Carbon chain growth processes

The hydrogenation of CO leads to surface CH species that act as monomers and chain initiators in chain growth processes. The initial step involves the dissociation of CO to form chemisorbed carbon, which is hydrogenated to surface methyl and methylene groups in subsequent steps (7-11). Chain growth occurs by stepwise addition of Ci monomers to a sur-fece alkyl group. 

The F-T process is a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. The distribution of products depends on the catalyst and the process operation conditions. The F-T chain-growth process is comparable with a polymerization process resulting in a distribution of chain-lengths of the products. The primary products of the F-T process are liquefied petroleum gas (LPG), naphtha, diesel, lubes, and waxes. 

Compared with that in the conventional supercritical phase reaction, the carbon chain growth was accelerated by the addition of alkenes with long carbon chains into the accompanying fluid. The essential prerequisite for this process is the rapid diffusion of these heavy added alkenes inside the catalyst pores to reach the metal sites, as well as effective diffusion of the heavy products produced from the interior active sites to the outer catalyst surface. Both diffusion processes are readily achieved in the supercritical phase. 

The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed CO2 selectivity as well in the alkene-added reaction, as CO2 is the byproduct from CO in the water-gas shift reaction. 

Anderson, et al., (6,7) extended the chain growth process to account for the production of straight carbon chains and chains with methyl branches ethyl-substituted species had not yet been found in synthesis products. Branching was postulated to be a part of the chain growth as depicted by the network in Table 1, in which carbons are added one at a time to the end or penultimate carbons at one end of the chain as indicated by the asterisks. 

PCs are usually synthesized by a step-growth process, that is, polycondensation, from phosgene or its derivatives and dihydroxy compounds. Instead of phosgene s method dialkyl- or diphenylcarbonate can also be applied for PC production. The ROP of cydic carbonates is an alternative method for the synthesis of both aliphatic and aromatic PCs. A comparison of these two methods is in favor of chain-growth process. The polycondensation affords rather limited molecular weight polymers, while high-molecular-weight PCs can be prepared in the ROP of cyclic monomers. 

Many addition polymerizations are chain-growth processes involving the carbon-carbon double bond, and most condensation reactions are step-growth processes. However, there are notable exceptions. For example, polyurethanes are made by the step-growth addition of glycols and diamines to the carbon-nitrogen double bond of diisocyanates. 

Environmental Considerations. Environmental problems in Ziegler chemistry alcohol processes are not severe. A small quantity of aluminum alkyl wastes is usually produced and represents the most significant disposal problem. It can be handled by controlled hydrolysis and separate disposal of the aqueous and organic streams. Organic by-products produced in chain growth and hydrolysis can be cleanly burned. Wastewater streams must be monitored for dissolved carbon, such as short-chain alcohols, and treated conventionally when necessary. 

A.luminum Jilkyl Chain Growth. Ethyl, Chevron, and Mitsubishi Chemical manufacture higher, linear alpha olefins from ethylene via chain growth on triethyl aluminum (15). The linear products are then used as oxo feedstock for both plasticizer and detergent range alcohols and because the feedstocks are linear, the linearity of the alcohol product, which has an entirely odd number of carbons, is a function of the oxo process employed. Alcohols are manufactured from this type of olefin by Sterling, Exxon, ICI, BASE, Oxochemie, and Mitsubishi Chemical. 

When the process of chain growth is satisfactorily completed, separation of the three hydrocarbon chains that are connected to the aluminum atom is accomplished by a displacement reaction. The chain-laden aluminum compound (called trialkyl aluminum compounds) is subjected to still higher temperatures and pressure. This causes an ethylene molecule to displace the long linear carbon chain. As the separation is made, triethyl aluminum is reformed, making a recyclable root for another go-around. 

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

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