Direct Hydrogenation and Hydrotreating of Neat Vegetal Oil into Renewable Diesel Using Alumina Binder with Zeolite

Hydrodeoxygenation of vegetal oil and methyl oleate studied using Pt and Pd supported with alumina-beta-zeolite (γ-Al2O3-H-β) and comparing its performance with NiMo/γ-Al2O3 and Pd/C in a semi-batch reactor, at 300 340 C and 2.0 MPa. This study has used Pd and Pt on H-β zeolites with alumina binder as extrudate and it examines over triglycerides hydrogenolysis. Pd/C showed higher rate of deoxygenation and the selectivity of C18/C17 was higher for NiMo-S/γ-Al2O3 and PdPt/γ-Al2O3-H-β. PdPt/γ-Al2O3-H-β enhanced the hydrogenation and hydrogenolysis, while conversion and yield in deoxygenation and decomposition has reached identicical performance as NiMo-S/γ-Al2O3 catalytic deoxygenation process. Both the thermal decomposition and the hydrogenation of unsaturated triglycerides were mainly affected by H2 partial pressure. Transition metals like Pd and Pt on H-β and alumina binder ratio of 40:60 wt% showed effective hydrodeoxygenation of vegetal oil without substituting sulfur feed.


Introduction
In the present scenario, few refineries are involved in producing biomass based diesel-like fuel, named as renewable diesel, to replace the conventional one [1][2][3][4][5]. In the last two decades, raw vegetal oils, animal fats and algal oils had been converted into renewable diesel by catalytic hydrotreatment techniques [5][6][7][8][9][10][11][12][13][14][15]. With that, three main methods, such as hydrogenolysis, catalytic cracking and thermal cracking were deeply investigated. Hydrogenolysis produces long straight-chain hydrocarbon with a high cetane index, while cracking produces straight-chain hydrocarbon with by-products of lighter hydrocarbons and aromatics which were in the range of gasoline and fuel gas [16].
High catalytic activity, high turnover frequency of active sites, chemical stability and a known quantity of catalysts requirement are mainly successful with the use of sulfiding transition metals. Here, substituting sulphur in feed for co-processing diesel and fatty acids makes additional operating cost [22,23]. However, sulphur on mono-or bi-metallic transition metals on the support metal-phosphide and metal-carbide were extensively investigated in hydroprocessing conditions [21,25]. https://doi.org/10.37358/RC.20.9.8321

Catalyst activation
The NiMo/Al2O3 contains 2-3 % Ni, 12-14 % Mo and 83-86% γ-Al2O3 support, extrudes at the sieve range of 10-30 DIN (aperture ~ 0.2-0.6 mm), for investigation of both sulphide and sulphide-free form of hydroprocessing catalyst. With reference to the previous studies, the sulphide form of NiMo/γ-Al2O3 (NiMo-S), calcined at 400 o C, and in-situ sulphidation was carried out with 10% by volume of S in H2S at 200 mL/min, for 3 h, in the trickle-bed reactor [6,11] and a sulphide-free form of NiMo/γ-Al2O3 (NiMo) carried in-situ activated in H2, at 400 o C. Similarly, the Pd/C was activated with H2 after drying in an oven at 110 o C for 4 h. The catalysts were kept under an N2 atmosphere to prevent from oxidation.
H-β and disperal (boehmite) were blended in a ratio of 40:60 wt%, and the hydrated blend was extruded in 1/32" and dried at 110 o C, for 5 h. After calcination at 300 o C for 4 h, 0.5 wt% of Pd and 0.5 wt% of Pt with respect to weight of support material (dry basis) are loaded on the extruder by wet impregnation method using Pt(NO3)2.2H2O and Pd(NO3)2 solution. Finally, the zeolite loaded with Pd and Pt (Pt-Pd/ γ-Al2O3-H-β) was calcined at 400 o C for 6 h. The Pt-Pd/ γ-Al2O3-H-β was crushed and sieved in 10-30 DIN, and H2 in-situ activation performed as like Pd/C and NiMo catalysts.

