acid strength.
1. INTRODUCTION The development of suitable methods for the treatment and recycling of polymeric and oil wastes is one of the challenges that the technology must face in the next years. Among the different alternatives to deal with this kind of wastes, chemical recycling present a high potential since it may allow to transform these residues in different products that can be used as raw materials for the preparation of fuels and chemicals. Currently, a number of processes are being investigated for the chemical recycling of plastic wastes" alcoholysis, hydrolysis, gasification, hydrogenation, pyrolysis and catalytic cracking. In the past, different types of acid catalysts have been applied for the catalytic cracking of polyolefinic plastics or for the conversion of heavy oils obtained by thermal cracking of the former. Among those catalysts, amorphous silica-alumina and different zeolites (X, Y and ZSM-5) have been typically used [1-3]. However, whereas zeolitic materials are advantageous in terms of acid sites distribution and strength, their application for the conversion of heavy wastes is limited by their pore size. In recent works [4,5], we have shown that these problems can be overcome with the use of MCM-41 materials as catalysts for the conversion of pure
438 polyolefins, since they present a balanced combination of medium acid strength with high surface area and uniform pore sizes in the mesopore range (1.5 - 10 nm). In the present work, the study of the potential use of MCM-41 materials for the catalytic conversion of polymeric and oil wastes into chemicals and hydrocarbon mixtures useful as feedstocks has been continued by using a mixture of polyolefins and a lube distillate as raw materials. In the last case, the evolution of the activity along the time on stream has provided information about the stability and the deactivation of MCM-41.
2. EXPERIMENTAL The MCM-41 sample used in this work has been synthesized at room temperature using tetratethylorthosilicate and aluminum isopropoxide as Si and AI sources, respectively. The alkoxides are hydrolyzed in a first step with an HCI solution also containing the surfactant, cetyltrimethylammonium chloride. Subsequently the formation of the solid mesoporous phase is accelerated through the addition of an ammonia solution. The solid product so obtained is separated by filtration, dried at I10~ and calcined in air at 550~ yielding the MCM-41 sample free of surfactant and directly in acid form, hence further ion exchange treatments are not necessary. The ZSM-5 sample has been synthesized from ethanol-containing gels according to a published procedure [6]. A commercial amorphous SiO2-AI203 sample was also used as reference (Stidchemie, KA-3). The three catalysts have been characterized by conventional techniques: X-ray fluorescence (XRF), X-ray diffraction (XRD), N 2 adsorption at 77 K, NH3 temperature programmed desorption (TPD), 27A1 MAS NMR, etc. Thus, the XRD and N2 adsorption measurements confirm that the MCM-41 sample has a regular mesoporosity with pore size around 2.9 nm, whereas 27A1 MAS-NMR measurements show that all the AI atoms present tetrahedral coordination in the as-synthesized sample. The main properties of these materials have been summarized in Table 1. Table 1. Physicochemical properties of the catalysts ,
,
Catalyst
Si/AI
Acidity (NHj TPD) (mmol/g) Tmax (~
Dp (nm) a
Saer (m2/g) .
,,.,
,
MCM-41
45.6
0.22
332
2.9
1177
ZSM-5
34.1
0.52
467
0.55
467
SiO2-A1203
2.0
0.14
304
2 -12
169
pore diameter
The catalytic experiments of polymer degradation have been carried out in a batch reactor at 400~ with continuous N2 flow. The reaction temperature was reached after 15 min heating from room temperature. The products leaving the reactor during 30 min were separated and accumulated as gaseous and liquid fractions for subsequent GC analysis. The following polyolefins were used as raw materials: high-density polyethylene (HDPE), low-
439 density polyethylene (LDPE), polypropylene (PP, isotacticity index = 93%) and a mixture of them having a composition that resembles the distribution of these polymers existing in municipal plastic wastes (LDPE" 46.5 wt%, HDPE" 25 wt% and PP: 28.5 wt%). The catalytic conversion of a lube distillate, formed by paraffinic hydrocarbons with boiling points in the range 450-550~ (C25- Cs0), has been studied in a down-flow fixed bed reactor at 400~ under atmospheric pressure. The lube distillate was fed by means of a syringe pump and mixed with a N2 stream previously to be introduced into the reactor. The products in the effluent stream were separated in a condenser at 0~ into gaseous and liquid phases, which were further quantified and analyzed by GC.
