Infrared Spectroscopic Studies on the Surface Chemistry of High‐Surface‐Area Gallia Polymorphs

Polymorphs α, β, and γ of Ga₂O₃ having hexagonal (corundum‐type), monoclinic and cubic (spinel‐type) structure, respectively, were prepared in a high‐surface‐area form, and characterized by powder X‐ray diffraction. Nitrogen adsorption at 77 K showed these gallia samples to have specific surface areas of 77 (α‐Ga₂O₃), 40 (β‐Ga₂O₃) and 120 m² g^?1 (γ‐Ga₂O₃). Fourier transform infrared spectroscopy of adsorbed carbon monoxide (at 77 K) and pyridine (at room temperature) showed that the three gallia polymorphs have a very similar surface Lewis acidity, regardless of their different crystal structures. This Lewis acidity was assigned, mainly, to coordinatively unsaturated tetrahedral Ga³⁺ ions situated on the surface of the small crystallites which constitute the different metal oxide varieties. Ga³⁺···CO adducts formed after CO adsorption gave (in all cases) a characteristic C–O stretching band at 2195–2200 cm^?1, while Lewis‐type adducts formed with adsorbed pyridine were characterized by IR absorption bands at 1610–1612 and 1446–1450 cm^?1. The three (partially hydroxylated) gallia polymorphs showed also a very weak Brønsted acidity, which they manifested by forming hydrogen‐bonded adducts with both CO and pyridine; however no protonation of adsorbed pyridine occurred.     

改性Y分子筛的酸碱性能及吸附性能的研究

吡啶、吡咯、苯、甲苯、乙苯作为探针分子，在Li＋, Na＋, K＋, Cs＋改性的Y型分子筛上进行吸附，用TPD及IR方法系统地研究了不同碱金属离子改性的Y型分子筛的酸碱性能和吸附性能的变化. 结果表明，按Li、Na、K、Cs的顺序, 随着碱金属离子半径的增大, 其L酸酸强度依次减弱，L碱的强度逐渐增强.由于改性Y型分子筛所含碱金属离子的不同，其对芳烃的吸附的强弱及吸附量的大小亦不同.随着骨架外的阳离子的半径逐渐增大，碱的强度逐渐增强，与芳烃的作用愈强烈，导致TPD脱附峰温增高及芳烃和侧链上的C－H伸缩振动谱带向低波数位移愈多.由于位阻的原因，对含同一种碱金属离子的分子筛来说，随着芳烃侧链C数的增加，芳烃的吸附量逐渐减小.     

A convenient large scale synthesis of 2,6-dimethyl-4-(trimethylstannyl)pyridine

Reaction of phosphorus oxychloride with 2,6-dimethylpyridine N-oxide hydrochloride ( 1 ) gave a mixture of 2-(chloromethyl)-6-methylpyridine ( 2 ) and 4-chloro-2,6-dimethylpyridine ( 3 ). Treatment of this mixture with triethylamine converted 2 to the quaternary salt 4 which was separated by water extraction leaving 3 which was subsequently reacted with trimethylstannyl sodium to yield 2,6-dimethyl-4-(trimethylstannyl)pyridine ( 6 ).     

离子液体酸性的红外光谱探针法研究

以吡啶和乙腈分子为探针, 使用红外光谱法研究了常用室温离子液体的酸性. 采用吡啶为探针分子时, 出现的～1450 cm^－1、～1540 cm^－1吸收带可以分别指示离子液体的Lewis、Brønsted酸性；采用乙腈为探针分子时, ～2253 cm^－1的C≡N伸缩振动向高波数移动并伴有新峰的出现，可以指示离子液体的Lewis酸性。可以通过比较吡啶探针～1450 cm^－1吸收带的峰位置对离子液体的Lewis酸强度进行排序，并且可以用乙腈探针更灵敏地区分出不同离子液体的Lewis酸的强度，使用该方法研究了离子液体的结构对其Brønsted / Lewis酸性的影响。     

Infrared spectroscopic study of the CO2 adsorption effect on the surface acidity of zinc oxide

Taking account the bands ν_a(O¹²CO) and ν_a(O¹³CO) (in natural abundance) due to linear species, the study by FT-infrared spectroscopy of the CO₂ adsorption on ZnO has shown particular sites which can be described as Zn²⁺ ions with two vacancies, with a reactive oxygen ion in an adjacent position. Using the usual probe molecules (CO, acetonitrile, ammonia, pyridine), we have compared the Lewis acidity of ZnO and ZnO which has been pretreated with CO₂. This preadsorption causes an increase of the Lewis acidity. Moreover we have found that it prevents the dissociative adsorption of acetonitrile, ammonia, pyridine, propene and butenes. The model site described above well accounts for these results. It is concluded that the increase of the acidity due to CO₂ preadsorption is related to the formation of bidentate carbonates.     

