Acid Site Density Research Articles

On the Chemistry of Iron Oxide Supported on γ-Alumina and Silica Catalysts.

Catalysts of iron oxide on γ-alumina and silica which were prepared by an incipient wetness impregnation technique have been investigated in an effort to understand how the surface chemical properties are influenced by the nature of the supports. Surprisingly, this is the first study to compare in depth the influence of the supports on physicochemical parameters such as acidity, site nuclearity, and reducibility. In this study, surface characterisation techniques including N2 physisorption at −196 °C, ammonia temperature-programmed desorption, inductively coupled plasma optical emission spectrometry, temperature-programmed reduction with hydrogen, CO-chemisorption, scanning electron microscopy, transmission electron microscopy, and NO adsorption by in situ Fourier transform infrared spectroscopy have been performed to understand the different surface reactions occurring over the two different supports. The aim of this study is to ascertain the primary differences between these two catalysts using several catalyst characterization techniques and correlate their chemical and structural differences to their catalytic activity in the conversion of 2-chlorophenol. The results disclose a higher density of acid sites, a smaller particle size of iron oxide, stabilization of Fe(II) aluminate after reduction on the alumina surface, and finally, the formation of isolated iron cations on the surface of alumina which are notably absent on the silica-supported catalyst.

Modified sulfonation method for converting carbonized glucose into solid acid catalyst for the esterification of palm fatty acid distillate

The sulfonation variables for the preparation of a sulfonated incomplete carbonized glucose solid acid catalyst were optimized using response surface methodology (RSM) with central composite rotatable design for the conversion of palm fatty acid distillate (PFAD) to fatty acid methyl esters (FAME). Effects of four sulfonation variables namely time (1–9 h), (NH4)2SO4 concentration (2–18%), concentrated H2SO4 (15–35 ml) and temperature (130–170 °C) were investigated. The optimum sulfonation conditions generated numerically by the RSM-CCRD were 5.34 h of time, 11.25% of (NH4)2SO4 concentration, 25.16 ml of concentrated H2SO4 and 151.15 °C of temperature which gave 93.30% FFA conversion and yield of 91.87% FAME. The optimum sulfonated catalyst was characterized using X-ray diffraction (XRD), fourier transform infrared (FTIR), thermogravimetric analyser (TGA), Brunauer Emmett Teller (BET), temperature programmed ammonia desorption (TPD-NH3), and field emission scanning electron microscope (FESEM) to establish its properties. The optimized sulfonated catalyst was further used for esterification to optimize the process conditions. The optimized catalyst had a BET surface of 4.47 m2/g and acid site density (Brønsted) of 5.92 mmol/g exhibited high catalytic activity as a result of efficient accessibility of PFAD and methanol to the sulfonic sites during the esterification reaction. The optimization of the esterification process conditions achieved 93.23% FFA conversion at optimum process conditions of 4 wt% of optimized sulfonated catalyst, 10:1 methanol to PFAD molar ratio, 65 °C temperature and 4 h of reaction time. The synthesized catalyst showed it could be reused up to five cycles maintaining a high catalytic activity. Therefore, the catalyst has proved to be a potential heterogeneous candidate for the conversion of high acid value feedstock.

Production of Biodiesel by Esterification of Free Fatty Acid over Solid Catalyst from Biomass Waste

Recently, low cost feedstocks have been utilized to replace vegetable oils in order to improve the economic feasibility of biodiesel. The esterification of free fatty acid (FFA) on Palm Fatty Acid Distillate (PFAD) with methanol using solid catalyst generated from bagasse fly ash is a promising method to convert FFA into biodiesel. In this research, the esterification of FFA on PFAD using the sulfonated bagasse fly ash catalyst was studied. The performances of the catalysts were evaluated in terms of the reaction temperatures, the molar ratios of methanol to PFAD, and the catalyst loading. The effects of the mass ratio of catalyst to oil (1-10%), the molar ratio of methanol to oil (6:1-12:1), and the reaction temperature (40-60°C) were studied for the conversion of PFAD to optimize the reaction conditions. The results showed that the optimum conditions were methanol to PFAD molar ratio of 12:1, the amount of catalyst of 10%wt. of PFAD, and reaction temperature of 6°C. The reusability of the solid acid carbon catalysts was also studied in this work. The catalytic activity decreased up to 38% after third cycle. The significant decline in catalyst esterification activity was due to acid site leaching. The physico-characteristics and acid site densities were analyzed by Nitrogen gas adsorption, surface functional groups by Fourier transform infrared spectroscopy (FT-IR), elemental analysis using X-ray fluorescent (XRF), and acid-base back titration methods for determination of acid density.