3. RESULTS AND DISCUSSION
3.1. Catalytic degradation of pure polyolefins MCM-41 was first tested in the catalytic conversion of three pure polyolefins: HDPE, LDPE and PP, the results obtained being compared with those corresponding to a ZSM-5 zeolite and a commercial amorphous silica-alumina. Different plastic/catalyst ratios were used for each polymer in order to obtain adequate polymer conversions. The activities are shown and compared in Figure l for the different catalysts and polymers. With the three polyolefins, the less active sample is the commercial amorphous silica-alumina, which leads to conversions just slightly higher than those observed by thermal cracking. This poor activity is probably originated by the weak acidity and the low surface area present in this material (see Table l).
Figure 1. Activity in the cracking of pure polyolefins (400~ 30 min., P/C = plastic/catalyst mass ratio) The comparison between the catalytic behavior of MCM-41 and ZSM-5 zeolite is more complex. The latter is clearly a superior catalyst for the degradation of highly linear polymers such as HDPE probably due to its stronger acidity, but it exhibits very low activity in the PP conversion, polyolefin with a high proportion of side methyl groups. In this case, the conversion obtained over MCM-41 is almost total, whereas the activity of ZSM-5 is very
440 close to that of the thermal cracking. An intermediate result is observed in the degradation of LDPE, linear polymer having a certain degree of branching, since both materials present similar activities. The order of activity exhibited by MCM-41, both in terms of overall conversion or conversion per unit mass of catalyst, follows the expected trend according to the proportion of highly reactive tertiary carbons present in the polymeric chains: PP > LDPE > HDPE. In this way, the lack of activity observed over ZSM-5 for the PP degradation is remarkable, which confirms previous results obtained at a higher plastic/catalyst ratio [5], indicating that in this case the presence of the side methyl groups in the polymeric chain hinders its access to the narrow zeolite pores. Thus, the low PP conversion observed over ZSM-5 must take place on the external acid sites that, given the crystal size of this sample (5~m), accounts just for a very small proportion of the total acidity. These steric and diffusional limitations are not present in the polymer degradation catalyzed by MCM~ due the larger pore size of this material. Concerning the product distribution, Figure 2 compares the selectivity towards different hydrocarbon groups obtained with the two most active catalysts, MCM-41 and ZSM-5, in the conversion of the three polymers. In all cases, MCM-41 leads mainly to liquid fractions having boiling points in the range of gasoline and middle distillates, with overall selectivities between 70 and 85%. In contrast, the main products obtained over ZSM-5 zeolite are gaseous hydrocarbons with a high proportion of olefins.
Figure 2. Product distribution in the catalytic cracking of pure polyolefins (reaction conditions as in Figure 1)
441 Similar conclusions can be obtained from the corresponding product distribution per carbon atom number. Thus, Figure 3 compares the distribution obtained in the LDPE cracking over MCM-41 and ZSM-5. For the latter, a strong maximum is observed between C3 and C6, with around 35 wt% of the products corresponding to C4 hydrocarbons. On the contrary, the distribution obtained over MCM-41 is much wider since, although a maximum is present also at C4, significant proportions of products are observed up to C20. From these results, two different cracking pathways can be envisaged" end-chain cracking leading to gaseous hydrocarbons and random cracking at any point in the chain, which yields a wide distribution of heavy hydrocarbons. The first mechanism is predominant in the polymer degradation over ZSM-5, while both pathways take place over MCM-41. These differences are probably originated by the narrow pores of the zeolite which promote an intensive cracking of the molecules as they enter and diffuse along the channels, although the higher strength of the zeolite acid sites may also favour the end-chain cracking. 35
9
30
--~-- MCM-41 -- 9
ZSM-5
25
~ 20
~ ~o
,:
,
|
I
,
I
|
9
,
i
Carbon atom number
Figure 3. Product distribution per carbon atom number in the LDPE catalytic cracking (reaction conditions as in Figure l) Likewise, it must be pointed out that the primary products of the cracking reactions may undergo subsequent transformations leading to a higher variety of hydrocarbons. In this way, in the two distributions shown in Figure 3, a second maximum is observed at Cs for both ZSM-5 and MCM-41, originated probably by the dimerization of C4 olefins.
