Hydrogenation of Glucose with Bimetallic Catalysts (NiM) of Raney Type

M. Guisnet et st. (Editors), Heterogeneous Cstelvsis and Fine Chemicals 1988 Elsevier Science Publishers B.V., Amsterdam - Printed In The Netherlands

189

HYDROGENATION OF GLUCOSE WITH BIMETALLIC CATALYSTS (NiM) OF RANEY TYPE

J. COURT, J.P. DAMON, J. MASSON and P. WIERZCHOWSKI Universite J. FOURIER de GRENOBLE Laboratoire d'Etudes Dynamiques Structurales de la Selectivite (LEDSS-l) - BP 53X - 38041 GRENOBLE CEDEX

et

ABSTRACT Hydrogenation of glucose in distilled water and in buffer solution has been performed with promoted Raney nickel catalysts. The catalysts have been prepared from Ni?_ I~ A1 1 (where M = Cr,Fe,Co,Cu,Mo and x ,,;;: 0.4). The promoter metal, with the- ~xtepcion of cobalt, favours aluminum retention in the catalyst, this high residual aluminum content could be partially responsible for the increase of the nickel activity observed in the presence of a metal M. Iron which is the most efficient promoter loses totally its activity in phosphate buffer at pH 6.1. The activity of Cr or Mo promoted nickel catalyst is about twice that of the unpromoted Raney nickel; in phosphate buffer the activity of these catalysts is enhanced but remains similar to that of the Raney nickel. Cobalt and copper present in small amounts do not change the properties of the Ni-Al system. INTRODUCTION We prepared bimetallic catalysts (NiM) of the Raney nickel type from where M=Cr,Fe,Co,Cu,Mo and x .s 0.4). several crystallized alloys (Ni 2_xMxA1 3, Studies of these catalysts in the hydrogenation of carbonyl and of ethylenic groups in an organic medium showed that che promoted metals enabled the hydrogenation selectivity of the carbonyl group as compared to the ethylenic group to be varied (1,2,3). To know more about their catalytic properties in an aqueous medium, we studied them in glucose hydrogenation. It has been shown that adding Mg (4), Ti (5), Fe (6), Fe and Mo (7), Cu (8) to Raney nickel improves the activity and the stabi 1ity of the catalyst in glucose hydrogenation. It is not yet possible to generalize the published results as many different methods have been used for addi ng the promoters, alloying and alkali leaching. Glucose solutions in water are slightly acidic at room temperature. As the ionisation constant increases with the temperature (9) and, given the oxidation-reduction potential value of nickel, its oxidation is possible during the reaction. We have studied therefore the activity of promoted Raney nickel in buffer solutions.

190 EXPERIMENTAL The preparation of the catalyst has already been described elsewhere (10). The bulk composition of each sample was determined by chemical analysis and expressed by the atomic ratios Al/Ni and MINi. The total surface area (SSET) was measured by nitrogen adsorption, the metallic surface area by the thiophene method (10), although 3-methyl thiophene was used for greater accuracy. The hydrogenation reactions in the liquid phase were carried out in a 250 ml static reactor, under constant hydrogen pressure. The experimental conditions were as follows: hydrogen pressure: 30 bars; temperature: 100°C; glucose concentration: 0.1 mol in 150 ml of water; catalyst mass: between 0.5 g and 0.55 g ; stirring speed: 900 rpm so that the diffusional limitation did not affect the reaction. The catalyst in water suspension had been pretreated at room temperature for 30 minutes at a pressure of 30 bars. The reaction kinetics were studied using two different methods - by measuring the hydrogen uptake and by G.L.C analysis (after silylation (11)) of samples withdrawn from the reaction mixture. The chromatographic measurement conditions were : wide bore capillary column Supelcowax 10, 30 m length, 0.75 mm ID, 1.0 m film, 5 ml/minute He flow rate, isothermal at 160°C, with inositol as an internal standard. Since the reaction is first order with respect to the glucose concentration, the activity of catalyst is given by the first order rate constant expressed per square -1 -2 meter of the meta11 i c surface (k in h m met)' The catalysts prepared from Ni 2A1 3 and Ni 2_xMxA1 3 will be referred to as Ni 2-3 and NiM x respectively. RESULTS AND DISCUSSION l/Influence of promoters Rate constants obtained with the different catalysts are given Table 1. As we can see, addition of a metal M to nickel always increases its activity except for Ni-Mo O. 4' Characteristics of each sample are also given in this table bulk composition (expressed by the atomic ratios Al/Ni and MINi) and surface areas (SBET and Smet)· The metal content (M) in the catalyst depends on the nature and the concentration of the said metal in the precursor alloy. This phenomenon is due to the elimination to a greater or lesser extent of the added metal occurring during the alkali leaching process, as already mentioned (12). We have shown previously that after alkali leaching, certain metals are in a metallic state and homogeneously distributed in the nickel catalyst i.e: Fe, Co and Cu (13,14) ; others are oxidized and strongly segregated to the surface i e : Cr and Mo (15,161.

