Acidic OH Groups Research Articles

CHEMICAL AND SPECTROSCOPIC CHARACTERIZATION OF THE HUMIC SUBSTANCES FROM SANDSTONE-DERIVED ROCK FRAGMENTS

The characterization of soil organic matter, and of humic substances in particular, has always been made on fine earth, i.e., the <2-mm fraction. The fraction larger than 2 mm, known as rock fragments or skeleton, is commonly discarded. We have extracted the humic acids (HAs) and fulvic acids (FAs) from fine earth, rock fragments, and rock fragments washing (i.e. the fine material adhering to the rock fragments) of the upper forest soil horizons, A1 and A2. The substances were characterized using wet-chemical analyses, FT-IR, and liquid state 13C NMR spectroscopies. HAs of the rock fragments have higher N and H content, paraffinic chains, aliphatic-OH, and highly substituted aromatic groups than those of the fine earth. These features indicate that carbohydrates, lipids, and proteinaceous residues are incorporated in the humic acids of the rock fragments. The fresh biological material residues present in the structure of the skeleton HAs may be attributable to: (i) the selective preservation from microbial and chemical attack offered by the skeletal environment; (ii) the more rapid cycling of the organic matter inside the rock fragments compared with that of the fine earth. The FAs show greater homogeneity than the HAs, probably because of their mobility among the different soil compartments. However, the FAs of the rock fragments have more carboxyl and acidic-OH groups than found in the fine earth. Greater differences are also observed between the two horizons. The FAs extracted from the A2 horizon show less mineralization than those from the A1 horizon. The humic substances extracted from the rock fragments washing show characteristics more similar to those of the skeleton than to those of the fine earth. This suggests that most rock fragments washing originate from the weathering of the rock fragments and, therefore, may be regarded as an intermediate phase between the skeleton and the fine earth.

Shape selective adsorption of olefins on Brønsted acidic OH (OD) groups on ferrierite studied by FT-IR

Adsorption of n-olefins (≤C 4) on a ferrierite (FER) zeolite was observed by infrared (IR) spectroscopy. Ethene and propene adsorbed selectively on the acidic OH (OD) groups in the small pores of FER, while butenes adsorbed also on the silanol groups on the external surface at ca. 200 K, a part of which transferred to the acidic sites upon increase in temperature. This indicates the existence of energy barriers for intercalation of butenes into the FER pores. The facility among butenes for migration to the FER pores was found to be in the order trans-2-butene > 1-butene > cis-2-butene.

How to make polyvinylphenol inhibitable by diazonaphthoquinone sulfonates

There is interest in phenolic resins that are transparent at 248 nm and the dissolution of which in alkaline aqueous developers can be inhibited by diazonaphthoquinone sulfonic acid derivatives, the standard photoactive compounds (PACs) of novolak resists. Poly(4-hydroxystyrene), i.e., poly(vinylphenol), has the requisite optical properties, but its dissolution is not very effectively inhibited by standard PACs. We have now found that poly(4-hydroxystyrene) can be made more inhibitable by increasing the acidity of its OH groups and, at the same time, lowering the concentration of the acidic OH groups in the resin.

Adhesion mechanisms in a cyanurate prepolymer on silicon and on aluminium

XPS, UPS, MIES, IR reflection spectroscopy and AFM are employed in a study of interaction mechanisms in thin films of a cyanurate prepolymer or various organic model compounds on different surface states of Si and oxidised Al. The trioxytriazine entity is identified as the only chemical part of the substances which can establish an adhesive interaction with the substrates. On the silicon surfaces, the mechanism is identified as a donor–acceptor interaction where the 2p electron lone pairs of N and O in the trioxytriazine act as donors for Si cations. On the Al oxide, the Lewis acidic OH-groups act as electron acceptors which withdraw electron density from the lone pairs at O and / or at N of the trioxytriazine. Moreover, a backdonation of electron density takes place from the metal Al beneath the oxide into the organic layer. The dicyanate monomer does not adhere at all, but desorbs quickly out of the interphase region on the substrates. Hence the thermosetting reaction of the prepolymer will be hampered and the resulting polymer network will be less dense near the substrate than in the bulk.

