Influence of Thermal Treatment of HUSY on Catalytic Pyrolysis of

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Influence of thermal treatment of HUSY on catalytic pyrolysis of polypropylene: An online photoionization mass spectrometric study Yizun Wang, Yu Wang, Yanan Zhu, Yang Pan, Jiuzhong Yang, Yuyang Li, and Fei Qi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00630 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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Influence of thermal treatment of HUSY on catalytic pyrolysis of polypropylene: An online photoionization mass spectrometric study Yizun Wanga, Yu Wangb, Yanan Zhub, Yang Panb,*, Jiuzhong Yangb, Yuyang Lic, Fei Qia,c,* a

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei,

Anhui 230026, P.R. China b

National Synchrotron Radiation Laboratory, University of Science and Technology of China,

Hefei, Anhui 230029, P.R. China c

Key Laboratory for Power Machinery and Engineering of Education, Shanghai Jiao Tong

University, Shanghai 200240, P.R. China

KEYWORDS. Thermal pretreatment; Polypropylene; HUSY; Catalytic pyrolysis; Photoionization mass spectrometry

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ABSTRACT.

Refuse-derived fuels or chemicals have attracted much attention because of their ecological and economical utilization of municipal solid waste. However, the related conversions involve complex chemical reactions that prevent the use of conventional analytical methods for characterization of the detailed profiles. In this study, the influence of the thermal pretreatment of HUSY on the catalytic pyrolysis of polypropylene (PP) was investigated using online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOFMS). The experiments were carried out in two modes: 1) In temperature-fixed mode, the mass spectra of the pyrolyzates of PP and PP/HUSY at different temperatures were obtained in real time. Moreover, the selectivity of the pyrolyzates and conversion of PP over HUSY zeolites calcined at different temperatures were studied. 2) In temperature-programmed mode, the formation temperatures of the PP pyrolysis products in the presence of HUSY were determined. HUSY zeolites that are pretreated at different temperatures in the range from 200 to 800 °C exhibit distinct catalytic properties. The results of catalytic pyrolysis with a pretreatment temperature of 200 or 800 °C differed compared with those obtained using a pretreatment temperature of 350, 500 or 650 °C. However, the performances of catalysts prepared using these three pretreatment temperatures were similar, consistent with the changes in the HUSY structures observed from the XRD results and the acidities measured by NH3-TPD. The different acidities of the HUSYs resulted in different catalytic activities and pyrolysis product distributions. The results of this study indicate that Py-PI-TOFMS is a powerful technique for the comprehensive study of pyrolysis products and their pyrolysis behaviors, as well as catalysts prepared using different pretreatment conditions.


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1. INTRODUCTION Currently, municipal solid waste (MSW) management is a challenge that many countries are facing. Rapid urbanization, new economic activity, and population growth place enormous pressure on solid waste management systems. Cities generated approximately 1.3 billion tons of MSW worldwide in 2010, and this amount is expected to increase to 2.2 billion tons by 2025 1. Plastics are one of the most important components in MSW. The huge demand for plastics results in treatment problems for plastic wastes. According to the data supplied by the World Bank 1, plastic waste accounts for 8–12% of the total MSW production in the world. Twenty million tons of plastic waste were produced in China in 2012; in Europe in 2010, this number was 24.7 million tons. In the US, approximately 30 million tons of plastic waste is generated each year. Polypropylene (PP) is a widely used plastic because of its excellent properties. In the US, approximately 14% of plastic products are made of PP, including medical bags, computer components, automotive components, pipes and general containers. Many studies have focused on the treatment of MSW

2-6

. Landfill, incineration and biological

treatment are three main approaches for processing MSW 1. However, landfill is not a proper way to deal with plastic wastes for its vast amount and long term of degradation in environment. Incineration of plastic wastes produces lots of poisonous products including dioxin7. Biological treatment also faces exhaust emission as well as the similar problems with landfill. In comparison, pyrolysis has become an important method for plastic wastes (especially polyolefins) recycling and fuel-like hydrocarbon recovery 8-13. Akpanudoh et al.14 studied the degradation of polyethylene over ultrastable Y zeolite and the trend of liquid hydrocarbon formation was observed with the acidity content. Siddiqui et al.15 studied a number of conditions that affect the product distribution when recycling waste plastic into liquid fuel oils. The investigation taken by Hazrat et al.16 employed various types of zeolite catalysts, concluding that the thermos-catalytic


