Engineering the catalytic batchwise synthesis of H2O2 from its elements

Hydrogen peroxide is a versatile oxidizing agent with several industrial applications. It is also one of “greenest”, since its oxidation by-product is only water. The global demand of the peroxide is increasing, due to its recent usage in new large scale oxidation processes, such as the epoxidation of propylene to propylene oxide and the synthesis of caprolactam. Nowadays most of the world production of H2O2 is carried out by the anthraquinone autoxidation process. Though very safe (H2 and O2 are never in direct contact), the costs related to the high energy consumption for the extraction and purification of the peroxide produced, together with the usage and periodic replacement of toxic and expensive solvents, stimulated the interest in new production paths. Among the several alternatives proposed, the most fascinating one is the direct synthesis (DS) from H2 and O2. It is a environmentally friendly process that would be economically profitable for an in-situ production, requiring lower investments and operating costs. During the last thirty years this system has been under intensive study both by industries as well as academia. However, it has not been commercialized yet, mainly because of poor selectivity and safety concerns. While most of the efforts on improving DS must address the catalyst, there are reaction engineering aspects that deserve attention. DS is frequently carried out in solvents other than water, both to improve H2 solubility and isolate the undesired product (H2O). Further, CO2 is used for safety, H2 solubility and H2O2 stability. However, the lack of information about the solubility of the reagents makes it difficult to develop a realistic kinetic description of the reactions involved in the DS process. Hence, the first step of the research presented herein dealt with solubility measurements, at temperatures in the range 268-288 K and pressures between 0.37 and 3.5 MPa. Measurements were focused on H2, i.e. the limiting reagent during the reaction. At all conditions investigated a linear relation between hydrogen partial pressure and concentration was observed. Increasing the temperature resulted in an enhanced H2 solubility at the same H2 partial pressure. At constant H2 fugacity, the presence of CO2 favored the dissolution of hydrogen in the liquid phase. Correlation and generalization of the measurements were provided through an EoS-based thermodynamic model for the estimation of H2 solubility at reaction conditions. A batch apparatus for the direct synthesis of hydrogen peroxide was developed, to carry out activity measurements on new catalysts and develop a quantitative model of the kinetics. Hydrogenation, disproportionation and direct synthesis reactions were studied on a commercial 5 wt.% Pd/C catalysts at temperatures in the range 258-313 K and pressure up to 2 MPa. Separate experiments were performed to highlight the role of each reaction. An enhanced H2O2 production was obtained adopting different H2 feeding policies, although selectivity did not exceeded 30%. A model of the gas bubbling, batch slurry reactor for H2O2 direct synthesis was developed. A sensitivity analysis on the mass transfer coefficients excluded any limitations occurring at experimental conditions. Comparable temperature dependence was observed for H2O production, hydrogenation and disproportionation (activation energies close to 45 kJ mol-1), while H2O2 synthesis had a much lower activation energy (close to 24 kJ mol-1), suggesting that a higher selectivity is achievable at low temperature. Disproportionation reaction had a very limited influence on the overall peroxide production rate, while hydrogenation was the most rapid side reaction. Water formation was significant, prevailing at higher temperatures. Following these results, Pd and PdAu catalysts supported on SBA15 were prepared and investigated for H2O2 direct synthesis. Catalysts were doped with bromine, a promoter in the H2O2 direct synthesis. Productivity and selectivity decreased when bromine was incorporated in the catalysts, suggesting a possible poisoning due to the grafting process. A synergetic effect between Pd and Au was observed both in presence and absence of bromopropylsilane grafting on the catalyst. Three modifiers of the SBA15 support (Al, CeO2 and Ti) were chosen to elucidate the influence of the surface properties on metal dispersion and catalytic performance. Higher productivity and selectivity were achieved incorporating Al into the SBA15 framework, whereas neither Ti nor CeO2 improved H2O2 yields. The enhanced performance observed for the PdAu/Al-SBA15 catalysts was attributed to the increased number of Brønsted acid sites. Supported catalysts were also synthesized depositing Pd on a highly acidic, macroporous PS-DVB resin (Lewtit K2621). Catalysts with active metal content in the range 0.3-5 wt.% were tested batchwise for the direct synthesis of H2O2. Preliminary H2O2 measurements and X-ray photoelectron spectroscopy (XPS) analysis revealed that the reduced form of Pd was more selective than PdO towards the peroxide. Transmission electron microscopy (TEM) images showed that smaller nanoclusters favored the production of H2O, likely due to their O-O bond breaking aptitud

