Synthesis of a MoVNbTe shell catalyst for oxidative dehydrogenation of ethane to ethylene

Abstract

A novel coated catalyst having an outer shell which is composed of a catalyst material having high surface area and contains molybdenum, vanadium, tellurium and niobium, and the use of this catalyst for the oxidative dehydrogenation of ethane to ethene or the oxidation of propane to acrylic acid and also a process for producing the catalyst is disclosed.

Claims

1. A coated catalyst comprising an inert support and a catalytically active outer shell which comprises a mixed oxide material of the formula Mo.sub.iV.sub.aNb.sub.bTe.sub.cO.sub.x present in an M1 phase, where a is 0.2-0.3, b is 0.05-0.2, c is 0.05-0.25, and x is selected such that the overall charge of the empirical formula is zero; wherein the coated catalyst has a BET surface area of more than 30 m.sup.2/g.

2. The coated catalyst as claimed in claim 1, wherein the coated catalyst has a mercury pore volume of greater than 0.1 cm.sup.3/g.

3. The coated catalyst as claimed in claim 1, wherein the coated catalyst has a mercury pore volume in the shell of greater than 0.2 cm.sup.3/g, based on the mass of the shell.

4. The coated catalyst as claimed in claim 1, wherein the inert support is selected from the group consisting of silicon oxide, aluminum oxide, steatite, mullite and cordierite.

5. The coated catalyst as claimed in claim 1, wherein the coated catalyst has a catalytically active outer shell having a layer thickness in the range from 200 to 400 m.

6. A process for producing a coated catalyst as claimed in claim 1, comprising the steps: a) production of a mixture of starting compounds, which contains molybdenum, vanadium, niobium and a tellurium-containing starting compound in which tellurium is present in the oxidation state+4 and also oxalic acid and at least one further oxo ligand, b) hydrothermal treatment of the mixture of starting compounds at a temperature of from 100 to 300 C., to give a product suspension, c) isolation and drying of the mixed oxide material present in the product suspension resulting from step b), d) optionally calcination of the mixed oxide material obtained in step c) under inert gas at from 300 to 450 C., e) production of a coating suspension containing the mixed oxide material from step d) with addition of organic and/or inorganic binders, f) coating of an inert catalyst support with the coating suspension from step e) by spraying the coating suspension into an agitated bed of the inert catalyst supports and optionally g) calcination of the catalyst particles from step f) at a temperature of from 80 to 400 C.

7. The process as claimed in claim 6, wherein the tellurium-containing starting compound is tellurium dioxide or a compound of the formula M.sub.x.sup.n+TeO.sub.3 (where n=1 or 2 and x=2/n) where M is an alkali metal or an alkaline earth metal.

8. The process as claimed in claim 6, wherein the mixture of starting compounds is present as an aqueous suspension.

9. The process as claimed in claim 6, wherein the mixture of starting compounds contains a dicarboxylic acid, a diol or another compound having two hydroxy groups in adjacent positions as further oxo ligand.

10. The process as claimed in claim 6, wherein the mixture of starting compounds contains molybdenum trioxide.

11. The process as claimed in claim 6, wherein the mixture of starting compounds contains vanadium pentoxide.

12. The process as claimed in claim 6, wherein the mixture of starting compounds contains citric acid as further oxo ligand.

13. The process as claimed in claim 6, wherein the mixture of starting compounds contains citric acid and glycol as further oxo ligands.

14. A catalyst powder comprising a mixed oxide material of the formula Mo.sub.iV.sub.aNb.sub.bTe.sub.cO.sub.x present in an M1 phase, where a is 0.2-0.3, b is 0.05-0.2, c is 0.05-0.25, and x is selected such that the overall charge of the empirical formula is zero; wherein the catalyst powder has a BET surface area of more than 30 m.sup.2/g.

Description

WORKING EXAMPLES

(1) The invention will be illustrated by the following nonlimiting working examples.

(2) Comparative Example 1 describes an MoVTeNb catalyst which was activated according to the prior art at 600 C. and pressed by customary methods with addition of customary tableting additives such as graphite and stearic acid to give pellets.

