BIOFUEL PRODUCTION OVER FISCHER-TROPSCH SYNTHESIS: EFFECT OF Fe-Co/meso-HZSM-5 CATALYST WEIGHT ON PRODUCT COMPOSITION AND PROCESS CONVERSION

Fischer-Tropsch Synthesis (FTS) using Fe-Co/meso-HZSM-5 catalyst has been investigated. The impregnated iron and cobalt on HZSM-5 could be used as bifunction catalyst which combined polimerizing synthesis gas and long hydrocarbon cracking for making biofuel (saturated C5–C25 hydrocarbons as gasoline, kerosene and diesel oil). The study emphasized the effect of ca talyst weight on product composition and process conversion. The HZSM-5, had been converted from ammonium ZSM-5 through calcination, and then desilicated with NaOH solution. The Co(NO3)2⋅6H2O and Fe(NO3)3⋅9H2O were used as precursor for in cipient wetness impregnation (IWI) on amorphous meso-HZSM-5. The catalyst consisted of 10 % Fe and 90 % Co by weight, called 10Fe-90Co/meso-HZSM-5. All catalysts were reduced in situ in the continuous reactor with flowing hydrogen at 25 mL/min, 1 bar, 400 °C for 10 hours. The catalyst performance was observed in the same continuous fixed bed reactor at 25 mL/min synthesis gas (30 % CO, 60 % H2, 10 % N2), 250 °C, 20 bar for 96 hours. Various catalyst weight (1, 1.2, 1.4, 1.6 gram) were applied in FTS. The desilicated HZSM-5 properties (BET analysis) were 6.1–29.9 nm mesoporous diameter, 0.3496 cc/g average mesoporous volume, 526.035 cc/g pore surface area, and the EDX analysis gave 22.1059 Si/Al ratio and 16.11 % loading (by weight) on meso-HZSM-5. The reduced catalyst showed the XRD spectra of Fe (66°), Fe-Co alloy (44.50°) and Co3O4 (36.80°). The reaction using 1 gram of 10Fe-90Co/meso-HZSM-5 catalyst produced the largest composition and conversion. The 1 gram catalyst gave the largest normal selectivity of gasoline (19.15 %) and kerosene (55.18 %). While the largest normal diesel oil selectivity (24.17 %) was obtained from 1.4 gram of catalyst. The CO conversion per gram of catalyst showed similar value (CO conversion of 26–28 %) for all catalyst weight.


Introduction
Zeolites are used in the process of separation, adsorption, and catalytic (i. e. cracking and other hydrotreating process). Homogeneous micropores, high thermal stability, acidity, and unique ability to stabilize metallic species were several their unique properties [1]. The mesoporous structure provides greater space for metal catalysts to occupy more support pores. The mesopore can be obtained from micropore over desilication treatment. The alkali (NaOH) solution was used as desilication agent. Nitric acid solution was used to transform the mesoporous HZSM-5 into amorphous structure, which could improve the adhesion of the metal catalyst on the mesoporous HZSM-5 support [2][3][4][5]. The KOH solution induced greater mesoporous volumes, but it damaged on zeolite crystal structure. The NaOH solutions were able to increased mesopore surface area and mesopore volume of NaOH-desilicated HZSM-5. The Fe-Co alloys were observed with less oxide of iron (Fe 2 O 3 ) as well cobalt (Co 3 O 4 ) in the produced catalysts [6]. The Fischer-Tropsch synthesis (FTS) is an exothermic and irreversible polymerization reaction that converts a mixture of hydrogen (H 2 ) and carbon monoxide (CO) into linear hydrocarbons as liquid fuel. FTS always produces various products such as olefins, paraffins and oxygenated compounds (alcohols, aldehydes, acids and ketones). Water was the main product of the FT reaction. Water Gas Shift (WGS) activity occured significantly in the use of Fe catalyst, but this reaction could be ignored in reactions using Co or Ru catalysts. Iron and cobalt as polymerization catalyst were used at FTS. The impregnated iron and cobalt on HZSM-5 could be used as bifunction catalyst which combined polymerizing synthesis gas and long chain hydrocarbon cracking for making biofuel [3,[7][8][9][10]. The FTS preferred to use cobalt because of some advantages, instead of iron catalyst. Iron catalyst was added in little amount to improve the cobalt catalyst performance (improve olefin selectivity, reduce methane selectivity, reduce temperature and pressure sensitivity to product distribution and reduce the catalyst price) [11,12].
