Oxidative conversion of LPG to olefins with mixed oxide catalysts: Surface chemistry and reactions network

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

315

Oxidative Conversion of LPG to olefins with Mixed Oxide catalysts: Surface Chemistry and Reactions Network M.V.Landau a, M.L.Kaliya a, A.Gutman a, L.O.Kogan a, M.Herskowitz a and P.F. van den Oosterkamp b aBlechner Center for Industrial Catalysis and Process Development, Ben-Gurion University of the Negev POB 653, Beer-Sheva 84105, Israel Tel. (972-7)-6472141, Fax.(972-7)-6472902 bKinetics Technology International (KTI) B.V., POB 86, 2700 AB Zoetermeer,The Netherlands,Tel.31 (79)-3531453, Fax.31 (79)-3513561 The catalyic performance of three mixed oxide catalytic systems V-Mo-, V-Mg and RE-LiHalogen (RLH) in LPG oxidative conversion was measured at different O2/LPG ratios, temperatures and WHSV. At high LPG conversions V-Mo-based catalysts yielded low olefins selectivity and high LPG combustion (CB), V-Mg - medium olefins selectivity by oxidative dehydrogenation (ODH) route and medium LPG CB selectivity, while RLH catalysts displayed high olefins selectivity by ODH and cracking (CR) routes at low CB. TP-reaction experiments and the effects of oxygen partial pressure on catalytic performance indicated a dynamic interaction of surface oxygen in the ODH, CB and CR routes. ESCA and TPD measurements detected three types of surface oxygen with different nucleophility and bonding strength. Their distribution correlated with LPG conversion selectivities. A correlation between catalysts acidity, the surface exposed metal cations concentration and the productivity by the CR route was derived. The surface basicity was also significant in olefins productivity by the ODH and CR routes. The selectivity of LPG oxidative reactions were attributed to different intermediates formed on the surface as a result of interaction of C3-C4 paraffins with oxygen atoms of different nucleophility. Both the redox balance of surface metal cations and the acidity-basicity balance are proposed to be significant. 1. I N T R O D U C T I O N Catalytic oxidative conversion of low paraffins into olefins, a potential alternative to steam cracking, is one of the attractive optiopns that could decrease the process temperature, minimize the coke deposition at the reactors walls and increase the olefins productivity. Various catalytic processes for oxidative production of ethylene, propylene and butylenes have been published. A review of the published results measured with individual C2-C4 paraffins [1] allowed to select three most efficient oxide catalyst systems for the study: V-Mo- [2], V-Mg- [3] and Mg-RE-LiHalogen (Mg-RLH) [4]. Comparison of their performance in LPG oxidation showed that V-Mocatalyzed mainly the full paraffins CB, V-Mg- displayed average olefins selectivity producing a large amount of butadiene while the RLH - containing oxide systems showed the highest olefins selectivity at high LPG conversions producing substantial amounts of C2-C3 olefins by CR and ODH routes [ 1] The purpose of this work was to study the states of surface oxygen and relate them to the catalytic performance of selected catalysts: V-Mo, V-Mg and RLH.

2. EXPERIMENTAL

Preparation of Catalysts. V-Mo-catalysts were prepared according to procedure described in [2]. Ammonium metavanadate and paramolybdate were dissolved separately at 70~ third solution containing all the other metal components in form of nitrate salts was mixed with the first two evaporated by mixing. The catalyst material was crushed, sieved, dried at 120~ and calcined at 350oc for 5 h. V-Mg samples were prepared by mixing the MgO obtained by decomposition of Mg(NO3)2 or Mg(OH)2 (with addition of SiO2 or TiO2 powders in some cases) with water solution of ammonium metavanadate (containing metal nitrates in some cases), evaporation the suspension to dryness, dried at 120~ and calcined at 550~ for 6 h. The RLH- catalysts were prepared via an aqueous slurry containing LiNO3, NI-I4-halogen salt, Dy-oxide and the second

