Photochemistry of benzophenone adsorbed on MCM-41 surface

Microporous and Mesoporous Materials 84 (2005) 1–10 www.elsevier.com/locate/micromeso

Photochemistry of benzophenone adsorbed on MCM-41 surface J.P. Da Silva a

a,b,*

, I. Ferreira Machado a, J.P. Lourenc¸o b, L.F. Vieira Ferreira

a

Centro de Quı´mica-Fı´sica Molecular, Complexo Interdisciplinar, Instituto Superior Te´cnico, Av. Rovisco Pais, Pt-1049-001 Lisboa, Portugal b FCT, Universidade do Algarve, Campus de Gambelas, Pt-8005-139 Faro, Portugal Received 7 January 2005; received in revised form 19 April 2005; accepted 19 April 2005 Available online 16 June 2005

Abstract The photochemistry of benzophenone adsorbed on a channel-type solid support MCM-41 was studied by the use of diffuse reflectance and chromatographic techniques. For comparison purposes parallel studies were also conducted on silicalite and H-ZSM-5. The observed ground state absorption, luminescence and transient absorption spectra and photoproduct distribution strongly depend on the substrate under study. Triplet state luminescence was observed in all solids, hydrogen-bonded excited benzophenone emission was detected on MCM-41 while protonated excited benzophenone luminescence was observed only on H-ZSM-5. Transient absorption showed the triplet state in all solids and formation of the OH benzophenone adduct on MCM-41 and H-ZSM5. Ketyl radical formation occurs after 355 nm irradiation but is a minor degradation pathway. The formation of transients resulting from the reaction of benzophenone with molecular oxygen was also detected on MCM-41, in air-equilibrated conditions and benzoic acid is the main degradation product. 2-Hydroxybenzophenone is one of the major photodegradation products on MCM-41 and H-ZSM-5. Products resulting from the a-cleavage were also detected under 266 nm irradiation. Benzophenone was found to be very stable in silicalite under all used conditions.
2005 Elsevier Inc. All rights reserved. Keywords: Surface photochemistry; Diffuse reflectance; Time-resolved luminescence; Laser flash photolysis; Degradation Products

1. Introduction The application of flash photolysis techniques to opaque samples was developed by Wilkinson and co-workers in the 1980s [1–4]. Due to their well-known photophysical and photochemical behaviour, aromatic ketones were among the probes often used in the initial studies of time-resolved luminescence and transient absorption techniques for these powdered samples. Benzophenone microcrystals (BZP) and BZP adsorbed on silica surface are good examples of those initial studies

*

Corresponding author. Address: Centro de Quı´mica-Fı´sica Molecular, Complexo Interdisciplinar, Instituto Superior Te´cnico, Av. Rovisco Pais, Pt-1049-001 Lisboa, Portugal. Tel.: +351 21 841 92 52; fax: +351 21 846 44 55. E-mail address: [email protected] (J.P. Da Silva). 1387-1811/$ - see front matter
2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.012

using the diffuse reflectance laser flash photolysis technique [1,5]. Triplet–triplet absorption was observed in the absence of molecular oxygen. The triplet state of BZP is efficiently quenched by hydrogen donors, leading to the formation of the diphenylketyl radical on silica surfaces [6] and when included within the polymer chains of microcrystalline cellulose [7]. The photochemical behaviour of ketones in channel-type zeolites has also been studied using these techniques [8–13]. BZP is one of the probes used in our group to characterize new solid supports. Triplet–triplet transient absorption of BZP crystals, or adsorbed on silicalite or microcrystalline cellulose and/or its ketyl radical on cellulose are routinely used as calibration standards in our set-up for transient absorption studies. BZP was also particularly useful to study the photophysics and photochemistry of new supramolecular systems, particularly calixarenes inclusion complexes [12,14].

