eAQ Hydrogenation Research Articles

Ce-doped SBA-15 supported Pd catalyst for efficient hydrogenation of 2-ethyl-anthraquinone

Hydrogenation of 2-ethyl-anthraquinone (EAQ) is a key reaction in the anthraquinone process for industrial production of H2O2, a green chemical with a variety of applications. Herein, Ce-doped SBA-15 was synthesized via a one-step hydrothermal method with ethylenediamine as pH regulator and explored as the support of Pd catalyst for EAQ hydrogenation. Part of Ce species was incorporated into SBA-15 framework and the rest existed as extra-framework Ce and fluorite phase CeO2 particles. The framework Ce species had a suppressing effect on the aggregation of Pd particles. Their amount increased with lowering Si/Ce molar ratio (SCA) from 50 to 20 and reached a plateau at 30. The content of Ce3+ and the number of oxygen vacancies showed the same trend as the amount of framework Ce with SCA, but monotonously increased with raising reduction temperature from 200 to 600 °C. In addition, hydrogen spillover from metal to support and an increase in the binding energy of Pd were observed. Compared with Pd/SBA-15, Ce-doped catalysts presented significantly improved catalytic performance. Among them, the catalyst with SCA of 30 and reduced at 400 °C exhibited the highest activity (3.8 times of Pd/SBA-15) and selectivity to 2-ethyl-anthrahydroquinone (94.2 % from 91.8 % over Pd/SBA-15).

Pd/NiO/Al Array Catalyst for 2-Ethylanthraquinone Hydrogenation: Synergistic Effect Between Pd and NiO/Al Support

Manipulating the surface acidic/basic property and pore structure of support are two effective approaches to increase catalytic performance of Pd-based catalyst in anthraquinone (eAQ) hydrogenation. Herein, to combine two promoting approaches, array-typed NiO/Al supported Pd catalyst were synthesized. By regulating preparation method, three Ni(OH)2/Al support precursors showed different morphologies of nest-like, face-to-face packed and dandelion-like structure, respectively. After loading Pd, three Pd/NiO/Al catalysts exhibited different catalytic performance in eAQ hydrogenation, among which the nest-like catalyst possessed the highest H2O2 space time yield of 107.5 g gPd−1 h−1 with > 99% selectivity to active anthraquinone. Detailed characterizations were performed to investigate the pore structure, basic property and electronic structure caused by different morphologies of catalysts, to explain the structure-performance relationship. Specifically, on the basis of ensuring effective collision of reactant molecules, the outer opening pores (20–100 nm) could decrease diffusion barriers of eAQ/eAQH2, which improves active site accessibility for eAQ and benefits desorption of eAQH2. In addition, suitable amount of weak basic sites Ni2+–OH with high electronic density appropriately improves surface electronic density of Pd NPs, which moderately enhances H2 activation/dissociation but could not lead to over hydrogenation to give deeply hydrogenated byproducts.

Hydrogenation of 2-ethylanthraquinone with monolithic catalysts: An experimental and modeling study

A series of Pd/zeolite/cordierite (zeolite=MCM-41, SBA-15, Beta, or MCM-22; cordierite=COR) monolithic catalysts, as well as Pd/ZIF-8/COR, Pd/Al2O3/COR, and Pd/SiO2/COR, were prepared, characterized, and evaluated for the hydrogenation of 2-ethylanthraquinone (eAQ) to 2-ethyl-9,10-anthrahydroquinone (eAQH2). We found that a 0.8% Pd/MCM-41/COR catalyst exhibited the highest H2O2 yield (7.54gL−1) and selectivity (85.3%) toward active quinones for hydrogen peroxide (H2O2) production using the anthraquinone method. Notably, the Pd efficiency (1573gH2O2g−1Pdh−1) for the 0.8% Pd/MCM-41/COR monolithic catalyst was much higher than that of a commercial pellet catalyst (500gH2O2g−1Pdh−1). The intrinsic kinetics of hydrogenation of eAQ over 0.8% Pd/MCM-41/COR was measured, and the kinetic equation parameters were incorporated into a computational fluid dynamics (CFD) model. The mass transfer coefficients for the monolithic catalysts are 5–20 times higher than the pelleted catalyst. In addition, we found that the gas-liquid mass transfer is the controlling step, showing the unique advantages of monolithic catalysts for process intensification.

