Pre-treatment of metal oxide catalysts for alkane dehydrogenation

Abstract

Catalytic dehydrogenation of light alkane gases over metal oxide catalysts is achieved by (a) pretreating the metal oxide catalyst with dimethyl ether (DME); and (b) catalytically reacting the alkane gas over the catalyst in a dehydrogenation reaction under conditions wherein the pretreatment increases the yield of reaction products.

Description

Technical Field

[0001] This invention relates to a method for the catalytic dehydrogenation of light alkane gas on a metal oxide catalyst. Background Technology

[0002] Dehydrogenation of light alkanes has been shown to occur on abundant metal oxides on Earth. ZrO2 catalysts have been shown to catalyze propane dehydrogenation (PDH) and exhibit a yield of approximately 5 mol kg⁻¹ at 823 K. -1 h -1 The initial dehydrogenation activity, in the absence of co-feed H2, increased to approximately 11 mol kg after 7 h of operation. -1 h -1 (40 kPa C3H8 in N2) [1]. CO pretreatment with ZrO2 at 823 K (57 kPa) for 0.5 h resulted in a sevenfold increase in the PDH rate (823 K, 40 kPa C3H8 in N2). From ambient temperature to 1173 K (1.6 Ks) -1 CO temperature-programmed reduction (CO-TPR, 1 kPa CO) showed significant CO consumption between 723 K and 1173 K [1], a temperature range known to result in water-gas shift reactions [2]; this treatment removes strongly adsorbed surface water in its dissociated state [3, 4]. Despite this, previous studies have attributed these effects of heat treatment (in H2 or CO) on reactivity to the few coordinatingly unsaturated Zr sites formed during such treatment (although the enthalpy of formation of O- vacancies on ZrO2 is >500 kJ mol). -1 [1]). The measured activation energy barrier (>130 kJ mol) -1 It is also much higher than the activation energy barriers derived from the theory of ZrO2 surfaces with O vacancies (<30 kJ mol). -1 [1] Summary of the Invention

[0003] The inconsistency between the aforementioned statements and current literature led us to explore the targeted treatment of ZrO2 catalysts with propylene (0.5 kPa) or dimethyl ether (DME, 1–10 kPa); these treatments, after propylene (0.5 kPa, 723 K, 1.8 ks) and DME treatment (1–10 kPa, 413–823 K, 0.06–3.6 ks), resulted in a 2-fold and a 40-fold increase in PDH rates (723 kPa, 13.7 kPa propane, and 12.3 kPa H2), respectively. The maximum rates measured after DME pretreatment at 723 K, 13.7 kPa propane, and in the presence of H2 (12.3 kPa) were comparable to those initial rates previously measured at 823 K, 40 kPa propane, and in the absence of H2 [1]. Pretreatment generates Zr-O site pairs and active sites. Propylene formation from any DME-derived carbon deposit and the stoichiometric reaction between propane is not significant, as the carbon deposited during DME treatment, measured via post-reaction oxidation, is approximately 10 times less than that required for the stoichiometric propylene formation reaction. The increased rate from olefins / DME likely stems from the dehydroxylation / decarboxylation reaction of the ZrO2 catalyst via olefins / DME with H2O / CO2, which occurs at temperatures much lower than those required for treatments in oxidation [1], reduction [1], or inert environments [1], thus allowing for the retention of a higher reaction surface area by preventing sintering prevalent during treatments at such higher temperatures. The method described herein provides a novel approach for preparing high-surface-area oxides free from bound H2O or CO2 by avoiding the requirement for stringent thermal treatment of oxide surfaces for dehydroxylation / decarboxylation (i.e., treatment without olefins / ethers) [6].

[0004] In one embodiment, the present invention provides a method for catalytic dehydrogenation of light alkane gases (e.g., ethane, propane, n-butane, isobutane, pentane) on a metal oxide catalyst (e.g., ZrO2, TiO2, Al2O3), the method comprising: (a) pretreating the metal oxide catalyst with dimethyl ether (DME); and (b) reacting the alkane gas on the catalyst in a dehydrogenation reaction, wherein the pretreatment is performed to improve the product yield of the dehydrogenation reaction.

[0005] In the case of the metal oxides of this invention, catalyst pretreatment should be understood as any process that contacts the catalyst with chemicals, combinations of chemicals, or series of chemicals to achieve or restore it to a higher activity and / or selectivity state before the catalyst is used in the intended chemical process or at an intermediate time point during catalyst use, as exemplified by periodic DME treatment, which restores initial activity and includes stopping the PDH reaction and performing DME treatment again and returning to the PDH feed. Such pretreatment can be performed inside or outside the chemical reactor. When used at an intermediate point in the catalyst use process, such pretreatment attempts to restore all or part of the catalyst's activity and / or selectivity in a scheme that would be considered by those skilled in the art as catalyst regeneration treatment. When initially used when the catalyst is loaded into the reactor, they can be referred to as activation or selective activation schemes.

[0006] In the implementation plan:

[0007] Pretreatment was carried out at temperatures up to 900 K;

[0008] Pretreatment was carried out at temperatures up to 873 K;

[0009] Pretreatment was carried out at temperatures up to 823 K;

[0010] Pretreatment was carried out at temperatures up to 723 K;

[0011] Pretreatment was carried out at temperatures ranging from 323 to 900 K;

[0012] Pretreatment was carried out at temperatures ranging from 323 to 873 K;

[0013] Pretreatment was carried out at temperatures ranging from 323 to 823 K.

[0014] Pretreatment was carried out at temperatures ranging from 323 to 723 K.

[0015] The alkane is propane;

[0016] The metal oxide is ZrO2

[0017] Pretreatment increases product yield by at least 2 times compared to comparable reactions without a pretreatment step;

[0018] The reaction product is an olefin; and / or

[0019] The pretreatment steps also include oxygen pretreatment before or after DME pretreatment.

