Catalytic Systems

Both homogeneous and heterogeneous systems have been used in the literature for studying the catalytic cracking of polymers. In general, heterogeneous catalysts are the preferred choice due to their easy separation and recovery from the reacting medium.

2.1 HOMOGENEOUSCATALYSTS

Homogeneous catalysts used for polyolefin degradation have mostly been classical Lewis acids such as AlCl3, metal tetrachloroaluminates melts and more recently, new catalytic systems based on organic ionic liquids.

Ivanova et al. [30] degraded polyethylene catalytically with AlCl3 and electrophilic complexes at 370°C giving rise to higher yields of gaseous compounds (88.2 wt%) than thermal cracking of this same polymer (40 wt% of gases at 400°C). The main gaseous compounds obtained in the catalytic cracking were isobutane (42.5%) and isobutene (21.8%) and the amount of hydrocarbons heavier than C5 was practically negligible. Tetrachloroaluminate melts M(AlCl4)n (M = Li, Na, K, Mg, Ca, Ba; n = 1-2), which constitute ionic media, were also applied as catalysts for polyethylene cracking leading to 90-95% of C4 hydrocarbons.

Catalytic systems over ionic liquids are gaining increased attention worldwide as benign solvents for green chemistry processes due to their low volatility and ease of product separation. Recently, catalytic cracking of polyethylene (HDPE, LDPE) has also been carried out using organic ionic liquids, such as 1-ethyl-3-methylimidazolium chloride-aluminum (III) chloride [31]. Light alkenes (C3-C5), such as isobutene, and branched and cyclic alkanes were the major product components. The reported working temperatures are meaningfully lower (90-250°C) than those used over conventional heterogeneous catalysts although times of 1 -6 days of reaction were needed to obtain yields of 60-95 wt%.

2.2 HETEROGENEOUS CATALYSTS

A wide variety of heterogeneous catalysts have been tested for the catalytic cracking of polyolefins and polystyrene, which can be summarized as follows:

  • conventional acid solids used in the catalytic cracking of hydrocarbon feedstocks: zeolites, silica-alumina, aluminas, fresh and spent FCC catalysts [32-42];
  • mesostructured catalysts: MCM-41, FSM-16, Al-SBA-15 [5, 43-48];
  • aluminium pillared clays [49-51];
  • nanocrystalline zeolites (n-HZSM-5) [52];
  • superacid solids (ZrO2/SO42-) [53];
  • gallosilicates [54, 55];
  • metals supported on carbon [56, 57];
  • basic oxides (BaO, K2O, etc.), mainly for polystyrene cracking [58, 59].

Among the above-mentioned solids, zeolites have been certainly the most studied catalysts for polyolefin cracking. Zeolites are crystalline microporous aluminosilicates of groups IA or IIA elements (mainly sodium, potassium, magnesium, calcium) whose chemical composition may be represented by the following formula [60]:

where 2 < y < 10, n is the cation valence and w represents the amount of water.

The zeolite framework is built from the combination of SiO4 and AlO4 tetrahedra linked by sharing oxygen atoms which extend tridimensionally. The presence of aluminium into the framework brings about the appearance of a negative net charge which must be balanced by an extra-framework cation. This charged nature of zeolites provides them with the capacity for ion exchange as well as acid properties when the extra-framework cation is a proton. The required acidity may be tuned by a proper choice of the zeolite structure as well as their aluminium content [60]. Zeolites may exhibit a mono-, bi- or tridimensional channel network with pore sizes of definite dimensions (below 1.0 nm) and even interconnected microporous cavities depending on their topology. In this regard, zeolites are considered as molecular sieves since their pore dimensions are close to those of many molecules (usually 0.4-1.0 nm) showing the property called 'shape selectivity' which allows them to discriminate among different reactants, transition states or products.

At present more than 100 zeolitic structures (both natural and synthetic) have been reported and their number grows annually as new structures are continuously being discovered which opens up a wide range of possible applications [61, 62]. However, from a practical viewpoint, only a few zeolites are used as industrial catalysts such as Y, ZSM-5, Beta and mordenite (Table 3.1), mainly due to the cost and difficulties inherent to their preparation [60]. When zeolites are applied for the catalytic cracking of polymers, their microporous structure causes important diffusional and steric hindrances for the access of the bulky plastic molecules to the internal acid sites [5, 24].