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Studies including identification and quantification of the final photoproducts in surface photochemistry of ketones are unfortunately rare in the literature [10,11,14]. The combination of the spectroscopic data and final products identification is in many cases crucial for the proposed photodegradation mechanisms of several compounds at the solid/gas interface [14–18]. Recently, the identification of the final degradation products of BZP/O-propylated p-tert-butylcalix[4]erene complexes was decisive in the interpretation of the time-resolved luminescence results [14]. Since its discovery [19], the M41S family of mesoporous molecular sieves has attracted considerable attention due to their unique properties, namely, large pore size, uniform pore distribution, high surface area, and long range ordering of the pore packing, all of which are potentially important in catalysis involving large molecules. MCM-41 is the most extensively studied member of this class of mesoporous materials. It has been reported to be more hydrophobic than silicas, [20,21] and the insertion of metal centers was reported to be the most important modification [20]. The photoionization of a number of large molecules in mesoporous MCM-41 is a well-known process [20,22]. The yield of photoionization is strongly affected by the molecular sieve pore size, being the largest in pores whose size fits the molecular diameter of the adsorbed compound. It has been proposed that the MCM-41 framework acts as an electron acceptor upon photoionization of several incorporated organic compounds [20,22]. H-ZSM-5 is a pentasil zeolite with H+ as the chargecompensating cation, possessing vertical channels with ˚ , perpendicular to zigzag formed chansize 5.3 · 5.6 A ˚ . Silicalite is the dealunels with dimensions 5.1 · 5.5 A minated analogue of ZSM-5 zeolite. Due to the lack of aluminium, silicalite has no significant catalytic or ion exchange properties when compared with other ZSM-5 zeolites. As host materials zeolites are excellent candidates for modifying the photophysics and photochemistry of a given organic species. Modifications on the spectral localization and intensity of the emission and the formation of charge transfer complexes and radical cations are good examples of such behaviour [8– 12,15,23–27]. The BZP photochemistry has never been studied on MCM-41. Among zeolites, silicalite and ZSM-5 have already been analysed using this probe. However a detailed study of the relation between the final photoproducts of BZP and the physical and chemical properties of these hosts has never been made in detail. In this work we present a study of the time-resolved luminescence, transient absorption and photoproduct distribution of BZP adsorbed on MCM-41 using different irradiation conditions. For comparison purposes the same studies were made on H-ZSM-5 and silicalite surfaces.

2. Experimental 2.1. Materials Silicalite (Union Carbide), H-ZSM-5, Si/Al = 19 ˚ (Merck) were (UOP Molecular Sieves) and silica 40 A used as powdered solid supports. BZP (Koch-Light, Scintillation grade), 2-hydroxybenzophenone, benzoic acid, benzaldehyde, biphenyl, fluorenone, 4-phenylbenzophenone (Aldrich), and the solvents methanol, ethanol, acetonitrile, hexane and iso-octane (Merck, Lichrosolv) were first analysed by chromatography and used as received. Water was deionized and distilled. For the synthesis of MCM-41 the following reagents were used: NaCl, NaOH, NH4NO3 (Merck p.a.), acetic acid (Riedel-de-Hae¨n), Ludox HS-40 (DuPont, 40 wt% SiO2), hexadecyltrimethylammonium chloride (HTACL, Aldrich, 25% aq. sol.). 2.2. Synthesis of MCM-41 The synthesis of MCM-41 was carried out as follows [28]. First, 144.3 g of 1 M NaOH sol. was mixed with 43.5 g Ludox HS-40 at 75 C for 1 h. After cooling to room temperature, this solution was slowly added under vigorous magnetic stirring to a mixture of 96.0 g hexadecyltrimethylammonium chloride solution and 298.0 g of distilled water. Stirring was maintained for an additional hour after the addition was completed. The final gel, with the composition 4SiO:Na2O:HTACl: 400H2O was placed into a propylene bottle and kept in an pre-heated oven at 100 C for 24 h. The reaction mixture was then cooled to room temperature and the pH was adjusted to 10 using acetic acid. The reaction bottle was placed again into the oven at 100 C for another 24 h period. After that, it was added 13.0 g NaCl and the bottle was kept in the oven for more 10 days, followed by another pH adjustment as described above. To recover the product it was centrifuged, washed with distilled water and dried at 80 C overnight. The template was partially removed by extraction with a solution 0.1 M NH4NO3 in 96% ethanol at reflux temperature for 2 h. After drying, the product was calcined under a flux of dry air at 550 C for 10 h. The temperature was increased from room temperature to 550 C at 1 C/min. 2.3. Sample preparation The samples used in this work (50, 100, 250, 500 and 1000 lmol g
1) were prepared using the solvent evaporation method, which consists in the addition of a solution containing the probe to the powdered solid support, followed by mixing and evaporation of the solvent. The supports were dried under vacuum, ca. 10
3 mbar, at 100 C for 24 h before use. Iso-octane was used for silicalite and H-ZSM-5, and hexane was