The combination of the Fischer-Tropsch (FT) reaction and cracking was conducted by combining Fe-Co alloy (Fischer-Tropsch reaction catalyst) and HZSM-5 zeolite (cracking catalyst). The merging of the FTS and cracking was able to produce biofuel product directly [7]. The Fe/HZSM-5 catalyst was able to produce 24.2 % of C 5-19 hydrocarbon as biofuel fraction [8]. The Co/HZSM-5 catalyst produced 48 % of C 5-11 [3]. The use of Fe and Co catalysts together (bimetallic) could increase catalytic activity compared to the use single metal catalyst (monometallic). The combination of Fe and Co catalysts (bimetal catalysts) with conventional TiO 2 , Al 2 O 3 and SiO 2 supports had been investigated. They could increase the biofuel reaction conversion and fraction compared to the use of Fe and Co catalysts separately [13][14][15]. The variables that influenced product distribution were temperature, feed gas composition, pressure, catalyst type and promoter. The synthesis gas composition influenced the product selectivity. The lower the CO partial pressure, the lower the surface coverage of CH 2 radicals, the lower the chain growth chance and the higher the chance of CH 2 radicals desorption. The higher the H 2 partial pressure, increases the chance of radicals termination being paraffin. Increasing H 2 /CO ratio would increase the selectivity of short-chain and more saturated hydrocarbons [16][17][18].
The study emphasized the effect of Fe-Co/mesoHZSM5 catalyst weight on product composition and conversion. The data obtained from this study will be used to develop efficiency of the Fischer-Tropsch reaction condition.
The 10Fe-90Co/meso-HZSM-5 catalyst was prepared from ammonium salt of ZSM-5 over calcination, desilication, dispersion, acid treatment [6]. The solution of Co(NO 3 ) 2 ·6H 2 O and Fe(NO 3 ) 3 ·9H 2 O were used as precursor for incipient wetness impregnation (IWI) process on amorphous meso-HZSM-5. Loading metal catalyst was 10 %. The catalyst consisted of 10 % Fe (by weight) and 90 % Co (by weight). The impregnated HZSM-5 was kept in desiccator at room temperature for 24 hours, dried in oven at 120 °C for 12 hours and then calcined at 550 °C for 2 hours. The catalyst performance was observed in a continuous fixed bed reactor ( Fig. 1) using 10Fe-90Co/meso-HZSM-5 catalyst, 25 mL/min synthesis gas (30 % CO, 60 % H 2 , 10 % N 2 ) at 250 °C, 20 bar. Various catalyst weight (1; 1.2; 1.4; 1.6 gram) were used in the Fischer-Tropsch reaction. The catalyst was mixed with glass beads as much as 6 times of the catalyst weight. The mixture of catalyst and glass beads was inserted into the catalyst bowl which was held with glass wool at the bottom and top of the catalyst to prevent the spilling out of the catalyst during process [19][20][21][22]. All catalysts were reduced in situ in the reactor with H 2 gas flowing at 25 mL/min, 1 bar, 400 °C for 10 hours. At the end of the reduction procedure, the reactor was cooled to 250 °C and the hydrogen gas flow was stopped, then it was replaced by the synthesis gas flow. The modified pressure regulator was installed after the reactor to maintain 20 bar pressure. The flow rate was calibrated using bubble soap meter at 25 mL/min at the operating pressure and temperature. Some hydrocarbon fractions condensed through hot trap at 30-40 °C and 20 bar. The low boiling point hydrocarbon fraction would be condensed in a cold trap (1-10 °C, 1 bar). The reaction was carried out for 96 hours with gas sampling every 24 hours and liquid sampling at the end of the whole reaction. Gas samples were taken using a 10 mL syringe and injected into 10 mL vacuum bottle (vacutainer).

