FR3014432A1 - PROCESS FOR THE DEHYDROGENATION OF HYDROCARBONS BY A HETEROGENEOUS PHOTOCATALYST IN ABSENCE OF DIOXYGEN. - Google Patents

Abstract

The invention describes a process for dehydrogenating a hydrocarbon feedstock containing at least one molecule having at least 2 carbon atoms and at least one single or double CC bond in the presence of a heterogeneous photocatalyst under irradiation comprising the following steps: a) said hydrocarbon feedstock is brought into contact with a heterogeneous photocatalyst, b) the heterogeneous photocatalyst is irradiated with at least one irradiation source producing at least one wavelength suitable for activating said photocatalyst so as to dehydrogenate said hydrocarbon feedstock in the presence of said photocatalyst activated by said irradiation source for converting said feed molecule having at least said single or double CC bond into a molecule having a higher degree of unsaturation of said DC bond, so as to obtain an effluent containing at least one at least partially dehydrogenated molecule and hydrogen, the steps a) and b) being carried out in the absence of oxygen.

Description

The field of the invention is that of the dehydrogenation of hydrocarbons by the use of a heterogeneous photocatalyst under irradiation in the absence of oxygen. A wide variety of unsaturated molecules such as ethylene, propene, linear butenes, isobutene, pentenes, styrene, as well as unsaturated molecules containing up to about 20 carbon atoms find applications in the processes. petrochemicals, such as polymerization units. These unsaturated hydrocarbons are therefore of great interest from an industrial point of view. The main sources of unsaturated molecules are steam cracking and catalytic cracking processes. However, both these processes also produce by-products and the increasing demand is directed to specific alkenes that would be uneconomical to produce by cracking. The dehydrogenation reaction of saturated hydrocarbons such as paraffins, producing unsaturated hydrocarbons, such as olefins, and hydrogen is thus known. The dehydrogenation can be represented by the following equation: CnHm_ * CnHm-2 + H2 Conventionally, the dehydrogenation reactions are conducted in the presence of a catalyst generally based on metals or metal oxides of groups VIB and VIII of the periodic classification of the elements, at high temperature, between 400 and 800 ° C, and in high dilution with an inert or quasi-inert gas to shift the thermodynamic equilibrium to the formation of unsaturated hydrocarbons. This type of reaction can also be carried out in the presence of an oxidant, such as oxygen, which consumes the hydrogen formed during the dehydrogenation step and thus displaces the thermodynamic equilibrium towards the formation of unsaturated hydrocarbons. This type of implementation for the dehydrogenation reaction of hydrocarbons is nevertheless a major energy consumer because of the high thermal levels required but also because of the significant dilution of the charges. In addition, significant deactivation phenomena are known due in particular to the formation of coke on the surface of the active phase of the catalysts. Reviews of the literature largely explain the operating principles of this type of reaction (SF Hakonsen et al., "Oxidative dehydrogenation of alkanes" in G. Ertl et al., Handbook of Heterogeneous Catalysis, Vol 7, Wiley VCH Verlag, Weinheim, Germany (2008), p 3384-3400;

Caspary et al., "Dehydrogenation of alkanes", in G. Ertl et al. eds., Handbook of Heterogeneous Catalysis, Vol. 7, Wiley-VCH Verlag, Weinheim, Germany (2008), p 3206-3229). Moreover, it is known to carry out dehydrogenation reactions by photocatalytic route. Thus, it is known from the literature to dehydrogenate linear or cyclic alkanes by photolitic activation of CH bonds via the use of homogeneous photocatalysts using complexes of noble metals such as osmium complexes (H. Kunkely et al., Inorganic Chemistry Communications, 13, p 134, 2010), Iridium (MJ Burk et al., Journal of the American Chemical Society, 109, p 8025, 1987, MJ Burk et al., Journal of Chemical Society, Chemical Communications, p 1829 1985, B. Rabay et al., Dalton Transaction, 42, p 8058, 2013) or ruthenium (H. Itagaki et al., Bulletin of the Chemical Society of Japan, 67, p 1254, 1994; K. Nomura et al. Journal of Chemical Society, Chemical Communications, 161, 1988, JA Maguire et al., Journal of the American Chemical Society, 111, p 7088, 1989). However, the use of this type of homogeneous photocatalyst makes it possible to achieve very low quantum yields, claimed between 0.07% and 1.6%, inducing very low productivities. In addition, cases of deactivation by degradation of metal complexes under the effect of irradiation are sometimes mentioned.

Y. Wada et al. (Studies in Surface Science and Catalysis, 75, p 2163, 1993) have proposed to immobilize a rhodium complex, RhCI (CO) (PMe3) 2, on a porous glass support. By using this photocatalytic system, these studies show the dehydrogenation of isobutane to isobutene under UV irradiation. This system has a good stability, thus resolving the potential degradation of the photocatalyst, but the productivity remains very low due to always very low quantum efficiency. On the other hand, the oxidative photocatalytic dehydrogenation of cyclohexane to benzene by the use of a heterogeneous photocatalyst composed of a combination of semiconductor MoOx and TiO2 is known from the literature (Ciambelli P. et al., Catalysis Today, 99, p 143, 2005). The implementation of this reaction is in the presence of oxygen, oxidizing, playing the role of sacrificial element. However, the use of oxygen as a sacrificial element sometimes involves obtaining oxygenated compounds during the conversion of cyclohexane, cyclohexanone or cyclohexanol (CB Almquist et al., Applied Catalysis A: General, 214, p259, 2001), and still produces carbon dioxide, the ultimate degradation product of the hydrocarbon. This production of oxygenated compounds and CO2 implies decreases in selectivities towards the production of the desired unsaturated hydrocarbons. The object of the invention is to propose an alternative method of dehydrogenation of synthesis of unsaturated hydrocarbons by the use of a heterogeneous photocatalyst under irradiation in the absence of oxygen. This synthetic route makes it possible to achieve very high quantum efficiencies with respect to the performance of the photocatalytic processes described in the prior art. In addition, unlike the process of oxidative dehydrogenation by photocatalysis, in which the hydrogen from the dehydrogenation is consumed as and when oxygen introduced into the reaction medium, the method according to the invention makes it possible to produce hydrogen which is a valuable co-product. On the other hand, the synthetic route according to the invention is distinguished from the conventional catalytic dehydrogenation pathway by very low thermal levels, of the order of ambient temperature, and the possibility of converting hydrocarbon feeds without the use of thinners. In addition, the dehydrogenation according to the invention can be carried out in the liquid phase or in the gas phase, at high or at low pressure. More particularly, the present invention relates to a process for the dehydrogenation of a hydrocarbon feedstock containing at least one molecule having at least 2 carbon atoms and at least one single or double CC bond in the presence of a heterogeneous photocatalyst under irradiation comprising the the following steps: a) the said hydrocarbon feedstock is brought into contact with a heterogeneous photocatalyst; and b) the heterogeneous photocatalyst is irradiated with at least one irradiation source producing at least one wavelength suitable for activating said photocatalyst. in order to dehydrogenate said hydrocarbon feed in the presence of said photocatalyst activated by said irradiation source to transform said feed molecule having at least said single or double CC bond into a molecule having a higher degree of unsaturation of said CC bond, in order to obtain an effluent containing at least one molecule at least partially shydrogénée and hydrogen, steps a) and b) being carried out in the absence of oxygen.

