M APPLIED CATALYSIS A: GENERAL

ELSEVIER Applied Catalysis A: General 154 (1997) 17-27

Partial oxidation of methane to syngas over nickel-based catalysts modified by alkali

metal oxide and rare earth metal oxide

Qing Miao, Guoxing Xiong*, Shishan Sheng, Wei Cui, Ling Xu, Xiexian Guo

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO.Box 110, Dalian 116023. People's Republic of China

Received 19 February 1996; received in revised form 9 October 1996; accepted 14 November 1996

Abstract

The NiO/A1203 catalyst was modified by alkali metal oxide (Li, Na, K) and rare-earth metal oxide (La, Ce, Y, Sm) in order to improve the thermal stability and the carbon-deposition resistance during the partial oxidation of methane to syngas (POM) reaction at high temperature. The reaction performance, thermal stability, structure, dispersity of nickel and carbon-deposition of the modified NiO/A1203 catalyst and unmodified NiO/AlaO3 catalyst were investigated by a series of characterization techniques including flow-reaction, BET, XRD, CO chemisorption and TG analysis. The results indicated that the modification with alkali metal oxide and rare-earth metal oxide improves the dispersion of active component nickel and the activity for the POM reaction over the nickel-based catalysts, and enhances their thermal stability during high temperature reaction and the ability to suppress the carbon-deposition over the nickel-based catalysts during the POM reaction. The nickel-based catalysts modified by alkali metal oxide and rare-earth metal oxide have excellent POM reaction performance (CH4 conversion of 94.8%, CO selectivity of 98.1%, 2.7 x 104 1/kg.h), excellent stability and carbon-deposition resistance.

Keywords: Methane partial oxidation; Syngas; Nickel-based catalysts; Modification; Alkali metal oxide; Rare earth metal oxide; Stability; Carbon deposition

1. Introduction

The large amounts of natural gas (mainly methane) found worldwide has led to extensive research programs recently in the area of direct methane conversion. The

* Corresponding author.

0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0926-860X(96)00377-8

18 Q. Miao et aL/Applied Catalysis A: General 154 (1997) 17-27

oxidative transformations of methane, such as oxidative coupling of methane to C 2

hydrocarbons (OCM), selective oxidation of methane to methanol and formalde- hyde (SOM) and partial oxidation of methane to syngas (POM), are important routes for the effective utilization of the abundant natural gas resources. Unfortu- nately, OCM and SOM appear to be difficult to obtain on an economical scale. An alternative strategy for the elaboration of methane is to convert it into syngas (CO + H2), either by steam reforming or by partial oxidation. Those products can be used as a feedstock for methanol or ammonia synthesis, or converted to higher hydrocarbons, alcohols or aldehydes by Fischer-Tropsch Catalysis. Because of the considerable expense associated with maintaining the reaction conditions needed for conventional steam reforming [ 1 ], partial oxidation of methane has received intensive attention since 1990 [2,3]. The catalyst [3] used in POM reaction can be classified into two types: (1) group VIII transition metal (Ni, Co, Fe) catalysts; (2) noble metal (Pd, Rh, Ru, Pt, Ir) catalysts. The nickel-based catalysts emerged as the most practical one because of their fast turnover rates and inexpensive cost. The major technical problem for the nickel-based catalysts is carbon deposition on the catalysts which causes break up of the nickel metal particles over the catalyst [4], and deactivate the catalyst by coveting the nickel surface [5]. Carbon deposition on the nickel-based catalysts during POM reaction is therefore, one of the most important considerations before this process can be used on an industrial scale. In order to solve the problem of the loss or sintering of nickel, which is the active component of NiO/A1203 catalysts, and of the carbon deposition over the nickel-based catalysts, a series of new ANiBOx/A1203 (A=Li, Na, K; B=La, Sm, Ce, Y) catalysts were developed, which have a lower nickel content, excellent POM reaction performance, excellent stability and are resistant to carbon-deposition.

2. Experimental

2.1. Preparation of LiNiLaOx/ Al203 supported catalysts

The catalysts were prepared by impregnation method, namely, impregnating appropriate amounts of LiNO3 and Ni(NO3)2 on La(NO3)3 on 7-A1203 supports for 24 h and dried at 393 K and then calcined in air at 823-1073 K for 4 h.

