Improving the Antioxidant Activity, Yield, and Hydrocarbon Content of Bio-Oil from the Pyrolysis of Açaí Seeds by Chemical Activation: Effect of Temperature and Molarity

Abstract

Biomass-derived products are a promising way to substitute the necessity for petroleum-derived products, since lignocellulosic material is widely available in our atmosphere and contributes to the reduction of greenhouse gases (GHGs), due to zero net emissions of CO₂. This study explores the impact of temperature and molarity on the pyrolysis of açaí seeds (Euterpe oleracea, Mart.) activated with KOH and subsequently on the yield of bio-oil, hydrocarbon content of bio-oil, antioxidant activity of bio-oil, and chemical composition of the aqueous phase. The experiments were carried out at 350, 400, and 450 °C and 1.0 atmosphere, with 2.0 M KOH, and at 450 °C and 1.0 atmosphere, with 0.5 M, 1.0 M, and 2.0 M KOH, at laboratory scale. The composition of bio-oils and the aqueous phase were determined by GC-MS, while the acid value, a physicochemical property of fundamental importance in biofuels, was determined by AOCS methods. The antioxidant activity of bio-oils was determined by the TEAC method. The solid phase (biochar) was characterized by X-ray diffraction (XRD). The diffractograms identified the presence of Kalicinite (KHCO₃) in biochar, and those higher temperatures favor the formation peaks of Kalicinite (KHCO₃). The pyrolysis of açaí seeds activated with KOH show bio-oil yields from 3.19 to 6.79 (wt.%), aqueous phase yields between 20.34 and 25.57 (wt.%), solid phase yields (coke) between 33.40 and 43.37 (wt.%), and gas yields from 31.85 to 34.45 (wt.%). The yield of bio-oil shows a smooth exponential increase with temperature. The acidity of bio-oil varied between 12.3 and 257.6 mg KOH/g, decreasing exponentially with temperature, while that of the aqueous phase varied between 17.9 and 118.9 mg KOH/g, showing an exponential decay behavior with temperature and demonstrating that higher temperatures favor not only the yield of bio-oil but also bio-oils with lower acidity. For the experiments with KOH activation, the GC-MS of bio-oil identified the presence of hydrocarbons (alkanes, alkenes, cycloalkanes, cycloalkenes, and aromatics) and oxygenates (carboxylic acids, phenols, ketones, and esters). The concentration of hydrocarbons varied between 10.19 and 25.71 (area.%), increasing with temperature, while that of oxygenates varied between 52.69 and 72.15 (area.%), decreasing with temperature. For the experiments with constant temperature, the concentrations of hydrocarbons in bio-oil increased exponentially with molarity, while those of oxygenates decreased exponentially, showing that higher molarities favor the formation of hydrocarbons in bio-oil. The antioxidant activity of bio-oils decreases with increasing temperature, as the content of phenolic compounds decreases, and it decreases with increasing KOH molarity, as higher molarities favor the formation of hydrocarbons. Finally, it can be concluded that chemical activation of açaí seeds with KOH favors not only the yield of bio-oil but also the content of hydrocarbons. The study of process variables is of utmost importance in order to clearly assess reaction mechanisms, economic viability, and design goals that could be derived from chemically activated biomass pyrolysis processes. The study of the antioxidant properties of pyrolysis oils provides insight into new products derived from biomass pyrolysis.

1. Introduction

Waste management and its final destination present considerable challenges for modern society due to population growth and increasing waste generation, causing not only social but also environmental damage, thus becoming a complex problem to be resolved [1,2]. A class of waste to be considered, due to its physicochemical characteristics and huge volume generated, is lignin–cellulosic biomass, particularly those associated with agro-industrial processes [3], such as açaí (Euterpe oleracea, Mart.) seeds.

The açaí plant (Euterpe oleracea, Mart.) is a species of palm that is indigenous to the Amazon region of Brazil [4]. It grows in large numbers in the floodplains of the Amazon estuary [5,6]. The fruits of the açaí plant have significant economic value for both the agroindustry and for extractive activities conducted by rural communities in the state of Pará in the Brazilian Amazon [7].

When the açaí pulp and skin are processed with warm water, a thick, purple juice is produced [3,6]. This process also generates a residue, which consists of açaí seeds. These seeds are a valuable biomass residue that contains lignin and cellulose and has the potential to be used for energy and fuel production in both solid and liquid forms [8,9,10,11,12]. During the 2016–2017 crop season, Brazil produced approximately 1200–1274 million tons of açaí fruits, with the state of Pará being the primary producer (94%). This high level of production results in a significant amount of solid waste [7,13].

Pyrolysis is a process of thermochemical conversion that can transform biomass into energy and fuel by subjecting it to high temperatures in an inert environment [8,11]. This process produces gaseous byproducts such as CH₄, CO₂, and CO, as well as liquid bio-oil and solid biochar [8,11]. The nature of the biomass, the type of pyrolysis process (analytical pyrolysis, flash pyrolysis, and vacuum pyrolysis), the type of reactor (drop-tube, fixed bed, and fluidized bed), the operating mode (batch, semi-continuous, continuous), and the process parameters (temperature, catalyst, catalyst-to-biomass ratio, gas flow rate, weight hour space velocity, etc.) all affect the yield and properties of the resulting products [14,15,16].

Although research has been conducted on the pyrolysis of residual açaí seeds [8,11,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31], most of these studies have focused on producing activated carbon/bio-adsorbents [8,17,18,19,20,21,22,23,24,25,26,27,28,29]. These studies have used residual açaí seeds in their natural state [17,18,19] or activated residual açaí seeds [8,20,21,22,23,24,25,26,27,28,29,30,31] and have involved chemical activation with NaOH [8,21,27], KOH [20,22,23,24,25,26,27,28,29,30,31], H₃PO₄ [24,26], or HNO₃ [20,26] or physical activation with CO₂ [28,29]. Chemical activation by alkali improves bio-oil yields and physical properties [11], and even though chemical activation by potassium hydroxide (KOH) has been fairly studied in the academic literature [20,22,23], some focused on chemical activation of the biochar produced by carbonization or pyrolysis [20,23] or studied only the activated carbon produced using a high concentration of KOH through a fairly different method [22]. In a recent previous study, we compared chemical activation by alkali and acid treatments and their effects on process parameters and chemical composition [31]. However, to date, no systematic study has examined simultaneously the effects of temperature and alkali activation of residual açaí seeds on the yield, chemical composition, acidity, and antioxidant activity of bio-oil, as well as the chemical composition and acidity of the aqueous phase. Investigating these variables, such as temperature and KOH concentration, is essential to understand the behavior and reaction mechanisms of the process and to design an effective process. Due to the complexity of the topic and the wide range of conditions under which the process (chemical activation followed by pyrolysis) can be carried out, technical data from different biomass pyrolysis feedstocks, reactors, and conditions are of fundamental importance in understanding and optimizing the process.

In addition, temperature is one of the key factors influencing the thermochemical decomposition of biomass compounds during the production of bio-oil by pyrolysis. The bio-oil obtained by the pyrolysis of açaí seeds contains hydrocarbons and an oxygenate fraction rich in phenols [31,32], which are of great interest due to their potential applications as sources of antioxidants, nutraceuticals, and preservatives in the food industry [33,34]. In the food industry, phenolic compounds are recognized for their natural antioxidant properties. Phenolic compounds can eliminate free radicals and inhibit oxidation, thus preventing or delaying oxidative processes in food products [35].