Hydrogen Production Through Steam Reforming of Toluene Over Ni Supported on MgAl Mixed Oxides Derived from Hydrotalcite-Like Compounds

This work studied the performance of mixed oxides derived from hydrotalcite-like compounds with different Al/(Ni + Mg + Al) molar ratios for the steam reforming of toluene. All catalysts deactivated, except for Ni/MgO. The amount of carbon formed on the catalysts depended on the Ni particle size and density of acid sites.

Comparative study of CHA- and AEI-type zeolytic catalysts for the conversion of chloromethane into light olefins

Three pairs of CHA- and AEI-type zeolytic materials with similar crystallite size but with different framework atoms, i.e., silicoaluminophosphate SAPO-34 and SAPO-18, aluminosilicate SSZ-13 and SSZ-39, and titaniumin-corporated TiAPSO-34 and TiAPO-18, were prepared and their catalytic activities of chloromethane into light olefins were compared according to the structure types, as well as acidic properties. The AEI-type catalysts were found to have lower ethene/propene ratios, indicating relatively higher propene selectivity, because their aei-cage was larger compared to the cha-cage. However, all the CHA-type catalysts exhibited better activity and higher selectivity to light olefins. Both H-SAPO-34 and H-TiAPSO-34 showed good catalytic stability over all the reaction times studied here. When compared to H-SAPO-34, however, H-TiAPSO-34 exhibited ca. 20mol% higher selectivity to ethene and propene, despite a similar total density of acid site. This performance may be due to the higher strength of medium acid sites.

Continuous esterification of oleic acid to ethyl oleate under sub/supercritical conditions over γ-Al2O3

Esterification of oleic acid with ethanol was conducted under subcritical and supercritical conditions in a packed-bed reactor containing γ-Al2O3. The presence of γ-Al2O3 significantly improved the reaction rate such that the 42% yield achieved at 325 °C, 200 bar, and 1-min residence time without the alumina was increased to 98% at the same conditions when alumina was present. The catalytic capacity was attributed to Lewis acid sites on the surface of alumina, and non Brønsted acid sites were detected. Experiments to study the kinetics were executed at a pressure of 200 bar, elevated temperatures (200, 225, 275, 300, and 325 °C), and residence times of half to 8 minutes. Mass transfer limitations were estimated to be negligible via the Mears and Weisz-Prater criteria. Kinetic analysis based on the one-step model demonstrates that the overall reaction was endothermic, and an Eley-Rideal (ER) reaction mechanism was proposed to describe each elementary reaction step. The stability of γ-Al2O3 on product conversion was tested via a 25 h operation under 325 °C, 200 bar, and 1 min residence time, and decrease of the conversion was not observed. However, results of the catalyst analytical characterization shows a decrease of the acid site density and surface area, supporting the occurrence of catalyst degradation. The addition of water slightly decreased the yield, while the pressure change from 200 to 100 bar did not have an obvious effect on the conversion.