J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

used for the samples on MCM-41. The mixture was magnetically stirred for at least 24 h and the solvent evaporation was performed placing the stirred slurry in a fume cupboard. The final solvent removal was performed overnight in an acrylic chamber with an electrically heated shelf (Heto, Model FD 1.0–110) with temperature control (30 ± 1 C) and again under vacuum (10
3 mbar). 2.4. Methods 2.4.1. X-ray diffraction (XRD) and BET surface areas The MCM-41 hexagonal structure was identified by powder X-ray diffraction on a Rigaku diffractometer using Cu Ka radiation filtered by Ni. Nitrogen adsorption of the calcined sample was measured at
196 C with an ASAP 2010 Micromeritics apparatus. Prior to the measurements, the sample was outgassed at 350 C for 3 h. The surface area was obtained by the BET method. In the determination of the pore diameter the geometrical model used [29]. 2.4.2. Diffuse reflectance ground state absorption spectra (GSDR) Ground state absorption spectra for the solid samples were recorded using an OLIS 14 spectrophotometer with a diffuse reflectance attachment, as described previously [30]. 2.4.3. Diffuse reflectance laser flash photolysis (DRLFP) and laser-induced luminescence (LIL) systems Schematic diagrams of the DRLFP system and of the LIL systems are presented in Ref. [30]. Laser flash photolysis experiments were carried out with the third or the fourth harmonic of a Nd:YAG laser (355 and 266 nm, ca. 6 ns FWHM, 10–30 mJ/pulse) from B.M. Industries (Thomson-CSF, model Saga 12–10), in the diffuse reflectance mode. The light arising from the irradiation of solid samples by the laser pulse is collected by a collimating beam probe coupled to an optical fiber (fused silica) and is detected by a gated intensified charge coupled device (Andor ICCD detector, based on the Hamamatsu S5769-0907). The ICCD is coupled to a fixed imaging compact spectrograph (Oriel, model FICS 77440). The system can be used either by capturing all light emitted by the sample or in a time-resolved mode by using a delay box (Stanford Research Systems, model D6535). The ICCD has high speed gating electronics (2.2 ns) and intensifier and covers the 200–900 nm wavelength range. Time-resolved absorption and emission spectra are available in the nanosecond to second time range. Transient absorption data are reported as percentage of absorption (% Abs.), defined as 100DJt/Jo = (1
Jt/Jo)100, where Jo and Jt are diffuse reflected light from sample before exposure to the exciting laser pulse

3

and at time t after excitation, respectively. For the laser-induced luminescence experiments a N2 laser (PTI model 2000, ca. 600 ps FWHM, 1.0 mJ/pulse) was also used. 2.4.4. Irradiation and product analysis Photodegradation studies under lamp irradiation were conducted in a reactor previously used to study the photochemistry of several compounds at the solid/ gas interface [14,16–18]. The lamp irradiation was performed at 254 nm using a 16 W low-pressure mercury lamp (Applied Photophysics) without filters and without refrigeration. The samples were irradiated in a quartz cell placed at 1 cm from the lamp surface, under argon atmosphere and in air-equilibrated conditions, during 3.5 h. Laser irradiation at 355 nm and 266 nm, in argon atmosphere, was also performed. In this case the samples were irradiated in a quartz cell during 0.5 h at five pulses (30 mJ) per second. The samples were mixed every 5 min during the laser irradiation process. Nonirradiated and irradiated samples were analysed after extraction with acetonitrile (a known weight of sample in a known volume of solvent) followed by centrifugation. Photolysis was followed by HPLC using a Merck-Hitachi 655A-11 chromatograph equipped with detectors 655A-22 UV and Shimadzu SPD-M6A Photodiode Array. A column LiChroCART 125 (RP-18, 5 lm) Merck was used and the runs were performed using mixtures water/acetonitrile. The extracts were also analysed by GC–MS using a Hewlett Packard 5890 Series II gas chromatograph with a 5971 series mass selective detector (E.I. 70 eV). A Restek RTX-20 capillary column with 20 m and 0.18 mm ID was used. The initial temperature 70 C was maintained during 5 min and then a rate of 5 C/min was used until 250 C.

