Improving the physicochemical properties of Y zeolite for catalytic cracking of heavy oil via sequential steam-alkali-acid treatments

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Improving the physicochemical properties of Y zeolite for catalytic cracking of heavy oil via sequential steam-alkali-acid treatments Erfan Aghaei a, Ramin Karimzadeh a, *, Hamid Reza Godini b, Aleksander Gurlo c, Oliver Gorke c a

Faculty of Chemical Engineering, Tarbiat Modares University, P.O.Box, 14115-114, Tehran, Iran Inorganic Membrane and Membrane Reactors, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology (TU/e), Den Dolech 2, 5612AD, Eindhoven, the Netherlands c Department of Ceramic Materials, Institute for Material Science and Technologies, Technische Universit€ at Berlin, Hardenbergstrasse 40, D-10623, Berlin, Germany b

A R T I C L E I N F O

A B S T R A C T

Keywords: Steam treatment Alkali treatment Acid washing Alkali agent Sequential treatment Heavy oil cracking

In this study, modified Y zeolite with high catalytic activity and long lifetime was successfully prepared using sequential steam-alkali-acid treatments. Different alkali agents (NaOH, Na2CO3 and CaCO3) with various con­ centrations were used in order to improve the catalytic performance of the synthesized Y zeolite in heavy oil conversion process. The physicochemical properties of the prepared samples were investigated using XRD, FESEM, EDX, solid-state29Si and 27Al NMR, BET, XRF, ICP, FTIR, and NH3-TPD techniques. In addition, the properties of the deposited coke on the used catalysts were studied by FTIR and TGA-DTA techniques. Al extraction via steam treatment created Si-rich regions in the Y zeolite samples. Then, the Si elements were easily extracted from these Si-rich regions by using alkali treatment. In addition, acid washing with weak EDTA so­ lution resulted in removal of extracted elements from the zeolite structure and pore opening after steam and alkali treatments. Combination of these treatments stimulated mesopores formation and improvement in textural properties for the Y zeolite samples. Structural and textural properties of Y zeolite were found to be preserved after sequential treatments. Catalytic performance of the prepared catalysts was investigated for the heavy oil catalytic cracking process in a fixed bed reactor. Regardless of the type of alkali agent used for sequential treatments, the heavy oil conversion and gasoline selectivity were strongly improved for all the modified cata­ lysts in reference to the parent catalyst. The catalytic activity tests showed that sample treated with 1 M CaCO3 solution could secure a high heavy oil conversion (70%) and high selectivity towards gasoline and kerosene (45%). Furthermore, modification with CaCO3 inhibits coke formation significantly and enhanced the catalytic lifetime.

1. Introduction

structures, they are widely used as catalyst, membrane, ion exchange materials, and adsorbent [5–7]. Zeolites like ZSM-5, beta and Y have been applied as catalyst for hydrocarbons cracking [8–11]. Zeolite Y is a promising catalyst for catalytic cracking of heavy hydrocarbons and gasoline production [12]. For instance, it has been used as a primary active component of fluid catalytic cracking (FCC) since it was first commercialized in 50 years ago [13,14]. Also, it is widely used as a catalyst for hydrocracking and alkylation processes in refineries and petrochemical industries [15]. Zeolite Y has faujasite (FAU) structure with open three-dimensional channels constructed from tetrahedral AlO4 and SiO4 blocks [16]. Distinguished properties of Y zeolite including high surface area, high thermal/hydrothermal stability, large pore volume and tunable acidity lead to increasing the importance of

Nowadays, depletion of light oil sources with increase in the demand for light fuels, utilization of alternative sources like heavy and inferior oil for gasoline and diesel production has become inevitable [1]. For fuel production, catalytic cracking of heavy oil is more promising than the thermal cracking technology as it provide higher liquid yield, lower gas formation, lower reaction temperature and higher selectivity for light fuels. Zeolite as a solid acid catalyst is the best catalytic material choice for heavy oil cracking providing a high selectivity to gasoline and diesel [2,3]. Zeolites are crystalline aluminosilicate materials, which are composed of AlO4 and SiO4 tetrahedral linked with oxygen atoms [4]. Due to the tunable potentials of micropores and shape selectivity in their

* Corresponding author. E-mail address: [email protected] (R. Karimzadeh). https://doi.org/10.1016/j.micromeso.2019.109854 Received 4 August 2019; Received in revised form 1 October 2019; Accepted 28 October 2019 Available online 31 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.

Please cite this article as: Erfan Aghaei, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109854

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Fig. 1. XRD patterns of prepared samples: (a) PY, (b) SAEY (1, NaOH), (c) SAEY (0.5, NaOH), (d) SAEY (1, Na2CO3), (e) SAEY (0.5, Na2CO3), (f) SAEY (1, CaCO3), (g) SAEY (0.5, CaCO3) and (h) SEY.

this zeolite as a catalyst for refineries. Although, the microporous Y zeolite is an excellent catalyst for cracking processes, but its application for heavy hydrocarbons cracking has been limited mainly because of its structure restrict mass transfer of heavier reactants and products. Catalytic cracking of long-chain hy­ drocarbons requires a catalyst with large pore and accessible active sites. Therefore, different methods including synthesis of zeolite with meso­ pores [17], synthesis of nano-sized zeolites [18,19] and development of

mesopores in zeolites with intrinsic micropores have been suggested [20–22]. Introduction of mesopores in microporous Y zeolite by desili­ cation and dealumination is a simple and cheaper technique than other available methods, which can be implemented in industrial scale. Different methods including desilication and delamination with acid [23–25], alkali [25,26] and steam [27,28] have been reported in liter­ ature for creating the mesopores in microporous Y zeolite. During steam and acid treatment via hydrolysis of Al–O bonds, Al elements are 2

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Table 1 Alkali treatment conditions of the samples and relative crystallinity, crystallite sized and SiO2/Al2O3 ratio of the prepared catalysts. Sample

Alkali Agent

Conc. (Molar)

Relative Crystallinitya (%)