Cyclohexene to adipic acid synthesis using heterogeneous polyoxometalate catalysts

Polyoxometalates prepared with W or Mo precursors, have been used together with H2O2 as homogeneous and heterogeneous catalysts for the adipic acid synthesis from cyclohexene. In this context, the K3PMo12O40, Cs3PMo12O40, K3PW12O40 and Cs3PW12O40 heteropolysalts were synthesized and calcined at 600 °C, characterized by XRD, acid-base titration, Raman Spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, SEM-EDX and evaluated in the cyclohexene oxidation to adipic acid. The XRD and FT-IR analyzes showed that the polyoxometalates structures were stable even after calcination at 600 °C. The thermogravimetric results showed that the heteropolysalts present superior thermal resistance compared to heteropolyacids. However, the acidities of the heteropolysalts are lower than the heteropolyacids. The K3PW12O40 catalyst presented the best performance in the catalytic tests (77% yield in adipic acid) associated with its higher total acid sites density. The catalytic performance of this recycled catalyst was evaluated and the yield in adipic acid was maintained.

Tuning the material and catalytic properties of SUZ-4 zeolites for the conversion of methanol or methane

This work describes the synthesis and investigation of improved SUZ-4 catalysts (SZR framework) and their application in methanol-to-olefins and methane-to-methanol conversion. We report boron containing, dealuminated and desilicated SUZ-4 catalysts. Ammonia and pyridine probe molecules enabled the assignment of acid site densities to the 10-MR and 8-MR pores, respectively. The difference in between the theoretically expected and the for the probes accessible acid site density is a good descriptor for the accessibility of pores and acid sites. The access to the 8-MR SUZ-4 pores is sterically hindered by counter ions as well as extra framework aluminum (EFAl) species that account for up to 15% of the aluminum. Dealumination and desilication were successfully applied to optimize the diffusion properties and resulted in outstandingly high BET surface area, nSi/nAl-ratios and potassium exchange degrees. The improved accessibility led to an up to 2-fold increased MTO conversion capacity of the catalysts. Cu-exchanged SUZ-4 was applied in the selective oxidation of methane to methanol. By operando X-ray absorption spectroscopy (XAS) we showed that CuII ions are mainly coordinated in 2Al sites of 6-MR motifs and inhibited for redox-reactions.

Alkylation desulfurization of FCC gasoline catalyzed by glycerol-based solid acid catalyst

ABSTRACTA novel high acid site density carbon-based solid acid catalyst was prepared and first applied in the FCC gasoline alkylation desulfurization. The catalyst was synthesized by a facile approach and characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), Brunauer–Emmett–Teller analysis (BET) and Gas Chromatography-Mass Spectrometer (GC-MS). The desulfurization performance of this solid acid catalyst was investigated and optimized.

Application of Ti-doped MoO2 microspheres prepared by spray pyrolysis to partial oxidation of n-dodecane

The present investigation is focused on improving the performance of molybdenum dioxide (MoO2) by doping with Ti for the partial oxidation (POX) of n-dodecane. Ti-doped MoO2 nanoparticles were synthesized via solvothermal cracking of polycrystalline MoO3 microparticles prepared by ultrasonic spray pyrolysis in the presence of a Ti precursor. Partial oxidation of n-dodecane was conducted at 850 °C with an O2/C ratio of 0.5. The 6 at% Ti-doped MoO2 was fully converted into orthorhombic carbide phase (β-Mo2C) during the reaction. This carbide sample showed high catalytic activity and stability with conversion and H2 yield of 94.4% and 86.3% after 24 h on stream, respectively. On the other hand, un-doped MoO2 was partially converted into the carbide phase during the reaction, which led to mixed oxide and carbide phases. This mixed phase showed poor catalytic activity and rapid deactivation after only 6 h of operation. Our ammonia temperature programmed desorption (TPD) and pyridine diffuse reflectance infrared Fourier transform (DRIFT) tests suggest that the addition of Ti to MoO2 improves both the density and strength of Lewis acid sites, and hence improves hydrocarbon activation. This increased surface carbon activation would enhance the carburization process of the Ti-doped MoO2 catalyst and retain the carbide phase under the POX condition.

Alkali metal ion exchanged ZSM-5 catalysts: on acidity and methanol-to-olefin performance

The acid site density and counter ion nature impact the proportion of alkene- and aromatic-based MTO reaction cycles.

