Iron- and Cobalt-Doped CeriaâZirconia Nanocomposites for Catalytic

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Iron- and cobalt-doped ceria-zirconia nanocomposites for catalytic cracking of naphtha with regenerative capability Oluwole Olagoke Ajumobi, Oki Muraza, Idris A Bakare, and Adnan M. Al-Amer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01376 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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 free 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 accessible to all readers and 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.

Energy & Fuels 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 43

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

C6H14

Energy & Fuels

Hexane

Olefins Catalytic Cracking

Paraffin

Product Distribution

BTX

ACS Paragon Plus Environment

Hexane

Energy & Fuels

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

Iron- and cobalt-doped ceria-zirconia nanocomposites for catalytic cracking of naphtha with regenerative capability Oluwole O. Ajumobia,b, Oki Murazaa,b*, Idris A. Bakarea, Adnan M. Al Amerb a

Center of Research Excellence in Nanotechnology, bChemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

Abstract Series of nano-sized iron- and cobalt-doped ceria-zirconia nanocomposites were prepared using hydrothermal synthesis technique at 180 °C for 24 h, with the successful novel incorporation of both Co and Fe on ceria-zirconia, for n-hexane catalytic cracking. Effects of dopant ions on the improvement of intrinsic properties of ceria-zirconia nanocomposites were investigated using disparate characterization techniques. The synthesized ceria-zirconia nanocomposites exhibited similar X-ray diffraction (XRD) patterns, indicating full fusion of the metal ions into the ceria-zirconia lattice structure. The synthesized nanocomposite catalysts were tested for n-hexane cracking over 10 h time-onstream, with no previous study or report for catalytic cracking of hexane via ceria-zirconia nanocomposites. Relatively high ethylene and propylene selectivity (both > 62%), was obtained over CZ, FeCoCZa and FeCoCZb over time-on-stream. Comparatively, best catalytic activity and stability was exhibited by FeCoCZa with higher n-hexane conversion. Temperature and catalyst weight per feed flowrate (W/F) variations were investigated using the best catalyst (FeCoCZa). Higher conversions were obtained at higher temperature and lower W/F but varied product selectivity and yield, over time-on-stream. In addition, the spent catalysts were successfully regenerated after catalytic testing via calcination at 600 °C for 4 h, and re-used for two additional cycles.

*Corresponding Author (OM): E-mail: [email protected], Phone: +966 13 860 7612


Page 2 of 43

Page 3 of 43

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

Energy & Fuels

1.0

Introduction Much research and effort have been dedicated to developing and to modify metal

oxides as catalyst for several processes, both industrially and on laboratory scale. Cerium is a transition metal, abundant in nature and its oxide (cerium IV oxide, CeO2) has been widely used as a catalyst. Cerium (IV) oxide commonly known as ceria, is a mesoporous amphoteric oxide, with more basic sites of medium strength compared to other oxides1 . In addition, it has also been used for various extensive applications. Ceria (CeO2) and ceria-based composite oxides have found application in many areas such as water-gas shift reactions2, cracking of hydrocarbons3, pollution emission control for automobiles as a three-way catalyst (TWC)4, soot combustion5, alcohol fuels cells6, oxidation of alcohols and organic compounds7,8 and conversion of heavy organic molecules9. Also, it has been used for many other processes such as steam reforming, biomedical applications10, synthesis gas production and dehydration of alcohols. Cerium exists in two oxidation states of Ce3+ (ionic radius, 1.14 Å) and Ce4+ (ionic radius, 0.97 Å), highlighting its redox (reduction-oxidation) properties. Ceria and ceria-based composites are well known as oxygen storage materials, owing to their ability for oxygen storage and release capacity (OSC), which allows them to undergo redox cycle11. This is an important property in hydrocarbon cracking process as this minimizes the formation of coke, preventing sintering and catalyst deactivation at high temperature even with steam. According to literature, ceria has low surface area (mostly in the range of 1 – 4 m2/g)12, and its catalytic properties are immensely reduced when exposed to high temperatures and unfavourable reaction conditions. This leads to reduction of CeO2 which


Energy & Fuels

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

leads to reduction in Lewis surface acidity, loss of active surface area, decreased OSC and redox ability

13

. However, the intrinsic characteristics and morphology of ceria can be

improved by synthesis modifications and approaches for improved morphology and catalyst efficiency. Doping ceria with suitable cations of transition or rare-earth metals such as zirconium (Zr4+) improves its basic and physicochemical properties such as stability (thermally and in harsh reaction conditions), increases surface area, OSC, redox property and oxygen mobility within lattice structure14,15. This can be attributed to the dispersion of the dopant ions (such as Zr4+) into the ceria crystal lattice structure, thus causing lattice distortions, structural defects and morphology agglomeration changes, thereby improving the mobility of oxygen ions and availability of oxygen-vacant sites14,16. Ceria, like other known catalyst oxides, is prone to sintering and catalyst deactivation especially at high temperature and unfavourable reaction environments. Doping ceria with oxides metal ions such as Zr4+ enhances its stability against sintering and catalyst deactivation in the presence of halides, preventing the development of chlorinated species in excess oxygen17. According to Moser et al 18, incorporation of zirconia improves the redox properties of ceria/zirconia, leading to better catalytic performance of ceria. Light olefins are known to be attributed raw materials needed for production in the petrochemical industries, with increasing demand for ethylene and propylene around the world. These olefins are commonly produced from thermal and steam cracking of several hydrocarbon feedstocks such as naphtha19,20. These two approaches require high energy due to the endothermic condition of the reaction which occurs at high temperature (700 – 900 °C) according to Boyadjian et al. energy

20

, consuming about 30% of the overall refinement process

19

. With the increasing economic consideration and the growing demand for these

olefins, catalytic cracking offers a more favourable, less energy demand, and control over desired product selectivity and yield. Naphtha cracking has provided an alternative route to


Page 4 of 43

Page 5 of 43

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

Energy & Fuels

olefin production in the presence of catalysts such as zeolite ZSM-1221. Catalytic cracking can be carried out at lower temperature than that of steam- and thermal cracking, and parameters such as propylene/ethylene yield ratio can be controlled through β-scission mechanism22. Catalytic cracking of hydrocarbon has an ionic mechanism which is executed either by the classical β-scission (bimolecular) of carbenium-ions chain or protolytic (monomolecular) cracking mechanism23,24. For metal oxide catalysts, the surface metal ions and lattice oxygen execute the cracking process when they make contact with the hydrocarbon feed. In this regard, we propose the synthesis of ceria-zirconia nanocomposites for nhexane cracking into light olefins with relatively high selectivity approximately 75 %. There has been no previous study or reports on n-hexane cracking via ceria-zirconia nanocomposites. These catalysts possess lattice oxygen as well as the presence of Lewis acidity for catalytic activity. These nanocomposites were synthesized using hydrothermal synthesis technique, and doped with iron and cobalt to obtain Fe-Co-doped ceria/zirconia nanocomposites. Sample characterizations were carried out using required characterization techniques and equipment. The effect of composition will be investigated on the phase structure, crystallite and particle sizes, surface area, acidity and basicity. Catalytic cracking of n-hexane will be performed by loading known amount of each synthesized catalyst into the reactor column in a fixed bed. Temperature and time factor (W/F) variations will investigated to substantiate their effect on conversion, stability, product selectivity, and product yield. 2.