Reactor
The semi-batch reactor consists of a stainless steel vessel of 300 mL capacity. The reactor was connected with both inlet and outlet gas line. It consists of an eight-microprocessor based process controller with an integrated part to the thermocouple, heating mantle and magnetic stirrer [6]. After the catalyst activation, 1 g of catalyst taken in the vessel with a 150 mL solution containing 20 wt% of SVO in n-dodecane and 3 g of n-tetradecane as an internal standard [6]. The experiments were carried out from 320 and 340°C at 2 MPa, with the same stirring speed. Sampling point was connected at a gas outlet connection and an average of 1 mL of liquid sample (OLP) was extracted and stored in the 6 o C cooler for analysis.

Physico-chemical characterization
The specific area and pore volume of the samples were analysed using Tristar 3000 automated Gas Adsorption Analyzer, measured by N2 physisorption isotherms at -195 o C. Before the analysis, 1 g sample of catalyst was thermally dried in a vacuum chamber, at 90 o C for 4 h, in N2 gas flow and used for specific surface area and pore-volume measurements.
The spent catalyst was refined for 22 h to extract the solvent, using m-xylene in the Soxhlet apparatus. The refined spent catalyst was dried in a vacuum dryer at 125°C under N2, followed by the analysis of coke deposition by BELAB AB, Sweden, using an ASTM D5291 standard test method (Leco CHN-600 instrument).
Simple distillation of 500 mL of OLP sample distilled under N2 atmosphere (100 mL/min), at two desirable cut-off temperatures, i.e. 100 o C to remove water molecules in distillate and 200 o C to remove n-dodecane and lighter hydrocarbons in distillate and rest as residue. This distillation was performed to identify the water molecules formation and elemental analysis of OLP residues.
The Spectrum One from Perkin-Elmer Fourier Transform Infrared Spectroscopy (FTIR) was used to analyses OLP samples at room temperature. OLP sample is placed between two high purity NaCl salt plates (pair of demountable cell in 4 mm thickness) with care for ensuring that no air bubbles appeared and the reading noted for mid-infrared ranges from 4000 cm -1 to 400 cm -1 [7].
Based on simulated distillation methods of ASTM D2887 standard, OLP samples (5 μL) has injected in Varian 3400 Gas chromatography (GC), equipped with a packed column (10% silicon OV-101, 80-100 mesh, 1 m 1/8" 2.00 mm) and a Flame Ionization Detector (FID) and Varian 4279 integrator are used for data computing [7]. With the injector and detector temperature of 300 o C, the column oven temperature raised from 40 to 325 o C, at a heating ramp of 18 o C/min and the final temperature has been maintained for 10 min.
The outlet gas from the reactor was analysed using the Clarus 500 GC online, connected with 600 link switch controllers, with the signal integrated to receive data. Gas analyser consists of inlet and outlet https://doi.org/10.37358/RC.20.9.8321 sampling valves, actuated by N2 gas on 0.4 MPa, which analyses CO, CO2, CH4 and H2 at Thermal Conductivity Detector (TCD) and hydrocarbons in FID [6,7,11]. Elemental analyses in OLP such as C, H and O under ASTM D5291 standard test method are performed at Karlshamn Kraft AB, Sweden. The traces of S, metals and salts are exempted in the analytical test [7].
Bruker 600 NMR spectrometer, containing a 5 mm diffusion probe, was used to analyse under 300 MHz for 1 H. Pretreatment of SVO carried in vortex mix of 0.2 g in 400 microliter of deuterated chloroform, and the sample has placed in a 5-mm diameter ultra-precision.