By a molecule having "a higher degree of unsaturation of said CC bond" is meant the transformation of a molecule having a single CC bond (paraffin) into a molecule having a double CC (olefin) or triple (acetylenic) bond or a molecule having a cyclic CC bond or the transformation of a molecule having a double CC bond (olefin) into a molecule having a triple CC bond (acetylenic) or a molecule having a cyclic CC bond. According to one variant, the heterogeneous photocatalyst is selected from TiO2, CdO, Ce203, CoO, Cu2O, FeTiO3, In203, NiO, PbO, ZnO, Ag2S, CdS, Ce2S3 , Cu2S, CuInS2, In2S3, ZnS and ZrS2 According to one variant, the photocatalyst is doped with one or more ions chosen from metal ions, non-metallic ions, or a mixture of metal ions and non-metallic. According to one variant, the photocatalyst further comprises at least one co-catalyst chosen from a metal, a metal oxide or a metal sulphide. According to one variant, the irradiation source is a source of artificial irradiation emitting in the ultraviolet and / or visible spectrum. According to a variant, the irradiation source emits at a nominal wavelength at maximum 50 nm less than the maximum wavelength absorbable by the photocatalyst and produces an irradiation such that at least 50% by number of photons are absorbable. by the photocatalyst. According to one variant, the dehydrogenation process is carried out at a temperature of -10 ° C. to + 200 ° C. and at a pressure of between 0.01 NPa and 70 MPa. According to a variant, said hydrocarbon feedstock contains at least one paraffinic, naphthenic, alkylaromatic and / or olefinic molecule containing from 2 to 20 carbon atoms. According to one variant, the contacting is done in fixed bed crossed, in a fixed bed licking or in suspension in which the photocatalyst is in the form of particles. According to one variant, the dehydrogenation process is carried out in the liquid phase or in the gas phase. According to one variant, the placing in contact is in the presence of a gaseous or liquid diluent fluid. According to one variant, the effluent containing at least one at least partially dehydrogenated molecule and hydrogen is subjected to at least one separation step so as to separate the product hydrogen and to obtain an enriched fraction in at least one at least partially dehydrogenated molecule.

According to another variant, the effluent containing at least one at least partially dehydrogenated molecule and hydrogen is subjected to a selective hydrogenation step in the presence of a selective hydrogenation and hydrogenation catalyst under operating conditions. selective hydrogenation for obtaining an effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule. According to one variant, at least part of the hydrogen used in the selective hydrogenation stage is the hydrogen produced during stage c). According to one variant, the selective hydrogenation step is carried out before or after the separation step. In the following, groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC Press, editor in chief D.R. Lide, 81st edition, 2000-2001). For example, group VIII according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification. DETAILED DESCRIPTION OF THE INVENTION According to step a) of the process according to the invention, said hydrocarbon feedstock is brought into contact with a heterogeneous photocatalyst in the absence of dioxygen. The hydrocarbon feedstock treated according to the process of the invention contains at least one molecule having at least 2 carbon atoms and at least one single or double C-C bond. By double bond is meant an olefinic or allylic bond and not an aromatic bond. The filler generally contains at least one paraffinic, naphthenic, alkylaromatic and / or olefinic molecule containing from 2 to 20 carbon atoms. The filler may contain impurities such as trace water. The molecule having at least 2 carbon atoms and at least one single or double CC bond converted by dehydrogenation does not contain heteroatoms such as nitrogen, oxygen, sulfur or halogen. The hydrocarbon feed may come from a specific cut or a mixture of cut resulting from the atmospheric distillation of oil, natural gas or be derived from biomass. By way of example, the feedstock may contain a molecule such as ethane, propane, butanes, butenes, pentanes, hexanes, heptanes, octanes, or ethylbenzene.

The heterogeneous photocatalyst used according to the method of the invention comprises at least one inorganic, organic or hybrid organic-inorganic semiconductor, supported or not. The band gap width of inorganic, organic or hybrid organic-inorganic semiconductor is generally between 0.1 and 4 eV.

According to a first variant, the heterogeneous photocatalyst comprises at least one inorganic semiconductor. The inorganic semiconductor may be one or more selected from one or more Group IVA elements, such as silicon, germanium, silicon carbide or silicon-germanium. It can also be composed of elements of groups IIIA and VA, such as GaP, GaN, InP and InGaAs, or elements of groups IIB and VIA, such as CdS, ZnO and ZnS, or elements of groups IB and VIIA, such as CuCl and AgBr, or elements of groups IVA and VIA, such as PbS, PbO, SnS and PbSnTe, or elements of groups VA and VIA, such as Bi2Te3 and Bi203, or elements of groups IIB and VA, such as Cd3P2, Zn3P2 and Zn3As2, or elements of groups IB and VIA, such as CuO, Cu2O and Ag2S, or elements of groups VIIIB and VIA, such as CoO, PdO, Fe2O3 and NiO, or elements of groups VIB and VIA, such as MoS2 and WO3, or elements of groups VB and VIA, such as V205 and Nb2O5, or elements of groups IVB and VIA, such as TiO2 and HfS2, or of elements of groups IIIA and VIA, such as In2O3 and 1n253, or elements of VIA groups and lanthanides, such as Ce203, Pr203, Sr1253, Tb2S3 and La2S3, or elements of VIA groups and actinides, such as U O2 and UO3. Preferably, the heterogeneous photocatalyst is selected from TiO2, CdO, Ce203, CoO, Cu2O, FeTiO3, R203, NiO, PbO, ZnO, Ag2S, CdS, Ce2S3, Cu2S, CulnS2, Rins2S3, ZnS and ZrS2.

According to another variant, the heterogeneous photocatalyst comprises at least one organic semiconductor. Among the organic semiconductors, mention may be made of tetracene, anthracene, polythiophene, polystyrene sulphonate and fullerenes. According to another variant, the heterogeneous photocatalyst comprises at least one hybrid organic-inorganic semiconductor. Among the organic-inorganic hybrid semiconductors, mention may be made of crystalline solids of the MOF type (for Metal Organic Frameworks according to the English terminology). The MOFs consist of inorganic subunits (transition metals, lanthanides, etc.) connected to each other by organic ligands (carboxylates, phosphonates, imidazolates, etc.), thus defining crystallized hybrid networks, sometimes porous.