Result and discussion 1. Characterization of Catalyst Fe-Co/meso-HZSM-5
Ammonium ZSM-5 (Si/Al = 40) was converted to microporous HZSM-5 through calcination process in the tube furnace reactor. The formation of mesoporous structures was obtained through the desilication process in which silicon was partly-removed from zeolite framework using NaOH as desilication agent. The desilicated HZSM-5 properties obtained from BET analysis were 6.1-29.9 nm mesoporous diameter, 0.3496 cc/g average mesoporous volume, 526.035 cc/g pore surface area, and from EDX analysis gave 22.1059 Si/Al ratio.
The Fe and Co catalysts loading was 16.11 % weight on mesoporous HZSM-5. The loading of catalyst was calculated from EDX analyzing. The impregnated mesoporous HZSM-5 was reduced to obtain metal catalyst on the active surface. The XRD analysis was used to observe the reduction results. The diffraction angle position of the metal peaks referred to the spectral Tavasoli's database [23]. The spectra of Fe (66°), Fe-Co alloy (44.50°) and Co 3 O 4 (36.80°) were shown in Fig. 2.   Fig. 2. The XRD Spectra for Fe-Co/HZSM-5 catalyst consisted of 10 % Fe and 90 % Co after reduction (with H 2 gas flowing at 25 mL/min, 1 bar, 400 °C for 10 hours)

2. Effect of catalyst weight to product composition
The FTS reaction produced two layers liquid product, each of which has a pungent odor. The two layer liquid products were taken from hot trap chamber (30-40 °C). Both of layers had similar viscosity. The upper layer ass clear yellow, while the lower layer was colorless. From GCMS analysis, the upper layer contained alkanes (paraffin) and less alkene (olefin). Carboxylic acids, alcohols, esters and aromatic compounds were found in the lower layer products. The increasing catalyst weight gave the higher alkane selectivity. One gram of catalyst obtained 85.04 % alkanes and 7.56 % alkenes. The alkane composition rose to 96.33 % on 1.2 grams of catalyst and increased again to 99.96 % on 1.4 and 1.6 grams of catalyst. Straight chain hydrocarbon selectivity was quite large (57-72 %). The 1.6 gram catalyst gave the greatest selectivity (72.03 %) when compared to 57.14 % on 1 gram catalyst. The more catalysts used, the greater active surface involved in the reaction so that the catalyst's performance was better. The reactants contained double H 2 than CO (H 2 /CO ratio = 2). The greater active surface area increased the concentration of hydrogen free radicals, so the reaction mechanism lead to the formation of paraffins and carboxylic acids.
The combination of polymerization using metal catalyst (Fe-Co) and cracking (meso-HZSM-5) in the FTS intended to produce saturated C 5 -C 19 hydrocarbons (gasoline, kerosene and diesel oil). The product distribution based on the hydrocarbon chain length in the both layers could be seen in Fig. 3, 4. The upper layer was dominated by alkane hydrocarbons with C 7 -C 25 hydrocarbon chain length (Fig. 3). The use of 1 and 1.2 gram catalysts produced C 7 -C 19 hydrocarbon while the more catalyst weight produced less C 20 -C 25 hydrocarbon (below 1.7 %). The 1 and 1.2 gram catalyst might form C 20 -C 25 in such a small amount, so that GCMS could not detect this traces. Increasing catalyst weight would rise the reaction chance on the catalyst active surface so that the more product was obtained (higher conversion) and the presence of C 20 -C 25 hydrocarbon could be measured using GCMS. The FT reaction using 10Fe-90Co/meso-HZSM-5 catalyst at 250 °C and 1 bar produced gasoline, kerosene and diesel oil. Kerosene was the largest product (55.18-61.68 %), was obtained from the catalyst weight of 1-1.6 gram. The largest gasoline fraction (19.15 %) was obtained from 1 gram of catalyst and decreased with increasing catalyst weight. The diesel oil fraction increased with the catalyst weight. The largest solar fraction (19.34 %) was obtained from 1.6 gram of catalyst. The greater the catalyst weight, the greater the diesel fuel produced and the smaller the gasoline produced. The lower layer showed the FTS side reaction, that produced alcohol, carboxylic acid, ester, aromatic and cycloalkane compounds (Fig. 4). The carboxylic acid was formed at any weight variation. The alcohol was formed in large amount at 1.2 and 1.4 gram catalyst. This condition could be used to alternative alcohol production from synthesis gas with this catalyst. The more 10Fe-90Co/meso-HZSM-5 catalyst, which used in the FT reaction, would have the greater active surface area. They might increase the reaction chances that increased CO con- Chemical Engineering version (Fig. 5). The larger active surfaces would increase CO dissociation which initiated the polymerization process. The CO conversion on 1 gram of catalyst was 27.33 % which increased to 42.46 % on the use of 1.6 grams of catalyst. The residual gas consisted of CO, H 2 , CO 2 and CH 4 were measured by GC-TCD (Fig. 6). The decreasing CO fraction in the residual gas showed that CO consumption increased with more catalysts being used. The formation of a relatively small CO 2 indicated that the WGS reaction was not dominant in the condition and catalyst used. The greater Co catalyst (90 %) in the bimetal Fe-Co did not push WGS reaction. But with increasing catalyst weight, the Fe weight was also larger, so that more CO 2 gas formation (1-2 %) was obtained in the use of catalysts 1.2-1.6 gram. How ever, the larger Co in the Fe-Co would drive the methane formation. The analysis showed that methane was formed in all catalyst weight insignificantly. In the 1 gram of catalyst, only 1 % methane was formed, while the use of more catalysts produced 5-6 % methane. The bimetal combination on supported mesoporous HZSM-5 reduced methane formation significantly compared to previous studies [3,15,24], which produced 7.9-21 % methane.

