Types Of Acid Sites Research Articles

F−@d4r, a New Type of Acidic Catalytic Site in Zeolite

As a solid acid, zeolite has been widely used in many fields such as tail gas treatment, petrochemical engineering, and the fine chemical industry. F− has been widely used in the synthesis of pure silica or high silica zeolites. To balance the charge of organic structure directing agents (OSDA), F− is often found located at the center of the double-4-rings (d4r) of the as-made zeolites. During calcination, fluorine ion is removed with the OSDA. We screened a series of composition building units and found that d4r is capable to retain F− in zeolite structure. We introduce the F− back after the calcination to create an unprecedented type of acid site, i.e., F−@d4r in pure silica zeolites ITQ-12. The F−@d4r species is thermal stable up to 300 °C. ITQ-12 with F−@d4r shows substantial catalytic activity in Biginelli reaction. Furthermore, the catalytic performance is proved to be positive correlated with the presence of F−@d4r, indicating the mild acid catalytic property of F−@d4r.

Restraining deactivation of β-zeolite by modifying with MgAl2O4 spinel in gas-phase glycerol dehydration

MgAl2O4 dispersed on HBEA zeolite with different spinel contents were successfully synthesized and their glycerol dehydration performances were presented. The different solids were characterized by XRD, infrared (IR) and Raman spectroscopy, N2 physisorption, SEM-FEG, acidity by TPD-NH3 and DRIFTS-pyridine adsorption. It was not possible to observe the spinel phase in the diffractograms due to its high dispersion, while the reflections of the HBEA were evidenced in the different samples. The Raman and infrared spectra suggested the presence of stretches concerning the MgAl2O4 spinel. N2 isotherms and low-angle XRD indicated that the pore structure and textural properties were altered after impregnation, but the micro-mesoporosity of zeolite was maintained. The HBEA morphology did not undergo significant changes by modification with spinel and the metals are evenly distributed according to SEM and EDS spectral mapping. The addition of spinel on zeolite caused a change in the acidic properties of HBEA related to the type, number and strength of the sites according to the TPD profiles and the DRIFTS spectra. The 2% of MgAl2O4 on HBEA catalyst exhibited the best catalytic performance due to the effects of its acidic and textural properties, reaching 60% of glycerol conversion, 66% of acrolein selectivity in 6 h and 16% of coke deposited in 10 h, restricting deactivation compared to pure zeolite. The reaction pathway for the different products formed depending on the type of acid site were detailed in the proposed mechanism and correlated with electrostatic potential maps as well as the reactions for coke formation were described according to compounds identified by GC-MS after coke extraction.

Improving NH3-SCR denitrification performance over WaCo0.4TiOx catalysts: Effect of surface acidity due to W addition on low-temperature and high-temperature activity

A serial of WaCo0.4TiOx catalysts (a = 0.04, 0.06, 0.08, 0.10) was synthesized by sol-gel method, which exhibited high NH3-SCR denitrification performance in a wide activity temperature window. W0.08Co0.4TiOx achieve best SCR activity (over 90% NO conversion) at 190–430 °C and good resistance to H2O/SO2. Various characterizations were used to investigate the influence of W doping on physicochemical property of these catalysts. Especially, in-situ DRIFTS were used to evaluate the type of acid sites, NOx adsorption, the reactivity of surface adsorption species, and the influences of H2O/SO2 on adsorption of NOx and NH3. The doping of W enhanced the acidity and weakened the oxidation capacity of WaCo0.4TiOx, but higher surface acidity, especial more Brønsted acid sites, can compensate for the effect of the decrease in the oxidation capacity on low-temperature activity. The optimal activity over W0.08Co0.4TiOx can be derived from the synergistic effect of surface acidity and oxidation performance.

Dehydration of methanol and ethanol over ferrierite originated layered zeolites - the role of acidity and porous structure.

Ferrierites and their delaminated (ITQ-6) and silica intercalated (ITQ-36) forms, with the intended molar Si/Al ratios of zeolite layers of 30 and 50, were synthesized and tested as catalysts of methanol to dimethyl ether (DME) as well as ethanol to diethyl ether (DEE) and ethylene dehydration. It was shown that increased content of acid sites, especially of Brønsted type, resulted in more active catalysts of alcohol dehydration. Brønsted acid sites dominate in ferrierites and their delaminated forms (ITQ-6). Contribution of the Lewis type of acid sites increased in silica pillared ferrierites (ITQ-36) possibly by deposition of aluminium species on the surface of amorphous silica. Conversion of methanol to DME was not limited by internal diffusion of reactants in narrow pores of ferrierite. Such limitation was observed for synthesis of larger DEE molecules over ferrierites. The ITQ-6 catalysts with the opened interlayer structure presented better efficiency in ethanol to DEE conversion due to overcoming these diffusional restrictions. Moreover, selectivity to DEE over ITQ-6 was higher than in the presence of three-dimensional ferrierite.

