Zirconia-Based Nanocatalysts in Heavy Oil Upgrading: A Mini Review

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Review

Zirconia-Based Nanocatalysts in Heavy Oil Upgrading: A Mini review Ahmad Masudi, and Oki Muraza Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03264 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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H2O

‘Propose Mechanistic reaction’ 1). CxHy* + H → CxHy

Dopant with Ceria

1

2). CxHy + 2 O2 - → CxHyOz 1

3). CxHyOz+ 2 O2 - →C

CeO2ZrO2

Zirconia

‘High Oxygen Transfer’

Ceria

Olefin intermediate

C + H* → C(x-n)H(y-n) C CO2 + CO + H2 + H2O

Zirconia

Asphaltenes in Heavy Oil

Ceria

Dopant with Ni

‘High Stability in Supercritical Water’

Nickel

Zirconia

Nickel ACS Paragon Plus Environment

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Zirconia-Based Nanocatalysts in Heavy Oil Upgrading: A Mini review Ahmad Masudi1, Oki Muraza2 1

Department of Chemical Process Engineering, Malaysia-Japan International Institute of Technology (MJIIT) Universiti Teknologi Malaysia Kuala Lumpur, Jalan Sultan Yahya Petra, 54100 Kuala Lumpur, Malaysia 2 Center of Research Excellence in Nanotechnology and Chemical Engineering Department King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

*Corresponding author: [email protected]

Abstract Two main limitations in heavy oil are recovery and upgrading process, which emphasized aquathermolysis as an alternative approach to reduce viscosity and to increase oil quality simultaneously. In addition to facilitating solubilization of asphaltene, hot water (subcritical and supercritical) injected into the heavy oil reservoir also serve as hydrogen donor. The main purpose of this article is to review of catalytic aquathermolysis of heavy oil over zirconia, which is a robust oxide. Zirconia based nanocatalysts are promising due to their high oxygen storage capacity, acidity and stability in sub- and supercritical water. The article also elaborates on various methods to fabricate nanosized zirconia with different shapes and sizes by sol gel, spray pyrolysis, electrospinning and hydrothermal technique. Keywords: Heavy oil; aquathermolysis, supercritical water; subcritical water

1 ACS Paragon Plus Environment

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1. Introduction The race for more oil to encounter energy demand centralized unconventional oil position to energy market. Worldwide reserve of unconventional oil is appoximately 6 trillion. Advance technology development is needed to increase heavy oil production, which only 500 billion barrels with existing technology1. Heavy oil has larger viscosity than conventional oil namely greater than 100 cP2. This unconventional oil mainly consist of asphaltenes, resins and aromatics. These composition associated with heavy metal such as vanadium and nickel3. The challenge for heavy oil recovery is its high viscosity. This high viscosity hinders the movement of heavy oil from reservoir. Numerous technologies were studied including in situ combustion, SAGD (steam assisted gravity drainage) and aquathermolysis. Among these technologies, aquathermolysis receives growing interest in recent years. Aquathermolysis is an alternative technique, which involves the injection of steam and water at elevated temperature and pressure to reservoir4, 5. Steam injection breaks large organic compound into smaller fragment and helps to ease the solubilization of organic matter6. Viscosity is proportional to asphaltene content. Asphaltenes, which comprise of 7-20 wt.% of heavy oil, are large organic compounds with molecular weight in the range of 500-750 g.mol-1. The molecular structure of asphaltene consists of polycyclic heteretom (S, O, N) with diameter 25-30 nm7, 8. The main mechanism for aquathermolysis reaction is the C-S breakage of asphaltenes, which tends to decrease heavy oil viscosity. This mechanism was clarified through interpreting similar gaseous product from aquathermolysis of different nitrogen compounds9. The same amount of H2S product with wide range of sulphur content from below 0.5 wt.% until 2.5-5 wt.% has confirmed that C-S breakage is originated from asphaltenes10.


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Viscosity reduction must occur in a short period to minimize the investment. Water behaves distinctly in near and supercritical condition. The study of Golombok and co-workers 11 revealed that water below its critical point only showed small viscosity reduction. However, near supercritical state, significant viscosity reduction was achieved (until ca. 90%). This result was better than thermal pyrolysis. Another research attempted to study the interaction between water and heavy oil in a sequential increase of temperature and pressure. They found that asphaltenes content decreased, when aromatic and aliphatic increased12. Aquathermolysis performance increased in the presence of catalysts13, 14. Therefore, the catalysts design were investigated intensively including soluble catalysts and solid acid catalyst15-17. The first series of water soluble catalysts, which reported by Clark et al14, were ruthenium and iron. This experiment was conducted to process bitumen at 375-415 °C and the percentage of sulfur reduction was 21 and 18%, respectively. Recently, Chao et al

18

employed

0.2 wt.% of copper aromatic sulfonic complex catalyst and able to decrease heavy oil viscosity until 95.5% by heating bitumen at 280 °C for 24 h. One of the most the promising oil soluble catalysts is molybdenum oleate

19

. The catalyst was tested at 240 °C for 24 h. By mixing 0.5

wt.% of catalyst with heavy oil, the viscosity of heavy oil decreased 90 %. Furthermore, adding 0.1 wt.% of an emulsifier to molybdenum decreased 90 % of oil viscosity at 200 °C in the same period

20

. In another experiment, equal viscosity reduction was achieved at lower reaction

temperature namely at 180 °C using Ni and Co based catalysts with petroleum sulfonate as emulsifier 21. However, the catalysts instability at high temperature and pressure would increase the industrial cost to recover and reuse22-24. In addition, the study of two kinds of soluble catalysts showed that asphaltenes and resins were partly aggregated after aquathermolysis 25.


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Amphiphilic catalyst, such as cobalt dodecylbenzensulfonate, was thermally stable until 400 °C. The catalyst reduced the interfacial oil-water surface tension and improved the mixing between water and oil. Upgrading performance over this catalyst was studied using Sheng li heavy oil at 250 °C for 24 h. The viscosity reduction was 38 %. This catalyst was promising for heavy oil with high aromaticity and high oxygen content 26. Another alternative catalyst was an ionic liquid. One of the ionic liquid catalysts consist of 10 wt.% of iron, 2% wt.% of molybdenum and an ionic liquid namely deep eutectic solvent. The upgrading was performed at 300 °C for 24 h, which was able to decrease 43 % of oil viscosity and 32 % of sulphur content 27. However, the efficiencies of these amphiphilic and ionic liquid catalysts were lower than soluble catalyst. Solid catalysts showed a higher stability at oil field condition with a high efficiency22, 28, 29