Synergistic effects of second metals on performance of (Co, Ag, Cu)-doped Pd/Al2O3 catalysts for 2-ethyl-anthraquinone hydrogenation

Hydrogenation of 2-ethyl-anthraquinone (EAQ) has been studied over Al2O3-supported monometallic Pd and bimetallic Pd-Co, Pd-Ag, and Pd-Cu catalysts prepared by incipient wetness co-impregnation method. The catalytic performance of the bimetallic catalysts could be greatly affected by the reduction temperature and mass ratio of the second metal (M=Co, Ag, Cu) to Pd (M/Pd). The highest catalytic activity, along with the high selectivity is achieved over Pd-Co catalysts having M/Pd=0.25 reduced at a low temperature of 80°C, with an increase in an order of magnitude in the reaction rate compared to the monometallic Pd catalyst. This could be ascribed to the synergistic effects of the Pd-Mα+ structure formed at low reduction temperature, which is confirmed by the results of H2-TPR, FTIR of CO adsorbed, STEM-EDX and XPS characterizations. The experimental data were satisfactorily fitted to a Langmuir-Hinshelwood (L-H) model involving a surface reaction controlling mechanism for EAQ hydrogenation. The results revealed that the surface reaction rate constant remarkably increased over bimetallic Pd-Co and Pd-Ag catalysts, implying that their superior activity is due to the more facile activation of carbonyl bond of EAQ on Pd-Mα+ sites than on Pd sites.

Continuously photocatalytic production of H2O2 with high concentrations using 2-ethylanthraquinone as photocatalyst

We report a photocatalytic process for H2O2 synthesis by using 2-ethylanthraquinone (EAQ) as photocatalyst. H2EAQ (or HEAQ) was in-situ produced by photocatalytic hydrogenation of EAQ with various solvents as hydrogen-donors (H-donors), and thus the pre-production of power-intensive H2 was circumvented. The industrial hydrogenation of EAQ and the oxidation of H2EAQ with O2 here were combined into one pot for H2O2 photocatalytic production. EAQ molecule can be repeatedly used with high TON under both UV and visible light irradiation. When dual-phase reaction was adopted, the maximal H2O2 concentration of 574.0mM in aqueous phase was achieved with mesitylene as H-donor. As the produced H2O2 concentration in the industrial AO process is estimated as ca. 400mM in one cycle, our method with the dual-phase reaction greatly promotes the possibility for the practical production of H2O2 with photocatalytic method.

The role of MgO in the performance of Pd/SiO2/cordierite monolith catalyst for the hydrogenation of 2-ethyl-anthraquinone

Highly dispersed Pd/SiO2/cordierite monolith catalysts (PSC) were prepared to investigate the influence of support alkalinity on 2-ethyl-anthraquinone (eAQ) hydrogenation. The support alkalinity was adjusted by the MgO content in SiO2 washcoat. The behavior of the PSC catalysts was tested by liquid phase hydrogenation of eAQ in a continuous trickle bed reactor. PSC catalyst with 2wt.% MgO (Mg-2 sample) exhibited excellent for hydrogenation of eAQ. The support alkalinity directly influenced the electronic properties of the Pd nanoparticles as proved by Fourier transformed infrared spectroscopy with CO probe molecule and X-ray photoelectron spectroscopy. The electron density at lower binding energy of the supported Pd particles increased with the increasing alkalinity. PSC catalysts with alkali modifiers exhibited higher hydrogenation efficiency for benefiting the activation of CO group in eAQ and the adsorption of eAQ via the enhanced interaction of lone pair of CO electrons. However, increased support alkalinity contributed to higher Pd loss due to the weak coating strength. Besides, the active quinone selectivity decreased with the progressive MgO addition due to the tautomerization of 2-ethylanthrahydroquinone to 2-ethyloxoanthrone, which is the precursor of other degradations. Consequently, a volcano shape curve between hydrogenation efficiency (H2O2 yield) and MgO content was obtained.

