HETEROGENEOUS ALKANE DEHYDROGENATION CATALYST

The present application claims the benefit of U.S. Provisional Application No. 61/916,393, filed on Dec. 16, 2013.

This invention relates generally to a heterogeneous alkane dehydrogenation catalyst, particularly a heterogeneous alkane dehydrogenation catalyst wherein at least two metal oxides are dispersed on a catalyst support, more particularly a heterogeneous alkane dehydrogenation catalyst wherein two metal oxides are dispersed on a metal oxide catalyst support and one of the metals in the dispersed metal oxides is the same as the metal in the metal oxide catalyst support, and still more particularly heterogeneous alkane dehydrogenation that comprises, consists essentially of or consists of a combination of gallium oxide (Ga₂O₃) and aluminum oxide (Al₂O₃) on an alumina (Al₂O₃)-containing support.

In a typical Ga₂O₃on Al₂O₃heterogeneous alkane dehydrogenation catalyst, the active component, Ga₂O₃, is deposited, e.g. by a known technique such as aqueous impregnation, using a suitable Ga₂O₃precursor (e.g. a salt such as a nitrate) on a surface of the Al₂O₃or silica-modified alumina (SiO₂—Al₂O₃) support and then calcined to form an active Ga₂O₃layer on the Al₂O₃support.

As used herein, a silica-modified alumina support preferably has a silica content within a range of from greater than 0 wt % to less than 10 wt %, based on total weight of the support. A silica-modified alumina support is not a zeolite.

This invention demonstrates an improvement over the typical Ga₂O₃on Al₂O₃heterogeneous alkane dehydrogenation catalyst. The improvement comprises depositing an Al₂O₃precursor on the surface of the Al₂O₃or SiO₂—Al₂O₃support, before, after or, preferably, in conjunction with depositing the Ga₂O₃precursor on the surface of said support (also known as “co-depositing”, “co-loading” or “co-deposition”). Following calcination subsequent to deposition of both the Al₂O₃precursor and the Ga₂O₃precursor, the catalyst has an active layer that comprises both Ga₂O₃and Al₂O₃. For a discussion of impregnation techniques and co-loading, see Catalyst Handbook, 2nd Edition, edited by Martyn V. Twygg, Oxford University Press, ISBN 1-874545-36-7, and Pure & Appl. Chem., Vol. 67, Nos 8/9, pp. 1257-1306, 1995).

PCT Application (WO) 2010/107591 (Luo et al.) discloses a supported paraffin dehydrogenation catalyst that comprises a first component selected from tin, germanium, lead, indium, Ga, thallium and compounds thereof, a second component selected from Group VIII of the Periodic Table (e.g. platinum (Pt), palladium, iron, ruthenium, osmium, cobalt, rhodium, iridium or nickel), an alkali metal or alkaline earth metal or a compound thereof, and a support comprising Al₂O₃in gamma crystalline form.

M. Chen et al., in “Dehydrogenation of propane over spinel-type Gallia-alumina solid solution catalysts”, Journal of Catalysis 256 (2008) pages 293-300, discloses dehydrogenation of propane to propylene over a series of mixed Ga_xAl_10-xoxides (x varying from 0 to 10). In summarizing literature for Ga₂O₃-based catalysts, M. Chen et al. refers to Ga₂O₃catalysts that are dispersed on an inert oxide support such as titania (TiO₂) or Al₂O₃. M. Chen et al. appears to equate solid solutions to bulk metal oxide catalysts.

B. Xu et al., in “Support effect in dehydrogenation of propane in the presence of CO₂over supported gallium oxide catalysts”, Journal of Catalysis 239 (2006) pages 470-477, teaches dehydrogenation of propane to propene (propylene) in the absence or presence of CO₂over different supported Ga₂O₃catalysts including Ga₂O₃/TiO₂, Ga₂O₃/Al₂O₃, Ga₂O₃/ZrO₂, Ga₂O₃/SiO₂and Ga₂O₃/MgO, the latter two being ineffective for dehydrogenation of propane. Supported catalysts are prepared by impregnating a Ga precursor solution onto the second named oxide, e.g. TiO₂.

C. Areádet al, in “Synthesis and Characterization of Spinel-Type Gallia-Alumina Solid Solutions”, Z. Anorg. Allg. Chem 2005, 631, pages 2121-2126, presents teachings relative to mixed Ga₂O₃—Al₂O₃oxides that are solid solutions (bulk catalysts) with Ga:Al ratios between 9:1 and 1:9 that have utility in hydrocarbon dehydrogenation.