3. Results and discussion 3.1. Solid support characterization Samples of all pure solid supports were first studied by ground state absorption, time-resolved luminescence and transient absorption. Ground state absorption spectra showed significative absorption only below 250 nm. Fig. 1a shows the time-resolved luminescence spectra of MCM-41 upon laser excitation at 266 nm (30 mJ per pulse). Three well-resolved emission bands appear with maxima at about 400, 500, and 660 nm. The longer wavelength emission band has a shorter lifetime than the other two, suggesting that the detected luminescence results at least from two different sources. The photoluminescence of MCM-41 upon excitation above 250 nm has been reported [31–33]. These photoluminescence properties have been attributed to point defects in the SiO2 matrix and are dependent on the

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3.2. Ground state absorption

a

2500 2000 1 1500 2

1000 500 0 300

400

3 4 5 6 500

600

700

800

900

λ /nm

Intensity/a.u.

b (100)

(110) (200)

1.5

3.0

4.5

(210) 6.0

7.5

9.0

2θ (degrees) Fig. 1. (a) Laser-induced luminescence spectra of MCM-41 (30 mJ/ pulse, 266 nm) at 0 ls (1), 10 ls (2), 20 ls (3), 40 ls (4), 60 ls (5), 90 ls (6), after the laser pulse; (b) X-ray diffraction pattern of calcined MCM-41.

preparation conditions of the materials. Some examples among the many postulated defect structures are the non-bridging oxygen hole centers „Si–O
(where the three lines indicate bonds with three oxygen atoms), the peroxyradicals „Si–O–O
, the surface EÕ centers „Si, and the oxygen vacancies „Si–Si„. Zeolites, with high amounts of surface OH groups, show the same or very similar photoluminescence spectra [33]. Similar studies exciting at 337 nm and 355 nm showed a very weak residual luminescence. No significant transient absorption was detected for all the supports in the studied conditions. The XRD pattern of calcined MCM-41 (see Fig. 1b) shows that after template removal the mesoporous structure retains the long-range hexagonal order as indicated by the four distinct diffraction peaks indexed as (1 0 0), (1 1 0), (2 0 0) and (2 1 0) in the hexagonal symmetry. The BET surface area of 1008 m2/g, obtained by N2 adsorption, and the existence of a pore filling step within a narrow range of p/po along with the reversibility of the step (data not shown) are consistent with the presence of tubular pores of uniform size [29] and confirms the high crystallinity of the prepared material. The application of the geometrical model for the pore diameter determina˚. tion gave a value of 41.5 A

The ground state absorption spectra of BZP for the supports under study showed the typical n ! p* and p ! p* absorption bands of BZP. No absorption attributable to the oxygen-protonated benzophenone was detected, [34] indicating that the protonation of BZP in the ground state does not occur for all the solids under study. Fig. 2 shows a comparison of the n ! p* absorption bands of BZP (normalized at 315 nm) on the studied supports and on silica. In all cases the BZP concentration was 250 lmol g
1. The shift shown in Fig. 2 is characteristic of the n ! p* transition of BZP carbonyl group as already reported at the solid/gas interface [7,12]. This result suggests an increase of polarity in the environment of the guest ketone carbonyl group as going from silicalite to H-ZSM-5 and to MCM-41. MCM-41 behaves in a very similar way to silica as curve B of Fig. 2 shows. This explains the deviation of the absorption band to the blue and the simultaneous broadening of the spectra with loss of vibronic resolution in H-ZSM-5 and MCM-41. Water adsorption studies on MCM-41 demonstrated the presence of a very hydrophilic fraction of the surface, the dense patches of interacting silanols and the hydrophobic part, formed by siloxane bridges and isolated silanols [21]. The behaviour of the n ! p* transition in this support suggests that the carbonyl group interacts preferentially with the hydrophilic part of the MCM-41 surface. The p ! p* transition of BZP shows a bathochromic shift with the increase of the acidity of the surface sites being responsible for the phosphorescence emission of BZP on Ti–Al binary oxides [35,36]. In the studied supports, the absorption spectra below 300 nm showed a small bathochromic shift as going from silicalite to HZSM-5 and MCM-41. Ground state absorption spectra

1.5

D 1.2

Normalized F(R)

Luminescence emission / a.u.

3000

0.9

C

0.6

A 0.3

0 300

B 320

340

360

380

400

λ /nm Fig. 2. Remission function (normalized at 315 nm) of BZP on MCM˚ (B), H-ZSM-5 (C), and silicalite (D). The 41 (A), silica 40 A concentration of BZP is 250 lmol g
1 in all cases.

