LEAD SULFIDE AS ALKANE DEHYDROGENATION CATALYST

Abstract

A catalyst for the dehydrogenation of alkanes to alkenes comprises lead(II) sulfide (PbS) as catalytically active material supported on a carrier. The dehydrogenation is carried out at a temperature between 500 and 650 C. and at a pressure from 0.5 bar below ambient pressure to 5 bar above ambient pressure.

Claims

1. A catalyst for the dehydrogenation of alkanes to alkenes, said catalyst comprising a catalytically active material supported on a carrier, wherein the catalytically active material is lead(II) sulfide (PbS), and wherein the catalyst is regenerated in several steps.

2. Catalyst according to claim 1, wherein the steps for regeneration comprise (a) oxidation in dilute air, (b) conversion into the corresponding sulfate, and (c) conversion back to the sulfide by reduction in dilute hydrogen containing some hydrogen sulfide.

3. Catalyst according to claim 2, wherein the oxidation in step (a) is carried out at a temperature between 350 and 750 C.

4. Catalyst according to claim 1, wherein the carrier is treated with a dilute alkali compound and subsequently washed to remove acid sites.

5. Catalyst according to claim 4, wherein the dilute alkali compound is potassium carbonate or any other potassium compound.

6. A process for the dehydrogenation of alkanes to the corresponding unsaturated alkenes and hydrogen (H.sub.2) comprising contacting the alkane with a catalyst according to claim 1 supported on a carrier, said catalyst comprising lead(II) sulfide (PbS).

7. Process according to claim 6, wherein the dehydrogenation is carried out at a temperature between 500 and 650 C.

8. Process according to claim 6, wherein the dehydrogenation is carried out at a pressure from 0.5 bar below ambient pressure to 5 bar above ambient pressure.

9. Process according to claim 8, wherein the dehydrogenation is carried out at ambient pressure or at a pressure from 0.5 bar below ambient pressure up to ambient pressure.

10. Process according to claim 6, wherein the feed gas contains sulfur in an amount determined such that the equilibrium reaction PbS+H.sub.2.Math.Pb+H.sub.2S is shifted towards PbS throughout the reactor.

Description

EXAMPLE 1

[0043] 15 g Pb(NO.sub.3).sub.2 is dissolved in 37.5 g water. This solution is used to impregnate 50 g of a support (pv=1 ml/g). The sample is rolled for 1 hour, dried overnight at 100 C. and calcined at 500 C. for 2 hours (4 hours heating ramp).

[0044] The sample is then washed in 100 ml of a 2% K.sub.2CO.sub.3 solution for 1 hour (rolling board). Afterwards the sample is washed two times with 200 ml water (one hour each, rolling board). The sample is filtered and dried overnight at 100 C. The catalyst contains 14 wt % Pb and 0.8 wt % K.

EXAMPLE 2

[0045] 20 g Pb(CH.sub.3COO).sub.2.3H.sub.2O is dissolved in 37.5 g water. This solution is used to impregnate 50 g support (pv=1 ml/g). The sample is rolled for 1 hour, dried overnight at 100 C. and calcined at 500 C. for 2 hours (4 hours heating ramp).

[0046] The sample is then washed in 100 ml of a 2% K.sub.2CO.sub.3 solution for 1 hour (rolling board). Afterwards the sample is washed two times with 200 ml water (one hour each, rolling board). The sample is filtered and dried overnight at 100 C. The catalyst contains 18 wt % Pb and 0.8 wt % K.

EXAMPLE 3

[0047] 15 g Pb(NO.sub.3).sub.2 and 1.5 g KNO.sub.3 are dissolved in 37.5 g water. This solution is used to impregnate 50 g of a support (pv=1 ml/g). The sample is rolled for 1 hour, dried overnight at 100 C. and calcined at 500 C. for 2 hours (4 hours heating ramp). The catalyst contains 14 wt % Pb and 1 wt % K.

EXAMPLE 4

[0048] 20 g Pb(CH.sub.3COO).sub.2.3H.sub.2O and 1.6 g KNO.sub.3 are dissolved in 37.5 g water. This solution is used to impregnate 50 g of a support (pv=1 ml/g). The sample is rolled for 1 hour, dried overnight at 100 C. and calcined at 500 C. for 2 hours (4 hours heating ramp). The catalyst contains 18 wt % Pb and 1 wt % K.

EXAMPLE 5

[0049] 5.0 g of the catalyst prepared in Example 1 was placed in a plug flow tubular stainless steel reactor (1.0 m long and with an internal diameter of 15 mm). The catalyst was placed in the middle of the reactor and supported on a grid. In the top as well as in the bottom of the catalyst bed a thermocouple was placed.