U.S. Pat. No. 4,056,576 (Gregory et al.) relates to a process for dehydrogenating saturated hydrocarbons in the presence of a Ga catalyst (elemental Ga or a Ga compound deposited on a support) to produce unsaturated hydrocarbons. The support may be Al₂O₃or SiO₂with or without surface hydroxyl groups that may be exchanged by ions of metals selected from Ga, Al, iron and nickel.

EP 0 905 112 (Buonomo et al.) relates to production of styrene starting from benzene and ethane using a dehydrogenation catalyst such as one based on Ga and platinum (Pt) on Al₂O₃in delta or theta phase or in a mixture of delta+theta, theta+alpha or delta+theta+alpha phases.

European Patent Publication (EP) 0441430 (Iezzi et al.) discloses a process for catalytically dehydrogenating a two to five carbon atom (C₂-C₅) paraffin using a supported catalyst composition consisting of Pt, tin, and a supporting substrate selected from titanated Al₂O₃, titanated SiO₂and/or titanium silicate.

U.S. Pat. No. 3,198,749 (Gladrow et al.) relates to a SiO₂—Al₂O₃—Ga₂O₃catalyst and its preparation.

U.S. Pat. No. 5,308,822 and its divisional U.S. Pat. No. 5,414,182 (both Iezzi et al.) provide a process for activating a catalytic composition for paraffin dehydrogenation that contains Ga, Al₂O₃and, optionally SiO₂and/or one or more alkali metals or alkaline earth metals.

U.S. Pat. No. 7,235,706 (Iezzi et al.) relates to a process for preparing light olefins from corresponding paraffins by reacting the paraffins in a reactor with a catalytic system containing Ga, Pt, optionally one or more alkali metals or alkaline earth metals and a SiO₂—Al₂O₃support. Preferred procedures include impregnation by incipient wetness or immersing the support in a solution containing the precursors.

Bulk metal oxide alkane dehydrogenation catalysts prepared by, for example, a sol-gel procedure tend to have a selectivity to desired olefins lower than desired, even with a high loading of Ga on the order of 20 weight percent (wt %), based on total catalyst weight. Such a high loading makes the catalyst much more expensive than a lower loading on the order of, for example, three wt % to five wt %. In addition, the bulk metal oxide catalysts have physical properties such as density and attrition resistance that are often less than desired due, at least in part, to an inability to adjust such properties independent of the bulk composition that necessarily includes the active component(s) (e.g. Ga). By way of contrast, the support used in preparing a supported catalyst can be designed and prepared to optimize relevant properties such as stability, density or attrition resistance independent of the active component(s).

Bulk metal oxide catalysts tend to use active components (e.g. Ga₂O₃) less efficiently than supported mixed metal oxide catalysts as the bulk catalysts must, as skilled artisans understand them, have the active component distributed throughout the catalyst. This distribution effectively makes a significant portion of the active component inaccessible for catalytic service.

As compared to such bulk metal oxide alkane dehydrogenation catalysts, supported alkane dehydrogenation catalysts such as Ga₂O₃disposed on a catalyst support (e.g. Al₂O₃or SiO₂—Al₂O₃) offer more economical utilization of the active component (Ga₂O₃) without substantially sacrificing desirable physical characteristics of the support material (e.g. attrition resistance and density).

Skilled artisans seek improvements in catalyst stability and performance irrespective of whether the catalyst is a bulk metal oxide catalyst or a supported metal oxide catalyst (e.g. supported Ga₂O₃catalyst).

In some aspects, this invention is a heterogeneous alkane dehydrogenation catalyst consisting of a combination of a) both aluminum oxide and gallium oxide dispersed as an active layer on (or onto) b) an alumina support or a silica-modified alumina support. Such catalysts have utility in dehydrogenating alkanes such as propane to produce propylene.

In some aspects, this invention is a process for preparing such a heterogeneous alkane dehydrogenation catalyst, which process comprises loading precursors to Ga₂O₃and Al₂O₃simultaneously or sequentially onto an Al₂O₃support or a SiO₂—Al₂O₃support. Following completion of the gallium oxide and aluminum oxide precursor loading, calcine the loaded support. Calcining occurs at a temperature sufficient to decompose the metal precursor, such temperature being at least 400° C., preferably at least 550° C. and most preferably at least 700° C. Calcining occurs at a temperature below 1100° C., preferably less than 1000° C.

