Hydrocracking of Calophyllum inophyllum Oil Employing Co and/or Mo Supported on γ-Al2O3 for Biofuel Production

Cobalt and/or Molybdenum based catalysts were simply dispersed on γ-Al2O3 through wet impregnation. The set of prepared monometallic and bimetallic catalysts of aCoO/γ-Al2O3, aMoO/γ-Al2O3, aCoOaMoO/γ-Al2O3, and bCoObMoO/γ-Al2O3 were investigated and evaluated in the hydrocracking of Calophyllum inophyllum (CIO) which is not a food crop as well as rich in unsaturated fatty acid that potential to be converted into biofuel. Out of the prepared catalysts, aCoOaMoO/γ-Al2O3 with total metal content, acidity, and specific surface area of 13.62 wt%, 5.01 mmol.g-1, and 107.67 m2.g-1, respectively, showed the best catalytic performance. The high metal loading of aCoOaMoO/γ-Al2O3 is favorable by producing 65.56 wt% liquid fraction through carbocation formation mechanism. It was selective to produce 8.61 wt% gasoline and 5.01 wt% diesel. Copyright © 2020 BCREC Group. All rights reserved


Introduction
The rapid depletion of fossil fuel as our primary energy resource has become a concern for many people, especially researchers since many years ago. As a limited resource, the continuous use of fossil fuel may cause completely used up. Moreover, fossil fuel grows environmental damage in producing carbon dioxide (CO2) emissions [1] that contribute to global warming. At this stage, the search for alternative resources has been gaining much attention and currently from those sources, Calophyllum inophyllum oil (CIO) as a non-edible biomass resource is a potential source of biofuel. The kernels of CIO have very high oil content (75%) which is composed of approximately 71% of unsaturated fatty acids (oleic and linoleic acids) [9]. The appearance of CIO is similar to olive oil, with an aromatic odor and an insipid taste [10]. The conversion of CIO into biodiesel has been carried out via pre-treatment giving the yield of biodiesel from the CIO under the optimized conditions is found to be 89% [11]. Other studies [12,13] found that the characterization test of CIO biodiesel is very close agreement to the diesel oil which is potential to be directly used for diesel engines. Conversion of bio-oil, in this case CIO, can be conducted through hydrocracking which includes hydrogenation and cracking reactions that take place simultaneously [7]. To achieve an optimum conversion of CIO, the catalyst should present in the hydrocracking reaction.
Supported precious metals as well as transition metals have been studied for the hydrocracking reaction [14][15][16][17]. However, despite its good catalytic activity, the use of precious metal as a catalyst is considered as costineffective and the utilization of mere low-cost transition metals cannot compete with precious metals catalytic performance. Therefore, optimizing an appropriate catalytic system is required. One of the reported alternatives is by designing heterogeneous catalysts based on the transition metals supported on porous materials [18][19][20]. It was reported that VI B or VIII B group transition elements, such as W, Mo, Co, and Ni, are active components for hydrogenation catalysis [21]. In addition, designing bimetallic catalysts, such as Co-Mo, is reported to highly efficient for several catalytic performances [22][23][24]. Moreover, the bimetallic catalyst composed of Co-Mo has a prominent influence on catalytic activity [25].
Considering our previous investigation [26] by employing Ni and/or Mo based catalyst on γ-Al2O3 on hydrocracking of CIO that exhibited quite satisfying results, in this study, we employed Co and/or Mo based catalyst on γ-Al2O3 support material evaluated in the same reaction. The catalysts were prepared through a simple impregnation method. In addition, the metal loading was set distinct from the previous investigation.

Catalyst Preparation
The preparation of monometallic and bimetallic catalysts of Co and/or Mo was conducted by using a simple wet impregnation method. The monometallic catalysts of were set to have 10 wt% (marked as a) of metal content denoted as aCoO/γ-Al2O3 and aMoO/γ-Al2O3 while bimetallic catalyst was set to have 5 wt% (marked as b) and 10 wt% of each metal content denoted as aCoOaMoO/γ-Al2O3 and bCoObMoO/γ-Al2O3. The only difference between monometallic and bimetallic catalysts preparation was the mixing of Co and Mo salt precursors prior to the dispersion on γ-Al2O3 support. Salt precursors of Co and/or Mo were firstly dissolved with double-distilled water followed by dispersion on γ-Al2O3. The mixtures were stirred at 70 °C for 20 min. Water was then evaporated and the solids were dried in the oven at 100 °C for 24 h. The dried solids were calcined in N2 gas stream (20 mL.min -1 ) at 450 °C for 2 h.

Catalyst Characterization
The crystallinity of the prepared catalysts was analyzed by an X-ray diffractometer (Rigaku SmartLab Miniflex 600) in the 2θ range of 2-80°. The acidity of catalysts by the gravimetric method using NH3 gas as the basic adsorbate. Metal content(s) of the catalysts were determined by using Atomic Adsorption Spectrophotometer (Perkin Elmer® 5100 PC). Characteristics of the pores were evaluated according to Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) theories using a gas sorption analyzer (Quantachrome® NOVA touch). The surface morphologies of the most excellent catalyst were analyzed using a scanning electron microscope (SEM Hitachi SU-3500®).