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process can be industrially or nationally sponsored to reduce solid polymer wastes from environment and the fuel production was impressive. Muhammad and coworkers

17

investigated

the product yield from pyrolysis of mixed and pure waste plastics using a two-stage pyrolysiscatalysis fixed bed reactor in presence of HZSM-5. They found that different kinds of plastics had significant interaction which would get higher C2-C4 gas yield and higher aromatic content in the oils than the pure plastic samples. As can be seen, the most widely used catalysts in these studies were zeolites, and the results demonstrated that the pore structures and acid sites of the zeolites affected the yield and selectivity of the products 18. Zeolites are a class of environmentally friendly catalysts that can be used for the pyrolysis of polymers and petrochemicals

19

. Most unmodified zeolites are composed of aluminosilicates.

The crystal structures, pores and acid sites of zeolites differ depending on the framework building blocks that consist of silicon and aluminum atoms and are sensitive to the preparation conditions. Calcination is a basic method for modifying zeolites for use as catalysts. Calcination at a certain temperature can change the number of acid sites, alter the microstructure and clean up the micropores, thereby improving the performance of zeolites. An understanding of the influence of calcination on zeolites is important because their adsorption and desorption properties and their catalytic activities are related to the acid sites and pore structure. Cruciani 20 studied the structural changes and thermal stabilities of several types of zeolites under heat treatment. Liu et al.

21

investigated the effects of calcination on raw rectorite mineral and

reported that rectorite experienced four stages of structural change after being calcined in a temperature range from 25 to 1300 °C. Serrano et al. variations of ZSM-5 after calcination, and Lu et al.

22

23

reported a detailed investigation of the studied the changes in ZSM-5 samples

calcined at four temperatures and their corresponding performance in the catalytic cracking of nbutane. In both studies, the calcination affects the L/B acid ratio and product selectivity.


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Thermogravimetric analysis (TGA) and gas chromatography-mass spectrometry (GC-MS) are the most commonly used analytical methods for studying the pyrolysis or catalytic pyrolysis of polyolefins

4, 24-31

. TGA can be used to determine the reaction temperature range and overall

reaction activation energy, whereas GC-MS provides qualitative and quantitative information regarding the pyrolysis products. Nevertheless, most of the nascent products obtained during pyrolysis cannot be observed in real time. Recently, online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOFMS) was utilized to investigate the catalytic pyrolysis processes of PP in the presence of HZSM-5 zeolite

32

. In comparison to the traditional “hard” electron ionization (EI) method for gaseous

component analysis, photoionization produces few or no fragments, making the identification and interpretation of complex pyrolysis products in real time more feasible. In the current study, the influence of HUSY calcination on its catalytic pyrolysis behavior was studied using online Py-PI-TOFMS as well as other conventional methods. The results indicate that calcination changes the characteristics and catalytic activities of HUSYs to different extents, which will affect the yield and selectivity of the products. 2. MATERIAL AND METHODS 2.1 Sample Preparation The pure PP (isotactic, ρ = 0.91 g/cm3, softening temperature = 161 °C, particle size < 180 µm) used in this study was supplied by Shanghai Liyang Machinery & Electric Co., Ltd. A commercially available HUSY zeolite (Si/Al = 5.2) was purchased from Nankai University Catalyst Co., Ltd. Prior to thermal pretreatment, HUSY was carefully ground to break the caking parts, and about 2 g of the ground HUSY powder was placed into a crucible and calcined in a muffle furnace under atmospheric conditions. The thermal pretreatments of HUSY were


