Types Of Acid Sites Research Articles

Characterization and performance of hybrid catalysts for levulinic acid production from glucose

In this work, a set of hybrid catalysts were synthesized, characterized and tested for the conversion of glucose to levulinic acid. The hybrid catalysts with 1:1, 1:2 and 2:1 weight ratios of CrCl3 and HY zeolite were prepared via the wetness impregnation method. Characterization of the hybrid catalysts using XRD, FESEM, BET, FT-IR, TGA, NH3-TPD and FT-IR of adsorbed pyridine demonstrated the catalytic reaction of the hybrid catalysts was predominantly influenced by the type of acid sites (Lewis acid), amount of acid sites and strength, surface area, hierarchical porous structures and shape selectivity. Highly reactive acid sites greatly influenced the catalytic activities of glucose in producing levulinic acid. Experimental results revealed that the hybrid 1:1 catalyst exhibited the highest catalytic performance with 62% of levulinic acid yield at 160°C in 180min of reaction temperature and time. The hybrid 1:1 catalyst has potential to be applied for biomass conversion to levulinic acid under adequate process conditions.

Influence of properties of SAPO's on the one-pot conversion of mono-, di- and poly-saccharides into 5-hydroxymethylfurfural

Synthesis of 5-hydroxymethylfurfural (5-HMF) from biomass derived mono- and poly-saccharides is gaining importance because of its usefulness in the preparation of important chemicals. In our work, we have synthesized several silicoaluminophosphate (SAPO) catalysts, and have shown that in the absence of any other pH modifying reagents, those are active in converting mono- and poly-saccharides into 5-HMF under biphasic reaction condition at 175 °C. Particularly, SAPO-44 catalyst showed the best activity in the conversion of fructose to yield 78% 5-HMF with 88% selectivity. On the contrary, all other catalysts showed lower yields (H-MOR: 63%, SAPO-5: 32%, 2DCT: 60%). Over SAPO-44, good yields for 5-HMF were observed when glucose (67%), maltose (57%), cellobiose (56%) and starch (68%) were used as substrates. Recycle study carried out with SAPO-44 catalyst in the fructose conversion reaction showed marginal decrease in the activity up to 3rd run and then afterwards constant activity was observed up to 5th run (1st: 78%, 2nd: 71%, 3rd: 66%, 4th: 65%, 5th: 65%). Catalyst characterizations revealed that SAPO catalysts have higher hydrophilic nature than H-MOR (Si/Al = 10) and hence it is postulated that this property may help in achieving better results. Further studies on the catalyst characterizations revealed that SAPO-44 undergoes modifications in its structure. However, ICP-OES data suggests that Al and/or P are not leached out in the solution indicating that change in local environment around elements is possible. The influence of acid amount, type of acid site etc. on the catalytic activity is discussed and found out that strong acid sites are required to boost the 5-HMF yields.

Direct conversion of carbohydrates to 5-hydroxymethylfurfural using Sn-Mont catalyst

5-Hydroxymethylfurfural (HMF) is a very important intermediate in the fine chemical industry. This study aims to investigate the direct conversion of glucose or glucose-based carbohydrates, such as sucrose, cellobiose, inulin, starch and cellulose, to HMF by using a Sn-Mont catalyst. With the use of this catalyst, a HMF yield of 53.5% was achieved from glucose in a mono-phase medium of tetrahydrofuran (THF)/dimethylsulfoxide (DMSO) at 160 °C for 3 h. The success of one-step conversion of glucose to HMF is attributed to the Sn-Mont catalyst containing two types of acid sites, Lewis acid and Bronsted acid sites. The former one plays a crucial role in the isomerization of glucose to fructose and the latter one is active in the dehydration of generated fructose to HMF. Furthermore, Sn-Mont catalyst also demonstrated excellent activity in the conversion of disaccharides and polysaccharides and as high as 39.1% HMF was directly obtained from cellulose in a THF/H2O–NaCl bi-phasic system.

