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Langmuir 1992,8, 272&2723
Mercaptan Adsorption Capacity and Catalytic Oxidation Activity of Silica-SupportedPhthalocyanines H. Fischer and G. Schulz-Ekloff Institut fiir Angewandte und Physikalische Chemie, Universitat Bremen, 0-2800Bremen 33, FRG
T. Buck and D. Wdhrle Institut fiir Organische und Makromolekulare Chemie, Universitat Bremen, 0-2800Bremen 33, FRG
M. Vassileva and A. Andreev Institute of Kinetics and Catalysis, Academy of Sciences, Sofia, Bulgaria Received January 22,1992. In Final Form: June 15, 1992
Adsorption capacities of 2-mercaptoethanol at silica-supported phthalocyanatocobalt(I1) (CoPc) and (2,9,16,23-tetranitrophthalocyanato~cobalt(II) (CoPc(NOd4) are determined by a thermal desorption technique. The adsorption capacities correlate with the turnover frequencies for catalytic mercaptan oxidation. The thermal desorption spectra reveal two states of adsorption for thiol as well as for dioxygen at the CoP~(N02)~-derived sample, but one state only at the CoPc-derived one. Estimated activation energies of desorption depend on the type of phthalocyanine but not on the degree of diepereon. The catalytically active sites of a defined supported phthalocyanine are energetically uniform.
Introduction Supported phthalocyanines find increasing application for the oxidation of thiols in heavy petroleum distillates.lI2 If metal complexes like the phthalocyanines are immobilized a t solid carriers, e.g., by impregnation, association of the singular chelate molecules can be expected due to strong interaction between the planar ?r-electron system^.^ The formation of aggregates impedes a correlation between the catalytic activity and the number of active sites, i.e., the evaluation of turnover frequencies, as long as quantitative information about the size and the size distribution of the aggregates is missing. Only qualitative information about the stateof aggregation can be gleaned from scanning electron micrographs, X-ray diffiactograms, UV/vis spectra) or electron paramagnetic resonance (EPR) feature^.^ Furthermore, it is not clear whether the reaction mechanism or the nature of the catalytically active center is changed by the immobilization of the metal complex.6 In this paper, adsorption capacities of 2-mercaptoethan01 and adsorption energies of oxygen and the thiol are studied using two model catalysts, i.e., unsubstituted cobalt(I1) phthalocyanine (CoPc) and cobalt(I1) 2,9,16,23tetranitrophthalocyanine (CoPc(NOz)r, Figure 1)in different loadings on silica, to elucidate the influences (i) of degrees of dispersion and (ii) of substituents on the adsorption energies and the related catalytic activities. Experimental Section Samples. CoPc and CoPc(NOd4 with different loadings on silica (11.94.4 w t %) were used. These samples were prepared (1) Meyers, R.A.,Ed.Handbook ofPetroleum RefiningProcesses;Mc Graw Hill Book Co.: New York, 1986. (2) Leitao, A.;Costa, C.;Rodrigues, A. Chem. Eng. Sci. 1987,42,2291. (3) Leznoff, C. C., Lever, A. B. P. Eds., Phthalocyanines, Properties and Applications; VCH Publishers: New York, 1989. (4) Wdhrle, D.; Hirndorf, U.; Schulz-Ekloff, G.; Ignatzek, E. 2. Naturforsch. 1986,41b, 179-184. (6)Wijhrle, D.;Buck, T.;Hhdorf, U.; Schulz-Ekloff, G.; Andreev, A. Makromol. Chem. 1989,190,961-974. (6) Wdhrle, D.;Buck, T.;Schueider, G.; Schulz-Ekloff, G.; Fischer, H. J . Inorg. Organomet. Polym. 1991, I, 115-130.
