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In-situ transesterification of Chlorella vulgaris using carbon-dot functionalized strontium oxide as a heterogeneous catalyst under microwave irradiation Alex Tangy, Vijay Bhooshan Kumar, Indra Neel Pulidindi, Yael Kinel-Tahan, Yaron Yehoshua, and Aharon Gedanken Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02519 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016
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Energy & Fuels
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In-situ transesterification of Chlorella vulgaris
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using carbon-dot functionalized strontium
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oxide as a heterogeneous catalyst under
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microwave irradiation
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Alex Tangy,#† Vijay Bhooshan Kumar,#† Indra Neel Pulidindi,† Yael Kinel-Tahan,‡
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Yaron Yehoshua‡ and Aharon Gedanken†*
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† Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
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‡ The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University,
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Ramat-Gan 5290002, Israel
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ABSTRACT:
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The main goal of this study is to functionalize SrO with carbon dots (C-dots) and to
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explore the composite as a catalyst for fatty acid methyl esters (FAME) production
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using Chlorella vulgaris as feedstock. C-dots are synthesized by sonicating
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polyethylene glycol followed by sonochemical modification of Sr(NO3)2 (precursor
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for SrO) with C-dots. Sonication facilitates the adhesion of C-dots to the surface of
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Sr(NO3)2. The resulting material is calcined in an inert environment to form SrO-C-
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dot composite. The effect of functionalizing SrO with C-dots on the transesterification
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of the lipids in the alga with methanol is studied. The optimization of a one-stage
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process of conversion of the lipid fraction of microalga Chlorella vulgaris into FAME
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using direct transesterification under microwave irradiation is illustrated. A lipid
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conversion value of 45.5 wt. % is achieved using the SrO-C-dot catalyst after 2.5
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minutes of microwave (MW) irradiation. The catalyst displayed better activity than
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commercial SrO. Microwave irradiation accelerates the disruption of the microalgal
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cells and facilitates the release of lipid content into the reaction medium. The catalyst
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is characterized by a variety of physico-chemical techniques. The FAME product
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obtained from the alga is quantified using 1H NMR spectroscopy. The new catalyst,
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namely, SrO-C-dot nanoparticles (NPs), yielded 97 wt.% FAME from Chlorella
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vulgaris in 2.5 minutes of MW irradiation.
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KEYWORDS: Microalga, Chlorella vulgaris, Lipid, FAME, SrO-C dot
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1. INTRODUCTION
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Depletion of fossil fuels, environmental concerns, and growing energy requirements
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prompt the replacement of at least part of the petroleum-based fuels used for
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transportation by renewable energy sources. The transportation sector, representing 30
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% of the world’s total energy consumption, is the largest energy-consuming sector
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after the industrial sector. Almost 60 % of world’s oil demand is for the transportation
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sector, which will probably be the most energy-demanding sector in the future.1
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Biodiesel produced from renewable biological sources has gained importance as an
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alternative energy source to fossil-based transportation fuels such as petrol and
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diesel.2,3 Biodiesel is generally produced using vegetable oils as feedstock. The major
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drawback of using edible biomass as feedstock is that arable land is used to farm the
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oil seed crops, and this can lead to a food-Vs-fuel-Vs-land conflict. Moreover, it has
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been reported that a number of feedstock like algal oils, jatropha and waste cooking
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oil, can reduce 60-90 % of the biodiesel production cost.4
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Microalgae are photosynthetic microorganisms capable of converting sunlight,
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atmospheric CO2 and wastewater into a variety of high-energy molecules, including
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fatty acids (FA) and triacylglycerols (TAGs), the major feedstock used for biodiesel
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or FAME production.5,6 Microalgae, also called third-generation fuel, are considered
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one of the most promising feedstock for biofuels.7 The productivity of these
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photosynthetic microorganisms in converting CO2 into carbon-rich lipids greatly
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exceeds that of agricultural oleaginous crops, without competing for arable land and
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food crops.8 Microalgae can be grown on fresh or salt water, and even in a wastewater
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environment.6 Recent advances present opportunities to develop microalgae as
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feedstock for biodiesel production in a sustainable and economical way within the
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next 10 to 15 years.9,10 Microalgae can be cultivated from several aqueous systems 3 ACS Paragon Plus Environment
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such as ponds and photo-bioreactors. Microalgae have the highest growth rates among
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all the photosynthetic organisms.11 Approximately 46 tons of oil/hectare/year can be
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produced from diatom algae.12 These characteristics make them an attractive
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feedstock for biodiesel production.