3.2. Catalytic degradation of mixed polyolefins The three catalysts have been also checked for the degradation of a physical mixture of the three polyolefins since this may be a more realistic case when dealing with plastic wastes. The composition of the polymer mixture has been selected to be within the range of polyolefin distribution usually present in municipal plastic wastes. The results obtained in the catalytic degradation of this mixture have been summarized in Table 2.
442 The amorphous silica-alumina has a very low activity, as expected from the results obtained with the pure plastics. MCM-41 leads to a conversion close to 50%, remarkably higher than that obtained with the ZSM-5 sample. It is interesting to note that the ZSM-5 activity is appreciably lower than the one could be predicted based on the results with the pure polyolefins. Although ZSM-5 has shown not to be active for the PP degradation, the proportion of this polymer in the mixture (28.5 wt%) does not support the low overall plastic conversion observed with this catalyst. It seems that polypropylene has a deactivating effect on the ZSM-5 zeolite. A possible explanation to this result is the existence of a preferential contact of the ZSM-5 crystals with the PP particles, hindering the conversion of the other polyolefins present in the mixture. Table 2. Catalytic cracking of mixed polyolefins (LDPE=46.5,HDPE=25,PP--28.5 wt%) T=400~ t=30 rain., plastic/catalyst= 50 (mass ratio) Conversion (%) C~-C4 paraffins (%) C_,-C4 olefins (%) C5-Cj2 CI3-C22
C23-C40
MCM-41
ZSM-5
SiO2-AI20.~
49.0 3.9 10.1 54.2 29.4 2.4
6.8 17.3 33.2 49.5 . . . .
3.8 17.9 28.9 53.2 . .
. .
. .
. .
The product distribution obtained in the mixed polyolefin degradation over MCM-41 and ZSM-5 agrees well with that observed in the pure polymer cracking. In this case around 86% of the products obtained with MCM-41 are liquid hydrocarbons in the range of both gasoline and middle distillates.
3.3. Catalytic degradation of lube distillate The catalytic properties of MCM-41 for the chemical recycling of used lubricating oils have been explored using as raw material a lube distillate formed by paraffinic hydrocarbons in the range C25-C50, the results being compared with those obtained over the ZSM-5 sample. The experiments have been carried out in a continous flow system, which has allowed us to obtain information about the catalyst deactivation. The composition of the reactor effluent observed in the cracking of the lube oil at 400~ after 2 hours of time on stream is illustrated in Figure 4, being also included as reference the composition of the raw material. The conversion obtained in each case has been calculated from the decrease observed in the curve corresponding to the raw lube oil. Under these conditions, MCM-41 is quite more active than the ZSM-5 sample in spite of the stronger acidity of the latter, suggesting that at short times on stream the activity is determined by the pore size of the catalyst, the superior conversion over MCM-41 being related to the absence of diffusional limitations. In regards to the product distribution, the results obtained are very similar to those commented above in the polyolefin degradation. Thus, whereas the product from the ZSM-5 cracking is formed by 63 wt% gases, mainly C3 and C4 olefins, the lube oil conversion over MCM-41 leads to 83 wt% of the products being in the range C5-C28. Therefore, it can be also
443 concluded that ZSM-5 catalyzed cracking of the lube oil takes place preferently at the end of the chains, while both end-chain and random cracking are observed with MCM-41. 10
"
'-
I
raw lube oil
]
s
I
~ 4
Em o 0
lb
io
3b
20
carbon atom number Figure 4. Effluent composition in the catalytic cracking of lube oil (400~ WSHV=0.21 h w, t= 2 h) The evolution of the catalytic activity along the time on stream for the lube oil cracking over MCM-41 and ZSM-5 is shown in Figure 5. Both catalysts undergo a loss of activity, but it is more accentuated for the MCM-41 sample. Thus, after 6 hours of time on stream the conversion observed with MCM-41 is lower than that of the zeolite. Although these results indicate that MCM-41 suffers an important deactivation, it is remarkable that this material can stand several hours of reaction exhibiting significant lube oil conversions. Moreover, it must be taken into account that ZSM-5 zeolite is widely known by its high resistance to deactivation by coking since the absence of large cavities in its pore structure avoid bulky polyaromatic molecules to be formed. However, this is not the case for MCM-41 due to its larger pore size.
