13th International Colloquium Fuels - September 2021 83 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C 2 -C 4 olefins Constantin Fuchs Karlsruhe Institute of Technology, Institute of Catalysis Research and Technology, Eggenstein-Leopoldshafen, Germany Constantin.Fuchs@kit.edu Matthias Betz Karlsruhe Institute of Technology, Institute of Catalysis Research and Technology, Eggenstein-Leopoldshafen, Germany Dr. Ulrich Arnold Karlsruhe Institute of Technology, Institute of Catalysis Research and Technology, Eggenstein-Leopoldshafen, Germany Prof. Dr.-Ing. Jörg Sauer Karlsruhe Institute of Technology, Institute of Catalysis Research and Technology, Eggenstein-Leopoldshafen, Germany Summary Particle formation during engine combustion is favoured in the presence of aromatics. However they offer high octane ratings. Aromatic-free fuels consequently produce lower particulate emissions than currently used fuels made from crude oil. Such synthetic fuels may be produced by heterogeneously catalyzed oligomerization of olefins like ethene, propene and butenes. The oligomerization of higher olefins has so far been less in the focus of fuel synthesis, but is promising with respect to the degree of branching and the octane rating. In this paper the possibilities of producing fuels by oligomerization of C 2 -C 4 olefins will be shown. Sources of sustainable olefins like the methanol-to-olefins process are presented, followed by the state of the art regarding the oligomerization. Furthermore, the set-up of a lab-scale plant is introduced and recent results concerning the co-oligomerizations of propene and 1-butene are presented. It is visible that mixtures of these two components as feeds effect their conversions and the resulting product spectra due to varying reactivities. 1. Introduction The main source of airborne contaminates in the cities are products from the combustion of organic substances. The combustion of fossil fuels will lead to increase the CO 2 content of the air. Additionally, side products like soot, SO 2 , NOx etc. worsen the quality of air especially in large cities. Fuels currently used in combustion engines are mainly based on fossil raw materials, relatively easy to handle and are characterized by their high energy density [1]. For spark-ignition engines, currently aromatics are indispensable. They offer the advantage of high octane ratings [2], but are considered to be precursors of particle formation [3]. In aviation, kerosene also consists of high proportions of aromatics up to x Aromatics = 26,5 vol.% [4]. During combustion, aromatics-free may consequently reduce the particulate emissions compared to currently used fossil fuels. With regard to air pollution control, aromatics-free fuels based on sustainable sources represent a low-emission option for the continued use of internal combustion engines in urban areas [5, 6]. In addition, renewable fuels offer the possibility in all respects to operate vehicles with low emissions and also to be used in the existing fleets. A further advantage of such synthetic fuels is the possibility of adjusting specific physico-chemical fuel properties through targeted synthesis of individual components. However, to balance the octane rating in aromatics-free paraffinic fuels, 84 13th International Colloquium Fuels - September 2021 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins the degree of branching of the paraffin molecules must be increased. Nowadays, the production of high quality, aromatic-free gasoline is still proving difficult. The heterogeneously catalyzed oligomerization of propene, as well as butenes in addition to ethene has so far been less investigated regarding the synthesis of fuels, but considering the molecule’s degree of branching and thus as well the octane rating, it is promising. There are already industrial-scale processes that convert these olefins into fuels, but these were invented several decades ago and have not been pursued and further developed since then for economic reasons [7-9]. Furthermore the octane rating of the produced gasoline is not sufficient for today’s standards [7, 10]. The focus of own work is on the investigation of the heterogeneously catalyzed oligomerization of alkenes with carbon chain lengths between C 2 and C 4 . The aim is to produce high-octane gasoline fuels that meet the EN 228 standard from sustainable olefins. Yields of sideproducts like kerosene and diesel are to minimize. Subsequently, the current states of the art related to the production of sustainable olefins, followed by heterogeneous oligomerization are shown and completed by labscale experiments of the latter process. 