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Algal biomass contains macromolecular proteins, polysaccharides and lipids,
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along with inorganic components.13 Only the lipid content is generally targeted for
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FAME production. FAME production from microalgae can be divided into three main
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stages: a) cultivation, b) harvesting, and c) lipid extraction and transesterification for
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FAME production. The lipid content of microalgae is dependent on the microalgae
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species and cultivation conditions. Microalgae can accumulate approximately between
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20 and 70% of lipid content from the dry biomass.14 The usual method for extracting
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lipids from the whole algae biomass consists of mixing the microalgae with an
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organic solvent such as chloroform or hexane as co-solvent, or adopting other
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methods such as using a functional membrane coated with a cationic polymer15 or
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using ionic liquids.16 The extracted micro algal lipids are then converted to FAME by
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transesterification using alcohol and a catalyst. Recently, a combination of lipid
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extraction and FAME conversion in a one-step process, called direct (in-situ)
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transesterification, has been reported.17 This methodology simplifies the production
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process and improves the FAME yield compared to conventional lipid extraction step
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that involves lipid loss. Moreover, the number of stages in the biodiesel production
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process are also reduced in the in-situ extraction based processes. Direct
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transesterification has several advantages, such as cell disruption, lipid extraction and
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transesterification in a single step, and has been proved to be more useful than
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conventional methods.18 The conventional two-step preparation of biodiesel from
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microalgae has several disadvantages compared to in situ transesterification, including 4 ACS Paragon Plus Environment
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operation complexity, a longer process time, high energy consumption and relatively
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higher cost, which restricts the commercial production of biodiesel from microalgae.
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Higher FAME yield is obtained by the direct transesterification than in a two-stage
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process.17,20 Moreover, direct transesterification provides energy-efficient and
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economical routes for biodiesel production.20
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Despite the high conversion yields achieved using homogeneous catalysts such
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as NaOH, KOH, H2SO4, etc., catalysts may remain in the FAME phase and, therefore,
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a FAME refining or washing step is needed. In this sense, the use of heterogeneous
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catalysts is a suitable alternative, allowing easy separation of the final product and
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reusability. Metal-oxide materials play an important role in the catalytic reaction
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because of their obvious advantages, which include easy processing, low cost and
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easy modification into composites. Chang et al. have reviewed different types of
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catalytic systems mediated by solid mixed metal-oxides for biodiesel production.21
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Among the metal-oxide catalysts, alkaline earth metal oxides possess excellent
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basicity and exhibit significant activity and selectivity in the transesterification
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reaction.22 Specifically, the use of SrO, despite its lower surface area23 and the partial
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solubility of the metal ion in the reaction medium,24 results in almost complete
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conversion of waste cooking oil into FAME in 10 s under microwave irradiation.51,52
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Moreover, all the lipids in dry Nannochloropsis microalgae are fully converted to
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FAME via direct transesterification of the microalgae in 5 min of microwave
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irradiation.17
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The existence of both microalgae, catalyst dispersed in methanol and FAME at
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the end of the transesterification reaction when a heterogeneous catalyst is used, leads
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to diffusion constraints of various components present in the medium relative to the
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case when a homogeneous catalyst is used. This leads to lowering of the reaction rate. 5 ACS Paragon Plus Environment
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The reaction takes place in the interfacial region between the catalyst, lipids, and
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methanol. Thus, either a highly active catalyst or vigorous mixing is required to
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increase the area of interaction/contact between the two phases.
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The use of metal oxide as a host matrix for C-dots has opened a new
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technological gateway for the development of catalysts due to the novel inherent
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characteristics of carbon nanostructures as well as nano-metal oxides. It is expected
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that replacing the commercial micron-sized catalytic particles by nanosized particles
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of SrO and C-dots can lead to further improvement in the transesterification rate of
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the conversion of algal biomass. C-dots are a nanostructured material with a
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substantial fraction of carbon, oxygen and hydrogen. Carbon dots are a kind of quasi-
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zero-dimensional nanomaterial. The three dimensions are all 10 nm or less in size, so
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that the mobility of the internal electrons is restricted within the nanoscale dimensions
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in all directions.27 C-dots may be a promising catalytic material because of their high
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chemical stability and multifunctionality.28
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The efficiency of FAME production from microalgae can be improved by the
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use of microwaves.17 Microwave irradiation enhances the mass transfer rate between
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immiscible phases and also diminishes the reaction time.18 Moreover, the FAME
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production rate and yield from wet microalgae biomass obtained through the one-step
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process using microwave irradiation are 6-fold and 1.3-fold higher than those
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obtained through the two-step process using conventional heating.29
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Our main objective in using microwave irradiation for the conversion of algae
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to FAME is to reduce production costs and energy requirements while maximizing
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lipid productivity, increasing the utilization efficiency of the biomass by making use
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of all algal biomass components and reducing the process time. The current report
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focuses on the design of an innovative catalytic process for a single-step conversion of
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Chlorella vulgaris into FAME using microwave irradiation. The goal of our
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investigation is to develop a new active catalytic material for FAME production,
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namely, C-dot-modified by SrO obtained from Sr(NO3)2, and to study its physical,
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chemical and catalytic properties. To the best of our knowledge, there are no reports
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on the use of the SrO-C-dot composite as a catalyst for FAME production from
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microalga. This study is a continuation of our recent work30 on the synthesis of C-dots
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and the elucidation of their physicochemical properties.
