Converting Lignocellulosic Pentosan Derived Yeast Single Cell Oil

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Converting Lignocellulosic Pentosan Derived Yeast Single Cell Oil into Aromatics: Biomass to Bio-BTX Omvir Singh, Tripti Sharma, Indrajit Ghosh, Diptarka Dasgupta, Bhanu Prasad Vempatapu, Saugata Hazra, Alexander L Kustov, Bipul Sarkar, and Debashish Ghosh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02851 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Sustainable Chemistry & Engineering

Converting Lignocellulosic Pentosan Derived Yeast Single Cell Oil into Aromatics: Biomass to Bio-BTX Omvir Singh,a,b‡ Tripti Sharma,b,c‡ Indrajit Ghosh,a Diptarka Dasgupta,c Bhanu Prasad Vempatapu,d Saugata Hazra,e Alexander L. Kustov,f Bipul Sarkar,*a,b and Debashish Ghosh,*c a Chemical

& Material Sciences Division, CSIR-Indian Institute of Petroleum, Haridwar Road,

Dehradun, Uttarakhand-248 005, India b

Academy of Scientific and Innovative Research(AcSIR), CSIR-HRDC Campus, Joggers

Roadm K amla Nehru Nagar, Ghaziabad-201 002, India c

Materials Resource Efficiency Division, CSIR-Indian Institute of Petroleum, Haridwar Road,

Dehradun, Uttarakhand- 248 005, India d

Analytical Sciences Division, CSIR-Indian Institute of Petroleum, Haridwar Road, Dehradun,

Uttarakhand – 248 005, India e

Department of Biotechnology, Indian Institute of Technology-Roorkee, Haridwar Highway,

Roorkee, Uttarakhand – 247667, India f

N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Ave,

Moscow-119991, Russian Federation * Email: [email protected]; [email protected]

1 ACS Paragon Plus Environment

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KEYWORDS: sugarcane bagasse; yeast single cell oil; biomass to chemicals; aromatization; Ga-Al2O3 catalyst

ABSTRACT

The ever increasing demand for sustainable energy and chemicals in association with the declining reserve for fossil fuel, are stimulating the search for alternative feedstocks and processes. In line, the catalytic upgradation of non-edible oil’s into high-yield commodity chemicals, including saturates and aromatics, has been studied. Yeast single cell oil (SCO) was converted into benzene, toluene, xylenes (BTX) over a Ga-(γ)alumina catalyst, at 450°C. The yield of aromatics found to improve with the incorporation of Ga. The physiochemical properties of the catalyst were characterized using different analytical techniques and reaction parameters were optimized using design-of-experiment (DOE). It was observed, that the gallium promotes the dehydrogenation activity abd results overall increase in aromatic yield. Maximum 89.4% conversion of SOC, with aromatic selectivity up to 77.7% was achieved.

INTRODUCTION The massive consumption of fossil fuels and associated environmental issues are leading to an increased interest in biofuels and green petrochemicals.1 The aromatics mainly BTX (benzene, toluene and xylene), earlier produced from diminishing fossil crudes, is being generated from renewable resources, such as tree borne oil or microbial single cell oil. Thus, the production of these so-called aromatics in a sustainable manner, which can lead to the reduction of greenhouse gas emissions.2-3 Oleaginous yeasts accumulate lipids within its cell 2 ACS Paragon Plus Environment

