Increasing Acid Strength Research Articles

The effect of acid strength on the mechanism of catalytic pyrolysis reaction of n-hexane in ZSM5: A DFT study

Acid strength is an important factor affecting the reaction mechanism. The influences of different Brønsted acid strengths on the cracking path of n-hexane were systematically investigated using the density functional theory. Most of the hexane activation cracking transition states are carbenium ions with no intermediate products of five-coordinate hexanium due to the weak Brønsted acid sites were not conducive to the stabilization of hexanium ions. With increasing acid strength, the protonated cracking reaction (the rate-controlling steps of monomolecular paths) energy barrier was reduced by 38.0% and the β-scission reaction (the rate-controlling steps of bimolecular paths) energy barrier was reduced by 27.15%, where enhanced the selectivity of E&P by changing the ratio of monomolecular and bimolecular cracking. However, the optimal reaction paths, the rate-controlling steps and the order of the various reaction energy barriers haven’t been changed. In addition, decreasing the acid strength increased the energy barriers of aromatization reaction and inhibited the reaction. The effect of molecular size on aromatization was further explored. The intermolecular hydrogen transfer reaction between 2-propoxide and 3-ethylcyclohexene is the rate controlling step and the larger the cycloalkane molecule, the higher the reaction energy barrier. The catalyst design requires a reduced pore size to prevent the formation of cycloalkanes and thus inhibit side reactions to improve the selectivity of E&P.

Catalytic conversion of asphaltenes to BTXN using metal-loaded modified HZSM-5

HZSM-5 zeolites with Si/Al ratios of 20, 35, 50 and 65 were prepared by the directing crystallization process of silicalite-1 seeds. The influence of Si/Al ratios on the production of benzene, toluene, xylene and naphthalene (BTXN) originated from asphaltenes catalytic pyrolysis was explored by adopting Py-GC/MS. Modified Z5-50 zeolites were prepared by various metal ions (Ni 2+ , Mo 6+ , Fe 3+ , and Co 2+ ) with different loading rates (3% (mass), 5% (mass), 7% (mass), and 9% (mass)) and the physical and chemical properties of these zeolites were characterized by XRD, SEM, ICP-OES, XPS, NH 3 -TPD, FTIR, Py-IR and N 2 adsorption−desorption isotherm. In addition, they were employed to catalyze the conversion of asphaltenes pyrolysis production to BTXN using Py-GC/MS. Results show that the highest relative content of aromatics has been obtained over HZSM-5 with Si/Al ratio of 50 (Z5-50), reaching 61.87%. Besides, the loading of Ni, Mo, Fe, and Co on Z5-50 leads to an increase of acid strength and provides new active sites. The relative content of BTXN increases by 3.17% over 3Ni-Z5, which may be ascribed to that Ni promoted the conversion of polycyclic aromatic hydrocarbons (PAHs) to monocyclic aromatics due to the cracking of aliphatic side chains of PAHs and the decrease of phenolic activation energy. While under the catalysis of 5Mo-Z5, the relative content of aromatics and BTXN augmented by 5.75% and 4.02%, respectively. In addition, the highest relative content of aromatics reaches 70.09% when the loading rate of Fe was 7% (mass), and the relative content of BTXN increases from 25.87% to 29.42%. The results demonstrate that the active sites provided by different metal species expressed diverse effects on BTXN. Although the Brønsted/Lewis acid ratios of HZSM-5 modified by metal decreased, the acid strength and the relative content of BTXN both increased, which illustrated that there is a synergistic catalysis with the Brønsted acid sites and Lewis acid sites provided by metal species. In general, the performance of the catalyst is affected by the pore structure, acidity and metal active sites. Moreover, the possible formation mechanism of BTXN derived from asphaltenes catalytic pyrolysis was proposed on the basis of structural features and catalytic performances of a series of zeolites.

