Synthesis of Nickel-loaded Sulfated Zirconia Catalyst and Its Application for Converting Used Palm Cooking Oil to Gasoline via Hydrocracking Process

The synthesis of the nickel-loaded sulfated zirconia catalyst (Ni-SZ) and its application for the hydrocracking process have been carried out. This work has been conducted to determine the activity and selectivity from various Ni concentrations loaded on sulfated zirconia (SZ) in the hydrocracking of used palm cooking oil. The synthesis technique was preceded by sulfation of ZrO2 through incipient wetness impregnation method using H2SO4 solution and then continued with the impregnation of Ni via hydrothermal method employing NiSO4 · 6H2O precursor salt. The hydrocracking process was performed in a fix-bed microreactor at the optimum temperature (350 °C). The SZ loaded with 3 wt% of Ni (Ni-SZ 3) successfully produced the highest liquid product (44.25 wt%) and selectivity on gasoline (100 %). Besides, the gasoline fraction in the liquid product was dominated by unwanted aromatics compounds. The excellent performance of Ni-SZ 3 due to it has high acidity value, specific surface area, and Ni content.

metals [17]. The application of noble metals that are relatively expensive can reduce economic feasibility, so applying other alternative metals is considered necessary. Nickel is a metal potentially to replace the role of noble metals due to its high hydrogenation activity, and in terms of cost, it is cheaper [18]. Several studies show that modifications of catalysts with Ni could increase the hydrogenation reactivity, the acidity value, and the number of hydrocracking products [19,20]. Several studies also proved that modification catalyst using Ni is potentially applied in the hydrocracking of isopropyl benzene, crude oil coal, and petroleum sludge waste [21]. The Ni-SZ catalyst is very potential to be used in the conversion of used palm cooking oil into biogasoline through hydrocracking. Because in this catalyst, there is SZ which provides acidity (plays a role in the cracking process) and Ni which has hydrogenation activity (for the hydrogenation process). The conversion of used palm cooking oil to biogaoline can occur through several reaction stages, starting from the saturation of unsaturated triglycerides, followed by the decomposition of triglycerides into fatty acids. Furthermore, there are hydrodeoxygenation, decarboxylation and hydrodecarbonylation processes to produce hydrocarbons. The existing hydrocarbons then undergo cracking to produce biogasoline and other fractions [22]. Although its potential for hydrocracking of use palm cooking oil is there, however it has not yet been reported.
This work presents the synthesis of Ni-SZ with various Ni concentrations loaded on sulfated zirconia (SZ) for hydrocracking of used palm cooking oil to produce gasoline. This study aims to determine the nickel concentration that produces the highest activity and selectivity on the hydrocracking of used palm oil.

Methods 2.2.1 Synthesis of SZ
SZ was synthesized using an incipient wetness impregnation method. 10 g of ZrO 2 was stirred with 50 mL of 0.5, 1.0, and 1.5 M H 2 SO 4 solutions for 24 h at room temperature. The obtained colloidal suspension was centrifuged at 1800 rpm for 15 min. The solids that resulted from centrifugation were dried at 110 °C for 24 h. The resulting materials were labeled as SZ 0.5, SZ 1.0, and SZ 1.5, respectively. SZ X with the highest acidity value (SZ 1.5) was calcined at 500, 550, 600, 650, and 700 °C for 5 h. After that, the catalyst was crushed and sieved at 150 mesh. The products were labeled SZ X-500; 550; 600; 650; 700, respectively.

The synthesis of Ni-SZ
Synthesis of nickel-loaded sulfated zirconia (Ni-SZ) was carried out by the hydrothermal method. 5 g of calcined SZ X with the highest acidity value (SZ 1.5-500) was mixed with 50 mL NiSO 4 · 6H 2 O aqueous solution that corresponded to 1, 2, 3 wt% Ni in stainless steel autoclave without magnetic stirrer then heated in the oven at 110 °C for 5 h. The solids obtained were dried at 100 °C for 24 h and then calcined at 500 °C for 4 h (rate of heating and cooling is 25 °C/min). The next step was the reduction of 4 g catalyst sample in the reduction reactor under H 2 gas flow rate 20 mL/min, and the reactor was set up at 400 °C for 4 h. Then, the catalyst was crushed and sieved at 150 mesh. The obtained catalysts were labeled Ni-SZ 1, Ni-SZ 2, and Ni-SZ 3, respectively.

