Location of Mg cations in mordenite zeolite studied by IR spectroscopy and density functional theory simulations with a CO adsorption probe

The location of Mg cations in the channel of mordenite zeolite was studied using a combination of DFT simulations and IR spectroscopy of adsorbed CO. The calculated adsorption energies and frequencies of CO on Mg cations are in good agreement with the results from the IR spectra of adsorbed CO. It is found that the Mg cations can occupy the sites A, C, D and E in mordenite and the distribution of the Mg cations in these sites follows the priority order, site C > site A > site D and site E.     

CO2 sorption on MgO and CaO surfaces: a comparative quantum chemical cluster study

A comparative quantum chemical study of CO2 adsorption on MgO and CaO has been carried out. Theoretical infrared (IR) frequencies are calculated and compared with IR experiments from the literature. The results show that CO2 adsorbs as monodentate on edge sites and bidentate on corner sites on MgO. The former assignment contradicts the common assumption of adsorption of CO2 in a bidentate configuration. On CaO, CO2 adsorbs as monodentate on both edge and corner sites, which is a reinterpretation of earlier experimental work. On terrace (100) sites, none of the adsorption modes on MgO or CaO possess calculated frequencies in agreement with the experimental IR spectra. These experimental bands were tentatively assigned to some slightly perturbed double negatively charged carbonate ions at the surface, rather than the monodentate structure suggested in the literature.     

CO在载钯分子筛薄膜电极上的增强红外吸收

以NaY分子筛超笼为微型反应器,通过“瓶中造船”技术合成钯原子簇(Pd₁₃).并首次把粗糙的分子筛薄膜电极引入到电化学原位红外反射光谱研究,发现了一类新的增强红外吸收现象.与金属钯电极相比,CO红外吸收带的谱峰强度显著增加,半峰宽加宽.研究结果表明,这种增强红外吸收取决于钯原子簇的纳米尺度及分子筛超笼的特殊环境.     

Characterization of CuY zeolites after dehydration,oxidation and reduction with carbon monoxide An adsorption and 129Xe nuclear magnetic resonance spectroscopy study

The adsorption isotherms and ¹²⁹Xe nuclear magnetic resonance (NMR) chemical shifts of xenon and the adsorption isotherms of carbon monoxide of Cu(II)- and Cu(I)-exchanged zeolites NaY were measured. The former zeolites of 53, 75, and 95% exchange degrees were investigated after various pretreatment steps comprising dehydration, oxidation and reduction with CO at 420°C as well as long-term CO reduction at 470°C. The Cu(I)Y zeolite of 70% exchange degree was prepared via a solid-state exchange procedure with CuCl and subjected to dehydration at 420°C. In all cases, except the dehydrated zeolites, almost linear xenon adsorption isotherms and linear ¹²⁹Xe NMR chemical shift _versus xenon concentration curves running parallel to each other are obtained. In contrast, the chemical shift curves for the dehydrated zeolites are non-linear at low xenon concentrations turning towards negative chemical shift values at very low pressures. The whole body of the experimental xenon data can be explained quantitatively with a unifying approach on the basis of a site adsorption model where the sites are (i) two types of cuprous ions of much different adsorption strength and ¹²⁹Xe chemical shift, (ii) Na⁺ cations, (iii) Lewis acid sites generated through autoreduction and reduction of Cu²⁺ by CO, and (iv) framework sites free of cations. These five types of sites are each characterized by Langmuir adsorption isotherm constants and local ¹²⁹Xe NMR chemical shifts. The adsorption site concentrations in the various zeolites are evaluated. The supercage Cu(I) concentration values are in nice agreement with the results deduced from the CO adsorption isotherm measurements.     