SiO2/Al2O3b

SiO2/Al2O3c

SiO2/Al2O3d

PY SAEY (1, NaOH) SAEY (0.5, NaOH) SAEY (1, Na2CO3) SAEY (0.5, Na2CO3) SAEY (1, CaCO3) SAEY (0.5, CaCO3) SEY

– NaOH NaOH Na2CO3 Na2CO3 CaCO3 CaCO3 –

– 1 0.5 1 0.5 1 0.5 –

100 34 51 83 94 35 77 96

5.51 4.63 6.00 7.28 7.53 6.25 5.71 7.56

5.21 4.69 5.94 6.92 7.30 5.9 5.72 7.01

4.86 4.69 5.71 5.26 5.72 4.21 4.64 7.17

a b c d

ASTM D5758-01 was applied as XRD relative crystallinity. XRF. EDX. ICP.

extracted from the structure and consequently some mesopores would be created. Dealumination enhances by increasing the steam treatment temperature, acid concentration and treatment time [27]. On the other hand, Al in the structure are responsible for acidity and therefore, changing the Al content and Al distribution change the acidic properties of Y zeolite [29]. By using alkali treatment, Si elements are removed from the structure and some mesopores would be created. In contrary, Y zeolite due to its low Si/Al ratio and high Al content has high resistance for alkaline leaching, because high Al concentration preserves the structure against Si removal during alkaline treatment [30,31]. There­ fore, Al extraction is necessary before silica extraction for NaY zeolite. Then, the created defects in the aluminium removal step makes Si extraction more effective [25,32]. Shen et al. [33] prepared USY zeolite using hydrothermal and sequential alkaline-hydrothermal treatments. Both of these methods have resulted in the same framework Si/Al ratio, but using later one (sequential alkaline-hydrothermal treatment method) has caused an increase in mesopores volume. Le Van Mao et al. [34] desilicated ZSM-5, NaY and NaX zeolites using Na2CO3 solution. After desilication, due to the Si removal from structure, the Si/Al ratio has shown a decrease and Si(4Al) coordination has increased for various zeolite samples. This method was not efficient for NaA zeolite because of its low Si/Al ratio. Qin et al. [26] created some mesopores by using sequential desilication and dealumination. Their results showed that by using AHSF (ammo­ nium hexafluorosilicate), uniform dealumination occurred. Fathi et al. [35] modified the performance of ZSM-5 with NaOH, Na2CO3 and CaCO3 in methanol to gasoline process. They reported that alkali treatment reduced the Si/Al ratio and the modified ZSM-5 with Na2CO3 showed the best catalytic lifetime and higher gasoline selectivity. ZSM-5 modification with CaCO3 and Na2CO3 solutions and using the resulted material as a catalyst for methanol to gasoline process has also been studied [36]. Treatment of ZSM-5 with 0.1 M solution of CaCO3 and Na2CO3 has improved the selectivity towards producing hydrocarbons in the gasoline boiling point range. MFI and FAU have different crys­ talline structures, pore diameters and Si/Al ratios, which can contribute to different characteristics while being subjected to alkali treatment process. MFI structure with pentasil framework has medium and high Si/Al ratios, but in FAU structure, Si/Al ratio is low and there are Si atoms in different α-cage, β-cage and D6R sites. Hence, single alkali treatment couldn’t produce enough porosity in the FAU structure as reported elsewhere [9]. However, textural properties could be enhanced by alkali treatment of ZSM-5 with similar conditions (or even lower alkalinity) [37,38]. Therefore, it can be concluded that it is not possible to extend the results of alkali treatment of MFI structure to FAU struc­ ture or other structures with different topologies and Si/Al ratios. Having observed the potential of Na2CO3 and CaCO3 alkali agents in improving the catalytic performance of ZSM-5, the need for a systematic analysis of the contributions of different alkali agents with different alkalinities in modification of Y zeolite is highlighted. Modification of Y zeolite by desilication and dealumination for

catalytic cracking processes has been extensively investigated by various researchers from different points of view. Nevertheless, modification of microporous Y zeolite with preserved structure and high catalytic per­ formance for heavy oil cracking is still a challenge. Having reviewed the published reports in this field, it can be concluded that conducting a comprehensive study on the treatment of Y zeolite by different alkali agents with various concentrations and testing the resulted modified Y zeolite as a catalyst for heavy oil cracking is of significance importance. Herein, a strategy for improving the physicochemical properties of the Y zeolite using sequential steam-alkali-acid treatments is reported. In this research, effect of using various concentrations and types of alkali agents including NaOH, Na2CO3 and CaCO3 on the physicochemical properties and catalytic performance of the prepared catalysts have been investigated. 2. Materials and methods 2.1. Materials Commercial NaY zeolite with SiO2/Al2O3 ¼ 5.2 was purchased from Naike company. The used alkali agents NaOH, Na2CO3 and CaCO3 were purchased from Merck. Ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA, Sigma Aldrich) was used for acid washing. NH4NO3 (Merck) was used for zeolite ion exchange. Deionized water was used as a solvent for solution preparation. 2.2. Zeolite modification procedures Work-flow diagram of the sequence of the preparation steps of zeo­ lites is shown in Fig. 1S. In order to study the impacts of the type and concentration of alkali agent, a set of NaY samples were modified by conducting the sequential steam-alkali-acid treatments as following: NaY zeolite was ion exchanged with 1 M NH4NO3 solution at 80 � C for 3 h and then dried at 100 � C overnight to get NH4Y (Fig. 1S, step (a)). In order to ensure that Naþ ions were replaced with NHþ 4 , this step was repeated three times. Then, a part of the NH4Y was calcined at 550 � C for 4 h to get proton form of Y. The parent proton form of Y zeolite (PY) was used as a catalyst without any further treatment. For the other samples, the steam treatment process was conducted over the produced NH4Y at 600 � C for 2 h using saturated steam (step (b)). Nitrogen stream was used as a carrier in order to convey the water vapour. Then, the desilication process was conducted (step (c)) using NaOH, Na2CO3 and CaCO3 as alkali agents with various concentrations (0.5 and 1 M). Alkali treatment was applied at 70 � C for 1 h under stirring. The detailed specifications of the samples subjected to alkali treatment are shown in Table 1. Then the obtained solution was filtered and washed with warm water until reaching the pH ¼ 7–8 and dried overnight. After alkali treatment, the prepared samples were treated with 0.11 M Na2EDTA with powder to solution ratio of 1–10 at 80–85 � C for 3 h under stirring conditions (step (d)). Thereafter, the obtained solution was filtered and the solid 3

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Fig. 2. FESEM images of the prepared samples with different conditions: (a) PY, (b) SAEY (1, NaOH), (c) SAEY (0.5, NaOH), (d) SAEY (1, Na2CO3), (e) SAEY (0.5, Na2CO3), (f) SAEY (1, CaCO3), (g) SAEY (0.5, CaCO3) and (h) SEY.

product was dried in an oven overnight. For comparison, one sample was prepared by steam and acid treatment without any alkali treatment (SEY). In the next step (step (e)), the prepared zeolites converted to NH4Y by three successive ion exchange at 80 � C for 3 h. The obtained solution was washed with deionized water until reaching pH of 7–8 and then dried overnight. Afterward, the obtained zeolites were shaped to cylindrical particles with diameter of 2–3 mm and calcined under air flow at 550 � C for 4 h. The prepared samples were labeled according to the preparation procedure. For example, the sample with Steam treat­ ment and Alkali treatment using 0.5 M NaOH and EDTA washing was labeled as SAEY (0.5, NaOH) while the Steam treated and acid washed sample (without alkali treatment) was labeled as SEY.