Characterization methods
The identification of the functional groups in the catalysts was carried out using a Fourier Transform Infrared spectrometer (FTIR, Shimadzu FTIR 8201 PC) equipped with the data station in the range of 400-4000 cm −1 with a KBr disc technique. The acidity of the catalysts was determined by the gravimetric method based on NH 3 gas adsorption and followed by FTIR analysis. The crystallinity and crystal size were characterized by X-Ray Diffractometer (XRD, Shimadzu Model XRD 6000, λ = 1.54 Å, 40 kV, 30 mA) with the source of Cu Kα radiation (Ni filtered), patterns were recorded from 4-80° (2θ), step size 0.017° 2θ, scan step time 10.1500 s. Crystal size was estimated by Scherrer equation: D = (Kλ) / (β cosθ); where D is the average of crystallite size (nm), K is Scherrer constant (0.89), λ is the X-ray wavelength, β is the observed angular width at half maximum intensity of the peak and θ is Bragg's angle. Surface morphology, composition, and elements distribution in the catalysts were characterized by Scanning Electron Microscope-Energy Dispersive Spectrometer mapping (SEM-EDS Mapping, JEOL JED 2300 Analysis Station, 15 kV). The concentration of Ni within Ni-SZ was analyzed by an Inductively Coupled Plasma spectrometer (ICP, ICPE-9820). Sample preparation for analysis of ICP was done with 0.5 g sample was added with 15 mL of aquaregia. The suspension was then stirred on the heating plate for 3 h. The resulting solution was diluted to 100 mL, and the concentration of Ni determine by calibration curve (r = 0.99964). Meanwhile, the specific surface area, total pore volume, average pore diameter, and pore distribution were analyzed by Gas Sorption Analyzer (GSA, Quadra Sorb Station 2 7.01). For each GSA analysis, 0.3 g of sample was used. The sample was thoroughly outgassed for 2 h, at 300 °C, bath temperature of −195.85 °C, and equilibration time of 120/variable sec (ads/des), then the weight of the outgassed sample was used in calculations.

Total acidity test of catalyst
The acidity test was performed by heating the empty porcelain cup at 150 °C for 2 h then weighed; the mass was noted as W 0 . A total of 0.1 g of sample was put into the porcelain cup, then heated at 150 °C for 2 h and weighed. The mass of porcelain cup+sample was noted as W 1 . The porcelain cup with a sample was put into a desiccator that contains NH 3 gas and allowed for 24 h. The porcelain cup+sample that has adsorbed the NH 3 gas was then weighed and noted as W 2 . The calculation of total acidity of the catalyst was calculated with Eq. (1).

Hydrocracking process
The hydrocracking process was performed in a fix-bed microreactor system. Scheme of the reactor was depicted in Fig. 1 [23]. Before the hydrocracking process was carried out, the determination of optimum temperature must be performed by thermal cracking of used cooking oil at 325, 350, and 375 °C (heating and cooling rate were 20 °C/min) under H 2 gas flow condition at 10 mL/min for 1 h. The product was collected in the cold trap for 1 h. Optimum temperature was the temperature that produces the highest liquid product. The hydrocracking process of used cooking oil with catalysts ZrO 2 , SZ X-Y (X = 1.5, Y = 500), Ni-SZ 1, Ni-SZ 2, and Ni-SZ 3 was carried out under H 2 gas flow conditions at 10 mL/min, in optimum temperature (heating and cooling rate were 20 °C/min) and the mass ratio of catalysts and used cooking oil was 1:100 (catalyst mass was 0.1 g and feed mass was 10 g) for 1 h. The product collected in the cold trap for 1 h were weighed and characterized by gas chromatography-mass spectrometry (GC-MS Shimadzu QP 2010S), and Rtx 5 column (30 m length, 0.25 mm internal diameter), Helium (He) as the carrier gas, and electron ionization detector. Set up method in this analysis was the column oven temperature: 50.0 °C, injection temperature: 310.00 °C, injection mode: split flow control mode: pressure, pressure: 13.0 kPa, total flow: 40.0 mL/min, column flow: 0.55 mL/min, linear velocity: 26.8 cm/sec, purge flow: 3.0 mL/min, split ratio: 66. The conversion of feed to yield was calculated with Eqs. (2)- (6): Residue wt% WR WF Coke wt% WUC WFC WF Gas Product wt% WF WLP WR WC WF Selectivity of liquid yields  [24,25]. In the SZ spectra, there are bands at 1018-1126 cm −1 that corresponded to sulfate vibration mode groups [9]. These bands indicate that sulfate has already impregnated on the ZrO 2 . The higher H 2 SO 4 concentration that was used, the band at 1018-1126 cm −1 becomes sharper that indicates the increase of the amount of sulfate in the SZ. The FTIR spectra of SZ 1.5 that calcinated at 500 °C; 600 °C; 650 °C; 700 °C are showed in Fig. 3. Based on Fig. 3, the spectra of calcined SZ 1.5 show the decreasing band intensity at 1002-1126 cm −1 (sulfate vibration mode groups) when the calcination temperature increase. These results indicate that the increasing temperature decomposes the sulfate group more intensely [26]. The sulfate that weakly bonded with surface hydroxyl group will be loss when increasing the calcination temperature [27].
The FTIR spectra of SZ 1.5-500, SZ 1.5-500 that loaded with 1 wt% of Ni (Ni-SZ 1), 2 wt% of Ni (Ni-SZ 2), and 3 wt% of Ni (Ni-SZ 3) are depicted in Fig. 4. Spectra in Fig. 4 show that impregnation of Ni reduces the intensity at 1002, 1056, 1126, and 1219 cm −1 . These results reflect the decrease of the sulfate group caused by heat treatment in the reduction catalyst step. The intense band at 1103 and 1141 cm −1 appears in the Ni-SZ catalyst. These two bands indicate the presence of the solid superacid structure. The bands that appear around 1103 and 1141 cm −1 correspond to the symmetry O-S-O stretching of bidentate persulfate ion coordinated with Ni [28]. The intensity band at 1103 and 1141 cm −1 increase with increasing Ni concentration. This is indicated that the bidentate persulfate ion that coordinated with Ni increase.

