Adsorptive selectivity and mechanism of three different adsorbents for

Subscriber access provided by UNIV OF TEXAS DALLAS

Applied Chemistry

Adsorptive selectivity and mechanism of three different adsorbents for nitrogenous compounds removal from microalgae bio-oil Fanghua Li, Lynn Katz, and Siyao Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04934 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Adsorptive selectivity and mechanism of three different adsorbents for nitrogenous compounds removal from microalgae bio-oil Fanghua Lia*, Lynn Katzb*, Siyao Qiuc aDepartment

of chemical engineering, Monash University, Wellington Rd, VIC 3800, Australia.

E-mail: [email protected] bProfessor,

Department of Civil, Architectural and Environmental Engineering,

The University of Texas at Austin, 301 E Dean Keeton Street Stop C1786, Austin, TX 78712-1173. E-mail: [email protected] cSchool

of Chemistry, Faculty of Science, Monash University, Clayton, Victoria 3800, Australia

ABSTRACT Microalgae bio-oil is of interest as a potential alternative to traditional fossil-fuel based transportation fuels. However, the presence of nitrogenous compounds in the bio-oil can lead to the release of NOx (a polluting gas) during combustion, and is detrimental to the environment. Thus, it is necessary to selectively remove these compounds from the bio-oil prior to the biooil being used as a transportation fuel. This study investigates the adsorptive capacity, selectivity and mechanism of three adsorbents, Cu-zeolite Y, silica gel and activated carbon, for the removal of nitrogen-containing compounds from bio-oils derived from pyrolysis of microalga Tetraselmis suecica. Since Cu cations preferentially adsorb nitrogen over oxygen, the affinity of Cu-zeolite Y for nitrogen could allow denitrogenation of microalgae bio-oils. Moreover, when Cu-zeolite Y was used for selective denitrogenation, the presence of the compounds containing aromatic rings such as benzocyclobutene and phenol would not substitute indole or hexadecanamide. The adsorptive selectivity of the silica gel depends on the acid-base interaction. Hydrogen bond interaction and acid-base interaction play an important role in adsorptive denitrogenation over the activated carbon. This study found that activated carbon is a promising adsorbent for the removal of nitrogenous compounds from microalgae bio-oil.

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

Keywords: Adsorption; Denitrogenation; Microalgae bio-oil; Selectivity; Mechanism 1. Introduction Tetraselmis suecica is a marine microalga strain used for biofuel production, especially as a feedstock for pyrolysis [1, 2]. The bio-oils produced from Tetraselmis suecica are known to have high stability, high heating value and relatively low oxygen-containing compounds [1]. However, bio-oils contain nitrogenous compounds that are converted into NOx during combustion, causing negative environmental impacts such as acid rain and the poisoning of the catalysts typically employed in thermochemical catalytic converters to reduce SOx and CO [3, 4, 5]. Moreover, the presence of fatty acids and carboxylic acids in the bio-oil can result in the bio-oil being corrosive [6]. Consequently, it is necessary to remove nitrogen-containing and acidic compounds from the microalgae bio-oil produced from Tetraselmis suecica. The removal of nitrogenous compounds from petroleum-derived oils can be achieved by either catalytic hydrodenitrogenation (HDN) or adsorptive denitrogenation (ADN). Previous studies have shown that HDN can produce energy-dense hydrocarbons with high heating values. However, the need for hydrogen at high pressure and temperature is a drawback of the process. In the case of algal oil, the presence of higher concentrations of nitrogen and the strength of C-N bonds in microalgae bio-oils [3] suggest that the application of the HDN process for removing nitrogen would be cost and energy-intensive thus defeating the purpose of producing oils from renewable microalgae resources [7]. To overcome some of these disadvantages, ADN has attracted more attention as an alternative method for the removal of nitrogenous compounds in recent years [8, 9]. ADN involves selective adsorption of nitrogenous compounds onto active adsorbent sites, thus separating them from the remaining oil. The adsorptive process can be performed at room temperature and atmospheric pressure, and in many cases, regeneration of the adsorbents can be achieved through solvent washing or thermal treatment [3]. Adsorbents used for ADN


Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


include metal-exchanged zeolites, silica gel, activated carbon and silica-alumina [10]. The previous studies on nitrogen removal from commercial diesel oil and transportation fuels using Cu-zeolite Y [11, 12, 13], suggest that 99.8% of the nitrogen can be removed at ambient temperature and pressure [12]. Recent reports highlight the use of silica gel as an adsorbent in denitrogenation at room temperature for light cycle oil (97.5% N removed) [14], gasoline (98.22% basic N removed) [15], diesel (99.7% N extracted) [16], SK SRGO (82% N removed) [17] and coker gas oil (86% N removed) [17]. Activated carbon has been used in a wide variety of applications on the removal of organic and inorganic chemicals for deodorization, decolorization, purification of drinking water and treatment of wastewater due to its high surface area, large pore volume and affinity for a range of contaminants. Activated carbon shows high adsorptive capacity and selectivity for the removal of nitrogenous compounds from a model diesel fuel [5] and also shows a positive effect on the separation of nitrogenous compounds from light cycle oils and shale oils [18]. In summary, each of these adsorbents has demonstrated the potential for the removal of nitrogenous compounds from microalgae bio-oil. However, many of the studies cited focused on adsorptive denitrogenation of model fuels or a single compound and did not evaluate adsorption within a complex background containing hydrocarbons, oxygenous compounds and nitrogenous compounds. The quantitative analysis that compares adsorptive selectivity of nitrogenous oil components across a range of adsorbents and in the presence of various hydrocarbons and oxygen and nitrogen-containing compounds is needed to provide clarity with respect to both the potential of ADN as well as providing insight into possible adsorptive mechanisms for each adsorbent. Comprehensive development of ADN processes involved in the direct use or modification of commercially available adsorbents with high capacity and selectivity, thus it is necessary to investigate the relative adsorptive selectivity and the adsorptive mechanisms of the adsorbents for the removal of nitrogenous compounds. In this



Page 4 of 31

paper, the removal of nitrogen-containing compounds from microalgae bio-oil which contains the oxygenous compounds of hexadecanoic acid and phenol, nitrogenous compounds of hexadecanamide and indole, and hydrocarbons of benzocyclobutene and heneicosane was studied over three typical adsorbents of Cu-zeolite Y, activated silica gel and activated carbon in a fixed-bed adsorptive reactor at ambient temperature and pressure. The adsorptive capacity and selectivity were studied and compared on the basis of the breakthrough and saturation curves over the adsorbents.

The correlations between the electronic properties of the

compounds and the experimental results from the adsorptive capacity and selectivity over the adsorbents were investigated for a better understanding of the adsorptive mechanism. 2. Materials and methods 2.1. Materials NaY zeolite and silica gel were obtained from Zeolyst International, USA. The activated carbon was a catalytic and adsorptive carbon, provided by the Centaur-Calgon Carbon Corporation, USA. NaY zeolite was calcined in the furnace at 550°C overnight. Silica gel was activated at 150°C for 16 h prior to use. 2.2. Adsorbent preparation Cu supported on NaY zeolite (Si/Al=2.43) was prepared using the ion exchange (IE) method. The preparation of Cu-zeolite Y was as follows. A 0.01 mol/L aqueous solution of CuCl2 was used for the ion exchange of the NaY molecular sieve. The mixed solution (3.2 g NaY molecular sieve to 500 mL CuCl2 solution) was refluxed at 80°C with continuous stirring for 48 h. Reduced Cu(II)-zeolite Y was prepared at 450°C in pure helium to convert Cu2+ to Cu+, which is Cu(I)-zeolite Y and is of interest for π-complexation [11]. The catalyst solution was filtered using vacuum-assisted filtration to remove supernatant and then dried at 100°C for 24 h. The metal-exchanged catalyst was crushed and sieved into small particles (150-350 µm), and then calcined at 500°C for 5 h.


Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Characterization of adsorbents Adsorbents were characterized using surface area and porosity analyzer (3FLEX 3500) to determine the BET surface area, pore volume and pore size. About 120 mg of sample was loaded into the 3FLEX sample tube and degassed at 150°C for 8 h. The tube was then transferred to analyse sample surface area, pore volume and pore size at the analysis port at a liquid N2 temperature of -196°C. The physical properties of the three adsorbents are listed in Table 1. From the table, it is evident that the activated carbon has the largest surface area and pore volume. Besides, the silica gel has the largest pore size. Table 1 Physical properties of the adsorbents Adsorbent

Surface area

Pore size

Pore volume(cm3/g)

Particle size

(m2/g)

(nm)

Cu-zeolite Y

425

1.75

0.37

150-350

Silica gel

401

2.42

0.68

350

Activated carbon

1718

2.12

1.35

350

(µm)

