Role Of Acid Sites Research Articles

Aldol condensation of biomass-derived aldehydes and ketones followed by hydrogenation over Ni/HZSM-5 to produce aviation fuel: Role of acid sites

As the aviation sector grows and fossil fuels are utilized, the environment and economy depend on biomass-based aviation fuel. This study investigated acid sites in aldol condensation to produce aviation fuel from biomass-derived aldehydes and ketones over the bifunctional catalyst Ni/HZSM-5. Due to its excellent 10-member ring channels structure, HZSM-5 yielded 63.81% of the targeted condensates as aviation fuel precursor. At 200 °C for 9 h, aldehydes and ketones were converted to 90.32% and 69.53% of the required condensates. Sodium ion-exchange HZSM-5 with different Brönsted acid site quantities was tested for condensation. Brönsted acid sites were the active center for aldol condensation of aldehydes and ketones, increasing product yield. In γ-Al2O3, the overall amount of Brönsted acid and Lewis acid was equivalent to that of HZSM-5, the amount of Lewis acid was much more. The yield of the targeted condensates over γ-Al2O3 was only 46.11%, much lower than that of HZSM-5 (69.53%). Thus, Brönsted acid sites were better than Lewis acid sites for aldol condensation of aldehydes and ketones. The balance of active sites for aldol condensation and hydrogenation increased the intended alkane yield to 62.91% with 20 wt% Ni loading. This study involves improving biomass-based aviation fuel generation.

Cascade hydrogenation of nitrobenzene to dicyclohexylamine with Pd/γ-Al2O3: The role of acid sites

Direct hydrogenation of nitrobenzene (NB) to dicyclohexylamine (DCHA) is a green but challenging process due to the competition of cross-coupling reaction (to DCHA) and hydrogenation reaction (to byproduct). Herein, we report that the cross-coupling reaction to desired DCHA is greatly enhanced by tuning the acidity of supported Pd catalysts. It was found that Pd NPs loading on commercial γ-Al2O3 (γ-Al2O3-Com) with moderate acidity were more selective than those on neutral SBA-15 or strongly acidic γ-Al2O3 nanorods. The presence of acid site could promote the coupling reaction. By analyzing the correlation between hydrogenation activity and DCHA selectivity, it was found that the matched rate of hydrogenation and cross-coupling is the key factor to achieve high selectivity. This study may open a new avenue for the hydrogenation of nitroarenes to DCHA by regulating the acidity of supports and the reaction conditions.

Roles of acidic sites in alumina catalysts for efficient d-xylose conversion to lactic acid

Active Lewis acid sites in γ-Al2O3 promote the catalytic activity for lactic acid production from d-xylose.

Revisiting side-chain alkylation of toluene to styrene: Critical role of microporous structures in catalysts

Due to potential advantages such as large availability and low cost of reactant feedstocks, the side-chain alkylation of toluene with methanol has been investigated as an alternative pathway for producing styrene. In this study, the effects of acid-base properties and microporous structures of zeolite catalysts were comprehensively investigated. The results showed that the main active sites for this reaction are base sites, which are required for activating the methyl group in toluene and converting methanol into HCHO, an alkylating agent. On the other hand, the role of acid sites is to stabilize toluene via Lewis acid-base interactions. Our results indicated that a significant dilemma exists in the design of the catalyst. Catalysts having sufficient basicity generally have limited Lewis acidity, which is ineffective for stabilizing toluene under the reaction conditions at elevated temperatures. Fortunately, the micropores of zeolites (e.g., X zeolites) provide secondary interactions such as van der Waals forces to toluene, which substantially enhance toluene stabilization, and thereby, the alkylation activity. In contrast, the model catalysts synthesized using mesoporous A zeolite showed no toluene adsorption at all due to the absence of any accessible microporosity. Consequently, even though the A zeolite catalysts have similar acid-base properties to those of the X zeolite catalysts, they showed no detectable toluene alkylation activity and produced only the decomposition products of methanol. The results clearly showed the importance of secondary interactions (or solvation effects) in zeolite catalysis, and explained why basic zeolites are superior to the conventional basic metal oxides that lack microporosity.

The Role of Acid Sites in the Catalytic Performance of Tungsten Oxide during the Dehydration of Isopropyl and Methyl Alcohols

WO3 catalyst was prepared by thermal decomposition of ammonium metatungstate (AMT) in a static air atmosphere for 3h at 450, 550, 650 and 750℃. The techniques employed for characterization of the catalyst were TG, DTA, XRD, FTIR, N2-sorption measurements. The surface acidity of the catalyst was investigated by adsorption of pyridine and 2, 6-dimethyl pyridine. The catalytic properties of the catalyst were carried out for the dehydration of isopropyl and methyl alcohols. The results revealed that WO3 is more active toward isopropanol dehydration than methanol dehydration. Also reflect that the reaction mechanism and the yield of propene and dimethyl ether produced from dehydration of isopropyl and methyl alcohols are controlled by the strength of acid sites.

