SILICA PROMOTOR FOR PROPANE DEHYDROGENATION CATALYSTS BASED ON PLATINUM AND GALLIUM

Abstract

A catalyst for the catalytic dehydrogenation of alkanes to the corresponding alkenes consists of platinum, gallium and optionally potassium on an alumina carrier. Silica has been added to the catalyst, preferably in an amount of 5-10 wt %, as a promotor for the performance thereof.

Claims

1. A catalyst for the dehydrogenation of alkanes, where lower alkanes are dehydrogenated to the corresponding alkenes according to the reaction
C.sub.nH.sub.2n+2<->C.sub.nH.sub.2n+H.sub.2 in which n is an integer from 2 to 5, by feeding the alkane to a catalyst-containing dehydrogenation reactor, said catalyst consisting of platinum, gallium and optionally potassium on an alumina carrier, wherein silica has been added as a promotor for the performance of the catalyst.

2. The catalyst according to claim 1, which contains SiO.sub.2 in an amount of 1-40 wt %.

3. The catalyst according to claim 2, wherein the SiO.sub.2 content is 1-30 wt %.

4. The catalyst according to claim 1, which contains 0.5-1.5 wt % Ga, 1-100 ppm Pt and 0.05-0.5 wt % K.sub.2O.

5. A process for the dehydrogenation of alkanes to the corresponding alkenes according to the reaction
C.sub.nH.sub.2n+2<->C.sub.nH.sub.2n+H.sub.2 in which n is an integer from 2 to 5 in the presence of a catalyst according to claim 1.

6. The process of claim 5, wherein the catalyst is arranged in a fixed bed.

7. The process of claim 5 comprising periodic cycles of sequential oxidative regeneration steps and said dehydrogenation steps, optionally separated by vacuum or flushing steps, but without a separate reduction step, such as a step in which hydrogen is fed to the catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIGS. 1a-1c show the steady-state catalytic performance (5th cycle) regarding activity (FIG. 1a), selectivity (FIG. 1b) and ‘oil’ formation as indicated by the formation of 1-butene (FIG. 1c) of 1.5 g (0.3-0.5 mm) catalyst at a temperature of 570° C., a 12 Nl/h flow of 10% propane and a pressure of 5 bar.

[0030] FIG. 2 shows the TPO (temperature-programmed oxidation) of spent catalysts after testing.

DETAILED DESCRIPTION

[0031] The invention is described in further detail in the experimental section which follows.

[0032] Experimental

[0033] SiO.sub.2 has been identified as a promotor for the performance of Pt/Ga catalysts supported on Al.sub.2O.sub.3. The following procedure was used:

[0034] All carriers were impregnated according to the process as described below. Al.sub.2O.sub.3 with different contents of SiO.sub.2 were used as carriers.

[0035] Preparation of Impregnation Solution:

[0036] 4.0 g of a 5 wt % Ga solution, 0.20 g of a 0.5 wt % Pt solution and 0.10 g KNO.sub.3 are dissolved with 11 ml water. This solution is used to impregnate 20 g of the selected support. The sample is rolled for 1 hour to ensure complete pore volume impregnation, dried at 100° C. overnight and then calcined at 700° C. for 2 h with a 4 h heating ramp.

[0037] The support materials were the following: [0038] 1. Al.sub.2O.sub.3, no SiO.sub.2 [0039] 2. Al.sub.2O.sub.3, 5 wt % SiO.sub.2, low surface area (SA) [0040] 3. Al.sub.2O.sub.3, 5 wt % SiO.sub.2, medium SA [0041] 4. Al.sub.2O.sub.3, 5 wt % SiO.sub.2, higher SA [0042] 5. Al.sub.2O.sub.3, 10 wt % SiO.sub.2, high SA [0043] 6. Al.sub.2O.sub.3, 20 wt % SiO.sub.2, high SA [0044] 7. Al.sub.2O.sub.3, 30 wt % SiO.sub.2, high SA

[0045] Catalyst Performance:

[0046] The reactor used was an isothermal quartz reactor with a quartz thermal pocket over the thermocouple. The outlet gas stream was analyzed using a gas chromatograph with an FID and TCD detector. The gas chromatograph analyzes the C1 to C4 hydrocarbons. Conversion and selectivity are based on the analyzed product mixture. Catalyst performances are evaluated by loading 1.5 gram of catalyst with a sieve fraction of 0.3-0.5 mm into the reactor, and then exposing the catalyst to five cycles of the following sequence of gas flows and temperatures: 200 ml/min of 10% propane in nitrogen for 14 mins at 570° C., followed by 200 ml/min nitrogen flush for 60 mins while heating to 630° C., followed by regeneration with 50 ml/min 2% Oxygen in nitrogen for 30 mins at 630° C., followed by cooling in 50 ml/min 2% Oxygen in nitrogen for 30 mins to 570° C., followed by 200 ml/min nitrogen flush for 3 mins at 570° C. The dehydrogenation cycle is then started again, without including a reduction step. The tests were performed at a pressure of 5 bar.

[0047] The results appear from the figures, where:

[0048] FIGS. 1a-1i show the steady-state catalytic performance (5th cycle) regarding activity (FIG. 1a), selectivity (FIG. 1b) and ‘oil’ formation as indicated by the formation of 1-butene (FIG. 1c) of 1.5 g (0.3-0.5 mm) catalyst at a temperature of 570° C., a 12 Nl/h flow of 10% propane and a pressure of 5 bar, and

[0049] FIG. 2 shows the TPO (temperature-programmed oxidation) of spent catalysts after testing.

[0050] It can be seen in FIG. 1a that all SiO.sub.2-containing catalysts have a higher performance after 11 minutes on stream than the corresponding reference catalyst without SiO.sub.2. Two of the catalysts with 5 wt % SiO.sub.2 furthermore also have a higher initial activity after 1 minute on stream. It is thus seen that SiO.sub.2 is able to improve both the activity and the stability of the catalyst.

[0051] The catalytic activity of the catalyst seems to correlate very well with the Lewis acidity of the carriers (http://www.sasolgermany.de/fileadmin/doc/aumina/0271.SAS-BR-Inorganics_Siral_Siralox_WEB.pdf). The by-product formation (selectivity), the oil formation and the coke formation all seem to correlate with the Brønsted acidity of the carrier. Furthermore, the higher the SiO.sub.2 loading is, the harder the coke becomes (FIG. 2). It thus requires increasingly higher temperatures to remove the coke. In conclusion, Lewis acid sites introduced by SiO.sub.2 appear to be beneficial for the catalyst, whereas Brønsted acid sites cause side reactions. The optimum catalyst performance seems to be obtained with the 5 wt % SiO.sub.2 carrier.