Catalytic Selective Oxidation - American Chemical Society

Chapter 18

Hydrocarbon Partial Oxidation Catalysts Prepared by the High-Temperature Aerosol Decomposition Process Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch018

Crystal and Catalytic Chemistry William R. Moser Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609 The advantages of aerosol processes for the synthesis of complex metal oxide and supported metal catalysts are mainly in the high degree of homogeneity of the finished catalyst and their ease of preparation. The process demonstrated an unusual capability to synthesize ion modified catalysts over a wide range of modifying ion concentrations while maintaining homogeneous solid solutions structures. Catalytic investigations using these complex metal oxide catalysts showed that smooth changes in catalytic performance could be realized as functions of the concentration of the modifying ions. The method led to materials of superior catalytic properties for propylene oxidations by bismuth molybdates and iron modified bismuth molybdates, and methane activation catalysts in the perovskite series. A new phase was obtained from the synthesis of P-V-O catalysts for butane oxidation. Platinum-iridium and silver metal supported catalysts on a high surface area α-alumina displayed an exceptionally high degree of homogeneity.

A fundamental objective of modern catalyst synthesis is to produce solid state catalysts that are single phase and homogeneous solid solutions. This is especially important at the stage of catalyst discovery research where one usually attempts to establish linear relationships between catalyst composition vs chemical performance. It is also important in the fabrication of commercial scale catalysts to ensure reproducible catalytic performance in the process reactor. Classical methods of preparation such as co-precipitation, thermal fusion, spray drying, and freeze drying usually result in some multiple phase materials, a condition that becomes more severe as the concentrations of the modifying ions are increased. These methods are sometimes unreliable with regard to reproducibility. In addition, the synthesis of ion substituted homogeneous solid solution catalysts such as perovskites, scheelites, and spinels are difficult to obtain as single phase materials if a moderate to high surface area material is required. 0097-6156/93/0523-0244$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S. Ted, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch018

18.

MOSER

Hydrocarbon Partial Oxidation Catalysts

245

This report will describe an aerosol process that was developed in these laboratories over the past few years for the synthesis of complex metal oxide catalysts. Providing that the aerosol reactor has sufficient capabilities for process variability, the method was found in these laboratories to produce a wide range of catalysts not only for hydrocarbon partial oxidation, but also catalysts for a wide range of commodity and fuels chemical processes. Several years ago, we reported (1) the development of a down flow reactor configuration that was found effective for the synthesis of a variety of supported and unsupported catalysts, novel ceramics and superconductors. In the interim, the process was modified to an up flow configuration to afford an even greater latitude in increasing the residence time of the aerosol droplets within the heated zone of the reactor; but the up flow configuration also affords a more even decomposition of the aerosol. This configuration now permits the formation of metal oxide catalysts, ceramics and superconductors that could not be synthesized using the previously described down flow reactor configuration (1). The principal advantage of aerosol processes for metal oxide synthesis is that the process starts with an ideal precursor, an aqueous metal salt solution. This is converted to an aerosol in the gas phase followed by a rapid decomposition at high temperature over 1-5 seconds residence time to produce high surface area homogeneous metal oxide catalysts. The process has demonstrated capabilities superior to all other techniques for the synthesis of phase pure, high surface area catalysts in a reproducible way. The most recent configuration of the process schematic is illustrated in Figure 1, and the continuous process is amenable to the preparation of industrial scale catalysts. The origin of such aerosol processes is found in early patents (2) by Ebner in 1939. Similar processes were described in the mid 1970's by Roy and co-workers (3) for the synthesis of alumina and calcium aluminate. The contribution of this laboratory to aerosol synthesis was to design a process for controlling the injection and decomposition of the aerosol in a well controlled way using facilities which provide the process variability required for a general metal oxide synthesis. The processes which evolved is described as the High Temperature Aerosol Decomposition (HTAD) process; other similar processes are known as spray pyrolysis, and evaporative decomposition of solutions (EDS). The aerosol is formed as shown in Figure 1 by either an ultrasonic aerosol droplet generator or by a forced air spray nozzle. The residence time within the furnace may be controlled by make-up air fed into the bottom of the reactor to control the contact time for decomposition between 1 to 15 seconds. The temperature is regulated by a three zone furnace which may be controlled between ambient and 1350 °C, either with or without an axial gradient. The process operates under a slight vacuum, and may use a pulsed injection depending on the material to be synthesized. Several process parameters may be used to purposely adjust the surface areas of the desired catalysts. Usual surface areas are in the range of 10-80 m^/g, although other complex metal oxides, spinels, were synthesized in areas as high as 250 m^/g. Studies in these laboratories have resulted in the synthesis and catalytic evaluations on a wide range of perovskites and ion modified homogeneous solid solutions for Fischer-Tropsch catalysis, copper modified spinels for higher alcohol synthesis, ion substituted perovskites for methane activation, alkali modified metal