J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

Luminescence emission/a.u.

a

442 nm

10000

1

8000 6000

2

4000

3 4

2000

5 8

0 300

400

500

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λ /nm 6000

b

428 nm

5000 1

4000 3000

2

2000

3 4

1000

5 9

0 300

350

400

450

500

550

600

650

700

λ /nm Fig. 3. Laser-induced luminescence spectra of BZP on MCM-41. (a) 355 nm excitation (30 mJ/pulse, 500 lmol g
1): 1.5 ms (1), 3.0 ms (2), 4.5 ms (3), 6.0 ms (4), 7.5 ms (5), 9.0 ms (6), 10.5 ms (7), 12.0 ms (8), after the laser pulse. (b) 337 nm excitation (1.4 mJ/pulse, 100 lmol g
1): 0 ns (1), 2 ns (2), 4 ns (3), 6 ns (4), 8 ns (5), 10 ns (6), 12 ns (7), 14 ns (8), 18 ns (9), after the laser pulse, under airequilibrated conditions.

3500 1

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The luminescence of BZP adsorbed on solid surfaces results from three forms of the excited probe: the triplet np* state (3BZP*), which is observed peaking around 450 nm and shows a characteristic and well defined vibrational structure [12] hydrogen-bonded excited BZP (BZP. . .H), between the C@O group and the OH groups of the surface, which has the maximum emission between 430 and 440 nm and shows no vibrational structure [35] and finally protonated excited BZP (BZPH+) with maximum observed between 450 and 485 nm which gives in general non-structured emissions [10,35]. Previous studies on silicalite, exciting at 355 nm showed a structured emission band centred at 444 nm [12]. The results obtained under 266 nm excitation in this support showed the same emission band, which was assigned to BZP phosphorescence. Fig. 3a presents the time-resolved luminescence spectra of BZP on MCM-41. A broad band centred at about 442 nm is observed and retains its maximum in the millisecond time range. This result suggests that 3BZP* is the only luminescent species on MCM-41. This was unexpected since the ground state absorption spectra indicated that BZP is in a hydrophilic environment and therefore some formation of BZP. . .H should occur. The spectra presented in Fig. 3a were obtained in argon atmosphere. All these time-resolved luminescence studies were also performed in air-equilibrated conditions. In this latter case the maximum emission was centred around 430 nm. This result shows us the presence of BZP. . .H, in close contact with the hydrophilic surface of the support. In air-equilibrated conditions oxygen suppresses at least partially 3BZP* emission and we are able to observe BZP. . .H luminescence from protected molecules and/or less available to oxygen quenching, giving their typical broad emission centred at 430 nm. In argon atmosphere 3BZP* is not suppressed and its emission dominates the luminescence spectra. BZP. . .H appears as a shoulder in the luminescence spectra obtained in argon atmosphere but only at pulse end. Therefore a time-resolved luminescence study of concentrations lower than 500 lmol g
1 and within the first 20 ns could isolate the luminescence of BZP. . .H. This was accomplished using the N2 laser and the ICCD detection system described in the experimental section. Fig. 3b shows the time-resolved luminescence spectra obtained with a 100 lmol g
1 sample concentration in the first 20 ns (air-equilibrated conditions). The maximum emission in these conditions is located at 428 nm, confirming the presence of BZP. . .H.

Luminescence emission/a.u.

3.3. Room temperature laser-induced luminescence

12000

Luminescence emission/a.u.

indicate that following irradiation at 355 nm a np* state is formed while at 254 nm or 266 nm a pp* state is obtained.

5

500

3500

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3000 2500 2000 b c d e f g

1500 1000 500 0 350

450

550

λ/nm

600

550

650

650

700

λ /nm Fig. 4. Laser-induced luminescence spectra of BZP on H-ZSM-5 (355 nm excitation, 30 mJ/pulse, 250 lmol g
1): 50 ns (1), 100 ns (2), 150 ns (3), 200 ns (4), 250 ns (5), 300 ns (6), 350 ns (7), 450 ns (8), after the laser pulse. Inset: time-resolved luminescence spectra at longer times: 1 ls (a), 10 ls (b), 20 ls (c), 30 ls (d), 40 ls (e), 60 ls (f), and 90 ls (g).

Fig. 4 presents the time-resolved luminescence spectra of BZP on H-ZSM-5, excited at 355 nm.