Adsorption of mercury on lignin: Combined surface complexation modeling and X-ray absorption spectroscopy studies

Adsorption of mercury (Hg) on lignin was studied at a range of pH values using a combination of batch adsorption experiments, a surface complexation model (SCM) and synchrotron X-ray absorption spectroscopy (XAS). Surface complexation modeling indicates that three types of acid sites on lignin surfaces, namely aliphatic carboxylic-, aromatic carboxylic- and phenolic-type surface groups, contributed to Hg(II) adsorption. The bond distance and coordination number of Hg(II) adsorption samples at pH 3.0, 4.0 and 5.5 were obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy analysis. The results of SCM and XAS combined reveal that the predominant adsorption species of Hg(II) on lignin changes from HgCl 2 0 to monodentate complex –C–O–HgCl and then bidentate complex –C–O–Hg–O–C– with increasing pH value from 2.0 to 6.0. The good agreement between SCM and XAS results provides new insight into understanding the mechanisms of Hg(II) adsorption on lignin.

Synthesis and Characterization of 12-Tungstosilicic Acid Anchored to MCM-41 as well as Its Use as Environmentally Benign Catalyst for Synthesis of Succinate and Malonate Diesters

12-Tungstosilisic acid anchored to MCM-41 was synthesized and characterized by various physicochemical techniques such as thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), laser-Raman spectroscopy, diffuse reflectance spectroscopy (DRS), N2 adsorption–desorption, 29Si-magic-angle spinning (MAS) NMR, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The total acidity was determined by n-butyl amine titration. The types of acidic sites (acidic strength) were determined by potentiometric titration. The use of synthesized material was explored for esterification of diacarboxylic acids with butanol. Influence of various reaction parameters (catalyst concentration, acid/alcohol molar ratio and reaction time) on catalytic performance was studied. The catalyst shows high activity in terms of higher yields toward diesters, especially for dioctyl succinate and dioctyl malonate. The catalyst was also regenerated and reused for four cycles. All these characteristics imply the high potential of an environmentally benign catalyst for synthesis of succinate and malonate diesters.

Physico-chemical characterization of Ni/MCM-41 synthesized by a template ion exchange approach

A template ion exchange approach has been employed to prepare Ni/MCM-41 with varying nickel loadings. The synthesized samples were studied by nitrogen physisorption, X-ray diffraction, atomic absorption spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and Fourier transform infrared spectroscopy. Extensive compound formation was found to result from the adopted preparation method. The Ni(II) phase was elucidated to be a nickel silicate with layer structure. Corresponding layers were comprised of an octahedral Ni(II) sheet in between two tetrahedral SiO 4 sheets. Nickel compound formation was accompanied by simultaneous dissolution of the support material. The nickel phase was most likely deposited onto the external surface of the MCM-41 particles. Temperature-programmed desorption of ammonia was used to investigate the influence of nickel content on acidity. Moderate acidity was found for the undoped support material. Introduction of nickel led to the formation of two additional types of acid sites with weak and moderate acidity characteristics.

Influence of acid–base properties of cobalt–molybdenum catalysts supported on magnesium orthophosphates in isomerization of 3,3-dimethylbut-1-ene

Synthesis and physico-chemical characterization of a pure magnesium phosphate (MgP) prepared by coprecipitation, and MgP modified by introduction of cobalt–molybdenum (4–12wt.% of MoO3 with the Co/Mo ratio fixed at 0.5) have been carried out. The structural properties of these catalysts were characterized by X-ray diffraction, their textural properties were determined by N2 adsorption–desorption isotherms and the dispersion of cobalt–molybdenum was studied by XPS spectroscopy. Their acid properties have been investigated by in situ FT-IR spectroscopy of adsorbed molecules, often, 2,6-dimethylpyridine (pKa=6.7), pyridine (pKa=5.3). Co–Mo incorporation leads to a modification in the MgP acid–base properties, especially on the acid sites type and number. Thus, lower loading of cobalt–molybdenum species decreased the number of strong Lewis acid sites whereas higher loading increased it. It was found that Lewis acid sites on magnesium phosphates play an important role in the isomerization of 3,3-dimethylbut-1-ene.The 3,3-dimethylbut-1-ene (33DMB1) conversion increases with the reaction temperature from 493 to 653K for MgP, but decreases after 573K for MgP supported by Co–Mo. A linear relationship between both types of acid sites and conversion values was found. The deactivation of the catalysts appears at high reaction temperature (>573K).

