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Table of Contents
Abstract
1. Introduction
4. Conclusions
ACKNOWLEDGEMENTS
ASSOCIATED CONTENT
Subscriber access provided by Nottingham Trent University 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. Kinetics, Catalysis, and Reaction Engineering Self-pillared ZSM-5-supported Ni nanoparticles as an efficient catalyst for upgrading oleic acid to aviation-fuel-range-alkanes Fuxiang Feng, Li Wang, Xiangwen Zhang, and Qingfa Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02527 • Publication Date (Web): 05 Jul 2019 Downloaded from pubs.acs.org on July 11, 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. 1 Self-pillared ZSM-5-supported Ni nanoparticles as an efficient catalyst for upgrading oleic acid to aviationfuel-range-alkanes Fuxiang Feng a, Li Wang a,b*, Xiangwen Zhang a,b, Qingfa Wang a,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: wlytj@tju.edu.cn (L. Wang), qfwang@tju.edu.cn (Q. Wang). Page 1 of 30 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 2
Abstract
HZSM-5 zeolite has been extensively utilized in petrochemical and fine chemical industry owing to its excellent isomerization/cracking activity and hydrothermal stability. However, it still remains a great challenge for ZSM-5-supported catalysts to upgrade vegetable oils to aviation-fuel-range-alkanes (AFRAs). Herein, an efficient Ni/ZSM-5 catalyst composed of self-pillared (SP) nanosheets along with high external acid fraction and short characteristic diffusion length was synthesized and employed for this reaction. This SP Ni/ZSM-5 nanosheet catalyst shows high deoxygenation, cracking/isomerization activity and excellent catalytic stability for upgrading oleic acid to AFRAs, which is attributed to the SP nanosheet structure and reasonable acid distribution. At last, a high AFRA yield of 40% with an iso/n-alkanes ratio of 1.8 was achieved. Key words: Self-pillared ZSM-5; Accessibility; Acid distribution; Oleic acid; Aviation fuel Page 2 of 30 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 3
1. Introduction
Developing renewable aviation fuels has been considered as an efficient alternative to overcome the increasing energy crisis and global environmental problems.1,2 Aviation fuel produced from vegetable oils and their derivatives (fatty acids: FAs) has attracted more attention in recent years.3,4 And a single-step route to produce aviation fuel from vegetable oils or FAs has been extensively investigated over various bifunctional catalysts.5,6 Among them, the zeolite-supported metal catalysts showed promising catalytic performance on account of their appealing deoxygenation and isomerization activity.7,8 However, the USY-, SBA-15-, and MAM-41-supported metal catalysts needed high reaction temperature (> 360 ºC) for the hydroconversion of vegetable oils to aviation fuel because of their low intrinsic cracking activity.9,10 The conventional β- and ZSM-5-supported catalysts showed high cracking activity for the deoxygenated products even at lower reaction temperature (≤ 300 ºC). Whereas, the internal mass transfer limitation and shape selectivity of microporosity leaded to low selectivity (< 30%) of AFRAs and high selectivity (> 75%) of C4-C8 hydrocarbons.11-14 Therefore, decreasing the internal mass transfer limitation and shape selectivity of microporosity were crucial for ZSM-5-supported catalysts to efficiently upgrade vegetable oils to AFRAs at low reaction temperature (≤ 300 ºC). Previous investigations showed that decreasing catalyst size and/or introducing mesopore were the effective methods to mitigate the drawbacks of microporosity and kept the well defined structure and acid properties, thus improving the catalytic activity and product selectivity.15-17 However, the reported nano- and hierarchical ZSM-5-supported catalysts had Page 3 of 30 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 4 not shown desirable AFRA yield and iso/n-alkanes ratio.18,19 In our previous work, we designed a multilamellar Ni/ZSM-5 catalyst with paralleled nanosheet structure, which significantly increased the AFRA yield up to 41% (the maximum theoretical yield is 73.6%).20 Yet, the iso/n-alkanes ratio was still as low as 0.75, which was unsatisfactory compared to that of some real aviation fuels (1.2-2.0).21,22 Recently, it reported a self-pillared (SP) ZSM-5 zeolite,23 which remains the nanosheet structure but with different arrangement form and smaller particle size compare to the multilamellar one, leading to different acid distribution and catalytic performance.24-26 This SP ZSM-5 zeolite displayed high activity and stability for glucose/lactose isomerization, propane aromatization and bioethanol dehydration,24-26 and it was also highly active for mesitylene alkylation and benzyl alcohol self-etherification owing to the high accessibility of acid sites.23 Hence, this SP ZSM-5-supported Ni nanoparticles catalyst was speculated to be an efficient one for upgrading vegetable oils or FAs to AFRAs. In this context, for simultaneously improving the AFRA yield as well as the iso/n-alkanes ratio, the SP Ni/ZSM-5 catalysts with different Si/Al ratios (100, 200, and 300) were synthesized and employed for this conversion. The effect of the SP nanosheet structure on the activity and product selectivity was well investigated. 