Acidity test
Determination of the catalyst total acidity values is carried out by the gravimetric method based on the adsorption of ammonia ( NH 3 ) gas, shown in Table 1. The acidity of ZrO 2 comes from Zr 4+ that acts as Lewis acid [17]. Based on the data shown in Table 1, the higher H 2 SO 4 concentration for ZrO 2 modification increases the acidity value due to the formation of polydentate surface SO 4 species, conferring significant Brønsted acidity [28,29].
The other results show that a higher calcination temperature causes the decrease of acidity value because of the sulfate decomposition as a consequence of the decrease Brønsted and Lewis acid site in the SZ catalyst [30]. The amount of Ni that load in SZ 1.5-500 affects the acidity because Ni can increase the acidity of the catalyst. The partially-charged orbitals of Ni are effective in accepting an electron pair (Ni increase Lewis acid site) [11,31]. The acidity test results are also confirmed by FTIR characterization. Fig. 5 shows the FTIR spectra of ZrO 2 and SZ in different concentrations of H 2 SO 4 solution used for sulfation after the acidity test . These spectra show bands at 1118 and 1404 cm −1 . The vibration at 1118 cm −1 indicates the presence of NH 3 coordinated by Lewis acid sites, and asymmetric stretching vibration at 1404 cm −1 shows the presence of ions formed by proton transfer from the Brønsted acid site to NH 3 [32]. Fig. 5 shows that the band intensities at 1118 and 1404 cm −1 become sharper when the H 2 SO 4 solution concentrations get higher. These results indicate that the higher H 2 SO 4 concentration is able to increase the number of Brønsted and Lewis acid sites in the catalysts.
The FTIR spectra of SZ 1.5 with various calcination temperatures after the acidity test are presented in Fig. 6. These FTIR spectra show the decreasing intensity at 1118 and 1404 cm −1 in higher calcination temperatures, indicating the decrease of Bronsted and Lewis acid sites. . The absence of a significant peak shift of SZ indicates that sulfation followed with calcination does not change the crystal phase of ZrO 2 , and SZ 1.5-500 has a lower intensity of the three dominant peaks than ZrO 2 .    This data also shows that the higher calcination temperature can increase the intensity of these three peaks because of the decomposition of sulfate on the ZrO 2 surface at high temperatures.
Diffractograms of SZ 1.5-500, Ni-SZ 1, Ni-SZ 2, Ni-SZ 3 are presented in Fig. 9. Ni-SZ diffractogram shows that Ni impregnation decreases the peaks intensity at 2θ = 28.16°; 31.42°, and 50.05° because of the Ni coverage on the SZ 1.5-500 surface. The peak of Ni was not detected because the amount of Ni on the surface of the catalyst relatively low.
The crystal size calculation of the particle is carried out by the Scherrer equation are listed in Table 2. The result shows that sulfation followed by calcination and nickel impregnation process change the average crystal size of the ZrO 2 . The data indicate that higher calcination temperature and Ni loading can decrease the crystal size. These phenomena are occurred due to crystal defect that caused by high sulfation, and followed by calcination and Ni loading.

