Relations between structure, acidity, and activity of WOx/TiO2: influence of the initial state of the support, titanium oxyhydroxide, or titanium oxide

Two series of WO(x)/TiO(2) catalysts, containing W surface densities up to 4.4 W atoms/nm(2), were prepared by pore volume impregnation of two different supports, titanium oxyhydroxide (amorphous) or titanium oxide (crystallized, 100% anatase). The influence of W surface density and the nature of the support on the surface structure, development of the acidity, and catalytic performances were examined. The texture and structure of the catalysts were investigated by Brunauer-Emmett-Teller measurements, X-ray diffraction (XRD), and Raman and infrared spectroscopy. The catalytic activity was tested for 2-propanol dehydration and n-hexane isomerization. For catalysts obtained by impregnation of titanium oxide, XRD and Raman results showed that W was present as a surface phase. Infrared spectra indicated an increase in the degree of polymerization of W species with increasing W surface density. CO and lutidine adsorption, followed by infrared spectroscopy, showed an increase in the strength and abundance of Br?nsted acid sites (measured after lutidine desorption at 573 K) with the W surface density above a threshold of 1.3 W atoms/nm(2). The development of Br?nsted acidity correlated with the evolution of the infrared bands attributed to polymerized W species. A direct relationship was observed between the concentration of Br?nsted acid sites and the catalytic activity for 2-propanol dehydration. Catalytic activity, for n-hexane isomerization, appears to be associated with the presence of highly condensed W species. The catalysts synthesized by impregnation of titanium oxyhydroxide exhibited a comparable behavior. Hence, for a given W surface density, the W surface structure, concentration of Br?nsted acid sites, and catalytic performances were similar. Thus, no significant effect of the initial form of the support (titanium oxyhydroxide versus titanium oxide; 100% anatase) was evidenced.     

Correlations between acidity, surface structure, and catalytic activity of niobium oxide supported on zirconia

The development of the acidity and the relationship between acidity, catalytic activity, and the surface structure for niobium oxide supported on zirconia were investigated for a series of solids. The catalysts were active for 2-propanol dehydration only above a threshold in Nb loading. The acidity was studied by infrared spectroscopy of adsorbed 2,6-dimethylpyridine as a probe molecule, 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 also correlated with the catalytic activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric Nb surface species. These results were compared to those previously reported for the WO(x)/ZrO(2) catalysts.     

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.     

单斜及四方晶相ZrO2催化CO加氢反应性能的比较

研究了以纯单斜（ｍ）和四方（ｔ）晶相ＺｒＯ２为催化剂的ＣＯ加氢反应．尽管两种晶相催化剂均有较高的低碳烯烃的选择性,但是也发现了两者催化性能的显著差别．ｍ－ＺｒＯ２催化剂对异丁烯有突出的选择性,而ｔ－ＺｒＯ２催化剂,则只有乙烯和丙烯,几乎没有Ｃ４烯烃的选择性．室温下ＣＯ吸附的原位ＩＲ谱测试可见,只有在ｔ－ＺｒＯ２催化剂上观测到不可逆含氧吸附物种．吡啶吸附的Ｒａｍａｎ谱显示出它们之间表面性质的差别,ｍ－ＺｒＯ２催化剂表面存在等强度的Ｌｅｗｉｓ和Ｂｒｏｎｓｔｅｄ酸中心,而在ｔ－ＺｒＯ２催化剂表面几乎只有Ｂｒｏｎｓｔｅｄ酸中心．催化过程的一些模型分子电子结构计算也表明了ＺｒＯ２催化剂对低碳烯烃选择性内在的电子结构条件．我们推测ＣＯ在ｍ－ＺｒＯ２催化剂表面的孪式吸附物种可能是导致异丁烯产物的根源     

SO42-/ZrO2-Al2O3催化剂表面酸性质对正丁烷异构化反应性能的影响研究

采用“沉淀-浸渍”法制备一系列不同硫酸负载量的SO₄^2-/ZrO₂-Al₂O₃催化剂,利用N₂吸附-脱附、Py-FTIR、XRD等手段对催化剂进行表征。在常压、200 ℃、H₂：C₄=2：3和质量空速为3 h^-1的反应条件下,在固定床微型反应评价装置上考察了硫酸负载量对SO₄^2-/ZrO₂-Al₂O₃催化正丁烷异构化反应性能的影响。Py-FTIR结果表明,硫酸化处理为催化剂表面提供了丰富的Brønsted酸性位,其中,强Brønsted酸性位在正丁烷异构化反应中起重要作用,因此,硫酸化处理可显著提高正丁烷异构化活性,而Lewis酸性位与之没有直接关系。     