Effect of thermal treatment conditions on the acid properties of zeolite Beta

The acid properties of zeolite Beta were studied by IR spectroscopy with the use of adsorbed NH3 and CO probe molecules. It was found that the strength of the Bronsted acid sites (BASs) of zeolite Beta was the same as that of zeolite ZSM-5. Approximately a third of the total amount of BASs participated in the formation of hydrogen bonds at interdomain boundaries; thus, they were inaccessible to molecules larger than the ammonia molecule. The qualitative and quantitative compositions of acid sites in zeolite Beta can be regulated by changing the conditions of calcination of the initial zeolite form (temperature and/or gas atmosphere). Calcination under vacuum conditions afforded the highest acidity of the zeolite with respect to all types of acid sites. Calcination in a flow of air resulted in the lowest acidity of the sample, especially, with respect to the concentration of strong Lewis acid sites, because of the formation of an alumina phase. Calcination in an atmosphere of helium resulted in a decrease (as compared with a vacuum) in the rate of removal of the organic template decomposition products of the initial zeolite form from the channel volume. The resulting ethylene was mainly adsorbed at strong Lewis sites and converted into undesorbed condensation products.

Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER

Periodic DFT calculations were performed on H-FER models having Si/Al ratios of 71 : 1, 35 : 1 and 8 : 1, in order to investigate the effect of aluminium content on the properties of the zeolite Brønsted acid sites. Relative stability of these sites was found to be dependent on Si/Al ratio, which is the main factor dictating the relative concentration of Brønsted acid sites having different types of local configuration, to the point that some types of acid site are formed only when the aluminium content of the zeolite is relatively high. The number of AlO(4) tetrahedra sharing an oxygen with the SiO(4) tetrahedron involved in the Brønsted acid site determines the Si-O(H)-Al angle, O-H stretching frequency and deprotonation energy (and hence acid strength). For Brønsted acid protons not involved in intra-zeolite H-bonding, a correlation was found between Si-O(H)-Al angle and O-H stretching frequency.

A High-Throughput Experimentation Study of the Synthesis of Lactates with Solid Acid Catalysts

The conversion of dihydroxyacetone to ethyl lactate catalysed by different solid acids was investigated by means of High-Throughput Experimentation. A mesoporous amorphous aluminosilicate catalyst displaying very high selectivity and a good yield towards the lactate was identified. The correlation between the type of acid sites and the catalytic behaviour is discussed.

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.

Phenol alkylation with isobutene — influence of heterogeneous Lewis and/or Brønsted acid sites

Acidic solid catalysts with different types of acidity were used to study the liquid-phase alkylation of phenol with isobutene. A phosphonium ionic liquid immobilized on silica type carrier exhibiting pure Lewis acidity, Amberlyst 15 with pure Brønsted acidity as well as WO 3/ZrO 2 with both types of acid sites were used for this study. The active sites are postulated based on pyridine-FT-IR and NH 3-TPD studies, BET analyses, MAS NMR and XRD measurements. The different properties of the chosen catalysts are mirrored in the product distribution of the reaction mixture. It was found that WO 3/ZrO 2 is a very active and selective catalyst for the production of 2,4-di- tert-butylphenol under mild reaction conditions.

