Methanol synthesis

Methanol is inside the top 10 produced molecules. Methanol has been a common chemical feedstock for several important chemicals such as acetic acid, methyl ter-butyl ether (MTBE), formaldehyde and chloromethane. Moreover, methanol being a clean liquid fuel could provide convenient storage of energy for fuel cell applications, particularly in transportation and mobile devices. In additions, over the  last few decades, methanol-to-hydrocarbons (MTHC) technologies, in particular methanol-to-olefin (MTO) and methanol-to-gasoline (MTG), have been the focus for a large number of researcher dealing with the upgrading of natural resources beneficial both for the petrolchemistry and fuel industries. The process to synthesize methanol from carbon monoxide and hydrogen was introduced by BASF in 1923 and it was the second large-scale application of catalysis (after ammonia synthesis) and high-pressure technology (100-300 bar) to the chemical industry. Like the ammonia  process, methanol synthesis was dependent on the development of an effective catalyst, but unlike the ammonia synthesis catalyst, the methanol catalyst had to be selective as well as active. The reactions involved in the methanol synthesis are:

Reaction (B) and (C) combined are equivalent to reaction (A), so that either, or both, of the carbon oxides can be the starting point for methanol synthesis. Reactions (A)一(C) are exothermic; reactions (A) and (B) are accompanied by a decrease in volume. Hence, the value of the equilibrium constant decreases with temperature and increases with pressure (Kp=pCH₃OH/pcop2H₂). Thus, high conversions to methanol, given a sufficiently active catalyst, will be obtained at high pressures and low temperatures.

In addition to the synthesis of methanol, both carbon monoxide and carbon dioxide can take part in other hydrogenation reactions, producing by-products such as hydrocarbons, ethers and higher alcohols:

These reactions are much more exothermic than the methanol synthesis reactions and methanol is thermodynamically less stable and less likely to be formed from carbon monoxide and hydrogen than the other possible products, such as methane. Which of the products is formed is controlled by kinetics factors; that is, by the catalyst being selective in favoring a reaction path leading to the desired product.

The catalytic synthesis of methanol from syngas has been conventionally carried out in two-phase reactors with the syngas and products in the vapor phase and the catalyst as solid phase. The large exothermic heat of reaction in addition to the low heat capacity of the vapor increases the potential for thermal runaway and damage to the catalyst in the vapor phase, thus limiting the maximum operable reaction temperature.

methanol synthesis catalyst

There are two class of catalysts studied and used for methanol synthesis: high-pressure and low-pressure catalysts (Tab. 1 .1).

Hiqh-pressure catalysts

The catalyst used in the original process was derived by empirical methods. It contains zinc oxide and chromic and was used in the high-pressure process for 40 years. Zinc oxide alone was a good catalyst for methanol synthesis at high pressure and temperature above 350℃, but it was not stable and quickly lost its activity. It was found that die-off could be retarded by the incorporation of chromic, which acted as a stabilizer preventing the growth of the zinc oxide crystals.

Today zinc oxide, still a major component in synthesis catalysts, is known to have a defect structure with a non stoichiometric oxide lattice which is probably responsible for its catalytic activity. Stabilizers such as chromic probably prevent recrystallization of the zinc oxide and preserve the defect structure, as well as preventing crystal growth and loss of surface area. The most active zinc oxide was obtained from the zinc carbonate mineral smithsonite, containing traces of various impurities, which acted as "promoters", probably by forming solid solutions. The promoters may have both increased the specific activity of the zinc oxide by inducing lattice defects, and stabilized the zinc oxide by the inhibition of sintering.

