Sourn~lof~~lolecu~crCatalys~,11(1981)283- 291 @ Elsevier Sequoia S.A., iacsann~ - Printed in the Netherlands

283

PHOTOCHEMECAL OXIDATION WITH HETEROGENEZED PHOTOSENSITIZER

A. RIVA, F. TRIFIRi)

ktituto di Tecnologie Chimiche Speciali, Facoltti di Chimica Industriale, UniversitG di Bologna, Vkde Risorgimento 4 - 40136 Bologna (Italy)

and F. SANTARELLI

Istituto di Impiunti Chimici. FacoltG di Ingegneria. Universitti di Bologna, Wale Risorgi- men to . 2 - CO136 Bolognu (Ztaly)

Methylene blue (MB), a well known singlet-oxygen activator, has been supported on silica and used as an heterogeneous photosensitizer in the photo-oxidative reaction of tryptophan. The reaction was studied by measur- ing the effect of the following parameters on the initial disappearance rate of the tryptophan:

(i) the covered surface area of the silica; (ii) the MB concentration in the two-phase system with constant cover- age_ It was observed that the conversion decreased to the point where it was

cancelled out by high coverage as the degree of particle coverage was in- creased. This, together with an examination of the adsorbent-adsorbate and adsorbate-adsorbate interactions, led to the conclusion that cnly MB mono- mer on silica is active as a photosensitizer. As the quantity adsorbed was in- creased, the photoactivity decreased due to the progressive formation on the surface of inactive dirner forms and micehes with increasing degree of poly- merization. The photosensitizer remained stable after prolcnged u:e.

The order of the reaction rate with regard to the quantity of MB sup- ported (with equal degree of coverage) was 0.41. The explanation OZ this dependence (les than 1) aud the formulation of a plausible kinetic equation for the heterogeneous process, require a careful consideration of the inter- action between the radiation and the reaction mixture, with special regard to scattering phenomena, which must surely occur in the resulting hetero- geneous system.

Attempts to clarify the role of scattering are described based on results from the nranyl sulphate-photosensitized decomposition of ox&c acid. This was studied both in homogeneous phase (in which the reaction kinetics are completsly known) and under heterogeneous conditions in the same reactor used for the photooxidation of tryptophan. The experimental res&s were interpreted on the basis of the calculated values of the absorbed energy; the

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agreement between the experimental and calculated values proved optimal when the particles introduced were essentially opaque.

Introduction

Singlet oxygen can be produced photochemically provided a suitable photosensitizer is used [ l- ?]. Me’thylene blue (MB) has been used with this aim both in homogeneous and heteroge,ieous systems ]S - lo] .

In the latkr case, when MB is supported on silica particles, alI the ad- vantages of heterogeneous catalysis result, as was proved in the oxidation of olefins 111 - 13]_

The investigation of heterogeneous photocatalysis is extended here to oxidation in aqueous solutions. Reference has been made to a substrate - tryptophan (TRP) - for which oxidation with singlet oxygen is well known [14,15].

The oxidation of TRP has been investigated in a semi-batch reactor filled with the TRP solution and silica particles supporting the photocatalyst. The reactor, through which a gas stream containing Oz was continuously flowing, was irradiated in order to promote singlet oxygen production [16].

The inrluence of the most significant parame+ers which affect the reac- tion kinetics has been inferred quite straightforwardly from the experimental results, but a more comprehensive analysis of the reactor performance is not possible until the radiant energlr transport within the raactor is clearly under- stood. This presents difficulties owing to complex interactions between the radiation and the reacting mixture. In order to obti useful information on light propagation within an heterogeneous system the classical oxalic acid photodecomposition has been studied in the same reactor used for the kine- tic study with TRP. Both homogeneous and heterogeneous reaction condi- tions have been investigated, the latter by the addition of arCficial scattering centers, of different optical properties, to the reacting mixture. This actino- metric reaction was chosen on the basis of the following arguments:

(i) the kinetics of the homogeneous reaction are completely known

1171; (iij the conversion is proportional to the total absorbed energy. Experimental results can therefore give useful information on the

amount of energy which is wasted as a result of the scattering phenomena occurring within the reactor. Furthermore, the complete knowledge of the kinetics providsd a unique opportunity for fruitful use of mathematical models of radiative transport. The radiative transfer within the reacting mix- ture has been modelled according to 2 simplified scheme which included ah the parameters relevant ‘to the radiative transfer in heterogeneous media

]181.