Comparative Example 1

(3) 68.25 g of TeO.sub.2 (Alfa Aesar) and 200 g of distilled H.sub.2O were firstly weighed together into the ZrO.sub.2-coated milling vessel and milled in a planetary ball mill using 50 1 cm balls (ZrO.sub.2) at 400 rpm for 1 hour. The milled slurry 1 was subsequently transferred together with 500 ml of distilled H.sub.2O into a 2 l glass beaker. 56.83 g of Nb.sub.2O.sub.5 and 200 g distilled H.sub.2O were weighed together into the ZrO.sub.2-coated milling vessel and milled in the same ball mill under identical conditions to the TeO.sub.2. This milled slurry was subsequently transferred together with 500 ml of distilled H.sub.2O into a second 2 l glass beaker for 2 hours. After 20 hours, the mixture was heated to 80 C. and 107.8 g of oxalic acid dihydrate was added to the Nb.sub.2O.sub.5 suspension 2. The slurry 3 is formed and was stirred for approximately 1 h using a magnetic stirrer. 6 l distilled H.sub.2O were placed in an autoclave (40 l) and heated to 80 C. while stirring. After the water had attained the temperature, 61.58 g of citric acid, 19.9 g of ethylene glycol, 615.5 g of MoO.sub.3 (Sigma Aldrich D.sub.50=13.0 m), 124.5 g of V.sub.2O.sub.5, the milled TeO.sub.2 (slurry 1) and the milled Nb.sub.2O.sub.5 in oxalic acid (slurry 3) were added in succession. 850 ml of distilled H.sub.2O were used for transfer into the autoclave and rinsing of the vessels. The total amount of water in the autoclave was 8.25 l (speed of the stirrer 90 rpm). After the autoclave had been closed, the contents were blanketed with nitrogen under slightly superatmospheric pressure (4 bar) for 5 minutes. A hydrothermal synthesis was carried out in the 40 l autoclave at 190 C. for 48 l (heating time 3 h). After the synthesis (suspension has a temperature of less than 50 C.), the suspension was filtered under reduced pressure through a blue band filter and the filter cake was washed with 5 l of distilled H.sub.2O. The precursor material P1 was produced in this way. P1 was subsequently dried at 80 C. for 3 days in a drying oven. The precursor material P2 was produced in this way. P2 was subsequently milled in a beater mill. The precursor material P3 was produced in this way.

(4) Solids yield: 0.8 kg

(5) P3 was then calcined under the following conditions: heating rate 5 C./min, 280 C./4 h, air flow: 1 l/min. A precursor material P4 was produced in this way.

(6) P4 was then activated under the following conditions: activation was carried out at 650 C. for 2 hours (heating rate 10 C./min) under N.sub.2 (0.5 l/min) in a retort in a furnace. The catalyst K1 was produced in this way.

(7) The catalyst K1 has a BET surface area of 9 m.sup.2/g and an N.sub.2 pore volume of 0.04 cm.sup.3/g.

(8) This powder K1 was then used to produce catalyst pellets K2. For this purpose, 473 g of the powder K1 were intimately mixed with 9.65 g of graphite, 54.96 g of stearic acid and 54.96 g of fine silicon dioxide (Syloid C809). The catalyst powder K3 was produced in this way.

(9) The catalyst powder K3 was granulated twice (i.e. pressed and once again comminuted through a sieve to give a granular material comprising particles in the range of about 30-400 m, using a Powtec roller compactor). The catalyst granules K4 were produced in this way. The catalyst granules K4 were tableted in a tableting press (Rotab) using a pressing pressure of about 11 kN to give rings (diameter 5.4 mm, height 5 mm, internal diameter 2.5 mm). The shaped catalyst body K5 was produced in this way.

(10) After tableting, the stearic acid was burnt out from the shaped catalyst body K5 at 350 C. in air in a Nabertherm convection oven using a slow heating rate (<1 C./10 min). The comparative catalyst K6 was produced in this way.

(11) Comparative Example 2 describes a comparative catalyst in the case of which the catalyst powder was produced by the process of the invention but was tableted in the same way as Comparative Example 1.