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2. EXPERIMENTAL SECTION
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2.1. Materials and Methods
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2.1. Chemical reagents
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Polyethylene glycol-400 (PEG-400, 99.998%) and Sr(NO3)2 (≥ 99.0%) used as
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precursor of SrO are purchased from Sigma-Aldrich, Israel. Strontium oxide (SrO)
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(99.5%) is purchased from Alfa Aesar. Isopropyl alcohol (99.7%), acetone, methanol,
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and chloroform are purchased from Bio Lab and are used as received.
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2.2. Preparation of SrO-C-dots
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Recently, we have reported the formation of ultrafine C-dots30 by ultrasonication of
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PEG-400. The current synthesis (Figure 1) was performed as follows: Step 1) 20 mL
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of polyethylene glycol (PEG-400) was transferred into a quartz test tube that had been
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dipped in a water/Si-oil bath at 70 °C. The tip of an ultrasonic transducer (Sonics and
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Materials Inc., USA, model VCX 750, frequency 20 kHz, AC voltage 230 V) was
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dipped in the solution, about 2 cm above the bottom of the test tube. Ultrasonic
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irradiation was applied with a sonication amplitude of 70 % for 2.5 h. These
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conditions are found to be optimal for the formation of C-dots.30 Step 2) 10 g of
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Sr(NO3)2 is added to the test tube and the sonication was continued for another 30 min
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in a hot water bath at 50 °C. Subsequently, the product mixture was centrifuged. The
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Sr(NO3)2-C-dot nanoparticles were washed with double-distilled water and acetone
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(1:1) and then dried in vacuum at room temperature. The dried product was subjected
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to calcination for varying periods of time to determine the optimal conditions for the
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decomposition of Sr(NO3)2. The optimal condition was found by thermogravimetric
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analysis (TGA) measurements in the presence of N2 gas. The calcination was
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performed in an inert atmosphere to prevent the decomposition of the C-dots. This
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methodology aided the in-situ deposition of C-dots on the surface of Sr(NO3)2 via
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sonication and subsequent calcination of the material in an inert atmosphere resulted
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in the formation of SrO-C-dot composite. The amount of carbon dots in the reaction
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medium is much lower than that of Sr(NO3)2 (10 g). In the calcined product, the
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relative amount of SrO (93 wt. %) is much higher compared to carbon dots (7 wt. %).
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Moreover, the particle size of SrO (20-40 nm) is greater than that of the carbon dots (7
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nm). These factors confirm the possibility that SrO is a support upon which the carbon
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dots are functionalized in the composite material, SrO-C-dots.
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2.3 Characterization of SrO-C-dots
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The precise decomposition temperature of Sr(NO3)2-C-dot to SrO-C-dot was deduced
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from the TGA. The TGA curves were recorded using TGA-GC-MS (EI/CI) Clarus
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680/Clarus SQ 8C by Perkin Elmer over the temperature range of 25-900 °C in both
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nitrogen and air atmospheres at a heating rate of 10 °C/min. Powder X-ray diffraction
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(PXRD) study is conducted to probe the crystallographic nature of the product (SrO-
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C-dot) and to verify the transformation of Sr(NO3)2 to SrO. XRD patterns were
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collected using a Bruker AXS Advance powder X-ray diffractometer (CuKα 8 ACS Paragon Plus Environment
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radiation; λ = 0.15417 nm) operating at 40 kV/30 mA with a 0.0019 step size per 0.5 s
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over the range of 10-80° (2θ). The phases were identified using the power diffraction
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file (PDF) database (JCPDS, International Centre for Diffraction Data). Fourier
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Transform Infra-Red (FT-IR) spectra are recorded in KBr pallet mode on a Nicolet
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(Impact 410) FT-IR spectrophotometer under atmospheric conditions. The samples
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are scanned over the range of 400-4,000 cm−1. The morphology of the C-dots-SrO
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composite and the respective elemental mapping is evaluated using high-resolution
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scanning electron microscopy (HR-SEM) with a JEOL-JSM 700F instrument and an
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LEO Gemini 982 field-emission gun SEM (FEG-SEM). Due to the low conductivity
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of SrO, the sample was coated with Iridium (Ir). Elemental analysis was carried out
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using energy-dispersive x-ray spectroscopy (EDS) in conjunction with the HR-SEM
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instrument. Transmission electron microscope (TEM) images of the SrO-C-dot
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particles are obtained using a Tecnai G2 (FEI, Oregon USA), a high-contrast/cryo
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TEM equipped with a bottom CCD 1 Kx1 K camera to visualize and evaluate the
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shape, size and surface morphology of the SrO-C-dots. Samples for the TEM images
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were prepared by making a suspension of the particles in isopropyl alcohol, using
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water-bath sonication. Two small droplets of the sample containing SrO-C-dot
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particles are then applied on a TEM copper grid coated with a carbon film, and dried
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on a covered Petri dish in vacuum. The particle-size distribution of the SrO-C-dot
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nanoparticles is measured by the dynamic light-scattering (DLS) particle-size analysis
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model using a Zetasizer Nano SZ (Malvern Instruments Ltd., Worcestershire, UK).
