WO2000062926A1 - Vanadyl pyrophosphate oxidation catalyst - Google Patents

Vanadyl pyrophosphate oxidation catalyst Download PDF

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Publication number
WO2000062926A1
WO2000062926A1 PCT/US2000/009906 US0009906W WO0062926A1 WO 2000062926 A1 WO2000062926 A1 WO 2000062926A1 US 0009906 W US0009906 W US 0009906W WO 0062926 A1 WO0062926 A1 WO 0062926A1
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Prior art keywords
silicon carbide
vanadium
beta
particles
phosphate
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PCT/US2000/009906
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French (fr)
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Marc J. Ledoux
Henrich Baudouin
Jan Joseph Lerou
Claude Crouzet
Christophe Bouchy
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E.I. Du Pont De Nemours And Company
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Priority to AU43458/00A priority Critical patent/AU4345800A/en
Publication of WO2000062926A1 publication Critical patent/WO2000062926A1/en

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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/16—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation
    • C07C51/21—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen
    • C07C51/215—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen of saturated hydrocarbyl groups
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/24—Nitrogen compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/002—Mixed oxides other than spinels, e.g. perovskite
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/14—Phosphorus; Compounds thereof
    • B01J27/186—Phosphorus; Compounds thereof with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J27/195—Phosphorus; Compounds thereof with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium with vanadium, niobium or tantalum
    • B01J27/198—Vanadium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/20—Carbon compounds
    • B01J27/22—Carbides
    • B01J27/224—Silicon carbide
    • B01J35/30—
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/0009—Use of binding agents; Moulding; Pressing; Powdering; Granulating; Addition of materials ameliorating the mechanical properties of the product catalyst
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/16—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2523/00—Constitutive chemical elements of heterogeneous catalysts

Definitions

  • This invention relates to a beta-silicon carbide particles impregnated with vanadyl pyrophosphate oxidation catalyst and the process for the preparation thereof.
  • VPO vanadyl pyrophosphate
  • VPO catalysts are hydrochloric acid digestion of V 2 0 5 and H3PO 4 in either an aqueous solvent, as described, for example, in U.S. Patent 3,985,775, or non aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, vanadium hydrogen phosphate. VO(HOPO 3 ).(H 2 O)o 5 .
  • the precursor is then activated by heating, as described, for example, in U.S. Patent 3,864,280 and U.S. Patent 4,043,943. Further optimization of the preparation is described in U.S. Patent 4,132,670. whereby vanadium pentoxide is heated with a selected anhydrous unsubstituted alcohol, adding an orthophosphoric acid to form the catalyst precursor and calcining the precursor to obtain the catalyst having high intrinsic surface area. Further attempts to improve the VPO catalyst performance by the use of dopants and/or supports are described in U.S. Patent 4,442,226 and U.S. Patent 4,778,890.
  • Vanadium, phosphorus and oxygen can form a large number of distinct compounds which have been well characterized, e.g., ⁇ -VOPO ⁇ -VOPO VOHPO 4 , (VO) 2 P 2 0 7 , VO(PO 3 ) 2 and VO(H 2 PO 4 ) 2 .
  • the most active catalytic phase is believed to be (VO) 2 P 2 ⁇ 7 , which is also the predominant oxide phase in VPO catalysts.
  • VPO catalysts are usually referred to as "mixed oxides" in recognition of the probable presence of other oxide phases.
  • VPO catalysts typically have V:P atomic ratios in the range of 1 : 1 to 1:2 and have an average bulk vanadium oxidation state in the range of 4.0-4.3.
  • VPO catalysts were prepared from vanadyl hydrogen phosphate hemihydrate precursor that was free of microcrystalline or amorphous phases, such as VO(H 2 PO 4 )2 anc ⁇ ⁇ - and ⁇ -vanadyl (V) orthophosphates. It was disclosed that these undesirable components could be removed by washing either the precursor or the catalyst with boiling water.
  • An object of this invention is to further advance the technology of VPO catalysis by providing for a beta-silicon carbide supported VPO catalyst particularly effective for hydrocarbon oxidation.
