Catalytic Cracking of N-Hexadecane Using Carbon Nanostructures/Nano-Zeolite-Y Composite Catalyst

catalysts

Article Catalytic Cracking of n-Hexadecane Using Carbon Nanostructures/Nano-Zeolite-Y Composite Catalyst

Botagoz Zhuman 1 , Shaheen Fatima Anis 2, Saepurahman 3 , Gnanapragasam Singravel 4 and Raed Hashaikeh 2,*

1 Department of Chemical Engineering, Khalifa University of Science and Technology, P.O. Box 127788 Abu Dhabi, UAE; [email protected] 2 New York University Abu Dhabi (NYUAD) Water Research Center, NYUAD, P.O. Box 129188 Abu Dhabi, UAE; [email protected] 3 Department of Chemistry, Institut Teknologi Bandung, Jl. Ganesha no. 10, Bandung 40132, Indonesia; [email protected] 4 ADNOC Reﬁnery Research Centre (ARRC), P.O. Box 123593 Abu Dhabi, UAE; [email protected] * Correspondence: [email protected]; Tel.: +971-2-6284626

Received: 25 September 2020; Accepted: 20 October 2020; Published: 28 November 2020

Abstract: Zeolite-based catalysts are usually utilized in the form of a composite with binders, such as alumina, silica, clay, and others. However, these binders are usually known to block the accessibility of the active sites in zeolites, leading to a decreased effective surface area and agglomeration of zeolite particles. The aim of this work is to utilize carbon nanostructures (CNS) as a binding material for nano-zeolite-Y particles. The unique properties of CNS, such as its high surface area, thermal stability, and flexibility of its fibrous structure, makes it a promising material to hold and bind the nano-zeolite particles, yet with a contemporaneous accessibility of the reactants to the porous zeolite structure. In the current study, a nano-zeolite-Y/CNS composite catalyst was fabricated through a ball milling approach. The catalyst possesses a high surface area of 834 m2/g, which is significantly higher than the conventional commercial cracking catalysts. Using CNS as a binding material provided homogeneous distribution of the zeolite nanoparticles with high accessibility to the active sites and good mechanical stability. In addition, CNS was found to be an effective binding material for nano-zeolite particles, solving their major drawback of agglomeration. The nano-zeolite-Y/CNS composite showed 80% conversion for hexadecane catalytic cracking into valuable olefins and hydrogen gas, which was 14% higher compared to that of pure nano-zeolite-Y particles.

Keywords: nano-zeolite; carbon nanostructures; composite catalyst; cracking

1. Introduction Zeolite HY is a commercially important catalyst used widely in petroleum refining and petrochemical industries. H-Y zeolite has superior properties such as a 3D porous network containing channels and cages, high surface area, strong acidic sites, and high thermal and good hydrothermal stability [1]. These properties make zeolites a suitable catalyst for acid-catalyzed reactions, such as cracking and hydrocracking reactions of hydrocarbons [2,3]. In these processes, the zeolite is usually used in powder and extrudates forms [4], often containing and shaped in the presence of binders, such as alumina [5], clay [6], silica [7], titania [8], and phosphorus [9] or combinations of alumina and silica [10]. Those binders are generally present at higher than 20 wt.% to obtain an appropriate mechanical strength or to be shaped [11]. Alumina is mainly used as a zeolite binder in the preparation of catalyst extrudates, pellets, granules, flakes, and powder [12]. There are many limitations caused by conventionally used binders in the industry, the diffusion limitation being one of the important ones.

Catalysts 2020, 10, 1385; doi:10.3390/catal10121385 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1385 2 of 17

Poor diffusion can be caused by inhomogeneous distribution of the zeolite particles within the binding material, low surface area of the binders, and poor pore distribution within its structure [13]. Moreover, some binders can alter zeolite properties, in terms of acidity, which can affect overall selectivity and conversion [14]. Owing to the binder phases, catalysts in the extrudate form possess lower surface areas than bare zeolite particles. Usually, the surface area of alumina-supported industrial catalysts ranges from 60 to 100 m2/g [15]. Thus, most of the active area is lost in the binders. Catalytic reactions usually take place at high temperatures and pressures, thus the mechanical and thermal stabilities of the catalyst are important parameters. Mechanical properties are highly dependent on the type of binders used during the catalyst fabrication. However, commonly used binders for catalysts are brittle in nature, which makes them prone to fast attrition. Besides coking, attrition is found to be one of the reasons for fast catalyst deactivation [16]. Due to high surface area, less coking, and better mass transfer of the reactant and product molecules within the catalyst, nano-zeolite-Y has been reported to have a higher catalytic efficiency compared to zeolite-Y micro-particles [17,18]. Despite nano-zeolite having all these advantages, it tends to agglomerate during the catalytic process due to its high surface energy [19]. To overcome this limitation of nano-zeolites, they must be shaped into a different form, such as fibers [17]. Another approach of solving the agglomeration issue of nano-zeolites is using them in combination with various functional materials in the form of nanocomposites. Progress in nanomaterials has paved the way for various carbon nanomaterials including carbon nanotubes (CNTs) [20], carbon nanofibers (CNFs) [21], fullerenes [22], and novel structured carbons [23]. Due to their superior chemical and physical properties, resistance to both acid and alkaline media, controllable porosity, and surface chemistry, carbon-based nanomaterials have successfully been reported as catalyst supports for various heterogeneous catalyst reactions [24–26]. Moreover, carbon-based binders have different anchoring sites for the attachment of active components, which gives better dispersion of the particles compared to conventionally used alumina binders [27]. Despite the fact that CNTs-supported catalysts are widely studied in the literature [27–30], a new approach of synthesizing a low-cost, flexible, and porous structure named carbon nanostructure (CNS) has paved the way for CNS nanocomposites. CNS has been proposed and developed by Applied Nano Structured Solutions LLC, a subsidiary of Lockheed Martin [31]. CNS refers to interdigitated, branched, and crosslinked CNTs that share common walls with one another. They possess good chemical, mechanical, electrical, and thermal properties, together with flexibility of the fibrous material. CNS has been used as a binding material with lithium iron phosphate (LiFePO4) particles as battery electrodes [32,33]. A cathode with high loadings of the active material was achieved. Moreover, homogeneous distribution of LiFePO4 particles within the CNS network facilitated a fast charge transfer and increased the conductivity and diffusion coefficient of the cathode [32]. Therefore, it is expected that using CNS as a binding material provide homogeneous distribution of the particles due to its fibrous structure and can prevent blocking of its active sites. This can address the agglomeration issue of zeolite particles commonly caused by conventionally used binders, such as alumina. Another advantage of using CNS as a binder is to prevent a reduction in the surface area of the composite catalyst, as CNS has a higher surface area [32], compared to alumina binders. In addition, the flexibility of the fibrous structure and superior properties of CNS allow catalysts to be shaped in different forms, which increases its mechanical stability and prevents attrition and chipping of the catalyst during the catalytic process. In this work, CNS is used as a binding material for zeolite nanoparticles. These zeolite nanoparticles were produced through a ball milling approach, reported previously by our group [34]. The ball-milled nano-zeolite with a high Si/Al ratio of 15 has also been previously reported for hydrocracking [3,17], as an electrocatalyst [18], and in water treatment [35,36] applications. This work studies the catalytic activity of a CNS/nano-zeolite-Y composite catalyst and compares it with the catalytic activity of nano-zeolite-Y particles alone for n-hexadecane cracking reaction. Cracking of alkanes by zeolite plays a central role in the production of fuels from petroleum. Several parameters such as catalyst activity, Catalysts 2020, 10, 1385 3 of 17 selectivity of the products, stability of the catalyst, coking, and catalyst deactivation tendencies have been studied and reported. The following abbreviations are used hereafter in the paper: nano-zeolite-Y particles—“NHYZ”, and nano-zeolite-Y/CNS composite—“NHYZ/CNS”.

