The Influential Mechanism of Absorbers and Active Metal on Microwave-Assisted Pyrolysis of Sargassum

Abstract

Composite catalysts combining absorbers and active metal hold significant potential for improving the efficiency of biomass microwave-assisted pyrolysis (MAP). Compatibility optimization of composite catalysts can be facilitated through comparative analysis for the influential mechanisms of absorbers and catalysts. Therefore, decoupling experiments about the MAP of Sargassum and calculations based on density functional theory (DFT) were conducted in this research, to investigate the influential mechanisms of absorbers and active metal. The results show the introduction of both the absorbers (SiC) and active metal (MgO) increase the yields of high-value components, such as hydrogen and hydrocarbons. However, their influential mechanisms are different. The introduction of SiC enhances the heating rate within the reaction zone, shortening the duration of MAP and inhibiting the condensation of bio-oil and the interaction between bio-oil and bio-char, and thereby increasing the bio-oil yield by 4%. The introduction of MgO lowers the energy barriers for macromolecular decomposition and gas generation, promoting the decomposition of bio-char and bio-oil, and thus leading to a 12% increase in the yield of bio-gas. This research conclusion provides a theoretical basis for the optimization and design of composite catalysts.

1. Introduction

The increasing global demand for energy, combined with growing environmental challenges, has necessitated the development of sustainable and eco-friendly energy sources. Biomass energy, as a renewable resource with significant potential and promising prospects, has garnered considerable attention [1]. Algae, classified as third-generation biomass energy, have emerged as a primary focus in energy research due to their wide availability, rapid growth cycles, and high yields of bio-oil and gas [2,3]. Thermochemical conversion technologies for algae facilitate efficient energy conversion and the production of high-quality bio-oil. With advancements in pyrolysis techniques, the production of high-calorific-value fuels from algae has become achievable. Studies have demonstrated that algal-derived fuels can achieve calorific values comparable to petroleum (up to 42 MJ/kg) [4], positioning algal thermochemical conversion as one of the most promising technologies in the field [5].

To fully exploit the energy potential of algae, microwave-assisted pyrolysis (MAP) technology has been proposed to enhance bio-oil and gas yields and improve energy efficiency [6,7,8]. Microwave heating helps to achieve uniform heating and efficient pyrolysis. This method offers advantages such as homogeneous heating, high energy utilization, and precise controllability [9], making MAP of algae an increasingly prominent area of research in energy science.

Despite its potential, significant challenges remain in algal MAP, including the low microwave absorption efficiency and high heteroatom content in bio-oil [10]. To address these issues, catalytic pyrolysis has been introduced. Iturbides [11] investigated the microwave pyrolysis characteristics of Sargassum, and the results indicated that the addition of bio-char/silicon carbide significantly enhanced bio-gas production while increasing the volume fractions of H₂ and CO. Xian [7] demonstrated that activated carbon significantly increases the yield of aromatic hydrocarbons and hydrogen in Ascophyllum MAP volatiles. Francisco [12] investigated the effect of clinoptilolite on the pyrolysis of Sargassum, revealing that the addition of clinoptilolite significantly enhanced the pyrolysis conversion rate and the generation of aliphatic hydrocarbons. Chen [13] compared the catalytic performance of two composite additives (activated carbon with Na₂CO₃ and activated carbon with CaCO₃) in MAP, finding that composite additives reduce specific energy consumption, with the activated carbon–Na₂CO₃ mixture exhibiting superior catalytic activity.

In summary, catalysts enhance bio-oil/gas quality, while microwave absorbers improve energy transfer efficiency. The synergistic use of absorbers and catalysts offers a viable strategy to overcome bottlenecks in algal MAP. Dong [14] demonstrated that the composite catalyst composed of bio-char and steel slag significantly enhanced the component selectivity of Sargassum bio-oil, decreasing the content of nitrogen-containing compounds. Chen [15] investigated the effect of a composite catalyst composed of graphite and Fe₂O₃ on the MAP of Dunaliella salina, revealing the catalyst significantly enhanced the pyrolysis rate but effectively reduced the activation energy of the pyrolysis reaction. Nevertheless, existing studies predominantly focus on additive catalytic performance, with limited mechanistic insights into catalytic pathways or interactions between composite additive components.