In Catalytic Selective Oxidation; Oyama, S. Ted, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

246

CATALYTIC SELECTIVE OXIDATION CENTER UNE THERMOCOUPLE

PRESSURE GAGE REACTOR SIDE


HIGH TEMPERATURE OVEN FILTER GAS TEMPERATURE

INJECTOR GAS FLOW CONTROL

METAL SALT FEED SOLUTION

PUMP

PRESSURE GAGE PUMP SIDE

Figure 1. Aerosol process configuration for the continuous preparation of metal oxides by the High Temperature Aerosol Decomposition (HTAD) Process. oxides for methane activation, methanol synthesis catalysts, propylene oxidation catalysts, silver on alpha-alumina supported catalysts for hydrocarbon oxidation, metallic and bimetallic catalysts for selective dehydrogenations, and high surface area, non-acidic, alpha-alumina. The process also resulted in a wide variety of ceramics of exceptionally high phase purity, nano-phase materials, and the direct synthesis of the orthorhombic structure of the Y-Ba-Cu superconductor and its metallic silver modifications. This report will concentrate on the synthesis of a wide variety of hydrocarbon partial oxidation catalysts using the HTAD process. The objective is to provide an understanding of the range of advantages of the HTAD process for such catalyst fabrication for widely different oxidation catalysts rather than describing details for one specific system. The synthesis, characterization, and catalytic properties of HTAD oxidation catalysts will be described in the following systems: 1.) bismuth molybdates for the oxidation of propylene to acrolein; 2.) iron modified bismuth molybdate catalysts for propylene oxidation in the substitutional series, Bi(2-2x)Fe2 Mo30i2 where χ was varied from 0.0 to 1.0; 3.) ytterbium modified strontium cerium oxide perovskites for the oxidative dimerization of methane to ethane/ethylene; 4.) phosphorus modified vanadium oxides for butane oxidation to maleic anhydride; 5.) platinum, silver, and other noble metals for hydrocarbon oxidation; and Sr and Ca modified La(i_ )M FeO(3_ ), La(i_ )M CoO(3_y), and other perovskites for CO oxidation. x