2.Catalysts Results 2020, 10, x FOR PEER REVIEW 3 of 18

2.1.parameters Surface Morphology such as catalyst and Elemental activity, Mappingselectivity of the products, stability of the catalyst, coking, and catalystFigure deactivation1a–c shows SEMtendencies images have of NHYZ, been studied CNS, and and NHYZ reported./CNS. The The f SEMollowing image abbreviations of the composite are showedused hereafter that zeolite in nanoparticles the paper: are nano highly-zeolite entangled-Y particles within—“NHYZ” CNS bundles., and Zeolitenano-z nanoparticleseolite-Y/CNS werecomposite seen to— be“NHYZ/CNS”. present in di fferent sizes (Figure1a), as previously reported in [ 34]. From the SEM image of the NHYZ/CNS composite (Figure1c), it can be seen that using CNS as a binding material 2. Results can form a highly homogeneous structure which can bind nano-zeolite particles together, meanwhile creating a porous and open network for efficient diffusion of the feed molecules to the active sites of 2.1. Surface Morphology and Elemental Mapping zeolite. Further, SEM-EDS analysis of an industrial catalyst extrudate was performed in order to study the uniformityFigure 1a in–c distribution shows SEM of images the catalyst of NHYZ, and the CNS binder., and Figure NHYZ/CNS. S1 reveals The theSEM inhomogeneous image of the distributioncomposite showedof zeolite that particles zeolite within nanoparticles alumina. are In highly addition, entangled non-uniform within distribution CNS bundles. of tungsten Zeolite (sulphidenanoparticles form) were was alsoseen observed. to be present This in is onedifferent of the si majorzes (Figure causes 1 fora), theas previously inaccessibility reported of active in [ sites,34]. whichFrom isthe usually SEM image hindered of the because NHYZ/CNS of the bindingcomposite material. (Figure Meanwhile,1c), it can be the seen NHYZ that /usingCNS compositeCNS as a showedbinding great material dispersion can form of nano-zeolite-Y a highly homogeneous particles in structure the CNS matrixwhich (Figurecan bind2). Thenano detected-zeolite elementsparticles Al,together Si, and, Omeanwhile are due to thecreating zeolite, a while porous C wasand detected open network from CNS. for efficient diffusion of the feed molecules to the active sites of zeolite. Further, SEM-EDS analysis of an industrial catalyst extrudate 2.2.was Surface performed Area in order to study the uniformity in distribution of the catalyst and the binder. Figure S1 reveals the inhomogeneous distribution of zeolite particles within alumina. In addition, Figure3 shows the N adsorption–desorption curves for NHYZ particles, CNS, and NHYZ/CNS. non-uniform distribution2 of tungsten (sulphide form) was also observed. This is one of the major BET surface areas for NHYZ and CNS were 1190 and 223 m2/g, respectively. The surface area of the causes for the inaccessibility of active sites, which is usually hindered because of the binding composite catalyst is highly dependent on the surface area of its components and was found to be material. Meanwhile, the NHYZ/CNS composite showed great dispersion of nano-zeolite-Y particles 834 m2/g. It can be seen from the graphs that catalysts have a mesoporous structure, typical of most in the CNS matrix (Figure 2). The detected elements Al, Si, and O are due to the zeolite, while C was zeolites [37]. detected from CNS.

FigureFigure 1. 1.SEM SEM images images of (a ) of nano-zeolite-Y (a) nano-zeolite particles-Y particles (NHYZ), (NHYZ scale bar), =scale3 µ m, bar (b ) = carbon 3 µm nanostructure, (b) carbon (CNS),nanostructure scale bar (=CNS1 µ)m,, scale and bar (c) = NHYZ 1 µm,/ CNS,and (c scale) NHYZ/CNS, bar = 3 µm. scale bar = 3 µm.

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Figure 2. EDS analysis of the NHYZ/CNS catalyst, scale bar = 20 µm.

2.2. Surface Area

Figure 3 shows the N2 adsorption–desorption curves for NHYZ particles, CNS, and NHYZ/CNS. BET surface areas for NHYZ and CNS were 1190 and 223 m2/g, respectively. The surface area of the composite catalyst is highly dependent on the surface area of its components and was found to be 834 m2/g. It can be seen from the graphs that catalysts have a mesoporous structure, Figure 2. EDS analysis of the NHYZ/CNS catalyst, scale bar = 20 µm. typical of mostFigure zeolites 2. EDS [37] .analys is of the NHYZ/CNS catalyst, scale bar = 20 µm.

2.2. Surface Area

Figure 3. NFigure2 adsorption 3. N2 adsorption/desorption/desorption isotherm isotherm of of NHYZ,NHYZ, NHYZ/CNS, NHYZ/CNS, and andCNS, CNS,filled circles ﬁlled represent circles represent adsorptionadsorption and empty and circlesempty circles represents represents desorption, desorption, SASA == surfacesurface area area..

2.3. X-ray Di2.3.ﬀ Xraction-Ray Diffraction Analysis Analysis

Figure4 represents the XRD graph for NHYZ, NHYZ /CNS, and CNS materials. CNS shows two characteristic amorphous peaks at 25.7 and 42.7 degrees, whereas NHYZ has crystalline characteristic peaks of HY zeolite at 15.7, 18.7, 23.7, 27.1, 30.8, 31.5, and 34.2 degrees. NHYZ/CNS retained all of the characteristic peaks of NHYZ and has amorphous peak broadening around 25.5 degrees.

Figure 3. N2 adsorption/desorption isotherm of NHYZ, NHYZ/CNS, and CNS, filled circles represent adsorption and empty circles represents desorption, SA = surface area.

