[0001] None.

BACKGROUND [0002] An analysis of the technology development around alkane to alkene dehydrogenation from the early 1940s to the present has identified the following problems in the alkane dehydrogenation process: (i) Catalyst coking due to high process temperature; (ii) Catalyst deactivation due to high process and regeneration temperatures; (iii) Non-optimal use of feedstock; (iv) Low alkane (e.g., propane) conversion or conversion rate per reactor pass; and (v) Low alkene (e.g., propylene) selectivity from the dehydrogenation reaction.

[0003] Although at present only a few percent of the propylene volume is coming from catalytic dehydrogenation, it is expected that this percentage will grow significantly. Steam cracking is predominantly useful for ethane conversion and is thermo-dynamically limited in the amount of propylene it can render. Parafin→ Olefin + Hydrogen (1)

[0004] One of the reasons why industrial processes are operated at high temperatures (range of 500 to 700 °C) is that reaction 1 is highly endothermic. As an example the reaction of propane to propylene is shown:

C ₃ ¾→ C ₃ H ₆ + H ₂ ΔΗ° ₂₉₈ = 124kJ/mol (2) The other issue with reaction 1 is that it is an equilibrium. These reaction characteristics contributes to the above listed problems with the process.

[0005] Significant effort has gone into developing more effective and efficient catalyst systems which would allow for a lower process temperature and/or higher alkane conversion rates while maintaining or improving alkene selectivity as well as other characteristics such as catalyst life time. A recent comprehensive review of the state of the art in that area was compiled by Sattler et al. (Chem. Rev. 2014, 114: 10613-53). The review describes studies into platinum catalysts to which promoters like Sn, Na, Fe, Ce, Zn, Ga, Mg, In, and/or Ge had been added. Especially tin (Sn) addition appears to generate interesting performance features like reducing coke formation. In terms of metal oxides chromia has been evaluated and modified with vanadium, aluminum, zirconium, titanium and/or magnesium oxides. Other materials like the oxides of molybdenum, vanadium, gallium, and indium have been studied. Even carbon based catalysts have been evaluated.

[0006] One relatively obvious route to eliminate some of the problems associate with alkane dehydrogenation is to move from the endothermic equilibrium reaction 2 to the exothermic complete reaction 3, as given below for propane to propylene:

C ₃ ¾ + ½0 ₂ → C ₃ H ₆ + H ₂ 0 ΔΗ° ₂₉₈ = -118 kJ/mol (3)

This reaction is referred to as oxidative dehydrogenation, oxy-dehydrogenation (ODH), or in its milder forms where diluted forms of oxygen or other oxidants are used as selective hydrogen combustion (SHC), or selective dehydrogenation. The latter descriptions and terms are meant to inherently imply technologies which are designed to minimize combustion of alkanes and alkenes, while maximizing the removal of hydrogen (Sattler et al., Chem. Rev. 2014, 114: 10613-53; Karl Jozef Caspary et al., in Handbook of Heterogeneous Catalysis, Ertl, Knozinger, and Weitkamp (eds) Wiley-VHC Verlag GmbH: Weinheim, Germany, 2008; pp 3206-29; Industrial Catalysis and Separations: Innovations for Process Intensification, Raghavan and Reddy editors - Chapter 8: Cracking and Oxidative Dehydrogenation of Ethane to Ethylene, 287-328, Apple Academic Press 2014). The steam active reforming (STAR) process and Linde-BASF processes are versions of this technology. Both are described in detail by Caspary et al. (Handbook of Heterogeneous Catalysis; Ertl, Knozinger, and Weitkamp (eds) Wiley-VHC Verlag GmbH: Weinheim, Germany, 2008; pp 3206-29). The processes described is performed at a temperature of 500 to 700 °C, results in a C ₃ conversion per pass of about or less than 50%, and has a C ₃ selectivity of 90% or less. Thus, there is a need for additional methods and systems for more effective alkane dehydrogenation.