x

x

v

x

x


18. MOSER


247

Experimental Although this report describes the preparation of a wide variety of oxidation catalysts, only the iron modified bismuth molybdates will be described in detail. Other preparations are described in this section through an indication of the starting soluble salts and their synthesis temperatures which are the key process parameters. Other aspects of their HT AD preparations are similar to that described for the iron bismuth molybdates which follows. The iron substitution series, Bi(2-2x)Fe2xMo30i2, synthesized in the HT AD process using the up flow configuration shown in Figure 1 as well as the down flow configuration described before(l). The up flow configuration was found optimum for the synthesis of this type of compound and only those results will be presented here. Although a variety of process parameters were investigated resulting in different morphologies and surface areas, the optimum conditions for synthesis were as follows: Feed solution, Total metal ion molarity of 0.6 M in de-ionized water containing 10% v/v of nitric acid consisting of Βί(Νθ3)3·5Η2θ, Fe(N03)3«9H20, and (ΝΗ4)6Μθ7θ24*4Η2θ; Furnace temperature of 900°C and pre-heated make-up air temperature of 200°C flowing at 3-4 ft^/min into a mullite tube which was 2 3/4 inches inside diameter and 4ftlong. A spraying nozzle was used with a head pressure of 20 psig, and the solution was introduced to the reactor using a pulsed injection of 2 seconds on and 4 seconds off; the product was collected using either a Pall, porous stainless steelfilteror afiberfilter.The process was run under 1-5 in. H2O vacuum to ensure smooth flow of materials through the reactor section. All solid materials were analyzed by XRD, SEM, infrared, BET surface area, and inductive coupled arc plasma. All up flow samples and co-precipitation samples resulted in acceptable elemental analysis when analyzed for Bi, Fe, and Mo. The catalysis studies for propylene oxidations were carried out in a heated, well insulated micro-reactor consisting of a 1/2 inch i.d. stainless steel reactor section. Gases were fed by mass flow controllers, and water vapor was fed to the reacting stream by passing the air feed stream through a temperature controlled water saturator. All products were analyzed using an on-line GC. The upper portion of the reactor was filled with 20 mesh silanized quartz, and the catalyst bed consisted of 2.0 mL of the catalyst powder calcined at 450°C and mixed with an equal volume of the silanized quartz. Feed compositions and reactor conditions used were: propylene/02/N2/H20 = 5.4/14.5/55.7/25.4; The reaction was studied at atmospheric pressures using contact times of 10 seconds. The GC analysis determined effluent concentrations of CO, CO2, oxygen and hydrocarbons using a back flushed, dual column configuration and thermal conductivity detection for the permanent gases and FID for hydrocarbon and oxygenate analysis. The methane oxidation studies used a schematic similar to the above except that the reactor section was fabricated entirelyfromquartz and reaction temperatures from 600 to 1000°C were examined. The precise conditions for the experiments discussed are indicated in the figure captions.


w

a

s

American Chemicai

Society Library 1155 16th St. N. W Washington, D G,

20036


248

CATALYTIC SELECTIVE OXIDATION

is

1.0 0.9 ^

g

0.8 ζ

11 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch018

2

s

c £

0.6 i

§ I i

0.5 i

ϋ

a

01

s

A Ο

0.7 ζ

α

0.4 i 3 0.3 f

S ο

ο

? 0.2

ο - Co—precipitation π - Downflow Δ - Upflow

ο

0.1

s

3 / 0.0 ·& . I 1 111 Μ 111 H 1 ι Μ Ν 11 11 n I 1 1111 1 1 ι μ 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Sο

Μ 1.0

1111

1111

»ιi

Bismuth Concentration Atomic Fraction i n Preparation Solution

Figure 2. Compositional diagram for the preparation of bismuth molybdate catalysts using the HTAD process configuration shown in Figure 1 at 900°C using air as make up gas. Plot is of concentrations of bismuth used in the reacting solution vs arc plasma analyzed concentrations of thefinishedcatalysts directly from HTAD reactor: Circles: Co-precipitation prepared materials. Triangles: Up flow prepared aerosol materials. Squares*. Down flow prepared aerosol materials. Results Synthesis and Characterization Bismuth Molybdates And Iron Substituted Bismuth Molybdates The aerosol syntheses of the binary bismuth molybdate compounds using stoichiometrics evenly distributed over a bismuth atomfractionfrom0 to 1.0 and the iron modified series Bi(2-2x)^ 2x^°3^i2 » 0.0 to 1.0 were carried out at 900°C. To illustrate that the aerosol synthesis providesfinishedcatalysts having predictable metal ion concentration based on the feedstock compositions, Figure 2 shows that the up flow configuration resulted in correct metal ion concentrations. Elemental analyses for Bi, Fe and Mo by arc plasma on these samples gave satisfactory values. The figure also shows that the down flow preparation of these compounds led to high Bi concentrations. This was likely due to the fact that a water scrubber was used to collect the catalyst particles which selectively removed molybdenum oxide through e