2.2. Test of catalytic performance

Catalysts were tested by flow-reaction in an atmosphere pressure fixed-bed microreactor. Products of the reaction were analyzed by Gas Chromatography using a TCD detector. The reaction conditions were as follows: PCH4 : Po2 =2 : 1, weight of catalysts=0.1 g, GHSV=2.7× 10 4 l/kg.h. The conversion of methane, the selectivity and the yield of the products were calculated on the basis of carbon

Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27 19

numbers of the methane reacted. The amount of H2 was corrected by external standard method.

2.3. Characterizations of catalysts

The specific surface area of samples (previously evacuated at 623 K for 4 h)was determined by the BET nitrogen method in a volumetric equipment Micrometers ASAP 2000. XRD characterization was performed with a Riguku D/Max-RB X- ray diffractometer using a copper target at 40 kV x 100 mA and scanning speed of 8°/rain. CO adsorption measurements were carried out in a Micromeritics Che- misorb 2800 Instrument. The weight of sample is about 200 rag. The samples evacuated at 308 K were reduced by H2 at 873 K and then evacuated at 773 K for 30 rain to remove the physically adsorbed H2. CO was admitted to the sample at 308 K and the adsorption isotherm was measured. The first adsorption uptake is the total amount of CO adsorption. The amount of reversible adsorption was obtained by measuring the second adsorption uptake after evacuation at 308 K. The difference between those two adsorption uptakes is the amount of irreversible adsorption.TG test was performed with a Perkin-Elmer TGS-2 instrument. The TG profiles were recorded and treated by Perkin-Elmer 3600 work station at a programmed temperature velocity of 10 K/rain in air with the flow rate of 25 ml/min.

3. Results and discussion

3.1. Performance of LiNiLaOx/Ale03 catalyst

The effect of a cycle of sequential changes in reaction temperature on the performance of LiNiLaOx/A1203 is shown in Fig. 1. At the initial reaction temperature of 973 K, conversion of CH 4 was only 37%, and the main reaction products were CO2 and H20. As the temperature was increased stepwise to 1073 K, the CH4 conversion increased to 41%, but total oxidation to CO2 and H20 remained the dominant reaction. A further increase in reaction temperature to 1123 K, however, resulted in a sudden change in both activity and selectivity. Conversion of CH4 and selectivity increased to 90% and 98% respectively, which is due to the reduction of surface nickel to the reduced state and the reduced nickel metal is the active site for the POM reaction [6]. Further stepwise increases in reaction temperature to 1173 K caused little additional change in either activity or selectivity. When the reaction temperature was subsequently decreased from 1173 K to 873 K, the CH4 conversion and CO selectivity did not retrace the pathways observed during the initial temperature increase. With reaction temperature decreasing, because of re-oxidation of a portion of the reduced nickel, CH4 conversion and CO selectivity decreased gradually while maintaining

20 Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27

o ~ 100

90

"~ 80

70

60

50

40

30 J I I

173 973 1023 1073 1123 1173

T e m p e r a t u r e (K)

c.4 :o,on] co ]

Fig. 1. Performance of LiNiLaOx/Al203 catalyst at different temperatures.

a H2/CO product ratio of 2/1, until the reaction ceased at 873 K. Those results are in agreement with the report of Lunsford [7]. Under the constant space velocity by keeping the flow rate of CH4 (15 ml/min) and the total flow rate (25 ml/min) of 02 and He constant, the respective flow rates of 02 and He were changed to obtain different CH4]O2 ratio. The effect of CH4/O 2 ratio on the performance of LiNiLaOx/Al;O3 was investigated (see Fig. 2). At the CH4/O2 ratio of 1.56, CH4 conversion is almost 100%, but CO selectivity is only 89%. With CH4/O 2 ratio increasing, CO selectivity increased to virtually 99%, but CH4 conversion decreased drastically. There exists an optimum value at CH4/O2 ratio of 2.0 where CH4 conversion is 98.0% and CO selectivity is 98.5%. Under the reaction condition that CH4]O2 ratio is 2 : 1 and reaction temperature is 1123 K, the

100 A - ,, • 100

80

95

40 90 O o

2O CH4 Conversion CO Selectivity

o / . , ~ 85 1.6 2.0 3o. 5.1 15.7

C H 4 / O 2 ra t io

Fig. 2. Influence of C H 4 / O 2 ratio on the performance of LiNiLaOx/A1203 catalyst (at 1123 K).

Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27

100 t 100

~ . 8O

• ~ 60

~ 40

20

0

CH4 Conversion #

i

I CO Selectivity 60

I L

18 27 36 54 90

Space Velocity (xlO00)

9O

80 .~

70 ~ r ~

Fig. 3. Performance of LiNiLaOx/A1203 catalyst at different space velocity (at 1123 K).