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by more than 20% w/w regarding its dry cell mass. Such lipids, often referred to as single cell oil (SCO) or yeast lipid, finds compositional similarities against tree borne oil and can be envisaged as green feedstock for oleochemicals or petrochemicals. In order to attained materials resource efficiency, cultivation of yeast lipid using lignocellulosic biomass-derived sugars using fermentation platform finds promising advantages over plant-derived fatty oil or even algal oil for bio-BTX production. Biobased BTX may also provide the primary feedstocks for the petrochemical industry. The product stream can be integrated into the existing refinery stream for high-end chemicals and also be blended with aliphatic hydrocarbons to produce commercial grade gasoline.4 Heterogeneous catalysts are essential for upgrading fatty oil into transportation fuels as well as renewable aromatics.5 The upgraded hydrocarbons or other intermediates can be mixed for the production of BTX or “green” gasoline, diesel, and other industrial chemicals.6 In general, the upgrading process removes the oxygenated compounds from fatty oil (glycerides) in terms of H2O, CO, and CO2, while at the same time altering its chemical structures to reduce its molecular weight.7 Thermo-catalytic cracking is done in the presence of an acidic catalyst (mainly zeolite based), modified with transition metals via formation of carbocations. Fatty oil (long chain glyceride) upgrading involves multiple reaction steps like hydro-deoxygenation, decarbonylation, decarboxylation, hydrocracking, or hydrogenation etc. Each of the individual components in oil may play a specific role, during upgradation in a onepot reaction. The competitive reactions and impurities present in the fatty oil also deactivate the catalyst.8 The processing efficiency of fatty oil upgradation relies heavily upon the activity, selectivity, and energy efficiency of the type of catalyst used.9 Thus, multifunctional catalysts can be used to produce light hydrocarbon or aromatics. H-ZSM-5 (MFI) is vastly used for the aromatization of naphtha, n-alkane, e.g. propane, ethane to high BTX.10-12 The high activity of H-ZSM-5 catalyst in the aromatization is attributed to its high acid strength and suitable pore 3 ACS Paragon Plus Environment


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geometry.13 There are few reports on Ni-modified ZSM-5 or SiO2 for the production of aromatics by crude bio-oil (derived through pyrolysis).14-15 The depolymerization of lignin, hemicellulose and cellulose fraction of the biomass requires enhanced thermal steps and pressure. Additionally, the choice of H-ZSM-5 catalyst is unfeasible for feeds with high density and average molecular size because of the size restriction of the feed molecules to the pore, which may cause rapid deactivation and plugging of the catalyst bed. In this context, Al2O3 is an excellent choice because of its high surface area and acidic strength. However, the low dehydrogenation ability of Al2O3 will restrict the overall aromatics yield. Therefore, the introduction of Ga can modify the surface and improves the dehydrogenation capability.16 The gallyl ions (GaO+) known to favour the dehydrogenation step, hence improvise the aromatics yield and provide reasonably stability to the catalyst.17 The Objective of this experimental study was to find an alternative route to produce bio-BTX from pentosan derived yeast SCO. Lignocellulosic biomass is constituted of three major fractions, namely hemicellulose, cellulose and lignin. Deconstruction of biomass into fermentable monomeric sugars (chiefly, pentosans from hemicellulose and hexosans from cellulose), and their further processing through microbial fermentation, is well established. However, use of hexose fraction for yeast fermentation targeting 2G-ethanol, is already at the verge of commercialization, pentose mediated yeast fermentation, and especially biomassderived pentosan’s conversion is not established at large scale. In this contribution, aromatization of yeast SCO over Al2O3 and Ga-Al2O3 catalysts has been studied at 450°C under atmospheric pressure. Reaction condition was optimized using DOE, and 89.4% conversion with 77.7% aromatic selectivity was attained. MATERIALS AND METHODS


