Selective hydroconversion of oleic acid into aviation fuel range

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Kinetics, Catalysis, and Reaction Engineering

Selective hydroconversion of oleic acid into aviation fuel range alkanes over the ultrathin Ni/ZSM-5 nanosheets Fuxiang Feng, Li Wang, Xiangwen Zhang, and Qingfa Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00103 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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Industrial & Engineering Chemistry Research

Selective hydroconversion of oleic acid into aviation fuel range alkanes over the ultrathin Ni/ZSM-5 nanosheets Fuxiang Feng a, Li Wang a,b, Xiangwen Zhang a,b, Qingfa Wanga,b* a

Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China.



Corresponding author: Tel & fax: +86 22 27892340.

E-mail address: [email protected] (Q. Wang). 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

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Abstract Ultrathin

Ni/ZSM-5

nanosheet

catalysts

with

different

acid

concentration,

B/L

(Brønsted/Lewis acid sites) ratio and nanosheet thickness are designed for the hydroconversion of

oleic

acid

into

aviation

fuel

range

alkanes

(AFRA).

The

role

of

acid

concentration/distribution and nanosheet thickness was investigated. High acid concentration enhances HDO reaction and intrinsic deoxygenation activity due to the synergistic effect between acid sites and metal sites. Selective cracking reaction can be enhanced by tailoring nanosheet thickness and acid distribution. Thin nanosheet favors isomerization, selective cracking reaction as well as the generation of central branched isomers. External acid sites dominate the primary cracking and central C-C cracking of the deoxygenated products, but internal acid sites promote the deeper cracking and terminal C-C cracking. Formation of aromatics is thoroughly suppressed at low reaction temperature (≤ 533 K). And a high AFRA yield of 41.4% is achieved over the 3nm nanosheet catalyst with Si/Al=300 and B/L=0.18.

Key words: Aviation fuel; Oleic acid; Hydroconversion; Acid distribution; Ni/ZSM-5 nanosheets

2 ACS Paragon Plus Environment

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1. Introduction To develop the alternative aviation fuel from biomass has become an important research due to the incremental consumption with the rapid development of the aviation industry and the environmental problem derived carbon emission.1,2 For the stringent requirements (high energy content, low freeze point and viscosity) of the final products, two consecutive processes: catalytic deoxygenation of fatty acids or their derivatives and acid catalyzed reactions (isomerization and cracking) of the deoxygenated products are necessary.3,4 Recently, a single-step route has been developed to produce aviation fuel range hydrocarbons from vegetable oils and fatty acids over different types of bifunctional catalysts composed of metal and acid sites.5,6 With respect to the metal sites, catalysts supported noble metal (Pt, Pd, etc.) have been widely used because of their superior deoxygenation activity.7 However, the high cost and scarcity restrict their practical application. Sulphided transition metal (NiW, NiMo, CoMo) are alternative class catalysts but environmentally unfriendly.8 In recent years, metallic Ni has been extensively used because of its low cost and noticeable deoxygenation activity.9 But it usually needs high metal loading (>10 wt%) to achieve desired deoxygenation activity, which may result in a low metal dispersion due to the agglomeration at harsh treatments.10 As for the supports, moderate acidity and hierarchical porosity are beneficial for producing aviation fuel, because deeper cracking reaction of bulky molecular is facilitated by high acidity and diffusion limitation.11,12 Different types of catalysts have been investigated to produce aviation fuel by single-step route.13,14 For Al2O3, SiO2-Al2O3, SAPO11, MCM-41 and USY supported metal catalysts, a desired aviation fuel yield is usually obtained at a high reaction temperature (> 653 K) because of their low acidity and pore structure.15,16 But the Beta and ZSM-5 supported metal catalysts favor deeper cracking of long chain hydrocarbons as well as aromatization of the deeper cracking products even at a low reaction temperature (573 K).17,18 The aromatics selectivity in aviation fuel is relative high and 3 ACS Paragon Plus Environment


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increases with the elevation of reaction temperature over the beta and ZSM-5 catalysts.13,19 Generally, the conversion significantly decreases below 573 K due to the low activity of catalysts.20,21 Therefore, the production of aviation fuel range hydrocarbons with high conversion and low or no aromatic is a great challenge. In addition, although extensive studies on the hydroconversion of fatty acids into aviation fuel have been investigated, the mechanism of acid sites on the deoxygenation pathways is still unclear. And further research on the critical factors tailoring the selective cracking of long chain hydrocarbons is needed. In recent years, hierarchical zeolite catalysts have been rapidly developed because of its good diffusion characteristics and high activity.14,18 Ryoo et al.22 and Xiao et al.23 observed that bulky molecular reactions occurred at Al sites located at the external surface, and the catalytic activity is enhanced effectively by the introduction of mesopore.24,25 Choi et al.26 had reported a unit cell thick (2.5nm) MFI nanosheet zeolite with abundant mesoporosity and large number of external acid sites. Comparing to others hierarchical MFI catalysts, this MFI nanosheet zeolite catalyst has higher external acid distribution and pore connectivity due to its ultrathin nanosheet structure, which is more beneficial for isomerization of n-alkanes (C7 and C10)27,28 and catalyst lifetime.29,30 Moreover, the acid distribution of the catalyst can be easily tailored by varying the nanosheet crystal thickness. In this context, ultrathin Ni/ZSM-5 nanosheet catalysts with different Si/Al ratio (100, 150, 200 and 300) and crystal thickness (3, 12, 24 and 200nm) were synthesized. The role of acid concentration/distribution,

reaction

temperature

and

nanosheet

thickness

on

the

hydroconverison of oleic acid into aviation fuel range alkanes was systematically investigated. The possible deoxygenation pathways of oleic acid and the C-C bond breaking mechanism of the deoxygenated products were also proposed. 2. Materials and Methods 2.1. Chemicals. All the chemicals were used as received. 1-Bromooctadecane (≥ 99.0 wt%， 4 ACS Paragon Plus Environment