负载型ZrO2催化苯甲醛Meerwein-Ponndorf-Verley反应中的载体效应（英文）

采用浸渍法制备了Si-MCM-41和Al-MCM-41(Si/Al=50)介孔分子筛,SiO2,γ-Al2O3及MgO等负载的ZrO2催化剂,考察了其在以异丙醇为氢源苯甲醛Meerwein-Ponndorf-Verley(MPV)还原反应中的催化活性,并与纯ZrO2的催化活性进行对比.同时,采用X射线衍射、N2吸脱附法、X射线光电子能谱、紫外-可见漫反射光谱和吡啶原位吸附红外光谱等手段表征了催化剂.结果表明,ZrO2负载于Si-MCM-41,Al-MCM-41和SiO2后,催化活性明显提高,这归因于ZrO2与载体间存在强相互作用形成ZrOSi键,使催化剂表面ZrOH数量显著增多,Lewis酸中心强度增强,并出现Brnsted酸中心,三种催化剂的活性高低次序是5%ZrO2/Si-MCM-41>5%ZrO2/Al-MCM-41>5%ZrO2/SiO2.而5%ZrO2/Al2O3和5%ZrO2/MgO基本无催化活性,可归因为ZrO2与γ-Al2O3的弱相互作用使5%ZrO2/Al2O3的酸性与γ-Al2O3类似,ZrO2与MgO的强相互作用使5%ZrO2/MgO基本无酸性.     

Solvent-free preparation of nanosized sulfated zirconia with Brønsted acidic sites from a simple calcination

Nanosized sulfated zirconia with Br?nsted acidic sites has been prepared by a simple calcination in the absence of any solvent. XRD patterns reveal that the sulfated zirconia mainly consists of tetragonal crystalline zirconia with average size of about 7 nm, which is further confirmed by TEM images. N2 adsorption data show that the nanosized sulfated zirconia has high surface area (165-193 m2/g) and exhibits uniform pore distribution aggregated by zirconia nanoparticles. IR spectra of samples show that the sulfur species in the nanosized sulfated zirconia is a little different from that in conventional sulfated zirconia. Furthermore, IR spectra of adsorbed pyridine indicate that the nanosized sulfated zirconia contains relatively more Br?nsted acidic sites than conventional sulfated zirconia. Catalytic tests show that the nanosized sulfated zirconia exhibits much higher activity than conventional sulfated zirconia in catalytic esterification of cyclohexanol with acetic acid.     

Sn-Al-β分子筛酸性在葡萄糖转化反应中作用的固体NMR研究

制备了一系列具有不同酸性质的β分子筛催化剂, 通过固体核磁共振(NMR)探针分子技术对其酸性质进行了表征, 并考察了其催化葡萄糖转化为乙酰丙酸甲酯的性能. 吸附三甲基磷的³¹P NMR实验结果表明, 含有骨架Sn以及Al原子的Sn-Al-β催化剂同时具有Br?nsted与Lewis酸性. 通过2-¹³C-丙酮探针分子区分出 3种酸强度的Br?nsted酸位, 其中一种酸强度接近“超强酸”, 可能是由于空间邻近的Br?nsted酸位和Lewis酸位发生协同作用产生的. 葡萄糖转化为乙酰丙酸甲酯的催化反应结果表明, 相比于分别只含有Lewis酸位和Br?nsted酸位的Sn-β和Al-β样品以及两者的物理混合样品, Sn-Al-β分子筛催化剂具有高催化活性与产物选择性, 这主要是由于Br?nsted酸位和Lewis酸位的协同作用产生了强Br?nsted酸位, 这种强Br?nsted酸位进一步导致了更高的催化活性.     

A combined theoretical-experimental study on the acidity of WO(x)-ZrO(2) systems

This work provides a chemical approach to the relationship between structure and electronic behavior of the active surface of the WO(x)-ZrO(2) system as a function of W loads. This study shows that the electronic hardness (eta), the Lewis and Br?nsted acidity are functions of the local coordination and of the polymerization degree of the WO(x) domain. From theoretical calculations the observed behavior in the WO(x)-ZrO(2) system is explained: the Br?nsted acidity increases while the Lewis acidity decreases as the W centers go from tetrahedral to octahedral coordination and as the condensation degree of the WO(x) domain increases. Our results also indicate that not all the Br?nsted sites in the WO(x) domains are equally acid, and that as the W load increases the most acid sites decrease in number due to the condensation process. This finding also means a decrease on the average acidity per H site. Additionally, our results suggest that for surface densities in the 4-7 W nm(-2) range, mainly dimeric-tungstate species are present. A maximum in Br?nsted acidity was observed for a W surface density about 7 W nm(-2).     

