Catalytic Upgrading of Bioethanol to Fuel Grade Biobutanol: A Review

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Review

Catalytic Upgrading of Bioethanol to Fuel Grade Bio-butanol: A Review Ahmad Galadima, and Oki Muraza Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01443 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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Industrial & Engineering Chemistry Research

Catalytic Upgrading of Bioethanol to Fuel Grade Bio-butanol: A Review Ahmad Galadima1, Oki Muraza*1,2, 1

Center of Research Excellence in Nanotechnology, 2Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia *Corresponding Author (OM) E-mail: [email protected], Phone: +966 13 860 7612

Abstract Biobutanol is increasingly attracting industrial interest as fuel additive due to its numerous advantages over bioethanol that include higher calorific value, fuel efficiency and reducedengine problems. Therefore, bioethanol valorization to biobutanol provides a good option for the energy industry. The paper reviewed literature on the recent progress for the ethanol to butanol (ETB) process. Issues related to catalyst development and the role of compositional properties (i.e. catalyst nature and acidity-basicity) and reaction parameters in enhancing butanol yield were critically examined. The mini review also discussed the reaction mechanisms involved and identified new paths for further studies. Keywords: Bioethanol; biobutanol; catalysts, progress; research paths.

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1. Introduction Fossil fuels represent the major sources of energy and petrochemicals for the current global generation. However, the numerous challenges associated with their exploration and utilization necessitate the search for better alternatives. They are non-renewable and characterized by abnormal global distribution patterns, with some countries having huge reserves while others having limited or none (1-4). The emission of greenhouse gases such as CO2, CH4 and NOx and other environmental pollutants is an issue of critical global concerns for many years due to the persistent destruction to humans, plants and animals populations (5-9). Biomass exploration for conversion into fuels, fuel blends and petrochemicals provides one of the viable options given considerations for addressing the problems. Several advantages have been attributed to this shift. The biomass raw-materials are abundant and generally renewable, both the upgrading and utilization processes have limited contribution to environmental degradation and have the strong potentials to reduce country’s dependence on foreign oil (i.e. ensures economic sustainability of a country) (10-14). Among the biomass valorization strategies, ethanol blending with gasoline is a popular approach accepted in the United States and many other global regions. For this reason, the ethanol production have recently been on the increase in some key regions (see Figure 1) (15). In addition to bursting the octane properties of gasoline, the ethanol-gasoline fuel blends are characterized by limited emission of greenhouse gases and therefore reduced environmental pollution (16-18). However, some challenges have been reported for these blends (19). Conventional automobile engines are usually constructed with components that brittled with time. These components consequently disintegrate in the presence of alcohol, clogging the engine with sludge and triggering engine seizure (20). Configuration into modern engines with 2 ACS Paragon Plus Environment

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100% compatible ethanol blend properties will involve serious cost implications and can be considered difficult, especially for the low-income nations. As an excellent solvent, ethanol can dissolve resins in engines, causing complications including clogged injectors. Its ability to absorb water can also cause phase separation in engines. Similarly, application of ethanol blends in marine and aviation engines can be considered very difficult due to poor cold flow properties. An important option currently considered is the upgrading of the bioethanol into bio-butanol, which have better blending properties (21). Among other advantages, butanol has very good heat of combustion (~ 83% of that of gasoline) that is much higher than 65% for the ethanol (21, 22). Unlike ethanol that is miscible with water at all proportions, butanol shows low solubility in water due to the increased carbon chain. This factor reduces engine problems and also ensures recovery efficiency during butanol production (23). The less-corrosive nature of the butanol compared to ethanol means that, the current pipeline networks could be used for it safely transportation without the need for any reconfiguration. Other advantages include high octane number compatible with gasoline, possibility for blending with diesel and limited destruction to engine parts (21).

Please insert Figure 1 here.

Bio-butanol production from renewable resources and market capacity have in the recent years indicated a significant wave of increase with projections that the trend will continue for the future. By the year 2012, the world market accounted for 3,801 kT (24). Asia-Pacific region represented the major global consumer in the year with about 50% consumption. From the year



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2013 a growth rate of 4.6% has been predicted. Therefore, the global market can reach 4,980 kT by the year 2018. In this regard, the Asia-Pacific region is expected to have an annual growth rate of 7.7%, amounting to $4.3 billion in 2018. In North America, the butanol market accounted for 26.7% of the global capacity in 2013, valued at $1.8 billion. The capacity was also predicted to hit a value of $2.3 billion in 2018. In the European region, the market accounted for 17.5% of the global market in 2013 and was valued at $1.2 billion. With the predicted growth rate of 4.9%, the market will hit $1.5 billion in 2018 (24). The various statistics indicated butanol as becoming increasingly attractive for use in fuels and chemicals worldwide. Therefore, investigations on the various production alternatives and valorization paths are very important regarding energy and chemicals sustainability. This paper reviewed recent studies that targeted ethanol upgrading into butanol via heterogeneous catalysis. Background information was initially captured on the common bio-butanol production options. The roles of catalyst compositions (i.e. nature and basicity-acidity properties) and reaction parameters in enhancing product yield for the ethanolto-butanol (ETB) route and how these influence the reaction mechanisms were discussed. An outlook on the areas for further investigations was also presented. 2. Common Processes for Butanol Production Butanol otherwise called butyl-alcohol is among the light alkanols considered as fuel and intermediate for the chemical industry in the recent times. The butanol isomers which are secbutanol, tert-butanol and iso-butanol (i.e. 2-methyl-1-propanol) have also found similar applications in the industry. Two chemical methods are commonly employed for butanol production via the non-biomass route (Figure 2). The “oxo-synthesis” method involved the serial hydroformylation and hydrogenation of propylene as the primary feedstock using homogeneous catalysts based on Co or Rh (25, 26). The formylation reaction transforms 4 ACS Paragon Plus Environment

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propylene into butanaldehyde through interaction with CO and H2. In the subsequent step, the latter is hydrogenated to produce a mixture of n- and iso-butanol. The other chemical method called “crotonaldehyde hydrogenation” proceeds in three important stages. The aldol condensation and dehydration stages convert acetaldehyde into crotonaldehyde (27, 28), followed by hydrogenation into butanol using supported metal catalysts (29-32). This method normally produces butanol as the only reaction product. Non-biomass based methods are nonrenewable options and therefore challenging for the sustainable energy industry.