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TCI), N,N,N′,N′-tetramethyl-1,6-diaminohexane(≥99.0 wt%, Adamas Reagent Co.,Ltd) and 1bromohexane (≥99.0 wt%, Tianjin Guangfu Fine Chemical Research Institute) were adopted to synthesize the multiammonium surfactant structure-directing agents. The sodium hydroxide, aluminum sulfate octadecahydrate and tetraethyl orthosilicate (TEOS) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd and. Ni (NO3)2•6H2O (≥98.0 wt%, Alfa Aesar) were used as the Ni source. Oleic acid (98.0 wt%) was obtained from Shanghai Macklin Biochemical Co., Ltd. Commercial H-ZSM-5 zeolite with Si/Al=300 was purchased from Nankai university catalyst plant. 2.2. Catalyst Preparation and Characterizations. Gemini-type quaternary ammonium surfactants with a formula of C22H45–N+(CH3)2–C6H12–N+(CH3)2–C6H13 were synthesized via a two-step reaction between tetramethyl diaminohexane and corresponding bromoalkanes. ZSM-5 nanosheet zeolites were synthesized using the Gemini-surfactants as a structuredirecting agent (SDA) by one-step hydrothermal process. The nanosheet crystal thickness was tailored by changing the amount of SDA. Zeolites supported Ni catalysts were synthesized by the incipient wetness impregnation method. According to the nanosheet thickness and Si/Al ratio, the resultant samples were denoted as NSx(y), where x is the nanosheet crystal thickness (3, 12 and 24nm) and y is the Si/Al ratio (100, 150, 200 and 300). The commercial Ni/ZSM-5 catalyst with Si/Al=300 was named as C200(300). Ni/ZSM-5 catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), nitrogen adsorption/desorption (N2-BET), X-Ray Fluorescence (XRF), high-resolution

27Al

MAS NMR, ammonia

temperature programmed desorption (NH3-TPD), pyridine (Py) and 2,6-di-tert-butylpyridine (DBTPy)-IR. The detailed experimental procedures for catalyst preparation and characterizations were given in the Supporting Information file. 5 ACS Paragon Plus Environment


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2.3. Catalytic Conversion of Oleic Acid. The catalytic conversion of oleic acid was carried out with a fixed-bed flow reactor (1.0 cm i.d. and 45 cm in length). The reaction temperature was monitored with a thermocouple in the catalyst bed and controlled by three thermocouples on the reactor wall. 1.00 g of catalyst was loaded and fixed by SiC in the center of the reactor. Oleic acid (33.3 wt% in cyclohexane) was used as feedstock, which was injected into the reactor at a flow of 0.2 ml/min using a high-pressure pump. The reactions were carried out at corresponding reaction temperature under 3 MPa with a flowing H2 of 100 ml/min. Before the reaction, the loaded catalysts were reduced at 773 K under flowing H2 for 3 h. The gas and liquid products of each experiment were successively collected by each 1 h interval under the given conditions. The gaseous products were analyzed online with an Agilent 3000 gas chromatograph equipped with a TCD detector using three columns (molecular sieve, plot U and alumina). The liquid fraction was centrifuged in two parts: water and organic liquid products (OLPs). The OLPs were qualitatively analyzed with an Agilent 6890N gas chromatography/5975N mass spectrometry (GC/MS). A gas chromatograph (Bruker 456 GC, Bruker), equipped with a flame ionization detector (FID) and a commercially column (ZB-5 HT, 60 m × 0.25 mm × 0.25 m), was used to quantitatively analyze the hydrocarbons in OLPs. Icosane was used as internal standard to quantify the different products. The conversion of oleic acid, OLPs yield, and selectivity/yield of corresponding hydrocarbons (CxHy) in the OLPs were calculated according to Eq 1, 2, 3 and 4:15,18 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 = 𝑌𝑖𝑒𝑙𝑑(𝑂𝐿𝑃𝑠) =

𝑚𝑂𝐴𝑓𝑒𝑒𝑑 ― 𝑚𝑂𝐴𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

(1)

𝑚𝑂𝐴𝑓𝑒𝑒𝑑 𝑚𝑂𝐿𝑃𝑠

(2)

𝑚𝑂𝐴𝑓𝑒𝑒𝑑

𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦(𝐶𝑥𝐻𝑦) =

𝑚(𝐶𝐻)𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

(3)

∑(𝑚(𝐶𝐻)𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠)

𝑌𝑖𝑒𝑙𝑑(𝐶𝑥𝐻𝑦) = 𝑌𝑖𝑒𝑙𝑑(𝑂𝐿𝑃𝑠) ∗ 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦(𝐶𝑥𝐻𝑦)

(4)

