Recent advances in the catalytic pyrolysis of microalgae

Accepted Manuscript Title: Recent Advances in the Catalytic Pyrolysis of Microalgae Authors: Jechan Lee, Eilhann E. Kwon, Young-Kwon Park PII: DOI: Reference:

S0920-5861(19)30111-7 https://doi.org/10.1016/j.cattod.2019.03.010 CATTOD 12032

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

8 December 2018 25 January 2019 5 March 2019

Please cite this article as: Lee J, Kwon EE, Park Y-Kwon, Recent Advances in the Catalytic Pyrolysis of Microalgae, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.03.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Recent Advances in the Catalytic Pyrolysis of Microalgae

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Department of Environmental and Safety Engineering, Ajou University, Suwon 16499, Republic of Korea b

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Jechan Lee a,1, Eilhann E. Kwon b,1, and Young-Kwon Park c,*

Department of Environment and Energy, Sejong University, Seoul 05006, Republic of Korea

School of Environmental Engineering, The University of Seoul, Seoul 02504, Republic of Korea

These authors are co-first authors because they contributed equally to this work. Corresponding author: Y.-K. Park ([email protected])

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Graphical_Abstract

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Highlights

Recent catalytic pyrolysis making bio-oil with a high content of aromatics is discussed.

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Zeolites are most widely used for microalgae pyrolysis of to produce high-quality bio-oil.

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Challenges and future research in catalytic pyrolysis of microalgae are suggested.

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

Abstract

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The catalytic pyrolysis of microalgae offers a strategic means of increasing the value of pyrogenic products. Microalgae are a promising pyrolysis feedstock. This review summarizes the state-of-the-art catalytic pyrolysis processes to form bio-oil with a high proportion of aromatic

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compounds. The focus is on the current knowledge of the mechanisms to form aromatic hydrocarbon species from the catalytic pyrolysis of microalgae. In addition, that the effects of the reaction conditions and catalyst selection on the yield and composition of microalgal bio-oil are reviewed. The information shows that the catalyst and feedstock properties are closely associated with the formation of desired aromatic compounds. In addition, this review defines the technical

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challenges that need to be overcome and suggests future research for the further development of

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catalytic pyrolysis technologies for the production of aromatics from microalgae.

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Keywords: biorefinery; biofuels; microalgae; pyrolysis; thermochemical process

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1. Introduction

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Fossil fuels are pivotal to the modern life, and technological, social, and economic

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development would have not been possible without fossil fuels. On the other hand, combusting fossil fuels inevitably leads to global warming because anthropogenic carbon dioxide (CO2)

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emissions are far exceeding the Earth’s capacity to assimilate carbon via the natural carbon cycle. As a strategic way to alleviate the global environmental issue, several attempts to harness

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biofuels as an energy feedstock have been made because of its carbon neutrality [1-3]. Political support, such as the renewable fuel standard (RFS), have also helped expand the use of biofuels [4].

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Biofuels converted from edible crops have been commercialized (first-generation biofuels) [5]. Despite their numerous environmental benefits, the use of edible crops as a biofuel feedstock raises controversial issues, such as ethical dilemmas, increased crop prices, and potential stress

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on producing food commodities [6-8]. These side effects have prompted a great deal of research on the production of biofuels from inedible biomass (lignocellulosic biomass, organic wastes, etc.) [9, 10]. On the other hand, inedible biomass-derived biofuels (second-generation biofuels) have not been implemented fully because of the highly functionalized structures along with their heterogeneous matrices and technical incompleteness [11]. As an alternative to the second-

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generation biofuels, aquatic organisms, such as microalgae, have attracted considerable attention

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as a promising feedstock for biofuels (third-generation biofuels) [12, 13]. Microalgal biomass

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have advantages over lignocellulosic biomass owing to its rapid growth rate and high

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photosynthetic efficiency [14]. They do not require arable land for cultivation [14] and can be grown in fresh water, saline water, and even wastewater [15-17]. Moreover, microalgae can be

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used to mitigate CO2 emissions because they have a fast CO2 fixation rate [18]. Hence,

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exploiting microalgae for the production of biofuels is environmentally benign and commercially

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viable [19].

Among the various methodologies established for transforming microalgae to biofuels (e.g.,

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pyrolysis, hydrothermal carbonization, and liquefaction) [20-23], catalytic pyrolysis is a promising technology to convert microalgal biomass to liquid fuels efficiently [24]. Pyrolysis has

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advantages over the other technologies because it produces bio-oil with high fuel-to-feed ratios [25] relatively quickly and simply. Aquatic biomass feedstock can be converted to a range of hydrocarbon

mixtures

composed

of

aromatic

compounds

formed

via

dehydration,

decarboxylation, decarbonylation, deoxygenation, and aromatization reactions, which are

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promoted by porous catalysts such as zeolites and metal oxides [26-30]. Thus, the catalytic pyrolysis of microalgal biomass provides a strategic means of enhancing the quality and stability of bio-oil.

