INTERNATIONAL JOURNAL OF ENERGY RESEARCH, VOL. 20,763-766 (1996)

ACTIVITIES AND SELECTIVITIES OF

TEMPERATURES RELEVANT TO CHEMICAL

INTERCONVERSIONS

COPPER / METAL-OXIDE CATALYSTS AT

HEAT-PUMPS BASED ON ISOPROPANOL/ ACETONE

JOSEPH CUNNINGHAM. JAMES N. HICKEY AND ZHENYI WANG

Chemistry Department, Universiv College Cork, Republic of Ireland

SUMMARY

Results are presented concerning the possible suitability of various oxide supported copper catalysts for utilization in proposed chemical heat pumps, featuring isopropanol dehydrogenation as a ‘low’ temperature endothermic process coupled with acetone hydrogenation as a higher temperature exothermic process. For each of those processes, separate determinations have been made of the temperature ranges within which adequate levels of pseudo-steady- state activity and selectivity were maintained at vapour/solid interfaces over Cu/Al,O,, Cu/Cr,O,, Cu/SiO, and Cu/Ti02 catalysts. For acetone hydrogenation over Cu/Al,O, and Cu/SiO,, that range extended over 423-550 K. However, over Cu/Cr,O, and Cu/TiO, it was restricted to 423-473 K, limited by insufficient activity at the lower end, and at the upper end by a fall-off in selectivity caused by onset of acetone oligomerization to C,, C, and C,, ketones. Similar temperature ranges were found to be necessary for attainment of adequate levels of activity and selectivity for dehydrogenation of isopropanol vapour over oxide-supported copper catalysts. Again over Cu/TiO, materials, selectivity declined above 473 K owing to the formation of oligomeric ketone products. System modifica- tions to overcome the indicated difficulties are briefly considered.

KEY WORDS: chemical heat pump; acetone hydrogenation; isopropanol dehydrogenation; oxide-supported copper catalysts

1. INTRODUCTION

Consideration has been given in recent publications to the design and feasibility of chemical heat-pump systems for transforming low-grade heat (e.g. at 336-343 K) to upgraded waste heat (e.g. at 453-473 K) with the aid of two reversible chemical reactions over appropriate catalysts, viz. the endothermic dehydrogenation of isopropanol and the exothermic hydrogenation of acetone (Prevost and Beganel, 1980; Kato et af . 1986; Saito et a f . 1987). Effects of design variables on the energy performance of such heat transformer systems have been calculated on the basis of 0.32 MW upgraded heat (Gandia and Mortes, 1992), but it has been recognized that their future feasibility depends upon the availability of active and highly selective catalysts for these reactions in the indicated temperature ranges. As a follow-up to our previous publications dealing with the catalysis of isopropanol/acetone interconversions over copper and its oxides (Cunningham et af., 1986a, 1986b, 1987), recent experiments in these laboratories have separately examined the utility of oxide supported copper catalysts for the hydrogena- tion of acetone (Cunningham et af., 1993; Hickey, 1992), and for the dehydrogenation of isopropanol (Cunningham et af., 1994; Wang, 1995). Consideration is given here to previously unpublished data obtained in these studies which identify for both such conversions not only the activities and selectivities attained with such catalysts, but also the temperature ranges within which they apply.

CCC 0363-907X/96/090763-04 0 1996 by John Wiley & Sons, Ltd.