In recent years, there has been great interest in studying the antioxidant capacity/activity of biomass-pyrolysis-derived bio-oils from different materials including birch wood [33], coffee silverskin [34], oak and pine woods [35], Colombian spent coffee grounds [36], tobacco, tomato and coffee grounds [37], Japanese red pine [38], grape pomace [39], and ship wood [40]. However, until now, the influence of pyrolysis temperature on the antioxidant capacity of bio-oil has been investigated only in a few studies [34,37,39,40]. In addition, no study has simultaneously investigated the influence of temperature and chemical activation (molarity) on the antioxidant capacity of bio-oils from pyrolysis. In a recent previous work, we demonstrated the non-viability of energy products obtained through biomass pyrolysis in a technoeconomic assessment [41] showing the importance of conducting research in the area and the development of new products for non-energy applications such as the potential to design pharmaceutical and phytotherapy products, aggregating value to the pyrolysis production chain.

This study aims to investigate the impact of temperature and chemical activation with alkalis on the yield, hydrocarbon content, acidity, and antioxidant activity of bio-oil, as well as the chemical composition and acidity of the aqueous phase. To accomplish this goal, pyrolysis experiments were conducted with residual açaí seeds at 350, 400, and 450 °C and 1.0 atmosphere. The residual açaí seeds were subjected to chemical activation using aqueous solutions of 0.5 M, 1.0 M, and 2.0 M KOH. These experiments will provide the basis to examine the effects of temperature and alkali activation on the optimization of bio-oil yield and hydrocarbon content in order to gain insights into the potential applications of bio-oil from açaí seed pyrolysis.

2. Results

2.1. Process Analysis

2.1.1. Effect of Process Temperature

In order to understand how the process changes with increasing temperature, it is advisable to know the chemical composition of açaí seeds. According to the academic literature, dried açaí seeds are made up of 18.6% cellulose, 58.1% hemicellulose and 25.1% lignin [42,43]. This composition is important because it governs how decomposition of the material should occur with respect to process temperature. Hemicellulose and cellulose degrade early during pyrolysis (low temperature), whereas lignin tends to degrade later, and the organic bio-oil (primarily due to lignin decomposition) is produced in the later, higher temperature of pyrolysis. Pyrolysis experiments were conducted on activated açaí seeds at temperatures of 350, 400, and 450 °C and a pressure of 1.0 atm, after activation with a 2.0 M KOH solution. These experiments were carried out using a laboratory-scale borosilicate glass reactor. The results showed that the bio-oil yields ranged from 3.19% to 6.79% (wt.), while the aqueous phase yields ranged from 20.34% to 25.57% (wt.). The yields of biochar ranged from 33.40% to 43.37% (wt.), and the gas yields ranged from 31.85% to 34.45% (wt.). These findings are consistent with the bio-oil yields reported by Serrão et al. [30], who conducted a study on the pyrolysis of açaí seeds in a natural setting, and with the yields reported by Castro et al. [11], who investigated the pyrolysis of açaí seeds at pilot, bench, and laboratory scales. The biochar yields obtained in this study are also in agreement with similar data reported in the literature for the pyrolysis of açaí seeds, whether in their natural state [17,18,19] or chemically activated [8,20,21,22,23,24,25,26,27,28,29,30,31]. It is important to note that the observed increase in bio-oil yield with increasing temperature is attributed to the temperature range being within the range of maximum decomposition, where the original material is primarily decomposed into its initial products. These findings are summarized in

Table 1. Mass balance and yield of reaction products by pyrolysis of açaí seeds.

Process Parameters	2.0 M KOH
	350 °C	400 °C	450 °C
Mass of açaí seeds (g)	40.12	40.12	40.06
Cracking time (min)	62	67	72
Solid weight (coke) (g)	17.40	13.40	16.17
Liquid weight (bio-oil) (g)	1.28	2.64	2.72
Weight of H2O (g)	8.16	10.26	8.41
Weight of gas (g)	13.28	13.82	12.76
Bio-oil yield (wt.%)	3.19	6.58	6.79
Aqueous phase yield (wt.%)	20.34	25.57	20.99
Biochar yield (wt.%)	43.37	33.40	40.36
Gas yield (wt.%)	33.10	34.45	31.85
Acidity (mg KOH/g)	257.6	15.0	12.3

, which presents the yields of the different reaction products.

Figure 1 illustrates the yields of different reaction products, namely liquids, solids, H₂O, and gas, obtained from the pyrolysis of activated açaí seeds in a laboratory-scale borosilicate glass reactor. The pyrolysis was conducted at temperatures of 350, 400, and 450 °C, pressure of 1.0 atm, and the açaí seeds were activated using a 2.0 M KOH solution. The results demonstrate that the yield of bio-oil increases gradually as the temperature rises, and this trend was well-fitted with a first-order exponential decay model, yielding an r2 value of 1.00. These findings align with the observations made by Serrão et al. [30], who investigated the pyrolysis of açaí seeds in a natural environment at temperatures of 350, 400, and 450 °C and 1.0 atm on a pilot scale. Their study also reported an increase in bio-oil yield with temperature. Similar studies focusing on biomass pyrolysis have consistently reported an increase in bio-oil yield within the temperature range of 200 to 450 °C [44,45,46,47,48,49,50,51,52,53,54,55].

Since açaí seeds are mainly composed of cellulose and hemicellulose, these biopolymers are the first to be degraded, and we observed aqueous phase production as early as 150 °C. As the temperature increases, lignin is degraded, producing organic bio-oil. Thermogravimetry analyses of açaí seeds show that decomposition due to celluloses, hemicelluloses, and the initial part of lignin occurs between 180 and 370 °C [42]. Indeed, at lower temperatures, an initial small amount of bio-oil is formed along with a large quantity of aqueous phase. Hemicellulose is the easier to degrade, occurring at temperatures of 220–315 °C, with a maximum decomposition rate at 268 °C. Cellulose decomposes at higher temperatures between 315 and 400 °C with the maximum at 355 °C. Lignin exhibits a low rate of decomposition across a large temperature range, and even at 900 °C there is considerable presence of residue (over 45%) [56].

The data indicate that pyrolysis at 400 °C generates minimum char and maximum gas and water, suggesting that the pyrolysis mechanism is optimized by the reaction temperature of 400 °C when using chemical activation with KOH. The yield of biochar increases with a higher concentration of KOH, indicating a relationship between the mechanisms of gas and water formation and biochar formation. Previous research on açaí seed pyrolysis has shown that gas phase yields are associated with biochar formation [11], and alkaline salts serve as catalysts that affect gas and char reactions.