Identification and measurements of strong Brønsted acid site in ultrastable Y (USY) zeolite

By using the IRMS-TPD method in which IR (infrared) and MS (mass spectroscopy) worked together, acid sites of USY (ultrastable Y) zeolite were studied. A new band of OH playing a role of Br?nsted acid was clearly detected on Na2H2-ethylenediaminetetraacetic acid (EDTA)-treated USY at 3595 cm(-1) during an elevation in temperature after the adsorption of ammonia. MS-measured TPD (temperature-programmed desorption) of NH3 and IR-measured TPD of the NH4+ cation coincided well to show that this zeolite consisted of the Br?nsted acid sites. The MS-TPD profile at higher temperatures corresponded to the IR-TPD of the 3595-cm(-1) band, and therefore, this OH was identified as a strong acid site. From comparison between IR-TPD of OH and MS-TPD, numbers of three kinds of Br?nsted OH (i.e., those in super and sodalite cages of a Y zeolite structure) and created strong Br?nsted acid site were quantified. On the other hand, strength of the Br?nsted acid site DeltaH was determined individually by a simulation method, where the corrected IR-TPD of OH was simulated based on the proposed equation. Thus, a new strong Br?nsted acid site was identified in the EDTA-treated USY, and the amount and strength was measured quantitatively.     

草酸脱铝Y沸石的性质

草酸与草酸铵脱铝的Ｙ沸石，具有八面沸石结构和良好的结晶度。羟基红外光谱和吡啶吸附的红外光谱表明，在脱铝Ｙ沸石的骨架中存在着铝空位，这种缺铝Ｙ沸石中由３５５０ｃｍ＾－１表征的小笼羟基更具开放性，但稳定性较差，在升温过程中，它比３６４０ｃｍ＾－１表征的大笼羟基更易脱除。吡啶与ＣＯ双探针分子红外光谱的研究表明，升温脱羟基过程伴随着与Ｌｅｗｉｓ酸有关的非骨架铝的生成，低温Ｎ２吸附的结果显示，缺铝Ｙ沸石中还     

原位红外光谱法研究脱铝Y型沸石表面羟基及异丙苯裂解反应

本文用原位红外光谱法, 以自行设计的红外池, 采用流动体系研究了NH4Y及用EDTA配位脱铝的Y型沸石上的表面羟基, 苯及异丙苯在不同温度下的吸附, 以及用脉冲进样原位红外研究了异丙苯裂解反应。发现在脱铝Y型沸石(DAIY)中除Y型沸石有的3620和3535cm^-^1表面羟基峰外, 在3724cm^-^1处又出现一个与脱铝“空穴"Si-OH有关的吸收峰, 且此峰随脱铝量增加而增加, 脱氨过程的研究表明超笼中羟基具有酸性, 苯及异丙苯Y型或DNIY型沸石上均与超笼中羟基作用。在360℃时异丙苯裂解反应的活性中心主要是超笼中羟基, 催化剂失活主要是由于炭沉积在活性位上所致。     

Nitrogen as a Probe Molecule for the IR Studies of the Heterogeneity of OH Groups in Zeolites

One of the methods of IR studies of the heterogeneity of Si–OH–Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si–OH–Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si–OH–Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si–OH–Al of various number of Al near the bridge and of Si–OH–Al of various geometry.     

An IR Study on Adsorption of CO and NO on Copper Ion-exchanged Zeolite Beta

Thee adsorption of CO and NO on copper ion-exchanged zeolite Beta was investigated using IR method.It was found that the thermalvacuum pretreatment procedure could result in the reduction of Cu2 ions in zeolite Beta.The adsorption of CO on Cu sites in zeolite Beta closely follows Langmuir isotherm.Another Cu species may form during the reaction between water and CO.The catalytic decomposition of NO on the zeolite was observed at room temperature,indicating that the decomposition reaction may occur between two coordinated NO ligands of the same dinitrosyhc complex.Furthermore,the appearance of two series of NO adsorption bands reveals that copper ions existing at different cation sites may have different effect on the adsorption and decomposition of NO molecules.     