The photocatalyst may optionally be doped with one or more ions selected from metal ions, such as, for example, ions of V, Ni, Cr, Mo, Fe, Sn, Mn, Co, Re, Nb, Sb, La, Ce, Ta, Ti, non-metallic ions, such as for example C, N, S, F, P, or by a mixture of metallic and non-metallic ions. Optionally, the photocatalyst may contain in addition to the semiconductor at least one co-catalyst. Preferably, the photocatalyst contains a cocatalyst. The cocatalyst can be any metal, metal oxide or metal sulfide. The cocatalyst is preferably in contact with at least one constituent semiconductor of the photocatalyst. Among the metals, there may be mentioned, for example, Pt, Au, Pd, Ag, Cu, Ni, Rh and Ir. Examples of metal oxides that may be mentioned include Cr 2 O 3, NiO, PtO 2, RuO 2, IrO 2, CuO and Mn 2 O 3. Among the metal sulphides, mention may be made, for example, of MoS2, ZnS, Ag2S, PtS2, RuS2 and PbS. In a variant, the photocatalyst can be deposited on a non-activatable support by close UV irradiation (wavelength irradiation up to 280 nm). This support is either an electrical insulator, such as for example Al 2 O 3, SiO 2 and ZrO 2, or an electrical conductor such as, for example, carbon black, carbon nanotubes and graphite. The mode of synthesis of the photocatalyst may be any synthesis mode known to those skilled in the art and adapted to the desired photocatalyst. The mode of adding possible cocatalysts can be done by any method known to those skilled in the art. Preferably, the cocatalyst is introduced by dry impregnation, excess impregnation or by photo-deposition. According to step b) of the process according to the invention, the heterogeneous photocatalyst is irradiated in the absence of oxygen by at least one irradiation source producing at least one wavelength suitable for activating said photocatalyst so as to dehydrogenate said photocatalyst. hydrocarbon feedstock in the presence of said photocatalyst activated by said irradiation source for transforming said feed molecule having at least said single or double CC bond into a molecule having a higher degree of unsaturation of said DC bond, so as to obtain a effluent containing at least one at least partially dehydrogenated molecule and hydrogen.

Photocatalysis is based on the principle of activation of a semiconductor, or photocatalyst, using the energy provided by the irradiation. Photocatalysis can be defined as the absorption of a photon whose energy is greater than the forbidden bandgap or "bandgap" according to the English terminology between the valence band and the conduction band, which induces the forming an electron-hole pair in the semiconductor. We therefore have the excitation of an electron at the level of the conduction band and the formation of a hole on the valence band. This electron-hole pair will allow the formation of free radicals that will either react with compounds present in the medium or then recombine according to various mechanisms. Each photocatalyst has a difference in energy between its conduction band and its valence band, or "bandgap", which is its own. A photocatalyst can be activated by the absorption of at least one photon. Absorbable photons are those whose energy is greater than the bandgap, the photocatalyst. In other words, the photocatalysts can be activated by at least one photon of a wavelength corresponding to the energy associated with the bandgap width of the photocatalyst or of a lower wavelength. The maximum wavelength absorbable by the photocatalyst is calculated using the following equation: ## EQU1 ## With A max the maximum wavelength absorbable by the photocatalyst (in m), h the Planck constant (4,13433559.10 -15 eV. $), C the speed of light in a vacuum (299 792 458 ms-1) and Eg the band gap or "bandgap" of the photocatalyst (in eV). Any irradiation source emitting at least one wavelength suitable for activating said photocatalyst, that is to say absorbable by the photocatalyst can be used according to the invention. For example, it is possible to use natural solar irradiation or an artificial irradiation source of the laser, Hg, incandescent lamp, fluorescent tube, plasma or light emitting diode (LED) type (LED or Light-Emitting Diode). Preferably, the irradiation source is artificial. The irradiation source produces radiation of which at least a portion of the wavelengths is less than the maximum absorbable wavelength (λmax) by the photocatalyst. The irradiation source generally emits in the ultraviolet and / or visible spectrum, that is to say it emits a wavelength range greater than 280 nm, preferably 315 to 800 nm. According to a first variant, the irradiation source may be a monochromatic source. By monochromatic irradiation source is meant a source 5 emitting photons at a given wavelength, also called the nominal wavelength. In this case, the irradiation source is preferably of the laser type. According to a second variant, the irradiation source emits photons in a wavelength range of plus or minus 50 nm, preferably plus or minus 20 nm, with respect to the nominal wavelength. In this case, the irradiation source 10 is preferably of the light-emitting diode type (LED, or LED in English for Light-Emitting Diode). One or more sources of irradiation can be used. According to a first variant, the irradiation source can be centralized, as in the case of a single laser. According to another variant, the irradiation source may be dispersed, as in the case of a multitude of light-emitting diodes. The irradiation source provides a photon flux that irradiates the reaction medium containing the photocatalyst. The interface between the reaction medium and the light source varies depending on the applications and the nature of the light source. In a first variant, the irradiation source can thus be located outside the reactor, the interface between the two is then by means of an optical waveguide such as optical fiber. This variant is particularly indicated in the case of a laser source and this especially as the power of the source is large. According to a second variant, the irradiation source is located in the reactor, preferably near the photocatalyst to limit losses. This implementation is for example adapted to the use of LEDs. Whether for one or the other of the variants, the arrangement of the sources or waveguides will generally be preferred so as to maximize the surface of the interface between the irradiation source and the reaction medium by unit of photoreactor volume. The power of the source is such that it is greater than the conversion objective of the load in terms of response energy affected by the electrical efficiency of the source, namely the ratio of the irradiation power emitted by the source on the electric power required to generate it, the quantum efficiency, namely the ratio between the catalytic acts induced by photocatalysis and the number of photons absorbed by the photocatalyst affected stoichiometric coefficients of the photocatalyzed reaction and a holding optical efficiency account of the dispersion of the irradiation between the source and the photocatalyst, for example because of the absorbing nature of the solvent and the optical interfaces or during the conduct of the irradiation in a waveguide. The energy efficiency of the light source is defined by the following equation: 1 1 irradiation - Reflectric luminous flux with P luminous flux the power of the irradiation of the light source in watts and electric the power consumed by the light source. The energy yield of the irradiation source is preferably greater than 20%, preferably greater than 30%.

In order to optimize the overall energy efficiency of the process according to the invention, the irradiation source preferably produces a radiation whose wavelength is suitable for activation of the photocatalyst. According to a preferred variant of the process according to the invention, an irradiation source is used which emits at a nominal wavelength at most 50 nm less, preferably at most 20 nm below, the maximum wavelength absorbable by the photocatalyst (and therefore greater than the energy corresponding to the forbidden bandwidth) and which produces an irradiation such that at least 50% in number of photons are absorbable by the photocatalyst. By way of example, in the case of a TiO 2 photocatalyst having a band gap of 3.2 eV (ie a maximum absorbable wavelength of 390 nm), the irradiation source preferably produces an irradiation between 370 and 390 nm. Preferably, the irradiation generated is such that at least 50% by number of photons, preferably at least 80%, preferably at least 90%, very preferably at least 95% by number of photons are absorbable. by the photocatalyst. In other words, at least 50% of the photons, preferably at least 80%, preferably at least 90%, very preferably at least 95% of the photons have an energy greater than or equal to the forbidden band width of said photocatalyst.