Butanol production from biomass was first reported in 1862 by Louis Pasteur (33). The process commonly represented as ABE (i.e. acetone, butanol and ethanol) method utilizes microorganisms, the Clostridia species, to ferment cellulosic materials via a complex chemistry into ABE as the main reaction products (34). The process received wide acceptance during the world wars I and II due to the rise in consumptions of acetone and butanol for the production of explosive substances as well as fuels for transportations (35). The fermentation process comprises of two principal stages referred to as acidogenesis and solventogenesis (36, 37). In the former case, butyric and acetic acids are produced as the reaction products coupled with cell growth of the micro-organisms. The production of these acids usually lowers the medium pH to less than 5. During the second stage (i.e. solventogenesis), the micro-organisms utilize these acids as additional carbon source to produce a range of solvents with the ABE as dominant species. Several challenges have been identified with the ABE process (38, 39). Although it



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received good acceptance over the years, the overall yield of butanol is usually less than 2% (21). This is associated with solvent toxicity which retards reaction progress over time. For instance, the toxicity of butanol damages the cell membrane of micro-organisms, leading to total cell destruction (21, 40). The reaction mechanism is only partly understood due to its complex nature and therefore enhancing butanol yield by limiting the yield of acetone, ethanol and other products is still challenging. Recent attempts to address these issues include the engineering of different toxicity-resistant bacteria strains for catalyzing the process and the adoption of batch fermentation technology. In the latter technology, butanol-resistant bacteria is employed to transform biomass feedstock into mainly butyric acid under anaerobic conditions (41, 42). The butyric acid is then further fermented in another batch of experiment to produce crude butanol for purification to fuel grade. The batch fermentation can produce up to 40% butanol yield and is therefore given preference recently (21). However, the main mechanisms for improved bacteria resistance and activity are not yet resolved. Different companies are recently evaluating various alternatives for industrial biobutanol production (43). The Gevo Company Colorado is one of the key players that utilize modified yeast as catalyst for glucose upgrading into mainly butanol to very high yield (44). Test results have shown that, up to > 100 g/L of butanol could be achieved via the Gevo process. Unlike this company that gave emphasis to glucose upgrading, Butamax Advanced Biofuels and British Petroleum companies are investigating the commercial prospects of a range of biomass feedstocks (including cellulosic materials) for the fermentation process into butanol (43). Pilot operational plants are expected to be commissioned in the recent years. Cobalt Technologies California and ButylFuel LLC are developing micro-organisms of high resistance to butanol solvent coupled with continuous ABE fermentation process for upgrading cellulosic and starch


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materials to yields more than two times obtainable via the conventional ABE fermentation process (45). Many other companies are picking interest in butanol production. The Green Biologics company United Kingdom has developed thermophilic bacteria with resistance to 4% solvent concentration (43). In collaboration with Laxmi Organics, an Indian company, commercial demonstration plants are underway for butanol production from cellulosic and hemicellulosic feedstocks. The Canadian renewable energy company SyntecBiofuel planned to escalate the technology for municipal solid waste (MSW) and agricultural residues upgrading via thermochemical process. Catalyst development is a key priority considered by this company. The SuperButolTM Technology Company recently reported the upgrading of butylenes derived from petroleum refining for upgrading into butanol using an integrated large-scale pilot plant (46). Up to 97.4% of overall conversion was achieved for the operation conditions of 100-150 oC, 5 x 106 to 7 x 106 Nm-2 and various space velocities. The increasing interest by international companies for butanol production to high yield from different routes using affordable raw-materials and catalysts means that, the ethanol to butanol production route would be attractive for the near future.

3. Bioethanol to Bio-butanol The ethanol to butanol (ETB) process represents another biomass-based ethanol valorization option with potentials to produce butanol with higher selectivity than can be achieved using the ABE fermentation method discussed earlier (47, 48). The chemistry of ETB is believed to proceed by two identified reaction paths (i.e. the Guerbet mechanism and the bimolecular condensation mechanism) (49). In the Guerbet activation path, the initial step involved dehydrogenation of ethanol to produce acetaldehyde (Figure 3). In the subsequent steps, two 7 ACS Paragon Plus Environment


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molecules of acetaldehyde coupled via aldol condensation forming crotonaldehyde as an intermediate, to produce butanol as the reaction product (50). There is the possibility of further condensation reactions involving both butanol and acetaldehyde to form higher alcohols or aldehydes. However, the catalyst basicity and the selection of reaction parameters determine the reaction selectivity. This will be discussed in the subsequent sections of the paper. The bimolecular mechanism otherwise called the direct coupling mechanism proceeds by the condensation of –OH group of one ethanol molecule with a β-H of another ethanol molecule to eliminate water molecule and produce butanol (Figure 3) (51). Here, the basic sites strength is very critical factor for the –OH group activation and the subsequent dehydration process.


3.1. Role of Catalyst Composition A range of catalyst systems is considered as active materials for the ETB reaction. Although still debatable, the basic sites are accepted as the main active sites of a catalyst suitable for this reaction. The composition of the catalyst and the associated basic sites density and strength can predict the yield of butanol obtained from the reaction. Promoted zeolites, hydroxyapatites and the metal oxide systems are usually employed as catalytic materials for the reaction, yielding different activities and selectivities. 3.1.1. Hydroxyapatite (HAP) Catalysts


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The hydroxyapatite (HAP, Ca10(PO4)6(OH)2) catalysts are characterized by a unique property comprising of both acid and basic sites in a single structured crystal (52, 53). This property enabled their catalytic performance in many processes other than the ETB reaction (54, 55). The work of Tsuchida et al. (56) employed HAP catalysts with different Ca to P ratios for the reaction. The catalysts were synthesized by precipitation method using hydrated calcium nitrate and ammonium phosphates as precursors. The reaction was conducted at 400 oC and 10,000 h-1 using 20% of ethanol in He as the feed. The ethanol conversion increased with increasing the Ca/P ratio, reaching an optimal conversion of 23% for a ratio of 1.64. However, the conversion declined with further increased in the Ca/P ratio. The reaction products were mainly butanol, ethylene and acetaldehyde. The highest butanol selectivity of 62.4% was achieved with the Ca/P ratio of 1.64. A lower ratio of 1.52 produced the highest ethylene selectivity of > 80%, whereas the highest acetaldehyde selectivity of 25% was observed with the highest Ca/P ratio of 1.69. Increasing the Ca/P ratio reduces the concentration of acid sites and increases the amount of basic sites. This factor favours ethanol dehydrogenation and therefore the selectivity to acetaldehyde increased (57). It could be seen that, the lowest Ca/P ratio corresponding to the highest concentration of acid sites was more selective to ethylene due to ethanol dehydration associated with these sites. The results generally indicated that, there should be a balance between both acidic and basic sites for the HAP catalyst to produce an optimal ethanol conversion and butanol selectivity. Recently, Scalbert and co-workers (58) used a commercial grade HAP catalyst (A0312711) for the reaction at 400 oC, 14 h-1 and 15.2% of ethanol. The reaction products were mainly butanol and acetaldehyde with traces of butenols and ethylene. The yields of butanol and acetaldehyde were 8 and 3%, respectively, whereas the cumulative yield of other reaction products was < 3%. The ethanol conversion was < 20% with a butanol