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In this regard, the catalytic pyrolysis of microalgae has recently attracted considerable attention. Several review articles have outlined the thermochemical transformation of microalgae to biofuels [31-35]. On the other hand, these papers mostly reviewed hydrothermal processing of microalgae. To the best of the authors’ knowledge, there are few reports on the catalytic pyrolysis of microalgae. Therefore, a critical evaluation of the literature is needed given the vast

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information on this subject. Accordingly, this review discusses the recent achievements in

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catalytic pyrolysis of microalgae and highlights the current challenges and potential fields.

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2. Catalytic pyrolysis

Catalytic pyrolysis has been studied widely as a means to deoxygenate pyrolytic vapors and

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produce aromatic hydrocarbons, such as benzene, toluene, xylene (BTX), and phenol [36, 37].

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The demand for these chemicals by the petrochemical industries has increased. Zeolites are the

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most commonly used catalyst for the catalytic pyrolysis of biomass [38-41]. For zeolitecatalyzed biomass pyrolysis, anhydrous sugar is converted from cellulose via dehydration at the

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acid sites of the catalyst [42]. A combination of oligomerization, decarbonylation, and decarboxylation of the anhydrosugar produces olefins (C2 to C6), which undergo further

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aromatization yielding aromatic compounds (e.g., BTX). The pyrolysis of hemicellulose with non-crystalline characteristic leads to the production of xylose, which readily diffuses through zeolite pores. Xylose then forms low-molecular weight oxygenated species such as furans, acetol, formaldehyde, formic acid, and acetic acid. Furanic compounds can be transformed to mono- and

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polycyclic aromatic hydrocarbons via consecutive catalytic reactions (e.g., oligomerization and decarbonylation) and to olefins via a hydrocarbon pool mechanism [43-45]. Lignin is the most robust against pyrolysis because it has a complicated cross-linked three-dimensional structure of

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aromatic monomers (e.g., p-hydroxyphenyl, syringyl, and guaiacyl) [46-49]. C–O and C–C bond scission of these monomers generates intermediate species that are too large to diffuse into the catalyst pores. This results in coke formation on the catalyst surface due to re-polymerization. Coke formation on the surface is one of the most critical concerns of catalytic pyrolysis over zeolites [50]. Therefore, for zeolite-catalyzed pyrolysis, it is essential to understand the

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feedstock and the pore shape/size selectivity of zeolites.

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relationship between the chemical structure and characteristics of intermediates derived from the

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For the catalytic pyrolysis of biomass, continuous systems are more effective in obtaining a

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high yield of bio-oil than batch-type reactors [51]. Figure 1 presents a typical continuous reactor setup for the pyrolysis process. Catalytic pyrolysis can be divided into two types: ex situ and in

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situ catalytic pyrolysis [51, 52]. For ex situ catalytic pyrolysis, the feedstock is first converted to

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pyrolytic vapors, which then pass through the catalyst over and are transformed to bio-oil. For in

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situ catalytic pyrolysis, however, the feedstock and catalyst are mixed before pyrolysis. Ex situ operation may be preferred for the production of bio-oils under various operational temperatures

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and catalyst loadings, leading to a controlled product distribution in bio-oil [51]. The reaction conditions of catalytic pyrolysis have a strong effect on the properties of bio-oil. In particular,

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the fuel properties of bio-oil (e.g., heating value) are contingent on the oxygen content in bio-oil [53]. Xiao and co-workers reported that catalytic pyrolysis on HZSM-5 produces bio-oil with a higher pH (i.e., less acidic bio-oil) than non-catalytic pyrolysis [54]. Pyrolysis with ZSM-5 yielded an aromatic carbon yield of 31.1 % from cellulose [42]. The efficiency of the

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condensation system to condense pyrolytic vapor to bio-oil is critical for maximizing the yield of

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bio-oil [55].

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Figure 1. An example of continuous pyrolysis system setup. Reprinted from Rahman et al. [56],

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Copyright (2018), with permission from Elsevier.

3. Microalgae as a promising feedstock for biofuels

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To reduce vulnerability in the energy sector, a range of biofuels have been proposed to

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substitute for fossil fuels. For example, bioethanol and biodiesel from terrestrial biomass (e.g., energy crops and lignocellulosic biomass) have attract considerable attention, and are called 1st and 2nd generation biofuels [11]. On the other hand, the limited availability of land crops and plants and the land use associated with growing this feedstock make the biofuels produced from 1st and 2nd generation biomass unsustainable [57, 58]. Therefore, instead of terrestrial biomass,

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aquatic biomass, such as microalgae, has attracted considerable interest as a biofuel feedstock (i.e., 3rd generation biofuels). Microalgae (also known as microphytes) are microscopic unicellular organisms. They are

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generally found in marine and aquatic environments [59], existing individually or in groups/chains. Their sizes differ according to species, ranging from 30 to 400 µm [60]. Furthermore, microalgae differ from higher plants by having no leaves, stems, and roots. Microalgae can be a promising carbon-neutral biofuel feedstock because of its capability of performing photosynthesis to grow photo-autotrophically by consuming carbon dioxide and

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producing almost half of the oxygen in the atmosphere [61]. Microalgae have considerably more

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biodiversity than terrestrial plants and crops [59] (estimated number of species, approximately

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20,000 to 800,000 [62, 63]). In addition, microalgae are environmentally sustainable and

demand for transportation fuels [67].