2.4. Bio-oil sample In this study, the emphasis has been placed on the analysis of the dominant nitrogenous compounds of hexadacamide and indole, oxygenous compounds of hexadecanoic acid and phenol, and hydrocarbons of benzocyclobutene and heneicosane. The detailed information about these compounds in microalgae bio-oil is listed in Table 2 which shows the dominant compounds in microalgae bio-oil for this study. The mass concentration was calculated according to the standard curve of each compound via Gas chromatography-mass spectrometry. The molar concentration of each compound was calculated based on the following equations. Molar concentration (µmol/mL) = Mass concentration (µg/mL) / Molar weight (µg/µmol) 5 ACS Paragon Plus Environment


Page 6 of 31

Concentration N (ppmw) = Molar concentration (µmol/mL) × Molar weight N (µg/µmol) Table 2 The concentration of each compound in Tetraselmis suecica bio-oil Chemicals

Concentration N (µg/mL˖bio-oil)

Molar concentration (µmol/mL˖bio-oil)

Nitrogenous compounds Hexadecanamide

217.1

15.50

Indole

233.8

16.70

Total

450.9

32.20

Oxygenous compounds Hexadecanoic acid

21.10

Phenol

27.53

Hydrocarbons Benzocyclobutene

25.00

Heneicosane

3.700

2.5. Fixed-bed adsorption The schematic diagram of the experimental column setup is shown in supplementary data. An HPLC pump was used to provide a constant liquid flow rate of 0.5 mL/min. Adsorbents were packed into the column of 80 mm length and 10 mm diameter. Empty bed contact time was 10 min. The influent feed containing microalgae bio-oil was pumped through a fixed-bed reactor containing the selected adsorbent media. Syringe valves located at the influent and effluent of the column allowed sample collection. The column, tubes, and fittings were composed entirely of stainless steel (Nalge Nunc International, Rochester, New York) to avoid potential losses due to sorption by the column and appurtenances. Before each experimental run, the column assembly was dismantled, and all components were washed by distilled water and dried in an oven before reassembly. After the columns were packed, the entire system was flushed with ethyl acetate before an experiment was initiated. For each run, 1.0 g adsorbent was packed in the column. The two ends of the column were filled with glass 6 ACS Paragon Plus Environment

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wool fibres and porous frits to establish the flow path the column. The first microalgae bio-oil sample was taken from the outlet after 10 min. Subsequently, the bio-oil was continually sampled every 1-5 min until the saturation point was reached. The adsorptive capacity of each adsorbent was calculated based on the following equations. Amount of treated bio-oil = Volume flow rate of bio-oil (0.5 mL/min) × Time (min) Adsorptive capacity = Amount of treated bio-oil (mL-B/g-A) × Molar concentration (µmol/ mL) ×10-3 mmol/µmol Adsorptive capacity N = Amount of treated bio-oil (mL-B/g-A) × Concentration N (µg/mL ˑbio-oil) ×10-3 mg/µg 2.6. Characterization of bio-oil Both the quantitative and qualitative analysis (MassHunter Workstation Software) of the bio-oil was performed using a 5977A Gas chromatography-mass spectrometry (Agilent Technologies Inc) equipped with HP-5 capillary column (30m×0.25mm×0.25mm). The carrier gas was helium. The injector temperature was 250°C with an injector split ratio of 20:1. The temperature program was set as follows. A constant temperature of 40°C was kept for 4 min, then the temperature increased to 200°C at a heating rate of 10°C/min and held there for 5 min, and finally the temperature increased to 300°C at a heating rate of 50°C/min and held there for 3 min. Compounds corresponding to each peak in the bio-oil chromatography were identified by using the NIST 98 mass spectrometry database. Each compound (hexadecanamide, indole, hexadecanoic acid, phenol, benzocyclobutene and heneicosane, respectively) had its own quantitative method; standard curves were developed for quantitative analysis based on the concentration corresponding to the responses. The total nitrogen was identified by AntekMODEL 9000 Nitrogen Analyzer.