The contacting of the hydrocarbon feedstock and the photocatalyst can be done by any means known to those skilled in the art. Preferably, the contacting of the hydrocarbon feedstock and the photocatalyst is fixed bed crossed, fixed bed licking or suspension (also called "slurry" according to the English terminology). When the implementation is in fixed bed traversed, the photocatalyst is layered on a porous support, for example of ceramic or metallic sintered type, and the hydrocarbon feedstock to be converted into gaseous and / or liquid form is sent through the photocatalytic bed.

When the implementation is fixed bed licking, the photocatalyst is layered on a support and the hydrocarbon feedstock to be converted into gaseous and / or liquid form is sent to the photocatalytic bed. When the implementation is in suspension, the photocatalyst is in the form of particles suspended in a hydrocarbon liquid feedstock to be converted. In suspension, the implementation can be done in batch and continuously. The photocatalyzed dehydrogenation is conditioned by the provision of a photon adapted to the photocatalytic system for the reaction envisaged and is therefore not limited to a specific pressure or temperature range apart from those for providing the photocatalytic system. stability of the product (s). The temperature range employed for the photocatalytic dehydrogenation of the hydrocarbon feedstock is generally -10 ° C to + 200 ° C, more preferably 0 to 150 ° C. The pressure range employed for the photocatalytic dehydrogenation of the hydrocarbon feed is generally from 0.01 MPa to 70 MPa (0.1 to 700 bar), more preferably from 0.1 MPa to 2 MPa (1 to 20 bar). ). The process according to the invention can be carried out in the liquid phase or in the gas phase, and preferably in the liquid phase. The process according to the invention is carried out in the absence of oxygen, that is to say in an anoxic medium. For example, when steps a) and b) are in the liquid phase, the absence of oxygen is understood to mean that the photocatalyst is completely immersed in the reaction medium containing the charge, which can be placed under an inert atmosphere (nitrogen or argon). For example, when steps a) and b) are in the gas phase, the absence of oxygen means that the oxygen (or air) has been removed from the reaction chamber and then purged with an inert gas. (Nitrogen or argon) and in which the charging is brought into contact with the photocatalyst and the irradiation of said photocatalyst without adding dioxygen. When the process is carried out in the liquid phase or in the gas phase, a gaseous or liquid diluent fluid may be present in the reaction medium. The presence of a diluent fluid is not required for the realization of the invention, however it may be useful to add to the charge to ensure the dispersion of the charge in the medium, the dispersion of the photocatalyst, a control of the absorption of the reagents / products on the surface of the photocatalyst, the dilution of the products to limit their recombination and other similar parasitic reactions, the control of the temperature of the reaction medium by the choice of a suitable temperature which can compensate for the possible exo / endothermicity of the photocatalyzed reaction. The nature of the diluent fluid is chosen such that its influence is neutral on the reaction medium or that its possible reaction does not interfere with the achievement of the desired dehydrogenation. By way of example, nitrogen may be chosen as gaseous diluent or toluene in the case of dehydrogenation in the liquid phase and at low temperature. The effluent obtained after the dehydrogenation reaction contains, on the one hand, at least one at least partially dehydrogenated molecule resulting from the dehydrogenation reaction and hydrogen and, on the other hand, the unreacted filler, as well as the possible fluid. diluent, but also products of parallel reactions such as the products resulting from the continuation of the reaction beyond a simple dehydrogenation such as the formation of diolefinic or acetylenic molecules from a paraffinic molecule, and possible impurities such as than the water initially contained in the charge.

The dehydrogenation makes it possible to obtain from a feedstock containing a paraffinic molecule an effluent containing in particular an olefinic and / or diolefinic and / or acetylenic molecule or a naphthenic molecule. When the desired end product is an olefinic molecule, the effluent can be subjected to a selective hydrogenation step described below.

Dehydrogenation makes it possible to obtain, from a feedstock containing an olefinic molecule, an effluent containing in particular a diolefinic and / or acetylenic molecule or else an olefinic and / or acetylenic cyclic molecule.

Dehydrogenation makes it possible to obtain, from a feedstock containing a naphthenic molecule, an effluent containing in particular an olefinic and / or aromatic cyclic molecule. Dehydrogenation makes it possible to obtain from a feedstock containing an alkylaromatic molecule an effluent containing in particular an alkenylaromatic molecule. Optional Steps One or more optional steps may complete the process according to the invention in order to improve the energy efficiency of the process and the final yield of the desired product.

According to a first variant, the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of step c) of the process according to the invention may be subjected to at least one separation step allowing hydrogen to be separated. product and to obtain an enriched fraction in at least partially dehydrogenated molecule.

The separation step can be carried out by any method known to those skilled in the art, for example by vaporization, distillation, extractive distillation, extraction by solvent, by absorption, by adsorption on solid, by membranes or by a combination of these. techniques. Preferably, the effluent obtained in step c) is separated by distillation.

Preferably, the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of stage c) is sent to a system of distillation columns comprising one or more columns which makes it possible to separate, on the one hand, hydrogen and the enriched fraction of the desired dehydrogenated molecule, and secondly the unreacted filler and any impurities contained in the feed such as water, or products from parasitic reactions, or possibly the diluent fluid. The unreacted filler, as well as any diluent fluid may (wind) be advantageously recycled in step a) of the contacting. The hydrogen may advantageously be recycled at least partly in the optional selective hydrogenation step described below.

According to a second variant, the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of stage c) of the process according to the invention may be subjected to a selective hydrogenation step enabling the hydrogenation of the compounds to be selectively hydrogenated. the most unsaturated compounds to the corresponding alkenes or aromatics, avoiding total saturation and thus the formation of the corresponding alkanes or naphthenes by virtue of the hydrogen produced by the dehydrogenation and present in the effluent produced in step c). The selective hydrogenation therefore represents a "rehydrogenation" of the dehydrogenated bond previously during the dehydrogenation, but carried out selectively on a molecule having a degree of unsaturation that is too high relative to the desired product without, however, hydrogenating the dehydrogenated molecules having the degree of unsaturation sought. Generally, the selective hydrogenation aims to hydrogenate diolefinic or acetylenic molecules into monoolefinic molecules without hydrogenating the monoolefinic molecules resulting from the dehydrogenation. The selective hydrogenation step thus makes it possible to increase the yield of the desired olefinic product. Selective hydrogenation catalysts are well known to those skilled in the art and generally comprise an active phase based on Group VIII metals of the periodic table, preferably palladium or nickel and a refractory oxide support. The group VIII metal (s) is in the form of metal particles deposited on said support. In general, the metal content (ux) of the group VIII in the catalyst is between 0.01 and 50% by weight of the catalyst mass, preferably between 0.05 and 30% by weight of the catalyst mass.