191 TABLE 1 Characterization of the catalysts and their hydrogenation of glucose in aqueous solution.

All oy

Cata lyst

Bulk Composition Al/Ni MINi

Ni com Ni 2A1 3

Ni

Nil.91CoO.09A13 Nil.86CoO.14A13

NiCo O. 09 NiCo O. 14

0.24 0.17

Nil.93CrO.07A13 Ni 1.89CrO.llA13

NiCr O. 07 NiCr O. ll

Nil.95MoO.05A13 Ni 1.9MoO.1A13 Nil.8MoO.2A13 Nil.6MoO.4A13

0.10 0.26

catalytic

activity

Surf ace area BET Metallic

in

Activity k (*)

80 80

60 64

2.8 3.8

0.054 0.096

90 98

65 57

4.2 7.4

0.42 0.48

0.031 0.054

127 120

71 75

9.7 8.2

NiMo O. 05 NiMo O. 1 NiMo O. Z NiMo O. 4

0.28 0.36 0.66 1. 01

0.009 0.017 0.037 0.026

82 63 23

63 57 42 9

6.9 7.1 7.9 <2

Nil.9FeO.1A13 Nil.6FeO.4A13

NiFe O. l NiFeO. 4

0.38 0.51

0.073 0.26

78 112

60 58

7.8 24.1

Nil.9CuO.1A13

NiCu O. l

0.36

0.055

85

2_3

(*)

77

52.5

the

4.4

k expressed in 103h-lm-Z meta1

The Ni ZA1 3 phase favours the residual aluminum retention in the catalyst; this is increased if a metal is added (15). The results of Table 1 confirm this phenomenon for all the metals except for Co. In order to interpret these results, we have plotted the catalytic activity of each sample as a function of its aluminum content Al/Ni (Figure 1l. When comparing the two undoped samples N\om and Ni Z-3' the increase in activity observed can be attributed to the increase of residual aluminum content. This promoting effect of aluminum is unexpected : recent results for hydrogenation of carbonyl and ethylenic functions in non-aqueous media presupposed a rather inhibiting role of aluminum (3,17). The difference in the medium could explained this discrepancy in behaviour.

192

25

b¢i

2

15

15

b* 5

a

a.

a.

a¢ b. b •

.... *+/

10

I Cr 001

IFe;;--

MO O0 5

5

~

~i2-3



Ni 1-3

d

Ni com I

0.5

0

c. AIjNi

Fig. 1 Catalytic activity versus bulk aluminum content AllNi Bulk metal composition MINi of each sample Fe ¢ (a 0.073 - b 0.26) ; Cr ... (a 0.031 - b 0.054) ; 0.096) ; Co (a 0.054 - b Cu • (a 0.055); Mo • (a 0.09 - b : 0.017 c 0.037 - d : 0.026).

CUD'

0

Fig. 2 Catalytic activity in distilled water (-) and in phosphate buffer at pH 6.1 (- - -) .

*

A small amount of Co or Cu added to nickel (Co/Ni=0.054 and Cu/Ni= 0.055) would not appear to influence the promoting effect of aluminum. However with a greater cobalt content, although the aluminum content is low, activity increases. In this case, cobalt plays a doping role in addition to that of aluminum. Ni-FeO. l catalyst (FelNi = 0.073) gives similar results. Chromium and molybdenum have a particular behaviour. Only very small quantities of metal (MolNi = 0.009 or Cr/Ni = 0.031) are sufficient to produce a notable increase in activity. Any interpretation based only on the residual aluminum and the metal content in the bulk is difficult. As these two metals

