PRETREATMENT OF A Pt/Sn-BASED CATALYST

Abstract: This invention relates to a pretreatment procedure that reduces the required time to obtain the optimum activity of a Pt/Sn-based catalyst. The pretreatment activation consists of repeated cycles of 1) reduction by H2 at elevated temperatures, 2) alkane dehydrogenation at elevated temperatures, 3) oxidation/regeneration by a O2 containing gas at elevated temperatures.

Description: PRETREATMENT OF A Pt/Sn-BASED CATALYST Field of the invention. The present invention relates to a process of a pre-treatment activation of a Pt/Sn based catalyst and a catalyst activated by such a pre-treatment. Background of invention During industrial operation catalysts are permanently deactivated due to poisoning and sintering and have to be replaced at given time intervals, generally after 1 to 5 years in operation depending on feedstreams and reaction conditions. Usually the catalysts have to be treated according to a special pre-treatment procedure before the catalyst can operate at optimum performance (see e.s. WO 92/06784 where conditions for optimal reduction of a cobalt catalyst is described). Especially when catalysts have to be replaced at short intervals it is of major importance to reduce the pre-treatment time. The lifetime of a typical Pt/Sn based dehydrogenation catalyst is up to 3 years. The catalyst used in the STAR process (Pt/Sn on zink aluminate) has an expected lifetime of 1 to 2 years in commercial operation, while the catalyst used in the Oleflex process (Pt/Sn/Cs on Al 2 0 3 ) has an expected lifetime of 1 to 3 years [Catalytica (1993)]. Significant losses in alkene production can be foreseen if a long activation period is needed after each catalyst replacement. Such catalysts, or variations thereof, can also be used for a number of chemical processes that involve mainly or partly dehydrogenation or hydrogenation steps. A long activation period is also a disadvantage connected to unexpected stops and experimental development in the laboratory. The catalysts mentioned in the present invention are earlier described in patent applications number 932173 and 981126. Summary of the invention. There is a need for methods of quicker obtaining Pt/Sn based dehydrogenation catalysts at the maximal catalytic activity than has been possible with the prior art technology that may require several hundred hours on stream before the approximate maximal activity is obtained. Such a method would improve the total yields of dehydrogenation products obtained during the lifetime of a dehydrogenation catalyst. The disadvantages of the prior art technique are overcome by a process of a pretreatment activation of a Pt/Sn based dehydrogenation catalyst, wherein a Pt/Sn/Mg(AI)O catalyst is subjected to repeated cycles of 1.) reduction by H 2 at elevated temperatures. 2.) alkane dehydrogenation at elevated temperatures; 3.) oxidation/regeneration by a O 2 containing gas at elevated temperatures; Preferably the alkane dehydrogenation is effected at temperatures in the range of 500 to 700 °C . Also the oxidation/regeneration is preferable effected at temperatures in the range of 500 to 700 °C . Equally the reduction by H 2 is preferable effected in the range of 500 to 700 °C . Particularly the chemical process is dehydrogenation performed on a lower alkane. More preferably propane is dehydrogenated. In a preferred process the oxidation is effected by a gaseous O 2 /N 2 mixture gradually increasing the O 2 content. In a preferred process the final part of the oxidation is effected in air. With respect to the reduction step this is preferably effected in H 2 in a temperature range of 500 to 700 °C . With respect to the dehydrogenation time this is less than half of the dehydrogenation time during normal dehydrogenation process conditions. A typical dehydrogenation time is 2 hours. Good results have been obtained when the first part of the oxidation is effected for about 3 hours, whereas the final part of the oxidation is effected for about 1 hour. The reduction in H 2 is particularly