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bioweb.sungrant.org » Technical » Bioproducts » Bioproducts from Syngas » Mixed Higher Alcohols

Products fron Syngas—Mixed Higher Alcohols (Metal Catalyst)	Printer Friendly

In its simplest form, syngas (also called producer gas, town gas, blue water gas, and synthesis gas among others) is composed of carbon monoxide (CO) and hydrogen (H2), which provide the building blocks to produce a number of organic compounds. During the 1800’s, coal gasification was used for lighting and heating. The production of fuels and chemicals from syngas began in the early 20th century. Today, methanol and ammonia are produced from syngas, and in addition to hydrogen, constitute its major uses.    In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. Under today’s conditions, the least expensive feedstock is natural gas. Using numerous synthesis pathways, a large number of organic compounds can be produced from syngas (figure 1).            The oil embargos of the 1970’s spurred interest in the use of syngas to produce alcohols (methanol and higher alcohols) for blending with gasoline. Compared to methanol, higher alcohols (such as ethanol, butanol, etc.) have higher octane ratings (greater resistance to uncontrolled ignition in internal combustion engines), are less volatile, display a lower tendency toward phase separtation in the presence of water, and are more compatible with certain engine components (Mills, 1989). Using mixed alcohols (i.e., a mixture of methanol and higher alcohols) avoids the problems associated with the use of methanol alone. Additionally, mixed alcohols have lower vapor pressure, better solubility with hydrocarbon components, improved water tolerance, and higher overall heating value compared to methanol. And, when used as a diesel substitute at levels of 20-30% weight, the calorific value, lubrication properties, and ignition properties are improved compared to pure methanol (Höhlein, 1991). Mixed alcohols are also more compatible with the existing fuel infrastructure relative to methanol.   Historically, several processes have been developed and patented to make mixed alcohols from CO and H2, (e.g., Herman, 1991; Forzatti, 1991; Natta, 1957), however, commercial production has been hampered by poor selectivity and low product yields (typically around 10 percent of the syngas is converted to alcohol with methanol the most prevelant component) (Herman, 2000; Wender, 1996). Currently there are no commercial plants that produce mixed alcohols in the C2 to C6 range, largely due to the lack of appropriate catalysts.

Higher alcohol synthesis reactions
The mechanism for higher alcohol synthesis involves numerous reactions each with multiple pathways leading to a variety of products. The general reaction mechanism is shown in equation 1 (Hutchings, 1988; Wong, 1986).           nCO + 2nH2 → CnH2n+1OH + (n-1)H2O              (equation 1)                  (ΔHr = -61.2 kcal/mol)   with n typically ranging from 1 to 8 (Forzatti, 1991). The reaction stoichiometry suggests that the optimum CO/H2 ratio is 2, however, the reaction occurs simultaneously with the water-gas shift reaction (equation 2) which makes the optimum ratio closer to 1.            CO + H2O ↔ CO2 + H2                                  (equation 2)   The production of higher alcohols first involves the synthesis of methanol (equation 3).           CO + 2H2 ↔ CH3OH                                      (equation 3)   The first step in the production of higher alcohols involves the insertion of CO into the CH3OH to form a C-C bond. Production of linear alcohols involve stepwise C1 addition at the end of the chain to sequentially produce ethanol, propanol, butanol, etc. (Quarderer, 1986). Branched higher alcohols can be produced via a number of pathways including beta addition between C1 and Cn (n ≥ 2) to yield branched primary alcholhs such as isobutanol; beta addition between Cm (m = 2 or 3) and Cn (n ≥ 2); methyl ester formation via carboxylic acids formed from synthesized alcohols; and carbonylation of methanol to yield methyl formate (table 1) (Nunan, 1989; Xiaoding, 1987). Linear alcohols can proceed along the reaction path but branched alcohols are terminal products because they lack the 2 α-hydrogens required for chain growth (Hilmen, 1998).            