316

metal oxide (MgO,Ce-oxide or transition metal oxide). The water was evaporated, the paste dried at 130~ resulting solid was crushed,sieved and calcined at 500oc for 2h and at 750oc for 16h. Catalysts testing. A tubular titanium reactor 17 mm ID and 250 mm length supplied with the central thermowell was designed to test the catalysts over wide range of temperature and various feed compositions. Hydrocarbons - 25wt.% n-C4H10- 25wt.% i-C4H10- 50wt.%C3H8 (LPG artificial mixture) or its components, oxygen and nitrogen were fed separately by mass flow controllers (Brooks Instrument) and mixed in preheater at 450oc. The reactor was inserted into Carbolate tubulat oven, uniformly heated over a length about 50 mm. 1-5 g catalyst diluted with quartz pellets at 1:3 ratio was loaded between layers of quartz pellets. Axial temperature gradient in the catalyst layer during the tests was less than 5~ Homogeneous LPG oxidation in titanium reactor filled with quartz pellets at temperatures lower than 600oc was less than 5 wt.% conversion. The analysis of the reaction products excluding water was performed on line with GC HP-5890 that contained four columns - 45/60 Molecular Sieve 13X, 10 ft x 1/8"; 50 m x 0.53 mm Plot A1203; 80/100 Hysep Q 4 ft x 1/8" and 1 ft x 1/8", with internal switching valves and two detectors TCD and FID controlled by ChemStation analytical software. Selectivity was defined as wt of olefins in product divided by the wt of converted LPG feed. Catalysts characterizations. The catalysts composition was measured by energy -dispersive Xray (EDAX) - JEM-35, JEOL Co., link system AN-1000, Si-Li detector. The surface area was determined using BET method (ASTM 3663-84). Phase composition was measured by XRD in conventional, automated Philips PW 1050/70 diffractometer equipped with a long, fine focus Cu anode tube, 40 kW, 28 mA, a scintillation detector and a diffracted beam monochromator. The phase identification was carried out according to JCPDS-ICDD powder diffraction cards. PHI 549 SAM/AES/XPS apparatus with double CMA and Mg Ka X-ray source has been used for X-ray Photoelectron Spectroscopy (XPS) measurements of the catalysts. After recording general survey spectra, high resolution scans were taken at pass energy (25 eV) for the O ls peaks. The spectral components of O signals were found by fitting a sum of single component lines to the experimental data by means of non-linear least-square c.urve fitting to Gauss-Lorentz shape function using software provided by instruments manufacturer for peaks deconvolution. Care was taken to protect the calcined fresh samples from the contact with atmosphere by pressing them into 10 mm disks and transfering to the ESCA analytical chamber. The quantitative distribution of oxygen atoms with different O ls characteristics as well as total atomic surface concentrations of oxygen were calculated by conversion the peak areas into atomic compositions taking in account the sensitivity factors of all detected elements. Binding energies were referenced to the carbon ls line at 284.5 eV. The TPD and TP-reaction measurements were carried out in AMI-100 Catalyst Characterization System (Zeton-Altamira) equipped with quadrupol mass-spectrometer (Ametek1000). 3. RESULTS AND D I S C U S S I O N

3.1. Phenomenological description of observed catalytic effects Table 1 presents the olefins selectivitiy and productivity measured catalysts at about 30% LPG conversion. The measurements were temperature and O2/LPG ratio, keeping the LPG conversion constant by olefins selectivity is determined by a few basic components increasing in

with all the tested oxide carried out at constant varying the WHSV. The following sequence:

V-Mo- (5.1-8.4%) < V-Mg- (39.2-55.0%) < RLH (67.0-79.0%). The nature of promoters or components in RLH catalysts affected mainly the olefins productivity. The significance of different reaction routes is apparent in Table 2 that compares the CR and CB selectivities measured with selected representatives of the three catalyst groups: V-Mo-Nb-SbCa (Cat.A), 0.07V2Os-Mg(Cat.B) and Mg-Dy-Li-C1 (Cat.C). It also includes the results obtained with a catalyst that yielded a higher olefins productivity where the RLH composition was supported on a transition metal oxide (TM-RE-Li-C1, Cat.D). LPG was almost fully combusted on the V-Mo-catalyst. V-Mg-catalyst converted LPG mainly by ODH and CB routes with about equal efficiency. RLH catalysts enhance the ODH and CR routes with relatively low CB. Table 3 comoares the catalvtic oerformance of M~-Dv-Li-C1 catalyst in oxidation of individual LPG

317

components. All the hydrocarbons were converted mostly by ODH and CR routes, with CR selectivity increasing in the sequence: propane < n-butane < i-butane, so that the contribution of cracking products to total olefins yield was 55-65%. Figure 1 presents the olefins selectivity as a function of LPG conversion. Such plots are commonly used for comparison of low paraffins oxidation catalysts [5,6]. The V-based catalysts showed strong decrease in olefins selectivity with increasing conversion ( more expressed with V-Mo-) normally found with ODH catalysts [5.6], while the selectivity of RLH catalysts was almost independent on LPG conversion. Table 1 Compositions and performance of the tested catalyst belonging to the three selected groups Catalyst group

V-Mo

V-Mg

Catalyst composition

S.A., m2/g

0.09V205 0.74MOO3 0.02Nb205 0.02Sb203 0.13CaO 0.05V205 0.83MOO3 0.12CaO 0.17V205 0.83MOO3

Phase composition Olefins Olefins sel.*),% product. g/gCat., h

14

Sb204, Nb205, 8.4 SbNbO4 [Mo4011]O 5.1 MoO3,[Mo4011]O, 7.5 VMoO14

6 10

0.07V205 0.93MgO 0.07V205 0.93MgO 0.05V20 5 0.79MgO 0.16SIO2

60 100 90

0.05V205 0.94MGO 0.006TIO2 0.004Cr203 0.07V205 0.88MGO 0.05Li20 0.06V205 0.79MGO 0.05Li20 0.1C1

55

MgO, Mg3V208 MgO, Mg3V208 MgO, Mg3V208, Mg2SiO4 MgO, Mg3V208

57 52

Mg-RLH 0.8MgO 0.09Li20 0.002Dy203 0.1C1 0.7MgO 0.09Li20 0.002Ce203 0.21C1 0.39MgO 0.43Ce203 0.003Dy203 0.08Li20 0.1 C1 0.88MgO0.01Li20 0.001Dy203 0.1I 0.82MgO 0.1Li20 0.004Dy203 0.08Br 0.77MgO 0.09Li20 0.005Dy203 0.14F *) T = 585~

V-Mo catalysts T = 500~

0.03 0.028 0.027

44.9 44.3 43.5

0.15 0.15 0.14

39.2

0.13

MgO, Mg3V208 --

55.0 54.5

0.18 0.18

20 18 19

MgO,LiDyO2,Li20 -MgO, CeO2

77.3 79.0 78.5

0.08 0.1 0.1

-15 20

MgO,LiDyO2,Dy203 MgO, DyOBr,Dy203 MgO,LiDyO2,Li20

82.0 70.0 77.3

0.25 0.16 0.02

O2/LPG = 1; LPG conversion -- 30%

Table 2 Performance of selected representatives of the three catalyst groups in oxidation of LPG *) Catalyst

A

B d

a

b

C c

d

a

b

D

a

b

c

c

d

a

b

c

d

8.4

91.6

-- 0.03 44.9 55.1 3.1 0.15 77.3 22.0 39.00.08 74.7 28.0 36.2 1.03

*) Testing conditions as in Table 1; LPG conversion -30%; a-olefins selectivity,%, b -combustion selectivity,%, c -cracking selectivity (C1+C2),%, d - olefins productivity, g/g Cat.h A scheme of LPG reactions is proposed in Fig.2 to show the possible low paraffins transformations according to main three routes. It is based on measured products distributions and

318

90

.....

e

9 O'ql~

r

:

9

4~

9

4P,

41,~

9

;>

O r

,,, 30

0

.