The zinc oxide/chromic catalyst was tolerant of the impure synthesis gas, and could have a plant life of several years. It was not very selective and depending on synthesis conditions, as much as 2%of the inlet carbon oxide could be converted to methane, with a similar proportion to dimethyl ether. Because these side reactions are very exothermic, careful control of catalyst temperature was necessary. The catalyst was made by precipitation from zinc and chromium solutions or by impregnation of, for example, zinc carbonate with chromic acid or dichromate solution. In the catalyst produced by impregnation, the chromium was present in the hexavalent form, as CrO₃ rather than Cr₂O₃ as in the precipitated catalyst. Cr(VI) is particularly the catalyst. The chromate toxic and present a health hazard in the handling of also has to be reduced with great care to avoid a temperature runaway, because the reduction of CrO₃ to Cr₂O₃ with hydrogen is extremely exothermic:

Although the low pressure process based on high-activity catalysts has almost entirely taken the place of the high pressure process, there have been occasions when high pressure plant being supplied with pure synthesis gas could, to advantage, use a catalyst of higher activity. The copper/zinc oxide/alumina type was found to be very satisfactory, although at the synthesis pressure of 350 bar, with high partial pressure of water, sintering processes are accelerated.

Low-pressure catalysts

The methanol synthesis process has been operated at high pressure since its The methanol synthesis process has been operated at high pressure since its invention by BASF in 1920s. With the development of newer catalysts, the operating pressure was obviously decreased'S. Among those catalysts copper oxide appeared to have little activity itself, but was very effective when added to zinc oxide. This was also the case when copper was added to the zinc oxide/chromic catalyst, so it could be used at temperature as low as 300℃. However, the catalysts containing copper were not stable and lost activity. For example, a Zn/Cu/Cr catalyst in atomic proportions of 6/3/1 lost 40% of its activity in 72 hours. However, as a result of the ICI work on methanol catalysts, stable copper catalysts were produced. The loss of activity of the early copper catalysts in use was almost certainly due to loss of copper surface area and sintering, despite the presence of zinc oxide and other stabilizers. Although ZnO has activity for methanol synthesis, this is so much less than that of copper that acts as an inert diluent in copper/zinc oxide catalysts.

In an oversimplified model, the catalyst can be considered as particles of copper metal, surrounded and kept apart by stabilizer particles. The smaller the diameter of the refractory spacers, the higher is the practical copper loading and the higher the activity. High metal concentrations are used in industrial catalysts: in use, both metal and spacer dimensions increase and the rough relationship still holds. In the copper/zinc oxide/alumina, synthesis catalysts high activity and stability are obtained by optimizing the compositions and producing very small particles of the components in a very intimate mixture. By precipitation at a controlled pH, in which the acidic and alkaline solutions were mixed continuously, a catalyst of optimum composition and particle size is obtained. Although chromic had been effective as a stabilizer in the high pressure catalyst, alumina is superior to it as the third component in the low pressure catalyst. The alumina, present as a high surface area, poorly crystalline phase, is more effective than zinc oxide in preventing the sintering of copper crystallites. It might then be thought that a copper/alumina catalyst would be superior to the ternary catalysts, but there are several reasons why this is not so.

High area aluminas have acidic sites on the surface, which catalyze the parasitic reaction of methanol to dimethyl ether, so Cu/Al₂O₃ catalysts have poor selectivity. In the ternary catalysts, formation of dimethyl ether is very low, showing that the acidic sites are neutralized by the basic zinc oxide (either as a surface reaction only or in bulk reaction to zinc spinet).

The chemistry of the metal and support precursors, formed in the precipitation process, gives smaller copper crystallites with zinc oxide support than with alumina. Hence, the optimum combination of high initial activity and catalyst stability is obtained with the ternary Cu/ZnO/A1₂O₃ catalyst.

Although poisoning is not usually a significant problem in well-run methanol plants, the presence of a "poison-soak" in the support phase can be useful. Zinc oxide is much more effective than alumina in picking up and holding typical poisons, such as sulphur and chlorine compounds. Cu/ZnO/Al₂O₃ catalysts are still widely used and studied for the industrial synthesis of methanol. Various methods have been developed for the preparation of these kinds of catalysts. The difference in preparation methods, synthesis conditions, and pretreatment have a considerable influence on the structure of the catalysts, which finally leads to disparities in the catalytic performance. It is generally accepted that a large specific Cu surface area leads to an active methanol synthesis catalyst. In addition, the metal-support interaction plays a key role in this catalytic reaction.