Effect of Temperature on the Composition of Hydrocarbons and Oxygenates in Bio-Oil

Figure 2 and Figure 3 provide visual representations of the impact of process temperature on the composition of bio-oil derived from the pyrolysis of activated açaí seeds (Euterpe oleracea, Mart). The açaí seeds were chemically activated using a 2.0 M KOH solution, and the pyrolysis experiments were conducted at temperatures of 350, 400, and 450 °C, under a pressure of 1.0 atm, and at laboratory scale. The chemical composition of the liquid fraction of bio-oil was categorized into two main groups: hydrocarbons and oxygenates. Hydrocarbons encompass acyclic saturated/unsaturated hydrocarbons (alkanes + alkenes) as well as heterocyclic hydrocarbons (cycloalkanes + cycloalkenes + aromatics). On the other hand, oxygenates consist of phenols, ketones, and esters. This categorization allows for an assessment of the suitability of the designed process in producing chemical compounds that can serve as effective fuels. It is worth noting that oxygenates, due to their acidic nature, often do not burn efficiently, leading to the accumulation of residues in engines [11]. Therefore, the distinction between hydrocarbons and oxygenates serves as a valuable means of evaluating and comparing different thermochemical conversion processes for their potential fuel applications.

Supplementary Tables S1–S3 present the chemical functions, CAS numbers, sum of peak areas, and retention times of all molecules identified by GC-MS in the bio-oil obtained from the pyrolysis of activated açaí seeds (Euterpe oleracea, Mart) with a 2.0 M KOH solution at laboratory scale, using temperatures of 350, 400, and 450 °C and 1.0 atmosphere. The concentration of acyclic saturated/unsaturated hydrocarbons and heterocyclic hydrocarbons increases with temperature, particularly for the concentration of acyclic saturated/unsaturated hydrocarbons, as depicted in Figure 2.

The concentrations of both acyclic saturated/unsaturated hydrocarbons and heterocyclic hydrocarbons in the bio-oil obtained from the pyrolysis of activated açaí seeds were observed to rise as the process temperature increased. These results were effectively modeled using a first-order exponential growth model, with an r² value of 1.00 for both categories. These findings align with the research conducted by de Sousa et al. [32], who investigated the influence of process temperature on the concentration of hydrocarbons and oxygenates in bio-oil derived from açaí seeds using a pilot-scale pyrolysis setup. Their study, which examined temperatures of 350, 400, and 450 °C under a pressure of 1.0 atm, demonstrated that the concentration of hydrocarbons increased while the concentration of oxygenates decreased with rising temperature. Additionally, it was observed that the concentrations of alkanes, alkenes, and aromatics in the bio-oil increased with temperature, suggesting that higher pyrolysis temperatures promote the formation of hydrocarbons [32].

Figure 3 illustrates the concentration of oxygenates in bio-oil. The concentrations of phenols, esters, and ketones decrease with increasing pyrolysis temperature, showing a smooth exponential decay behavior, as shown in Figure 3. The results are according to those reported by de Sousa et al. [32]. The concentration of oxygenates in bio-oil was correlated with a first-order exponential decay model, exhibiting in all the cases a root-mean-square error (r²) of 1.00. According to de Sousa et al. [32], the concentrations of cresols, phenols, and ketones decrease with increasing pyrolysis temperature because of deoxygenation reactions converting phenols, esters, and ketones into hydrocarbons, carbon monoxide, carbon dioxide, and water (decarboxylation and decarbonylation). The mechanism of deoxygenation is not well understood, but it seems that the reaction rate of deoxygenation is influenced positively by reaction temperature. Studies show that adipic acid (dicarboxylic acid) can be converted into cyclopentanone via deoxygenation reactions, and at higher temperatures a considerable amount of cyclo-pentene is formed [15,16].

Figure 4 depicts the acid value, which represents the acidity of the bio-oils obtained from the pyrolysis of activated açaí seeds. The results reveal a notable reduction in bio-oil acidity as the pyrolysis temperature increases, following a sharp exponential decay trend. The acid value of the bio-oil ranged from 257.6 to 12.3 mg KOH/g and exhibited a strong correlation with a first-order exponential decay model, demonstrating an r² value of 1.00. These findings are in line with the observations presented in Figure 4, suggesting that higher concentrations of oxygenates in the bio-oil contribute to elevated levels of acidity.

Effect of Temperature on Chemical Composition of Products

Table 2. Chemical composition of aqueous phase obtained by pyrolysis of açaí seeds (Euterpe oleracea, Mart), identified by GC-MS.

Chemical Composition Ci (Area.%)	2.0 M KOH
	350 °C	400 °C	450 °C
Alcohols	2.34	20.74	26.62
Carboxylic Acids	4.05	15.02	9.23
Ketones	52.81	44.38	19.69
Oxygenates	40.80	19.86	44.46
${\displaystyle \sum _i^n}Ci$	100.00	100.00	100.00
Acidity (mg KOH/g)	118.9	26.8	17.9

presents the chemical composition and acidity of the aqueous phase generated from the pyrolysis of açaí seeds, and these data are depicted graphically in Figure 5. The determination of the chemical components and acidity of the aqueous phase was carried out using GC-MS analysis. Supplementary Tables S4–S6 provide detailed information on the chemical functions, sum of peak areas, CAS numbers, and retention times of all identified molecules.

Similar studies reported in the literature have identified the major oxygenate compounds in the aqueous phase as alcohols, carboxylic acids, and ketones, with remaining oxygen compounds such as phenols, aldehydes, and amines grouped under the category of oxygenates. Zhang et al. [57], found carboxylic acids, ketones, phenols, and furans in the aqueous phase produced by sawdust pyrolysis, while Torri and Fabbri [58], identified carboxylic acids, aldehydes, phenols, furans, sugars, and N-compounds in the aqueous phase produced by corn stalk pyrolysis. Similarly, Zhou et al. [59], reported the presence of carboxylic acids, aldehydes, phenols, ketones, furans, furfurals, sugars, and amines in the aqueous phase produced by corn stover pyrolysis.

The contribution of alcohols to the acidity of the aqueous phase is generally considered small or negligible due to their weak acidic nature. However, the α-hydrogen atoms of ketones exhibit acidic properties, which contribute to the overall acidity of the aqueous phase. Additionally, carboxylic acids with low carbon chain lengths are known to have a significant impact on the acidity of the aqueous phase. As the concentration of ketones decreases, it is expected that the acidity of the aqueous phase will also diminish. The acidity of the aqueous phase can be correlated with a first-order exponential decay model, indicating a decrease in acidity with increasing pyrolysis temperature.

Effect of Temperature on the Antioxidant Activity of Bio-Oil

Figure 6 demonstrates the effect of pyrolysis temperatures on the total antioxidant capacity of bio-oil produced by the pyrolysis of açaí seeds (Euterpe oleracea, Mart) with a 2.0 M KOH solution, 1.0 atmosphere, at laboratory scale. The effect of temperature on the average antioxidant capacity of pyrolysis bio-oil is shown in

Table 3. Antioxidant capacity of bio-oils produced by pyrolysis of açaí seeds (Euterpe oleracea, Mart).