ZSM-5 synthesis by the assistance of biomass and biomass-derivate compounds

ZSM-5 zeolites with tailored properties were successfully synthesized in the presence of biomass-derivative compounds following conventional hydrothermal route. The strategy consisted of introducing, along with silica and alumina precursors, either sugar cane bagasse itself or biomass-derivate compounds obtained by hydrolysis of the sugar cane. As-prepared ZSM-5 catalysts were thoroughly characterized by XRD, XRF, SEM, nitrogen sorption and H/D isotope exchange techniques. The catalysts were evaluated both in the cracking of n-hexane and the methanol conversion into hydrocarbons. Our approach yielded solely ZSM-5 phase with broad differences in their intrinsic properties: external surface area (between 51 and 153 m2 g−1); 8 < Si/Al < 29, thus leading up to high Brønsted acid sites density; crystal size (from 0.3 to 13 μm) and peculiar morphologies. The high Al-content ZSM-5 led to achieve the higher n-hexane cracking activity. The ZSM-5 with the highest external area and higher Si/Al (SAR) converted methanol mainly into light olefins, whilst, the ZSM-5 material with the lower external area and lower SAR yielded gasoline formation.

Co-Cluster-Based Metal-Organic Frameworks as Selective Catalysts for Benzene Tandem Acylation-Nazarov Cyclization to Benzocyclopentanone.

One-step selective benzene acylation-Nazarov cyclization is an attractive, yet challenging method for the synthesis of the benzocyclopentanone skeleton, which is key intermediate of many natural products. Herein, two metal-cluster-based metal-organic frameworks (MCOFs) {(H3 O)2 [Co4 (pbcd)2 (μ3 -OH)2 ]⋅CH3 CH2 OH⋅4 H2 O}n (1; pbcd=9,9'-(propan-1,3-diyl)bis(9H-carbazole-3,6-dicarboxylic acid) and {[Co5.25 (mcd)2 (HCO2 )(μ2 -O)0.5 (μ3 -OH)0.5 (H2 O)4 ]⋅6 H2 O⋅5 DMF}n (2; mcd=9,9'-methylenebis(9H-carbazole-3,6-dicarboxylic acid) were developed to catalyze a tandem Nazarov cyclization reaction of 1,3-dimethoxybenzene with α,β-unsaturated carboxylic acids for the synthesis of cyclopentenone[b]benzenes. MCOFs 1 and 2, which were constructed from tetranuclear CoII cluster [Co4 (μ3 -OH)2 ] and hexanuclear CoII cluster [Co6 (HCO2 )(μ2 -O)(μ3 -OH)], respectively, exhibit high catalytic activity arising not only from their suitable channel size and accessible catalytic sites, but also because of their high density of Lewis acidic sites within the frameworks and the synergic activity among CoII ions. In contrast, {[Co2 (pbcd)(bpe)]⋅2 H2 O⋅2 DMF}n (3; bpe=1,2-bis(pyridin-4-yl)ethane) containing binuclear CoII and having large pore windows is a highly selective catalyst for obtaining exclusively the acylation products. Easy product separation, simple reaction procedures, and catalyst recycling make the catalyst system attractive. This work highlights the synergistic effect among ions of MCOFs in interacting with substrates in a sequential or cooperative manner to achieve tandem catalysis.

Quantification of acid site densities on zeolites in the presence of solvents via determination of extinction coefficients of adsorbed pyridine