Engineering the catalytic batchwise synthesis of H2O2 from its elements

Hydrogen peroxide is a versatile oxidizing agent with several industrial applications. It is also one of “greenest”, since its oxidation by-product is only water. The global demand of the peroxide is increasing, due to its recent usage in new large scale oxidation processes, such as the epoxidation of propylene to propylene oxide and the synthesis of caprolactam. Nowadays most of the world production of H2O2 is carried out by the anthraquinone autoxidation process. Though very safe (H2 and O2 are never in direct contact), the costs related to the high energy consumption for the extraction and purification of the peroxide produced, together with the usage and periodic replacement of toxic and expensive solvents, stimulated the interest in new production paths. Among the several alternatives proposed, the most fascinating one is the direct synthesis (DS) from H2 and O2. It is a environmentally friendly process that would be economically profitable for an in-situ production, requiring lower investments and operating costs. During the last thirty years this system has been under intensive study both by industries as well as academia. However, it has not been commercialized yet, mainly because of poor selectivity and safety concerns. While most of the efforts on improving DS must address the catalyst, there are reaction engineering aspects that deserve attention. DS is frequently carried out in solvents other than water, both to improve H2 solubility and isolate the undesired product (H2O). Further, CO2 is used for safety, H2 solubility and H2O2 stability. However, the lack of information about the solubility of the reagents makes it difficult to develop a realistic kinetic description of the reactions involved in the DS process. Hence, the first step of the research presented herein dealt with solubility measurements, at temperatures in the range 268-288 K and pressures between 0.37 and 3.5 MPa. Measurements were focused on H2, i.e. the limiting reagent during the reaction. At all conditions investigated a linear relation between hydrogen partial pressure and concentration was observed. Increasing the temperature resulted in an enhanced H2 solubility at the same H2 partial pressure. At constant H2 fugacity, the presence of CO2 favored the dissolution of hydrogen in the liquid phase. Correlation and generalization of the measurements were provided through an EoS-based thermodynamic model for the estimation of H2 solubility at reaction conditions. A batch apparatus for the direct synthesis of hydrogen peroxide was developed, to carry out activity measurements on new catalysts and develop a quantitative model of the kinetics. Hydrogenation, disproportionation and direct synthesis reactions were studied on a commercial 5 wt.% Pd/C catalysts at temperatures in the range 258-313 K and pressure up to 2 MPa. Separate experiments were performed to highlight the role of each reaction. An enhanced H2O2 production was obtained adopting different H2 feeding policies, although selectivity did not exceeded 30%. A model of the gas bubbling, batch slurry reactor for H2O2 direct synthesis was developed. A sensitivity analysis on the mass transfer coefficients excluded any limitations occurring at experimental conditions. Comparable temperature dependence was observed for H2O production, hydrogenation and disproportionation (activation energies close to 45 kJ mol-1), while H2O2 synthesis had a much lower activation energy (close to 24 kJ mol-1), suggesting that a higher selectivity is achievable at low temperature. Disproportionation reaction had a very limited influence on the overall peroxide production rate, while hydrogenation was the most rapid side reaction. Water formation was significant, prevailing at higher temperatures. Following these results, Pd and PdAu catalysts supported on SBA15 were