Stabilized magnesia: A novel catalyst (support) material

Applied Catalysis, 54 (1989) 79-90 Elsevier Science Publishers B.V., Amsterdam -

79

Printed in The Netherlands

Stabilized Magnesia: a Novel Catalyst (Support) Material H. SCHAPER*, J.J. BERG-SLOT and W.H.J. STORK Koninhlijke/Shell-L,aboratorium, Anasterdam (Shell Research B. V.) P.0. Box 3003, 1003 AA Amsterdam (The Netherlands) (Received 17 March 1989)

ABSTRACT Magnesium aluminium hydroxycarbonate compounds with the hydrotalcite structure (“Feitknecht compounds”) and molar ratios of Mg-to-Al of 5-10 have been prepared without the formation of considerable amounts of separate magnesium compounds. Subsequent calcination yields magnesia-rich mixed oxides, which combine high surface areas with good stability towards heat and steam. These “stabilized magnesia” compounds have a pronounced basic character and are very active in the double-bond isomerization of 1-pentene. They may find application as solid base catalysts or as catalyst supports.

INTRODUCTION

In recent years, interest in the use of catalyst supports other than silica, alumina and mixtures thereof has increased. A promising alternative is magnesia, a material with a strongly basic character. Magnesia-based catalysts might find application in processes where the acidic character of common supports leads to undesired side reactions. In addition, magnesia could be applied as a catalyst for base-catalysed reactions such as aldol condensations or double-bond isomerization. It is known that magnesia with a high surface area (several hundreds of square metres per gram) can be prepared by calcination of magnesium hydroxide in vacua [ 11. Calcination in air, which is preferable from a practical point of view, often results in low surface areas, because of sintering of the newly formed small magnesium oxide crystallites in the presence of the water vapour produced upon decomposition [ 21. Furthermore, subjection of such a high surface area magnesia to small amounts of steam at elevated temperatures (for instance, during calcination of impregnated samples) is sufficient to cause a dramatic loss in surface area [3]. Incorporation of a suitable promoter could lead to a stabilized high surface area magnesia. In order to avoid calcination in vacua, such a promoter should preferably be present during the development of the surface area, i.e. during

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the decomposition of magnesium hydroxide crystallites. The preferred solution would be to incorporate a promoter into the magnesium hydroxide crystallites. This can be accomplished by using a “Feitknecht compound” [ 41 as a precursor. These compounds contain bivalent ions (e.g. Mg, Ni, Cu, Zn) and trivalent ions (e.g. Al, Cr, Fe). They have the hydrotalcite structure, consisting of brucite (magnesium hydroxide) type layers, in which part of the bivalent ions are replaced by trivalent ions, alternated by interlayers, which contain water and anions (preferably carbonate) to compensate for the excess charge of the trivalent ions. As to the Ni/Al Feitknecht compound (the precursor of coprecipitated nickel/alumina catalysts [5] ) it is known that calcination at moderate temperatures (below 600” C ) results in the formation of a high surface area mixed oxide, with a highly disordered structure, containing both types of cation randomly distributed in an oxygen anion lattice [ 6,7]. The intrinsic interaction between the cations in this phase results in a high stability. Therefore, it is possible that calcination of the related Mg/Al Feitknecht compound (hydrotalcite, i.e. magnesium aluminium hydroxycarbonate ) will lead to a stabilized high surface area “magnesia”. The main objection to using a Mg/Al Feitknecht compound for this purpose is the observation that these structures only exist for a limited range of M (II) to-M (III) molar ratios. Theoretically this range is from 2 to 3; in practice metastable homogeneous Feitknecht compounds with M (II) -to-M (III) ratios of 1 to 4 have been reported. Outside these ranges, the excess metal ions have been observed to precipitate as a separate hydroxide (carbonate, hydroxycarbonate) phase. It can be argued that addition of aluminium ions to magnesium compounds in such rather large quantities is bound to change the character of the resulting magnesia in terms of basicity. It may even introduce acidic sites, should the calcined material not be homogeneous, although this was not observed for the related Cu/Zn/Al Feitknecht compound [ 81. In view of these considerations, it would be desirable to prepare a Mg/Al Feitknecht compound with much higher Mg-to-Al ratios. Recent experiments within the Ni/Al system indicate that this could be possible [ 7,9]. Moreover, minerals are known with a similar structure and M (II) -to-M (III) ratios of 5 to 8 [lo]. These coalingite structures consist of two or three brucite type layers for every interlayer. Whether such structures can be prepared artificially remains to be investigated. We have attempted to prepare Mg/Al Feitknecht compounds with molar ratios between 5 and 15 by applying controlled conditions (constant pH and temperature) during the coprecipitation step. In addition, some reference materials were prepared. The samples were characterised by XRD, 27A1solid state NMR, surface area measurements and basicity determinations. Further, the catalytic activities of the materials for a base-catalysed reaction, the doublebond shift of 1-pentene, were determined. This reaction was chosen on the