The types of reactions that occur are affected by the operating conditions and the types of catalysts used. No kinetic analysis has been published that is capable of globally predicting product compositions over ranges of operating conditions (Beretta, 1996). Methanol formation is favored at low temperatures and high pressures (Courty, 1990). At high pressures, increasing temperature increases production of higher alcohols. Higher alcohol production is maximized when the H2/CO ratio is close to 1. Lower H2/CO ratios favor CO insertion and C-C chain growth. Methanol can be recycled to increase the yields of higher alcohols provided the catalyst used demonstrates good hydrocarbonylation activity (Courty, 1990; Quarderer, 1986). Thermodynamic constraints limit the theoretical yields of higher alcohols, and the heat generated during the chemical reactions must be removed to maintain control of process temperatures (Courty, 1984). Compared to methanol, production of higher alcohols generates more heat.   Water and carbon dioxide are produced as by-products of higher alcohol production. The water-gas shift reaction plays a major role in the types of compounds produced (e.g., depending on the catalyst’s shift activity, dehydration of alcohols can occur to produce higher alcohols, esters, and ethers) (Courty, 1990). Secondary reactions can occur, resulting in the production of hydrocarbons such as aldehydes and ketones (Courty, 1984; Courty, 1990). Methane is also frequently produced in substantial quantities (Roberts, 1992).    Production of higher alcohols occurs in reactors similar to those used in methanol and Fischer-Tropsch processes, and like those processes, removal of the large amount of excess heat generated during reactions is essential to maintaining control of the process temperature, to maximizing yields, and to minimizing catalyst deactivation by sintering. The use of slurry phase reactors is being evaluated at a pilot scale to produce isobutanol using a Cs-promoted Cu/Zn/Al2O3 catalyst in hydrocarbon oil (40 wt% slurry) at 12.5 MPa and 350°C (ChemSystems, 1990; Herman, 2000). Reactors with a “double bed” configuration are also being examined. These reactors are designed to optimize production of methanol from syngas in the first reactor using a Cu-based catalyst at a lower temperature, and then to produce higher alcohols (particularly isobutanol) in the second reactor which operates at a slightly higher temperature and uses a non-Cu Zn-chromite based catalyst to maximize the C-C forming steps (Verkerk, 1999).

Alcohol reaction catalysts
Catalysts play a pivotal role in syngas conversion reactions. The basic concept of a catalytic reaction is that reactants adsorb onto the catalyst surface, rearrange, and combine into products that then desorb from the catalyst surface. One of the fundamental functional differences between syngas synthesis catalysts is whether or not the adsorbed CO molecule dissociates on the catalyst surface. For higher alcohol synthesis, CO dissociation is a necessary reaction condition. For methanol synthesis the CO bond remains intact. The catalysts used to produce higher alcohols are bifunctional base-hydrogenation catalysts. An alkali metal is typically added to the catalyst to provide a basic site for the aldol condensation reaction to occur by activating the surface of the catalyst to adsorb CO and enhance the formation of the formate intermediate (Herman, 1991). Higher alcohol catalysts are classified into several groups based on their composition and include modified high pressure methanol synthesis catalysts, modified low pressure methanol synthesis catalysts, modified Fischer-Tropsch catalysts, and modified sulfide catalysts.   