effected for about 2 hours. The best results are achieved when the number of cycles are 6 to 12, but fewer cycles also have a significant effect. It is expected that an optimum pretreatment activation procedure to a large extent will depend on the exact composition and preparation method of the catalyst. This means that there will be large variations in the preferred number of cycles, gas compositions and required times for each step. A further object of the present invention is a catalyst activated by the pre-treatment set forth above. Detailed description of invention As already mentioned above, the objective of this invention is to define a pretreatment procedure which decreases the time required to reach an optimum conversion level in an alkane dehydrogenation unit, using a Pt/Sn/Mg(AI)O catalyst described in patent NO 179131. In this previous invention we describe a pre- treatment procedure which led to an optimum dehydrogenation activity of a Pt/Sn/Mg(AI)0 catalyst obtained by impregnating Pt and Sn on a pre-calcinated Mg(AI)O carrier material. The pre-treatment procedure used in the previous invention consisted of in situ reduction, followed by oxidation and a new reduction period (ROR). In the present invention, it was surprisingly observed that by using a slightly modified metal impregnation procedure, the ROR pre-treatment procedure did not lead to an optimum dehydrogenation activity. During testing of the dehydrogenation catalyst it was observed that the conversion level after the first regeneration was higher than the initial conversion level after the ROR. After the next test cycle the conversion level was even higher. A stable catalyst performance was not obtained before after 300h on stream, when the catalyst had been subjected to 9 cycles consisting of a dehydrogenation period followed by regeneration and reduction. A pre-treatment procedure for the catalyst that dramatically decreased the required time to obtain optimum conversion level in dehydrogenation of propane was developed. The following examples will serve to illustrate the advantages of using the herein described pre-treatment procedure for the activation of the Pt/Sn-Mg(AI)O dehydrogenation catalyst. The examples should not, however, limit the scope of the present invention. General The catalyst was prepared according to a slightly modification of the procedure described in patent NO 179131 , using a water based acidic solution in the deposition of platinum and tin on hydrotalcite or precalcined hydrotalcite. The catalysts used in the examples have an Mg/AI-ratio of 4.8. The content of platinum and tin are ca. 0.25 and 0.5 wt% respectively. The catalyst was calcinated at 560°C after the impregnation. The catalyst was pressed to tablets, crushed and sieved to a pellet size of 0.64-1.0 mm prior to testing. All pre-treatment procedures were performed in situ in a laboratory scale fixed bed titanium reactor with subsequent testing for propane dehydrogenation. The inner diameter of the reactor was 9 mm. A titanium tube with an outer diameter of 3 mm was located in the centre of the reactor. The reactor temperature was controlled by a thermocouple placed in the 3 mm tube inside the reactor. The catalyst pellets ( approximately 3 g) was placed on a titanium sinter in the reactor. The total pressure in the reactor was 1.1 bar, and the reactor temperature was 600°C. However, due to the endothermic nature of the reaction a temperature gradient was observed in the reactor. The reactor temperature was regulated to 600°C at a distance of 1/3 of the total catalyst bed-length from the top of the catalyst bed, while the temperature at the bottom of the catalyst bed, measured at a distance of approximately 1/3 of catalyst bed length from the bottom of the catalyst bed, was in the range of 590 to 615°C. The GHSV was 1000h "1 based on propane, and the reaction gas contained 4.5% hydrogen, 32% propane and rest steam on mole basis. The dehydrogenation periods usually lasted approximately 20 hours and were followed by a regeneration of the catalyst. The