Experimental

2.1

Hydrothermal synthesis Ceria Zirconia nanocomposites with different compositions as thus; CZ (Ce:75 wt.%;

Zr: 25 wt.%), FeCoCZa (Ce: 60 wt.%; Zr: 20 wt.%; Fe: 10 wt.%; Co: 10 wt.%) and FeCoCZb


Energy & Fuels

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

(Ce: 70 wt.%; Zr: 20 wt.%; Fe: 5 wt.%; Co: 5 wt.%) were prepared via hydrothermal synthesis. Precursor salts of Ce(CH3CO2)3.H2O (Sigma-Aldrich, 99.9 %), ZrO(NO3)2.H2O (Sigma-Aldrich, 99 %), Fe(NO3)3.9H2O (PRS Panreac, 98 %) and CoCl2.6H2O (BDH Chemicals, 99 %) were weighed accordingly (depending on the nanocomposite to be prepared) and dissolved in deionized water and magnetically stirred at 500 rpm for 1 h. Simultaneously, KOH (Panreac, 85%) solution was slowly added to the respective precursor solutions as precipitant. Further stirring for 3 h at room temperature to enhance aging and nucleation. The obtained solutions were poured into PTFE-lined steel autoclaves and placed in a hydrothermal oven at synthesis temperature of 180 oC for 24 h and a tumbling speed of 40 rpm. The obtained solutions centrifuged and washed for sample collection. The collected samples were dispersed in ethanol and dried in air at room temperature for 18 h, followed by drying at 75 oC for 10 h and calcined at 600 oC for 6 h. 2.2

Characterizations of Nanocomposites X-ray diffraction (XRD) was performed on all prepared samples before and after

calcination, in order to see the effect on calcination (which causes particle agglomeration due to dehydration) on crystallite size and lattice parameter after calcination. A Rigaku Miniflex II Desktop X-ray diffractometer with an accuracy of ± 0.02° with silicone standard calibration was used at 2Ɵ range of 3 o to 80 o, a scan speed of 3.00 and sampling step size of 0.03o. Raman spectroscopy was carried out to verify the presence extra peaks and the impact of Fe and Co ions on peak intensity of ceria-zirconia. This was carried out using Yvon Jobin Horiba Raman spectrometer (iHR320) with charge-couple device (CCD) detector at a spectrum window of 30 to 2000 cm-1, laser (green type, 532 nm) intensity of 50 % and an exposure time of 15 s. Surface morphology analysis was carried out by Field Emission Scanning Electron Microscopy (FE-SEM) using Oxford Instrument, X-Max Scanning Electron Microscope at high voltage of 20 kV, magnification of 200 – 300 kx and imaging


Page 6 of 43

Page 7 of 43

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

Energy & Fuels

scaling of 500 nm. Physicochemical properties of the synthesized samples were carried out by surface area analysis using Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Temperature programmed desorption (TPD) for acidity and basicity measurement was carried out using N2-adsorption technique was employed using liquid nitrogen for a downtime of 24 h at 180 °C. NH3- and CO2-TPD were carried out using Chemisorb 2750 Micrometrics Chemisorption Analyzer. For both acidity and basicity analysis, 0.1 g of each catalyst samples were loaded in form of a catalyst bed using quartz wool support at both ends. Pretreatment was carried out by purging at 550 °C and 25 mL min-1 constant high purity Helium flow for 1 h. This was followed by cooling to 100 °C and 45 °C for NH3- and CO2-TPD respectively. NH3 and CO2 were respectively adsorbed at 100 °C and 45 °C for 30 min at 25 mL min-1. Acidity and basicity measurement were performed by increasing the temperature to 900 °C at 10 °C min-1 in all cases. Fourier transform infrared (FT-IR) for functional group identification was carried out on the base (CZ) and the doped nanocomposites using Thermo Scientific Nicolet 6700 spectrometer. Thin Translucent pellet were made by mixing known weight of each nanocomposites and KBr. Spectra reading was carried out in the scanning range 200 – 4000 cm-1. 2.3.

Catalytic Cracking Catalytic cracking of hydrocarbon feed (n-Hexane, Sigma-aldrich, 97%) was carried

out at atmospheric pressure in a fixed bed reactor column. The n-hexane was used as a model compound for naphtha. The cracking was carried out at a temperature range of 600 - 650 °C in the presence of N2 (nitrogen) gas flow as a carrier gas, and without steam. 0.3 g of each catalyst sample was weighed, pelletized, crushed and sieved to particle size range of 100 – 300 nm. The pelletized samples were loaded and supported with quartz wool on both ends to make a catalyst bed in a reactor column of 7.92 mm internal diameter, heated length of 152.4 mm and overall length of 300 mm. Prior to injection of feed for catalytic test, the catalysts


Energy & Fuels

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 8 of 43

were pretreated in N2 flow at 20 mL min-1 for 90 min at 650 °C. A type K chromel-alumel thermocouple was inserted into the reactor column to detect and measure reaction temperature while another thermocouple was placed outside, in contact with the reactor column. Catalytic reactions were carried out by supplying feed flow using a HPLC pump (LabAlliance, Series II), for 10 h time-on-stream (TOS), 20 mL min-1 nitrogen gas flow, and time factor (W/F: W; catalysts weight/g, F; Feed rate/g h-1) = 0.065 - 0.131 h. The hexane feed was vaporized inside the reactor column and transported by the carrier gas (N2) to make contact with the catalyst bed for reaction. The gaseous product analysis was carried out by an on-line gas chromatograph (GC, Agilent Technologies, 7890A) with back and front Flame ionization detectors (FID). The products were injected into the gas chromatograph every 1 h for a period of 10 h TOS. Carbon balance was used to estimate hexane (C6) conversions, yield and product selectivity according to the equations below. Hexane Conversion =

, ,

,

∑

Product Selectivity = 2.4.

C-mol% C-mol%

(1) (2)

Catalyst Regeneration In this work, we will regenerate the best performing catalyst and CZ (base catalyst)

after being used in hexane cracking, followed by comparison and necessary deductions. The spent catalysts were characterized to see the effect of coking on the surface morphology and structure of the catalyst, and effect of dopant ions on carbon deposition. This was followed by calcination at 600 °C for 4 h to regenerate the catalysts. The regenerated catalysts were characterized, tested and the results obtained was compared with that of the fresh and spent samples. 3.0.

Results and discussion


Page 9 of 43

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

Energy & Fuels

3.1.

Phase identification and composition Figure 1 shows the XRD peaks obtained for the prepared samples before and after

calcination. These peaks were indexed to (111), (200), (220), (311), (222), (400) and (331), specifying a face-centered cubic structure with a space group of Fm3m. The patterns obtained from the doped Please insert Figure 1 here samples (FeCoCZa and FeCoCZb) showed no extra peaks when compared to that of CZ. The absence of extra peaks indicates full incorporation of dopant ions (Fe3+ and Co2+) into the crystal lattice structure of CZ and the phase structure of the CZ nanocomposite was unaltered4,25. Thus, we can infer that Zr4+, Fe3+ and Co2+ substitute Ce4+ ions in the ceria lattice. This is possibly favored by the solubility of these dopants ions, invariably enhancing co-doping with each other on the crystal lattice structure in the formation of ceria-zirconia nanocomposites. In addition, careful observation of the XRD diffraction patterns shows a slight shift in position of the diffraction peaks of FeCoCZa and FeCoCZb. The peaks moved slightly toward a higher 2Ɵ angle due to incorporation of dopant ions and calcination temperature. Furthermore, XRD patterns of FeCoCZa and FeCoCZb also exhibited wider peaks than CZ and reduced estimated lattice parameter than that of CZ, before and after calcination (Table 1), which is in accordance with Vegard’s law. Mean crystallite sizes of the CZ nanocomposites were estimated using Scherrer’s equation and peak broadening at full width half maximum (FWHM) of the major (111) peak while the lattice parameters (Å) were evaluated using Bragg’s equation. Please insert Table 1 here Crystallite size =