2. State of the art 2.1 Sources of sustainable olefins Starting from sustainable methanol, olefins can be produced via the methanol-to-olefins (MtO) process. This conversion takes place according to the following simplified equation: n CH 3 OH  [CH 2 ] n + n H 2 O (1) Figure 1: Double cycle reaction mechanism in the case of MtO catalysts [25] From DME, via the DtO process, this is also viable [11]. At lower pressures (p = 1 - 30 bar) and higher temperatures (T = 450 - 600 °C) compared to the methanol-to-gasoline (MtG) process [12, 13], the synthesis of shorter hydrocarbons is favoured [14, 15]. Both methanol-based processes were developed in the 1980s [15, 16]. In addition to Mobil, UOP and Norsk Hydro (now Ineos) brought the process to market readiness in the 1990s, using H-SAPO-34 as a catalyst in a low-pressure fluidised bed reactor. This concept enables efficient temperature control and continuous catalyst regeneration [17]. ZSM-5 zeolites and SAPO zeotypes are commercially used for the MtO process. Both catalysts have Brønsted acid sites in structured micropores. The pore system of the aluminosilicate ZSM-5 is threedimensional and has an average pore diameter of d Pore = 5,5 Å. H-SAPO-34 is a silica aluminophosphate with a cage structure and pore sizes of only d Pore = 3,8 Å. The small pore diameters favour the formation of small molecules and prevent the diffusion and formation of larger molecules. This explains the high selectivity to linear, light olefins. Aromatics and higher branched hydrocarbons are too large to diffuse out of the pore structure [18-20]. However, the smaller the pores of the catalysts, the faster this leads to their blockage and thus deactivation due to the deposition of coke. For the MtO reactions, catalysts with moderate acid strength and density of the acid centres are required. A high acid strength has a beneficial effect on the activity of the catalyst, but it is also deactivated faster due to the likewise increased formation of coke [21-23]. The formation of the olefins takes place through the formation of alkyl residues on aromatics and their subsequent spin-off. The aromatics are trapped in the catalyst structure, cannot diffuse out due to their size and serve as a co-catalyst. Side chains are formed on the aromatics through the addition of methanol. The separation of these leads to the formation of ethene. Propene and butenes are formed by subsequent methylation of ethene [24]. This double-cycle mechanism was consolidated by Björgen et al. [25] and is shown in Figure 1.More information on the synthesis of olefins from methanol and the catalysts that can be used for this purpose, as well as other influencing parameters, can be found in the review articles [15, 16]. Process flow diagrams of various other MtO processes are also shown. Alternatively, the dehydration of fermentative alcohols offers the possibility of producing their corresponding olefins on a regenerative basis. This is further explained below using iso-butanol as an example. The conversion of iso-butanol to iso-butene takes place by means of acid dehydration via an elimination mechanism, as shown in Figure 2 [26]. Figure 2: Elimination reaction of iso-butanol to isobutene [26]. By protonation of the OH group and formation of a carbenium ion, water splits off and iso-butene is formed. The reaction preferably takes place at p < 7 bar via γ-Al 2 O 3 catalysts [27]. SAPO, H-ZSM-5 or heteropolyacid catalysts can also be used. [28]. With alcohols such 13th International Colloquium Fuels - September 2021 85 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins as n-butanol, the elimination takes place at different positions, resulting in butene mixtures. With isobutanol, however, iso-butene is always formed due to symmetry. To overcome the activation energy of the reaction, temperatures of T = 250 - 350 °C are advan-tageous [26, 29, 30]. Direct conversion of ethanol to butenes is also possible. On a zinc-zirconium oxide catalyst at temperatures of T = 450 °C and p = 1 bar, selectivities S and yields Y of above 80% are achieved with complete conversion X of ethanol to iso-butene. The reaction requires water as a co-feed so that it proceeds via intermediates such as acetaldehyde and acetone to iso-butene. The ZnO in the catalyst reduces the acidity of the Lewis and Brønsted acid sites of the ZrO 2 and thus increases the basicity. This suppresses undesirable side reactions such as the dehydration of ethanol and the polymerisation of acetone. The dehydration of ethanol to acetaldehyde and its conversion to acetone via aldol condensations and dehydrations is promoted by the basic centres. The Brønsted sites are responsible for the final acid-catalysed conversion of acetone to iso-butene [31]. Furthermore, with Ag-ZrO 2 / SiO 2 catalysts, the direct conversion of ethanol to a butene-rich (1-, 2-butenes and butadiene) olefin mixture is possible at conversions of X = 99% and a selectivity of S = 88%. By adjusting the co-feed to hydrogen, the ratio of butenes to butadiene can be regulated [32]. A direct fermentation of olefins from biomass is also possible. Global Bioenergies, for example, has developed a process for the fermentative production of isobutene and is currently operating a first pilot plant in Leuna. The conversion takes place in stirred tank reactors at p = 4 bar and T = 32 °C using gene-modified E. coli bacteria, which produce iso-butene directly from sugars [33, 34]. The bacteria are currently specialised in the conversion of glucose. However, the straw hydrolysate is largely composed of glucose and xylose, and also contains inhibiting substances from the biomass. The adaptation of the bacteria to these conditions still needs further research, but the conversion of xylose to iso-butene is already possible. Lignin, on the other hand, cannot be converted [35]. 