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Raman spectra of the SrO-C-dots are recorded on a Renishaw inVia Raman
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microscope equipped with RL785 and RL830 Class 3B wavelength stabilized diode
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lasers and a Leica DM2500 M (Leica Microsystems) materials analysis microscope. A
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powder sample was sprinkled on the glass slide and the Raman spectra are recorded.
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The fluorescence properties of the SrO-C-dots are studied by a Varian Cary Eclipse
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Fluorescence spectrophotometer. The specific surface area of the particles was
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measured by the Brunauer–Emmett–Teller (BET) method at 77 K under liquid
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nitrogen on a Micromeritics instrument (Gemini 2375) after the samples had been
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evacuated at 25 °C for 12 h by an evacuator (Micromeritics, Flow Prep 060).
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2.4. Cultivation and harvesting of Chlorella vulgaris
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Freshwater microalga C. vulgaris cultures are grown in the laboratory in a 3 L sterile
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flask using a Bristol medium (https://utex.org/products/bristol-medium), with air bub-
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bling, under 70 µmol photons m-2s-1 and continuous LED light, at 22 °C with three
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replicates. After a week, the algae are transferred outdoors to a 30 L flat-panel photo-
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bioreactor (PBR). The light, temperature, oxygen and pH were monitored by a
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computerized system for 12 days. The light is monitored by a Li-COR QUANTUM
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sensor; the temperature and dissolved oxygen is monitored by LDO sensors (HACH
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LANGE); the pH was monitored by pHD sc Digital Differential pH/ORP sensors
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(HACH LANGE). The daylight (12 h) reached up to 2,200-2,500 µmol photons m-2s-
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1
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range was 6.1-7.8. During the first 7 days of the experiment, the initial inoculum
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reached maximum culture density. Then nutrient deficiency diverted the biosynthesis
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toward lipid accumulation in the algae cells. The algal culture was harvested by
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centrifugation and freeze-dried in a lyophilizer.
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2.5. In-situ transesterification of Chlorella vulgaris
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The transesterification reaction was conducted under domestic microwave oven
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(DMWO) irradiation (Figure S1 in the Supplementary Information). The DMWO was
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operated at 2.45 GHz in batch mode. The output of the domestic microwave reactor
. The temperature reached up to 29-42 °C at noon and 16-24 °C at night. The pH
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was 1,100 W .The microwave oven was operated at 70 % power (cycle mode of 21 s
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on and 9 s off), a cycle mode function provided by the DMWO manufacturer. The
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microwave oven was modified to allow the distillation column to pass through it (for
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enhanced safety of operation) and a stirring facility during the reaction. The
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modification was performed by replacing the bottom part of the oven by a rounded
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aluminum plate. The plate was carefully attached to the framework in a manner that
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enables magnetic stirring. The reaction temperature obtained as a result of the
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microwave irradiation was measured using a Pyrometer (Fluke, 65 Infrared
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Thermometer) after the irradiation was completed. The reaction temperature was
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found to be 333 K immediately after completion of the irradiation.
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The transesterification reactions are performed by a DMWO equipped with a
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condenser and carried out in a 50 mL round-bottom flask. A typical batch process of
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the transesterification reaction consists of taking 1 g of dried microalgae mixed with
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10 mL of chloroform, 5 mL of MeOH (1:2 v/v) (based on the procedure of Folch et
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al.31) and 0.3 g of catalyst, SrO-C-dots or commercial SrO, and irradiating the content
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in a microwave oven for a certain time. First, the catalyst was dispersed in MeOH by
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high magnetic stirring to ensure good dispersion of the catalyst into the MeOH. The
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microalgae and chloroform are subsequently added and the mixture was irradiated. At
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the end of the reaction, the temperature of the mixture is measured by a pyrometer and
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was found to be 333 K, owing to the presence of the condenser which allowed the
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cooling of the reaction. After the completion of the reaction, the samples are
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centrifuged and filtered under vacuum to separate the methanol-chloroform phase that
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contained the FAME from the microalgae, the glycerol and the catalyst. The solution
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of methanol-chloroform was evaporated in a rotary evaporator, and the FAME content
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was determined gravimetrically.17 The oil content was expressed by its weight relative
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to the weight of the microalgae biomass.