ZSM-5型分子筛的表面酸性与催化活性

合成了一批不同硅铝比的HZSM－5沸石分子筛，并用CO62 ,Fe^3 离子对其进行改性，测定了它们在醛氨缩合反应合成吡啶中的活性和选择性。对照它们的NH3－TPD，吡啶吸附红外光谱，研究了催化剂的表面酸性与醛氨缩合催化活性的关系。研究表明，硅铝比较小时，HZSM－5的酸中心较多，但过多的酸中心会引发其它的裂解反应和缩合反应，从而降低催化剂的活性和选择性；当硅铝比为120左右时，催化活性达到最大，吡啶碱产率达60％；若继续增大硅铝比，则无足够的酸中心进行反应，用Co^2 ,Fe^3 离子对HZSM进行改性，其B酸中心变化不明显，L酸中心减少，对氨气的吸附能力有所下降，从而保证了适当的酸中心暴露，有利于反应，采用CoZSM-5催化剂，吡啶碱产率可达78％。     

氯铝酸离子液体酸性的吡啶探针红外光谱研究

利用吡啶探针红外光谱法研究了氯铝酸离子液体的酸性,发现当氯铝酸离子液体A lC l3的摩尔分数x为0.4~0.5时,离子液体表现出弱Lew is酸的红外特性。对吡啶探针吸附Lew is酸位A l2C l7-、A lC l4-、A l2C l6和A lC l3的各振动模式红外特征峰峰位进行了归属,通过红外特征峰峰位置和峰面积可以指示离子液体的酸强度。利用吡啶探针递增吸附法考察了吡啶递增吸附对氯铝酸离子液体酸性位的影响。研究发现,当吡啶量增加到一定程度时,Py-A l2C l7-配位络合物容易与吡啶发生反应生成Py-A lC l4-和Py-A lC l3配位络合物,从而导致离子液体的吡啶吸附红外特征峰发生变化。     

铁硅酸盐分子筛乙苯氧化脱氢催化性能研究Ⅱ．FeZSM－5的酸碱性质和积炭对反应活性的影响

本文应用碱性气体吸附的红外光谱和ＴＰＤ技术研究了ＦｅＺＳＭ－５（Ｆ＿４）的酸性，酸性顺序为：Ｈ－Ｆ＿４＞Ｆｅ＿２Ｏ＿３／Ｆ＿４＞Ｆ＿４原粉＞水热处理的Ｋ－Ｆ＿４＞Ｌｉ－Ｆ＿４＞ＮａＦ＿４＞Ｋ－Ｆ＿４＞ＮａＯＨ／Ｆ＿４，比较了酸强度和积炭对乙苯氧化脱氢性能的影响，以及催化剂酸碱性质与积炭的关系．结果表明，乙苯氧化脱氢反应是在酸碱协同作用下进行的，积炭也参与了氧化脱氢反应，催化剂表面上Ｌ酸中心产生的具有一定Ｃ、Ｈ、Ｏ比的积炭具有高的氧化脱氢活性．碱金属阳离子改性的具有Ｌ酸中心的ＦｅＺＳＭ－５是乙苯氧化脱氢的高活性催化剂。     

FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions

The acidity of two dealuminated HY zeolites, obtained either by isomorphous substitution or by hydrothermal treatment at 550°C followed by acid leaching, was characterized by IR spectroscopy in dynamic or static modes. The probe molecules used were 2,6-dimethylpyridine, pyridine and deuterated acetonitrile. They showed that the steamed sample presented very strong acidic Brønsted sites, characterized by OH groups giving rise to a ν (OH) band at 3600 cm⁻¹, and Lewis acidity. The steamed sample was found active for n-hexane cracking at 400°C. Use of an IR cell working as a flow reactor and specific poisoning of hydroxyl groups by 2,6-dimethylpyridine evidenced the catalytic role played by the 3600 cm⁻¹ OH groups. On the other hand, cyclohexene conversion, a much less demanding reaction, occurred on both types of zeolite. All the accessible acidic hydroxyls appeared to be active and results were discussed according to the various reactions observed: cyclohexene isomerization, hydrogen transfer and cracking. The intensity of a band at 1586 cm⁻¹, assigned to carbonaceous deposits, was correlated to catalyst deactivation. The results showed that the 3600 cm⁻¹ OH groups were first consumed under cyclohexene flow, due to a rapid poisoning by coke. Adsorption of probe molecules on deactivated samples evidenced that the Lewis acid sites were almost unaffected whatever the reaction, suggesting that they do not play a significant role. This was confirmed by comparison of activity measurements on both types of zeolite towards cyclohexene conversion.     

Acidity, surface structure, and catalytic performance of WO(x) supported on monoclinic zirconia

The relationship between the acidity, catalytic activity, and surface structure for tungsten oxide supported on zirconia was investigated for a series of solids prepared by equilibrium adsorption on monoclinic zirconia. The catalysts were active for propanol dehydration only above a threshold in W loading. The acidity was studied by infrared spectroscopy of adsorbed probe molecules (2,6-dimethylpyridine and CO), and the onset of activity was correlated with that of the formation of relatively strong Br?nsted acid sites. The variation in the abundance of these sites correlated with the catalytic activity. Lewis sites were present but could not be directly associated with the activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric W surface species.     

Caractérisation part FT-IR des propriétés supercicielles et acido-basiques de la zircone phosphatée

FT-IR Characterization of superficial and acidic properties of phosphated zirconia. The nature of phosphate species formed by impregnation of zirconia by (NH₄)₂HPO₄ followed by decomposition at 400°C was studied by IR spectroscopy. These species present hydroxyl groups characterized by a ν(OH) band at 3666 cm⁻¹ due to the P-OH stretching vibration. The structure of phosphate species was examined using H₂¹⁸O isotopic exchange. Acid-base properties of phosphated zirconia was investigated by CO₂, pyridine and acetonitrile adsorption followed by FT-IR spectroscopy. The introduction of phosphated species, creates Brønsted acid sites on zirconia. A slight increase in Lewis acidity and a strong decrease in basicity is also detected.     

IR spectroscopy of N-methylpyrrole adsorbed on oxides--a probe of surface acidity

IR spectra of N-methylpyrrole (NMP) have been measured following adsorption on, and subsequent desorption from, SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3), H-mordenite, and sepiolite. Three modes of adsorption have been observed: (i) hydrogen bonding to surface hydroxyl groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Br?nsted acidic surface sites. Protonation of NMP was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Br?nsted acidic sites with pK(a) values 相似文献   

Revisiting acido-basicity of the MgO surface by periodic density functional theory calculations: role of surface topology and ion coordination on water dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Br?nsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.     

Adsorption of 2-chloropyridine on oxides--an infrared spectroscopic study

The adsorption of 2-chloropyridine on SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3) and H-mordenite has been studied by IR spectroscopy. The different modes of interaction with oxide surfaces, i.e. hydrogen-bonding and adsorption at Br?nsted or Lewis acid sites, was modelled by ab initio calculations at the B3LYP/DZ+(d) level. Adsorption on SiO(2) results in hydrogen bonding to surface hydroxyl groups, whereas the spectra obtained following adsorption on TiO(2) and ZrO(2) display evidence for electron transfer at Lewis acidic surface sites. Protonation of 2-chloropyridine at Br?nsted acidic sites was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Br?nsted acidic sites on these oxide surfaces with pK(a) values 相似文献   

Intrinsic acidity of dimethylhalonium ions: evidence for hyperconjugation in dimethylhalonium ylides in the gas phase