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Information related to lignocellulosic biomass, oleaginous yeast cultivation, lipid production, recovery and characterization techniques etc. has been detailed in the supporting information. Synthesis of catalyst The catalysts were prepared via wet impregnation technique. Typically, required amounts of metal salts i.e. Ga(NO3)3 and Co(No3)3.6H2O) was taken in 60ml deionized water and kept for homogenization. In continuation, 5g of γ-alumina was added and the resulting solution was stirred at 70°C until dry. The obtained material was then calcined at 450°C in air for 6 h. Reactor setup and operating procedure: The aromatization of SCO was performed in a continuous downflow fixed bed 8 mm quartz reactor. Typically, the reactor temperature was varied between 300-550°C and the process temperature was measured by the K-type thermocouple in contact with the catalyst bed. Typically, 1.0 g of catalyst was loaded in between the silicon carbide bed into the reactor. Pre-treatment of the catalyst was carried at 500°C using 20ml/min He flow. Liquid hourlyspace velocity (LHSV) of SCO was kept between 0.5-2.5 h-1 and introduced via a syringe pump. The water phase was collected and extracted using ethyl acetate followed by analyzed using the GC-DHA. The conversion and selectivity were calculated considering the data obtained from both aqueous and organic phase formed during the aromatization reaction The FAME analysis was done with Scion Instrument-435 gas chromatography with a biscyanopropyl polysiloxane column (100 m, 0.25 mm, 0.2 μm). A Bruker gas chromatograph (456-GC), equipped with an Rt-QPLOT column connected to a flame Ionization detector (FID) was used for quantification of hydrocarbons (DHA) with a Carboxen 1000 packed Column (60/80, 15 ft × 1/8 in. × 2.1 mm SS, Supelco). The conversion, selectivity and the yield of C6-C8 aromatics is defined as



𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑜𝑓 𝑆𝐶𝑂 𝑜𝑖𝑙 (%) = =

∑𝑦𝑡𝑛𝑡 ― ∑𝑦𝑡𝑝𝑛𝑡𝑝 ∑𝑦𝑡𝑛𝑡

𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝐶6 ―𝐶8 aromatics(%) =

𝑌𝑖𝑒𝑙𝑑 𝑜𝑓 𝐶6 ―𝐶8 aromatics(%) =

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(1)

× 100

∑𝑦𝐶

𝑛 6 ―𝐶8 aromatics 𝑡 ∑𝑦𝑝𝑛𝑝

× 100

(2)

𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 × 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 100

(3)

Where yt, ytp and yp are the weight fraction of triglycerides in feed, unconverted triglycerides in the product and all products obtained respectively, while nt, ntp and np represent the related carbon numbers. RESULTS AND DISCUSSION Yeast cultivation, lipid production, recovery and characterization The oleaginous yeast (RMIIPL32) was cultivated via two-stage fermentation process. Oil accumulation within the yeast cell was accentuated by depleting nitrogen in the fermentation system. Hence, initially with carbon limiting condition (under a moderately low carbon and nitrogen molar ratio; C/N ~ 19), and high aeration (0.75 vvm), yeast cell biomass was generated. Presence of excess nitrogen within the growth medium helped in protein as well as nucleotide synthesis, and thus in cell division, which in turn enhanced cell biomass formation.18 In order to induce lipid accumulation within the cell, the culture was fed with excess carbon (by adding concentrated sugar hydrolysate and in turn starved with nitrogen) under low aerated condition (0.25 vvm). The increased molar ratio of carbon and nitrogen > 40 channelized the carbon flux into lipogenesis and induced lipid accumulation within the cells. Yeast lipid generated under single stage fermentation, with applied nitrogen stress from the initial stage, reported to show high lipid quantity initially concerning cell dry mass. However, the carbon uptake level significantly decreased and resulted in lower quantitative lipid yield with respect to sugar conversion.19 Yeasts, unlike algae, being a heterotrophic organism, the quantitative 6 ACS Paragon Plus Environment

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lipid yield was calculated with respect to converted sugar, whereas in microalgae, being autotrophic organism and atmospheric CO2 being sole carbon source, the lipid quantity was measured with respect to dry cell mass.20 Heterotrophic yeasts surpassed autotrophic algae in terms of lipid productivity and titer, where yeasts bore more advantages like controlled and steady cultivation and oil production irrespective of climate and country, less prone to contamination, less land requirement, high scalability, short life cycle etc.21 Thus, in the present study, yeast lipid was produced through fed-batch fermentation process.22 In the first phase, 8.15 g/L yeast biomass was generated under the high aerated condition with a balanced C/N ratio of ~19; where sugarcane biomass pre-treated xylose-rich stream (19.61 g/L) was used as carbon source. Lipid accumulation phase was triggered by feeding concentrated sugarcane bagasse pre-treated stream to the fermentation medium and SCO yield and titer of 0.161 g/g (xylose based) and 3.71 g/L (non-polar lipid fraction) were achieved respectively under low aerated condition. The fermentation batch of 36 h (after yeast biomass generation time of 24 h) yielded 48.3% w/w lipid on dry yeast cell biomass basis, given in Figure 1. The lipid yield and titre of RMIIPL32 from sugarcane bagasse pre-treated broth were comparable with modified Yarrowia lipolytica polg3023, and Lipomyces starkeyi DSM 70296,24 and signified the successful production strategy to achieve high lipid generation, with fermentation time.