  • the invention provides beta-silicon carbide particles impregnated with a VPO catalyst.
  • the beta-silicon carbide particles can be either grains (>0.3 mm) in the samples "D", or in fine powder (40-80 mm) in the samples "L”.
  • the catalysts comprise a beta-silicon carbide "core”, a VPO catalyst "shell”, and a transition phase intermediate the core and the shell which contains Si, C, V, P and O.
  • the invention comprises a process for preparing beta- silicon carbide particles impregnated with a VPO catalyst, comprising the steps of: (a) forming a heterogeneous reaction mixture of suspended vanadium (IV) phosphate, (b) adding particulate beta-silicon carbide to the heterogeneous reaction mixture under agitation at a moderated temperature between 40°C and 120°C to form particulate beta-silicon carbide impregnated with vanadium (TV) phosphate,
  • Figure 1 is an X-ray diffraction pattern for hemihydate on beta-silicon carbide prior to aqueous washing (sample L2).
  • Figure 2 is an X-ray diffraction pattern for hemihydate on beta-silicon carbide after aqueous washing (sample L2).
  • Figure 3 is an X-ray diffraction pattern for activated catalyst (sample L2).
  • Figure 4 is an X-ray diffraction pattern for activated catalyst (sample DI).
  • Figure 5 is an X-ray diffraction pattern for "used" catalyst (sample L2).
  • Figure 6 is an X-ray diffraction pattern for "used" catalyst (sample DI).
  • Figure 7 is a transition electron micrograph of a beta-silicon carbide particle impregnated with a VPO catalyst (activated DI) in accordance with the invention (after activation but before being used), particularly illustrating the transition phase between the beta-silicon carbide core and the VPO shell.
  • Figure 8 is an X-ray diffraction pattern for a conventionally prepared bulk VPO catalyst.
  • Figure 9 is a plot of rate of formation of maleic anhydride versus reaction temperature.
  • Figure 10 is a plot of the yield of maleic anhydride versus reaction temperature.
  • Figure 11 is a plot of conversion of butane versus reaction temperature.
  • Figure 12 a plot of the selectivity to maleic anhydride versus reaction temperature.
  • Figure 13 is a plot of the selectivity to maleic anhydride versus butane conversion.
  • Figure 14 is a plot of rate of the selectivity to CO + CO 2 versus reaction temperature.
  • Figure 15 is a plot of the X-ray diffraction pattern of the pure support.
  • Figure 16 is a plot of the X-ray diffraction pattern for the comparative example, which comprises alpha-silicon carbide on the support.
  • Figure 17 is a plot of the pore volume data for beta-silicon carbide.
  • Figure 18 is a plot of rate of disappearance of n-butane normalized to the weight fraction of VPO in each catalyst sample.
  • a heterogeneous reaction mixture of vanadium (IV) phosphate is formed.
  • This step of forming the suspended hemihydrate can be accomplished by the hydrochloric acid digestion of V 2 O 5 and H3PO 4 in an aqueous solvent, as described, for example, in U.S. Patent 3,985,775, the disclosure of which is incorporated herein by reference.
  • the preferred method of forming the hemihydrate is to prepare a mixture of vanadium pentoxide and at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds, and heating the mixture to form a feed of vanadium pentoxide reduced to a valence between 4 and 4.6.
  • the feed is then contacted with a solution of orthophosphoric acid and at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds.
  • These two steps form a heterogeneous reaction mixture of suspended vanadium (IV) phosphate as taught in U.S. Patent 4,132,670, the disclosure of which is incorporated herein by reference. Any commercially available vanadium pentoxide, orthophosphoric acid and anhydrous alcohol of the type described above can be used in the practice of this process.