2.3. X-Ray Diffraction Analysis

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Figure 4 represents the XRD graph for NHYZ, NHYZ/CNS, and CNS materials. CNS shows two characteristic amorphous peaks at 25.7 and 42.7 degrees, whereas NHYZ has crystalline characteristic peaks of HY zeolite at 15.7, 18.7, 23.7, 27.1, 30.8, 31.5, and 34.2 degrees. NHYZ/CNS Catalysts 2020, 10retain, 1385ed all of the characteristic peaks of NHYZ and has amorphous peak broadening around 25.5 5 of 17 degrees.

NHYZ NHYZ/CNS CNS

Peak Intensity, (a.u.) Intensity, Peak

0 10 20 30 40 50 60 70 80 Angle, degree

Figure 4.FigureXRD 4. XRD graph graph of of NHYZ, NHYZ, CNS CNS,, and and NHYZ/CNS. NHYZ /CNS.

2.4. Catalytic Performance2.4. Catalytic Performance and Coke and Formation Coke Formation Percentage ofPercentage total conversion of total conversion for n-hexadecane for n-hexadecane is is shown shown in Table Table 1,1 while, while Figure Figure 5 shows5 shows the the total total conversion over time for both catalysts. The NHYZ/CNS composite has a higher conversion conversion overrate time(80%) forduring both the catalysts.1-h time on stream The NHYZ (TOS) compared/CNS composite to the NHYZ has catalyst a higher (66%). conversionThe activity rate (80%) during the 1-hof time NHYZ/CNS on stream is maintained (TOS) compared over 3 h, as tothe the conversion NHYZ rate catalyst decreases (66%). by only The 4%. activity Moreover, of NHYZ/CNS is maintainedNHYZ/CNS over 3 h, has as a thelower conversion deactivation compared rate decreases to NHYZ which by only was 4%.10% over Moreover, 3 h. Deactivation NHYZ is /CNS has a mainly caused by blockage of the active sites by coke. Similar catalyst deactivation has been reported lower deactivationin [17]. Considering compared that to cracking NHYZ reactions which are was happening 10% over at high 3 h.temperatures, Deactivation it is important is mainly to caused by blockage of thehighlight active that sites thermal by coke. cracking Similar could catalystalso be involved. deactivation However, has in beenour study, reported thermal in cracking [17]. Considering that crackinggave reactions several are by-products, happening such at as high n-hexane, temperatures, pentadecane it, and is important hexadecene, to with highlight only 4% that thermal hexadecane conversion at 514 °C . cracking could also be involved. However, in our study, thermal cracking gave several by-products, such as n-hexane, pentadecane, and hexadecene, with only 4% hexadecane conversion at 514 ◦C. Table2 show the product range for the n-hexadecane cracking reaction over 3 h including gaseous and liquid products (GP and LP) and coke. Figure6 presents the amount of each product formed after each hour of catalytic cracking by NHYZ and NHYZ/CNS catalysts. NHYZ/CNS had two times higher formation of LP (11.5 wt.%) and 9 wt.% higher cracked GP compared to NHYZ during the first hour of TOS. Selectivity of products is different for both of the catalysts, as can be seen in Table3 and Figures7 and8. NHYZ /CNS showed a high production of hydrogen gas around 40 wt.% over 3 h of TOS (Figure8), whereas NHYZ had para ffin as its major products, equal to 36.6 wt.% during the first hour of TOS (Figure7). This is due to the di fferent mechanisms of cracking of n-hexadecane for NHYZ/CNS and NHYZ catalysts. The NHYZ/CNS composite is found to be the superior catalyst for hydrogen production and had an active dehydrogenation process compared to NHYZ.

Table 1. Conversion rates for NHYZ and NHYZ/CNS catalysts.

Catalyst Name NHYZ NHYZ/CNS Hours Conversion % 1 66 80 2 62 77 3 56 76 Catalysts 2020, 10, x FOR PEER REVIEW 6 of 18

Table 1. Conversion rates for NHYZ and NHYZ/CNS catalysts.

Catalyst name NHYZ NHYZ/CNS Hours Conversion % 1 66 80 2 62 77 Catalysts 2020, 10, 1385 3 56 76 6 of 17

90 NHYZ NHYZ/CNS 80

Conversion, wt % 10

0 1 hour 2 hour 3 hour TOS, hour

Figure 5. Conversion rates for NHYZ and NHYZ/CNS catalysts.

Table 2. Gaseous andFigure liquid 5. Conversion products rates (GP for NHYZ and LP) and for NHYZ/CNS NHYZ andcatalysts. NHYZ /CNS catalysts.

TOS GaseousTable 2 Productsshow the product(GP), wt.% range for Liquid the n-hexadecan Productse (LP), cracking wt.% reaction Coke over after 3 h including 3 h TOS, wt.% gaseous and liquid products (GP and LP) andNHYZ coke. Figure 6 presents the amount of each product formed after each hour of catalytic cracking by NHYZ and NHYZ/CNS catalysts. NHYZ/CNS had 1 59.3 6.3 two times higher formation of LP (11.5 wt.%) and 9 wt.% higher cracked GP compared to NHYZ 2 55.6 6.4 0.8 duringCatalysts 20the20 , first10, x FORhour PEER of TOS REVIEW. Selectivity of products is different for both of the catalysts, as can7 of be 18 3 46.2 9.8 seen in Table 3 and Figures 7 and 8. NHYZ/CNS showed a high production of hydrogen gas around NHYZ/CNS 40 wt.% over 3 h of TOS (Figure 8), whereasNHYZ/CNS NHYZ had paraffin as its major products, equal to 36.6 11wt.% during the first 68.268.2 hour of TOS (Figure 7). This is due11.5 11.5 to the different mechanisms of cracking of 22n -hexadecane for 63.6NHYZ/CNS63.6 and NHYZ catalysts. The13.1 13.1 NHYZ/CNS composite is found3.4 to3.4 be the 33superior catalyst for 62.662.6 hydrogen production and had an 13.1 13.1active dehydrogenation process compared to NHYZ.

Table 2.80 Gaseous and liquid NHYZ products GP ( GP and LP ) for NHYZ NHYZ and NHYZ/CNS LP catalysts. NHYZ/CNS GP NHYZ/CNS LP TOS Gaseous Products70 (GP), wt.% Liquid Products (LP), wt.% Coke after 3 h TOS, wt.% NHYZ 1 6059.3 6.3 2 55.6 6.4 0.8 3 5046.2 9.8

Conversion, wt.% Conversion, 20

0 1 2 3 TOS, hours

Figure 6. GP and LP for NHYZ and NHYZ/CNS catalysts.

Table 3. Selectivity of NHYZ and NHYZ/CNS catalysts.