1

Energy & Fuels

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

Å = =

Page 10 of 43

2

Scherrer constant k was taken to be 0.94 which is synonymous for cubic phase structure, β and Ɵ represents the value for full width half maximum and the peak position of the (1 1 1) of the prominent (1 1 1) peak respectively, and λ was taken to be 0.1542 nm. Crystallite sizes with a range of 7.59 – 9.62 nm before calcination and 9.34 – 12.13 nm after calcination were estimated using scherrer’s equation (equation 1) while the lattice parameters were also estimated according to equation 2. We observed that the crystallite size of FeCoCZb was smaller than that of CZ while FeCoCZa gave a larger crystallite size before and after calcination. The shrinkage in the lattice parameter and reduced crystallite size of FeCoCZb can be attributed to the lattice pressure/strain. This is caused by the incorporation of Fe and Co ions with smaller ionic radii (0.64 Å and 0.65 Å, respectively) into the lattice structure of CZ. Also, the dispersion of Fe and Co oxides on the surface of CZ and the higher electronegativity of Fe and Co ions (1.83 and 188 respectively) cause change in bond length within the crystal lattice4,26. On the other hand, the larger crystallite size of FeCoCZa may be due to the high content of Fe and Co (10 wt.% each) in the composition. Fe have smaller ionic radius and more amorphous Fe oxide may be present on the surface of FeCoCZa compared to FeCoCZb, which contains 5 wt.% each of Fe and Co in its composition. Thus, we observed that the crystallite size of the doped samples is directly proportional to the weight percentage loading of the dopants ions. 3.2.

Raman Spectroscopy We have employed Raman spectra for obtaining more information on the crystal

structure, lattice defects, oxygen vacancies, and molecular orbital (M-O) bond layout. A fluorite cubic structure such as ceria-zirconia nanocomposite with a space group of Fm3m is expected to have F2g Raman band (or active mode) in its spectrum11. Generally, these


Page 11 of 43

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

Energy & Fuels

structures are composed of lattice vibrations of oxygen perceptive to their crystalline structure. From the obtained spectra in Figure 2, CZ exhibited a strong F2g Raman band at 461.06 cm-1 which is attributed to the breathing mode of oxygen ions attached to the Ce4+ in the crystal lattice4,26,27. FeCoCZa and FeCoCZb exhibited different spectra from CZ, with lower band intensity and the presence of minor peaks. The change in the band intensities of Raman spectra of the doped samples is as a result of the Fe-Ce and Co-Ce interaction and presence of dopants ions in CZ lattice25. In addition, the shift in the position of the F2g Raman band can be attributed to the incorporation of Fe and Co ions, change in crystallite sizes and lattice parameters, leading to lattice distortions and signals for oxygen vacancies. Both FeCoCZa and FeCoCZb exhibited very similar Raman spectra with the major F2g band depicting ceria-zirconia shifting to lower frequency around 459 cm-1 with broader peaks, however, FeCoCZa exhibited lower Raman band intensities than FeCoCZb. The peak at 459 cm-1 is as a result of the cubic fluorite-like lattice distortions leading to an increased Ce-O bond length. Thus, this lattice distortion can enhance oxygen storage and release capacity by generating an oxygen vacancy to compensate the effect of Ce4+ ions replaced by the dopant ions of Co and Fe to balance the ionic charges 4. Thus, the reduced peak intensity may speculate the distortion in the ceria-zirconia lattice structure, an evidence of the presence of Fe3+ and Co2+,27 especially when the composition of Fe is greater than 5% in the nanocomposites. Also, minor peaks around 280, 287 and 620 cm-1 observed in the Raman spectra of CZ, FeCoCZa and FeCOCZb can be linked to the Ce-Zr, Fe-Zr and Co-Zr interactions in the system, which causes tetragonal phase formation in the solid solution26,27. Also, the presence of similar minor peaks can be related to the presence of similar cubic phase in the samples. According to Kang et al., the weak bands may be associated to tetragonal displacement of oxygen species from the cubic structural positions as well as


Energy & Fuels

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

greater disarray of the anion sublattice28. Thus, doping influences lattice distortions similar to tetragonal phase development. Please insert Figure 2 here 3.3.

Surface Morphology Figure 3 shows the crystalline morphology and dense nanoparticle agglomeration of

CZ, FeCoCZa and FeCoCZb nanocomposites. Interstitial spaces within ceria-zirconia nanocomposite particle agglomerations of the samples created pores, although the porosity level is rather moderate. Morphology of CZ, FeCoCZa and FeCoCZb showed dense agglomeration of nanoparticles. The nanoparticle agglomerations can be attributed to direct effect of dehydration by calcination, which may lead to strong or weak interactions of the nanoparticles. Closer morphological observation of the images in Figure 3 shows that CZ nanoparticles have distinct uniform spherical shape. However, FeCoCZa and FeCoCZb appeared as rough spherical-shaped nanoparticles which can be attributed to the presence of dopant ions of Fe and Co in the crystal lattice structure of the nanocomposites, thus influencing their morphologies and surface structure. The smaller ionic radii of Fe and Co ions may probably affect the particle size diameter and hence, affecting the shape due to irregular ionic radii of the species present in the crystalline structure of the nanocomposite. Please insert Figure 3 here 3.4.

Textural properties The surface area of each prepared catalyst nanocomposite was measured using N2-

adsorption BET (Brunauer-Emmett-Teller) surface area analyzer. The prepared CZ nanocomposites are mesoporous and these are supported by the isotherm plot indicating a type 4 isotherm plot as shown in Figure 4. From Table 2, the results show that the incorporation of Fe and Co into CZ lattice structure increases the BET surface area,


Page 12 of 43

Page 13 of 43

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

Energy & Fuels

micropore volume, total pore volume, and BJH (Barrett-Joyner-Halenda) desorption average pore size of the nanocomposites. Please insert Figure 4 here Please insert Table 2 here

Total pore volume as well as micropore volume of the prepared samples are in the order FeCoCZa > FeCoCZb > CZ, with FeCoCZa having the largest total pore volume (0.173 cm3/g), micropore volume (0.0021 cm3/g) and average pore size (11.79 nm). The average pore size and distribution also followed the same trend and the pore size distribution in Figure 5 shows that CZ, FeCoCZa and FeCoCZb have uniform distribution. It is obvious that FeCoCZa and FeCoCZb have larger BET surface area, average pore size and pore size distribution compared to CZ. This is probably due to the presence of iron which may be responsible for the wider diameter in the pores of FeCoCZa and FeCoCZb, as well as their surface area and pore size. In the work done by Wang et al., incorporation of Co led to reduction surface area of Ce0.67Zr0.33O225, hence we can suggest that Fe ions are mostly responsible for the increased surface area of the doped samples. In addition, larger hysteresis loop was observed in the Langmuir isotherm plot for CZ while FeCoCZa and FeCoCZb have smaller/narrow hysteresis loop which is comparable to the BET surface area, average pore size and pore size distribution. Please insert Figure 5 here 3.5.

FT-IR Spectroscopy

Figure 6 shows the FT-IR spectra obtained for CZ, FeCoCZa and FeCoCZb after calcination. The doped samples exhibited similar spectra to that of the base sample (CZ) which comprise two major peaks. Although, more peaks with higher intensity were present in the spectra of


Energy & Fuels

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

FeCoCZa and FeCoCZb when compared to CZ spectrum. The absorption band at 756 cm-1 in CZ spectrum and at 550 – 650 cm-1 band range in the spectra of doped nanocomposites are attributed to the metal-oxide skeletal bond vibration26–28. In addition, we observed adsorption bands at 980 cm-1 and 1100 cm-1 in the doped nanocomposite spectra respectively. These adsorption bands are attributed to the vibrations of C-O stretching29 and effect of dopants on CZ lattice structure can be observed in the shift to higher wavenumbers. Also, the FeCoCZa and FeCoCZb nanocomposites exhibited Ce-OH stretching and hydroxyl group bending vibrations at 1330 and 1640 cm-1 respectively, which may have resulted from absorption of H2O molecules27,28. We also observed that both FeCoCZa and FeCoCZb exhibited metal oxide vibrations at 1500 cm-1 which is synonymous to OH-group bending vibrations. FeCoCZb also exhibitedbroad band with peak positioned at 3420 cm-1. This can be attributed to the hydroxyl group stretching vibration30, consequential to the physical absorption of water according to Zhang et al.29 Please insert Figure 6 here 3.6.

