Process for extending mo-zeolite catalyst lifetime for methane dehydroaromatization

Abstract

Methods for methane dehydroaromatization using Mo-MFI zeolite catalysts and methods for regenerating Mo-MFI zeolites are provided. One method for regenerating Mo- MFI zeolites that have been used for methane dehydroaromatization includes obtaining a spent Mo-MFI zeolite catalyst that has at least one deactivated framework Al site; mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst to provide a physical mixture of the two catalysts; and heating the mixture at a temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst. This method for regenerating MFI zeolites is particularly useful in methane conversion processes, including methane dehydroaromatization.

Description

PROCESS FOR EXTENDING MO-ZEOLITE CATALYST LIFETIME FOR METHANE DEHYDROAROMATIZATIONGOVERNMENT LICENSE RIGHTS

[0001] This invention was made with government support under Cooperative Agreement No. EEC-1647722 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUNDField of the Invention

[0002] Embodiments provided herein relate to zeolites and method for making same. More particularly, embodiments provided herein relate to Mo-zeolite catalysts and methods of using such Mo-zeolite catalysts for methane dehydroaromatization (methane DHA), and methods for extending Mo-zeolite catalyst lifetime for methane DHA.Description of the Related Art

[0003] Methane dehydroaromatization (DHA) is a promising pathway to directly convert methane into higher liquid density chemicals such as aromatics (e.g., benzene) while coproducing hydrogen. Methane is essentially activated by dehydrogenation and then oligomerized and aromatized. The reaction is endothermic and constrained by thermodynamics, so it requires high temperatures (>873 K) to obtain CH4 conversions >5%. The catalyst is typically molybdenum supported on an aluminosilicate zeolite, of which the MFI framework structure is most commonly used. Mo-MFI zeolites used in methane DHA can provide near-equilibrium conversion (-10% at 973 K) with high aromatics selectivity (-80%).

[0004] Zeolites contain tetrahedral silicon atoms linked by oxygen, forming charge-neutral crystalline structures. The isomorphous substitution of trivalent aluminum atoms for tetravalent silicon introduces a negative charge imbalance that can be compensated for by protons or metal cations, which can act as active sites for catalysis. Typical methods to synthesize Mo-MFI catalysts involve the deposition of MoOa onto MFI zeolites either through incipient wet impregnation (IWI) or by creating a physical mixture of the two solids. The MoOs-MFI composite material is then typically heated under flowing air at high temperatures (>773 K) totransform MoOs domains into ion-exchanged Mo species located on or within MFI zeolite crystallites, which are referred to as the active site precursors. These ion-exchanged Mo species carburize under reaction conditions (e.g., 950 K, 90 kPa CH4) to form Mo carbide domains, which catalyze the methane DHA reaction.

[0005] One of the main challenges to developing a catalytic process for methane DHA using Mo-MFI is the relatively fast catalyst deactivation (~10 h), which over time leads to an irreversible loss in activity. This deactivation can be overcome, in part, by developing a continuous process of reaction and regeneration cycles in which the catalyst is continuously recirculated between the reactor unit, where it reacts with methane and deactivates, and a separate regeneration unit. To realize this process, however, the catalyst must sustain large numbers of reaction-regeneration cycles. Reaction and regeneration cycles (using air or O2) have been performed with Mo-MFI catalysts but it has been observed that the aromatics yield decreases systematically with increasing numbers of these cycles.

[0006] To improve the number of reaction-regeneration cycles the Mo-MFI catalyst can withstand, various strategies has been investigated, including lowering the Mo content on the Mo-MFI catalyst and lowering both the reaction time and regeneration temperature in these cycles. These have shown to be effective in delaying the deactivation process (i.e., it leads to an increase in the number of reaction-regeneration cycles the catalyst can withstand), however, Mo-MFI eventually deactivates irreversibly, and it has to be replaced with fresh catalyst. This irreversible deactivation occurs because the MFI zeolite support undergoes structural degradation due to framework dealumination during the regeneration cycle, and a loss of framework Al atoms from the zeolite support that prevents Mo species from being able to form the precursor sites to the active Mo-carbide phase during the DHA reaction cycle.

[0007] Additional relevant information can be found in: Wang, L., et al. Catalysis Letters. 21 (1993) 35-41; Ma, S., et al. Journal of Energy Chemistry. 22 (2013) 1-20; Zhang, Z. Carbon Resources Conversion. 2 (2019) 157-174; Cook, B., et al. Applied Catalysis A. 365 (2009) 34- 41; Han, S. et al. Applied Catalysis B. 241 (2019), 305-318 ; and Kosinov, N., et al. J. Catal. 346 (2017) 125-133, all of which are incorporated by reference herein in their entireties.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarizedabove, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are, therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. It is emphasized that the figures are not necessarily to scale and certain features and certain views of the figures can be shown exaggerated in scale or in schematic for clarity and / or conciseness.

[0009] FIG. 1 shows the IF temperature programmed reduction (TPR) profile for Mo-MFI- 0.25 after air treatment at 823 K.

[0010] FIG. 2 shows methane conversion and carbon yield for a single methane DHA run. Reaction conditions: 950 K, 60 / 40 (vol / vol) CI F / Ar, 115 kPa total pressure, 11.8 (mol Mo) s - (mol CPU)1contact time.

