A Mild and Effective Method for the Reactivation or Maintenance of the Activity of Heterogeneous Catalysts

Abstract

able ( Figure 2). This places further constraints on asymmetric catalysis: (a) only the neutral histidine is effective, and (b) phosphate buffer (predominant counterion HP0:-0.2 t 0.4 -Lg k 0.6 0.8 1 .0 1.2 1.4 ? i' 7 8 9 PH -Fig. 2 Dependence of Ig k on the pH of the solution in the hydrolysis of (21 catalyzed by(12);[(12)]=1.08~ l0-'MM,[/2)]=5x 10-'M,[buffer]=0.02 M,0.47 vol.-% acetone in water; 0 (S,S)-(12), (S)-(2), borate buffer; __ phosphate buffer; D (R,S)-(lZj. (S)-V), ---borate buffer; (3 (S.SHl2). (R)-(2), borate buffer, __ phosphate buffer; + (R.S)-(IZj, (R)-/.?), _ _ _ borate buffer.at p H 7.5) is more efficacious than borate buffer (predominant counterion CIrather than H3B0, at p H 9). A model which displays these features is presented by formula (13).

Hydrogenation through Partial Oxidation of Hydrocarbons in Supercritical Water

“…Some researchers reported some specific features for solid catalyzed reactions in supercritical fluids (SCFs) Tadafumi ADSCHIRI, Susumu OKAZAKI, Makoto MOCHIDUKI, Shutaro KUROSAWA and Kunio ARAI (Tilscher et al, 1981: Yokota et al, 1989, 1991a, b: Saim and Subramaniam, 1991: Baptist Ngyugen and Subramaniam, 1992: Adschiri et al, 1991, 1995 . SCFs can dissolve solutes that are insoluble at ambient conditions and can supply a homogeneous reaction atmosphere for fluid phases, and thus reduce interphase transport limitations on reaction rates.…”

Coking and activity of porous catalysts in supercritical reaction media

“…A drawback with the model, however, is that it predicts steady states in catalyst effectiveness factor that are not always observed. While Tiltscher et al (1981) observed constant activity during supercritical 1-hexene isomerization on a macroporous catalyst, Saim and Subramaniam (1991) observed a gradual decrease in activity for the same reaction on a micro-to mesoporous industrial catalyst. The temporal decrease in catalyst activity may be modeled by considering a coking network, where different coke compounds with varying solubilities in the reaction mixture are formed.…”

“…The increased desorption of heavy hydrocarbons in dense supercritical reaction mixtures has been exploited to control both catalyst activity and product selectivity. Tiltscher et al (1981) demonstrated that it is indeed possible to extract heavy hydrocarbon byproducts in situ with supercritical reaction mixtures and thereby maintain catalyst activity. One of the reactions they studied was the &-/transisomerization of 1-hexene on a low surface area (5 m2/g), low activity, a-Al,O,/Al metal catalyst.…”

“…In recent years, supercritical reaction media have been employed to extract heavy hydrocarbon or coke compounds in situ from porous catalysts (Tiltscher et al, 1981;Saim and Subramaniam, 1990;Yokota and Fujimoto, 1991;Adschiri et al, 1991). Supercritical fluids (SCFs) display unique solvent Correspondence concerning this article should be addressed 10 B. Subramaniam and transport properties.…”

Supercritical catalytic dehydrogenation of toluene

“…Much less has been published on the effect of supercritical pressures and temperatures on reactions, particularly heterogeneous catalytic reactions. It has been demonstrated (Tilscher et al, 1981) that solid catalysts can be regenerated by treatment with a solvent a t supercritical conditions. Also, Koll and Metzger (1978) have shown that increased yields of polysaccharides can be obtained by carrying out the decomposition of cellulose in a supercritical solvent (e.g., acetone) rather than by pyrolysis.…”