285

MB was supported on silica particles by blending, under mild mixing conditions, a known volume of an MB solution and a carefuhy weighed quantity of silica (surface area 500 m’/g)_ After 5 h the solution was filtered and the particles washed with distilled water and dried at 80 “C. The time re- quired to obtain maximum MB adsorption was evaluated in some prebmi- nary tests.

The samples of suppoi+ted MB have been ident-died through the two following indices:

W = weight (in mg) of adsorbed MB/weight (in g) of adsorbing silica; CG = surface area of the support covered by MB/surface area of the

support. They were evaluated u&g:

w = (C, - C) V/m

where C, and C =e initial and final concentrations (mg/l) in solution, Y is the volume of the solution and m is the weight of the adsorbent solid (g);

where MSXB is the molecular weight of ME, N is the Avogadro number, bXrs is the area coveEd by a single molecule of adsorbate (in this case QarB = 1.20 nm’ [19], and (L is the surface per unit weight of the supporting silica.

The reaction was carried out in an elliptical reflector-photoreactor, i.e., an assembly in which the lamp (Hanau TN 250W) and the reactor (a Pyrex tube, i-d. = 2.6 cm, 25 cm long) were placed in one of the foci of an elliptical reflecting surface (height 40 cm, principal axes of the ellipse 63 cm and 87 cm, respectively).

The oxygen was supplied to the reacting system by a gas sbeam, fed through a sir&red glass septum located at the bottom of the reactor in such a way that an even distribution of gas resulted. The gas flow and a magnetic stirrer ensured good mixing of the reacting mixture.

In the TRP oxidation, variable ratios of O2 and N, were used in the gas feed (2 I/mm). The TRP concentration was measured as a function of time by following the change in fhe maximum of adsorbance intensity at 269 nm.

In the oxa& acid photodecomposition the gas stream was pure N,, fed at a rate of 2.5 I/mm. The oxalic acid concentration was measured as a Cum- tion of tlime by the classical titration with a O.OIN KMnOc solution [ZO, 21) _

Results and Gcussion

In order to obtain a general expression for the reaction rate, attention has been focussed on the initial rate of the recction (V,). The dependence on each of the parameters which are expected to affect the reaction rate has been investigated by undertaking several runs at different values of each parameter, & the others remaining unchanged. The dependence on CD can

0

0

-\

0 5 10 15

COVERAGE DEGREE-:000

Fig. 1. Initial reaction rate us. degree of coverage_ TRP solution: 1 X 10m3h5; catalyst (MB on silica) weight: 1 g; oxygen flow rate: 2.0 !/min.

1 I 1 I 1

350 400 5co 600 x0 BOO * Cnm)

Fig. 2. Diffuse reflectance visible spec’ka of two samples of supported !k’B; ( -) active.

(----) inactive_

he seen in Fig. 1 I V, decreases when CD increases, and hecc <sl Wcu~i @y small at high values of CD. This trend is due to the differer! cataIytic ark’- ities of the different ME species which may be present on the-.s-u._face, depending on the value of CD. This conclusion is supported by the diffuse- reflectance spectra of two samples of supported MB, the former active (Fig. 2, full line) and the latter inactive (Fig. 2, dashed Line).

In the sample with a lower value of CD (active catalyst) only the mono- mer form of MB (X,,, = 615 run) occurs, while at the highest values of CD

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50 100 OXYGEN CONCENTZATION (%I

Fig. 3. Initial reaction rate us. oxygen concentration. TFW solution: L x 10-3i%l,1; catalyst weight: I g; coverage degree: 2.07 X 10d3; gas flow rater 2-O I/min.

(inactive catalyst) both the dimeric form and polymeric units of varying lengths are present (X,,, = 665 run). The results of adsorption and desorp- tion tests of ME on silica show that the adsorbate-adsorbent interactions are very complex and do not follow the classical theories of adsorption equi- librium, since ‘the adsorption of MB on silica occurs through a sequence of steps. Ai; first only the monomer is adsorbed on the silica support, svhile polymolecular units of l&III3 appear later on specific sites. Smce only the monomeric form of MB is active in singlet oxygen:production, this interpre- tation of the results of Fig. I appears to be soundly supported_

Con&try to results reported elsewhere [13], the ectivity of the sup- ported photosensitizer was found to be very stable, since freshly prepared and intensively used samples both exhibited the same catalytic activity and had the same diffuse reflectance spectra.

Figure 3 shows the dependence of V, on the oxygen concentration at constant gas flow rate to the reactor. At low oxygen concentration (yo, = 0.65) V, is found to be 0.75 order with respect to the oxygen concentration, while at higher values of yo, a pseudo-zero order results. As far as the TRP concentration is concerned, V, has been found to be first order over the entie investigated range.