The intrinsic acidity of dimethylhalonium ions has been determined, both by theoretical methods and by gas-phase reactions of the isolated ions with pyridine bases. The calculated geometry of the dimethylhalonium ions shows a bent structure with the C-X-C angle decreasing in the order Cl > Br > I. Thermochemical calculations for the reaction of the dimethylhalonium ions with pyridine, 2,6-dimethylpyridine, and 2,6-di-tert-butylpyridine indicate that proton transfer, with the formation of the dimethylhalonium ylide is endothermic, whereas methyl transfer, with formation of methylhalide, is exothermic. The endothermicities for proton transfer are, nevertheless, dependent on the steric hindrance of the base. The bulkier the bases, the less endothermic the proton-transfer reaction is. Experimental gas-phase reactions support the calculations, showing that methyl transfer is the major reaction of dimethylchloronium and dimethyliodonium with pyridine, whereas proton transfer, as well as single electron transfer, is observed for the bulkier bases. The calculations also indicate that acidity increases in the order chloronium > bromonium > iodonium. NBO calculations predict that hyperconjugation with the sigma carbon-halogen orbital plays a role in stabilizing the halonium ylide species in the gas phase.     

丝光型Si-Fe沸石表面酸碱性的研究

本文藉助红外光谱(IR)、程序升温脱附—质谱(TPD—MS)等手段研究了丝光型Si—Fe沸石分子筛表面的酸碱性质。结果表明:丝光沸石骨架中的铝原子被铁原子取代以后,其表面酸性大为减弱,但其碱性相对增强,Si—Fe沸石分子筛大部分表面羟基的酸性很弱,仍有一定量的L酸中心存在于沸石的表面上。     

Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

Reactions of the β-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg(n)Bu], with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH(3) has been found to result in dearomatisation of the N-heterocyclic compounds. This reaction is proposed to occur through the formation of an unobserved N-heterocycle-coordinated magnesium hydride and subsequent hydride transfer via the C2-position of the heterocycle prior to hydride transfer to the C4-position and formation of thermodynamically-favoured magnesium 1,4-dihydropyridides. This reaction is kinetically suppressed for 2,6-dimethylpyridine while the kinetic product, the 1,2-dihydropyridide derivative, was isolated through reaction with 4-methylpyridine (4-methylpyridine), in which case the formation of the 1,4-dihyropyridide is prevented by the presence of the 4-methyl substituent. X-ray structures of the products of these reactions with 4-methylpyridine, 3,5-dimethylpyridine and iso-quinoline comprise a pseudo-tetrahedral magnesium centre while the regiochemistry of the particular dearomatisation reaction is determined by the substitution pattern of the N-heterocycle under observation. The compounds are all air-sensitive and exposure of the magnesium derivatives of dearomatised pyridine and 4-dimethylaminopyridine (DMAP) to air resulted in ligand rearomatisation and the formation of dimeric μ(2)-η(2)-η(2)-peroxomagnesium compounds which have also been subject to analysis by single crystal X-ray diffraction analysis. An unsuccessful extension of this chemistry to N-heterocycle hydrosilylation is suggested to be a consequence of the low basicity of the silane reagent in comparison to the pyridine substrates which effectively impedes any further interaction with the magnesium centres.     

Quantitative evaluation of Lewis acidity of metal ions derived from the g values of ESR spectra of superoxide: metal ion complexes in relation to the promoting effects in electron transfer reactions

The g values of ESR spectra of superoxide-metal ion complexes (O2(*-)-Mn+, n = 1, 2, 3) are determined in acetonitrile at 143 K. The binding energies (deltaE) of metal ions with O2*- have been evaluated from deviation of the gzz values from the free spin value. The deltaE values are well correlated with the catalytic reactivities of metal ions in electron transfer from cobalt(II) tetraphenylporphyrin to O2 and p-benzoquinone, which does not occur in the absence of metal ions under otherwise the same experimental conditions. The deltaE values can thereby be used as the first quantitative measure for Lewis acidity of metal ions in relation with the catalytic reactivities in electron transfer reactions.     

Surface acid-base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides

In this paper, surface physiochemical properties of three typical aluminas, gamma-Al(OH)3, gamma-Al2O3, and alpha-Al2O3, were investigated by means of XRD, SEM, TEM, BET surface area, TG/DTA, and potentiometric titration techniques. Based on the titration data, surface protonation and deprotonation constants were determined using the constant capacitance model (CCM). The emphasis of this research was laid on the comparison of the crystal structure, surface hydration/dehydration and acid-base properties of these three typical alumina minerals. The calculation results revealed that the surface acidity of the aluminas is in the order of alpha-Al2O3>gamma-Al(OH)3>gamma-Al2O3 after being hydrated for 1 h. The correlation between the hydration/dehydration mechanisms of alumina and its acid/base properties is discussed.