Figure 5. Catalyst deactivation during the lube oil cracking (reaction conditions as in Figure 4)
444 Changes in the product distribution are also observed along the time on stream. In the case of MCM-4 l, the relative proportion of gaseous hydrocarbons decreases as the catalyst is deactivated, whereas the formation of products higher than Ci0 is less affected. These results show that the catalyst deactivation influences in a higher extension the end-chain cracking reactions. Since coke deposition should also take place preferably on the stronger acid sites of the catalysts, it seems possible to establish a link between the acid strength and the cracking pathway. According to this, it can be proposed that the end-chain cracking reactions are favored and promoted by an stronger acidity. This conclusion would also explain the high amount of gaseous products observed in the polyolefins and lube oil degradations over ZSM-5 as a consequence of both the narrow pore size and the high acid strength of this material.
4. CONCLUSIONS MCM-41 presents interesting catalytic properties for the degradation and feedstock recycling of both plastic wastes and used lubricating oils. For all the raw materials investigated in this work, MCM-41 leads to activities quite superior than those observed with an amorphous silica-alumina sample. Compared to ZSM-5, the zeolite is a better catalyst than MCM-41 for the conversion of linear polymers, such as HDPE, due to its higher acid strength. However, the opposite is observed in the conversion of PP, a mixture of HDPE + LDPE + PP, and a lube oil, which can be assigned to the absence in MCM-41 of steric and diffusional hindrances. Regarding the catalyst deactivation along the time on stream, it has been observed during the lube oil cracking that MCM-41 is deactivated faster than ZSM-5 since the formation of bulky coke precursors within the mesopores is not restricted as in the zeolite. Nevertheless, MCM-41 leads to significant lube oil conversions during several hours of time on stream. Two different cracking pathways have been observed: end-chain cracking leading to gaseous hydrocarbons and random scission at any point in the chain, which yields a wider distribution of heavier hydrocarbons. Both mechanism take place in the reactions catalyzed by MCM-41, whereas mainly end-chain cracking is observed over ZSM-5. This difference has been related to the narrow pores and the high acid strength of the zeolite.
REFERENCES 1. Y. Uemichi, Y. Kashiwaya, M. Tsukidate, A. Ayame and H. Kanoh, Bull. Chem. Soc. Jpn., 56 (1983) 2768. 2. A.R. Songip, T. Masuda, H. Kuwuhara and K. Hashimoto, Appl. Catal. B' Environ., 2 (1993) 153. 3. R. Lin and R.L. White, J. Appl. Polym. Sci., 58 (1995) l 151. 4. J. Aguado, D.P. Serrano, M.D. Romero and J.M. Escola, Chem. Commun., (1996) 725. 5. J. Aguado, J.L. Sotelo, D.P. Serrano, J.A. Calles and J.M. Escola, Energy Fuels, I l (1997) 1225. 6. M.A. Uguina, A. de Lucas, F. Ruiz and D.P. Serrano, Ind. Eng. Chem. Res., 34 (1995) 451.