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performed in the temperature range from 200 to 800 °C at 150 °C intervals; the duration for each calcination was 2 h. In the mass spectrometric measurements, the calcined HUSYs and powdered pure PP were mixed well in a mass ratio of 1:1. The weight of each sample used in pyrolysis was 40 mg, which was measured using an analytical balance (AL 204, Mettler Toledo, Switzerland). 2.2 X-Ray Powder Diffraction (XRD) The XRD analyses were conducted on an X-ray diffractometer (TTR-III Rigaku, Tokyo, Japan) equipped with a Cu Kα radiation source operated at 40 kV and a current of 200 mA. The scanning rate was 10°/min over the 2θ range from 5 to 80° in steps of 0.02°. 2.3 Online Py-PI-TOFMS The online experiment was carried out using a homemade Py-PI-TOFMS apparatus that has been previously reported in detail

32-34

. A brief illustration is shown in Fig. 1; the experimental

setup comprises a tubular furnace, a transfer line, and a PI-TOFMS.

Figure 1. Schematic setup for Py-PI-TOFMS. The pyrolysis temperature of the tubular furnace was controlled by a temperature controller (SKY Technology Development Co., Ltd., China). A K-type thermocouple (1) was used to


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measure and feed back the temperature of the furnace. Another thermocouple (2) positioned near the sample boat was used to measure the temperature of the sample in real time. Prior to the experiment, the system was purged with flowing nitrogen for 15 min to ensure an inert atmosphere. The flow rate of the carrier gas (nitrogen) was maintained at 200 standard cubic centimeters per minute (SCCM). When the temperature reached the set value, the sample, with a weight of 40mg, was introduced into the middle position of the furnace using a quartz sample boat. The pyrolysis products were transferred through a deactivated fused-silica capillary (I.D. 250 µm) inside the heated transfer line (250 °C) to reach the ionization chamber (0.75 Pa), where the products were ionized by ultraviolet light emitted from a Kr lamp with a photon energy of 10.6 eV (PKS106, Heraeus, Ltd., Germany). To remove the fine particles from the product gas stream, a glass fiber filter with a pore size of 1.2 µm was placed between the transfer line and the furnace. The formed ions were mass analyzed by TOFMS. The ion signal was amplified with a VT120C preamplifier (ORTEC, Oak Ridge, USA) and recorded by a P7888 multiscaler (FAST Comtec, Oberhaching, Germany). The pyrolysis/photoionization mass spectrometric measurements were conducted in both temperature-fixed mode and temperature-programmed mode. In the first mode, the temperature of the furnace was fixed at a specific value in the range from 300 to 600 °C. In the latter mode, the heating rate of the furnace was set to 10 °C /min and the acquisition time for each spectrum was 10 s. 2.4 Temperature-Programmed Desorption (TPD) The TPD measurements were recorded using the PI-TOFMS apparatus described in Fig. 1. In a typical TPD measurement, HUSY (60 mg) was first pretreated in the furnace at 150 °C with N2 as the purge gas (200 SCCM) for 30 minutes. Then, NH3 (7 SCCM) and N2 (200 SCCM) were introduced into the sample tube for 40 minutes to ensure that the zeolite was fully adsorbed with


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ammonia. The signal change for ammonia was detected by online PI-TOFMS to ensure that 40 min was sufficient for adsorption. Finally, to desorb the physically adsorbed ammonia, HUSY was purged with N2 (400 SCCM) for 1 h at a furnace temperature of 80 °C. NH3-TPD was performed using Py-PI-TOFMS from 80 to 700 °C at a heating rate of 10 °C/min. The acquisition time for each spectrum was 10 s. 3. RESULTS AND DISCUSSION 3.1 Characterization of HUSY As shown in Fig. 2, the crystallinity of HUSY catalysts was not affected by calcination. The difference among each sample is the slight deviations of the diffraction peak positions. As the pretreatment temperature was increased, the diffraction peaks shifted toward the high-angle direction. The shifts of the diffraction peaks were not continuous. As shown in Fig. 2b, HUSY zeolites calcined at 200 and 800 °C exhibited peaks at 23.85 °and 23.98 °, respectively, and the diffraction peaks obtained using pretreatment temperatures of 350, 500, and 650 °C were located at nearly the same position. According to Bragg’s law, a diffraction peak shift to a higher angle indicates an increase in the lattice parameter and a corresponding shrinkage of the unit cell. It is noteworthy that the 2θ shifting is very small (0.13°), which would not influence the cell parameter significantly. The similar shifting can be obtained in the modification of zeolite with cation exchange. According to the data applied by Deng 35, the cell parameter only reduced 0.5% when the 2θ increased 0.1°. Therefore, little change of the framework structure can be safely concluded, and the cause of the shift is probably due to the chemical transformation of the surface. The characterization indicates two structural changes during the thermal pretreatment. The first change between 200 and 350 °C was possibly due to the dehydration of silanols, forming Si-O-Si and isolated silanol (Si-OH)36. The hydrated silanols is tended to form H-bonds and normally ACS Paragon Plus Environment