[0020] This invention includes all combinations of the specific embodiments described herein, as if each combination had been painstakingly described. Attached Figure Description

[0021] Figure 1 From 323 K to 873 K (0.167 Ks) -1 The molar rate of H2O desorption as a function of temperature during temperature-programmed oxidation (TPO) of ZrO2 catalyst (0.2 g, treated with He at 873 K and H2 at 25 kPa) prepared from 873 K to 1173 K.

[0022] Figure 2 The area PDH rate (r) during propane dehydrogenation on a ZrO2 catalyst (13.7 kPa C3H8, 12.3 kPa H2, 723 K, I723-O7.2) f,d The colors indicate the treatments used before the catalytic reaction (magenta: O2; dark yellow: propylene); the dashed lines indicate the stable activity measured on ZrO2 at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K before any subsequent treatments (multiple subsequent treatments).

[0023] Figure 3a -c. a) During the propane dehydrogenation reaction on a ZrO2 catalyst (13.7 kPa C3H8, 12.3 kPa H2, 723 K, I723-O7.2), PDH(r f,d (solid symbol) and C1 formation (r) C1 a) the area rate, and b) the C1 to C2 ratio during propane dehydrogenation on ZrO2 catalyst (13.7 kPa C3H8, 12.3 kPa H2, 723 K, I723-O7.2); colors indicate treatments seen before the reaction (magenta: O2; brown: DME); the magenta dashed line indicates the stable activity measured on ZrO2 at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K before any subsequent treatment (multiple subsequent treatments). Figure 2 c) Arrhenius plots of non-oxidizing PDH rates per unit mass based on Cr (square symbol, 40 kPa C3H8), Ru (triangle symbol, 40 kPa C3H8), Pt (diamond symbol, 100 kPa C3H8), and ZrO2 (black circle, 40 kPa C3H8) as reported in the literature; yellow indicates ZrO2 pretreatment in a 57 kPa CO catalyst, and commercial ZrO2 without DME treatment (black circle, 13.7 kPa C3H8) measured in this study, and ZrO2 with DME treatment in this study (brown circle, 13.7 kPa C3H8), where hollow and solid symbols represent 0 and 12.3 kPa H2 co-feeds, respectively.

[0024] Figure 4a -c. a) Propane dehydrogenation rate per unit initial catalyst surface area (r) during propane dehydrogenation on ZrO2 catalyst from 20 ks to 55 ks. f,d (solid symbol) and C1 formation rate (r C1 a) C1 to C2 ratio during propane dehydrogenation on ZrO2 catalyst from 20 ks to 55 ks (13.7 kPa C3H8, 12.3 kPa H2, 723 K, I723-O7.2); c) time-averaged C1 to C2 ratio and deactivation rate constant plotted as a function of time period. Figure 4a The magenta dashed line in the figure represents the stable activity measured on ZrO2 at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K prior to any subsequent treatment (multiple subsequent treatments).

[0025] Figure 5a -c. The area PDH (r) measured after the first DME treatment (i.e., T723-DME1.8) during propane dehydrogenation on a) ZrO2 catalyst (I723-O7.2) at 723 K. f,d (solid symbol) and C1 formation rate (r C1 (a) Hollow symbol); b) Area PDH (r) measured after a second DME treatment (i.e., T723-DME1.8(2)) during propane dehydrogenation on a ZrO2 catalyst (I723-O7.2) at 723 K. f,d (solid symbol) and C1 formation rate (r C1 (c) Area PDH (r) measured after the third DME treatment (i.e., T723-DME0.3(1), brown) and the oxidation treatment (i.e., T723-O3.6(4), magenta) during propane dehydrogenation on ZrO2 catalyst (I723-O7.2) at 723 K. f,d (solid symbol) and C1 formation rate (r C1 Hollow symbol); circle symbol (13.7 kPa C3H8, 12.3 kPa H2), star symbol (13.7 kPa C3H8, 0 kPa H2), triangle symbol (0 kPa C3H8, 12.3 kPa H2); Figures 5a-5c The magenta dashed line in the figure represents the stable activity measured on ZrO2 at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K prior to any subsequent treatment (multiple subsequent treatments).

[0026] Figure 6The C1 to C2 ratio during propane dehydrogenation on ZrO2 catalyst, measured after the first (i.e., T723-DME1.8(1), brown) and third (i.e., T723-DME0.3(1), black) DME treatments.

[0027] Figure 7 From 323 K to 723 K (0.03 Ks) -1 The ZrO2 catalyst (0.2 g, He-treated at 323 K) prepared by [the catalyst] was subjected to a 1 kPa dimethyl ether concentration (0.83 cm⁻¹). 3 s -1 The desorption molar rates of H2O (blue) and methanol (yellow) as a function of temperature during TPD (at 0.5 kPa Ar, balanced with He).

[0028] Figure 8a -b. a) Area PDH ratio (rc) measured on ZrO2 catalyst at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K after initial O2 pretreatment at 873 K (I873-O7, magenta), after 10 kPa DME treatment at 723 K (i.e., T823-DME0.3(x), x=1 or 2, brown), and after O2 treatment at 723 K (T723-O3.6(1), magenta). f,d b) and d) are the area PDH ratios (r0) measured on ZrO2 catalysts at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K, after 10 kPa DME treatment at 723 K (i.e., T823-DME0.3(x), x=3 or 4, brown) and after O2 treatment at 873 K (T873-O3.6(y), y=1 or 2, magenta). f,d The magenta dashed lines in Figures 8 and 8b represent the stable activity measured on ZrO2 at 13.7 kPa C3H8, 12.3 kPa H2, and 723 K, exactly after pretreatment in O2 for 7.2 ks (I723-O7.2) at 723 K. Figure 2 As shown.

[0029] Figure 9 The PDH rate constants, plotted as the reciprocal of temperature (13.7 kPa and 12.3 kPa inlet propane and H2 pressures), were obtained on prepared ZrO2 (1 kPa DME, 0.3 ks) under DME treatment. These are measurements of the PDH area rate normalized to the mean propane pressure.