2. Materials and Methods 2.1. Materials. Tetra(n-butyl)ammonium hydroxide solution (TBAOH, 40 wt% in water) was purchased from Sigma-Aldrich (USA). Sodium hydroxide (99 wt%), aluminum sulfate octadecahydrate (98 wt%), tetraethoxysilane (TEOS, 98 wt%) was supplied by Shanghai Macklin Biochemical Co., Ltd. (China). Ni(NO3)2·6H2O (98 wt%, Alfa Aesar, China) were used as Ni sources. Oleic acid (96 wt%) was purchased from Shanghai Aladdin Bio-Chem Page 4 of 30 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 5 Technology Co., Ltd. (China). The conventional H-ZSM-5 (Si/Al = 100) was purchased from Nankai University catalyst factory (China). 2.2. Catalysts Preparation. The SP ZSM-5 zeolites were prepared by a process described in the literature.23 Tetra(n-butyl)ammonium hydroxide solution, water, aluminum sulfate octadecahydrate and sodium hydroxide were mixed and added dropwise into TEOS under vigorous stir. After further stir for 48 h, a clear sol with a composition of 100 SiO2: 0.50/0.25/0.17 Al2O3: 30 TBAOH: 2 NaOH: 1000 H2O: 400 EtOH was obtained. The sol was sealed in a 100 mL Teflon-lined stainless steel autoclave and heated at 120 ºC for 48 h. The solid products were centrifuged and washed with distilled water until the pH value of the final supernatant was lower than 9. The final precipitates were dried at 120 ºC for 12 h and then calcined at 550 ºC (1 ºC/min) for 12 h in flowing air. A 10.0 g portion of calcined samples were ion-exchanged to H+ form with 250 mL 1 M NH4NO3 aqueous solution at 80 ºC for three times and subsequently calcined at 550 ºC. Ni (10 wt%) was loaded on the SP ZSM-5 and conventional ZSM-5 zeolites by incipient wetness impregnation with the Ni(NO3)2 aqueous solution. After impregnation, the samples were placed at 25 ºC for 12 h and then dried at 120 ºC for 12 h. At last, the samples were calcined at 450 ºC (1 ºC/min) for 6 h. On the basis of Si/Al ratios, the final SP Ni/ZSM-5 catalysts were denoted as Ni/SZ(x), x was the Si/Al ratio. And the conventional Ni/ZSM-5 catalyst was labeled as Ni/CZ(100). The regeneration of catalysts was carried out in air at 500 ºC for 1 h. 2.3. Catalyst Characterizations. X-ray diffraction (XRD) was used to determine the structural properties of the catalysts on a D/MAX-2500 X-ray diffraction with (Cu Kα, λ = Page 5 of 30 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 6 1.541 Å) radiation at 40 kV and 140 mA in the 2θ range from 5˚ to 90˚ at a speed of 5°/min. The relative crystallinity of the samples were determined based on the intensity of the characteristic peaks in the range of 6.0°-9.0° and 22.5°-26.0°. The Si/Al ratios and Ni loadings of the catalysts were measured by X-Ray Fluorescence (XRF, S4 Pioneer, Bruker). The morphology and structure were determined by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2100F). Porous properties of the catalysts were determined by N2 adsorptiondesorption (N2-BET, Micrometrics Tristar, Micromeritics). Before the test, the samples were evacuated under vacuum at 300 ºC for 8 h. The pore size distribution of the zeolites was calculated by NLDFT equilibrium model. The amount of carbonaceous deposit on the spent catalysts was determined using thermogravimetric analysis (TGA Q50) in flowing air of 40 mL/min at a heating rate of 10 °C/min. The weight of each sample measured was adopted as about 15.0 mg. Before the test, the samples were washed by cyclohexane to remove the weakly adsorbed reactants, and then dried at 120 ºC for 5 h. Ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperatureprogrammed desorption (H2-TPD) were carried out with a Chemisorption Physisorption Analyzer (AMI-300, Altamira Instruments) equipped with a thermal conductivity detector (TCD). For each NH3-TPD test, 100 mg of the reduced sample was pre-treated at 450 °C in He for 3 h. After cooled to 50 °C, NH3 adsorption was performed by a mixture of NH3 and He (20% NH3 in He) at 50 °C for 30 min, and then He was passed through the reactor for 60 min to remove the weakly adsorbed NH3 from the catalyst. Desorbed NH3 was monitored in the Page 6 of 30 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 7 range of 50-600 °C at 10 °C/min. For each H2-TPD analysis, 100 mg as-synthesized sample was reduced by 10% H2/He at 500 °C for 3 h. After cooled to 50 °C, H2 adsorption was performed in the atmosphere of 10% H2/He for 30 min, and then pure He was passed for 60 min to remove the weakly adsorbed H2 from the catalyst. Desorbed H2 was monitored in the range of 30-800 °C. The relative concentrations of Brønsted and Lewis acid sites were measured by infrared spectroscopy (Vertex 70, Bruker) of adsorbed pyridine (Py-IR) at 150 and 300 ºC. And the external Brønsted acid sites concentration was determined by infrared spectroscopy of adsorbed 2,6-di-tert-butyl pyridine (DTBPy-IR). The concentration of the total Brønsted acid sites, Lewis acid sites and external Brønsted acid sites was calculated according to the literature [20]. 2.4. Catalytic Hydroconversion of Oleic Acid. The catalytic hydroconversion of oleic acid to AFRAs over the SP and conventional Ni/ZSM-5 catalysts was carried out with a fixedbed flow reactor (1.0 cm i.d. and 45 cm in length). The reaction temperature was controlled by three thermocouples on the reactor wall and monitored with a thermocouple in the catalyst bed. A 2.00 g portion of catalyst was loaded and fixed by SiC to get sufficient catalyst-bed length (about 4 cm) in the reactor. Oleic acid was dissolved in cyclohexane to get a 33.3 wt% solution as the feedstock and injected into the reactor at a flow of 0.40 mL/min using a high-pressure pump. For each catalyst, the reaction was carried out at 260, 280, and 300 ºC under 3.0 MPa with the H2/Oil (Nml·ml-1) ratio of 750. Before each reaction, the sample was reduced under H2 at 500 ºC and 3.0 MPa for 3 h. The liquid products were successively collected for 1 h in each experiment. The gaseous fraction was analyzed online with an Agilent 3000 gas chromatograph equipped with three columns (molecular sieve, plot U, and alumina) and a TCD Page 7 of 30 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 8 detector. The liquid products were centrifuged in two parts: water and organic liquid products (OLPs). Icosane was used as the internal standard to quantify the OLPs with a gas chromatograph (Bruker 456 GC, Bruker), equipped with a commercial column (ZB-5 HT, 60 m × 0.25 mm × 0.25 mm) and a flame ionization detector (FID), and an Agilent 6890N gas chromatography/5975N mass spectrometry (GC/MS) instrument. The conversion of oleic acid, yield of OLPs, and selectivity/yield of corresponding hydrocarbons (CxHy) in the OLPs were calculated based on the eqs in the literature [20]. A blank experiment was also conducted at the same reaction condition over each catalyst. According to the results given in Figure S1, the effect of solvent in yield and selectivity calculations could be neglected. The critical molecular diameter (σ) mentioned it this work is estimated from the properties of the fluid at the critical point (c), shown in eq (1) according to Bird et al.27 σ=0.841Vc1/3 (1) Where Vc is the critical volume (cm3/mol), which is obtained from the CRC Handbook.28 3. Results and Discussion 3.1. Texture Properties of the SP Ni/ZSM-5 Catalysts. The SEM and TEM images of the SP and conventional ZSM-5 zeolites are illustrated in Figure 1. The images showed that the SP samples had a particle size of 120-150 nm (SEM) and consisted of nanosheets with an average thickness of 2-3 nm along the b-axis (TEM). In addition, the nanosheets arranged perpendicularly to each other, forming the SP structure with defined uniform mesopore (3-5 nm). This SP nanosheet structure improved the active sites accessibility during the catalytic reactions. The conventional ZSM-5 zeolite had an average thickness of 200 nm and particle size about 1.5 μm as shown in Figure S2, which was significantly higher than that of the SP Page 8 of 30 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 9 ones. Additionally, the characteristic diffusion lengths (xp) of the series catalysts decreased as the following: Ni/CZ(100) >> Ni/SZ(200) > Ni/SZ(100) > Ni/SZ(300) (determined by TEM), indicating that the SP Ni/ZSM-5 catalysts had much shorter micropore diffusion length than the conventional one. Figure 1. SEM and TEM images of the SP ZSM-5 zeolites. The XRD patterns of all the Ni/ZSM-5 catalysts are shown in Figure 2. All the samples showed the characteristic peaks in the range of 6º-9° and 22.5°-25.0°, which were assigned to the MFI framework (JCPDS No. 44-0003).20 However, the characteristic peaks of the Ni/SZ(x) catalysts were wider than that of the Ni/CZ(100) because