Active metals enhance the quality of bio-oil and bio-gas, while absorbers improve the utilization efficiency of microwaves. The combined use of absorbers and active metals offers a more effective solution to the bottleneck issues in algae MAP. The ratio of active metals to absorbers in a composite catalyst must be optimized based on the desired target products. However, the regulatory mechanisms of active metals and absorbers on the bio-product components in microwave pyrolysis are not yet fully understood. Further research is needed to clarify these mechanisms and their differences, enabling the optimized design of composite catalysts.

The mechanisms and differences in the effects of absorbers (SiC) and active metals (MgO) on algal MAP were investigated in this research. SiC is a typical microwave absorber, commonly used to enhance the microwave pyrolysis of materials with low microwave transparency. Moreover, SiC has relatively weak reactivity, making it suitable as a carrier material for composite catalysts. In contrast, MgO is a typical alkaline metal compound with high catalytic activity. Consequently, SiC and MgO serve as representative materials in studies aimed at investigating the effects of microwave absorbers and active metals on biomass pyrolysis. Furthermore, Sargassum was chosen for this study due to its environmental impact and potential for sustainable biomass utilization. As a highly productive marine algae, it accumulates along coastlines, causing ecological disruptions. Converting Sargassum into valuable fuels via microwave-assisted pyrolysis (MAP) addresses both environmental challenges and the growing demand for renewable energy. The increasing focus on Sargassum’s conversion aligns with global efforts to explore alternative energy sources and reduce marine waste [11,12]. Decoupling experiments and theoretical calculations were used to analyze the release patterns of bio-oil and -gas. The findings are expected to advance the development of high-value composite catalysts for algal MAP.

2. Materials and Methods

2.1. Experimental Materials

The study utilized Sargassum sourced from Shandong Province, China. Sargassum was placed in a drying oven and dried under hot air at 105 °C for 24 h to remove the moisture, eliminating the potential impact of environmental humidity on the experimental results. After drying and grinding, particles with a size range of 0.105–0.5 mm were sieved for experiments. Prior to testing, the biomass was dried for 24 h to eliminate moisture interference from ambient humidity. Proximate analysis was conducted following the Chinese National Standard GB/T 28731-2012 [7], while elemental composition was determined using an elemental analyzer (Flash 2000, Thermo Fisher Scientific, Waltham, MA, USA). The results are summarized in

Table 1. Proximate and ultimate analysis of Sargassum [16].

Sample	Proximate Analysis (wt.%)			Ultimate Analysis (wt. %)
	FCd	Vd	Ad	Cd	Hd	Od	Nd	Sd
Sargassum	6.32	60.01	33.67	31.47	4.21	28.82	0.82	1.01

Note: FC represents fixed carbon, V represents volatile matter, and A represents ash content. The subscript “d” denotes the dry basis.

. Silicon carbide (SiC, purity > 99%) was procured from Jinan Zhongye Advanced Materials Co., Ltd., Jinan, China. SiC particles larger than 1 mm served as bed material, while powdered SiC (50 μm) was blended as a microwave absorber. Magnesium oxide (MgO, purity > 98%) was supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

2.2. Experimental Setup

The experiments were conducted on a custom-built microwave pyrolysis platform, as shown in Figure 1 [16]. A vertical quartz reactor tube was housed within a microwave cavity (Model CY-CR1000C-M, Changyi Instrument Co., Ltd., Changsha, Hunan Province, China) to heat Sargassum to the target temperature (600 °C), at the operating frequency of 2.45 GHz. An inert atmosphere was maintained using high-purity nitrogen (99.999%), with gas flow regulated by a rotameter. The biomass temperature was monitored using a K-type thermocouple positioned at the center of the biomass layer, while the reactor pressure was measured via a pressure sensor. Temperature and pressure data were recorded and analyzed using a central control system. To prevent bio-oil condensation, the transfer line connecting the reactor outlet to the condenser was heated to 300 °C. Bio-oil was rapidly quenched in a condenser cooled to −15 °C by a circulating coolant system. Non-condensable gases were collected at the exhaust port for quantification.