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Figure 1. Cultivation and lipid production by Rhodotorula mucilaginosa IIPL32 yeast on xylose stream derived from acid hydrolyzed sugarcane bagasse. It shows changes in cell biomass (), total lipid (), and residual sugar ()as a function of fermentation time. Recovered lipid was characterized for fatty acid composition as per AOAC 996.06 and resulted in 12.98% saturated fatty acid (SFA; 0.35% C14, 9.75% C16, 2.06% C18, 0.82% C24); 74.35 % mono unsaturated fatty acid (MUFA; 0.91% C16:1, 0.35% 72.14% C18:1, 0.48% C20:1) and 9.25% poly unsaturated fatty acid (PUFA; 8.92 % C18:2, 0.33% C18:3) by weight. The elemental composition (CHON) of the SCO was found as C-71.06%, H-7.011%, O-19.36% and N-2.36% respectively. Catalyst Characterization


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Figure 2. (a) XRD data of pure γ-alumina, fresh and spent catalyst and adsorption-desorption isotherm of (b) pure γ-alumina, (c) 10Ga-Al2O3 and (d) spent 10Ga-Al2O3. [Inset- pore size distribution carve respectively] The powder XRD patterns for γ-alumina and Ga supported γ-alumina catalyst was shown in Figure 2a. The major peaks at 13.9, 28.5, 38.3, 49.3, 55.2 and 64.7° corresponded to (111), (113), (024), (422), (225) and (440) crystal planes [JCPDS card No. 10-0425] of pure γalumina. In Ga supported catalyst (10Ga-Al2O3), the additional diffraction peaks at 2θ of 23.7, 27.7, 38.2, 45.8, 52.5, and 59.4° corresponded to (201), (113), (310), (312), (601), and (603) planes [JCPDS No.41-1103] for β-Ga2O3. Whereas, (310), (312), (601), (603) crystal planes indicated the presence of metallic gallium in FCC (JCPDS No. 41-1103) type unit cell structure. The XRD pattern for spent catalyst showed a loss in peak intensity, which might be due to the deposition of carbonaceous materials over the catalyst surface. Table 1. Physiochemical Properties of the Ga modified γ-Alumina Catalysts.



Catalyst

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Pore volume Total acidity[a] 3 (cm /g)

SBET

Pore size

(m2/g)

(nm)

γ-Al2O3

257.82

130.84

0.439

0.32

10Ga-Al2O3

276.16

118.14

0.464

0.61

10Ga-Al2O3[c]

178.21

102.09

0.259

-

[a] Total acidity (in mmol/g) was measured via NH3-TPD, [b] spent catalyst; recovered after 3rd h time on stream

Adsorption-desorption isotherm and pore size distribution were shown in Figure 2b-d. N2 sorption isotherm showed type-IV isotherm curve with a hysteresis loop H1 at relative pressure in the range of 0.50-0.92. BET results (in Table 1) depicted a slight increase in the surface area from 257 to 276 m2/g. The pore size distribution for the Ga-Al2O3 catalysts reflected the mesopores at a diameter between 60 to 70 Å (Table 1) along with a pore volume between 0.0.439 to 0.464 cc g-1. The spent catalyst showed a decrease in surface area and was found to be 178 m2/g for the catalyst collected after the 3rd h on time on stream. The SEM images of γalumina and Ga-(γ)alumina catalysts (Figure S1a-c) were more of sediment layers of fine and large alumina particles, settle out simultaneously.25 The morphology of the used catalyst (in Figure S1d-f) revealed no change even after 5th h of continuous operation. The elemental composition of the fresh and spent catalyst could be observed by elemental mapping and energy dispersive X-ray (EDX) analysis (Figure S1-2). The HR-TEM described polycrystalline porous γ-alumina with a narrow range of crystallite size distribution within 10-20nm (Figure 3, b). The high-resolution images of 10Ga-Al2O3 (Figure 3, c-d) showed the (111) crystallite planes of γAl2O3 with a d spacing of 4.6Å. Gallium oxide (Ga2O3) also marked with exposed (201) plane with a d spacing at about 3.6Å. The catalyst reduced with 10%H2 balance He, (Figure 3, e-f) shows a d spacing of 2.8Å, which corresponds to the presence of metallic Ga. The HR-TEM image of fresh catalyst indicates the presence of Ga2O3 with particle size 10-12 nm. The


