Photocatalytic synthesis in membrane reactors is still at a preliminary research stage, despite the great potentiality of photocatalytic processes, especially when they are coupled with a membrane separation system. The possibility of using a membrane photoreactor for organic synthesis, developing a hybrid system in which the photocatalytic reaction and the separation of the desired product occurs in one step is of high interest. Some results obtained in a photocatalytic membrane contactor (PMC) for the one-step synthesis of phenol, and its simultaneous separation, are reported by Molinari et al. (2009). TiO2 has been used as catalyst, benzene as both reactant and extraction solvent, and a polypropylene membrane to separate the organic phase from the aqueous reactive environment. To avoid secondary products and to obtain an efficient phenol production, the use of a membrane system, with high phenol permeability and complete rejection of the catalyst, coupled with the photocatalytic process, seems a useful solution. The membrane photoreactor built in this study consists of an external lamp placed on a batch reactor containing the aqueous solution with the catalyst in suspension; by means of a peristaltic pump the solution is withdrawn from the photocatalytic reactor to a membrane contactor module in which a benzene solution is present as strip phase. A set of preliminary experiments were performed in order to select the operating conditions to be employed in the batch and membrane systems. Due to the low solubility of benzene in water and to its high volatility, it was necessary to work with an excess of substrate in solution. By comparing the data in batch experiments obtained at pH values equal to 5.5 and 3.1, it was observed that the more acidic pH condition allowed to obtain a slightly increase of phenol production in the aqueous phase and a constant flux in the organic phase after 2 h (1.27 mmol/h m2). Moreover, the area of the three main HPLC peaks detected at retention times of 3.0, 3.6 and 3.9 min indicated that a lower formation and extraction of these intermediates occurred at the acidic pH. Other works concerning photocatalytic conversions are reported in the following. Selective oxidation of various primary and secondary alcohols was studied by Pillai and Sahle-Demessie (2002) in a gas-phase photochemical reactor using immobilized TiO2 catalyst. An annular photoreactor was used at 463 K with an average contact time of 32 s. The system was found to be specifically suited for the selective oxidation of primary and secondary aliphatic alcohols to their corresponding carbonyl compounds. Benzylic alcohols gave higher conversions, however, with more secondary reaction products. The reaction mechanism for various products formed was explained. The effects of different reaction parameters, such as O2/alcohol ratio, water vapor, UV light, and contact time, were studied. The presence of oxygen was found to be critical for the photooxidation. Water vapor in the feed was also found to be beneficial, although it was not as critical as in hydrocarbon oxidation, where it was necessary for hydroxylating the catalyst surface and sustaining its activity. In alcohol oxidation, surface hydroxylation could be partially provided by the hydroxyl groups of the alcohol itself. Catalyst deactivation was also observed and it was attributed to the surface accumulation of reaction products. However, the catalyst regained its original activity after regeneration by calcination in air for 3 h at 723 K. Selective photocatalytic oxygenation of various substrates was achieved using organic photocatalysts via photoinduced electron-transfer reactions of photocatalysts with substrates and dioxygen under visible light irradiation (Fukuzumi and Ohkubo, 2012). Photoinduced electron transfer from benzene to the singlet-excited state of the 3-cyano-1-methylquinolinium ion enabled the oxidation of benzene by dioxygen with water to yield phenol selectively. Alkoxybenzenes were obtained when water was replaced by alcohols under otherwise the same experimental conditions. Photocatalytic selective oxygenation reactions of aromatic compounds was also achieved using an electron donor–acceptor, 9-mesityl-10-methylacridinium ion acting as a photocatalyst and dioxygen as an oxidant under visible light irradiation. The most recent advances in the application of heterogeneous photocatalysis to synthesize valuable compounds by selective oxidation and reduction are reported by Palmisano et al.(2010). The photocatalytic oxidation of 4-methoxybenzyl alcohol in water was performed in a fixed bed continuous annular reactor by using a home-prepared TiO2 catalyst supported on Pyrex glass beads (Yurdakal et al, 2010). The investigation was aimed to modeling the complex kinetics of the photoprocess which occurs through two parallel pathways: (i) partial oxidation to the corresponding aldehyde and (ii) total oxidation to CO2 and H2O. On these grounds, the influence of liquid flow rate, inlet concentrations of alcohol and oxygen, catalyst amount, and irradiation power on the photoreactivity were studied. A kinetic model like that of Langmuir-Hinshelwood satisfactorily fitted the experimental results and allowed determination of the values of model parameters. It was found that all these parameters are positively affected by an increase of radiant energy absorbed by the catalyst. All the reactivity results indicate that the partial oxidation pathway is favored by the low flux of absorbed photons and by low oxygen coverage on the TiO2 surface; opposite conditions favor the mineralization pathway. The