Structural and composition enhancement of Indian Kachchh kaolin clay: characterisation and application as low-cost catalyst

ABSTRACT The effect of acid treatment on kaolin clay from the Kachchh region of Gujarat, India, was explored to prepare a low-cost (−$0.6/kg) catalyst. The clay was treated with 10 M H2SO4 which increased the Si/Al ratio from 1.3 to 2.4, surface area from 10.322 to 54.193 m2/g and pore volume from 0.041 to 0.152 cm3/g. The XRF analysis indicated that the metal oxides in treated clay decreased with increasing acid strength; consequently increasing the SiO2 content. SEM micrographs displayed gradual disaggregation and decrease in the size of clay structure with an increase in acid strength. TG-DTA study confirmed the removal of structural water. FTIR and X-ray diffraction analysis indicated metal removal creating a large degree of structural disorder resulting in reduced crystallinity and leading towards the amorphous phase. BET study confirmed the formation of additional mesopores. The acid-treated clay was applied as a catalyst for the esterification of succinic acid (SA) with ethanol. The conversion of SA increased from 27% to 56% revealing higher activity and selectivity in terms of higher yield (96%) of valuable diester. The catalyst was reused up to three cycles with a slight loss of catalytic activity.

Exploring the Potential of Acid Treated Bentonite Clay for Various Catalytic Applications

Clays are widely used as catalysts and catalyst supports for a broad range of chemical processes due to their unique layered structure and low cost. Potential of Bentonite clay from Gujarat region (India) as a catalyst is evaluated in this work. Beneficial properties such as high cation exchange capacity, high absorption capacity, stickiness, expandable layer etc. makes Bentonite clay suitable for a wide range of catalytic applications in chemical process industries. Treatment of Bentonite clay with 5 - 10M sulphuric acid (H2SO4) followed by calcination at 600 0C showed removal of metallic impurities which increased porosity. The physical and chemical characteristics of modified clay were checked by WET process analysis and FTIR. Results showed that increase in acid strength led to better removal of Al3+ ion and other metal oxides thereby increasing silica-alumina ratio. Better surface characteristics indicate that Gujarat Bentonite clay is suitable for use as catalysts and catalyst supports.

On the pH-dependent strontium adsorption by Al-PILCs, impregnated with carboxylate groups of various acid strengths

The impregnation of a PILC with carboxylate groups is proposed as a means to improve the adsorption properties of the material for strontium ions. The role of solution pH and carboxylate acid strength in the functionalization of the organic groups is probed by base titrations. In comparing the pristine-PILC with PILCs carrying either oxalate or acetate impregnates, enhanced strontium uptake is evidenced by the modified solids, initiated at pH 6 and 8 in the presence of oxalate and acetate, respectively. The effect on uptake is higher in the former case, amounting to an increase in strontium adsorption by a factor between two and three, depending on the pH range. To further elucidate the significance of the carboxylate acid strength in cation uptake phenomena, strontium adsorption isotherms are presented for PILCs carrying acetate, oxalate, malonate and citrate groups. The results demonstrate that, at constant pH, adsorption increases with increasing acid strength

Screening of solid acid catalysts for etherification of glycerol with isobutene under identical conditions

We compared the performance of more than 70 solid acid catalysts with Brønsted acid function for the etherification of glycerol with isobutene under identical reaction conditions of 75 °C, 10 bar, 6 h and with an isobutene/glycerol molar ratio of 3/1. The catalysts were selected from a wide range of solid acid catalysts including ion exchange resins, zeolites, silica, and heteropolyacids, and their counterparts modified by simple treatments, with more than half of them being investigated for the first time for this reaction. For the modified commercial acid catalysts, the desired glycerol tertiary butyl ether (DTBGE and TTBGE) selectivity improved from 75 to 87 wt% upon partial cation exchange of the sulfonic acid protons in Amberlyst™ type ion exchange resins with Na+ cations; from 66 to 85 wt% by hydrothermal steam treatment of zeolite H-Y (SiO2/Al2O3 = 80); and from 75 to 80 wt% with partial La+-exchange of zeolite H-Beta (SiO2/Al2O3 = 300), all at high (90–100%) glycerol conversion. Impregnation of the heteropolyacids, tungstosilicic acid (TSA) and tungstophosphoric acid (TPA), on silica at a loading of 50 wt% provided a glycerol conversion of higher than 65 wt% and with a desired ether selectivity in the range of 62 to 76 wt%. In general, total ether selectivity increased with decreasing acid capacity for ion exchange resins, while the desired ether selectivity was enhanced with increasing acid strength for zeolites and supported heteropolyacids on mesoporous silica. Data present a detailed guideline for the selection of solid acid catalysts for the etherification of glycerol with isobutene.