2.3. Experimental Procedure

Pyrolysis experiments were conducted under a reactor gauge pressure of 100 Pa with a nitrogen flow rate of 0.2 L/min. Coarse SiC particles were placed at the reactor bottom as fixed bed material to enhance biomass heating rates and preheat the nitrogen stream. The biomass layer, containing 36 g of Sargassum, was positioned above the SiC bed. Three configurations were used to reveal the influential mechanisms and differences of SiC/MgO: (1) individual MAP of Sargassum; (2) MAP with 10% (mass ratio) SiC powder; (3) MAP with 10% (mass ratio) MgO powder.

Prior to pyrolysis, the reactor was purged with nitrogen (0.2 L/min) for 20 min to ensure inert conditions. Microwave heating was then initiated with a two-stage power protocol: a constant 1250 W during the heating phase, followed by modulated power (0–600 W) to stabilize temperature. The pyrolysis phase commenced at 200 °C and lasted 40 min. The temperature of the Sargassum layer rose to the set temperature in the second minute after the pyrolysis started, and then fluctuated slightly at the set temperature. The heating-up curve was similar to the previous study [16]. Post-reaction, the microwave generator was deactivated, and residual solids were cooled to ambient temperature (20–25 °C) under nitrogen flow. All experimental conditions were conducted in triplicate, and the error bars shown in the relevant figures represent the standard deviation of the measured values.

2.4. Analytical Methods

The major gaseous products (H₂, CO, CH₄, and CO₂) were quantified using a gas chromatograph (GC N6, FULI Inc., Wenling, Zhejiang Province, China) equipped with a thermal conductivity detector (TCD) [17]. The argon carrier gas (99.999%) was set at a flow rate of 1.0 mL/min. Organic components in bio-oil were identified using gas chromatography–mass spectrometry (GC-MS Crystal 9000, FULI Inc., China) with a BP-5ms capillary column. Helium (99.999%) was used as the carrier gas with a flow rate of 1.0 mL/min. The oven temperature program consisted of the following: 35 °C (held for 5 min) → ramp to 150 °C at 1 °C/min (held for 10 min) → ramp to 200 °C at 3 °C/min (held for 5 min) → final ramp to 250 °C at 3 °C/min (held for 5 min).

2.5. Computational Methods

Bio-oil compound concentrations were calculated using GC-MS peak area ratios. Gas yields correspond to the cumulative outputs of H₂, CO, CH₄, and CO₂.

Geometries of reactants, transition states, intermediates, and products were optimized with the M06-2X functional [18] and the def2-SVP basis set [19] in the ORCA program [20]. Frequency analyses confirmed stationary points, and intrinsic reaction coordinate (IRC) calculations verified transition state connectivity. Gibbs free energies were computed using the Shermo module [21]. Reaction barriers were defined as the Gibbs energy differences between transition states and reactants.

3. Results and Discussion

3.1. Microwave Pyrolysis Characteristics of Sargassum

During individual MAP of sargassum, bio-oil and bio-gas yields reached 22% and 27%, respectively (Figure 2a). The gas evolution profiles (Figure 2b) showed that H₂, CO, and CH₄ yields initially rose and then declined, whereas CO₂ decreased monotonically [16]. Over 84% of the total gas was released within the first 10 min, with 95% emitted by 20 min. The organics in bio-oil comprised oxygenates (61%), nitrogen-containing compounds (30%), hydrocarbons (8%), and sulfur-containing compounds (1%) (Figure 2c). The dominant oxygenates included alcohols, phenols, furans, ketones, esters, and acids.