  • the particulate beta-silicon carbide can be any beta-silicon carbide (including commercially available grades) having a specific surface area that is at least 10 m 2 /gram, preferably 27-35 m 2 /gram. Most preferred are grains of beta-silicon carbide prepared according to one of the techniques described in EP-A-0,313,480, EP-A-0,440,569, EP-A-0,511,919, EP-A-0,543,751 and
  • EP-A-0,624,560 (corresponding to U.S. Patents 4,914,070; 5,217,930; 5,196,389; 5,427,761 and 5,429,780 respectively), the disclosure of which are incorporated herein by reference. Additionally, the beta-silicon carbide used should show no significant microporosity, i.e., the pore volume (in cc/g) of pores with diameters below 40 A is essentially zero.
  • the particulate beta-silicon carbide is added to the heterogeneous reaction mixture under agitation at moderated temperature between 40°C and 120°C to form vanadium (IV) phosphate supported on the particulate beta-silicon carbide.
  • the rapid crystallization of the hemihydrate is to be avoided, as that would result in a mixture of crystallized hemihydrate and beta-silicon carbide particles, rather than the impregnated beta-silicon carbide particles of this invention.
  • reaction mixture As the reaction continues, some of the solvents will be eliminated and the reaction mixture will begin to thicken. At this point the reaction mixture is placed under partial vacuum at a temperature above 125°C to dry the mixture to the consistency of a non-dry mud.
  • the resulting material ( Figure 1) is washed with water to extract the VO(H 2 P0 4 ) 2 phase from the precursor.
  • the presence/absence of the VO(H 2 P0 4 ) 2 phase is monitored by the use of X-ray diffraction in accordance with Guliants et al., Catalysis Today, 28 (1996), pages 275-295, incorporated herein by reference.
  • the material consists essentially of catalyst precursor, vanadium hydrogen phosphate, VO(HOPO 3 ).(H 2 O)o. 5 on beta-silicon carbide ( Figure 2).
  • the beta-silicon carbide supported catalyst is then formed from the precursor by heating the precursor in air, followed by heating in a mixture of air and hydrocarbon in accordance with the procedure described in the aforementioned U.S. 4,132,670. To insure that the catalyst is fully stabilized for use in the oxidation of hydrocarbons, it is preferred that the supported VPO catalyst be exposed to a mixture of air and hydrocarbon for a period of at least 50 hours and preferably at least 100 hours ( Figure 3).
  • the catalysts of this invention may be further processed to impart attrition resistance by methods known in the art, such as, for example, by applying a coating of SiO 2 in accordance with U.S. Patent 4,677,084, the disclosure of which is incorporated herein by reference. This further process is particularly applied to the fine powder supported material.
  • the catalysts of this invention are well suited for hydrocarbon oxidations in any type of reactor: fixed bed, fluidized bed and recirculating solids reactor.
  • the beta-silicon carbide support acts as a heat sink, the catalysts of this invention can be utilized at higher temperatures than the unsupported corresponding catalyst. More specifically, this catalyst is well suited to be efficiently utilized in fixed bed reactors with improved selectivity at high butane concentrations.
  • Figure 18 the rate of disappearance of n-butane was normalized to the weight fraction of VPO in each catalyst sample.
  • W weight of VPO in each catalyst sample (not total weight of catalyst).
  • VPO vanadium phosphorus oxide
  • P V precursor phase no washing of vanadium phosphorus oxide precursor/ to remove 2:1 P V precursor phase
  • 30 wt % vanadium phosphorus oxide/ with the washing step The data shows that washing greatly improves the activity of the VPO/ phase at all temperatures. It also shows that the rate of reaction, normalized to the VPO in each catalyst sample, deviates (i.e., improves) for the sample at heat treatment temperatures greater than 350°C.
  • catalytic tests can be conducted at this point, it is preferable to stabilize the activated catalysts by further subjecting the activated catalysts to up to 200 hours of the air/butane flow at 420°C.
  • Catalytic tests were run on the stabilized samples relative to a conventionally prepared bulk VPO catalyst.
  • the X-ray diffraction pattern for the conventionally prepared bulk VPO catalyst is shown in Figure 8.
  • the X-ray difraction patterns for the used catalysts are shown in Figures 5 and 6, for the fine powder and the grains respectively.
  • the catalytic performance of the catalysts relative to the conventionally prepared bulk VPO catalyst was determined by monitoring the oxidation of n-butane to maleic anhydride.