21

influence of space velocity on the performance of LiNiLaOrdAlaO3 was also investigated. The results (Fig. 3) indicate that influence of space velocity was not appreciable. LiNiLaOrdA1203 kept excellent performance at the wide range of high space velocity.

3.2. Performance of catalysts with different components

A series of catalysts with different components were prepared with the same preparation method and under the same condition in order to investigate the action of different components. The results are presented in Table 1. There were strik- ingly different performances when Fe or Co displaced Ni component in LiNiLaOx/ A 1 2 0 3 catalyst. CO selectivity and C H 4 conversion of Co or Fe containing catalysts were much lower than those of Ni-containing catalyst, and s o m e C2H4 was produced. The different performance among Fe, Co, Ni may be possibly attributed to the difference in their structure of d-orbitals, ability to remove hydrogen, redox ability and the interaction between transition metal and A1203 support. Zhang [8] had done comparative study of the POM catalytic performance of the first series transition metals (Mn, Fe,Co,Ni,Cu). They claimed that the different performance of those transition metals containing catalysts were due to their different redox ability determined by standard of their different electric potentials. POM activity and selectivity are most probably related to the changeable valence of M°/M 2+ sites. As far as the LiCLaOx/A1203 (C=Ni,Fe,Co) catalysts were concerned, except for their different standard inherent electric potentials, the interaction between transition metal and A1203 support was one of the most important factors that determined the redox ability. The nickel is the most suitable for POM reaction according to the results obtained in Table 1.

22 Q. Miao et aL/Applied Catalysis A: General 154 (1997) 17-27

Table 1 Comparison of performance of different catalysts at 1123 K

Sample CH4 : 02 CH 4 CO CO 2 C2H4 C2H 6 conversion selectivity selectivity selectivity selectivity (%) (%) (%) (%) (%)

LiNiLaOx/Al203 2.0 91.5 99.1 0.9 0.0 0.0 LiCoLaOx/Al203 2.0 39.2 30.7 52.3 17.0 0.0 LiFeLaOx/A1203 2.0 35.2 33.4 57.2 9.4 0.0

LiNiLaOx/A12 03 1.8 97.0 97.0 3.0 0.0 0.0 NaNiLaOx/A1203 1.8 98.6 95.2 4.8 0.0 0.0 KNiLaOx/A1203 1.8 97.6 94.5 5.5 0.0 0.0

LiNiCeOx/Al203 1.8 97.6 95.8 4.2 0.0 0.0 LiNiYOx/A1203 1.8 97.2 96.8 3.2 0.0 0.0 LiNiSmOx/A1203 1.8 97.9 95.2 4.8 0.0 0.0

GHSV=2.7x 104 l/kg h.

There is no obvious difference in CH4 conversion among the catalysts with different alkali metal oxide ANiLaOx/A1203 (A=Li, Na, K). However, their CO selectivity decreased slightly according to the sequence LiNiLaOx/A1203 > NaNiLaOx/A1203 > KNiLaOx/A1203 (see Table 1). Those results maybe related to their different steric factors and electronic modifications. The influence of alkali metals on the Lewis acid sites of 7-A1203 has been studied with NMR and IR of adsorbed CO by Castro [9]. The physical properties of A1203 are slightly changed by the addition of low alkali metal concentrations. Li and K are adsorbed selectively on tetrahedral Lewis A13+ sites of the alumina surface. These effects on the tetrahedral cations are more marked with increasing molar concentration and ionic radius of the alkali metal added to AlaO3. Results demonstrate that the influence of the alkali metals is not only due to steric factors but also due to possible electronic modifications. However, the performances of LiNiCOx/A1203 (C=La, Ce, Sm, Y) catalysts are little different from one another (see Table 1). All of them have excellent POM performance. The effects of rare earth oxides may be to increase the stability of catalysts during high temperature and avoid the sintering of nickel. Stability is one of the most important criteria to evaluate catalysts. Nickel-based catalysts supported on "y-A1203 are usually unstable at high tem- perature. The stability of 7-A1203 supported catalysts during high temperature reaction is due to the thermal deterioration of the 7-A1203 support that causes sintering and leads to pore closing and reduction in surface area as well as phase transformation into a-A1203 which changes an active surface layer and promotes a low surface area structure [10]. Preventing the alumina support from thermal deterioration is studied by many authors [ 11-13] and a large number of additives to the 7-A1203 support have been shown to inhibit sintering and phase transforma-

Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27 23

tion. Such additives include SiOz, BaO, CeO2, La203, etc and other rare metal oxides. Another reason causing the deactivation of NiO/AIzO3 catalyst is the loss and sintering of nickel on the NiO/A1203 catalyst and carbon deposition over the NiO/A1203 catalyst. Schmidt found the loss of nickel on the 3wt% NiO/AI203 catalyst during the POM reaction [14]. The CH4 conversion and H2 selectivity decreased with 2%. The serious carbon deposition over the NiO/AI203 catalyst that filled the catalyst pores and caused the granules to disintegrate into a fine powder at CH4 " 0 2 ratios _>2 was reported by Lunsford [7]. Only at CH4 " 0 2 ratios=l.78, the steady-state concentration of surface carbon caused no observable decrease in catalytic activity after 50 h of continuous reaction at 800°C. The serious carbon deposition resulted in the pore closing and break up of the nickel metal particles over the catalyst, and deactivate the catalyst by covering the nickel surface [5]. So, the stability of NiO/AI203 catalyst is one of the important considerations before this process can be used on an industrial scale.

The introduction of rare earth oxides can increase the thermal stability of NiO/'~- A1203 and prevent the sintering of nickel. The stability of LiNiLaOx/A1203 catalyst was studied under the condition with reaction temperature at 1123 K, CH4/O2 ratio of 1.96 and space velocity of 2.7×1041/kg.h (see Table 2). The reaction attained steady-state condition with CH4 conversion of 92.1% and CO selectivity of 98.2% after 10 min initial reaction. During the 50 h lift-test, CH4 conversion kept exceeding 96.5% and CO selectivity kept exceeding 95%. Those results possibly indicate that the LiNiLaOx/Al203 catalyst was quite stable during high temperature reaction and the introduction of Li20 and La203 can improve the thermal stability of NiO/AI203. Further tests are under way.

3.3. Comparative study of the POM performance between NiO/Al20~ and LiNiLaOx/Al20 3 catalysts

Comparative study of the POM catalytic performance between NiO/A1203 and LiNiLaOx/AI203 catalysts that contain the same amount of nickel was performed

Table 2 Life test of LiNiLaOx/A1203 catalysts at 1123 K

Reaction time CH4 conversion (%) CO (%) H2/CO

10 min 92.1 98.2 1.92 l h 97.1 94.8 2.08 5 h 96.5 97.5 1.99

10 h 98.2 95.6 2.07 20 h 97.7 96.6 2.03 30 h 97.5 97.0 2.02 40 h 97.4 97.1 2.01 50 h 96.6 97.8 1.98

Reaction conditions: CH4+O2=45 ml/min, CH4 : O2=1.96, weight of sample: 100 mg.

24 Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27

Table 3 Comparison of performance between LiNiLaOx/Al203 and NiO/AI203 at 1123 K

Sample CH4 conversion (%) CO selectivity (%) CO yield (%) H2/CO

CH4 0 2

LiNiLaOx/A1203 91.5 99.8 99.1 90.7 1.93 NiO/A1203 88.7 99.7 99.0 87.9 1.89

CH4 : 02=2: 1, GHSV=2.7×104 1/kg h.

to investigate the effects of alkali metal oxide Li20 and rare-earth metal oxide La203 on the POM reaction performance. Their catalytic performance, physical properties and carbon deposition over catalysts after POM reaction were com- pared.

Comparison of performance between LiNiLaOx/A1203 and NiO/A1203 at 1123 K was shown in Table 3. The CH4 conversion of LiNiLaOrdA1203 is slightly higher than that of NiO/A1203. These may be due to the modification by Li and La that improves the interaction between active component and support, and enhances the activity of POM reaction. Choudhary [15] also reported that supported nickel catalysts using the supports (containing SiO2 and/or A1203) precoated with MgO, CaO or rare-earth oxide show very much higher activity, selectivity and produc- tivity in methane-to-syngas conversion reactions, than the catalysts prepared using catalyst carriers without any precoating.

In order to demonstrate the hypothesis that there may be an improved interaction between Ni and support in the LiNiLaOrdA1203 catalyst, a series of characteriza- tion, including BET, XRD and CO chemisorption, were performed. In Table 4, it is observed that there is no remarkable difference on their surface area and crystal phase structure between the two samples. The surface area of NiO/A1203 is just somewhat higher than that of LiNiLaOxJA1203. The XRD results indicate that their crystal phases are the same and both exhibit the "y-A1203 crystal phase.