Modified high pressure methanol synthesis catalysts are catalysts that operate at temperatures ranging from 300 to 425°C and at pressures ranging from 12.5 to 30 MPa. They are used to produce branched primary alcohols and are composed of ZnO/Cr2O3 with an alkali added. Use of these catalysts to produce higher alcohols is referred to as oxygen retention reversal (ORR) aldol condensation with β-carbon (adjacent to the alcohol oxygen) addition (Herman, 2000). Between 1935 and 1945, these catalysts were used to commercially produce mixed alcohols from syngas, but demand decreased as petroleum became readily available and the desire for neat alcohols for manufacturing chemicals increased (Forzatti, 1991). Much of this early work is detailed in Natta, 1957.   A number of papers describe the properties and effectiveness of promoted Zn/Cr higher alcohol synthesis catalysts (Epling, 1997; Epling, 1998; Epling, 1999; Hoflund, 1997; Hoflund, 1999; Minahan,  1998 a,b). Cu-Zn-Cr oxides, without the addition of an alkali, have been shown to produce greater higher alcohol yields at low chromium (Cr) levels (15-21 wt% Cr). The chromia does not provide an active catalytic site but in small amounts, acts as a structural promoter that increases the surface area of the catalyst and helps inhibit Cu sintering (Campos-Martin, 1995). Methanol synthesis is fast compared to higher alcohol synthesis using a 3% K2O/ZnCr catalyst, but is still equilibrium limited, even at high space velocities. High CO2 has been shown to inhibit higher alcohol synthesis (e.g., a 3-fold decrease in C2+ alcohols at 400°C and 6% CO2) (Tronconi, 1989). Higher alcohol production was maximized under conditions of high temperatures, no CO2, a H2/CO ratio of 1, and a CO conversion rate of about 5 to 20%. The addition of Cs (at 0.3-0.5 mol% Cs; a surface concentration of 15-25%) has been shown to increase the ethanol synthesis rate and enhance the formation of higher alcohols (at 310°C, 7.6 MPa, H2/CO ratio of 0.45) (Nunan, 1989).   Modified low pressure methanol synthesis catalysts are used at temperatures ranging from 275 to 310°C and at pressures ranging from 5 to 10 MPa. They consist of Cu/ZnO or Cu/ZnO/Al2O3 with alkali added and are used to produce primary alcohols. Many of the early processes used these catalysts. The Octamix catalyst (25-40 wt% CuO, 10-18 wt% Al2O3, 30-45 wt% ZnO, and 1.7-2.5 wt% K2O; a Cu:Zn ratio of 0.4-1.9; and 3-18 wt% mix of oxidic promoters such as Cr, Ce, La, Mn, or Th) operating under conditions of 250 to 400°C, 10 MPa, a gas hourly space velocity of 1000-10,000/h, and a starting gas composition of 25-30% CO, 0-8% N2, 0-5% CO2, and 0-5% CH4 in a balance of H2, resulted in between 21-29% of the CO converted to alcohol with 29 to 45% of the alcohols being C2+ alcohols and 17 to 25% of the CO converted to CO2 (Xiaoding, 1987). Methanol is the most abundant (~80%) oxygenated product produced when using these catalysts and the oxygenated products produced typically have fewer carbons compared with products produced using modified high temperature catalysts. Several articles detail the effectives of these catalysts including Elliott, 1988; Nunan, 1989; Smith, 1984, 1992.   Modified Fischer-Tropsch catalysts, CuO/CoO/Al2O3 operate at temperatures between 260 and 340°C and pressures of 6 to 20 MPa and catalyze the production of linear primary alcohols that follow ASF distribution. Developed by the Institut Français du Pétrole (IFP), modified Fischer-Tropsch catalysts are homogeneous mixed-oxide formulations containing Cu and Co on an alumina support which forms the active components for higher alcohol synthesis. It is modified with Zn and alkali metals. The catalysts were designed for process conditions similar to those of low temperature methanol synthesis conditions (5-15 MPa, T = 220-350°C, H2/CO = 0.5-4 with