. The earliest study of solid catalyst was based on the superacid catalyst namely HFBF3. The

first heavy oil sample was Alberta’s reservoir, which was processed at 170-190 °C for 1 h in the presence of hydrogen. This catalyst was able to convert 56% of bitumen to volatile oganic compounds30. However, as the recovery of the catalysts was very difficult, ther acid catalysts were explored, for instance, nano-keggin catalyst namely K3PMo12O40 showed a high conversion up to 90% at 280 °C. The catalyst has both acidity and redox activity, which demonstrated a synergetic effect to upgrade heavy oil31. This heteroply solid acid exhibited low tolerancy to water. It was established that incorporation of metal could increase the catalyst tolerance to water22, 32, 33. As an example, doping cesium to the heterpoly acid could modify catalyst surface hydrophobicity. From this experiment, the highest water tolerant was achieved with doping 2.5 wt.% of cesium to the catalyst

34

. On the other hand, the screening of some metal oxides and

their mixed oxides to water tolerance

28, 35, 36

showed that MoO3-ZrO2 and WO3-ZrO2 were the


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most stable mixed oxides in water. However, since MoO3-ZrO237 showed better conversion to the esterification of fatty acids, this catalyst was considered as one of the most potential mixed oxiedes for upgrading heavy oil38. Zirconia is widely used for solid oxide fuel cell39, peat pyrolysis40 and clean-up gasification gas41. For instance, in biomass gasification, zirconia is more resistant to Sulphur, while at the same time, zirconia removes tar and ammonia42. Moreover, zirconia showed a good stability at high temperature and in sub- and supercritical water43. The article highlights state of art of the synthesis methods and highlights the size and morphology control of zirconia nanoparticles. The article also emphasizes the potential use of zirconia-related catalysts for catalytic aquathermolysis. 2. Upgrading heavy oil using nanosized oxides Since heavy oil has low H/C, this type of oil require upgrading process before obtaining transportation fuel. Generally, upgrading process can be splitted into two categories, namely hydrogen addition and carbon rejection. Sometimes, the research and technology developers combine these two technologies to obtain desired result44. The use of nanoparticles for heavy oil upgrading is relatively novel approach. Nanomaterials with size lower than 100 nm have good dispersibility and high activity due to their high surface area to volume ratio. Moreover, nanoparticles easily penetrates porous medium such as reservoir rock45. Hence, resulting a more efficient contact with heavy oil. Among nanoparticles, nano-oxides gained strong interest over the last few years, with their capability to transfer oxygen ions

46

and to provide high affinity to asphaltenes47. Nano

sized CeO2 with controlled morphology has a large oxygen storage capacity, which further crack the asphaltenes. Meanwhile, the study of adsorption properties of acid, amphoter and basic metal


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oxide to asphaltenes also showed interesting results. This research revealed that surface interaction is a dominant key in reaction mechanism. According to this study, the extracted asphaltenes was a basic, hence it was easy to have a better affinity with an acidic oxide. Metal oxide from transitional elements also served as potential catalysts. NiO which was prepared by co-precipitation using ethylene glycol has average size 60-70 nm. This oxide decreased 20% of oil viscosity48. On the other hand, hematite, α-Fe2O3, which the most abundant of iron source, showed high activity to break C-S bond. This activity was concluded from hematite reaction with thiopene with various concentrations. From this experiment, incorporation of 0.1 wt.% of hematite decomposed 26.3 % of thiophene at 120 °C. The mechanistic reaction was also strengthened by absence of C-S bond in IR spectrometry. This reaction also produce rich SO2 and CO2. The result could be an indication that C-S bond was cracked and confirmed existence of active oxygen in the catalyst. In this reaction, hematite was reduced at initial reaction, then re-oxidized by the presence of water49. Meanwhile, another iron based catalyst namely Fe3O4 also reported to upgrade heavy oil at 200 °C for 6 h. This study showed that viscosity of heavy oil reduced up to 71.3%50. Finally, a series of metal oxides consist of NiO, Co3O4 and Fe2O3 were studied by Nassar et al51 as nanocatalysts for heavy oil upgrading. The average sizes of NiO, Co3O4 and Fe2O3 were 12 nm, 22 nm and 43 nm, respectively. The results exhibited that percentage of viscosity reduction in 300 °C were 37, 20 and 30%, respectively. This result confirmed that affinity of metal oxide to asphaltenes was one crucial criteria to achieve significant viscosity reduction.


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3. Synthesis of zirconia and zirconia nanocomposites There are many routes to prepare zirconia nanoparticles and its composites. This article only focuses on bottom-up approach namely sol gel, spray pyrolysis, electrospinning and hydrothermal as described in Fig. 1.

Please insert Fig. 1 here. (Fig. 1.Synthesis of zirconia nanoparticle and zirconia nanocomposites).

3.1 Sol-Gel Sol-gel is a common technique to prepare bulk metal oxide. This technique is a simple way to obtain gel from sol through hydrolysis and polycondensation. However, this method has numerous challenges to produce nanosized metal oxide. The major limitation is to control simultaneous hydrolysis and condensation reaction, which lead to agglomeration. The complexity is a result from fast hydrolysis reaction rate to the precursor. To solve the problem, non-aqueous medium is used due to its slower reaction as consequences of medium strength of carbon bond52, 53. Generally, non-aqueous sol-gel synthesis can be assisted with solvent or surfactant. Zirconia nanoparticle with crystallite size in a range of 10-36 nm was produced from reaction of zirconium alkoxide in present of alcoholic solvent 54. In addition, Suciu et al 55 clarified the effect of various solvent in precursor Zr(NO3)2. The solvents were glycerol, ethylene glycol, citric acid and sucrose-pectin. The smallest crystallite size of 33 nm was obtained from sucrose pectin. This method is less toxic than surfactant and high surface purity. However, surfactant facilitate easier 7 ACS Paragon Plus Environment

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method to control over morphology, size and size distribution. As example, different zirconia properties were achieved with the same precursor as reported by Nesamani et al56 and Siddiqui et al 57. These two experiments confirmed that properties of zirconia were highly influenced by the surfactant source. Zhan et al

58

produced composite of zirconia-silica from alkoxide source and ketone via

aldol like condensation. Another composite such as YSZ (yttria stabilized zirconia) also received growing interest. The common precursors were Zr(NO3)2, ZrOCl2 and ZrCl4 with identical source of yttria namely Y(NO3)359. In the process, the same organic solvents such as sucrosepectin and citric acid-ethylene glycol were used. From this experiment, smaller crystallite size of this composite as compared to pure zirconia was obtained. Even, the crystallite size of zirconia was ca. 17 nm, while ratio of zirconia to sucrose was set in a range of 1 to 4. Another composite, zirconia-scandia was obtained with large organic surfactant namely acrylamide and N, N’methylene-bisacrylamide, ammonium persulfate. The average size of this composite was 5-8 nm60. One of the most important factors to control the shape and the size of zirconia through sol-gel is the selection of precursor

57

. The experiment showed clear morphology distinction

from two different zirconia sources. ZrOCl2 precursor produced rod shape and Zr(SO4)2 favored flower like-morpholgies. Moreover, the product crystallite size originated from ZrOCl2 was twice than Zr(SO4)2.