.

.

.

I~

I-

0

A

I. . . . . . .

20

40 LPG c o n v e r t ; I o n ,

A

,.A

~.. 60

8O

%

Figure 1. Olefins selectivity vs. LPG conversion plots for all the testcd catalysts

"~ C3H 8 ~ - - . E . _ ~ - C " H ? ~

9 i-C4Hzo

8. ~ ,.,,

~ 7.

~ *oa

~

, O ~ , ,...~6 ~._,0.

~.~" .n__,_

.

~.

CzH4~

~

n-C4HI0 . ~ ' "

tg.1

C2H6

L ,-,-

~

7-- co~_

n-C4H~ ~~ cis,trans-C4H8 t6. H24-O2 - - ~ H20 ~7.CH4 + O 2 ~ CO + H20 ~ i-C4H8 l,.C H 4+O 2 ~ CO 2 + H 2

Cn_xH2(n.x),CH4,Cn_xH2(n.x)+2,H20

CRI 1.3,6,8,15,19 ODH CnH2n+2 CB

CO

,.~ ,o2

r.- CnH2n,H20

2,4,7,14 5,9,10,11,12,13,16,17,18

CO,CO/,H20,Hz Figure 2. Reactions network in oxidative conversion of LPG

319

Table 3 Performance of Mg-RLH catalyst C in oxidation of LPG components *) Paraffin

n-Butane

i-butane

a

b

c

d

a

b

72.2

26.2

50.3

0.13

76.1

20.1

c

propane d

a

58.4 0.19

78.2

b

c

d

21.5

60.1

0.18

*) Testing conditions as in Table 1" Hydrocarbons conversion -- 30%. a - olefins selectivity,%, b combustion selectivity,%, c - cracking selectivity,%, d - olefins productivity, g/g Cat.h kinetic studies. The molar amount of hydrogen detected in products was higher than the amount of olefins could produce without a change in the number of carbon atoms while the amount of consumed oxygen was lower than needed for combustion of hydrogen stochiometrically. Therefore reactions like 5, 10, 13 and 19 in Fig.2 were included in the reactions network assuming production of hydrogen as a result of partial combustion. 3.2. Surface oxygen role

in

oxidative conversion of light

alkanes

Lattice oxygen in metal oxides reacted in catalytic cycles is replenished by reoxidation [7-9]. The effect of O2/LPG ratio on the catalytic performance of three selected catalysts shown in Fig.3 indicates that oxygen from gas phase is a reactant in all the three routes of catalytic conversion. Molecular oxygen could react with adsorbed hydrocarbons or oxygen bonding and activation at the catalysts surface could be nesessary.

90

?

60

Catalyst A

/

~

80

3

90

1

atalyst B

2 ~

3

60

40

3O

30 0 --

0

0.5

1

1.5

2

OxygenlLPG molar rallo

2.5

0

I

0.5

--I .....

1

t-

1.5

OxygenlLPG molar ratio

t---

2

----t--

0

I

0.4

0.8

1.2

OxygenlLPG molar rallo

Figure 3. Effect of O2/LPG ratio on performance of selected catalysts in LPG oxidation at 585~ 1-LPG conversion, 2 - olefins selectivity, 3 - oxygen conversion, 4 - cracking selectivity Three consecutive runs of n-C4H 10-TP-reaction experiments were carried out with selected catalysts A,B,C and D. 25 cm3/min mixture 9%.vol. n-C4H10-He was fed to the reactor of the AMI-100 Catalysts Characterization System containing 3 g catalyst after heating to 200oc in He flow. Then the temperature was gradually increased at 5OC/min up to 600oc (Cat.A), 750oc (Cat.B,C) and 800oc (Cat.D). After reaching the required temperature, the gas flow was switched to He, catalysts were purged for 1 h,cooled to 200oc. Then the procedure of the first run was repeated. Before the third run, performed at the same conditions, the catalysts were reoxidized in 5%vol Oa-He flow at 550oc for 2 hours with subsequent cooling to 200oc. During the n-C4H10-TP-reaction runs the concentration of n-C4H10 in effluent gas as well as