Preferably, when the active phase comprises palladium, the palladium content is advantageously between 0.01 and 5% by weight of the catalyst mass, preferably between 0.05 and 2% by weight of the catalyst mass, and more preferably between 0.05 and 1% by weight of the catalyst mass. Preferably, when the active phase comprises nickel, the nickel content is advantageously between 1 and 50% by weight of the catalyst mass, more preferably between 5 and 40% by weight of the catalyst mass and even more preferably between 5 and 30% by weight of the catalyst mass. The promotion of selective hydrogenation catalysts based on palladium or nickel has frequently been proposed in order to improve the performance of selective hydrogenation. The active phase of said catalyst may thus additionally comprise at least one additional metal chosen from Group VIII metals (other than palladium or nickel), Group IB metals and / or tin. Preferably, the additional metal of group VIII is chosen from platinum, ruthenium and rhodium, as well as palladium (in the case of a nickel-based catalyst) and nickel (in the case of a catalyst). based on palladium). Advantageously, the additional metal of group IB is chosen from copper, gold and silver. The said additional metal (s) of the group VIII and / or of the group IB is (are) preferably present in a content representing from 0.01 to 20% by weight of the mass of the catalyst, preferably from 0.05 to 10% by weight of the catalyst mass and even more preferably from 0.05 to 5% by weight of the mass of said catalyst. The tin is preferably present in a content representing from 0.02 to 15% by weight of the mass of the catalyst, such that the ratio Sn / metal (ux) of group VIII is between 0.01 and 0.2, preferably between 0.025 to 0.055, and even more preferably between 0.03 to 0.05.

The support on which said active phase is deposited is advantageously formed of at least one refractory oxide preferentially chosen from metal oxides of groups IIA, IIIB, IVB, IIIA and IVA according to the CAS notation of the periodic table of elements. Preferably, said support is formed of at least one single oxide selected from alumina (Al2O3), silica (SiO2), titanium oxide (T102), cerine (CeO2) and zirconia (ZrO2) . Preferably, said support is chosen from aluminas, silicas and silica-aluminas. Very preferably, said support is an alumina. The pore volume of the support is generally between 0.1 cm3 / g and 1.5 cm3 / g, preferably between 0.5 cm3 / g and 1.3 cm3 / g. The specific surface area of the support is generally between 10 m 2 / g and 250 m 2 / g, preferably between 30 m 2 / g and 220 m 2 / g. The total pore volume is measured by mercury porosimetry according to ASTM D4284-92 with a wetting angle of 140 °, as described in the book Rouquerol F .; Rouquerol J .; Singh K. "Adsorption by Powders & Porous Solids: Principle, methodology and applications", Academic Press, 1999, for example by means of an Autopore IIITM model apparatus of the Microméritics ™ brand, the specific surface is determined by the BET method, described in the same book. Said porous support is advantageously in the form of balls, extrudates, pellets, or irregular and non-spherical agglomerates, the specific shape of which may result from a crushing step. Very advantageously, said support is in the form of balls or extrudates.

The preparation of the selective hydrogenation catalyst can be carried out by any method known to those skilled in the art. It generally comprises the subsequent or simultaneous impregnation of the metals on the support, a drying, a calcination and then a reduction. Optionally, a drying step and / or a calcination step may be performed between the consecutive impregnation steps. Prior to the introduction of the catalyst and its subsequent use in the catalytic reactor and the implementation of the selective hydrogenation step, the catalyst is subjected to at least one reducing treatment step, by contact with a gas reducing agent, for example with hydrogen, pure or diluted, at high temperature, typically greater than or equal to 50 ° C for a duration greater than or equal to 2 hours. This treatment makes it possible to activate said precursor and to form particles of metal, in particular of group VIII metal, in the zero-valent state. Said reducing treatment can be carried out in situ or ex situ, that is to say before the catalyst is loaded into the selective hydrogenation reactor. In a general manner, the selective hydrogenation step is carried out at a temperature of between 0 ° C. and 500 ° C., a pressure in the range 0.1 to 20 MPa, a volume hourly velocity VVH (defined as the ratio the volume flow rate of the feedstock over the volume of the catalyst per hour) of between 0.1 and 200 h -1 for a liquid feed, between 100 and 50 000 h -1 for a gaseous feedstock, and a molar ratio of hydrogen / (polyunsaturated compounds) to hydrogenate) between 0.1 and 200. More particularly, in the case of a hydrogenation reaction in the liquid phase, the pressure is generally between 1 and 6.5 MPa, more preferably between 1 and 5 MPa, the temperature is between 2 and 200 ° C and the molar ratio hydrogen / (polyunsaturated compounds to be hydrogenated) is between 0.1 and 10, preferably between 1 and 8. The hourly volume rates are between 1 and 200 h -1 . In the case of a hydrogenation reaction in the gas phase, the pressure is generally between 1 and 3 MPa, the temperature is between 40 and 120 ° C and the molar ratio hydrogen / (polyunsaturated compounds to be hydrogenated) is between 0.1 and 200. The hourly volume speeds are between 100 and 50 000h-1. The technological implementation of the selective hydrogenation step is carried out, for example, by injection, in ascending or descending current, of the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of step c. ) of the process according to the invention and hydrogen in at least one fixed bed reactor. Said reactor may be of the isothermal or adiabatic type. An adiabatic reactor is preferred. The injected effluent may advantageously be diluted by one or more re-injection (s) of the effluent from said selective hydrogenation reactor where the selective hydrogenation reaction occurs at various points of the reactor located between the inlet and the reactor outlet. The technological implementation of the selective hydrogenation process can also be advantageously carried out by the implantation of a supported catalyst in a reactive distillation column or in reactor-exchangers. The flow of hydrogen can be introduced at the same time as the feedstock to be hydrogenated and / or at a different point of the reactor. The selective hydrogenation step may be carried out before or after the separation step.

Thus, according to a third variant, the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of step c) of the process according to the invention can be subjected to a separation step making it possible to separate the hydrogen product and to obtain a fraction enriched in at least one dehydrogenated molecule, then said fraction enriched in at least one dehydrogenated molecule is subjected to a selective hydrogenation stage making it possible to obtain an effluent enriched in at least one unsaturated molecule at the degree of lower unsaturation with respect to the effluent containing at least one at least partially dehydrogenated molecule. The effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation thus contains unsaturated molecules with the desired degree of unsaturation obtained by the dehydrogenation (for example an olefin from a paraffin) and unsaturated molecules at the d desired unsaturation obtained via "over" dehydrogenation (for example a diolefinic or acetylenic molecule from a paraffin) but "rehydrogenated" by selective hydrogenation to olefinic molecule). This variant is particularly advantageous when the conversion of the photocatalytic dehydrogenation is low and the effluent of the dehydrogenation contains a large fraction of unreacted filler. This large fraction of unreacted filler is thus advantageously separated from the fraction enriched in at least one dehydrogenated molecule before the latter is subjected to the selective hydrogenation step. Advantageously, the unreacted filler fraction is at least partly and preferably entirely recycled in step a) of the process according to the invention. Advantageously, the hydrogen separated during the separation step is at least partly introduced into the selective hydrogenation step. According to a sub-variant, the effluent obtained after the selective hydrogenation enriched in unsaturated molecule with a lower degree of unsaturation may be subjected to an additional separation step making it possible to separate the desired product from any impurities such as water or products resulting from parasitic reactions. This separation step can be carried out by any method known to those skilled in the art, for example by vaporization, distillation, extractive distillation, extraction by solvent, by absorption, by adsorption on solid, by membranes or by a combination of these techniques. . Preferably, the effluent obtained in the selective hydrogenation step is separated by distillation.