  • Figures 15 and 16 show the powder x-ray data for the alfa-SiC and its hydrate, respectively.
  • Figure 16 shows the material to be macroscopically inhomogeneous.

Abstract

Beta-silicon carbide particles impregnated with a vanadyl pyrophosphate oxidation catalyst are prepared by forming a heterogeneous reaction mixture of suspended vanadium (IV) phosphate (via hydrochloric acid digestion of V2O5 and H3PO4 in an aqueous solvent or via heating vanadium pentoxide with at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds to form a feed of vanadium pentoxide reduced to a valence between 4 and 4.6, and then contacting the feed with a solution of orthophosphoric acid and at least one unsubstituted alcohol as described above), adding particulate beta-silicon carbide to the hererogeneous reaction mixture under agitation at moderated temperature between 40 °C and 120 °C, followed by drying, washing and calcining the particles thus formed.

Description

TITLE VANADYL PYROPHOSPHATE OXIDATION CATALYST FIELD OF THE INVENTION This invention relates to a beta-silicon carbide particles impregnated with vanadyl pyrophosphate oxidation catalyst and the process for the preparation thereof.
BACKGROUND OF THE INVENTION Maleic anhydride is used as a raw material for numerous products, including agricultural chemicals, paints, paper sizing, food additives and synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed for its production, the most.successful of which involves the vapor phase oxidation of n-butane to maleic anhydride in the presence of a vanadyl pyrophosphate ("VPO") catalyst. Since the development of this method in the 1970's, research has continued to improve the reaction conditions and, particularly, the VPO catalysts.
A review of the improvements made in this technology is given by G. J. Hutchings, Applied Catalysis, 72(1991), Elsevier Science Publishers B. V. Amsterdam, pages 1-31. The preferred method of preparation of VPO catalysts is the hydrochloric acid digestion of V205 and H3PO4 in either an aqueous solvent, as described, for example, in U.S. Patent 3,985,775, or non aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, vanadium hydrogen phosphate. VO(HOPO3).(H2O)o 5. The precursor is then activated by heating, as described, for example, in U.S. Patent 3,864,280 and U.S. Patent 4,043,943. Further optimization of the preparation is described in U.S. Patent 4,132,670. whereby vanadium pentoxide is heated with a selected anhydrous unsubstituted alcohol, adding an orthophosphoric acid to form the catalyst precursor and calcining the precursor to obtain the catalyst having high intrinsic surface area. Further attempts to improve the VPO catalyst performance by the use of dopants and/or supports are described in U.S. Patent 4,442,226 and U.S. Patent 4,778,890.
Vanadium, phosphorus and oxygen can form a large number of distinct compounds which have been well characterized, e.g., α-VOPO γ-VOPO VOHPO4, (VO)2P207, VO(PO3)2 and VO(H2PO4)2. The most active catalytic phase is believed to be (VO)2P2θ7, which is also the predominant oxide phase in VPO catalysts. Nevertheless, VPO catalysts are usually referred to as "mixed oxides" in recognition of the probable presence of other oxide phases. VPO catalysts typically have V:P atomic ratios in the range of 1 : 1 to 1:2 and have an average bulk vanadium oxidation state in the range of 4.0-4.3.
Guliants et al., Catalysis Today, 28 (1996), pages 275-295, studied the effect of the phase composition of VPO catalysts on their effectiveness as catalysts for the oxidation of n-butane to maleic anhydride. This work indicated that the best performing VPO catalyst was prepared from vanadyl hydrogen phosphate hemihydrate precursor that was free of microcrystalline or amorphous phases, such as VO(H2PO4)2 anc^ δ- and γ-vanadyl (V) orthophosphates. It was disclosed that these undesirable components could be removed by washing either the precursor or the catalyst with boiling water.
An object of this invention is to further advance the technology of VPO catalysis by providing for a beta-silicon carbide supported VPO catalyst particularly effective for hydrocarbon oxidation.