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have very low acidity, whereas isolated Si-OH is a mild Bronsted acid. By heating, large number of hydrated silanols were condensated to little isolate silanol. The second change occurred from 650 °C to 800 °C might be the dealumination from the framework and of course the loss of strong acidities.

Figure 2. XRD patterns of HUSY calcined at different temperatures. For further information of the acidity of the HUSY zeolites, NH3-TPD experiments were carried out. Figure 3 shows the normalized NH3-TPD results for HUSYs prepared under different thermal pretreatment conditions. Each profile contains two broad bands, except in the case of the sample pretreated at 800 °C. In general, the peak at approximately 220 °C is due to a weak acid, and the peak at approximately 390 °C corresponds to a strong acid 37. HUSY that was pretreated at 200 °C contained the largest amount of weak acid sites. However, the acidity comes from hydrated silanols, which should be very weak. The TPD profiles of HUSY calcined at 350 and 500 °C exhibited a slight decrease in weak acid, and the intensities of the strong acid sites remained at a relatively high level. During calcination, dehydration occurred and the amount of silanols was greatly reduced. However, the weak acidity did not decrease too much. This is


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because of the formation of isolated silanols as mentioned before. The appearance of similar profiles for the samples pretreated at 350 and 500 °C indicates that HUSY pretreated in a temperature range from 350 to 500 °C reaches a steady state. When the pretreatment temperature was increased to 650 °C, the intensities of the strong/weak acids decreased, indicating that some of the strong/weak acid sites in HUSY were eliminated at higher temperatures. At a pretreatment temperature of 800 °C, the number of acid sites decreased compared to those of HUSYs pretreated at lower temperatures. The significant decrease in strong acid sites was caused by dealumination during calcination

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as well as by blocking of these sites by cationic oxidic

aluminum species 39-42. The amount and strength of the acid sites strongly affect the performance of HUSY in catalytic reactions. The catalytic pyrolysis of PP with HUSY calcined at various temperatures will be discussed in a later section.

Figure 3. NH3-TPD results for HUSY calcined at different temperatures. 3.2 Photoionization Mass Spectra Fig. 4 shows the photoionization mass spectra of the pyrolysis products of PP and PP/HUSY 50 wt% samples at different temperatures. Because of the “soft” near-threshold photoionization characteristics, nearly all of the mass peaks were assignable as parent ions of the pyrolysis


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products with few or no fragments. All of the species shown in Fig. 4 are classified as alkanes, alkenes, dienes, or aromatics, as listed in Table 1.

Figure 4. Photoionization mass spectra of the pyrolysis products of pure PP and PP/HUSY 50 wt% at different temperatures. HUSY was calcined at 500 °C prior to the catalytic pyrolysis experiments. As shown in Fig. 4(a, e), the initial decomposition temperature of PP decreased in the presence of HUSY. Moreover, the product distributions were substantially simplified by the addition of HUSY. Additionally, at the same temperature, more low-molecular-weight alkanes and aromatics were generated in the presence of the HUSY catalyst, and the production of dienes was substantially reduced. The acidity of HUSY results in the catalytic conversion of heavy carbons chains (carbon numbers greater than 10) to light carbon chains in the temperature range from 300 to 500 °C. The dienes were likely cyclized to aromatics in the 12-membered rings in the HUSY zeolite. However, pyrolysis at 600 °C in the presence of HUSY produced heavier carbon chains