Dilution	TEAC (µM/L) 2.0 M KOH
	350 °C	400 °C	450 °C
1:20	2.35	2.33	2.32
1:40	2.31	2.29	1.74
1:80	2.30	2.24	1.33
1:160	2.33	1.36	1.05
1:320	2.06	1.01	0.71
1:640	1.25	0.36	0.33

. As previously described, pyrolysis is a complex process involving the thermal degradation of biomass, and reaction conditions, particularly temperature, play a critical role in determining the chemical composition of the bio-oil [31]. At different pyrolysis temperatures, various intermediate reactions occur, leading to the formation of different chemical compounds within the bio-oil but also the production of a gaseous and a solid phase with different compositions and characteristics [9,30,31,32]. The results presented in Table 3 indicate that the strong antioxidant activity of the bio-oil at 350 °C may be associated with its high concentration of oxygenated compounds at this temperature, particularly phenolic compounds, which account for 50.077 (area.%) of the total oxygenated compounds, as shown in Figure 4. Recently, Valdez et al. [31] demonstrated that higher pyrolysis temperatures (450 °C) of açai seeds activated with a 2.0 M KOH solution do not favor the formation of oxygenates in the bio-oil, leading to the formation of hydrocarbon-rich bio-oils. On the other hand, lower pyrolysis temperatures (350 °C) favor the formation of oxygenates, particularly phenolic compounds, in the açai seed bio-oil [31]. In this context, phenolic compounds are strongly associated with high antioxidant activity, as described in the literature [60,61,62]. The results are according to similar studies reported by del Pozo et al. [34], who investigated the effect of temperature on the antioxidant capacity of the pyrolysis bio-oil of coffee silverskin, and by del Pozo et al. [39], who investigated the effect of temperature on the antioxidant capacity of the pyrolysis bio-oil of grape pomace, showing that a higher pyrolysis temperature leads to bio-oils with lower antioxidant capacity.

In general, by thermochemical degradation of biomass, higher temperatures result in the degradation of valuable compounds within the bio-oil, including phenolic compounds, while lower temperatures favor not only the formation but also preserve phenols in bio-oil [31]. Furthermore, bio-oil is the reaction product most abundant in valuable chemical compounds, making it particularly significant for the food, cosmetic, and pharmaceutical industries. Among the essential components found in bio-oil are phenolic compounds. These compounds have gained substantial interest due to their potential applications as natural sources of antioxidants, nutraceuticals, and preservatives in the food industry [34,38].

Phenolic compounds possess strong antioxidant properties, which help in neutralizing harmful free radicals and reducing oxidative stress that can delay or inhibit the oxidation of DNA, proteins, and lipids [61]. As a result, they can extend the shelf life of food products by inhibiting the oxidation of fats and oils, preventing rancidity, and maintaining the overall quality and freshness of the food [60]. In addition, these compounds may confer health benefits beyond basic nutrition when incorporated into a diet. Nutraceuticals derived from phenolic compounds have been associated with various health-promoting effects, such as anti-inflammatory, anticancer, and cardioprotective properties [60,61,62]. Therefore, the temperature at which pyrolysis occurs is a critical parameter that influences both the quantity and quality of the produced bio-oil.

Understanding the impact of pyrolysis temperature is essential to maximize the desired compounds, such as phenolic compounds and/or hydrocarbons, and ensure that the bio-oil meets specific industrial and application requirements, whether in applications such as biofuels or chemical products with high added value in the food, cosmetics, and pharmaceutical industries.

2.1.2. Effect of KOH Solution Molarity

Table 4. Mass balance by pyrolysis of activated açaí seeds at 450 °C, 1.0 atm, and activated with 0.5 M, 1.0 M, and 2.0 M KOH.

Process Parameters	450 °C
	0.5 M	1.0 M	2.0 M
Mass of açaí seeds (g)	33.285	40.040	40.06
Cracking time (min)	72	72	72
Solid weight (coke) (g)	9.650	13.080	16.17
Bio-oil weight (g)	3.431	2.720	2.72
H2O weight (g)	12.290	11.99	8.41
Gas weight (g)	7.914	12.25	12.76
Bio-oil yield (wt.%)	10.31	6.79	6.79
H2O yield (wt.%)	36.92	29.94	20.99
Biochar yield (wt.%)	29.99	32.67	40.36
Gas yield (wt.%)	23.78	30.59	31.85
Acidity (mg KOH/g)	112.7	103.7	12.3

illustrates the process parameters, mass balances, yields of reaction products (bio-oil, biochar, H₂O, and gas), and acidity of bio-oils produced by the pyrolysis of activated açaí seeds at 450 °C, 1.0 atm, and activated with 0.5 M, 1.0 M, and 2.0 M KOH.

In laboratory-scale pyrolysis experiments, activated açaí seeds were subjected to different concentrations of KOH solution (0.5 M, 1.0 M, and 2.0 M) and pyrolyzed at 450 °C and 1.0 atm in a borosilicate glass reactor. The yields of bio-oil, aqueous phase, biochar, and gas were determined and found to range from 6.79% to 10.31%, 20.99% to 36.92%, 29.99% to 40.36%, and 23.78% to 31.85% (wt.), respectively. The bio-oil yields obtained were higher than those reported by Serrão et al. [30], but similar to those reported by Castro et al. [11]. The yields of biochar were also consistent with those reported in previous studies of açaí seed pyrolysis chemically activated with NaOH [8,21,27], KOH [20,22,23,24,25,26,27,28,29,30,31], H₃PO₄ [24,26], and HNO₃ [20,26]. Chemical activation of biomass with alkalis may enhance the bio-oil yield as reported by Wang et al. [63], as well as promote cracking of long-chain molecules into smaller gas molecules. This effect is attributed to the presence of alkaline and alkaline earth salts, which act as catalysts for the cracking process. The XRD analysis of the biochar indicated the presence of KOH and its salts, which may have contributed to the observed yields. Figure 7 depicts the data in chart form. The yield of biochar increases linearly with increasing solution molarity and a root-mean-square error (r²) of 0.99, while that of gas increases exponentially and a root-mean-square error (r²) of 1.00, showing that higher molarities favor gas production. The results show that bio-oil yields tend to increase and reach a plateau around 1.0 M; however, physicochemical analysis shows that acidity is heavily lowered when using 2.0 M, and higher amounts of water are obtained, indicating that the mechanism of basic pre-treatment on biomass pyrolysis further deoxygenates the bio-oil, trapping oxygenates in water and biochar phases.

Effect of Molarity in Chemical Composition of Bio-Oil

Figure 8 demonstrates the effect of different molarities of KOH solution on the concentration of hydrocarbons and oxygenates in the bio-oil produced by pyrolysis of açaí seeds (Euterpe oleracea, Mart) at 450 °C and 1.0 atmosphere, at laboratory scale.

Supplementary Tables S7 and S8 display the chemical functions, sum of peak areas, CAS numbers, and retention times of all the molecules identified in the aqueous phase by GC-MS during the pyrolysis of açaí seeds (Euterpe oleracea, Mart), activated with 0.5 M and 1.0 M KOH solutions, at 450 °C, 1.0 atmosphere, and at laboratory scale. The concentrations of hydrocarbons, including alkanes, alkenes, aromatics, and cyclic hydrocarbons, increase with molarity and follow a first-order exponential growth model, while those of oxygenates, including alcohols, amine, carboxylic acids, phenols, ketones, furans, and others, decrease with molarity and follow a first-order exponential decay model. The results are consistent with those of de Castro [64], who studied the pyrolysis of açaí seeds at 450 °C, 1.0 atmosphere, and activated with 0.5 M, 1.0 M, and 2.0 M NaOH, at pilot scale, respectively, and observed that higher NaOH solution molarities favored the formation of hydrocarbons. Both the hydrocarbon and oxygenate concentrations were found to have a high correlation, as evidenced by their root-mean-square error (r²) values of 0.999.