[0011] FIG. 3 shows data on Mo-MFI-0.25 for the initial benzene formation rate measured after a given number of reaction-regeneration cycles normalized by the initial benzene formation rate measured in the 1strun. Reaction conditions: 950 K, 60 / 40 (vol / vol) CFU / Ar, 115 kPa total pressure, 17.5 (mol Mo) s (mol CH4)1contact time. Regeneration conditions: 823 K, air, 6 h. Argon flush in between reaction-regeneration steps.

[0012] FIG. 4 shows H? TPR profiles of Mo-MFI-0.25 before reaction (black line), after 7 (grey line) reaction-regeneration cycles and after 10 (light grey line) reaction-regeneration cycles. Samples were pretreated in air at 823 K for 2 h before the H2 TPR measurement.

[0013] FIG. 5 shows XRD patterns of a) the “fresh” Mo-MFI-0.25 catalyst and b) “spent” Mo-MFI-0.25 after 10 reaction-regeneration cycles.

[0014] FIG. 6 shows H2 TPR profiles of the “fresh” Mo-MFI-0.25 (black line), “spent” Mo-MFI-0.25 after 7 reaction-regeneration cycles (grey line), and “regenerated” Mo-MFI- 0.09-mix (light line). Samples were pretreated in air at 823 K for 2 h before TPR.

[0015] FIG. 7 shows the initial benzene formation rate measured after a given number of reaction-regeneration cycles normalized by the initial benzene formation rate measured in the 1strun for Mo-MFI-0.24 and Mo-MFL0.08-mix.SUMMARY

[0016] Methods for methane dehydroaromatization using supported catalysts and method for regenerating supported catalysts are provided. The supported catalysts can be ZSM-5 (MFI) type zeolites. In at least one embodiment, a method for regenerating supported catalysts includes: obtaining a spent Mo-MFI zeolite catalyst that has at least one deactivated frameworkA1 site; mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst to provide a physical mixture of the two catalysts: and heating the mixture at a temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst. Additional embodiments include any one or more of the following: a) wherein the physical mixture of the catalysts can have a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Al fresh : Mo spent) of between 0.01—10; b) wherein the physical mixture of the catalysts can have a mass ratio of fresh catalyst to spent catalyst of from 3:1 to 1 :1; c) wherein the spent Mo-MFI zeolite catalyst can be obtained from a methane dehydroaromatization process; d) wherein the mixture can be heated at a temperature of 700K-950K for at least 15 minutes and up to one hour; e) wherein the mixture can be heated at a temperature of 700K-950K in dry air; and / or f) any one or more of the foregoing a) through e).

[0017] In at least one other embodiment, a method for methane dehydroaromatization includes: a) reacting methane in the presence of a Mo-MFI zeolite catalyst to produce an aromatic product and at least partially deactivated Mo-MFI zeolite catalyst, wherein the aromatic product is produced at an initial rate; b) treating the at least partially deactivated Mo- MFI zeolite catalyst with hot air to regenerate the Mo-MFI zeolite catalyst; c) repeating steps a) and b) until the initial rate for producing the aromatic product falls below 90%, whereby the Mo-MFI zeolite catalyst is spent; d) mixing a fresh MFI zeolite catalyst with the spent Mo- MFI zeolite catalyst; and e) heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst to a regeneration temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst. The foregoing method can further include any one or more of the following embodiments 1 to 9:1) repeating steps a) and b) anywhere from 4-10 times until the at least partially deactivated Mo-MFI zeolite catalyst becomes spent;2) mixing the fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst can include adding the fresh MFI zeolite catalyst to obtain a mass ratio of fresh catalyst to spent catalyst of from 3: 1 to 1 : 1 ;3) wherein the mixture of the fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst can have a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Al fresh : Mospent) is between 0.01-10;4) wherein the methane can be reacted at a temperature of 750-1 ,000K;5) heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst can include heating the mixture at a regeneration temperature of 700K-950K;6) heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst can include heating the mixture in dry air;7) wherein the aromatic product can include one or more aromatics selected from the group consisting of benzene, toluene, xylene, and naphthalene;8) wherein the methane can be derived from shale gas; and / or9) any or more of the foregoing 1 ) through 8).DETAILED DESCRIPTION

[0018] It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure can repeat reference numerals and / or letters in the various embodiments and across the figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and / or configurations. Moreover, the exemplary embodiments presented below can be combined in any combination of ways, i.e. , any element from one exemplary embodiment can be used in any other exemplary embodiment, without departing from the scope of the disclosure.

[0019] Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities can refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function.

[0020] Furthermore, in the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean“including, but not limited to.” The phrase “consisting essentially of’ means that the described / claimed composition does not include any other components that will materially alter its properties by any more than 5% of that property, and in any case does not include any other component to a level greater than 3 mass%.

[0021] Unless otherwise indicated, all numerical values are "about" or "approximately" the indicated value, meaning the values take into account experimental error, machine tolerances and other variations that would be expected by a person having ordinary skill in the art. It should also be understood that the precise numerical values used in the specification and claims constitute specific embodiments. Efforts have been made to ensure the accuracy of the data in the examples. However, it should be understood that any measured data inherently contains a certain level of error due to the limitation of the technique and / or equipment used for making the measurement.