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The FAME product is analyzed by 1H NMR spectroscopy (Bruker Avance 300
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spectrometer). Chloroform (CDCl3) was used as a solvent for 1H NMR sample
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preparation. The conversion is calculated directly from the integrated areas of the
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methoxy group in the FAME at 3.65 ppm (singlet) and from the integrated areas of
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the α-carbonyl methylene protons present in the triglyceride derivatives at 2.26 ppm
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(triplet). Equation 1 was used to estimate the conversion of the lipid content to
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FAME:
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where
is the integrated value of the protons of the methyl esters and
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integrated value of the methylene protons. The conversion ratio of the lipids in the
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microalgae to the resultant FAME was obtained by dividing IMe by ICH2. The factors 2
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and 3 are derived from the fact that methylene carbon possesses two protons and
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methyl carbon has three attached protons.52
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3. RESULTS AND DISCUSSION
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3.1. Characterization of SrO-C-dot catalyst
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3.1.1. Thermal analysis of Sr(NO3)2-C-dots and SrO-C-dot composite
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The TGA analysis of the Sr(NO3)2@C-dots before and after calcination for 4 h at 650
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°C is shown in Figure 2. The TGA of the Sr(NO3)2-C-dots before calcination was
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performed in a nitrogen environment. The resulting material, SrO-C-dots, was further
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analyzed by TGA in an air atmosphere to evaluate the temperature decomposition of 12 ACS Paragon Plus Environment
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the C-dots. As indicated in Figure 2, in the case of the Sr(NO3)2-C-dots, the first
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weight loss (~4 wt.%) occurs between 200-325 °C due to dehydration and loss of
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water. The second and major wt. loss (48.5 wt. %) occurs between 650-660 °C,
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corresponding to the decomposition of the Sr(NO3)2 to SrO. 32 Pure Sr(NO3)2 melts at
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571 °C and decomposes at 658 °C.33 Our observation of the conversion of Sr(NO3)2 to
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SrO is consistent with the literature.68 Moreover, the calcination temperature of 650
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°C is reported as being the most suitable temperature for preparing solid-base
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catalysts from Sr(NO3)2, as a result of its thermal property.32,34 The temperature must
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be high enough to decompose Sr(NO3)2 to the active oxide species, yet not high
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enough to hinder the sintering of the catalyst particles. Therefore, a calcination
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temperature of 650 °C was subsequently used for the decomposition of the Sr(NO3)2-
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C-dots to produce the SrO-C-dot catalyst. The TGA of the SrO-C-dot composite in air
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indicates a weight loss of 7% at ~ 450 °C. The weight loss is attributed to the
300
decomposition of the C-dots or the loss of OH, CO and CO2 from the surface
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functional groups of the C-dots. TGA analysis also confirms that the remaining 93 wt.
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% is SrO and 7% is C-dots. Moreover, the plateau achieved at a temperature higher
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than 500 °C indicates the thermal stability of the SrO material. According to the
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decomposition formula of Sr(NO3)2 to SrO, the expected weight loss is 48.96 wt. %,
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which is very close to the observed weight loss (48.5 wt. %).
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3.1.2. XRD analysis of SrO-C-dot composition and determination of calcination
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time
308
The crystal structure of the SrO-C-dot composite was analyzed using XRD. A
309
calcination time of 2-3 h at 650 °C in a N2 environment cannot completely convert all
310
the Sr(NO3)2-C-dots to SrO-C-dots (Figure 3). Intense signals of a strontianite
311
(SrCO3) (JCPDF file no. 84-1778) with orthorhombic phase belonging to a Pmcn 13 ACS Paragon Plus Environment
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space group were identified (2θ = 25.3°, 26.0°, 37.3°, 44.5°, 46.8° and 48.0°) in
313
addition to the SrO phase. The formation of strontianite is probably due to the reaction
314
of the Sr(NO3)2 with the C-dots at elevated temperatures, even in an inert atmosphere,
315
due to the high reactivity of the carbon dots. The SrCO3 peaks are observed in the
316
samples that were calcined for a period of over 3 h. However, the intensity of the
317
peaks typical of Sr(CO3)2 decrease with calcination time. At the same time, the
318
intensity of the signals corresponding to oxide phase increased. A pure SrO phase is
319
observed in the material after calcination for 4 h at 650 °C (JCPDF file no. 06-0520;
320
cubic structure with a lattice constant value of 5.16 Å). The diffraction peaks at the 2θ
321
values of 30.0°, 34.8°, 50.2°, 59.4° and 62.4° respectively, correspond to the (111),
322
(200), (220), (311) and (222) planes of SrO. Signals typical of carbon structure were
323
not observed at ~25 and ~45° due to the much smaller amount of carbon dots in the
324
SrO-C-dot composite. Although, from the TGA analysis, the amount of carbon dots is
325
~7 wt. %, the carbon dots comprise of elemental carbon, oxygen and hydrogen
326
existing as surface functional groups like the carboxyl and phenolic groups.