SUMMARY OF THE INVENTION In one aspect, the invention provides beta-silicon carbide particles impregnated with a VPO catalyst. The beta-silicon carbide particles can be either grains (>0.3 mm) in the samples "D", or in fine powder (40-80 mm) in the samples "L". Essentially, the catalysts comprise a beta-silicon carbide "core", a VPO catalyst "shell", and a transition phase intermediate the core and the shell which contains Si, C, V, P and O.
In another aspect, the invention comprises a process for preparing beta- silicon carbide particles impregnated with a VPO catalyst, comprising the steps of: (a) forming a heterogeneous reaction mixture of suspended vanadium (IV) phosphate, (b) adding particulate beta-silicon carbide to the heterogeneous reaction mixture under agitation at a moderated temperature between 40°C and 120°C to form particulate beta-silicon carbide impregnated with vanadium (TV) phosphate,
(c) drying the vanadium (IV) phosphate impregnated beta-silicon carbide particles,
(d) optionally, but preferably, washing the vanadium (IV) phosphate impregnated beta-silicon carbide particles with water; and
(e) calcining the vanadium (IV) phosphate impregnated beta-silicon carbide particles at elevated temperature to obtain beta-silicon carbide particles impregnated with a VPO catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an X-ray diffraction pattern for hemihydate on beta-silicon carbide prior to aqueous washing (sample L2). Figure 2 is an X-ray diffraction pattern for hemihydate on beta-silicon carbide after aqueous washing (sample L2).
Figure 3 is an X-ray diffraction pattern for activated catalyst (sample L2).
Figure 4 is an X-ray diffraction pattern for activated catalyst (sample DI). Figure 5 is an X-ray diffraction pattern for "used" catalyst (sample L2).
Figure 6 is an X-ray diffraction pattern for "used" catalyst (sample DI).
Figure 7 is a transition electron micrograph of a beta-silicon carbide particle impregnated with a VPO catalyst (activated DI) in accordance with the invention (after activation but before being used), particularly illustrating the transition phase between the beta-silicon carbide core and the VPO shell.
Figure 8 is an X-ray diffraction pattern for a conventionally prepared bulk VPO catalyst.
Figure 9 is a plot of rate of formation of maleic anhydride versus reaction temperature. Figure 10 is a plot of the yield of maleic anhydride versus reaction temperature.
Figure 11 is a plot of conversion of butane versus reaction temperature.
Figure 12 a plot of the selectivity to maleic anhydride versus reaction temperature. Figure 13 is a plot of the selectivity to maleic anhydride versus butane conversion.
Figure 14 is a plot of rate of the selectivity to CO + CO2 versus reaction temperature.
Figure 15 is a plot of the X-ray diffraction pattern of the pure support. Figure 16 is a plot of the X-ray diffraction pattern for the comparative example, which comprises alpha-silicon carbide on the support.
Figure 17 is a plot of the pore volume data for beta-silicon carbide.
Figure 18 is a plot of rate of disappearance of n-butane normalized to the weight fraction of VPO in each catalyst sample. DETAILED DESCRIPTION OF THE INVENTION
In the first steps of preparing the VPO catalyst impregnated beta-silicon carbide, a heterogeneous reaction mixture of vanadium (IV) phosphate is formed. This step of forming the suspended hemihydrate can be accomplished by the hydrochloric acid digestion of V2O5 and H3PO4 in an aqueous solvent, as described, for example, in U.S. Patent 3,985,775, the disclosure of which is incorporated herein by reference. The preferred method of forming the hemihydrate, however, is to prepare a mixture of vanadium pentoxide and at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds, and heating the mixture to form a feed of vanadium pentoxide reduced to a valence between 4 and 4.6. The feed is then contacted with a solution of orthophosphoric acid and at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds. These two steps form a heterogeneous reaction mixture of suspended vanadium (IV) phosphate as taught in U.S. Patent 4,132,670, the disclosure of which is incorporated herein by reference. Any commercially available vanadium pentoxide, orthophosphoric acid and anhydrous alcohol of the type described above can be used in the practice of this process.