Catalyst NHYZ NHYZ/CNS TOS 1 2 3 1 2 3 Paraffins 36.6 33.3 30.5 17.5 12.7 12.9 Olefins 21.3 21.9 19.1 14.1 10.3 8.9 Aromatics 4.2 3.2 3.3 7.9 6.2 7.1 Hydrogen 3.3 3.2 2.7 39.4 46.6 45.9

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Table 3. Selectivity of NHYZ and NHYZ/CNS catalysts. Catalysts 2020, 10, x FOR PEER REVIEW 8 of 18 Catalyst NHYZ NHYZ/CNS TOS 1 2 3 1 2 3 Paraﬃns 36.6 33.3 30.5 17.5 12.7 12.9 Oleﬁns 21.3 21.9 19.1 14.1 10.3 8.9 Catalysts 2020, 10, x FOR PEER REVIEW 1 hour 8 of 18 35 Aromatics 4.2 3.2 3.3 7.9 6.2 7.1 Hydrogen 3.3 3.2 2.7 39.4 46.6 45.9 2 hour

30 3 hour

25 1 hour 35 2 hour

2030 3 hour

1525

10 Conversion, wt.% Conversion, 20

155

100 Conversion, wt.% Conversion, Paraffins Olefins Aromatics Hydrogen 5 Products

0 Paraffins Olefins Aromatics Hydrogen Figure 7. Selectivity of the NHYZ catalyst. Products

Figure 7. Selectivity of the NHYZ catalyst. 50 Figure 1 hour 7. Selectivity of the NHYZ catalyst. 45 2 hour 40 3 hour 50 35 1 hour 45 30 2 hour 40 25 3 hour 35 20 30 15

Conversion, wt.% Conversion, 25 10 20 5 15 0

Conversion, wt.% Conversion, 10 Paraffins Olefins Aromatics Hydrogen

5 Products 0 Figure 8. Selectivity of the NHYZ/CNS catalyst. Paraffins Olefins Aromatics Hydrogen Figure9 shows the carbonFigure number 8. Selectivity of products of the NHYZ/CNS cracked at catalys the endt. of each hour for both the catalysts. The carbon number of the products agreesProducts well with the literature [38,39]. The majority of Figure 9 shows the carbon number of products cracked at the end of each hour for both the catalysts. The carbon number of the products agrees well with the literature [38,39]. The m ajority of the products are in the diapasonFigure of 8.C 1Selectivity and C7, while of the theNHYZ/CNS products catalys with t.a carbon number higher than 7 were less than 1%. Isobutane was the major cracked product for NHYZ during the first hour of Figure 9 shows the carbon number of products cracked at the end of each hour for both the

catalysts. The carbon number of the products agrees well with the literature [38,39]. The majority of the products are in the diapason of C1 and C7, while the products with a carbon number higher than 7 were less than 1%. Isobutane was the major cracked product for NHYZ during the first hour of

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Catalysts 2020, 10, x FOR PEER REVIEW 9 of 18 the products are in the diapason of C1 and C7, while the products with a carbon number higher than 7 TOSwere (12.49 less than wt.%) 1%., w Isobutanehereas propylene was the major was the cracked major product hydrocarbon for NHYZ (HC) during product the for first NHYZ/CNS hour of TOS (9.66(12.49 wt.% wt.%), during whereas the first propylene hour of was TOS) the and major was hydrocarbon found to be (HC)the dominant product for product NHYZ after/CNS the (9.66 initial wt.% hourduring for theboth first the hour catalysts. of TOS) C3 and and was C5 foundparaffins to bewere the dominantfound to be product the second after themajor initial product hour fors after both propylenethe catalysts. for both C3 and catalysts. C5 para Carbonffins were numbers found tofor be paraffins the second and major olefins products are shown after propylenein Tables S1 for and both S2catalysts., respectively. Carbon numbers for paraffins and olefins are shown in Tables S1 and S2, respectively.

14 NHYZ 14 NHYZ 1 hour 1 hour NHYZ/CNS NHYZ/CNS 12 12

10 10

8 8

6 6

Olefins Weight % Weight Olefins 4 4

Paraffins Weight % Weight Paraffins

2 2

0 0 C2 C3 iC4 C1 C2 C3 iC4 C5 iC5 C6+1 Carbon Number Carbon Number

14 NHYZ 2 hour 14 NHYZ 2 hour NHYZ/CNS NHYZ/CNS

12 12

10 10

8 8

6 6

4 % Weight Olefins Paraffins Weight % Weight Paraffins 4

2 2

0 0 C1 C2 C3 iC4 C5 iC5 C6+1 C2 C3 iC4 Carbon Number Carbon Number

NHYZ 14 3 hour 14 NHYZ 3 hour NHYZ/CNS NHYZ/CNS

12 12

10 10

8 8

6 6

4 % Weight Olefins Paraffins Weight % Weight Paraffins 4

2 2

0 0 C1 C2 C3 iC4 C5 iC5 C6+1 C2 C3 iC4 Carbon Number Carbon Number

Figure 9. Paraﬃns and oleﬁns carbon number in GP of NHYZ and NHYZ/CNS catalysts. Figure 9. Paraffins and olefins carbon number in GP of NHYZ and NHYZ/CNS catalysts. Coke formation has been studied using TGA (Figure 10) of fresh and spent catalysts. Here, 3.4 andCoke 0.8 wt.% formation of coke has were been calculated studied using for NHYZ TGA /(CNSFigure and 10) NHYZ of fresh catalysts, and spent respectively. catalysts. Here, The TGA3.4 andgraph 0.8 ofwt. CNS% of alone coke isw shownere calculated in Figure for S2 NHYZ/CNS and the decomposition and NHYZ temperature catalysts, respectively is equal to 581. The◦C TGA (Table graph of CNS alone is shown in Figure S2 and the decomposition temperature is equal to 581 °C (Table S3). Higher formation of coke for NHYZ/CNS can be explained by the overall higher conversion of the feed compared to NHYZ and the more significant hydrogen transfer process.

Catalysts 2020, 10, 1385 9 of 17 Catalysts 2020, 10, x FOR PEER REVIEW 10 of 18

S3).CokedCatalysts Higher catalysts 2020 formation, 10, x wereFOR PEER ofanalyzed coke REVIEW for using NHYZ SEM/CNS as canshown be explainedin Figure by11. theFigure overall 11a highershows conversionthat the spent10 of 18 theNHYZ feed catalyst compared formed to NHYZ agglomerates and the after more the signiﬁcant cracking hydrogenreaction. From transfer Figure process. 11b, it Coked can be catalystsobserved Coked catalysts were analyzed using SEM as shown in Figure 11. Figure 11a shows that the spent werethat nano analyzed-zeolite using particles SEM asremained shown attached in Figure to 11 CNS. Figure bundles. 11a shows that the spent NHYZ catalyst NHYZ catalyst formed agglomerates after the cracking reaction. From Figure 11b, it can be observed formed agglomerates after the cracking reaction. From Figure 11b, it can be observed that nano-zeolite that nano-zeolite particles remained attached to CNS bundles. particles remained attached to CNS bundles.

Figure 10. TGA analysis of fresh and spent NHYZ and NHYZ/CNS catalysts. Figure 10. TGA analysis of fresh and spent NHYZ and NHYZ/CNS catalysts. Figure 10. TGA analysis of fresh and spent NHYZ and NHYZ/CNS catalysts.