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selectivity of ~50%. Meunier et al. (59), employed similar catalyst (i.e. HAP, A0312711) with a BET surface area of 82 m2/g and monitored the reaction kinetics at a similar temperature of 400 o

C. Butanol was produced as the main reaction product with a maximum selectivity of 55%.

Acetaldehyde dominated the side reaction products, but its selectivity is much lower than that of butanol. The energy required to activate butanol formation (i.e. 133 kJ/mol) was considerably higher than that for acetaldehyde (88 kJ/mol). Similarly, the reaction order for butanol formation was positive (+ 0.4) whereas negative (i.e. – 0.1) for the acetaldehyde. Therefore, on the basis of this variation, it could be established that, the two species were associated with different ratedetermining steps for their production. The main issue to be observed here is that, the aldol condensation pathway for butanol production from acetaldehyde (i.e. self-aldolization route) cannot account for the reaction mechanism. The authors argued that butanol was formed directly from ethanol and therefore ruled out the possibility of Guerbet mechanism illustrated in Figure 3. These observations were also corroborated by Scalbert and co-workers (60), who demonstrated that temperatures ≥ 400 oC does not favour the aldol condensation pathway. Tsuchida et al. (61), developed a set of HAP catalysts with Ca/P ratios in the range of 1.59 to 1.67 using an earlier validated precipitation method (62, 63). The density of the acidic and basic sites was dependent on the Ca/P ratio. Increasing the Ca/P ratio raised the basic sites density from 0.01 to 0.53 µmol/m2 whereas the acidic sites density reduced from 0.038 to 0.006 µmol/m2. Consistent with some previous studies (64-67), the Ca/P ratio at the catalyst surfaces was lower than those observed in the bulk. The surface ratios ranged between 1.40 and 1.50 but consistently increased with increase in the bulk ratios stated above as observed from the XPS studies. Lowering the concentration of Ca ions by reducing the Ca/P ratio creates new acidic sites in these catalysts as also reported elsewhere (68-70). When ethanol upgrading was


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performed, increasing the Ca/P ratio lowers the reaction temperature required to achieve an optimal ethanol conversion of 20%. With HAP (Ca/P = 1.59), the required temperature was 387 o

C but reduced to 296 and 298 oC for the HAP (Ca/P = 1.65) and HAP (Ca/P = 1.67) catalysts,

respectively. Therefore, increasing the catalyst basicity lowers the required temperature for ethanol activation. Different reaction products were generated with ethylene, butanol and acetaldehyde as the dominant species. The butanol selectivity increased with increasing Ca/P ratio (see Figure 4). HAP (Ca/P = 1.59) does not produce any butanol, but was 87.3% selective to ethylene. On the other hand, the most basic HAP (Ca/P = 1.67) produced the highest butanol selectivity of ~70% with < 1% selectivity to ethylene. The HAP (Ca/P = 1.62) catalyst produced the highest acetaldehyde selectivity of ~8%, but this catalyst was also 39% selective to butanol. According to these results, changing the catalyst composition plays an important role in defining the selectivity to particular reaction product. Similarly, the participation of both basic and acidic sites could be proposed. The authors (61) argued that, the mechanism of the reaction is more complex than that presented in Figure 3. They proposed that, vaporized ethanol undergo dissociative adsorption involving ethoxide and proton intermediates on the Lewis acidic and Brønsted basic sites, respectively. This subsequently produces aldehydes, enolate carbocations, crotonaldehyde and finally butanol. However, this proposal failed to account for the production of olefins and other species like diethyl ether and aromatics detected during the reaction.




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Ogo and co-workers (71) conducted a study with modified HAP catalysts developed through hydrothermal procedures adopted by some other authors (72-74). Four catalyst systems (Ca-P, Sr-P, Ca-V and Sr-V) were developed such that, Ca-P represents the unmodified HAP catalyst whereas the other catalysts indicated systems where Ca and/or P of the parent catalyst was replaced by the incorporated metal species. Compositional characterization showed a closer molar elemental ratio of 1.69 for the Ca/P, Sr/P, Ca/V and Sr/V cases. However, the various modifications reduced the BET specific surface area of the parent Ca-P catalyst, with the most severe reduction of more than 65% observed with Sr-V (i.e. Sr10(VO4)6(OH)2) catalyst. Ethanol upgrading was performed at 300 oC using 2.0 g of catalyst and 16.1% of ethanol in Ar as feed. The Sr-P (i.e. Sr10(PO4)6(OH)2) catalyst produced the highest ethanol conversion of 7.6%, whereas the Sr-V catalyst produced the lowest conversion of 5.8%. Therefore, replacement of Ca ions by Sr ions without alteration to the P species was more favorable for ethanol activation. The Sr-P catalyst increased butanol selectivity from 74.5% for the parent Ca-P catalyst to 81.2% whereas the Ca-V (i.e. Ca10(VO4)6(OH)2) and Sr-V (i.e. Sr10(VO4)6(OH)2) showed a selectivity decay to 22 and 8%, respectively. Even at the equivalent conversion of 20%, the Sr-P catalyst produced 80% butanol selectivity compared to other modified catalysts that showed complete selectivity decay. Sr-P catalyst improved the selectivity of crotonaldehyde at the intermediate (i.e. aldol condensation) stage. This enhanced the chances of butanol formation and retarded catalyst coking or selectivity to products like hydrocarbons and butenols. The work of Ogu et al. (75) evaluated the effect of varying Sr/P ratio on the activity of the Sr-P (i.e. strontium modified HAP) catalyst. The Sr/P ratios were varied between 1.58 and 1.70. However, varying the ratio does not alter the BET surface area because the value was relatively constant at ~30 m2/g. When ethanol upgrading was performed at 300oC and 130 h.g/mol, the conversion increased with


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increasing Sr/P ratio. At the lowest ratio of 1.58, the conversion was 1.1% but increased to 5.9, 7.9 and 11.3% when the ratio was raised to 1.64, 1.67 and 1.70, respectively. A similar trend was also observed for the butanol selectivity because it increased from 69 to 86.4% (see Figure 5). All the catalysts produced negligible yield of acetaldehyde, indicating that this had been consumed at the aldol condensation stage of the reaction. Consistent with the previous studies (62, 76), higher (i.e. C6 and C8) alcohols were produced by high Sr/P ratios. Acidity-basicity studies conducted demonstrated the density of the basic sites to increase with increasing the Sr/P ratio. At the ratio of 1.58, the density was 0.37 µmol/m2 but increased linearly to 1.0 µmol/m2 when the ratio reached 1.70. However, the acidic sites density does not show any clear trend. Therefore, the basic sites were the main active sites required to achieve high ethanol conversion and butanol selectivity. It could clearly be established that, their density correlated with the activity parameters. Please insert Figure 5 here.

