Co-upgrading of biomass and polyethylene -derived volatiles for organic liquid over Ru, Ti, Sn/HZSM-5 coupled with NTP technology

Highlights

• Co-upgrading of biomass and plastic vapors was conducted by zeolites coupled with NTP.

• The synergy of Ru, Ti species and NTP enhanced formation of active atomic radicals.

• Integration of mixed volatiles was strengthened to lower oxygen and increase yield.

• TiHZ5 gave the maximum MAHs selectivity (64.19 %) and minimum PAHs selectivity (7.95 %).

• Metal modified HZSM-5 induced by NTP had high coke-resistance and removed coke easily.

Abstract

Co-upgrading of biomass and polyethylene -derived volatiles was conducted over HZSM-5 and its metal-modified versions, and NTP was introduced to enhance the conversion. The effects of metal species on the active radicals, organic yields, properties and compositions were explored. The Ru and Ti species, especially the low-valence titanium-oxides, increased atomic radicals, relieved the bottleneck of hydrogen transfer, and induced more volatiles into the radicals scale. The synergy of acid sites, metal species and NTP contributed to the integration of volatiles and exhibited promising potential in lowering oxygen and increasing organic yield. Specifically, TiHZ5 yielded the organic liquid of 58.73 % with the HHV of 38.73 MJ/kg, and RuHZ5 produced 51.70 % of organic liquid with the HHV of 36.96 MJ/kg. The synergy between SnHZ5 and NTP was so weak that it sacrificed partial yield to promote HHV. The Ru, Ti, and Sn modification increased the aromatic selectivity from 23.38 %–48.9 %, 72.14 % and 35.66 %, respectively, which caused the decrease of effective hydrogen to carbon ratios. Particularly, TiHZ5 gave the maximum MAHs selectivity of 64.19 % and minimum PAHs selectivity of 7.95 %. Besides, Ru, Ti and Sn modification decreased coking rate from 16.97 % to 7.04 %, 4.62 % and 8.46 %, respectively, and removal of coke became easier. This study provided a new waste-energy refinery way to reduce the disposal of waste plastics and relieve the dependence of fossil fuels.

Introduction

Recently, for the reasons of energy and environment issues, combined conversion of biomass and plastics had become a research hotspot [1,2]. Most researches tried to directly mix biomass and plastics for pyrolysis conversion, and they were mainly focused on two aspects: (I) The synergy or interaction between biomass and plastic components, for examples, cellulose and low-density polyethylene had a significant synergy that increased valuable petrochemicals and decreased undesired coke [3], a synergistic aromatic formation was caused by the interaction between terrefied yellow poplar and high-density polyethylene [4], the synergy between biomass and polyethylene was consistent, regardless of pyrolysis temperatures [5]; (II) The catalytic effect of various catalysts, such as HZSM-5 and modified versions [6,7], sodium-based catalysts [8], and porous carbon catalysts [9]. Meanwhile, someone pointed out that no notable synergy was found in the co-pyrolysis process, while an obvious synergy was observed during the catalytic co-pyrolysis [10]. However, the in-situ catalysis method of directly mixing catalysts with raw materials had defects in catalyst recycling, temperature controlling and sufficient contact between feedstocks and catalysts. Hence, it had become an important trend to separate catalysts from feedstocks, and perform ex-situ catalysis [11,12]. Also, many researchers found that the type of feedstocks had a great impact on the synergy. For example, the pyrolysis intervals of cellulose and low-density polyethylene had a high degree of coincidence, so the synergy was significant [3]. Anyway, no report had been reported about separately conducting the pyrolysis of biomass and plastics, and then feeding the both pyrolysis products together for co-upgrading.

Generally, higher aromatics in the bio-oil was strongly beneficial in terms of its possible use as fuels, and metal modified HZSM-5 had obvious advantages [6,7,13]. So far, related researches found that the synergy between metal species and acid sites could provide an ideal environment for typical aromatization, enhancing the formation of aromatics [14,15]. Compared with catalytic upgrading of biomass pyrolysis volatiles, two aspects should be taken in consideration for co-upgrading of mixed products from biomass and plastic pyrolysis: (I) The aromatization of ethylene from plastic pyrolysis would compete with the aromatization of biomass pyrolysis products. Because ethylene had a lower bond-energy and it was an important intermediate product in the aromatization, its aromatization had no hydrogen transfer bottleneck; (II) the scale of ethylene-based products from plastic pyrolysis was smaller than that of many oxygen-containing organics from biomass pyrolysis. Based on the scale of ‘hydrocarbon pool’ mechanism proposed by Dahl et al. [16], small-molecule olefins were the intermediate products in the hydrocarbon pool, which might hinder the higher-scale compounds to transform. In other words, the integration was weak in the co-upgrading of mixed pyrolysis volatiles, and it mainly depended on the interaction between pyrolysis products under certain conditions [17,18].

To overcome above problems, NTP technology was introduced to activate reactants and form synergy with acid sites and metal species. NTP could directly input energy to the chemical bonds of reactants to activate and dissociate the molecules, and it was often used to enable or accelerate some reactions that were difficult to perform under normal conditions, such as CH₄ reforming and coupling, CO₂ reforming with hydrocarbons, and NO decomposition [19]; besides, the acid sites and metal species on the catalyst played a directional induction role, and the mixed pyrolysis products could be transformed to the active atoms (or molecular fragments) radicals under the synergy of the metal species, acid sites and NTP. However, the upgrading effects varied with the modified metals. So far, many metals had been applied to modify HZSM-5 and catalyze biomass-derived volatiles for aromatics [20]. After reviewed related reports, we found that ruthenium (Ru), titanium (Ti) and tin (Sn) modification showed the higher activity in other research fields, for examples, Ru/HZSM-5 was acted as an excellent selective and reusable catalyst for the reduction amination of furfural [21], HZSM-5 loaded a suitable TiO₂ exhibited excellent stability and conversion performance in paraffin cracking reforming [22], Sn/HZSM-5 was superior to other catalysts in glycerol into aromatics in term of aromatics yield and catalyst stability [23]. However, there was little report involved in the field of biomass or plastic pyrolysis conversion.

Therefore, to strongly improve the integration of biomass and plastic -derived products, selectively produce more aromatics and simultaneously suppress catalyst coking, HZSM-5 was modified with Ru, Ti and Sn, and NTP technology was introduced to form synergy. After characterized by SEM(EDS), XRD, NH₃-TPD, and textural properties, the HZSM-5 and its metal-modified versions were used to catalyze mixed volatiles from biomass and plastic pyrolysis. In this study, microwave pyrolysis induced by silicon carbide (SiC) was applied to overcome the inefficient heating of traditional heat and mass transfer, and realize the rapid and efficient heating [24]. OES was employed to investigate the formation characteristics of active radicals. Quantitative analysis was conducted to determine the products yields, and elemental composition and calorific value of organic liquid. Moreover, GCMS and ¹H NMR semi-quantitative analysis were carried out to detect the chemical compositions and functional groups. Besides, the spent catalysts were collected and characterized by nitrogen (N₂) adsorption and desorption, TGA and TEM to evaluate the stability.