of their smaller size and nanosheet structure. The relative crystallinity (RC) of the Ni/SZ(x) catalysts was obviously lower than that of the conventional one due to the smaller crystallites,29 and it increased with the increasing Page 9 of 30 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 10 Si/Al ratio. Moreover, only the {h0l} reflections were observed over the self-pillared samples, implying the thin b-axis framework thickness and wide a-c planes, which was in accordance with the result of TEM.23 The reflections at 2θ = 44.5°, 51.9° and 76.5° indicated the existence of cubic metal Ni.30 The reflections of metal Ni on Ni/CZ(100) catalyst was significantly stronger than that on Ni/SZ(x) catalysts, suggesting that smaller Ni particles was formed attributed to the high specific surface area of the SP ZSM-5s, which was consistent with the result of H2-TPD as shown in Table 1. The XRF result presented that the Ni loadings of all the catalysts were about 10 wt%. 10 20 30 40 50 60 70 80 90 Ni/SZ(100) Ni/SZ(200) Ni/SZ(300) Ni/CZ(100) ★ Ni 20 2 10 2 40 0 00 2 30 1 5 03 50 1 20 0 42.3% 52.6% 56.1% ★ In te ns ity (a .u .) 2( ★ ★ ★ RC=100% 10 1 ★ ★ ★ ★ ★ ★ ★ ★ Figure 2. XRD patterns of the SP and conventional Ni/ZSM-5 catalysts. Reflections of Nickel oxide are highlighted with a star (★). N2-BET was employed to determine the porosity of the SP Ni/ZSM-5 catalysts, their N2 adsorption–desorption isotherms and pore size distributions are presented in Figure 3. As shown in Figure 3a, the inflection points of the catalysts at relative pressure from 0 to 0.1 was the fingerprint of a microporous structure.17 All the SP Ni/ZSM-5 catalysts showed the type IV isotherm with a steep hysteresis loop at relative pressure from 0.8 to 1.0, suggesting that the mesoporosity was derived from the lamellar spacing or interparticle voids,24 which was agree with the results of SEM and TEM. The pore size distributions (PSDs) of the SP samples Page 10 of 30 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 11 indicated that the mesopore size ranged from 3 to 5 nm (Figure 3b). The pore properties are summarized in Table 1, the Ni/SZ(x) catalysts had much higher external surface areas and mesopore volumes compared to Ni/CZ(100). In addition, the SP Ni/ZSM-5 catalysts had similar micropore specific surface areas/volumes but lower mesopore specific surface areas/volumes after loading 10 wt% Ni. However, a manifestly reduced micropore specific surface area/volume and similar mesopore specific surface area/volume was obtained over the Ni/CZ(100) catalyst compare to the conventional ZSM-5 zeolite. These results indicated that a part of the Ni nanoparticles were encapsulated into the micropore of Ni/CZ(100) catalyst, but most of the Ni nanoparticles were loaded on the external surface of Ni/SZ(x) catalysts, which increased the metal sites accessibility. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ni/CZ(300) Ni/CZ(200) Ni/CZ(100) Ni/CZ(100) A bs or be d V ol um e (S T P) (c m 3 / g) P/P0 a 2 3 4 5 6 7 8 Ni/SZ(300) Ni/SZ(200) Ni/SZ(100) Ni/CZ(100) Pore size (nm) dV /d lo gD (c m 3 / g▪ nm ) b Figure 3. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of the SP and conventional Ni/ZSM-5 catalysts. Page 11 of 30 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 12 Table 1. Physicochemical properties of the SP and conventional Ni/ZSM-5 catalysts. samples Ni a (wt %) Ni Size b (nm) SBET (m2/g) Sext (m2/g) Smicro (m2/g) Vmeso (cm3/g) Vmicro (cm3/g) xp c (nm) CZ(100) - - 305 86 219 0.051 0.141 100 SZ(100) - - 605 333 272 0.585 0.125 1.2 Ni/CZ(100) 9.9 19.1 (17.5) 245 72 173 0.047 0.115 100 Ni/SZ(100) 10.1 14.1 (12.1) 542 284 258 0.553 0.118 1.2 Ni/SZ(200) 10.0 14.8 (12.8) 508 245 263 0.545 0.122 1.3 Ni/SZ(300) 10.1 15.4 (12.6) 546 300 246 0.573 0.113 1.1 a Determined by XRF analysis; b Ni size was Calculated by Scherrer equation31 and H2-TPD (assumed one H2 molecule to be desorbed per surface Ni atom)8; c The characteristic diffusion length, xp (nm), is defined as half the thickness of the zeolite crystal along the b-axis.32,33 3.2. Acidity Properties of the SP Ni/ZSM-5 Catalysts. The acid properties of all the catalysts were determined by NH3-TPD, Py-IR and DBTPy-IR. As shown in Figure 4, both the NH3 desorption peaks of the SP catalysts shifted to lower temperature as the Si/Al ratio increased, suggesting that increasing Si/Al ratio weakened the acid strength. The IR spectra of Py and DTBPy adsorption are given in Figure S3, and the acid properties are listed in Table 2. For the SP Ni/ZSM-5 catalysts, the Brønsted and Lewis acid concentration significantly decreased with the increasing Si/Al ratio, but the Brønsted/Lewis acid sites (B/L) ratio was similar. In addition, the SP Ni/SZ(x) catalysts had much lower B/L ratio and Brønsted acid concentration than the conventional one, which made their acidity features similar to that of the mesoporous zeolites (MCM-41, SBA-15 at al.).34 Ni/SZ(100) catalyst had similar total acid concentration compared to Ni/CZ(100) catalyst, while its external acid concentration and external acid site fraction was significantly higher. Moreover, the external acid site fraction of the Ni/ZSM-5 catalysts (determined by DTBPy-IR) increased with the reduction of Page 12 of 30 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 13 characteristic diffusion length (Figure 4b), indicating the higher acid sites accessibility of the SP Ni/ZSM-5 catalysts than the conventional one. 100 150 200 250 300 350 400 Ni/SZ(300) Ni/SZ(200) Ni/SZ(100) Ni/CZ(100) T C D Bed temperture (C) a 1 10 100 0.1 1 1 1.1 1.2 1.3 1.4 0.35 0.36 0.37 0.38 0.39 0.4 0.41 Ni/SZ(200) Ni/SZ(100) Ex te rn al ac id si te fr ac tio n f ex t Characteristic diffusion length xp (nm) Ni/SZ(300) Ex te rn al a ci d sit e f ra ct io n f ex t Characteristic diffusion length xp (nm) b Ni/CZ(100) Figure 4. (a) NH3-TPD curves of the SP and conventional Ni/ZSM-5 catalysts and (b) external acid site fractions of Ni/ZSM-5 catalysts as a function of the characteristic diffusion lengths. Table 2. Acid properties of the SP and conventional Ni/ZSM-5 catalysts. strong (μmol/g) weak (μmol/g) catalysts B (py) L (py) B (py) L (py) total (μmol/g) B/L fext a Ni/CZ(100) 54 25 43 68 190 1.03 0.04 Ni/SZ(100) 17 40 10 109 176 0.18 0.38 Ni/SZ(200) 10 25 6 53 94 0.21 0.36 Ni/SZ(300) 4 14 3 27 48 0.17 0.40 a External acid site fraction, (fext), defined as the amount of acid sites quantified by DTBPy-IR to the amount of acid sites measured by Py-IR. 3.3. Catalytic Hydroconversion of Oleic Acid 3.3.1 Effect of the SP nanosheet structure on deoxygenation activity and pathways. The hydroconversion of oleic acid was carried out at different reaction temperatures (260, 280, and 300 ºC) to investigate the effect of the SP nanosheet structure on the deoxygenation activity and pathways. As shown in Figure 5a, 100% conversion of oleic acid was achieved over all the catalysts at 300 ºC. As the reaction temperature declined to 280 ºC, Ni/SZ(x) catalysts still kept Page 13 of 30 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 complete conversion of oleic acid, while the oleic acid conversion reduced to 86% over Ni/CZ(100) catalyst. With a further decrease of the reaction temperature to 260 ºC, only the Ni/SZ(100) catalyst achieved 100% conversion of oleic acid. For Ni/SZ(200) and Ni/SZ(300) catalysts, the oleic acid conversion slightly declined to 95% and 92%, respectively. But only 60% conversion of oleic acid was obtained over Ni/CZ(100) catalyst. In addition, as the Si/Al ratio decreased from 300 to 100, the conversion of oleic acid increased from 92% to 100% over the SP catalysts at 260 ºC. These results indicated that Ni/SZ(x) catalysts were extremely active for hydrodeoxygenation of oleic acid compare to Ni/CZ(100). The high activity of the Ni/SZ(x) catalysts was attributed to the SP nanosheet structure, which resulted in high Ni dispersion and high accessibility of the active sites (metal and acid sites). Wang et al.20 and Lercher et al.35 reported that acid sites, especially the external acid sites, facilatated the C-O bond cleavage by adsorbing the basic oxygen atom, which increased the deoxygenation activity. As illustrated in Figure 5b, the hydrodeoxygenation (HDO) selectivity of the SP catalysts was higher than that of the conventional one (22 mol%), and it increased with the decline of Si/Al ratio. This result concluded that the increasing external acid concentration improved the deoxygenation activity by accelerating the HDO reaction. 0 20 40 60 80 100 300 C280 C Ni/CZ(100)Ni/SZ(300) 260 C Ni/SZ(200) C on ve rs io n % Ni/SZ(100) a 0 10 20 30 40 50 Se le ct iv ity (m ol % ) HDO DCO2 DCO Ni/SZ(300) Ni/CZ(100)Ni/SZ(200)Ni/SZ(100) b 0 10 20 30 40 50 60 70 E xt er na l a ci d si te s ( m ol /g ) Page 14 of 30 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 15 3.3.2 Effect of the SP nanosheet structure on product distributions. The effect of the SP nanosheet structure on the product distributions was then explored at different reaction temperatures (260, 280, and 300 ºC). As illustrated in Figure 6a, the SP Ni/SZ(x) catalysts had higher OLP yield than the conventional Ni/CZ(100) catalyst, especially at 260 and 280 ºC attributed to the higher oleic acid conversion and lower cracking degree. For Ni/SZ(100) catalyst, the OLP yield reduced as the reaction temperature increased from 260 to 300 ºC due to the enhanced secondary cracking of the deoxygenated products at higher reaction temperature.34 But the OLP yield over Ni/SZ(200) and Ni/SZ(300) catalysts firstly increased owing to the increased oleic acid conversion and then decreased as a result of the enhanced secondary cracking. The OLP yield over Ni/CZ(100) catalyst successively increased mainly due to the increasing oleic acid conversion. With the increment of reaction temperature, the organic gas products (OGP) yield increased over all the catalysts by reason of the incremental conversion, enhanced secondary cracking, or both (Figure 6b). 