Understanding the Effect of Acid Strength on the Alkane-Alkoxide Hydride Transfer Reaction over Solid Acid Catalysts: Insights from Density Functional Theory

Hydride transfer as the key step in the alkylation reaction harbors burgeoning research interest. Herein, the influence of acid strength on hydride transfer was investigated via density functional theory. Based on energetic analysis, increasing acid strength can stabilize the carbenium ion and destabilize alkoxide species, resulting in significant improvement of hydride transfer performance. Notably, the hydride transfer reaction cannot occur at a weak acid site owing to difficulty in forming a stable H-shared intermediate, which perfectly coincides with experiments. More interestingly, the changing extent of energy for intermediates and transition states correlates well with the ionic character at various acid strengths. A stronger ionic character leads to a larger changing extent of energy. Therefore, the changing trends of activation energy barriers to acid strength can be predicated by comparing the strength of ionic character between reactants and transition states. The results are of referential sig...

Assessment of acid strength in sodium-exchanged resin catalysts: Consequences on glycerol etherification with isobutene in batch and flow reactors

Consequences of a decrease in the number of acid sites by the cation exchange on the acid strength of the resin catalysts were assessed using ammonia as a probe molecule. IR spectra illustrated that ν(NH) bands on NH3-saturated Amberlyst 15™ shift linearly to higher frequencies with an increase in the degree of sodium exchange associated with an increase in the Brønsted acid strength. These results were further confirmed by density functional theory calculations illustrating that the deprotonation energy of a sulfonated styrene group decreases upon Na+-exchange on its neighboring counterparts. Consequences of these changes in acid strength were investigated on glycerol etherification with isobutene. Batch reactor measurements at high conversions illustrated that selectivity to desired glycerol ethers increases as that of isobutene dimerization is suppressed with an increase in acid strength. The effects of acid strength on the complex reaction network were further investigated using a once-through flow reactor specifically focusing on low conversions. These measurements showed that mono-tert-butyl glycerol ether and di-isobutene were the primary products on pristine Amberlyst 15™, while di- and tri-tert-butyl glycerol ethers also become a primary product on sodium-exchanged counterpart catalysts.

Obtaining scratch-free specimens of dental enamel prepared by sectioning, grinding, polishing, and acid etching for scanning electron microscopy.

The aim of the present study was to identify the stage during acid etching of dental enamel where scratches introduced by sectioning and grinding procedures were eliminated. Nitric acid was used as etchant. Four hundred 2-3 mm-thick longitudinal sections of human third molar crowns were assigned to 80 groups, each group differing with respect to grinding procedure (silicon carbide paper grit 600; 1,000; 1,200; and 1,200 + polishing), acid concentration (0.1%, 1%, 2.5%, and 5%) and etching time (15, 30, 45, 90, and 180 s). Observation of the specimens with scanning electron microscopy (SEM) showed that elimination of scratches was facilitated by increasing fineness of grinding paper and polishing, and by increasing acid concentration and increasing etching time. An acid concentration of 0.1% did not remove scratches totally irrespective of grinding procedure or etching time, 1% acid removed most scratches after 45-90 s, while 2.5% and 5% acids removed most scratches after 30-45 s. It is proposed that the scratches are eliminated/obscured through a combined effect of the acid and the inherent structure of the enamel. The results of the present study may serve as a guide in choosing the best procedures for obtaining scratch-free enamel surfaces through acid etching with the least possible loss of enamel substance. Research highlights Sectioning and grinding produce scratches in dental enamel. Scratches are eliminated by acid etching prior to SEM studies of enamel structure. Scratch elimination increases with increasing acid strength and etching time and is facilitated by finer grit grinding paper and polishing.