Acidity and Basicity Measurement Ceria is known to be an amphoteric oxide which possesses both acidic and basic

properties

31

. NH3- and CO2-TPD were performed to investigate the presence of acid and

basic sites accordingly, on the surface of prepared nanocomposite samples. Distribution and strength of these sites on the catalysts influence their performance for catalytic cracking of hydrocarbons. The knowledge of the strength, type and distribution of these sites helps in the optimization of these nanocomposites for selective and desired reactions. From CO2-TPD profiles in Figure 7a, we categorized the basic site strength into weak (40 – 300 °C), medium (310 – 500 °C) and strong (> 500 °C) basic sites. All the samples exhibited weak basic sites. We observed similar distribution and the presence of only weak


Page 14 of 43

Page 15 of 43

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

Energy & Fuels

basic sites in the profile exhibited by CZ depicted by desorption peaks at 84 °C ad 93 °C respectively. Furthermore, FeCoCZb showed the presence of mostly weak basic sites, around 120 °C. However, FeCoCZa possess basic sites in the whole experimental region from 40 – 900 °C, with the presence of significant of desorption peaks attributed to weak, medium and strong basic sites. In addition, FeCoCZa exhibited the highest strength of basicity (0.24 mmol g-1) according to Table 3. Hence, doping CZ with Fe and Co increases its basic strength. Please insert Figure 7 here From the NH3-TPD profiles of the ceria-zirconia nanocomposites, all the samples have weak acid strength (see Figure 7b). Comparing the NH3-TPD profiles of CZ and the doped samples, we observed the presence of a single peak in the rich acidity region of CZ. Also, from Table 3, the total acidity of all the nanocomposite samples was low. In addition, all the nanocomposites have significantly lower total acidity compared to their basicity. Thus, doping Co and Fe ions into the lattice structure of CZ improves its acid strength slightly, while the total basicity remarkably increased. Overall, the total acid strengths of the catalysts are extremely low, as shown by the obtained total acidity. Hence, these catalysts fit well to the family of oxidation catalyst and not acid catalysts32,33. Please insert Table 3 here 3.7.

Catalytic Cracking Reaction of n-Hexane

3.7.1. Performance of doped nanocomposites on catalytic test The developed ceria-zirconia (base) catalysts were tested for hexane cracking at 650 °C, at a feed flow composition of flowrate of 17.5% hexane feed + 82.5% N2, and W/F of 0.087 h. The doped nanocomposite catalysts were also tested at these same conditions to outline the effect of dopant ions on the performance of CZ on catalytic cracking reaction of n-


Energy & Fuels

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

hexane. The results are summarized in Table 4, highlighting the initial conversion, selectivity and yield of selected products and subsequent results after 5 h and 10 h time-on-stream for each catalyst. All the doped nanocomposite catalysts showed higher initial (1 h reaction time) conversion than CZ. All catalysts systems exhibited appreciable conversions with improved stability over 10 h time-on-stream according to Table 4. The catalysts also showed appreciable and stable selectivity towards C2= and C3= olefins as well as stable initial and subsequent C1 and C2 paraffin selectivity over 10 h reaction time. Figure 8 shows that FeCoCZa gave stable total olefins (C2 + C3 + C4 + C5) selectivity over reaction TOS, and the highest C2= and C3= olefins yields at 10 h reaction time. Other selected product yields for olefins and paraffin are shown as well. Consequently, higher conversion and stability was obtained with ceria-zirconia co-doped with Fe and Co. However, based on the obtained results, FeCoCZa (10 wt.% Fe and Co each) gave better catalytic performance (in terms of conversion and product yield) than FeCoCZb (5 wt.% Fe and Co each). The better activity observed in FeCoCZa is probably due to higher loading compositions of the dopant ions than in FeCoCZb, which has improved the characteristic properties of CZ such as higher surface acidity and basicity, increased surface area and average pore size, which invariably influences higher availability of active sites. Thus, the higher surface area will allow more contact with hexane molecules for catalytic decomposition by the available active sites, leading to higher conversion. Hence, we can possibly postulate that increasing the percentage loading composition of Fe and Co on ceria-zirconia nanocomposite catalyst to 10 wt.% improves its catalytic performance. In comparison, promoting ceria-zirconia with dopant ions of Fe and Co (at 10 wt. % each) improved the catalytic cracking of the hydrocarbon (C-H) bond and resulted in better nhexane conversion. Furthermore, the cracking of the C-H bond must have proceeded via the


Page 16 of 43

Page 17 of 43

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

Energy & Fuels

lattice oxygen of the catalysts, which is also complemented by the Lewis acid sites on the catalysts’ surface, for oxidative cracking of the C-H bond of n-hexane. In addition, many commercial catalysts such as ZSM-5 have shown higher total surface acidity, surface area and higher conversions but lower total olefin and individual ethylene selectivity, as shown in the work done by Konno et al 19. The authors reported high initial conversions of about 94 Cmol% and the highest total olefin and ethylene selectivity obtained were ~65 C-mol% and 24.7 C-mol% respectively. Also, n-hexane conversion was carried out by Boyadjian et al. in the presence of artificial suypply of oxygen and rapid catalyst deactivation was observed in the first 1 h of reaction

20

. The authors reported a maximum total olefin selectivity of

approximately 60 mol% (carbon based), with approximately 23 mol% and 26 mol% selectivity for ethylene and propylene respectively, and negligible conversion in the absence of oxygen supply. Mohammed et al. also carried out n-hexane cracking in a fixed-bed flow reactor via steam-assisted technique using modified ZSM-12 zeolites at a reaction temperature of 650 °C 21. The authors obtained conversions slightly higher than 50 C-mol% when modified with La and Ce and a reduced conversion of 20 C-mol% when modified with Boron. However, ethylene selectivity was lower than 20 C-mol% in all cases. Also, the maximum combined selectivity of C2= + C3= olefin obtained over ZSM-12 zeolites was 55.1 C-mol%, which is lower than 65 C-mol% obtained for CZ nanocomposites in our research study. High initial n-hexane conversion (~81%) was obtained over nanosized ZSM-22 (TON) zeolites at 650 °C and W/F = 0.125 h, in the work done by Jamil et al

21

. However, the

maximum total olefin selectivity reported by the authors was not more than 61 %. Comparatively, CZ nanocomposites exhibited ethylene and propylene selectivity greater than 30 C-mol%, with a mean combined value greater than 65 C-mol% and higher total olefins selectivity (~75 C-mol%) than commercial catalysts at 650 °C. This is higher than most reported cracking catalysts for n-hexane in literatures. Also, there is no need for additional


Energy & Fuels

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

supply of oxygen or steam to aid the cracking reaction for CZ nanocomposite catalysts, unlike common and commercial catalysts. This is because of the redox and OSC capability of CZ nanocomposites to supply active lattice oxygen specie on their surface. In addition, it is noteworthy that CZ nanocomposites exhibited good catalytic activity for naphtha (n-hexane) cracking, even though they possess lower surface area and acid strength, compared to zeolites and most commercial catalysts. Please insert Table 4 here Please insert Figure 8 here 3.7.2. Influence of temperature variation on the performance of FeCoCZa FeCoCZa was selected as the best catalyst based on the results reported and the comparisons made in 3.7.1 in terms of conversion, yield and stability. Thus, we investigated the effect of temperature variation on catalytic cracking of n-hexane and product distribution using FeCoCZa. Catalytic activity was exhibited at both 600 °C and 650 °C, with better performance obtained at 650 °C under the same reaction conditions. Conversion was lower at 600 °C when compared to that at 650 °C over 10 h time on stream. Also, the total olefin selectivity was lower at 600 °C with higher BTX selectivity, higher methane yield (see Figure 9), as well as higher paraffin selectivity, than at 650 °C (see Table 5). Detailed observations of the reported results show stable olefin selectivity at 650 °C. In addition, significant drop of 86.5% was estimated in the total olefin selectivity after 10 h time-onstream at 600 °C, which is evident in the drop in C2= and C3= olefins selectivity after 10 h. Similar trend of product selectivity was observed in the combined product yield reported in Table 5 at 600 °C, while both C2=-C3= olefin and C1-C2 paraffin combined yields decrease over time-on-stream at 650 °C. At 600 °C, C2=-C3= olefin yield decreases with timeon-stream while C1-C2 paraffin yield improved. A possible explanation for this is the