Metal-reinforced sulfonic-acid-modified zirconia for the removal of trace olefins from aromatics

Metal-reinforced sulfonic-acid-modified zirconia catalysts were successfully prepared and used to remove trace olefins from aromatics in a fixed-bed reactor. Catalysts were characterized by ICP-OES, N₂ adsorption–desorption, X-ray diffraction, thermogravimetric analysis (TGA), and pyridine-FTIR spectroscopy. Different metals and calcination temperatures had great influence on the catalytic activity. Alumina-reinforced sulfated zirconia exhibited outstanding catalytic performance, stable regeneration activity, and giant surface area, and are promising in industrial catalysis. TGA showed that the decomposition of methyl could be attributed to Brønsted acid sites, and pyridine-FTIR spectroscopy proved the weak Brønsted sites on these synthesized metal-reinforced sulfated zirconia. Also, a relation between the reaction rate and weak Brønsted acid density is proposed.     

热处理过程中磷钨杂多酸修饰ZrO2气凝胶固体酸的结构及性能演变

基于N₂物理吸附、X射线衍射(XRD)、傅里叶变换红外(FTIR)光谱、吸附吡啶红外(Py-FTIR)光谱、氨程序升温脱附(NH₃-TPD)等物理化学表征及催化四氢呋喃(THF)聚合反应实验结果, 探讨了120-750 °C空气气氛热处理对磷钨杂多酸(TPA)修饰ZrO₂气凝胶(ZA)(TPZA)固体酸结构、酸性质及酸催化性能的影响. 研究发现, 对于固载量(TPA质量分数)为25%的TPZA材料, 随热处理温度升高, ZrO₂表面发生了Keggin-杂多阴离子结构畸变、杂多阴离子分解、低聚合度表面多钨酸根物种形成以及多钨酸根物种沿载体表面二维方向聚合的钨物种演变过程; 材料相应地经历了其Brönsted酸性及酸催化活性失而复得的变化. 在实验考察范围内, 材料明显的Brönsted酸性及催化THF聚合反应活性发生在120-300 °C及550-750 °C两个热处理温度段. 其间, 材料的Brönsted酸性分别源于与Keggin-杂多阴离子及在载体表面二维方向上具有一定聚合度的聚钨物种相联系的酸性质子. 在350-450 °C处理温度范围内, 绝大多数Keggin-杂多阴离子的分解及钨物种沿载体表面方向聚合度的不足使得样品中难以形成明显数量的Brönsted酸中心. 此外, 在实验考察的整个温度范围内, 材料显示明显的Lewis酸性, 这应该与样品表面存在配位不饱和的Zr⁴⁺阳离子及磷的氧化物有关.     

苯乙酮氢转移反应中ZrO2/MCM-41和ZrO2/AC的催化性能对比

采用浸渍法制备了分别以活性炭(AC)和全硅MCM-41介孔分子筛负载的ZrO2催化剂,并对其进行了XRD、氮气吸附-脱附、X射线光电子能谱、差热-热重分析和吡啶吸附原位红外光谱等表征,考察了其在以异丙醇为氢源还原苯乙酮为α-苯乙醇的Meerwein-Ponndorf-Verley(MPV)反应中的催化活性,并与水合ZrO2进行对比.研究了载体对催化剂活性的影响.结果表明,ZrO2经MCM-41负载后,与载体发生强相互作用,可能形成Si—O—Zr键,ZrO2在载体表面呈高分散的无定形态,Zr—OH数目显著增加,L酸性增强,并形成B酸中心,使催化剂活性显著高于水合ZrO2;ZrO2负载在AC上后,与载体未发生强相互作用,ZrO2在载体表面未呈高分散状态,增加的Zr—OH数目相对较少,L酸性较弱,未形成B酸中心,催化活性未明显增加,但在较高焙烧温度(400～600℃)下,其仍能保持稳定的催化活性,这可归因于ZrO2/AC中AC孔道疏通及AC石墨层对苯乙酮上苯环的吸附作用,使活性位附近的反应底物浓度显著增大.     