Finally, the effect of the zrnount of supported MB has been investi- gated using different quantities of an active catalyst with constant CD. The resulting trend for V, is given in Fig. 4: an unusually low value (0.41) results for the reaction order with respect to this variable.

The analysis of such a result requires a more comprehensive breakdown of the radiation transfer within the reacting medium. QuantitativeIy, it can be reasoned that:

(i) since the attenuation of tie radiation is not governed by a linear law, a proportional increase of the absorbed energy cannot be expected when tie concentration of the absorbing species is increased;

I 1 I

2 3 c i WEIGHT OF CkTALYST Igl

Fig. 4. Initial reaction rate L’S_ MB weight. TFtP solution: ? X 10e3M; coverage degree: 2.07 x 1o-3 ; 0xyge;l flow rate: 2.0 I/min.

0 50 %J TIME (mm)

Fig. 5. Uranyl oxalate ancentration us. irradiation time. (- - - -) homogeneous case; (- ) heterogenecus cases: (1) silica particles covered by MB; (2) gas bubbk; (3) gas bubbles + glass powder; (4) (1) + gas bubbles.

(ii) the heterogeneity of the reacting system (three phases) causes the radiation to titeract with the reacting medium, not only through a simple absorption proceq but also throu.gh scattetig at cenkes of heterogeneity (solid particles and gas bubbles) which -Je present in thz system. The result- ing light dispersion can considerably reduce the amount of the radiant energy which is absorbed within the reactor.

In or&r to giwe these statements a stronger quantitative basis, the photodecon;posiGon of oxalic acid has been studied ti the same reaction

assembly, both in the usual homogeneous phase and under some artiEcial.ly induced heterogeneous conditions.

Results are given in Fig. 5 and confirm that, when the conversion is not tao high, the reaction rate is zero order with respect to the concentration of the ox&c acid, and is proportional to the radiant energy which has been ab- sorbed within the reactor. The reduction of the conversion and of the ab- sorbed energy, as a result of scatterirzg phenomena, is apparent. An attempt to relate this reduction to the nature of the scattering centers has been made by comparing the ratio between the obsenred reaction rates in the hetero- geneous and homogeneous cases with the ratio of the radiant energy ab- sorbed in these two cases. The terms for the latter ratio cannot be measured, but can he easily calculated from a modei developed for the radiative traus- fer proces once the radient energy b&nce equation has been solved. The radiative energy balance equation has been considered for an absorbing, iso- tiopically scattering, plane slab, thereby reducing the problem to a uni- dimensional one, as has been common practice in kinetic studies of homo- geneous photoreactions in eliipticd photoreactors [22. 23]_ The solution hti been obtaiued here following the same procedure as given in ref. 18.

The assumed geometry is an idealization of the actual situation, but the cylindrical symmetry of the reactor, and the good mixing in it, make the resuit applicable to the situation occurring in any longitudinal plane through the axis of the reactor. In the situation examined, the significant optical parameters to he considered are:

(i) the optical thickness, m, which gives the ratio between a geometric characteristic length and the mean free path of a photon;

(ii) the single scattering albedo, c, which gives the probability of scatter occurring. These are defined as

where p and c are the absorption and the scattering coefficients, respectively. A knowledge of p and (T is therefore required to obtain the desired values of m and c. (T is related only ‘to the heterogeneous particles, while p is related both to the liquid solution and to the heterogeneity centers and must there- fore be evaluated as fl = (I - E)& + BP, where s is the particle hold-up and p, and p, are the absorption coefficients of the solution and the heterogen- eous center, respectively.

The correct value of & can be evaluated quite straightforwardly, as shown in the Appendix, but the evaluation of pP and G = ~~ is more cumber- some. They cm be evaluated following the procedure given in ref. 24; since the particles are large enough to make ED/A > 5, the principles of geometric optics apply. pP and (T~ can therefore be evaluated as

where At is the total surface xea of the particIes in a unit volume, eh and fib being the hemispherical emissivity and reflectivity, respectively.

290

eh and p h can, in tu.rn, be evaluated through the equation

where r is the fraction of the incident radiation which is transmitted through the particles.

The previous relationship can be used in a simple way only if the par- ticles behave as opaque, diffuse reflectors, since in that case I? = 0 and ph can be easily measured in an integrating sphere. A comparison of the experi- mental results with the computed ones has therefore been possible only for Curve 1 of Fig. 5. The agreement, as shown in the Appendix, yvas found quite satisfactory and gives some confidence in the validity ,,f the model adopted. Unfortunately, in all the other situations, where I’ + 0, the evalua- tion of Ed and p,, is not so straightforward and is possible only m the realm of the iviie theory of scattering. The complex refraction indices which are required for the latter, are not easily available. A knowledge cf them is a critical point in the optical characterization of the reacting mixture and can be a prohibitive obstacle to the practical use of any mathematir-al model of the radiative transfer process.