CuCl改性Y型分子筛的CO吸附性能研究

利用自发单层分散原理,讨论了在NaY,NH4Y,CuY分子筛上CuCl分散改性情况,并对所得到的改性分子筛进行了CO吸附性能研究。实验结果表明,在低CuCl担载量时,CuCl可在这三种载体表面达到原子水平分散,其分散容量分别为0.58g/gNaY,0.68g/gNH4Y和0.41g/gCuY。由于CO与Cu^+可生配位化合物,经CuCl改性的Y型分子筛对CO显示出较高吸附性能。其中,0.6gCuCl/gNH4Y分子筛表现出最高的CO吸附容量,在室温、常压下可达123mL/gNH4Y.     

Acidic properties of [Al], [Ga] and [Fe] isomorphously substituted zeolites. Density functional model cluster study of the complexes with a probe CO molecule

The Brønsted acid strength and related characteristics of bridging hydroxyl groups in [Al]-, [Ga]- and [Fe]-framework-substituted zeolites have been studied using a model cluster density functional approach based on a gradient-corrected exchange-correlation energy functional. The acidity is found to decrease in the order Al(OH)Si > Ga(OH)Si > Fe(OH)Si in agreement with existing experimental and theoretical results. The present quantification is based on the calculated deprotonation energy of H₃Si(OH)TH₃, on the adsorption energy of a CO probe molecule as well as on the changes of the vibrational frequencies and absolute IR intensities of the O---H and of the C---O modes induced by CO adsorption. The vibrational parameters of the carbonyl adsorption complex in [Fe]-zeolites are predicted.     

Identity of two types of strong Brønsted acid sites in mazzite revealed by CO probe: IR study and periodic DFT modeling

The nature of catalytic Brønsted sites in mazzite is clarified by molecular modeling combined with spectroscopy. Density Functional Theory study for periodic models of high-silica mazzite evidence that most stable bridging hydroxyls, noticeably binding CO probe, fall into two categories: Brønsted sites located in larger channels, characterized by higher OH frequency of bare hydroxyl with very large redshift upon CO interaction, and lower-frequency sites located in smaller channels, showing lower redshift. This fully corresponds to two bands obtained for OH stretch in IR spectra. Very good agreement between theory and experiment found in this work not only confirms that Brønsted sites studied here belong to the strongest acid sites among known zeolites but also clarifies their identity in mazzite. Location of sites with exceptionally large red shift upon CO adsorption at 12-T wide channel very well conforms to both intuitive expectations and predictions for other zeolites from former studies.     

Vibrational Spectra of Zeolite Y as a Function of Ion Exchange

Zeolite Y is one of the earliest known and most widely used synthetic zeolites. Many experimental investigations verify the valuable ion exchange capability of this zeolite. In this study, we assessed the effects of ion exchange on its vibrational spectra. We applied classical lattice dynamics methods for IR and Raman intensity calculations. Computed spectra of optimized zeolite Y structures with different cations were compared with experimental data. The spectra obtained in this study are in agreement with previous experimental and computational studies on zeolites from the faujasite group.     

Density functional theory of water-gas shift reaction on molybdenum carbide

The density functional theory (DFT) of the water-gas shift (WGS) reaction over molybdenum carbide was studied with the aim of understanding the dissociation of H(2)O, the OH group, and CO to determine on what sections of molybdenum carbide CO(2) and H(2) formed and whether they played a role in the reaction. The energy diagram of each elementary step, the reaction of the hydrogen and oxygen atoms with CO, and the transition state for this elementary step were also studied. The IR spectra of the CO adsorption was experimentally analyzed for the identification of several candidates of the CO adsorption modes. The adsorptions of the threefold Mo site (a) with and (b) without the underlying C atom of the second layer have the second and highest adsorption energies of -281.59 and -321.00 kJ/mol, respectively. The IR data showed that the bands at 1626 cm(-1) from the IR experiments are (a) the nearest adsorption of the threefold Mo site with the underlying C atom at the calculated/corrected band of 1621 cm(-1). The calculated/corrected threefold adsorption (b) had the highest adsorption energy but exhibited an IR band at 1147 cm(-1) which was not observed in the experimental data. The C-O bond length increased to 1.49 from 1.36 after the H(2)O adsorption (b), suggesting the dissociation of C-O after the H(2)O coadsorption. The WGS reaction on the beta-Mo(2)C(001) slab carbide was calculated and took place as follows: H(2)O was dissociated into OH and H on the Mo(2)C surface and the OH subsequently dissociated into H and O atoms. CO approached the O atom to form CO(2).     