The particulate beta-silicon carbide can be any beta-silicon carbide (including commercially available grades) having a specific surface area that is at least 10 m2/gram, preferably 27-35 m2/gram. Most preferred are grains of beta-silicon carbide prepared according to one of the techniques described in EP-A-0,313,480, EP-A-0,440,569, EP-A-0,511,919, EP-A-0,543,751 and
EP-A-0,624,560 (corresponding to U.S. Patents 4,914,070; 5,217,930; 5,196,389; 5,427,761 and 5,429,780 respectively), the disclosure of which are incorporated herein by reference. Additionally, the beta-silicon carbide used should show no significant microporosity, i.e., the pore volume (in cc/g) of pores with diameters below 40 A is essentially zero.
The particulate beta-silicon carbide is added to the heterogeneous reaction mixture under agitation at moderated temperature between 40°C and 120°C to form vanadium (IV) phosphate supported on the particulate beta-silicon carbide. During this step, the rapid crystallization of the hemihydrate is to be avoided, as that would result in a mixture of crystallized hemihydrate and beta-silicon carbide particles, rather than the impregnated beta-silicon carbide particles of this invention.
As the reaction continues, some of the solvents will be eliminated and the reaction mixture will begin to thicken. At this point the reaction mixture is placed under partial vacuum at a temperature above 125°C to dry the mixture to the consistency of a non-dry mud.
The resulting material (Figure 1) is washed with water to extract the VO(H2P04)2 phase from the precursor. The presence/absence of the VO(H2P04)2 phase is monitored by the use of X-ray diffraction in accordance with Guliants et al., Catalysis Today, 28 (1996), pages 275-295, incorporated herein by reference. At this point in the process, the material consists essentially of catalyst precursor, vanadium hydrogen phosphate, VO(HOPO3).(H2O)o.5 on beta-silicon carbide (Figure 2). The beta-silicon carbide supported catalyst is then formed from the precursor by heating the precursor in air, followed by heating in a mixture of air and hydrocarbon in accordance with the procedure described in the aforementioned U.S. 4,132,670. To insure that the catalyst is fully stabilized for use in the oxidation of hydrocarbons, it is preferred that the supported VPO catalyst be exposed to a mixture of air and hydrocarbon for a period of at least 50 hours and preferably at least 100 hours (Figure 3).
The catalysts of this invention may be further processed to impart attrition resistance by methods known in the art, such as, for example, by applying a coating of SiO2 in accordance with U.S. Patent 4,677,084, the disclosure of which is incorporated herein by reference. This further process is particularly applied to the fine powder supported material.
The catalysts of this invention are well suited for hydrocarbon oxidations in any type of reactor: fixed bed, fluidized bed and recirculating solids reactor. In particular because the beta-silicon carbide support acts as a heat sink, the catalysts of this invention can be utilized at higher temperatures than the unsupported corresponding catalyst. More specifically, this catalyst is well suited to be efficiently utilized in fixed bed reactors with improved selectivity at high butane concentrations. In Figure 18 the rate of disappearance of n-butane was normalized to the weight fraction of VPO in each catalyst sample. The units of this rate expression are mole (n-butane)/[(gram VPO in catalyst)(sec)], as calculated according the following formula, which is appropriate for an apparent first order reaction: rate = (1- alpha)ln (1/(1 -alpha) F/W where: alpha = fraction of butane remaining (1 -alpha = fraction of butane converted or fractional butane conversion) E = flow rate of n-butane in mole/sec
W= weight of VPO in each catalyst sample (not total weight of catalyst).
