Several Technologies for In Situ HPPO with TS1Supported Pd Catalysts

A process for the epoxidation of propene with in situ generation of HP was proposed in the 1990s by the Tosoh Corporation [23]. It was suggested that PO could be made via a direct reaction between hydrogen and oxygen in the presence ofpropene, using a catalyst made of Pd supported on crystalline titanium silicate, in a flow system. The solvent for the reaction was t-butanol, and the reaction temperature was 45 °C. A conversion of 0.8% was reported, with a selectivity for PO of 99%.

ARCO Chem Tech (now Lyondell) has reported catalysts that show enhanced selectivity and yield for PO [24]. Again, the catalyst was made of Pd impregnated over TS-1, but the addition of either NH4OH, benzothiophene or triphenylphosphine in the reaction medium enabled an increase in the PO yield with respect to propene and H2. Selectivity was also improved due to the suppression of the propene hydrogenation reaction.

BASF has also claimed the use of metal-modified TS-1 catalysts [25]. Various catalyst compositions have been described, including (i) titanium or vanadium silicalites containing rare earth ions [25a] and (ii) titanium or vanadium silicalites containing noble metals (Ru, Rh, Pd, Os, Ir, Pt, Re, Au, Ag). In these systems the

Table 6.4 Performance data for BASF catalysts in HPPO with generation of HP by means of H2 oxidation with O2 (DSHP) [25]a.

Catalyst

C3H6 conversion (%)

Selectivity PO/C3H8 (%)

Solvent

Reaction time (h)

0.49% Pd/TS-1

1.8

5.2/94.7

Methanol

17

0.49% Pd/TS-1

1.4

94.0/5.9

Water

3

0.49% Pd/TS-1

1.1

91.1/nr

Water

20

^Reaction conditions: 1 g catalyst, 60 mL solvent, gassed with 0.45 Lh~ hydrogen for 0.5 h, then feed: 0.17% propene (0.08% when water is the solvent), 18.7% hydrogen, 18.7% oxygen, remainder nitrogen, T = 45-50°C, P = 1atm.

^Reaction conditions: 1 g catalyst, 60 mL solvent, gassed with 0.45 Lh~ hydrogen for 0.5 h, then feed: 0.17% propene (0.08% when water is the solvent), 18.7% hydrogen, 18.7% oxygen, remainder nitrogen, T = 45-50°C, P = 1atm.

metal is (i) highly dispersed, to avoid metal-metal interaction, and (ii) present in at least two different oxidation states, that is, Pd0, Pd +1 or Pd + 2 [25b]. A procedure for compacting/molding this catalyst has also been reported [25c]. Table 6.4 compiles some relevant results claimed in these patents.

In more recent patents the two steps, that is, HP generation and PO synthesis, are carried out separately, each one with a specific catalyst and optimized reaction conditions [25d]. The gaseous stream exiting the PO synthesis reactor, containing the unconverted propene and the oxygen generated by HP decomposition, can be fed to the HP generation reaction. The reactions of (i) propane dehydrogenation to propene, (ii) HP generation by reaction between hydrogen and oxygen and (iii) PO generation by epoxidation of propene with HP are combined in an integrated process (Figure 6.9) [25e].

In BASF's two-stage integrated approach for PO production, HP is first produced by the direct combination of hydrogen and oxygen in a 6-7 wt% concentration in a

Figure 6.9 Process integration in BASF technology for PO production with DSHP-HPPO.

methanolic or hydro-alcoholic solution. The catalyst system consists of alternately laid corrugated and flat steel nets made from fine wires, and rolled into a cylindrical monolith. This monolith is impregnated with a palladium salt. The reactor vessel is flooded with methanol and hydrogen is bubbled in it as fine droplets from multi-leveled feed points. Enriched oxygen is fed from the bottom of the reactor. The oxygen-to-hydrogen ratio in the reactor is within safe limits. Hydrogen conversion is 76% and selectivity for HP is 82mol.%. Reaction conditions are 40-50 °C and 50-54atm; the reactor temperature is maintained by external cooling.

In the second step, a dilute HP-methanol solution is introduced in a fixed-bed epoxidation reactor. Make-up propene, recycled propene and HP from the product purification stage are fed into the reactor. The reaction is catalyzed by titanium silicalite, and takes place at 40-50 0C and 300 psi. HP per-pass conversion is initially 96% but drops down to 63% after 400 hours. PO selectivity is 95 mol.%; propene perpass conversion is 39.8%. This technology gives capital savings compared to conventional hydroperoxidation technologies; however, it is likely that the operating costs of such a plant are higher than that of the latter.

Hoechst [26] (now Sanofi-Aventis) studied the epoxidation of propene with in situ generated HP, using a Ti silicate support with Pt/Pd metals. A very active system was reported, which at 43 0C (methanol and water solvent) converts 25% propene in 2 hours in a batch system, at 60 atm pressure, with a selectivity for PO (with respect to all the organic products formed) of 46%, and therefore a yield (with respect to propene) of 11.7% [26b,c]. The catalyst was made of 1 wt% Pd and 0.02 wt% Pt co-supported on TS-1. In general, the 1% Pd/TS-1 system exhibited a lower PO yield from the reaction between propene and HP than TS-1, due to the ability of the metal to decompose HP. However, it was possible to obtain similar results to those obtained with TS-1 due to a peculiar impregnation and reduction procedure; specifically, the catalyst was prepared by impregnation of TS-1 with Pt and Pd tetramine ligands. The best performance was obtained when the catalyst was reduced simply by treating the sample in a N2 flow at 150 0C (autoreduction). This was attributed to the fact that this reducing treatment favored the formation of small Pd clusters, and a high fraction of Pd2 + species, which seem to play an important role in the reaction. The addition of small amounts of Pt also favors the development of Pd2 + species. Performance was greatly improved by the addition of NaBr as a promoter [26a]; with a doped TS-1/Pd/Pt catalyst, a selectivity for PO of 87.3% was achieved at 19.4% propene conversion. Without the NaBr promoter, the selectivity dropped to 34%.

To our knowledge, however, all these technologies have not gone beyond the pilot-plant stage. From an investment point of view, the in situ DSHP-HPPO would be the most attractive option, because a single reactor is needed. However, the reaction conditions for the two desired reactions are hardly compatible: the HP synthesis needs low temperature (<20 0C), strong Bronsted acids as co-catalysts and halides as selective catalyst poisons, while the epoxidation needs higher temperatures, usually above 40 0C, strong acids lead to the formation ofby-products from PO and bromide can be oxidized to bromine, leading to brominated by-products. Moreover, the precious metal catalyst needed to make HP should not catalyze the hydrogenation of propene to propane. The best catalytic systems found give good selectivities of PO

based on propene at moderate to low conversions, but the selectivity of PO based on H2 is usually very low, <30%. Therefore, to make this approach an improvement in the direction of better sustainability, much higher selectivity should be achieved.

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