Where mOAfeed and mOAproducts are the mass of oleic acid in the feed and in the products, 6 ACS Paragon Plus Environment

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respectively. m(CH)n products is the mass of corresponding hydrocarbons in the OLPs. mOLPs is the mass of OLPs. 3. Results and Discussion 3.1. Texture Properties of Ni/ZSM-5 Nanosheet Catalysts. The XRD patterns of the series catalysts with different Si/Al ratio and nanosheet thickness are illustrated in Figure 1. The catalysts present characteristic reflections of MFI framework zeolites (JCPDS-44-0003) at 2θ = 8.3°, 9.2°, 23.5° and 24.3° and the cubic structure nickel oxide (JCPDS-47-1049) at 2θ = 37.2°, 43.3°, 62.9°, 75.4° and 79.4°.12 Besides, only the reflections of {h0l} are sufficiently sharp for indexing, which indicates that the zeolites possess wide a-c planes with large coherent domains and small b-axis framework thickness.26 The relative crystallinity (RC) of all the samples is calculated based on the peak areas of four main characteristic peaks. Figure 1 shows that the relative crystallinity increases with the increase of nanosheet thickness, and the nanosheet catalysts has much lower crystallinity than the commercial catalyst due to their thin nanosheet structure. Moreover, the

27Al

MAS NMR analysis demonstrates that no non-

framework aluminum is detected in the different nanosheet thickness catalysts with Si/Al=100 (Figure S1).


Page 8 of 34

NS24(300) 



55.9% 



NS24(100)

58.0% NS12(300)

49.4 %

Intensity(a.u.)

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

101 200 102 301 400 103 501 303 104 503


NS12(100)

49.1% NS3(300)

40.6% NS3(200)

42.1% NS3(150)

41.1% NS3(100)

43.2% C200(300)

RC=100% 10

20

30

40

50

2

60

70

80

90

Figure 1. XRD patterns of the series Ni/ZSM-5 nanosheet catalysts with different Si/Al ratio and nanosheet thickness. Nickel oxide reflections are highlighted with a square (◆). The SEM and TEM images of calcined ZSM-5 zeolite samples are given in Figure 2. The obtained ZSM-5 nanosheet zeolites are three dimensional intergrowth of laminas (SEM) composed of several alternating MFI nanosheets or single one nanosheet (TEM). The average thickness of ZSM-5 zeolite nanosheets along b-axis is about 3, 12 and 24 nm for NS3, NS12 and NS24 samples, respectively. The results indicate that the nanosheet thickness is well tailored from single unit cell thickness to multiple unit cell thickness along b-axis. The mesoporosity formed from interlayer space between nanosheets is retained by the intergrowth of laminas and slight deviations of the crystal orientation in the a-c planes.26 The commercial ZSM-5 zeolite has an average thickness about 200nm. 8 ACS Paragon Plus Environment

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Figure 2. SEM (left) and TEM (right) images of the series ZSM-5 zeolites. The pore properties of Ni/ZSM-5 nanosheet catalysts determined by N2 adsorptiondesorption are shown in Figure 3. There is an inflection point at the low pressure from 0 to 0.1 atm (Figure 3a), suggesting the existence of microporous structure.11 At the relatively high pressure from 0.45 to 0.90 atm, all the nanosheet catalysts show the type IV isotherm with a remarkable hysteresis loop, a known fingerprint of a hierarchical porous system. This means that abundant mesoporous structures exist in the samples.9,23 The N2 adsorption-desorption isotherms of all the samples indicate that both micropore and mesopore exist in the nanosheet 9 ACS Paragon Plus Environment


catalysts. The mesopore size distributions calculated from the adsorption branch of isothermal curves are illustrated in Figure 3b, and the structural properties of the Ni/ZSM-5 catalysts are listed in Table 1. The 3 nm nanosheet catalysts show similar mesopore size distribution at around 4.0 nm. As the nanosheets thickness increases, the mesopore specific surface area and mesopore volume of Ni/ZSM-5 catalysts reduce gradually, and the mesopore size distribution broadens because of the further crystal growth of 3 nm MFI nanosheets, causing the uniform mesopore structure gradually reassembling in a randomly interconnected manner.31 According to the definition of the hierarchy factors (HF),32 the 3 nm nanosheet samples are very mesoporous materials with low microporosity, while the NS12(y) and NS24(y) samples are composed of zeolite nanocrystals with improved HF values, and the commercial C200(300) catalyst is micropore material, which is consistent with the result in Figure 3. NS3(100)

a

b

NS3(100)

NS3(150)

NS3(150)

NS3(200)

NS3(200)

NS3(300)

NS3(300)

dV/dlog(D)

Quantity Adsorbed (cm3/g STP)

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 10 of 34

NS12(100)

NS12(300)

NS12(100)

NS12(300)

NS24(100)

NS24(100)

NS24(300)

NS24(300)

C200(300)

C200(300)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

10

P/P0

Pore size (nm)

100

Figure 3. (a) N2 adsorption and desorption isotherms and (b) pore size distributions of the series Ni/ZSM-5 catalysts. 10 ACS Paragon Plus Environment

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Table 1. Physicochemical properties of the series Ni/ZSM-5 catalysts Catalysts

a, b

Si/Al

a

Ni b

SBET c

Sext/mes

SMicro

Vmeso d

VMicro

(wt%)

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

HFe

NS3(100)