Unraveling the Role of Metal Oxide Catalysts in the CO2 Desorption Process from Nonaqueous Sorbents: An Experimental Study Carried out with 13C NMR

The CO2 desorption process from a CO2-saturated nonaqueous sorbent, 2-(2-aminoethoxy)ethanol (DGA) in diethylene glycol monomethyl ether (DEGMME), was investigated in the absence and presence of four different metal oxides, namely, V2O5, TiO2, WO3, and ZnO, aiming at identifying acid catalysts with the potential to reduce the energy demand for sorbent regeneration. The desorption performances of the DGA-DEGMME solutions with and without catalysts were evaluated in terms of CO2 desorption rate, overall CO2 desorbed, and thermal energy consumption. The desorption mechanism was understood by the 13C NMR speciation study of the solutions during the regeneration experiments. As a result, metal oxide catalysts, depending on the number and types of acidic sites on their surface, can accelerate the carbamate breakdown and the subsequent CO2 release. In particular, WO3 and TiO2 can be considered good candidates for implementation in CO2 capture processes with nonaqueous sorbents, as they greatly improved the desorption performance, and their structures remain unchanged before and after use. The combination of nonaqueous sorbents with catalysts, here reported for the first time, provides an effective strategy that could inspire the design of advanced sorbents for energy-efficient CO2 capture.

Tuning of Selectivity for Sustainable Production of Acrolein from Glycerol

AbstractThe catalytic dehydration of glycerol into acrolein is a rapidly growing area of both scientific interest and technological importance. The main problem faced by researchers is enhancement of selectivity while maintaining the stability and activity of a catalyst. This minireview is centered on key approaches for tuning of selectivity: changing of physicochemical properties (type of acid sites, its amount, pore and channel size, and doping with noble metals) and selection of optimal reaction conditions (temperature, contact time, feedstock composition, co‐feeding O2 or air). This overview also proposes and discusses the use of metal‐organic frameworks as novel catalysts to overcome the main challenges in the dehydration of glycerol.

Sulfated ordinary clay for acid-catalyzed conversion of biomass derivatives: Impacts of abundance and types of acidic sites on catalytic performance

Ordinary clay is an abundantly available material for use as catalyst support. In this study, sulfuric acid was used as sulfonating agent to prepare sulfated clay-based super-acid catalyst. The effects of sulfuric acid concentration and calcination temperature on physiochemical properties, interaction of sulfur species with the oxides in clay, distribution of Brønsted acid sites and Lewis acid sites were characterized with XRD, pore structure analysis, FT-IR, elemental analysis, NH3-temperature programmed desorption (NH3-TPD) and Pyridine-DRIFTS etc. The catalytic performance of different catalysts was evaluated by conversion of furfuryl alcohol (FA) to ethyl levulinate (EL). The results showed that sulfuric acid reacted with alumina in clay to form Al2(SO4)3, thus changing structure and crystal phase of the catalyst. Increasing calcination temperature enhanced the interaction between sulfate species and clay, the retaining of sulfur on clay surface and the formation of more Brønsted acid sites while less Lewis acid sites. In addition, balanced distribution of the acidic sites was crucial for selective conversion of FA to EL. Over the clay catalyst impregnated with 4 ​M ​H2SO4 and calcined at 300 ​°C, the yield of EL could reach 90%. The catalyst was not stable in polar solvent like ethanol due to leaching of sulfur species, while in aprotic solvent like dimethyl sulfoxide, the catalyst maintained a good catalytic stability.

Precisely regulating acid density and types to promote the stable two-step conversion of methanol to aromatics via light hydrocarbons

Acidic properties of ZSM-5 zeolites for the aromatization of light hydrocarbons strongly determined the final catalytic selectivity and stability for two-step methanol conversions of aromatics via light hydrocarbons. Herein, the density and types of acid sites of ZSM-5 were precisely adjusted to explore their influences on the final catalytic performance. Different from the indistinctive effect of SiO2/Al2O3 ratios on traditional one-step methanol conversion to aromatics, decreasing acid density by increasing SiO2/Al2O3 ratios from 36 to 70 decelerated coke formation and significantly extended the catalytic lifetime from 46.5 to 190.8 h. Further modification with low Zn loading decreased B/L ratio from 19.0 to 1.7 and established a good catalytic synergy between Zn-Lewis and Brønsted acid sites for aromatization of light hydrocarbons, which increased the aromatics selectivity from 27.5% to 31.4%. The decreased catalytic stability after introducing Zn species was attributed to the coke coverage on acid sites but not the final blockage of pore channels as mentioned in previous studies, which was indicated by the similar coke rate of the deactivated catalyst and their gradually increased surface area with increasing Zn content. This work provides more insights into the regulation of acid properties for aromatization catalyst to boost the conversion of methanol to aromatics.

Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

H-ZSM-5 zeolite is a typical catalyst for methanol-to-olefins (MTO) conversion. Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework, the catalytic roles of the acid sites in different pore channels of the H-ZSM-5 zeolite are not well understood. In this study, the MTO reaction network, involving the aromatic cycle, alkene cycle, and aromatization process, and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel, sinusoidal channel, and intersection cavity of H-ZSM-5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations. The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels. In contrast, the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels, whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities. Consequently, the catalytic performance of H-ZSM-5 zeolite for MTO conversion, including activity and product selectivity, can be regulated properly through the purposive alteration of the acid site distribution, viz., the location of Al in the zeolite framework. This study helps to elucidate the relation between the catalytic performance of different acid sites in the H-ZSM-5 zeolite framework for MTO conversion, which should greatly benefit the design of efficient catalyst for methanol conversion.

Utilization of renewable resources: Investigation on role of active sites in zeolite catalyst for transformation of furfuryl alcohol into alkyl levulinate

A bio-derived furfuryl alcohol transformation into various high-value chemicals is a growing field of interest among researchers. This study reports an exclusive investigation of the porosity and active sites responsible for the efficient alcoholysis of furfuryl alcohol to alkyl levulinate by the aid of zeolite catalyst. Alkyl levulinate is a promising platform chemical potentially used as a fuel additive and also for the production of chemicals. A detailed study using well-characterized HZSM-5 catalyst on the influence of acidity and post synthesis modification like desilication, dealumination, metal ion exchange and phosphate modification revealed the most desired type of acid sites required to catalyze this reaction. Among the HZSM-5 catalysts tested, HZSM-5 (SAR 95) showed the best performance of ≥ 99 % furfuryl alcohol conversion and 85 % butyl levulinate selectivity under optimum conditions. The catalyst exhibited good recyclability additionally addressing all the challenges reported in the previous literature fulfilling the green chemistry principles.

Methanol Dehydration to Dimethyl Ether over Modified γ-Al2O3 with Acid, Base and Zeolite (NaA and NaX)

The effect of acids, bases, zeolite NaA and zeolite NaX impregnation to g-Al2O3 on the catalyst characteristics and activity against methanol dehydration reaction were investigated. The catalyst characteristics include N2 physisorption, X-ray diffraction (XRD), and temperature-programmed desorption of ammonia (NH3-TPD) in addition to catalytic dehydration of methanol performed in a micro fixed-bed reactor at 270°C and 1 atm. The results of XRD characterization showed no changes related to the modification of alumina over acids, bases, and zeolite NaA and zeolite NaX. Therefore, the modification did not have any effect on the crystalline structure of alumina. The textural and surface acidity of g-Al2O3 changed post addition of acids, bases, zeolite NaA and zeolite NaX. NH3-TPD analysis results demonstrated that synthesized g-Al2O3 has three types of acid sites: weak, medium, and strong; however, the weak acid sites were not observed on alumina catalysts modified phosphate, KOH, zeolite NaA, and zeolite NaX. Furthermore, the concentration of strong acid sites increased in the catalyst containing KOH. The catalytic test results showed that the untreated g-Al2O3 catalyst gave prominent activity in dehydration of methanol compared to the treated catalyst following the number and strength of acid sites.

MCM-41 grafted with citric acid: The role of carboxylic groups in enhancing the synthesis of xanthenes and removal of heavy metal ions