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than pyrolysis in the absence of HUSY. At 600 °C, the HUSY catalyst may undergo severe condensation and carbonization. Table 1. Major pyrolysis products of PP m/z

Formula

Species

m/z

Formula

Species

28

C2H4

Ethylene b

98

C7H14

1-Heptene a

42

C3H6

Propylene b

100

C7H16

Heptane

54

C4H6

1,3-butadiene b

106

C8H10

Xylene b

56

C4H8

2-methyl-1-propene b

112

C8H16

4-methyl-2-heptene b

68

C5H8

1,4-Pentadiene a

114

C8H18

Octane

2-Methyl-1,3-butadiene a

120

C9H12

Mesitylene c

70

C5H10

1-pentene a

124

C9H16

2,6-Dimethyl-2,4-heptadiene a

72

C5H12

2-Methyl-butane a

126

C9H18

2,4-dimethyl-1-heptene b

78

C6H6

Benzene b

82

C6H10

1,3-Pentadiene a

128

C9H20

Nonane

84

C6H12

2-methyl-1-pentene b

140

C10H20

2,4,6-trimethyl-1-heptene b

4-methyl-2-pentene b

154

C11H22

4,6-trimethyl-2-nonene b

2-Methyl pentane a

168

C12H24

2,4,6-trimethyl-1-nonene b

86

C6H14

4,6-dimethyl-2-heptene b

Hexane a

4,6,8-trimethyl-2-nonene b

92

C7H8

Toluene b

96

C7H12

2,4-Hexadiene a

a

182

C13H26

2,4,6,8-tetramethyl-1-nonene b

Ref. 43. b Ref. 32. c Ref. 44.

3.3 Temperature-Programmed Pyrolysis Temperature-programmed pyrolysis based on photoionization mass spectrometry is a powerful tool for comprehensively studying the release of pyrolysis products. The profiles of four typical pyrolysis products of PP (i.e., pentane (m/z = 72), pentene (m/z = 70), hexadiene (m/z = 82), and xylene (m/z = 106)) are plotted as a function of temperature in Fig. 5. The profiles of these four


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types of products generated from PP/HUSY at different calcination temperatures are also shown in Fig. 5. Overall, the presence of HUSY substantially decreased the reaction temperature. The reaction temperature of HUSY calcined at 200 °C was not as low as those of HUSY calcined at other temperatures. However, this sample produced more olefins than the other HUSY catalysts. The trends for the products catalyzed by HUSY calcined at 350, 500, and 650 °C were similar to each other.

Figure 5. Temperature-programmed pyrolysis results for representative species at different calcination temperatures: a) C5H12, b) C5H10, c) C6H10, and d) C8H10.


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Figure 6. Relationship between the initial appearance temperatures and the pyrolysis products of PP and PP/HUSY at different calcination temperatures: a) alkanes, b) alkenes, c) dienes, and d) aromatics. The formation temperatures of different groups of PP pyrolysis products were also obtained from the temperature-programmed decomposition experiments. Fig. 6 shows the relationship between the initial appearance temperatures and pyrolysis products of PP and PP with HUSY catalysts at different pretreatment temperatures. The pyrolysis of pure PP requires the highest decomposition temperature for alkanes, alkenes, dienes and aromatics but does not exhibit a clear trend among the different types of products. In the case of HUSY calcined at 200 °C, the decomposition temperature was lower than that of pure PP but relatively higher than those of HUSY pretreated at other temperatures because of the hydrated surface and therefore lacking of weak acidity. For HUSY calcined at 350, 500 and 650 °C, the formation temperatures and trends for the different product groups were approximately the same, even though the number of acid sites on HUSY calcined at 650 °C was approximately 17% lower than those on HUSY calcined at 350 and 500 °C. This result is consistent with those obtained from the XRD and NH3-TPD measurements. Although the number of acid sites on HUSY calcined at 800 °C decreased substantially, the formation temperatures of the catalytic pyrolysis products for PP/HUSY were not significantly higher than those for HUSY calcined at 350, 500 and 650 °C, indicating that the weak acid sites will reduce the pyrolysis temperature, as well as that dehydration of the zeolite catalyst will significantly affect the acidity and the pyrolysis activity. The appearance temperatures of the alkanes, alkenes, dienes, and aromatics shown in Fig. 6 are also informative. The softening temperature of PP used in this study was approximately 161 °C. However, most of the products, especially the alkanes and alkenes in the presence of HUSYs calcined at 350 to 800


