The present invention relates to a regeneration procedure for transition metal-based dehydrogenation catalysts used in processes for the dehydrogenation of alkanes to the cor responding alkenes.

The catalytic dehydrogenation of lower alkanes is a simple, but yet important reaction, which can be illustrated by the dehydrogenation of propane to propylene in accordance with the reaction:

C ₃ H ₈ <-> C ₃ H ₆ + ¾

With the ever growing demand for light olefins, i.e. lower aliphatic open-chain hydrocarbons having a carbon-carbon double bond, catalytic dehydrogenation is growing in im portance. Especially the dehydrogenation of propane and isobutane are important reactions which are used commer cially for the production of propylene and isobutylene, re spectively. Propylene is an important fundamental chemical building block for plastics and resins, and the worldwide demand for propylene has been growing steadily for decades. It is expected that the demand growth for propylene will soon be equal to or even higher than that for ethylene. One of the major applications of isobutylene is that it can be used as feedstock in the manufacture of methyl-tert-butyl ether (MTBE) . The process shown above is endothermic and requires approx imately 125 kJ/mole in heat of reaction. Thus, in order to achieve a reasonable degree of conversion, the dehydrogena tion process is taking place at a temperature around 600°C. The dehydrogenation of isobutane is similar to that of pro pane in every respect, apart from requiring a slightly lower temperature.

Today there are 4 major processes for alkane dehydrogena tion in commercial use: The Catofin process which uses a chromia catalyst supported on alumina, the Oleflex process which uses a platinum-tin catalyst supported on alumina, the STAR process which uses a platinum catalyst supported on ZnAl C spinel and the Snamprogetti-Yarzintez process which uses a chromia catalyst. The differences between these processes primarily relate to the supply of the heat of reaction, but also to the kind of catalyst used in each process .

In most (if not all) catalytic alkane dehydrogenation pro cesses, the conditions at which dehydrogenation takes place typically cause a deactivation of the catalyst due to sin tering of the active component and/or carbon formation and/or phase transformations, any and all of which can lead to loss of activity and/or selectivity of the catalyst. Thus, all catalytic alkane dehydrogenation processes in clude periodic regenerations of the catalyst. The frequency of the catalyst regeneration varies greatly for the indus trial processes mentioned, e.g. in the Catofin process, the catalyst is regenerated around 50 times per day, whereas, for the Oleflex process, the catalyst is regenerated after around 10 days. A typical regeneration procedure could in clude an oxidative regeneration to remove carbon, although there can also be additional steps in a regeneration phase which are necessary to a specific process or catalyst, such as chloro-regeneration for re-dispersion of active metal in the Oleflex process with Pt-Sn based catalysts and reduc tive regeneration in the Catofin process for converting un- selective Cr(VI) to selective Cr(III) . This means that re generation can entail one or more different steps, together forming a regeneration sequence. Those who practice alkane dehydrogenation, especially propane dehydrogenation (PDH) , will understand that as the activity of a PDH catalyst de creases, the alkene production also decreases with a conse quent negative impact on the process economics.

Thus, the regeneration and stabilization of dehydrogenation catalysts is the subject of a number of prior art docu ments, such as EP 2 000 450 A1 which describes a method for greatly extending the useful life of a catalyst bed used in the catalytic dehydrogenation of alkylaromatic hydrocarbons while maintaining a high level of conversion and of selec tivity and without the need to interrupt the conversion process .

US 9.861.976 B2 discloses a process for the regeneration of an oxidative dehydrogenation catalyst in an alternate or separate regeneration reactor by employing controlled steam/air and time/pressure/temperature conditions. The re generated catalyst is an iron-based oxide catalyst with or without a content of zinc. In WO 2017/151361 Al, an air-soak containing regeneration process is disclosed, which comprises several steps, i.e. removing surface carbon species from a gallium-based alkane dehydrogenation catalyst in a combustion process in the presence of a fuel gas, conditioning the catalyst in an air-soak treatment at 660-850°C with a flow of an oxygen- containing gas having 0.1-100 ppmv of a chlorine source and achieving a pre-determined alkane conversion percentage for the gallium-based alkane dehydrogenation catalyst.

The present invention relates to an alternative regenera tion procedure for transition metal-based dehydrogenation catalysts used in processes for the dehydrogenation of al kanes to the corresponding alkenes.