The bio-gas yield slightly exceeds that of bio-oil during microwave-assisted pyrolysis. Microwaves help to improve the heating mode of biomass particles, facilitating the formation of small-molecule gases [22]. Additionally, higher pyrolysis temperatures enhance secondary decomposition reactions of bio-oil, further increasing bio-gas production. The release patterns of H₂, CO, and CH₄ in gaseous products differ from those of CO₂, primarily due to distinct formation pathways. At low temperatures, bio-gas evolution is predominantly associated with thermally unstable aliphatic compounds.

Table 2. Possible path for bio-gas generation at a low temperature (473 K).

Reaction	Decarboxylation (R1)	Decarbonylation (R2)	Methyl Ether Cleavage (R3)	Carbon-Chain Scission for H Production (R4)	Alcohol Scission for H production (R5)	O-H bond Cleavage for H Generation (R6)
Energy barriers (kJ/mol)	296.1	364.4	393.6	468.6	359.1	389.7

illustrates the thermal decomposition pathways and corresponding energy barriers for carboxyl, aldehyde, methoxy, and hydroxyl groups, which are functional groups characteristic of algal biomass. The energy barrier of the decarboxylation reaction is the lowest (287.4 kJ/mol at 473 K), and thus the CO₂ production peaks at low temperatures and declines as the carboxyl group content diminishes. Conversely, the low yield of CH₄ is mainly due to the relatively high energy barrier for the decomposition of methoxy groups [16].

In liquid products, oxygenated compounds constitute the primary organic components, consistent with the elemental analysis (Table 1), which shows oxygen as the dominant element in algae. Nitrogen and sulfur exhibit comparable contents, yet sulfur-containing species in bio-oil remain scarce. This suggests that sulfur preferentially migrates to the gas phase through decomposition or becomes immobilized in the solid phase via interactions with inorganic salts. Wang et al. [23] employed DFT calculations to elucidate sulfur migration mechanisms, proposing that algal sulfur predominantly exists as sulfonic acid groups in polysaccharides. These groups exhibit low decomposition barriers, facilitating gaseous sulfur release. Additionally, the high ash content in algae enhances self-sulfur fixation through alkali/alkaline earth metals, promoting sulfur radical immobilization in solids rather than the formation of stable organic sulfur compounds in bio-oil. The hydrocarbon content remains low due to the high energy barriers of hydrodeoxygenation and denitrogenation—key pathways for converting algal acids, sugars, and proteins into hydrocarbons [24]. Without catalysts, these reactions proceed inefficiently, limiting hydrocarbon yields.

3.2. Effects of SiC and MgO on Microwave Pyrolysis of Algae

Figure 3a illustrates the influence of additives on product yields. The introduction of SiC increased the bio-oil yield from 22% to 25%, but had little impact on the bio-gas yield, which remained around 27%. In contrast, the incorporation of MgO significantly enhanced the bio-gas yield, increasing it from 27% to 39%, while simultaneously reducing the bio-oil yield from 22% to 18%. However, the addition of either SiC or MgO resulted in a decrease in the bio-char yield, while also improving the pyrolysis rate. Figure 3b details the temporal gas release patterns. The introduction of additives did not alter the general gas yield trend (initial increase followed by decline), with peak gas production consistently occurring within 0–16 min. All additives elevated peak gas yields, with MgO demonstrating the most pronounced enhancement.

With the addition of SiC, rapid gas release occurs during the early stage (0–7 min), followed by a sharp decline to levels slightly below those of the additive-free case. The addition of MgO resulted in a bio-gas release pattern similar to that observed without the additive. After reaching the predetermined temperature (starting from the second sampling point), the bio-gas yields were consistently higher than those without the additive. Figure 3c indicates the introduction of MgO significantly increased the yield of all bio-gas components, while the incorporation of SiC increased the yields of H₂ and CO, but reduced the CO₂ yield. As for CH₄, due to its relatively low yield, the presence or absence of additives had little impact on its production. Figure 3d reveals minimal additive-induced changes in bio-oil organic composition: oxygenated species remain dominant, followed by nitrogen-containing compounds, while hydrocarbons and sulfur-containing species are negligible. The additives reduced the content of oxygenated species and hydrocarbons while increasing nitrogen-containing compounds. Further analysis shows elevated levels of alcohols, ketones, acids, and ethers, but a reduction in aromatic oxygenates (e.g., furans and phenols).