Catalyst Test
Catalytic activity and selectivity of the prepared catalysts were evaluated via hydrocracking of the CIO to produce biofuel (Scheme 1 Appendix). A 21.0 cm length semi-batch stainless steel reactor with an inner diameter of 2.80 cm and an outer diameter of 3.10 cm was used. The catalyst-to-feed weight ratio of 1:100 was determined for the hydrocracking process and placed inside the reactor. The reactor was sealed and streamed by H2 gas (30 mL.min -1 ) at 550 °C for 2 h to ensure that there is no O2 gas involved in the reaction. Three fractions of products which are liquid, gas, and solid have resulted after hydrocracking reaction ended. The aim product (liquid fraction) was condensed and collected in the heart-shaped-flask. It was further analyzed using gas chromatography-mass spectroscopy (GC-MS Shimadzu® QP2010S). The liquid fraction was further classified according to the number of composed carbons which are gasoline fraction (C6-C12) and diesel fraction (>C12).

Catalyst Preparation and Characterization
With the aim to develop an alternative catalyst for the conversion of biomass, the prepared catalysts show the suitability for hydrocracking of CIO. Table 1 shows the results of total metal loading in the catalysts show that the metal(s) were not fully deposited on the catalyst support as the wt% of the total loading is lower than intended to be. This might be happened due to the mass loss during the preparation process. The loading of metal(s) on support material changed its properties. It caused a decrease in surface area and pore volume because the metal particles filled up the pores. The aCoOa-MoO/γ-Al2O3 catalyst with the highest metal content shows a significant decrease in surface area and pore volume.
The presence of metal on the support material has been reported can increase its acidity as it can provide Lewis acid site which is in accordance with this study [25]. This Lewis acid site is preferable in many chemical reactions including hydrocracking reaction. The acidity of the prepared catalysts is shown in Table 1. Without the impregnation of metal(s), the acidity of γ-Al2O3 was 2.08 mmol.g -1 . Based on Table 1, the acidity of γ-Al2O3 was increased by the presence of metal species. In addition, the total amount of metal loaded will also affect the acidity. A higher total amount of metal will give higher acidity. However, not all prepared catalysts are in line with this theoretical approach. The acidity of aCoOaMoO/γ-Al2O3 catalyst which has the highest metal loading was only 5.01 mmol.g -1 , lower than bCoObMoO/ γ-Al2O3 catalyst which has only 7.35 wt% metal loading. This phenomenon might be occurred due to the big particle metal species generated on aCoOaMoO/γ-Al2O3 decreases the active site of the catalyst. Moreover, the generated big particles may also block the pore of γ-Al2O3 which also decreases the active sites. This phenomenon is confirmed by surface analysis results.
The adsorption-desorption analysis utilizing N2 gas molecule with Brunauer-Emmett -Teller (BET) theory allows us to identify the physical properties of the catalyst. In addition, by employing the Barrett-Joyner-Halenda (BJH) method, the pore distribution of the catalyst can be observed. According to the physical sorption analysis shows in Figure 1, all prepared catalyst generated hysteresis loop which indicates the pore sizes are in the range of mesoporous material (identified as type IV adsorption). As shown in Figure 1, the volume of N2 gas adsorbed is decreased after the impregnation of metals(s). The lowest volume of N2 gas adsorbed in aCoOaMoO/γ-Al2O3 was due to the high metal loading that impregnated on the support. As a result, the volume of N2 gas adsorbed will be decreased.
There was quite a decrease in specific surface area in aCoOaMoO/γ-Al2O3 while there was no significant gap in other catalysts. The more metals loading might cause more aggregates in the metal particles that lead to the inhomogeneous deposition on the support or even might block the pore. In addition, the pore volume and average pore diameter are in accordance with the specific surface area which is decreased with the deposition of metal particles. The evaluation of pore radius by BJH method ( Figure 2) shows that without experiencing the impregnation process, pure Al2O3 has the most homogeneous pore distribution indicated by the high and narrow peak [26]. After metal(s) deposition, the homogeneity of the pore radius is slightly changed while maintaining the radius range. The low homogeneity of pore radius was due to the distinct amount of the loaded metals. In this sense, the more metal(s) loaded will tend to decrease the homogeneity of the pores. With that being said, it is important to control the preparation method to generate more homogeneous metal(s) dispersion which is expected to have better catalytic performance.
The impregnated metal(s) species were characterized using X-ray diffractometer. As shown in Figure 3, the XRD pattern of γ-Al2O3 support material indicated by the broad peak at 2θ started around 20° showing its amorphous character. In addition, it was also indicated at 2θ around 45° and 67°. The trend of γ-Al2O3 characteristic peaks was observed in all prepared catalyst denoting that the impregnation of metal(s) did not damage the support materials.
In the monometallic catalyst of aCoO/γ-Al2O3, the characteristics of cobalt species in the form of oxide were present at 2θ around 27°, 38°, 56°, 59°, and 63° (JCPDS no. 00-002-0770). While in the MoO/γ-Al2O3, the molybdenum oxide species were present at 2θ around 37°, 42°, and 60° (JCPDS no. 01-074-4517). On the other hand, in the bimetallic catalysts of aCoOaMoO/γ-Al2O3 and bCoObMoO/ γ-Al2O3, the new peaks correspond to the Co-Mo-oxide species were detected at the similar 2θ around 18°, 25°, 32°, 37°, 56°, and 59° (JCPDS no. 00-021-0869). This shows that the impregnation of bimetallic CoMo was proceeded generating distinct XRD patterns from a monometallic metal impregnation. SEM analysis was conducted to see the morphology of the catalysts. Moreover, the effect of impregnation can be observed by the SEM images of γ-Al2O3 ( Figure 4A) and after impregnation of metals represented by aCoOaMoO/ γ-Al2O3 catalyst ( Figure 4B). Based on the images, the impregnation of metals resulted in a very distinct morphology of the support surface indicating that the metals have successfully deposited on the support material.