2.3. Applied characterization techniques X-ray diffraction patterns of the prepared samples were obtained using a PANalytical X’Pert powder diffractometer with CoKα radiation in the 2θ range from 6.5 to 50� . Relative crystallinity was calculated according to the surface area of the peaks at 2θ ¼ 7.2, 11.8, 13.8, 18.18, 21.7, 23.7, 27.5.31.5 and 36.6� and NaY considered to be 100% crys­ talline. The chemical composition of the prepared samples was measured using an energy dispersive spectrometer (EDX, Zeiss Gemini Leo 1530), X-ray fluorescence (an autosampler PW 4400 sequential wavelength X-ray spectrometer, Panalytical, Netherlands) and induc­ tively coupled plasma emission spectrometer (ICP, Ultima Expert, 4

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2.4. Experimental setup used for catalytic cracking

Table 2 Properties of feed (iso-diesel). Properties

Amount

Viscosity (cSt) Density (g.cm 3) Refractive Index Sulphur content (ppm)

6.73 0.8398 1.465 80

Distillation Initial boiling point (� C) 10% 20% 50% 90% Final boiling point

304 389.5 403.3 428.1 467.9 519.3

Schematic diagram of the experimental setup used for analysing the catalytic performance of the samples is shown in Fig. 2S. Catalytic performance of the parent sample and the prepared/modified catalysts were evaluated in a cylindrical stainless steel fixed bed reactor at 550 � C and atmospheric pressure. Heavy oil from Shazand Oil Refinery (Iran), with the properties reported in Table 2 was used as a feed. 1 g of each shaped catalyst sample was placed at the center of the reactor close to the thermocouple. The quartz chips were added at the beginning of the reactor prior to the catalyst bed in order to increase the feed temperature, acquire the required reaction temperature, and uni­ form distribution of the feed stream. A K-type thermocouple was used for temperature measurement. Nitrogen with a flow rate of 50 cc/min was used as a carrier gas. Prior to the reaction, the catalyst was activated at 550 � C for 1 h under 50 cc/min N2 flow. A preheater with temperature of 200 � C was used to preheat the feed. Heavy oil with a flow rate of 0.5 mL min 1 was introduced to the preheater by the syringe pump and then, the heated feed with the aid of carrier gas was introduced into the reactor at the reaction temperature (550 � C). The weight hourly space velocity (WHSV) was 25 gfeed.h 1.g 1cat. After the reaction, liquid and gaseous products were separated in a condenser. Gaseous components were analyzed using an on-line gas chromatography (Agilent, 7890 A) with an FID detector. The liquid product was collected, weighted and analyzed using a SIMDISS analyzer equipped with FID and a capillary column (10 m � 530 μm) according to ASTM method D2887. The liquid yield was calculated by dividing the obtained liquid product mass for 1 h to the mass of injected feed into the reactor at the same time (equation (1)). Coke yield was calculated in the same way and the rest was considered to be the gas yield (equations (2) and (3)).

Horiba Scientific) analysis. Si29 and Al27 MAS NMR of the prepared samples was conducted by using Bruker Avance 400 MHz Solid State NMR in order to identify the different coordination of Si and Al in zeolite structure after treatments. The nitrogen adsorption-desorption test was conducted with BELsorp - mini II (Japan) at 77 K. The total specific surface area was determined with BET method. The micropore volume and external surface area were calculated with t-plot method. The total pore volume was measured from the amount of N2 adsorbed at p/ p0 ¼ 0.99. The pore size distribution was determined with BJH method. The morphology and macrostructure of the parent Y and the modified samples were investigated using Field Emission Scanning Electron Mi­ croscopy (ZEISS MERLIN FE-SEM). The functional groups of the pre­ pared catalysts and the spent catalysts were analyzed in an EQUINOX 55 (Bruker, Germany) operating in attenuated total reflection (ATR) mode with a resolution of 1 cm 1. Characterization of the acid sites of the samples was performed using an AutoChem II 2920 Analyzer (Micro­ meritics, USA). Before analysis, 0.1 g of each calcined sample was pre­ heated at 550 � C for 60 min under a 50 mL min 1 helium gas flow. Ammonia adsorption is made of a mixture of 5% (Molar basis) of ammonia in helium at a total flow rate of 50 mL min 1 at 100 � C. After adsorption of ammonia, the samples were kept under a helium gas flow at 100 � C to remove physically adsorbed gases. Finally, the helium flow (50 mL min 1) is passed through the sample with increasing tempera­ ture up to 800 � C at a rate of 10 K min 1. In order to measure the amount of the coke deposited over catalysts, thermogravimetric analysis of the prepared samples was performed in a STA 409 (Netzsch, Germany) analyzer from room temperature to 800 � C.

Fig. 3.

29

Si (a) and

27

YLiquid¼ (MLiquid/MFeed)*100

(1)

YCoke¼ (MCoke/MFeed)*100

(2)

YGas ¼ 100 – YLiquid – YCoke

(3)

Where MFeed, MLiquid and MCoke are mass of injected feed, obtained liquid and coke, respectively. In addition, YLiquid, Ygas and YCoke are calculated yield of obtained liquid, gas and coke products, respectively. Heavy oil conversion was defined as part of the feed that was converted to hy­ drocarbons with the boiling temperature range lower than 345 � C. The obtained liquid products were divided into five cuts including gasoline (30–205 � C), kerosene (205–275 � C), light gas oil (275–345 � C), heavy

Al (b) NMR spectra of PY and treated samples. 5

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Fig. 4. N2 adsorption-desorption hysteresis (a) and pore size distribution (b) for the prepared catalysts.

gas oil (345–455 � C) and vacuum gas oil (455–545 � C) based on their boiling temperature ranges. In addition, other properties of the obtained liquid including density (10 mL pycnometer (Isolab, Germany)), vis­ cosity (Normalab, France) and refractive index (Atago, Japan) were measured. Riazi equation was performed in order to estimate the mo­ lecular weight of the liquid product [39].