Results and discussions
The experimental investigation has been performed for Pd/C, bi-metallic catalysts like NiMo on alumina and PtPd on the alumina-zeolite. The pore size of the investigated catalysts, as shown in Figure 1 and Table 1, appears at the range between 50 Ǻ and 80 Ǻ. Mainly, NiMo has an average adsorption pore diameter of 75.65 Ǻ and Pt-Pd/ γ-Al2O3-H-β had 54.24 Ǻ. Pd/C had an average adsorption pore diameter of 51.03 Ǻ. However, Pd/C in HDO might have a DCA/DCO mechanism with intermediates of oleate or stearaldehyde, and HDO can be enhanced with active sites using a fresh H2 supply. But, coke deposition over active sites can harm the activity and performance of catalyst life-span. On this NiMo regeneration ability, coke deposition over catalyst can deteriorate pore mouth shrinking and decreases the surface area. The spent NiMo-S average adsorption pore diameter was 64.2 Ǻ, and the surface area was 54.5 m 2 /g, whereas fresh NiMo-S had 157.6 m 2 /g. The trend of N2 absorption and desorption appeared as Langmuir type IV isotherms for both fresh ( Figure 1) and spent catalyst, but the quantity of adsorption diminished with the coke deposition. The total pore volume for adsorption of fresh and spent catalyst reduced from 0.34 cm 3 /g to 0.12 cm 3 /g. Hence, the coke deposits might be involved in the rupture of S-metal and further formation of hydrogen sulphide had been reduced to a fresh supply of sulphur and hydrogen for the active sites. From the literature, it is evident that the moderate pore size of NiMo-S, NiMo and Pt-Pd/ γ-Al2O3-H-β catalysts can control the secondary cracking [29]. Langmuir adsorption isotherms is used to investigate different materials are indicated in Figure 1, in which the hydroprocessing study of Pt-Pd/ γ-Al2O3-H-β reduces the pore diameter of 69.6 Å to 44.3 Å. The result of lower coke deposition in Pt-Pd/ γ-Al2O3-H-β was an indication of a lower rate of deactivation and also, the selectivey and yield indicator of the HDO can be used as the study for the catalyst performance.

Oxygenated products
The study examined over triglycerides hydrogenation and HDO using NiMo, NiMo-S, Pd/C and Pt-Pd/γ-Al2O3-H-β, which are carried out at 320 o C and 2.0 MPa, either in the absence or presence of H2. As there are different materials with different shapes and structures in these catalyst studies, it is required to identify the minimum mass transfer resistance [32]. So, the effect of mass transfer resistance on the external catalyst surface can be determined by performing different temperatures and sterrier speeds. The conversion rate for 3 h run in batch reactor, for 20 wt% vegetal oil (20-SVO) in n-dodecane solvent of 320 o C and 2.0 MPa shows identical product formation at 650 rpm to 1000 rpm. The sterrier speed of 700 rpm is used for all the test-run with understanding of negligeble external diffusion, which has not influence the rate-controlling step. Also, under similar condition, hydrogenolysis of SVO is investigated. In this examining, for 10 o C raise in operating temperature (between 300 and 35 o C), it tends to double the HDO conversion rate. This indicates the negeligible pore diffusing effect over the catalyst surface [31,32]. The formation of oxygenated compounds, such as esters, carboxylic acids, aldehydes or ketones in OLP, appeared by the dissociation of -C-Oin glycerides and -C-O-(CO)into free fatty acid/aldehydes and glycerol [15][16][17][18][19][20].   (Figures 2 and 3) observes that the aldehydes and alcohol formation shows an increase in concentration, at a wavelength between 2800 cm -1 and 3100 cm -1 , which mainly shows the CHX level of the fatty ester group by HDO. A wavelength of about 3200 cm -1 -3400 cm -1 shows the presence of carboxyl acids (-(C=O)-OH) or alcohol (O-H) or water molecules. The wavelength around 2731 cm -1 shows the formation of aldehyde or ketone group and a peak at 1713 cm -1 -1715 cm -1 wavelength shows the presence of carboxyl acids or carbonyl compounds, represented in C=O stretching. The wavelength peak at 1748 cm -1 -1742 cm -1 in catalytic hydroprocessing indicates the formation of esters and a peak at 920 cm -1 indicated the appearance of the aromatic and dimer group. Here, the radicals of carboxylic esters could form dimer compounds and it can be confirmed at the wavelength of 1280 cm -1 and an increase in C=C stretch at 1650 cm -1 , as observed in