Fourier transform infrared spectroscopic study of surface acidity by pyridine adsorption on Mo/ZrO2-SiO2(Al2O3) catalysts

Acidity of the oxidic molybdenum catalysts supported on mixed ZrO2-SiO2 and ZrO2-Al2O3 carriers was investigated by Fourier transform infrared spectroscopy of pyridine adsorption. Deposition of molybdenum on ZrO2-SiO2 and ZrO2-Al2O3 supports leads to formation of surface Br?nsted acid sites. The number of the Br?nsted and Lewis acid sites in supported-molybdenum catalysts depends on both the ZrO2 content and the type of the support. With increasing ZrO2 content, the Lewis acid sites increase for both series of catalysts. The Br?nsted acid sites are higher for Mo/ZrO2-SiO2 samples compared to those for Mo/ZrO2-Al2O3 and also increase with zirconia.     

Influence of synthesis methods on tungsten dispersion, structural deformation, and surface acidity in binary WO3-ZrO2 system

WO3-ZrO2 catalysts were synthesized by precipitating the aqueous solutions of zirconium oxynitrate and ammonium metatungstate with ammonium hydroxide. The white slurry precipitate was treated under three different conditions. In the as-made materials, the amorphous phase was formed in the aged and refluxed samples, while well-crystallized tetragonal and monoclinic phases were obtained in the hydrothermally treated sample. The real amount of tungsten loaded in the samples was similar for the three samples, independently of the treatments; however, the tungsten surface atomic density in the annealed WO3-ZrO2 samples varied between 6 and 9 W atoms/nm2. Two different contrast types of aggregates were determined by scanning electron microscopy, the white particles which are rich in W, and the gray ones which are rich in zirconium; both of them were formed in the calcined solids prepared under aging or reflux condition. A very high dispersion of tungsten species on the zirconia surface was achieved in the hydrothermally treated sample. The degree of the interaction between WO(x) and ZrO2 surface strongly modified the Zr-O bond lengths and bond angles in the structure of tetragonal zirconia as proved by X-ray diffraction analysis and the Rietveld refinement. The catalyst obtained under hydrothermal condition exhibited the highest dispersion of tungsten species in the zirconia, which in turn causes strong structural deformation of the tetragonal ZrO2 phase responsible of the strongest surface acidity and, consequently, the optimum catalytic activity for n-hexane isomerization.     

Effect of Brønsted/Lewis Acid Ratio on Conversion of Sugars to 5‐Hydroxymethylfurfural over Mesoporous Nb and Nb‐W Oxides

A series of mesoporous Nb and Nb‐W oxides were employed as highly active solid acid catalysts for the conversion of glucose to 5‐hydroxymethylfurfural (HMF ). The results of solid state ³¹P MAS NMR spectroscopy with adsorbed trimethylphosphine as probe molecule show that the addition of W in niobium oxide increases the number of Brønsted acid sites and decreases the number of Lewis acid sites. The catalytic performance for Nb‐W oxides varied with the ratio of Brønsted to Lewis acid sites and high glucose conversion was observed over Nb₅W₅ and Nb₇W₃ oxides with high ratios of Brønsted to Lewis acid sites. All Nb‐W oxides show a relatively high selectivity of HMF , whereas no HMF forms over sulfuric acid due to its pure Brønsted acidity. The results indicate fast isomerization of glucose to fructose over Lewis acid sites followed by dehydration of fructose to HMF over Brønsted acid sites. Moreover, comparing to the reaction occurred in aqueous media, the 2‐butanol/H₂O system enhances the HMF selectivity and stabilizes the activity of the catalysts which gives the highest HMF selectivity of 52% over Nb₇W₃ oxide. The 2‐butanol/H₂O catalytic system can also be employed in conversion of sucrose, achieving HMF selectivity of 46% over Nb₅W₅ oxide.     

Fast and selective sugar conversion to alkyl lactate and lactic acid with bifunctional carbon-silica catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Br?nsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Br?nsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Br?nsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Br?nsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Br?nsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.     