IR spectroscopic investigation of structural hydroxyl groups in W−Si heteropolycompound supported on Al2O3

Structural hydroxyl and deuteroxyl groups within the K4[SiW12O40]/Al2O3 and K6[SiW11PdO39]/Al2O3 systems were studied by diffuse-reflectance FTIR spectroscopy in a spectral range of fundamental stretching vibrations, first overtones, and combination bands of stretching and bending vibrations. For hydroxyl groups, the region of combination vibrations is the most informative. The calculated frequencies of bending vibrations of hydroxyl groups (865 and 730 cm−1) are characteristic of acidic OH groups.

Structure of Dimerized Alkoxy Species of 2-Methylpropene on Zeolites and Silica−Alumina Studied by FT-IR

Adsorption and reaction of 2-methylpropene (isobutene) on zeolites (ZSM-5 and faujasite (Y zeolite)), a silica−alumina, and a silica were investigated below room temperature by infrared (IR) spectroscopy. On silica, isobutene molecules that adsorbed on silanol groups simply desorbed below 230 K in vacuum, while isobutene π-bonded to the acidic OH groups (π-complex) on acid catalysts reacted to dimer alkoxy (2,4,4-trimethylpentoxy) groups. The structure of the reaction product, dimer alkoxy groups, was found to be restricted by the pore size of zeolites; 2,4,4-trimethyl-2-pentoxy species were identified on silica−alumina and Y zeolite similarly to those on mordenite, while the reaction of isobutene resulted in 2,4,4-trimethyl-1-pentoxy species on the acidic OH groups in the pores of ZSM-5. Therefore, the space restriction of the pore size of zeolites on the reaction product was directly observed by the low-temperature IR study.

Stable Dimerized Alkoxy Species of 2-Methylpropene on Mordenite Zeolite Studied by FT-IR

Adsorption and reactions of 2-methylpropene (isobutene) on mordenite were investigated between 150 and 300 K by infrared (IR) spectroscopy. The methyl groups of isobutene interacted with the acidic OH groups of mordenite below 160 K, which was converted to the strongly interacted π-bonded isobutene to the acidic OH groups (π-complex). Above 200 K, the π-bonded isobutene dimerized even under evacuation condition. The stable 2,4,4-trimethyl-2-pentoxy species was identified by FT-IR by comparison with the spectra of the adsorbed C8 olefins. The expected monomeric reaction intermediate, the 2-methyl-2-propoxy (tert-butoxy) species, was not observed. This is probably due to the rapid migration and reaction of the adsorbed isobutene molecules.

Effect of Broensted and Lewis sites in ferrierites on skeletal isomerization of n-butenes

Skeletal isomerization of n-butenes to isobutene at 620 K was followed over NaH-ferrierites (FER) with different concentration of Broensted sites, prepared by NH 4 + vs. Na + ion exchange, and over H-FER containing both, the Broensted and Lewis sites formed by zeolite dehydroxylation at temperatures ranging from 670 to 990 K. Quantitative analysis of IR spectra of OH groups and adsorbed d 3-acetonitrile on ferrierite enabled determination of the concentration of Broensted and Lewis sites, and detected also those present in 8-ring ferrierite channels. With H-FER, the yield of isobutene is linearly proportional to the concentration of acidic OH groups. Formation of C 3 and C 5 olefins, C 2–C 5 paraffins and higher molecular weight products, resulting from di-, oligomerization of butenes followed by cracking/hydrogen transfer reactions, is enhanced by the presence of Lewis sites, if simultaneously present with the Broensted sites, that is in a substantial change of the selectivity of the reaction. On the other hand, both the conversion and yield of isobutene are nearly constant at decreasing concentrations of OH groups in NaH-FER, and only at low-concentration range of acidic hydroxyls a decrease in conversion was found. This indicates that the exchanged Na + ions are placed preferably in small ferrierite channels, as also evidenced by 23Na MAS NMR, which size is not large enough to accommodate isobutene molecules. Thus, besides the well-known positive effect of the inner geometry of ferrierite channels on isobutene selectivity, it has been shown that the reaction of n-butenes to isobutene is proportional to the number of hydroxyls located in large channels of ferrierite, and simultaneous presence of Lewis sites enhances di-,oligomerization/cracking reactions leading to higher yields of by-products.