Young and co-workers (77) reported a comparative study using MgO and HAP as catalysts. Aldol condensation of acetaldehyde and ethanol activation to butanol were conducted to establish more insights on the reaction mechanism. Both catalysts produced 100% selectivity to crotonaldehyde during acetaldehyde condensation under variable conditions, but the HAP catalyst was more active, producing an equivalent condensation rate to MgO at a 120 oC lower temperature. For example, the acetaldehyde condensation rate of MgO at 400 oC and 20,000 Nm2

of acetaldehyde was 7.5 x 10-8 mol/m2s, but the same rate was achieved with the HAP catalyst

at 280 oC and similar partial pressure. A similar trend was observed for the ethanol activation to butanol. The reaction rate of HAP catalyst was more than three times that of MgO catalyst and 13 ACS Paragon Plus Environment


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could be achieved at 50 oC lower. MgO produced 12% ethylene, 67% acetaldehyde and 13% of butanol. When the reaction was repeated with HAP catalyst, the selectivity of butanol increased to 67%, whereas the selectivities of ethylene and acetaldehyde declined to 1 and 32%, respectively. The results indicated ethanol activation to occur through the Guerbet mechanism, of which aldol condensation plays an important role. HAP catalyst being more active for the aldol condensation due to its unique acid-base properties was therefore more active for butanol production. The high selectivity of MgO catalyst to acetaldehyde indicated its low-ability for further acetaldehyde condensation and hydrogenation to butanol. The HAP catalysts have recently been demonstrated by some authors as very good candidates for the aldol condensation of aldehyde and ketones (78-81). However, unanimous detail mechanisms of how the acid-base pairs influence the catalytic activity are still being investigated.

3.1.2 Oxide and Zeolite Catalysts In the recent years, metal oxides have attracted much interest as catalysts for the industry. Their compositional and basicity-acidity properties are very important for catalyzing a range of reactions, including the condensation of light alcohols into higher alcohols (82-84). Their behaviours in ethanol upgrading to butanol could be due to their C-C coupling ability as observed from their roles during direct methane coupling to ethylene (85-87). Therefore, ETB studies conducted with these catalysts targeted details on the reaction mechanism and the possibility of enhancing butanol yield. Ndou et al. (88) conducted a study with oxides of group II metals at 450 oC using 0.5 g of catalyst. BaO catalyst produced 21% ethanol conversion but was inactive for the formation of butanol. This catalyst produced 7 and 11% yields of acetaldehyde and butanal, respectively, as the reaction products. When the reaction was repeated with CaO,


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the conversion dropped to 15% with production of 1% butanol. MgO demonstrated the best catalytic properties. Ethanol conversion and butanol yield with this catalyst were 56 and 18%, respectively. The results indicated the less basic MgO as the most active catalyst for butanol formation. For confirmation, different metal ions (Ca, Zn, Pd and Cu) were separately incorporated into MgO and the reactions repeated under similar conditions. Modification with these ions improved the surface basicity of MgO but both ethanol conversion and butanol selectivity declined. Therefore, increasing the surface basicity of catalyst does not favor butanol production under these conditions. Previously Ueda and co-workers (89) documented that, the basicity of MgO catalyst could be improved by incorporation of metal ions with ionic radius larger than that of the Mg2+ ion. These authors also observed the increased basicity to lower conversion and propanol yield during ethanol and methanol condensation with metals modified MgO catalysts (90, 91). Consistent with the works of Iglesia and co-workers (92-95), the reaction pathway for the MgO catalyst was believed to be bimolecular (i.e. direct coupling) otherwise called “ethanol dimerization mechanism”. Guerbet mechanism could not be considered favourable from the products distribution. Ethylene was not detected and acetaldehyde was produced as by-product. Similarly, crotonaldehyde was not detected, screening out the possibility of aldol condensation that usually occur with the Guerbet pathway. Recently, Riittonen et al. (96) conducted the reaction with metals modified alumina as catalysts. Nitrates of Co, Cu and Ni were used as precursors for the metals incorporation via depositionprecipitation method. Catalyst acidity was dependent on the metal and its loading. The 19 wt.% Ni/Al2O3 catalyst contained 17 and 63 mmol/g of Brønsted and Lewis acid sites, respectively, whereas 14 and 76 mmol/g for the 16 wt.% Co/Al2O3 catalyst. With Cu modified catalysts, both Brønsted and Lewis acid sites increased with increasing Cu loading. The 1.8 wt.% Cu/Al2O3



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catalyst contained respectively 16 and 140 mmol/g of these sites that increased to 25 and 225 mmol/g for the 4.5 wt.% Cu/Al2O3 catalyst. Ethanol upgrading performed at 240 oC and 4.3 h-1 revealed both conversion and butanol selectivity to be dependent on the catalyst composition. The 16 wt.%Co/Al2O3 catalyst produced the highest conversion of 28%, followed by 19 wt.% Ni/Al2O3 catalyst with 25%. The two Cu modified catalysts produced similar initial ethanol conversion of 14% but the low loading (1.8 wt.%) showed a decay to 12% after 100 h. The Ni and Cu modified catalysts produced comparable butanol selectivity in the range of 60-65% but negligible values were observed with the Co modified catalyst. The activity of the Ni modified catalyst could be correlated to the work of Yang et al. (51), who found 19% conversion and 64% butanol selectivity. The Co modified catalyst produced mainly ethyl acetate via the dehydrogenative dimerization mechanism reported is some previous works (97-100). Other side reactions involved the production of ethylene, methane and diethyl ether (see Figure 6).