The heterogeneous alkane dehydrogenation catalyst comprises, consists essentially of or consists of an inactive support that has an active layer comprising, consisting essentially of or consisting of Ga and Al, preferably in the form of Ga₂O₃and Al₂O₃, dispersed thereon. The active layer has a molar ratio of Ga to Al within a range of from greater than 0.5:1 to less than 15:1, preferably from 1:1 to 10:1 and more preferably from 1.5:1 to 5:1. Ga, expressed as Ga₂O₃, on the support and in the active layer, is present in an amount within a range of from less than 14 wt %, more preferably less than 10 wt % and still more preferably less than 5 wt %, in each case greater than 0 wt % and based on total catalyst weight. Al, expressed as Al₂O₃, on the support and in the active layer, is present in an amount within a range of from 0.05 wt % to 10 wt %, preferably from 0.05 wt % to 5 wt %, each wt % being based upon total catalyst weight. The support is preferably substantially free of Ga and more preferably completely free of Ga prior to having the active layer dispersed thereon. The support is preferably selected from Al₂O₃and SiO₂—Al₂O₃. The SiO₂—Al₂O₃has a SiO₂content within a range of from 0.1 wt % to 10 wt %, preferably from 0.1 wt % to 5 wt %, in each case based upon total weight of the support.

The above heterogeneous alkane dehydrogenation catalyst has a dehydrogenation performance that exceeds that of comparative catalysts such as a bulk mixed oxide catalyst or a supported catalyst wherein only Ga₂O₃is dispersed as a supported metal oxide.

Ga₂O₃precursors are suitably selected from soluble gallium salts, such as gallium (III) nitrate, gallium (III) acetylacetonate, gallium (III) chloride, with gallium (III) nitrate being preferred.

Al₂O₃precursors are suitably selected from soluble aluminum salts, such as aluminum (III) nitrate, aluminum (III) acetylacetonate, aluminum (III) chloride, with aluminum (III) nitrate being preferred.

In a replication of work presented by Chen et al. in the 2008 Journal of Catalysis article noted above, mix together concentrated aqueous ammonia (28 wt % ammonia, Aldrich, Catalogue No. 221228, based upon total weight of concentrated aqueous ammonia) and ethanol in a 50:50 volume ratio. Add this mixture dropwise to an ethanol solution of gallium nitrate hydrate (99.9 percent purity, Aldrich, Catalogue No. 289892) and aluminum nitrate hydrate (at least 98 percent purity, Aldrich, Catalogue No. 237973) until solution pH reaches 8.5 and no further visible precipitation is observed. The ethanol solutions each contain 15 grams (g) of gallium nitrate hydrate and varying amounts of aluminum nitrate hydrate, with CEx A containing 13.2 g, CEx B containing 6.6 g, and CEx C containing 3.3 g. Filter gel from the solution and wash the gel with ethanol before drying it overnight at 373° Kelvin (100° C.) and then calcining it at 773° K (500° C.) for six hours.

Use aqueous incipient wetness impregnation to prepare a supported catalyst using 20 g of SIRALOX™ 1.5/70 (Sasol, 1.5 wt % silica, based on total weight of the support, and a surface area (S.A.) of 79 square meters per gram (m²/g) as a catalyst support. Pre-dry the catalyst support at a temperature of 350° C. for a period of two hours. Spray a solution with a targeted amount of metal precursor (gallium nitrate hydrate and aluminum nitrate as in CEx A-C and potassium nitrate (at least 99% purity, Aldrich, Catalogue No. 221295) and solution volume sufficient to match 95% pore volume (PV) (0.25 milliliters per gram (mL/g) onto the pre-dried support. Age the sprayed support at ambient temperature for two hours before drying it in an electric muffle furnace at 175° C. for one hour and then calcining it at 750° C. for one hour. Metal precursor amounts are as follows: Ex 1—1.72 g gallium nitrate hydrate, 1.53 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 2—1.72 g gallium nitrate hydrate, 0.76 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 3—1.72 g gallium nitrate hydrate, 0.38 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 4—1.72 g gallium nitrate hydrate, 0.18 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 5—12.06 g gallium nitrate hydrate, 3.11 g aluminum nitrate hydrate and 0.15 g potassium nitrate; CEx D—1.72 g gallium nitrate hydrate and 0.13 g potassium nitrate; and CEx E—11.87g gallium nitrate hydrate and 0.15 g potassium nitrate.