Scanning Electron Microscopy-Energy Dispersive Spectrometry (SEM-EDS) mapping
The SEM images with magnitude 5000× present in Fig. 10 indicate that all the catalysts have a rough surface and irregular shape. The modification of ZrO 2 with sulfation and Ni loading does not change the surface structure significantly. The characterization of EDS based on the overall map listed in Table 3 shows that the sulfation process increases the mass percent of S, likewise with the impregnation of Ni, which increases Ni mass percent.   These results indicate that sulfate and Ni impregnation have been successfully carried out. The amount of Ni detected by EDS in all Ni-SZ is lower than the amount of Ni that impregnated. This phenomenon occurred due to inadequate mixing between Ni precursor and SZ as a result of the absence of a stirring process during the impregnation process. The distribution of S on SZ 1.5-500 and Ni on Ni-SZ is present in the mapping images shown in Fig. 11. Based on the mapping result, it can be seen that sulfur on SZ 1.5-500 and Ni on Ni-SZ 1, Ni-SZ 2, Ni-SZ 3 are equally dispersed on the catalyst surface.

Inductively Couple Plasma (ICP)
The data of the Ni percentage analysis using the ICP method are listed in Table 4. Based on the ICP analysis, it can be known that the impregnation of Ni in the SZ 1.5-500 matrix with the hydrothermal method is successfully carried out with an average loading value of 60 %. This data shows that the concentration of impregnated Ni is smaller than the initial concentration used. These results may be due to inadequate mixing between Ni precursor and SZ 1.5-500 due to the absence of the stirring step during the impregnation process. Besides that, Utami et al. [23] explain that the lower metal contents than the initial concentrations are caused by metal competition to bind in the catalyst. Some of the weakly bonded metal will be released from SZ 1.5-500 matrix [23].

Gas Sorption Analysis (GSA)
The specific surface area, total pore volume, and average pore diameter of catalysts are listed in Table 5. The BET isothermal nitrogen ( N 2 ) adsorption-desorption method is applied to derive the specific surface area data, and the BJH desorption method has been used to calculate the average pore diameter. SZ 1.5-500 has a lower surface area than the ZrO 2 catalyst because of the sulfate agglomeration in the pore of the catalysts [23]. On the other hand, the impregnation of Ni increases the specific surface area of the catalysts as a consequence of Ni dispersion in the pore and SZ surface. The average pore diameter of Ni-SZ drops significantly as a result of pore-blocking by Ni impregnation. The isotherm curves of N 2 adsorption-desorption are presented in Fig. 12.
The isotherm curves of all catalysts follow the isotherm curve type 4. This type of isotherm presents a hysteresis loop related to capillary condensation in mesopores and has a limiting uptake over a range of high p/p 0 . In addition, this isotherm curve has four types of hysteresis loops showing the characteristics of pore material [33]. The shape of ZrO 2 and SZ 1.5-500 isotherm curves show hysteresis type 3 that does not exhibit adsorption limits at high p/p 0 , and there is an aggregate of plate-like particles forming slit-shaped pores. For Ni-SZ 1, 2 and 3 catalysts are classified to hysteresis type 4 suggests the presence of narrow pore in the material. The difference in pore   shape between Ni-SZ 1, 2, 3, and ZrO 2 , SZ 1.5-500 may be caused by the dispersion of Ni narrowing of the pore. The pore size distribution curve based on BJH desorption method presented in Fig. 13 shows the catalysts pore predominantly of mesopores (2-50 nm).

Analysis of hydrocracking products
Used palm cooking oil as feed has been transesterified with methanol to form fatty acid methyl ester before analyzed with GC-MS. The spectrum data is shown in Fig. 14.
The data from GC-MS analysis and the distribution of fatty acid of the triglyceride are presented in Table 6. These analyses indicate that used palm cooking oil contained C 17 and C 19 compounds. The distribution of fatty acid of the triglyceride was calculated with Eq. (7).