[0030] Figure 10a-c. Area PDH rate (r0.06) measured on ZrO2 catalyst (I723-O7.2) after DME (10 kPa) treatment at 823 K (i.e., T823-DME0.06, brown) and O2 treatment at 823 K (i.e., T823-O3.6(1), magenta), at 13.7 kPa C3H8, 12.3 kPa (circles) or 0 kPa (squares) H2 and 823 K. f,d b) After O2 treatment at 723 K (i.e., T723-O3.6(5), magenta) and DME treatment at 823 K (1 kPa DME, T823-DME1kPa0.06(x), x=1-3, purple), the area PDH rate (r) was measured at 13.7 kPa C3H8, 12.3 kPa H2 and 823 K. f,d The area PDH rate (r) measured at 13.7 kPa C3H8, 12.3 kPa H2, and 823 K after O2 treatment at 823 K (i.e., T823-O3.6(y), y = 2 or 3, magenta) and DME treatment at 573 K (1 kPa DME, T573-DME1kPa0.06 or T573-DME1kPa0.6, blue) f,d ). Detailed Implementation

[0031] Unless otherwise specified or limited, the terms “a” and “an” mean one or more, and the term “or” means and / or, throughout these descriptions and the specification. It should be understood that the embodiments and implementations described herein are for illustrative purposes only, and various modifications or variations based on these embodiments and implementations are to be suggested to those skilled in the art, and such modifications or variations are to be included within the spirit and scope of this application and the appended claims. All publications, patents, and patent applications cited herein, including their citations, are incorporated herein by reference in their entirety for all purposes.

[0032] In these embodiments, we demonstrate a strategy for removing irreversible titrants such as H₂O and CO₂ bound to zirconium oxide (ZrO₂) due to the synthetic scheme used to form the catalyst or during subsequent exposure to ambient air; this removal ultimately increases the activity of ZrO₂ during propane dehydrogenation (PDH) reactions. ZrO₂ catalysts bind H₂O and CO₂ titrants from the air; as our temperature-programmed experiments show, these substances cannot be completely removed even at temperatures above 1173 K. It has also been shown that the required high-temperature heat treatment blocks the ZrO₂ catalyst and reduces the accessible surface area, resulting in lower dehydrogenation reactivity on ZrO₂ after such high-temperature treatment, despite the removal of these strong surface titrants. Unbound by any particular theory, the reaction of alkenes (e.g., propylene) and ethers (e.g., dimethyl ether) with H₂O / CO₂ allows for the removal of surface H₂O / CO₂ titrants, thereby releasing exposed Zr-O sites for alkane dehydrogenation reactions without disrupting the porous structure and active exposed surface area associated with high-temperature treatment. Propane dehydrogenation (PDH) is used here as an example, but is generally suitable for any reaction catalyzed by stoichiometric Zr-O site pairs. At 723 K, the activity of the ZrO₂ catalyst increased by approximately 2-fold and 40-fold, respectively, after treatment with DME (1–10 kPa) at 723 K for 1.8 ks. The observed promotion of the PDH rate did not originate from the decomposition of carbon deposits or a stoichiometric reaction between carbon deposits and propane, because propylene formation was immediately inhibited when the propane feed was shut off, and the carbon deposited on ZrO₂ via DME treatment was >10 times less than the carbon required for the stoichiometric propylene formation reaction. Specifically, DME treatment of the ZrO2 catalyst increased its activity to 5 mol kg⁻¹ in propane at 723 K and 13.7 kPa. -1 h -1 (with 12.3 kPa H2) and 10 mol kg -1 h -1 (No H2), which is comparable to the value previously obtained for the ZrO2 catalyst at 823 K (7 mol kg). -1 h -1 No H2 was present at 40 kPa propane[1]. After DME treatment, the PDH reactivity measured at 823 K and 13.7 kPa propane became 28 mol kg. -1 h -1(With H2 at 12.3 kPa), this is more than twice as high as that of Pt and Cr-based catalysts, even though the latter were measured at 823 K under higher propane pressure (40 kPa) and in the absence of H2 inhibitors. Temperature-programmed reaction studies using DME (1 kPa) confirmed the successful removal of surface hydroxyl groups via DME hydration, as methanol, a hydration product of DME, continues to be generated at temperatures above 520 ± 5 K. The PDH activation barrier measured on the DME-treated ZrO2 catalyst was 92 kJ mol. -1 It is far lower than the value reported in the literature (> 130 kJ mol). -1 [1], but the value calculated theoretically (106 kJ mol) -1 These findings confirm that the PDH reaction is catalytically carried out on a stoichiometric ZrO2 surface, which is exposed with the aid of olefin / ether pretreatment. The current strategy results in reproducible PDH reactivity after treatment with or without oxidation-treated DME. Our method can be applied to other metal oxide catalysts (e.g., TiO2, Al2O3) to remove H2O / CO2 as site titrants, thereby exposing their bare surfaces for catalytic reactions without the risk of sintering and loss of active surface area.

[0033] Catalyst synthesis and catalytic rate and selectivity measurement:

[0034] ZrO2 material was prepared using a hydrothermal scheme previously described in reference [1], involving mixing an aqueous solution of ZrO(NO3)2·xH2O (12.3 g in 30 ml of deionized water) and urea (21.6 g in 30 ml of deionized water), followed by urea hydrolysis, pH increase, and crystallization of ZrO2 powder (453 K, 20 h), which was then dried overnight in ambient air at 383 K. The ZrO2 material was further prepared by heating to 723 K (at 0.167 Ks). -1 (Down), keep for 2 hours, then use flowing He (2 cm) 3 g -1 s -1 UHP, Praxair) purging to remove residual O2 from the reactor, in a flowing mixture of O2 and He (2 cm) 3 g -1 s -1The synthesized catalyst was treated with 4% O2 (Praxair) equilibrated with He. C3H8 (50% C3H8, 10% Ar internal standard, equilibrated with He, Praxair) and hydrogen (99.999%, Praxair) were introduced into the reactor at 723 K with a He stream (UHP, Praxair). The effluent stream was analyzed by online mass spectrometry (MS, MKS) and gas chromatography (GC; Agilent 6890A) after separation (GS-GASPRO column, Agilent).

[0035] ZrO2 is oxidized to 1173K using a temperature program.