The PY and other treated samples showed standard XRD patterns of the faujasite structure (JCPDS number 01-077-1551). All the prepared samples revealed the FAU type structure without any impurity except the samples modified with CaCO3. For these samples, some impurity related to the calcite (CaCO3, JCPDS number: 00-001-0837) was observed that is attributed to the remained alkaline agent. After steam and alkali treatment, relative crystallinity of all prepared samples reduced. This reduction was much more significant for the treated samples with high concentration of NaOH and CaCO3. The relative crystallinity of the prepared samples is shown in Table 1. The relative crystallinity of the PY and SEY was 100 and 96%, respectively. After alkali treatment with 1 and 0.5 M NaOH, relative crystallinity reduced to 34 and 51%, respectively. This indicates the high desilication and destruction of the structure in presence of NaOH. In case of Na2CO3

3. Results and discussions 3.1. Physicochemical characterizations 3.1.1. XRD analysis The XRD patterns of the parent and prepared/modified samples are shown in Fig. 1. 6

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Table 3 Structural properties of the parent and the modified samples. Sample name

PY SAEY (1, NaOH) SAEY (0.5, NaOH) SAEY (1, Na2CO3) SAEY (0.5, Na2CO3) SAEY (1, CaCO3) SAEY (0.5, CaCO3) SEY a b c d e f

Characteristic SBET (m2 g 1)a

SMicro (m2 g 1)b

SMeso (m2 g 1)c

VTotal (cm3g

704.5 340.5 642.1 760.4 766.4

643.1 238.4 574.1 658.7 662.7

61.4 102.1 68 101.7 103.7

0.3115 0.2351 0.3300 0.4154 0.4203

475.1 685.7 785.2

432.7 626.8 672.1

42.4 58.9 113.1

0.2522 0.3530 0.4258

VMeso (cm3 g 1)f

Average pore diameter (nm)

Acidity (mmol/ g)

0.2449 0.0949 0.2221 0.2517 0.2535

0.0666 0.1402 0.1079 0.1637 0.1668

1.7686 2.762 2.0558 2.1851 2.1936

2.54 1.65 2.52 2.20 2.49

0.1642 0.2372 0.2571

0.088 0.1158 0.1687

2.1239 2.0591 2.1692

2.06 2.38 2.47

VMicro (cm3g

1 d

)

1 e

)

The BET method using adsorption data were applied for determination of total surface areas in P/P0 ranging from 0.005 to 0.1. Micropore surface area evaluated by t-plot method. Mesopore surface area calculated using SBET–SMicro. Total pore volumes were estimated from the adsorbed amount at P/P0 ¼ 0.99. Micropore volume calculated by t-plot method. Mesopore volume calculated using VTotal–VMicro.

treatment, this reduction was observed to be lower mainly because of the relatively lower alkalinity of Na2CO3 in comparison to NaOH. For sample desilicated with 1 M CaCO3, relative crystallinity was also low. Because of the low alkalinity of CaCO3 solution, the observed reduction of intensity in case of CaCO3 treated sample could be attributed to the calcite particles coated on the Y crystals which usually causes the reduction of intensity. For the preparation of this sample, high content of CaCO3 was used and after ion exchange, some CaCO3 particles remained as observed through the XRD analysis results.

the other samples which is due to the high concentration of the NaOH used for desilication of this sample. In addition, surface of the treated samples becomes significantly rough and some hollows and defects were observed. For the sample desilicated with 1 M CaCO3, some agglomer­ ated small particles were observed (different magnification images are shown in Fig. 3S) that are related to the calcite particles. These results are in line with the XRD results implying the intensity reduction and impurity production for this sample. 3.1.4. Solid-state 29Si and 27Al NMR analysis Solid-State 29Si and 27Al NMR spectra of the PY and the treated samples are illustrated in Fig. 3. The 29Si NMR spectra showed five peaks between 110 and 85 ppm which are attributed to Si(nAl), n ¼ 0–4. The PY showed highintensity peaks for Si(4Al), Si(3Al) and Si(2Al) because of low Si/Al ratio (high Al content) that is in line with the results of elemental analysis. Also, for the dealuminated sample (SEY), Si(0Al) and Si(1Al) with high intensity appeared which confirm that the dealumination occurs during the steam treatment and acid washing. For the SEY sample, a shoulder peak at 115 ppm appeared that could be attributed to the amorphous silica or aluminosilicate species formed in the silica-rich region during steam treatment and acid washing [4,40]. However, for the alkali treated samples, due to desilication and silica dissolution in silica-rich region, the intensity of this peak was low. For the alkali treated sam­ ples with high NaOH and CaCO3 concentration, peaks related to aluminum-rich structure unit were sharp, which is similar to the re­ ported results in the literature for desilicated Y zeolite [33]. A peak at 62 ppm was observed which is assigned to four-coordinate framework aluminium. After desilication and dealumination, this peak becomes broader that indicates the appearance of some disorder in the tetra-coordinated Al after the creation of mesopores through sequential treatments [41]. This is a known phenomenon commonly observed for faujasite and Pentasil structure after steaming and is consistent with the results reported by García-Martínez et al. [42]. In addition, another peak at 0 ppm appeared that is related to octahedrally coordinated aluminum. For the treated samples, especially with NaOH and CaCO3, the tetra­ hedrally coordinated peak was broad which indicates a wide range of T-O-T angle. As can be seen here, extra-framework aluminum species located at 0 ppm were observed for the both treated and parent samples except for the sample treated with CaCO3. This indicates that the amount of extra-framework aluminum was high or concentration of the used acid for washing was low. Due to the strong alkalinity of the NaOH solution, during desilication of Si-rich species, extra-framework Al removed from the structure and the broad peak with low intensity was appeared at 0 ppm.