101

9.9

575

390

185

0.477

0.095

0.113

NS3(150)

146

9.8

603

452

151

0.516

0.075

0.095

NS3(200)

192

10.4

586

430

156

0.500

0.079

0.100

NS3(300)

302

10.5

563

379

183

0.480

0.094

0.110

NS12(300)

308

10.1

486

305

180

0.375

0.093

0.125

C200(300)

308

9.9

304

32

272

0.032

0.152

0.087

NS24(300)

297

10.0

430

213

216

0.232

0.100

0.150

NS12(100)

108

10.2

494

299

196

0.324

0.104

0.147

NS24(100)

97

9.7

437

226

0.242

0.102

0.143

c

211 d

Determined by XRF analysis. Specific surface area. Pore volume.

e Hierarchy

factor (HF): determined as the product

(Vmicro/Vpore) * (Smeso/SBET).

The acid properties of all the samples were investigated by NH3-TPD and Py-IR, and the number of accessible Brønsted acid sites was determined by DBTP-IR.33 The results are demonstrated in Figure 4 and Figure S2. As the Si/Al ratio decreases from 300 to 100, the ammonia desorption peak shifts to high temperature, indicating the increase of acid strength. Consequently, the total acid sites as well as B/L ratio also increase (Table 2). For the catalysts with the same Si/Al ratio (100 or 300) but different nanosheet thickness (3, 12 and 24 nm), the total acid concentrations are almost the same. But the strong Brønsted acid sites and B/L ratio increase rapidly with the increased nanosheet thickness. The acid sites accessibility factor (AF) is similar for the identical nanosheet thickness catalysts, meanwhile it increases as the nanosheet thickness decreases attributed to the increased external surface and pore mouth amount. The C200(300) catalyst has the least accessible acid sites. The results of DTBPy-IR indicate that the accessible Brønsted acid sites decrease with the increase of Si/Al ratio and nanosheet thickness (Figure S2b).



NS24(100) NS12(100)

NS3(100)

TCD %

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 12 of 34

NS3(150) NS3(200) C200(300) NS24(300) NS12(300) NS3(300)

100 150 200 250 300 350 400

Bed temperature (C)

Figure 4. NH3-TPD curves of the series Ni/ZSM-5 catalysts Table 2. Acidity properties of the series Ni/ZSM-5 catalysts

Catalysts

Strong acid

Weak acid

Accessible acid

(μmol/g)

(μmol/g)

(μmol/g)

Total B/L

AF

(μmol/g)

b

L (py) B(DTBPy) L(DTBPy)a

B (py)

L (py)

B (py)

NS3(100)

28.5

54.5

25.1

90.0

35.9

96.8

198.1

0.37

0.57

NS3(150)

18.2

37.8

12.2

66.9

17.0

58.7

135.1

0.29

0.56

NS3(200)

13.4

28.5

5.67

50.9

11.1

46.0

98.4

0.24

0.58

NS3(300)

8.3

18.6

3.1

32.8

6.0

32.8

62.6

0.18

0.62

NS12(300)

15.6

15.6

9.4

29.1

6.3

11.2

69.7

0.56

0.25

NS24(300)

26.7

15.0

10.3

21.2

2.6

2.5

73.3

1.02

0.07

C200(300)

20.0

9.3

15.7

25.3

1.0

1.1

73.1

1.03

0.03

NS12(100)

41.6

49.5

25.3

92.0

16.1

33.9

208.4

0.64

0.24

NS24(100)

90.3

39.4

34.7

55.4

10.0

7.6

219.8

1.32

0.08

a

Assume Lewis acid had the same spatial distribution as Brønsted acid. b The accessibility factor (AF) was defined as the

number of sites detected by adsorption of DTBP (accessible acid sites) divided by the total amount of acid sites quantified by pyridine absorption.

3.2. Catalytic Conversion of Oleic Acid. The selective hydroconversion of oleic acid into 12 ACS Paragon Plus Environment

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aviation fuel range alkanes over the Ni/ZSM-5 nanosheet catalysts with different Si/Al ratio (100, 150, 200 and 300) was investigated at 533 K. The conversion of oleic acid and yield of organic liquid products (OLPs) over the series ZSM-5 nanosheet catalysts are given in Figure 5a. Almost complete conversion is achieved over all the catalysts. This indicates that the catalysts have high deoxygenation activity even with a low acid concentration. The OLPs yield is 60.1% over the NS3(100) catalyst and gradually increases up to 75.4% with the decrease of Si/Al ratio to 300, suggesting that decreased Si/Al ratio increases the OLPs yield due to the suppressed deep cracking of long chain hydrocarbons.34 To explore the variation of the deoxygenation pathways, the distribution of H2O, CO2 and CO on different catalysts was analyzed and is illustrated in Figure 5b. The distribution of these oxygen-containing products is significantly influenced by the Si/Al ratio. Over the NS3(100) catalyst, H2O (47 mol%) and CO2 (32 mol%) are the main oxygen-containing products. As the Si/Al ratio increases, the selectivity of H2O decreases but the selectivity of CO2 increases significantly. The maximum selectivity of CO2 (40 mol%) is obtained over the NS3(300) sample, indicating that DCO2 becomes the dominant reaction at high Si/Al ratio. Meanwhile, the CO selectivity only slightly increases from 21% to 25 mol%. These results imply that decreased acid concentration inhibits the HDO reaction. This is because the basic oxygen atoms of oleic acid can interact with Lewis acid sites of catalysts by chemisorption as well as the Brønsted acid sites via H-bonding.35,36 The interaction between acid sites and oleic acid activates the C-O bonds, and thus favors its hydrogenation towards water by active H generated on the adjacent metallic Ni. Moreover, the Brønsted acid sites provide an additional delivery of the proton by hydrogen spillover, thus increasing the concentration of active H around metallic Ni particle for the hydrogenation process.35 The result suggests that acid sites participate the HDO reaction, and their synergetic interaction with adjacent Ni sites enhances the HDO reaction. Meanwhile, the deoxygenation of oleic acid via DCOx reaction mainly 13 ACS Paragon Plus Environment