327
Therefore, the actual amount of elemental carbon is even less than 7 wt. %. This may
328
be the reason for the absence of carbon signals in the XRD analysis (Figure 3). The
329
FT-IR spectrum of the SrO-C-dots (Figure S2) shows a broad and strong absorption
330
band at 592 cm-1, with a shoulder at 620 cm-1 attributed to the Sr–O stretching
331
vibration. The observed peak at 592 cm-1 attests to the formation of metal-oxide (Sr–
332
O) bonds.52,70,35 For comparison, commercial SrO was analyzed using FT-IR
333
spectroscope and a significant attenuation of typical absorption bands is observed,
334
probably due to the micron-size particles of commercial SrO that absorbs far less than
335
the nanometric SrO powder in the SrO-C-dot composite.
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3.1.3. Surface morphology and composition of SrO-C-dot composite 14 ACS Paragon Plus Environment
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The surface morphology of the SrO-C composite was deciphered from HR-SEM
338
analysis and the chemical composition of the composite was determined from EDS
339
analysis. The SEM images, the EDS spectrum and the elemental mapping recorded on
340
the SrO-C-dot composite are shown in Figure 4. Functionalization of SrO with C-
341
dots generates a high density of irregular small particles with a rough surface. In
342
contrast, commercial SrO comprises large dense particles, exposing smooth planes or
343
regularly facetted steps with micron-sized particles.36 The results of the SEM analysis
344
pertaining to the particle size and morphology of the SrO-C-dot composite is
345
consistent with the BET sorptometry results. The specific surface area of the SrO-C-
346
dot composite is 3.5 m2g-1, whereas that of commercial SrO is 0.5 m2g-1. BET specific
347
surface area value of the SrO-C-dot composite is 7 times greater than that of
348
commercial SrO.49 The particle size distribution of the catalyst was also evaluated by
349
HRSEM and the average particle size is 20-30 nm. Elemental mapping of the SrO-C-
350
dot composite shows uniform distribution of Sr, O and elemental carbon on the
351
surface. This indicates the complete decomposition of the Sr(NO)3-C-dot to SrO-C-
352
dot composite as well as the homogeneous surface functionalization of SrO with
353
carbon dots. The elemental composition shown by the EDS analysis is considered
354
only for qualitative purpose and not as a quantitative estimate of carbon owing to the
355
fact that the analysis is carried out by spreading the sample on a carbon tape placed on
356
a Cu stub.
357
3.1.4. TEM and DLS analysis of SrO-C-dots
358
The TEM image of SrO NPs surrounded by carbon dots displays a porous (Figure 5).
359
The average particle size of carbon dots on the SrO surface is ~ 7 nm. The average
360
particle size of the SrO is also measured in an isopropyl alcohol suspension by DLS
361
(Figure S3). The suspension was formed using a bath sonicator. The results of the 15 ACS Paragon Plus Environment
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362
DLS analysis show that the size of the SrO particles in the composite is ~ 200 nm.
363
The particle size measured by DLS is about 10 times greater than that measured by
364
HRSEM. The larger particle size values obtained by DLS measurement are due to
365
particle aggregation.
366
3.1.5. Raman analysis
367
To confirm the presence of C-dots on the surface of SrO in the SrO-C-dot composite,
368
Raman analysis was performed. Figure S4 shows the appearance of two prominent
369
peaks at 1,380 and 1,549 cm-1, corresponding to the D- and G-bands respectively. The
370
peak pattern is characteristic of the formation of carbon dots.37 Moreover, in the case
371
of the SrO-C-dot composite, the signal intensity of the graphic band increases
372
compared to the D-band, which is attributed to the increase in the crystallinity of the
373
carbon structure formed at a high temperature calcination (650 °C for 4 h in an N2
374
atmosphere), during which a variety of reactions such as dehydration, cyclization and
375
condensation of the surface functional groups of carbon dots can take place.