The introduction of additives significantly influences the pyrolysis behavior of algae. Under the conditions without additives, the pyrolysis of Sargassum began in the high-temperature region at the bottom. Because SiC has a stronger microwave absorption capacity than biomass, it facilitates rapid heating by absorbing microwaves, while the upper layers of the biomass, with lower microwave absorption capability, experience slower heating, thus extending the pyrolysis time. Under the condition with SiC, the SiC particle is uniformly mixed with Sargassum, and multiple hotspots can form in the microwave field, increasing the heating rate in the reaction zone. As a result, as shown in Figure 2b, the bio-gas yield at the first sampling point with SiC added is higher than that without additives, but after 7 min, the bio-gas yield becomes lower than that under the no-additive condition, indicating that the presence of SiC accelerates the heating rate of the biomass layer and thus the release of bio-gas. In contrast, under the condition with MgO, the bio-gas yield at the first sampling point is significantly lower than that without additives, mainly because MgO has weak microwave absorption properties and does not form hotspots in the biomass layer, potentially hindering the absorption of microwaves by Sargassum. However, the addition of MgO significantly increases the bio-gas yield, which may be attributed to the catalytic effect of MgO. The incorporation of active metals promotes the decomposition of macromolecular substances and the formation of small-molecule compounds [23,24,25,26], thereby enhancing bio-gas generation.

The introduction of additives not only affects the release patterns of bio-gas but also significantly influences the composition of the bio-gas components. The addition of SiC accelerates the heating rate of the overall pyrolysis region. As discussed in the previous section, the formation of CO₂ primarily arises from decarboxylation reactions, which have a relatively low energy barrier and dominate at lower temperatures. However, as the temperature increases, the equilibrium of the water–gas shift reaction shifts towards the production of CO, thereby promoting the consumption of CO₂. Furthermore, the rapid increase in temperature also facilitates the progression of high-energy barrier reactions, such as decarbonylation (the generation of CO) and the condensation reactions of carbon chains or hydroxyl groups (which lead to H₂ production). These reactions contribute to an increase in the yield of both H₂ and CO.

In addition to influencing the bio-gas yield, the introduction of additives also affects the release of bio-oil. The inclusion of SiC accelerates the decomposition of Sargassum by increasing the overall heating rate, which in turn facilitates the formation of bio-oil. However, as shown in Figure 3a, the introduction of SiC only modestly increased the bio-oil yield, with little effect on the bio-gas yield. This suggests that, in the absence of additives, microwave pyrolysis of Sargassum experiences uneven temperature distribution, with the lower layers heating more quickly than the upper layers. As a result, bio-oil produced in the upper biomass layers may condense, where active functional groups such as hydroxyl groups, carboxyl groups, and unsaturated carbon–carbon bonds may react with the char, leading to the re-conversion of bio-oil into bio-char [27]. However, the addition of SiC inhibits the condensation process and the conversion of bio-oil into bio-char. Furthermore, the faster heating rate in the reaction zone induced by SiC accelerates the volatilization of bio-oil, thereby reducing the likelihood of bio-oil undergoing decomposition reactions on high-temperature surfaces, which would typically convert it into bio-gas. Thus, while SiC accelerates the decomposition of Sargassum, it only slightly enhances the bio-oil yield and does not significantly increase the bio-gas yield. On the other hand, MgO, as an alkaline catalyst, alters the pyrolysis reaction pathway, which promotes the breakdown of larger molecules. Unlike SiC, MgO does not inhibit the condensation of bio-oil, indicating that it facilitates both the decomposition of bio-char and bio-oil, and promotes the formation of bio-gas.