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morphology and the size of gallium oxide do not alter even after regeneration of the 10GaAl2O3 catalyst.

Figure 3. HR-TEM images of (a-b) pure Al2O3; (c-d) 10Ga-Al2O3; (e-f) 10Ga-Al2O3reduced; (gh) 10Ga-Al2O3spent and related SEAD images of (i) Al2O3 and (j) 10Ga-Al2O3. The total acidity of the catalysts was measured by NH3-desorption experiments. The NH3TPD data shows moderate acidity of alumina support (Figure 4a) at a Tmax around 199°C. In 10% Ga loaded catalyst, the desorption peak observed to shift a bit, toward higher temperature at 213°C. From the amount of desorbed ammonia, the support Al2O3 shows total acidity of 0.332 mmol/g (in Table 1). Addition of Ga on γ-Al2O3 (10Ga-Al2O3) shows an almost twofold increase in total acidity (0.611 mmol/g), which can also be attributed from the peak intensity in Figure 4a. The nature of the acidic sites present in the catalysts was distinguished using pyridine-IR and shows in Figure S3. The IR bands at 1445 cm-1 correspond to the coordinated pyridine species with Lewis acid sites. While the bands at 1552 cm-1 can be



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designates as pyridinium ions associated with the Brønsted acid sites. Except for Brønsted and Lewis acid sites, the IR-band related to both B+L sites was also noticed at 1490 cm-1. X-ray photoelectron spectroscopy (XPS) analysis showed that the material was a mixture of gallium oxides (Figure 5a). All the data were normalised taking C 2p at 284.8 eV. Curve fitting was done by measuring the bare Al2O3 support and the parameters obtained were used to fit the Ga species. The Ga 3d3/2 spectra indicated the presence of Ga in mixed oxidation state (Ga3+ and Ga2+). However, a chemical shift at +20.9eV and +18.7eV from the bulk Ga peak was observed, the same type of shift was also observed by Mukherjee et al.26 The two peaks at binding energies 22 and 17 eV, attributes to Ga2+ and Ga3+ (Ga2O3) respectively.27 The O 2s XP spectrum (Figure 5c) at 22.0 eV, and 23.7 eV for Ga-Al-O and Ga-O-Al respectively, attributed to the formation of mix bonding at the interface.28

Figure 4. (a) NH3-TPD of γ-alumina and 10Ga-Al2O3; (b) H2-TPR of 10Ga-Al2O3 (fresh catalyst) and 10Ga-Al2O3spent (spent catalyst; recovered after 3rd time on stream). The H2-TPR of γ-alumina and gallium supported catalyst were shown in Figure 4a. 10GaAl2O3 catalyst showed three reduction peaks, spanning from 350°C to 570°C. The 1st peak at Tmax 382°C was shouldered to the 2nd peak at Tmax 462°C, and the 3rd peak was spotted at Tmax 564°C. The peaks at 382°C and 462°C corresponded to the reduction of Ga3+→Ga2+→Ga°. We 12 ACS Paragon Plus Environment

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believed the relatively low intense 1st peak was because of free Ga species on the catalyst surface. In the spent catalyst, both the peaks shifted toward lower value, which could be due to the weak interaction between Ga-Al2O3 in the spent catalyst. The same was experienced by Chi et al. for Ni-ZeO2 catalyst.29