depends on metallic Ni due to its high C-C hydrogenolysis activity of metallic Ni,37,38 which is consist with the previous work.9,39-41 So as the acid concentration decreases, the HDO reaction selectivity decreases but the DCOx reaction selectivity increases. The OLPs distribution over the series catalysts was further determined. As shown in Figure 5c, no C16-C18 hydrocarbons but high C4-C8 hydrocarbons yield (42.7%) is obtained over the NS3(100) catalyst with only 17.4% of C9-C15 hydrocarbons. This indicates that the NS3(100) catalyst has high deep cracking activity. As the Si/Al ratio increases, the yield of C9C15 hydrocarbons and C16-C18 hydrocarbons significantly increases with the decrease of C4-C8 hydrocarbons yield. The NS3(300) catalyst shows the maximum C9-C15 hydrocarbons yield of 38.1% with 15.2% of C16-C18 hydrocarbons and 22.1% of C4-C8 hydrocarbons. This is possible that high Si/Al ratio decreases the acid concentration as well as the B/L ratio, resulting in lower Brønsted acid site concentration, and thus restrains the deeper cracking but enhances the selective cracking of the deoxygenated products into aviation fuel range alkanes. Figure 5d depicts that low carbon number (C9, C10) hydrocarbons are the main compounds of aviation fuel range alkanes over NS3(100) and NS3(150) catalysts. As the Si/Al ratio increases, high carbon number (> C11) hydrocarbons selectivity increases. The experiment and DFT calculation results of Song et al.42 narrated that ZSM-5 with low Si/Al ratio favored the central cracking over terminal cracking due to the increased adjacent Brønsted acid sites. And high Brønsted acid site concentration enhanced deeper cracking of long chain hydrocarbons (> C11). Therefore, the catalysts with low Si/Al ratio result in more low carbon number hydrocarbons in aviation fuel range alkanes.


Page 14 of 34

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a

Conversion of oleic acid

b

Yield of OLPs

Selectivity %

Conversion and Yield %

c

DCO

80 60 40

NS3(100)

NS3(150)

NS3(200)

30 20

0

NS3(300)

d

C4-C8 Hydrocarbons C9-C15 Hydrocarbons

50

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0 NS3(100)

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C9

NS3(300)

C10

C11

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Figure 5. (a) Conversion of oleic acid and yield of OLPs, (b) distribution of H2O, CO and CO2, (c) hydrocarbons distribution of OLPs and (d) carbon number distribution of C9-C15 hydrocarbons over NS3(100), NS3(150), NS3(200) and NS3(300) catalysts at 533 K. As for the NS3(300) catalyst, the selective hydroconversion of oleic acid to aviation fuel range alkanes at different reaction temperatures was further investigated. Figure 6a elucidates that the oleic acid conversion over the NS3(300) catalyst is 92% with an OLPs yield of 72% at 513 K. As the reaction temperature increases up to 523 K, oleic acid conversion reaches 100% and the OLPs yield increases up to 79.1%, then decreases to 75.4% at 533 K due to the enhanced deeper cracking of deoxygenated products. Figure 6b depicts that the HDO selectivity decreases with an increased DCOx selectivity as the reaction temperature increases. The result indicates that increased reaction temperature favors the DCOx reaction but restrains the HDO reaction. This is because HDO reaction is an exothermic reaction, while DCO and DCO2 are endothermic reactions.43,44 15 ACS Paragon Plus Environment


In contrast to the increased yield of C4-C8 hydrocarbons, the C16-C18 hydrocarbons yield decreases as the reaction temperature increases from 513 K to 533 K (Figure 6c), and the maximum aviation fuel range hydrocarbons yield of 41.4% is obtained at 523 K. Moreover, the aviation fuel is dominated by n-alkanes and their isomers, no aromatic is detected (Figure S3). This is probably due to the low reaction temperature and short micropore path which thoroughly restrains the aromatization of olefins.13,45 For the NS3(100) catalyst, as the reaction temperature increases from 513 K to 533 K, the selectivity of C16-C18 hydrocarbons decreases from 28.1% to zero (Figure S4). The selectivity of C4-C8 hydrocarbons increases with the reaction temperature increasing from 513 K to 573 K but decreases at 613 K, which is contrary to the variation of C9-C15 hydrocarbons selectivity. The aromatics content in C9-C15 hydrocarbons increases from 75% to 100% as the reaction temperature increases from 573 K to 613 K. In our previous work, it was found that the olefins from cracking and dehydrogenation of long chain hydrocarbons could be converted into aromatics on acid sites.13 The increased reaction temperature promotes the aromatization of short chain olefins thus decreasing the selectivity of C4-C8 hydrocarbons but increasing aromatics selectivity in C9-C15 hydrocarbons. The carbon number distribution of C9-C15 hydrocarbons was further determined and is shown in Figure 6d. As the reaction temperature increases, the low carbon number hydrocarbons (< C12) selectivity increases and high carbon number hydrocarbons selectivity decreases. Because high reaction temperature favors the central C-C bonds cracking over terminal cracking and enhances the deeper cracking of long chain hydrocarbons to get low carbon number hydrocarbons.


