Direct conversion o f propane to iso-propanol over bi-functional catalyst

Following on from the previous work, experiments were conducted using the C0 3O4

nanoparticles in combination with the best hydrating catalyst from the previous experiment. The direct catalytic partial oxidation o f propane to iso-propanol would require a duel functioning catalyst that is able to activate propane and introduce oxygen via hydration at the lowest possible temperature in order to avoid gas phase

and unselective reactions. It was envisaged that by combination o f the low

temperature and highly active C 0 3 O 4 nanoparticles with a suitable hydrating catalyst

the direct conversion o f propane to iso-propanol could be achieved. The catalyst studied in the following section was made by direct combination o f the C0 3O4

nanoparticles with the phosphomolybdic acid catalyst. The arrangement within the bed and the precise reaction conditions are given in experimental section 2.4.1. The catalyst was heated up to 70°C in a He/ 0 2 mix. Once the temperature had been

allowed to stabilise propane and H2O were introduced to the system. The sample

cylinder was heated in 10°C steps and allowed to stabilise at each temperature for approx. 15-20 minutes before sample injection. Three injections were done at each temperature and the average taken. Carbon dioxide, propene and iso-propanol were the only products present. There was no evidence o f any other hydration products in the reaction effluent. The results for propane conversion as a function o f water concentration are given in figure 5.20.

\

\

0 •

0 S 10 15 20 25

Water partial pressures kPa

Figure 5.20 Propane conversion to iso-propanol over Ct^CVPhosphomolybdic acid mixture.

Conversion as a function o f water partial pressure. Reaction at 100°C.

With water in the feed propane conversion over the mixed catalyst was very low with a maximum o f 0.36% conversion to products. The conversion is seen to decrease with increasing water pressure. Generally the iso-propanol is present in trace amounts with carbon dioxide being the primary product. The exact conversion values are given in table 5.5.

Table 5.5 Propane conversion to iso-propanol as a function o f water vapour pressure

Water temperature/°C Vapour pressure/KPa Conversion/%

2 0 1 . 2 0.33

30 2.3 0.19

40 4.2 0.17

50 7.4 0.13

60 12.3 0 . 1 0

The bifunctional catalyst is relatively inactive with propane conversion being suppressed by both water in the feed and by dilution o f the active redox catalyst with the relatively inactive phosphomolybdic acid. Increasing the water concentration has a negative effect on the C0 3O4 catalyst limiting propene production and hence limiting

the total iso-propanol yield.

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The selectivities as a function o f water temperature are given in figure 5.21.

60 oo

^ sn nn

Water partial pressure/kPa

F ig u re 5.21 Selectivity to iso-propanol as a function o f water partial pressure over

Co30 4/phosphom olybdic acid catalyst: ( + ) C 0 2 (□ ) C3H6(A )C3H5OH

The typical propane conversion over the nanocrystaline C0 3O4 at 100°C is

approximately 1% with a 97% selectivity to propene. This was not the case with the cobalt oxide used in this experiment. Tests done with no water in feed showed results o f less than 0.5% conversion with only a 10-20% selectivity to propene. The primary product under these conditions was carbon dioxide with the catalyst remaining 80% selective to CO2 throughout the whole reaction. W hether this is due to the presence of

the acid catalyst or due to the cobalt oxide will have to be established. Varying the concentration o f water in the system had little effect on product selectivity. The catalyst showed a maximum 14% selectivity to iso-propanol and varied little with water concentration. The same was true for the propene selectivity which was no greater than 10% throughout the reaction. Interestingly it is worth noting that even with no water in the feed, trace amounts o f iso-propanol are still present. Removal of propane from the feed results in the disappearance o f this peak. This could indicate

that there is sufficient water from the initial reaction to hydrate the silica catalyst according to:

CnH2n+2 + 0.5O2 —CnH2n + H20

Alternatively it may just be the case that the acid catalyst is itself sufficiently hydrated from the outset. Given that there is very little variation in activity with increasing water concentration this may well be the case

2 0 15

A-V—A-

w ater partial p re ssu re / kPa

Figure 5.22 product yields as function o f water partial pressure over Co30 4/Phosphomolybdic acid

catalyst

The product yields as function o f water temperature are given in figure 5.22. Due to the poor conversions and selectivities the iso-propanol yields are very poor. There are trace amounts o f propene and iso-propanol in the feed but nothing significant. All yields are below 0.5% with iso-propanol yields not exceeding 0.03%. Such low traces

o f product are disappointing but not unexpected given the low temperatures employed, coupled with the limited activity of the catalysts used.

Given that the propane conversion over C0 3O4 is suppressed by the presence o f

water it not surprising that the conversion is so low. The maximum yield o f propene obtainable over the C0 3O4 in a wet atmosphere at 100°C is just 0.5%. This means that

there is only a trace amount o f propene within the feed that can go on to react with the acid catalyst to produce isopropanol. The total iso-propanol production is limited by the activity o f the redox catalyst in the system. Nevertheless, iso-propanol is still present as a product in the reaction mixture and although the conversions and selectivities are extremely low there is definite proof that the concept works. By combination o f a dehydrogenation and acid catalyst the direct conversion o f propane to iso-propanol was possible at 100°C.

5.4 Discussion