Catalyst plotted: VPO (bulk), 30 wt % vanadium phosphorus oxide/ (no washing of vanadium phosphorus oxide precursor/ to remove 2:1 P V precursor phase), and 30 wt % vanadium phosphorus oxide/ with the washing step. The data shows that washing greatly improves the activity of the VPO/ phase at all temperatures. It also shows that the rate of reaction, normalized to the VPO in each catalyst sample, deviates (i.e., improves) for the sample at heat treatment temperatures greater than 350°C. EXAMPLE A suspension containing 5.0013 g of V2O5 (99.5%), 20 ml of isobutanol (99.5%) and 13 ml of benzyl alcohol was heated under reflux at 120-130°C for three hours to generate V2O in accordance with the following reactions:
isobutanol + V2O5 -» isobutanal + V2O4 + H2O benzyl alcohol + V2O5 → benzaldehyde + V2O4 + H2O
The water generated from the reactions was removed by a conventional Dean-Stark trap. The resulting solution was then cooled down to 20°C and a solution of 7.448 g of H3PO4 (98%) in 10 ml of isobutanol was added dropwise to the solution with stirring. The solution was then heated to Ϊ20-130°C under reflux until a strong blue-green color appeared in accordance with the reaction below.
2 H3PO4 + V2O5 → 2 (VO)HPO4»0.5 H2O + H20
Hemihydrate
Then, under strong stirring, 10 g of SiC at 80°C (so that it is closer in temperature to that of the reflux medium) is added to the solution while maintaining the temperature at about 130°C. The reaction carries on to completion and some of the solvents start to be eliminated. When the temperature of the mixture reaches about 135°C, the drying process is engaged under partial vacuum to obtain a "mud", which is placed in a glass vessel and dried at 220°C for 12 hours in air. The dried material is then crushed and sieved through a 40 mm (4 x 10~5 meter) sieve to eliminate particles less than 40 mm in size. At this point the product is the hemihydrate and VO(H2P04)2 supported on silicon. An X-ray diffraction pattern of this material is shown in Figure 1.
This sample is washed four times in hot water (90°C) to extract the VO(H2P04)2 phase which appears on the X-ray diffraction pattern of the unwashed hemihydrate (Figure 1). After the four washes, the VO(H2P0 )2 phase disappeared. The X-ray diffraction pattern of this washed material is shown in Figure 2.
The above sample were subjected to activation in accordance with the teachings of U.S. 4,132,670 by heating the samples to a temperature of 380°C at 3°C per minute under air flow of 1.5 cc per minute per cc of sample for 2 hours.
The samples were then heated to temperature of 480°C at 3°C per minute under air/butane (1.5% by volume of butane) flow of 3 cc per minute per cc of sample for 15 hours, then the temperature was allowed to cool to 420°C under air/butane
(1.5% by volume of butane) flow of 17 cc per minute per cc of sample for 100 hours. This produced "Activated Catalyst". X-ray diffraction patterns for this sample are shown in Figures 3 and 4, Figure 3 corresponding to the fine powder support and Figure 4 to the bigger grains. A transmission electron micrograph ("TEM") of the VPO catalyst-impregnated SiC particles is shown in Figure 7. As seen therein, the particles comprise a distinct transition phase (see arrows in Figure 7) located between the SiC "core" (denoted " 10" in Figure 7) and the VPO "shell" (denoted "20" in Figure 7).
Although catalytic tests can be conducted at this point, it is preferable to stabilize the activated catalysts by further subjecting the activated catalysts to up to 200 hours of the air/butane flow at 420°C. Catalytic tests were run on the stabilized samples relative to a conventionally prepared bulk VPO catalyst. The X-ray diffraction pattern for the conventionally prepared bulk VPO catalyst is shown in Figure 8. The X-ray difraction patterns for the used catalysts are shown in Figures 5 and 6, for the fine powder and the grains respectively. The catalytic performance of the catalysts relative to the conventionally prepared bulk VPO catalyst was determined by monitoring the oxidation of n-butane to maleic anhydride. The reaction was carried out as described above for activation at a ratio of O2 to butane of 1.4-1.5: 1 and from temperatures ranging from 310°C to 470°C. The results of the tests are shown in Figures 9-14. Figure 17 shows no pore volume below about 40 A, which is one of the criteria to distinguish beta-SiC from the alpha form (i.e., no microporosity). The surface area is about 30 m2/g (27-35 m /g preferred).
COMPARATIVE EXAMPLE Attempts at supporting VPO on alpha-SiC, showed "heterogeneity" in the material. XRD pattern (attached) essentially only the hemihydrate phase, with only a very small amoung of alpha-SiC; the VPO did not adhere to the SiC, and has separated.