Page 18 of 43

Page 19 of 43

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

Energy & Fuels

increasing side reactions (e.g. aromatization and isomerization) of the intermediates on the surface of the catalyst to form more BTX and RCH2 (paraffin) with reaction time-onstream34. Furthermore, selectivity towards ethylene formation was higher than that of propylene at 650 °C and vice versa at 600 °C, over the 10 h reaction time-on-stream. The higher selectivity to ethylene at elevated temperatures is due to the further cracking of C3 and C4 olefins to ethylene20. Also, ethylene is majorly formed from protolytic (monomolecular) cracking of carbenium-ions chain on catalyst acid sites which is favoured at higher temperature23. However, higher propylene formation and selectivity at 600 °C can be attributed to bimolecular chain cracking mechanism which is favoured by lower temperature and hydride-transfer, followed by β-scission on the formed intermediate or carbeniumion22,24. Please insert Table 5 here Please insert Figure 9 here

3.7.3. Influence of time factor (W/F) variation Effect of time factor variation (in terms of catalyst weight per feed flowrate, W/F) on conversion and selectivity was investigated by varying the amount of n-hexane in the total feed flow at constant catalyst weight. According to Table 5, conversion increased as the W/F decreased, over the entire time-on-stream. At W/F of 0.131 h and 0.087 h, we observed difference in conversion but similar trend in selectivity stability for ethylene and propylene. In terms of product yield, W/F of 0.087 h gave higher C2=, C3= olefins and C1, C2 paraffin yield than W/F = 0.131 h as shown in Figure 10 and Table 5, both initially and after 10 h time-on-stream. Compared to W/F = 0.131 h and 0.087 h, W/F = 0.065 h gave the highest conversion, and lowest initial selectivity to ethylene and propylene. In addition, at higher


Energy & Fuels

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 20 of 43

reaction time-on-stream, product selectivity towards ethylene and propylene was further reduced to approximately zero from 5 to 10 h time-on-stream (see Figure 10 and Table 5). Hence, we infer that W/F should be greater than 0.065 h to obtain appreciable ethylene and propylene selectivity. Please insert Figure 10 here 3.8

Oxygen Storage and Release Capability of CZ Nanocomposites

Temperature programmed reduction (TPR) analysis was performed to study the oxygen mobility in the lattice structure of the CZ nanocomposite catalysts and their reducibility, after calcination. The results shown by the TPR profiles in Figure 11 shows that CZ exhibited one reduction peak at 684 °C within 400 – 800 °C range, which is synonymous to the reduction of surface and bulk oxygen 25,35. This shows that the reduction of Ce4+ to Ce3+ in CZ occurred at 684 °C. However, for FeCoCZa and FeCoCZb, the reduction of Ce4+ to Ce3+ occurred at lower temperatures of 576 °C and 495 °C respectively. The positional shift of the reduction peaks in TPR profiles of FeCoCZa and FeCoCZb is due to the dopant ions of Fe and Co present in the lattice structure of CZ. This led to the weakening of the Ce-O bond for better reducibility of the CeO2 specie in the solid solutions of FeCoCZa and FeCoCZb

4,25,36

. In

addition, the incorporation of varied metal ions with different valence affects the metal-bond (M-O-M´) strength of the surface and bulk oxygen 37. Hence, we can infer that both FeCoCZa and FeCoCZb have higher oxygen mobility as a result of their lower reduction temperatures 28,38

and increased interdependent interaction between Ce-O, Fe-O and Co-O 4. The total OSC

of the catalysts in terms of hydrogen consumption were estimated. The highest H2 consumption of 6.58 mmol g-1 was exhibited by FeCoCZa while FeCoCZb and CZ gave 5.36 mmol g-1 and 3.82 mmol g-1 respectively. Due to its lowest reduction temperature, we expected FeCoCZa to have the highest oxygen mobility and OSC. Thus, we can postulate the availability of active lattice oxygen specie on the surface of the CZ nanocomposite catalysts,


Page 21 of 43

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

Energy & Fuels

improved oxygen vacancies and reducibility for FeCoCZa and FeCoCZb due to the presence of Fe and Co ions. Furthermore, the higher catalytic activity of FeCoCZa for n-hexane conversion can also be justified by its higher OSC estimate in terms of H2 consumption, compared to CZ and FeCoCZb. Thus, we can infer that FeCoCZa has highest available active oxygen specie and vacancies, leading to improved oxidative cracking reaction of n-hexane to light olefins. Overall, the TPR result justifies that CZ nanocomposite catalysts fit well to the family of oxidation catalyst, as stated earlier in section 3.6. Please insert Figure 11 here 3.9

Catalytic Mechanism of Naphtha Cracking over FeCo/CZ Nanocomposites Naphtha cracking mechanism involves β-splitting or isomerization of the formed

alkyl radical (intermediate) from the reaction of metal ions and the lattice oxygen with the hydrocarbon feed. In our research, we observed the dominance of β-splitting of the formed intermediate in the gas phase at W/F = 0.087 h and 650 °C formed are light olefins (ethylene and propylene)

21

, since the major products

34

. Hence, we believed the naphtha (n-

hexane) molecule reacts with active lattice oxygen specie and metal ions on the surface of FeCo/CZ nanocomposites to form the alkyl radical intermediate, according to Yoshimura et al 21. This is followed by the gas phase oxidative cracking via β-scission of the carbon chain of the desorbed intermediate radical (carbenium-ion) from the FeCo/CZ catalyst surfaces. Hence, the formation of light olefins and other hydrocarbon products with shorter carbon chain and aromatics (mainly BTX with very low selectivity at 650 °C). Both classical βscission (bimolecular) and protolytic (monomolecular) cracking of the formed intermediate occurred in the cracking reaction via CZ nanocomposites23,24. This is justified by the formation and appreciable selectivity obtained for both ethylene and propylene as the major products formed.