prepared and investigated for H2O2 direct synthesis. Catalysts were doped with bromine, a promoter in the H2O2 direct synthesis. Productivity and selectivity decreased when bromine was incorporated in the catalysts, suggesting a possible poisoning due to the grafting process. A synergetic effect between Pd and Au was observed both in presence and absence of bromopropylsilane grafting on the catalyst. Three modifiers of the SBA15 support (Al, CeO2 and Ti) were chosen to elucidate the influence of the surface properties on metal dispersion and catalytic performance. Higher productivity and selectivity were achieved incorporating Al into the SBA15 framework, whereas neither Ti nor CeO2 improved H2O2 yields. The enhanced performance observed for the PdAu/Al-SBA15 catalysts was attributed to the increased number of Brønsted acid sites. Supported catalysts were also synthesized depositing Pd on a highly acidic, macroporous PS-DVB resin (Lewtit K2621). Catalysts with active metal content in the range 0.3-5 wt.% were tested batchwise for the direct synthesis of H2O2. Preliminary H2O2 measurements and X-ray photoelectron spectroscopy (XPS) analysis revealed that the reduced form of Pd was more selective than PdO towards the peroxide. Transmission electron microscopy (TEM) images showed that smaller nanoclusters favored the production of H2O, likely due to their O-O bond breaking aptitudeIl perossido di idrogeno è un potente agente ossidante, molto usato nella pratica industriale. E’ uno dei meno tossici, dal momento che l’unico sottoprodotto della sua ossidazione è l’acqua. A livello mondiale, la domanda di H2O2 è in costante aumento, non da ultimo grazie a recenti usi in nuovi processi ossidativi, quali l’epossidazione del propilene e la sintesi del caprolattame. Attualmente l’acqua ossigenata viene prodotta quasi esclusivamente attraverso l’auto-ossidazione dell’antrachinone. Sebbene molto sicuro (non vi è mai contatto diretto tra idrogeno ed ossigeno), questo processo presenta alcuni svantaggi, quali ad esempio gli alti costi di esercizio, dovuti in particolare all’alta richiesta energetica per la separazione e la purificazione del perossido prodotto. Si tratta inoltre di un processo potenzialmente inquinante, in quanto fa uso di costosi solventi tossici, e dagli alti costi d’investimento, essendo economicamente vantaggioso solo per grandi produzioni (>4*104 tonnellate all’anno). Pertanto l’H2O2 è attualmente prodotta in pochi grandi impianti e trasferita per grandi distanze all’utente finale. Il trasporto aggiunge costi e rischi, in quanto soluzioni concentrate di H2O2 possono decomporre violentemente. Nelle ultime decadi vi è stato un notevole interesse nella ricerca di nuovi processi di produzione del perossido di idrogeno, che fossero contemporaneamente meno costosi ed inquinanti. Tra le varie alternative proposte, la più affascinante è sicuramente la sintesi diretta a partire da H2 ed O2. Si tratta di un processo “verde”, che si propone di eliminare i sottoprodotti inquinanti e, allo stesso tempo, ridurre i costi di produzione, rendendo economicamente vantaggiosa la produzione in situ presso l’utilizzatore finale. Nonostante il grande interesse sia industriale che accademico suscitato da tale processo negli ultimi trent’anni, a tutt’oggi non vi è nessuna applicazione industriale. Il motivo di ciò è da ricercarsi principalmente nei problemi di sicurezza e selettività che a tutt’ora restano irrisolti. La mancanza di informazioni sulla solubilità dei reagenti alle condizioni di reazione rende difficoltoso ottenere una descrizione cinetica precisa delle reazioni coinvolte nella sintesi diretta. Pertanto i primi passi della ricerca qui presentata sono stati mossi con l’obiettivo di raccogliere dati di solubilità alle condizioni di reazione (temperatura compresa tra i 268 e i 288 K e pressione tra 0.37 e 3.5 MPa). In particolare, si era interessati