Interpretation of IR spectra of adsorbed compounds on tectosilicate surfaces

Interaction of pyridine, benzene and methanol with the surface OH groups of a novel type of mesoporous material was investigated. No acidic OH groups were evidenced by pyridine adsorption on any of the materials. From the reversible character of benzene adsorption weak, physical adsorption was concluded. Chemisorption was only proved for methanol adsorption.

Catalytic properties of zeolite CeY modified with NH 4F

Substantial increase in catalytic activity for cumene cracking of zeolite Y modified by exchanging of ∼50% of original Na cations with Ce 3+ (CeY) has been attained by its treatment with NH 4F solution followed by calcination. Enhanced activity was also noticed, when cerium loading was increased up to 70% of ion exchange degree and the Ce modified zeolite was impregnated with NH 4F and then calcined. The catalytic results are consistent with the IR spectra of the zeolite acidic OH groups.

IR spectroscopy study of CO and NOx adsorption on a Cu/Zr-HMS catalyst

Abstract The adsorption of CO, NO and NO 2 and the co-adsorption of CO+NO and NO+O 2 on Cu/Zr-HMS have been investigated by infrared spectroscopy. Carbon monoxide and nitrogen monoxide are adsorbed selectively on Cu + and Cu 2+ cations, respectively. Owing to back π -donation, the Cu + –CO complexes [ ν (CO) at 2132 cm −1 ] are relatively stable towards evacuation and water vapor treatment. On the contrary, the Cu2 + –NO nitrosyls [ ν (NO) at 1888 cm −1 ] are easily decomposed during evacuation. Adsorption of NO 2 and co-adsorption of NO and O 2 cause the formation of weakly adsorbed N 2 O 4 (a band at 1740–1730 cm −1 ) and N 2 O 3 (bands at 1904 and 1540 cm −1 ) as well as strongly bound nitrates (bands at 1620–1550 cm −1 ). The latter possess a high oxidation ability, oxidizing NO at room temperature and methane at 200°C. The results obtained show that the coordination state of copper ions in Cu/Zr-HMS is closer to their state in Cu/SiO 2 and Cu/Al 2 O 3 than to that in Cu-ZSM-5, which is due to the absence of strongly acidic OH groups in Zr-HMS.

Activation of Small Alkanes on Solid Acids. An H/D Exchange Study by Liquid and Solid-State NMR: The Activation Energy and the Inhibiting Effect of Carbon Monoxide

The regioselective hydrogen/deuterium exchange taking place between isobutane and acidic OH groups of strong solid acids has been investigated both byin-situsolid state1H MAS NMR with perdeuteroisobutane and H-ZSM5 and by liquid1H and2H NMR with isobutane on various D2O-exchanged solid acids. The activation energy of the process was estimated from the variable temperaturein-situ1H MAS NMR kinetic experiments. A drastic inhibition of the hydrogen exchange was observed in all cases when carbon monoxide was present which is rationalized by the interaction of CO with carbocationic reaction intermediates.

Acidic and basic sites in NaX and NaY faujasites investigated by NH3, CO2 and CO molecular probes

The acid-base properties of samples of NaY and NaX faujasites have been investigated by adsorbing different probe molecules and measuring IR spectra. In the case of the NaY sample, only Na+ ions were found to be involved in the adsorption of CO2 and CO, confirming the overwhelming Lewis acid character of this material. In contrast, carbonate-like species were formed by adsorbing carbon dioxide on the NaX sample, due to the reaction of basic framework oxygen atoms with CO2 molecules polarised on neighbour Na+ ions. The spectroscopic analysis of NaX also showed evidence of Bronsted acid hydroxyls and OH groups bonded to extra-framework Al atoms. Ammonia adsorption revealed that the amount of Bronsted acid hydroxyls is significantly lower than the Lewis acid Na+ countercations. Moreover, small oxide particles, carrying carbonate-like species on their surface, are present in the zeolitic cavities. These particles could be responsible for the basic reactivity towards CO observed after outgassing the NaX sample at high temperature.

