Catalytic hydrothermal liquefaction of nutrient-stressed microalgae for production of high-quality bio-oil over Zr-doped HZSM-5 catalyst

Highlights

• Nutrient stressing altered the microalgae compositions especially reduction of protein by ∼50%.

• The presence of Zr/HZSM-5 catalyst was beneficial and improved the HTL bio-oil yield and quality.

• The nitrogen content in HTL bio-oil from stressed algae was within the range for petroleum crude and the most abundant fatty acid was found to be omega-7 fatty acid.

• High heating value (43.56 MJ kg⁻¹) within the range for petroleum crude (42–48 MJ kg⁻¹) was achieved.

• Suitability of nutrient stressed algae in enhancing the quality of HTL bio-oil was established.

Abstract

The Hydrothermal liquefaction (HTL) of a nutrient-stressed microalgae (Scenedesmus obliquus) (NSM) with and without the use of zirconium-doped HZSM-5 catalyst was investigated under temperature conditions ranging from 250 to 350 ^°C. The wet impregnation method was used to prepare the catalyst, and HTL experiments were conducted on the unstressed microalgae (CM) for comparison. Under the stressed condition, the protein content of the microalgae was reduced from 42.35% to 22.08% while the carbohydrate and lipid contents were increased from 25.36% to 42.55% and 17.16%–21.62% respectively. The maximum HTL bio-oil yield of 52.8 wt% and 24.27 wt% were found for NSM and CM respectively at 350 ^°C with addition of Zr-HZSM-5 catalyst. Higher denitrogenation and deoxygenation was achieved with NSM compared to CM. At high temperature of 350 ^°C, the most abundant fatty acid in NSM was found to be cis-vaccenic acid (omega-7- fatty acid), and this could be explored for the possibility of extracting products of great value from the bio-oil for applications other than biofuels. Mainly, the use of Zr-doped HZSM-5 catalyst on nutrient-stressed S. obliquus microalgae resulted in enhanced bio-oil yield and characteristics which compared well with petroleum crude.

Introduction

Hydrothermal liquefaction (HTL) is considered to be a preferable thermal conversion method for the production of bio-oil from aquatic biomass such as microalgae because biomass drying-step is not required, thus avoiding the high energy costs associated with biomass drying [1,2]. Besides, HTL is an interesting technique that utilizes wet microalgae biomass for bio-oil production with low energy consumed in comparison to pyrolysis [3,4]. Consequently, HTL has attracted growing interest as a promising technique to produce biofuels from microalgae. Despite the above-stated advantages, maximizing the yield and quality of bio-oil products from the HTL of microalgae has been a challenge. For instance, due to the high nitrogen and oxygen content found in HTL bio-oils, there is need for the bio-oils to be further processed before usage as fuel [5].

“One of the most widely investigated approaches employed as a control strategy for addressing the problem of yield and quality of HTL bio-oils is the utilization of various catalysts which improve the hydrothermal liquefaction process. Catalysts played an essential role in improving the quality and yield of bio-oil produced from HTL of microalgae [6,7]. Past studies have reported a beneficial effect from the utilization of catalysts in reducing the oxygen content of bio-oil product as well as improving the yield of bio-oil [8,9]. Majority of the earlier works were focusing on homogeneous catalysts using alkali or metallic salts for the generation of bio-oils from the HTL conversion of microalgae [10,11]. Very recently, researchers have explored the use of heterogeneous catalysts because they are easy to separate, reusability and selectivity [2,[12], [13], [14], [15]]. Due to their high selectivity and activity, zeolitic based catalysts have been given priority above other heterogeneous catalysts for HTL and bio-oil upgrading. For example, Xu et al. [8] found increased yield of bio-oil (from 32% to 38%) with HZSM-5 as catalyst in the HTL of Chlorella pyrenoidosa. The authors further studied the effect of doping cerium on HZSM-5 (Ce-HZSM-5 catalyst) and found significant improvement in the yield of bio-oil (52%) when compared to the use of HZSM-5 alone (38%). Previous study by Ma et al. [9] investigated the HTL of Ulva prolifera with three different zeolites catalysts (Mordenite, Y-Zeolite and ZSM-5) and it was found that ZSM-5 catalyst showed superior performance in comparison to the other zeolite catalyst having the highest bio-oil yield (29.3 wt %). The authors also found that the oxygen content in HTL bio-oil was greatly reduced with the addition of ZSM-5 when in comparison to the use of mordenite and Y-zeolite as catalysts for the liquefaction process.”