Figure 9 demonstrates the acid value (acidity) of bio-oils obtained from the pyrolysis of açaí seeds (Euterpe oleracea, Mart) that were activated with a 2.0 M KOH solution at 450 °C and 1.0 atmosphere, as well as activated with 0.5 M, 1.0 M, and 2.0 M KOH solutions at laboratory scale. The data indicate that the acidity of the bio-oil decreases as the molarity of the KOH solution increases, following a sharp exponential decay pattern. The acid value of the bio-oil ranged from 113.7 to 12.3 (mgKOH/g) and can be correlated with a first-order exponential decay model, with an r² value of 0.999. This finding aligns with the results presented in Figure 8, suggesting that higher concentrations of hydrocarbons in the bio-oil lead to lower acidity.

Effect of Molarity on Antioxidant Activity of the Bio-Oil

Table 5. Antioxidant capacity of bio-oils produced by pyrolysis of açaí seeds (Euterpe oleracea, Mart), with different molarities (0.5–2.0 M KOH solution), at 450 °C.

Dilution	TEAC (mmol/L) 450 °C
	0.5 M KOH	1.0 M KOH	2.0 M KOH
1:20	2.40	2.40	2.32
1:40	2.40	2.40	1.74
1:80	2.37	2.37	1.33
1:160	2.29	2.29	1.05
1:320	1.62	1.53	0.71
1:640	0.90	1.03	0.33

shows the antioxidant capacity of bio-oils produced by pyrolysis of açaí seeds (Euterpe oleracea, Mart), with different molarities (0.5–2.0 M KOH solution), at 450 °C, 1.0 atmosphere, and at laboratory scale, and these data are depicted graphically in Figure 10. Data clearly show that the bio-oil produced by pyrolysis of 0.5 and 1.0 M KOH solutions at 450 °C exhibited higher antioxidant activity compared to the bio-oils produced with 2.0 M of KOH solution. These findings indicate that the high antioxidant activity of the bio-oil from residual açai seeds found at lower molarities of KOH solution (0.5 and 1.0 M) is associated with an increased formation of oxygenated compounds under these conditions, particularly phenolic compounds, which accounts for 94.33 (area.%) of the oxygenated compounds at this molarity, as shown in Figure 8. In this context, our previous study also demonstrated that alkali activation of residual açai seeds during pyrolysis for bio-oil production altered the yield, content of hydrocarbons and oxygenates, and acidity, as well as the chemical composition of the bio-oil [31].

2.1.3. Characterization of Biochar by XRD and Reaction Mechanism

Figure 11 illustrates the XRD of biochar produced by the pyrolysis of açaí seeds at 350 (a), 400 (b), and 450 °C (c), 1.0 atmosphere, and activated with 2.0 M KOH, at laboratory scale, respectively. The diffractogram of biochar illustrated in Figure 11 shows the presence of 01 (one) peak (K2) of high intensity on the position 2θ: 30.0 (100%), and 02 (two) peaks of medium intensity, the first (K1) on the position 2θ: 24.2 (66.8%) and the second (K3) on the position 2θ: 40.6 (68.6%), being all the peaks associated with Kalicinite (KHCO₃), the dominant crystalline phase in biochar. The findings from Prakongkep et al. [65], who conducted a study on the chemical properties, elemental analysis, and nutrient content, as well as the morphological and crystalline characteristics of biochar obtained from the pyrolysis of durian shell at 350 °C and at laboratory scale, align with the results of this study. Prakongkep et al. [65], observed that the dominant crystalline phase identified in the biochar was Kalicinite (KHCO₃).

The diffractogram of biochar illustrated in Figure 11b shows the presence of 02 (two) peaks of high intensity, the first observed on the position 2θ: 24.1 (81.73%) and the second on the position 2θ: 30.0 (100%), and 01 (one) peak of medium intensity on the position 2θ: 31.3 (62.2%), being all the peaks associated with Kalicinite (KHCO₃), the dominant crystalline phase in biochar, as reported by Prakongkep et al. [65]. The diffractogram of biochar illustrated in Figure 11c shows the presence of 03 (three) peaks of high intensity, the first observed on the position 2θ: 30.2 (100.0%), the second on the position 2θ: 31.3 (79.9%), and the third on the position 2θ: 34.2 (92.1%), being all the peaks associated with Kalicinite (KHCO₃), the dominant crystalline phase in biochar, as reported by Prakongkep et al. [65].

Furthermore, Han Lee and co-workers [66], investigated the XRD patterns of biochar that had been chemically modified using K agents. Despite undergoing thermochemical transformations at much higher temperatures (600–900 °C), they observed similar behavior, which is illustrated by the diffractograms shown in Figure 11. Han Lee et al. [66], proposed that XRD patterns changing with increasing temperature is due to thermal decomposition of KHCO₃ into other oxides, such as K₂O and K₂CO₃, producing CO₂ and H₂O. In a study conducted by Díaz-Terán et al. [67], the chemical activation of lignocellulosic material using KOH was investigated. XRD analysis of the activated material showed the presence of K₂CO₃ at a position of 2θ: 31.0. It was observed that the intensity of the peak increased with temperature, indicating a temperature-dependent effect on the formation of K₂CO₃. Similar results can be observed in Figure 11. Díaz-Terán et al. [67] proposed the possibility of CO₂ formation by reaction of K₂CO₃ with biochar, close to 470 °C, as well as the possibility of the formation of K salts that could oxidize and form potassium oxides and even KOH again, such that it is expected that the presence of KHCO₃ (Kalicinite) and K₂CO₃ crystalline phases in biochar after pyrolysis of açaí seeds activated with 2.0 M KOH may show a maximum peak intensity over the diffractogram on the position 2θ.

The presence of alkaline and alkaline earth ions in biomass is known to act as a catalyst for the formation of smaller-chain compounds. Pyrolysis products from biomass can be broadly characterized as being formed from hemicellulose and cellulose decomposition and those obtained through lignin pyrolysis. Products derived from hemicellulose and cellulose can be classified into anhydro-sugars, furan-ring derivatives (furfural, hexamethyl furfural, furanone, and others), low-molecular-weight compounds (glycolaldehyde, formic, and acetic acids), non-condensable gases (CO, CO₂), water, and char [68]. In the work of Mahadevan et al. [68], it was found out that increasing the presence of Na and K ions in the biomass suppresses the formation of levoglucosan and glycolaldehyde while increasing concentrations of acetic acid, acetol, and acetaldehyde. Furthermore, higher alkali ion concentration reduced HMF formation and ketone formation while increasing CO₂ levels in the gas phase [68]. The increase in acetic acid concentration was not observed in this work, corroborated by the low acid value exhibited by KOH-treated biomass. Chen et al. [69], studied alkaline additives in biomass pyrolysis and were not able to identify the presence of acetic acid in pyrolysis products due to the acid–base neutralization that occurs with excess alkali. They identified increased phenol production with alkaline doping [69]. We also did not observe the presence of levoglucosan with increased molarity of KOH activating solution corroborating the work of Mahadevan et al. and others [68,69,70,71]. Patwardhan et al. [70], studied the influence of alkaline ions in model cellulose compounds and observed that lower-molecular-weight compounds such as glycolaldehyde, acetol, formic acid, and acetic acid are formed through cleavage of the bonds in the pyranose ring [70]. Yang et al. [71], studied the influence of K ions during pyrolysis and concluded that they promote the homolytic scission of several bonds in the pyranose ring in different positions, resulting in compounds with the corresponding number of carbon atoms. Levoglucosan is formed through the cleavage of glycosidic bonds in cellulose. However, the scission at the pyranose ring competes with the cleavage of glycosidic links [71].