[0036] At 4 kPa O2 (equilibrated with He, 3.35 cm) 3 g -1 s -1 ZrO2 was prepared by treatment in He and in H2 / He at 873 K prior to temperature-programmed oxidation (TPO) in [the study]. The temperature was maintained at 0.167 K s. -1 The concentration was increased from 323 K to 873 K in 4 kPa O2 and held for 7.2 ks, then decreased by 0.167 K s. -1 The temperature was eventually increased from 873 K to 1173 K. After inert and reducing treatment (i.e., 25 kPa H2 in He), H2O continued to be generated from the sample, indicating that H2O molecules strongly titrate the Zr-O Lewis acid-base pair, consistent with previous reports [4, 7]. In fact, raising the temperature to 1173 K did not remove all the bound water, as water continued to be generated. Previous reports have shown that high-temperature surface annealing removes surface hydroxyl groups, but it also leads to pore collapse and surface area loss. Therefore, water [8] adsorbed freely on ZrO2 as surface hydroxyl groups titrates the Zr-O site pairs, especially those on the most active monoclinic and tetragonal ZrO2 surfaces.

[0037] Effect of propylene pretreatment at 723 K on propane dehydrogenation rate

[0038] In the following text, we denote the initial pretreatment of ZrO2 catalysts not exposed to gases other than air at ambient temperature as "I" and the treatment of ZrO2 previously exposed to gases other than air at ambient temperature as "T". The treatment temperature and duration are denoted as t (t = 723-873 K) and δ (δ = 0-7.2 ks), respectively. The treatment gas conditions are denoted as α (α = O, py, DME, abbreviations for O2, propylene, and dimethyl ether (DME), respectively); unless otherwise specified, the partial pressures of O2, propylene, and DME used in these treatments are 4 kPa, 0.5 kPa, and 10 kPa, respectively. The number of times the catalyst undergoes the same treatment is denoted as "(i)" (i = 1-5). Thus, It-αδ and Tt-αδ(i) represent the initial pretreatment in α gas at temperature t for duration δ and the treatment repeated under the same conditions but for the i-th time. As an example, T723-O3.6(2) represents a second O2 treatment at 723 K for 3.6 ks. Table 1 summarizes the treatment conditions applied in the following discussion.

[0039]

[0040] The forward rate r of propane dehydrogenation f,d Defined as

[0041] (1a)

[0042] in (1b)

[0043] in , , and The values ​​represent the measured net rate, near-equilibrium, reaction quotient, and equilibrium constant of the PDH reaction at the target temperature, respectively. Propane dehydrogenation occurred on a ZrO2 catalyst (pretreated at 723 K for 2 h in a 4% O2 / He mixture, denoted as I723-O7.2), with an initial area rate of 3.7 μmol m⁻¹. -2 h -1 The rate decayed rapidly, stabilizing at 2.5 μmol m after approximately 1 ks. -2 h -1 ,like Figure 2 As shown. Propylene treatment (0.5 kPa, equilibrated with He, denoted as T723-py1.8 at 723 K) intended to remove surface-bound H2O / CO2 resulted in a two-fold increase in the initial area PDH rate (i.e., from 3.7 μmol m). -2 h -1 Up to 5.5 μmol m -2 h -1Intentional removal of propane (13.7 kPa to 0 kPa) from the inlet stream (5.6 ks to 6.7 ks time-on-stream) immediately inhibited propylene formation, indicating that the enhanced reactivity conferred by propylene treatment reflects the actual reaction between the active sites and propane, rather than the stoichiometric decomposition of any potential carbonaceous residues formed during propylene treatment into propylene. We hypothesize that propylene reacts with H2O / CO2 to release Zr-O site pairs. As an example, propylene can react with and remove strongly bound H2O to form propanol, a hydration product of propylene.

[0044]

[0045] The rate rapidly decreased to 2.8 μmol m within 3 ks. -2 h -1 This is comparable to the value derived from pristine ZrO2 (I723-O7.2) without propylene treatment. Treatment in O2 for 3.6 ks (denoted as T723-O3.6(1)) restores the steady-state rate to 2.6 μmol m -2 h -1 ,like Figure 2 As shown.

[0046] Effect of dimethyl ether (DME) treatment at 723 K on propane dehydrogenation rate

[0047] Following oxidation, the catalyst was exposed to DME (10 kPa, 723 K, for 1.8 ks; T723-DME1.8(1)). This treatment resulted in 110 μmol m -2 h -1 The area PDH rate, which is higher than that of O2 treatment (i.e., I723-O7.2 and T723-O3.6 (1), 2.6 μmol m -2 h -1 Those observed after treatment were approximately 40 times higher. DME treatment also increased C1 (i.e., methane, at a rate expressed as r). C1 C1 and C2 (i.e., ethylene and ethane, the rate is expressed as r) C2 The formation rate of ). For example, Figure 3a As shown, compared with the sample without DME treatment (0.06 μmol m -2 h -1 Compared to [previous data], the surface methane formation rate increased by approximately 280 times, reaching 18 μmol m. -2 h -1 The C1 / C2 molar ratio is close to one ( Figure 3bThis indicates that methane is formed from propane via hydrogenolysis and cracking, as shown in equations 3a and 3b, respectively, rather than via stoichiometric consumption of any DME-derived organic residues.

[0048]

[0049] The removal of H2 co-feed (14.5 ks to 15.1 ks) resulted in a further twofold increase in the PDH rate (to 246 μmol / m³). -2 h -1 This reflects the kinetic inhibition of the rate by H2 on the PDH reaction on ZrO2 previously shown at 823–873 K[5]. Figure 3c The dehydrogenation rates (normalized by mass) on the ZrO2 catalyst after DME treatment, as well as on previously reported catalysts based on Cr [9], Pt [10-12], Ru [9] and ZrO2 [1], are shown. Initial PDH rates (10 mol kg⁻¹) after treatment with T723-DME1.8(1) (13.7 kPa C3H8, no H2 added at 723 K) are shown. -1 h -1 Similar to those reported at much higher temperatures (40-100 kPa, Pt: 14.5 molkg) on ​​other catalysts at 823-828 K and even higher C3H8 pressures. -1 h -1 Cr: 13 mol kg -1 h -1 Ru: 10.7 mol kg -1 h -1 ZrO2: 7.1 mol kg -1 h -1 ).