Production of middle distillate in a dual-bed reactor from synthesis gas through wax cracking: Effect of acid property of Pd-loaded solid acid catalysts on the wax conversion and middle distillate selectivity

Selective production of middle distillate (C10–C20) from synthesis gas (CO+H2) was carried out in a dual-bed reactor. Co-based catalysts were used in the first-bed reactor to produce wax (C21+) from synthesis gas, and Pd-loaded solid acid catalysts were used in the second-bed reactor to produce middle distillate from wax through hydrocracking. Co/TiO2 catalyst in the first-bed reactor produced more than 35wt% middle distillate and more than 20wt% wax. These products served as a suitable feed for the production of middle distillate through hydrocracking in the second-bed reactor. Pd-loaded solid acid catalysts used in the second-bed reactor retained three types of acid sites (weak-, medium-, and strong-acid sites). Correlations between medium acidity and catalytic activity of Pd-loaded solid acid catalyst revealed that wax conversion and increment of middle distillate selectivity in the second-bed reactor were increased with increasing medium acidity of the catalyst. Total acidity of Pd-loaded solid acid catalyst was increased with increasing medium acidity of the catalyst. Thus, the medium acidity (total acidity) of Pd-loaded solid acid catalyst served as a crucial factor in hydrocracking of wax. Among the second-bed catalysts, the Pd/mesoporous alumina (Pd/MA) catalyst with the highest medium acidity (total acidity) showed the best catalytic performance (ca. 83% wax conversion and ca. 15wt% increment of middle distillate selectivity).

Deactivation of WNiPd/TiO 2Al 2O 3 catalyst during the upgrading of LCO

The deactivation of a novel catalyst during the production of a high-quality low-emission diesel component has been studied experimentally in a hydrotreating microplant. This process is compared with a commercial operation that used a conventional HDT catalyst. Catalysts were tested by incrementing temperature as a function of cycle length to keep the sulfur constant in the product, maintaining all other operating conditions constant. Spent catalysts were characterized using solid 13CNMR and XPS spectroscopy, pyridine adsorption, GC–MS of the coke extracted, and chemical analysis. Effective pore diffusivity was measured with pentane, and the catalyst activity and selectivity were measured with LCO and probe molecules to verify naphthenic ring opening capability. An important decline in hydrogenation and ring opening activity was detected along the cycle length due to acid site deactivation. The deactivation is attributed to LCO poly-aromatics adsorption on a particular type of acid site. A commercially deactivated HDT catalyst that operated with LCO presents a similar type of coke to that of the pilot plant.

Molecular simulation studies of adsorption of hydrogen cyanide and methyl ethyl ketone on zeolite NaX and activated carbon

In the present research study the primary aim is to understand and characterize the physical adsorption of polar molecules namely, hydrogen cyanide and methyl ethyl ketone (MEK) in zeolite NaX, and activated carbon through detailed Monte–Carlo simulations and computational quantum chemistry techniques. The sorption capacity and affinity of the zeolite is compared with activated carbon with different acid site concentrations, type of acid site, and pore sizes by simulating single component isotherms and Henry's constant at 25 °C. The role/contribution of certain types of electrostatic interactions namely charge–dipole, charge–induced dipole with zeolite NaX and activated carbon, as well as dipole–dipole interactions among polar molecules is analyzed and discussed.

Synthesis of camphene from α-pinene using SO32− functionalized MCM-41 as catalyst

MCM-41 and Aerosil-200 functionalized with different quantity of SO3− groups were used as catalysts for the heterogeneous synthesis of camphene from α-pinene. The characterization of the catalysts was made by thermodesorption of NH3 and physisorption of nitrogen. Three types of acid sites were found on the functionalized MCM-41, whereas on Aerosil-200 mainly strong acid sites were found. The conversion of α-pinene increased with the quantity of medium strength acid sites. The highest conversion was 100% with yield to camphene of 39.3% for catalyst with the higher content of SO3−. The highest yield to camphene was 42.2% with a conversion of 95% and was reached for moderate acid treatment of catalyst. The Aerosil-200 based catalyst did not show conversion of α-pinene. Other terpenes as limonene, terpinolene and δ-terpinene were obtained as by products.