Roles of Acid Sites on Pt/H-ZSM5 Catalyst in Catalytic Oxidation of Diesel soot

Pt/H-ZSM5 and Pt/Al2O3 with similar surface Pt particle sizes and chemical states were prepared by incipient wetness impregnation as catalysts for soot oxidation. Pt/H-ZSM5 exhibits obviously higher activities in both O2 and NO + O2 than Pt/Al2O3. On the basis of the results obtained with in situ DRIFT and other structural and surface property characterizations, the high catalytic activity of Pt/H-ZSM5 is currently attributed to two main factors. First, the acidic H-ZSM5 support inhibits NO2 adsorption on the catalyst, leading to a preferential adsorption of NO2 on the surface of soot and providing more chances for NO2–soot reactions. Second, the strong acid sites on the surface of Pt/H-ZSM5 can participate in the catalytic formation and decomposition of surface oxygenated complexes. Consequently, a high catalytic efficiency for soot oxidation is achieved on the Pt/H-ZSM5 catalyst.

Role of acid sites and surface hydroxyl groups in isophthalonitrile hydrogenation catalyzed by supported Ni–Co catalysts

For hydrogenation of isophthalonitrile, the metal sites catalyzed the hydrogenation reactions, while the acid sites (including the original and new ones) and the surface hydroxyl groups catalyzed the condensation side reactions.

Role of acidic sites in beta-hexachlorocyclohexane (β-HCH) adsorption by activated carbons: molecular modelling and adsorption–desorption studies

The role played by acidic groups on the removal of β-HCH from contaminated water by adsorption on activated is shown.

Role of acid sites and selectivity correlation in solvent free liquid phase dehydration of sorbitol to isosorbide

A number of Brönsted acids (methanesulfonic acid, p-toluene sulfonic acid, triflic acid, sulfamic acid, citric acid, NaHSO4, and boric acid) and Lewis acids (metal sulfate/triflates) were employed in solvent free dehydration of sorbitol and their influence on anhydroalcohols selectivity has been investigated. The outcome indicated that all the acid catalysts produced first mono-dehydrated product sorbitan followed by second dehydration of 1,4-sorbitan to isosorbide. However, the formation and yield isosorbide were found to depend on the nature of acid sites and their acidic strength. The Brönsted acids are more efficient to convert sorbitol to isosorbide than Lewis acids. The Brönsted acids having lower pKa value (i.e. strong acid) exhibited high catalytic activity as well as yield of isosorbide. In the case of Lewis acids, the catalytic activity and selectivity were radically depended on which metal used and their stability during the reaction. The water formed during reaction induced Brönsted acidity on Lewis acid metal site. The Lewis-assisted Brönsted acid site enabled high yield of isosorbide up to 70% at moderate temperature (160°C).

An Integrated Catalytic and Transient Study of Sulfated Zirconias: Investigation of the Reaction Mechanism and the Role of Acidic Sites in n‐Butane Isomerization

AbstractThe transient temporal analysis of products (TAP) pulse method has been applied to investigate the isomerization of n‐butane on sulfated zirconias at very low pressure. By combining these results with findings from XRD, XPS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature‐programmed desorption (TPD), and catalytic studies at atmospheric pressure, a wide range of pressure conditions can be accessed to study the influence of surface sulfate groups on the isomerization activity and the mechanism of this reaction. The catalytic activity of powder catalysts has been correlated to the surface site density of sulfates, with pyrosulfate structures playing a major role in the initiation of the reaction. The complex interplay between sulfate‐free and sulfate‐covered zirconia surfaces for the adsorption of alkanes has been investigated in detail, with TAP pulse experiments showing a monomolecular reaction pathway at low n‐butane partial pressures. Moreover, TAP pulse experiments have allowed detection of the reaction products of this initiation process, including butene and water, and have shown their influence on the deactivation of sulfated zirconias. Heats of adsorption have been calculated from the TAP pulse experiments.