[0050] The increased rate of H2 removal also led to a two-fold decrease in the Cl formation rate. Figure 3a The C1 / C2 ratio increased from approximately one to approximately 1.5. Figure 3b The reasons for these trends remain unclear, as H2 is reported to preferentially remove reaction-derived organic residues, such as CH4, from zeolite solid acids, which suggests an alternative dehydrogenation route for such residues

[13] .

[0051] The reintroduction of H2 (12.3 kPa) restored the dehydrogenation and C1 (and C2) formation rates to those measured before H2 was removed from the inlet stream. Figure 3a The C1 / C2 molar ratio returned to near one and increased slightly with operating time (from 0.97 to 1.07), while the dehydrogenation and C1 formation rates decreased slightly in a manner consistent with the first-order deactivation process.

[0052]

[0053] Here, r i , and t m Let i represent the rate (i = f, d and C1 respectively for the dehydrogenation rate and the C1 rate), the deactivation rate constant of reaction i, and the running time m at any time. and The values ​​are 4.1 × 10 -2 ks -1 and 3.5×10 -1 ks -1 Compared to the dehydrogenation rate, the C1 formation rate decreases more significantly with operating time, leading to a corresponding increase in dehydrogenation selectivity (i.e., the instantaneous selectivity ratio). During the period from 11 to 19 ks, the number of cases increased from 6 to 72.

[0054] After 20 ks (20-40 ks) of operation, the PDH and methane formation rates continued to decrease as a function of operating time, although methane formation was more rapid. Figure 4a The PDH rate eventually asymptotically approaches values ​​comparable to those measured without DME / propylene treatment. When stoichiometric cracking and hydrogenolysis (in Equations 3a and 3b, respectively) are considered, excess methane formation results in a C1 / C2 molar ratio greater than one at 20–40 ks of operation. This excess methane indicates that carbonaceous deposits are catalytically removed in the form of methane by Zr-O sites, as previously observed in propane dehydrogenation over zeolites under similar conditions

[13] . These carbonaceous deposits likely originate from a combination of olefins and deep dehydrogenation products, representing the amount of olefin products that do not escape the catalyst surface but accumulate in a semi-intermittent manner. After 40 ks of operation, the deactivation rate decreases, and the C1 / C2 molar ratio becomes less than one ( Figure 4b The reason is still unclear.

[0055] We define the time intervals of operation as: 0) 11-19 ks, 1) 20-25 ks, 2) 25-30 ks, 3) 30-35 ks, 4) 35-40 ks, 5) 40-45 ks, and 6) 45-51 ks. Figure 4c Average within each time period The molar ratio of C1 to C2 is plotted as a function of the corresponding time period. From Figure 4cIt can be seen that the deactivation rate constant increases and decreases almost proportionally (except in time period 4) with the value of the C1 to C2 molar ratio. These observations lead us to speculate that the carbonaceous sediment titration of the Zr-O site pair is the cause of operational deactivation. The deactivation rate constant and the C1 to C2 molar ratio observed in time periods 0) to 3) also increase with the initial increase in the amount of carbonaceous sediment. Then, with the titration of the Zr-O site pair in catalytic PDH, propane cracking / hydrogenation, and carbonaceous sediment hydrogenolysis, the PDH rate, The molar ratio of C1 to C2 decreased, approaching a steady-state value during the time period from 4) to 6). The PDH rate eventually decreased to the activity measured after the initial oxidation treatment, indicating that the carbonaceous deposits did not titrate all ZrO2 sites.

[0056] In summary, these results suggest that olefin / DME treatment may lead to an increased rate via a reaction between olefin / DME and H2O / CO2. The decreased rate after DME treatment may reflect the gradual accumulation of carbonaceous deposits.

[0057] Effect of subsequent dimethyl ether (DME) treatment at 723 K on propane dehydrogenation rate

[0058] After 3.6 ks at 4% O2 / He (T723-O3.6(2)), the repeatability of the DME treatment was examined by subsequent treatment (DME, 10 kPa, for 1.8 ks, T723-DME1.8(2)). Figure 5b and Figure 5c The PDH area rate and Cl formation rate measured after T723-DME1.8 (2) and T723-DME0.3 (1) are shown respectively; those measured after the first such treatment are shown in Figure 5a (also displayed in the middle) Figure 4a The T723-O3.6(2) and T723-DME1.8(2) schemes resulted in similar initial rates (after the first and second treatments, the dehydrogenation and C1 rates were 130 μmol m). -2 h -1 and 27 μmol m -2 h -1 For 110 μmolm -2 h -1 and 18 μmol m -2 h -1Removing propane from the inlet stream (57.2–59.6 ks) immediately inhibited the formation of all propylene, but not all C1, consistent with the presence and gradual removal of reaction-derived organic residues such as C1. Restoring the propane flow rate (from 0 kPa to 13.7 kPa) restored the propane dehydrogenation rate (continuing the previous rate trend, increasing from 0 to 75 μmol m). -2 h -1 The methane formation rate (continuing the previous rate trend, increasing from 0 to 1.2 μmol m) and the methane formation rate (continuing the previous rate trend, increasing from 0 to 1.2 μmol m) -2 h -1 ).