photoproduction of vanillin was studied in aqueous medium starting from trans-ferulic acid, isoeugenol, eugenol or vanyllyl alcohol by using both commercial and home prepared TiO2 samples as photocatalysts (Augugliaro et al, 2012). The visible-light-induced oxidation of trans-ferulic acid by TiO2 photocatalysis was also carried out under visible light via the formation of a charge-trasfer complex (Parrino et al, 2012). A drawback for this type of photoreaction is the relatively low selectivity observed and the presence of other products. The combination of photocatalysis and pervaporation by using non-porous membranes has been suggested to separate the vanillin or other aldehydes from the irradiated suspension to avoid their subsequent oxidation, increasing in such a way the selectivity (Camera Roda et al., 2010; Camera Roda et al, in press). The mild experimental conditions used suggest that heterogeneous photocatalysis could be in some cases an alternative green route for replacing environmentally hazardous processes with safe and energy efficient routes. References Augugliaro V, Camera Roda G, Loddo V, Palmisano G, Palmisano L, Parrino F, Puma M A (2012) Synthesis of vanillin in water by TiO2 photocatalysis. Applied Catalysis B: Environmental 111-112:555-561 Camera Roda G, Augugliaro V, Cardillo A, Loddo V, Palmisano G, Palmisano L (in press) A pervaporation photocatalytic reactor for the green synthesis of vanillin. Chemical Engineering Journal, in press Camera Roda G, Augugliaro V, Loddo V, Palmisano L, Palmisano G (2010) Production of aldehydes by oxidation in aqueous medium with selective recovery of the product by means of pervaporation. Patent RM10A000319 Fukuzumi S, Ohkubo K (2013) Selective photocatalytic reactions with organic photocatalysts. Chem. Sci. 4:561-574 Molinari R, Caruso A, Poerio T ( 2009) Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor. Catalysis Today 144:81–86 Parrino F, Augugliaro V, Camera Roda G, Loddo V, López-Muñoz M J, Márquez-Álvarez C, Palmisano G, Palmisano L, Puma M A (2012) Visible light induced oxidation of trans-ferulic acid by TiO2 photocatalysis. Journal of Catalysis 295:254-260 Pillai UR, Sahle-Demessie E(2002) Selective oxidation of alcohols in gas phase using light-activated titanium dioxide. Journal of Catalysis 211:434-444 Palmisano G, García-López E, Marcì G, Loddo V, Yurdakal S, Augugliaro V, Palmisano L(2010) Advances in selective conversions by heterogeneous photocatalysis. Chemical Communications 46:7074-7089 Yurdakal S, Loddo V, Palmisano G, Augugliaro V, Berber H, Palmisano L(2010) Kinetics of 4-Methoxybenzyl Alcohol Oxidation in Aqueous Solution in a Fixed Bed Photocatalytic Reactor. Industrial & Engineering Chemistry Research 49: 6699-6708
Photocatalytic membrane reactor in the conversion of organic compounds
MOLINARI, Raffaele
2013-01-01
Abstract
Photocatalytic synthesis in membrane reactors is still at a preliminary research stage, despite the great potentiality of photocatalytic processes, especially when they are coupled with a membrane separation system. The possibility of using a membrane photoreactor for organic synthesis, developing a hybrid system in which the photocatalytic reaction and the separation of the desired product occurs in one step is of high interest. Some results obtained in a photocatalytic membrane contactor (PMC) for the one-step synthesis of phenol, and its simultaneous separation, are reported by Molinari et al. (2009). TiO2 has been used as catalyst, benzene as both reactant and extraction solvent, and a polypropylene membrane to separate the organic phase from the aqueous reactive environment. To avoid secondary products and to obtain an efficient phenol production, the use of a membrane system, with high phenol permeability and complete rejection of the catalyst, coupled with the photocatalytic process, seems a useful solution. The membrane photoreactor built in this study consists of an external lamp placed on a batch reactor containing the aqueous solution with the catalyst in suspension; by means of a peristaltic pump the solution is withdrawn from the photocatalytic reactor to a membrane contactor module in which a benzene solution is present as strip phase. A set of preliminary experiments were performed in order to select the operating conditions to be employed in the batch and membrane systems. Due to the low solubility of benzene in water and to its high volatility, it was necessary to work with an excess of substrate in solution. By comparing the data in batch experiments obtained at pH values equal to 5.5 and 3.1, it was observed that the more acidic pH condition allowed to obtain a slightly increase of phenol production in the aqueous phase and a constant flux in the organic phase after 2 h (1.27 mmol/h m2). Moreover, the area of the three main HPLC peaks detected at retention times of 3.0, 3.6 and 3.9 min indicated that a lower formation and extraction of these intermediates occurred at the acidic pH. Other works concerning photocatalytic conversions are reported in the following. Selective oxidation of various primary and secondary alcohols was studied by Pillai and Sahle-Demessie (2002) in a gas-phase photochemical reactor using immobilized TiO2 catalyst. An annular photoreactor was used at 463 K with an average contact time of 32 s. The system was found to be specifically suited for the selective oxidation of primary and secondary aliphatic alcohols to their corresponding carbonyl compounds. Benzylic alcohols gave higher conversions, however, with more secondary reaction products. The reaction mechanism for various products formed was explained. The