3.1.2. Chemical composition analysis The SiO2/Al2O3 ratio was determined by three different methods including, XRF, EDX and ICP (Table 1). Nearly all of the applied calcu­ lation methods and the used agents resulted the same trend with regard to the SiO2/Al2O3 ratio. After steam treatment and EDTA washing (SEY sample), the SiO2/Al2O3 ratio increased due to the selective elimination of Al. For all treated samples, regardless of the type of used alkali agent, SiO2/Al2O3 ratio was lower than the ones observed for SEY sample which demonstrates that Si extraction from the structure using these alkali agents was effective. XRF results showed that, only the SiO2/Al2O3 ratio for the SAEY (1, NaOH) is lower than for the PY sample that can be attributed to the high desilication for this sample due to high basicity of NaOH. Among alkali modified samples, the samples treated with Na2CO3 showed higher Si/Al ratio that indicates a low Si removal. ICP analysis showed lower values for Si/Al ratio than the ones measured by XRF and EDX. This is due to the gradient of Si/Al ratio from surface to the core of the particles which can be attributed to the accumulation of Si on the surface and the outer surface of particles. Final acid washing removed Al from the outer layers of the particles and caused increase in Si/Al ratio in XRF and EDX analysis, which are not bulk analysis techniques. 3.1.3. FESEM analysis FESEM images of the parent and dealuminated-desilicated samples are shown in Fig. 2. The PY sample had octahedral morphology with well-defined parti­ cles. After steam treatment and washing with EDTA, this morphology remained the same and particles with sharp edges were observed (Fig. 2h). For the desilicated samples, octahedral morphology of Y zeolite was observed but some irregular shape particles were also detected. These irregular shape particles were formed because of the extracted elements during steam-alkali-acid treatments. These results are in good agreement with XRD analysis where intensity reduction was observed for the desilicated samples. For the SAEY (1, NaOH) sample, the number of the produced irregular shape particles was higher than 7

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788 791 782 798 798 804 812 809 812 810 811 807 812 804 813 812 721 733 725 732 729 731 734 756 745 765 737 734 734 735 743 752

– – – – – – – – – – 874 874 – – – –

994–1143 1010–1140 1009–1143 1026–1148 1018–1148 1026–1148 1022–1166 1019–1168 1022–1166 1016–1169 1023–1164 1024–1140 1020–1162 1021–1140 1022–1167 1022–1170

– – – – – – – – – – 1445 1443 – – – –

– 1587 – 1586 – 1589 – 1588 – 1586 – 1590 – 1589 – 1588

1632 1640 1638 1640 1631 1640 1631 1642 1633 1642 1634 1633 1636 1645 1636 1643

3364 3405 3369 3451 3372 3417 3374 3523 3372 3591 3403 3497 3378 3590 3378 3612

3.1.5. BET analysis N2 adsorption-desorption provides valuable information about the textural properties of the prepared catalysts. Inner surface area repre­ sents the surface area associated to the micropores and cages, while external surface area reflects the mesopores surface area and the outer surface area of the particles and crystals. Providing a sufficient external surface area and mesopore volume to ensure an efficient processing of large hydrocarbon molecules of heavy oil is very important. Since these large hydrocarbons cannot enter into the micropores, access to the Brønsted acidic sites located in the micropores is limited. Nevertheless, these large molecules can be cracked over the acidic sites located at the external surface area of the crystals and inside the large pores of the particles. In comparison to the micropores, blockage of these large pores due to coke formation takes a longer time. The results of N2 adsorption-desorption, pore size distribution and textural properties of the PY and dealuminated-desilicated samples are shown in Fig. 4 and Table 3. The PY sample with high N2 adsorption in low relative pressure (P/ P0 < 0.1) showed type I isotherm that is representative of the micropo­ rous materials. For all of the desilicated-dealuminated samples, a hys­ teresis loop at higher relative pressures (more than 0.4) was observed that is related to the type IV isotherm representing mesoporous mate­ rials. After alkali treatment with 1 M NaOH, surface area and the pore volume strongly reduced to 340.5 m2 g 1 and 0.2351 cm3 g 1, respec­ tively. While in case of 0.5 M NaOH treated sample, the reduction was moderate. These results are in line with the relative crystallinity reduction and structure destruction observed in the XRD and FESEM analyses. For the samples desilicated with Na2CO3, surface area and pore volume are high and comparable with SEY. This is because of the mild alkali treatment conditions in the presence of Na2CO3 that caused low destruction of the structure. In addition, EDTA washing removed amorphous materials and hence reduced pore blockage. For the SAEY (1, CaCO3) sample, according to the XRD and FESEM analyses some ag­ gregation of calcite nanoparticles was detected, surface area and pore volume reduced because of pore blockage with these particles. For the SAEY (0.5, CaCO3) treated under milder conditions, a major part of calcite particles removed during acid washing and ion exchange steps and consequently high surface area and pore volume observed. After dealumination-desilication, average pore diameter and meso pore vol­ ume increased for all the modified samples. That indicates the efficiency of the sequential treatments for increasing the pore diameter and mes­ opores formation. The BJH pore size distribution of the prepared sam­ ples are shown in Fig. 4b. After dealumination-desilication some pores with diameters around 2–10 nm can be seen. These pores are very important for the performance of heavy oil cracking catalysts because they can facilitate access of large molecules of feed to the acid sites of catalyst. 3.1.6. FTIR analysis The results of the FTIR analysis on the parent and the treated catalyst samples are shown in Table 4. All the samples showed identical spectra, which is typically repre­ senting the faujasite structure of Y zeolite. The band around 3375 cm 1 is attributed to the hydroxyl groups of Y zeolite. The band at 448 cm 1 is assigned to T-O-T bending vibration. The band at 570 cm 1 is ascribed to double six-membered rings (D6R) in the micropores of zeolite Y. The intensity of this peak was low (see Fig. 4S) for the treated samples with NaOH that indicates change in structure during NaOH treatment. The band at 721 cm 1 is due to the symmetric stretching vibration of T-O-T. The peak at 788 cm 1 is assigned to internal tetrahedral symmetrical stretching and the peaks located at 994 and 1143 cm 1 are attributed to the asymmetric stretching vibration of framework T-O-T (T ¼ Si and Al). Due to the higher frequency of the Si–O bond than the Al–O bond (because of higher electronegativity of Si) [43], vibration frequency of these bonds shifted to higher wavenumbers for the treated samples showing higher Si/Al ratios than PY. The band at 1632 cm 1 is

SAEY (1, NaOH) SAEY (0.5, NaOH) SAEY (1, Na2CO3) SAEY (0.5, Na2CO3) SAEY (1, CaCO3) SAEY (0.5, CaCO3) SEY

F: Fresh catalyst. S: Spent catalyst.