x=



18. MOSER


249

solubilization. Results were in agreement with those obtained from very careful preparations of the same materials by co-precipitation, except that the HTAD syntheses take considerably shorter preparation times compared to the co-precipitated materials. The as prepared HTAD materials demonstrated BET surface areas which were between 1 to 14 m^/g depending on bismuth concentrations. The surface areas of the iron modified series started out at low Fe concentrations, 0.03 atomfractionFe, of 1 m^/g and steadily increased to 7 m^/g at an iron atom fraction of 0.40. All of these samples were calcined to the maximum temperature (450°C) used in the catalytic investigation. This calcination led to a decrease in the surface areas of all of the catalysts into the league of 1-2 m^/g, and were nearly the same as that determined on the co-precipitation series of 1-2.5 m^/g. SEM analysis showed that the particles had a morphology of 1-10μπι spherical particles and were hollow. Upon calcination, these particles sintered with the above mentioned loss in surface area without changing size or spherical morphology of the parent particle. The XRD patterns for the material where the iron substitution was x= 0.40 as obtained from the HTAD reactor (top curve) and after calcination in air at 450°C (bottom curve) are shown in Figure 3. These X-Ray diffraction patterns are typical of the entire two series of bismuth molybdates. An examination of the XRD patterns of the HTAD materials in comparison to the patterns collected on identical compositions by co-precipitation and fusion showed that the phase purities observed for the HTAD materials were equal or better than either the co-precipitated or fusion prepared materials. Over the entire iron substitution series, the XRD reflections broadened as the iron concentration in the synthesis increased. This suggests a hindrance to crystal particle growth as evidenced by the substantially broadened reflections. The phase analysis of the HTAD iron substitutional series showed that the mole fraction of the compound containing no bismuth, Fe3Mo30i2 continuously increased as the iron substitution increased; however, high concentrations of iron led to separate phase M 0 O 3 and Fe2Û3. Thus, the synthesis did not provide a homogeneous solid ion substitution by iron over the entire composition range studied. However, substitution may have occurred into the α-Βΐ2Μθ3θΐ2 host oxide at low concentrations below 5 atom % (based on total metal ions). Silver, Platinum and Noble Metals Supported On Alumina The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, PtIr, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for ccalumina were evident. The low temperature required for calcination to the alphaphase along with TEM results suggest that this material was formed as nano-phase, aalumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. Seven compositions of Ag-alumina were synthesized between 0-65% w/w silver at 900 °C by the HTAD process. Characterization of these materials by XRD



250


15

20

25

30

35

40

45

50

55

60

65

70

DEGREES 2Θ Figure 3. XRD patterns for the iron substituted composition having the empirical formula of Bi(2-2x)^ 2x^°30i2 where χ = 0.40. The top powder diffraction pattern is for the aerosol prepared material as directly obtained from the reactor. The bottom diffraction pattern is for the material after calcining in air for 4 hr at 450°C. e

showed that only in the case of the 65% material did reflection for metallic silver appear. Phase analysis on all lower compositions of 1, 5, 15, 30, and 50% w/w Ag showed no reflections for any known silver or silver-alumina compound either as the reduced metal or oxide form. When all of the compositions were calcined at 900°C, near the melting point of metallic silver, strong reflections appeared for metallic silver and α-alumina. These data suggest that well dispersed metallic, supported catalysts may be directly synthesized by the aerosol process to very high concentrations.



18. MOSER


251

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. Phosphorus-Vanadium Oxide Catalysts for Butane to Maleic Anhydride Oxidation To illustrate the capabilities of the aerosol process for the synthesis of solid state oxidation catalysts not normally obtainedfromclassical synthesis, the preparation of a series of P-V-0 catalysts was examined. The original objective was to determine whether the synthesis of the reported (4) active phase, β-(νθ)2Ρ2θ7> of the catalyst responsible for the selective oxidation of butane to maleic anhydride could be synthesized by the aerosol technique. Although a variety of synthesis, compositions and reactor parameters were studied, the P-V-0 catalysts in the temperature series were synthesized in the up flow HTAD reactor using a 0.12 M solution of ammonium vanadate in water which contained the required amount of 85% phosphoric acid to result in a 1.2/1.0 P/V atom ratio. This atom ratio is normally preferred for the most selective oxidation of butane to maleic anhydride. Table I shows that the P/V atom ratios obtained for the analyzed, finished (green colored) catalysts were approximately the same as the feed composition when a series of preparations were studied between 350°C and 800°C. This was typical for all of the catalysts synthesized under a variety of conditions. Table I Analytical Data on HTAD Synthesized P-V-O Catalysts P/V Ratio P/V Ratio Reactor Température Catalyst Feed T°C 350 1.20 1.18 1.13 1.20 400 1.19 1.20 425 1.26 1.20 450 1.21 1.20 500 1.22 1.20 550 1.23 1.20 650 1.19 1.20 700 1.20 1.19 800