437
Catalytic properties of MCM-41 for the feedstock recycling of plastic and lubricating oil wastes D.P. Serrano, J. Aguado, J.L. Sotelo, R. Van Grieken, J.M. Escola and J.M. Mendndez Chemical Engineering Department, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain
The potential application of Al-containing MCM-41 for the catalytic degradation of polyolefinic plastics and used lubricating oils has been investigated and compared with the behavior of ZSM-5 zeolite and commercial amorphous SiO:AlzO3. For all the studied raw plastics, MCM-41 presents significant higher activity than the amorphous catalyst, and even superior to that of ZSM-5 for the degradation of pure polypropylene, a mixture of three polyolefins (high density polyethylene + low density polyethylene + polypropylene) and a lube distillate. On the contrary, the ZSM-5 zeolite leads to higher conversions in the degradation of pure polyethylenic plastics and shows a slower deactivation during the lube oil cracking. Regarding the product distribution, while ZSM-5 leads mainly to light hydrocarbons (C_,-C5), MCM-41 cracks these wastes into liquid fractions (gasoline and middle distillates), which suggests the cracking pathway is not the same with these two materials, being governed by their pore size and/or their acid strength.
1. INTRODUCTION The development of suitable methods for the treatment and recycling of polymeric and oil wastes is one of the challenges that the technology must face in the next years. Among the different alternatives to deal with this kind of wastes, chemical recycling present a high potential since it may allow to transform these residues in different products that can be used as raw materials for the preparation of fuels and chemicals. Currently, a number of processes are being investigated for the chemical recycling of plastic wastes" alcoholysis, hydrolysis, gasification, hydrogenation, pyrolysis and catalytic cracking. In the past, different types of acid catalysts have been applied for the catalytic cracking of polyolefinic plastics or for the conversion of heavy oils obtained by thermal cracking of the former. Among those catalysts, amorphous silica-alumina and different zeolites (X, Y and ZSM-5) have been typically used [1-3]. However, whereas zeolitic materials are advantageous in terms of acid sites distribution and strength, their application for the conversion of heavy wastes is limited by their pore size. In recent works [4,5], we have shown that these problems can be overcome with the use of MCM-41 materials as catalysts for the conversion of pure
438 polyolefins, since they present a balanced combination of medium acid strength with high surface area and uniform pore sizes in the mesopore range (1.5 - 10 nm). In the present work, the study of the potential use of MCM-41 materials for the catalytic conversion of polymeric and oil wastes into chemicals and hydrocarbon mixtures useful as feedstocks has been continued by using a mixture of polyolefins and a lube distillate as raw materials. In the last case, the evolution of the activity along the time on stream has provided information about the stability and the deactivation of MCM-41.
2. EXPERIMENTAL The MCM-41 sample used in this work has been synthesized at room temperature using tetratethylorthosilicate and aluminum isopropoxide as Si and AI sources, respectively. The alkoxides are hydrolyzed in a first step with an HCI solution also containing the surfactant, cetyltrimethylammonium chloride. Subsequently the formation of the solid mesoporous phase is accelerated through the addition of an ammonia solution. The solid product so obtained is separated by filtration, dried at I10~ and calcined in air at 550~ yielding the MCM-41 sample free of surfactant and directly in acid form, hence further ion exchange treatments are not necessary. The ZSM-5 sample has been synthesized from ethanol-containing gels according to a published procedure [6]. A commercial amorphous SiO2-AI203 sample was also used as reference (Stidchemie, KA-3). The three catalysts have been characterized by conventional techniques: X-ray fluorescence (XRF), X-ray diffraction (XRD), N 2 adsorption at 77 K, NH3 temperature programmed desorption (TPD), 27A1 MAS NMR, etc. Thus, the XRD and N2 adsorption measurements confirm that the MCM-41 sample has a regular mesoporosity with pore size around 2.9 nm, whereas 27A1 MAS-NMR measurements show that all the AI atoms present tetrahedral coordination in the as-synthesized sample. The main properties of these materials have been summarized in Table 1. Table 1. Physicochemical properties of the catalysts ,
,
Catalyst
Si/AI
Acidity (NHj TPD) (mmol/g) Tmax (~
Dp (nm) a
Saer (m2/g) .