448 436 443 444 446 450 447 450 446 452 450 447 448 450 447 450 F S F S F S F S F S F S F S F S PY

570 575 572 562 582 591 582 596 586 595 580 580 580 582 585 600

– – – – – – – – – – 713 712 – – – –

Internal tetrahedral symmetrical stretching of T-O-T Symmetric stretching vibration of T-OT CO3 out of plane deformation Double sixmembered rings T-O-T bending vibration Sample

Functional group

Table 4 Functional groups for the fresh and spent catalysts.

Asymmetric CO stretching in CaCO3

Asymmetric stretching vibration of framework T-O-T

OCO bending inplane deformation

C¼C bond stretching vibration

Hydroxyl group of physically adsorbed er

Hydroxyl groups

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Fig. 5. NH3-TPD analysis of the prepared samples.

represent the weak acid sites and those at 240–500 � C correspond to the medium and strong acid sites. The PY sample had high amount of acid sites. After modification with alkali agents, different behavior was observed for the modified samples. For the samples treated with high alkali concentration, a decrease in strong acid sites was observed while the opposite trend was observed for the samples treated with low alkali concentration and SEY. By using high concentration of alkali agents, SiO2/Al2O3 ratio decreased and Si elements with higher Al neighbor­ hood (Si(4Al), Si(3Al) and Si(2Al)) increased, as discussed in elemental analysis and NMR results. As the Brønsted acid sites with low Al neighborhood have high acidity strength [45,46], hence lower strong acid site concentration of these samples is justified. The SEY and samples treated under mild alkali conditions, due to the higher Si/Al ratio and lower Al neighborhood, had higher strong acidic sites. The same ascending trend for the weak and strong acid sites was observed for the NaY sample modified with NaOH [21]. These findings indicated that NH3-TPD results for acidity are in line with the results of elemental and NMR analyses. Strong acid sites are responsible for the coke formation and catalyst deactivation. Therefore, neutralization of the strong acid sites by using higher alkali concentration treatment can increase the catalyst lifetime. Fig. 6. Obtained liquid hydrocarbons yield for the different catalysts with time on stream.

3.2. Catalytic performance study toward heavy oil conversion

corresponded to the hydroxyl group of physically adsorbed water. The bands at 2330 and 2359 cm 1 are related to CO and CO2 respectively, adsorbed from the surrounding atmosphere. For the SAEY (1, CaCO3) sample, the bands at 713, 874 and 1445 cm 1 are assigned to out of plane deformation of CO3, asymmetric CO stretching in CaCO3 and in-plane bending deformation of OCO respectively [44]. In addition, the band at 873 cm 1 could be related to Si–O–Ca2þ. Presence of these bands in this sample is in good agreement with the XRD results.

3.2.1. Liquid yield with time on stream (TOS) Catalytic performance of the prepared samples was evaluated in heavy oil cracking process. Catalytic cracking was performed in a fixed bed reactor operating at 550 � C and atmospheric pressure. The obtained liquid yield for the prepared catalysts as a function of TOS are plotted in Fig. 6. The liquid yield of PY was higher than the other modified samples throughout the reaction. Higher liquid yield can indicate a lower activity of catalyst or lower cracking and gas formation. The SEY sample had lower liquid yield at the beginning of reaction and it increased quickly with TOS due to the coke formation. The samples treated with CaCO3 and Na2CO3 showed lower liquid yield that can illustrate higher cata­ lytic activity of these modified samples.

3.1.7. NH3-TPD analysis Results of ammonia desorption for prepared catalysts are illustrated in Fig. 5 and Table 3. The observed acid sites in the range of 90–240 � C are considered to 9

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Fig. 7. Heavy oil conversion (a) and different hydrocarbons cut selectivity (b) for the prepared catalysts (TOS ¼ 120 min and T ¼ 550 � C).

3.2.2. Effect of sequential treatment conditions on heavy oil conversion and products selectivity Heavy oil conversion and product selectivity of the PY and the pre­ pared catalysts with different treatment conditions are displayed in Fig. 7. Heavy oil conversion for all the modified catalysts was higher than for the PY that could be attributed to the increase of their mesopore volume and pore diameter after different treatments. For the SEY sample and the treated samples with NaOH and Na2CO3, the external surface area, mesopore volume and pore diameter were high that caused the better catalytic performance. The catalysts treated with CaCO3 showed an excellent improvement on the heavy oil conversion and the resulted liquid products properties. Products yield and physical properties of the obtained liquid over different catalysts are shown in Table 5.

Due to the higher activity of the modified catalysts, high amount of gas products was formed. A straightforward correlation was not observed between the acidity and the potential of gas formation. The prepared samples with lower acidity showed high gas formation. It seems that final catalytic performance is combination of interaction of different parameters. As can be seen in Table 5, refractive index, density, viscosity and molecular weight of the modified samples, especially the samples modified with CaCO3, were lower than ones measured for PY. They had higher gasoline and kerosene yield and lower heavy liquid products. This was due to the improvement of the textural and acidic properties due to CaCO3 modification. More than 70% of the heavy feed oil converted over the SAEY (1, CaCO3) showing 45% selectivity towards gasoline and kerosene. That illustrates the excellent catalytic perfor­ mance of this sample for heavy oil conversion. 10

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Table 5 Products yields of catalytic test and refractive index, density, viscosity and molecular weight of the obtained liquid product for the different catalysts. Sample Product Yield, wt% Gas Liquid Coke Total Liquid Properties Refractive Index Density (g.cm 3) Viscosity (cSt) Molecular weight (g. gmol)

2h 4h 2h 4h 2h 4h

PY

SAEY (1, NaOH)

SAEY (0.5, NaOH)

SAEY (1, Na2CO3)

SAEY (0.5, Na2CO3)

SAEY (1, CaCO3)

SAEY (0.5, CaCO3)