The CO chemisorption is a useful method to characterize the interaction between metal and supports and the distribution of metal on the supports. The results of CO chemisorption are shown in Table 5. A1203 and LiLaO2/A1203 adsorbed little CO. In comparison with NiO/A1203, the total amount of CO uptake over LiNiLaOrd A1203 is higher than that over NiO/A1203, but the amount of irreversible CO uptake over the former is far lower than that over the latter. Those results indicated that the

Table 4 Comparison of surface area and crystal phase structure between LiNiLaOx/A1203 and NiO/A1203 catalysts

Sample Surface area (m2/g) XRD determination

AI203 149.1 "y-A1203 NiO/A1203 160.5 'y-AI203 LiNiLaOx/A1203 147.8 "y-Al203

Q. Miao et al./Applied Catalysis A: General 154 (1997) 1~27

Table 5 CO chemisorption on the nickel-based catalysts after 873 K reduction

25

Sample Treatment Total uptake Reversible Irreversible conditions (ml/g) uptake (ml/g) uptake (ml/g)

A1203 773 K evacuation 0.00 0.00 0.00 873 K reduction 0.36 0.36 0.00

LiLaO2/AleO3 773 K evacuation 0.22 0.22 0.00 873 K reduction 0.40 0.40 0.00

NiO/AI203 773 K evacuation 0.31 0.31 0.00 873 K reduction 4.65 2.39 2.26

LiNiLaOx/A12Os 773 K evacuation 0.27 0.27 0.00 873 K reduction 6.98 6.10 0.88

amount of weak CO adsorption site on the LiNiLaOx/A1203 was much larger than that on the NiO/A1203 and the amount of strong CO adsorption site on the LiNiLaOx/AI203 was much less than that on the NiO/AI203. As the distribution of strong CO adsorption site and weak CO adsorption site is related to the existing states of nickel over supports [16], namely CO was adsorbed very strongly on the poorly dispersed or crystalline nickel as the bridged adsorbed species and was adsorbed weakly on the well-dispersed nickel as subcarbonyl adsorbed species, it can be concluded that the nickel over LiNiLaOx/A1203 catalyst is more well- dispersed than the nickel over NiO/A1203 catalyst. The results of CO chemisorp- tion demonstrate that the introduction of Li and La improves the dispersion of active component nickel.

The introduction of Li and La not only improves the thermal stability and activity of the nickel-based catalyst, but more importantly, the incorporation of Li and La also improves the ability of carbon-deposition resistance. The deposition of surface carbon over the NiO/AI203 catalyst during the POM reaction is undesirable and resulted in the deactivation of NiO/AlzO3 [7,14,17]. It is well known that the acidity of catalyst surface favors carbon-deposition and basicity of the catalyst surface prevents carbon deposition [18]. So, the addition of basic component Li and La oxides should be beneficial to prevent carbon-deposition over the catalyst surface [19,20]. The TG results of the samples after the POM reaction for a period of 10 h indicate that the carbon-deposition resistant ability of LiNiLaOx/AI203 is much better than that of NiO/A1203. There was almost no carbon deposition over LiNiLaOx/A1203 (see Fig. 4) during the period of 10 h POM reaction,. It was shown that the introduction of Li and La oxides can suppress the carbon-deposition of nickel-based catalysts. Tang also studied the carbon deposition of nickel catalysts for methane reforming with CO2. They found that the content of carbon

26 Q. Miao et al./Applied Catalysis A: General 154 (1997) 17-27

\ b

o

, i O i i t i

273 373 473 573 673 773 873 973 1073

Temperature (K)

Fig. 4. TG profiles of NiO/AI203 and LiNiLaOx/Al203 catalysts after POM reactions. (a) NiO/A1203; (b) LiNiLaOx/A1203.

deposition decreased with the increase in base strength of the catalysts. The increase in base strength on the surfaces of catalysts can inhibit carbon deposition [20].

4. Conclusions

Modification with alkali metal oxide and rare-earth metal oxide not only improves the activity for the partial oxidation of methane to syngas (POM) reaction and the thermal stability of the nickel-based catalysts during high temperature reaction, but also enhances the ability to suppress the carbon- deposition over the nickel-based catalysts during the POM reaction. The nickel-based catalysts modified by alkali metal oxide and rare-earth metal oxide have excellent POM reaction performance, excellent stability and are resistant to carbon-deposition.

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