376
3.2.
377
transesterification of lipids of Chlorella vulgaris
378
The catalytic activity of the SrO-C-dot composite during the transesterification
379
reaction is tested. A one-step direct transesterification reaction is carried out by
380
irradiating the mixture of methanol and chloroform (1:2 v/v) and Chlorella vulgaris in
381
the presence of SrO-C-dot catalyst in a domestic microwave oven at various
382
irradiation times (2 to 5 min). The effect of the microwave irradiation time on the
383
conversion of the lipids and the yield of FAME is depicted in Figure 6. A lipid
384
conversion value of 97 wt. % and a corresponding FAME yield of 45.5 wt. %,
385
calculated gravimetrically, were achieved in a short reaction time of 2.5 min. The
Evaluation
of
the
catalytic
activity
of
16 ACS Paragon Plus Environment
SrO-C-dots
for
the
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values of conversion of lipids and yield of FAME are achieved with a modest amount
387
of catalyst (0.3 g) comprising 93 wt. % SrO (0.279 g) and 7 wt. % (0.021 g) C-dots.
388
For comparison, commercial micrometer-sized SrO particles were also used as
389
the catalyst for the direct transesterification of Chlorella vulgaris under identical
390
reaction conditions. A plot of the variation in the conversion of the lipid and yield of
391
FAME using a commercial SrO catalyst is depicted in Figure 7. With the commercial
392
SrO catalyst, an irradiation time of 6 min is required to reach similar conversion and
393
yield values obtained using the SrO-C-dot composite catalyst in 2.5 minutes. Thus,
394
the transesterification is accelerated by approximately 2.4 times using the SrO-C-dot
395
catalyst, indicating its improved performance over the conventional SrO catalyst. The
396
improved activity of the SrO-C-dot composite for the acceleration of the
397
transesterification of lipid content is attributed to the unique structural features of the
398
C-dots. C-dots are extremely hydrophilic owing to their extensive surface
399
functionalization. Surface functionalities such as carboxylic and phenolic groups are
400
acidic in nature. These moieties play a crucial role in the esterification of free fatty
401
acids (FFA) in the algal biomass. Thus, the synergistic role of the acidic sites of the
402
C-dots as well as the highly basic sites of SrO result in a 2.5-fold acceleration in the
403
transesterification reaction compared to pure SrO. Another factor that is responsible
404
for the enhanced activity of the SrO-C-dot composite is the surface area value, which
405
is 7 times higher than that of commercial SrO.
406
A typical 1H NMR spectrum of the FAME product obtained from Chlorella
407
vulgaris using the SrO-C composite upon irradiation for 2.5 min is shown in Figure
408
8. The yield of FAME is estimated gravimetrically by calculating its weight relative to
409
the weight of the dry microalgae biomass. The conversion percentage of oil to FAME
410
is calculated by integrating the 1H NMR peaks as shown in Figure 8 using Eq. 1. 17 ACS Paragon Plus Environment
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411
When SrO-C-dot composite is used as catalyst optimal conversion is achieved after
412
irradiation for 2.5 min resulting in an oil weight percentage of 45.5%, which
413
corresponds to the lipid conversion value of 97 wt. %, as deduced from the 1H NMR
414
spectrum (Figure 8). Conversion values greater than 97 wt. % could not be achieved
415
even after irradiation for a longer time (Figure 6). This is probably due to the
416
presence of FFA in the microalgae.38
417
Besides the characteristic peak at 3.6 ppm, which arises from the methyl-ester,
418
and the triplet at 2.3 ppm related to α-carbonyl methylene, which are characteristic
419
peaks confirming the presence of methyl esters in FAME, the 1H NMR spectrum
420
shows a strong peak at 1.226 ppm arising from methylene protons of the carbon chain
421
of esters and a triplet at 0.851 ppm is related to terminal methyl-hydrogen protons.
422
The other peak, which appears at 1.576 ppm, is attributed to β-carbonyl methylene
423
protons.39 The absence of a signal between 4.05-4.37 ppm in the 1H NMR spectrum is
424
attributed to the absence of CH2 groups (–CH2–O–COR) of triglycerides, indicating
425
high yield of FAME and high catalytic activity of the SrO-C-dots.25 Thus, a novel
426
composite of SrO and C-dots has been designed and tested for the accelerated
427
production of FAME in a single step from Chlorella vulgaris.