Figure 3. The dimers from the polymerisation of the carboxyl group show high concentration intensity for Pd/C, but other catalysts show only traceable amount. The band identified functional groups are listed in Table 2 based on using studies carried by Tegou et al [33]. To understand thermal cracking in the catalytic hydrotreating technology, a non-catalytic thermal treatment needs to be carried out. Thermal cracking of 20-SVO triglycerides at 320 o C and 2.0 MPa in N2 atmosphere is investigated and table 2 represents the identified major spectroscopy peak. The wavelength of 1710 cm -1 indicates the carbonyl structure, either -(CO)H or -(CO)-. Also, the wavelength of 2730 cm -1 implies the aldehydes or ketones, which show a broader spectrum indication. These broadband wavelengths of 2730 cm -1 and 1710 cm -1 show the formation of carboxyl acids that was due to the α-carbon bond dissociation from the propane group. The FTIR result shows that wavelength from 3010 cm -1 to 2800 cm -1 has a broad intensity as noticed in Figure 3. This more widespread intensity denotes the presence of a high concentration of glycerol or hydroxyl group, because of CHx scission between α-and β-carbon in glyceride ester. The C-O bond of glyceride breaks at low enthalpy by leaving aldehyde or acid, or by leaving the both. In the Pd/C hydrotreatment, the intensity in the OLP sample at 1595 cm -1 and 1578 cm -1 indicates aromatic/cyclic compounds. These arene compounds formation from the coupling of α-carbon radical and carbonyl group are either formed from ester or propane dissociation in glycerides. The aromatic/cyclic compounds, as indicated by the wavelength of 1032 cm -1 and 701 cm -1 , confirmed the presence of β-hydroxyl carbonyl compounds. Table 2 reports the band assignment in OLP and the carboxylic and carbonyl compounds for all the tested catalysts that had resembled in HDO product formation and also its reaction route. Table 2 and Figure 3 show the presence of trans-and cis-hydrocarbon chain. PdPt//γ-Al2O3-H-β has the strong and moderate intensity of trans-and cis-hydrocarbon, NiMo-S has only strong intensity at 966 cm -1 indicating trans-alkenes and Pd/C has strong intensity at 665 cm -1 reporting cis-alkenes, whereas thermal cracking of trans-alkenes are observed on strong intensity at 966 cm -1 , however, in the presence of H2 at thermal cracking induces in cis-alkenes formation.
It is found that the products from thermal treatment initiate bond breaking on esters. The breakage of the -C-O-bond, either on the glycerol or ester chain can be expected. Primarily, the thermal vibration over the C-C bonding initiates the decomposition of the saturated structure rather the unsaturated group. So, C=O and C=C has the stability on high bond energy [34] and inhibits secondary cracking on the saturated group; this implies that the catalytic activity can induce the carbonyl or pi-bonding at the active sites. The α-carbon had weak stability and had low bonding energy, when compared to the β-carbon with its adjacent, so β-carbon can withstand the bond breaking to its adjacent of α-carbon and γ-carbon compounds, as also mentioned in [35]. Thus, the expected bond break in α-carbon can produce radicals which can saturate its valence using abstraction of -H atom from near-by carbon or in the hydrocarbon chain. This abstraction induces the unsaturation in the hydrocarbon between C3 and C15 carbon chains, and this can also initiate aromatic or cyclic compound formation.

Hydrogenation and Hydrogenolysis
In H2 atmosphere, pure SVO and 20-SVO in a semi-batch reactor are tested at 320 o C and 2.0 MPa by NiMo-S, Pd/C and PdPt//γ-Al2O3-H-β catalysts. Here, the sampling of SVO hydroprocessed has tested with 1 H-NMR, which indicated the formation of hydrocarbons of C17 and C18 in hydroprocessing. This can be noticed in Figure 4. The peak identified 1 H-NMR at 2.5-3 f1(ppm) for glycerides are disappeared in the OLP. It is revealed that the hydrogenation of the triglycerides formed, initially. Dissociation of carboxyl compounds initiates, and further the formation of saturated C17 and C18 hydrocarbons can be proved as the desired products of glycerides hydrogenolysis. The composition of carboxylic compounds in the intermediates using Pd/C and Pt-Pd/γ-Al2O3-H-β catalytic process shows a side reaction in the formation of esters, carbonyl, carboxylic and lighter hydrocarbons (less