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economically viable feedstock for biofuels [64-66]. Microalgal biofuels can meet the global

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Microalgae have a number of advantages as an energy feedstock [68, 69]. First, most

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microalgae are an inedible resource with a relatively simple cellular structure. Second,

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microalgae grow fast, leading to a rapid production rate [70]. In addition, they can be grown in salty and wastewater, and consume CO2 in the atmosphere as the carbon source for their growth

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[70]. Moreover, microalgae-derived biofuels are environmentally beneficial in terms of air

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pollution controls (APCs) because of the absence of sulfur [70].

4. Catalytic pyrolysis of microalgae The catalytic pyrolysis of microalgae has received less attention than that of conventional biomass. Over the last three years, however, the number of papers published on the catalytic

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pyrolysis of microalgae has increased rapidly, highlighting the substantial attention given to the production of value-added products from microalgae via pyrolysis. At the early stages of the catalytic pyrolysis of microalgae, zeolites were the most-widely used catalysts. More recently, a

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range of oxides (e.g., metal oxides) and a combination of zeolites and oxides have been used for pyrolysis to valorize pyrolytic products (e.g., bio-oil). This will be outlined in the section discussing the catalytic pyrolysis of different microalgal biomass over zeolite catalysts and other catalytic materials.

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4.1. Pyrolysis of microalgae with zeolites

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Ansah et al. conducted the catalytic pyrolysis of two samples of Chlamydomonas debaryana

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(C. debaryana) (i.e., raw and hydrothermally treated ones) over beta-zeolite (β-zeolite) [24].

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They reported that β-zeolite is active for the production of total hydrocarbons, including aliphatic and aromatic compounds, in the pyrolysis of both raw and hydrothermally treated C. debaryana

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(~55% for the raw sample and ~65% for the treated sample at 700 °C). The denitrogenation of

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bio-oil is critical to improving the quality of microalgal bio-oil [71, 72] and reducing coke

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formation on a zeolite catalyst [73]. The hydrothermal carbonization of C. debaryana at 200 °C for 6 h reduced its nitrogen (N) content from 9.5 to 5.2 wt.% [24]. The use of a β-zeolite catalyst

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for the pyrolysis of hydrothermally treated C. debaryana resulted in an 8.3% yield of nitrogenous species.

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Jafarian and Tavasoli synthesized a hybrid composite consisting of hexagonal mesoporous

silicate (HMS) and ZSM-5 zeolite (HMS-ZSM5) [74]. The composite was used as a catalyst support onto which Ni, Fe, or Ce was impregnated (10 % metal loading). The Fe catalyst had the highest total acidity (measured by ammonia (NH3) adsorption-desorption; 7.99 mmol NH3 g−1)

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among the catalysts tested, whereas the Ni catalyst had the lowest total acidity (3.49 mmol NH3 g−1). The use of a Fe catalyst enhanced deoxygenation in the pyrolysis process (i.e., less oxygenate formation) compared to the other two, possibly because deoxygenation occurs on the

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acid sites [75, 76]. Raw Arthrospira platensis (A. platensis) was pyrolyzed and 500 °C was found to be the optimal pyrolysis temperature. Acid-washed feedstock increased the bio-oil yield by impeding char formation. The pyrolysis of acid-washed A. platensis was conducted over the three catalysts (Ni/HMS-ZSM5, Fe/HMS-ZSM5, and Ce/HMS-ZSM5). Among them, Fe/HMSZSM5 was the most effective in producing hydrocarbons and hydrogen (3.8 mmol g−1) by

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accelerating denitrogenation and deoxygenation. This was attributed to the larger number of acid

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sites present on the Fe catalyst. Iron was dispersed over the support surface with no replacement

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of protons, allowing acidic hydrogen atoms to remain at the pyrolysis temperature (e.g., 500 °C).

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Isochrysis sp. was pyrolyzed over a commercial lithium low-silica X-type (Li-LSX) zeolite to produce aliphatic and aromatic hydrocarbons [56]. Li-LSX zeolite-catalyzed pyrolysis

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produced a five times higher yield of aromatic compounds than non-catalytic pyrolysis. The

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pyrolysis of Isochrysis sp. over the catalyst (feed/catalyst = 1 (w/w)) at 500 °C yielded 29 wt.%

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bio-oil containing 23.1% aliphatic hydrocarbons and 11.8% aromatic compounds by expediting the denitrogenation of the pyrolytic products. As the pyrolysis temperature was increased from

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400 to 500 °C, the bio-oil yield increased from 20 to 29 wt.%; however, the bio-oil yield decreased to 24 wt.% when the pyrolysis temperature was increased to 700 °C. This might have

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been due to the cracking of pyrolysis vapors at this temperature. For pyrolysis at 500 °C, an increase in catalyst loading from 0.75 to 4.5 g (amount of feedstock was the same) decreased the bio-oil yield from 30 to 23 wt.% but increased the gas yield from 34 to 43 wt.%. This was attributed to a longer contact time between the pyrolytic vapors and catalyst bed.