Order mesoporous MCM-41 was synthesized via a sol-gel method and then grafted by adding the appropriate amount of citric acid (CA). The structure and morphology of pure and grafted mesoporous MCM-41 were investigated by XRD, BET, TEM, and FT-IR measurements. TEM measurements confirm that MCM-41 particles display an ordered structure with uniform mesoporous arranges into a hexagonal, honeycomb-like lattice and still preserved their mesoporous structure after grafting with different weight contents of CA. The pore sizes and volume of the CA/MCM-41 samples were found to vary markedly within CA contents. The surface acidity and the type of acid sites (Brønsted and Lewis) were determined using the potentiometric titration method and the FTIR of pyridine adsorption. In this work, it was found that the textural, acidic properties and the activities of catalysts could be improved by citric acid treatment The results illustrated that the surface acidity and acidic strength of CA/MCM-41 gradually increased with increasing the CA amount up to 50 wt.%. Also, the prepared catalysts showed both Brönsted and Lewis acid sites and the number of Brönsted acid sites increased largely compared with Lewis acid sites with increasing the CA content which indicates the role of carboxylic groups in enhancing the Brönsted acid sites. The catalytic performance of the samples was tested by the synthesis of 14-Phenyl-14H-dibenzo [a, j] xanthene (xanthene). In this reaction, the sample with 50 wt.% of CA displayed the highest acidity and catalytic activity. The adsorption activity was examined for removing Cd2+ and Pb2+ from aqueous solution with varying parameters such as pH, concentration, temperature and time. The equilibrium adsorption data obtained fitted with the Langmuir model and the adsorption kinetics favored the pseudo 2nd order model. Also, the adsorption was spontaneous and exothermic.

Not all acid sites are created equal: Alkene isomerization and dimerization occur on separate sites on Amberlyst®15 catalyst

AbstractLinear 1‐alkenes undergo concurrent dimerization and double bond isomerization on the solid acid catalyst Amberlyst®15. Kinetic data show that double bond isomerization occurs on a site where the alkene adsorption coefficient is small, whereas alkene dimerization occurs on a site where the alkene adsorption coefficient is large. The fraction of isomerization sites occupied by alkene is small, whereas the dimerization sites are practically saturated with alkene. Logically, two different types of acid sites must exist on the surface of Amberlyst®15 catalyst. On the dimerization sites, substantial skeletal rearrangement occurs, likely via rapidly isomerizing carbenium ions, π‐complexes, and/or protonated cyclopropyl intermediates.

Effect of crystalline phases and acid sites on the dehydration of 1-octadecanol to 1-octadecene over TiO2–ZrO2 mixed oxides

TiO2–ZrO2 mixed oxides with different amounts of TiO2 were prepared by co-precipitation method and used to synthesize 1-octadecene from 1-octadecanol. The results show that the doping of TiO2 leads to the formation of Lewis acid sites and Bronsted acid sites on the TiO2–ZrO2 mixed oxides. For catalysts with TiO2 doping < 3 wt.%, the catalysts are in an intermediate state from the monoclinic and tetragonal zirconia crystalline phases to the amorphous form, and no Ti–O–Zr bond is formed. For catalysts with TiO2 doping ≥ 3 wt.%, an amorphous structure and Ti–O–Zr bond are formed. The crystalline phase of metal oxides, amount and type of acid sites simultaneously affect the performance of the catalysts. The acid sites on TiO2–ZrO2 mixed oxides with monoclinic and tetragonal zirconia crystalline phases have much lower dehydration activity than those with an amorphous form. Lewis acid sites are responsible for both the dehydration of 1-octadecanol to form 1-octadecene and the double carbon bond migration of 1-octadecene to form 2-octadecene. Bronsted acid sites mainly catalyze the double carbon bond migration of 1-octadecene.

Ketonic decarboxylation and esterification of propionic acid over beta zeolites

Abstract The present work reports the nanocrystalline Beta zeolites synthesis from the high concentration hydrogels and their behavior in the propionic acid esterification with cyclohexanol and in the ketonic decarboxylation of propionic acid. The solids were characterized by X-ray diffraction, N2-adsorption, transmission and scanning electron microscopy, diffuse reflectance infrared spectroscopy. Three types of acid sites has been evidenced in Beta zeolites: external silanol groups (Si–OH) (with weakly acidity), Si–OH–Al groups (with strong Bronsted acidity) and silanol nests associated with the defect sites (with Lewis acidity). In the propionic acid esterification with cyclohexanol, the highest cyclohexyl propionate yield was obtained over the sample with the highest specific surface area, the highest mesopore volume and the highest Bronsted acidity. In the propionic acid ketonic decarboxylation, the catalyst with the lowest ratio Bronsted acid sites/Lewis acid sites showed the highest activity and selectivity towards 3-pentanone compared to the other two which are more selective towards propanol.