[0022] In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” The phrase “consisting essentially of’ means that the described / claimed composition does not include any other components that will materially alter its properties by any more than 5% of that property, and in any case, does not include any other component to a level greater than 3 wt%.

[0023] The term “or” is intended to encompass both exclusive and inclusive cases, i.e., “A or B” is intended to be synonymous with “at least one of A and B,” unless otherwise expressly specified herein.

[0024] The indefinite articles “a” and “an” refer to both singular forms (i.e., “one”) and plural referents (i.e., one or more) unless the context clearly dictates otherwise. For example, embodiments using “an olefin” include embodiments where one, two, or more olefins are used, unless specified to the contrary or the context clearly indicates that only one olefin is used.

[0025] The term “oligomerization” refers to the formation of an oligomer from molecules of lower relative molecular mass.

[0026] The terms “alkane” and “paraffin” are interchangeably and both refer to any saturated molecule containing hydrogen and carbon atoms only, in which all the carbon-carbon bonds are single bonds and are saturated with hydrogen. Such saturated molecules can be linear, branched, and / or cyclic.

[0027] The terms “alkene” and “olefin” are interchangeably and both refer to any unsaturated molecule containing hydrogen and carbon atoms only, in which one or more pairsof carbon atoms are linked by a double bond. Such unsaturated molecules can be linear, branched, or cyclic, and can include one, two, three or more pairs of carbon atoms linked by double bounds (i.e. mono-olefins, di-olefins, tri-olefins, etc).

[0028] The term “wt%” means percentage by weight, “vol%” means percentage by volume, “mol%” means percentage by mole, “ppm” means parts per million, and “ppm wt” and “ppmw” are used interchangeably and mean parts per million on a weight basis. All concentrations herein, unless otherwise stated, are expressed on the basis of the total amount of the composition in question.

[0029] The term “hydrocarbon stream” refers to any stream that naturally contains methane and one or more other hydrocarbons. Illustrative hydrocarbon streams can be or can include a raw shale gas stream or raw natural gas stream or other raw hydrocarbon gaseous stream that is obtained directly (i.e. without processing to remove water and / or acid gas) from a reservoir, wellhead, or pipeline. Suitable hydrocarbon streams can also originate from a refinery, such as from a FCC, coker, steam cracker, and pyrolysis gasoline (pygas). Suitable hydrocarbon streams can also be or can include coal gas. Illustrative hydrocarbon streams can also be or can also include a gas stream that has been treated for acid gas and water removal.

[0030] The term “hydrocarbon” refers to a compound that consists of carbon and hydrogen atoms only.

[0031] The term “liquid hydrocarbon” refers to a hydrocarbon that is liquid at room temperature and ambient pressure, and primarily includes Cs+ hydrocarbons. Moreover, this term can also refer to hydrocarbons that are liquid at room temperature but require high pressure such C4 alkane and alkenes.

[0032] The term “shale gas” refers to natural gas that is produced from a shale or other tight formation, and is a gaseous phase mixture containing natural gas liquids, acid gases, water, nitrogen (N2), and possibly trace amounts of contaminants. A suitable shale gas (or natural gas) contains at least 50 mol% CH4 and up to 50 mol% of C2H6, C3H8, C4H10, and / or C5+ hydrocarbons. For example, a suitable shale gas (or natural gas) contains about 60 mol% to about 95 mol% CH4 and about 5 mol% to about 45 mol% of C2H6, C3H8, C4H10, and / or C5+ hydrocarbons (or collectively referred to as “C2+ hydrocarbons” or “C2+ alkanes”). Among the C2+ hydrocarbons, C2H6 is generally the highest concentration followed by C3H8 then C4H10. Nitrogen gas (N2) can also be present in the shale gas.

[0033] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specifiedin the claims. Depending on the context, all references to the “invention” may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims. Each of the inventions will now be described in greater detail below, including specific embodiments, versions and examples, but the inventions are not limited to these embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the inventions, when the information in this disclosure is combined with publicly available information and technology.

[0034] Embodiments described herein provide methods for methane DHA and for restoring catalytic activity of a partially deactivated or spent Mo-MFI catalyst. It has been surprisingly discovered that a Mo-MFI catalyst that has been exposed to various cycles of methane DHA reaction and regeneration can be physically mixed with a “fresh” zeolite catalyst to restore benzene formation rates to at or near initial values, such as at least 95%, 96%, 97%, 98%, or 99% of the initial values. This is a significant finding that can be used to develop a continuous reaction-regeneration process where “fresh” zeolite catalyst can be introduced to extend the Mo-MFI catalyst activity over larger runs of methane DHA reaction and regeneration cycles.

[0035] The catalytic activity of a partially deactivated Mo-MFI catalyst can be significantly enhanced by creating a physical mixture of the partially deactivated or “spent” catalyst with a fresh zeolite containing some amount of framework Al atoms (hereafter referred to as “fresh” catalyst), and then subjecting this physical mixture to a regeneration cycle (e.g., hot air). The “fresh” catalyst can be nominally in its aluminosilicate form or can be in a Mo-zeolite composite form. The fresh catalyst can be added in amounts such that the molar ratio of framework Al atoms in the “fresh” catalyst and Mo atoms in the “spent” catalyst (Akesh : Mospem) is between 0.01-10. The fresh catalyst also can be added in amounts such that a mass ratio of the fresh catalyst to the spent catalyst ranges from 1: 1 to 3: 1, such as 2:1, 1.8:1, 1.5: 1 or 1.2:1.