27Al

NMR spectra of Al2O3 and Ga-Al2O3 catalysts are shown in

Figure 5d. Characteristic asymmetric peaks ascribed to Al atoms in the octahedral (AlO6) and tetrahedral (AlO4) environments could be observed at chemical shifts of 11 and 70 ppm, respectively. Chemical shift corresponded to AlO4 and AlO6 of pure γ-alumina and 10Ga-Al2O3 showed an integration ratio of 1:5 and 1:3.5. That could either indicate the decrease in Brønsted acids side (BAS) or increase in Lewis acid side (LAS). As the NH3-TPD confirmed the increase in total acidity (Table-1), hence it should be the increase in LAS by the addition of Ga and which was also experienced by Petre et al.30 An isotropic chemical shifts form Al octahedrally to tetrahedrally coordinated by oxygen was observed for the spent 10Ga-Al2O3 catalyst. The framework octahedral alumina is converted to tetrahedral alumina after the reaction.31 Most probably, the unsaturated Al centres are located on or near the surface of the particles being expected to show structural disorder. The phenomenon can be explained as, Ga doping on γ-alumina interact with the tetrahedral AlO4 signal, and replace the hydroxyl located at AlO5 and AlO6 sites.32 This was due to the Ga3+ attract the electronegative AlO6 sites. This caused the integration ratio for the chemical shifts corresponded to AlO4 and AlO6 of the spent catalyst (6:1) to reverse.



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Figure 5. XP spectra of (a) Ga 3d; (b) Al 2p (c) O 1s and (d) 27Al MAS NMR spectra of γalumina and different Ga-Al2O3 catalyst. Note- (spent catalyst; recovered after 3rd h time-onstream). Figure S4-S5 depicted the differential weight loss as a function of time and percentage of weight loss as a function of the temperature of the catalysts. All the catalysts including pure γalumina and Ga supported γ-alumina were stable up to 800°C. The pre-calcined support (i.e. commercial γ-alumina) shows ~4.05% weight loss, which may be due to the moisture gained during the storage of the material. For, Ga-(γ)alumina catalyst, the first endothermic weight loss at 90-150°C is related to the removal of physically adsorbed water. While the second weight loss at above 150-400°C is attributed to dehydroxylation process and urea decomposition (used in the catalyst preparation process). The broad peak after 400°C with a very consistent loss in weight in TGA curve may be attributed to crystallization of transition alumina and or decomposition of residual organics and further continuation of dehydroxylation process.33 For spent Ga-(γ)alumina catalyst, two weight loss steps are observed. The first step 14 ACS Paragon Plus Environment

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started at 90-250°C followed by consecutive steps in the range 250–600°C. The letter exothermic peak came from the release of chemisorbed carbon loss, which could be better observed by DTG peak. An overall weight loss of 22.1% was observed in the temperature range of ambient to 800°C. Table 2. Detail hydrocarbon analysis of aromatisation of Lipid oil over various γ-alumina catalysts. Entr y no

Catalyst

Total Total Total Product selectivity Convers aromatic (C6-C8) Benzene Toluene ompEthylion s xylene xylene xylene benzene (%) (%) 54.1 40.1 35.9 20.2 2.3 0.2 8.6 1.0 3.6

1

5Co-MFI

2

10Co-MFI

56.7

41.7

41.0

21.5

3.9

0.2

9.8

1.4

4.2

3

5Ga-Al2O3

74.9

73.7

71.9

5.0

66.0

0.2

0.2

0.3

0.2

4

5Ga74.2 74.1 5.4 64.0 1.2 1.7 1.1 0.7 82.3 [a] Al2O3 5 10Ga86.1 76.1 72.6 3.4 62.0 1.3 0.9 4.1 0.9 Al2O3 6 10Ga89.4 77.7 74.7 4.0 62.3 1.2 1.5 4.7 1.0 Al2O3[a] 7 10Ga84.1 75.7 67.8 3.5 58.0 1.1 1.2 3.5 0.5 [b] Al2O3 Reaction condition: catalyst wt.-1 g (pelletized); reaction temp.-450°C; LHSV-2 (h-1); GHSV-=20 h-1; time-after 3h , [a] preheating of yeast SCO before reaction; [b] data obtained after 2nd cycle.