,,.,
,
MCM-41
45.6
0.22
332
2.9
1177
ZSM-5
34.1
0.52
467
0.55
467
SiO2-A1203
2.0
0.14
304
2 -12
169
pore diameter
The catalytic experiments of polymer degradation have been carried out in a batch reactor at 400~ with continuous N2 flow. The reaction temperature was reached after 15 min heating from room temperature. The products leaving the reactor during 30 min were separated and accumulated as gaseous and liquid fractions for subsequent GC analysis. The following polyolefins were used as raw materials: high-density polyethylene (HDPE), low-
439 density polyethylene (LDPE), polypropylene (PP, isotacticity index = 93%) and a mixture of them having a composition that resembles the distribution of these polymers existing in municipal plastic wastes (LDPE" 46.5 wt%, HDPE" 25 wt% and PP: 28.5 wt%). The catalytic conversion of a lube distillate, formed by paraffinic hydrocarbons with boiling points in the range 450-550~ (C25- Cs0), has been studied in a down-flow fixed bed reactor at 400~ under atmospheric pressure. The lube distillate was fed by means of a syringe pump and mixed with a N2 stream previously to be introduced into the reactor. The products in the effluent stream were separated in a condenser at 0~ into gaseous and liquid phases, which were further quantified and analyzed by GC.
3. RESULTS AND DISCUSSION
3.1. Catalytic degradation of pure polyolefins MCM-41 was first tested in the catalytic conversion of three pure polyolefins: HDPE, LDPE and PP, the results obtained being compared with those corresponding to a ZSM-5 zeolite and a commercial amorphous silica-alumina. Different plastic/catalyst ratios were used for each polymer in order to obtain adequate polymer conversions. The activities are shown and compared in Figure l for the different catalysts and polymers. With the three polyolefins, the less active sample is the commercial amorphous silica-alumina, which leads to conversions just slightly higher than those observed by thermal cracking. This poor activity is probably originated by the weak acidity and the low surface area present in this material (see Table l).
Figure 1. Activity in the cracking of pure polyolefins (400~ 30 min., P/C = plastic/catalyst mass ratio) The comparison between the catalytic behavior of MCM-41 and ZSM-5 zeolite is more complex. The latter is clearly a superior catalyst for the degradation of highly linear polymers such as HDPE probably due to its stronger acidity, but it exhibits very low activity in the PP conversion, polyolefin with a high proportion of side methyl groups. In this case, the conversion obtained over MCM-41 is almost total, whereas the activity of ZSM-5 is very
440 close to that of the thermal cracking. An intermediate result is observed in the degradation of LDPE, linear polymer having a certain degree of branching, since both materials present similar activities. The order of activity exhibited by MCM-41, both in terms of overall conversion or conversion per unit mass of catalyst, follows the expected trend according to the proportion of highly reactive tertiary carbons present in the polymeric chains: PP > LDPE > HDPE. In this way, the lack of activity observed over ZSM-5 for the PP degradation is remarkable, which confirms previous results obtained at a higher plastic/catalyst ratio [5], indicating that in this case the presence of the side methyl groups in the polymeric chain hinders its access to the narrow zeolite pores. Thus, the low PP conversion observed over ZSM-5 must take place on the external acid sites that, given the crystal size of this sample (5~m), accounts just for a very small proportion of the total acidity. These steric and diffusional limitations are not present in the polymer degradation catalyzed by MCM~ due the larger pore size of this material. Concerning the product distribution, Figure 2 compares the selectivity towards different hydrocarbon groups obtained with the two most active catalysts, MCM-41 and ZSM-5, in the conversion of the three polymers. In all cases, MCM-41 leads mainly to liquid fractions having boiling points in the range of gasoline and middle distillates, with overall selectivities between 70 and 85%. In contrast, the main products obtained over ZSM-5 zeolite are gaseous hydrocarbons with a high proportion of olefins.