Page 8 of 31

2.7. Computational details of the semi-empirical quantum chemical method The geometries of the molecules have been optimized by molecular dynamics (MD) and molecular mechanics (MM2) through Chem3D software. The geometries and properties were further calculated by using a semi-empirical method via PM6 [19]. These calculations were carried out by Gaussian 09 (Revision A.02) [20]. 3. Results and discussion The purpose of this investigation was to examine the removal of nitrogen-containing compounds from microalgae bio-oils that can cause environmental problems, as well as fuel instability problems that can degrade fuels and affect engine performance. Bio-oils were treated with different adsorbents, Cu-zeolite Y, silica gel and activated carbon and the adsorptive performance was examined. As shown in Figures 1, 2 and 3, the area between the line Ct/Co=0 (breakthrough curve) and before the line C/Co=1 (saturation curve) constitutes the number of the adsorbed compounds, and the area between the line C/Co=1 and after the line C /Co=1 is regarded as the number of the substituted compounds. The adsorptive capacity of each compound on the three adsorbents was calculated and listed in Table 3. The breakthrough curves are discussed in the following parts to compare the adsorptive capacities of heneicosane (HEN), benzocyclobutene (BEN), phenol, hexadecanoic acid (HAA), indole and hexadecanamide (HAM) over three adsorbents of Cu-zeolite Y, silica gel and activated carbon, respectively. Additionally, the adsorptive selectivities are calculated based on the data of the adsorptive capacities, and the results are listed in Table 4. 3.1. Adsorptive performance of the three adsorbents The breakthrough curves of HEN, BEN, phenol, HAA, indole and HAM over Cuzeolite Y at 5.0 h-1 LHSV are presented in Figure 1. The initial two breakthrough molecules


Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were HEN and BEN, with approximately the same volume amount of the bio-oil inlet, 1.5 mL per gram of the adsorbent (mL-B/g-A). After the breakthrough, the C/Co value for the two hydrocarbons increased rapidly to over 1.0. While the conditions for the two oxygen-containing compounds were different. HAA was the third breakthrough compound with the amount of bio-oil of 3.0 mL-B/g-A. Phenol subsequently broke through with the amount of bio-oil of 5.0 mL-B/g-A, and the breakthrough amount was approximately 1.7 times higher than that of HAA. The amount of the bio-oil correlating with the saturation point was 4.0 and 9.0 mL-B/g-A for HAA and phenol, respectively. After the saturation point, the C/Co value for HAA rose rapidly to C/Co=1.4, while the C/Co value for phenol increased gradually to 1.16. Following these two oxygen-containing compounds, the breakthrough curves of Indole and HAM can be seen at a treated bio-oil amount of 10.0 and 15.0 mL-B/g-A, respectively. It is clear that Cu-zeolite Y had higher adsorptive capacity towards these two nitrogen-containing compounds.

Figure 1. Adsorptive performance of hydrocarbons, oxygenous and nitrogenous compounds over Cu-zeolite Y

On the basis of the breakthrough order, the adsorptive selectivity for the six adsorbates increases in the order of HEN < BEN < HAA < phenol < indole < HAM. The breakthrough and saturation capacities for each compound are calculated and the results are listed in Table



Page 10 of 31

3. In this study, a selectivity factor was defined based on the breakthrough point to quantitatively discuss the relative adsorptive selectivity, which is expressed as: ai-m=Capi/Capm

(1)

Capi : the adsorptive capacity of compound ‘i’; Capm : the adsorptive capacity of BEN as the reference compound; Since the three adsorbents showed quite low adsorptive capacities for HEN and the concentration of HEN in microalgae bio-oil was also considerably low, HEN was neglected for the calculation of the selectivity factor. It should be noted that the breakthrough capacities were used rather than the equilibrium capacities in Eq. (1), therefore the selectivity factor was for breakthrough selectivity instead of the equilibrium selectivity. The calculated relative selectivity factors are presented in Table 4. The ai-m values are 1.0, 1.69, 3.67, 4.45 and 6.20 for BEN, HAA, phenol, indole and HAM, respectively. Table 3 Adsorptive capacities (mmol/g) of the three adsorbents Adsorbate Adsorbent

Hexadecanami

Indole

Hexadecanoic

de

Phenol

Benzocyclobutene

Henelcosane

acid

Cu-zeolite Y Breakthrough

0.2325

0.167

0.0633

0.1377

0.0375

0.00555

Saturation

0.3100

0.2672

0.0844

0.2478

0.050

0.00740

Breakthrough

0.3255

0.0501

0.3587

0.0826

0.0375

0.00555

Saturation

0.4340

0.0835

0.4642

0.1377

0.075

0.0111

Breakthrough

0.7595

0.334

0.7596

0.3575

0.1

0.0185

Saturation

0.8680

0.4175

0.8651

0.5506

0.175

0.0333

Silica gel

Activated carbon


Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4 Selectivity factors (ai-m) for each compound Selectivitya