The

high butanol selectivity of 65% achieved with 4.5 wt.% Cu/Al2O3 (i.e. with denser Lewis acid sites) compared to 55% for the 1.8 wt.% Cu/Al2O3 catalyst, indicated the Lewis acid sites as favourable for the reaction. The authors (96) proposed that, inverse spinel metal aluminate structures provided the active centers required for enhanced butanol selectivity. These structures contained metal ions with octahedral configuration due to weak metal-support interaction and were dominant with Ni and Cu modified catalysts. Therefore, in addition to basicity-acidity properties, the mode of metal incorporation into the alumina frameworks plays an important catalytic role. With Co modified catalyst, the Co ions were tetrahedrally coordinated due to stronger metal-support interaction. This consequently favoured the selectivity to ethyl acetate formation via the dehydrogenative dimerization pathway (Figure 6). However, further studies are necessary to clarify these proposals. Riittonen and co-workers (101) have conducted a similar


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study, employing Ni, Ag and Ru modified alumina as catalysts. The reaction was conducted at 250 oC using 50 mg of catalyst. The 20.7 wt.%Ni/Al2O3 catalyst produced the best activity of 25% conversion and 80% butanol selectivity. The Ag modified catalysts containing 2-6 wt.% of Ag produced the worst ethanol conversion of 1% and maximum butanol selectivity of 20%. These catalysts were mainly selective to the formation of acetaldehyde (45-49%). With Ru modified catalysts, both conversion and butanol selectivity increased with increasing Ru loading. For the 1 wt.% Ru loading, the conversion was 8% with no any butanol selectivity, but the ethanol conversion and butanol increased to 12 and 9%, respectively. These authors argued that, the most possible reaction pathway with all the catalyst was the Guerbet mechanism involving aldol condensation of acetaldehyde followed by subsequent hydrogenation to butanol. Therefore, the high selectivity of the Ni modified catalyst could be attributed to high aldol condensation and hydrogenation properties. Although details on the catalyst coordination structure were not provided, it could be proposed that, the Ni modified catalyst possessed the octahedral coordination associated with weak metal-support interaction as reported in the above study (96).


A range of other modified oxide catalysts have recently been employed for the ETB reaction. Kozlowski and Davis (102) carried out the reaction with Na modified ZrO2 catalysts. Incorporation of 0.1 wt.% showed complete retention of BET surface area (12 m2/g) and pore volume (0.09 mL/g) of the pure monoclinic ZrO2 catalysts, but a higher loading of 1.0 wt.% reduced these values to 10 m2/g and 0.085 mL/g, respectively. However, the basic sites density



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increased from 1.4 to 3.6 µmol/m2 whereas the acid sites density decreased from 3.3 to 1.7 µmol/m2 following the incorporation of 1.0 wt.% Na. When ethanol conversion was carried out at 340oC, all the catalysts produced very low ethanol conversion that also reduced with increasing Na content (i.e. catalyst basicity). A similar trend was observed on the rate of butanol formation. It reduced from 1.1 nmol/m2.s for the pure ZrO2 to 0.4 and 0 nmol/m2.s for the 0.1 wt.% Na/ZrO2 and 1.0 wt.% Na/ZrO2 catalysts, respectively. The production of crotonaldehyde also obeys a similar pattern but that of acetaldehyde increased from 15 to 21 nmol/m2.s. These results can provide further insight into the reaction mechanism. The detection of acetaldehyde, crotonaldehyde and butanol in these trends indicated the possibility of the reaction to proceed via the Guerbet mechanism. Modification with Na reduces the chance of acetaldehyde transformation via aldol condensation to crotonaldehyde. Possibly, the Na particles destroyed the most active acid-base pairs in the pure ZrO2 which are required for this transformation. Padilla and workers (103) employed Co and Ni as modifiers instead of Na. 5 wt.% each of the metals was incorporated onto ZrO2 using nitrate precursor via impregnation method. The severe reaction conditions (700 oC and 74,000 h-1) adopted shifted the reaction mainly towards the formation of hydrogen (70-72%) and COx (x = 1, 2). All the catalysts produced traces of acetaldehyde, ethylene, acetone and diethyl ether with no butanol. Earley et al. (104) employed entirely different catalysts (i.e Cu/CeO2, Cu/ TiO2 and Cu/Al2O3) and conducted the reaction at 330 oC and 1.97 h-1 in the presence of CO2 ( 1 mL/min). The Cu/CeO2 catalyst demonstrated the best activity (84% conversion and 31% butanol selectivity) whereas the worst activity was observed with the Cu/TiO2 catalyst (9% conversion and 10% butanol selectivity). The Cu/Al2O3 catalyst produced high conversion of 82%, but the butanol selectivity declined significantly to 5%. The enhanced activity of the Cu/CeO2 catalyst was attributed to its ability to favourably


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promote aldol condensation step in the Guerbet mechanism. This was also favoured by the presence of CO2, which increases the surface acidity by re-oxidation of Ce into CeO2 without alteration to the active Cu particles.

Please insert Table 1 here.

In Table 1, results from some recent studies involving mixed oxide catalysts are presented. The results generally indicated the catalytic activity and butanol selectivity to be dependent on the catalyst composition and associated basicity properties. According to Marcu et al. (105), the content of Cu in a Cu-Mg-Al mixed oxide can influence catalyst basicity and ethanol conversion activity. The amount of basic sites decreased from 1.85 mmol/g when the Cu/Mg ratio was 1 to 0.89 mmol/g when the ratio reached 20. When ethanol upgrading was performed at 200 oC using 0.5 g of catalyst, ethanol conversion increased from 2.5% when the ratio was 1 to 4.5% when the ratio reached 10 before it later dropped to 3.8% when the ratio was further raised to 20. On the other hand, selectivity to butanol decreased, whereas the formation of acetaldehyde increased. The catalyst Cu5MgAl(3)O with Cu/Mg ratio of 5 produced the optimal butanol yield of 16.4%, therefore moderate concentration of Cu ions appeared most suitable for the reaction. Carvalho et al. (106) conducted a similar with mixed MgO-Al2O3 catalysts at 350 oC using 0.3 g of catalyst. Ethanol conversion declined from 40 to 37% when the Mg/Al ratio was raised from 1:1 to 3:1. However, the butanol selectivity increased from 22 to 38%. Although the MgO-Al2O3 (3:1) with lower basicity of 0.031 mmol/g showed higher selectivity to acetaldehyde (20%), its butanol yield was higher by > 5%. Therefore, this composition corresponds to best catalytic performance.



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It enhanced acetaldehyde transformation to butanol at the intermediate stages of the reaction. Nair and co-workers (107) employed MgOx-Al2O3 and VOx-Al2O3 as catalysts for the reaction at 180oC using 0.022 g of catalyst. Both catalysts were non-selective to butanol but produced 60 and 78% of acetaldehyde, respectively. However, the MgOx-Al2O3 catalyst was 2.5 times more active. Although the basicity data was not reported, it could be stated that the catalysts does not contain appropriate combination of basic and acid sites for butanol production. According to an in situ infrared study by Carvalho et al. (108), reaction with the mixed oxides proceed by Guerbet mechanism described earlier, therefore aldol condensation of acetaldehyde to form crotonaldehyde and subsequent hydrogenation to butanol play an important role in defining catalytic activity. The most active mixed oxides are those that favour these transformations. Butanol yield can be reduced due to the possibility of the catalytic active sites catalyzing further reactions involving the produced butanol. As shown in Figure 7, butanol can react with unreacted ethanol in the reaction stream to form C6 alcohols like 1-hexanol or undergo self-condensation to form C8 alcohols. The production of high (i.e. C4, C6 and C8) olefins can also not be ruled out, as these species have successfully been derived from oxide catalysts (109-111).