than C8) in the OLP, as shown in the Figure 5 and 6. But, SVO HDO by NiMo-S shows the highest yield in obtaining C17 and C18 hydrocarbons with minor quantity of carbonyl compounds. Product selectivity is reduced due to a low partial pressure of H2 in the involvement of -H protonation in the deoxygenation mechanism. Metal loaded on the catalyst was mainly beneficial for proton donor of DCA/DCO. While substituting of -H donor on HDO for carbonyl compounds results in the formation of water molecules and hydrogenolysis compounds such as (both saturated and unsaturated) C18. The abstraction of -H from hydrocarbon gives additional protonation on metal sites [7,11]. But, lower electrophilicity on Ni metal over Mo, results in favour of HDO rather than direct scission of (C=O)-O and C=O bonds. Further sulphides improved the deoxygenation by metal effects, https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 71 (9) like Ni dispersion, and they gains direct deoxygenation with high selective compounds in hydroprocessing. In using PtPd/H-β, the formation of both straightchain and branched C17 and C18 from hydrogenating carbonyl compounds are noticed. The deoxygenation associated with branching of hydrocarbon can be expected for hydroprocessing with the support of zeolite. Similarly, this branching can confirm with the higher unsaturated gases hydrocarbon such as ƩC3, ƩC4 and ƩC5 mentioned in the Figure 6. However sulfur-free catalyst of Pt-Pd/γ-Al2O3-H-β has weak acidity to undergo for deoxygenation with proton donor of -H atoms and the role on surface area and specific hydrogenation metals induces hydrogenolysis. This hydrogenation and hydrogenolysis are the primary reaction mechanism involved in hydrotreating and, collectively, this catalytic and thermal hydrotreating undergones hydroprocessing in the production of renewable diesel.

Catalytic hydroprocessing of triglycerides
The hydroprocessing of the triglycerides with saturated and unsaturated C18 compounds examined at temperature 320 o C and 2.0 MPa represented in Figure 5. The notable results in Figure 5 shows that the formation of intermediate such as carboxylic acid (C17-COOH), ketones and aldehydes (C17-CO-). So, this intermediate produces C17 and C18 saturated hydrocarbons. Figure 5b indicates the actual product distribution in OLP for Pd/C hydroprocessing. The decomposition of C17-COOH or C17-CHO formed C16 and C17 in the OLP with partial hydrogenation and, this by-product can produce CO, CO2, methane (C1) and ethane/ene in the gas outlet. The total concentration of CO, CO2, C1 and ethane/ene have produced 20, 13.5 and 14 mol% at gas outlet through decomposition mechanism in NiMo-S, Pd/C and Pt-Pd/γ-Al2O3-H-β, has been observed in Figure 6a. Also, the same condition at MO has produced total concentration of CO, CO2 and ethane/ene are 2.44 mol%, 2.26 mol% and 2.27 mol%, has been observed in Figure 6b. These results confirm the C-C cracking of α-and β-carbon at fatty acids. The CO and CO2 in DCO and DCA mechanism evolved from hydrotreating of glycerides and it can also reduce the cetane value of diesel [36]. The HDO of NiMo-S, Pd/C and Pt-Pd/γ-Al2O3-H-β have produced propane/ene from SVO, as the compositions of gases 8.2, 7.1 mol% and 8.1 mol%, while methane from MO gives the compositions of 7.0, 6.2 mol% and 7.1 mol%. The hydrogenation of CO and CO2 into C1 might happen in fuel gas of hydrotreatment. Using Pt-Pd/γ-Al2O3-H-β, the hydrogenation of CO and CO2 to C1 can attain hydrogenation at a low temperature over lower thermodynamically equilibrium condition. This driving mechanism might also enable the hydrogenation of CH3-COOH and CH3-CHO into ethane/ene. However, the selectivity of C17/C18 mole ratio from triglycerides through both NiMo-S and Pd/C relatively appears between 0.7 and 0.85; Pt-Pd/γ-Al2O3-H-β can produce 0.95 as indicated in the Figure  5d. Similar comparison of results on the mol ratio selectivity of carboxylic to carbonyl hydrocarbon indicates the NiMo-S and Pd/C below 0.2, but Pt-Pd/γ-Al2O3-H-β shows nearly 1.1. The DCO and DCA mechanism in hydroprocessing produces C17, whereas the rate of this mechanism on NiMo-S [7,11] and Pt-Pd/γ-Al2O3-H-β may reduce the moderate temperature. This result confirms the formation of free fatty acids as intermediate and has a higher rate of C18 formed over sulphur-free zeolite. In the case of carbonyl group from glycerides, it