Figure 11. SEM images of coked catalysts: (a) NHYZ, and (b) NHYZ/CNS, scale bar = 5 µm. Figure 11. SEM images of coked catalysts: (a) NHYZ, and (b) NHYZ/CNS, scale bar = 5 µm. 3. Discussion 3. DiscussionFigure 11. SEM images of coked catalysts: (a) NHYZ, and (b) NHYZ/CNS, scale bar = 5 µm. Using CNS as a binding material can form a highly homogeneous structure (Figure1c), and can overcome3. DiscUsingussion the CNS limitations as a binding caused material by agglomeration can form a highly of active homogeneous components. structure CNS binds(Figure nano-zeolite 1c), and can particlesovercome together, the limitations meanwhile caused creating by agglomeration a porous and openof active structure components. for efficient CNS di ffbindsusion nano of the-zeolite feed moleculesparticlesUsing together to theCNS active ,as meanwhile a sitesbinding of zeolite. creatingmaterial This, acan porous in form essence, anda highly open overcomes homogeneous structure the limitationsfor efficient structure caused diffusion (Figure bycommonly 1ofc) the, and feed can usedmoleculesovercome binders tothe [ 40 the limitations]. The active macroporous sites caused of zeolite.by structure agglomeration This, of CNSin essence of can active benefit, overcomes components. the mass the transport CNS limitations binds process nano caused of-zeolite the by commonlyparticles together used binders, meanwhile [40]. Thecreating macroporous a porous and structure open strucof CNSture canfor efficientbenefit thediffusion mass of transport the feed processmolecules of the to feed the activeto acti ve sites sites of of zeolite. nano- zeolite This, -inY. essenceIn addition,, overcomes the composite the limitations catalyst ha causeds a high by commonly used binders [40]. The macroporous structure of CNS can benefit the mass transport

process of the feed to active sites of nano-zeolite-Y. In addition, the composite catalyst has a high

Catalysts 2020, 10, 1385 10 of 17 feed to active sites of nano-zeolite-Y. In addition, the composite catalyst has a high surface area of 834 m2/g, and it is higher compared to common alumina-supported industrial catalysts [4] which can range from 60 to 100 m2/g [15]. Table4 represents surface areas of nano- and micro-zeolite-Y used as ﬂuid catalytic cracking (FCC) catalysts shaped with Al2O3, SiO2, or SiO2/Al2O3 as binders. The surface area of an industrial catalyst is highly dependent on the surface area of its binders and ranges from 200 2 2 to 350 m /g, while the surface area of Al2O3 itself ranges between 200 and 270 m /g [41,42]. However, using SiO2/Al2O3 binders for nano-zeolite-Y results in a loss of active surface area and coverage of the nano-particles with the binding material. The developed NHYZ/CNS composite has three times higher surface area compared to the reported industrial FCC cracking catalysts. This shows that using CNS as a binding material can form a highly eﬃcient composite with a high surface area.

Table 4. Surface areas of nano- and micro-zeolite-Y-based ﬂuid catalytic cracking (FCC) catalysts.

No FCC Catalyst Surface Area, m2/g Reference

1 Nano-Y zeolite in SiO2/Al2O3 matrix 230 [43] 2 Nano-zeolite-Y in SiO2 binder 315 (nano-zeolite-Y original surface area 570) [44] 3 Micro-zeolite-Y-supported Al2O3 341 (zeolite-Y original surface area 672) [45]

Moreover, using CNS as a binding material does not alter and can preserve the crystalline structure of NHYZ (Figure4). XRD analysis shows all nano-zeolite-Y characteristic peaks for the NHYZ/CNS composite. Unlike while using Al2O3 as a binder, all the crystalline characteristic peaks of nano-zeolite-Y have disappeared, showing only Al2O3 peaks for the composite catalyst after shaping with a binder [46]. On the other hand, using CNS as a binder creates a highly accessible porous structure for nano-zeolite-Y while retaining the crystallinity of the catalyst. Additionally, the homogeneous dispersion of nano-zeolite particles, maintained crystalline structure, and high surface area of the catalyst can serve for increased efficiency of the overall catalyst. A high conversion of 80% for NHYZ/CNS can be another evidence of the highly dispersed and homogeneous structure of the composite created after the ball milling process [34,47] which provided a higher accessibility to active sites for the reaction with feed molecules. This could also be explained due to the faster diffusion for nano-sized zeolite particles created after the ball milling process [34,47]. Meanwhile, NHYZ had lower conversion (66%) compared to NHYZ/CNS, possibly due to having nano-zeolite particles agglomerated during the catalytic process. Besides, NHYZ had a higher deactivation rate of 10% during the 3 h of TOS, which is 6% higher than NHYZ/CNS. Despite NHYZ having a much higher surface area than NHYZ/CNS, the conversion rates showed an inverse trend. This might be due to the fact that NHYZ agglomerates during the catalytic cracking reaction, which will not allow the feed to reach all of the active surface area of NHYZ. Moreover, this also shows that nano-zeolite has higher catalytic efficiency in the form of the NHYZ/CNS composite which prevents nanoparticles from agglomeration and creates a more efficient catalyst with higher accessibility of active sites of nano-zeolite to the feed. Both catalysts gave a different selectivity of products (Figures7 and8). Figure8 shows that NHYZ/CNS serves as an efficient catalytic cracking catalyst for the production of highly valuable alkenes and hydrogen gas. Product distribution of NHYZ/CNS and NHYZ catalysts can be interpreted by protolytic cracking [48], β scission, dehydrogenation, and hydrogen transfer [38]. Primarily n-hexadecane is adsorbed into the pore of zeolite, then it undergoes cracking at Bronsted (H+) and Lewis acid (tricoordinated aluminium or/and AlO+) sites [49]. Protolytic cracking is kinetically dominant in the initial steps and it is originated at the Bronsted acid (H+) site of zeolite to form penta-coordinated carbonium ion. Further C-C bond breaking results in paraffin in gaseous form and adsorbed carbenium ion. Major products for both the catalysts are in the gaseous form for the total TOS, as shown in Table2 and Figure6. Both catalysts form an overall higher concentration of paraffins compared to olefins during the 3 h of the cracking reaction, as can be seen in Table3 and Figures7 and8 . Further rearrangement of carbenium ion leads to the formation of branched paraffins. Catalysts 2020, 10, x FOR PEER REVIEW 12 of 18