[0059] Repeated O2 treatment for 3.6 ks (T723-O3.6(3)) followed by a shorter duration DME treatment (10 kPa) of 0.3 ks (T723-DME0.3(1)) resulted in 189 μmol m -2 h -1 and 38 μmol m -2 h -1 The initial PDH and methane area rates, such as Figure 5c As shown, this is even higher than those obtained after previous DME treatments of longer durations (i.e., T723-DME1.8(1) and T723-DME1.8(2)). This trend and observation may reflect the hydrodynamic delay at the introduction of reactants, resulting in initial rate measurements that do not reflect the expected steady-state propane and H2 pressures. Compared to those after DME treatment for 1.8 ks, the catalyst deactivation reflected by the decrease in PDH area rate was less significant after DME treatment for 0.3 ks. Figure 6 The C1 / C2 ratios measured after T723-DME1.8(1) and T723-DME0.3(1) were plotted as a function of operating time. Although the C1 / C2 molar ratios started at the beginning in both cases, they were reached much more slowly and to a lesser extent after T723-DME0.3(1) treatment than after T723-DME1.8(1) treatment, indicating that the accumulation of carbonaceous deposits was less significant in the T723-DME0.3(1) treatment, resulting in a longer catalyst lifetime. The effects of DME partial pressure and treatment duration on PDH rate and catalyst lifetime will be part of our future research.

[0060] Following T723-DME0.3 (1) and PDH rate measurements, the ZrO2 catalyst was regenerated via oxidation treatment (T723-O3.6 (4)). This was compared to the initial oxidation treatment (T723-O7.2, 2.6 μmol m -2 h -1Compared to those steady-state rates measured later, the propane dehydrogenation rate showed 2.2 μmol m -2 h -1 A slightly lower area rate.

[0061] The CO2 generated during the oxidation processes (i.e., T723-O3.6(2), T723-O3.6(3), and T723-O3.6(4)) provides quantitative information about the cumulative amount of reaction-derived organic residues formed and remaining on the catalyst by (i) DME treatment and (ii) propane dehydrogenation. Table 2 shows the amount of CO2 generated during each O2 treatment. 2.5 × 10⁻⁶ ks of CO2 was deposited on the ZrO2 catalyst during the DME treatment (0.3–1.8 ks) in conjunction with the propane dehydrogenation reaction. -4 mol (18 C nm) -2 ) to 2.8×10 -4 mol (21 Cnm) -2 The carbon in the solution is directly treated with DME for 0.3 ks (i.e., T723-DME0.3(2)), without propane dehydrogenation, resulting in the formation of 1.2 × 10⁻⁶ carbon atoms. -4 mol of carbon (9 C nm) -2 ).

[0062]

[0063] Equation 5 defines the molar amount of excess propylene (χ) formed by the catalyst after DME treatment:

[0064] (5)

[0065] in , and The values ​​are the forward dehydrogenation rate after tDME treatment at any time during operation, the steady-state forward dehydrogenation rate after O2 pretreatment (i.e., I723-O7.2), and the catalyst surface area, respectively. For T723-DME1.8(1), χ is 3.5 × 10⁻⁶. -3 mol (i.e., 263 nm) -2 The carbon content is more than an order of magnitude higher than that deposited by DME treatment (Table 2, Item 4). If the reaction between the carbonaceous deposit and propane occurs via a stoichiometric reaction, the H atoms derived from propane dehydrogenation are sufficient to destroy all the deposited carbon from DME treatment (i.e., 9 nm). -2 Hydrogenation to CH4, excess 6.0 × 10⁻⁶ -3 mol. Therefore, we conclude that propane dehydrogenation occurs catalytically on the ZrO2 surface.

[0066] The impact of dimethyl ether (DME) treatment on water removal, as revealed by temperature-programmed desorption (TPD).

[0067] Prior to TPD, the prepared ZrO2 was treated in He at 323 K for 7.2 ks. Then, it was subjected to DME at 1 kPa (equilibrated with He, 0.83 cm⁻¹). 3 s -1 In this context, the temperature is 0.03 K s. -1 The temperature was increased from 323 K to 723 K. The H₂O and methanol precipitation curves plotted relative to the temperature are shown below. Figure 7 As shown, water desorption occurs at 373 K and 523 K with increasing temperature, while methanol formation begins at 520 K. The formation of water indicates that the stoichiometric reaction between DME and surface hydroxyl groups occurs via the following:

[0068]

[0069] Methanol precipitation clearly demonstrates the successful removal of water via the DME hydration reaction. This water removal exposes stoichiometric Zr-O site pairs, as shown in Equation 6. Figure 1 Conversely, both the methanol and water signals decreased below the detection limit at 723 K, reflecting the complete removal of water (and surface hydroxyl groups) via DME hydration at 723 K.

[0070] The reasons for the increased rate from DME treatment and the apparent barrier measurement of PDH rate on DME-cleaned ZrO2 surfaces

[0071] As shown in Figure 8, the catalyst (I723-O7.2) was pretreated in O2 at 723 K for the same duration. Figure 1 2.6 μmol m -2 h -1 In comparison, heat treatment of the newly supported ZrO2 catalyst at 873 K for 7.2 ks (I873-O7.2) (i.e., treatment in the absence of olefins / ethers) resulted in an approximately 10-fold increase in initial propane dehydrogenation, reaching 26 μmol m. -2 h -1 Subsequently, a DME treatment at 723 K for 0.3 ks (T723-DME0.3(1)) increased the rate to 94 μmol m. -2 h -1 This is slightly smaller than that in T723-DME1.8(1)(110 μmol m -2 h -1 , Figure 2a) Rates measured in the previous loading of the sample after pretreatment at 723 K. We hypothesize that high-temperature heat treatment can generate more active Zr-O site pairs via water / CO2 removal, such as... Figure 1 As clearly shown, this high-temperature treatment simultaneously collapses the porous structure of ZrO2 and leads to a reduction in surface area, as previously reported for ZrO2 [6]. As a result, for the newly supported catalyst (Fig. 8, initial oxidation treatment at 873 K), the PDH area rate after DME treatment at 723 K is lower than that for the previously supported catalyst ( Figure 2 The area rates of those PDHs were measured (after initial oxidation treatment at 723 K).