conversion levels were calculated from on-line GC analysis. The analysis were taken with approximately 1 hours intervals. The conversion levels presented were obtained after 5h in the dehydrogenation period. The regeneration of the catalyst was done by burning of the formed coke using air diluted with nitrogen. The content of oxygen was initially reduced to approximately 2% and was stepwise increased to a final level of 21 % where pure air was used. The regeneration period was followed by catalyst reduction using hydrogen. Both the regeneration and the reduction of the catalyst were done at 600°C. Example 1 (ROR) The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. Heating from ambient temperature to 600°C in nitrogen, 100ml/min. 2. Reduction in H 2 . 20ml/min, 600°C, 2h. 3. Oxidation in a flow containing 40ml/min N 2 and 10ml/min air. 600°C. 1 h. 4. Oxidation in air. 50 ml/min., 600°C, 1 h. 5. Reduced in H 2 . 600°C. 2h. 20ml/min. The results from the subsequent propane dehydrogenation tests are shown in Figure 1 and Table 1. Example 2: (ROR - (PDH(600°C)-OR)*X) The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. The catalyst is first pre-treated according to the ROR-procedure described in Example 1. 2. PDH ( Propane Dehydrogenation) at 600°C. in a stream consisting of C 3 H 8 (70ml/min). H 2 (10ml/min) and steam (140 ml/min) for 2h. 3. Oxidation/regeneration using an 0 2 /N 2 -mixture with a gradually increasing O 2 content. (1 , 5 and 10%). Each step lasted 1h. Different O 2 content was obtained by diluting air with N 2 . Total flow:84ml/min., T=600°C. 4. Oxidation in air. 80ml/min., 600°C, 1 h. 5. Reduction in H 2 . 20ml/min., 600°C, 2h. Step 2 to 5 was repeated 9 times, when no further increase in the conversion level of propane in the PDH period was observed. The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1. Example 3 (R-OR*X) The catalyst was subjected to a pre-treatment procedure consisting of the following steps:: 1. Heating from ambient temperature to 600°C in nitrogen, 100ml/min. 2. Reduction in H 2 . 20ml/min., 600°C, 2h. 3. Oxidation/regeneration using an O 2 /N 2 mixture with a gradually increasing O 2 content (1. 5. and 10%). 1 h on each step. Different O 2 content was obtained by diluting air with N 2 . Total flow: 84ml/min. T=600°C. 4. Oxidation in air. 80ml/min., 600°C, 1 h. ? 5. Reduction in H 2 . 20ml/min., 600°C, 2h. Step 3 to 5 are repeated 12 times. The results from the following PDH conversion tests are shown in Figure 1 and Table 1. Example 4: (ROR - (PDH-OR)*X) The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. The catalyst is first pre-treated according to the ROR-procedure described in Example 1. 2. PDH ( Propane Dehydrogenation) at 650°C. in a stream consisting of C 3 H 8 (70ml/min). H 2 (10ml/min) and steam (140 ml/min) for 2h. 3. Oxidation/regeneration in an 0 2 /N 2 mixture with a gradually increasing O 2 content (1. 5. and 10%). 1h on each step. Different O 2 content was obtained by diluting air with N 2 . Total flow:84ml/min., T=600°C. 4. Oxidation in air. 80ml/min., 600°C, 1h. 5. Reduction in H 2 . 20ml/min., 600°C, 2h. Step 2 to 5 was repeated 6 times, when no further increase in the conversion level of propane in the PDH period was observed. The results from the subsequent propane dehydrogenation tests are shown in Figure 1 and Table 1. Example 5: R-(PDH-OR)*X The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. Heating from ambient temperature to 600°C in nitrogen, 100ml/min. 2. Reduction in H 2 . 20ml/min., 600°C, 2h. 3. PDH ( Propane Dehydrogenation) at 630°C. in a stream consisting of C 3 H 8 (70ml/min). H 2 (10ml/min) and steam (140 ml/min) for 2h. 4. Oxidation/regeneration using an O 2 /N 2 -mixture with a gradually increasing O 2 content. (1. 5. and 10%) 1h on each step. Different 0 2 content was obtained by diluting air with N 2 . Total flow:84ml/min. T=600°C. 5. Oxidation in air. 80ml/min., 600°C, 1h. 6. Reduction in H 2 . 