Formation of branched species of olefins is attributed to the participation of Lewis acid sites during the initiation of cracking, forming as a result a carbenium ion which desorbs as a branched olefin, if this carbenium ion has four or more carbon atoms. Formation of iso-butene is higher for NHYZ during the first hour compared to NHYZ/CNS, whereas the concentration of isobutene increases for NHYZ/CNS after the first hour of TOS (Figure 9 and Table S2). CatalystsHydride2020, 10, 1385 transfer between the feed molecule (n-hexadecane) and adsorbed carbenium ion11 ofleads 17 to further cracking via β scission, causing a chain reaction [50,51]. The chain reaction gives a higher concentration of olefins. Table 3 shows an increase in the concentration of olefins for 2-h TOS for NHYZ.Formation However, of branched it is not species the case of olefinsfor NHYZ/CNS is attributed due to to the the participation significant ofdehydrogenation Lewis acid sites or duringformation theinitiation of hydrogen of cracking, gas. Figure forming 8 shows as a result the concentration a carbenium ion of which formed desorbs hydrogen as a gas branched for the olefin,NHYZ/CNS if this carbenium catalyst. Production ion has four of orhydrogen more carbon gas was atoms. observed Formation as one of of iso-butene the major is products higher for for NHYZNHYZ/CNS during thecompared first hour to comparedNHYZ. During to NHYZ the /1CNS,-h TOS, whereas 39.4 wt.% the concentration of H2 was obtained of isobutene as a product. increases It forincreased NHYZ/CNS until after 48–47 the wt.% first during hour of 2 TOS- and (Figure 3-h of9 TOS and, Tablewhereas S2). NHYZ H2 accounted for 3.3–2.7 wt.% duringHydride the 3 transfer-h TOS. betweenFormation the of feed hydrogen molecule gas (n-hexadecane) can be a result and of adsorbedthe C-H bond carbenium cleavage ion leadsby the toBronsted further cracking acid (H+) via siteβ ofscission, zeolite causingwhich results a chain in reaction formation [50 o,51f H].2 The and chaincarbenium reaction ion gives [50]. aThe higher other concentrationpossible source of olefins. of hydrogen Table3 gasshows is the an increase(-OH) groups in the concentrationof the catalyst. of The olefins selectivity for 2-h TOSresults for NHYZ.(Table 3) However,suggest that it is protonation not the case is for happening NHYZ/CNS at the due C- toH thebond significant and is dominant dehydrogenation for the NHYZ/CNS or formation catalyst of hydrogenwhich gives gas. a Figure higher8 showsamount the of concentrationH2, whereas primary of formed protonation hydrogen for gas NHYZ for the is NHYZ initiating/CNS on catalyst. the C-C Productionbond, resulting of hydrogen in a higher gas wasconcentration observed asof oneparaffins. of the Formation major products of molecular for NHYZ hydrogen/CNS compared (H2) during to NHYZ.the cracking During reaction the 1-h TOS,of alkanes 39.4 wt.% by superacid of H2 was catalysts obtained has as abeen product. extensively It increased reported until by 48–47 Oleh wt.% et al. during[52–54 2-]. and 3-h of TOS, whereas NHYZ H2 accounted for 3.3–2.7 wt.% during the 3-h TOS. Formation + of hydrogenFormation gas can of be aromatics a result of and the C-H coke bond can cleavage be explained by the byBronsted a secondary acid (H reaction) site of zeolite or hydrogen which resultstransfer. in formationIf the paraffin/ of H2oandlefin carbenium (P/O) ratio ion is higher [50]. The than other 1, it possible signifies source a hydrogen of hydrogen transfer gas process. is the (-OH)Consi groupsdering ofthe the P/O catalyst. ratio of The NHYZ selectivity, hydrogen results transfer (Table3 )is suggest more significant that protonation for NHYZ is happening compared at to theNHYZ/CNS. C-H bond and However, is dominant if the for concentration the NHYZ/CNSs of coke catalyst and which aromatics gives are a higher considered amount, then of H NHYZ/CNS2, whereas primaryhas the protonationmore dominant for NHYZhydrogen is initiating transfer compared on the C-C to bond, NHYZ. resulting On the in other a higher hand, concentration NHYZ/CNS ofhas parahigherffins. total Formation conversion of molecular of the feed, hydrogen which will (H2) result during in the higher cracking concentration reaction ofs of alkanes coke and by superacid aromatics. catalystsThe proposed has been reaction extensively mechanism reported by is Oleh depicted et al. [ in52 –Figure54]. 12 and all products formed after n-hexadecaneFormation ofcracking aromatics are andshown coke in canFigure be explained13. by a secondary reaction or hydrogen transfer. If the paraGenerallyffin/olefin, for (Pboth/O) ratioof the is catal higherysts, than hydrogen 1, it signifies transfer a hydrogen is dominant transfer during process. the first Considering hour and theleads P/O to ratio the formation of NHYZ, of hydrogen unsaturated transfer species is more such significantas aromatics for and NHYZ coke, comparedas shown in to Tables NHYZ 2/ CNS.and 3. However,[55]. However, if the concentrations it is less dominant of coke after and the aromatics first hour are since considered, the concentration then NHYZ of /aromaticsCNS has the decreases more dominantfor both of hydrogen the catalysts, transfer as can compared be seen tofrom NHYZ. Figures On 7 the and other 8. hand, NHYZ/CNS has higher total conversionProduct of the dis feed,tribution which showswill result that in higherpropylene concentrations was the majorof coke HC and aromatics. product formed The proposed for the reactionNHYZ/CN mechanismS composite is depicted catalyst in during Figure 12the and 3-h all TOS. products It is important formed after to n-hexadecanenote that the production cracking are of shownlight olefins in Figure such 13 .as propylene, n-butene, and isobutene is highly desirable and is one of the global targetsGenerally, for fluid for both catalytic of the cracking catalysts, (FCC) hydrogen units. transfer These is types dominant of olefins during can the be first useful hour andfor further leads toetherification the formation and of unsaturatedalkylation processes species suchto produce as aromatics more env andironmentally coke, as shown friendly in Tables gasoline2 and products.3[ 55]. However,The aromatic it is less products dominant produced after the were first toluene, hour since benzene, the concentration naphthalene, of aromaticscymene, and decreases xylenes, for where both ofbenzene the catalysts, was the as candominant be seen product. from Figures 7 and8.

Figure 12. Proposed reaction mechanism of n-hexadecane cracking.

Catalysts 2020, 10, x FOR PEER REVIEW 13 of 18 Catalysts 2020, 10, 1385 12 of 17 Figure 12. Proposed reaction mechanism of n-hexadecane cracking.