0743-7463/92/2408-2720$03.00/0
R
-I-
R
Figure 1. Structures of the cobalt phthalocyaninea. For CoPc, R = H; for CoPc(NOz)r, R = NOz.
by reaction of Co(I1)-loadedsilica (Riedelde Hah, art.no.39806, surface area 283 m2/g,grain size 5-20 pm, pore diameter 6 nm, pH 6.6-7) with 1,2-dicyanobenzene or 4-nitro-1,2-dicyanobn~ene.~fi The extents of loading of the support by the complexee were determined by quantitative W/vis spectroecopy (PE 664) after detaching the complex from the silica with concentrated sulfuric acid or pyridine.' The characteristic patterne of the visible spectra of the phthalocyanines and their changea were followed by diffuee reflectance spectroscopy on a PE Lambda 9 spectrometerequippedwith an integrating sphere attachement. IR spectroscopy (Digilab FTS-7 FTIFt) was ale0 used for characterizations. DerorptionMeasurements. Apparatus. Thetemperatureprogrammed desorption (TPD)experimentswere carried out in flowing He carrier gaa aa described by Cvetanovi6 and Amenomiya.' The adsorbent (up to 300 mg) was placed on a stainlean steel sinter pot of a Swagelok gas filter system. The desorbate were detected by a thermal conductivitycell (PESigma 4B) and recorded by an integrator (Merck D-2000). Linear heating r a w in the range 4-30 K-min-' could be applied by an adapted temperature programmer. For the temperature-programmed desorption experiments helium (Meseer Grieeheim, FRG,'He 4.6", conknt 99.996% He), cleaned by a Meseer Griesheim (7) CvetanoviC, R. J.; Amenomiya, Y. Adv. Catal. 1967,17,103-149.
Q 1992 American
Chemical Society
Langmuir, Vol. 8, No. 11, 1992 2721
Mercaptan Adsorption Capacity
Oxysorb system (H& I0.5 vol ppm; 0 2 I0.1 vol ppm), oxygen (Meeser Griesheim, FRG, '02 4.6", content 99.99% 02),and 2-mercaptoethnnol (Riedelde Haen, art. no. 62736,content 99%) were used. Thermal Pretreatment. Prior to the adsorption each adsorbent sample was kept in the pure He stream at 620 K for 12 h and finally heated to 750 K for 10 min to ensure complete water desorption. Subsequently, the adsorbent was cooled to room temperature. Adsorption and Desorption Procedures. From a series of blank experiments an optimum amount of 2-mercaptoethanol was evaluated, which was dosed into the He stream (flow rate 20 "in-1) using a Hamilton syringe. Then the samplewas heated to 750 K,and the desorption of 2-mercaptoethanolwas recorded. The experiment was repeated with different heating rates. Oxygenadsorption experiments were carried out using the method reported by Tsuchiya et al? The oven chamber was cooled to 110K by intducing liquid nitrogenand kept at that temperature, while oxygen was dosed into the He stream. Subsequently,the sample wae heated to 750 K, detecting the oxygen desorption. This experiment was also repeated with different heating rates. Deaorption experiments with coadsorbed thiol and oxygen were carried out by adsorption of 2-mercaptoethanolat room temperature and subeequent0 2 adsorption at 110K. The adsorbate ratio RSHO2 = 4 was dosed to the adsorbent, according to the stoichiometry shown in eq 1.
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4HOCH2CH&3H+ 0, 2HOCH2CH2SSCH2CH20H + 2H20 (1) For the analysis of reaction products from the coadsorption, the adsorption chamber was connected to the gas inlet value of a mass spectrometer (Finnigan8222). The desorbing (1 K-min-l) compounds were detected using up to 80 ecanslmin. Catalytic Measurements. Aqueous suspensionsof the silicasupported phthalocyanines (containing 1 P mol of phthalocyanine) in the reaction vessel (100 mL) of a Marhan apparatus (Normag) are saturated with dioxygen under bubbling and stirring. A 10-mL sample of a buffer solution (pH 9, Merck) containing 0.5 mL (7.1 X 10-9 mol) of 2-mercaptoethanolwas added. The kinetics of dioxygen consumption were measured volumetrically by the 50-mL buret at 25 O C as described before.6 The catalyticactivitiesare defined asturnover frequencies(TOF), i.e., moles of converted thiol divided by moles of phthalocyanine units per minute.
Results Sample Characterization. The carrier silica contains CoPc and CoPc(N0z)r in the range of 0.4-11.9 wt % .The phthalocyanines exist on the surface of the support. X-ray powder diffraction and scanning electron microscopy of the samples showed increasing dispersion with decreasing loading of the phthalocyanines on the sili~a.~9~ For loadings