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°C, were generated before PP was fully melted. One explanation is that the small branches on the surface of the PP particles were softened and could enter the pores of HUSY to contact more acidic sites. The probability of generating carbocations on the branches of PP would be much greater than on those parts only in contact with the surface of HUSY, which is beneficial for the formation of small products. The idea of branches entering the pores of zeolites has been previously proposed by Pinto et al.

18

, Sakata et al.

45

and Marcilla et al.

46, 47

on the basis of

studies of the catalytic pyrolysis of polymers with different degrees of branching. When the temperature was higher than the melting point, longer chains entered the pores of HUSY and products with larger carbon numbers were generated. In comparison to alkanes and alkenes, the formation temperatures of dienes and aromatics were higher, indicating that the activation energies of the corresponding reactions were high. 3.4 Temperature-Fixed Pyrolysis The selectivity of the pyrolysis products and conversion of PP in the presence of HUSY was studied using Py-PI-TOFMS in temperature-fixed mode. The yields of the major products were obtained by integrating the mass peaks in the spectra measured at various pyrolysis and calcination temperatures. Fig. 7 shows a comparison of the variations in the relative ion intensities for the alkanes, alkenes, dienes and aromatics obtained at pyrolytic temperatures of 300, 400, 500, and 600 °C and in the presence of HUSY catalysts pretreated at 200, 350, 500, 650 and 800 °C. As shown in Fig. 7(a-d), the alkane yields were substantially improved in the presence of HUSY. The product distributions obtained using catalysts pretreated at 350, 500, and 650 °C were similar to each other but differed from those obtained using catalysts pretreated at 200 and 800 °C, especially in the case of pyrolysis temperatures of 300 and 400 °C. This phenomenon implies that the number/strength of valid acid sites (e.g. isolated silanols and Si-O-Al) is higher in HUSY


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pretreated at 350, 500 and 650 °C. Lower or higher pretreatment temperatures reduce the catalytic activity of HUSY toward the generation of alkanes. The more valid acid sites, the lower activation energy in catalytic pyrolysis of PP. So that the sample with HUSY pretreated at 350, 500 and 650 °C generated more low-molecular-weight products when the pyrolysis temperature is lower (Fig. 7(a, b)). And when the pyrolysis temperature was high enough to induce dehydration of the zeolite, the samples displayed a similar distribution of alkane products, as shown in Fig. 7(d).

Figure 7. Product distributions for the catalytic pyrolysis of PP and PP/HUSY at different pyrolysis temperatures, which are labeled at the top, and different HUSY calcination temperatures shown in (c). For pure PP, alkenes cannot be formed until a relatively high temperature of 400 °C is reached, as demonstrated in Fig. 7(e-h). The presence of HUSY can substantially promote the formation of low-molecular-weight alkenes, especially those with carbon numbers ranging from C3 to C7.