SUMMARY

[0007] Embodiments of the invention are directed to methods for performing alkane dehydrogenation to alkenes in a more effective manner. In certain aspects the methods include dehydrogenation of propane to propylene. In certain aspects the methods use a ceramic membrane assisted process operated at temperatures in the range of 350 to 500 °C. In certain aspects the membrane can be based on polysiloxane silica precursors, crosslinked by subjection to pyrolysis at 700 C under inert atmosphere. Embodiments of the invention provide higher alkane conversions (at least or about 15% higher), improved alkene selectivity (by at least or about 8%), and minimized catalyst coking, thereby removing the need for catalyst regeneration.

[0008] Certain embodiments are directed to a process for alkane dehydrogenation comprising introducing a hydrocarbon source comprising at least one alkane into an alkane dehydrogenation reactor, the reactor comprising an alkane dehydrogenation catalyst bed operably coupled to a hydrogen permeable mixed-matrix ceramic membrane having palladium (Pd) or Pd alloy embedded in the ceramic membrane, the dehydrogenation reaction being performed at a temperature of 350 to 500 °C producing a dehydrogenation effluent, wherein the alkane conversion is at least 45% and alkene selectivity is at least 85%. In certain aspects the embedded Pd or Pd alloy are comprised in nanoparticles. The nanoparticle can have a core comprising Pd or Pd alloy, and a shell comprising a microporous ceramic. In certain aspects the core has a diameter or average diameter of at least or about 1, 2, 4, or 6 nm. In a further aspects the Pd or Pd alloy core is non-catalytic in regard to dehydrogenation of an alkane. In certain aspects the Pd or Pd alloy is dispersed in a layer of at least or about 400, 500, or 600 nm in depth. In a further aspect the nanoparticle shell averages about 3, 6, or 9 nm in thickness. In certain aspects the process described herein results in an alkane conversion is at least 45, 50, 55, 60, 65, 70, or 75% and the alkene selectivity is at least 85, 90, 95, 96, 97, 98, or 99%. In a further aspect the process or reaction is performed at, at least, or about 300, 350, 375 to 380, 400, or 420 °C. In certain embodiments the at least one alkane is propane and the effluent comprises propylene. In other aspects the hydrogen permeance of the mixed-matrix ceramic membrane is at least or about 0.25, 0.5, 1.0, or 1.25 m ³ /(m ² .h.bar) at 400 °C. In still further aspects the mixed-matrix ceramic membrane has a hydrogen/propane permselectivity of at least 50, 75, 100, 125, or 150 at 400 °C. In certain aspects the mixed-matrix ceramic membrane has less than or about a 20%) reduction in hydrogen flux after 75, 100, to 200 hours of processing time. In certain aspects the process or reaction is performed at about 4, 5, 6, 7 to 8, 9, 10 bar. In further aspects the alkane dehydrogenation catalyst is a NiO dehydrogenation catalyst.

[0009] Certain embodiments are directed to an alkane dehydrogenation reactor comprising a fixed alkane dehydrogenation catalyst bed operably coupled to a hydrogen permeable mixed-matrix ceramic membrane comprising embedded palladium (Pd) or Pd alloy, the membrane having a catalyst surface and a hydrogen processing surface, wherein the hydrogen processing surface is in contact with a gas sweep. In certain aspects the catalyst and mixed-matrix ceramic membrane are configured as a tube reactor. In further aspects the catalyst forms the core of the tube reactor and the mixed-matrix ceramic membrane forms the outer wall of the tube reactor. In certain instances the hydrogen processing surface of the membrane is operably coupled to a combustion catalyst for processing hydrogen permeate by combustion. In certain aspects the hydrogen permeance of the mixed-matrix ceramic membrane is at least 0.5 m ³ /(m ² .h.bar) at 400 °C and a hydrogen/propane permselectivity of at least 100 at 400 °C.