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basis of a recent paper by Reichle [111,who showed that a calcined hydrotalcite (with molar ratio of Mg-to-Al of 3 ) is not sufficiently active for this reaction. EXPERIMENTAL

Preparation The Mg/Al samples were prepared by coprecipitation at constant pH and temperature, in a continuous process. A flow (2l/h) of an aqueous solution of magnesium nitrate and aluminium sulfate, with a total cation concentration of 1 M, was mixed with vigorous stirring in a 50-ml glass vessel with an alkaline flow (KOH/K,CO,, with the carbonate concentration adjusted to give a molar CO;--to-Al ratio of 0.5). Molar ratios of Mg-to-Al were varied from 5 to 15. The pH was kept constant, typically at values between 8.5 and 9.5, by adjusting the flow rate of the alkaline solution. The temperature was kept constant, typically in the range of 60 to 7O”C, by placing the mixing chamber, as well as the feed pipes, in a thermostatted water bath. The precipitate was continuously removed, filtered, washed thoroughly with hot distilled water to remove adsorbed potassium compounds, and dried at 80 oC overnight. The dried precipitate was calcined in air at 400-700’ C for periods of 2 to 18 h, applying heating rates of 100 to 500”C/h. For some samples, the dried product was pelletized at a pressure of 1000 to 2000 bar. The pellets were crushed to obtain a 30-80 mesh fraction, which was then calcined as described above. As a reference sample, an unpromoted high surface area magnesia was prepared by calcination of magnesium hydroxycarbonate at 500 to 700” C for 2 h, applying heating rates of 500 oC/h. A sample supposed to be representative of the calcined hydrotalcite material used by Reichle [111 was prepared by precipitating an aqueous solution of magnesium nitrate and aluminium nitrate (molar ratio of Mg-to-Al was 3 ) with a sodium hydroxide and sodium carbonate solution (decreasing pH method), as described in ref. 11.The dried sample was calcined at 600 ‘C, as described above. To investigate the steam stability, some calcined samples were subjected to a 50:50 steam-to-nitrogen mixture of atmospheric pressure for 20 hours at temperatures of 400 to 600” C. Characterisation Uncalcined samples were characterised by X-ray diffraction (XRD) . Calcined samples were, in addition to XRD, characterised by surface area determination (N, ) and 27A1solid state NMR. The basicity of the calcined samples was determined by the indicator method: a slurry of the sample in toluene (100

82 TABLE 1 Indicators used for basicity determinations Indicator

pKa

Colour (acidic)

Colour (basic)

Bromothymol blue 2,4_Dinitroaniline 4-Chloro-2-nitroaniline I-Nitroaniline 4-Chloroaniline

7.1 15.0 17.2 18.4 26.5

yellow light yellow light yellow yellow colourless

purple wine red yellow-orange yellow-orange pink

mg/l), with a small amount of indicator, was titrated with a 0.1 N solution of benzoic acid in toluene. Table 1 lists the indicators used. Testing Double-bond isomerisation (DBI ) experiments were carried out in conventional microflow equipment. A portion of about 3 g of freshly calcined catalyst (30-80 mesh) was mixed with 4 g of inert Sic (0.1 mm) and placed in the tubular reactor. The flow dynamic properties of the resulting fixed bed should satisfy the plug flow concept. The temperature of the reactor was increased to 300°C in 1 h under a flow of 6 Nl/h of nitrogen. Subsequently, 1-pentene was mixed in at a volume ratio of 1: 10. Analysis of the 1-pentene used showed it to be 97.4% pure, the main impurities being 2-methyl-1-butene (1.6%), n-pentane (0.3% ) and trans-2-pentene (0.3% ). Sampling (in duplicate ) was started after 16 h to approximate a steady state operation. Samples were analysed by gas chromatography. RESULTS AND DISCUSSION