Amorphous silica-alumina (SiO2-Al2O3) has been also tested for the catalytic cracking of polyolefins [36, 37, 43]. This acid solid is featured by having a broad distribution of pore sizes, which is determined by the synthesis procedure. Moreover, the occurrence of a bimodal pore size distribution (e.g. meso-macroporous) is usually present. The aluminium

Table 3.1 Structural features of common zeolites used as catalysts in plastic cracking

Zeolite

Structure

Pore size (nm)

Si/Al ratio

ZSM-5

MFI

0.53 x 0.56, 0.51 x 0.55

10-1000

Y

FAU

0.74

1.5-3

Beta

BEA

0.64 x 0.76, 0.55 x 0.55

8-1000

Mordenite

MOR

0.65 x 0.7

5

content of these materials can be varied within a broad range (0.1-30 wt%) and their acid strength is of medium type, lower than that of the majority of zeolites, containing acid sites of both Bronsted and Lewis nature. Amorphous silica-alumina is also employed as a component of the formulation of FCC catalysts for the cracking of heavy feedstocks as these macromolecules can enter through the large meso/macropores of this material.

Both fresh and especially spent FCC catalysts have been the object of recent attention for polymer cracking [41, 42]. Although they are less active than silica-alumina or meso-porous catalysts [41], spent FCC catalysts still maintain enough activity to be considered as a good choice regarding the fact that their cost is basically zero and they are continuously being disposed of from FCC units. In addition, it has been proved [42] that the unavoidable metal contamination coming from their accumulation after the processing of heavy feedstocks (mostly Ni and V in amounts within 3000-6000 ppm) catalysts did not affect the obtained products over spent FCC catalysts. These same authors carry out an economic evaluation of a catalytic system based on used FCC catalysts, concluding that the cost seems comparable to that of a commercial thermal cracking plant.

The design of catalysts capable of overcoming steric hindrances by having more accessible acid sites located either into larger pores (mesoporous catalysts, pillared clays) [5, 43-51] or over the external surface (nanozeolites) [52] have been successfully tested for the cracking of polyolefins. The first mesoporous aluminosilicates (M41S) were discovered in the early 1990s by Mobil Oil researchers [63]. These aluminosilicates show a uniform mesopore size which can be tailored within the 1.5-10.0 nm range by a suitable choice of synthesis conditions (template, temperature and medium composition). Their BET surface areas are around 1000 m2 g-1 and their pore volumes about 0.8 cm3 g-1. MCM-41, which exhibits a hexagonal array of unidimensional mesopores, has been the most studied catalyst of the M41S family. Other mesoporous aluminosilicates such as FSM-16 have also been tested for the catalytic cracking of polyethylene. FSM-16 shows textural properties close to those of MCM-41 materials in terms of BET surface area and pore size, the main difference residing in that a layered silicate (kanemite) is used as silica source in its synthesis [46]. Both MCM-41 and FSM-16 have shown activity for the cracking of polyethylene even in their pure silica form [46, 47, 64]. Recently, another mesoporous silicate (SBA-15) was synthesized with BET surface areas around 600-800 m2 g-1 and uniform mesopores of dimensions adjustable within the 3.0-30.0 nm range [65]. SBA-15 has the advantage of its wider pore walls (>2.0 nm), that provides this material with higher hydrothermal and thermal stability compared to MCM-41, an important feature from the point of view of the catalyst regeneration. Catalytic cracking of polyethylene over Al-SBA-15 has also been carried out, yielding similar results in terms of product distribution to those of Al-MCM-41 catalysts [48]. Figure 3.5 shows the transmission electron micrographs of both Al-MCM-41 and Al-SBA-15 wherein their respective hexagonal array of mesopores is clearly appreciated. All the reported mesoporous alumi-nosilicates possess amorphous pore walls and a medium acid strength distribution, quite similar to that of silica-alumina.

Both natural clays and their aluminium oxide pillared analogues have also been tested for the catalytic cracking of polyethylene [49-51]. The clays investigated include mont-morillonite and saponite. They possess a layered structure which can be converted into a two-dimensional network of interconnected micropores by intercalation of molecular moieties. In the case of aluminium pillared clays, these materials show a mild acidity

Pillared Clay
Figure 3.5 TEM micrographs of MCM-41 (a) and SBA-15 (b) mesoporous materials

and an accessible pore size structure. The consideration of pillared clays as possible catalysts for plastic cracking is mainly supported by the fact that their acidity is weaker in strength than that of zeolites. Accordingly, they show a lower cracking activity, but also the catalyst deactivation by coke formation takes place to a lower extent compared with zeolitic catalysts. Moreover, the liquid products obtained over the clay catalysts are heavier, as the strong acidity of zeolites is responsible for plastic overcracking reactions with the production of light hydrocarbons. Likewise, the mild clay acidity leads to a lower occurrence of hydrogen-transfer reactions compared with US-Y zeolite, which in turn causes the formation of alkenes as the main products of the polyethylene cracking over clay catalysts.

