Study On Thermal Cracking Of Polypropylene-based Wood Plastic Composite Under Modified HZSM-5 Catalysis

Wood plastic composite (WPC) has been rapidly developed in recent years. The upcoming problem we face is the harmless disposal of WPC wastes. WPC is mainly composed of biomass and plastics with high value as fuel. The presence of PP makes it difficult to degrade via microbial hydrolysis. However, WPC wastes can be converted into high-grade biofuels via fast pyrolysis technology. WPC pyrolysis products contain a large number of oxygenated compounds derived from biomass pyrolysis such as acids, aldehydes/ketones, furans, phenols, and C5-C30 hydrocarbons derived from plastic pyrolysis. And the synergy between biomass and plastics are not clear. Obviously, further upgrading of these WPC pyrolytic products are needed before its use for a transportation fuel. HZSM-5 zeolite is an efficient catalyst for both biomass deoxygenation and polyolefins cracking in pyrolysis process because of its strong acidity and unique pore structure. However, coke formed rapidly in the catalytic pyrolysis process covers the acid sites and blocks the pores, consequently resulting in the deactivation of the HZSM-5 catalyst. In addition, polycyclic aromatic hydrocarbons (PAHs) are generated in the catalysis of HZSM-5, which are carcinogenic/mutagenic and environmentally unfriendly. For these reasons, modification of HZSM-5 zeolite is needed to modify its acidity and structure, resist its deactivation, and improve product distribution. In this paper, the synergistic effects between biomass and PP, and the product composition and distribution from the pyrolysis of PP-based composite (WPP) with or without modified HZSM-5 zeolites were studied. The main contents and results of experiments are as following.1) α-Cellulose, hemicellulose and lignin were isolated from poplar wood, and then they were mixed with PP at a ratio of 1:1 (w:w) in the torque rheometer to form WPCs, respectively. The pyrolytic behaviors of four WPCs and their components were studied in a thermogravimetric analyzer (TGA). Hemicellulose shows the lowest thermal stability. Lignin decomposes slowly over a broad temperature range. Cellulose has the highest initial decomposition temperature and lowest char yield. The pyrolytic behavior of poplar wood could be considered as the sum of the pyrolytic behaviors of each individual chemical component. In comparison with poplar wood, PP shows a higher thermal stability, and decomposes over a relative narrow temperature range giving lower fixed carbon contents. The thermal decomposition of four WPCs could be basically divided into two stages ascribing to the thermal decomposition of biomass components and PP, respectively. The weight loss rates of biomass were decreased slightly in the presence of PP. The char formed from biomass components could act as a thermal stabilizer for PP, increasing its decomposition temperature. Expect for lignin-PP composites, the experimental char yields of WPCs are higher than that of theoretical values, indicating that some interactions between biomass components and PP occurred during thermogravimetric processes.2) The pyrolysis conditions of WPP were studied by using Py-GC/MS. The product composition, distribution and mechanisms of biomass components, PP and their composites pyrolyses were studied. The interactions between biomass components and PP were also investigated. The optimal pyrolysis conditions of WPP are pyrolyzed at 600℃,60 s with a heating rate of 20℃/ms. The pyrolysis of cellulose produces higher yield of aldehydes and ketones, while levoglucosan (LG) produces more yields of furans and other anhydrosugars. Pyrolysis of hemicellulose produces a lot of acids, especially acetic acid. Lignin generates a high yield of phenols, which mainly composed of guaiacyl phenols. In addition, there are small amounts of acids and aldehydes. Several kinds of industrial lignins such as enzymatic hydrolysis lignin, alkali lignin and sulfonated lignin can be successfully converted into phenols or other value-added chemicals by fast pyrolysis. Besides cellulose-PP composite, some new products such as cyclopentenones and 2-alkenes are identified in the pyrolyses of hemicellulose-PP and lignin-PP composites. Furthermore, the experimental contents of organic acids, aldehydes/acetones, furans and phenols are lower than theoretical values, while the yields of aliphatic hydrocarbons, especially those hydrocarbons with a carbon number of 3n, are significantly higher than theoretical values. These suggest that there are synergistic effects between biomass components and PP in the pyrolysis