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Table 1. Synthesis of zirconia and zirconia nanocomposite using sol-gel technique. Precursor Solvent or Experiment Crystallite size Ref Surfactant zirconium

n- ethanol,

butoxide

NH4OH

and Phase

• Adjusted pH until 10 Mix of tetragonal and after dissolve zirconium n-butoxide in ethanol

• Calcinedat 400, 600 and 800 °C for 4 h

Zr(OC3H7)4,

acetylacetone,

TEOS

2-propanol

54

monoclinic T(°C) T

M

400

11.9 10

600

22.3 26

800

36

30

• Covered with plastic film Not reported

58

after hydrolysis reaction start

• Calcined at 500-900 °C for 4 h ZrOCl2·8 H2O benzyl alcohol, • Calcined at 600 °C for 5 ZrOCl2·8 H2O = 18.1 or

sodium lauryl

Zr(SO4)2·H2O

sulphate

h

57

nm Zr(SO4)2·H2O = 9.7 nm

Mixed of tetragonal and monoclinic ZrOCl2

benzyl Alcohol, • Adjusted pH until 10 2-10 nm sodium

with aqueous NH3 to monoclinic which stable

dodecylsulphate

precursor

56

up to 1100 °C


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• Calcined at 750 °C for 2 h Zr(NO3)2·H2O

glycerol

tetragonal + monoclinic

• Calcined at 900 °C ethylene glycol

55

42.43 nm tetragonal + monoclinic 34.81 nm

citric acid

tetragonal + monoclinic 38.62 nm

ZrOCl2·6H2O,

citric

acid, • Calcined at 1000 °C for 2 Average: 29 nm

Y(NO3)3·6H2O

ethylene glycol

h

61

Monoclinic

• Lab and Industrial scale show same conversion. The difference only gel formation time ZrCl4,

sucrose, pectin

Y(NO3)3·6H2O

• Calcined at 700 °C for 4 31; 23; 17; 17; 18; 20 h

59

nm

• Various sucrose to Zr Cubic molar ratio is applied Zr(OH)4·nH2O, acrylamide, N, • Calcined at 500 °C

5-8 nm

Sc2O3

Flourite cubic

N’-methylene-

60

bisacrylamide, ammonium persulfate


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3.2 Spray Pyrolysis Spray pyrolysis is a route involving gas or volatile solution, which is injected to series of chamber. Initially, atomization will occur with pneumatic, electrostatic or ultrasonic. This aerosol-solution is then evaporated and the solution is precipitated. Finally, the droplet changes to dense solid through drying and sintering62. The schematic illustration of this technique is presented in Fig. 2.

Please insert Fig. 2 here. (Fig. 2. Schematic illustration of spray pyrolysis technique62).

Nanomaterial zirconia was produced via spray pyrolysis of ZrO(NO3)2 using two fluid nozzles as atomizer. The dominant phase of product was monoclinic with only 4% tetragonal. To identify the impact of feed rate, other experimental condition variables such as air pressure, hot blower rate, liquid density were maintained constant. From this experiment, the optimum condition was a feed rate 0.15 L h-1 with an average particle size of 30 nm and a surface area of 280 m2 g-1 with a production rate of 3.6 g.h-163. Zirconia-alumina nanocomposite was achieved from nitrate salt of zirconia and alumina. The ratio of zirconia and alumina was 1:1. Ultrasonic homogenizer was used to completely mix zirconia and alumina. The product has crystallite size of 27.4 nm stabilized zirconia (YSZ), was reported by Perednis et al

65

64

. Another composite, yttria

. The precursor was zirconium (IV)

acetylacetonate and yttrium (III) acetate. These solutions were dissolved in ethanol with a constant stirring rate. The final composite was tetragonal with crystallite size 8-10 nm after


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calcination at 950 °C. Meanwhile, composite zirconia-scandia with low content of manganese was prepared by mixing its nitrate solution and produced large particle diameter66. Morphology control in spray pyrolisis method was also investigated recently. Two composites consisted of YSZ and ceria from acetate solution were used as sample. The YSZ composite was spherical with smooth surface. On the other hand, ceria was bowl-like and bumpy. From this research, it was considered that large difference in solubility further differentiated the morphology. The solubility of YSZ and ceria was ca. 8000 and ca. 260 g/L, respectively67.


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Table 2. Preparation of zirconia and zirconia nanocomposites with spray pyrolysis. Precursor

Condition

Result

Ref

ZrO(NO3)2

Air pressure: 30-40 psi

Output: 72±5%

63

Temperature: 773 K

96% monoclinic; 4%: tetragonal

Speed of hot blower: 1800 Average crystallite size: 18 nm rpm

Grain size of tetragonal: 26 nm

Liquid density: 1.031 g.cm-1

Feed rate Effect:

Operational: 10 h

0.10L h-1: APS: 28 nm; SA: 283 m2 g-1 0.15L h-1: APS: 30 nm; SA: 280 m2 g-1 0.20 L h-1:APS: 46 nm; SA: 195 m2 g-1 0.25 L h-1:APS: 93 nm; SA: 141 m2 g-1 0.30 L h-1:APS: 112 nm; SA: 98 m2 g-1 The optimum condition was 0.15 L h-1

Al(NO3)3;

- Ultrasonic homogenizer

Zr(NO3)4

- Deposited at SiO2 substrate The at 300 °C

64

Crsytallite size: 27.4 nm composite

showed

good

stability


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- Heated 500 °C for 5 h (C5H7O2)4Zr;

Atomizer:collision nebulizer Size: 8-10 nm

(CH3CO2)3 YH2O and airbrush

65

3%YSZ : tetragonal

in methanol

Furnace: 950 °C

Zr(NO3)2xH2O,

- Mixed

equimolar

citric Particle diameter: 0.67 µm

Sc(NO3)2xH2O,

acid and ethylene glycol to The material was promising for solid

Mn(NO3)24H2O

nitrate solution

66

oxide fuel cells

- Charred at 240 °C - Calcined at 750 °C for 10 h - Atomizer:

ultrasonic

nebulizer

Zr(NO3)43H2O,

- Mixed with citric acid and (La,Gd)2Zr2O7: 0.607 nm

La(NO3)36H2O,

polyethylene

Gd(NO3)36H2O

solvent

glycol

68

as (ZrGd)O2 : 0.261 nm

- Atomizer: steel nozzle - Pyrolized at 500 °C - Annealed product at 9001400 °C for 10-24 h APS: Average particle size; SA: Surface area