Based on this background, zeolites as catalysts have been shown to be promising for enhancing the quality and yield of HTL bio-oil derived from microalgae, and also some studies have shown that the chances of making fuel-grade bio-fuels could be enhanced with zeolite catalysts especially with HZSM-5 [16]. However, there is a likelihood of catalyst deactivation during the HTL process because of a decrement in the number of active sites related to the zeolite frameworks [17]. Also, the zeolitic framework degradation resulting from the synergistic impact of water and heat during hydrothermal conditions and the blockage of pores by carbonaceous deposits have been identified as possible challenges that could affect the efficiency of the catalyst [17]. To address these challenges, some approaches have been suggested, which include the incorporation of metals in the catalyst or the use of fluoride media in the zeolite preparation [6]. Metal promoters (such as Ni, Zr or Ru) and incorporating support material like ZrO₂ could improve catalyst efficiency [6].

Another approach yet to be explored with the thermal conversion process may involve the use of nitrogen stressed condition to alter the biomass feedstock composition for improved bio-oil product yield and quality. Microalgae have variable compositions that depends largely on the species types, growth and environmental conditions [18]. As protein accounts for the major intracellular nitrogen pools in microalgae, hence, the amount of nitrogen nutrient used during cultivation may have a direct effect on the protein content of the microalgae biomass. It is believed that most of the nitrogen present in the bio-oil product probably originates from the protein content of the microalgae [19]. Consequently, the quality of bio-oil product derived from microalgae may also depend greatly on the microalgae's biochemical composition [1]. Studies have shown that the most notable changes under nitrogen limitation condition involve the accumulation of carbohydrates and/or lipids, coupled with a reduction of protein content [[20], [21], [22], [23]]. Thus, nitrogen alteration is seen as an approach that could provide the needed solution towards developing an efficient and sustainable HTL processes for quality biofuel production from various species of microalgae.

“Scenedesmus obliquus microalgae was chosen in this study because of its high protein composition [24,25] and to stand in as baseline for high-quality HTL bio-oil production from high protein microalgae. Till date, very few studies are available for the catalytic HTL of S. obliquus microalgae [13,25,26]. Recently, Kohansal et al. [26] and Arun et al. [13] reported the use of heterogeneous catalysts such as Ni/AC, Ni/CeO₂, Ni/AC-CeO₂ and clamshell derived catalyst for the HTL of S. obliquus into bio-oil. The major finding from both works was higher bio-oil yield, but high nitrogen content in the biofuel product remained a challenge, thereby affecting the quality of bio-oil produced. Hence, further research efforts are needed to identify better heterogeneous catalysts that can improve the quality of HTL bio-oil by simultaneously reducing oxygen and nitrogen content of the bio-oil product to meet desirable standard. To the best of the authors’ knowledge, no previous work has documented the suitability of nutrient stressed approach combined with the use of catalyst as a tool for enhancement of yield and quality of bio-oil produced from hydrothermal liquefaction of microalgae. In this study, hydrothermal liquefaction (HTL) of nutrient-stressed Scenedesmus obliquus microalgae was investigated under temperature conditions ranging from 250 to 350 ^°C with and without the use of Zr-HZSM-5 catalyst. HTL experiments were conducted on the unstressed Scenedesmus obliquus microalgae (CM) for comparison. The wet impregnation method was used to prepare the catalyst and the synthesized catalyst was characterized by X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), high-resolution scanning electron microscopy and energy dispersive spectroscopy (HRSEM/EDS).