[0072] For the new catalyst support (i.e., I873-O7.2), subsequent O2 treatment at 723 K (T723-O3.6(1)) resulted in a decrease in the PDH rate (13.7 kPa propane, 12.3 kPa H2, 723 K) to 2.5 μmol m -2 h -1 This is consistent with the previous initial O2 pretreatment at 723 K (i.e., 2.6 μmol m -2 h -1 I723-O7.2 Figure 1 The rates measured afterward were comparable. This is because the oxidation process removes carbon deposits derived from previous DME treatments and / or PDH rate measurements as H2O and CO2. Figure 8b The results showed that (i) the H2O and / or CO2 generated during the oxidation treatment (T723-O3.6) titrated Zr-O sites, and (ii) the oxidation treatment at 873 K released more empty Zr-O site pairs than the oxidation treatment at 723 K. However, a subsequent DME treatment of 0.3 ks (T723-DME0.3(2)) restored the propane dehydrogenation activity to the same level as the previous DME treatment (T723-DME0.3(1)), independent of the previously measured PDH activity after the oxidation treatment. This observation further confirms that DME can regenerate Zr-O site pairs in a repeatable manner.

[0073] Attempts were made to restore Zr-O site pairs by oxidation treatment with He at 873 K for 3.6 ks (T873-O3.6(1)) or overnight (T873-O3.6(2)), resulting in 10 μmol m -2 h -1 and 17 μmol m -2 h -1 The area PDH rate. These oxidation treatments at 873 K did not fully recover the initial oxidation rate (i.e., 26 μmol m -2h -1 The PDH rate (after I873-O7.2) was measured. We hypothesize that the degree of regeneration of Zr-O site pairs via oxidation and subsequent He treatment at 873 K depends on (i) the amount of carbonaceous residue formed by previous treatments (multiple previous treatments) and the PDH rate measurement, which determines the amount of CO2 and H2O formed during the oxidation treatment, and (ii) the duration of catalyst residence in He. The inconsistent rates measured after oxidation treatment reflect the amount of inconsistency in the regenerated Zr-O sites after these treatments. However, DME treatment at 723 K (T723-DME0.3(i), i = 1–4) always resulted in approximately 90 μmol m -2 h -1 The reproducible reactivity was observed, regardless of the PDH rate measured after the preceding oxidation treatment. These observations further confirm that DME can remove the irreversible H2O and CO2 titrant generated during the oxidation process and expose the ZrO2 surface to the same extent.

[0074] We further confirmed the effectiveness of DME treatment by measuring the apparent PDH barrier on the DME-treated ZrO2 catalyst. PDH rates were measured from 723 K to 873 K at 13.7 kPa propane pressure and 12.3 kPa H2 pressure. The PDH rate constant obtained by normalizing the measured PDH rates to the arithmetic mean of the inlet and outlet propane pressures was plotted in an Arrhenius diagram. Figure 9 This measurement is more than 40 kJ mol smaller than the previously reported values ​​on ZrO2 catalysts [1]. -1 However, it seems to correspond to the potential barrier (106 kJ / mol) of PDH on a stoichiometric monoclinic ZrO2(-111) surface derived from density functional theory (DFT-D3) calculations. -1 The values ​​are comparable. We suspect that previous measurements without DME treatment could not expose Zr-O site pairs at lower temperatures (e.g., <823 K). Therefore, these previous reports underestimated PDH turnover to a greater extent at lower temperatures, thus overestimating the apparent PDH barrier. The consistency between the experimental and theoretically derived barriers confirms that the DME (and olefin) hydration reaction removes water bound to the Zr-O site pairs and exposes these site pairs as active sites for the PDH reaction.

[0075] The effect of dimethyl ether (DME) treatment on propane dehydrogenation rate at 823 K and operation in cyclic mode

[0076] A DME (10 kPa) treatment duration of 0.06 ks at 823 K (T823 - DME0.06) resulted in an initial PDH rate of 643 μmol m at 13.7 kPa propane, 12.3 kPa H2, and 823 K. -2 h -1 ,like Figure 10a As shown. The catalyst then rapidly deactivates, with a first-order deactivation rate constant of 2.2 × 10⁻⁶. -1 ks -1 Increasing the H2 partial pressure from 12.3 kPa to 50 kPa resulted in an increase in the rate from 218 μmol / m³. -2 h -1 Instantaneously decreased to 144 μmol m -2 h -1 ( Figure 10a This is consistent with the H2 inhibition observed at 723 K ( Figure 3a After oxidation treatment at 823 K for 3.6 ks (T823-O3.6(1)), the propane dehydrogenation rate increased to 168 μmol m. -2 h -1 Then, the PDH rate gradually increased to a maximum of 375 μmol m -2 h -1 Then, it decayed to 318 μmol m over time. -2 h -1 Oxidation at 723 K for 3.6 ks (T723-O3.6(5)) resulted in a PDH rate of approximately 220 μmol m at 823 K. -2 h -1 ,like Figure 10b As shown. The PDH rate is comparable to the initial rate obtained after T823-O3.6(1) (168 μmol m -2 h -1 , Figure 10a However, it did not experience a similar transient after T823-O3.6(1). These inconsistent area PDH rates obtained after oxidation treatment suggest that the number of exposed Zr-O site pairs may vary depending on the previous treatment conditions or the duration of the PDH rate measurement.

[0077] Figure 10b The following three direct DME treatments, using lower DME pressures (1 kPa, denoted as T823DME1kPa(x), where x = 1, 2, or 3), each increased the PDH rate to approximately 420 μmol m -2 h -1After each DME treatment, the PDH rate rapidly decreased to approximately 300 μmol mJ within 1.5 ks. -2 h -1 ,like Figure 10b As shown; the first-order inactivation rate constant is approximately 2.2-2.5 × 10⁻⁶. -1 ks -1 These observations lead us to conclude that (i) the deactivation was partly mediated by water poisoning caused by ppm-level water in the reactant stream, and (ii) the DME treatment was highly reproducible, regenerating Zr-O site pairs even without oxidation. However, the PDH rate could not be restored to the rate measured after the first DME treatment at 823 K (i.e., 643 μmol m). -2 h -1 (T823-DME0.06)

[0078] Subsequent oxidation treatment (T823-O3.6(2)) resulted in 267 μmol m -2 h -1 The area PDH rate. Subsequent DME treatments at 573 K of 0.6 ks (T573-DME1kPa0.6) or 0.06 ks (T573-DME1kPa0.06) resulted in 611 μmol m² / g of PDH. -2 h -1 and 837 μmol m -2 h -1 The area PDH rate was comparable to (or even higher than) that obtained after T823-DME 0.06. These results indicate that a sustained DME treatment of 0.06 ks at 573 K and 1 kPa is effective and leads to the removal of H2O and CO2. Finally, an oxidation treatment of 3.6 ks at 823 K (T823-O3.6(3)) resulted in approximately 175 μmol / m³ of surface PDH. -2 h -1 The PDH rate. Again, oxidation treatments (e.g., T823-O3.6(1), T823-O3.6(2) and T823-O3.6(3)) do not reset the ZrO2 catalyst to a consistent starting point, but instead exhibit a memory effect.