Normalization of Selectivity and Conversion
Normalization of liquid products selectivity and reaction conversion to catalyst weight was conducted to determine the efficiency and effectiveness of the catalysts use. The product selectivity and conversion were normalized by the catalyst weight that was varied in this study. This norma lization data could be used to take better decision in the catalyst efficiency and effectiveness before commercializing.
Normalization of biofuel selectivity (gasoline, kerosene and diesel) to the 10Fe-90Co/meso-HZSM-5 catalyst weight in the FT reaction of 250 °C, 20 bar using synthesis gas (30 % CO, 60 % H 2 , 10 % N 2 ) for 96 hours was shown in Fig. 7. The 1 gram catalyst weight gave the largest normal selectivity to gasoline (19.15 %) and kerosene (55.18 %). While the largest normal diesel oil selectivity (24.17 %) was obtained from the use of 1.4 gram of catalyst weight.
Normalization of CO reaction conversion to the 10Fe-90Co/meso-HZSM-5 catalyst weight in the 250 °C, 20 bar FT reaction using synthesis gas (30 % CO, 60 % H 2 , 10 % N 2 ) for 96 hours was shown in Fig. 8. The resulting normal conversion showed similar value for all catalyst in the range of 0.26-0.28. All catalyst weight variation showed similar normal conversion over 96 hours, so the use of 1 gram of catalyst was more economically beneficial. These results were in line with the normal selectivity of gasoline and kerosene products that reached the highest value on the use of 1 gram of catalyst.   nitrogen with each rate controller could be used to reduce raw material price. Coal or biomass that have the lowest price raw material, could be interesting for the next research. The catalyst development need further research to improve the reaction conversion in simple reactor and mild operation condition (temperature, pressure and time). Continuously data will give better reaction kinetics analysis for gaining best condition. The residual gas analysis as conversion indicator need to be measured continuously with integrated GC-TCD, instead of separately one. Gas sensor with Arduino program could be use as alternative to collect large data every seconds or minutes. The carbon monoxide sensor (MQ-7), carbon dioxide sensor (MG-811) and methane sensor (MQ-4) should be developed and integrated with Fischer-Tropsch reactor system.
The Fischer-Tropsch reaction using 1 gram of 10Fe-90Co/meso-HZSM-5 catalyst and synthesis gas (30 % CO, 60 % H 2 , 10 % N 2 ) at 250 °C, 20 bar, 96 hours in the continuous fixed-bed reactor, showed the largest composition and conversion. The combination of polymerization using metal catalyst (Fe-Co) and cracking (meso-HZSM-5) was intended to produce saturated C 5 -C 25 hydrocarbons (gasoline, kerosene and diesel oil). The 1 gram catalyst weight gave the largest normal selectivity to gasoline (19.15 %) and kerosene (55.18 %). While the largest normal diesel oil selectivity (24.17 %) was obtained from 1.4 gram of catalyst. The CO conversion per gram of catalyst showed similar value (26-28 %) for all catalyst weight.