[0036] It is believed this strategy is effective for regenerating any zeolite framework topology, not just MFI frameworks, such as ZSM-5, BEA, MOR, Y, A1PO-5, for example. For simplicity and ease of description, however, embodiments of the present invention will be further described with reference to MFI zeolite frameworks.

[0037] Not wishing to be bound by theory, it is believed the formation of catalytically active Mo precursor sites on the MFI zeolite support involves a volatile Mo intermediate, which converts aggregated MoO3 domains to ion-exchanged Mo species that charge-compensate theframework Al sites in the zeolite. In this manner, the “fresh” catalyst provides a reservoir of Mo ion-exchange sites in separate zeolite crystallites, which regenerates the Mo species to its active phase.

[0038] During the regeneration step (e.g., hot air treatment (700K -l,000K for 1-24 hrs), the deactivated forms of Mo on the “spent” catalyst (which can be a variety of species, including molybdenum oxides (MoOs) and aluminum molybdates (Ah MoC h)) decompose to generate volatile Mo species. These volatile Mo species then react with the acid sites (H+sites that charge-compensate framework Al atoms) located on the “fresh” catalyst to form ion- exchanged Mo species, which are the precursors to the active Mo-carbide phase that forms during the methane DHA reaction step. The “fresh” catalyst replaces the framework Al atoms that are gradually removed in the “spent” catalyst through dealumination, such that the Mo sites can be regenerated.

[0039] Said another way, deactivated forms of Mo will volatilize during the hot air regeneration treatment, and these volatilized Mo species need an anchoring location on the zeolite to form an ion-exchanged Mo species. These anchoring locations, however, are progressively removed from the “spent” zeolite due to dealumination, so providing new anchoring locations on the fresh catalyst allows the Mo species to be anchored and regenerated, providing a site for the ion-exchange reaction to occur.

[0040] By ‘ ‘spent”, it is meant that the catalyst has been exposed to at least one (1) reaction cycle. A “spent” catalyst has less framework Al atoms (caused by dealumination) than a “fresh” catalyst, which refers to a catalyst that has not been exposed to at least one (1) reaction cycle. Accordingly, the “spent” catalyst will have fewer H+sites than a “fresh” catalyst. A “regenerated” catalyst refers to a spent catalyst that has been exposed to air or oxygen (i.e., O2 molecules) at temperatures at or above 700K, to reactivate the catalyst, and by “reactivate” it is meant that the number of H+sites on the spent catalyst is increased and / or the amount of surface carbon deposits is decreased due to the exposure to the O2 molecules provided by the air and / or oxygen gas at the elevated temperature.

[0041] In certain embodiments, the dehydroaromatization can be performed in any suitable reactor, such as a gas phase reactor or a fixed bed gas phase reactor. A dual bed circulating fluidized bed reactor system can be used to allow for continuous regeneration of deactivated Mo / H-MFI catalyst using hot air or H2. The reaction feed for the dehydroaromatization can include argon gas in addition to methane. The methane can be derived from any hydrocarbon stream, such as natural gas or shale gas. The dehydroaromatization can be performed at 600to 800°C in the presence of the catalyst described herein. The resulting aromatic products can be selected from the group consisting of benzene, toluene, xylene, naphthalene, and coke. An illustrative reaction run includes a mixture of methane and argon in a 2:1 volume ratio at a GHSV of 4,500 ml / h g cat and temperature of about 750°C in the presence of the Mo-MFI catalyst for about 1 hour.

[0042] The present disclosure is not limited by the method of catalyst preparation. All suitable methods should be considered to be within the scope herein. Conventional methods include co-precipitation from an aqueous, an organic, or a combination solution-dispersion, impregnation, dry mixing, wet mixing or the like, alone or in various combinations. In general, any method can be used which provides compositions of matter containing the prescribed components in effective amounts. According to an embodiment the substrate is charged with active metal via an incipient wetness impregnation. Other impregnation techniques such as by soaking, pore volume impregnation, or percolation can optionally be used. Alternate methods such as ion exchange, wash coat, precipitation, and gel formation can also be used.

[0043] The Si: Al ratio of the zeolite can range from a low of about 10, 20, 30 or 40 to high of about 50, 80, 100, or 200. The source of aluminum can be aluminum hydroxide, aluminum nitrate, aluminosilicate, and any combinations or derivatives thereof. The source of silicon can be colloidal silica, a silicon alkoxide compound, fumed silica, amorphous silica, aluminosilicate, and any combinations or derivatives thereof.

[0044] The crystallization process preferably occurs at about 130°C to about 150°C, but can range from as low of about 60°C, 70°C, 80°C or 90°C to a high of about 150°C, 180°C, 200°C or 240°C. The crystallization process can also take place at about 100°C, 130°C, 140°C, 150°C, 160°C, 170°C, or 180°C.

[0045] The high temperature air calcination treatment can occur at about 450 to about 550°C. The air calcination treatment can also range from as low of about 400°C, 410°C, or 420°C to a high of about 500°C, 550°C or 600°C.