Comparative Example 2

(12) 116.06 g of TeO.sub.2 (Alfa Aesar) were firstly slurried in 1000 g of distilled H.sub.2O by means of a precision glass stirrer and milled in a MicroCer ball mill (Netsch) using 0.8 mm balls (ZrO.sub.2). The portion was subsequently transferred together with 750 ml of distilled H.sub.2O into a glass beaker and stirred by means of a magnetic stirrer. 96.64 g of Nb.sub.2O.sub.5 and 183.35 g of oxalic acid dihydrate were slurried in 1000 g of distilled H.sub.2O by means of a precision glass stirrer and milled in the same ball mill. The portion was subsequently transferred together with 750 ml of distilled H.sub.2O into a 3 1 glass beaker and stirred by means of a magnetic stirrer. After 20 hours, both suspensions were heated to 80 C. and stirred for about 1 hour. 1046.7 g of MoO.sub.3 (Sigma Aldrich; somewhat larger particles) were suspended in 8.5 1 of water and likewise milled quickly by means of this ball mill (D.sub.50=12.7 m). This 8.5 l of MoO.sub.3 suspension were placed in an autoclave (40 l) and heated to 80 C. while stirring. After the water had attained the temperature, 14.61 g of citric acid, 33.85 g of ethylene glycol, 211.61 g of V.sub.2O.sub.5, the milled TeO.sub.2 and the milled Nb.sub.2O.sub.5 in oxalic acid were added in succession. The total amount of water in the autoclave was 14 l (speed of the stirrer 90 rpm). After the autoclave had been closed, the contents were blanketed with nitrogen under slightly superatmospheric pressure (4 bar) for 5 minutes. A hydrothermal synthesis was carried out in the 40 l autoclave at 190 C./48 h (heating time 3 h). After the synthesis (i.e. when the suspension has a temperature of less than 50 C.), the suspension was filtered under reduced pressure through a blue band filter and the filter cake was washed with 5 l of distilled H.sub.2O. The filter cake was then dried at 80 C. in a drying oven for 3 days and subsequently milled in a beater mill. The solids yield was 0.8 kg.

(13) The solid was subsequently activated: it was calcined at 400 C./2 h (heating rate 10 C./min) under N.sub.2 (0.5 l/min) in a retort in a furnace.

(14) The activated solid has a BET surface area of 27 m.sup.2/g and an N.sub.2 pore volume of 0.116 cm.sup.3/g.

(15) This powder was used to produce catalyst pellets. These pellets were produced as described in Comparative Example 1.

(16) Example 3 describes the catalyst according to the invention in the case of which only 20% by weight of catalyst composition were applied to an inert support.

Example 1

(17) 116.06 g of TeO.sub.2 (Alfa Aesar) were firstly slurried in 250 g of distilled H.sub.2O and milled in a ball mill. The portion was subsequently transferred together with 7500 ml of distilled H.sub.2O into a glass beaker. 96.64 g of Nb.sub.2O.sub.5 were slurried into 250 g of distilled H.sub.2O and milled in a ball mill. The portion was subsequently transferred together with 500 ml of distilled H.sub.2O into a glass beaker. Next morning, the mixture was heated to 80 C., 183.35 g of oxalic acid dihydrate was added to the Nb.sub.2O.sub.5 suspension and the mixture was stirred for about 1 hour. 1046.7 g of MoO.sub.3 (Sigma Aldrich) were suspended in 8.5 l of water and milled (D.sub.50=2.9 m) using a MicroCer ball mill for 4 hours with circulation. This 8.5 1 of MoO.sub.3 suspension was placed in an autoclave (40 l) and heated to 80 C. while stirring. After the water had attained the temperature, 14.61 g of citric acid, 33.85 g of ethylene glycol, 211.61 g of V.sub.2O.sub.5, the milled TeO.sub.2 and the milled Nb.sub.2O.sub.5 in oxalic acid were added in succession. The total amount of water in the autoclave was 14 l (speed of the stirrer 90 rpm). The contents of the autoclave were subsequently blanketed with nitrogen. A hydrothermal synthesis was carried out in the 40 l autoclave at 190 C./48 h. After the synthesis, the mixture was filtered under reduced pressure through a blue band filter and the filter cake was washed with 5 l of distilled H.sub.2O. The filter cake was subsequently dried at 80 C. in a drying oven for 3 days and subsequently milled in a beater mill (small IKA laboratory mill). The solids yield was 1.4 kg.

(18) The solid obtained was subsequently calcined: calcination was carried out at 400 C./2 h, (heating rate 10 C./min) under N2 (0.5 l/min) in a retort in a furnace.

(19) The activated solid has a BET surface area of 50 m.sup.2/g and an N2 pore volume of 0.27 cm.sup.3/g.