all’H2, in quanto reagente limitante del processo. A tutte le condizioni indagate, è stata riscontrata una relazione lineare tra la pressione parziale e la concentrazione di H2. Contrariamente a quanto normalmente avviene, l’incremento di temperatura ha avuto l’effetto di aumentare la solubilità nella fase liquida (a parità di pressione parziale). Inoltre, a parità di fugacità di H2, la presenza di CO2 ha favorito la concentrazione dell’H2 nel liquido. I risultati ottenuti sono stati generalizzati sviluppando un modello per stimare la solubilità dell’H2 alle condizioni di reazione. E’ stato poi realizzato un apparato batch per la sintesi diretta di acqua ossigenata. Un catalizzatore commerciale a base di Pd (5 wt.%) su carbone è stato utilizzato per studiare le reazioni di idrogenazioni, dismutazione e sintesi a temperature comprese tra 258 e 313 K e pressioni fino a 2.0 MPa. Il ruolo di ciascuna reazione è stato studiato attraverso esperimenti specifici. Appropriate politiche di alimentazione dell’H2 hanno permesso di realizzare un aumento di produzione rispetto a condizioni tipicamente batch. Tuttavia il catalizzatore testato ha rivelato limiti di selettività, non superando valori del 30% ca. Per studiare le cinetiche di reazione, è stato sviluppato un modello per il reattore batch. Un’analisi di sensitività sui coefficienti di trasporto di materia (sia dalla fase gassosa alla liquida che dalla liquida al catalizzatore) ha permesso di escludere ogni limitazione tra le fasi coinvolte nelle reazioni. Le reazioni indesiderate (formazione di H2O, dismutazione ed idrogenazione) hanno rivelato una simile dipendenza dalla temperatura (con un’energia di attivazione di circa 45 kJ mol-1). Una minore energia di attivazione è stata ottenuta per la reazione di sintesi diretta di H2O2 (24 kJ mol-1), il che suggerisce che la selettività è favorita alle basse temperature. Un confronto tra le velocità delle reazioni coinvolte ha permesso di identificare la dismutazione come la reazione più lenta di distruzione del perossido. Inoltre, la formazione di acqua era sempre significativa, compromettendo la selettività. A seguito di questi risultati, si è deciso di focalizzare l’attenzione sul catalizzatore. Catalizzatori mono e bi-metallici sono stati realizzati depositando Pd e PdAu su SBA15, una silice macroporosa e strutturata. Tali catalizzatori sono stati anche dopati con l’aggiunta di bromo, un noto promotore della reazione di sintesi diretta. Sia la selettività che la produttività sono diminuite modificando i catalizzatori con l’alogenio, probabilmente a causa di un avvelenamento durante la procedura di innesto del bromo. Una sinergia tra i metalli Pd e Au è stata osservata sia nei catalizzatori con e che senza bromo. Tre modifiche sono state apportate al miglior catalizzatore sviluppato (PdAu/SBA15) per evidenziare l’influenza delle proprietà superficiali sulla reazione di sintesi diretta. Tre modificatori sono stati incorporati nel supporto: Al, CeO2 e Ti. Un aumento sia di selettività che di produttività è stato riscontrato solo con l’aggiunta di Al. Tale risultato è stato attribuito al maggior numero di siti acidi di Brønsted riscontrati su questo catalizzatore. Un'altra famiglia di catalizzatori, con un contenuto di metallo attivo variabile tra lo 0.3 ed il 5 wt.%, è stata sintetizzata depositando del Pd su una resina acida e macroporosa, miscela di PS e DVB. I risultati preliminari dei test catalitici e delle analisi di spettroscopia fotoelettronica a raggi X (XPS) hanno rivelato che lo stato di ossidazione del palladio più selettivo verso il perossido è quello ridotto, mentre il PdO porta più facilmente alla formazione di H2O. Le immagini al microscopio elettronico a trasmissione (TEM) hanno mostrato che i nanocluster di Pd più piccoli portato alla formazione preferenziale di H2O, il che è probabilmente legato alla loro propensione alla rottura del legame O-