[0079] In summary, DME treatment promotes PDH rate, but not significantly at 823 K; repeated DME treatment resets PDH rate in a consistent manner; DME treatment is able to remove water and regenerate active sites at temperatures as low as 573 K with low DME exposure (i.e., 1 kPa).

[0080] References

[0081] [1] Y. Zhang, Y. Zhao, T. Otroshchenko, H. Lund, M.-M. Pohl, U.Rodemerck, D. Linke, H. Jiao, G. Jiang, E.V. Kondratenko, Control ofcoordinatively unsaturated Zr sites in ZrO 2 for efficient C–H bondactivation, Nature communications, 9 (2018) 1-10.

[0082] [2] M.-Y. He, J.G. Ekerdt, Temperature-programmed studies of theadsorption of synthesis gas on zirconium dioxide, Journal of Catalysis, 87(1984) 238-254.

[0083] [3] J. Kondo, H. Abe, Y. Sakata, K.-i. Maruya, K. Domen, T. Onishi,Infrared studies of adsorbed species of H2, CO and CO 2 over ZrO 2, Journalof the Chemical Society, Faraday Transactions 1: Physical Chemistry inCondensed Phases, 84 (1988) 511-519.

[0084] [4] J. Kondo, Y. Sakata, K. Domen, K.-i. Maruya, T. Onishi, Infraredstudy of hydrogen adsorbed on ZrO2, Journal of the Chemical Society, FaradayTransactions, 86 (1990) 397-401.

[0085] [5] T.F.J.C.D.E. Iglesia, Stabilization of zirconium oxide catalystsfor paraffin dehydrogenation by co-feed hydrogen Unpublished results.

[0086] [6] S. Xie, E. Iglesia, A.T. Bell, Water-assisted tetragonal-to-monoclinic phase transformation of ZrO2 at low temperatures, Chemistry ofmaterials, 12 (2000) 2442-2447.

[0087] [7] M.-Y. He, J.G. Ekerdt, Methanol formation on zirconium dioxide,Journal of Catalysis, 90 (1984) 17-23.

[0088] [8] P. Lackner, J. Hulva, E.-M. Köck, W. Mayr-Schmölzer, J.I.J. Choi,S. Penner, U. Diebold, F. Mittendorfer, J. Redinger, B. Klötzer, Wateradsorption at zirconia: from the ZrO 2 (111) / Pt 3 Zr (0001) model system topowder samples, Journal of Materials Chemistry A, 6 (2018) 17587-17601.

[0089] [9] T. Otroshchenko, S. Sokolov, M. Stoyanova, V.A. Kondratenko, U.Rodemerck, D. Linke, E.V. Kondratenko, ZrO2‐Based Alternatives toConventional Propane Dehydrogenation Catalysts: Active Sites, Design, andPerformance, Angewandte Chemie International Edition, 54 (2015) 15880-15883.

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[10] P.L. De Cola, R. Gläser, J. Weitkamp, Non-oxidative propanedehydrogenation over Pt–Zn-containing zeolites, Applied Catalysis A: General,306 (2006) 85-97.

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[11] J.J. Sattler, I.D. Gonzalez‐Jimenez, L. Luo, B.A. Stears, A.Malek, D.G. Barton, B.A. Kilos, M.P. Kaminsky, T.W. Verhoeven, E.J. Koers,Platinum‐promoted Ga / Al2O3 as highly active, selective, and stable catalystfor the dehydrogenation of propane, Angewandte Chemie, 126 (2014) 9405-9410.

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Claims

1. A method for catalytically dehydrogenating light alkane gas on a metal oxide catalyst, the method comprising the following steps: (a) Pretreatment of a metal oxide catalyst with dimethyl ether (DME), wherein the pretreatment is performed by the DME reacting with H2O and CO2 on the surface of the metal oxide to remove H2O and CO2 as site titrants, thereby exposing the bare surface of the metal oxide for catalytic reaction. and (b) Under conditions in which pretreatment is performed to improve the yield of reaction products, the alkane gas is catalyzed in the dehydrogenation reaction on a pretreated catalyst. The metal oxides are selected from ZrO2, TiO2, and Al2O3. The pretreatment described therein is carried out at temperatures up to 900K, and The pretreatment increases the product yield by at least two times compared to comparable reactions without a pretreatment step.

2. The method of claim 1, wherein the pretreatment is performed at a temperature up to 873 K.

3. The method of claim 1, wherein the pretreatment is performed at a temperature up to 823 K.

4. The method of claim 1, wherein the pretreatment is performed at a temperature up to 723 K.

5. The method of claim 1, wherein the pretreatment is performed at a temperature in the range of 323-900K.

6. The method of claim 1, wherein the pretreatment is performed at a temperature in the range of 323-873 K.

7. The method of claim 1, wherein the pretreatment is performed at a temperature in the range of 323-823 K.

8. The method of claim 1, wherein the pretreatment is performed at a temperature in the range of 323-723 K.

9. The method according to claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the alkane is propane.

10. The method of claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the reaction product is an olefin.

11. The method of claim 9, wherein the reaction product is an olefin.

12. The method of claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the pretreatment step further comprises pretreatment with oxygen before or after pretreatment with DME.

13. The method of claim 9, wherein the pretreatment step further comprises pretreatment with oxygen before or after pretreatment with DME.

14. The method of claim 11, wherein the pretreatment step further comprises pretreatment with oxygen before or after pretreatment with DME.

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