Catalytic Activity
The catalytic performance of the prepared catalysts was evaluated on the hydrocracking of the CIO carried in a fixed-bed reactor. CIO as a bio-oil is a thick-dark green liquid. After it   is converted into biofuel by heating and streaming with H2 gas, it turns into the thinner and lighter green to yellowish liquid. As summarized in Table 2, thermal hydrocracking as well as hydrocracking with pure support material were conducted to have a clearer observation. During the hydrocracking process, three fractions of products (liquid, gas, and coke) were generated. A previous reported investigation [28] stated that thermal hydrocracking prefers to produce gases because the reaction is carried out via the formation of radical ions in high temperature without catalyst generating short carbon chains (mostly gases). On the other hand, the catalytic hydrocracking proceeds through the formation of carbo-cations to produce longer carbon chains (liquids). This is aligned with the present investigation. Table 2 summarized the product distribution of the hydrocracking of the CIO. As the prepared catalyst is aimed to produce biofuel (liquid), the more liquid fraction produced is desirable. In this case, all prepared catalyst shows mild to good catalytic performance on hydrocracking of CIO. However, aCoOaMoO/ γ-Al2O3 gave the most satisfying result by producing 65.56 wt% of liquid fraction and selected Copyright © 2020, BCREC, ISSN 1978-2993 , 15 (3), 2020, 747 as the best catalyst in this study with the highest metal content. In this case, the higher liquid product is affected by high metal loading. This is because the presence of catalyst favors to the carbocations formation mechanism that will lead to produce longer carbon chains (liquid). In contrast, proceeding the reaction without a catalyst will favor gas production through the formation of radical ions mechanism that generating a short carbon chain. Moreover, in terms of undesired coke formation, this catalyst also showed the lowest amount of coke with only 0.32 wt%. As shown in Figure 5, there are no significant differences in metal particle size in the SEM images indicating that a very small yield of coke was left on the catalyst. As the formation of coke could deactivate the catalyst and decrease activity, no wonder that aCoOaMoO/γ-Al2O3 catalyst with the smallest coke production gave the best catalytic performance on this reaction.

Bulletin of Chemical Reaction Engineering & Catalysis
Comparing to our previous report [26] on similar work employing Ni and Mo as catalyst based, the catalytic conversion was quite similar. However, the present study was more superior as the metals were set in a lower total loading (10 wt% for monometallic catalyst; 20

The Selectivity of Liquid Product
The generated liquid product was characterized by GC-MS and the compositions were grouped into gasoline fraction (C6-C12) and diesel fraction (>C12). Based on the previous reports [28,29], the retention time for gasoline was <30 minutes while diesel was >30 minutes. The major product of the hydrocracking reaction is summarized in Table 3. According to Table 3, each catalyst shows different compound compositions. In terms of selectivity, the bCoObMoO/γ-Al2O3 catalyst is likely to have better selectivity toward the liquid product composition with 9.11 wt% gasoline and 7.52 wt% diesel compared to aCoOaMoO/γ-Al2O3 catalyst with the highest yield liquid product composed of 8.61 wt% gasoline and 5.01 wt%  diesel. Despite its low production of liquid fraction, the product of hydrocracking process employing γ-Al2O3 without metal species favor long carbon chains. On the other hand, employing metal(s) impregnated on the support favor to the gasoline fraction.

Conclusions
The deposition of metal(s) on γ-Al2O3 was successfully conducted through simple wet impregnation. The prepared monometallic catalysts of aCoO/γ-Al2O3 and aMoO/γ-Al2O3, as well as bimetallic catalysts of aCoOaMoO/γ-Al2O3 and bCoObMoO/γ-Al2O3, were characterized and evaluated in the hydrocracking of CIO. Out of the prepared catalysts, aCoOaMoO/γ-Al2O3 with total metal content, acidity and specific surface area of 13.62 wt%, 5.01 mmol.g -1 , and 107.67 m 2 .g -1 , respectively, showed best catalytic performance by producing 65.56 wt% liquid fraction. This high production of liquid product was due to its high metal content that favors the carbocations formation mechanism that generates longer carbon chains (liquid phase). It was selective to produce 8.61 wt% gasoline and 5.01 wt% diesel.