428
Designing green and sustainable methods for extraction is a vital research
429
topic in multidisciplinary areas. Recently, Crampon et al., reported effective
430
extraction of neutral lipids from the microalgae Spirulina platensis using super critical
431
CO2.40 Chemat et al., introduced the concept of six principles of green-extraction that
432
could be applied to industrial process for intensification and energy saving.41 From
433
the perspective of reduced energy consumption,26 safe and high quality extraction of
434
lipids from the algae using microwave irradiation as well as ultrasonic waves. Even
435
though, microalgae is a versatile feedstock for biofuel production due to the tenability 18 ACS Paragon Plus Environment
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Energy & Fuels
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of chemical composition (for instance obtaining high lipid content algae), higher
437
productivity and photosynthetic efficiency,42-45 for the biodiesel production process to
438
be sustainable and competitive several challenges like resource constraints (CO2,
439
water, nutrients), development of innovative large scale production technology require
440
immense research efforts and remains a challenge.46-50
441
4. CONCLUSION
442
A novel SrO-C-dot catalyst is prepared from Sr(NO3)2 and PEG400 as precursors for
443
Sr and C, respectively, under sonochemical conditions. Homogeneous surface
444
functionalization of SrO nanoparticles with carbon dots is achieved using the
445
synthetic strategy developed based on sonication and subsequent calcination in an
446
inert atmosphere at 650 °C for 4 h. The developed catalyst is used for the single-step
447
conversion of Chlorella vulgaris to FAME in a microwave oven. A modest lipid
448
conversion value of 97 wt. % and a FAME yield of 45.5 wt. % are achieved using the
449
SrO-C-dot composite catalyst in a short reaction time of 2.5 min. The SrO-C-dot
450
catalyst shows improved performance for the transesterification of lipids compared to
451
the conventional SrO catalyst. The enhanced catalytic activity (2.4-fold) of the SrO-C-
452
dot composite is attributed to the unique structural features of carbon-dots with a
453
variety of oxygen, carboxylic and phenolic functionalities that promote the
454
esterification of FFAs in the algal biomass. Moreover, the higher surface area of SrO-
455
C-dots compared to commercial SrO is another factor that enhances the catalytic
456
activity with an increased number of accessible active reactant sites (lipids and
457
MeOH). Thus, a SrO-C-dot composite catalyst is designed in an innovative way with
458
very high potential for the fast and enhanced production of FAME from Chlorella
459
vulgaris. This new methodology can be used for developing a CaO/BaO/MgO-C-dot
460
composite exclusively for biodiesel production. 19 ACS Paragon Plus Environment
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461
ASSOCIATED CONTENT
462
Supporting Information.
463
Image of the modified microwave oven (Figure S1), FT-IR spectra of SrO-C-dots and
464
commercial SrO catalysts (Figure S2), dynamic light scattering (DLS) trace of SrO-C
465
dot composite (Figure S3) and Raman spectra of C-dots and SrO-C-dot composite
466
(Figure S4) are provided in the supporting information. This material is available free
467
of charge via the internet at http://pubs.acs.org.
468
AUTHOR INFORMATION
469
Corresponding Author
470
*Phone: 972-3-5318315; Fax: 972-3-7384053.
471
E-mail:
[email protected].
472
Notes
473
The authors declare no competing financial interest.
474
Author Contributions
475
# Alex Tangy and Vijay Bhooshan Kumar have contributed equally for this work.
476
ACKNOWLEDGEMENTS
477
Gedanken thanks the Israeli Ministry of Science, Technology and Space for the
478
research grant (206712) supporting this work. Grateful thanks are due for the India-
479
Israel cooperative scientific research grant (203768) supporting this research.
480
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LIST OF FIGURES:
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Figure 1. Schematic overview of the sonochemical synthesis of the Sr(NO3)2-C-dot composite.
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Figure 2. TGA curve of Sr(NO3)2-C-dots in N2 atmosphere and of the SrO-C-dot composite in air after calcination of Sr(NO3)2-C-dots at 650 °C for 4 h in an N2 environment at a constant heating rate of 10 °C/min.
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Figure 3. Effect of calcination time on the conversion of Sr(NO3)2-C-dots to SrO-Cdots (T = 650 °C; t = 1 – 4 h).
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Figure 4. HR-SEM images (a and b), EDS spectrum (c) and elemental mapping (d) of the SrO-C-dot composite.
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Figure 5. TEM of SrO-C-dot composite
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Figure 6. Effect of microwave irradiation time on FAME yield and conversion of lipids of Chlorella vulgaris to FAME using SrO-C-dot composite as catalyst.
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Figure 7. Effect of microwave irradiation time on FAME yield and conversion of lipids of Chlorella vulgaris to FAME using a commercial SrO catalyst.
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Figure 8. 1H NMR spectrum of FAME obtained from the direct transesterification of Chlorella vulgaris by microwave irradiation for 2.5 min using the SrO-C-dot composite catalyst.
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