Characterisation of the uncalcined materials The XRD results for a sample with a Mg-to-Al molar ratio of 5 show a diffuse pattern that is characteristic of a Feitknecht compound (Fig. 1) . There is no evidence of separate magnesium phases. In samples with a Mg-to-Al ratio of 10, hydromagnesite was usually observed in addition to the Feitknecht structure; however, even at this very high Mg-to-Al ratio, samples have been prepared where separate magnesium phases are almost absent, by lowering the temperature of precipitation to 40°C (Fig. 2). For samples with a Mg-to-Al ratio of 15, a large quantity of hydromagnesite was always observed in addition to a Feitknecht compound. A coalingite type phase [lo] could not be detected in any of these samples. It seems unlikely that under these conditions separate magnesium phases

83

50.0

ANGLE

PEAK

D SPACING

(deg)

l/l

(ANG)

60.0

max

10.90

8.110

86.4

i%)

Ipm

700 28

hkl 003

22.08

4.023

69.9

006

34.27

2.614

76.4

w9/012

38.16

2.357

23.0

015

44.94

2.016

14.5

018

60.14

1 s37

47.6

110

61.24

1.512

52.9

113

84.81

1.437

4.4

116

Fig. 1. XRD pattern of a coprecipitated sample with a Mg-to-Al molar ratio of 5.

1,. 00

100

200

300

400

50.0

600

‘PZX 60.0

70.0 28

288

1. 00

10.0

20.0

00

100

200

30.0

‘:z

j.

jr

400 500 HYDROTALCITE,

,,

.

(

600 SYN

700 22-

800 700

60.0

700

800

600 %8

, 30.0

40.0

500

HYDROMAGNESITE

25-613

Fig. 2. XRD pattern of a coprecipitated sample with a Mg-to-Al molar ratio of 10.

84

I -I5

I

I

I

I

20

25

30

35

AL / (Mg

+ At)

%

mot/mot

Fig. 3. Comparison of lattice parameters of a hydrotalcite structure with Mg-to-Al molar ratio of 5 and literature data.

can be formed that are XRD amorphous. Therefore, the virtual absence of such separate magnesium phases in the XRD pattern strongly indicates that Feitknecht compounds with very high Mg-to-Al ratios (5-10) can be formed. This is further supported by a comparison of the lattice parameters for sample with Mg-to-Al ratio of 5 with those published for the usual range of Mg-to-Al ratios [ 121, as shown in Fig. 3. A reasonable correlation between lattice parameters and Mg-to-Al ratio is obtained. Characterisation of the calcined materials The XRD results for all calcined samples show a diffuse, distorted magnesium oxide pattern. The 27A1solid state NMR results for a sample with Mgto-Al ratio of 5, calcined at 400 ’ C, show a clear predominance of octahedrally coordinated aluminium ions, presumably indicating that these ions mainly occupy sites in a magnesium oxide lattice (Fig. 4). In this context, it is important to note that in a similarly prepared Zn/Al sample, aluminium ions are predominantly present in a tetrahedral coordination, in line with the tetrahedral zinc oxide structure [ 131. It would appear that calcination at 400’ C does not result in a clear phase separation, in agreement with the disordered oxide structures observed in similar nickel/aluminium oxides [ 6,7]. In support of this, we have

85

4

1

I

I

I

300

200

100

0

-100

1

-200

I

-300

Fig. 4. 27A1solid state NMR spectrum for a sample with Mg-to-Al molar ratio of 5, calcined at 400°C. IA

tm’/g)

260r 240220

-

200

-

ieo

-

160

-

140

-

120

-

100

-

60

-

60

-

40-

520

660

600

640 680 720 760 CALCINATION TEMPERATURE, C

Fig. 5. Thermal stability of stabilized magnesia compared with that of pure magnesia.

found that the calcined materials can be reconverted to the hydrotalcite structure by contact with aqueous solutions. The surface areas of the calcined samples were found to depend on the Mgto-Al ratio and the calcination temperature, but not (within the limits of this investigation) on the duration of calcination, the heating rate or pelletizing. Figs. 5 and 6 show some typical data. The steam stability of a sample with a Mg-to-Al ratio of 5 is demonstrated in Fig. 7. These figures also contain data for pure magnesia, for comparison. These results show that “stabilized magnesia” compounds with high surface areas (typically 200-270 m2/g) and a reasonable stability towards heat and steam can indeed be prepared from coprecipitated Feitknecht-type Mg/Al compounds, with Mg-to-Al molar ratios of 5 to 10. The gain over pure high surface area magnesia in this respect is impressive.