252


One interesting aspect of the synthesized catalysts was that their XRD patterns measured immediately after synthesis resulted in broad reflections as illustrated in Figure 4a for the catalyst synthesized at 600°C using an 8 seconds residence time of the aerosol in the reactor and a feed concentration of 0.8 M in vanadium. When the fresh catalyst was immediately calcined in nitrogen at 450°C for 3 hrs, the XRD shown in Figure 4b still showed only broad bands along with three reflections emerging at 21.3, 29.0, and 35.9 degree 2Θ. However, when the uncalcined material was allowed to stand in an air tight container for 14 days at room temperature, the well defined XRD pattern shown in Figure 4c was recorded, demonstrating peaks at 12.1, 24.7, and 29.2 degree 2Θ. The optimum process parameters for the synthesis of this type of material was at 700°C, using a vanadium concentration of 0.12 M and a residence time of 8 seconds, and a P/V ratio of 1.2. The XRD of the material synthesized under these conditions is shown in Figure 5 exhibiting only three reflections at 12.3, 25.1, and 29.1 degree 2Θ. This P-V-0 phase is not the same as the catalytically active species, P-(VO)2P2Û7, obtainedfromclassical synthesis where the reported (4) XRD reflections for the three main peaks were at 22.8, 28.3, and 29.9 degree 2Θ. It is also not the same as the reported (5) alcohol solvent prepared precursor. Aerosol preparations at different conditions resulted in similar but not identical reflection positions for all of the compounds after standing in closed containers for more than a week, where the positions of the three bands reported above were either shifted or of different intensities. Our interpretation of the large dspacing for the principal reflection, its variability with process conditions and crystal development on standing is that the catalyst synthesis afforded disordered lamella compounds which were microcrystalline immediately after synthesis but slowly crystallized upon standing. The surface areas of the compounds prepared between 600 and 800°C were between 6-14 m^/g, and the measurements on the freshly prepared materials were the same as those after calcining in nitrogen at 450°C for 3 hrs. Another series of synthesis studies showed that the addition of small amounts of oxalic acid to the synthesis solution resulted in an ability to systematically vary the oxidation state of the vanadium ion between 4 and 5. A wide variety of compositions and materials of slightly different crystal properties were prepared for evaluation as butane oxidation catalysts. These materials were active for butane oxidation, and the activity and selectivity varied according to the process parameters used for synthesis. This chemical performance data will be reported separately. Perovskite Catalysts for Methane Conversion Two series of catalysts were synthesized for subsequent evaluation as methane dimerization catalysts. The first series was alkali modified zinc oxide (6) and magnesium oxide catalysts (7), which were reported to be active for methane activation, while the second series was ion modified perovskites described by Machida and Enyo (8). The objective of the present study was to determine whether the aerosol technique could provide a wide range of ion substitutions as homogeneous solid solutions, and to determine whether moderately high surface area catalysts could


18. MOSER

253

Hydrocarbon Partial Oxidation Catalysts 7.31


3.06

7.03

I

1

1

1

1

1

1

1

1

10

15

20

25

30

35

40

45

50

D E G R E E S

r"—I

55

60

2 Θ

Figure 4. X-Ray diffraction patterns for phosphorous-vanadium oxide catalysts prepared by aerosol technique at 600°C, 8 seconds residence time, and 0.8 M V in feed: a. Catalyst analyzed by XRD immediately after synthesis, b. catalyst calcined at 450°C in nitrogen for 3 hrs immediately after synthesis, c. Catalyst allowed to stand for 14 days in an air tight container at ambient temperature without calcining.