2.2 Oligomerization The produced olefins are then fed to the oligomerization process. There, the lower olefins are converted into higher olefins, only low contents of aromatics will be generated. As an example, this is shown below in a general formula for ethene, propene and butene. a C 2 H 4 + b C 3 H 6 + c C 4 H 8  w C 5 H 10 + x C 6 H 12 + y C 7 H 14 + z C 8 H 16 … (2) For the conversion, acid sites especially Brønsted sites are important to achieve a first protonation to form carbenium-ions [36]. The subsequent chain growth is the result from this initial step. Exemplary, Table 1 summarizes three oligomerization processes, which are industrially realised at scale and briefly described below. In the 1980s, the Mobil Olefins to Gasoline and Distillate (MOGD) process was developed by Mobil Oil Corp. (now ExxonMobil). In this process, olefins from the methanol-to-olefins process are converted to gasoline, kerosene and diesel on an acidic ZSM-5 catalyst (d Pore = 5.5 Å) in several reactors connected in series. In the MOGD process, four fixed-bed reactors are operated simultaneously, one of which is regenerated due to coke formation while the other reactors are in operation (Figure 3). The process takes place at T = 150 - 200 °C and p = 7 - 12 bar [7]. By adjusting the process conditions, the ratio of gasoline to distillate can be varied between 0.2 and 100. Table 1: Overview of the MOGD, Polynaphtha and Catpoly processes with regard to reaction conditions and reaction control. MOGD (Exxon- Mobil) [7] Polynaphtha (Axens IFP) [8] Catpoly (UOP) [9] Catalyst Zeolite ZSM-5 Silica-Alumina Solid phosphoric acid T [°C] 150 - 200 150 - 200 180 - 200 p [bar] 7 - 12 30 - 50 30 - 40 LHSV [m³/ h / m³ Catalyst] 1 - 2 0,3 - 0,5 0,5 - 1 Conversion to fuels [%] > 90 95 - 98 90 - 95 Reactor type Adiabatic fixed bed reactor Fixed bed reactor Hordes reactor Table 2: Properties of fuels from MOGD process [7]. Gasoline Kerosene Diesel Relative density* [-] 0,73 0,78 0,78 Research octane number [-] 92 Motor octane number [-] 79 Cetane number [-] 52 Composition Paraffins[wt.%] 4 95 Olefins [wt.%] 94 1 Aromatics [wt.%] 2 4 *related to the density of water at T = 4 °C 86 13th International Colloquium Fuels - September 2021 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins The advantage of this synthesis is the higher product quality of the gasoline compared to a methanol-togasoline process, since, for example, it does not or only contain minor quantities below 5 wt.% any particlepromoting aromatics. Also durol is not present, a quadruple-methylated C 10 -aromatic with a high melting point (T melting,Durol = 78 - 81 °C) [37, 38]. To remove the heat of the exothermic reaction, intercoolers are used after the reactors. Table 2 shows fuel-relevant key properties as well as the composition according to substance groups of the gasoline, kerosene and diesel fractions. Kerosene and diesel were previously hydrotreated with hydrogen. The polynaphtha process developed by the Institute Français du Pétrol (now Axens IFP) uses C 3 -C 5 olefins as reactants. These are converted to oligomeric gasoline and gas oil on a silica-alumina catalyst in the same temperature range, but at significantly higher pressures (p = 30 - 50 bar) than in the case of the MOGD process. In order to obtain higher yields of longer oligomers in the form of gas oil, parts of the LPG fraction, as well as the oligomeric gasoline, is recycled and oligomerized again. Accumulating deposits on the catalysts can be burnt off by adding an air/ steam mixture and thus, the catalysts can be regenerated [8]. Honeywell UOP’s Catpoly process was already developed in the 1930s. It uses solid phosphoric acid (SPA) as a catalyst and is run with comparable operating windows to the Polynaphta process. However, the conversion of the C 3 and C 4 olefins takes place in a horde reactor and not in several fixed-bed reactors connected in series. [9]. Light olefins are recycled for further oligomerization to higher oligomers. In addition to the processes of fuel synthesis from olefins, processes based on alcohols have been developed. These also use the principle of oligomerization and offer the advantage of sustainable production of the educts by fermentative processes based on sugar-containing biomass. Sustainable alcohols are dehydrated to olefins, which than undergo oligomerization primary to molecules in the kerosene range. These “Alcohol to Jet” processes (short AtJ) are continuously developed and optimized for example by Lanzatech/ PNNL and Gevo [39]. Figure 3: Flow scheme of the MOGD process [7]. 