20ml/min, 600°C, 0.5h. Step 3 to 6 were repeated 5 times, when it was found no further increase in conversion level in the PDH period (Step 3) are observed. The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1. Example 6: R-(PDH-OR)*X The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. Heating from ambient temperature to 600°C in nitrogen, 100ml/min. 2. Reduction in H 2 . 20ml/min., 600°C, 2h. 3. PDH ( Propane Dehydrogenation) at 630°C. in a stream consisting of C 3 H 8 (70ml/min). H 2 (10ml/min) and steam (140 ml/min) for 2h. 4. Oxidation/regeneration using an O 2 /N 2 -mixture with a gradually increasing 0 2 content. (1. 5. and 10%) 1 h on each step. Different O 2 content was obtained by diluting air with N 2 . Total flow:84ml/min. T=600°C. 5. Oxidation in air. 80ml/min., 600°C, 1h. 6. Reduction in H 2 . 20ml/min, 600°C, 2h. Step 3 to 6 are repeated 6 times, when it was observed no further increase in conversion level in the PDH period (Step 3). The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1. Table 1 Normalised conversion of propane with time on stream. The conversion level are obtained 5 hours after finishing the regeneration/reduction period. The results are also shown in Figure 1. Normalised conversion data from Ex 1 to Ex 6 are shown in Figure 1. The conversion level data for Example 1 to Example 6 are all given as normalised values, where the stabilised conversion level which are obtained on the completely pre-treated catalyst is given the value 100%. A typical example of the obtained conversion level of propane to propene is given in Table 2. Similar conversion levels are also obtained in the remaining examples. However, the conversion level shows small variation due to minor changes in the reaction conditions as for instance the temperature profile in the reactor and different partial pressure in the gases entering the reactor. Table 2 Conversion data from Example 5 Time on 40,3 65,2 90,2 115,1 140 164,9 stream conversion 53,4 56,6 58,5 58,8 59,1 59,4 level [%] The results show that by using the ROR pre-treatment procedure described in patent NO 179131 on the present catalyst the activity of the catalyst increases after each conversion cycle, consisting of a dehydrogenation period followed by regeneration/oxidation and reduction. The catalyst does not operate at an optimum performance until after 300h on stream. Using a pre-treatment procedure consisting of repeated cycles of reduction and oxidations and a final reduction (Example 3), the time to reach the optimum performance is reduced to approximately 50-150h. However, several dehydrogenation-regeneration cycles have to be conducted after the pre-treatment procedure in order to give a high and stable conversion level. A pre-treatment procedure which included the aforementioned ROR procedure followed by several cycles consisting of a short dehydrogenation period (2h, at 600°) followed by regeneration (oxidation) and reduction in hydrogen (Example 2) reduced the pre-treatment time to approximately 80h. When the short dehydrogenation period during the pre-treatment was conducted at higher temperature, i.e. 630- 650°C even shorter pre-treatment time was obtained (Example 4). The initial ROR pre-treatment could also be excluded using this kind of catalyst pre-treatment. (Example 5 and 6). A pre-treatment time of 40h was obtained using a pre-treatment consisting of an initial reduction followed by 5 cycles consisting of dehydrogenation, regeneration and reduction. A detailed description of this procedure is given in example 5. After this pre-treatment the catalyst operate at a conversion level of 90% of the optimum conversion level. Within two conventional 20 hours dehydrogenation periods at 600°C, the catalyst performance was at an optimum level. Unsuccessful pretreatment Example 7: OR The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 1. Heating from ambient temperature to 600°C in nitrogen, 100ml/min. 2. Oxidation in air. 50ml/min., 600°C, 10h. 3. Reduction in H 2 . 