Adsorption of metal ions on lignin

This study investigated the adsorption of the heavy metal ions Pb(II), Cu(II), Cd(II), Zn(II), and Ni(II) on a lignin isolated from black liquor, a waste product of the paper industry. Lignin has affinity with metal ions in the following order: Pb(II) > Cu(II) > Cd(II) > Zn(II) > Ni(II). The adsorption kinetic data can be described well with a pseudosecond-order model and the equilibrium data can be fitted well to the Langmuir isotherm. Metal ion adsorption was strongly dependent on pH and ionic strength. Surface complexation modelling was performed to elucidate the adsorption mechanism involved. This shows that lignin surfaces contain two main types of acid sites attributed to carboxylic- and phenolic-type surface groups and the phenolic sites have a higher affinity for metal ions than the carboxylic sites.

Characterization and catalytic activity of vanadia supported on rice husk silica promoted samaria

Abstract A series of supported vanadia catalysts (2–10 wt% V2O5) were prepared by wet impregnation method using rice husk silica promoted samaria as the support. Analysis of the catalysts was done by various physicochemical techniques like energy dispersive X-ray analysis (EDX), powder X-ray diffraction (XRD), BET surface area measurements, thermal analysis (TGA/DTA) and UV–Vis diffused reflectance spectroscopy (DR UV–Vis). Temperature programmed desorption (TPD-NH3) of ammonia was employed to quantify the various types of acid sites on the surface of the catalysts. Catalytic activity of these catalysts was studied in liquid phase oxidation of benzyl alcohol with H2O2 as oxidant. The structural characterization of the catalysts suggests the formation of an orthovanadate (SmVO4) type phase at higher loadings of V2O5. The catalytic activity increases with vanadia loading up to 6 wt% V2O5, and further increase reduces the conversion rate as well as selectivity.

The Effect of Type of Acid Sites in Molecular Sieves on Activity and Selectivity in Acylation Reactions

The role of the type of acid site (Broensted vs. Lewis) on the activity and selectivity of molecular sieve catalysts was investigated in ferrocene and toluene acylation. H-, Zn-, Fe-, Al- and La-forms of zeolite Beta, USY and mesoporous molecular sieves (Al)MCM-41, (Al)SBA-15 were tested. It was observed that addition of metal cations acting as Lewis acid sites can increase the acidity of various molecular sieve catalysts. No general relationship between the type of cation and conversion of individual substrate was found. While the highest activity in ferrocene acylation was observed after addition of Zn, in the case of toluene acylation Al-forms of catalysts were the most active. The results indicate that the acid strength of cationic Lewis sites controls their activity in acylation reactions.

Manganese Oxide Catalysts Supported on TiO2, Al2O3, and SiO2: A Comparison for Low-Temperature SCR of NO with NH3

A series of TiO2-, Al2O3-, and SiO2-supported manganese oxide catalysts were prepared, characterized, and catalytically tested for selective catalytic reduction (SCR) of NO with NH3 in the presence of excess oxygen at low temperatures (373−523 K). Various commercial supports were used in this study to find out the influence of surface area, support nature (acidic, basic), and crystalline phase on SCR activity. XRD studies reveal the presence of anatase and rutile phases for titania supports and the existence of γ-alumina in the case of alumina support. Silica support was amorphous. No independent lines corresponding to the crystalline MnO2 were observed on pure anatase and rutile samples. However, the presence of MnO2 was confirmed on other supports by XRD. BET surface area values suggest that specific surface area of the supports was decreased after impregnating with MnO2. The FT-IR and ammonia TPD studies indicate the presence of two types of acid sites on these catalysts, and the acidic strength of the...