Nanocrystalline zeolites, mainly HZSM-5, have also shown remarkable activity for polyolefin cracking [52]. These materials are zeolites synthesized by procedures leading to nanometer crystal size (<100 nm), which provides them with a high share of external surface area fully accessible to the whole of the bulky polymer macromolecules. For instance, nanocrystalline HZSM-5 (n-HZSM-5) with a crystal size around 60 nm presents an external surface area of 82 m2 g-1 (almost 20% of the total surface area) and consequently, a high amount of external acid sites. This catalyst has shown a high activity in the cracking of polyolefins, even when working at temperatures as low as 340°C and high plastic/catalyst ratios (P/C = 100).

Catalytic degradation of a standard polyolefin mixture made up of LDPE (46.5%), HDPE (25%) and PP (28.5%) was performed in a semi-batch reactor over a variety of acid solids, differing both in textural properties and acid strength distribution [43]. The conversions and the selectivities by groups and carbon atom number obtained in the cracking of the polyolefin mixture at 400°C for 0.5 h and using a plastic/catalyst mass ratio (P/C) of 50 are illustrated in Figures 3.6, 3.7 and 3.8, respectively. The highest conversions were obtained in the following order: n-HZSM-5 > HBeta > HMCM-41 > HZSM-5. However, the acid strength of the catalysts decreased as follows: HZSM-5 > n-HZSM-5 > HBeta > HMCM-41 > HY - SiO2 - Al2O3. The high performance of HMCM-41 was ascribed to both its large surface area and mesopore size while for the case of zeolite HBeta, its relatively large micropore size and medium acid strength are the main reasons to explain its good catalytic behaviour. It is noteworthy to observe the huge differences between the conversions obtained over the micrometer-size (HZSM-5) and nanocrystalline

Thermal cracking ^

0 20 40 60 80 100

Conversion (wt%)

Figure 3.6 Conversions obtained in the catalytic cracking of a model polyolefin mixture (semi-batch reactor, P/C = 50; T = 400°C, t = 30 min) [43]. (Reproduced by permission of the American Chemical Society)

Figure 3.6 Conversions obtained in the catalytic cracking of a model polyolefin mixture (semi-batch reactor, P/C = 50; T = 400°C, t = 30 min) [43]. (Reproduced by permission of the American Chemical Society)

Semi Batch Reactor Model With Recycle

C1—C4 C2_C4 paraffins olefins

C5-C12 C6-C9 C13

aromatics

C13 C3

C1—C4 C2_C4 paraffins olefins

C5-C12 C6-C9 C13

aromatics

C13 C3

Figure 3.7 Selectivities by groups obtained in the catalytic cracking of a model polyolefin mixture (semi-batch reactor, P/C = 50; T = 400°C, t = 30 min) [43]. (Reproduced by permission of the American Chemical Society)

Carbon atom number

Figure 3.8 Selectivities by carbon atom number obtained in the catalytic cracking of a model polyolefin mixture (semi-batch reactor, P/C = 50; T = 400°C, t = 30 min) [43]. (Reproduced by permission of the American Chemical Society)

Carbon atom number

Figure 3.8 Selectivities by carbon atom number obtained in the catalytic cracking of a model polyolefin mixture (semi-batch reactor, P/C = 50; T = 400°C, t = 30 min) [43]. (Reproduced by permission of the American Chemical Society)

(n-HZSM-5) zeolites, which have been directly related with the higher accessibility of the acid sites in this last material. The selectivity by groups and atom carbon number distribution of the products (Figures 3.7 and 3.8, respectively) are mainly related to the catalyst acid strength. The stronger the acid sites, the higher the amount of gaseous C1 -C4 formed by the end chain scission mechanism (mainly C3-C4). In contrast, the amount of C5-C12 hydrocarbons increases over catalysts with medium acid strength as they promote random scission reactions of the polymer backbone.

Super acid solids (ZrO2/SO42-) have been tested [53] in the catalytic cracking of HDPE using a thermogravimetric equipment (TG). These solids present higher acidity than 100% sulphuric acid and it is also superior to that of zeolites. The results obtained pointed out the following reactivity order: ZrO2/SO42- > zeolite HZSM-5 > silica-alumina, with a high share of volatile hydrocarbons, mainly C4-C5 alkenes, being obtained.

A different approach was followed by Uemichi et al. [56] that used metals (Pt, Fe, Mo, Zn, Co, etc.) supported over activated carbons as catalysts for the catalytic degradation of polyethylene at 300°C towards aromatic compounds (mostly benzene). The most effective metals being Pt, Fe and Mo that produced around a 45% yield of aromatics. The role played by the metals was the hydrogen desorption while its abstraction occurs primarily over the carbon sites. In line with the same target products (aromatics), gallosilicates were also employed for the catalytic cracking of polyethylenes at 425°C [54]. The catalyst was actually an HZSM-5 zeolite with Ga incorporated into the framework as heteroatom

(Si/Ga = 25) instead of aluminium, and proved to be highly effective for the production of aromatics (BTX yields ~ 50%).

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