193 are scrongly segregated to the surface, it is therefore not surprising that there is such a promoting effect of the doping metal even with small bulk contents. We observed that Ni-Mo O. 4 has a very low activity. We must point out that all the characteristics of this sample (surface area, composition) differ from the others. (Table 1). Ni-Fe catalyst shows an activity markedly higher than that of all the O. 4 not ignoring the synergetic effect other samples (6 times that of Ni 2_ 3).Though of metallic iron on nickel, the difference between oxidation reduction potentiel of these two metals must be taken into account. By its own oxidation in the reaction medium, iron may well prevent nickel from oxidizing. The promoting effect of aluminum previously detected for this reaction, probably results from the same cause. Nevertheless, it is difficult to generalize the promoting effect on this basis alone: for example, surfaces of the highly active Ni-Cr or Ni-Mo catalysts are essentially composed of nickel, aluminum and the oxides of chromium or molybdenum resulting from the catalyst preparation. 2/The influence of the medium In order to shed light on the influence of acidity on the activity of the catalysts we hydrogenated the glucose in buffer solutions with a concentration of 0.1 mol.l- l. The Ni catalyst activity (Table 2) is always much greater 2_3 with a phosphate buffer than in pure water and we measured a five fold increase when pH = 6.1. TABLE 2 Catalytic activity of Ni 2_3 catalyst in buffer solution and selectivity of the sorbitol formation.

Buffer

pH

without acetate phosphate phosphate phosphate

4.3 5.2 5.2 6.1 7

Activity k(a) 3.8 2.3 12.8 18.3 10.2

k expressed in 103h-lm-2 metal

Sorbito 1 Sorbitol+Mannitol 1 1 1 0.99 0.88

194 Some authors have found that when pH = 8 the time required for complete hydrogenation of glucose can be halved (18). We must point out that even with the same pH (5.Z) and with the same cation have a very different (Na+) the pairs CH and P0 3COOH/CH 3COO 4H 3/P04H Z influence on the reactivity of Ni The phosphate ions thus have a catalytic Z_ 3' effect of thei r own in additi on to the one resulti ng from the change in pH. This result has to be related to a patented process to increase the reactivity of supported nickel catalysts when nickel phosphate is added (19). The promoted catalysts were examined in the conditions which led to the greatest change in Ni activity i.e a 0.1 mol 1-1 phosphate buffer at pH 6.1 2_3 Their activities in these conditions were compared to those observed in distilled water (pH 4.3) (Fig. 2). A great increase in activity was noticed with the presence of phosphate with low levels of chromium or molybdenum : the and NiMo O. 05) in the buffer medium activity of the two catalysts (NiCr O. 07 became similar to those of the reference catalyst (Ni On the other hand 2_3). the buffer produces no effect on the activity of NiCr O. ll' The activity of NiCu O. l although doubled when in contact with phosphate at pH 6.1, is still less than for Ni Z_ 3' NiFe O. 4 behaves in a very different way compared to other catalysts. Its activity at pH 6.1 is four times less than with distilled water. Phosphates and phosphori c acid can thus present quite the opposite effects accordi ng to the nature of the promoter. These results illustrate the complexity of the prob1em with many factors such as the effect of pH, the acid-base pair used, the heterogeneity of the catalytic surfaces, etc. The role of these different parameters is currently being investigated. 3/Sorbitol selectivity We did not monitor any formation of mannitol with any of the catalysts used in distilled water. The Raney nickel, whether promoted or not, remains very selective with a transformation rate of at least 50 %. We studied (Table 2) the selectivity of the sorbitol formation according to It is excellent at pH 5.2 with the two the pH for the reference catalyst Ni 2_3. and P0 An equally good acid-base pairs employed (CH 3COOH/CH3COO4H/P04H 2-). selectivity was recorded with phosphates at pH 6.1. In this case the selectivity was 99 % whi1st the activity of the catalyst was multiplied by about 5. A significant fall was observed at pH 7. Selectivity is still good with promoted catalysts in contact with a phosphate buffer at pH 6.1 i.e. 99%.

195 CONCLUSION Results of this study show that it is possible to increase Raney nickel activity by addition of other metals. phase favours aluminum retention; the promoters, with the The Ni 2A1 3 exception of cobalt, enhance this phenomenon. We suggest that this increase in aluminum content is partially responsible for the increase of activity. The metal Mcan also take part in the promoting effect. The extent of its role is function of its nature and also of its surface concentration. Chromium and molybdenum have a particular behaviour : they are oxidized during the alkali leaching period which modifies their distribution in the catalyst and therefore their surface concentration. Iron has a very pronounced promoting effect. One explanation could be that this metal is oxidized during the catalytic reaction and thus maintains a clean surface of metallic nickel, avoiding a possible oxidation of the active nickel sites. Such oxidations could possibly occur in the case of glucose hydrogenation in aqueous media. The pH of the solution modifies the activity of the catalysts, but the buffer used seems also to play an important role. The greatest activity enhancement is observed with phosphates at pH 6.1 for all catalysts except with iron promoted Raney nickel. Further investigations are necessary to understand the special behaviour of iron. ACKNOWLEDGMENTS assistance.

We are indebted to C. LAMBEAUX and P. CIVIDINO for technical

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