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J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

A non-structured emission band centred around 452 nm appears at early times and then shifts to longer wavelengths up to 460 nm, 450 ns after the laser pulse. The luminescence in the microsecond time scale (see inset of Fig. 4) is even more red-shifted up to 470 nm, 90 ls after the laser pulse. The time evolution of the emission maximum in both times scales clearly indicate the presence of 3BZP* and BZPH+. At early times a mixture of the two forms is present, the emission from 3BZP* being dominant. In the microsecond time scale BZPH+ emission prevails, giving rise to the typical broad emission band of this species. The formation BZPH+ was already reported in ZSM-5 [10,11]. Zeolite activation above 50 C allowed the detection of this species for low concentration samples (0.1 lmol g
1) on H-ZSM-5 [10]. Our time-resolved luminescence results are in agreement with this behaviour and allowed us the separation and identification of the main luminescent transients for high loading of BZP (ca. 500 l mol g
1). 3.4. Diffuse reflectance laser flash photolysis Transient absorption spectra on silicalite exciting at 266 nm showed only 3BZP* absorption and are similar to those obtained with 355 nm excitation [12]. Triplet– triplet T1 ! T2 and T1 ! T3 transient absorptions were observed and identified, peaking at 520 nm and 320–330 nm, respectively. Fig. 5 presents the transient absorption results for BZP on MCM-41 excited at 266 nm in argon atmosphere. The differences to silicalite are a broad band between 300 nm and 450 nm and much lower intensity of the triplet–triplet absorption. 3BZP* decays within a few microseconds while the main absorption band remains after 20 ms. This absorption is mainly due to the BZPOH
radical, a transient already observed for BZP on other solid surfaces [14]. As it will become clear

15

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from the transient absorption results on H-ZSM-5 and from the photoproduct distribution, this absorption band is also due to the benzoyl radical. In the presence of molecular oxygen the triplet state is suppressed and its absorption bands were not observed (see inset of Fig. 5). These conditions revealed an absorption centred around 410 nm. This band shifts to the absorption region of the BZPOH
radical after 2 ls and shows similar intensity 20 ms after the laser pulse. The absorption band between 330 and 480 nm was also observed in a laser photolysis study of benzoyl radical precursors (aromatic ketones) in oxygen-containing solutions [38]. The obtained absorption was therefore assigned to transients involved in the reaction of BZP with molecular oxygen. Fig. 6 presents the transient absorption results for BZP on H-ZSM-5 excited at 266 nm in argon atmosphere. Two main differences were observed in comparison with silicalite: the presence of a shoulder around 380 nm and the broadening of the absorption band in the region of the T1 ! T2 absorption, in the nanosecond time scale. Time-resolved luminescence results indicated that BZPH+ is present at longer times. This transient was reported to have absorption bands at 320 nm, 385 nm and 500 nm, being the two latter ones of similar intensity [34]. BZPH+ should contribute therefore to the absorption at 385 nm and to the broadening of the absorption band centred around 520 nm. However the transient spectrum observed after 1 ms shows that the absorption at 385 nm is still observed while the one near 500 nm almost disappears. This result indicates that, although present on this solid support, BZPH+ has little contribution to the observed transient absorption spectra. Another radical that could lead to the broadening of the absorption between 400 and 600 nm is the benzophenone ketyl radical. The formation of this transient (maximum absorption at about 550 nm) from BZPH+ was proposed on H-ZSM-5 and other acid surfaces

575

675

0.5 µs 10 25 µs 5

1000 µs

775

λ /nm

Fig. 5. Transient absorption spectra of BZP on MCM-41 (500 lmol g
1) excited at 266 nm in argon atmosphere. The inset shows the correspondent spectra obtained at 0.5 ls (a), 1.0 ls (b), and 2.0 ls (c), after the laser pulse (355 nm) in air-equilibrated conditions.

0 275 -5

375

475

575

675

775

λ /nm

Fig. 6. Transient absorption spectra of BZP (500 lmol g
1) on HZSM-5 excited at 266 nm in argon atmosphere.

J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

[10,11,35,36]. The comparison with the spectra obtained on silicalite, were only the triplet state is present, and on cellulose, where both the triplet and the ketyl radical are present [7,12] indicates that the ketyl radical is a minor transient on H-ZSM-5. The shoulder between 350 nm and 400 nm indicates that the BZPOH
radical is also present on H-ZSM-5. The transient absorption spectra exciting at 355 nm (not shown) present the same main features. However the bands observed below 400 nm, due to the T1 ! T3 transition and the BZPOH
radical, are well resolved, indicating the formation of other absorbing species when exciting at 266 nm. This result suggests that the photochemistry depends on the excitation wavelength. From the comparison of the photoproduct distribution obtained at 266 nm and 355 nm (see photodegradation products section) it is clear that the a-cleavage of BZP is an important degradation pathway in the former conditions. The presence of benzoyl radical, which has a maximum absorption around 360 nm and a lower intensity absorption band near 500 nm, [16,37] is therefore expected under 266 nm irradiation and contributes to the broadening of the spectra. The transient absorption obtained after 1000 ls (see Fig. 6) shows an excellent spectrum of this radical.