INTERACTION OF THE Fe/γ-Al2O3 CATALYTIC SYSTEM WITH PROBE MOLECULES III. STUDY OF THE INTERACTION OF AMMONIA WITH γ-Al2O3 OXIDE AND THE Fe/γ-Al2O3 SYSTEM

The work is devoted to the study of the interaction of heterogeneous catalytic systems with adsorbedmolecules. It presents the results of the interaction of the initial γ-Al2O3 oxide and the Fe/γ-Al2O3 system withammonia obtained using IR spectroscopy and the method of temperature programmed desorption.Lewis and Bronsted acid centers were detected on the surface of both γ-Al2O3 oxide and the Fe/γ-Al2O3 systemduring their interaction with ammonia in the temperature range 293-773K using IR spectroscopy. The relativeintensity of the absorption bands corresponding to these centers depends on the temperature of interaction withammonia.It was found by ammonia TPD that adsorbed NH3 is desorbed in the form of five peaks from the γ-Al2O3 surfaceon a temperature scale, and in the form of seven peaks from the surface of the Fe/γ-Al2O3 system. It was shown thatthe total amount of desorbed ammonia decreases markedly with increasing adsorption temperature. Moreover, forindividual temperature peaks, the amount of adsorbed ammonia can either decrease, pass through an extremum, orremain approximately constant. The appearance of additional desorption peaks for the Fe/γ-Al2O3 system isassociated with iron deposited on alumina.Proceeding from the temperature range of existence of desorption peaks for γ-Al2O3 oxide and the Fe/γ-Al2O3system it was established that they contain weak acid centers (desorption temperature up to 523K), acidic centers ofmedium strength (desorption temperature from 523 to 613K) and strong acid sites (desorption temperature above613K). The volume of desorbed ammonia in the indicated temperature ranges can serve as a quantitative measure ofvarious types of acid sites.

Two-Dimensional Zeolite Materials: Structural and Acidity Properties.

Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si4+) and aluminum (Al3+) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2–3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues’ structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed.

Selectivity Control in Tandem Catalytic Furfural Upgrading on Zeolite-Encapsulated Pt Nanoparticles through Site and Solvent Engineering

Selectivity control is a pressing challenge in developing selective tandem catalytic processes. In this work, we demonstrate that tailoring types of acid sites of zeolite-encapsulated Pt nanopartic...

Kinetic modeling of oleic acid esterification with UiO-66: from intrinsic experimental data to kinetics via elementary reaction steps

This work reports the experimental conversion of oleic acid (OA) into methyl oleate via UiO-66 MOF catalyst. Strong focus is on the kinetic modeling: 67 models, based on elementary reaction steps for Eley-Rideal, Langmuir-Hinshelwood and Hattori kinetic mechanisms, are proposed. Via the application of initial reaction rate theory and nonisothermal kinetic modeling, one model adequately describes the experimental data. It is a variant on the Eley-Rideal type of model, such that the surface reaction includes the oleic acid adsorbate, methanol reacts from the liquid phase and one additional active site is considered to make the ester and water as surface reaction products, so that overall two active sites are used in the surface reaction. An activation energy of 54.9 ± 1.8 kJ mol−1 is reported and the desorption of the ester was found to be irreversible. In addition, NH3 TPD revealed at least two types of acid sites (#) on the UiO-66 MOF, from which turnover rates were calculated in the range of 2.54 ± 0.33 to 8.82 ± 0.87 mmolOA mol#−1 s−1. Since the catalyst is recyclable and no additional acids have to be used, this system can be considered as a green alternative for the well-known homogeneously catalyzed esterification reaction.

Conversion of levulinic acid using CuO/WO3(x)-Al2O3 catalysts

The CuO/WO3(x)-Al2O3 catalysts were tested for the conversion of levulinic acid into chemicals of high added-value compounds. The WO3(x)-Al2O3 modified supports with 5 different WO3 contents (2, 4, 6, 8, and 10 wt%) were prepared by the wet impregnation method, consequently 5 wt% CuO was impregnated on the modified supports. The optimum catalytic activity was obtained with CuO/WO3(6%)-Al2O3 catalyst, which was directly related to the highest acidity of the catalyst showed by NH3-TPD and acid strength by titration analysis. The distribution of products observed on CuO/WO3(x)-Al2O3, was correlated with a change in acid site type and acid strength. The changes in the acidity by the addition of CuO over modified support was attributed to structural changes (distortions) stabilizing the active phase. This distortion is increasing the acidity strength, and favoring a higher catalytic activity with changes in the selectivity in function of CuO content over WO3(x)-Al2O3 modified support, obtaining the maximum formation of 2-MTHF and possibly other higher value-added chemicals from GVL such as: 1,4-pentanediol, pentenoic acid, pentanoic acid, 2-pentanol, and gaseous products were on CuO/WO3(6%)-Al2O3 catalyst.