Acknowledgment

The research was supported by the C.N.R_ (Italy).

References

1 X_ Gollnick, Adu. Photochem., 6 (19G8) 1. 2 C. S. Foote, Science. 162 (1968) 963. 3 T. Matsuura, N. Yoshimura, A Nishinaga and I. Saito, Tetrahedron Left., 2Z (1969)

1669. 4 R. P. Waine, Ado. Fhotochem., 7 (1969) 311. 5 M. L Kaplan, Chem. Technol. Z (1971) 621. 6 P. Lehtken, Chem. Ztg., 8 (1974) 2. 7 M. Fischer, ringew. Chem., Znt. 2d. En&, Z 7 (1578) 16. 8 C. H. Giles and R. B. McKay, Text. Res. J., 33 (1963) 527. 9 F. C. Schaefer and W. D. Zimmermann, Xzture (London). 220 (1968) 66.

10 13. Brkic, P. Forzatti, I. Pasquon and F. T&i&,, J. Phofochem, J (1976) 23. 11 R. Nilsson and D. A. Keams, Photochem. PhofobioL, 29 (1974) 181. 12 -4. P. Schaap, A. L. Thayer, E. C. Blosey and D C. Neckers, J. Am. Chem. Sot.. 97

(1975) 3741.

13 D. Brkic, P. Forzatti, I. Pasquon lnd F. Trifirb,, J. Afol. Cola!., 3 (1977/78) 173. 13 R. Nilsson, P. B. Merkel and D. R. Kearns. Photochem. Phofobiol., 16 (1972) 117. 15 I. Saito, T. Matsuura, &I. Nakagawa and T. Hino, Act. Chem. Res., 10 (1977) 346. 16 D. Brkic, P. Forzatti and F. Trifirb, Chem. Eng. Sci., 33 (1978) 353. 17 D. H. Volman and J. R. Seed, J. Am. Chem.. Sot.. 86 (1964) 5095.

18 C. Stramigioli, G. Spadoni and F. Santare!li. fnL_ J. HeaL !ifass ‘Prunsfer. 2Z (1978) 660.

19 C. H. Giles and A P. D’Silve, ‘~YuRs. Faraday Sot., 65 (1969) 2516. 20 J. G. Cab:ert and J. N. Pit& Photochemistry. Wiley, New York, 1966. p_ 786. 21 J. N. Pitts, J. D. Margerum, R. P. Taylor and W. Brim, J. Am. Chem. Sot.. 77 (1955)

5499.

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22 A. E. -0 and J. M. Smith. AICkEJ. 12 (Is&?) 1124. 23 E. Eoval and J. &I_ Smith. AICAEJ.. 16 (1970) 553. 24 H. C_ Hottel 2nd A. F. Sarof~n, Pndfatiue Transfer, McGraw-Hill, New York, 1967,

ch. 12.

Appendix

(1) The homogeneous case 0, has been evaluated as the average value of B in the range-of the use-

ful wavelengths emitted by the lamp, weighted by the energies Eh emitted by the lamp 2t uly X.

P, = r A&h J%

= 1.67 cm-r EA

Assuming the reactor diameter as I,,

m = fl,l, = 4.28

resulted.

(2) The heterog;eneous case Only case 1 (r = 0) of Fig. 5 has been considered. ph has been evaluated

as the weighted

13APh.A h It

Ph= xx EA

and then

average of the ph.h measured, in the useful X range, with

= 0.2

Eh = 1 -ph = 0.8.

in the experimental conditions, the particle hold-up, E = 0.056, and the average diameter of a particle D, = 0.19 mm. It followed that

BP = 3.6 cm-r ; CT,, = 0.9 cm-’

and then

/3 = (I- ~)a, i E& = 1.778 cm-l ; cr = up = 0.9 cm-l

giving 0

m = (/3 + o)l, = 6.85 ; c= - = 0.33. P *CT

These values of m and c have been used in the solution of the radiant energy balance equation, and the ratio i ‘Ihet/i ‘;I, has been evaluated (E ” is the energy absorbed in the vohune associated with a unit interfacial area of the slab). This ratio was compared with the ratio c+&h- of the reaction rates as derived horn the curves of Fig. 5 for the homogeneous case (dashed Ene) and for the heterogeneous case (line 1). The agreement was quite good since

?het - = 0.94

ahet -- *,, = 0.95 E hc.m whom