CO2 also as a reactant) (Xiaoding, 1987). Patented IFP catalyst are, on an element basis, 10-50% Cu, 5-25% Co, 5-30% Al, and 10-70% Zn and the alkali/Al ratio is 0-0.2, the Zn/Al ratio is 0.4-2.0, the  Co/Al ratio is 0.2-0.75, and the Cu/Al ratio is 0.1-3.0 (Xiaoding, 1987). Greater homogeneity leads to improved catalyst performance. Use of IFP catalysts yield mainly saturated, straight-chained terminal alcohols that follow an Anderson-Shultz-Flory-type (ASF) distribution for chain growth. At optimal conditions, 5 and 30% of the CO and CO2 is converted to a liquid product containing 30-50% higher alcohols with hydrocarbons byproducts. The lack of long-term stability and low activity of these catalysts hinders their commercial application. At a pilot-scale, catalyst lifetimes of up to 8000 hours have been achieved with little deactivation, caused mainly by coke formation and sintering that decreases the homogeneity of the catalyst (Xiaoding, 1987). Further discussion of IFP-type catalysts can be found in Courty, 1984; Courty, 1990; Courty, 1982; Dai, 1989; and Dalmon, 1992.   Modified sulfide catalysts (mainly MoS2) are used at temperatures between 260 and 350°C and pressures between 3 and 17.5 MPa and result in the production of linear alcohols. They were independently developed by both Dow Chemical and Union Carbide in 1984 (Herman 1991). Sulfide catalysts are well known hydrogenation catalysts, but the addition of alkali metals as dopants shifts production from hydrocarbons to alcohols. The alkali metal suppresses the hydrogenation activity of the molybdenum (Mo) active site and provides additional active sites for alcohol formation. Cesium (Cs) is the most effective alkali promoter (Herman, 2000), although addition of potassium (K) has also been extensively studied. The catalysts selectively (75-90%) produce higher alcohols from syngas with a 10% CO conversion efficiency when the H2/CO ratio is one (Herman, 1991). Both higher alcohols and hydrocarbons formed over sufide catalysts follow a similar ASF molecular weight distribution. Sulfide catalysts are extremely resistant to sulfur poisoning, and in fact, require 50-100 ppm sulfur in the feed gas to maintain the sulfidity of the catalyst (Courty, 1990). H2S in the feed gas is also thought to moderate the hydrogenation properties of the catalyst and improve selectivity of higher alcohols by reducing methanol production (Forzatti, 1991). Sulfide catalysts are less sensitive to CO2 in the syngas than other catalysts, and the presence of moderate to low (7%) CO2 concentration in the syngas does not significantly impact CO conversion, but the presence of large amounts (>30%) retards their activity (Herman, 1991). However, even low levels of CO2 shift the reaction toward methanol rather than higher alcohols (Herman, 1991). Adding cobalt (Co) to alkalized MoS2 catalysts increases the production of ethanol and other higher alcohols due to promotion of homologation of methanol to ethanol by Co (Forzatti, 1991). Sulfide catalyst activity can be affected by the support material (Avila, 1995; Bian, 1998; Iranmahboob, 2002 a,b; Li, 1998).   Rhodium (Rh) based catalysts are another group of catalysts that are not specifically used to produce higher alcohols but have been developed for selective ethanol synthesis. Other C2 oxygenates (i.e., acetaldehyde and acetic acid) as well as increased levels of methane production are also synthesized using Rh-based catalysts (Nirula, 1994). However, the high cost and limited availability of rhodium significantly affects its commercial potential (Xiaoding, 1987).   A major hurdle to the commercial production of higher alcohols from syngas is the need to improve the selectivity and productivity of catalysts (Fierro, 1993). To date, modified methanol and modified FT catalysts have been more effective in the production of mixed alcohols; the sulfide-based catalysts tend to be less active than the oxide-based catalysts (Herman, 2000).

