Understanding catalytic mechanisms of HZSM-5 in hydrothermal liquefaction of algae through model components: Glucose and glutamic acid

Highlights

• HTL of glucose, glutamic acid and the binary mixtures with HZSM-5 were studied.

• HZSM-5 improved bio-oil formation from HTL of glucose or binary mixtures.

• H/C ratio and HHVs of the bio-oils increased greatly in the presence of HZSM-5.

• Deep degradation of the liquefied intermediates was promoted by HZSM-5.

Abstract

Hydrothermal liquefaction (HTL) of algae has attracted great interest as a thermal conversion for biofuels. In an effort to understand the catalytic mechanism by which algae undergoes HTL in the presence of HZSM-5, the different roles of the major components (carbohydrates and proteins) were investigated. Glucose and glutamic acid were selected as model compounds of carbohydrates and proteins, respectively. Glucose, glutamic acid, and their binary mixtures were processed under hydrothermal conditions in the presence of HZSM-5 over a temperature range of 220–330 °C. The products were analyzed using a combination of elemental analysis (EA), thermal gravimetric analysis (TGA), gas chromatography mass spectrometry (GC-MS), and Fourier transform infrared (FT-IR) spectroscopy to characterize the physicochemical properties. The effects of HZSM-5 on the yield and quality of bio-oil were investigated. Results indicated that the addition of HZSM-5 did not significantly affect the bio-oil yield of glutamic acid but increased the bio-oil yields from glucose and their binary mixtures. The hydrogen-to-carbon (H/C) ratio and the higher heating values (HHVs) of the bio-oils were all greatly increased in the presence of HZSM-5. The addition of HZSM-5 reduced the contents of undesirable oxygenated compounds, nitrogenous compounds, and increased the hydrocarbon content. Maillard reactions between glucose and glutamic acid were also strengthened by HZSM-5.

Introduction

Recent studies have demonstrated good prospects for the use of algae as bio-oil feedstock. The hydrothermal liquefaction (HTL) of algae presents a recently developed approach to upgrading algae into potentially useful biofuels [1]. HTL is the thermochemical conversion of biomass into liquid fuels by processing in a hot, pressurized water environment for sufficient time to break down the solid bio-polymeric structure to mainly liquid components [2]. Typical conditions of HTL process are 200–450 °C of temperature and operating pressures from 4 to 22 MPa [3,4]. HTL is especially suitable for converting algae into valuable liquid products because it eliminates the need for drying the feedstock therefore could minimize the energy consumption required during other biomass conversion technologies [5]. Algae are composed of macromolecular polymers, such as carbohydrates and proteins. The obtained bio-oils from HTL of algae is a dark brown organic liquid having various chemical compositions, such as acids, phenols, alcohols, ketones, esters. Despite being a promising alternative energy source, bio-oils which have relatively high HHVs in the range of 30–40 MJ/kg display poor fuel qualities due to their high oxygen content, high acidities (pH 2–3), high viscosities, and low heating value [4,6]. On the other hand, the high temperature of HTL is conducive to algae decomposition, but the acidity of the water is not high enough to decompose the algae completely, so it is essential to add a catalyst [7]. As a consequence, upgrading processes are necessary for improving the quality of the bio-oils to meet the specifications of commercial fuels [1,8,9].

Catalytic methods are efficient approaches to turning bio-oils into high-quality fuels. Catalysts significantly influence bio-oil yields and quality during HTL processes. Acid catalysts (i.e. H₂SO₄, HCl etc.) and alkali catalysts (i.e. Na₂CO₃, K₂CO₃, KOH, and NaOH etc.) have been widely used in biomass liquefaction; however, the use of homogeneous acids and alkali catalysts is corrosive to liquefaction equipment. In addition, homogeneous catalysts do not extensively affect the intermediate reactions which are the key step for obtaining high-quality bio-oils from algae liquefaction [10,11].

Heterogeneous solid catalysts are almost fully separable and recoverable from the final reaction mixtures, and are easy disposal with low risk. Conventional homogeneous catalysts are expected to be replaced by more environmentally friendly heterogeneous catalysts to improve bio-oil yields and quality in biomass liquefaction. Among the heterogeneous catalysts examined, zeolites and metal-supported zeolites are the most extensively studied for improving yield and qualities of bio-oils in the direct hydrothermal liquefaction of algae [12,13].

Solid acid catalysts, such as zeolites, have been extensively investigated in HTL processes applied to lipids which are beneficial for the production of lighter fractions [14]. ZSM-5 is normally used in the petrochemical industry because of its acidity ad shape selective nature due to its micro and mesoporous structure. ZSM-5 also possesses a high concentration of Lewis acid and Brønsted acid sites [14]. The HZSM-5 catalyst is a hydrothermally stable catalyst used in bio-oil upgrading, which was found to show high selectivity to aromatic hydrocarbons [15]. Results have found that HZSM-5 promotes the production of hydrocarbons in the hydrothermal catalytic process of palmitic acid at 400 °C [16]. Overall, because of the moderate acid strength, HZSM-5 zeolites are widely reported to be suitable for the transformation of oxygenated chemicals and other components into hydrocarbons [17]. HZSM-5 with higher acidities produced higher hydrocarbon yields compared to HZSM-5 of lower acidities. On the other hand, several studies showed that zeolites are not limited to the production of alkanes and aromatics in HTL process of algae [12,14,16]. HZSM-5 could remove heteroatom such as S, N, O by promoting the valorization process which converts the derivatives into corresponding hydrocarbons [13]. The results of Duan et al. (2016) [11] inferred that zeolites with more channel structures were more selective during HTL of algae.

On the whole, previous studies have focused on the effects of HZSM-5 and their structural changes, whereas the specific performances of HZSM-5 applied to the HTL of different algae species, especially their performances on sugars and proteins, have been less concerned. In this article, HZSM-5 was used in the HTL process of glucose, glutamic acid, and their binary mixtures. The product yields, compositions, and reaction pathways of the HTL of major algal components were investigated to assess the HZSM-5 effects on the HTL of algae.