In addition, the introduction of additives also leads to changes in the composition of the bio-oil. Both SiC and MgO increased the nitrogen compound content, indicating that these additives promoted the decomposition of proteins in Sargassum. Both SiC and MgO also facilitated a reduction in oxygenated components and an increase in hydrocarbons in the bio-oil, suggesting that both additives promoted the deoxygenation reactions in the bio-oil. Hydrodeoxygenation, a common deoxygenation process in bio-oil [28], benefits from the faster heating rate induced by SiC, which accelerates the deoxygenation reactions. Furthermore, the presence of active metals (MgO) lowers the reaction energy barriers, further facilitating the deoxygenation process.

In summary, the introduction of additives such as SiC and MgO has distinct effects on the pyrolysis of Sargassum. SiC accelerates the heating rate, promotes the formation of bio-oil, and inhibits the condensation of bio-oil into bio-char. However, its impact on bio-gas yield is minimal. MgO, on the other hand, enhances both bio-oil and bio-gas yields through its catalytic effects, promoting the breakdown of larger molecules and the formation of bio-gas. Both additives also play a key role in altering the composition of bio-oil, primarily by promoting the decomposition of proteins and the deoxygenation of the bio-oil.

3.3. Reaction Mechanism of MgO Introduction

Based on the previous sections, the role of silicon carbide (SiC) in the pyrolysis of Sargassum is primarily attributed to its ability to accelerate the heating rate within the pyrolysis zone. This enhanced heating promotes the rapid decomposition of the biomass and suppresses secondary reactions of the bio-oil, thereby altering the distribution of pyrolysis products. In contrast, magnesium oxide (MgO) modifies the product distribution mainly by directly influencing the reaction pathways through catalytic activity. Given this fundamental difference, it is necessary to conduct a more in-depth analysis of the reaction mechanism associated with MgO.

Density functional theory (DFT) serves as an effective tool for exploring reaction mechanisms at the molecular level. However, due to the inherent complexity of pyrolysis reactions, which involve a vast number of intermediate species and competing pathways, directly modeling real pyrolysis reactions using DFT is highly challenging and computationally intensive. To address this, and to conserve computational resources while still capturing the essential reactivity of the system, simplified models were adopted. These models omit long carbon chains that do not actively participate in the reactions and instead focus on the key reactive sites representative of those found in biomass-derived molecules.

Moreover, it is important to note that the influence of MgO on pyrolysis primarily manifests at elevated temperatures. Therefore, unlike the analytical temperature used in Table 2, the DFT calculations were conducted at a representative final pyrolysis temperature of 600 °C. At this temperature, the energy barriers (activation energies) of the key reaction pathways, as illustrated in Figure 4, were computed to investigate the catalytic effect of MgO. This approach allows for a clearer understanding of how MgO facilitates specific reactions during high-temperature pyrolysis and contributes to the observed changes in product distribution.

Alkali and alkaline earth metals disrupt original electron distributions, thereby enhancing reactant activity and reducing reaction energy barriers [25]. According to Figure 3d, oxygenated organic compounds are the predominant organics in bio-oil. Among them, acids and alcohols can be converted into bio-gas through the pathways shown in R1 and R3, respectively. Esters, ketones, and ethers can undergo hydrogen transfer reactions to produce acids, aldehydes, and alcohols, which can be subsequently decomposed into bio-gas through the pathways shown in R1, R2, and R3, respectively. In addition, phenol and furan are typical aromatic oxygenate organics in bio-oil. Due to the high thermal stability of the benzene ring, phenolic compounds are more resistant to decomposition, whereas furan may undergo ring-opening reactions to generate small gaseous molecules. Therefore, the decomposition behaviors of acids, aldehydes, alcohols, and furan were investigated though DFT calculation, with particular emphasis on the catalytic role of active metal oxides (MgO).

According to Figure 4, MgO incorporation dramatically reduces the hydroxyl dehydrogenation barrier (R5) from 367.2 kJ/mol to 281.2 kJ/mol, and reduces the decarboxylation reaction (R1) barrier from 285.8 kJ/mol to 275.1 kJ/mol, though it increases the decarbonylation barrier (R2) from 363.7 kJ/mol to 394.2 kJ/mol. Theoretically, the addition of MgO should increase H₂ yield while decreasing CO yield. However, the results shown in Figure 3c reveal that the addition of MgO boosts both H₂ and CO yields. This can be attributed to the reverse water–gas shift reaction (CO₂ + H₂ → CO + H₂O), the increase of H₂ yield promoting the conversion of CO₂ to CO, and thus increasing the yield of CO. Additionally, extended gas residence time in fixed-bed reactors further facilitates homogeneous gas-phase reactions.