Figure 2. Product distribution in the catalytic cracking of pure polyolefins (reaction conditions as in Figure 1)
441 Similar conclusions can be obtained from the corresponding product distribution per carbon atom number. Thus, Figure 3 compares the distribution obtained in the LDPE cracking over MCM-41 and ZSM-5. For the latter, a strong maximum is observed between C3 and C6, with around 35 wt% of the products corresponding to C4 hydrocarbons. On the contrary, the distribution obtained over MCM-41 is much wider since, although a maximum is present also at C4, significant proportions of products are observed up to C20. From these results, two different cracking pathways can be envisaged" end-chain cracking leading to gaseous hydrocarbons and random cracking at any point in the chain, which yields a wide distribution of heavy hydrocarbons. The first mechanism is predominant in the polymer degradation over ZSM-5, while both pathways take place over MCM-41. These differences are probably originated by the narrow pores of the zeolite which promote an intensive cracking of the molecules as they enter and diffuse along the channels, although the higher strength of the zeolite acid sites may also favour the end-chain cracking. 35
9
30
--~-- MCM-41 -- 9
ZSM-5
25
~ 20
~ ~o
,:
,
|
I
,
I
|
9
,
i
Carbon atom number
Figure 3. Product distribution per carbon atom number in the LDPE catalytic cracking (reaction conditions as in Figure l) Likewise, it must be pointed out that the primary products of the cracking reactions may undergo subsequent transformations leading to a higher variety of hydrocarbons. In this way, in the two distributions shown in Figure 3, a second maximum is observed at Cs for both ZSM-5 and MCM-41, originated probably by the dimerization of C4 olefins.
3.2. Catalytic degradation of mixed polyolefins The three catalysts have been also checked for the degradation of a physical mixture of the three polyolefins since this may be a more realistic case when dealing with plastic wastes. The composition of the polymer mixture has been selected to be within the range of polyolefin distribution usually present in municipal plastic wastes. The results obtained in the catalytic degradation of this mixture have been summarized in Table 2.
442 The amorphous silica-alumina has a very low activity, as expected from the results obtained with the pure plastics. MCM-41 leads to a conversion close to 50%, remarkably higher than that obtained with the ZSM-5 sample. It is interesting to note that the ZSM-5 activity is appreciably lower than the one could be predicted based on the results with the pure polyolefins. Although ZSM-5 has shown not to be active for the PP degradation, the proportion of this polymer in the mixture (28.5 wt%) does not support the low overall plastic conversion observed with this catalyst. It seems that polypropylene has a deactivating effect on the ZSM-5 zeolite. A possible explanation to this result is the existence of a preferential contact of the ZSM-5 crystals with the PP particles, hindering the conversion of the other polyolefins present in the mixture. Table 2. Catalytic cracking of mixed polyolefins (LDPE=46.5,HDPE=25,PP--28.5 wt%) T=400~ t=30 rain., plastic/catalyst= 50 (mass ratio) Conversion (%) C~-C4 paraffins (%) C_,-C4 olefins (%) C5-Cj2 CI3-C22
C23-C40
MCM-41
ZSM-5
SiO2-AI20.~
49.0 3.9 10.1 54.2 29.4 2.4
6.8 17.3 33.2 49.5 . . . .
3.8 17.9 28.9 53.2 . .
. .
. .
. .
The product distribution obtained in the mixed polyolefin degradation over MCM-41 and ZSM-5 agrees well with that observed in the pure polymer cracking. In this case around 86% of the products obtained with MCM-41 are liquid hydrocarbons in the range of both gasoline and middle distillates.