254


be obtained by the aerosol method in a geometry having low micro-porosity. It was previously shown (1) that catalysts prepared by this technique generally demonstrate moderate to high surface areas depending on synthesis conditions, and the materials so obtained have no micro-porosity. The surface area is generally a result of the external surface of the particles which are formed either as thin walled hollow spheres with one or more holes through the walls or sub-micron fragmented particles. It is expected that this characteristics would be favorable for methane conversion to ethane and ethylene without over oxidation of the products. The first set of alkali modified materials was Li Zn(i_ )0(i_x/2). The value of χ was 0.00, 0.03, 0.07, 0.10, 0.20, and 0.40. These compounds were synthesized from aqueous solutions of their nitrates using the down flow reactor reported previously (1) but using a porous stainless steel filter and a HTAD process temperature of 1300°C. Processing conditions were adjusted so that a hollow spherical morphology resulted for all of the materials synthesized. Particle sizes of the spheres ranged from 0.5 to 6 microns. Surface areas varied from 4-8 ttfl/g. Phase analysis of the entire series by XRD showed that only reflections for hexagonal zinc oxide were observed except in the sample of highest lithium concentration. This material showed a trace of an unidentified separate phase (intensity ratio to the main ZnO reflection was 1.1%) with the only observable reflection at 42.9 degree 2Θ. Thus, the aerosol method evidently resulted in a well dispersed form of lithium within the ZnO matrix. Computation of the lattice parameters for ZnO for all of the compounds using 5 seconds scans at a step rate of 0.05 degree per minute between 55 and 65 degree 2Θ showed that the lattice parameters varied little. This suggests that either 1.) the compounds in this series did not result in homogeneous solid solutions of lithium in zinc oxide, but the structure is that of exceptionally well dispersed lithium oxide within the ZnO host, or 2.) the lithium ion substituted into the interstices of the host crystal giving little change in the lattice parameters. Synthesis of solid state materials within the substitutional series Mq \ZriQ 9O0.95 where M was Li, Na, K, Rb, and Cs resulted in well dispersed alkali metals for Li and Na and progressively more intense separate phase reflections for M2O progressing down the alkali metal series for K, Rb, and Cs. The data suggest that the alkali metal component in the LiZnO series and Li and Κ in this series are stabilized by the ZnO host and not simply physically admixed micro-crystallites of the alkali metal component in ZnO. Preparation of these same materials where a water collector was used in the process resulted in hollow spheres which consisted of pitted, porous walls as demonstrated by SEM analysis. The holes in the walls were most visible for the Cs and Rb compounds at a magnification of 5,000 and were not visible at this magnification for the Li, Na, and Κ compounds. EDX analysis of the Cs and Rb materials did not detect either element indicating that the pores were make in the walls of the sphere due to the leaching effect of the water scrubber of large crystallites after the synthesis. Materials prepared by the dry collection method exhibited no pores in the walls of the spheres and they demonstrated normal alkali metal analyses. Methane dimerization catalysts in the perovskite substitutional series SrCe(i_ χ)Υ°χΟ(3-χ/2) where χ was varied from 0.0 to 1.0 were synthesized by the HTAD process. These same catalysts were synthesized by Machida and Enyo (8) by fusion