Hexadecanamide

Indole

Hexadecanoic

Phenol

Benzocyclobutene

acid Cu-zeolite Y Breakthrough

6.20

4.45

1.69

3.67

1

Saturation

6.20

5.34

1.69

4.96

1

Breakthrough

8.68

1.34

9.57

2.20

1

Saturation

5.79

1.11

6.19

1.84

1

Breakthrough

7.60

3.34

7.60

3.58

1

Saturation

4.96

2.39

4.94

3.15

1

Silica gel

Activated carbon

aThe

relative selectivity factor was used as defined by Eq. (1)

The adsorptive performance of the six compounds over the silica gel at 5.0 h-1 LHSV are presented in Figure 2. It can be seen that there are mainly three categories corresponding to three pairs of compounds which showed the similar breakthrough trend. They are two hydrocarbons of BEN and HEN, two aromatic heterocyclic organic compounds of phenol and indole and the third group of HAA and its derivative HAM. Both BEN and HEN initially appeared at the outlet at a bio-oil inlet amount of 1.5 mL-B/g-A similar to the amount for Cuzeolite Y. Subsequently, the C/Co ratio for the two hydrocarbons increased rapidly to about 1.4, and finally, arrived back to 1.0 at the saturation point of phenol and indole. Phenol and indole broke through with approximately the same amount of the bio-oil inlet (3.0 mL-B/g-A). After the breakthrough point, the C/Co ratio for both molecules increased to 1.1, and subsequently remained at this ratio until HAA appeared at the outlet. The C/Co ratio for the two molecules then dropped to 1.0 while the HAA reached the saturation point. HAA appeared at an amount of bio-oil inlet of 17.0 mL-B/g-A. After the breakthrough point, the C/Co ratio for indole and phenol increased to 1.16 and subsequently returned to 1.0 when HAM reached



Page 12 of 31

the saturation point. The final breakthrough compound was HAM with a breakthrough amount of 21.0 mL-B/g-A, and the treated bio-oil amount based on the end point was 28.0 mL-B/g-A. The adsorptive capacities of the six molecules over the silica gel showed an increase in order of HEN < BEN < indole < phenol < HAM < HAA. The adsorptive selectivity for the six molecules over the silica gel showed an increase following the order of HEN< BEN < indole < phenol < HAM < HAA. The relative selectivity factors (ai-m) are 1.0, 1.34, 2.20, 8.68 and 9.57 for BEN, indole, phenol, HAM and HAA, respectively.

Figure 2. Adsorptive performance of hydrocarbons, oxygenous and nitrogenous compounds over silica gel.

The adsorptive performance over activated carbon at 5.0 h-1 LHSV is presented in Figure 3. It can be seen that the adsorptive performance of the six compounds over activated carbon is quite different from Cu-zeolite Y and silica gel. BEN appeared at a treated bio-oil amount of 4.0 mL-B/g-A. After the breakthrough point, the C/Co value raised rapidly to more than 1.4, and gradually returned to 1.0 at the bio-oil inlet amount of 17.5 mL-B/g-A. HEN was observed at a treated bio-oil amount of 5.0 mL-B/g-A, and subsequently, the C/Co value increased rapidly to more than 1.3. Phenol molecules appeared in the outlet when the treated bio-oil amount was 13.0 mL-B/g-A. Subsequently, the C/Co value for phenol increased rapidly to around 1.4 and subsequently decreased to 1.0 until HAA broke through. HAA broke through


Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at a treated bio-oil amount of 36.0 mL-B/g-A, and subsequently increased rapidly to 1.0 and remained at this value. The C/Co value of the two oxygenous molecules dropped to 1.0 when activated carbon was saturated by indole. Indole and HAM initially appeared when the bio-oil inlet amount was 20.0 and 49.0 mL-B/g-A, respectively, and the saturation amount of the treated bio-oil was 25.0 and 56.0 mL-B/g-A, respectively. The adsorptive capacities of the major compounds in microalgae bio-oil over activated carbon followed the order of HEN < BEN < indole < phenol < HAM ≈ HAA. The relative selectivity factors (ai-m) are 1.0, 3.34, 3.58, 7.60 and 7.60 for BEN, indole, phenol, HAM and HAA, respectively.

Figure 3. Adsorptive performance of hydrocarbons, oxygenous and nitrogenous compounds over activated carbon.