Controllable preparation and catalytic performance of Pd/anodic alumina oxide@Al catalyst for hydrogenation of ethylanthraquinone

Anodic alumina oxide (AAO) supports with different pore sizes and depths were controllable prepared by a two-step low-potential anodizing process. The prepared AAO supports with θ-ring shape were utilized to prepare supported Pd catalysts by impregnation method for the first time. The catalytic performance of the Pd/AAO@Al catalysts were investigated in the catalytic hydrogenation of 2-ethylanthraquinone (eAQ) to 2-ethylanthrahydroquinone, a key step in the anthraquinone process for the manufacture of hydrogen peroxide. Compared with Pd supported on spherical Al2O3 catalyst, Pd/AAO@Al catalysts showed much higher anthraquinone hydrogenation activity and preferable selectivity due to the highly ordered array of cylindrical shaped pores, short diffusion distance and confinement effect of the AAO supports. Furthermore, the effect of pore size and depth to the catalytic performance towards hydrogenation of eAQ has also been studied. Pd nanoparticles supported on AAO support with 24.5nm pore size and 18μm depth exhibited the best catalytic performance towards hydrogenation of eAQ.

The role of alkali modifiers (Li, Na, K, Cs) in activity of 2%Pd/Al 2O 3 catalysts for 2-ethyl-9,10-anthraquione hydrogenation

Present research concentrates on the role of alkali modifiers (Li, Na, K, Cs) in activity of 2%Pd/Al 2O 3 catalyst for 2-ethyl-9,10-anthraquinone (eAQ) hydrogenation. The catalysts with various content of alkali modifier (Me/Pd molar ratio ranges from 0.5 up to 4, Me-alkali metal) were prepared by impregnation of pre-reduced 2%Pd/Al 2O 3 catalyst with appropriate alkali metal carbonates. The XPS, EDS and TEM measurements show that alkali promoters are introduced into alumina matrix. The microcalorimertic experiments of CO adsorption prove the interaction of CO with catalysts leading to stronger bonding of carbon monoxide by alkali doped catalysts. The presence of alkali promoters in Pd/Al 2O 3 catalyst plays an essential role in the whole eAQ hydrogenation process. The nature of alkali promoter and its content (Me/Pd atomic ratio) in catalyst are of importance. As the alkalinity of promoter increases going from Li to Cs all the effects caused by their presence become stronger. In the presence of alkali doped catalysts the content of 2-ethyloxoanthrone (OXO, isomer of 2-ethyl-9,10-anthrahydroquinone) formed is higher than that on un-doped 2%Pd/Al 2O 3. On the other hand, reactions in the “deep hydrogenation” stage comprising the formation of 2-ethyl-5,6,7,8-tetrahydro-9,10-anthraquinone (H 4eAQ) and the transformation of OXO to 2-ethylanthrone and other degradation products are remarkably inhibited. In particular, the formation of 2-ethylanthrone via hydrogenolysis of OXO isomer is strongly suppressed. The Cs-doped catalyst exhibits the highest activity to OXO among all the catalysts tested whereas the ability of Cs-doped catalysts to the formation of anthrone is most effectively inhibited. The role of alkali modifiers is considered to be associated with stronger interactions between the catalyst and quinone reagents, and in particular OXO isomer. Moreover, in the reagent adsorption the centres of support nearby the palladium particles may also participate by affecting the mode of reagents adsorption.