Note: The product stream contains saturates (cyclic, Iso and normal), di-or tri-aromatics and little amount of unsaturates apart from the total aromatics.

Aromatization using γ-alumina as a support The dehydrogenation of yeast SCO has performed over gallium-(γ)alumina catalyst in a fixed bed down-flow reactor in the range of 300-450°C and at atmosphere helium pressure of 20 h-1 space velocity (GHSV). Metal supported MFI (H-ZSM-5) shows average conversion with insignificant yield aromatics (Table 2, entry 1-2). Bare H-ZSM-5 and γ-alumina show significant cracking rather than aromatization and end up with more gaseous product than 15 ACS Paragon Plus Environment


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liquid yield. Though the selectivity to C6-C8 improves with the loading of cobalt (Table 2, entry 1-2) but high benzene contains makes it less feasible for blending. Ga supported γ-alumina showed an increase in SCO conversion, along with improved yield of total aromatic (Table 2, entry 4-7). The maximum yield of aromatic was obtained up to 77.7% at 450°C with 89% SCO conversion as shown in Table 2. The yield of aromatics increased with increase in the gallium metal concentration, due to an increase in acidity (as shown in Table 2) of the catalyst. The yield of aromatics was also found to increase with the use of pre-heater. The pre-heater was optimized at 120°C, before the reaction. The overall liquid product yield obtained over 10GaAl2O3 is ~88.1%. The total gas produced during the aromatization reaction was calculated by a gas meter (MilliGascounters, Ritter GmbH) and analyzed by GC-RGA (refinery gas analyzer). The gaseous product obtained during the reaction mainly consists of C1-C5 alkane and olefins with a sizable quantity of hydrogen, CO and CO2. Quantified GC-RGA result of the gases is tabulated in Table S1. Optimization of reaction parameters


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Figure 6. Effect of reaction temperature (up) and liquid hour space velocity (LHSV in h-1) in respect to conversion of lipid oil and selectivity of total aromatics. Reaction condition: catalyst wt.-1 g (pelletized); reaction temp.- 400-550°C; LHSV-0.5-2 (h-1); GHSV-1200 h-1; time-after 3h. The conversion of oil started after 250°C, and the majority of the product was observed to be di- and higher aromatics. This could be due to the low intense cracking, which resulted in the higher mono and di-olefins. Above 350°C, the relative abundance of mono-aromatics started to increase while the conversion reached up to 70.9% with 70.2% total aromatics selectivity (Figure 6). The maximum yield of BTX was 74.7% (in Table 2 and Figure 6) was achieved at 450°C with 89.4% oil conversion. Although the conversion goes near 95% at 550°C, the overall liquid yield goes down and gaseous product appears dominantly. Therefore, looking at the BTX yield as major concern, 450°C was selected as optimum temperature for all the reactions. The helium flow used for constant reactor flux had no effect within the range of 1200-1800 h-1 and therefore, it was omitted during the rest of the reactions and kept at 1200 h-1 GHSV. While the liquid hour space velocity (i.e. LHSV) was taken at an increment of 0.5 h-1 and studied from 0.5 to 2 h-1. The conversion towards BTX tend to decrease with the feed rate, (Figure 6) but the liquid yield increased continuously. This was because of the higher degree of cracking at a low flow rate. We experienced a reasonable conversion of SCO with the high liquid yield at 2 h-1. Recyclability of the catalyst was also tested by giving hexane wash of the used catalyst (to remove the absorbed yeast SCO) and drying in a standard oven at 100°C. The pre-dried catalyst then regenerated using air at 750°C and used as it was. In the first recycle, we experienced a slight decrease in conversion. This could be due to the loss of active surface area to a certain, resulting from the prolonged exposure to high gas-phase temperatures and