86

OL 0

I

I

5

I

I 20

15

10

hip/Al

RATIO,

moL/moL

Fig. 6. Surface areas of stabilized magnesia samples calcined at 4OO”C, as a function of Mg-to-Al molar ratio.

400

500

600

Fig. 7. Steam stability of stabilized magnesia compared with that of pure magnesia: surface areas after steam treatment at indicated temperatures.

Basicity of the calcined materials We have found during the titration experiments that colour changes in these systems are generally small and slow. Therefore, we did not determine the amount of basic sites in this way. Still, by observing the colours in the presence of various indicators, the basicity could be estimated in terms of pKa ranges. The results are summarised in Table 2, which also contains the surface areas of these samples. According to the literature [ 14-171, the strength of the basic

87 TABLE 2 Basicities and surface areas of calcined samples Basicity range

Sample

Calcination temp. (“C)

MgO

700

50

15.0-17.2

SM (5/l)”

700

190

17.2-26.5

SM (50) SM (5/l) SM (10/l) MgO

500 600 700 600

240 210 150 90

SM (lo/l) SM (10/l) Mgo

500 600 500

210 160 170

7.1-15.0

> 26.5

“SM (z/l)

Surface area (m*/g)

stands for stabilized magnesia with a molar ratio of Mg-to-Al of x.

sites of pure magnesia is in the range 18.4-26.5. This corresponds rather well with the values we found. The data in Table 2 show that the stabilized magnesia samples are about as basic as pure magnesia. This holds for samples calcined at 500 or 600’ C. For samples calcined at 700” C, the basicity of the stabilized magnesia samples is clearly higher, which is presumably related to the higher thermal stability. It can be argued that sintering, as evidenced by the surface area loss upon calcination at 700” C, will probably eliminate the strongest basic sites first. Activity for the double-bond isomerisation of 1-pentene Preliminary test runs with a reactor filled with Sic showed no conversion of l-pentene under reaction conditions, indicating the absence of thermal contributions and the inertness of reactor wall and Sic. Fig. 8 shows the results for stabilized magnesia samples with Mg-to-Al ratios of 5 and 10, calcined at different temperatures, as well as for pure magnesia and a calcined hydrotalcite with a conventional Mg-to-Al ratio of 3. The data demonstrate that stabilized magnesia samples with a molar Mgto-Al ratio of 5 are very active for DBI. In some cases even the thermodynamic equilibrium conversion under these circumstances was reached. It should be noted that these equilibrium data deviate from those given by Reichle [ 111, who apparently omitted to correct for the temperature of reaction. There is a strong dependence of the activity on the calcination temperature, as was already indicated by the basicity data in Table 2. Calcination at 700 oC causes a drop in activity, which cannot be accounted for by a similar loss of the surface

88 100

CONVERSION,

% 240

210

i30

150

160

90

210

__________________._.________._______________________________~~__ THERMODYNAMIC

*) W/Al 3) HT-CALCINED

EOUILIERIUM

HYDROTALCITE

j 450

500

STABILIZED

600

700,

MAGNESIA (Mg/AL

5)

,600

700

1610 Mg/Al

IO)

,

600 M90

600 HT ‘j

T CALCINATION,

C

Fig. 8. Results of double-bond isomerisation of l-pentene at 300’ C.