The work of Marcu et al. (112) compared the activity of Cu-Mg-Al mixed oxide with that of some other transition metal (i.e. M-Mg-Al) mixed oxides, containing similar 5 mol.% of metal ion in each case. The acidity-basicity properties were dependent on the catalyst composition. The Cu-Mg-Al catalyst possessed 1.58 and 2.30 mmol/g of basic and acidic sites, respectively. Pd-


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Mg-Al catalyst has higher amount of basic sites (1.94 mmol/g) but lower concentration of the acid sites (1.74 mmol/g) than the Cu-Mg-Al catalyst. The Mn-Mg-Al catalyst contained the lowest amount of basic sites (1.36 mmol/g) with the highest concentration of the acidic sites (3.37 mmol/g). On the other hand, the Yb-Mg-Al catalyst possessed 1.68 and 1.54 mmol/g of acidic and basic sites, respectively. When ethanol upgrading was carried out at 200 oC using 0.5 of catalyst for 5 h, the Cu-Mg-Al catalyst produced the highest ethanol conversion of 4.1% with 40.3 and 9.9% selectivities to butanol and acetaldehyde, respectively. The Pd-Mg-Al catalyst produced a slightly lower conversion of 3.8%, but was more selective to butanol formation (72.7%) and 12.3% of acetaldehyde. The other catalysts produced similar activities of < 2% conversion and 53 and 20% selectivities to butanol and acetaldehyde, respectively. According to the overall results, the Pd-Mg-Al catalyst produced the highest yield of butanol followed by the Cu-Mg-Al catalyst. Correlating with the acidity-basicity characteristics, it could be established that the Pd-Mg-Al catalyst contained closer concentrations of basic and acidic sites that were mainly of moderate strengths. This unique property enhanced its selectivity to butanol formation. The product distribution showed the reaction to proceed via the Guerbet mechanism, with limited side reactions for the formation of C6 and C8 alcohols and olefins (105, 113). Therefore, the PdMg-Al catalyst was more active for ethanol dehydrogenation to acetaldehyde followed by rapid aldol condensation and subsequent hydrogenation into butanol. The Mn-Mg-Al and Yb-Mg-Al catalysts were less active for these sequential reactions than the other catalysts due to their poor combination of the acidic and basic site pairs.

Zeolites can simply be described as hydrated aluminosilicate materials that possessed interlinked tetrahedra of alumina (AlO4) and silica (SiO4). They are usually characterized by exceptional



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stability properties than the HAP and oxide catalysts. The application of zeolite catalysts in ethanol upgrading relates to their topological and acidity/basicity properties (114, 115). The ability of these catalysts to exist in different crystalline structures with different pore sizes permits the possibility of enhancing the selectivity to desired reaction products. The shapeselectivity characteristics could also be modified by the incorporation of metals or modification to the zeolite synthesis route. Zeolite catalysts are also known to possess unique acidity-basicity properties and have demonstrated good activities in the acidity and basicity dependent reactions. However, only few studies have been conducted with zeolites for butanol production from ethanol. Yang and Meng (50) conducted a study with alkali metals modified X zeolite catalysts with similar Si/Al ratio of ~ 1.40. Ethanol upgrading conducted at 420 oC and 5.6 g.h/mol showed the Na-X and Li-X catalyst to demonstrated the highest reaction rates of 9 x 104 and 17.5 x 104 mol/min.g, respectively. However, these catalysts were 100% selective to the formation of gaseous products. No any butanol was detected or acetaldehyde was detected. These catalysts were therefore in-active for the condensation reaction of ethanol. The K-X catalyst was the least active material with a reaction rate of 4.8 x 104 mol/min.g but produced trace quantities of butanol (< 2%). The bimetallic Rb-Li-X and Rb-Na-X catalysts demonstrated similar ethanol transformation rates of ~ 6 x 104 mol/min.g with different selectivity to butanol of 40.9 and 36.6%, respectively. The results therefore indicated modification with bimetallic systems as better promising alternative for improving butanol selectivity. Reaction with these catalysts obeyed the Guerbet mechanism discussed earlier and the bimetallic composite allowed the formation of acid-base pairs for enhancing acetaldehyde condensation by aldol mechanism and subsequent hydrogenation to butanol. Recent studies conducted with protonic zeolites like HZSM-5 and H-BEA showed these catalysts as mainly selected to the production ethylene, higher


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olefins and aromatics (116-119). The catalyst acidity and topological properties shifted the reaction to ethanol dehydration, oligomerization and dehydroclyclization, without any evidence of condensation as no trace of butanol, acetaldehyde or crotonaldehyde was detected. Therefore, the protonic zeolites are not good candidates for the ethanol to butanol reaction. However, the acidity of these catalysts can be reduced by incorporation of metal modifiers (120, 121). This factor can also improve the density of basic sites in the catalysts (122). Detail studies with these catalysts are therefore required to evaluate the possibility of improving ethanol conversion and butanol yield. 3.2. Role of Reaction conditions The selection of suitable reaction conditions like temperature, ethanol/water concentrations, space velocity and contact time can have pronounce effect on the catalytic-activity, stability and selectivity to butanol versus side reaction products. Tsuchida et al. (56) evaluated the effect changing contact time on the catalytic performance of HAP catalyst with Ca/P ratio of 1.64 at 300 oC. Increasing the contact time from 0.02 to 0.89 s raised the ethanol conversion from 0.6 to 10.7%. The selectivity to butanol increased from 3.2 to 75.0%, whereas the selectivity of acetaldehyde declined from 61.7 to 4.4%. Raising the contact time creates more active acid-base pairs with potential to favour catalytic activity and butanol production. The results also showed the selectivity of C1-C3 hydrocarbons to decline, whereas the production of C6, C8 and C10 alcohols increased slightly with increasing the contact time. Therefore, the new active sites improved successive ethanol propagation to higher alcohols and that Guerbet reaction pathway was unaltered by changing the contact time. As shown in Figure 8, the propagation mechanism involved the initial adsorption of ethanol on the catalyst sites followed by subsequent interaction with pre-generated alcohol to form another alcohol of two carbons higher, with elimination of 23 ACS Paragon Plus Environment