Replicate Ex 1-5 and CEx D-E with changes to prepare four catalysts using high purity Al₂O₃(at least 99.5% pure, CATALOX™ 5/70, Sasol) as the support. Metal precursor amounts are as follows: Ex 6—1.72 g gallium nitrate hydrate, 1.78 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 7—1.72 g gallium nitrate hydrate, 0.89 g aluminum nitrate hydrate and 0.13 g potassium nitrate; Ex 8—1.72 g gallium nitrate hydrate, 0.44 g aluminum nitrate hydrate and 0.13 g potassium nitrate; and CEx F—1.72 g gallium nitrate hydrate and 0.13 g potassium nitrate.

Replicate Ex 2-3 with changes to prepare two catalysts by sequentially loading first the gallium nitrate hydrate and potassium nitrate and second the aluminum nitrate hydrate precursors. After the first loading step with gallium and potassium precursors, age the obtained material for two hours at ambient temperature, dry the aged at 175° C. for one hr, and then calcine the dried material at 750° C. for 1 hour before loading the aluminum nitrate hydrate precursor. After completing the aluminum precursor loading, dry the material and calcine it in the same manner as after the first loading step.

Admix 0.5 g of each catalyst with 1.0 g silicon carbide, then subject the catalyst to a number of dehydrogenation reaction/catalyst reactivation/catalyst rejuvenation cycles as detailed below. In the dehydrogenation reaction step, pass a feed stream (95 mole percent (mol %) propane and 5 mol % nitrogen through a catalyst for a period of 60 seconds at a temperature of 625° C. and a propane weight hourly space velocity (WHSV) of 8 reciprocal hours (hr⁻¹) under ambient pressure (e.g. one atmosphere). Collect data for propane conversion and propane selectivity approximately 6 seconds after initiating contact between the feed stream and the catalyst. After the 60 second period lapses, ramp reactor temperature to 730° C. at a rate of 20° C. per minute in the presence of helium (He) flowing through the catalyst at a rate of 120 standard cubic centimeters per minute (sccm). Maintain the temperature at 730° C. while contacting the catalyst with a simulated CH₄combustion products stream (4 mol % oxygen, 8 mol % carbon dioxide, 16 mol % water vapor and 72 mol % He) at a flow rate of 150 sccm for a period of three minutes. Subsequent to treatment with the simulated combustion products stream, pass 100% air through the catalyst at a flow rate of 150 sccm for a period of 15 minutes. After air treatment and before starting another PDH reaction cycle, cool the reactor to the reaction temperature (625° C.) and stabililze the temperature of the system over a period of 20 min under flowing He (flow rate of 120 sccm) to effect stripping of labile oxygen from the catalyst and make the temperature of the catalyst bed substantially uniform before the next reaction/regeneration cycle.

Summarize catalyst test results for catalysts prepared in CEx A-C after 15, 30 and 50 cycles in terms of % propane (C₃H₈) conversion, % propylene (C₃H₆) selectivity and product % selectivity for propylene (C₃H₆in Table 2 below. In Tables 3A-3C below, do the same for Ex 1-5, 9, 10, CEx D and CEx E.

The conversion, selectivity and yield are all based on mol %.

The data presented in Tables 2 and 3A through 3C provide support for a number of observations. First, as shown in Table 2, bulk mixed metal oxides, even with Ga₂O₃loadings in excess of 65 wt % (see Table 1 for CEx A-C), provide a propane conversion of no more than 40.3% (Table 2, CEx A, 15 cycles). Second, also as shown in Table 2, the maximum selectivity to propylene for bulk mixed metal oxides is 83.9% (Table 2, CEx C, 50 cycles). Third, Table 3C shows that propane conversion, propylene selectivity and propylene yield are somewhat higher for a relatively low Ga₂O₃loading (2.2 wt % for CEx D) than for a relatively higher Ga₂O₃loading (13.2 wt % for CEx E). Fourth, addition of Al₂O₃to the active layer (along with the Ga₂O₃), either in a one-step procedure (Ex 1-5) or a sequential procedure (Ex 9-10), leads to a marked increase in propylene selectivity relative to what one can obtain with bulk mixed metal oxides where the same oxides are used but with Ga₂O₃loadings significantly lower for the supported catalysts than for the bulk mixed metal oxides. Fifth, the amount of Al₂O₃included in the active layer also affects catalyst performance, with a Ga/Al molar ratio preferred to range from greater than 0.5:1 to less than 15:1, and more preferably 1:1 to less than 10:1, and most preferably 1.5:1 to 5:1. For catalyst to have good activity and selectivity, gallium oxides loading is preferably to be greater 0 wt % and lower than 14 wt %, and more preferably be greater 0 wt % and lower than 10 wt %, and most preferably greater 0 wt % and lower than 5 wt %.