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a

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Figure 6. (a) Conversion of oleic acid and yield of OLPs, (b) distribution of H2O, CO and CO2, (c) hydrocarbons distribution of OLPs and (d) carbon number distribution of C9-C15 hydrocarbons over NS3(300) catalyst at 513, 523 and 533 K. The selective hydroconversion of oleic acid into aviation fuel range alkanes over the Ni/ZSM-5 nanosheet catalysts and commercial Ni/ZSM-5 catalyst was investigated. The conversion of oleic acid and yield of OLPs over the series Ni/ZSM-5 catalysts with different nanosheet thickness are given in Figure 7a. Almost complete conversion of oleic acid is achieved over the NS3(300) catalyst. As the nanosheet thickness increases from 3 to 12, 24 and 200 nm, oleic acid conversion gradually decreases to 80.2%, 60.4% and 45.7% over the NS12(300), NS24(300) and C200(300) catalysts, respectively, due to the decreased Ni dispersion and more wrapped Ni nanoparticles by micropore compared to the NS3(300) catalyst (Figure S5). Moreover, Figure S6a shows that the conversion over NS12(100) and NS24(100) catalysts is unexpectedly higher than that over the NS12(300) and NS24(300) 17 ACS Paragon Plus Environment


catalysts in spite of the similar Ni dispersion (Figure S5) and pore structure (Table 1). The result reveals that high acid concentration enhances the deoxygenation activity, which is in accordance with previous work.9,35,46 As discussed above, the activated C-O bonds on acid sites are easily hydrogenated by active H around adjacent Ni particle. So the synergy between acid sites and metal sites accelerates the deoxygenation of oleic acid, resulting in high deoxygenation activity. As the nanosheet thickness increases from 3 nm to 200 nm, the OLPs yield significantly decreases from 79.1% to 35.1% due to the decreased conversion of oleic acid. The distribution of H2O, CO2 and CO on the Ni/ZSM-5 catalysts with different nanosheet thickness is illustrated in Figure 7b. As the nanosheet thickness increases from 3 nm to 200 nm, the HDO selectivity gradually decreases from 50% to 32%. But the DCO and DCO2 selectivity gradually increases from 19% to 26% and 31% to 42%, respectively. The result indicates that decreased nanosheet thickness favors HDO reaction over DCOx reaction. The decreased nanosheet thickness increases the accessible acid sites, which is beneficial for HDO reaction. Moreover, the decreased nanosheet thickness reduces Ni particles size, thus creating more Ni stepped sites to favor C-O hydrogenation towards water.41 Therefore the NS3(300) catalyst demonstrates the highest HDO selectivity. As the nanosheet thickness increases from 3 nm to 200 nm, the long chain hydrocarbons (C16-C18) selectivity firstly decreases and then increases, which is consist with the variation of accessible Brønsted acid sites rather than the total Brønsted acid sites. But the selectivity of C4C8 hydrocarbons (deeper cracking products) increases continuously, consisting with the variation of internal Brønsted acid sites (Figure 7c). This result suggests that the primary cracking of long chain C16-C18 hydrocarbons is likely dominated by external Brønsted acid site, and internal Brønsted acid sites (in micropore) enhances the deeper cracking reaction. In order to further confirm the role of external acid sites, the iso/n-alkanes ratio of the long chain 18 ACS Paragon Plus Environment

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hydrocarbons (C16-C18) was investigated. Figure S6b shows that the iso/n-alkanes ratio of C16C18 hydrocarbons significantly decreases from 0.4 to 0.01 as the nanosheet thickness increases from 3 nm to 200 nm due to the decrease of external acid sites. The results indicate that external acid sites dominate the isomerization and primary cracking of the deoxygenated products, and internal acid sites contribute to the deeper cracking. So the acid spatial distribution tailors the selective cracking of deoxygenated products into aviation fuel range alkanes. 100

HDO DCO2 DCO

80 60 40 20

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Figure 7. (a) Conversion of oleic acid and yield of OLPs, (b) distribution of H2O, CO and CO2, (c) hydrocarbon distribution of OLPs and (d) carbon number distribution of C9-C15 hydrocarbons over NS3(300), NS12(300), NS24(300) and C200(300) catalysts at 523 K. As shown in Figure 7d, C12 hydrocarbons have the highest selectivity among aviation fuel range hydrocarbons over the nanosheet catalysts (3, 12 and 24 nm), and the C12 hydrocarbons selectivity increases with the increase of nanosheet thickness. Figure 7d shows that increased nanosheet thickness accelerates the generation of non-central C-C bonds cracking products (C6 19 ACS Paragon Plus Environment