(20) This powder was then used to produce a coating suspension. For this purpose, 181 g of the powder were suspended in 1047 g of water, and 39.93 g of Bindzil 2034DI silica sol, 4.52 g of Syloid C809 and 13.57 g of Coconit were added. This suspension was homogenized using an Ultra-Turrax stirrer (5 min/6000 rpm). 54.30 g of EP16 vinyl acetate adhesive from Wacker was subsequently added and the total mixture was then stirred for 1 hour by means of a magnetic stirrer. 600 g of steatite rings (4 mm diameter, 2 mm internal diameter, 4 mm height) were then coated with the coating suspension in a coating plant from Httlin. Here, the bed of the rings was set into rotating motion by means of an air stream of from 198 m.sup.3/h to 260 m.sup.3/h (70 C.) from below through oblique slots in a plate. The coating suspension was sprayed through nozzles into this rotating bed (0.3 bar). (Coating loss via the air stream: 14.5%; proportion of the porous layer (silica+catalyst): 21.5%; proportion of the active composition: 19.2%)

(21) After coating of the shaped catalyst support bodies, the vinyl acetate adhesive was burnt out at 320 C. in air in a furnace.

(22) Example 2 describes a test for catalytic activity in the oxidative dehydrogenation of ethane to ethylene at various temperatures.

Example 2

(23) The catalysts were tested in a test for activity in the oxidative dehydrogenation of ethane.

(24) From 380 to 420 ml (373 g of pellets or 460 g of coated rings) were introduced into a tube reactor heated by means of a salt bath (diameter 2.54 cm, length 1 m, isothermally heated zone 80 cm).

(25) The catalyst was heated to 290 C. under a stream of nitrogen. Steam, air and subsequently ethane were then additionally introduced until the following flow rates had been achieved: N.sub.2=48 sl/h; water=0.7 ml/min (reported as liquid, water was vaporized); air=35.3 sl/h and ethane 13.7 sl/min. The salt bath temperature was then increased in steps of 2 C. to the temperatures listed in Table 1 (the unit [sl] corresponds to the standard liter, i.e. one liter at 1.0133 bar and 0 C.)

(26) The inlet gas compositions and the outlet gas compositions were analyzed. For this purpose, a substream was drawn off through heated conduits by means of a vacuum pump. This analysis substream was firstly drawn through a sample valve of a GC, through a gas cooler and then dried and through an NDIR analyzer (from Rosemount). In the GC, ethane, ethene, acetic acid and water were analyzed by means of an Rt U-BOND column having a temperature profile of from 45 C. to 190 C. in 8.4 minutes at a gas flow of 10 ml/min. The NDIR analyzer (Rosemount) contains NDIR cells for CO, CO.sub.2, ethane, ethene and also a paramagnetic oxygen sensor.

(27) The conversion of ethene was calculated by comparison of the inlet and outlet gas compositions and is shown in Table 1.

(28) It can be seen from the results in Table 1 that the catalyst of Comparative Example 2 is significantly more active than the comparative catalyst 1 according to the prior art at 330 C., since it achieves the same ethane conversion of 67% at as low as 302 C. The salt bath temperature for the catalyst of Comparative Example 2 cannot be set to 330 C. since an uncontrolled temperature rise in the exothermic reaction (runaway reaction) is observed under these conditions. For this reason, the catalyst of Comparative Example 2 cannot be utilized optimally.

(29) The catalyst of Example 3 according to the invention, on the other hand, has only 20% of the active mass in the same reactor volume and achieves an ethane conversion of 50% at 330 C.

(30) TABLE-US-00001 TABLE 1 N.sub.2 pore BET surface Hg pore BET of volume of Cat. Proportion area of volume of Salt bath T.sub.Max in the powder the powder shape of active catalyst catalyst temperature Ethane calcination catalysts catalysts external composition particles particles in ODH conversion Exp. [ C.] [m.sup.2/g] [cm.sup.3/g] [mm] [%] [m.sup.2/g] [cm.sup.3/g] [ C.] [%] Comp. 1 600 9 0.037 pellet 5.4 90 44 0.23 330 C. 67 Comp. 2 400 27 0.12 pellet 5.4 90 57 0.28 302 C. 63 Ex. 1 400 50 0.27 coated 4 19.2 65 0.115 345 C. 64 (coating 0.54) 330 C. 50

(31) Table 1 compares the BET surface areas and the pore volumes of the catalyst according to the invention of Example 3 with the other comparative examples.