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3.3 Electrospinning Electrospinning is a method to prepare fibrous material utilizing a high electric field in presence of certain polymers to prevent fiber structural collapse, which may occur during calcination. Since nanofiber has a large ratio of surface area to volume, this morphology is attractive to various application such as catalysis and optoelectronic. This technique is extensive approach to prepare nanofiber ceramic with high yield and low cost69. Early electrospinning discoveries were commonly focused to fabricate nanoporous polymer. Then, after the invention of ceramic nanofibers, zirconia received great interest. Shao et al

70

reported the first study to prepare nanofiber zirconia through electrospinning. Zirconia

precursor and the polymer must be mixed before being injected to electrospinning reactor. The combination of zirconium oxy-chloride and PVA produced pure tetragonal zirconia at 800 °C with diameter between 50 and 200 nm. Zirconia fiber was also accomplished from zirconium acetate solution. However, the diameter was bigger than previous report namely 300-500 nm at 800 °C

71

. PVP that has a larger building block than PVA was responsible to form larger

diameter. To improve catalytic properties, building block for electrospinning can be modified from templating agent, not limited to polymer as reported by Yin et al

72

. The templating agent

such as triblock copolymer (such as Pluronic series were used to produce average fiber diameter in the range of 100-400 nm. Zirconia composite such as YSZ was also achieved through electrospinning 73. Yttria and zirconia precursor with certain ratio must be completely dissolved before mixing with the polymer. The experiment used two stages of calcination until 1500 °C to obtain monoclinic 8YSZ. In another report, crystallite formation of fiber NiO/ZrO2 was accomplished through


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electrospinning with calcination at 550 °C. The average fiber diameter was 106 ± 25 nm with the assistance of PVA74. Table 3. Fabrication of zirconia and zirconia nanocomposites with electrospinning. Materials zirconium

Experiment

- 10 wt.% of PVA

oxychloride,

Electrospinning condition

polyvynyl

-

alcohol (Mw: 80,000)

Product

Ref

Tetragonal

70

Voltage: 20 kV

- Dried at 70 °C for 12 h under vacuum

- Calcined at 800 °C for 10 h Product: ZrO2 zirconium

with rate 240°C h-1 71

- 6 wt.% of PVP

acetate solution

-

FR: 0.1-0.4 mL/h

Temp Cryst. Size

(Solvent: acetic

-

TCD 15 cm

(°C)

(nm)

acid, Zr 16

-

Voltage: 10 kV

400

14.5

Tetr

600-

14.9

mon.

%wt), PVP (Polyvinyl

- Calcined at 200-1000 °C for 2 h in a muffle furnace

Pyrrolidone)

800 1000

Phase

11.5% 26.9

Mw: 1,300,000

mon. 85.4%

Product: nanofiber ZrO2 zirconium

- Age until ≈ 7000 cps

Temp Cryst. Size

Phase

72


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oxychloride,

Electrospin condition

(°C)

(nm)

ethanol, acetic

-

Voltage: 30 kV

450-

2-3

acid, pluronic

-

Dry at 100 °C overnight

900

EO133PO5EO133

-

Calcined at 450 °C for 6 h

-

Calcined at 900 °C for 3 h

Mix of tetr and mon.

Product: Nanofiber ZrO2 zirconyl

15% wt.% of PVA

chloride, nickel

Ni:Zr= 1:1

acetate, PVA

Voltage: 12-25 kV

74

TCD: 12 cm Product:

- Calcination: 400 and 550°C

NiO/ZrO2 zirconyl

- Mixed

ZrOCl2

and Cubic solid solution

chloride, ytrium

Y(NO3)3·6H2O with stir for 2 h

nitrate, PVP

(A)

(Mw≈3x106)

73

- Dissolved 15 wt.% PVP in ethanol (B)

- Blended 1:1 (A) and (B) to Product: YSZ

electrospinning reactor Electrospinning condition -

Voltage: 12-15 kV

-

FR: 0.03 mL h-1


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-

TCD: 3 inches

- Fired fiber via this sequence: *from 25 to 500 °C for 1 h *from 500 to1500 °C for 1 h *from 1500 to 25 °C FR: Flowrate; TCD: Tip to collector distance

3.4 Hydrothermal Synthesis Hydrothermal route is a simple approach to synthesize nanomaterials with a narrow size. In this technique, crystal nucleation and growth occur simultaneously at liquid phase. By surpassing calcinaton, hydrothermal able to minimize agglomeration and crystallize in relatively low temperature and pressure. Zhang et al 75 produced nanosized zirconia using zirconium oxychloride as precursor with cetyltrimethylammonium bromide (CTAB) as surfactant. After mixing with strong agitation, ammonia solution was added as precipitating agent. The research showed that two phases of zirconia namely tetragonal and monoclinic were obtained at a temperature lower than 250 °C. Above this temperature, pure tetrahedral phase was obtained with an average size 22 nm after heating for 18 h. In this study, CTAB was used not only to prevent agglomeration of ZrO2, but also facilitate lower temperature for tetragonal formation. In another research, the smaller crystallite size of zirconia namely 1-20 nm was achieved with the same zirconia precursor, but the pressure was much higher at 25 bar. However, the products were still combination of monoclinic and tetragonal 76.


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Other reported zirconia nanocomposites were La2O3/ZrO2, TiO2/ZrO2 and CeO2/ZrO2 76. This article showed that morphology of composites were monoclinic, amorphous and cubic solid solution with the surface area 77, 209, and 96 m2/g, respectively. The experiment also exhibited that TiO2/ZrO2 was a potential mixed oxide for acid-base catalysis. Mineralizer is often used to control size of zirconia with hydrothermal method. Type and concentration of mineralizer were highly influenced zirconia product. Mineralizer such as NH4F produced better crystallinity of zirconia. Higher concentration of NH4F also increased the crystallite size. To identify the effect of mineralizer type, several mineralizers such NH4Cl, NH4Br, KF, NaF were explored. The result showed that mineralizer without F- resulted in products with the same characteristic with pure zirconia. On the other hand, F- mineralizer promoted better crystallinity to zirconia 77. Doping zirconia is a method to stabilize zirconia at high temperature. Another alternative was through particle size stabilization. The particle size stabilization was achieved via hydrothermal at high temperature and pressure with supercritical water. Moreover, this technique was able to produce pure phase within a short period. Several factors such as temperature, pressure, pH and precursor concentration were the keys to control zirconia size and morphology. The study of hydrothermal mechanism revealed that intermediate ion was the key in directing the morphology. Zr(OH)22+ was an intermediate ion to form monoclinic, which occured at low pH. Meanwhile, tetragonal phase was produced via Zr(OH)5, which appear with high pH78. Pure monoclinic zirconia was reported by Becker et al

79

with crystallite size of ca. 5-6 nm, rapidly.