320

concentrations of C4H8 (ODH product), C2H4,CH4 (CR products), CO2, H20 and H2 (CB productg) were monitored by MS. TP-reaction spectra for butane consumption (similar in shape for all catalysts) is shown in Fig.4a. It could be divided into three parts reflecting different catalysts performance as the temperature increases: I - no butane consumption at low temperature, II increasing butane consumption by fresh and reoxidized catalyst and no consumption with reduced catalyst, III- increasing butane consumption in all the runs that could be a result of other reaction routes (e.g. homogeneous reactions with oxygen evoluted by oxides decomposition). In the second series of TP-reaction experiments, the temperature during butane flow was changed in a ramp mode: it was increased in the same way as in previous series up to value a little higher than it corresponded to the end of the part II and kept constant for 1 hour. In this case (Fig.4b) the butane concentration spectra with fresh and reoxidized catalysts showed a minimum as a result of gradual conversion of surface oxygen while the reduced catalyst did not display defined peaks. The concentrations of all the other compounds monitored by MS displayed a -

(a)

F~gure 4. n-C4H10-TP-reaction spectra recorded with Mg.RLHcatalyst B

321

maximum over the same time pcricxt. The normalized MS peaks intensities lot butane, butylene, ethylene, methane, water, carlx~n dioxide and hydrogen measured at maximum butane consumption lot catalysts A,B,C and D are presented in Fig.5. The peaks normalization was done separately for every experiment, so their relative intensities shown in Fig.5 for different catalysts could not be compared. In all cases the products distribution with fresh and reduced catalysts were close to those measurcd in steady-state experiments, excluding high CO2 evolution with the fresh RLH catalysts. Reducing the V-Mg and RLH catalysts in butane flow almost fully depressed their ODH and CB activity shifting the products distribution in the direction of CR and dehydrogenation while the V-Mo- catalyst in reduced form produced the same CB products as fresh and reoxidized form with lower efficiency. These results are evident for the need for adsorbed oxygen species in the reaction cycles producing products according to the three main conversion routes detected in steady state experiments. Then the differences in performance of the three selected catalysts groups in LPG o~dation is probably caused by different states and concentrations of the surface lattice oxygen atoms. It is widely accepted 18-101 that the ability, of surface oxygens Os to react with hydrocarbons and the type of reaction depend on the distribution of Os among the different species: O2(gas) ~ O2(ads) w," O2-(ads) ~-*'20-(~s) w-~'202-(lattice). The performance of the most strongly bonded lattice oxygen that could be removed at high temperatures by the reaction with hydrocarbons in catalytic cycles is governed by their nucleophilicity being directly related to the effective negative charge and bonding strength [8-11]. Those characteristics together with surface concentrations of different oxygen forms for selected catalysts A-D were measured by TPD and ESCA.

Fi~zure 5. n-C4Hlo-TP-reaction products distribution with catalysts A-D at butane consumption

322

3.3. Surface chemistry characterizations The TPD experiments were carried out with 3 g catalysts A,B,C and D in He flow 25 cm3/min monitoring by MS the evolution of 02, CO2 and H20 over a temperature range 200-800oc ( for VMo- catalyst 200-600oc), heating at 5~ The results presented in Table 4 showed that only V-Mo- catalyst contains comparatively weakly bonded oxygen that could be partially desorbed at the temperatures used in steady-state catalytic tests. The oxygen bonding strength corresponding to Table 4 He-TPD of fresh catalyst Catalysts

Desorbed species:

