Solid base catalytic hydrothermal liquefaction of macroalgae: Effects of process parameter on product yield and characterization

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Highlights

  • Different solvents and solid base catalysts was used in macroalgae HTL.

  • Maximum bio-oil (33.0 wt%) was obtained with CaO/ZrO2 (10 wt%) at 280 °C.

  • Solid base catalyst enhanced the compound(s) selectivity.

  • Catalytic HTL bio-oil showed the higher carbon content and HHV.

Abstract

The hydrothermal liquefaction (HTL) of Sargassum tenerrimum (ST) macroalgae was carried out for 15 min, over various solid base catalysts (CaO supported on CeO2, Al2O3, and ZrO2) at different reaction temperatures (260–300 °C), different catalyst quantities (5–25 wt%) and using different solvent systems. Maximum bio-oil (BO) yields for the non-catalytic HTL with single solvent water, ethanol, and water-ethanol co-solvent were 3.3 wt%, 23.3 wt%, and 32.0 wt%, respectively, at 280 °C. Ethanol as single solvent elicited highest BO yield of 25.2 wt% with CaO/ZrO2 (10.0 wt%) catalyst. However, the highest BO yield (33.0 wt%) accompanied by higher conversion (70.5%) was obtained with CaO/ZrO2 (10.0 wt%) under water-ethanol co-solvent. The selectively higher percentage of ester functional compounds (87.8%) was found with CaO/ZrO2 catalyst under water-ethanol co-solvent. Also, the bio-oil obtained from catalytic liquefaction showed a higher high heating value (HHV) compared to that from the non-catalytic HTL reaction.

Introduction

Worldwide, there has been increased use of non-renewable fossil fuel resources for economic development, for which reason they have been found continuously depleting (Zhang et al., 2019, Xu et al., 2011). It is thus essential to look at alternatives to fossil resources (Yan et al., 2019, Zhang et al., 2019). Biomass such as agricultural, forest, municipal solid waste, animal wastes, waste from food processing and aquatic residues are promising alternatives to fossil resources because biomass is renewable and can be used for both energy and chemicals production (Raikova et al., 2019b, Aysu and Sanna, 2015). Many different technologies have been used for biomass utilization which could be biological, physical or thermochemical conversion. Among the thermochemical processes such as pyrolysis, hydrothermal liquefaction (HTL) and gasification, the hydrothermal method is most widely used for aquatic biomass conversion into useful products as it does not require feed drying process and can be commercialized in industrial perspectives (Ma et al., 2019, Raikova et al., 2019a). The process parameter is an important factor during biomass conversion into products (Xue et al., 2016: Guo et al., 2015). Researchers have been using various parameters such as temperature, reaction environment, reaction holding time, catalyst, reaction heating rate, etc., for the optimization and to produce higher yields of bio-oil (Yuan et al., 2019, Kandasamy et al., 2020, Ma et al., 2019).

While HTL of macroalgae has been carried out over various homogeneous catalysts such as KOH, K2CO3, NaOH, Na2CO3, H2SO4, HCl etc., (Shakya et al., 2015, Yan et al., 2019, Kandasamy et al., 2020, Egesa et al., 2018, Yang et al., 2014), the use of homogeneous catalyst might not be useful for industrial applications as it is hard to separate and reuse (Ma et al., 2019, Wang et al., 2018). Hence the need to initiate use of solid catalyst for the liquefaction process. Wang et al. (2018) investigated HTL of the microalgae Nannochloropsis over various transition metals M/TiO2 (M = Fe, Co, Ni, Mo, or Mn) as catalysts and showed that Ni/TiO2 was the most effective for higher bio-oil yield. The maximum bio-oil yield of 48.23% was obtained at 300 °C. They observed that upon addition of a catalyst, the quality and composition of the bio-oil also significantly changed. Another research group (Yuan et al., 2019) carried out a catalytic HTL of macroalgae with the co-solvent system. They observed that with the addition of ZSM-5 catalyst, the bio-oil yields (46.75%) as well as the conversion (95.5%) in the HTL reaction were enhanced significantly. The catalytic bio-oil thus obtained was majorly composed of ester compounds compared to that from the non-catalytic reaction. Kandasamy et al. (2020) examined the catalytic HTL reaction of Spirulina platensis macroalgae with nano-catalyst (CeO2). Maximum bio-oil (26.0%) with higher conversion was observed at 250 °C. The bio-oil being largely composed of amino acids and nitrogen-containing compounds, the need arises to introduce a suitable catalyst which could reduce its nitrogen content. Ma et al. (2019) added zeolite-based catalysts for HTL of macroalgae and found ZSM-5 to be most suitable in terms of bio-oil yield and quality. Maximum bio-oil yield (29.3 wt%) was observed at 280 °C. They found that use of the zeolite catalyst could reduce the oxygen content in the bio-oil. However, the production of selective functional group compounds or quality bio-oil would call for further catalytic screening of algal biomass.

While homogenous base catalytic HTL of macroalgae has been reported by several researchers, the use of solid base catalyst as a more economically viable choice has not been examined. Hence the present study, wherein various solid base catalysts were synthesized for application in HTL of Sargassum tenerrimum (ST) at different reaction temperatures, catalyst amounts and involving the use of co-solvent. We examined the catalytic effect on product yield and composition changes in the bio-oil by GC–MS, FT-IR, NMR, and elemental analysis. The prepared catalysts were characterized using various techniques: BET, SEM, TEM, XRD, and CO2-TPD. The possible reaction mechanism of functional compound formation was also investigated.

Section snippets

Materials and methods

Sargassum tenerrimum (ST) was collected from the west coast region of Goa, India. The sun-dried sample was crushed and sieved to a size between 0.5 and 2 mm.

Feed characterizations

Table 1 depicts the proximate and ultimate analyses as well as higher heating value (HHV) of ST macroalgae. The ST biomass composition showed the presence of volatile matter (61.5%), fixed carbon (6.3%), and highest ash content (26.5%). Elemental analysis indicated that 32.0% carbon, 4.7% hydrogen, 0.93% nitrogen, 1.55% sulfur, and a higher amount of 60.7% oxygen were present in the ST biomass. The stretching of functional groups such as Osingle bondH, Csingle bondH, Cdouble bondC, Cdouble bondO, and Csingle bondO was most abundant in ST biomass.

Conclusions

In this study, choice of catalyst, catalyst amount, reaction temperature and solvent system have been investigated while using solid based catalysts for HTL of ST. The HTL reaction using CaO/ZrO2 catalyst in a water-ethanol co-solvent system produced maximum bio-oil yield of 33.0 wt%. Higher percentages of ester functional compounds were obtained with CaO/ZrO2 catalyst in ethanol solvent. Catalytic liquefaction reaction reduced the nitrogen and oxygen content in the bio-oil, thereby enhancing

CRediT authorship contribution statement

Bijoy Biswas: Conceptualization, Investigation, Methodology, Writing - original draft. Avnish Kumar: Data curation, Methodology. Alisha C. Fernandes: Procurement of algal biomass, Investigation, Formal analysis. Komal Saini: Formal analysis, Resources. Shweta Negi: Investigation. Usha D. Muraleedharan: Writing - review & editing. Thallada Bhaskar: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the Director, CSIR-Indian Institute of Petroleum, Dehradun for his constant encouragement and support and AcSIR for granting permission to conduct this research work at CSIR-IIP. Bijoy Biswas thanks CSIR, New Delhi, India, for his Senior Research Fellowship (SRF). Authors also thank the Analytical Science Division (ASD) of CSIR-IIP.

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