Hydroformylations high-pressure operation

Supercritical co2 Solubility of product/extract in supercritical C02 Hydroformylation [198, 199], hydrogenation [200] Increased selectivity High pressure, operational safety, C02 emission... 

The rate of hydroformylation increases with increasing hydrogen and decreases with increasing carbon monoxide partial pressures (9), suggesting that rates of hydroformylation would be satisfactory at high H2 and low CO partial pressures. In industrial practice, however, high pressures of both H2 and CO ate required in order to stabilize the HCo(CO)4 catalyst at the temperatures necessary for practical rates (10). Commercial processes, for example, operate at >24 MPa (3480 psi) and >140 C. 

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

For this reaction, the early investigations of Reppe pointed out the need for catalyst precursors to operate at high pressure [2], It is necessary to work at 150-300 bar of CO in order to stabilize the two catalytic species [Co(H)(CO)4] or [Ni(H)(X)(CO)2] that adopt a mechanism analogous to the cobalt-catalyzed hydroformylation [44,45]. Many industrial applications have been reported [28,46,47] for the synthesis of plasticizers and detergents. Similarly, the two-step methoxycarbonylation of 1,3-butadiene has been explored by BASF and other companies to produce dimethyl 1,6-hexanedioate (adipate) directly from the C4 cut [28,48]. The first step operates at 130 °C and... 

The industrial hydroformylation of short-chained olefins such as propene and butenes is nowadays almost exclusively performed by so-called LPO (low-pressure oxo) processes, which are rhodium-based. In other words, the former high-pressure technology based on cobalt has been replaced by the low-pressure processes, which cover nearly 80% of total C4 capacity due to their obvious advantages (cf. [8]). Nevertheless, some cobalt processes are still in operation for propene hydroformylation, for example as second stages in combination with a low-pressure process serving as the first stage [8, 9]. 

There are several problems with cobalt catalysed hydroformylation. The pressures required are high, leading to high capital costs. The selectivity (n iso) is rather low and there are side reactions. Catalyst losses arise through its volatility and also by decomposition to metallic cobalt. The metal has to be removed with acids from time to time, causing corrosion. Fortunately, cobalt is cheap. As better processes are now available it is unlikely that new plants of this type will be built. It is economic, however, to keep plants running which are already in operation. This is still the major industrial route to butanal. 

Ligand-Modified Rhodium Process. The triphenylphosphine-modified rhodium oxo process, termed the LP Oxo process, is the industry standard for the hydroformylation of ethylene and propylene as of this writing (ca 1995). It employs a triphenylphosphine [603-35-0] (TPP) (1) modified rhodium catalyst. The process operates at low (0.7—3 MPa (100—450 psi)) pressures and low (80—120°C) temperatures. Suitable sources of rhodium are the alkanoate, 2,4-pentanedionate, or nitrate. A low (60—80 kPa (8.7—11.6 psi)) CO partial pressure and high (10—12%) TPP concentration are critical to obtaining a high (eg, 10 1) normal-to-branched aldehyde ratio. The process, first commercialized in 1976 by Union Carbide Corporation in Ponce, Puerto Rico, has been ficensed worldwide by Union Carbide Corporation and Davy Process Technology. 

However, (Ph3P)2Rh(CO)Cl on alumina or activated carbon were effective hydroformylation catalysts under more severe conditions 108). At 148°C and a pressure of 49 atm (CO 37.5 mol%, H2 37.5, propylene 25), good activity was found. The propylene conversion was 30% at a contact time of 0.92 cm3 of reactor void space/cm3 of feed per minute. Isomer ratios of 1.3 to 1.9 1 n iso were realized. By-product formation was low, with <1% conversion to alcohols plus alkanes and 2.2% high-boiling materials. This system was stable for a 300 hour operating time, with no detectable loss of activity or selectivity. 

Despite some differences, the mechanism of rhodium-catalyzed hydroformylation is very similar to that of the cobalt-catalyzed process.39-42 Scheme 7.1 depicts the so-called associative route which is operative when the ligand is in excess. Rhodium metal and many Rh(I) compounds serve as precursor to form21,22—in the presence of triphenylphosphine, CO and H2—the active species [RhH(CO)(PPh3)3] (5). At high CO partial pressure and low catalyst concentration without added PPh3, the [RhH(CO)2(PPh3)] monotriphenylphosphine complex instead of 6 coordinates the alkene and participates in the so-called dissociative route.21,39... 

Union Carbide invented the industrial use of highly active ligand-modified rhodium complexes.90-93 [RhH(CO)(PPh3)3], the most widely used catalyst, operates under mild reaction conditions (90-120°C, 10-50 atm). This process, therefore, is also called low-pressure oxo process. Important features of the rhodium-catalyzed hydroformylation are the high selectivity to n-aldehydes (about 92%) and the formation of very low amounts of alcohols and alkanes. Purification of the reactants, however, is necessary because of low catalyst concentrations. 

The introduction of rhodium has allowed the development of processes which operate under much milder conditions and lower pressures, are highly selective, and avoid loss of alkene by hydrogenation. Although the catalyst is active at moderate temperature, plants are usually operated at 120°C to give a high n/iso (linear/ branched) ratio. The key to selectivity is the use of triphenylphosphine in large excess which leads to >95% straight chain anti-Markovnikov product. The process is used for the hydroformylation of propene to n-butyraldehyde, allyl alcohol to butanediol, and maleic anhydride to 1,4-butanediol, tetrahydrofuran, and y-butyrolactone. 

In the 1960s, Shell commercialized a one-step process using a phosphine modified cobalt catalyst. This process operates at a much lower pressure, provides a high selectivity for the normal aldehyde isomer, and carries out the hydroformylation and hydrogenation sequentially without isolating the aldehyde. 

The oldest catalysts for hydroformylation are derived from Co carbonyls, which under reaction conditions give the active form of the catalysts, HCo(CO)4 Co catalysts can be considerably stabilized, e.g., by introducing phosphine ligands. However, even with them, the pressures required for operation are rather high, always above 100 atm. Rhodium can be used as,... 

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