Influencing the selectivity of zeolite Y for triglycine adsorption

In prior work we studied the adsorption of triglycine on zeolite Y under reference conditions. This study aims to solve the question of which adsorbent properties and process conditions influence the adsorption triglycine from an aqueous solution by zeolite Y. Relevant zeolite parameters to study are: the amount of acidic sites (Si/Al(2) ratio), counter ion, micropore structure. Process conditions that may influence triglycine adsorption are pH, other components such as sugars, amino acids and salts, and temperature. Adsorption of triglycine on zeolite HY is dominated by ionic interaction. The capacity and selectivity of zeolite HY for triglycine can be changed by choosing different Si/Al(2) ratios or changing the counter ion. The presence of cations and basic anions in solutions reduces triglycine adsorption. Fructose and glycine have no significant influence on triglycine adsorption. Temperature only has a slight influence. The pore structure of zeolite Y is not a critical factor for triglycine adsorption, provided pores are accessible to triglycine and in the micropore range (<2 nm). While this allows other zeolites than zeolite Y to be applied, the presence of the supercage structure of zeolite Y is beneficial to obtain better adsorption of triglycine in its neutral form.     

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).     

Molecular water on exposed Al3+ cations is a source of acidity in silicoaluminas

Evidence about the remarkable acidic nature of molecular water adsorbed at the surface of mesoporous MCM-41 silicoaluminas (MSA) with a ratio of Si/Al = 25 is reported, coming from both infrared spectroscopy and ab initio calculations. By using CO as a probe, OH stretching and HOH bending modes of water adsorbed on coordinatively unsaturated Al ions (Lewis sites) at the surface of MSA have been detected for the first time. CO adsorption causes OH stretching frequencies to suffer a bathochromic shift of about -200/-250 cm(-1), whereas the HOH bending frequency undergoes a hypsochromic shift by about +10/+20 cm(-1). B3LYP/6-31+G(d,p) calculations on model clusters designed to mimic both Br?nsted and Lewis sites validate the assignments, showing that water adsorbed on the surface Lewis site, when interacting with CO, shows an acidity comparable to that of a classical zeolitic Br?nsted site.     

Effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites

Zeolites NaY and Ce(IV)Y were employed as adsorbents to remove organic sulfur compounds from model gasoline(MG) solutions with and without toluene in static adsorption experiments at room temperature(RT) and atmospheric pressure.The adsorbents were characterized by XRD,XRF and pyridine infrared spectrum(IR).The adsorption experiments show that the desulfurization performance of Ce(IV)Y is much better than that of NaY.The sulfur removal over both NaY and Ce(IV)Y decreases with the increase of toluene concentration in MG,however,the decline tendency on Ce(IV)Y is smooth,and it is steep on NaY.FT-IR spectra of thiophene adsorption indicate that thiophene molecules are mainly adsorbed on NaY via π electron interaction,but on Ce(IV)Y,in addition to the π electron interaction,both Ce4+-S direct interaction and protonation of thiophene also play important roles.Toluene molecules are adsorbed on NaY also via π electron interaction.Although the amount of Brnsted acid sites is increased due to the introduction of Ce4+ ions into NaY zeolite,it is not found to influence the adsorption mode of toluene over Ce(IV)Y.Compared with NaY zeolite,the improved desulfurization performance over Ce(IV)Y for removing organic sulfur compounds from MG solution,especially those containing large amount of aromatics,may be ascribed to the direct Ce(IV)-S interaction,which is much resistant to the influence resulted from toluene adsorption.