The breakthrough curves for the total nitrogen removal over different adsorbents at 5.0 h-1 LHSV are presented in Figure 4. Nitrogenous compounds initially appeared at the treated bio-oil amount of 15.0, 16.0 and 42.0 mL-B/g-A for the silica gel, Cu-zeolite Y and the activated carbon, with an adsorptive capacity of 0.38, 0.40 and 1.09 mmol-N/g-A or 5.32, 5.60 and 15.26 mg-N/g-A, respectively (Table 5). The saturation adsorptive capacity was 0.52, 0.58 and 1.29 mmol-N/g-A (or 7.28, 8.12 and 18.06 mg-N/g-A), respectively. The adsorptive capacity of the total nitrogen removal followed the order of the silica gel < Cu-zeolite Y < activated carbon. 13 ACS Paragon Plus Environment


Page 14 of 31

Figure 4. Adsorptive performance for the total nitrogen over three different adsorbents Table 5 Adsorptive capacities for the total nitrogen Adsorbent

Based on weight

Based on the surface area

Total N mmol/g

Total N mg∙N/g

µmol/m2

Cu-zeolite Y Breakthrough

0.40

5.60

0.940

Saturation

0.58

8.12

1.363

Breakthrough

0.38

5.32

0.947

Saturation

0.52

7.28

1.296

Breakthrough

1.09

15.26

0.634

Saturation

1.29

18.06

0.751

Silica gel

Activated carbon

The adsorptive performance of an adsorbent is generally dependent on both the physical properties such as surface area, pore volume and pore size and the surface chemical properties such as the number of active sites and their density [21, 22]. In this study, it was found that the adsorptive capacities based on the weight of the adsorbents were in the order of Cu-zeolite Y


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


< silica gel < activated carbon for the total nitrogen, suggesting that activated carbon is more favorable adsorbent of the three adsorbents. Activated carbon presented an adsorptive capacity of around three times higher than that of Cu-zeolite Y and silica gel. By contract in terms of the adsorptive capacity based on per square meter of the adsorbents, silica gel and Cu-zeolite Y have relatively higher adsorptive capacities than that of the activated carbon. This indicates that from the perspective of the surface chemical property, silica gel and Cu-zeolite Y are the more advantageous adsorbents than activated carbon. In addition, it suggests that activated carbon has the highest adsorptive capacity partly because of its large surface area which is 4.0 times higher than that of the Cu-zeolite Y, and 4.3 times higher than that of the silica gel. This explanation is supported by the findings of carbon adsorptive denitrogenation of real gas oil where it was found that the performance of the activated carbon on the removal of nitrogenous compounds is determined by the large surface area and the presence of surface oxygen functional groups [23]. In addition, the high performance of activated carbon can be attributed to its wide-open microstructure coupled with the dominant presence of the low size mesopores [23]. Furthermore, in the study of Jiang et al. on carbon adsorption of dibenzothiophene from fuel oils, the researchers indicate that mesopore volume played an important role in the process of dibenzothiophene removal [24]. The adsorptive selectivities of the three adsorbents for the hexadecanamide, indole, hexadecanoic acid, phenol and benzocyclobutene are given in Table 4. As can be seen, the selectivity factors ai-m of HAM are 3.76, 5.26 and 4.61 for Cu-zeolite Y, silica gel and activated carbon, respectively, and the selectivity factors ai-m of indole are 4.45, 1.34 and 3.34 for Cuzeolite Y, silica gel and activated carbon, respectively. It is clear that Cu-zeolite Y has the highest selectivity for removing indole, and silica gel has the highest selectivity for removing HAM. The activated carbon has a moderate selectivity for both HAM and indole. For the oxygenous compounds, silica gel shows the highest adsorptive selectivity for HAA (ai-m=9.57)



Page 16 of 31

and Cu-zeolite Y shows the highest adsorptive selectivity for phenol (ai-m=4.25). Additionally, the activated carbon presents about 0.2 times lower adsorptive selectivity for HAA (ai-m= 7.60) than silica gel and about 0.03 times lower adsorptive selectivity for phenol (ai-m= 4.15) than the Cu-zeolite Y. Since the adsorbent used for denitrogenation needs to have a high affinity towards nitrogen-containing compounds and a low affinity towards other compounds in microalgae bio-oil, therefore, regarding both adsorptive capacity and selectivity for nitrogencontaining compounds, activated carbon is of interest as an adsorbent for the removal of the two dominant nitrogen-containing compounds (i.e. HAM and indole) existing in microalgae bio-oil. In summary, related to the experiments carried out in the fixed-bed column, the results of this study showed that the three selected adsorbents were effective in removing the nitrogencontaining compounds and their use can result in fuels that exhibited excellent storage stability. Thus, these simple adsorption methods can be independent of the refining process and do result in an environmentally cleaner burning fuel [16]. It was evident that the activated carbon exhibited better performance than the adsorbents of Cu-zeolite Y and silica gel. In this work, the activated carbon achieved the highest total N-adsorption capacity at the established experimental conditions. However, it is necessary to develop new adsorbents with a higher Nadsorption capacity in order to enhance adsorption of indole and make the breakthrough time longer for a commercial application of this adsorption process [25]. 3.2. Adsorptive mechanism over three different adsorbents It is evident that adsorptive selectivities of the three adsorbents for the six compounds are different. This difference indicates that the adsorption of the nitrogenous, oxygenous compounds and hydrocarbons on different adsorbents may follow different adsorptive mechanisms. For a deep understanding of the fundamental adsorptive mechanisms, it is