According to a fourth variant, the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of stage c) of the process according to the invention is subjected to a selective hydrogenation stage allowing the selective hydrogenation of molecules. having a degree of unsaturation that is too high relative to the desired product so as to obtain an effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule, and then said effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation is subjected to a separation step which makes it possible to separate, on the one hand, a fraction enriched in at least one unsaturated molecule to the lower degree and, on the other hand, the unreacted charge and any impurities such as water, or products resulting from spurious reactions, or possibly the e diluent fluid. The fraction enriched in at least one lower unsaturated molecule with respect to the effluent containing at least one at least partially dehydrogenated molecule comprises at least one unsaturated molecule hydrogenated selectively during the selective hydrogenation but also at least one molecule at least partially dehydrogenated from unreacted dehydrogenation during selective hydrogenation. The hydrogen required for the selective hydrogenation may come directly from the effluent containing at least one at least partially dehydrogenated molecule obtained at the end of step c). Advantageously, the unreacted filler fraction is at least partly and preferably entirely recycled in step a) of the process according to the invention. Figures 1 to 3 show schematically the process according to the invention: Figure 1 describes the dehydrogenation process according to the invention, followed by an optional separation step. A hydrocarbon feed (100) to be dehydrogenated is introduced into a photoreactor (1000) in which the contacting of the feedstock and the heterogeneous photocatalyst is carried out in the absence of oxygen (step a). The conditions of pressure and temperatures are adapted to the dehydrogenation that is desired. This photoreactor is characterized in that it is adapted to the desired operating pressure and in that it allows the control of the temperature of the reaction by the provision of hot or cold utilities. A diluent fluid (300) can be introduced into the photoreactor. The photocatalyst may either be suspended in the feedstock, in the diluent or in the reaction medium. The photocatalyst is arranged in the photoreactor so that it is both in contact with the reaction medium and subjected to irradiation of the source. Irradiation is provided by an irradiation source (2000). This source (2000) provides a photon flux (200) which irradiates the heterogeneous photocatalyst by at least one irradiation source producing at least one wavelength suitable for activating said photocatalyst (step b). The interface between the reaction medium containing the photocatalyst and the light source varies according to the applications and the nature of the light source, the source (2000) being able to be located outside the reactor (1000) or in the reactor ( 1000). The filler (100) is at least partially dehydrogenated in the presence of said photocatalyst activated by the irradiation source (2000) to transform said filler molecule having at least said single or double CC bond into a molecule having a higher degree of unsaturation of said bond CC, so as to obtain an effluent (101) containing at least one molecule at least partially dehydrogenated, hydrogen, the unreacted filler, optionally the diluent fluid and impurities from parasitic reactions. The effluent (101) is then subjected to a separation step, for example in a system of distillation columns (4000) making it possible to separate, on the one hand, the product hydrogen (600) and the fraction enriched in at least one molecule dehydrogenated sought (103), and secondly the unreacted filler (400) in the presence of the optional diluent fluid and any impurities (500) such as water or products from parasitic reactions. The unreacted filler (400) and the optional diluent fluid are advantageously recycled to the photoreactor (1000). FIG. 2 describes the dehydrogenation process according to the invention, followed by an optional selective hydrogenation step and an optional separation step. The references of FIG. 2, identical to those of FIG. 1, designate the same elements. The effect (101) leaving the dehydrogenation step is subjected to a selective hydrogenation step (3000) under the operating conditions of selective hydrogenation in the presence of a selective hydrogenation catalyst and optionally hydrogenation. (301) and for selectively hydrogenating the molecules having a degree of unsaturation too high relative to the desired product so as to obtain an effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation compared to the effluent containing at least one at least partially dehydrogenated molecule. The effluent of the selective hydrogenation step (102) is then subjected to a separation step, for example in a system of distillation columns (4000) making it possible to separate, on the one hand, the unconsumed hydrogen (600). and the fraction enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule (103), and secondly the unreacted filler (400) in the presence of the possible diluent fluid and any impurities (500) such as water or products from parasitic reactions. The unreacted filler (400) and the optional diluent fluid are advantageously recycled to the photoreactor (1000). At least a portion of the hydrogen (601) may be advantageously recycled to the selective hydrogenation step (3000). It should be noted that the fraction enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule (103), the unreacted filler (400) or the The product hydrogen (600) can be supplied at a pressure suitable for subsequent use or recycling in the process, for example by recompression (not shown). FIG. 3 describes the dehydrogenation process according to the invention, followed by a first optional separation step, then an optional selective hydrogenation step and then a second optional separation step. The references of FIG. 3, identical to those of FIG. 1, designate the same elements. The fraction enriched in at least one dehydrogenated molecule (103) obtained after the first separation (4000) (as described for example in FIG. 1) is subjected to a selective hydrogenation step (3001) under the operating conditions of selective hydrogenation and in the presence of a selective hydrogenation catalyst, hydrogen (303) resulting from a hydrogen booster or the recycling of hydrogen produced and separated (601) (605) during the separation steps (4000) and / or (4001) for selectively hydrogenating the molecules having a degree of unsaturation that is too high relative to the desired product so as to obtain an effluent enriched with at least one unsaturated molecule with a lower degree of unsaturation compared to effluent containing at least one at least partially dehydrogenated molecule (104). The hydrogen (601) required for the hydrogenation can at least partly come from the separation step (4000). The effluent of the selective hydrogenation step (104) is then subjected to a second separation step, for example in a system of distillation columns (4001) making it possible to separate, on the one hand, the hydrogen that is not consumed during the selective hydrogenation (604) and the fraction enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule (105), and secondly any impurities such as water or products resulting from parasitic reactions (501). At least a portion of the hydrogen (605) may be advantageously recycled to the selective hydrogenation step (3001). The following examples illustrate the invention without limiting its scope. EXAMPLES Example 1: Photocatalyst A (TiO2) Photocatalyst A is a commercial TiO2 semiconductor (Aeroxide® P25, Aldrich, purity> 99.5%). The particle size of the photocatalyst is 21 nm and the specific surface area measured by BET method is equal to 52 m 2 / g. By diffuse reflection absorption analysis, the bandgap of photocatalyst A is measured at 3.23 eV. Example 2 Photocatalyst B (CdS) Photocatalyst B is a semiconductor based on commercial CdS (Aldrich ()). The specific surface area measured by BET method is equal to 70 m 2 / g. By absorption analysis in diffuse reflection, the bandgap of photocatalyst B is measured at 2.49 eV. Example 3 Photocatalyst C (Pt / T102) Photocatalyst C is composed of TiO2 semiconductor and platinum as co-catalyst. To prepare the photocatalyst C, a volume of 50 ml of distilled water is prepared in which is introduced 7,12 mg platinum precursor H2PtC16,6H20 (Aldrich®, 37.5% Pt basis) and 3 ml of methanol. 250 mg of TiO2 (Aeroxide® P25, Aldrich, purity> 99.5%) are added to the mixture. The mixture is then subjected to UV irradiation, using a 125 W Hg HPKTM lamp, for 2 hours with stirring. The mixture is then centrifuged for 10 minutes at 8000 rpm. The recovered solid then undergoes 2 successive washings with distilled water, each washing being followed by a centrifugation step under the same conditions as those mentioned above. The recovered solid is then dried in an oven at 110 ° C. for at least 12 hours. The solid obtained contains 0.93% by weight of platinum on TiO 2 and corresponds to photocatalyst C.