Composite of zirconia such as ceria-ziconia and yttria stabilized zirconia (YSZ) were also reported elsewhere80.


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Precursor ZrOCl2 8H2O, CTAB

Table 4. Hydyrothermal synthesis of zirconia and zirconia nanocomposites. Experiment Result Ref - zirconia concentration: 0.1 250 °C: tetragonal 298 via water bath for 2 h

Size: 22 nm

- Added NH3 dropwise until pH 9 - Heated at (150, 200. 250) °C for 18 h in autoclave ZrOCl2 8H2O;

- Added NH4OH dropwise to ZrO2 = SA: 65 m2 g-1; Mix of

TiCl4;

precursor until pH 10

monoclinic + tetragonal; 10-20 nm TiO2/ZrO2 = SA: 209 m2 g-1;

LaCl3 6H2O; CeCl3 7H2O

76

Hydrothermal

Amorph; Potential for acid base

- 220 °C; 25 bar for 4 h under catalysis CeO2/ZrO2 = SA: 96 m2 g-1; cubic

stir

- Dry at 120 °C and calcined solid solution at 500 °C for 10 h

La2O3/ZrO2 = SA: 77 m2 g-1; monoclinic

Zr(NO3)4 5H2O, - Mixed in DW with continuous The more NH4F, The bigger its NH4F

stir

77

crystallite size

- NH4F conc (0.125 M; 0.25 M; 0.3 M)


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Hydrothermal: - Autoclave 180 °C for 72 h CTAB: cetyltrimethylammonium bromide

4. Zirconia-based catalysts in heavy oil upgrading Zirconia is an amphoteric catalyst. However, zirconia can be converted to a superacid solid catalyst with sulfate ion to produce SO42-/ZrO281. The most frequent method to prepare this catalyst was through sulfating zirconium hydroxide. The sulfation induced acid site of Lewis and Brønsted on the surface this mixed oxide as presented in Fig. 337. Modified sulfated zirconia catalysts with Ni2+ and Sn2+ for Sheng li oil viscosity reduction were studied intensively. These catalysts were investigated at 240 °C and 3-4 MPa for 24 h. Doped SO42-/ZrO2 with Ni2+ was a better viscosity reducer than Sn2+ with a conversion 57% and 20%, respectively. Nevertheless, the conversion decreased from 57% to 20% in presence of 5% water due to solubilization of sulfate ion82. A common method to enhance durability due to sulfate leaching was the facbrication of mixed oxide. The composite consisted of MoO3-Nd2O3 showed high resistance in water 83. Please insert Fig. 3 here. (Fig. 3. The production of lewis and Brønsted acid site in zirconia sulfate in37 which was adapted from84). Tungstate zirconia was also investigated for viscosity reduction of heavy oil. The composite was prepared through precipitation and impregnation with various ratio of tungsten to zirconia. Number of acid sites and acid strength are highly influenced by tungsten percentage. Hydrothermal synthesis produced better crystalized zirconia-tungsten and more acidic than 21 ACS Paragon Plus Environment

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Energy & Fuels

impregnation method. This research showed that the catalyst was able to lower the viscosity through C-C breaking, which was identical to catalytic cracking. The catalysts were evaluated at 220 °C and 2 MPa for 6 h. The best viscosity reduction was reached (82%). This finding demonstated an alternative method to upgrade heavy oil with low or no water content85. Recently, FeOx based catalyst, namely CeO2-ZrO2-Al2O3-FeOx was applied as the catalyst to upgrade Canadian heavy oil to evaluate the optimum condition to produce the most dominant desired lighter fractions. In these experiments, ratio of mixed oxide was kept constant; CeO2:ZrO2:Al2O3 = 2.5%:7.5%:7%, respectively. The pressure of the reaction was varied from 2.8 to 24 MPa to find the optimum pressure condition. The optimum pressure was observed at 19 MPa. Below this pressure, the number of coke was greater than 22%, which decreased the amount of VGO and gas oil. Meanwhile, above this pressure, the desired lighter fraction decreased from 70 wt.% to 45 wt.% due to the recombination of lighter fraction at catalyst surface, which was called as 'cage effect'. Therefore, the temperature of this reaction was varied in the range of 350-420 °C, to evaluate its optimum temperature. The feed of this reaction consisted of vacum residue of 65 mol.% C of bitumen and some heavy component with average molecular weight higher than 500 g/mol and 2000 g/mol, respectively. At the temperature above 400oC, the viscosity of bitumen decreased significantly above 400 °C to 300 g/mol. Hence, it ease the interaction with catalyst active site and produce higher desired lighter fraction. From this condition, the lighter component reached 70 wt.%, while carbonaceous residue decreased from 20 wt.% to 10 wt.%. In this research, catalyst stability was evaluated by reacting bitumen at its optimum condition; 19 MPa and 420 °C for 6 h. The product of this condition consist of 65% of lighter component and coke yield 10 %. The catalyst was then reused after removing its surface through calcination at 500°C for 2 h. The amount of lighter fraction and carbonaceous


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residue are similar with fresh catalyst. This result showed that the mixed oxide has high stability and durability86.

4.1 The stability of zirconia in subcritical water Hot water is a cheap and safe solvent for heavy oil. Hot water eases the interaction of large molecules with catalysts surface. However, the long term catalyst activity in this medium are still challenging during past decade. Therefore, the search for stable catalyst in the hot water become key stage which would accelerate heavy oil global realization. Among the catalysts, metal oxides are still the first option for catalyst support at high temperature of hydrothermal condition due to their higher mechanical strength as compared to carbide catalyst. Meanwhile, catalyst promotor must be a metal oxide with intermediate acid strength, which would be dissolved in hot water. This conclusion was derived from electronegativity of metal oxide which confirmed solubility of metal oxide or hydroxde in water. Moreover, pourbaix diagram which display stable metal phase in varied pH and electrochemical potential was used as second criteria. Based on this diagram, the stable metal oxides in hot water (200 °C) are only zirconia, alumina and titania. Zirconia and alumina are transformed into its hydroxide form. Meanwhile, titania sustained its oxide form. From these two criteria, zirconia is promising candidate as support and promoter 87. Pourbaix diagram of zirconia in hot water is presented in Fig. 4.


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Energy & Fuels

Please insert Fig. 4 here. (Fig. 4. Pourbaix diagram of zirconia87).