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


necessary to combine the adsorptive selectivity and the electronic properties of the six compounds. Some electronic properties such as bond order, dipole magnitude, ionization potential and partial atomic charge of the HAM, indole, HAA, phenol, HEN and BEN were calculated using the semi-empirical method, and the results are shown in Table 6. As can be seen that: (1) The highest bond order of the compounds followed the order of HEN < HAA < HAM < phenol < indole < BEN. (2) The dipole of the compounds followed the order of HEN < BEN < phenol < indole < HAA < HAM. (3) The ionization potential followed the order of indole < HAM < BEN < phenol < HEN < HAA. (4) The number of N atoms plus C (sp2) atoms in the compounds followed the order of HEN < HAA < HAM < phenol < BEN < indole.


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 18 of 31

Table 6 The electronic properties of the adsorbates Adsorbates

Ionization (eV)

Mulliken

Number of N + C

charge on

(sp2)

The highest bond order

Dipole magnitude (D)

Molecular Size (pm)

Na (a.u.)

horizontal

Nitrogenous compounds Hexadecanamide

7.77

-1.1166

2

1.670

4.134

1727

Indole

8.36

-0.5545

9

2.207

1.945

627

Oxygenous compounds Hexadecanoic acid

10.25

1

1.630

1.9848

1686

Phenol

8.36

6

2.104

1.3584

572

Benzocyclobutene

8.03

8

2.318

0.1542

615

Heneicosane

9.76

0

1.544

0.0935

2447

Hydrocarbons

Note: aNet atomic charge Number of C (sp2): According to the double-bond numbers in the optimized structure

All the data collected in the table are according to a semi-empirical calculation by the software Gaussian (PM6) [20].


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


BEN

HAM HAA

Indole

Phenol

Note: neutral regions in green, electronegative areas in orange-red, and the electropositive areas in blue Figure 5. The distribution of the electrostatic potential in the examined molecules



Page 20 of 31

The electrostatic potential on the electron density with colour boundaries for the compounds of HAM, indole, HAA, phenol and BEN are shown in Figure 5. HEN has no electrostatic potential map due to its non-polarity. It is evident that the negative electrostatic potential is predominantly settled on the middle of the molecular planes and the side near O atom, and the negative electrostatic potential follows the order of HEN (-7.19) < HAA (-6.74) < HAM (-6.66) < indole (-5.57) < BEN (-5.21) < phenol (-4.26). Phenol has the highest electrostatic potential in all assessed adsorbates, and the negative domain is distributed on the O atom and the middle of the molecular plane (benzene ring). It is also evident that the positive electrostatic potential is predominantly settled on the sides of the molecular planes and the side near H (–OH) or N (–NH) atom and the value of the positive electrostatic potential follows the order of HEN (-3.95) < HAM (-3.84) < indole (-2.77) < BEN (-2.65) < HAA (2.12) < phenol (8.22). Phenol also has the highest positive electrostatic potential in all assessed adsorbates, and such positive domain is located near H atom of the –OH group and the side of the molecular plane. 3.2.1. Adsorption mechanism over Cu-zeolite Y As for the adsorption on Cu-zeolite Y, the adsorptive selectivity follows the order of HEN < BEN (1.0) < HAA (1.69) < phenol (3.67) < indole (4.45) < HAM (6.20), as presented in Table 4. This selectivity order has a good agreement with the highest bond order (HEN < HAA < HAM < phenol < indole < BEN) and the number of N + C (sp2) (HEN< HAA < HAM < phenol < BEN < indole) without regard to BEN and HAM. This indicates that there is a close relationship between this selectivity order and the observed molecular properties of the highest bond order and the number of C (sp2)+N, suggesting that the molecular properties may influence the adsorptive mechanisms of Cu-zeolite Y. The first two components were hydrocarbons which appear when the amount of the treated bio-oil per gram of adsorbent (mL-B/g-A) reached 1.5 mL. The results suggest that Cu-