Fourier transform infrared (FTIR) spectroscopy is frequently used to characterize properties of catalytic sites, identify reaction intermediates, and probe interactions between adsorbates and surfaces, making it a versatile tool for elucidating reaction mechanisms in heterogeneous catalysis. While this technique is typically regarded as qualitative or semiquantitative in nature, quantitative results can be obtained by determining integrated molar extinction coefficients (IMECs), or the amount of signal per mole of adsorbed or bulk species. IMECs for vibrational modes corresponding to pyridine adsorbed onto Brønsted and Lewis acid sites have been determined in the vapor phase; however, the effect of solvents on catalytic sites cannot be captured. Liquid-phase IMECs enable quantification of adsorbates in the presence of solvents, allowing quantitative evaluations of the effect of solvents on catalysts. A method is outlined for determining IMECs of pyridine adsorbed on zeolites in the liquid phase in an attenuated total reflection (ATR) configuration. The method can also be applied to substrates other than pyridine. The IMECs for pyridine adsorbed onto Brønsted and Lewis acid sites in water, acetonitrile, and ethanol are determined. The IMEC values are shown to be dependent on solvent choice, but independent of zeolite framework.

Facile Formation of Lactic Acid from a Triose Sugar in Water over Niobium Oxide with a Deformed Orthorhombic Phase

Hydrothermal treatment of NH4[NbO(C2O4)2(H2O)2]·nH2O in water at 448 K for 3 days produced crystalline Nb2O5 with a deformed orthorhombic structure and a high surface area (208 m2 g–1). Fourier-transform infrared spectroscopy measurements of pyridine adsorption revealed that the Nb2O5 catalyst has both high densities of Bronsted and Lewis acid sites that can work in the presence of water. One feature of the Nb2O5 catalyst is its high density of water-compatible Lewis acid sites (0.21 mmol g–1), which is much larger than that of Nb2O5·nH2O (0.03 mmol g–1). The Nb2O5 catalyst was studied as a solid acid catalyst for the formation of lactic acid from 1,3-dihydroxyacetone and pyruvaldehyde in water at 373 K, and was determined to be a highly active and selective catalyst, compared with typical acid catalysts (H2SO4, Sc(OTf)3, and Nb2O5·nH2O). A high Lewis acid density with moderate acid strength is a crucial factor for the high catalytic performance exhibited for the former reaction. High densities of both Br...

Sustainable Production of 2,3-Pentanedione: Catalytic Performance of Ba2P2O7 Doped with Cs for Vapor-Phase Condensation of Lactic Acid

Sustainable production of 2,3-pentanedione from lactic acid over Ba2P2O7 doped with Cs was investigated. Varying Cs components and calcination temperature can efficiently tune the acid–base properties of catalysts. It was found that both base site density and acid site density had a marked impact on catalytic performance for condensation of lactic acid to 2,3-pentanedione. Enhanced base site density and acid site density favor the catalytic activity. Furthermore, base/acid molar ratio has also played an important role on catalytic performance. From the observations based on the effect of Cs doped amount and calcination temperature, an appropriate base/acid molar ratio is around 8. Low reaction temperature as well as low LA LHSV favors selective formation of 2,3-pentanedione from lactic acid over the catalyst of 1.0CsNO3/2.5Ba2P2O7 calcined at 600 °C. Encouragingly, the condensation reaction of lactic acid proceeds efficiently at around 28 h on stream, and 2,3-pentanedione selectivity remains around 60%.

Tuning solid catalysts to control regioselectivity in cross aldol condensations with unsymmetrical ketones for biomass conversion

Several families of catalysts characterized by different types of acid and base sites were developed to steer regioselectivity in the aldol condensation of unsymmetrical ketones. The catalysts were propyl sulfonic-acid (PrSO3H) functionalized SBA-15 with various acid loadings and with or without hydrophobization by octyl groups (Oc); isomorphously substituted BEA zeotypes; and sulfated zirconias with various sulfate loadings. Benzaldehyde and 2-butanone served as test reactants at a temperature of 140°C. Sulfonic acid-functionalized catalysts preferentially formed branched condensation product and metal-substituted BEA zeotypes selectively formed linear condensation product. Maximum condensation product yields were 65% branched at 98% conversion with the highest site density Oc/PrSO3H/SBA-15 and 76% linear at 94% conversion with Sn-BEA. Sulfated zirconia catalysts were less selective. Product distributions analyzed at different conversions revealed how kinetics and equilibria of addition and dehydration govern regioselectivity. Further observations were: 1. Increasing the acid site density significantly increased the turnover frequency for PrSO3H/SBA-15, suggesting a cooperative effect between adjacent acid sites. 2. Brønsted acid sites catalyzed an undesired side reaction, the cleavage of the branched addition product to β-methyl styrene and acetic acid. This reaction was suppressed by hydrophobizing the surface through octyl co-functionalization. 3. Deactivation upon re-use was generally slight, and Sn-BEA could be fully regenerated by calcination.