0 20 40 60 80 100 300 C280 C Ni/CZ(100)Ni/SZ(300) 260 C Ni/SZ(200) Y ie ld o f O L Ps % Ni/SZ(100) a 0 4 8 12 16 20 300 C280 C Ni/CZ(100)Ni/SZ(300) 260 C Ni/SZ(200) Y ie ld o f O G Ps % Ni/SZ(100) b Figure 6. Yield of (a) OLPs and (b) OGPs over the SP and conventional Ni/ZSM-5 catalysts at 260, 280, and 300 ºC. The OLP distributions over the Ni/ZSM-5 catalysts at different reaction temperatures (260, 280, and 300 ºC) are displayed in Figure 7. For the SP Ni/ZSM-5 catalysts, as the Si/Al ratio decreased from 300 to 100, the C16-C18 hydrocarbons yield decreased but the C4-C8 Page 15 of 30 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 16 hydrocarbons yield increased because of the enhanced secondary cracking caused by incremental acid concentration, especially the Brønsted acid concentration.35 And the optimum reaction temperature for producing AFRAs (C9-C15 hydrocarbons) over the SP catalysts declined from 300 to 260 ºC with the Si/Al ratio decreasing from 300 to 100, indicating that increased acid concentration reduced the optimum reaction temperature for the cracking of the deoxygenated products into AFRAs. This was because that both the acid sites and reaction temperature were the driving forces for the cracking of alkanes.34,35 Moreover, the Ni/SZ(x) catalysts achieved a much higher AFRA yield (36-40% vs 10%) with higher iso/n-alkanes ratio ( 1.8 vs 0.8) and selective cracking ratio than Ni/CZ(100) catalyst (Figure 8). 0 10 20 30 40 50 60 300 C4-C8 Hydrocarbons C9-C15 Hydrocarbons C16-C18 Hydrocarbons 280 Y ie ld % 260 a 38.7 % 0 10 20 30 40 50 60 300 C4-C8 Hydrocarbons C9-C15 Hydrocarbons C16-C18 Hydrocarbons 280260 b 35.5 % 0 10 20 30 40 50 60 300 C4-C8 Hydrocarbons C9-C15 Hydrocarbons C16-C18 Hydrocarbons 280 Y ie ld % 260 Temperature (C) c 39.7 % 0 10 20 30 40 50 60 300 C4-C8 Hydrocarbons C9-C15 Hydrocarbons C16-C18 Hydrocarbons 280260 Temperature (C) d 10.1% Figure 7. OLP distributions over (a) Ni/SZ(100), (b) Ni/SZ(200), (c) Ni/SZ(300), and (d) Ni/CZ(100) catalysts at 260, 280, and 300 ºC. Page 16 of 30 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 17 0 1 2 3 4 is o/ nal ka ne s r at io 260 °C 280 °C 300 °C Ni/SZ(200) Ni/SZ(300)Ni/SZ(100)Ni/CZ(100) a 0.8 1.81.81.7 0.0 0.6 1.2 1.8 2.4 3.0 260 °C 280 °C 300 °C SC R Ni/SZ(200) Ni/SZ(300)Ni/SZ(100)Ni/CZ(100) b Figure 8. (a) iso-/n-alkanes ratio of C9-C15 hydrocarbons and (b) selectivity cracking ratio (SCR: defined as the ratio of C9-C15 selectivity/C4-C8 selectivity) over the SP and conventional Ni/ZSM-5 catalysts at 260, 280, and 300 ºC. For analysis of the different AFRA yields and iso/n-alkanes ratios between the SP and conventional Ni/ZSM-5 catalysts, the carbon number distribution and the 2-methyl isomer ratio of the AFRAs were further investigated. As shown in Figure 9, upon comparison with the conventional catalyst, the SP ones showed lower selectivity of long-chain (C16-C18) and shortchain (C4-C8) hydrocarbons but higher selectivity of AFRAs (C9-C15 hydrocarbons). Generally, the deoxygenated products (C17/C18) are too big (critical molecular diameter about 10 Å) to enter into the micropore (5.5×5.6 Å) of ZSM-5 zeolites. So the SP catalysts with high external acid concentration could enhance the isomerization and cracking of the deoxygenated products into AFRAs on the catalyst external surface and pore mouth. The higher critical molecular diameter of the generated AFRA isomers inhibited their diffusion into the micropore for secondary cracking. In addition, the SP nanosheet structure increased the proximity between Ni and Brønsted acid sites, which could weaken the secondary cracking of the linear AFRAs.38 And the shorter characteristic diffusion length of the SP nanosheet, which significantly reduced the internal mass transfer limitation, favored the generated AFRA isomers escape from the catalyst. As a result, the SP catalysts showed high AFRA yield and high iso/n-alkanes ratio Page 17 of 30 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 18 attributed to the nanosheet structure. 