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Adamakis et al. examined the effects of the microalgal lipid content on the composition of bio-oil produced via catalytic pyrolysis with zeolite [77]. In this study, Chlorella vulgaris (C. vulgaris) with different lipid contents was converted to bio-oil via pyrolysis with ZSM-5. Lipid

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extraction from C. vulgaris (i.e., the residual sample after extraction is composed mainly of carbohydrates) prior to catalytic pyrolysis afforded a biocrude containing more aromatic compounds (~70% based on gas chromatograph/mass spectrometer (GC/MS) peak area) than the biocrude derived from non-extracted C. vulgaris (50% based on GC/MS peak area). The results suggest that microalgae with a lower lipid content can lead to the production of more aromatics

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through catalytic pyrolysis.

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Nannochloropsis sp. was co-pyrolyzed with scum originating from a wastewater treatment

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facility in the presence of HZSM-5 zeolite (prepared by calcining a commercial ZSM-5 at 500 ºC)

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using a microwave heater (750 W) [78]. The pyrolysis temperature, feed/catalyst ratio, and microalgae/scum ratio were the key parameters. A co-pyrolysis temperature of 550 ºC,

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feed/catalyst ratio of 0.5, and microalgae/scum ratio of 0.5 were found to be the optimal

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conditions to provide the highest bio-oil yield (25.2 wt.%) with the maximum proportion of

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aromatic species (73.2 area%). The authors also considered the effects of effective hydrogen index (EHI; an indicator reflecting a relative content of hydrogen in different biomass feedstock)

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of the feed mixture of microalgae and scum. Scum acted as a hydrogen donor, increasing the EHI of the feed mixture. At EHI value > 0.7, the co-pyrolysis of microalgae and scum worked

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synergistically to obtain bio-oil in high yield with a high proportion of aromatic compounds. Guo and co-workers reported other co-pyrolysis work for microalgae over HZSM-5 [79].

They mixed Nannochloropsis salina (N. salina) and polypropylene and used the mixture as the feedstock to produce aromatic hydrocarbons. Compared to the pyrolysis of the individual

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feedstock, co-pyrolysis of the feed mixture had a synergistic effect on forming aromatic compounds at 800 °C with a N. salina/polypropylene ratio of 1:1 (w/w). For example, under the comparable reaction conditions, co-pyrolysis yielded 45 wt.% bio-oil, whereas the pyrolysis of N.

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salina and polypropylene individually yielded 37.5 and 32.5 wt.% bio-oil, respectively. The reaction mechanism of aromatics production via co-pyrolysis process was also proposed. As shown in Figure 2, olefins (mainly produced from cracking of polypropylene) were major intermediates in the formation of aromatic compounds from the co-pyrolysis of N. salina and polypropylene. Aromatic compounds could be produced via three different pathways. First,

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dehydration and Diels-Alder reactions between furanic compounds (derived from carbohydrate

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parts of N. salina) and olefins (derived from polypropylene) occur. HZSM-5 reinforces the

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Diels-Alder reaction, which accelerates the production of aromatics by consuming olefins [80,

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81]. The second mechanistic route involves the entry of olefins from triglycerides (via thermal decomposition, decarbonylation, and cracking) and proteins (via deamination and cracking) in

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the microalgae as well as the olefins derived from polypropylene into the hydrocarbon pool over

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HZSM-5. The other route involves the oligomerization, cyclization, and aromatization of the

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alkenes derived from polypropylene. Table 1 lists catalytic pyrolysis of microalgae over zeolites.

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Figure 2. Plausible reaction pathways for the formation of aromatic hydrocarbons from the mixture of microalgae and polypropylene. Reprinted from Qi et al. [79], Copyright (2018), with permission from Elsevier.

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Table 1. Catalytic pyrolysis of microalgae over zeolite catalysts. Catalyst

Pyrolysis conditions

Catalyst performance

Reactor type

Reference

Chlorella vulgaris

HZSM-5 (Si/Al = 23)

Carbon yield of aromatics = 24% (total selectivity toward aromatics = 75%)

In situ

[82]

Nannochloropsis sp. + scum

HZSM-5 (Si/Al=30)

Yield of bio-oil = 25.2 wt.% (the maximum proportion of aromatic species = 73.2 area%)

In situ

[78]

Isochrysis sp.