Late-Transition-Metal-Modified β-Mo2C Catalysts for Enhanced Hydrogenation during Guaiacol Deoxygenation

Molybdenum carbide has been identified as a promising bifunctional catalyst in the deoxygenation of a variety of pyrolysis vapor model compounds. Although high deoxygenation activity has been demonstrated, complementary hydrogenation activity has been limited, especially for lignin-derived, aromatic model compounds. The ability to control the relative site densities of acidic and hydrogenation functionalities represents a catalyst design challenge for these materials with the goal to improve hydrogenation activity under ex situ catalytic fast pyrolysis (CFP) conditions. Here we demonstrate that the addition of Pt and Ni to β-Mo2C resulted in an increase in the H*-site density with only a minor decrease in the acid-site density. In contrast, the addition of Pd did not significantly alter the H*- or acid-site densities. High conversions (>94%) and high selectivities to 0-oxygen products (>80%) were observed in guaiacol deoxygenation under ex situ CFP conditions (350 °C and 0.44 MPa H2) for all catalysts. Pt...

Catalytic transformation of furfural into bio-based succinic acid via ultrasonic oxidation using β-cyclodextrin-SO3H carbon catalyst: A liquid biofuel candidate

The selective transformation of furfural (FF) into bio-based succinic acid (SA) promoted by co-existence of H2O2 and SO3H-CD-carbon catalyst was investigated using an ultrasonic system. This research was proposed on the novel catalytic system with the environmental friendly for SA production. A new SO3H-CD-carbon catalyst was synthesized via in-situ hydrothermal carbonization with sulfonation using β-cyclodextrin (CD) as a carbon source and hydroxyethylsulfonic acid as a sulfonating agent. The results showed that the synergistic effects of SO3H-CD-carbon catalyst and H2O2 on the FF oxidation had a significant importance for improving the SA yield. A highest SA yield of 81.2% was successfully achieved through the optimum conditions obtained from 2k factorial and Box-Behnken designs. The SO3H-CD-carbon catalyst also exhibited the better catalytic performance when compared with various commercial sulfonic acid catalysts such as Amberlyst-35, Nafion SAC-13 and SiO2-Tosic acid. Moreover, the spent catalyst was be easily regenerated recovering its initial density of acid sites which resulted in the well recyclability up to 13 cycles.

Catalytic dehydrofluorination of 1,1,1,3,3-pentafluoropropane to 1,3,3,3-tetrafluoropropene over fluorinated NiO/Cr 2 O 3 catalysts

Abstract Catalytic dehydrofluorination of 1,1,1,3,3-pentafluoropropane to 1,3,3,3-tetrafluoropropene was performed on a series of fluorinated NiO/Cr2O3 catalysts. The NiO/Cr2O3 catalysts were more active than the Cr2O3 because the new acid sites provided by NiF2 had higher turnover frequencies (9.43 × 10−3 − 12.08 × 10−3 s−1) than those on the Cr2O3 (4.55 × 10−3 s−1). Also, the NiO/Cr2O3 was more stable than the Cr2O3 due to its lower density of surface acid sites, which alleviated the coke deposition on the catalyst as evidenced by the Raman spectroscopic results. The kinetic results revealed that the15NiO/Cr2O3 had much lower activation energy (63.6 ± 4.5 kJ mol−1) than the Cr2O3 (127.6 ± 3.8 kJ mol−1). Accordingly, different reaction pathways on the two catalysts were proposed, which involved the cleavage of the C F and C H bonds on the surface acid and base sites, respectively.