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


zeolite Y has a very low affinity for hydrocarbon adsorption. HAA initially appeared with the bio-oil amount of 3.0 mL-B/g-A. While the next was phenol at 5.0 mL-B/g-A. Unlike hydrocarbons and HAA, whose concentration rapidly reached C/Co=1, phenol composition gradually increased, suggesting Cu-zeolite Y has a residual affinity to adsorb phenol. However, nitrogen-containing compounds indole and HAM were observed only after 10.0 and 15.0 mLB/g-A, respectively, suggesting that Cu-zeolite Y has a very strong adsorptive affinity towards nitrogen-containing compounds. This can be attributed to the fact that N can form bonds or complexes with Cu by donating the long pair of electrons to the vacant of orbital Cu. The molecular sizes for BEN (615 pm), indole (627 pm) and phenol (572 pm) are similar as shown in Table 6. However, the adsorptive selectivities are quite different, indicating that the adsorptive capacity for these compounds can be determined by the number of active pore sites or surface functional groups of Cu-zeolite Y adsorbent. The adsorptive capacity of Cu-zeolite Y (1750 pm) for BEN (615 pm) and HEN (2447 pm) is limited by its pore size or pore volume, working as a filter. Since the electron-rich areas of HAM and HAA are in the zones of nitrogen atoms and hydroxyl (–OH), as presented in Figure 5, it is rational to postulate that a direct attraction between the N atoms or hydroxyl and the surface copper on Cu-zeolite Y may play a crucial role in the selectivity of nitrogenous compounds. The data-based results support further that the adsorptive mechanism involves direct interaction between the nitrogen atoms and the surface copper, which was also found in Kim et al’s study [26]. Cu-zeolite Y selectively adsorbs indole and phenol over HAM and HAA, suggesting that the acid-base effect might not play a crucial role in the process of Cu-zeolite Y adsorption. Hydrogenation of indole is likely to occur on the copper surface due to the fact that there are possibly active H atoms on copper surface as was reported in Kim et al’s study where the hydrogenation of 1-octene and benzothiophene on the nickel surface occurred due to the presence of hydrogen atoms on the nickel surface [26]. This hydrogenation contributes to the



Page 22 of 31

formation of dinitroindole or dinitrophenol, which enhances the negative electrostatic potential in indole and phenol. This may be the reason that Cu-zeolite Y exhibits relatively high selectivity for phenol and indole. Another interesting finding in this study was the continuous increase in initial concentrations of some compounds, in particular, HEN and HAA, in microalgae bio-oils by more than 40% (C/Co>1.4) after the saturation point (C/Co=1) was reached. From this it can be deduced that the adsorption of these molecules is probably partial reversible and the molecules have a lower affinity for adsorption than the subsequent breakthrough compounds, leading to the partial replacement of the compounds with lower adsorptive affinity by the compounds with higher adsorptive affinity. The adsorption of Cu-zeolite Y (Figure 1 and Table 3) shows that some of the sorbed BEN and HEN can be exchanged by oxygenous compounds. Approximately 70% of HAA molecules in the adsorption column can be substituted by phenol, indicating that this partial adsorption of HAA is reversible, which further suggests that the adsorptive attraction of phenol to Cu-zeolite Y is stronger than that of HAA, probably because of the steric hindrance of the – CH3 in HAA. It is evident from Figure 1 that most of the adsorbed phenol molecules (>87%) could not be replaced by nitrogenous compounds, indicating that the adsorption affinity of phenol on Cu-zeolite Y was too strong to be substituted by nitrogenous molecules. This is consistent with a previous discussion that a significant amount of the hydroxyl molecules such as phenol is adsorbed on the copper-based adsorbent by the direct interaction between the hydroxyl group and the copper. This interaction may lead to the formation of surface copper oxide [26]. In summary, there may have two adsorption configurations for nitrogenous compounds on copper-based adsorbents: side adsorption and terminal adsorption. In the side adsorption, the nitrogenous compound is adsorbed flatly on the active adsorption site, and the p-electron


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the aromatic ring may play an essential role, as has been observed for indole adsorption [7]. In the terminal adsorption configuration, the nitrogen atoms in the nitrogen-containing compounds may directly interact with the surface copper atoms [27]. 3.2.2. Adsorption mechanism over silica gel The adsorptive selectivity of various substances was also examined, and it was found that it increases in the order of HEN

Adsorptive selectivity and mechanism of three different adsorbents for

Recommend Documents