process. The redicals derived from biomass pyrolysis could interact with PP redicals, resulting in the reduction of oxygen-containing compounds and enhancement of 3n light hydrocarbons formation.3) The effects of WPP to HZSM-5 ratios, the Si/Al ratios of HZSM-5 and different zeolites on product composition and distribution of WPP pyrolysis were investigated using Py-GC/MS, and the effects of HZSM-5 on pyrolysis products of poplar wood and it’s main components, PP and their composites were also studied. The yields of aromatic compounds increase significantly at the expense of oxygenated compounds when WPP to HZSM-5 ratios decrease. The Si/Al ratios of HZSM-5 affect its abilities of deoxygenation and aromatization. In addition, zeolites with different structures have different selectivities for aromatics. The main pyrolysis products of PP are altered from alkenes to monoaromatics, and the carbon numbers of products are narrowed from C5-C31 to C4-C12. The catalytic pyrolyses of cellulose and LG over HZSM-5 zeolite suggest that linear aldehydes/ketones were more favorable to form aromatics through deoxygenation than furans and anhydrosugars. Acetic acid, furfural and guaiacol are typical products from poplar wood pyrolysis. Monoaromatics especially toluene and p-xylene are the main products from the catalytic pyrolysis of acetic acid in the presence of HZSM-5. The pyrolysis of furfural over HZSM-5 is more favorable to produce naphthalenes instead of benzenes, indicated that furans are the main source of PAHs. Guaiacol has high thermal stability, and only a small amount of aromatics is generated from pyrolysis of guaiacol with HZSM-5 zeolite. It can be concluded that aromatics from lignin pyrolysis are mainly derived from the side chains of phenylproane units, and the deoxygenation of some phenols produce only few aromatics. The experimental yields of oxygenated compounds from four composites pyrolyses over HZSM-5 are lower than theoretical values. In contrast, the experimental yields of aromatics and alkenes were higher than theoretical values. It is indicated that there are interactions between the pyrolysis vapors of biomass components and PP in the catalysis of HZSM-5. Biomass-derived carbonyl and furanic compounds can react with PP-derived olefins to form aromatics via HZSM-5-catalyzed Diels-Alder reactions.4) A series of modified HZSM-5 zeolites were prepared by wet impregnation method, such as phosphorus modification, alkali/alkaline (K, Mg and Ba) modification, rare earth (La and Ce) modification and transition metal (Zn, Mo and Ni) modification. Metal oxides (ZnO、 CaO, Fe2O3 and nano TiO2) were mixed with HZSM-5 zeolite at a ratio of 1. The properties of modified zeolites were determined by XRD, N2 adsorption and NH3-TPD techniques. The effects of these modified zeolites on product composition and distribution of WPP pyrolysis were studied using Py-GC/MS. XRD patterns show that these modified HZSM-5 zeolites arewell crystallized as the MFI structure. However, their relative crystallinities are decreased with the increasing amounts of modifying elements. No new diffraction peaks are identified in the XRD spectra of P, K and La modified zeolites. In other spectra of modified zeolites some new diffraction peaks corresponding to their oxide crystallites respectively are observed. The introduction of modifying elements block some micropores of HZSM-5 leading to the reduction of BET surface and pore volume. NH3-TPD results show that the amounts of strong acid sites decreased remarkably and even disappeared after modifying HZSM-5. The changes of weak acid sites are affected by different modifying elements. For example, the amounts of weak acid sites decreased when P or Zn modified HZSM-5 while increased in the K and La modification. Compared with parent HZSM-5 zeolite, P, K, La and Zn modified HZSM-5 zeolites show a shift in selectivity from aromatics (benzenes, naphthalenes and indenes, etc.) to low oxygenated and valuable products (actone, furan and alkylphenols, etc.) during the catalytic fast pyrolysis of WPP. Furthermore, the yields of light aliphatic hydrocarbons with a carbon number range from C5 to C15 are increased significantly.In brief, WPC pyrolysis has unique advantages compared with biomass pyrolysis alone. There are significant interactions between biomass’s main components and PP in thermal degradation, fast pyrolysis and HZSM-5 catalytic pyrolysis processes. The interaction intensity is related to the component of biomass. In-line catalytic pyrolyses of WPP pyrolysis vapors over modified HZSM-5 improve the product compositions and distributions, promoting the quality of bio-oil.