As with Pt—based or chromia-based dehydrogenation cata lysts, a transition metal-based dehydrogenation catalyst needs frequent regeneration due to carbon deposition. For the utilization of transition metal-based catalysts in de hydrogenation processes, it has been discovered that after an oxidative regeneration to remove carbon, it is benefi cial to include a sulfidation step to the regeneration se quence, as this step leads to a clear enhancement of the catalyst selectivity in subsequent dehydrogenation steps in comparison to purely reduced or purely oxidized Fe-based catalysts .

So the present invention relates to a regeneration proce dure for a transition metal-based dehydrogenation catalyst used in processes for the dehydrogenation of alkanes to the corresponding alkenes, said procedure including - an oxidative regeneration using an oxygen containing gas to remove carbon deposited on the catalyst due to dehydro genation, an optional reduction step, in which a hydrogen-contain ing gas is fed to the reactor, and

- a subsequent sulfidation step, in which a gas containing sulfur, such as ¾S, DMDS or DMS, together with hydrogen is fed to the reactor, where the catalyst reacts with the sul fur species to form metal sulfide, thereby ensuring the presence of a certain sulfur load on the catalyst obtained by adding sulfur in the last stage of the regeneration procedure.

In the following, what happens to transition metal-based catalysts during dehydrogenation will be explained:

Transition metal-based dehydrogenation catalysts contain metal oxides (denoted as MeO) when they are first put into service. The catalysts can optionally be reduced to form metals :

MeO + ¾ <—> Me + ¾0

During dehydrogenation, the catalyst is exposed to alkanes, such as propane, and also hydrogen and sulfur, and here several phases are possible:

Me + ¾S <—> MeS + ¾

MeO + ¾S <—> MeS + ¾0 9 MeS + C ₃ H ₈ + 5 H ₂ <-> 3 Me ₃ C + 9 ¾S

At the typical dehydrogenation conditions, there is often not sufficient ¾S in the feed gas to keep the catalyst as sulfide, but carbide formation will take place.

In parallel, coke is deposited on the catalysts, which causes deactivation. When the catalysts are sufficiently deactivated, the production becomes too low to continue de hydrogenation, and the catalysts are then regenerated to remove coke.

During regeneration, the catalysts are exposed to an oxygen containing gas at high temperatures, typically at 400-700 °C, which will cause oxidation of coke on the catalysts ac cording to the reactions:

C + 0 ₂ <-> C0 ₂

Me ₃ C + O2 <—> MeO + CO2

MeS + 2 0 ₂ <-> MeS0 ₄

In some cases, the regeneration temperature is sufficiently high to cause decomposition of the metal sulfates according to the reaction:

MeS0 ₄ <-> MeO + S0 ₃

SO2 can also be formed by a decomposition of sulfates.

SO ₂ /SO ₃ , when formed, will leave the reactor together with the flowing regeneration gas, causing a loss of sulfur from the catalyst. It has been shown that metal sulfides are more active and stable dehydrogenation catalysts than the corresponding metals .

The present invention deals with an alternative regenera tion procedure where the catalyst is optionally reduced af ter the oxidative regeneration step and further sulfided using a sulfur containing gas.

The invention is illustrated further in the following exam ple .

Example 1

This example shows a comparison of selectivities to propene during propane dehydrogenation for an Fe-based catalyst following three different regeneration sequences, finishing with sulfidation, reduction or oxidation, respectively. The results, depicted as selectivity in percent vs. hours on stream, are shown in Fig. 1, and the results are given in the table below.

The details of the test were as follows: A catalyst con taining 10 wt% Fe, impregnated on transition alumina spheres, was tested in a tubular reactor under propane de hydrogenation conditions with a 03¾:H2 ratio of 2, a WHSV (weight hourly space velocity, which is defined as the weight of feed flowing per unit weight of the catalyst per hour) of 3.5 h ^_1 , 200 ppmv ¾S, 620°C, 1 barg, the catalyst being exposed to the dehydrogenation condition for 7 hours at a time. Prior to each propane dehydrogenation exposure, the catalyst underwent: a) oxidative regeneration in 1% Cy for 2 hours followed by reduction/sulfidation using a gas consisting of 20% hydro gen in nitrogen and approximately 2000 ppmv ¾S, b) oxidative regeneration in 1% Cy for 2 hours followed by reduction using a gas consisting of 20% hydrogen in nitro gen and no ¾S, and c) oxidative regeneration in 1% Cy for 10 hours.