Lignin is formed by a complex network of phenylpropanoid units formed through oxidative polymerization of coumaryl, coniferyl, and sinapyl alcohols. The chemical pathway of lignin degradation is not well understood, and since the degree of methoxylation differs between lignin precursors, various types of intermolecular linkages can exist. During pyrolysis, depolymerization of the cross-linked chains occurs at carbon–carbon and aryl–ether linkages. The presence of K and Na ions was shown to have a clear impact in lignin-derived products, increasing the yield of phenols in the work of Mahadevan et al. [68]. Indeed, the amount of bio-oil produced increased with chemical activation as well as the concentration of phenolic compounds.

Table 6. Chemical composition of bio-oil obtained through pyrolysis of açaí seeds at 450 °C.

Chemical Compound	Bio-Oil (Area.%)
Linear hydrocarbons	9.64
Undecane	1.12
Tridecane	2.48
Pentadecane	2.29
Dodecane, 5,8-diethyl	1.63
6-tridecene	2.12
Cyclic and aromatic hydrocarbons	11.89
Cyclohexene, 6-(2-butenylidene)-1,5,5-trimethyl-	1.85
Naphthalene	4.40
Naphthalene, 1-methyl	2.39
1H-Indene, 1-ethylidene	3.25
Carboxylic acids	8.53
Dodecanoic acid	4.31
Tetradecanoic acid	4.22
Esters	4.07
Undecanoic acid, 10-methyl-, methyl ester	1.10
Methyl tetradecanoate	2.97
Ketones	3.53
2-Pentanone, 4-hydroxy-4-methyl	1.88
2-Cyclopenten-1-one, 2,3-dimethyl	1.65
Phenolic compounds	55.70
Phenol	15.93
Methyl phenol	20.53
Dimethyl phenol	10.09
Methoxy phenol	4.58
Ethyl methoxy phenol	4.57
Furans	5.75
Benzofuran, 2-methyl	1.88
Furan, 2-(2 furanylmethyl)-5-methyl	2.09
Benzofuran, 4,7-dimethyl	1.78
Aldehydes	0.91
Cinnamaldehyde, β-methyl-	0.91
Total	100

displays the chemical composition of the bio-oil obtained through pyrolysis at 450 °C without any chemical activation, and

Table 7. Chemical composition of bio-oil and aqueous phase obtained through pyrolysis of chemically activated açaí seeds with KOH 2.0M solution at 450 °C.

Chemical Compound	Bio-Oil (Area.%)	Aqueous Phase (Area.%)
Alcohols	10.43	26.61
2,3,4,5,6-Pentamethyl benzyl alcohol	1.74	-
2-Furanmethanol	1.58	8.94
Benzenemethanol, α-ethyl-4-methoxy-	1.80	-
Cyclohexanol, 5-methyl-2-(1-methylethyl)-, (1α,2α,5β)-	3.80	-
2,4-Dimethyl-2-oxazoline-4-methanol	-	17.67
1-Hexadecanol, 2-methyl-	1.51
Linear Hydrocarbons	12.13	3.39
Decane	1.23	-
Undecane	1.59	-
Tridecane	2.60	-
Tetradecane	2.33	-
7-Tetradecene	2.86	3.39
Nonadecane	1.52	-
Cyclic and aromatic hydrocarbons	13.59	-
Bicyclo[4.2.0]octa-1,3,5-triene	3.13	-
Ethylbenzene	2.21	-
Toluene	1.95	-
1,2,4,4-Tetramethylcyclopentene	1.30	-
1,3-Cyclopentadiene, 5-(1-methylpropylidene)-	0.89	-
Cyclohexane	3.25	-
Cyclohexane, 1,2,4-tris(methylene)-	0.86	-
Nitrogenated compounds	4.38	13.05
4-(2,5-Dihydro-3-methoxyphenyl)butylamine	2.44	-
Tricyclo[3.1.0.0(2,4)]hex-3-ene-3-carbonitrile	1.94	-
N-Tert.-butyl-N-(2-propenyl)amine	-	6.05
2-Propen-1-amine, N,N-bis(1-methylethyl)-	-	3.62
Aziridine, 2-(1,1-dimethylethyl)-1-ethyl-3-methyl-, trans-	-	3.38
Carboxylic acids	0.97	9.23
Butanoic acid, 4-hydroxy-	0.97	-
Butanedioic acid, methylene-	-	2.49
Butanoic acid, 4-hydroxy-	-	6.74
Esters	1.29	3.32
Acetic acid, 7-hydroxy-1,3,4,5,6,7-hexahydro-2H-naphthalen-4a-ylmethyl ester	1.29	-
Carbamic acid, phenyl ester	-	3.32
Ketones	7.07	44.38
2-Cyclopenten-1-one, 2,3-dimethyl-	1.86	-
2-Cyclopenten-1-one, 2-methyl-	0.90	-
4-(3,7,7-Trimethyl-2-oxabicyclo[3.2.0]hept-3-en-1-yl)but-3-en-2-one	3.02	-
Spiro[2.3]hexan-5-one, 4,4-diethyl-	1.29	-
2-Pentanone, 4-amino-4-methyl-	-	32.54
2-Propanone, (1-methylethylidene)hydrazone	-	2.29
4-Piperidinone, 2,2,6,6-tetramethyl-	-	9.55
Phenolic compounds	42.98	-
Phenol	5.73	-
Methyl phenol	5.60	-
Dimethyl phenol	9.74	-
Trimethyl phenol	8.26	-
Ethyl methyl phenol	4.84	-
Dimethoxy phenol	2.90	-
Ethyl phenol	4.316	-
Ethyl methoxy phenol	2.94	-
Non identified fraction	5.81	-
Total	100	100

displays the chemical composition in area.% of the GC-MS chromatogram of both liquid phases (aqueous and organic bio-oil phases) formed by pyrolysis of 2.0 M KOH-activated açaí seeds at 450 °C.

As proposed by Mahadevan et al. [68], and other authors, no levoglucosan was identified in the bio-oil or aqueous phase obtained from the pyrolysis of KOH-activated pyrolysis. Remarkably, the number of carboxylic acids in the bio-oil was reduced from 8.53 to 0.97%, and according to what is proposed by those authors, carboxylic acids as acetic acids should increase in the liquid fraction [68,70,71]. Chen et al. [69], could not identify the presence of acetic acid and suggested that acid–base neutralization of the alkaline additive was the cause. There was an increase in the presence of ketone compounds from 3.53 to 7.07%, and Mahadevan et al. [68], observed that acetic acid concentration could be affected by ketonization reactions of carboxylic acids to ketones. Ketones can be further deoxygenated to hydrocarbons through ketonic decarboxylation, generating the hydrocarbons present in the bio-oil [72].

