Eni Chem Approach TS1 Allows the Integration of HP and PO Synthesis

Since the early 1980s, EniChem has been a pioneer in the development of the process, holding a portfolio of patents [10c,d]. The integration of HP synthesis by means of alkylanthraquinone/alkylanthrahydroquinone (RAQ/RAHQ) cycle technology, with PO production, by means of propene epoxidation with HP, is possible because of the peculiar properties of the TS-1 catalyst (Scheme 6.5). TS-1 can selectively epoxidize propene using diluted HP [12]. A water- methanol mixture is the solvent for the epoxidation; the alcohol is necessary to obtain a sufficient reaction rate. Therefore, a cost-saving feature in this process is the fact that the crude HP produced can be used directly in the epoxidation of propene. Moreover, integration of the two processes is also allowed by the easily accomplished separation of propene and PO from the water-methanol mixture. Methanol, after separation and purification, can be recycled to the epoxidation step.

Compared to existing PO process technologies, HPPO is claimed to offer unique benefits from economic and environmental points of view. New PO plants built using HPPO technology are claimed to be more economical because they require up to 25% less capital. The simplicity of the process derives from the fact that there is no need for

6.2 PO-only Routes: Several Approaches for Sustainable Alternatives j 331 .0

+ H2O2 -- + H20

Air H202

Scheme 6.5 RAQ/RAHQ process for the generation of HP, and integration with propene epoxidation.

additional infrastructure or markets for coproducts, and also because a simpler raw material integration is required. The environmental benefits derive from the reduction of wastewater by up to 70-80% and of energy usage by 35%.

EniChem first developed the concept of process integration for PO production [7f]. The main advantages of the process, compared to the conventional industrial technologies, were reported to be:

1. The high selectivity for PO and the generation of limited amounts of by-products, which can be easily disposed of.

2. The independence from an external supply of HP, since hydrogen and oxygen can be produced at most chemical sites by existing technologies, helping to improve the flexibility of the process in relation to the market, and avoiding the coproduction of a second chemical.

The reaction is carried out at 50-60 ° C, with methanol as solvent (it dissolves both HP and the alkene), and a pressure of 10-15 atm. The reaction is kinetically more favorable in the most polar solvents; water might be even better than methanol: however, in this case, the TS-1 rapidly decays. EniChem chose a mixture of water and methanol as the solvent for the development of the process, as did other companies later on. It is now accepted that the solvent has a role in the partition of the reagents between the external medium and the catalyst pores; however, other factors may overlap with the substrate partition effect. In fact, several lines of evidence exist for the adsorption of protic molecules (i.e., methanol) on Ti sites [13]. The solvation of the olefin by less polar solvents decreases its concentration in the proximity of active sites; the concept of zeolites as solid solvents, introduced by Derouane, is useful in this regard [14].

A PO selectivity on a propene basis as high as 98%, and a yield on a HP basis of around 90%, have been reported (conversion close to 100%, while that of the alkene was lower than 50%) [10e]. The main by-products were obtained due to consecutive reactions involving the epoxide, such as ring opening by methanol (the solvent) to yield the glycol monomethyl ether, or by water to yield the glycol, and glycol ketal. The addition of ppm basic compounds is important for obtaining an almost quantitative selectivity.

The rate of catalyst deactivation is a function of the TS-1 crystal size [12a, 13a]; with larger crystals, the slow diffusion of the epoxide solvolysis products (especially with more hindered products) makes the blockage of pores more likely. In propene epoxidation, polyethers are mainly responsible for this phenomenon, when the catalyst is used in consecutive reaction cycles. The activity of the catalyst can be restored by washing it with a solvent or by calcining it at temperatures higher than 5000 C.

Two different process configurations were proposed by EniChem [15]. The first configuration involves the in situ generation of HP, through the classical RAQ/ RHAQ route, in the same medium where the epoxidation reaction occurs (in situ HPPO). Scheme 6.6 shows the reactions involved.

The first reaction, hydrogenation of the alkylanthraquinone, is catalyzed by Pd. The second, the epoxidation of propene by the HP generated by air oxidation of the RAHQ, is catalyzed by TS-1. This is possible because TS-1 activity is not affected by the polynuclear compounds forming the redox couple, since they do not enter the zeolite cages due to steric hindrance (the average diameter of the channel system of TS-1 and TS-2, with MFI and MEL type structures, respectively, is 0.55 nm; the cross

Reactions With Propene
Scheme 6.6 Reactions involved in the EniChem process for propene epoxidation with in situ generation of HP.

section of alkylated anthraquinone is greater than 0.6 nm). Therefore, access of these bulky molecules to the Ti active sites is hindered, and potential oxidative degradation or interference with the catalytic processes is prevented.

A mixture of solvents is used: 2-methylnaphthalene (22vol.%) to dissolve the alkylanthraquinone, a polar compound, preferably methylisobutylcarbynol (68vol.%) to dissolve the alkylanthrahydroquinone, and methanol (10 vol.%). Methanol is also a co-catalyst, since the rate ofreaction is much accelerated in the presence ofthis solvent. The best yield to PO, based on starting ethylanthraquinone, was 78%, at 30 °C, with 3 atm propene, 2 atm air and 0.31 wt% TS-1 as the catalyst, in a 1.5 h reaction time. For this to happen, autoxidation and epoxidation must occur at the same temperature, that is to say, a moderate temperature, to prevent the degradation of anthraquinone. The disadvantage of the process is that the optimal conditions for the generation of HP are not the same as for the epoxidation reaction.

The second process configuration is also based on process integration, but the RAQ/RAHQ and the epoxidation reactions occur in different reactors. The solvent is a methanol-water mixture, which extracts HP from the organic solvent of the redox process, and which is also the solvent for the solution fed to the epoxidation reaction. Once again, the advantage of this process over a conventional one, in which HP is produced separately, is the elimination of the expensive HP purification and concentration steps. Organic impurities that may have been extracted along with HP are precluded from diffusing inside TS-1 by their relatively large molecular size. The quantity of methanol passing in the working solution is small enough not to interfere with the cycle of reactions in the RAQ process. A yield to PO of 71.3% with respect to charged anthraquinone has been reported [15c]. The RAQ/RAHQ cycle for the production of HP is performed in a mixture of a hydrophobic (e.g., xylene) and a hydrophilic (water + methanol) phase.

The oxidation and reduction steps in the RAQ/RAHQ cycle are performed in two separate reactors. A bubble column is applied for the oxidation of the RAHQ, during which HP is produced. For the Pd-catalyzed hydrogenation of the quinones, a slurry, fixed-bed or monolith reactor can be used. After the reactor and L/L settler, a diluted HP-containing water-methanol stream is finally obtained. After the epoxidation step, crude PO is separated and the water-methanol mixture is returned to the HP synthesis process, thus realizing an efficient process integration.

Figure 6.4 shows a simplified flow-sheet of the two process configurations proposed by EniChem.

Continue reading here: From the Dream Reaction to the Real Process the Implemented HPPO Process

Was this article helpful?

0 0