[0010] The term "embed" or "embedded" as used herein refers to a spatial relationship of an item (e.g., a nanoparticle) relative to a structure (e.g., membrane matrix) in which the item is at least partially enclosed within the structure. In particular, when used in connection to spatial relationship of nanoparticle with reference to a matrix the term "embed" refers to the nanoparticles being at least partially enclosed by the matrix in a suitable configuration within the matrix material. [0011] As used herein, "selectivity" with regard to the reaction means that an alkane feedstock is converted to an alkene product with the same carbon number. For example, at a selectivity of 90% or greater with a propane feedstock, 90% or greater of the propane feedstock is converted to an alkene product with three carbons (e.g., propylene product). The selectivity indicates that the alkane feedstock and alkene product have the same carbon number. In certain embodiments, the dehydrogenation reaction disclosed herein provides a selectivity of 90% or greater, or 95% or greater, at a reasonable conversion rate of above 50%.

[0012] Conversion can be based on the mole or weight % of alkane converted to alkene. The conversion of the process described herein can be 50, 55, 60, 70, 75% or greater.

[0013] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. [0014] The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0015] Throughout this application, the term "about" is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.

[0016] The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or." [0017] As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. [0018] Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

DESCRIPTION OF TH E DRAWINGS

[0019] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0020] FIG. 1 Schematic diagram of one embodiment of a mixed matrix membrane.

DESCRIPTION

[0021] Certain embodiments described herein provide improvements in one or more of the following areas of the dehydrogenation process: (i) reduced catalyst coking; (ii) reduced catalyst deactivation; (iii) optimizing use of feedstock; (iv) improved alkane conversion per reactor pass; and/or (v) improved alkane selectivity.

[0022] Certain embodiments are directed to a low temperature membrane based process for alkane dehydrogenation comprising introducing a hydrocarbon source comprising at least one alkane into an alkane dehydrogenation reactor, the reactor comprising an alkane dehydrogenation catalyst bed operably coupled to a hydrogen permeable mixed-matrix ceramic membrane having palladium (Pd) or Pd alloy particles embedded in the ceramic membrane.

[0023] Hydrogen selectivity has made palladium (Pd) based membranes attractive candidates for dehydrogenation reactions. Pd membranes have certain technical disadvantages in terms of stability and durability. Especially poisoning by chemicals such as sulfur, CO, and olefins, which can become an issue by decreasing the hydrogen flux. Alloying Pd with, for example, silver and/or copper appears to alleviate these problems to some extent and also using composite membranes, where the Pd (alloy) is deposited on top of a mesoporous zirconia or alumina layer is thought to create improvements. Pd membranes can be fouled as a result of autocatalytic polymerization of adsorbed propylene followed by further reactions which lead to coal tars. Currently Pd membranes are not considered ideal and are in need of further improvement. Certain publications recommend an operating temperature for Pd based membranes not in excess of 300 °C to avoid fouling. For alkane dehydrogenation operating temperature of less than 300 °C is only possible if the dehydrogenation reaction and the membrane hydrogen separation are performed in sequence.

[0024] The process described herein employs a mixed-matrix membrane concept to address some of the outstanding issues with Pd membranes. This approach improves the selectivity of a polymer membrane by embedding high selectivity molecular sieving materials. The basic idea is shown in FIG. 1. Since polymeric materials are not appropriate for high temperature applications, in the present case the embedding material, microporous ceramic materials can be used as the matrix for embedding Pd or Pd alloy particles. In certain aspects Pd or Pd alloy particles are embedded in the ceramic membranes. Appropriate hydrogen selectivity can be achieved by using Pd or a Pd-alloy as the embedded material. In certain aspects the Pd or Pd alloy is in the form of a nanoparticle. Nanoparticles can be used to create the optimum balance of high surface area versus loading. [0025] In a further aspect the Pd or Pd alloy nanoparticle can comprises a Pd or Pd alloy core surrounded by a shell. The shell can be a microporous ceramic material. In one aspects the nanoparticles can be relatively large nanoparticles (NPs) with a thin shell of only a few nm. In certain aspects a Pd core of approximately 2, 50, 100, 200, 300, 400, 500, 600 or more nm is sufficient for adequate selectivity. In certain aspects a metal oxide like silica or alumina can be used as a shell material. The nanoparticles can comprise a Pd core (2 to 600 nm in diameter) surrounded by a 1 to 6 nm thick porous shell.