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argon atmosphere conditions benzoic acid was detected only in trace amounts, suggesting the involvement of molecular oxygen on its formation. 2-OHBZP and benzoic acid were also detected after lamp irradiation (254 nm) of BZP/calixarene complexes [14]. The presence of benzaldehyde and Ph-BZP confirm the BZP acleavage in these conditions. Since the C–CO bond energy of benzophenone is higher than the excitation energy of the lowest triplet state, benzophenone in the T1 state cannot dissociate. Therefore this reaction must occur from a higher excited triplet state, as already reported in solution, [39] and a wavelength dependence of the photoproduct distribution should be found. 3.5.2. Laser irradiation at 266 and 355 nm A study of the photoproduct distribution was therefore made under 266 nm and 355 nm laser irradiation, in argon atmosphere. Fig. 7 shows the photoproduct distribution on MCM-41 at 266 nm and 355 nm. The main degradation products under both irradiation conditions are shown in Table 1. As observed under lamp conditions, products derived from the a-cleavage were also detected. The product with retention time (rt) 45.89 is also significant. The obtained data were not enough to propose a structure to this compound. As expected, under 355 nm irradiation benzaldehyde and

3.5. Photodegradation products 3.5.1. Lamp irradiation at 254 nm The photoproducts were initially studied in air-equilibrated conditions under lamp irradiation (254 nm). With the used experimental set-up, BZP was found stable in silicalite. In H-ZSM-5 2-hydroybenzophenone (2OHBZP) was the main degradation product while on MCM-41, besides 2-OHBZP, benzoic acid was also found (see Table 1). Trace amounts of benzaldehyde, biphenyl, isomers of 2-OHBZP (m/z = 198) and of compounds with m/z = 258, assigned to phenylbenzophenones (Ph-BZP), were also found on MCM-41. In

Table 1 Main photodegradation products in different experimental conditions Solid support

MCM-41

Experimental conditions Lamp, 254 nm Air equilibrated

Laser, 266 nm Argon

Laser, 355 nm Argon

2-OHBZP

OH-BZPs Ph-BZPs Benzaldehyde Biphenyl Rt = 45.89 mina

OH-BZPs

OH-BZPs Biphenyl Rt = 45.89 mina

OH-BZPs Benzhydrol Rt = 45.89 mina

Benzoic acid

H-ZSM-5

2-OHBZP

Benzopinacol Rt = 45.89 mina

2-OHBZP: 2-hydroxybenzophenone; OH-BZPs: hydroxybenzophenones; Ph-BZPs: phenylbenzophenones. a Compound with of 45.89 min retention time.

Fig. 7. GC–MS chromatograms of the extracts of irradiated samples of BZP on MCM-41 obtained at 266 nm (a), and at 355 nm (b), under argon atmosphere, in both cases.

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J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

absorption (of BZPOH
radical) or through the photoproduct distribution. However, the detection of water using BZP is only possible when this reaction is one of the main pathways, which is the case for MCM-41 and H-ZSM-5. The a-cleavage of BZP was detected in MCM-41 and H-ZSM-5, when using radiation able to promote the p ! p* transition (254 nm or 266 nm). This is in agreement with previous results [39] and indicates that the reaction competes with other deactivation pathways of this state. This primary photoreaction process is in agreement with the formation of benzaldehyde, biphenyl and Ph-BZP(s), and with the detection of the benzoyl radical and consequent broadening of the transient absorption observed under 266 nm excitation. The formation of benzhydrol and benzopinacol was detected on ZSM-5 and other acidic solid surfaces [11,10,36,37]. The former degradation product was in fact detected on H-ZSM-5 under 355 nm excitation. The used conditions do not allow for a good chromatographic separation of benzhydrol from BZP because the latter compound is present in much large concentration (the products were analysed at conversions lower than 5%) and they show near retention times. However single m/z = 184 ion chromatogram (the molecular ion of benzhydrol) showed unequivocally the formation of this compound, although in low amount. Benzopinacol was not detected on H-ZSM-5. This was expected since the stereochemical restrictions imposed by the channel size ˚ ) do not allow the formation of this com(Bint 6 6 A pound. However its possible formation at the external surface of the zeolite cannot be excluded, in spite of