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180 5 10 15 20 Ni/CZ(100) Ni/SZ(100) Ni/SZ(200) Ni/SZ(300) Se le ct iv ity % Carbon number Figure 9. Carbon number distribution of OLPs over the SP and conventional Ni/ZSM-5 catalysts. Additionally, the maximum AFRA yield over the Ni/ZSM-5 catalysts showed a linear increase with the increasing external acid fraction, but it decreased linearly with the increased characteristic diffusion length (Figure 10). This further confirmed that the SP nanosheet structure with high external acid fraction and short characteristic diffusion lengths favored the generation of AFRAs. The 2-methyl isomer ratio increases linearly with the increasing characteristic diffusion length but decreases with the increment of external acid fraction, which was because that high external acid fraction favored alkanes isomerize on the catalyst external surface to generate random n-methyl isomers (n=2-6), while the reduced characteristic diffusion length weakened the shape-selectivity for generating 2-methyl isomers by reason of the declined internal mass transfer limitation. Moreover, it was observed that the 2-methyl isomer ratio of the high carbon number hydrocarbons (≥ C12, critical molecular diameter above 5.6 Å) was significantly lower than that of the low carbon number hydrocarbons (≤ C11). This was because that the high carbon number hydrocarbons mainly isomerized on the catalyst external surface, which further confirmed the above results and was consistent with the previous work.39 Page 18 of 30 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 19 0.0 0.1 0.2 0.3 0.4 10 15 20 25 30 35 40 Ni/CZ(100) Ni/SZ(200) Ni/SZ(100) M ax im um A FR A y ie ld External acid fraction fext Ni/SZ(300) a 0.3 0.4 0.5 0.6 0.7 0.8 R2=0.998R2=0.996 2- m et hy l i so m er r at io 1 1.1 1.2 1.3 1.4 80 85 90 95 100 105 10 15 20 25 30 35 40 Ni/CZ(100)Ni/SZ(200)Ni/SZ(100) M ax im um A FR A y ie ld Characteristic diffusion length xp (nm) Ni/SZ(300) b 0.3 0.4 0.5 0.6 0.7 0.8 2- m et hy l i so m er r at io Figure 10. The maximum AFRA yield and 2-methyl isomers ratio over the SP and conventional Ni/ZSM-5 catalysts as a function of (a) external acid fraction and (b) characteristic diffusion length. At last, we compared the SP Ni/ZSM-5 catalyst with the multilamellar one in our previous work 20 both in the preparations and catalytic results. The multilamellar ZSM-5 zeolites were synthesized at 150 °C for 120 h with tumbling at 30 rpm, while the SP ones were synthesized at only 120 °C for 48 h and with no tumbling. Additionally, the expensive structure-directing agent (SDA, C22-6-6(Br)2) of the multilamellar ZSM-5 zeolites need to be synthesized by two steps using toluene as the solvent. But the SDA of the SP ones was TBAOH, which was cheaper and accessible from the market. These indicated that the SP catalysts could be obtained in lower cost. The resulted SP Ni/ZSM-5 catalysts inherited the nanosheet structure with the intergrown pattern rather than the paralleled arrange as the multilamellar ones. And the SP Ni/ZSM-5 catalyst obtained similar AFRA yield (40% vs 41%) but much higher iso/n-alkanes ratio (1.8 vs 0.75) compare to the multilamellar one. Accordingly, we further concluded that the high AFRA yield originated from the contribution of the nanosheet structure, which had short characteristic diffusion length. The higher iso/n-alkanes ratio over the SP Ni/ZSM-5 catalyst may be result from its reasonable acid distribution (low concentration of strong Brønsted acid, slightly low external acid fraction and low B/L ratio as shown in Table S1), which was derived Page 19 of 30 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 20 from the different synthesize conditions and nanosheet structure patterns. 3.3.3. Stability and Cokes. For investigating the catalytic stability of the SP and conventional Ni/ZSM-5 catalysts, the oleic acid conversion and OLP distribution with time on stream at 300 °C and 3 MPa was further investigated. As displayed in Figure 11a, Ni/SZ(x) catalysts showed an oleic acid conversion of 100% even after 360 min. However, the oleic acid conversion began to decrease after 180 min and dropped to 86.3% at 360 min over Ni/CZ(100) catalyst. In addition, the OLP distribution over Ni/SZ(300) catalyst almost unchanged at first 240 min with a slight increase of C16-C18 hydrocarbons selectivity after 360 min run. This was the result of the accumulation of intermediate products on the catalyst surface, which covered part of the acid sites. This result implied that the SP Ni/ZSM-5 catalysts with the nanosheet structure had excellent durability for the hydroconversion of oleic acid to AFRAs. In order to study the reason