induces higher decomposition to form C17, CO and CO2, thus the selectivity of C17/C18 had lower value for NiMo-S and Pd/C. Especially, Pd/C has a higher rate of DCA and DCO reaction, which form low C18 hydrocarbons. With a lower selectivity ratio of C17 and C18, the cetane value can be enhanced in renewable diesel. Figure 7a points out that the rate of disappearing of triglycerides was higher for Pd/C than other catalysts. But, its product selectivity is 20 to 30% lower than NiMo-S and Pt-Pd/γ-Al2O3-H-β, respectively, and this value can be observed in Figure 7b and Table  3. As compared to NiMo-S, Pt-Pd/γ-Al2O3-H-β has received 7% to 10% lower product selectivity in OLP, which indicates the branching of hydrocarbon tend to secondary cracking to form smaller fragment hydrocarbon. This hydrocarbon may not produce oligomeric products at the α-carbon of any hydrocarbon chain, for both OLP of 20-SVO and MO. Hence, in disproportion mechanism, product hydrocarbon contain branches have been suitable for diesel compounds if its carbon chain within in its composition and boiling point ranges has became an advantage. So, as indicate in Table 3, it indicates that the OLP have the formation of isomeric compounds using Pt-Pd/γ-Al2O3-H-β which is beneficial for fuel properties. Also, the low concentration of cyclic and aromatic compounds noted in zeolite HDO. But, the hydroprocessing of oxygenated hydrocarbon might need higher residence time for the complete hydration. So, it is necessary to have complete hydroprocessing with smaller flow velocity of direct SVO in continuous process. Initially, the hydroprocessing of SVO in the presence of H2 has involved in the hydrogenation, rather than the effect of hydration, which happens if the saturation effectively consumes H2. So, the rate of deoxygenation slows down due to weak -H proton density on the surface of active sites or inadequate H2 supply through the gas film. However, the presence of -H proton on transition metals can suppress the dehydrogenation mechanism. So, suppressed dehydrogenation can critically affect the initiation of https://doi.org/10.37358/RC.20.9.8321 branching of the carbenium ions. But, in this study, the -H proton deficiency enhances the dehydrogenation and increases the carbenium formation. So, this mechanistic rate of hydrogenation and dehydrogenation can increase the carbocation and secondary cracking. In addition to partial hydrogenation, cracked hydrocarbon can consume H2. In this case of reduced -H proton for longer residence time, the decrease in the slope of selectivity can be identified due to the catalyst proton deficiency, which is observed in Figure 7b. Hence, it increases the rate of dissociation, either the dehydrogenation from alcohol or DCO of carbonyl group into C17 and CO or CO2.

Conclusions
This study reveals that C18 and C17 are the final products in hydroprocessing of all sulphide and nonsulphide catalysts, excluding high concentration of C16 formation in Pd/C. NiMo-S can achieve high catalytic activity and selectivity; instead, the use of NiMo can initiate DCA and DCO pathways only. The reduction of H2 partial pressure involves in deactivation either by -S rupture on active sites or reduced dehydration with no proton supply in the HDO mechanism. So, H2 consumption for hydrogenation of unsaturated glyceride ester group is notable for all tested catalysts. The unsaturated SVO contains mainly cis-group esters, which can be converted into trans-group esters. Multilayer adsorption over Pd/C catalyst involved in α-and β-carbon cracking as the intrinsic mass transfer control on the surface for desorption might break of the molecules into lighter hydrocarbon. Also, DCA and DCO can be extended to higher order in thermal dissociation due to smaller pore volume. Langmuir IVtype adsorption isotherm of catalysts such as NiMo-S and Pt-Pd/γ-Al2O3-H-β has enhanced HDO with higher pore volume, which can give better selectivity in C17 and C18 formation. However, Pt-Pd/γ-Al2O3-H-β resembles in the reaction rate with NiMo-S and forms relatively similar coke deposition and deactivation. Also, branching of carboxylic and carbonyl compounds in hydroprocessing through Pt-Pd/γ-Al2O3-H-β is beneficial, but higher temperature can induce the equilibrium and increases the decomposition at a disproportion route to make hydrocarbon branching such as isomerisation and oligomerisation.