and Cm-6). It has been reported that the large pore preferred central cracking over terminal cracking due to the weakened spatial constraints.48-50 The increased nanosheet thickness enhanced the spatial constraints, thus favoring the non-central C-C bonds (H13C6-C12H25) cracking. The result indicates that external acid sites favor the cleavage of central C-C bonds in long chain hydrocarbons because of the weakened spatial constraints of external surface. And pore mouth acid sites and internal acid sites promote the breaking of non-central C-C bonds (13H6C-C12H25) resulting in high C12 alkanes selectivity due to the shape selectivity of micropore. To investigate the catalytic stability and reusability of the catalysts with different nanosheet thickness, oleic acid conversion with time on stream was explored. Figure S7a demonstrates that catalytic activity remains constant for 360 min, 270 min, 210 min and 120 min over the NS3(300), NS12(300), NS24(300) and C200(300) catalysts, respectively. Maria Milina et al.31,32 reported that high pore connectivity and opened mesopore enhanced the catalyst lifetime. The nanosheet structure can increase pore connectivity, and the mesopore derived from the nanosheet space is opened one. So the decreased nanosheet thickness increases the catalytic stability. Moreover, the regenerated NS3(300) catalyst (calcined at 773 K for 4 h) shows almost the same catalytic activity as the fresh one. Figure S7b indicates that the weight loss of the catalysts increases with the decrease of nanosheet thickness. And the main weight loss is at 573-673 K, which is corresponding to the deoxygenated intermediate products. This indicates that thin nanosheet increases catalyst carbon tolerance. 3.3. Proposed reaction mechanism. To investigate the key factors influencing the selective cracking, the selective cracking ratio (SCR) defined as the ratio of C9-C15 hydrocarbons selectivity to C4-C8 hydrocarbons selectivity was calculated, and the effects of acid concentration, B/L ratio, reaction temperature and nanosheet thickness on SCR values were discussed. 20 ACS Paragon Plus Environment

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As shown in Figure 8a, the SCR value almost linearly increases with the increase of Si/Al ratio. As for alkanes cracking, it is generally considered to be proceeded via the isomerization of alkanes.50,51 So the iso/n-alkanes ratio was also investigated. It can be found that the iso/nalkanes ratio also presents a linear decrease with the decrease of acid concentration, especially the pore mouth acid concentration.52,53 This indicates that low acid concentration would result in decreased isomerization activity, and then inhibit the deep cracking reaction. So, the selective cracking can be enhanced by tailoring the acid concentration. Furthermore, the influence of acid distribution on the selective cracking reaction of deoxygenation products was also investigated. Figure 8b shows that the B/L ratio is negative linear relationship with the selective cracking ratio, but is positive linear relationship with the iso/n-alkanes ratio. The reduced B/L ratio (from 0.37 to 0.18) decreases the Brønsted acid concentration as well as the pairs of adjacent Brønsted acid sites and adjacent Brønsted-Lewis acid sites, which enhances the isomerization and cracking of alkanes.41,54,55 Therefore, decreasing B/L ratio can inhibit isomerization and deeper cracking reaction. This result indicates that the B/L ratio is also a critical factor for the selective cracking of deoxygenated products into aviation fuel range alkanes. As the reaction temperature increases from 513 K to 533 K, the iso/n-alkanes ratio of C9C15 hydrocarbons increases from 0.56 to 0.90, but the selective cracking ratio decreases from 2.8 to 1.7 (Figure 8c). The result suggests that higher reaction temperature results in higher isomerization activity, thus leading to higher deep cracking due to the high cracking activity of isomers compare to n-alkanes.56 Therefore, the selective cracking of deoxygenated products into aviation fuel range alkanes can be further optimized by tailoring the reaction temperature.



3.0

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1.5

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Figure 8. (a) The near-linear relationship between the acid sites concentration and SCR & iso/n-alkanes ratio, (b) The near-linear relationship between B/L ratio and SCR & iso/n-alkanes ratio and (c) SCR and iso/n-alkanes ratio at different reaction temperature over NS3(300) catalyst. Figure 9a illustrates that the ratio of 2-methyl isomers to the total isomers in C9-C15 hydrocarbons increases with the increase of nanosheet thickness over the Si/Al=300 catalysts, especially when the nanosheet thickness increases from 24 nm to 200 nm. This is consistent with the result of Si/Al=100 catalysts (in Figure S8a). This is because the formation of 2-methy isomers is diffusion-based, and thin nanosheet offer too short a diffusion path to generate this selectivity.55 The result indicates that thin nanosheet weakens the diffusion resistance, and increases the selectivity of central branched isomers. Moreover, as shown in Figure 9b and Figure S8b, decreased nanosheet thickness improves both the SCR values and iso/n-alkanes ratio of C9-C15 hydrocarbons, which can’t be achieved 22 ACS Paragon Plus Environment

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by tailoring the acid properties and reaction conditions. As discussed above, thin nanosheet weakens the diffusion resistance, so the primary cracking products and branched hydrocarbons can easily escape from the thin nanosheet catalyst before deeper cracking,27,57 resulting in high SCR value and iso/n-alkanes ratio. This result indicates that decreased nanosheet thickness enhances both the selective cracking reaction of long chain hydrocarbon (C17/18) and the iso/nalkanes ratio of C9-C15 hydrocarbons due to the short diffusion path in micropore. 0.7