The role of acidic sites and the catalytic reaction pathways on the Rh/ZrO2catalysts for ethanol steam reforming

Rh catalysts supported on ZrO(2)-based oxides were studied for ethanol steam reforming (SR) reaction. Pure ZrO(2) as the support resulted in higher H(2) production yield compared to the ZrO(2) oxide decorated with CeO(2), Al(2)O(3), La(2)O(3) or Li(2)O at the reaction temperature of 300 degrees C. Above 450 degrees C, all the catalysts exhibited similar catalytic activity. However, at low reaction temperatures (below 400 degrees C), a significant enhancement in the catalytic activity, selectivity and stability was achieved by replacing the ZrO(2) support prepared by a precipitation method (ZrO(2)-CP) with that prepared by a hydrothermal method (ZrO(2)-HT). A deactivation was observed during the EtOH SR reaction at 300 degrees C on the two catalysts of Rh/ZrO(2)-CP and Rh/ZrO(2)-HT. NH(3)-TPD experiments confirmed that the ZrO(2)-HT support had two types of acidic sites while the ZrO(2)-CP support had only one type of weak acidic sites. DRIFTS studies showed that the absorption of EtOH molecules was strong on the Rh/ZrO(2)-HT catalyst and a number of C(2) oxygenates were accumulated on the catalyst surface. Meanwhile, the EtOH absorption on the Rh/ZrO(2)-CP catalyst was weak and the accumulation of CO, carbonate and CH(x) was observed. It is concluded that the relatively strong Lewis acidic sites in the Rh/ZrO(2)-HT catalyst is responsible for the strong absorption of EtOH molecules, and the subsequent C-H breakage step (formation of acetaldehyde or called as dehydrogenation reaction) is a fast reaction on it; on the Rh/ZrO(2)-CP catalyst, the EtOH adsorption was weak and the C-C breakage was the dominating reaction which led to the accumulation of surface CO, CH(x) and CO(2) species. Therefore, it is believed that, in order to promote the absorption of EtOH molecules and to reduce the formation of metastable carbonaceous species (C(2) oxygenates) during the reaction, the catalyst should be enhanced both with Lewis acidity and with C-C bond breakage function. Also, it was found that the Rh particle size and distribution, as well as the surface area of the catalyst, were not important factors in determining the catalytic performance.

Acid site-assisted vapor-phase oxidation of ethene to acetic acid over palladium with silica-supported tungstosilicic acid and tungstated zirconia

Abstract The selective oxidation of ethene to acetic acid in the vapor phase over solid acid-supported Pd catalyst was investigated to elucidate the reaction mechanism and the roles of Pd and acidic sites in promoting the reaction. Two catalysts, Pd–H 4 SiW 12 O 40 /SiO 2 and Pd/WO 3 –ZrO 2 , exhibited high catalytic activity. Variations in product distribution with contact time, the requirement for the presence of water vapor for the reaction to occur, and the reactivities of possible reaction intermediates indicated that acetic acid is formed via an acetaldehyde intermediate which is produced in a Wacker-type reaction, while the role of the acid sites is to re-oxidize Pd 0 into Pd 2+ in the presence of O 2 . It was concluded that catalytic efficiency is dependent on maximization of the interface between metallic Pd and acidic surface by establishing an appropriate ratio of Pd and acidic sites.

Catalytic and FT-IR study on the reaction pathway for oxidation of propane and propylene on V- or Mo–V-based catalysts

A series of V- and Mo–V-based catalysts, i.e. undoped and K-doped VO x /Al 2O 3, MoVNbO, MoVSbO and MoVTeNbO mixed metal oxides, have been tested in the oxidation of propane and propylene. All the catalysts are active and relatively selective in the partial oxidation of propane and propylene, although the nature of reaction products changes depending on the characteristics of the catalysts. Thus, acrylic acid (MoVTeNbO), acetic and acrylic acid (MoVSbO), acetic acid (MoVNbO) and propylene and CO x (supported catalysts) were the most important products obtained from both propane and propylene. FT-IR spectroscopy of NH 3 indicates the presence of Brønsted and Lewis acid sites, although the acidity decreases in the order: MoVNbO > VO x /Al 2O 3 > MoVSbO > MoVTeNbO > K-doped VO x /Al 2O 3. FT-IR studies of propylene adsorbed and desorbed at different temperatures allowed us to propose three different intermediates: (i) a π-allylic compound, intermediate in the selective oxidation of propylene to acrylic acid; (ii) an enolic-type compound, intermediate in the hydration/oxidation of the olefin to form acetone and acetic acid; (iii) a π-bonded propylene species interacting with Lewis acid sites, precursor in the deep oxidation of propylene. Accordingly, the key factors in the achievement of an active and selective catalyst for the oxidation of propane to acrylic acid as MoVTeNbO mixed oxides is tentatively proposed. The role of acid sites in selective and non-selective pathways are also discussed.