Hydrogenation of 2-ethyl-9,10-anthraquinone on Pd/SiO 2 catalysts : The role of humidity in the transformation of hydroquinone form

The role of humidity in the hydrogenation of 2-ethyl-9,10-anthraquinone (eAQ) and especially in the further transformation of 2-ethylanthracen-9,10-diol (eAQH 2) primary and desirable product formed on eAQ hydrogenation was studied. Three 0.5% Pd/SiO 2 catalysts differing in the content of Na 2CO 3 and thus in alkaline properties were used. They were obtained by successive removing of Na 2CO 3 from the catalyst prepared by precipitation of palladium hydroxide onto Na 2CO 3 pre-impregnated SiO 2. Hydrogenation experiments were carried out in “standard” and subsequently in “wet” systems at 62 °C, atmospheric pressure of H 2 in mixture of 2-octanol–xylene (1:1) as the solvent. Humidity influences the whole hydrogenation process, fundamental reaction, the hydrogenation of eAQ to eAQH 2 (quinone–hydroquinone stage) and further transformation of eAQH 2. A complicated effect of humidity is observed. Humidity has an effect on consecutive reactions transforming eAQH 2 via OXO-tautomer to intermediate product 2-ethyl-9,10-dihydro-9,10-dihydroxyanthracene (INT), precursor of 2-ethylanthrone (eAN). The influence of humidity varies depending on the content of Na 2CO 3 in catalysts and thus their alkaline properties. The observed effects are correlated with adsorption configuration of reagents and especially eAQH 2 and INT. Highly alkaline medium as well as humidity facilitated adsorption in the carbonyl group—bonded configuration thus enhancing the transformation of eAQH 2 by hydrogenolytic reactions and especially the formation of eAN.

Pd/polyaniline(SiO 2) a novel catalyst for the hydrogenation of 2-ethylanthraquinone

A novel palladium catalyst supported on silica grains coated with polyaniline (PANI) (24.4 wt% of polymer) was used for the hydrogenation of 2-ethylanthraquinone (eAQ). This 2%Pd/PANI(SiO 2) catalyst exhibited much better selectivity in the hydrogenation of eAQ to active quinones than that of 2%Pd/SiO 2 prepared by a conventional precipitation method. It is suggested that the modification of properties of Pd centres by PANI matrix, has an effect on their reactivity. The weakening of the strength of hydrogen bonding and hydrophobic character of polymer both can be considered as factors remarkably improving performance of Pd/PANI(SiO 2) catalyst.

Physicochemical and catalytic properties of palladium supported on poly( o-methoxyaniline)

Palladium was introduced into a conjugated polymer poly( o-methoxyaniline) (POM) by reacting the powdered polymer with aqueous solution of PdCl 2 of low acidity (PdCl 2: 2.3 × 10 −3 mol/dm 3, HCl: 0.66 × 10 −3 mol/dm 3). Various Pd 2+ complexes with Cl −, H 2O, OH − ligands coexisted in this solution but predominated [PdCl 2(H 2O) 2] ones. Several techniques like X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, extended X-ray absorption fine structure have been used to characterize the poly( o-methoxyaniline)–Pd systems. In particular, the state of Pd species in the Pd/POM of various content of palladium (2–8 wt.% Pd) and chemical changes in the polymer matrix induced by insertion of palladium were studied. The protonation and redox reactions involved on palladium incorporation resulted in palladium ions and Pd metal in the final samples. Metallic Pd produced due to spontaneous reduction of palladium ions by the polymer formed large crystalline particles 200–1000 nm in size. The Pd 2+ species in the form of anionic complexes like [PdCl 4] 2− acted as the counterions at low content of palladium (2–4 wt.% Pd). At high palladium content (8 wt.% Pd), several atoms like Cl, N and/or O were identified by extended X-ray absorption fine structure (EXAFS) technique in the nearest environment of Pd atoms. The structural groups of POM (like N groups and/or OCH 3) as well as H 2O, OH − molecules are, therefore, considered as probable species in the coordination sphere of palladium. The catalytic properties were studied for the as-prepared Pd/POM and the samples additionally reduced with aqueous solution of NaH 2PO 2. They were used in the hydrogenation of C C bonds in maleic acid (MAC) and C O groups in 2-ethylanthraquinone (eAQ) at 60 °C and atmospheric pressure of hydrogen using xylene–octanol-2 or water medium. The correlation between Pd/POM activities and the content of Pd metal was found. Activation of the as-prepared Pd/POM with NaH 2PO 2 improved their catalytic properties. Much higher and much stable activities were then obtained in both MAC and eAQ hydrogenation reactions.