SEY

7.391 92.363 0.246 100

11.848 87.997 0.155 100

8.887 90.895 0.218 100

11.034 88.632 0.333 100

14.066 85.576 0.358 100

19.413 80.416 0.171 100

15.166 84.544 0.289 100

14.361 85.338 0.302 100

1.461 0.8234 0.8303 2.7 3.6 190.1 212.5

1.460 0.8191 0.8223 2.55 2.93 185.6 196.7

1.459 0.8200 0.8236 2.61 2.90 187.6 195.8

1.462 0.8217 0.8273 2.65 2.94 188.5 196.8

1.460 0.8197 0.8226 2.34 2.68 178.5 194.8

1.455 0.8051 0.8123 1.45 1.74 140.7 154.7

1.458 0.8140 0.8192 1.99 2.47 165.4 182.9

1.458 0.8225 0.8300 2.49 3.02 183.5 198.8

Gas products selectivity over different catalysts is illustrated in Fig. 8. For all the treated catalysts and the PY sample, propylene and C4þ hydrocarbons were the main gaseous products. It is worth noting that propylene selectivity over PY was observed to be reduced rapidly during TOS that can indicate a fast catalytic deactivation of this catalyst. Nevertheless, propylene selectivity over the modified catalysts was constant or slightly reduced with TOS. In addition, propylene/ethylene ratio and C4þ hydrocarbons selectivity were observed to be higher for the catalysts modified with CaCO3.

treated with Na2CO3 and is lower for the SEY, PY, SAE (0.5, CaCO3) and samples treated with NaOH(see Fig. 5S). The SAE (1, CaCO3) had the least intensity for this peak that can represent lower aromatics formation and coke deposition. These findings are in good agreement with the amount of weight loss in the TGA analysis. In addition, the presence of band at 3000-3400 cm 1 is related to C–H bond in aromatic rings, which wasn’t observed for the spent catalysts. It can be concluded that there is a high unsaturation degree of the aromatic rings formed as coke over catalysts [53]. On the basis of the peak observed at 1581 cm 1 and absence of C–H bond in aromatics, it can be concluded that deposited coke over catalysts is hard coke with aromatic nature. These results are in line with TGA analysis. Intensity of the OH groups in the region 3200-3600 cm 1 reduced after the reaction that indicates condensation of OH groups with coke during the catalytic test.

3.2.3. Spent catalysts characterization TGA-DTA analysis of the coked catalysts was carried out and results are shown in Fig. 9. For all of the prepared samples, a weight loss at lower temperatures (<200 � C) was observed that is related to the physically adsorbed water and organic components of the feed. This reduction was sharp for the PY sample because of its lower Si/Al ratio and higher affinity to water adsorption than the alkali treated samples. Weight loss at temperatures lower than 400 � C that is because of soft coke deposition was very low for the alkali treated samples. The next weight loss from 400 to 650 � C observed for all of the samples is related to the hard coke formation [47]. The trend of weight loss for the alkali treated samples with CaCO3 was different compared to the other modified samples. For the CaCO3 modified samples, hard coke decomposition starts at 400 � C while for other alkali treated samples it started at 500 � C. This indicates that coke formed on the CaCO3 modified samples is softer and can be removed at lower temperature. That is very important from an industrial point of view because less energy is needed for the regeneration of these cata­ lysts. It is worth nothing that for the SAEY (1, CaCO3) another weight loss was observed at 650 � C. Based on the DTA results, this weight loos is endothermic and can’t be related to coke combustion. XRD and elemental analyses showed presence of CaCO3 particles and this weight loss is because of the endothermic decomposition of CaCO3 (to CaO and CO2) and gas formation (CO2). This point has been reported and dis­ cussed in the literature [44,48,49]. According to this analysis, SEY and alkali treated samples with Na2CO3 showed high coke deposition and the sample treated with 1 M NaOH due to low surface area and acidity showed the lowest coke formation. In addition, the SAEY (1, CaCO3) showed low weight loss (<14%) and low temperature for coke burning that is important for good catalytic performance. Moreover, it has been reported that modification of ZSM-5 with CaCO3 and CaO caused improvement of coke resistance in methanol to olefins conversion [50]. Basicity properties of CaCO3 and CaO poison strong Brønsted acidic sites of catalyst and increase resistance to coke formation [50]. The results of FTIR analysis of the spent catalysts are illustrated in – C bond stretching Table 4. The peak at 1587 cm 1 is attributed to the C– vibration in polycyclic aromatic compounds of deposited coke over spent catalysts [51,52]. The intensity of this peak is high for the samples

4. Comparison of the results of the current study with the published data Table 6 provides a comparison of the observed physicochemical properties and catalytic performance results in the current study with the ones published in literature. Different treatment methods have been applied for increasing the mesoporosity of Y zeolite containing different initial Si/Al ratios, which have resulted in different catalytic performances. Since different feeds and experimental conditions have been applied in those studies, the reported results there cannot be simply compared. Some of the so far reported studies focusing on desilication and dealumination of Y zeolite and using it as a catalyst for cracking of large molecules have been summarized in Table 6. As it can be seen in this table, the obtained re­ sults in current study are comparable with the reported results in the literature in terms of the observed enhancement in mesoporosity of Y zeolite and their catalytic performance. The observed catalytic perfor­ mance of the modified sample in this study demonstrates its superiority in comparison to the reported efforts in the literature in terms of securing higher conversion of heavy oil cracking. 5. Conclusions Various alkali agents with different alkalinity were used for the modification of Y zeolite and the prepared materials were used for cat­ alytic cracking of heavy oil. Type and concentration of alkali agent had diverse effects on the textural and catalytic properties of Y zeolite. After sequential steam-alkali-acid treatments, the relative crystallinity was reduced but crystalline structure of the Y zeolite was preserved. High concentration of the strong alkali agent (NaOH) caused strong desili­ cation with formation of some amorphous materials and surface area reduction. Modification with different alkali agents increased mesopore volume and pore diameter. Using higher concentration of alkali agents reduced the strong Brønsted acid sites. After these sequential 11

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Fig. 8. Gaseous components selectivity of the prepared catalysts (550) with time on stream (T ¼ 550 � C): (a) SAE (1, NaOH), (b) SAE (0.5, NaOH), (c) SAE (1, Na2CO3), (d) SAE (0.5, Na2CO3), (e) SAE (1, CaCO3), (f) SAE (0.5, CaCO3), (g) SEY and (h) PY.