[0046] The support material can have a surface area in the range of from about 10 nr / g to about 700 nr / g, a pore volume in the range of from about 0.1 cc / g to about 4.0 cc / g and an average particle size in the range of from about 5 pm to about 500 pm. More preferably, the support material can have a surface area in the range of from about 50 m2 / g to about 500 m2 / g, pore volume of from about 0.5 cc / g to about 3.5 cc / g and average particle size of from about 10 pm to about 200 pm. The surface area can range from a low of about 50 m2 / g, 150 m2 / g, or300 m2 / g to a high of about 500 m2 / g, 700 m2 / g, or 900 m2 / g. The surface area also can range from a low of about 200 m2 / g, 300 m2 / g, or 400 m2 / g to a high of about 600 nr / g, 800 m2 / g, or 1 ,000 m2 / g. The average pore size of the support material can range from about 10 A to 1000 A, about 50 A to about 500 A, about 75 A to about 350 A, about 50 A to about 300 A, or about 75 A to about 120 A.EXAMPLES

[0047] Embodiments discussed and described herein can be further described with the following examples. Although the following examples are directed to specific embodiments, they are not to be viewed as limiting in any specific respect.

[0048] Commercial NHC-MFI (Si / Al = 13) was obtained from Zeolyst International (CBV 2314) and converted into H-MFI by flowing dry air ( 1 .67 cm3s’1g’1, Air Zero, Indiana Oxygen) in a muffle furnace at 773 K for 4 h (0.0167 K s’1).

[0049] Mo-MFI (Mo / Al = 0.25) was prepared by incipient wet impregnation (IWI). To form the Mo-MFI, an aqueous solution was prepared by adding (NH4)6Mo?O24 (Fischer Scientific, 99.9%) salt to deionized water. Around 0.37 grams of the (NH4)6Mo?O24 salt was mixed with 4.65 grams of deionized water to create an aqueous molybdenum solution with a concentration of 0.078 grams of salt per gram of water (about 1 wt% Mo). Then, around 0.60 grams of this solution was added to around 1.10 grams of H-MFI in a dropwise manner while stirring. The sample was then treated in flowing dry air (1.67 cm3s’1g’1, Air Zero, Indiana Oxygen) in a muffle furnace at 823 K for 6 h (0.0167 K s’1).Methane DHA Reaction

[0050] Mo-MFI was pelletized and sieved to obtain particles between 180-250 pm. Catalyst samples (0.3-0.75 grams) were placed within tubular quartz reactors (12.7 mm ID) and supported between quartz wool (Fischer Scientific) plugs. Reactor temperature was controlled using a clam-shell furnace connected to and monitored by a concentric Type K thermocouple (Omega) above the catalyst bed and an additional thermocouple located at the external surface of the reactor. Mo-MFI catalysts were heated in Argon (UHP, 99.999%, 0.5 cm3s’1, Indiana Oxygen) to 950 K (0.167 K s’1) and held for 0.5 h to allow stabilization of temperature. Then, a CH4 / Ar (95 / 5 vol%, Certified Mixture, Indiana Oxygen) was diluted with Ar to introduce a stream of 60 vol% CH4 and 40 vol% Ar to the reactor. The total flow rate and catalyst loading were adjusted to achieve Mo contact times between 10-18 (mol Mo)- s- (mol CH4)1. Transfer lines from the reactor outlet were heated (>423 K) to prevent condensation ofproducts (i.e., benzene, naphthalene). Reactor effluent analysis and quantification was performed using a gas chromatograph (GC Agilent 6890) equipped with a capillary column (DB- 1 , 50 m x 320 pm x 5 pm) connected to a flame ionization detector (FID) for hydrocarbons and a packed pre-column (Supelco , 3 m x 3.2 mm x 2.1 mm) followed by a packed column (Supelco 60 / 80 Carboxen 1000, 4.5 m x 3.2 mm) connected to a thermal conductivity detector (TCD) for Ar, CH4, CO, and H2. The GC pre-column was flushed with He after 0.16 h of the injection to remove accumulated hydrocarbons.

[0051] Methane DHA catalytic measurements were performed on Mo-MFI by flowing 60 kPa CH4 (in balance Ar) to the catalyst bed and allowing it to deactivate until -80-90% of the initial rate was observed. Then, the reactor was flushed with Ar (UHP, 99.999%, 0.5 cm3s’1, Indiana Oxygen) and temperature was cooled to 823 K (0.25 h) before introducing air (0.25 cm3s’1, Air Zero, Indiana Oxygen) for 6 h into the reactor to regenerate the catalyst.

[0052] After the regeneration step, temperature was increased to 950 K (0.167 K s’1) under flowing Ar and held for 0.5 h before introducing the flow of 60 kPa CH4 to start the next reaction cycle. After various reaction-regeneration cycles (4, 7, and 10) the temperature was cooled to room temperature to perform characterization analysis on the “spent” catalysts, and to mix “spent” catalysts with “fresh” H-MFI to recover the Mo active site precursors.

[0053] “Spent” Mo-MFI (around 0.25 grams) was physically mixed with “fresh” H-MFI (around 0.45 grams) in a mortar and pestle for 0.5 h to obtain a solid mixture (mass ratio of 1 :1.8 of Mo-MFI spent: fresh H-MFI), which was then treated in dry air (1.67 cm3s’1g’1, Air Zero, Indiana Oxygen) in a muffle furnace at 973 K for 1 h (0.167 K s’1).

