Oxygen carrying materials with surface modification for redox-based catalysis and methods of making and uses thereof

Abstract

Redox catalysts having surface medication, methods of making redox catalysts with surface modification, and uses of the surface modified redox catalysts are provided. In some aspects, the redox catalysts include a core oxygen carrier region such as CaMnO3, BaMnO3-δ, SrMnO3-δ, Mn2SiO4, Mn2MgO4-δ, La0.8Sr0.2O3-δ, La0.8Sr0.2FeO3-δ, Ca9Ti0.1Mn0.9O3-δ, Pr6O11-δ, manganese ore, or a combination thereof; and an outer shell having an average thickness of about 1-100 monolayers surrounding the outer surface of the core region. The outer shell can include, for example a salt selected such as Li2WO4, Na2WO4, K2WO4, SrWO4, Li2MoO4, Na2MoO4, K2MoO4, CsMoO4, Li2CO3, Na2CO3, K2CO3, or a combination thereof.

We claim: 
     
         1 . A redox catalyst comprising:
 (a) a core region having an outer surface, the core region comprising an oxygen carrier, and   (b) an outer shell having an average thickness of about 1-100 monolayers surrounding the outer surface of the core region, the outer shell comprising a metal salt.   
     
     
         2 . The redox catalyst according to  claim 1 , wherein the outer shell comprises an alkaline earth metal tungstate selected from the group consisting of tungstates having a formula BWO 4 , B 2 WO 5 , B 3 WO 6 , and combinations thereof, where B is selected from the group consisting of Mg, Ca, Sr, and Ba. 
     
     
         3 . The redox catalyst according to  claim 1 , wherein the outer shell comprises an alkali metal tungstate selected from the group consisting of Li 2 WO 4 , Na 2 WO 4 , K 2 WO 4 , Cs 2 WO 4 , and a combination thereof. 
     
     
         4 . The redox catalyst according to  claim 1 , wherein the outer shell comprises a halide salt having a formula AX,
 where A is Na, K, Li, Rb, or Cs, and   where X is F, Cl, Br, or I.   
     
     
         5 . The redox catalyst according to  claim 1 , wherein the outer shell comprises a molybdate salt having a formula A 2 MoO 4 ,
 where A is Li, Na, K, or Cs.   
     
     
         6 . The redox catalyst according to  claim 1 , wherein the outer shell comprises a molybdate salt having a formula BMoO 4 ,
 where B is Mg, Ca, Sr, Ba, a transition metal, or a rare earth oxide.   
     
     
         7 . The redox catalyst according to  claim 1 , wherein the outer shell comprises a metal carbonate, metal phosphate, metal vanadate, metal sulfate, metal halide, a combination thereof, or a combination thereof with one or more other mixed oxides. 
     
     
         8 . The redox catalyst according to  claim 1 , wherein the low-temperature oxygen carrier comprises perovskites of the form Mo oxides, V oxides, mixed oxides, or any combination thereof. 
     
     
         9 . The redox catalyst according to  claim 1 , wherein the low-temperature oxygen carrier comprises a perovskite or other material containing Dy, Pb, Bi, Pr, Ferrites, or any combination thereof, that exhibit low temperature (<750° C.) oxygen donation or uncoupling materials, and wherein the low-temperature oxygen carrier comprises Dy 2 0 3 , PrOx, BiO x , or any combination thereof. 
     
     
         10 . The redox catalyst according to  claim 1 , wherein the oxygen carrier comprises mixed manganese silica oxides, optionally wherein the mixed manganese silica oxides are synthesized in such a way that a substantial portion of the Mn and Si exist in a mixed Mn x Si y O z  phase, optionally wherein the mixed phase comprises Mn 7 SiO 12 , and wherein the mixed manganese silica oxides provide improved redox kinetics, oxygen capacity, or both over a SiO 2  supported MnO x  phase; and wherein the mixed manganese silica oxides comprise Mn loading of >30% so that the mixed manganese silica oxides are formed in an oxygenated environment and resulting in improved usable oxygen capacity. 
     