Li-LSX-zeolite

In situ

[56]

Chlorella vulgaris

5 wt.% Ni/zeoliteY (Si/Al=30)

800 °C; catalyst/feed ratio = 20 800 °C (microwave heating); feed/catalyst ratio = 0.5; microalga/scum ratio = 0.5 500 °C; feed/catalyst ratio = 1 400 °C; feed/catalyst ratio = 0.5

Ex situ

[83]

Arthrospira plantensis

10% Fe/HMSZSM5

500 °C; acid washing pretreatment

In situ

[74]

In situ

[79]

Yield of bio-oil = 59 wt.%

In situ

[84]

Relative composition of monoaromatics = ~25 area%

In situ

[85]

Yield of aromatics = 25 wt.%

In situ

[86]

Carbon yield of aromatics-rich liquid = 19.4%

Ex situ

[87]

Yield of BTX = 10.1 wt.% of feed

In situ

[88]

Relative composition of hydrocarbons

Ex situ

[89]

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Feedstock

HZSM-5 (Si/Al = 25)

Nannochloropsis

HZSM-5 (Si/Al = 80)

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Nannochloropsis salina + polypropylene

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Arthrospira platensis

Chlorella vulgaris Chlorella pyrenoidosa Chlorella vulgaris

Tetraselmis suecica

Zeolite-β hydrogen (Si/Al = 360) HZSM-5 (Si/Al = 50) ZSM5 (Si/Al = 45) HZSM-5 (Si/Al = 30) CBV 720 (Si/Al =

800 °C; microalga/polypropylene ratio = 1 500 °C (microwave heating); catalyst ratio = 0.5 600 °C; feed/catalyst ratio = 0.02 500 °C; feed/catalyst = 0.11 650 °C; feed/catalyst = 0.05 550 °C; feed/catalyst = 0.2 400 °C; feed/catalyst

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Yield of bio-oil = 29 wt.% (23.1% aliphatics and 11.8% aromatics) Yield of bio-oil = 10% (Selectivity toward hydrocarbons = 36 area%) Yield of bio-oil = 30 wt.% (Selectivity toward aromatics and hydrocarbons = ~50 area%) Yield of bio-oil = 45 wt.% (yield of monoaromatic hydrocarbons = 0.1223 g/g of feed)

ratio = 5 600 °C; feed/catalyst ratio = 0.05 250 °C; feed/catalyst ratio = 0.2

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Chlorella vulgaris

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Desmodesmus communis

30) HZSM-5 (Si/Al = 45) 5 wt.% Cu/HZSM5 (Si/Al = 30)

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= 9.8 area% Relative composition of BTEX = 61.8 area%

Yield of aromatics = 21.2 wt.% of feed

Ex situ

[90]

In situ

[91]

4.2. Pyrolysis of microalgae with catalysts other than zeolites Sanna and co-workers produced bio-oil from Pavlova sp. microalgae via pyrolysis over different supported metal catalysts [92, 93]. Among the TiO2-supported metal oxide catalysts

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(oxide form of Ce, Ni, and Co) examined, Ni/TiO2 produced the highest yield of bio-oil (22.6 wt.%) from Pavlova sp. at 500 °C [92]. Bio-oils made by the pyrolysis of Pavlova sp. with these catalysts had gross calorific values, ranging from 35 to 37 MJ kg−1. The deoxygenation performance of the three TiO2-supported catalysts during pyrolysis was in the following order: Ni > Ce > Co, producing more aromatic and aliphatic compounds. The superior performance of

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the Ni/TiO2 catalyst was attributed to the strong interaction of Ni with TiO2. The use of ceria-

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based catalysts supported on alumina (Al2O3) to pyrolyze Pavlova sp. feedstock was also tested

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[93]. Compared to non-catalytic pyrolysis, the Mg-Ce/Al2O3 catalyst decreased the oxygen

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content of biocrude from 14.1 to 9.8 wt.%. In particular, nitrogen was removed partially as gaseous HCN and NH3 from the liquid pyrolytic product produced with the Mg-Ce/Al2O3

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HCN and NH3 [94].

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catalyst. Ceria was reported to play a role in cracking the protein parts of microalgae to generate

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Shahbazi and co-workers carried out the catalytic pyrolysis of raw or hydrothermally treated C. debaryana over activated carbon [24]. They reported that activated carbon is active for the

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aromatization of oxygenated species during the pyrolysis of both raw and hydrothermally treated C. debaryana; the monoaromatic hydrocarbon yield was >43% for both types of feedstock.