Hydrocarbons and higher alcohol are thermodynamically more stable than methanol and could be formed. The extent to which this occurs is controlled by the selectivity of the catalyst. In this respect, the Cu catalysts are superior to the ZnO/Cr₂O₃ catalyst. Methanation needs to be minimized, both because it is a loss of feedstock and, as a highly exothermic reaction, it can cause temperature runaways. As the by-products are formed in parasitic (parallel) reactions and not by any intrinsic inefficiency of the synthesis reaction itself, improved activity for methanol synthesis alone, also give improved selectivity. Thus,the copper catalysts are conditions, but more selective than ZnO catalysts under the same process there is a further benefit. The higher activity allows the use of lower process temperatures, so by-products formation is further decreased relative to methanol synthesis, because most of the parasitic reactions have higher activation energies. The formation of by products is influenced markedly by any impurities in the catalyst, either left in the catalyst during its manufacture or introduced during use. Thus, alkaline impurities can result in the production of higher alcohols and, in addition, cause some decrease in activity. Similarly, acidic impurities (for example silica) can lead to the formation of high molecular weight waxes on the catalyst, which can cause loss of activity by blocking some of the smaller pores. The weaker acidity found on the surface of high area aluminas does not give waxes, but it catalyses the dehydration of methanol to dimethyl ether. These potential problems are  eliminated by careful design and manufacture of the catalyst.

The presence of VIII group metals, such as iron, is particularly undesirable, as they increase hydrogenation activity and promote the dissociation of carbon monoxide and dioxide, leading to formation of methane and long-chain paraffins and/or waxes by Fischer-Tropsch type reaction. Methanation was always a problem with the high-pressure catalyst, and it is thought that this was mainly caused by iron impurities. Other VIII group metals, as cobalt or nickel, can also catalyze methanation but they have been found to have a further deleterious effect. Methanol synthesis itself is inhibited, probably by surface coverage of copper crystallites with support oxide. Thus, alkali, acidic species and VIII group metals can be regarded as poisons for synthesis catalysts. In addition, traces of sulphur and chlorine can reduce the activity of the copper catalyst by reacting with the active surface. Chlorine is particularly undesirable because, in addition to poisoning of copper surface, the copper chloride produced is mobile and cause rapid sintering of the metal. Chlorine and sulphur also react with the free zinc oxide in the catalyst, so that much of the uptake occurs on the catalyst near the inlet to the bed, which therefore acts as a guard for the rest of the bed. Poisoning of synthesis catalyst is not normally a problem except as a result of maloperation.

Although extensive studies have been carried out, controversies remain concerning the role of active sites involved in the catalysts, the effect of the additions of various promoters and the reaction mechanism's. The active sites in Cu/ZnO catalysts have long been studied. On this issue, researchers are roughly divided in two groups: one insists that metallic Cu atoms are homogeneously active for the methanol synthesis, and the other claims that special sites (or active sites) exist in addition to metallic Cu atoms.

Also for ZnO the model proposed are divided in four groups (i) ZnO increases the dispersion of Cu; (ii) ZnO acts as a reservoir for atomic hydrogen spilling over onto the Cu surface to promote hydrogenation process; (iii) ZnO stabilizes some active planes of Cu or the morphology of Cu particles and (iv) ZnO creates active sites on the Cu surface.

Several non-conventional catalyst types have attracted attention because of their high activity for methanol synthesis. These include:

Allovs: The Raney methods has been utilized for catalyst preparation, dissolving aluminum from CuAl₂ alloys with or without the presence of dissolved zinc. Catalysts so prepared have high activity and selectivity. Properties depended on leaching conditions, and it was found that greatly improved catalysts could be obtained by low temperature leaching in caustic solutions that contain near-saturation levels of sodium zincate.