02

A B C D

H20

CO 2

a

b

c

a

b

c

a

b

c

>600 680 705 ND

>20 40 50 ND

480 560 650 ND

ND 700 720 480

ND 500 80 8

ND 510 640 420

ND 710 720 580

ND 30 300 90

ND 580 600 550

a - Temperature of peaks maximum,~ : b - normalized MS peaks intensity c - Temperature of initial product desorption, oC; ND - not detected the temperatures of initial oxygen desorption and its maxima, increased in catalysts sequence: A
A

B

C

D

3 1 8 220 100 22500

1 ND ND 2 1 100

1 ND ND ND ND 30

1 ND ND 1 ND 130

323

absent in reoxidized catalysts (Fig.5). The hydroxyls are formed during the reaction of butane with fresh or reoxidized catalysts. The amount of evoluted water in butane-TP-reaction experiments was comparable with the amounts of other products (Fig.5). After switching the butane flow to He substantial amount of water was desorbed at the purging stage at a much higher concentration compared to the other products (Table 5). Taking into account that no water evolution was detected at the purging stage as well as during butane-TP-reaction with reduced catalysts (except V-Mo- ) it appears that at least part of surface lattice oxygen reacts with hydrocarbons forming hydroxyl groups that are removed at the purging stage as a result of nonreductive dehydroxylation. The ESCA measurements of the O ls electrons BE carried out with fresh catalysts A-D detected one, two or three bands in RFE-spectra depending on catalysts origin corresponding to the O ls electrons BE range 528.4-529.3 eV (OI), 529.9-530.3 eV (OII) and 531.0-531.8 eV (OIII). The values of total oxygen surface concentrations, O ls electrons BE corresponding to different oxygen states and the relative amounts of those oxygen species exposed at the catalysts surface are shown in Table 6. Decreasing the O ls electrons BE in the sequence OIII >OII>OI reflects increasing of electron density or effective negative charge on oxygen atoms. This corresponds to increasing of oxygen atoms nucleophility (basicity or ability for proton abstraction from hydrocarbon molecule). Other observations showed that: Ar sputtering with increased duration removes from the RFES spectra of RLH catalysts the peaks corresponding to OIII species leaving the OI species unchanged; in case of Vcontaining catalysts Ar sputtering does not affect the shape of the spectra and the O ls characteristic was very close to 530.0 eV observed with pure V205. The ESCA measurements with separate individual oxides, hydroxides and carbonates of the elements building the catalysts compositions showed that the O ls electrons BE values of OIII oxygen species correspond to carbonates, hydroxyls, magnesia or lithia. It could be concluded that OIII oxygen atoms with low nucleophility being included mainly in subsurface species cannot play significant role in catalytic cycles. According to ESCA measurements carried out with vanadium oxide, consistent with the data presented in [ 12], the O ls characteristic of OII oxygen species is very close to the V==O doubly bonded oxygen atoms exposed at the surface of (010) planes of V205 crystals. The oxygen species with O ls characteristic of OI do not exist at the surface of individual main components of catalysts A,B vanadia, molybdena, or exist in small concentration at the surface of magnesia or TM. They -

-

-

Table 6 Characteristics of surface oxygen species in selected catalysts according to ESCA Catalysts A Oxygen surface concentration, % at. 80 Metal cations surface concentration, % at. 20 O Is characteristics of oxygen species,eV: OI -OII 530.3 OIII -Normalized oxygen species concentrations: OI 0 OII 100 OIII 0 1- (OII/total) 0

B 70 30

C 55 48

D 36 55

529.2 530.3 531.8

529.3 -531.0

528.8 530.2 531.5

47 40 13 0.6

48 0 52 1

58 20 22 0.8

form mainly as a result of interaction of those components with additives: V-Mg, Mg-RLH or TM-RLH.This is illustrated in Table 7 for Mg-RLH catalyst. From the results of catalytic tests in butane oxidation it is evident that existence of all the components is essential for the performance of this catalytic system. In case of Mg-RLH system magnesia displayed two RFES peaks of O III oxygen at 530,5 and 531.5 eV corresponding to MgO and Mg(OH) 2 in agreement with [13,14] and less than 10% of oxygen in form of O I (529.7 eV) which could be attributed to cationic