Liquid-phase synthesis of isoprene from MTBE and formalin using cesium salts of silicotungstic acid

A single-stage synthesis of isoprene from methyl tertiary-butyl ether (MTBE) and formalin in an organic-aqueous two-phase system were studied by using Cs-exchanged silicotungstic acid catalysts (CsXH4-XSiW12O40, abbreviated as CsX-STA). The CsX-STA catalysts (x=2.0, 2.5, 3.0, and 3.5) were prepared using Cs2CO3 as Cs source and water as solvent. The catalysts were characterized by using XRF, FTIR, FT-Raman, XRD, N2 sorption, light scattering, TGA, and potentiometric titration techniques to relate catalytic activities to catalyst properties. The characterization results showed that the Keggin structure and cubic unit cell of all Cs-STA catalyst samples were well preserved. The increase in acid strength and total acid sites with decreasing Cs content (x) resulted in increasing formaldehyde conversion and isoprene selectivity. However, at low Cs content (x≤3.0), the high contribution of homogeneous catalysis also played a role in the catalytic activities. The highest activity (isoprene formation normalized with total acid sites) of Cs2.5-STA catalyst was found to be attributed to its high surface area. For the organic solvent effect, the Cs2.5-STA catalyst activity increased with the increasing solvent polarity in the order: n-hexane < cyclohexane < toluene.

Biocompatible microcrystalline cellulose particles from cotton wool and magnetization via a simple in situ co-precipitation method

This investigation describes the preparation of magnetically doped degradable microcrystalline cellulose (MCC) nanocomposite particles with application potential in biotechnology, solid support for biomolecule/water purification, oil recovery from water and beyond. MCC was first extracted from cotton wool, the most abundant biocompatible polymer, by sulfuric acid hydrolysis and the effect of acid strength was examined. The size of the elongated fiber structure was reduced with increasing acid strength. MCC particles extracted by treatment with 70% sulfuric acid were used to prepare magnetic MCC nanocomposite particles. The nanocomposite particles named as MCC/Fe3O4 were prepared via in situ co-precipitation of Fe+3/Fe+2 from their alkaline solution. The precipitated Fe3O4 nanoparticles are expected to be bonded with MCC particles via hydrogen bonding. The nanocomposite dispersion was colloidally stable and the particles responded when external magnetic field was applied. It was possible to control the magnetic property by regulating the content of iron oxide.

The effect of acid strength and postacidification reaction time on the determination of chlorophyllain ethanol extracts of aquatic periphyton

Quantification of chlorophyll a (Chl a) is essential to the study of aquatic ecosystems, yet differences in methodology may introduce significant errors to its determination in ethanol extracts. Insufficient acidification slows the conversion of Chl a to pheophytin a leading to an underestimate of Chl a concentration. Furthermore, slight differences in the postacidification reaction time can introduce greater errors in calculated Chl a and impede our ability to make cross-study comparisons. We used known concentrations of pure Chl a from the blue-green algae Anacystis nidulans dissolved in 95% ethanol to evaluate the effect of acid strength and postacidification reaction time on the spectrophotometric determination of Chl a. Increasing acid strength resulted in more rapid stabilization of calculated Chl a concentration. At reaction times less than 120 s estimates of Chl a deviated from known concentrations by as much as 84.8%. The magnitude of error in the calculated Chl a values were dependent on acid strength and reaction time, which allowed us to develop predictive equations to correct Chl a measurements that were insufficiently acidified or read prior to reaction completion. We validated our predictive equations using benthic periphyton samples from northern Alaska and northwestern Vermont, U.S.A. Our results indicate that under-acidified samples with known reaction times can be easily corrected so results from different methods can be standardized. For future analyses we recommend acidifying ethanol-extracted algal samples to 0.008 mol HCl L−1 and allowing samples to react for 30–60 min to ensure accurate and consistent results.