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The molecular weights of alkenes obtained using catalysts pretreated at 200 and 800 °C were relatively higher than those obtained using the other catalysts, especially in the case of pyrolysis at 300 °C. The number of acid sites and acid type or the structure of HUSY calcined at 200 and 800 °C were more suitable for generating alkenes. More/stronger acid sites did not lead to more alkene products although they contribute a lot in generating alkanes. As previously mentioned, HUSY can selectively suppress the production of dienes. As shown in Fig. 7(i-l), the quantities of dienes obtained at 600 °C were approximately 10 times higher than those obtained at 300 °C for the pyrolysis of pure PP. With the HUSY/PP mixture, the ion intensities of dienes in a catalytic pyrolysis temperature range of 300-600 °C remained at a relatively low level. Because of the inefficient or excessive pretreatment of HUSY at 200 and 800 °C, the suppression of dienes by HUSY was not as high as in the case of HUSY pretreated at 350, 500, or 650 °C. The production of aromatics in pure PP pyrolysis required a relatively high temperature. Catalytic pyrolysis of PP with HUSY promoted the formation of aromatics. As shown in Fig. 7(m-p), the total aromatic yields were much higher when HUSY was added. The catalytic pyrolysis product distributions at six pretreatment temperatures exhibited the same trends, and the dominant mass peak was observed at C9H12 (m/z = 120). A low pretreatment temperature of 200 °C favored the production of aromatics, especially those with carbon numbers ranging from C8 to C12.

4. CONCLUSION In this study, online pyrolysis photoionization time-of-flight mass spectrometry was applied to study the catalytic pyrolysis of PP with HUSY pretreated at various temperatures. First, HUSY zeolites pretreated at 200, 350, 500, 650 and 800 °C were characterized using XRD and NH3ACS Paragon Plus Environment

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TPD measurements. The XRD patterns indicated that the crystallinity of HUSY was not collapse after calcination. The diffraction peaks of HUSY shifted toward higher angles with two stages as the thermal pretreatment temperature was increased. The stages of shifting were attributed to the dehydration of silanols and dealumination from the framework, respectively. The NH3-TPD profiles demonstrate that the intensities of the strong/weak acids decreased with increasing temperature. In the temperature-fixed mode, the photoionization mass spectra of PP and PP/HUSY at different pyrolysis temperatures were obtained in real time. Major products, such as alkanes, alkenes, dienes and aromatics, differed for PP and PP/HUSY at different pyrolysis temperatures. In the temperature-programmed mode, the formation temperatures of the PP pyrolysis products in the presence of HUSY calcined at different temperatures were obtained. The presence of HUSY substantially decreased the pyrolysis temperature. However, the selectivity and yields for alkanes, alkenes, dienes and aromatics with HUSY under different thermal treatment conditions differed substantially. HUSY pretreated at 350, 500, and 650 °C exhibited similar catalytic performances, consistent with the structure changes of HUSY characterized by XRD and NH3TPD. The activity of HUSY pretreated at 200 and 800 °C was lower compared to the activities of the samples pretreated at 350, 500, and 650 °C, especially in the case of the catalyst pretreated at 200 °C. However, the yield of alkenes, dienes and aromatics was higher. Different HUSY pretreatment temperatures are recommended for different purposes for the catalytic pyrolysis of PP.


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ACKNOWLEDGMENTS This work has been supported by grants from the National Natural Science Foundation of China (91545120 and U1432128), National Basic Research Program of China (973 Program) (2013CB834602 and 2012CB719701), Chinese Academy of Sciences, and Chinese Universities Scientific Fund.

AUTHOR INFORMATION Corresponding Authors Fax: +86 551 65141078. E-mail: [email protected] (YP); E-mail: [email protected] (FQ). ABBREVIATIONS MSW, municipal solid waste; Py-PI-TOFMS, pyrolysis photoionization time-of-flight mass spectrometry; PP, polypropylene; GC/MS, gas chromatography/mass spectrometry; TGA, thermogravimetric analysis; EI, electron ionization; SCCM, standard cubic centimeters per minute; XRD, X-ray powder diffraction; TPD, temperature-programmed desorption REFERENCES (1) Hoornweg, D.; Bhada-Tata, P. What a waste, a global review of solid waste management; World Bank: Washington, D.C., 2012; pp 16-21. (2) Tan, S. T.; Hashim, H.; Lim, J. S.; Ho, W. S.; Lee, C. T.; Yan, J., Energy and emissions benefits of renewable energy derived from municipal solid waste: Analysis of a low carbon scenario in Malaysia. Appl. Energy 2014, 136, 797-804.


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Influence of Thermal Treatment of HUSY on Catalytic Pyrolysis of

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