area. Moreover, we have found that one stabilized magnesia sample (Mg-toAl ratio of 5 ) , steamed at 400’ C, followed by a recalcination at 600 *C, showed a 60% drop in conversion, although the surface area had only decreased by 10%. Apparently, a large number of active basic sites are eliminated during the first annealing of the material. Again, this is in line with the basicity data. The stabilized magnesia samples with molar Mg-to-Al ratio of 10 are less active than their 5/l counterparts. At least part of this difference can be ascribed to their lower surface areas. The 10/l sample calcined at 600°C is still substantially more active than the pure magnesia; the latter sample has the lowest surface area of all samples investigated. Compared to a calcined hydrotalcite with a conventional Mg-to-Al ratio of 3, the stabilized magnesia samples with Mg-to-Al ratio of 5 are considerably more active. As they have similar surface areas after calcination at 600’ C, this difference is presumably due to a higher intrinsic basicity at higher Mg-to-Al ratios. Surprisingly, the performance of the material with a Mg-to-Al ratio of 3 is clearly better than that stated by Reichle, who reported a conversion of only 9% under similar conditions for such a sample [ 111. This difference could be related to difficulties in reproducing the exact precipitation conditions, with many variables (e.g. stirring rate) being unknown. Furthermore, Reichle applied a calcination temperature of 450 +_10 ‘C. We have found for our stabilized magnesia samples that the activity is strongly dependent on calcination temperature in this region, with no activity at all being observed for samples cal-

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cined at 400°C in spite of the high surface areas. This can be ascribed to incomplete removal of hydroxide and carbonate groups from the surface. For samples with a higher aluminium content (and consequently higher hydroxide and carbonate contents), calcination at 4502 10°C may not be sufficient. With respect to selectivity, we have found no evidence of skeletal isomerisation, presumably indicating the absence of acidic sites. The ci.s/trans ratios in the 2pentene products were approximately the equilibrium ratios, although sites with a high intrinsic basicity should in principle favour the formation of the cis isomer [ 141. Still, this is in line with results for the DBI of 1-pentene over calcium oxide at 250°C [ 181. Apparently, under these conditions, the cIs/ tram isomerisation reactions are sufficiently fast to ensure a thermodynamic equilibrium in this respect. CONCLUSIONS

(1) It appears possible to prepare Feitknecht compounds for molar Mg-toAl ratios of 5 to 10, without the formation of considerable amounts of separate magnesium compounds. (2) Upon calcination, these Feitknecht compounds yield disordered mixed oxides, which combine high surface areas ( > 200 m2/g) with an improved stability towards heat and steam, when compared to pure magnesia. (3) These “stabilized magnesia” compounds have a strongly basic character, similar to that of pure magnesia. (4) Stabilized magnesia compounds are promising solid base catalysts, as evidenced by their performance in the double-bond isomerization of l-pentene. In a subsequent paper, we plan to present results on the shaping of these stabilized magnesia compounds into extrudates and on their use as catalyst supports. ACKNOWLEDGEMENTS

The authors would like to thank Ms. G.G.M. Harmsen for the XRD measurements, Mr. J. van Amstel and co-workers for the surface area determinations and Mr. A.E. Wilson for the NMR measurements. Stimulating discussions with Dr. P.H.M. de Korte (Billiton Research Arnhem) are gratefully acknowledged. REFERENCES 1 2 3 4 5

R.I. Razhouk and R.S. Mikhail, J. Phys. Chem., 63 (1959) 1050. J. Green, J. Mater. Sci., 18 (1983) 637. P.J. Anderson and R.F. Hohrldck, Trans. Faraday Sot., 58 (1962) 1993. R. Allmann, Chimia, 24 (1970) 99. E.C. Kruissink, Thesis, Delft (1981).

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DC. Puxley, I.J. Kitchener, C. Komodromos and N.D. Parkyns, Stud. Surf. Sci. Catal., 16 (1983) 237. P.H.M. de Korte, Thesis, Delft (1988). S. Gusi et al., J. Catal., 94 (1985) 120. H.G.J. Lansink Rotgerink, Thesis, Twente (1988). J. Pastor-Rodriquez and H.F.W. Taylor, Mineral. Mag., 38 (1971) 286. W.T. Reichle, J. Catal., 94 (1985) 547. G.W. Brindley and S. Kikkawa, Am. Mineral., 64 (1979) 836. H. Schaper and J.J. Berg-Slot, unpublished results. K. Tanabe, Solid Acids and Bases, Academic Press, New York, London, 1970. J. Take, N. Kikuchi and Y. Yoneda, J. Catal., 21 (1971) 164. H. Hattori, N. Yoshii and K. Tanabe, Proc. 5th Int. Congress on Catalysis, Elsevier, 1973, p. 233. J.L. Lemberton, G. Perot and M. Guisnet, J. Catal., 89 (1984) 69. Y. Schaechter and H. Pines, J. Catal., 11 (1968) 147.