[0054] Mo / Al ratios were measured for Mo-MFI samples after IWI and air treatment. Mo / Al ratios were also measured for Mo-MFI samples after mixing “spent” catalyst with “fresh” H-MFI. Table 1 reports these measured Mo / Al ratios. Sample codes are referred to as Mo-MFI-X, where X denotes the Mo / Al ratio. Sample codes for catalysts that were prepared by physically mixing spent catalyst and fresh H-MFI are denoted as Mo-MFI-X-mix, where X denotes the Mo / Al ratio in the final physical mixture.

[0055] Table 1. Catalyst properties of Mo-MFI samples.Sample Si / Al Mo / AlMo-MFIb13 0.08aCatalysts after IWI of Mo and air treatment (823 K, 6 h).bCatalysts after mixing “spent” Mo-MFI with “fresh” H-MFI and air treatment (973 K, 1 h).

[0056] In addition, H2 TPR was performed to characterize the number of ion-exchange Mo species. FIG. 1 shows the H2 TPR profile for Mo-MFI-0.25 after air treatment at 823 K. The high temperature feature (-1260 K) is unique to the ion-exchanged Mo species and quantification of total H2 normalized by Mo content (H2 / M0) indicates complete reduction of MO6+to Mo°

[0057] H2 temperature programmed reduction (H2 TPR) experiments were performed using a Micromeritics Autochem II 2920 equipped with a thermal conductivity detector (TCD) to quantify the amount of H2 consumed. The quantification method was validated with a Ag2O standard, which was able to quantify -88% of the theoretical value. Mo-MFI samples (0.04- 0.075 grams) were loaded into a quartz U-tube reactor supported between two plugs of quartz wool, and a clam-shell furnace controlled the temperature measured by a thermocouple above the sample bed. A slurry mixture of 2-propanol (Sigma Aldrich, 99.9%) and liquid N2 was placed after the reactor and before the thermal conductivity detector (TCD) to remove water produced during the reduction of Mo. Samples were pretreated in flowing air (0.5 cm3s'1, Air Zero, Indiana Oxygen) to a range of temperatures (773-1023 K) for 2 h, cooled to 353 K. Then, a flow of 5% H2 in balance Ar (0.416 cm3s'1, Certified Mixture, Indiana Oxygen) was sent through the sample until a TCD baseline was obtained followed by a temperature ramp to 1273 K (0.167 K s1, hold 0.5 h).

[0058] Benzene formation rates were measured for Mo-MFI-0.25 over 10 successive methane DHA reaction-regeneration cycles. During initial time-on-stream, methane carburizes the ion-exchanged Mo species, which is evidenced by the low values of the carbon balance and the formation of CO (see FIG. 2). Carbon yield was calculated for all reactions using all the hydrocarbon products (i.e., ethane, ethylene, benzene, toluene, and naphthalene) normalized by the inlet methane flow rate. The time corresponding to the maximum value of the carbon yield is taken as the initial benzene formation rate, and this value is compared for each successive reaction-regeneration cycle. Moreover, the initial benzene formation rate was normalized by the rate measured on the first methane DHA run, and this rate ratio is shown in FIG. 2 for each cycle performed. These data evidence the systematic decrease of benzene formation rates with increasing number of reaction-regeneration cycles.

[0059] FIG. 3 shows data on Mo-MFI-0.25 for the initial benzene formation rate measured after a given number of reaction-regeneration cycles normalized by the initial benzene formation rate measured in the 1strun. Reaction conditions: 950 K, 60 / 40 (vol / vol) CPU / Ar, 115 kPa total pressure, 17.5 (mol Mo) s (mol CH4)"1contact time. Regeneration conditions: 823 K, air, 6 h. Argon flush in between reaction-regeneration steps.

[0060] FIG. 4 depicts H2 TPR profiles of Mo-MFI-0.25 for before reaction (black line) and after 7 (grey line) and 10 (light grey line) reaction-regeneration cycles. Samples were pretreated in air at 823 K for 2 h before the H2 TPR measurement.

[0061] Bulk characterization (XRD and elemental analysis) and H2 TPR experiments were performed to assess the structural changes in Mo speciation on Mo-MFI after 4, 7, and 10 reaction-regeneration cycles. XRD (FIG. 5) patterns did not reflect a significant change in crystallinity, as can be observed when comparing the “fresh” catalyst and the “spent” catalyst after 10 cycles.

[0062] Zeolite frameworks were determined by powder X-ray diffraction (XRD) using a Rigaku SmartLab X-ray diffractometer with Cu Kot radiation source. XRD patterns were collected over a range of 4-40° 20 with a scan rate of 0.04° s-1using a low-dead volume sample holder (Rigaku).

[0063] Quantifying the Mo content (Table 2) evidences no loss of Mo by sublimation from the catalyst during reaction-regeneration cycles. Importantly, from H2 TPR experiments (FIG. 6), the reduction feature at 1260 K that is uniquely indicative of ion-exchanged Mo species is observed to decrease systematically with increasing numbers of reaction-regeneration cycles. In addition, H2 / M0 values shown in Table 2, also decrease systematically with increasing numbers of reaction-regeneration cycles, indicating that Mo species are transforming into an inactive phase. Taken together, the decrease in the fraction of ion-exchanged Mo species correlates with the decrease of initial benzene formation rate with successive reactionregeneration cycles.