[0026] The alloying component in a palladium-alloy core may be any chemical or chemicals capable of combining with palladium and that does not include palladium. For example, the alloying component may be carbon, silicon, silicon oxide, alumina, a metal, a polymer or polymer end-product, a dendrimer, a natural-based product such as cellulose, and so on. The alloying component in the palladium-alloy core can be a metal or combination of metals not including palladium. For example, the metal in the palladium-metal alloy may be an alkali, alkaline earth, main group, transition, lanthanide, or actinide metal. In certain aspects the alloying metal or metals in the palladium-alloy core are transition metals. In a further aspect the alloying component is one or more 3d transition metals, particularly nickel (Ni), cobalt (Co), and/or iron (Fe). Gold (Au) and its combination with other metals, particularly, Ni, Co, and Fe, are other preferred alloying components.

[0027] In certain instances the mixed matrix membranes have embedded nanoparticles in which the concentration of nanoparticles in the matrix can be between about 1, 5, 10, 15, 20 to 30, 40, 50 wt % of the membrane weight as determined by, for example, x-ray photoelectron spectroscopy of the membranes. In certain aspects the concentration of nanoparticles can be between about 1 and 10 wt %. In certain aspects the concentration of nanoparticles is about or at least 10, 20, 30, 40 up to 50 wt % of the membrane. [0028] In certain aspects the membranes described herein can have a homogeneous distribution throughout the membrane. In other aspects some (greater than about 5%) nanoparticles can be present as clusters of nanoparticles. In still other aspects, the particles can be discrete and not detectable as clusters.

[0029] The Pd embedded membrane can have one or more of the following advantages: (a) The microporous ceramic shell prevents propane and propylene from migrating to the Pd core and protects it from fouling, (b) The support and silica part of the structure would allow for relatively unrestricted permeance to the actual membrane, which is the Pd nanostructure. (c) As Pd is extremely hydrogen selective, the hydrogen selectivity would very high, (d) Due to the nano scale thickness, the permeance could be higher than what has been achieved with any Pd-based membrane to date, (e) The selection of the shell material, for example Ce0 ₂ , imparts a catalytic activity to enable H ₂ combustion on the non-dehydrogenation side of the membrane.

[0030] Certain embodiments can use microporous silica membranes. Microporous silica membranes can be used to improve the alkane dehydrogenation process (U.S. Patent 5,430,218; Juttke et al., Chemical Engineering Transactions, 2013, 32: 1891-96; Kiwi- Minsker et al., Chemical Engineering Science, 2002, 57:4947-53). To enable molecular sieving of hydrogen versus slightly larger species like propane, the pores have to be extremely small and very uniformly distributed. The kinetic diameter of certain relevant molecules are as follows: H ₂ (0.29 nm); C0 ₂ (0.33 nm); 0 ₂ (0.35 nm); N ₂ (0.36 nm); CH ₄ (0.38 nm), n-propene (0.44 nm) n-propane (0.43 nm); n-butene (0.45 nm); and n-butane (0.47 nm), giving an indication of the membrane pore size requirements. The thickness of the membrane is typically from 10 to around 150 microns (Li et al., Journal of Membrane Science, 2010, 354:48-54).