Example 4 Photocatalyst D (Au / T102) Photocatalyst D is composed of TiO2 semiconductor and gold as cocatalyst. To prepare photocatalyst D, a volume of 50 ml of distilled water is prepared in which 5.10 mg of HAuCl 4, xH 2 O gold precursor (Aldrich®,> 49% by weight) and 3 ml of methanol are introduced. 250 mg of TiO2 (Aeroxide® P25, Aldrich, purity> 99.5%) are added to the mixture. The mixture is then subjected to UV irradiation, using a 125 W Hg HPKTM lamp, for 2 hours with stirring. The mixture is then centrifuged for 10 minutes at 8000 rpm. The recovered solid then undergoes 2 successive washings with distilled water, each washing being followed by a centrifugation step under the same conditions as those mentioned above. The recovered solid is then dried in an oven at 110 ° C. for at least 12 hours. The solid obtained contains 0.97% weight of gold on TiO2 and corresponds to photocatalyst D.35. Example 5 Photocatalyst E (Ru / CdS) Photocatalyst E is composed of CdS semiconductor and ruthenium as co-catalyst . To prepare the photocatalyst E, a volume of 50 ml of distilled water is prepared in which 5.29 mg of ruthenium precursor RuCl3, xH2O (Aldrich®,) and 3 ml of methanol are introduced. 250 mg of CdS (Aldrich®) are added to the mixture. The mixture is then subjected to UV irradiation, using a 125 W Hg HPKTM lamp, for 2 hours with stirring. The mixture is then centrifuged for 10 minutes at 8000 rpm. The recovered solid then undergoes 2 successive washings with distilled water, each washing being followed by a centrifugation step under the same conditions as those mentioned above. The recovered solid is then dried in an oven at 110 ° C. for at least 12 hours. The solid obtained contains 0.81% by weight of ruthenium on CdS and corresponds to photocatalyst E. Example 6: Photocatalytic dehydrogenation of ethylbenzene to styrene Photocatalysts A, B, C, D and E are subjected to a photocatalytic test in the liquid phase of dehydrogenation of ethylbenzene to styrene and dihydrogen in a semi-open stirred Pyrex reactor with a jacket to regulate the test temperature. To do this, 150 mg of photocatalyst are suspended in 50 ml of anhydrous ethylbenzene (Aldrich®, purity 99.8%). The tests are carried out at 25 ° C. under atmospheric pressure with an argon flow rate of 10 mL / min to entrain the hydrogen gas product, which gas is analyzed by gas chromatography. By periodic sampling, the liquid phase is also analyzed by gas chromatography to measure the formation of styrene. The irradiation source is provided by a UV LED panel centered around 365 nm (350-380 nm) delivering 415 W / m2 through a 4.15 cm2 optical window. Before switching on the irradiation source, the argon flow is left for 2 hours. Table 1 below summarizes the performance of the photocatalysts drawn from the examples.

Table 1: Styrene production and apparent quantum yields Photocatalyst Production of apparent styrene quantum yield (pmol / h) (%) A (TiO2) 11 1.2 B (CdS) 59 6.2 C (Pt / TiO2) 335 35, 4D (Au / TiO2) 318 33.6 E (Ru / CdS) 382 40.3 The values listed in this table are initial performance measured after 10 minutes of testing. The apparent quantum yield is calculated by the ratio of the number of moles of H2 formed multiplied by 2 and the number of moles of incident photons, because it takes 2 photons to generate a molecule of styrene and a molecule of H2. The incident photon flux was calculated at 1.89 × 10 -3 E / h. EXAMPLE 7 Photocatalytic dehydrogenation of octene n-octane Photocatalysts A, B, C, D and E are subjected to a photocatalytic liquid phase test for the dehydrogenation of n-octane to octene and dihydrogen in a semi-open pyrex stirred reactor. equipped with a double jacket to regulate the test temperature. For this purpose, 150 mg of photocatalyst are suspended in 50 ml of anhydrous octane (Aldrich®, purity> 99%).

The tests are carried out at 25 ° C. under atmospheric pressure with an argon flow rate of 10 mL / min to entrain the hydrogen gas product, which gas is analyzed by gas chromatography. By periodic sampling, the liquid phase is also analyzed by gas chromatography to measure octene formation. The irradiation source is provided by a UV LED panel centered around 365 nm (350-380 nm) delivering 415 W / m2 through a 4.15 cm2 optical window. Before switching on the irradiation source, the argon flow is left for 2 hours. Table 2 below summarizes the performance of the photocatalysts from the examples. Table 2 Octene Productions and Apparent Quantum Yields Photocatalyst Octene Production Quanal Yield (pmol / h) apparent (%) A (TiO 2) 7 0, 74 B (CdS) 2.11 C (Pt / TiO2) 212 22.4 D (Au / TiO2) 276 29.2 E (Ru / CdS) 139 14.7 The values listed in this table are initial measured performances after 10 minutes of testing. The apparent quantum yield is calculated by the ratio of the number of moles of H 2 formed multiplied by 2 and the number of moles of incident photons, because it takes 2 photons to generate an octene molecule and a molecule of H2. The incident photon flux was calculated at 1.89 × 10 -3 E / h. Example 8 Photocatalytic dehydrogenation of butene to 1,3-butadiene Photocatalysts A, B, C, D and E are subjected to a photocatalytic test in the gas phase of dehydrogenation of 1-butene to 1,3-butadiene and dihydrogen in a reactor fixed bed provided with a quartz optic window of 4.15 cm 2 and provided with a ceramic sinter on which is deposited in a thin layer 250 mg of photocatalyst. The tests are carried out at ambient temperature, ie 23 ° C. and under atmospheric pressure, with a flow rate of 1-butene (Air Liquide, purity> 99%) of 3 mL / min, which gas is analyzed at the outlet of the reactor by gas chromatography. . The irradiation source is provided by a UV LED panel centered around 365 nm (350-380 nm) delivering 415 W / m2 through the optical window. Before igniting the irradiation source, the flow of 1-butene is left for 2 hours at 10 ml / min. Table 3 below summarizes the photocatalyst performance from the examples. Table 3: 1,3-Butadiene Productions and Apparent Quantum Yields Photocatalyst Production of 1,3-butadiene quantum yield (pmol / h) apparent (%) A (TiO2) 32 3.4 B (CdS) 113 11.9 C (Pt / TiO2) 567 59.9 D (Au / TiO2) 498 52.0 E (Ru / CdS) 602 63.6 The values listed in this table are initial performance measured after 10 minutes of testing. The apparent quantum efficiency is calculated by the ratio of the number of moles of H 2 formed multiplied by 2 and the number of moles of incident photons, because it takes 2 photons to generate a molecule of 1,3-butadiene and a molecule of H2. The incident photon flux was calculated at 1.89 × 10 -3 E / h. EXAMPLE 9 Photocatalytic dehydrogenation of ethylbenzene to styrene by optimizing the yield of styrene via selective hydrogenation This example is based on the description of FIG. 2. A charge (100) consisting of ethylbenzene is considered. In this example it is not necessary to provide diluent fluid (300). The irradiation source is here a laser for example a XeF type excimer laser of wavelength 351 nm, and whose output is 30% between the electric energy supplied to the source and the energy transported by the flux of photons. A 30% quantum yield is considered for the dehydrogenation reaction of ethylbenzene to styrene on a heterogeneous Au / TiO 2 catalyst (prepared as described in the article by X. Wang et al., Advanced Materials Research, Vols. -149, 1258, 2011). The ethylbenzene feed (100) is introduced into the dehydrogenation photoreactor at a rate of one ton per hour at a pressure of 1 MPa (10 bar) and at a temperature of 50 ° C. The system includes an unconverted (400) load recycle of 5.8 t / h. The composition of the feedstock (100) supplemented with the unreacted recycled feedstock (400) is 99.8% ethylbenzene, 1.4% styrene and 0.01% phenylacetylene.