3. Experimental 3.1 Lab-scale plant Experiments for the oligomerization of C 2 -C 4 olefins are performed in a laboratory plant with a maximum production volume of 2.5 l/ week. Ethene is supplied in gaseous form, propene and butenes are pumped into the reactor as liquids. Argon is also available as an inert gas. The fixed-bed reactor has an inner diameter of d i = 16 mm and is divided into 3 zones. The inlet and outlet bed only consist of silicon carbide of particle size d = 250 - 500 µm. Both areas are separated by the reaction zone, in which SiC is also present, but mixed with catalysts. In general, tests at up to T = 350 °C and p = 70 bar are possible. Liquid products are subsequently condensed out, collected and after the experiments analysed offline with a gas chromatograph (Agilent 6890 with DB-1 column). Gas is sent to an online gas chromatograph (HP 5890 with RT-Alumina BOND/ Na 2 SO 4 column) and analysed there. 3.2 Catalysts and catalyst support materials Silica alumina support materials impregnated with transition metals like nickel are used as catalysts for the oligomerization. This combination has already proven to be very effective regarding ethene oligomerization [40, 41]. Furthermore it enables high olefin conversions at mild reaction conditions (T = 80 - 200 °C, p Olefins = 10 - 50 bar, WHSV = 2 - 10 h -1 ). Boehmite SIRALOX ® with varying Al 2 O 3 / SiO 2 ratios and thus also different strengths and numbers of Lewis and Brønsted acid sites are used as catalyst support. Table 3 shows exemplary some properties of SIRALOX ® 20. Table 3: Properties of SIRALOX ® 20 catalyst support [42]. SIRALOX ® 20 Al 2 O 3 / SiO 2 [%] 80 / 20 BET [m²/ g] 420 Pore volume [ml/ g] 0,8 The support material is doped with nickel up to a certain amount using the incipient wetness impregnation method, described below. First, the catalyst support is calcined at T = 550 °C for t = 8 h -1 . Afterwards a nickel salt is dissolved in water and applied according to the desired mass fraction of nickel. Finally, the impregnated catalyst support is dried and calcined again and then brought to the same particle size as the SiC. For the first experiments in the plant, m = 5 g of the SIRALOX ® 20 loaded with 2 wt.% Ni are used each time. The reactor is heated to T = 120 °C and pressur- 13th International Colloquium Fuels - September 2021 87 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins ized to p total = 20 bar. Always argon is used as inert gas with a molar fraction of x Ar = 20 mol.% to control the thermal output of the exothermal reaction and maintain a constant bed temperature. The oligomerization of the pure olefins 1-butene and propene is performed with a catalyst loading of WHSV = 2 h -1 . Due to instabilities in the feed-dosing, evoked by too low mass flows for the pumping units, the co-oligomerization of butene and propene is held with doubled WHSV of 4 h -1 to prevent fluctuation in the feeding system. 4. Results and discussion 4.1 1-Butene oligomerization The first experiments were performed with 1-butene. Figure 4 shows the conversion X, the mass fractions of the produced liquids corresponding to their individual carbon lengths are presented in Figure 5. Figure 4: 1-Butene conversion over time-on-stream (TOS) during oligomerization (T = 120 °C, p C4H8 = 16 bar, WHSV = 2 h -1 ). Due to educt accumulation in the reactor during the initial period, no 1-butenes or isomers of it, formed by the thermal equilibrium, leave the reactor. Therefore, the calculation of butene conversion, which is coupled to the inlet and outlet streams, provides a complete conversion of X = 100% at the start of the experiment (Figure 4). After around t = 2 h, the reaction begins to enter a steady state, where the conversion is stabilizing on an almost constant level (X ~ 70%). After t = 6 h the experiment is finished, the olefin feeding is turned off, followed by a flushing phase under Argon flow to rinse out remaining products from the reactor. The mass composition (Figure 5) shows a high selectivity in the liquid of around S = 60% to C 8 , the dimer of butenes. The rest of the converted butenes primarily form trimers or tetramers, represented by C 12 and C 13+ . In total, 118 different liquid components are generated with high selectivities about S > 85% to hydrocarbons in the gasoline and kerosene range. Figure 5: Liquid mass fractions of the 1-butene oligomerization (T = 120 °C, p C4H8 = 16 bar, WHSV = 2 h -1 ). 4.2 Propene oligomerization Afterwards, the propene oligomerization was held under the same reaction conditions as before. In Figure 6 the propene conversion is portrayed, Figure 7 illustrates the liquid mass fractions. In the gaseous phase, only propene is detected. Figure 6: Conversion of propene during oligomerization (T = 120 °C, p C3H6 = 16 bar, WHSV = 2 h -1 ). 