[0064] Table 2. Mo content and H2 (per Mo) quantified by H2 TPR for catalysts used during methane DHA reaction-regeneration cyclescontent).

[0065] Elemental analysis was performed by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Thermo Scientific iCAP 7000 Plus Series ICP-OES. Around 0.02-0.04 grams of sample was digested in 2.5 grams of hydrofluoric acid (48 wt %, Alfa Aesar) for 48 h and then diluted in 50 grams of deionized water (18.2 ohms). For ICP analysis, 10 grams of the digested sample was mixed with 0.3 grams of HNO3 (70 wt.%, Sigma-Aldrich) before analysis.

[0066] Mo-MFI-0.25 was used for 7 cycles and then mixed with “fresh” H-MFI to create a physical mixture (sample code: Mo-MFI-0.09-mix). The Mo-MFI-0.09-mix sample was then subjected to an air treatment at 973 K. H2 TPR experiments comparing the “fresh” Mo-MFI- 0.25, “spent” Mo-MFI-0.25 (after 7 cycles), and “regenerated” Mo-MFI-0.09-mix are shown in FIG. 6. The fraction of ion-exchanged Mo determined by the high-temperature reduction feature (-1260 K) and total H2 consumed (per Mo) increased for the Mo-MFI-0.09-mix sample (Table 3), indicating that more ion-exchanged Mo species were able to form once additional ion-exchange sites were introduced (via “fresh” H-MFI) in the physical mixture. This observation indicates that Mo species can form a volatile precursor that can redisperse onto other zeolite crystallite particles in the solid mixture.

[0067] Table 1 : Mo content and H2 (per Mo) quantified by H2 TPR for the initial Mo- MFI zeolite sample, a partially deactivated Mo-MFI zeolite sample (after 7 reaction-regeneration cycles; “spent”), and for a mixture of “spent” Mo-MFI and “fresh” H-MFI catalyst.aMo / Al was determined with ICP-OES.bH2 / Mo was quantified by H2 TPR and ICP-OES (Mo content).

[0068] Methane DHA reactions were conducted with Mo-MFI-0.24 for 4 successive reaction-regeneration cycles. “Spent” catalyst was then mixed with “fresh” H-MFI to create a physical mixture (Mo-MFI-0.08-mix) that was then treated in air at 973 K for 1 h. The treated Mo-MFI-0.08-mix was then loaded into the reactor and methane DHA reaction-regeneration cycles were performed. Initial benzene formation rates increased and were recovered to rates similar to those measured in the first reaction cycle on Mo-MFI-0.24 (FIG. 7). Taken together, kinetic measurements along with H2 TPR characterization data indicate that larger amounts of ion-exchanged Mo sites were formed during the air treatment step, upon introduction of an additional reservoir of ion-exchange sites supplied by “fresh” catalyst particles, and consequently leading to a recovery in the benzene formation rates during the methane DHA reaction cycle.

[0069] The above results show that mixing partially deactivated Mo-MFI with “fresh” H- MFI restores the ability to form active site precursors (ion-exchanged Mo), which otherwise become irreversibly lost with increasing numbers of reaction -regeneration cycles. This leads to an increase in the methane DHA reaction rates (normalized per Mo) to restore the initial activity of the “fresh” Mo-MFI catalyst. This can be used as a strategy to develop a continuous reaction-regeneration process, in which continuously adding “fresh” H-MFI in the regeneration cycle can extend the Mo-MFI catalyst lifetime.

[0070] Aspects of the present invention further include the following numbered Embodiments 1-15:

[0071] Embodiment 1. A method for methane dehydroaromatization, comprising: a) reacting methane in the presence of a Mo-MFI zeolite catalyst to produce an aromatic product and at least partially deactivated Mo-MFI zeolite catalyst, wherein the aromatic product isproduced at an initial rate; b) treating the at least partially deactivated Mo-MFI zeolite catalyst with hot air to regenerate the Mo-MFI zeolite catalyst; c) repeating steps a) and b) until the initial rate for producing the aromatic product falls below 90%, whereby the Mo-MFI zeolite catalyst is spent; d) mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst; and e) heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst to a regeneration temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst.

[0072] Embodiment 2: The method according to Embodiment 1 , wherein mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst comprises adding the fresh MFI zeolite catalyst to obtain a mass ratio of fresh catalyst to spent catalyst of from 3:1 to 1:1.

[0073] Embodiment 3: The method according to Embodiment 1 or 2, wherein the mixture of the fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst has a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Alfrcsh : Mospent) is between 0.01-10.

[0074] Embodiment 4: The method according to any Embodiment 1 to 3, wherein the methane is reacted at a temperature of 750-l,000K.

[0075] Embodiment 5: The method according to any Embodiment 1 to 4, wherein heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst comprises heating the mixture at a regeneration temperature of 700K-950K.

[0076] Embodiment 6: The method according to any Embodiment 1 to 5, wherein heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst comprises heating the mixture in dry air.

[0077] Embodiment 7 : The method according to any Embodiment 1 to 6, wherein the steps a) and b) are repeated 4-10 times until the at least partially deactivated Mo-MFI zeolite catalyst becomes spent.