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water molecules. Yang and Meng (50) conducted the reaction at 320 oC using a Rb-Li modified X zeolite catalyst (i.e. Rb-Li-X) at the contact times in the range of 3 to 15 gh/mol. Here, raising the contact time lowers the rate of ethanol activation. At the contact time of 3 g.h/mol, the rate was 0.6 mmol/min.g but dropped to 0.3 mmol/min.g when the contact time was raised to 15 g.h/mol. On the other hand, the selectivity to butanol initially increased from 35 to 42% when the contact time was raised from 3 to 5.6 g.h/mol before dropping to 28% when the contact time was further raised to 15 g.h/mol. Therefore, the contact time of 5.6 g.h/mol was the optimal butanol production condition. It ensures adequate acidity-basicity properties for this catalyst. At longer contact times, the production of acetaldehyde dominated the reaction, indicating that its further transformation into butanol via aldol condensation and subsequent hydrogenation was retarded.


Tsuchida and co-workers (56) studied the influence of reaction temperature on the activity of a HAP catalyst with Ca/P ratio of 1.64 using a constant contact time of 1.78 s. Raising the temperature from 300 to 450 oC increased ethanol conversion from 14.7 to 95.3%. However, this factor had a negative effect on the production of both butanol and acetaldehyde. The selectivity of butanol dropped from 76.3 to 6.0% whereas that of acetaldehyde declined from 3.0 to 1.3%. Therefore, the lower reaction temperature of 300 oC was the most suitable for butanol production with this catalyst. At this temperature, further reactions between ethanol and butanol and other side reactions were suppressed. However, at higher reaction temperatures of 400-450 oC, reactions such as ethanol-butanol condensation, dehydration of butanol and dehydrocyclization can take place. Therefore, the higher yield of aromatics (11.2%) and other hydrocarbon species


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obtained at high temperatures could be attributed to these side reactions. Earley et al. (104) conducted the reaction with Cu/Al2O3 and Cu/CeO2 as catalysts at variable temperatures of 190 to 330oC. The catalytic performance was both temperature and catalyst dependent. The Cu/Al2O3 catalyst produced 7% ethanol conversion and 6% butanol selectivity at 190 oC. When the reaction temperature was raised to 330 oC, ethanol conversion rapidly increased to 82%, but the selectivity to butanol dropped to 3%. Reactions conducted with Cu/CeO2 produced 16% ethanol conversion and 60% butanol selectivity at 190 oC that changed to 84 and 31% at 320 oC, respectively. The results therefore indicated the rise in temperature as an unfavorable factor for butanol production and the effect being more pronounced for the most active Cu/CeO2 catalyst. The production of high molecular weight alcohols and hydrocarbons increased as the temperature was raised over both catalysts, indicating that the side reactions were favoured by high temperatures.

Yang and Meng (50) reported the rate of ethanol conversion to increase with increase in reaction temperature when the reaction was conducted with Rb-Li modified X zeolite as catalyst using a contact time of 5.6 gh/mol. At 400 oC, the rate was 0.3 mmol/min.g but increased to 1.8 mmol/min.g when the temperature reached 480 oC. On the other hand, the selectivity to butanol declined. At 400 oC, the selectivity was 45%, but dropped to 15% at the temperature of 480 oC. The production of acetaldehyde, gaseous hydrocarbons and C6-products (including alcohols) increased remarkably as the temperature increases. Therefore, side reactions became dominant at elevated temperatures. Kolzowski and Davis (102) monitored the kinetics of butanol and acetaldehyde production from ethanol upgrading at various temperatures using 0.1 wt.% Na/ZrO2 as catalyst and ethanol flowrate of 0.39 µmol/m2s. The ethanol conversion was very



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low ( 70% selective to hydrogen production compared to < 50% for the bulk materials. They were also completely


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stable for 15 h period. Sun et al. (130) found nano-Au/SiO2 catalysts (3-5 nm) as very efficient catalysts for ethanol oxidation to acetic acid, acetaldehyde and acetyl acetate with 100% selectivity and 37-58% conversion at 210oC and 60,7950 Nm-2. The catalysts were completely stable for 6 h period. Viswanadham et al. (131) demonstrated nano-HZSM-5 catalyst (~ 30 nm) as more active and selective for ethanol transformation into gasoline range hydrocarbons than the bulk H-ZSM-5 catalyst at 450 oC and 2.0 h-1. The nano-catalyst produced 74% of gasoline range hydrocarbons compared to 59% for the bulk catalyst under similar conditions. Octane rating studies also indicated the composition of the two catalysts to have 91 and 87 RON values, respectively. There are some other issues that need to be addressed with the view of enhancing butanol selectivity/yield, catalyst lifetime and understanding the overall reaction chemistry. Only few studies have been reported with the zeolite catalysts. Therefore, further studies would be useful in handling some of these challenges. Catalyst preparation options and reaction conditions that can improve butanol selectivity and ensure longer catalyst lifetime must be appropriately explored. The possibility of mitigating coke deposition and limiting side reaction interferences should be adequately studied. The role of acidic sites during the reaction is still debatable whereas basic sites are considered as very active for high butanol selectivity. Further investigations should target the specific role of the acid sites and the acid-base pairs in enhancing catalytic activity. There are some arguments that, at low temperature below 400 oC the reaction proceeds through the Guerbet pathway, whereas through the direct bimolecular/coupling mechanism at higher temperatures. These proposals should be further evaluated as contradicting results have been reported. According to Tsuchida et al. (56), C6, C8 and C10 alcohols are produced from ethanol propagation process. This proposal should adequately be evaluated as the



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mechanism was first reported by these authors. The effect of coordination structure (i.e. tetrahedral or octahedral) of metal modified alumina catalysts on butanol selectivity needs further investigations as the few reported studies do not provide correlation data to ascertain the proposal that, the octahedral structure was more favourable for butanol production due to low metal-support interaction. The origin of carbonaceous deposits and their effects on butanol versus side reaction products selectivity was only partly discussed by the recent literature and must therefore be explored for proper resolution of the process mechanism. 5. Conclusion Bioethanol upgrading to biobutanol represents another option being viewed for handling the engine and efficiency problems associated with the utilization of ethanol as fuel blends. The reaction also has strong potentials to mitigate the problems of very low butanol yields encountered with the alternative biomass fermentation methods. The nature of the catalyst under consideration and the associated acidic versus basic sites strength can predict the yield of butanol obtainable from the reaction. Promoted zeolites, hydroxyapatites and the oxide systems were employed as catalytic materials for the reaction, yielding different activity/selectivity trends. The low yields (< 30%) of butanol achievable with these catalysts so far indicated that more studies are still required to improve system performance for commercialization. The ethanol conversion of ~40% and ~50% butanol selectivity obtained with the mixed oxide catalyst represent the optimal combination reported so far. Therefore, further investigations with the view of enhancing both the selectivity and conversion are necessary. There are some other issues that should be critically evaluated. These include the role of the acidic sites, details on the overall reaction chemistry and possible options for improving catalyst stability by enhancing resistance to poisons and the limiting coke formation. Another factors to be critically explored are the 30 ACS Paragon Plus Environment