H2 solubility in methanol in the presence of CO2 and O2

Hydrogen solubility in methanol, (methanol + carbon dioxide) and (methanol + carbon dioxide + oxygen) was measured and correlated at different temperatures (268 < T/K < 288) and pressures (0.37 < P/MPa < 3.5). Hydrogen content in the liquid phase was measured using a gas absorption method and Fugatron HYD-100 instrument. Experiments were performed in a fixed volume cell at constant temperature and hydrogen content was varied with subsequent loadings in the cell environment. At all conditions investigated a linear relation between hydrogen partial pressure and concentration was observed. Results were correlated and generalized as Henry's constants for H 2, as a function of temperature and CO 2/methanol overall ratio. Correlation and generalization of the measurements was provided through a thermodynamic model, based on Peng-Robinson equation of state with van der Waals mixing rules and Boston-Mathias \u3b1-function. H 2 solubility in methanol was confirmed to grow with temperature and amount of CO 2; at constant H 2 partial pressure, O 2 does not affect H 2 solubilit

Direct synthesis of H2O2 over Pd supported on rare earths promoted zirconia

in this work Pd (0.3 or 0.6wt.%) was supported on both ZrxM1-xO2 (M=La, Y, Ce) and on mechanical mixtures of CeO2 and ZrO2. The synthesized catalysts were characterized by XRD, TPR, AAS and CO chemisorption and tested for the direct synthesis of hydrogen peroxide in a high pressure semibatch apparatus. The reactants conversion was limited in order to avoid mass-transfer limitations. No selectivity enhancers of any kind were used and the all the materials were halide free. Small metal particles were obtained (1-2.6nm). Supports with smaller pore diameters leaded to larger Pd particles, which in turn were found to preferentially support the formation of the peroxide. Moreover, supports with higher reducibility favored the production of H2O2, probably due to an easier reduction of the active metal, essential to achieve high selectivity. Notwithstanding the absence of enhancers, the specific activity and selectivity recorded were very high

The influence of catalyst amount and Pd loading on the H2O2 synthesis from hydrogen and oxygen

Palladium catalysts with an active metal content from 0.3 to 5.0 wt.% and supported on a strongly acidic, macroporous resin were prepared by ion-exchange/reduction method. H2O2 direct synthesis was carried out in the absence of promoters (acids and halides). The total Pd amount in the reacting environment was varied by changing A) the catalyst concentration in the slurry and B) the Pd content of the catalyst. In both cases, smaller amounts of the active metal enhance the selectivity towards H2O2, at any H2 conversion, with option B) better than A). In case A), the Pd(II)/Pd(0) molar ratio (XPS) in the spent catalysts was found to decrease at lower catalyst Pd content. With these catalysts and this experimental set-up the dynamic HL2/Pd molar ratio, the metal loading and the metal particle size were the key factors controlling the selectivity, which reached 57% at 60% H2 conversion, and 80% at lower conversion

Kinetics and Mechanism of H₂O₂ Direct Synthesis over a Pd/C Catalyst in a Batch Reactor

Kinetic experiments of the decomposition, hydrogenation, and direct synthesis of hydrogen peroxide were performed on a commercial Pd/C catalyst. Temperature effects and subsequent hydrogen addition were investigated without using promoters. The hydrogen amount in the liquid phase was measured online by using a Fugatron Instrument to investigate the effect of the gas on the direct synthesis. Decomposition and hydrogenation reactions were affected differently by the temperatures used during the experiments. The formation of hydrogen peroxide showed different behaviors with different hydrogen feeding policies. The hydrogen dissolved in the liquid phase measured experimentally was correlated with the hydrogen peroxide production. As the amount of dissolved hydrogen increases in the liquid phase the direct synthesis rate increases, while the reaction slows down as the hydrogen pressure is decreased. The selectivity is also affected by the H₂ recharges. Every time that hydrogen is recharged in the reactor (during the direct synthesis) the selectivity toward H₂O₂ increases. Two different methods to recharge H₂ during the reaction were analyzed. The first method consists in feeding the hydrogen when it is totally consumed, the second one in refilling hydrogen in the reactor before its total consumption. The hydrogen solubility was found as an important parameter for the direct synthesis. An explanation on hydrogen peroxide formation was given taking into account the H₂/Pd ratio