载体对Pt/WO_3-ZrO_2催化临氢异构反应性能的影响

以平衡吸附过氧钨酸的水合氧化锆为前驱体,经焙烧得到WO_3-ZrO_2固体酸,并采用XRD、UV-vis、NH_3-TPD等手段考察了过氧钨酸吸附液浓度及焙烧温度对WO_3-ZrO_2固体酸组成、结构及酸性的影响。通过BET、H_2-TPR、H_2-TPD等表征手段和正戊烷临氢异构反应,考察了负载铂后相应催化剂的结构、还原与氢吸附性质及其催化正戊烷临氢异构反应的性能。结果表明,焙烧温度为700℃时,随着吸附液浓度的增加,所得载体酸度及相应催化剂比表面积均先增加后减小,且在吸附液浓度为82 mmol W/L时达到最大值。吸附液浓度为59 mmol W/L时,随着焙烧温度的升高,所得载体四方相氧化锆含量、酸度及相应催化剂比表面积均降低。吸附液浓度为82 mmol W/L、焙烧温度为700℃所得载体负载0.5%(质量分数)铂后催化活性最高。该催化剂在250℃常压临氢操作、n(H_2)/n(n-C_5H_(12))为3、WHSV为1.0 h~(-1)的条件下,催化正戊烷异构反应中异戊烷收率可达57.7%。     

Structure and catalytic properties of sulfated zirconia foams

Porous sulfated zirconia foams were manufactured by a simple methodology based on the sol–gel process combined with a liquid foam template that used a surfactant mixture. A block copolymer (Pluronic F-127) and an anionic surfactant [sodium dodecylsulfate (SDS)] were mixed in different proportions in order to optimize the porous and surface properties of the ceramic material. By adjusting the SDS/Pluronic ratio, it was possible to obtain sulfated zirconia with a combination of macropores and mesopores that provided high porosity (≈90 %) and surface area (≈80 m² g^?1). The sulfate groups linked to the zirconia surface stabilized the tetragonal phase, to the detriment of the thermodynamically stable monoclinic phase. The sulfate groups and the tetragonal phase decreased as a function of the amount of SDS in the liquid foam template. The combined porous and structural characteristics, together with surface acidity, provided enhanced catalytic activity when the sulfated zirconia foams were employed in the isopropanol dehydration reaction. A further benefit was the selectivity towards propene and negligible formation of acetone.     

The Effect of Sulfate Ion on the Isomerization of n-Butane to iso-Butane

The effect of sulfate ion (SO42-) loading on the properties of Pt/SO42- -ZrO2 and on the catalytic isomerization of n-butane to iso-butane was studied. The catalyst was prepared by impregnation of Zr(OH)4 with H2SO4 and platinum solution followed by calcination at 600℃. Ammonia TPD and FT-IR were used to confirm the distribution of acid sites and the structure of the sulfate species. Nitrogen physisorption and X-ray diffraction were used to confirm the physical structures of Pt/SO42--ZrO2. XRD pattern showed that the presence of sulfate ion stabilized the metastable tetragonal phase of zirconia and hindered the transition of amorphous phase to monoclinic phase of zirconia. Ammonia TPD profiles indicated the distributions of weak and medium acid sites observed on 0.1 N and 1.0 N sulfate in the loaded catalysts. The addition of 2.0 N and 4.0 N sulfate ion generated strong acid site and decreased the weak and medium acid sites. However, the XRD results and the specific surface area of the catalysts indicated that the excessive amount of sulfate ion collapsed the structure of the catalyst. The catalysts showed high activity and stability for isomerization of n-butane to iso-butane at 200℃under hydrogen atmosphere. The conversion of n-butane to iso-butane per specific surface area of the catalyst increased with the increasing amount of sulfate ion owing to the existence of the bidentate sulfate and/or polynucleic sulfate species ((ZrO)2SO2), which acts as an active site for the isomerization.     

以水热法合成的水合氧化锆为载体的Pt／WO3-ZrO2上的正己烷异构化

通过水热法合成了一系列水合氧化锆,以之为载体采用浸渍法制备了Pt/WO3-ZrO2催化剂,考察了氢氧化锆的水热温度对Pt/WO3-ZrO2异构化活性的影响.通过X射线衍射、NH3程序升温脱附及H2程序升温还原表征了样品的晶相结构、酸性及还原性能.结果表明,水合氧化锆及以此为载体的催化剂的晶相结构均随着氢氧化锆水热温度的变化而变化,水热温度升高,四方相氧化锆比例下降.具有一定晶相结构的水合氧化锆为载体的催化剂具有较无定形氢氧化锆为载体的催化剂更多的强酸中心和更高的异构化催化活性.高的异构化活性可能与催化剂上更多的强酸中心有关。