The commercially important process for producing CHDM is the hydrogenation of dimethyl terephthalate (DMT) via dimethyl hexahydroterephthalate (DMHT) in a methanolic solution or in the molten state. The hydrogenation plant consists of two reactors. In the fifirst reactor, a continuous circulation of product DMHT and of DMT is maintained. The molten DMT is pumped into the inlet of this reactor, which operates at 30 – 48 MPa and 160 – 180 ℃ with a commercial supported Pd catalyst. Temperature control is achieved by operating at suffificiently high cross-sectional loadings of the mixture of ca. 10 % in product DMHT. This allows the radial dissipation of heat via the reactor walls and largely avoids high temperature peaks. The cooled reactor efflfluent, which contains only minor amounts of unreacted DMT, is divided: 8 – 10 parts are recycled via a pump and heat exchanger to the reactor inlet. The remaining 1-2 parts (the exact amount corresponds to the amount of fresh DMT) are fed continuously into the second reactor for the fifinal Pd-catalyzed hydrogenation.
An advantage of this process is that DMT can be hydrogenated as ca. 10 % dilute feed without handling large amounts of liquid. The yield of DMHT is typically 97 – 98 %, with methyl 4-methyl-4- cyclohexanecarboxylic acid methyl ester and some 1-hydroxymethyl-4-methylcyclohexane as main byproducts. 1,4-Bis(hydroxymethyl)cyclohexane (CHDM) is formed in the second stage of the hydrogenation.
The industrial processes use commercial copper chromite catalysts. The effects of catalyst, residence time, and temperature must be carefully adjusted in order to achieve a cis/trans ratio of 1/3 to 1/4 in the CHDM product. A constant isomer ratio is important for further processing of CHDM in polyester formation since the crystal structures of polyesters of cisand trans-CHDM differ and thus inflfluence the melting range and density of polyester fifibers.
In addition to the side products of the fifirst hydrogenation stage, 4-methyloxymethylhy-droxymethylcyclohexane and bis(4-hydroxymethylcyclohexyl) ether can be formed in the second stage of ester hydrogenation. The Eastman process avoids the formation of high-boiling compounds and thus saves cumbersome separation of side products by fractionation in vacuum. The diol is purifified by simply removing methanol and low-boiling compounds. The CHDM of fifiber quality obtained by this method is ca. 99 % pure. Pure CHDM may also be obtained by recycling waste poly(1,4-cyclohexylene dimethylene terephthalate) by fifirst cleaving the polyester in the presence of low molecular mass alcohols and then subjecting the resulting mixture to a hydrogenation analogous to that described above.