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Activated carbon allowed the formation of fewer polyaromatichydrocarbons (PAHs) (an indicator of coke formation during catalytic pyrolysis [95]) for the pyrolysis of C. debaryana than β-zeolite (e.g., ~17% PAHs with β-zeolite while ~6% PAHs with activated carbon for the pyrolysis of the raw sample at 500 °C). The use of activated carbon as a catalyst also resulted in

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a lower yield of N-containing species (e.g., nitriles) (negligible amounts of nitrogenous species were identified) from the pyrolysis of hydrothermally treated C. debaryana. Gautam and Vinu compared the performance of Nannochloropsis oculata (N. oculata)

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pyrolysis with and without a Co-Mo catalyst (Mo:Co = 6:1 (w/w)) supported on gamma-alumina (γ-Al2O3) [96]. The metals existed as oxides on the support (the catalyst was not reduced prior to the reaction). The catalyst was prepared using the typical incipient wetness impregnation of Co and Mo precursors on a commercial γ-Al2O3 support. Without a catalyst, the pyrolysis of N. oculata at 500 °C produced aliphatic hydrocarbons (C10 to C20), mono- and poly-aromatic

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hydrocarbons, and nitriles. On the other hand, the production of long-chain nitriles (>C15) and

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aliphatic alkenes was promoted using the Co–Mo/γ-Al2O3 catalyst at 500 °C with a feedstock to

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catalyst ratio of 1:3 (w/w). Among the products, the selectivity toward dimethylketene and 1-

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isocyanobutane was 35%. As shown in Figure 3, the catalyst promoted C–H addition, ammoniation, ketonization, decarboxylation, dehydration, and isomerization reactions during the

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catalytic pyrolysis of N. oculata.

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Figure 3. Proposed reaction pathways of pyrolysis of N. oculata in the presence of Co–Mo/γAl2O3 catalyst. Reprinted from Gautam and Vinu [96], Copyright (2018), with permission from

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Elsevier.

Simão et al. used HY-340 niobic acid for the catalytic pyrolysis of Arthrospira maxima (A. maxima) [97]. The catalytic performance of HY-340 was compared with that of HZSM-5 zeolite. Pyrolysis over HZSM-5 resulted in 5 times more aromatic compounds (mainly toluene,

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ethylbenzene, and o-xylene; 80%) than non-catalytic pyrolysis (15%) based on the GC/MS area. In contrast, HY-340 did not promote the formation of aromatic hydrocarbons compared to noncatalytic pyrolysis. The difference in the production of aromatics between HZSM-5 and HY-340

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was attributed to the lower Brønsted acidity of HY-340 than HZSM-5 [98]. Vieira and co-workers used hand-made hydrotalcite as a catalyst in the pyrolysis of Chlamydomonas reinhardtii (C. reinhardtii) [99]. Hydrotalcite is a basic material with high thermal stability and large surface area [100]. The hydrotalcite catalyst was effective in decreasing the nitrogen content in the C. reinhardtii-derived biocrude. For example, at 650 °C,

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nitrogenated species (pentanenitrile, methylisocyanide, butanetrile, and benzenepropanenitrile)

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produced via the pyrolysis without any catalyst was ~41 area% (based on GC/MS), while those

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with the catalyst was ~17.5 area%. This decrease was likely caused by the formation of hydrogen

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cyanide (HCN) or NH3. They also used solar thermal energy as a heat source for the pyrolysis of C. reinhardtii in the presence of the hydrotalcite [101]. Figure 4 presents a schematic view of the

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solar thermal pyrolysis equipment. The authors optimized the reaction conditions to obtain bio-

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oil with the highest yield (55 wt.%): 1.98 g feed loading, 9.9 min reaction time, and 22.9%

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catalyst loading. The use of hydrotalcite enhanced the denitrogenation of the nitrogenated compounds contained in the C. reinhardtii-derived bio-oil, thereby increasing the yield of the

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corresponding hydrocarbons.

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Figure 4. Experimental apparatus for solar thermal pyrolysis of microalgae. Reprinted from

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Andrade et al. [101], Copyright (2018), with permission from Elsevier.

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Magnetite (Fe3O4) was also used as a catalytic material for the pyrolysis of two different

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microalgae (Chlorella and Spirulina) heated by microwave [102]. Fe3O4 served as a microwave receptor and expedited the production of nitrogenous aliphatic species while impeding the

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production of nitrogenous aromatic species. Lauramide and indole were the major value-added

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nitrogenous compounds in the bio-oil produced via microwave pyrolysis with magnetite.

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Lauramide was produced in three steps during pyrolysis. First, deamination or decarboxylation of amino acid was initiated. Second, the intermediates produced in the first step polymerized to

CC

form lauric acid. Third, lauric acid was transformed to lauramide. Homolysis of the aliphatic chain contained in tryptophan led to the formation of indole.

A

Yu et al. also performed catalytic pyrolysis of commercially available microalgae (microalgal

species was not provided) using oil shale as a catalyst in a commercial pyrolyzer equipped with a GC/MS to examine how the pyrolysis temperature and the catalyst loading affect the product distribution of bio-oil and selectivity toward aromatic compounds [103]. The use of oil shale increased the yields of ketones, aromatics, and hydrocarbons with a decrease in the yields of

19

alcohols and acids. With a 3 % loading of oil shale, a 35 % yield of aromatics was achieved at 600 °C, which was found to be the optimal temperature for pyrolysis. As the oil shale loading was increased from 1 to 3 wt.%, the acid content in the pyrolytic product decreased from ~56 to

SC RI PT

~41% (based on GC/MS peak area), while the aromatic content increased from ~15 to ~35%. In the presence of oil shale, the content of oxygenated species was also decreased by 24%, which was attributed to aldol condensation and ketonization.