Received 21 November 1994 Revised 5 Januaty 1995

764 J. CUNNINGHAM, J. N. HICKEY AND ZHENYJ WANG

2. RESULTS

2.1 Acetone Hydrogenation

Full details are given elsewhere (Hickey, 1992) of the ex-copper-hydrosol preparative method developed to achieve cv. 13 wt% copper upon TiO,, Cr203, SiO, or Al,O, supports. Prior to comparisons of their catalytic activities on a conventional continuous-flow microcatalytic reactor, samples were pre-reduced in situ in flowing H, at 473 K. Sample temperature was then decreased to 448 K before establishing a reactant flow having P (Ac): P (H,) : P (Ad = 30 : 380: 350 Torr and a flow rate of 20 ml min-'. The following bracketed values represent mean values of pseudo-steady-state (P.s.s.) conversions observed with > 98% selectivity to isopropanol in these conditions at 40-180 min on-stream over: Cu/Al,O, (60%), Cu/Cr,O, (60%), Cu/SiO, (57%) and Cu/Tio, (46%). Similar results were obtained at temperatures in the range 448-473 K. The close approach to equilibrium-limited conversion over the first three of those materials, at temperatures corresponding to the lower end of the range envisaged for heat-pump application appeared encouraging, but it was desirable to test if such activity and selectivity was retained over prolonged periods of exposure to net reducing conditions. For the purpose of accelerated testing of this point, the P.S.S. catalytic activity was again tested at 448 K, but with pre-reduction after 2 h in flowing H, at 773 K (HTR,,,). This completely eliminated the activity of Cu/Cr,O, and Cu/TiO, at T, = 448, whereas conversion over Cu/SiO, was only slightly decreased (from 57 to 50%), and that of CU/AI,O, declined from 61% to 38%. Such HTR,,,-induced elimination of hydrogenation activity of Cu/Cr,O, and Cu/TiO, - allied to subsequent observations that activity could be restored by re-oxidation for 2 h at 773 K and re-reduction at 473 K - pointed to the occurrence of some type of hydrogenation-inhibiting effect, originating from interactions between supported Copper and the HTR-reduced TiO, or Cr,O, supports. Metal/support interactions capable of giving rise to such reversible inhibition of hydrogenation activity in these systems included (i) blockage by reduction-induced defects upon the metal oxide surface of pathways for reverse hydrogen spillover back to metal particles for release to the gas phase (Courbon et al. 1981) or (ii) decoration/encapsulation of the copper particles by reduced TiO,-, or CrZ03-x (Chen et al., 1986; Boccuzzi et al. 1993). Conversely, the non-occurrence of a strongly inhibiting effect of HTR,,, upon the hydrogentation activity of Cu/SiO, could be understood in terms of the non-occurrence of any such inhibiting effect in that material involving a non-reducible metal oxide support.

When T, was increased to 509 f 8 K for samples pre-reduced only at 473 K, S(i.p.1, the selectivity for conversion to acetone at T, - 510 K was as low as 11% over Cu/TiO, after 50 min on-stream before rising to just 37% in P.S.S. conditions. Likewise, that for Cu/Cr,O, was initially 20% before rising to 70%. However, S(i.p.) for Cu/SiO, remained constant at 65 f 3%. Extent of loss of selectivity at 510 K varied in the sequence Cu/TiO, > Cu/Cr,O, > Cu/Al,O, > > Cu/SiO,, which correlated with the experimentally observed extents of formation of colourless oil products of increased boiling point and molecular weight (IMWP). Collection of these relatively abundant oligomeric products over Cu/TiO,, and their analyses by capillary GC and GC/MS, showed them to consist mainly of saturated C, and C, ketones. Their formation could be understood in terms of successive involvements of acetone in aldol-condensation steps over the catalysts, followed by secondary dehydration plus hydrogenation of products from such aldol condensations (Cunningham et al., 1993; Hickey, 1992). Clearly, it would be essential to avoid the formation of such IMWP products from acetone hydrogenation in any waste-heat- transformer system. Ways in which formation of these unwanted products from acetone oligomerization at T, 2 473 K might be totally averted - e.g. through modifications of the catalysts and the use of halogenated acetone reactants - are under active investigation, especially over Cu/SiO, catalysts, which is most promising for acetone hydrogenation at 473 k 30 K.

2.2 Dehydrogenation of Isopropanol to Acetone

Efficient operation of this endothermic reaction at substantially lower temperature than the complemen- tary exothermic hydrogenation of acetone would be essential in proposed heat-pump applications. Since

CHEMICAL HEAT PUMP CATALYSTS 765

that requirement was not satisfied by the above-mentioned catalysts featuring ca. 13% copper deposited as colloidal copper onto the supports, different preparative methods were adopted in efforts to prepare copper/metal-oxide catalysts having the requisite high activity and selectivity for isopropanol dehydro- genation at such lower temperatures. Catalyst precursors having only monolayer-equivalent amounts of Cu(I1) ions highly dispersed upon TiO, (which corresponded to ca. 3 wt% copper) were first prepared by wet impregnation with copper malonate or high purity copper acetate. For purposes of eventual comparisons with the Cu/TiO, materials, MgO and CeO, were likewise wet-impregnated with 3 wt% Cu(I1) ions. After drying these Cu(II)/TiO, , Cu(II)/MgO and Cu(II)/CeO, precursors, the copper ions supported thereon were reduced by immersion into deaerated hydrazine solutions. Resulting materials were filtered, washed, dried in a vacuum-oven at 383 K and stored in dry air. Reoxidation of the highly dispersed copper occurred during storage, as evidenced by temperature programmed reduction (TPR) profiles characterized by i'&)/Z&ax) values as follows: 'as stored' Cu/TiO,, Cu/CeO, and Cu/MgO materials - 415/458 K, 493/504 K and 511/526 K, respectively. These values, and also comparison with unsupported CuO powder, confirmed significantly enhanced ease of reduction for highly-dispersed copper species upon Cu/TiO,.