The concentration of phenolic compounds decreased from 55 to 43%, contrary to what was proposed by these authors, where phenolic compounds formation is favored by the presence of alkaline ions. The number of hydrocarbons was greater for the KOH-activated pyrolysis and suggests that some phenolic compounds further reacted and formed hydrocarbons. It is important to note that compound identification through GC-MS is a hard task and even more so when complex molecules or matrices are involved, as in the case of the organic bio-oil. As suggested by Mahadevan et al. [68], the KOH-activated bio-oil did not contain furans, where the non-activated contained 5.75% furan derivatives. Mahadevan et al. [68], commented about alkali metals suppressing the formation of HMF in the bio-oil. Considering their observations, the following reaction mechanism is proposed for KOH-activated lignin depolymerization, as shown in Figure 12.

3. Materials and Methods

3.1. Methodology

The methodology employed to obtain the results of this study is shown in Figure 13. Initially, it was necessary to pre-treat and characterize açaí seeds obtained from a local market. The seeds were dried, milled, and sieved before being subjected to chemical activation with diluted solutions of KOH with 3 different concentrations (0.5, 1.0, and 2.0 M). The activated biomass was filtered, washed, and dried before being milled again to reduce large chunks of biomass to desired particle size. The resulting activated biomass was subjected to pyrolysis experiments at different temperatures (350, 400, and 450 °C). Biochar produced was analyzed by XRD. Liquid fraction was characterized by AOCS acid value method, and chemical composition was analyzed by GC-MS and FT-IR.

3.2. Materials

Açaí seeds (Euterpe oleracea Mart.) that were discarded on the sidewalks and streets by a local açaí store in the Jurunas District of Belém, Pará, Brazil, were collected and stored in plastic bags. Figure 14 visually displays the açaí seeds (Euterpe oleracea Mart.) that were found scattered on the sidewalks and streets near the açaí store in the Jurunas District of Belém, Pará, Brazil.

3.2.1. Pre-Treatment of Açaí Seeds (Euterpe oleracea, Mart.)

The açaí (Euterpe oleracea, Mart.) seeds were subjected to drying in an oven controlled by an analog device (DeLeo, Porto Alegre, Brazil, Model: DL-SE) at a temperature of 105 °C for a duration of 24 h. Afterwards, the dried seeds were ground using a laboratory knife cutting mill (TRAPP, Jaraguá do Sul/SC, Brazil, Model: TRF 600). Then, the dried and ground açaí seeds were sieved using a set of sieves of 28, 35, 48, and 60 mesh in order to remove the excess fiber material, as well as to decrease the specific particle diameter. The drying, grinding, and sieving of açaí seeds are shown in Figure 15.

3.2.2. Chemical Activation of Açaí Seeds (Euterpe oleracea, Mart.)

The prepared açaí seeds underwent chemical activation using 0.5 M, 1.0 M, and 2.0 M KOH solutions. The procedure involved the following steps: Approximately 60 g of dried, ground, and sieved açaí seeds was gently mixed with 120 mL of 0.5 M, 1.0 M, or 2.0 M KOH solutions (1:2 mass/volume ratio) for 30 min in a 250 mL becker. The impregnation process was conducted at room temperature. The formed suspension was then transferred to a paper filter, washed with 120 mL of distilled water, and left to rest for 24 h, following a previously described method [31]. Subsequently, it was dried at 100 °C ± 5 °C for 24 h. Finally, the dried and impregnated seeds were ground using a porcelain pestle and mortar. Figure 16 depicts the chemical activation process of açaí seed fine powders using a 2.0 M KOH solution.

3.3. Experimental Apparatus and Procedures

3.3.1. Experimental Apparatus

A diagram in Figure 17 shows a simplified setup of a small borosilicate glass reactor, which was used in the laboratory experiments. The system consists of a cylindrical reactor made of borosilicate glass with a diameter of 3.3 cm, a height of 22.8 cm, and a volume of 195 mL. The reactor is placed inside a cylindrical oven with a ceramic heating system of 800 W, and the temperature is controlled by a digital temperature controller. The temperature inside the reactor is measured with a K-type thermocouple. A Liebig condenser is connected to the exit of the glass reactor using a Y-shaped connection, and the liquid products are collected in a 50 mL borosilicate glass flask. The non-condensable gases are sent to the flare system through an opening in the 90° curve, coupled between the Liebig condenser and the glass separating funnel. The experiments were conducted in atmospheric pressure. Figure 18 shows the laboratory-scale pyrolysis reactor used in the experiments.

3.3.2. Experimental Procedures

By the pyrolysis of chemically activated açaí seeds, approximately 40.0 g was weighed using a semi-analytical balance (Marte Científica e Instrumentação Industrial Ltd.a, São-Paulo, Brazil, Model: AD330). Then, the chemically activated açaí seeds were placed inside the glass reactor. After connecting the glass reactor to the Liebig condenser, using a Y-shaped connection, and the condenser to the separating funnel, the cooling system was turned on, and the water temperature was set to 10 °C. No inert gas is injected into the system. Due to the small size of the reactor and since most air is flushed out of the reactor, the degree of oxidation of the sample is quite small and can be neglected. Then, the desired heating rate (10 °C/min) and temperature (350, 400, or 450 °C) were set up. After the desired reactor temperature (set-point temperature) was reached, the reactor set-point temperature was maintained for 30 min. The temperature of the reactor was monitored at 10–15 min intervals. The weight of the liquid phase (consisting of bio-oil and aqueous phase) and biochar was measured, and the weight of gas was calculated by subtracting the total weight of the liquid and biochar from the total weight of the feedstock. The bio-oil was separated from the aqueous phase through decantation in the separation funnel. The bio-oil was then analyzed for its physical and chemical properties, including acid value.

3.4. Characterization of Products

3.4.1. Acidity of Liquid Fractions

The acid value of the bio-oils and aqueous phases was determined according to the official methods (AOCS Cd 3d-63), as detailed in previous studies [73,74,75,76].

3.4.2. Antioxidant Activity of Bio-Oils

The total antioxidant capacity was determined according to the Trolox^® equivalent antioxidant capacity (TEAC) method proposed by Miller et al. [77] and modified by Re et al. [78], being adapted for conditions of temperature, proportions of reagents, and reaction time. TEAC is a colorimetric technique based on the reaction between ABTS (Sigma A1888) and potassium persulfate (K₂S₂O₈), producing the ABTS• (cation radical), a green/blue chromophore, with maximum absorbance at 645, 734, and 815 nm. Addition of antioxidants to the cation radical reduces it again to ABTS. The reaction was measured spectrophotometrically by observing the changes in absorbance measured at 734 nm over a time interval of 05 (five) minutes, using a spectrophotometer (Spectrum, Shanghai, China, Model: SP-2000UV). Thus, the extent of discoloration, defined as an index of inhibition of the ABTS• (cation radical), was measured, being equal to the total antioxidant activity of the sample. The TEAC results were expressed in micromoles per liter (µmoles/L). Initially, the AC-ABTS standard curve was constructed by successive dilution of standard Trolox solution (2.5 mM), and each point was measured in triplicate to compute a mean absorbance value. Afterwards, 2970 μL of ABTS was read, corresponding to initial time (τ₀). Then, 30 μL of sample (bio-oil) was added. The reading of absorbance was carried out after 05 (five) minutes (τ₅).