Commercial production of mixed alcohols
Currently there are no commercial plants that produce mixed alcohols in the C2 to C6 range. Table 2 summaries work by companies that have actively pursued mixed alcohols research. In terms of commercial development, Dow, IFP and Snamprogetti are the most advanced (Nirula, 1994).   Snamprogetti (also referred to as SEHT - Snamprogetti, Enichem, and Haldor Topsoe), operated a 12,000 tonne/yr pilot plant in Pisticci, Italy from 1982-1987 using a process similar to that for methanol production but using a different catalyst and a higher operating temperature (Olayan, 1987; Courty, 1990; Mills, 1989; Nirula, 1994). The syngas was produced via POx of natural gas and the mixed alcohols synthesized in a fixed bed adiabatic reactor. The alcohols are purified using 3 distillation columns-the first removes methanol and ethanol, the second removes water, and the third recovers the C3+ alcohols using azeotropic distillation with cyclohexane. The process reduces the water content from 20% to less than 0.1 wt% (El Sawy, 1990; Fox, 1993; Ricci, 1984). The alcohol mixture (called MAS, Metanolo piu Alcoli Superiori – methanol plus higher alcohols) was blended into gasoline at about 5 vol% and marketed successfully as a premium gasoline known as SUPER E (Mills, 1989; El Sawy, 1990). Blends of 10 vol% were also tested in 13 different types of European cars and were similar to conventional gasoline with respect to driveability, acceleration, octane, and fuel economy (Ricci, 1984). Exhaust emissions varied depending on the type of car, but generally, CO and unburned hydrocarbons were reduced (as much as 40% and 18% respectively), with slightly higher NOx emissions (Ricci, 1984). Extensive research was largely discontinued because of the availability of large amounts of cheap petroleum.   Dow tested 2 high pressure reactors (one fluid bed and one fixed bed) using catalysts that are resistant to sulfur and contain molybdenum (Mo), tungsten (W), or rhenium (Re), either free or combined with alkali or an alkaline-earth metal, with an optional support (Nirula, 1994). Using a catalyst containing 21% Mo and 1.5% potassium (K) on a carbon support achieved a 23.4% CO conversion under conditions of a H2/CO mixture of 0.84, a temperature of 262°C, and pressure of 7.2 MPa. The product mix consisted of 25 mol% methanol, 12.4 mol% ethanol, 6.8 mol% 1-propanol, 2 mol% 1-butanol, and 0.3 mol% C5 alcohols (Quarderer, 1984). In addition to the mixed alcohols, a substantial amount of CO2, methanol, and hydrocarbons were also produced (Xiaoding, 1987). Adjusting the process conditions altered the relative amount of methanol and higher alcohols produced with the higher alcohols composed mostly of straight chain C2-C5 alcohols. The reported water content is low (0.2% to 2-3 wt%) depending on study (Quarderer, 1984, 1986).            The Octamix process, developed by the Institute of Energy Process Engineering at the Research Centre Julich in cooperation with Lurgi was tested in a 2 tonne/day facility in 1990 (Goehna, 1989; Höhlein, 1991). The process was similar to a low pressure methanol synthesis process, except that CO2 was removed and the product purified. The syngas is produced by a combination of steam reforming and autothermal reforming with a tubular reactor used to synthesize the mixed alcohols. The water content of the crude mixture is only 1-2% which permits the use of a stabilizer column instead of distillation or molecular sieves to dry the product (El Sawy, 1990; Fox, 1993). The water content of the final product is 0.1-0.3 wt% (Goehna, 1989). The Octamix product was granted an EPA waiver to be used as a gasoline additive on February 1, 1988 (El Sawy, 1990).   The IFP (also called Substifuel) process is in the laboratory stage and involves a 20 BPD facility built in Chiba, Japan (Courty, 1990; El Sawy, 1990). The process uses steam reforming followed by multibed quench synthesis reactors and methanol distillation, extractive distillation with diethylene glycol (DEG), and distillation to recover the DEG (El Sawy, 1990; Fox, 1993). The final product contains about 0.2 wt% water.   The EcaleneTM process to produce mixed alcohols is currently at the bench scale but is being scaled up to a 500 gallon/day pilot plant. The project is a collaborative effort between Western Research Institute (WRI) and Power Energy Fuels, Inc. (PEFI), who developed the technology. The composition of the alcohol mixture, which can vary depending on process parameters, is shown in table 3. Ethanol is the principle alcohol. The synthesis reactor operates at 290-360°C and 145-1,595 psi (Lucero, 2001).            A number of studies have examined the cost of producing higher alcohols using natural gas as the feedstock. Key elements of these studies are summarized in table 4. Generally, about 50 percent of the costs of producing alcohols from syngas are for the capital costs associated with syngas production, with 29% of the costs attributed to alcohol synthesis, 17% for CO2 removal, and 4% for alcohol fractionation (Courty, 1990). Economies of scale would improve the economics of mixed alcohol synthesis but it is also extremely important to reduce energy losses as well as overall heat and momentum transfer duty (Lange, 1996).

References
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