20ml/min, 600°C, 2h. The conversion after 5 hours propane dehydrogenation (same test conditions as described in example 1 to 6, 600°C) was 11%, and the selectivity was 94%. Example 8: O-Steam-R The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 4. Oxidation/regeneration of the catalyst from example 7 using an O 2 /N 2 -mixture with a gradually increasing 0 2 content. (1. 5. and 10%) 1h on each step. Different 0 2 content was obtained by diluting air with N 2 . Total flow: 84ml/min. T=600°C. 5. Steaming. 140ml/min., 600°C, 10h. 6. Reduction in H 2 . 20ml/min, 600°C, 2h. The conversion after 5 hours propane dehydrogenation (same test conditions as described in example 1 to 6, 600°C) was 26%, and the selectivity was 95%. Example 9: O-Steam-R The catalyst was subjected to a pre-treatment procedure consisting of the following steps: 5 7. Oxidation/regeneration of the catalyst from example 8 using an 0 2 /N 2 -mixture with a gradually increasing 0 2 content. (1. 5. and 10%) 1h on each step. Different 0 2 content was obtained by diluting air with N 2 . Total flow: 84ml/min. T=600°C. 8. Steaming. 140ml/min., 700°C, 10h. ιo 9. Reduction in H 2 . 20ml/min, 600°C, 2h. The conversion after 5 hours propane dehydrogenation (same test conditions as described in example 1 to 6, 600°C) was 7.5%, and the selectivity was 93.5%. i5 Conclusion The activation period for the Pt/Sn/Mg(AI)O dehydrogenation catalyst is reduced from 300h to approximately 50 h by using the pre-treatment procedure developed in the present invention. The pre-treatment procedure consists of an initial reduction of the catalyst using hydrogen, followed by several cycles which includes a short 20 dehydrogenation period. Literature cited Catalytica Oxidative Dehydrogenation and Alternative Dehydrogenation Processes Study No.4192 OD; Catalytica Studies Division: Mountain View, CA, 1993 25 Akporiaye, D., Rønnekleiv, M., Hasselgard, P., NO 179131 , 1996, assigned to Statoil.

Claims: Patent Claims 1. A process of a pre-treatment activation of a Pt/Sn based catalyst, wherein a Pt/Sn/Mg(AI)0 catalyst is subjected to one or more cycles of 1.) reduction by H 2 at elevated temperatures. 2.) alkane dehydrogenation at elevated temperatures; 3.) oxidation/regeneration by a 0 2 containing gas at elevated temperatures; 2. The process of the claim 1 , wherein the alkane dehydrogenation is effected at temperatures in the range of 500 to 700 °C . 3. The process of the claim 1 , wherein the oxidation/regeneration is effected at temperatures in the range of 500 to 700 °C . 4. The process of the claim 1 , wherein the reduction by H 2 is effected in the range of 500 to 700 °C . 5. The process of the claims 1 and 2, wherein a lower alkane is dehydrogenated. 6. The process of the claims 1 , 3, and 5, wherein propane is dehydrogenated. 7. The process of the claims 1 and 3, wherein the oxidation is effected by a gaseous mixture gradually increasing the O 2 content. 8. The process of the claims 1 and 7, wherein the final part of the oxidation is effected in air. 9. The process of the claim 1 , wherein the reduction in H 2 is effected in a temperature range of 500 to 700 °C . 10. The process of the claims 1 , 2, 5, and 6, wherein the dehydrogenation is effected for about 2 hours. 11. The process of the claimsl , 3, 7, and 8, wherein the first part of the oxidation is effected for about 3 hours, whereas the final part of the oxidation is effected for about 1 hour. 12. The process of the claim 1 , wherein the reduction in H 2 is effected for about 2 hours. 13. The process of the claims 1 to 12, wherein the number of cycles are more than one. 14. The process of the claims 1 to 12, wherein the number of cycles are 6 to 12. 15. A catalyst activated by the pre-treatment of the claims 1 to 13.

Inventors: JENSEN STAALE FOERRE, ROENNEKLEIV MORTEN, RYTTER ERLING, SOERAKER PAAL, JENSEN, STAALE, FOERRE, ROENNEKLEIV, MORTEN, RYTTER, ERLING, SOERAKER, PAAL
Applicants: JENSEN STAALE FOERRE, NORSKE STATS OLJESELSKAP, ROENNEKLEIV MORTEN, RYTTER ERLING, SOERAKER PAAL, DEN NORSKE STATS OLJESELSKAP A.S, JENSEN, STAALE, FOERRE, ROENNEKLEIV, MORTEN, RYTTER, ERLING, SOERAKER, PAAL
Application year: 2000
Main classification: B01J38
IPC codes: B01J23, B01J37, B01J38, C07C5