324

vacancies at the MgO surface. Introduction of lithia creates additional O I oxygen species (Table 7) while subsequent introduction of CI and Dy strongly shifts the distribution of surface oxygens into OI direction with increasing the effective negative charge at those atoms. The formation of highly nucleophilic oxygens is a result of changes in coordination and chemical bonds polarity of lattice oxygen atoms caused, for example, by formation of new phases like Mg3V208 where oxygen ions became bridged between V and Mg ions [15], substituting Li into MgO lattice [ 16] or formation of LiC1 crystals covered by thin lithia layer[17]. Decreasing the total oxygen surface concentration in the catalysts sequence A --> D ( Table 6) expose more metal cations and chlorine that behave as electron acceptors. Thus increasing the amount of highly nucleophilic oxygen atoms in the same row as electron donors should be accompanied by substantial changes in catalysts acidity-basicity. Those characteristics were measured by NH3- and CO2-TPD after saturation the catalysts samples with corresponding gases at 40oc. The results are shown in Table 8. V-Mo- catalyst displayed the lowest acidity corresponding to the lowest metal cations concentration but about 50% of the acid sites were strong desorbing ammonia at >250~ The other catalysts contain few strong acid sites but the total acidity strongly increased in the sequence B
Catalysts

529.7 MgO 529.2 0.94MgO-Li20 . . . . 0.994MgO-Dy203 -0.93MgO-0.006Dy203-Li20 529.2 0.87MgO-0.06Li20-C1 0.86MgO-0.006Dy203529.3 0.06Li20-C1 *) T= 550~

--530.3 ---

530.5; 531.5 530.5; 531,5 531.2 531.3 530.6;531.5 531.0

11.4 4.5 25.0 5.2 18.0

55 74 56 70 69

38.0

70

WHSV = 0.33 h -1, O2/C4H10 = 1

Table 8 Acis-base characteristics of selected catalysts Catalysts Acidity: total, ~M NH 3/g - > 250oC/total Basicity: total, l.tM CO 2/g >250oC/total -

-

-

A

B

C

D

30 0.5

81 0.002

140 0.04

340 0.03

0.5 0

7 0.3

4 0.4

4 0.6

to the absence of highly nucleophilic oxygen species. The basicity of other catalysts was about one order of magnitude higher: V-Mg and Mg-RLH catalysts displayed about equal distribution between strong and weak basic sites while at the surface of catalyst D the relative amount of strong basic sites was more than twice higher. It corresponds to apperance of highly nucleophilic oxygen species and increasing their nucleophility from C to D (Table 6).

325

3.4. R o l e of d i f f e r e n t

s u r f a c e s p e c i e s in catalytic cycles

Comparison the surface characteristics of selected representatives of the three catalysts groups with their catalytic performance in LPG oxidation show: i - at temperatures less than 600oc all three LPG oxidative conversion cycles - ODH, CR and CB, are controlled by interaction of hydrocarbons with surface lattice oxygen atoms OI and OII, that form surface OH-groups being removed by dehydroxylation before reoxidation, as indicated from the results of TP-reaction and TPD experiments discussed in the part 3.2. ii - combination of OII oxygen species with low nucleophilicity (basicity) bonded to easy reducible metal cations (V,Mo) with acid sites leads to CB increasing with increased acid sites strength; it was indicated by direct correlation between olefins selectivity measured with catalysts A-D (Table 2) and parameter [ 1-OII/Ototal] (Table 6)reflecting decrease of CB selectivity with decrease of the fraction of OII in all the surface oxygen atoms and furthermore by substantial increase of the strong acid sites concentration from V-Mg to V- Mo (Table 8). iii - combination of OI oxygen species with high nucleophility (basicity) bonded to hardly reducible cations (Mg,RE) with weak acid sites leads to ODH and CR increasing with increased basicity of OI atoms, as indicated by comparing changes in the fraction of OI (Table 6) and their basic strength (Table 8) from catalyst A to catalyst D with CR and olefins selectivities of those catalysts presented in Table 2. iiii - the efficiency of CR conversion route increases with increased weak acidity of the catalyst as indicated from the direct correlation between CR productivity of A-D catalysts that could be easily estimated from the data of Table 2 and surface concentrations of metal cations and chlorine given in Table 6. Based on this information two different modes of paraffins activation are assumed, leading to CB or ODH-CR products depending on catalysts surface chemistry that are consistent with generally accepted models [8-11]. V-Mo- catalyst containing strong acid (electron-acceptor) sites, easy reducible cations andweak nucleophilic (proton-acceptor) oxygen atoms could adsorb the hydrocarbon molecule as a result of hydride-ion abstraction by acid sites. Reduction of metal cation with splitting of one of metal-oxygen bonds and stabilizing the proton and carbanion in form of OH and alkoxy species: CnH2n+2 ?-