x

x


18. MOSER


methods and were shown to be active for methane dimerization. The objective of the present studies was to determine 1.) whether the HTAD process provided catalysts which were substantially free from separate phases, and 2.) whether the Yb substitution into the host perovskite could be extended to high levels of substitution. The perovskites were synthesized at 1300°C using total solution molarities in each preparation of 0.30 M and residence times of 2-4 seconds. No further calcination was necessary to obtain the pure perovskite phase. When χ was varied in SrCe(i. )Yb O(3_x/2) fro 0.0 to 1.0, and the resulting materials examined for their phase purity and composition, it was found that the materials were composed of either one of two phases which predominated the composition at each end of the compositional series. When χ was between 0.00 and 0.30, the perovskite structure dominated the composition, and when χ was 0.40 to 1.00, the fluorite structure dominated the phase composition. The substitution of the Yb ion in the perovskite portion of the substitutional series (x=0.00 to 0.40) gaveriseto a homogeneous solid solution of Yb in SrCeC>3 as evident from the linear change of the d-spacing vs Yb concentration for the principal reflectionfrom3.05 at x=0.00 to 2.96 at x=0.40. Furthermore, a plot of d-spacing vs Yb concentration for x=0.60 to 1.00 showed that the Ce ion formed a homogeneous solid solution in the fluorite structure, SrYb2Û4, as shown by the linear decrease in a main reflection from 2.054 at x=0.06 to 2.016 at x=1.00. No separate phases for SrO, CeC>2, or Yb2U3. were detected in the catalysts by XRD analyses. By adjusting the process parameters, the materials were synthesized as hollow spheres having surface areas between 2-4 m^/g. The surface areas smoothly increased as the mole fraction of Yb was increasedfrom0.00 to 1.00. The preparation of this series of compounds by classical means of fusion at 1300°C resulted in surface areas of 0.9 to 1.4 m^/g. This method generally results in low surface areas in the range of 0.1 to 1 m^/g. High temperature fusion techniques also usually result in the formation of separate phases of the component metal oxides along with the desired perovskite phase unless the materials are extensively thermally processed at high temperatures for several days. This processing usually leads to very low surface areas near 0.1 m^/g. The synthesis of these materials having thin walls to aid in the dissipation of heat from the particle coupled with a low particle micro-porosity should be favorable for the selective conversion of methane to ethane and ethylene The catalytic properties for methane conversion were examined using this series of solid state materials mainly between 710 and 810°C in a quartz reactor with on-line GC analysis using an inlet feed composition of He/CH4/02/N2 =11.5/2.0/1.0/0.5. Figure 6 illustrates the effect on methane conversion of a variation in the concentration of Yb from x=0.00 to 1.00 at 810°C. The figure shows that conversions normalized to unit surface areas could be increased smoothly up to 60% across the substitutional series under these conditions. On the other hand the selectivity of C-2 products rose smoothly as a result of homogeneous substitution in the perovskite structure (x=0.00 to 0.40) from 34% to 46%. However, during homogeneous substitution in the fluorite material (x=0.60 to 1.00) the selectivity was surprisingly flat between 30 to 33%. Non-selective products were CO and CO2. Although the maximum C-2 yields under optimum conditions were around 20%, the interesting observation is the fact that the chemical performance could be m

x


255

x



256


DEGREES

SO

Figure 5. XRD of optimum synthesis of P-V-0 catalyst by HTAD process at 700°C, 12 seconds residence time, using feed solution of 0.12 M in V. 80.0

0.0

|ΐΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΙΙΙΙΙΙΙΙ|ΙΜΜΙΙΙΙ|ΙΙΙΙΙΙΜΐ|

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

X MOLE FRACTION Yb SUBSTITUTED

Figure 6. Methane conversions (normalized to unit surface area) as a function of the mole fraction for the substitution (x) of ytterbium in SrCe(i_ )Yb O(3.. /2). Conditions for catalytic experiment were: 810°C in a quartz reactor using an inlet feed composition of He/CH4/C>2/N2 =11.5/2.0/1.0/0.5 x