     
         11 . The redox catalyst according to  claim 1 , wherein the oxygen carrier comprises manganese ores comprising one or more minerals selected from pyrolusite (Mn0 2 ), braunite, psilomelane, and Birnessite. 
     
     
         12 . The redox catalyst according to  claim 1 , wherein the oxygen carrier comprises bulk oxides including M 2-x SiO 4  structured materials, wherein the M 2-x SiO 4  structured materials comprise olivines, wherein M is selected from Mn, Fe, Mg, or a mixture thereof, in an amount effective to enhance the physical strength of the redox catalyst particles, to provide additional oxygen carrying capacity, to catalyze thermal naphtha cracking, of any combination thereof. 
     
     
         13 . The redox catalyst according to  claim 1 , wherein the oxygen carrier is active for oxidative dehydrogenation of methane, ethane, or propane at a temperature of from about 500° C. to about 850° C. 
     
     
         14 . The redox catalyst according to  claim 1 , wherein a ratio of cation to anion in the outer shell is about ¼ to 4 times a stoichiometric cation to anion ratio. 
     
     
         15 . The redox catalyst according to  claim 1 , wherein the oxygen carrier comprises enhanced oxygen release/decomposition properties to allow significantly reduced (>10%), near neutral, or exothermic heat of reaction in oxidative dehydrogenation. 
     
     
         16 . The redox catalyst according to  claim 1 , wherein the redox catalyst is active for oxidative dehydrogenation (ODH) of methane, ethane, or propane or oxidative cracking of naphtha at a temperature of about 500° C. to about 850° C. via a two-step, reduction-oxidation process comprising:
 donating a lattice oxygen of the core region for the ODH or oxidative cracking reaction; and 
 in a subsequent step, regenerating the lattice oxygen in a suitable oxidizing atmosphere (including CO 2 , air, or O 2 ) thereby producing heat that substantially offsets the net-endothermic reaction(s) in the ODH/oxidative cracking step. 
 
     
     
         17 . A method of making the redox catalyst according to  claim 1 , the method comprising
 (a) forming a precursor comprising the oxygen carrier and the metal salt, wherein the metal salt comprises an alkaline or rare earth tungstate selected from the group consisting of BWO 4 , B 2 WO 5 , and B 3 WO 6  where B is Mg, Ca, Sr, Ba, or a rare earth element; and wherein the oxygen carrier is substantially free of alkali metals and metal oxides;   (b) heating the precursor to an elevated temperature above a Tamman temperature of the salt to allow facile surface transport and wetting of the salt to form the shell that fully or partially covers the surface of the core.   
     
     
         18 . The method according to  claim 17 , wherein the outer shell is layered onto an outer surface of the core via one or more of the following steps:
 (a) high temperature annealing,   (b) addition of a molten alkali salt or alkaline earth salt such a lithium chloride or strontium chloride that either acts a flux during heating, or forms a molten phase at elevated temperatures that dissolves the molybdate, vanadate, phosphate, sulfate, alkali earth or rare earth tungstate in the salt to form the shell; and   (c) annealing under reducing, oxidizing, or redox conditions.   
     
     
         19 . The method according to  claim 18 , further comprising in step (b) washing the molten alkali salt or alkaline earth salt from the shell after heating or removing the salt in a non-molten state though evaporation at annealing temperature, wherein washing the molten alkali salt from the outer shell after heating leaves a non-molten salt shell. 
     
     
         20 . The method according to  claim 18 , wherein the outer shell comprises a combination of a first alkali salt and a second non-alkali salt, wherein the first alkali salt is selected such that the first alkali salt melts and dissolves the second non-alkali salt at elevated temperatures to wet the outer surface of the core, wherein the method further comprises creating a melt of a mixture of the first alkali salt and the second non-alkali salt at a temperature lower than a melting point of each of the individual salts in the mixture.