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commercialization prospects of the ethanol to butanol route compared to other butanol production methods and the associated ecomomic implications. Acknowledgements The authors would like to acknowledge the funding provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit in Center of Research Excellence in Nanotechnology at King Fahd University of Petroleum & Minerals (KFUPM) for supporting this work through project No. 13-NAN1702-04 as part of the National Science, Technology and Innovation Plan.

List of Tables Table 1. Recent ethanol to butanol reactions with mixed oxide catalysts. Table 2. Role of reaction parameters on the behaviors of some catalysts during ETB reaction.

List of Figures. Figure 1. Recent ethanol production in the USA. Figure 2. A scheme for oxo-synthesis and crotonaldehyde hydrogenation methods for butanol production. Figure 3. A scheme of mechanisms for catalytic ethanol activation to butanol. Figure 4. Effect of Ca/P ratio of HAP catalyst on butanol selectivity at 20% equivalent conversion. Figure 5. Effect of Sr/P ratio on butanol selectivity for Sr modified HAP catalyst. 31 ACS Paragon Plus Environment


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Figure 6. Dehydrogenative dimerization of ethanol and other side reactions with potentials to lower butanol selectivity during ethanol upgrading. Figure 7. A scheme for high alcohols and olefins formation from butanol. Figure 8. Ethanol propagation scheme for C6, C8 and C10 alcohols production via the mechanism proposed by Tsuchida et al. (56).

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131. Viswanadham, N., Saxena, S.K., Kumar, K., Sreenivasulu, P., Nandan, D. Catalytic performance of nano crystalline H-ZSM-5 in ethanol to gasoline (ETG) reaction. Fuel, 2012, 95, 298–304.

Table 1. Recent ethanol to butanol (ETB) reactions with mixed oxide catalysts. Catalyst Reaction Ethanol Butanol Acetaldehyde Total conditions conversion selectivity, selectivity, % basicity, % mmol/g o Cu1MgAl(3)O 200 C, 0.5 2.5 43 8 1.85 g, 5h. Cu5MgAl(3)O 200oC, 0.5 4.1 40 10 1.58 g, 5h. Cu10MgAl(3)O 200 oC, 0.5 4.5 28 13 1.2 g, 5h. Cu20MgAl(3)O 200 oC, 0.5 3.8 18 4 0.89 g, 5h. MgO-Al2O3 350oC, 0.3 40 22 4 0.048 (1:1) g catalyst, 12% ethanol in N2, total flow = 40 mL/min. MgO-Al2O3 350 oC, 0.3 37 38 20 0.031 (3:1) g catalyst, 12% ethanol in N2, total flow = 40 40 ACS Paragon Plus Environment

Ref

(105) (105) (105) (105) (106)

(106)

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Al2O3

MgO

MgOx-Al2O3 VOx-Al2O3

mL/min. 350 oC, 0.3 g catalyst, 12% ethanol in total N2, flow = 40 mL/min. 350 oC, 0.3 g catalyst, 12% ethanol in total N2, flow = 40 mL/min. o 180 C, 0.022 g of catalyst. o 180 C, 0.022 g of catalyst.

90

None

None. 100% 0.043 selective to ethylene

(106)

05

None

80

0.063

(106)

Rate = 1.6 x None 103/mol.s

60

Rate = 0.6 x None 103/mol.s

78

Mg metal (107) density = 2 atoms/nm2 V metal (107) density = 2 atoms/nm2

Table 2. Role of reaction parameters on the behaviors of some catalysts during ETB reaction. Catalyst Reaction Ethanol Butanol Acetaldehyde Ref. conditions conversion selectivity, selectivity, % % Cu5MgAl(1)O 200 oC, 0.5 7.3 0.8 4.6 (105) g, 24h. 100% ethanol as the feed. Cu5MgAl(1)O 200 oC, 0.5 1.5 4.1 36.8 (105) g, 24h. 96% ethanol + 4% water as the feed. Cu7MgAl(3)O 0.5 g of 0.5 1.0 12 (105) catalyst, 200 oC, 5 h. Calcination temp. = 250 o C Cu7MgAl(3)O 0.5 g of 4.5 40 9 (105) catalyst, 200 oC, 5 h. Calcination 41 ACS Paragon Plus Environment


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Pd5MgAl(1)O Pd5MgAl(1)O

20.7 wt.% Ni/Al2O3 (commercial, HTC-500) 20.7 wt.% Ni/Al2O3 (commercial, HTC-500)

temp. = o 550 C 0.5 g of 4.2 catalyst, 200 oC, 5 h. 0.5 g of 17.5 catalyst, 260 oC, 5 h. 250oC, 7 x 5 106 Nm-2. Reaction time = < 1 h 250 oC, 7 x 26 106 Nm-2. Reaction time = 72 h.

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72

12

(112)

78

8

(112)

10

24

(101)

82

15

(101)

Figure 1. Recent ethanol production in the USA. Data source: Ref (15).


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Figure 2. A scheme for oxo-synthesis and crotonaldehyde hydrogenation methods for butanol production.



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Figure 3. A scheme of mechanisms for catalytic ethanol activation to butanol.


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Figure 4. Effect of Ca/P ratio of HAP catalyst on butanol selectivity at 20% equivalent conversion. Data source: Ref (61).



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Figure 5. Effect of Sr/P ratio on the butanol selectivity for Sr modified HAP catalyst (i.e. Sr10(PO4)6(OH)2). Reaction conditions; 300oC, 2.0g catalyst and 130 hg/mol. Data source: Ref (75).


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Figure 6. Dehydrogenative dimerization of ethanol and other side reactions with potentials to lower butanol selectivity during ethanol upgrading.



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Figure 7. A scheme for high alcohols and olefins formation from butanol.


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Figure 8. Ethanol propagation scheme for C6, C8 and C10 alcohols production via the mechanism proposed by Tsuchida et al. (56).


Catalytic Upgrading of Bioethanol to Fuel Grade Biobutanol: A Review

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