Alloys: Cu- rare earths-Binary copper-thorium alloys were prepared, followed by oxidation of the thorium to thoria, then reducing the copper oxide formed to copper metal. This catalyst was active for methanol synthesis over a range of conditions. Catalysts derived from rare earth-copper intermetallic alloy precursors have been found to exhibit extraordinarily high activity for CO hydrogenation to methanol. Typical precursor alloys were prepared by melting Cu with Ce or La. Methanol synthesis over Cu一La catalysts was observed at temperatures as low as 100℃.

Supported platinum, group VIII metals-Catalysts containing Pd can produce sizeable amounts of methanol when operated under conditions where methanol is thermodynamically stable. Of the platinum group metals, rhodiumcatalysts have received most attention. Supported rhodium  catalysts containing large amounts of molybdenum are particularly active. While selectivity towards alcohols is enhanced by molybdenum additions to about 60% alcohols and 40% hydrocarbons at 250℃ such selectivity is not attractive commercially.

Reaction mechanism

The methanol synthesis reaction has been the subject of many mechanistic studies since the process was first introduced. Bridger and Spencer in a detailed analysis, reported in the Catalyst Handbook, show how the results of many studies are often conflicting due to the properties of different type of catalysts (which often depend on details of preparation), to the different bulk phases present in the operating catalyst (varying with process conditions), to the reaction conditions and poisons.

All industrial plants operate with CO/CO₂/H₂ mixtures and it is not obvious which carbon oxide is the source of methanol. In most studies, it was concluded or assumed that adsorption of carbon monoxide is the starting point, but also evidence has been obtained indicating that methanol is synthesized directly from carbon dioxide. Certainly, the presence of CO₂ in the reacting gas has a market effect in increasing reaction rate, and the effect is reversible, even if it was found

also that using a zinc/chromic catalyst the carbon dioxide decrease the synthesis rate.

On Cu/Zn/A1₂O₃ catalysts, the role of carbon dioxide on the methanol synthesis reaction is very important. It was observed that the optimal loading of carbon dioxide depends upon the operating temperature. The reactions describing the methanol  synthesis over Cu/Zn/Al₂O₃ catalyst primarily involve the CO₂ hydrogenation reaction and the forward water gas shift reaction:

The CO₂ concentration in the syngas feedstock is very important to the right balance of the two reactions. If the CO₂ concentration is too high, the direction of the WGS reaction is reversed and carbon dioxide is competitively consumed in both the reactions. If the CO₂ concentration is too low, the potential for carbon deposition and the reduction of catalyst oxides is increased, thereby increasing the catalyst deactivation. The catalytic activity drops significantly and progressively when it is exposed to a CO₂-free syngas feed. The catalyst deactivates in absence of carbon dioxide.

For what concern the state of the active metals in a working catalyst, it was demonstrated that the catalyst surface is in a dynamic state. The support oxides play a minor role in the reaction mechanism, but it has been observed that the coverage of adsorbed oxygen varies with different support oxides.

A surface formate is the pivotal intermediate over both zinc oxide and supported copper catalysts. Surface formate is made by the hydrogenation of adsorbed carbon dioxide, and the rate determining step in methanol synthesis appears to be hydrogenolysis of the formate intermediate first to methoxy and then to methanol. The remaining adsorbed oxygen is removed by carbon monoxide or hydrogen, depending on the reaction conditions, to give CO₂ or H₂O. The

concurrent water gas shift reaction probably occurs via adsorbed oxygen rather than formate intermediate:

The active sites are on the surface of the copper metal crystallites. The surface of copper is mobile under reaction conditions and it is partially covered by a mobile layer  of  adsorbed  oxygen.  Adsorbed  hydrogen  mostly  is  present  on  the uncovered copper metal (even if requiring O_(ads) for adsorption), whereas carbon dioxide adsorbs on the oxidized surface. Thus, the site of reaction consists of bare copper metal atoms next to an oxide surface site, i.e. a Cu(0)/Cu(I) site, but this occur in different parts of the surface as the reaction proceeds.

The mechanism of the synthesis of methanol on copper catalysts from CO₂-free gas mixtures is not well understood. Formate intermediates, made from residual O_(ads),  may be involved again or successive additions of H_(ads) to adsorbed carbon monoxide may give first adsorbed methoxy and then methanol.