Influences of the confinement effect and acid strength of zeolite on the mechanisms of Methanol-to-Olefins conversion over H-ZSM-5: A theoretical study of alkenes-based cycle

Methanol-to-Olefins (MTO) conversion over acidic zeolite catalysts has become the most important non-petrochemical route for the production of light olefins. The ‘dual-cycle’ mechanism (i.e., alkenes-based cycle and aromatics-based cycle) over H-ZSM-5 zeolite has been generally accepted for olefins generation from methanol conversion. However, the relationship between the catalytic performance and the confinement effect/acid strength of the catalyst is still unclear. Herein, the methylation, isomerization and cracking processes involved in the alkenes-based cycle are discussed in-depth by density functional theory (DFT) calculations. The calculation results predicted that the transition states can be considerably stabilized by the van der Waals (vdW) interactions from the zeolite framework, resulting in the reduction of the activation barriers. And acid strength can also enhance the reaction activities. However, the catalytic reactivity of all elementary steps in the alkenes-based cycle can be improved at a different degree with increasing the acid strength. In addition, the ethene formation, transformation and the precursor of ethene formation need higher energy. And increasing acid strength can sharply decrease the activation barriers of ethene formation of cracking reaction, indicating that ethene formation may need strong acid strength.

Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst

Kinetic analysis of hydrogen production catalyzed by Co(dmgBF2)2(CH3CN)2 (dmgBF2 = difluoroboryl-dimethylglyoxime) was performed in acetonitrile with a series of para-substituted anilinium acids. It was determined that the mechanism of hydrogen evolution is governed by three elementary steps; two are acid concentration and pKa dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H2 release. The kinetics of the first proton transfer step, the protonation of the singly reduced catalyst, were evaluated using foot-of-the-wave analysis, as well as current–potential analysis for voltammograms displaying total catalysis behavior. Analysis of the total catalysis peak shift required the empirical determination of a new equation for the ECEC′ catalytic mechanism using digital simulations. The kinetics of the second proton transfer step—assigned to protonation of the doubly reduced, singly protonated species—and the acid-independent step were determined by analyzing the plateau current of the catalytic wave over a range of acid concentrations. Both proton transfer steps follow linear free energy relationships of log(k) vs acid pKa. These linear relationships give slopes of −0.94 and −0.77 for the first and second proton transfers, respectively, indicating that both steps become faster with increasing acid strength.

Promotional effect of H3PO4 on ceria catalyst for selective catalytic reduction of NO by NH3

A series of H3PO4-modified CeO2 samples were prepared by impregnation of CeO2 with H3PO4 solution, and evaluated for the selective catalytic reduction of NOx by NH3. The samples were characterized by X-ray diffraction, N2 adsorption-desorption, infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of NH3, and temperature-programmed reduction of H2. The results showed that more than 80% NO conversion was achieved in the temperature range 250–550 °C over the H3PO4-CeO2 catalyst. The enhanced catalytic performance could be ascribed to the increase in acidic strength, especially Brönsted acidity, and reduction in redox properties of the CeO2 after H3PO4 modification.

Hydrolysis of cyclohexyl acetate to cyclohexanol with high selectivity over SO3H-functionalized ionic liquids

The catalytic performance of acidic ionic liquids (ILs) was evaluated for the hydrolysis of cyclohexyl acetate to cyclohexanol. Cyclohexyl acetate conversion increased with increasing acid strength of the ILs, but the five SO3H-functionalized ILs with different acidity had similarly high activity. All of the ILs exhibited high selectivity to cyclohexanol. This can be attributed to the high polarity of the ILs, which inhibits the formation of the transition state for the side reaction, pyrolysis of cyclohexyl acetate to cyclohexene, and then increases cyclohexanol selectivity. Under optimized conditions, cyclohexyl acetate conversion was 80.2 with 96.6 % cyclohexanol selectivity over [HSO3bmim]HSO4 (bmim = butyl-3-methyl-imidazolium). Moreover, [HSO3bmim]HSO4 also showed high reusability.