Characterization and catalytic activity of differently pretreated Pd/Al 2O 3 catalysts: the role of acid sites and of palladium–alumina interactions

The effect of catalyst pretreatment on the performance of chlorine-free γ-alumina-supported palladium catalysts with metal loadings in the range of 0.3 to 2.77 wt% has been investigated in the hydroconversion of n-hexane (nH) and 2,2-dimethylbutane (22DMB) at 290 °C and atmospheric pressure. The catalyst properties were modified by varying the catalyst pretreatment: low-temperature reduction (LTR) at 300 °C; high-temperature reduction (HTR) at 600 °C; and oxidation of highly reduced catalysts followed by LTR (regeneration). The isomerization activity for both nH and 22DMB conversion is enhanced by HTR of the Pd/Al 2O 3 as compared to LTR and regeneration. It is concluded that HTR increases the Lewis acidity of the alumina and the participation of Lewis acid sites improves catalytic performance. The influence of Lewis acid catalyst sites on the macroscopic kinetics overshadows the modest catalytic contribution from palladium. Ammonia effectively blocks the acid centers and limits the overall catalytic performance to metal-only catalysis. The role of support acid sites in the conversion of C 6-hydrocarbons was further demonstrated by isomerization activity associated with 22DMB hydroconversion catalyzed by an alumina-supported Ni catalyst. Based on the results of FTIR studies of adsorbed CO and results of temperature-programmed palladium hydride phase decomposition (TPHD), it was concluded that a part of the alumina-supported Pd transforms into a Pd–Al alloy during the HTR pretreatment. The catalytic performance of the regenerated catalysts in the C 6-alkane conversion is consistent with the suggestion that the Pd–Al alloy formed during HTR decomposes during the regeneration treatment to form Al 2O 3 moieties on the Pd surface. These moieties, most likely, interact strongly with nearby palladium species, creating sites which are active for alkane isomerization and, simultaneously, resistant to coking.

The role of acidic properties of metal oxide catalysts in the catalytic oxidation

Acidic properties of mixed oxide catalysts sometimes play an important role in the catalytic oxidations of both saturated and unsaturated lower hydrocarbons, where reactions, especially the selectivities to the target products, are strongly affected by the functions of acidic sites. Such typical reactions are summarized and the roles of acidic sites are explained. The concept mentioned above is practically utilized for the design of industrial catalysts.

The role of acid sites in cobalt zeolite catalysts for selective catalytic reduction of NO x

The role of the acidic support in ion-exchanged cobalt-zeolite, lean NOx catalysts has been determined by studying the individual steps in the selective reduction pathway. At a GHSV of 10,000 and reaction temperatures below 400°C, NO oxidation is not sufficiently rapid to obtain equilibrium over, for example, 1–4 wt% Co-mordenite catalysts. The NO oxidation rate increases in the order H+Co2+ Co oxide, and neither the number, nor the strength of the acid sites affects the specific rate of the Co2+ ions. For reduction of NO2 by propylene at 300°C and methane at 400°C, the formation of N2 is suggested to occur at support protons sites. In addition, the rate of N2 formation increases linearly with an increase in the number of acid sites, and the specific activity increases with an increase in acid strength. Cobalt (2+) ions do not contribute significantly to the formation of N2, but do non-selectively reduce NO2 to NO. It is proposed that the formation of N2 occurs by protonation of the reducing agent followed by attack of the carbocation by gas phase NO2. Thus, the selective reduction of NO requires two catalytic functions, metal and acid sites.

Kinetics, acid sites and deactivation of h-mordenite during the SCR of NOx with CH4

H-mordenites are active for the SCR reaction but they suffer irreversible partial deactivation after being on stream for one hour at 650 degrees C. The reaction orders and activation energies are not significantly affected by deactivation. This indicates that deactivation originates in a decrease in the number of active sites due to dealumination and possible pore blockage. The NO disappearance rate correlates with TPD NH3 between 300 and 700 degrees C:; FTIR confirms these results. Xe-129 NMR of adsorbed xenon shows that pore blockage occurs and is due to the presence of aluminum species in the main zeolite channels. The overall deactivation process and the role of acid sites is discussed in terms of the current literature.

Coke formation in high-silica zeolites

Abstract The formation of coke during hydrocarbon transformations over acid zeolite catalysts has important practical consequences for a growing number of industrial processes. This review presents available material on the formation, nature and properties of coke in high silica zeolite catalysts, and on the resulting deactivation and regeneration of the catalysts. Techniques for analyzing and characterizing coke, the role of acid sites in coke formation and the chemistry of coke formation are discussed, with particular emphasis on the methanol to gasoline process.

Experimental evidence for the role of acidic sites in the side-chain alkylation of alkylbenzenes with methanol

Experimental evidence for the role of acidic sites in the side-chain alkylation of alkylbenzenes with methanol