0.6

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a 2-methyl isomers ratio

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0.3

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Figure 9. (a) The ratio of 2-methyl isomers to the total isomers in C9-C15 hydrocarbons and (b) SCR & iso/n-alkanes ratio over different nanosheet thickness catalysts with Si/Al=300. According to the above results and discussion, the possible deoxygenation mechanism of oleic acid and the selective cracking scheme of deoxygenated products (C18/C17) were suggested. As shown in Scheme 1, the oxygen atom removed from oleic acid is presented in water, carbon monoxide and carbon dioxide, meaning that the three oxygen-removal pathways (hydrodeoxygenation (HDO), decarbonylation (DCO), and decarboxylation (DCO2)) occur simultaneously during the hydrodeoxygenation of oleic acid.34 HDO reaction pathway is enhanced by the synergy between metal sites and acid sites through adsorbing the carboxylic group at acid sites of catalyst surface to activate the C-O bonds and subsequently hydrogenated by active H generated on adjacent metallic Ni. DCO2 reaction mainly depends on metallic Ni due to its high C-C hydrogenolysis activity.



Scheme 1. Proposed pathways for the deoxygenation of oleic acid. A possible isomerization and cracking mechanism for the deoxygenated products is summarized in Scheme 2. The deoxygenated products firstly isomerize at metal sites and external acid sites (Route 1). Low acid concentration, B/L ratio and reaction temperature inhibit the deeper cracking by tailoring the isomerization reaction and favor the selective cracking of isomers into aviation fuel range alkanes (AFRA), thus increasing the AFRA yield and high carbon number hydrocarbons content in AFRA but decreasing iso/n-alkanes ratio (Route 2). On the contrary, high acid concentration, B/L ratio and reaction temperature promote the deeper cracking by tailoring the isomerization reaction, resulting in low AFRA yield and low carbon number hydrocarbon content in AFRA but high iso/n-alkanes ratio (Route 3). Thin nanosheet accelerates the primary cracking products and isomers escaping from the catalyst before deeper cracking through shortening the diffusion path in micropore, thus improving both the SCR and the iso/n-alkanes ratio of C9-C15 hydrocarbons (Route 4). Thick nanosheet favors the deeper cracking reaction, leading to low SCR and iso/n-alkanes ratio (Route 5), the generation of 2-methyl isomers and C12 hydrocarbons due to long micropore path and shape selectivity (Route 6). High reaction temperature (≥ 573 K) causes the aromatization of deeper cracking products, resulting in high aromatics content in aviation fuel (Route 7).


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Scheme 2. Proposed isomerization and cracking mechanism for the deoxygenated products.

4. Conclusions A series of ultrathin Ni/ZSM-5 nanosheet catalysts with different Si/Al ratio (100, 150, 200 and 300) and crystal thickness (3, 12, 24 and 200 nm) were successfully synthesized, and the role of acid concentration/distribution and nanosheet thickness on selective hydroconversion of oleic acid into aviation fuel range alkanes (AFRA) was systematically investigated. The HDO reaction activity and Ni intrinsic deoxygenation activity was enhanced by the synergy between metal sites and acid sites. Initially, C-O bonds were activated by acid sites and hydrogenated by active H generated on adjacent metallic Ni. DCOx reaction was dominated mainly by metallic Ni due to its high C-C hydrogenolysis activity. 25 ACS Paragon Plus Environment


The isomerization and selective cracking of the deoxygenated products into aviation fuel range alkanes could be tailored by acid spatial distribution, B/L ratio, reaction temperature and nanosheet thickness. Low acid concentration, B/L ratio and reaction temperature restrained the deeper cracking but favored the selective cracking of the deoxygenated products into AFRA, resulting in increased AFRA yield and high carbon number hydrocarbon content in AFRA but decreased iso/n-alkanes ratio. Thin nanosheet satisfactorily improved both the SCR and the iso/n-alkanes ratio of C9-C15 hydrocarbons by accelerating the primary cracking products and isomers escaping from the catalyst before deeper cracking through shortening the diffusion path in micropore. Thin nanosheet also favored the generation of central branched isomers rather than 2-isomers due to the short micropore length. External acid sites dominated the primary cracking and central C-C cracking of the deoxygenated products owing to the weakened spatial constraints, but internal acid sites promoted the deeper cracking and terminal C-C cracking due to the shape selectivity of micropore. Formation of aromatics was thoroughly suppressed at low reaction temperature (523 K). The NS3(300) nanosheet catalyst exhibited excellent selective cracking activity (AFRA yield of 41.4%) and catalytic stability compare to the C200(300) catalyst. This work provided a guide for designing highly active and stable catalyst for producing aviation fuel from biomass.

AUTHOR INFORMATION Corresponding Author *Tel./Fax: 86-22-27892340. E-mail: [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The financial supports by National Natural Science Foundation of China (GrantNo.21476169, 21476168) are gratefully acknowledged. The authors thank Dr. Hao Chen for his help on catalytic evaluation.

ASSOCIATED CONTENT Supporting Information. Detailed procedures for catalyst preparation and characterizations, 27Al

MAS NMR spectra of Si/Al=100 catalysts, (DTBP)Py-IR spectra, gas chromatogram of

OLPs over NS3(300), more OLPs distribution over NS3(100), Ni size distribution, more conversion, iso/n-alkanes ratio, 2-methyl isomers ratio and SCR values, stability test and thermogravimetric analysis data.

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Selective hydroconversion of oleic acid into aviation fuel range

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