4.2 The stability of zirconia in supercritical water Carbon residue is serious problem for heavy oil catalytic upgrading. Coke formation will poison catalyst active site, hence its activity decreased drastically. In heavy oil, which mainly consists of asphaltenes, the probability to yield carbon residue is higher than in the conventional oil. One strategy to decrease coke formation is by reacting the heavy oil at high temperature and pressure with sub- and supercritical water. Supercritical water (SCW) is a promising solvent for heavy oil upgrading. Almost all bitumen moieties dissolved in SCW at 440 °C, 29 MPa with ratio of bitumen to SCW 1:2.5 88. The agglomeration asphaltenes are also avoided because their tendency to form microemulsion which surpass coke formation

89

. Furthemore, SCW produced

more active in-situ hydrogen than hydrogen molecule 90. The stability of many ceramics in SCW was reported previously, carbon, α-Al2O3 and zirconia were the only stable materials in SCW. Zirconia showed the biggest fracture toughness 91

. According to Adam et al 92, monoclinic is the stable form of zirconia crystal. The same result

also showed by other references

93, 94

. The stability of zirconia in SCW was enhanced with

metallic elements. The Ni/ZrO2 in SCW as gasification catalyst was stable for 85 h and no futher


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decreased on surface area

43

Page 26 of 49

. Moreover, Ni/ZrO2 produced the highest hydrogen yield in

swicthgrass gasification 92. Monoclinic zirconia has a lower surface area than the tetragonal structure. Several researchers reported the method to stabilize tetragonal zirconia in SCW. Zöhrer al

95

added

1.77% HfO2 and La2O3 9.7% to ZrO2. Matsui et al 96 added the second stabilizer (Al2O3 or GeO2) in YSZ and the composite was stable until 4 years at 140 °C of hot water.

5. Acidity and oxygen storage properties of zirconia based catalysts and their roles in hydrothermal (subcritical and supercritical water) upgrading of heavy oil

Upgrading heavy oil, which has low H/C ratio, mainly require hydrogen addition or oxygen species. However, providing hydrogen or oxygen from external source is expensive. Therefore, the technique and the material selection to supply hydrogen and oxygen become a crucial concern. One method to provide internal hydrogen is to select material with high acidiity, which will easily transfer hydrogen. Among acid compound, tungstate zirconia showed best performance for viscosity reduction 97. The type of acid cites in zirconia can be clarified with FTIR spectra after pyridine adsorption. Tetragonal form of nanosized zirconia has only Lewis acid sites, while monoclinic has both Lewis and Brønsted site. In addition, monoclinic zirconia has a larger acid density which is approximately double than tetragonal zirconia. The number of acid sites were proportional to the number of Zr4+ in its surface as calculated with density functional theory (DFT)98.


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Energy & Fuels

Zirconia is a bifunctional acid-base catalyst with Zr4+ as acid site and O2- as basic site. Zirconia has a wide range application in energy and chemical industry. Among the process, zirconia was utilized for hydrogenation of

benzoate acid to benzaldehyde, which was

commercialized by Mitsubshi Kasei Corp. Many types of metal oxide including MgO, TiO2, ZnO, γ-Al2O3 and zirconia were tested in presence of hydrogen. Zirconia showed the best performance with selectivity up to 97% and conversion of 53% at 400 °C. Moreover, doping ultrasmall Cr2O3 to ZrO2 with ratio Cr/Zr 0.05 enhanced the conversion significantly to 98% at 350 °C99. The mechanism for direct decarboxylation is presented in Fig. 5.

Please insert Fig. 5 here. Fig. 5. The mechanism for direct decarboxylation of benzoate acid to benzaldehyde100.

Another industrial application of zirconia is in Meerwein Pondorf Varley (MPV) reduction. This process aim to reduce aldehyhde, ketones, carboxyclic acid or ester with alcohol. The study of direct reduction of levulinic acid to γ-valerolactone was perfomed with Beta-zeolite containing zirconia. The best catalytic performance was accomplished with Si/Zr 100 with high conversion and selectivity, ca. 100 and 96%, respectively. The reaction was investigated at reflux 118 °C using 2-pentanol as a reactant. This best performance due to its moderate Lewis acid sites, which were contained in Zr-zeolite Beta. Meanwhile, doping with Al, which is Brønsted acid site to Zr-beta zeolite decreased the selectivity to 71% 101. Natural zeolites

102

were also investigated for bitumen upgrading and cracking. The

performance of a natural zeolite, namely chabazite, was better than commercial zeolite Y, which was wellknown as a fluid catalytic cracking catalyst. The use of zeolite Y, only showed slight


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viscosity reduction of Athabasca bitumen. Meanwhile, modified chabazite with Ca changed viscosity until 97%

103

. External surface acidity and crystallinity of chabazite played an

important role for heavy oil upgrading. Ca-chabazite showed a high acid strength with both Hammet indicator and ammonia TPD, namely -3.0 and (-5.6) and 3.66 meq/g. This catalyst was able to reduce viscosity until 81% in the presence of 3% water 104. Spectroscopic analysis is used to predict basic mechanism for viscosity reduction, the spectroscopic measurement of chabazite at 350-650 °C confirmed that acid site was mainly Brønsted acid within framework and nonframework

105

. Meanwhile, acid site of WOx/ZrO2 consists of Lewis and Brønsted. In

addition, the study of desulfurization confirmed that catalysts performance was in linear correlation with Brønsted acid site

106

. Based on the theoretical aspect, the optimal zirconia

performance is one Lewis acid site with maximum Brønsted sites. The optimum for converting viscosity is 20% of tungsten with strong acid amount namely 0.35 mmol/g and Hammet indicator of -14.5 and -16.1

107

. Hence, it confirmed that chabazite acid site originated from its CHA

framework, while the acid sites in WOx/ZrO2 were originated from high dispersion of tungsten on zirconia. Acid sites in zirconia-silica composite was scrutinized by experiment and theoretical approach. Characterization with XRD validated that the composite consist of tetragonal zirconia and amorphous silica. Acid strength measurement with NH3-TPD showed that initially high the heat adsorption was high and then decreased when the surface area became larger. Moreover, pyridine adsorption verified that only Lewis acid site existed on tetragonal zirconia. Based on theoretical approach, the lower heat of adsorption was originated from NH3-NH3 repulsion. In addition, strong acidity in the early adsorption stage was attributed to the adsorbed NH3 on Zr site with neighboring oxygen deficient 108.


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Table 5. Acidity of zirconia and zirconia nanocomposites. Materials

Acidity

Comment

ZrO2 tetragonal

Lewis acid sites

The

number

Ref of

acid

sites

is

98

Acid Surface Density: 1.7 proportional to number of Zr4+ on NH3/nm2 ZrO2 monoclinic

the surface

Both Lewis and Brønsted acid site Acid surface density: 3.4 NH3/nm2

10 WO3/ZrO2

0.36 mmol/g

Both Brønsted and lewis acid sites

15 WO3/ZrO2

0.46 mmol/g

are

20 WO3/ZrO2

0.53 mmol/g

strength due to W=O interaction at

25 WO3/ZrO2

0.51 mmol/g

ZrO2 to WO3

SO42-/ZrO2

Hammet

Indicator:

confirmed.