Methanol plants

Although the chemistry of methanol manufacture is simple, CO+2H₂ → CH₃OH, several process steps are required for a modern plant for methanol production. Conversion of syngas to methanol per pass in present commercial operation is limited to 25 vol.%or less by thermodynamic restrictions, even with modern catalysts and plant design, costly recycle is required.

Synthesis gas of an appropriate composition (ideally: ) is supplied recycled to the loop and circulated continuously, so that unreacted gas over the catalyst. From the converter the gas passes to a condenser, which removes the crude methanol liquor, and then to the circulator, which takes the gas back to the converter. Fresh "make-up gas" is supplied continuously to maintain the pressure in the loop as the synthesis proceeds. This gas always departs from the ideal composition to some degree: methane and other inert gases are present and frequently there is excess hydrogen, depending on the feedstock and processes used to manufacture the make-up gas. To prevent these gases from building up in the loop and diluting the reactants, a continuous purge is taken off. Depending on process economics, the purge may be used as fuel for the reformer or it gives a supply of hydrogen after passage through a pressure swing adsorption unit.

The crude methanol is distilled to separate the methanol from water and impurities such as  higher alcohols, ethers, etc., that are present in low concentrations.

The use of low conversion per pass together with the recycling of the unreacted gas around the loop facilitates the control of temperature in the catalyst bed. Nevertheless, the highly exothermic nature of the reaction requires the use of special reactor designs.

Part of the circulating gas is preheated and fed to the inlet of the reactor. The remainder is used as quench gas and is admitted to the catalyst bed through lozenge distributors in order to control bed temperatures. Temperature control within the catalyst bed can also be achieved using tube-cooled or steam raising reactor designs. The tube-cooled reactor combines a lower capital cost than the quench reactor with simplicity of operation and very flexible design. Circulating gas is pre-heated by passing it through tubes in the catalyst bed. This removes the heat of reaction from the catalyst bed. Temperature control is achieved by using a gas bypass around the catalyst bed and by controlling heat recovery in a feed-effluent heat exchanger immediately downstream. Two configurations are possible with the steam-raising type of reactor. The catalyst may be contained within the tubes of a shell-and-tube heat exchanger, with boiling water acting as coolant on the shell side. Alternatively, the catalyst may be contained within the shell with boiling water in the tubes, and this arrangement is similar to that used in the tube-cooled reactor design, as in both reactors the catalyst bed is cooled by fluid flow through tubes in the bed. Since the design with catalyst on the shell side allows the heat transfer surface area to be reduced to as little as one-seventh that of the design with catalyst in the tubes, the former is the preferred configuration.

In the ICI steam-raising reactor design, circulating gas enters through a vertical distribution plate and flows transversely across the catalyst bed. This allows the steam coils to be placed asymmetrically within the catalyst bed, so optimizing heat removal, and giving a small pressure drop. In plants based on natural gas, steam-raising reactors are less efficient at recovering the heat of reaction than quench or tube-cooled designs. This is because heat can be recovered into boiler feed water from the quench or tube-cooled designs, which can then be used to generate high pressure rather than intermediate-pressure steam. Chem Systems first developed the liquid phase methanol synthesis process (LPMeOH^TM) in the late 197Os. The novel feature of this technology is that methanol is derived from synthesis gas over a finely powdered commercial Cu/Zn/A1₂O₃ catalyst dispersed in an inert liquid. The slurry phase operation facilitates easy heat removal, thus enabling isothermal conditions in the reactor system. High agitation rates in the reactor provide for a reaction environment without temperature and concentration gradients. The salient features of the liquid phase process are:

    (i)   use of CO rich gas;

    (ii)   enhanced heat transfer of exothermic heat;

    (iii)   high once-through conversion of syngas.

The liquid acts as a heat sink and, by making possible efficient heat exchange, limits temperature increase. The catalyst size must be 1-10μm and need in situ activation.