No significant activity of the 'as stored' 3% cu/metal-oxide materials for isopropanol dehydrogenation was found at T, I 453 K prior to their in situ reduction in flowing H, for 2 h. Overall percentage conversion of isopropanol and the selectivity for acetone production over pre-reduced 2.7% Cu/TiO,, under a 20 ml min-' inlet flow of reactant consisting of 20 Torr isopropanol plus 740 Torr argon, varied with reaction temperature as illustrated by plots (a) and (b) of Figure 1. Those plots illustrate an onset of dehydrogenation activity at ca. 410 K followed by rapid increase in conversion at T, = 410-423 K, with acetone being the dominant hydrocarbon product detected. Despite this favourable high selectivity towards acetone at ca. 423 K, the small difference of this lowest temperature for efficient and selective

Mole Converalon and Selectlvity (%) 100

80

60

40

20

0 373 423 473 623 673 623 673

Trxn./ K Figure 1. Temperature dependence of activity and selectivity in isopropanol conversion over 2.7% Cu/TiO,: (a) conversion of isopropanol; (b) selectivity for acetone; (c) selectivity for propylene; (d) selectivity for IMWP. GHSV 85800 h- ' , 8.8 Tom

isopropanol

766 J. CUNNINGHAM. J. N. HICKEY AND ZHENYJ WANG

dehydrogenation of isopropanol from the temperature range of 448-473 K noted above as appropriate for selective hydrogenation of acetone over other copper/metal-oxide catalysts, indicated that substantial up-grading of waste heat using acetone/isopropanol interconversions would not be feasible utilizing any combination of the oxide-supported copper catalysts here reported. Indeed, this may be implied by the principle of microscopic reversibility in situations involving correlated operation of forward and reverse processes over similar catalysts. Thus it is not surprising that modified heat pump proposals (Saito et al., 1987) envisage the isopropanol dehydrogenation step as occurring in a different (liquid) phase and as being physically and thermodynamically separated from the acetone hydrogenation step. Present results lend indirect support to that concept and furthermore point to a need for (i) the development of homogeneous catalysts capable of selective dehydrogenation of isopropanol at low temperatures in the liquid phase; and (ii) further improvements in heterogeneous catalysts with a view to achieving increased activity and 100% selectivity for hydrogenation at the temperatures of interest in waste-heat upgrading. Special attention must be given to the avoidance of acetone oligomerization, since plot (d) of Figure 1 illustrates that, even when starting with isopropanol as the sole reactant over Cu/TiO,, onward oligomerization of the primary product, acetone, was not averted but rather represented a set of substantial secondary reactions capable of diverting large amounts of isopropanol and acetone into undesirable IMWT products at reaction temperatures as low as 450 K.

REFERENCES

Boccuzzi, F., Baricco, M. and Guglielminotti, E. (1993). Appl. Surface Sci., 70/71, 147. Chen, H.-W., White, White, J. M. and Ekhart, J. C. (1986). J. Carafysis, 99, 293. Courbon, H., Henmann, J.-M. and Pichat, P. (1981). Journal of Catalysis, 72, 129. Cunningham, J., Al-Sayyed, G. H., Cronin, J. A., Fierro, J. L. G., Healy, C., Ilyas, M. and Tobin, J. P. (1986). J. Catalysis, 102, 160. Cunningham, J., Al-Sayyed, G. H., Cronin, J. A., Healy, C. and Hirschwald, W. (1986a). Appl. Catalysis, 25, 129. Cunningham, J., Hickey, J. N., Brown, M. D. and Meenan, B. J. (1993). J. Materials Chemistry, 3, 743. Cunningham, J., McNamara, D., Fierro, J. L. G. and OBrien, S. (1987). Appl. Carafysis, 95, 381. Cunningham, J., O’Neill, M. Patrick, G., Hickey, J. N. Wang, Z., Galwey, A. L. and Fierro, J. L. G. (1994). J. Thermal Analysis, 41,

Gandia, L. M. and Montes, M. (1992). Inr. J. Energy Research, 16, 851. Hickey, J.N. (1992). M.Sc. thesis, National University of Ireland. Kato, Y., Kameyama, H. and Saito, Y. (1986). WorM Congres I l l of Chemical Engineering, Tokyo Japan, pp. 676-679. Prevost, M, and Beganel, R. (1980). Proc. Inf. Seminar on Thermochemical Energy Storage, Stockholm, Sweden, Vol. 95, p. 111. Saito, Y., Kameyama, H. and Yoshida, K. (1987). Int. J Energy Research, 11, 549. Wang, Z., (1995) Ph.D thesis, National University of Ireland.

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