88 13th International Colloquium Fuels - September 2021 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins Immediately visible is the significantly lower conversion of propene (X ~ 20%) in comparison to butene. Due to its molecular structure, propene is not as reactive as butene. Futhermore there are no isomers of propene like the 2-butenes or iso-buten in the case of butene. Those isomers may easier form higher, more reactive carbenium-ions. As conclusion, propene needs higher amounts of Brønsted acid sites, whereas butene oligomerization also takes place on surfaces with lower density and acidity of acid sites. Figure 7: Liquid mass fractions of the propene oligomerization (T = 120 °C, p C3H6 = 16 bar, WHSV = 2 h -1 ). In Figure 7 is shown, that dimerisation to C 6 almost does not take place, respectively is only an intermediate to higher olefins like the preferably formed trimer C 9 . The formation of hydrocarbons which are no integer multiples of propene is related to metathesis reactions. In contrast to the butene oligomerization, with 208 liquid components, the propene coupling leads to almost twice as many different molecules. In the analyzed product range, due to its smaller carbon chain lenght higher degrees of oligomerization of propene are conceivable than of butene. For example to produce a C 12 olefin, 3 butene but 4 propene molecules are needed, The resulting increased coordination possibilities for oligomers of the same carbon chain length may lead to a higher quantity of different isomers. 4.3 Co-oligomerization of propene and 1-butene Figure 8: Olefin conversion during the cooligomerization of 1-butene (T = 120 °C, pC3H6 = 8 bar, pC4H8 = 8 bar, WHSV = 4 h-1). The Figures 8 and 9 illustrate the results of the cooligomerization of propene and butene. The conversion of C 3 H 6 is shown as yellow rhombs, C 4 H 8 is marked with red circles. Both conversions drop down after the start quite significant and settle at around X = 25 - 30 mol.%. For propene this behaviour is already shown in the section before, but for butene the conversion is significantly lower than in the sole butene oligomerization. Due to the co-oligomerization of molecules with different carbon chain lenght and more possibilities for their coupling, a wider product range arises. This is also reflected in the once more increased quantity of 248 liquid components. The products of di-, triand tetramerization reactions of propene and butenes (mainly C 6 , C 8, C 9 and C 12 ) amount in total to 42,5 wt.%. Compounds of chain lenghts formed by both educts, propene and butene, like C 7 , C 10 or C 11 , amount to more than 50 wt.%. Concluded from this, oligomerization reactions of various olefins as feed are at least as, if not slightly, preferential to reactions of one sole sort of olefins. 13th International Colloquium Fuels - September 2021 89 Production of sustainable fuels by heterogeneously catalyzed oligomerization of C2-C4 olefins Figure 9: Liquid mass fractions of the cooligomerization of propene and 1-butene (T = 120 °C, p C3H6 = 8 bar, p C4H8 = 8 bar, WHSV = 4 h -1 ). Noteworthy, propene dimerization to C 6 as product is still not as present as butene dimerization. Apart from this, the trimer C 9 is formed with about 10 wt.%. Furthermore the C 10 -fraction has, with almost 17 wt.%, a ten times higher mass fraction due to cooligomerization of propene and butene. Consequently, C 6 has to be an intermediate which then undergoes higher oligmerization processes. Even the synthesis of C 7 oligomers by coupling of one butene and one propene molecule is more favoured compared to the formation of C 6. Thus, butenes promote the reaction of propene, but on the other hand, in presence of propene the conversion of butene is significantly reduced by more than 60%. This effect has to be investigated in future experiments and compared with results from feed-compositions provided by MtO processes. 5. Summary and Outlook The heterogeneously catalyzed oligomerization represents great potential for renewable fuels with emission-reduced properties. A broad spectra of fuels (gasoline, kerosene or diesel) may be synthesized with high selectivities. In comparison to the Fischer-Tropsch process with low selectivities [43], this is benficial and favoured for industrial application. Employing a lab-scale plant, first results showed high gasoline selectivities. Even kerosene is producable with high selectivity. In general, the catalyst shows high activity and can be adapted to the production of desired fuel ranges. Recent results show, that compared with butenes, propene requires a higher density of acid sites as the carbenium-ion formed is not as reactive as the one formed by butenes. In addition, the effects of the feed composition in respect to liquid product fractions were shown. A higher quantity of components can be observed during propene oligomerization. Attributed to higher degrees of oligomerization and increased coordination possibilities, a broader spectra of olefins and a higher quantity of different isomers is formed. 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