The irradiation source (2000) provides 1.61 MW in the form of a photon flux (200) with a wavelength of 351 nm. This energy will induce the conversion of 15% of ethylbenzene to styrene and 1.3% of styrene to phenylacetylene. This results in an effluent (101) at 73.9 kmol / h of composition: molar percentage (%) Ethylbenzene C6H6-CH2-CH3 73.6 Styrene C6H6-CH = CH2 13.0 Phenylacetylene C6H6-CE CH 0.2 Dihydrogen 13.2 The effluent (101) is then subjected to a selective hydrogenation step. The hydrogen present in the effluent (101) is not separated beforehand and it is chosen not to supply additional hydrogen ( 301). The hydrogenation reactor (3000) contains a selective hydrogenation catalyst based on palladium. This reaction is exothermic, resulting in a flow (102) selectively hydrogenated at 100 ° C for a flow rate of 73.1 kmol / h. This corresponds to a conversion of 93% phenylacetylene and 7.5% styrene and results in a composition of: Percent molar (%) Ethylbenzene 75.5 Styrene 12.3 Phenylacetylene 0.01 Dihydrogen 12.2 This selective hydrogenation leaves the phenylacetylene in the form of traces of the order of 0.01 mol%. The styrene is then cooled, separated and recompressed as follows: - 18.8 kg / h of dihydrogen (600) at 50 ° C. and 1 MPa (10 bar) with a purity corresponding to 99.9 % mol dihydrogen and 0.1% styrene, - 983 kg / h styrene (103) at 50 ° C and 1 MPa (10 bai) with a purity of 94% styrene, 6% ethylbenzene and 0, 01% phenylacetylene, 5810 kg / h recycle (400) ethylbenzene 99.8 mol%, 1.4% styrene and 0.01 mol% phenylacetylene at 50 ° C and 1 MPa (10 ber) . 20,

Claims (15)

REVENDICATIONS1. A process for dehydrogenating a hydrocarbon feedstock containing at least one molecule having at least 2 carbon atoms and at least one single or double C-C bond in the presence of a photocatalyst. heterogeneous under irradiation comprising the following steps: a) said hydrocarbon feed is brought into contact with a heterogeneous photocatalyst, b) the heterogeneous photocatalyst is irradiated with at least one irradiation source producing at least one wavelength adapted to the heterogeneous photocatalyst; activating said photocatalyst so as to dehydrogenate said hydrocarbon feed in the presence of said photocatalyst activated by said irradiation source to transform said feed molecule having at least said single or double CC bond into a molecule having a higher degree of unsaturation of said bond CC, so as to obtain an effluent containing at least one at least partially dehydrogenated molecule and hydrogen, steps a) and b) being carried out in the absence of oxygen. 2. The dehydrogenation process as claimed in claim 1, in which the heterogeneous photocatalyst is chosen from TiO2, CdO, Ce203, CoO, Cu20, FeTiO3, In203, NiO, PbO, ZnO, Ag2S, CdS, Ce2S3, Cu2S, CuInS2, In2S3, ZnS and ZrS2. 3. dehydrogenation process according to one of claims 1 to 2, wherein the photocatalyst is doped with one or more ions selected from metal ions, non-metallic ions, or a mixture of metal ions and non-metallic . 4. dehydrogenation process according to one of claims 1 to 3, wherein the photocatalyst further comprises at least one cocatalyst selected from a metal, a metal oxide or a metal sulfide. 5. dehydrogenation process according to one of claims 1 to 4, wherein the irradiation source is a source of artificial radiation emitting in the ultraviolet spectrum and / or visible. 6. Dehydrogenation process according to one of claims 1 to 5, wherein said irradiation source emits at a nominal wavelength at maximum 50 nm less than the maximum wavelength absorbable by the photocatalyst and produces a irradiation such that at least 50% in number of photons are absorbable by the photocatalyst. 7. dehydrogenation process according to one of claims 1 to 6, which is carried out at a temperature of -10 ° C to 4- 200 ° C and a pressure between 0.01 MPa to 70 MPa. 8. The dehydrogenation process according to one of claims 1 to 7, wherein said hydrocarbon feed contains at least one paraffinic, naphthenic, alkylaromatic and / or olefinic molecule containing from 2 to 20 carbon atoms. 9. dehydrogenation process according to one of claims 1 to 8, wherein the contacting is in fixed bed crossed, fixed bed licking or suspended in which the photocatalyst is in particle form. 10. dehydrogenation process according to one of claims 1 to 9, which is carried out in liquid phase or gas phase. 11. The dehydrogenation process according to one of claims 1 to 10, wherein the bringing into contact is in the presence of a gaseous or liquid diluent fluid. 12. dehydrogenation process according to one of claims 1 to 11, wherein the effluent containing at least one molecule at least partially dehydrogenated and hydrogen is subjected to at least one separation step so as to separate the hydrogen product and to obtain a fraction enriched in at least one molecule at least partially dehydrogenated. 13. dehydrogenation process according to one of claims 1 to 11, wherein the effluent containing at least one molecule at least partially dehydrogenated and hydrogen is subjected to a selective hydrogenation step in the presence of a catalyst of d selective hydrogenation and hydrogen under selective hydrogenation operating conditions making it possible to obtain an effluent enriched in at least one unsaturated molecule with a lower degree of unsaturation relative to the effluent containing at least one at least partially dehydrogenated molecule. The dehydrogenation process according to claim 13, wherein at least a portion of the hydrogen used in the selective hydrogenation step is the hydrogen produced in step o). 15. The dehydrogenation process according to claim 13, wherein the selective hydrogenation step is carried out before or after the separation step.