Amphoteric oxides, such as MgO and ZrO2, were used as catalysts to promote the ketonization reaction to form more long-chain ketones during the pyrolysis of Schizochytrium

U

limacinum (S. limacinum) [28]. The ketonization reaction between myristic acid and palmitic

N

acid present in S. limacinum led to the formation of long-chain ketones, including 14-

A

heptacosanone and 16-hentriacontanone. For example, the pyrolysis of S. limacinum at 400 ºC

M

with MgO (catalyst/feedstock = 1 w/w) produced 22.3% 14-heptacosanone and 29.9% 16hentriacontanone (based on the relative GC/MS area). With ZrO2, however, 4.7% 14-

D

heptacosanone and 6.9% 16-hentriacontanone were produced under comparable pyrolysis

TE

conditions.

EP

Biochar, a porous carbon-rich solid material produced by the slow pyrolysis of biomass and organic waste [104], has attracted considerable interest for its many applications, such as

CC

environmental remediation, soil amendment, and carbon capture and storage (CCS) [105-107]. Biochar has recently been applied to a range of catalytic reactions [108-111]. Chen and co-

A

workers used biochar (made from bamboo waste via pyrolysis at 600 °C) as a catalyst for the copyrolysis of microalga (A. platensis or Nannochloropsis sp.) and bamboo waste [112]. The use of a biochar catalyst in co-pyrolysis increased pyrolytic gas yield whilst maintaining a bio-oil yield of 35 to 37 wt.%. The biochar catalyst enhanced the cracking of oxygenates and long-chain fatty

20

acids, forming more aromatic compounds. The oxygen content of the biocrude was 7 to 9 wt.%. The oxygen decomposed over the biochar catalyst was released as H2O, CO, and CO2. This was attributed to surface –COOH, –OH, and O–C=O groups acting as catalytic sites for co-pyrolysis.

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This study showed the potential of biochar as an effective catalyst to decrease the oxygen content in bio-oil. On the other hand, the biochar catalyst lost its surface area after co-pyrolysis (from 19.2 to 0.1 m2 g−1) due mainly to poisoning of the pores by the oxygenated or nitrogenated

A

CC

EP

TE

D

M

A

N

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species. Table 2 lists the catalytic pyrolysis of microalgae over catalysts other than zeolites.

21

N U SC RI PT

Table 2. Catalytic pyrolysis of microalgae over catalysts other than zeolites. Microalgae

Catalyst

Pyrolysis conditions

Chlorella vulgaris

Activated carbon

800 °C

Activated carbon

Ni/TiO2

Nannochloropsis oculata

Co–Mo/γ-Al2O3 (Mo/Co = 6)

PT

ED

Pavlova sp.

CC E

Nannochloropsis sp. + bamboo waste

Bamboo wastederived biochar

Pavlova sp.

Mg–Ce/Al2O3

Chlamydomonas reinhardtii

Hydrotalcite

Microalgae

Oil shale

Tetraselmis and Isochrysis

Ni–Ce/ZrO2

Nannochloropsis sp.

Ni–Ce/Al2O3

A

500 °C (microwave heating) 500 °C (microwave heating)

A

Magnetite

M

(1) Chlorella (2) Spirulina (1) Chlorella (2) Spirulina

500 °C; feed/catalyst ratio = 1 500 °C; feed/catalyst ratio = 3 600 °C; feed/catalyst ratio = 2; microalga/bamboo waste ratio = 1 500 °C; feed/catalyst ratio = 1 650 °C; feed/catalyst ratio = 2 600 °C; 3% loading of oil shale 500 °C; feed/catalyst ratio = 0.3 500 °C; feed/catalyst ratio = 2.3

22

Catalyst performance

Yield of gas and liquid = 89.2 wt.% (syngas concentration = 159735 ppmv CO; 75213 ppmv H2) (1) Yield of bio-oil = 53.8 wt.% (2) Yield of bio-oil = 48.4 wt.% (1) Yield of bio-oil = 49.4 wt.% (2) Yield of bio-oil = 46.4 wt.% Yield of bio-oil = 22.6 wt.% (relative proportion of monoaromatics = 47.8 area%; relative proportion of aliphatics = 14.2 area%) Selectivity toward 1‑ isocyanobutane and dimethylketene = 35% Yield of bio-oil = 35 to 37 wt.% (oxygen content = 7 to 9 wt.%) Yield of bio-oil = 20 wt.% (oxygen content = 9.8 wt.%) Relative proportion of aromatic hydrocarbons = 18.5 area% Yield of aromatics = 35 area% Yield of bio-oil = 26 wt.% (oxygen content = 9 to 15%) Yield of bio-oil = 23.3 wt.% (oxygen content = 13.7 wt.%)