Structure and Density of Active Zn Species in Zn/H-ZSM5 Propane Aromatization Catalysts

Exchanged Zn cations increase propane conversion turnover rates, hydrogen formation rates, and selectivity to aromatics on H-ZSM5.In situZnK-edge X-ray absorption studies show that aqueous ion exchange with H-ZSM5 (Si/Al=14.5) leads to isolated Zn2+cations with tetrahedral symmetry. These Zn species reside at cation exchange sites as monomeric cations and form directly from nitrate solutions during ion exchange. X-ray absorption and temperature-programmed reduction studies show that Zn2+cations in Zn/H-ZSM5 do not reduce to zero-valent species during propane reactions at 773 K. Extrazeolitic ZnO crystals form on samples prepared by impregnation techniques. In contrast to exchanged Zn2+cations, these bulk ZnO crystals reduce to Zn metal and elute from the catalyst bed as Zn vapor under typical propane aromatization conditions. Condensation reactions of (Zn2+OH)+species with acidic OH groups appear to lead to the formation of the active Zn2+cations interacting with two Al sites (O−–Zn2+–O−). This active Zn species is supported by acid site density measurements from NH3titration and isotopic titration of remaining Brønsted acid sites with deuterium in Zn/H-ZSM5.

Infrared Study of Ozone Adsorption on CeO2

Ozone (O3) adsorption on CeO2 pretreated under different conditions and characterized by low-temperature CO adsorption was studied by Fourier transform infrared (FTIR) spectroscopy at 77−300 K. Preliminary exposure to CO2, pyridine, acetonitrile, or methanol at 293 K or to CO at 77 K, as well as adsorption of 18O substituted O3 were used to clarify the nature of adsorption sites and the structure of the surface species. In addition to physisorption and weak H-bonding with the most acidic OH groups of the ceria surface, O3 forms coordinate complexes with two kinds of Lewis acid sites. These complexes have the bands of the ν1 and ν3 modes of O3 at 1096 and 1018−1015 cm-1, or 1104 and 1012−1008 cm-1, respectively. The coordinated O3 molecule is no longer symmetric but is bound to a surface Ce4+ ion via one of its terminal oxygen atoms. Basic surface sites of the samples pretreated at 773 K account for O3 decomposition, which occurs almost explosively at 77 K but could be inhibited if O3 adsorption is perform...

Physicochemical and catalytic properties of a new type of as-synthesized aluminium-deficient Y zeolite

Physicochemical (IR spectroscopy, 27Al and 29Si MAS NMR spectroscopy, XRD, adsorption measurements) and catalytic properties of a new type of low-alumina Y zeolite (LAY) prepared by direct synthesis (SiO 2/Al 2O 3 molar ratios 5.5–7.2) without using any templating agents or post-synthesis dealumination procedures were studied in comparison with the conventional or dealuminated Y zeolites. The low-alumina NaH-LAY zeolites are characterized by the higher thermal stability of acidic OH groups as compared with dealuminated Y zeolite. The unit cell parameter measured by XRD remains virtually unchanged and close to 2.463 nm for Na-LAY zeolite with SiO 2/Al 2O 3 ratios of 5.85–7.2. Extraframework aluminium was not found in decationized LAY zeolite samples by 27Al MAS NMR until the residual Na 2O content reached values below 1 wt.%. In addition, it is possible to tune the activity in the catalytic cracking of n-hexane by varying the extent of decationization. The content of isomer products increases with a growth of the residual Na 2O content in LAY zeolite. Introduction of this new zeolite into the cracking catalysts provides the maximum yield of gasoline at lower catalyst/feed ratios, compared to catalysts containing dealuminated Y zeolites. The case of the commercial use of the NaH-LAY zeolite is discussed.

Structural and Textural Studies of ZnO Modified with Dodecyltrimethylammonium Chloride

The effect of dodecyltrimethylammonium chloride — a cationic surfactant — on the texture and structure of ZnO, which is commonly used in the photocatalytic degradation of surfactants, has been investigated together with the effect of thermal treatment of the resulting materials at temperatures ≤350°C. Structural changes were followed by combining diffraction and infrared (IR) spectroscopic measurements while textural variations were monitored via low-temperature nitrogen adsorption studies. In addition, the effect of soaking the original sample of ZnO in water for 5 d has been monitored together with the effect of heat treatment on the resulting products. Bands corresponding to a carbonate complex were observed in the IR spectrum of all samples subjected to thermal treatment below 275°C. This complex gave rise to new bands in the XRD patterns of such samples situated at d-distances of 2.339, 2.023 and 1.431 Å, respectively, which disappeared when the samples were subjected to heat treatment above 275°C. The intensities of such bands decreased when increasing surfactant concentrations were employed in the initial treatment of the ZnO solid. Acid OH groups on the surface of the solid appear to provide sites for the formation of the carbonate complex. Such sites are attacked either by the aqua complex [Zn(H2O)6]2+ formed from dissolved ZnO in the presence of water alone or by the cationic surfactant when the latter is present in the system. Attack of the surfactant on the surface probably occurs through exchange with an H+ ion. The cationic surfactant was also adsorbed on to basic sites present on the surface of the solid since the concentration of such sites diminished as the surfactant concentration was increased. Measurements of the specific surface areas and pore structural analyses for all the samples studied indicated the presence of micro- and meso-pores in most cases, the former being absent from the parent ZnO employed. A comparison between the application of the V l versus t plot and the α s-plot for such pore structural analyses is presented together with a discussion of the criteria necessary for the application of the αs-method.