[0031] Polymer derived ceramic membranes, such as polysiloxane based membranes, can be used for the purpose of membrane assisted dehydrogenation. In certain aspects the silica precursor is Polysiloxane XP RV 200 from Evonik Industries AG. The membranes can be supported, e.g., γ-Α1 ₂ 0 ₃ supported membranes, and can be crosslinked and subjected to pyrolysis at 700 °C under inert atmosphere. The H ₂ permeance and the perm selectivity H ₂ /C ₃ H ₈ can be tested in an appropriate temperature range. The permeance (m ³ /(m ² /h/bar)) and permselectivity can be plotted as a function of temperature - both increase with temperature. In certain aspects the Pd or Pd alloy nanoparticles and the matrix material (e.g., polysiloxane) can be mixed prior to pyrolysis.

[0032] There are several side reactions possible in the dehydrogenation process: propane conversion to ethylene and methane by either thermal cracking, but more likely at these reactions temperatures by catalytic cracking; and hydrogenolysis of propane to ethane (shown as reaction 4 below) (Sattler et al., Chem. Rev. 2014, 114: 10613-53; Sahebdelfar et al., Chemical Engineering Research and Design, 2012, 90(8): 1090-97). C ₃ H ₈ + H ₂ <→ C ₂ H ₆ +CH ₄ ΔΗ° ₂₉₈ = -63 kJ/mol (4)

[0033] To minimize the hydrogenolysis side reaction the membrane should allow for the fast removal of hydrogen, minimizing reaction 4. This could (in part) explain higher propylene selectivity for the membrane reactor set-up. To facilitate its flow through the membrane hydrogen should be removed or converted as soon as it reaches the outside of the membrane. Combustion, conversion, or hydrogen capturing and storing can be used to remove or convert the hydrogen permeate. In certain aspects hydrogen combustion is used to convert the hydrogen permeate to water. In a further aspect the dehydrogenation part of the reactor can be operated at relatively low temperatures (350-400 °C) to avoid coking and the need for catalyst regeneration.

[0034] The auto-ignition temperature of hydrogen/oxygen for the stoichiometric mixture (2: 1) at atmospheric pressure is 570 °C. So for temperatures below 500 °C the presence of a suitable catalyst is required for hydrogen combustion. There are many options for a hydrogen combustion catalyst, some of which include, but are not limited to CeO, CuO, NiO, Co ₃ 0 ₄ , and Mn0 ₂ . One advantage to low temperature hydrogen combustion is that there is no NO _x generation and a reduced risk of creating a fire (Haruta and Sano, Int. Hydrogen Energy, 1981, 6(6):601-8). Preferably the catalyst should allow the use of air versus pure oxygen. Using air is not only less expensive but also minimizes the possibility of creating explosive mixtures in case of mechanical failure of the membrane.

[0035] The alkane feedstock can comprise at least one alkane. As used herein, the term "alkane" refers to a branched or straight chain, saturated hydrocarbon having 3 to 100 carbons. Exemplary alkanes include propane, n-butane, isobutane, n-pentane, isopentane, and neopentane. In certain embodiments, the alkane has 3 to 10 carbons. The alkane can be, for example, propane, butane (e.g. all isomers of butane, including, for example, n-butane, 2- methylpropane, and the like), a pentane (e.g. all isomers of pentane, including, for example, n-pentane, 2-methylbutane, and the like), an octane (e.g. all isomers of octane, including, for example, n-octane, 2,3-dimethylhexane, 4-methylheptane, and the like). The alkane feedstock can comprise a single alkane or a mixture of alkanes. As such, the alkane to be dehydrogenated can be a single alkane or a mixture of alkanes. The alkane can be a mixture of isomers of an alkane of a single carbon number. The alkane feedstock can comprise hydrocarbons in addition to the alkane or mixture of alkanes to be dehydrogenated. A hydrocarbon feed composition from any suitable source can be used as the alkane feedstock. Alternatively, the alkane feedstock can be isolated from a hydrocarbon feed composition in accordance with known techniques such as fractional distillation, cracking, reforming, dehydrogenation, etc. (including combinations thereof).