Distribution
Area of coumpound Total area = × X 100 % The optimum hydrocracking temperature was determined based on the thermal cracking temperature, producing the largest conversion into liquid products. The thermal cracking analysis (Fig. 15) indicates that the highest conversion to liquid fuel is achieved at 350 °C. The temperature at 325 °C gives the smallest amount of liquid fuel because of non-optimum feed evaporation, and this is proved by the large residue that exists. Temperature 375 °C produces a lower liquid yield than 350 °C. However, the increasing temperature until 375 °C decreases the liquid product because the evaporation of feed is too fast, so it could not be adequately condensed. The other reason is that coke formation increases, and it covers the active sites, consequently inhibit the mass transfer of the feed to the active site of the catalyst, and it lowered activity to form the liquid product [34]. The hydrocracking reaction is applied with ZrO 2 , SZ 1.5-500, Ni-SZ 1, Ni-SZ 2, and Ni-SZ 3 catalyst. The result of their catalytic activity test is depicted in Fig. 16. Based on this activity test, SZ 1.5-500 produces the lowest liquid product conversion compared to the other catalysts. This is due to its lowest surface area and its properties that are easily deactivated by coke. Coke formation on the SZ 1.5-500 blocks the active site and decreases its activity to produce a liquid product [35]. The hydrocracking using Ni-SZ catalyst with a higher concentration of Ni produces a higher liquid product due to the high specific surface area and the acidity of Ni-SZ. The acidity of the catalyst directly proportional to its catalytic activity for the hydrocracking reaction [36].
In general, liquid products from the hydrocracking of used cooking oil consist of hydrocarbons and non-hydrocarbons. The non-hydrocarbon products are the uncracked feed, while the hydrocarbon products are derived from the cracked fatty acids.
The selectivity of catalyst to light ( C 1 -C 4 ), gasoline ( C 5 -C 12 ), diesel ( C 13 -C 20 ), and non-hydrocarbon fraction showed in Table 7. This analysis (Table 7) shows that the liquid products of all catalysts predominantly by gasoline fractions and the highest gasoline produced by Ni-SZ 3. This phenomenon is due to Ni-SZ 3 having the highest acidity, surface area, and stability of the catalysts.
Several treatments have been carried out to produce gasoline from used cooking oil as listed in Table 8.
Based on Table 8, we can see that the activity of the Ni-SZ 3 inferior against Ni-Mo Alumina [37] but it more superior against ZSM-5 [38] and Mo-SBA-15 [39] although a lower temperature operation was carried out. This is indicated that Ni-SZ 3 is better catalyst to convert used cooking oil to gasoline than Mo-SBA-15 and ZSM-5 while worse than Ni-Mo Alumina.  Five main compounds from the liquid products are listed in Table 9, and most of them are aromatic hydrocarbons. The formation of aromatic hydrocarbon from triglyceride occurred through nine interconnected reactions of triglyceride molecules and intermediates: 1. hydrogenolysis, forming fatty acids and propane; 2. dehydrogenation of diene chains producing triene fatty acids; 3. conjugation of double bonds system in triene chain; 4. migration of double bonds (conjugated system) along the chain of fatty acid towards the terminus of hydrocarbon chain (carboxylic or methyl group); 5. deoxygenation of carboxylic groups in fatty acids; 6. intermolecular Diels-Alder reaction of conjugated triene system forming a cyclohexene ring inside or at the end of hydrocarbon chain; 7. dehydrogenation of cyclohexene to aromatic ring; 8. hydrocracking of alkyl chains; 9. isomerization of alkyl chains [40].   . 15 The results of the thermal cracking Aromatic hydrocarbons are not expected to appear in fuels because of their high combustion energy and toxicity. Further research is needed to increase the conversion value of liquid products and reduce the content of aromatic compounds in the gasoline fraction of hydrocracking products so that potentially be used in the future as a safe, economical, and environmentally friendly fuel source. One of the problems the cause of a large amount of aromatics product is the low hydrogenation activity of  [41][42][43]. To solve this problem, we suggested increasing the hydrogenation activity of the catalyst by insert two kinds of metals that have high hydrogenation activity to SZ to form a bimetallic catalyst.

Conclusions
Synthesis and application of Ni-SZ for the hydrocracking process were successfully performed. The data showed that sulfation on ZrO 2 increases the total acidity, but on the other hand, this reduces crystallinity, average crystal size, and specific surface area. The increase of calcination temperature can decrease the acidity value, crystallinity, and average crystal size. The impregnation of Ni can increase the acidity value and specific surface area of the catalyst. Furthermore, in the hydrocracking process, this impregnation is able to increase the activity and selectivity in gasoline fractions as well. The sulfated zirconia catalyst loaded with 3 wt% of (Ni-SZ 3) has proven successfully producing the highest liquid yield (44.25 wt%) and selectivity in gasoline fractions (100 %) compared to the other catalysts. Although the selectivity in the gasoline fraction is 100 %, the compounds in it are dominated by unwanted aromatic compounds, so it is necessary to improve the catalyst to solve this problem.