3.4.3. Chemical Composition of Bio-Oils and Aqueous Phase

CG-MS was used to determine the chemical composition of bio-oils and aqueous phases, following the procedures described by Castro et al. [11]. A 1.0 µL sample was injected in split mode into a gas chromatographer (Agilent Technologies, Santa Clara, CA, USA, GC-7890B), equipped with a fused silica capillary column (SLBTM-5ms) coupled to a mass spectrometer (Agilent, Santa Clara, CA, USA, MS-5977A). The peak intensity, retention times, and identification of compounds were analyzed based on the NIST mass spectra library. Since no internal standard was used, the concentrations were expressed in area.%. The presence of chemical groups was confirmed by Fourier-transform infrared spectroscopy (FT-IR) (BRUKER, Ettlingen, Germany, Model: VERTEX 70v) at the Laboratory of Vibrational Spectroscopy and High Pressure (LEVAP-PPGF/UFPA), and the procedures applied to determine the qualitative FT-IR analysis are described elsewhere [79,80,81,82].

3.4.4. Characterization of Biochar by XRD

The crystalline and mineralogical analysis of chemically activated açaí seeds after pyrolysis was conducted using an X-ray diffraction (XRD) technique. The XRD measurements were carried out using a BRUKER D8 ADVANCE (Billerica, MA, USA) diffractometer with Bragg–Brentano geometry, coupled with a LynxEye detector. The analysis was performed at the Laboratory of X-ray Diffraction (PPGF/UFPA), and the equipment used had the following specifications: a generator with a maximum power of 3 kW, a tube voltage of 40 kV, a tube current of 40 mA, and a Cu X-ray tube with Kα1 wavelength of 1.540598 Å. The optical setup of the instrument included fixed divergence, scattering, and receiving slits, with a Kβ filter of Ni and a graphite monochromator. The Soller slit was set to 2.5°, and the divergent slit was 0.6 mm. The goniometer used was a vertical model, allowing a scanning range of 5–110° (2θ) with a scanning speed of 0.2°/min (2θ) and an accuracy of ±0.02°. The XRD data were recorded with an angular step of 0.02° and a scan rate of 0.2°/min at room temperature. The identification of crystalline phases in the chemically activated açaí seeds was performed following established procedures described in the literature [9,15,16,31,83,84].

3.5. Mass Balances by Pyrolysis of Açaí Seeds

An overall mass balance was performed in order to calculate the quantity of gas formed. Basically, the weights of feed, bio-oil, aqueous phase, and coke formed are recorded, and a global integral mass balance calculation yields the quantity of gas formed by difference [85,86]. A differential mass balance of the considered system is described by Equation (1):

$$\frac{dM}{dt}={\dot{M}}_i-{\dot{M}}_o$$

(1)

where ${\dot{M}}_i$ and ${\dot{M}}_o$ are the mass flow rates in and out of the control volume, respectively. Multiplication of Equation (1) by dt and integrating yields:

$${\int }_{t_i}^{t_f}\frac{dM}{dt}dt={\int }_{t_i}^{t_f}{\dot{M}}_{I}dt-{\int }_{t_i}^{t_f}{\dot{M}}_{O}dt$$

(2)

$$M_{t_f}-M_{t_i}={\int }_{t_i}^{t_f}{\dot{M}}_{I}dt-{\int }_{t_i}^{t_f}{\dot{M}}_{O}dt$$

(3)

where $M_{t_f}$ is the remaining material in the reactor after pyrolysis, i.e., the biochar formed, and $M_{t_i}$ corresponds to the feed weight loaded. Since there is no mass being added to the reactor, ${\dot{M}}_I=0$. The mass flowing out of the reactor, ${\dot{M}}_O$, is represented by the vapors formed during the pyrolysis process and condensed or not afterwards as bio-oil, aqueous phase, and non-condensable gases. Then, Equation (2) becomes:

$$M_{char}-M_{feed}=-\left[{\int }_{t_i}^{t_f}{\dot{M}}_{bio-oil}dt+{\int }_{t_i}^{t_f}{\dot{M}}_{aqueous phase}dt+{\int }_{t_i}^{t_f}{\dot{M}}_{gas}dt \right]$$

(4)

$$M_{feed}-M_{char}={\int }_{t_i}^{t_f}{\dot{M}}_{bio-oil}dt+{\int }_{t_i}^{t_f}{\dot{M}}_{aqueous phase}dt+{\int }_{t_i}^{t_f}{\dot{M}}_{gas}dt$$

(5)

Assuming constant behavior of the flow rates of bio-oil, aqueous phase, and gas phases, Equation (4) yields the final form of mass balance for the pyrolysis process.

$$M_{feed}-M_{char}-M_{bio-oil}-M_{aqueous phase}=M_{gas}$$

(6)

4. Conclusions

The X-ray diffraction (XRD) analysis of biochar derived from the pyrolysis of activated açaí seeds treated with a 2.0 M KOH solution at 350, 400, and 450 °C, 1.0 atm, and at laboratory scale indicates that Kalicinite (KHCO₃) is the dominant crystalline phase present. The yield of bio-oil increases smoothly as the pyrolysis temperature rises, and this increase can be correlated with a first-order exponential decay model. The concentration of hydrocarbons, particularly acyclic saturated/unsaturated hydrocarbons and heterocyclic hydrocarbons, in the bio-oil also increases with temperature, while the concentration of oxygenates (such as cresols, phenols, and ketones) decreases. This suggests that higher pyrolysis temperatures promote the formation of hydrocarbons while suppressing the formation of oxygenates. The significant reduction in oxygenate concentration leads to a sharp decrease in bio-oil acidity from 257.6 to 12.3 (mgKOH/g) as the temperature increases. The aqueous phase generated during the pyrolysis of activated açaí seeds contains carboxylic acids, ketones, alcohols, phenols, and other compounds. The acidity of the aqueous phase decreases sharply with temperature, mainly due to the decreased concentration of ketones. The yields of biochar increase linearly with higher molarities of the KOH solution, while the yields of gas and solid phases (biochar) increase exponentially and linearly, respectively, with increasing molarity. Higher molarities of the KOH solution favor the production of gas and biochar. At 450 °C and 1.0 atm, the concentration of hydrocarbons in the bio-oil increases exponentially with the molarity of the KOH solution, while the concentration of oxygenates decreases exponentially. Moreover, the acidity of the bio-oil decreases sharply with increasing molarity, exhibiting a sharp exponential decay behavior. These findings are consistent with similar studies reported in the literature [71,72,73]. In the work of Amaral et al. [41], a technoeconomic assessment of chemically NaOH activated açaí seeds pyrolysis was performed using data from a pilot plant conducting pyrolysis at 450 °C, and it was found that production cost was superior to product revenue, indicating the need for designing new products that could be derived from pyrolysis processes, such as the use of bio-oil for pharmaceutical and phytotherapy applications [41]. The study of the antioxidant properties of bio-oil and its relation to process variables is essential for this goal to be achieved.