A suspension containing 5.0051 g of V2O5 (99.5%), 20 ml of isobutanol (99.5%>) and 13 ml of benzyl alcohol was heated under reflux at 120-130°C for three hours to generate V2O4 in accordance with the following reactions:
isobutanol + V2O5 — isobutanal + V2O4 + H2O
benzyl alcohol + V2O5— benzaldehyde + V2O4 + H2O
The water generated from the reactions was removed by a conventional Dean-Stark trap. The resulting solution was then cooled down to 20°C and a solution of 7.026 g of H3PO4 (98%) in 15 ml of isobutanol was added dropwise to the solution with stirring. The solution was then heated to 120-130°C under reflux until a strong blue-green color appeared in accordance with the reaction below.
2 H3PO4 ÷ V2O4 — 2 (VO)HPO4 • 0.5 H2O + H2O Hemihydrate
Then, under strong stirring, 10.033 g of SiC in grains between 36 μm and 150 μm (SiC Norton 5132 washed with KOH to extract any oxides bulk or on the surface) at 80°C added to the solution while maintaining the temperature at about 130°C. The reaction carries on to completion and some of the solvents start to be eliminated. When the temperature of the mixture reaches about 135°C, the drying process is engaged under partial vacuum to obtain a "mud", which is placed in a glass vessel and dried at 220°C for 12 hours in air. After this treatment the solid is heterogeneous, a fine powder contains mainly the hemihydrate and bigger grains are mainly composed of SiC (carborundum) (confirmed by X-ray diffraction). Different other treatments and processes have been tried to impregnate carborundum or alfa-SiC with hemihydrate (or VPO) without success. Figures 15 and 16 show the powder x-ray data for the alfa-SiC and its hydrate, respectively. Figure 16 shows the material to be macroscopically inhomogeneous.

Claims

What is claimed is:
1. Particles of beta-silicon carbide impregnated with a vanadyl pyrophosphate oxidation catalyst.
2. The impregnated particles of Claim 1 , wherein the particles are substantially free of VO(H2PO4)2.
3. The impregnated particles of Claim 1 , wherein the particles have a size greater than 0.3 mm or a size in the range 40-80 mm.
4. A process for preparing particles of beta-silicon carbide impregnated with a vanadyl pyrophosphate oxidation catalyst, comprising the steps of: (a) forming a heterogeneous reaction mucture of suspended vanadium
(IV) phosphate,
(b) adding particulate beta-silicon carbide to the heterogeneous reaction mixture under agitation at a temperature between 40°C and 120°C to form vanadium (IV) phosphate supported on the particulate beta-silicon carbide, (c) drying the vanadium (IV) phosphate supported on the particulate beta-silicon carbide,
(d) optionally washing the vanadium (IV) phosphate supported on the particulate beta-silicon carbide with water; and
(e) calcining the vanadium (IV) phosphate supported on the particulate beta-silicon carbide at elevated temperature to obtain the beta-silicon carbide particles impregnated with a vanadium pyrophosphate oxidation catalyst.
5. The process of Claim 4, wherein forming the heterogeneous reaction mixture of vanadium (IV) phosphate comprises heating vanadium pentoxide with at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds to form a feed of vanadium pentoxide reduced to a valence between 4 and 4.6, and then contacting the feed with a solution of orthophosphoric acid and at least one substantially anhydrous unsubstituted alcohol having 1-10 carbon atoms, 1-3 hydroxyl groups and free from olefinic double bonds.
6. The process of Claim 4, wherein forming the heterogeneous reaction mixture of suspended vanadium (IV) phosphate comprises the hydrochloric acid digestion of V2O5 and H3PO4 in an aqueous solvent.
7. The process of Claim 4, wherein step (b) is conducted at a temperature of 80°C.
PCT/US2000/009906 1999-04-15 2000-04-14 Vanadyl pyrophosphate oxidation catalyst WO2000062926A1 (en)

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