x


x

18. MOSER


257

systematically changed as a function of substitution in both the perovskite and fluorite regions of the substitutional series. Perovskite Catalysts for CO Conversion A series of perovskites catalysts was synthesized to illustrate the capabilities of the process to produce materials as homogeneous solid solutions and in unusually high phase purities. This type of catalyst was reported by Voorhoeve and co-workers (9) to be active for CO oxidation. Compounds in the substitutional series L a n . )M CoO(3_y) d ^ (1-χ)Μχ*βΟ(3-γ) were synthesized by the HTAD process (1) where M was Ca and Sr and χ = 0.0 to 1.0. In addition, nine lanthanide based perovskites were synthesized in the series LnFe03 where Ln was La, Ce, Pr, Nd, Sm, Gd, Dy, Er, and Yb. An examination of the XRD patterns for all of the compounds synthesized demonstrated reflections only for the perovskite. Surface areas recorded for the Ca and Sr substitutional series, LaQ_ )M FeO(3_ ), varied from 10 to 25 m^/g, and a few experiments using different process conditions to produce fragmented materials rather than hollow spheres resulted in perovskites having 60 m^/g. All of these compounds required 1200°C HTAD reactor temperatures using residence times between 2-4 seconds, for synthesis of the pure phase perovskite. Figure 7 illustrates a typical XRD pattern for the Lao. gCao. 2^03 perovskite. It is seen from the figure that the material was synthesized without phase impurities, and a plot of d-spacings for the main reflection vs Ca molefractionled to a linear decrease in d-spacings as Ca increased. This series of catalysts was not studied for CO oxidation but was extensively examined for CO-H2 conversions to hydrocarbons where systematic changes in chemical performance were observed for both the Ca and Sr substitutional series across the entire substitutional series.


a n

x

α

x

x

x

v

Catalytic Studies using Bismuth Molybdate Compositions The catalytic reactivity of the HTAD bismuth molybdates using a series of 14 compositions varying the bismuth ion concentration from 0 to 100% and the iron substitutional series of Bi(2-2x)Fe2xMo30i2 evaluated for the partial oxidation of propylene between 352 and 452°C. The conditions used were: propylene/02/N2/H20 = 5.4/14.5/55.7/25.4 at atmospheric pressure and contact times of 10 seconds. Figure 8 shows the unusual reactivity data as a function of iron substitution in the latter series. The reactivityfroman iron atom substitution of 5% to 35% iron (based on the total g. at of metal atoms) is similar to data obtained in other laboratories using classically prepared Fe-Bi-Mo-0 catalysts. However, the sharp spike in rate in the substitutional range of 0-5% is noteworthy, and may point to an important relationship between structure and reactivity. The XRD of these materials in this low range of iron substitution showed only reflections for the host oxide aB12M03O12 indicating that the iron was well dispersed. The data do not permit the conclusion that the iron, even at this low concentration, entered the lattice as a homogeneous solid solution since the concentration range is only moderately above the detection limits for a separate iron oxide phase by XRD. w e r e



258


I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

10 16 20 26 30 36 40 46 60 66 60 66 70 76 ΘΟ

DEGREES 2Θ Figure 7 X-Ray diffraction pattern for the aerosol synthesis of the calcium substituted perovskite Lao sCao^FeC^ 1200°C using a 3 seconds residence time. a t

35

Iron Concentration A t o m i c P e r c e n t of Total

Metals

Figure 8. Propylene consumption rate data for aerosol prepared catalysts having the composition of Bi(2-2x)Fe2 Mo30i2 where χ = 0.0 - 1.0. Top curve represents data at a catalytic reactor temperature of 450°C and the bottom curve represents data taken at 400°C. The catalytic reactor conditions were: propylene/02/N2/H20 = 5.4/14.5/55.7/25.4 at atmospheric pressure and contact times of 10 seconds. x



18. MOSER


259

The selectivity to acrolein as a function of iron substitution is shown in Figure 9. It is interesting to note that the selectivity profile vs iron concentration follows that observed for rates vs iron concentration. The unusual aspect of these data is the fact that selectivity increased in the regime where the catalyst was performing at highest conversion. Finally, Arrhenius treatments of the catalytic data were examined for the HTAD synthesized substitutional series, Βί(2-2χ)^β2χΜθ3θΐ2 > and the binary bismuth molybdate series where Bi/Mo ratios were variedfrompure Mo oxide to pure Bi oxide. The noteworthy aspect of the oxidation results is that in the most reactive regime of χ = 0-5% atomfractionFe, before separate phase Fe3Mo3

Catalytic Selective Oxidation - American Chemical Society

Recommend Documents