Spatial arrangement and acid strength effects on acid–base cooperatively catalyzed aldol condensation on aminosilica materials

Weak acids are known to enhance the activity of amines in aldol condensation reactions on silica-based catalysts. The effects of acid strength and arrangement of the promoting site with respect to a secondary amine have been investigated in the aldol condensation of 4-nitrobenzaldehyde with acetone. Changing the substituent of this secondary amine from a methyl to an ethyl group decreases the activity. An intramolecular OH function provided by a primary alcohol incorporated on the β-carbon of the amine substituent exhibits a similar cooperativity as an intermolecular OH function provided by neighboring surface silanols. A maximum activity was achieved when the secondary amine with the same primary alcohol-containing substituent was surrounded by surface silanols, indicating the potential advantage of simultaneously activating both reactants by the formation of a hydrogen bond in contrast to the consecutive activation when there is only one promoting site in the vicinity of the amine. Changing the alcohol to stronger acids resulted in a reduced cooperativity with increasing acid strength. After removing the silanols from the surface, the activity of the catalysts which exhibit an intramolecular cooperativity retained about 68–83% of their activities while the activity of the conventional secondary amine was reduced by a factor of four compared to its intermolecularly cooperative counterpart.

Inhibitive action of the acid extract of Luffa aegyptiaca leaves on the corrosion of mild steel in acidic medium

The inhibitive effect of the acid extract of Luffa aegyptiaca (LA) leaves on the corrosion of mild steel (MS) in the hydrochloric acid medium was studied using mass loss, colorimetric and electrochemical methods. The inhibition efficiency, increased with increase in concentration of the extract, but decreased with a raise in temperature and increase in acid strength. Polarization measurement studies revealed that LA extract act as a mixed-type inhibitor but predominantly control the anodic reaction. Furthermore, the inhibition efficiency synergistically increased with the addition of halide ions. Physical adsorption mechanism is proposed from the trend in inhibition efficiency with the change in temperature and from thermodynamic parameters. It has been found that the adsorption of LA on MS complies with Langmuir adsorption isotherm and follows the kinetic thermodynamic model of El-Awady at all temperatures studied. Fourier transform infrared spectroscopy and optical profiler images confirmed the adsorption of LA on MS surface.

Hydration of cyclohexene to cyclohexanol over SO3H-functionalized imidazole ionic liquids

SO3H-functionalized imidazole ionic liquids [HSO3-bmim]CF3SO3, [HSO3-bmim]-HSO4, [HSO3-bmim]CH3SO3 and [HSO3-bmim]p-CH3(C6H4)SO3H were used as catalysts for the hydration of cyclohexene to cyclohexanol. It was found that the catalytic activity of the ionic liquids increased with increasing acid strength, and [HSO3-bmim]HSO4 simultaneously showed high activity and high cyclohexanol selectivity. Under the optimized conditions, cyclohexene conversion was 10.1 % with 98.7 % cyclohexanol selectivity. When [HSO3bmim]HSO4 was separated and reused for the fifth time, cyclohexene conversion was still 10.2 % with 99.0 % cyclohexanol selectivity. The good stability may be attributed to the little effect of the side-products with high-boiling points on the ionic liquid.

Corrosion Inhibition of Mild Steel in Sulphuric Acid Using Luffa Aegyptiaca Leaves Extract

The corrosion inhibition of mild steel (MS) in sulphuric acid using Luffa aegyptiaca (LA) leaves extract was investigated by weight loss studies, colorimetric measurements, potentiodynamic polarisation and electrochemical impedance spectroscopy. The adsorption of LA leaves extract was confirmed by Fourier transform infrared spectroscopy and optical profilometry. The inhibition efficiency increased with increasing concentration of the extract but decreased with rise in temperature and with the increase in acid strength. Furthermore, the inhibition efficiency synergistically increased on the addition of halide ions. Polarisation studies revealed that the LA extract acts as mixed- type inhibitor. Physical adsorption mechanism is proposed from the trend in inhibition efficiency with the change in temperature and from thermodynamic parameters. The adsorption of LA on MS surface obeys Langmuir adsorption isotherm and follows the kinetic thermodynamic model of El-Awady at all temperatures studied.