Energy & Fuels

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

3.10. Catalyst Regeneration and Reusability The amount of carbonaceous deposit (coke) on CZ was less than that of FeCoCZa. This was probably due to the presence of Fe and Co ions on present in the crystal lattice structure of FeCoCZa. Also, XRD analysis was carried out on the spent samples of CZ and FeCoCZa and the presence of amorphous carbon on CZ while filamentous carbon and presence of Fe3C (between 2Ɵ = 42° – 46°) peaks were observed on FeCoCZa. However, their phase structure remained unchanged. Also, from the micrograph images, we observed amorphous carbon on spent CZ (8.5% coke) while filamentous carbon in form of rods and nanofibers (48.1% coke) were observed on FeCoCZa. The large amount of carbon present on spent FeCoCZa is probably due to the presence of iron and cobalt ions present in its chemical composition. In addition, the amorphous carbon present on spent CZ is due to the adsorption of n-hexane feed on the metal ions on its active sites, causing decomposition of the hexane feed to form carbon deposit with time39,40. Also, the formation of fibrous-structured carbon and rods deposited on spent FeCoCZa may have resulted from the disintegration of n-hexane feed on the metal ions, enhancing the formation of filamentous carbon39,40. Furthermore, part of the large amount of filamentous carbon deposited on spent FeCoCZa formed carbon rods which were visible at 50 µm micrograph image scale (Figure 12). This can be explained by the further build-up of amorphous carbon which were accumulated on the catalyst surface and reactor wall during time-on-stream in a process known as thickening, according to Terry and Baker40. Please insert Figure 12 here Please insert Figure 13 here Regeneration of the spent catalyst samples was then carried out by calcining in air at 600 °C for 4 h. XRD analysis of the regenerated samples showed no extra peaks and were similar in patterns to those of their respective fresh samples (see Figure 12 and 13). In


Page 22 of 43

Page 23 of 43

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

Energy & Fuels

addition, the SEM micrographs show that no carbon deposition were present on the surface of the catalyst samples after regeneration. Also, careful observation of the SEM images showed that the morphology of both samples was restored after regeneration. The regenerated catalysts displayed excellent regenerative capability, although, there has been no previous report or data on the performance and regenerative capability of ceria-zirconia nanocomposites for hexane cracking. FeCoCZa showed 9.1% loss and 16.9% loss in catalytic activity based on n-hexane conversions after first and second regeneration cycle, as compared to the fresh FeCoCZa sample. The decline in the FeCoCZa activity after regeneration can be attributed to the gradual changes in the surface morphology. This can be observed in the slight change of the nanoparticles shapes of the regenerated samples and the increasing agglomeration of the nanoparticles after each regeneration cycle by calcination (See Figure 12). Hence, increasing agglomeration rate will eventually lead to reduction in porosity and surface area, thus, causing a gradual reduction in the catalyst activity due to loss of available active sites by pore constriction. Appreciable ethylene and propylene selectivity and yield were obtained after using the regenerated FeCoCZa for two cycles. However, the product yield of ethylene and propylene reduced after each regeneration cycle. This is due to reduction in catalytic activity of the catalyst which was stated earlier. However, based on the obtained results, FeCoCZa displayed appreciable regenerative and reusability capability. Please insert Table 6 here

Conclusions Cubic phase-structured Fe-Co co-doped CZ has been successfully synthesized via hydrothermal synthesis and the characterization results obtained were compared. Fe and Co were successfully incorporated in the crystal lattice structure of CZ


Energy & Fuels

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

according to the XRD patterns obtained. Improved intrinsic characteristics of ceriazirconia due to doping with iron and cobalt was obtained and verified by the characterization results. Thus, we conclude that doping CZ with Fe and Co improves its characteristic properties such as improved physicochemical properties, and enhanced surface acidity and basicity. The prepared nanocomposite catalysts exhibited catalytic activity for hexane cracking. Comparing the catalytic performance, FeCoCZa gave the best performance with good stability and relatively high and stable selectivity for olefin. Thus, we can infer that doping CZ with Fe and Co improved its catalytic activity and stability. Furthermore, conversion improved with increase in temperature and W/F. However, from the results obtained for this work, we can infer that lower temperature favors high propylene selectivity while higher temperature favors ethylene. Also, we conclude that the W/F should be greater than 0.065 h due to high paraffin selectivity and yield (mostly methane) obtained after 3 h time-on-stream, even though the conversion was high. In addition, the spent catalysts were characterized, analyzed and successfully regenerated by calcination. The regenerated catalyst showed good catalytic performance after two regeneration cycles. Thus, the developed catalyst nanocomposites can be regenerated, maintaining their respective properties with good reusability performance.

Acknowledgements

The authors would like to appreciate the provision of funds by Saudi Aramco for supporting this work through project contract number 6600011900 as part of the Oil Upgrading theme at King Fahd University of Petroleum and Minerals. The authors


Page 24 of 43

Page 25 of 43

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

Energy & Fuels

acknowledge the contribution from S. Adewale for Raman spectroscopy analysis and M. Qamaruddin on the analysis of textural properties.

References (1)

Vivier, L.; Duprez, D. ChemSusChem 2010, 3 (6), 654–678.

(2)

Trovarelli, A. In ACS Journal; Imperial College Press, 2002; Vol. 2, pp 12923– 12924.

(3)

Mehdi Dejhosseini, Tsutomu Aida, Masaru Watanabe, Seiichi Takami, Daisuke Hojo, Nobuaki Aoki, Toshihiko Arita, Atsushi Kishita, T. A. ACS J. 2013, 27, 4624–4631.

(4)

Wang, J.; Zhang, B.; Shen, M.; Wang, J.; Wang, W.; Ma, J.; Liu, S.; Jia, L. J. Sol-Gel Sci. Technol. 2011, 58 (1), 259–268.

(5)

Shimizu, K.; Kawachi, H.; Satsuma, A. Appl. Catal. B Environ. 2010, 96 (1–2), 169–175.

(6)

Tapan, N. A.; Cacan, U. B.; Varışlı, D. Int. J. Electrochem. Sci. 2014, 9, 4440– 4464.

(7)

Olmos, C. M.; Chinchilla, L. E.; Villa, A.; Delgado, J. J.; Pan, H.; Hungria, A. B.; Blanco, G.; Calvino, J. J.; Prati, L.; Chen, X. Appl. Catal. A Gen. 2016, 525, 145– 157.

(8)

Tsoncheva, T.; Ivanova, R.; Henych, J.; Dimitrov, M.; Kormunda, M.; Kovacheva, D.; Scotti, N.; Dal, V.; Stengl, V. Appl. Catal. A Gen. 2015, 502 (5), 418–432.

(9)

Dar, B. A.; Khalid, S.; Wani, T. A.; Mir, M. A.; Farooqui, M. Green Sustain. Chem. 2015, 5 (February), 15–24.

(10)

Fatima, B.; Najam-ul-Haq, M.; Jabeen, F.; Majeed, S.; Ashiq, M. N.; Musharraf, S. G.; Shad, M. a; Xu, G. Analyst 2013, 138 (17), 5059–5067.

(11)

Kullgren, J.; Hermansson, K.; Broqvist, P. J. Phys. Chem. Lett. 2013, 4 (4), 604– 608.

(12)

Akbari-Fakhrabadi, A.; Meruane, V.; Jamshidijam, M.; Gracia-Pinilla, M. A.; Mangalaraja, R. V. Mater. Sci. Eng. A 2016, 649, 168–173.

(13)

Reddy, B. M.; Bharali, P.; Thrimurthulu, G.; Saikia, P.; Katta, L.; Park, S. E. Catal. Letters 2008, 123 (3–4), 327–333.

(14)

Pudukudy, M.; Yaakob, Z. Der Pharma Chem. 2014, 6 (1), 188–216.

(15)

Piumetti, M.; Bensaid, S.; Russo, N.; Fino, D. Appl. Catal. B Environ. 2016, 180, 271–282.

(16)

Thammachart, M.; Meeyoo, V.; Risksomboon, T.; Osuwan, S. Catal. Today 2001, 68 (1–3), 53–61.

(17)

Moser, M.; Mondelli, C.; Schmidt, T.; Girgsdies, F.; Schuster, M. E.; Farra, R.;


Energy & Fuels

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

Szentmiklosi, L.; Teschner, D.; Perez-Ramirez, J. Appl. Catal. B Environ. 2013, 132–133, 123–131. (18)

Moser, M.; Vil, G.; Colussi, S.; Krumeich, F.; Teschner, D.; Szentmiklsi, L.; Trovarelli, A.; Prez-Ramirez, J. J. Catal. 2015, 331, 128–137.

(19)

Konno, H.; Okamura, T.; Kawahara, T.; Nakasaka, Y.; Tago, T.; Masuda, T. Chem. Eng. J. 2012, 207–208, 490–496.

(20)

Boyadjian, C.; Van Der Veer, B.; Babich, I. V.; Lefferts, L.; Seshan, K. Catal. Today 2010, 157 (1–4), 345–350.