Reactor type

Reference

In situ

[113]

In situ

[102]

In situ

[102]

In situ

[92]

In situ

[96]

Ex situ

[112]

In situ

[93]

In situ

[99]

In situ

[103]

In situ

[114]

In situ

[115]

5. Outlook and challenges Catalytic pyrolysis technologies for the production of microalgal bio-oil have been conducted extensively. Despite the success in the conversion of microalgae to value-added products such as

SC RI PT

bio-oil with a high content of aromatic hydrocarbons via catalytic pyrolysis, there are many critical technical hurdles that remain to be overcome before the pyrolysis processes can be commercialized. The aim of this paper was to encourage further interest in this area by providing a critical review of state-of-the-art pyrolysis processes to convert microalgae to value-added chemicals (e.g., bio-oil).

U

The use of model compounds of microalgae would be useful for studying the catalytic

N

pyrolysis of microalgae because the identification and quantification of the pyrolytic products

A

from real feedstock are often complicated. A recent study used model compounds of microalgae

M

for non-catalytic pyrolysis [116]. This type of study can be suggested as a methodology that can potentially be applied to future research to better understand the reaction chemistry in the

D

catalytic pyrolysis of microalgae.

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N-containing compounds are expected to be present in microalgal bio-oil derived mainly

EP

from amino acids (i.e., protein) [65]. This is an important concern while considering microalgaederived bio-oil for biofuel applications because the presence of N-containing compounds will

CC

lead to adverse effects, such as the release of NOx when bio-oil is combusted. Therefore, lowering the content of N-containing species in microalgal bio-oil will be necessary to make it a

A

more environmentally friendly option. Overall, zeolites (mostly HZMS-5) are the most widely used catalyst for the catalytic

pyrolysis of microalgae. Non-zeolite catalysts, such as supported metal catalysts and carbon materials, have also been applied, but the selectivity towards aromatic compounds was generally

23

higher over zeolite catalysts than non-zeolite catalysts. On the other hand, more coke formed on the zeolite catalysts than the other catalysts (e.g., activated carbon) during the catalytic pyrolysis of microalgae. Therefore, the development of a new class of catalytic materials that combines the

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improved catalytic performance (i.e., high conversion of microalgae, high selectivity towards aromatic compounds in bio-oil, and production of bio-oil with low N content) with reduced coke formation would be a highly desirable research direction.

Coking is a severe problem for catalytic pyrolysis, particularly with zeolite catalysts. The rate of coke formation on zeolites is strongly associated with the pore shape and pore size, which is

U

available in proximity to its active sites, active site characteristics, and pyrolysis conditions [117].

N

Coke formation can be reduced by controlling the operation conditions carefully to limit the

A

formation of coke-indicator molecules, such as PAHs and olefins. The use of a zeolite catalyst

M

with low-density acid sites would help reduce coking. Coked zeolites can be regenerated by the oxidation of the coke in air or oxygen. On the other hand, water has detrimental effects on the

D

catalytic sites of zeolites at high temperatures. Therefore, new methods to regenerate zeolites

TE

under mild conditions will be needed.

EP

The yields of microalgal bio-oil are not high enough (< 50 wt.% in most cases) considering that the catalytic pyrolysis of plant biomass feedstock leads to bio-oil yields of up to 80 wt.% [55,

CC

118]. Thus, pyrolysis processes for the treatment of microalgae need to be developed to increase the selectivity towards bio-oil by optimizing the operational parameters and employing suitable

A

catalysts.

To improve the selectivity toward aromatic compounds, it is essential to understand the

reaction pathways for the conversion of intermediates to the desired products during the pyrolysis of microalgae. To this end, it is desirable to quantify the actual concentration of

24

aromatic hydrocarbons in microalgal bio-oil. In most cases, however, the composition of biocrude derived from microalgae was reported based on the GC/MS peak areas. Although the GC/MS peak area% may reflect the relative difference between a compositional matrix of bio-oil

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samples, it does not mean the actual concentrations of individual species contained in the bio-oil. Modification of the pyrolyzer setup would be helpful for improving the quality of bio-oil (i.e., high yield of aromatics). For example, microwave-assisted catalytic pyrolysis may be an effective approach to increase the proportion of aromatic species in bio-oil. In microwaveassisted pyrolysis, the selection of an appropriate catalyst that also serves as a microwave

U

receptor is important. The use of a high-pressure pyrolysis reactor to obtain a high-quality

N

biocrude from microalgae may be considered.

A

Thus far, a good understanding of mechanisms of the formation of aromatics has been

M

achieved. On the other hand, the development of separation methods has not been emphasized. Therefore, more studies on efficient methods of separating desirable products (e.g., aromatic

EP

TE

chemicals from microalgae.

D

hydrocarbons) from bio-oil will be needed to enable the commercial-scale production of such

Acknowledgements

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This work was supported by the National Research Foundation of Korea (NRF) funded by Korea

A

government (MSIT) (No. 2018R1A2B2001121).

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