for the different catalytic stability of the SP and conventional Ni/ZSM-5 catalysts, the amount of carbonaceous deposition over all the catalysts was measured since carbonaceous deposit was the main reason for catalyst deactivation.29,40 As shown in Figure S4a, the slight weight loss below 100 °C mainly derived from the water evaporation. The rapid weight loss between 300 and 400 °C may ascribe to the combustion/desorption of the reactant and/or intermediate products adsorbed on the catalyst surface.41 And the SP Ni/SZ(x) catalysts had more carbonaceous deposition than Ni/CZ(100) catalyst. Furthermore, the nature of the carbonaceous deposition was determined by FT-IR to analysis the probable intermediate products, and the results are shown in Figure S4b. Compared with the fresh and regenerated Ni/SZ(300) catalysts, the spent Ni/ZSM-5 catalysts had extra peaks at 2920 and 2860 cm-1, which were ascribed to the -CH2- bond and -CHO bond, respectively.42,43 There was Page 20 of 30 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 21 also a slight absorption at 1746 cm-1 assigned to C=O bond in -COOH over the spent catalysts. Therefore, the weight loss between 300 °C and 400 °C may be assigned to the intermediate products (such as acids/aldehydes) accumulated on the catalyst surface. The further weight loss above 400 °C was because of the combustion of aromatic coke species.44 And Ni/SZ(x) catalysts had a higher weight loss than that of Ni/CZ(100) catalyst (2 wt% vs 1.2 wt%). This was because that the mesopore of the SP catalysts structure favored the cyclization of generated olefins,19,45 which were the precursor for aromatic coke.46,47 Although higher amount of the carbonaceous deposition was observed over the SP Ni/ZSM-5 catalysts, they demonstrated better catalytic stability owing to the SP nanosheet structure. As shown in Table S2, the spent Ni/SZ(300) catalyst had a manifestly lower mesopore specific surface area and similar micropore specific surface area compare to the fresh one. However, the spent Ni/CZ(100) catalyst showed lower micropore specific surface area than the fresh one. This was because that the SP nanosheet structure facilitated the coke species transfer to the catalyst external surface from the micropore channel due to the short characteristic diffusion length, thus improving the catalytic stability, which was in accordance with the previous work.48 60 120 180 240 300 360 60 70 80 90 100 C on ve rs io n % TOS (min) Ni/SZ(100) Ni/SZ(100) Ni/SZ(200) Ni/CZ(300) a 60 120 180 240 300 360 0 20 40 60 80 100 C4-C8 C16-C18 C9-C15 Se le ct iv ity % TOS (min) b Figure 11. (a) Oleic acid conversion over the SP and conventional Ni/ZSM-5 catalysts and (b) OLP distribution over Ni/SZ(300) catalyst with time on stream at 300 °C, 3.0 MPa and 3.6 h-1. Page 21 of 30 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 22
4. Conclusions
The SP ZSM-5-supported Ni nanoparticles catalysts had high Ni dispersion, high accessibility of active sites (metal/acid sites) and short characteristic diffusion length attributed to the SP nanosheet structure. As a consequence, the SP catalysts showed outstanding catalytic activity (conversion ≥ 92%) even at low reaction temperature (260 C) and excellent catalytic stability. The SP nanosheet structure promoted the isomerization and cracking of the deoxygenated products into AFRAs owing to the high external acid fraction and shortened characteristic diffusion length, which also weakened the secondary cracking of the linear AFRAs and favored the generated isomers escape from the catalyst attributed to the decreased internal mass transfer limitation. This work provided some insights for the design of highly efficient catalysts for producing high performance aviation fuel from vegetable oils and FAs in a single-step route. AUTHOR INFORMATION Corresponding Author *Tel./Fax: 86-22-27892340. E-mail address: wlytj@tju.edu.cn (L. Wang), qfwang@tju.edu.cn (Q. Wang). Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
The financial supports by National Natural Science Foundation of China (GrantNo.21476169, Page 22 of 30 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 23 21476168) are gratefully acknowledged. The authors thank Prof. Xinhua Liang in Missouri University of Science & Technology and Prof. Sheng Dai in Oak Ridge National Laboratory (USA) for their help on the English.
ASSOCIATED CONTENT
Supporting Information. Blank experimental results, More TEM image, (DTBP)Py-IR spectra, thermogravimetric analysis data and FT-IR spectroscopic results, comparison of the SP and multilamellar Ni/ZSM-5 catalysts, more N2-BET data.
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