3.3. Catalytic degradation of lube distillate The catalytic properties of MCM-41 for the chemical recycling of used lubricating oils have been explored using as raw material a lube distillate formed by paraffinic hydrocarbons in the range C25-C50, the results being compared with those obtained over the ZSM-5 sample. The experiments have been carried out in a continous flow system, which has allowed us to obtain information about the catalyst deactivation. The composition of the reactor effluent observed in the cracking of the lube oil at 400~ after 2 hours of time on stream is illustrated in Figure 4, being also included as reference the composition of the raw material. The conversion obtained in each case has been calculated from the decrease observed in the curve corresponding to the raw lube oil. Under these conditions, MCM-41 is quite more active than the ZSM-5 sample in spite of the stronger acidity of the latter, suggesting that at short times on stream the activity is determined by the pore size of the catalyst, the superior conversion over MCM-41 being related to the absence of diffusional limitations. In regards to the product distribution, the results obtained are very similar to those commented above in the polyolefin degradation. Thus, whereas the product from the ZSM-5 cracking is formed by 63 wt% gases, mainly C3 and C4 olefins, the lube oil conversion over MCM-41 leads to 83 wt% of the products being in the range C5-C28. Therefore, it can be also
443 concluded that ZSM-5 catalyzed cracking of the lube oil takes place preferently at the end of the chains, while both end-chain and random cracking are observed with MCM-41. 10
"
'-
I
raw lube oil
]
s
I
~ 4
Em o 0
lb
io
3b
20
carbon atom number Figure 4. Effluent composition in the catalytic cracking of lube oil (400~ WSHV=0.21 h w, t= 2 h) The evolution of the catalytic activity along the time on stream for the lube oil cracking over MCM-41 and ZSM-5 is shown in Figure 5. Both catalysts undergo a loss of activity, but it is more accentuated for the MCM-41 sample. Thus, after 6 hours of time on stream the conversion observed with MCM-41 is lower than that of the zeolite. Although these results indicate that MCM-41 suffers an important deactivation, it is remarkable that this material can stand several hours of reaction exhibiting significant lube oil conversions. Moreover, it must be taken into account that ZSM-5 zeolite is widely known by its high resistance to deactivation by coking since the absence of large cavities in its pore structure avoid bulky polyaromatic molecules to be formed. However, this is not the case for MCM-41 due to its larger pore size.
Figure 5. Catalyst deactivation during the lube oil cracking (reaction conditions as in Figure 4)
444 Changes in the product distribution are also observed along the time on stream. In the case of MCM-4 l, the relative proportion of gaseous hydrocarbons decreases as the catalyst is deactivated, whereas the formation of products higher than Ci0 is less affected. These results show that the catalyst deactivation influences in a higher extension the end-chain cracking reactions. Since coke deposition should also take place preferably on the stronger acid sites of the catalysts, it seems possible to establish a link between the acid strength and the cracking pathway. According to this, it can be proposed that the end-chain cracking reactions are favored and promoted by an stronger acidity. This conclusion would also explain the high amount of gaseous products observed in the polyolefins and lube oil degradations over ZSM-5 as a consequence of both the narrow pore size and the high acid strength of this material.
4. CONCLUSIONS MCM-41 presents interesting catalytic properties for the degradation and feedstock recycling of both plastic wastes and used lubricating oils. For all the raw materials investigated in this work, MCM-41 leads to activities quite superior than those observed with an amorphous silica-alumina sample. Compared to ZSM-5, the zeolite is a better catalyst than MCM-41 for the conversion of linear polymers, such as HDPE, due to its higher acid strength. However, the opposite is observed in the conversion of PP, a mixture of HDPE + LDPE + PP, and a lube oil, which can be assigned to the absence in MCM-41 of steric and diffusional hindrances. Regarding the catalyst deactivation along the time on stream, it has been observed during the lube oil cracking that MCM-41 is deactivated faster than ZSM-5 since the formation of bulky coke precursors within the mesopores is not restricted as in the zeolite. Nevertheless, MCM-41 leads to significant lube oil conversions during several hours of time on stream. Two different cracking pathways have been observed: end-chain cracking leading to gaseous hydrocarbons and random scission at any point in the chain, which yields a wider distribution of heavier hydrocarbons. Both mechanism take place in the reactions catalyzed by MCM-41, whereas mainly end-chain cracking is observed over ZSM-5. This difference has been related to the narrow pores and the high acid strength of the zeolite.
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