Figure 13. Products of n-hexadecane catalytic cracking. Figure 13. Products of n-hexadecane catalytic cracking. Product distribution shows that propylene was the major HC product formed for the NHYZ/CNS In order to endure the collisions happening inside the reactor in an industrial catalytic cracking composite catalyst during the 3-h TOS. It is important to note that the production of light olefins unit, the catalyst should have enough hardness and yet need to be flexible to prevent attrition from such as propylene, n-butene, and isobutene is highly desirable and is one of the global targets for happening. Apart from that, the composite catalyst should maintain its structure after the cracking fluid catalytic cracking (FCC) units. These types of olefins can be useful for further etherification reaction which can prove its mechanical stability under harsh conditions. Usually industrial FCC and alkylation processes to produce more environmentally friendly gasoline products. The aromatic catalysts are shaped in the form of extrudates, spheres, granules, etc. However, catalysts in this form products produced were toluene, benzene, naphthalene, cymene, and xylenes, where benzene was the tend to attire and chip under harsh conditions inside the reactor. Commonly used industrial dominant product. catalysts are brittle in nature and lack flexibility. Therefore, using a flexible, fibrous, and In order to endure the collisions happening inside the reactor in an industrial catalytic cracking mechanically stable binding material allows the catalyst to be shaped into different forms, while unit, the catalyst should have enough hardness and yet need to be flexible to prevent attrition from reducing its tendency to chip and attire. Moreover, it can increase the mechanical stability of the happening. Apart from that, the composite catalyst should maintain its structure after the cracking composite catalyst -by effectively forming a highly bonded structure. SEM images of coked catalysts reaction which can prove its mechanical stability under harsh conditions. Usually industrial FCC (Figure 11) illustrate that the composite catalyst is mechanically stable under harsh conditions. This catalysts are shaped in the form of extrudates, spheres, granules, etc. However, catalysts in this form proves the efficiency of CNS as a binding material for the shaping of catalysts and that it could be tend to attire and chip under harsh conditions inside the reactor. Commonly used industrial catalysts used as a successful physical functionalization method for the fabrication of zeolite/CNS composites. are brittle in nature and lack flexibility. Therefore, using a flexible, fibrous, and mechanically stable Additionally, it proves that CNS can provide the matrix which prevents nanoparticles from binding material allows the catalyst to be shaped into different forms, while reducing its tendency agglomeration. It can be seen that NHYZ/CNS forms a highly bonded composite. The physical-form to chip and attire. Moreover, it can increase the mechanical stability of the composite catalyst -by NHYZ/CNS catalysts can be observed to be intact, with no major changes, after the catalytic cracking effectively forming a highly bonded structure. SEM images of coked catalysts (Figure 11) illustrate reaction in the micro-catalytic reactor, thus attesting to the physical stability of the NHYZ/CNS that the composite catalyst is mechanically stable under harsh conditions. This proves the efficiency catalyst. of CNS as a binding material for the shaping of catalysts and that it could be used as a successful Another important factor that needs to be considered is that CNS would be removed through physical functionalization method for the fabrication of zeolite/CNS composites. Additionally, it proves the calcination step during the regeneration process of the catalyst. Therefore, more studies are that CNS can provide the matrix which prevents nanoparticles from agglomeration. It can be seen required in order to lower the concentration of CNS during the fabrication process while retaining that NHYZ/CNS forms a highly bonded composite. The physical-form NHYZ/CNS catalysts can be the same efficiency and homogeneous distribution within the composite. observed to be intact, with no major changes, after the catalytic cracking reaction in the micro-catalytic reactor, thus attesting to the physical stability of the NHYZ/CNS catalyst. 4. Materials and Methods Another important factor that needs to be considered is that CNS would be removed through the calcination4.1. Materials step during the regeneration process of the catalyst. Therefore, more studies are required in order to lower the concentration of CNS during the fabrication process while retaining the same efficiencyUltra and-stable homogeneous Zeolite-Y distribution (CBV-720) within with a theSiO composite.2/Al2O3 ratio of 30 was procured from Zeolyst International and used as a parent zeolite. Nano-sized ball-milled zeolite-Y was obtained by ball milling (high-energy ball mill Emax, Retsch, Haan, Germany) the parent zeolite particles with CNS.

Catalysts 2020, 10, 1385 13 of 17

4. Materials and Methods

4.1. Materials

Ultra-stable Zeolite-Y (CBV-720) with a SiO2/Al2O3 ratio of 30 was procured from Zeolyst InternationalCatalysts 2020, 10 and, x FOR used PEER as REVIEW a parent zeolite. Nano-sized ball-milled zeolite-Y was obtained by14 ball of 18 milling (high-energy ball mill Emax, Retsch, Haan, Germany) the parent zeolite particles with CNS. The procedure reported in [34] was followed. Ethanol, ≥99%, was purchased from Sigma Aldrich. The procedure reported in [34] was followed. Ethanol, 99%, was purchased from Sigma Aldrich. Deionized (DI) water was produced using the Suez/Purite≥ Neptune Ultimate Water Purification Deionized (DI) water was produced using the Suez/Purite Neptune Ultimate Water Puriﬁcation Machine. n-Hexadecane, ≥99%, was purchased from Sigma Aldrich and used for catalytic testing. Machine. n-Hexadecane, 99%, was purchased from Sigma Aldrich and used for catalytic testing. Carbon nanostructures (CNS)≥ were supplied by Lockheed Martin with a bulk density of 0.015 g/cm3 Carbon nanostructures (CNS) were supplied by Lockheed Martin with a bulk density of 0.015 g/cm3 and incorporated CNTs with a length of 1–10 µm. and incorporated CNTs with a length of 1–10 µm.

4.2.4.2.Preparation Preparation ofof CompositeComposite Catalyst Catalyst Using Using Ball Ball Milling Milling NHYZNHYZ was was obtained obtained through through a ball amilling ball millingprocess, reportedprocess, previously reported previouslyby our group by [17 our,18, 34 group–36]. [17,18,34The NHYZ–36]. /CNS composite was fabricated using the following procedure depicted in Figure 14. Ball millingThe NHYZ was carried/CNS composite out in zirconia was fabricated jars using using 2 mm the zirconia following balls. procedure Commercial depicted zeolite-Y in Figure particles 14. andBall CNS milling were was used carried in the out ratio in zirconia of 75:25 byjars weight using in2 mm a solvent zirconia ratio balls. of water Commercial/ethanol zeolite of 1:1 by-Y volume.particles Balland milling CNS were was performed used in the at 1000 ratio rpm of 75:25 for 1 h. by Ball-milled weight in samples a solvent were ratio centrifuged of water (Thermo/ethanol Scientiﬁc, of 1:1 by Herarussvolume. MegafugeBall milling 40R, was Langenselbold, performed at Germany) 1000 rpm atfor 4000 1 h. rpm Ball for-milled 5 min, samples and the were bottom centrifuge part ofd the(Thermo sample Scientific, was selected Heraruss for this Megafuge study. The 40R, bottom Langenselbold, part was selectedGermany due) at to4000 having rpm afor higher 5 min, yield, and surfacethe bottom area, and part crystallinity of the sample of the was zeolite selected particles. for this Zeolite study. yield, The surface bottom area, part and was crystallinity selected due of the to materialhaving a have higher been yield, previously surface reportedarea, and by crystallinity our group [of34 ].the Centrifugation zeolite particles. was Zeolite followed yield, by dryingsurface area, and crystallinity of the material have been previously reported by our group [34]. under vacuum at 80 ◦C overnight. Centrifugation was followed by drying under vacuum at 80 °C overnight.