effects of different reaction parameters, such as O2/alcohol ratio, water vapor, UV light, and contact time, were studied. The presence of oxygen was found to be critical for the photooxidation. Water vapor in the feed was also found to be beneficial, although it was not as critical as in hydrocarbon oxidation, where it was necessary for hydroxylating the catalyst surface and sustaining its activity. In alcohol oxidation, surface hydroxylation could be partially provided by the hydroxyl groups of the alcohol itself. Catalyst deactivation was also observed and it was attributed to the surface accumulation of reaction products. However, the catalyst regained its original activity after regeneration by calcination in air for 3 h at 723 K. Selective photocatalytic oxygenation of various substrates was achieved using organic photocatalysts via photoinduced electron-transfer reactions of photocatalysts with substrates and dioxygen under visible light irradiation (Fukuzumi and Ohkubo, 2012). Photoinduced electron transfer from benzene to the singlet-excited state of the 3-cyano-1-methylquinolinium ion enabled the oxidation of benzene by dioxygen with water to yield phenol selectively. Alkoxybenzenes were obtained when water was replaced by alcohols under otherwise the same experimental conditions. Photocatalytic selective oxygenation reactions of aromatic compounds was also achieved using an electron donor–acceptor, 9-mesityl-10-methylacridinium ion acting as a photocatalyst and dioxygen as an oxidant under visible light irradiation. The most recent advances in the application of heterogeneous photocatalysis to synthesize valuable compounds by selective oxidation and reduction are reported by Palmisano et al.(2010). The photocatalytic oxidation of 4-methoxybenzyl alcohol in water was performed in a fixed bed continuous annular reactor by using a home-prepared TiO2 catalyst supported on Pyrex glass beads (Yurdakal et al, 2010). The investigation was aimed to modeling the complex kinetics of the photoprocess which occurs through two parallel pathways: (i) partial oxidation to the corresponding aldehyde and (ii) total oxidation to CO2 and H2O. On these grounds, the influence of liquid flow rate, inlet concentrations of alcohol and oxygen, catalyst amount, and irradiation power on the photoreactivity were studied. A kinetic model like that of Langmuir-Hinshelwood satisfactorily fitted the experimental results and allowed determination of the values of model parameters. It was found that all these parameters are positively affected by an increase of radiant energy absorbed by the catalyst. All the reactivity results indicate that the partial oxidation pathway is favored by the low flux of absorbed photons and by low oxygen coverage on the TiO2 surface; opposite conditions favor the mineralization pathway. The photoproduction of vanillin was studied in aqueous medium starting from trans-ferulic acid, isoeugenol, eugenol or vanyllyl alcohol by using both commercial and home prepared TiO2 samples as photocatalysts (Augugliaro et al, 2012). The visible-light-induced oxidation of trans-ferulic acid by TiO2 photocatalysis was also carried out under visible light via the formation of a charge-trasfer complex (Parrino et al, 2012). A drawback for this type of photoreaction is the relatively low selectivity observed and the presence of other products. The combination of photocatalysis and pervaporation by using non-porous membranes has been suggested to separate the vanillin or other aldehydes from the irradiated suspension to avoid their subsequent oxidation, increasing in such a way the selectivity (Camera Roda et al., 2010; Camera Roda et al, in press). The mild experimental conditions used suggest that heterogeneous photocatalysis could be in some cases an alternative green route for replacing environmentally hazardous processes with safe and energy efficient routes. References Augugliaro V, Camera Roda G, Loddo V, Palmisano G, Palmisano L, Parrino F, Puma M A (2012) Synthesis of vanillin in water by TiO2 photocatalysis. Applied Catalysis B: Environmental 111-112:555-561 Camera Roda G, Augugliaro V, Cardillo A, Loddo V, Palmisano G, Palmisano L (in press) A pervaporation photocatalytic reactor for the green synthesis of vanillin. Chemical Engineering Journal, in press Camera Roda G, Augugliaro V, Loddo V, Palmisano L, Palmisano G (2010) Production of aldehydes by oxidation in aqueous medium with selective recovery of the product by means of pervaporation. Patent RM10A000319 Fukuzumi S, Ohkubo K (2013) Selective photocatalytic reactions with organic photocatalysts. Chem. Sci. 4:561-574 Molinari R, Caruso A, Poerio T ( 2009) Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor. Catalysis Today 144:81–86 Parrino F, Augugliaro V, Camera Roda G, Loddo V, López-Muñoz M J, Márquez-Álvarez C, Palmisano G, Palmisano L, Puma M A (2012) Visible light induced oxidation of trans-ferulic acid by TiO2 photocatalysis. Journal of Catalysis 295:254-260 Pillai UR, Sahle-Demessie E(2002) Selective oxidation of alcohols in gas phase using light-activated titanium dioxide. Journal of Catalysis 211:434-444 Palmisano G, García-López E, Marcì G, Loddo V, Yurdakal S, Augugliaro V, Palmisano L(2010) Advances in selective conversions by heterogeneous photocatalysis. Chemical Communications 46:7074-7089 Yurdakal S, Loddo V, Palmisano G, Augugliaro V, Berber H, Palmisano L(2010) Kinetics of 4-Methoxybenzyl Alcohol Oxidation in Aqueous Solution in a Fixed Bed Photocatalytic Reactor. Industrial & Engineering Chemistry Research 49: 6699-6708I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