Interaction of water with a fatty acid Langmuir film

Using sum-frequency vibrational spectroscopy we have studied the structure of the fatty acid monolayer–water interface as a function of pH of water. At low pH values, the monolayer is neutral and the surface water structure is disordered by hydrogen bonding of water molecules with the acid head-groups. At high pH values, acid head-groups are ionized and the resulting surface field helps the surface water molecules form a more ordered hydrogen-bonding network. At pH ∼3, a small change in pH leads to a reorientation of the acid OH groups while the alkyl chains remain conformationally ordered at all pH values.

IR study of the low temperature adsorption of CO on tetragonal zirconia and sulfated tetragonal zirconia

In situ FTIR spectroscopy has been used to study the adsorption at ∼77 K of CO on (i) tetragonal zirconia (t-ZrO 2), (ii) non-calcined sulfated tetragonal zirconia (t-SZ) and (iii) calcined sulfated tetragonal zirconia ([t-SZ] C). CO adsorption at low temperature turned out to be a suitable probe to test the surface charge-withdrawing properties of the various zirconia-based systems. On t-ZrO 2 it has been possible to distinguish the H-bonding adsorption of CO on at least two families of acidic surface OH groups and the coordinative adsorption of CO on surface Lewis acidic sites (i.e. coordinatively unsaturated Zr 4+ surface centres) located either in regular patches of low-index crystal planes (the `top' termination of ZrO 2 crystallites) or in defective terminations of the particles (the `side' termination of ZrO 2 crystallites). On t-SZ, virtually only CO uptake by H-bonding on some surface OH groups is observed, as virtually all of the non-hydroxylated parts of the surface are occupied and maintained coordinatively saturated by sulfate groups. On [t-SZ] C, the calcination process selectively eliminates the sulfates that were initially located in the defective crystal terminations. As a consequence CO uptake reveals, besides two well resolved families of OH groups yielding adsorption by H-bonding, two well resolved families of Lewis acid sites located in defective crystal terminations (strong Lewis sites), whereas no CO uptake occurs in the regular terminations of the crystallites. The latter crystal positions remain occupied by a family of sulfates, that turn out to be responsible for the catalytic activity of [t-SZ] C systems.

Theoretical Study of the Mechanism of Zeolite-Catalyzed Isomerization Reactions of Linear Butenes

Density functional theory is used to study the mechanism of double-bond isomerization and skeletal isomerization of linear butenes catalyzed by a protonated zeolite, which is simulated by a cluster consisting of two Si and one Al tetrahedra. The study includes complete geometry optimization and characterization of reactants, products, reaction intermediates and transition states, and calculation of the activation energies for the different processes involved. It is shown that the double bond isomerization proceeds by a concerted mechanism which does not involve the formation of either ionic or covalent alkoxy intermediates. According to this concerted mechanism, in one step the acid OH group of the zeolite protonates the double bond of adsorbed but-1-ene and the basic neighboring O atom of the cluster abstracts a hydrogen from the olefin, restoring the zeolite active site and yielding adsorbed but-2-ene. However, the mechanism of skeletal isomerization of linear butenes consists of three elementary steps: protonation of adsorbed but-1-ene to give a secondary alkoxy intermediate, conversion of the secondary alkoxy intermediate into a branched primary one through a cyclic transition state in which the transferring methyl group is halfway between its position in the linear and in the branched species, and decomposition of the primary alkoxy intermediate to give adsorbed isobutene. The activation barriers calculated for the two reactions are in good agreement with experimental data.