(21)

Sanhoob, M. A.; Muraza, O.; Taniguchi, T.; Tago, T.; Watanabe, G.; Masuda, T. Energy & Fuels 2016, 30 (11), 9679–9685.

(22)

Wang, Y.; Yokoi, T.; Namba, S.; Kondo, J. N.; Tatsumi, T. Appl. Catal. A Gen. 2015, 504, 192–202.

(23)

Wielers, A. F. H.; Vaarkamp, M.; Post, M. F. M. J. Catal. 1991, 127 (1), 51–66.

(24)

Haag, W. O.; Dessau, R. M.; Lago, R. M. Stud. Surf. Sci. Catal. 1991, 60 (C), 255– 265.

(25)

Wang, J.; Shen, M.; Wang, J.; Gao, J.; Ma, J.; Liu, S. J. Rare Earths 2012, 30 (9), 878–883.

(26)

Laguna, O. H.; Pérez, A.; Centeno, M. A.; Odriozola, J. A. Appl. Catal. B Environ. 2015, 176–177, 385–395.

(27)

Laguna, O. H.; Centeno, M. A.; Boutonnet, M.; Odriozola, J. A. Appl. Catal. B Environ. 2011, 106 (3–4), 621–629.

(28)

Kang, M.; Wu, X.; Zhang, J.; Zhao, N.; Wei, W.; Sun, Y. RSC Adv. 2013, 5583– 5590.

(29)

Zhang, Y.; Zhang, L.; Deng, J.; Dai, H.; He, H. Inorg. Chem. 2009, 48 (5), 2181– 2192.

(30)

Machmudah, S.; Akmal Hadian, M.; Samodro K., L.; Winardi, S.; _ W.; Kanda, H.; Goto, M. Mod. Appl. Sci. 2015, 9 (7), 134–139.

(31)

Venkatesh, S.Z. Mohamed, N. M. M. Indian J. Chem. 2015, 54 (July), 843–850.

(32)

Kondoh, H.; Nakasaka, Y.; Kitaguchi, T.; Yoshikawa, T.; Tago, T.; Masuda, T. Fuel Process. Technol. 2016, 145, 96–101.

(33)

Kondoh, H.; Tanaka, K.; Nakasaka, Y.; Tago, T.; Masuda, T. Fuel 2016, 167, 288–294.

(34)

Wilczura-Wachnik, H. Catalytic cracking of hydrocarbons; Warsaw, 1989; Vol. 56.

(35)

Deng, J.; Zhou, Y.; Cui, Y.; Lan, L.; Wang, J.; Yuan, S.; Chen, Y. J. Mater. Sci. 2017, 52 (9), 5242–5255.

(36)

Liang, C.; Ma, Z.; Lin, H.; Ding, L.; Qiu, J.; Frandsen, W.; Su, D. J. Mater. Chem. 2009, 19 (10), 1417.

(37)

Constantinou, D. A.; Efstathiou, A. M. Appl. Catal. B Environ. 2010, 96 (3–4), 276–289.

(38)

Song, H.; Mirkelamoglu, B.; Ozkan, U. S. Appl. Catal. A Gen. 2010, 382 (1), 58–64.


Page 26 of 43

Page 27 of 43

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

Energy & Fuels

(39)

Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26 (1), 51–62.

(40)

Terry, R.; Baker, K. ACS Div. Fuel Chem. Prepr. 1996, 41 (2), 521–523.


Energy & Fuels

111

FeCoCZb

200

b 220

311 222

400

331

Intensity (a.u)

a

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

Page 28 of 43

FeCoCZa

CZ

2 Ɵ (°)

2 Ɵ (°)

Figure 1. XRD patterns showing diffraction peaks for CZ, FeCoCZa and FeCoCZb: (a) before calcination (b) after calcination. ACS Paragon Plus Environment

Page 29 of 43

a

b

461

459

Intensity (cnt)

287

Intensity (cnt)

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

Energy & Fuels

FeCoCZb 280

620

FeCoCZa CZ

Raman Shift (cm-1)

Raman Shift (cm-1)

Figure 2. Raman spectra: (a) CZ (b)Plus FeCoCZa and FeCoCZb nanocomposites. ACS Paragon Environment

b

Energy & Fuels

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

CZ

500 nm

FeCoCZa

500 nm

Figure 3. FESEM images for CZ, FeCoCZa and FeCoCZb nanocomposites. 500 nm 500 nm


Page 30 of 43

FeCoCZb

500 nm

Page 31 of 43

120

40

70

110

CZ

35

FeCoCZa

25 20 15 10

90

Quantity Adsorbed [cm³/g STP]

30

80 70 60 50 40 30 20

5

FeCoCZb

60

100



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

Energy & Fuels

50 40 30 20 10

10 0 0

0.2

0.4

0.6

0.8

Relative Pressure [p/p°]

1

0

0 0

0.2

0.4

0.6

0.8



1

0

0.2

0.4

0.6

0.8


Figure 4. Adsorption-Desorption Isotherm Plot showing Hysteresis Loop for CZ, FeCoCZa and FeCoCZb

1

Energy & Fuels

0.006

CZ FeCoCZa

0.005

Pore Volume (cm³/g·Å)

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

Page 32 of 43

FeCoCZb 0.004 0.003 0.002 0.001 0 0

-0.001

50

100

150

Pore Width (Å)

ACS Paragon Plus Environment Figure 5. Pore Size Distribution of CZ, FeCoCZa and FeCoCZb.

200

Page 33 of 43

FeCoCZb

Transmittance (%)

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

Energy & Fuels

FeCoCZa

CZ

Wavenumbers (cm-1) Figure 6. FT-IR Spectra of CZ and CZ showing functional group distribution. ACSnanocomposites Paragon Plus Environment

Energy & Fuels

a

FeCoCZb

b

FeCoCZb

FeCoCZb TCD (a.u.)

FeCoCZa

TCD (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

Page 34 of 43

CZ CZ

Temperature (oC)

Temperature (oC)

Figure 7. (a) CO2-TPD and NHPlus of ceria-zirconia nanocomposites ACS(b) Paragon Environment 3-TPD

Page 35 of 43

Product Yield C2=

Total Olefin Selectivity CZ

FeCoCZ_A

FeCoCZ_B

80

C3

C4=

C1

b C2

C3

C4

12

a

10

75

70

8

Yield

Selectivity (%)

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

Energy & Fuels

65 60 55

6 4

50

1

5 TOS (h)

10

2 0

CZ

FeCoCZa

FeCoCZb

Figure 8. (a) Total olefin selectivity over time on stream (b) Selected product yield after 10 h time on stream. ACS Paragon Plus Environment Reaction conditions: 650°C, time factor, W/F= 0.087 h, 0.3 g catalyst and 17.5% Hexane + 82.5% N2 feed flow

Energy & Fuels

Page 36 of 43

Product Yield C2=

C3=

C4=

C1

b C2

C3

C4

Total Olefin Selectivity 600 °C

12

650 °C

80

a

70

10

60

8

50

Yield

Selectivity (%)

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

40

6

30 4

20 10

2 0 1

5

TOS (h)

10 0

600 °C

650 °C Temperature (oC)

Figure 9. Influence of temperature variation on catalytic performance of FeCoCZa (a) Total olefin selectivity over TOS (b) Paragon Plus Environment Selected product yields after 10 h time on stream. ACS Reaction conditions: 650°C, time factor, W/F= 0.087 h, 0.3 g catalyst and 17.5% Hexane + 82.5% N feed flow

Page 37 of 43

Energy & Fuels

b

Product Yield C2=

Total Olefin Selectivity 0.131

0.087

a

C3=

C4=

C1

C2

C3

C4

60

0.065

80

50 70

Selectivity (%)

60

40

50

Yield

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

40

30

30

20

20 10

10

0

1

5

TOS (h)

10 0 0.131

0.087

0.065

Time factor, W/F (h)

Figure 10. Influence of time factor (W/F) variation on catalytic performance of FeCoCZa (a) Total olefin selectivity over time ACS Paragon Plus Environment on stream (b) Selected product yields after 10 h time on stream.