[0036] The alkene product can comprise at least one alkene. As used herein, in connection with the alkene product, the term "alkene" refers to a branched or straight chain, unsaturated hydrocarbon having 3 to 100 carbons and one or more carbon-carbon double bonds. In certain embodiments the alkene product comprises 3 to 10 carbons. In certain embodiments the alkene product comprises 3 to 10 carbons and one or two double bonds. The alkene can be, for example, propylene (propene), a butene (e.g., all isomers of butene, including, for example, 1 -butene, 2-butene, 2-methyl-l -propene, and the like), or a pentene (e.g., all isomers of pentene, including, for example, 1-pentene, 2-pentene, and the like). In an embodiment, the alkene comprises a propene. In certain aspects the alkene is selected from the group consisting of a propene, a butene, a pentene, an octene, a nonane, a decane, a dodecene, and mixtures thereof. Primarily, the alkene is the same carbon number as the feed. The alkene can comprise a single alkene or a mixture of alkenes. The alkene can be a mixture of isomers of an alkene of a single carbon number.

[0037] In certain embodiments the process or dehydrogenation reaction takes place in the presence of a solid alkane dehydrogenation catalyst. Alkane dehydrogenation catalyst can include, but are not limited to platinum catalysts with promoters like Sn, Na, Fe, Ce, Zn, Ga, Mg, In, and/or Ge; V-Mo-Nb-Te catalyst; nickel oxide (NiO) catalyst in the presence of Ti, Ta, Nb, Hf, W, Y, Zn and combinations of these metals (e.g., Nio.63 bo.19Tao.i8Ox) (see US Patents 7,498,289; 7,626,068; and 7,674,944 each of which is incorporated herein by reference); and the like.

[0038] In certain embodiments the dehydrogenation catalyst is operably coupled to a hydrogen permeable membrane. Because of the typical dehydrogenation reaction temperatures in the 400 to 600 °C range, so-called microporous or dense membranes are most preferred. Microporous membranes can be made from silica, alumina, zirconia, titania, zeolites or even carbon, and the transport mechanism to obtain selectivity towards hydrogen is molecular sieving. Palladium (Pd), Pd alloys, and perovskites are the materials of choice for dense hydrogen membranes. In the special case of Pd and its alloys the mechanism is solution/diffusion which creates near perfect hydrogen selectivity (» 1000). Both microporous and dense membranes have an upper use temperature of about 600 °C. [0039] In certain aspects the membrane reactor is a tube reactor design. There are two options for the propane dehydrogenation: inside the tubes or outside the tubes, i.e., having a catalytic core or gas sweep core respectively. In certain aspects the reactor tubes will have a dehydrogenation catalyst core and a hydrogen permeable wall with the hydrogen migrating thru the membrane which surrounds the dehydrogenation catalyst so it is combusted externally or on the external surface of the tube within the reactor vessel. Membrane permeance is one of the key properties that determines whether or not a membrane will be able to fulfill the requirements of a production facility or system. Varying the permeance input in a spreadsheet is used to determine reactor volume as a function of hydrogen permeance, which can return the following result: (i) from a separation point of view, the reactor volume can be inversely proportional to the hydrogen permeance; (ii) below a certain permeance the reactor volume and the number of tubes required become impractical; (iii) the threshold hydrogen permeance can be 0.5 m ³ /(m ² /t^ar), where the reactor volume needed is 160 m ³ with approximately 1,000 tubes in the reactor; (iv) at a hydrogen permeance of 1 m ³ /(m ² /h/bar) the reactor volume needed would be approximately 80 m ³ and about 500 tubes. Given this guidance an appropriate reactor can be constructed having the appropriate number tubes in the appropriate configuration given the characteristics of the membrane. These calculations are one example and provide a rough indication of possible requirement for one embodiment. Other parameters can be considered and optimized by one of skill - like reaction residence time, space between the tubes, hydrogen radial concentration gradient within the tubes, requirements related to heat management, and the like. Reactor design and process modeling can be used to determine the optimum performance.

[0040] Microporous carbon and zeolite membranes, with their combination of low selectivity and rather low permeance appear as less attractive candidates.