2-Ethyl-9,10-anthraquinone hydrogenation over Pd/polymers: Effect of polymers–Pd(II) chlorocomplexes interactions

Two polymers, namely poly(4-vinylpyridine) (PVP) and polyaniline (PANI), were used as the supports for palladium catalysts acting in 2-ethyl-9,10-anthraquinone (EAQ) hydrogenation, a key step in the industrial production of H 2O 2. The nature of PVP and PANI interactions with various chlorocomplexes of Pd(II) coexisting in PdCl 2–H 2O–HCl solutions was studied using IR (mid-IR, far-IR), UV–Vis and XPS spectroscopies. It was found that the type of interactions involving nitrogen atoms of the polymers depended mainly on the acidity of PdCl 2 solution. Protonation of polymers (via acid–base reactions) as well as coordination of Pd 2+ ions by nitrogen atoms of the polymers took place in highly acidic PdCl 2 solution (2 M HCl) containing predominantly anionic [PdCl 4] 2−, [PdCl 3(H 2O)] − complexes (series A of experiments). In the weakly acidic PdCl 2 solutions (0.66×10 −3 M HCl ) (series B of experiments) containing predominantly electrically neutral [PdCl 2(H 2O) 2] complexes, hydrolysis of the complex proceeded as the main process resulting in precipitation of palladium oxide on PVP. In the case of PANI in solution B, the redox mechanism was involved resulting in the reduction of some Pd 2+ to Pd 0 accompanied by partial oxidation of the polymer chain. As a consequence of various mechanisms of polymers reactions with Pd 2+ ions, surface morphology of the final catalysts, characterized by XRD and SEM methods, was different. It was found that dispersion of palladium in Pd/PVP and Pd/PANI catalysts (1–10 wt.% Pd) influenced the course of EAQ hydrogenation. The presence of large palladium particles promoted reactions leading to the formation of the so-termed “degradation products” not capable of hydrogen peroxide formation. Pd/PVP catalysts (series B) exhibited higher activity. Selectivity of EAQ hydrogenation in their presence was better than that seen for Pd/PANI catalysts.

Liquid-Phase Hydrogenation of 2-Ethylanthraquinone over Pd/Polyaniline Catalysts

The conjugated polymer polyaniline (PANI) has been used as the support for palladium catalysts. It was found that the dispersion of Pd in Pd/PANI catalysts depends on the nature of Pd2+–PANI interactions proceeding upon catalyst preparation. The type of interaction (protonation or oxidation–reduction process) was influenced mainly by the acidity of the precursor solution containing PdCl2–H2O–HCl. Under various conditions of acidity, two series of catalysts, A and B (0.8–10 wt% Pd), were prepared. In catalysts of series A, prepared using a strongly acidic solution (2 M HCl), the dispersion of Pd was high. On the other hand, in catalysts of series B, prepared using a weakly acidic PdCl2 solution (0.66×10−3 M HCl), large Pd particles (100–600 nm) were present. They were formed as the result of an oxidation–reduction process proceeding with the participation of PANI chain. The physicochemical properties of catalyst were characterized by XRD and SEM methods. All catalysts exhibited activity in 2-ethylanthraquinone (eAQ) liquid-phase hydrogenation. The catalysts of series A were more active and the selectivity of eAQ hydrogenation in their presence was higher than that seen for catalysts of series B.