Energy & Fuels

FeCoCZa 495 oC

TCD (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

Page 38 of 43

FeCoCZb

576 oC

684 oC

CZ

Temperature (oC)

Figure 11. Temperature programmed reduction profiles and reducibility of CZ, ACS Paragon Plus Environment FeCoCZa and FeCoCZb.

CZ

Page 39 of 43

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

Energy & Fuels

FRESH

500 nm

SPENT

REGENERATED

500 nm

500 nm

FeCoCZa FRESH

500 nm

SPENT

REGENERATED

50 mm ACS Paragon Plus Environment

Figure 12. FESEM Micrograph of Fresh, Spent and Regenerated CZ and FeCoCZa Samples

500 nm

Energy & Fuels

Cf

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

Page 40 of 43

Fe3C

Ca

2 Ɵ (°) ACS Paragon Plus Environment

Figure 13. XRD Analysis of Fresh, Spent and Regenerated CZ and FeCoCZ_A Samples (Ca: Amorphous carbon, Cf: Filamentous carbon).

Page 41 of 43

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

Energy & Fuels

Table 1. Peak angles, lattice parameter and crystallite size of CZ, CoCZ, FeCZ,

FeCoCZa and FeCoCZb. Accuracy = ± 0.02° 2Ɵ (deg)

Sample

Lattice Parameter (Å)

Crystallite Size (nm)

Before calcination

After calcination

Before calcination

After calcination

Before calcination

After calcination

CZ

28.98

28.99

5.33

5.33

9.62

11.79

FeCoCZa

29.14

29.25

5.30

5.28

10.45

12.13

FeCoCZb

29.19

29.34

5.29

5.27

7.59

9.34

Table 2. BET Surface Area, Pore Volume and Pore Size Analysis of CZ and DopedCZ Nanocomposites. Error = ±2.1 % Sample SBETa STOTALb VMICROc VMESOd Average 2 3 3 3 (m /g) (cm /g) (cm /g) (cm /g) Pore Size (nm) CZ 32 0.053 0.0004 0.052 4.217 FeCoCZa

43

0.173

0.0021

0.171

11.790

FeCoCZb

43

0.091

0.0016

0.089

5.960

a

SBET: BET Surface Area; VTOTAL: Total Pore Volume; c VMICRO: Micropore Volume; d VMESO: Mesopore Volume. b

Table 3. Total Acidity and Basicity Estimation of Prepared Nanocomposites. Sample CZ FeCoCZa FeCoCZb

Total Basicity (mmol g-1) 0.03 0.24 0.16

Total Acidity (mmol g-1) 0.07 0.51 0.76


Energy & Fuels

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 42 of 43

Table 4. Performance of CZ nanocomposite catalysts on n-hexane cracking (Testing conditions: 650 °C, W/F = 0.087 h. Error = ±5% Catalyst

TOS (h)

Light Olefins Selectivity (%)

Conv (C-mol%)

=

CZ

FeCoCZa

FeCoCZb

Paraffin Selectivity

BTX (%)

C2

C3=

Others

C1

C2

Others

C2= C3=

Yield (%) + C1 C2

+

1

31.2

35.2

33.8

5.8

13.1

9.7

2.2

0.2

21.5

7.1

5

23.9

35.4

33.9

5.3

13.3

9.8

2.3

0.0

16.5

5.5

10

21.5

36.0

33.9

4.8

13.3

9.8

1.9

0.3

15.1

4.9

1

49.0

34.0

31.8

7.0

14

10.9

2.0

0.3

32.2

12.2

5

41.7

34.5

31.9

7.1

13.5

10.1

2.0

0.9

27.7

9.9

10

33.3

34.8

32.7

6.1

13.5

10.3

2.3

0.4

22.5

7.9

1

32.4

31.9

28.9

12.8

12.9

9.4

2.4

1.7

19.7

7.2

5

24.5

34.5

33.0

6.1

13.4

10.7

1.8

0.5

16.5

5.8

10

20.2

34.5

33.8

5.2

13.3

10.7

2.3

0.2

13.8

4.8

TOS: Time on stream Conv: n-hexane conversion BTX: Benzene+Toluene+Xylene selectivity C1: methane C2: ethane C2=: ethylene C3=: propyylene


Page 43 of 43

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

Energy & Fuels

Table 5. Effect of temperature and time factor (W/F) variation on conversion, selectivity and yield of FeCoCZa on n-hexane cracking. Error = ±5% TOS (h)


Conv (C-

Parafins Selectivity (%)

BTX (%)

Yield (%)

mol%)

Temperature (°C) 1 33.9 600

650

C2=

C3=

Others

C1

C2

Others

29.8 24.5 3.1

14.1 10.6 2.5

7.4 13.2 48.6

10.4 14.9 11.7

14.7 8.3 17.7

C2= + C3=

C1+ C2

8.9 11.3 15

15.1 9.7 2.0

6.0 6.5 12.3

5 10

23.1 20.2

14.7 17.2 1.4

1 5 10

49.0 41.7 33.3

34.0 34.5 34.8

31.8 31.9 32.7

7.0 7.1 6.1

14.0 13.5 13.5

10.9 10.1 10.3

2.0 2.0 2.3

0.3 0.9 0.4

32.2 27.7 22.5

12.2 9.9 7.9

30.5 30.1 28.3

8.8 9.8 11.1

11.2 13.9 14.7

6.2 6.5 5.9

6.3 3.5 2.6

1.3 1.8 1.8

17.1 12.4 11.9

4.5 3.9 3.8

Time factor W/F (h) (h) 1 25.8 0.131 5 10

19.3 18.6

35.7 34.4 35.6

0.087

1 5 10

49.0 41.7 33.3

34.0 34.5 34.8

31.8 31.9 32.7

7.0 7.1 6.1

14.0 13.5 13.5

10.9 10.1 10.3

2.0 2.0 2.3

0.3 0.9 0.3

32.2 27.7 22.5

12.2 9.9 7.9

0.065

1 5 10

78.1 75.2 57.4

22.3 0.0 0.1

29.8 0.1 0.1

9.2 0.6 1.1

15.2 96.3 98.4

14.8 2.5 0.1

7.7 0.4 0.0

1.0 0.1 0.2

40.7 0.0 0.1

23.5 70.4 56.5

TOS: Time on stream Conv: n-hexane conversion BTX: Benzene+Toluene+Xylene selectivity

Table 6. Regenerative capability of FeCoCZ_A for two cycle regeneration runs on n-hexane cracking. Cycle

TOS (h)

Conv (C-


Parafins Selectivity (%)

BTX (%)

Yield (%)

mol%)

Fresh

1

2

C2=

C3=

Others C1

C2

C2=+C3= C1+C2

Others

1

49.0

34.0

31.8

7.0

14.0

10.9

2.0

0.3

32.2

12.2

10

33.3

34.8

32.7

6.1

13.5

10.3

2.3

0.4

22.5

7.9

1

44.5

23.8

28.8

6.8

12.3

11.9

9.2

7.2

23.4

10.8

10

33.5

33.8

32.9

6.8

13.4

10.4

2.7

0.0

22.4

8.0

1

41.7

30.5

30.8

8.2

13.8

12.0

0.2

4.5

15.2

7.8

10

30.0

30.5

32.5

8.0

13.2

11.5

4.2

0.1

18.9

7.4


Iron- and Cobalt-Doped CeriaâZirconia Nanocomposites for Catalytic

Recommend Documents

Iron- and Cobalt-Doped CeriaâZirconia Nanocomposites for Catalytic