O H!1

/z,,

CnH2n+ 1 +

OH OCnH2n 1 (n-l)+ M e - O_ MIe (m--l; . _

0

I] m+

n+Me 0 lvle !

! !

| i

!

RLH catalysts do not contain strong acid sites and easy reducible metal cations but have strongly nucleophilic (proton-acceptor) oxygen atoms and weak acid sites. The hydrocarbon molecule could be adsorbed as a result of proton abstraction by strongly nucleophilic lattice oxygen without splitting the metal-oxygen bond and stabilization of proton and carbcation in form of OH and alkyl species: CnH2n+2 O~ CnH2n+ I O - M e (n'l)+ - 0 - M e n+ - 0 ~ ! i

! i

CnH2n+l O - M e (n-l)+ i i

H O - M e n+ - 0 | i

The subsequent transformations of alkoxy radicals containing strong C-O bonds at the surface of V-Mo- catalyst with weakly bonded oxygen atoms yields preferentially formation of full CB products with some hydrogen evolution, while alkyl radicals stabilized on acid sites at the surface of RLH catalysts as a result of C-H bonds polarization in the strong field of metal-

326

oxygen ion pairs should be preferentially transformed to olefins as a result of further hydrogen abstraction (ODH) or CR. The fraction of CR products in olefins depends on catalysts acidity increasing the lifetime of alkyl radicals on the catalyst surface.The V-Mg- catalyst contained the both types of surface oxygen OI and OII in about equal amounts (Table 6) displaying average acidity and basicity (Table 8) and including easy reducible (V) as well as hardly reducible (Mg) metal cations. As a sequence it showed an average catalytic performance. In both cases the catalytic reaction cycle became closed as a result of dehydroxyation of catalysts surface and further oxygen adsorption-insertion in the oxide lattice that in case of V- or V-Mo-containing catalysts is accompanied by increasing of metals oxidation extent. In addition to further reacting of alkyl and alkoxy intermediates at the catalysts surface with dynamic lattice oxygen they could be desorbed into gas phase and react there homogeneously with gas oxygen as it was demonstrated in [ 13] for V-Mg-catalyst. Testing the RLH catalysts in fixed-bed reactor with void fraction of catalysts layer varied from 28 to 43% showed that this route became significant at temperatures higher than 590oc but no substantial changes in products distribution were observed. 4.

SUMMARY

The RLH-based catalysts display high olefins selectivity at high LPG conversions producing olefins by oxydative dehydrogenation and oxidative cracking. The last charactristics allow them to produce ethylene from LPG that is the main product of steam cracking. Supporting the RLH system at different carriers affects mostly the catalysts productivity. The RLH-based catalysts display about 50% olefins yield with productivity per reaction volume close to steam cracking. The high selectivity of RLH-catalysts to olefins is a result of a definite combination of surface oxygen state, oxygen / metal cations ratio, redox properties of metal cations and acidity-basicity balance. Further studies are needed in order to understand the role of the support and the proper functioning of RE-Alkali-Halogen systems in oxidation of low paraffins. REFERENSES

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