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regeneration in high a temperature.34 The activity and liquid yield then remained constant up to 2 recycle. Mechanistic pathway The product distribution results indicated the role of Brønsted acid for catalytic cracking followed by dehydrogenation for yeast SCO conversion over Lewis acid site. At 1st the long chain triglyceride got cracked to give olefin, which undergoes into olefin inter-conversion over the Lewis acid site.35 From the NH3-TPD and Py-IR analysis, it could be concluded that the surface of unmodified γ-alumina catalyst existed in a majority of Lewis acid sites with a small amount of Brønsted acid sites. Incorporation of Ga enriched the amount of the Lewis acid site and along with Al that could crack the long chain triglyceride to formed carbonium ions. The carbonium ion formed as an intermediate36 gave raise in mono & di-olefins via β-scission reaction. Olefins and di-olefins got dimerized and then skeletal isomerized in on the B+L acid sites.37-38 In general, aromatics might be formed via two possible paths (a) oligomerization followed by cyclization of olefins, and (b) cycloaddition reaction of alkene and conjugated diene that was known as Diels-alder reaction to formed aromatic. As no cycloalkanes were detected in the product stream which could justify the formation of BTX via Diels-alder reaction, hence we assumed the formation of yeast SCO into aromatics followed the oligomerization and cyclization path. It is well known that Ga-(γ)alumina catalyst enriches with Lewis acid sites and have strong hydro-dehydrogenation activity.39 They catalyse the dehydrogenation/ oligomerization of intermediates formed via cracking followed by cyclization of those cyclic intermediates into the corresponding aromatics. 40 The results indicate that for improving the yields of aromatics, it is inevitable to increase the participation of the dehydrogenation reactions as well as in the formation of the corresponding aromatics from cyclic intermediates. 18 ACS Paragon Plus Environment

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CONCLUSION In conclusion, this work depicted the feasibility of Ga incorporated γ-alumina could be used for the upgradation of yeast SCO for the production of aromatics/ BTX. The presented work could be summarized as follows: (a) Microbial oil cultivated from lignocellulosic biomass derived pentosans mediated yeast fermentation, could be used as an alternative feedstock for the production of bio-BTX. (b) Catalyst modified by Ga metal had higher conversion and selectivity as compared to pure γ-alumina and H-ZSM-5. Incorporation of gallium increased the amount of Lewis acidic site and promoted the aromatization reaction via cycloaddition and dehydrogenation of yeast SCO (largely containing long chain triglyceride). (c) The incorporated gallyl ions (GaO+) increased the degree of dehydrogenation, which indirectly favoured the formation of aromatic hydrocarbons. The results indicate that for improving the yields of aromatics, it is inevitable to increase the participation of the dehydrogenation reactions as well as in the formation of the corresponding aromatics from cyclic intermediates. (d) Optimized conversion was achieved at 450°C under atmospheric pressure. Maximum conversion of SCO up to 89% with 77.7% aromatic selectivity was obtained at 2 h-1 LHSV. ASSOCIATED CONTENT Supporting Information. Information related to lignocellulosic biomass, oleaginous yeast cultivation, lipid production, recovery and characterization techniques etc. has been detailed in the supporting information.



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AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Author Contributions ‡These authors contributed equally. ACKNOWLEDGMENT O.S acknowledges DST, India and T. S acknowledges GAP-3509 and GAP-3512 for this study. AcSIR and CSIR-IIP are specially acknowledged for providing all the facilities and PhD registration. The authors acknowledge DIIP for his support. Analytical Sciences Division, CSIR-Indian Institute of Petroleum is especially acknowledging for analytical services. REFERENCES 1.

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Graphical Abstract

Synopsis


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Yeast single cell oil (SCO) was converted into benzene, toluene, xylenes over a Ga-alumina catalyst, at 450°C. Gallium found to promote the dehydrogenation activity, which result an overall increase in aromatic yield.



141x59mm (150 x 150 DPI)

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Converting Lignocellulosic Pentosan Derived Yeast Single Cell Oil

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