FigureFigure 14. 14.Composite Composite catalyst catalyst fabrication fabrication procedure procedure using using ball ball milling. milling. Ball-milled Ball-milled zeolite zeolite particles particles are indicatedare indicated by yellow by yellow dashed dashed circles, circles, scale s barcale= bar500 = nm.500 nm.

4.3.4.3.Characterization Characterization SurfaceSurface morphology morphology and and elemental elemental analysis analysis were analyzedwere analyzed using high-resolution using high-resolution scanning scanning electron microscopyelectron microscopy (HRSEM) (Nova (HRSEM) Nano (Nova SEM, FEI, Nano Eindhoven, SEM, FEI, The Eindhoven Netherlands), The and Netherlands) an energy-dispersive and an energy-dispersive X-ray spectrum (EDS) (EDXS, FEI, Eindhoven, The Netherlands). Surface area was measured by multi-point Brunauer–Emmett–Teller (Quantachrome, NOVA 2000e, FL, USA) (BET) surface and pore size analyzer. Surface area (BET) was calculated using P/P0 regimes from 0.05 to 0.35. Thermo-gravimetric analysis (TGA) of the coked and fresh catalysts was performed using Perkin Elmer TGA 4000 (Waltham, MA, USA). A total of 10–20 mg of the sample was heated until 900 °C under oxygen flow of 20 mL/min at a 10 °C /min heating rate. XRD analysis was performed using XRD PANalytical Empyrean (Almelo, the Netherlands), and the scan was performed between 5 and 70° at a scan rate of 3 degrees per min.

Catalysts 2020, 10, 1385 14 of 17

X-ray spectrum (EDS) (EDXS, FEI, Eindhoven, The Netherlands). Surface area was measured by multi-point Brunauer–Emmett–Teller (Quantachrome, NOVA 2000e, Boynton Beach, FL, USA) (BET) surface and pore size analyzer. Surface area (BET) was calculated using P/P0 regimes from 0.05 to 0.35. Thermo-gravimetric analysis (TGA) of the coked and fresh catalysts was performed using Perkin Elmer TGA 4000 (Waltham, MA, USA). A total of 10–20 mg of the sample was heated until 900 ◦C under oxygen ﬂow of 20 mL/min at a 10 ◦C/min heating rate. XRD analysis was performed using XRD PANalytical Empyrean (Almelo, the Netherlands), and the scan was performed between 5 and 70◦ at a scan rate of 3 degrees per min.

4.4. Catalytic Testing n-Hexadecane catalytic cracking was carried out in a steady-state fixed-bed micro-catalytic reactor (custom made by Sigma Enterprises LLC, Abu Dhabi, United Arab Emirates). Thermocouples were placed in the top, middle, and bottom of the catalytic bed to measure the temperature distribution, whereas the flow rates of the gas and feed were controlled by mass flow controllers. The catalyst 1 mass of 0.86 g was used for all experiments, with a weight hourly space velocity (WHSV) of 2.68 h− . NHYZ particles and the NHYZ/CNS composite were tested in powder form. The catalyst activation process was carried out at 514 ◦C in a helium atmosphere. After 3 h of activation, liquid feed was inserted along with helium. The temperature was raised to 514 ◦C and kept constant thereafter. The rate of hexadecane feed was set at 0.05 cc/hr. The reaction was carried out for 3 h at an atmospheric pressure of 1 bar. Liquid and gaseous products were collected separately. The weight of LP was recorded at every 1 h interval and was analyzed offline using a gas chromatograph mass spectrometer (GC 7890B-MS5977A) equipped with a 5-column nonpolar column. The GP were also collected at every hour interval and analyzed by an offline GC system (7890A GC system, 8-column RGA with a FID detector). Percentage conversion and selectivity were calculated using the following equations:

Initial feed (g) Final feed (g) % Conversion = − 100 (1) Initial feed (g) ×

Mass of individual product (g) % Selectivity = 100 (2) Initial feed (g) ﬁnal feed (g) × − Carbon number in the products was calculated based on the weight of the individual products. The mass balance was calculated in accordance with GP and LP and coke that formed, and the remaining accounted for loss of HC products during the testing and collection of the samples which is evident from the results.

5. Conclusions In this study, CNS was used as a binding material for nano-zeolite-Y. Using CNS as a binding material can be beneficial for catalytic applications due to its large external surface area, flexible fibrous structure, and chemical resistance to both acid and basic media. A nano-zeolite-Y/CNS catalyst was successfully fabricated using a ball milling procedure, while creating a highly homogeneous structure with a high surface area of 874 m2/g compared to the commonly used alumina-supported industrial catalysts. The fabricated composite catalyst was tested for n-hexadecane cracking reaction and was compared to the catalytic activity of nano-zeolite-Y particles. Nano-zeolite-Y/CNS showed higher conversion towards n-hexadecane (80%) compared to nano-zeolite particles alone (66%). This showed that using CNS as a binding material for nano-zeolite-Y could solve the main drawback of nanoparticle agglomeration while creating a composite with homogeneous distribution of nano-zeolite-Y particles within the CNS matrix. High mechanical stability, and high accessibility to the catalytic sites of nano-zeolite-Y was achieved. Moreover, using CNS as a binder created a porous composite with a retained crystalline structure of nano-zeolite-Y particles. The nano-zeolite-Y/CNS composite showed high conversion of n-hexandecane and production of highly valuable alkenes and hydrogen gas. Catalysts 2020, 10, 1385 15 of 17

Most of the hydrocarbon products were between C1 and C7, propylene being the major hydrocarbon product. The aromatic products were toluene, benzene, naphthalene, cymene, and xylene. Moreover, zeolite-Y nanoparticles were seen to remain attached to CNS after being subjected to harsh cracking conditions inside the reactor, hence proving the catalyst’s mechanical stability.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/12/1385/s1, Figure S1: EDS analysis of an industrial hydrocracking catalyst extrudate received from local refinery in Abu Dhabi, United Arab Emirates, Figure S2: TGA graph of CNS, Table S1: Paraffins Carbon number in of NHYZ/CNS and NHYZ catalysts, Table S2: Olefins Carbon number in of NHYZ/CNS and NHYZ catalysts, Table S3: Decomposition Temperature of CNS. Author Contributions: Supervision, conceptualization, visualization, investigation, and project administration was carried out by R.H. Catalytic testing was performed at the premises of ADNOC refining research center (ARRC) under supervision of G.S. Composite catalyst was developed and characterized by joint efforts of Saepurahman and B.Z. Data analysis and original draft preparation were carried out by cooperative efforts of S.F.A. and B.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially funded by Abu Dhabi National Oil Company Research Center (ARRC). Acknowledgments: The authors would like to thank ARRC for providing facilities for the catalytic reaction. Conflicts of Interest: The authors declare that they have no known competing interest that could have appeared to influence the work reported in this paper.

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