In addition to promoting the conversion of oxygenated organics of bio-oil, such as acids, aldehydes, and alcohols, into bio-gas, the introduction of MgO also facilitates the decomposition of aromatic oxygenates in bio-oil. Further investigation into MgO’s impact on aromatic structures shows that typical aromatic systems (e.g., benzene) exhibit reduced C-C bond orders from 1.52 to 1.49 upon MgO adsorption. Under MgO catalysis, the ring-opening barrier of oxygen-containing aromatic structures (e.g., furan, R11) decreases from 382.3 kJ/mol to 353.1 kJ/mol, with a concomitant reduction in the hydrogen transfer barrier in furan rings from 298.3 kJ/mol to 232.9 kJ/mol. These findings confirm MgO’s dual functionality in promoting structural disruption of macromolecular organics while inhibiting aromatic condensation, thereby promoting the decomposition of organics in bio-oil, suppressing the polycondensation/aromatization of macromolecular compounds in bio-oil and further conversion into bio-char, and enhancing bio-gas production.

Moreover, for bio-oil composition, MgO promotes fatty acid decarboxylation (R1), particularly crucial for algae-derived unsaturated fatty acids predominantly in cis configuration, effectively converting carboxylic acids to hydrocarbons and increasing aliphatic content in bio-oil.

Comparative analysis reveals distinct mechanisms: SiC primarily elevates the bulk temperature to inhibit volatile–char interactions (e.g., addition reactions) while promoting the secondary decomposition of macromolecules, thereby enhancing bio-oil yields. Conversely, MgO operates through barrier reduction pathways, facilitating bio-gas generation and bio-oil cracking while destabilizing aromatic systems to enhance oxygenated compound decomposition and hydrocarbon production.

4. Conclusions

To elucidate the mechanistic roles of Sargassum MAP, systematic investigations combining a mechanistic experiment and density functional theory (DFT) calculations were conducted, yielding the following conclusions:

• H₂, CO, and CH₄ exhibit similar evolution trends (initial increase followed by decline), contrasting with the monotonic decrease of CO₂. Bio-oil composition is dominated by oxygenated compounds (61%) and nitrogenous species (30%).
• SiC enhances bio-oil yields through an improved heating rate in the pyrolysis region, whereas MgO selectively promotes bio-gas yield (increase by 12%) via catalytic cracking. MgO reduces the content of oxygenated compounds and aromatics while increasing hydrocarbons.
• SiC accelerates MAP reactions via bulk heating effects, while MgO lowers reaction barriers in gas-forming pathways, and decreases the bond order of aromatics. Therefore, in the design of composite catalysts, increasing the content of SiC helps to shorten the MAP duration and increase the yield of bio-oil, while increasing the content of MgO contributes to the production of high-value hydrogen and hydrocarbons.

5. Limitations and Outlook

The optimized combination of a microwave absorber (SiC) and active metal oxide (MgO) shows promising potential as an in situ catalyst system for the microwave pyrolysis of algae. By adjusting the proportions of SiC and MgO, the catalyst composition can be tailored to favor the selective production of bio-oil or bio-gas, depending on the desired application. After pyrolysis, the catalyst can be separated from the bio-char by simple combustion, enabling efficient recovery and reuse. This process suggests good scalability and recyclability of the catalytic system. In future work, deploying such composite catalysts in advanced pyrolysis setups such as fluidized bed reactors could further enhance catalyst utilization and process efficiency. Additionally, investigating ternary additive systems, expanding to other types of algal or lignocellulosic biomass, and conducting long-term catalyst stability and regeneration studies would contribute to the practical application of this strategy in large-scale bioenergy conversion.