biphenyl were not detected and only one of the m/ z = 258 compounds appears, indicating that the a-cleavage is a minor degradation pathway at this wavelength. The photoproducts found on H-ZSM-5 are presented in Table 1. Benzopinacol was not detected on this solid support in the studied conditions. All the identifications were made analysing authentic samples. 3.6. Reaction pathways Except for silicalite, where BZP was found stable, the main degradation pathway observed in all conditions and solid supports is the one that leads to 2-OHBZP. The transient species also present under all conditions in the reactive supports is the BZPOH
radical. Therefore, all data suggest that one of the main degradation pathways is the one that leads to 2-OHBZP through the formation of the BZPOH
radical (see Fig. 8). This OH adduct can result from the BZP radical cation, after reaction with water [40]. However no absorption assignable to this transient was detected between 300 nm and 800 nm, probably due to the low absorption in this spectral range and/or to its fast reaction [40]. In the studied solids we were not able to confirm this pathway for the OH adduct formation. The participation of water in this reaction pathway is however clear. Both OHBZP(s) and their precursor (BZPOH
) were not detected on silicalite, which has a hydrophobic character, and were present on MCM-41 and H-ZSM-5, which have water molecules on their hydrophilic surfaces. BZP is therefore a good probe to detect water on solid supports either by transient

O

O

..

O

BZP +

.

+

H

Ph-BZPs Benzoyl radical 266 nm 254 nm

O

OH

O

.

BZP

hν 254 nm

* BZP

.

+ 2H

BZPOH .

H2O

OHBZPs

OH

OH 355 nm H

.

H

H

266 nm

Benzhydrol (on H-ZSM-5)

OH

.

355 nm

OH

.

O2

Ketyl radical OH

O OH OH Benzopinacol (on MCM-41)

Fig. 8. Photoreaction scheme of BZP on MCM-41 and H-ZSM-5.

J.P. Da Silva et al. / Microporous and Mesoporous Materials 84 (2005) 1–10

the fact that the external surface of H-ZSM-5 is very reduced when compared with the internal channel surface [23,24]. Benzhydrol was not detected on MCM-41 in any of the studied conditions. However benzopinacol was found on this support under 355 nm excitation. This indicates that the formation of the ketyl radical occurs but is a minor degradation pathway. In this support ˚ average pore size, allowing the channels have 41.5 A the coupling of the formed ketyl radicals of BZP. The low concentration of benzhydrol or benzopinacol is in agreement with the transient absorption results since no significative amount of the BZP ketyl radical, the precursor of these compounds, was detected in our flash-photolysis studies. The formation of benzoic acid and of transients involved on its formation (see inset of Fig. 5) are an indication of the presence of molecular oxygen. However on H-ZSM-5 benzoic acid was detected only in trace amounts in air-equilibrated conditions, indicating that other photoreaction pathways prevail. The formation of benzoic acid is associated with the presence of oxygen. 4. Conclusions The photochemical behaviour of benzophenone on MCM-41 was assessed using diffuse reflectance and chromatographic techniques. Ground state absorption spectra showed a decrease of polarity in the environment of the guest molecule as going from MCM-41 to H-ZSM-5 and to silicalite. The triplet state was detected in all studied solid supports by both time-resolved luminescence and transient absorption. Time-resolved luminescence studies also revealed the presence of hydrogen-bonded excited benzophenone on MCM-41 and protonated excited benzophenone on H-ZSM-5. The formation of 2-hydroxybenzophenone through the OH benzophenone adduct is one of the main photodegradation pathways on MCM-41 and H-ZSM-5. a-cleavage was observed under 266 nm irradiation, leading to the benzoyl radical formation and to benzaldehyde, biphenyl and phenylbenzophenones as the main final photodegradation products. Transient absorption showed that ketyl radical formation is a minor degradation pathway. Photochemistry studies on MCM-41 in air-equilibrated conditions indicated the formation of transients resulting from the reaction of benzophenone with molecular oxygen. Under these conditions benzoic acid is the main degradation product. Acknowledgement We thank Prof. Filipa Ribeiro for the nitrogen adsorption measurements. Post-Doctoral fellowship SFRH/BPD/15589/2001 is gratefully acknowledged.

9

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