12

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Fig. 9. TGA (a) and DTA (b) analysis of the spent catalyst after 4 h reaction.

modifications, the performance of the catalytic system in terms of heavy oil conversion and light fuels (gasoline and kerosene) selectivity were improved. Modification with CaCO3 had a significant effect on the heavy oil conversion and light fuel selectivity. Treatment with CaCO3

improved its resistance to coke formation and also its catalytic lifetime due to strong Brønsted acid sites modification. In addition, a different type of coke, which needs lower regeneration temperature, was pro­ duced over the samples modified with CaCO3. This lower needed 13

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Table 6 Textural properties and catalytic performance indicators of treated Y zeolites: Comparing the results of this study with the ones published earlier. Treatment type

SExt(M/P) & Vmeso(M/P)

Reaction conditions

Catalytic results improvement (%)

Ref.

Sequential NaOH and AHFS treatment (Si/Al ¼ 10.9)

SExt(M/P) ¼ 3.38 Vmeso(M/ P) ¼ 3.16

Conversion ¼ 0.6%

[26]

NaOH treatment of steam-acid treated Y (Si/Al ¼ 28.4)

SExt(M/P) ¼ 2.08 Vmeso(M/ P) ¼ 2.31

T ¼ 423 K, WHSV ¼ 121 1 Time ¼ 240 s Feed ¼ TIBP T ¼ 360–390 � C, LHSV ¼ 2.42 1 Time ¼ 200 h VGO hydrocracking T ¼ 653 K N2/Hydrocarbon ¼ 60 Feed ¼ Isooctane T ¼ 550 � C, Time ¼ 7 min Cat/Oil ¼ 8.5 Feed ¼ Bio-Oil T ¼ 530 � C, Cat/oil ¼ 6 Rate ¼ 1.2 g min 1 Feed ¼ Venezuelan heavy oil T ¼ 530 � C, Cat/oil ¼ 6 Rate ¼ 1.2 g min 1 Feed ¼ Venezuelan heavy oil T ¼ 500 � C, Time ¼ 16 s Feed ¼ 0.1ml/0.4 g TIBP T ¼ 527 � C, Time ¼ 30 s Feed ¼ 3.2 g/min, VGO T ¼ 423 K, Time ¼ 240 s Feed ¼ 12 L/h saturated N2 with TIBP T ¼ 120 � C, Time ¼ 2 h Feed ¼ cyclohexanone T ¼ 500 � C, Time ¼ 12 s Cat/ Oil ¼ 4.7 Feed ¼ TIPB T ¼ 120 � C, Time ¼ 2 h Feed ¼ cyclohexanone T ¼ 550 � C, WHSV ¼ 32 1 Time ¼ 255 min Feed ¼ middle distillate T ¼ 550 � C, WHSV ¼ 32 1 Time ¼ 255 min Feed ¼ middle distillate T ¼ 550 � C, WHSV ¼ 25 1 Time ¼ 120 min Feed ¼ heavy oil

Activity ¼ 13% SKerosene: 5.2% SDiesel: 4.7% Conversion ¼ 55%

[3] [54]

SHydrocarbons: 29%

[55]

Conversion ¼ 4.1%

[1]

Conversion ¼ 0.4%

[1]

Conversion ¼ 53%

[56]

Sgasoline ¼ 8%

[57]

Conversion ¼ 4.3%

[58]

Conversion ¼ 0.6%

[15]

Conversion ¼ 45.2%

[59]

Conversion ¼ 141%

[4]

Sgasoline ¼ 68% Viscosity reduction ¼ 33%

[9]

Sgasoline ¼ 146% Viscosity reduction ¼ 45%

[53]

Conversion ¼ 44% Sgasoline ¼ 149% Viscosity reduction ¼ 46%

This work

Ammonium fluorosilicate treatment (Si/Al ¼ 2.47) Sequential NaOH and Steam treatment (Si/Al ¼ 30) Sequential NaOH and Steam treatment (Si/Al ¼ 2.9) Sequential NaOH and Steam treatment (Si/Al ¼ 2.4)

SExt(M/P) ¼ n.m Vmeso(M/

P) ¼ n.m

SExt(M/P) ¼ 2 Vmeso(M/P) ¼ 2.3 SExt(M/P) ¼ 1.5 Vmeso(M/

P) ¼ 1.2

SExt(M/P) ¼ 1.2 Vmeso(M/

P) ¼ 1.1

NaOH treatment (Si/Al ¼ 30)

SExt(M/P) ¼ 2 Vmeso(M/P) ¼ 2.3

Sequential NH4OH-CTAB and Steam treatment (Si/Al ¼ 30)

SExt(M/P) ¼ 4.2 Vmeso(M/ P) ¼ 3.2 SExt(M/P) ¼ 1.3 Vmeso(M/ P) ¼ 1.7

NaOH and Steam treatment (Si/Al ¼ 2.6) Sequential acid and NaOH-CTAB and steam treatment (Si/ Al ¼ 2.7) Steam-Acid-NaOH Sequential acid, NaOH and steam (Si/Al ¼ 2.5) Alkali (Si/Al ¼ 2.6) La modification (Si/Al ¼ 2.6) Steam-alkali-acid treatment (Si/Al ¼ 2.6)

SExt(M/P) ¼ 9.7 Vmeso(M/P) ¼ 6 SExt(M/P) ¼ 2 Vmeso(M/P) ¼ 2.3 SExt(M/P) ¼ 13.1 Vmeso(M/ P) ¼ 8.6 SExt(M/P) ¼ 1.05 Vmeso(M/ P) ¼ 1.26 SExt(M/P) ¼ 1.18 Vmeso(M/

P) ¼ 1.25

SExt(M/P) ¼ 1.85 Vmeso(M/

P) ¼ 2.53

SExt(M/P): ratio of external surface area of modified catalyst to the parent catalyst. Vmeso(M/P): ratio of mesopores volume of modified catalyst to the parent catalyst. TIBP: 1,3,5-Tri-isopropylbenzene. AHFS: ammonium hexafluorosilicate. Sgasoline: Selectivity of Gasoline. n.m: not mentioned.

temperature for regeneration has positive implication from industrial point of view. According to the results of TGA and FTIR analysis, aro­ matics rings with high saturation degree (low C–H branch) were the main products deposited as coke over the catalysts.

Erasmus þ programme of the European Union for the financial support and for making this research possible. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109854.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements The authors gratefully acknowledge Tarbiat Modares University for the financial support of the research as well as Iran Nanotechnology Initiative Council for complementary financial supports. We would like to thank the Iranian Ministry of Science, Research and Technology for €flich and Dr. Jan Dirk financial support of this research. Dr. Katja Ho Epping are acknowledged for technical assistance with measurements (FESEM and Si and Al MAS NMR). The authors wish to thank the 14

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