The

high

97

acid

107

-14.5

and -16.1 Zr-Zeolite

Beta 45 mol/g

Adding Brønsted acid will lower

100 Chabazite

101

acid strength Hammet Indicator :-3.0 and

104

-5.6 NH3-TPD: 3.66 meq/g


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Recent method to advance zirconia performance was reported recently through impregnation to red mud with varied Zr content. Red mud comprises mainly of iron oxide and alumina with low content of silica and titania. The material was evaluated in steam catalytic cracking of vacuum residue at 470 °C and 0.1 MPa for 2 h. The highest conversion and liquid yield, namely 85% and 35%, respectively was reached over 3% wt. of Zr. Both conversion and liquid yield decreased significantly, when the zirconia content was above 3 wt.%. Higher content of zirconia led to a lower catalyst surface area and decreased catalyst active site. Hence, the amount of coke and liquid yield also decreased. Meanwhile, the side product of this reaction contained low carbon dioxide. Therefore, it was confirmed that catalyst active site was not from oxygen lattice. Meanwhile, H/C ratio of product was relatively high (1.76). This was an indication that catalyst donated some of hydrogen to vacuum residue. Addition of zirconia increased the catalyst acidity and facilitated more hydrogenation reaction. Moreover, the catalyst showed a high durability after the second and the third repetition with a slightly decrease of activity after the first usage. Finally, this composite showed a better activity and durability as compared to Al-FeOx catalyst109. Another novel improvement of zirconia oxide compound for heavy oil upgrading at superheated steam was reported recently. First, titania zirconia with varied composition was evaluated as catalytic cracking of 10 wt.% atmospheric residue (AR). Therefore, the optimum composition was investigated for catalytic upgrading Canadian Oil sand bitumen. Among the variation, equimolar zirconia titania catalyst showed the best perfomance with high lighter fraction and low carbon residue. This compound has the highest acidity namely 0.3 mmol NH3/ g, which mainly consisted of Lewis acid. The total acidity of zirconia-titania was approximately a half of zeolite H-BEA (Si/Al = 13.5) acidity. However, zeolite has a lower tolerancy to carbon 29 ACS Paragon Plus Environment

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Energy & Fuels

deposition at high temperature. Then, the composites were evaluated to decompose 10% of Canadian oil sand bitumen. The reaction was carried out at 470 °C and modified time factor 88 m2.h.g-1. The produced lighter fraction (VGO and gas oil) and coke were 61% and 20%, respectively. The product of this reaction mainly consisted of lighter hydrocarbon, volatile and olefin. Therefore, it was evidence that the mechanism of this reaction occured via carbon-carbon breakage and polycyclic aromatic opening110. This titania-zirconia catalyst showed a high tolerancy to carbon residue in superheated steam, hence the composite was potential as a catalyst in heavy oil upgrading111. Another significant properties in heavy oil upgrading was the ability for catalyst substance to produce internal oxygen. The ability of ceria zirconia to produce hydrogen and oxygen via water splitting at 500 °C was reported recently 112. Zirconia is a stable metal oxide, which cannot transfer oxygen species. Doping zirconia lattice will lead to a crystal defect. More positive ion in zirconia lattice with rapid charge transfer would lead oxygen vacant condition to maintain electroneutrality. Hence, some non-stoichiometric oxygen species would be produced. In a catalytic system of FeOx-ZrO2, Fumoto et al, showed that ZrO2 produced labile oxygen generated from H2O. Increasing ZrO2 content at constant FeOx, increased the CO2 product 36. On the other hand, the ZrO2-FeOx was evaluated to transform 2-metoxy phenol to phenol under steam condition. The catalyst was investigated at 400 °C and 0.1 MPa for 2 h. This catalyst yielded a low conversion of 15% without hydrogen addition. Adding Yttria also initiated anionic vacancy and stabilized tetragonal phase. The amount of oxygen vacant was dependent on Yttrium oxide content. Even 1% of Yttria could generate O2· and increased surface area. The optimum range for oxygen donor was 1 to 10% of Yttria. In this narrow range, 5% of Yttria store more Zr3+, hence more labile oxygen were accumulated 113.


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Oxygen migration in YSZ was evaluated with DFT method. The result confirmed that the cations in sharing edge were inverse proportional to the oxygen migration barrier. The smaller cation in sharing edge, the bigger oxygen energy migration barrier, which was achieved through Zr4+-Zr4+ interaction114. Another dopant, such as silica also improved tetragonal stability. Generally, anionic deficient was formed to stabilize crystal. In case of silica-zirconia, silica stabilized zirconia crystal by limiting its growth. Finally, Del monte et al, concluded that this composite cannot transferred oxygen species115. Incorporation of Zr4+ into CeO2 lattice increased the amount of OSC (oxygen storage capacity). Doping with 50% Zr, increased OSC to 122 µmol O2/g ceria 116. Meanwhile, Meng et al reported that Ce0.5Zr0.5O2 increased the OSC to 80 µmol O2/g

117

. Ceria ability to produce

oxygen vacant was originated from the reduction reaction occur from Ce4+ to Ce3+. Introduction of Zr4+ with cubic fluorite structure and small size led to the formation of Ce3+ in CeO2 lattice. This hypothesis was strengthen with a computational method, which confirmed that oxygen vacant prefer to have a neighboring position with Zr site. The increase of the OSC is resulted from Zr role to increase oxygen mobility. Moreover, according to thermodynamic aspect, doping ceria to zirconia decreased both enthalpy and entropy redox reaction. At low Zr content with 0-5 wt.%, the percent of enthalpy decreased to around 17%112. Enhancing OSC in ceria was also achieved through doping with rare-earth elements. Comparison of ceria modified with lanthanum oxide and zirconia showed that the OSC difference was 63 µmol O2/g with the OSC of lanthanum was the largest116. In addition, preparation method also affected the OSC properties. The OSC of Ce0.5Zr0.5O2 was 250 (ultrasonic assisted membrane reactor, UAMR) and 164 µmol O2/g (coprecipitation)

117

. CeO2


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cubic form with particles size of 8 nm, which was prepared in supercritical water, yielded the OSC of 340 µmol O2/g. Moreover, synthesis of Ce0.65Zr0.35O2 under supercritical water resulted the OSC of 680 µmol O2/g80. The role of zirconia in mixed oxides for heavy oil upgrading in supercritical water was similar to the behavior of CeO246. The gaseous products of bitumen feed reacted with CeO2 in the absence of hydrogen addition were investigated with gas chromatography. The gaseous products consist of methane, alkene and others. After reacting with cubic CeO2, the product mainly consisted of CO2, alkene and small amount of hydrogen. The generation of rich CO2 was an indication that CeO2 was able to produce active oxygen and oxygen from water splitting. The water splitting phenomenon also strengthen by the transformation of alkane to alkene. This process occured through ionic mechanism, hence the formation of radical, which tended to polymerize was minimized. In case of zirconia, the OSC amount of zirconia in mixed oxide was larger than pure CeO2. Hence, the performance to crack asphaltene and reduce viscosity was increased. The propose mechanism to describe the role of CeO2 nanoparticle in a reaction with bitumen in supercritical water is presented in Fig. 6.