[0050] The inlet and exit pressures were recorded by pressure transducers. Before the catalytic tests, blind tests were carried out, and the results from the blind tests were subtracted from the later catalytic tests. The blind tests typically showed 4% conversion at 560 C. and 12% conversion at 600 C., both with a selectivity to propene of 50%.

[0051] The catalyst was initially reduced and sulfide in a gas consisting of 50 Nl/h of N.sub.2, 4.5 Nl/h of H.sub.2 and 0.5 Nl/h of H.sub.2S, being heated in this gas from room temperature to the reaction temperature of 600 C. over a period of 60 minutes.

[0052] At 600 C., the catalyst was tested in a gas containing 45 Nl/h of N.sub.2, 5 Nl/h of C.sub.3H.sub.8, 1.8 Nl/h of H.sub.2 and 0.2 Nl/h of H.sub.2S. It showed a formation (after subtraction of the reactor contribution) of 0.2 Nl/h of propene corresponding to 40 Nl/h propene/kg cat/h. The subtraction of the reactor contribution showed a selectivity of 100% within the experimental error. The experiment was conducted at a pressure of 0.26 MPa, and the measurements were recorded after 10 hours of reaction.

EXAMPLE 6

[0053] After 40 hours of reaction, the catalyst was subjected to regeneration for 6 hours at 560 C. in the presence of a gas containing 49.5 Nl/h of N.sub.2 and 0.5 Nl/h of 0.sub.2. Then it was reduced and sulfided for 2 hours in a gas containing 50 Nl/h of N.sub.2, 9 Nl/h of H.sub.2 and 1 Nl/h of H.sub.2S. It was then tested for 20 hours at 560 C. in a gas containing 45 Nl/h of N.sub.2, 5 Nl/h of propane, 0.9 Nl/h of H.sub.2 and 0.1 Nl/h of H.sub.2S. A formation (after subtraction of the reactor contribution) of 0.21 Nl/h of propene was found at a selectivity of approximately 100% corresponding to 124 Nl/h propene/kg cat/h. The pressure was 0.22 MPa.

[0054] The temperature was increased to 600 C. using the same reaction mixture. After a reaction time of 85 hours in total, a formation of propene (after subtraction of the reactor contribution) of 0.62 Nl/h was found, corresponding to 124 Nl/h propene/kg cat/h. The pressure was 0.22 MPa.

[0055] The catalyst was regenerated after more than 50 consecutive hours in the propane-containing gas. The amount of CO.sub.2 was recorded, and it was concluded that less than 1% of the converted propane had ended up as carbon on the catalyst and the reactor wall.

[0056] The catalyst was reduced and sulfide and then tested again at 600 C. using the same conditions as above. It still displayed the same performance.

[0057] After cooling in nitrogen, the catalyst was analyzed by means of X-ray powder diffraction. Apart from the carrier material, only PbS was seen. It had an average crystallite size of 64 nm.

[0058] The catalyst of the invention is deactivated slowly by carbon deposition and therefore it needs to be regenerated, just like the commercially available catalysts based on platinum or chromium oxide. The regeneration takes place by combustion in dilute air, i.e. 1% O.sub.2 and 99% N.sub.2, at 560-600 C.

[0059] Regeneration of most metal sulfides using N.sub.2 with 1% O.sub.2 will lead to formation of the corresponding sulfate. In order to conserve sulfur on the catalyst, regeneration should start at 400 C. followed by a carbon removal at 600 C.

[0060] The regeneration takes the catalyst through two phase transition stages, from sulfide to sulfate or oxide and back again to sulfide. The phase transitions involve not only structural transformations, but also volume changes. It is expected that sintering/dispersion of the system will reach steady state after a number of regenerations.

[0061] During dehydrogenation, some carbon is deposited on the catalyst, resulting in a slow deactivation. Dehydrogenation takes place for some hours followed by catalyst regeneration in N.sub.2 containing 1% O.sub.2. This is typically followed by a sulfidation or a direct return to dehydrogenation. In this case, a direct reaction between sulfate and propane takes place, resulting in a large CO.sub.2 formation.

[0062] The carrier is treated with a dilute alkali compound and subsequently washed to remove acid sites. Preferably the dilute alkali compound is potassium carbonate or any other potassium compound.

[0063] In the experiments, the carrier has been dipped in a dilute potassium carbonate solution followed by a two-step wash in demineralized water, resulting in a potassium content of 0.15 wt %. Acid sites have been removed, but not necessarily all of them. The results indicate a pressure influence on the carbon formation, and they also indicate that carbon formation takes place from propylene, not propane. Furthermore, the results indicate that there is a complete carbon removal during regeneration.