[0078] Embodiment 8: The method according to any Embodiment 1 to 7, wherein the aromatic product comprises one or more aromatics selected from the group consisting of benzene, toluene, xylene, and naphthalene.

[0079] Embodiment 9: The method according to any Embodiment 1 to 8, wherein the methane is derived from shale gas.

[0080] Embodiment 10: A method for regenerating MFI zeolites, comprising: obtaining a spent Mo-MFI zeolite catalyst that has at least one deactivated framework Al site; mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst to provide a physical mixtureof the two catalysts; and heating the mixture at a temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst.

[0081] Embodiment 11: The method according to Embodiment 10, wherein the physical mixture of the catalysts has a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Alfresh '■ Mospent) of between 0.01-10.

[0082] Embodiment 12: The method according to Embodiment 10 or 11, wherein the physical mixture of the catalysts has a mass ratio of fresh catalyst to spent catalyst of from 3:1 to 1 :1.

[0083] Embodiment 13: The method according to any Embodiment 10 to 12, wherein the spent Mo-MFI zeolite catalyst is obtained from a methane dehydroaromatization process.

[0084] Embodiment 14: The method according to any Embodiment 10 to 13, wherein the mixture is heated at a temperature of 700K-950K for at least 15 minutes and up to one hour.

[0085] Embodiment 15: The method according to any Embodiment 10 to 14, wherein the mixture is heated at a temperature of 700K-950K in dry air.

[0086] All patents and patent applications, test procedures (such as ASTM methods, UL methods, and the like), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this disclosure and for all jurisdictions in which such incorporation is permitted.

[0087] Certain embodiments and features have been described using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges including the combination of any two values, e.g., the combination of any lower value with any upper value, the combination of any two lower values, and / or the combination of any two upper values are contemplated unless otherwise indicated. Certain lower limits, upper limits and ranges appear in one or more claims below. All numerical values are “about” or “approximately” the indicated value, meaning the values take into account experimental error, machine tolerances and other variations that would be expected by a person having ordinary skill in the art.

[0088] The foregoing has also outlined features of several embodiments so that those skilled in the art can better understand the present disclosure. Those skilled in the art should appreciate that they can readily use the present disclosure as a basis for designing or modifying other methods or devices for carrying out the same purposes and / or achieving the same advantages of the embodiments disclosed herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the presentdisclosure, and that they can make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure, and the scope thereof is determined by the claims that follow.

[0089] Various terms have been defined above. To the extent a term used in a claim is not defined above, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Furthermore, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions in which such incorporation is permitted.

[0090] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

CLAIMSWhat is claimed is:

1. A method for methane dehydroaromatization, comprising: a) reacting methane in the presence of a Mo-MFl zeolite catalyst to produce an aromatic product and at least partially deactivated Mo- MFI zeolite catalyst, wherein the aromatic product is produced at an initial rate; b) treating the at least partially deactivated Mo-MFI zeolite catalyst with hot air to regenerate the Mo-MFI zeolite catalyst; c) repeating steps a) and b) until the initial rate for producing the aromatic product falls below 90%, whereby the Mo-MFI zeolite catalyst is spent; d) mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst; and e) heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst to a regeneration temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst.

2. The method of claim 1 , wherein mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst comprises adding the fresh MFI zeolite catalyst to obtain a mass ratio of fresh catalyst to spent catalyst of from 3:1 to 1 :1.

3. The method of claim 1, wherein the mixture of the fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst has a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Alfresh : Mospent) is between 0.01-10.

4. The method of claim 1, wherein the methane is reacted at a temperature of 750- l,000K.

5. The method of claim 1 , wherein heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst comprises heating the mixture at a regeneration temperature of 700K-950K.

6. The method of claim 1 , wherein heating the mixture of the fresh MFI zeolite catalyst and the spent Mo-MFI zeolite catalyst comprises heating the mixture in dry air.

7. The method of claim 1, wherein the steps a) and b) are repeated 4-10 times until the at least partially deactivated Mo-MFI zeolite catalyst becomes spent.

8. The method of claim 1, wherein the aromatic product comprises one or more aromatics selected from the group consisting of benzene, toluene, xylene, and naphthalene.

9. The method of claim 1 , wherein the methane is derived from shale gas.

10. A method for regenerating MFI zeolites, comprising: obtaining a spent Mo-MFI zeolite catalyst that has at least one deactivated framework Al site; mixing a fresh MFI zeolite catalyst with the spent Mo-MFI zeolite catalyst to provide a physical mixture of the two catalysts; and heating the mixture at a temperature of 700K-950K to reactivate the spent Mo-MFI zeolite catalyst.

11. The method of claim 10, wherein the physical mixture of the catalysts has a molar ratio of framework Al atoms in the fresh MFI zeolite catalyst to Mo atoms in the spent catalyst (Alfresh : Mospent) of between 0.01-10.

12. The method of claim 10, wherein the physical mixture of the catalysts has a mass ratio of fresh catalyst to spent catalyst of from 3: 1 to 1:1.

13. The method of claim 10, wherein the spent Mo-MFI zeolite catalyst is obtained from a methane dehydroaromatization process.

14. The method of claim 10, wherein the mixture is heated at a temperature of 700K- 950K for at least 15 minutes and up to one hour.

15. The method of claim 10, wherein the mixture is heated at a temperature of 700K- 95 OK in dry air.

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