Please insert Fig, 6 here. Fig. 6. The propose mechanistic reaction of CeO2 nanoparticle to bitumen in supercritical water [adapted after 23].


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Table 6. Redox properties of zirconia and zirconia nanocomposites. Ceramic

Comment Ref Activated and transferred oxygen in H2O over ZrO2 surface.

ZrO2

36

Therefore, its increased the possibility to oxidize organic compound ZrO2

nano-

structure

Several nanosized (Fe2O3, Co3O4, NiO) and nanostuctured metal

51, 118

oxides (Ni, Fe) were studied to upgrade heavy oil. The result revealed that all materials can upgrade heavy oil in the absence of hydrogen.

ZrO2-Al2O3FeOx Silica

Al2O3 improved ZrO2-FeOx catalyst activity and stability in

35

steam environment. Short route in nanocrystal site enhanced the charge and mass

Supported

transport. No report related to oxygen transfer in main open literature.

Zirconia

Meanwhile, Del Monte reported no oxygen vacancies in SiO2-ZrO2.

115

Hence, the capability to upgrade heavy oil is doubted . Ce-Zr

Doping Zr into Ce created oxygen vacancies in Ce crystal. Bulk

116,

oxygen was then easily moved to other substancies at low temperature.

117

Thus, the usage for heavy oil upgrading is promising. YSZ

Ytria stabilized zirconia crystal even in high temperature. The presence of Y2O3 in ZrO2 lattice, enriched oxygen vacancies in this

113

composite. Consequently, more organic molecules are potentially oxidized.


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6. Conclusions and Outlook Some challenges are still remained in the upgrading of heavy oil. Supercritical water, a promising solvent for heavy oil upgrading due to its ability to suppress coke formation and to produce active hydrogen, is currently being explored in combination with robust catalysts. Despite high pressure system required to generate supercritical water, to meet industrial demand, the investigation on the complex diagram of water-hydrocarbon, the stable catalysts with ability to crack hydrocarbon, and the optimum condition to produce different middle distillates with an existing market penetration are also essential for efficient refineries. At last, the knowledge on the functional supercritical water devices is required. Zirconium oxides and zirconia nanocomposites for heavy oil upgrading can be fabricated with different methods. Successfull preparation of zirconia nanocomposites will open different opportunities not only for heavy oil upgrading, but also for biomass upgrading and waste valorization. Crucial aspects in each manufacturing method must be studied to produce zirconiabased catalysts in a economical way with desirable composition, uniform morphology and size. As one of the most promising stable catalysts for heavy oil upgrading, documented methods to produce zirconia are a crucial. A number of preparation techniques such as sol gel, electrospinning, spray pyrolysis and hydrothermal needs to be investigated to establish the optimum condition. Some factors such as precursor source, concentration and the type of metal oxide must be considered to obtain the desired properties. Zirconia still needs futher enhancement before it can be applied commercially as a heavy oil catalyst. Monoclinic zirconia is stable in supercritical water. However, monoclinic zirconia is a weak acid. Therefore, zirconia was modified with tungsten, which tend to form a strong acid. The best performance was achieved, when tungsten was highly dispersed in zirconia surface. In addition, zirconia needs 34 ACS Paragon Plus Environment

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Page 36 of 49

stabilizer such as rare earth to maintain its monoclinic form. Other acidic modifications were reported in zirconia-titania and zirconia modified with red mud. Meanwhile, zirconia with a high oxygen storage capacity (OSC) was fabricated rapidly by a hydrothermal method in supercritical water. The potential composite was ceria-zirconia. However, since the basic mechanism for heavy oil upgrading with zirconia is still soughted, further research to serve acid site with a high tolerancy in water and stable composite with high OSC is still needed. Meanwhile, further studies are still required to elaborate the optimum synthesis conditions and to improve catalyst stability.

Acknowledgement The authors would like to thank the funding provided 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.


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114. Plata, J. J.; Márquez, A. M.; Sanz, J. F., Understanding the Interplay of Dopants, Interfaces, and Anionic Conductivity in Doped Ceria/Zirconia Heteroepitaxial Structures. Chemistry of Materials 2014, 26, (11), 3385-3390. 115. del Monte, F.; Larsen, W.; Mackenzie, J. D., Stabilization of Tetragonal ZrO2 in ZrO2–SiO2 Binary Oxides. Journal of the American Ceramic Society 2000, 83, (3), 628-634. 116. Katta, L.; Sudarsanam, P.; Thrimurthulu, G.; Reddy, B. M., Doped nanosized ceria solid solutions for low temperature soot oxidation: Zirconium versus lanthanum promoters. Applied Catalysis B: Environmental 2010, 101, (1-2), 101-108. 117. Meng, L.; Liu, L.; Zi, X.; Dai, H.; Zhao, Z.; Wang, X.; He, H., Preparation of ceria-zirconia solid solution with enhanced oxygen storage capacity and redox performance. Frontiers of Environmental Science & Engineering in China 2010, 4, (2), 164-171. 118. Greff, J.; Babadagli, T., Use of nano-metal particles as catalyst under electromagnetic heating for in-situ heavy oil recovery. Journal of Petroleum Science and Engineering 2013, 112, 258-265.


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Fig. 1. Synthesis of zirconia and zirconia nanocomposites. ACSnanoparticle Paragon Plus Environment

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Fig. 2. Schematic illustration of spray pyrolysis technique [47].

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Fig. 3. The production of lewis and Brønsted acid site in zirconia sulfate in [23] which was drawen from [69]

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Fig. 4. Pourbaix diagram of zirconia [72]. ACS Paragon Plus Environment

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Fig. 5. The mechanism for direct decarboxylation of benzoate acid to benzaldehyde [85]. ACS Paragon Plus Environment

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Fig. 6. The propose mechanistic reaction of CeO2 nanoparticle to bitumen in supercritical water (after Dejhosseini et al [46]). ACS Paragon Plus Environment

Zirconia-Based Nanocatalysts in Heavy Oil Upgrading: A Mini Review

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